Cooling of large cores
For large core diameters (40 mm and larger), a positive transport of coolant must be ensured. This can be done with inserts in which the coolant reaches the tip of the core through a central bore and is led through a spiral to its circumference,

and between core and insert helically to the outlet. This design weakens the core significantly.
Cooling of slender core

Cooling of cylinder cores and other round parts should be done with a double helix, The coolant flows to the core tip in one helix and returns in another helix. The wall thickness of the core should be at least 3 mm in this case.

If the diameter or width is very small (less than 3 mm), only air-cooling is feasible. Air is blown at the cores from the outside during opening or flows through a central hole from inside. This procedure, of course, does not permit maintaining an exact temperature.

Better cooling of slender cores (those measuring less than 5 mm) is accomplished by using inserts made of materials with high thermal conductivity, such as copper or beryllium-copper materials. Such inserts are press-fitted into the core and extend with their base, which has a cross section as large as is feasible, into a cooling channel.