Numerical Simulation of Wood-Polymer Composites Extrusion

Abstract

The combination of fillers and polymers is used to influence the mechanical, optical and processing properties of the base material and is finding more and more applications. The addition of wood to polymers results in wood-polymer composites (WPC), whereby the wood content can be up to 80 wt. %. This composite is mainly processed to deckings by means of extrusion. The extrusion die is an essential part of the processing procedure, which determines both process parameters and the final product. A special feature of WPC extrusion dies is cooling plate, which leads to partial solidification of the melt before leaving the die. This solidification is necessary to ensure dimensionally stable extrusion without strand breakage. For the design of WPC extrusion dies, numerical simulations are increasingly used in addition to empirical data, whereby OpenFOAM is used in the present investigations. Based on rheological measurements, both the shear thinning flow behavior and the temperature dependence of the viscosity are modeled. The partial solidification of the melt at the cooling plate before the outlet is modeled in a single-phase over a step in viscosity. In addition, the solidified melt slips on the cooling wall, which is also taken into account in the model. In the parallel zone, there is a pure shear flow, while in the transition elements such as the flange to the extruder or at the mandrel, uniand biaxial extension components are dominant due to the crosssectional changes. High-density polyethylen (HDPE) with wood flour exibits a shear thinning and extensional thickening behavior, so that different models are necessary for description. To calculate the stresses, interpolation is performed between the different models with respect to the invariants according to B¨ohme [1], depending on the type of flow present. To validate the numerical simulations, an extrusion die for a square hollow profile with pressure and temperature sensors along the flow direction is subject to experimental measurements. The aim of the numerical simulations is to investigate the flow in more detail and to give optimization hints.

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