Hydrodynamic lubrication and coating of wire using a polymer melt during drawing process.

A device based on an adaptation of the Christopherson tube is investigated for the lubrication and other effects of employing a polymer melt as the lubricant during the wire drawing process. The device is heated to convert the polymer feed into a viscous melt and the pressure required is generated by a hydrodynamic action produced by the motion of the wire. On the basis of experimental evidence, it is apparent that deformation commences before the wire reaches 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. To take this aspect of the process into account, a mathematically described effective die shape is used in the present analysis. The plastic strain hardening properties and the strain rate sensitivity of the wire material are also incorporated into the analysis. The study utilises an empirical expression relating shear stress and rate of shear together with an experimentally derived pressure coefficient of viscosity, in determining the coat thickness possible on the wire. The theory contains the effect of a limiting value to the shear stress, which exhibits itself as slip in the polymer. An alternative theory is also presented which assumes that shear stress is zero at the polymer/tube interface. This much simplified analysis allows the length of the deformation zone to be determined. An extensive series of experimental studies have shown that the coat thickness reduces both as speed increases and as the wire material strength increases. Predictions of coat thickness from the analysis tend to be lower than those obtained experimentally. At low drawing speeds a coat defect was observed which gave the coated wire a "bamboo" shape. It is probable that this defect is caused by the slip-stick nature of the polymer melt in the Christopherson tube. The assumed die shape and predicted pressure distributions are verified by experiment.