The mold can be thought of as a heat exchanger because heat passes into the mold and transfers out of it.
The mold's primary energy input is hot plastic injected into the cavity. Hot runners can also be a an energy source.
During mold filling, heat is lost by the following three mechanisms:
- Coolant flow in the cooling circuits
- About 80-95 percent of the heat introduced into the mold by the plastic melt is transferred by conduction through the metal to the surface of the cooling channels and dissipated into the heat transfer fluid.
- Conduction to the injection molding machine mold
- Most of the heat will be extracted by coolant flow. When running the mold hot, the heat lost through the mold and to the atmosphere may exceed heat input from the plastic melt.
- Convection and radiation to the atmosphere
- Convection on the mold surfaces and conduction into the molding machine are of only minor importance, accounting for typically 5-15 percent of the total heat flow. The radiative heat transfer should be considered only when the temperature of the mold is high, that is, greater than 85°C, because the radiative heat flow is typically less than 5 percent of the total amount.
In addition to the heat input from the plastic melt, the hot runners and manifolds can also contribute heat to the mold. Cases where the coolant is at a temperature well above the ambient temperature also contribute heat.
Air gaps
A layer of air can impair the effective transfer of heat. Therefore, take steps to eliminate any air gaps between the mold insert and molding plates, and any air pockets in the cooling channels.
Temperature difference
The temperature difference on opposite sides of the part should be kept to a minimum and should not exceed 10°C for parts that require a tight tolerance.