Solids compaction and conveying in a single-screw extruder by discrete particle modeling

To improve our understanding of granular flow in the solids-conveying zone of a single-screw extruder, a discrete particle model has been developed. The discrete particle approach allowed us to simulate polymer compaction without using the isotropic stress assumption used by continuum models. An elastic-fully plastic contact force displacement model was validated by simulating HDPE and PS under compaction in a batch cell and comparing against reported data in the literature. The particle model and experimental results showed good agreement for PS which exhibited elastic behavior up to the tested limit of 12 MPa. The simulation for HDPE showed marked deviation above 4 MPa, over predicting the plastic dissipative losses. The extruder simulations showed exponential pressure development along the axial length of the screw, similar to continuum models, but indicated that bulk density in the screw was affected by attributes of the flow field besides just pressure. The non-isotropic stress distribution of the solids in the screw was examined, with the stresses found to be concentrated at different boundaries depending on pressure development.