Effect of Powder Breakdown Additives on Properties of Ester-Hardened Sodium Silicate Bonded Ceramic Sand

The sand systems containing sodium silicate binder have excellent casting quality and eco-friendly advantages. However, the poor collapsibility of sodium silicate bonded sand (SSBS) makes it difficult for foundries to recycle used sand. Thus, many investigations have been performed recently to improve the collapsibility of SSBS. In this study, the powder breakdown additives alumina (Al2O3), silicon carbide (SiC) and boron nitride (BN) were used as modifiers to further improve the collapsibility of ester-hardened sodium silicate bonded ceramic sand (SSBCS) on the basis of modified sodium silicate, and the specific parameters were optimized by orthogonal experiments. The results of orthogonal experiment showed the optimum additive amount of Al2O3, SiC and BN were 0.4%, 0.3% and 0.1% of ceramic sand, and the addition of modifiers effectively increased the 1h tensile strength and further reduced the residual strength of the SSCBS. The experimental results indicated that the difference of the linear expansion coefficient between the modifiers, the new phase AlN formed by the modified sand sample and the phases Na3PO4, Na4B2O5 formed by the unmodified sand sample effectively accelerated the generation of many coarse pores and very big cracks on the bonded bridge in the process of heating at 950 °C and cooling, thereby resulting in the improvement in the collapsibility of SSBCS.