In foundries, sand is used for making moulds. Natural sand found on the bed and banks of rivers provides an abundant source, although high quality silica sand is also mined. Sand is chemically SiO2–silicon dioxide in granular form. Ordinary river sand contains a contain percentage of clay, moisture, non-metallic impurities and traces of magnesium and calcium salts besides silica grains. This sand, after suitable treatment, is used for mould making. A good, well prepared moulding sand should have the following properties:
(i) Refractoriness i.e., it should be able to with stand high temperatures.
(ii) Permeability i.e., ability to allow gases, water vapour and air to pass through it.
(iii) Green sand strength i.e., when a mould is made with moist sand, it should have sufficient strength, otherwise mould will break.
(iv) Good flowability i.e., when it is packed around a pattern in a moulding box, it should be able to fill all nooks and corners, otherwise the impression of pattern in mould would not be sharp and clear.
(v) Good collapsibility i.e., it should collapse easily after the casting has cooled down and has been extracted after breaking the mould. It is particularly important in case of core making.
(vi) Cohesiveness i.e., ability of sand grains to stick together. Without cohesiveness, the moulds will lack strength.
(vii) Adhesiveness i.e., ability of sand to stick to other bodies. If the moulding sand does not stick to the walls of moulding box, the whole mould will slip through the box.
Properties like permeability, cohesiveness and green strength are dependent upon size and shape of sand grains, as also upon the binding material and moisture content present in sand. Clay is a natural
binder. Chemical binders like bentonite are sometimes added if clay content in natural sand is not enough.
Standard tests have been devised by foundry men to determine properties of sand. Generally fresh moulding sand prepared in the foundry has the following composition:
Silica 75% (approx.)
Clay 10–15%
Bentonite 2–5% (as required)
Coal dust 5–10%
Moisture 6–8%
Core sand has oil as the main binding material. A core gets surrounded by molten metal which causes the oil to vaporise. This increases collapsibility of sand and makes it easy to remove sand from the holes in the casting.
(i) Refractoriness i.e., it should be able to with stand high temperatures.
(ii) Permeability i.e., ability to allow gases, water vapour and air to pass through it.
(iii) Green sand strength i.e., when a mould is made with moist sand, it should have sufficient strength, otherwise mould will break.
(iv) Good flowability i.e., when it is packed around a pattern in a moulding box, it should be able to fill all nooks and corners, otherwise the impression of pattern in mould would not be sharp and clear.
(v) Good collapsibility i.e., it should collapse easily after the casting has cooled down and has been extracted after breaking the mould. It is particularly important in case of core making.
(vi) Cohesiveness i.e., ability of sand grains to stick together. Without cohesiveness, the moulds will lack strength.
(vii) Adhesiveness i.e., ability of sand to stick to other bodies. If the moulding sand does not stick to the walls of moulding box, the whole mould will slip through the box.
Properties like permeability, cohesiveness and green strength are dependent upon size and shape of sand grains, as also upon the binding material and moisture content present in sand. Clay is a natural
binder. Chemical binders like bentonite are sometimes added if clay content in natural sand is not enough.
Standard tests have been devised by foundry men to determine properties of sand. Generally fresh moulding sand prepared in the foundry has the following composition:
Silica 75% (approx.)
Clay 10–15%
Bentonite 2–5% (as required)
Coal dust 5–10%
Moisture 6–8%
Core sand has oil as the main binding material. A core gets surrounded by molten metal which causes the oil to vaporise. This increases collapsibility of sand and makes it easy to remove sand from the holes in the casting.
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