Extrusion is a way of producing metal and plastic components of constant cross-section (e.g., tubes, sheets) by forcing the material through a die. In cold extrusion a billet of metal is surrounded by liquid lubricant in a suitable chamber; hydrostatic pressure is increased, forcing the metal through orifices at one end of the chamber. In hot extrusion, used for plastics, the material is melted throughout before being forced through a die and rapidly cooled.