The whole system of mould-making, melting of metals, casting of the melt
in mould, solidifying the cast metal to produce an object in the form of the mould, and final
dressing and finishing of the object is called foundry. Moulds may be of two types:
(i) Those made of some metal (e.g., zinc) in which case the mould is called “die” and it
is permanent; the casting operation is called “die casting”.
(ii) Those made of sand which are called “sand mould” or simply “mould”, and they are
generally of the “use-and-throw” kind. The casting operation is called “sand
Clay – Bentonite 93
A sand mould (hereinafter referred to as mould) for solid cast is made with moist wellbonded
sand rammed into the desired pattern by hand or machine, and is suited to casting of
metals or alloys. Bentonite is used as a binding agent in casting of both ferrous and
nonferrous metals, but more particularly iron. Plasticity when wet, and fusion are the key
criteria. High plasticity ensures good binding power, while high fusion point is necessary to
ensure that the molten metal/alloy cast does not melt it. Both sodium and calcium types of
bentonite can be used.
Depending on the quality of casting, the Indian foundry industries use bentonite with
varying physical characteristics depending on the final product, by and large following the
specifications of the American Foundrymen’s Association. Nevertheless, sodium bentonite is
preferred to calcium bentonite. The parameters specified by them are: swelling capacity 12-45
ml; gel time instant to 10 minutes; gel index 20-72; base-exchange capacity 60-100 milliequivalents
per 100 gm; green compressive strength 7-12 psi.