Silica sol coating is a mixture of adhesive silica sol, refractory powder and a small amount of functional additives. The process of silica sol investment casting shell making is essentially the gelation process of silica sol coating, and then the gel changes from wet gel to dry gel. Drying is a key process. During the drying process, the capillary force causes a series of physical and chemical changes in the internal microstructure of the shell, which can be roughly divided into four stages, but there is no strict boundary between each stage, either gradual or overlapping.