Which ejection temperature should I enter?

Use the Moldflow ejection temperature for your grade, or the heat deflection temperature or slightly below it. The default PC grade lists 128 °C ejection with an 85 °C mold, so enter 43.

Which thermal diffusivity value should I use?

Use an effective value: averaged conductivity divided by density times specific heat over the cooling range. All three change with temperature, so a 25 °C handbook value like PC's 0.144 mm²/s is a starting point.

Does the result include pack and hold time?

Yes. The clock starts at end of fill, and the part cools during pack and hold as well as under the cooling timer. Compare it with hold time plus cooling timer, not the cooling timer alone.

Should I add a margin before setting the cooling timer?

Yes, for production. A common rule of thumb adds about 20% to the cooling timer to absorb incoming material variation and machine shifts, and more for tight-tolerance parts.

Injection Molding calculator

Mold Cooling Time Calculator

Estimate how long the thickest wall needs before its center cools to ejection temperature, and check it against your cycle. You need wall thickness, thermal diffusivity, and melt, mold and ejection temperatures.

What this calculator does

Time a molded wall needs to cool to ejection temperature, and whether your cycle gives it that long.

Formula used

Cooling time = wall² ÷ (π² × thermal diffusivity) × ln[(4 ÷ π) × (melt − mold) ÷ (ejection − mold)]
Cooling headroom = cooling time in your cycle − cooling time
Thickest wall your cycle cools = wall × √(cooling time in your cycle ÷ cooling time)
Cooling time per wall thickness squared = cooling time ÷ wall²

Inputs explained

Maximum Wall Thickness: Thickest nominal wall, rib base or boss, from CAD.
Thermal Diffusivity: Moldflow or supplier data, or conductivity ÷ (density × specific heat).
Melt Minus Mold Temperature: Melt temperature less mold surface temperature, from the setup sheet.
Ejection Minus Mold Temperature: Ejection temperature (HDT or Moldflow value) less mold temperature.
Cooling Time in Your Cycle: Pack and hold time plus cooling timer, from the setup sheet.

How to use the result

Best suited to checking a quoted cycle before cutting steel, setting a first cooling timer, arguing wall thickness in a design review.
Cooling line layout changes real cooling time; this plate model assumes a uniform mold surface. Diffusivity varies with temperature, so use an effective value averaged over the cooling range. Semi-crystalline resins such as PP release crystallization heat; include it in specific heat or the estimate runs short.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

Which ejection temperature should I enter? Use the Moldflow ejection temperature for your grade, or the heat deflection temperature or slightly below it. The default PC grade lists 128 °C ejection with an 85 °C mold, so enter 43.
Which thermal diffusivity value should I use? Use an effective value: averaged conductivity divided by density times specific heat over the cooling range. All three change with temperature, so a 25 °C handbook value like PC's 0.144 mm²/s is a starting point.
Does the result include pack and hold time? Yes. The clock starts at end of fill, and the part cools during pack and hold as well as under the cooling timer. Compare it with hold time plus cooling timer, not the cooling timer alone.
Should I add a margin before setting the cooling timer? Yes, for production. A common rule of thumb adds about 20% to the cooling timer to absorb incoming material variation and machine shifts, and more for tight-tolerance parts.

Last reviewed 2026-10-01.