What Really Drives the Cost of an Injection Molded Part
Two suppliers can quote the same part at 0.42 USD and 0.71 USD and both be honest. The gap is almost never margin — it is a chain of decisions made months earlier, most of them in the first conversation about the design.
What is actually inside a piece price
- Material — resin grade, colourant, and the scrap you generate
- Machine time — cycle time multiplied by the hourly rate of the press size your part needs
- Labour and overhead — operators, QC, packaging, factory cost
- Secondary operations — insert loading, welding, printing, assembly
- Yield loss — the parts you make and do not ship
- Packing and freight — usually quoted separately, sometimes hidden inside
Tooling is a one-time cost and should be quoted as one. When a supplier folds tooling into the piece price, you lose the ability to compare anything.
The five levers that move unit price most
1. Wall thickness
Cycle time scales roughly with the square of wall thickness: the plastic has to cool all the way through. Taking a 3 mm wall down to 2.4 mm is often the single largest saving available on a part, and it is free if the design allows it.
2. Cycle time
Cycle time is cooling, plus injection, plus hold, plus ejection. Cooling is usually the biggest share, which is why cooling layout is a design decision and not a mould-making detail.
3. Cavitation
More cavities per tool means more parts per hour and a lower piece price, but a more expensive tool and a bigger press. The right number depends on annual volume: a four-cavity tool that only ever runs two cavities is a tool you overpaid for.
4. Tolerances
This is the classic overspend. Tightening a non-functional dimension from plus or minus 0.2 mm to plus or minus 0.05 mm does not make a better part; it makes a more expensive one, because it forces slower cycles, more inspection and a higher scrap rate. Mark the dimensions that matter and let the rest breathe.
5. Material grade
Moving from an unfilled grade to a glass-filled one can cut cost through stiffness, and add cost through mould wear and shorter tool life. It is a trade, not a saving, and it should be calculated over the life of the tool rather than the first order.
What drives the tooling price
| Driver | Effect |
|---|---|
| Number of cavities | Each additional cavity multiplies cavity and core work |
| Steel grade and hardness | Sets tool life and how well it holds a polished surface |
| Hot runner | Higher tool cost, shorter cycles, no runner waste |
| Side actions and sliders | Undercuts cost real money; a design change may remove the need |
| Surface finish and texture | Polishing and texturing time, and it is not reversible |
| Tolerance on the part | Tighter tolerances mean more fitting and more trial cycles |
How to compare quotes honestly
- Ask for tooling and piece price separately
- Ask which press size the piece price assumes
- Ask what cavitation the piece price assumes
- Ask whether material certificates, dimensional reports and packing are included
- Ask what the price is valid for, and what happens to it if resin moves
Four quotes on those five answers will sort themselves out. The cheap one usually turns out to have assumed a smaller machine, a single cavity and no inspection.
The cheapest change is usually a design change
Almost every part we quote has one or two features that could be removed, relaxed or moved for a meaningful saving at no cost to function. That is what a DFM review is for: finding those before the steel is cut, when they are still free.
Send us a part and we will show you the trade-offs behind the number. Request a quote, or read how we build the tool.