Calculations

How to Calculate Cure Time, Shrinkage, and Batch Yield in Rubber Manufacturing

The core rubber-process formulas worked in real units: cure time scaling, shrinkage allowance, batch yield, calender throughput, and die swell.

Start with cure time, the master variable in molding. Rheometer data gives you t90 (time to 90 percent of maximum torque) at a reference temperature, say 8.5 minutes at 160 C. To scale to a hotter press you apply the Arrhenius rule of thumb: cure rate roughly doubles per 10 C rise. Moving from 160 C to 180 C is 20 C, so divide 8.5 by 2^(20/10) = 4, giving about 2.1 minutes of flat cure. Then add heat-penetration time, estimated as thickness squared over diffusivity: a 12 mm part at k near 0.15 mm squared per second needs 144/0.15 = 960 seconds thermal lag at the core. The Tire Cure Time calculator combines both terms so you do not under-cure thick sections.

Shrinkage allowance sizes the mold cavity above the target part dimension. Rubber shrinks as it cools from cure temperature, typically 1.5 to 3.0 percent linear for most compounds, higher for peroxide-cured EPDM. The formula: cavity dimension = part dimension divided by (1 minus shrink fraction). For a seal that must finish at 50.00 mm with 2.2 percent shrink, cut the cavity at 50.00 / (1 - 0.022) = 51.12 mm. Get the fraction from a certified cure-and-measure test, not the datasheet, because filler loading and post-cure both shift it. The Rubber Shrinkage Allowance calculator inverts this so you enter finished size and shrink percent and read cavity size directly.

Banbury batch size depends on chamber volume and fill factor. Net mixing volume equals rotor-free chamber volume times fill factor, usually 0.70 to 0.78 for productive mixing. A nominal 250 liter chamber at 0.72 fill gives 180 liters of working volume. Convert to mass with compound density: a carbon-black-loaded SBR at 1.15 g per cc yields 180,000 cc times 1.15 = 207 kg per drop. If you overfill past 0.80 you lose dispersion and spike ram pressure; below 0.65 you waste cycle time. The Banbury Mixer Capacity and Mixing Batch Yield calculators turn chamber size, fill factor, and density into kilograms per batch.

Calender throughput is a rate calculation from sheet geometry and line speed. Mass flow equals width times thickness times speed times density. Running a 1.4 m wide sheet at 1.8 mm gauge, 25 m per minute, compound density 1.12 g per cc: 1400 mm times 1.8 mm times 25,000 mm per minute equals 63.0 million cubic mm per minute, times 0.00112 kg per cubic cm (1.12 g per cc) gives about 70.6 kg per minute, or 4,230 kg per hour. Bank size and roll temperature cap the practical speed. The Calender Throughput calculator does this conversion and flags when gauge tolerance forces you to slow the line.

Extrusion output pairs a volumetric rate with die swell. Volumetric rate from screw geometry is roughly drag flow: Q equals a constant times screw diameter squared times channel depth times RPM, minus pressure back-flow. For quoting, most shops measure it empirically: weigh 60 seconds of extrudate, divide by density for cc per minute. Die swell then corrects the die opening: swell ratio B equals extrudate dimension over die dimension, commonly 1.20 to 1.50 for unfilled gum, dropping toward 1.05 to 1.15 with 50 phr carbon black. To hit a 6.0 mm profile at B of 1.25, cut the die at 6.0 / 1.25 = 4.8 mm. Use the Extrusion Swell and Rubber Extrusion Rate calculators together.

Mold cavity output ties cure time to press capacity. Parts per hour equals cavities times 3600 divided by total cycle time in seconds. Total cycle is cure time plus load, close, open, and demold, often 20 to 40 seconds of non-cure overhead. A 16-cavity tool with 120 second cure and 30 second handling runs 3600 / 150 = 24 cycles per hour, times 16 = 384 parts per hour. Press availability at 85 percent drops that to about 326 good parts. The Mold Cavity Output calculator lets you test how shaving 15 seconds off cure, worth 24 extra parts per hour here, pays back against scorch risk.

Scrap-adjusted yield converts gross output into sellable units. Effective yield equals gross parts times (1 minus scrap fraction). A press-cure line at 4 percent scrap and 384 gross parts per hour nets 369 good parts. Material yield differs: subtract flash, sprue, and runner mass from the shot. If each shot is 95 g and finished part is 82 g, material utilization is 82 / 95 = 86 percent, so 14 percent goes to trim that you may reclaim or lose. The Elastomer Scrap Cost and Mixing Batch Yield calculators track both piece-count scrap and mass utilization so your kilograms-in matches your parts-out.

Tie the chain together to size a job. Suppose an order needs 50,000 gaskets at 82 g finished. At 86 percent material utilization each consumes 95 g, so you mix 50,000 times 0.095 equals 4,750 kg net, or 4,940 kg accounting for 4 percent line scrap. At 207 kg per Banbury drop that is 24 batches. On the press at 369 good parts per hour you need 136 press hours, roughly 17 shifts at 8 hours. Every number here traces back to one measured input: t90, shrink percent, fill factor, density, or swell ratio. Nail those five and the rest of the calculations follow cleanly.

Published 2026-07-01.