Theoretically, cooling time is proportional to the square of the heaviest part wall thickness and the largest runner diameter raised to the power of 1.6, and inversely proportional to the thermal diffusivity of the polymer melt.
Cooling time
These relationships are given by:
is the cooling time, 
is the thickness at the thickest part of the part wall, 
is the diameter of the largest runner, and 
is the thermal diffusivity.
In other words, doubling the wall thickness quadruples the cooling time.
The thermal diffusivity of polymer melt is defined as:
where:
where:
is the thermal conductivity 
is the density 
is the specific heat constant volume
Reynolds number and coolant flow
The type of coolant flow can be determined by the Reynolds number, 
, as listed in the following table.
| Reynolds number () |
Type of flow |
|---|---|
| 10,000 < |
Turbulent |
| 2,300 < < 10,000 |
Transition |
| 100 < < 2,300 |
Laminar |
| < 100 |
Stagnated |
The Reynolds number is defined as:
where:
where:
- is the density of the coolant
is the averaged velocity of the coolant 
is the diameter of the cooling channel 
is the dynamic viscosity of the coolant