Application of Silica Sol in Investment Casting

1. Silica sol characteristics

The colloidal solution is milky white and completely transparent. When making porcelain shells, it uses magnesium chloride colloidal solution for bonding, just like liquid sodium silicate and magnesium chloride ethyl ester hydrolyzate. Its primary characteristic is amorphous SiO2, and the particles have a large specific surface area, are negatively charged, and have a hydrated silanol layer. During the drying process, wet strength is provided. When the shell is fired, most of the free water and structural water evaporate, and a high strength can be obtained to withstand the influence of casting molten aluminum alloy can be obtained. One of the most important characteristics of silica sol investment casting is its dependability. The pH value is related to reliability, and the pH is 8.5–10 Z stable. A pH of 2 to 4 is relatively stable, while a pH of 7 or higher is extremely unstable. The silica sol's electrolytic medium affects the charge density of the electric double-layer structure and reduces its reliability. Alkalization, imbibition, ion exchange, peptization, hydrolysis, and other methods are used to create silica sol.

2. Silica sol selection

The silica sol performance index value includes SiO2 content, relative density, thickener content, PH value, SiO2 particle size, and other factors. The surface of the casting sticky sand must be modified or surfactant added.

3. Silica sol configuration

Because the ambient temperature of aluminum alloy casting has different requirements for the lithofacies composition of different porcelain shells, for castings such as heat-resistant alloy, steel, titanium metal, aluminum alloy profiles, and alloy copper have different requirements. However, fireproof granular materials must be able to be bonded by silica sol investment casting, and causing the slurry to gel when the coating is placed at room temperature is difficult. Aluminum oxide, zirconia ceramics, zirconia corundum, aluminum ore, and quartz glass tube powder with particle sizes ranging from 0.075mm to 0.075mm are frequently used. It is usually necessary to add suspending agents, anti-settling agents, and other auxiliary materials to improve the spraying process of the coating.Double distilled water or ethanol are two common paint thinners. To avoid gelation of silica sol caused by excessive moisture absorption by some granular materials, granular materials must be added while stirring. Once the recipe is chosen, the viscosity and hardness of the foundry product coating determine its performance. Its performance must change with the application of paint pre-dip. Because water evaporation decreases and viscosity increases, it is necessary to add a small amount of silica sol and double-distilled water according to the calculated amount to maintain the original paint's SiO2 content and viscosity.