Injection Molding calculator

Mold Cavitation Calculator

Find the fewest cavities a mold needs to meet annual demand after downtime and scrap, before you quote the tool. You need demand, press hours, cycle time, uptime, scrap rate and your planned cavity count.

What this calculator does

  • The fewest cavities a mold needs to meet annual demand after uptime and scrap, and whether your planned count covers it.

Formula used

  • Shots per year = available hours × 3,600 ÷ cycle time × uptime % ÷ 100
  • Exact cavities = annual demand ÷ (shots per year × (1 − scrap % ÷ 100))
  • Minimum cavities = exact cavities rounded up to a whole cavity
  • Good parts per year = shots per year × planned cavities × (1 − scrap % ÷ 100)
  • Press hours needed = available hours × exact cavities ÷ planned cavities; press load = exact cavities ÷ planned cavities × 100

Inputs explained

  • Annual Part Demand: Good parts per year from the customer forecast or contract.
  • Available Production Hours per Year: Press hours for this mold after holidays, maintenance and changeovers.
  • Cycle Time per Shot: Mold close to mold close, from the quote or a similar mold.
  • Uptime: Share of those hours the press is cycling, from downtime logs.
  • Scrap Rate: Rejected share of parts molded, from scrap records on similar parts.
  • Cavities Planned: Cavity count in your quote or tool design.

How to use the result

  • Best suited to choosing cavitation when quoting a new mold, checking an existing tool against higher demand.
  • Does not check clamp force, shot size, tie-bar spacing or platen size at the cavity count. Treats demand as level through the year; seasonal peaks need a monthly check.

Current U.S. benchmarks

  • The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% from a year earlier. Quotes priced off last quarter's material cost miss this move.
  • The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

  • Is one 8-cavity mold better than two 4-cavity molds? Not always. Two four-cavity tools keep half the output running while one is in repair; one eight-cavity tool suits stable, high-volume demand. Compare total cost, including maintenance and the risk of a bad cavity.
  • What uptime should I enter for a mold that does not exist yet? Use the run time your downtime log shows for similar molds on the same press class, as a share of available hours. Changeovers are already out of available hours, so do not count them again.
  • Why does a 2% scrap rate change the cavity count? Demand counts good parts, but every reject still fills a cavity. At 2% scrap you mold about 102 parts for every 100 you ship, which can tip the requirement over a whole cavity.
  • How much does a faster cycle cut the cavity count? The exact requirement is proportional to cycle time, so going from 25 to 20 seconds cuts it by 20%. Prove the faster cycle on a similar mold before quoting fewer cavities.

Related guides

Last reviewed 2026-10-01.