New method to analyze the throughput-pressure behavior with the corrected twin screw channel geometry and the induced drag flow of screw elements by using 3D FEM simulation

The mathematical modeling of the throughput-pressure behavior of the co-rotating twin screw extrusion process was subject of a variety of publications. Due to limited options within the area of numerical simulations, most of the modeling was based on analytical simplifications and adaptions. For that reason, commonly known models for describing the throughput behavior of co-rotating twin screws are based on the concept of the kinematical inversion principle. Within this concept, in comparison to the real kinematic behavior supposing the screws are rotating with a rotational velocity, the barrel is inducing the drag flow with an approximated translational velocity in the system. Further, the channel geometry is transferred to a rectangular shape with a constant channel height and is set in two-dimensional level for a simplified calculation. However, with regards to the geometrical diversity of the co-rotating systems with varying ratios of outer to inner diameter of the screws, the resulting channel geometry indicates different heights which have a significant influence on the inducing drag flow within the process. The model for the melt throughput-pressure behavior allows a variation of general screw and barrel geometry ratios but neglects the significance of the induced drag flow depending on the varying channel height. For that reason, a new method to measure the influence of the non-inverted drag flow by using 3D FEM simulations are presented in this paper. With the presented data, it is possible to extend the conventional model by considering the varying channel height and the correct induction of the drag flow with different screw ratios. The results indicate a systematical deviation between the analytical model and the data of this study. Overall, the consideration of the varying channel height increase the accuracy of the induced drag flow within the calculation of the throughput-pressure gradient.