Modelling PTFE paste extrusion: The effect of an objective flow type parameter

Abstract

In a recent article [P.D. Patil, J.J. Feng, S.G. Hatzikiriakos, Constitutive modelling and flow simulation of polytetrafluoroethylene paste extrusion, J. Non-Newtonian Fluid Mech. 139 (2006), 44–53], a rheological constitutive equation was proposed to model the incompressible flow of poly-tetra-fluoro-ethylene (PTFE) paste and its structural development (fibrillation between the PTFE particles) during flow. The constitutive model was a combination of shear-thinning and shear-thickening viscosity terms, depending on a structural parameter, ξ, which obeyed a convective-transport equation. In the latter, a non-objective flow type parameter, ψ, was employed, which depended on the magnitudes of the rate-of-strain and vorticity tensors. In the present work, an objective flow type parameter is used instead [R.G. Larson, Flows of constant stretch history for polymeric materials with power-law distributions of relaxation times, Rheol. Acta 24 (1985), 443–449]. Despite its complexity and the severe slip at the wall, good convergence has been obtained even for very high apparent shear rates. New results are shown as contours of the ξ-parameter in the flow field as well as contours of the new objective flow type parameter, ψ*, and compared with previous results. It is shown that the corrected model predicts higher structural levels (more fibrils) in PTFE paste flow through extrusion dies and that the new results are more consistent with experimental observations.