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Plastic Injection Molding Cost Calculator: The Formula and a Worked Example
A plastic injection molding cost calculator in four lines: resin, machine time, tooling amortization and extras, worked through for a 60 cm³ ABS part.
by the HardwareMap editors3 min read
Contents
A plastic injection molding cost calculator is four lines of arithmetic: resin, machine time, tooling amortization and extras, each divided down to one part. This note gives the formula, the inputs a molder will ask for, and a fully worked example for a small ABS enclosure part. For what drives each input, start with the parent note on injection molding cost.
An interactive version of this calculator is planned. Until it ships, the table below is the calculator: copy it into a spreadsheet and change the inputs.
The plastic injection molding cost calculator formula
Per-part cost is the sum of four terms:
- Material = part volume × resin density × (1 + runner and scrap allowance) × resin price per kg.
- Machine = press hourly rate ÷ parts per hour, where parts per hour = (3,600 ÷ cycle time in seconds) × cavities.
- Tooling = mold cost ÷ units you amortize it over.
- Extras = secondary operations (inserts, pad printing, ultrasonic welding), packing and freight per part.
If scrap is high, divide the first two terms by yield. Most quotes fold labor and overhead into the press rate, so the formula does not count them twice.
Inputs a molder will ask for
Volume is not weight. Multiply by density: 60 cm³ of ABS is 62.4 g, and with a 5% runner and scrap allowance the shot charge per part is about 65.5 g.
Worked example: a 60 cm³ ABS enclosure part
Same part, two molds, three lifetime volumes. All prices are the example inputs above.
Three things fall out of the table.
- Tooling dominates at low volume. At 10,000 units the mold is two thirds of part cost on the single-cavity tool and 90% on the 4-cavity tool. Material is under 10%.
- Cavities trade tooling for machine time. The 4-cavity tool saves $0.375 per part in press time and costs $15,000 more. Break-even is $15,000 ÷ $0.375 = 40,000 units. Below that, buy the simpler mold.
- Resin price barely matters here. A 20% resin increase adds about 3 cents. A 5-second cycle time cut on the single-cavity tool saves about 8 cents. Spend design effort on wall thickness before you haggle over resin.
For how mold price itself is built up (steel, cavities, slides, texture), see injection molding tooling cost.
How to sanity-check a molding quote
Ask each molder to break the quote into mold cost, cavities, cycle time, resin and grade, shot weight and part price. Then rebuild the part price with the formula. If your rebuild and their number differ by more than a few cents, one input is different, usually cycle time or tool life, and that is the question to ask.
Two inputs deserve extra scrutiny. Cycle time is mostly cooling, and cooling time grows roughly with the square of wall thickness, so a part with a 3 mm boss and 1.5 mm walls runs at the speed of the boss. Tool life decides how far the mold amortizes: an aluminum tool may not last a 50,000-unit run, while hardened steel lasts far longer at a higher price. Mold makers state the rated shot count; get it in writing. Protolabs publishes its design guidelines and quotes online, which is a useful reference point for prototype-volume tools.
Plug the result into your bill of materials as the enclosure line, then carry it into hardware unit economics. Part design changes that shorten cycle time are cheapest before tooling starts, which is the case for a design for manufacturing review.
Frequently asked questions
How do you calculate injection molding cost per part?
Add four terms. Material is shot weight times resin price, with runner and scrap included. Machine time is the press hourly rate divided by parts per hour, where parts per hour is 3,600 divided by cycle seconds times cavities. Tooling is mold cost divided by the units you will amortize it over. Then add secondary operations and packing. Divide by yield if scrap is significant.
What drives injection molding cycle time?
Cooling, in most parts. The plastic must solidify before ejection, and cooling time grows roughly with the square of wall thickness, so a 3 mm wall cools in about four times the time of a 1.5 mm wall. Fill, pack and mold opening add a few seconds each. Thin, uniform walls are the cheapest cycle-time fix and they cost nothing at the design stage.
When is a multi-cavity mold worth it?
When the per-part machine savings pay back the extra tooling cost inside your lifetime volume. Divide the extra mold cost by the machine-cost saving per part to get the break-even quantity. In the worked example here, a 4-cavity mold beats a single cavity at about 40,000 units. Below that, the cheaper single-cavity mold wins even though each part costs more to run.
Why do quotes from different molders vary so much?
Each input moves. Mold makers in different countries price tool steel and labor differently. Press rates depend on machine size and region. Molders assume different cycle times, scrap rates and tool life. Ask every quote to break out mold cost, cavities, cycle time, resin and part price, so you can drop each figure into the same formula and compare like with like.
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