Investigation of the influence of material and pellet shape on the dissipation in the solids conveying zone of single-screw extruders based on the discrete element method (DEM)

Abstract

In polymer processing, there are existing currently established models for the conveying and melting behavior of polymer materials. These models record the majority of relevant industry applications well. In the field of conveying solids the models especially relate to Darnell and Mol (1956), who postulate a plug flow in the solid area. The following melting process implies a heat input exclusively from heat conduction at the barrel and from dissipation energy which results from the contact between pellets and screw / barrel. The frictional heat, resulting from relative motion in the solid bed, is being neglected in the models. This is why an overestimation of melting efficiency by the actual models is to be expected. Furthermore, the models do not take the influence of pellet shape and size into consideration, which is why there are expected higher internal frictions from sharp-edged pellets than from round ones with a smooth surface. In terms of cylindrical pellet, alignment and unroll effects play an important role. In order to mathematically describe and analyze complex motions in a solid bed correctly, numerical simulations based on the Discrete Element Method (DEM) have been carried out. In order to do so, a 60 mm solid conveying test stand has been built up in the simulation. The test stand can be fed with pellets of various shapes and sizes. Pellet and material parameters, which have been defined in preliminary investigations and were implemented in the simulation, play a decisive role. Because of the variation of dimensionless geometry and process index, a modelling approach arises, with which now the description of the influence of various pellet sizes and shapes on the dissipation in the solid conveying area is possible for common extruder sizes.