Sourcing Custom Plastic Injection Moulded Parts From China: Tooling, Tolerances and the Questions Nobody Asks First
The first injection moulding quote I ever got from China was 3,800 USD for the tool. The second, for the identical drawing, was 11,500 USD. Same part, same resin, same city. If you do not know what drives the number, you are just picking the middle figure and hoping.
So here is what actually moves a plastic tooling quote, in the order it matters.
Tooling steel and cavity count
The price is dominated by two decisions you make, not the factory. The first is the steel. P20 pre-hardened is the standard workhorse for most consumer parts and is fine for runs in the tens of thousands to low hundreds of thousands. S136 or 718H stainless is what you pay for when the resin is corrosive (PVC, flame-retardant grades) or when you need optical clarity and long life. A quote using P20 will look cheap; ask what steel is in it, and for a hardness check.
The second is cavities. One cavity makes one part per cycle. A 2-cavity or 4-cavity tool roughly doubles or quadruples output per cycle but the tool cost climbs too, and so does the tonnage of the press you need. For a first run of 5,000 parts, a single-cavity family tool is often the sane choice. For 500,000 parts a year, the multi-cavity tool pays for itself within a couple of months. This is a real financial calculation, and it depends on your volume forecast more than anything the factory says.
Hot runner or cold runner
Cold runners create sprues that must be trimmed and often reground. Hot runner systems eliminate that waste and improve cycle consistency, but add tooling cost and a controller. On high-value engineering resins (PC, POM, glass-filled nylon), the reduced waste usually justifies hot runner. On cheap commodity parts, it often does not. Ask which one is quoted, because a cheap quote is almost always a cold runner.
Tolerances: the part that ends up costing you
Plastic shrinks as it cools, and it shrinks differently in the direction of flow versus across it. A supplier quoting ±0.05mm across a 100mm part in glass-filled nylon is either very good or not being honest. General moulding tolerances follow roughly DIN 16742 or a supplier's own table; the finer the tolerance, the more the tool has to be built for it and the higher the reject rate, which shows up in unit price.
Draft angles are non-negotiable. No draft means the part sticks, the ejector pins scar the surface, and you get short shots. If your CAD has vertical walls that should have draft, the factory may quietly add it — and now your part does not fit. Design the draft in, or discuss it up front.
Who owns the mould
This is the question buyers forget, and it is the expensive one. If you paid for the tool, you own it — but if it sits in the supplier's plant, you own an object you cannot easily retrieve. Get it in writing that the mould is your property, that you may take it with a written notice, and ideally keep a tooling certificate with the mould's serial number and photos. If you ever want to move production, a mould you cannot get back is a very expensive hostage.
Every year, buyers lose tooling to suppliers who "forgot" who paid for it. Paper trails are cheap. Re-cutting a mould is not.
Resin grade is the quiet margin
PP is not PP. A virgin PP homopolymer and a regrind-heavy blend behave differently, colour differently and last differently. Insist the resin grade and brand be stated on the spec — many factories will use whatever is cheapest that week unless you pin it down. If you need UV stability, food contact, or flame retardance, that is a specific grade with a specific additive package, not a generic one.
What to send the factory
- A STEP or IGES file, not just a PDF drawing.
- Material with grade, and any flammability/UV/food-contact requirements.
- Critical dimensions marked — do not tolerance everything equally.
- Colour reference (Pantone or a physical chip) and surface finish (SPI A/B/C/D).
- Annual volume estimate and first-order quantity.
- Whether you require first article inspection and a PPAP-style report.
Get the steel grade, the cavity count, the resin grade and the mould ownership into the quote response before you compare prices. Otherwise you are not comparing tools, you are comparing guesses.