BUILDUP AND RELAXATION OF MOLECULAR-ORIENTATION IN INJECTION-MOLDING .1. FORMULATION

A numerical simulation model for the analysis of the buildup and the relaxation of molecular orientation in injection-molded products has been developed. The constitutive equations of the material are described using an extended version of the viscoelastic material model developed by Leonov. The volumetric responses are described using two different equations of state, the Tait equation and an equation developed by Spencer and Gilmore. A WLF-type of equation is used for the temperature and pressure dependence. Stresses calculated with the model are coupled to birefringence by means of the stress-optical rule. Birefringence, in turn, is used to characterize the molecular orientation. The model is used to investigate the influence of the processing conditions, the mold elasticity, and the pressure dependence of the material functions on the pressure and birefringence profiles. The material data used are for polystyrene, PS 678E, and are determined experimentally.