In the centrifugal casting, molten metal is poured into mold while they are rotating. The metal falling into the center of mold at the axis of rotation is thrown out by the centrifugal force under sufficient pressure towards the periphery, and the contaminants or impurities present being lighter in weight are also pushed towards the center. This is often machined out any way. Solidification progresses from the outer surface inwards, thus developing an area of weakness in the center of wall. This is caused by the meeting of the grain boundaries at final solidification and the entrapment of impurities in the central section. The grain is refined and the castings are completely free from any porosity defect by the forced movement of the molten metal, thus making dense and sound castings which are less subject to directional variations that static castings. The use of gates, feeders, and cores is eliminated, making the method less expensive and complicated.
Hollow cylindrical bodies such as cast iron water supply and sewerage pipes, steel gun barrels, and other symmetrical objects such as gears, disk wheels, pulleys, are conveniently cast without core by the centrifugal casting.
Centrifugal casting can be classified into three general types: True centrifugal, Semi-Centrifugal, and Centrifuged.
True Centrifugal casting : This employs mold of rotational symmetry made of steel (with a refractory mold wash or even a green - or dry-sand lining) or of graphite. The melt is poured while the mold rotates at its axis, which may be horizontal, vertical or inclined at any suitable angle between 0 to 90*, although horizontal axis of rotation is a more common practice. While rotating, the molten metal is carried to walls of the cavity by centrifugal force. The metal then solidifies forming a hollow casting without the use of a central core. The outside of the mold is water-cooled to accelerate solidification.
By proper control of flow rates and movement of pouring orifice, long and large tubes of very uniform quality and wall thickness can be cast. If desired, the outer contour of the casting can be varied, while the inside remains cylindrical.
The method is ideal for hollow cylindrical castings such as bushings, gun barrels, cast iron pipes, etc.
Semi-centrifugal Casting : This is a means of forming symmetrical shapes about the rotative axis, which is usually vertical in a balanced state. The molten metal is introduced though a gate which is placed on the axis, and flows outward to the rim by the centrifugal force. If a central bore is required in casting , a dry sand core is best suited. The central gate acts as a riser for the hub portion.
Since the speeds are low, high pouring pressures are not produced and the impurities are not rejected towards the center as effectively as in the true centrifugal casting.
This method is generally employed for making large-sized castings which are symmetrical about their own axis such as gears, disked wheels, propellers and pulleys.
Centrifuge: In this process several identical or nearly similar molds are located radially about a vertically arranged central riser or sprue which feeds the metal into the cavities through a number of radially gates. The entire mold is rotated with the central sprue which acts as the axis of rotation. Thus, it is not a purely centrifugal process.
This type of casting is suitable for small, intricate parts where feeding problems are encountered. This method can be used to advantage for stack molding of six or more molds mounted one above the other.
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