extrusion

Extrusion is squeezing metal through a shaped hole to give it a cross-section, exactly like pushing toothpaste out of a tube or Play-Doh through a star-shaped nozzle. Whatever the opening in the die looks like, the metal comes out with that profile, and it keeps coming as long as you push, so you get long lengths of constant cross-section. The aluminium frame of a window, a heat sink with its fins, a curtain rail, a hollow tube: all are extrusions cut to length.

A heated billet is loaded into a chamber and a ram forces it through a steel die whose opening is the part's cross-section. In direct extrusion the ram and metal move the same way; in indirect extrusion the die moves onto a stationary billet to reduce friction. Aluminium is the classic extrusion metal because it is soft when hot and flows readily; you can make astonishingly complex hollow shapes with internal webs in a single pass, which would be very expensive to machine.

As with forging and rolling, the deformation aligns the grain structure along the push direction, so an extrusion is anisotropic, strongest along its length. That is usually welcome because extrusions are loaded along their length. The honest limits are that extrusion makes only constant cross-sections (you cannot vary the profile along the length), and thin, sharp, or wildly asymmetric shapes are hard to fill evenly. Note that polymers are extruded too, for pipe and film, but by melting rather than plastic flow of a solid.