Molding Math

How to Calculate Injection Molding Cycle Time, Clamp Tonnage, and Shot Size

Work through the five core injection molding formulas with real units and numbers: clamp tonnage, shot size, cycle time, part weight, and cavitation.

Clamp tonnage is the first number to get right because it sizes the machine. The rule of thumb is 2 to 5 tons per square inch of projected area, depending on resin and wall geometry. Take a part with a projected area of 30 square inches molded in ABS at a mid factor of 3 tons per square inch: 30 times 3 equals 90 tons, then add roughly 10 percent for runners, so about 99 tons. You would specify a 150 ton press to keep working load near 66 percent of capacity. The Clamp Tonnage calculator lets you swap the tonnage factor by material; glass-filled nylon can push 4 to 5, while polyethylene sits near 2.

Shot size defines how much melt the barrel must deliver per cycle. It equals total part weight plus runner weight, then divided by the machine's usable shot capacity to confirm fit. Say four cavities each weigh 12 grams and the cold runner weighs 9 grams: shot equals (4 times 12) plus 9, or 57 grams. Machines are rated in grams of polystyrene, so correct for resin density by multiplying by the ratio of your resin's density to 1.05. For polypropylene at 0.905, the corrected shot is 57 times (0.905 divided by 1.05), about 49 grams. The Shot Size calculator does this density correction automatically.

Part weight itself comes before shot size, and you often need it before a mold exists. Weight equals part volume times melt density. A part with 18 cubic centimeters of solid volume in polycarbonate at 1.20 grams per cubic centimeter weighs 18 times 1.20, or 21.6 grams. Pull volume from your CAD model's mass properties, and always use melt or solid density from the resin datasheet rather than a generic value. The Plastic Part Weight and Runner Weight calculators keep these separate so you can see the runner-to-part ratio, which you want under 20 percent on a cold runner tool to limit waste.

Cycle time is the throughput driver and breaks into fill, pack and hold, cooling, and mold open. Cooling dominates and scales with the square of wall thickness: roughly t equals (s squared divided by (pi squared times alpha)) times a log term, where s is wall thickness and alpha is thermal diffusivity near 0.09 square millimeters per second for many resins. A 2 millimeter wall cools in about 8 to 12 seconds; double the wall to 4 millimeters and cooling roughly quadruples to 30 to 45 seconds. Add 1 to 2 seconds fill, 3 to 6 seconds pack, and 2 to 4 seconds mold open. The Injection Molding Cycle Time calculator sums these stages.

Cavitation ties production volume to tooling investment. Cavities equal (annual volume times cycle time) divided by (available seconds per year times target efficiency). For 2 million parts per year at a 30 second cycle, with 6,000 machine hours giving 21.6 million seconds at 85 percent efficiency: (2,000,000 times 30) divided by (21,600,000 times 0.85) equals about 3.3, so you round up to a 4 cavity mold. The Mold Cavitation calculator handles the rounding and lets you test how a faster cycle reduces required cavities, which directly lowers tooling cost.

Parts per hour converts cycle time into a rate you can schedule against. It equals 3,600 seconds divided by cycle time, times cavity count. A 4 cavity mold on a 30 second cycle yields (3,600 divided by 30) times 4, or 480 parts per hour at 100 percent uptime. Apply a realistic 85 percent OEE and you plan around 408 good parts per hour. The Mold Parts Per Hour calculator chains directly off your cycle time and cavitation results, so a two second cooling improvement on that job adds roughly 30 parts per hour.

Cushion percentage is a small but critical process check, not a design number. Cushion is the residual melt left in front of the screw at the end of hold, and it should sit near 5 to 10 percent of shot size so the screw never bottoms out. On a 57 gram shot, target a cushion around 3 to 6 grams, or about 3 to 6 millimeters of screw travel on a typical 40 millimeter screw. Too little cushion means no pressure transfer and short shots; too much wastes barrel residence time. The Cushion Percentage calculator flags when you drift outside that window.

Chain the calculators in order and every downstream number stays consistent. Start with Plastic Part Weight and Runner Weight, feed those into Shot Size and Clamp Tonnage to pick the press, then set cavitation from annual volume and cycle time, and finish with Mold Parts Per Hour for scheduling. A single input error, like using 1.05 density for a 1.35 gram per cubic centimeter filled resin, cascades into an undersized shot and a stalled fill. Keep every density, area, and wall value sourced from the datasheet or CAD, and re-run the chain whenever the part design changes.

Published 2026-07-01.