Abstract
In this paper there will be no report concerning the relation between experimental values and theoritical values, though some theoretical considerations have been made in the past about thermal conductivity of coatings for metal mold casting. This is caused by the fact that it is comparatively difficult to quantitatively measure the intricate coating layers. In this report, the thermal conductivity of the system silica flour-diatomaceous earth coatings were obtained from the thermal resistance measurements which have already been discussed in the previous paper and the relation between porosity and thermal conductivity of coatings were considered. It was also discussed that porosity of coatings influences durability. Durability was measured from the decreasing rate of coatings by the repeated metal mold castings. The results obtained were as follows. (1) Porosity of coatings was maximum when using only diatomaceous earth as the basic material, and the decrease of porosity becomes smaller with addition of silica flour. About the binders, phosphate proved to have more porosity and smaller pore size than silicate. And dilution of the binder increased the porosity of coatings. (2) Relations between porosity and thermal conductivity were clarified. In case of coatings with porosity over 70% (with over 50% diatomaceous earth), thermal conductivity decreased rapidly. This was also proven from the results of casting tests. The relation between porosity and thermal conductivity can be surmised from Russell’s equation, and more over the relation between volume fractions of the binary system basic materials and thermal conductivity can also be surmised from Kingery’s equation. (3) Acid treatment on metal mold surface improved the adhesiveness of coating for metal mold. Durability of coating increased by adding 30% diatomaceous earth in silica flour.
