IPE 141

Industrial & Production Engineering

Shahjalal University of Science & Technology, Sylhet
 Comprehensive lecture notes on Manufacturing Processes I

Lecture 17 Casting Process

Plaster Mold Casting

17.1 Introduction
Plaster mold casting is similar to sand molding except that plaster is
substituted for sand. Plaster of Paris, or simply ‘plaster’, is a type of building
material based on calcium sulfate hemihydrate, nominally 2CaSO4-H2O. In
plaster mold casting, a plaster is mixed using talc, sand, sodium silicate, and
water to form a slurry and to control contraction and setting time, reduce
cracking, and increase strength. This slurry is poured over the polished
surfaces of the pattern halves (usually plastic or metal) in a flask and allowed
to set. The slurry sets in less than 15 minutes to form the mold.

17.2 Definition
Plaster mold casting, also called rubber plaster molding (RPM), is a method
of producing silver, gold, magnesium, copper, aluminum, or zinc castings by
pouring liquid metal into plaster (gypsum) molds. The molding material is a
mixture of fine silica sand, asbestos, and Plaster of Paris as a binder. Water
is added to the mixture until a creamy slurry is obtained, which is then
employed in molding. The drying process should be very slow to avoid
cracking of the mold.

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The slurry sets in less than 15 minutes to form the mold. The mold halves
are extracted carefully from the pattern and then dried in an oven at a
temperature range of 120 to 260°C (248 to 500°F). The mold halves are
carefully assembled, along with the cores. The molten metal is poured in the
molds. After the metals cool down, the plaster is broken, and the cores
washed out. The processing steps of plaster molding is shown in Fig. 17.1.

1 5 6

2-3

1
1 4 7
1

Fig. 17.1 Plaster mold casting process.

17.3 Process
The processing steps are discussed below:
1. Construct the pattern from the customer drawing or CAD file. The
pattern is engineered to include: metal shrinkage, mold taper (if
required), and machine stock (if required). Patterns can be made
from: metal, plaster, wood, or thermosetting plastic. However, wood
has a limited life due to water absorption from the plaster slurry.

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2. Negative molds are made from the model. A positive resin cope-and
-drag pattern is now made from the negative molds.
3. Insert the core or core boxes that are made from the core plugs.
Gating, runner system, and flasks are added as necessary.
4. A liquid plaster slurry is poured around the cope and drag pattern and
into the core boxes. But, the composition of plaster slurry varies.
Additives are sometimes used to control mold expansion and fibers
added to improve mold strength.
5. The plaster mold is next removed from the cope and drag patterns.
The plaster mold and cores are then baked to remove moisture.
Molten metal is prepared by degassing, and a spectrographic sample
is taken to check the chemical analysis.
6. The molten metal is then poured into the assembled plaster mold.
The plaster is removed by mechanical knock-out and high-pressure
water-jet. When the casting has cooled, the gates and risers are then
removed.
7. The raw castings are inspected and serialized. Castings may then
require heat treatment and NDT testing. After inspection, casting is
ready for-machining, painting, or other special finishing operations.

17.4 Applications
This process is typically used for nonferrous metals such as aluminum, zinc,
or copper-based alloys, as shown in Fig. 17.2. It cannot be used to cast
ferrous material because the sulfur in gypsum slowly reacts with iron.
Typical applications include:

IPE 141 pg. 185

 • metal molds for plastic and rubber molding;

• pump and impellers, and parts of intricate geometry;

• waveguide components (for use in microwave applications);

• lock components, gear blanks, valve parts, etc. and

• molds for plastic and rubber processing, i.e., tyre molds.

Fig 17.2 Typical parts produced by the plaster molding process: machine
components (left) and the housing of a machine (right).

17.5 Advantages

• Products are having excellent surface finish and better dimensional

accuracy.

• Capability to make thin cross-sections in the casting.

• Low scrap losses and the wastes can be recycled.

• Easy to change design during production.

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• Tooling and equipment costs are low to moderate. But, skilled labor
is needed.

• Suitable for small batches of 100 and medium-volume production.

• Casting sizes range from less than one ounce to several hundred
pounds, but less than 20 lb is most common.

17.6 Disadvantages

• The mold is not permeable, thus limiting the escape of gases from
the mold cavity. The problem can be solved by: evacuating air from
the mold cavity before pouring or aerating the plaster slurry prior to
mold making to disperse voids, or using a particular mold
composition and treatment known as Antioch Process.

Antioch process involves using about 50% sand mixed with the plaster,
heating the mold in an autoclave (an oven that uses superheated steam under
pressure), and then drying.

• Curing of plaster mold at high production, and

• Limited to the casting of low melting point alloys, such as Al, Mg,
Gold, Silver, and copper alloys, etc.
• Lead time is high, and it can be several days to weeks.
• Direct labor costs are moderate to high due to high skilled labor.

IPE 141 pg. 187

 Lecture 18 Casting Process

Permanent Mold Casting

18.1 Introduction
Basic permanent mold casting is a generic term used to describe all
permanent mold casting processes. The main similarity of this group being
the employment of a permanent mold that can be used repeatedly for
multiple metal castings. The mold, also called a die, is commonly made of
steel or iron, but other metals or ceramics can be used. Parts that may be
manufactured in the industry using this metal casting process include
cylinder blocks, cylinder heads, pistons, connecting rods, parts for aircraft
and rockets, gear blanks, and kitchenware.

18.2 Description
A permanent mold can be used repeatedly for producing a casting of the
same form and dimensions. Permanent molds are usually made of steel, cast
iron, or even graphite. Each mold is generally made of two or more pieces
that are assembled by fitting and clamping. When molten metal is poured
into metal molds and subjected to only to hydrostatic pressure, the process
is called permanent mold casting. When complex cores are required, they
are usually made of sand or plaster, and the mold is said to be semi-
permanent.

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18.3 Process
The basic permanent mold casting process is described in Fig. 18.1. In
preparation for casting, the mold is preheated, and one or more coatings are
sprayed on the cavity. Preheating facilitates metal flow through the gating
system and into the cavity.
Hydraulic Movable mold Stationary Spray Core
Cavity
cylinder section mold section nozzle

(a) (b)

F V

(c) (d) (e)

Fig. 18.1 Steps in permanent mold casting: (a) Mold is preheated and coated; (b)
Cores (if used) are inserted, and mold is closed; (c) molten metal is poured into the
mold; (d)Mold is opened; and (e) Finish part.

IPE 141 pg. 189

When planning to manufacture using a permanent mold manufacturing
process, the first step is to create the mold. The sections of the mold are most
likely machined from two separate metal blocks. These parts are
manufactured precisely. They are created so that they fit together and may
be opened and closed easily and accurately.
The mold life depends on the following interrelated factors, such as-
(a) the mold material;
(b) the metal to be cast; and
(c) the operating temperature of the mold.
The gating system, as well as the part geometry, is machined into the casting
mold. However, once created, a permanent mold may be used tens of
thousands of times before its mold life is up. Due to the continuous repetition
of high forces and temperatures, all molds will eventually decay to the point
where they can no longer effectively manufacture quality metal castings.
Before pouring the metal casting, the internal surfaces of the permanent
mold are sprayed with a slurry consisting of refractory materials suspended
in a liquid. This coating serves as a thermal gradient, helping to control the
heat flow and acting as a lubricant for easier removal of the cast part.
The two parts of the mold must be closed and held together with force, using
some sort of mechanical means. Most likely, the mold will be heated prior
to the pouring of the metal casting. A possible temperature that a permanent
metal casting mold may be heated to before pouring.
The metal cast part is usually removed before much cooling occurs, to
prevent the solid metal casting from contracting too much in the mold. Cores
are often employed in a permanent mold metal casting process.

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18.4 Advantages
The advantages of permanent mold castings are:

• substantial increase in productivity (i.e., mold does not have to be made

for each casting);

• close tolerances;

• superior surface finish;

• improved mechanical properties of the castings;

• noticeable reduction in the percentage of rejects; and

• economically feasible for mass production.

18.5 Disadvantages
The disadvantages of permanent mold casting include:

• not feasible for all size of production; and

• not all alloys are suitable for this process.

18.6 Applications
Common casting metals are aluminium, magnesium, and copper alloys.
Other materials include tin, zinc, lead alloys, and iron and steel, are also cast
in the graphite molds.
Permanent mold castings are used for their wide range of desirable
mechanical and physical properties, including but not limited to: high
strength-to-weight ratio, reduced machining costs due to superior castability,
premium surface finish and precision machining surfaces.

IPE 141 pg. 191

 Lecture 19 Casting Process

Die Casting

19.1 Introduction
Die casting is a permanent mold casting process in which the molten metal
is injected in the mold cavity under high pressure. The pressure is maintained
during solidification, after which the mold is opened, and the part is
removed. Molds in this casting operation are called dies; hence, the names
die casting. The use of high pressure to force the metal into the die cavity is
the most notable feature that distinguishes this process from others in the
permanent mold category. Die castings operations are carried out in special
die casting machines. Modern die casting machines are designed to hold and
accurately close the two halves of the mold and keep them closed while the
liquid metal is forced into the cavity.

19.2 Description
Conventional die casting is a net-shape manufacturing process using a
permanent metal die that produces components ranging in weight from a few
grams to moderately large (up to 25 kg) quickly and economically. Usually,
die casting is not used to produce large products; but car door frame or
transmission housing can be produced using die casting technologies.
Conventional die cast components can be produced in a wide range of alloy

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systems, including aluminum, zinc, magnesium, lead, and brass. There are
two main types of die casting machines:

1. hot chamber die casting, and

2. cold chamber die casting; they are differentiated by how the molten
metal is injected into the cavity.

Fig. 19.1 Graphical illustration of a hot-chamber die casting machine.

19.2.1 Hot chamber die casting

In hot-chamber die castings, the metal is melted in a container attached to
the machine, and a plunger is used to inject the liquid metal under high

IPE 141 pg. 193

pressure into the die. A schematic of a hot-chamber die casting machine is
shown in Figure 19.1. Typical injection pressures are 7 to 35 MPa. A
significant portion of the metal injection system is immersed in the molten
metal at all times. Production rates up to 500 parts per hour are most common
with cycle times varying from less than 1 sec for small components weighing
less than a few grams to 30 sec for castings of several kilograms. This helps
keep cycle times to a minimum, as molten metal needs to travel only a very
short distance for each cycle. The process is therefore limited in its
applications to low melting point metals that do not chemically attack the
plunger and other mechanical components. These metals include zinc, tin,
lead, and sometimes magnesium. Higher melting point metals, including
aluminum alloys, cause rapid degradation of the metal injection system.

19.2.2 Cold-chamber die casting

Cold-chamber die casting machine, as illustrated in Fig. 19.2, is typically
used to conventionally die cast components using brass and aluminum
alloys. Unlike the hot chamber machine, the metal injection system is only
in contact with the molten metal for a short period. In this process, molten
metal is poured into an unheated chamber from an external melting
container, and a piston is used to inject the metal under high pressure into
the die cavity. Injection pressures used in these machines are typically 14 to
140 MPa. Compared to hot chamber machines, cycle rates are not usually as
fast because of the need to ladle the liquid metal into the chamber from an
external source. Nevertheless, this casting process is a high-production
operation. Cold-chamber machines are typically used for casting aluminum,
brass, and magnesium alloys. Low melting point alloys (i.e., zinc, tin, lead)
can also be cast on cold chamber machines, but the advantages of the hot-
chamber process usually favor its use on these metals.
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Fig. 19.2 Graphical illustration of a cold-chamber die casting machine.

19.3 Process

Similar to other casting processes, the process of die casting consists of five
main stages, these are: (i) clamping the mold, (ii) injection of molten metal,
(iii) cooling, (iv) ejection, and (v) trimming.
Clamping: The first step is the preparation and clamping of the two halves
of the die. Each die half is cleaned and lubricated to facilitate the ejection of
the next part. However, lubrication may not be required after each cycle, but
after 2 or 3 cycles, depending upon the material. After lubrication, the two
die halves are closed and securely clamped together. Sufficient force is
applied to the die to keep it securely closed while the metal is injected.

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Injection of molten metal: In this stage, the molten metal is transferred into
a chamber where it can be injected into the die. The method of transferring
the molten metal is dependent upon the type of die casting machine, whether
a hot chamber or cold chamber machine is being used. Once transferred, the
molten metal is injected at high pressures into the die. This pressure holds
the molten metal in the dies during solidification.
In the hot chamber die casting, molten metal is forced through the cavity of
a pre-shaped mold using pressure, as shown in Fig. 19.3 a. A goose-neck
channel attached to the hot chamber casting machines transfers the molten
metal to the mold cavity.

Fig. 19.3 The injection process in die casting: (a) in the hot chamber process,
molten metal flows into the chamber by the plunger force through the
goose-neck channel, and (b) in the cold chamber, molten metal is poured
into the chamber by a ladle and a ram forces metal to flow into die cavity.

Cooling: The molten metal that is injected into the die will begin to cool and
solidify once it enters the die cavity. When the entire cavity is filled, and the
molten metal solidifies, the final shape of the casting is formed. The die can
not be opened until the cooling time has elapsed, and the casting is solidified.
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Ejection: After the predetermined cooling time has passed, the die halves
are opened, and an ejection mechanism pushes the casting out of the die
cavity. The ejection mechanism must apply some force to eject the part
because during cooling, the part shrinks and adheres to the die.
Trimming: The excess material, along with any flash that has occurred, must
be trimmed from the casting either manually via cutting or sawing, or using
a trimming press. The scrap material that results from this trimming is either
discarded or can be reused in the die casting process.

19.4 Advantages
The advantages of die casting include:
1. high production rates are possible;
2. it is economical for large production quantities;
3. close tolerances are possible on the order of ± 0.003 in. (±0.076 mm)
on small parts;
4. good surface finish;
5. thin sections are possible, down to about 0.020 in. (0.5 mm); and
6. rapid cooling, which provides small grain size and good strength to
the casting.
But, the advantages of hot chamber die casting includes:

• high production rate

• less danger to labor.
The advantages of cold chamber die casting includes:

IPE 141 pg. 197

 • it can process more denser metal castings.
• process has a low maintenance cost.
• produces parts with lower overhead costs.

19.5 Disadvantages
The limitations of this process include:
1. shape restrictions of the to be cast; the part geometry must be such
that it can be removed from the die cavity;
2. formation of flush is common in die casting; and
3. casting with sprue and gating system incorporated with the casting.
Specifically, the disadvantages of hot chamber die casting includes:

• only low melting materials and alloys can be cast.

• plunger is always dipped into the molten metal due to its less life.
The disadvantages of cold chamber die casting includes:

• processing has slower production cycles compared to a hot chamber;

• the molten metal can cool down even before injection.
• the molten metal in the cold chamber is more exposed to oxidation
and other contaminants.

19.6 Applications
Conventional die-cast components can be produced in a wide range of alloy
systems, including aluminum, zinc, magnesium, lead, and brass. Some
examples of its application would include: golf equipment, camera housings,
electronic housings, locks and deadbolts, pumps and compressors, hospital
bed control panels, dental workstation baseplates.
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 Lecture 21 Casting Process

Slush casting

21.1 Introduction
Slush casting is a variation of permanent mold casting that is used to produce
hollow parts. In this method, neither the strength of the part nor its internal
geometry can be controlled accurately. This metal casting process is used
primarily to manufacture toys and parts that are ornamental in nature, such
as lamp base, statue, etc.

21.2 Description
Slush casting is a traditional method of the permanent mold casting process,
where the molten metal is not allowed to solidify in the mold completely.
When the desired thickness is obtained, the remaining molten metal is
poured out.
In this process, a metal mold in two or more sections is used. The mold is
filled with molten metal. After partial solidification of the liquid metal on
the surface in the desired thickness, the mold is inverted in order to drain out
the still-liquid metal at the center, resulting in a hollow casting. The mold
halves then are opened, and the casting removed. This is a relatively
inexpensive process for small production runs and generally is used only for
low-melting lead and zinc-based metals and to produce ornamental items
that need not be strong, such as statues, lamp pedestals, and toys.

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21.3 Process
Slush casting can be done in an open or closed mold. Mostly pewter (a gray
alloy of tin with copper and antimony, formerly, tin and lead) is cast using
the slush casting technique. Slush molding can be a simple hand operation
for limited production or an elaborate conveyorized system for long runs.
This process can be a one pour method, where finished or semi-finished
products can be made by one slushing step or a multiple-pour method where
two or more slushing steps are used. The processing steps of slush casting
are shown in Fig. 21.1.

Fig. 21.1 Slush casting process.

• Firstly, a pattern is made using plaster or wood. Then the pattern is

placed on a cardboard or wooden board. A mold box is kept around the
pattern.
• The unwanted space that is formed is the mold box can be eliminated
by placing a board. When the mold is set, the pattern is withdrawn from
the mold.

IPE 141 pg. 203

 • The metal melted completely and poured into the mold, which is
shaped in the desired form. Rotate the mold to coat the sides.
• Solidification begins at the walls because they are relatively cool; it
then works inward, so the thickness of the shell is controlled by the
amount of time allowed before the mold is drained.
• When the metal settles in the mold, the remaining liquid metal is
poured out of the mold. Thus, a hollow skin metal is formed inside the
mold.
• If the cast needs to be thicker, once again, molten metal is poured into
the mold and poured out. This process is repeated until the desired
thickness is achieved.
• When the metal hardens, the mold is broken to remove the castings.
The inside of each cast retains molten textures while the exterior is
smooth and shiny.

21.4 Applications
Slush molding is an excellent method of producing open, hollow objects,
including rain boots, shoes, toys, dolls, and automotive products, such as
protective skin coatings on the arm-rests, head-rests, and crash pads. Vinyl
powder compound will reproduce the surface finish of the mold, whether
matte or glossy. Mold porosity, depending upon the severity, may cause such
detrimental effects as surface gloss reduction, pin-holing, and voids in the
molded part.
Decorative and ornamental objects that are cast are as vase, bowls,
candlesticks, lamps, statues, jewelries, animal miniatures, various
collectibles, etc. A variety of exquisitely designed casting can be cast for

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small objects and components for the industry like tankard handle, handles
for hollow wares, etc. An example of a fabricated toy car is shown in Fig
21.2.

Fig. 21.2 Fabricated toy car; inside of toy (left) and the exterior of the toy (right).

21.5 Advantages
The advantages of slush casting include:

• Slush casting is used to produce hollow parts without the use of cores.
• The desired thickness can be achieved by the controlled pouring of
the molten metal. The leftover molten metal can be reused.
• Lightweight
• Uses less material

21.6 Disadvantages
The disadvantages of slush casting are:

• The process is time consuming;

• Requires manual labor;

IPE 141 pg. 205

 • Casting wall thickness can vary by man to man. A typical example is
shown in Fig. 21.3.

Fig. 21.3 A typical example of wall thickness depending on the time passed.

21.7 Typical Uses

• Props and displays;

• Art objects;
• Bonded bronze pieces;
• Prototypes;
• Exquisitely designed small objects e.g., statues, jewelries, animal
miniatures, handles, etc.

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Lecture 22 Casting Process

Centrifugal Casting

22.1 Introduction
Centrifugal casting, also called ‘spin casting’ or ‘rotocasting’, is a metal
casting process in which the mold is rotated at high speed so that centrifugal
force distributes the molten metal to the outer regions of the die cavity. This
differs from most metal casting processes, which use gravity or pressure to
fill the mold. In centrifugal casting, a permanent mold made from steel, cast
iron, or graphite is typically used. However, the use of expendable sand
molds is also possible.

Centrifugal casting is used to produce axially symmetric parts, such as

cylinders or disks, which are typically hollow. Due to the high centrifugal
forces, these parts have a very fine grain on the outer surface and possess
mechanical properties approximately 30% greater than parts formed with
static casting methods. These parts may be cast from ferrous metals such as
low alloy steel, stainless steel, and iron, or non-ferrous alloys such as
aluminum, bronze, copper, magnesium, and nickel. Centrifugal casting is
performed in a wide variety of industries, including aerospace, industrial,
marine, and power transmission. Typical parts include bearings, bushings,
coils, cylinder liners, nozzles, pipes/tubes, pressure vessels, pulleys, rings,
and wheels.

IPE 141 pg. 207

22.2 Description
In the centrifugal casting, a permanent mold is rotated continuously at high
speeds (300 to 3000 rpm) as the molten metal is poured. The molten metal
spreads along the inside mold wall, where it solidifies after cooling. The
casting is usually a fine-grained casting with an especially fine-grained outer
diameter, due to the rapid cooling at the surface of the mold. Lighter
impurities and inclusions move towards the inside diameter and can be
machined away following the casting.
There are two types of centrifugal casting processes:
a. Vertical, and
b. Horizontal.
Vertical centrifugal casting: In the vertical centrifugal casting process, the
molten metal is poured into a vertically oriented preheated rotating die. The
centrifugal forces in the vertical spinning mold cause the molten metal to
spread horizontally and vertically along the inner diameter of the mold so
that it assumes the specified shape.
As the mold and die spin, the denser elements of the molten metal are pushed
toward the outer diameter. The mold acts as a cooling surface, which
solidifies the metal from the outer diameter as it moves towards the bore.
This process, shown in Fig. 22.1(a), causes impurities to float toward the
inner diameter, from which they can be machined away when the casting
process is complete.
Vertical centrifugal casting can also be used to cast non-cylindrical parts
such as valves and propellers by using specialty castings. Other products that
are commonly produced with vertical centrifugal casting methods include:
balls for ball valves, gear blanks, short bushings, flanges, sprockets, etc.
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Metal pouring

Mold cavity

Outer shell

Mold spin

(a) (b)
Fig. 22.1 Setup for centrifugal casting: (a) Vertical, and (b) Horizontal machines.

Horizontal centrifugal casting: In the horizontal centrifugal casting process,

as shown in Fig. 22.1 (b), the die rotates about the horizontal axis. This
process is especially suited for large cylindrical parts where the casting
length is significantly longer than its outside diameter. This includes straight
tube sections, long cylinders with end flanges, or short parts such as rings or
flanges where multiple parts. Although vertical and horizontal castings can
produce products of similar quality, castings with vertical orientations are
more suitable.

The group of centrifugal casting includes:

1. True centrifugal casting,

2. Semi-centrifugal casting, and
3. Centrifuge casting.

IPE 141 pg. 209

1. True centrifugal casting: In the true centrifugal casting, molten metal is
poured into a rotating mold to produce a tubular part. Examples of parts
made by this process include pipes, tubes, bushings, and rings. The possible
setup is illustrated in Fig. 22.2.

Fig. 22.2 The basic set up of the true centrifugal casting process.

Molten metal is poured into a horizontal rotating mold at one end. The high-
speed rotation results in centrifugal force that cause the metal to take the
shape of the mold cavity. Thus, the outside shapes of the casting can be
round, octagonal, hexagonal, or other. However, the inside shape of the
casting is (theoretically) perfectly round, due to the radially symmetric
forces at work.

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Centrifugal force is defined by the mathematical equation as:

mv 2
F=
R

where F = force, N (or lb); m = mass in kg, (or lb); v = velocity m/s (or
ft/sec); and R = inside radius of the mold, m (or ft). The force of gravity is
its weight W = mg, where W is given in kg, and g = acceleration of gravity
(9.8 m/sec2). The G-factor, GF, is the ratio of centrifugal force divided by
weight, is given by:

F mv 2 v2
GF = = =
W Rmg Rg

Velocity ‘v’ can be expressed as 2RN/60 = RN/30, where N = rotational

speed, rev/min. Substituting this expression, we obtain

 N 
2

R 
GF =  30 
g

Rearranging this to solve for rotational speed N and using diameter D rather
than radius in the resulting equation, we have

30 2 gGF
N=
 D

where D = inside diameter of’ the mold, m (or ft). If the G-factor (60 to 80
are found to be appropriate for horizontal centrifugal casting) is too low in
centrifugal casting, the liquid metal will not remain forced against the mold
wall during the upper half of the circular path but will ‘rain’ inside the cavity.

IPE 141 pg. 211

2. Semi-centrifugal casting: In this method, centrifugal force is used to
produce solid castings rather than tubular parts. The difference is that in the
semi-centrifugal casting, the mold is filled completely with molten metal,
which is supplied to the casting through a central sprue. The centripetal force
acting on the casting’s material during the fabrication process of semi-
centrifugal casting plays a significant role in determining the properties of
the final cast part. Therefore, the density of metal in the final casting is higher
in the outer sections than at the center of rotation.

Fig. 22.3 The schematic representation of semi-centrifugal casting.

The process, shown in Fig. 22.3, is often used on parts in which the center
of the casting is machined away, thus eliminating the portion of the casting
where the quality is lowest. Wheels and pulleys for tracked vehicles are
examples of this type of castings made by this process.

3. Centrifuge casting: In the centrifuge casting, the mold is designed with

part cavities located away from the axis of rotation so that the molten metal

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poured into the mold is distributed to these cavities by centrifugal force. This
provides a means of increasing the filling pressure within each mold and
allows for the reproduction of intricate details. The process is used for
smaller parts, and the radial symmetry of the part is not a requirement as it
is for the other two centrifugal casting methods. This method is often used
for the pouring of investment casting patterns. The basic arrangement of
centrifuge casting is illustrated in Fig. 22.4.

Final product

Fig. 22.4 Basic arrangement of centrifuge casting.

22.3 Process
Similar to other casting processes, the processing operation includes the
steps: (i) mold preparation, (ii) molten pouring, (iii) cooling, (iv) ejection,
and (v) finishing.
Mold preparation: The walls of a cylindrical mold are coated with a
refractory ceramic coating, which involves a few steps (application, rotation,
drying, and baking). Once prepared and secured, the mold is rotated about
its axis at high speeds (300-3000 rpm), typically around 1000 rpm.

IPE 141 pg. 213

Pouring: Molten metal is poured directly into the rotating mold, without the
use of runners or a gating system. The centrifugal force drives the material
towards the mold walls as the mold fills.
Cooling: With all of the molten metal in the mold, the mold remains spinning
as the metal cools. Cooling begins quickly at the mold walls and proceeds
inwards.
Casting ejection: After the casting has cooled and solidified, the rotation is
stopped, and the casting can be removed.
Finishing: While the centrifugal force drives the dense metal to the mold
walls, and less dense impurities or bubbles flow to the inner surface of the
casting. As a result, secondary processes such as machining, grinding, or
blasting, are required to clean and smooth the inner diameter of the part.

22.4 Advantages
The advantages of centrifugal casting are:

• Castings acquire high density, high mechanical strength, and fine -

grained structure;
• Impurities are pulled toward the inside surface and can be easily
machined; therefore, inclusions and impurities are lighter;
• Gates and risers are not needed;
• Formation of hollow interiors without cores;
• High output, and ultimately cost-effective;
• Both ferrous and non-ferrous metals can be used;
• Castings with good dimensional accuracy and quality are produced;
• Can produce castings with up to 10 ft in diameter and 50 ft in length.

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22.5 Disadvantages
The limitations of centrifugal casting are:

• Skilled labors are to be employed for this process;

• The castings are having an inaccurate diameter of the inner surface;
• Only some shapes can be generated by this casting process;
• Not all alloys can be cast in this way;
• Centrifugal castings require very high investments;
• Vibration defects occur due to improper mounting and faulty
equipment;
• The mold rotating too slowly or the pouring rate too fast can result
in the metal falling down from the top of the rotation onto the bottom,
called raining effect.

22.6 Applications
Most metals suitable for static casting are suitable for centrifugal casting: all
steels, iron, copper, aluminum, and nickel alloys. Also, glass, thermoplastics,
composites, and ceramics (metal molds sprayed with a refractory
material) can be molded by this method.
Typical applications include- pipes, brake drums, pulley wheels, train
wheels, flywheels, gun barrels, gear blanks, large bearing liners, engine-
cylinder liners, pressure vessels, nozzles, etc.

22.7 Problems and Solution

Problem 1. A horizontal true centrifugal casting operation will be used to
make copper tubing. The lengths will be 1.5 m with outside diameter 15.0

IPE 141 pg. 215

cm, and inside diameter 12.5 cm. If the rotational speed of the pipe = 1000
rev/min, determine the G factor. [Ref: Groover, 4th ed, pp. 256]
Solution: From the equation of GF, we know

 N 
2

R 
GF =  30 
g

7.5(3.14× 1000⁄30)2
Putting the values in equation, 𝐺𝐹 = = 83.8 (Ans.)
981

Problem 2. A true centrifugal casting operation is to be performed in a

horizontal configuration to make cast iron pipe sections. The sections will
have a length = 42.0 in, outside diameter = 8.0 in, and wall thickness = 0.50
in. If the rotational speed of the pipe = 500 rev/min, determine the G-factor.
Is the operation likely to be successful? [Ref: Groover, 4th ed, pp. 257]
Solution:

Given that, The GF,

Outside wall of casting, GF = v2/Rg
R = 0.5(8)/12 = 0.333 ft. = (17.45)2/ (0.333 × 32.2)

v = RN/30 = (.333)(500)/30 = 28.38 (Ans)

= 17.45 ft/sec.

Since the G-factor is less than 60, the rotational speed is not sufficient, and
the operation is likely to be unsuccessful.
Problem 3. A horizontal true centrifugal casting process is used to make
brass bushings with the following dimensions: length = 10 cm, outside

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diameter = 15 cm, and inside diameter = 12 cm. (i) Determine the required
rotational speed in order to obtain a G-factor of 70. (ii) When operating at
this speed, what is the centrifugal force per square meter (Pa) imposed by
the molten metal on the inside wall of the mold? [Ref: Groover, 4e, pp. 257]
Solution:
(i) The outside wall diameter of the casting, which is equal to the inside wall
diameter of the mold, D = 15 cm

Therefore, N = (30/)(2g × 70/15)0.5 = 913.7 rev/min.

(ii) Use 1.0 cm of mold wall thickness,

Area of mold wall, A = DoL = (15 cm)(1 cm) = 15(10)-4 m2.

Volume of cast metal, V = ( R02 − Ri2 ) 1 = 63.62 cm2

Mass, m = (8.62g/cm3)(63.62 cm3) = 548.4 g = 0.5484 kg

v = RN/30, using mean radius R = (7.5 + 6.0)/2 = 6.75 cm,

v = (6.75)(913.7)/30 = 645.86 cm/s = 6.4585 m/s

Centrifugal force per square meter on mold wall = Fc/A where Fc = mv2/R
mv 2 (0.5484  6.4586)2
Fc = = = 338.9 kg-m/s2
R 6.75 10−2
Given that, 1 N = 9.81 kg-m/s2, Fc = 338.9/9.81 = 34.55 N
Fc/A = (34.55 N)/(15 x 10-4 m2) = 0.7331(104) N/m2 = 7331 Pa (Ans.)

Problem 4. True centrifugal casting is performed horizontally to make large

diameter copper tube sections. The tubes have a length = 1.0 m, diameter =
0.25 m, and wall thickness = 15 mm. (i) If the rotational speed of the pipe =

IPE 141 pg. 217

700 rev/min, determine the G-factor on the molten metal. (ii) Is the rotational
speed sufficient to avoid ‘‘rain?’’ (iii) What volume of molten metal must
be poured into the mold to make the casting if solidification shrinkage and
contraction after solidification are considered? Solidification shrinkage for
copper = 4.5%, and solid thermal contraction = 7.5%.
[Ref: Groover, 4e, pp. 257]
Solution:
(i) GF = v2/Rg g = 9.8 m/s2

v = RN/30 = (.125)(700)/30 = 9.163 m/s

N ( )
2

v2 R 30
GF = = = 68.54
Rg g

(ii) G-factor lies in 60 to 80, therefore, G-factor is sufficient for a successful

casting operation.

(iii) Volume of final product after solidification and cooling is

V=
( (0.25) 2
− (0.03) 2 )   10
= 0.011074 m3
4
For solidification shrinkage = 4.9% and solid thermal contraction = 7.5% for
copper.
Taking these factors into account, V = 0.011074/(1- 0.049)(1- 0.075)
= 0.0126 m3 (Ans.)
Problem 5: Horizontal true centrifugal casting is used to make aluminum
rings with length = 5 cm, outside diameter = 65 cm, and inside diameter =

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60 cm. (i) Determine the rotational speed that will provide a G-factor = 60.
(ii) Suppose that the ring was made out of steel instead of aluminum. If the
rotational speed computed in part (i) were used in the steel casting operation,
determine the G-factor and (iii) Centrifugal force per square meter (Pa) on
the mold wall. (iv) Would this rotational speed result in a successful
operation? The density of steel = 7.87 g/cm3. [Ref: Groover, 5e, pp. 285]
Solution: (i) Using the inside diameter of mold in the Equation, we get,
D = Do = 65 cm, and g = 981 cm/s2,

30 2 gGF 30 2  981 60
Rotational speed, N = = = 406.4 rev/min.
 D  65
(ii) Rotational speed would be the same as in part (i) because mass does not
enter the computation of rotational speed. N = 406.4 rev/min
(iii) Area of the mold wall, A = πDoL = π×(65 cm)×(5 cm) = 1021 cm2

 65 2  60 2 
( )
Volume of metal, V =  R02 − Ri2 L =    −     5 = 2454.4 cm3
 2   2  

 7.87
As the density of steel ρ = 7.87 g/cm3, Mass, m = = = 19316 gm
V 2454.4
Velocity, v = πRN/30; using mean radius R = (65 + 60)/4 = 31.25 cm
 RN   31.25  406.4
⸫ velocity, v = = = 1329.9 cm/s = 13.299 m/s
30 30

Fc mv 2
Centrifugal force per square meter on mold = where Fc =
A R
Where, Fc = (19.316 kg)(13.299 m/s)2/(0.3125 m) = 10,932.1 kg-m/s2

IPE 141 pg. 219

Given that 1 N = 9.81 kg-m/s2, Fc = 10,932.1/9.81 = 1114.4 N

(19.316 13.299 )
2

Fc 0.3125 = 10,914.7 N/m2

Centrifugal force = =
A 102110−4
(iv) The G-factor of 60 would probably result in a successful casting
operation.

Problem 6: For the steel ring of preceding Problem 5, determine the volume
of molten metal that must be poured into the mold, given that the liquid
shrinkage is 0.5%, solidification shrinkage = 3%, and solid contraction after
freezing = 7.2%.
Solution:

 65 2  60 2 
V =  ( R − R ) L =    −     5 = 2454.4
2
0 i
2

Volume of casting,  2   2   cm3

2454.4
V= = 2740.2
0.8957
Given that the molten metal shrinkage = 0.5%, and from Table 10.1, the
solidification shrinkage for steel = 3% and the solid contraction during
cooling = 7.2%, the total volumetric contraction is
(1―0.005)×(1―0.03)×(1―0.072) = 0.8957
The required starting volume of molten metal
2454.4
V= = 2740.2 cm3
0.8957

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Lecture 23 Casting Process

Casting Defects

23.1 Introduction
It is an unwanted irregularity that appears in the casting during the metal
casting process. There are various reasons or sources which are responsible
for the defects in the cast metal. Here in this section, we will discuss all the
major types of casting defects. Some of the defects produced may be
neglected or tolerated, and some are not acceptable; it must be eliminated for
better functioning of the parts.
There are numerous defects occur in the foundry process. Some defects are
common to any or all casting processes. These defects are discussed below.

23.2 Different types of defects

A properly designed casting, a properly prepared mold, and correctly malted
metal should result in a defect-free casting. However, if proper control is not
exercised in the foundry, a variety of defects may result in the casting. The
common defects in casting process are:
Misruns: A misrun (Fig. 23.1 a) is a casting that has solidified before
completely filling the mold cavity.

IPE 141 pg. 221

Fig. 23.1 Some common defects in castings: (a) Misruns; (b) Cold shut; (c) Cold
shots; (d) Shrinkage cavity; (e) Micro-porosity; and (f) Hot tears.

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Typical causes include-

(a) fluidity of molten metal is insufficient,
(b) pouring temperature is too low,
(c) pouring is done too slowly, and
(d) cross-section of the mold cavity is too thin or improper gating system.
Remedies:
i. Increasing the pouring temperature of the molten metal increases the
fluidity.
ii. Proper gating system design,
iii. Avoiding the too thin sections.

Cold Shut: A cold shut (Fig. 23.2 b) occurs when two portions of the metal
flow together, but there is a lack of fusion between them due to premature
freezing. Its causes are similar to those of a misrun.
Typical causes include-
(a) Poor gating system,
(b) Low melting temperature
(c) Lack of fluidity
Remedies include:
i. Improved gating system.
ii. Proper pouring temperature.

Cold Shots: When spattering occurs during pouring, solid globules of metal

IPE 141 pg. 223

are formed that become entrapped in the casting, as shown in Fig. 23.1 (c).
Pouring procedures and gating system designs that avoid spattering can
prevent this defect.
Typical causes include-
(a) Formation of oxide skin which prevents that flow of metal,
(b) Inclusion of other impurities,
(c) Use of high moisture content sand.
Remedies include:
i. By reducing the moisture content of the molding sand.
ii. Improving the gating system.
iii. Providing proper pouring temperature.

Shrinkage Cavity: This defect (Fig. 23.1 d) is the depression in the surface
or an internal void in the casting caused by solidification shrinkage that
restricts the amount of molten metal available in the last region to freeze. It
often occurs near the top of the casting, in which it is referred to as a pipe.
Typical causes include-
(a) Uneven or uncontrolled solidification of molten metal.
(b) Pouring temperature is too high.
Remedies include:

i. This defect can be removed by applying the principle of directional

solidification in mold design.
ii. Usage of chills (a chill is an object which is used to promote
solidification in a specific portion of a metal casting) and padding.

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 Comprehensive lecture notes on Manufacturing Processes I

iii. The problem can often be solved by proper riser design.

Micro-porosity: This refers to a network of small voids distributed

throughout the casting (Fig. 23.1 e) caused by localized solidification
shrinkage of the final molten metal in the dendritic structure. The defect is
usually associated with alloys, because of the protected manner in which
freezing occurs in these metals.
Typical causes include-
(a) Use of high moisture content sand.
(b) Absorption of hydrogen or carbon monoxide gas by molten metal.
(c) Pouring of steel from wet ladles or not sufficiently gasified.
The remedies:
i. By reducing the moisture content of the molding sand.
ii. Good fluxing and melting practices should be used.
iii. Increasing permeability of the sand.
iv. By doing a rapid rate of solidification.

Hot Tears: Also called hot cracking, occurs when the metal is hot. It is a
weak part of the casting, and the residual stress (tensile) in the material
causes the casting fails when the molten metal cools down. In this case, the
failure of casting looks like a crack and called as hot tears or hot cracking
(Fig. 23.1 f). The defect is manifested as a separation of the metal at a point
of high tensile strength stress caused by the metal’s inability to shrink
naturally.
Typical cause is the improper mold design.

IPE 141 pg. 225

Remedies include:
i. Proper mold design that can easily eliminate these types of casting
defects.
ii. Elimination of residual stress from the material of the casting.
iii. In sand casting and other expandable mold processes, it is prevented
by compounding the mold to be collapsible.

However, some defects are related to only sand molds, as illustrated in Fig.
23.2, therefore they occur only in sand castings. These are-
(a) Sand blow, (i) Swell,
(b) Pin-holes, (j) Drop,
(c) Sand wash, (k) Slag inclusion,
(d) Scabs, (l) Fins,
(e) Penetration, (m) Warpage,
(f) Mold shift, (n) Dirt,
(g) Core shift, (o) Metal penetration,
(h) Mold crack, (p) Rattail, etc.

(a) Sand blow: This defect consists of a balloon-shaped gas cavity

caused by the release of mold gases during pouring. It occurs at the
below of the casting surface near the top of the casting. Low
permeability, poor venting, and high moisture content of the sand
mold are the usual causes.

(b) Pinholes: A defect similar to a sand blow involves the formation of

many small gas cavities at or slightly below the surface of the casting.

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(a) Sand blow (b) Pinholes (c) sand wash

(d) Scabs (e) Penetration (f) Mold shift

(g) Core shift (h) Mold crack (i) Swell

(j) Drop (k) Slag Inclusion (l) Fins

Fig. 23.2 Different casting defects that are usual in sand casting.

IPE 141 pg. 227

 They are very small holes of about 2 mm in size, which appears on
the surface of the casting. This defect happens because of the
dissolution of the hydrogen gases in the molten metal.
(c) Sand wash: A wash is an irregularity in the surface of the casting that
results from erosion of the sand mold during pouring. The contour of
the erosion is imprinted into the surface of the final cast part.

(d) Scabs: This is a rough area on the surface of the casting due to
encrustations of sand and metal. It is caused by portions of the mold
surface flaking off during solidification and becoming embedded in
the casting surface.

(e) Penetration: When the fluidity of the liquid metal is high, it may
penetrate into the sand mold or sand core. After freezing, the surface
of the casting consists of a mixture of sand grains and metal. Harder
packing of the sand mold helps to alleviate this condition.

(f) Mold shift: This is manifested as a step in the cast product at the
parting line caused by sidewise displacement of the cope with respect
to the drag.

(g) Core shift: A similar movement can happen with the core, but the
displacement is usually vertical. Core shift and mold shift are caused
by the buoyancy of the molten metal.

(h) Mold crack: If mold strength is insufficient, a crack may develop into
which liquid metal can sweep to form a fin on the final casting.

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(i) Swell: It is the enlargement of the mold cavity because of the molten
metal pressure, which results in localized or overall enlargement of
the casting. This type of problem happens due to the defective or
improper ramming of the mold.

(j) Drop: Drop defect occurs when there is cracking on the upper surface
of the sand, and sand pieces fall into the molten metal. Soft ramming
and low strength of sand, insufficient fluxing, and inclusion of
impurities are the main causes for the drop.

(k) Slag inclusion: This defect is caused when the molten metal
containing slag particles is poured in the mold cavity, and it gets
solidifies. Remove slag particles from the molten metal before
pouring it into the mold cavity.

(l) Fins: A thin projection of metal, not considered as a part of casting

is called as fins or fin. It is usually occurring at the parting of the
mold or core section. Incorrect assembling of mold and cores, the
insufficient weight of the mold, or improper clamping of the flask
may produce the fins.
Other types of defects include:
(m) Warpage: Warping, as shown in Fig. 23.3, is an unwanted casting
deformity that can occur over time, which results in a change in the
dimensions of the final product. It can happen during or after
solidification.

IPE 141 pg. 229

Fig. 23.3 Sand casting defect: warpage (left) and metal penetration (right).

(n) Metal penetration: These casting defects (Fig. 23.3) appear as an

uneven and rough surface of the casting. When the size of sand grains
is larges, the molten fuses into the sand and solidifies, giving metal
penetration defect. It is caused due to low strength, large grain size,
high permeability, and soft ramming of sand. Because of this, the molten
metal penetrates in the molding sand, and rough or uneven casting
surface can be produced.

(o) Dirt: The embedding of particles of dust and sand in the casting
surface, results in dirt defect. Cursing of mold due to improper
handling and sand wash and the presence of slag particles in the
molten metal cause dirt addition.

(p) Rattail: Rattails (Fig. 23.4), or veins, appear as an

irregular line or crack on the casting when the
surface of the molding sand buckles up. Rattails
usually occur on the surface of the mold bottom,
an area covered with molten material. Rattails
occur when excessive heat of the metal causes the
sand to expand.
Fig. 23.4 Rattail
pg. 230 Manufacturing Processes I
I believe, the duties of a university teacher are teaching, research
and service. As a first part of my job, the cardinal aim of my
teaching is to carry out lectures on different subjects through the
effective utilizations of different teaching aids in the classroom
environment so that the students can understand subject matters
easily. Beyond striving to ensure, students learn the fundamental
content of the courses and in this way they get to facilitate the
acquisition of lifelong learning skills, to help students develop
evidence-based clinical problem-solving strategies, and to prepare
students to function as highly skilled and competent manpower
across the scope of practical field.

Industrial & Production Engineering

Shahjalal University of Science & Technology, Sylhet