In order to control the temperature of the mold, it is common to use water as the coolant if the temperature is less than 100°C. The cooling water whose temperature is controlled by the re-circulating pump of the temperature controller, circulates inside the cooling water holes provided inside the mold, and stabilizes the temperature of the mold by heat transfer and radiation.

Normally, it is not possible to see from the outside of the mold the way that the cooling water is circulating (it can be seen if the mold is transparent, though). In actuality there are two patterns for the flow of cooling water in terms of fluid mechanics, namely, "laminar flow" and "turbulent flow".

In order to efficiently carry out temperature control of the mold, the desirable condition for the flow of the cooling water is a "turbulent flow". The condition to make the flow turbulent can be roughly calculated by the indices called the dynamic coefficient of the viscosity of the fluid, the hole diameter, and the Reynolds number that is determined by the flow speed. Limiting the discussion only to water, it is possible to realize a turbulent flow of the cooling water by making the water flow at more than a certain speed. In other words, if the diameter of the cooling water hole of the mold is determined, in order to make the flow turbulent, it is sufficient to make the supply flow rate from the circulating pump higher than a certain value. In this case, since the dynamic coefficient of viscosity changes with the water temperature, it is necessary to change the flow rate in proportion to the water temperature.

The data that are indices for a turbulent flow are listed in the following Table.

Table: Turbulent flow region limiting flow rate of cooling water (L/min)