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1
CASTING PROCESS
Madan Lal Chandravanshi
Assistant Professor
Mechanical Engineering Department
Indian School of Mines Dhanbad
2
Introduction
Casting is a manufacturing process by which a
molten material such as metal or plastic is
introduced into a mold, allowed to solidify within
the mold, and then ejected or broken out to make
a fabricated part.
Casting is used for making parts of complex
shape that would be difficult or uneconomical to
make by other methods, such as cutting from
solid material.
3
Introduction
Casting may be used to form hot, liquid
metals or melt-able plastics (called
thermoplastics), or various materials that
cold set after mixing of components such as
certain plastic resins such as epoxy, water
setting materials
Substitution is always a factor in deciding whether
other techniques should be used instead of
casting.
4
Fundamentals of Casting
Six basic factors involved in the
casting process:
Mold cavity
Melting process
Pouring technique
Solidification process
Part removal process
Post processing
5
Casting Terms
Pattern
Flask
Cope
Drag
Core
Core Box
Core Print
Mold Cavity
6
Casting Terms
Riser
Gating System
Pouring Cup
Sprue
Runner
Gate
Parting Line
Draft
7
View of the Mould
8
Cope, Drag and the pattern
9
Placement of Runner and Riser
10
Cope and drag
11
Placement of runner and riser
12
Initial view of the cope and drag
13
14
POURING OF MOLTEN METALS FROM LADLE
15
Pouring of molten metal
16
Pouring of White hot molten metal
17
Steps in Casting
18
1. Drag portion of pattern is placed in flask
 The diagrams to the
left shows the pattern
on a flat board and a
casting box called a
‘drag’ being placed
over it
19
2. Drag is packed with sand
 Special casting sand will soon be
packed around the pattern.
 To ensure the easy removal of pattern
from the sand, parting powder is
sprinkled over and around it. (parting
powder is similar to talcum powder).
 It stops the casting sand sticking to
the pattern and pulling away with it
when the pattern is finally removed
from the sand
20
2. Drag is packed with sand (cont.)
 Casting sand is then shaken
through a sieve.
 only fine particles fall around
the pattern. This is called
facing sand and it must be
fine so that detail on the
pattern shows up on the final
casting
21
3. Packing of sand with ram
 The drag is then packed with
more casting sand. It is a good
idea to sieve all the sand being
placed above the pattern and
then ram it down firmly using a
ramming tool.
 The tool has two ends,
 Cylindrical (used for general
packing down of the sand)
 Pointed (Used for packing sand
close up to the pattern).
22
4. Leveling
 When the drag is packed fully it is leveled off
(called ‘strickled off’) using a straight steel bar
23
5. Turn Over Drag
 The entire drag and its
contents are then turned
over so that the base of the
pattern can be seen.
24
6. Fixing and positioning Locating Pin
 A top box called a ‘cope’ is
then placed on top of the
drag and locating pins are
put in position so that the
casting boxes cannot move
sideways
25
7. Insert Sprue and Risers
 Sprue pins are positioned. One usually on the
back of the pattern and the other to the side.
These will eventually provide an entrance and
exit for the molten aluminum when it is poured
into the sand.
 The sand is packed/rammed into the cope in
the same way as the drag. Parting powder is
first applied, followed by facing sand.
 The sprue pins should be taller than the box
and stand out from the sand when it is leveled
with a strickling bar
26
8. Pack the cope with Sand
 Small depressions are dug
into the sand at the top of
the two sprue pins. These
are useful when the
aluminum is poured.
 The depressions are called
the pouring basin and
feeder
27
9. Gates and runners cut into mold
 The top box (the cope) is then removed
and if all is well the cope with the sand
inside should lift off the drag (bottom
box) without the sand falling out. A
small ‘gate’ is cut below the position of
one of the sprue pins.
 This will help the molten aluminium flow
into the cavity left by the mould. Small
tools are available or can easily be
made to dig a variety of shapes in the
casting sand. They are similar to small
trowels
28
10. Removal of Pattern
 The pattern is removed using a
‘spike’. The end of the spike can be
threaded and so it can be screwed
into the softwood pattern.
 Before removing the pattern it is a
good idea to gently tap the spike so
that it loosens the pattern from the
sand. It can then be lifted away
from the casting box (drag).
29
11. Closing cope and drag
 The cope (top casting box) is
placed back on top of the
drag and the locating pins put
in position.
 Before this is done vents can
be created using a thin piece
of welding rod, pushing it
through the sand . This
allows gases to escape once
the aluminium is poured
30
12. Pouring of Molten Metal
 The aluminium is poured with great
care.
 The aluminium is poured down the
hole left by the first sprue pin (now
called the ‘runner’).
 As it runs down the runner it flows
through the ‘gate’ cut by the trowel,
into the cavity left by the pattern and
up the riser (the hole left by the
second sprue pin).
 The casting should be left for
sufficient time before removal from
the sand.
31
13. Final Casting
 When removed from
the sand, the runner
and riser are cut away
and the casting is
ready for machining
32
Process Factors
Molten metal problems
Reaction of the metal and its environment can
lead to poor quality castings. Oxygen and
molten metal react to form “slag” or “dross.”
These impurities can become trapped in
castings to impair surface finish, machinability,
or reduce the mechanical properties of the
castings.
33
Process Factors
Fluidity
Molten metal must flow then freeze into the desired
shape. Incorrect flow characteristics can result in
“short” shots, incorrect part tolerances, cracks in
castings, voids, etc.
Gating System
Correct design of the gating system is a must.
Gating system controls the speed, rate, and delivery
of molten material into the mold cavity.
34
Process Factors
Patterns
Shrinkage allowance
Cast Iron = 1/10 - 1/8 in/ft
Aluminum = 1/8 - 5/32 in/ft
Brass = 3/16 in/ft
Amount of draft
Finish material allowance
Final dimensional accuracy of the casting
35
Classification
Casting Process
Expendable mold Multiple-use mold
Sand Casting
Shell Casting
Investment casting
Lost foam casting
Die casting
Permanent mold casting
Slush Casting
36
Expendable Molding Process
Process for class discussion
Sand
Investment
37
1. Sand Casting
Introduction
38
39
Sand Casting Steps
40
Sand Casting Steps
Drag portion of pattern is placed in flask
 Drag is packed with sand
 Mold turned over
Insert sprue and riser
Pack with sand
 Flask is separated - pattern removed
 Gates and runners cut into mold
 Similar process steps performed on cope
 Cope & drag reassembled
Possibly a core is added
41
Sand Casting Steps
Molten metal poured into mold
Casting solidifies
Mold opened……..distorted
Part removed
Post Processing
42
Sand casting
Sand casting requires a lead time of days for production
at high output rates (1-20 pieces/hr-mold), and is
unsurpassed for large-part production. Green (moist)
sand has almost no part weight limit, whereas dry sand
has a practical part mass limit of 2300-2700 kg.
The sand is bonded together using clays (as in green
sand) or chemical binders, or polymerized oils.
Sand in most operations can be recycled many times and
requires little additional input.
43
Sand casting
Preparation of the sand mold is fast and requires
a pattern which can "stamp" out the casting
template.
 Typically, sand casting is used for processing
low-temperature metals, such as iron, copper,
aluminum, magnesium, and nickel alloys.
Sand casting can also be used for high
temperature metals where other means would be
unpractical. It is by far the oldest and best
understood of all techniques.
44
Sand Casting Advantages &
Disadvantages
Disadvantages
Part tolerances +/- .01 - .015”
Poor surface finish
Limited design freedom
In hand ramming, process can be labor
intensive
Single use of mold
45
Sand Casting Advantages &
Disadvantages
Advantages
General tooling costs are low
Sand in most cases can be reused in some
form
Can handle a wide variety of metals
Relatively easy process to obtain net shape or
near-net shape
46
47
Impellers
49
2. Investment Casting or lost
wax casting
50
Investment Casting (The Lost Wax Process)
This casting process has been practiced for
Hundred of years.
 bees wax was used to form the pattern,
today’s high technology waxes, refractory
materials and specialist alloys, the castings
ensure high quality components are produced
with the key benefits of accuracy, repeatability,
versatility and integrity.
51
Valve of a nuclear reactor manufactured through
investment casting
52
Investment Casting (The Lost Wax Process)
The process is suitable for repeatable
production of net shape components, from a
variety of different metals and high
performance alloys. Although generally used
for small castings, this process has been used
to produce complete aircraft door frames, with
steel castings of up to 300 kg and aluminium
castings of up to 30 kg.
53
Investment Casting (The Lost Wax Process)
 Compared to other casting processes such
as die casting or sand casting it can be an
expensive process, however the components
that can be produced using investment
casting can incorporate intricate contours,
and in most cases the components are cast
near net shape, so requiring little or no
rework once cast.
Investment casting (lost wax casting)
(a) Wax pattern
(injection molding)
(b) Multiple patterns
assembled to wax
sprue
(c) Shell built 
immerse into ceramic
slurry
 immerse into fine sand
(few layers)
(d) dry ceramic
melt out the wax
fire ceramic (burn
wax)
(e) Pour molten metal (gravity)
 cool, solidify
[Hollow casting:
pouring excess metal before
solidification
(f) Break ceramic shell
(vibration or water
blasting)
(g) Cut off parts
(high-speed friction
saw)
 finishing (polish)
55
Investment Casting
 Process steps:
Produce master
pattern of desired
casting
Produce master die
Produce wax patterns
56
Investment Casting
 Process steps continued:
Assemble wax patterns on
a common sprue
sometimes called a tree
57
Investment Casting
 Process steps continued:
- Coat “tree” with an initial
investment material
Vibrate to remove air and
settle material around patterns
Apply dry refectory grains
(Fused silica/Zircon) on the
liquid ceramic coating
58
Investment Casting
• Process steps
Finish coat
Allow investment to harden
Process is repeated wit
gradually increased grains in
slurry.
Fire investment to finish
hardening process and melt
our wax patterns
Preheat mold
59
Investment Casting
 Process steps
-Pour molten metal into mold
cavity
Allow metal to solidify
Remove castings
60
Investment Casting
• Process steps:
Post processing
61
62
63
Advantages & Disadvantages
 Advantages
Wide variety of metals can be cast including high
temperature alloys
Excellent surface finish (60-220 μ in.)
Good dimensional accuracy (+/- .003” up to ¼”)
Tooling cost average
Complex shapes with fine details can be made
thin sections are possible
weights from <1 ounce to 100 lb >.
no parting lines
can be automated
many parts can be made at once providing lower per piece
cost
64
Advantages & Disadvantages
Disadvantages
Price per unit costs can be high
One mold per batch
less strength than die cast parts
process is slow
more steps are involved in production
65
Investment Casting
Typical large applications are:
 - large propellers
 - large frames
 - nozzles
 - cams
 - valve parts
 - dental
 - jewelry
 - orthopedic surgical implants
 - camera components
66
Multiple Use Mold
67
Permanent mold casting
Permanent mold casting requires a set-up time
on the order of weeks to prepare a steel tool,
after which production rates of 5-50 pieces/hr-
mold are achieved with an upper mass limit of 9
kg per iron alloy item (cf., up to 135 kg for many
nonferrous metal parts) and a lower limit of about
0.1 kg.
 Steel cavities are coated with refractory wash of
acetylene soot before processing to allow easy
removal of the work piece and promote longer
tool life.
68
Permanent mold casting
Permanent molds have a life which varies
depending on maintenance of after which
they require refinishing or replacement.
 Cast parts from a permanent mold
generally show 20% increase in tensile
strength and 30% increase in elongation
as compared to the products of sand
casting.
69
Permanent mold casting
The only necessary input is the coating
applied regularly.
Typically, permanent mold casting is used
in forming iron, aluminum, magnesium,
and copper-based alloys.
The process is highly automated.
70
Multiple Use Mold
Advantages
Mold is reusable
Generally, a good surface finish is obtained
Dimensional accuracy can be as good as +/-
.003”
Control of mold temperatures
71
Multiple Use Mold
Disadvantages
Majority of molds use low-melt alloys
Mold costs can be high
Mold life varies
Temperature of alloy being poured
Mold material
Mold temperature
Thermal shock
Mold configuration
72
3. Die Casting
73
1. Gravity Die Casting
74
2. Pressure Die Casting
 The Process
Molten metal is forced
into the die cavity under
pressure. The metal is
kept under pressure
until it solidifies.
 In a hot chamber process the pressure chamber is connected to the die
cavity is immersed permanently in the molten metal. The inlet port of the
pressurizing cylinder is uncovered as the plunger moves to the open
(unpressurized) position. This allows a new charge of molten metal to fill the
cavity and thus can fill the cavity faster than the cold chamber process. The
hot chamber process is used for metals of low melting point and high fluidity
such as tin, zinc, and lead that tend not to alloy easily with steel at their melt
temperatures.
DIE CASTING
HOT CHAMBER
76
Die Casting
Process Steps:
 Lubrication of dies
 Closing and locking of dies
 Molten metal is forced into the die cavity. The molten
metal is injected through a runner and gate with high
pressures.
 air escapes into overflow wells, and out vents, and metal
fills the molds
 Held under pressure until it solidifies
 Die opens. knockout pins eject the part
 the parts are cut off the runners and sprues
77
78
Process Parameters
 Normal Minimum Section Thickness:
Al: .03" Small Parts: .06" Medium Parts
Mg: .03" Small Parts: .045" Medium Parts
Zinc: .025" Small Parts: .040" Medium Parts
 Tolerances:
Al and Mg ± .002"/in.
Zinc ± .0015"/in.
Brass ± .005"/in.
 Metals: Aluminum, Zinc, Magnesium, and Brass
79
Advantages
 Fine section detail (.003”)
 Excellent dimensional accuracy (+/- .002”)
 High production rates (cycles less than 1 minute)
 Excellent surface finish
 Control of process temperatures
Extended mold life
Limited part defects
intricate parts possible
inserts feasible
minimum finishing operations
80
Disadvantages
 Part size (up to 25 lbs.)
 Limited to low melt alloys
 Tooling Cost is high
 long setup times
 $5000-200,000 for machine
 metal melting point temperature must be
lower than die
81
Applications:
automotive parts
appliances
office machines
bathroom fixtures
outboard motors
toys
clocks
tools
82
4. Centrifugal Casting
83
basic process Step:
1. a mold is set up and rotated along a vertical, or
horizontal axis.
2. The mold is coated with a refractory coating.
3. While rotating molten metal is poured in.
4. The metal that is poured in will then distribute
itself over the rotating wall.
5. During cooling lower density impurities will tend
to rise towards the center of rotation.
6. After the part has solidified, it is removed and
finished.
84
85
86
87
88
variants of process :
 true centrifugal casting - long molds are rotated
about a horizontal axis. This can be used to make long
axial parts such as seamless pipes.
 Semi-centrifugal casting - parts with a wide radial
parts. parts such as wheels with spokes can be made
with this technique.
89
Centrifugal and semi-centrifugal casting
used for axis-symmetric parts (internally).
 Parts from 6" to 5' in diameter can be
made, but typical diameters are 10' to 30'.
 Long tubes can be made that could not
normally be rolled.
90
 Typical metals cast
are,
 - steel
 - nickel alloys
 - copper
 - aluminum
 Typical applications are
1. - train wheels
2. - jewelry
3. - seamless pressure
tubes/pipes
91
Advantages,
 - good uniform metal properties
 - no sprues/gates to remove
 - the outside of the casting is at the required
dimensions
 - lower material usage
 - no parting lines
 - low scrap rates
92
Disadvantages,
extra equipment needed to spin mold
the inner metal of the part contains
impurities
93
Casting Defects
Gas Defects
Shrinkage cavities
Molding material defects
Pouring material defects
Metallurgical defects
94
1. Gas Defects
Caused due to entrapped gases in mold
due to
Lower venting
Poor permeability of mold (due to finer grain
size, higher clay content, higher moisture
content, excessive ramming of mold)
Improper design of casting
High pouring temperature
95
Blow holes / Open blows
 Spherical, flattened or
elongated cavities present
inside the casting (BLOW
HOLES) or on the surface
(OPEN BLOW).
96
Air Inclusions
 due to absorption of
atmospheric and
other gases in the
molten metal in
Furnace.
97
Pin Hole Porosity
 Caused by hydrogen in
the molten metal.
 Picked up in the furnace
or by the dissociation of
water inside the mold
cavity.
 at lower temp solubility of
gas decreases and
therefore hydrogen
escapes. It leaves a pin
like structure at the
surface.
98
Scar
 A shallow blow,
usually found on the
flat casting surface.
99
Blister
 Scar covered by thin
layers of a metal.
100
Scab
 Rough, thin layer of a metal, protruding above the casting
surface, on top thin layer of sand.
 The scan results the up heaved sand is separated from the
mold surface and the liquid metal flow into the space between
the mold and the displaced sand.
101
Wash
 A low projection on
the drag surface of a
casting commencing
near the gate is called
wash.
 Due to erosion of
sand by the high
velocity liquid metal in
bottom gating.
102
Misrun
 Freezing of molten
metal before reaching
the farthest point of
the mold.
 Due to insufficient
superheating of
metal.
103
Cold Shut
 Misrun at the centre
in case of two gates is
called Coldshut.
104
Hot Tear (Crack due to residual stresses)
105
Thank you

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castings (1).pdf

  • 1. 1 CASTING PROCESS Madan Lal Chandravanshi Assistant Professor Mechanical Engineering Department Indian School of Mines Dhanbad
  • 2. 2 Introduction Casting is a manufacturing process by which a molten material such as metal or plastic is introduced into a mold, allowed to solidify within the mold, and then ejected or broken out to make a fabricated part. Casting is used for making parts of complex shape that would be difficult or uneconomical to make by other methods, such as cutting from solid material.
  • 3. 3 Introduction Casting may be used to form hot, liquid metals or melt-able plastics (called thermoplastics), or various materials that cold set after mixing of components such as certain plastic resins such as epoxy, water setting materials Substitution is always a factor in deciding whether other techniques should be used instead of casting.
  • 4. 4 Fundamentals of Casting Six basic factors involved in the casting process: Mold cavity Melting process Pouring technique Solidification process Part removal process Post processing
  • 6. 6 Casting Terms Riser Gating System Pouring Cup Sprue Runner Gate Parting Line Draft
  • 8. 8 Cope, Drag and the pattern
  • 12. 12 Initial view of the cope and drag
  • 13. 13
  • 14. 14 POURING OF MOLTEN METALS FROM LADLE
  • 16. 16 Pouring of White hot molten metal
  • 18. 18 1. Drag portion of pattern is placed in flask  The diagrams to the left shows the pattern on a flat board and a casting box called a ‘drag’ being placed over it
  • 19. 19 2. Drag is packed with sand  Special casting sand will soon be packed around the pattern.  To ensure the easy removal of pattern from the sand, parting powder is sprinkled over and around it. (parting powder is similar to talcum powder).  It stops the casting sand sticking to the pattern and pulling away with it when the pattern is finally removed from the sand
  • 20. 20 2. Drag is packed with sand (cont.)  Casting sand is then shaken through a sieve.  only fine particles fall around the pattern. This is called facing sand and it must be fine so that detail on the pattern shows up on the final casting
  • 21. 21 3. Packing of sand with ram  The drag is then packed with more casting sand. It is a good idea to sieve all the sand being placed above the pattern and then ram it down firmly using a ramming tool.  The tool has two ends,  Cylindrical (used for general packing down of the sand)  Pointed (Used for packing sand close up to the pattern).
  • 22. 22 4. Leveling  When the drag is packed fully it is leveled off (called ‘strickled off’) using a straight steel bar
  • 23. 23 5. Turn Over Drag  The entire drag and its contents are then turned over so that the base of the pattern can be seen.
  • 24. 24 6. Fixing and positioning Locating Pin  A top box called a ‘cope’ is then placed on top of the drag and locating pins are put in position so that the casting boxes cannot move sideways
  • 25. 25 7. Insert Sprue and Risers  Sprue pins are positioned. One usually on the back of the pattern and the other to the side. These will eventually provide an entrance and exit for the molten aluminum when it is poured into the sand.  The sand is packed/rammed into the cope in the same way as the drag. Parting powder is first applied, followed by facing sand.  The sprue pins should be taller than the box and stand out from the sand when it is leveled with a strickling bar
  • 26. 26 8. Pack the cope with Sand  Small depressions are dug into the sand at the top of the two sprue pins. These are useful when the aluminum is poured.  The depressions are called the pouring basin and feeder
  • 27. 27 9. Gates and runners cut into mold  The top box (the cope) is then removed and if all is well the cope with the sand inside should lift off the drag (bottom box) without the sand falling out. A small ‘gate’ is cut below the position of one of the sprue pins.  This will help the molten aluminium flow into the cavity left by the mould. Small tools are available or can easily be made to dig a variety of shapes in the casting sand. They are similar to small trowels
  • 28. 28 10. Removal of Pattern  The pattern is removed using a ‘spike’. The end of the spike can be threaded and so it can be screwed into the softwood pattern.  Before removing the pattern it is a good idea to gently tap the spike so that it loosens the pattern from the sand. It can then be lifted away from the casting box (drag).
  • 29. 29 11. Closing cope and drag  The cope (top casting box) is placed back on top of the drag and the locating pins put in position.  Before this is done vents can be created using a thin piece of welding rod, pushing it through the sand . This allows gases to escape once the aluminium is poured
  • 30. 30 12. Pouring of Molten Metal  The aluminium is poured with great care.  The aluminium is poured down the hole left by the first sprue pin (now called the ‘runner’).  As it runs down the runner it flows through the ‘gate’ cut by the trowel, into the cavity left by the pattern and up the riser (the hole left by the second sprue pin).  The casting should be left for sufficient time before removal from the sand.
  • 31. 31 13. Final Casting  When removed from the sand, the runner and riser are cut away and the casting is ready for machining
  • 32. 32 Process Factors Molten metal problems Reaction of the metal and its environment can lead to poor quality castings. Oxygen and molten metal react to form “slag” or “dross.” These impurities can become trapped in castings to impair surface finish, machinability, or reduce the mechanical properties of the castings.
  • 33. 33 Process Factors Fluidity Molten metal must flow then freeze into the desired shape. Incorrect flow characteristics can result in “short” shots, incorrect part tolerances, cracks in castings, voids, etc. Gating System Correct design of the gating system is a must. Gating system controls the speed, rate, and delivery of molten material into the mold cavity.
  • 34. 34 Process Factors Patterns Shrinkage allowance Cast Iron = 1/10 - 1/8 in/ft Aluminum = 1/8 - 5/32 in/ft Brass = 3/16 in/ft Amount of draft Finish material allowance Final dimensional accuracy of the casting
  • 35. 35 Classification Casting Process Expendable mold Multiple-use mold Sand Casting Shell Casting Investment casting Lost foam casting Die casting Permanent mold casting Slush Casting
  • 36. 36 Expendable Molding Process Process for class discussion Sand Investment
  • 38. 38
  • 40. 40 Sand Casting Steps Drag portion of pattern is placed in flask  Drag is packed with sand  Mold turned over Insert sprue and riser Pack with sand  Flask is separated - pattern removed  Gates and runners cut into mold  Similar process steps performed on cope  Cope & drag reassembled Possibly a core is added
  • 41. 41 Sand Casting Steps Molten metal poured into mold Casting solidifies Mold opened……..distorted Part removed Post Processing
  • 42. 42 Sand casting Sand casting requires a lead time of days for production at high output rates (1-20 pieces/hr-mold), and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of 2300-2700 kg. The sand is bonded together using clays (as in green sand) or chemical binders, or polymerized oils. Sand in most operations can be recycled many times and requires little additional input.
  • 43. 43 Sand casting Preparation of the sand mold is fast and requires a pattern which can "stamp" out the casting template.  Typically, sand casting is used for processing low-temperature metals, such as iron, copper, aluminum, magnesium, and nickel alloys. Sand casting can also be used for high temperature metals where other means would be unpractical. It is by far the oldest and best understood of all techniques.
  • 44. 44 Sand Casting Advantages & Disadvantages Disadvantages Part tolerances +/- .01 - .015” Poor surface finish Limited design freedom In hand ramming, process can be labor intensive Single use of mold
  • 45. 45 Sand Casting Advantages & Disadvantages Advantages General tooling costs are low Sand in most cases can be reused in some form Can handle a wide variety of metals Relatively easy process to obtain net shape or near-net shape
  • 46. 46
  • 47. 47
  • 49. 49 2. Investment Casting or lost wax casting
  • 50. 50 Investment Casting (The Lost Wax Process) This casting process has been practiced for Hundred of years.  bees wax was used to form the pattern, today’s high technology waxes, refractory materials and specialist alloys, the castings ensure high quality components are produced with the key benefits of accuracy, repeatability, versatility and integrity.
  • 51. 51 Valve of a nuclear reactor manufactured through investment casting
  • 52. 52 Investment Casting (The Lost Wax Process) The process is suitable for repeatable production of net shape components, from a variety of different metals and high performance alloys. Although generally used for small castings, this process has been used to produce complete aircraft door frames, with steel castings of up to 300 kg and aluminium castings of up to 30 kg.
  • 53. 53 Investment Casting (The Lost Wax Process)  Compared to other casting processes such as die casting or sand casting it can be an expensive process, however the components that can be produced using investment casting can incorporate intricate contours, and in most cases the components are cast near net shape, so requiring little or no rework once cast.
  • 54. Investment casting (lost wax casting) (a) Wax pattern (injection molding) (b) Multiple patterns assembled to wax sprue (c) Shell built  immerse into ceramic slurry  immerse into fine sand (few layers) (d) dry ceramic melt out the wax fire ceramic (burn wax) (e) Pour molten metal (gravity)  cool, solidify [Hollow casting: pouring excess metal before solidification (f) Break ceramic shell (vibration or water blasting) (g) Cut off parts (high-speed friction saw)  finishing (polish)
  • 55. 55 Investment Casting  Process steps: Produce master pattern of desired casting Produce master die Produce wax patterns
  • 56. 56 Investment Casting  Process steps continued: Assemble wax patterns on a common sprue sometimes called a tree
  • 57. 57 Investment Casting  Process steps continued: - Coat “tree” with an initial investment material Vibrate to remove air and settle material around patterns Apply dry refectory grains (Fused silica/Zircon) on the liquid ceramic coating
  • 58. 58 Investment Casting • Process steps Finish coat Allow investment to harden Process is repeated wit gradually increased grains in slurry. Fire investment to finish hardening process and melt our wax patterns Preheat mold
  • 59. 59 Investment Casting  Process steps -Pour molten metal into mold cavity Allow metal to solidify Remove castings
  • 60. 60 Investment Casting • Process steps: Post processing
  • 61. 61
  • 62. 62
  • 63. 63 Advantages & Disadvantages  Advantages Wide variety of metals can be cast including high temperature alloys Excellent surface finish (60-220 μ in.) Good dimensional accuracy (+/- .003” up to ¼”) Tooling cost average Complex shapes with fine details can be made thin sections are possible weights from <1 ounce to 100 lb >. no parting lines can be automated many parts can be made at once providing lower per piece cost
  • 64. 64 Advantages & Disadvantages Disadvantages Price per unit costs can be high One mold per batch less strength than die cast parts process is slow more steps are involved in production
  • 65. 65 Investment Casting Typical large applications are:  - large propellers  - large frames  - nozzles  - cams  - valve parts  - dental  - jewelry  - orthopedic surgical implants  - camera components
  • 67. 67 Permanent mold casting Permanent mold casting requires a set-up time on the order of weeks to prepare a steel tool, after which production rates of 5-50 pieces/hr- mold are achieved with an upper mass limit of 9 kg per iron alloy item (cf., up to 135 kg for many nonferrous metal parts) and a lower limit of about 0.1 kg.  Steel cavities are coated with refractory wash of acetylene soot before processing to allow easy removal of the work piece and promote longer tool life.
  • 68. 68 Permanent mold casting Permanent molds have a life which varies depending on maintenance of after which they require refinishing or replacement.  Cast parts from a permanent mold generally show 20% increase in tensile strength and 30% increase in elongation as compared to the products of sand casting.
  • 69. 69 Permanent mold casting The only necessary input is the coating applied regularly. Typically, permanent mold casting is used in forming iron, aluminum, magnesium, and copper-based alloys. The process is highly automated.
  • 70. 70 Multiple Use Mold Advantages Mold is reusable Generally, a good surface finish is obtained Dimensional accuracy can be as good as +/- .003” Control of mold temperatures
  • 71. 71 Multiple Use Mold Disadvantages Majority of molds use low-melt alloys Mold costs can be high Mold life varies Temperature of alloy being poured Mold material Mold temperature Thermal shock Mold configuration
  • 73. 73 1. Gravity Die Casting
  • 74. 74 2. Pressure Die Casting  The Process Molten metal is forced into the die cavity under pressure. The metal is kept under pressure until it solidifies.
  • 75.  In a hot chamber process the pressure chamber is connected to the die cavity is immersed permanently in the molten metal. The inlet port of the pressurizing cylinder is uncovered as the plunger moves to the open (unpressurized) position. This allows a new charge of molten metal to fill the cavity and thus can fill the cavity faster than the cold chamber process. The hot chamber process is used for metals of low melting point and high fluidity such as tin, zinc, and lead that tend not to alloy easily with steel at their melt temperatures. DIE CASTING HOT CHAMBER
  • 76. 76 Die Casting Process Steps:  Lubrication of dies  Closing and locking of dies  Molten metal is forced into the die cavity. The molten metal is injected through a runner and gate with high pressures.  air escapes into overflow wells, and out vents, and metal fills the molds  Held under pressure until it solidifies  Die opens. knockout pins eject the part  the parts are cut off the runners and sprues
  • 77. 77
  • 78. 78 Process Parameters  Normal Minimum Section Thickness: Al: .03" Small Parts: .06" Medium Parts Mg: .03" Small Parts: .045" Medium Parts Zinc: .025" Small Parts: .040" Medium Parts  Tolerances: Al and Mg ± .002"/in. Zinc ± .0015"/in. Brass ± .005"/in.  Metals: Aluminum, Zinc, Magnesium, and Brass
  • 79. 79 Advantages  Fine section detail (.003”)  Excellent dimensional accuracy (+/- .002”)  High production rates (cycles less than 1 minute)  Excellent surface finish  Control of process temperatures Extended mold life Limited part defects intricate parts possible inserts feasible minimum finishing operations
  • 80. 80 Disadvantages  Part size (up to 25 lbs.)  Limited to low melt alloys  Tooling Cost is high  long setup times  $5000-200,000 for machine  metal melting point temperature must be lower than die
  • 81. 81 Applications: automotive parts appliances office machines bathroom fixtures outboard motors toys clocks tools
  • 83. 83 basic process Step: 1. a mold is set up and rotated along a vertical, or horizontal axis. 2. The mold is coated with a refractory coating. 3. While rotating molten metal is poured in. 4. The metal that is poured in will then distribute itself over the rotating wall. 5. During cooling lower density impurities will tend to rise towards the center of rotation. 6. After the part has solidified, it is removed and finished.
  • 84. 84
  • 85. 85
  • 86. 86
  • 87. 87
  • 88. 88 variants of process :  true centrifugal casting - long molds are rotated about a horizontal axis. This can be used to make long axial parts such as seamless pipes.  Semi-centrifugal casting - parts with a wide radial parts. parts such as wheels with spokes can be made with this technique.
  • 89. 89 Centrifugal and semi-centrifugal casting used for axis-symmetric parts (internally).  Parts from 6" to 5' in diameter can be made, but typical diameters are 10' to 30'.  Long tubes can be made that could not normally be rolled.
  • 90. 90  Typical metals cast are,  - steel  - nickel alloys  - copper  - aluminum  Typical applications are 1. - train wheels 2. - jewelry 3. - seamless pressure tubes/pipes
  • 91. 91 Advantages,  - good uniform metal properties  - no sprues/gates to remove  - the outside of the casting is at the required dimensions  - lower material usage  - no parting lines  - low scrap rates
  • 92. 92 Disadvantages, extra equipment needed to spin mold the inner metal of the part contains impurities
  • 93. 93 Casting Defects Gas Defects Shrinkage cavities Molding material defects Pouring material defects Metallurgical defects
  • 94. 94 1. Gas Defects Caused due to entrapped gases in mold due to Lower venting Poor permeability of mold (due to finer grain size, higher clay content, higher moisture content, excessive ramming of mold) Improper design of casting High pouring temperature
  • 95. 95 Blow holes / Open blows  Spherical, flattened or elongated cavities present inside the casting (BLOW HOLES) or on the surface (OPEN BLOW).
  • 96. 96 Air Inclusions  due to absorption of atmospheric and other gases in the molten metal in Furnace.
  • 97. 97 Pin Hole Porosity  Caused by hydrogen in the molten metal.  Picked up in the furnace or by the dissociation of water inside the mold cavity.  at lower temp solubility of gas decreases and therefore hydrogen escapes. It leaves a pin like structure at the surface.
  • 98. 98 Scar  A shallow blow, usually found on the flat casting surface.
  • 99. 99 Blister  Scar covered by thin layers of a metal.
  • 100. 100 Scab  Rough, thin layer of a metal, protruding above the casting surface, on top thin layer of sand.  The scan results the up heaved sand is separated from the mold surface and the liquid metal flow into the space between the mold and the displaced sand.
  • 101. 101 Wash  A low projection on the drag surface of a casting commencing near the gate is called wash.  Due to erosion of sand by the high velocity liquid metal in bottom gating.
  • 102. 102 Misrun  Freezing of molten metal before reaching the farthest point of the mold.  Due to insufficient superheating of metal.
  • 103. 103 Cold Shut  Misrun at the centre in case of two gates is called Coldshut.
  • 104. 104 Hot Tear (Crack due to residual stresses)