Rubber Part Weight and Compound Charge Calculator
Rubber is bought and priced by weight but designed by drawing, so every quote starts with converting geometry into grams of compound. This calculator does that for rings, washers, bushings and discs, and adds shrink and flash so you get the real charge weight, not just the net part weight.
Your numbers
Nominal molded outside diameter of the ring, washer, grommet or bushing body.
Use 0 for a solid disc or button.
From the compound data sheet. Typical: EPDM about 1.1 to 1.3, NBR about 1.2 to 1.4, FKM about 1.8 to 2.0.
Linear post-cure shrinkage used to size the cavity. Varies by compound and cure; confirm with a first-article measurement.
Extra compound over part weight needed to fill the cavity and bleed to the flash land.
Delivered cost of mixed, sheeted compound.
Your results
Geometry here is a simple annular cylinder. For molded profiles, use CAD volume and enter an equivalent height. Always confirm charge weight with a trial shot.
Material cost per part vs flash allowance
Sweeps Flash and sprue allowance from 0 to 40, holding your other inputs fixed.
Pull part weights and compound cost from your ERP
Optional: we can review how your estimating gets weight and compound cost, and show a read-only way to pull it from your ERP.
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How to use it
Enter the molded outside diameter, inside diameter and height from the drawing, then the specific gravity from the compound data sheet. Set shrinkage and a flash allowance based on your own history for the tool style. The primary results are the charge weight per cavity, which is the preform you will cut or extrude, and the compound cost per good part. Use 0 for the inside diameter on a solid disc.
- Use molded dimensions, not post-trim or machined dimensions.
- Take specific gravity from the actual compound, not a generic family value.
- Check the flash allowance against a real weighed shot when the mold exists.
The formulas behind it
Volume of an annular cylinder is pi/4 times the difference of the squared diameters times the height. Mass is volume times specific gravity, and one cm3 of a compound at SG 1.0 weighs one gram. Rubber shrinks on cooling after cure, so the cavity is cut larger by the linear shrink rate; because volume scales as the cube of length, the cavity volume is the part volume times (1 + shrink) cubed. Charge weight is part weight times one plus the flash allowance.
Reading the results
The default 50 mm by 30 mm by 6 mm ring at SG 1.2 weighs about 12 g and needs roughly 13 g of charge at 8 percent flash. Across eight cavities that is about 0.1 kg per press cycle. The what-if chart shows how quickly material cost rises with flash allowance, which is why tool design details such as flash grooves, overflow volume and preform shape matter on high-cost compounds like FKM.
Common mistakes
The usual errors are using a family-level specific gravity, forgetting that compound density changes with filler loading, and quoting on net weight without any flash. Another is double counting shrink by also enlarging the drawing dimensions. Remember too that compression molds often need a deliberate overcharge of 3 to 10 percent, while transfer and injection tools carry sprue and runner volume that cold-runner designs cannot recycle.
Frequently Asked Questions
What specific gravity should I use for rubber?
Use the value on the compound data sheet or certificate of analysis. Unfilled natural rubber is about 0.92, but filled compounds typically range from 1.1 to 1.5, and fluoroelastomers run near 1.8 to 2.0. A wrong SG shifts weight and cost directly.
How much flash allowance is typical?
Compression molding usually runs 3 to 10 percent over part weight, but small parts and tight flash lands can need more. Weigh a trial shot and trimmed parts to find your real figure. Cold-runner injection adds sprue and runner volume on top.
Can I reuse cured rubber flash?
Generally no. Vulcanized rubber is crosslinked and cannot be remelted like thermoplastic regrind, so flash is a true cost. A few operations grind cured scrap into low-percentage filler, but it is not a normal costing assumption.
Does shrinkage change part weight?
No. Shrinkage changes the cavity size you need to cut, not the finished part weight. The cavity is larger so that after cure and cooling the part reaches drawing dimensions. Charge weight is based on the finished part plus flash.
If your ERP already holds BOM weights and compound costs, Netray can build the charge-weight and material-cost logic straight into your quoting flow. Ask for a short review of how your estimating pulls this data today.
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Free tools
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Guides
- Rubber Compound Cost and Scrap Calculation for MoldersRubber compound cost and scrap calculation: convert part volume to shot weight, add flash, runners and start-up scrap, and track yield variance in your ERP.
- ERP for Rubber Molding and Extrusion Manufacturers: What to RequireERP for rubber molding and extrusion manufacturers: what to require for compound lots, cure data, mold tracking, scrap and costing, and how common systems compare.
- Rubber Compound Lot Traceability and Shelf Life ControlControl rubber compound shelf life and lot traceability: storage, re-test dates, FEFO issue, rheometer release and genealogy from batch to shipped part.