Optimization of injection-molding processing conditions for plastic double-convex Fresnel lens using grey-based Taguchi method

Abstract

Injection molding is an ideal technique for the low-cost mass production of plastic optical lenses. However, the optical quality of the lens is significantly dependent on the processing parameters employed in the molding process. In particular, the parameters should be set in such a way as to minimize both the shrinkage displacement-induced warpage (in order to reduce optical aberrations) and the residual stress (in order to reduce the birefringence). However, in practice, the two performance targets are mutually exclusive. That is, a smaller warpage results in a greater residual stress, and vice versa. Thus, in the present study, Taguchi experiments are first performed to determine the processing parameters which individually minimize the warpage and retardation of a symmetric plastic double-convex Fresnel lens, respectively. A grey relational analysis technique is then applied to the Taguchi results to establish the processing parameters which achieve the optimal tradeoff between the two performance objectives. The validity of the proposed method is demonstrated by means of mold-flow analysis simulations. The results show that in simultaneously optimizing both the warpage and the retardation, the injection molding control factors are ranked in order of decreasing influence as follows: packing pressure, packing time, melt temperature, filling time, cooling time and mold temperature. Given the optimal process parameter settings, the warpage and retardation are reduced by 16.42% and 74.74%, respectively, compared to the original design. In other words, the proposed grey-based Taguchi method provides a viable technique for optimizing both performance targets for the considered double-convex Fresnel lens.