A coolant is a fluid which flows through the cooling channels to regulate the temperature of the molten plastic in the mold. An ideal coolant has high thermal capacity, low viscosity, is low-cost, and is chemically inert, neither causing nor promoting corrosion of the cooling system.
The following table lists the most commonly used coolants:
| Coolant | Temperature |
|---|---|
| Ordinary water from mains or a cooling tower | 20°C–25°C |
| Ordinary water from a heater/circulator unit | > 30°C |
| Cold water from a chiller | > 10°C |
| Water mixed with antifreeze from a chiller | > ‒5°C |
| Oil-from a heater/circulator unit | > 80°C |
An important parameter that the mold designer must know is the pressure of coolant available at the plant. If the available pressure is less than the pressure that the cooling channels require, the cooling of the part could be ineffective due to non-turbulent flow of coolant.
Adding glycol (antifreeze) to the coolant will raise its viscosity, resulting in higher pumping pressures being required, or a lower flow rate. Using a chilled water and glycol mix may be less effective cooling than a well-designed circuit with plain tower water circulating at the optimum flow rate. Oil is generally used only when very high mold temperatures are required. The available pump capacity must be greater than that required by the cooling circuits in the mold.