METAL CASTING PROCESSES

1. Sand Casting

2. Other Expendable Mold Casting Processes

3. Permanent Mold Casting Processes

4. Foundry Practice
METAL CASTING PROCESSES
 Two Categories of Casting
Processes
1. Expendable mold processes - mold is sacrificed to
remove part
 Advantage: more complex shapes possible
 Disadvantage: production rates often limited by
the time to make mold rather than casting itself
2. Permanent mold processes - mold is made of metal
and can be used to make many castings
 Advantage: higher production rates
 Disadvantage: geometries limited by need to
open mold
 Overview of Sand Casting

 Most widely used casting process, accounting for a

significant majority of total tonnage cast
 Nearly all alloys can be sand casted, including metals
with high melting temperatures, such as steel, nickel,
and titanium
 Castings range in size from small to very large
 Production quantities from one to millions
 Sand casting weighing over 680 kg (1500 lb) for an
air compressor frame (photo courtesy of Elkhart
Foundry).
 Steps in Sand Casting

1. Pour the molten metal into sand mold

2. Allow time for metal to solidify
3. Break up the mold to remove casting
4. Clean and inspect casting

 Separate gating and riser system

5. Heat treatment of casting is sometimes required to
improve metallurgical properties
 Making the Sand Mold

 The cavity in the sand mold is formed by packing sand

around a pattern, then separating the mold into two
halves and removing the pattern
 The mold must also contain gating and riser system
 If casting is to have internal surfaces, a core must be
included in mold
 A new sand mold must be made for each part
produced
 Sand Casting Production
Sequence
 Production sequence in sand casting, including
pattern‑making and mold‑making
 The Pattern

Full‑sized model of part, slightly enlarged to account for

shrinkage and machining allowances in the casting
 Pattern materials:
 Wood - common material because it is easy to
work, but it warps
 Metal - more expensive to fabricate, but lasts
longer
 Plastic - compromise between wood and metal
 Types of Patterns

 Types of patterns used in sand casting: (a) solid

pattern, (b) split pattern, (c) match‑plate pattern, (d)
cope and drag pattern
 Core

Full‑scale model of interior surfaces of part

 Inserted into mold cavity prior to pouring
 The molten metal flows and solidifies between the mold
cavity and the core to form the casting's external and
internal surfaces
 May require supports to hold it in position in the mold
cavity during pouring, called chaplets
 Core in Mold

 (a) Core held in place in the mold cavity by chaplets,

(b) possible chaplet design, (c) casting
 Properties of Mold

 Strength ‑ to maintain shape and resist erosion

 Permeability ‑ to allow hot air and gases to pass
through voids in sand
 Thermal stability ‑ to resist cracking on contact with
molten metal
 Collapsibility ‑ ability to give way and allow casting to
shrink without cracking the casting
 Reusability ‑ can sand from broken mold be reused to
make other molds?
 Other Expendable Mold
Processes
 Shell Molding
 Investment Casting
 Shell Molding

Casting process in which

the mold is a thin shell
of sand held together by
thermosetting resin
binder
 Steps: (1) A metal
pattern is heated and
placed over a box
containing sand mixed
with thermosetting resin
 Steps in Shell Molding

 (2) Box is inverted so

that sand and resin
fall onto the hot
pattern, causing a
layer of the mixture to
partially cure on the
surface to form a
hard shell
 Steps in Shell Molding

 (3) Box is
repositioned so
loose uncured
particles drop
away
 Steps in Shell Molding

 (4) Sand shell is

heated in oven for
several minutes to
complete curing
 Steps in Shell Molding

 (5) shell mold

is stripped
from pattern
 Steps in Shell Molding

 (6) Two halves of the shell

mold are assembled,
supported by sand or
metal shot in a box, and
pouring is accomplished
Shell Moulding

TWO shells are clamped together inside a mould box

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Shell Moulding
Molten metal
poured

22
Shell Moulding
Molten
Metal

23
Shell Moulding
Solidified

24
Shell Moulding

Break the mould

25
Shell Moulding

Take the casting out side

26
Shell Moulding
Remove the unwanted
projections

27
Shell Moulding

The final product is ready

28
Shell Moulding
Steps involved in shell moulding – pictorial representation

29
 Shell Molding: Advantages and
Disadvantages

 Advantages:
 Smoother cavity surface permits easier flow of
molten metal and better surface finish
 Good dimensional accuracy
 Mold collapsibility minimizes cracks in casting
 Can be mechanized for mass production
 Disadvantages:
 More expensive metal pattern
 Difficult to justify for small quantities
 Other Expendable Mold
Processes
 Shell Molding
 Investment Casting
 Investment Casting
(Lost Wax Process)

A pattern made of wax is coated with a refractory material

to make the mold, after which wax is melted away prior
to pouring molten metal
 "Investment" comes from a less familiar definition of
"invest" - "to cover completely," which refers to coating
of refractory material around wax pattern
 It is a precision casting process
 Capable of producing castings of high accuracy and
intricate detail
Investment Moulding
Wide range of application -

Metallurgical Combustion
arts chamber of jet
aircrafts

33
Investment Moulding
X 5 nos.

Final product

A wax pattern used to create a jet

engine turbine blade

WAX Pattern (5 Nos.)

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Investment Moulding

WAX
Investment Moulding

Align the patterns to form a

complete WAX TREE

Complete WAX TREE

36
Investment Moulding

Dip the Wax tree in to the

Dump Box filled with
Investment slurry

Plaster of Paris
+
Binder
+
Powdered Silica

37
Investment Moulding
WAX tree is coated
with a very thin shell of
investment coating

38
Investment Moulding

Heated
Heated

WAX drains off…

Heated

Heated

39
Investment Moulding

Thin shell of investment

coating remains
Investment Moulding

Heated Heated Again heated to cure the

investment coating and remove
all the remaining traces of wax

Heated

Heated

41
Investment Moulding

The investment coating is

inverted

Ready for pouring molten

metal
Investment Moulding

Molten metal poured in

Investment Moulding

Allowed to solidify
Investment Moulding

Break the mould by mechanical

vibrations
or
water blasting
Investment Moulding

Complete metal tree

Investment Moulding

Cut off the required part from the

metal tree

(Fettling process)
Investment Moulding

Final product (5 Nos.)

Investment Moulding

MOVIE -1
49
 Steps in Investment Casting

 (1) Wax patterns are

produced
 (2) Several patterns
are attached to a
sprue to form a
pattern tree
 Steps in Investment Casting

 (3) Pattern tree is

coated with a thin
layer of refractory
material
 (4) Full mold is
formed by covering
the coated tree with
sufficient refractory
material to make it
rigid
 Steps in Investment Casting

 (5) Mold is held in an

inverted position and
heated to melt the wax
and permit it to drip out
of the cavity
 (6) Mold is preheated
to a high temperature,
the molten metal is
poured, and it solidifies
 Steps in Investment Casting

 (7) Mold is broken

away from the finished
casting and the parts
are separated from
the sprue
 Investment Casting:
Advantages and Disadvantages
 Advantages:
 Parts of great complexity and intricacy can be cast
 Close dimensional control and good surface finish
 Wax can usually be recovered for reuse
 This is a net shape process
 Additional machining is not normally required
 Disadvantages:
 Many processing steps are required
 Relatively expensive process
 Permanent Mold
Casting Processes
 Economic disadvantage of expendable mold casting:
 A new mold is required for every casting
 In permanent mold casting, the mold is reused many
times
 The processes include:
 Basic permanent mold casting
 Die casting
 Centrifugal casting
 The Basic Permanent Mold
Process

Uses a metal mold constructed of two sections

designed for easy, precise opening and closing
 Molds used for casting lower melting point alloys are
commonly made of steel or cast iron
 Molds used for casting steel must be made of
refractory material, due to the very high pouring
temperatures
 Steps in
Permanent Mold Casting
 (1) Mold is preheated and coated for lubrication and
heat dissipation
 Steps in
Permanent Mold Casting
 (2) Cores (if any
are used) are
inserted and
mold is closed
 Steps in
Permanent Mold Casting

 (3) Molten metal is

poured into the mold,
where it solidifies
 Permanent Mold Casting:
Advantages and Limitations

 Advantages of permanent mold casting:

 Good dimensional control and surface finish
 Rapid solidification caused by metal mold results in
a finer grain structure, so castings are stronger
 Limitations:
 Generally limited to metals of lower melting point
 Simpler part geometries compared to sand casting
because of need to open the mold
 High cost of mold
 Applications and Metals for
Permanent Mold Casting
 Due to high mold cost, process is best suited to high
volume production and can be automated accordingly

 Typical parts: automotive pistons, pump bodies, and

certain castings for aircraft and missiles
 Metals commonly cast: aluminum, magnesium,
copper‑base alloys, and cast iron
 Unsuited to steels because of very high pouring
temperatures
 Die Casting

A permanent mold casting process in which molten

metal is injected into mold cavity under high pressure

 Pressure is maintained during solidification, then

mold is opened and part is removed
 Molds in this casting operation are called dies; hence
the name die casting
 Use of high pressure to force metal into die cavity is
what distinguishes this from other permanent mold
processes
 Die Casting Machines

 Designed to hold and accurately close two mold

halves and keep them closed while liquid metal is
forced into cavity
 Two main types:
1. Hot‑chamber machine
2. Cold‑chamber machine
 Hot-Chamber Die Casting

Metal is melted in a container, and a piston injects liquid

metal under high pressure into the die
 High production rates
 500 parts per hour not uncommon
 Applications limited to low melting‑point metals that
do not chemically attack plunger and other
mechanical components
 Casting metals: zinc, tin, lead, and magnesium
 Hot-Chamber Die Casting

 Hot‑chamber die
casting cycle: (1)
with die closed
and plunger
withdrawn,
molten metal
flows into the
chamber
 Hot-Chamber Die Casting

 (2) plunger forces

metal in chamber to
flow into die,
maintaining pressure
during cooling and
solidification.
 Hot-Chamber Die Casting

 (3) Plunger is
withdrawn, die is
opened, and
casting is ejected
 Cold‑Chamber Die Casting
Machine

Molten metal is poured into unheated chamber from

external melting container, and a piston injects metal
under high pressure into die cavity
 High production but not usually as fast as hot‑chamber
machines because of pouring step
 Casting metals: aluminum, brass, and magnesium alloys

 Advantages of hot‑chamber process favor its use on low

melting‑point alloys (zinc, tin, lead)
 Cold‑Chamber Die Casting Cycle

 (1) With die closed and ram withdrawn, molten metal is

poured into the chamber
 Cold‑Chamber Die Casting Cycle

 (2) Ram forces metal to flow into die, maintaining

pressure during cooling and solidification
 Cold‑Chamber Die Casting Cycle

 (3) Ram is withdrawn, die is opened, and part is

ejected
 Molds for Die Casting

 Usually made of tool steel, mold steel, or maraging

steel
 Tungsten and molybdenum (good refractory qualities)
used to die cast steel and cast iron
 Ejector pins required to remove part from die when it
opens
 Lubricants must be sprayed onto cavity surfaces to
prevent sticking
 Die Casting:
Advantages and Limitations
 Advantages:
 Economical for large production quantities
 Good accuracy and surface finish
 Thin sections possible
 Rapid cooling means small grain size and good
strength in casting
 Disadvantages:
 Generally limited to metals with low metal points
 Part geometry must allow removal from die
 Centrifugal Casting

• Used for casting AXI - SYMETRICAL components.

• The Inertial force of rotation plays the major role.

(Centrifugal force)

• Hence the name; “Centrifugal Castings”

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Centrifugal Casting

Molten metal poured

Molten metal is thrown to the walls
of mould cavity

Rotating Mould Box

Axis of rotation
(Vertical)
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Centrifugal Casting
Rotation
Solidification
maintained
Complete
until
complete Casting
Centrifugal solidification
Process

Rotating Mould Box

Axis of rotation
(Vertical)
76
 Centrifugal Casting
Solidification Complete

MOVIE

Rotating Mould Box

Axis of rotation
(Vertical)
Rotating Mould Box
 Centrifugal Casting
Solidification Complete

Axis of rotation
(Horizontal) Mainly for castings with
Uniform diameter

Rotating Mould Box 78

Centrifugal Casting

79
 Centrifugal Casting

Seamless pipes
 Surface Cleaning and Inspection

Removal of sand from casting surface and otherwise

enhancing appearance of surface
 Cleaning methods: tumbling, air‑blasting with coarse
sand grit or metal shot, wire brushing, buffing, and
chemical pickling
 Surface cleaning is most important for sand casting
 In many permanent mold processes, this step
can be avoided
 Defects are possible in casting, and inspection is
needed to detect their presence
 Casting Quality

 There are numerous opportunities for things to go

wrong in a casting operation, resulting in quality
defects in the product
 The defects can be classified as follows:
 General defects common to all casting
processes
 Defects related to sand casting process
 General Defects:
Misrun

 A casting that has

solidified before
completely filling mold
cavity
 General Defects:
Cold Shut

 Two portions of metal

flow together but there
is a lack of fusion due
to premature freezing
 General Defects:
Cold Shot

 Metal splatters during

pouring and solid
globules form and
become entrapped in
casting
 General Defects:
Shrinkage Cavity

 Depression in surface
or internal void caused
by solidification
shrinkage that restricts
amount of molten
metal available in last
region to freeze
 Sand Casting Defects:
Sand Blow

 Balloon‑shaped gas
cavity caused by
release of mold
gases during pouring
 Sand Casting Defects:
Pin Holes

 Formation of many small

gas cavities at or slightly
below surface of casting
 Sand Casting Defects:
Penetration

 When fluidity of liquid metal

is high, it may penetrate
into sand mold or core,
causing casting surface to
consist of a mixture of sand
grains and metal
 Sand Casting Defects:
Mold Shift

 A step in the cast

product at parting
line caused by
sidewise relative
displacement of
cope and drag