Modeling and simulation of bulk viscoelasticity for amorphous polymers in injection molding

Bulk viscoelasticity is not well studied and understood in the field of polymer processing. Its behavior in solid mechanics applications, such as time-dependent bulk modulus and time-dependent thermal expansion, was rarely considered but started to receive attention recently. Bulk viscosity (bulk viscoelasticity in fluid mechanics formulation) has been ignored in polymer processing for decades. Bulk viscosity could play an essential role in compressible polymer melts that undergo substantial volume changes caused by variations in temperature and mechanical pressure during fluid motion and solidification. This study investigates the bulk viscosity of an amorphous polymer, polystyrene (PS), through measurements, modeling, and implementation in an injection molding simulation. Simulation results of cavity pressures and shrinkages are validated with experimental data in a three-plate mold case (part size 300 x 100 x 3 mm(3)). Results demonstrate that the effects of bulk viscosity reduced mechanical pressure variations during the packing stage in injection molding. However, the cavity pressure predicted by GNF (generalized Newtonian fluid) models with bulk viscosity drops too fast during the holding stage. The current GNF model can neither accurately describe isothermal pressurization (bulk creep) experiment data. A three-element-based constitutive model is proposed to describe bulk viscoelasticity in isobaric cooling and isothermal pressurization PVT (pressure-specific volume-temperature) measurements. This proposed model's predictions of cavity pressure, part weight, and shrinkage agree with the experiments and show significant improvement over the GNF model.