Abstract
In casting cast iron, the use of metal mold shows a remarkable difference from the ordinary sand technique. The thermal conductivity of the mold metal is very much higher than that of refractories used for sand mold. Accordingly, problems on the use of metal mold may possibly be related to heat transfer in molten metal, mold coating, and metal mold. The properties of mold coating in metal mold casting of cast iron may be classified to the three groups which are represented by graphite, zircon, and diatomaceous earth. The respective characteristics of these mold coating materials are : 1) Graphite type : Heat flux density at the coating layer is remarkably high. The differece in surface temperatures between the mold coating and metal mold being very small. The temperature attainable is the highest among the three coating materials. 2) Diatomaceous earth type : Heat transfer through the diatomaceous earth coating is much suppresed as compared to the case of graphite coating. The temperature difference obtained between the mold coating and the metal mold was more than 200°C, when the surface temperature of mold coating was maintained at higher temperatures than 700°C for a long duration. 3) zircon type : The properties of zircon type are intermediate, but its surface temperature obtained was the lowest among those materials in question. The boundary air gap formation and its duration which affect the heat transfer between molten metal and mold coating depended on the kind of mold coating materials and the thickness of metal mold. The air gap formation was most delayed and the duration of the gap was the longest when the diatomaceous earth was used and the thickness of metal mold was 10 to 20mm. When the metal mold thickness was 5 or 60mm, air gaps were repeatedly formed and vanished. In conclusion, diatomaceous earth was most effective as a mold coating material in question on metal mold casting for cast iron.
