casting

1. UNIT-3 Special Casting Process

5. Centrifugal Casting
• In this casting process, molten metal
is poured into a revolving mold and
allowed to solidify molten metal by
pressure of centrifugal force.
• This process makes hollow product.
• There are three main types of
centrifugal casting process:
1. True centrifugal casting process
2. Semi centrifugal casting process
3. Centrifuging process.

6. Centrifugal Casting
1. True Centrifugal casting process:
• This is normally used for the making of hollow pipes, tubes, hallow bushes, etc.
Which are axisymmetric with a concentric hole.
• The axis of rotation can be either horizontal, vertical or any angle between.
• Very long pipes are normally cast with horizontal axis, whereas short pieces are
more conveniently cast with a vertical axis.

7. Process
• In centrifugal casting, a permanent mold is rotated continuously about its axis at high
speeds (300 to 3000 rpm) as the molten metal is poured.
• The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies
after cooling.
• The casting is usually a fine-grained casting with a very fine-grained outer diameter, owing
to chilling against the mould surface.
• Impurities and inclusions are thrown to the surface of the inside diameter, which can be
machined away.
• Casting machines may be either horizontal or vertical-axis. Horizontal axis machines are
preferred for long, thin cylinders, vertical machines for rings.
• Most castings are solidified from the outside first. This may be used to
encourage directional solidification of the casting, and thus give useful metallurgical
properties to it. Often the inner and outer layers are discarded and only the

8. • Features of centrifugal casting
• Castings can be made in almost any length, thickness and diameter.
• Different wall thicknesses can be produced from the same size mold.
• Eliminates the need for cores.
• Resistant to atmospheric corrosion, a typical situation with pipes.
• Mechanical properties of centrifugal castings are excellent.
• Only cylindrical shapes can be produced with this process.
• Size limits are up to 3 m (10 feet) diameter and 15 m (50 feet) length.
• Wall thickness range from 2.5 mm to 125 mm (0.1 - 5.0 in).
• Tolerance limit: on the OD can be 2.5 mm (0.1 in) on the ID can be 3.8 mm (0.15
in).
• Surface finish ranges from 2.5 mm to 12.5 mm (0.1 - 0.5 in) rms.

9. 9
Advantages
Good mechanical properties can be achieved
No cores are required for making concentric holes in the
case of true centrifugal casting.
There is no need for gates and runners, Which increases
the casting yield , reaches to almost 100%.
Limitations
Castings which are axi-symmetric and having
concentric holes are suitable.
Equipment is expensive.

10. Materials
• Typical materials that can be cast with this process
are iron,
• steel,
• stainless steels,
• glass, and
• alloys of aluminum,
• copper and nickel.

11. • Typical parts made by this process are
• pipes,
• boilers,
• pressure vessels ,
• flywheels,
• cylinder liners and
• other parts that are axi-symmetric.
• It is notably used to cast cylinder liners and sleeve valves for
piston engines, parts which could not be reliably manufactured
otherwise.

14. Semi - Centrifugal Casting
• Semi –centrifugal casting is used for jobs which are more
complicated than those possible in true centrifigal casting, but are
axisymmetric in nature.
• The casting like symmetrical shape, pulley, wheel, disk, gear like
big shape products manufactured.
• More than one casting achieved.
• Here vertical axis machine is used. In center the hub is provided to
create hollow core.

16. Centrifugal Casting
3. Centrifuging:
• This process is used for
non-symmetrical castings
having intricate details
and also for precision
castings.
• The centrifugal force
provides high-fluid
pressure to force the
molten metal into mould
cavity.
• A number of similar
components can be cast
simultaneously.

18. Die Casting
• Die casting utilizes two
blocks of heat resistant metal
machined to meet along the
plane of the parting line.
• Fine vents are cut to allow
air to escape as the metal
centers.
• Molten or semi molten metal
is either poured under
gravity or is forced under
high pressure into a die and
pressure is maintained till
solidification stage.

19. Die Casting
1. Gravity Die Casting:
• feeder is incorporated so
that when the level of the
metal in the feeder is
above that at the highest
point of the casting.
• head of liquid in feeder
forces the metal into all
parts of the mould
cavity.
• Aluminum, Copper,
Lead, Magnesium, Zinc
products are produced.

20. Die Casting
2. Pressure die casting:
• In pressure die casting metal flows under high pressure
• Also as the die is metallic, the casting rate is high and thus mass
production is possible.
The following are the types of pressure die casting
(A) Hot chamber die-casting
(1) Submerged plunger die casting
(2) Air blown or goose neck die casting
(B) Cold chamber die casting.

21. Die Casting
(A)Hot chamber die-
casting:
(1)Submerged plunger
die casting:
• Generally 125
kg/cm2- pressure is
applied.
• The castings from
zinc, tin and lead
alloys are produced
having low melting

23. Die Casting
(2) Air blown or
goose neck die
casting machine:
• with this process
the metals
having low
melting
temperature like
lead, tin, zinc
ore to be cast.
• In this machine

24. Advantages of die casting
• Advantages:
– Thin section (0.5 mm thickness) can be easily made.
– Impression or complicated design can be achieved on component
walls.
– The production rate is high (300 per hour approx).
– Die set can be used many times.
– All non-ferrous products are produced with this method.
– Better surface finish is achieved.

25. Disadvantages of die casting
• It is not economic for small quantity of
production.
• It is used for only small casting (10 kg weight
approx).
• Initial cost is high for die and other equipment.
• Only non-ferrous products are casted.
• If proper care is not taken then the defects like
blow hole can be possible.

26. Investment Casting
• This process is called the lost-wax
process or precision casting. The
following are the different stages
(1) Die making
(2) Making wax pattern
(3) Precoating the wax pattern
assembly
(4) Investment the wax pattern in
mould box
(5) Removal of wax pattern
(6) Pouring molten metal
(7) Cleaning of casting

27. Advantages Investment Casting
(1)Very smooth surface of casting without parting lines
are achieved.
(2) Machining of intricate shape can be manufactured.
(3) Die casting can be replaced when short runs are
involved.
(4) Castings are sound and have large grains as the rate of
cooling is slow.
(5) It represents the only method suitable for manufacture
of precision shaped castings of high melting point metals
which would cause too rapid die failures is normal die
casting process.

28. Disadvantages Investment Casting
(1) It is expensive process and hence is adopted
only where small number of intricate and highly
accurate parts particularly high melting point
alloys are to be manufactured.
(2) This process is suitable for small size parts.
(3) This presents some difficulties when cores
are to be used.

29. Defects in Casting
• Casting defects, their causes & remedies:
(1) Blow holes
Holes on casting surface is blow holes. This defect occur
because of air comes out from surface.
Causes: Moisture level is high in mould sand, improper
baking of core. unnecessaryy carbonic binder, unwanted
ramming, small vent hole, fine sand etc.
Remedies: Proper moisture level, proper baking of core,
proper use of binder, proper ramming, proper vent hole with
vent rod, selection of sand particle.

30. Blow holes Casting Defects

31. Casting defects, their causes &
remedies
(2) Shrinkage
When metal transfer from liquid to sold its volume will
decrease. During this process if it will not get more
molten metal then in the internal surface of casting voids
are developed, that is known as shrinkage.
Causes: Defective runner, gate and riser, molten metal's
pouring temperature.
Remedies : Proper arrangement of runner, riser and gate
for proper directional solidification. If required, the design
can be changed, maintaining proper molten metal
temperature.

32. Shrinkage Casting Defects

33. Casting defects, their causes &
remedies
• (3) Crack :
• Due to solidification metal shrinks in mould.
The improper shrinkage develops stress on
surface, due to that cracks propagates on
surface also known as pull.
• Causes: Non availability of collapsibility
property of mould and core, improper design,
hard ramming of mould.
• Remedies: Collapsibility property can be
improved, design can improved. properly

34. Crack casting defect

35. Casting defects, their causes &
remedies
• 4) Inclusions :
• Unwanted ingredients such as metal oxide, slag, sand
particles give defects known as inclusions in metal
castings.
• Causes: Improper gating system, improper pouring,
low quality mould and core sand, improper ramming,
impurity in metal charge.
• Remedies: By modifying gating system, turbulence
free pouring, using good quality mould and core, proper
ramming of mould sand, proper and pure metal charge
should be used and by using oxide free molten metal
crucible.

37. Casting defects, their causes &
remedies
• (5) Lift and shift :
• Some part of casting gets distortion known as lift and shift.
• Causes: Improper alignment of pattern parts, improper support of core, improper clamping of mould box,
improper strength of mould sand.
• Remedies: By aligning the mould box with help of dowel pin, properly supporting core in mould,
properly clamping of mould box, providing proper strength mould and core sand.
• (6) Swell
• Due to molten metal the some part of mould cavity become large so the casting becomes larger than
required which known as swell.
• Causes : Pressure of molten metal on surface, improper ramming of sand, low strength of core sand.
• Remedies: By properly ramming of sand, by increasing core strength so molten metal can easily flow in
mould.
• (7) Fins
• Thin edge on casting known as fins. Generally it is on parting line.
• Causes: Improper clamping of mould b0x, improper arrangement of mould and core.
• Remedies: Properly clamping of cope and drag, proper assembly of mould and core.

38. Casting defects, their causes &
remedies
• (8) Misrun and cold shut :Molten metal cannot reach in all the parts of mould, so this improper filled casting
known as misrun. Molten comes from different sides, sometimes cannot mix each other. This defect known as
cold shut.
• Causes: Defective design of gating system, low fluidity of molten metal, thin wall of casting, non-continues
pouring of metal.
• Remedies: Modification of gating system, increasing temperature of molten metal, continuously pouring of
molten metal, increasing porosity of sand.
• (9) Metal Penetration: Surface of casting becomes rough due to metal penetration.
• Causes: Bigger size of sand, less ramming, low strength of moulding sand and core, higher permeability.
• Remedies : Fine grain sand, proper ramming of mould sand, proper mixture should be used to increase
moulding and core porosity.
• (10) Hard spot: Some part of casting solidifies very fast and that surface becomes tough, that known as hard
spot.
• Causes: Bad casting design, improper métal composition, improper use of chills.
• Remedies: Proper design of castings so metal solidifies in same time, proper metal Composition, proper use of
chills in design.

39. Casting defects, their causes &
remedies
• (11) Run out: While pouring, molten metal comes out (leaks out) from casting known as run out.
• Causes: Defective mould box, defectively moulding process.
• Remedies Moulding box should be changed, modification in moulding process.
• (12) Drop: A drop occurs when cope surface cracks and breaks, thus the pieces of sand fall into the
molten metal.
• Causes Due to either low green strength or improper ramming of the cope flask, improper
reinforcement.
• Remedies: Proper mixing of binder for strength improvement, proper arrangement of steel rod for
reinforcement in core and mould, proper ramming.
• (13) Warpage: After or before solidification casting may twist or change shape known as warpage.
• Causes: Improper design, lack of directional solidification, internal stress.
• Remedies: Proper design of casting to get directional solidification. By proper heat treatment the stress
can be removed.