Centrifugal Casting:
Centrifugal
casting is done by pouring molten metal into a rotating mould. The
centrifugal force acting on the mould helps in feeding and positioning
the metal in the mould. Mould rotation is continued till after the metal
is solidified.
Centrifugal
casting results in denser and cleaner metal as heavier metal is thrown
to parts of the mould away from the centre of rotation and the lighter
impurities like slag, oxides and inclusion are squeezed out to the
centre.
The castings produced have a close grain
structure, good detail, high density and superior mechanical properties.
Elaborate gating and risering systems are not required as very simple
systems will do the job. There is also a considerable saving of
material.
Types of centrifugal casting:
Centrifugal
casting can be divided into three categories namely true centrifugal
casting, semi centrifugal casting and centrifuging.
True centrifugal casting:
The
true centrifugal method of casting is used to produce hollow castings
with a round hole. The characteristic feature of this process is that
the hole is produced by the centrifugal force alone and no cores are
used.
The
mould is rotated about the axis of the hole with the axis held
horizontal, inclined or vertical. The outside surface of the job may be
round, square, hexagonal etc. and should be symmetrical with the whole
axis. The central hole should 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 relatively short lengths are
poured with moulds rotated about an inclined or vertical axis. Rotation
about the vertical or inclined axis is convenient but the central hole
produced will be slightly parabolic with smaller diameter at the bottom
because the metal has a tendency to settle down due to gravity. The
speed of rotation for true centrifugal casting should be high enough to
hold the metal on to the mould wall till 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 may result 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
suitable. The moulds used for the process may be metal moulds or
refractory or sand lined moulds. Common products produced by true
centrifugal casting include pipes, oil engine cylinders, piston ring
stock, gear blank stock, bearing bushes and the like.
Semi-centrifugal casting:
In
semi-centrifugal casting process 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.
Centrifuging:
Centrifuging
or centrifuge casting is employed 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. Centrifuging helps in proper feeding of castings
resulting in clean, close grained castings.
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