CASTING

NAME- ARNAB HAZRA

ROLL NO.- 34900721009
SUBJECT- MANUFACTURING PROCESSES (PC-ME302)

DEPARTMENT OF MECHANICAL ENGINEERING

2nd YEAR
COOCHBEHAR GOVERNMENT ENGINEERING COLLEGE
 MANUFACTURING PROCESS
DEFINITION: The term manufacturing refers to the processing of raw materials or parts into
finished goods through the use of tools, human labor, machinery, and chemical processing.
The word ‘MANUFACTURING’ comes from latin word ‘MANUFACTUS’, which means ‘MADE
BY HANDS’.
There are three types of manufacturing processes-
1. Zero process:- In this process the mass of the object remains same. Ex.- Casting, Metal
forming.

2. Positive process:- In this process the mass of object is increased. Ex.- Welding, Soldering.

3. Negative process:- In this process the mass of object is increased. Ex.- Machining
operation.
 CASTING PROCESS
• Casting process is one of the earliest metal shaping techniques known to human
being. It means pouring molten metal into a refractory mold cavity and allows it
to solidify. The solidified object is taken out from the mold either by breaking or
taking the mold apart. The solidified object is called casting and the technique
followed in method is known as casting process.
• Foundry engineering deals with the process of making in molds prepared by
patterns. The whole process of producing castings may be classified into five
stages-
1. Pattern making
2. Molding & Core making
3. Melting & Casting
4. Fettling
5. Testing & Inspection
 PATTERN
 The first step in casting is pattern making. A pattern is a model or the replica of
the object (to be casted) with some allowances. It is embedded in molding sand
and suitable ramming of molding sand around the pattern is made. The pattern is
then withdrawn for generating cavity (known as mold) in molding sand. Thus it is
a mold forming tool. It may be defined as a model or form around which sand is
packed to give rise to a cavity known as mold cavity in which when molten metal
is poured, the result is the cast object. When this cavity (mold) is filled with
molten metal, molten metal solidifies and produces a casting (product).
 OBJECTIVES OF A PATTERN:
I. Pattern prepares a mould cavity for the purpose of making a casting.
II. Pattern possesses core prints which produces seats in form of extra
recess for core placement in the mould.
III. Properly constructed patterns minimize overall cost of the casting.
IV. Properly constructed patterns minimize overall cost of the casting.
COMMON PATTERN MATERIALS
1. Wood: Wood is the most popular and commonly used material for pattern
making. It is cheap, easily available in abundance, repairable and easily
fabricated in various forms using resin and glues. It is very light and can produce
highly smooth surface. Wood can preserve its surface by application of a shellac
coating for longer life of the pattern. But, in spite of its above qualities, it is
susceptible to shrinkage and warpage and its life is short because of the reasons
that it is highly affected by moisture of the molding sand. After some use it warps
and wears out quickly as it is having less resistance to sand abrasion. It can not
withstand rough handily and is weak in comparison to metal. In the light of above
qualities, wooden patterns are preferred only when the numbers of castings to be
produced are less. The main varieties of woods used in pattern-making are
shisham, kail, deodar, teak and mahogany.
o Advantages of wooden patterns:
a) Wood can be easily worked. It is light in weight.
b) It is easily available,very cheap and easy to join.
c) It is easy to obtain good surface finish.
d) It can be easily repaired.
o Disadvantages of wooden patterns:
a) It is susceptible to moisture & tends to warp.
b) It wears out quickly due to sand abrasion.
c) It is weaker than metallic patterns.

2. Metal: Metallic patterns are preferred when the number of castings

required is large enough to justify their use. These patterns are not much
affected by moisture as wooden pattern. The wear and tear of this
pattern is very less and hence posses longer life. Moreover, metal is easier
to shape the pattern with good precision, surface finish and intricacy in
shapes. It can withstand against corrosion and handling for longer period.
It possesses excellent strength to weight ratio. The main disadvantages of
metallic patterns are higher cost, higher weight and tendency of rusting.
It is preferred for production of castings in large quantities with same
pattern. The metals commonly used for pattern making are cast iron,
brass and bronzes and aluminum alloys. Aluminum alloy patterns are more
popular and best among all the metallic patterns because of their high
light ness, good surface finish, low melting point and good strength. They
also possesses good resistance to corrosion and abrasion by sand and
there by enhancing longer life of pattern.
3. Plastic:
 Plastics are getting more popularity now a days because the patterns
made of these materials are lighter, stronger, moisture and wear
resistant, non sticky to molding sand, durable and they are not affected
by the moisture of the molding sand. Moreover they impart very smooth
surface finish on the pattern surface. These materials are somewhat
fragile, less resistant to sudden loading and their section may need metal
reinforcement. The plastics used for this purpose are thermosetting
resins. Phenolic resin plastics are commonly used.This Plastic material has
a very low solid shrinkage.

4. Plaster:
 This material belongs to gypsum family which can be easily cast and
worked with wooden tools and preferable for producing highly intricate
casting. The main advantages of plaster are that it has high compressive
strength and is of high expansion setting type which compensate for the
shrinkage allowance of the casting metal. It is also preferred for
production of small size intricate castings and making core boxes.
5. Wax:
 Patterns made from wax are excellent for Investment casting
process. The materials used are blends of several types of
waxes, and other additives which act as polymerizing agents,
stabilizers, etc. The commonly used waxes are paraffin wax,
shellac wax, bees-wax, cerasin wax, and micro-crystalline
wax. The properties desired in a good wax pattern include
low ash content up to 0.05 per cent, resistant to the primary
coat material used for investment, high tensile strength and
hardness, and substantial weld strength. Such patterns are
generally used in the process of investment casting where
accuracy is linked with intricacy of the cast object.
Casting
 TYPES OF PATTERN
 One piece or solid pattern
 Two piece or split pattern
 Loose piece pattern
 Match plate pattern
 Follow board pattern
 Gated pattern
 Sweep pattern
1. Single-piece or solid pattern:
 Solid pattern is made
of single piece without
joints, partings lines or
loose pieces. It is the
simplest form of the
pattern.
 Used for large size
simple castings.
 It is inexpensive
pattern.
2. Two-piece or split pattern
 When solid pattern is
difficult for withdrawal
from the mold cavity, then
solid pattern is splited in
two parts. Split pattern is
made in two pieces which
are joined at the parting
line by means of dowel pins.
The splitting at the parting
line is done to facilitate the
withdrawal of the pattern.
3. Loose-piece Pattern
 Loose piece pattern is used when
pattern is difficult for withdrawl
from the mould. Loose pieces are
provided on the pattern and they
are the part of pattern. The main
pattern is removed first leaving
the loose piece portion of the
pattern in the mould. Finally the
loose piece is withdrawal
separately leaving the intricate
mould.
4. Match plate pattern
 This pattern is made in two
halves and is on mounted on
the opposite sides of a
wooden or metallic plate,
known as match plate. The
gates and runners are also
attached to the plate. This
pattern is used in machine
molding
 By using this we can
eliminate mismatch of cope
& drag cavities.
5. Follow board pattern
 When the use of solid or
split patterns becomes
difficult, a contour
corresponding to the exact
shape of one half of the
pattern is made in a
wooden board, which is
called a follow board and it
acts as a molding board for
the first molding operation
as shown in Fig.
 Used for structurally weak
patterns.
6. Gated pattern
 In the mass production of
casings, multi cavity molds
are used. Such molds are
formed by joining a number
of patterns and gates and
providing a common runner
for the molten metal, as
shown in Figure.These
patterns are made of
metals, and metallic pieces
to form gates and runners
are attached to the
pattern.
7. Sweep pattern
 Sweep patterns are used for
forming large circular molds of
symmetric kind by revolving a
sweep attached to a spindle as
shown in Fig. Actually a sweep
is a template of wood or metal
and is attached to the spindle at
one edge and the other edge
has a contour depending upon
the desired shape of the mold.
 Sweep patterns avoids the
necessity of making a
full,large circular & costly
three dimensional pattern.
 PATTERN ALLOWANCES
 1. Shrinkage Allowance
In practice it is found that all common cast metals shrink a
significant amount when they are cooled from the molten state. The
total contraction in volume is divided into the following parts-
Liquid contraction, i.e. the contraction during the period in which
the temperature of the liquid metal or alloy falls from the pouring
temperature to the liquidus temperature.
Contraction on cooling from the liquidus to the solidus temperature,
i.e. solidifying contraction.
Contraction that results there after until the temperature reaches
the room temperature. This is known as solid contraction.
 The first two of the above are taken care of by proper gating and
Risering. Only the last one, i.e. the solid contraction is taken care
by the pattern makers by giving a positive shrinkage allowance.
This contraction allowance is different for different metals.
2. Machining Allowance
 It is a positive allowance given to compensate for the amount of
material that is lost in machining or finishing the casting. If this
allowance is not given, the casting will become undersize after
machining. The amount of this allowance depends on the size of
casting, methods of machining and the degree of finish.
3. Rapping or Shake Allowance
 Before withdrawing the pattern it is rapped and thereby the size
of the mould cavity increases. Actually by rapping, the external
sections move outwards increasing the size and internal sections
move inwards decreasing the size. This movement may be
insignificant in the case of small and medium size castings, but it
is significant in the case of large castings. This allowance is kept
negative and hence the pattern is made slightly smaller in
dimensions.
4. Draft or Taper Allowance
 When a pattern is drawn from a
mold, there is always some
possibility of injuring the edges of
the mold. This danger is greatly
decreased if the vertical surfaces
of a pattern are tapered inward
slightly. This slight taper inward
on the vertical surfaces of a
pattern is known as the draft.
Draft may be expressed in
millimeter per meter on a side, or
in degrees. It is also a positive
allowance.
5. Distortion Allowance
 This allowance is applied to the
castings which have the tendency
to distort during cooling due to
thermal stresses developed. For
example a casting in the form of
U shape will contract at the
closed end on cooling, while the
open end will remain fixed in
position. Therefore, to avoid the
distortion, the legs of U pattern
must converge slightly so that the
sides will remain parallel after
cooling. It is a Zero allowance.
Difference Between Casting and Molding:
 A mold is an impression of an object or organism. It precedes a cast which
is a final step in making a replica of the object or the organism. The mold
is used to capture the entire details of the object before a cast is made. A
mold can also be synthetically made where a hollow block or cavity is
filled with a pliable material. The process of making a mold is
called molding, which is basically the process of creating an impression of
any object to be filled with an assortment of materials. Usually, the
material can be spread or poured into or onto the particular molded
object to allow it to set or harden. There are many materials that are
used to make molds, and these depend on the desired shape and the size
of the object.
 A cast is the final step the follows a mold. No cast can be made in the
absence of a mold. Before a cast is made, the casting material is spread
or poured into the mold of the organism or object to make a final 3D
imprint. In engineering, a metal is heated to extremely high temperatures
until it turns into a liquid. This liquid can be poured into a mold to make
any required product of a desired shape and size. That is how jewelries
and equipment are made in metalworking. The liquid will be left to
harden before a cast can be made.
 CASTING DEFECTS
 It is an unwanted irregularities that appear in the casting during metal
casting process. There is various reason or sources which is responsible for
the defects in the cast metal. Here in this section we will discuss all the
major types of casting defects. Some of the defects produced may be
neglected or tolerated and some are not acceptable, it must be eliminated
for better functioning of the parts.

1. Shift or Mismatch: The defect caused due to misalignment of upper and

lower part of the casting and misplacement of the core at parting line.
Remedies: (i) Proper alignment of the pattern or die part, molding boxes.
(ii) Correct mountings of pattern on pattern plates.
(iii) Check the alignment of flask.
2. Swell: It is the enlargement of the mold cavity because of the molten
metal pressure, which results in localised or overall enlargement of the
casting.
Remedies: (i) The sand should be rammed properly and evenly.
 CASTING DEFECTS(cont.)
3. Blowholes: When gases entrapped on the surface of the casting
due to solidifying metal, a rounded or oval cavity is formed called
as blowholes.
Causes: (i) Excessive moisture in the sand.
(ii) Low Permeability of the sand.
(iii) Sand grains are too fine.
(iv) Too hard rammed sand.
(v) Insufficient venting is provided.
Remedies: (i) The moisture content in the sand must be controlled
and kept at desired level.
(ii) High permeability sand should be used.
(iii) Sand of appropriate grain size should be used.
(iv) Sufficient ramming should be done.
(v) Adequate venting facility should be provided.
 CASTING DEFECTS(cont.)
4. Drop: Drop defect occurs when there is cracking on the upper surface of
the sand and sand pieces fall into the molten metal.

Causes: (i) Soft ramming and low strength of sand.

(ii) Insufficient fluxing of molten metal. Fluxing means addition of
a substance in molten metal to remove impurities. After fluxing the
impurities from the molten metal can be easily removed.
(iii) Insufficient reinforcement of sand projections in the cope.

Remedies: (i) Sand of high strength should be used with proper ramming
(neither too hard nor soft).
(ii) There should be proper fluxing of molten metal, so the
impurities present in molten metal is removed easily before pouring it into
the mold.
(iii) Sufficient reinforcement of the sand projections in the cope.
 CASTING DEFECTS(cont.)
5. Metal Penetration: These casting defects appear as an
uneven and rough surface of the casting. When the size of
sand grains is larges, the molten fuses into the sand and
solidifies giving us metal penetration defect.

Causes: It is caused due to low strength, large grain size, high

permeability and soft ramming of sand. Because of this the
molten metal penetrates in the molding sand and we get
rough or uneven casting surface.

Remedies: This defect can be eliminated by using high

strength, small grain size, low permeability and soft
ramming of sand.
 CASTING DEFECTS(cont.)
6. Pinholes: They are very small holes of about 2 mm in size
which appears on the surface of the casting. This defect
happens because of the dissolution of the hydrogen gases in
the molten metal. When the molten metal is poured in the
mold cavity and as it starts to solidify, the solubility of the
hydrogen gas decreases and it starts escaping out the
molten metal leaves behind small number of holes called as
pinholes.

Remedies: (i) By reducing the moisture content of the molding

sand.
(ii) Good fluxing and melting practices should be
used.
(iii) Increasing permeability of the sand.
(iv) By doing rapid rate of solidification
 CASTING DEFECTS(cont.)
7. Shrinkage Cavity: The formation of cavity in the casting due
to volumetric contraction(liquid & solidification
shrinkage) is called as shrinkage cavity.

Causes: (i) Uneven or uncontrolled solidification of molten

metal.
(ii) Pouring temperature is too high.

Remedies: (i) This defect can be removed by applying

principle of directional solidification in mold design.
(ii) Wise use of chills (a chill is an object which is
used to promote solidification in a specific portion of a
metal casting) and padding.
 CASTING DEFECTS(cont.)
8. Cold Shut: It is a type of surface defects and a line on the
surface can be seen. When the molten metal enters into the
mold from two gates and when these two streams of molten
metal meet at a junction with low temperatures than they do
not fuse with each other and solidifies creating a cold shut
(appear as line on the casting).
Causes: (i) Poor gating system
(ii) Low melting temperature
(iii) Lack of fluidity

Remedies: (i) Improved gating system.

(ii) Proper pouring temperature.
 CASTING DEFECTS(cont.)
9. Misrun: When the molten metal solidifies before
completely filling the mold cavity and leaves a space in the
mold called as misrun.

Causes: (i) Low fluidity of the molten metal.

(ii) Low temperature of the molten metal which
decreases its fluidity.
(iii) Too thin section and improper gating system.

Remedies: (i) Increasing the pouring temperature of the

molten metal increases the fluidity.
(ii) Proper gating system
(iii) Too thin section is avoided
 CASTING DEFECTS(cont.)
10. Slag Inclusion: This defect is caused when the molten metal containing
slag particles is poured in the mold cavity and it gets solidifies.

Remedies: Remove slag particles form the molten metal before pouring it into
the mold cavity.

11. Hot Tears or Hot Cracks: when the metal is hot it is weak and the residual stress
(tensile) in the material cause the casting fails as the molten metal cools down. The
failure of casting in this case is looks like cracks and called as hot tears or hot
cracking.

Remedies: (i) Proper mold design can easily eliminate these types of casting defects.
(ii) Elimination of residual stress from the material of the casting
 CASTING DEFECTS(cont.)
15. Honeycombing or Spongines: It is an external defect in which
there is a number of small cavities in close proximity present in
the metal casting.

Causes: (i) It is caused due to dirt and scurf held mechanically in

the suspension of the molten metal.
(ii) Due to imperfect skimming in the ladle.

Remedies: (i) Prevent the entry of dirt and scurf in the molten
metal.
(ii) Prevent sand wash.
(iii) Remove slag materials from the molten metal by
proper skimming in the ladle.
CASTING DEFECTS(cont.)
 DIFFERENT CASTING PROCEDURE:

1. SAND CASTING PROCESS.

2. GRAVITY DIE PROCESS
3. PRESSURE DIE CASTING
4. INVESTMENT CASTING
5. PLASTER CASTING
6. CENTRIFUGAL CASTING
7. LOST-FOAM CASTING
8. VACUUM CASTING
9. SQUEEZING CASTING
10. CONTINUOUS CASTING
11. SHELL MOLDING.
 SAND CASTING PROCESS
Sand casting is a versatile casting process that can be
used to cast any metal alloy, ferrous or non-ferrous.
It is widely used for mass production in industrial units,
such as automotive metal casting parts, like engine
blocks, cylinder heads, crankshafts, etc.The process
uses a mold that’s made of silica-based materials,
such as naturally-bonded or synthetic sand form the
smooth mold surface. The mold surface has 2 parts,
cope (the upper half) and drag (the lower half).
Molten metal is poured into the pattern using a pouring cup and left to solidify to
take the final shape. Finally, trimming off extra metal is done for the finishing of the
final metal casting product

USES:- •air compressor pistons.

•bearings.
•blowers & impellers.
•bushings.
 GRAVITY DIE CASTING

Gravity die casting, also typically known as permanent mold casting, uses reusable molds made of metal,
like steel, graphite etc. to fabricate metal and metal alloys. This type of metal casting can manufacture
various parts like gears, gear housing, pipe fittings, wheels, engine pistons, etc.
In this process, the direct pouring of molten metal into the mold cavity takes place under the effect
of gravity. For better coverage, the die can be tilted to control the filling. The molten metal is then allowed
to cool and solidifies within the mold to form products. As a result, this process makes casting of materials
like lead, zinc, aluminum, and magnesium alloys, certain bronzes, and cast iron more common.
This casting process works on the bottom-up approach to fill the mold, in contrast, to other
pressure casting processes. Although, the process has a higher casting rate than sand casting. But, it is
relatively higher in cost due to expensive metal molds. Check its advantages and disadvantages below.
USES:- turbos, brake calipers, knuckles, engine cylinder heads, engine blocks and pistons.
 PRESSURE DIE CASTING
There are two types of pressure die casting depending on the
pressure. Namely, the low-pressure die casting and the high
-pressure die casting. High pressure die casting is more popular
for mass production of complex geometries that require high
precision. Whereas, for large and simple parts production,
low-pressure die casting is a preferable casting process.
In this type of metal casting, non-ferrous metals and
alloys such as zinc, tin, copper and aluminum are injected into
a reusable mold coated with a lubricant at high pressure. Thus, high pressure is maintained throughout the
rapid injection process to avoid metal hardening. Finally, after completion of the process, extraction of
casting and finishing takes place to remove any excess material.
Pressure die casting differs from gravity die casting in the process of filling up the mold. Where
pressure die casting uses high-pressure injection of molten metal into the metal mold. The molten metal is
then solidified rapidly to produce the required product.
USES:- engine blocks, gearbox casings, oil sumps, engine mounts and structural parts like cross-car
beams.
 INVESTMENT CASTING

Investment casting, also known as lost-wax casting,

is a process that invests wax pattern with refractory
material and a binding agent to shape a disposable
ceramic mold, and then molten metal is poured into
the mold to make metal castings. Investment
casting is an expensive and labor-intensive process
and can be used for mass production or in complex
castings to produce metal casting products ranging
from gears, bicycle trunks, moto discs, and spare
parts in blasting machines.
aerospace and power generation industries to produce turbine blades with complex
USES:- shapes or cooling systems.
 PLASTER CASTING
The plaster casting process is similar to sand casting,
except that the mold is made of a mixture called
‘Plaster of Paris’. Due to the low thermal conductivity
and heat capacity of plaster, it cools the metal more
slowly as compared to sand, which helps in attaining
high accuracy, especially for thin cross-section parts.
However, it is not suitable for high-temperature ferrous
materials.
Particularly, small castings of 30 grams, as well
as large castings of 45 kilograms, can be manufactured
through the plaster mold casting process. For example,
the image below shows a cast component of a section
thickness of 1 mm made by this process.
USES:- to make lock components, gears, valves, ornaments, fittings, and toolings.
 CENTRIFUGAL CASTING
Centrifugal casting, also known as roto casting, is a process for industrially
manufacturing cylindrical parts with centrifugal forces. This type of metal
casting uses a preheated spinning die in which the molten metal is poured.
The centrifugal forces help to distribute the molten metal within the die at
high pressure.
Three variations of centrifugal casting exist; true centrifugal
casting process, semi centrifugal casting process, and vertical centrifugal
casting process. Semi centrifugal casting, unlike true centrifugal casting,
has the mold completely filled using a sprue. However, in true centrifugal
casting, molten metal sticks to the sides due to continuous rotation.
On the contrary, vertical centrifugal casting, as the name suggests,
uses directional molding following the same process as true centrifugal casting.
Typically, centrifugal casting produces rotational shapes like cylinders. In particular, parts like bush bearings,
clutch plates, piston rings, and cylinder liners, etc. Also, the pouring of metal in the center of the mold helps in reducing
defects such as blow holes, shrinkage, and gas pockets. However, it is not suitable for all kinds of metal alloys.

USES:- jet engine compressor cases, petrochemical furnace tubes, many military and defense
components, and other applications requiring high reliability
 LOST-FOAM CASTING
Lost-foam casting method is similar to investment casting with
the difference that it uses foam for the pattern instead of wax.
Once the pattern is formed, coating with a refractory ceramic
takes place by dipping, coating, spraying, or brushing. Then,
the molten metal is poured into the mold to form the desired
product.
This technique can be used for various materials like alloy
steel, carbon steel, alloy cast iron, ferrous alloy, etc. In
particular, products like pump housing, fire hydrants, valves,
and fittings are made through the lost-foam casting
process.Lost foam casting possesses numerous advantages
which include high precision and high accuracy production.
However, there are a few limitations of this process that makes
it unsuitable for low-volume applications.

USES:- to create complex metal pieces and parts in which molten metal evaporates a foam mold being
held still with sand.
 VACUUM CASTING
As the name suggests, is the type of casting where
production occurs under vacuum pressure of 100 bar or less
to exhaust gas from the mold cavity. In this process, molten
metal is poured into the mold cavity inside a vacuum
chamber in order to eliminate bubbles and air pockets. The
vacuum evacuation of the die cavity reduces the entrapment
of gases within the cavity during the metal injection process.
Finally, the metal is cured in a heating chamber and removed
from the mold.
The vacuum die casting process is popular in
various industries including automobiles, aerospace,
electronics, marine, telecommunication, etc. As a result,
some components fabricated by this manufacturing process
include structural chassis components and automotive body
parts.
USES:- for the production of small series of functional plastic parts
 SQUEEZING CASTING
Liquid forging or squeeze casting is a hybrid metal forming
process that merges permanent mold casting and die forging
in a single step. In this process, a specific amount of molten
metal alloy is injected into a die, and pressure is applied to
shape it. Then, the metal part is heated over melting
temperature and extracted from the die.
Particularly, squeezing die casting is a potential
casting process for safety-critical parts in automotive systems.
For example, space frame joints, aluminum front steering
knuckles, chassis frames, brackets, or nodes.This type of
metal casting combines the benefits of casting and forging
processes. For example, the high pressure applied during solidification helps prevent shrinkage
and porosities. However, due to specific tooling requirements, it is not as popular as other casting
processes for mass production.
USES:-
 CONTINUOUS CASTING
As the name suggests, it allows consistent mass production of
metal profiles with a constant cross-section. This type of
casting is popular in the production of steel bars. Also, the
vertical cast creates semi-continuous casting like billets,
ingots, bars, etc.
In this process, molten metal is poured at
a calculated rate in a water-cooled, open-ended mold that
allows a surface of solid metal to form on the liquid metal
in the center. Metal solidification, thus, happens from the
outside in. After this process finishes, strands of metal can
be continuously extracted from the mold. Predetermined lengths of products can be cut off by
mechanical shears or traveling oxyacetylene torches.Generally, the products created using continuous
casting are homogeneous, consistent, and dense. However, it also limits its use to such applications.
Some other advantages and disadvantages are:

USES:- allows metals and metal alloys to be shaped then solidified without interruption
 SHELL MOLDING

Shell molding is an expendable mold casting process. It is similar to the sand

casting process except for that the hardened shell of sand forms the mold
cavity instead of a flask of sand. The sand used is finer than that utilized
in sand casting and it is mixed with a resin so that it can be heat treated
and hardened into the shell around the pattern
Shell molding makes industrial products, for example,
gearbox housing, connecting rod, small-size boats, truck hoods, cylindrical
heads, camshafts, valve bodies, etc.The casting products possess a good
surface finish and dimensional accuracy. However, the limitation of this kind of casting includes the size
and weight limitation of the parts. Some other advantages and disadvantages are:

USES:- for both ferrous and non-ferrous metals.

Topics covered ahead:

 Introduction to solidification
 Concept of solidification on casting
 Solidification of pure metals
 Nucleation And Growth
Introduction to solidification:

 The process of transformation of a substance from

liquid to solid state in which the crystal lattice forms
and crystals appear. Solidification mechanism is essential
for preventing defects due to shrinkage.
 In Pure metals and eutectic alloys solidification takes
place at constant temperature, but in solid solution
alloys proceeds over a temperature range.
Concept of solidification on casting….

 Solidification mechanism is essential for preventing

defects due to shrinkage.As soon as the molten metal is
poured in a sand mold, the process of Solidification starts.
During solidification, cast forms develops cohesion and
acquires structural Characteristics.
 The mode of solidification affects the properties of the
casting acquires a metallographic structure which is
determined during solidification. Themetallographic
structure consists of:
1. Grain size, shape and orientation
2. Distribution of alloying elements
3. Underlying crystal structure and its imperfections
4. Liquid contraction (shrinkage): liquid contraction occurs
when the metal is in liquid state.
5. Liquid contraction (shrinkage)
6. Solidification contraction (shrinkage)
7. Solid contraction (shrinkage)
Neucleation and dendritic growth…

A metal in molten condition possesses high energy . As the molten metal

cools, it loses energy to form crystals . Since heat loss is more rapid near
the mold walls than any other place, the first metal crystallites called
‘nuclei’ form here.Nuclei formed as above tend to grow at the second
stage of solidification. The crystal growth occurs in a dendrite
manner.Dendrite growth takes place by the evolution of small arms on
the original branches of individual dendrites
1. Slow cooling makes the dendrites to grow long whereas fast cooling
causes short dendrite growth.
2. Since eventually dendrites become grains, slow cooling results in large
grain structure and fast cooling in small grain structure in the solidified
metal.
Solidification of pure metals:

Pure metals generally posses

 Excellent thermal and electrical conductivity(e.g. Cu and Al).
 Higher ductility, higher melting point, lower yield point and tensile strength, and
 Better corrosion resistance, as compared to alloys.
As metals posses high melting points, they exhibit certain difficulties in
Casting
 Difficulties during pouring
 Occurrence of several metal-mold reactions
 Greater tendency toward cracking
 Their mode of solidification, which may produce defective castings.Above freezing
point the metal is liquid and below freezing point, it is in solid.
From the cooling curve(shown in prev. slide )the following
observations can be made:
 Liquid metals cools from A to B
 From B to C, the melt liberates latent heat of fusion; temperature
remains constant.
 The liquid metal starts solidifying at B and it is partly solid at any point
between B and C and at C metal is purely solid.
 From C to D, the solid metal cools and tends to reach room
temperature.
 The slopes of AB and CD depend upon the specific heats of liquid and
solid metals respectively.
Note
Solidification time:
 Whether the casting is pure metal or alloy, solidification takes time. The total
solidification time is the time required for the casting to solidify after pouring. This
time is dependent on the size and shape of the casting by an empirical relationship
known as Chvorinov’s rule, which states that freezing time Is proportional to (V/A)²
ratio , where (V/A) is the ratio of the volume of the casting to its surface area.

Where ,

TTS=total solidification time, min;

V=volume of the casting, cm³
A=surface area of the casting, cm³
N=is an exponent usually taken to have a value= 2
Cm is the mold constant
 DESIGN OF GATING SYSTEM :

 Introduction : The main function of gating system is to lead

clean molten metal from ladle to the casting cavity ensuring
smooth, uniform and complete filling.
The flow of molten metal after being poured is a transient
phenomena accompanied by :
 Turbulence,
 Separation of the flow from the boundaries,
 Dividing and combined flow at the junction,
 Simultaneous heat transfer during the flow and onset of
solidification.
BASIC COMPONENTS OF A GATING
SYSTEM :
 Pouring Basin.
 Sprue.
 Sprue well.
 Runner.
 Ingate.
BASIC COMPONENTS OF A GATING
SYSTEM(Contd.)
POURING BASIN.
It is the funnel-shaped opening, made at the top of the mould.
The main purpose of the pouring basin is to direct the flow of
molten metal from ladle to the sprue.

SPRUE WELL.
It is located at the base of the sprue. It arrests the free fall of
molten metal through the sprue and turns it by a right angle
towards the runner and also acts as a reservoir for the
runners and gates as they fill.
BASIC COMPONENTS OF A GATING
SYSTEM(Contd.)
SPRUE.
• Connects the pouring basin to
the runner or ingate.
• Generally tapered downward to
avoid aspiration of air.
• The round sprue has a minimal
surface area exposed to cooling
and offers the lowest resistance
to the flow of metal.
• The square or rectangular sprue
minimizes the air aspiration and
turbulence.
BASIC COMPONENTS OF A GATING
SYSTEM(Contd.)
RUNNER :
In case of large casting, the
fluidity length of the molten
metal is less than the maximum
distance required to be travelled
by the molten metal along the
flow path. So it is necessary to
provide the multiple ingates to
reduce the maximum flow
distance needed to be travelled
by the molten metal.
BASIC COMPONENTS OF A GATING
SYSTEM(Contd.)
GATE(INGATES)
It is a small passage which connects the runner to the
mould cavity. The cross section is square, rectangular and
trapezoidal.
GUIDELINES TO DESIGN A GATING
SYSTEM :
 The size of the sprue fixes the flow rate.
 The sprue should be located at certain distance from the
gates so as to minimum velocity of molten metal at
ingates.
 Rectangular cross-section sprue is better than circular
one with the same cross-sectional area, since critical
velocity for turbulence is much less for circular sections.
 In addition, vertex formation tendency in a sprue with
circular cross section is higher.
GUIDELINES TO DESIGN A GATING
SYSTEM(Contd.):
 Sprue should be tapered by approximately 5% to
avoid aspiration of the air and free fall of the metal.
 Ingates should be located in thick regions.
 Multiple gating is frequently desirable.
 Rectangular cross-section of runners and ingates
are generally preferred in sand castings.
 BERNOULLI’s THEOREM
APPLICATION :
Bernoulli’s theorem
𝑝𝑝 𝑣𝑣2
h+ + = constant
𝜌𝜌𝑔𝑔 2𝑔𝑔 2
𝑝𝑝1 𝑣𝑣 1 𝑝𝑝2 𝑣𝑣22
h1 + + = h2 + + +f
𝜌𝜌𝑔𝑔 2𝑔𝑔 𝜌𝜌𝜌𝜌 2𝑔𝑔

Assuming f=0 and system

remain at atmospheric
pressure.
No turbulence and friction.
BERNOULLI’s THEOREM
APPLICATION(Contd.) :
𝑣𝑣21 𝑣𝑣22
h1 + = h2 +
2𝑔𝑔 2𝑔𝑔

CONTINUITY EQUATION
Q= A1V1 = A2V2
Q= Volumetric flow rate
A= Cross section area
V= Velocity
BERNOULLI’s THEOREM
APPLICATION(Contd.) :
Applying Bernoulli’s
Equation at 2 and 3.
𝑣𝑣22 𝑝𝑝2 𝑣𝑣32 𝑝𝑝3
h 2+ + = h 3+ +
2𝑔𝑔 𝜌𝜌𝑔𝑔 2𝑔𝑔 𝜌𝜌𝜌𝜌
(𝑣𝑣32−𝑣𝑣22)𝜌𝜌𝜌𝜌
p2 = +p3-h2𝜌𝜌𝑔𝑔
2𝑔𝑔
A2v2 = A3v3
𝑝𝑝2 = 𝑝𝑝3 − ℎ2𝜌𝜌𝜌𝜌
BERNOULLI’s THEOREM
APPLICATION(Contd.) :
𝑣𝑣22 𝑝𝑝2 𝑣𝑣32 𝑝𝑝3
h2+ + = h3+ + (1)
2𝑔𝑔 𝜌𝜌𝑔𝑔 2𝑔𝑔 𝜌𝜌𝜌𝜌
h3 = 0
p2 = p3 = atm. Pressure
𝑣𝑣22 𝑣𝑣32
h2 + =
2𝑔𝑔 2𝑔𝑔
𝐴𝐴2𝑣𝑣2 = 𝐴𝐴3𝑣𝑣3
𝑣𝑣2 = 𝑅𝑅𝑅𝑅3
𝑏𝑏𝑏𝑏𝑏𝑏
𝑣𝑣3 = √(2𝑔𝑔𝑔𝑔𝑔)
𝑣𝑣𝑣 = √(2𝑔𝑔𝑔𝑔𝑔 )
Then put the values in the equation (1).
BERNOULLI’s THEOREM
APPLICATION(Contd.) :

From previous page,

𝐴𝐴
But R = 3
𝐴𝐴2
Thus,
𝐴𝐴3 ℎ
= √( 𝑐𝑐 )
𝐴𝐴2 ℎ𝑡𝑡
𝐴𝐴 ℎ
( 3)2 = ( 𝑐𝑐 )
𝐴𝐴2 ℎ𝑡𝑡

This is look like an equation

of a parabola.
CHOKE AREA :
The choke is a control area
placed in the sprue well to
control the flow of liquid
metal flowing into the mold
cavity so that the mold is
filled in calculated pouring
time . The choke area plays
an important role in gating
systems because the area
allows metal to pass through
at a consistent and constant
flow.
 GATING RATIO
Areas of Sprue : Runner : Gate(s)
The base of the Sprue and Choke are the same.
The ratios between the cross-sectional area can be
grouped into either Pressurized or Non-pressurized.

Pressurized: The pressurized gating system is a gating

system whose cross-sectional surface area decreases
gradually towards the mould cavity .The in-gate area is
minimized to put pressure on the system.
Non-pressurized: The non-pressurized gating system is a
gating system whose total surface of the doors increases
gradually towards the mould cavity.
RISERS :-
Risers are reservoirs of molten metal. They feed this
material to sections of the mould to compensate for
volumetric shrinkage as the casting solidifies.
There are different classifications for risers.

i. Top risers:- Risers that feed the metal casting

from the top.
ii. Side risers:- Risers that feed the metal casting
from the side.
RISERS :-
Top riser & Side riser
TYPES OF GATE :-
A gate is a passage or channel through which the molten
metal flows from the runner to the mould cavity. However, it
should be located where it can easily removed without any
loss to the casting. As per their position in the mould cavity,
gates may be classified as follows:

 Parting line gate

 Top gate
 Bottom gate
PARTING LINE GATE :-

o It is the simplest type of gate and the molten metal enters

the mould cavity at the parting line. Such type of gate is
cut by hand when the cope and drag are separated or it
can be formed by an attached gate to the pattern.

o Parting line allows the use of devices that can effectively

trap any slag, dirt, or sand, which passes with the metal
down the sprue.
PARTING LINE GATE(Contd.) :-

o It is the simplest type of gate and

the molten metal enters the mould
cavity at the parting line. Such
type of gate is cut by hand when
the cope and drag are separated or
it can be formed by an attached
gate to the pattern.

o Parting line allows the use of

devices that can effectively trap
any slag, dirt, or sand, which
passes with the metal down the
sprue.
TOP GATE :-

In this of gate the molten metal

from the top flows, down directly
into the mould. As all the molten
metal enters the casting at the
top, the hottest metal comes to
rest at the top of casting with
the result, proper temperature
gradient formed to enable
directional solidification of
casting from the bottom side
towards the riser.
TOP GATE(Contd.) :-

o The principle advantages of top gating are its simplicity for

moulding, its low consumption of additional metal and,
above all the generation of temperature gradients
favourable to feeding from top heads.
o The main disadvantages of this type of gating is the erosion
of the mould, which takes place by the failing metal.
o The cavity of mould, therefore should be much harder and
strong to resist this impact.
BOTTOM GATE :-

In this type of gate, the molten

metal from the pouring basin
flows down and enter the
mould cavity at or near its
bottom. Bottom type of gate
facilities the mould to be
prepared in two moulding
boxes. During pouring the
molten metal, bottom type of
gate enable to reduce the
erosion of mould and core and
minimize the turbulence of
metal.
Thanking you