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Manufacturing Technology Unit I - R2017 Anna University Syllabus
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Unit I
Metal Casting Processes
SAND CASTING:
Sand casting is the process used for making
components of complicated shapes in large
quantities.
It is the process of producing metal parts by
pouring molten metal into the mould cavity of
the required shape and allowing the metal to
solidify.
Plant where castings are made is called
“foundry”.
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SAND MOULD:
Mould is the cavity of the required shape made
using moulding sand or other suitable materials.
Pattern is the model of the required casting made
in wood, metal or plastics.
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Important processes involved in foundry are:
1. Pattern making
2. Mould making
3. Casting
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PATTERN AND PATTERN MAKING
A pattern is one of the important tool used for
making cavities in the mould.
Factors are useful for the selection of patterns:
1. Size and complexity of the shape.
2. Number of components to be produced.
3. Method of casting to be used.
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Types of patterns:
1. Solid or single piece pattern
• These types of patterns are made of single solid piece
without joints, partings or loose piece.
• It is made exactly into the desired casting to be
produced with some allowances. Removal from sand
is easy.
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2. Split pattern:
• A pattern which is having complex geometry cannot be
removed from mould of they are made by a single
piece.
• Split pattern made into two parts.
• One part is used to produce the lower half of the
mould whereas the other part is used to produce the
upper half of the mould.
• Two parts are joined by dowel pins.
• Line separating these two parts is called parting line.
• If split pattern are made of three pieces, its called as
three piece pattern.
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3. Sweep pattern:
• Sweep patterns are mainly used to generate the
surfaces of revolution such as cylinder, cone and
sphere in large castings.
• It have rotational symmetry. It is a section of a
pattern made of wood or metal to the required cross
section.
• It rotate about one edge or spindle.
• Sand is approximately rammed around the mould
cavity. The sweep is rotated to form the mould cavity
in the sand.
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4.Segmental pattern:
• A segmental pattern is a segment of whole pattern.
• Pattern is used for making circular moulds.
• A vertical spindle is fixed at the center of the drag box.
Bottom of the mould is rammed and leveled.
• Then the pattern is attached to the spindle.
• Moulding sand is filled and rammed on the inner and
outer sides of the pattern. After ramming the pattern is
moved to form the next segment.
• Like this full mould is completed.
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Skeleton pattern:
• In some cases solid pattern made of wood requires a
large amount of wood. Hence it is very expensive.
• Here we are using skeleton pattern.
• This shape is made of wooden strips with a lot of
openings and fitted to end supports.
• First the skeleton pattern is filled with loam sand.
Ramming is done inside the openings in the pattern.
• Moulds for water pipes, turbine castings and pipe
bends are made by a skeleton pattern.
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Shell pattern:
Shell pattern is a hollow pattern. Its outer shape is used
for making the mould. The core is prepared using the
inner surface of the pattern itself.
The pattern is made into two halves and joined
accurately by dowels.
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Loose piece pattern:
• If a pattern is made from a single piece having
projections or back drafts which lie above or below the
parting plane, it is impossible to withdraw it from the
mould.
• In such cases the pattern is split up into solid pattern
and loose piece.
• After making the mould, the solid pattern is first
removed and the loose pieces are removed without
breaking the mould.
• Loose pieces are attached to the main body of pattern
by pins.
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Match plate pattern:
A pattern which is made into two halves mounted on
both sides of a plate.
It is accurately placed between cope and drag flasks by
means of locating pins.
For small castings, many patterns can be mounted on
the same match plate.
Match plate patterns are used in machine moulding. It
used for small, accurate size and large number of
castings.
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Pattern materilas
Following factors are to be considered,:
• Design of casting.
• Number of castings to be produced.
• Degree of accuracy and surface finish.
• Shape, complexity and size of the castings.
• Casting method adopted.
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Mostly used pattern materials are,:
1.Wood:
• Commonly used pattern material.
• Teak, mahogany, rosewood are used for making
pattern.
• Laminated wooden sheets are also used for getting
accuracy, surface finish and long life.
• Wood should not contain 10% moisture.
• Metal spray coating up to 0.25 mm thick may be given
on wooden pattern.
• Zinc and aluminium are used for coating the on the
wooden surface to avoid moisture absorption and good
surface finish.
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i) Advantages:
• Light in weight, cheap and easily available.
• Easy to work, easy to cut and easy to fabricate.
• Can easily repaired.
• Can be smoothened by varnishes and paints.
• Good surface finish can be obtained.
ii) Limitations:
• Absorbs water from sand and changes its shape.
• High wear and tear by sand. Can’t be used for mass
production
• Cannot be used in machine moulding.
• Not possible to achieve high accuracy.
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2. Metals:
The metal pattern is used when a large number of
castings to be made.
Generally used in machine moulding.
i) Advantages:
Long life and accurate in size.
Has smooth surface.
Mass production possible.
Does not absorb moisture and deform in size.
Can be used for rough handling.
Resistance to wear and tear.
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ii) Limitations:
Costlier and heavier than wood.
Cannot be easily repaired.
Ferrous patterns can be rusted.
Metals used for making pattern:
2.1 Cast Iron:
• Cast iron having fine grain size. High resistance to sand
abrasion and has smooth surface but it heavier.
• Cost is less and more durable than other metals.
• Brittle can be easily broken. Will get rusted by moisture
unless it is protected.
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2.2 Brass:
• It may be easily worked and built up by soldering or
brazing.
• It is used for small size patterns due to high cost.
• Not affected by moisture.
• Provides good surface finish and withstands wear and tear.
• Product can be made with very high accuracy.
2.3 Aluminium:
• Light in weight, strong and easily machined.
• Can be made with high accuracy and good surface finish.
• Not affected by moisture and gets rusted.
• Very soft and easily damaged by rough handling.
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3. Plaster:
• Plaster of paris or gypsum is used for making hollow boards for
moulding the work.
• Can easily made difficult shapes.
• Has high compressive strength. Used only for small patterns.
• Can affect by moisture.
4. Plastics:
• Plastic pattern has many advantages over other materials.
• Plastic pattern is cast from a wooden pattern called master
pattern.
• Light in weight but strong.
• Not affected by moisture and more resistant to wear.
• Does not shrink much, dimensional accuracy. Smooth glassy
surface.
Widely used plastics:
1. Poly acrylates 2. Poly ethylene 3.Poly vinyl chloride.
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5. Wax:
• Wax pattern is primarily used in investment castings.
• Commonly used waxes are paraffin wax, shell wax and
microcrystalline wax.
• Has good surface finish and high dimensional accuracy.
• It will not absorb moisture and easy to work. The cost
is low but it can used for making small patterns only.
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Pattern allowances
• Patterns are not made into the exact size of the
castings to be produced. patterns are made slightly
larger than the required castings.
• This extra size given on the pattern is called pattern
allowance.
• The pattern allowances are given for the purpose of
compensating the metal shrinkage to provide extra
metal which is to be removed in machining.
• If allowances are not given on the pattern, the
casting will become smaller than the required size.
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Various types of allowances:
1. Shrinkage allowance:
• Metals shrink on solidification and contracts further
on cooling to room temperature. To compensate it,
the pattern is made larger than the required casting.
• This extra size provided on the pattern for metal
shrinkage is called shrinkage allowance.
• If it is not given, the casting will become smaller after
it is cooled.
Materials Shrinkage Allowance
C.I 10.4 mm
Aluminium 17 mm/m
Brass 15.3 mm/m
Steel 20.8 mm/m
Zinc, Lead 25 mm/m 30
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2. Machining or Finishing Allowance:
• All castings are to be machined to get the required
surface finish on the metal.
• During machining, some of the metals are removed
from the casting.
• For this purpose the pattern is made larger than the
required casting.
• The amount of finish allowance depends on the
material, size of casting, volume of production, method
of machining, degree of finishing.
• Machining allowance is larger for hand moulding when
compare to machine moulding.
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3. Draft or Taper allowance:
• If the vertical faces of pattern are perpendicular to
the parting line, the edges of mould may be
damaged when the pattern is removed.
• The vertical faces are made into taper for easy
removal of pattern.
• The slight taper provided on the vertical sides of
pattern is called draft allowance.
https://nptel.ac.in/courses/112107144/16
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The amount of taper depends upon
1. Height and size of pattern.
2. Moulding method
3. Mould materials.
Common draft 1 to 3 degree.
Taper for external surface 10 to 25 mm/m
Taper for internal surface 40 to 65 mm/m
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4. Distortion or camber allowance:
• Sometimes castings get distorted, during
solidification, due to their typical shape.
• For example, if the casting has the form of the letter
U, V, T, or L etc. it will tend to contract at the closed
end causing the vertical legs to look slightly inclined.
• This can be prevented by making the legs of the U, V,
T, or L shaped pattern converge slightly (in opposite)
so that the casting after distortion will have its sides
vertical.
• The distortion in casting may occur due to internal
stresses. These internal stresses are caused on
account of unequal cooling of different section of the
casting.
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5. Rapping or shake allowance:
• To remove the pattern out of mould, it is slightly
shaked to detach it from the mould cavity. This is
called rapping.
• Due to rapping the mould cavity become large.
• To avoid this the pattern is made slightly smaller
than the required casting.
• This allowance given on the pattern is called shake
or rapping allowance.
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MOULDING SAND
Special type of sand used for making mould.
Moulding sand essentially contains three constituents
1. Sand
2. Binder
3. Additive
Reason for using moulding sand:
1. It maintains shape at very high temperature.
2. It makes a mould porous.
3. Can be used again and again.
4. It is inexpensive.
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1. Sand:
Silica sand is widely used as moulding sand has 80 to 90%
silicon dioxide. It gives refractoriness to the sand.
Advantages:
1. Cheap and easily available.
2. Can be easily be moulded and reusable.
3. Has high thermal stability.
According to clay content moulding sand is classified into
a) Silica sand- 2% clay
b) Lean or weak sand – 2 to 10% clay
c) Moderately strong sand – 10 to 20% clay.
d) Strong sand – up to 30 % clay.
e) Loam sand – up to 50 % clay.
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a) Natural sand:
It is available from natural deposits. Needs 5 to 8%
water. Available at riverbeds, contains 80 to 90% silica,
5 to 10% alumina or clay and small percentage of lime
and magnesia.
Natural sand also prepared by crushing and milling the
soft yellow sand stone. This sand has less refractoriness
than synthetic sand.
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Advantages:
1. Cheap and easily available.
2. Easy to repair.
3. Wide range of grain sizes and shapes are
available.
Limitations:
1. Has less refractoriness.
2. Has high expansion ratio.
3. May be fused with metals.
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b) Synthetic sand:
The moulding sand prepared artificially by mixing clay free
sand having specified grain type with a specified type of
clay binder as well as water and other additives.
This sand prepared with the desired required properties.
It is used for both machine moulding and high pressure
moulding.
Advantages:
Has more uniform grain size.
Required properties can be obtained.
Has higher refractoriness.
Can be easily mouldable.
Limitation:
More expensive
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c) Special sands:
Special sand is prepared for obtaining specific properties
such as refractoriness, high heat conductivity and low
expansion ratio.
It is prepared for applying in a particular place of mould.
We can get good quality casting with fine surface finish.
i) Zircon sand: (Zr Si O4)
It is mainly used for making cores of brass and bronze
castings. It is also used as facing sand.
Does not react with moulding sand.
ii) Chromite sand:
Used for making chilled castings. It may be used as facing
sand in steel casting. It has good refractoriness, high heat
conductivity and low expansion ratio.
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2. Binder:
Binders are used to bring cohesiveness to the
sand. They bind the sand grains together and
give strength to the moulding sand.
Classified as:
1. Organic binder.
2. Inorganic binder.
a) Organic binder:
Organic binders are mainly used for core
making. They are cereal, resins, pitch, drying
oils, molasses.
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b) Inorganic binders:
Clay binders are the most common type of
inorganic binder.
Clay is formed by weathering and
decomposition of rocks.
Common type clay are 1. Fire clay 2. Kaolinite
3. Bentonite.
Kaolinite and bentonite clays are the most
popular clays because they have high
thermochemical stability.
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3. Additives:
• Additives are added to the moulding sand to improve
the properties such as strength, refractoriness and
permeability.
• These additives may be necessary to give a good
surface finish to the casting or to eliminate the casting
defects.
• Additives are not used for binding purpose.
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a) Sea coal:
• Sea coal is finely powdered bituminous coal. It is
used to obtain smoother and cleaner surfaces of
castings.
• It reduce the adherence of sand particle to the
casting.
• Used for make ferrous castings. Up to 8% can add.
• When metal poured coal dust burns and gives
volatile substances such as CO2 and CO which form
space between mould walls and metals.
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b) Saw dust:
• It improves the permeability and deformability of the
moulds.
• Should be dry, otherwise peat contain 70-73%
volatile matter should be used.
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c) Pitch:
• It is distilled from soft coal. It improves hot
strength.
• Gives fine surface finish for ferrous castings.
•
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d) Cereals:
• It is finely ground corn flour. It increase green
strength and dry strength of the moulding sand.
• Used about 1%.
e) Silica flour:
• It is very fine powdered silica. Generally mixed
twice with the moulding sand to make facing sand
and used around the pattern.
• Improves surface finish of the casting.
• Increases hot strength and reduces sand expansion
defects.
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f) Special additives:
• Fuel oil, dextrin, molasses and iron are added to
moulding sand.
• Fuel oil-Improves mouldability.
• Dextrin-Increases both collapsibility and strength.
• Molasses-Improve dry strength and collapsibility.
• Iron oxide-improves hot strength of moulding sand.
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Types of Moulding Sand
1. Green sand.
2. Dry sand.
3. Facing sand.
4. Loam sand.
5. Backing sand.
6. Parting sand.
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1. Green sand.
• Sand which is in moist state is known as green sand.
• 5 to 8% of water and 16 to 30% of clay.
• Can retain any shape and have good damping
capacity.
• Used for small, simple and medium size castings.
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2. Dry sand:
• Moulding sand prepared in dry stage.
• Mould form by dry sand is called dry sand mould.
• Used for making large castings.
• Does not cause defects due to moisture.
• It has greater strength and rigidity.
• If the portion around the cavity only dries, it is
called skin dry mould.
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4. Loam sand:
• Loam sand consists of fine silica sand, clay, graphite,
fibre and water.
• Loam sand is sand containing up to 50 % clay which
has been worked to the consistency of builder
mortar.
• This sand is used for loam sand moulds for making
very heavy castings usually with the help of sweeps
and skeleton patterns.
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5. Backing sand:
• Backing sand is the bulk of the sand used to back up
the facing sand and to fill up the volume of the flask.
• It consists mainly of old, repeatedly used moulding
sand which is generally black in colour due to
addition of coal dust and burning on contact with hot
metal.
• Because of the colour backing sand is also
sometimes called black sand.
• The main purpose for the use of backing sand is to
reduce the cost of moulding.
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6. Parting sand:
• Parting sand is usually applied when a casting is
made up of two halves with cope and drag.
• To avoid sticking of cope and drag, parting sand is
sprinkled over the parting surface.
• Parting sand consists of fine grained clay free dried
silica sand, sea sand or burnt sand with some parting
compounds.
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Properties of moulding sand
A good casting can be produced only with the use of good
quality moulding sand. For this the moulding properties
of the sand have to be controlled.
These properties are
1. Porosity or permeability.
2. Plasticity or flowability
3. Adhesiveness.
4. Strength or cohesiveness
5. Refractoriness.
6. collapsibility
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1. Porosity or permeability.
• Permeability is a measure of moulding sand by
which the sand allows the steam and gases to
pass through it.
• When molten metal is poured into the mould,
steam and gases are formed due to moisture,
binder and additives present in the sand.
• Even though vent holes and riser are provided, all
of these gases will not escape through it.
• To escape the remaining gases, the moulding
sand should have good gas permeability.
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Permeability depends on
a) Quality and quantity of clay.
b) Moisture content.
Affecting factor of permeability
a) Clay content is less permeability will be more.
b) Grain size larger, permeability will be more.
c) Soft ramming improve permeability.
d) Higher silica content lower the permeability.
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2. Plasticity or flowability:
• It is ability of moulding sand to flow and pack all
around the pattern and take up the required shape.
• It gives the shape of the pattern and retains the
shape after removing the pattern.
• This property may be improved by adding clay and
water to silica sand.
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3. Adhesiveness:
• This is the property of moulding sand by which it sticks
to another body.
• The moulding sand should stick to the sides of the
moulding boxes.
• So it does not fall out when the flasks are lifter and
turned over.
• This property depends on the type and amount of
binder used in the sand mix.
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4. Strength or cohesiveness:
• It is the property of moulding sand by which it sticks
together.
• A moulding sand should have sufficient strength so
that the mould does not collapse or partially
damaged during shifting and pouring the molten
metal.
• Because of pouring the molten metal into the mould
cavity, its bottom and walls are subjected to high
pressure.
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5. Refractoriness:
• This is the property of moulding sand to withsand
the temperature of the molten metal to be poured
so that it does not get cracked and fused with the
metal.
• This property depends on the purity of the sand
particles and their size.
• Rough and large grain in moulding sand increase
the refractoriness.
• Poor refractoriness will result the rough surface in
casting.
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6. Collapsibility:
• This property permits the moulding sand to
collapse easily after the casting solidifies.
• If the mould does not collapse, it causes cracks on
the casting.
• This property depends on the amount of quartz
and binders.
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● On the top of the housing, an enclosure along
with infrared heating bulb is connected.
● Bubb controlled by an switch and entire housing
mounted on the base.
● Measured quanity of sand taken in the pan
provided on the testing apparatus.
● Pan is closed and bulb switched on.
● Infrared heating bulb emits heating rays towards
the pan.
● Heating is done 2 to 3 minutes. Moisture content
evaporated form the sand.
● Sand is taken from the pan and reweighted.
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● Moisture content = W1-w2.
● W1 – Weight of the sand before drying.
● W2 - Weight of the sand after drying.
● Percentage of moisture content = W1-w2
w1
X 100
Advantages:
1. Simple process.
2. Testing cost is very less.
Disadvantages:
1.Time consumption is more.
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b) Moisture teller method:
•Weighted quanity of moulding sand sample is taken and put
into the pan.
•The pan has nearly 500 mesh screen in the bottom.
•Now the pan is perfectly closed by the lid and hot air blown
through the sand for 5 minutes.
•Due to this moisture content gets removed.
•The sand is reweighted in the same pan teller, the loss in
weight is indicated on the dial as moisture content which is
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c) Moisture teller based on chemical reaction:
●In the same teller machine, the weighted
quantities of both sand and calcium carbide are
put in the pan without mesh screen.
●Calcium carbide reacts only with moisture in the
sand not with the sand composition.
CaC2 + 2H20 C2H2+Ca(OH)2
●Depending upon the amount of acetylene gas
and its pressure the moisture content is
determined.
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2. Clay content test:
●Weighted quanity of sand sample, distilled water and 1%
sodium hydroxide(NaOH) solution are put in the mixing
device.
●The mixture is stirred nearly 5 minutes and allowed to settle
down for 10 minutes. Dirty water removed from the top.
●Distilled water and 1% NaOH solution are again added, the
same procedure repeated till the water at the top becomes
clean.
●Finally the water drained and sand is dried well. Dried sand is
reweighted. Loss in weight indicates the clay content.
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● This test is carried out on completely dry and
clay free sand.
● Test apparatus has a set of known values of
sieves placed one over the other.
● Top sieve is rough and the bottom one is finest.
● Pan is placed under the bottom most sieve.
● The shaker vibrates the sieve up and down and
the sand placed on the top sieve gets screened
and collects on different sieves.
● Normally eleven sieves are placed one above the
other and under the last sieve a pan is placed.
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4. Permeability test:
● Permeability defined as the amount of air which passes
through the standard specimen.
● Weighted quanity of moulding sand is taken.
● Standard specimen of size 50.8 * 50.8 mm prepared by a
specimen rammer.
● Standard method:
● The permeability tester consists of water seal, inverted bell
jar, mercury seal and pressure manometer.
● In this method 2000 cc of air is filled in the inverted bell jar.
● The air is allowed to pass through the sand specimen.
● The level of mercury manometer changes with respect to the
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● At some point, the air entering the specimen becomes
equal to the air escaped through the specimen.
● Due to this stabilized pressure the manometer level
didn't change.
● Time taken for passing 2000 cc air through the
specimen is noted.
● Permeability number = VH / APT
● V – volume of air passed (2000 cc)
● H – Height of the specimen.
● A- Area of the specimen. Π/4 d²
● T – Time taken for 2000cc to pass.
● P – Air pressure measured by manometer.
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5. Strength test.
● This test is mainly carried out to measure the holding
power or bonding power of sand.
● Compressive strength test is very much important. Test
carried out in universal testing machine.
● Specimen size 50.8 mm x 50.8 mm (dia x length)
● Indicators of high and low strength, wheel, dial indicator
and jaws are provided in the testing machine.
● Test specimen is just held between the jaws. Now hand
wheel is rotated to actuate the hydraulic mechanism.
● Hydraulic pressure is applied and the deformation starts
slowly on the specimen.
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● Two types indicators are used, one for
measuring high strength and the other for
measuring low strength.
● Each indiactor consists of three scales for
measuring compressive, tensile and shear
strength individually.
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6. Deformation and toughness test:
● Deformation is defined as the plasticity of sand that can
be tested by reducing the length of specimen by
applying compressive force.
● Higher deformation indicates the better capacity of the
mould on volumetric contraction.
● This deformation of green compressive strength
provides the quality of sand termed as toughness.
● Sand toughness number = Deformation x green
compressive strength.
● Toughness refers the ability of sand to withsand rough
handling.
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7. Hot strength test:
● Cylindrical specimen with double end rammed
about 28 x 50 mm is prepared on the sand
specimen tester.
● The specimen is placed in a dilatometer and
tested.
● For testing in the dilameter, the specimen is
heated from 500 to 2500°F and compressive
force is applied over the specimen.
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8. Refractoriness test:
● Refractoriness- ability to withstand at high temperature.
● Segercones, Different cones of different materials whose
softening temperature are known, are heated along with the
specimen.
● At a particulat temperature, the tip of the segercone will
melt.
● The melting point is noted, it gives the refractoriness of the
sand.
● Another test – heating sand to 1500 c and noting the
appearance and dimensional changes of the specimen.
● Good refractory sand must not expand more than 7%.
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9. Mould hardness test:
● The hardness of the moulding sand tested in an
indentation hardness test.
● Specimen prepared, and placed where the indentation is
to be formed.
● The intenter placed at the bottom of the plunger.
● The intentation formed on the specimen by rotating the
manual loading device.
● The plunger is moved down, force is applied on the
specimen and it maintained for the specified time and
then it is released.
● Then the specimen is taken out and diameter of
indentation is measured.
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Melting furnaces
● Blast furnace – For iron
● Cupola furnace – for C.I
● Open hearth furnace – For steel
● Crucible furnace.
● Pot furnace.
● Electric furnace.
Depends upon type of metal and the quanity of metal
to be melted.
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● The purpose of blast furnace is to reduce and convert iron
oxides into liquid iron.
● Blast furnace developed very high temperture inside the
furnaces by forcing a blast of heated air.
● Its height is about 30 m and interior dia of 8 m.
● Coke, limestone and iron ore are poured in at the top.Air is
blown in through passage near the base.
● This blast allows the combustion of fuel.
● It reduces the oxide in the metal which is being heavier
sinks into the bottom of the furnace.
● Fe2O3 + 3CO 2Fe + 3CO2
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● Compressed air blown into the furnace reacts with the
carbon to produce carbon monoxide which then reacts
with iron oxide, reacting chemically to produce iron and
carbon dioxide which leaks out of the furnace.
● Calcium oxide reacts with various acadic impurities in the
iron forming a slag containing calcium silicate CaSio3.
● Slag floating over the molten metal is removed through
the slag notch.
● Heavier pig iron is taken out using a tap hole at the bottom
of the furnace.
● https://youtu.be/hBqhGHfzQFQ?t=363
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● It is a vertical and cylindrical shell made of 10mm thick
steel plate. Lined with refractory bricks inside.
● Two bottom doors close the bottom of the cupola.
● Sand bed laid over the bottom doors sloping towards the
hole.Molten metal stays over this bed.
● Opening called tuyeres are provided one meter above the
bottom.
● Fuel is supplied through these tuyeres for making
complete combustion.
● There is Wind box and blower for the supply of air into the
furnace.
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Preparation stage:
● Slag and waste cleaned, broken bricks are replaced.
Bottom door closed.
● Sand bed with sloping towards tap hole is prepared up to a
height of 200 mm.
● Slag hole is prepared.
● Firing stage:
● Oil waste and wooden pieces are placed at the bottom and
fire is started, sufficient amount of air is supplied.
● The coke is charged at several portions. Now the coke start
burns.
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Charging and melting:
● Pig iron and Metal scrap are added into the furnace
through the charging door. Limestone is added to
remove the impurities.
● The ratio of pig iron to limestone and pig iron to coke
are 25:1 and 10:1 respectively.
● Iron is soaked for one hour in the furnace, after that
the blast turned on.
● The molten metal will begin to collect at the sand bed.
● Slag removed by slag hole.
● https://youtu.be/8vq6bB3egVY?t=619
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Special casting processes
Shell mould casting:
https://youtu.be/5Kkv8udoLYI?t=19 101
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● It is a process in which, the sand mixed with a thermosetting resin
is allowed to come into contact with a heated metallic pattern
plate.
● So that a thin and strong shell of mould is formed around the
pattern.
● The shell is removed from the pattern and the cope and drag are
removed together and kept in a flask with the back up material.
● And then the molten metal is poured into the mould.
● Dry and clay free fine sand is used for preparing the shell moulding
sand.
● Synthetic resin used in shell moulding are essentially
thermosetting resins, which get hardened by heat.
● Combined with the sand they have very high strength and
resistance to heat.
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● The heated pattern melts and hardens the resin. It results
in bonding the sand grains closely together and forms a
shell around the pattern.
● After 20-30 s, the pattern and sand are inverted.
● The thickness of the shell can be controlled by the time of
contact.
● Then the shell mould is heated in an oven at 300 c for 15-
60 sec.
● When pouring the molten metal, the two halves are
clamped down together by clamps.
● After cooling and solidification, the shells are broken or
shaken away from the castings.
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Advantages:
● High accuracy castings.
● Good surface finish can be obtained.
● Complex parts can be made by this method.
● Less sand is used compared to other methods.
● Moulds can be stored for long time.
● Permeability of shell moulds is high.
● Limitation:
● Only small size castings can be made.
● Cost is more.
● More equipeents is needed for handling the shell mouldings.
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● The first step in this process is the preparation of the pattern.
● To do this, molten wax which is used as the pattern material
is injected under pressure of 2.5 Mpa into a metallic die
which has the cavity of the casting to be made.
● To make mould, the prepared pattern is dipped into a slurry
made by suspending fire ceramic materials in a liquid such as
ethyl silicate or sodium silicate.
● Thus a small shell is formed around wax pattern. The process
of dipping with ceramic slurries continued.
● Finally when a shell thickness reached 6 to 15 mm, the mould
is ready for further processing.
● The next step in the process is to remove the pattern from
the mould, which is done by heating the mould to melt the
pattern.
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● The melted wax is completely drained through the sprue.
● The mould are then pre heated to a temperature of 100 to
1000°c, depending on the size,complexity and the metal
of the casting.
● The molten metal is poured into the mould under gravity.
● https://youtu.be/TVsJlWEzZY8?t=20
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Advantages:
● Complex shapes can be made.
● Very close tolerance and better surface finish can be
produced.
● Castings produced by this process are ready for use with
little or no machining required.
● Very fine and thin sections can be produced by this
process.
● Limitations:
● The process is normally limited by the size and mass of the
casting.
● This is more expensive process because of larger manual
labour involved.
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Ceramic mould casting:
• Ceramic slurry is first prepared by mixing fine
grained refractory powders of zircon, alumina,
fused silica.
• Slurry is applied over the pattern to form thin
coating around it. Then it is Baked in a less
expensive fire clay.
• Pattern removed form the mould, and the mould
heated about 1000°C.
• Then molten metal is poured into the mould
cavity through the sprue to produce castings.
• https://youtu.be/CpFCaHMjlhA?t=404
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Pressure die casting:
• Most of the casting methods using disposable
moulds where it must be broken in order to
obtain the castings.
• In the die casting process, the mould called as
die is used for making casting which is
permanent.
• Molten metal is forced into the mould cavity
under high pressure.
• Used for casting of low melting temperature
materials.
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• In hot chamber die casting the melting
furnace is an integral part of the mould.
• Molten metal forced into the die by using
plunger.
• The die is immediately cooled by water and
sufficient cooling is provided for solidification.
• Then the movable die is moved some distance
and finished casting is removed by ejectors.
• The plunger and die are operated by hydraulic
systems.
• Operating pressure is 15MN/m².
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2. Cold chamber die casting:
• The metal melting unit is not an integral part of the machine.
The metal is melted and brought to the machine for pouring.
• Working same as hot chamber die casting.
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Application:
1. Household equipment such as washing machine parts,
vacuum cleaner body, fan case, etc.
2. Automobile parts – fuel pump, carburetor body, horn, wiper
and crank case.
3. Toys – electric trains, model aircraft etc.
Advantages:
1. Very accurate casting can be produced.
2. Good surface finish can obtain.
3. Rate of production is high.
4. Casting with varying thickness can be made.
5. Casting defects are less.
6. Die has long life – 75,000 castings
7. Sprue, runners and gates can be remelted.
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• Mould is generally made of two halves.
• They can clamp together.
• Permanent mould is made of heat resisting cast iron, alloy
steel, graphite or other suitable material.
• Pouring cap, sprue, gates and riser are made in this mould
itself.
• Mould preheated and refractory coating is done.
• This coating protects mould surfaces form sticking and very
useful for easy removal.
• Molten metal is fed into the mould with the help of
gravitational force.
• Almost all metals can be cast in this mould.
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• Centrifugal casting is primarily used for making hollow
castings such as pipe without using core, metal mould
is made to rotate.
• The metal is poured into the mould,then it is rotated by
an electric motor as well as it moves axially on the rails.
• Due to centrifugal force, the molten metal is thrown to
the walls of the mould.
• The outside of the mould is water cooled. So the
molten metal immediately solidifies.
• Centrifugal casting method is used for producing
cylindrical and symmetrical objects.
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• In this process, molten metal is poured from a laddle
continuously into a long vertical mould.
• Mould is made of copper, brass or graphite.
• The mould is water cooled. Hence the molten metal
immediately solidified. Solidified casting comes down
continuously.
• Saw or oxy acetylene flame is used to cut the casting and x
ray unit controls the pouring rate of molten metal.
• Lubricating oil is used to reduce friction between casting
and mould.
• Guide rolls keep on pulling the casting to match with
cooling rate.
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122. N Suresh ApMech
• Co2 Casting is a kind of sand casting process. In this process
the sand moulding mixture is hardened by blowing gas over
the mould.
• In addition, one can be sure of getting dimensionally accurate
castings with fine surface finish. But, this process is not
economical than green sand casting process.
• The process is basically a hardening process for moulds and
cores.
• The principle of working of the co2 process is based on the
fact that if co2 gas is passed through a sand mix containing
sodium silicate, the sand immediately becomes extremely
strongly bonded as the sodium silicate becomes a stiff gel.
• This gel is responsible for giving the necessary strength to the
mould.
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• When the packing is complete, co2 is forced into the
mould at a pressure of about 1.45 kgf/cm²(142 kn/m²) .
The gas is inert upto 15 to 30 seconds.
• The volume of co2 required can be calculated if the
quantity of sodium silicate present is known.
• For every 1 kg of sodium silicate, 0.50-0.75 kg of gas is
required.
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Advantages:
• Compared to other casting methods cores and moulds
are strong.
• Reduces fuel cost since gas is used instead of to other
costly heating generating elements. Operation is
speedy. Moulds and cores can be immediately after
processing.
• Great dimensional accuracy can be attained than other
moulding or core making process.
• Semi-skilled labour can be used.
• This process can be fully automated
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• Stir casting is a liquid state method of
composite materials fabrication in which a
dispersed phase is mixed with a molten matrix
metal by means of mechanical stirring.
• Solidification of the melt containing
suspended particles to obtain the desired
distribution of the dispersed phase in the cast
matrix.
• The addition of particles to the melt changes
its viscosity of the melt and it has implications
for casting processes.
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Advantages:
• Its advantages lie in its simplicity, flexibility and
applicability to large scale production.
• The liquid metallurgy technique is the most economical
of all available routes for metal matrix composite
production.
• This method allows very large size components to be
fabricated.
• The cost of preparing composite materials using a
casting method is about one third to one half of other
method. For high volume production possibility for
further reduction of cost.
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128. N Suresh ApMech
Cleaning of castings:
Cleaning operations are the operations required in sand casting
after the casting is removed from the mould.
Cleaning of castings refers to the removal of gates, risers and
sand. Also cleaning may involve machining or abrasive finishing
of the cast product.
The cleaning operations are
1. Removal of gates, riser and feeder:
In case of brittle material, the gates and feeder can be removed
by impact force.
Other processes that may be used to cut off the metallic parts
by band saws, grinding machine and cutting torches.
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2. Surface cleaning:
• There are several methods to remove the adhered sand
form the castings.
• Tumbling is done in a barrel like machine called tumbling
mill which helps to remove the sand by rubbing action of
the cast parts with each other.
• In shot blasting, abrasive particles are thrown on the
surface of the casting with a carrying medium. Air is most
common medium used in this process.
• Blasting processes include air blasting, centrifugal blasting
and hydro blasting etc.
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3. Finishing:
The finishing refers to the final cleaning.
Depending upon the requirement of the end
product final finish is provided by machining,
polishing, buffing, chemical treatment etc.
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Defects in sand casting
1. Shrinkage:
• It is depression on the casting surface.
Causes:
• Improper solidification
• incorrect pouring temperature.
• faulty gating, runner and riser systems.
Remedies:
• Provide proper solidification
• Pour at correct temperature
• Modify gating, runner and riser systems.
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2. Blow holes:
When the molten metal is poured, gases and
steam are formed. If these gases could not come out
below holes are formed.
Causes:
• Excess moisture in sand.
• Hard ramming
• Improper venting
• Excess binder.
Remedies:
• Control moisture
• Ram properly
• Provide sufficient vent hole.
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3. Scab:
it is the erosion or breaking down of the mould
and the recess filled with metal.
Causes:
• Uneven ramming.
• High velocity of pouring.
Remedies:
• Provide uniform ramming
• Pour at correct velocity
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Defects in sand casting:
Defects Causes remedies
4. Swell:
Swell is the deformation
of vertical mould surface
1. Soft ramming
2. Quick pouring
3. Mould is not properly
supported
1. Ram properly
2. Pour at correct
velocity
3. Provide support to the
mould.
5. Hard spots:
Some spots on the
surface become hard.
1. Rapid cooling
2. Pouring at low
temperature
1. Provide uniform
cooling
2. Pour at correct
temperature
6. Runout:
It is the leakage of metal
out of the mould while
pouring
1. Faulty moulding
2. Improper parting line.
3. Improper gating
system
1. Modify moulding
system
2. Provide proper parting
line.
3. Provide proper gating
system
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7. Honey combing:
Number of small
cavities present on the
casting surface
1. Soft ramming
2. Faulty gating
3. Faulty pouring
1. Provide correct
ramming
2. Provide correct
gating system
3. Pour at correct
temperature.
8. Cracks:
Small cracks appear on
the corner of the
casting
Due to sharp corners Provide taper or round
corners.
9. Shift:
Mismatching of casting
sections.
1. Worn out or bent
clamping pins in the
moulding box.
2. Loose dowels in
patterns
3. Misalignment of
moulding boxes.
4. Improper box
locating of core.
1. Repair or replace
the pins and dowel
pins.
2. Tighten dowels in
pattern.
3. Align the moulding
boxes properly.
4. Provide proper box
locating of core.
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10. Misrun or cold shut:
It is the incomplete
filling of the mould
cavity at one pouring.
1. Low pouring
temperature.
2. Too small gate.
3. Insufficient molten
metal in the ladle.
1. Pour at correct
temperature
2. Provide correct
gating
3. Pour sufficient
molten metal.
11. Fins:
It is a thin projection of
metal at the parting
line.
1. Incorrect assembly
of core and moulds.
2. Improper pouring
temperature.
3. Improper gating
system
1. Assemble the
moulds and cores
correctly.
2. Pour at correct
temperature
3. Provide correct
gating
12. dress:
Dress is lighter
impurities appear on
the top surface of a
casting.
Improper using of
strainer
Use strainer properly
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Cores
Core is a body made of sand which is used to
make a cavity or a hole in the casting. The
shape of the core is similar to the required
cavity in the casting to be made.
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