Abstract: For a method of making foundry cores and moulds using a resin-coated sand in a hot box process, a crystalline unsaturated polyester resin for making the binder uses an unsaturated dicarboxylic acid, a symmetrical dicarboxylic aromatic acid and a glycol which is predominantly 1,6 hexanediol, with the possible addition of ethylene glycol and other symmetrical glycols. The molecular weight of the resin is at least 2000 and it is mixed with solid monomer such as methylene bis acrylamide or a low-viscosity liquid monomer such as diallyl phthalate or a mixture of the two. The aromatic acid is between 30 and 55 mole percent of the total acid and the acid value before addition of the monomer is below 30 mg KOH/g to result in a viscosity of not more than 30 poise at 125.degree. C.