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Write short notes on: Characteristics of moulding sand

Write short notes on:

a. Characteristics of moulding sand

b. Centrifugal casting

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Characteristics of moulding sand

The basic characteristics of molding sand and core sand are adhesiveness, cohesiveness, collapsibility, flow ability, dry strength, green strength, permeability, refractoriness

  1. Adhesiveness: Property of molding sand to get the stick or adhere to foreign material such sticking of molding sand with the inner wall of molding box.
  2. Cohesiveness: property of molding sand by virtue which the sand grain particles interact and attract each other within the molding sand.
  3. Collapsibility: After the molten metal in the mould gets solidified, the sand mould must be collapsible so that free contraction of the metal occurs and this would naturally avoid the tearing or cracking of the contracting metal. In absence of collapsibility property the contraction of the metal is hindered by the mold and thus results in tears and cracks in the casting. This property is highly required in cores.
  4. Dry strength: As soon as the molten metal is poured into the mould, the moisture in the sand layer adjacent to the hot metal gets evaporated and this dry sand layer must have sufficient strength to its shape in order to avoid erosion of mould wall during the flow of molten metal. The dry strength also prevents the enlargement of mould cavity cause by the metallostatic pressure of the liquid metal.
  5. Flow ability: the ability of the sand to get compacted and behave like a fluid. It will flow uniformly to all portions of pattern when rammed and distribute the ramming pressure evenly all around in all directions. Flow ability increases with decrease in grain size of sand. The flow ability also varies with moisture and clay content in sand.
  6. Permeability: Also called as porosity of the moulding sand. It allows the escape of any air, gases or moisture present or generated in the mould when the molten metal is poured into it. Permeability is a function of grain size, grain shape, and moisture and clay contents in the molding sand.
  7. Refractoriness: It is the ability of molding sand to withstand high temperatures without breaking down or fusing thus facilitating to get sound casting. It is a highly important characteristic of molding sands. Refractoriness can only be increased to a limited extent.

Centrifugal casting

  • The manufacturing process of centrifugal casting is a metal casting technique, that uses the forces generated by centripetal acceleration to distribute the molten material in the mold.
  • In centrifugal casting, to manufacture a cast, it is necessary to rotate the mold using some mechanical means which here is achieved by rollers.
  • The mold is rotated about its axis at a predetermined speed.
  • Molds for smaller parts may be rotated about a vertical axis.
  • However, most times in true centrifugal casting manufacture the mold will be rotated about a horizontal axis. The effects of gravity on the material during the metal casting process make it particularly necessary to cast longer parts with forces generated from horizontal rather than vertical rotation.
  • The molten material for the cast part is introduced to the mold from an external source, usually by means of some spout.
  • The liquid metal flows down into the mold. Once inside the cavity, the centrifugal forces from the spinning mold force the molten material to the outer wall. Molten material for the casting may be poured into a spinning mold or the rotation of the mold may begin after pouring has occurred.
  • The metal casting will harden as the mold continues to rotate.
  • This casting process is very well suited for the manufacture of hollow cylindrical tubes.
  • The forces used in this technique guarantee good adhesion of the casting material to the surface of the mold.
  • Thickness of the cast part can be determined by the amount of material poured.
  • Centrifugal casting is a great manufacturing process for producing hollow cylindrical parts.
  • The metal casting's wall thickness is controlled by the amount of material added during the pouring phase.
  • Centrifugal casting is a manufacturing process that is capable of very high rates of productivity.
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