FD Type I fast-drying silica sol, after being aluminum-modified, can shorten the gelation time. During the drying process of the gel, due to the high supersaturation of SiO2 colloidal particles, a large number of gel nuclei can be generated. Because this modified gel nuclei are not easy to grow, they can form gels with larger specific surface energy. The gel skeleton is dense, with fewer surface defects and very high strength, so the compactness of the shell mold is also good. However, due to the smaller capillary pore diameter, the breathability of the shell mold is slightly poorer. FD type fast-drying silica sol is made by adding organic polymers to FD Type I fast-drying silica sol. Adding polymers to FD Type I silica sol greatly reduces the probability of cracking or crazing of the shell mold because at the critical point of drying, flexible organic polymer long-chain macromolecules impart flexibility to the rigid gel skeleton, thereby buffering the contraction pressure on the tube wall caused by capillary shrinkage. Therefore, FD type fast-drying silica sol has higher wet strength, and when baked at high temperatures, new breathable pores will be generated after the organic matter burns off. In conclusion, FD fast-drying silica sol can give the shell mold high wet strength, low residual strength, and good breathability.