Discrete element simulation of the solids conveying zone of a single-screw extruder.

摘要

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 that 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. The DEM technique was implemented in this work to allow fundamental study of the local transport phenomena within the screw channel. 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.; To improve application of DEM to polymer processing applications it was necessary to evaluate the contact mechanics of a selection of commonly used polymers. The contact behaviours of high-density polyethylene (HDPE), polystyrene (PS) and polycarbonate (PC) were revealed from a series of impact studies where spherical polymeric particles struck a steel anvil at various angles of incidence and impact velocity. The coefficients of restitution and friction were calculated from high-speed video analysis of individual impacts. The collected data was used to evaluate the relevance of several popular normal contact force-displacement models to determine their suitability for the tested semi-crystalline and glassy polymers. The influence of the normal force-displacement models on the solids transport zone of an extruder was subsequently discussed based on the findings from the impact study.; To improve our understanding of the heat transfer within granular polymer beds, both experimental trials and numerical simulations are presented for systems with and without shear. Both amorphous polymer, polystyrene (PS), and a semi-crystalline polymer, high-density polyethylene (HDPE) of varying particle size from 6mm to 25mm were examined. The estimated effective thermal conductivity data from the experimental trials was found to fit two simple semi-empirical models allowing one to readily approximate the property based on porosity of the bed and solid thermal properties. Discrete particle simulations, i.e. DEM, were used to recognize the importance of heat transport through the stagnant interstitial gas phase in comparison to particle-particle conduction. The results underscore the importance of including an effective thermal conductivity for granular solids in an extruder when using continuum models.; The compaction of polymer spheres within the solids conveying zone of a single-screw extruder and a cylindrical test cell was simulated with a three-dimensional Discrete Element Method. The simulations demonstrated emergent behaviour, such as an exponential pressure rise in the screw channel, and variable pressure ratios, in addition to providing reasonable predictions for the mass flow rate. Simulations in the cylindrical test cell provided validation of the force-displacement model chosen for this work by comparing the simulated pressure-density curves with data from the literature.