抄録
Grasping the retained strength change in sand molds is important for improving collapsibility of mold. In this work, the retained strength changes along the metal-mold interface in various sand molds (with facing) bonded with bentonite and Hibushi clay binders were investigated.
There is no difference between bentonite and Kibushi clay with regard to the retained strength change in sand molds. However, as the bentonite swells with moisture, well-compacted sand molds bonded with bentonite have higher compressive strength than those with Kibushi clay. Consequently, the retained strength curves in sand molds with bentonite slide up to a higher strength side than in case of Kibushi clay.
The primary factor working on the retained strentgh is the thermal expansion of molding sand. The molding sand which shows a larger thermal expansion has a larger volume change with heating and cooling, As a results, the bond between sand grains are disrupted and that lowers the retained strength of the part heated up to a high temperature. On the other hand, as molding sand with smaller thermal expansion shows intergranular sintering at high temperatures and moreover, as the bonds of sand grains are strengthened with fused binder, the retained strength increases rapidly at the part where it has been heated up to a high temperature. Silica sand and olivine sand belong to the former type, and chromite sand and zircon sand belong to the latter type. But, as olivine sand mold fuses above 1,300°C, the retained strength increases near the metal-mold interface. In chromite sand and zircon sand molds, rapid rises in the retained strength near the matal-mold interface take place without regard to binders, and it, therefore, seems difficult to improve collapsibility of such molds.
