Look around any room and you will find dozens of injection-moulded parts: the casing of a remote control, a toothbrush handle, a plug, a storage box lid. Injection moulding is one of the most widely used ways to make plastic parts, because once it is set up it can produce the same shape again and again, quickly and accurately.
The basic idea
Plastic pellets are melted, forced under pressure into a closed metal mould, cooled until solid, and then pushed out as a finished part. The mould is a precise negative of the part, often made from hardened steel or aluminium.
The machine
- Hopper: holds the plastic pellets, sometimes mixed with colourant.
- Barrel and screw: heaters and a rotating screw melt and mix the plastic and move it forward.
- Clamping unit: holds the two halves of the mould shut against the injection pressure.
- Ejector system: pins that push the cooled part out of the mould.
One cycle, step by step
- Clamping: the mould closes.
- Injection: molten plastic is pushed in until the cavity is full.
- Holding: pressure is maintained so the part doesn't shrink away from the mould as it cools.
- Cooling: the plastic solidifies; this is often the longest step.
- Ejection: the mould opens and the part is released.
Cycle times can range from a few seconds for small, thin parts to much longer for thick ones.
Common materials
Most injection moulding uses thermoplastics — plastics that soften when heated and harden on cooling — such as polypropylene, polyethylene, ABS, polycarbonate and nylon. Some thermosets can be moulded too, with different equipment. Our explainer on thermoplastics and thermosets covers the difference.
Typical defects and why they happen
- Short shots: the cavity doesn't fill completely.
- Sink marks: small dips where thick sections shrink while cooling.
- Flash: thin excess plastic that escapes along the mould's parting line.
- Warping: uneven cooling bends the part.
Good part design — even wall thickness, draft angles so parts release easily, and rounded corners — avoids many of these problems.
When it makes sense
Moulds are expensive to design and make, so injection moulding is usually chosen for larger production runs where the tooling cost is spread across many parts. For prototypes or small quantities, 3D printing or machining is often more practical.
For a wider view of the materials side, see our pages on innovative plastics and polymer and plastic technology.
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