After ejection, the die surface cools. During this time the compressive stress levels are reduced. If the die surface is sprayed, it experiences very rapid cooling as the water contained in the die lubricant evaporates. This leads to the stresses becoming tensile, peaking just after the end of spraying, as the surface tries to contract but is constrained by the now hotter subsurface areas.
This process repeated over a few thousand cycles causes thermal fatigue cracking in the surface of the die and the appearance of heat checking.
Though the analysis of temperatures and stresses in the die during casting, simulation can determine how the cyclical stress loading will influence the onset of heat checking.