Moulding sand (Foundry sand) is used to make mould in sand casting process. Molding sand is found on the bottom and banks of rivers and lakes. It is formed by breakdown of rocks due to natural forces like wind, rain, water currents and heat.
Main constituents of Moulding sand:
Silica sand – It is the major constituent of molding sand containing 80 to 90% silicon oxide.
Binder – The purpose of binder is to provide bonding action and strength. The binder decreases permeability. Clay is mostly used as a binder. The amount of clay content varies from 6 to 10%.
Water – Water helps clay to develop plasticity and strength. The water is absorbed by the clay up to a certain limit. The additional water acts as lubricant to make the sand more moldable but the strength may decrease.
Additives – Some more additives are also there to improve the specific properties like mouldability, collapsibility, surface finish and hot strength of molding sand. Example – silica flour, wood flour, iron oxide, fuel oil, graphite, molasses, perlite etc.
Properties of moulding sand:
Permeability (Porosity) – The property of sand to allow gases, water and steam vapors to pass through it is known as permeability or porosity. The permeability of sand depends upon the following factors.
- Grain size of sand and shape of the grain. The round shape is more favorable for escaping of gases and steam vapors.
- More is the moisture content, less is the permeability.
Adhesiveness – The sand particles should stick to the other bodies particularly to the molding box so that the sand mass can be held properly in box and does not fall while moving the mould.
Cohesiveness – The sand particles should have ability to stick together. Cohesion must be retained when the mould is filled with molten metal and it should not wash away during pouring. Cohesiveness depends upon the shape and size of grain. It also depends upon distribution of binding material in the sand.
Plasticity – Plasticity is the property of material to acquire desired shape under pressure and to retain it after the removal of pressure. Sand should have good plasticity in order to have good impression of pattern in mould.
Green strength – The strength of sand in its wet stage is called green strength. The sand should have adequate strength in its wet and dry state. When we pour the molten metal, the sand adjacent to metal dries and it should not erode.
Refractoriness – The ability of sand to withstand high heat without breaking down or burning is called refractoriness. The refractoriness can be improved by removing the impurities.
Chemical resistivity – The sand should not chemically react with the molten metal so that it can be used for long time to make mould.