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Injection Molding Cycle Time Calculator

Cycle time drives part cost more than almost any other molding variable, and cooling is usually the largest slice. This calculator estimates cooling from wall thickness and temperatures, then builds up to cycle time, parts per hour and shift output.

Your numbers

mm
mm2/s

Roughly 0.07 to 0.10 for ABS, PC and PS, about 0.10 to 0.15 for PP and PE.

deg C
deg C
deg C

Average part temperature at which it is stiff enough to eject.

s
s
s
%

Your results

Cycle time
20.4047
Seconds per shot. Hold is treated as part of cooling.
Parts per hour
706
Estimated cooling time
13.9047
Seconds, measured from the start of fill.
Cooling share of cycle
68.1%
Shots per hour
176.43
Good output per 8 hour shift
4,799

The cooling formula is a one-dimensional estimate for a flat section. Real cooling depends on channel layout, coolant temperature, material and part geometry.

Save results:

Cycle time (s) vs wall thickness

Sweeps Maximum wall thickness from 1 to 6, holding your other inputs fixed.

Compare routing cycle times with real press data

Optional: a short review of how to feed actual cycle times from your presses back into ERP routings.

Optional. The tool and every export work without this. No spam.

How to use it

Enter the thickest wall that governs cooling, the resin's thermal diffusivity, and the three temperatures: melt, mold wall and ejection. Add fill time, hold time and the dry cycle (mold open, eject and close). Enter cavity count and a realistic efficiency for the job. The tool returns cooling time, total cycle, parts per hour and an eight hour output. Use it during quoting, then compare against actual press data once the job runs.

  • Use the thickest wall that is not a rib or boss.
  • Ejection temperature is the part average, not the surface.
  • Check the dry cycle against the press datasheet.

The method

The estimate uses the classic one-dimensional heat conduction result for a slab: cooling time equals wall thickness squared, divided by pi squared times thermal diffusivity, times the natural log of four over pi times the ratio of melt minus mold temperature to eject minus mold temperature. Cooling grows with the square of thickness, so a 3 mm wall takes about 1.44 times as long as a 2.5 mm wall. Hold time overlaps cooling, so the cycle takes the larger of the two after fill.

Worked example and reading results

With the defaults (2.5 mm ABS-like wall, 230 C melt, 60 C mold, 90 C eject) cooling comes out near 14 seconds, giving a cycle of roughly 20 seconds. Four cavities then make about 700 parts per hour, or about 4,800 good parts per shift at 85 percent efficiency. If measured cooling on the press is far longer, look at coolant temperature, flow rate, channel distance to the cavity and hot spots rather than just adding time.

Common mistakes

Using nominal wall rather than the thickest section understates cooling. Setting the mold temperature to the chiller setpoint instead of the actual wall temperature hides poor channel design. Counting hold and cooling as additive double counts time. Finally, remember that plasticizing time can become the limit on large shots with slow screw recovery, so compare recovery time to the cooling time before trusting the result.

Frequently Asked Questions

How is injection molding cooling time calculated?

A common estimate is wall thickness squared divided by pi squared times thermal diffusivity, multiplied by the natural log of the temperature ratio between melt, mold and ejection temperature. It is a one-dimensional approximation for a flat wall.

What share of the cycle is cooling?

For many parts cooling is 50 to 80 percent of the total cycle, rising with wall thickness. Thin-walled packaging is closer to fast fill and short cooling, while thick technical parts are dominated by cooling.

How do I reduce cycle time?

Look at cooling first: colder coolant, turbulent flow, conformal or closer channels, thinner walls and a higher allowable ejection temperature. Then trim hold time using a gate-seal study and tighten the dry cycle.

Why is my real cycle longer than the estimate?

The estimate ignores robot or degating time, slow ejection, screw recovery, thick ribs and bosses, and cooling circuit inefficiency. Compare each element against a timed cycle breakdown on the press.

If your ERP routings still carry guessed cycle times, we can help compare them with real press data. Reach out for a short review.

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