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Moulded pulp products manufacturing with thermoforming
Didone, Mattia; Tosello, Guido Published in: Packaging Technology and Science
Link to article, DOI: 10.1002/pts.2412
Publication date: 2019
Document Version Peer reviewed version
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Citation (APA): Didone, M., & Tosello, G. (2019). Moulded pulp products manufacturing with thermoforming. Packaging Technology and Science, 32(1), 7-22. https://doi.org/10.1002/pts.2412
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Molded pulp products manufacturing with thermoforming Mattia Didone, Guido Tosello
Technical University of Denmark, Department of Mechanical Engineering, 2800 Kgs. Lyngby, Denmark
Abstract Over the past years, eco-friendly packaging solutions such as molded pulp has resonated with a growing number of consumers. Among all of them, the thermoformed products make use of the most recent manufacturing approach that produces high quality, thin-walled items. However, it remains an under-researched area, and the development of an efficient and precise manufacturing process is fundamental in order to increase the implementation of sustainable packaging. With the purpose of setting a step towards in the standardization of design and testing practices of eco-friendly packaging, this work focused on the characterization of the thermoforming process of molded pulp products and their characteristics. Three different analysis were carried out for this purpose, covering the dewatering efficiency of the process, a quantification of the molding geometrical accuracy and an analysis of the internal microstructure of the parts. Experimental results and statistical analysis show that the dewatering efficiency is mainly governed by the mold’s temperature while the duration of the contact time is not influential. In the second investigation, the geometrical accuracy of the moldability of micro-features was assessed. The process appeared to be dependently related to the pulp type employed. Finally, the internal microstructure was documented using computed tomography. The analysis shows an increase in the internal void fraction linked with an increase in the mold’s temperature. The role of the water change of phase in the thermoforming process was also discussed by reference to the work conducted on impulse drying.
Keywords: Molded pulp, molded fiber, thermoforming, impulse drying, 3D structure of paper.
Introduction In recent years, economic, environmental and social concerns have resulted in an increasing demand for sustainable packaging products 1. Molded pulp is an environmentally friendly packaging material that is recyclable, compostable and eventually biodegradable, and could be used as an alternative to oil- based packaging products such as EPS (expanded polystyrene) or vacuum formed PET (polyethylene terephthalate). In fact, molded pulp consists mainly of water and wood fibers, which are primarily composed of cellulose.
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Molded pulp versatility is remarkable, but its diffusion depends also on the development of an efficient, accurate and robust manufacturing process. The most advanced manufacturing approach in this category is thermoforming 1. In this process, the initial partially formed product is pressed in a heated mold for a period in the order of tens or hundreds of seconds, where it is densified and dried. The resulting items have good dimensional accuracy and smooth surfaces that resemble the ones of plastic products. Nevertheless, the thermoforming of molded pulp is an energy intensive operation, and a research effort is needed in this direction for making the process more sustainable. Just as in the conventional papermaking industry, a large portion of resources is consumed during the drying process. Very few reports of recent innovations within the field can be found in the literature 2– 4, and none of them addresses specifically the thermoforming technology. On the other hand, a commonly known technology in the pulp and paper industry that is similar to it in nature is impulse drying, introduced by Wahren 5 in the 1980s. Impulse drying is an advance drying technique in which water is removed from a wet paper web by the combination of mechanical pressure and intense heat. In this process, the paper web is subjected to a temperature between 100 °C and 400 °C, and simultaneously pressed at 1.5 MPa to 8 MPa 6. The dwell, or nip, residence time (i.e. the time in which the paper web is subjected to the combine effect of heat and pressure) is around 20 ms to 50 ms. This time frame is a consequence of the design of a conventional press nip for the papermaking industry, which is a continuous flow process. Impulse drying technology proved to offer enhanced dewatering and physical properties of paper sheets 7,8. Its mechanism is complex and the physics underlying the phenomenon was never fully described. It was suggested that if the heat transfer rate is sufficiently high, a high-pressure steam is generated on the surface of the paper web next to the hot medium. The steam expansion could then aid in the displacement/removal of the water held between the fibers 5,9,10. This effect was most recently studied and termed “flashing-assisted displacement dewatering” by Lucisano et al. 11. In their research, it was experimentally proved that the displacement of liquid water by the action of an expanding steam layer plays a significant role in the physics of dewatering during an impulse drying event. An undesired and secondary phenomenon that is also linked to the expanding steam is called delamination 6,7. It is mainly caused by an imbalance between the pressure within the sheet and the external, atmospheric, pressure 12. When the drag forces resulting from the escaping vapor overcome the cohesive forces holding the paper web together, the sheet delaminates 13. From the perspective of the products characteristics and quality assurance, the geometrical accuracy of the parts and their internal structure are two important aspects. In a molding process, when evaluating the capability of molding features at the micro- scale, the accuracy of the replicated features with respect to the master geometry has to be quantified 14. For this purpose, various dimensional measurement solutions can be applied 15. Among all of them, optical instruments are strongly emerging because of their distinctive advantages 16. Their contact-less and non-destructive nature makes them suitable for measuring soft components: in fact, even a small force would invalidate the measurement by damaging the molded pulp surface. They are also
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typically faster than contact instruments such as coordinate-measuring machines and profilometers 17. The internal structure of molded pulp products determines instead transport properties such as permeability to water and moisture, electrical and thermal conductivity. It is thus of importance for assessing the packaging durability and whether there is the need for selecting an appropriate coating to be combined with the molded pulp item. Computed tomography (CT) is an established technique for acquiring 3D images that can be used to characterize the internal structure of a wide variety of materials, including paper. At the pore scale, a fundamental structural unit can be assumed to repeat itself. This fundamental repeating unit has often been referred to as representative elementary volume (REV), and by assessing the properties of various REV geometries, it is possible to scale the transport properties of the entire porous media 18,19. The pore structure of paper is complex and presents significant interconnectivity, as exemplified by the reconstruction of paper using synchrotron beam radiations CT reported in 20–25. Despite the advantages in terms of resolution of acquisition of synchrotron CT, conventional CT setup is widely used and constantly improved. It was in fact used among others by Wernersson et al. 26, Sharma et al. 27 and Miettinen et al. 28.
With the purpose of setting a step towards in the standardization of design and testing practices of eco-friendly packaging, this work focused on the characterization of the thermoforming process of molded pulp products and their characteristics. In particular, this work comprises of three investigations. The first one aims at characterizing the process while the other two focus on the geometrical accuracy and internal structure of the parts. In the first investigation, the dewatering efficiency of a thermoforming process was quantified and the influence of the process parameters discussed. Inspired by the findings around the impulse drying technology, the molded pulp samples of this work were thermoformed at temperatures above the normal boiling point of water (around 100 °C) and at a pressure of 1.9 MPa and 2.5 MPa. Although these conditions resemble the ones of the impulse technology, and because of the different tooling design of a molded pulp machine, the samples were dried for periods in the order of seconds. Consequently, the technology employed in this work is not addressed as impulse. The goal of the first study was to gain a better understating of the thermoforming process for making it optimized and more sustainable. The second investigation focused on the capability of molding features at the micro- scale. Different type of geometrical micro-features were molded on the surface of the thermoformed molded pulp samples. A laser-scanning confocal digital microscope was thus employed for measuring the geometry of the master and of the replicated features. The two measurements were compared and a quality indicator was defined. In the third analysis, CT inspections were carried out using a conventional setup in order to evaluate the voids or pores distributions within a specified REV of selected thermoformed molded pulp samples. For clarity for the reader, each chapter is subdivided into the three experimental works, namely:
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- Effect of the process parameters on the dewatering efficiency. - Replication quality of micro-features. - Void analysis with computed tomography. An overall discussion on the results of the three analysis is given in the conclusive chapters. The experiments described in this work represent a first step in understanding the thermoforming process of molded pulp products manufacture. The experiments are limited to one type of fiber and to a relatively flat products geometry. The main purpose is in fact to present methods and techniques that can be employed for improving on the technology level of molded pulp production. This topic have received scarce attention in the literature and no direct investigations are available. The aim of this study is to contribute in filling this gap and to consolidate best practices in order to increase the diffusion of environmentally friendly packaging products.
Methods Figure 1 shows the scheme of the laboratory-scale thermoforming machine employed throughout this work. The machine thermoforms pre-formed molded pulp disks with a diameter of 100 mm (Figure 2). The paper disks were formed with a custom-made sheet former that resembles the TAPPI T 205 test method 29. The disk were formed with an initial dryness level around 25 %, and a basis weight of 600 g/m2. The samples were made of commercially available bleached chemical pulp. The pre-formed disks had a thickness around 10 mm, which reduced to half after being thermoformed. The system consists of a double acting cylinder that presses two halves of a mold together. The upper half of the mold is fixed to the outer frame and is heated via two cartridge heaters. The bottom half of the mold is moved upwards by the double acting cylinder and the upward movement stops when the imposed level of pressure is reached. The pre-formed paper disks are placed in the bottom half of the mold on top of a fine metal wire mesh. The mesh covers a series of holes that allows for the removing of the water/steam via vacuum suction during the process. The laboratory-scale machine was operated as follows: i) The upper half of the mold was heated to the imposed level of temperature. ii) A wet pre-formed paper disk was positioned in the bottom half of the mold and this was moved upwards by the double-acting cylinder until the imposed level of mechanical pressure was reached. iii) The mechanical pressure and the temperature were kept constant for a defined time, named contact time. iv) After the contact time was elapsed, vacuum suction was applied for a defined time, named vacuum time. v) The mechanical pressure was released; the molded disk was collected and weighted for assessing the dryness level. Highlighted in bold are the process parameters object of the first experimental investigation: temperature, pressure, contact time and vacuum time.
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Fixed half of Cartridge the mold heaters
Drainage channels Metal wire mesh Vacuum suction
Moving half Double-acting of the mold cylinder
Outer frame
Figure 1: Scheme of the laboratory-scale thermoforming machine.
(a) (b)
Figure 2: (a) Pre-formed wet paper disk. Initial thickness around 10 mm. (b) Thermoformed molded paper pulp disks. Final thickness around 5 mm. Diameter (before and after the thermoformed event) equals to 100 mm.
Effect of the process parameters on the dewatering efficiency The dewatering efficiency was assessed by measuring the weight of the molded disks after the drying event and after they were bone-dried; the dryness level was consequently calculated according to equation (1).
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