Excerpt from the study published in the Journal of Materials Processing Technology 195 (2008) 267–274 by Adrian Sabau and Ralph Dinwiddie
During the high pressure die casting process, lubricants are sprayed in order to cool the dies and facilitate the ejection of the casting. The cooling effects of the die lubricant were investigated using thermogravimetric analysis (TGA), heat flux sensors (HFS), and infrared imaging. The evolution of the heat flux and pictures taken using a high-speed infrared camera revealed that lubricant application was a transient process. The short time response of the HFS allows the monitoring and data acquisition of the surface temperature and heat flux without additional data processing. A similar set of experiments was performed with deionized water in order to assess the lubricant effect. The high heat flux obtained at 300° C was attributed to the wetting and absorbent properties of the lubricant. Pictures of the spray cone and lubricant ﬂow on the die were also used to explain the heat flux evolution.
During the die casting process, the dies are sprayed with a lubricant, dies are closed, and liquid metal is injected into the die cavity under high pressures. Net shape parts are produced after subsequent metal solidiﬁcation and cooling, dies are opened, and parts are ejected. Lubricants facilitate the ejection of the finished product, reduce the soldering effects (Fraser and Jahedi, 1997), and cool the dies (Piskoti, 2003). The lubricant ﬁlm thickness on the die surface was used to quantify the lubricant adhesion performance. The lubricant ﬁlm thickness was usually determined indirectly, using optical or X-ray techniques (Fraser and Jahedi, 1997) or die temperature (Piskoti, 2003).
Channels are drilled into the dies for heating or cooling in order to maintain temperature levels that will yield progressive solidiﬁcation and uniform cooling of the parts. In order to minimize casting defects, the metal delivery and heating/cooling systems are designed based on the analysis of heat transfer and solidiﬁcation phenomena. One of the parameters required for the die design is the amount of heat removed during lubricant application. Data on heat transfer coefficients or heat flux evolution during lubricant application are used to characterize the heat removal capability of lubricants and perform numerical simulations of the die casting process (Liu et al., 2000).
Spray cooling was mainly studied for other applications than the die casting process. There is significant effort on the study of spray impingement for power electronics using water and refrigerants, and steel industry using water and oils (Stewart et al., 1995). Few predictive models for heat transfer exist other than those for critical heat flux (CHF) (Pautsch and Shedd, 2005).