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Modeling and flow simulation of polytetrafluoroethylene (PTFE) paste extrusion Patil, Pramod Dhanaji

Abstract

Constitutive rheological equations are proposed for the paste extrusion of polytetrafluoroethylene (PTFE) that take into account the continuous change of the microstructure during flow, essentially through fibril formation. The mechanism of fibrillation is captured through a microscopic model for a structural parameter,ξ , that is the percentage of fibrillated domains in the paste. This model essentially represents a balance of fibrillated and unfibrillated domains through a first order kinetic differential equation. The rate of fibril formation is assumed to be a function of the strain rate and a flow type parameter, which describes the relative strength of straining and rotation in mixed type flows. The proposed constitutive equation consists of a shear-thinning and a shear-thickening terms, the relative contribution of the two being a function of ξ. To improve the physics of the constitutive equation and in order to develop a truly predictive flow model, another constitutive equation is proposed which consist of a viscous (shear-thinning) and an elastic (strain-hardening) term. A modified Mooney-Rivlin model is used to model the elastic behavior of the paste. The viscous and elastic parameters are determined by using shear and extensional rheometrical data on the paste. Finite element simulations using the proposed constitutive relations predict accurately the variation of the extrusion pressure with the apparent shear rate and die geometrical parameters. An approximate analytical mathematical model for polytetrafluoroethylene (PTFE) paste extrusion through annular dies is also developed. This model takes into account the elastic-plastic and viscous nature of the material in its non-melt state due to the formation of fibrils and presence of lubricant. The radial flow hypothesis (RFH) has been used to describe the flow kinematics of PTFE paste in the conical annular section of the die. The validity of this hypothesis is demonstrated by performing numerical simulations using the developed shear thinning and shear thickening model. Model predictions are presented for various cases and are found to be consistent with experimental results of macroscopic pressure drop measurements in rod and tube extrusion.

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