Process · Changes the form, not the material

Extrusion

Push a softened material through a shaped hole and it comes out that shape, continuously — pipe, film, fibre, window frame, and the same idea in aluminium.

Material is forced through a die that gives it a constant cross-section, and the product emerges as a continuous length that is then cooled and cut. It is the natural process for anything long and uniform, and it is the opposite trade to moulding: cheap tooling, one profile, unlimited length.

The same principle serves polymers, aluminium, clay and food. A plastic pipe, an aluminium window frame, a brick and a piece of pasta are all extrusions, and the differences are in temperature and in whether the material leaves the die as a melt or a plastic solid.

Uses

Pipe and tubing, window and door profiles, cable insulation, sheet and film — blown film for bags is an extrusion inflated as it leaves the die — synthetic fibre, and the granulation step that turns raw polymer into the pellets every other process takes as input.

In metals: aluminium profiles, which is why aluminium is the metal of window frames and heat sinks. A complex constant cross-section that would be expensive to machine is nearly free to extrude.

History

First patented for lead pipe in 1797 and used through the nineteenth century for lead and for gutta-percha insulation on submarine telegraph cable — which is the first mass application of a polymer extrusion, and it laid the Atlantic cable.

Screw extrusion of thermoplastics arrives in the 1930s, and is the direct ancestor of the injection moulding machine's screw.

Economic significance

Tooling is a fraction of a mould's cost and a die can be changed in an hour, so extrusion is where short runs and custom profiles live. It is also the process that makes the intermediate forms almost everything else starts from — pellets, sheet, film and fibre are extruded before they are anything.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

takes as input

Polyvinyl chloride — material · pipe and window profile, which is most of what PVC is by tonnage and nothing to do with the packaging it is blamed for
Polyethylene — material · blown film for bags and sheet, and pipe — the extruded forms are the majority of polyethylene's use
EPDM — material · the door and window seal profiles on every car, made as continuous lengths and cut to fit
Polyethylene terephthalate — material · as fibre and sheet; the bottle is blow moulded from an injection-moulded preform rather than extruded
Aluminium — element · window frames, heat sinks and structural profile — a complex constant cross-section costs almost nothing to extrude and a great deal to machine
Clay — material · the standard way a brick is formed before firing: a continuous column pushed through a die and wire-cut into units
6000 series aluminium alloy — alloy · the family's defining process — it flows through a die faster than any other aluminium, which is why architectural and structural profile is this alloy

Sources

Wikidata
Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
Material World
Our own writing

Where it comes from, and what it becomes

Follow Extrusion back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

Extrusion → takes as input (window frames, heat sinks and structural profile — a complex constant cross-section costs almost nothing to extrude and a great deal to machine) → Aluminium → is extracted from (via alumina — bauxite is refined by the Bayer process before smelting) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is sourced from (partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt) → Peridotite → is composed of (the mineral that defines the rock and gives it its name — a peridotite is olivine-dominant by definition) → Olivine
Extrusion → takes as input (blown film for bags and sheet, and pipe — the extruded forms are the majority of polyethylene's use) → Polyethylene → is produced by (from ethylene; chain length and branching decide whether it is a milk bottle or a fibre) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
Extrusion → takes as input (pipe and window profile, which is most of what PVC is by tonnage and nothing to do with the packaging it is blamed for) → Polyvinyl chloride → is sourced from (the chlorine arriving from chlor-alkali electrolysis of salt) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as the brine the cell electrolyses, and as the source of the hydrogen that comes off the cathode) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
Extrusion → takes as input (as fibre and sheet; the bottle is blow moulded from an injection-moulded preform rather than extruded) → Polyethylene terephthalate → is produced by (a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
Extrusion → takes as input (the door and window seal profiles on every car, made as continuous lengths and cut to fit) → EPDM → is produced by (ethylene and propylene copolymerised with a Ziegler-Natta catalyst, plus a few per cent of a diene to give vulcanisation something to work with) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
Extrusion → takes as input (the family's defining process — it flows through a die faster than any other aluminium, which is why architectural and structural profile is this alloy) → 6000 series aluminium alloy → is produced by (and the composition window is narrow: the magnesium-to-silicon ratio decides whether the precipitate forms usefully) → Alloying and melting → takes as input (the base metal of both brass and bronze) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite

These are the most distinct paths back. Extrusion can be traced through others besides.