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1.0 FUNDAMENTALS OF METAL FORMING
Metal forming processes can be classified into two basic categories: (1) bulk deformation
processes and (2) sheet metalworking processes. Each category includes several major
classes of shaping operations, as indicated in Figure 1.0
Figure 1.0: Classification of metal forming operations.
1.1 Bulk Deformation Processes
Bulk deformation processes are generally characterized by significant deformations and
massive shape changes, and the surface area-to-volume of the work is relatively small. The
term bulk describes the workparts that have this low area-to-volume ratio. Starting work
shapes for these processes include cylindrical billets and rectangular bars. Figure 1.1 illustrates
the following basic operations in bulk deformation:
Figure 1.1: Bulk Deformation Processes (a) Rolling, (b) Forging, (c) Extrusion, (d) Drawing.
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Rolling, this is a compression deformation process in which the thickness of a slab orplate is
reduced by two opposing cylindrical tools called rolls. The rolls rotate so as todraw the work
into the gap between them and squeeze it.
Forging. In forging, a work piece is compressed between two opposing dies, so that thedie
shapes are imparted to the work. Forging is traditionally a hot working process, butmany types
of forging are performed cold.
Extrusion. This is a compression process in which the work metal is forced to flowthrough a
die opening, thereby taking the shape of the opening as its own cross section.
Drawing. In this forming process, the diameter of a round wire or bar is reduced bypulling it
through a die opening.
1.2 Sheet Metalworking
Sheet metalworking processes are forming and related operations performed on metal
sheets, strips, and coils. The surface area-to-volume ratio of the starting metal is high; thus,
this ratio is a useful means to distinguish hulk deformation from sheet metal processes. Press
working is the term often applied to sheet metal operations because the machines used to
perform these operations are presses (presses of various types are also used h other
manufacturing processes). A part produced in a sheet metal operation is often called a
stamping.
Sheet metal operations are always performed as cold working processes and are
accomplished using a set of tooling called apunch and die. The punch is the positive portion
and thedie is the negative portion of the tool set. The basic sheet metal operations are
sketched in Figure1.2:
Figure 1.2: Basic sheet metal operations (a) Bending, (b) Drawing, (c) Shearing.
Bending. Bending involves straining of a metal sheet or plate to take an angle along a
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(usually) straight axis.
Drawing. In sheet metalworking, drawing refers to the forming of a flat metal sheet into a
hollow or concave shape, such as a cup, by stretching the metal. A blankholder is used to hold
down the blank while the punch pushes into the sheet metal, as shown in Figure 1.2(b). To
distinguish this operation from bar and wire drawing, the terms cup drawing or deep
drawing are often used.
Shearing. This process is somewhat out-of-place in our list of deformation
processes,because it involves cutting rather than forming. A shearing operation cuts the
workusing a punch and die, as in Figure 1.2(c). Although it is not a forming process, it
isincluded here because it is a necessary and very common operation in sheet metal-working.
1.3 MATERIAL BEHAVIOR IN METAL FORMING
The typical stress-strain curve for most metals is divided into an elastic region and a plastic
region. In metal forming, the plastic region is of primary interest because the material is
plastically and permanently deformed in these processes.
The typical stress-strain relationship for metal exhibits elasticity below the yield point and
strain hardening above it;the metal's behavior is expressed by the flow curve.
𝝈 = 𝑲𝝐𝐧
Where K = the strength coefficient, MPa ; and n is the strain hardening exponent. The stress
𝝈and strain 𝝐 in the flow curve are true stress and true strain. The flow curve is generally valid
as a relationship that defines a metal's plastic behavior in cold working.
Flow Stress: The flow curve describes the stress-strain relationship in the region in which
metal forming takes place, it indicates the flow stress of the metal — the strength property that
determines forces and power required to accomplish a particular forming operation. The
stress required to continue deformation must be increased to match this increase in strength.
Flow stress is defined at the instantaneous value of stress required to continue deforming the
material — to keep the metal "flowing."
 f  K n ………………………………………………………….. (1.1)
Average Flow Stress The average flow stress (also called the mean flow stress) is the average
value of stress over the stress-strain curve from the beginning of strain to the final
(maximum) value that occurs during deformation. The value is illustrated in the stressstrain plot of Figure 1.
The average flow stress is determined by integrating the flow curve equation, Eq. (1.1),
between zero and the final strain value defining the range of interest. This yields the equation:
K n
 avf 
……………………………………………………….. (1.2)
1 n
Where  avf = average flow stress, MPa; and  = maximum strain value during the
deformation process. Given values of K and n for the work material, a method of computing
final strain will be developed for each process. Based on this strain, Eq. (1.2) can be used to
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determine the average flow stress to which the metal is subjected during the operation.
Figure 1.3: Stress-Strain curve indicating location of average flow stress in relation to yield
strength  y and final flow stress
1.4 TEMPERATURE IN METAL FORMING
The flow curve is a valid representation of stress-strain behavior of a metal during plastic
deformation, particularly for cold working operations. For any metal, the values of K and n
depend on temperature. Both strength and strain hardening are reduced at higher
temperatures. In addition, ductility is increased at higher temperatures. These property
changes are important because any deformation operation can be accomplished with lower
forces and power at elevated temperature. There are three temperature ranges: cold, warm,
and hot working.
Cold Working: Cold working (also known as cold forming) is metal forming performed at
room temperature or slightly above. Significant advantages of cold forming compared to hot
working are: (1) better accuracy, meaning closer tolerances; (2) better surface finish; (3)
strain hardening increases strength and hardness of the part; (4) grain flow during
deformation provides the opportunity for desirable directional properties to be obtained in
the resulting product; and (5) no heating of the work is required, which saves on furnace and
fuel costs and permits higher production rates to be achieved. Owing to this combination of
advantages, many cold forming processes have developed into important mass-production
operations. They provide close tolerances and good surfaces, minimizing the amount of
machining required and permitting these operations to be classified as net shape or near net
shape processes.
There are certain disadvantages or limitations associated with cold forming operations: (1)
higher forces and power are required to perform the operation; (2) care must be taken to
ensure that the surfaces of the starting workpiece are free of scale and dirt; and (3) ductility
and strain hardening of the work metal limit the amount of forming that can be done to the
part. In some operations, the metal must be annealed in order to allow further deformation to
be accomplished. In other cases, the metal is simply not ductile enough to be cold worked.
To overcome the strain hardening problem and reduce force and power requirements,
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many forming operations are performed in elevated temperatures. There are two elevated
temperature ranges involved, giving rise to the terms warm working and hot working.
Warm Working: Because plastic deformation properties are normally enhanced by increasing workpiece temperature, forming operations are sometimes performed at temperatures somewhat above room temperature but below the recrystallization temperature. The
term warm working is applied to this second temperature range. The dividing line between
cold working and warm working is often expressed in terms of the melting point for the metal.
The dividing line is usually taken to be 0.3Tm, where Tm ,is the melting point (absolute
temperature) for the particular metal.
The lower strength and strain hardening, as well as higher ductility of the metal at the
intermediate temperatures, provide warm working with the following advantages over cold
working: (1) lower forces and power, (2) more intricate work geometries possible, and (3)
need for annealing may be reduced or eliminated.
Hot Working: Hot working (also called hot forming) involves deformation at temperatures above the recrystallization temperature. The recrystallization temperature for a given
metal is about one-half of its melting point on the absolute scale. In practice, hot working is
usually carried out at temperatures somewhat above 0.5Tm. The work metal continues to
soften as temperature is increased beyond 0.5Tm, thus enhancing the advantage of hot working
above this level. However, the deformation process itself generates heat, which increases work
temperatures in localized regions of the part. This can cause melting in these regions, which is
highly undesirable. Also, scale on the work surface is accelerated at higher temperatures.
Accordingly, hot working temperatures are usually maintained within the range 0.5Tm to
0.75Tm.
The most significant advantage of hot working is the capability to produce substantial plastic
deformation of the metal—far more than is possible with cold working or warm working. All
of this results in the following advantages relative to cold working: (1) the shape of the
workpart can be significantly altered; (2) lower forces and power are required to deform the
metal; (3) metals that usually fracture in cold working can be hot formed; (4) strength
properties are generally isotropic because of the absence of the oriented grain structure
typically created in cold working; and (5) no strengthening of the part occurs from work
hardening. This last advantage may seem inconsistent, since strengthening of the metal is
often considered an advantage for cold working. Disadvantages of hot working include lower
dimensional accuracy, higher total energy required (due to the thermal energy to heat the
workpiece), work surface oxidation (scale), poorer surface finish, and shorter tool life.
1.5 FRICTION AND LUBRICATION IN METAL FORMING
Friction in metal forming arises because of the close contact between the tool and work
surfaces and the high pressures that drive the surfaces together in these operations. In most
metal forming processes, friction is undesirable for the following reasons: (1) metal flow in the
work is retarded, causing residual stresses and sometimes defects in the product; (2) forces
and power to perform the operation are increased, and (3) tool wear can lead to loss of
dimensional accuracy, resulting in defective parts and requiring replacement of the tooling.
Since tools in metal forming are generally expensive, tool wear is a major concern. Friction
and tool wear are more severe in hot working because of the much harsher environment.
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Friction in metal forming is different from that encountered in most mechanical systems,
such as gear trains, shafts and bearings, and other components involving relative motion
between surfaces. These other cases are generally characterized by low contact pressures, low
to moderate temperatures, and ample lubrication to minimize metal-to-metal contact. By
contrast, the metal forming environment features high pressures between a hardened tool
and a soft workpart, plastic deformation of the softer material, and high temperatures (at
least in hot working). These conditions can result in relatively high coefficients of friction in
metal working, even in the presence of lubricants. Typical values of coefficient of friction for
the three categories of metal forming are listed in Table 1.
If the coefficient of friction becomes large enough, a condition known as sticking occurs.
Sticking in metalworking (also called sticking friction) is the tendency for the two surfaces in
relative motion to adhere to each other rather than slide. It means that the friction stress
between the surfaces exceeds the shear flow stress of the work metal, thus causing the metal
to deform by a shear process beneath the surface rather than slip at the surface. Sticking
occurs in metal forming operations and is a prominent problem in rolling.
Metalworking lubricants are applied to the tool-work interface in many forming
operations to reduce the harmful effects of friction. Benefits include reduced sticking, forces,
power, and tool wear; and better surface finish on the product. Lubricants also serve other
functions, such as removing heat from the tooling. Considerations in choosing an appropriate
metalworking lubricant include: type of forming process (rolling, forging, sheet metal drawing,
and so on), whether used in hot working or cold working, work material, chemical reactivity
with the tool and work metals (it is generally desirable for the lubricant to adhere to the
surfaces to be most effective in reducing friction), ease of application, toxicity, flammability,
and cost.
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2.0 BULK DEFORMATION PROCESSES IN METALWORKING
Bulk deformation processes are performed as cold, warm, and hot working operations. Cold
and warm working is appropriate when the shape change is less severe, and there is a need to
improve mechanical properties and achieve good finish on the part. Hot working is generally
required when massive deformation of large workparts is involved.
The commercial and technological importance of bulk deformation processes derives from the
following:



When performed as hot working operations, they can achieve significant change in the
shape of the workpart.
When performed as cold working operations, they can be used not only to shape the
product, but also to increase its strength through strain hardening.
These processes produce little or no waste as a by-product of the operation.
Some bulk deformation operations are near net shape or net shape processes; they achieve
final product geometry with little or no subsequent machining.
The bulk deformation processes covered are (1) rolling, (2) forging, (3) extrusion, and (4)
wire and bar drawing
2.1 ROLLING
Rolling is a deformation process in which the thickness of the work is reduced by compressive
forces exerted by two opposing rolls. The rolls rotate as illustrated in Figure 2.1 to pull and
simultaneously squeeze the work between them. The basic process shown in our figure is flat
rolling, used to reduce the thickness of a rectangular cross section. A closely related process is
shape rolling, in which a square cross section is formed into a shape such as an I-beam.
Most rolling processes are very capital intensive, requiring massive pieces of equipment, called
rolling mills, to perform them. The high investment cost requires the mills to be used for
production in large quantities of standard items such as sheets and plates. Most rolling is
carried out by hot working, called hot rolling, owing to the large amount of deformation
required. Hot-rolled metal is generally free of residual stresses, and its properties are
isotropic. Disadvantages of hot tolling are that the product cannot he held to close tolerances,
and the surface has a characteristic oxide scale.
Figure 2.1: The rolling process (specifically, flat rolling).
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The work starts out as a cast steel ingot that has just solidified. While it is still hot, the ingot is
placed in a furnace where it remains for many hours until it has reached a uniform temperature
throughout, so that the metal will flow consistently during rolling. For steel, the desired temperature
for rolling is around 12008C (2200°F). The heating operation is called soaking, and the furnaces in
which it is carried out are called soaking pits.
From soaking, the ingot is moved to the rolling mill, where it is rolled into one of three intermediate
shapes called blooms, billets, or slabs. A bloom has a square cross section 150 mm x 150 mm (6 x 6) or
larger. A slab is rolled from an ingot or a bloom and has a rectangular cross section of width 250 mm (10 in)
or more and thickness 40 mm (1.5 in) or more. A billet is rolled from a bloom and is square with dimensions
40 mm (1.5 in) on a side or larger. These intermediate shapes are subsequently rolled into final product
shapes.
Blooms are rolled into structural shapes and rails for railroad tracks. Billets are rolled into bars and rods.
These shapes are the raw materials for machining, wire drawing, forging, and other metalworking processes.
Slabs are rolled into plates, sheets, and strips. Hot-rolled plates are used in shipbuilding, bridges, boilers,
welded structures for various heavy machines, tubes and pipes, and many other products. Figure 19.2 shows
some of these rolled steel products. Further flattening of hot-rolled plates and sheets is often accomplished
by cold rolling, in order to prepare them for subsequent sheet metal operations. Cold rolling strengthens the
metal and permits a tighter tolerance on thickness. In addition, the surface of the cold-rolled sheet is absent
of scale and generally superior to the corresponding hot-rolled product. These characteristics make coldrolled sheets, strips, and coils ideal for stampings, exterior panels, and other parts of products ranging from
automobiles to appliances and office furniture.
FIGURE 2.1: Some of the steel products made in a rolling mill
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2.1.1 Flat Rolling and Its Analysis
Figure 2.1.1: Side view of flat rolling, indicating before and after thickness, work velocities,
angle of contact with rolls, and other features
Flat rolling involves the rolling of slabs, strips, sheets, and plates—workparts of rectangular cross section in
which the width is greater than the thickness. In flat rolling, the work is squeezed between two rolls so that
its thickness is reduced by an amount called the draft:
d  t o  t f ……………………………………………. (2.1.1)
Where d=draft, mm; t o = starting thickness, mm; and t f = final thickness, mm. Draft is sometimes
expressed as a fraction of the starting stock thickness, called the reduction:
r
d
to ……………………………………………………… (2.1.2)
Where r=reduction. When a series of rolling operations are used, reduction is taken as the sum of the drafts
divided by the original thickness.
In addition to thickness reduction, rolling usually increases work width. This is called spreading, and it
tends to be most pronounced with low width-to-thickness ratios and tow coefficients of friction.
Conservation of matter is preserved, so the volume of metal exiting the rolls equals the volume entering:
t o wo Lo  t f w f L f ………………………………………………… (2.1.3)
Where wo and w f are the before and after work widths, mm; and Lo and L f are the before and after
work lengths, mm. Similarly, before and after volume rates of material flow must be the same, so the before
and after velocities can be related:
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t o wo vo  t f w f v f ………………………………………..………… (2.1.4)
Where vo and v f are the entering and exiting velocities of the work.
The rolls contact the work along a contact arc defined by the angle  . Each roll has radius R, and its
rotational speed gives it a surface velocity vr . This velocity is greater than the entering speed of the work
vo and less than its exiting speed v f . Since the metal flow is continuous, there is a gradual change in
velocity of the work between the rolls. However, there is one point along the arc where work velocity equals
roll velocity. This is called the no-slip point, also known as the neutral point. On either side of this point,
slipping and friction occur between roll and work.
The amount of slip between the rollsand the work can be measured by means of the forward slip;
a term used in rolling that is defined:
s
v f  vr
vr
………………………………………………… (2.1.5)
Where s= forward slip; v f = final (exiting) work velocity, m/s; and vr = roll speed, m/s.
The true strain experienced by the work in rolling is based on before and after stock
thicknesses. In equation form,
t
tf
  ln o …………………………………………………… (2.1.6)
The true strain can be used to determine the average flow stress  avf applied to the work
material in flat rolling. Recall from the previous Eq. (1.2), that
K n
 avf 
……………………………………………………….. (2.1.7)
1 n
The average flow stress is used to compute estimates of force and power in rolling.
Friction in rolling occurs with a certain coefficient of friction, and the compression force of the
rolls, multiplied by this coefficient of friction, results in a friction force between the rolls and the
work. On the entrance side of the no-slip point, friction force is in one direction, and on the
other side it is in the opposite direction. However, the two forces are not equal. The friction
force on the entrance side is greater, so that the net force pulls the work through the rolls. If
this were not the case, rolling would not be possible. There is a limit to the maximum possible
draft that can be accomplished in flat rolling with a given coefficient of friction, given by:
d max  µ 2 R……………………………………. (2.1.8)
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Coefficient of friction in rolling depends on lubrication, work material, and working
temperature. In cold rolling, the value is around 0.1; in warm working, a typical value is
around 0.2; and in hot rolling, µis around 0.4. Hot rolling is often characterized by a
condition called sticking, in which the hot work surface adheres to the rolls over the contact
arc. This condition often occurs in the rolling of steels and high-temperature alloys. When
sticking occurs, the coefficient of friction can be as high as 0.7, The consequence of sticking is
that the surface layers of the work are restricted to move at the same speed as the roll speed
vr ; and below the surface, deformation is more severe in order to allow passage of the piece
through the roll gap.
Given a coefficient of friction sufficient to perform rolling, roll force F required to
maintain separation between the two rolls can be computed by integrating the unit roll
pressure (shown as p in Figure 2.1.1) over the roll-work contact area. This can be expressed:
L
F  w pdL …………………………………….. (2.1.9)
0
WhereF—rolling force, N; w=the width of the work being rolled, mm; p =roll pressure, MPa;
and L = length of contact between rolls and work, mm. The integration requires two separate
terms, one for either side of the neutral point. Variation in roll pressure along the contact
length is significant. A sense of this variation can be obtained from the plot in Figure 2.1.2.
Pressure reaches a maximum at the neutral point, and trails off on either side to the entrance
and exit points. As friction increases, maximum pressure increases relative to entrance and exit
values. As friction decreases, the neutral point shifts away from the entrance and toward the
exit in order to maintain a net pull force in the direction of rolling. Otherwise, with low
friction, the work would slip rather than pass between the rolls.
Figure 2.1.2: Typical variation in pressure along the contact length in flat rolling
An approximation of the results obtained by Eq. (2.1.9) can be calculated based on the average
flow stress experienced by the work material in the roll gap. That is,
F   avf wL ……………………………………………. (2.1.10)
Where  avf = average flow stress from Eq. (2.7), MPa; and the product wLis the roll-work contact area,
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mm2. Contact length can be approximated by
L  R (t o  t f ) …………………………………………………….. (2.1.11)
The torque in rolling can be estimated by assuming that the roll force is centered on the work as it passes
between the rolls, and that it acts with a moment arm of one-half the contact length L. Thus, torque for each
roll is
T  0.5 FL ………………………………………………………. (2.1.12)
The power required to drive each roll is the product of torque and angular velocity. Angular velocity is 2N,
where N = rotational speed of the roll. Thus, the power for each roll is 2NT. Substituting Eq. (2.12) for torque
in this expression for power, and doubling the value to account for the fact that a rolling mill consists of two
powered rolls, we get the following expression:
P  2NFL ………………………………………………………… (2.1.13)
Where P = power, J/s or W; N= rotational speed, rev/sec; F=rolling force, N; and L = contact length, m.
Inspection of Eqs. (2.1.10) and (2.1.13) indicates that force and/or power to roll a strip of a given width and
work material can be reduced by any of the following: (1) using hot rolling rather than cold rolling to reduce
strength and strain hardening (K and n) of the work material; (2) reducing the draft in each pass; (3) using a
smaller roll radius R to reduce force; and (4) using a lower rolling speed N to reduce power.
2.1.2 Shape Rolling
In shape rolling, the work is deformed into a contoured cross section. Products made by shape rolling include
construction shapes such as I-beams, L-beams, and U-channels; rails for railroad tracks; and round and
square bars and rods.
2.1.3 Rolling Mills
Various rolling mill configurations are available to deal with the variety of applications and technical
problems in the rolling process. The basic rolling mill consists of two opposing rolls and is referred to as a
two-high rolling mill. The rolls in these mills have diameters in the range 0.6-1.4 m (2.0-4.5 ft). The two-high
configuration can be either reversing or non-reversing. In the non-reversing mill, the rolls always rotate in
the same direction, and the work always passes through from the same side. The reversing mill allows the
direction of roll rotation to be reversed, so that the work can be passed through in either direction. This
permits a series of reductions to be made through the same set of rolls, simply by passing through the work
from opposite directions multiple times.
EXAMPLE:
A 300-mm-wide strip 25 mm thick is fed through a rolling mill with two powered rolls
Rolling'each of radius =250 mm. The work thickness is to be reduced to 22 mm in one pass ata roll speed
of 50 rev/min. The work material has a flow curve defined by K =275 MPa and n =0.15 and the coefficient of
friction between the rolls and the work is assumed to be 0.12. Determine if the friction is sufficient to
permit the rolling operation to be accomplished.If so, calculate the roll force, torque, and
horsepower.(Rolling operation feasible, 1,444,254N, 19,786Nm, 207.2Kw (278hp)).
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2.2FORGING
Forging is a deformation process in which the work is compressed between two dies,
using either impact or gradual pressure to form the part. Forging is an important
industrial process used to make a variety of high-strength components for
automotive, aerospace, and other applications. These components include engine
crankshafts and connecting rods, gears, aircraft structural components, and jet
engine turbine parts. In addition, steel and other basic metals industries use forging
to establishthe basic form of large components that are subsequently machined to final
shape and dimensions.
Forging is carried out in many different ways. One way to classify it is by working temperature.
Most forging operations are performed hot or warm, owing to thesignificant deformation
demanded by the process and the need to reduce strength and increase ductility of the work
metal. However, cold forging is also very common for certain products. The advantage of cold
forging is the increased strength that results from strain hardening of the component.
Either impact or gradual pressure is used in forging. The distinction derives more from the
type of equipment used than differences in process technology. A forging machine that
applies an impact load is called a forging hammer, while one that applies gradual pressure is
called a forging press.
Another difference among forging operations is the degree to which the flow of the work
metal is constrained by the dies. By this classification, there are three types of forging
operations, shown in Figure 2.2: (a) open-die forging, (b) impression-die forging, and (c)
flashless forging.
Figure 2.2: (a) open-die forging, (b) impression-die forging, and (c) flashless forging.
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2.2.1 Open-Die Forging
The simplest case of open-die forging involves compression of a workpart of cylindrical cross
section between two flat dies, much in the manner of a compression test. This forging
operation, known as upsetting or upset forging, reduces the height of the work and increases
its diameter.
Analysis of Open-Die Forging If open-die forging is carried out under ideal conditions of no
friction between work and die surfaces, then homogeneous deformation occurs, and the radial
flow of the material is uniform throughout its height, as pictured in Figure 2.2.1. Under these
ideal conditions, the true strain experienced by the work during the process can be
determined by
h
h
  ln o …………………………………………………… (2.2.1)
Where ho = starting height of the work, mm; and h=the height at some intermediate point in
the process, mm. At the end of the compression stroke, h= its final value h f , and the true
strain reaches its maximum value.
Figure 2.2.1: Homogenous deformation of a cylindrical workpart under ideal conditions in
an open-die forging operation.
Estimates of force to perform upsetting can be calculated. The force required to
continue the compression at any given height h during the process can be obtained by
multiplying the corresponding cross-sectional area by the flow stress:
F   f A ……………………………………………………… (2.2.2)
Where F = force, N; A = cross-sectional area of the part, mm2; and  f =flow stress
corresponding to the strain given by Eq. (2.2.1), MPa. Area A continuously increases during the
operation as height is reduced. Flow stress  f also increases as a result of work hardening,
except when the metal is perfectly plastic (e.g., in hot working). In this case, the strain
hardening exponent n = 0, and flow stress  f equals the metal's yield strength Y. Force
reaches a maximum value at the end of the forging stroke, when both area and flow stress are
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at their highest values.
An actual upsetting operation does not occur quite as shown in Figure 2.2.1 because friction
opposes the flow of work metal at the die surfaces. This creates the barreling effect. When
performed on a hot workpart with cold dies, the barreling effect is even more pronounced.
This results from a higher coefficient of friction typical in hot working and heat transfer at
and near the die surfaces, which cools the metal and increases its resistance to deformation.
The hotter metal in the middle of the part flows more readily than the cooler metal at the
ends. These effects are more significant as the diameter-to-height ratio of the workpart
increases, due to the greater contact area at the work-die interface.
All of these factors cause the actual upsetting force to be greater than what is predicted by
Eq. (2.2.2). As an approximation, we can apply ashape factor to Eq. (2.2.2) to account for
effects of the D/h ratio and friction:
F  K f  f A ……………………………………………………… (2.2.3)
Where F,  f and A have the same definitions as in the previous equation; and K f is the
forging shape factor, defined as
Kf = 𝟏 +
𝟎.𝟒µ𝑫…………………………………………. (2.2.4)
𝐡
Where µ= coefficient of friction; D = workpart diameter or other dimension representing
contact length with die surface, mm; and A= workpart height, mm.
EXAMPLE:
A cylindrical workpiece is subjected to a cold upset forging operation. The starting piece is 75
mm in height and 50 mm in diameter. It is reduced in the operation to a height of36 mm.
The work material has a flow curve defined by K = 350 MPa and n =0.17. Assume a coefficient
of friction of 0.1. Determine the force as the process begins (strain assumed as 0.002), at
intermediate heights of 62 mm, 49 mm, and at the final height of 36 mm.(245,330N,
649,303N, 955,642N, 1,467,422N)
2.3 EXTRUSION
Extrusion is a compression process in which the work metal is forced to flow through a die
opening to produce a desired cross-sectional shape. The process can be likened to
squeezing toothpaste out of a toothpaste tube. There are several advantages of the modern
process: (1) a variety of shapes are possible, especially with hot extrusion; however, a
limitation of the geometry is that the cross section of the part must be uniform throughout its
extruded length; (2) grain structure and strength properties are enhanced in cold and warm
extrusion; (3) fairly close tolerances are possible, especially in cold extrusion; and (4) in some
extrusion operations, little or no wasted material is created.
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2.3.1 Types of Extrusion
Extrusion is carried out in various ways. One way of classifying the operations is by physical
configuration, in which the two principal types are direct extrusion and indirect extrusion.
Another classification it by working temperature: cold, warm, or hot extrusion. Finally,
extrusion is performed as either a continuous process or a discrete process.
Direct extrusion (also called forward extrusion) is illustrated in Figure 2.3.1. A metal billet is
loaded into a container, and a ram compresses the material, forcing it to flow through one or
more openings in a die at the opposite end of the container. As the ram approaches the die, a
small portion of the billet remains that cannot be forced through the die opening. This extra
portion, called the butt, is separated from the product by cutting it just beyond the exit of the
die.
Figure 2.3.1: Direct Extrusion
One of the problems in direct extrusion is the significant friction that exists between the work
surface and the walls of the container as the billet is forced to slide toward the die opening.
This friction causes a substantial increase in the ram force required in direct extrusion. In hot
extrusion, the friction problem is aggravated by the presence of an oxide layer on the surface
of the billet. This oxide layer can cause defects in the extruded product. To address these
problems, a dummy block is often used between the ram and the work billet. The diameter of
the dummy block is slightly smaller than the billet diameter, so that a narrow ring of work
metal (mostly the oxide layer) is left in the container, leaving the final product free of oxides.
Hollow sections (e.g., tubes) are possible in direct extrusion by the process setup in Figure
2.3.2. The starting billet is prepared with a hole parallel to its axis. This allows passage of a
mandrel that is attached to the dummy block. As the billet is compressed, the material is
forced to flow through the clearance between the mandrel and the die opening. The resulting
cross section is tubular. Semi-hollow cross-sectional shapes are usually extruded in the same
way.
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Figure 2.3.2: (a) Direct extrusion to produce a hollow or semi-hollow cross section; (b) hollow
and (c) semi-hollow cross sections
The starting billet in direct extrusion is usually round in cross section, but the final shape is
determined by the shape of the die opening. Obviously, the largest dimension of the die
opening must be smaller than the diameter of the billet.
In Indirect extrusion, also called backward extrusion and reverse extrusion, Figure 2.3.3 (a),
the die is mounted to the ram rather than at the opposite end of the container. As the ram
penetration into the work, the metal is forced to flow through the clearance in a direction
opposite to the motion of the ram. Since the billet is not forced to move relative to the
container, there is no friction at the container walls, and the ram force is therefore lower than
in direct extrusion. Limitation of indirect extrusion are imposed by the lower rigidity of the
hollow ram and the difficulty in supporting the extruded product as it exits the die.
Indirect extrusion can produce hollow (tubular) cross sections, as in Figure 2.3.3 (b). In this
method, the ram is pressed into the billet, forcing the material to flow around the ram and
take a cup shape. There are practical limitations on the length of the extrudedpart that can
be made by this method. Support of the ram becomes a problem as work length increases.
Figure 2.3.3: Indirect extrusion to produce (a) a solid cross section and (b) a hollow cross
section
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Hot extrusion involves prior heating of the billet to a temperature above its re-crystallization
temperature. This reduces strength and increases ductility of the metal, permitting more
extreme size reductions and more complex shape to be achieved in the process. Additional
advantages include reduction of ram force, increased ram speed, and reduction of grain flow
characteristics in the final product.
Cold extrusion and warm extrusion are generally used to produce discrete parts, often in
finished (or near finished) form. The term Impact extrusion is used to indicate high-speed
cold extrusion. Some important advantages of cold extrusion include increased strength due
to strain hardening, close tolerances, improved surface finish, absence of oxide layers, and
high production rates. Cold extrusion at room temperature also eliminates the need for heating
the starting billet.
2.3.2 Analysis of Extrusion
Let us use Figure 2.3.4 as a reference in discussing some of the parameters in extrusion. The
diagram assumes that both billet and extrudate are round in cross section. One important
parameter is the extrusion ratio, also called the reduction ratio. The ratio is defined:
rx 
A0
A f ………………………………………………………. (2.3.1)
Where rx = extrusion ratio; Ao =cross-sectional area of the starting billet, mm2; and A f = final
cross-sectional area of the extruded section, mm2. The ratio applies for both direct and
indirect extrusion.
Figure 2.3.4: Pressure and other variables in direct extrusion
The value of rx , can be used to determine true strain in extrusion, given that ideal
deformation occurs with no friction and no redundant work:
  ln rx  ln
AO
……………………………………………… (2.3.2)
Af
Under the assumption of ideal deformation (no friction and no redundant work), the
pressure applied by the ram to compress the billet through the die opening depicted in our
figure can be computed as follows:
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p   avf ln rx ………………………………………………………. (2.3.3)
Where  avf = average flow stress during deformation, Mpa. For convenience, we restate Eq,
(2.1.7)
K n
 avf 
……………………………………………………….. (2.1.7)
1 n
In fact, extrusion is not a frictionless process, and the previous equations grossly
underestimate the strain and pressure in an extrusion operation. Friction exists between the
die and the work as the billet squeezes down and passes through the die opening. . In
direct extrusion, friction also exists between the container wall and the billet surface. The
effect of friction is to increase the strain experienced by the metal. Thus, the actual pressure
is greater than that given by Eq. (2.3.3), which assumes frictionless extrusion.
The following empirical equation proposed by Johnson for estimating extrusion strain has
gained considerable recognition:
 x  a  b ln rx ………………………………………………………. (2.3.4)
Where  x = extrusion strain; and a and b are empirical constants for a given die angle.
Typical values of these constants are: a = 0.8 and b = 1.2 to 1.5. Values of a and b tend to
increase with increasing die angle.
The ram pressure to perform indirect extrusion can be estimated based on Johnson's
extrusion strain formula as follows:
p   avf  x ………………………………………………………. (2.3.5)
Where  avf is calculated based on ideal strain from Eq. (2.3.2), rather than extrusion strain in
Eq. (2.3.4).
In direct extrusion, the effect of friction between the container walls and the billet causes the
ram pressure to be greater than for indirect extrusion. We can write thefollowing expression
which isolates the friction force in the direct extrusion container:
p f Do
4
2
 p c DO L ……………………………………………….. (2.3.6)
Where p f = additional pressure required to overcome friction, MPa;
Do
= billet cross4
sectional area, mm2;  = coefficient of friction at the container wall; p c = pressure of the
billet against the container wall, MPa; and Do L = area of the interface between billet and
container wall, mm2. The right-hand side of this equation indicates the billet-container
friction force, and the left-hand side gives the additional ram force to overcome that friction.
In the worst case, sticking occurs at the container wall so that friction stress equals shear yield
strength of the work metal:
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pcDO L   sDo L ……………………………………………….. (2.3.7)
Where  s = shear yield strength, MPa. If we assume that  s =  avf /2;then p f reduces to
the following:
p f   avf
2L
…………………………………………………………. (2.3.8)
Do
Based on this reasoning, the following formula can be used to compute ram pressure in direct
extrusion:

2L 
 …………………………………………………………. (2.3.9)
p   avf   x 
D
o 

Where the term 2L/Doaccounts for the additional pressure due to friction at the container-billet
interface. L is the portion of the billet length remaining to be extruded, and Dois the original
diameter of the billet. Note thatp is reduced as the remaining billet length decreases during
the process.
Ram force in indirect or direct extrusion is simply pressure p from Eqs. (2.3.5) or (2.3.9),
respectively, multiplied by billet area Ao:
F  pAO ………………………………………………………… (2.3.10)
Where F=ram force in extrusion, N. Power required to carry out the extrusion operation is
simply:
P  Fv ………………………………………………………… (2.3.11)
Where P=power, J/s; F=ram force, N; and v=ram velocity, m/s.
EXAMPLE:
Abillet75 mm long and 25 mm in diameter is to be extruded in a direct extrusion operation with
extrusion ratio rx = 4.0. The extrudate has a round cross section. The die angle (half-angle) = 90°.
The work metal has a strength coefficient=415 MPa, and strain hardening exponent=0.18. Use the
Johnson formula with a =0.8 and b = 1.5 to estimate extrusion strain. Determine the pressure
applied to the end of the billet as the ram moves forward from 75, 50, 25 to 0.(3312Mpa,
2566Mpa, 1820Mpa, 1074Mpa).
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2.4 WIRE AND BAR DRAWING
In the context of bulk deformation, drawing is an operation in which the cross section of a
bar, rod, or wire is reduced by pulling it through a die opening, as in Figure 2.4. The general
features of the process are similar to those of extrusion. The difference is that the work is
pulled through the die in drawing, whereas it is pushed through the die in extrusion.
Although the presence of tensile stresses is obvious in drawing, compression also plays a
significant role because the metal is squeezed down as it passes through the die opening. For this
reason, the deformation that occurs in drawing is sometimes referred to as indirect
compression. Drawing is a term also used in sheet metalworking. The term wire and bar
drawing is used to distinguish the drawing process discussed here from the sheet metal
process of the same name.
The basic difference between bar drawing and wire drawing is the stock size that is
processed. Bar drawing is the term used for large diameter bar and rod stock, while wire
drawing applies to small diameter stock. Wire sizes down to 0.03 mm are possible in wire
drawing. Although the mechanics of the process are the same for the two cases, the methods,
equipment, and even the terminology are somewhat different.
Bar drawing is generally accomplished as a single-draft operation—the stock is pulled
through one die opening. Because the beginning stock has a large diameter, it is in the form of
a straight cylindrical piece rather than coiled. This limits the length of the work that can be
drawn, necessitating a batch type operation. By contrast, wire is drawn from coils consisting of
several hundred (or even several thousand) feet of wire and is passed through a series of draw
dies. The number of dies varies typically between 4 and 12. The term continuous drawing is
used to describe this type of operation because of the long production runs that are achieved
with the wire coils, which can be butt-welded each to the next to make the operation truly
continuous.
Figure 2.4.1: Drawing of bar, rod, or wire
In a drawing operation, the change in size of the work is usually given by the area reduction, defined as
follows:
r
Ao  A f
Ao
……………………………………………………….. (2.4.1)
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Where r=area reduction in drawing; Ao = original area of work, mm2; and A f =final area, mm2. Area
reduction is often expressed as a percentage.In bar drawing, rod drawing, and in drawing of large
diameter wire for upsetting and heading operations, the term draft is used to denote the before and after
difference in size of the processed work. The drqftis simply the difference between original and final
stock diameters:
d  Do  D f ……………………………………………………….. (2.4.2)
Where d = draft, mm; Do = original diameter of work, mm; and D f = final work diameter, mm.
2.4.1 Analysis of Drawing
If no friction or redundant work occurred in drawing, true strain could be determined as follows:
  ln
AO
1
 ln
……………………………………………… (2.4.3)
Af
1 r
Where Ao and A f are the original and final cross-sectional areas of the work, as previously defined; and r=
drawing reduction as given by Eq. (2.4.1). The stress that results from this ideal deformation is given by:
   avf    avf ln
AO
…………………………………………………………….. (2.4.4)
Af
Where  avf = average flow stress based on the value of strain given by Eq. 2.4.3.
Because friction is present in drawing and the work metal experiences inhomogeneous deformation, the
actual stress is larger than provided by Eq. (2.4.4). In addition to the ratio Ao / A f , other variables that
influence draw stress are die angle and coefficient of friction at the work-die interface. A number of
methods have been proposed for predicting draw stress based on values of these parameters. We present
the equation suggested by Schey:


 d   avf 1 
A
 
 ln O …………………………………………………………….. (2.4.5)
tan  
Af
where  d = draw stress, MPa; µ= die-work coefficient of friction; α = die angle (half-angle) as defined in
Figure 2.4; and  is a factor that accounts for inhomogeneous deformation which is determined as follows
for a round cross section:
  0.88  0.12
D
Lc ………………………………………………………… (2.4.6)
where D = average diameter of work during drawing, mm; and Lc = contact length of the work with the
draw die in Figure 2.4.1, mm. Values of D and Lccan be determined from the following:
D
Do  D f
2
……………………………………………………… (2.4.7)
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Lc 
Do  D f
2 sin 
2022
……………………………………………………… (2.4.8)
The corresponding draw force is then the area of the drawn cross section multiplied by the draw stress:
A
 

F  A f  d  A f  avf 1 
 ln o ………………(2.4.9)
Af
 tan  
Where F = draw force, N; and the other terms are defined above. The power required in a drawing operation
is the draw force multiplied by exit velocity of the work.
Why is more than one step required to achieve the desired reduction in wire drawing? Why not take the
entire reduction in a single pass through one die, as in extrusion? The answer can be explained as follows.
From the preceding equations, it is clear that as the reduction increases; draw stress increases. If the
reduction is large enough, draw stress will exceed the yield strength of the exiting metal. When that
happens, the drawn wire will simply elongate instead of new material being squeezed through the die
opening. For wire drawing to be successful, maximum draw stress must be less than the yield strength of the
exiting metal.
Let us assume a perfectly plastic metal (n=0), no friction, and no redundant work. In this
ideal case, the maximum possible draw stress is equal to the yield strength of the work
material. Expressing this using the equation for draw stress under conditions of ideal
deformation, Eq. (2.4.4), and setting  avf   y (because n = 0),
 d   avf ln
This means that ln Ao
Af
AO
A
1
  y ln o   y ln
  y ………………….. (2.4.10)
Af
Af
1 r
 ln 1
 1 . Hence
1 r
Ao
Af
1
 1 must equal the natural
1 r
logarithm base e. That is, the maximum possible strain is 1.0;
 max  1.0 ………………………………………………………… (2.4.11)
The maximum possible area ratio is:
Ao
Af
 e  2.7183 …………………………………………….. (2.4.12)
and the maximum possible reduction is:
rmax 
e 1
 0.632 ……………………………………………… (2.4.13)
e
The value given by Eq. (2.4.13) is often used as the theoretical maximum reduction possible in a
single draw, even though it ignores (1) the effects of friction and redundant work, which
would reduce the maximum possible value, and (2) strain hardening, which would increase the
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maximum possible reduction because the exiting wire would be stronger than the starting
metal. In practice, draw reductions per pass are quite below the theoretical limit. Reductions
of 0.50 for jingle-draft bar drawing and 0.30 for multiple-draft wire drawing seem to be the
upper limits in industrial operations.
EXAMPLE:
Wire is drawn through a draw die with entrance angle = 15°. Starting diameter is 2.5 mmand final diameter
= 2.0 mm. The coefficient of friction at the work-die interface = 0.07. The metal has a strength
coefficient K=205 MPa and a strain hardening exponent n=0.20. Determine the draw stress and draw
force in this operation.(94.3Mpa, 296N)
3.0 SHEET METAL WORKING
Sheet metalworking includes cutting and forming operations performed on relatively thin
sheets of metal. Typical sheet-metal thicknesses are between 0.4 mm and 6 mm. When
thickness exceeds about 6 mm, the stock is usually referred to as plate rather than sheet. The
sheet or plate stock used in sheet metalworking is produced by rolling
Most sheet-metal processing is performed at room temperature (cold working). The
exceptions are when the stock is thick, the metal is brittle, or the deformation is significant.
These are usually cases of warm working rather than hot working.
The three major categories of sheet-metal processes are (1) cutting, (2) bending, and (3)
drawing. Cutting is used to separate large sheets into smaller pieces, to cut out part perimeters,
and to make holes in parts.Bending and drawing are used to form sheet-metal parts into their
required shapes.
Most sheet-metal operations are performed on machine tools called presses. The term
stamping press is used to distinguish these presses from forging and extrusion presses. The
tooling that performs sheet metalwork is called a punch-and-die: the term stamping die is
also used. The sheet-metal products are called stampings. To facilitate mass production, the
sheet metal is often presented to the press as long strips or coils.
Cutting of sheet metal is accomplished by a shearing action between two sharp cutting edges.
There are three principal operations in press working that cut metal by the shearing
mechanism just described: shearing, blanking, and punching.
Shearing: is a sheet-metal cutting operation along a straight line between two cutting edges.
Shearing is typically used to cut large sheets into smaller sections for subsequent pressworking
operations. It is performed on a machine called a power shears, or squaring shears. The
upper blade of the power shears is often inclined, to reduce the required cutting force.
Blanking: involves cutting of the sheet metal along a closed outline in a single step to
separate the piece from the surrounding stock. The part that is cut out is the desired product in
the operation and is called the blank. Punching or piercingis similar to blanking except that
the separated piece is scrap, called the slug.The remaining stock is the desired part.
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3.1 CUTTING OPERATIONS
3.1.1 Engineering Analysis of Sheet-Metal Cutting
Important parameters in sheet-metal cutting are clearance between punch and die, stock
thickness, type of metal and its strength, and length of the cut.
The clearance c in a shearing operation is the distance between the punch and die. Typical
clearances in conventional press working range between 4% and 8% of the sheet-metal
thickness. If the clearance is too small, then the fracture lines tend to pass each other, causing
a double burnishing and larger cutting forces. If the clearance is too large, the metal becomes
pinched between the cutting edges and an excessive burr results. In special operations
requiring very straight edges, such as shaving and fine blanking, clearance is only about 1% of
stock thickness.
The correct clearance depends on sheet-metal type and thickness. The recommended
clearance can be calculated by the following formula:
c  Ac t
Where c = clearance, mm; A = clearance allowance; and t = stock thickness, mm. The
clearance allowance is determined according to type of metal. For convenience, metals are
classified into three groups given in Table 3.1, with an associated allowance value for each
group.
Table 3.1: Clearance allowance value for three sheet-metal groups
Whether to add the clearance value to the die size or subtract it from the punch size depends
on whether the part being cut out is a blank or a slug as shown on figure 3.1. Because of the
geometry of the sheared edge, the outer dimension of the part cut out of the sheet will be
larger than the hole size. Thus, punch and die sizes for a round blank of diameter Db are
determined as:
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Figure 3.1: Die and punch size relationship
Blanking punch diameter  Db  2c
Blanking die diameter  Db
Punch and die sizes for a round hole of diameter Dh are determined as:
Hole punch diameters = Dh
Hole die diameter  Dh  2c
In order for the slug or blank to drop through the die, the die opening must have an angular
clearance of 0.25° to 1.5° on each side.
Estimates of cutting force are important this force determines the size (tonnage) of the press
needed. Cutting force F in sheet metalworking can be determined by:
F   .tL
 = shear strength of the sheet metal, Mpa, t = stock thickness, mm, and L = length of the
cut edge, mm. In blanking, punching, slotting, and similar operations, L is the perimeter
length of the blank or hole being cut. The minor effect of clearance in determining the value of
L can be neglected.
If shear strength is unknown, an alternative way of estimating the cutting force is to use the
tensile strength, as follows:
F  0.7 t .tL
 t = ultimate tensile strength, Mpa.
3.2 BENDING OPERATIONS
Bending in sheet-metal work is defined as the straining of the metal around a straight axis. During the
bending operation, the metal on the inside of the neutral plane is compressed, while the metal on the outside
of the neutral plane is stretched. The metal is plastically deformed so that the bend takes a permanent set
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upon removal of the stresses that caused it. Bending produces little or no change in the thickness of the
sheet metal.
3.2.1 V-Bending and Edge Bending
Bending operations are performed using punch and die tooling. The two common bending methods and
associated tooling are V-bending, performed with a V-die; and edge bending, performed with a wiping die.
These methods are illustrated in Figure 3.2.
In V-bending, the sheet metal is bent between a V-shaped punch and die. Included angles ranging from very
obtuse to very acute can be made with V-dies. V-bending is generally used for low-production operations.
It is often performed on a press brake and the associated V-dies are relatively simple and inexpensive.
Edge bending involves cantilever loading of the sheet metal. A pressure pad is used to apply a force Fh to
hold the base of the part against the die, while the punch forces the part to yield and bend over the edge of
the die.
3.2.2 Engineering Analysis of Bending
Some of the important terms in sheet-metal bending are identified in Figure 3.2.2. The metal of thicknesst is
bent through an angle called the bend angle  . This results in a sheet-metal part with an included angle  ' ,
where    '  180 0 . The bend radius R is normally specified on the inside of the part, rather than at the
neutral axis, and is determined by the radius on the tooling used to perform the operation. The bend is made
over the width of the work piecew.
If the bend radius is small relative to stock thickness; the metal tends to stretch during bending. It is
important to be able to estimate the amount of stretching that occurs, if any, so that the final part length will
match the specified dimension. The problem is to determine the length of the neutral axis before beading to
account for stretching of the final bent section. This length is called the bend allowance, and it can be
estimated as follows:
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Ab  2

360
2022
R  Kbat 
Where Ab = bend allowance, mm;  = bend angle, degrees; R = bend radius, mm; t = stock thickness, mm;
and K ba , is factor to estimate stretching. The following design values are recommended for K ba :
If R  2t , K ba = 0.33; and R  2t , K ba = 0.50. The values of K ba , predict that stretching occurs only if bend
radius is small relative to sheet thickness.
When the bending pressure is removed at the end of the deformation operation, elastic energy remains in
the bent part, causing it to recover partially toward its original shape. This elastic recovery is called
springbuck, defined as the increase in included angle of the bent part relative to the included angle of the
forming tool after the tool is removed.
Compensation for springback can be accomplished by several methods. Two common
methods are overbending and bottoming. In ovcrbendlng, the punch angle and radius are
fabricated slightly smaller than the specified angle on the final part so that the sheet metal
springs back to the desired value. Bottoming involves squeezing the part at the end of the
stroke, thus plastically deforming it in the bend region.
The force required to perform bending depends on the geometry of the punch-and-die and
the strength, thickness, arid length of the sheet metal. The maximum bending force can-be
estimated by means of the following equation:
F
K bf  t wt 2
D
Where F = bending force, N;  t = tensile strength of the sheet metal, Mpa, w = width of part
in the direction of the bend axis, mm; t = stock thickness, mm; and D = die opening
dimension. K bf is a constant that accounts for differences encountered in an actual bending process. Its value
depends on type of bending: for V-bending, K bf = 1.33 and for edge bending, K bf = 0.33.
Other bending operations include flanging, hemming, seaming and curling.
EXAMPLE:
A sheet-metal blank is to be bent as shown in figure below. The metal has a modulus of
elasticity = 205 GPa, yield strength = 275 MPa, and tensile strength=450 MPa. Determine (a)
the starting blank size and (b) the bending force if a V-die is used with a die opening
dimension=25 mm.(44.5mm wide, 69.08mm long; 10,909N)
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3.3 DRAWING OPERATION
Drawing is a sheet-metal-forming operation used to make cup-shaped, box-shaped, or other
complex-curved, hollow-shaped parts. It is performed by placing a piece of sheet metal over a
die cavity and then pushing the metal into the opening with a punch, as in Figure 3.3. The
blank must usually be held down flat against the die by a blank holder. Common parts made by
drawing include beverage cans, ammunition shells, sinks, cooking pots, and automobile body
panels.
Figure 3.3: Drawing of a cup-shaped part
3.3.1 Mechanics of Drawing
Drawing of a cup-shaped part is the basic drawing operation, with dimensions and parameters
as pictured in Figure 3.3. A blank of diameter Db is drawn into a die by means of a punch of
diameter Dp. The punch and die must have corner radii, given by Rp and Rd. If the punch and
die were to have sharp corners (Rp and Rd = 0), a hole-punching operation (and not a very good
one) would be accomplished rather than a drawing operation. The sides of the punch and die
are separatedby a clearance c. This clearance in drawing isabout 10% greater than the stock
c  1.1t
thickness:
As the punch proceeds downward toward its final bottom position, the work experiences a
complex sequence of stresses and strains as it is gradually formed into the shape defined by the
punch and die cavity. As the punch first begins to push into the work, the metal is subjected to a
bending operation. As the punch moves further down, a straightening action occurs in the
metal that was previously bent over the die radius. The metal at the bottom of the cup, as well
as along the punch radius, has been moved downward with the punch, but the metal that was
bent over the die radius must now be straightened in order to be pulled into the clearance to
form the wall of the cylinder. At the same time, more metal must be added to replace that being
used in the cylinder wall. This new metal comes from the outside edge of the blank. The metal
in the outer portions of the blank is pulled or drawn toward the die opening to resupply the
previously bent and straightened metal now forming the cylinder wall. This type of metal flow
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through a constricted space gives the drawing process its name.
The holding force applied by the blankholder is now seen to be a critical factor in deep drawing. If it is too
small, wrinkling occurs. If it is too large, it prevents the metal from flowing properly toward the die cavity,
resulting in stretching and possible tearing of the sheet metal. Determining the proper holding force
involves a delicate balance between these opposing factors.
Progressive downward motion of the punch results in a continuation of the metal flow caused by drawing
and compression. In addition, some thinning of the cylinder wall occurs. The force being applied by the
punch is opposed by the metal in the form of deformation and friction in the operation. A portion of
thedeformation involves stretching and thinning of the metal as it is pulled over the edge of the die opening.
Up to 25% thinning of the side wall may occur in a successful drawing operation, mostly near the base of the
cup.
3.3.2 Engineering Analysis of Drawing
One of the measures of the severity of a deep drawing operation is the drawing ratio DR. This is most easily
defined for a cylindrical shape as the ratio of blank diameter Db, to punch diameter Dp. In equation form:
DR 
Db
Dp
The drawing ratio provides an indication, of the severity of a given drawing operation. The greater the ratio, the
more severe the operation. An approximate upper limit on the drawing ratio is a value of 2.0. The actual limiting
value for a given operation depends on punch and die corner radii (Rpand Rd,), friction conditions, depth of
draw, and characteristics of the sheet metal (e.g., ductility, degree of directionality of strength properties in the
metal).
Another way to characterize a given drawing operation is by the reduction r, where
r
Db  D p
Db
It is very closely related to drawing ratio. Consistent with the previous limit on DR (DR < 2.0), the value of
reduction r should be less than 0.50.
A third measure in deep drawing is the thickness-to-diameter ratio t/Db, (thickness of the starting blank t divided
by the blank diameter Db). Often expressed as a percent, it is desirable for the t/Db ratio to be greater than 1%.
As t/Db, decreases, tendency for wrinklingincreases.
In cases where these limits on drawing ratio, reduction, and t/Db, ratio are exceeded by the design of the drawn
part, the blank must be drawn in two or more steps, sometimes with annealing between the steps.
The drawing force required to perform a given operation can be estimated roughly by the
formula:
D

F  D p t t  b  0.7 
D

 p

Where F = drawing force, N; t = original blank thickness, mm;  t = tensile strength, MPa;
and DpandDbare the starting blank diameter and punch diameter, respectively, mm. The
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constant 0.7 is a correction factor to account for friction. The drawing force varies throughout
the downward movement of the punch, usually reaching its maximum value at about one-third
the length of the punch stroke.
The holding force is an important factor in a drawing operation. As a rough approximation,
the holding pressure can be set at a value = 0.015 of the yield strength of the sheet metal.
This value is then multiplied by that portion of the starting area of the blank that is to be held
by the blankholder. In equation form:

Fh  0.015 y Db  D p  2.2t  2 Rd 
2
2

Where Fh = holding force in drawing, N;  y = yield strength of the sheet metal, MPa; t =
starting stock thickness, mm; Rd = die corner radius, mm; and the other terms have been
previously defined. The holding force is usually about one-third the drawing force.
3.3.3 Defects in Drawing
A number of defects can occur in a drawn product, following is a list of common defects, with
sketches in Figure 3.3.3:
(a) Wrinkling in the flange. Wrinkling in a drawn part consists of a series of ridges
thatform radially in the undrawn flange of the workpart due to compressive buckling.
(b) Wrinkling in the wall. If and when the flange is drawn into the cup, these
ridgesappear in the vertical wall.
(c) Tearing. Tearing is an open crack in the vertical wall, usually near the base of thedrawn
cup, due to high tensile stresses that cause thinning and failure of the metalat this
location. This type of failure can also occur as the metal is pulled over a sharpdie corner.
(d) Earing. This is the formation of irregularities (called ears) in the upper edge of a
deep drawn cup, caused by anisotropy in the sheet metal. If the material is perfectly
EXAMPLE:
A drawing operation is used to form a cylindrical cup with inside diameter=75 mm and height = 50 mm.
The starting blank size = 138 mm and the stock thickness=2.4 mm. Determine (a) drawing force and (b)
holding force, given that the tensile strength of the sheet metal (low-carbon steel) = 300 MPa
and yield strength = 175 MPa. The die corner radius = 6 mm.(193,396N, 86,824N)
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4.0 SHAPING PROCESSES FOR PLASTICS
Plastics can be shaped into a wide variety of products, such as molded parts, extruded
sections, films and sheets, insulation coatings on electrical wires, and fibers for textiles. In
addition, plastics are often the principal ingredient in other materials, such as paints and
varnishes; adhesives; and various polymer matrix composites. The commercial and
technological importance of these shaping processes derives from the growing importance of
the materials being processed.
Several reasons why the plastic-shaping processes are important:

The variety of shaping processes, and the ease with which polymers can be processed,
allows an almost unlimited variety of part geometries to be formed.
 Many plastic parts are formed by molding, which is a net shape process; further shaping
is generally not needed.
 Although heating is usually required to form plastics, less energy is required than for
metals because the processing temperatures are much lower for plastics.
 Because lower temperatures are used in processing, handling of the product is simplified during production. Because many plastic processing methods are one-step operations (e.g., molding), the amount of product handling required is substantially reduced
compared to metals.
 Finishing by painting or plating is not required (except in unusual circumstances) for
plastics.
Plastic-shaping processes can be classified as follows according to the resulting product
geometry: (1) continuous extruded products with constant cross section other than sheets,
films, and filaments; (2) continuous sheets and films; (3) molded parts that are mostly solid;
(4) hollow molded parts with relatively thin walls.
4.1 EXTRUSION
Extrusion is one of the fundamental shaping processes, for metals and ceramics as well as polymers. Extrusion
is a compression process in which material is forced to flow through a die orifice to provide long continuous
product whose cross-sectional shape is determined by the shape of the orifice. As a polymer shaping
process, it is widely used for thermoplastics and elastomers (but rarely for thermosets) to mass produce items
such as tubing, pipes, hose, structural shapes (such as window and door molding), sheet and film, and coated
electrical wire and cable, For these types of products, extrusion is carried out as a continuous process; the
extrudate(extruded product) is subsequently cut into desired lengths.
4.1.1 Process and Equipment
In polymer extrusion, feedstock in pellet or powder form is fed into an extrusion barrel where it is heated
and melted and forced to flow through a die opening by means of a rotating screw, as illustrated in Figure
4.1.1 (a). The two main components of the extruder are the barrel and the screw. The die is not a component
of the extruder; it is a special tool that must be fabricated for the particular profile to be produced.
The internal diameter of the extruder barrel typically ranges from 25 to 150 mm. The barrel is long relative
to its diameter, with L/D ratios usually between 10 and 30. The higher ratios are used for thermoplastic
materials, while lower L/D values are for elastomers.
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Figure 4.1.1(a): Components, features of a single screw extruder for plastics and elastomers
The material is conveyed through the barrel toward the die opening by the action of the extruder screw, which
rotates at about 60 rev/mm. The screw serves several functions and is divided into sections that correspond to
these functions. The sections and functions are (I) feed section, in which the stock is moved from the hopper
port and preheated; (2) compression section or transition section, where the polymer is transformed into
liquid consistency, air entrapped amongst the pellets is extracted from the melt, and the material is
compressed; and (3) metering section, in which the melt is homogenized and sufficient pressure is
developed to pump it through the die opening.
The operation of the screw is determined by its geometry and speed of rotation. Typical extruder screw
geometry is depicted in figure 4.1.1 (b). The screw consists of spiraled flights (threads) with channels between
them through which the polymer melt is moved. The channel has a width wc and depth dc. As the screw
rotates, the flights push the material forward through the channel from the hopper end of the barrel toward the
die. The flight diameter is smaller than the barrel diameter D by a very small clearance—around 0.05 mm.
The function of the clearance is to limit leakage of the melt backward to the trailing channel. The flight land
has a width w f and is made of hardened steel to resist wear as it turns and rubs against the inside of the
barrel.
Figure 4.1.1 (b): Details of an extruder screw inside the barrel
The screw has a pitch whose value is usually close to the diameter D. The flight angle A is the helix angle of
the screw and can be determined from the relation:
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tan A 
2022
p
Where p is pitch of the screw.
D
The increase in pressure applied to the polymer melt in the three sections of the barrel is determined
largely by the channel depth dc. In Figure 4.1.1 (a), dc, is relatively large in the feed section to allow large
amounts of granular polymer to be admitted into the barrel. In the compression section, dcis gradually
reduced, thus applying increased pressure on the polymer as it melts. In the metering section, dc, is small
and pressure reaches a maximum as flow is restrained by the screen pack and backer plate. The three
sections of the screw are shown as being about equal in length in Figure 4.1.1 (a); this is appropriate for a
polymer which melts gradually, such as low-density polyethylene. For other polymers, the optimal section
lengths are different. For crystalline polymers such as nylon, melting occurs rather abruptly at a specific
melting point, and therefore a short compression section is appropriate. Amorphous polymers such as
polyvinylchloride melt more slowly than LDPE, and the compression zone for these materials must take
almost the entire length of the screw. Although the optimal screw design for each material type is different, it
is common practice to use general-purpose screws. These designs represent a compromise among the
different materials, and they avoid the need to make frequent screw changes, which result in costly
equipment downtime.
Progress of the polymer along the barrel leads ultimately to the die zone. Before reaching the die, the melt
passes through a screen pack—a series of wire meshes supported by a stiff plate (called a breaker plate)
containing small axial holes. The screen pack assembly functions to (1) filter contaminants and hard lumps
from the melt; (2) build pressure in the metering section; and (3) straighten the flow of the polymer melt
and remove its "memory" of the circular motion imposed by the screw. This last function is concerned with
the polymer's viscoelastic property; if the flow were left unstraightened, the polymer would play back its
history of turning inside the extrusion chamber, tending to twist and distort the extrudate.
4.1.2 Analysis of Extrusion
As the screw rotates inside the barrel, the polymer melt is forced to move forward toward the die. The
principal transport mechanism is drag flow, resulting from friction between the viscous liquid and two
opposing surfaces moving relative to each other: (1) the stationary barrel and (2) the channel of the turning
screw. The velocity of the screw is given by:
v  DN cos A and
wc  D tan A  w f cos A
Where D = screw flight diameter, m; N = screw rotational speed, rev/s; wc = screw channel width, m; A =
flight angle; and w f =flight land width, m. If we assume that the flight land width is negligibly small, then the
last of these equations reduces to:
wc  D tan A cos A  D sin A
The volume drag flow rate is given by:
Qd  0.5 2 D 2 Nd c sin A cos A
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Where Qd = volume drag flow rate, m3/s; dc = screw channel depth, m.
If no forces were present to resist the forward motion of the fluid, this equation would provide a reasonable
description of the melt flow rate inside the extruder.However, compressing the polymer melt through the
downstream die creates a backpressure in the barrel that reduces the material moved by drag flow. This flow
reduction, which we shall call the back pressure flow, depends on the screw dimensions, viscosity of the
polymer melt, and pressure gradient along the barrel. These dependencies can be summarized in this
equation:
Dd c 3 sin 2 A  dp 
Qb 
 
12
 dl 
Where Qb = back pressure flow, m3/s;  = viscosity, N-s/m2; dp
= the pressure gradient, MPa/m; and
dl
the other terms were previously defined. The actual pressure gradient in the barrel is a function of the shape
of the screw over its length; If we assume as an approximation that the profile is a straight line, then the
pressure gradient becomes a constant p/L, and the previous equation reduces to:
pDd c sin 2 A
Qb 
12L
3
where p = head pressure in the barrel, MPa; and L = the length of the barrel, m. Recall that this back
pressure flow is really not an actual flow by itself; it is a reduction in the drag flow.Thus, we can compute
the magnitude of the melt flow in an extruder as the difference between the drag flow and the back pressure
flow:
Q x  Q d  Qb
pDd c sin 2 A
Qx  0.5 D Nd c sin A cos A 
12L
3
2
2
Where Qx = the resulting flow rate of polymer melt in the extruder. The above Equation assumes that there is
minimal leak flow through the clearance between flights and barrel. Leak flow of melt will be small compared
to drag and back pressure flow except in badly worn extruders.
The Equationcontains many parameters, which can be divided into two types: (1) design parameters, and (2)
operating parameters. The design parameters are those that define the geometry of the screw and barrel:
For a given extruder operation, these factors cannot be changed during the process. The operating parameters
are those that can be changed during the process to affect output flow; they include rotational speed, head
pressure and melt viscosity.
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4.2 INJECTION MOLDING
Injection molding is a process in which a polymer is heated to a highly plastic state and forced
to flow under high pressure into a mold cavity, where it solidifies. The molded part, called a
molding, is then removed from the cavity. The process produces discretecomponents that are
almost always net shape. The production cycle time is typically in the range 10 to 30 sec,
although cycles of 1 min or longer are not uncommon. Also, the mold may contain more than
one cavity, so that multiple moldings are produced each cycle.
As illustrated in our schematic in Figure 4.2.1, an injection molding machine consists of two
principal components: (1) the plastic injection unit and (2) the mold clamping unit. The
injection unit is much like an extruder. It consists of a barrel that is fed from one end by a
hopper containing a supply of plastic pellets. Inside the barrel is a screw whose operation
surpasses that of an extruder screw in the following respect: in addition to turning for mixing
and heating the polymer, it also acts as a ram which rapidly moves forward to inject molten
plastic into the mold.
Figure 4.2.1: Injection molding machine
The clamping unit is concerned with the operation of the mold. Its functions are to (1) hold
the two halves of the mold in proper alignment with each other; (2) keep the mold closed
during injection by applying a clamping force sufficient to resist the injection force; and(3)
open and close the mold at appropriate times in the molding cycle.
The cycle for injection molding of a thermoplastic polymer proceeds in the following sequence,
illustrated in Figure 4.2.2. Let us pick up the action with the mold open and the machine ready
to start a new molding: (1) The mold is closed and clamped. (2) A shotof melt, which has been
brought to the right temperature and viscosity by heating and by the mechanical working of the
screw, is injected under high pressure into the mold cavity. The plastic cools and begins to
solidify when it encounters the cold surface of the mold.Ram pressure is maintained to pack
additional melt into the cavity to compensate for contraction during cooling. (3) The screw is
rotated and retracted with the nonreturn valve open to permit fresh polymer to flow into the
forward portion of the barrel. Meanwhile, the polymer in the mold has completely solidified.
(4) The mold is opened, and the part is ejected and removed.
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Figure 4.2.2: Typical mold cycle (1) mold is closed (2) melt is injected into cavity (3) screw is
retracted and (4) mold opens, and part is ejected.
4.2.1 The Mold
The mold is the special tool in injection molding; it is custom-designed and fabricated for
the given part to be produced.A mold consists of (1) one or more cavities that determine part
geometry, (2) distribution channels through which the polymer melt flows to the cavities, (3) an
ejection system for part removal, (4) a cooling system, and (5) vents to permit evacuation of
air from the cavities.
A mold must have a distribution channel through which the polymer melts flows from the
nozzle of the injection barrel into the mold cavity. The distribution channel consists of (1) a
sprue, which leads from the nozzle into the mold; (2) runners, which lead from the sprue to
the cavity (or cavities); and (3) gates that constrict the flow of plastic into the cavity. There are
one or more gates for each cavity in the mold.
An ejection system is needed to eject the molded part from the cavity at the end of the
molding cycle. Ejector pins built into the moving half of the mold usually accomplish this
function. The cavity is divided between the two mold halves in such a way that the natural
shrinkage of the molding causes the part to stick to the moving half. When the mold opens,
the ejector pins push the part out of the mold cavity.
A cooling system is required for the mold. This consists of an external pump connected to
passageways in the mold, through which water is circulated to remove heat from the hot plastic.
The conventional two-plate mold is illustrated in figure 4.2.3 below.
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Figure 4.2.3: Details of two-plate mold for thermoplastic injection molding (a) closed and (b)
open.
Polymers have high thermal expansion coefficients, and significant shrinkage occurs during
cooling of the plastic in the mold. Some thermoplastics undergo volumetric contractions of
around 10% after injection into the mold. Contraction of crystalline plastics tends to be
greater than for amorphous polymers. Shrinkage is usually expressed as the reduction in
linearsize that occurs during cooling to room temperature from the molding temperature for
the given polymer. Appropriate units are therefore mm/mmof the dimension under
consideration. Typical values for selected polymers are given in Table 4.2.3.
Table 4.2.3: Typical values of shrinkage for moldings of selected thermoplastics
To compensate for shrinkage, the dimensions of the mold cavity must be made larger than the
specified part dimensions. The following formula can be used:
Dc  D p  D p S  D p S 2
WhereDc= dimension of cavity, mm; Dp = molded part dimension, mm, and S=shrinkage
values obtained from Table 4.2.3.
4.2.2 Defects in Injection Molding
Injection molding is a complicated process, and many things can go wrong. Here are some of the
common defects in injection molded parts:
Short shots—As in casting, a short shot is a molding that has solidified before completely
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filling the cavity. The defect can be corrected by increasing temperature and/or pressure. The
defect may also result from use of a machine with insufficient shot capacity, in which case a
larger machine is needed.
Flashing—flashing occurs when the polymer melt is squeezed into the parting surface between
mold plates; it can also occur around ejection pins. The defect is usually caused by (1) vents and
clearances in the mold that are too large; (2) injection pressure too high compared to clamping
force; (3) melt temperature too high; or (4) excessive shot size.
Sink marks and voids—These are defects usually related to thick molded sections. A sink
mark occurs when the outer surface on the molding solidifies, but contraction of the internal
material causes the skin to be depressed below its intended profile. A voidcaused by the same
basic phenomenon; however, the surface material retains its form and the shrinkage manifests
itself as aninternal void due to high tensile stresses on the still-molten polymer. These defects
can be addressed by increasing the packing pressure following injection. A better solution is to
design the part to have uniform section thicknesses and to use thinner sections.
Weld lines—Weld lines occur when polymer melt flows around a core or other convex detail in
the mold cavity and meets from opposite directions; the boundary thus formed is called a weld
line, and it may have mechanical properties that are inferior to those in the rest of the part.
Higher melt temperatures, higher injection pressures, alternative gating locations on the part,
and better venting are ways of dealing with this defect.
4.3 COMPRESSION AND TRANSFER MOLDING
4.3.1 COMPRESSION MOLDING
Compression molding is an old and widely used molding process for thermosetting plastics. Its
applications also include rubber tires, and various polymer matrix composite parts. The
process, illustrated in Figure 4.3.1 for a TS plastic, consists of (1) loading a precise amount of
molding compound, called the charge, into the bottom half of a heated mold; (2) bringing the
mold halves together to compress the charge, forcing it to flow and conform to the shape of the
cavity; (3) heating the charge by means of the hot mold to polymerize and cure the material
into a solidified part; and (4) opening the mold halves and removing the part from the cavity.
Figure 4.3.1: Compression molding for thermosetting plastics: (1) charge is loaded; (2) and
(3) charge Is compressed and cured; and (4) part is ejected and removed
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The initial charge of molding compound can be in any of several forms, including powders or
pellets, liquid, or preform. The amount of polymer must be precisely controlled to obtain
repeatable consistency in the molded product. It has become common practice to preheat the
charge prior to its placement into the mold; this softens the polymer and shortens the
production cycle time. Preheating methods include infrared heaters, convection heating in an
oven, and use of a heated rotating screw in a barrel. The latter technique (borrowed from
injection molding) is also used to meter the amount of the charge.
Compression molding presses are oriented vertically and contain two platens to which the
mold halves are fastened. The presses involve either of two types of actuation: (1) upstroke of
the bottom platen or (2) downstroke of the top platen, the former being the more common
machine configuration. They are generally powered by a hydraulic cylinder that can be
designed to provide clamping capacities up to several hundred tons.
Molds for compression molding are generally simpler than their injection mold counterparts.
There is no sprue and runner system in a compression mold and the process itself is generally
limited to simpler part geometries due to the lower flowcapabilities of the starting
thermosetting materials.
Materials for compression molding include phenolics, melamine, urea-formaldehyde,
epoxies, urethanes. and elastomers. Typical TS plastic moldings include electric plugs, sockets,
and housings; pot handles, and dinnerware plates. Advantages noted for compression
molding in these applications include: molds that are simpler, less expensive, and require low
maintenance; less scrap; and low residual stresses in the molded parts
4.3.2 TRANSFER MOLDING
In this process, a thermosetting charge (preform) is loaded into a chamber immediately ahead
of the mold cavity, where it is heated; pressure is then applied to force the softened polymer to
flow into the heated mold where curing occurs. There are two variants of the process,
illustrated in Figure 4.3.2: (a) pot transfer molding, in which the charge is injected from a
"pot" through a vertical sprue channel into the cavity; and (b) plunger transfer molding, in
which the charge is injected by means of a plunger from a heated well through lateral channels
into the mold cavity. In both cases, scrap is produced each cycle in the form of the leftover
material in the base of the well and lateral channels, called the cull. In addition, the sprue in
pot transfer is scrap material. Because the polymers are thermosetting, the scrap cannot be
recovered.
Transfer molding is closely related to compression molding, because it is utilized on the same
polymer types (thermosets and elastomers). One can also see similarities to injection
molding, in the way the charge is preheated in a separate chamber and then injected into the
mold. Transfer molding is capable of molding part shapes that are more intricate than
compression molding but not as intricate as injection molding. Transfer molding also lends
itself to molding with inserts, in which a metal or ceramic insert is placed into the cavity prior
to injection, and the heated plastic bonds to the insert during molding.
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Figure 4.3.2 (a) Pot transfer molding, and (b) plunger transfer molding. Cycle in both
processes is: (1) charge is loaded into pot, (2) softened polymer is pressed into mold
cavity and cured, and (3)part is ejected.
4.4 BLOW MOLDING AND ROTATIONAL MOLDING
Both of these processes are used to make hollow, seamless parts out of thermoplastic
polymers. Rotational molding can also be used for thermosets. Parts range in size from small
plastic bottles of only 5 ml (0.15 oz) to large storage drums of 38,000-liter (10,000-gal)
capacity. Although the two processes compete in certain cases, generally they have found their
own niches. Blow molding is more suited to the mass production of small disposable
containers, while rotational molding favors large, hollow shapes.
4.4.1 BLOW MOLDING
Blow moldingis a molding process in which air pressure is used to inflate soft plastic into a
mold cavity. It is an important industrial process for making one-piece hollow plastic parts
with thin walls, such as bottles and similar containers. Because many of these items are used for
consumer beverages for mass markets, production is typically organized for very high
quantities. The technology is borrowed from the glass industrywith which plastics compete in
the disposable and recyclable bottle market.
Blow molding is accomplished in two steps: (1) fabrication of a starting tube of molten plastic,
called a parison (same as in glass-blowing); and (2) inflation of the tube to the desired final
shape. Forming the parison is accomplished by either of two processes: (1) extrusion or (2)
injection molding.
Course Lecturer: Eng. Muhamed Swaleh Tel: 0721 868 758
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MPEN 511: MATERIAL FORMING TECHNOLOGY
2022
Extrusion Blow Molding This form of blow molding consists of the cycle illustrated in
Figure 4.4.1. In most cases, the process is organized as a very high production operation for
making plastic bottles. The sequence is automated and usually integrated with downstream
operations such as bottle filling and labeling.
Figure 4.4.1: Extrusion blow molding: (1) extrusion of parison, (2) parison is pinched at the
top and sealed at the bottom around a metal blow pin as the two halves of the mold come
together; (3) the tube is inflated so that it takes the shape of the mold cavity; and (4) mold
is opened to remove the solidified part.
Injection Blow MoldingIn this process, the starting parison is injection molded rather than
extruded. A simplified sequence is outlined in Figure 4.4.2. Compared to its extrusion-based
competitor, the injection blow-molding process has a lower production rate, which explains why it is
less widely used.
In a variation of injection blow molding, called stretch blow molding, the blowing rod extends
downward into the injection molded parison during step 2, thus stretching the soft plastic and
creating a more favorable stressing of the polymer than conventional injection blow molding or
extrusion blow molding. The resulting structure is more rigid, with higher transparency and better
impact resistance. The most widely used material for stretch blow molding is polyethylene
terephthalate (PET), polyesterthat has very low permeability and is strengthened by the stretch-blowmolding process. The combination of properties makes it ideal as a container for carbonated
beverages.
4.4.2 ROTATIONAL MOLDING
Rotational molding uses gravity inside a rotating mold to achieve a hollow form. Also called
rotomolding, it is an alternative to blow molding for making large, hollow shapes. It is used
principally for thermoplastic polymers, but applications for thermosets and elastomers are
becoming more common. Rotomolding tends to favor more complex external geometries,
larger parts, and lower production quantities than blow molding. The process consists of the
following steps: (1) A predetermined amount of polymer powder is loaded into the cavity of a
Course Lecturer: Eng. Muhamed Swaleh Tel: 0721 868 758
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MPEN 511: MATERIAL FORMING TECHNOLOGY
2022
split mold. (2) The mold is then heated and simultaneously rotated on two perpendicular
axes, so that .the powder impinges on all internal surfaces of the mold, gradually forming a
fused layer of uniform thickness. (3) While still rotating, the mold is cooled so that the plastic
skin solidifies. (4) The mold is opened, and the part is unloaded. Rotational speeds used in
the process are relatively slow. It is gravity, not centrifugal force that causes uniform coating
of the mold surfaces.
Molds in rotational molding are simple and inexpensive compared to injection molding or
blow molding, but the production cycle is much longer, lasting perhaps 10 min or more. To
balance these advantages and disadvantages in production, rotational molding is often
performed on a multicavity indexing machine, such as the three-station machine shown in
Figure 4.4.2. The machine is designed so that three molds are indexed in sequence through
three workstations. Thus, all three molds are working simultaneously. The first workstation is
an unload-load station where the finished part is unloaded from the mold, and the powder for
the next part is loaded into the cavity. The second station consists of a heating chamber
where hot-air convection heats the mold while it is simultaneously rotated. Temperatures
inside the chamber are around 375°C (700°F), depending on the polymer and the item being
molded. The third station cools the mold, using forced cold air or water spray, to cool and
solidify the plastic molding inside.
Figure 4.4.2:Rotational molding cycleperformed on a three-station indexing machine: (1)
unload-load station; (2) heat and rotate mold; (3) cool the mold.
Course Lecturer: Eng. Muhamed Swaleh Tel: 0721 868 758
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