"Measurement and Numerical Analysis of Mixing and Melt Temperature Homogeneity in a Screw Injection Unit"
Measurement and Numerical Analysis of Mixing and Melt Temperature Homogeneity in a Screw Injection Unit
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In polymer processing both the thermal homogeneity and the optical-mechanical homogeneity (mixing quality) of the polymer melt play a vital role for the part quality. Thus, the quality of the product strongly depends on the ability of the plasticizing unit to generate a homogeneous melt pool in the screw chamber. Previous attempts to accurately measure the real polymer melt temperature in the screw chamber as well as in the screw channels have failed on account of the challenging metrological boundary. In this thesis, a novel ultrasound system was used for the online-measurement of these essential process parameters. The measured ultrasonic velocity between the screw root and barrel wall was combined with pressure measurements to calculate the mean temperature in the screw channel or the chamber in front of the screw. Utilizing the above-described system, axial profiles of the mean temperature in the screw chamber were measured. An application of this measurement system is the development of strategies to minimize the axial temperature gradients in the screw chamber. Besides the well-known method to vary the process parameters screw circumferential velocity and back pressure, a starve-fed mode of operation was used to improve the melt temperature homogeneity of the melt. It was shown that this operation mode provides an additional degree of freedom influence the melt temperature and is superior compared to the variation of the process parameters. To get a deeper insight into the melt flow in polymer processing equipment, numerical methods were used. More concrete, the open source CFD toolbox OpenFOAM® was extended to describe the non-isothermal flow of fluids with non-Newtonian temperature dependent viscosity. Based on these developments the OpenFOAM® toolbox was further extended to calculate measures to characterize mixing in polymer processing.