The quality of mold (core) sand has a great influence on the quality of casting. For example, the defects of sand hole, air hole, sand inclusion and crack are often caused by the unqualified quality of mold sand. Influence of molding sand performance on casting quality: molding sand shall have the following properties:
(1) Strength under the action of external force, its performance that is not easy to be destroyed is called strength. It is very important that this kind of property does not deform and damage under the influence of mould manufacturing, transportation, liquid metal impact and pressure, otherwise it will cause the defects such as box collapse, sand washing and sand hole.
(2) Air permeability molding sand is called air permeability because there is a gap between the sand particles.
(3) Refractoriness molding sand does not soften or melt under the action of high temperature metal liquid. When the fire resistance is not enough, the sand will burn and stick to the surface of the casting to form a layer of hard skin, which will accelerate the tool wear during cutting. In order to make up for the lack of fire resistance of molding sand, a layer of coating is applied on the surface of mold cavity.
(4) The volume of the core sand is compressed with the cooling shrinkage of the casting. After pouring, the lower the high temperature strength of the sand, the better the yield, and the less the mechanical resistance of the casting. The internal stress of the casting decreases; on the contrary, when the shrinkage is blocked, the internal stress is large, and even cracks are produced. In addition, there are reusability and gas generation. For resin sand, there are micropowder, burning reduction, residual alkali of basic resin and so on.