Environmental-Friendly Sand Casting Technique using Frozen Mold

摘要

The frozen mold is a type of sand mold, which is simply produced by freezing the mixture of just sand and water. Sand casting process using frozen molds bring some industrial advantages. The biggest benefit is the reduction of environmental load. Basically, the ice bridging between sand particles works as bonding agent in the frozen mold. Vibration, noise and dust due to shaking out of mold almost never cause since the frozen mold collapses spontaneously after casting. Sand mold with no organic binders inhibits foul odor and simplifies sand reclamation process. The positive effect as a cold source is another unique feature. The frozen mold which is supplied at low temperature accelerates the cooling rate of cast products, possibly promising finer cast structure. In this study, effective freezing technique for sand mold with complicated geometry, including cores, is examined. Copper alloy and aluminum alloy are cast into frozen mold and various casting properties such as mold strength, flowability, dimensional accuracy and so on are evaluated. Recently, the advanced freezing method in which the cold air was forcibly injected into the sand mold was developed. This method enabled to provide frozen mold in short time and at low cost. We have successfully modified this method to apply to core molds which usually required uniform condition for entire lateral surfaces. A frozen mold produced from the mixture of silica sand and 5 mass% water exhibited excellent compressive strength, being competitive with organic self-hardening mold. This performance suggested a possible application to large sized casting products. Not only matrix molds, core molds were successfully replaced with frozen molds. The reliability and the availability of frozen core mold were confirmed by actual casting tests. A thin frozen core rod of 50 mm in length and 4 mm in diameter has never broken during casting work of aluminum alloy, and the cavity corresponding to the core mold was well obtained in the casting product. Also, the core sand was ejected very easily from the product, since a frozen core satisfactory collapsed after casting. It was confirmed that the frozen mold enabled to improve the flowability of castings. This welcome behavior is explained as follows. Frozen molds generate more water vapor compared to conventional green sand molds. This water vapor lies between molten metal and the mold surface as a thin film, and decreases heat-transfer coefficient from the state of those direct contact. As a result, the temperature drop of molten metal just after pouring becomes suppressed and the flowability can be improved. Good aeration property in a frozen mold boosts up the above improvement since the back pressure in the cavity must decrease. In contrast, after the solidification, the cooling rates of cast products reversed and that for a frozen mold got faster because the heat capacity as a cold source was much bigger in a frozen mold. It was confirmed that the cast structure in copper alloy became finer by the above rapid cooling effect.