Q. Why we need additives explain?

Additives are the materials added in small quantities to the moulding sand in order to enhance its exiting properties and to impart to it special properties. These additives may be necessary to give a good surface finish to the casting or to eliminate casting defects that's arise from either the expansion of the moulding sand as it is heated or the contraction of the casting as it cools in the mould. There may be some overlapping between an additive and a binder because many people include organic binders also in the category of additives. Metal penetration or burning on is penetration of metal between sand grains and also strong adhesion of the fused or sintered sand to the metal of the casting surface. The burnt on sand involves difficulty in cleaning operations and is responsible for rapid wear of cutting tools used to machine the castings.