The manufacturing science of thermoplastic composites

The structural properties of a thermoplastic composite are strongly dependent on the crystallinity and the autohesive strength between the prepreg tapes during consolidation. Recently there has been increasing interest in the development of an automated consolidation process. The rapid rates and non-isothermal conditions of this process place a prime importance on developing an understanding of factors controlling crystallinity and interply bonding. The relationship of these properties to operating conditions, therefore, will become a critical factor in the design of an automated consolidation process. In this study, experiments were designed to determine the effects of time, temperature and pressure on the autohesive strength of polystyrene, polyethylene and polypropylene. A strong dependence on time and temperature was noted for polystyrene. There was also some dependence on pressure. The semi-crystalline polymers, on the other hand, exhibited little dependence on any of the variables. The results, which compared well with theory, indicate that autohesion could be an important factor in the automated consolidation of amorphous polymers. Finite element analysis was used to model an automated consolidation process. This is accomplished through the use of a finite element computer program called FIDAP. Crystallization kinetics, utilizing a differential form of the Nakamura non-isothermal crystallization equation, are incorporated into the model. An investigation was carried out to determine the effects of velocity and postheating on the thermal history and crystallinity distribution of polypropylene and polyetheretherketone (PEEK). Other factors such as crystalline nucleation rate were also analyzed. The predictions of temperature and crystallinity distribution can be used to determine the optimum operating conditions for the consolidation process.