Computational and Experimental Modelling of Transport Phenomena in Single Screw Plasticating Units under Consideration of the Melt Quality
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The present work presents semi-analytical models for the description of flow processes in the plasticating unit of single-screw extruders. It also explains how the melt quality is determined during the screw extrusion process.
In the first part of the study models for the calculation of the melting and metering section are presented. Available calculation models are thereby extended and a new one for barrier sections is introduced. The models are compared mathematically on the basis
of both literature data and experimental results. The melting model is also validated with the help of measurements made by a novel, non-invasive, ultrasound-based system. The enhancements carried out in the metering section calculation are based on the
calculation of the two-dimensional pressure-/throughput behavior of generalized Newtonian fluids and stored Newton-Raphson approximation data. For the calculation of complex barrier screw geometries, a systematic approach based on networks that use
tensor algebra and numerical methods for the calculation of barrier screw geometries in
terms of pressure, mass flow and temperature is presented. The model uses a melt
dominant calculation, whereby the pressure- and drag flow, as well as polymer melting
are taken into account.
In the second part, parameters affecting the melt quality were monitored, checked and
adjusted using selected measurement tools. The influence of process, machine, and
material parameters on polymer melt quality is elucidated using selected examples from
the film and pipe industry. To determine the quality of the material, samples were
characterized inline, online as well as offline. Measuring systems which continuously
monitor the melt quality (elongation die, gas sensor), determine the fiber length and
orientation (X-CT, OCT) and facilitate process optimization (rheology, SEC) were also
dealt with. It is shown that product quality is influenced by many processing parameters, e.g. the pressure, the screw speed, and the temperature, and that a well-designed
plasticating unit increases the throughput without impairing the melt quality and
therefore reduces production costs.