Last updated: 2026-09-04 · 6 min read
How Does a Mold’s Price Become a Per-Part Number? — Tooling Amortization and the True Cost of Ownership
A mold is a fixed cost spread across every part it produces. The number that matters is not the tool invoice but the fully-loaded cost per part: tool price plus lifetime upkeep plus scrap cost, divided by accepted pieces shipped. A $40,000 tool over 40,000 parts “costs” a dollar each — until you add maintenance, yield loss and risk that volume never arrives. Amortization is a lever you pull with volume and yield, not optimism.
Key Facts
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Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.
What Actually Goes Into Cost Per Part?
Five cost elements, each with its own behavior:
| Cost Element | Behavior | How to Handle It |
|---|---|---|
| Tool price | Fixed one-time cost | Divide by accepted volume, not aspirational |
| Piece conversion | Material + cycle + machine + labor | Scales linearly; does not fall with volume |
| Tooling upkeep | PM, wear parts, repairs over tool life | Estimate per thousand shots, add to the pool |
| Scrap and yield loss | Start-up, purge, down-parts | Divide by accepted good parts only |
| Storage and standby | Idle tooling still costs space | Amortize over realistic campaign length |
Only one line — tool price — is genuinely fixed. Every other line either scales with parts or quietly taxes the good ones. Conversion cost per part is essentially constant, so more parts do not make the resin and machine-hour per piece any smaller; what amortization shrinks is only the tool-price slice. That is why the per-part total falls steeply at first and then flattens into a hard floor set by conversion plus normal scrap.
The denominator matters as much as the numerator: accepted parts shipped, not shots fired. A tool that makes 100,000 pieces but yields 80,000 good ones has to carry its whole price on 80,000. Improving yield lowers cost per part more cheaply than any negotiation over the tool invoice.
How Do You Turn Tool Price Into a Defensible Per-Part Figure?
Four steps, in order:
- Amortize over a committed quantity band, not a single guess. Model a low, expected and high volume band and compute the per-part tool cost at each. If the tool is a dollar at expected count but three at the pessimistic one, the real question becomes what volume you can defend, not what the spreadsheet average says. Tying the band to actual purchase commitments keeps the exercise honest.
- Load the fixed cost, not just the invoice. Add recurring ownership before you divide: scheduled preventive maintenance, expected wear-part swaps over the projected shot count, periodic repair, and storage across campaign gaps. Express it as a per-thousand-shots allowance so it scales with real use. Skipping these is what lets a quoted per-part number quietly inflate by a third over the program.
- Divide by accepted good parts, and treat yield as a cost lever. The denominator should be pieces that pass inspection and ship, not gross shots. Estimate realistic yield — start-up scrap, purge at color change, down-parts caught at final gate — and let the tool price rest only on the good ones. Closing a 5-point yield gap lowers amortized cost, conversion cost and material cost at once.
- Compare against alternatives at your real volume. Test the per-part figure against a soft or single-cavity tool with lower fixed but higher conversion, against printing for very low counts, and against a higher-cavity tool with more fixed but cheaper per-piece. Each option is a different fixed-versus-variable split, so the winner flips with volume. Run all of them on the same accepted-parts denominator.
Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.
You manage three dials, not one: the tool price, the yield you actually hold, and the volume you commit to. The honest per-part number is (tool + upkeep + scrap) ÷ accepted pieces — kept alongside the conversion floor so you know how much is truly fixed.
Frequently Asked Questions
Should the mold be paid for up front or rolled into the part price?
Both are the same money with different risk. Paying the tool up front makes it your asset — you own it, you can move it, and your per-part price carries no tooling slice, but you eat the whole cost if the program dies early.
Rolling the tool into the part price defers cash but usually costs more over the run because the recovery carries a risk margin. The disciplined middle path is a separate tooling invoice you own, plus a transparent conversion price with no hidden tool slice.
At what volume does a bigger, more expensive tool pay off?
At the crossover where the extra fixed cost is recovered by lower per-part conversion. A more cavitated tool costs more to build but makes four or eight parts per shot instead of one or two, so conversion drops per piece.
Multiply the per-part saving by the volume you can defend and set it equal to the extra tool price — that breakeven is the count above which the larger tool is genuinely cheaper all-in. Below it, the smaller tool wins despite a higher piece price.
Does a mold that never fully amortizes waste its cost?
Only the unamortized remainder, and that is a reason to size the tool to the volume you can defend rather than to a hope. If a tool is priced for 100,000 shots and the program ends at 20,000, the stranded slice is the real waste.
The goal is not to spend the least on the tool but to avoid paying fixed cost for capacity you never convert into accepted, shipped parts. A modest, well-yielded tool that pays for itself early is a better result than a large tool half-used.
Key Takeaways
- The honest per-part formula is (tool + lifetime upkeep + scrap) ÷ accepted good parts — not tool ÷ expected pieces.
- Only the tool-price slice falls with volume; conversion stays flat, so the per-part curve hits a hard floor set by resin and cycle time.
- Three dials manage the number: tool price, yield, and committed volume — improving yield usually lowers cost more cheaply than renegotiating the tool.
A mold’s price is a fixed number; the per-part cost is a managed outcome.
The arithmetic is simple once you load all the cost elements and divide by the right denominator. The discipline is in the inputs: committed volume, not hoped-for volume; accepted parts, not gross shots; and the full lifetime cost, not just the invoice.
Sources
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Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet