Injection Molding

Molded Part Cost Estimation: Key Variables and Common Mistakes

Molded part cost depends on resin use, cycle time, cavitation, and tooling recovery. Here is how to build an accurate cost model and where most estimates go wrong.

Molded part cost equals material cost plus machine time cost plus labor cost plus tooling amortization plus overhead. Material cost equals shot weight in grams divided by 1000, multiplied by resin price per kg, then divided by cavitation count. For a 50 gram ABS part at $2.20/kg on a 2 cavity tool, material cost is (50 / 1000 x 2.20) / 2 = $0.055 per part. Machine time cost equals machine hourly rate divided by 3600, multiplied by cycle time. On an $80 per hour press running a 22 second cycle, that machine component is small per cavity, but across millions of parts it becomes one of the biggest quote assumptions to audit.

Tooling amortization is often the input that creates the biggest quote gap between suppliers. A $120,000 two-cavity mold amortized over 500,000 parts adds $0.24 per part, while over 1,000,000 parts it drops to $0.12. Scrap is the multiplier many teams forget, because good part cost equals total part cost divided by yield. At 3% scrap, a $0.35 piece becomes $0.35 / 0.97 = $0.361. Resin purchase price, scrap rate, and annual volume usually come from purchasing records, production history, and the approved quote package rather than from engineering estimates alone.

The most common estimation error is using resin list price instead of the actual purchased price. List and actual often differ by 15% to 30% based on contract terms, volume, and timing. Another frequent miss is leaving out runner and sprue weight in a cold runner mold. In many cold runner tools, runners are 20% to 40% of total shot weight and belong in the material calculation even if some regrind is reused. Hot runner systems reduce that loss, but they often add $15,000 to $40,000 to tooling cost and need to be evaluated as a full business case.

Build the model in layers: first material, then machine time, then labor, then tooling, then overhead. That structure shows which cost driver actually matters most on your part. Thin-wall parts with short cycles are often machine-time dominated, while large technical parts made in nylon, PEEK, or PC-ABS blends are usually material dominated. Use the result to test cost reduction ideas before chasing them. If the model says material is 55% of total cost, reducing cycle time by 5% will not move the quote much.

When comparing quotes, always ask what cycle time, resin price, scrap rate, and annual volume the supplier assumed. A quote that looks 15% cheaper may be built on a resin number from last year or on a cycle they cannot sustain after launch. Also review cavitation, expected uptime, and tooling life because those assumptions can shift the cost by pennies that add up fast at volume. Related checks such as molded part weight, regrind policy, and tool maintenance frequency help explain why two quotes with the same nominal cycle still land at different prices. A transparent cost stack reduces both sourcing surprises and margin erosion after SOP.

Published 2026-05-28.