Residual stresses and birefringence in injection molding of semicrystalline polymer - Part I: Theoretical formulation

A composite model for the buildup and the relaxation of residual stresses and birefringence during the injection molding of semicrystalline polymer is proposed. This model takes into account the flow-induced crystallization occurring under nonisothermal conditions. The effects of crystallization on the variations of mechanical and physical properties are considered. The viscoelastic constitutive equation due to Giesekus and Leonov is employed for the calculation of the buildup and relaxation of flow stresses, whereas the Morland-Lee constitutive equation of linear viscoelasticity is utilized far the calculation of the buildup and relaxation of thermal stresses during the molding. The overall residual stresses in the molding are determined based upon the addition of residual flow and thermal stresses. The multilayered microstructure in the moldings arising due to the flow-induced crystallization is determined. Flow birefringence is related to flow stresses according to the stress-optical rule. An expression is proposed to evaluate the overall residual birefringence based upon the resultant microstructure pattern and residual flow birefringence in the moldings. As a result, numerical simulations for idealized injection molding process are formulated to calculate the residual stresses and birefringence in molded semicrystalline polymers. Based upon the above theoretical approach, the numerical simulation schemes are formulated, and a computer program is developed. Based upon the simulations, the effects of molding conditions and molecular weights on the overall residual stresses and birefringence are further evaluated.