Most single-screw extruders used in the plastics processing industry are plasticizing extruders, designed to melt solid pellets or powders within the screw channel during processing. In many cases, the efficiency of the melting process acts as the primary throughput-limiting factor. If the material melts too late in the process, it may not be sufficiently mixed, resulting in substandard product quality. Accurate prediction of the melting process is therefore essential for efficient and cost-effective machine design. A practical method for engineers is the modeling of the melting process using mathematical–physical models that can be solved without complex numerical methods. These models enable rapid calculations while still providing sufficient predictive accuracy. This study revisits the modified Tadmor model by Potente, which describes the melting process and predicts the delay-zone length, extending from the hopper front edge to the point of melt pool formation. Based on extensive experimental investigations, this model is adapted by redefining the flow temperatures at the phase boundary and accounting for surface porosity at the beginning of the melting zone. Additionally, the effect of variable solid bed dynamics on model accuracy is examined. Significant model improvements were achieved by accounting for reduced heat flow into the solid bed due to the porous surface structure in the solid conveying zone, along with a new assumption for the flow temperature at the phase boundary between the solid bed and melt film.

The accurate prediction of throughput and the melting process in single-screw extruders are crucial for the design and optimization of plasticizing units, as the melting process is often the rate-limiting factor and markedly influences final product quality. While the design of these units has long relied on empirical values and simplified analytical models, Computational Fluid Dynamics (CFD) now offers a powerful alternative for process analysis. However, its predictive accuracy strongly depends on the precision of the underlying physical models. This work aims to improve and validate a numerical two-phase simulation model for plasticizing extruders. To this end, extensive experimental investigations were conducted using screw pull-out tests, following the method of Maddock, on an extruder with a 30 mm diameter. Utilizing High-Density Polyethylene (HDPE) and two different three-zone screws, a broad dataset was generated through a statistical design of experiments by systematically varying screw speed and barrel temperature. Based on this experimental data, targeted adjustments were made to a CFD model that builds upon the continuum approach by Karrenberg and further advanced by Dörner and Schall. The key improvements include the implementation of models for the bulk density and thermal conductivity, which account for the porosity of the bulk material in the screw channel as a function of pressure and temperature. The validation of the improved model shows good agreement between simulation and experiment. Throughput is predicted with high precision, and the simulated melting profiles reproduce the expected melting mechanism, although the melting lengths appear to be underestimated when compared with the cross-sections from the screw pull-out experiments, the accuracy of which is subject to discussion. Analyses at the process limits, where unmelted material was observed exiting the extruder, likewise revealed that the model overestimates the melting performance, although the extent of overestimation is smaller. This study demonstrates that by adapting the physical sub-models, particularly by considering porosity, the predictive accuracy of CFD simulations can be markedly increased, especially with respect to throughput, thus providing an alternative tool for extruder design.