Extrusion

In this process, the billet to be extruded is held in a container and pushed against a stationary die (direct or forward extrusion), or the die penetrates the billet (indirect or reverse extrusion), either hot or cold. Friction over the container and die surfaces has a decisive influence on material flow and dies pressures.

Only lower-melting alloys, including aluminum alloys, can be extruded hot without a lubricant, and then deformation occurs with shearing the alloy at the die billet interface. Two-dimensional shapes of great complexity and thin walls (e.g., architectural extrusions) can be made at a relatively low cost. In the absence of a lubricant, divided material strains can be reunited and welded in a bridge-type die, allowing extrusions with one or more closed cavities, in a very wide size range (multihole tubes are extruded with walls as thin as 0.25 mm, or 0.010 in.). Hot extrusion of higher-melting temperature alloys requires a high-temperature lubricant, often glass; however the product shapes are limited and the wall thickness is greater. The purpose may be that of producing a two-dimensional semi- manufactured product or, if part of the billet is retained at the starting diameter, that of making near-net-shape three-dimensional products such as large valves and fasteners. The skill of the die designer, in this case, plays an important role in ensuring a sound product.

Cold extrusion is always conducted with an effective lubricant (often with the lubricant superimposed on or reacted with a conversion coating on the metal) and is normally used to create near-net-shape products such as fasteners, automotive components, and so forth. Impact extrusion is a term employed for the cold extrusion of thin walled products such as toothpaste tubes.