The use of fused silica materials in precision casting shells in developed countries such as the United States and Japan continues to increase year by year, especially in the surface layer of silica sol shells. Zircon materials have made great breakthroughs in the use and price ratio, and they are ideal materials for engineering applications.
Fused silica is made by melting natural high-purity silica in an electric furnace at a temperature higher than 1760°C, and then rapidly cooling it. This process transforms crystalline silica into amorphous glass melt.
The melting temperature of fused silica is about 1713℃, the thermal conductivity is low, and the thermal expansion coefficient is almost the smallest among all refractory materials, so it has high thermal shock stability. Therefore, the fused silica shell is rarely cracked due to temperature changes during the firing and pouring process. It is an ideal refractory material for investment casting and can be used as a refractory material for surface or back coatings and as a sanding material.
Fused silica will partially or improve the shell performance. Fused silica has a small thermal expansion coefficient, which helps prevent the shell from cracking and deforming during the dewaxing and roasting process, and helps to ensure the dimensional stability of the casting. The purity of fused silica is high, and the stability of the coating is good; the high temperature creep resistance of the shell is improved.
When the temperature of fused silica is low, the thermal conductivity is poor, and the heat capacity is only half that of zircon sand. Most molten metal has poor wettability to it, which makes it easy to produce gaps between the solidified metal layer and the inner surface of the shell. The conductivity is further reduced, which is conducive to the filling of thin-walled castings. Fused silica has high transparency at high temperatures and can transfer heat through radiation, making it more thermally conductive than aluminum silicate shells. It makes the castings cool faster, and it is easier to obtain sound castings.
When the casting is cooled, the cristobalite changes from a high temperature type to a low temperature type. At the same time, the volume changes suddenly, causing numerous cracks in the mold shell and a sharp drop in strength, which is beneficial to the shelling. Fused silica is acidic, and the shell can be removed by chemical sand cleaning methods such as alkaline boiling and alkaline explosion.
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