Studies on single-screw extrusion of wood-polymer composites with yield stress and slip effects

Abstract

Studies on rheology and processing of wood-polymer composites are very limited in the literature [1]. These are pseudoplastic and viscoelastic materials that exhibit yield stress and wall slipping. Slip effects and flow with yield stress have been investigated numerically for single screw extrusion using ANSYS-Polyflow software. Fully three-dimensional non-Newtonian analysis has been performed both in the extruder (slip on the screw/barrel surfaces) and in the die. The model of Ostwald-de Waele has been used for flow modeling, the generalized law of Navier has been applied for a slip analysis, and Bingham law has been used for an analysis of the flow with yield stress. An effect of the parameters of these models on the flow in the extruder/die system has been studied. Screw pumping characteristics and die characteristics (i.e. flow rate vs. pressure gradient) have been computed and implemented into the recently developed [2] global model of the single screw extrusion process of wood-polymer composites. This model allows to predict various process parameters, e.g. flow rate, pressure and temperature profiles, melting profile. The effect of slipping and yield stress on the extruder operation, i.e. location of the operating point on the screw/die characteristics has been discussed.