Checking Out Heat Checking

Die casting tooling usually has a limited lifetime due to thermal fatigue cracking or ‘heat checking’ of the die surface, caused by high cyclical thermal loads. The surface of a die is exposed to a rapid increase in temperature as metal fills the die and the casting solidifies. As the die heats up, compressive stresses develop in the surface as it tries to expand but is constrained by cooler subsurface areas.

Development of stresses in the die surface during one casting cycle

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.

Comparison of the calculated die life (right) with the actual crack pattern in two areas of the die (left)

* Courtesy of Volkswagen AG, Germany

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