In this paper, the influence of the tool surface on the polymer melt flow was studied. For this purpose, a novel rheological extrusion slit die combined with a lab scale single screw extruder was employed in which the die surface can be varied through exchangeable inserts. The die is instrumented with pressure transducers for the measurement of the pressure loss at a certain flow rate and to determine wall shear stress. Thermocouples allow the determination of melt temperature when entering the flow channel. In the experiments, tool surfaces with different roughness were used to study the influence of the flow channel surface on polymer melt flow. Tests with different channel heights were carried out to achieve data about wall slip of the polymer melts. The lip length of the region with constant channel height was varied to determine the entrance pressure loss. Wall slip was evaluated using the concept of reduced volume flow rate and the wall slip velocity was calculated with the Mooney method. Furthermore, surface analysis of the die inserts was performed before and after the extrusion tests to be able to detect possible changes of the die surface due to the extrusion process. Polymethylmethacrylate shows wall slip on polished steel. There is an increase in the wall slip velocity with higher wall shear stress and temperature. On a rough surface the reduced flow rate curves for the different channel heights are the same and no wall slip occurs. Compared to data achieved with a high pressure capillary rheometer differences occur which may be attributed to temperature rise under extrusion conditions. It could be confirmed through contact angle measurements that there is an increase in the polar fraction of surface energy after the extrusion tests with polymethylmethacrylate. Roughness measurements which were carried out with a confocal microscope revealed only a slight smoothing of the rougher die inserts.