Ask where a molded part’s price comes from and the common answer is “the resin, plus a markup.” That answer is wrong in a way that costs people money — both molders who under-quote and buyers who can’t tell a fair price from a fishy one. The piece price is a build-up of several costs that have nothing to do with the material, and on a lot of parts the resin is not even the biggest one.
This article walks through how a molder actually builds up a per-part cost, in logic only — no rates, no currency. The point is to understand the structure and the levers, because that’s what tells you why one part costs what it does and where the cost can actually be moved.
The machine is sold by the hour — and the hour isn’t flat
The core of part cost is machine time. A press is costed at an hourly rate, and the key fact most people miss is that the rate rises with clamp tonnage: a small press is the cheapest hour on the floor, and the largest presses cost several times more per hour. The machine portion of the part is essentially:
machine-hour rate × cycle time
Two consequences fall straight out of that. First, cycle time dominates the machine cost — every second of cooling you carry is multiplied across every part for the life of the job. Second, press size matters, so running a small part in an oversized machine quietly inflates its cost. Getting the part onto the right-sized press at the shortest sound cycle is the single biggest lever on the machine portion of the price.
Fixed cost doesn’t disappear — it gets allocated
A plant has costs that exist whether or not a given job is running: building and equipment depreciation, maintenance, finance cost, management and admin, utilities and the transformer. These don’t belong to any one part, but they have to be recovered by all of them.
Shops handle this by allocating fixed cost down to a usable basis. The logic: sum the annual fixed costs, divide to a per-machine and per-day figure, then to a per-gram-per-day basis using the plant’s total shot capacity multiplied by a realistic utilization factor (commonly around 75%, since no plant runs every machine every minute). That gives a rate that can be applied to any job by its shot weight and cycle. The takeaway isn’t the arithmetic — it’s that fixed cost is real and it lands on every part, which is why a shop running at low utilization has higher per-part cost even with identical machines.
The per-part build-up
Put the pieces together and a self-made part’s cost is a stack, not a single number:
| Component | How it’s built | Notes |
|---|---|---|
| Processing cost | (labor + electricity + fixed-cost allocation) ÷ good shots per day | ”Good shots” applies a yield factor (commonly ~95%) — scrap raises every other part’s cost |
| Tooling amortization | tool cost ÷ tool life in shots | Omitted when the customer supplies the tool |
| Material | resin unit cost × part gross weight | Gross weight includes the runner and process loss, not just the part |
| Auxiliary material | colorant, additives | — |
| Packaging | boxes, trays, dunnage | — |
| Freight | shipping to the customer | — |
| Secondary operations | assembly, printing, inserts, etc. | Often a large, overlooked share |
Totals depend on who owns the tool: a self-made part = processing + tooling amortization + material + auxiliary + packaging + freight + secondary; a customer-supplied-tool job drops the tooling amortization line. Margin and tax are added on top per the shop’s own structure.
Two lines deserve a second look. Yield is a multiplier on everything: at a 95% yield, the 5% that scraps doesn’t just lose its own cost — it loads the machine time, labor, and material onto the good parts that remain. And gross weight, not net part weight, drives material cost, because the runner and any process loss are resin you bought and have to account for.
The levers a shop actually controls
Once you see the build-up, the levers become obvious — and they’re mostly not “buy cheaper resin”:
- Cycle time. Cooling dominates the cycle, and cooling is driven by the thickest wall. Faster, balanced cooling and thinner, more uniform walls cut machine time on every shot. (This is also why mold temperature and cooling discipline pay back so directly.)
- Gross weight. Trimming runner mass — or reclaiming it as controlled regrind — directly cuts material cost.
- Yield. Every point of scrap reduction lowers the cost of all the good parts. Scrap is a cost lever disguised as a quality metric.
- Right-sizing the press. Matching the part to the smallest sound machine lowers the machine-hour rate it’s charged against.
- Secondary operations. Designing them out, or automating them, removes a cost that’s often larger than the molding itself.
And the biggest lever of all sits upstream of the molder entirely: design. The decisions made on the part design — wall thickness, material, tolerances, how many secondary operations it forces — lock in the large majority of a part’s eventual cost long before the first shot. A molder can optimize cycle and yield, but they can’t out-process a part that was designed to be expensive.
FAQs
Why isn’t a molded part’s price just material plus a markup?
Because material is only one line in a multi-part build-up. The price also includes machine time (an hourly rate times cycle time), an allocation of the plant’s fixed costs, tooling amortization if the molder owns the tool, auxiliary material, packaging, freight, and any secondary operations — all before margin and tax. On many parts the machine time and secondary operations together exceed the resin cost. Treating price as “resin plus markup” misses most of what actually drives it.
Why does press size affect the price of a small part?
Because presses are costed at an hourly rate that rises with clamp tonnage — a large machine costs several times more per hour than a small one. Since the machine portion of a part is essentially the machine-hour rate times the cycle time, running a small part in an oversized press charges it against an unnecessarily high rate and inflates its cost. Matching the part to the smallest press that can run it soundly is one of the most direct ways to lower the piece price.
What’s the difference between net part weight and gross weight in costing?
Net part weight is just the part; gross weight includes the runner and any process loss — the resin you actually bought and consumed per shot. Material cost is driven by gross weight, not net, because the sprue and runner are real material even if they’re reground afterward. Costing on net weight understates material cost and is a common way molders quietly lose money. Trimming runner mass or reclaiming it as controlled regrind is a legitimate lever on the gross-weight line.
How does scrap rate affect part cost?
Scrap loads its cost onto the good parts. If a job runs at 95% yield, the 5% that’s scrapped still consumed machine time, labor, and material — and that cost has to be recovered across the parts that actually ship, raising each one’s cost. That’s why yield is a cost lever, not just a quality number: every point of scrap reduction lowers the effective cost of all the good parts, often more than shaving the material price would.
What has the biggest influence on what a part costs?
Design, by a wide margin. Wall thickness, material choice, tolerances, and how many secondary operations the part forces are decided before molding and lock in the large majority of the eventual cost. A molder can optimize cycle time, yield, and runner mass, but those work within the envelope the design set. The cheapest place to reduce part cost is on the design side, early — long before the tool is cut or the first shot is run.