Modelling the solids inflow and solids conveying of single-screw extruders using the discrete element method

A new solids-conveying model for the single-screw extruder based on the Discrete Element Method (DEM) is proposed in this work. The polymer solids are treated as spherical particles moving in a 3-D environment which includes the feed hopper, the solids-inflow zone, and the solids-conveying region of an extruder, without inclusion of the plug flow assumption common to continuum models. Normal and tangential forces resulting from inelastic collisions with neighboring particles and surfaces dictate how each polymer pellet is conveyed through the model extruder. The DEM technique was implemented in this work to allow fundamental study of the local transport phenomena within the screw channel. Reported in this paper are results examining the cross- and down-channel velocity profile of solids in the screw; the residence time distribution; the cross-channel temperature profile; and the coordination number distribution. Two exit conditions were evaluated by the model: i) the open-discharge case where no compaction of the solids occurred; and ii) the restricted case where the axial pressure increased as the solids flowed towards the barrel exit. The predictions of the DEM simulations allowed for detailed observations of the solids movement in the screw, providing insight into the inherent flow fluctuations of extrusion systems.