Measurement and modeling of bulk viscosity for polystyrene melts

Bulk viscosity describes the irreversible resistance to the rate of volume change. Bulk viscosity, which is more than ten thousand times higher than shear viscosity, has been ignored in the field of polymer processing for the past decades. Bulk viscosity may play an important role for compressible polymer melts undergoing strong compression during processing, especially during the packing and holding stage in injection molding. In this study, bulk viscosity of an amorphous Polystyrene melt is investigated through measurements, modeling, and implementation in an injection molding simulation. The results demonstrated that bulk viscosity can be derived from a cooling rate-controlled PVT (pressure-specific volume–temperature) measurement. A new pressure-specific volume–temperature–cooling rate model was developed to obtain smooth and reliable bulk viscosity results. Furthermore, a Cross-William–Landel–Ferry–Arrhenius model was found capable of describing the dependence of temperature, rate of volume change, and mechanical pressure on bulk viscosity of this polymer melt. The proposed modeling was first verified using the non-equilibrium PVT and then was implemented into an injection molding simulation. Simulation results showed that the effects of bulk viscosity not only prevent the material from changing its size but also reduce mechanical pressure variations during the injection molding packing stage.