Unit 4 : Metal Casting Processes Dr. Abdul Khader A A P a g e | 41
UNIT IV : METAL CASTING PROCESSES
1. State uses of metal casting (Nov-Dec-2017)
Uses of Casting
Casting process can be used for
• Automobile engine blocks,
• Cylinder blocks,
• Pistons, piston rings,
• Machine tool beds and frames,
• Mill rolls, wheels and housings of steam and hydraulic turbines, turbine vanes and aircraft jet
engine blades,
• Water supply and sewer pipes,
• Sanitary fittings etc
2. Explain metal casting process (April-May 2016)
Process of casting
1. Take the pattern (the material of the pattern may be wood, metal or plastic).
2. Prepare the moulding sand.
3. With the help of pattern, prepare the mould and necessary cores.
4. Melt -the metal or alloy to be cast.
5. Pour the molten metal/ alloy into mould cavity.
6. Allow the molten metal to cool and solidify.
7. Remove the casting from the mould. This operation is called 'shake out'.
8. Clean and finish casting. The operation is known as 'fettling'.
9. Test and inspect the casting.
10. Remove the defects if any and if possible (salvaging the casting).
11. Stress relieve the casting by heat treatment.
12. Again inspect the casting.
13. The casting is ready for use.
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3. Define pattern and list the materials used for pattern (April-May 2019, Nov-Dec-2018)
4. List different pattern making materials (Nov-Dec 2016)
Pattern
• In casting, a pattern is a replica of the object to be cast, used to prepare the cavity into which
molten material will be poured during the casting process.
Pattern making materials
Wood : White pine, Mahogany, Maple, Birch and Cherry, Teak, Shisham, Kail and Deodar.
Metal : Cast iron, Brass, Alluminium, White metal.
Plastic : Plastics of epoxy resins, acrylates, phenol formaldehyde and polyester resins.
Quick setting compounds : Gypsum, resin-impregnated materials, waxes
5. Explain briefly cope, drag and core used in casting process (Nov-Dec-2018, Nov-Dec
2016)
Cope and drag
In foundry work, the term cope and drag refer
respectively to the upper and lower parts of a two-part
casting flask used in sand casting as shown in fig.
In the production of large castings, the complete moulds
are too heavy to be handled by a single operator.
Therefore, cope and drag patterns are used to ease this
problem to efficient operation.
The patterns are made in halves, split on a convenient
joint line and separate cope and drag patterns are built
and mounted on individual plates or boards.
This arrangement permits one operator or group of
operators to prepare the cope half of the mould while another operator or group worked on the
drag half. This increases the production capacity
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Core
A core is a device used in casting and
moulding processes to produce internal
cavities as shown in fig.
The core is normally a disposable item that is
destroyed to get it out of the piece.
6. List any 10 patterns used in casting (April – May 2018, April May 2017, April-May
2016)
Types of pattern
1. Single piece or solid pattern.
2. Split pattern.
3. Match plate pattern.
4. Cope and drag pattern.
5. Loose piece pattern.
6. Gated pattern.
7. Skeleton pattern.
8. Sweep pattern.
9. Shell pattern.
10. Segmental pattern.
11. Boxed-up pattern.
12. Built up pattern.
13. Lagged-up pattern.
14. Left and right-hand pattern
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6. List pattern making allowances (Nov-Dec-2017, Nov-Dec 2015)
7. Explain casting allowance (April – May 2018, Nov-Dec 2017, April-May 2016)
Pattern allowances
Shrinkage allowance.
1. Machining allowance.
2. Pattern draft or taper allowance.
3. Corners and fillets allowance.
4. Rapping or shake allowance.
5. Distortion allowance.
Shrinkage allowance
• As the metal shrinks on solidification and contracts on cooling to room temperature.
• To compensate this, linear dismissions of patterns are increased in respect of those of the
finished casting to be obtained, which is known as shrinkage allowance.
• It is given as mm/ m
Steel = 20 mm/m
C.I. / Malleable Iron =10 mm/m
Brass, Cu, Al = 15 mm/m
Zinc, Lead = 25 mm/m
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Machining allowance
Usually, rough surfaces of castings have to be machined to improve surface finish. So size of the
casting must be slightly more than the finished part represented in drawings. This extra amount of
metal provided on the surfaces to be machined is called machining
1. Kind of metal to be used.
2. Size and shape of the casting and
3. Methods of moulding.
Typical machining allowances
Draft allowance
A certain taper is given on the pattern surfaces that
are parallel to the direction in which the pattern is
withdrawn from the mould.
This is to avoid damage to the mould during
withdrawal of the pattern
The draft allowance depends on
1. Length of vertical side.
2. Intricacy of the pattern
3. The method of moulding.
Generally, draft is about 10 to 20 mm/ m on exterior surfaces and 40 to 60 mm/ m on interior
surfaces.
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Corner of fillet allowance
• The intersection of surfaces in casting should be smooth and form no sharp angles. For this,
the external and internal corners of patterns are suitably rounded.
• They are known as rounded corners or fillets.
• Fillets facilitates the removal of the pattern from the mould, prevents the formation of
cracks and shrink holes in the casting.
Rapping allowance
• To take pattern out of the mould cavity it is slightly rapped to detach it from the mould
cavity.
• Due to this, the cavity in the mould increases slightly. So, the pattern is made slightly
smaller.
Distortion allowance
• Some
castings, because of their size, shape and type of metal, tend to warp or distort during the
cooling period.
• This is a result of uneven shrinkage and is due to uneven metal thickness causing it to cool
more rapidly.
• The shape of the pattern is thus bent in the opposite direction to overcome this distortion
as shown in fig.
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8. Explain with a sketch, a split pattern (April-May 2019, Nov-Dec-2017, Nov-Dec 2015,
April-May 2017
Single piece pattern
• This pattern is made without joints, partings or any loose pieces and it is not attached to a
frame or plate, as shown in fig.
• This pattern is exactly like a desired casting. For making mould, the pattern is
accommodated either in cope or drag.
• This moulding process is quite inconvenient and time consuming. Therefore these are used
for large castings, for example, stuffing box of steam engine
Split pattern
These patterns are split along the parting
plane to facilitate the withdrawal of the
pattern out of the mould before the pouring
operation.
• Fig. shows the split pattern for casting a
bush.
• The two parts of the pattern are joined
together with the help of dowel pins.
• For a more complex casting, the pattern
may be split in more than two parts as shown
in lower fig.
• These are used for casting of spindles,
cylinders, small pulleys, steam valve bodies
etc.
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9. Explain sweep pattern with a sketch (Nov-Dec-2018)
Swept pattern
A sweep is a section or wooden board of proper contour that is
rotated about one edge to shape mould cavities having shapes of
rotational symmetry as shown in fig.
Sweep patterns are used when a large sized castings are to be
produced in a short time.
A complete pattern is not necessary and becomes very expensive
for very large castings.
The moulds are made manually, either in a pit or on the foundry
floor. Therefore, these are referred as pit moulding or floor
moulding. In this case loam sand is used and a brick or wooden frame work supports the loam
sand.
• Once the mould is ready, the sweep pattern and the post about which it rotates, are removed
before pouring the molten metal into the mould cavity.
• Large kettles of C.I. are made by sweep patterns:
10. Explain loose piece pattern with a sketch (April – May 2018)
Loose piece pattern
• Some patterns are produced as assemblies of loose component pieces. When a one piece
solid pattern has projections which lie above or below the parting plane, it is impossible to
remove the pattern from the mould. With such patterns, the projections are made with the
help of loose pieces. A loose piece is attached to the main body of the pattern by a pin or
dovetail slide
• While moulding, sand is rammed securely around the loose piece. Then pins are removed.
The sand is then packed and rammed around the total pattern. When the main pattern is
removed, the loose pieces remain in the mould. These are then carefully rapped and drawn
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11. State properties of moulding sand (Nov-Dec-2017, April May 2017, April-May 2016,
Nov-Dec 2015)
Moulding
• Moulding is the process of creating mould cavities of different shapes by using metal or sand.
• The shape of mould cavity corresponds to the shape of the casting required except in
dimension.
• In general, a mould is referred as the exact replica of the casting.
• A hot molten metal is poured into the mould cavity and allowing it to solidify for getting the casting.
• In most of the foundries, sand is used as moulding material. Sand moulds are prepared by
using materials such as base sand, binder, water and other ingredients.
• Moulding process may be carried out on the floor or bench known as floor moulding or bench
moulding. The moulding process may be conducted with hand tools by the moulder known
as hand moulding process or with the help of machine known as machine moulding process
Moulding sand
• Sand is the principal material used in the foundry shop for moulding process. Sand is obtained
from river bed, sea, lake and deserts.
• The sand should possess the properties which are vital for foundry purposes
Properties of moulding sand
1. The sand should have adequate strength in its green, dry and hot states.
2. He sand should have high permeability.
3. The sand should have high thermal stability.
4. It should have good refractoriness.
5. It should have good flowability.
6. It should have uniform sand texture.
7. It should be cheap and reusable.
8. It should have good thermal conductivity
9. It should have low collapsibility
10. It should be easy to prepare and control
11. It should have good adhesiveness
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12. State ingredients of foundry sand (April-May 2019, Nov-Dec 2016)
Principal ingredients of moulding sand
• Silica sand grains - 80 to 90%.
• Clay - 5 to 20%
• Moisture - 2 to 8 %
• Miscellaneous materials - below 2%
i) Oxides of iron
ii) Limestone
iii) Magnesia soda and
iv) Potash
13. List types of foundry sand (Nov-Dec 2016, Nov-Dec 2015)
Types of moulding sandGreen sand
1. Dry sand
2. Loam sand
3. Facing sand
4. Backing sand
5. System sand
6. Parting sand
7. Core sand
8. C02 sand
9. Shell sand or synthetic sand
10. Oil and molasses sand
11. Mould washes.
14. With a neat sketch, explain centrifugal casting (April-May 2019, Nov-Dec-2017)
Centrifugal casting
• In centrifugal casting, the molten metal is poured into moulds while they are rotating.
• The molten metal falling into the centre of the mould is thrown out by the centrifugal force
under high pressure towards the periphery and the impurities which are light in weight are
pushed towards centre.
• The solidification progresses from the outer surface to the inwards. The grains are refined
and castings are completely free from any porosity defect by the forced movement of the
molten metal, thus making the dense and sound castings.
• The use of gates, feeders and cores are eliminated, hence this method becomes less
expensive and complicated
• In this method, hollow cylindrical bodies like cast iron water supply pipes, sewerage pipes,
steel gun barrels and other symmetrical objects such as pulleys, gears, disk wheels can be
produced easily
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True Centrifugal casting
Centrifuging
In this process several identical or nearly
similar moulds are located radially about
a vertically arranged central riser or sprue
which feeds the metal into the cavities
through a number of radially arranged
gates.
The entire mould is rotated with the
central sprue which acts as the axis of
rotation as shown in fig.
Therefore, it is not a purely centrifugal
process. As the molten metal is poured
through central sprue, the metal enters
into the different gates and fills the
radially arranged cavities by means of
centrifugal force due to the high speed of
rotation.
This method is suitable for small, intricate
parts where feeding problems are
encountered or it can be used for stack moulding.
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15. Explain with sketches slush casting (Nov-Dec-2017, Nov-Dec 2015)
Slush casting
• The slush casting is a variation of permanent mould casting that is used to produce hollow
parts. The process is based on the solidification of molten metal by the chilling effect.
• It is used for casting low melting temperature metals and alloys such as gold, silver,
aluminium, zinc, lead and their alloys etc. It is Also used for making hollow castings without
the use of cores.
• In this process, the molten metal is poured into a metallic mould.
• The metal is retained in the mould long enough for the outer skin to solidify. Finally, the mould
is turned over to remove metal still in molten condition.
• Due to the chilling effect, only a thin layer of the metal sticks to the mould surface which is
taken out by opening the halves of the mould and which is the required product as shown in
fig.
This method is only adopted for ornaments and toys of non-ferrous alloys.
Applications
1. Used for decorative and ornamental objects.
2. Used for bowls, candle sticks, lamps and statues.
3. Used for production of jewelleries, animal miniatures, handles for hollow wares etc.
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16. Explain briefly die casting with its applications (Nov-Dec-2017, April-May 2017)
Die casting
• Die casting is the process of rapidly producing accurately dimensioned parts by forcing the
molten metal under pressure into a split metal dies.
• The molten metal fills the entire die within a fraction of second and solidifies quickly due to
the low temperature of the dies as they are water cooled.
• The casting is ejected by separating the die halves. If the parts are small, several parts may be
cast at one time known as multiple cavity die.
Advantages of die casting
1. More economical for mass production.
2. Close dimensional tolerances can be achieved.
3. Very high rate of production.
4. Unit cost is minimum.
5. Good surface finish can be achieved.
6. Very fine details can produced
7. Very thin sections of the order of 0.5mm ran be cast.
8. Longer die-life is obtained.
9. Less floor space is required.
10. High strength and excellent mechanical properties can be achieved.
Disadvantages of die casting
1. It is uneconomical for nonferrous alloys.
2. Not economical for small runs.
3. The heavy castings cannot be cast and maximum size of the casting is limited.
4. High cost of die and die casting equipment.
5. Complicated die design.
6. Usually the die castings contain some porosity due to the entrapped air.
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17. Explain with a sketch, die casting
Hot chamber die casting
• It is a submerged plunger type machine, in which plunger operates in one end of a gooseneck
casting which is submerged in the molten metal as shown in fig
• When the plunger moves in the upper position, the molten metal flows by gravity into this
casting through holes just below the plunger.
• When plunger moves down, the holes are closed and the entrapped liquid metal is forced into
the die through the gooseneck channel and in gate. As the plunger retracts, the channel is
again filled with the right amount of molten metal. The plunger made of refractory material
may be operated by manually or mechanically and hydraulically
• Heating is continued throughout the operation to keep the molten metal in liquid state.
18. Explain briefly “Runner” and “Raiser” used in casting process (Nov-Dec 2016)
Runner and riser
• Runner : In large castings, molten metal is usually carried from the sprue base to several gates
around the cavity through a passageway called the runner.
• Runner is generally preferred in the drag as shown in fig. Sometimes it may be located in the
cope depending upon the shape of the casting.
• Riser : It is also known as feeder. Riser is a reservoir of molten metal built into a metal casting
mould to prevent cavities due to shrinkage, by supplying this material to sections of the mould
to compensate for any shrinkage during cooling
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19. Differentiate between sand casting and die casting (April-May 2016, Nov-Dec 2017)
20. List defects in casting (Nov-Dec 2015)
Casting defects
1. Shift
2. Warpage
3. Fin
4. Swell
5. Blowholes
6. Drop
7. Dirt
8. Honeycombing or sponginess
9. Metal penetration and rough surface
10. Sand holes
11. Pin holes
12. Scabs
13. Shrinkage cavity
14. Hot tears (pulls)
15. Cold shut and missions
16. Poured short
17. Internal air pocket.
Sand casting Die casting
1 From pattern mould is prepared
in sand
Pattern itself is mould
2 Pattern is made of wood, metal
or plastic
Made up of hardened steel
2 Suitable for one time
production
Suitable for mass production
3 Not accurate dimensional
tolerance
Close dimensional tolerance
4 Not good surface finish Good surface finish
5 Very thin sections can not be
cast
Thin sections can be cast
6 Mould is prepared for each
piece
Long die life
7 Molten metal will not come in
contact with pattern
Comes in direct contact
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S No
Defect Cause Remedies
1. Shift Mismatching of top and bottom
parts of the casting
Maintaining proper alignment of
pattern or die
2. Warpage
It is undesirable deformation in casting due to different rates of solidification.
Proper casting design.
3. Fin
It is a thin projection of metal at
the parting line due to improper
assembly or clamping.
Correct assembling and clamping.
4.
Swell It is an enlargement of mould
cavity by metal pressure, results
in overall enlargement of casting.
The sand should be rammed
properly and evenly.
5. Blowholes
These are small holes below the surface of casting caused by excessive moisture in the sand
Adjusting the moisture content
6.
Drop
Surface of mould cracks and pieces fall into the molten metal leading to low strength
Providing sufficient strength and proper ramming of sand.
7. Honeycombing
or sponginess
These are small cavities very close
to each other caused by dirt in
molten metal
Removing the slag particles in
molten metal
8 Internal air pocket
Pouring boiling metal or rapid
pouring of metal, faulty and poor
quality metal, excess moisture in
sand.
Correct pouring and using right quality metal and dry sand.
21. Explain the following casting defects (i) Blow hole (ii) Shrinkage cavity (iii) Misrun (iv)
Cold shut (v) Mismatch) (Nov-Dec-2018)
Blow hole
When gases entrapped on the surface of the casting due to solidifying metal, a rounded or oval
cavity is formed called as blowholes. These defects are always present in the cope part of the
mold.
Causes
(i) Excessive moisture in the sand.
(ii) Low Permeability of the sand.
(iii) Sand grains are too fine.
(iv) Too hard rammed sand.
(v) Insufficient venting is provided
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Shrinkage Cavity
The formation of cavity in the casting due to volumetric contraction is called as shrinkage
cavity.
Causes
(i) Uneven or uncontrolled solidification of molten metal.
(ii) Pouring temperature is too high.
Cold Shut
It is a type of surface defects and a line on the surface can be seen. When the molten metal
enters into the mold from two gates and when these two streams of molten metal meet at a
junction with low temperatures than they do not fuse with each other and solidifies creating a
cold shut (appear as line on the casting). It looks like a crack with round edge.
Causes
(i) Poor gating system
(ii) Low melting temperature
(iii) Lack of fluidity
Misrun
When the molten metal solidifies before completely filling the mold cavity and leaves a space
in the mold called as misrun.
Causes
(i) Low fluidity of the molten metal.
(ii) Low temperature of the molten metal which decreases its fluidity.
(iii) Too thin section and improper gating system
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Shift or Mismatch The defect caused due to misalignment of upper and lower part of the casting and misplacement of the core at parting line.
Cause:
(i) Improper alignment of upper and lower part during mould preparation.
(ii) Misalignment of flask (a flask is type of tool which is used to contain a mould in metal
casting. it may be square, round, rectangular or of any convenient shape.)
Classification of moulding sand
• Natural moulding sands
• Artificial or synthetic or high silica sands.
• Special sands.
Natural moulding sand
• Natural moulding sands are also called as green sands, and are taken from river beds or dug
from pits.
• They contain appreciable amount of clay about 5 to 20% which acts as a bond between the
sand grains and they are used as received with water added.
• The quantity and type of clay mineral present will affect the strength, toughness and
refractoriness of the sand.
Synthetic moulding sand
• These are basically high silica sands containing about 95 to 98% of silica and less than 2% of
clay or no clay (binder) in its natural form.
• They are made in foundry by crushing quartzite sandstones and then washing and grinding to
get desired grain size.
• The desired strength and bonding properties can be obtained by adding the binders such as
bentonite, water and other materials as required.
• The synthetic sands are more expensive than natural sand.
Special moulding sand
• Special sands are zircon, chromite, olivine, chamotte and chrome-magnesite.
• They possess the special characteristics which are not ordinarily obtained in other sands.
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• Zircon sands are used for cores of brass and bronze castings, chamotte is used for heavy steel
castings.
• Chrome-magnesite sands are particularly useful for steel castings and olivine sand for non-
ferrous castings of intricate shape.
Advantages of casting
1. Parts (both small and large) of intricate shapes can be produced.
2. A part can be made almost of the finished shape before any machining is done.
3. Almost all the metals and alloys and some plastics can be casted.
4. Good mechanical and service properties.
5. Mechanical and automated casting processes decrease the cost of casting.
6. Casting provides freedom in the design process.
7. Excellent vibration damping capacity.
8. Casting provides uniform directional properties.
Disadvantages of casting
1. High initial cost.
2. Casting of thin sections becomes difficult.
3. Great care is to be taken while handling molten metals and chemicals.
4. Casting is not economical for small number of production.
5. Casting is the tedious process.
6. Great care should be taken to control the cooling rate to get defect free castings.
7. Castings may have internal defects such as shrinkage, blowholes, etc