Air evacuation and resin impregnation in semi-pregs: effects of feature dimensions

Prepregs with discontinuous resin (semi-pregs) impart robustness to vacuum-bag-only processing of composites. Limited guidance exists for evaluating advantageous resin patterns (i.e. dry space dimensions required to achieve both efficient air evacuation and full resin infiltration during cure). A flow front model was developed based on resin cure kinetics and rheological behavior, and then determined maximum dry space dimensions for semi-pregs under a range of realistic manufacturing conditions. Model predictions were validated in situ. Under controlled laboratory cure conditions, small surface openings (similar to 3.7mm) resulted in full resin infiltration. Under adverse conditions (resin with accrued out-time), the maximum opening size dropped 40% (to similar to 2.2mm). Using a mathematical model, air evacuation time was calculated for various feature sizes using permeability measurements. Model predictions were tested and verified via fabrication of laminates. This methodology can be applied to other resin systems to guide vacuum-bag-only prepreg design and support robust production of composites.