Moulding sand is used in foundry shops in making mould. It in obtained from lakes, rivers, sea and deserts. There are mainly two types of sands used in foundry shop.
Natural sand: This type of sand contains sufficient amount of binding clay so there is no need to add binding material to it.
Silica sand: The silica sand does not contains any clay content so suitable binders are added to it to make it suitable for foundry work. They are also known as synthetic sands, when they are mixed with binders and additives.
Characteristics of moulding sand:
Refractoriness: The ability of moulding sand to withstand high temperature of the molten metal without fusing. If the moulding sand does not possess this property then it will fuse on coming in contact with the molten metal. It will form a slag on the surface of casting, thus causing defect in casting.
Permeability: It is the property of moulding sand which allows the gases and steam to escape through the sand mould. If gases are not allowed to escape it can result in defects like blow holes in casting. Permeability is also known as Porosity.
Flowability: The moulding sand must have property to pack properly around the pattern to acquire the desired shape. This property can be improved by addition of clay and water to the moulding sand.
Adhesiveness: The property of moulding sand which enables it to adhere to the surfaces of other materials. This property helps in holding bulky sand projections of the mould.
Cohesiveness: The property of moulding sand due to which its rammed particles bind together firmly which allows the pattern to be removed without damaging the edges and surfaces of mould.
Collapsibility: The property of moulding sand due to which the sand mould collapse automatically after solidification of the casting to allow free contraction of metal.