The cooling technique also presents problems during the drilling of the passages.This method allows drilled cooling passages to be placed precisely in the mold;

often adjacent to difficult-to-cool casting sections within the mold cavity. The major disadvantage of this design is the inability to set up uniform or differential cooling and to avoid hot or cold spots in other regions of the mold.

Since the passages must be straight and perpendicular to each other, elaborate drilling patterns are required to cool the mold. Then, after drilling the paths, the excess openings must be closed with special threaded plugs. However, these plugs may still leak due to differences in thermal expansion between the mold and plug material.

Once plugged, the mold needs to be pressure tested to ensure against leaks. If this testing shows a leak formed by porosity in the mold material, the mold must be scrapped. However, the leak may not be evident during testing as some leaks only are discovered when the mold is heated during use, increasing the size of a small defect in the material to a large (and dangerous) problem. Since the drilling and testing follows machining of the mold, the cost of machining also would be lost with the scrapped mold.
Instead of drilled passages, another option for permanent mold foundries is to have the tool maker cast prefabricated steel pipe into the mold.

Prefabricated Piping in the Permanent Mold-Prefabricated cooling passages may be designed with one water circuit or two separate circuits. In this method, steel pipe is prefabricated to the desired cooling pattern and then cast into the iron during mold production. This method allows water to pass directly across any areas that would need cooling without restriction to straight-line distances. Another advantage of this system in comparison to the drilled passages is the virtual elimination of problems related to the mold leakage.