Residual stresses and birefringence in injection molding of semicrystalline polymer - Part II: Experiment and simulation

The viscoelastic process simulation of the idealized injection molding proc ess for various isotactic polypropylenes having different molecular weights are conducted by using the theoretical formulation developed in the first part of the present study. Molding experiments are performed under a variet y of molding conditions, and birefringence measurements are carried out on the molded samples. Based upon the experimental and simulated results of th e residual stresses and birefringence, the effects of molding conditions an d molecular weights are studied. The influences of flow-induced crystalliza tion kinetics on the buildup and relaxation of residual stresses and birefr ingence in the moldings are discussed. It has been found that, due to the a bsence of the flow-induced crystallization in the core region, the flow str esses and birefringence introduced in that region during the filling stage relax shortly after the cavity is filled. Moreover, the flow stresses and b irefringence get frozen-in due to the occurrence of the flow-induced crysta llization are relatively large and located within a very narrow region clos e to the mold wall. The main contribution to the overall residual stresses is found to be due by the residual thermal stresses that are introduced by the progression of crystallization during the postfilling stage. The distri bution of the residual birefringence in the moldings is closely related to that of the crystalline microstructure pattern and less dependent upon the level of overall residual stresses.