Experimental characterisation of the consolidation of a commingled glass/polypropylene composite

Commingled fabrics offer a cost-effective solution to the manufacture of thermoplastic composites with aligned reinforcements. Consolidation and subsequent void reduction would appear to be the rate-determining stage during processing. Models for consolidation have concentrated on transverse flow of matrix into the reinforcement tows. Experimental studies for other materials have suggested alternative mechanisms, in particular dissolution of entrapped air into the polymer matrix under continued application of pressure. In this study a series of experiments were used to determine the effects of rate, temperature and holding pressure on the consolidation of a glass/polyproplyene commingled fabric. Results are presented in terms of consolidation pressure, void content and micrographs to illustrate the evolution of the microstructure. Increasing rate resulted in increased consolidation pressure as expected, although significant shear thinning occurred even at modest mould closure speeds. Increased rate also led to an increase in void content at the end of the consolidation phase (prior to dissolution into the matrix). The application of pressure during cooling resulted in a dramatic decrease in void content, with levels of less than 1% observed for 3 MPa (regardless of the rate of consolidation). Experiments where the material was heated to 200°C and then cooled to the required temperature prior to consolidation indicated a significant effect of supercooling. This resulted in almost no change in the required consolidation pressure with increasing material temperature above 150°C. Observations of the microstructure at various stages during consolidation suggested that pre-heating of the material resulted in pools of coalesced matrix both within and between the tows. At the end of consolidation the remaining voids were predominantly in the matrix rich regions between tows.