Viscoelastic Flow Simulation of Polytetrafluoroethylene  (PTFE) Paste Extrusion 

Pramod D. Patil, Isaias Ochoa, James J. Feng & Savvas G. Hatzikiriakos

J. Non-Newtonian Fluid Mech. 153, 25-33 (2008)

Abstract - A viscoelastic constitutive equation is proposed for the paste extrusion of  polytetrafluoroethylene (PTFE) that takes into account the continuous change of the paste  microstructure during flow, through fibril formation. The mechanism of fibrillation is  captured through a microscopic model for a structural parameter, ξ that represents the  percentage of fibrillated domains of the paste. The proposed viscoelastic constitutive  equation consists of a viscous shear-thinning term (Carreau model) and an elastic term  (modified Mooney-Rivlin model), the relative contribution of the two depending on ξ.  The viscous and elastic parameters of the model are determined by using shear and  extensional rheometry on the paste. Finite element simulations based on the proposed  constitutive relation with the measured model parameters, predict reasonably well the  variations of the extrusion pressure with the apparent shear rate and the die geometrical  characteristics.