Extrusion

For the process that creates volcanic rock, see extrusive (geology).

Extrusion is a manufacturing process where a material, often in the form of a billet, is pushed and/or drawn through a die to create long objects of a fixed cross-section. Hollow sections are usually extruded by placing a pin or mandrel in the die, and in some cases positive pressure is applied to the internal cavities through the pin. Extrusion may be continuous (producing indefinitely long material) or semi-continuous (repeatedly producing many shorter pieces). Some materials are hot drawn whilst others may be cold drawn.

Extrusion of a round blank through a die

The feedstock may be forced through the die by various methods: by an auger, which can be single or twin screw, powered by an electric motor; by a ram, driven by hydraulic pressure (for steel alloys and titanium alloys for example), oil pressure (for aluminum ) or in other specialised processes such as rollers inside a perforated drum for the production of many simultaneous streams of material.

Commonly extruded materials are copper (pipe for plumbing), aluminium (various extrusion profiles for tracks, frames, rails, mullions), steel (rod, track), titanium (aircraft components, including seat tracks, engine rings, structural parts), rubber (tire tread and sidewalls), a multitude of plastics (tubing, pipes, rods, rails, seals), and grains such as wheat, corn, and rice for food products.

Steel and titanium extrusions use glass powder as a lubricant. This process was invented in France, in 1935 by Mr Séjournet. This solution was patented throughout the world and helped launch industrial steel extrusion. The process was later applied to titanium.

Plastic extrusion commonly uses plastic chips or pellets, which are dried in a hopper before going to the feed screw. The polymer resin is heated to molten state as it is fed to the extrusion tooling. The die molds the heated mass into the desired shape. The heated mass is cooled and solidified as it is pulled through the die or water tank. In some cases (such as fibre reinforced tubes) the extrudate is pulled through a very long die, in a process called pultrusion.

Extrusion simulation tools help to understand extrusion process and to optimize development of tools and products.

Food extrusion

Extrusion has found a great application in food processing. Various products like pastas, breakfast cereals, and ready to eat snacks are now manufactured by extrusion. Softer foods such as meringue have long been piped using pastry bags.

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