What matters
- Use accepted parts, not shots, in the denominator.
- Separate recurring unit cost from optional tooling amortization.
- Cavity count helps only when the mold fills, cools, ejects, and maintains reliably.
A useful cost-per-part formula
Tooling amortization can be added as a separate line: tooling and qualification divided by the number of accepted parts in the commercial horizon. Keeping it separate prevents a paid-off tool from appearing to have the same economics as a new one.
Use the complete shot weight for material, including runners when they are not fully recovered. Use the demonstrated cycle rather than a target cycle that has not been proven in the selected resin and mold.
Turn cycle time into machine cost
A simplified machine component is the hourly molding rate multiplied by cycle seconds, divided by 3,600 and then divided by saleable cavities. The word saleable matters: planned scrap, cavity imbalance, and downtime do not disappear because the mold has more impressions.
| Input | Example | Why it matters |
|---|---|---|
| Machine rate | $65/hour | Machine size, region, automation |
| Cycle | 32 seconds | Cooling usually dominates |
| Cavities | 2 | Two parts per accepted shot |
| Machine cost/part | $0.289 | $65 × 32 ÷ 3,600 ÷ 2 |
Material cost is more than part weight
Convert resin price and total shot weight into the same units, then account for runners, purge, start-up scrap, moisture conditioning, color concentrate, and any regrind limit. Medical, optical, flame-rated, or customer-specified grades may also carry lot-control and handling costs.
- Confirm whether the quote uses gross shot weight or net finished-part weight.
- Ask whether resin is fixed, indexed, or passed through at invoice cost.
- State whether runner regrind is allowed and at what percentage.
- Use a measured scrap rate after process qualification; early models should show the assumption.
Why the curve flattens
The steep early drop comes from spreading tooling and setup over more units. Once those lines are small, resin, machine time, packaging, and inspection form a floor. At that point, another order-size increase may help cash efficiency more than physical manufacturing cost.
For a cavity-count decision, compare total annual cost rather than the quoted piece price alone. A larger mold can require a larger press, a more complex runner, and more expensive maintenance even when it produces more parts per cycle.
Use this as a design review, not a production release
These guides support early planning. The selected resin supplier, toolmaker, and molder still need to confirm the final geometry, process window, tolerances, safety factors, validation plan, and commercial assumptions for your application.