Centrifugal castings

• It works on fundamental principle of centrifugal force on a rotating part. During this method a mould is rotated regarding its central axis when the molten metal is poured into it.

• A centrifugal force acts on molten metal due to this rotation that forces the metal at outer wall of mould. The mould rotates till the whole casting solidifies.

• The slag oxide and other inclusions are lighter and gets separated from metal then segregate towards the center.

• As the molten metal solidifies from the outside in a casting with dense close grain structure is formed.

• Because close grain structure, the centrifugal method offers the products with good physical properties than static casting method.

• Centrifugal casting products advantages are

  • Smoother Surface

  • Lighter Weight

  • Thinner Walls

• Combining correct mold design, mold coatings, mold spinning speeds, pouring speeds, cooling rates and metal chemistry leads to castings with higher yields, fewer impurities and greater strength.

TYPES OF CENTRIFUGAL CASTING :

• Centrifugal castings are classified into 3 categories. These are

  • True centrifugal casting

  • Semi centrifugal casting

  • Centrifuging

TRUE CENTRIFUGAL CASTING :

• This method is used to produce hollow castings with a round hole.

• The characteristic feature of this method is that the hole is produced by the centrifugal force.

• The mould is rotated regarding the axis of the hole with the axis held horizontal, inclined or vertical.

• The outside surface of the job may be round, square, hexagonal and may be symmetrical with the hole axis.

• The central hole ought to be round to be formed without cores.

• Long castings like cast iron, soil pipes are cast with the moulds rotated about a horizontal axis.

• Castings with comparatively short lengths are poured with moulds rotated about an inclined or vertical axis.

• Rotation regarding the vertical or inclined axis is convenient however the central hole made are slightly parabolic with smaller diameter at the bottom.

• The speed of rotation for true centrifugal casting ought to be high enough to hold the metal on to the mould wall until it solidifies.

• A low speed of rotation would result in raining or slipping of the metal inside the mould.

• Too large a speed of rotation on the other hand could end in internal stresses and possible hot tears.

• A speed which would provide a centrifugal force of 60 to 75 times the force of gravity on horizontal moulds and 100 times force of gravity for vertical moulds is found to be appropriate.

• The moulds used for the process could also be metal moulds or refractory or sand lined moulds.

SEMI CENTRIFUGAL CASTING :

• In this method no attempt is made to produce a hole without a core.

• The centrifugal force resulting from rotation of the mould is used to properly feed the casting to produce a close grained clean casting.

• The process is suitable for large axis symmetrical castings like gear blanks, fly wheels and track wheels.

• Any hole round or otherwise is made with the use of a core. The mould is clamped to a turn table with casting axis along the axis of rotation.

• The metal is poured along or near the axis to feed the points farthest from the axis of rotation under pressure.

• If made solid the central portion tends to be porous and with inclusion which are removed in subsequent machining.

CETRIFUGING CASTING :

• Centrifuging is used to force metal under pressure into moulds of small castings or castings not symmetrical about any axis of rotation.

• The moulds are made around a central axis of rotation to balance each other.

• The metal is poured along this axis of rotation through a central sprue and made to flow into mould cavities through radial ingates cut on the mould interface.

• Stack moulds can be used to advantage in centrifuging of castings required in very large numbers.

• Stack moulds can be used to advantage in centrifuging of castings required in very large numbers.

ADVANTAGES :

• Formation of hollow interiors in cylinders without cores.

• Less material required for gate.

• Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and porosity.

DISADVANTAGES :

• More segregation of alloy component during pouring under the forces of Rotation.

• Contamination of internal surface of castings with non-metallic Inclusions.

• Inaccurate internal diameter.

APPLICATIONS:

• Pipes, oil engine cylinders, piston ring stock, gear blank stock, bearing bushes.