What are the main Constituents of Molding Sand?
The main constituents of molding sand involve silica sand, binder, moisture content, and additives.
1.1 Silica sand
Silica sand in form of granular quartz is the main constituent of molding sand having enough refractoriness which can impart strength, stability, and permeability to molding and core sand. But along with silica small amounts of iron oxide, alumina, limestone (CaCO3), magnesia, soda, and potash are present as impurities. The chemical composition of silica sand gives an idea of the impurities like lime, magnesia, alkalis, etc. present. The silica sand can be specified according to the sand grain size and the shape (angular, sub-angular, and rounded) of the sand.
1.2 Binder
Binders can be either inorganic or organic substances. Binders included in the inorganic group are clay sodium silicate and portland cement etc. In a foundry shop, the clay acts as a binder which may be Kaolinite, Ball Clay, Fire Clay, Limonite, and Bentonite. Binders included in the organic group are dextrin, molasses, linseed oil. It may include resins like phenol-formaldehyde, urea-formaldehyde, etc. Binders of the organic group are mostly used for core making. Among all the above binders. The bentonite variety of clay is the most commonly used. However, this clay alone cannot develop bonds among sand grins without the presence of moisture content in molding sand and core sand.
1.3 Moisture
The amount of moisture content in the molding sand varies from 2% to 8%. This amount is added to the mixture of clay and silica sand for developing bonds. This is the amount of water required to fill the pores between the particles of clay without separating them. This amount of water is held rigidly by the clay and is responsible for developing strength. The effect of clay and water decreases permeability with increasing clay and moisture content. The green compressive strength first increases with the increase in clay content. But after a certain value, it starts decreasing.
1.4 Additives
Additives are the materials generally added to the molding and core sand mixture to develop some special property in the sand. Some commonly used additives for enhancing the properties of molding and core sands are coal dust, cornflour, dextrin, sea-coal, pitch, wood flour, silica flour.
