From CAD Volume to Shot Weight
Start with part volume from the model, multiply by compound specific gravity from the data sheet, and you have net part weight. Specific gravity varies a lot between polymers and fillers, with silicone, EPDM, NBR and FKM all different, so never use an average. Then add the material the cavity cannot keep: flash that squeezes out of the parting line, preform overhang, and for transfer or injection the sprue, cull and runner.
Preform weight is typically set a few percent above cavity volume to ensure fill, and that overage is real cost. Weigh actual preforms and flash from a first article to replace estimates.
Building the Scrap Factor
A single scrap percentage hides too much. Separate the sources so you can see which one to attack. Start-up scrap covers the first cycles after a mold change, when you dial in temperature and cure.
Process scrap covers rejects from blisters, backrinding, knit lines, short shots and sticking. Material scrap covers preforms or slab that sat too long and scorched, out-of-spec compound lots and mill trim. Inspection scrap covers parts that fail dimensions, durometer or visual standards.
Each source has a different owner, so separate reason codes are far more useful than one blended number.
- Start-up and changeover scrap
- Process defects: blisters, short shots, knit lines, sticking
- Aged or scorched compound and preform losses
- Inspection and customer return scrap
In-House Mixing Versus Buying Compound
If you mix on a mill or in an internal mixer, your compound cost includes raw polymer, fillers, plasticizers, curatives, mixing labor, power and mixer scrap, plus the cost of QC on every batch (rheometer cure curve, hardness and specific gravity). If you buy mixed slab or strip, you pay a compounder price per pound but may also pay minimum batch quantities and carry inventory that ages. Compare on a fully loaded basis, including shelf life and the cost of lots that fail incoming inspection.
A shop that mixes only a few custom compounds may find outsourcing cheaper once the true mixer overhead is counted.
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Forging Billet Weight and Yield Calculator
Work out the billet weight needed for a forged part given flash, scale and butt losses, then see yield and net material cost per piece.
Tracking Yield Variance in the ERP
To know real compound cost, issue compound to the job by weight and compare it to the standard shot weight times good parts. The difference is yield variance. In ERPs such as Infor SyteLine, DELMIAworks, Epicor and NetSuite, material issued against a job and good quantity reported against it produce this variance if you record both consistently.
The usual failure is that operators issue compound in bulk and the job never closes against actual weight, or scrap is reported without a reason code. Weigh preforms at the press, use backflush carefully, and review variance by compound and by mold weekly.
When the Detailed Approach Is Overkill
If compound is a small fraction of part cost, as with low-cost EPDM on labor-heavy parts, detailed yield tracking may not pay back, and a single scrap factor reviewed quarterly is fine. The effort is worth it for FKM, fluorosilicone, HNBR, or any job where material is more than about a third of cost. Start with your five most expensive compounds and your ten highest-volume molds, and expand only if the data uncovers real variance.
Frequently asked questions
How do I calculate rubber part weight from volume?
Multiply the part volume by the compound specific gravity from the data sheet, using consistent units. That gives net weight. Then add flash, sprue, cull, runner and preform overage to get the shot weight you actually consume. Confirm by weighing a few real shots.
What is a normal scrap rate in rubber molding?
It varies widely with polymer, part geometry, mold quality and process, so there is no single normal figure. Simple parts in stable processes can run low single digits, while complex bonded or tight-tolerance parts run much higher. Track your own rate by mold and reason code instead of benchmarking against a general number.
Can scrap rubber be reground and reused?
Cured thermoset rubber cannot be remelted like thermoplastic. Some cured scrap is ground and used as filler in lower-grade products, but it generally cannot go back into the same compound. Uncured scrap such as mill trim or unused preforms can sometimes be reworked, if cure state and customer spec allow.
Netray can build a private assistant over your ERP that flags jobs where compound usage or scrap drifts from standard, with the query shown so you can verify every figure.