Up to now, existing mathematical models can only describe the throughput behaviour of compounding extruders assuming the condition of a wall adhering melt. However there is a variety of polymer melts, as well as polymer suspensions, ceramic materials and food stuffs, which in the course of manufacturing are wall-slipping. A mathematical model describing the behaviour of the isothermal flow of non-Newtonian materials is developed. In addition to the development of the analytical model, which is based on Flumerfelt et al. [1] and Hirshberger [2], the flow behaviour of wall-slipping polymer melts is also analysed using Finite-Element-Simulations (FE). Comparison of the results for pressure, throughput and velocity profiles along the channel depth shows a very high degree of agreement.