Study of thermal homogeneity in single-screw plastication process

Abstract : Single-screw plastication, used in extrusion and in injection molding, is a major way of processing commodity thermoplastics. In injection molding, a high level of reliability is usually achieved that makes this process ideally suited to mass market production. Nonetheless, process fluctuations still appear that make molded part quality control an everyday issue. In this work, a combined modeling of plastication, throughput calculation, and laminar dispersion are used, to investigate how thermal fluctuations could generate and propagate along the screw length and affect the melt homogeneity at the end of the metering section. To do this, we used plastication models to relate changes in processing parameters to changes in the plastication length. Moreover, a simple model of throughput calculation is used to relate the screw geometry, the polymer rheology, and the processing parameters to get a good estimate of the mass flow rate. Hence, we found that the typical residence time in a single screw is around one-tenth of the thermal diffusion timescale. This residence time is too short for the dispersion coefficient to reach a steady state but too long to be able to neglect radial thermal diffusion and resort to a purely convective solution. Therefore, a full diffusion/convection problem has to be solved with a base flow described by the classic pressure and drag velocity field. The major importance of the processing parameters in the breakthrough curve of arbitrary temperature fluctuation at the end of the metering section of the injection molding screw is demonstrated. When the flow back-pressure is high, the temperature fluctuation is spread more evenly with time, whereas a pressure drop in the flow will results in a breakthrough curve which presents a larger peak of fluctuation. A commercial Computational Fluid Dynamics (CFD) software, ANSYS Polyflow, was used to verify the model. Moreover, a thermal and structural analysis has been performed on an existing injection molding barrel, featuring 3 optical glass blocks for visualization, with the aim of analyzing the effect of operating conditions on the safety factor of the existing glass windows. This analysis was performed using ANSYS Workbench (Mechanical), with contact problems included.
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Liangxiao Bu. Study of thermal homogeneity in single-screw plastication process. Materials. Université de Lyon, 2019. English. ⟨NNT : 2019LYSEI014⟩. ⟨tel-02308436⟩

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