The discussion of the effect of scale on the die cast process begins with considering the reasons that we design in thermal passages in the die. Benefits of thermal passages can include

Cycle time reduction

Ordering of solidification

More effective die preheating

Refinement of grain structure

Transfer of heat around die

Reduction of poor fill / knit lines

Die solder control

Vent freeze off

Even though it is possible using die spray technique to run many dies without thermal passages, the yield reduction usually makes this an uneconomic choice.

As a young die cast engineer it took a while to understand why a die that started making good parts deteriorated into making scrap. Die casting is a thermal process. Scale build up in the thermal passages within the die upsets that process. And no, you do not need fancy instrumentation to measure the impact of scale build up. I have seen cores for making the fine grain structure needed to create castings with dry seal pipe threads accumulate enough scale to stop the flow in one shift. This was because boiling within the passage deposited the accumulated tower water minerals. Ostrich engineering applies. If you stick your head in the sand, you will not look for the scale build up within the thermal passages in the die.

Once you admit that your die cast process creates scale, it is possible to implement improvements. I have found that the a major reduction in scale formation is possible. Next I rank ordered a list of thermal strategies from worst to best

Tower water- steel pipes-atmospheric outlet pressure

City water- steel pipes-atmospheic outlet pressure

Hot oil-steel pipes-pressurized outlet

Closed loop hot water- stainless pipes- pressurized outlet

Closed loop de-ionized water- stainless pipes-pressurized outlet

(stainless pipes reduce rust build up)

(pressurized outlets control boiling within the die)

Do not feel bad if your plant currently uses a less favourable strategy. I can make a list because I have been there.