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.