In extrusion, a bar or metal is forced from an enclosed cavity via a die orifice by a compressive force applied by a ram. Since there are no tensile forces, high deformations are possible without the risk of fracture of the extruded material. The extruded article has the desired, reduced cross-sectional area, and also has a good surface finish so that further machining is not needed. Extrusion products include rods and tubes with varying degrees of complexity in cross-section.
Examples of metals that can be extruded include lead, tin, aluminium alloys, copper, brass and steel. The minimum cross-sectional dimensions for extruded articles are approximately 3 mm in diameter for steel and 1 mm in diameter for aluminium. Some metals such as lead alloys and brass lend themselves to extrusion rather than drawing or rolling.
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Hot extrusion is carried out at a temperature T of approximately 0.6Tm and the pressures required range from 35 to 700 MPa. Under these demanding conditions, a lubricant is required to protect the die. Oil and graphite lubricants function well at temperatures up to 150°C, but borate glass or hexagonal boron nitride powders are favoured at higher temperatures where carbon-based lubricants oxidise.
Cold extrusion is performed at temperatures significantly below the melting temperature of the alloy being deformed, and generally at room temperature. The process can be used for most materials, provided that sufficiently robust machinery can be designed. Products of cold extrusion include aluminium cans, collapsible tubes and gear blanks.