What is injection moulding?
Injection moulding is a forming technique in which a molten material, usually plastic, is injected under high pressure into a closed mould (the tool or die), after which the part cools and is ejected. Mainly thermoplastics are processed this way, and to a lesser extent thermosets and rubbers. Metals such as aluminium and zinc can be shaped in a similar way; this is usually called die casting. The Dutch term is spuitgieten.
How does it work?
- Melting. The injection moulding machine heats plastic granules in a barrel with a rotating screw until they form a fluid mass.
- Injection. The screw forces the melt under high pressure through the gate into the mould cavity.
- Holding pressure and cooling. Pressure is held briefly to compensate for shrinkage, while cooling channels in the mould let the part solidify.
- Ejection. The mould opens, ejector pins push the part out, and the next cycle starts.
The mould is expensive to make, so injection moulding only pays off for large production runs. In return you get great freedom of shape and every part is identical within tight tolerances.
Application
On site you meet injection-moulded parts everywhere: fittings for plastic pipework, weep vents for brickwork, rebar spacers, wall plugs, clips and cable trunking. Zinc or aluminium door furniture is often die-cast. Pipes and profiles themselves, by contrast, are extruded.
- Use fittings and pipe from the same system and material. A PVC fitting is joined with PVC solvent cement; PP fittings cannot be glued and use a push-fit socket with a rubber seal.
- Check for moulding defects. Flash (thin fins along the mould parting line) can damage a sealing ring. Remove it carefully or use another fitting.
- Mind UV and temperature. Not every plastic part is meant for outdoor use or high temperatures. Check what the manufacturer states on the product or packaging.
- Custom parts are expensive. A bespoke moulded part needs its own mould; for small quantities, 3D printing or machining is usually cheaper.
Common mistake: joining a plastic fitting with the wrong adhesive or without knowing the system. The joint looks fine but starts leaking later.
Related terms
- Plastic
- Mould
- Extrusion
- Blow molding
- PVC
- Spacer
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