Material extrusion-based additive manufacturing, strictly speaking of the Fused Filament Fabrication (FFF), is characterized by using a thermoplastic polymer in form of a solid filament as a built material. The filament melts inside of the heated hotend and is subsequently extruded under the pressure generated by the filament feeding force of the feed rollers. The processes in the heated extrusion channel are complex and difficult to model, as the melt properties are a nonlinear function of temperature and shear rate, and there is also a phase transition from solid filament to liquid melt. In this paper, the required feeding force and resulting extrudate temperature at the nozzle outlet at different feeding rates, liquefier temperatures and nozzle geometries of two ABS and PP materials were investigated. For a better understanding of the melting process in the hotend, the degree of melting of the strands was determined via dead-stop experiments at selected operating points. Two process windows could be identified: at low feed rates, the feed force increases linearly with increasing velocity. At a certain point, the force increases rapidly and fluctuates. The melting investigations showed that this is related to the increasingly unmolten material reaching the nozzle outlet. The semi-crystalline PP showed a smaller processing window for stable flow compared to ABS and lower extrudate temperatures due to its different behavior at the phase transition. Based on the experimental studies, Computational Fluid Dynamics (CFD) simulations were performed to predict the pressure and temperature distributions inside the channel. For modelling the shear-dependent viscosity the Carreau model was used, while the temperature dependency was described by a jump function with a solid viscosity of 106 Pas. It is shown that the numerical model can predict the feed force with good accuracy and represent the change between process windows at increasing speeds.