What Goes Into the Cost of One Molded Part?

Last updated: 2026-09-01 · Reading time: about 10 min

Every piece-price estimate, however polished the spreadsheet, reduces to the same short equation. Cost per good part = resin + machine time + labor + tool amortization + secondary operations + scrap allowance + packaging — and the engineering conversation is about the drivers behind each term, not the arithmetic itself. Buyers who know the formula stop comparing numbers and start comparing assumptions, which is the only comparison that survives contact with production.

Key facts

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  • The seven terms of the piece-cost equation each have exactly one or two physical drivers — cycle time, part weight, cavitation, labor content, tool value, yield and packaging method — and all of them are knowable at DFM.
  • Machine time per part equals the cycle time divided by the number of cavities, times the machine rate — which is why cavitation and cycle discipline dominate everything else on stable programs.
  • Resin cost follows part weight plus runner weight, with a recovery credit for reprocessable runner scrap — the term with the most hidden assumptions in casual quotes.
  • Scrap is a multiplier on the whole equation: every percent of yield loss raises the effective cost of every other term, which is why process stability is a cost conversation, not a quality afterthought.
  • Estimates fail on the edges — packaging, secondary handling, tool maintenance inside the amortization rate — so a serious piece cost names every term before it defends the total.

    • Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. See also related guide.

    What is the full formula for molded part cost?

    Cost per good part
    The total program cost divided by the parts that actually ship: resin consumed (net of regrind credit) + machine time + direct labor + tool amortization + secondary operations + scrap allowance + packaging, all divided by good-part count. Seven terms, one number, every term traceable.

    Written out, the equation is short enough to hold in your head: cost per good part = resin + machine + labor + amortization + secondary + scrap + packaging. The arithmetic is trivial; the engineering lives in what fills each slot.
    Resin is part weight plus runner weight, times the resin price, minus the credit for runner material that returns to the press as regrind — where the regrind ratio decision sets how much credit is real. Machine is the cycle time divided by cavities, times the machine-hour rate — where the cavitation decision and the cooling design do their quiet work. Labor is the handling content the cell does not automate.
    Amortization is the tool value divided by its expected lifetime shots — where the tool life assumptions either hold or quietly lie. Secondary is everything after the press: degating, finishing, assembly, inspection. Scrap is the yield multiplier. Packaging is what protects the part on its way to the customer. Seven terms — and every one of them is a decision someone made on purpose, or failed to.

    Which term dominates the number?

    Term Primary drivers Who moves it
    Resin Part weight, runner weight, resin price, regrind credit. Design (wall thickness) and material selection.
    Machine time Cycle time, cavitation, machine rate. Tool design (cooling), process engineering.
    Labor Automation level, handling content, inspection intensity. Cell design at quotation.
    Amortization Tool value, expected lifetime shots, maintenance cost. Cavitation, steel choice, volume honesty.
    Secondary Degating, finishing, assembly, decoration. Design decisions at DFM.
    Scrap Process capability, tool condition, inspection policy. Process validation discipline.
    Packaging Protection class, returnable vs expendable, density. Logistics requirements.

    On most stable programs the machine-time term and the resin term carry the majority of the piece cost, which is why the two loudest conversations in molding are cycle time and part weight.
    But the quiet terms decide the outliers: a part that looks affordable at press-side can price badly once secondary handling and packaging enter, and a program whose scrap assumption was optimism will discover the real yield as a permanent surcharge on every term at once. The professional habit is to ask which terms the quote does not mention — because the missing terms are always recoverable later as either disputes or surcharges.

    How do the terms interact?

    The terms are not independent, and the interactions are where naive piece-cost models mislead.
    Cycle time and cavitation multiply together into machine time per part, so a slower cycle can be offset by more cavities — until the bigger tool raises the amortization term and the machine steps up a size, moving the hourly rate; the honest answer comes from the full equation, never from one term alone.
    Scrap multiplies everything: a five-percent yield loss means five percent more resin, machine, labor and secondary cost per good part, which is why process validation is part of the price and not a courtesy. The regrind credit interacts with scrap in the opposite direction — runner returns reduce resin cost — but only up to the ratio the part tolerates, which is a materials decision with a price tag.
    Even packaging interacts: denser packaging lowers freight per part, which may or may not appear in the piece price depending on who owns the freight terms. This is why the equation is run over the program, not at the press: the interactions cross shop boundaries, and each boundary is a place where an unowned assumption hides.

    How do you run a sensitivity check?

    1Fill the seven terms with the quote’s own assumptions. Cycle time, cavitation, shot weight, labor content, tool value, lifetime shots, yield, packaging — each number written beside the term it feeds, from the itemized quote, not from habit.
    2Stress the two big terms first. Move cycle time and resin price by realistic amounts and watch the total — this shows which assumptions the business case actually depends on.
    3Stress the honest terms. Move yield down and maintenance cost up; the gap between the nominal and stressed numbers is the risk price of an unvalidated process.
    4Compare suppliers on the terms, not the total. Two quotes with different cycle assumptions are two different machines and two different tools; align the terms and the totals become comparable — the same discipline as any quote comparison.
    5Write the assumptions into the record. The sensitivity sheet that survives is the one with names and dates beside the numbers — the document the program argues from at the first capacity review.

    Where do piece-cost estimates usually go wrong?

  • Theoretical cycles — the estimate uses the fastest cycle anyone has seen, not the cycle the cooling design supports; the error surfaces as a permanent gap between promise and invoice.
  • Runner weight ignored — parts with large cold runners carry hidden resin cost; the runner is part of the shot whether or not it appears in the model.
  • Amortization without maintenance — tool value spread over shots with no upkeep line understates the true tooling cost per part across the program.
  • Yield assumed, never validated — the scrap term copied from a previous program is a guess; validated yield is a measurement, and the difference compounds on every part.
  • Secondary operations discovered late — degating, fixture time and finishing that exist in production but not in the estimate; a DFM pass exists to name them early.
  • Packaging treated as free — protection class and freight density move real money; terms that belong to logistics still belong to the program’s cost.

    • “A piece price is a claim about the future. Seven terms, each with a name on it — that is the difference between a claim and an engineering estimate.”
    — Arthur, Tooling Engineer at aCatMould

    Frequently asked questions

    Why do two suppliers’ piece prices differ for the same part?

    Because the seven terms hide different assumptions — different cycle estimates, cavitation plans, yield expectations and labor content. The totals differ because the terms differ; align the terms and the gap either closes or becomes a real difference in approach worth discussing. This is the same normalization discipline used for comparing tool quotes.

    How much does a second of cycle time actually cost?

    It depends on cavitation and machine rate, and the honest way to know is arithmetic on your own numbers: machine term per part = cycle ÷ cavities × rate, so one second moves the total by that fraction — multiplied by every part the program ever ships. On high-volume programs the second is not small; on low-volume programs it is noise. Run the term, do not guess it.

    Should tool amortization appear in the piece price at all?

    Yes — but stated as a term, not dissolved into the number. A piece price that hides tooling cost cannot be renegotiated when volumes change, cannot be compared across suppliers who amortize differently, and cannot survive a program extension. The term stays visible; the recovery schedule stays a separate, written agreement.

    Does regrind really lower the piece cost?

    It lowers the resin term by the value of the runner material that re-enters the process — up to the ratio the part tolerates. The credit is real when the ratio is engineered and documented, and false when it is assumed; the regrind conversation is where the credit either survives or becomes a quality liability with a price tag.

    What is a reasonable scrap allowance to build in?

    None as a default — the term should come from validated process data on similar parts, or be explicitly flagged as an assumption in the estimate. A copied allowance is either generous, in which case it hides cost, or optimistic, in which case it guarantees one; the validation work that produces a real yield number pays for itself the first time the program argues with a budget.

    How does the landed cost of the tool connect to piece cost?

    Through the amortization term: what the tool actually costs delivered — quote plus crating, freight, duties and brokerage — is the number that should be spread over lifetime shots, not the bare toolmaker’s invoice. Programs that amortize the landed value produce piece costs that match the ledger; programs that amortize the quote produce a gap someone pays for later.

    How does volume change the piece cost?

    Through the amortization term and through cavitation: higher volume spreads tool cost over more parts and justifies more cavities, which cuts the machine term — so the piece price falls with volume in a way that is calculable, not magical. But the same math punishes thin volume with a tool that waits; the honest piece cost always names the volume it assumes, and the sensitivity sheet shows what happens when the volume moves.

    Where does the machine rate come from?

    From the supplier’s cost structure — press tonnage class, energy, labor, overhead — and it is a legitimate question to ask at quotation. Two shops with different rates can still produce viable piece costs if their cycles and yields differ; the rate matters only inside the full equation. Ask for the tonnage class the part runs on; a part quoted on an oversized press is a quiet surcharge on the machine term.

    Key takeaways

    1. Seven terms — resin, machine, labor, amortization, secondary, scrap, packaging — and every serious piece cost names all of them.
    2. Machine term = cycle ÷ cavities × rate; cavitation and cooling discipline are the piece-cost levers on stable programs.
    3. Scrap multiplies every other term; validated yield is a cost asset, assumed yield is a liability.
    4. Run sensitivities on the terms, compare suppliers on the terms, and write the assumptions down with names and dates.
    5. Amortize the landed tool value, not the bare quote — the piece cost should match the ledger, not flatter it.

    If you are staring at a piece price that needs to be defended, send us the part, the quotes and the volumes. You will get the seven-term breakdown with each assumption named, the sensitivity check run on the terms that carry your business case, and the questions that turn any estimate into an engineering figure — from a named tooling engineer, before any deposit leaves your account.

    Sources
    Plastics Industry Association — industry resources on plastics processing practice
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization
    MatWeb — material property database, reference for resin grades and densities
    HASCO — moldmaking components, reference for tool component documentation

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet