Manufacturing Process - Casting

ME 1107
Manufacturing Process
Casting
Akib Jabed Shovon
Department of Mechanical Engineering
Khulna University of Engineering & Technology
Khulna 9203
Roll: 1705006

Casting
A K I B J A B E D S H O V O N `
Casting:
It is a process of pouring molten metal into a mould in a cavity of the shape to be made & allowing it to
solidify. As this is done in foundry shop, so it is also called founding.
Basic requirements of Casting:
1. A pattern.
2. Melting process.
3. Pouring process.
4. Solidification.
5. Mould removal.
6. Clearing & finishing.
Advantages of Casting:
1. Molten material can flow into very small section so that intricate shapes can be made.
2. Any material can be cast (Ferrous and Non-Ferrous).
3. Necessary tools are simple and inexpensive.
4. Size and weight is not a limitation.
5. Better dimensional accuracy and surface finish.
Disadvantages of Casting:
1. Labor intensive process.
2. Defects are unavoidable.
3. Dimensional accuracy and surface finish is not good in case of sand casting.
Components of a Mould:
Figure: Mould Components

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• Ladle: Carries molten metal
• Flask: Supports & contains mold.
• Cope: Top part of the mould.
• Drag: Bottom part of the mould.
• Parting line: Partition between cope & drag.
• Molten metal is poured into pouring basin until there is metal in riser.
• Molten metal flows down through sprue & fills mold cavity.
• Gate & running system controls metal flow into mold cavity.
• Chaplets are used to support a core.
• Cores are inserts made from sand. They form hollow regions of final casting.
• Vents carry off gases produced when molten metal contacts sand.
• Much gas permeates out through sand.
Pattern:
A pattern may be defined as a model of casting, constructed in such a way that it can be used for forming
an impression (mould) in moulding sand.
Classifications of Patterns:
According to use patterns can be classified as-
1. Removable Pattern
2. Disposable Pattern
1. Removable Pattern:
Advantages of Removable Patterns:
1. Reusable.
2. Can be used in machine moulding.
3. Cavity produced can be inspected.
4. Easy to handle.
Disadvantages of Removable Patterns:
1. Time required more.
2. Finishing is not good.
3. More metal is needed.
4. Complex pattern with loose piece is difficult to handle.
2. Disposable Pattern:
Advantages of Disposable Patterns:
1. Time required less.
2. Less metal needed.
3. Mould making is simple.
4. Finishing is good.

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Disadvantages of Disposable Patterns:
1. Not reusable.
2. Cavity produced can’t be inspected.
3. Machine moulding is not possible.
4. Patterns are difficult to handle.
According to physical structure patterns can be classified as-
1. Single Piece Pattern
2. Split Pattern
3. Gated Pattern
4. Loose Piece Pattern
5. Follow Board Pattern
6. Match Plate Pattern
7. Sweep Pattern
8. Skeleton Pattern
9. Shell Pattern
1. Single Piece Pattern:
This type of pattern is made without joints, partings or any loose pieces. Such a pattern is also called as a
loose piece, since it is not attached to a plate. The single piece pattern is generally used for large casting
of simple shape & for limited production.
Figure: Single Piece Pattern
2. Split Pattern:
Many patterns can’t be made in single piece because of the difficulties encountered in removing them
from the mould. To eliminate this difficulty, some patterns are made in two parts, so that half of the
pattern will rest in the lower part of the mould & half in the upper part. Two parts of the pattern are
aligned with dowel pins. Sometimes, pattern is constructed in three or more parts for complicated casting.
Such a pattern is called multi-piece pattern.
Figure: Split Pattern

Casting
3. Gated Pattern:
In production work where several castings are required gated pattern are used. Such patterns are made
of metal to give them strength & to eliminate any warping tendency. To save time, a number of castings
are produced in a single multi-cavity mould by joining a group of patterns. The gates or runners for the
molten metal are incorporated in the integrated pattern. By this arrangement the time ordinary spent by
the moulder in cutting gates & drawing patterns is eliminated. These groups of patterns with gate formers
attached to them are called gated pattern.
Figure: Gated Pattern
4. Loose Piece Pattern:
It is a pattern with loose pieces which are necessary to facilitate withdrawal of the pattern from the mould.
It is used to produce undercuts. Loose pieces are removed separately through the cavity formed on main
pattern, after the pattern pieces are removed.
Figure: Loose Piece Pattern
5. Follow Board Pattern:
The patterns having thin sections, tend to get distorted or collapse during ramming. Sagging of this pattern
due to ramming can be easily overcome by constructing a supporting block (follow board) which may fit
inside the pattern to serve as a support.
Figure: Follow Board Pattern

Casting
6. Match Plate Pattern:
Match plate pattern is made by fastening each half of a split pattern to the opposite sides of one plate.
This plate provides a substantial mounting for patterns & is widely used in machine moulding. The grates
& runners are also attached in their correct positions.
Figure: Match Plate Pattern
7. Sweep Pattern:
Many patterns of symmetrical & regular shape, usually of large size may be constructed by the use of
sweep pattern which sweeps the desired shape into the sand mould thus eliminating the need for costly
three dimensional patterns. The sweep pattern is arranged to rotate about a central axis on a needle.
Figure: Sweep Pattern
8. Skeleton Pattern:
For vary large castings solid patterns would require a tremendous amount of timber, which may not be
economical particularly if the casting required are less. In such cases the pattern is made of wooden frame
& rib construction (skeleton) so that it will form a partially exterior or interior outline of the casting &
provide general contour & size of the desired casting.
Figure: Skeleton Pattern

Casting
9. Shell Pattern:
Shell pattern is usually mounted on a plate & parted along the centre line, the two halves being accurately
doweled together. Usually, these short bends are moulded & cast in pairs. This type of pattern is largely
used for drainage fittings & pipe work.
Figure: Shell Pattern
Pattern Allowance:
The patterns are not made the exact size of the desired casting as such a pattern would produce undersize
casting. To compensate for any dimensional and structural changes which might happen during the
casting process, allowances are usually made in the pattern. There are 5 types of pattern allowances and
they are described below.
1. Shrinkage Allowance
2. Draft Allowance
3. Finishing Allowance
4. Distortion Allowance
5. Shaking Allowance
1. Shrinkage Allowance:
Generally, metals shrink in size during solidification and cooling in the mould. So casting becomes smaller
than the pattern and the mould cavity. Therefore, to compensate for this, mould and the pattern should
be made larger than the casting by the amount of shrinkage. The amount of compensation for shrinkage
is called the shrinkage allowance When metal cools, it naturally shrinks in size. The total contraction of a
casting comprises of three elements- the contraction of liquid from pouring temperature to freezing
temperature, the contraction on account of change from liquid to solid & the lasting contraction of solid
casting from freezing temperature to surroundings.
Figure: Shrinkage Allowance

Casting
2. Draft or Taper Allowance:
When a pattern is drawn from a mould, there is always a possibility of damaging the edges of the mould.
Draft is taper made on the vertical faces of a pattern to make easier drawing of pattern out of the mould.
This tapering of the sides of the pattern, known as draft, is done to provide a slight clearance for the
pattern as it is lifted up. The draft is expressed in millimeters per meter on a side or in degrees. The amount
of draft needed depends upon the shape of casting, depth of casting, moulding method, intricacy of
*pattern, moulding material etc.
Figure: Draft Allowance
3. Finishing or Machining Allowance:
In case the casting designed to be machined, they are cast over-sized in those dimensions shown in the
finished working drawings. Additional metal is provided so that there will be some metal to machine. After
casting the additional metal is removed by machining. For average sized casting, allowance is 3 mm for
Ferrous and 1.5 mm for Non-Ferrous metals. Finishing allowance is given due to the following reasons:
 Castings get oxidized inside mould and during heat treatment.
 Scale thus formed requires to be removed.
 For removing surface roughness, slag, dirt and other imperfections from the casting.
 For obtaining exact dimensions on the casting.
 To achieve desired surface finish on the casting.
4. Distortion or Camber Allowance:
This allowance applies only to those castings of irregular
shapes such as U-shape or those having large flat areas,
which are distorted in the process of cooling as a result of
metal shrinkage. This is due to the uneven shrinkage of
different parts of the casting. Expecting the amount of
warpage, a pattern may be made with allowance of
warpage. It is called camber. For example, a U-shaped
casting will be distorted during cooling with the legs
diverging, instead of parallel.
Figure: Distortion Allowance

Casting
5. Rapping or Shaking Allowance:
When the pattern is shaken for easy withdrawal, the mould cavity, hence the casting is slightly increased
in size. In order to compensate for this increase, the pattern should be initially made slightly smaller. For
small and medium sized castings, this allowance can be ignored. But for large sized and precision castings,
however, shaking allowance is to be considered. It’s a negative allowance.
Figure: Shaking Allowance
Moulding Sand:
Moulding sand is a moulding material that is used to maintain the shape of the mold cavity until the
molten metal solidifies. It is the principle raw material used in moulding sand because it provides several
major characteristics that may not be obtained from other materials. Moulding sand is defined as granular
particles resulting from the breakdown of rocks, due to the action of natural forces, such as frost, wind,
rain, heat & water currents.
Principal constituents of Moulding Sand:
 Silica (SiO2) 86-90%
 Alumina (Al2O3) 4-8%
 Iron oxide (Fe2O3) 2-5%
 Smaller amount of Ti, Mn, Ca some alkaline component.
Classifications of Moulding Sands:
According to the source moulding sand may be classified as-
 Natural Sand:
Obtained from natural resources like lake, river. It contains water as the only binder. It has
advantage of maintaining moisture content for a long period.
 Synthetic Sand:
Artificial sand is obtained by mixing clay free sand, binder & other materials as required. It is a
better moulding sand as its properties can be controlled easily.
According to the use sand is classified as-
 Green Sand:
When sand is in its natural state more or less moist state, it is referred to as green sand. It is a
mixture of silica with 18-30% clay & 6-8% water. This clay & water give bonding strength to green
sand.
 Dry sand:
Dry sand is prepared by drying or baking green sand to remove all moisture contents.

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 Loam sand:
It is a mixture of clay & sand milled with water to a thin plastic paste from which moulds are built
up on a baking of soft bricks. It contains up to 50% clay & also contains fire clay. The moisture
content is 18-20%.
 Facing sand:
It is used directly next to the surface of the pattern & it comes into contact with molten metal. It
must possess high strength & refractoriness. It is made of silica sand & clay without the addition
of used sand. It contains finely divided bituminous coal 2-8%.
 Backing sand:
The old, repeatedly used moulding sand, black in colour due to addition of coal dust & burning or
coming in contact with molten metal is known as backing sand or floor sand or black sand. It is
used to fill in the mould at the back of facing layer.
 Parting sand:
The moulding boxes are separated from adhering to each other by spreading a fine sharp dry sand
called parting sand. Parting sand is also used to keep the green sand from sticking to the pattern.
 Core sand:
It is used for making cores. It is silica sand mixed with core oil.
 System sand:
This is used in machine moulding to fill the whole flask. Its strength, permeability & refractoriness
must be higher.
Properties of Moulding Sand:
 Permeability:
The passage of gaseous materials, water & steam vapor through the moulding sand is related to
porosity or in other words permeability.
 Cohesiveness:
The ability of sand particles to stick together is termed as cohesiveness or strength of moulding
sand.
 Adhesiveness:
The sand particles must be capable of sticking to other bodies particularly to the moulding box &
it is called adhesiveness.
 Plasticity:
It refers to the condition of acquiring predetermined shape under pressure & to retain it when
the pressure is removed
 Refractoriness:
It is the ability of the silica sand to withstand high heat without breaking down or fusing.
 Chemical resistivity:
Moulding sand should not chemically react or combine with molten metal so that it can be used
again & again.
 Binding property:
Binder allows sand to flow to take up the pattern shape. It must not be so strong that break out
becomes difficult, nor should it be so weak that it allows surface skin of casting to break.
 Flow ability:
It is the ability of sand to take up the desired shape, similar to plasticity.

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 Green strength:
It depends on- (a) Grain size, (b) Shape & distribution of sand grains, (c) Type & amount of clay or
other binder & (d) Moisture content.
Dry compressive strength of a moulding sand mixture increases as the moisture is added until the
sand is too wet to be workable.
Sand Testing:
Periodic tests are necessary to determine the essential qualities of foundry sand. The most important tests
to be conducted-
1. Grain Fineness Test
2. Permeability Test
3. Strength Test
4. Moisture Content Test
5. Clay Content Test
6. Hardness Test
7. Flowability Test
Permeability Test:
1. Base Permeability Test
2. Green Permeability Test
3. Dry Permeability Test
4. Baked Permeability Test
Figure: Permeability Meter Figure: Schematic Sketch of Permeability Meter
Specimen cross section 20.26 cm2
, height 5.08 cm 2000 cc of air passes through the specimen. Need to
measure the time required to pass the air.

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The permeability number P can be found mathematically by the formula given below:
𝑃 =
𝑣 ℎ
𝑝 𝑎 𝑡
P = Permeability number to be determined
v = Volume of air passing through the specimen in cm2
h = Height of the specimen in cm (5.08cm)
p = Pressure of air in gm/cm2
(10 gm/cm2
)
a = Cross sectional are of specimen in cm2
(20.26 cm2
)
t = Time for air to pass in minutes
𝑃 =
200 × 5.08 × 60
10 × 20.26 × 𝑡
=
3007.2
𝑡𝑖𝑚𝑒 𝑖𝑛 𝑠𝑒𝑐𝑜𝑛𝑑𝑠
No. Type of Sand Permeability Number
01 Loam Sand (15% moisture) <5
02 Moulding mixture for Cast Iron 0 – 80
03 Moulding mixture for Bronze 35
04 Moulding mixture for Aluminum 20 – 40
05 Steel-dry Sand 60 – 100
06 Steel-green Sand 150 – 300
Core:
A core is a predetermined shaped mass of dry sand which is made separately from mould. A core is
sometimes defined as “any projection of the sand into the mold”. This projection may be formed by
pattern itself or made outside and introduced into the mold after the pattern is withdrawn.
Purpose of Using Core:
 To obtain the desired cavities which otherwise could not be obtained by normal moulding.
 In pit moulding, the entire mould is made of cores.
 To reduce metal erosion in gates, runners and pouring basin.
Classifications of Cores:
According to sand used:
 Green Sand Core
 Dry Sand Core
According to the position or use:
 Horizontal Core
 Vertical Core
 Balanced Core
 Drop Core

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Figure: Classifications of cores
Properties of Good Core:
 Permeability:
A core is supposed to be more permeable than the mould itself. Vents are also provided in core
for giving easy escape of hot gases that are generated during casting.
 Gas Generation:
Core should produce minimum amount of gas when in contact with molten metal.
 Collapsibility:
A core should be capable of collapsing shortly after the molten metal has solidified around the
core. Collapsibility provides freshness in the contraction of the metal.
 Thermal Stability:
According to service conditions to which a core is subjected, core sand should be highly refractory,
& able to withstand high temperatures of molten metal otherwise defects like rough surface &
metal penetration etc. may occur.
 Strength:
Core must have high strength & be capable of being handled after drying. If required, wires may
be inserted in the core while it is being moulded to give added strength. It must be able to
withstand force of molten metal.
Classifications of Casting Processes:
Casting Processes can be classified into following four categories:
 Conventional Moulding Process:
1. Green Sand Moulding
2. Dry Sand Moulding
3. Flaskless Moulding
 Chemical Sand Moulding Process:
1. Shell Moulding
2. Sodium Silicate Moulding
3. No Bake Moulding

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 Permanent Mould Casting:
1. Gravity Die Casting
2. Low and High Pressure Die Casting
 Special Casting Process:
1. Lost Wax Casting
2. Centrifugal Casting
3. Continuous Casting
4. Ceramic Shell Moulding
5. Evaporative Pattern Casting
6. Vacuum Sealed Moulding
Green Sand Moulding:
Green sand is the most diversified molding method used in metal casting operations. The process utilizes
a mold made of compressed or compacted moist sand. The term "green" denotes the presence of
moisture in the molding sand. The mold material consists of silica sand mixed with a suitable bonding
agent (usually clay) and moisture.
Figure: Green sad moulding process
Advantages Green Sand Moulding:
1. A reasonably priced process, green sand casting enables great flexibility in the choice of molds
and patterns.
2. This is an environmental friendly process where there mold aggregated can be treated and
repeatedly used.
3. This process enables speedy, economical and expeditious execution of the casting process
4. Easily adaptable to automated and semi-automated machines for some process.
5. Most metals can be cast by this method.
6. Pattern costs and material costs are relatively low.
7. No Limitation with respect to size of casting and type of metal or alloy used.
Limitations of Green Sand Moulding:
Surface finish of the castings obtained by this process is not good & machining is often required to achieve
the finished product. aggregate and adhesives are readily available at reasonable rates.

Casting
Applications of Green Sand Moulding:
Green sand casting is used largely for heavy casting like casting bells, where the castings are to be cleaned
and polished for fine surface finish.
Shell Moulding:
It is a recent invention in casting techniques for mass production & smooth finish. It is as expendable
mould casting process that uses a resin covered sand to form the mould. The patterns for making the
shell mould are made of metal as they are heated to 540°C during this process. The sand is used is fine
dry silica sand, preferably round grained, free from organic impurities & with not more than 1% clay.
Zircon sand is another raw material used for manufacturing shell. Zircon sand produces better surface
finish of its small round grain.
Figure: Steps in shell moulding process
A. Hot pattern is clamped to the dump box containing a mixture of sand & thermosetting plastic
resin.
B. Pattern & box are inverted & kept in this position for some time. Hot pattern melts resin in contact
with it.
C. Box & pattern are again inverted & brought original position. A thin shell of resin bonded sand
sticks to pattern & the rest falls.
D. Patterns with shell is placed in oven & heated to cure resin bond.
E. Shells is stripped from the pattern with the help of ejector pins.
F. Two shells are assembled, clamped & properly backed with sand in a suitable box. This forms a
shell mould ready to receive the metal.
Advantages of Shell Moulding:
1. Greater surface finish and dimensional accuracy. Tolerance about ± 0.03 to ± 0.13 mm.
2. Suitable for casting thin sections of high definition, for example, petrol engine cylinder.
3. Machining and Cleaning cost is negligible.
4. Reduced sand handling difficulties because Very small amount of sand needs to use.
5. Less floor space and greater production rate.
6. Less skilled labor needed.
7. Moulds can be stored until required.

Casting
Limitations of Shell Moulding:
1. Patterns are very expensive.
2. The size of the casting is limited <450 Kg.
3. Highly complicated shape can’t be obtained.
4. More sophisticated equipments are needed.
5. Resin binder is expensive.
6. Minimum thickness can be cast is 4 mm.
7. Suitable for large quantities of production.
Applications of Shell Moulding:
1. Used for casting pipe fittings, valves etc.
Die Casting:
Die casting is a process in which molten metals are forced by pressure into a permanent metal mold is
known as a die. The pressure is maintained until the liquid metal solidifies. It is also called pressure die
casting. Die consists of two parts: (1) Stationary & (2) Movable or Ejector.
Classifications of Die Casting:
There are mainly two types of die casting-
1. Hot chamber die casting:
Heating chamber is an integral part of the machine unit. It is used for lead, tin, zinc & low melting
point alloys. Hot chamber machine has two types of arrangements:
1. Submerged Plunger Type
2. Direct Air Pressure Type
2. Cold chamber die casting:
Heating chamber is outside the machine unit. Metal is melted there & then poured into machine.
Used for aluminum, magnesium, copper base alloys & high melting point no-ferrous alloys.
Hot Chamber Die Casting:
1. Submerged Plunger Type Die Casting:
Figure: Submerged plunger type casting machine

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In submerged plunger type machine, the goose neck type container always remains immersed in the metal
pot. The die is made in two parts (stationary die platen & movable die platen). The molten metal from the
metal container is forced inside the die with the help of a plunger submerged in the molten metal &
operated hydraulically. The plunger moves inside the cylinder having a hole in its bottom. When the
plunger moves up, the molten metal comes up & fills the cylinder which is then forced into the die cavity
through the nozzle in the goose neck of the container in the down stroke of the plunger. The movable die
platen also moves hydraulically & the movements of plunger & movable die platen are synchronized such
that when metal is being forced, the two die halves are in match & on solidification when plunger is
moving up, the movable die platen moves away & the casting is removed. The pressure used is of the
order of 100-150kg/cm2
.
2. Direct Air Pressure Type Die Casting:
Figure: Direct air pressure type die casting machine
In direct air pressure type die casting machine, direct pressure (40 kg/cm2
) is used for forcing the metal
inside the die. The goose neck container is operated by a lifting mechanism. Initially it is submerged in the
molten metal & is filled by gravity. Then it is raised so as to bring the nozzle in contact with the die opening
& is locked in that position. Compressed air then forces the metal into the die & the pressure is maintained
till solidification. When solidification is complete, the goose neck is lowered down & casting is removed
by ejector pins after opening the dies & withdrawing the cores, if any.
Cold Chamber Die Casting:
The machine is used for casting alloys which requires high pressures & have high melting temperatures
such as brass, aluminum & magnesium. In this machine the metal melting unit is not an integral part of
the machine & metals are melted in a self-contained pot in an auxiliary furnace. The molten metal is ladled
in the plunger cavity next to the dies & forced into the die cavity by a hydraulically operated plunger or
horizontal plunger & pressure is maintained till solidification. These machines can either have vertical
plunger or horizontal plunger for forcing molten metal into die & are built very strong & rigid to withstand
the heavy pressures. The process of casting mainly consists of four steps.
i. Pouring the molten metal below the plunger & placing the movable platen & cores in position.
ii. Forcing the molten metal into the dies by means of plunger.
iii. Withdrawing of cores & opening dies.
iv. Ejecting the casting from the movable die platen.

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Figure: Cold chamber die casting process.
Advantages of Die Casting:
1. Less floor space needed.
2. Precision manufacturing & less machining cost.
3. Thin sections of the order of 0.5 mm is possible.
4. Improved surface finish.
5. Less rejection, strong & dense metal structure.
6. High production rate (800 castings per hour).
Limitations of Die Casting:
1. Cost of die & equipments is high.
2. Life of Die decreases rapidly due to high temperature.
3. Only non-ferrous alloys can be casted.
4. Size of castings is limited.
5. Special skill required for die maintenance.
6. Usually contains porosity due to the entrapping of air.
Investment or Lost Wax Casting:
This process is called the lost wax process or precision casting. This process uses wax pattern which is
subsequently melted from the mould, leaving a cavity having all the details of the original pattern.
Generally, this process is used for producing light & intricate parts. This process doesn’t need a parting
line or any form of split mould.
The process of investment casting consists of two stages. First of all, a master pattern is made of steel or
brass & it is replica of the part to be cast. Around it, a split mould is formed from gelatine or an alloy of
low melting point. This alloy is poured over the master pattern. After solidification master mould is
obtained. This master mould is used for making the wax or lost pattern. The following are the materials
used for preparing master mould:

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1. Plaster of Paris or gypsum products for non-ferrous castings.
2. Ethyl silicate, Sodium silicate and phosphoric acid for steel castings.
3. Sometimes fine grain silica sand with binders.
The Master mould is then filled with either liquid wax or thermoplastic polystyrene resin which when
solidified forms a wax pattern. This wax pattern is used for making the final casting. Then the process of
investment of the pattern is followed which consists of casting the wax pattern with slurry consisting of
silica sand or graphite mixed with water. This wax pattern is used for making the final mould in the same
fashion as the conventional moulding process. This mould is then dried in air for 2-3 hours & then baked
in an oven so that the wax may melt out. To improve resistivity, the mould is further heated up to 1000°C
called sintering of the mould & finally cooled to 100°C for obtaining the casting.
Figure: Lost wax casting process
Advantages of Lost wax casting:
1. Very smooth surface & high dimensional accuracy.
2. Reproduction of surface details & dimensions with precision.
3. No cores & loose pieces.
4. Very thin sections can be cast.
5. Complex shape is easily obtained.
6. No or little machining is needed.
Disadvantages of Lost wax casting:
1. Limited by the size and mass of the casting.
2. Expensive process.
3. Long time process.
4. Large floor space needed.

Casting
Centrifugal Casting:
In this casting molten metal is solidified while mould is revolving. The metal solidifies under the pressure
of centrifugal force which cause the metal to take up the impression of the mould cavity. Metals towards
the periphery & gases and other materials towards the axis of rotation.
Classifications of Centrifugal Casting Processes:
1. True Centrifugal Casting
2. Semi-centrifugal Casting
3. Centrifuged or Pressure Casting
1. True Centrifugal Casting:
In this process, the castings are made in a hollow, cylindrical mould rotated about an axis common to
both casting & mould; the axis may be horizontal, vertical or inclined. Usually the mould rotated in
horizontal plane. The most commonly cast parts by this process are cast-iron pipes, liners, bushes &
cylinder barrels etc. This process is limited only for symmetrical-shaped objects, such as pipes, rolls,
cylinder sleeves, & liners, piston-ring stock, bearings bushings etc.
Figure: True centrifugal casting process

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2. Semi Centrifugal Casting:
This also known as profiled centrifugal casting. It is nearly similar to true centrifugal casting with only
difference that central core is used to form the inner surface. The particular shape of the casting is
produced by the mould & core shapes & not by centrifugal force. However, centrifugal force aids in proper
feeding of mould cavities like rim sections & eliminates the possibility of porosity in the castings.
Rotational speed of this casting process is lower than that of true centrifugal casting process. In this
process the axis of spin is always vertical. This process is used only for symmetrical objects. Symmetrical
objects like wheel having arms etc are good examples of this casting. Parts produced by this process
include gears, fly wheels, track wheels etc.
Figure: Semi-centrifugal casting process
3. Centrifuged or Pressure Casting:
This process is used for non-symmetrical castings having intricate details & also for precision castings. The
centrifugal force provides high fluid pressure to the force the molten metal into mould cavity. A number
of similar components can be cast simultaneously. The mould cavities are positioned about its own axis &
also about the axis of a central down sprue. The metal is introduced at the center & fed into the moulds
through radial in gates. Centrifuging is possible only in vertical direction.
Figure: Centrifuged Casting Process
Advantages of Centrifugal Casting:
1. Castings produced are sounder with dense structure, cleaner & the foreign inclusions are
eliminated completely.
2. Mass production is possible with less rejections.

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3. Use of runners, risers & cores is eliminated.
4. Mechanical & physical properties are improved.
5. Low machining cost.
6. Thinner sections can be cast.
7. Any metal can be cast by this process.
Disadvantages of Centrifugal Casting:
1. Limited to only cylindrical & annular parts with a limited range of sizes.
2. It involves high initial cost.
3. Too high speed may result in surface cracks.
Permanent Mould Casting:
Permanent mould casting or gravity die casting like sand casting, this method also consists of pouring
molten metal but in a metallic mould without external pressure. 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 applied but
hydrostatic pressure created by the risers is mainly responsible for casting of metal in the mould. As no
external pressure is applied, this process in sometimes called gravity die casting.
The metal used for the mould should have such a composition as to withstand high temperature.
Generally, they are made of grey cast iron (having high resistance to thermal shocks), alloy steels (for very
high temperatures and withstanding surface erosion), or non-ferrous alloys. Inner surfaces of the mould
are coated first with a refractory & then with lamp black or core oil. This is done in order to reduce the
chilling effect on the cast metal and to facilitate the removal of casting, and prevent the adherence of the
molten metal to the mould. The moulds also have the facilities of setting cores for hollow castings and
ejector pins for ejecting out casting from the mould after solidification. The moulds are generally made in
two halves which are hinged at one end & have provision for clamping at the other end.
The heat of the molten metal is removed very rapidly because of fast conduction through metallic mould.
The metal thus shrinks very fast, leaving an air space between the mould surface and casting.
After solidification, the casting tends to cling to one of the mould halves. Spring loaded ejector pins are
used to eject the casting from the mould.
Figure: Permanent mould casting process.

Casting
Advantages of Permanent Mould Casting:
1. Free from sand & good surface finish.
2. Superior in hardness & mechanical properties.
3. Heavier density than sand casting.
4. Less skilled labor needed.
5. Production rate is high.
6. Requires less space.
7. Number of rejection is less.
Disadvantages of Permanent Mould Casting:
1. Not suitable for alloy of high melting point and large size.
2. Limited to mass production of identical product.
3. Defects like stress and surface hardness produced due to surface chilling effect.
4. Not flexible.
5. Mould maintenance cost is high.
Continuous Casting:
Continuous Casting Continuous casting has proved itself to be a most economical way of casting
wherever feasible and several methods have been devised and successfully used. In this process the
molten metal is continuously poured into a mould around which there are faculties for rapidly chilling
the metal to the point of solidification. The solidified metal is then continuously removed from the
mould at the calculated rate. The following processes have been successfully used for continuous
casting of metals:
1. Reciprocating Mould Process:
In this process molten metal is poured
into a holding furnace. At the bottom of
this holding furnace there is a needle
vaIve arrangement by which the quantity
of flow can be changed. Metal from this
needle valve flows to a mould and is
distributed all along the mould surface.
The level of the metal in the mould is
kept constant at all times. The mould is
surrounded all around by cooling water.
The water cooled mould is reciprocated
up and down. The down-stroke is
synchronized with the discharge rate of
Slab. As the mould moves up after its
down limit, it is again filled with molten
metal and again in down stroke, cooling Figure: Reciprocating continuous casting process
starts and this process continues.

Casting
2. Asarco Process:
It is a modification of the reciprocating process. In
this process, the forming die is an integral part of
the furnace and thus there is no problem of
controlling the flow of metal. The metal is fed by
gravity into the mould from the furnace as it is
continuously solidified and withdrawn by the rolls
below. The die is water cooled & self-lubricating
and thus has excellent resistance to thermal
shocks. The upper end of die is in molten metal
and thus serves the function of riser and acts as
path for dissipation of evolved gases. At the time.
of starting, a similar rod is put into the die till its
upper end melts and forms continuity with
molten metal, and it is then withdrawn by rolls
and the process becomes continuous. By this
process, rounds, tubes, squares and special
shapes can be conveniently produced & it is
particularly suited to alloys like phosphorised
copper and standard bronzes. The physical properties of castings produced are superior to other
processes. In continuous casting, solidifying zone being relatively small, practically all problems
encountered with feeding and shrinkage are overcome. Problems due to fast cooling in mould zone
are faced (segregation and cracking). A greater degree of control is, therefore, required in comparison
to batch casting.
Direct casting of sheets has also been tried but has not proved to be very successful due to some
drawbacks like segregation problems (variation of metal analysis from place to place) and
solidification at ends being faster than at center results in greater thickness at the edges than desired.
Casting Defects:
There are three types of casting defects:
1. Major or Most Severe Defects
2. Intermediate Defects
3. Minor Defects
Different casting defects are described below:
 Surface Roughness:
Caused due to too coarse moulding sand or high pouring temperature. In steel casting, iron
is oxidized which reacts with silica to form rough compounds.
 Scabs or Buckles:
Due to sand shearing from cope surface. Occurs due to too fine sand, uneven ramming, high
moisture, low running of molten metal.
Figure: Continuous Asarco casting process

Casting
 Blow Holes:
Internal voids as a result of excessive gaseous materials. Caused by hard ramming, excessive
moisture, low permeability, improper venting, excessive gas producing ingredients.
 Pinholes:
Surface reactions cause surface porosity or pinholes. Magnesium reacts with water vapor
and forms H2.
 Hot Tears:
Cracks during solidification. Caused by discontinuity of casting, excessive mould hardness,
improper metallurgical and pouring temperature control etc.
 Cold Shots:
Two metal stream meeting together are too cold to fuse properly. Caused by slow pouring,
improper gate design.
 Run Outs:
Drainage of metal from cavity. Caused by too large pattern, inadequate mould weights and
excessive pouring pressure.
 Fins:
Usually occurs at the parting of mould and core section. Caused by run out of metals.
 Internal Air Pockets:
Caused by pouring boiling metals.
 Misruns:
Some portion is not filled with metals. Caused due to low pouring temperature, lack of
fluidity of metals, too small gate etc.
There are many other defects resulting from improper ramming, slow feeding, poor design, metal
contraction, gas generations etc.

Manufacturing Process - Casting

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