The composition of white corundum is α – Al2O3, its content is over 98% – 99%, the impurity is very little, the fire resistance is up to 2000 ℃ – 2050 ℃, containing Al2O3 385% – 90%, and it is brown crystal. The density of corundum is 3.85-3.9g/cm3, the Mohs hardness is higher than grade 9, and the melting point is above 1800 ℃. The chemical composition of chromite powder used for casting coating is Cr2O3 > 36%, Cao < 2%, SiO2 < 7%, FeO = 12% – 18%, MgO = 13% – 17%, Al2O3 = 8% – 12%, ignition loss < 2%.
Graphite is divided into natural and artificial. There are two forms of natural graphite: amorphous graphite and flake graphite. Its fixed carbon is not more than 84%. It can still be used in coatings when the fixed carbon is as low as 65% and the ash content is as high as 30%. However, the lower the fixed carbon is, the coarser the graphite powder is, because there is more carbon in it, which is easy to aggregate into large particles. Graphite (density 2.5g / cm3) is neutral. It has the highest chilling ability to metal and stable chemical properties. Because of its shape characteristics, flake graphite has a strong covering force on the surface of sand mold. That is to say, when the dense coating is formed, the powder weight consumed is the smallest.
Adding talcum powder into graphite coating can improve the high temperature cracking resistance, suspension, fluidity and thickness of coating, but make the surface strength and leveling of coating worse. The flow coating with quartz powder as the main refractory powder has poor leveling property and resistance to drop marks. In addition, the addition of feldspar powder to the coating also has a good melting effect.
Alcohol based coating has quick drying coating, which is applied to sand mold (core), ignited and dried with fire, and then poured in a closed box.