Process · Changes the form, not the material

Injection moulding

Melt, inject, cool, eject — in seconds, identically, a hundred million times. How most plastic objects come to exist.

Plastic granules are melted in a heated barrel by a rotating screw, the screw is driven forward as a ram to inject the melt into a closed steel mould at high pressure, the part cools until rigid, and the mould opens and ejects it. Cycle times run from a couple of seconds for a bottle cap to a minute or two for a thick part.

The cost structure is what defines it. The mould is expensive, precise and slow to make; each part afterwards is close to the cost of its material and the seconds of machine time. Nothing else makes a complex plastic shape so cheaply at volume, and nothing else is so unforgiving of low volume.

Uses

Almost every rigid plastic object of any complexity: bottle caps, housings, connectors, syringes, crates, toys, gears, car interior parts. The LEGO brick is the standard demonstration — moulded to a few micrometres of tolerance so that bricks made decades apart still clutch.

It shapes thermoplastics rather than thermosets, because the process depends on the material melting and re-solidifying without changing chemically.

History

Patented in 1872 by John Wesley Hyatt for celluloid, and constrained for decades by the plunger machines of the time. The screw-injection machine, developed in the 1940s, is what made it general: a screw both melts the polymer by shearing it and meters the shot, which a plunger cannot do evenly.

Economic significance

The economics of the tool decide the economics of the product. A steel mould for a mass-market part costs as much as a house and pays back over millions of shots, which is why plastic products are cheap in volume and why changing a moulded design is a capital decision rather than a drawing change.

That asymmetry shapes what gets designed. It rewards long production runs and standardisation, penalises variety, and is the reason 3D printing found its market in the volumes below the point where a mould can be justified rather than in competition with moulding itself.

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

ABS — material · the standard material for a moulded housing, and the LEGO brick's tolerance is the demonstration of what the process can hold
Polypropylene — material · closures, tubs and living hinges — polypropylene's fatigue resistance is why a moulded hinge can flex a million times
Polystyrene — material · cheap rigid mouldings, and the reason so much disposable product was polystyrene before it was polypropylene
POM — material · small precision parts — gears, clips and cams — where dimensional stability is the requirement
Polycarbonate — material · headlamp lenses and optical discs, where the process has to reproduce a surface accurately enough to work optically
PEEK — material · at melt temperatures around 400 °C, which puts the tooling and the machine beyond ordinary specification
Plastic — material · which is how most rigid plastic objects come to exist

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 Injection moulding 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

Injection moulding → takes as input (closures, tubs and living hinges — polypropylene's fatigue resistance is why a moulded hinge can flex a million times) → Polypropylene → is produced by (polymerised from propylene with a Ziegler-Natta catalyst, which is what controls the arrangement of the side groups and so whether the product is a structural plastic or a gum) → 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
Injection moulding → takes as input (cheap rigid mouldings, and the reason so much disposable product was polystyrene before it was polypropylene) → Polystyrene → is produced by (straightforward addition polymerisation of styrene) → 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
Injection moulding → takes as input (the standard material for a moulded housing, and the LEGO brick's tolerance is the demonstration of what the process can hold) → ABS → is produced by (styrene and acrylonitrile polymerised in the presence of polybutadiene rubber, so the rubber phase is grafted in rather than blended) → 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
Injection moulding → takes as input (small precision parts — gears, clips and cams — where dimensional stability is the requirement) → POM → is produced by (formaldehyde polymerised and then end-capped — the capping is the invention, because an uncapped chain unzips from its ends) → 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
Injection moulding → takes as input (headlamp lenses and optical discs, where the process has to reproduce a surface accurately enough to work optically) → Polycarbonate → is produced by (from bisphenol A and a carbonate source — and the bisphenol A is the material's standing regulatory problem) → 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
Injection moulding → takes as input (at melt temperatures around 400 °C, which puts the tooling and the machine beyond ordinary specification) → PEEK → is produced by (a step-growth polymerisation between aromatic monomers at high temperature in a polar solvent) → 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

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