Injection Molding Cooling Time Calculator
Cooling is usually the biggest slice of an injection molding cycle, and it grows with the square of wall thickness. This calculator gives a quick estimate of the cooling time so you can quote, compare resins or judge whether a design change will pay back in cycle time.
Your numbers
Use the thickest section that must be solid at ejection.
Common thermoplastics fall roughly between 0.05 and 0.12 mm2/s. Check the resin datasheet.
Often near the heat deflection temperature of the resin. Must be above the mold temperature and below the melt temperature.
Everything in the cycle that is not cooling after pack.
Your results
One-dimensional estimate. Cooling channel layout, hot spots, ribs and bosses change the real result. Validate with mold flow analysis or trial data.
Cooling time (s) vs wall thickness (mm)
Sweeps Maximum wall thickness from 0.8 to 6, holding your other inputs fixed.
Compare standard cycles with actual cycles
Optional: we can review how routings and actual run times are held in your ERP and show where standards drift.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the thickest wall that must be rigid at ejection, a thermal diffusivity for the resin, and the three temperatures: melt, mold wall and the centerline temperature at which the part can be ejected without distortion. Add the time spent in fill, pack and mold movement to see the full cycle and shots per hour. The chart sweeps wall thickness so you can see the penalty of adding a millimeter.
The formula
For a flat plate cooled from both sides, cooling time t = h squared divided by (pi squared x alpha) x ln of [4/pi x (Tmelt - Tmold) / (Teject - Tmold)], where h is the wall thickness and alpha the thermal diffusivity. Doubling the wall thickness roughly quadruples the cooling time, which is why coring out thick sections is the strongest cycle reduction lever. Lowering mold temperature or raising allowed ejection temperature also helps, but each has quality limits.
- Thickness is squared, so small thickness changes matter
- Higher ejection temperature shortens cooling but risks warpage and ejector marks
- Mold temperature affects crystallinity and surface finish, not just time
Worked example
A 2.5 mm polypropylene-type wall with alpha 0.09 mm2/s, melt 230, mold 50 and ejection 90 deg C gives a log term of about 1.75 and a cooling time near 12 seconds. With 9 seconds of other cycle time, the shot takes about 21 seconds and cooling is close to 58 percent of it. Thinning the wall to 2.0 mm cuts cooling to roughly 8 seconds.
Common mistakes
Using the nominal wall instead of the thickest section, mixing Celsius and Fahrenheit, and treating the result as exact. Real parts have ribs, bosses and cooling channel gaps that stretch the time. Also remember the screw must recover during cooling, so cooling time cannot drop below the plasticizing time of the machine.
Frequently Asked Questions
Why does thicker wall cooling time rise so fast?
Heat has to conduct to the mold surface, and the time for conduction scales with the square of the distance. A wall twice as thick takes about four times as long to cool to the same centerline temperature.
What ejection temperature should I use?
Pick a centerline temperature at which the part is stiff enough to resist ejector forces and warpage, commonly near or below the heat deflection temperature of the resin. Your resin supplier or a trial run gives the best value.
Is this accurate enough for quoting?
It is a sound first estimate for flat walls. Complex geometry, conformal cooling and hot spots change the result. Compare against trial cycle times and adjust the effective diffusivity until the estimate matches your data.
Can cooling time be shorter than the screw recovery time?
No. Screw recovery runs during cooling, so the cycle cannot go below the plasticizing time for the shot. If recovery is the limit, a larger screw or a smaller shot size is needed, not faster cooling.
Netray can compare estimated cycle times against actual cycle times captured from your MES or ERP routings, so quotes stop drifting from shop floor reality.
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