Modification of inorganic binder used for sand core-making in foundry practice

Abstract

With modified water glass as binder and the introduction of micro silicon powders into the core-making process, an improvement was made to the tensile strength and collapsibility of the sand core. The potassium hydroxide, sodium hexametaphosphate and white sugar were applied as the modifiers of water glass. The optimum proportion of the modifiers was determined through the combination of single factor test and orthogonal test. The optimum proportion of water glass, potassium hydroxide, sodium hexametaphosphate and white sugar is 1000: 40: 5: 5 (wt.). In terms of weight, modified binder and micro silicon powders accounted for 2% and 0.6% of sand, respectively. The sand core was hardened by going through a warm core box process, in which the temperature of core box was 150 °C, and the compressed hot air was blown at 120 °C for 40 s under a pressure of 0.2 MPa. As for the sand core bonded with modified water glass, the tensile strength is 2.46 MPa at room temperature (σ0) and 2.49 MPa at 25 °C and 40% RH for 24 h (σ24), which are 2 times more than that with unmodified binder. The bonded strengths of sand core are increased as a result of the reaction between −OH groups from addition of potassium hydroxide and SiO2 particles widely distributed in the sand core. Comparing with the sand core bonded with unmodified water glass, the high temperature residual tensile strength (σr) of sand core bonded with modified water glass under 600 °C for 5 min, is sharply reduced from 0.20 MPa to 0.01 MPa. By the comparison with unmodified water glass, the dynamic viscosity of the modified water glass and the flowability of molding sand using modified water glass are increased from 74 mPa·s and 2.15 g to 80 mPa·s and 2.21 g, respectively. As revealed by FT-IR analysis, new groups including PO3− , PO43− , and Si-O-C appear in the molecular structure of modified water glass, which are beneficial to the collapsibility of sand core.