Mechanisms of inter-ply void formation during vacuum bag-only cure of woven prepregs

The removal of inter-ply air is critical for limiting porosity in laminates. In this study, an in situ monitoring technique was employed to observe inter-ply air evolution during vacuum bag-only cure. Observations showed that reduced vacuum resulted in inefficient inter-ply air evacuation, a more rapid bubble expansion rate, and formation of new air bubbles. A modified tow impregnation model showed that resin infiltration was impeded at reduced vacuum conditions due to the presence of intra-tow air. However, the cured laminates showed that tows were fully impregnated in all cases, indicating that the entrapped intra-tow air migrated to inter-ply regions during cure. The interactions between intra-tow and inter-tow air at deficient vacuum conditions were revealed. Findings led to the conclusion that air remaining in intra-tow regions contributed more to the increase of inter-ply voids than the reduction in consolidation pressure difference associated with reduced vacuum.