Extrusion is the process by which long straight metal parts can be produced. The cross-sections that can be produced vary from solid round, rectangular, to L shapes, T shapes. Tubes and many other different types. Extrusion is done by squeezing metal in a closed cavity through a tool, known as a die using either a mechanical or hydraulic press.

Extrusion produces compressive and shear forces in the stock. No tensile is produced, which makes high deformation possible without tearing the metal. The cavity in which the raw material is contained is lined with a wear resistant material. This can withstand the high radial loads that are created when the material is pushed the die.

In cold extrusion, which is used for the manufacture of special sections and hollow articles, the material is generally made to flow in the cold condition by the application of high pressure, which forces it through the cavity enclosed between a punch and a die. A distinction is to be made between forward extrusion (Fig.2), in which the extruded metal flows in the direction of movement of the punch, and backward extrusion (Fig.1), which is characterized by the opposite direction of flow. The two techniques may be used in simultaneous combination (Figs 3 & 4), Cold extrusion can be used with any material that possesses adequate cold work–ability; e.g., tin, zinc, copper and its alloys, aluminum, and its alloys and it is for these metals that the process is most widely adopted. Low carbon soft-annealed steel can also be cold-extruded. If the product cannot be fully shaped in a single operation, the extrusion process may be performed in several stages. The solid or hollow products that can be made by cold extrusion are relatively limited in size.

Initially cold extrusions are produced consists of round blanks, lengths cut from bars, or specially performed blanks. The punches and dies used in cold extrusion are subject to severe working conditions and are made of wear-resistant tools steels; e.g., high-alloy chromium steels. To reduce friction, the tool surfaces are polished. In the cold extrusion of steel the blank may additionally be given a phosphate coating to minimize friction.

A widely used cold extrusion method is impact extrusion. It is used for making collapsible tubes of lead, tin, zinc and aluminum. E.g., toothpaste tubes. Zinc battery cans are also produced in this way. A pointed punch descends swiftly on to a disc-shaped blank arranged in a die, causing the metal to flow upwards and around the punch.