Development of press forming techniques for thermoplastic composites

Abstract

Thermoplastic composites are very attractive for aerospace structures due to their high specific strength and stiffness, enhanced toughness and high temperature resistance. One of the manufacturing methods used to produce thermoplastic composite parts is press forming. Press formed parts typically have constant thickness and fibre orientation. However, composite parts may be optimized by tailoring the stiffness and strength, thereby reducing structural weight. In this thesis, the processing techniques for this manufacturing method were developed by investigating the feasibility of a multiple step forming approach for the production of parts with variable thickness and fibre orientation. This manufacturing approach was proven from both a formability and structural integrity viewpoints. By using an efficient blankholder system with clamps and springs, thermoplastic composite laminates can be multiple step formed into three-dimensional shapes with no occurrence of visual forming defects such as wrinkling, fibre buckling or ply folding. Furthermore, good quality bonds can be achieved between laminates formed and co-consolidated in different steps. To achieve a high degree of bonding between GF/PEI composite laminates, the co-consolidation process window was estimated to be between 270ºC and 300ºC. The use of a post-consolidation stage in a hot press was needed to improve the degree of bonding between co-consolidated laminates formed in different steps, which led to very long production cycle times. For future research, the maximum temperature limit of the metal mould should be increased and tooling active cooling mechanisms must be incorporated to allow the mould to be rapidly cooled down, thereby decreasing production cycle times. With these implementations, a better insight on the feasibility of the process in an industrial environment may be gained.