A THERMOVISCOELASTIC MODEL FOR RESIDUAL-STRESS IN INJECTION MOLDED THERMOPLASTICS

The main sources for residual stress build-up and warpage in injection-molded thermoplastic parts are the holding pressures at the gate and the asymmetry in temperature at the mold walls. Accurate computation of these stresses requires a viscoelastic model since creep and stress relaxation behavior of polymers is magnified at the temperature range for filling, packing, and cooling stages of the injection molding process. Furthermore, injection molded parts can rarely be treated as isotropic; orientation, crystallinity, and presence of fibers require a more general material-model presentation. In this paper, we present a transversely isotropic thermoviscoelastic model for injection-molded thermoplastic parts, provide a description of the material properties, and briefly discuss measurement techniques and their limitations.