casting.pdf .

Department of Mechanical & Manufacturing Engineering, MIT, Manipal 1 of 47
MANUFACTURING TECHNOLOGY
CHAPTER 2
CASTING

Types of casting:

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SPECIAL CASTING METHODS
1. PERMANENT MOULD CASTING
2. SHELL MOULD CASTING
3. PLASTER MOULD CASTING
4. INVESTMENT CASTING
5. SLUSH CASTING
6. PRESSURE DIE CASTING
7. CENTRIFUGAL CASTING
8. CONTINUOUS CASTING
Department of Mechanical & Manufacturing Engineering, MIT, Manipal

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Permanent Mould casting

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• In the sand castings the moulds are destroyed after
solidification of castings
• Here, moulds are generally made of cast iron (or) steel
which are not destroyed or rebuilt after every casting.
• Since these moulds last for long periods, the process
is called permanent mould casting.
• In this process, no external pressure is applied, this
process in sometimes called Gravity die casting.

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• This requires a mould material that has a sufficiently high
melting point to withstand erosion by the liquid metal at
pouring temperature.
• A high enough strength not to deform in repeated use, and
high thermal fatigue resistance (resists formation of
cracks)
• The material used for making moulds (dies) may be cast
iron, although alloy steels are the most widely used.

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• Refractory metal alloys like molybdenum alloys can be used
• The resistance of the mould to the melt can be increased
with refractory coatings or lining of sodium silicate,
phosphoric acid, soap stone and talc.
• Castings to be produced by permanent mould methods
should be relatively simple in design.
• Aluminium alloys, magnesium alloys, copper alloys are some
of the materials that are normally cast.

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Advantages:
• Closer dimensional tolerance
• Better surface finish & improved quality
• Lower rate of rejections
• More economical in mass production

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Disadvantages:
• Expensive
• Difficulty in removing the castings (moulds
cannot be broken up)
• Not suitable for metals and alloys of very high
temperature
• Impractical for large size castings & complicated
shapes
Applications:
Automobile pistons, stators, gear blanks, etc.

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Shell Mould casting

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Shell Mould casting

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Shell Mould casting

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1. Also known as CRONING or ‘C’ process.
2. A metal pattern is heated to180 – 2500C. Prior
to heating, a silicone release agent is sprayed
on the pattern and plate.
3. Pattern is then turned over face down and
clamped to the open end of the “Dump Box”.
4. The Dump box contains a mixture of clay free
sand (90 to 140 GFN) and thermosetting
resin.
Shell Mould casting

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5. The Dump box is inverted so that dry sand-resin
mixture falls on to the face of the pattern.
6. The resin-sand mixture in contact gets heated, & the
resin softens to form a soft &uniform shell (6mm
thickness ) on the surface of the pattern.
7. Dump box is inverted to initial position, excess sand-
resin mixture falls down leaving a shell adhering to
the pattern.
Shell Mould casting

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Shell Mould casting
8. The pattern along with the shell is passed directly into
an oven for 1 – 2 minutes where the shell cures and
acquires rigidity.
9. The shell is then stripped from the pattern with help of
ejector pins which may be an integral part of the pattern.
10. After the shells have cooled, 2 mating shells are
securely fastened either by mechanical clamping or by
using resin as an adhesive to form a complete mould.
11. The heat of molten metal starts burning resin binder of
the mould and the gases evolved escape through the
permeable shell walls.

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Shell Mould casting
12. By the time the casting has solidified, the binder has completely
burnt out and on tapping, the shell mould disintegrates easily. The
loose sand is then removed from the casting.
Applications:
• Small mechanical parts requiring high precision (gear housing),
Cylinder heads,& connecting rods.

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Shell Mould casting
Advantages:
• High quality of casting with smoother surface finish possible
because of finer grain sand.
• Complex shapes can be produced.
• Closer dimensional tolerance
• Machining of castings is reduced / eliminated due to
reduced/negligible draft.
• Saves material
• Very less sand required for moulding
• Unskilled labour can be employed

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Shell Mould casting
Limitations:
• Expensive (pattern cost/resin cost/equipment cost)
• Not economical for low volume production.
• Maximum casting size and weight are limited
• Relative inflexibility in gating and risering as these are
provided in the shell itself

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Plaster Mould Casting

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• The mould is prepared in gypsum / plaster of Paris
• Plaster of Paris is mixed with talc, asbestos, fibres,
silica flour, and a controlled amount water to form a
slurry.
• This plaster slurry is poured over the metallic pattern
confined in a flask.
• The mould is vibrated and slurry allowed to set

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• The pattern is removed after about 30mins, when the setting
is complete and the mould is dried and baked by slowly
heating it to about 200deg c in an oven.
• Cope and drag matched by guide pins.
• Molten metal is poured into the mould
• Then the casting is cooled and removed by destroying the
mould.

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Advantages:
• High degree of dimensional accuracy & surface finish
• Warping and distortion of thin sections can be
avoided(plaster has lower rate of heat conductivity)
Disadvantages:
• Low permeability
Applications: Ornaments, statues, jewelry etc.

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A. A metal die (2 halves) is made with
cavities for making expendable wax
patterns. Patterns are made by injecting
wax under pressure ranging 7 to 70
kg/cm2.
B. Gates and sprues are formed in the
same manner and attached to the
pattern assembly. The pattern may be
pre-coated with a refractory coating
material. Eg. Silica flour in ethyl silicate.
C. The wax pattern is covered with a metal
flask.
Investment casting (Lost wax process or Precision casting)

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D. Ceramic slurry (sand +water +calcium
phosphate+ Magnesium oxide) is
poured & allowed to harden.
E. The flask is heated. The wax melts & is
removed.
F. The mould is then inverted, molten
metal is poured inside the mould and
allowed to solidify.

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Before pouring the mould is pre-heated,
to vaporize any remaining wax, for easy
flow of molten metal and to compensate
for shrinkage of casting during
solidification.
G. The castings are removed by breaking
the mould and sent for finishing.

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Investment casting --- cont’d
Advantages:
• Good surface finish can be obtained (machining
eliminated)
• Extremely thin section can be cast successfully
• Defect free castings may be obtained
Disadvantages:
• Unsuitable for castings of more than about 5Kg
weight
• Expensive
• Precise control is required in all stages of casting

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Slush Casting

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Slush Casting
1. Molten metal is poured into a metal mould.
2. After desired thickness of solidified skin (thin walled
section or shell) is obtained, the mould is inverted.
3. The remaining liquid is poured out.
4. Mould halves are opened and the casting is
removed.
5. For making ornamental and decorative objects and
toys from low melting -point alloys like zinc, tin
&lead.
Merits:
• Good appearance
• Hollow castings can be obtained without the use of
cores

Pressed casting
• Also known as Corthias process.

Pressed casting
Process
• The hot liquid metal, after putting in the open ended mould, is
pressurized to enter the mould cavity.
• Due to this, the space around the mould cavity fills with molten metal.
• As soon as metal solidifies, the core is pulled out and casting removed.
• Generally used for ornamental castings only.

PRESSURE DIE CASTING
HOT CHAMBER
DIE CASTING
COLD CHAMBER
DIE CASTING

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HOT CHAMBER DIE CASTING

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Goose neck (or) Air injection type
• The cast iron gooseneck is so placed that it can be dipped beneath the
surface of the molten metal to receive the same when needed.
• The molten metal fills the cylindrical portion & the curve passageways of
the gooseneck.
• Gooseneck is then raised & connected to an air line which supplies
pressure to force the molten metal into the closed die.
• After the casting has solidified, the gooseneck is again dipped beneath the
molten metal to receive molten metal again for the next cycle.
• In the meantime, die halves open out, casting is ejected & die closes. Cycle
then repeats.

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COLD CHAMBER DIE CASTING

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COLD CHAMBER DIE CASTING--- cont’d
• Consists of a pressure chamber or cold chamber of cylindrical shape
fitted with a ram or piston usually operated by hydraulic pressure.
• A measured quantity of molten metal is brought in a ladle and poured
into the chamber after the die is closed and all the cores are locked in
position.
• The ram forces the metal into the die.
• Once the casting has solidified, the moveable half of the die slides
away and the die opens.
• Cores are withdrawn, ram moves back and the ejector forces out the
casting.

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DIE CASTING
Advantages:
• Die casting can produce parts with good strength
and complex shapes.
• Parts have good dimensional accuracy and surface
details.
• Production rate is very high due to metal mould
and high pressure involved in the process
• Because of the high pressures involved, wall
thickness of as small as 0.5mm can be prodcued.
• No risers are used in die casting process.

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DIE CASTING
Limitations:
• Since very high pressures are involved, dies
will have to be made stronger.
• Lack of permeability
• High initial cost – hence not economical for
low volume production.

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Difference between Hot chamber and cold chamber die-casting
Hot Chamber die-casting
• The melting unit forms an integral
part of the machine
• This is suitable for low melting
point alloys like zinc, lead etc.
• Molten metal is filled by gravity to
the goose-neck container operated
by a lifting mechanism
• Direct air pressure is used for
forcing the molten metal inside the
die
Cold Chamber die-casting
• The melting unit is not an integral
part of the machine
• This is suitable for high melting
point alloys like brass, aluminium,
magnesium etc.
• The molten metal is ladled in the
plunger cavity
• Molten metal is forced in to the die
cavity by a hydraulically operated
plunger

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Centrifugal Casting

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Centrifugal Casting -- cont’d
• The mold is rotated rapidly about its central axis as the metal is poured
into it.
• Because of the centrifugal force, a continuous pressure will be acting on
the metal as it solidifies.
• The slag, oxides and other inclusions being lighter get separated from the
metal and segregate towards the center.
• The mold can be rotated about a vertical, horizontal or an inclined axis .
• The length and outside diameter are fixed by the mold cavity dimensions
while the inside diameter is determined by the amount of molten metal
poured into the mold.

Centrifugal casting--- cont’d
Advantages:
•Formation of hollow interiors in cylinders without cores.
•Fine grained structure at the outer surface of the casting free of gas,
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.

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Continuous casting

Continuous casting

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Continuous casting
• The continuous casting process, also called strand casting.
• Molten steel is poured from a ladle into a temporary
container called a tundish, which dispenses the metal to one
more continuous casting mouId.
• The steel begins to solidify at the outer regions as it travels
down through the water-cooled mould.
• Water sprays accelerate the cooling process.

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• The steel begins to solidify at the outer regions
as it travels down through the water cooled
mold.
• While still hot and plastic, the metal is bent
from vertical to horizontal orientation.
• It is then cut into sections or fed continuously
into a rolling mill where it is formed into plate
or sheet or tubes.
Continuous casting

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Advantages:
It can produce longer continuous lengths
castings.
A single mould is sufficient to produce a large
number of pieces, as a result cost, energy and
scrap are reduced
Limitations:
• Difficulties in the design of mould due to high
temperature
Continuous casting

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