Extrusion shapes metal, plastic and pasta the same way you squeeze toothpaste
Push soft material through a shaped opening and it comes out with that exact profile, as long as you like. Extrusion makes window frames, pipes, tubes and even food this way. When the process first worked on lead pipe in 1820, people called it squirting.
The method's great strengths are that it can form very intricate cross-sections and handle brittle materials, because the material is only squeezed, never stretched. Its cousin, drawing, pulls material through a die instead, which limits how much the shape can change in one pass; drawing is how most wire is made. Extrusion works on metals, plastics, ceramics, concrete, modelling clay and foodstuffs, and it can run continuously or produce batches of separate pieces. Squeezing metal this way can even make it stronger.
Joseph Bramah patented the first extrusion process in 1797, forcing preheated soft metal through a die with a hand plunger to make pipe. In 1820 Thomas Burr applied it to lead pipe using a hydraulic press, another Bramah invention. Alexander Dick extended it to copper and brass in 1894, and steel followed in 1951. Newer variants include friction extrusion, patented by Britain's Welding Institute in 1991, and micro-extrusion, which yields parts slim enough to pass through a square one millimetre wide.
In a typical run, a heated billet goes into the press, a ram pushes against a dummy block behind it, and the profile emerges from the die, after which it is stretched straight. Hot extrusion takes place above the temperature at which the metal recrystallises, keeping it soft. Cold extrusion near room temperature avoids surface oxidation and gives stronger parts, tighter tolerances and a better finish; it produces items such as collapsible tubes, fire extinguisher cases and shock absorber cylinders.
Hollow sections need special dies with depth, or a mandrel inside a hollow billet. A key measure is the extrusion ratio, the starting cross-section divided by the finished one, and the process can achieve very high ratios while still making good parts.
Source: Extrusion
