<P> Molding sand, also known as foundry sand, is a sand that when moistened and compressed or oiled or heated tends to pack well and hold its shape . It is used in the process of sand casting for preparing the mould cavity . </P> <P> Green sand is an aggregate of sand, bentonite clay, pulverized coal and water . Its principal use is in making molds for metal casting . The largest portion of the aggregate is always sand, which can be either silica . There are many recipes for the proportion of clay, but they all strike different balances between moldability, surface finish, and ability of the hot molten metal to degas . The coal, typically referred to in as sea - coal, which is present at a ratio of less than 5%, partially combusts in the surface of the molten metal leading to offgassing of organic vapors . </P> <P> Sand casting is one of the earliest forms of casting practiced due to the simplicity of materials involved . It still remains one of the cheapest ways to cast metals because of that same simplicity . Other methods of casting, such as those using coquille, boast higher quality of surface finish, but have higher cost . </P>

What are the main constituents of moulding sand
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