1 1

CHAPTER 1

INTRODUCTION

1-Definition: Casting is a liquidus for process, includes melting the metal, pouring it in a pre-shaped mould

and allowing it to solidify.

OR: It is the process of pouring a molten metal into a pre-shaped cavity, allowing the metal to

solidify and finally taking out the cast product.

2-Why casting used:

It can be used when:

1- Casting huge and complex engineering shapes

2-Provides great design freedom

3-Producing shapes or products, which cannot be produced with other methods

4-More economical, specially in mass production

5-Casting the metals and alloys, which can be difficulty, machined as cast iron.

3-Disadvantages of casting:

1-Low dimensional accuracy

2-Ineconomic for small production

3-Low surface finish

4-Most products needs other forming processes.

4-Casting stages:

Metal

melting

Mould preparation

Pouring

Solidification

Cast extraction, fettling, cleaning and inspection

Final product Machining

2 2

5-The important factors of casting operations are:

1-The flow of the molten metal into the mold cavity

2-Heat transfer during solidification

3-Influence of type of mold material

4-Solidification of the metal from its molten state.

6- Casting products:

1-Machines frames, beds and body

2-Engine block, valves and pistons

3-Pump casing and impeller

4-Turbine vans

5-Under ground water pipes

7- Types of casting:

1- Sand casting 2- Shell-mold casting

3- Expandable pattern casting (Lost Foam) 4- Plaster-mold casting

5- Ceramic mold casting 6- Investment casting

7- Vacuum casting 8- Permanent mold casting

9- Slush casting 10- Pressure casting

11- Die casting 12- Centrifugal casting

13- Squeeze casting 14- Continuous casting

3 3

CHAPTER 2

1-Solidification of metals:

After molten metal is poured into a mold, a series of events takes place during solidification of

the casting and it’s cooling to ambient temperature. These events greatly influence the, size,

shape, uniformity, and chemical composition of the grains formed throughout the casting,

which in turn influence its overall properties. The significant factors affecting these events

are:1) the type of metal, 2) the thermal properties of both the metal and the mold, 3) the

geometric relationship between volume and surface area of the casting, 4) and the shape of

the mold.

1.1-Pure metals solidification:

Because a pure metal has a clearly defined melting or freezing point, it solidifies at a constant

temperature. Pure aluminum, for example, solidifies at 660 ºC, iron at 1537 ºC, and tungsten at

3410 ºC. When the temperature of the molten metal is reduced to its freezing point, its

temperature remains constant while the latent heat of fusion is given off. The solidification front

moves through the molten metal, solidifying from the mold wall in towards the center. The

solidified metal, which we now call the casting, is then taken out of the mold and begins to cool

to ambient temperature.

The grain structure of a pure metal cast in a square mold is shown in Fi.1. At the mold walls,

the metal cools rapidly since the walls at ambient temperature. Rapid cooling produces a solid

skin, or shell, of fine equiaxed grains. The grains grow in the direction opposite to the heat

transfer out through the metal. Those grains that have favorable orientation will grow

preferentially and are called columnar grains, Fig.2. As the driving force of the heat transfer is

reduced away from the mold walls, the grain becomes equiaxed and coarse.

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Fig.1 Schematic illustration of three cast structures of metal solidified in a

square mold: (a) pure metals; (b) solid-solution alloys; and structure oriented by

using nucleating agents.

Fig.2 Development of a preferred texture at a cool mold wall. Note that only

favorable oriented grains grow away from the surface of the mold.

1.2 Alloys solidification:

Solidification in alloys begins when the temperature drops below the liquidus, TL, and is complete

when it reaches the solidus, TS, Fig.3. Within this temperature range, the alloy is in a mushy or

pasty state with columnar dendrites.

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Fig.3 Schematic illustration of alloy solidification and temperature distribution

in the solidifying metal. Note the formation of dendrites in the mushy zone.

Note the presence of liquid metal between the dendrite arms. Dendrites have three-dimensional

arms and branches (secondary arms) and they eventually interlock, as shown in Fig.4. The width of

the mushy zone, where both liquid and solid phases are present, is an important factor during

solidification. This zone can be described in terms of a temperature difference, known as the

freezing range, as follows:

Freezing range = TL – TS.

In Fig.3, it can be seen that the pure metals have a freezing range that approaches zero and that the

solidification front moves as a plane front, without forming a mushy zone. Eutectics solidify in a

similar manner with an approximately plane front.

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Fig.4 (a) Solidification pattern for gray cast iron in a square casting. Note that

after 11 min of cooling, dendrites reach each other, but the casting is still mushy

throughout. It takes about two hours for this casting to solidify completely. (b)

Solidification of carbon steels in sand and shell mold. Note the difference in

solidification patterns as carbon content increases.

13 Effect of cooling rates:

1-Slow cooling rates, on the order of 102 K/s, or long local solidification times result

in coarse dendritic structures with large spacing between the dendrite arms.

2-For faster cooling rates, on the order of 104 K/s, or short local solidification times,

the structure becomes finer with smaller dendrite arm spacing.

The structures developed and the resulting grain size, in turn, influence the properties

of the casting. As grain size decreases, (a) the strength and ductility of the cast alloy

increases, (b) micro porosity (indendritic shrinkage voids) in the casting decreases,

and (c) the tendency for the casting to crack (hot cracking) during solidification

decreases.

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1.4Fluidity of molten metal:

Fluidity is a term commonly used to describe the capability of the molten metal to

fill mold cavity. This term consists of two basic factors: (1) characteristics of the

molten metal and (2) casting parameters.

1-Effect of molten metal characteristics on fluidity:

a) Viscosity: As viscosity and its sensitivity to temperature increases, fluidity

decreases.

b) Surface tension: A high surface tension of the liquid metal reduces fluidity.

Oxides films developed on the surface of the molten metal thus have a significant

adverse effect on fluidity. For example, the oxide film on the surface of pure molten

aluminum triples the surface tension.

c) Inclusions: As insoluble particles, inclusions can have a significant adverse effect

on fluidity. This effect can be verified by observing the viscosity of a liquid such as

oil with and without sand particles in it; the former has higher viscosity.

d) Solidification pattern of the alloy: The manner in which solidification occurs can

influence fluidity. Moreover, fluidity is inversely proportional to the freezing range.

Thus the shorter the range, the higher the fluidity becomes. Consequently, alloys with

long freezing ranges have lower fluidity.

2-Effect of casting parameters on fluidity:

a) Mold design: The design and dimensions of components such as the sprue,

runners, and risers all influence fluidity.

b) Mold material and its surface characteristics: The higher the thermal

conductivity of the mold and the rougher its surfaces, the low the fluidity of the

molten metal becomes. Heating the mold improves fluidity, even though it slows

down solidification of the metal and the casting develops coarse grains; hence it has

less strength.

c) Degree of superheat: It is defined as the increment of temperature above the

melting point of the alloy. Superheat improves fluidity by delaying solidification.

d) Rate of pouring: The slower the rate of pouring the molten metal into the mold,

the lower the fluidity becomes because of the faster rate of cooling.

e) Heat transfer: This factor directly affects the viscosity of the molten metal.

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Fluidity tests:

Although none is accepted universally, several tests have been developed to quantify fluidity. One

such test is shown in Fig.5, where the molten metal is made to flow along a channel at room

temperature. Obviously this length is a function of the thermal properties of the metal and the mold,

as well as the design of the channel. The fluidity index is the length of the solidified metal in the

spiral passage. The greater the length of the solidified metal, the greater is its fluidity.

Fig.5 A test for fluidity using a spiral mold.

1.5Solidification time:

During the early stages of solidification, a thin solidified skin begins to form at the cool mold walls

and, as time passes, the skin thickens. With flat mold walls, this thickness is proportional to the

square root of time. Thus doubling the time will make the skin (2)0.5

=1.14 times, or 41 percent,

thicker.

The solidification time is a function of the volume of a casting and its surface area.

Solidification time = C ( volume / surface area)2, (1)

Where C is a constant that reflects mold material, metal properties and

temperature. Thus large sphere solidifies and cools to ambient temperature at a

much slower rate than dose a smaller sphere. The reason is that the volume of the

sphere is proportional to the cube of its diameter, and the surface area is proportional

to the square of its diameter.

The effects of mold geometry and elapsed time on skin thickness and shape are

shown in Fig.6. As illustrated, the un-solidified molten metal has been pored from

them the mold at different time intervals, ranging from 5 s to 6 min. note that the skin

thickness increases with elapsed time but the skin is thinner at internal angles

(location A in Figure) than at external angles (location B). Slower cooling at internal

angles than at external angles causes this latter condition. A process called slush

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casting, which is based on this principle, makes hollow ornamental and decorative

objects.

Fig.6 A steel casting solidified skin. The remaining molten metal is poured out at

the times indicated in the figure.

Example:

Solidification times for different shapes:

Three pieces being cast have the same volume but different shapes. One is a

sphere; one is a cube, and the other a cylinder with a height equal to its diameter.

Which piece will solidify the fastest and which one the slowest?

Solution:

The volume is unity, so we have from equation (1):

Solidification time 1 / surface area

The respective surface areas are:

Sphere: V = (4/3) лr3, r = (3/4 л)1/3

,

And A = 4 лr2 =4 л (3/4 л)

2/3 = 4.84;

Cube: V = a3, a = 1,

A = 6a2 = 6;

Cylinder: V = лr2b = 2 лr

3, r = (1/2 л)1/3, and

A = 2 лr2 + 2 лrb = 6 лr

2 = 6 л(1/2 л)

2/3 = 5.54

Thus the respective solidification time’s t are

tsphere = 0.043 C, tcube = 0.028 C, and t cylinder = 0.033 C.

Hence the cube-shaped casting will solidify the faster and the sphere-shaped casting will solidify

the slowest.

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1.6 Shrinkage:

Because of their thermal expansion characteristics, metal shrink (contract) during

solidification and cooling. Shrinkage, which causes dimensional changes- and,

sometimes, cracking-is the result of:

a) Contraction of the molten metal as it cools prior to its solidification

b) Contraction of the molten metal during phase change from liquid to solid

c) Contraction of the solidified metal (the casting) as its temperature drops to

ambient temperature.

The largest amount of shrinkage occurs during cooling of the casting. The amount of

contraction for various metals during solidification is shown in Table 2.1.

Table 2.1 Solidification contraction for various cast metals

Metal or alloy

Volumetric

solidification

contraction %

Metal or alloy

Volumetric

solidification

contraction %

Aluminum 6.6 70% Cu-30% Zn 4.5

Al-4.5% Cu 6.3 90% Cu-10% Al 4.0

Al-12% Si 3.8 Gray Iron Expansion to 2.5

Carbon steel 2.5-3.0 Magnesium 4.2

1% Carbon steel 4.0 White iron 4.0-5.5

Copper 4.9 Zinc 6.5

1.7Defects:

As we well see in this section, various defects can result in manufacturing processes,

depending on factors such as materials, part design, and processing techniques.

While some defects affect only the appearance of parts, others can have major

adverse effects on the structural integrity of parts made.

The following defects can develop in castings:

A. Metallic projections, consisting of fins, flash, or massive projections such as

swell and rough surfaces.

B. Cavities, consisting of rounded or rough internal or exposed cavities, including

blowholes, pinholes, and shrinkage cavities.

C. Discontinuities, such as cracks, cold or hot tearing, and cold shuts, as shown

in Figs.7&8.

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D. Defective surface, such as surface folds, laps, scars, adhering sand layers, and

oxide scale.

E. Incomplete casting, such as misruns, insufficient volume of metal poured, and

run out.

F. Incorrect dimensions or shape, owing to factors such as improper shrinkage

allowance, pattern mounting error, irregular contraction, deformed pattern, or

warped casting.

G. Inclusions, which form during melting, solidification, and molding. Generally

nonmetallic, they are regarded as harmful because they act like stress risers and

reduce the strength of the casting.

Fig.7 Examples of hot tears in castings. These defects occur because the

casting cannot shrink freely during cooling, owing to constrains in various

portions of the molds and cores

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Fig.8 Examples of common defects in castings. These defects can be

minimized or eliminated by proper design and preparation of molds and

control of pouring procedures.

13 13

CHAPTER 3

METAL-CASTING PROCESSES

1-Introduction:

This chapter focuses on the major metal-casting processes and their principles,

advantages, and limitations. Two trends currently are having a large impact on the

casting industry. The first is continuing mechanization and automation of the

casting process, which has led to significant changes in the use of equipment and

labor. Advanced machinery and automated process-control systems have replaced

traditional methods of casting. The second major trend affecting the casting

industry is the increasing demand for high-quality castings with close tolerances.

This demand is spurring the further development of casting processes that produce

high-quality castings, see Table3.1.

Table 3.1 General characteristics of casting processes

Process

Typical

material

cast

Weight (kg)

min max

Typical

surface

finish

(m, Ra)

Porosity* Shape

complexity*

Dim.

Accuracy*

Section

Thickness

mm

Min max

Sand All 0.05 N.L. 5-25 4 1-2 3 3 N.L.

Shell All 0.05 100+ 1-3 4 2-3 2 2 -----

Expandable

pattern

All 0.05 N.L. 5-20 4 1 2 2 N.L.

Plaster Al, Zn,

Mg, Cu

0.05 50+ 1-2 3 1-2 2 1 ------

Investment All 0.005 100+ 1-3 3 1 1 1 75

Permanent

mold

All 0.05 300 2-3 2-3 3-4 1 2 50

Die Al, Cu,

Zn, Mg

<0.05 50 1-2 1-2 3-4 1 0.5 12

Centrifugal All --- 5000+ 2-10 1-2 3-4 3 2 100

Relative rating: 1 best, 5 worst.

2-Sand Casting:

The traditional method of casting metals is in sand molds and has been used for

millennia. Although the origin of the sand casting dates to ancient times, it is still

the most prevalent form of casting. Simply, sand casting, consists of:

1- Placing a pattern (having the shape of the desired casting0 in sand to make an

imprint

2- Incorporating a gating system

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3- Filling the resulting cavity with molten metal

4- Allowing the metal to cool until it solidifies

5- Breaking away the sand mold

6- Removing the casting

Fig.1 shows an outline of the typical sand casting operation steps.

Furnaces solidification Shakeout, removal Additional heat Defects, pressure

of risers and gates treatment Dimensions

Fig.1 Outline of production steps in a typical sand-casting operation.

2-1 Sands:

Most sand casting operations use silica sand (SiO2), because it is inexpensive and

is suitable as mold material because of its resistance to high temperatures. There

are two general types of sand: naturally bonded and synthetic sand. Because its

composition can be controlled more accurately most foundries prefer synthetic

sand.

Several factors are important in the selection of sand for sand molds. Sand having

fine, rounded grains can be closely packed and forms a smooth mold surface.

Good permeability of molds and cores allows gases and steam evolved during

casting to escape easily.

The selection of sand involves certain tradeoffs with respect to properties. For

example, fine-grained sand enhances mold strength, but the fine grains also lower

mold permeability. Sand is typically conditioned before use. Mulling machines

are used to uniformly mull (mix thoroughly) sand with additives. For example

Molding Pattern

making

Core making

Gating

system

Mold Sand

Metal

melting

Pouring

into

mold

Casting Heat

treatment Cleaning

and

finishing

Inspection

15 15

clay 9bentonite) is used as a cohesive agent to bond sand particles, giving the sand

strength.

Zircon (ZrSiO4), olivine (Mg2SiO4), and iron silicate (Fe2SiO4) sands are often

used in steel foundries for their low thermal expansion. Chromate (FeCr2O4) is

used for its high heat transfer property.

2.2 Types of sand molds:

There are three types of sand molds:

1- Green sand mold: It is the most common mold material. The term green

refers to the fact that the sand in the mold is moist or damp while the metal is

being poured into it. Green mold sand is a mixture of sand, clay, and water.

Greensand molding is the least expensive method of making molds. In the

skin-dried method, the mold surfaces are dried, either by storing the mold in air

or drying it with torches. Skin-dried molds are generally used for large castings

because of their higher strength.

2- Cold-box mold: In this process, various organic and inorganic binders are

blended into the sand to bond the grains chemically for greater strength. These

molds are dimensionally more accurate than green sand molds but more

expensive.

3- No-back mold: In this process, a synthetic liquid resin is mixed with the sand,

and the mixture hardens at room temperature. Because molding of the mold in

this and the cold-box process takes place without heat, they are called cold-

setting processes.

Major components of sand molds as shown in Fig.2 are:

1- The mold itself, which is supported by a flask. A two-piece mold consists of a

cope on top and a drag on the bottom. When more than two pieces are used,

the additional parts are called cheeks.

2- A pouring basin or pouring cup, into which the molten metal, is poured.

3- A sprue, through which the molten metal flows downward.

4- The runner system, which has channels that carry the molten metal from the

sprue to the mold cavity.

5- Gates are the inlets into the mold cavity.

6- Risers, which supply additional metal to the casting as it shrinks during

solidification. Fig.2 shows two different types of risers: a blind riser and an

open riser.

7- Cores, which are inserts made from sand. They are placed in the mold to form

hollow regions or otherwise define the interior surface of the casting.

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8- Vents, which are placed in the molds to carry off gases produced when the

molten metal comes into contact with the sand in the molds and cores. They

also exhaust air from the mold cavity as the molten metal flows into the mold.

Fig.2 Schematic illustration of a sand mold showing various features.

2.3 Patterns:

Patterns are used to mold the sand mixture into the shape of the casting. They may

be made of wood, plastic, or metal. The selection of a pattern material depends

on:

1- The size and shape of the casting

2- The dimensional accuracy

3- The quantity of castings required

4- The molding process to be used as shown in Table 2.

Because patterns are used repeatedly to make molds, the strength and durability of

the material selected for pattern must reflect the number of the castings that the mold

will produce. They may be made of a combination of materials to reduce wear in

critical regions. Patterns are usually coated with a parting agent to facilitate their

removal from the molds.

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Table 2 Characteristics of pattern materials

Characteristic Wood Aluminum Steel Plastic Cast iron

Machine-

ability

E G F G G

Wear

resistance

P G E F E

Strength F G E G G

Weight E G P G P

Repair-ability E P G G P

Resistance to:

Corrosion

Swelling

E

P

E

E

P

E

E

E

P

E

E, excellent; G, good; F, fair; P, poor

2.3.1 Types of patterns:

1- One-piece patterns, also called loose or solid pattern, are generally used for

simple shapes and low-quantity production. They are generally made of wood

and are inexpensive.

2- Split patterns are two pieces patterns made so that each forms a portion of the

cavity for the casting. In this way castings having complicated shapes can be

produced.

3- Match-plate patterns are a popular type of mounted pattern in which two-

piece patterns are constructed by securing each half of one or more split

patterns to the opposite sides of a single plate, as shown in Fig.3. This type of

patterns is used most often in conjunction with molding machines and large

production runs to produce smaller castings.

Fig.3 A typical metal match-plate pattern used in sand casting.

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2.3.2 Pattern design:

Pattern design is a crucial aspect of the total casting operation.

The design should provide for:

1- Metal shrinkage,

2- Ease of removal from the sand mold by means of a taper or draft, Fig.4, and

3- Proper metal flow in the mold cavity.

These topics will be discussed in greater details in the next chapter.

Fig.4 Taper on patterns for ease removal from the sand mold.

2.4 Cores:

Cores are used for castings with internal cavities or passages. Cores are placed in the

mold cavity before casting to form the interior surfaces of the casting and are

removed from the finished part during shakeout and further processing. Like molds,

cores must possess strength, permeability, ability to withstand heat, and

collapsibility. The cores are anchored by core prints. These are recesses that are

added to the pattern to support the core and to provide vents for the escape of the

gasses, Fig.5.

Cores are generally made in a manner similar to that used in making molds, and the

majority are made with shell, no-bake, or cold-box processes. Cores are formed in

core–boxes, which are used much like patterns are used to form sand molds. The

sand can be packed into the boxes with sweeps or blown into the box by compressed

air from core blowers. Core blowers have the advantages of producing uniform cores

and operating at a very high production rate.

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Fig.5 Different shapes of sand cores supported by core prints.

2.5 Sand-molding machines

The oldest known method of molding, which still used for simple castings, is to compact the sand

by hand hammering or ramming it around the pattern. For most operations, however, the sand

mixture is compacted around the pattern by molding machines, Fig.6. These machines have the

following advantages:

1- Eliminate labor cost,

2- Offer high quality casting by improving the application and distribution of forces,

3- Manipulate the mold in a carefully controlled fashion,

4- Increase the rate of production

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Fig.6 Various designs of squeeze heads for mold making: (a) conventional flat

head; (b) profile head; (c) equalizing squeeze pistons; and (d) flexible

diaphragm.

2.5.1 Automatic molding methods

1- Jolting the assembly. Jolting the assembly can further assist mechanization of

the molding process. The flask, molding sand, and pattern are placed on a pattern

plate mounted on an anvil, and jolted upward by air pressure at rapid intervals, as

shown in Fig.7. The inertial forces compact the sand around the pattern. Jolting

produces the highest compaction at the horizontal parting line, whereas in squeezing,

compaction is highest at the squeeze head, Fig.6. Thus more uniform compaction can

be obtained by combining them, as shown in Fig.7b.

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Fig.7 (a) Schematic illustration of a jolt-type mold-making machine. (b)

Schematic illustration of a mold-making machine combines jolting and squeeze.

2- Vertical flaskless molding. In this method, the halves of the pattern form a

vertical chamber wall against which sand is blown and compacted, Fig.8. Then the

mold halves are packed horizontally, with the parting line oriented vertically and

moved along a pouring conveyor.

Fig.8 Vertical flaskless molding. (a) Sand is squeezed between two halves of the

pattern. (b) Assembled molds pass along on assembly line for pouring.

3- Sands lingers molding. In this process the flask is filled uniformly with sand

under a stream of high pressure. They are used to fill large flasks and are typically

operated by machine. An impeller in the machine throws sand from its blades or

cups at such high speeds that the machine not only places the sand but also rams it

approximately.

4- Impact molding. In the impact molding process, the sand is compacted by

controlled explosion or instantaneous release of compressed gasses. This method

produces molds with uniform strength and good permeability.

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5- Vacuum molding. The process is also known as “V” process. In this process, the

molding process can be done in several steps:

1) The pattern is covered tightly by a thin sheet of plastic

2) A flask is placed over the coated pattern and is filled with dry sand

3) A second sheet of plastic is placed on top of the sand

4) A vacuum action hardens the sand so that the pattern can be

withdrawn

5) Both halves of the mold are made this way and then assembled

During pouring, the mold remains under a vacuum but the casting cavity does not. When the metal

has solidified, the vacuum is turned off and the sand falls away, releasing the casting. Vacuum

molding produces castings having very good detail and accuracy. It is especially will suited for

large, relatively flat castings.

2.6 The sand-casting operation:

Casting procedure:

1) After the mold has been shaped

2) The cores have been placed in position

3) The two halves (cope and drag) are closed

4) The two halves should be clamped

5) Also the two halves should be weighed down to prevent the

separation of the mold sections under the pressure exerted when the

molten metal is poured into the mold cavity

The complete sequence of operations in sand casting is shown in Fig.9: 1) Mechanical drawing (a) of the part is used to generate a design of the pattern

2) Considerations such as shrinkage and draft must be belt into the drawing (b-c)

3) Patterns have been mounted on plates equipped with pins for Guidant. Note the

presence of core prints designed to hold the core in place

4) Core boxes (d-e) produce core halves, before and after pasted together

5) The cores will be used to produce the hollow area of the part shown in (a)

6) The cope and the mold is assembled by taking the cope pattern plate, securing it to

the flask through aligning pins (f),

7) The drag half is produced in a similar manner, with the pattern inserted (h). A

bottom board is under the drag and aligned with pins

8) The pattern, flask, and bottom board are inverted, and the pattern is withdrawn

leaving the appropriate imprint (i)

9) The core is set in place (j) within the drag cavity

10) The mold is closed (k) by putting the cope on top of the drag and securing the

assembly with pins

11) The metal is left to solidify (l)

After the metal solidifies the following steps should be preceded: a)- the casting is removed from the mold,

23 23

b)-the sprue, gates and risers are cut off its mold (m) by oxyfuel-gas cutting, sawing,

shearing and abrasive wheels and recycled,

c)- the casting is cleaned from sand and oxide layers adhering to the casting by vibration or

by sand blasting,

d)- castings may be cleaned by electrochemical means or by pickling with chemicals to

remove surface oxides,

e)-depending on the metal used, the casting may subsequently be heat- treated to improve

certain properties needed,

f)- finishing operations may involve straightening or forging with dies to obtain final

dimensions.

2.6 Factors that should be taken into consideration during casting:

1) The design of the gating system is important for proper delivery of the molten metal

into the mold cavity

2) Turbulence must be minimized

3) Air and gases must be allowed to escape by such means as vents

4) Proper temperature gradients must be established and maintained to minimize

shrinkage and porosity

5) The design of risers is also important in order to supply the necessary molten metal

during solidification of the casting.

2.7 Disadvantages of sand casting:

1)- The surface finish obtained is largely a function of the materials used in making the mold

2)- Dimensional accuracy is not so good as that of other casting processes

2.8 Uses and advantages:

It can be used to produce intricate shapes, such as cast-iron engine blocks and very large propellers

for ocean liners.

Sand casting can be economical for relatively small production runs, and equipments cost are

generally low.

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Fig.9 Schematic illustration of the sequence of operations for

sand casting

3-Shell-Mold Casting:

Shell-mold casting was first developed in 1940s and has grown significantly because:

1)- It can produce many types of castings

2)- The castings produced having close tolerances and good surface finishes and low cost

3.1 Steps for making the process:

1- A mounted pattern made of ferrous metal or aluminum is heated to 175-350 ˚C

2- The pattern is coated with a parting agent such as silicon

3- Then clamped to a box or chamber containing a fine sand containing 2.5 to 4.0 %

thermosetting resin binder as phenol-formaldehyde

4- This makes the sand particles to be coated with the binder

5- The box is either rotated upside down, Fig.10, or the sand mixture is blown over the

pattern, allowing it to coat the pattern

25 25

6- The assembly is then placed in an oven for a short period of time to complete the

curing of the resin

7- The shell hardens around the pattern and is removed from the pattern using built-in

ejector pins

8- The two halve-shells are made in this manner

9- The two halves are then bonded or clamped together in preparation for pouring.

Fig.10 Common method of making shell molds, called dump-box

technique

The thickness of the shell can be accurately determined by controlling the time that the pattern is in

contact with the mold. The shells are light and thin, usually 5-10 mm. Shell molds are generally

poured with the parting line horizontal and may also be supported with sand.

3.2 Limitations of the process:

1- Shell sand have much lower permeability than sand used for green-sand molding,

because a sand of much smaller grain size is used for shell molding

2- The decomposition of the shell-sand binder also produces a high volume of gas

3- Unless the molds are properly vented, trapped air and gas can cause serious problems

in shell molding of ferrous castings

26 26

3.3 Advantages of the process:

1- The walls of the molds are relatively smooth

2- This offers low resistance to flow of the molten metal

3- Producing castings with sharper corners

4- Thinner sections with smaller projections than are possible in green-sand

molds can be produced

5- Shell-mold casting may be more economical than other casting processes

6- The high quality of the finished casting can significantly reduce cleaning,

machining, and other finishing costs

7- Complex shapes can be produced with less labor, and the process can be

automated fairly easily.

4-Expandable Pattern Casting (Lost Foam):

The expandable pattern casting process uses a polystyrene pattern, which

evaporates upon contact with the molten metal to form a cavity for the casting. The

process is also known as evaporative-pattern or lost-pattern casting, and under the

trade name Full-Mold process.

4.1 Casting procedure:

1- Raw expandable polystyrene (EPS) bend, containing 5 to 8 percent petane are

placed in a preheated die, usually made of aluminum

2- The polystyrene expands and takes the shape of the cavity

3- Additional heat is applied to fuse and bond the beads together

4- The die is then cooled and opened, and the polystyrene pattern is removed

5- The pattern is then coated with a water-base refractory slurry, dried, and placed

in a flask

6- The flask is then filled with loose fine sand. The sand may be dried or mixed

with bonding agents to give it additional strength.

7- Then, without removing the polystyrene pattern, the molten metal is poured

into the mold. This action immediately vaporizes the pattern and fills the mold

cavity.

4.2 Advantages of the evaporative pattern process:

1- It is relatively simple process because there are no parting lines, cores, or riser

system; hence it has design flexibility

2- Inexpensive flasks are sufficient for the process

27 27

3- Polystyrene is inexpensive and can be easily processed into patterns having

very complex shapes, various sizes, and fine surface detail

4- The casting requires minimum finishing and cleaning operations

5- The process is economical for long production runs

6- The process can be automated

4.3 Typical applications of the process

The process can be used to produce cylinder heads, crankshafts, brake components

and manifolds for automobiles, and machine bases. The aluminum engine blocks and

other components of the automobile are made by this process.

5- Plaster-Mold Casting:

In the plaster mold casting process, the mold is made of plaster of pairs (gypsum, or calcium

sulfate), with the addition of talc and silica flour to improve strength and control the time required

for the plaster to set.

5.1 Process procedure:

1- The above components are mixed with water, and then the resulting slurry is poured

over the pattern

2- After the plaster sets, usually within 15 minutes, the pattern is removed and the mold

is dried at 120 –260 ˚C to remove moisture

3- The mold halves are then assembled to form the mold cavity

4- Then the assembly is preheated to about 120 ˚C

5- The molten metal is then poured into the mold

5.2 Pattern materials and process uses:

Patterns materials used for plaster molding are generally made of aluminum alloys, thermosetting

plastics, brass, or zinc alloys. Wood patterns are not suitable for making a large number of molds,

because the patterns are repeatedly subjected to the water-based slurry.

Process uses: Since there is a limit to the maximum temperature that the plaster mold can

withstand, generally about 1200 ˚C, plaster-mold casting is used only for aluminum, magnesium,

zinc, and some copper-base alloys.

5.3 Advantages and disadvantages of the process:

Advantages:

1- The castings have fine details with good surface finish

2- Because of the low thermal conductivity of the mold, the casting cool slowly, and

more uniform grain structure is obtained.

28 28

3- High dimensional accuracy and good surface finish obtained

Disadvantages:

Because plaster molds have very low permeability, gasses evolved during solidification of the metal

cannot escape.

5.4 Casting dimensions and weight:

Wall thickness of the parts produced can be 1-2.5 mm. Casting usually weigh less than 10 kg and

are typically in the range of 125-250 g, although parts as light as 1.0 g have been made.

6. Ceramic-Mold Casting:

The ceramic-mold casting process is similar to the plaster-mold casting process, with the

exception that it uses refractory mold materials suitable for high-temperature applications. The

process is also called cope-and-drag investment casting.

6.1 Ceramic material composition:

The slurry is a mixture of fine-grained zircon (ZrSiO4), aluminum oxide, and fused silica, which are

mixed with bonding agents.

6.2 Mold preparation:

1- The above mixture is poured over the pattern which may be made of wood or metal,

which has been placed in a flask

2- After setting, the molds are removed and dried

3- Then the mold is burned off to remove the volatile matter, and then baked

4- The molds are clamped firmly and used as all-ceramic molds.

The sequence of the operation is shown in Fig.11.

The high temperature resistance of the refractory molding materials allows these molds to be

used in casting ferrous and other high-temperature alloys, stainless steels, and tool steels.

29 29

Fig.11 Sequence of operations in making a ceramic mold.

6.3 Advantages, disadvantages and uses:

Advantages:

The castings have good dimensional accuracy and surface finish over a wide range of sizes and

shapes.

Disadvantages:

But the process is somewhat expensive

Uses:

Typical parts made are impellers, cutters for machining, dies for metal forming, and molds for

making plastic or rubber components. This process has cast parts weighing as much as 700 kg.

7. Investment Casting:

The investment casting process, also called lost-wax process is one of the first used casting

processes.

7.1 Process procedure:

The pattern is made of wax or a plastic such as polystyrene. The sequences involve in the

investment casting are shown in Fig.12, and as follows:

1- The pattern is made by injecting molten wax into a metal die in the shape of the pattern.

30 30

2- The pattern is then dipped into a slurry of refractory material, such as a very fine silica,

including water, ethyl silicate, and acids.

3- After this initial coating has dried

4- The pattern is coated repeatedly to increase its thickness

5- The one-piece mold is dried in air and heated to a temperature of 90-175 ˚C in an inverted

position to melt out the wax for about 12 hours.

6- The mold is then fired to 650-1050 ˚C for about 4 hours, depending on the metal to be cast

7- After the mold has been poured and the metal has solidified, the mold is broken up and the

casting is removed. A number of patterns can be joined to make one mold, called a tree

(Fig.12c), thus increasing the production rate.

7.2 Advantages, part dimensions and uses:

Advantages:

Although the labor and material involved make the lost-wax process costly;

1- It is suitable for casting high-melting alloys

2- With good surface finish and close tolerance.

3- Thus little or no finishing operations are required.

4- This process is capable of producing complex shapes

Part dimensions and uses:

The produced parts weighing from 1 g to 35 kg, for a wide variety of ferrous and nonferrous metals

and alloys. Typical parts made are components for office equipment and mechanical components

such as gears, cams, valves and ratchets.

7.3 Ceramic-Shell Investment Casting:

A variation of the investment-casting process is ceramic-shell casting. It uses the same type of wax

or plastic pattern, which is:

1- Dipped first in ethyl silicate gel and then dipped into a fluidized bed of fine-grained

fused silica flour.

2- The pattern is then dipped into coarser-grained silica (to build up additional coatings

and thickness to withstand the thermal shock of pouring).

3- The rest of the procedure is similar to investment casting.

Uses:

This process is economical and is used extensively for precision casting of steels and high-

temperature alloys.

31 31

Fig.12 Schematic illustration of the investment casting process.

8. Permanent-Mold Casting:

In the permanent-mold casting process, also called hard-mold casting, two halves of a

mold are made from materials such as cast iron, steel, bronze, or refractory metal alloys. The

mold cavity and gating system are machined into the mold and thus become an integral pert of it.

8.1 Core materials:

To produce castings with internal cavities, cores made of metal or sand are placed in the mold

prior to casting. Typical core materials are oil-bonded or resin-bonded, plaster, graphite, gray iron,

low carbon steel, and hot-work die steel. Gray iron is the most commonly used, particularly for

large molds of aluminum and magnesium castings.

To increase the life of the permanent molds:

The surfaces of the mold cavity are usually coated with refractory slurry, such as sodium

silicate, and clay, or sprayed with graphite every few castings. This coating also serves as parting

agents and as thermal barriers, controlling the rate of cooling of the casting. Mechanical ejectors,

32 32

such as pins located in various parts of the mold, may be needed for removal of complex castings.

Ejectors usually leave small round impressions on the castings.

8.2 Procedure:

1- The molds are clamped together by mechanical means and heated to about 150-200

˚C to facilitate metal flow and reduce thermal damage to the dies.

2- The molten metal is then poured through the gating system

3- After solidification, the molds are opened and the casting is removed.

4- Special means employed to cool the mold include water or the use of fins, similar to

those found on motorcycle.

8.3 Advantages and uses:

Advantages:

1- This process produces castings with good surface finish

2- Close tolerances

3- Uniform and good mechanical properties

4- And at high production rates

5- Although the permanent-mold casting operation can be performed manually, the

process can be automated for large production runs

6- Although equipment costs can be high because of die costs, the process can be

mechanized, thus keeping labor costs low.

7- Permanent-mold casting is not economical for small production runs.

Uses:

This process is used mostly for aluminum, magnesium, and copper alloys and gray cast iron

because of their generally lower melting points. Steels can also be cast using graphite or heat-

resistant metal molds.

9. Slush Casting:

We noted in one of the figures of the last previous chapter that the solidified skin first develops in a

casting and that this skin becomes thicker with time. Hollow castings with thin walls can be made

by permanent-mold casting using this principle. This process is called slush casting.

9.1 Procedure:

1- The molten metal is poured into the metal mold

2- After the desired thickness of solidified skin is obtained, the mold is inverted or

slung

3- The remaining metal is poured out

4- The mold halves are then opened and the casting is removed.

33 33

Uses:

This process is suitable for small production runs and is generally used for making ornamental and

decorative objects and toys from low-melting-point metals, such as zinc, tin, and lead alloys.

10. Pressure Casting:

In the two permanent-mold processes that we just described, the molten metal flows into the mold

cavity by gravity. In the pressure-casting process, also called pressure pouring or low-

pressure casting, Fig.13, the molten metal is forced upward by gas pressure into a graphite or

metal mold. The pressure is maintained until the metal has completely solidified in the mold. The

molten metal may also be forced upward by a vacuum, which also removes dissolved gases and

gives the casting lower porosity. Pressure casting is generally used for high-quality castings. An

example for this process is steel railroad-car wheels.

Fig.13 a)- Bottom-pressure casting utilizes graphite molds for production of steel

railroad wheels. B)- Gravity-pouring method of casting a railroad wheel.

11. Die Casting:

The die casting process, is a further example of permanent-mold casting. The molten metal is

forced into the die cavity at pressures ranging from 0.7-700 MPa. Typical parts made by die-casting

are carburetors, motors, business-machine and appliance components, hard tools, and toys. The

weight of most casting ranges from less than 90 g to about 25 kg.

There are two basic types of die-casting machines: hot-chamber and cold-chamber.

34 34

11.1 Hot-chamber process:

The hot-chamber process, Fig.14, involves the use of a piston, which traps a certain volume of

molten metal and forces it into the die cavity through a gooseneck and nozzle. The pressures range

up to 35 MPa, with an average of about 15 MPa. The metal is held under pressure until it solidifies

in the die.

Fig.14 Sequence of steps in the die casting of a part in the hot-chamber process

To improve the die life and to aid in rapid metal cooling - thus reducing the cycle time - dies

are usually cooled by circulating water or oil through various passageways in the die block. Cycle

times usually range up to 900 shots per hour for zinc, although very small components can be cast

at 18000 shots per hour. This process commonly casts low melting-point alloys such as zinc, tin

and lead.

11.2 Cold-chamber process:

In the cold-chamber process, Fig.15, molten metal is poured into the injection cylinder (shot

chamber) with a ladle. The shot chamber is not heated – hence the term cold chamber. The metal

is forced into the die cavity at pressures usually ranging from 20 MPa to70 MPa, although they may

be as high as 150 MPa. The machines may be horizontal, Fig.16, or vertical, in which the shot

chamber is vertical and the machine is similar to a vertical press.

35 35

Fig.15 Sequence of operations in die casting of a parting the cold-chamber

process

Fig.16 Schematic illustration of a die-casting machine

The horizontal machines are large compared to the size of the casting because large forces are

required to keep the two halves of the dies closed. Otherwise, the pressure of the molten metal in

the die cavities may force the dies apart.

11.2.1 Method application

High-melting-point alloys of aluminum, magnesium, and copper are commonly cast by this method,

although other metals (including ferrous metals) can also be cast in this manner. Molten-metal

temperatures start at about 600 ˚C for aluminum and magnesium alloys and increase considerably

for copper-base and iron-base alloys.

36 36

11.3 Process capabilities and machine selection:

Because of the high pressure used, the dies have a tendency to part unless clamped together tightly.

Die-casting machines are rated according to the clamping force that can be exerted to keep the dies

closed.

11.3.1 Machine selection:

The capacities of the commercially available machines range from 25 tons to 3000 tons. Other

factors involved in the selection of die-casting machines are die size, piston stroke, shot pressure,

and cost.

11.3.2 Types of dies:

Die-casting dies, Fig.17, may be made single-cavity dies, multiple-cavity dies (with several

identical cavities), combination-cavity dies (with several different cavities), or unit dies (simple

small dies that can be combined in two or more units in a master holding die).

Typically, the ratio of die weight to part weight is 1000 to 1. Thus the die for a casting weighing 2

kg will weigh about 2000 kg. Dies are usually made of hot-work die steel or mold steels. Die-wear

increases with the temperature of the molten metal. Heat checking of dies (surface cracking from

repeated heating and cooling of the die) can be a problem. When die materials are selected and

maintained properly, dies may last more than half a million shots before any significant die wear

takes place.

Fig.17 Various types of cavities in die-casting dies.

11.3.4 Die design:

Die design includes taper (draft) to allow the removal of the casting, Fig.4. The sprues and runners

may be removed either manually or by using trim dies in a press. The entire die casting process can

be highly automated. Lubricants (parting agents) are usually applied, as thin coatings on die

surfaces. Alloys except magnesium alloys generally require lubricants. These are usually water-base

lubricants either graphite or other compounds in suspension. Because of the high cooling capacity

of water, these fluids are also effective in keeping die temperatures low.

37 37

11.4 Advantages, disadvantages, uses and cost:

Advantages:

1- Die casting has the capability for high production rates with good strength

2- High quality parts with complex shapes

3- It also produces good dimensional accuracy and surface details

4- Thus requiring little or no subsequent machining or finishing operations (net-shape

forming)

5- Because of the high pressure involved, wall thicknesses as small as 0.5 mm are

produced and are smaller than those obtained by other casting methods

6- The castings have a fine-grained, hard skin with higher strength.

Disadvantages:

1- Ejector marks remain

2- Also do a small amount of flash (thin material squeezed out between the dies) at the

parting line.

Uses:

Components such as pins, shafts, and threaded-fasteners can be die cast integrally.

Cost:

Equipment costs, particularly the cost of dies, are somewhat high, but labor costs are generally low

because the process is semi-or fully automated. Die-casting is economical for large production runs.

The properties and typical applications of common die-casting alloys are given in Table 3.

Table 3 Properties and typical applications of common die-casting alloys

Alloy

Ultimate

Tensile

Strength

(MPa)

Yield

Strength

(MPa)

Elongation

In 50 mm

(%)

Applications

Al-3.5% Cu-

8.5%Si

320

160

2.5

Automotive components,

electrical motor frames and

housing

Al-12%Si

300

150

2.5

Complex shapes with thin walls,

rats requiring strength at higher

temperatures

Brass 858 (60

Cu)

380 200 15 Fixtures, lock hardware,

bushing, ornamental castings

Mg+9A+-0.7

Zn

230 160 3 Power tools, automotive parts

Zinc+3% Al 280 ___ 10 Automotive parts, office

equipment, building hardware

Zn+4Al+1Cu 320 ___ 7 Automotive parts, building

hardware, business equipment

38 38

12. Centrifugal Casting:

The centrifugal-casting process utilizes the inertial forces caused by rotation to distribute the

molten metal into the mold cavities. There are three types of centrifugal casting: true centrifugal

casting, semi-centrifugal casting, and centrifuging.

12.1 True centrifugal casting:

In the true centrifugal casting, hollow cylindrical parts, such as pipes, gun barrels, and streetlamp

posts, are produced by the technique shown in Fig.18, in which molten metal is poured into a

rotating mold. The axis of rotation is usually horizontal but can be vertical for short workpieces.

Molds are made of steel, iron, or graphite and may be coated with a refractory lining to increase

mold life. The mold surfaces can be shaped so that pipes with various outer shapes, including

square or polygonal, can be cast. The inner surface of the casting remains cylindrical because the

molten metal is uniformly distributed by centrifugal forces. However, because of density

differences, lighter elements such as dross, impurities, and pieces of the refractory lining tend to

collect on the inner surface of the casting.

Fig.18 Schematic illustration of the centrifugal casting process.

39 39

12.1.1 Advantages and casting dimensions:

Advantages:

1- Castings of good quality

2- Dimension accuracy

3- External surface detail are obtained by this process

Casing dimensions:

Cylindrical parts ranging from 13 mm to 3 meter in diameter and 16 meter long can be cast

centrifugally, with wall thicknesses ranging from 6 mm to 125 mm

12.2 Semi-centrifugal casting:

An example of semi-centrifugal casting is shown in Fig.19a. This method is used to cast parts with

rotational symmetry, such as a wheel with spokes.

12.3 Centrifuging:

In centrifuging, also called centrifuge casting, mold cavities of any shape are placed at a

certain distance from the axis of rotation. The molten metal is poured from the center and is forced

into the mold by centrifugal forces, as shown in Fig.19b. The properties of the castings vary by

distance from the axis of rotation.

Fig.19 (a) Schematic illustration of the semi-centrifugal casting process.

(b) Schematic illustration of casting by centrifuging.

40 40

13. Squeeze Casting:

The squeeze casting, or liquid-metal forging, process involves solidification of the molten

metal under high pressure, Fig.20. The machinery includes, die, punch, and ejector pin. This

process combines the advantages of casting and forging.

Fig.20 Sequence of operations in the squeeze casting process.

13.1 The pressure applied:

The pressure applied by the punch has the following advantages:

1- Keeps the entrapped gasses in solution

2- The contact under high pressure at the die-metal interface promotes rapid heat

transfer

3- Due to this rapid cooling, a fine microstructure obtained

4- Good mechanical properties

5- The application of pressure also overcomes feeding problems that can arise when

casting metals with along freezing range

13.2 Process advantages:

1- Parts can be made to near-net shape

2- It can be used to produce complex shapes with fine surface detail, from both ferrous

and nonferrous alloys.

14. Continuous Casting:

This method is used to produce ingots of both ferrous and nonferrous alloys. This process can be

used to cast tubes or pipes and plates.

41 41

14.1 Casting procedure: 1- Metal is poured into the mold when the base is firstly upward at the bottom of the

mold

2- After the first part of metal is solidified, the base is moved down using the piston

3- The piston takes the solidified metal and in the same time the molten metal is being

continuously poured

4- It is important to control the piston speed carefully with the solidification rate

Fig.21 Continuous casting machine

14.2 Advantages of the process:

1- Minimum losses

2- Produces steel ingots of large sizes and lengths

3- Produce ingots of similar and regular specifications

4- Low cost of production

Solidified

metal

Base

Piston

Ladl

e

Molten metal

Quenching

water

42 42

15. Inspections and Testing of Castings:

15.1 Visual inspection: (Nondestructive test) It can be done by using naked eye, or by using lens, or microscope of low magnification.

15.2 Dimensional inspection: (nondestructive test) This test can be done by using measuring tools (i.e. venires, micrometer, …….)

15.3 Mechanical testing: (Destructive test) A specimen is cut from the cast, machined to the standard size required for the special test, and

mechanical test (i.e. tension, compression, impact, bending, ….) were performed. The test results

should be analyzed to know if the casting has the required properties or not.

15.4 Metallurgical test: (Destructive test) A specimen is cut from the cast and prepared for metallugraphic test. The specimen is examined on

an optical microscope of high magnification or an electron microscope. Grain size, shape and

porosity can be seen in this test.

15.5 Sound test: (Nondestructive test) In this test a chain suspends the cast, then it left to swing freely. A hummer then strikes the cast.

Notice the obtained sound.

15.6 Pressure test: (Nondestructive test) This test is used for valves or tubes that exposed to internal pressure. The cast is put under a

pressure equal to twice the working pressure. The pressure can be obtained from a pump. If the

casting contains any defects then it will explode and rejected.

15.7 Ultrasonic test: (Nondestructive test) The specimen is exposed to the ultrasonic waves and the echo of these waves should be recorded.

The wave should have the same size and shape. If there is any change in the wave size at any

position this means that there is a defect at this point.

Probe

Echo pulse

Time

43 43

15.8 Radiographic test: (X-ray test) (Nondestructive test) The cast is exposed to X-ray or γ-ray. A film is put under the cast. The defects will appear on the

film as dark areas.

Sound cast Defect cast

Probe

Echo pulse

Time

Defected

echo

X- ray X-ray

Film

Cast

44 44

15.9 Fluorescent-penetrate test: (Nondestructive test) The test procedure is as follows:

1- The surface of the cast should be cleaned 2- Apply the penetrate

3- Then wash the surface of the cast

4- Apply the developer

5- Look at the surface of the cast to see any surface defects easily

Unacceptable, rejected or defective castings are re-melted for reprocessing. Because of the major

economic impact, the types of defects present in the castings and their causes must be investigated.

Control of all stages during casting, from mold preparation to the removal of castings from molds or

dies, is important in maintaining good quality.

16. Melting Practice and Furnaces: Melting practice is an important aspect of casting operations, because it has a direct bearing on the

quality of castings. To protect the surface of the molten metal against atmospheric reaction and

contamination –and to refine the melt- the pour must be insulated against heat loss. Insulation is

usually provided by covering the surface or mixing the melt with compounds that form a slag. In

casting steels, the composition of the slag includes CaO, SiO2, MnO, and FeO.

16.1 Furnace charge:

Furnaces are charged with melting stock consisting of metal, alloying elements and various other

materials such as flux and slag-forming constituents.

Fluxes are inorganic compounds that refine the molten metal by removing dissolved gases and

various impurities. Fluxes may be added manually or can be injected automatically into the molten

metal.

Fluxes have several functions, depending on the metal. For example, for aluminum alloys there

are: (a) cover fluxes (to form a barrier to oxidation), (b) cleaning fluxes, (c) drossing fluxes, (d)

refining fluxes, (e) and wall cleaning fluxes.

Fluxes for aluminum typically consists of chlorides, fluorides, and borates of aluminum, calcium,

magnesium, potassium, and sodium.

Flux for magnesium consists of a composition of magnesium chloride, potassium chloride, barium

chloride, and calcium fluoride.

For copper alloys, there are: (a) oxidizing fluxes, (b) refining fluxes, (c) and mold fluxes for semi-

continuous casting (to prevent oxidation and improve lubricity).

For zinc alloys, such as in die-casting typical fluxes contain chlorides of zinc, potassium, and

sodium.

Fluxes for cast iron typically include sodium carbonate and calcium fluoride.

45 45

16.2 Melting furnaces:

The melting furnaces commonly used in foundries are: electric-arc, induction, crucible, and cupolas.

16.2.1 Electric-arc furnaces:

In the electric arc furnaces, the source of heat is a continuous electric arc formed between the

electrodes and the charged metal, Fig. 22. Temperatures as high as 1925 ˚C are generated in this

type of furnaces. There are usually three graphite electrodes, and they can be as large as 750 mm in

diameter and 1.5 to 2.5 meter in length. Their height in the furnace can be adjusted depending on

the amount of metal present and wear of the electrodes.

Working steps: 1- Steel scrap and small amount of carbon and limestone are dropped into the electric furnace

through the open roof

2- Electric furnaces can also be charged with 100 percent scrap

3- The roof is then closed

4- The electrodes are lowered

5- Power is turned on

6- Within a period of about two hours the metal melts

7- The current is shut off, and the electrodes are raised

8- The furnace is tilted and the molten metal is pored into a ladle

Electric furnace capacities range from 60 to 90 tons of steel per day. The quality of steel produced

is better than that of open-hearth or basic-oxygen steels.

Fig. 22 Schematic illustration of types of electric furnaces: (a) direct arc,

(b) indirect arc, and (c) induction.

For smaller quantities, electric furnaces are of the induction type. The metal is placed in a crucible,

a large pot made of refractory material and surrounded with a copper coil through which alternating

46 46

current is passed, Fig.22c. The induced current in the charge melts the metal. These furnaces

are also used for re-melting metal for casting.

16.2.2 Basic-oxygen furnace:

The basic-oxygen furnace (BOF) is the newest and fastest steel-making process. The vigorous

agitation of the oxygen refines the molten metal by an oxidation process in which iron oxide is first

produced. The oxide reacts with the carbon in the molten metal, producing carbon dioxide. The

BOF process is capable of refining 250 tons of steel in 35 to 50 minutes. Most BOF steels, which

are of better quality than open-hearth furnace steels and have low impurity levels, are processed into

plates, sheets, and various structural shapes, such as I-beams and channels.

Working procedure: 1- Typically, 200 tons of molten pig iron and 90 tons of scrap are charged (fed) into a vessel,

Fig.23a.

2- Pure oxygen is then blown into the furnace for about 20 minutes under a pressure of about

1250 kPa, through a water-cooled lance, which is a long tube as shown in Fig.23b.

3- Fluxing agents, such as lime, are added through a chute.

4- The lance is retracted and the furnace is taped by tilting it. Note the opening in Fig.23c for

the molten metal

5- The slag is then removed by tilting the furnace in the opposite direction

16.2.3 Vacuum furnaces:

Steel may be also be melted in induction furnaces from which the air has been removed, similar to

the one shown in Fig. 22c. Because the process removes gaseous impurities from the molten metal,

vacuum melting produces high-quality steels.

16.2.4 Induction furnaces:

As shown in Fig.22c, these furnaces are especially useful in smaller foundries and produce

composition-controlled smaller melts. There are two basic types.

1- The coreless induction furnace consists of a crucible completely surrounded

with a water-cooled copper coil through which a high frequency current passes.

Because there is a strong electromagnetic stirring action during induction heating,

this type of furnaces has excellent mixing characteristics for alloying and adding new

charge of metal.

2- The core or channel furnace which uses low frequency (as low as 60 Hz) and

has a coil that surrounds only a small portion of the unit. It is commonly used in

nonferrous foundries and is particularly suitable for superheating (heat above normal

casting temperature to improve fluidity), holding (keeping the molten metal at a

constant temperature for a period of time, thus making it suitable for die-casting

applications).

47 47

Fig. 23 Schematic illustration showing (a) charging, (b) melting, and

(c) pouring of molten metal in a basic-oxygen process.

16.2.5 Crucible furnaces:

These furnaces have been used extensively throughout history, as shown in Fig.24, are heated with

various fuels, such as commercial gases, fuel oil, as well as electricity. They may be stationary,

tilting, or movable. Many ferrous and nonferrous metals are melted in these furnaces.

48 48

Fig.24 Crucible furnace

16.2.6 Cupola furnaces:

These furnaces are basically refractory-lined vertical steel vessels that are charged with alternating

layers of metal, coke, and flux, Fig.25. Although they require major investments and are being

replaced by induction furnaces, cupolas operate continuously, have high melting rates, and

produce large amounts of molten metal.

16.3 Furnaces selection:

Furnaces selection requires careful consideration of several factors that can significantly influence

the quality of castings, as well as the economics of casting operations. So proper selection of a

furnace depends on:

1- Economic considerations, such as initial cost and operating and maintenance costs

2- The composition and melting point of the alloy to be cast and ease of controlling

metal chemistry

3- Control of the furnace atmosphere to avoid contamination of the metal

4- Capacity and the rate of melting required

5- Environmental considerations, such as air pollution and noise

6- Power supply and its availability and cost of fuel

7- Ease of superheating the metal

8- Type of charge material that can be used

49 49

Fig.25 Copula furnace

16.4 Safety in foundries:

As all other manufacturing operations, safety in foundries is an important consideration. Safety is

particularly important in these operations because of the following factors: 1- Dust from sand and other compounds used in casting, thus requiring proper

ventilation and safety equipment for the workers

2- Fumes from molten metals, as well as splashing of the molten metal during transfer

or pouring

3- The presence of fuels for furnaces, the control of their pressure, the proper operation

of valves, etc.

50 50

4- The presence of water and moisture in crucibles, molds, and other locations, since it

rapidly converts to steam, creating severe danger of explosion

5- Improper handling of fluxes, thus absorbing moisture and creating a danger

6- Inspection of equipment, such as pyrometers, for accuracy and proper calibration

7- The need for proper personal safety equipment such as gloves, face shields, and

shoes.

51 51

CHAPTER 4

CASTING DESIGN, MATERIALS, AND ECONOMICS

1-Introduction: The successful casting practice requires careful control of a large number of variables. These

variables pertain to the particular characteristics of the metals and alloys cast,

1- Method of casting

2- Mold and die materials

3- Mold design

4- Various process parameters

5- The flow of the molten metal in the mold cavity

6- Gating systems

7- The rate of cooling

8- The gases evolved during casting

2-Design considerations: As in all engineering practice and manufacturing operations, certain guidelines and design

principles pertaining to casting have been developed over many years. Although these principles

were established primarily through practical experience, analytical methods and computer-aided

design and manufacturing techniques are now coming into wider use, improving productivity and

the quality of castings. Moreover, careful design can result in significant cast savings.

2.1 Designing for expandable-mold casting: The following guidelines generally apply to all types of castings. The most significant design

considerations are identified and addressed.

2.1.1 Corners, angles, and section thickness:

1- Sharp corners, angles, and fillets should be avoided, Fig.1, as they may cause

tearing and cracking during solidification of the metal. Note that sharp corners are

avoided to reduce stress concentrations.

Fig. 1 Suggested design modifications to avoid defects in castings

52 52

2- Fillet radii should be selected to reduce stress concentrations and to ensure

proper liquid-metal flow during the pouring process. Fillet radii usually range from 3

mm to 25 mm. On the other hand, if the fillet radii are too large, the volume of the

material in those regions is also large and, consequently, the rate of cooling is less.

3- Section changes in the castings should smoothly blend into each other. The

location of the largest circle that can be inscribed in the particular region is critical so

far as shrinkage cavities are concerned, as shown in (Figs.2a&b). Because the

cooling rate in regions with the large circle is less, they are called hot spots. These

regions could develop shrinkage cavities and porosity, (Figs.2c&d). cavities at hot

spots can be eliminated with small cores, (Fig.2e).

Fig.2 Examples of designs showing the importance of maintaining uniform

cross-sections in castings to avoid hot spots and shrinkage cavities. Other examples of design principles that can be used to avoid shrinkage cavities are shown

in Fig.3.

Fig.3 Examples of design modifications to avoid shrinkage cavities in castings

53 53

4- Although they increase the cost of production, metal paddings in the mold can

eliminate or minimize hot spots. These paddings act as external chills. Such as that

shown for casting of a hollow cylindrical part with internal ribs in Fig.4.

Fig.4 The use of metal padding (chills) to increase the rate of cooling in

thick regions in a casting to avoid shrinkage cavities.

2.1.2 Flat areas:

Large flat areas (plain surfaces) should be avoided. They may warp because of temperature

gradients during cooling or develop poor surface finish because of uneven flow of metal during

pouring. Flat surfaces can be broken up with ribs and serrations.

2.1.3 Shrinkage:

Allowances for shrinkage during solidification should be provided for, so as to avoid cracking of

the casting. Fig.5a depicts a wheel with spokes. If the spokes are curved, the tensile stress in them

resulting from contraction during solidification-and hence the tendency for cracking-is reduced.

Another example is shown in Fig.5b, in which the original design has been altered slightly. Pattern

dimensions should also provide for shrinkage of the metal during solidification and cooling.

Allowance for shrinkage, also known as patternmaker’s shrinkage allowance, usually range from

about 10 mm/m to 20 mm/m. Table1 shows the normal shrinkage allowance for some metals cast in

sand molds.

54 54

Fig.5 Two examples of poor and good casting design practice to avoid tears

caused by contraction during cooling.

Table 1 Normal shrinkage allowance for some metals cast in sand molds

Metal Percent

Gray cast iron 0.83-1.3

White cast iron 2.1

Malleable cast iron 0.78-1.0

Aluminum alloys 1.3

Yellow brass 1.3-1.6

Magnesium alloys 1.3

Phosphor bronze 1.0-1.6

Aluminum bronze 2.1

High-manganese steel 2.6

2.1.4 Parting line: Recall that the parting line is the line, or plane, separating the upper (cope) and the lower (drag)

halves of the molds, as shown in Fig.6. In general, it is desirable for the parting line to be along a

flat plane, rather than contoured. Whenever possible, the parting line should be at the corners or

edges of castings, rather than on flat surfaces in the middle of the casting. In this way, the flash at

the parting line (material squeezing out between the two halves of the mold) will not be as visible.

The location of the parting line is important because it influences:

1- Mold design

2- Ease of molding

3- Number and shape of cores

4- Method of support

5- The gating system

Three examples of casting design modifications are shown in Fig.7.

55 55

Fig.6 Redesign of a casting by making the parting line straight to avoid defects.

Fig.7 Examples of casting design modifications

2.1.5 Draft:

As we saw in the last chapter, a small draft (taper) is provided in sand-mold patterns to enable

removal of the pattern without damaging the mold. Typical drafts range from 5 mm/m to 15

mm/m. Depending on the quality of the pattern, draft angles usually range from 0.5º to 2.0º. The

angles on inside surfaces are typically twice this range. They have to be higher than those for

outer surfaces because the casting shrinks inward towards the core.

56 56

2.1.6 Tolerances:

Tolerances-the permissible variation in the dimensions of a part- depend on:

1- The particular casting process

2- The size of the casting

3- The type of the pattern used Tolerances should be as wide as possible, within the limits of good part performance; otherwise the

cost of the casting increases. In commercial practice, tolerances usually are in the range of 0.8

mm for small castings and increase with the size of castings, say to 6 mm for large castings.

2.1.7 Machining allowance: Because most expandable-mold castings require some additional finishing operations, such as

machining, allowance should be made in casting design for these operations. Machining

allowances, which are included in pattern dimensions, depend on the type of casting and

increase with the size and section thickness of castings. Allowances usually range from 2 mm to 5

mm for small castings, to more than 25 mm for large castings.

2.1.8 Residual stress:

The different cooling rates within the body of a casting cause residual stresses. Stress reliving may

thus be necessary to avoid distortions in critical applications.

2.2 Casting Alloys:

2.2.1 Nonferrous casting alloys:

2.2.1.1Aluminum-base alloys:

Alloys with an aluminum base have a wide range of mechanical properties.

Advantages: 1- The alloys have various hardening mechanisms and heat treatments that can be used

with them.

2- Their fluidity depends on oxides and alloying elements in the metal

3- These alloys have high electrical conductivity

4- They have generally good atmospheric corrosion resistance. They are nontoxic and

light weight

5- They have good machinability.

Disadvantages: 1- Their resistance to some acids and all alkalis is poor and care must be taken to

prevent galvanic corrosion

2- They have generally low resistance to wear and abrasion, except for alloys with

silicon

57 57

Applications: 1- Aluminum-base alloys have many applications, including architectural and

decorative use

2- Engine blocks of some automobiles are made of aluminum-alloy castings

2.2.1.2Magnesium-base alloys:

Advantages: 1- The lowest density of all commercial casting alloys are those made from the

magnesium-base group

2- They have good corrosion resistance

3- They have moderate strength, depending on the particular heat treatment used

2.2.1.3Copper-base alloys:

Although somewhat expensive, copper-base alloys have many advantages.

Advantages: 1- Good electrical and thermal conductivity

2- Good corrosion resistance

3- The alloys are nontoxic

4- The alloys have wear resistance suitable for bearing materials

5- The mechanical properties and fluidity are influenced by the alloying elements

2.2.1.4Zinc-base alloys:

Advantages: 1- The alloys have a low-melting point

2- Zinc-base alloys have good fluidity

3- The alloys have sufficient strength for structural applications

4- These alloys are commonly used in die-casting.

2.2.1.5High-temperature alloys:

High-temperature alloys have a wide range properties and typically require temperatures of up to

1650 ºC for casting titanium and super alloys-and higher for refractory alloys. Special techniques

are used in casting these alloys into parts for jet-and rocket-engine components. Some of these

alloys are more suitable and economical for casting than for shaping by other manufacturing

methods, such as forging. The following table shows the properties and typical applications of cast

nonferrous alloys.

58 58

2.2.2 Ferrous casting alloys:

2.2.2.1 Cast irons:

Cast iron represents the largest amount of all metals cast. They generally possess several desirable

properties, such as wear resistance, hardness and good machinability. The term cast iron refers to

a family of alloys. They are classified as gray cast iron (gray iron), ductile (nodular or spherical)

iron, white cast iron, and compacted graphite iron.

Properties and typical applications of cast nonferrous alloys

ALLOYS

CONDITION

ULTIMATE

TENSILE

STRENGTH

MPa

YIELD

STRENGTH

MPa

ELONGATION

IN 50 MM (%)

TYPICAL

APPLICATIONS

ALUMINUM ALLOYS

195

(A01950)

Heat treated 220-280 110-220 8.5-2 Sand casting

319

(AO3190)

Heat treated 185-250 125-180 2-1.5 Sand casting

356

(AO3560)

Heat treated 260 185 5 Permanent mold

casting

COPPER ALLOYS

Red brass

(C83600)

Annealed 235 115 25 Pipe fitting, gears

Yellow

brass

(C86400)

Annealed

275

95

25

Hardware,

ornamental

Manganese

bronze

(C86100)

Annealed

480

195

30

Propeller hubs,

blades

Leaded tin bronze

(C92500)

Annealed

260

105

35

Gears, bearings, valves

Gun metal

(C90500)

Annealed 275 105 30 Pump parts,

fittings

Nickel

silver

(C97600)

Annealed

275

175

15

Marine parts,

valves

MAGNESIUM ALLOYS

AZ91A F 230 150 3 Die casting

AZ63A

T4

275

95

12

Sand and

permanent mold

casting

AZ91C T6 275 130 5 High strength

EZ33A T5 160 110 3 Elevated temp.

HK31A T6 210 105 8 Elevated temp.

QE22A T6 275 205 4 Highest strength

59 59

a- Gray cast iron: Gray cast irons are specified by two-digit ASTM designation. Class 20, for example, specifies that

the material must have a minimum tensile strength of 20 ksi (140 MPa). The mechanical properties

for several classes of gray cast iron are shown in the next table.

Mechanical properties of gray cast irons

ASTM

Class

Ultimate

tensile

strength

(MPa)

Compressive

strength

(MPa)

Elastic

modulus

(Gpa)

Hardness

(HB)

20 152 572 66 to 97 156

25 179 669 79 to 102 174

30 214 752 90 to 113 210

35 252 855 100 to 119 212

40 293 965 110 to 138 235

50 362 1130 130 to 157 262

60 431 1293 141 to 162 302

Advantages: Castings of gray cast iron have relatively few shrinkage cavities and little porosity.

Uses: 1- Typical uses of gray cast iron are for engine blocks

2- Machine bases

3- Electric-motor housings

4- Pipes

5- Wear surfaces for machines

b- Ductile (nodular) iron:

Ductile irons are specified by a set of two-digit numbers. Thus, for example, class or grade 80-55-

06 indicates that the material has a minimum tensile strength of 80 ksi (550 MPa), a minimum yield

strength of 55 ksi (380 MPa), and 6 percent elongation in 50 mm.

Uses: Typically used for machine parts, pipes, and crankshafts.

c- White cast iron:

Because of its extreme hardness and wear resistance, white cast iron is used mainly for liners

for machinery to process abrasive materials, rolls for rolling mills, and railroad-car shoes.

60 60

d- Malleable iron: Malleable irons are specified by a five-digit designation. Thus 35018, for example, indicates that

the yield strength of the material is 35 ksi (240 MPa), and its elongation is 18 percent in 50 mm.

The principal uses of malleable iron is for railroad equipment and various types of hardware.

e- Compacted graphite iron: Compacted graphite iron has the properties that all between those of gray and ductile irons.

Whereas gray iron has good damping and thermal conductivity but low ductility and ductile iron

has poor damping and thermal conductivity but high tensile strength, compacted graphite iron has

damping and thermal properties to gray cast iron and strength and stiffness comparable to those of

ductile iron.

Advantages: 1- Because of its strength, parts made of compacted graphite iron can be lighter

2- It is easy to cast

3- Its machineability is better than ductile iron

Properties and typical applications of cast irons

Cast iron

Type

Ultimate

tensile

strength

(MPa)

Yield

strength

(MPa)

Elongation

in 50 mm

(%)

Typical applications

Gray

Ferritic 170 140 0.4 Pipe, sanitary ware

Pearlitic 275 240 0.4 Engine block,

machine tools

Martensitic 550 550 0 Wearing surfaces

Ductile

(nodular)

Ferretic 415 275 18 Pipe, general services

Pearlitic 550 380 6 Crankshafts, highly

stressed parts

Tempered

martensite

825 620 2 H.S machine parts,

wear resistant parts

Malleable

Ferretic

365

240

18

Hardware, pipe

fittings, general

engineering service

Pearlitic 450 310 10 Railroad equipment,

couplings

Tempered

martensite

700 550 2 Railroad equipment,

gears, connecting rods

White Pearlitic 275 275 0 Wear-resistant parts,

mill rolls

61 61

2.2.2Cast steels:

Because of the high temperatures required to melt cast steels, up to 1650 ºC, their casting requires

considerable knowledge and experience. The high temperatures involved present difficulties in the

selection of mold materials-particularly in view of the high reactivity of steels with oxygen-in

melting and pouring the metal. Steel castings possess properties that are more uniform than those

made by mechanical working processes. Cast steels can be welded; however, welding alters the

cast microstructure in the heat-affected zone, influencing the strength, ductility, and toughness

of the cast metal. Subsequent heat treatment must be performed to restore the mechanical

properties of the casting. Cast weldments have gained importance where complex configurations, or

the size of the casting, may prevent casting the part economically in one place.

2.2.3Cast stainless steels:

Casting of stainless steels involves considerations similar to those for steels in general. Stainless

steels generally have:

1- A long freezing range and high melting temperatures.

2- They develop various structures, depending on their composition and the process

parameters

3- Cast stainless steels are available in various compositions

4- It can be heat-treated and welded

5- These cast products have high heat and corrosion resistance

6- Nickel-base casting alloys are used for severely corrosive environments nd very high

temperature service.

2.3Economics of casting:

When looking at various casting processes, you will note that:

1- Some casting processes require more labor than the others

2- Some processes require expensive dies and machinery

3- And some take a great deal of time to complete.

These important characteristics are outlined in the following table. Each of thee individual factors

listed affects to varying degrees the overall cost of a casting operation. As we can see from the

following table, relatively little cost is involved in molds for sand casting. On the other hand, die-casting dies

require expensive materials and a great deal of machining and preparation.

The cost of a product involves: 1- The cost of the materials

2- The labor cost

3- The tooling cost

4- The equipment cost

5- Preparation for casting a product include making molds and dies that are require raw

materials, time, and effort, which can be translated into cost

6- In addition to molds and dies, facilities are required for melting and poring the

molten metal into the molds or dies. These facilities include furnaces and related

machinery; their cost depend on the level of automation desired

62 62

7- Finally, costs are involved in heat treating, cleaning, and inspecting the castings.

General cost characteristics of casting processes

Process

Cost Production

rate Pc/hr) Die Equipment Labor

Sand L L L-M Less than 20

Shell-mold L-M M-H L-M Less than 50

Plaster L-M M M-H Less than 10

Investment M-H L-M H Less than

1000

Permanent

mold

M M L-M Less than 60

Die H H L-M Less than 200

Centrifugal M H L-M Less than 50

Heat-treating is an important part of the production of many alloy groups, especially ferrous

castings, and is necessary to produce improved mechanical properties. However, heat-treating also

introduces another set of production problems, such as scale formation and warpage, and can be a

significant part of the production costs.

The amount of labor required for these operations can vary considerably, depending on the

particular process and level of automation. Investment casting, for example, requires a great deal of

labor because the large number of steps involved in this operation. On the other hand, operations

such as highly automated die-casting can maintain high production rates with little labor required.

It can be noted from the above table that, however, that the cost of equipment per casting (unit cost)

will decrease as the number of parts increases. Thus sustained high production rates can justify the

high cost of dies and machinery. Thus not all manufacturing decisions are based purely on

economic considerations, but also on the quality of the produced casting from the different casting

processes. Then, if the part can be produced by more than one or two processes, the final decision

rests on both economic and technical considerations.