Viscosity is an index of the resistance of fluid to flow. The viscosity of simple fluids, such as, water and oil, is usually a constant value at constant temperature. These fluids are referred as Newtonian fluids. Unlike simple fluids, the viscosity of thermoplastic depends on its chemical structure, composition, and processing conditions. At a specific chemical structure and formulation, viscosity of the thermoplastics depends on the temperature, shear rate, and pressure.

The volume of the thermoplastics changes significantly with change in the temperature and pressure. It is therefore essential to characterize the pressure-volume-temperature (PVT) relation to calculate the compressibility of a material during the injection molding analysis. A good PVT model describes the dependence of specific volume on temperature and pressure.