Injection Molding

How to Calculate Injection Molding Cycle Time

Injection molding cycle time determines your press output and part cost. This guide walks through the formula, typical benchmarks, and where estimates usually go wrong.

Injection molding cycle time equals fill time plus pack or hold time plus cooling time plus mold open, close, and eject time. For a thin-walled part at 2 mm wall thickness, cooling alone is typically 60% to 70% of total cycle, often 8 to 15 seconds on a 20 to 25 second cycle. Fill time is usually 0.5 to 2 seconds, and pack or hold is often 2 to 8 seconds depending on resin and geometry. Mold motion adds another 2 to 5 seconds depending on press size and automation. Total cycle drives parts per hour, press loading, and the machine cost built into every piece price.

Cooling time is the most variable input and the one most often underestimated. A simplified cooling formula is t = (s^2 / (pi^2 x alpha)) x ln((4/pi) x (Tmelt - Tcoolant) / (Teject - Tcoolant)), where s is wall thickness, alpha is resin thermal diffusivity, and the T terms are melt, coolant, and ejection temperatures. For polypropylene with a 3 mm wall, this often works out to roughly 18 to 22 seconds of cooling. Thermal diffusivity varies by resin, roughly 0.07 mm2/s for PP and about 0.14 mm2/s for nylon, so resin choice changes cycle time even when part thickness stays the same. Fill and hold values usually come from machine data, process sheets, and first article trials rather than from mold design assumptions alone.

The most common mistake is using the process engineer's target cycle instead of the actual measured average on the floor. Target and actual often differ by 10% to 20% because of weak cooling circuits, inconsistent ejection, or machine acceleration limits. Another common miss is ignoring mold motion time on large-tonnage presses. A 1000 ton press may need 4 to 6 seconds just for clamp open and close, while a 250 ton press may only need 2 to 3 seconds. Those missing seconds can cost hundreds of parts per shift when quoting a high-volume job.

Once you have cycle time, divide 3600 seconds by cycle time to get theoretical parts per hour. Then multiply by cavity count and uptime to get realistic output. For a 24 second cycle with 8 cavities at 85% uptime, output is (3600 / 24) x 8 x 0.85 = 1020 parts per hour. That is the number you use for capacity planning, labor loading, and quote validation. If actual output is consistently below this number, investigate uptime losses and cavity imbalance before assuming cycle time is the main issue.

A 2 second reduction on a 24 second cycle is an 8.3% throughput gain. On a press running 6000 hours per year, that gain recovers about 500 hours of capacity. Compare that recovery against the cost of better cooling inserts, conformal cooling, faster ejection, or improved automation. Also review related variables such as clamp tonnage margin, shot size, and actual cooling water temperature because they can cap the cycle improvement you can realistically keep in production. Accurate baseline cycle data makes those improvement payback decisions straightforward.

Published 2026-05-28.