Effect of 3D-printed sand molds on the soundness of pure copper castings in the vicinity of as-cast surfaces

Although the use of three-dimensional (3D)-printed sand molds enables both the formation of near-net shapes and the manufacture of ultra-high-mix and ultra-low-volume pure copper castings, detailed clarification is required regarding the correspondence between the use of such molds and the generation of casting defects. In this study, the effects of 3D-printed molds on the soundness of pure copper castings in the vicinity of as-cast surfaces were investigated. For castings with high and medium oxygen contents, multiple cavities were observed in the vicinity of the as-cast surfaces of the portions of the castings obtained from 3D-printed molds made of artificial mullite sand and furan binder. However, cavities were not generated in castings with low oxygen content (irrespective of the mold materials) or those obtained from CO2 molds made of silica sand and water glass via the CO2 process (irrespective of the oxygen content). The analyses of 3D-printed molds show that gases such as combustion gases of organic compounds and water vapor from the dehydration of hydrates were generated by heating. Moreover, for castings with high oxygen content, the generation of cavities was prevented using 3D-printed molds impregnated with water glass and held at 400 degrees C. However, our analyses demonstrated the occurrence of chemical reactions between molten copper or Cu2O and SiO2, which caused the burning of silica sand on as-cast surfaces (i.e., penetration into the CO2 molds) in castings with high and medium oxygen contents. These results indicate that the adequate deoxidization of the melt and the rapid formation of a solidified shell after pouring are effective in enabling the utilization of 3D-printed molds and that combining granular sand that is non-reactive to molten copper with an inorganic binder produces ideal molds for pure copper castings.