3.expendable mold casting

details on expandable mold casting

1. METAL CASTING
PROCESSES
EXPENDABLE MOLD
CASTING
1

2. CAST PRODUCTS
Gray-iron castings including
transmission valve body (left)
and hub rotor with disk-brake
cylinder (front). Source:
Courtesy of Central Foundry
Division of General Motors
Corporation.
A cast transmission housing
Polaroid PDC-2000 digital
camera with a AZ91D die-cast,
high purity magnesium case
Two-piece Polaroid camera case
made by the hot-chamber die
casting process.
Source: Courtesy of Polaroid
Corporation and Chicago White
Metal Casting, Inc.
2

3. COVERAGE
Introduction
Sand Casting
Types of sand
Properties of molding sand
Cores and Core making
Patterns and their type
Expendable-Mold processes-Multiple use pattern
Expendable-Mold processes-Single use pattern
Shakeout Cleaning and Finishing
Summary
3

4. INTRODUCTION
Metal casting process begins by creating a mold,
‘reverse’ shape of the part, made from a refractory
material,
Molten metal is poured into the mould cavity and
allowed to cool until it solidifies.
The solidified metal part is removed from the mould.
Examples: carburettors , engine blocks, automotive components,
crankshaft, crankcase , gun barrels , agricultural and railroad
equipments, architectural components, pies and plumbing fixtures,
power tools, frying pans and large components of hydraulic turbines.
carburetors , engine blocks, automotive components, crankshaft,
crankcase , gun barrels , agricultural and railroad equipments,
architectural components, pies and plumbing fixtures, power tools,
frying pans and large components of hydraulic turbines. 4

5. INTRODUCTION
A large number of metal components used every day
are made by casting.
The reasons for this include:
Casting can produce very complex geometry parts with
internal cavities and hollow sections.
It can be used to make small (few hundred grams) to very
large size parts (thousands of kilograms)
It is economical, with very little wastage: the extra metal
in each casting is re-melted and re-used
Cast metal is isotropic – it has the same
physical/mechanical properties along any direction.
5

6. INTRODUCTION
Factors to consider for castings
Desired dimensional accuracy
Surface quality
Number of castings
Type of pattern and core box needed
Cost of required mold or die
Restrictions due to the selected material
Three categories of molds
Single-use molds with multiple-use patterns
Single-use molds with single-use patterns
Multiple-use molds
6

7. INTRODUCTION
Suitability of casting operation for a given material
depends on:
Melting temp. of job and mold material
Solubility of the job and mold material
Chemical reaction between job and mold material
Solubility of atmosphere in the job material at different
temperatures encountered in the casting
Thermal properties such as conductivity and Coefficient
of linear expansion of both the job and the mold
materials
7

8. DIFFERENT CASTING PROCESSES, THEIR
ADVANTAGES , DISADVANTAGES AND
APPLICATIONS
8

9. SAND CASTING
Sand casting is the most common and versatile form
of casting
Granular material is mixed with clay and water
Packed around a pattern
Gravity flow is the most common method of
inserting the liquid metal into the mold
Metal is allowed to solidify and then the mold is
removed
9

10. SAND CASTING
Fig. 1. Work flow in typical sand-casting foundries [source: www.p2pays.org]
10

11. SAND CASTING
Fig. 2 Schematic showing steps of the sand casting process (Source: Kalpakjian and Schmid)11

12. SAND CASTING
Fig. 2 (Contd.) Schematic showing steps of the sand casting process (Source: Kalpakjian and Schmid)
12

13. SAND CASTING
Fig. 3 Sequential steps in
making a sand casting. a)
A pattern board is placed
between the bottom (drag)
and top (cope) halves of a
flask, with the bottom side
up. b) Sand is then packed
into the bottom or drag half
of the mold. c) 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. d) The
upper or cope half of the
mold is then packed with
sand.
13

14. SAND CASTING
Fig.3 e) The mold is opened,
the pattern board is drawn
(removed), and the runner and
gate are cut into the bottom
parting surface of the sand. e’)
The parting surface of the upper
or cope half of the mold is also
shown with the pattern and pins
removed. f) The mold is
reassembled with the pattern
board removed, and molten
metal is poured through the
sprue. g) The contents are
shaken from the flask and the
metal segment is separated
from the sand, ready for further
processing.
14

15. PATTERNS AND PATTERN
MATERIALS
First step in casting is to design and construct
the pattern
Pattern selection is determined by the number of
castings, size and shape of castings, desired
dimensional precision, and molding process
Pattern materials
Wood patterns are relatively cheap, but not
dimensionally stable
Metal patterns are expensive, but more stable and
durable
Hard plastics may also be used
15

16. TYPES OF PATTERNS
The type of pattern is selected based on the number
of castings and the complexity of the part
One-piece or solid patterns are used when the shape
is relatively simple and the number of castings is
small
Split patterns are used for moderate quantities
Pattern is divided into two segments
oMatch Plate Pattern
oTo produce large quantities of duplicate molds
o Cope and Drag pattern
oUsed for production of large quantities of identical parts or if
casting is too large 16

17. TYPES OF PATTERN
Split pattern,
Follow-board,
Match Plate,
Loose-piece,
Sweep,
Skeleton
pattern
17

18. PATTERN GEOMETRY
18
(a) solid pattern or one piece pattern, (b) split pattern, (c)
match plate pattern (d) cope and drag pattern
‑

19. TYPES OF PATTERNS
Match-plate patterns
Cope and drag segments of a split pattern are permanently
fastened
Pins and guide holes ensure that the cope and drag will be
properly aligned on reassembly
Cope and drag patterns
Used for large quantities of castings
Multiple castings can occur at once
Two or more patterns on each cope and drag
19

20. TYPES OF PATTERNS
Fig.5 Below) Method of using a follow
board to position a single-piece pattern
and locate a parting surface. The final
figure shows the flask of the previous
operation (the drag segment) inverted in
preparation for construction of the upper
portion of the mold (cope segment).
Fig.4 (Above) Single-piece
pattern for a pinion gear.
20

21. MATCH PLATE PATTERN
21

22. COPE AND DRAG PATTERN
22

23. TYPES OF PATTERNS
Fig.6 Split pattern, showing the two sections
together and separated. The light-colored
portions are core prints.
Fig.7 Match-plate pattern used to produce
two identical parts in a single flask. (Left)
Cope side; (right) drag side. (Note: The
views are opposite sides of a single-pattern
board.
23

24. COPE AND DRAG PATTERNS
Fig.8 Cope-and-drag pattern for producing two heavy parts. (Left) Cope section;
(right) drag section. (Note: These are two separate pattern boards.)
24
Used for large quantities of identical parts or when casting is too large
Enable independent molding pg cope and drag segments of mold
Large molds can be handled more easily in separate segments and
smaller molds can be produce at a faster rate

25. LOOSE -PIECE PATTERN
Loose piece patterns are expensive, require careful maintenance, slow
the molding process and increase molding costs
Enable the complex shape sand casting which may otherwise require the
full-mold,l ost foam or investment casting
25
Fig. Loose piece pattern for molding a large
worm gear. After sufficient sand is packed
around the pattern to hold the pieces in
position, the wooden pins are withdrawn.
Mold is completed then and the pieces of
Pattern are sequentially removed.
When the geometry of product does not permit easily with drawl of
single or two piece patterns, a loose piece pattern is used
Separate pieces are joined to a primary pattern segment by beveled
grooves or pins and after molding the primary segment of pattern is
withdrawn.
The hole created permits the removal of remaining segments
sequentially

26. 26
FOUNDRY SANDS
Silica (SiO
2
) or silica mixed with other minerals
Good refractory properties capacity to endure high temperatures
‑
Small grain size yields better surface finish on the cast part
Large grain size is more permeable, allowing gases to escape during
pouring
Irregular grain shapes strengthen molds due to interlocking,
compared to round grains
Disadvantage: interlocking tends to reduce permeability
Binders
Sand is held together by a mixture of water and bonding clay
Typical mix: 90% sand, 3% water, and 7% clay
Other bonding agents also used in sand molds:
Organic resins (e g , phenolic resins)
Inorganic binders (e g , sodium silicate and phosphate)
Additives are sometimes combined with the mixture to increase
strength and/or permeability

27. 27
TYPES OF SAND MOLD
Green sand molds
‑
- mixture of sand, clay, and water;
“Green" means mold contains moisture at time of
pouring
Dry sand mold
‑
- organic binders rather than clay
And mold is baked to improve strength
Skin dried mold
‑
- drying mold cavity surface of a
green sand mold to a depth of 10 to 25 mm, using
‑
torches or heating lamps

28. SANDS AND SAND CONDITIONING
Four requirements of sand used in casting
Refractoriness-ability withstand high temperatures
Cohesiveness-ability to retain shape
Permeability-ability of a gases to escape through the sand
Collapsibility-ability to accommodate shrinkage and part
removal
Size of sand particles, amount of bonding agent,
moisture content, and additives are selected to
obtain sufficient requirements
28

29. FOUNDRY SAND
Ingredients of moulding sand
70-85% silica sand (SiO2)
10-12% bonding material e.g., clay etc.
3-6% water
Requirements of molding sand
Refractoriness – ability to remain solid at high temp
Cohesiveness – bonding
Permeability – gas flow through mould
Collapsibility – ability to permit metal to shrink after solidification
Factors affecting mold performance
Permeability
Green strength
Dry strength
29

30. PROCESSING OF SAND
Green-sand mixture is 88% silica, 9% clay, and 3% water
Each grain of sand needs to be coated uniformly with
additive agents
Muller kneads, rolls, and stirs the sand to coat it
Fig. 9 Schematic diagram of a
continuous (left) and batch-type
(right) sand muller. Plow blades
move and loosen the sand, and
the muller wheels compress and
mix the components. (Courtesy
of ASM International. Metals
Park, OH.)
30

31. SAND TESTING
Blended molding sand is characterized by the following
attributes
Moisture content, clay content, compactibility
Properties of compacted sand
Mold hardness, permeability, strength
Standard testing
Grain size
Moisture content
Clay content
Permeability
Compressive strength
Ability to withstand erosion
Hardness
Compactibility
31

32. SAND TESTING EQUIPMENT
Fig.10 Schematic of a permeability tester in operation. A
standard sample in a metal sleeve is sealed by an O-ring
onto the top of the unit while air is passed through the
sand. (Courtesy of Dietert Foundry Testing Equipment
Inc, Detroit, MI)
Fig.11 Sand mold hardness tester.
(Courtesy of Dietert Foundry Testing
Equipment Inc., Detroit, MI)
32

33. SAND PROPERTIES AND SAND-
RELATED DEFECTS
Silica sand
Cheap and lightweight but undergoes a phase
transformation and volumetric expansion when it is
heated to 585°C
Castings with large, flat surfaces are prone to sand
expansion defects
Trapped or dissolved gases can cause gas-related
voids or blows
33

34. SAND PROPERTIES
Penetration occurs when the sand grains become
embedded in the surface of the casting
Hot tears or crack occur in metals with large
amounts of solidification shrinkage
Tensile stresses develop while the metal is still partially
liquid and if these stresses do not go away, cracking can
occur.
34

35. 35
DESIRABLE PROPERTIES OF SAND
BASED MOLD MATERIAL
Inexpensive in bulk quantity
Retain properties through transportation and
storage
Uniformly fills a flask or container
Compacted or set by simple methods
Sufficient elasticity to remain undamaged during
pattern withdrawal
Withstand high temperatures and maintain its
dimensions until metal solidifies
Sufficient permeable to allow the escape of gases

36. 36
DESIRABLE PROPERTIES OF SAND
BASED MOLD MATERIAL
Sufficiently dense to prevent metal penetration
Sufficiently cohesive to prevent washout of mold
material into the pour stream
Chemically inert to metal being cast
Good collapsibility to permit easy removal and
separation of casting
Can be recycled

37. EFFECT OF MOISTURE, GRAIN SIZE
AND SHAPE ON PROPERTIES OF
MOLDING SAND
37

38. THE MAKING OF SAND MOLDS
Hand ramming is the method of packing sand to
produce a sand mold
Used when few castings are to be made
Slow, labor intensive
Nonuniform compaction
Molding machines
Reduce the labor and required skill
Castings with good dimensional accuracy and
consistency
38

39. THE MAKING OF SAND MOLDS
Molds begin with a pattern and a flask
Mixed sand is packed in the flask
Sand slinger uses rotation to fling sand against the
pattern, slinger is manipulated to deposit sand into the
mold progressively,
It’s a common method to attain uniform compaction in a large
mold and large casting
Jolting is a process in which sand is placed over the flask
and pattern and they are all lifted and dropped to
compact the sand
Squeezing machines use air and a diaphragm
For match plate molding, a combination of jolting
and squeezing is used
39

40. METHODS OF COMPACTING SAND
Fig. 12 (Above) Jolting a mold
section. (Note: The pattern is on the
bottom, where the greatest packing
is expected.)
Fig.13 (Above) Squeezing a sand-filled mold
section. While the pattern is on the bottom, the
highest packing will be directly under the
squeeze head.
40

41. Fig. 14 a)(Left) Schematic diagram showing relative sand densities obtained by flat-
plate squeezing, where all areas get vertically compressed by the same amount of
movement (left) and by flexible-diaphragm squeezing, where all areas flow to the
same resisting pressure (right).
41

42. Fig. 14 b) Various designs of squeeze heads for mold making: (a) conventional flat head;
(b) profile head; (c) equalizing squeeze pistons; and (d) flexible diaphragm. Kalpakjian
and Schmid Manufacturing Engg. And Technology.
42

43. Fig. 14 c). Vertical flaskless molding. (a) Sand is squeezed between two halves of the
pattern. (b) Assembled molds pass along an assembly line for pouring.
Kalpakjian and Schmid Manufacturing Engg. And Technology. 43
VERTICAL FLASKLESS
MOLDING

44. ALTERNATIVE MOLDING
METHODS
Stack molding
Molds containing a cope impression on the bottom and a
drag impression on the top are stacked on top of one
another vertically
Common vertical sprue
Large molds
Large flasks can be placed directly on the foundry floor
Sand slingers may be used to pack the sand
Pneumatic rammers may be used
44

45. GREEN-SAND, DRY-SAND, AND
SKIN-DRIED MOLDS
Green-sand casting
Process for both ferrous and nonferrous metals
Sand is blended with clay, water, and additives
Molds are filled by a gravity feed
Low tooling costs
Least expensive
Automated system capable of producing 300 molds/hr
Design limitations
Rough surface finish
Poor dimensional accuracy
Low strength
45

46. GREEN SAND CASTING
46

47. DRY-SAND MOLD
Mold is heated to a temperature between 150 to 300
0
C and baked to
drive off the moisture which helps in strengthening the mold and
reducing the volume of gas generated during pouring.
Dry-sand molds are durable
Long storage life
Not popular because of long time required for drying
SKIN-DRIED MOLDS
Dries only the sand next to the mold cavity, torches may be used
to dry the sand
Used for large steel parts
Binders like molasses, linseed oil or corn flour may be added to
enhance the strength of the skin-dried layer
Can be given a high silica wash prior to drying to increase the
refractoriness 47

48. CAST PARTS
Fig.15 A variety of sand cast aluminum parts. (Courtesy of Bodine
Aluminum Inc., St. Louis, MO)
48

49. SODIUM SILICATE-CO
2

MOLDING
Molds and cores can receive strength from the addition of
3-6% sodium silicate
Remains soft and moldable until it is exposed to CO
2
CO
2
is nontoxic, nonflammable, odorless and needs no heating to
initiate the reaction.
Sand achieve TS of about 0.3 MPa in just 5 sec of CO
2
gassing with
strength rising to (0.7 -1.4) MPa after 24 hrs of aging.
Hardened sands have poor collapsibility
Shakeout and core removal is difficult
Heating makes the mold stronger
Additives that are used burnout during pouring to
enhance collapsibility
49

50. NO-BAKE, AIR-SET, OR
CHEMICALLY BONDED SANDS
Organic and inorganic resin binders can be mixed
with the sand before the molding operation
Curing reactions begin immediately
Cost of no-bake molding is about 20-30% more than
green-sand molding
High dimensional precision and good surface finish
50

51. NO-BAKE SANDS
No-bake sand can be compacted by light
vibrations
Wood, plastic, fiberglass, or Styrofoam can be used as
patterns
System selections are based on the metal being
poured, cure time desired, complexity and
thickness of the casting, and the possibility of
sand reclamation
Good hot strength
High resistance to mold-related casting defects
Mold decomposes after the metal has been
poured providing good shakeout
51

52. SHELL MOLDING
Basic steps
Individual grains are sand are precoated with a thin
layer of thermosetting phenol (phenol formaldehyde)
resin and heat sensitive liquid catalyst.
This is then dumped, blown or shot onto a heated (230
to 315
0
C) metallic pattern
Heat from the pattern partially cures a layer of material
Pattern and sand mixture are inverted and only the
layer of partially cured material remains
The pattern with the shell is placed in an oven and the
curing process is completed
Hardened shell is stripped from the pattern
Shells are clamped or glued together with a thermoset
adhesive
Shell molds are placed in a pouring jacked and
surrounded by sand, gravel, etc. for extra support
52

53. SHELL MOLDING
Advantages:
High productivity, low labor costs, smooth surfaces, high level
of precision
High quality of casting significantly reduces cleaning,
machining and other finishing costs.
Smoother cavity surface permits easier flow of molten metal and
better surface finish on casting
Good dimensional accuracy
Machining often not required
Mold collapsibility usually avoids cracks in casting
Can be mechanized for mass production
Disadvantages:
More expensive metal pattern
Difficult to justify for small quantities
53

54. DUMP-BOX SHELL MOLDING
Fig.16 Schematic of the dump-box version of shell molding. a) A heated pattern is placed
over a dump box containing granules of resin-coated sand. b) The box is inverted, and the
heat forms a partially cured shell around the pattern. c) The box is righted, the top is
removed, and the pattern and partially cured sand is placed in an oven to further cure the
shell. d) The shell is stripped from the pattern. e) Matched shells are then joined and
supported in a flask ready for pouring.
54

55. SHELL-MOLD PATTERN
Fig.17 (Top) Two halves
of a shell-mold pattern.
(Bottom) The two shells
before clamping, and the
final shell-mold casting
with attached pouring
basin, runner, and riser.
(Courtesy of Shalco
Systems, Lansing, MI.)
55

56. SHELL-MOLD CASTING
56

57. SHELL MOLDING
57

58. OTHER SAND-BASED MOLDING
METHODS
V-Process or Vacuum Molding
Drape a thin sheet of heat-softened plastic over a
specially vented pattern, apply vacuum on the a pattern
so that sheet is drawn tight to the surface of the pattern
A vacuum flask is then put over the pattern and flask is
filled with vibrated dry, unbonded sand
Sprue and pouring cup are formed and a second sheet of
plastic is placed over the pattern
Vacuum is then drawn on the flask itself compacting the
sand to provide necessary strength and hardness
When the vacuum is released, the pattern is withdrawn
58

59. V-PROCESS
Fig.18 Schematic of the V-process or vacuum molding. A) A vacuum is pulled on a pattern,
drawing a heated shrink-wrap plastic sheet tightly against it. b) A vacuum flask is placed over
the pattern and filled with dry unbonded sand, a pouring basin and sprue are formed; the
remaining sand is leveled; a second heated plastic sheet is placed on top; and a mold vacuum
is drawn to compact the sand and hold the shape. c) With the mold vacuum being maintained,
the pattern vacuum is then broken and the pattern is withdrawn. The cope and drag segments
are assembled, and the molten metal is poured.
59

60. ADVANTAGES AND
DISADVANTAGES OF THE V-
PROCESS
Advantages
Absence of moisture-related defects
Binder cost is eliminated
Sand is completely reusable
Finer sands can be used
Better surface finish
No fumes generated during the pouring operation
Exceptional shakeout characteristics
Disadvantages
Relatively slow process
Used primarily for production of prototypes
Low to medium volume parts
More than 10 but less than 50,000
60

61. EFF-SET PROCESS
Wet sand with enough clay to prevent mold collapse
Pattern is removed
Surface of the mold is sprayed with liquid nitrogen
Ice that forms serves as a binder
Molten metal is poured into the mold while the surface
is in frozen condition
Low binder cost and excellent shakeout
NOT used in commercially
61

62. 3. CORES AND CORE MAKING
Complex internal cavities can be produced with
cores
Cores can be used to improve casting design
Cores may have relatively low strength
If long cores are used, machining may need to be
done afterwards
Green sand cores are not an option for more complex
shapes
62

63. DRY-SAND CORES
Produced separate from the remainder of the mold
Inserted into core prints that hold the cores in
position
Dump-core box
Sand is packed into the mold cavity
Sand is baked or hardened
Single-piece cores
Two-halves of a core box are clamped together
63

64. DRY-SAND CORES
Fig.19 V-8 engine block (bottom
center) and the five dry-sand
cores that are used in the
construction of its mold.
(Courtesy of General Motors
Corporation, Detroit, MI.)
64

65. ADDITIONAL CORE METHODS
Core-oil process
Sand is blended with oil to develop strength
Wet sand is blown or rammed into a simple core box
Hot-box method
Sand is blended with a thermosetting binder
Cold-box process
Binder coated sand is packed and then sealed
Gas or vaporized catalyst polymerizes the resin
65

66. ADDITIONAL CORE METHODS
Fig.21 (Right) Upper Right; A dump-
type core box; (bottom) core halves for
baking; and (upper left) a completed
core made by gluing two opposing
halves together.
Fig.20 (Left) Four methods of making a hole
in a cast pulley. Three involve the use of a
core.
66

67. ADDITIONAL CORE
CONSIDERATIONS
Air-set or no-bake sands may be used
Eliminate gassing operations
Reactive organic resin and a curing catalyst
Shell-molding
Core making alternative
Produces hollow cores with excellent strength
Selecting the proper core method is based on the
following considerations
Production quantity, production rate, required
precision, required surface finish, metal being poured
67

68. CASTING CORE
CHARACTERISTICS
Sufficient strength before hardening
Sufficient hardness and strength after hardening
Smooth surface
Minimum generation of gases
Adequate permeability
Adequate refractoriness
Collapsibility
68

69. TECHNIQUES TO ENHANCE
CORE PROPERTIES
Addition of internal wires or rods
Vent holes
Cores can be connected to the outer surfaces of the
mold cavity
Core prints
Chaplets- small metal supports that are placed
between the cores and the mold cavity surfaces and
become integral to the final casting
69

70. CHAPLETS: USED TO AVOID CORE
SHIFTING
Fig,.22 (Left) Typical chaplets. (Right) Method of supporting a core by use of
chaplets (relative size of the chaplets is exaggerated).
70

71. MOLD MODIFICATIONS
Cheeks are second parting lines that allow parts to be
cast in a mold with withdrawable patterns
Inset cores can be used to improve productivity
Fig.24 (Left) Method of making a reentrant angle or inset
section by using a three-piece flask.
Fig.23 (Right) Molding an
inset section using a dry-
sand core.
71

72. 4. OTHER EXPENDABLE-MOLD
PROCESSES WITH MULTIPLE-USE
PATTERNS
Plaster mold casting
Mold material is made out of plaster of paris (Calcium
Sulphate or Gypsum)
Slurry is poured over a metal pattern
Improved surface finish and dimensional accuracy
Limited to the lower-melting-temperature nonferrous alloys
Antioch process
Variation of plaster mold casting
50% plaster, 50% sand
72

73. PLASTER MOLD CASTING
73

74. PLASTER MOLD CASTING
74
Advantages:
Good dimensional accuracy and surface finish
Capability to make thin cross sections in casting
‑
Disadvantages:
Moisture in plaster mold causes problems:
Mold must be baked to remove moisture
Mold strength is lost when is over-baked, yet moisture content
can cause defects in product
Plaster molds cannot stand high temperatures, so limited to
lower melting point alloys

75. CERAMIC MOLD CASTING
Mold is made from ceramic material as it withstands higher
temperatures
Greater mold cost than other casting methods
Shaw process
Reusable pattern inside a slightly tapered flask
Pour mixture of refractory aggregate, hydrolyzed ethyl silicate,
alcohol and gelling agent, it sets to a rubbery state that allows
the part and flask to be removed
Mold surface is then ignited with a torch, burning thus the
volatiles and producing a 3D network of microcracks
(microcrazing) in ceramic
The gaps are small enough to prevent metal penetration and large
enough to provide venting (permeability) and to accommodate
both the thermal expansion of ceramic particles during pour and
the subsequent shrinkage of the solidified metal. 75

76. Fig.25 Group of intricate cutters
produced by ceramic mold casting.
(Courtesy of Avnet Shaw Division of
Avnet, Inc., Phoenix, AZ)
76
Ceramic Mold Casting

77. OTHER CASTING METHODS
Expendable graphite molds
Some metals are difficult to cast
Titanium
Reacts with many common mold materials
Powdered graphite can be combined with additives
and compacted around a pattern
Mold is broken to remove the product
Rubber-mold casting
Artificial elastomers can be compounded in liquid
form and poured over the pattern to produce a
semirigid mold
Limited to small castings and low-melting-point
materials
77

78. 5. EXPENDABLE-MOLD PROCESSES
USING SINGLE-USE PATTERNS
Investment casting
One of the oldest casting
methods
Products such as rocket
components, and jet
engine turbine blades
Complex shapes
Most materials can be
casted
Fig.26 Typical parts produced by investment casting.
(Courtesy of Haynes International, Kokomo, IN.)
78

79. INVESTMENT CASTING
Sequential steps for investment casting
Produce a master pattern
Produce a master die
Produce wax patterns
Assemble the wax patterns onto a common wax sprue
Coat the tree with a thin layer of investment material
Form additional investment around the coated cluster
Allow the investment to harden
Remove the wax pattern from the mold by melting or dissolving
Heat the mold before pouring to 550 to 1100
0
C to ensure
complete removal of mold wax, curing to add strength and to
allow molten metal to retain heat and flow more readily into all
of the thin sections and details.
Pour the molten metal
Remove the solidified casting from the mold
79

80. INVESTMENT CASTING
Fig.27 Investment-casting steps for the flask-cast method. (Courtesy of Investment Casting
Institute, Dallas, TX.)
80

81. INVESTMENT CASTING
Complex shapes can be
cast
Thin sections can be cast
Machining can be
eliminated or reduced
Complex process
Can be costly
Advantage Disadvantage
81

82. INVESTMENT CASTING
Fig.28 Investment-casting steps for the shell-casting procedure.(Courtesy of Investment
Casting Institute, Dallas, TX.)
82

83. INVESTMENT CASTING
Fig.28 83

84. PROCESS STEPS
Pattern creation - The wax
patterns are typically
injection molded into a metal
die and are formed as one
piece.
Several of these patterns are
attached to a central wax
gating system (sprue,
runners, and risers), to form
a tree-like assembly.
The gating system forms the
channels through which the
molten metal will flow to the
mold cavity.
84

85. PROCESS STEPS: SHELL MAKING
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.
85

86. PROCESS STEPS: 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.
Pouring is typically
achieved manually under
the force of gravity, but
other methods such as
vacuum or pressure are
sometimes used.
86

87. PROCESS STEPS: COOLING AND REMOVAL
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.
87

88. PROCESS STEPS: REMOVAL AND FINISHING
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. 88

89. COUNTER-GRAVITY
INVESTMENT CASTING
Pouring process is upside down
Vacuum is used within the chamber
Draws metal up through the central sprue and into the mold
Free of slag and dross, Low level of inclusions
Little turbulence
Improved machinability
Mechanical properties approach those of wrought material
Gating does not need to control turbulence so simpler gating
systems is employed which also results in high yield as 60 to 95% of
the withdrawn metal becomes cast product s against 15 to 50% in
gravity pouring.
Lower pouring temperatures, result in improved grain structure
and better surface finish
89

90. VACUUM CASTING
Also known as counter gravity low pressure (CL) process
Mixture of fine sand and urethane is molded over metal dies and
cured with amine vapor
Hold the mold with robot arm, partially immersed in molten metal
held in induction furnace
If metal is melted in Air (CLA) and If in vacuum (CLV) process
Vacuum reduces the air pressure inside mold to about 2/3
rd
of atm.
Pressure and draws the molten metal into the mold cavity through
the gate in the bottom
Molten metal is kept at a temperature 55
0
C above liquidus, so that
metal starts solidifying in fraction of second
Its an alternative to investment, shell mold and green sand casting
Suitable for thin walled (0.7 mm) complex shapes with uniform
properties
CLA parts are made at high volume and relatively low cost
CLV parts usually contain reactive metals like Ti, Al, Zr, Hf..these
parts in form of superalloys for gas turbine may be 0.5 mm thick
90

91. VACUUM CASTING
91
Schematic illustration of the vacuum-casting process. Note that the mold has a bottom
gate. (a) Before and (b) after immersion of the mold into the molten metal. Source:
From R. Blackburn, "Vacuum Casting Goes Commercial," Advanced Materials and
Processes, February 1990, p. 18. ASM International.

92. EVAPORATIVE PATTERN (FULL-
MOLD AND LOST-FOAM)
CASTING
Reusable patterns can complicate withdrawal
May mandate design modifications
Evaporative pattern processes
Pattern is made of expanded polystyrene (EPS) or
polymethylmethacrylate (EPMMA)
Pattern remains in the mold until the molten metal melts away the
pattern
If small quantities are required, patterns may be cut by hand
Material is lightweight
Preformed material in the form of pouring basin, sprue, runner
segments and risers can be attached with hot-melt glue to form
complete gating and patterns assembly. Small products can be
assembled into clusters or trees,
92

93. EVAPORATIVE PATTERNS
Metal mold or die is used to mass-produce the
evaporative patterns
Hard Polystyrene beads are first preexpended ans stabilized
and then injected into a preheated die or mold made of
Aluminium.
A Steam cycle further expends these beads to fill the die and
fuse before getting cooled in the die.
Resulting pattern is 2.5% polymer and 97.5% air
For multiple and complex shapes, patterns can be divided
into segments or slices assembled by hot-melt gluing
Once polystyrene gating system is attached to pattern
there are several options for mold like lost foam process
‑
,
lost pattern process, evaporative foam process
‑
, and full mold
‑
process
93

94. FULL MOLD PROCESS

Schematic of the full mold process. (Left) An uncoated EPS
pattern is supported by bonded sand to produce a mold.
(Right) Hot metal progressively vaporizes the EPS pattern and
fills the resulting cavity.
94
Uses a mold of sand packed around a polystyrene foam pattern which
vaporizes when molten metal is poured into mold

95. - Styrofoam pattern
- dipped in refractory slurry  dried
- sand (support)
- pour liquid metal
- foam evaporates, metal fills the shell
- cool, solidify
- break shell  part
1 Expanded polystyrene casting
process: pattern of polystyrene is
coated with refractory compound;

96. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
EXPANDED POLYSTYRENE PROCESS
Expanded polystyrene casting
process: (2) foam pattern is
placed in mold box, and sand is
compacted around the pattern;
Expanded polystyrene casting process:
(3) molten metal is poured into the
portion of the pattern that forms the
pouring cup and sprue. As the metal
enters the mold, the polystyrene foam is
vaporized ahead of the advancing liquid,
thus the resulting mold cavity is filled.

97. LOST FOAM PROCESS
Fig.29 Schematic of the lost-foam casting process. In this process, the polystyrene
pattern is dipped in a ceramic slurry, and the coated pattern is then surrounded with
loose, unbonded sand.
97

98. LOST FOAM PROCESS
98
Polystyrene assembly is dipped into water based ceramic
that forms a refractory coating thin enough and
sufficiently permeable to permit escape of the molten and
gaseous pattern material but rigid to prevent mold
collapse
Suspend assemble in flask surrounded by sand and
vibrate it to compact the sand
Molten metal vaporises polystyrene and coating separates
metal from sand
Solidified casting removed after dumping loose sand from
the flask

99. ADVANTAGES OF THE FULL-
MOLD AND LOST-FOAM
PROCESS
Sand can be reused
Can produce castings of any size with ferrous or non ferrous
metals
No draft since no withdrawl of pattern
Complex shapes can be produced that would otherwise
need cores, loose piece patterns or extensive finishing
High precision and smooth surface finish so machining or
finishing minimized or totally eliminated
Pattern need not be removed from the mold
Fragile or complex cores not required
Absence of parting line eliminates need to remove fins or
associated lines
Simplifies and expedites mold making, since two mold
‑
halves (cope and drag) are not required as in a conventional
green sand mold
‑
99

100. DISADVANTAGES
A new pattern is needed for every casting
Economic justification of the process is highly dependent on
cost of producing patterns
100

101. LOST-FOAM CASTING
Fig.30 The stages of lost-foam casting, proceeding counterclockwise from the lower left:
polystyrene beads→ expanded polystyrene pellets → three foam pattern segments → an
assembled and dipped polystyrene pattern → a finished metal casting that is a metal
duplicate of the polystyrene pattern. (Courtesy of Saturn Corporation, Spring Hill, TN.)
101

102. LOST-FOAM CASTING
102

103. 6. SHAKEOUT, CLEANING, AND
FINISHING
Final step of casting involves separating the molds
and mold material
Shakeout operations
Separate the molds and sand from the flasks
Punchout machines
Vibratory machines
Rotary separators
Blast cleaning
103

104. SUMMARY
Control of mold shape, liquid flow, and solidification
provide a means of controlling properties of the
casting
Each process has unique advantages and
disadvantages
Best method is chosen based on the product shape,
material and desired properties
104