UNIT – I

CASTING
 Module-I
Casting processes: Classification, Metal mould casting processes, advanced casting
processes, investment casting, Rheocasting, mould and core making materials and their
characteristics. Technology of Selected casting Processes: Clay bonded, synthetic resin
bonded, inorganic material bonded mould and core making, sand additives, mould coating,
continuous casting process, centrifugal casting process.

1. Introduction to Casting processes

Metal casting process begins by creating a mold, which is the ‘reverse’ shape of the part
we need. The mould is made from a refractory material, for example, sand. The metal is
heated in an oven until it melts, and the molten metal is poured into the mould cavity. The
liquid takes the shape of cavity, which is the shape of the part. It is cooled until it solidifies.
Finally, the solidified metal part is removed from the mould.

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

Common examples: door handles, locks, the outer casing or housing for motors, pumps,
etc., wheels of many cars. Casting is also heavily used in the toy industry to make parts,
e.g. toy cars, planes, and so on. Typical metal cast parts are shown in Fig.1

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 Fig.1: Typical metal cast parts

Table 1 summarizes different types of castings, their advantages, disadvantages and

examples.
Table 1

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1.1 Sand Casting: Sand casting uses natural or synthetic sand (lake sand) which is mostly
refractory material called silica (SiO2). The sand grains must be small enough so that it can
be packed densely; however, the grains must be large enough to allow gasses formed
during the metal pouring to escape through the pores. Larger sized molds use green sand
(mixture of sand, clay and some water). Sand can be re-used, and excess metal poured is
cut-off and re-used also.

Typical sand molds have the following parts (Fig 2):

• The mold is made of two parts, the top half is called the cope, and bottom part is the
drag.
• The liquid flows into the gap between the two parts, called the mold cavity. The
geometry of the cavity is created by the use of a wooden shape, called the pattern. The
shape of the patterns is (almost) identical to the shape of the part we need to make.
• A funnel shaped cavity; the top of the funnel is the pouring cup; the pipe-shaped neck
of the funnel is the sprue– the liquid metal is poured into the pouring cup, and flows
down the sprue.
• The runners are the horizontal hollow channels that connect the bottom of the sprue
to the mould cavity. The region where any runner joins with the cavity is called the gate.

Fig 2: Schematic representation of a typical sand mould cross-section

Some extra cavities are made connecting to the top surface of the mold. Excess metal
poured into the mould flows into these cavities, called risers. They act as reservoirs; as

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 the metal solidifies inside the cavity, it shrinks, and the extra metal from the risers flows
back down to avoid holes in the cast part.
• Vents are narrow holes connecting the cavity to the atmosphere to allow gasses and the
air in the cavity to escape.
• Cores: Many cast parts have interior holes (hollow parts), or other cavities in their shape
that are not directly accessible from either piece of the mold. Such interior surfaces are
generated by inserts called cores. Cores are made by baking sand with some binder so
that they can retain their shape when handled. The mold is assembled by placing the
core into the cavity of the drag, and then placing the cope on top, and locking the mold.
After the casting is done, the sand is shaken off, and the core is pulled away and
usually broken off.
Gating System: Channel through which molten metal flows into cavity from outside of
mold consists of a downsprue, through which metal enters a runner leading to the main
cavity. At top of down-sprue, a pouring cup is often used to minimize splash and
turbulence as the metal flows into down-sprue.

1.2 Shell-mold Casting:

In this process the moulds and cores are prepared by mixing the dry free flowing sand
with thermosetting resins and then heating the aggregate (mixture of fine sand (100-150
mesh) and thermosetting resins) against a heated metal plate. Due to the heat, the resin
cures, which causes the sand grains to get bonded with each other and it forms a hard
shell around the metallic pattern. The inside portion of the shell is the exact replica of
the pattern against which the sand aggregate is placed before heating. The shape and
dimension of the inside portion of the shell thus formed is exactly the same as that of
the pattern. If the pattern is of two pieces then the other half of the shell is also prepared
the same way. Two halves of the shells prepared are placed together after inserting the
core, if any, to make the assembly of the mould. The assembly of the shell is then
placed in a molding flask and backing material is placed all around the shell mould
assembly to give its assembly the sufficient strength. Now the shell mould is fully ready
for pouring the liquid metal.

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Sand
The dry free flowing sand used in the shell mould must be completely free of clay
content. The grain size of the sand used in shell molding is generally in the range of
100150 meshes, as the shell casting process is recommended for castings that require
good surface finish. However, depending on the requirement of surface finish of the final
casting, the grain size of the sand can be ascertained. Also, if the grain size is very fine,
it requires large amount of resins, making it expensive.

Resin and Catalyst

The resins most widely used, are the phenol formaldehyde resins, which are
thermosetting in nature. Combined with sand, they give very high strength and
resistance to heat. The resin initially has excess phenol and acts like a thermoplastic
material. In order to develop the thermosetting properties of the resin, the coating of the
sand is done with resin and a catalyst (Hexa-methylene-tetramine, known as Hexa).
The measure of resin is 4-6% of sand by weight, the catalysts 14-16% of sand by
weight. The curing temperature of the resin along with the catalysts is around 150o C
and the time required for complete curing is 50 – 65 seconds. The sand composition to
be used in making various casting of different materials can be seen from the relevant
standards.
The resins available are of water-bourn, flake, or the granular types. The specifications
of liquid, flakes or powder resins can be obtained from IS 8246-1976, IS 11266-1985,
and IS 10979-1981 respectively.

Shell-mold casting yields better surface quality and tolerances. The process is
described as follows:
- The 2-piece pattern is made of metal (e.g. aluminum or steel), it is heated to between
175°C-370°C, and coated with a lubricant, e.g. silicone spray.
- Each heated half-pattern is covered with a mixture of sand and a thermoset
resin/epoxy binder. The binder glues a layer of sand to the pattern, forming a shell. The
process may be repeated to get a thicker shell (Fig 3).
- The assembly is baked to cure it. 6

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- The patterns are removed, and the two half-shells joined together to form the mold;
metal is poured into the mold.
- When the metal solidifies, the shell is broken to get the part.

Fig 3: Making the shell-mold and Shell mold casting

1.3 Expendable-pattern casting (lost foam process)

The pattern used in this process is made from polystyrene (this is the light, white packaging
material which is used to pack electronics inside the boxes). Polystyrene foam is 95% air
bubbles, and the material itself evaporates when the liquid metal is poured on it.
The pattern itself is made by molding – the polystyrene beads and pentane are put inside
an aluminum mold, and heated; it expands to fill the mold, and takes the shape of the
cavity. The pattern is removed, and used for the casting process, as follows:
- The pattern is dipped in slurry of water and clay (or other refractory grains); it is dried to
get a hard shell around the pattern.
- The shell-covered pattern is placed in a container with sand for support, and liquid metal
is poured from a hole on top.
- The foam evaporates as the metal fills the shell; upon cooling and solidification, the part is
removed by breaking the shell. 7
The process is useful since it is very cheap, and yields good surface finish and complex
geometry. There are no runners, risers, gating or parting lines – thus the design process is

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simplified. The process is used to manufacture crank-shafts for engines, aluminum engine
blocks, manifolds etc.

Fig 4: Expendable mold casting

Details
The minimum wall thickness for a full-mold casting is 2.5 mm (0.10 in). Typical dimensional
tolerances are 0.3% and typical surface finishes are from 2.5 to 25 µm (100 to 1000 µin)
RMS. The size range is from 400 g (0.88 lb) to several tonnes (tons).
Full-mold casting is often used to produce cylinder heads, engine blocks, pump housings,
automotive brake components, and manifolds. Commonly employed materials include
aluminium, iron, steel, nickel alloys, and copper alloys.

Advantages and disadvantages

This casting process is advantageous for very complex castings that would regularly
require cores. It is also dimensionally accurate, requires no draft, and has no parting
lines so no flash is formed. As compared to investment casting, it is cheaper because it
is a simpler process and the foam is cheaper than the wax. Risers are not usually
required due to the nature of the process; because the molten metal vaporizes the foam
the first metal into the mold cools more quickly than the rest, which results in natural
directional solidification.

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The two main disadvantages are that pattern costs can be high for low volume
applications and the patterns are easily damaged or distorted due to their low strength.
If a die is used to create the patterns there is a large initial cost.

1.4 Full Mold Process / Lost Foam Process / Evaporative Pattern Casting Process

The use of foam patterns for metal casting was patented by H.F. Shroyer on April 15,
1958. In Shroyer's patent, a pattern was machined from a block of expanded
polystyrene (EPS) and supported by bonded sand during pouring. This process is
known as the full mold process. With the full mold process, the pattern is usually
machined from an EPS block and is used to make primarily large, one-of-a kind
castings. The full mold process was originally known as the lost foam process.
However, current patents have required that the generic term for the process be full
mold.

In 1964, M.C. Flemmings used unbounded sand with the process. This is known today
as lost foam casting (LFC). With LFC, the foam pattern is molded from polystyrene
beads. LFC is differentiated from full mold by the use of unbounded sand (LFC) as
opposed to bonded sand (full mold process).

Foam casting techniques have been referred to by a variety of generic and proprietary
names. Among these are lost foam, evaporative pattern casting, and cavity less casting,
evaporative foam casting, and full mold casting.

In this method, the pattern, complete with gates and risers, is prepared from expanded
polystyrene. This pattern is embedded in a no bake type of sand. While the pattern is
inside the mold, molten metal is poured through the sprue. The heat of the metal is
sufficient to gasify the pattern and progressive displacement of pattern material by the
molten metal takes place.

The EPC process is an economical method for producing complex, close-tolerance

castings using an expandable polystyrene pattern and unbonded sand. Expandable
polystyrene is a thermoplastic material that can be molded into a variety of complex,

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rigid shapes. The EPC process involves attaching expandable polystyrene patterns to
an expandable polystyrene gating system and applying a refractory coating to the entire
assembly. After the coating has dried, the foam pattern assembly is positioned on loose
dry sand in a vented flask. Additional sand is then added while the flask is vibrated until
the pattern assembly is completely embedded in sand. Molten metal is poured into the
sprue, vaporizing the foam polystyrene, perfectly reproducing the pattern.

1.5 Plaster-mold casting

The mold is made by mixing plaster of paris (CaSO4) with talc and silica flour; this is a fine
white powder, which, when mixed with water gets a clay-like consistency and can be
shaped around the pattern (it is the same material used to make casts for people if they
fracture a bone). The plaster cast can be finished to yield very good surface finish and
dimensional accuracy. However, it is relatively soft and not strong enough at temperature
above 1200°C, so this method is mainly used to make castings from non-ferrous metals,
e.g. zinc, copper, aluminum, and magnesium.
Since plaster has lower thermal conductivity, the casting cools slowly, and therefore has
more uniform grain structure (i.e. less warpage, less residual stresses).

1.6 Ceramic mold casting

Similar to plaster-mold casting, except that ceramic material is used (e.g. silica or
powdered Zircon ZrSiO4). Ceramics are refractory (e.g. the clay hotpot used in Chinese
restaurants to cook some dishes), and also have higher strength that plaster.
- The ceramic slurry forms a shell over the pattern;
- It is dried in a low temperature oven, and the pattern is removed 8
- Then it is backed by clay for strength, and baked in a high temperature oven to burn off
any volatile substances.
- The metal is cast same as in plaster casting.
This process can be used to make very good quality castings of steel or even stainless
steel; it is used for parts such as impellor blades (for turbines, pumps, or rotors for motor-
boats).

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1.7 Investment casting (lost wax process)
The investment casting process, which is commonly referred to as the “lost wax method”,
originated in and around the fourth millennium B.C. It is evidenced through the architectural
works found in the form of idols, pectorals and jewelry in remains of the ancient Egypt and
Mesopotamia. The investment casting process initiates with the production of wax replicas
or patterns of the required shape of castings. Each and every casting requires a pattern to
be produced. Wax or polystyrene is made used as the injecting material. The assembly of
large number of patterns are made and attached to a wax sprue centrally. Metallic dies are
used to prepare the patterns. The pattern is immersed in refractory slurry which completely
surrounds it and gets set at room temperature forming the mold. The mold is further
heated, so that the pattern melts and flows out, leaving the required cavity behind. After
heating, the mold gets further hardened and molten metal is poured while it is still hot. After
the casting gets solidified, the mold is broken and it is taken out.

The basic steps of the investment casting process are as shown in Fig 5:
1. Preparing the heat-disposable wax, plastic or polystyrene patterns in a die. 2. Assembly
of the prepared patterns onto a gating system 3. “Investing,” (covering) the pattern
assembly with a refractory slurry which builds the shell.
4. Melting the pattern assembly (burning out the wax) by firing, for removing the traces of
the pattern material 5. The metal in molten state is poured into the formed mold. 6. Once
the metal solidifies, the shell is removed (knocked out). 7. Fettling (cutting off) of the
pouring basin and gates followed by finishing operations to get the desired dimensional
tolerances and finish.

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 Fig 5: The Basic Steps of the Investment Casting Process

Fig 5a: Steps in the investment casting process

This is an old process, and has been used since ancient times to make jewellery –
therefore it is of great importance to HK. It is also used to make other small (few grams,
though it can be used for parts up to a few kilograms). The steps of this process are shown
in the Fig 5a.

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An advantage of this process is that the wax can carry very fine details – so the process not
only gives good dimensional tolerances, but also excellent surface finish; in fact, almost
any surface texture as well as logos etc. can be reproduced with very high level of detail.

1.8 Vacuum casting

This process is also called counter-gravity casting. It is basically the same process as
investment casting, except for the step of filling the mold. In this case, the material is
sucked upwards into the mould by a vacuum pump. The figure 6below shows the basic
idea – notice how the mold appears in an inverted position from the usual casting process,
and is lowered into the flask with the molten metal (Fig 6).
One advantage of vacuum casting is that by releasing the pressure a short time after the
mold is filled, we can release the un-solidified metal back into the flask. This allows us to
create hollow castings. Since most of the heat is conducted away from the surface between
the mold and the metal, therefore the portion of the metal closest to the mold surface
always solidifies first; the solid front travels inwards into the cavity. Thus, if the liquid is
drained a very short time after the filling, then we get a very thin walled hollow object, etc.
(Fig 7).

Fig 6: Vacuum casting

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 Fig 7: Draining out metal before solidification yields hollow castings

1.9 Permanent mold casting

Here, the two halves of the mold are made of metal, usually cast iron, steel, or refractory
alloys. The cavity, including the runners and gating system are machined into the mold
halves. For hollow parts, either permanent cores (made of metal) or sand-bonded ones
may be used, depending on whether the core can be extracted from the part without
damage after casting. The surface of the mold is coated with clay or other hard refractory
material – this improves the life of the mold. Before molding, the surface is covered with a
spray of graphite or silica, which acts as a lubricant. This has two purposes – it improves
the flow of the liquid metal, and it allows the cast part to be withdrawn from the mold more
easily. The process can be automated, and therefore yields high throughput rates. Also, it
produces very good tolerance and surface finish. It is commonly used for producing pistons
used in car engines, gear blanks, cylinder heads, and other parts made of low melting point
metals, e.g. copper, bronze, aluminum, magnesium, etc.

In the pressure casting process the molten material is forced upward by gas pressure
into a graphite mould or metallic mould Fig 8. The pressure is maintained until the melt
has completely solidified in the mould. The molten material may also be forced upward
by a vacuum, which also removes dissolved gases ahead of the rising melt and
produces a casting with lower porosity.

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 Fig 8: Pressure Casting Process

Variations of this method include Vacuum Riserless Casting (VRC) and Pressure
Riserless Casting (PRC). These techniques are capable of producing a range of
structural and high performance castings exhibiting excellent mechanical attributes and
microstructure refinements in an economical manner. While VRC process uses vacuum
to draw the liquid material up into a mould cavity, PRC uses pressure applied to a
molten bath to force melt into a mould cavity. Yet another approach combines both
techniques to achieve appropriate casting conditions.

Squeeze casting developed in the 1960s, involves solidification of the molten material
under high pressure Fig 9. Thus it is a combination of casting and forging. The
machinery includes a die, punch, and ejector pin.

Fig 9: Sequence of operations in squeeze-casting:

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1. Bring a ladle filled with liquid material close to the dies
2. Pour liquid in the bottom die cavity
3. Close dies and applies pressure
4. Open dies and ejects the solidified product

The pressure applied by the punch keeps the entrapped gases in solution, and the high-
pressure contact at the die-product interface promotes rapid heat transfer, resulting in a
fine microstructure with good mechanical properties. Parts can be made to near-net
shape, with complex shapes and fine surface detail, from both nonferrous and ferrous
alloys. Typical products: automotive wheels and mortar bodies (a short-barreled
cannon). The pressures required in squeeze casting are lower than those for hot or cold
forging.

Die casting
Die casting is a very commonly used type of permanent mold casting process. It is used for
producing many components of home appliances (e.g rice cookers, stoves, fans, washing
and drying machines, fridges), motors, toys and hand-tools – since Pearl river delta is a
largest manufacturer of such products in the world, this technology is used by many HK-
based companies. Surface finish and tolerance of die cast parts is so good that there is
almost no post-processing required. Die casting molds are expensive, and require
significant lead time to fabricate; they are commonly called dies. There are two common
types of die casting: hot- and cold-chamber die casting.
• In a hot chamber process (used for Zinc alloys, magnesium) the pressure chamber
connected to the die cavity is filled permanently in the molten metal. The basic cycle of
operation is as follows: (i) die is closed and gooseneck cylinder is filled with molten metal;
(ii) plunger pushes molten metal through gooseneck passage and nozzle and into the die
cavity; metal is held under pressure until it solidifies; (iii) die opens and cores, if any, are
retracted; casting stays in ejector die; plunger returns, pulling molten metal back through
nozzle and gooseneck; (iv) ejector pins push casting out of ejector die. As plunger
uncovers inlet hole, molten metal refills gooseneck cylinder. The hot chamber process is

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used for metals that (a) have low melting points and (b) do not alloy with the die material,
steel; common examples are tin, zinc, and lead (Fig 10a).
• In a cold chamber process, the molten metal is poured into the cold chamber in each
cycle. The operating cycle is (i) Die is closed and molten metal is ladled into the cold
chamber cylinder; (ii) plunger pushes molten metal into die cavity; the metal is held under
high pressure until it solidifies; (iii) die opens and plunger follows to push the solidified slug
from the cylinder, if there are cores, they are retracted away; (iv) ejector pins push casting
off ejector die and plunger returns to original position. This process is particularly useful for
high melting point metals such as Aluminum, and Copper (and its alloys) (Fig 10b).

Fig. 10: (a) Hot chamber die casting (b) Cold chamber die casting

The Die-casting process is a typical example of permanent-mould casting. The molten

material is forced into the die cavity at pressures ranging from 0.7 to 700 MPa. Typical
products are carburettors, motor housings, business machine and appliance
components, hand tools and toys. The weight of most castings ranges from less than 90
g to about 25 kg.

The Hot-chamber process involves the use of a piston, which traps a certain volume
of melt and forces it into the die cavity through a gooseneck and nozzle Fig 10c.

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 Fig 10c: Hot-chamber process

The pressures range up to 35 MPa. The melt is held under pressure until it solidifies. To
improve die life and to aid in rapid heat transfer, thus reducing the cycle time, dies are
cooled by circulating water or oil through passageways in the die block. Cycle times
usually range up to 900 shots per hour for zinc, (very small components such as zipper
teeth can be cast at 18,000 shots per hour). This process commonly casts low-melting-
point alloys of metals such as zinc, tin, and lead.

In the Cold-chamber process molten metal is poured into the injection cylinder with a
ladle Fig 10d. The shot chamber is not heated. The melt is forced into the die cavity at
pressures ranging from 20 MPa to 70 MPa, (in extremes 150 MPa). The machines may
be horizontal or vertical.

Process capabilities and machine selection: High-melting-point alloys of Al, Mg, and Cu
are cast by this method; ferrous alloys can also be cast in this manner. The dies have a
tendency to part unless clamped together tightly. Die casting machines are rated
according to the clamping force and range from 25 t to 3000 t. A further factor in the
selection of die-casting machines is the piston stroke which delimits the volume of fluid
injected into die cavity.

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 Fig 10d: Cold-chamber process

Dies may be made for single or multiple cavities. Dies wear increases with the
temperature of the fluid. Heat cracking of the die surface from repeated heating and
cooling can be a problem. However dies may last more than half a million shots before
die wear becomes significant.
The entire die-casting and finishing process can be highly automated. Lubricants are
applied, as parting agents on die surfaces. Alloys (except Mg alloys) generally require
lubricants. Die-casting has the capability for high production rates with good strength,
high-quality parts with complex shapes, good dimensional accuracy and surface detail,
thus requiring little or no subsequent machining or finishing operations. Components
such as pins, shafts, and fasteners can be cast integrally. Ejector marks remain, as do
small amounts of flash (thin material squeezed out between the dies) at the die parting
line.

Die-casting can compete favourably in some products with other manufacturing

methods, such as metallic-sheet stamping or forging. Because the molten material
chills rapidly at the die walls, the casting has a fine-grain, hard skin with higher strength
than in the centre. The strength-to-weight ratio of die-cast parts increases with
decreasing wall thickness. With good surface finish and dimensional accuracy, die-
casting can produce bearing surfaces that would normally be machined. The cost of
dies is somewhat high, but die-casting is economical for large production runs.

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1.10 Centrifugal casting
Centrifugal casting uses a permanent mold that is rotated about its axis at a speed between
300 to 3000 rpm as the molten metal is poured. Centrifugal forces cause the metal to be
pushed out towards the mold walls, where it solidifies after cooling. Parts cast in this
method have a fine grain microstructure, which is resistant to atmospheric corrosion; hence
this method has been used to manufacture pipes. Since metal is heavier than impurities,
most of the impurities and inclusions are closer to the inner diameter and can be machined
away. The surface finish along the inner diameter is also much worse than along the outer
surface.

Fig 11: Centrifugal casting schematic

The essential feature of centrifugal casting is the introduction of molten metal into a mold
which is rotated during solidification of the casting. The centrifugal force is relied upon for
shaping and feeding the molten metal with the utmost of detail as the liquid metal is thrown
by the force of gravity into the designed crevices and detail of the mold. (Fig. 11)

The concept of centrifugal casting is by no means a modern process. This technique which
lends clarity to detail was used by Benvenuto Cellini and others in the founding arts during
the 16th century. The mention of actual centrifugal casting machines is first recorded when
a British inventor, A.G. Eckhardt, was issued a patent in the year 1807. His method utilized
the placing of the molds in an upright position on pivots or revolving bases (sometimes
referred to today as a "vertical" centrifugal casting machine). In 1857 a U.S. patent

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described wheel molds which presumably were used for the centrifugal casting of railroad
car wheels.

The centrifugal casting of railroad car wheels was one of the first applications involving
controlled variations in chemical composition from the outside periphery of the car wheel as
compared to the balance of the casting. As the casting was poured, a quantity of
ferromanganese was introduced with the first metal to enter the mold. This formed a high
manganese wear resistant tread and car wheel flange, as compared to the softer second
portion of the molten metal which became the center portion and the hub of the wheel.
Although this practice is no longer used, similar applications do exist since, in principle, true
solutions will not be separated in the centrifugal casting process.

Centrifugal casting utilizes inertial forces caused by rotation to distribute the molten
material into mould cavities. Variations of this manufacturing method include:
True centrifugal casting,
Semi-centrifugal casting, and
Centrifuging (also called centrifuged or spin casting).

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 Fig 11a: Centrifugal Casting Methods

It is important to remember, however, that materials such as iron or copper that are
immiscible in certain ranges are apt to segregate badly, such as lead in certain bronzes.
Tubing with alloy modifications on the inside diameter which are designed to meet specific
corrosion resistant characteristics have been successfully produced using the centrifugal
casting technique.
Centrifugal casting remained a casting method for large objects until 1907 when Dr.
Taggart, a dentist, introduced it to other dentists who experimented with the method hoping
to perfect cast inlays for teeth that would replace malleting flake gold into prepared cavities.
A Dr. Campbell in Missouri used a Hoosier cowbell as a casting flask. A wire loop such as
an extra-long bucket bail was added to the bell, the clapper was removed, and the model
and its sprues were embedded in the investment plaster.

After the mold had been heated, the prepared molten metal was poured into the sprue and
the bell swung first in pendulum style, then in a circular motion, to force the metal into all
areas of the pattern chamber. This action resembled the old trick of swinging a bucketful of
water over one's head in a circular motion. After 1920, the process began to be used for the
manufacturing of cast iron water pressure pipe, and use of the process has been extended
to a much wider range of shapes and alloys.
In centrifugal casting, the mold may spin about a horizontal, inclined or vertical axis. The
outside shape of the casting is determined by the shape of the mold. The inside contour is

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determined by the free surface of the liquid metal during solidification. The centrifugal force
produced by rotation is large compared with normal hydrostatic forces and is utilized in two
ways.
The first of these is seen in pouring, where the force can be used to distribute liquid metal
over the outer surfaces of a mold. This provides a means of forming hollow cylinders and
other annular shapes. The second is the development of high pressure in the casting
during freezing. This, in conjunction with directional solidification, assists feeding and
accelerates the separation of non-metallic inclusions and precipitated gases. The
advantages of the process are therefore twofold: suitability for casting cylindrical forms and
high metallurgical quality of the product,
The effectiveness of centrifugal force in promoting a high standard of soundness and
metallurgical quality depends above all on achieving a controlled pattern of solidification,
this being governed by the process used and by the shape and dimensions of the casting.
High feeding pressure is no substitute for directional freezing, which remains a primary aim
of casting technique.
Considering first the casting of a plain cylinder, conditions can be seen to be highly
favorable to directional solidification owing to the marked radial temperature gradient
extending from the mold wall. Under these conditions the central mass of liquid metal,
under high pressure, has ready access to the zone of crystallization and fulfills the function
of the feeder head used in static casting. The steepest gradients and the best conditions of
all occur in the outermost zone of the casting, especially when a metal mold is employed.

Another important factor is the length to diameter ratio of the casting, a high ratio
minimizing heat losses from the bore through radiation and convection. Under these
conditions, heat is dissipated almost entirely through the mold wall and freezing is virtually
unidirectional until the casting is completely solid; the wall of the casting is then sound
throughout.
The casting of a plain pipe or tube is accomplished by rotation of a mold about its own
axis—the bore shape being produced by centrifugal force alone, and the wall thickness
determined by the volume of metal introduced. This practice is widely referred to as "true
centrifugal casting." (Fig. 11b)

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 Fig 11b: True Centrifugal Casting

Fig 11c: True Centrifugal Casting

In true centrifugal casting, Fig 11c, hollow cylindrical parts, e.g. pipes and lampposts, are
produced by pouring liquid into a rotating mould. The axis of rotation is usually horizontal
but can also be vertical. Moulds are made of steel, cast iron, or graphite and may be coated
with a refractory lining to increase mould life. Pipes with various outer shapes, (including
polygonal) can be cast. The inner surface of the casting remains cylindrical because the
molten material is uniformly distributed by centrifugal forces. Because of density
differences, lighter particles such as dross and impurities tend to collect on the inner
surface of the casting.

Cylindrical parts ranging from ∅13 mm to 3 m in diameter and 16 m long can be produced
with wall thickness ranging from 6 mm to 125 mm. The acceleration generated by the
centrifugal force is high, as much as 150 g, and is necessary for casting thick-walled parts.
This process enables good dimensional accuracy, and external surface detail. Typical
products are pipes, bushings, engine cylinder liners, and bearing rings with or without

24
flanges. Apart from metallic products some glass and ceramic products (e.g. TV picture
tubes and ceramic membrane tubes) are also manufactured using this technique.
In the case of a component of varying internal diameter or irregular wall thickness, a central
core may be used to form the internal contours, feeder heads then being introduced to
compensate for solidification shrinkage. A further step away from the original concept is the
spacing of separate shaped castings about a central downsprue which forms the axis of
rotation. These variations are referred to respectively as "semi-centrifugal casting and
centrifuging or pressure casting." In both cases, since the castings are shaped entirely by
the mold and cores, centrifugal force is used primarily as a source of pressure for feeding.

Semi-Centrifugal Casting
Such items as wheels and pulleys are occasionally cast in a semi-centrifugal setup as
illustrated in (Fig. 11d). This type of mold need not be rotated as fast as in the case of a
true centrifugal casting for only enough force is needed to cause the metal to first flow to
the outer rim. As the wheel rotates around its hub core, the mold cavity is filled from rim to
hub not from bottom to top as is the case of common gravity pouring. This action promotes
the direction of solidification from rim to hub and provides the required feeding by using
only one central reservoir. Pouring and feeding on the center hub increases the yield
especially when casting high shrinkage alloys. Here, as in other centrifugal setups, the
centrifugal force helps force lightweight nonmetalhc inclusions and trapped gas toward the
center and into the feeder for elimination.

Fig. 11d Semi-Centrifugal Casting

25
In true or open bore casting, circumferential velocity is imparted from mold to metal by
frictional forces at the mold surface and within the liquid. In horizontal axis casting, the
metal entering the mold must rapidly acquire sufficient velocity to prevent instability and
"raining" as it passes over the upper half of its circular path, because of slip, the generation
of the necessary minimum force of 1G in the metal requires a much greater peripheral mold
velocity than would be the case if metal and mold were moving together. (Fig. 11e)

Fig 11e: Schematic representation of True Centrifugal Casting Machine

Semi-centrifugal casting is used to cast parts with rotational symmetry, such as

wheels with spokes and central hub. This technique can be applied to most expendable
and permanent moulds.
In Centrifuging mould cavities of odd shape are placed at a certain distance from the axis
of rotation. The molten material is forced into the mould by centrifugal forces. The attributes
within the castings vary with the distance from the axis of rotation, Fig 11f.

Fig 11f: Centrifuging

26
Vertical Centrifugal Casting
Vertical castings are produced by pouring a given weight of metal into a mold that rotates
about a vertical axis. The metal is picked up and distributed on the inside surface of the
mold. Dross, slag and other nonmetallics are centrifuged to the inside. Unlike the horizontal
casting, it is not possible to obtain a uniform bore. Depending on the rotational speed of the
mold, the inside will have varying amounts of taper. The inside surface will be that of the
parabola of revolution. The paraboloid "A" in Fig. 11g shows the shape of the cavity formed
by a relatively high rotational speed and paraboloid "B" shows the approximate shape of
the cavity that would be formed at a lower speed. This fact can be utilized advantageously
in the production of certain conically shaped parts.

Fig 11g: Vertical Centrifugal Casting

The vertical axis centrifugal casting method is not suited to the production of pipelike
shapes because of the inherent taper on the inside. Likewise, it is not suited to the
production of very long parts. It finds its greatest application in the production of ringlike
shapes. Because the inside contour can be controlled to some extent, the method is
particularly useful in producing tapered sections. Also, because the rotational speeds can
be lower than in the horizontal axis machine, there is greater latitude in modifying the
outside shape.

Vertical casting machines consist of a rotating table on which a mold is centered and
fastened. The machine must be constructed to withstand static and dynamic loads imposed
on it. The dynamic loading is the most critical. Speed controls are infinitely variable and
speed regulation should be good. For safety's sake the machines are often mounted below

27
floor level. They are provided with adequate shields for protection in case of runout or
machine failure.

1.11 Rheocasting processes

Semi-solid metal casting is a near net shape variant of die casting. The process is used
with non-ferrous metals, such as aluminium, copper, and magnesium. The process
combines the advantages of casting and forging. The process is named after the fluid
property thixotropy, which is the phenomenon that allows this process to work. Simply,
thixotropic fluids shear when the material flows, but thicken when standing. The process of
thixocasting offers a number of advantages, such as improved mechanical properties, good
surface finish, near net shape and so on. However, the thixocastingprocess has also a
number of disadvantages, such as the need for special feedstock with near spherical
primary crystals. In order to cast such special billets for thixocasting one has to pay a more
expensive premium than normal. Eliminating this additional specialized casting step leads
to savings in both costs and time. A product can be cast into a near net shape part directly
from the molten metal state as in rheocasting, where the need of special billet is removed.
Therefore, rheocasting is advantageous from an energy and cost saving point of view when
compared to thixocasting. In the early days of semisolid casting research, mechanical
stirring was used in order to achieve the right microstructures. More recently, electric
stirring has used. There are two kinds of rheocasting process using a cooling slope and a
process using low superheat casting, respectively. In the process using the cooling slope,
the metal is in the semisolid condition when it flows into the die. In the low superheat
casting process, the seed of the crystals are generated at the die surface. The casting is
carried out before the crystal seeds could be re-melted. The crystal seeds could then grow
to become spherical primary crystals. In the processes described only pouring of the
molten metal into the die has been needed for the semisolid casting to take place. In
conventional semisolid casting, the solid metal fraction content is usually 50%, however, in
this process; casting has been tried at lower than 50% fraction solids.The primary crystal
size becomes smaller as the solid rate becomes lower. In thixocasting, metal handling is
difficult at fraction solids lower than 50%. However, in rheocasting, casting metal with lower

28
fraction solids is easy because the product, which is thin, can be cast at low fraction solids.
Fig. 10 shows the two rheocasting processes devised in this discussion.

The molten metal was poured into the lower die half via the cooling slope. The molten
metal became semisolid slurry on the cooling slope. The cooling slope, which is very
compact and simple, is made from mild steel, it is water-cooled and as a package offers
both low equipment costs and low running costs. Thecooling slope can be easily mounted
as part of any conventional casting machine. In conventional semisolid casting process, a
typical fraction solid of about 50% is required, however, the present study aimed at fraction
solids lower than 50%. The primary crystal size in the product becomes smaller as the
fraction solid is reduced. The solidification rate of the semisolid slurry after flowing through
on the cooling slope was about 10%. Casting was done immediately after pouring without
holding the slurry in order not to increase the solidification rate. Therefore, there was no
need of a system that controls the rate of solidification; this simplified the processes
investigated in the present study. Fig. 10(b) shows the rheocasting process that used low
superheat casting. The superheat of the molten metal was 10 0C. The crystal seeds are
generated at the lower die surface, and the upper die is inserted into the lower die before
the metal solidifies. When the superheat of the molten metal is low, the crystal seeds do not
melt and if sufficient crystal seeds remain, they can grow into spheroidal primary crystals.
The low superheat casting is simpler than the cooling slope process.

Fig. 12 Two kinds of Rheocasting process

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1.12 Continuous Casting
Continuous casting transforms molten metal into solid on a continuous basis and
includes a variety of important commercial processes. These processes are the most
efficient way to solidify large volumes of metal into simple shapes for subsequent
processing. Most basic metals are mass-produced using a continuous casting process,
including over 500 million tons of steel, 20 million tons of aluminum, and 1 million tons
of copper, nickel, and other metals in the world each year. Continuous casting is
distinguished from other solidification processes by its steady state nature, relative to an
outside observer in a laboratory frame of reference. The molten metal solidifies against
the mold walls while it is simultaneously withdrawn from the bottom of the mold at a rate
which maintains the solid / liquid interface at a constant position with time. The process
works best when all of its aspects operate in this steady-state manner. Relative to other
casting processes, continuous casting generally has a higher capital cost, but lower
operating cost. It is the most cost- and energy- efficient method to mass-produce
semifinished metal products with consistent quality in a variety of sizes and shapes.
Cross-sections can be rectangular, for subsequent rolling into plate or sheet, square or
circular for long products, and even “dog-bone” shapes, for rolling into I or H beams.
In the continuous casting, molten steel is poured from the tundish in the water cooled mold
and partially solidified bloom/billet or slab (hereafter called strand) is withdrawn from the
bottom of the mold into water spray so that solidified bloom/billet or slab is produced
constantly and continuously. Continuous casting is widely adopted by steelmakers. The
advantages of continuous casting over ingot casting are
 Quality of the cast product is better
 No need to have slabbing / blooming or billet mill as required when ingot casting is
used.
 Higher extent of automation is possible
 Width of the slab can be adjusted with the downstream strip mill.
 Continuously cast products show less segregation.
 Hot direct charging of the cast product for rolling is possible which leads to energy
saving.

30
The essential components of a continuous casting machine are tundish, water cooled mold,
water spray and torch cutters. Tundish, mold and water spray are arranged such that
molten stream is poured from tundish to mold and solidified strand (billet/bloom/billet) is
produced continuously. The required length of the strand is cut by torch cutter. In Fig. 13,
the arrangement of tundish, mold and water spray is shown. Various continuous casting
processes are shown in Fig 13a.

Fig 13: Arrangement of tundish, mold and water spray in a curved mold machine

Fig. 13a Various continuous casting processes

31
Tundish

Tundish is a refractory lined vessel. Liquid steel is usually tapped from ladle into tundish.
The stream is shrouded as it enters from ladle to tundish. The functions of the tundish are:

Reservoir of molten steel

Tundish acts as a reservoir for molten steel. It supplies molten steel in presence of a slag
cover to all continuous casting molds constantly and continuously at constant steel flow
rate. The flow rate is maintained constant by maintaining a constant steel bath height in the
tundish through teeming of molten steel from the ladle. The number of mold is either one or
more than one. Normally bloom and billet casting machines are multi strand i.e. number of
molds are either 4 or 6 or 8. Slab casters usually have either single or two molds. During
sequence casting and ladle change over periods, tundish supplies molten steel to the
molds.

Distributor

Tundish distributes molten steel to different molds of the continuous casting machine at
constant flow rant and superheat which is required for stand similarly with reference to
solidification microstructure. Control of superheat is required in all the moulds to reduce
break out. Location of ladles stream in the tundish is important. It may be located
symmetric or asymmetric to the centre of the tundish depending on the number of mold.
For single strand machines, molten stream enters from one side and exits the other side of
the tundish. In multi strand tundishes, ladle stream is either at the centre of the tundish or
displaced to the width side of the tundish.

32
 Fig. 13b: Tundish with flow control device, namely weir and slotted dam

Inclusion removal
Tundish helps to remove inclusions during the process of continuous casting. For this
purpose liquid steel flow in the tundish is modified by inserting dams, weirs, slotted dams
etc. The whole idea is to utilize the residence time available before steel leaves the tundish.
For example, if capacity of tundish is 40 tons and casting speed is 5 tons/min, then the
average residence time of molten steel in the tundish is 8 minutes. During this average
residence time, inclusion removal can be exercised .For this purpose flow of steel melt in
the tundish has to be modified so as to accelerate the inclusion removal. The Inclusion
removal is a two-step step unit operation, namely floatation and absorption by a flux added
on the surface of the tundish. Flux is usually rice husk, or fly ash or some synthetic powder.

Mold:

Mold is the heart of continuous casting. In the water cooled mold, molten stream enters
from the tundish into mold in presence of flux through the submerged nozzle immersed in
the liquid steel. Solidification of steel begins in the mold. The casting powder is added onto
the top of molten steel in the mold. It melts and penetrates between the surface of mold
and the solidifying strand to minimize friction as shown in Fig 13c. Control of height of
molten steel in the mould is crucial for the success of the continuous casting machine. The

33
solidification begins from the meniscus of steel level in the mould. Mold level sensors are
used to control the meniscus level in the mould.

Fig13c: Role of flux in continuous casting mold

As seen in the figure, flux melts and enters into the gap between mold surface and
solidified strand. Molds are made of copper alloys. Small amounts of alloying elements are
added to increase the strength. Mold is tapered to reduce the air gap formation. Taper is
typically 1% of the mold length. For cross section of mold the taper is about 1mm for 1m
long mold. The cross section of the mold is the cross section of the slab/bloom/billet.
Length of the mold is around 0.7 and is more for large cross sections. Mold cross section
decreases gradually from top to bottom. Mould extracts around 10% of the total heat.

The mold is oscillated up and down to withdraw the partially solidified strand (strand is
either billet or bloom or slab).The oscillated frequency can be varied. At Tata steel slab
caster frequency is varied in between 0 and 250cycles/min and the stroke length from 0 to
12mm.

Steel level in mould is controlled, that is the meniscus for smooth caster operation. Sensors
are used to control the meniscus level.

34
The functions of mold flux are.

 Inclusion absorption capability

 Prevention of oxidation
 Minimization of heat losses
 Flux on melting enters into the air gap and provides lubrication

For the above functions the flux should have the following properties.
 Low viscosity
 Low liquidus temperature
 Melting rate of flux must match with the speed of the continuous casting.

2. The Capabilities of major casting processes are compared in Table 2.

Table 2: Capabilities of major casting processes

The hierarchical classification of various casting processes are summarized in Fig. 14.

35
 Fig.14 Hierarchical classification of various casting processes

2.1 Sand mould and core making

Sand casting is the most common productiontechnique, especially for ferrous castings.
Sand ismixed with clay and water or with chemical binders and then packed or rammed
around the pattern to form a mould half. The two halves are joined together to make the
mould - a rigid cavity that provides the required shape for the casting, as shown in Fig. 15
below. Variations on this technique include the use of plaster in place of sand and the
recently invented Pattern less process, where the mould is machined directly out of a sand
block without the need for a pattern.
36
Cores are produced by blowing, ramming or in heated processes, investing sand into a
core box. The finished cores, which can be solid or hollow, are inserted into the mould to
provide the internal cavities of the casting before the mould halves are joined. Sand cores
are also widely used in die-casting, where permanent metal moulds are employed.

Fig 15: Assembled Mould with Core Inserted Ready for Casting

Sand Preparation: Moulding sand should have good flowability (for better reproduction of
pattern details), adequate green strength (to prevent its collapse during moulding), dry
strength (to prevent its collapse during mould filling), sufficient refractoriness (to withstand
molten metal temperature), enough permeability (to allow entrapped air and gases
generated inside the mould to escape) and collapsibility (for ease of shakeout).
These are achieved by a suitable composition of sand, binders, additives and moisture.
Silica sand is the most widely available and economical. Special sands include zirconsand
(lower thermal expansion, higher refractoriness and higher thermal conductivity, butmore
expensive), olivine sand (with properties in between silica and zircon sand) and
chromite/magnesite sand (high thermal conductivity). The most widely used binder is
bentonite clay (sodium or calcium bentonite), which imparts strength and plasticity tosilica
sand with the addition of water. Additives include coal dust (to improve surfacefinish by gas
evolution at metal-mould interface), iron oxide (for high temperatureresistance), dextrin (for
improved toughness and collapsibility) and molasses (for highstrength and collapsibility).
Modern sand plants automatically carry out mulling, mixing, aeration and testing of the
sand. They also reclaim used sand through magnetic separation(to remove metal

37
particles), crushing of lumps and finally removal of excess fines andbond (usually by
washing in hot water or by mechanical impact).

Core Making: Cores are surrounded by molten metal, and have higher requirement
compared to mould sand in terms of strength (to support their own weight and the
buoyancy force of metal), permeability and collapsibility (especially for curved holes,
otherwise they will be difficult to clean out). The most widely used binder for core sands is
vegetable oil (linseed and corn oil, sometimes mixed with mineral oils), which is
economical, but requires heating in an oven to about 240 C for 2-3 hours to develop
sufficient strength. Another widely used process uses sodium silicate binder mixed in dry
sand free of clay; the sand mixture hardens immediately when CO2 gas is passed through
it. The process is highly productive. The core develops high compressive strength but has
poor collapsibility. Other processes are based on organic binders; mainly thermosetting
resins such as phenol, urea and furan. This includes hot box and cold box processes. The
core sand mixed with binder is filled into a core box either manually or using a sand slinger.
For higher productivity core blowing machines are used, in which core boxes are mounted
in the machine and sand is forced and pressed into the core box under a stream of high
velocity air. This is followed by appropriate heating of the core box to impart the desired
properties to the core.

Moulding: This involves packing the moulding sand uniformly around a pattern placed in a
moulding box (or flask). Most foundries are equipped with jolt-squeeze machines operated
by compressed air. The combination of jolting and squeezing action gives good compaction
of sand near the pattern (by jolting the sand into crevices) as well as the top where the
squeeze plate comes in contact with the mould. Many modern foundries have high
pressure moulding equipment, which use air impulse or gas injection to impact the sand on
the pattern. These machines produce relatively less noise and dust compared to jolt and
squeeze machines and has much higher productivity (several moulds per minute). A
special type of high pressure moulding machine is the flask less moulding machine
pioneered by Disamatic, in which the parting plane is vertical and the mouldcavity is formed
between consecutive blocks of mould.

38
2.2 Heating the Metal
• Heating furnaces are used to heat the metal to molten temperature sufficient for casting
• The heat required is the sum of:
1. Heat to raise temperature to melting point
2. Heat of fusion to convert from solid to liquid
3. Heat to raise molten metal to desired temperature for pouring

Pouring the Molten Metal

•For this step to be successful, metal must flow into all regions of the mold, most
importantly the main cavity, before solidifying
•Factors that determine success: Pouring temperature, pouring rate, Turbulence

2.3 Solidification of Metals

Transformation of molten metal back into solid state •Solidification differs depending on
whether the metal is a pure element or an alloy
A pure metal solidifies at a constant temperature equal to its freezing point (same as
melting point)

Fig 16 - Cooling curve for a pure metal during casting

Solidification of Pure Metals

•Due to chilling action of mold wall, a thin skin of solid metal is formed at the interface
immediately after pouring

39
•Skin thickness increases to form a shell around the molten metal as solidification
progresses
•Rate of freezing depends on heat transfer into mold, as well as thermal properties of the
metal

Figure 16a - Characteristic grain structure in a casting of a pure metal, showing

randomly oriented grains of small size near the mold wall, and large columnar grains
oriented toward the center of the casting

Most alloys freeze over a temperature range rather than at a single temperature

Figure 17 - (a) Phase diagram for a copper-nickel alloy system and (b) associated
cooling curve for a 50%Ni-50%Cu composition during casting

40
Figure 18 - Characteristic grain structure in an alloy casting, showing segregation of
alloying components in center of casting

Solidification Time
• Solidification takes time
• Total solidification time TST = time required for casting to solidify after pouring
•TST depends on size and shape of casting by relationship known as Chvorinov's Rule

where TST = total solidification time; V = volume of the casting; A = surface area of
casting; n = exponent usually taken to have a value = 2; and Cm is mold constant
Mold Constant in Chvorinov's Rule
•Cm depends on mold material, thermal properties of casting metal, and pouring
temperature relative to melting point
•Value of Cm for a given casting operation can be based on experimental data from
previous operations carried out using same mold material, metal, and pouring
temperature, even though the shape of the part may be quite different

A casting with a higher volume-to-surface area ratio cools and solidifies more slowly
than one with a lower ratio
 To feed molten metal to main cavity, TST for riser must greater than TST for
main casting

41
• Since riser and casting mold constants will be equal, design the riser to have a larger
volume-to-area ratio so that the main casting solidifies first
 This minimizes the effects of shrinkage

Figure 19 - Shrinkage of a cylindrical casting during solidification and cooling: (0)

starting level of molten metal immediately after pouring; (1) reduction in level caused by
liquid contraction during cooling (dimensional reductions are exaggerated for clarity in
sketches)

Figure 20 - (2) reduction in height and formation of shrinkage cavity caused by

solidification shrinkage; (3) further reduction in height and diameter due to thermal
contraction during cooling of the solid metal (dimensional reductions are exaggerated
for clarity in our sketches)

Solidification Shrinkage
•Occurs in nearly all metals because the solid phase has a higher density than the liquid
phase
•Thus, solidification causes a reduction in volume per unit weight of metal

42
•Exception: cast iron with high C content
--Graphitization during final stages of freezing causes expansion that counteracts
volumetric decrease associated with phase change

Shrinkage Allowance
•Patternmakers account for solidification shrinkage and thermal contraction by making
mold cavity oversized
•Amount by which mold is made larger relative to final casting size is called pattern
shrinkage allowance
•Casting dimensions are expressed linearly, so allowances are applied accordingly

Directional Solidification
•To minimize damaging effects of shrinkage, it is desirable for regions of the casting
most distant from the liquid metal supply to freeze first and for solidification to progress
from these remote regions toward the riser(s)
Thus, molten metal is continually available from risers to prevent shrinkage voids
The term directional solidification describes this aspect of freezing and methods by
which it is controlled

Achieving Directional Solidification

•Desired directional solidification is achieved using Chvorinov's Rule to design the
casting itself, its orientation in the mold, and the riser system that feeds it
•Locate sections of the casting with lower V/A ratios away from riser, so freezing occurs
first in these regions, and the liquid metal supply for the rest of the casting remains open
•Chills - internal or external heat sinks that cause rapid freezing in certain regions of the
casting

43
Figure 21 - (a) External chill to encourage rapid freezing of the molten metal in a thin
section of the casting; and (b) the likely result if the external chill were not used

Riser Design
• Riser is waste metal that is separated from then casting and remelted to make more
castings
• To minimize waste in the unit operation, it is desirable for the volume of metal in the
riser to be a minimum
• Since the geometry of the riser is normally selected to maximize the V/A ratio, this
allows reduction of riser volume as much as possible

Melting: Most widely used melting equipment include cupula, oil/gas fired furnaces
(including crucible and rotary furnaces), direct arc furnace and induction furnace. The
cupola is the simplest and the most economical, and most suited for grey iron. Layers of pig
iron, coke and flux (limestone) are charged into the cupola; air for combustion is blown
through several openings (tuyeres). Use of hot air blast and double row tuyeres improves
cupola efficiency. Oil or gas fired crucible furnaces are suitable for melting small quantities
of metal, usually non-ferrous. The crucible is usually made of graphite and clay. Rotary
furnaces are made of steel shells lined with refractory, turning at a rate of 1-2 rpm. The
charge is placed through a door in the middle; one end of the furnace is heated (by firing oil
or gas) and the melt is taken out through the other end. Electric furnaces include direct arc
and induction furnaces, which are more widely preferred by newer foundries owing to ease
of control over temperature and composition, and high melting rate. In arc furnace, the heat
is generated between the electrodes and transferred to the metal. In induction furnace, the
heat is generated in the metal itself by eddy currents. Induction furnaces can be classified

44
depending on the location of the induction coil (cored and coreless), and frequency of
current (high or medium).

Molten metal is prepared in a variety of furnaces, the choice of which is determined by the
quality, quantity and throughput required.
Electric induction furnaces are the most common type used for batch melting of ferrous,
copper and super alloys. This method involves the use of an electrical current surrounding
a crucible that holds the metal charge. Furnace sizes range from < 100 kg up to 15 tons.
For production of super alloys and titanium, melting may be undertaken in a vacuum
chamber to prevent oxidation.
Cupolas are used solely by iron foundries for continuous production of molten iron. The
cupola consists of a shaft in which a coke bed is established. Metal, coke and limestone
are alternately charged into the furnace from the top. Molten metal trickles through the
coke bed picking up essential carbon, while impurities react with the limestone to form
waste slag. Both metal and slag are continuously tapped out at the bottom. Metal
throughputs of 1 to 45 tons per hour are achieved in the UK.
Electric arc furnaces are still used by a few ferrous foundries in the UK, mainly producing
steel castings, although most have been replaced by induction furnaces. Furnaces of 3 to
100 tons capacity are in use in the UK. The design involves the use of a holding bath into
which electrodes are inserted. The heat generated by creating a charge between the
electrodes causes the metal to melt.
Rotary furnaces are relatively uncommon in the UK but are used in some iron foundries.
The furnace consists of a horizontal cylindrical steel shell mounted on rollers and lined with
refractory material. The furnace is fired using gas or oil from one end and the furnace body
is slowly rotated during melting.

Gas-fired shaft and resistance furnaces are used for melting of aluminium. Shaft furnaces
provide a continuous melting and tapping capability, useful at high production facilities.
Resistance furnaces are employed for melting of small batches.
Gas and oil-fired crucible furnaces are used for small batch melting of copper and
aluminium alloys, although oil-fired units are less common now and tend to be limited to

45
smaller foundries. Unlike the larger furnaces where molten metal is tapped into a ladle for
casting, the crucible is lifted out (or pops out) of the heating chamber and the molten metal
can be poured directly into the mould.

2.4 Casting Applications

Castings can range in size: from a few grams (for example, watch case) to several tones
(marine diesel engines), shape complexity: from simple (manhole cover) to intricate (6-
cylinder engine block) and order size: one-off (paper mill crusher) to mass production
(automobile pistons). The desired dimensional accuracy and surface finish can be achieved
by the choice of process and its control. Castings enable many pieces to be combined into
a single part, eliminating assembly and inventory and reducing costs by 50% or more
compared to machined parts. Unlike plastics, castings can be completely recycled. Today,
castings are used in virtually all walks of life. Major areas of applications are given below
(see Fig. 1.3). The transport sector and heavy equipment (for construction, farming and
mining) take up over 50% of castings produced.

Transport: automobile, aerospace, railways and shipping

Heavy equipment: construction, farming and mining
Machine tools: machining, casting, plastics moulding, forging, extrusion and forming
Plant machinery: chemical, petroleum, paper, sugar, textile, steel and thermal plants
Defense: vehicles, artillery, munitions, storage and supporting equipment
Electrical machines: motors, generators, pumps and compressors
Municipal castings: pipes, joints, valves and fittings
Household: appliances, kitchen and gardening equipment, furniture and fittings
Art objects: sculptures, idols, furniture, lamp stands and decorative items

Virtually any metal or alloy that can be melted can be cast. The most common
ferrousmetals include grey iron, ductile iron, malleable iron and steel. Alloys of iron and
steel are used for high performance applications, such as temperature, wear and
corrosion resistance. The most common non-ferrous metals include aluminum, copper,
zinc and magnesium based alloys. The production and application of ductile iron and

46
aluminum castings are steadily increasing. Aluminum has overtaken steel in terms of
production by weight. The consumption of magnesium alloys is rapidly increasing in
automobile and other sectors, owing its high strength to weight ratio. Important and
emerging metal titanium is stronger than steel, but has found limited applications owing
to thedifficulty in casting and machining. Table 3 lists the major metals in use today (by
weight) along with their main characteristics and typical applications.

Table 3: Major cast metals

47