A Tailored Modelling Approach to Predict the Three-Dimensional Flow of Polymer Melts in Helical Screw Channels
Sprache des Vortragstitels:
38th International Conference of the Polymer Processing Society
Sprache des Tagungstitel:
High-performance screws such as barrier, double-wave, and energy-transfer screws are becoming more and more popular for the polymer industry to meet the steadily increasing requirements on productivity and melt quality in single screw extrusion. A special feature of these screws are regions with deep channels and multiple flights, where the channel curvature and the flight flanks affect the melt flow significantly. Hence, the traditional flat plate model, which neglects both effects, becomes invalid in these regions. Aiming for a more realistic description of the local flow conditions, we present a novel calculation routine, in which the balance and constitutive equations for creeping flows are formulated in a helical reference frame, scaled to a dimensionless representation, and solved numerically using the finite element method. The modified equations include the three-dimensional flow pattern inside a confined helical screw channel, coupled with the shear thinning behaviour of the melt using a power law model. Focusing on fully developed flow conditions in channels of constant dimensions allows to reduce the flow domain to short segments of unit length. Furthermore, the flow is treated as isothermal and incompressible. As a result, computation becomes faster and requires less memory compared to a full computational fluid dynamics (CFD) simulation. This makes the routine particularly suitable for comprehensive parametric design studies, in which the influencing factors may be varied systematically in a wide range and in numerous levels to derive precise process models using data processing algorithms. By predicting the dimensionless flow rate and viscous dissipation rate for selected industrial use cases, we found a pronounced influence of curvature and aspect ratio in deep channel sections. Additional full CFD simulations support the plausibility of the modelling approach.