casting, sand casting, pattern materials, types of pattern allowances, principles of gating system, riser design, solidification of metals and alloys, die casting, centrifugal casting, investment casting, casting defects,

1. CASTING
GUNASHEKAR.G
ASSISTANT PROFESSOR
MECHANICAL ENGINEERING
JBIET

2. Introduction
• Casting is one of the oldest manufacturing
process and even today it is the first step in
manufacturing most products.
• In this process the material is first liquefied by
properly heating it in a suitable furnace.
• The liquid is poured into a previously prepared
mould cavity where It is allowed to solidify.
• The product is taken out of the mould cavity,
trimmed and cleaned to shape.
• the solidified product is called casting.

3. Casting classification

4. Steps involved in making a casting
• Pattern making
• Preparation of mould
• Melting and pouring of the liquefied metal.
• Solidification and further cooling to room temperature
• Defects and inspection

5. Pattern and Mould making
• A pattern is made of wood or metal, is a replica of the final product and is
used for preparing mould cavity
• Mould cavity which contains molten metal is essentially a negative of the
final product
• Mould material should posses refractory characteristics and with stand
the pouring temperature
• When the mold is used for single casting, it made of sand and known as
expendable mold
• When the mold is used repeatedly for number of castings and is made of
metal or graphite are called permanent mould
• For making holes or hollow cavities inside a casting, cores made of either
sand or metal are used.

6. Melting and Pouring
• Several types of furnaces are available for melting metals
and their selection depends on the type of metal, the
maximum temperature required and the rate and the
mode of molten metal delivery.
• Before pouring provisions are made for the escape of
dissolved gases. The gating system should be designed to
minimize the turbulent flow and erosion of mould
cavity.The other important factors are the pouring
temperature and the pouring rate.

7. Solidification and Cooling
• The properties of the casting significantly depends on the
solidification time cooing rate.
• Shrinkage of casting, during cooling of solidified metal
should not be restrained by the mould material, otherwise
internal stresses may develop and form cracks in casting.
• Proper care should be taken at the design stage of casting
so that shrinkage can occur without casting defects.

8. Removal, Cleaning, Finishing and
Inspection
• After the casting is removed from the mould it is
thoroughly cleaned and the excess material usually along
the parting line and the place where the molten metal was
poured, is removed using a potable grinder.
• White light inspection, pressure test, magnetic particle
inspection, radiographic test, ultrasonic inspection etc. are
used

9. Advantages of casting
• Molten metal flows into any section in the mould cavity
hence complex shape can be produced.
• Tools required are simple and inexpensive.
• Ideal method for producing small quantities.
• Due to uniform cooling rate from all directions, the
properties of casting are same in all direction.
• Casting of any size and weight even up to 200 tons can be
made.
• Practically any material can be casted be it ferrous or non-
ferrous
• casting is often the cheapest and most direct way of
producing a shape with certain desired mechanical
properties.

10. Disadvantages of casting
• Dimensional accuracy and surface finish achieved
by normal sand casting process is poor.
• Sand casting is labour intensive
• Defects are inevitable
Applications of casting
• Cylindrical blocks
• Machine tool beds
• Ship propeller
• Pistons and piston rings
• Turbine blades

11. Sand casting
• Sand casting uses ordinary sand as the primary mould
material.
• The sand grains are mixed with small amounts of other
materials, such as clay and water, to improve
mouldability and cohesive strength, and are then
packed around a pattern that has the shape of the
desired casting.
• The pattern must be removed before pouring, themold
is usually made in two or more pieces.
• An opening called a sprue hole is cut from the top of
the mold through the sand and connected to a system
of channels called runners.
Contd….

12. • The molten metal is poured into the sprue hole,
flows through the runners, and enters the mold
cavity through an opening called a gate.
• Gravity flow is the most common means of
introducing the metal into the mold.
• After solidification, the mold is broken and the
finished casting is removed.
• The casting is then “fettled” by cutting off the
ingate and the feeder head.
• Because the mold is destroyed, a new mold must
be made for each casting.

13. Sequential steps in making a sand
casting
• A pattern board is placed between the bottom (drag)
and top (cope) halves of a flask, with the bottom side
up.
• Sand is then packed into the drag half of the mold.
• A bottom board is positioned on top of the packed
sand, and the mold is turned over, showing the top
(cope) half of pattern with sprue and riser pins in place.
• The cope half of the mold is then packed with sand.

14. • The mold is opened, the pattern board is
drawn (removed), and the runner and gate are
cut into the surface of the sand.
• The mold is reassembled with the pattern
board removed, and molten metal is poured
through the sprue.
• The contents are shaken from the flask and
the metal segment is separated from the sand,
ready for further processing.

18. Casting Terms
• Flask: A moulding flask is one which holds the sand
mould intact. It is made up of wood for temporary
applications or metal for long‐term use.
• Drag: Lower moulding flask.
• Cope: Uppermoulding flask.
• Cheek: Intermediate moulding flask used in threepiece
moulding.
• Pattern: Pattern is a replica of the final object to be
made with some modifications.
• Parting line: This is the dividing line between the two
molding flasks that makes up the sand mould.
Contd…

19. • Bottom board: This is a board normally made of wood,
which is used at the start of the mould making.
• Molding sand: The freshly prepared refractory material
used for making the mould cavity. It is a mixture of
silica, clay and moisture in appropriate proportions.
• Backing sand: This is made up of used and burntsand.
• Core: Used for making hollow cavities in castings.
• Pouring basin: A small funnel‐shaped cavity at the top
of the mould into which the molten metal is poured.
• Sprue: The passage through which the molten metal
from the pouring basin reaches the mould cavity.
Contd…

20. • Runner: The passage ways in the parting plane through
which molten metal flow is regulated before they reach
the mould cavity.
• Gate: The actual entry point through which molten
metal enters the mould cavity in a controlled rate.
• Chaplet: Chaplets are used to support cores inside the
mould cavity.
• Chill: Chills are metallic objects, which are placed in the
mould to increase the cooling rate of castings.
• Riser: It is a reservoir of molten metal provided in the
casting so that hot metal can flow back into the mould
cavity when there is a reduction in volume of metal
due to solidification

21. Pattern
• A pattern is a replica of the object to be made
by the casting process, with some
modifications.
The main modifications are
• The addition of pattern allowances,
• The provision of core prints.
• Elimination of fine details, which cannot be
obtained by casting and hence are to be
obtained by further processing

22. Types of Pattern
1.Single piece pattern.
2.Split piece pattern.
3.Loose piece pattern.
4.Match plate pattern.
5.Sweep pattern.
6.Gated pattern.
7.Skeleton pattern
8.Follow board pattern.
9.Cope and Drag pattern

23. Single Piece Pattern
• These are inexpensive and the simplest type of patterns. As
the name indicates, they are made of a single piece. It is best
suited for limited(very small) production or in proto type
development. This pattern is expected to be entirely in drag

24. Split Pattern or Two Piece Pattern
• This is the most widely used type of pattern for intricate
castings. When the contour of the casting makes its
withdrawal from the mould difficult, or when the depth of the
casting is too high, then the pattern is split into two parts so
that one part is in the drag and the other in the cope. The two
halves should be aligned properly by use of dowel pins.

25. Loose Piece Pattern
• This type of pattern is used when the contour
of the part is such that withdrawing the
pattern from the mould is not possible.

26. Match Plate Pattern
• The cope and drag patterns along with the
gating and the risers are mounted on a single
matching metal or wooden plate on either
side.

27. Sweep Pattern
• It is used to sweep the complete casting by means of
a plane sweep. These are used for generating large
shapes, which are axi‐symmetrical or prismatic in
nature such as bell‐shaped or cylindrical. This greatly
reduces the cost of a 3D pattern

28. Skeleton Pattern
• A skeleton of the pattern made of strips of wood is used for
building the final pattern by packing sand around the
skeleton. After packing the sand, the desired form is obtained
with the help of a stickle. This type of pattern is useful
generally for very large castings, required in small quantities
where large expense on complete wooden pattern is not
justified.

29. Gated Pattern
• Gating and runner system are integral with the
pattern. This would eliminate the hand cutting of the
runners and gates and help in improving the
productivity of a moulding.

30. Follow Board Pattern
• This type of pattern is adopted for those castings
where there are some portions, which are
structurally weak and if not supported properly are
likely to break under the force of ramming.

31. Cope and Drag Pattern
• These are similar to split patterns. In addition to splitting the
pattern, the cope and drag halves of the pattern along with
the gating and riser systems are attached separately to the
metal or wooden plates along with the alignment pins. They
are called the cope and drag patterns.

32. Pattern Materials
Wood:
• It is easily available, low weight, easily shaped and relatively cheap.
• For very large castings wood is the only practical pattern material
• Commonly used wood pattern are teak, white pine and mahogany wood.
• The disadvantage of wood is absorption of moisture as a result of which
dimensional change occur
Metal :
• Due to their durability and smooth surface finish they are used for large
scale casting production and for closer dimensional tolerances.
• patterns are more expensive
• Commonly used metals are Cast Iron, Brass, aluminium and white metal.
• Aluminium is light, easily workable and are corrosion resistant.
White metal has very small shrinkage
Plastics
• They are low weight, easier formability, smooth surface and durable.
• They don't absorb moisture so they are dimensionally stable and can be
cleaned easily .
• Epoxy resin, PVC, Nylon, Polystyrene are commonly used.

33. Pattern Allowances
1. Shrinkage or contraction allowance
2. Draft or taper allowance
3. Machining or finish allowance
4. Distortion or camber allowance
5. Rapping allowance

34. Shrinkage allowance
• Shrinkage allowance is the allowance provided in the pattern
to compensate the solid shrinkage taking place during the
cooling of material from solidus temperature to room
temperature
• Shrinkage of metal during casting will take place in 3 stages
i.e. liquid shrinkage, solidification shrinkage, solid shrinkage
• Liquid shrinkage refers to the reduction in volume when the
metal changes temperature from pouring to solidus
temperature in liquid state. To account for this, risers are
provided in the moulds.
• Solidification shrinkage refers to the reduction in volume
when metal changes from liquid to solid state at the solidus
temperature. To account for this, risers are provided in the
moulds.

35. • Solid shrinkage is the reduction in volume caused,
when a metal loses temperature in the solid state. The
shrinkage allowance is provided to take care of this
reduction
• The first two shrinkages will be taken care by providing
riser during casting. But the third will be provided as
shrinkage allowance in patern
• All metals shrink when cooling except perhaps
bismuth.
• Grey cast iron has negative shrinkage allowance
• The shrinkage allowance is always to be added to the
linear dimensions. Even in case of internal dimensions.

36. Draft Allowance
• To reduce the chances of the damage of the mould cavity at the time of
pattern removal, the vertical faces of the pattern are always tapered from
the parting line. This provision is called draft allowance. It is provided for
easy removal of pattern from the mould
• Inner surfaces of the pattern require higher draft than outer surfaces.
• In general 5° to 8° draft is given for internal surfaces and 1° to 3° is given
for external surfaces.
• Draft is always provided as an extra metal.

37. Machining allowance
• The extra material provided on the pattern is
called machining allowance.
• Machining allowance is provided for getting
good surface finish.
• Ferrous materials would have scales on the
skin which are to be removed by machining
process.
• It is also called as finish allowance.

38. Shake Allowance
• At the time of pattern removal, the pattern is
rapped all around the vertical faces to enlarge the
mould cavity slightly to facilitates its removal.
• As is reduces the dimension of pattern so It is
taken as negative allowance and is to be applied
only to those dimensions, which are parallel to
the parting plane.
• Shake allowance is provided to avoid mold
damages taking place due to adhesiveness
property of molding sand.
• It is also called as rapping allowance.

39. Distortion Allowance
• A metal when it has just solidified is very weak and
therefore is likely to be distortion prone.
• This is particularly so for weaker sections such as long flat
portions, V, U sections or in a complicated casting which
may have thin and long sections which are connected to
thick sections.
• The foundry practice should be to make extra material
provision for reducing the distortion.

40. Types of sand
1. Molding sand: it is freshly prepared refractory
material used for preparing the mould cavity. It is a
mixture of silica, clay, and moisture in appropriate
proportions. It cnsists of 3 basic elements
• Silica sand particles(75-80%):- used for producing
required strength of the mold
• Clay: Acts as binding agents mixed to the moulding
sands
Kaolinite or fire clay (Al2O3 2SiO2 2H2O), and
Bentonite (Al2O3 4SiO2 H2O nH2O).
• Water: Clay is activated by water.

41. 2. Facing sand: The small amount of carbonaceous
material sprinkled on the inner surface of the
mold cavity to give a better surface finish to the
castings.
3. Backing sand: It is what constitutes most of the
refractory material found in the mould. This is
made up of used and burnt sand.
4. Green Sand: The molding sand that contains
moisture is termed as green sand. The green sand
should have enough strength so that the
constructed mould retains its shape.
5. Dry sand: When the moisture in the molding
sand is completely expelled, it is called dry sand.

42. Moulding Sand Properties
• Porosity or Permeability: Permeability or
porosity of the moulding sand is the measure of
its ability to permit air to flow through it.
• Strength: It is defined as the property of holding
together of sand grains. A moulding sand should
have ample strength so that the mould does not
collapse or get partially destroyed during
conveying, turning over or closing.
• Refractoriness: It is the ability of the moulding
sand mixture to withstand the heat of melt
without showing any signs of softening or fusion

43. • Collapsibility: This is the ability of the moulding
sand to decrease in volume to some extent under
the compressive forces developed by the
shrinkage of metal during freezing and
subsequent cooling.
• Adhesiveness: This is the property of sand
mixture to adhere to another body (here, the
moulding flasks). The moulding sand should cling
to the sides of the moulding boxes so that it does
not fall out when the flasks are lifted and turned
over. This property depends on the type and
amount of binder used in the sand mix.

44. Gating System
In casting process the gating system refers o
all those elements which are connected with
the flow of molten meal from pouring basin to
the mould cavity. It consists of 4 basic
elements.
• Pouring basin
• Sprue
• Runner
• In gate

45. Characteristics of idle gating system
• The time taken for pouring should be minimum
• To minimize turbulence to avoid trapping gasses into
the mold.
• The flow is laminar
• The velocity of molten metal is as high as possible
• To get enough metal into the mold cavity before the
metal starts to solidify
• A proper thermal gradient should be maintained so
that the casting is cooled without any shrinkage
cavites.
• No aspiration effect should take place.
• Avoid the sand erosion.

46. 1. Pouring basin:
• A small funnel shaped cavity at the top of the
mould into which the molten metal is
poured.
• The pouring basin seperates the impurities
by skim core
• It reduces the momentum of liquid flowing
into mould by settling first into it.

47. 2. Sprue:
• The passage through which the molten metal
from the pouring basin, reaches the mould
cavity.
• it controls the flow of metal into the mould.
• It is always vertical with straight tapered circular
section.
• He height of the sprue is mainly responsible for
producing the required velocity of molten metal
into gating system
• Straight tapered sprue is selected to avoid the
aspiration affect.

48. 3. Runner:
• A runner is commonly a horizontal channel
which connects the sprue with ingate, thus
allowing the molten metal to enter the mould
cavity.
• The runners are of larger cross‐section and often
streamlined to slow down and smooth out the
flow, and are designed to provide approximately
uniform flow rates to the various parts of the
mould cavity.
• It is mainly used for minimizing the sand erosion
in casting process.
• Runners are commonly made trapezoidal in
cross‐section

49. 4. Ingate:
• A channel through which the molten metal
enters the mould cavity.

50. Accessories in gating system
• Skim core: it is used t stop the slag and dirt from
entering the mould cavity.
• Strainer: it acts as a filter for separating the
impurities present in the molten metal . It is
made by using ceramic material with high
porosity.
• Splash core: it is used for avoiding sand erosion
from bottom of sprue. It is made by using ceramic
with low porosity.
• Skim bob: it is a semi circular cut given to the
runner. It is used for separating impurities
present in molten metal.

51. • Gating Ratio: it is the proportion of the cross
sectional areas between sprue runner and
ingate
Gating ratio = ratio (Sprue area: Runner area:
Ingate area)

52. Classification of Gating system
Based on pressure above molten metal :
• Pressurized gating system
• Un‐pressurized gating system
Based on the position of ingate
1. Top gate
2. Bottom gate
3. Parting gate
4. Step gate

53. Pressurized Gating System
• If the top of the pouring basin is closed and the pressure above molten
metal in pouring basin is maintained greater than atmospheric pressure is
called pressurized gating system i.e. P>Patm
• The total cross sectional area decreases towards the mold cavity
• Back pressure is maintained because the ingate area is smallest
• Flow of liquid (volume) is almost equal from all gates
• Back pressure helps in reducing the aspiration as the sprue always runs full
• Because of the back pressure the metal flows at high velocity leading to
more turbulence and chances of mold erosion.
• It can be advantageously used for ferrous castings
• Gating ratio of a typical pressurized gating system is as follows
sprue:runner:ingate::1:2:1

54. Un‐Pressurized Gating System
• If the pressure above molten metal in gating system is
equal to atmospheric pressure, it is called as non-
pressurized gating system P=Patm.
• It is easy to maintain Patm just by keeping top of pouring
basin to atmosphere.
• The total cross sectional area increases towards the mold
cavity.
• Flow of liquid (volume) is different from all gates
• Aspiration in the gating system as the system never runs full
• There is no pressure existing in the metal flow thus it helps
to reduce turbulence.
• It is used for casting aluminium and magnesium alloys.
• Gating ratio of typical non-pressurized gating system is as
follows sprue:runner:ingate::1:4:4

55. • Top gate: Causes turbulence in the mould
cavity, it is prone to form dross, favorable
temperature gradient towards the gate, only
for ferrous alloys.
• Bottom gate: No mould erosion, used for very
deep moulds, higher pouring time, Causes
unfavorable temperature gradients.
• Parting Gate: most widely used gate, easiest
and most economical in preparation.
• Step Gate: Used for heavy and large castings,
size of ingates are normally increased from
top to bottom

57. Risers
• Risers are added reservoirs designed to feed
liquid metal to the solidifying casting as a
means of compensating for solidification
shrinkage.

58. Functions of riser
• Store sufficient liquid metal and supply the same
to the casting it solidifies there by avoiding
volumetric shrinkage of the casting.
• the risers must solidify after the casting.
• The riser must be kept open to the atmosphere
and laced in such a location that it maintains a
positive pressure of liquid metal on all portions of
the casting it is intended to feed
• Riser facilitates visual inspection to know
whether the mould cavity is filled or not.

59. Types of riser
1. Open riser(top riser)
• The top surface of the riser will be open to the atmosphere.
• The open riser is usually placed on the top of the casting.
• Gravity and atmospheric pressure causes the liquid metal in the riser to
flow into the solidifying casting.

60. 2. Side riser
• If the riser is located between runner and casting it is known
as side riser.
• The side riser receives the molten metal directly from the
runner before it enters the mould cavity and is more effective
than top riser.

61. 3. Blind riser
• A riser which does not break to the top of the
cope and is entirely surrounded by moulding
sand is known as blind riser.

62. Riser Design
• The volume of riser should be at least 3 times the
shrinkage volume of casting
• The solidification time of molten metal in the riser must
be at least equal to the solidification time of molten
metal in the casting cavity.
• The shape of the riser is selected such a way that
solidification time of molten metal in the riser be as
maximum as possible. Best preferable shape of the riser
is cylindrical shape.
• Directional solidification: the rate of cooling is not
uniform, which results in formation of voids and cavities
in casting regions, to avoid voids the solidification starts
from extreme ends and continue towards the riser.

63. Solidification Time For Casting
• Solidification of casting occurs by loosing heat from the surfaces
and amount of heat is given by volume of casting.
• Cooling characteristics of a casting is the ratio of surface area to
volume. Higher the value of cooling characteristics faster is the
cooling of casting.
• Chvorinov’s rule state that solidification time of casting is
proportional to the square of the ratio of volume to surface area of
casing.
• Total solidification time
Where
Ts=Solidification time V= Volume of casting
SA=Surface area K= mould constant

64. Methods of Riser Design
Following are the methods for riser design:
1.Caine’s Method
2.Modulus Method
3.NRL Method

65. Caine’s Method
Caine’s equation
Where
X = Freezing ratio
Y = Riser volume / Casting volume
A, b and c = Constant
Freezing ratio

66. Modulus Method
• Modulus is the inverse of the cooling
characteristic ( surface area/ Volume) and is
defined as
• Modulus(M) = Volume / Surface area
• For sound casting modulus of riser should be
greater than the modulus of casting by a
factor of 1.2. Therefore Mr= 1.2 Mc

67. NRL Method
• NRL stand for Naval research Laboratory.
• NRL method is essentially a simplification of
Caine’s method.
• In this method shape factor is used in place of
freezing ratio.
Shape factor =

68. Chills
• External chills are masses of high‐heat‐capacity
high‐thermalconductivity material that are placed in the
mould (adjacent to the casting) to accelerate the cooling of
various regions. Chills can effectively promote directional
solidification or increase the effective feeding distance of a
riser. They can often be used to reduce the number of risers
required for a casting.
• Internal chills are pieces of metal that are placed within
the mould cavity to absorb heat and promote more rapid
solidification. Since some of this metal will melt during the
operation, it will absorb not only the heat‐capacity energy,
but also some heat of fusion. Since they ultimately become
part of the final casting, internal chills must be made from
the same alloy as that being cast.

69. Solidification
• Solidification is the process where liquid metal
transforms into solid upon cooling.
• The structure produced by solidification,
particularly the grain size and grain shape, affects
to a large extent the properties of the products.
• 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.

70. Solidification of pure metal
• Pure metals solidifies at a constant temperature equal
to its freezing point, which same as its melting point.
• The solidification starts at the sides of the mould
providing sufficient cooling to molten metal.
• The change form liquid to solid does not occur all at
once. The process of solidification starts with
nucleation, the formation of stable solid particles
within the liquid metal.
• fine equi-axed grains are formed near the wall of the
mold and columnar grain growth takes place up to the
centre of the ingot.
• Columnar grains have their axes perpendicular to
mould face.
.

72. Solidification of alloy
• During solidification of alloys the transition
from liquid to solid occurs during the change
in temperature.
• The solidification starts at the sides of the
mould providing sufficient cooling to molten
metal.
• Solidification of an alloy results in the
formation of crystals having dendritic
structure

74. Die Casting
• Molten metal is injected into closed metal dies under
pressures ranging from 100 to 150 MPa.
• Pressure is maintained during solidification. This high
pressure forces the metal into intricate details,
produces smooth surface and excellent dimensional
accuracy
• After which the dies separate and the casting is ejected
along with its attached sprues and runners.
• Cores must be simple and retractable and take the
form of moving metal segments
• Most die castings are made from non-ferrous metals,
specifically zinc, copper, aluminium, magnesium, lead,
and tin based alloys

76. Graphite+oil

77. • Die casting machines can be
1. Hot chamber
2. Cold chamber

78. Hot chamber die casting
• The molten metal for casting is placed in the holding
furnace at the required temperature adjacent to the
machine.
• The injection mechanism is placed within the holding
furnace and most of its part is in constant touch with
the molten metal.
• When pressure is transmitted by the injection piston,
the metal is forced through the gooseneck into the die.
• On the return stroke, the metal is drawn towards the
gooseneck for the next shot.
• Good for low melting point metals like zinc, tin, lead.
(approx. 400°C)
• Faster than cold chamber machines

80. Cycle in Hot Chamber Casting

81. Cold chamber die casting
• In cold chamber process the molten metal is poured
into the piston sleeve by manually or automatically.
• The metal is pushed into die by a hydraulically
operated plunger.
• After solidification the plunger is withdrawn and ready
for next shot.
• Cold chamber machines are typically used for casting
aluminum, brass, and magnesium alloys.
• Compared to hot chamber cycle rate, this process is
not usually as fast because of the external source of
molten metal.
• Casts high melting point metals ( > 600°C)

83. Cycle in Cold Chamber Casting

84. Advantages
• Extremely smooth surfaces (1 μm)
• Excellent dimensional accuracy
• Rapid production rate
• Better mechanical properties compared to
sand casting
• Intricate parts possible
• Minimum finishing operations
• Thin sections possible

85. Disadvantages
• High initial die cost
• Limited to high‐fluidity nonferrous metals
• Part size is limited
• Porosity may be a problem
• Some scrap in sprues, runners, and flash, but
this can be directly recycled.
• Metals of high melting point are not suitable
• No flexibility of processing

86. Applications
• Carburetors
• Crank cases
• Automotive parts of scooters, motorcycles.
• Bathroom fixtures
• Handle bar housings
• Toys

87. Investment casting process
Basic steps:
• Wax patterns are produced by injection
molding with metal die.
• Multiple patterns are assembled to a central
wax sprue.
• A shell is built by immersing the assembly in a
liquid ceramic slurry.
• Then the assembly immerse into the
extremely the sand.

88. • The ceramic is dried, the wax is melted out,
ceramic is fired to burn all wax.
• The shell is filled with molten metal by gravity
pouring.
• On solidification the parts, gates, sprue and
pouring cup become one solid casting.
• Hollow casting can be made by pouring out
excess metal before it solidifies.
• After metal solidifies, the ceramic shell is broken
off by vibration or water blasting.
• The parts are cut away from the sprue using a
high speed friction saw.
• Minor finishing gives final part.

91. Advantages of investment casting
• Parts of greater complexity and intricacy can be
cast.
• Close dimensional control.
• Good surface finish.
• The lost wax can be reused.
• Additional machining is not required in normal
course.
• Intricate shapes can be cast.
• Fairly high production rate
• High melting point alloy can be cast, almost any
• metal can be cast

92. Limitations of investment casting:
• Time consuming process.
• High labour cost.
• High tooling cost.
• Long lead time possible
• Costly patterns and moulds
• Limited size

93. Applications of Investment casting:
• Turbine blades.
• Pipe fittings.
• Lock parts.
• Hand tools.
• Jewellery.
• Aerospace and rocket components.
• Surgical instruments

95. Centrifugal Casting
• Centrifugal casting uses a permanent mold that is
rotated about its axis at a speed between 300 to 3000
rpm as the molten metal is poured.
• Centrifugal forces cause the metal to be pushed out
towards the mold walls, where it solidifies after
cooling.
• Parts cast in this method have a fine grain
microstructure, which is resistant to atmospheric
corrosion.
• The mold is coated with a refractory coating.
• During cooling lower density impurities will tend to rise
towards the center of rotation.

97. Advantages
• Suitable for all casting metals & alloys
• Fly wheels, hollow cylindrical castings.
• Good mechanical properties of the products.
• Fine grained structure at the outer surface of the casting free
of gas and shrinkage cavities and porosity
• Formation of hollow interiors in cylinders without cores
• Can produce a wide range of cylindrical parts, including ones
of large size.
• Good dimensional accuracy, soundness, and cleanliness
• There is no need for gates and runners, which increases the
casting yield, reaching almost 100 %.

98. Disadvantages
• Not suitable for intricate castings
• High manufacturing cost
• More segregation of alloy component during
pouring under the forces of rotation
• Contamination of internal surface of castings with
nonmetallic inclusions
• Inaccurate internal diameter
• Shape is limited.
• Spinning equipment can be expensive
• Poor machinability

99. Centrifugal casting can be classified into three
general types:
• True centrifugal casting
• Semi-centrifugal casting
• Centrifuging casting

100. True centrifugal casting:
• Molten metal is poured into a rotating mould. The axis of
rotation may be horizontal, vertical or inclined at any
suitable angle between 0-90°.
• Moulds are made with steel, iron or graphite and may be
coated with refractory lining for increases life of mould.
• Mould surfaces can be shaped so that pipes with various
outer shapes, including square or polygonal can be cast.
• Inner surface of the casting remains cylindrical, because
molten metal is uniformly distributed by centrifugal forces.
• Most of the impurities and inclusions are closer to the inner
diameter and can be machined away.

103. Advantages:
• Quick and economical than other methods.
• Risers, feed heads, cores are eliminated.
• Ferrous and non-ferrous metals can be cast
• Good surface finish.
Disadvantages:
• Mechanical composition of alloys are notuniform.
• Mechanical properties of centrifugal castings are superior to
sand castings but gravity segregation is encountered in some
alloys.
• Good for production of cylindrical parts only.
Applications:
• Pipes, bushings, gears, flywheels.

104. Semi centrifugal casting:
• It is also known as profited centrifugal casting.
• It is nearly to true centrifugal casting with only
difference is central core is used in the inner
surface.
• The moulds are rotated about a vertical axis and
metal enters the mould through central pouring
basin.
• This method is used to cast parts which are more
complicated but are axi-symmetry in nature.
• Rotational speeds are lower than for true
centrifugal casting

107. Limitations:
• Rotational speed is less compare to the true
centrifugal process.
• This process is used only for symmetrical parts.
• Yield value is less when compare to the true
centrifugal process
Applications:
• It is used for making Gears, Fly wheels and Track
Wheels etc.

109. Centrifuge casting
• The centrifuging process is used in order to obtain higher
metal pressures during solidification when casting shapes are
not axisymmetrical.
• This is suitable for small jobs of any shape.
• A number of small jobs are joined together by means of radial
runners with a central sprue on revolving table.

110. Advantages:
• Irregular shapes can be cast [ like bearing caps, small
brackets.]
• Dental profession use this process for casting gold inlays.
Disadvantages:
• This type of casting is possible only in vertical direction.
Applications:
• Bearing caps,Lamp Bracket etc.

112. Casting Defects
The following are the major defects, which are
likely to occur in sand castings
• Gas defects
• Shrinkage cavities
• Molding material defects
• Pouring metal defects
• Mold shift.

113. Gas Defects
• A condition existing in a casting caused by the
trapping of gas in the molten metal or by mold
gases evolved during the pouring of the casting.
• The defects in this category can be classified into
blowholes and pinhole porosity.
• Blowholes are spherical or elongated cavities
present in the casting on the surface or inside the
casting.
• Pinhole porosity occurs due to the dissolution of
hydrogen gas, which gets entrapped during
heating of molten metal.

114. 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 mold.
• Sprues may be too thin, too long or not attached
in the proper location, causing shrinkage cavities.
• It is recommended to use thick sprues to avoid
shrinkage cavities.

115. Molding Material Defects
• Cuts and washes,
• Scab
• Metal penetration,
• Fusion
• Swell

116. Metallurgical defects
• Hot tears or hot cracking, cause of this defect is that
stresses and strains built up during solidification are
too high compared to the actual strength of the
semisolid material. This type of defects occurs in the
lower part of the solidification range, close to the
solidus, when the alloy has a wide solidification
temperature range and a small amount of liquid,.
Proper mould design prevents this type of defect.
• Hot spots are areas on the surface of casting that
become very hard because they cooled more quickly
than the surrounding material.

117. Assignment Questions
1. With a neat sketch explain the working of
crucible melting.
2. Explain the working of cupola furnace with
neat sketch.
3. Write short notes on steel making processes.