New Wave-Screw Concepts in High-Speed Extruder

Research Project Contents: Single-screw extruders are indisputably the most important machines in the polymer processing industry. During plasticating extrusion, polymer is fed to the extruder in the solid-state and the material is melted as it is conveyed by the rotating extruder screw from the feed port to the die. Based on the economic framework, the primary objective of machine manufacturers is to increase the output capacity while guaranteeing high melt quality. Meeting these demands requires proper screw design and thus a thorough understanding of the transport phenomena governing the extrusion process. Focusing on solid bed breaking screw concepts, this research project will investigate wave- and energy-transfer-sections in detail placing special emphasis on their application in the melting zone of high-speed single-screw extruders. In order to take advantage of their great potential to combine both high output rates and excellent melt quality, new design strategies based on mathematical-physical models will be developed and validated by experimental studies carried out on fast running single- screw extruders. This will yield a sustainable contribution for the development of high-speed extrusion concepts in combination with solid bed breaking screw sections. Hypothesis: The research work will provide mathematical-physical process models needed to optimize wave- and energy-transfer-sections in the melting zone of high-speed single-screw extruders. This will help to meet the high demands made on the single-screw extruder regarding output capacity and melt quality. Methods: Experimental studies will be carried out on both a novel screw simulator allowing the detailed analysis of the melting process in solid bed breaking flow channels and on a conventional as well as a grooved feed high-speed single-screw extruder. To extend existing process models, analytical as well as numerical methods will be applied comprising the network theory and the finite volume method in particular. Innovation and novelty: The few scientific papers dealing with the above mentioned topic still leave many questions unanswered and show that the contents of this proposal are highly relevant. Previous studies have proven that increased output rates caused by an early breaking of the solid bed in high-speed extrusion can be combined with the high requirements in terms of melt quality. Especially the application of wave- and energy-transfer-sections in the melting zone is of high interest, as these zones show the potential to induce solid bed breaking. However, as wave- or energy-transfer-zones are usually found in the melt conveying zone, their application in the melting zone requires new design strategies based on mathematical-physical process models.

The extruder is the most important processing machine in the polymer industry. Based on the economic framework, the primary objective of machine manufacturers is to increase the output rate while guaranteeing excellent melt quality. To meet the ever-increasing demands on the machinery, the process requires further optimization and thus a deeper understanding of the transport mechanisms in the plasticating unit governing physical operation. This research project investigated the operation of so-called wave-dispersion zones that were implemented to allow the extruder to operate at higher output rates without causing excessive temperatures and irregularities in the discharge. The term wave-dispersion zone refers to screw zones consisting of two or more parallel flow channels that oscillate cyclically in depth over a plurality of cycles. With its geometrical configuration, wave-dispersion screws are intended to provide better metering, mixing, and melting characteristics than conventional extruder screws. Despite their recognized performance, very few scientific analyses have examined the flow in wave-dispersion zones. As a result, the screw concepts are still not properly understood and their current designs offer potential for optimization. To close this scientific gap, this research project analyzed the process behavior of wave-dispersion screw both theoretically and experimentally to provide new strategies for the design and optimization of wave-dispersion zones in single-screw extruders. To allow a systematic optimization of the screw geometry, novel mathematical process models were developed. Using various polymer resins, experimental studies were carried out on both a novel extrusion die and on various grooved-feed single-screw extruders. During the experiments, the flow of polymer melts in wave-dispersions zones was systemically investigated by measuring characteristic process parameters such as e.g. screw speed, throughput, pressure behavior, or energy input. These results were then used to optimize existing screw concepts and to develop new design criteria for wave-dispersion zones in single-screw extruders. To predict the complex flow of polymer melts in wave-dispersion zones, we developed an analytical melt-conveying model for polymer melts. The novelty of the melt-conveying model lies in the coupling of the shear-thinning flow behavior of polymer melts and the underlying three-dimensional flow field in the screw channel. Removing the need for numerical simulations, the new model predicts the conveying characteristics of both pressure-generating and pressure-consuming wave-dispersion zones. The high accuracy of the new model was numerically and experimentally confirmed.