When the mold is in production, deposits are likely to form on the cooling channel surfaces, especially in hard water areas. This is called fouling and it greatly reduces the effectiveness of heat transfer from the metal to the coolant.
The thermal conductivity of mineral can be 98 percent less than tool steel so 1 mm of fouling deposit gives the equivalent thermal resistance to 50 mm of tool steel.
Fouling is a production maintenance problem, and the effect of fouling should not be considered at the design stage.