Injection Molding Cost Breakdown: Understanding Where Your Budget Goes
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A molded part’s cost splits into five buckets: material, machine time, tooling amortization, secondary operations and packaging, and quality and overhead. Which bucket dominates depends on the part and the program: a large part in an expensive resin is material-driven, a small part on a long cycle is machine-driven, and a low-volume part with a complex mold is tooling-driven. The useful exercise is to build the breakdown for the actual part with quoted or measured inputs, because that shows which lever (design, resin, cavitation, cycle time, or volume) moves the price, rather than relying on industry averages that may not describe the job.
The Five Buckets and What Drives Each
| Bucket | Driver | Main levers |
|---|---|---|
| Material | part weight, runner weight, scrap, resin price, regrind policy | wall thickness, resin choice, hot runner, scrap reduction |
| Machine time | cycle time, cavities, press rate, uptime, yield | cooling design, cavitation, press size, automation |
| Tooling | mold price divided by parts over its life or program | mold class, cavitation, actions, life matched to volume |
| Secondary and packaging | assembly, decorating, inspection, packing labor and materials | design for assembly, in-mold features, automation |
| Quality and overhead | inspection, documentation, engineering, facility | process monitoring, capability, fewer manual checks |
A Worked Breakdown
For example, assume (illustrative inputs, not market prices):
- part weight 50 g, two cavities, cold runner 10 g per shot, runner not reground;
- resin $2.20 per kg; scrap 3% of the parts molded (97% yield);
- 30-second cycle; press rate $60 per hour; uptime 85%;
- mold $60,000 amortized over 500,000 parts;
- packaging and handling $0.03 per part.
- Material per good part. Shot weight 2 × 50 + 10 = 110 g, or 55 g per part; divided by 97% yield, 56.7 g per good part; at $2.20 per kg, about $0.125.
- Machine time per good part. 2 cavities × 120 cycles per hour = 240 parts per hour at full speed; × 0.85 uptime × 0.97 yield = about 198 good parts per hour; $60 ÷ 198 = about $0.303.
- Tooling. $60,000 ÷ 500,000 = $0.120.
- Packaging and handling. $0.030.
- Subtotal. About $0.578 per part for these four lines. The $60 press rate is assumed to be fully burdened, so plant overhead sits inside the machine line; quality costs (inspection labor, gauges, scrap handling beyond the yield already counted) are not included and would be added as a fifth line from the plant’s own records.
| Bucket | Cost per part | Share of the four-line subtotal |
|---|---|---|
| Material | $0.125 | 21.6% |
| Machine time | $0.303 | 52.5% |
| Tooling | $0.120 | 20.8% |
| Packaging | $0.030 | 5.2% |
In this four-line subtotal machine time is about half the cost, so a shorter cycle or more cavities would move the price more than a cheaper resin. With a different part, the ranking changes; the point is to compute it. The step-by-step method, including how scrap and uptime are handled, is in calculating per-part cost.
Why Weight Savings Are Smaller Than They Look
Take the same part and cut its weight by 10%, from 50 g to 45 g, with the runner unchanged:
- New shot: 2 × 45 + 10 = 100 g, or 50 g per part; ÷ 0.97 = 51.5 g per good part.
- Material cost falls from about $0.125 to $0.113, a 9.1% saving, not 10%, because the runner and scrap did not shrink with the part.
If the thinner wall also shortens the cooling time, the machine-time saving can be larger than the material saving, which is why wall thickness reductions are evaluated on both buckets.
Tooling: Price Versus Cost per Part
A cheaper mold is not always cheaper per part. For example, a $30,000 mold that lasts 50,000 parts contributes $0.60 per part, while an $80,000 mold that lasts 500,000 parts contributes $0.16 per part, if the program needs that many. The comparison only works when the expected program volume is stated, which is why mold class and life belong in the RFQ, as covered in mold design fundamentals for buyers and injection mold tooling costs.
Secondary Operations and Quality
Every step after the press adds labor, handling, and a chance of damage: trimming, assembly, inserts, printing, inspection, packing. Some can be designed out (molded-in features instead of assembled ones, a hot runner instead of degating) or automated. Quality costs include inspection labor, gauges, documentation, and the cost of scrap and returns. Process monitoring that catches drift at the press reduces both scrap and inspection effort, which is covered in quality control methods.
Reading a Quote Through the Breakdown
Ask suppliers to show, or at least confirm, the inputs behind the price:
- part and runner weight and the resin price used;
- cycle time and cavities assumed;
- press size and rate;
- scrap or yield assumption;
- how tooling is charged (separately, amortized into piece price, or both);
- packaging and any secondary operations.
Two quotes that differ by 20% can differ for a reason that matters (a faster cycle the supplier has proved) or for one that does not (an optimistic scrap rate). The breakdown shows which.
Where the Levers Are
- Design: wall thickness, part consolidation, features that remove secondary operations.
- Material: grade choice, regrind where allowed, hot runner where runner waste is costly.
- Tooling: cavitation and mold class matched to volume.
- Process: cycle time from cooling and gate-freeze studies, scrap reduction, uptime.
- Volume and scheduling: longer runs spread setup cost; demand certainty supports tooling investment.
The cost bucket that is largest for this part is where to start, and only a breakdown built from the part’s own numbers shows which one that is.
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