UNIV. PROF. PROF. H. C. MGU DR. ING. DIPL. WIRT. ING. THOMAS GRIES, is Head of Institute at the Institut für Tex- tiltechnik der RWTH. Aachen University.
M AT ER IA L S | PR EPR E G S
Cost Efficiency through Load-optimised and Semi-impregnated Prepregs The manufacturing of prepreg based fibre composite parts generally requires multiple process steps and curing via autoclave. This leads to long cycle times and high investment costs. RWTH Aachen University demonstrates how to overcome these problems by the usage of novel partially impregnated prepregs based on tailored textiles.
Authors
High Component Prices CARSTEN UTHEMANN, M. SC., is Research Assistant at the Institut für Textiltechnik der RWTH Aachen University (Germany).
LENNART JACOBSEN, M. SC., is Research Assistant at the Institut für Textiltechnik der RWTH Aachen University (Germany).
UNIV.PROF. PROF. H. C. MGU DR.ING. DIPL.WIRT. ING. THOMAS GRIES, is Head of Institute at the Institut für Textiltechnik der RWTH Aachen University (Germany).
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In the steadily growing market for carbon fibre reinforced plastics (CFRP), pre-impregnated reinforcement textiles, so-called prepregs, play a major role. With a market share of approximately 45 %, prepreg-based processes generated a worldwide turnover in 2015 of approximately € 4.4 billion [1]. The technology is mainly applied for the production of parts with high quality requirements and for series smaller than 10.000 pieces/ year [2]. For up to 1000 pieces/year, the deposition and the stacking of the prepregs are generally done manually. For this, UD-pre-
pregs or prepregs based on woven fabrics are cut into shape and then layered to form the specific component. For quantities greater than 1000 pieces/year, automated deposition systems can be used. In general, the layers are then cured under the influence of temperature and pressure using an autoclave. This allows fibre volume contents of about 60 %. In combination with the specific alignment of the reinforcement fibres within the individual layers, the manufactured components show high quality and excellent mechanical properties. In return, the coating process, the deposition and the use of the
Figure 1 Comparison of the conventional process chain and the Tailored Prepreg process chain (© ITA) WORLDWIDE
cess costs can be reduced by up to 20 % compared to the conventional prepreg process, Figure 1.
Production of Tailored Non-Crimp Fabrics Figure 2
Schematic representation of a modular TNCF system (© ITA)
autoclave cause high investment and energy costs and require long cycle times. In addition, the waste resulting from the stacking process is expensive to recycle due to the already applied matrix system. These circumstances lead to high process costs and component prices. For that, the use of prepregs is currently only suitable for certain high-performance applications [2]. For instance, the estimated costs of carbon fiber reinforced plastic within the aerospace sector, which is currently the main user of the prepreg technology, averaged 260 €/kg in 2016 [1].
Tailored Prepregs One promising approach to reduce the process costs lies in the application of Tailored Prepregs. These semi-impregnated prepregs are based on load-optimised reinforcements
Figure 3 Volume 10
textiles, so-called tailored textiles. At the Institut für Textiltechnik (ITA) at the RWTH Aachen University, such an approach is pursued using tailored non-crimp fabrics (TNCF). TNCF are locally reinforced noncrimp fabrics, which are produced in an automated single-step production process. The produced TNCF are then partially impregnated via transfer coating and subsequently processed to composite parts. The use of multi-layered fabrics reduces the costs of depositing by up to 80 % compared to the processing of single-layer UD-prepregs. Furthermore, the load-optimised design of the TNCF guaranties a reduction of fibre waste by up to 20 %. The partial impregnation allows the curing out-of-autoclave, which shortens the cycle times and reduces the necessary investment costs. Due to the shorter process chain, the high degree of automation and the efficient use of the material, the pro-
TNCF with locally applied carbon fibre reinforcement patches (© ITA)
The TNCF technology was originally developed at ITA [3]. It is a development of the warp knitting process for the production of conventional non-crimp fabrics (NCF). Compared to conventional NCF, the mechanical properties and the drapability of TNCF can be customised according to specific requirements. This is achieved through the local deposition of additional reinforcement structures, the local variation of the knitting pattern, as well as the adjustment of the knitting yarn tension. The necessary machine modifications are modular and can be retrofitted on conventional warp knitting machines. The modular structure of a TNCF system is shown in Figure 2. The TNCF process is divided into several steps. In a first step, a base layer structure is built up using conventional weft insertion systems. Depending on the component geometry and the respective load case, additional reinforcement structures are then applied. These structures can be roll material, tailored patches or special stringer structures. The positioning of the reinforcing structures can be performed automatically by means of a storage module. If necessary, the reinforcement structures can be fixed on the base layer using various binder technologies. The layers are then connected using a knitting unit. An adaptive hold-down bar is used to fix the local reinforcements and the base layer during the knitting process. The bar is self-adjusting by means of damping elements, so that local thickness variations can be compensated. An example for the result of the knitting process is shown in Figure 3 by means of a hybrid TNCF with a glass fibre base layer and locally applied carbon fibre reinforcement patches. The binding type can be varied by means of an electromechanically driven guide bar, Figure 4. The movement of the guide bar is carried out by a linear motor in accordance to the main shaft rotational speed. In addition, the warp yarn tension can be adjusted
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by means of an electromechanical warp beam drive. By varying the knitting parameters, the drapability of the TNCF can be locally adjusted. Thus, the layer structure of the TNCF can be fixed by means of a narrow-meshed knitting pattern for two-dimensionally curved areas of the composite parts, whereas a high drapability can be achieved in three-dimensionally curved areas by open-meshed patterns. [4] The TNCF technology was originally designed for the discontinuous processing via Liquid Composite Molding (LCM). For this purpose, an automated cutting and stacking unit is implemented into the warp knitting machine. However, the continuous coating process for the production of Tailored Prepregs requires the production of roll material. The varying thickness of the TNCF leads to an uneven surface pressure distribution between the layers during the winding. This causes fibre distortion, which can lead to an intolerable reduction of the mechanical properties of the composite part. At ITA, an alternative draw-off strategy is being developed. This strategy includes the additional feeding of a flexible compensating material within the winding process, Figure 5. The compensating material compensates the thickness variation within the TNCF and thus guarantees an even surface pressure distribution and thus a distortion-free winding.
Figure 4 TNCF with local carbon fibre reinforcement and varying knitting pattern (© ITA)
urated area on the bottom of the fabric. The impregnation is carried out on a continuous coating plant, Figure 6. The TNCF are being unwound, the compensating material removed and returned to the fabric production. The TNCF is then placed on a separat-
tensile stress in the textile, resulting in a force in the radial direction, which forces the TNCF into the resin film. The heating of the table causes a reduction in the resin viscosity, which additionally supports the impregnation. The linear take-down of the coating
The process costs can be reduced by up to 20 % compared to the conventional prepreg process.
Partial Impregnation Following the TNCF production, the partial impregnation of the textile semifinished product is carried out. In this case, the semifinished product is only coated with a resin system on one side. This results in a dry layer on the top of the fabric and a resin-supersat-
ing film with a resin film applied on it. The resin film thickness can be precisely matched to the desired fibre volume ratio in the entire component. The coated textile is then pulled over a curved heating table. This causes a
Figure 5 Schematic representation of the feeding of a compensation material during the winding of locally reinforced TNCF (© ITA) 20
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system is designed in such a way that the TNCF is not damaged in the area of the thickness steps. Finally, the Tailored Prepregs are diced. Due to the partial impregnation, the further processing of the tailored prepregs is possible by vacuum build-up. In the construction of the prepreg layers, the dry side of the prepreg forms a channel through which the air contained can be evacuated. After the evacuation, the layer structure is heated, whereby the resin system becomes flowable. This is followed by the complete impregnation of the layer structure in the thickness direction. In Figure 7 the impregnation processes based on the conventional infusion and the transfer coating process are compared.
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Summary and Outlook
Figure 6
Schematic representation of the system for partial impregnation (© ITA)
Figure 7
Schematic comparison between classical infusion and transfer coating (© ITA)
Within the research project “Tailored Prepreg”, a new process chain is being developed at the Institut für Textiltechnik at the RWTH Aachen University, in which load-optimised non-crimp fabrics are produced by singlestep manufacturing and are then partially impregnated in a continuous coating process and subsequently consolidated. For this purpose, the existing process for the manufacturing of TNCF is extended by a continuous draw-off, which allows a distortion-free winding of fabrics with local reinforcements. In a subsequent transfer coating process, the reinforcement fabrics are partially impregnated, which ensures rapid consolidation with a high fibre volume ratio without the use of an autoclave. The investigated process chain allows the reduction of manual effort, fibre waste and investment costs, which considerably reduces the costs for prepreg components. |
References Due to the transfer coating, the necessary flow path is only a few mm depending on the thickness of the buildup. Compared to conventional infusion methods, in which the entire textile layer has to be penetrated, the time for complete impregnation is thus significantly reduced. Moreover, the precise introduction of the required amount of resin onto a resinous base in a vacuum construction can be dispensed with. On top of that, the use of an autoclave becomes unnecessary.
Figure 8 shows the bottom of the vacuum assembly. The white area in front of the flow front represents a flow aid, which becomes transparent when impregnated and shows the black reinforcement fabric on top. Both sections were simultaneously vacuumed. In the transfer coating, the impregnation has already been completed, while the flow front of the infusion has passed the textile only halfway through.
[1] Kraus, T.; Kühnel, M.; Witten, E.: Composites Marktbericht 2016. Marktentwicklungen, Trends, Ausblicke und Herausforderungen, Carbon Composites e.V., 2016 [2] Lässig R.; Eisenhut M.; Arne M.: Serienproduktion von hochfesten Faserverbundbauteilen. Studie Perspektiven für den deutschen Maschinen- und Anlagenbau, Roland Berger, 2013 [3] Schnabel, A.: Entwicklung von lokal angepassten textilen Halbzeugen für die Großserienfertigung von faserverstärkten Kunststoffen. Aachen: Shaker Verlag, 2013 [4] Krieger, H.; Gries, T.; Stapleton, S.E.: Design of Tailored Non-Crimp Fabrics Based on Stitching Geometry. In: Applied Composite Materials, Springer Netherland, 2017
Thanks
Figure 8
Comparison between classical infusion and transfer coating (© ITA)
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The IGF project “Tailored Prepreg” (No. 194 41 N) of Forschungsvereinigung Forschungskuratorium Textil eV, Reinhardtstraße 12-14, 10117 Berlin is funded by AiF within the program for the promotion of industrial joint research (IGF) by the Federal Ministry of Economics and Energy BMWi on the basis of a decision of the German Bundestag. We would like to thank the participating project partners as well as the members of the project accompanying committee for their cooperation and the support of the research work.
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