Molding 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
Main constituents of Molding 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
– 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 molding sand:
Permeability (Porosity) –
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.
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.
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 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.
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 –
sand should not chemically react with the molten metal so that it can be
used for long time to make mould.