Influence From Process Parameters on Void Formation in Resin Transfer Molding

The influence of different process variables on the void content in resin transfer molding (RTM) has been investigated experimentally. The moldings were made in a flat mold filled by a parallel flow from one edge of the laminate to the other. The voids were found concentrated in a narrow region close to the ventilation side of the laminate. The void volume fraction in this region was almost constant and dropped over a short distance to basically no voids in the rest of the laminate. Micrographs from cross sections in different directions revealed that the voids were of two different types, long cylindrical bubbles inside the fiber bundles and larger spheroidal bubbles in the space between the fiber bundles. An efficient way of reducing the void content was to use vacuum assistance during mold filling. This technique was beneficial both for the magnitude of the void content and for the extent of the void region. The void content with the highest level of vacuum assistance (= 1 kPa absolute pressure), was practically negligible. Strong indications for void generation by mechanical entrapment at the flow front was found. The lowering of the void content with vacuum assistance can be interpreted as a result of compression of voids when the vacuum is released and a higher mobility of voids created at a lower pressure.