The drying process is non-uniform at the microscopic level. During the skeleton aging stage, the inner walls of capillary pores unevenly distribute block-like thin layers of liquid. As evaporation proceeds, the distance from the outer surface to the evaporation front inside the pores increases, and the driving force for evaporation decreases. Consequently, liquid flow gradually disappears, and the distribution of liquid on the outer surface becomes increasingly discontinuous. At this point, drying enters the surface dry phase. In this phase, evaporation occurs entirely within the bulk phase, making the evaporation rate less sensitive to external conditions. Liquids near the outer surface within capillary pores are in a discontinuous state, and the transport of liquids primarily occurs through diffusion. As drying progresses to this stage, the total stress acting on the shell significantly diminishes. However, since the compressive stress on the undried interface of the skeleton is greater than that on the dried interface, uneven drying can lead to differential stresses that may cause bending deformation of the shell. This phase corresponds to the stacking phase of investment casting shells. The drying rate by diffusion is relatively slow, so a longer stacking time is required, but it is less dependent on environmental humidity. After stacking, the shell loses free water and some adsorbed water from the gel. Heating to 350°C can remove all water molecules present in the shell. When heated to temperatures above 700°C, the shell will lose almost all hydroxyl water, with some hydroxyl groups converting into siloxane bonds, resulting in higher shell strength.