Start With the Drawing Callouts, Not the Weight
Before any math, read the print for the facts that set cost. The polymer family (EPDM, NBR, FKM, silicone, HNBR, neoprene, natural rubber), the durometer in Shore A, and the ASTM D2000 line callout or customer spec decide which compounds you can run and how long they cure. Tolerances per RMA drawing practice (A1, A2, A3 commercial and precision classes) decide whether you need a flashless or precision mold.
Bonded metal inserts mean adhesive, cleaning, priming and insert prep labor. Post cure for FKM and silicone is a separate oven step with its own queue. Missing any of these at quote time is where most underquoted jobs begin.
- Polymer, durometer and any ASTM D2000 or customer spec line
- Tolerance class and whether flash lines are cosmetic or functional
- Bonded inserts, adhesive system and insert cleaning requirements
- Post cure, secondary trimming and special packaging
- Annual volume, release quantity and PPAP or FAI expectations
Material Cost: Part Weight Is the Smallest Piece
Compound is bought or mixed by the pound, but parts are priced from the cubic inch, so you need the compound specific gravity to convert the CAD volume into shot weight. Then add what the cavity does not hold: preform trim loss, flash, and in transfer or injection the sprue, cull or cold runner. A part that weighs 20 grams may consume 28 grams of compound per cavity once flash and runner are counted.
Apply a scrap factor for start-up, rejected parts and out-of-spec preforms, and an uplift for compound that ages out before use. Also check whether the customer wants a specific compounder or a controlled formulation, since that changes the price per pound.
Press Time and Cavity Count
Press cost per part is cycle time divided by cavities, times the loaded press rate. Cycle time in rubber is dominated by cure, which comes from the compound t90 at mold temperature, adjusted for part thickness, plus load, close, bump or breathe cycles, and unload. Thick sections cure slowly because rubber is a poor heat conductor, so a 1/2 inch cross section can run several times longer than a thin gasket.
Cavity count is limited by press platen size, tonnage on projected area, and how much hand loading one operator can keep up with. Quote from a realistic cavities-per-operator figure, not the theoretical mold capacity.
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Molded Part Quote Cost Build-Up and Margin Worksheet
One row per part: enter weight, cavities, cycle time, resin price, press rate, tool cost and target margin. The sheet builds cost per 1,000 parts, quoted price and annual gross profit.
Secondary Operations and Tooling Amortization
Deflash, tumble or cryogenic trimming, post cure, 100 percent visual inspection, leak testing, bonding rework and special packaging each add labor or outside processing. Price them from routings, not guesses. For tooling, divide mold cost by the quantity you will actually ship, not the forecast, and decide whether the customer pays up front, pays in installments or you amortize into the piece price.
Add mold maintenance, since compression molds need cleaning and venting work on a schedule. Customer-owned tooling still consumes press setup, storage and repair time that someone must charge for.
Where Quotes Go Wrong and When a Spreadsheet Is Enough
Quotes drift because the estimator uses the last job's cycle time, ignores that this compound scorches earlier, or forgets that the mold is a 12 cavity tool needing two operators. The fix is to compare each quote to actual job cost from your ERP after the first production run and feed the real cure time, scrap and labor back into your standard rates. If you quote fewer than a dozen new parts a month and run mostly repeat work, a well-built spreadsheet with a compound table and press rate table is enough.
The effort of integrating quoting with the ERP pays off when RFQ volume is high or when estimators leave and take the rules with them.
Frequently asked questions
What is the formula for pricing a rubber molded part?
Piece price is roughly compound cost per part (shot weight times price per pound times one plus scrap), plus press cost (cycle time divided by cavities times the press rate), plus secondary operations, plus tooling amortization, plus overhead and margin. The shot weight must include flash and runner waste, and cycle time must be driven by the compound cure time, not the part weight.
How many parts should I amortize a rubber mold over?
Use the quantity the customer has committed to or released, not the forecast, and set a minimum such as one year of orders. If the customer will not commit, bill the mold as a separate line or require a tooling deposit. Amortizing over an optimistic volume is a common way rubber shops lose money on short-lived programs.
Do I need ERP to quote rubber parts accurately?
Not at low volume. A spreadsheet with compound specific gravity, press rates and standard cure times works for a small shop with mostly repeat work. ERP matters once you need actual cure time, scrap and labor from past jobs to calibrate quotes, or when several estimators must quote consistently.
If your quote rules live in one estimator's head or a workbook nobody trusts, Netray can build a private quoting assistant that drafts from your own job history and routes every draft to an estimator for approval.