Metal extrusion is a manufacturing process in which a billet is forced through a shaped die to produce a component with a substantially constant cross-section. The process is widely used to manufacture pipes, tubes, structural sections and complex engineering profiles.
Extrusion is particularly important in the production of aluminium and copper-alloy components, although a much wider range of metals can be extruded using suitable process temperatures, equipment and tooling. Extrusions may be solid or hollow and can incorporate complex cross-sectional features which would otherwise require several manufacturing operations.
Direct extrusion, also known as forward extrusion, is the most conventional extrusion arrangement. The billet is placed inside a container and a ram pushes it towards a stationary die at the opposite end of the press. The extruded material flows through the die in the same general direction as the movement of the ram.
As the billet moves relative to the container wall, friction occurs between the billet and container. This friction contributes to the force required during the extrusion operation.
In indirect extrusion, also known as backward extrusion, the die moves relative to the billet and the extruded material flows in the opposite direction to the movement of the die and supporting stem.
Because there is substantially less relative movement between the billet and the container wall, container friction is reduced compared with direct extrusion. This can reduce the extrusion force required for an otherwise comparable operation.
The arrangement has practical limitations associated with supporting the die and accommodating the emerging extrusion, which is one reason direct extrusion remains widely used.
Hot extrusion is carried out with the billet at an elevated temperature. Heating reduces the flow stress of the material and allows substantial plastic deformation to take place with lower forming forces than would normally be required for cold working.
The billet temperature, die temperature, extrusion speed and lubrication must be controlled appropriately for the material being processed. Excessive temperature or unsuitable processing conditions can adversely affect surface quality, dimensional control and material properties.
Warm extrusion is performed at an intermediate temperature and can provide a compromise between the forming-force requirements of cold extrusion and some of the dimensional and surface limitations associated with hot working.
Cold extrusion is performed without heating the workpiece to conventional hot-working temperatures. For suitable materials and component geometries it can provide good dimensional accuracy and surface finish.
Cold extrusion requires greater forming forces and places substantial loads on the dies and production equipment. The material must also possess sufficient ductility for the required deformation.
Aluminium and aluminium alloys are particularly well suited to extrusion and are extensively used to produce complex structural and engineering profiles. Copper and copper alloys are also commonly extruded, including the production of tube, bar and engineering sections.
Other metals, including magnesium alloys, steels, titanium alloys and other engineering materials, can also be extruded where appropriate equipment, tooling, lubrication and processing temperatures are available. The difficulty and economics of extrusion vary substantially between materials.
Extruded sections may be solid, semi-hollow or hollow depending on the material and die arrangement. Relatively complicated profiles can be produced because the cross-sectional geometry is principally established by the extrusion die.
Typical extruded products include:
This ability to incorporate functional features into the cross-section can reduce the need for subsequent machining, fabrication and assembly.
Industrial extrusion presses use substantial compressive force to push or otherwise force the billet through the die. Hydraulic presses are widely used, particularly where high forces and controlled ram movement are required.
Presses may be arranged horizontally or vertically, although horizontal arrangements are common for the production of long extruded sections. The required press force depends on factors including billet material, billet diameter, processing temperature, extrusion ratio, friction, die geometry and extrusion speed.
An important parameter in extrusion is the relationship between the initial cross-sectional area of the billet and the cross-sectional area of the resulting extrusion. This is commonly expressed as the extrusion ratio.
A greater reduction in cross-sectional area generally requires greater material deformation. Extrusion ratio therefore influences the required force, material flow, process conditions and the practicality of producing a particular profile.
Extrusion can produce complicated cross-sections economically, but a profile should still be designed with the behaviour of the material and the manufacture of the die in mind.
Important considerations include:
A well-designed extrusion can significantly reduce the amount of material which must subsequently be removed by machining. Grooves, ribs, channels and other features can often be incorporated into the extruded profile, leaving only local precision features to be machined.
Bearing locations, threaded holes, precision bores, sealing faces and other features requiring close tolerances will normally require appropriate secondary manufacturing operations.
The engineer should therefore consider the extrusion and subsequent machining operations together rather than designing the extruded section in isolation.
The development of extrusion as an industrial process dates back more than two centuries. Joseph Bramah patented a method of producing lead pipe in 1797 in which soft metal was forced through a die. Subsequent development of extrusion equipment and processes enabled the technique to be applied to an increasingly wide range of metals and engineering products.
Metal extrusion is a forming process in which metal is forced through a shaped die to produce a length having a substantially constant cross-section.
In direct extrusion the ram pushes the billet towards a stationary die and the billet moves relative to the container. In indirect extrusion the die moves relative to the billet and there is substantially less relative movement between the billet and container wall, reducing container friction.
Aluminium and copper alloys are extensively extruded, while magnesium, steel, titanium and other metals can also be processed using suitable equipment, temperatures, dies and process conditions.
Extrusion can produce long and relatively complex profiles efficiently, including solid and hollow sections. Incorporating functional geometry directly into the cross-section can also reduce subsequent machining and fabrication.