Casting Process and different types of casting defects

PREPARED BY – MR S P UDGAVE
SHARAD INSTITUTE OF TECHNOLOGY COLLEGE OF
ENGINEERING ICHALKARANJI
CASTING
PROCESSES

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12. CENTRIFUGAL CASTING
• Centrifugal casting is a process in which the molten metal is
poured and allowed to solidify in a revolving mould.
• The centrifugal force due to the revolving mould holds the
molten metal against the mould wall until it solidifies.
• The material used for preparing moulds may be cast iron,
steel, sand or graphite (for non-ferrous castings).
• The process is used for making castings of hollow cylindrical
shapes.
• The various centrifugal casting techniques include:
a) True centrifugal casting
b) Semi-centrifugal casting and
c) Centrifuge casting.

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12.a. True Centrifugal casting
 True centrifugal casting is used to produce parts that are symmetrical
about the axis like that of pipes, tubes, bushings, liners and rings.
 The outside shape of the casting can be round, octagonal,
hexagonal etc., but the inside shape is perfectly (theoretically) round
due to radially symmetric forces.
 This eliminates the need for cores for producing hollow castings.
 Figure shows the true centrifugal process.
Figure: True centrifugal process

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Steps involved in the process
1. The mould of the desired shape is prepared with metal and the walls are
coated with a refractory ceramic coating.
2. The mould is rotated about its axis at high speeds in the range of 300 - 3000
rpm. A measured quantity of molten metal is poured into the rotating mould.
3. The centrifugal force of the rotating mould throws the liquid metal towards
the mould wall and holds the molten metal until it solidifies.
4. The casting cools and solidifies from its outer surface towards the axis of
rotation of the mould thereby promoting directional solidification.
5. The thickness of the casting obtained can be controlled by the amount of
liquid metal being poured.
 An inherent quality of true centrifugal castings is based on the fact that, the
non-metallic impurities in castings being less dense than the metal, are
forced towards the inner surface (towards the axis) of the casting due to
the centrifugal forces. These impurities can be machined later by a suitable
machining process (say boring operation).
 The mould may be rotated horizontally or vertically.
 When the mould is rotated about horizontal axis, a true cylindrical inside
surface is produced; if rotated on a vertical axis, parabolic inside surface is
produced.
 Cores and gating/risering systems are not required for this process.

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12.b. Semi-centrifugal casting
 Semi-centrifugal casting process is used to produce solid castings and
hence, requires a core to produce hollow cavities.
 The process is used only for symmetrically shaped objects and the axis of
rotation of the mould is always vertical.
 Gear blanks, sheaves, wheels and pulley are the commonly produced parts
by this process.
 Figure shows the process to produce a wheel shaped casting.

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 The mould is prepared in the usual manner using cope and drag box.
 The mould cavity is prepared with its central axis being vertical and
concentric with the axis of rotation.
 The core is placed in position and the mould is rotated at suitable speeds,
usually less than true centrifugal casting process.
 The centrifugal force produced due to the rotation of the mould causes the
molten metal to fill the cavity to produce the desired shape.

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12.c. Centrifuging Process
 In true and semi centrifugal process, the axis of the mould/cavity
coincide with the axis of rotation.
 Where as in centrifuging process, the axis of the mould cavity does
not coincide with the axis of rotation.
 The mould is designed with part cavities located away from the axis
of rotation.
 Hence, this process is suitable for non-symmetrical castings.
 Figure shows the centrifuging process.

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1. Several mould cavities are arranged in
a circle and connected to a central
down sprue through gates.
2. The axis of the down sprue is common
to the axis of rotation of the mould.
3. As the mould is rotated, the liquid
metal is poured down the sprue which
feeds the metal into the mould cavity
under centrifugal force.
4. The rotational speed depends on a
number of factors such as, the
moulding medium (sand, metal or
ceramic), size of the casting, type of
metal being poured and the distance of
the cavity from the central axis (sprue
axis).
5. Centrifuging is done only about a
vertical axis.

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1. Jolt Machine
• A jolt machine consists of a flat table mounted on a piston-cylinder
arrangement and can be raised or lowered by means of compressed
air.
• In operation, the mould box with the pattern and sand is placed on
the table. The table is raised to a short distance and then dropped
down under the influence of gravity against a solid bed plate. The
action of raising and dropping (lowering) is called 'Jolting'.
• Jolting causes the sand particles to get packed tightly above and
around the pattern. The number of 'jolts' may vary depending on the
size and hardness of the mould required. Usually, less than 20 jolts
are sufficient for a good moulding.
• The disadvantage of this type is that, the density and hardness of
the rammed sand at the top of the mould box is less when compared
to its bottom portions.

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2. Squeeze Machine
 In squeeze machine, the mould box with pattern and sand in it is placed on
a fixed table as shown in figure
 A flat plate or a rubber diaphragm is brought in contact with the upper
surface of the loose sand and pressure is applied by a pneumatically
operated piston.
 The squeezing action of the plate causes the sand particles to get packed
tightly above and around the pattern.
 Squeezing is continued until the mould attains the desired density.
 In some machines, the squeeze plate may be stationary with the mould box
moving upward.
 The disadvantage of squeeze machine is that, the density and hardness of
the rammed sand at the bottom of the mould box is less when compared to
its top portions.

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3. Jolt Squeeze Machine
 Jolt squeeze machine combines the operating principles of 'jolt' and
'squeeze' machines resulting in uniform ramming of the sand in all portions
of the moulds
• The machine makes use of a match plate
pattern placed between the cope and the
drag box.
• The whole assembly is placed on the table
with the drag box on it.
• The table is actuated by two pistons in air
cylinders, one inside the other. One piston
called 'Jolt piston' raises and drops the table
repeatedly for a predetermined number of
times, while the other piston called 'squeeze
piston' pushes the table upward to squeeze
the sand in the flask against the squeeze
plate. In operation, sand is filled in the drag
box and jolted repeatedly by operating the jolt
piston.
• After jolting, the complete mould assembly is rolled over by hand.
• The cope is now filled with sand and by operating the squeeze piston, the mould
assembly is raised against the squeeze plate. By the end of this operation, the sand
in the mould box is uniformly packed.
• The match plate is now vibrated and removed. The mould is finished and made
ready for pouring.

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4. Sand slinger
 A sand slinger is an automatic machine equipped with a unit that throws
sand rapidly and with great force into the mould box. Figure shows a sand
slinger. Sand slinger consists of a rigid base, sand bin, bucket elevator, belt
conveyor, ramming head (sand impeller) and a swinging arm.
• In operation, the pre-mixed sand
mixture from the sand bin is
picked by the bucket elevator and
is dropped on to the belt conveyor.
• The conveyor carries the sand to
the ramming head, inside which
there is a rotating impeller having
cup shaped blades rotating at high
speeds (around 1800 rpm).

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 The force of the rotor blades imparts velocity to the sand particles and as a
result the sand is thrown with very high velocity into the mould box thereby
filling and ramming the sand at the same time.
 The density of the ramming sand can be controlled by varying the speed of
the impeller. Rest of the operations, viz., removal of pattern, cutting gates
etc., are done manually.
 In the initial stages of ramming, the blades are rotated at slow speeds;
around 1000 - 1200 rpm to avoid damage to the pattern due to the abrasive
action of the high velocity sand particles.

Operation of Cupola
The cupola is charged with wood at the bottom. On the top of the wood a
bed of coke is built. Alternating layers of metal and ferrous alloys, coke,
and limestone are fed into the furnace from the top. The purpose of
adding flux is to eliminate the impurities and to protect the metal from
oxidation. Air blast is opened for the complete combustion of coke.
When sufficient metal has been melted that slag hole is first opened to
remove the slag. Tap hole is then opened to collect the metal in the
ladle.

CUPOLA FURNACE
For many years, the cupola was the primary method of melting used in iron foundries. The
cupola furnace has several unique characteristics which are responsible for its widespread use as
a melting unit for cast iron.
Cupola furnace is employed for melting scrap metal or pig iron for production of various cast
irons. It is also used for production of nodular and malleable cast iron. It is available in good
varying sizes. The main considerations in selection of cupolas are melting capacity, diameter of
shell without lining or with lining, spark arrester.
Shape
A typical cupola melting furnace consists of a water-cooled vertical cylinder which is lined with
refractory material.
Construction
The construction of a conventional cupola consists of a vertical steel shell which is lined with a
refractory brick.
The charge is introduced into the furnace body by means of an opening approximately half
way up the vertical shaft.
The charge consists of alternate layers of the metal to be melted, coke fuel and limestone flux.
The fuel is burnt in air which is introduced through tuyeres positioned above the hearth. The
hot gases generated in the lower part of the shaft ascend and preheat the descending charge.

Various Zones of Cupola Furnace
Various numbers of chemical reactions take place in different zones of cupola. The
construction and different zones of cupola are :
1. Well
The space between the bottom of the tuyeres and the sand bed inside the cylindrical
shell of the cupola is called as well of the cupola. As the melting occurs, the molten
metal is get collected in this portion before tapping out.
2. Combustion zone
The combustion zone of Cupola is also called as oxidizing zone. It is located between
the upper of the tuyeres and a theoretical level above it. The total height of this zone is
normally from 15 cm. to 30 cm. The combustion actually takes place in this zone by
consuming the free oxygen completely from the air blast and generating tremendous
heat. The heat generated in this zone is sufficient enough to meet the requirements of
other zones of cupola. The heat is further evolved also due to oxidation of silicon and
manganese. A temperature of about 1540°C to 1870°C is achieved in this zone. Few
exothermic reactions takes place in this zone these are represented as:
C + O2 → CO2 + Heat
Si + O2 → SiO2 + Heat
2Mn + O2 → 2MnO + Heat

3. Reducing zone
Reducing zone of Cupola is also known as the protective zone which is located
between the upper level of the combustion zone and the upper level of the coke
bed. In this zone, CO2 is changed to CO through an endothermic reaction, as a
result of which the temperature falls from combustion zone temperature to
about 1200°C at the top of this zone. The important chemical reaction takes
place in this zone which is given as under.
CO2 + C (coke) → 2CO + Heat
Nitrogen does not participate in the chemical reaction occurring in his zone as
it is also the other main constituent of the upward moving hot gases. Because
of the reducing atmosphere in this zone, the charge is protected against
oxidation.
4. Melting zone
The lower layer of metal charge above the lower layer of coke bed is termed as
melting zone of Cupola. The metal charge starts melting in this zone and
trickles down through coke bed and gets collected in the well. Sufficient
carbon content picked by the molten metal in this zone is represented by the
chemical reaction given as under.
3Fe + 2CO → Fe3C + CO2

5. Preheating zone
Preheating zone starts from the upper end of the melting zone and continues
up to the bottom level of the charging door. This zone contains a number of
alternate layers of coke bed, flux and metal charge. The main objective of this
zone is to preheat the charges from room
temperature to about 1090°C before entering the metal charge to the melting
zone. The preheating takes place in this zone due to the upward movement of
hot gases. During the preheating process, the metal charge in solid form picks
up some sulphur content in this zone.
6. Stack
The empty portion of cupola above the preheating zone is called as stack. It
provides the passage to hot gases to go to atmosphere from the cupola
furnace.
Charging of Cupola Furnace
Before the blower is started, the furnace is uniformly pre-heated and the
metal and coke charges, lying in alternate layers, are sufficiently heated up.
The cover plates are positioned suitably and the blower is started.
The height of coke charge in the cupola in each layer varies generally from
10 to 15 cms. The requirement of flux to the metal charge depends upon the
quality of the charged metal and scarp, the composition of the coke and the
amount of ash content present in the coke.

Advantages
It is simple and economical to operate.
A cupola is capable of accepting a wide range of materials without reducing melt
quality. Dirty, oily scrap can be melted as well as a wide range of steel and iron. They
therefore play an important role in the metal recycling industry
Cupolas can refine the metal charge, removing impurities out of the slag.
From a life-cycle perspective, cupolas are more efficient and less harmful to the
environment than electric furnaces. This is because they derive energy directly from coke
rather than from electricity that first has to be generated.
The continuous rather than batch process suits the demands of a repetition foundry.
High melt rates
Ease of operation
Adequate temperature control
Chemical composition control
Efficiency of cupola varies from 30 to 50%.
Less floor space requirements comparing with those furnaces with same capacity.
Limitations
Since molten iron and coke are in contact with each other, certain elements like si, Mn
are lost and others like sulphur are picked up. This changes the final analysis of molten
metal.
Close temperature control is difficult to maintain

Arc Furnace Process:
•An electric arc furnace mainly consists of the spherical shaped furnace, a retractable roof with
electric rods and a refractory material dressed hearth for collecting the molten metal. The scrap
steel or the iron is placed in the furnace and the electric rod made of graphite is passed into it.
The scrap or metal is melted by the electric current passed through the rods and the radiant
energy produced by the arc. Once the metal is melted the doors on the sides are opened to
remove the alloy, slag and the oxygen that is formed.
After charging, the roof is swung back over the furnace and meltdown commences. The
electrodes are lowered onto the scrap, an arc is struck and the electrodes are then set to bore into
the layer of shred at the top of the furnace. Lower voltages are selected for this first part of the
operation to protect the roof and walls from excessive heat and damage from the arcs.
Advantages:
•Electric arc furnaces are are pollution free and have outstanding metallurgical control.
•The advantage of a electric metal casting furnace is that temperature can go up to 1800 Celsius.
•Large reduction in specific energy (energy per unit weight) required to produce the steel.
•Another benefit is flexibility: EAFs can be rapidly started and stopped.
Some of the disadvantages:
•Lots of sound levels can be produced only in place energy is available in a large scale and the
movement of scrap through the locality can cause health hazards.
•Electric arc steelmaking is only economical where there is plentiful electricity, with a well-
developed electrical grid.

CASTINGS
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 Fettling is the name given to cover all those operations which help giving the
casting a good appearance after the same has been shaken out of the sand
mold.
 Fettling includes
1. Removal of cores from the casting.
2. Removal of adhering sand and oxide scale from the casting surface (surface
cleaning).
3. Removal of gates, risers, runners etc. from the casting.
4. Removal of fins, and other unwanted projections from the castings.
1. Removal of Cores
 It may be difficult to remove dry sand and hardened cores in the absence of
suitable equipment.
 Hammering or vibrations imparted to cores does loosen and break them up.
 Sand portions sticking inside the castings are removed by poking action using
a metal rod.
 Cores from larger castings may be removed effectively by pneumatic rapping
and hydro blasting.
.

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2. Cleaning of Casting Surfaces
• The outside and inside surfaces of castings are cleaned of adhering refractory
(sand) particles and oxide scale and they (i.e., surfaces) look smooth and pleasing.
• The extent of surface cleaning required depends upon the metal/ alloy of the
casting and size of the casting.
• Steel castings (because of their high melting and pouring temperatures and
consequent burning of the sand in contact with the molten metal) require
considerable more cleaning than those of iron and brass.
• Aluminium castings are virtually free from burned-on sand.
• Since heavy castings suffer more than light castings from the burning-on of sand,
their cleaning is more difficult.
• Sand may be removed from the surfaces of castings using hand methods or
mechanical equipment
3. Removal of gates and risers
• Numerous methods are available for removing feeding and gating systems.
• The choice of a particular method depends upon the type of metal/alloy,
— size of the casting,
— size of runners, gates and risers.
• A few commonly used methods are given below:
1. Chipping hammers
2. Flogging (knocking off).
3. Shearing.
4. Sawing
5. Abrasive wheel slitting
6. Machining.

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4. REMOVAL OF FINS AND OTHER UNWANTED PROJECTIONS
FROM CASTINGS
• Castings are trimmed to remove fins, chaplets, wires, parting line
and the stumps of feeder heads and ingates. All these unwanted
projections are dressed flush with the surface.
• The methods employed to remove unwanted projections from the
castings are
– Chipping
– Sawing
– Flame cutting
– Flame gouging and flame scarfing.
– Grinding
– Abrasive belt machining
– Rotary tools cutting
– Trimming and sizing.

CASTING DEFECTS
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 Casting process involves a number of variables and a loss of control in
any of these variables can cause defects under certain circumstances.
 Some of the common casting defects, their features and remedies to
prevent such defects are discussed below.
1. Shrinkage defect
 Shrinkage is a void on the surface of the castings resulting from
concentrated contraction or shrinkage of metal during solidification.Refer
figure.
 Although a riser is used to over come the shrinkage effect, in some cases
it fails to feed the molten metal efficiently to the casting as it solidifies.
Remedies
• Use large sprue and riser to promote directional
solidification.
• Locate risers and gating systems in correct positions.
• Gates to be cut as wide as possible.

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2. Porosity defect (Blow hole and Pin hole)
• Molten metal absorb gases from various sources such as fluxes, moisture in sand,
binders, additives and normal atmospheric gases like oxygen and nitrogen.
• If these gases are not allowed to escape, they get entrapped in the mould cavity
forming small balloon shaped voids or cavities leading to porosity defect in
castings.
• Two types of gas related defects occur in castings. They are:
– blow hole
– pin hole defect.
• Blow holes occur below the surface of the castings and are not visible from the
outside surface.
• Pin holes are small gas cavities, many in number at or slightly below the surface of
the casting.
Remedies
• Avoid excess ramming of mould.
• Provide proper vent holes.
• Avoid use of excess carbonaceous or
other organic material in the sand/core
binders, because these materials react
with the molten metal producing large
amount of gases.

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3. Misrun
 Misrun occur when the mould cavity is not completely filled with molten
metal.
 it is a defect wherein a casting solidifies before the molten metal
completely fills the cavity.
Remedies
• Fluidity of metal should be high.
• Pouring rate and time should be
controlled.
• Thin sections should be suitable
designed.
4. Penetration
When fluidity of liquid metal is high, it may
penetrate into the sand mould/core (into the
voids between the sand particles) causing a
fused aggregate of metal and sand on the
surface of the casting leading to defect.
Remedies
• Sand should be properly rammed. . . .
• Moulding sand/core sand should not be too coarse to promote metal
penetration.
• Control proper metal temperature.

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5. Mould shift
 It is a step in the cast product at the parting line caused by sidewise
relative displacement of cope and drag box.
Remedies
 Proper alignment of cope and drag box.
 Proper handling of assembled cope and drag box during operations.
6. Cold shut
 Two portions of metal flow together, but lack of fusion due to premature
freezing results in a defect known as cold shut.
Remedies
 Place gates and risers at proper locations.
 Metal fluidity should be high.
Mould shift Cold shut

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7. Hot tears
 A hot tear is an internal or external ragged discontinuity formed in the
casting due to the pulling action of the metal just after it has solidified.
Remedies
 Provide adequate fillets at sharp corners.
 Proper metallurgical and pouring temperature.
 Place gates and risers at proper locations.

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