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Download Mould & Core Material for the Steel Foundry by A. D. Sarkar, J. G. Pearce PDF

By A. D. Sarkar, J. G. Pearce

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17) and a maximum attractive force is at an optimum distance of separation x. There will be many such dipoles in a clay-water medium which will be oriented selectively forming a network binding the hydrated clay particles together. Depending on the type of clay, that is on the magnitude of charge on its broken surface, a maximum degree of hydration will be necessary to develop a dipole completely. This is why strengths of clay-bonded sands increase with increasing amount of water addition to a maximum value.

One advantage in using sodium montmorillonite is that optimum green and dry strengths are achieved at low amounts of clay. Of the kaolinitic clays, fireclay is an example and is a mixture of clay mineral, sand and silt. The ratio of sand to silt varies according to their geographical location and an average composition is 30% clay mineral, 42 % sand and 28 % silt. The bonding and refractory properties of fireclay from various sources vary widely and a very large amount of the material is needed to develop an optimum bond.

Starch is also known to increase dry strengths of moulding materials. THE PRINCIPAL BINDERS 49 Sulphite Lye and Molasses. Sulphite lye and molasses are water soluble and are used as additives in moulding sands. They increase dry strength and tend to migrate to the surfaces giving high surface hardness to dry moulds. Hard surfaces mean a lack of friability in the exposed mould areas and consequently mould erosion during metal flow is minimized. The proprietary core creams are mixtures blended from oils, starch, sulphite lye, etc.

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