Estimate injection molding cooling time from wall thickness, material thermal diffusivity, and melt, mold, and ejection temperatures.
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Injection Molding Cooling Time Formula
The calculator estimates cooling time from wall thickness, thermal diffusivity, and the temperature drop needed before ejection. Wall thickness is converted to millimeters, and temperatures entered in °F are converted to °C before the formula is applied.
t_c = (s^2 * ln((4/pi) * ((T_m - T_w) / (T_e - T_w)))) / (4*pi^2*alpha)
- tc = estimated cooling time, in seconds
- s = wall thickness, in millimeters
- ln = natural logarithm
- pi = 3.14159
- Tm = melt temperature, in °C
- Tw = mold wall or mold temperature, in °C
- Te = ejection temperature, in °C
- alpha = thermal diffusivity of the plastic, in mm²/s
The material selector fills in a typical thermal diffusivity value. If you choose custom, you can enter your own alpha value in mm²/s. The thickness unit selector converts inches to millimeters using 1 in = 25.4 mm. The temperature unit selector converts °F to °C so the temperature ratio is consistent.
The temperature inputs must follow this order: melt temperature must be highest, mold temperature must be lowest, and ejection temperature must fall between them.
Typical Thermal Diffusivity Values for Injection Molding Plastics
These are the preset alpha values used by the calculator. Actual values can vary by grade, filler content, moisture, and processing conditions.
| Material | Thermal diffusivity alpha | General note |
|---|---|---|
| ABS | 0.11 mm²/s | Common amorphous molding resin |
| PP | 0.14 mm²/s | Semi-crystalline resin with moderate cooling rate |
| HDPE | 0.20 mm²/s | Higher diffusivity, often cools faster for the same wall |
| PS | 0.10 mm²/s | Common amorphous resin |
| PC | 0.12 mm²/s | Often processed at higher melt and mold temperatures |
| PA6 | 0.13 mm²/s | Nylon grade and moisture can affect behavior |
| POM | 0.15 mm²/s | Semi-crystalline engineering resin |
Input Rules and Result Sensitivity
| Input or condition | Effect on cooling time |
|---|---|
| Wall thickness increases | Cooling time rises with thickness squared. Doubling wall thickness gives about 4 times the cooling time, if other inputs stay the same. |
| Thermal diffusivity increases | Cooling time decreases because heat moves through the material faster. |
| Ejection temperature is set lower | Cooling time usually increases because the part must cool further before ejection. |
| Mold temperature is raised | Cooling time can increase because the mold removes heat less aggressively. |
Example Problems
Example 1: ABS part in metric units
Suppose you have a 2.5 mm ABS wall, alpha = 0.11 mm²/s, melt temperature = 230 °C, mold temperature = 60 °C, and ejection temperature = 100 °C.
t_c = (2.5^2 * ln((4/pi) * ((230 - 60) / (100 - 60)))) / (4*pi^2*0.11)
The estimated cooling time is about 2.43 seconds, or 0.0405 minutes.
Example 2: PP part using inches and °F
Suppose you have a 0.12 in PP wall, alpha = 0.14 mm²/s, melt temperature = 430 °F, mold temperature = 100 °F, and ejection temperature = 180 °F.
First, 0.12 in converts to 3.048 mm. The temperatures are converted to °C before calculation.
t_c = (3.048^2 * ln((4/pi) * ((221.11 - 37.78) / (82.22 - 37.78)))) / (4*pi^2*0.14)
The estimated cooling time is about 2.79 seconds, or 0.0465 minutes.
FAQ
Why does wall thickness affect cooling time so much?
Cooling time is proportional to wall thickness squared. A small increase in wall thickness can make a large difference in cycle time. For example, increasing a wall from 2 mm to 4 mm does not double the estimated cooling time. It increases it by about 4 times, assuming the same material and temperatures.
What ejection temperature should you use?
Use the temperature where the part is rigid enough to eject without warping, sticking, or leaving ejector marks. This depends on the resin, part geometry, mold design, and acceptable dimensional change. The ejection temperature must be above the mold temperature and below the melt temperature for the formula to work.
Is this the same as total cycle time?
No. Cooling time is only one part of the injection molding cycle. Total cycle time also includes injection, packing, screw recovery, mold opening, ejection, and mold closing. Cooling is often the largest part of the cycle, but it is not the full cycle by itself.
