Process for preforming continuous fibre-reinforced thermoplastic semi-finished products

The German government's environmental policy targets, such as reducing emissions by at least 40% by 2020 and by 80 to 95% by 2050 in comparison with figures for 1990, require a reduction in energy consumption, which also correlates with smaller moving masses. However, the weight of modern vehicles continues to rise. The only way to reverse this spiralling, environmentally harmful weight problem in large-scale automotive production is through consistent implementation of lightweight construction. This is why fibre-reinforced plastics (FRP), alongside known lightweight materials such as aluminium and high-strength steels, are increasingly coming to the attention of developers of new generations of vehicles. A significant advantage of FRPs over conventional sheet metals is the variety of characteristics and possible shapes. Dry, complex-shaped technical textiles with thermosetting resin systems have been state-of-the-art for several years and can be produced economically for large-scale production, as strikingly demonstrated by the BMW i3, for example. Nevertheless, the automotive industry is calling for thermoplastic solutions, as thermoplastics have better impact behaviour thanks to their greater toughness. Furthermore, thermoplastics have particular advantages when it comes to recycling, and cooling of the melt occurs far more quickly than chemical crosslinking processes, allowing for shorter processing times. Another advantage of continuous fibre-reinforced thermoplastics (CFT) is based on the linear and branched arrangement of the polymer warp compared to crosslinked thermosetting systems. These can be melted, thereby allowing preforming in chronologically distinct steps, as well as welding. The preforming of standardized CFT materials - such as thin tapes with unidirectional (UD) reinforcement and thick-walled individual layers made from fibre-reinforced multi-layer thermoplastics (organic sheets) - represents a major challenge in terms of the formability of flat multi-layer composites into three-dimensional structures with multiple bends. The consolidated material prevents free dislocation of the fibres, which would otherwise lead to wrinkling, fibre breakage, unwanted fibre reorientation and inhomogeneities in the component thickness. Although established procedures such as variothermal temperature controls in the tool or in-situ impregnation methods can handle such issues, these are not economical for large-scale production processes. Novel approaches for the production of CFT preforms using folding devices specifically designed for this application permit shaping of a prototype into complex high-performance components for applications in large-scale production. With additional support from existing drape simulations and drape coefficients determined specifically for this purpose, it is possible for preforms having multi-layer structures to be created in layers from UD tapes and glass-fibre reinforced organic sheets and conditioned for use with the tool.