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The viscoelastic process simulation of the idealized injection molding process 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 variety of molding conditions, and birefringence measurements are carried out on the molded samples. Based upon the experimental and simulated results of the residual stresses and birefringence, the effects of molding conditions and molecular weights are studied. The influences of flow-induced crystallization kinetics on the buildup and relaxation of residual stresses and birefringence in the moldings are discussed. It has been found that, due to the absence of the flow-induced crystallization in the core region, the flow stresses and birefringence introduced in that region during the filling stage relax shortly after the cavity is filled. Moreover, the low stresses and birefringence get frozen-in due to the occurrence of the low-induced crystallization are relatively large and located within a very narrow region close 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 distribution 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.

Residual Stresses and Birefringence in Injection Molding of Semicrystalline Polymer: Part II: Experiment and Simulation