Modelling of Residual Stresses in Rubber Pressed Composite Products

Continuous-fibre-reinforced thermoplastic composites are increasingly used in demanding applications in, for example, the aerospace industry. The use of specialty thermoplastics such as polyetherimide (PEI), polyphenylenesulfide (PPS) and polyether-etherketone (PEEK) reinforced with glass, carbon or aramid fibres results in solvent and corrosion resistant products with high specific stiffness and high melting temperature. These composites can be moulded into complicated shapes with the rubber pressing process where a relatively hot laminate is shortly pressed between a cold rubber mould and a cold steel tool. The process is fast compared to the forming of thermosetting polymer matrix composites where extensive curing times are involved. During the moulding, residual stresses build up that cause unwanted distortions of the final product. These stresses are caused by anisotropic shrinkage and non-symmetric process conditions. The anisotropic shrinkage can be thermally induced or it can be a result of crystallization. The upper and lower mould induce non-symmetrical process conditions: the former is a highly deformable elastomeric with a very low thermal conductivity, whereas the latter is made from high stiffness steel with a high thermal conductivity. As a result, the laminate exhibits a non-uniform temperature profile through the thickness during moulding, and has different mechanical boundary conditions on the to