Effects of strain hardening and strain rate sensitivity of the wire material during drawing under non-Newtonian plastohydrodynamic lubrication conditions

Abstract

This is an experimental and theoretical study of the process of drawing wire through a device based on an adaptation of the Christopherson tube and employing a polymer melt as the lubricating agent. Previous analyses have assumed Newtonian behaviour for the melt producing simplified but useful relationships for the process. The present study utilises an empirical expression, relating shear stress and rate of shear together with an experimentally derived pressure coefficient of viscosity, in determining the coating thickness possible on the wire. On the basis of experimental evidence, it is apparent that deformation commences before the die in the Christopherson tube itself, with the die effectively acting only as a seal. Under these conditions, the die geometry becomes of secondary importance and the deformation actually takes place as if an effective die of continuously changing die angle is being used. In this study a theoretical analysis of the deformation is presented which takes account of material strain hardening and strain-rate sensitivity and assumes a mathematically described effective die shape. Extensive experimental studies have shown that the coating thickness reduces as both speed and wire material strength increase. Predictions of coating thickness from the analysis tend to be lower than those obtained experimentally. The assumed die shape and predicted pressure distributions are also compared with those obtained experimentally.