Theoretically, cooling time is proportional to the square of the heaviest part wall thickness or the power of 1.6 for the largest runner diameter. That is:
where the thermal diffusivity of polymer melt is defined as
In other words, doubling the wall thickness quadruples the cooling time.
Reynolds number and coolant flow
Whether or not the coolant flow is turbulent can be determined by the Reynolds number (Re), as listed in Table 1. The Reynolds number is defined as:
where ρ is the density of the coolant, U is the averaged velocity of the coolant, d is the diameter of the cooling channel, and η is the dynamic viscosity of the coolant.
TABLE 1. Coolant flow types and corresponding
Reynolds number ranges