After the silica sol coating soaks the mold, due to the evaporation of water on the coating surface, a humidity difference will be generated between the surface and the interior. Driven by the humidity difference, the internal water will continue to diffuse to the surface in the form of liquid water. At the same time, the silica sol particles are continuously precipitated in a colloidal dispersion state. The precipitated silica particles are combined with hydrogen bonds to form a skeleton, forming a connected polymer capillary structure, and finally completely lose fluidity and turn into gel. At this time, the gel is a wet gel.