Enhanced Hydrodynamic Lubrication in Hydrostatic Extrusion Using a Modified Die

A conical die with a short, thin wedge inlet was designed to improve lubrication in hydrostatic extrusion. An analysis for the die indicates a critical billet speed above which hydrodynamic lubrication prevails. The extrusion pressure — speed variation is found to be very similar to the friction characteristic of a journal bearing. The extrusion pressure has a minimum value at a speed higher than the critical speed. At speeds above that for minimum extrusion pressure the process would be free of the stick-slip instability associated with simple conical dies at low speeds. Effects of the fluid properties, yield stress, work-hardening and the extrusion ratio are discussed. Lubrication with this die is compared with that of a simple conical die. Experiments using a modified die indicate improved lubrication in hydrostatic extrusion.