Evaluation of the influence of the tool surface on polymer melt flow using a novel rheological extrusion slit die

In this paper, the influence of the tool surface on polymer melt flow is studied. For this purpose, a novel rheological extrusion slit die in conjunction with a lab-scale single screw extruder in which the die surface can be varied using exchangeable inserts was employed. In the experiments, tool surfaces of different roughness were used to study the influence of the flow channel surface on wall slip of a polymethylmethacrylate (PMMA) melt. 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 caused by the extrusion process. PMMA shows wall slip on polished steel. There is an increase in the wall slip velocity with rising wall shear stress. On a rough steel surface no wall slip was observed. Compared to data obtained with a high pressure capillary rheometer good qualitative correspondence was obtained, but quantitative differences of the wall slip velocity values between the two measuring methods were found. It was confirmed through contact angle measurements that there is an increase in the polar fraction of surface energy after the extrusion tests. Roughness measurements carried out with a confocal microscope revealed only slight smoothing of the rougher die inserts.