Practical investment casting technology
2026,01,05
A friend asked me on WeChat, "Professor Venda, I've encountered a problem. Why does the strength of the mold shell decrease after firing, and it breaks easily when poured, even after half an hour without casting? The red shell is fine." This is actually a question about the high-temperature strength and residual strength of the mold shell. Today, I'll specifically discuss my views on this issue.
I've actually researched a lot of materials, and the book *Practical Investment Casting Technology* by Hiroki Yamaya discusses high-temperature strength and residual strength extensively. He explains this issue very thoroughly. Domestic books generally only briefly mention this topic. Below is an excerpt from Hiroki Yamaya's book. Please read and consider it carefully.
The physical properties of the mold shell before and after firing differ significantly, changing with the high-temperature variations of the refractory materials and binders that make up the mold shell. While there is considerable existing data on firing strength tests, the actual requirement is strength capable of withstanding the thermal shock of molten metal. Therefore, firing strength values at higher temperatures are necessary, but testing equipment meeting this requirement is lacking. Firing strength is primarily influenced by the firing temperature. Figure 6-2 shows the relationship between firing temperature and flexural strength of silica sol shells. The firing strength of silica sol shells increases with temperature, reaching a maximum at 1050℃, after which the strength tends to decrease. Ethyl silicate shells exhibit a similar strength variation. The high-temperature residual strength of the shell after firing and cooling to room temperature is approximately 50% lower than the high-temperature strength, especially the residual strength after high-temperature firing above 1000℃, which is almost close to the dry strength. Therefore, considering the requirement that the shell must withstand the impact stress of the molten metal during actual pouring and the residual strength after shell collapse, a firing temperature of 1050℃ is recommended.
As shown in the graph above, the high-temperature residual strength of the silica sol mold shell decreases significantly, by approximately 50%, after calcination and cooling to room temperature. Furthermore, the residual strength drops sharply after calcination at temperatures above 1000℃, almost reaching its dry strength. This explains the problem encountered by the person mentioned earlier. If the mold shell is not poured and left to cool for half an hour, its temperature will likely have dropped to near room temperature. Therefore, its strength will be roughly equivalent to its wet strength, making it unable to withstand the impact of high-temperature metal.
Another curve can be referenced here. This is the furnace cooling characteristic curve of the mold shell after calcination.
