EFFECT OF SODIUM SILICATE MODIFICATION BY NANO-SiO2 ON COATING PROPERTIES USED IN INVESTMENT CASTING

Tóm tắt

Sodium silicate is widely used as a binder in coating slurry in investment casting technology to produce components with high dimensional accuracy and good surface smoothness. However, this slurry still presents several limitations, including low permeability, high residual strength, and a propensity for surface cracking. The aim of this study is to introduce a sodium silicate modification method to overcome these disadvantages. The nano-SiO₂ modifier, which was synthesized by the sol-gel method, consists of spherical SiO₂ with low tendency to agglomerate and an average diameter of approximately 20 nm. Sodium silicate was modified by nano-SiO₂ at concentrations of 2.5%, 5.0%, and 7.5% by weight relative to the unmodified binder. Each modified binder formulation was subsequently mixed with zircon powder at binder-to-powder ratios of 60:40; 57.5:42.5, and 55:45, was evaluated based on properties such as viscosity, adhesion performance, and permeability. The optimum parameters for the coating slurry are as follows: the sodium silicate with a modulus of 2.9 modified by 2.5% nano-SiO₂; a binder-to-powder ratio of 55/45; and 0.5% Carboxymethyl cellulose used as an additive. According to theory of investment casting, the sample was cast in a vacuum chamber, using a pumps to maintain the low-pressure environment. In this study, the applied pressure is 0.4 kg/cm² which can improve the casting quality.