Artificial spherical sands were cured using water glass as a binder with or without addition of porous silica to prepare the test piece and aluminum alloy casting core. In order to clarify the curing and collapsibility mechanism of inorganic sand mold, the cross-linked structure between their particles were evaluated by SEM observation. A three-dimensional network of crosslinking bridges derived from water glass formed between the spherical particles enables the preparation of a core with sufficient strength for casting. The water glass added to cure the core requires less than 2wt% by weight for the sand prepared by the melting method and about a third as much as that prepared by the sintering method. The high collapsibility of such an inorganic mold is due to the rapid elongation of crosslinking bridges between the particles when heated above a certain temperature so that the particles are separated from each other. Especially for spherical particles with a smooth surface, the relatively small amount of water glass for curing would cause collapsibility accelerated by the thinning of the crosslinked body wall occurring simultaneously with elongation. Further more, the addition of porous silica would induce elongation at a lower temperature as compared to when poroussilica is not added, and is extremely effective for improving the collapsibility of the inorganic sand mold.
