Why does cooling time increase with the square of wall thickness?

In most parts, more than half of the cycle time goes to cooling. So the most effective way to shorten the cycle is usually to understand cooling time.

Heat leaves through the wall by conduction

Plastic is a poor heat conductor. The heat at the center of the part reaches the mold wall by conduction, and the time is roughly proportional to:

t ∝ (wall thickness)² / thermal diffusivity (α)

That's why:

• If you halve the wall thickness, cooling time drops to roughly a quarter.
• If you double the wall thickness, the time rises to roughly four times.

This shows why keeping wall thickness unnecessarily thick at the design stage is expensive. For example, reducing thickness from 3 mm to 2 mm shortens the time by a factor of (3/2)² = 2.25, i.e. roughly 56%.

Mold temperature and ejection temperature

The time depends on melt temperature, mold temperature, and the temperature at which the part can be ejected. As mold temperature rises, the difference between part and mold shrinks and cooling takes longer. However, in semi-crystalline materials (POM, PA, PBT, etc.) a higher mold temperature improves surface quality and dimensional stability; so the choice isn't "the coldest mold possible" but the part quality that's actually needed.

Latent heat of crystallization

Semi-crystalline materials release additional heat as they solidify, separate from the temperature drop. If this heat isn't accounted for, the calculated cooling time comes out considerably short. For example, in a 2 mm-wall POM part this effect can more than double the time. Amorphous materials (PC, ABS, PS) don't have this effect.

Does water flow rate determine the time?

No; it determines the required flow rate. Cooling time is limited by conduction through the wall. If the water isn't cold enough or isn't flowing enough, the mold heats up and the time lengthens. The correct approach:

1. Find the time from wall thickness.
2. Calculate the heat that must be removed in that time.
3. Provide the flow rate at which the water's inlet-outlet temperature difference stays within a few degrees, and ensure turbulent flow (Reynolds number above roughly 4000).

Practical checks

• If the water inlet-outlet difference exceeds a few °C, the flow rate is insufficient or a channel is clogged (scale).
• If the flow in the channel is laminar, heat transfer is poor; reduce the channel diameter or increase the flow rate.
• Measure the mold surface temperature with a thermometer or infrared sensor; water temperature and mold surface temperature are not the same.

Note: The ratios above are based on a flat-plate model; corners, ribs and thick regions extend the time. The result should be confirmed during the trial run.

The calculation tool calculates cooling time from wall thickness, latent heat and water flow rate separately, and shows which one is the limiting factor.