Effects of material and process parameters on void evolution in unidirectional prepreg during vacuum bag-only cure

Void reduction during composites manufacturing is critical for successful processing. In this study, we perform a parametric study to determine the mechanisms of interply void evolution in unidirectional prepregs during vacuum bag-only cure and to identify the key factors that affect interply air removal. We employ an in situ visualization setup for direct, real-time observation of air removal for prepregs during cure. Results showed that super-ambient dwell at 50℃ effectively promoted interply air removal in unidirectional prepregs, reduced vacuum quality (80% vacuum) had negligible effects on part quality, and that an increase in moisture content of the laminate notably increased void content. Prepreg moisture content was tracked by the inspection of laminate water content at different times during the cure cycle, and the data was combined with a diffusion-based analytical model to predict void size and to improve the understanding of void evolution mechanisms. Results indicated that moisture content of the laminate decreased markedly as cure progressed, providing insights into bubble behavior (expansion and shrinkage) observed during cure. The modified model predictions aligned with experimental data, especially during the second stage, confirming that the observed void growth results from moisture diffusion.