Predicting the Pumping Capability of Single-Screw Extruders: A Comparison of Two- and Three-Dimensional Modeling Approaches

Pumping is a key processing step in all single-screw extrusion operations. Forcing the polymer resin through the die at the discharge end of the processing machine requires the extruder screw to generate pressure, which may develop at various levels in all functional zones, depending on the design of the screw-barrel configuration. The total pumping capability of the single-screw extruder is therefore determined by the geometry of the entire screw. Since the geometry of each screw section varies, the validity of existing melt-conveying models must be critically reexamined when predicting the pumping capability of a single-screw extruder. Numerous theories for the prediction of flow rate and pressure build-up in extruders have been published: While most are based on the flat-plate approximation, only a few include the effect of channel curvature. Assuming flow of power-law fluids, this work compares screw characteristic curves of flattened and curved screw channels for two- and three-dimensional flows, in which the influence of the screw flights is either ignored or considered, respectively. By solving the governing equations for these flow situations, we demonstrate the distinct differences between these modeling approaches. We further analyze the usefulness of each method in relation to the underlying screw geometries.