The principal material used in moulding is the moulding sand’ which is also referred as “Green Sand”. This sand must possess the following qualities.
- Cohesiveness – The ability to retain its shape when the pattern has been removed during the pouring and solidification, of the metal.
- Refractivity – The ability to withstand the heat of the molten, metal without fusing.
- Porosity – his enables the free escape of gases which form as the hot metal contacts the moisture in the mould.
- Re-Tempering Qualities – The quality that enables it to be re tempered for use in many successive moulds.
Cohesiveness or bonding properties are imparted to moulding sands chiefly by the presence of 2 to 6 percent of clay, The clay in moulding sand must be ideally distributed in the form of a thin layer on the individual grains, and this distribution and complete mixing of the water and coal dust is done by milling the sand mixture in various mills. The sand is finally riddled by pressing it. Bentonite which is colloidal clay of volcanic origin, has been found superior to ordinary colloidal clay imparting cohesiveness to moulding sand.
The mould as a whole must also be porous, that is, the grains of sand must not be packed too closely together and space must exist between them. Thus the porosity of a mould depends largely on the degree of ramming and this is a matter on which the moulder has to exercise skill acquired by experience. The adequate venting of a mould however generally requires the provision of venting channels and these are formed by pricking the sand of the cope with a wire after it has been rammed up, thus forming a number of holes or vents.
The sand selected for moulding must withstand the heat from molten metal. This property is called “refractoriness”, It is obvious that metals which have high melting points must be poured into moulds employing sands of greater refractoriness.
Backing sand – This is normally the reconditioned foundry sand and is used for ramming the bulk of the moulding flask. The moulding flask is completely filled with backing sand after the pattern is covered with a thin layer of facing sand. Since the casting is not affected to any great extent by the backing sand, it usually contains the burnt facing sand, moulding sand and clay.
Parting Sand – This is the material which is sprinkled on the pattern and to the parting surfaces of the mould halves before they are prepared, to prevent the adherence of the moulding sand. This helps in easy withdrawal of the pattern and easier separation of the cope and drag flasks at parting surface. It is essentially a non sticky material such as washed silica grains.
Facing Sand – Facing sand which is used next to the surface of the pattern is made of finely riddled moulding sand, to which 40 to 60 percent of new sand and bond have been added. The finer texture of facing sand will produce a smoother surface on castings.