29th Annual Technical Conference of the American Society for Composites, ASC 2014; 16th US-Japan Conference on Composite Materials; ASTM-D30 Meeting, California, United States Of America, 8 - 10 September 2014
Out-of-Autoclave (OOA) thermoset prepreg manufacturing of aerospace quality parts is performed under low pressures, which makes it more susceptible to void formation and growth as compared to high pressure autoclave processing. Thus, OOA prepregs are intentionally partially impregnated with resin, with the goal to distribute the resin such that a completely connected network of empty channels is formed in the initial material. This network serves as pathways for evacuation of gases entrapped in the laminate before consolidation and cure. This work investigates how mechanically entrapped air can be removed from partially impregnated OOA prepreg laminates before oven curing. First, a model of void evacuation time is derived to estimate the time necessary to apply vacuum to remove air from within the laminate before placing it in the oven. Next, a flow visualization technique is presented where the resin film of the partially impregnated OOA prepreg is pressed into the fabric, while recording the resin flow on the dry side. This quantifies the degree of resin impregnation with processing time. A relationship between degree of resin impregnation and gas permeability is presented which influences the evacuation time necessary to remove air from the system. A large panel (1.2m long) was fabricated in which low and high evacuation times were employed based on the model physics. The void content was quantified along the length via sectioning/polishing/image analysis and shown to have a void content of 1%. The results of this study should prove useful to develop optimal vacuum application times and temperature and pressure cycles for void reduction and removal during processing of prepregs.