A statistical experimental study of the injection molding of optical lenses

In the injection molding of plastic optical lenses, the processing conditions have critical effects on the quality of the molded lenses. Since there are many process parameters involved in an injection molding process, and more importantly, an optical lens needs precisely controlled surface contours, determination of the processing conditions for lens molding is very complicated. The objective of this work is to investigate experimentally some effects of the molding conditions on the surface contours of injection molded lenses. A spherical lens was molded using polycarbonate. The surface profiles of the lenses molded under different processing conditions were measured using a laser interferometer. The birefringence of the lenses was measured using a specially designed polarimeter to characterize the residual stress in the lenses. Statistical methods were employed in the experimental studies in order to systematically analyze the effects of various process parameters on the lens contour errors. The process parameters studied include injection speed, holding pressure and mold temperature, etc. The contour errors were correlated to the mold shrinkage and the residual stress in the molded lenses. The study shows that in addition to the mold shrinkage the stress also plays a vital role in determining the lens surface contours.