Dependence of bulk viscosity of polypropylene on strain,strain rate,and melt temperature

The relations between bulk viscosity, strain, strain rate, and melt temperature of polypropylene are investigated in uniaxial compression similar to the measurement of PVT diagram of a polymer. A new and more practical bulk viscosity model for the simulation of injection molding is proposed. The bulk viscosity increased with the increase of compression deformation and decreased when the compression speed and the melt temperature are higher. This dependency is attributed to the orientation and occupancy of the molecule within the free interstitial space. In addition, the magnitude of the bulk viscosity was found to be significantly greater than the shear viscosity at high compression deformation and low strain rate. Thus, not only shear viscosity but also the bulk viscosity should be considered in the simulation of the packing‐holding stage of the plastic injection molding process. POLYM. ENG. SCI., 57:830–837, 2017. © 2016 Society of Plastics Engineers