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Controlling the relevant process parameters of the casting coating process to improve the quality of precision castings is aimed at reducing the workload of casting repair, polishing, and finishing, improving labor productivity, and meeting customer requirements for product quality. The coating slurry is a key process for quality control in the silica sol process. Surface coating refers to the direct contact between surface coating and molten metal, which determines the surface quality of castings. Surface coatings are generally composed of binders, refractory materials, wetting agents, and defoamers. As the surface coating is applied to the wax mold and directly replicated onto the casting surface, in order to ensure that the water-based silica sol coating can be applied well, a wetting agent is added to the surface coating to improve its coating performance. After adding wetting agents, the surface tension of the coating decreases, and air is easily drawn in during the stirring process to form bubbles, so defoamers need to be added.

The meaning of precision casting silica sol surface coating is that defoamers are generally organic additives such as silicone resin and alcohols. Defoamer can stretch the facial mask on the bubble surface and burst the bubbles. The dosage of wetting agent and defoamer is generally around 0.1% to 0.3% of silica sol, usually added to the surface slurry. Wetting agents and defoamers are organic compounds, and adding too much will breed bacteria in the coating, which will deteriorate the stability of the coating. The surface coating directly replicates the surface of the casting. The larger the powder to liquid ratio of the surface coating slurry, the denser the inner surface of the produced shell, and the better the surface quality of the casting. Therefore, while ensuring the coating adhesion, the powder to liquid ratio of the surface coating slurry should be increased as much as possible. Non testing coatings need to withstand the impact of metal liquid and may react with metal liquid and oxides, so the selection of raw materials is relatively strict.

The surface coating generally uses silica sol with high w (sio2) content, such as w (sio2)=30% silica sol, and the refractory powder generally uses fused alumina with high refractoriness. Silica sol is mainly used as a binder for precision casting coatings, and can be used as a surface coating and reinforcement coating for shells. Applying small particle silica sol to precision casting instead of ethyl silicate not only reduces costs but also improves working conditions. When used for precision casting of thin shells, it can achieve high shell strength, high dimensional accuracy, good casting smoothness, better molding quality than water glass, and lower cost than ethyl silicate. This is particularly suitable for large castings, castings with high surface finish requirements, and castings that require no or minimal allowance. Using silica sol as a high-temperature resistant coating for molds can give the coating good heat resistance and wear resistance, reduce the loss of molten metal on the mold at high temperatures, and help with demolding.