Casting of the metals

Casting (Foundry)
Unit-IV RME402
By
Akhileshwar Nirala
Mechanical Engineering
Galgotias College of Engineering
AkhileshwarNirala,GCETGreater
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1. Flask: A metal or wood frame which holds the sand mould. Depending upon
the position of the flask in the moulding structure, it is referred to by various
names such as;
> Cope – upper moulding flask,
> Check- middle part of the flast
> Drag – lower moulding flask,
2. Pattern: It is the replica of the final object to be made by casting process
with some modifications. Pattern is used to make the mould cavity.
3. Mould cavity (or mould): It is the impression in a mould produced by the
removal of the pattern. When filled with molten metal it forms a casting.
4. Pouring Basin: A small funnel-shaped cavity at the top of the mould into
which the molten metal is poured.
5. Sprue: The passage through which the molten metal, from the pouring basin,
reaches the mould cavity. In many cases it controls the flow of metal into the
mould.
6. Sprue well: This is a reservoir for the molten metal at the bottom of the
sprue. The molten metal, as it moves down in the sprue, gains in velocity. The
sprue well is provided to reduce the momentum or kinetic energy of the
flowing molten metal.

7. Runner: The horizontal channel which connects the Sprue to the gates is called
runner. Through runner molten metal is carried from the Sprue to the gate. It also
regulates the flow of molten metal before they reach the mould cavity.
8. Runner extension: It is used to collect the impurities like slag and dross
flowing with molten metal.
9. Gate: The actual entry point (end of the runner) through which molten metal
enters the mould cavity.

10. Riser: It is a reservoir of molten metal provided in the casting so that it can
feed molten metal in to the mould cavity when there is a reduction in volume
of metal due to solidification.
11.Vent: Vents are narrow opening which connect the cavity to the atmosphere.
The main function of vent is to allow gasses and the air in the cavity to escape
as the metal fills the mould.
12. Core: It is used for making hollow cavities in the casting. After the casting
is done, core is pulled away and usually broken off.
13. Core prints: For supporting the cores in the mould cavity an impression in
the form of a recess is made in the mould. This impression is known as core
print. The impression in the mould is made with the help of projection on
pattern.
14. Chaplet: Chaplets are metallic objects which are used to support the core
inside the mould cavity. Chaplets are used when core print cannot provide
sufficient support to hold the core at proper position.
15. Chill: Chills are cooled metallic objects, which are placed in the mould to
increase the cooling rate of casting. These are mainly used to achieve
directional solidification of the casting.
16. Choke: It is that part of the gating system which regulates the flow of metal
in to the mould cavity.

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Types of Pattern Allowances:
(A) Positive allowance
(1) Shrinkage or Contraction allowance
(2) Machining or Finishing allowance
(3) Draft or Tapper allowance
(4) Distortion or camber allowance
(B) Negative allowance
(1) Shake or rapping allowance

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3. Draft or Tapper allowance

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4. Distortion or camber allowance

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Usually during removal of the pattern from the mold cavity, the pattern is rapped
all around the faces, in order to facilitate easy removal. In this process, the final
cavity is enlarged. To compensate for this, the pattern dimensions need to be
reduced. There are no standard values for this allowance, as it is heavily
dependent on the personnel. This allowance is a NEGATIVE ALLOWANCE,
and a common way of going around this allowance is to increase the draft
allowance. Shaking of the pattern causes an enlargement of the mould cavity and
results in a bigger casting.
(B) Negative allowance
(1) Shake or rapping allowance

Moulding Sand
• In a foundry shop sand is the principal moulding material and is used for all
types of castings. Silica sand found in natural deposits is best suited for
moulding purpose because it can withstand high temperature without fusing.
Types of moulding sand: Foundry sand can be grouped as;
1. According to clay contents
> Natural sand
> Synthetic sand
2. According to use
> Facing sand
> Backing sand
> Core sand

Elements of a Gating System
The various elements that are connected with a gating system are
1. Pouring Basin
2. Sprue
3. Sprue base well
4. Runner & Runner extension
5. Gates or in-gate
6. Choke
7. Riser

Solidification of casting
• In a casting process, the material is first heated to completely melt and then
poured into a cavity of the mold. As soon as the molten metal is in the
mold, it begins to cool. When the temperature drops below the freezing
point (melting point) of the material, solidification starts. Solidification
involves a change of phase of the material and differs depending on
whether the material is a pure element or an alloy.
• A pure metal solidifies at a constant temperature, which is its melting point
(freezing point).
• For alloys, the solidification occurs over a temperature range depending
upon the composition. A typical cooling curve for Ni-Cu system is given in
Figure

• As temperature drops, solidification begins at a temperature indicated by
liquidus and is completed when the solidus is reached. It is also of interest
to examine the metallic grain formation and growth during the
solidification process. The metal which forms the initial skin has been
rapidly cooled by extraction of heat through the mold wall. This causes the
grains to be fine, equiaxed, and randomly oriented. As cooling continues,
further grain formation and growth occur in a direction away from the heat
transfer and dendrite structure generated. Characteristic grain structures of
in a casting of pure metal and in an alloy is provided in Figure 6.3.

Shrinkage during solidification
• The total shrinkage of a casting, from pouring temperature to room
temperature, takes place in three stages.
• Liquid Shrinkage: It is the shrinkage of molten metal from pouring
temperature to solidification temperature.
• Shrinkage due to change of phase: It is the shrinkage associated with
the change of phase from liquid to solid state (latent heat of fusion).
• Solid Shrinkage: It is the shrinkage of solid casting from solidification
temperature to room temperature.
• In metal casting riser is used to compensate liquid shrinkage as well as
shrinkage due to change in phase. To compensate solid shrinkage,
allowance is provided in pattern.
• Depending on the metal, the shrinkage from pouring to freezing
temperature (liquid shrinkage + shrinkage due change in phase) is varies
from 2.5 to 7.5%.

Types of Riser
Various types of riser used in metal casting are;
• Open Riser
• Blind Riser
• Top Riser
• Side Riser

• Top Riser: It is located on the
top of the casting. Because of
their location, top risers have
shorter feeding distance. It
also offers additional pressure
head on the molten metal in
the mould.
• Side Riser: It is located
adjacent to the mould cavity
and receives the molten metal
directly from the runner. It is
filled at last and contains the
hottest metal. Due to this, a
side riser is more effective
than a top riser.

Open Riser
• An open riser has its upper surface open to the atmosphere. The top
of the molten metal in the riser should not be allowed to freeze
before casting. This is because, atmospheric pressure needed to force
the molten metal inside the casting.
• An open riser should be cylindrical in order to have a minimum of
surface area in contact with the mould.
• Open riser gives lower casting yield. This is because, open riser are
larger in size than blind riser, therefore, a large amount of metal is
lost.

Blind Riser
• A blind riser is closed at its upper surface and completely
surrounded by the moulding sand. However, a vent at the top
of the riser may be provided to admit atmospheric pressure.
Advantages:
• A blind riser gives higher casting yield than open riser.
• Being surrounded by the moulding sand from all sides, the
metal in riser cools slowly.
• A blind riser can be removed more easily from a casting.
Disadvantage:
• A blind riser is difficult to mould than open riser.
• It cannot provide visual check to ensure filling up of mould
cavity.

Riser Design
• Design Requirements of Risers: The design requirement of risers
depends on a great extent upon the type of metal to be cast and the
complexity of the casting. The following are the basic requirements of a
riser.
• It should be solidify after the casting.
(V/A)2
Riser > (V/A)2
casting
• Its volume must be sufficient to compensate for metal shrinkage within the
casting.
• To achieve higher casting yield, size of the riser should be as small as
possible.

Methods for Riser Design:
The size of a riser can be estimated by using following
methods;
1. Chvorinov’s Rule:
2. Caine’s Method
3. Naval Research Laboratory Method
4. Modulus Method

1. Chvorinov’s Rule
• Based on heat transfer analysis, the amount of heat content is
proportional to the volume of the casting and the rate of heat transfer
depends on the surface area of the casting. According to this rule,
• The solidification time of a casting is proportional to the square of
the ratio of volume-to-surface area of the casting.
Solidification time ts = k (V/As)2
• Where V = volume of the casting, As = surface area of the casting
and k = mould contestant which depends on the pouring
temperature, mould and casting thermal characteristics.
• Minimum riser size: The minimum size of a riser can be calculated
from Chvorinov’s rule with assumption that the riser takes 25%
longer time to solidify than the casting. Thus;
(ts)riser = 1.25 (ts)casting
• (V/As)2
riser = 1.25 (V/As)2
casting

2. Caine’s Method:
• This method of calculating riser size is based on experimentally
determined hyperbolic relationship between freezing ratio and volume
ratio of the casting and riser.
• Freezing Ratio: Freezing ratio is defined as the ratio of cooling
characteristic of casting to the riser.
Freezing ratio X =
• Volume Ratio: It is defined as the ratio of the volume of the riser to the
casting.
• Volume ratio Y =
• Where A-surface area, V-volume

• The Caine’s relationship between freezing ratio and volume ratio is given
as:
X = {a/(y-b)} – c
• Where,
a = freezing characteristics constant for the metal
b = contraction ratio from liquid to solid
c = relative freezing rate of riser and casting
The above equation when plotted will be as shown in figure. The line shows
the locus of the points that separate the sound casting and the casting with
shrinkage.

3. Naval Research Laboratory Method
• This method is a simplification of Caine’s method. It defines a shape factor
to replace the freezing ratio. The shape factor is defined as;
Shape Factor = (L + W) / T
Where L = maximum length of the casting, W = maximum width of the
casting and T = maximum thickness of the casting.

4. Modulus Method:
• Modulus method is also used for finding the optimum riser
size. It is practically established that if the modulus of the riser
exceeds the modulus of the casting by a factor of 1.2, the
feeding during solidification would be satisfactory. Thus;
MRiser ≥ 1.2MCasting
• Modulus: Modulus is the inverse of cooling characteristics
(surface area / volume). So
Modulus M = Volume / Surface area

• In steel castings, it is generally preferable to choose a cylindrical riser with
a height-to-diameter ratio (H/D) of 1.
Volume of the riser V = (π/4) D2H = (π/4) D3 (since D =H)
For a top riser, the bottom end of the riser is in contact with the casting and
thus does not contribute to the calculation of surface area.
Surface area of the top riser SA = (π/4) D2 + πDH
= (π/4) D2 + πD2
The modulus of such a cylindrical riser MR would be;
MR = 0.2D
But for satisfactory feeding, MR = 1.2 MC
Or 0.2D = 1.2MC
Or D = 6MC

Gate
• These are the actual entry point (end of the runner) through which molten
metal enters the mould cavity. The shape and the cross section of the gate
should be such that it allows the metal to enter quickly into the mould
cavity.
Types of gate: The various types of gates are;
1. Top Gate (or vertical gate)
2. Bottom Gate (or horizontal gate)
3. Parting Gate
4. Step Gate
The selection of a proper gate is depends on the Casting material, casting
shape and size.

Top Gate (or vertical gate)
In top gating, the molten metal is poured vertically to fill the mould cavity.
Advantages:
• Since, first the molten metal entering the gate reaches the bottom and the hotter
metal is always at the top. Due to this, a favourable temperature gradient
towards the gate is achieved.
• The mould is filled very quickly.
• It gives higher casting yield.
Disadvantages:
• Since the metal is falls directly into the mould cavity through a height, it is
causes mould erosion.
• Since the molten metal falls through a height, when it strikes to the mould, it
creates splashing and turbulence. Because of splashing and turbulence, the
chances of slag and dross formation increases.

Applications:
• To gate is suitable only for small castings
which having simple shapes.
• It cannot be used for large and deep
castings. This is because, as the height of
top gate increases, the momentum of
molten metal increases which results in
mould erosion and slag formation.
• To gate is not used for the casting of Non-
Ferrous materials. This is because non-
ferrous metals forms slag more easily.
• To gate is used only for the casting of
Ferrous metals.

Bottom Gate (or horizontal gate)
It is made in the drag portion of the mould. The molten metal enters the mould
cavity from the bottom and rises slowly upward in the mould.
Advantages:
• It gives minimum turbulence and splashing, therefore, the chance of slag and
dross formation are reduces.
• It gives minimum mould erosion.
• It gives good surface finish to the castings.
Disadvantages:
• A bottom gate gives unfavourable temperature gradient because hotter metal is
at the bottom and cooler metal at the top of the mould.
• Due to unfavourable temperature gradients, it is difficult to achieve direction
solidification especially when the bottom gate has a riser at the top of the
casting.

• Bottom gates are best suited for large sized steel casting.

Parting Gate:
• Parting gate is a compromise between the top gate and bottom gate. The gate is
cut in the parting plane. In this a part of the casting is in cope reason while the
other part is in drag reason.
• For the mould cavity in Drag, it acts as a top gate.
• For the mould cavity in Cope, it acts as a bottom gate.
• Parting gate is the most widely used gate in sand casting.

Properties of moulding sand
The properties that are generally required in moulding sand are as follows;
1. Refractoriness
2. Permeability or porosity
3. Flowability or plasticity
4. Strength
> Green strength
> Dry strength
> Hot strength
5. Adhesiveness
6. Cohesiveness
7. Collapsibility
8. Chemical resistivity
9. Coefficient of expansion

• Refractoriness: It is the ability of a moulding material to withstand the high
temperature of the molten metal so that fusion does not occur.
• Permeability: It is the property of the sand which allows the gases and stream
to escape easily from the mould.
During pouring and subsequent solidification of a casting, a large amount of
gases and steam is generated. These gases are those that have been absorbed by
the metal during melting, air absorbed from the atmosphere and the steam
generated by the moulding and core sand. If these gases are not allowed to
escape from the mould, they would be entrapped inside the casting and cause
casting defects. To overcome this problem the moulding material must be
porous.

• Flowability: It refers to the ability of the sand to flow around and over the
pattern when the mould is rammed to give desired shape.
• Green Strength: The moulding sand that contains moisture is known as green
sand. The green sand should have enough strength so that the constructed
mould retains its shape.
• Dry Strength: It is the strength of dry sand. A mould should possess enough
dry strength to retain the exact shape of the mould cavity and to withstand
pressure of the molten metal.
• Hot strength: As soon as the moisture is eliminated, the sand would reach at a
high temperature when the metal in the mould is still in liquid state. At this
stage, the strength of the sand that is required to hold the shape of the cavity is
called hot strength.

• Adhesiveness: This is the property of sand to adhere to another body. Due
to this property the sand particles stick to the surface of the moulding box.
• Cohesiveness: It is defined as the property which holds the sand grains
together. Due to this property, the mould faces gets sufficient strength to
withstand the pressure of the molten metal.
• Collapsibility: The moulding sand should also have collapsibility so that
during the contraction of the solidified casting it does not provide any
resistance, which may result in cracks in the castings. Collapsibility
provides free contraction of the metal during solidification.
• Chemical resistivity: It is the property of the sand due to which it resists
any chemical reaction with the molten metal.
• Coefficient of expansion: The sand should have low coefficient of
thermal expansion.

Casting Defects
A casting defect is an irregularity in the metal casting that is undesired.
Some defects can be tolerated while others can be repaired using methods
such as welding and metallization. In casting process defects may occur
due to one or more of the following reasons:
• Fault in design of casting pattern
• Fault in design of mould and core
• Fault in design of gating system and riser
• Improper choice of moulding sand
• Improper metal composition
• Inadequate melting temperature and rate of pouring

Classification of casting defects: The casting defects can be classified into four main
categories:
1. Gas defects
> Blow holes & Open blows
> Porosity
> Pin hole
2. Moulding material defects
> Cut &Washes
> Metal penetration
> Fusion
> Rate tail & Buckles
> Shift or mismatch (Mould shift and core shift)
> Swell
> Drop
> Scar
> Scab
> Blister
3. Pouring metal defects
> Misruns
> Cold shuts
> Solid inclusions (sand & slag)
4. Metallurgical defects
> Hot tears
> Cold cracks
> Shrinkage cavities

1. Gas Defects:
Blow holes & Open blows: These are the spherical or elongated cavities
present inside or on the surface of the casting.
• When spherical or elongated cavity is present “on the surface” of the casting,
they are called open blows.
• When spherical or elongated cavity is present “inside the surface” of the
casting, they are called blow holes.
• These defects are caused by the moisture left in the mould and the core.
Because of the heat in the molten metal, the moisture is converted into steam;
part of which when entrapped in the casting results as blow holes or open
blows. Apart from the presence of moisture, they also occur due to lower
venting and lower permeability and hard ramming of the mould.

• Pin Hole Porosity: It occurs as small and long holes uniformly dispersed
throughout a casting.
• It is caused by the dissolved gases in the molten metal. As the molten metal
gets solidified, it loses temperature which decreases the solubility of gases,
thereby expelling the dissolved gases. The gases while leaving the solidifying
metal would cause very small (2 mm in diameter) and long holes showing the
path of escape.
• Pin hole: These are tinny blow holes occur just below the casting surface.

2. Moulding material defects
• Cut &Washes: It is the low projection of excess metal near the gate. This is
caused by the erosion of moulding sand due to the high velocity of the flowing
molten metal in bottom gating.
This defect can be eliminated by altering the gating design, by reducing
turbulence in the metal, by increasing the size of the gate or by using multiple
in-gates.

Metal penetration: If the mould surface is too rough and porous, the
liquid metal may enter between the sand particles up to a certain distance (in
the mould). This causes rough projections on the surface of the casting. This
defect is called penetration.
• This occurs because the sand is coarse or no mould wash was applied on the
surface of the mould. The coarser the sand grains more the metal penetration.

Fusion: This is caused by the fusion of the sand grains with the molten metal,
giving a brittle, glassy appearance on the casting surface. The main reason for
this is that the clay or the sand particles are of lower refractoriness or that the
pouring temperature is too high.
Rate tail & Buckles:
• Rate Tail: Rat tail is a long, shallow, angular depression. This is caused by
compression failure of the skin of the mould cavity, when the pouring
temperature is too high.
• Buckle: Buckle is a v-shaped depression on the surface of a flat casting. This is
caused by the expansion of the moulding sand, when the pouring temperature is
too high.

• Shift or Mismatch: The mould or core shift defect occurs when two halves
of a mould (cope and drag) or moulding boxes have not been properly
aligned.
• Swell: Under the influence of the metallostatic forces, the mould wall may
move back causing a swell in the dimensions of the casting. This defect is
found in vertical surfaces of a casting.
The main cause of this defect is faulty mould making procedure. A proper
ramming of the mould should correct this defect.

• Drop: An irregular projection on the cope surface of a casting is called a
drop. The dropping of loose moulding sand normally from the cope surface
into the mould cavity is responsible for this defect. This can be eliminated
by proper ramming of the cope surface.
• Scar: A shallow blow, usually found in flat casting surface, is called as
scar.
• Blister: A scar covered with a thin layer of metal is called blister. These are
due to improper permeability or venting.

• Scab: These are rough projections on the casting surface. A scab results
when a portion of the mould face lifts and liquid metal flows into the space
between the mould and the displaced sand.
• Dirt: Sometimes sand particles dropping out from the cope surface; get
embedded on the top surface of the casting. When removed, these sand
particles leave small angular holes, known as dirt.

3. Pouring Metal Defects
Misrun: Misrun is caused when the molten metal is unable to fill the
mould cavity completely and thus leaves unfilled cavity.
The reasons for this defect are:
• Lower fluidity of molten metal caused by low pouring temperature.
• When pouring rate is too slow.
• When the section thickness of the casting is very small.

• Cold Shut: A cold shut is caused when two streams while meeting in the
mould cavity, do not fuse together properly thus forming a discontinuity in
the casting.
The main reasons for this defect are lower pouring temperature, slow
pouring rate etc.
• Solid inclusions (sand & slag): Particles of slag, refractory materials sand
or de-oxidation products are trapped in the casting during pouring
solidification. The provision of choke in the gating system and the pouring
basin at the top of the mould can prevent this defect.

4. Metallurgical defects
• Hot tears: Hot tear is the crack in the casting caused by high residual
stresses. This defect occurs soon after the casting material start solidifying.
The main cause for is poor casting design.
• Cold cracks: These cracks occur due to presence of hydrogen and high
cooling stresses. Cold crack occurs below 430ºC.
• These cracks can be prevented by designing the casting so that it does not
develop too high cooling stresses and no chance be given to gases,
particular hydrogen, to get dissolved in molten metal.

• Shrinkage cavities: These are caused by liquid shrinkage occurring during
the solidification of the casting. To compensate for this, proper feeding of
liquid metal is required. For this reason risers are placed at the appropriate
places in the mould.

Special Casting Technique
• Die casting
• Centrifugal casting
• Investment casting
• Stir casting
• CO2 Casting
• Continuous casting

Die Casting
• Die casting is process in which molten metal is forced by external pressure into
a metal mould known as die. The molten metal kept under pressure during the
process of solidification.
• Because of high pressure involved in die casting, any narrow sections, complex
shapes and fine surface finish can easily be produced.
• The usual pressure employed is from 10 – 15 Mpa.
• Die casting is most suitable for non-ferrous metals with relatively low melting
points (around 800ºC), such as lead, zinc, aluminium, copper, magnesium etc.
• Casting metals with high melting points like steel, iron and other ferrous
metals, reduces die life. Thus ferrous metal can be cast to a limited extent.
Types of die casting process:
1. Cold Chamber Die Casting
2. Hot Chamber Die Casting

Cold Chamber Die Casting
• In this process, the metal is melted in a separate furnace. With the help of
ladle, molten metal is transported to the die casting machine and measured
quantity is fed into the unheated chamber. Then plunger forces the metal
into the die cavity and maintains the pressure till it solidifies. In the next
step, the die opens and the casting is ejected. At the same time, the plunger
returns to its original position completing the operation.
• High melting point alloys are normally cast by this method.
• The main disadvantage of cold chamber process is the longer cycle time
needed compared to hot chamber process. This is because the molten metal
must be transferred, from holding furnace to the shot chamber, for each
cycle. Also, since the metal is transferred into the machine, it may lose the
superheat and some time may cause defects such as cold shut.

Comparison between hot and cold chamberdie casting
Cold chamber die casting
• The metal is melted in a separate
furnace. With the help of ladle,
molten metal is transported to the
die casting machine (shot chamber).
• Shot chamber remains cold during
the process.
• Longer cycling time
• High melting point alloys of
aluminium, magnesium and
copper are normally cast by this
method.
Hot chamber die casting
• The molten metal is injected from the
same chamber in which it is melted.
There is no handling or transfer of
molten metal (shot chamber remains
submerged in the reservoir of molten
metal).
• Shot chamber remains hot during the
process.
• Fast cycling time
• The hot chamber die casting process
is used for most of the low melting
temperature alloys such as zinc, lead
and tin.

Advantages of die casting
• Large quantities of identical parts can be produced rapidly and
economically.
• It gives very good surface finish, therefore, little machining is required on
the parts produced.
• The parts having thin and complex shapes can be casted accurately and
easily.
• It produces almost defect free casting.
• Closer dimensional tolerance can be achieved.
• It gives better mechanical properties because of high pressure and rapid
cooling rate involved. Rapid cooling provides small grain size and good
strength to casting
• It is very economical for mass production.

Disadvantages of die casting
• The cost of equipment and die is high.
• The maximum size of casting is limited (4 – 15 kg).
• This is not suitable for all materials. It is best suited only for low melting
point non-ferrous metals. It is not suitable for steel, iron and other high
melting point alloys.
• The air in the die cavity gets trapped inside the casting, causing porous
casting.
• Maintenance cost is high.

Applications of die casting
• The typical products made by die casting are automobile parts like
carburettor bodies, speedometer, ignition system parts, chassis, tools etc.

Casting of the metals

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