BINDERS FOR FOUNDRY CORES AND MOULDS

A binder for a foundry core or mould consists of an alkali metal salt of a polybasic organic acid or of a polymerised monobasic organic acid, for example sodium polyacrylate, an alkaline earth metal hydroxide such as calcium hydroxide, one or more polyvalent metal oxides, preferably magnesium oxide, and calcium citrate, together with water. The mixture may also include another polyvalent metal oxide, for example zinc oxide. The inclusion of calcium citrate in the binder composition, particularly in the presence of magnesium oxide, has been found to reduce the problem of 'softening back' in which the strength of the core or mould interior deteriorates over storage periods of a few days due to the continued absorption of carbon dioxide from the atmosphere in damp conditions.