When molten metal is poured into a mold, the solidified layer forming on the interface between the molten metal and the mold, raises the gas pressure inside the mold during gas is generated in the mold. Such high pressure gas may breaks the solidified layer and penetrates into molten metal. Consequently small quantity of gas remainder which fails to float up to the surface results in the occurrence of blow hole or pin hole in castings. One of the objects of our experiment is to review the behavior of gas generated within various types of mold, particularly within self-hardening mold using dicalcium silicate and sodium silicate as its principal hardening agents. The test results are as follows: Gas generation, to a large extent, is influenced by the remainder of inactive sodium silicate. In other words, the more inactive sodium silicate remains, the more gas is generated. In addition to this, sand grain expansion and elevated pressure following temperature rise maylower the permeabitity, thus resulting in the elevation of gas pressure in mold. As sodium silicate mold is in fluid state at elevated temperature, striking lowering of permeability is observed. Test results are summerized below: 1) In the case of using sodium silicate equal in molecular ratio and density, the less is sodium silicate added, the less are generated gas volume and gas pressure, with the greater permeability. 2) In use of sodium silicate of higher molecular ratio and lower density lessens both gas volume and gas pressume. These cause to improve the permeability. 3) The more C2S is added, the less are the volume and gas pressure. This causes to obtain better peameability during pouring. 4) As a result of our study on generated gas, it is concluded that this type of self-hardening mold has less deleterious effect on molten metal than CO2 mold.
