The process of hydrostatic extrusion is known to have many advantages as compared with conventional lubricated extrusion. The large amount of oil subject to high pressure brings about several important process problems including stick-slip effect and large energy loss of fluid.
The present study is concerned with hydrofilm extrusion in which the amount of oil is limited to a minimum to eliminate the drawbacks in conventional hydrostatic extrusion. For the analysis of hydrofilm extrusion a modified upper-bound theory is used in combination with the theory of hydrodynamic lubrication. According to the concept of energy minimization the process parameters are optimized such that the total energy composed of metal deformation energy and fluid shear energy dissipation is minimized with respect to the chosen parameters. As a die profile to be optimized, a fourth-order polynomial is chosen both for computation and experiment. Strain-hardening effect is incorporated in the analysis of metal deformation, and variation of viscosity due to pressure change is taken into account in the fluid analysis.
The study is structured in three stages according to the product shape.
(1) Hydrofilm extrusion of solid round rods.
(2) Hydrofilm extrusion of circular tubes.
(3) Hydrofilm extrusion of elliptic sections. The proposed theory can predict the pressure distribution of fluid at the die surface and film thickness at the outlet as well as the mean extrusion pressure. The optimal die profile can also be determined at the minimum extrusion pressure.
In order to confirm the validity of the proposed theory, experiments are carried out for each model. Billets of mild steel (AISI 1015) are extruded using the extrusion set-up at room temperature and castor oil is used as the lubricant and pressure-transmitting medium. The theoretical extrusion pressures are in good agreement with experimental results. The extruded products show beautiful surface finish with no defects found on the s...