Effect of external pressure on the manufacturing quality of vacuum infused composite structures

Vacuum assisted resin infusion (VARI) molding is an attractive low-cost manufacturing technique for large-scale composite structures. However, its relatively limited curing pressure typically results in inferior manufacturing quality compared with autoclave processing of prepregs. For the VARI process, a series of improvement strategies based on external pressure regulation have been developed, including permanent magnets, external pressure chambers, and inflatable bladders, to improve laminate quality. Despite these efforts, the pressure-dependent evolution of laminate structural quality in composite structures fabricated by vacuum infusion has not been systematically clarified, which limits further process optimization and industrial application. To address this issue, external pressures ranging from 0 to 0.9 MPa were applied during the curing process after resin infusion was completed for composite structures using a vacuum bag and an autoclave. The results showed that appropriate pressure reduces the void content while enhancing laminate compactness and mechanical performance. In contrast, excessive pressure leads to significant resin loss, indicating a competition between void suppression and resin depletion. SEM fractography confirmed that excessive resin loss depletes the interlaminar resin layer, shifting the failure mode from ductile yielding to brittle debonding. Furthermore, a multi-scale coupling mechanism framework was proposed to elucidate the physical origin of this trade-off. Considering these competing effects, an external pressure of approximately 0.3 MPa was identified as providing an optimal balance for manufacturing quality and reliability, offering practical guidance for cost-effective VARI process optimization.