Numerical simulation of flow processes of wood‐polymer in extrusion dies

Fabian Liese,Olaf Wünsch

doi:10.1002/pamm.202000221

Abstract

AbstractThe addition of wood to polymers results in wood‐polymer composites (WPC), whereby the wood content can be up to 80 %. This composite is mainly processed in extruded deckings. The extrusion die is an essential part of the process, which determines both process parameters and the final product. A characteristic of WPC extrusion dies is a partial solidification of the melt before leaving the die. This solidification is necessary to ensure a dimensionally stable extrusion. For the design of WPC extrusion dies numerical flow field calculations are increasingly used. Based on rheological measurements, both the shear thinning flow behaviour and the temperature dependence of the viscosity are modelled. In the parallel zone a pure shear flow is present, while in the transition elements such as the flange to the extruder or the mandrel due to the cross‐sectional changes strain rates are dominant. High‐density polyethylene (HDPE) with wood fibers has shear thinning and strain thickening behavior, so different models are needed to describe it. The results of the numerical simulations are compared with experimental values of an extrusion die for a square hollow profile.

Highlights

The market for wood-polymer composites has been growing for years, so that the design of the extrusion dies plays an increasingly important role
Simulation methods are already often used for standard polymers, but rarely for wood-polymer composites (WPC), so that cost- and time-intensive design based on experience is still common
In order to determine the influence of strain viscosity in the extrusion die, numerical simulations are carried out and compared with experimental data of the pressure at the points sp0–sp3, which were collected by SKZ - Das Kunststoffzentrum

Summary

Introduction

The market for wood-polymer composites has been growing for years, so that the design of the extrusion dies plays an increasingly important role. Simulation methods are already often used for standard polymers, but rarely for WPC, so that cost- and time-intensive design based on experience is still common. The shear-thinning behavior in material modeling is considered in the literature [1]. In an extrusion die there are shear-dominant flow areas such as the parallel zone and strain-dominant areas, especially at the transition areas to the extruder and in front of the mandrel. Due to the strain-thickening behaviour, viscosity is increased in these areas, which will be investigated in more detail in this work

Material model

Numerical simulations

Results and discussion