In the first-generation moulding methods, the final mould strength is achieved by mechanical compaction of the clay bonded moulding aggregate, either manually by ramming, or using a machine moulding. Greensand is the most widely used moulding medium consisting of the base aggregate, bonded with a mixture of clay and water. Its ‘greenness’ refers to its weak, plastic condition in contrast to that of the dried moulds which are extremely hard and rigid. Greensand is low cost, environmentally friendly and recyclable, and production of the moulds is extremely fast. A modern greensand plants can produce about 600 moulds per hour.
Dry sand practice – the strength of greensand moulds can be further increased by drying or surface drying. The moulds are dried at the temperature of about 600 °C in case of steel castings, and at about 400 °C in case of grey iron castings. The high temperature drying increases the strength of the clay bounded moulding aggregate up to ten times. However, high temperature drying is lengthy and energy-consuming, and so very costly. For example, heat consumption is several times higher than that in metal melting for the casting itself. Dry sand moulds have now been practically entirely superseded by the moulds made by the second-generation moulding methods, even for heavy castings.
Compaction of the moulding aggregate and its uniformity throughout the mould volume has a great influence on the casting quality. Insufficient compaction of the moulding aggregate may, on the other hand, cause deformation of the mould cavity by additional compaction owing to the metallostatic pressure of the cast metal, and the erosion of the moulds surface caused by the flow of the liquid metal. Therefore, the optimum degree of the aggregate compaction must be chosen before making the mould or core.
Machine moulding methods of the moulding aggregate: