Analysis of Residual Stress and Birefringence in a Transparent Injection Molded Article for Molding Condition

Abstract

Residual stresses are developed during injection molding process and these cause a warpage and invoke a bire- fringence in a molded article. The levels of residual stresses are dependent of molding conditions, such as melt tem- perature, mold temperature, cooling condition, and packing condition. Among the residual stress measurement methods, the photoelasticity method is simple and convenience to measure compared with other methods. However this method can be utilized only for a transparent product. Birefringence and residual stress have been observed in injection molded polycarbonate (PC), polystyrene (PS), and polymethylmethacrylate (PMMA) specimens according to injection molding conditions. Computer simulation has been performed and compared with experimental observation of birefringence. PS specimen showed the highest birefringence whereas PMMA showed the lowest birefringence among them. Locations close to the gate showed higher residual stress than locations far from the gate. Long cooling time, high injection tem- perature, and long packing time reduced residual stress.