Investigation on the Residual Stress Relaxation according to Annealing Condition for Transparent Injection Molded Part

Residual stress is developed in the injection molded articles during the molding process due to temperature variation and shear stress. The residual stress causes the deformation and warpage in the injection molded parts shortly within several days or after several years. Therefore, the injection molding conditions should be optimized to reduce the residual stress. And residual stress in the part should be also relaxed after molding process to maintain its shape. According to the annealing conditions, such as relative humidity, temperature and time, this study investigates the relaxation of residual stress generated in the transparent injection molded specimens. Through the experimental results, it was realized that the residual stress was relaxed at a relative humidity higher than 50%. Utilizing photoelasticity equipment, it was found that the residual stress was rapidly relaxed near glass transition temperature. Additionally, we recognized that the specimen shrunk along the flow direction but expanded to the perpendicular direction of the flow during the annealing processes, which resulted in the warpage of the specimen.