A novel technique for tube sinking.

A new technique for tube sinking has been developed which should, in a number of ways, help to solve the problems associated with conventional tube sinking processes, eg die wear and the need for a swaged down leading end for easy insertion through the die. The conventional reduction die is altogether replaced by a die-less reduction unit of stepped bore configuration. The deformation is induced by means of hydrodynamic pressure and drag force generated in the unit due to the motion of the tube through the viscous fluid medium (polymer melt). The dimensions of the die-less reduction unit are such that the smallest bore size is dimensionally greater than the nominal diameter of the undeformed tube, thus metal to metal contact and hence wear, should no longer be a problem. As no conventional reduction die is used, the need for a reduced diameter leading end is also eliminated. Experimental results show that greater reduction in tube diameter and the coating thickness were obtained at slower drawing speeds (about 0.1 m/s). The maximum reduction in diameter noted was about 7 per cent.Analytical models have been developed, assuming with Newtonian and non-Newtonian characteristics of the pressure medium, which enabled prediction of the length of the deformation zone, percentage reduction in diameter and drawing stress. In the non-Newtonian analysis account was taken of the pressure coefficient of viscosity, derived from the available data; the limiting shear stress, which manifests itself as slip in the polymer melt and the strain hardening and the strain rate sensitivity of the tube material.The percentage reduction in diameter predicted using the Newtonian analyses appear to differ considerably from the experimental results both in trend and magnitude. Non-Newtonian analysis predicted theoretical results which are much closer to the ones observed experimentally.