ƒT½áƒFí 2

ƒT½áƒFí 2

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Here the molten metal is being poured into the moulds. The most important factor affecting the fluidity of the molten metal is the pouring temperature or the amount of superheat , although, freezing temperature and freezing range are also important factors affecting fluidity. The higher the pouring temperature, the higher the fluidity. However at excessively high pouring temperatures , metal-mould reactions are accelerated and penetration of the small voids is possible, between the sand particles in a sand mould, which would leave particles of sand embedded in the casting itself, rendering it mechanically unsound. Fundimentals of Casting In casting , a solid is melted , heated to proper temperature, and sometimes treated to modify its chemical composition. The molten material, generally metal, is then poured into a cavity or mould , which contains it in proper shape during solidification. Thus, in a single step, simple or complex shapes can be made
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from any metal that can be melted. The resulting product can have virtually any configuration the designer desires. In addition, the resistance to working stresses can be optimised, directional properties can be controlled, and a pleasing appearance can be produced. Cast parts range in size from a fraction of an inch and a fraction of an ounce (such as the individual teeth on a zipper), to over 30 feet (10 metres) and many tons (such as the huge propellers and stern frames of ocean liners). Moreover, casting has marked advantages in the production of complex shapes, of parts having hollow sections or internal cavities, of parts that contain irregular curved surfaces (except those made from thin sheet metal), of very large parts, and of parts made from metals that are difficult to machine. Because of these obvious advantages, casting is one of the most important of the manufacturing processes. Today, it is nearly impossible to design anything that cannot be cast by means of one or more of the available casting processes. However, as in all manufacturing techniques, the best results and economy are achieved if the designer understands the various options and tailors the design to use the most appropriate process in the most efficient manner. The various processes differ primarily in the mould material (whether sand, metal, or other material) and the pouring method (gravity, vacuum, low pressure, or high pressure). All of the processes share the requirement that the materials solidify in a manner that would maximize the properties, while simultaneously preventing potential defects, such as shrinkage voids, gas porosity, and trapped inclusions. Six basic factors are involved in casting processes:-
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Mould Cavity A mould cavity, having the desired shape and size, must be produced with due allowance for shrinkage of the solidifying metal. Any complexity of shape desired in the finished casting must exist in the cavity. Consequently, the mould material must be able to reproduce the desired detail and also must have a
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ƒT½áƒFí 2

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