Design and Optimization of Wave-Dispersion Screws

From a quantitative point of view, extrusion is the most important process in the polymer industry. Every year, this technique converts more than 114 million tons of polymeric materials. T his amount corresponds roughly to one third of the global industrial production of polymer resins. Despite its widespread industrial use, the elementary processing steps of a plasticating extrusion process are generally the same: Solid polymeric material, typically in the form of pellets or powders, is compressed, molten and homogenized by a rotating screw mounted a heated barrel. The polymer melt is then pumped through a die, where the shaping process takes place. Single-screw extruders are us ed in various continuous manufacturing processes to produce semi-finished products such as films, pipes, profiles, sheets, and fibers. Furthermore, the processing machine is used in recycling, injection molding, and blow molding. Given the economic framework, the primary objective of machine manufacturers and plastic converters is to increase the output rate while guaranteeing excellent melt quality. To meet the ever - increasing demands of the plastic industry, a variety of extruder screws have emerged over recent decades. While some of these concepts have been analyzed intensively by numerous scientific studies, others are still not properly understood, and their current designs offer potential for optimization. To close this scientific gap, this research project carried out in cooperation with Kunststofftechnik Paderborn (Paderborn University) investigates the operation of so-called wave-dispersion screws. This term refers to extruder screws usually consisting of two or more screw c hannels that os cillate periodically in depth, with alternating wave peaks and valleys. The wave cycles in adjacent c hannels are out of phase, i.e., the channel depth of one channel decreases while the other increases, thereby improving the mixing performance of the screw and hence guaranteeing excellent melt homogeneity even at high outputs. Despite their recognized industrial relevance, only a few scientific studies have examined the performance of wave-dispersion screws. As a result, traditional screw designs were mainly based on experimental trial-and-error procedures rather than the principles of extrusion theory. This research project intends to develop novel process models for wave-dispersion screws, using computational flow simulations, hybrid modeling techniques, and experimental studies that allow a systematic design and process optimization. Special attention will be attached to the processing s teps of melting, melt conveying, and mixing. The results of this research project increase the understanding of the physical transport phenomena in wave-dispersion screws and provide the tools need for systematic model-based analysis of the screws.

Single-screw extruders are used for shaping various plastic products such as pipes, hoses, films, and sheets. The growing demands placed on these machines in terms of productivity, product quality and energy efficiency become increasingly difficult to realise with conventional plasticising screws. One promising solution is the use of so-called wave-dispersion screws: Thanks to their wave-like channel depth profile, they allow for good mixing of the plastic melt at relatively low energy consumption and high production rates. For an optimised design of these screws, however, there has been a lack of calculation models that map the flow conditions in the extruder with sufficient accuracy and provide useful results in a reasonable time frame. As a result, the high potential of wave-dispersion screws has not been fully utilised yet in the plastics processing industry. Aiming for closing this research gap, an existing calculation approach has been further improved during this research project. This approach maps melt conveying in single-screw extruders via a network of interconnected screw elements, being able to capture the cross-sectional changes in the conveying direction and the flow across the screw flights in wave-dispersion screws. At the same time, the calculation times required are considerably shorter compared to fully three-dimensional flow simulations. To describe the operating behaviour of the individual sections along the screw more accurately, new approximation equations for the pumping capability and the energy input with an extended range of validity were derived. These equations are based on fully three-dimensional computational fluid dynamics simulations that consider the shear-thinning behaviour of polymer melts, cover a wide processing window including pressure-generating and pressure-consuming screw zones, and, for the first time, fully represent the influence of the channel curvature. Both the melt flow in the channel and the leakage flow over the screw flights can be modelled far more realistically by the new equations, which has been proven by an error analysis. In addition, by employing the novel regression models, the approximation of experimentally determined process data by means of network-based calculations is improved. The fast predictions and the improved accuracy of the network-based calculation tool open up promising opportunities for assessing the operating behaviour of wave-dispersion screws. Applying this tool will accelerate both troubleshooting of running extrusion processes and the development of new screws. The obtained improvements in efficiency and quality will contribute to a more economic and sustainable processing of plastics.