A trial device for measuring gas pressure in molds and cores at casting was designed and its feasibility was studied in shell mold casting. The device mainly consisted of a thin gauge steel tube, one end for embedding in motds for the introduction of pressured gas and the other end mounted with a gas pressure gauge made of a silicon diaphragm. Gas pressure in cores were measured by the use of semiconductor pressure transducers, while iron was cast at 1,400°C. The cores used were shell mold cores 100 mm wide, each with a core print for degassing on its open side, and were varied in the following factores : length and thickness of core, resin content and grain size of sand.
Hydrostatic pressure measurement conducted for the purpose of calibration, showed that the output electrical signal and the pressure were in linear relationship in the range of 0-0.3 kg/cm
2. The maximum gas pressure in an unvented shell mold core wholly surrounded with molten cast iron was found to be equal to hydrostatic pressure of the molten cast iron at the top of the core. When gas pressure at the core surface exceeded the hydrostatic pressure of the molten metal, gas blew out into the molten metal and formed gas bubbles.
The gas pressure distribution was not uniform in the core. The pressure increased with the distance from the core print. For cores with the same thickness, the gas pressure increased with the core length. The gas pressure at the tip of the 200 mm core did not agree with but was higher than the estimation obtained by extrapolation of the gas pressure curve plotted with the 100 mm and 150 mm cores. Even at the same distance from the core print, the gas pressure in the longer core was found to be higher. For the cores with the same length, the gas pressure increased with the decrease in the core thickness. For the core made ot sand with AFS grain size number 29.2, the gas pressure increased with resin content, while for the core made of sand with AFS number 52.7, the gas pressure showed only a slight increase irrespective of the increase in resin content. When the structural configuration and the resin content were kept constant, the gas pressure increased with the decrease in the grain size of the sand used, and the gas pressure increase was inversely proportionai to the permeability of the core.
This device has thus been proved to facilitate measurement of gas pressure at various parts of the core, even those with a complicated configuration, and the resulting gas pressure-time curve is greatly effective in preventing the casting defects attributed to gas bubbling.
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