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Concept
Extrusion
Summary
Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex cross-sections; and to work materials that are brittle, because the material encounters only compressive and shear stresses. It also creates excellent surface finish and gives considerable freedom of form in the design process.
Drawing is a similar process, using the tensile strength of the material to pull it through the die. It limits the amount of change that can be performed in one step, so it is limited to simpler shapes, and multiple stages are usually needed. Drawing is the main way to produce wire. Metal bars and tubes are also often drawn.
Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). It can be done with hot or cold material. Commonly extruded materials include metals, polymers, ceramics, concrete, modelling clay, and foodstuffs. Products of extrusion are generally called extrudates.
Also referred to as "hole flanging", hollow cavities within extruded material cannot be produced using a simple flat extrusion die, because there would be no way to support the centre barrier of the die. Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section. The die shape then internally changes along its length into the final shape, with the suspended center pieces supported from the back of the die. The material flows around the supports and fuses to create the desired closed shape.
The extrusion of metals can also increase their strength.
In 1797, Joseph Bramah patented the first extrusion process for making pipe out of soft metals. It involved preheating the metal and then forcing it through a die via a hand-driven plunger. In 1820 Thomas Burr implemented that process for lead pipe, with a hydraulic press (also invented by Joseph Bramah).
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