In hydro-static extrusion the billet is smaller in diameter than the chamber, which is filled with a fluid and the pressure is transmitted to the billet by a ram. Billet does not come in contact with the piston and the material is deformed due to development of hydro-static pressure within the fluid. Since hydro-static pressure increases the ductility, very good quality products can be extruded by this process. In this process large hydro-static pressure applied to the billet, the product in the receiving chamber is maintained under a lower pressure. There is no friction to overcome along the container walls.
In Hydro-static extrusion, the billet in the container is extruded through the die by the action of a liquid pressure medium rather than by direct application of the load with a ram. The process of pure hydro-static extrusion differs from conventional extrusion in that the billet is completely surrounded by a fluid, which is sealed off and is pressurized sufficiently to extrude the billet through the die. Hydro-static extrusion can be done hot, warm, or cold and can be used to extrude brittle materials that cannot be processed by conventional extrusion Hydro-static extrusion also allows greater reductions in area (higher extrusion ratios) than either cold or conventional hot extrusion. The primary advantages of simple hydro-static extrusion over conventional hot or cold extrusion methods are:
- There is no friction between the billet and the container. Therefore, the pressure at the beginning of extrusion is much lower, and billets of any length can theoretically be extruded.
- Friction at the die can be significantly reduced by a film of pressurized lubricant between the deforming metal and the die surface.
- The lower extrusion pressures and the reduced die friction of hydro-static extrusion allow the use of either higher extrusion ratios or lower extrusion temperatures.
- The uniform hydro-static pressure in the container means that billets do not have to be straight colled wire can also be extruded.
Limitations of the hydro-static extrusion process include:
- Containment of the fluid under high pressure (up to 2 GPa, or 290 ksi) requires reliable seals between the container bore surface and both the ram and die. The technology required to achieve dependable seals at these points is widely available. However, also sealing between the billet nose and the die can easily be achieved by chamfering or tapering the billet nose to match the entry angle of the die.
- In addition to being tapered to match the die opening angle, the billet is also usually machined all over to remove surface defects that would otherwise reappear on the extruded product. This is especially true when cast billets are being used.Development of Non Traditional Machining Processes