Part Costing

What Actually Drives Injection Molded Part Cost, and How to Quote It

How resin, machine time, scrap, and tooling amortization stack into a per-part price, and where molding quotes quietly lose money.

A molded part's cost splits into four buckets: material, machine time, tooling amortization, and overhead plus margin. On a typical commodity part, material runs 40 to 60 percent of piece price, machine time 25 to 40 percent, and tooling amortization the remainder until the mold is paid off. Get the split wrong and you either lose the bid or lose margin. Start every quote by pinning the resin cost per part and the machine rate, because those two lines move the total more than anything else and they are the numbers buyers push back on hardest.

Material cost is resin price times part weight, plus runner that does not get reground, divided by yield. Resin at 1.80 dollars per pound and a 21.6 gram part is 21.6 divided by 453.6 pounds times 1.80, about 0.086 dollars of resin per part. Add cold runner: a 9 gram runner across 4 cavities is 2.25 grams per part, and if only half returns as regrind you carry roughly 1.1 grams of pure loss. The Resin Cost Per Part calculator handles the gram-to-pound and cavity math so you are not quoting off a spreadsheet that silently drops the runner.

Machine time cost is the hourly rate divided by parts per hour. A 150 ton press might carry a fully burdened rate of 45 to 70 dollars per hour including operator, energy, and depreciation. At 408 good parts per hour that is 60 divided by 408, about 0.147 dollars of machine cost per part. This is why cycle time is a cost lever, not just an engineering number: shaving a 30 second cycle to 27 seconds lifts the rate near 453 parts per hour and drops machine cost to roughly 0.132, a 10 percent cut on the largest controllable line.

Scrap and regrind are where quotes quietly bleed. If your process runs 4 percent scrap, every 100 good parts actually consumed material and machine time for about 104 shots, so multiply both your material and machine lines by 1 divided by (1 minus scrap), here 1.042. Regrind can claw some of it back: blending 15 percent regrind at near-zero material cost trims the material line, but most specs cap regrind at 10 to 25 percent for cosmetic or structural parts. The Regrind Percentage calculator shows the allowable blend, and the yield adjustment is the single most-forgotten factor in bad quotes.

Tooling is a separate capital line that you either bill up front or amortize into the piece price. A 4 cavity mold at 40,000 dollars spread over a committed 500,000 part program adds 0.08 dollars per part; spread over only 100,000 parts it adds 0.40, which can double a low-cost part. Always quote the amortization against the customer's committed volume, not their optimistic forecast, and state the breakpoint. Cavitation choices feed straight into this: an 8 cavity tool costs more but halves cycles, so model tooling and machine cost together rather than optimizing either alone.

Overhead and margin sit on top. Fixed plant overhead not captured in the machine rate, quality, packaging, and freight typically add 10 to 20 percent, then margin of 10 to 30 percent depending on the market. Build the piece price as material plus machine plus tooling amortization, multiply by (1 plus overhead), then by (1 plus margin). Skipping the overhead multiply is a classic error that leaves a quote looking competitive while it fails to cover the building. Show these as explicit percentages so a buyer negotiating volume knows exactly which line has room.

A worked full quote: 0.086 material plus 0.147 machine equals 0.233 direct, times 1.042 for 4 percent scrap is 0.243, plus 0.08 tooling amortization is 0.323. Apply 15 percent overhead to reach 0.371, then 20 percent margin for a 0.445 dollar piece price. Change one assumption, say resin jumps to 2.40 dollars per pound, and material rises to 0.115, pushing the quote past 0.48. This is why locking material index clauses and stated volume breakpoints into the quote protects you when resin or forecasts move against you.

The most defensible quotes state their assumptions on the same page as the number: resin grade and price index, cavity count, cycle time, scrap rate, overhead percent, and the committed volume that sets tooling amortization. When a customer asks for a lower price, that transparency turns a haggle into a trade, more volume for lower amortization, or a cheaper resin for a lower material line. Estimates go wrong when these levers are hidden inside one blended number, because then every negotiation attacks margin instead of the underlying cost driver.

Published 2026-07-01.