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Plaster slurry readily flows around pattern, capturing its fine details and good surface finish. The slurry thus prepared is poured over the metal pattern kept in cope, as shown in Figure 1(b). When mixed with water, the gypsum re-crystallizes as it dehydrates, which causes the plaster to set by the interlocking of crystals. The process consists of preparing plaster slurry by mixing gypsum with silica flour, silica sand and water (see Figure 1(a)). Plaster mould casting process is illustrated in Figure 1. Wood patterns not generally used due to extended contact with water. Patterns for plaster mould casting process can be made from wood, metal, plastic etc (preferably brass match plate pattern). In this process, moulds are prepared by pouring plaster (or gypsum, CaSO 4-2H 2O) around the pattern. This process is used for casting objects of aluminium, lead, tin, magnesium, bronze alloys. Plaster mould casting process is also known as ceramic mould casting process. We discuss some of the widely used special casting processes. However, special casting techniques are not preferred always owing to their high initial investment requirement. Ability to produce extremely thin sections etc. Minimum need for further machining of castings,Ħ. Special casting processes have the following advantages over the sand casting processes:ĥ. To able classify casting processes on the basis of mould (permanent / expendable) and pattern.Ĥ.Ěble to select various special casting processes on the basis of different applications. To gain knowledge about the various special casting process.ģ. To understand the need of special casting process.Ģ. In these situations, special casting processes are used for making high precision castings. In addition, sand casting process does not always produce good and consistent results and it cannot be used in situations when more accurate and precise castings are to be made. It becomes, therefore, obvious that the use of a permanent mould would do a considerable saving in time and labour cost of mould making. The sand moulds are single use moulds as they are destroyed after the casting has been removed from the moulding box. Sand casting processes, which we have studied so far, find wide applications and give satisfactory results at lowest cost in many cases.