Molding Calculations

How to Calculate Yield, Cavitation, and Cure Time for Molded Seals

Step-by-step formulas for the five calculations that govern molded seal, gasket, and O-ring production, with units and worked examples.

Rubber blank yield is the first number to nail down because it feeds every downstream estimate. Yield equals net part volume times cavity count, divided by charge weight per shot. Take a 40mm O-ring, 3.53mm cross-section: torus volume is 2 times pi squared times R times r squared, so 2 times 9.87 times 20mm times (1.765mm) squared equals about 1,229 cubic mm, or 1.23 cubic cm. At 1.35 g/cc for a nitrile compound, that is 1.66 g of cured part. If your preform charge is 2.1 g, blank yield is 1.66 divided by 2.1, or 79 percent. The Rubber Blank Yield calculator handles multi-cavity charges directly.

Mold cavitation output converts cavity count and cure cycle into parts per hour. Output equals cavities times 3,600 divided by total cycle seconds, times a good-parts factor. A 48-cavity tool at 210 seconds cure plus 40 seconds load and unload runs a 250-second cycle: 48 times 3,600 divided by 250 equals 691 parts per hour gross. Apply a 0.94 first-pass yield and you plan on 650 saleable parts per hour. Run this in the Mold Cavitation Output calculator, then check it against Cure Press Capacity to confirm your press tonnage supports the projected number of cavities at the required 3,000 to 5,000 psi cavity pressure.

Batch cure time comes from the compound rheometer, not a guess. The rule of thumb is t90 at mold temperature plus a section-thickness adder of roughly 1 minute per additional 1mm of the thickest cross-section beyond the reference specimen. If the MDR reports t90 of 4.5 minutes at 180C for a 2mm disc and your part peaks at 6mm, add about 4 minutes, giving a 8.5-minute press cure. Every 10C rise roughly halves cure time for most sulfur systems, so moving from 170C to 180C can cut a 9-minute cure to near 4.5 minutes. The Batch Cure Time calculator applies the Arrhenius shift for you.

Material shrinkage sets the mold cavity size, and getting it wrong scraps the whole tool. Cavity dimension equals nominal part dimension times (1 plus shrink fraction). A compound with 2.0 percent linear shrink means a 40.00mm target ring needs a cavity cut to 40.00 times 1.020, or 40.80mm. Shrinkage is temperature dependent: a part demolded at 180C and measured at 23C sees the bulk of its contraction from thermal plus cure shrink combined. Fillers cut shrink; a 60 durometer black may run 1.6 percent while an unfilled silicone runs 3.0 to 4.0 percent. The Material Shrinkage calculator lets you enter compound-specific rates per axis.

Seal groove fill verifies the O-ring actually seals without over-stuffing the gland. Fill percent equals O-ring cross-section area divided by gland cross-section area, times 100. For a 3.53mm cord in a groove 2.8mm deep by 4.8mm wide, ring area is pi times 1.765 squared, or 9.79 square mm; gland area is 2.8 times 4.8, or 13.44 square mm. Fill is 9.79 divided by 13.44, or 73 percent. Target 60 to 85 percent: below 60 you risk leak paths, above 90 you risk extrusion and groove damage under thermal expansion. The Seal Groove Fill calculator flags out-of-range glands instantly.

Compression set margin tells you whether the seal still pushes back after service. Compression set percent equals (original thickness minus recovered thickness) divided by (original thickness minus spacer thickness), times 100. A ring compressed 25 percent for 70 hours at 100C that recovers to 3.35mm from a 3.53mm original, using a 2.65mm spacer, gives (3.53 minus 3.35) divided by (3.53 minus 2.65), or 0.18 divided by 0.88, equal to 20 percent set. Your margin is the spec limit minus actual: a 25 percent spec with 20 percent measured leaves 5 points. The Compression Set Margin calculator converts test data straight to remaining sealing force headroom.

Tie the numbers together before you commit a tool. Suppose you need 500,000 nitrile O-rings a year. At 650 good parts per hour from a 48-cavity mold running 6,000 hours, one press yields 3.9 million a year, so a single tool with spare capacity covers the volume with margin for the 6 percent scrap already baked into the yield factor. Cross-check charge weight against the Rubber Blank Yield figure of 2.1 g per part times 500,000, or 1,050 kg of compound before scrap, roughly 1,120 kg after a 6.7 percent flash and reject allowance.

A quick unit-discipline checklist prevents the most common math errors. Keep volume in cubic cm and density in g/cc so mass falls out in grams. Convert durometer to nothing; it is a hardness index, not a modulus, so never plug it into a force equation directly. Cure times live in minutes at the mold surface temperature, not the setpoint, which can differ by 5 to 12C on a cold platen. Shrinkage is dimensionless as a fraction but must be applied per axis for anisotropic parts. Run each figure through the matching calculator and the units reconcile on their own.

Published 2026-07-01.