Injection molding melts a thermoplastic and forces it into a steel or aluminium tool, then repeats the cycle every few seconds for as long as the tool lasts. Almost the entire cost of the process arrives before the first good part exists, which is why a usable quote always carries two numbers: what the tool costs to cut, and what each part costs to shoot.
The two prices, and why one of them is fixed
Tooling is a capital purchase: cavity count, steel grade, side actions and expected life move it far more than the size of the part does. The per part price is material plus a few seconds of machine time plus a share of overhead, and it barely moves with quantity. So the decision is never really about the part price, it is about how many parts will amortise the tool.
Choosing the plastic before the shape
Commodity resins such as polypropylene, polyethylene and polystyrene cover most housings and containers cheaply. Engineering grades such as ABS, nylon, polycarbonate and acetal buy stiffness, impact strength, heat resistance or wear. Glass filling raises stiffness and lowers toughness. Every choice reshapes the part, because shrinkage, wall thickness and warp behaviour differ across the whole list.
What the process demands of the design
Uniform wall thickness, generous draft on every face that leaves the tool, ribs at a fraction of the wall thickness so they do not sink, radii at internal corners, and undercuts either designed out or paid for with a side action. A design that ignores these does not fail loudly. It produces parts with sink marks, warp and short shots that nobody can tune away.
When it is the wrong process
Below a few hundred parts the tool rarely pays for itself, and machining, printing or cast urethane will land the job sooner and cheaper. Very large parts, very thick sections and very short runs all push in the same direction. A good moulder says this early, because a tool cut for a programme that never reaches volume is a loss on both sides.
Questions people ask about plastic injection molding
What is plastic injection molding in one sentence?
Molten plastic is injected under pressure into a closed metal tool, cooled until it holds its shape, and ejected, with the cycle repeating automatically.
How many parts justify a tool?
It depends on the part price gap between moulding and the alternative, but a few thousand is a common crossover for small parts, and complex parts with expensive alternatives can justify a tool much sooner.
Which plastic should I choose?
Start from what the part must survive: temperature, load, impact, chemicals, sunlight and any regulatory requirement. Narrow to a family on that basis, then let cost and moulding behaviour choose the grade.
What is a typical cycle time?
Anything from a few seconds for a thin small part to a minute or more for a thick one, because cooling dominates and thick sections cool slowly. Wall thickness is the biggest lever a designer has on part price.