Abstract
When aluminum powder is added to monoaluminum phosphate solution, a gelling reaction takes place. This reaction can be applied for self-hardening sand molding. For example, a mixture of 100 silica sand, 5 monoaluminum phosphate and 0.25 aluminum powder by weight hardened by itself and a test piece of 1-1/8 in dia. x 2 in gave a compressive strength of 20kg/cm2 after it had been let alone for 8 hours (Fig. 1). The similar reaction could be made by using iron or zinc powder. The compressive strength of the molded sand was increased with increasing monoaluminum phosphate content while it was less influenced by the amount of metal powder added excessively. The properties of the mold was not changed when it was stored and there was no tendency for the mold to be wet by absorbing moisture in air. It needed not dry the mold, because the sand mixture originally contained little water. The collapsibility of the sand mold was good, because the mold lost its compressive strength when it was red heated. The mold had a disadvantage that when it was heated at high temperatures, it caused veining. Therefore, it needed to add a small content of dextrin for preventing the mold from veining.
