Injection molding cost per part: shot weight, cycle time and cavitation

Injection molding is unusually tractable to cost, because almost everything traces back to three numbers: how much plastic goes into a shot, how long the cycle takes, and how many parts each cycle produces.

It is also unusually easy to get badly wrong, for exactly the same reason — an error in any of the three moves the entire sheet rather than one line of it.

The three numbers everything hangs on

The ten categories, in process order

1. Material — shot weight divided by parts per cycle

Take the full shot weight, including runner and sprue, and divide by the number of parts that shot produces. Colorant and additives are separate lines at their own price, because they are usually far more expensive per unit weight than the base resin.

Material that comes back as regrind is handled as a recovery — see the dedicated guide. Do not reduce shot weight to account for it; that is the same double subtraction the sheet metal guide warns about.

2. Machine time — cycle divided by parts per cycle

Machine hourly rate times cycle time, divided by parts per cycle. Rates differ sharply by clamping force, so use the rate for the press the job actually runs on, not a plant average.

Include the drying time the material needs if it occupies equipment. It is easy to forget because the press is not the thing being occupied.

3. Labor

Often small on an automated cell and never zero. Include part removal if it is not automatic, degating, inspection, and the operator's share if one person tends several presses.

4. Tooling and mold amortization

Two different things that must not be merged:

• Consumables and wear items — divided by parts produced between replacements.
• The mold itself — a one-off cost spread over the parts it will produce. This has its own guide, because how many parts you divide by is a decision, not a fact.

5. Scrap

Short shots, flash, sink, burns, contamination. Startup scrap at the beginning of a run is real and belongs here — it is charged to the run that produced it, spread across that run's good parts.

6. Rework

Degating, trimming flash, refinishing. If parts are scrapped rather than reworked, enter zero — not blank.

7. Outside processing

Painting, plating, printing, ultrasonic welding, assembly. Vendor price per lot divided by pieces, plus transport both ways.

8. Packaging

Bags, boxes, dividers, returnable containers. Molded parts are often light and bulky, so packaging is a bigger share here than in metalworking.

9. Freight

Light and bulky means the truck fills up before it loads up. Cost a shipment, divide by the parts in it, and note the assumption.

10. Plant overhead

Space, chilled water, compressed air, granulators, supervision. If power is already inside the machine hourly rate, do not charge it again here.

Three checks

1. Material is full shot weight including runner, divided by parts per cycle.
2. Parts per cycle is the cavities actually running.
3. Recovered material is credited once, and shot weight was not reduced to account for it.