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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