Spatially homogeneous gelation in liquid composite molding

On-line mixing of the resin with its curing agents prior to injection into a mold is a common industrial technique for fabricating composite parts. For vinyl-ester resins that cure via free radical polymerization, the concentrations of retarder, accelerator, and initiator are pre-selected and cannot be changed during the injection. Hence, the resin that enters the mold the earliest has cured longer than the resin that enters the mold later, since the gel time for the resin is the same, owing to the fixed ratio of the curing agents. This approach leads to inhomogeneous cure of the resin and consequently to longer residence time of the resin in the mold. It requires an additional 50 to 75 percent of the filling time before a part can be de-molded. In this study, it is shown that by adjusting the concentration of curing agents during the injection, a more homogeneous gel time throughout the mold can be achieved. The time to de-mold is reduced to 18-24 percent of the filling time. Sensors that measure the conductivity of the resin were used to detect the location and monitor the cure of vinyl-ester. This approach could be extended to other resin systems to control the spatial curing of the resin in the mold.