Reverse forming thermoplastic composites: Design and process development

Structural reuse is a promising alternative to recycling of composite materials; it preserves material composition while liberating the materials for reuse in secondary applications. Thermoplastic reinforced composite materials have the potential to expand reuse opportunities by adapting their shape, or reversing them to a laminate blank. In this study, we evaluated reverse forming of glass fibre-polypropylene (GF-PP) laminates by developing a processing method, testing material properties and the effect of three design parameters: forming strain, laminate architecture and material type. Forming strain relates to the deformation mechanism of inter-ply slip, and is imposed through varying the contour depth and bending radius. Laminate architecture relates to resin redistribution, and is imposed by using an orthogonal as well as quasi isotropic layup. Finally, the material type affects both Inter-ply slip as well as resin redistribution, and is imposed by using plain and twill weaves. GF-PP blanks were prepared using a heated platen press and subsequently formed and flattened using convection heating (<165 °C) and vacuum pressure in a novel moulding process. The samples had typical values for flexural strength of 91 - 113 MPa and flexural modulus of 9–16 GPa. Using a Design of Experiments analysis the process was deemed robust for the given boundary conditions. These results demonstrate the feasibility of reverse forming for cases where inter-ply slip is the governing deformation mechanism. The presented reverse forming process and design parameters can be used to create new thermoplastic composite parts, anticipating for structural reuse through reverse forming.