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

Abstract 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.