Injection molding turns a hopper of plastic granules into millions of identical parts. It is the workhorse of mass production — and like most workhorses, it rewards anyone who understands the basics. This article walks through the cycle, the materials and the volumes where the process makes sense.
The four-stage cycle
Every molded part passes through the same loop, usually in 15 to 60 seconds:
- Clamp. The mold halves close under pressure — up to 3,000 tons on the largest machines, which is exactly the range we run in our workshop.
- Inject. Melted polymer is pushed into the cavity at high speed and pressure.
- Cool. The part solidifies. Cooling takes most of the cycle, which is why wall thickness drives cost.
- Eject. The mold opens and pins push the part out, ready for the next shot.
Choosing a thermoplastic
The material decision shapes everything downstream. Commodity resins such as polypropylene and ABS are cheap, forgiving and fine for housings and brackets. Engineering resins — nylon, PBT, POM, PC/ABS — add strength, heat resistance or chemical resistance at a higher price. The right choice is a balance between performance, cosmetic requirements and cost, and it should be locked before tooling begins.
Where injection molding pays off
Molds are a capital investment, so the process earns its keep in volume. Above roughly 5,000 to 10,000 parts a year, the per-part economics usually beat machining or printing — and above a million, nothing else comes close. If you are unsure whether your part belongs in an injection mold at all, a manufacturability review will tell you in days, not months.
Injection molding is one of the four core technologies we run alongside die casting and stamping — which means one accountable partner from design to serial production.