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 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
A large sand casting
▪ Production quantities from one to weighing over 680 kg
millions (1500 lb) for an air
compressor frame (photo
courtesy of Elkhart
Foundry).
 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
 Other Expendable Mold Processes
▪ Shell Molding
▪ Investment Casting
▪ Expanded Polystyrene Process
 1. Shell Molding
Casting process in which the mold
is a thin shell of sand held
together by thermosetting resin
binder

Steps in shell-molding:

(1) a match-plate or cope-and-drag

metal pattern is heated and
placed over a box containing
sand mixed with thermosetting
resin.
 1. 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

(3) box is repositioned so that loose

uncured particles drop away;
 1. Shell Molding
(4) Sand shell is heated in oven for
several minutes to complete
curing

(5) shell mold is stripped from the

pattern;
 1. Shell Molding
(6) two halves of the shell mold are
assembled, supported by sand or
metal shot in a box, and pouring is
accomplished;

(7) the finished casting with sprue

removed.
 1. Shell Molding
▪ Advantages of shell molding:
▪ Smoother cavity surface permits easier flow of molten metal and better
surface finish
▪ Good dimensional accuracy - machining often not required
▪ Can be mechanized for mass production

▪ Disadvantages:
▪ More expensive metal pattern
▪ Difficult to justify for large quantities
▪ The size of casting obtained by shell casting is limited.

▪ Application:
▪ Cylinders and cylinder heads for air-cooled IC engine, Cast tooth bevel
gear, Small crank shaft
 2. Investment Casting (Lost Wax Process)
A pattern made of wax is coated with a refractory material to make
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
 Limitation
Basic Steps Advantages
Application

Metallic die
1) Pattern Making :-
▪ Molten wax which is used as pattern material is
injected in metallic die
Pattern
▪ Wax after solidify would produce pattern
▪ Pattern is ejected from the die Wax injection

2) Assembly :- Sprue
▪ A number of patterns (depending on size and
complexity) are attached to a central wax
stick, or sprue, to form a casting cluster, or
assembly by applying heat
Assembly
 Limitation
Basic Steps Advantages
Application

3) Shell Building :-

▪ To make the mould, the assembly of prepared pattern

is dipped into a slurry made by suspending fine
ceramic materials in a liquid such as ethyl silicate or Slurry
sodium silicate
▪ Slurry will get coated around the pattern forming
shell Shell building

▪ The shell is cured and process of dipping is continued with ceramic slurry
▪ The shell thickness required depends on the casting shape and mass, type of
ceramic and the binder used
 Limitation
Basic Steps Advantages
Application

4) Dewax / Burnout : -

▪ In this step, wax pattern is removed from the

mould, which is done by heating the mould to
melt the pattern

▪ The melted wax is completely drained

Melted
through the sprue by inverting the mould
wax
Dewax
 Limitation
Basic Steps Advantages
Application

5) Gravity Pouring :- Molten metal

▪ In the conventional gravity pouring method,

metal is poured into the shell through a funnel-
shaped pour cup and flows by gravity down the
sprue channel, through the gates and into the
part cavities.

▪ As the metal cools, the parts, gates, sprue and

pouring cup become one solid casting. Gravity Pouring
 Limitation
Basic Steps Advantages
Application

6) Knock out :-
▪ After the casting has cooled, the ceramic shell is
broken off

7) Cut – off : -
▪ The parts are cut from the sprue using a high
speed friction saw.

8) Finished casting :-
▪ After minor finishing operations, the castings,
which are identical in configuration to the wax
patterns which shaped them, are ready for
certification and shipment to the customer
 Limitation
Basic Steps Advantages
Application

1. Complex shapes which are difficult to produce by any other method are
possible
2. Very fine details and thin sections can be produced by this process
3. Very close tolerances and better surface finish can be produced
4. Casting produced by investment casting process are ready for use with little
or no machining required
 Limitation
Basic Steps Advantages
Application

1. The process is normally limited by the size and mass of casting. The upper
limit on the mass of a casting may be of the order of 35 Kg
2. This is more expensive process because of large manual labour involved in
the preparation of the pattern and the mould.

Application
1. In old days this process was used for the preparation of jewellery and surgical
instruments
2. Presently it is used for production of vanes and blades for gas turbines,
mechanical Components such as gears, cams, valves etc., wave guide for
radars, bolts and triggers for fire arms, stainless steel valve bodies and
impellers for turbo chargers
 3. Expanded Polystyrene Process
▪ Other names:
lost-foam process,
lost pattern process,
Evaporative-foam process,
Full-mold process

▪ Uses a mold of sand packed around a polystyrene foam pattern which

vaporizes when molten metal is poured into mold

▪ Polystyrene foam pattern includes sprue, risers, gating system, and

internal cores (if needed)
 3. Expanded Polystyrene Process
1) Foam pattern is placed in mold 2) Molten metal is poured into the
box, and sand is compacted portion of the pattern that forms the
around the pattern; 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.
 3. Expanded Polystyrene Process
▪ Advantages of expanded polystyrene process:
▪ Pattern need not be removed from the mold
▪ Simplifies and speeds mould making, because two mold
halves are not required as in a conventional green-sand mold
▪ Disadvantages:
▪ A new pattern is needed for every casting
▪ Economic justification of the process is highly dependent on
cost of producing patterns
▪ Applications:
▪ automobile engines
 Type 2] 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:

2.1 Permanent mold casting/Gravity casting
2.2 Die casting
2.3 Centrifugal casting
 2-1. Permanent mould casting
Uses a metal mold constructed of two sections designed for easy,
precise opening and closing

▪ Mould material used, for casting, lower melting point alloys, are
commonly made of steel or cast iron

▪ Moulding material used, for casting steel, must be made of refractory

material, due to the very high pouring temperatures

▪ It is also called gravity mould casting since the metal enters the
mould cavity under gravity.
 2.1 Permanent Mold Casting
Steps in permanent mold casting:

(1) mold is preheated and coated

 2.1 Permanent Mold Casting
(2) cores (if used) are inserted and mold is closed,

(3) molten metal is poured into the mold, where it solidifies.

(4) Gray cast iron used for mould material(High thermal fatigue).
 Advantages and Limitations
▪ Advantages of permanent mold casting:
▪ Good dimensional control and surface finish
▪ More rapid solidification caused by the cold 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.
▪ Maximum size of the casting that can be produced is limited because of
the equipment
▪ Low melting point metals AL, Zn, Magnesium, Brass.
▪ Application:
▪ Automobile pistons, connecting rod, aircraft fittings, cylinder block etc.
 2.2 Die Casting (Pressure die-casting)
It is casting technique which involves, injection of molten metal into
permanent metallic mould under high pressure (100-150 MPa) .

▪ It is also called as pressure die casting

▪ 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
▪ Two main types:

a) Hot-chamber machine
b) Cold-chamber machine

The main difference between these two types is that ------

In the hot chamber machine, the holding furnace for the liquid metal
is integral with the diecasting machine

In cold chamber machine, the metal is melted in a separate furnace

and then poured into the diecasting machine with a ladle for
each casting cycle
 2.2 (a)Hot-Chamber Die Casting
Steps in hot chamber die casting:

(1) Initially die are closed and

plunger withdrawn, molten
metal flows into the chamber

(2) plunger forces metal in

chamber to flow into die,
maintaining pressure during
cooling and solidification.

▪ Gooseneck is used to pump liquid metal into die cavity. The gooseneck
is submerged in the holding furnace containing liquid molten metal. It is
made up of grey alloy or ductile iron or cast steel.
 2.2 (a) 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

▪ Applications limited to low melting point metals because -----

- For materials like Al and brass, they have high Melting
temperature
- Gooseneck of hot chamber is continuously in contact with
molten metal
- Liquid ‘Al’ would attack the gooseneck material

▪ Casting metals: zinc, tin, lead, and magnesium

 2.2 (b) Cold-Chamber Die Casting
Cycle in cold-chamber casting:

(1) with die closed and ram withdrawn, molten metal is poured into the
chamber
 2.2 (b)Cold-Chamber Die Casting
(2) ram forces metal to flow into die, maintaining pressure during cooling
and solidification.
 2.2 (b)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 (High M.P.)

 2.2 Molds for Die Casting
▪ Usually made of tool steel, mold steel, gray cast iron

▪ Tungsten and molybdenum (good refractory qualities) used to die

cast steel and cast iron
 2.2 Die Casting (Pressure die-casting)
▪ Advantages of die casting:
▪ Economical for large production quantities
▪ Good accuracy and surface finish
▪ Thin sections are possible due to injection of liquid metal with
pressure
▪ Rapid cooling provides small grain size and good strength to
casting

▪ Disadvantages:
▪ Generally limited to metals with low metal points
▪ Part geometry must allow removal from die
▪ The dies and machines are very expensive
Limitation:
▪ High initial die cost
▪ Part size limited
▪ Porosity may be a problem
▪ Some scrap in sprue, runner this can be directly
recycled.
Application:
▪ Carburettors
▪ Automotive parts
▪ Toys
▪ Bathroom fixtures
 2.3 Centrifugal Casting
A family of casting processes in which the mold is rotated at high
speed so centrifugal force distributes molten metal to outer regions
of die cavity

▪ The group includes:

2.3 (a)True centrifugal casting
2.3 (b) Semi centrifugal casting
2.3 (c) Centrifuge casting
 2.3 (a)True Centrifugal Casting
Molten metal is poured into rotating mold to produce a tubular part

▪ Parts: hollow pipes, tubes, bushings, and rings which are axi-
symmetric with concentric hole

▪ Since the metal is pushed outwards because of centrifugal force, no

core needs to be used for making concentric hole

▪ Outside shape of casting can be round, octagonal, hexagonal, etc ,

but inside shape is (theoretically) perfectly round, due to radially
symmetric forces
2.3 (a)True Centrifugal Casting
2.3 (a)True Centrifugal Casting
 2.3 (a)True Centrifugal Casting

Advantages:

1. Mechanical properties are better than any other casting process, because ----
the inclusions such as slag and oxides get segregated towards the centre
and can be easily removed by machining.

2. Upto certain thickness of object, proper directional solidification can be

obtained starting from the mould surface to the centre.

3. No core are required for making concentric holes in the case of true
centrifugal casting

4. There is no need for gates and runners, which increases the casting yield,
reaching almost 100%
 2.3 (b)Semi-centrifugal Casting
▪ This casting technique can produce parts solid parts which may not
require central hole
▪ The mould made of sand or metal are rotated about vertical axis and
metal enters the mould through the central pouring basin
▪ Molds are designed with sprue at center to supply feed metal
▪ Density of metal in final casting is greater in outer sections than at
center of rotation
▪ Often used on parts in which center of casting is machined away, thus
eliminating the portion where quality is lowest
▪ Examples: wheels and pulleys
2.3 (b)Semi-centrifugal Casting
 2.3 (c) Centrifuge Casting
▪ This technique is same as that of semi-
centrifugal casting only difference is ---
▪ Centrifuge casting is used when casting
shapes are not axi-symmetrical, means
axis of casting and axis of rotation are
not coincide
▪ Number of such jobs are joined together
be means of radial runners with central
sprue on a revolving table

▪ Used for smaller parts

▪ Radial symmetry of part is not required
as in other centrifugal casting methods
 Furnaces for Casting Processes
▪ Furnaces most commonly used in foundries:
▪ Cupolas
▪ Crucible furnaces
Bale –out type
Stationary type
tilting type
▪ Electric-arc furnaces
▪ Induction furnaces
General Defects
 Gas Defects

Defects due to evolution of gases (Gas defects) :

1. Blowholes
2. Pin hole porosity
3. Blister
 Open blow / Blow holes

Cavities on the surface are called open blows and inside the casting called
blow holes
Causes:

1. Excessive moisture in the mould

2. Slag in the metal reacts with carbon
in the metal and liberates Co
3. Lower permeability of the mould.

Remedies:

1. Provide vent holes

2. Avoid excessive compaction of mould
3. Avoid excessive moister in the moulding sands
4. Extra care to be taken to segregate slag from liquid metal
5. Avoid using rusted chills and chaplets.
 Pin hole porosity
Causes:
1. Hydrogen is absorbed by molten
metal inside the furnace and also
inside the cavity

2. As the melt gets solidified, it loses

the temperature and liberates
dissolved hydrogen

Remidies:

1. Vaccum melting
2. Vaccum degassing
3. Avoid very high pouring temperature
Blister
 Defects due to pouring of the melt:
(Pouring metal defects)

1. Mis-run
2. Cold shut
3. Slag or dross inclusion
 Misrun
A casting that has solidified before completely filling mold
cavity
Causes:

1. Insufficient fluidity
2. Low pouring temperature
3. Too small ingates
4. Low pouring speed

Remedies:

1. Increase pouring temperature

2. Increase pouring speed
3. Make ingates larger
 Cold Shut
Two portions of metal flow together
but there is a lack of fusion due to
premature freezing
Causes:
1. Longer distance between the
ingates
2. Large surface area to volume
ratio

Remedies:
1. Use more number of ingates
2. Increase the pouring
temperature
 Inclusion

Causes:
1. Impurities present in the molten
metal
2. Sand cracked and broken from
gating system and mould cavity

Remedies:
1. Skimming of molten metal before
pouring
2. Choosing a moulding sand with
adequate hot strength
Defects causes by moulding material
(Moulding material defects)

1. Metal penetration
2. Flash
3. Run-out
4. Swell
 Metal 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

Causes:
1. Larger sand grains
2. Insufficient compaction of sand

Remedies:

1. Use fine sand grains

2. Reduce casting temperature
3. Apply sufficient compaction of mould
 Flash or Fins

Causes:
1. Sand is not properly
compacted along paring line

2. Small gap exists between

cope and drag

Remedies:
1. Moulding sand should be
leveled properly along the
parting line
 Run-out

A run out is caused when molten metal leaks out of the mould due to
metallostatic forces.
Causes:
1. Fault mould making
2. Faulty moulding flask

Remedies:
1. Prepare moulding flask
properly
2. Place a weight over the cope
 Defects due to metallurgical factors:
(Metallurgical defects)

1. Hot tears: Hot tears is a macroscopic separation

due to differential contraction of the casting during
solidification.
2. Due to long freezing range of alloy.
3. High sulphur content promotes hot tearing.
Hot tear
Remedies:

1. Use exothermic pads / Chills.

2. Control the composition. Using
less sulphur.
3. Use grain refiners.
Defects due to other factors:

1. Mismatch
 Mismatch / Mould shift
A step in cast product at parting
line caused by sidewise
relative displacement of cope
and drag

Causes:
Misalignment of moulding
boxes

Remedies:
Ensure proper alignment of
moulding boxes
Defects due to shrinkage

1. Shrinkage cavity
 Shrinkage Cavity

Depression in surface or internal void caused by

solidification shrinkage that restricts amount of molten
metal available in last region to freeze

Figure Some common defects in castings: (d) shrinkage cavity

 References
▪ O. P. Khanna, “A text book for production
technology-Foundry”

▪ M P Groover, “Fundamentals of Modern

Manufacturing” John Wiley & Sons, Inc. 2007

▪ Sunil Jha, “Metal casting-

web.iitd.ac.in/~suniljha”, IIT delhi.

▪ Kalpakjin S, “Manufacturing Engineering and

Technology”, Pearson publication