Constitutive relations for the extrusion of siloxane gum and silicone rubber

Silicones are widely used in many industries and in many forms, and extrusion is the most common fabrication method used to produce a range of products. However, the material properties and the extrusion process are not well understood and a generalised model including prediction of material properties and process simulations is not available. It is envisioned that computational fluid dynamic (CFD) modelling will be applicable to providing a better description of the extrusion process. Constitutive relations required for a CFD model were developed from viscosity data for silicone gum and rubber compounds. For the generalised Newtonian fluid (GNF) relations, the Yasuda model gave good prediction of the shear viscosity for all materials. Only the non‐linear differential models were capable of describing the viscoelastic behaviour. The Phan Thien–Tanner (PTT) model correctly modelled the viscosity and dynamic moduli for silicone gum, but for the rubber compounds only the viscosities could be adequately predicted. Underprediction was in evidence at low shear rates. The irreversible Papanastasiou–Scriven–Macosko (PSM) form of the Kaye–Bernstein–Kearsley–Zapas (KBKZ) was the most successful of the integral viscoelastic models. Good prediction of shear viscosity and dynamic moduli was achieved for all silicone materials. The first normal stress and extensional behaviour were predicted at high to medium shear rates. The constitutive equations presented here will enable a detailed CFD modelling study to be undertaken of the extrusion process, and this will be presented in a subsequent paper. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.