Compression molding of chopped woven thermoplastic composite flakes: a study on processing and performance

Abstract

Continuous fiber reinforced composites with high-performance thermoplastic polymer matrices have an enormous potential in terms of performance, production rate, cost efficiency and recyclability. The use of this relatively new class of materials by the aerospace and automotive industry has been growing steadily during the last decade. However, the use of continuous reinforcements limit the complexity of the shape of the end products, as defects such as wrinkles can form during processing. Moreover, a significant amount of process waste is generated, which lowers the efficiency of the conventional production processes involving a prepreg cutting stage and/or final trimming stages. The overall efficiency of the manufacturing value chain of composites can be improved by adjoining a complementary manufacturing process that utilizes the process waste incurred in the primary process. The discontinuous form of reinforcements in the reclaimed material will provide a means to manufacture complex 3D geometries with near-net edges and at the same time use the raw material efficiently. However, the discontinuities in the reinforcing phase lead to a reduced mechanical performance compared to continuous reinforcements. Suitable applications include semi-structural parts and non-load bearing structures. This thesis focuses on the processing of planar discontinuous reinforcements and the associated mechanical performance. Chopped thermoplastic semi-preg with a woven fabric reinforcement, referred to as flakes, is considered as a standalone molding compound in this study. A compression molding process is chosen to manufacture parts, as it allows for complex geometries while retaining a long fiber length. Although processing of different types of discontinuous reinforcements has been studied in the past, the processing of the specific class of planar reinforcements with a woven architecture and high fiber content has not been explored so far. The principal objective is to develop a strategy for manufacturing woven-flake reinforced parts with good quality and consistent mechanical properties. For achieving this goal, the flow behavior of chopped woven material, process induced jamming of the material and the mechanical properties of molded plates are experimentally investigated and are explained with theoretical models. The work presented in the thesis shows the multidisciplinary nature of the problem, with strong correlations between the material, process and the design of the part leading to the final part performance. Therefore, a processing strategy is proposed which takes into consideration the aforementioned three basic blocks to manufacture consistent parts. Finally, the proposed strategy is validated by manufacturing a full-scale part with typical design features, which successfully demonstrates the processing capabilities of the material and the developed process.