 | This article contains promotional content. (November 2026) |
Injection moulding (also spelled injection molding) is a way of making plastic parts by pushing melted plastic into a closed metal mould. The plastic cools and hardens, the mould opens, and the part is pushed out. The same mould is then used again for the next part. This is why the method suits items made in large numbers that must match each other closely, such as bottle caps, switch housings and car interior parts.
Process
Before any plastic is melted, the part and the mould have to exist. A designer checks the part drawing for problems such as uneven wall thickness, missing draft angles or awkward gate positions. This check is called a design for manufacturability (DFM) review. The mould is then machined from tool steel, with channels for cooling water and pins to eject the part.[1]
Once the mould is ready, each part goes through the same cycle:
- Melting. Plastic pellets fall from a hopper into a heated barrel. A rotating screw melts them and moves the melt forward.
- Clamping. The two halves of the mould are closed and held shut against the pressure of the melt.
- Injection. The screw pushes the melt through the sprue, runner and gate into the cavity.
- Packing and holding. Plastic shrinks as it cools, so extra melt is pushed in to make up for it. Pressure is held until the gate freezes.
- Cooling. The part cools in the mould until it is rigid enough to handle.
- Ejection. The mould opens and the part is pushed out. In many moulds the sprue and runner come out attached and are trimmed off.
Some sources group these steps differently, into four to six stages.[1]
Materials
Some resins, including nylon, PET and polycarbonate, absorb moisture. If they are not dried first, the water damages the polymer in the hot barrel. The result is weaker parts with silver streaks on the surface, called splay. Polyethylene and polypropylene take up very little moisture and usually go into the hopper as supplied.[1]
Common defects
- Flash: a thin film of plastic at the seam, often from too little clamping force.
- Short shot: the cavity does not fill completely.
- Sink marks: small dents on the surface, linked to packing and cooling.
- Warpage: the part bends as it cools unevenly. Cooling time rises roughly with the square of wall thickness, so thick or uneven walls make it worse.[1]
References
- ↑ 1.0 1.1 1.2 1.3 "Injection Molding Process Steps: Pellets to Parts". Radiant Design. Retrieved 11 November 2026.
External links
- Radiant Design, an injection mold manufacturer in India
- Injection molding process steps explained