Shrinkage Analysis on Convex Shellby Injection Molding

Abstract

Abstract Injection molding of a convex shell has induced divergent flow front directions and therefore the shrinkage of a convex shell is difficult to controll. In some optical applications, such as ophthalmic devices of soft contact lens, that can be cast molded with a set of two convex shells made by injection molding there is a demand in maintaining dimensional stability. This research investigates the optimization of injection molding parameters for the minimum shrinkage of a convex shell of polypropylene (PP). The MPI 5.0 software is used first to simulate the filling stage of the convex shell and find the feasible operational ranges of parameters. Experiments were performed by an electric injection machine. Short shot experiments have been tested to compare with results of mold flow simulation. Taguchi's method is then used to find the optimal parameters for the minimum shrinkage of convex shell. Four parameters or factors, including mold temperature, injection temperature, holding pressure, and cooling time are considered in this study. An ANOVA table has been obtained for checking the significance of parameters. Results of simulation and experiments have been compared and the holding or packing pressure is found as the most significant parameter for the minimum shrinkage of a convex shell. Cooling time and injection temperature have been found as the second and third most significant parameters in this study. The optimal parameters have been established and then verified by the optimal injection molding experiment and the minimum shrinkage of convex shell was obtained as 72 μm.