Features: the surface of the casting is bonded with a layer of molding material that is difficult to clean. There are two kinds of bonded sand: mechanical bonded sand and chemical bonded sand. The difference between them is that mechanical bonded sand is formed by the penetration of high temperature metal solution into the pores between sand particles; chemical bonded sand is a low melting point compound formed by metal oxidation and molding materials, which is firmly bonded with castings.
(1) The SiO2 content (mass fraction) of sand with high refractoriness should be higher than 92%, and better than 95%.
(2) Zircon sand (ZrSiO4) or chromite sand (FeCr2O4) can be used for castings with higher requirements, which can achieve better results.
(3) Lower pouring temperature and increase pouring speed properly, weaken the thermodynamic and chemical effect of metal liquid on sand mold.
(4) The compactness of sand mould should be high (the hardness of B-type hardness tester should be higher than 85, preferably higher than 90), and it should be uniform. The repair of sand core shall be in place, without local looseness, and at the same time, coating peeling shall be prevented.