Presented at the Investment Casting Institute 55th Technical Conference and Expo, Oct. 14 – 17, 2007, Cleveland, Ohio
Successful Pyrometry in Investment Casting
The FAR Expert System SpectroPyrometer has accurately measured solid and liquid metal temperatures in investment casting. The problems it has solved are significant and explain the skepticism investment casters have toward non-contact temperature measurement. The problems are caused by the nature and behavior of the target: a solid metal charge changes phase into a turbulent liquid. Liquids, especially turbulent liquids, give conventional pyrometry problems because the changing microscopic shape of the surface governs the radiation characteristics, i.e., the emissivity. Metals as a class, whether solid or liquid, are the most difficult for pyrometry due to the behavior of their emissivity: it changes with wavelength (color) in addition to all the other variables that affect it. Put the two together and no instrument that requires emissivity information beforehand or assumes constant emissivity (or constant relative emissivity) has a chance of success. The SpectroPyrometer has shown exact agreement with dip thermocouples for extended periods, confirming accurate temperature measurement in this demanding environment. The improved control resulting from continuous, accurate temperature measurement has effected a 41% decrease in non-fill, a key process variable.