(677f) CFD-FEA Analysis for Scale-up of a Twin Screw Extruder Kneading Block Section

The kneading block sections of co-rotating twin screw extruders induce both distributive and dispersive mixing of the components of the fluid stream as the material is sheared and elongated by the clearances between the intermeshing kneading blocks and the barrel walls. Narrow kneading discs tend to provide more dispersive (and consequently distributive) mixing, and are the focus of this study. In practice, the fluid stream will often be a multi-phase mixture where the primary phase is a molten polymer and the added phases may be solid particles, or immiscible fluids. In the current investigation, the fluid steam mixture will be treated as pseudo-homogeneous, high-viscosity, shear-thinning material (i.e., polymeric melt). Neutral (non-forwarding) kneading blocks can be treated as fluid filled, which is the case for the current study.

Although the mixing performance is of obvious importance, this study is focused on the mechanical integrity of the narrow kneading blocks under various operating conditions. If the fluid forces acting on the kneading block lobes is too high, the lobes can bend, twist, or break causing significant damage to the twin screw extruder at significant cost of both capital and production capacity.

This study evaluated multiple methods to simulate the flow and pressure characteristics of a polymeric fluid in the intermeshing co-rotating system at different geometric scales (and resulting disc cantilever lengths) from development scale to production plant. The resulting fluid pressure contours on the kneading discs were applied to the disc surfaces within a finite element analysis to evaluate the structural deformation and overall structural integrity of these kneading elements (by comparing the resulting stresses with the yield stress of the disc material). If the risk of damage is determined to be high, this analysis approach can be used to modify the kneading block design to withstand the anticipated stresses.