What is the formula for injection molded part cost?

Part Cost = Resin per Part + Press Rate x Cycle Time / 3,600 / Cavities + Tooling per Part. With $0.095 of resin, a $55 press hour, a 22 s cycle, 8 cavities and $0.017 of tooling, the part costs $0.154.

How do I calculate the press time cost per part?

Multiply the press rate per hour by the cycle time in seconds, divide by 3,600 and by the cavities running. A $55 press on a 22 s cycle with 8 cavities is 55 x 22 / 3,600 / 8 = $0.042 per part.

What should the press rate include?

The full cost of one running press hour: the machine's depreciation, power, maintenance and floor space, the operator's share, and plant overhead. Build it with the injection machine hour rate calculator rather than using a rate from another shop.

How is the mold cost spread over the parts?

Divide the mold cost by the parts it will make over the program or its life, whichever is shorter, or quote it separately as a one-time tooling charge. The tooling amortization formula covers both ways.

How do cavities change the part cost?

They divide the press time. Going from 8 to 4 cavities on the example doubles press time per part from $0.042 to $0.084 and raises the part from $0.154 to $0.196, with the same resin and tooling per part. A mold with more cavities costs more, so compare both totals.

Cost and Quoting

Injection Molded Part Cost Formula

An injection molded part costs its resin, its share of press time and its share of the mold: part cost = resin + press rate x cycle time / 3,600 / cavities + tooling per part. An 8-cavity closure on a 22 s cycle and a $55 an hour press costs $0.154. Use it to price a part, check a quote, or see whether resin, press time or tooling matters most.

Formula

Part Cost = Resin per Part + Press Rate x Cycle Time / 3,600 / Cavities + Tooling per Part

Variables

Resin per Part: Resin cost per good part, including its share of runner and scrap not reground
Press Rate: Cost of one press hour with its operator and overhead, per hour
Cycle Time: Seconds per shot
Cavities: Cavities that actually run in the mold
Tooling per Part: Mold cost amortized over the parts it will make, per part

Understanding the Injection Molded Part Cost Formula

A molded part's cost has three parts. Resin is what the part and its share of the runner weigh, times the resin price, allowing for scrap. Conversion is the press hour, with its operator and overhead, shared by the parts one hour makes: press rate x cycle time / 3,600 / cavities. Tooling is the mold's cost spread over the parts it will make.

Which part dominates changes the right question. In the example resin is 62% of the $0.154, so a cheaper grade or a lighter runner moves the price more than a faster cycle. On large parts or single-cavity molds, conversion usually dominates and every second of cycle time counts.

Cavitation is the strongest lever on conversion. The same press hour shared by 8 cavities costs $0.042 a part; shared by 4 it costs $0.084, and with the same resin and tooling per part the part rises from $0.154 to $0.196. More cavities cost more to build, so tooling per part and press time trade against each other.

Worked Example

An 8-cavity closure mold runs a 22 s cycle on a press that costs $55 an hour with its operator. Resin is $0.095 per good part and the mold is amortized at $0.017 per part. The customer's target price is $0.17.

Press time per part = 55 x 22 / 3,600 / 8 = $0.0420
Part cost = 0.095 + 0.0420 + 0.017 = $0.154
Margin at the $0.17 target = 0.17 - 0.154 = $0.016, 9.4% of the price
Parts per hour = 3,600 / 22 x 8 = 1,309

Result: $0.154 per part: resin 62%, press time 27%, tooling 11%

Common Mistake

Dividing press time by the cavities on the drawing instead of the cavities that run. Blocking one cavity of an 8-cavity mold cuts output by an eighth and raises the press time per part by a seventh. Count resin per good part too, with the runner and any scrap that is not reground.

Frequently Asked Questions

What is the formula for injection molded part cost?: Part Cost = Resin per Part + Press Rate x Cycle Time / 3,600 / Cavities + Tooling per Part. With $0.095 of resin, a $55 press hour, a 22 s cycle, 8 cavities and $0.017 of tooling, the part costs $0.154.
How do I calculate the press time cost per part?: Multiply the press rate per hour by the cycle time in seconds, divide by 3,600 and by the cavities running. A $55 press on a 22 s cycle with 8 cavities is 55 x 22 / 3,600 / 8 = $0.042 per part.
What should the press rate include?: The full cost of one running press hour: the machine's depreciation, power, maintenance and floor space, the operator's share, and plant overhead. Build it with the injection machine hour rate calculator rather than using a rate from another shop.
How is the mold cost spread over the parts?: Divide the mold cost by the parts it will make over the program or its life, whichever is shorter, or quote it separately as a one-time tooling charge. The tooling amortization formula covers both ways.
How do cavities change the part cost?: They divide the press time. Going from 8 to 4 cavities on the example doubles press time per part from $0.042 to $0.084 and raises the part from $0.154 to $0.196, with the same resin and tooling per part. A mold with more cavities costs more, so compare both totals.