Apart from the rate of deformation, the viscosity of polymer melts also depends on the temperature. In increasing the temperature, the volume of the polymer increases due to thermal expansion. In having more space, the long polymer chains slip more easily which results in a decrease of the viscosity.
For temperatures 100K above the glass transition temperature Tg, the exponential Arrhenius equation is used to describe the temperature influence. This is for example the case when simulating the injection stage of injection moulding processes.
For temperatures close to the glass transition temperature Tg, the Williams-Landel-Ferry (WLF) equation is suitable. For simulating the packing stage in injection moulding processes for example, the WLF equation is normally applied