For the mathematical modeling of single screw extruders crucial simplifications are frequently made. In order to estimate the quality of these models it is necessary to evaluate the differences resulting from these simplifying presumptions. This study refers to the influence of two frequently used simplifications, the power-law model and the assumption of an isothermal flow The differences due to the application of the power-law show an evident dependency on the descriptive material parameters. Depending on the Nahme-number the isothermal simplification proves partly clear differences from non-isothermal flows. These differences however are considerably, reduced in the range of high Peclet-numbers.