3D Printed Sand Tools for Thermoforming Applications of Carbon Fiber Reinforced Composites—A Perspective

materials

Perspective 3D Printed Sand Tools for Thermoforming Applications of Carbon Fiber Reinforced Composites—A Perspective

Daniel Günther 1, Patricia Erhard 1 , Simon Schwab 1 and Iman Taha 1,2,*

1 Fraunhofer Institute for Casting, Composite and Processing Technology, Am Technologiezentrum 2, 86159 Augsburg, Germany; [email protected] (D.G.); [email protected] (P.E.); [email protected] (S.S.) 2 Faculty of Engineering, Ain Shams University, El-Sarayat Street 1, Cairo 11517, Egypt * Correspondence: [email protected]; Tel.: +49-(0)821-90678-252

Abstract: Tooling, especially for prototyping or small series, may prove to be very costly. Further, prototyping of fiber reinforced thermoplastic shell structures may rely on time-consuming manual efforts. This perspective paper discusses the idea of fabricating tools at reduced time and cost compared to conventional machining-based methods. The targeted tools are manufactured out of sand using the Binder Jetting process. These molds should fulfill the demands regarding flexural and compressive behavior while allowing for vacuum thermoforming of fiber reinforced thermoplastic sheets. The paper discusses the requirements and the challenges and presents a perspective study addressing this innovative idea. The authors present the idea for discussion in the additive manufacturing and FRP producing communities.

Keywords: binder jetting; sands; vacuum thermoforming; ﬁber reinforced composite

Citation: Günther, D.; Erhard, P.; Schwab, S.; Taha, I. 3D Printed Sand Tools for Thermoforming 1. Introduction Applications of Carbon Fiber A main industrial focus today lies in energy-efficient and resource-saving manufactur- Reinforced Composites—A ing. This is one way to meet relevant ecological and economic targets as well as to ensure Perspective. Materials 2021, 14, 4639. the competitiveness of products in the long term. As a result, lightweight components of https://doi.org/10.3390/ma14164639 high mechanical performance are increasingly attracting attention. Thus, fiber reinforced polymers (FRP), especially involving carbon fibers, are currently in the spotlight of develop- Academic Editor: Tuhin Mukherjee ments. Related manufacturing processes, equipment and tools must be adapted to the new material and functionality requirements. A significant amount of time in the optimization Received: 19 July 2021 process of a component lies in the creation of initial prototypes. The production of small Accepted: 13 August 2021 Published: 18 August 2021 series often proves to be particularly cost-intensive. These are primarily manufactured in manual processes. Depending on part complexity, costly and time-consuming tool manufacturing might be necessary. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Additive manufacturing provides a good solution regarding the rapid fabrication of published maps and institutional affil- tools. Next to functional integration, additively manufactured molds can combine complex- iations. ity and lightweight. As an example, additive manufacturing makes it possible to realize complex cooling channels within the tool without the need of parting the mold. Some additive manufacturing techniques, such as the Binder Jetting process have a high volume throughput compared to the established CNC machining, thus allowing for lower manufacturing time and energy consumption. Further, additive manufacturing contributes to Copyright: © 2021 by the authors. the conservation of material resources, since it relies on generating the final structure layer Licensee MDPI, Basel, Switzerland. This article is an open access article by layer as opposed to the material removal concept in established machining processes. distributed under the terms and For processing FRPs, both tools for low temperature applications (such as hand layup or conditions of the Creative Commons Resin Transfer Molding) as well as high temperature applications (such as autoclave curing Attribution (CC BY) license (https:// or compression molding) are relevant. Generally, tools do not require high strength [1,2]. creativecommons.org/licenses/by/ In high pressure processes, as in the case of autoclave curing, compression molding or 4.0/).

Materials 2021, 14, 4639. https://doi.org/10.3390/ma14164639 https://www.mdpi.com/journal/materials Materials 2021, 14, 4639 2 of 20

vacuum-assisted processes, loads are primarily compressive. In fact, these loads are often more favorable for additively manufactured parts, as compared to tensile loads [2]. Several studies have been concerned with the fabrication of tools by additive manufacturing from different materials. Brotan et al. [3] fabricated innovative tools out of Marlok C1650 steel powder in a Powder Bed Fusion (PBF) process to realize complex gra- dient structures to increase the thermal fatigue resistance and allow for a defined thermal management of the tools. The target applications of these low weight tools are injection molding and die casting. Similarly, Fette et al. [4] compared common additive manufacturing techniques—Additive Layer Manufacturing (ALM), Electron Beam Melting (EBM) and Selective Laser Melting (SLM)—with conventional methods to manufacture metal tooling. The authors claim that the use of integrated heating channels close to the mold surface allows the even dissipation of heat. Warden [5] studied the application of Fused Deposition Modeling (FDM) to manufacture tools for compression molding thermoplastic multiaxial prepreg systems. The author proposed a low-temperature curing cycle in order to reduce the thermal degradation of the FDM tooling. Further, Bere et al. [6] processed Carbon Fiber Reinforced Polymers (CFRP) through vacuum bagging and oven curing using an FDM polylactic acid (PLA) versus acrylonitrile butadiene styrene (ABS) mold. The PLA mold was treated with a layer of epoxy that contains aluminum powder to enhance the bonding with a subsequently applied polyester gel coat, which further acts as a release agent and facilitates demolding. Hassen et al. [7] proposed the use of the extrusion-based Big Area Additive Manufacturing (BAAM) process to fabricate CFRP tools for further molding purposes. The main advantage lies in the increased throughput (~16,400 cm3/h) and the possibility to flexibly process material from granule form [8,9]. Further attempts have been made to reduce tooling costs for the thermoforming of non-reinforced plastics by additive manufacturing. Laser-sintered metal parts can be excluded due to their high costs. Although manufacturing costs can be reduced to as low as 14% compared to conventional tooling methods (milling), although this is offset by the high metal powder costs of 167% [10]. For the medical field, additive manufacturing of molds from calcium sulfate and gypsum powders was successfully demonstrated and even implemented for thermoforming of plastic structures [11,12]. In addition, Junk et al. [10] reported the application of the inkjet technology for thermoforming mold fabrication out of polymer gypsum. This was tested for a case study, in which a fairing made of ABS for an unmanned aerial vehicle was produced. Among other things, this study aimed to integrate vacuum channels in the manufacturing step without the need for additional post-processing (e.g., drilling). The study also explained the specific technical challenges, such as the demolding of undercuts or the separation of the tool. Results showed that the tool had sufficient strength for the subsequent thermoforming process. Another advantage of additive manufacturing is the design flexibility of both the overall geometry and the internal structure to influence strength and other physical properties for optimized vacuum guidance [11,13]. Little work has been carried out so far to elicit the advantages of using silica sand for tooling purposes. In this perspective, the authors claim that sand structures may prove to be efficient for tooling in general and for vacuum thermoforming processes in specific. Sand structures are thought to be cost-effective. Rough calculations evidenced a tooling cost of EUR 600 to 1000 for an aluminum or steel omega shaped tool compared to less than EUR 100 for a sand tool; the outer tool dimensions are 300 mm × 260 mm × 60 mm. In addition, their low thermal conductivity is favorable for the thermoforming process. First, maintaining the tool at constant temperature allows uniform manufacturing conditions for all parts that are to be formed. Second, when forming thermoplastics, the sheets need to be heated above their glass transition (for amorphous polymers) and slightly below their melting point (for semi-crystalline polymers). Within the process, it is necessary to ensure that forming is completed before the sheet temperature falls below these levels. Achieving that through low heat conduction through the tool may assist in allowing an Materials 2021, 14, x FOR PEER REVIEW 3 of 20

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energy-efficient thermoforming process. In the case of thermoforming, vacuum channels may be directly printed, eliminating the necessity of subsequent drilling steps. In addition, bonding of individualenergy-efficient particles thermoformingallows a porous process. structure, In thewhich case might of thermoforming, be advantageous vacuum channels for vacuum drawing.may be directly printed, eliminating the necessity of subsequent drilling steps. In addition, This manuscriptbonding presents of individual a novel particles idea and allows gives a an porous insight structure, regarding which current might and be advantageous planned researchfor activities vacuum drawing.targeting the use of sand molds for vacuum thermoforming of FRPs. The targeted manufacturingThis manuscript process presents for athe novel mold idea is Binder and gives Jetting. an Further, insight regarding the ap- current and proach for moldplanned modification research for activitiesthe anticipated targeting application the use ofis presented. sand molds In foraddition, vacuum the thermoforming challenges regardingof FRPs. the The vacuum targeted thermoformin manufacturingg of FRPs process are fordiscussed the mold and is Bindera systematic Jetting. Further, the procedure is followedapproach to foraddress mold them. modification for the anticipated application is presented. In addition, the challenges regarding the vacuum thermoforming of FRPs are discussed and a systematic 2. Thermoformingprocedure of Fiber is followedReinforced to addressComposites them. One of the2. established Thermoforming methods of in Fiber the automotive Reinforced sector Composites to fabricate FRP thin-walled structures is the Resin Transfer Molding (RTM) process. This involves the layup of multi- One of the established methods in the automotive sector to fabricate FRP thin-walled axial textiles into stacks, followed by draping into a 3-dimensional shape by matched tool structures is the Resin Transfer Molding (RTM) process. This involves the layup of multi- pressing and finallyaxial textilesresin infusion. into stacks, This followed process by is drapingmostly limited into a 3-dimensional to thermosets shape due to by matched tool their low viscositypressing (below and finally1000 mPa resin∙s) infusion. compared This to process thermoplastics is mostly limited(beyond to thermosets100,000 due to their mPa∙s) and is ablelow to viscosity fulfill the (below demands 1000 mPafor good·s) compared surface quality. to thermoplastics A drawback (beyond lies in 100,000 the mPa·s) and long cycle timesis ableneeded to fulfill for part the curing demands (standard for good RTM surface processes quality. may A drawback require several lies in the long cycle hours or days fortimes curing; needed enhanced for part RTM curing proce (standardsses 2–10 RTM min) processes [14], thus may limiting require the severaluse of hours or days thermosets for massfor curing; production. enhanced In contrast, RTM processes the use 2–10of thermoplastic min) [14], thus matrices limiting makes the use short of thermosets for cycle times (60–140mass s) production. [15] more feasible. In contrast, Thermoplastics the use of thermoplastic further have matricesthe advantages makes shortof cycle times providing improved(60–140 toughness s)[15] more behavior feasible. and Thermoplastics allow for recyclability. further haveFor fiber the advantagesreinforced of providing thermoplastics improvedthe thermoforming toughness process behavior is applicable. and allow The for process, recyclability. as illustrated For fiber in reinforcedFig- thermo- ure 1, implies formingplastics a the flat thermoforming laminate into a process3D geometry is applicable. under the The influence process, of as tempera- illustrated in Figure1, ture. For heatingimplies IR lamps forming or convection a flat laminate ovens into are aoften 3D geometry used [16]. under The transfer the influence time to of temperature. the press has toFor be heatingkept short IR in lamps order or to convection avoid extensive ovens laminate are often cooling, used [16 which]. The would transfer time to the prohibit adequatepress forming. has to For be kept3D forming short in of order FRPs,to both avoid matched-die extensive forming laminate as cooling, well as which would deformable dieprohibit forming adequate is applicable. forming. During For shaping, 3D forming the oflaminate FRPs, bothis held matched-die in place using forming as well as a support framedeformable or a blank die holder. forming This is applicable.is crucial in During order shaping, to keep thethe laminatelaminate is under held in place using a tension and accordinglysupport frame prevent or athe blank creation holder. of Thiswrinkles. is crucial Further, in order the mold to keep is thekept laminate closed under tension during deformationand accordingly to avoid heat prevent loss and the to creation maintain of wrinkles.high pressures Further, (10–40 the moldbar), target- is kept closed during ing proper compactiondeformation and tofull avoid consolidation heat loss and[17]. to The maintain final part high is pressuresthen left to (10–40 cool within bar), targeting proper the mold, until compactionit reaches a temperature and full consolidation below the [glass17]. The transition final part temperature is then left of to the cool ma- within the mold, trix. until it reaches a temperature below the glass transition temperature of the matrix.

Figure 1. SchematicFigure illustration 1. Schematic of the thermoforming illustration of the process. thermoforming process.

Schug [17] summarizedSchug [17 ]the summarized main influencing the main parameters influencing on parameters the forming on the behavior forming behavior and part quality. The author claims that the tool temperature has a major impact on the cooling and part quality. The author claims that the tool temperature has a major impact on the time and morphology of the material. Further, the forming speed should be selected in such cooling time and morphology of the material. Further, the forming speed should be se- a way that allows a compromise between timely shaping and shear thickening response lected in such a way that allows a compromise between timely shaping and shear thick- of the thermoplastic melt. The press load, where a high pressure favors consolidation ening response of the thermoplastic melt. The press load, where a high pressure favors on the one hand but may lead to dry spots and matrix flow on the other hand, mainly consolidation on the one hand but may lead to dry spots and matrix flow on the other governs the surface quality. Further typical defects are fiber undulations, gap formation, hand, mainly governs the surface quality. Further typical defects are fiber undulations, out-of-plane wrinkles and folds [18,19]. Insufficient contact between mold surface and gap formation, out-of-plane wrinkles and folds [18,19]. Insufficient contact between mold laminate at the end of the forming process may be the reason for rough surface areas. In surface and laminate at the end of the forming process may be the reason for rough surface the case of extensive tension of the laminate within the support frame, a fiber breakage and areas. In the case of extensive tension of the laminate within the support frame, a fiber thus drastic losses in mechanical performance can be expected. breakage and thus drasticIn the caselosses of in single mechanical and double performance diaphragm can forming,be expected. pneumatic pressure is applied. The function of the diaphragm is to transfer the mechanical loads to the laminate and

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Materials 2021, 14, 4639 4 of 20 In the case of single and double diaphragm forming, pneumatic pressure is applied. The function of the diaphragm is to transfer the mechanical loads to the laminate and keeping a biaxial tension for wrinkle and undulation prevention. In single diaphragm forming,keeping the a biaxial laminate tension is placed for wrinkle over the and mold undulation having the prevention. diaphragm Inon singletop. The diaphragm vacuum isforming, applied the and laminate the laminate is placed is forced over the against mold th havinge tooling the diaphragmto take its shape. on top. In The double vacuum dia- is phragmapplied forming and the laminate the laminate is forced is sandwiched against the toolingbetween to two take flexible its shape. membranes In double diaphragmand com- pactedforming by the vacuum. laminate Further is sandwiched vacuum is between applied twoto press flexible the membranessandwiched and stack compacted against the by mold.vacuum. This Further technique vacuum allows is a applied greater to degree press theof double sandwiched curvature stack than against single the diaphragm mold. This formingtechnique [20]. allows For amore greater intricate degree geometries of double curvatureand smaller than fillets single the diaphragm hydroforming forming can [ 20be]. adopted.For more Here, intricate a fluid geometries pressure and is applied smaller filletsto a rubber the hydroforming diaphragm canto force be adopted. the laminate Here, stacka fluid to pressurethe mold. is Hydroforming applied to a rubber and diaphr diaphragmagm forming to force theeliminate laminate the stack need to of the mutual mold. conformingHydroforming tools and and diaphragm hence, the forming tooling eliminatecost is lowered the need [20]. of mutual conforming tools and hence,Harrison the tooling et al. cost [21] is investigated lowered [20 ].the use of springs instead of friction-based blank- holdersHarrison to induce et al.in-plane [21] investigated tension in the laminate use of springs in order instead to prevent of friction-based the formation blank- of wrinkles.holders toThis induce technique in-plane was tensionevaluated in as the bein laminateg flexible, in ordereasy to to use prevent and naturally the formation facili- tatingof wrinkles. heat transfer This techniqueinto the laminate was evaluated prior to asforming. being flexible,However, easy the to authors use and report naturally that thefacilitating focused application heat transfer of into tension the laminatemight lead prior to in-plane to forming. buckling However, and localized the authors zones report of that the focused application of tension might lead to in-plane buckling and localized zones high shear. of high shear. The use of vacuum thermoforming for pure plastic sheets is a widespread process. In The use of vacuum thermoforming for pure plastic sheets is a widespread process. In 2018, the market size for vacuum forming was estimated at USD 11.69 billion [22]. Various 2018, the market size for vacuum forming was estimated at USD 11.69 billion [22]. Various plastics (e.g., PA, PE, PP, PC or PET) are often applied. The basic process (Figure 2a) im- plastics (e.g., PA, PE, PP, PC or PET) are often applied. The basic process (Figure2a) implies plies forming a sheet into a 3D geometry by heating it to a formable state, pressing it forming a sheet into a 3D geometry by heating it to a formable state, pressing it against against a mold and holding it in that position until the sheet is cooled below the glass a mold and holding it in that position until the sheet is cooled below the glass transition transition temperature. Finally, the part is trimmed to final shape [23]. In most cases, the temperature. Finally, the part is trimmed to final shape [23]. In most cases, the processing processing temperatures are below the melting temperature and above the glass transition temperatures are below the melting temperature and above the glass transition temperature temperature (e.g., around 240–250 °C for PA6) [24,25]. Heating is mostly carried out by (e.g., around 240–250 ◦C for PA6) [24,25]. Heating is mostly carried out by radiation, for radiation, for example by using IR-heaters. Accordingly, bringing the plastic sheet to a example by using IR-heaters. Accordingly, bringing the plastic sheet to a viscoelastic state viscoelasticdepends on state the polymer’sdepends on ability the polymer’s to absorb ab theility long to absorb infrared the wavelength long infrared energy. wavelength Plastics energy.with low Plastics crystallinity with low are saidcrystallinity to have betterare said formability to have better [25] since formability the crystals [25] hindersince the the crystalsflow behavior hinder ofthe the flow viscous behavior fluid. of the viscous fluid.

(a)

(b)

(c)

FigureFigure 2. 2. (a(a) )Basic Basic vacuum vacuum thermoforming thermoforming process, process, (b (b) )vacuum vacuum thermoforming thermoforming with with pre-stretching pre-stretching viavia inflation inﬂation of of a a bubble, bubble, ( (cc)) vacuum vacuum thermoforming thermoforming using using a a plug-assist. plug-assist.

Despite its high process flexibility and good suitability for prototype and series production, thermoforming is associated with some challenges. When using single-sided tools, the component thickness can vary significantly, due to varying stretching according to the geometry. Sheet areas first touching the mold are hindered in their biaxial motion by

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Despite its high process flexibility and good suitability for prototype and series production, thermoforming is associated with some challenges. When using single-sided Materials 2021, 14, 4639 5 of 20 tools, the component thickness can vary significantly, due to varying stretching according to the geometry. Sheet areas first touching the mold are hindered in their biaxial motion by friction and thus remain thicker than other areas that continue to witness stretching. frictionThis is andalso thusassociated remain with thicker faster than cooling other areas by heat that conduction continue to witnessfrom the stretching. touching Thisarea isto alsothe mold. associated This withuneven faster deformation cooling by and heat cooling conduction behavior from can the touchinglead to residual area to stresses the mold. in Thisthe final uneven component deformation [23]. andAs a coolingremedy, behavior a pre-stretching can lead step to residual may be stressesintroduced in the after final the componentheating step. [23 In]. that As a case remedy, a clamping a pre-stretching tool fixes stepthe edges may be of introducedthe sheet and after air the pressure heating is step.applied In thatto inflate case a the clamping sheet into tool fixesa bubble the edges(Figure of 2b) the sheet[26]. In and the air next pressure step, isforming applied is toachieved inflate theby sheetvacuum into pressure. a bubble Nevertheless, (Figure2b) [ 26 the]. In thickness the next distribution step, forming in is the achieved final com- by vacuumponents pressure.largely depends Nevertheless, on the theviscoelastic thickness properties distribution of inthe the polymer final components materials. largely dependsIn plug-assisted on the viscoelastic (Figure properties 2c) thermoforming, of the polymer the heated materials. sheet is pre-stretched through a mechanicalIn plug-assisted plug. Next, (Figure actual2c) thermoforming, forming is completed the heated through sheet the is pre-stretchedapplication of through vacuum apressure mechanical [27]. plug. Chen Next, et al. actual [28] observed forming that is completed thickness through at the sidewalls the application increased of vacuum with in- pressurecreasing [mold27]. Chentemperature, et al. [28 preheating] observed temp thaterature, thickness plug at the depth sidewalls and holding increased time, with but increasingdecreased moldwith increasing temperature, plug preheating speed. At temperature, a thermoforming plug depthtemperature and holding above time, the glass but decreasedtransition withthe sheet increasing material plug becomes speed. sticky At athermoforming and thus unable temperature to slip over abovethe plug the surface glass transitiondue to increased the sheet friction material [27,29,30]. becomes sticky and thus unable to slip over the plug surface due to increased friction [27,29,30]. 3. Binder Jetting of Sand Tools 3. Binder Jetting of Sand Tools Conventional tool manufacturing relies on subtractive (machining) process starting Conventional tool manufacturing relies on subtractive (machining) process starting from a material block. Such tools are often metallic (e.g., tool steel type AISI-SAE 1045 from a material block. Such tools are often metallic (e.g., tool steel type AISI-SAE 1045 [31]) [31]) or in some cases made from polymers (e.g., RAKU®Tool [32]) (RAMPF Tooling Solu- or in some cases made from polymers (e.g., RAKU®Tool [32]) (RAMPF Tooling Solutions tions GmbH & Co. KG, Grafenberg, Germany) and foams. Considering material costs and GmbH & Co. KG, Grafenberg, Germany) and foams. Considering material costs and machining time, the fabrication of tools may prove to be extensively time consuming and machining time, the fabrication of tools may prove to be extensively time consuming and costlycostly [[31].31]. InIn contrast contrast to to the the subtractive subtractive manufacturing, manufacturing, additive additive manufacturing manufacturing builds builds the struc- the turestructure in a layer-by-layer in a layer-by-layer technique, technique, applying applyi materialng material only there only where there it iswhere needed. it isFigure needed.3 illustratesFigure 3 illustrates the Binder the Jetting Binder process Jetting according process according to ASTM to [33 ASTM]. Here, [33]. the Here, following the following process stepsprocess are steps repeated are untilrepeated the desired until the component desired iscomponent created: A is build created: platform A build is lowered platform by ais layerlowered thickness by a layer of ranging thickness between of ranging 10 µm tobetween 400 µm. 10 The µm hereby-created to 400 µm. The free hereby-created space is then filledfree space with powderedis then filled material with powdered using a re-coater. material In using the third a re-coater. step, a binder In the is third selectively step, a depositedbinder is selectively using an inkjet deposited print headusing to an bond inkjet the print individual head to particles bond the together. individual This particles creates atogether. bond within This the creates layer anda bond with within the layer thebelow. layer and In the with case the of Binderlayer below. Jetting In of sand,the case sand of grainsBinder are Jetting used of as sand, the building sand grains material. are used as the building material.

FigureFigure 3.3.Schematic Schematic illustrationillustration ofof the the steps steps characterizing characterizing the the Binder Binder Jetting Jetting process: process: First—lower First—lower platform,platform, Second—recoat Second—recoat layer, layer, Third—jet Third—jet binder. binder.

ThisThis setupsetup makesmakes Binder Jetting Jetting systems systems partic particularlyularly easy easy to to scale scale in interms terms of ofperfor- per- formance.mance. The The number number of nozzles of nozzles correlates correlates with with the theoverall overall performance performance [34]. [ 34Likewise,]. Likewise, sev- severaleral layers layers can can be be applied applied simultaneously. simultaneously. This This feature feature cannot cannot be be achieved withwith laser-laser- basedbased systemssystems usingusing beambeam deﬂection.deflection. SuchSuch uniqueunique arrangementarrangement leadsleads toto extraordinaryextraordinary buildbuild rates rates and, and, consequently, consequently, to special to special cost-effectiveness. cost-effectiveness. Binder JettingBinder currently Jetting currently achieves aachieves maximum a maximum build-up ratebuild-up of approximately rate of approximately 400 L/h (Datasheet 400 L/h (Datasheet ExerialTM Exerial3D) (ExOne,TM 3D) Gersthofen,(ExOne, Gersthofen, Germany) Germany) printerand printer costs and signiﬁcantly costs significantly less than less 10 than€/l. 10 The €/l. basis The basis of the of Binder Jetting can be various particle materials. Plastic particles, metal powders or inorganic materials such as sand or ceramics can be used. The particles are adapted to the layer thickness to be achieved, and the spectrum of average particle sizes ranges from d50 = 20 µm to d50 = 400 µm. Silica sand is particularly cost-effective for use as tools. Here, the raw material costs are often less than 100 €/t. Materials 2021, 14, x FOR PEER REVIEW 6 of 20

the Binder Jetting can be various particle materials. Plastic particles, metal powders or inorganic materials such as sand or ceramics can be used. The particles are adapted to the Materials 2021, 14, 4639 layer thickness to be achieved, and the spectrum of average particle sizes ranges from6 of d 2050 = 20 µm to d50 = 400 µm. Silica sand is particularly cost-effective for use as tools. Here, the raw material costs are often less than 100 €/t. The components manufactured by Binder Jetting can be used either directly or with The components manufactured by Binder Jetting can be used either directly or with post-treatment as a tool. Post-treatment usually increases the strength and wear resistance post-treatment as a tool. Post-treatment usually increases the strength and wear resistance of the product. Used directly, sand binder systems achieve a maximum tensile strength of of the product. Used directly, sand binder systems achieve a maximum tensile strength of about 10 MPa (identified by preliminary examinations and further referred to as the base about 10 MPa (identified by preliminary examinations and further referred to as the base material). The strength can be significantly increased, for example, by infiltration with an material). The strength can be significantly increased, for example, by infiltration with an epoxy resin. The sand or the underlying particle material can be sintered or an impression epoxy resin. The sand or the underlying particle material can be sintered or an impression cancan bebe mademade withwith aa higher-strengthhigher-strength materialmaterial (e.g.,(e.g., withwith aa polymerpolymer cement)cement) whenwhen eveneven higherhigher strengthsstrengths areare required. required. TheThe strengthstrength ofof thethe basebase materialmaterial isis affectedaffected byby different different aspects aspects related related to to material material andand process,process, such as the the strength strength of of a a sand sand grain, grain, the the density density of ofthe the sand, sand, the the binder binder ad- adhesionhesion and and cohesion cohesion as well as well as the as amount the amount of binder. of binder. The sand The grains sand grainsare packed are packedmore or moreless densely or less denselyduring the during coating the process, coating process,depending depending on the technique on the technique [35]. This [ 35results]. This in resultsa volume in aratio volume of air ratio to sand of air of to about sand of50% about. Depending 50%. Depending on the binder on the system, binder approxi- system, approximatelymately 2–5 wt% 2–5 binder wt% binderpenetrates penetrates into the into loos thee sand loose during sand duringthe deposition the deposition of the binder. of the binder.The capillary The capillary action causes action causesliquid to liquid accumula to accumulatete at the contact at the contact points pointsof the ofsand the grains sand grains(Figure (Figure 4), which4 ), whichsolidify solidify by drying by drying or polymerization, or polymerization, depending depending on the binder on the system. binder system.The cavity The that cavity now that remains now remains can be filled can be by filled infiltration by infiltration with another with another material material system, system,thus raising thus the raising strength the strengthdue to the due higher to the amount higher of amount binder ofcompared binder comparedto the base to mate- the baserial. material.

Figure 4. SEM-Micrograph of the connection between two grains. Figure 4. SEM-Micrograph of the connection between two grains. The properties of printed sand molds can be used especially in applications where The properties of printed sand molds can be used especially in applications where low strength is of secondary importance or even helpful. For example, a casting core needs low strength is of secondary importance or even helpful. For example, a casting core needs to be as strong as necessary for safe handling but, after the casting process, has to be easily removedto be as strong out of as the necessary cavity. Also for safe relevant handling here bu aret, coresafter the for casting hollow process, structures has that to be have easily to beremoved removed out after of the the cavity. lamination Also processrelevant (washouts) here are cores [36]. for hollow structures that have to be removedTools for after casting the lamination purposes are process also manufactured(washouts) [36]. using tools fabricated by Binder Jetting.Tools Very for different casting casting purposes materials are also can manufactured be used in this using process. tools For fabricated example, by concrete Binder canJetting. be molded Very different to produce casting structures materials in can the constructionbe used in this industry process. (Figure For example,5)[ 37,38 concrete]. The moldscan be must molded be pre-treated to produce for structures casting by in sealing the construction the surface toindustry prevent (Figure the ingress 5) [37,38]. of mixing The water.molds Aftermust be the pre-treated concrete has for set,casting the moldby sea isling opened the surface and removed to prevent from the ingress the structural of mix- member.ing water. Depending After the concrete on whether has theset, structurethe mold isis undercut,opened and the removed molds can from be usedthe structural once or severalmember. times. Depending This is alsoon whether the case, the with structure laminating is undercut, molds where the molds the mold can isbe used used only once for or aseveral few impressions times. This in is prototypingalso the case, applications. with laminating molds where the mold is used only for a fewSand impressions molds are in further prototyping widely applications. spread in the foundry industry for the production of metal castings. Here, individual parts with small and large dimensions are often realized with printed molds. In particular, parts that conventionally require the storage of large molds over long periods can be produced economically using Binder Jetting [39]. On the other hand, large series in engine construction can nowadays also be realized with sand molds from 3D printers [40].

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Figure 5. Concrete part formed on a Binder-Jetted tool [38].

Sand molds are further widely spread in the foundry industry for the production of metal castings. Here, individual parts with small and large dimensions are often realized with printed molds. In particular, parts that conventionally require the storage of large molds over long periods can be produced economically using Binder Jetting [39]. On the other hand, large series in engine construction can nowadays also be realized with sand Figure 5. Concrete part formed on a Binder-Jetted tool [38]. moldsFigure from 5. 3DConcrete printers part [40]. formed on a Binder-Jetted tool [38]. VariousVariousSand sand molds sand types are types have further have been beenwidely qualified qualified spread for the forin theusage the foundry usage in Binder in industry Binder Jetting Jetting for of thesand of production sand molds molds of and andcoresmetal cores [41]. castings. [Silica41]. Silica Here,sand sand isindividual most is most commonly parts commonly with used small used in foundries and in foundries large to dimensions make to make molds moldsare (amongst often (amongst realized otherother withmethods methodsprinted also molds. alsothrough through In additiveparticular, additive manufact parts manufacturing), thaturing), conventionally due dueto its to costrequire its costeffectiveness. the effectiveness. storage The of Thelarge sandsand morphologymolds morphology over long and periodsparticle and particle sizecan be(Figure size produced (Figure 6) distribution econ6) distributionomically are known using are known Binderto strongly toJetting strongly affect [39]. the affectOn the resultingtheother resulting packing hand, packinglargedensity series [35] density inand engi [surface35ne] and construction properties surface properties can[42]. nowadays The [packing42]. The also density packing be realized influences density with influ- sand bothences mechanicalmolds both from mechanical strength 3D printers and strength [40].permeability. and permeability. In general, by In general,choosing by a sand choosing with aa sandsmaller with a d50 mediumsmallerVarious dgrain50 medium sandsize thantypes grain a havereference size been than qualifiedone, a reference a higher for one, the casting usage a higher surface in Binder casting quality Jetting surface can ofquality besand ex- molds can pectedbeand [43]. expected cores However, [41]. [43]. Silica However,this sandgrain is thissize most grainimplies commonly size low implies permeability used low in permeabilityfoundries to air and to makegases. to air molds andThe gases.shape (amongst The shape of the sand grain also has a decisive influence. Sharp edged grains, on the one hand, of theother sand grainmethods also also has athrough decisive additive influenc manufacte. Sharp edgeduring), grains, due to on its the cost one effectiveness. hand, have The have the least contact with each other in a compacted structure and thus make the sand the leastsand contact morphology with each and other particle in a size compac (Figureted 6)structure distribution and thusare known make the to strongly sand highly affect the highly permeable to gases. On the other hand, they cannot be packed to the optimum permeableresulting to gases. packing On densitythe other [35] hand, and they surface cannot properties be packed [42]. to The the packingoptimum density extent influencesdur- extent during Binder Jetting and structures made from them have low strength [44]. ing Binderboth mechanicalJetting and strengthstructures and made permeability. from them In have general, low strengthby choosing [44]. a sand with a smaller d50 medium grain size than a reference one, a higher casting surface quality can be expected [43]. However, this grain size implies low permeability to air and gases. The shape of the sand grain also has a decisive influence. Sharp edged grains, on the one hand, have the least contact with each other in a compacted structure and thus make the sand highly permeable to gases. On the other hand, they cannot be packed to the optimum extent during Binder Jetting and structures made from them have low strength [44].

(a) (b)

FigureFigure 6. SEM-Micrographs 6. SEM-Micrographs of different of different qualities qualities of sand of suitable sand suitable for the forBinder the Jetting Binder of Jetting tools: of(a) tools: Silica Sand type GS14 RP by Strobel Quarzsand GmbH Freihung, Germany, and (b) Bauxitsand by (a) Silica Sand type GS14 RP by Strobel Quarzsand GmbH Freihung, Germany, and (b) Bauxitsand Hüttenes Albertus GmbH Düsseldorf, Germany. by Hüttenes Albertus GmbH Düsseldorf, Germany. CurrentCurrent binder binder systems systems in combination in combination with withsand sand as a molding as a molding material material are inexpen- are inexpen-

sive siveepoxy, epoxy, furan furan or phenolic or phenolic(a) resin–based resin–based systems systems that thatbring bring sufficient sufficient( bthermal) thermal stability. stability. EnvironmentallyEnvironmentally friendly friendly inorganic inorganic binders binders are nowadays are nowadays the subject the subject of research of research and andare are preferredpreferredFigure within 6. SEM-Micrographs within this thisstudy study [43,44]. of [43 different ,44]. qualities of sand suitable for the Binder Jetting of tools: (a) Silica Sand type GS14 RP by Strobel Quarzsand GmbH Freihung, Germany, and (b) Bauxitsand by For 3DFor printing 3D printing of tools, of tools, the mechanical the mechanical properties properties of the of 3D-printed the 3D-printed part partprior prior to to Hüttenes Albertus GmbH Düsseldorf, Germany. infiltrationinfiltration might might be of be minor of minor importance. importance. Thus Thus,, a sufficient a sufficient strength strength for secure for secure handling handling during the infiltration process is expected to be acceptable considering the overall process. during theCurrent infiltration binder process systems is expected in combination to be acceptable with sand considering as a molding the materialoverall process. are inexpen- The permeability, however, is essential to ensure a sufficient infiltration depth and thus The permeability,sive epoxy, furan however, or phenolic is essential resin–based to ensure syst a emssufficient that bring infiltration sufficient depth thermal and thus stability. to provide the basis for infiltrated sand tools of enhanced mechanical properties. Sand Environmentally friendly inorganic binders are nowadays the subject of research and are qualities that will be investigated within this study are silica sands obtained from natural preferred within this study [43,44]. reservoirs of varying powder size distributions as well as artificial sands produced by For 3D printing of tools, the mechanical properties of the 3D-printed part prior to sintering. In contrast to natural sands, artificially produced sands typically show regular, infiltration might be of minor importance. Thus, a sufficient strength for secure handling round shapes and narrow particle size distributions. during the infiltration process is expected to be acceptable considering the overall process. The permeability, however, is essential to ensure a sufficient infiltration depth and thus

Materials 2021, 14, x FOR PEER REVIEW 8 of 20

to provide the basis for infiltrated sand tools of enhanced mechanical properties. Sand qualities that will be investigated within this study are silica sands obtained from natural reservoirs of varying powder size distributions as well as artificial sands produced by sintering. In contrast to natural sands, artificially produced sands typically show regular, Materials 2021, 14, 4639 round shapes and narrow particle size distributions. 8 of 20

4. Sand Tools for Vacuum Forming of FRP

4. SandThe Tools main fortarget Vacuum of the Forming investigations of FRP is to develop an innovative process chain for the rapid and cost-effective production of large-area thermoplastic-based shell structures The main target of the investigations is to develop an innovative process chain for the made of fiber reinforced plastics—especially during the product development phase, or rapid and cost-effective production of large-area thermoplastic-based shell structures made where small part numbers are needed. In the solution approach, the advantages of addi- of fiber reinforced plastics—especially during the product development phase, or where tively manufactured sand molds are combined with the possibilities of a fast thermoform- small part numbers are needed. In the solution approach, the advantages of additively ing process. This results in synergy effects that significantly advance prototype and small manufactured sand molds are combined with the possibilities of a fast thermoforming series production. In the following the main requirements regarding sand tools for vac- process. This results in synergy effects that significantly advance prototype and small uum thermoforming applications are given: series production. In the following the main requirements regarding sand tools for vacuum • thermoforming Compression applications strength exceeding are given: 20 MPa in order to stand the specific process loads; • Acceptable wear resistance to withstand at least ten forming cycles; • Compression strength exceeding 20 MPa in order to stand the specific process loads; • Surface finish with arithmetic mean roughness Ra < 20 µm to minimize indentations • Acceptable wear resistance to withstand at least ten forming cycles; on visible surfaces; • Surface finish with arithmetic mean roughness Ra < 20 µm to minimize indentations • Porouson visible structure surfaces; to ensure a safe application of vacuum; • • TemperaturePorous structure resistance to ensure up ato safe 280 application°C; of vacuum; • • Cost-effectiveTemperature resistance in terms of up engineering to 280 ◦C; and production comparable with state-of-the- • artCost-effective tools. in terms of engineering and production comparable with state-of-the- Inart order tools. to address the aforementioned requirements, a segmented approach is ap- pliedIn (Figure order to7). address In the first the aforementionedstage, the focus requirements,is laid on the sand a segmented mold, where approach materials is applied and process(Figure7 parameters). In the first will stage, be thedefin focused. isAfter laid 3D on printing the sand by mold, Bin whereder Jetting, materials a mockup and process tool is furtherparameters treated will to be fulfill defined. the Afterrequirements 3D printing regarding by Binder mechanical Jetting, a properties mockup tool and is surface further quality.treated toIn fulfillparallel, the the requirements feasibility of regarding vacuum mechanicalthermoforming properties is studied, and surfaceaddressing quality. the mainIn parallel, challenges the feasibilitysuch as the of applicable vacuum thermoformingclamping mechanism is studied, for FRP addressing laminate theto ensure main vacuumchallenges pressure such as on the the applicable one hand clamping and draping mechanism of the laminate for FRP laminateunder tension to ensure on the vacuum other hand.pressure on the one hand and draping of the laminate under tension on the other hand.

FigureFigure 7. InvestigationInvestigation approachapproach targetingtargeting thethe fabricationfabrication ofof sandsand toolstools forfor vacuumvacuum formingforming ofof FRP.FRP.

4.1. Initial Approaches and Results in Binder Jetting of Sand Tools For preliminary preliminary tests, tests, the the silica silica sand sand GS14 GS14 (Strobel (Strobel Quarzsand Quarzsand GmbH, GmbH, Freihung, Freihung, Ger- many)Germany) in combination in combination with with an aninorganic inorganic binder binder system system (IOB (IOB by by voxeljet voxeljet AG, AG, Friedberg, Friedberg, Germany) was selected. The The 3D 3D printing test testss were carried out using increased binder weight contentscontents (150%, (150%, 200% 200% and and 300% 300% with with respect respect to conventionally to conventionally used binder used amount) binder amount)in order toin order investigate to investigate the feasible the feasible limits andlimits their and effects their effects on the on mechanical the mechanical properties prop- ertiesconsidered considered relevant relevant for sand for tool sand fabrication tool fabrication for vacuum for vacuum forming forming of FRP. of Further, FRP. Further, during during3D printing, 3D printing, the binder the content binder iscontent to be increased is to be increased globally on globally the one on hand the andone selectivelyhand and selectivelyvaried locally varied on thelocally one on hand. the Localone hand. variation Local (further variation referred (further to referred as the skin-core to as the setup) skin- coreimplies setup) the implies fabrication the offabrication a skin layer of a of skin 5 mm layer thickness of 5 mm with thickness high binder with high content binder relative con- tentto the relative core segment. to the core This segment. allows for This superior allows mechanicalfor superior properties mechanical at theproperties mold surface. at the In contrast, in the case of global binder application at high binder content, the entire sand structure is printed at the aforementioned increased values. Two sets of specimens were produced: bars with a cross-section of 22.4 × 22.4 mm for 3-point ﬂexure tests and cylinders

D50 × 50 mm for compression tests and density evaluation. Five specimens were tested for every parameter set. Figures8–10 show the results of the preliminary investigations, indicating that the increase of binder content can be directly correlated with an increase in density, compressive strength and bending strength for globally increased binder contents MaterialsMaterials 20212021,, 1414,, xx FORFOR PEERPEER REVIEWREVIEW 99 ofof 2020

moldmold surface.surface. InIn contrast,contrast, inin thethe casecase ofof globglobalal binderbinder applicationapplication atat highhigh binderbinder content,content, thethe entireentire sandsand structurestructure isis printedprinted atat thethe aforementionedaforementioned increasedincreased values.values. TwoTwo setssets ofof specimensspecimens werewere produced:produced: barsbars withwith aa cross-sectioncross-section ofof 22.422.4 ×× 22.422.4 mmmm forfor 3-point3-point flexureflexure teststests andand cylinderscylinders D50D50 ×× 5050 mmmm forfor compressioncompression teststests andand densitydensity evaluation.evaluation. FiveFive spec-spec- imensimens werewere testedtested forfor everyevery parameterparameter set.set. FiFiguresgures 8–108–10 showshow thethe resultsresults ofof thethe prelimi-preliminary investigations, indicating that the increase of binder content can be directly corre- Materials 2021, 14nary, 4639 investigations, indicating that the increase of binder content can be directly corre- 9 of 20 latedlated withwith anan increaseincrease inin density,density, compressivecompressive strengthstrength andand bendingbending strengthstrength forfor globallyglobally increasedincreased binderbinder contentscontents (a)(a) andand skin–coreskin–core sesettingsttings (b).(b). TheThe specimens’specimens’ densitiesdensities printedprinted usingusing thethe skin-coreskin-core strategystrategy werewere foundfound toto bebe higherhigher thanthan thosethose ofof thethe respectiverespective speci-speci- mensmens withwith overalloverall(a) increasedincreased and skin–core binderbinder settings contentscontents (b). (F(F Theigureigure specimens’ 8).8). ThisThis effect densitieseffect waswas printed attributedattributed using toto the thethe skin-core strategy factfact thatthat thethe printedprintedwere binderbinder found ofofto thethe be underlyinunderlyin higher thangg layerlayer those andand of itsits the residualresidual respective moisturemoisture specimens impedesimpedes with thethe overall increased depositiondeposition ofof newnewbinder powderpowder contents andand itsits (Figure compressibilicompressibili8). Thisty.ty. effect TheThe was compressivecompressive attributed strengthsstrengths to the fact ofof that thethe spec-spec- the printed binder of imensimens waswas significantlysignificantlythe underlying reducedreduced layer byby thethe and skin–coreskin–core its residual settingsetting moisture (Figure(Figure impedes 9),9), whilewhile the deposition thethe bendingbending of new powder and strengthstrength slightlyslightly itsincreasedincreased compressibility. (Figure(Figure 10).10). The ThisThis compressive waswas relatedrelated strengths toto thethe maximum ofmaximum the specimens bendingbending was stress,stress, signiﬁcantly reduced whichwhich increasesincreases indirectlyindirectlyby the skin–core proportionalproportional setting toto (Figure ththee geometricalgeometrical9), while the momentmoment bending ofof strength inertiainertia slightlyofof thethe cross-cross- increased (Figure 10). sectionsection thatthat isis smallersmallerThis was forfor hollowhollow related rectangularrectangular to the maximum cross-sectionscross-sections bending stress, comparedcompared which toto increases filledfilled rectangularrectangular indirectly proportional to the geometrical moment of inertia of the cross-section that is smaller for hollow rectangular areas.areas. cross-sections compared to ﬁlled rectangular areas.

((aa)) ((bb)) Figure 8. Density of specimens produced at varying binder contents. The binder content was in- Figure 8. FigureDensity 8. of Density specimens of specimens produced produced at varying at binder varying contents. binder Thecontents. binder The content binder was content increased was (in-a) globally and creased (a) globally and (b) only in an outer skin of 5 mm. (b) only increased an outer (a) skin globally of 5 mm.and (b) only in an outer skin of 5 mm.

Materials 2021, 14, x FOR PEER REVIEW ((aa)) ((bb)) 10 of 20

Figure 9.FigureFigureCompressive 9.9. CompressiveCompressive strength strengthstrength of specimens ofof specimensspecimens produced producedproduced at varying atat varyingvarying binder binderbinder contents. contents.contents. The binder TheThe binderbinder content con-con- was increased (a) globallytenttent and waswas (b increasedincreased) only in an ((aa) outer) globallyglobally skin andand of 5 (( mm.bb)) onlyonly inin anan outerouter skinskin ofof 55 mm.mm.

(a) (b)

Figure 10.Figure3-point 10. bending 3-point bending strength strength of specimens of specimens produced produc at varyinged at bindervarying contents. binder contents. The binder The content binder was increased (a) globallycontent and (wasb) only increased in an outer (a) globally skin of and 5 mm. (b) only in an outer skin of 5 mm.

Especially the dimensional accuracy in building direction decreased significantly with increasing binder contents. While the specimens’ dimensions (measured by a Vernier caliper) in building direction roughly comply with targeted dimensions for moderately increased binder contents (mean deviations of +0.83% for 150% binder content and +1.56% for 200% binder content), the dimensions for 300% binder content deviated extremely from the targeted values (mean deviation of +3.91). The upper application limit of binder content was therefore identified to be 200% for the observed sand–binder combination. Especially the print strategy (e.g., the local density of the jetted binder), were found to play an important role and will be further investigated.

4.2. Initial Approaches and Results in Infiltration of Sand Tools The requirements regarding the mechanical properties cannot be achieved with the basic material system. A conflict of targets exists regarding strength, penetrability and final permeability. This conflict can only be solved by considering the process, i.e., including printing and infiltration. Therefore, the 3D-printed sand structures were subsequently impregnated with various resins. In a preliminary test, first promising results were gained by infiltrating two D15 × 20 mm specimens. The samples were Binder-Jetted using GS14 sand and furan resin (VX-2C by voxeljet AG, Friedberg, Germany). The Epoxy resin (IH16 by Ebalta Kunststoff GmbH, Rothenburg ob der Tauber, Germany) was applied as the infiltration medium. Infiltration was carried out manually at room temperature using a brush. Figure 11a shows two infiltrated specimens: at the top of the figure, a radial cut shows the full impregnation of the sand specimen; at the bottom, the second specimen, previously tested for compression strength, is shown. The fully infiltrated specimen reached a compressive strength of ~70 MPa, thus exceeding the targeted value of 20 MPa. However, when altering the specimen dimensions to D50 × 50 mm, it was observed that only a comparably limited penetration depth could be achieved. Figure 11b demonstrates that such samples witnessed a spalling effect during compression testing. This was accompanied by a reduced mean compression strength of 5.9 MPa at a standard deviation of 0.3 MPa. When doubling the amount of infiltration medium for another five specimens of D50 × 50 mm, spalling could still be observed, while the compression strength increased to a mean compression strength of 12.1 MPa at a standard deviation of 3.1 MPa. These first experiments were intended to show the potential and challenges associated with the infiltration of 3D-printed sand structures. Further examinations with a larger quantity of specimens including a broader variety of materials are planned for future investigations. A rough evaluation of the surface quality achievable by grinding the epoxy-infiltrated structure showed Ra of 7 µm, fulfilling the requirements stated above. However, hand-grinding using sand 120-grit paper shows a low level of automation. Thus, alternative methods are also scope of the future investigations.

Materials 2021, 14, 4639 10 of 20

Especially the dimensional accuracy in building direction decreased signiﬁcantly with increasing binder contents. While the specimens’ dimensions (measured by a Vernier caliper) in building direction roughly comply with targeted dimensions for moderately increased binder contents (mean deviations of +0.83% for 150% binder content and +1.56% for 200% binder content), the dimensions for 300% binder content deviated extremely from the targeted values (mean deviation of +3.91). The upper application limit of binder content was therefore identiﬁed to be 200% for the observed sand–binder combination. Especially the print strategy (e.g., the local density of the jetted binder), were found to play an important role and will be further investigated.

4.2. Initial Approaches and Results in Infiltration of Sand Tools The requirements regarding the mechanical properties cannot be achieved with the basic material system. A conflict of targets exists regarding strength, penetrability and final permeability. This conflict can only be solved by considering the process, i.e., including printing and infiltration. Therefore, the 3D-printed sand structures were subsequently impregnated with various resins. In a preliminary test, first promising results were gained by infiltrating two D15 × 20 mm specimens. The samples were Binder-Jetted using GS14 sand and furan resin (VX-2C by voxeljet AG, Friedberg, Germany). The Epoxy resin (IH16 by Ebalta Kunststoff GmbH, Rothenburg ob der Tauber, Germany) was applied as the infiltration medium. Infiltration was carried out manually at room temperature using a brush. Figure 11a shows two infiltrated specimens: at the top of the figure, a radial cut shows the full impregnation of the sand specimen; at the bottom, the second specimen, previously tested for compression strength, is shown. The fully infiltrated specimen reached a compressive strength of ~70 MPa, thus exceeding the targeted value of 20 MPa. However, when altering the specimen dimensions to D50 × 50 mm, it was observed that only a comparably limited penetration depth could be achieved. Figure 11b demonstrates that such samples witnessed a spalling effect during compression testing. This was accompanied by a reduced mean compression strength of 5.9 MPa at a standard deviation of 0.3 MPa. When doubling the amount of infiltration medium for another five specimens of D50 × 50 mm, spalling could still be observed, while the compression strength increased to a mean compression strength of 12.1 MPa at a standard deviation of 3.1 MPa. These first experiments were intended to show the potential and challenges associated with the infiltration of 3D-printed sand structures. Further examinations with Materials 2021, 14, x FOR PEER REVIEW 11 of 20 a larger quantity of specimens including a broader variety of materials are planned for future investigations.

(a) (b)

FigureFigure 11. 11.TheThe 3D-printed 3D-printed specimens specimens infiltrated inﬁltrated with with epoxy epoxy resin: resin: (a) fully (a) fully infiltrated inﬁltrated specimens specimens of of dimensionsdimensions D15 D15 × 20× mm,20 mm, (b) spalling (b) spalling effect effect during during compression compression testing testing for specimens for specimens of dimen- of dimensions D50 × 50 mm. sions D50 × 50 mm.

4.3. IntendedA rough Future evaluation Investigations of the Regarding surface quality Sand Tool achievable Fabrication by grinding the epoxy-inﬁltrated µ structureThe results showed of the R afirstof 7 investigationsm, fulﬁlling theshowed requirements that the requirements stated above. regarding However, the hand- tools can in principle be met. The preliminary tests have to be refined in order to prove the feasibility of the production of suitable tools. The planned study implies the evaluation of the mechanical performance, dimensional accuracy, density, permeability and surface quality of the sand structures with respect to the targeted requirements. Table 1 presents the applied test methods for evaluation.

Table 1. Materials testing methods for further investigations.

Material Testing Method Standard 3-point flexure test VDG P71 Compression test DIN EN ISO 126 Dimensional accuracy DIN 862 Permeability test VDG P41 Density determination DIN 862, DIN 8128-1 Roughness measurement ISO 4288

Various sands are to be considered for the future study in order to investigate the effect of particle size and shape on density, strength, roughness, penetrability and permeability. Three different particle size distributions of natural silica sand will be investigated (GS14, GS19 and GS25 by Strobel Quarzsand GmbH, Freihung, Germany 3D) as well as one synthetic sand composed of aluminium silicate (Cerabeads ES650 by Hüttenes-Alber- tus Chemische Werke GmbH, Düsseldorf, Germany). Printing will be carried out using two different binder systems: a furan resin (VX-2C by voxeljet AG, Friedberg, Germany) that is the most frequently used for Binder Jetting of sand molds and an inorganic binder system (IOB by voxeljet AG, Friedberg, Germany), which is gaining increased attention due to its low emissions during casting. Further, a graded structure that allows for a controlled permeability for subsequent impregnation and the final vacuum forming purposes may be applicable in sand tools. Therefore, the binder content and the printing strategy will be further investigated in accordance with the preliminary tests. In addition, the lower limit regarding binder content will be identified. This is expected to result in lower densities, which may enhance the penetrability of the 3D-printed sand structure by the infiltration media. Figure 12 sums up the planned investigations with the various material systems and 3D printing parameters.

Materials 2021, 14, 4639 11 of 20

grinding using sand 120-grit paper shows a low level of automation. Thus, alternative methods are also scope of the future investigations.

4.3. Intended Future Investigations Regarding Sand Tool Fabrication The results of the ﬁrst investigations showed that the requirements regarding the tools can in principle be met. The preliminary tests have to be reﬁned in order to prove the feasibility of the production of suitable tools. The planned study implies the evaluation of the mechanical performance, dimensional accuracy, density, permeability and surface quality of the sand structures with respect to the targeted requirements. Table1 presents the applied test methods for evaluation.

Table 1. Materials testing methods for further investigations.

Material Testing Method Standard 3-point ﬂexure test VDG P71 Compression test DIN EN ISO 126 Dimensional accuracy DIN 862 Permeability test VDG P41 Density determination DIN 862, DIN 8128-1 Roughness measurement ISO 4288

Various sands are to be considered for the future study in order to investigate the effect of particle size and shape on density, strength, roughness, penetrability and permeability. Three different particle size distributions of natural silica sand will be investigated (GS14, GS19 and GS25 by Strobel Quarzsand GmbH, Freihung, Germany 3D) as well as one synthetic sand composed of aluminium silicate (Cerabeads ES650 by Hüttenes-Albertus Chemische Werke GmbH, Düsseldorf, Germany). Printing will be carried out using two different binder systems: a furan resin (VX-2C by voxeljet AG, Friedberg, Germany) that is the most frequently used for Binder Jetting of sand molds and an inorganic binder system (IOB by voxeljet AG, Friedberg, Germany), which is gaining increased attention due to its low emissions during casting. Further, a graded structure that allows for a controlled permeability for subsequent impregnation and the final vacuum forming purposes may be applicable in sand tools. Therefore, the binder content and the printing strategy will be further investigated in accordance with the preliminary tests. In addition, the lower limit regarding binder content will be identified. This is expected to result in lower densities, which may enhance the penetrability of the 3D-printed sand structure by the infiltration Materials 2021, 14, x FOR PEER REVIEW 12 of 20 media. Figure 12 sums up the planned investigations with the various material systems and 3D printing parameters.

FigureFigure 12. 12.Planned Planned variations variations regarding regarding 3D 3D printing printing of of sand sand tools. tools.

FurtherFurther investigations investigations target target the the enhancement enhancement of of the the penetration penetration depth depth using using alterna- alter- tivenative resins resins of higherof higher penetrating penetrating capacity capacity in in addition addition to to tailoring tailoring the the permeability permeability of of the the 3D-printed3D-printed structure structure by by locally locally altering altering the the binder binder content. content. Figure Figure 13 13 shows shows the the suggested suggested specimenspecimen geometrygeometry designeddesigned forfor investigating investigating the the depth depth of of penetration. penetration.The The sample sample is is cylindricalcylindrical withwith aa central hole hole making making an an indentation indentation of of 5 ml 5 ml volume. volume. Accordingly, Accordingly, a pre- a pre-meteredmetered amount amount of infiltration of inﬁltration media media is to is tobe beapplied applied into into the the indentation, indentation, which which will will allow a comparability of the impregnation efficiency with varying infiltration media at varying temperatures and printing material systems (involving powder and binder) and parameters. The penetration depth will be examined microscopically across the cross-section.

Figure 13. Suggested specimen geometry for the determination of the penetration depth.

Further investigations will include surface modifications, as smooth surfaces will not only enhance the surface quality of the FRP part but also are expected to reduce forces when demolding. Investigations will concern known application methods like grinding and coating of the infiltrated tools as well as the modification of the impregnation media.

4.4. Thermoforming of Organosheets The implementation of vacuum thermoforming for FRPs requires a step-by-step approach to overcome the first obvious challenges. These mainly lie in the lack of ductility of the fibers in contrast to the surrounding thermoplastic matrix. Heating the organosheet above the melting point of the matrix allows the matrix to flow and thus to be formed. During this stage, the matrix is stretched within the blank holder or the support frame. In the case of endless fibers, these are tensed but remain unable to stretch. This leads to the withdrawal of the organosheet from the support frame or excessive folds. Accordingly, a flexible holder is necessary; this should allow the sheet to flow and feed new material necessary to envelope the 3D mold geometry. A further challenge lies in the load available for deformation. It can be expected that the vacuum pressure alone is insufficient for deforming FRP sheets. Although the main target of the study is to keep the process as simple as possible, it is kept in mind that an application of a diaphragm might prove to be inevitable. Other than in the case of conventional thermoforming with unreinforced polymer sheets, it is assumed that heat must be applied to both surfaces of the sheet in order to achieve a homogeneous melt of the matrix throughout the thickness. This implies that the setup of a thermoforming unit must be

Materials 2021, 14, x FOR PEER REVIEW 12 of 20

Figure 12. Planned variations regarding 3D printing of sand tools.

Further investigations target the enhancement of the penetration depth using alternative resins of higher penetrating capacity in addition to tailoring the permeability of the 3D-printed structure by locally altering the binder content. Figure 13 shows the suggested specimen geometry designed for investigating the depth of penetration. The sample is cylindrical with a central hole making an indentation of 5 ml volume. Accordingly, a pre- metered amount of infiltration media is to be applied into the indentation, which will allow a comparability of the impregnation efficiency with varying infiltration media at varying temperatures and printing material systems (involving powder and binder) and pa- Materials 2021, 14, 4639 rameters. The penetration depth will be exam12ined of 20 microscopically across the cross-section. allow a comparability of the impregnation efﬁciency with varying inﬁltration media at varying temperatures and printing material systems (involving powder and binder) and parameters. The penetration depth will be examined microscopically across the cross-section.

Figure 13. Suggested specimen geometry for the determination of the penetration depth. Further investigations will include surface modifications, as smooth surfaces will not Figureonly 13. enhance Suggested the surface quality specimen of the FRP partgeometry but also are expected for the to reduce determination forces of the penetration depth. when demolding. Investigations will concern known application methods like grinding and coating of the infiltrated tools as well as the modification of the impregnation media. Further4.4. Thermoforming investigations of Organosheets will include surface modifications, as smooth surfaces will not The implementation of vacuum thermoforming for FRPs requires a step-by-step only approachenhance to overcome the the surface first obvious challenges. quality These mainlyof the lie in theFRP lack of ductilitypart but also are expected to reduce forces of the fibers in contrast to the surrounding thermoplastic matrix. Heating the organosheet whenabove demolding. the melting point of Investigations the matrix allows the matrix will to flow andconcern thus to be formed. known application methods like grinding During this stage, the matrix is stretched within the blank holder or the support frame. In and coatingthe case of endless of fibers,the theseinfiltrated are tensed but remain tools unable as to well stretch. This as leads the to themodification of the impregnation media. withdrawal of the organosheet from the support frame or excessive folds. Accordingly, a flexible holder is necessary; this should allow the sheet to flow and feed new material necessary to envelope the 3D mold geometry. 4.4. ThermoformingA further challenge lies of in theOrganosheets load available for deformation. It can be expected that the vacuum pressure alone is insufficient for deforming FRP sheets. Although the main Thetarget of implementation the study is to keep the process of as simplevacuum as possible, thermoformin it is kept in mind that g for FRPs requires a step-by-step ap- an application of a diaphragm might prove to be inevitable. Other than in the case of conventional thermoforming with unreinforced polymer sheets, it is assumed that heat proachmust to be appliedovercome to both surfaces the of the first sheet inobvious order to achieve challenges. a homogeneous melt These mainly lie in the lack of ductility of the matrix throughout the thickness. This implies that the setup of a thermoforming of theunit fibers must be adaptedin contrast to allow heating to both the sheet surrounding surfaces in addition to the thermoplastic mold surface matrix. Heating the organosheet in order to avoid rapid cooling of the sheet during deformation. Figure 14 shows the aboveenvisaged the machinemelting setup. Anpoint IR-heating of unit the is moved matrix into the thermoforming allows unit,the to matrix to flow and thus to be formed. simultaneously heat both sides of the sheet and the mold surface. The heating unit is then Duringwheeled this out andstage, the thermoforming the matrix process takes is place. stretched within the blank holder or the support frame. In the case of endless fibers, these are tensed but remain unable to stretch. This leads to the withdrawal of the organosheet from the support frame or excessive folds. Accordingly, a flexible holder is necessary; this should allow the sheet to flow and feed new material necessary to envelope the 3D mold geometry. A further challenge lies in the load available for deformation. It can be expected that the vacuum pressure alone is insufficient for deforming FRP sheets. Although the main target of the study is to keep the process as simple as possible, it is kept in mind that an application of a diaphragm might prove to be inevitable. Other than in the case of conventional thermoforming with unreinforced polymer sheets, it is assumed that heat must be applied to both surfaces of the sheet in order to achieve a homogeneous melt of the matrix throughout the thickness. This implies that the setup of a thermoforming unit must be

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adaptedadapted toto allowallow heatingheating bothboth sheetsheet surfacessurfaces inin additionaddition toto thethe moldmold surfacesurface inin orderorder toto avoidavoid rapidrapid coolingcooling ofof thethe sheetsheet duringduring defdeformation.ormation. FigureFigure 1414 showsshows thethe envisagedenvisaged ma-ma- chinechine setup.setup. AnAn IR-heatingIR-heating unitunit isis movedmoved inintoto thethe thermoformingthermoforming unit,unit, toto simultaneouslysimultaneously Materials 2021, 14, 4639 13 of 20 heatheat bothboth sidessides ofof thethe sheetsheet andand thethe moldmold susurface.rface. TheThe heatingheating unitunit isis thenthen wheeledwheeled outout andand thethe thermoformingthermoforming processprocess takestakes place.place.

FigureFigure 14.14. PreliminaryPreliminary conceptconcept ofof thethe heatingheating andand thermoformingthermoforming unit. unit.

InInIn orderorder toto tackletackle thesethese challenges,challenges, first,first, first, thethe focusfocus was was set set on on the the development development of of the the sandsand mold mold forfor vacuumvacuumvacuum thermoformingthermoformingthermoforming applications applicationsapplications in inin general. general.general. Hence, Hence,Hence, the thethe first firstfirst tools toolstools devel- de-de- velopedvelopedoped in theinin thethe above-mentioned above-mentionedabove-mentioned stage stagestage were werewere tested testedtested for their forfor theirtheir feasibility feasibilityfeasibility for pure forfor thermoplasticpurepure thermo-thermo- plasticplasticsheets. sheets.sheets. In that stage,InIn thatthat an stage,stage, ABS fenderanan ABSABS was fenfen vacuumderder waswas thermoformed.vacuumvacuum thermoformed.thermoformed. The sand mold,TheThe sandsand illustrated mold,mold, illustratedillustratedin Figure 15 inina, FigureFigure is binder-jetted 15a,15a, isis binder-jettedbinder-jetted as a hollow asas structure aa hollowhollow in orderstructurestructure to save inin orderorder weight toto (finalsavesave weight weight (final(final30 kg )weightweight and equipped 3030 kg)kg) andand with equippedequipped vacuum withwith channels. vacuumvacuum The channels.channels. mold was TheThe further moldmold impregnated waswas furtherfurther usingimpreg-impreg- an natednatedepoxy usingusing resin toanan increase epoxyepoxy resinresin the stability toto increaseincrease of the thethe surface ststabilityability layer. ofof thethe For surfacesurface improved layer.layer. surface ForFor improved finish,improved the surfacesurfacemold was finish,finish, ground thethe moldmold using waswas sandpaper groundground (grit usingusing size sandpapersandpaper 120). The (grit(grit process sizesize proved120).120). TheThe to processprocess be feasible provedproved and totothe bebe surface feasiblefeasible quality, andand thethe as surfacesurface presented quality,quality, in Figure asas prpr 15esentedesentedb,c, was inin foundFigureFigure acceptable. 15b,c,15b,c, waswas foundfound acceptable.acceptable.

((aa)) ((bb)) ((cc)) FigureFigure 15.15. (((aaa))) Binder-jettedBinder-jetted Binder-jetted sandsand sand moldmold mold forfor for therther thermoformingmoformingmoforming thermoplasticthermoplastic fenders,fenders, (( bb)) thermoformedthermoformed fenderfenderfender mademade fromfrom ABS,ABS, ( (cc)) applicationapplication of of the the ABS ABS fender. fender.

InIn thethe nextnext phase, phase, thethe processing processing ability ability of of FRPs FRPs is is studied studied using using metallic metallic molds molds equippedequipped withwith conventionalconventional vacuumvacuum channels.channels. Here,Here, issuesissues likelike vacuumvacuum pressure,pressure, heatingheating temperature,temperature, heatingheating heating timetime time andand setupsetup areare exam examined.examined.ined. TheThe The difficultydifficulty difficulty levellevel level isis increasedincreased by by first considering randomly oriented short fiber reinforced sheets. Then, the fiber length firstfirst consideringconsidering randomlyrandomly orientedoriented shortshort fibefiberr reinforcedreinforced sheets.sheets. Then,Then, thethe fiberfiber lengthlength and the orientation are altered step by step. When considering endless fibers, the behavior andand thethe orientationorientation areare alteredaltered stepstep byby step.step. WhenWhen consideringconsidering endlessendless fibers,fibers, thethe behaviorbehavior of woven textiles will be observed in contrast to multi-axial fabrics. These experiments ofof wovenwoven textilestextiles willwill bebe observedobserved inin contrastcontrast toto multi-axialmulti-axial fabrics.fabrics. TheseThese experimentsexperiments areare are to be carried out on different levels of geometrical complexities. Figure 16 shows the toto bebe carriedcarried outout onon differentdifferent levelslevels ofof gegeometricalometrical complexities.complexities. FigureFigure 1616 showsshows thethe en-en- envisaged geometries of the mold. visagedvisaged geometriesgeometries ofof thethe mold.mold.

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Figure 16. Targeted designs of mockup molds to tackle the challenges in various complexity levels. FigureFigure 16. 16.Targeted Targeted designs designs of of mockup mockup molds molds to to tackle tackle the the challenges challenges in in various various complexity complexity levels. levels.

AsAs mentioned mentioned above, above, the the support support frame frame has has to to fulfill fulfill several several functions functions and and resembles resembles oneone of of the the key key factors factors for for successful successful thermoforming. thermoforming. First, First, the the frame frame has has toseal to seal the the forming form- areaing toarea ensure to ensure vacuum vacuum build-up. build-up. Further, Further, it should it should act as a blankact as holdera blank that holder has tothat provide has to enoughprovide degrees enough of degrees freedom of to freedom allow for to feeding allow fo sufficientr feeding sheet sufficient material sheet to compensatematerial to com- the desiredpensate 3D the shape. desired Only 3D ifshape. both featuresOnly if both are providedfeatures are by provided the clamping by the mechanism clampinga mecha- good moldingnism a good process molding and part process quality and can part be quality expected. can be expected. InIn order order to to be be able able to to identify identify the the necessary necessary clamping clamping force, force, preliminary preliminary evaluation evaluation ofof the the frictional frictional forces forces between between the the above-mentioned above-mentioned materials materials and and the the anticipated anticipated frame frame materialsmaterials were were conducted. conducted. The The investigations investigations were were performed performed using using a self-constructed a self-constructed test rig,test as rig, depicted as depicted in Figure in Figure 17. The17. The rig rig consists consists of aof carriage a carriage for for material material A A and and a a further further fixturefixture for for material material B. B. Material Material A A is is cut cut to to the the dimensions dimensions 180 180 mm mm× ×50 50 mm mm and and is is clamped clamped atat both both ends ends into into the the carriage. carriage. Material Material B B is is prepared preparedprepared to toto the thethe dimensions dimedimensions 110 110 mm mm× ×50 50 mm. mm. TheThe test test setup setup involves involves sandwiching sandwiching two two layers layers of materialofof materialmaterial A between AA betweenbetween two twotwo single singlesingle layers layerslayers of materialof material B on B each on each side side (Figure (Figure 17 magnification). 17 magnification). Hence, Hence, a contact a contact area of area50 mm of 50× mm50 mm × 50 ismm created. is created. The normal The normal (press) (press) force force is controlled is controlled by weights. by weights. For the For measurements, the measurements, the testthe rigtest is rig mounted is mounted into ainto universal a universal testing testing machine. machine. The pullingThe pulling rope rope is attached is attached to the to crossheadthe crosshead and and translates translates the the vertical vertical movement movement into into a horizontala horizontal movement movement between between materialmaterial A A and and B B through through the the idler idler pulleys. pulleys. When When the thethe carriage carriagecarriage is isis moved movedmoved back backback and andand forth, forth,forth, a aa frictionfriction between between A A and and B B can can be be measured. measured.

FigureFigure 17. 17.Test Test rig rig for for measurement measurement of of frictional frictional forces forces between between FRP FRP sheet sheet and and clamp clamp materials. materials.

TheThe candidate candidate materials materials for for the the support support frame frame in in direct direct contact contact with with the the sheets sheets were were selectedselected to to be be polytetrafluoroethylene polytetrafluoroethylene (PTFE) (PTFE) and and aluminum. aluminum. Regarding Regarding the the anticipated anticipated sheetsheet materials, materials, aa unidirectionalunidirectional carbon carbon fibe fiberr and and another another short-recycled short-recycled carbon carbon fiber fiber re- reinforcedinforced polyamide polyamide tape tape were investigated.investigated. TheTheThe materialmaterialmaterial combinationscombinationscombinationsexamined examinedexamined are areare summarizedsummarized in in Table Table2. 2. For For this this test test campaign, campaign, the the universal universal test test machine machine ZwickRoell ZwickRoell Z100Z100 in in combination combination with with a a KMD KMD 5 5 KN KN load load cell cell was was used. used. For For creating creating a a specific specific press press forceforce between between materials materials A A and and B, B, a a 1 1 kg kg mass mass was was applied applied on on the the materials. materials. The The crosshead crosshead waswas moved moved with with a a constantconstant speedspeed ofof 100100 mm/minmm/min and and stopped stopped after after a adisplacement displacement of of 80

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mm. All tests were performed at room temperature. The load-displacement data were Materials 2021,recorded14, 4639 for three replicates, where only material A was replaced by a new one after each 15 of 20 examination. To calculate the friction coefficients, two forces were evaluated from the load-displacement curves. The static and dynamic coefficients of friction were calculated according to Equation80 mm. (1), All where tests were µPT performedis the coefficient at room of temperature. friction, F Theis the load-displacement recorded force, data were FN is the applied recordednormal force, for three m replicates,is the mass where and onlyg the material gravitational A was replacedacceleration. by a new one after each In the case ofexamination. the static friction To calculate coefficient the friction µPTs, the coefficients, force F is two identified forces were at the evaluated begin- from the ning of motion asload-displacement the maximum force curves. valu Thee between static and 0 and dynamic 10 mm coefficients displacement, of friction whereas were calculated µ for the dynamic frictionaccording coefficient to Equation µPTd (1), F whereis givenPT byis the coefficientaverage force of friction, value, F between is the recorded 10 force, and 60 mm displacement.FN is the applied The calculated normal force, values m is are the presented mass andg in the Table gravitational 2. acceleration. Table 2. OverviewF of the material combinations, the measured forces and calculated friction values. μ ; F m ∗ g 1 kg ∗ 9.81 m/s^2 9.81 N (1) 2∗F µ µ Material Combination Average Average PTs PTd Max. Sliding (Static (Dynamic Material A Material B Force [N] Force [N] Friction) Friction) Table 2. Overview of the material combinations, the measured forces and calculated friction values. UD CF-PA Teflon 4.78 3.06 0.243 0.156 UD CF-PA Aluminium 6.91 5.08 0.352 0.259 Material Combination Average 𝝁𝑷𝑻𝒔 𝝁𝑷𝑻𝒅 UD CF-PAAverage UD CF-PA 4.68 3.7 0.238 0.189 Sliding Force (Static Fric- (Dynamic Fric- Material A MaterialTeflon B Max. AluminiumForce [N] 8.71 6.3 0.444 0.321 rCF_PA Teflon[N] 5.89 4.35tion) 0.300tion) 0.222 UD CF-PA TeflonrCF_PA Aluminium4.78 12.373.06 7.020.243 0.624 0.156 0.358 UD CF-PA Aluminium 6.91 5.08 0.352 0.259 In the case of the static friction coefficient µ , the force F is identified at the beginning UD CF-PA UD CF-PA 4.68 3.7 PTs 0.238 0.189 of motion as the maximum force value between 0 and 10 mm displacement, whereas for Teflon Aluminium 8.71 6.3 0.444 0.321 the dynamic friction coefficient µPTd F is given by the average force value, between 10 and rCF_PA Teflon60 mm displacement. 5.89 The calculated 4.35 values are presented 0.300 in Table2. 0.222 rCF_PA Aluminium 12.37 7.02 0.624 0.358 F µ = ;F = m ∗ g = 1 kg ∗ 9.81 m/sˆ2 = 9.81 N (1) PT 2 ∗ F N A simple model for the calculation of theN clamping force was developed based on the coefficients of frictionA µ simplePT, the modelachievable for the vacuum calculation pressure of the and clamping the required force was forming developed ge- based on ometry. The modelthe is coefficients presented of in friction moreµ PTdet, theail achievablein the Appendix vacuum A. pressure Through and the the requiredknown forming pressure differencegeometry. ∆𝑝 (between The model vacuum is presented and ambient in more detailpressure) in the and Appendix the surfaceA. Through area theA known pressure difference ∆p (between vacuum and ambient pressure) and the surface area A of of the sheet, the resulting force Fvac can be calculated. With this force and the assumption the sheet, the resulting force F can be calculated. With this force and the assumption that that the tape behaves like a rope (inelasticvac material, unable to transfer moments), the the tape behaves like a rope (inelastic material, unable to transfer moments), the clamping clamping force Fs can be estimated. Accordingly, Figure 18 gives a simplified estimation force Fs can be estimated. Accordingly, Figure 18 gives a simplified estimation of the of the forming processforming at process its initial at itsstate, initial where state, the where pliable the pliable sheet sheetis not is yet not in yet contact in contact with with any of any of the mold surfacesthe mold except surfaces the except top thesurf topace surface (indicated (indicated by the by plate the plate in Figure in Figure 18). 18 ).

Figure 18.Figure Side view 18. Side of the view test of thestand test and stand acting and actingforces forces and top and view top view of the of thetest test stand. stand.

The test rig is furtherThe test illustrated rig is further in Figure illustrated 19. inKnowing Figure 19 the. Knowing inclination the inclination angles and angles and dimensions of thedimensions test rig, the of surface the test area rig, theof a surface clamped area foil of acan clamped be calculated. foil can beFurther, calculated. the Further, the vacuum force Fvac applied to the foil can be determined and can be decomposed in vacuum force Fvac applied to the foil can be determined and can be decomposed in its horizontal and vertical force components, for each area segments (A1–A4 in Figure 18).

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Based on the assumption that the tape behaves like a rope and that the tape does not transport any momentum in addition to the boundary condition, that Fs × µPTs < FFoil (where FFoil is the tangential tensile force in the foil), the parameters for all area segments can be calculated. To validate the model and calculations, a test (Figure 19) was performed, using the aforementioned setup, while integrating a load cell in an exact copy of the described model (Figure 18). A PA6 foil (commonly used in vacuum infusion processes) was used. A vacuum cleaner (Wieland IS-46h) was used to create the desired pressure difference of 270 mbar. After assembling the test stand and clamping the foil in the frame the load cell

Materials 2021, 14, 4639 is tared, and the vacuum was applied. A nominal force of 0.91 kN was generated16 of 20 and was further used to validate the model. The first trials indicated that with the vacuum cleaner and the test stand the expected pressure difference of 270 mbar could not be achieved. However, the pressure difference was not measured, and it remains unclear what the ab- its horizontal and vertical force components, for each area segments (A1–A4 in Figure 18). Basedsolute on achieved the assumption vacuum that pressure the tape would behaves lead like to, arope regarding and that the the pressure tape does difference not between ambient and vacuum. This difference is vital for the calculations. Hence, in the next transport any momentum in addition to the boundary condition, that Fs × µPTs < FFoil (wherestep, a F Foilpressureis the tangentialsensor is tensileintegrated force into in the the foil), test the stand parameters to raise for the all necessary area segments data for val- canidation. be calculated.

FigureFigure 19. 19.Test Test setup setup for thefor validationthe validation of the of developed the developed model. model.

ToOnce validate the thetheoretical model and model calculations, is validated, a test (Figure it can 19be) applied was performed, to construct using thethe support aforementionedframe and to setup,define while the necessary integrating clamping a load cell inforces. an exact These copy aspects of the described will further model be consid- (Figureered for 18). a Aprototype PA6 foil (commonly vacuum thermoforming used in vacuum infusion stand. The processes) products, was used.produced A vacuum with the pro- cleaner (Wieland IS-46h) was used to create the desired pressure difference of 270 mbar. totype machine, are to be evaluated regarding their dimensional accuracy, the fiber dis- After assembling the test stand and clamping the foil in the frame the load cell is tared, and thetribution vacuum and was local applied. fiber A volume nominal forcecontent. of 0.91 kN was generated and was further used to validateFirst theexperimental model. The firsttrials trials have indicated proved that the with above-mentioned the vacuum cleaner challe andnges. the test Trials with standendless the fiber expected reinforced pressure thermoplastic difference of 270 sheets mbar (Figure could not20a) be confirmed achieved. However,the demand the for a new pressureclamping difference concept was and not a measured,two-sided and heating it remains system unclear with what short the transportation absolute achieved routes from vacuumthe heating pressure unit would to the lead forming to, regarding unit. Further, the pressure the differencetrials have between shown, ambient that the and short fiber vacuum.material This can differencebe challenging, is vital as for it the tends calculations. to loft (Figure Hence, 20b) in the and next thus step, generate a pressure voids in the sensorprocess, is integrated which in into turn the negatively test stand toaffect raise th thee generation necessary data of the for required validation. vacuum pressure. Once the theoretical model is validated, it can be applied to construct the support frame and to define the necessary clamping forces. These aspects will further be considered for a prototype vacuum thermoforming stand. The products, produced with the prototype machine, are to be evaluated regarding their dimensional accuracy, the fiber distribution and local fiber volume content. First experimental trials have proved the above-mentioned challenges. Trials with endless fiber reinforced thermoplastic sheets (Figure 20a) confirmed the demand for a new clamping concept and a two-sided heating system with short transportation routes from the heating unit to the forming unit. Further, the trials have shown, that the short fiber material can be challenging, as it tends to loft (Figure 20b) and thus generate voids in the process, which in turn negatively affect the generation of the required vacuum pressure.

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(a) (b)

FigureFigure 20. 20.(a) ( Vacuuma) Vacuum thermoforming thermoforming of a multiaxial of a multiaxial endless CFRP endless sheet CFRP under non-optimalsheet under conditions. non-optimal (b) Vacuum con- thermoformedditions. (b) Vacuum short CFRP thermoformed sheet and resulting short lofting CFRP effect. sheet and resulting lofting effect.

4.5. Summary of Technical Challenges 4.5. Summary of Technical Challenges Preliminary investigations showed that 3D-printed sand specimens meet the require- Preliminary investigationsments for the showed application that of FRP3D-p vacuumrinted formingsand specimens tools regarding meet strength the require- and surface ments for the applicationquality of on FRP laboratory vacuum scale. forming It was shown tools thatregarding it is possible strength to customize and surface density and quality on laboratorystrength scale. by It adapting was shown the 3D printingthat it parametersis possible and to through customize post-processing density and by infiltration, while varying impregnation parameters and media. strength by adapting theTechnical 3D printing challenges parame in developingters and 3D-printedthrough post-processing sand tools for vacuum by forminginfiltra- of FRP tion, while varying impregnatiderive in particularon parameters from increased and mechanical media. and thermal loads when forming FRP sheets. Technical challengesProcess in related developing mechanical 3D-printed peak loads aresand expected tools for to occur vacuum during forming forming andof FRP ejecting of derive in particularthe from formed increased part resulting mechan in multi-axialical and stresses thermal on theloads tool. when Thus, aforming geometry-dependent FRP sheets. Process relatedstability mechanical of the tool peak material load is needed.s are expected However, to a residualoccur during permeability forming is expected and to be advantageous for the vacuum-forming process. Especially the homogeneity of the material ejecting of the formedproperties part resulting within the in tool, multi-axial strongly influenced stresses byon thethe infiltration tool. Thus, process, a geometry- are expected to dependent stability playof the an importanttool material role for is the needed. long-term However, stability of a the residual tools. permeability is expected to be advantageousMoreover, for temperaturethe vacuum-f increasesorming in process. the tool arise Especially along with the an homogene- increased number ity of the material propertiesof production within cycles the as thetool, sheets strongly solidify influenced in direct contact by the with infiltration the tool surface. pro- How- ever, using 3D printing offers extensive possibilities to overcome constraints by including cess, are expected toenhanced play an functionalities,important role such for as the undercut long-term cooling stability and vacuum of the channels. tools. Moreover, temperatureRegarding increases the vacuum in the forming tool arise of FRPs, along the with main an challenges increased lie innumber the construction of production cycles asof the a suitable sheets sheet solidify holder in thatdirect allows contact material with feed the to tool compensate surface. its However, draping over the using 3D printing offersmold andextensive that provides possibilities enough to tension overcome to prohibit constraints wrinkles by in theincluding sheet. In en- addition, hanced functionalities,rapid such cooling as undercut against the cooling mold has and to be vacuum prevented channels. up to the point, where the geometry is accurately mapped. Further, the forming process itself, the dynamic process and the Regarding the vacuumreproducibility forming of the of material FRPs, feedingthe main behavior challenges will be thelie keyin the challenges construction for successful of a suitable sheet holdervacuum that forming. allows material feed to compensate its draping over the mold and that provides enough tension to prohibit wrinkles in the sheet. In addition, rapid cooling against the mold has to be prevented up to the point, where the geometry is accurately mapped. Further, the forming process itself, the dynamic process and the reproducibility of the material feeding behavior will be the key challenges for successful vacuum forming. In view of the transfer to an industrial application, the central key challenge is defin- ing an economically, sustainable and preferably automatable process for the production of tools of consistent quality.

Materials 2021, 14, 4639 18 of 20

In view of the transfer to an industrial application, the central key challenge is deﬁning an economically, sustainable and preferably automatable process for the production of tools of consistent quality.

Author Contributions: The individual contributions of the authors are as follows. Conceptualization, I.T. and D.G.; methodology, P.E. and S.S.; investigation, P.E. and S.S.; Formal analysis, P.E. and S.S.; validation, P.E. and S.S.; writing—original draft preparation, I.T., D.G., P.E. and S.S.; writing—review and editing, I.T.; supervision, D.G. and I.T.; project administration, I.T.; funding acquisition, I.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Bavarian Research Foundation, grant number 1400-19. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors acknowledge the support of the partners within the scope of the aforementioned funded project: voxeljet AG (Binder Jetting Process), BBG GmbH (construction of sheet holder and machine), Miedl Kunststoff & Design GmbH (vacuum forming expertise), Gierl DCP GmbH (alternative mold materials) and solar velomobil pedilio (deﬁtion of product requirements). Materials 2021, 14, x FOR PEER REVIEW 18 of 20

Conﬂicts of Interest: The authors declare no conﬂict of interest.

AppendixAppendix A A

Figure A1. Model for the Calculation of the Necessary Clamping Force. Figure A1. Model for the Calculation of the Necessary Clamping Force. Author Contributions: The individual contributions of the authors are as follows. Conceptualiza- tion, I.T. and D.G.; methodology, P.E. and S.S.; investigation, P.E. and S. S.; Formal analysis, P.E. and S.S.; validation, P.E. and S.S.; writing—original draft preparation, I.T., D.G., P.E. and S.S.; writing—review and editing, I.T.; supervision, D.G. and I.T.; project administration, I.T.; funding acquisition, I.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Bavarian Research Foundation, grant number 1400-19. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors acknowledge the support of the partners within the scope of the aforementioned funded project: voxeljet AG (Binder Jetting Process), BBG GmbH (construction of sheet holder and machine), Miedl Kunststoff & Design GmbH (vacuum forming expertise), Gierl DCP GmbH (alternative mold materials) and solar velomobil pedilio (defition of product requirements). Conflicts of Interest: The authors declare no conflict of interest.

References 1. Francis, F.L. Solid Processes. In Materials Processing–A Unified Approach to Processing of Metals, Ceramics and Polymers, 1st ed.; Academic Press: Cambridge, MA, USA, 2016; Chapter 4, pp. 251–342.

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