Modeling and experimental study of birefringence in injection molding of semicrystalline polymers

The prediction of birefringence developed in injection moldings is very important in order to satisfy required specification of molded products. A novel approach for the numerical simulation of the flow-induced crystallization and frozen-in birefringence in moldings of semicrystalline polymers was proposed. The approach was based on the calculation of elastic recovery that becomes frozen when the flow-induced crystallization occurred. The flow effect on the equilibrium melting temperature elevation due to the entropy reduction between the oriented and unoriented melts was incorporated to model crystallization. To find the entropy reduction and the frozen-in elastic recovery during crystallization, a non-linear viscoelastic constitutive equation was used. From the ultimate elastic recovery the crystalline orientation function was calculated. The crystalline and amorphous contributions to the overall birefringence were obtained from the crystalline orientation function and the flow birefringence, respectively. The birefringence profiles were measured and predicted in moldings of polypropylenes of different molecular weights obtained at various melt temperatures, injection speeds, holding times and mold temperatures. The resulting predictions were in fair agreement with corresponding experimental data.