Development of a Stamping Unit and Analysis of the Heating Process for the Preforming of Textile Materials

Universidad Politécnica de Madrid Escuela Superior de Ingenieros Industriales

Cátedra de Proyectos

Proyecto final de carrera

Nº: 16408318

Marina Moya Sánchez

Nº matrícula: 08318

Marzo 2016

José Rios Chueco

Prof. Dr.-Ing. Jürgen Fleischer

RESUMEN Inmersa en una sociedad cada vez más consciente de la necesidad de preservar el medio ambiente, la industria de la automoción se enfrenta al reto de reducir las emisiones de CO2 de sus vehículos. La estrategia con la que afrontar este problema difiere de unas empresas a otras, si bien todas ellas coinciden en que es necesaria una reducción del peso de los vehículos para reducir el consumo de combustible, y con ello las emisiones a la atmósfera. Esta necesidad de reducir el peso total está guiando a las empresas automovilísticas hacia la utilización de materiales compuestos, y más concretamente a la utilización de materiales compuestos reforzados con fibra de carbono (CFRP, del inglés Carbon FIber Reinforced Polymers). Junto con el aluminio, los CFRP se están posicionando como los materiales de los vehículos del futuro, debido a sus buenas propiedades mecánicas acompañadas de una reducción de peso de más de un 50%, en comparación con los materiales actuales. Desde un enfoque de investigación, el wbk – Instituto de tecnología de la Producción de la Universidad de Karlsruhe – está desarrollando una cadena de producción totalmente automatizada de componentes de fibra de carbono mediante el proceso de inyección de resina (RTM). Una de las etapas fundamentales de la cadena de fabricación por RTM es la etapa de preformado. El preformado es el proceso que tiene lugar antes de la inyección de resina, por el cual el conjunto de telas que van a componer el refuerzo del material compuesto pasan de su estado bidimensional a su estado tridimensional, adquiriendo una forma muy próxima a la de la pieza definitiva. Este proceso de preformado se lleva a cabo en la estación del wbk por estampación. Una resina aglutinante se aplica a las telas previamente al proceso de preformado, para garantizar la inmovilidad de las fibras durante el preformado y la inyección, así como para facilitar su manipulación. El producto final obtenido del proceso de preformado es la preforma, que se introducirá en la estación de RTM para la inyección de resina. La precisión de la forma definitiva y la inexistencia de defectos en la superficie tras el preformado tienen un impacto directo en la calidad final del componente de fibra de carbono. El objetivo final es obtener una etapa de preformado totalmente automatizada que garantice la calidad óptima de la preforma. Esta tesis se centra en el diseño de una nueva unidad de estampación para la estación de preformado del wbk que mejore los resultados obtenidos en la actual estación.

Tabla de Contenidos

1 Presentación de la Universidad de Acogida 1 1.1. Karlsruhe, Alemania 1 1.2. Karlsruher Institut für Technologie (KIT) 2 1.3. wbk, Instituto de Tecnología de la Producción 2 2 Introducción 3 2.1. Motivación 3 2.2. Objetivo 5 3 Fundamentos 6 3.1. Carbon Fiber Reinforced Polymers (CFRP) 6 3.2. Fabricación de componentes de fibra de carbono 6 3.3. Resin Transfer Molding Process (RTM) 7 4 Estado del Arte 8 5 Metodología 12 6 Diseño de la Unidad de Estampación de la Estación de Preformado del wbk 13 6.1. Análisis del proceso de preformado y aislamiento de problemas 13 6.2. Propuesta y selección de alternativas 15 6.3. Descripción y evaluación del diseño 18 6.3.1 Unidad de estampación 18 6.3.2 Mesa de la preforma 20 6.3.3 Marco exterior 21 7 Validación del Diseño 22 7.1. Análisis térmico del proceso de preformado 22 7.1.1 Descripción de los elementos implicados en el análisis térmico 23 7.1.2 Elaboración del modelo y simulación 24 7.1.3 Resultado del análisis térmico 25 7.2. Presupuesto de diseño 27 8 Conclusiones 28 9 Proyecto Original Anexo I. Estimación de costes de diseño, fabricación y montaje Anexo II. Elementos Análisis Térmico

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Presentación de la Universidad de Acogida

1 Presentación de la Universidad de Acogida 1.1. Karlsruhe, Alemania Karlsruhe es la segunda ciudad más grande del estado de Baden-Wurtemberg en el suroeste de Alemania. Cuenta con una población de aproximadamente 300.000 habitantes y, pese a su tamaño medio, es sede de importantes instituciones para la vida pública alemana, como el Bundesverfassungsgericht o Tribunal Constitucional Federal Alemán, y una de las cinco Casas de Moneda alemanas, conocida como Staatliche Münze. Karlsruhe está situada en el corazón de Europa; a 15km de la frontera con Francia, 80km al norte de Estrasburgo, 63 km al oeste de Stuttgart y a 125 km al sur de Frankfurt. Dispone de una amplia red ferroviaria con buenas conexiones con grandes ciudades europeas cercanas (Basilea, Hamburgo, Estrasburgo, Lyon). También dispone de su propio aeropuerto regional (Baden-Airpark) así como de un importante puerto de mercancías en el Rhin, con más de 5,52 millones de toneladas transportadas por año. Además de la Universidad Técnica de Karlsruhe (KIT), la ciudad cuenta con la Universidad Internacional de Karl, la Universidad Tecnológica y Económica de Karlsruhe, diversas academias y escuelas de arte y múltiples institutos y centros de investigación, que la convierten en un referente en educación e investigación a nivel nacional e internacional.

Figura 1 Palacio de Karlsruhe, Alemania

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Presentación de la Universidad de Acogida

1.2. Karlsruher Institut für Technologie (KIT) El Karlsruhe Institue of Technology (KIT) es una de las más grandes y prestigiosas instituciones educativas y académicas en Alemania, reconocida mundialmente por su labor de investigación en el campo de la tecnología avanzada. El KIT fue creado en 2009, cuando la Universidad de Karlsruhe – fundada en 1825 como Universidad Pública centrada en la investigación – se fusionó con el Centro de Investigación de Karlsruhe (Forschungszentrum Karlsurhe), creado en 1956 como Centro Nacional de Investigaciones Nucleares. El KIT forma parte del “TU9” desde 2006, iniciativa alemana de excelencia universitaria, orientada a la promoción de la investigación científica. El KIT es un Centro de Excelencia Educativa con impecable reputación tanto en Alemania como en el extranjero. Ocupa el primer puesto a nivel nacional entre las Escuelas de Ingeniería y Ciencias Naturales, encontrándose entre las diez primeras a nivel europeo. El KIT destaca por la calidad de sus proyectos de investigación y sus tesis. Con más de 24.700 estudiantes (45% estudiantes de ingeniería, 32% estudiantes de matemáticas y ciencias naturales) así como con más de 9.400 empleados (63% investigadores), y con un presupuesto de 844 millones de euros, el KIT goza de una gran reputación en ingeniería industrial, física, ingeniería mecánica, ingeniería eléctrica, informática y economía de la información. 1.3. wbk, Instituto de Tecnología de la Producción El wbk, Institut für Produktionstechnik (Instituto de Tecnología de la Producción), con cerca de 100 empleados, es uno de los institutos más grandes de la facultad de Ingeniería Mecánica del KIT. Sus tres departamentos, Fabricación e Ingeniería de Materiales, Máquinas, Equipamiento y Automatización de Procesos y Sistemas de Producción, tienen como bases fundamentales la investigación, la educación y la innovación tecnológica. Además de su labor de investigación en las áreas convencionales de ingeniería mecánica y de planta, el wbk pone especial énfasis en el desarrollo de tecnología de producción tales como la ingeniería energética, la movilidad eléctrica y el estudio, desarrollo y producción de materiales ligeros como los compuestos de fibra de carbono o vidrio. Por una parte, en el instituto se estudia la aplicación de sistemas de producción convencionales a los nuevos materiales y por otra, se estudian y desarrollan nuevos procesos, con el objetivo de asentar las bases de la tecnología de producción del futuro. El wbk ofrece, tanto a sus trabajadores como a sus estudiantes, unas condiciones de trabajo excelentes ya sea para estudios teóricos o experimentales.

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Introducción

2 Introducción 2.1. Motivación Durante los últimos años, la evolución de la demanda de fibra de carbono ha experimentado un crecimiento continuo y constante. Según datos del Composites Market Report 2014, entre 2009 y 2013 la demanda de fibra de carbono creció desde las 26.500 a las 46.500 toneladas. La mayor parte de la fibra de carbono producida mundialmente se emplea, en combinación con una matriz aglutinante, para la fabricación de compuestos de fibra de carbono. Así pues, una tendencia similar a la observada en la demanda de fibra de carbono puede observarse en la demanda de compuestos de fibra de carbono (CRP, del inglés, Carbon Reinforced Polymers). La Figura 2 muestra la evolución de la demanda global de CRP en toneladas desde 2008, y la evolución prevista para los próximos años. Se estima que este crecimiento se mantenga durante los próximos años, pudiéndose prever una demanda global de compuestos de fibra de carbono de 140 toneladas para el año 2020.

Figura 2 Demanda global de CRP en 1000 toneladas entre 2008 y 2020 (*estimado) [CMR- 14]

En 2014 la Unión Europea aprobó la normativa que obliga a los países miembros a reducir las emisiones de CO2 de los vehículos desde los actuales 130 g CO2/km a 95 g CO2/km. Otros países, como Estados Unidos o Japón, también están tomando medidas que tienen por finalidad la reducción de las emisiones de CO2. Estas medidas suponen un gran reto para la industria automovilística mundial, que se encuentra actualmente en la necesidad de reinventarse y encontrar alternativas a sus actuales diseños para reducir las emisiones de sus vehículos. Las estrategias que las compañías automovilísticas planean seguir para alcanzar estos niveles de emisión son muy variadas y dependen del tipo de vehículo y la compañía en cuestión. En la Figura 3 se muestra un resumen de estas estrategias. Como se puede observar, la principal medida a tomar es la mejora de la eficiencia de los motores empleados. En segunda posición se encuentra la

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Introducción reducción del peso total de los vehículos, de forma que menos combustible sea necesario para la propulsión de los mismos. Con este objetivo de generalizar la producción de vehículos de peso reducido, el aluminio y los compuestos de fibra de carbono se están posicionando como los materiales de los coches del futuro.

Figura 3 Estrategias para el cumplimento de la normativa europea de emisiones [CMR-14]

El motivo por el que la industria de los materiales compuestos no se ha hecho un hueco todavía en el sector automovilístico es la falta de automatización en los procesos de fabricación de componentes de fibra de carbono y, consecuentemente, el elevado coste que supone su introducción. De ahí que uno de los objetivos principales, y también uno de los mayores retos, de las compañías automovilísticas hoy en día sea el desarrollo de una cadena de producción de componentes de fibra de carbono totalmente automatizada. Junto con la necesidad de automatizar el proceso y reducir los costes de fabricación, se encuentra la necesidad de desarrollar una tecnología capaz de fabricar componentes sin ningún tipo de restricción de tamaño o geometría. Es en este contexto en el que el wbk – Institute of Production Science, en el Karlsruher Institute für Technologie (Karlsruhe, Alemania), está desarrollando su cadena de producción completamente automatizada para la fabricación de componentes de fibra de carbono. El proceso que está siendo desarrollado en el wbk se centra en la fabricación de componentes mediante el proceso de fabricación de inyección de resina RTM (Resin Transfer Molding). Dadas las necesidades actuales del mercado automovilístico, y la falta de desarrollo y automatización de la tecnología existente, la optimización de cada etapa del proceso de fabricación supone un paso hacia delante en la búsqueda del objetivo fundamental de incluir la producción en masa de componentes de fibra de carbono en la industria automovilística.

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Introducción

2.2. Objetivo Tomando como referencia la unidad de estampación que se encuentra actualmente en la estación de preformado para materiales compuestos del taller del wbk, el objetivo de este proyecto es analizar el proceso de preformado, identificar las variables que podrían ser mejoradas y proponer un nuevo diseño de la unidad de estampación que suponga una mejora en la calidad de las preformas obtenidas, así como una reducción de los tiempos de fabricación. Como segundo objetivo de este proyecto se busca la validación, tanto técnica como económica, del diseño propuesto, para que en proyectos futuros se profundice en su desarrollo y se proceda a su fabricación y posterior montaje en la actual estación de preformado. Un tercer objetivo surge durante el desarrollo del proyecto que se presenta, e implica la simulación de los procesos térmicos que tienen lugar durante el proceso de preformado, para estudiar la viabilidad de la utilización del diseño propuesto en el presente proyecto.

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Fundamentos

3 Fundamentos 3.1. Carbon Fiber Reinforced Polymers (CFRP) El conjunto de todos los materiales a disposición del ser humano se puede dividir en cuatro grandes grupos: materiales metálicos, poliméricos, cerámicos y materiales compuestos. Estos últimos son combinaciones de los tres primeros, y dan como resultado materiales con mejores cualidades que sus componentes empleados de manera individual. Al contrario que las aleaciones metálicas, cada componente que conforma un material compuesto mantiene sus características químicas, físicas y mecánicas, lo que permite a la ciencia desarrollar materiales con las propiedades óptimas para el trabajo específico para el que están pensados. Dos fases conforman el seno de un material compuesto: una fase continua, la matriz, y una fase discontinua, el refuerzo. La matriz de un material compuesto es la fase continua del compuesto y puede ser un polímero, un metal o una cerámica. La matriz aporta las propiedades físicas y químicas del material, y se encarga de funciones imprescindibles como son el mantenimiento de las fibras en la posición adecuada y la protección frente a la abrasión y las condiciones externas. Por su parte, el material de refuerzo tiene carácter discreto y le aporta al compuesto las propiedades mecánicas. En la mayor parte de las ocasiones, el material de refuerzo es más duro, más resistente y más rígido que la matriz. El refuerzo puede encontrarse en forma de fibras – ya sean continuas o discontinuas – o en forma de partículas. Los materiales empleados como refuerzo son, en su mayor parte, fibras de vidrio, carbono, aramida o boro. Dentro del gran abanico de posibilidades que ofrecen los materiales compuestos, ocupan los materiales poliméricos reforzados con fibra de carbono un lugar privilegiado. Los materiales poliméricos reforzados con fibra de carbono – desde ahora CFRP, del inglés Carbon Fiber Reinforced Polymers – son materiales compuestos en los que un polímero – en la mayor parte de las ocasiones termoestable – desempeña las funciones de matriz, mientras que fibras de carbono tejidas o trenzadas previamente en forma de capas de tejido constituyen el refuerzo. El polímero empleado habitualmente es la resina epoxi, aunque en ocasiones pueden emplearse también termoplásticos como el poliéster o el viniléster. Sin embargo, el uso de estos materiales está hoy en día limitado por el elevado coste que supone para altos volúmenes de producción. 3.2. Fabricación de componentes de fibra de carbono Como se ha indicado previamente, el refuerzo de fibras de carbono se dispone en el seno del material compuesto en forma de capas de tela. Las fibras se entretejen mediante diversos tipos de procesado (woving, brading, knitting o stitching), dando lugar a los tejidos precursores que una vez superpuestos y

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Fundamentos preformados se inyectan de resina para dar lugar al compuesto definitivo. La orientación y disposición de las fibras en el compuesto definen los distintos tipos de CFRP. La selección de telas necesarias para la elaboración de un CFRP depende de las características de la pieza que ha de ser fabricada y la función que va a desempeñar. La variedad de telas disponibles es directamente proporcional a la variedad de requisitos que de ellas se exige. Así, las propiedades de las distintas telas de fibra de carbono varían en función del patrón de tejido, de la cantidad y diámetro de las fibras, de la orientación o incluso del refuerzo con otros materiales, como la fibra de vidrio. Como característica común, todas las telas deben ser lo suficientemente estables como para ser manipuladas, cortadas y transportadas al molde, así como lo suficientemente plegables para adaptarse al contorno y forma del mismo. La fabricación de componentes de fibra de carbono es un campo muy amplio en el que existe una gran variedad de alternativas. Como primera clasificación, los métodos de fabricación se pueden dividir en dos grandes categorías: procesos en molde abierto, y procesos en molde cerrado. La metodología de molde abierto lleva desde hace ya tiempo establecida en la industria de compuestos de matriz polimérica. La principal ventaja de este tipo de procesos es la simplicidad y la viabilidad económica, pero tienen el inconveniente de ser fundamentalmente procesos manuales, que requieren de operarios especializados e implican bajos niveles de producción. Por otra parte, los métodos de molde cerrado se refieren a los procesos en los que la pieza es fabricada en el interior de la cavidad de un molde formado por dos o más herramientas (macho y hembra). Dentro de este tipo de procesos, encontramos dos categorías fundamentales: los procesos de moldeado por compresión, y los procesos de moldeado por inyección. El uso de los primeros solo está justificado por la necesidad de grandes niveles de producción (en torno a los cientos de miles), debido a su elevado coste. La alternativa son los procesos de moldeado por inyección de líquido. Entre estos últimos, tres procesos son de especial relevancia: el RIM (Reaction Inyection Molding), SRIM (Structural Reaction Inyection Molding) y el RTM (Resin Transfer Molding). 3.3. Resin Transfer Molding Process (RTM) La mejor de las alternativas entre los métodos de molde cerrado es el método de trasferencia de resina (RTM, del inglés Resin Transfer Molding). El proceso de inyección de resina tiene lugar después de colocar el material de refuerzo (las telas de fibra de carbono) sobre el molde. El proceso RTM produce componentes con dureza, resistencia y peso próximos a los obtenidos en procesos de molde abierto. Los tiempos de ciclo varían entre unos pocos minutos para piezas pequeñas, y varias horas para piezas de gran tamaño. El proceso RTM está representado en la Figura 4 y consiste en la inyección bajo presión (en torno a los 700 kPa) de la resina en la cavidad del molde, una vez éste se ha cerrado con el material de refuerzo ya en su interior. Tanto las

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Estado del Arte herramientas del molde como la resina se precalientan antes de la inyección. Cuando la inyección ha finalizado, se incrementa la temperatura del molde para curar la resina. En muchas ocasiones, se emplea un sistema de generación de vacío para eliminar el aire atrapado en el molde y acelerar el proceso. El grosor medio de componentes elaborados mediante el proceso RTM es de 4mm, con un contenido del 30% de fibra de carbono.

Figura 4 Proceso de fabricación RTM [AVK-10] 4 Estado del Arte Actualmente ya existen vehículos cuya carrocería o algunos de sus componentes están siendo fabricados en fibra de carbono, como es el caso de los modelos i3 e i8 de BMW, en los que el compartimento de pasajeros está hecho enteramente de fibra de carbono, el Chevrolet Corvette Stingway, que tiene su capó hecho en fibra de carbono, o el Tejin Concept Car, que, pese a ser por ahora un prototipo, tiene una carrocería de 47 kg de peso fabricada completamente en fibra de carbono. Uno de los factores que está haciendo que, sin embargo, estos casos sigan siendo aislados, es la dificultad de obtener una preforma de calidad para la etapa posterior de inyección de la resina. El preformado es el proceso que tiene lugar antes de la inyección de resina, por el cual el conjunto de telas que van a componer el refuerzo del material compuesto pasan de su estado bidimensional a su estado tridimensional, adquiriendo una forma muy próxima a la de la pieza definitiva. Los métodos de preformado actuales se dividen entre métodos de preformado directo, y métodos de preformado secuencial. El preformado directo incluye aquellos procesos que requieren una única etapa para la obtención de la preforma definitiva, partiendo directamente de las fibras. El mayor inconveniente de este tipo de procesos deriva de su poca adaptabilidad a formas complejas y grandes dimensiones, reduciéndose su utilización a piezas axisimétricas.

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Estado del Arte

Por otra parte, el preformado secuencial es aquel que requiere al menos dos etapas para la obtención de la preforma definitiva. El primero de los pasos implica la fabricación de las telas de fibra, que en pasos posteriores se moldea a su forma definitiva a través de diferentes métodos como pueden ser el método de cilindro, diafragma o estampación. Para la fabricación de componentes de formas complejas, los procesos de preformado secuencial han demostrado ser más apropiados Diferentes líneas de investigación están siendo seguidas en universidades y centros de investigación del mundo para optimizar los actuales procesos de preformado. Ejemplos de instituciones que están conformando el actual estado del arte del preformando y de la automatización del proceso de fabricación de componentes de fibra de carbono son la empresa norteamericana GFM, el Instituto de Tecnología Textil (ITA) de la universidad RWTH de Aachen (Alemania) o el Instituto Fraunhofer de Tecnología Química (ICT, Pfinztal, Alemania) en colaboración con la Universidad de Ontario del Oeste (Canadá). En el Instituto de Ciencias de la Producción (wbk), en la Universidad Técnica de Karlsruhe (Alemania), se desarrolla en la actualidad otra cadena totalmente automatizada de fabricación de componentes de fibra de carbono basada en el RTM, que conforma también al actual estado del arte de la fabricación de componentes de fibra de carbono. El taller de wbk cuenta con una cadena completa de fabricación RTM, con una estación de preformado enteramente diseñada en el instituto que permite el preformado tanto por diafragma como por estampación, aunque estudios recientes han llevado a considerar el método de estampación como el método óptimo para la fabricación de preformas. Este método es un método ideal para el preformado de componentes de no muy elevado espesor y formas complejas, compatible con altos volúmenes de fabricación. Una característica propia del proceso de preformado del wbk es su proceso de estampación secuencial. Esto es, el molde superior de la preforma, en lugar de descender entero sobre el molde inferior, está subdividido en componentes independientes que cierran el molde de manera gradual, lo que busca asemejar el preformado al preformado manual que llevaría cabo una persona. Lo que se quiere conseguir con este preformado secuencial es disminuir las arrugas que se generan en la preforma al descender el molde en bloque. El preformado en el wbk comienza con el corte del material (que llega al taller en forma de rollos de tela) en partes más pequeñas. Una vez el material se ha cortado en sus dimensiones finales, se apilan varias capas de material que le aportarán el espesor al componente. Tras esto, se le aplica a las telas una capa de material aglutinante, que fija las fibras y evita alteraciones de su orientación y posición durante el proceso. Posteriormente, el conjunto de material se transporta a la mesa de calentamiento, donde se calientan las telas hasta una temperatura superior a la temperatura de activación del aglutinante. Es de gran importancia

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Estado del Arte que en todo lo que queda de proceso, hasta que la preforma está totalmente constituida, la temperatura de las fibras no descienda por debajo de la temperatura de activación del aglutinante, ya que esto supondría alteraciones y defectos en la calidad final de la preforma. Una vez calentado el conjunto de material, éste es transportado al área de carga de la estación de preformado, donde se colocan sobre la herramienta macho del molde. Para evitar un enfriamiento rápido del material, la mesa de la preforma ha sido previamente calentada y se mantiene a una temperatura constante de 50ºC. La mesa de la preforma se mueve a lo largo de la estación de preformado hasta el área de estampación de la misma, donde se detiene. A continuación tiene lugar el cierre secuencial del molde. Tras el estampado, el molde permanece cerrado el tiempo suficiente como para que la temperatura de las telas vuelva a establecerse por debajo de la temperatura de activación del molde, y a continuación se retiran las herramientas del molde superior para proceder al control de calidad de la preforma generada. Tras el control de calidad, la preforma definitiva se lleva a la unidad de RTM.

Figura 5 Proceso de estampación secuencial instalado en el wbk

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Estado del Arte

Figura 6 Estación de preformado en el wbk (Karlsruhe, Alemania)

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Metodología

5 Metodología Para analizar de manera más organizada el proceso de preformado, y con ello facilitar la identificación de requisitos, necesidades y funciones a optimizar, se va a trabajar siguiendo la metodología de resolución SPALTEN. El método STALPEN es un método de resolución de problemas desarrollado por el Instituto de Desarrollo de Productos (IPEK) de la universidad técnica de Karlsruhe. El término SPALTEN es un acrónimo alemán cuyas siglas representan cada uno de los pasos a seguir durante la resolución de un problema de cualquier índole, aunque está especialmente encaminado a la resolución de problemas de ingeniería. Cada una de las etapas de dicho proceso se encuentra reflejada en la Figura 7.

El alcance de este proyecto, sin embargo, no completa el ciclo completo de resolución, abarcando únicamente los cinco primeros pasos, desde el análisis de la situación hasta la evaluación de la solución elegida, dejando para proyectos futuros la fabricación e implementación de la misma.

Figura 7 Método SPALTEN de resolución de problemas

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Diseño de la Unidad de Estampación de la Estación de Preformado del wbk

6 Diseño de la Unidad de Estampación de la Estación de Preformado del wbk 6.1. Análisis del proceso de preformado y aislamiento de problemas Un primer análisis de la estructura funcional del proceso de preformado concluye una estructura como la indicada en la Figura 8.

Preformado

Trasporte y automatización

Preforma Materia para prima Calentamiento Estampación Enfriamiento RTM

Figura 8 Estructura funcional del proceso de preformado

Como se puede observar, se identifican tres funciones principales:

- La función térmica, que recoge todos aquellos procesos de calentamiento y enfriamiento que tienen lugar durante el preformado, ya sean de las herramientas o del material. - La función mecánica de estampación (y la de preformado por diafragma, aunque no se incluya en este estudio), que se refieren al diseño y mecánica propiamente dichos de la estación de preformado. - La función de transporte y automatización, que se refiere a todos los elementos que permiten el funcionamiento del sistema: brazos mecánicos, actuadores neumáticos… así como la automática a ellos asociada.

De estas tres funciones identificadas, la tercera queda fuera del alcance de este proyecto.

Dado que el objetivo fundamental del proyecto es el diseño de la unidad de estampación, como primera aproximación se van a considerar las funciones térmica y mecánica independientes, y se va a diseñar la unidad bajo la hipótesis de influencia nula de los procesos térmicos en el diseño, para posteriormente, validar dicha hipótesis con un análisis de la cadena térmica del proceso. Así pues, en el punto que sigue se describe el proceso de diseño seguido.

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Diseño de la Unidad de Estampación de la Estación de Preformado del wbk

Un análisis más en profundidad de la unidad de estampación instalada en la estación de preformado del wbk, antes de su sustitución permite identificar y definir el conjunto de características que se desean cambiar o mejorar.

PROBLEMAS DETECTADOS OJETIVOS DE DISEÑO

Defectos en la preforma fabricada: pliegues originados como consecuencia de los espacios existentes entre los distintos componentes del molde superior. Se establece como objetivo diseñar un mecanismo que permita el El uso repetitivo de la unidad de posicionamiento de los cilindros, tanto estampación provoca el aflojamiento de inicial como después de varios usos, los tornillos que sujetan los cilindros, y para mantener los defectos en el mínimo con ello el desplazamiento de los posible. mismos, lo que provoca que los espacios diseñados entre los componentes varíe, y con ello aparezcan defectos en la preforma.

Rediseñar completamente la estructura Baja eficiencia del sistema de sujeción de de sujeción de los cilindros, que garantice los cilindros hidráulicos que se encargan su inmovilidad durante el proceso de de la estampación de cada una de las estampación, o que mantenga su partes del molde. movilidad en el mínimo posible.

Encontrar una solución que reduzca la Nula adaptabilidad de la unidad instalada necesidad de rediseñar una nueva uniad a variaciones del diseño del componente de estampación cada vez que una nueva a fabricar, o a la fabricación de otros preforma va a fabricarse. Buscar una componentes. solución lo más flexible posible a otras geometrías y tamaños.

Diseñar un sistema de sujeción del Falta de sujeción de las telas, previa a la material que evite su movimiento durante estampación, lo que trae consigo el su traslado desde la etapa de movimiento de las mismas antes y calentamiento hasta la de estampación, durante la estampación, y variaciones así como durante el proceso de geométricas de la preforma definitiva. estampación propiamente dicho.

Tabla 1 Resumen de problemas detectados y objetivos de diseño

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Diseño de la Unidad de Estampación de la Estación de Preformado del wbk

A la hora de comenzar con el diseño, se establecen ciertos requisitos que deben ser cumplidos, a saber:

1. Es necesario emplear el mismo modelo de cilindros que ya se emplean actualmente en la unidad de estampación. No se van a comprar nuevos cilindros, por lo que cualquier diseño que se lleve a cabo debe adaptarse a las dimensiones y características actuales. 2. El diseño debe garantizar la posición relativa entre los cilindros y entre las distintas partes del molde. 3. Se requiere adaptar las dimensiones del diseño a las condiciones de contorno dimensionales y geométricas establecidas por la estación de preformado. El diseño definido no debe suponer variación alguna en la estructura global de la estación de preformado. 4. Se deben mantener los costes de fabricación en el mínimo posible. Esto es, siempre que sea posible, se deben emplear conectores y perfiles estandarizados que abaraten costes y reduzcan tiempos de fabricación y montaje. 5. El diseño debe realizarse de forma que se siga garantizando el mantenimiento térmico de las telas por encima de la temperatura de activación del aglutinante durante todo el proceso de estampación.

6.2. Propuesta y selección de alternativas Una vez definidos los objetivos y los requisitos de diseño, se procede a la propuesta de diferentes alternativas de diseño, para posteriormente proceder a seleccionar la que mejor se ajuste a las necesidades, buscando siempre un equilibrio entre economía, funcionalidad y facilidad de fabricación y montaje. Para facilitar el diseño, se divide la unidad de estampación en tres funciones (sistema de posicionamiento, estructura de sujeción y sistema de sujeción de las telas), y para cada una de estas funciones, se proponen dos o más alternativas que permitan evaluar, comparar y seleccionar la más adecuada. Para que la selección de las alternativas sea lo más objetiva posible, y con el objetivo de llevar a cabo un análisis y posterior selección preciso y organizado, se siguen las pautas establecidas por la VDI 2225 (Design engineering methods – Engineering design at optimum cost – Simplified calculation of costs). La VDI (Verein Deutscher Ingenieure ó Asociación de ingenieros alemanes) es una asociación de ingenieros que promueve el avance de la tecnología y representa los intereses de ingenieros y de empresas de ingeniería en Alemania. Una de las múltiples tareas que desempeña esta organización es la de establecer reglas y pautas para definir la metodología, requisititos y limitaciones en los diferentes aspectos que dan forma al amplio rango de técnica y tecnología que

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Diseño de la Unidad de Estampación de la Estación de Preformado del wbk conforman ingeniería hoy en día. Una de estas múltiples normativas desarrolladas por la VDI es la VDI 2225 (1998), que define un método de decisión para diseños de ingeniería. Según este método cada alternativa individual se evalúa de acuerdo con los requisitos específicos asociados a la misma. Así, cada función de diseño debe quedar definida por una serie de parámetros que caracterizan su finalidad, a los cuales posteriormente se les asocia un coeficiente de influencia, en función del papel que desempeñan en la misma. A continuación cada alternativa es evaluada según los distintos parámetros en una escala de puntuación del 0 al 4 (tal y como se define en la Tabla 2), según el grado de satisfacción con respecto a dichos parámetros.

Grado de satisfacción de las alternativas Muy buena (ideal p = 4 puntos Buena p = 3 puntos Suficiente p = 2 puntos Mala p = 1 puntos Insatisfactoria p = 0 puntos

Tabla 2 Escaña de evaluación [VDI – 98]

Pij representa la puntuación otorgada a la alternativa “j” relativa al criterio de evaluación “i”. Por su parte, gi representa el coeficiente de influencia asociado al criterio “i”. Con esto, se define Gij como la puntuación corregida asociada a la alternativa “j”, en relación al criterio “i”, y se define Gges,j como el valor final asociado a la alternativa “j”. Por otra parte, si se divide este valor final entre el máximo valor alcanzado en un criterio y se multiplica por la suma de los coeficientes de influencia asociados a la alternativa “j”, se obtiene la calidad relativa Wges,j que cuantifica el grado de validez de una alternativa.

퐺푖푗 = 𝑔푖 · 푃푖푗 Fórmula 1

퐺푔푒푠,푗 = ∑ 퐺푖푗 Fórmula 2

퐺푔푒푠,푗 푊푔푒푠,푗 = Fórmula 3 푃푚푎푥·∑ 푔푖푗

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Diseño de la Unidad de Estampación de la Estación de Preformado del wbk

De acuerdo con la normativa VDI 2225, el valor de calidad define una alternativa como:

- Insatisfactoria, cuando Wges,j < 0,6

- Buena, cuando Wges,j ∈ (0,6 – 0,7)

- Muy buena, cuando Wges,j > 0,7

Como resultado del análisis definido previamente, se concluye el diseño representado en la Figura 9. Para una descripción más organizada, se decide subdividir el diseño en tres partes diferenciadas: la unidad de estampación propiamente dicha, sus componentes localizados en la mesa de la preforma (el molde inferior) y por último el marco exterior.

1. Unidad de estampación 2. Marco exterior 3. Mesa de la preforma

Figura 9 Diseño definitivo de la unidad de estampación

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Diseño de la Unidad de Estampación de la Estación de Preformado del wbk

6.3. Descripción y evaluación del diseño

6.3.1 Unidad de estampación

El diseño final de la unidad de estampación es la que se muestra en la Figura 10. El material que va a emplearse para la fabricación tanto del molde como del resto de componentes del diseño es el aluminio, debido a sus buenas propiedades térmicas y mecánicas, así como a su bajo peso. Como solución tanto al problema de sujeción de los cilindros como al de posicionamiento de los mismos se propone un nuevo diseño de la unidad de estampación que se basa en la utilización de una placa gruesa de aluminio como soporte de los cilindros, en sustitución de los perfiles estandarizados que sustentaban los mismos en su diseño original, y que no les permitían ningún tipo de ajuste posicional. Esta placa juega también un papel fundamental en el sistema de posicionamiento diseñado para los cilindros. Cada cilindro neumático tiene asociadas dos piezas de sujeción y posicionamiento. Estas piezas permiten el desplazamiento de los cilindros en ambas direcciones del plano. Por otra parte, las ranuras localizadas en ambos extremos de la placa permiten su fijación al marco exterior de la estructura. Así, a la hora de instalar la unidad de estampación en la estación de preformado, se fija primero la posición global de la placa en relación al marco exterior, para posteriormente ajustar individualmente cada cilindro. La posición de los cilindros en la placa se ha estudiado para obtener la mejor distribución del espacio posible, con el objetivo de maximizar la posibilidad de desplazamiento de los mismos.

Figura 10 Unidad de estampación

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Diseño de la Unidad de Estampación de la Estación de Preformado del wbk

Con el requisito en mente de no alterar la estructura global de la estación de preformado y adaptar el diseño a las restricciones dimensionales establecidas inicialmente, surge el problema de que, con la unidad de estampación tal cual está diseñada, si se atornillan los cilindros directamente a los componentes del molde, la carrera de los cilindros disponibles no es suficiente para completar el proceso de cerrado del molde. Por otra parte, resulta imposible bajar la unidad de estampación, debido a que interfiere con los raíles instalados para la unidad de drapeado por diafragma. Por este motivo, es necesario que cada uno de los componentes del molde superior se fabrique con una altura superior a la del diseño inicial, de manera que el cerrado del molde pueda completarse correctamente. Esta solución, sin embargo, supone un incremento en el coste de fabricación ya que triplica la cantidad de material necesaria para la fabricación. Por este motivo y pese a que difiere de los requisitos establecidos, con el fin de obtener el mínimo coste de fabricación se evalúan también la posibilidad de rediseñar la estructura de la unidad de preformado por diafragma – y con ello reestructurar la estación de preformado – así como la posibilidad de adquirir nuevos actuadores neumáticos de mayor carrera.

Alternativa I. Respetar las condiciones Cantidad Coste (€) dimensionales iniciales Material bruto (aluminio) 90 5,50 € 495,00 € Máquina Fresado + Taladrado 10 10,00 € 100,00 € Operario 10 8,00 € 80,00 € Herramientas 10 3,50 € 35,00 €

710,00 €

Alternativa II. Rediseñar estructura de la estación Cantidad Coste (€) Material bruto (kg de aluminio) 30 5,50 € 165,00 € Máquina Fresado + Taladrado 3,5 10,00 € 35,00 € Operario 3,5 8,00 € 28,00 € Rediseño (licencia Siemens NX) 50 1,50 € 75,00 € Rediseño (salario ingeniero) 50 30,00 € 1.500,00 € Montaje 15 8,00 € 120,00 €

1.923,00 € Alternativa III. Compra de nuevos actuadores Cantidad Material bruto (aluminio) 30 5,50 € 165,00 € Actuadores neumáticos 7 300,00 € 1.050,00 € 1.215,00 €

Tabla 3 Comparación de alternativas para el molde superior

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Diseño de la Unidad de Estampación de la Estación de Preformado del wbk

En base a lo observado en la Tabla 3, se concluye que, pese a suponer un incremento del tiempo de fabricación y del coste, la alternativa elegida inicialmente es la más adecuada.

Por otra parte, el movimiento relativo de los cilindros sobre la placa soporte es posible gracias al empleo de dos piezas de posicionamiento que se fabrican también específicamente para este diseño. Cada una de estas piezas permite el movimiento del cilindro en una de las direcciones principales del plano. Los tornillos empleados para fijar ambas piezas a cilindro y placa soporte son tornillos estandarizados M5 y M8 respectivamente. Por último, una evaluación del proceso de fabricación de la pieza laquesea concluye la eliminación de los nervios laterales de la misma para simplificar el proceso de fabricación, que de esta manera implica únicamente una etapa de doblado y una posterior etapa de taladrado y fresado, reduciendo el tiempo de fabricación y los costes de material que se derivarían del diseño inicial.

6.3.2 Mesa de la preforma

El diseño de la unidad de estampación se ha visto muy limitado debido a la imposibilidad de hacer variaciones en la mesa de la preforma. Muchas de las alternativas barajadas pasaban por la necesidad de modificar las dimensiones del orificio de la mesa de la preforma para adaptarla a la nueva unidad. Sin embargo, el wbk considera por ahora un gasto innecesario la modificación de la mesa, y por lo tanto el diseño mantiene el concepto básico que tenía en origen, si bien ha sido necesario realizar varias modificaciones para conseguir un sistema de sujeción de las telas que impida su movimiento durante el transporte y la estampación. Así, se ha estudiado la utilización de elementos de sujeción (como los empleados por los brazos robóticos para el transporte de las telas) que, mediante un mecanismo de succión sustenten las telas durante el preformado. El tipo de elemento de sujeción que se va a utilizar se basa en el efecto Coanda y se va a localizar en el exterior del molde. De esta manera se reduce la complejidad de fabricación que supone la inclusión del elemento de sujeción en el seno del molde. Así, se diseña el molde inferior con dos características fundamentales. La primera de ellas es la inclusión de un canal de aire en el interior que atraviesa el molde desde su parte inferior, bajo la mesa de preformado, hasta su superficie superior, sobre la que se depositan las telas. El diseño consta también, por tanto, de una pieza plástica que, junto con la junta indicada en la figura, constituyen el cierre de dicho canal y el punto de contacto con las telas. La segunda de las características del diseño del conjunto inferior de la unidad de estampación es la pieza de adaptación, cuyo diseño se ha tenido que adaptar a la entrada de aire necesaria para el funcionamiento del conjunto. La Figura 11 representa el diseño definitivo de la mesa de la preforma, la

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Diseño de la Unidad de Estampación de la Estación de Preformado del wbk herramienta macho del molde y la pieza de adaptación a la mesa..

Figura 11 Mesa de la preforma

6.3.3 Marco exterior Por último, el diseño definitivo del marco exterior se intenta mantener lo más simple posible. Todos los elementos empleados son perfiles y conectores estandarizados que reducen los costes de fabricación así como los tiempos de montaje. Se ha modificado el diseño del marco para adaptarlo a la nueva unidad de estampación

Figura 12 Marco exterior

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Validación del Diseño

7 Validación del Diseño 7.1. Análisis térmico del proceso de preformado Un factor fundamental a la hora de garantizar la calidad de la pieza que saldrá fabricada del proceso de fabricación RTM es que durante todo el proceso de preformado el conjunto de telas de refuerzo no disminuyan su temperatura por debajo de los 80ºC. Esta temperatura representa la temperatura de fusión del aglutinante de resina que se le aplica a las telas con el objetivo de mantener el tejido perfectamente alineado durante la manipulación y permitir que durante la inyección RTM el material no se deshilache ni pierda su forma, reduciendo así los defectos en el componente definitivo. Es importante que el aglutinante se encuentre en todo momento en estado fundido, para garantizar una distribución homogénea del mismo y una buena impregnación de las telas.

Como se indicó al comienzo de este proyecto, el diseño de la unidad de estampación se ha realizado bajo la hipótesis de influencia nula de los procesos térmicos en la misma. Sin embargo, existen dos factores que afectan directamente a la temperatura de las telas durante el preformado. Por una parte el contacto a temperatura ambiente de cada uno de los componentes del molde superior con las telas durante el estampado y por otra, la presencia de una corriente de aire sobre las telas debida a la introducción del elemento de sujeción en el diseño del molde inferior.

Por este motivo, se procede a realizar un análisis de la cadena de calor durante el proceso de preformado que permita verificar la validez del diseño o, en su defecto, ayude a determinar las medidas a tomar para garantizar las condiciones térmicas del preformado.

Es importante recalcar que el análisis que se va a realizar no pretende ser un estudio en profundidad que refleje minuciosamente cada etapa del proceso de preformado, sino un análisis orientativo inicial que permita evaluar el proceso y validar el concepto de diseño propuesto.

Una representación gráfica de los procesos térmicos que tienen lugar durante el preformado puede verse en la Figura 13. Dicho proceso comienza con el calentamiento de la mesa de calentamiento hasta que su temperatura superficial alcanza los 115ºC. A continuación se le aplica una fina capa del aglutinante de resina al conjunto de telas que conformarán el refuerzo de la pieza. Una vez aplicado el aglutinante, el conjunto de telas es transportado manualmente – aunque esta función la realizará en un futuro un brazo robótico – hasta la mesa de calentamiento, donde se calientan hasta alcanzar una temperatura superior a la temperatura de fusión del aglutinante. Tras el calentamiento, las telas son

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Validación del Diseño transportadas nuevamente hasta el área de carga de la estación de preformado, depositándolas sobre la herramienta macho del molde, que previamente se ha calentado hasta una temperatura de 50ºC. La mesa de la preforma se desplaza a lo largo de la estación de preformado hasta el área de drapeado, donde finalmente tiene lugar la estampación. Una vez finalizada la estampación, se mantiene el molde cerrado durante dos minutos, para que la temperatura descienda nuevamente por debajo de la temperatura de fusión del aglutinante, para, una vez alcanzado este estado estable, abrir el molde y sacar la preforma que pasará posteriormente a una etapa de acabado – en la que se recorta el material sobrante – y por último pasará un control de calidad.

Figura 13 Cadena de calor durante el preformado

7.1.1 Descripción de los elementos implicados en el análisis térmico  Mesa de calentamiento: La mesa de calentamiento de las telas consiste en una placa de aluminio de 80 centímetros cuadrados y 20 centímetros de espesor, a la que se le aplica calor a través de cuatro cilindros de aluminio a los que se han conectado cuatro resistencias de boquilla. La Figura 14 muestra una fotografía del conjunto.

 Resistencias de boquilla: Las resistencias empleadas son WEMA DH 200, con capacidad calorífica máxima de 6 W/cm2 y una temperatura máxima de 300ºC. Para mayor información técnica, consultar el Anexo II.  Material de refuerzo: Por su parte, el tipo de telas que se emplea para el ensayo son las mismas con las que se está trabajando en la actualidad para la fabricación de componentes en la cadena del wbk, esto es, telas de fibra de carbono unidireccionales, con un peso de 333 g/m2.

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Validación del Diseño

 Aglutinante de resina: se emplea para la fabricación de componentes de fibra de carbono el aglutinante EPIOKTE Resin 05390, cuya temperatura de fusión es de 80ºC.

Figura 14 Vista inferior de la mesa de calentamiento

7.1.2 Elaboración del modelo y simulación Para una correcta simulación de los procesos térmicos durante el preformado, se comienza diseñando un modelo empleando el programa Abaqus 6.14, en el que quedan definidas tanto las propiedades físicas y térmicas de la mesa de calentamiento, como las de las telas de refuerzo.

La elevada cantidad de contenido bibliográfico referente a las propiedades térmicas del aluminio permiten estimar las propiedades de la mesa sin complicaciones, mientras que la estimación de las propiedades del conjunto de telas resulta más tediosa debido, por una parte, a la falta de información aportada por los fabricantes y por otra, a la dificultad de estimar la conductancia térmica entre las distintas capas de tela. Por este motivo, se acaba por hacer una simplificación del modelo que pasa a entender el conjunto de telas como un único elemento en lugar de seis superpuestos. En este modelo se define la conductividad del conjunto como una propiedad ortotrópica en la que se define una conductividad equivalente en la dirección perpendicular al plano que permite simular el comportamiento térmico a través del conjunto de telas, si bien no define su comportamiento de manera individual.

Una vez caracterizados los distintos componentes que intervienen en el proceso, se procede a la simulación. De dicha simulación se extraen datos correspondientes a la temperatura tanto de la mesa durante su calentamiento desde temperatura ambiente, como de las telas de refuerzo.

Para validar el modelo definido, se realizan ensayos por una parte, durante

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Validación del Diseño calentamiento de la mesa y por otra durante el calentamiento de las telas.

- Ensayo 1. Se mide la temperatura en las superficies superior e inferior de la placa de la mesa de calentamiento. Se toman temperaturas durante el calentamiento cada 5 minutos en siete puntos distintos. - Ensayo 2. Una vez la mesa ha alcanzado la temperatura superficial de 115ºC, se miden las temperaturas en distintos puntos de la superficie superior de una capa de tela de fibra de carbono durante su calentamiento. Se toman medidas cada cinco segundos en tres puntos distintos de la superficie. - Ensayo 3. Se repite el proceso de medida anterior, midiendo la temperatura sobre la superficie de un conjunto de tres telas.

Se comparan gráficamente los resultados obtenidos en los ensayos descritos con los datos extraídos del modelo definido en Abaqus, concluyéndose la validez del mismo. Una vez validado el modelo, se procede con el resto de la simulación, obteniéndose las temperaturas en distintos puntos del conjunto de telas de refuerzo tras los procesos de transporte, estampación y enfriamiento, así como las gráficas de la evolución térmica durante los mismos.

7.1.3 Resultado del análisis térmico La simulación del proceso completo trae como resultado fundamental la gráfica que se muestra en la Figura 15. En ella se reflejan la temperatura media alcanzada en la capas superior (fría), intermedia e inferior (caliente) del conjunto de telas – seis – que van a constituir el refuerzo del componente a fabricar.

La primera de las etapas constituye el calentamiento de las telas hasta que éstas alcanzan un estado estable. Como puede observarse, la capa más fría, la superior, al finalizar el calentamiento apenas alcanza los 80ºC necesarios para fundir el aglutinante. La lógica disminución térmica durante el transporte hacia la unidad de estampación implica que una parte de la superficie de las telas se encuentre durante la estampación por debajo de la temperatura necesaria para que la impregnación de la resina sea homogénea y válida. Tras el proceso de estampación, es necesario esperar a que todos los puntos del material de refuerzo disminuyan su temperatura de nuevo hasta encontrarse todos por debajo de los 80ºC. Esta situación se da un minuto después de completar el proceso de estampación. Sin embargo, resulta evidente la necesidad de estudiar nuevamente el proceso de calentamiento ya que la calidad de las piezas se está viendo mermada por un pobre proceso de calentamiento.

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Validación del Diseño

Figura 15 Evolución térmica del material de refuerzo durante el proceso de preformado

La Figura 16 refleja la temperatura final de la preforma tras finalizar la etapa de estampación, junto con las herramientas del molde inferior y superior.

Figura 16 Temperatura final tras el preformado

En base a los resultados obtenidos en este primer análisis, se decide no realizar el segundo análisis que implicaba la inclusión del nuevo diseño del molde, y el efecto negativo que podría suponer para la temperatura de las telas la presencia de la corriente de aire asociada a la utilización del elemento de sujeción para sustentar las mismas. El motivo de esta decisión se fundamenta en que, viendo los inesperados resultados obtenidos en el anterior análisis, y entendiendo entonces que en la situación actual las telas no alcanzan la temperatura requerida, la introducción del elemento de sujeción sólo supone un agravante de la situación.

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Validación del Diseño

7.2. Presupuesto de diseño Se muestra a continuación una estimación de presupuesto que incluye, por una parte, el trabajo ya empleado tanto en diseñar como en realizar el estudio térmico y, por otra, el coste que supondrá la fabricación y montaje de la unidad diseñada. Información más detallada del cálculo de cotes puede encontrarse en el Anexo I. Cantidad (h) Coste (€/h) Costes de Diseño Coste horario licencia Siemens NX 240 1,50 € 360,00 € Tutor del proyecto 5 30,00 € 150,00 € Alumno 240 0,00 € 0,00 € 510,00 €

Compra de Componentes Cantidad Coste (€) Perfiles de aluminio 45x45 (1000 mm) 10 26,00 € 260,00 € Perfiles de aluminio 60x45 (m) 1 50,00 € 50,00 € Conectores universales (y tornillería) 26 7,00 € 182,00 € Brackets (60x60) 8 10,00 € 80,00 € Tapa de plástico 1 5,00 € 5,00 € Elemento de sujeción 1 122,60 € 122,60 € Manguera elemento de sujeción 1 10,00 € 10,00 € 709,60 €

Costes de Fabricación Cantidad Coste (€) Componentes herramienta hembra del molde 7 34,32 € 240,25 € Herramienta hembra del molde 1 237,20 € 237,20 € Pieza de adaptación a la mesa 1 171,45 € 171,45 € Placa soporte 1 127,20 € 127,20 € Pieza posicionamiento forma de S 7 1,61 € 11,30 € Pieza posicionamiento plana 7 2,54 € 17,75 € 805,15 €

Costes de Montaje Cantidad (h) Coste (€/h) Operarios 15 8,00 € 120,00 € 120,00 €

Estudio Adicional Térmico Cantidad (h) Coste (€/h) Coste horario licencia Abaqus 6.14 180 1,50 € 270,00 € Salario ingeniero responsable 3 30,00 € 90,00 € Salario estudiante 180 0,00 € 0,00 € Maquina 15 15,00 € 225,00 € Telas de fibra de carbono - 0,00 € 0,00 € 585,00 €

TOTAL 2.729,75€

Marina Moya Sánchez 27

Conclusiones

8 Conclusiones La metodología de resolución SPALTEN ha permitido concluir un diseño válido tanto técnica como económicamente, que podría suponer una mejora en el camino por alcanzar el objetivo último de implantar una cadena de producción RTM plenamente automatizada y con una calidad de producción equiparable a la obtenida de manera manual. La introducción del diseño propuesto persigue dar un salto de calidad en la fabricación de las piezas – derivada de la mayor precisión del estampado y mejor sujeción de las telas durante el mismo – así como una reducción de los tiempos de ciclo al minimizarse el tiempo de puesta a punto y calibrado por unidad.

Tnp = Tiempo no productivo Tciclo = Tf + Tnp + Tp Tp = Tiempo de puesta a punto y calibrado Tf = Tiempo de fabricación

Por otra parte, el estudio económico del diseño y fabricación del prototipo concluyen la viabilidad de fabricación del mismo. Al tratarse de un nuevo diseño, aunque el análisis y resultados de simulación inicial resultan favorables, la funcionalidad real o no de la misma no puede más que validarse mediante la fabricación de un prototipo que permita evaluar su viabilidad y funcionalidad en condiciones de trabajo reales, así como detectar la aparición de problemas derivados de su funcionamiento e identificar posibles mejoras. En manos de futuros proyectos de investigación queda la fabricación y evaluación del funcionamiento real de este diseño. Por otra parte, el estudio de la cadena de calor durante el proceso de preformado arroja varias conclusiones fundamentales. Se concluye que el actual sistema de calentamiento de las telas cuenta con varias desventajas que necesitan ser solucionados en pos de la calidad del producto final. Dos defectos en el calentamiento son hallados durante el estudio de la cadena de calor: el calentamiento irregular de las telas – asociado a la mala instalación de las fuentes en la mesa – y la insuficiencia del sistema disponible para calentar las telas hasta la temperatura deseada. Como solución a estos problemas se sugiere mejorar el sistema de calentamiento actual mediante la utilización de una plancha de aluminio, precalentada por encima de la temperatura de fundición del aglutinante, que permita el calentamiento de las telas no sólo desde su superficie inferior, sino también desde la superficie superior, lo que permitiría alcanzar mayores temperaturas en las telas sin necesidad de cambiar las fuentes de la mesa, así como reducir los tiempos de ciclo derivados del calentamiento de las mismas. Como consecuencia de los inesperados resultados de este análisis inicial, se concluye que es necesario realizar un análisis más detallado de la cadena de calor para poder realizar una toma de decisiones final.

Marina Moya Sánchez 28

Diplomarbeit

cand. mach. Marina Moya Sánchez

Matrikelnr.: 1797857

Development of a stamping unit and analysis of the heating process for the preforming of textile materials

(Entwicklung einer Stempeleinheit und

Analyse des Aufheizprozesses für das wbk Institute of Production Science Preforming von textilen Karlsruhe Institute of Technology (KIT) Kaiserstraße 12 Materialien) 76131 Karlsruhe

Prof. Dr.-Ing. Jürgen Fleischer Prof. Dr.-Ing. Gisela Lanza Prof. Dr.-Ing. habil. Volker Schulze

Statement of Originality

This thesis contains no material that has been accepted for the award of any other degree or diploma in any other university. To the best of my knowledge, this thesis contains no material previously published or written by another person, except where reference is made in the text.

Karlsruhe, August 31st 2015

______

Marina Moya Sánchez

Acknowledgement

I would like to extend thanks to my supervisor Mr. Sven Coutandin for the help and guidance during the course of this thesis work. I would also like to thank the wbk – Institute of Production Science under the leadership of Professor Dr. Jürgen Fleischer for the opportunity to conduct this work at the institute. I would also like to thank every person that contributed, greater or lesser extent, in drawing the big smile that I’ve been wearing since I put my feet on Germany. I would like to thank my friends from Spain, because we’ve proved that no matter the distance in between us and no matter the time that passes without seeing each other, our friendship gets stronger every day. Thank you for knowing me, for supporting me and for being part of my life. Thanks to my family, and especially to my “hermiga”, Alicia. And thanks to Music, for everything.

Thank you

Marina Moya

Abstract

Within a society increasingly aware of the need for preserving the environment, the automotive industry is doing its own labor by researching and utilizing new lighter materials in order to reduce to a minimum the CO2 emissions. This low-weight final objective is leading the industry to composite, and especially to carbon fiber reinforced polymers (CFRP), as future substitutes for the actual vehicle materials. CFRP appear to be the best alternative for metal parts, due to the more than 50% weight reduction in addition to the equivalent mechanical properties. Within the scope of the research at the wbk Institute of Production Science a fully automated process for resin transfer molding (RTM) is to be investigated. The properties of textile pre-products for RTM manufacture are more complex than conventional materials and are yet to be optimally mastered. One of the critical steps in the RTM-chain is the preforming. The preforming station installed in the wbk is based on a stamping method, and it is responsible for the transformation of two dimensional mats into multi-curved three dimensional preforms. A binder resin is applied to the textiles during this process to keep the fabrics perfectly aligned and make them easier to handle. The resulting sub-product is then ready to feed the RTM press for the resin transfer process to be completed. The accuracy on the final contour obtained and the absence of gaps and defects after the preforming has a direct impact on the quality of the final carbon fiber part. The final objective is achieving an automated preforming process that guarantees a high quality preform. This thesis is focused on two main objectives: the design of a new stamping unit that can improve the preform quality results, and the thermal modeling of the process, in order to assure the quality of the binder resin curing.

wbk Institute of Production Science Table of Contents

Table of Contents

Table of Abbreviations 1

1 Introduction 1 1.1 Motivation 2 1.2 Objective 2 1.3 Structure of the Thesis 2

2 Fundamentals 4 2.1 Composites 4 2.2 Carbon Fiber Reinforced Polymers (CFRP) 6 2.2.1 Types of Matrix 6 2.2.2 Types of Textile Reinforcement 6 2.3 Manufacture of CFRP 8 2.3.1 Resin Transfer Molding Process (RTM) 9 2.4 Heat Transfer Fundamentals 14 2.4.1 Conduction 14 2.4.2 Convection 16 2.4.3 Radiation 17 2.4.4 Heat Capacity 17 2.5 Product Development Method, VDI 17

3 State of the Art 19

4 Own approach 23 4.1 Design of the Stamping Unit 23 4.2 Heat Transfer Analysis 24

5 Stamping Unit Design 26 5.1 Design Boundary Conditions 26 5.1.1 Current Preforming Station 26 5.2 Functional Structure and Requirements 31 5.2.1 Functional Structure 31 5.2.2 Design Requirements 33 5.3 Concepts 35 5.3.1 Stamping System Concepts 35 5.3.2 Fixing Interface Concepts 38 5.3.3 Fabrics Holding Concepts 39 5.4 Evaluation Methodology and Criteria 40 5.4.1 Evaluation Method 40 5.4.2 Evaluation Criteria 41 Marina Moya Sánchez Page I wbk Institute of Production Science Table of Contents

5.5 Alternative Selection for the Stamping Unit 43 5.5.1 Stamping System 43 5.5.2 Fixing Interface 45 5.5.3 Handling System 46

6 Results: Stamping Unit Design 48 6.1 Stamping Unit 49 6.2 Profile Table 51 6.3 Stamp Module 52

7 Heat Transfer Chain Analysis 53 7.1 The Heat Chain during the Preforming 53 7.1.1 Heating Table 54 7.1.2 Unidirectional Carbon Fiber Textile Products 55 7.2 Model Definition 55 7.2.1 Heating Table Model 56 7.2.2 Unidirectional Fabrics Model 58 7.3 Data Collection 59 7.3.1 Experiments Description 59 7.3.2 Experiments Results 62 7.4 Model Validation and Optimization 64 7.4.1 Heating Table Model Validation 64 7.4.2 Fabrics Model Validation 67

8 Results: Analysis of the Heat Transfer Chain 72 8.1 Table and Fabrics Heating 72 8.2 Fabrics Transport 73 8.3 Stamping and cooling 75

9 Summary and Outlook 77 9.1 Summary 77 9.1.1 Stamping Unit Design 77 9.1.2 Thermal Analysis 77 9.2 Outlook 79 List of Figures I List of Tables IV 10 References V

Marina Moya Sánchez Page II wbk Institute of Production Science Table of Abbreviations

Table of Abbreviations

Symbol Measurement Unity 2D Two-dimensional

3D Three-dimensional cp Specific heat J/kg·°C 2 hc Thermal contact conductance W/m ·°C CAD Computer Aided Design

CFRP Carbon Fiber Reinforced Polymers h Film coefficient W/m2·°C RTM Resin Transfer Molding

T Temperature °C Association of German Engineers (Verein Deutsche VDI Ingenieure) wbk Institut für Produktionstechnik

ε Emissivity - κ Thermal conductivity W/m·°C σ Stefan-Boltzman constant W/m2·K4 t time s

Marina Moya Sánchez Page 1 wbk Institute of Production Science 1 Introduction

1 Introduction During the last few years, the development of carbon fiber demand has enjoyed a steady and continuous growth. During this period, demand for carbon fiber grew from 26,500 t in 2009 to 46,500 t in 2013 with high annual growth rates starting at an initial level of over 20% and easing to a current level of nearly 7%. [CMR–14] Most of the carbon fiber produced is used worldwide in combination with a binding matrix to produce carbon composites. As a result, the growth trends observed in carbon fiber and CRP are very similar. Figure 1.1 shows the development of global CRP demand in tons. Growth in CRP consumption is forecast to continue at 10.6% until 2020, essentially matching that of carbon fiber. [CMR-14]

Figure 1.1 Global CRP demand in 1,000 tons 2008-2020 (*estimated) [CMR-14]

An overview of the distribution of CFRP use by application is shown in Figure 1.2. As seen, aerospace & defense industries entail the 50% of the CFR global revenues, followed by sport industry and wind turbines.

Figure 1.2 Global CF revenues in US$ million by application (2013) [CMR-14]

Marina Moya Sánchez Page 1 wbk Institute of Production Science 1 Introduction

1.1 Motivation

In 2014 European Union approved the standard to reduce to 95 g CO2/km emissions from cars in 2020. Nowadays this standard is stablished in 130 g CO2/km. The US, Japan and other countries are also defining emission limits for the vehicles to come. For automotive industry this involves a big challenge. One of the top strategies that car companies are planning to follow, in order to achieve this fuel efficiency and emission standards is to reduce the weight of the vehicle, so less fuel will be needed to propel it. [AG-14] With the objective of achieving a widespread light weight vehicles production, aluminium and composites are positioning for being the material of the cars of the future. The reason why composites industry has not yet made a dent in the automotive world is the lack of automation of the manufacturing processes of CFRP parts, and the consequent high prices that composites parts suppose. The development of a fully automated production chain to enable mass production of carbon fiber parts is the main objective of the car companies to achieve weight reduction, without imposing a disproportionate cost. Added to the need of automation and to the reduction of costs is the need of technology capable of manufacturing parts with no limitations of geometry and size. This is the context in which the wbk – Institute of Production Science is developing its resin transfer molding chain process. Seeing the current market needs, and the lack of development of technology in this environment, optimizing each step of the manufacturing process is a step forward towards the sought objective to include mass production of carbon fiber parts in the automotive industry. 1.2 Objective The first objective of this thesis is to provide a general overview of the RTM process and a detailed analysis of the preforming step. Taking advantage of the need to replace the stamp unit in the preform station available in the wbk hall, which takes a step forward in the geometry of the preforms to manufacture, and with the aim of continuous improvement always present, the operations that take place during the preforming step will be analyzed in order to identify variables that could be improved. Two major processes take place during the preforming. On the one hand, the heating process that involves all the heat transfer operations during the draping, and on the other hand, the stamping process, which allows the stack of fabrics to achieve its final shape. Considering these two processes, the thesis is going to be subdivided in two different parts with a common objective: achieving solutions for a better performance of the preforming, which could be translated into the obtaining of better quality preforms and the reduction of the process duration. Considering each part of the thesis separately, an entirely new stamping module design is going to be developed in the first part of the thesis, while on the second part of it a thermal analysis of the heating process is going to be performed in order to characterize the fabrics used in the preforming, to analyze the heat transfer during the stamping process and to identify possible problems and solutions. 1.3 Structure of the Thesis The basis of composites and carbon fiber reinforced polymers, together with the RTM process and the preforming process are explained during chapter two. In chapter two Marina Moya Sánchez Page 2 wbk Institute of Production Science 1 Introduction also can be found a brief introduction to heat transfer and finally an explanation of the norm used for this thesis. The actual state of the art is summarized in chapter three. After this, this thesis is going to be divided in two different parts. Chapters five and six are going to be focused on the design and development of the stamp unit in the preforming station, while chapters seven and eight are going to be focused on the heat transfer chain analysis. Chapter four summarizes the methodology used for the development of this thesis. Thereby, chapter five focuses on giving a precise vision of the current station installed in the wbk, stablishes the boundary conditions and determines the requirements needed. In chapter five are also defined the different functions in which the process can be divided and the different design alternatives. Following the leads defined in previous points, the end of this chapter summarizes the evaluation of every alternative and the final design is represented in chapter six. Following with the second part of the thesis, chapter seven sum up the heating transfer processes during the preforming, explains the methodology followed to obtain the final model of the process in Abaqus, offering a final solution for the characterization of the fabrics and the parts involved in the process, and finally chapter eight summarizes the results obtained after the simulations. Chapters eight, nine and ten are reserved for exposing final summarized conclusions and defining the guidelines for possible futures works on this theme.

Marina Moya Sánchez Page 3 wbk Institute of Production Science 2 Fundamentals

2 Fundamentals Most of the man-made materials can be classified in four classes: metals, polymers, ceramics and composites. Combinations of the first three classes, which result in better properties than those of the individual components, used alone, leads to the fourth class, composites. In contrast to metallic alloys, each material that conform a composite retains its separate chemical, physical and mechanical properties, which allows the technic to develop materials with the optimal properties for the specific work they are designed for.

Nowadays, the constant search for solutions to reduce CO2 emissions has led to composites as the future of aerospace, automotive and energy industries. The weight saving to increase payload and the reductions of the cost/time of the production cycle are imperative targets. For these reasons, aerospace companies, which are traditionally based on the use of metal alloys, have been focusing for past decade on composite materials. In the last years, automotive companies have joined this research too. The main advantages of composites with respect to metals, that are resistance to corrosion and fatigue and high performance/weight ratios, are a set of qualities for winning the current and future applications. Obviously, this is possible only through the development of economically competitive technologies. [Lau-12] Once the technology available is capable of mass producing composite parts in an economic way, actual materials used in industry could be substituted by lighter and equally resistant composites (Fig. 2.1).

Figure 2.1 Comparison of specific strength and modulus of high-strength composites and some aerospace alloys [Cam-10] 2.1 Composites Composites are characterized by being multiphase materials within which the phase distribution and geometry have been deliberately tailored to optimize one or more properties [Bar-11]. The main advantages of composite materials are their high strength and stiffness, combined with low density, when compared with bulk materials, allowing for a weight reduction in the finished part [Cam-10]. In composite materials two distinct phases can be distinguished: a continuous phase, the matrix, and a discontinuous phase, the reinforcement.

Marina Moya Sánchez Page 4 wbk Institute of Production Science 2 Fundamentals

The matrix is a polymer, metal, or ceramic. Polymers have low strength and stiffness, metals have intermediate strength and stiffness but high ductility, and ceramics have high strength and stiffness but are brittle. The matrix performs several critical functions, including maintaining the fibers in the proper orientation and spacing and protecting them from abrasion and the environment. On the other hand, the reinforcing phase provides the strength and stiffness. In most cases, the reinforcement is harder, stronger, and stiffer than the matrix. The reinforcement is usually a fiber or a particulate [Cam-10]. Particulate reinforcements are characterized by their approximately equal size in all directions. Spherical, platelets or any other geometry shaped particulates are used. The properties of the particulate composites are usually worse than fiber composites. They tend to be weaker and less stiff, but on the other hand they are usually cheaper. Fiber reinforcements, meanwhile, can be continuous or discontinuous. Continuous fibers have long aspect ratios, while discontinuous fibers have short aspect ratios. Continuous-fiber composites normally have a preferred orientation, while discontinuous fibers generally have a random orientation. Fibers produce high- strength composites because of their small diameter; they contain far fewer defects (normally surface defects) compared to the material produced in bulk. Typically glass, carbon, aramid and boron fibers are used. The fiber diameter ranges from 5 μm to 25 μm. For carbon fiber the diameter is in the 5 to 8 μm range. As a general rule, the smaller the diameter of the fiber, the higher its strength, but often the cost increases as the diameter becomes smaller. [Cam-10] These fibers can be woven, braided or knitted into precursor textile forms before infiltration by a matrix. The fiber precursor is drawn, extruded or spun as a group of individual fibers. These groups referred to as filaments, strands, ends or tows, depending on the industry. Glass fiber groups are called “strands”, whereas carbon fiber groups are called “tows”. Twisted strands are called yarns. Filaments and yarns are the first elements of manufacturing composites. [Cam-10]

Figure 2.2 Discretization in textile composites representing scales of textile, yarn and filament [Roy-04]

Marina Moya Sánchez Page 5 wbk Institute of Production Science 2 Fundamentals

2.2 Carbon Fiber Reinforced Polymers (CFRP) Recently, carbon-based materials have received much attention for their many potential applications. The carbon fibers are very strong, stiff, and lightweight, enabling the carbon materials to deliver improved performance in several applications such as aerospace, sports, automotive, wind energy, oil and gas, infrastructure, defense, and semiconductors. However, the use of carbon fibers in cost-sensitive, high-volume industrial applications is limited because of their relatively high costs. However, its production is expected to increase because of its widespread use in high-volume industrial applications. [Soo-15] CFRP are characterized by a polymeric matrix that is usually a thermosetting or a thermoplastic plastic, and a reinforcement that generally consists on the superposition of carbon fiber fabrics. The orientation of the fibers on the fabrics, and the mode in which the fibers are held together define each kind of fabric. The fabrics selection for a specific part manufacturing depends on the characteristics of the part that has to be manufactured, and the function it is going to perform. 2.2.1 Types of Matrix The matrix polymers used in CFRP production can be further divided into thermoplastics and thermosetting plastics. Thermosetting plastics continue to be the polymer matrix used most commonly with carbon fiber. The polymer determines the chemical resistance, surface resistance, flammability and the electric characteristics of the composite. The matrix selection is therefore performed based on these characteristics including environmental, cost, performance and manufacturing requirements. [Sig-13] A number of factors have contributed to the more established market position of thermosetting plastics, such as their good mechanical properties, temperature resistance, low moisture absorption, lower material costs and large selection of matrix systems, material manufacturers and manufacturing processes. Thermoplastics, on the other hand, offer advantages, which will probably lead to them being more widely adopted in future. Some of these advantages are their short processing times (no chemical reaction required, unlike thermosetting plastics), their resistant, high damage tolerance, good formability and weldability, ease to storage and ease to recycle. So far, elastomer matrices are not widely used although this may change in the future, e.g. for elastic, hingeless shaft connections in mechanical engineering applications [CMR-14]. 2.2.2 Types of Textile Reinforcement Depending on the final shape of the part, its physical requirements and the manufacturing method to be used, different types of fabrics can be found. Most commonly used methods for the fabrics manufacturing are weaving, knitting, braiding and stitching. Fabrics refer to all flat sheets, roll goods, irrespective of whether they are strictly fabrics. The fabric must be inherently stable enough to be handled, cut, and transported to the mold, but pliable enough to conform to the mold shape and contours. Two basic types of fiber fabrics can be defined: woven fabrics and Non-Crimp Fabrics (NCF). The fundamental difference between both types stems in their formation. While woven fabrics have a weave formation, NCF don’t. This makes it possible for NCF’s to Marina Moya Sánchez Page 6 wbk Institute of Production Science 2 Fundamentals ideally transmit the forces that arise in the stress direction using targeted fiber alignment. This makes reduced component weight despite identical mechanical values or higher loads for the same component weight compared to standard reinforcement textiles (woven fabric, etc.) possible.

Figure 2.3 Woven and non-crimp fabrics structure

Woven Fabrics Of all the different ways of working with carbon fiber, woven fabrics are the most common and versatile. Their manufacturing is achieved by weaving bi-directional bunches of carbon fiber yarns, forming a 2D single layer. The manner in the yarns are interwoven is the weave pattern. The weave pattern of the woven fabrics determines the distribution of the fiber in a particular direction. While unidirectional weave pattern has 95% of the fibers in the 0º direction, plain –weave pattern has 50% of the fibers in 0º and the other 50% in 90º. There are also different patterns depending on the number of yarns that are interwoven together. The two most commonly used weaving patterns are the 2x2 twill weave (Figure 2.4 a) and the plain (1x1) weave (Figure 2.4 b). 2x2 twill weave is by far the most common carbon fiber fabric used in the automotive industry. This fabric type follows a distinct diagonal pattern. The pattern goes over two intersecting warps and under two. This makes the fabric more pliable and looser, which means it can be applied more easily because it can be stretched to curves and contours with fewer complications. The plain or 1×1 weave is the second most used fabric type in the industry. The weave pattern goes up one and down one. Plain weave is then a tighter knit fabric and is easier to handle without making any distortions.

(a) (b) (c)

Figure 2.4 Most common woven patterns

To improve the quality of the final composite, different materials, as glass fiber, can be interwoven also. Woven fabrics are among the most widely used textile reinforcements, in a variety of applications. They exhibit good stability in 0º and 90º directions. Also three-dimensional textile preforms can be woven by using triaxial weaving.

Marina Moya Sánchez Page 7 wbk Institute of Production Science 2 Fundamentals

Finally, in unidirectional fabrics (Figure 2.4 c), fibers are all aligned parallel in one direction and uncrimped providing the highest mechanical properties. The fibers are held together by stitching with polyester yarns. Composites using unidirectional tapes or sheets have high strength in the direction of the fiber. Unidirectional sheets are thin, and multiple layers are required for most structural applications. In some composite designs, it may be necessary to provide a corrosion or weather barrier to the surface of the product. [Soo-15]

Non-Crimp Fabrics Non-crimp fabrics are made of superposed layers of carbon fiber set in different directions and held together by fiber stitching or knitting. A thermoplastic polymer out of nylon or polyester material is usually used as the stitching fiber. Depending on the requirements of the final composite, sometimes glass or aramid fibers are used as stitching fiber also. Non-crimp fabrics have significant advantages over woven fabrics and prepregs. They have increased strength and better off-axis reinforcement. Orientated unidirectional layers are structurally more efficient than crimped woven fabric. Their fatigue and impact resistances are also improved and they are quicker to wet than woven fabrics. These kinds of fabrics are used in high performance composite cars, GRP power boats, blades for wind energy and FRP pipelines. The multilayer setup has the potential to out-perform prepregs-tape laminates in addition to the normally straight fibers it can withstand a greater amount of out of plane stresses [Bak-04]. The combination of non-crimp material and thermoset binder can be used in the preforming process. This configuration is easier to drape than using resin pre- impregnated textiles (prepregs). Prepregs are flat forms made out of fiber or woven fabrics and are impregnated with resin [Bak-04].

Figure 2.5 Non-crimp fabrics structure [Lan-12]

2.3 Manufacture of CFRP Manufacture of carbon fiber is a very broad field and has a wide variety of alternatives. As a first classification, manufacturing methods can be divided into two types: open molding and closed molding processes.

Marina Moya Sánchez Page 8 wbk Institute of Production Science 2 Fundamentals

Open molding is the method that has been established in the polymer-matrix composite industry for the longest time. The main advantages of open molding methods are their simplicity and economic viability, but they have the disadvantage of being mostly manual methods, which requires specialized operators and implies low production rates. The molding method involves placing reinforcements and liquid resin onto the surface of an open mold. The hand layup version involves applying the reinforcements and the resin by hand, while the spray-up version uses tailored spray equipment to deposit both the reinforcements and the resin on the mold or an alternative substrate. Producing large complicated shapes as well as smaller and simpler composite products is possible. [Soo-15] The open mold process is able to cure at room temperature without added heat or pressure, and the actual process will require 4–6 h from start to finish for each molding produced. Wet layup, hand layup, spray-up, tape layup, filament winding and autoclave curing are the most common open mold methods. On the other hand, close molding methods comprises the fabrication processes in which the part is produced in within a mold cavity by using two or more tool pieces. There is a wide variety of molding processes that can be classified in two main categories: compression molding and liquid injection molding. Using compression molding is only justified by the need of large number of parts production (tens of thousands), due to its high cost. The alternative are the liquid injection molding processes, including RIM, structural reaction injection molding (SRIM) and Resin Transfer Molding (RTM). 2.3.1 Resin Transfer Molding Process (RTM) The best alternative for close mold production is RTM. For RTM, the liquid resin injection follows once the mold is first loaded with materials such as the reinforcement fibers and cores. RTM processes produce parts with strengths, stiffness, and weights that most closely resemble typical open mold parts. Cycle times can range from a few minutes for small parts to several hours for large parts. A wide variety of resin chemistries are used according to the design end use requirements. RTM is a very simple process. The stack of fabrics that is going to work as the reinforcement in the composite is placed into the mold cavity and the mold is closed. The resin is injected into the cavity under pressure. The injection pressure is around 700kPa. The tooling and resin are preheated before injection. When the injection is complete the heating temperature of the mold is increased to resin cure temperature. Very often a vacuum is used to remove the entrapped air and speed up the process. Once the resin fills the cavity, it cures. Typically, thermoset polymers such as epoxy, vinyl ester, methyl methacrylate, polyester, or phenolic resin are used with fiberglass, carbon fiber, aramid, and synthetic fiber reinforcements, either individually or in combination with each other. [Soo-15] In most common applications using the RTM process, parts are molded with a cross- sectional thickness of 4 mm with 30% fiber content of the total laminate. Figure 2.6 represents the entire RTM process chain and its steps. Specifically in Fig 2.6 it is shown the chain installed in the wbk department. As seen, besides the injection itself, the process requires some preparation steps that are also significant, namely, the draping step and the handling tasks. On the other hand, Figure 2.7 shows the specific steps followed during the resin transfer injection process.

Marina Moya Sánchez Page 9 wbk Institute of Production Science 2 Fundamentals

Figure 2.6 RTM Process and process chain steps [Fle-13]

Figure 2.7 The Resin Transfer Molding (RTM) process [AVK-10]

2.3.1.1 Preforming Preforming is carried out before the RTM resin injection process. A preform is a 2D or 3D form that is of a near end-geometry in form and dimension. The form is to be inserted into the mold cavity of a RTM Press where a resin will be injected into the cavity to obtain the end, hardened composite part [Bak-04].

Marina Moya Sánchez Page 10 wbk Institute of Production Science 2 Fundamentals

In a general RTM process, the reinforcement can be either “preformed” to the exact shape of the molding tool in a previous operation or be hand-tailored during the loading process in the molding tool. Furthermore, the reinforcement preform must not have creases or any other possible defects due to the preforming process. [Bak-04] In Table 1 are summarized the different methods of preform manufacturing. As seen, a first classification can be done by dividing the methods into direct preform production and sequential preform production. Direct preform production involves all the manufacturing processes that require only one step for obtaining the final preform starting directly from the carbon fibers. That means that the preforms are manufactured, for example, by 3D weaving, knitting or braiding to their final shape. The greatest disadvantage of these methods is the small range of preform shapes that actual technology is able to manufacture, being usually limited to axisymmetric shapes. The sequential preforming processes require at least two steps. Usually a first step involves the manufacturing of the textile reinforcement in form of a sheet, which in further steps is going to be shaped into its final contour by different methods such as diaphragm forming, stamping, etc.

Table 1. Methods of Preforming Manufacturing [Hen-11]

For manufacturing complex part preforms, sequential preform production has been proved to be more appropriate. Different preforming methods have been developed in order to find out the best way to obtain the most precise preform that guarantees good quality on the final part. The most common sequential preforming methods are the diaphragm method, which obtains the final preform by spreading the semi-finished textile, and the stamp method, which obtains the final preform by pressing the semi- finished textile with the help of a stamp. The draping of semi-finished products by means of membrane takes place not only in the preform, but also for example in the processing of prepregs. The shaping is in all variants of the diaphragm methods achieved by a differential pressure, so that only a positive or negative half-mold is required. After heating the stack of fabrics to the required temperature (depending on the type of diaphragm, fabrics and binder being used), a pressure difference is applied and the membrane is pressed against the mold and shapes the semi-finished product into its final contour [AVK-10]. The different variants of diaphragm methods differ both in the type of differential pressure generation as well as in the number of diaphragms used. Figure 2.8, represents a diaphragm preforming station, with two diaphragms. If only a single membrane (single diaphragm) is used, the semi-finished product is in direct contact with the mold. This allows easy positioning of the semi-finished

Marina Moya Sánchez Page 11 wbk Institute of Production Science 2 Fundamentals products, however, finds a rapid cooling of previously heated semi-finished products instead.

Figure 2.8 Draping by double diaphragm method [AVK-10]

The stamp forming process, on the other hand, is ideally suited to the large volume production of thinwalled thermoplastic composite products with complex shapes. The process involves a small number of steps, as shown in Figure 2.9. Usually, a stack of differently oriented uni-directional (UD) or textile fiber reinforced plies is used for this kind of preform manufacturing. The stack is positioned within a gripping frame and transported to a heating device, such as an infra-red oven, or a heating table, like the one available in the wbk. The stack is transported towards the tooling, after a sufficiently high temperature above the polymer melting point has been reached. The tooling consists of a positive male and a negative female part. Both matched-metal and rubber-metal configurations are used in practice, for which at least one of the tools is usually pre-heated to control the cooling process of the formed laminate. The preform is formed by closing the tooling, after which a high consolidation pressure is applied. The formed preform is released after 1 to 2 minutes of cooling. Subsequently, a trimming operation is applied to remove the excess material.

Figure 2.9 Draping by stamping method

2.3.1.2 Industrial Handling Industrial handling plays an important paper during the whole process. To achieve the fully automated process the importance of selecting the most proper grippers to carry out the handling tasks is fundamental. The handling of semi-finished textile products, which are dimensionally unstable and permeable to air, is nowadays a big challenge. Most of handling tasks (supplying and feeding of semi-finished textile products) are

Marina Moya Sánchez Page 12 wbk Institute of Production Science 2 Fundamentals still carried out manually. A significant amount of research is being carried out in the field of handling of the textile products and progress is being made. When handling semi-finished textile products, the main objectives are nearly damage, free gripping and high process reliability during the picking. [Fle-13] These requirements have to be taken into consideration when selecting the gripping principle. An overview of common gripping principles is shown in Figure 2.10. At present, needle grippers, Coanda grippers and Bernoulli grippers are used for handling semi- finished textile products. Electrostatic grippers and grippers that work according to the principle of adhesion forces, like freezing grippers, are in contrast still a subject matter of current research activities.

Figure 2.10 Typical grippers for textile semi-finished products [Fle-13]

First of the three main types of fabrics gripper is the needle gripper. It is the most simple and effective of them. Its performance is based on the action of a group of needles that cross the stack of fabrics and allow its transport. The main problem of this kind of gripper is the damage caused in the fabrics. On the other hand non-contact low pressure grippers such as Coanda and Bernouilli grippers are the best alternatives for needle grippers. Even though their effectiveness is not as good as the needle ones, these kinds of grippers don’t suppose any damage to the fabrics. The Bernouilli gripper is based on the Bernouilli principle. The Bernoulli principle states that if a high velocity air stream is passed over the surface of an object, the local pressure will drop. If a higher pressure exists on the other side of the object a net force towards the high velocity side is produced. If this force exceeds the resistance to motion (typically weight) the object will be drawn towards the low pressure side. Because airflow is outwards from the gripper there is no risk of blockage as with suction cups. Commercially available Bernoulli grippers are typically targeted at handling rigid products such as silicon wafers and circuit boards. Finally, the Coanda gripper is based on the Coanda principle. The Coanda principle explains that when there is a big amount of air hitting a curved surface, the air has a tendency to follow that curved surface. Utilizing this concept, the grippers eject air in a circular pattern through an annular gap around a curved surface, which creates a low pressure point in the middle that draws in air. This creates a vacuum that engages the part. [Ada-14] a) Needle gripper b) Bernouilli gripper c) Coanda gripper

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Table 2. Most common types of grippers 2.4 Heat Transfer Fundamentals Heat transfer (or heat) is thermal energy in transit due to a spatial temperature difference. Whenever a temperature difference exists in a medium or between media, heat transfer must occur [Ber-11]. As shown in the Figure 2.11, heat transfer can be divided in three different processes or modes: conduction, convection and radiation.

Figure 2.11 Conduction, convection and radiation heat transfer modes [Ber-11]

Heating transfer processes take place all along the preforming process. During the heating of the heating table and the preform table heat is transferred by conduction across the different aluminium parts and in between them. Also conduction can be found across the fabrics during their warming. Interaction between the plates, fabrics and surrounding is present by radiation and convection processes.

2.4.1 Conduction Conduction may be viewed as the transfer of energy from the more energetic to the less energetic particles of a substance due to interactions between the particles. [Ber- 11] Convection processes occur within solids, liquids or gases, provided there is a temperature difference between different points of the substance, and until a new equilibrium is reached. Every heat transfer process can be quantified in terms of the appropriate rate equation. For heat conduction the Fourier’s law quantifies the amount of energy being transferred per unit time. 휕푇 푞′′ = −푘 · Formel 2.1 휕푥 Being q’’ the heat flux (W/m2) in the x-direction per unit area perpendicular to the direction of transfer, 휕푇/휕푥 the temperature gradient and k (W/m·K) the thermal conductivity which is characteristic of the given material and has dependence on the Marina Moya Sánchez Page 14 wbk Institute of Production Science 2 Fundamentals temperature. The minus sign is a consequence of the fact that heat is transferred in the direction of decreasing temperature.

8.1.1.1.1.1 Contact Resistance and Contact Conductance When a junction is formed by pressing two similar or dissimilar materials together, only a small fraction of the nominal surface area is actually in contact because of the non-flatness and roughness of the contacting surfaces. If a heat flux is imposed across the junction, the uniform flow of heat is generally restricted to conduction through the contact spots, as shown in Figure 2.12. The limited number and size of the contact spots results in an actual contact area which is significantly smaller than the apparent contact area. This limited contact area causes a thermal resistance, the contact resistance or thermal contact resistance. [Hew-15] The inverse of this property is termed thermal contact conductance.

Figure 2.12 Contact surface between two parts pressed together and temperature variation due to contact conductance [DS-15]

Through the interface between the two contacting faces, two modes of heat transfer exist. The first is conduction through points of solid-to-solid contact (Qconduction) which is very effective. Secondly, conduction through the gas filled gaps (Qgap) which, due to its low thermal conductivity, can be very poor. To treat the thermal contact resistance, an interfacial conductance, hc, is placed in series with the conducting media on both sides as shown in the next figure. [DS-15]

Figure 2.13 Heat flow between two solids in contact and the temperature distribution [DS-15]

From considerations of energy conservation, the heat flow between the two bodies in contact is found as:

푇0−푇푓 푞 = ∆퐿 1 ∆퐿 Formel 2.2 2 + + 1 퐴·푘2 ℎ푐 퐴·푘1

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One may observe that the heat flow is directly related to the thermal conductivities of the bodies in contact, k1 and k2, the contact area A, and the thermal contact resistance, 1/hc , which, as previously noted, is the inverse of the thermal conductance coefficient, hc.

The interfacial conductance, hc, depends on the surface finish of the contacting faces, the material of each face, the pressure with which the surfaces are forced together and the substance in the gaps between the two contacting faces. 2.4.2 Convection Convection heat transfer occurs between a fluid in motion and a bounding surface when the two are at different temperatures. Consider fluid flow over the heated surface of Figure 2.14. A consequence of the fluid–surface interaction is the development of a region in the fluid through which the velocity varies from zero at the surface to a finite value u∞ associated with the flow. This region of the fluid is known as the hydrodynamic, or velocity, boundary layer. Moreover, if the surface and flow temperatures differ, there will be a region of the fluid through which the temperature varies from Ts (y=0) to T∞ in the outer flow. This region, called the thermal boundary layer may be smaller, larger, or the same size as that through which the velocity varies. In any case, if Ts > T∞, convection heat transfer will occur from the surface to the outer flow. [Ber-11]

Figure 2.14 Fluid temperature distribution over a heated surface [Ber-11]

Depending on the nature of the fluid flow, two types of convections can be distinguished. - Forced convection, when the flow is caused by external means, such a fan, a pump or atmospheric winds. - Free (or natural) convection, when the flow is induced by buoyancy forces, due to density differences caused by temperature variations. The equation that quantifies the heat convection is the Newton’s equation.

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푞′′ = ℎ · (푇푠 − 푇∞) Formel 2.3

2 Where, q’’ (W/m ) is the convective heat flux, Ts is the surface temperature, T∞ is the fluid and h (W/m2·K) is the convection heat transfer coefficient (or film coefficient). This coefficient depends on conditions in the boundary layer, which are influenced by surface geometry, the nature of the fluid motion, and an assortment of fluid thermodynamic and transport properties. Any study of convection ultimately reduces to a study of the means by which h may be determined.

2.4.3 Radiation Radiation may be attributed to changes in the electron configurations of the constituent atoms or molecules. The energy of the radiation field is transported by electromagnetic waves (or alternatively, photons). 2.4.4 Heat Capacity The heat capacity rate is heat transfer terminology used in thermodynamics and different forms of engineering denoting the quantity of heat a flowing fluid of a certain mass flow rate is able to absorb or release per unit temperature change per unit time. [Ber-11]

푑푚 퐶 = 푐 · Formel 2.4 푝 푑푡

Where C is the heat capacity rate of the fluid of interest, dm/dt is the mass flow rate of the fluid of interest and cp is the specific heat of the fluid of interest. 2.5 Product Development Method, VDI The VDI (Verein Deutscher Ingenieure or Association of German Engineers) is an engineering association that promotes the advancement of technology and represents the interests of engineers and of engineering businesses in Germany. One of the multiple labors of the VDI is to define standards and guidelines considering methodology, requirements and limitations in the different aspects that shape the wide range of technic and technology present nowadays in engineering. Today, approximately 200 Standards based on the latest technical developments are produced by the VDI’s technical divisions per year. That way the VDI has systematically built up a set of technical regulations, which today contains more than 2000 valid VDI Standards extensively covering the broad field of technology. Today’s topics range from securing loads on road vehicles to testing of optical fibers up to biomimetic and monitoring the consequences of genetically modified organisms. [VDI- 15] VDI Standards play a very important role in German engineering and as pioneers for international standardization. The VDI Standards are considered accessible for every market participant; they allow savings by reducing information and control costs as well as costs for adjustment and trading. Another advantage of VDI standard is the faster spread of technical knowledge. Standards thereby facilitate contractual arrangements, commercial transactions, and serve to reduce technical trade barriers. With the VDI Standards the VDI fulfils its primary function: the transfer of technical knowledge as a service to engineers and students. [VDI-15]

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One of the multiple guidelines developed by the VDI is the VDI 2225 (1998), which provides a decision method for engineering designs. VDI 2225 proposes a simple approach, based on a five-point scale to score the alternatives. VDI 2225 also considers that the criteria may have different importance in the alternatives, and therefore different weights should evaluate them. VDI do not specify or recommend the scale to weight the alternatives. This Standard not only rates the alternatives based on their technical value, but also offers a decision method based on their costs. The computation of costs is done separately from the technical computation. Further, the VDI 2225 proposes a graphic approach to evaluate the alternative, plotting the technical value x versus the economic value y, defining a point s, in the s-diagram (graph x versus y). VDI suggests that the best solutions have a balanced relationship between cost and technical skills, thus, being nearly the diagonal (traced) line of the s- diagram (Figure 2.15). The s-diagram is also useful to accomplish the evolution of a product. The values s1, s2 and s3 could represent respectively the first, second and third edition of a product. For the development of this thesis, only the technical aspect is going to be taken in consideration, leaving the economic aspect for possible future investigations. [And-11]

Figure 2.15 S-Diagram for alterative evaluation [VDI-98]

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3 State of the Art Nowadays in some sectors of the industry the use of composite materials is already widespread. In the automotive world, although it still hasn’t been introduced widely, the use of composite materials to replace the currently used materials (aluminum, steal...) is an alternative that has been in the sector for some years now. However, in recent years, due to the reasons explained in previous points, the introduction of composites in mass production vehicles is becoming a necessity, more than a possible alternative. For this reason, research in this field is growing exponentially. In fact, the first vehicles whose structure, body or some of their parts are made of carbon fiber are being already manufactured. This is the case of models i3 and i8 of BMW in which all the passenger compartment is made of carbon fiber. Another example is the Chevrolet Corvette Stingray, which has introduced composite materials for the manufacturing of the hood or the Teijin Concept Car, which for now is just a prototype but has a 47 kg weigh fully composite manufactured structure. As seen, even though composites are entering the automotive industry, there are still only a limited number of examples of vehicles using CFRP, and composite materials technology still has a long way to go to meet the current needs of the automotive industry. Composites offer great potential to reduce automotive structural weight, yet they’re currently not economically competitive with conventional materials, from a production point of view, due to the manual or semi-automated labor involved. One of the factors that are slowing the growth of the automated CFRP manufacturing is the preforming. The obtaining of a good quality preform for the RTM process is one of the big challenges of composites engineering. In the last years, engineered preforms have been developed through the use of automated knitting and weaving machinery. These two- and three-dimensional constructions are increasingly capable of reinforcing high-performance structural composite parts, but most have failed to enter the manufacturing mainstream in the automotive industry due to their perceived high cost, the auto industry’s change- averse culture and some difficult-to-surmount engineering hurdles. [Bla-13] For this reason, during the last years equipment and material suppliers have been developing a variety of preforming methods. An example is the American GFM. A prototype developed recently was centered on the manufacturing of the front bumper for a car using a biaxial hybrid glass fiber/carbon fiber fabric. The fabric was located on a mold tool that rested on a movable table; the table then moved the tool into a second position under a matching air-controlled “forming template” that compressed the fabrics from above. After the draping, the table moved again the tool to a third position, where the cure of the binder took place. Another project is being developed at the Institut für Textiltechnik (ITA) at RWTH Aachen University (Aachen, Germany). ITA has developed a wide range of automated preforming technologies. Recently the manufacturing of a 1,230 mm by 760 mm composite roof segment that could replace a portion of the current steel roof of the BMW 3 Series convertible has proved their applicability to large-scale vehicle production. The size and complexity of some parts make it difficult to manufacture in a single preforming step. For these reason, also multistep preforming machines are being developed. The Fraunhofer Institute for Chemical Technology (ICT, Pfinztal, Germany) in collaboration with the University of Western Ontario (London, Ontario, Canada), develops preforming processes in a fully automated PreformCenter

Marina Moya Sánchez Page 19 wbk Institute of Production Science 3 designed by Dieffenbacher GmbH (Eppingen, Germany, and Windsor, Ontario, Canada). The PreformCenter comprises several modules: the first is a CNC cutting table to cut the plies necessary to make the part preform. A robot arm e then transfers the cut plies from the cutting table to the binder application module, a cabinet containing spray equipment with nozzles that spray an epoxy-based binder upward onto the preform’s bottom surface. The robot then places the tacky laminate stack on a “draping” module, which automatically forms the 2-D layup into a 3-D shape. [Bla-13]

Figure 3.1 PreformCenter designed by Dieffenbacher GmbH (Eppingen, Germany, and Windsor, Ontario, Canada).

Separately, A five-year-old startup, EELCEE AB (Trollhättan, Sweden, and Lausanne, Switzerland), is marketing a uniquely different approach to preforms. An automated process cell pulls multiple continuous fiber rovings from a creel through a series of dies that wet out the fiber with resin. Then, a robotically controlled layup head creates an open, tailored 3-D “skeleton,” trademarked QEE-FORM, by rapidly placing the fiber/resin strands, in any shape or size, around integral metallic bushings or fastener points. The resulting QEE-FORM frame is then robotically placed in either an injection molding machine or in a compression molding press, where it provides additional reinforcement for glass mat thermoplastic (GMT) or sheet molding compound (SMC). [Bla-13] Even though research is being made in different countries and institutions, for now BMW (Munich, Germany) is staying ahead of the rest by developing the most complex preforming process to date: the automated preform lines at its Dingolfing and Leipzig plants, which are currently producing the all-carbon Life Module body for the i3 electric car. Carbon multiaxial fabrics with binder are preformed in a multistep process. [Bla- 13] This research to find the best solutions for preforms manufacturing is also taking place in the wbk – Institut of Production Science at the Karlsruher Institut für Technologie (KIT, Karlsruhe, Germany). A fully automated RTM process chain is being developed, in which a state of the art preforming station is installed. The preforming station counts with both multi-step stamping and diaphragm methods for performing the preforming operations. Figure 3.2 represents a layout of the hall where the full RTM manufacturing process takes place.

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Figure 3.2 RTM Process Chain Layout in wbk. (1- Cutting, 2- Fabrics heating, 3- Stamping, 4- Quality control, 5- RTM)

The process begins with the cutting of the textile products on the cutting station. Once the fabrics are cut into their final dimension, a special binder is applied to them and then they are heated in the heating table until they reach the binder activation temperature. Then an industrial robot arm moves the fabrics to the preforming station, where the draping takes place. The preforming station developed in the wbk has the special characteristic of performing both diaphragm and stamping draping methods. This thesis is focused on the second one, which has been proved in better final quality results. Figure 3.3 represents the three areas in which the preforming station can be subdivided.

Figure 3.3 Draping Station Areas

As seen on the Figure 3.3, the preforming station is divided into three major sections namely the loading area, the draping area and the quality assurance (QA) area.

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After the heating process, and when the fabrics have reached the binder melting point, the two-dimensional CFRP sheets are loaded in the preforming station, moved by a robotic arm that locates them on the male tool. Once the fabrics are correctly located, the profile table moves to the draping module, where the stamping punches are located, and the stamping takes place. After the stamping process, the punches are left in their lowest position until the preform has reached a stable point in which all the fabrics have a lower temperature than the binder melting temperature. Then the punches are retired and the profile table moves again to the quality assurance area, where the final preform is studied, and possible defects analyzed. The preform station is designed also for using the diaphragm process as an alternative preforming process, although the contour geometry obtained after this process has shown to be less accurate. After the quality assurance has been done, the final preform is brought to the RTM press, where the resin injection is made. An important difference between the stamping process developed in wbk and any other industrial stamping process (for example the sheet metal stamping) resides in the multiple punches used for the process, which on the one hand allow the process to obtain better contour results by trying to simulate the manual draping but, on the other hand, the multiple punches are also the root of a number of problems that have to be evaluated and solved. Figure 3.4 represents this process.

Figure 3.4 Sequential stamping process

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4 Own approach In this section the procedure of the thesis is going to be defined. As it is divided in two defined parts, a different procedure is followed for each one. 4.1 Design of the Stamping Unit For the design to be made with the most thoroughness possible, the SPALTEN problem solving method is going to be followed. SPALTEN is a methodical problem solving system developed by the Istitute for Product Development (IPEK) at the Karlsruhe Institute of Technology. The word SPALTEN is a German acronym which represents every step that has to be followed to achieve a good final solution. The SPALTEN acronym refers to: Situation Analysis (Situationsanalyse), Problem Isolation (Problemeingrenzung), Alternatves Presentation (Alternativen aufzeigen), Solution Selection (Lösungsauswahl), Scope Analyse (Tragweite analysieren), Decision and Implementation (Entscheidung und Umsetzung) and Debriefing and learning (Nachbereitung und Lernen). This sequence can be understood as a guide for the methodological solution of any problem. Applied to the specific work in this thesis, the following flowchart shows the working procedure that is going to be followed.

Figure 4.1 SPALTEN problem solving method for the stamping unit design

On this basis, the proceeding order is the one specified:

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 As an initial step, research of the basis of the process and the state of the art is carried out. Then, the requirements and boundary conditions are going to be defined, with the objective of establishing the limits of the design and defining the functions that must be specially evaluated.  Once this limits are stablished, the problem to solve is going to be structured as a functional structure, in which the important functions that must be considered are determined.  Different alternatives for every function are going to be presented, for a further selection of the best alternative.  The alternatives of every function are going to be evaluated considering different influential factors, and an alternative for every function is going to be selected, giving a final design concept and an evaluation of its characteristics.  Considering the fact that the construction is not going to take place during the development of this thesis, the two last steps of the SPALTEN method are not going to be followed. The realization of these steps, together with an improvement of the final design obtained after the work of this thesis, is left for further thesis to be carried out. 4.2 Heat Transfer Analysis Being the heat transfer analysis an independent part of this thesis, a brand new proceeding system is applied, considering it a completely different type of work. Figure 4.2 summarizes de procedure that is going to be followed during the developing of this part of the thesis.

Figure 4.2 Methodology followed for the heat transfer analysis

Considering this, the proceeding order is the one that follows:  After an initial step of reading and research over fundamentals and state of the art regarding this topic, the current heating process that takes place in the wbk Marina Moya Sánchez Page 24 wbk Institute of Production Science 4 Own approach

preforming station is going to be defined, as well as the interactions in between the involved elements, the boundary conditions and the applied loads.  Temperature and time measures are going to be taken in the preforming station and heating table in the wbk hall to compare the results to the defined model.  An initial FE model is going to be defined in Abaqus, with theoretical approximate values for the different parameters that affect the heat transfer in the different parts involved in the process as well as the fabrics used in the preforming.  The model results are going to be compared with the data obtained in the experiments.  A loop procedure then is going to be followed, in which adjustments are made in the model and compared to the experiment data, until a final solution that closely represents the real behavior of the components is found.  A simulation of the whole process is going to be run, from the table heating to the fabrics stamping, and temperature variation during the process is going to be studied.

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5 Stamping Unit Design 5.1 Design Boundary Conditions As it was said in previous points, the target of the wbk is to achieve a fully automated RTM process chain fit for industrial application, in which high quality composite parts are manufactured.

Handling Handling Cutting Preforming Inyection Finishing textiles textiles

Figure 5.1 RTM Process

The critical step in the RTM-chain is the preforming. The final shape of the shell obtained in the preform process affects directly to the accuracy obtained in the final carbon fiber part. The contour accuracy and the absence of gaps and folds are the most important quality requirements that must be considered, in order to achieve high quality parts at the end of the chain. 5.1.1 Current Preforming Station The preforming station is responsible for the draping process during the RTM chain. In the longitudinal dimension three areas can be distinguished, the loading area, the draping area and the quality assurance area. Meanwhile, in the vertical direction there are also three areas, the bottom being the base where the profile table moves. The next vertical stage is where the infrared heater and diaphragm are fixed, both parts belonging to the diaphragm draping method. The third vertical stage is where the stamp unit stays when it is in its top-end position. In the stamping process at issue, the loading area of the station recieves the fabrics from the previous heating stage. With the use of two pneumatic guides, the profile table then moves the fabrics to the stamping position in which the stamp module stands. Again using pneumatic guides, de module is moved to its working position, and then a sequentian stamping takes place. After a waiting period to low the fabrics temperature, the stamps are retired and the table moves again to the final quality assurance area in which a countor scann can be used to validate the quality of the preform. For the diaphragm method application, a membrane and a heating unit are installed, as well as a vacuum pump responsible for creating the preassure difference for the diaphragm drapping, explained previously in this text. Figure 5.2 shows the current preforming station. The entire station has its CAD representation. The software used for the already existing CAD representation, and the one that is going to be used during the development of this thesis is Siemens NX 9.0. The frame is the static support to hold all components and movement axes together and therefore gives the preforming its basic form and structure. The maximum dimensions of the frame are 3476 mm in length, 1860 mm in width and 3051 mm in height [Göb-12]. Being the station a prototype in which changes are made constantly, the frame is made out of extruded aluminium profiles that allow the easy attachment of new modules or peripherals.

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Figure 5.2 Preforming Station

The Stamp Module

The stamp module is a fundamentally structural component of the stamping station, and it have to main functions. Firstly, it is responsible of the correct subjection of the Marina Moya Sánchez Page 27 wbk Institute of Production Science 5 Stamping Unit Design stamping unit to the main frame of the station, and secondly it allows the movement of the unit in the vertical direction, permitting the unit to be in its working position when the stamp draping method is being used, and then locating the unit on its top rest position when the diaphragm draping method is functioning. Stamping unit it is considered the moveable part of the module that holds the cylinders assembly and that is going to be modified during the course of this thesis.

The stamp module is fully made out of aluminum profiles and standardized connectors. It has two pneumatic guides in each side, which are responsible of the vertical movement of the module from its top position to its low positon, where it meets the profile table. The guides are manufactured by FESTO, and its characteristics are: Model FESTO Linear Drive DGC Piston diameter 25 mm Stroke 160 mm Cushioning Flexible cushioning rings/pads on both sides Positioning sensing For proximity sensor Temperature Resistance Heat-resistant seals max. 120ºC

Table 3. Stamp module pneumatic guides characteristics

The stamping module is represented in Figure 5.3. The stamping unit that is going to be designed has to fit the available space in the module. The module has maximum dimensions of 1538mm x 850mm and it has a middle clearance reserved for the cylinders assembly of 830x760 mm.

Figure 5.3 Stamp module

The Profile Table

The profile table has a determinant role during the whole process, being part of every step of the preforming. It is responsible of moving the mold and the fabrics along the different areas of the station, beginning in the loading area, passing through the

Marina Moya Sánchez Page 28 wbk Institute of Production Science 5 Stamping Unit Design draping area and finding its end position in the quality assurance area. The movement of the profile table is possible thanks to two longitudinal pneumatic guides located on the extreme sides of the table. The guides are manufactured by FESTO, and its characteristics are: Model FESTO Linear Drive DGC Piston diameter 40 mm Stroke 900 mm Cushioning Pneumatic cushioning, adjustable Positioning sensing For proximity sensor Guide Recirculation ball bearing guide

Table 4. Profile table pneumatic guides characteristics

The profile table has the surface area of 1000 mm by 1000 mm and it is made also of aluminium. It holds the mold and the work-piece during the preforming process The table is designed with an upper drilled surface that allows the air flow during the diaphragm process vacuuming. In the middle of the table a space is reserved for fixing the male tool. On its lower side the table is heated by six heating sources that will increase its temperature to a constant temperature of 50 º C during the stamping process. The stopper defines the position of the longitudinal table when it is in the draping area. The stopper is retracted when the table is to be moved into the quality assurance area.

Figure 5.4 Profile table

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Current Stamping Unit Figure 5.5 represents the stamping unit as it is installed now in the wbk preforming station.

Figure 5.5 Current stamping unit

The current stamping unit that is installed in the preforming station is based on an aluminium profile structure, as it is seen on the image. This structure is fixed to the rest of the stamping module by standardized connectors. The cylinders responsible of the stamping process are fixed directly to the aluminium profiles by using only one screw per cylinder. The cylinders used for this assembly are the same ones that will be used for the design of the new stamping unit, and their characteristics are summarized in Table 5.

FESTO DFM Guide Cylinder B Model Series

Piston diameter 25 mm

Stroke 160 mm

Flexible cushioning rings/pads on Cushioning both sides

Positioning sensing For proximity sensor

Temperature Heat-resistant seals max. 120ºC Resistance

Table 5. Stamping unit pneumatic cylinders characteristics

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5.2 Functional Structure and Requirements

5.2.1 Functional Structure The preforming stage that is going to be improved is now going to be subdivided in a functional structure, in order to isolate the different requirements, and to facilitate a more organized design. Considering structural and operational characteristics of the stamping unit, three major functions have been identified. - The stamping system, which involves all the cylinders and the way they are fixed to the stamping module and to the female tools. - The fixing interface, which refers to the way the cylinders-mold assembly, is fixed to the stamping unit frame. - The fabrics holding system, which will evaluate the possibility of implementing a holding system for the fabrics in the positive shaped mold. The heating process has been added as a fourth function to the functional structure diagram, considering that, even it has no influence on the design that is going to be developed in the following points, it is an important factor during the process. Both the stamping system function and the fabrics holding function have been also subdivided in two separated sub-functions, as shown on Figure 5.6. Each of these functions is going to be studied separately and design concepts given and evaluated, in order to find an optimal final design.

Figure 5.6 Stamping unit functional structure

5.2.1.1 Stamping System The final objective of the research that is being made in the preforming station is to obtain the most precise final preform after the sequential stamping process. One of the biggest problems that wbk is facing right now, is to achieve this accurate contour shape. The fact that seven different stamps are being used for the same mold, has an important disadvantage, which is the difficulty to define an appropriate positioning system that guarantees the relative position between the punches and between the negative and the positive shapes of the mold.

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Considering this, one of the challenges is to develop an accurate positioning system that allows every cylinder to adjust its position independently in both planar directions.

Figure 5.7 Schematic view of the stamping unit

Figure 5.7 shows a schematic view of the stamping unit. As seen, two different types of parts can be distinguished, on the one hand, the fixed parts and on the other, the movable parts. They are considered fixed only during the stamp process. As it has been said before, both the table and the stamp module have defined movements along the preforming station. The figure permits the understanding of the two sub-functions that are going to be considered. The first one, the positioning, that implies the modification of the relative position of the movable parts, either between the cylinders and the plate or between the plate and the stamp module. The second one is the positioning reference definition, which will try to define a proper reference that guarantees that the positioning is done correctly.

5.2.1.2 Fixing Interface This function is intended to define the way the plate is fixed to the stamping unit frame. An adequate and simple solution has to be found to include the new stamping unit in the already existing preforming station.

5.2.1.3 Fabrics Handling Handling operations occur all along the RTM process. Specifically in the preforming stage, handling is necessary to move the multilayers 2D preform to the heating table and from there to the preforming station. After the preforming, handling is also necessary to move the 3D preform to the RTM station. In the current station, no holding system is included during the stamping process itself. In order to keep to a minimum the superficial defects of the final preform, a holding system is going to be introduced in the lower mold. This improvement will enable the process better final shape results, by preventing the relative movement of the fabrics on the mold during the draping.

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5.2.2 Design Requirements

5.2.2.1 General Requirements General requirements are considered the ones that can be applicable to any machine or engineering product design. In this specific context, general requirements will be those who must be present during the entire design process and are applicable to all design functions to be defined in next sections. Summarizing, the final concept design must be functional, safe, economic, manufacturing viable, environmentally friendly, competitive and marketable. Greater or lesser extent, all this requirements should be considered during de design process.

5.2.2.2 Quality Requirements The preformed part must fulfill the predefined geometry specifications for this production stage. Figure 5.8 shows the objective part that is going to be manufactured during the process.

Figure 5.8 Final preform

The preform should have minimum creases if the draping is optimal. A quality assurance station is available on the preforming station and geometry measurements are made using a laser surface scanner. Visual inspection for visible (macro) defects is also carried out in this station. The quality of the preform has an influence on the final part quality after the complete RTM production process. The 3D preform obtained must be as close as possible to the final part shape, and the design should seek the optimal continuous thickness.

5.2.2.3 Function Requirements Finally, the requirements associated to each of the described functions are here specified.

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Stamping System The stamping system must be included in the existing stamping module, so both the dimensional and geometrical specifications explained in point 5.1.1 should be considered. The adaptability to other mold designs for future preform shapes is an important factor to consider. The process, as it is developed to date, requires a complete new design of the stamping system in every new part to be made. Even so, it is important to define the most adaptable system possible, so that the minimum amount of changes has to be made in the future. The ease of assembly is also an important agent, in order to reduce cycle times of the whole chain and improve productivity. Finally, the system components must have good wear resistance and any maintenance action should be possible with minimum effort. For this new design of the stamping station, the existing cylinders have to be used.

Fixing Interface The fixing should be preferably done with standardized connectors and profiles. Ease of assembly and disassembly are important factors to take care of, just as the adaptability to other mold shapes. A factor that can also be considered while designing the interface fixing is the possibility of using the plate itself as a primary positioning system. It is preferably to do the less changes to the current stamping module, in order to keep in a minimum the expenses and the complexity of the design.

Handling System The handling has to be made with no damage of the fabrics and energetically efficiently. As the other functions, the most adaptable for future shapes the design is, the better.

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5.3 Concepts In the following section the alternative solutions for every function are going to be described. 5.3.1 Stamping System Concepts The currently installed fastening system for the cylinders is based on the use of aluminium profiles. With this kind of fastening, the tensions after every punch are concentrated on the only screw that holds the cylinder to the profile, as seen in Figure 5.9. This implies that the screw that fixes the cylinder to the bar has to be tightened after every few stamping operations, because of its tendency to loosen. A first design decision is to replace this present system, and use an alternative one that permits a better distribution of the forces after every punch. This alternative is the use of a clamping plate, specially designed for this application and this specific preform shape.

Figure 5.9 Current cylinder fastening system

The cylinders positioning on the plate is directly dependent on the size and distribution of the different parts in which the negative shaped part of the mold is divided. As a first step of the CAD design, the cylinders are fixed into the positions they must have, so that every part of the mold stays in its initial relative position. The final position of the cylinders is decided as a result of the best possible combination of these two factors: - Cylinders should be as separate as possible from each other, so there is more design freedom. - Cylinders should be as centered as possible when fixed to the mold punch they are attached to, so that the forces are equally distributed and the preform quality does not decrease. Consequently, the second design decision that has to be made is that the center cylinder has to be fixed to the plate, because there is too little space in its surroundings for a positioning system. Therefore, the center cylinder is going to be used in all alternatives as the origin from which the relative position of the remaining cylinders is going to be measured.

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5.3.1.1 Cylinder Positioning On the basis of this plate-based design, the different design alternatives for positioning the cylinders are as follows:

Alternative I – Fixed to the plate (P1) Each cylinder is fixed directly to the plate, with the help of brackets that can be bought to an external supplier. The only difference with the existing assembly would be the change of the bar fastening method to the plate one. No cylinder-positioning system is included in this alternative.

Figure 5.10 Cylinders positioning: Alternative I

Alternative II – Fully adjustable (P2) Assuming the middle cylinder fixed, as explained before, this alternative implies two degrees of freedom for every cylinder. The positioning system for every cylinder consists of two parts that have to be manufactured specifically for this design. The x- positioning bracket is screwed to the cylinder in its lateral and top surfaces, and permits the cylinder movement in the “x” direction, according to the local coordinate system. When the cylinder is located in the required position, this part is fixed to the y- positioning bracket, which allows movement only in the “y” direction. After the second positioning, the cylinder is fixed to the plate. The plate is also specifically designed for this concept, and it is thought to be manufactured by milling.

Figure 5.11 Cylinders positioning: Alternative II

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Alternative III – Semi-fixed cylinders (P3) Assuming again the middle cylinder fixed to the plate, and considering it the origin for the positioning of the remaining cylinders, this alterative consists on an auxiliary halved plate. Each half of the plate allows the movement in its local “y” direction of the three cylinders that it holds. Besides, the lateral cylinders of each half can move relatively to the auxiliary plate in its “x” direction, while the center cylinders are fixed.

Figure 5.12 Cylinders positioning: Alternative III

Alternative IV – Fully adjustable (upper holding) (P4) A last positioning idea consists on holding each cylinder on its upper surface and attaching a mechanism like the one shown in figure 5.13 that would use two screws to adjust the position of each cylinder. Even though the quality of the positioning using this system is very high, the difficult design is a big issue. First of all, there should be space for the bars of the cylinder to pass through the device, and secondly, a complex holding system for the mechanism to an upper plate should be designed. For these reasons, this alternative is finally dismissed, even though should be considered again for future improvements.

Figure 5.13 Cylinders positioning: Alternative IV

5.3.1.2 Positioning Reference For the positioning to be correctly made, it would be better if there were a reference. As it is done actually, the reference is only manual. The punches are located in the position that the operator thinks fits the best. Even though this is a good method, it is not as precise as it should be, given the quality that is required. For a better positioning of the mold, two reference alternatives are presented. Marina Moya Sánchez Page 37 wbk Institute of Production Science 5 Stamping Unit Design

Alternative I – Via mold (R1) Consist on redesigning the profile table making the cavity for positioning the mold a bit wider, so that there is space in the adaptation part to include positioning pins. It also implies manufacturing wider punches, so they can incorporate the corresponding holes. In this case, the plate should be positioned so that the center cylinder is located in its stamping position, and then the surrounding cylinders can be adjusted so that they match with the pins on the profile table.

Figure 5.14 Positioning reference: Alternative I

Alternative II – Guide bars (R2) Although both the profile table and the stamp unit are already part of the draping station, and their relative position is assumed correct, the inclusion of guide bars in between them is proposed, as an improvement of the current draping station and a method to guarantee a better performance of the entire process. As seen on the image, the alternative consists on the inclusion of four guide bars (two per side) that are attached to the profile table as seen. The attachment method has been decided that way, in order to make the fewest possible changes in the table.

Figure 5.15 Positioning reference: Alternative II 5.3.2 Fixing Interface Concepts The alternatives for the fixing interface have been defined depending on the degrees of freedom of the plate before it is screwed to the stamping module frame.

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Alternative I – One Degree of Freedom (F1) It allows the plate the longitudinal positioning. The plate is located on the two bars that play the function of guides and it can be pushed over them until the best position is found. Then, the plate is screwed.

Figure 5.16 Fixing Interface: Alternative I

Alternative II – Two Degrees of Freedom (F2) It implies using the plate itself as a preliminary positioning system. It has the advantage that it can be used as a positioning system for the center cylinder, giving it the two degrees of freedom that it missed when fixed to the plate.

Figure 5.17 Fixing Interface: Alternative II

5.3.3 Fabrics Holding Concepts For the holding concepts to sub-functions were defined: on the one hand, the gripper selection, and on the other hand, the disposal of it in the mold. Three types of grippers have been taken in consideration: needle gripper, Coanda gripper and Bernouilli gripper. As they have been already described in the

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Fundamentals chapter of this thesis, no further information is going to be given in this point. Even though the needle gripper was considered as an alternative for this function, the damage that this kind of gripper causes to the fabrics makes it impossible to use it and it has been finally dismissed. On the other hand, the complication that means including any of the grippers inside the mold, leads to the decision of placing it outside of it. Channels for the air flow are going to be included in the mold, and a plastic piece is going to be used as a cover. Two alternatives are thought for the disposal of the gripper and air flow channels. In the first one, the fabrics are hold thanks to one only air channel, the second one includes two air channels. It is also important to consider the effect of the air flow on the fabrics temperature. This fact is going to be studied in the second part of the thesis.

Figure 5.18 Mold air flow channels 5.4 Evaluation Methodology and Criteria For the selection to be as objective as possible and in order to carry out an organized and precise evaluation and posterior alternative selection, the VDI 2225 (Design engineering methods - Engineering design at optimum cost - Simplified calculation of costs) guidelines are going to be followed. 5.4.1 Evaluation Method Every individual alternative has to be evaluated according to specific requirements. Each function is going to be defined by a group of parameters that define it and that characterize its purpose. Since not all the parameters that define a function have the same influence on it, every one of them is going to be assigned to a weighting factor. The individual punctuations are assessed according to their degree of satisfaction with respect to the evaluation criteria. Based on the points and the respective weightings the concepts can be compared and the selection can be made. The rating scale used for the scoring is done in accordance with VDI-2225 guideline: Approximation to the ideal realization very good (ideal) p = 4 points good p = 3 points enough p = 2 points bad p = 1 points unsatisfactory p = 0 points

Table 6. Grading Scale [VDI – 98]

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As seen, every alternative is going to be evaluated with points from 0 to 4 according to their degree of compliance of the evaluated criterion. Pij represents the evaluation of the alternative “j” relative to the criterion “i”. Also, gi represents the weighting factor for the criterion “i”. Given this, Gij is defined as the individual final value of an alternative j regarding a criterion i, and Gges,j is the final value of an alternative j. Moreover, dividing the total value to the maximum possible value multiplied by the sum of the weighting factors for an alternative “j”, the relative quality grading value Wges,j is obtained.

퐺푖푗 = 𝑔푖 · 푃푖푗 Formel 5.1

퐺푔푒푠,푗 = ∑ 퐺푖푗 Formel 5.2

퐺𝑔푒푠,푗 푊푔푒푠,푗 = Formel 5.3 푃푚푎푥·∑ 푔푖푗

According to VDI-2225 regulation, the relative value can be classified in:

- Unsatisfactory, when Wges,j < 0,6

- Good, when Wges,j ∈ (0,6 – 0,7)

- Very good, when Wges,j > 0,7

5.4.2 Evaluation Criteria Before grading the alternatives, the evaluation criteria and their associated weighting factor have to be defined. In the following points detailed explanations for the elected criteria and weighting in every function and subfunction are given.

5.4.2.1 Stamping System For redesigning the stamping unit, and installing a positioning device for the cylinders, two subfunctions were defined. For the positioning system to be evaluated, the most important factor to consider is the positioning accuracy. Ease of manufacturing refers to the type of components the device requires. It is important to consider the availability of them in the industrial market or on the contrary, the need of manufacturing, the complexity of the manufacturing process and the costs that that would suppose. The ease of assembly is also a determinant factor. Mounting and dismounting the assembly in the shortest time and with the less effort is important in order to assure easy maintenance operations and mold changes. Having always in mind an ideal universal system, which wouldn’t require any changes if a new different preform wants to be manufactured, adaptability is also a factor to consider even though it is not considered so important. At last, maintainability is also considered, which represents the predicted frequency that the system would require for maintenance operations, positioning readjusting and parts changing. On the other hand, for a correct reference selection, if used, also the positioning accuracy is considered as the most important parameter. The adaptability of the reference system to other future mold shapes is considered as an important factor too,

Marina Moya Sánchez Page 41 wbk Institute of Production Science 5 Stamping Unit Design considering the fact that the reference system should be usable for every mold shape and only changes in the cylinder-plate assembly should be made. As in the first subfunction, ease of manufacturing and assembly are considered important factors, followed by the costs that would suppose integrating the idea in the current station and their maintainability. Given this, the criteria that are going to be used to define the stamping system function are summarized in Table 7.

Cylinder Positioning Positioning accuracy 4 Ease of manufacturing 3 Ease of assembly 3 Adaptability 2 Maintainability 1

Positioning Reference Positioning accuracy 4 Integration cost 4 Adaptability 3 Ease of manufacturing 2 Ease of assembly 2 Maintainability 1

Table 7. Evaluation criteria for the stamping system

5.4.2.2 Fixing Interface The criteria that are going to be evaluated for the fixing interface are the ones shown in Table 8.

Fixing Interface Joint stiffness 4 Adaptability 3 Integration cost 3 Positioning accuracy 3 Ease of manufacturing 2 Ease of assembly 2

Table 8. Evaluation Criteria for the fixing interface

The most important factor is the stiffness of the fixing, that means, the grade in which the union can guarantee the minimum deformation of the plate and frame while the stamping process takes place. The rest of the criteria that are going to be considered refer more or less to the same explanation given in the previous function. Namely adaptability, integration cost, ease of manufacturing, ease of assembly and maintainability are going to be considered. Also the positioning accuracy is a factor to consider.

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5.4.2.3 Holding System The selection of the gripper is made following the guidelines of a previous study made in the wbk [Feb-13]. For the gripper selection then, the evaluating criteria are as shown in Table 9. Gripper selection Non damage gripping 4 Multi-Layer Gripping 4 Material flexibility 3 Holding normal force (per unit area) 2 Holding shear force (per unit area) 2 Separating 1 Energy efficiency 1

Table 9. Evaluation criteria for the gripper selection

Given the fact that the fabrics are going to be located on top of the suction surface, the forces the gripper has to transmit to the fabrics are not as important as in a usual holding operation. That is the reason why the holding force transmitted to the fabrics has been decided to be a criterion as important as the ease of manufacturing and the layers adaptation to the mold.

Gripper disposal Holding force 3 Ease of manufacturing 3 Layers adaptation to mold 3 Fabrics temperature holding 3 Cost 2 Maintainability 2 Holding force 3 Ease of manufacturing 3

Table 10. Evaluation criteria for the gripper disposal 5.5 Alternative Selection for the Stamping Unit This section presents the detailed punctuation given to the different alternatives based on the criteria previously defined and justifies the final selection.

5.5.1 Stamping System

5.5.1.1 Cylinders Positioning As none of the proposed alternatives has perfect positioning accuracy, none of them has been given four points. However, three points have been given to the alternatives three and four, three to the second alternative and only one to the first one, which reduces its positioning capacity to the plate positioning. Considering the ease of manufacturing, alternative I is considered the best due to the simplicity of the plate and the fact that the rest of the parts are simple parts that can be bought to an external supplier. The rest of the alternatives need for a more difficult manufacturing process of the plate and require special parts designed specifically for this concept, so less punctuation has been given. Regarding the ease of assembly, also the first

Marina Moya Sánchez Page 43 wbk Institute of Production Science 5 Stamping Unit Design alternative is the best option, considering that there are only 4 screws per cylinder. Due to the bigger amount of parts composing the other three alternatives and the complexity of their designs, their punctuation regarding the ease of assembly is lower. As a first design, none of the alternatives are thought to be used with future mold designs, and for a future preform to be manufactured, a fully new stamping unit should be designed. Even so, alternatives three and four could be used for a different preform as far as the new mold is adapted to the restrictions imposed by the cylinders positioning movement, while alternative one is restricted to this only preform shape. Finally, referring to maintainability, only one point has been given to the first alternative, due to the only two screws that fix each cylinder to the plate, and that very likely would give the same problem as the current station has: loose of the screws after a few preforming processes, and the consequent need of tightening. After all this, the results of the analysis are given in the Table 11, which shows alterative III as the best of the proposed alternatives, being the other three, according to VDI-2225, unsatisfactory.

P1 P2 P3 P4

Criteria gij Pij Gij Pij Gij Pij Gij Pij Gij Positioning accuracy 4 1 4 2 8 3 12 3 12 Ease of manufacturing 3 3 9 2 6 2 6 1 3 Ease of assembly 3 3 9 2 6 2 6 2 6 Adaptability 2 1 2 1 2 2 4 2 4 Maintainability 1 1 1 3 3 3 3 2 2 Total Value - 25 25 31 27 Relative Value - 0,48 0,48 0,60 0,52

Table 11. Alternative selection of the cylinder positioning system

5.5.1.2 Positioning reference Including alternative one in the station would imply an enormous quality increment of the final preform thanks to the much better accuracy regarding the relative position of the different mold parts. On the other hand, alternative two is a good option for improving the general quality of the preforming station, but its impact on the cylinders positioning is almost nonexistent. The integration cost is the second important factor associated to the reference function. In both cases changes should be made in the profile table and parts should be changed. This implies extra costs and time and that is the reason why both alternatives have low punctuation regarding this criterion. Adaptability in both cases has a high punctuation. For alternative two, the design is fully adaptable to future preform shapes and there would be no need of changing it in the future. On the other hand, alternative one is not fully adaptable to future preform shapes and its adaptability is limited to preforms of a similar size of the actual one. Even so, most of the parts that are thought to be manufactured in this preforming station have a similar size, so the adaptability of this alternative is considered also high. Ease of assembly is high rated in both cases, due to the simplicity of the design, while the ease of manufacturing is rated a little bit lower, considering that the parts are

Marina Moya Sánchez Page 44 wbk Institute of Production Science 5 Stamping Unit Design specifically designed for these concepts. Finally, maintainability is not considered a problem and punctuations given are high in both cases. R1 R2

Criteria gij Pij Gij Pij Gij Positioning accuracy 4 4 16 2 8 Integration cost 4 1 4 2 8 Adaptability 3 3 9 4 12 Ease of manufacturing 2 2 4 1 2 Ease of assembly 2 3 6 3 6 Maintainability 1 3 3 3 3 Total Value 42 39 Relative Value 0,66 0,61

Table 12. Alternative selection for the positioning reference

As seen on Table 12, both alternatives are considered “good” according to VDI-2225, being the first one the best of them. Even so, a final decision has been made not to include any of them on the final design, due to the changes that would be necessary in the profile table and that weren’t planned in the first place. The ideas are presented to be considered for possible future improvements of the station. 5.5.2 Fixing Interface Both the joint stiffness and the adaptability criteria are considered “very good” in the proposed alternatives. The cost that implies including the alternatives in the station is substantial but necessary. Even so, the intention is to do as few changes as possible in the stamp unit. Continuing with the evaluation, the fixed condition of alternative one implies the 0 points given to the positioning accuracy criterion, opposite to the four points given to the second alternative, which allows movement in the two planar directions. Ease of manufacturing and ease of assembly in both cases are good rated because of the use of mostly standardized elements in the design. F1 F2

Criteria gij Pij Gij Pij Gij Joint stiffness 4 4 16 3 12 Adaptability 3 4 12 4 12 Integration cost 3 2 6 2 6 Positioning accuracy 3 0 0 4 12 Ease of manufacturing 2 3 6 2 4 Ease of assembly 2 4 8 3 6 Total Value - 48 52 Relative Value - 0,71 0,76

Table 13. Alternative selection of the fixing interface

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5.5.3 Handling System

5.5.3.1 Gripper Selection The gripper evaluation is based on a previous study written in the wbk (see Fle-13 in the bibliography). The most important aspect to consider is the non-damaging nature of the gripper. While needles grippers are punctuated with zero points, the Coanda and Bernouilli have been valued with four and three points respectively, due to their non-contact nature. On the other hand, other aspects of the two non-contact grippers have been valued with lower points than the needle ones, namely the multi-layer gripping capacity, the handling forces or the energy efficiency: while needle grippers are really efficient grippers, Coanda and Bernouilli require more energy to develop their work. Coanda Bernouilli Needles

Criteria gij Pij Gij Pij Gij Pij Gij Non damage gripping 4 4 16 3 12 0 0 Multi-Layer Gripping 4 2 8 2 8 3 12 Material flexibility 3 4 12 3 9 3 9 Holding normal force (per unit 2 2 4 2 4 3 6 area) Holding shear force (per unit 2 2 4 2 4 3 6 area) Separating 1 1 1 1 1 1 1 Energy efficiency 1 0 0 0 0 3 3 Total Value - 45 38 37

Relative Value - 0,66 0,56 0,54

Table 14. Alternative selection of the holding system

As seen on Table 14, the most valid of the grippers is the Coanda one, being the other two, according to VDI 2225, not valid.

5.5.3.2 Gripper Disposal Finally an evaluation of the possible air channels on the mold is made. As seen on Table 15, the difference in between having one or two channels, regarding the holding force, is nonexistent. The ease of manufacturing is obviously higher in the simpler alternative, for this reason three points were given to the first alternative, while only two were given to the second. On the other hand the alternative with two channels of air assures that the fabrics are going to be fully attached to the mold and the final shape of the preform will be more close to the ideal shape. Regarding the fabrics temperature, the fact that two air flows go across the fabrics can imply a drop of the temperatures that could cause failure on the curing of the binder. Finally, maintainability is not a big issue regarding the lower mold.

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One channel Two Channels

Criteria gij Pij Gij Pij Gij Holding force 3 2 6 2 6 Ease of manufacturing 3 3 9 1 6 Layers adaptation to mold 3 1 3 3 9 Fabrics temperature holding 3 3 9 1 3 Cost 2 3 6 3 6 Maintainability 2 4 8 4 8

Total Value - 41 35

Relative Value - 0,64 0,55

Table 15. Selection of the gripper disposal

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6 Results: Stamping Unit Design

For an easier representation of the final design, it has been represented considering the same three main modules defined in 5.1.1 for the current station description.

Stamp Module

Stamping Unit Profile Table

Figure 6.1 CAD final stamp unit design

1. Stamping Unit – formed by the cylinders, the female mold tools, the plate and all the parts involved in their fixing. 2. Stamp Module – formed by the structure that integrates the stamping system in the preforming station. 3. Profile Table – formed by the profile table, the male mold tool and the parts involved in their fixing.

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6.1 Stamping Unit The stamping unit is represented in Figure 6.2

Figure 6.2 Stamping unit final design

The representative parts of the module are shown in Table 16. The most complex part, and the key for the correct functioning of the positioning system, is the plate (a), which has to be precisely manufactured to avoid gaps in between the plate and the positioning parts that hold the cylinders. Four grooves on each side of the plate have been added for the positioning and fixing of the plate to the frame. Two brackets are needed for fixing the middle cylinder to the plate (d). For fixing the remaining cylinders, two parts are designed: an S-shaped part that adjust the position in one direction (b) and a flat part that adjust the positon in the other direction (c). The screws used to fix the cylinders to the S-shaped part are standardized M5 screws, while the ones used to fix the flat part to the plate are M8. To fix the positioning parts in the reduced available space in between cylinders, the flat adjusting part has been designed specifically for each cylinder, having two of them the groove for the screws on the right side, while the rest of them have it on the left side. Finally, considering the dimension of the cylinders in their extended position and the distances in between the mold and the profile table, it is necessary to attach to each mold part an extension of the cylinder bars. The cylinders are screwed to the mold by four M5 screws. A representative mold punch is shown in the table (e).

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b) c)

Table 16. Most representative parts of the stamping unit

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6.2 Profile Table The profile table is represented in Figure 6.3

Figure 6.3 Profile table final design

Due to the decision of not including the gripper on the mold, but locating it outside of it, a specific plastic part is designed to be located on top of the mold, and a joint is included to guarantee perfect closure between the mold and the cover part. An air channel crosses the mold and the entrance of air is located in the base of the mold. The air from the gripper will be introduced from under the profile table. The previous design of the table wasn’t thought for a gripper mold, so the part that holds the mold and prevents from its movement has to be redesigned. The shape and size of the new preform also require a brand new part for adjusting the mold to the table.

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6.3 Stamp Module The stamp module is represented in Figure 6.4

Figure 6.4 Stamp module final design

The final module involves the more basic, but not less important. It is formed of standardized profiles and connectors. It has been necessary to make a few changes on the structure of the previous unit. First of all, the available space for supporting the plate wasn’t’ wide enough, so it has been changed. Also it has been added a structure on the top to support the pipes and complements of the cylinders. For fixing the plate four short standard profiles have been used. As a difference with the rest of the structure, instead of using 45x45 mm profiles, 60x45 mm profiles have been used, to compensate the high of the plate. Finally an S-shape part has been designed to locate the final position of the plate on the structure.

Figure 6.5 Detailed view of the plate positioning

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7 Heat Transfer Chain Analysis It is considered part of the heating process every step of the preforming in which heating transfer processes take place. In the following points the heating chain is going to be described, as well as the requirements for the FE analysis and the definition of the models. The objective is to analyze the thermal behavior of the fabrics and the table during the process, and the thermal behavior during the stamping, in order to determine the quality of the heating, the times in which the process takes place and finally drawing conclusions and offering possible quality and time reducing solutions. 7.1 The Heat Chain during the Preforming The entire heating process that takes place during the preforming is summarized in Figure 7.1.

Figure 7.1 Heating Chain during the Preforming

The process takes places as follows:

1. The heating table is warmed up from its initial room temperature (Tamb) to an approximate average temperature of 115ºC on its top surface.

2. The layers that are going to form the final preform are cut to their final dimensions and stacked in their required orientation order, which depends on the application and requirements of the final composite part. In between layers of carbon fiber fabrics, an amount of binder is applied to keep the fabrics perfectly aligned and make them easier to handle.

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3. The stack of layers is located on of the heating table, and then warmed up from its initial Tamb temperature until its temperature is higher than the melting point of the resin binder.

4. When the fabrics have reached this temperature they are moved to the loading area in the preforming station, and put on the male shaped mold. The profile table and the mold have also been previously warmed up to 50ºC (Tmold). The profile table moves the fabrics from the loading area to the draping area of the preforming station.

5. When the mold is correctly positioned, the stamping process takes place.

6. The punches used to do the stamping stay in their lower position the necessary time for the preform to cool down until a final stable temperature (under the binder melting point) is reached. Then they are retired and the preform is moved to the quality assurance area to be inspected. During the whole process, it is important that the fabrics temperature doesn’t get lower than the binder melting point temperature, so that the quality of the preform can be assured. 7.1.1 Heating Table The available heating table consists on an 80 square centimeters and 20 centimeters thick aluminium plate. It is sustained by an aluminium frame that is isolated from the plate by four non-conducting plastic parts. To the lower surface of the plate are screwed four aluminium cylinders (with 60 mm diameter and 50 mm length), into which the heating from the sources is applied. The sources are connected to a thermal regulator that allows modifying the temperatures achieved by the sources.

Figure 7.2 Bottom view of the heating table

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The thermal sources used are WEMA DH200, whose characteristics are summarized in the following table. Housing material Brass Insulation material Mica Heating capacity Max. 6 W/cm2 Maximum operating Tº 300ºC Supply voltage 230V

Table 17. Thermal sources WEMA DH 200 characteristics

7.1.2 Unidirectional Carbon Fiber Textile Products Stitch-bonded unidirectional carbon fabrics are being used currently in the RTM process in wbk. The amount of layers that are needed depends on the geometry, size and technical specifications of the final part. In the case of the parts that are now being manufactured in the wbk, six layers of the defined fabrics are being used. Type of fabrics Panex® 35 UD300 0° Carbon Panex® 35 50K 309 90° Glass 34 dtex 10 Polyester Stitch 76 dtex 6 Binder Resin Powder 8 Total Fabric Weight 333 g/m2

Table 18. Textile fabrics Panex UD300 characteristics

Fabrics are stabilized by adding powder binders that are heat-activated before the preforming. These binders enable the consolidation of fabric stacks and the fashioning of stable preforms that improve handling but also prevent deformation when the resin is injected into the mold. It is important that the binder is compatible with the resin and does not diminish the fabric’s permeability. The binder used in the study process is EPIKOTE Resin 05390, a thermoplastic binder powder based on bisphenol-A. Its melting temperature is around 80-90°C. Type of binder EPIKOTE Resin 05390 Aspect White powder Necessary quantity 3 -15 g/m2 Melting Temperature 80-90 ºC Characteristic Repeatedly thermoformable

Table 19. Binder EPIOKTE 05390 characteristics

7.2 Model Definition For the heat transfer simulation the software Abaqus 6.13-4 is going to be used. The simulation will be an uncoupled heat transfer analysis, used to model solid body heat conduction with general, temperature-dependent conductivity, internal energy (including latent heat effects), and quite general convection and radiation boundary conditions, including cavity radiation [CAE-12]. It is important to emphasize that all the units used for the finite elements simulation are given in the IS, with the exception of temperatures, which are given in Celsius degrees.

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7.2.1 Heating Table Model The table is modeled in the part module of Abaqus. As the plate of the table is separated from the rest of the frame by insulating parts, only the plate and the heating cylinders are modeled. Furthermore, the screws that fix the plate to the frame (one on each corner of the plate) and the ones that fix the cylinders to the plate are omitted. Given this, five parts are defined for the table heating analysis, the plate and four equal cylinders. As the plate and the cylinders are different parts, the contact in between them is defined by a contact conductance property in the model. During the warming up of the table, heat is applied in the lateral surface of the cylinders and heat transfer processes take place in its three basic forms, conduction, convection and radiation. Firstly, heat is transferred through the aluminum cylinders and plate due to conduction. Convection and radiation processes also take place in the free surfaces of the table and cylinders, due to the interaction with the surrounding air.

Figure 7.3 Heating table model

For the analysis three main problems have to be solved: the material properties definition, the contact properties between the plate and the cylinders and the interaction between the table and the surrounding air. Given the big amount of bibliography about aluminium thermal properties, the following values are given to aluminium’s density and conductivity [Ber-11]. Density 2698 kg/m³ Conductivity 237 W/m·ºC Even though conductivity values variate with temperature, the interval of temperatures in which this process takes place (from 25ºC to 250ºC approximately) is small enough to consider it constant. The specific heat is set as a temperature dependent property, and the values given are shown in in Table 20. Temperature (ºC) Specific Heat (J/kg·ºC) -100 720 20 900 100 960 300 1020 500 1110

Table 20. Specific heat values of aluminium depending on temperature [Efu-15]

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Regarding the heat transfer methods, besides conduction, which has already been defined, radiation and convection are modeled. Both of them are defined in the external surfaces of the table, and the lower surface of the cylinders. For the radiation definition, the following values are stablished [Ber-11]. Absolut Zero -237 ºC Stefan-Boltzman Constant 5.68·10-8 Emissivity 0.06 Convection, on the other hand requires a more deep analysis to determine appropriate values. In the Table 21 typical values of the film coefficient are given for different types of convection. In the case of study, there is free convection being the fluid the air surrounding the table. That is, the value of the film coefficient should be between 5 and 25 W/m2·ºC. As a first approximation, a value of 20 W/m2·ºC is given.

Type h (W/m2·ºC)

Free Convection – Air 5-25

Forced Convection – Air 10-500

Free Convection – Water 500-1000

Forced Convection – Water 100-15000

Table 21. Typical values of film coefficient [Ber-11]

Finally, an estimation of the contact conductance between the cylinders and the plate is made. Typical contact conductance values are shown in Table 22. Considering the aluminium-aluminium contact between the parts, a first contact conductance value of 2400 W/m2·K is stablished.

2 Type hc (W/m ·ºC)

Iron/aluminium 45000

Copper/copper 10000-25000

Aluminium/aluminium 2200-12000

Stainless steel /stainless steel 2000 - 3700 Stainless steel /stainless steel 200-1100 (evacuated gaps) Ceramic/ceramic 500-3000

Table 22. Typical values of contact conductance [DS-15]

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7.2.2 Unidirectional Fabrics Model Once the table has been modeled, the fabrics that are going to be used for the manufacturing of the preform have to be modeled as well. As an initial idea, the stack of layers is defined as a six separate part assembly, with their corresponding contact conductance value in between them, and in between the first of the layers and the aluminium plate surface. Parallel the contact conductance property it is necessary also to define carbon fiber textile properties such as conductivity, density and specific heat, and heat transfer properties such as radiation and convection with the surrounding air.

Figure 7.4 Unidirectional carbon fibers model

A first model is defined with the values shown in Table 24. Density is a property given by the fabrics supplier (Table 18). Conductivity and specific heat hypothesis, on the other hand, are made after a research of other equivalent carbon fiber textile values. Taking in consideration the investigation made by the Department of Mechanical Engineering in Wichita State University (USA), the values given to conductivity and specific heat of the fabrics are as follows [Jov-12].

Conductivity (k22 = k33) 0.8 W/m·°C

Conductivity (k11) 10 W/m·°C

Specific Heat 1000 J/kg·ºC

Neither conductivity nor specific heat are going to be defined as temperature dependent, due to the small variations of these parameters in the temperature interval of study (25ºC – 150ºC). As it is shown in Figure 7.5 the heat conductivity in the carbon fiber is different in the radial and axial direction. Nevertheless, as the heat is transferred from the table to the fabrics only in the radial direction of the fibers, and due to the small thickness of the layers, the difference between radial and axial conductivity should not influence the final temperatures achieved by the fabrics, For this reason, a simplification of the model is made an the thermal conductivity of the fabrics is considered as an isotropic property in this context. Even so, a test is going to be made on the fabrics to corroborate this hypothesis. Marina Moya Sánchez Page 58 wbk Institute of Production Science 7 Heat Transfer Chain Analysis

Figure 7.5 Conductivity in carbon fibers

Regarding the heat transfer properties, research is made to find typical values of emissivity and film coefficient in the study conditions, as it was made to define the plate values. Finally, and considering that the contact in between two fabrics, or in between the fabrics and the table, is a very irregular contact, the value for the contact conductance is stablished in a very low value, as seen in Table 24.

Density 333 kg/m³ Conductivity 0.8 W/m°C Specific Heat 1000 J/kg·ºC Emissivity 0.7 Film Coefficient on fabrics surface 10 W/m2·ºC Contact Conductance Fabrics-Aluminium 15 W/m2·°C Contact Conductance Fabrics-Fabrics* 16 W/m2·°C

Table 23. First approximation of the model parameters of the fabrics as a six part assembly

With all the data summarized in the previous paragraphs, the model is already finished and ready to run.

7.3 Data Collection

7.3.1 Experiments Description Measurement of the increasing temperatures on the table

For the validation of the model different experiments have been made in the heating table, as well as in the fabrics that are going to be used in future preform manufacturing. To validate the model defined in the previous point, temperature measurements have been taken in the indicated points of the table in Figure 7.7. On the one hand, it has been measured the heating in the middle contact point between each heating cylinder and the plate (Figure 7.7, bottom view). The temperature of these four points is continuously monitored and showed in the screen of the thermal regulator available on the table (THERMOPLAY TH-M6, Figure 7.6), so the obtained results are considered very precise. On the other hand, temperatures are taken manually on the surface of the heating table using a digital thermometer (Voltcraft K101, Figure 7.6). The measurements are taken on the points shown in Figure 7.7, top view. As seen, only three temperatures are considered necessary to characterize the table temperatures, due to the double

Marina Moya Sánchez Page 59 wbk Institute of Production Science 7 Heat Transfer Chain Analysis symmetry of the problem. Considering this, seven temperatures are measured during the warming of the table to characterize it: S1, S2, S3, S4, P1, P2 and P3.

Figure 7.6 Digital thermometer Voltcraft K101 (left) and thermal regulator THERMOPLAY TH- M6 (right)

Figure 7.7 Temperature measurement points on the heating table

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To begin the experiment, the power controller is configured to heat the table until the heating sources achieve 250ºC and measurements are taken every five minutes until the temperature of the table stabilizes. Measurement of the increasing temperatures on the carbon fiber layers After measuring the heating of the table, the rising temperatures of the fabrics during the heating have also been measured. For this experiment, measures have been taken manually using the digital thermometer used in the previous experiment. Temperatures of the fabrics have been taken on the surface of a single layer and on the surface of a stack of three layers. The fabrics used to do the experiment are square shaped layers of 30 mm2. Temperatures have been measured on the top surface of the fabrics, in the points specified in Figure 7.8 (L1 and L2). In the definition of the model, a hypothesis was made that considered the conductivity of the fabrics as an isotropic property. That hypothesis determines that the orientation of the fabrics regarding this context doesn’t influence the final temperatures achieved by the fabrics. If the hypothesis is valid, the temperature in points L1 and L2 should be approximately the same at the end of the experiment.

Figure 7.8 Schematic view of the unidirectional fiber fabrics

For the realization of the experiment, the fabric (or fabrics) is located on the heating table after it has been warmed up, and measurements are taken every five seconds until the temperature of the top surface stabilizes.

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7.3.2 Experiments Results Table heating results Figure 7.9 shows the temperatures of the cylinders during the heating process.

250

200

150 S1 S2 100 S3

Temperature (ºC) Temperature 50 S4

0 0 500 1000 1500 2000 2500 3000 3500 Time (s)

Figure 7.9 Temperature – time in heating sources

As seen in Figure 7.9, the four sources have different behavior during the heating, and none of them achieves the desired final 250ºC. The experiment is stopped after 55 minutes, when the temperature variation with time is very low. These differences of temperatures are associated to problems in the contact in between the power sources and the cylinders. As the sources don’t get to the same final temperature, the temperature distribution in the surface of the table can’t be assumed to be symmetric, as it was supposed to be. For this reason, to have a homogeneous warming of the fabrics, and an accurate model of the process, the following experiments are modified and the sources temperatures are set to 180ºC, which is the temperature in which sources three and four stabilizes, and which guarantees that the table surface temperatures are symmetric. Figure 7.10 shows the temperatures achieved on the table surface in P1, P2 and P3 after setting the sources final temperature in 180ºC.

160 140

120 100 P1 80 P2 60 P3

Temperature (ºC) Temperature 40 20 0 0 500 1000 1500 2000 2500 3000 3500 4000 Time (s)

Figure 7.10 Temperature-time in table surface

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Fabrics heating results Once the temperature in the sources has reached 180 ºC, the temperature measurements on the fabrics surface can be made. Another advantage of doing the experiment at lower temperature is that the heating of the fabrics is slower, and given the fact that the measurements are made manually, the error committed in the measurement is reduced. The experiment is firstly made with only one layer and then with a stack of three. Temperatures are taken every 5 seconds in points L1 and L2 (fiber and through-the- thickness fiber directions). Results are shown in figures 7.11 and 7.12.

Surface temperature - One Layer 120

100

60 L1

40 L2 Temperature(ºC)

0 0 50 100 150 200 Time (s)

Figure 7.11 Temperatures on top surface of one layer

Surface temperature - Three Layers 100 90

70 60 50 L1 40

30 L2 Temperature(ºC) 20 10 0 0 50 100 150 200 250 Time (s)

Figure 7.12 Tempeartures on top surface of a stack of three layers

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As it was supposed, temperatures achieved in both points (L1 and L2) are almost equal, and it can be assumed that the small existent differences are caused by the error committed during the measurement. For this reason, the hypothesis about isotropic conduction behavior during the warming can be validated. As a conclusion, the temperatures reached in the top surface of the first and the third layer after the worming is shown in Figure 7.13.

120

100

60 First Layer

40 Third Layer Temperature Temperature (ºC)

0 0 50 100 150 200 Time (s)

Figure 7.13 Temperature-time in fabrics surface 7.4 Model Validation and Optimization

7.4.1 Heating Table Model Validation For the model to be validated, the analysis is configured to extract graphics of the temperature variation of the equivalent model points in which the measurements were taken during the experiments (P1, P2, P3, S1, S2, S3 and S4), in their load conditions, that is, configuring the sources to achieve an equal 180ºC temperature. The power is set as the 78% of the full power and is configured to be achieved instantaneously. The curves represented in Figures 7.14, 7.15 and 7.16 show the temperatures of the sources during the warming in the given case of load, and the effect on the curve of variations in the representative parameters while keeping the remaining parameters constant. Even though the results obtained in the first approximation are positive, some changes are made in the initial parameters, to obtain a more accurate solution. Variations of specific heat, considering the remaining properties constant, don’t imply wide changes in the temperature curve, while variations of conductivity and film coefficient affect directly to the final temperatures reached in the sources. Even so, the present high slope for the first seconds of the process, that isn’t so high on the real process, doesn’t vary with any of the these parameter variations. It is concluded that the reason for this is the non-instantaneous power applied in the real process. As seen in Figure 7.16, the necessary time to achieve the final power affects to the initial slope of the curve, but not to the final temperatures. The figure shows the temperature variation of the sources considering the cases of achieving full power instantaneously, after twenty seconds and after 500 seconds.

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Figure 7.14 S1 temperature curve modifications due to conductivity variations

Figure 7.15 S1 temperature curve modifications due to film coefficient variations

Figure 7.16 S1 temperature curve modifications due to retard on achieving full power

Finally, after optimizing the model considering the effect of the parameter variations, a final model for the heating table is achieved, whose characteristics are summarized in Table 23.

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Model I Model II Density 2698 kg/m³ 2698 kg/m³ Conductivity 237 W/m°C 230 W/m·°C Specific Heat Table 14 Table 14 Emissivity 0.06 0.06 Film Coefficient on surface 20 W/m2·°C 11.5 W/m2·°C Contact Conductance Aluminium-Aluminium 2400 W/m2·°C 2650 W/m2·°C

Table 24. Model parameters of the heating table, before and after optimization

Figure 7.17 show the comparison between the temperatures achieved with the model simulation (P1_M, P2_M, P3_M) and the ones achieved in the real experiment on the plate surface (P1, P2, P3).

160

140

120 P1 100 P2 80 P3

60 P3_M

Temperature (ºC) Temperature P1_M 40 P2_M 20

0 0 500 1000 1500 2000 2500 3000 3500

Time (s)

Figure 7.17 Comparison between the model and the experiment table surface temperatures

Once the aluminium, contact and surface properties are defined, the power curve applied in each source can be modified to simulate the real conditions in which the fabrics heating takes place. Figure 7.18 represents the temperature curve of the four sources obtained in the real process (S1, S2, S3, S4), and the curves obtained in the simulation (S1_M. S2_M, S3_M, S4_M).

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250

200

S1 150 S1_ M S2 100 S2_

Temperature (ºC) Temperature M 50 S3

0 0 500 1000 1500 2000 2500 3000 Time (s)

Figure 7.18 Temperature variation of the sources during the heating

As seen, the similitudes between the experimental results and the model results allow validating the heating table model. 7.4.2 Fabrics Model Validation As it has been done for the heating table, the validation of the fabrics model is now compared and validated with the experimental results. In the following graphics, the temperature results obtained for the surface of the fabrics layers on the simulation are represented in red. As it happened with the heating table, the obtained results are positive and similar to the experimental values, but for a more accurate solution of the problem some parameters should be modified. Graphics are extracted showing the effect of changing the influential parameters on the final fabric surface temperature curve. The effect of these variations is summarized in Table 25, graphics a) to f). All graphics represent the temperature variation on the surface of the first layer, except graphic f), which represents the surface temperature variation of both the first and third layer. As seen in the graphics, a big problem is found while developing this model: the temperature variation with time depends mostly on the contact conductance in between the layers and the table and on the contact conductance in between the fabrics, but variations of conductivity don’t imply any changes. This problem can be associated to the fact that temperatures on the table are very high for the thin layers that are being modeled, and the effect of conductivity in the normal-to-surface direction can’t be observed.

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a) Effect of conductivity (k) variation on T b) Effect of specific heat (cp) variation on T

c = 10 k = 1000 p

c = 1000 k =1 p

c) Effect of contact conductance Al-CF (h ) c d) Effect of emissivity (ε) variation on T variation on T

hc = 1000 ε = 0.5

ε = 0.7

ε = 0.9

hc = 15

f) Effect of contact conductance CF-CF (h ) e) Effect of film coefficient (h) variation on T c variation on T

h = 15 hc = 16

h = 10

hc = 1000

Table 25. Effect of model properties variations on temperature-time curve for the six part model

For this reason, the decision is made to do a simplification of the model and represent the stack of fabrics as an only part divided in six partitions, instead of six different parts. Even though it will not define the thermal properties of the material as it is, it can define the thermal behavior of the fabrics during the process. In this case, the conductivity of the fabrics is defined as an orthotropic property in which conductivity in Marina Moya Sánchez Page 68 wbk Institute of Production Science 7 Heat Transfer Chain Analysis planar directions is equal (as proved in the experiments) and a different conductivity is defined in the normal-to-fabrics direction. This conductivity will be an “equivalent” conductivity that will have implicit the contact conductance in between layers. Considering this new simplification, and after a first optimization of the parameters, the model is redefined as follows: Model I Model II Density 333 kg/m³ 333 kg/m³ Conductivity (k = k ) 10 W/m°C x y 0.8 W/m°C Conductivity (kz) 0.1 W/m°C Specific Heat 1000 J/kg·ºC 1700 J/kg·ºC Emissivity 0.7 0.5 Film Coefficient on fabrics surface 10 W/m2·ºC 15 W/m2·ºC Contact Conductance Fabrics-Aluminium 15 W/m2·°C 100 W/m2·°C Contact Conductance Fabrics-Fabrics 16 W/m2·°C -

Table 26. Model parameters of the fabrics, before and after model simplification

A final optimization of this model (Model II) is made. For this purpose, same procedure as for the heating table is followed to adjust the parameters. Table 28 summarizes the effect of varying each of the involved parameters, keeping the rest of them constant (curve in blue represents the temperature variation under Model II conditions). As seen on figure a), variations of conductivity in the planar direction of the fabrics doesn’t suppose any observable variations of the temperature curves. For this reason, the final value is stablished in 10 W/m·K, which is a common value found in other similar fabrics. On the other hand, the equivalent conductivity defined in the z direction (b) implies great changes in the temperature curves. As influent as the conductivity in z direction are also the film coefficient on the fabrics surface (e), and the contact conductance coefficient between the fabrics and the aluminium plate (f). On the other hand, specific heat changes (c) imply variations on quickness in which the fabrics reach their equilibrium temperature, but not in the final value of this one. Finally, variations of the emissivity value imply visible changes, but not as determinant as the ones seen in b), e) and f). Considering this behavior of the fabrics, the model is optimized to a final solution reflected in Table 27 (Model III). Firstly the parameters that imply the biggest changes are adjusted, and secondly emissivity and specific heat are also modeled to reach the most close to reality solution. The final characterization of the stack of fabrics is defined in Table 27. Model I Model II Model III Density 333 kg/m³ 333 kg/m³ 333 kg/m³ Conductivity (k = k ) 10 W/m°C 10 W/m°C x y 0.8 W/m°C Conductivity (kz) 0.1 W/m°C 0.05 W/m°C Specific Heat 1000 J/kg·ºC 1700 J/kg·ºC 1800 J/kg·ºC Emissivity 0.7 0.5 0.3 Film Coefficient on fabrics surface 10 W/m2·ºC 15 W/m2·ºC 10 W/m2·ºC Contact Conductance Fabrics-Aluminium 15 W/m2·°C 100 W/m2·°C 125 W/m2·°C

Table 27. Model parameters of the heating fabrics

a) Effect of conductivity (kx=ky) variation on T b) Effect of conductivity (kz) variation on T

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c) Effect of specific heat (cp) variation on T d) Effect of emissivity (ε) variation on T

f) Effect of contact conductance Al-CF (h ) e) Effect of film coefficient (h) variation on T c variation on T

Table 28. Effect of model properties variations on temperature-time curve for the one part model

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Finally, Figure 7.19 represents the comparison between the surface temperatures obtained in the model, due to the conditions summarized in Table 27, and the temperatures obtained in the experiment of the fabrics heating, being C1 the temperatures obtained on the surface of the first layer, and C3 the temperatures obtained on the surface of the third layer.

120

100

80 C1 60 C3 C1_M 40

Temperature (ºC) Temperature C3_M 20

0 0 50 100 150 200 Time (s)

Figure 7.19 Comparison between the model and de essay layers surface temperatures

As seen, the similitudes between the experimental results and the model results allow validating the fabrics model. A final temperatures distribution of the fabrics after the essay can be represented now on the surfaces of the stack of fabrics.

a) b)

II)

Figure 7.20 Heat flow (I) and final temperature (II) on the sixth (a) and first (b) layer of the stack of fabrics after the heating in essay conditions (180ºC in heating sources)

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8 Results: Analysis of the Heat Transfer Chain Once the heating table and the fabrics are characterized, a simulation of the heating chain during the preforming can be done, to determine the behavior of the fabrics. As it was explained, the heating process can be summarized in five steps: table heating, fabrics heating, fabrics transport, stamping and cooling. 8.1 Table and Fabrics Heating As a first simulations step, the temperature distribution on the heating table after fifty five minutes is shown in Figure 8.1. The average temperature obtained on the surface of the table after this time is 90ºC.

Figure 8.1 Temperature distribution on the heating table

After the fifty five minutes of warming, the fabrics are loaded on the table and their temperature raises until an equilibrium point is reached. In these conditions, the average temperature in the upper surface of the sixth layer is 78 ºC. This temperature is too low guaranteeing the melting of the binder, so first decision is to increment the heating time of the table to an eighty minutes. In these conditions, the average temperature achieved in the surface of the table is 120ºC. Temperature distribution on the fabrics then, after 80 minutes of heating the table, and 200 seconds of warming the fabrics, is as shown in figure 8.2. Figure also shows the heat flow distribution after the heating process. In these conditions the highest temperature is reached in the lowest surface of the stack of fabrics and has a value of 111,219 ºC, while the lowest is reached in the top surface of the sixth layer, and has a value of 78ºC. On the other hand, average temperature on the warmest surface (Figure 8.2b) is 107,04 ºC, while the average temperature on the coldest surface (Figure 8.2a) is 82,29 ºC. Even though this temperature is on the limit of the valid temperatures required by the binder resin, changes on the heating time of neither the table nor the fabrics imply high increments on the final temperature of the fabrics, because the equilibrium point has already been achieved for the power applied by the sources. For this reason, solutions should be found to achieve higher temperatures, like warming the fabrics also from their top surface, or using higher power heat sources.

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a) b)

II)

Figure 8.2 Heat flow (I) and final temperature (II) on the sixth (a) and first (b) layer of the stack of fabrics after the heating 8.2 Fabrics Transport

The transport from the heating table to the stamping point is divided in two steps: a first step that involves the movement from the heating table to the profile table, which is located on the loading area of the preforming station, and a second step which involve the movement of the profile table from the loading area to the stamping area. Even though in this second step the fabrics are already in touch with the profile table, which is 50ºC, this contact is going to be dismissed, and the transport is going to be considered as an only 15 seconds step, in which the only interactions are the convection and radiation previously mentioned. For the transport, it is considered that it is done manually by an operator of the preforming station, as it is done actually. As the process is intended to be fully automated, in a future this operation will be carried out by a robotic arm, and considerations should be taken over the different convection conditions due to the velocity of the fabrics during the movement and the effect of the air flow of the grippers that hold the fabrics. For the transport simulation the contact interaction that was stablished between the fabrics and the heating table is deactivated, as well as the load on the heating cylinders and every interaction of the heating table. The only interactions left during this step are the convection and radiation between the fabrics and the surrounding air.

A simulation is run to see the decrease of the layers temperature until they get to ambient temperature. Figure 8.3 shows the nodal temperature of all the nodes present in the warmest and the coldest surface of the stack of layers. As seen, more or less after eight minutes the assembly has returned to the initial 25ºC.

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Figure 8.3 Decreasing temperature of fabrics when submitted to free air convection on surface

Figure 8.4 shows the temperature evolution of the fabrics during the process. To simplify the graphic, it represents only the nodal temperature of the nodes that reach maximum and minimum temperature in both first and sixth layer. As seen, while temperature on the first layer suffers a big decrease, temperature on the sixth layer suffers almost any changes. As it is obvious, temperatures of every point of the stack of fabrics are located in between the maximum and the minimum curves.

160

140

120

100 Max - First Layer 80 Min - Sixth Layer

60 Min - First Layer Temperature(ºC) Max - Sixth Layer 40

0 0 25 50 75 100 125 150 175 200 Time (s)

Figure 8.4 Temperature variations of the fabrics during the heating a transport processes

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8.3 Stamping and cooling Finally the stamping process has to be simulated. If a complete and very precise analysis were intended to be done, for the simulation there should be taken in consideration factors such as the tensions and deformations of the fabrics, and their influence on their thermal behavior, the movement of the punches and the effect of the pressure on the fabrics and the effect of the discontinuities and defects of the fabrics during the preforming. Nevertheless, all these factors stay out of the limits of the work of this thesis, which is going to be focused only on the heat transfer problem, leaving aside other important factors that, on the other hand, may have big influence on the thermal problem and should be analyzed in further projects. Considering this, the draping of the fabrics is not going to be simulated and the simulation is going to represent only the heat transfer that takes place during the time in which the punches are in its lowest position. This process lasts between fifty and sixty seconds, and after that the punches are retired and the fabrics, now shaped as the final preform, can be moved to the quality assurance area. Because the stamping process itself is not going to be modeled, it is difficult to associate the temperature distribution of the fabrics (in their planar condition) obtained in previous steps to the already shaped fabrics that are going to be used in this step. For this reason, the decision is made to give the bottom and the top surfaces of the fabrics in their shaped form the average temperature of the fabrics obtained previously in the equivalent surfaces. Given this, the top surface initial temperature of the fabrics is set to 95°C and the bottom surface is set to 107 ºC. The intermediate points are stablished as a constant gradient in between the extreme temperatures. Continuing with the model definition, the punches are defined with an initial temperature of 25ºC (no heating is applied to them before or during the stamping), and the mold initial temperature is set in its constant 50ºC (Figure 8.5). For contact conductance between the fabrics and the aluminium parts (punches and mold), the same value obtained in previous analysis is used. In this simulation also convection and radiation of the parts are defined, even though their influence in the process is minimal.

Figure 8.5 Temperature distribution on the preform and mold tools after the stamping

The following figure represents the final temperature distribution obtained in the fabrics after the stamping process.

Marina Moya Sánchez Page 75 wbk Institute of Production Science 8 Results: Analysis of the Heat Transfer Chain

Figure 8.6 Temperature distribution on the preform after the stamping

The simulation was configured to continue until the fabrics achieved a temperature lower than the binder melting temperature (80ºC). From this temperature, the stack of fabrics has achieved a stable situation and the preform can be released and can be moved to the quality assurance area. The duration of this process after the simulation was three minutes, and after that point, almost the entire stack of fabrics had achieved this limit value, remaining in higher temperatures only some points of the exterior surface of the stack, that doesn’t have an influence on the binder.

Marina Moya Sánchez Page 76 wbk Institute of Production Science 9 Summary and Outlook

9 Summary and Outlook 9.1 Summary In this chapter, the advantages and disadvantages of the developed solutions are pointed out and the results obtained in the thermal analysis are evaluated. For this purpose, initially the components of the stamping unit are analyzed and evaluated. Since the commissioning of the plant has not yet been carried out, and the design should be improved before its implementation, the assessments are limited to the practical suitability of concepts chosen. 9.1.1 Stamping Unit Design The complications that imply installing a precise positioning system together with the cost of manufacturing each part independently require that the device proposed has to suppose a notable advance for the preforming station so that the system is worth the investment. The proposed final design was focused on obtaining a simple positioning system, that won’t imply a big investment or complicated manufacturing processes, and that will allow to realize first tryouts of installing this kind of system so that further study on the feasibility or not to continue in this line of research can be made. Every change that takes place in the preforming station must be economically viable and technically efficient. Due to the investment required to implement the changes, it must be sure that the changes involves shape differences between consecutive preforms. Replacing the current standardized profiles used for holding the cylinder by a plate is an important development, especially in ensuring greater rigidity. The main drawback is the need for specific design of the plate for each new preform shape. Still, being now the preforming station a prototype in research, the new preform will not be replaced by a more complex one for another two years, so the design of the plate and the other components will be amortized. The positioning system of the plate relatively to the outer frame is based on the plate system used by most of the normal sheet metal stamping machines, but simplified. Its implementation is very simple and has got good predictable results. Meanwhile, the introduction of the gripper in the lower mold is an improvement referring to the final quality of the preform. Improved clamping of the fabrics during preforming ensures less movement after each punch, so that defects and folds of the material are greatly reduced. Moreover, a disadvantage of the introduction of fastening system is the increase in energy consumption and the possible decrease in temperature of the fabrics during preforming. As it mentioned, the correction, if necessary, and the subsequent production of a prototype system will be necessary to check the results and determine whether or not to install the positioning system in the preforming station. 9.1.2 Thermal Analysis During the thermal analysis, a simulation of the heating process has been made, starting with the heating of the table, and then simulating the thermal behavior of the fabrics during the process. The simulation of the first step of the process, the heating of the table, has determined a time of eighty minutes to achieve temperature high enough to heat the fabrics. After

Marina Moya Sánchez Page 77 wbk Institute of Production Science 9 Summary and Outlook this time the table does not reach much higher temperatures. A first conclusion is drawn from this data: the need to fix the contact conditions in between the sources and the cylinders. Substituting the sources, as a second alternative for achieving higher temperatures on the table, would also reduce both heating time table, and thereby the cycle time. After the table heating simulation, the entire heating process of the fabrics has been simulated, and a graphic of the process that characterizes the temperature evolution of the fabrics during the preforming has been extracted. This graphic shows the four steps in which the process has been divided, namely, fabrics heating, transport, stamping and cooling. The final result obtained is shown in figure 9.1.

Figure 9.1 Temperature evolution of the fabrics during the preforming process

In the figure, the highest and the lowest temperatures achieved by the fabrics are shown. In red, the binder melting point is indicated. The heating of the fabrics takes about 200 seconds until the temperatures stabilize again. The temperatures reached by the fabrics are suitable for the activating the binder. Still, the introduction of a stream of hot air onto the fabrics to facilitate heat transfer and to achieve higher temperatures, or using infrared light to heat the upper layers is proposed to reduce the heating time. Moreover, without actually using such complex solutions, the simple use of an aluminum plate on the fabrics to apply pressure on the stack would decrease the time it takes to reach the fabric temperature balance by increasing the thermal contact conductance, and therefore will imply a reduction of the cycle time. During the transport, the most important decrease of temperature takes place in the points with high temperatures on the fabrics, due to the greater temperature gradient with the surrounding air. For this reason, the temperature decrease on the bottom surfaces is almost nonexistent. After the transport, the stamping process takes place. After the stamping the fabrics have to low their temperature again under the binder melting point, which is 80ºC. Even though the achieved solution can be considered valid, and the necessary time

Marina Moya Sánchez Page 78 wbk Institute of Production Science 9 Summary and Outlook for the fabrics to achieve this temperature is concluded to be 240 seconds, the results obtained after the simulation aren’t the expected and the model should be reviewed. This fact is observed in the unusual linear decreasing of the fabrics for the points of high temperatures, which indicates a possible error during the simulation. The main complication in what refers to this thermal analysis is the limited amount of literature found on the thermal characteristics of the fabrics. Thus, due to the difficulty of making assumptions about the thermal properties, the results of this analysis should be used as an initial approach to the possible thermal behavior during preforming, but are subject to errors and future analysis should be conducted to refine the results. 9.2 Outlook Since the commissioning of the designed stamping unit is still pending, is for further study to improve to conceptual design given and the construction and implementation of it. Subsequently, experiments can be carried out that could not be realized. The complexity of the design would require a prototype to be developed and proves should be made to determine if the option chosen is valid, and an analysis to determine if the time and economic costs compensate the advantages of its implementation. Improvements on the design should also be made to obtain solutions that approach the ideal final design that could adapt to any preform shape. Regarding the heating analysis, a further work should be made that including on the simulation the effect of the mechanical properties of the fabrics on their thermal behavior, and also a study of the influence of installing grippers on the male tool on the fabrics temperature.

Marina Moya Sánchez Page 79 wbk Institute of Production Science List of Figures

10 List of Figures Figure 1.1 Global CRP demand in 1,000 tons 2008-2020 (*estimated) [CMR-14] 1 Figure 1.2 Global CF revenues in US$ million by application (2013) [CMR-14] 1 Figure 2.1 Comparison of specific strength and modulus of high-strength composites and some aerospace alloys [Cam-10] 4 Figure 2.2 Discretization in textile composites representing scales of textile, yarn and filament [Roy-04] 5 Figure 2.3 Woven and non-crimp fabrics structure 7 Figure 2.4 Most common woven patterns 7 Figure 2.5 Non-crimp fabrics structure [Lan-12] 8 Figure 2.6 RTM Process and process chain steps [Fle-13] 10 Figure 2.7 The Resin Transfer Molding (RTM) process [AVK-10] 10 Figure 2.8 Draping by double diaphragm method [AVK-10] 12 Figure 2.9 Draping by stamping method 12 Figure 2.10 Typical grippers for textile semi-finished products [Fle-13] 13 Figure 2.11 Conduction, convection and radiation heat transfer modes [Ber-11] 14 Figure 2.12 Contact surface between two parts pressed together and temperature variation due to contact conductance [DS-15] 15 Figure 2.13 Heat flow between two solids in contact and the temperature distribution [DS-15] 15 Figure 2.14 Fluid temperature distribution over a heated surface [Ber-11] 16 Figure 2.15 S-Diagram for alterative evaluation [VDI-98] 18 Figure 3.1 PreformCenter designed by Dieffenbacher GmbH (Eppingen, Germany, and Windsor, Ontario, Canada). 20 Figure 3.2 RTM Process Chain Layout in wbk. (1- Cutting, 2- Fabrics heating, 3- Stamping, 4- Quality control, 5- RTM) 21 Figure 3.3 Draping Station Areas 21 Figure 3.4 Sequential stamping process 22 Figure 4.1 _ SPALTEN problem solving method for the stamping unit design 23 Figure 4.2 Methodology followed for the heat transfer analysis 24 Figure 5.1 RTM Process 26 Figure 5.2 Preforming Station 27 Figure 5.3 Stamp module 28 Figure 5.4 Profile table 29 Figure 5.5 Current stamping unit 30 Figure 5.6 Stamping unit functional structure 31

Marina Moya Sánchez Page I wbk Institute of Production Science List of Figures

Figure 5.7 Schematic view of the stamping unit 32 Figure 5.8 Final preform 33 Figure 5.9 Current cylinder fastening system 35 Figure 5.10 Cylinders positioning: Alternative I 36 Figure 5.11 Cylinders positioning: Alternative II 36 Figure 5.12 Cylinders positioning: Alternative III 37 Figure 5.13 Cylinders positioning: Alternative IV 37 Figure 5.14 Positioning reference: Alternative I 38 Figure 5.15 Positioning reference: Alternative II 38 Figure 5.16 Fixing Interface: Alternative I 39 Figure 5.17 Fixing Interface: Alternative II 39 Figure 5.18 Mold air flow channels 40 Figure 6.1 CAD final stamp unit design 48 Figure 6.2 Stamping unit final design 49 Figure 6.3 Profile table final design 51 Figure 6.4 Stamp module final design 52 Figure 6.5 Detailed view of the plate positioning 52 Figure 7.1 Heating Chain during the Preforming 53 Figure 7.2 Bottom view of the heating table 54 Figure 7.3 Heating table model 56 Figure 7.4 Unidirectional carbon fibers model 58 Figure 7.5 Conductivity in carbon fibers 59 Figure 7.6 Digital thermometer Voltcraft K101 (left) and thermal regulator THERMOPLAY TH-M6 (right) 60 Figure 7.7 Temperature measurement points on the heating table 60 Figure 7.8 Schematic view of the unidirectional fiber fabrics 61 Figure 7.9 Temperature – time in heating sources 62 Figure 7.10 Temperature-time in table surface 62 Figure 7.11 Temperatures on top surface of one layer 63 Figure 7.12 Tempeartures on top surface of a stack of three layers 63 Figure 7.13 Temperature-time in fabrics surface 64 Figure 7.14 S1 temperature curve modifications due to conductivity variations 65 Figure 7.15 S1 temperature curve modifications due to film coefficient variations 65 Figure 7.16 S1 temperature curve modifications due to retard on achieving full power 65

Marina Moya Sánchez Page II wbk Institute of Production Science List of Figures

Figure 7.17 Comparison between the model and the experiment table surface temperatures 66 Figure 7.18 Temperature variation of the sources during the heating 67 Figure 7.19 Comparison between the model and de essay layers surface temperatures 71 Figure 7.20 Heat flow (I) and final temperature (II) on the sixth (a) and first (b) layer of the stack of fabrics after the heating in essay conditions (180ºC in heating sources) 71 Figure 8.1 Temperature distribution on the heating table 72 Figure 8.2 Heat flow (I) and final temperature (II) on the sixth (a) and first (b) layer of the stack of fabrics after the heating 73 Figure 8.3 Decreasing temperature of fabrics when submitted to free air convection on surface 74 Figure 8.4 Temperature variations of the fabrics during the heating a transport processes 74 Figure 8.5 Temperature distribution on the preform and mold tools after the stamping 75 Figure 8.6 Temperature distribution on the preform after the stamping 76

Marina Moya Sánchez Page III wbk Institute of Production Science References

11 List of Tables Table 1. Methods of Preforming Manufacturing [Hen-11] 11 Table 2. Most common types of grippers 14 Table 3. Stamp module pneumatic guides characteristics 28 Table 4. Profile table pneumatic guides characteristics 29 Table 5. Stamping unit pneumatic cylinders characteristics 30 Table 6. Grading Scale [VDI – 98] 16 Table 7. Evaluation criteria for the stamping system 42 Table 8. Evaluation Criteria for the fixing interface 42 Table 9. Evaluation criteria for the gripper selection 43 Table 10. Evaluation criteria for the gripper disposal 43 Table 11. Alternative selection of the cylinder positioning system 44 Table 12. Alternative selection for the positioning reference 45 Table 13. Alternative selection of the fixing interface 45 Table 14. Alternative selection of the holding system 46 Table 15. Selection of the gripper disposal 47 Table 16. Most representative parts of the stamping unit 50 Table 17. Thermal sources WEMA DH 200 characteristics 55 Table 18. Textile fabrics Panex UD300 characteristics 55 Table 19. Binder EPIOKTE 05390 characteristics 55 Table 20. Specific heat values of aluminium depending on temperature [Efu-15] 56 Table 21. Typical values of film coefficient [Ber-11] 57 Table 22. Typical values of contact conductance [DS-15] 57 Table 24. First approximation of the model parameters of the fabrics as a six part assembly 59 Table 23. Model parameters of the heating table, before and after optimization 66 Table 25. Effect of model properties variations on temperature-time curve for the six part model 68 Table 26. Model parameters of the fabrics, before and after model simplification 69 Table 27. Model parameters of the heating fabrics 69 Table 28. Effect of model properties variations on temperature-time curve for the one part model 70

Marina Moya Sánchez Page IV wbk Institute of Production Science References

12 References

Books:

[AVK-10] AVK – Industrievereinigung Verstärkte Kunststoffe e.V. (2010), Handbuch Faserverbundkunststoffe.

[Bak-04] Baker, A.; Dutton, S. & Kelly, D. (2004), Composite Materials for Aircraft Structures, Second Edition, American Institute of Aeronautics and Astronautics.

[Bar-11] Barzin Mobasher (2011), Mechanics of Fiber and Textile Reinforced Cement Composites. ISBN: 9781439806609.

[Ber-11] Bergman, T. L.; Lavine, A. S.; Incropera, F. P. & Dewitt, D. P. (2011), Fundamentals of Heat and Mass Transfer. ISBN: 978-0470-50197-9.

[Cam-10] Campbell, F. C. (2010), Structural Composite Materials. ISBN: 978-1- 61503-037-8.

[CAE-12] Dassault Systèmes (2012), Abaqus/CAE User's Manual.

[Hen-11] Henning, Frank; Moeller, Elvira (2011), Handbuch Leichtbau - Methoden, Werkstoffe, Fertigung.

[Lau-12] Laurenzi, S. & Marchetti, M. (2012) , 'Advanced Composite Materials by Resin Transfer Molding for Aerospace Applications' in Composites and Their Properties, ed Ning Hu. ISBN: 978-953-51-0711-8.

[Roy-04] Roye, A., Gries, T (2004), Design by Application – Maßgeschneiderte Abstandskettengewirke für den Einsatz als Betonbewehrungen (Contoured spacer fabrics for use as textile reinforced in concrete structures).

[Soo-15] Soo-Jin Park (ed.) (2015), Carbon Fibers, Springer. ISBN: 978-94-017- 9477-0.

[Vic-11] Vicente, A. & Oliveira Gomes, J. d. (2011), 'Selection of Additive Manufacturing Technologies Using Decision Methods' in Rapid Prototyping Technology - Principles and Functional, ed Dr. M. Hoque. ISBN: 978-953- 307-970-7.

Diplomarbeit/Thesis:

[Has-12] Hasselström, A. K. J. & Nilsson, U. E. (2012), Thermal Contact Conductance in Bolted Joints, Chalmers Univeristy Of Technoloy, Gotheborg, Sweden, Department of Materials and Manufacturing Technology.

[Hau-13] Manuel Hauer (2013), Konstruktion und Inbetriebnahme einer stempelbasierten Vorrichtung für die endkonturnahe Drapierung von

Marina Moya Sánchez Page V wbk Institute of Production Science References

textilen Halbzeugen, Karlsruher Institut für Technologie, wbk, Institut für Produktiontechnik.

[Goe-12] Michael Göb (2012), Konzipierung, Konstruktion und Inbetriebnahme einer Anlage für die endkonturnahe Drapierung textiler Halbzeug innerhalb des RTM-Prozess, Karlsruher Institut für Technologie, wbk, Institut für Produktiontechnik.

[Sig-13] Mlungisi Sigasa, B. (2013), Development and implementation of a control system for a preforming facility for fiber-reinforced composite parts, Karlsruher Institut für Technologie, wbk, Institut für Produktiontechnik.

Scientific Papers, Reports and Publications

[CMR-14] Thomas Kraus; Michael Kühnel & Dr. Elmar Witten (2014), Composites Market Report 2014: Market developments, trends, challenges and opportunities.

[Fle-14] Fleischer, J.; Förster, F. & Crispieri, N. V. (2014), 'Intelligent gripper technology for the handling of carbon fiber material', Production Engineering, vol. 8, no. 6, pp. 691–700.

[Fle-13] J. Fleischer, A. Ochs, F. Förster (2013), 'Gripping Technology for Carbon Fibre Material'.

[Paq-14] Paquin, R. (2014), 'How Prepared is the Automotive Industry? Solutions for Meeting Fuel Efficiency and Emissions Standards'.

[Fle-12] Prof. Dr.-Ing. J. Fleischer, Prof. Dr.-Ing. G. Lanza (2012), Overcoming the challenges of automated preforming of semi-finished textiles, Munich.

[Jov-12] R. Joven, R. Das, A. Ahmed, P. Roozbehjavan, B. Minaie (2012), 'Thermal Properties of Carbon Fiber - Epoxi Composites with Different Fabrics Weaves'.

[VDI-98] VDI-Richtlinie (1998): Konstruktionsmethodik – Technisch-wirtschaftliches Konstruieren –Technisch-wirtschaftliche Bewertung. VDI 2225 Blatt 3. Düsseldorf: Verein Deutscher Ingenieure, 1998.

Internet Sources

[Bla-13] Black, S. (2013), 'Structural preform technologies emerge from the shadows', Composites Technology, vol. 18, no. 5, pp. 22–29. http://ct.epubxp.com/i/178128-oct-2013.

[DS-12] Dassault Systèmes (2012), SolidWorks Help Guide. http://help.solidworks.com/2012/English/SolidWorks/sldworks/r_welcome_s w_online_help.htm.

[Efu-15] EFunda - Engineering Fundamentals. http://www.efunda.com.

Marina Moya Sánchez Page VI wbk Institute of Production Science References

[Hew-15] Hewitt, G. F. & Barbosa, J. R. , Thermopedia. A-to-Z Guide to Thermodynamics, Heat & Mass Transfer, and Fluids Engineering. http://www.thermopedia.com/.

[Lan-12] LBIE. Lance Brown Import-Export (2012). http://www.lbie.com/vyarn.htm.

[Ada-14] Larry Adams (2014), 'Air grippers grasp thin, delicate parts', vol. 7, no. 5, pp. 20–22. http://www.micromanufacturing.com/sites/all/files/xap_backissues/SeptOct 2014.pdf.

Marina Moya Sánchez Page VII

ANEXO I.

ESTIMACIÓN DE COSTES DE DISEÑO, FABRICACIÓN Y MONTAJE Costes de diseño 510,00 € 1.095,00 € Costes de simulación 585,00 € Costes de fabricación 805,15 € 2.729,75 € Costes de componentes 709,60 € 1.634,75 € Costes de montaje 120,00 €

Costes de diseño Cantidad (h) Coste (€/h) Coste horario licencia Siemens NX 240 1,50 € 360,00 € Tutor del proyecto 5 30,00 € 150,00 € Alumno 240 0,00 € 0,00 € 510,00 €

Costes de fabricación Componentes de la herramienta macho del molde Cantidad (kg) Coste(€/kg) Material bruto (Aluminio) 30 5,50 € 165,00 €

Cantidad (h) Coste (€/h) Máquina Fresado + Taladrado 10,00 € 35,00 € Operario 3,5 8,00 € 28,00 € Herramientas 3,50 € 12,25 € 240,25 €

Herramienta hembra del molde Cantidad (kg) Coste(€/kg) Material bruto (Aluminio) 40 5,50 € 220,00 €

Cantidad (h) Coste (€/h) Máquina Fresado + Taladrado 10,00 € 8,00 € Operario 0,8 8,00 € 6,40 € Herramientas 3,50 € 2,80 € 237,20 €

Pieza de adaptación a la mesa Cantidad (kg) Coste(€/kg) Material bruto (Aluminio) 30 5,50 € 165,00 €

Cantidad (h) Coste (€/h) Máquina Fresado + Taladrado 10,00 € 3,00 € Operario 0,3 8,00 € 2,40 € Herramientas 3,50 € 1,05 € 171,45 €

Placa Soporte Cantidad (kg) Coste(€/kg) Material bruto (Aluminio) 20 5,50 € 110,00 €

Cantidad (h) Coste (€/h) Máquina Fresado 10,00 € 8,00 € Operario 0,8 8,00 € 6,40 € Herramientas 3,50 € 2,80 € 127,20 € Pieza posicionamiento en forma de S (7 uds) Cantidad (kg) Coste(€/kg) Material bruto (Aluminio) 0,1 5,50 € 0,55 €

Cantidad (h) Coste (€/h) Máquinas taladrado + Doblado 10,00 € 5,00 € Operario 0,5 8,00 € 4,00 € Herramientas 3,50 € 1,75 € 11,30 €

Pieza posicionamiento plana (siete uds) Cantidad (kg) Coste(€/kg) Material bruto (Aluminio) 0,1 5,50 € 0,55 €

Cantidad (h) Coste (€/h) Máquina Fresado 10,00 € 8,00 € Operario 0,3 8,00 € 6,40 € Herramientas 3,50 € 2,80 € 17,75 €

Compra de componentes Cantidad Precio Perfiles de aluminio 45x45 (1000 mm) 10 26,00 € 260,00 € Perfiles de aluminio 60x45 (m) 1 50,00 € 50,00 € Conectores universales (con tornillos) 26 7,00 € 182,00 € Brackets (60x60) 8 10,00 € 80,00 € Plastic gripper cover 1 5,00 € 5,00 € Elemento de sujeción 1 122,60 € 122,60 € Manguera gripper 1 10,00 € 10,00 € 709,60 €

Costes de montaje horas coste horario Operarios 15 8,00 € 120,00 € 120,00 €

Costes estudio adicional térmico cantidad coste horario Coste horario licencia Abaqus 180 1,50 € 270,00 € Salario ingeniero responsable 3 30,00 € 90,00 € Salario estudiante 180 0,00 € 0,00 € Maquina 15 15,00 € 225,00 € Telas de fibra de carbono 0,00 € 0,00 € 585,00 €

ANEXO II.

ELEMENTOS ANÁLISIS TÉRMICO NEU Epoxidharze Binderpulver

Binderpulver

Bei der Serienherstellung von Faserverbundbauteilen Binderpulver werden mittels geeigneter Anlagen im RTM- oder Pressverfahren werden sogenannte großflächig maschinell oder auch partiell von Hand Preforms verwendet. Das sind Vorformlinge, die aus auf die Textilien aufgerieselt und thermisch aktiviert. den exakt zugeschnittenen Faserlagen und einem Dabei kommt der Wahl des Binders eine entschei- Binder aus EP-Pulver in einer Pressform unter Tempe- dende Bedeutung zu. Der Binder verhindert das Aus- ratur hergestellt werden. Die Vorformlinge können fransen der Zuschnitte und verbindet die einzelnen stapelweise bis zur Weiterverarbeitung zwischenge- Faserlagen miteinander. Durch sein thermoplasti- lagert werden. Durch die hohe Eigensteifigkeit lassen sches Verhalten kann der Preform unter Wärme wei- sich Preforms mit computergesteuerten Greifwerk- terhin umgeformt werden. Während der Harzimprä- zeugen bewegen und positionieren. gnierung im Werkzeug hält er die Fasern in Position.

Textile Verstärkung, Mehrlagiges Aufschichten und Binder auftragen Pressen Entformen z. B. Gewebe, Gelege, UD

Lay-up

Schema der Preformherstellung mit EP-Binderpulver

EPIKOTE Resin 05390 EPIKOTE System 620

Thermoplastisches Binderpulver auf Basis Bisphenol-A. Vernetzendes Binderpulver zur Herstellung von Erweichungstemperatur 80 – 90°C. Bleibt dauerhaft Preforms, die auch unter Prozesswärme dauerhaft thermoplastisch, dadurch sind bebinderte Textilien formstabil bleiben. Unterhalb 100°C verhält sich mehrfach thermisch aktivierbar, z.B. in mehrstufigen dieser Binder thermoplastisch. Bei Erwärmung über Umformprozessen. Keine Beeinträchtigung der Lami- 120°C beginnt die Vernetzung. nateigenschaften. Vorteile System 620

• das Preformwerkzeug kann isotherm betrieben werden, zum Entformen ist kein Herunterkühlen mehr nötig • kurze Vernetzungszeiten möglich, z. B. 3 Min. bei 140°C • einfaches Einlegen und Schließen des Werkzeugs, da sich der Vorformling unter Wärme nicht mehr entspannt • Verstärkungsfasern behalten während des Füllvor- gangs ihre Position und Orientierung

98 www.lange-ritter.de Epoxidharze Epoxidharze NEU Binderpulver Binderpulver

Ohne Binder

Die Fasern werden vom fließenden Harz verschoben. In unmittelbarer Nähe des Angusses ist die Desorientierung der Fasern am größten. Es kommt zum Zusammenschieben der Fasern und im ungünstigen Fall zur Blockade des Harzflusses.

Thermoplastischer Binder

Die Fasern halten zunächst ihre Position und Orientierung. Bei langen Injektionszeiten kann es zum Schmelzen des Binders kommen, die Fasern beginnen im Harzstrom zu wandern. Da der Effekt zeitverzögert einsetzt, sind die Auswirkungen weniger gravierend.

Vernetzter Binder

Auch unter hohen Prozesstemperaturen und bei großen Volumenströmen hält der Binder die Fasern exakt in Position.

Binderpulver Branchen

Auftrags- Erweichungs- Packungs- Produkt menge temp. größen EP Artikel-Nr. bezeichnung Aussehen g/m2 °C Charakteristik kg

EPIKOTE weißes 05E05390.20 3 – 15 90 mehrfach thermoverformbar 20 Resin 05390 Pulver 90 – 100°C mehrfach umformbar; EPIKOTE weißes 05E620.20 3 – 15 95 >100°C beginnende Vernetzung; 20 System 620 Pulver z.B: 3 Min. bei 140°C; 90 Sek. bei 160°C

www.lange-ritter.de 99 Düsenheizbänder Nozzle Heater Bands INFO INFO

DH 200 (Messing, Mikanit-isoliert) (Brass, Mica insulated)

DH 400 (Edelstahl, Mikanit-isoliert) (Stainless steel, Mica insulated)

DH 600 (Edelstahl, Magnesiumoxid-isoliert) (Stainless steel, MgO insulated)

DH 200 DH 400 DH 600

Anwendung: Application: • Beheizung von Düsen an Spritzgießmaschinen. • Heating of nozzles on injection molding machines. • Vielfältige Einsatzgebiete im Maschinen- und Formen- • Various applications possible. bau. • Heating of pipelines. • Beheizung von Rohrleitungen.

DH 200, DH 400 DH 200, DH 400 Besondere Merkmale: Technical features: • Hohe Lebensdauer bei einfacher Montage. • Long durability, easy to install. • Völlig geschlossene, kunststoffdichte Ausführung. • Fully sealed housing against plastic penetration. • Mit eingebautem Thermofühler lieferbar. • Available with built-in thermocouple. • Variable Kabellängen und -abgänge lieferbar. • Available with different cable lengths and lead exits.

DH 600 DH 600 Besondere Merkmale: Technical features: • Schnelleres Aufheizen durch hohe Leistungsdichte. • Faster heating-up because of high power density. • Lange Lebensdauer der Heizung durch sehr große • Long durability through mechanical strength. mechanische Stabilität. • Less machine down whilst higher productivity. • Kürzere Maschinenstillstandzeiten, höhere Produktivität. • Complies with IP68 if ordered with stainless steel hose. • Bei Ausführung mit Edelstahl-Wellschlauch, auch IP68 • Available with built-in thermocouple. ﬂüssigkeitsdicht lieferbar. • Available with different cable lengths and lead exits. • Mit eingebautem Thermofühler lieferbar. • Variable Kabellängen und -abgänge lieferbar. ]

1 1 / 1 3 H1.1

[ WEMA GmbH • D-58515 Lüdenscheid • Kalver Str. 28 • Tel. +49 2351 9395-0 • Fax +49 2351 9395-33 • [email protected] • www.wema.de Düsenheizbänder Nozzle Heater Bands INFO INFO

Technische Speziﬁkationen: Technical speciﬁcations:

Heizbandtyp DH 200 DH 400 DH 600 Type

Mantelmaterial Messing Edelstahl Edelstahl

Housing material Brass Stainless steel Stainless steel

Isoliermaterial Mikanit Magnesiumoxid Insulation material Mica MgO Ausführung völlig geschlossen, kunststoffdicht

Technical feature fully sealed housing against plastic penetration Betriebstemperatur, max °C 300 350 600 Maximum operating temperature Heizleistung W/cm2 max. 6 max. 12 Heating capacity Anschlussspannung 230 oder 400 (andere auf Anfrage) V Supply voltage 230 or 400 (other available) Hochspannungsfestigkeit V-AC 1500 High-voltage strength Isolationswiderstand, kalt 1 (bei 500 V-DC) MOhm Insulation resistance 1 (at 500 V-DC) Thermofühler, eingebaut auf Wunsch (Standard: FeCuNi, Typ J, potenzialhaltig; Sonderausführung: potenzialfrei)

Built-in thermocouple available (Standard: FeCuNi, Type J, grounded; Optional: not grounded Innendurchmesser d mm 25 - 110 Internal diameter

Heizbandbreite b mm 20 - 80 20 - 70 22 - 60 Width Standard-Kabellängen 300, 500, 800, 1000, 1500, 2000, 2500, andere auf Anfrage mm Cable length 3000, 3500, … other available Kabeltyp 3 x 0,75 mm2 Reinnickel, glasseidenisoliert, metallgeﬂecht ummantelt Type of cable 3 x 0,75 mm2 heat resistant nickel wire Schutzart IP40 Protection class

Schutzart mit Edelstahl-Wellschlauch – IP68 Protection class with stainl. steel hose Spannschrauben M5x20 mm, mit verdrehsicherer Mutter M5x30 mm, m. verd. Bolzen

Tensioning screws M5x20 mm, anti-turn nut M5x30 mm, with bolt Anschlusskappe stabile Stahlausführung

Terminal cap Steel Norm CE Compliance

Ø10 18

24 30 13

4 9 18

d max.110 d min.25, min.25, max.110 12 12

b b DH 200/DH400 DH 200: min.20, max.80 DH 600 min.22, max.60 DH 400: min.20, max.70 ]

H1.2 1 1 / 1 3

WEMA GmbH • D-58515 Lüdenscheid • Kalver Str. 28 • Tel. +49 2351 9395-0 • Fax +49 2351 9395-33 • [email protected] • www.wema.de [ Düsenheizbänder Nozzle Heater Bands

Lieferbare Standard-Ausführungen in 230 V Available standard versions (230 V)

d b P [W] d b P [W] d b P [W] d b P [W] Ø DH 200 DH 400 DH 600 Ø DH 200 DH 400 DH 600 Ø DH 200 DH 400 DH 600 Ø DH 200 DH 400 DH 600 25 20 90 90 – 48 20 180 180 – 60 35 370 370 600 80 60 800 800 1000 22 – – 200 22 – – – 40 450 450 650 65 800 – – 25 100 100 200 25 200 200 – 45 480 – – 70 850 850 – 30 140 140 250 30 270 270 – 50 520 520 800 80 900 – – 28 20 100 100 – 35 300 300 – 55 550 – – 85 20 300 300 – 25 100 100 – 40 320 320 – 60 600 600 800 22 – – – 30 140 140 – 45 350 – – 65 650 – – 25 340 340 – 30 20 100 100 – 50 420 420 – 70 700 700 – 30 500 500 – 22 – – 200* 55 430 – – 80 800 – – 40 600 600 – 25 120 120 250 60 450 450 – 65 20 270 270 – 45 650 – – 30 160 160 300* 65 500 – – 22 – – 400 50 700 700 – 40 240 240 350 70 550 550 – 25 300 300 500 55 750 – – 45 250 – – 80 650 – – 30 370 370 500 60 900 900 – 50 250 250 500 50 20 180 180 – 35 400 400 500 65 900 – – 55 260 – – 22 – – 350 40 450 450 650 70 1000 1000 – 60 300 – 500 25 210 210 400 45 480 – – 80 1000 – – 32 30 180 180 300 30 270 270 500* 50 520 520 700 90 20 300 300 – 35 20 120 120 – 34* 310 310 500 55 550 – – 22 – – 500 22 – – 250* 40 350 350 600* 60 600 600 800 25 350 350 700 25 150 150 300 45 370 – – 65 650 – – 30 500 500 700 30 180 180 350 50 430 430 600 70 700 700 – 40 600 600 800 40 240 240 400 55 450 – – 80 800 – – 45 650 – – 45 250 – 500 60 500 500 750 70 20 200 200 – 50 700 700 900 50 270 270 500 65 550 – – 22 – – 400 55 750 – – 55 300 – – 70 600 600 – 25 280 280 550 60 900 900 1000 60 320 – 600 80 700 – – 30 400 400 650 65 900 – – 40 20 160 160 – 55 20 200 200 – 35 420 420 750 70 1000 1000 – 22 – – 250* 22 – – 300 40 500 500 800* 80 1000 – – 25 180 180 300 25 240 240 450 45 550 – – 100 20 300 300 – 30 250 250 350 30 320 320 500 50 600 600 800 22 – – 600 35 270 270 400 35 350 350 500 55 650 – – 25 350 350 650 40 300 300 400* 40 400 400 650 60 720 720 800 30 550 550 800 45 320 – 500 45 440 – – 65 720 – – 40 700 700 900 50 350 350 600 50 480 480 650 70 750 750 – 45 750 – – 55 370 – – 55 500 – – 80 800 – – 50 800 800 1000 60 400 400 800 60 550 550 800 75 20 230 230 – 55 850 – – 65 450 – – 65 600 – – 22 – – 450 60 900 900 1100 70 500 500 – 70 650 650 – 25 300 300 550 65 950 – – 80 600 – – 80 700 – – 30 450 450 650 70 1000 1000 – 42 20 160 160 – 58 20 200 200 – 40 550 550 800 80 1000 – – 22 – – 250 25 250 250 – 45 550 – – 110 20 300 300 – 30 250 250 350 30 320 320 – 50 600 600 800 22 – – 650 45 20 180 180 – 35 350 350 – 55 650 – – 25 350 350 650 22 – – 300 40 400 400 – 60 750 750 800 30 550 550 800 25 200 200 350 45 440 – – 65 750 – – 40 700 700 900 30 250 250 400* 50 480 480 – 70 800 800 – 45 750 – – 35 270 270 450 55 500 – – 80 850 – – 50 800 800 1000 40 320 320 500 60 550 550 – 80 20 320 320 – 55 850 – – 45 350 – 600 65 600 – – 22 – – 450 60 900 900 1000 50 400 400 700 70 650 650 – 25 340 340 600 65 950 – – 55 420 – 700 80 700 – – 30 450 450 600 70 1000 1000 – 60 450 450 800 60 20 210 210 – 40 550 550 800 80 1000 – – 65 500 – – 22 – – 350 45 600 – – 550 550 – 260 260 400 * = Kabellänge 800 mm 70 25 50 650 650 800 Typ 1, 45° 80 650 – – 30 350 350 500* 55 650 – –

Andere Abmessungen oder Heizleistungen auf Anfrage! Various dimensions and heating capacities upon request!

= ab Lager lieferbar = ex stock item Bei Bestellungseingang bis 10:00 Uhr erfolgt der If ordered prior to 10 am, the requested ex-stock Versand unter üblichem Vorbehalt am gleichen Tag. heater will be shipped the same day. Offer subject to Zwischenverkauf vorbehalten! prior sale!

Bei der Bestellung unbedingt angeben: Essential ordering information: Heizbandtyp, Innen-ø, Breite, Anschlussspannung, Leistung, Type, internal diameter, width, supply voltage, heating capacity, Kabellänge und Kabel-Abgangsvariante. Ohne weitere Angaben cable length and cable exit. We will supply the nozzle heater wird das Düsenheizband ohne Stecker und ohne band without connector and thermocouple unless you request Thermofühler geliefert. something different. ]

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[ WEMA GmbH • D-58515 Lüdenscheid • Kalver Str. 28 • Tel. +49 2351 9395-0 • Fax +49 2351 9395-33 • [email protected] • www.wema.de Technical Datasheet Panex® 35 Uni-Directional Fabrics Stitch-Bonded Uni-Directional Carbon Fabrics

Description

Panex® 35 Stitch-Bonded Uni-Directional Carbon Fabrics are produced from our Panex® 35 50K Continuous Tow Carbon Fiber. Unique ber spreading techniques are utilized to obtain a wide range of UD fabric weights for a varied set of composite part applications. Quick composite part build-up is cost eectively achieved with our diverse weight range of low-cost carbon fabric products.

Material Overview UD150 UD200 UD300 UD400 UD500 UD600 UD900V

0° Carbon Panex® 35 50K 158 200 309 403 500 600 865

90° Glass 34 dtex 10 10 10 10 10 10 -

Polyester Veil ------30

Polyester Stitch 76 dtex 6 6 6 6 6 6 5

Binder Resin Powder 8 8 8 - - 8 -

Total Fabric Weight 182 g/m 224 g/m 333 g/m 419 g/m 516 g/m 624g/m 900 g/m 5.37 oz/yd 6.61 oz/yd 9.82 oz/yd 12.36 oz/yd 15.22 oz/yd 18.40 oz/yd 26.54 oz/yd

Average values shown

Fabric Construction UD150 UD200 UD300 UD400 UD500 UD600 UD900V

Stitch Length 3.6 mm 5.5 mm

Stitch Pattern Tricot ½ Tricot

Cure Thickness .21 mm .25 mm .37 mm .46 mm .57 mm .69 mm 1.00 mm

Roll Width 30 cm - 61 cm - 122 cm 122 cm

Roll Length 100 m 50 m 30 m

Average values shown

Composite Properties SI US Method

Tensile Strength 1,449 MPa 210 ksi DIN EN ISO 527

Tensile Modulus 126 GPa 18.3 msi DIN EN ISO 527

Compressive Strength 809 MPa 117 ksi ASTM D694

Compressive Modulus 107 GPa 15.5 msi ASTM D695

Typical Fiber Volume Fraction (FVF) is 55%. Standard Epoxy Resin System

Typical Packaging Certi cation

Wound on cardboard cone, sealed in polyethylene Panex® 35 Fabrics are manufactured in accordance bag, and placed in cardboard box. Rolls stacked with Zoltek’s written and published data. A Certi - horizontally on pallets when shipping. cate of Conformance is provided with each ship- ment. + Requirements other than standard widths and roll lengths should be speci ed by purchase order. Safety

Approval Obtain, read, and understand the Material Safety Data Sheet (MSDS) before use of this or any other Germanischer Lloyd (GL) has Zoltek product. granted GL approval to Panex® 35 Uni-Directional Fabrics for use in wind energy applications. CERTIFIED Germanischer Lloyd Approved, Approval No. WP 1030011 HH

ISO 9001 AS9100 PANEX® is a registered trademark BUREAU VERITAS Certification of Zoltek Corporation CERTIFIED No US09000636 Zoltek Corporation | 3101 McKelvey Rd. | St. Louis, MO 63044 P: 314-291-5110 | F: 314-291-8536 | E: [email protected] | W: www.zoltek.com 0812