casting

1. UNIT-3 Special Casting Process

5. Centrifugal Casting
• In this casting process, molten metal
is poured into a revolving mold and
allowed to solidify molten metal by
pressure of centrifugal force.
• This process makes hollow product.
• There are three main types of
centrifugal casting process:
1. True centrifugal casting process
2. Semi centrifugal casting process
3. Centrifuging process.

6. Centrifugal Casting
1. True Centrifugal casting process:
• This is normally used for the making of hollow pipes, tubes, hallow bushes, etc.
Which are axisymmetric with a concentric hole.
• The axis of rotation can be either horizontal, vertical or any angle between.
• Very long pipes are normally cast with horizontal axis, whereas short pieces are
more conveniently cast with a vertical axis.

7. Process
• In centrifugal casting, a permanent mold is rotated continuously about its axis at high
speeds (300 to 3000 rpm) as the molten metal is poured.
• The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies
after cooling.
• The casting is usually a fine-grained casting with a very fine-grained outer diameter, owing
to chilling against the mould surface.
• Impurities and inclusions are thrown to the surface of the inside diameter, which can be
machined away.
• Casting machines may be either horizontal or vertical-axis. Horizontal axis machines are
preferred for long, thin cylinders, vertical machines for rings.
• Most castings are solidified from the outside first. This may be used to
encourage directional solidification of the casting, and thus give useful metallurgical
properties to it. Often the inner and outer layers are discarded and only the
intermediary columnar zone is used.

8. • Features of centrifugal casting
• Castings can be made in almost any length, thickness and diameter.
• Different wall thicknesses can be produced from the same size mold.
• Eliminates the need for cores.
• Resistant to atmospheric corrosion, a typical situation with pipes.
• Mechanical properties of centrifugal castings are excellent.
• Only cylindrical shapes can be produced with this process.
• Size limits are up to 3 m (10 feet) diameter and 15 m (50 feet) length.
• Wall thickness range from 2.5 mm to 125 mm (0.1 - 5.0 in).
• Tolerance limit: on the OD can be 2.5 mm (0.1 in) on the ID can be 3.8 mm (0.15
in).
• Surface finish ranges from 2.5 mm to 12.5 mm (0.1 - 0.5 in) rms.

9. 9
Advantages
Good mechanical properties can be achieved
No cores are required for making concentric holes in the
case of true centrifugal casting.
There is no need for gates and runners, Which increases
the casting yield , reaches to almost 100%.
Limitations
Castings which are axi-symmetric and having
concentric holes are suitable.
Equipment is expensive.

10. Materials
• Typical materials that can be cast with this process
are iron,
• steel,
• stainless steels,
• glass, and
• alloys of aluminum,
• copper and nickel.

11. • Typical parts made by this process are
• pipes,
• boilers,
• pressure vessels ,
• flywheels,
• cylinder liners and
• other parts that are axi-symmetric.
• It is notably used to cast cylinder liners and sleeve valves for
piston engines, parts which could not be reliably manufactured
otherwise.

14. Semi - Centrifugal Casting
• Semi –centrifugal casting is used for jobs which are more
complicated than those possible in true centrifigal casting, but are
axisymmetric in nature.
• The casting like symmetrical shape, pulley, wheel, disk, gear like
big shape products manufactured.
• More than one casting achieved.
• Here vertical axis machine is used. In center the hub is provided to
create hollow core.

16. Centrifugal Casting
3. Centrifuging:
• This process is used for
non-symmetrical castings
having intricate details
and also for precision
castings.
• The centrifugal force
provides high-fluid
pressure to force the
molten metal into mould
cavity.
• A number of similar
components can be cast
simultaneously.

18. • WHAT IS A DIE?
• A die is a specialized tool used in manufacturing industries
to cut or shape material mostly using a press tool , mould
& die casting. Like molds, dies are generally customized to
the item they are used to create. Products made with dies
range from simple paper clips to complex pieces used in
advanced technology
• PRESS TOOL MOULD DIE CASTING
•

19. DEFINETION OF DIE CASTING
• Die casting is a metal casting process that is characterized by
forcing molten metal under high pressure into a mold
cavity. The mold cavity is created using two hardened tool
steel dies which have been machined into shape and work
similarly to an injection mold during the process.
• Most die castings are made from non-ferrous metals,
specifically
1.) zinc
2.) copper
3.) aluminium
4.) magnesium
5.) lead
6.) tin based alloys
o Depending on the type of metal being cast, a hot- or cold-
chamber machine is used.

20. 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.
1. Gravity Die Casting:

21. 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.

22. Permanent Mold Casting
Typical parts include gears, splines, wheels, gear housings,
pipe fittings, fuel injection housings, and automotive engine

23. Die Casting
2. Pressure die casting:
• In pressure die casting metal flows under high
pressure
• Also as the die is metallic, the casting rate is high
and thus mass production is possible.
The following are the types of pressure die casting
(A) Hot chamber die-casting
(B) Cold chamber die casting.

24. HOT CHAMBER DIE CASTING
In hot chamber die casting manufacture, the supply of molten
metal is attached to the die casting machine and is an integral
part of the casting apparatus for this manufacturing
operation.
The metal for casting is maintained at an appropriate temperature
in a holding furnace adjacent to, if not part of, the machine.
The injection mechanism is located within the holding furnace and
a substantial part of it is therefore in constant contact with the
molten metal.
Pressure is transmitted to the metal by the injection piston, which
forces it through the gooseneck and into the die. On the return
stroke metal is drawn into the gooseneck for the next shot.
In this process there is minimum contact between air and the metal
to be injected, thus minimizing the tendency for turbulent
entrainment of air in the metal during injection. Due to the
prolonged contact between the metal and parts of the injection
system hot chamber is restricted to zinc-base alloys.
The Zinc alloys are the most widely used in the die casting
process. They have very desirable physical, mechanical and
casting properties. They also have the ability to be readily finished
with commercial electroplated or organic coatings.

26.  Some applications of Zinc Die Castings:
• Automotive Industry
• Fuel Pumps
• Carburetor Parts
• Valve Covers
• Handles
hot-chamber machines are primarily
used with zinc, tin, and lead based
alloys.

27. • The essential feature of this process is the
independent holding and injection units. In
the cold chamber process metal is
transferred by ladle, manually or
automatically, to the shot sleeve.
• Actuation of the injection piston forces the
metal into the die. This is a single-shot
operation.
• This procedure minimizes the contact time
between the hot metal and the injector
components, thus extending their operating
life.
• However, the turbulence associated with
high-speed injection is likely to entrain air in
the metal, which can cause gas porosity in
COLD CHAMBER DIE CASTING

28. • Next to zinc aluminum is the most widely used die-casting
alloy. The primary advantage is it light weight and its high
resistance to corrosion. Magnesium alloy die-castings are also
produced and are used where a high strength–to–weight ratio is
desirable.
• The mold has sections, which include the “cover” or hot side
and the “movable” or ejector side. The die may also have
additional moveable segments called slides or pulls, which are
used to create features such as undercuts or holes which are
parallel to the parting line. The machines run at required
temperatures and pressures to produce a quality part to near net-
shape.

29. Die Casting
• Cold Chamber
Cold-chamber machines are used with
a large composition of aluminium,
magnesium and copper.
Some application for
Aluminum Die Castings:
Automotive industry
Home Appliances
Communication Equipment
Sports & Leisur

30. Advantages of die casting
• Advantages:
– Thin section (0.5 mm thickness) can be easily made.
– Impression or complicated design can be achieved on
component walls.
– The production rate is high (300 per hour approx).
– Die set can be used many times.
– All non-ferrous products are produced with this method.
– Better surface finish is achieved.

31. Disadvantages of die casting
• It is not economic for small quantity of
production.
• It is used for only small casting (10 kg weight
approx).
• Initial cost is high for die and other equipment.
• Only non-ferrous products are casted.
• If proper care is not taken then the defects like
blow hole can be possible.

32. Investment Casting
• Investment casting is one of the oldest manufacturing
processes, dating back thousands of years, in which molten
metal is poured into an expendable ceramic mold.
• Investment casting is often referred to as "lost-wax casting"
because the wax pattern is melted out of the mold after it
has been formed.
• The mold is formed by using a wax pattern - a disposable
piece in the shape of the desired part. The pattern is
surrounded, or "invested", into ceramic slurry that hardens
into the mold.
• However, since the mold is destroyed during the process,
parts with complex geometries and intricate details can be

33. Investment Casting
• The following are the different
stages in Investment Casting
(1) Die making
(2) Making wax pattern
(3) Precoating the wax pattern
assembly
(4) Investment the wax pattern in
mould box
(5) Removal of wax pattern
(6) Pouring molten metal
(7) Cleaning of casting

34. • Investment casting requires the use of a metal
die, wax, ceramic slurry, furnace, molten
metal, and any machines needed for
sandblasting, cutting, or grinding. The process
steps include the following:

35. Process
• Pattern creation - The wax patterns are typically
injection molded into a metal die and are formed
as one piece. Cores may be used to form any
internal features on the pattern.
• Assembly of Patterns - The Patterns are attached to
a central wax stick called a sprue to from a
casting cluster or assembly.

36. • Mold creation - This "pattern tree" is dipped into a slurry of fine
ceramic particles, coated with more coarse particles, and then dried to
form a ceramic shell around the patterns and gating system. This
process is repeated until the shell is thick enough to withstand the
molten metal it will encounter.
• The shell is then placed into an oven and the wax is melted out
leaving a hollow ceramic shell that acts as a one-piece mold, hence the
name "lost wax" casting.

37. Pouring - The mold is preheated in a furnace to approximately
1000°C (1832°F) and the molten metal is poured from a ladle into
the gating system of the mold, filling the mold cavity..
Cooling - After the mold has been filled, the molten metal is
allowed to cool and solidify into the shape of the final casting.
Cooling time depends on the thickness of the part, thickness of the
mold, and the material used.

38. • Casting removal - After the molten metal has cooled, the mold
can be broken and the casting removed. The ceramic mold is
typically broken using water jets, but several other methods
exist. Once removed, the parts are separated from the gating
system by either sawing or cold breaking (using liquid
nitrogen).
• Finishing - Often times, finishing operations such as grinding
or sandblasting are used to smooth the part at the gates. Heat
treatment is also sometimes used to harden the final part.

40. Products
• Turbine blades for aerospace
and power industries.
• Firearm parts such as triggers
and hammers
• Artificial orthopedic surgical
implants and parts for artificial
limbs
• Industrial pumps,, automobiles,
components, pipe fittings and
pipe elbows.

41. • Investment casting can make use of most metals,
most commonly using aluminum alloys, bronze
alloys, magnesium alloys, cast iron, stainless
steel, and tool steel
• This process is beneficial for casting metals with
high melting temperatures that can not be molded in
die casting.
• Parts that are typically made by investment casting
include those with complex geometry such as
turbine blades or firearm components.

42. •Excellent surface finish.
•Tight dimensional tolerances.
•Complex and intricate shapes may be produced.
•Capability to cast thin walls.
•Low material waste.
•No parting lines
•A wide variety of material can be cast using this
method
Advantages

43. Disadvantage
• Limitations on size of casting
• Higher casting costs make it important to take
full advantage of the process to eliminate all
machining operations.
• Individual pattern is required for each
casting.

44. Casting
Defects

45. Definition
A casting defect is an undesired irregularity in
a metal casting process. Some defects can be
tolerated while others can be repaired,
otherwise they must be eliminated.
The Following are the major defects which are likely to occur in sand castings.
(i) Gas defects
(ii) Shrinkage cavities
(iii) Molding Material defects
(iv) Pouring Metal defects
(v) Metallurgical defects

46. Defects in Casting
• (I)Gas defects
• Casting defects, their causes & remedies:
(1) Blow holes
These are the spherical, flattened or elongated cavities present inside
the casting or on the surfaces.
Causes: Moisture level is high in mould sand, improper baking of
core. unnecessary carbonic binder, unwanted ramming, small vent
hole, fine sand etc.
Remedies: Proper moisture level, proper baking of core, proper use
of binder, proper ramming, proper vent hole with vent rod, selection
of sand particle.

47. PIN HOLES
 Formation of many small gas cavities at or slightly below
surface of casting is called as pin holes.
 Causes :
• Sand with high moisture content.
• Absorption of hydrogen/carbon monoxide gas in the metal.
• Alloy not being properly degassed.
• Sand containing gas producing ingredients.
 Remedies :
• Reducing the moisture content & increasing permeability of moulding sand.
• Employing good melting and fluxing practices.
• Improving a rapid rate of solidification.
4
7

48. Blow holes Casting Defects

49. Casting defects, their causes &
remedies
(II) Shrinkage cavities
When metal transfer from liquid to sold its volume will
decrease. During this process if it will not get more
molten metal then in the internal surface of casting voids
are developed, that is known as shrinkage.
Causes: Defective runner, gate and riser, molten metal's
pouring temperature.
Remedies : Proper arrangement of runner, riser and gate
for proper directional solidification. If required, the design
can be changed, maintaining proper molten metal
temperature.

50. Shrinkage Casting Defects

51. Casting defects, their causes & remedies
• (1) Metal Penetration: Surface of casting becomes rough due to metal penetration.
• Causes: Bigger size of sand, less ramming, low strength of moulding sand and core, higher
permeability.
• Remedies : Fine grain sand, proper ramming of mould sand, proper mixture should be used to
increase moulding and core porosity.
• (2) Lift and shift :
• Some part of casting gets distortion known as lift and shift.
• Causes: Improper alignment of pattern parts, improper support of core, improper clamping of mould
box, improper strength of mould sand.
• Remedies: By aligning the mould box with help of dowel pin, properly supporting core in mould,
properly clamping of mould box, providing proper strength mould and core sand.
• (3) Swell
• Due to molten metal the some part of mould cavity become large so the casting becomes larger than
required which known as swell.
• Causes : Pressure of molten metal on surface, improper ramming of sand, low strength of core sand.
• Remedies: By properly ramming of sand, by increasing core strength so molten metal can easily flow
in mould.
(III) Moulding Material defects

52. Casting defects, their causes & remedies
• (4) Run out: While pouring, molten metal comes out (leaks out) from casting known as run out.
• Causes: Defective mould box, defectively moulding process.
• Remedies Moulding box should be changed, modification in moulding process.
• (5) Drop: A drop occurs when cope surface cracks and breaks, thus the pieces of sand fall into the
molten metal.
• Causes Due to either low green strength or improper ramming of the cope flask, improper
reinforcement.
• Remedies: Proper mixing of binder for strength improvement, proper arrangement of steel rod for
reinforcement in core and mould, proper ramming.

53. Casting defects, their causes & remedies
• (1) Misrun and cold shut :Molten metal cannot reach in all the parts of
mould, so this improper filled casting known as misrun. Molten comes from
different sides, sometimes cannot mix each other. This defect known as
cold shut.
• Causes: Defective design of gating system, low fluidity of molten metal,
thin wall of casting, non-continues pouring of metal.
• Remedies: Modification of gating system, increasing temperature of molten
metal, continuously pouring of molten metal, increasing porosity of sand.
• (2) Warpage: After or before solidification casting may twist or change
shape known as warpage.
• Causes: Improper design, lack of directional solidification, internal stress.
• Remedies: Proper design of casting to get directional solidification. By
proper heat treatment the stress can be removed.
(IV) Pouring metal defects

54. 4) Inclusions :
• Unwanted ingredients such as metal oxide, slag, sand particles give defects known as
inclusions in metal castings.
• Causes: Improper gating system, improper pouring, low quality mould and core sand,
improper ramming, impurity in metal charge.
• Remedies: By modifying gating system, turbulence free pouring, using good quality mould
and core, proper ramming of mould sand, proper and pure metal charge should be used and by
using oxide free molten metal crucible.

55. Casting defects, their causes & remedies
• Hard spot: Some part of casting solidifies very fast
and that surface becomes tough, that known as hard
spot.
• Causes: Bad casting design, improper métal
composition, improper use of chills.
• Remedies: Proper design of castings so metal
solidifies in same time, proper metal Composition,
proper use of chills in design.
(V) Metallurgical defects

58. MELTING FURNACES
• Before pouring into the mold, the metal to be casted has to be in the molten or
liquid state.
• Furnace is used for carrying out not only the basic ore refining process but
mainly utilized to melt the metal also.
• A blast furnace performs basic melting (of iron ore) operation to get pig iron,
cupola furnace is used for getting cast iron and an electric arc furnace is used for
re-melting steel.
• Different furnaces are employed for melting and re-melting ferrous and
nonferrous materials.

59. What is Furnace???
• Heating media or device.
• Used for heating and melting.
• For providing heat to chemical reactions for
processes like cracking.
• The furnace may be heated by fuel as in many
furnaces coke is used as a fuel.
• some are operated by electrical energy e.g.
electric arc furnace.

60. Furnaces for Casting Processes
• Furnaces most commonly used in foundries:
– Cupolas
– Direct fuel-fired furnaces
– Crucible furnaces
– Electric-arc furnaces
– Induction furnaces

61. FURNACES FOR MELTING DIFFERENT
MATERIALS
Grey Cast Iron
(a) Cupola
(b) Air furnace
(c) Rotary furnace
(d) Electric arc furnace
Non-ferrous Metals
(a) Reverberatory furnaces (fuel fired) (Al, Cu)
(i) Stationary
(ii) Tilting
(b) Rotary furnaces
(i) Fuel fired
(ii) Electrically heated
(c) Induction furnaces (Cu, Al)
(i) Low frequency
(ii) High frequency.
(d) Electric Arc furnaces (Cu)
(e) Crucible furnaces (AI, Cu)
(i) Pit type
(ii) Tilting type
(iii) Non-tilting or bale-out type
(iv) Electric resistance type (Cu)
(f) Pot furnaces (fuel fired) (Mg and AI)
(i) Stationary
(ii) Tilting
Steel
(a) Electric furnaces
(b) Open hearth furnace

62. CUPOLA FURNACE
• Cupola furnace is employed for melting scrap metal or pig iron for
production of various cast irons.
• It is also used for production of nodular and malleable cast iron.
• It is available in good varying sizes.
• The main considerations in selection of cupolas are melting
capacity, diameter of shell without lining or with lining, spark
arrester.

63. Cupola Furnace
• Cupola was made by Rene-Antoine around 1720.
• Cupola is a melting device.
• Used in foundries for production of cast iron.
• Used for making bronzes.
• Its charge is Coke , Metal , Flux.
• Scrap of blast furnace is re melted in cupola.
• Large cupolas may produce up to 100 tons/hour
of hot iron.

64. Construction
• Cupola is a cylindrical in
shape and placed vertical.
• Its shell is made of steel.
• Its size is expressed in
diameters and can range from
0.5 to 4.0 m.
• It supported by four legs.
• Internal walls are lined with
refectory bricks.
• Its lining is temporary.

65. Parts of Cupola
• Spark arrester.
• Charging door.
• Air box.
• Tuyeres.
• Tap hole.
• Slag hole.

66. Zones
Well
• The space between the bottom
of the Tuyeres and the sand bed.
• Molten metal collected in this
portion.
Combustion zone
• Also known as oxidizing zone .
• Combustion take place in this
zone.
• It is located between well and
melting zone.
• Height of this zone is normally
15cm to 30cm.

67. Zones
• In this zone the temperature
is 1540°C to 1870°C.
• The exothermic reactions
takes place in this zone
these are following .
• C + O2 → CO2 + Heat
• Si + O2 → SiO2 + Heat
• 2Mn + O2 → 2MnO + Heat
Reducing zone
• Locate between upper level
of combustion zone and
upper level of coke bed.

68. Zones
• In this zone temperature is
about 1200°C.
• In this zone CO2 change in to
CO.
CO2 + C (coke) → 2CO
Melting zone
• In this zone the melting is done.
• It is located between preheating
zone and combustion zone.
• The following reaction take
place in this zone.
• 3Fe + 2CO → Fe3C + CO2 .

69. Zones
Preheating zone
• This zone is starts from the upper
end of the melting zone and
continues up to the bottom level of
the charging door .
• Objective of this zone is preheat the
charges from room temperature to
about 1090°C before entering the
metal charge to the melting zone.
Stack
• The empty portion of cupola above
the preheating zone is called as stack.
It provides the passage to hot gases
to go to atmosphere from the cupola
furnace.

70. Charging of Cupola Furnace
• Before the blower is started, the furnace is uniformly
pre-heated and the metal, flux and coke charges, lying
in alternate layers, are sufficiently heated up.
• The cover plates are positioned suitably and the blower
is started.
• The height of coke charge in the cupola in each layer
varies generally from 10 to 15 cm . The requirement of
flux to the metal charge depends upon the quality of the
charged metal and scarp, the composition of the coke
and the amount of ash content present in the coke.

71. Working of Cupola Furnace
• Its charge consist of
scrap, coke and flux.
• The charge is placed
layer by layer.
• The first layer is coke,
second is flux and third
metal.
• Air enter through the
bottom tuyeres.
• This increases the energy
efficiency of the furnace.
• Coke is consumed.

72. Working of Cupola Furnace
• The hot exhaust gases rise up
through the charge, preheating it.
• The charge is melted.
• As the material is consumed,
additional charges can be added to
the furnace.
• A continuous flow of iron emerges
from the bottom of the furnace.
• The slag is removed from slag hole.
• The molten metal achieved by tap
hole.

73. Operation of Cupola
• Preparation of cupola.
• Firing the cupola.
• Soaking of iron.
• Opening of air blast.
• Pouring the molten metal.
• Closing the cupola.

74. Preparation of cupola
• Slag and metal adhere to the cupola lining
from the previous run is removed and lining of
cupola is re made.
• The bottom plates are swung to closing
position supported by prob.
• The sand bed is then prepared with molding
sand such that its slopes to towards the tap
hole.

75. Firing the Cupola
• The cupola is fired by kindling wood at the
bottom.
• This should be done 2.5 to 3 hours before the
molten metal is required.
• On the top of the kindling wood a bed of coke
is built.
• The height of the coke bed is may be vary
from 50cm to 125cm according to the size of
cupola.

76. Soaking of Iron
• When the furnace is charged fully it is
maintain for about 45 minutes.
• The charge is slowly heated.
• During the stage the air blast is shut off and
iron is soaked.

77. Opening of blast air
• At the end of the soaking period the air blast is
opened.
• The taping hole is closed by a plug when the
melting proceeds and molten metal is collect at
the bottom.

78. Pouring of molten metal
• When the sufficient amount of metal has
collected in the hearth the slag hole is opened
and the slag is removed.
• Then taping hole is opened and molten metal is
flows out in the table.
• The same procedure is repeated until the
charge is melted and the operation is over.

79. Closing the cupola
• When the operation is over the air blast is shut
off .
• The bottom of furnace is opened by removing
the prop.

80. Advantages
• It is simple and economical to operate .
• Cupolas can refine the metal charge, removing
impurities out of the slag.
• High melt rates .
• Ease of operation .
• Adequate temperature control .
• Chemical composition control .
• Efficiency of cupola varies from 30 to 50%.
• Less floor space requirements.

81. Disadvantages
• Since molten iron and coke are in contact with
each other, certain elements like si , Mn are
lost and others like sulphur are picked up. This
changes the final analysis of molten metal.
• Close temperature control is difficult to
maintain

82. Factors responsible for the selection of furnace:-
(i) Considerations of initial cost and cost of its operation.
(ii) Relative average cost of repair and maintenance.
(iii) Availability and relative cost of various fuels in the particular locality.
(iv) Melting efficiency, in particular speed of melting.
(v) Composition and melting temperature of the metal.
(vi) Degree of quality control required in respect of metal purification of
refining,
(vii) Cleanliness and noise level in operation.
(viii) Personnel choice or sales influence.

83. Electric-Arc Furnaces
Charge is melted by heat generated from an electric arc
• High power consumption, but electric-arc furnaces can be designed
for high melting capacity
• Used primarily for melting steel