Extrusion, Drawing, Forging and Sheetmetal working processes

Metal Processing Technology (MEng 4273)
Mulualem Hailu (MSc.)
School of Mechanical and Industrial Engineering
Mechanical Engineering Department
Dire Dawa University Institute of Technology
Dire Dawa, Ethiopia
December, 2021
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Extrusion and Drawing process
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Cont…
 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;
(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.
However, a limitation is that the cross section of the extruded part must be
uniform throughout its length.
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TYPES OF EXTRUSION
• Extrusion is carried out in various ways. One important distinction is between
direct extrusion and indirect extrusion. Another classification is 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)
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.
Fig. Direct Extrusion
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Indirect extrusion
 also called backward extrusion and reverse extrusion, the die is mounted to
the ram rather than at the opposite end of the container.
 As the ram penetrates 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.
 Limitations 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.
Fig. Indirect extrusion to produce (a) a solid cross section and (b) a hollow cross section.
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Cont…
 In the worst case, sticking occurs at the container wall so that friction stress
equals shear yield strength of the work metal:
 Based on this reasoning, the following formula can be used to compute ram
pressure in direct extrusion:
where the term 2L/Do accounts 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 Do is the original diameter
of the billet.
Note that: p is reduced as the remaining billet length decreases during the process.
 Ram force in indirect or direct extrusion is simply pressure p, respectively,
multiplied by billet area Ao:
F= pAo
where F = ram force in extrusion, N.
Power required to carry out the extrusion operation is simply: P = Fv
where P = power, J/s ; F = ram force, N; and v = ram velocity, m/s.
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Extrusion Dies and Presses
 Extrusion Dies : Important factors in an extrusion die are die angle and orifice
shape. Die angle, more precisely die half-angle, is shown on fig (a). below.
 For low angles, surface area of the die is large, leading to increased friction at
the die–billet interface. Higher friction results in larger ram force.
 On the other hand, a large die angle causes more turbulence in the metal flow
during reduction, increasing the ram force required. Thus, the effect of die angle
on ram force is a U-shaped function, as shown on Fig (b) below. An optimum
die angle exists, as suggested by our hypothetical plot. The optimum angle
depends on various factors (e.g., work material, billet temperature, and
lubrication) and is therefore difficult to determine for a given extrusion job.
Fig (a) Definition of die angle in direct extrusion; (b) effect of die angle on ram force.
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EXTRUSION PRESSES
 Extrusion presses are either horizontal or vertical, defined by the
orientation of the work axis. Horizontal types are more common. The
presses are usually hydraulically driven, which is especially suited to
semi continuous production of long sections, as in direct extrusion.
 Mechanical drives are often used for cold extrusion of individual parts,
such as in impact extrusion.
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Defects In Extruded Products
 Owing to the considerable deformation associated with extrusion operations, a
number of defects can occur in extruded products. The defects can be classified
into the following categories,
Fig. Some common defects in extrusion: (a) centerburst, (b) piping, and (c) surface cracking.
(a) Centerburst. This defect is an internal crack that develops as a result of tensile
stresses along the centerline of the work part during extrusion. It is an internal defect
that is usually not noticeable by visual observation. Conditions that promote
centerburst are high die angles, low extrusion ratios, and impurities in the work
metal that serve as starting points for crack defects. The difficult aspect of
centerburst is its detection. Other names sometimes used for this defect include
arrowhead fracture, center cracking, and chevron cracking.
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Cont…
(b) Piping. Piping is a defect associated with direct extrusion. it is the formation
of a sink hole in the end of the billet. The use of a dummy block whose diameter
is slightly less than that of the billet helps to avoid piping. Other names given to
this defect include tailpipe and fishtailing.
(c) Surface cracking. This defect results from high workpart temperatures that
cause cracks to develop at the surface. They often occur when extrusion speed is
too high, leading to high strain rates and associated heat generation. Other
factors contributing to surface cracking are high friction and surface chilling of
high temperature billets in hot extrusion.
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Cont…
 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 shown in Figure below.
 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.
• 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 (0.001 in) are possible in wire drawing.
Fig. Drawing of bar, rod, or wire. 16

Cont…
 The corresponding draw force is then the area of the drawn cross
section multiplied by the draw stress:
where F = draw force, N .
 The power required in a drawing operation is the draw force
multiplied by exit velocity of the work.
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Problem on drawing
#1. A spool of wire has a starting diameter of 2.5 mm. It is drawn through
a die with an opening that is to 2.1 mm. The entrance angle of the die is
18. Coefficient of friction at the work–die interface is 0.08. The work
metal has a strength coefficient of 450 MPa and a strain-hardening
coefficient of 0.26. The drawing is performed at room temperature.
Determine (a) area reduction, (b) draw stress, and (c) draw force required
for the operation.
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Forging process
 Forging is a deformation process in which the work is compressed between
two dies, using either impact or gradual pressure to form the part.
 Today, 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.
 Forging is carried out in many different ways. One way to classify the
operations is by working temperature. Most forging operations are performed
hot or warm, owing to the significant 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.
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Cont…
• 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 19.9: (a)
open-die forging, (b) impression-die forging, and (c) flashless forging.
• In open-die forging, the work is compressed between two flat (or almost
flat) dies, thus allowing the metal to flow without constraint in a lateral
direction relative to the die surfaces.
• In impression-die forging, the die surfaces contain a shape or impression
that is imparted to the work during compression, thus constraining metal
flow to a significant degree.
• In flashless forging, the work is completely constrained within the die and
no excess flash is produced
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Cont…
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Three types of forging operation illustrated by cross-sectional sketches: (a) open-die forging, (b) impression-die forging,
and (c) flashless forging.

Cont…
 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 flat
rolling
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.
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CUTTING OPERATIONS
 Cutting of sheet metal is accomplished by a shearing action between
two sharp cutting edges. As the punch begins to push into the work,
plastic deformation occurs in the surfaces of the sheet.
 As the punch moves downward, penetration occurs in which the punch
compresses the sheet and cuts into the metal. This penetration zone is
generally about one-third the thickness of the sheet. As the punch
continues to travel into the work, fracture is initiated in the work at the
two cutting edges.
 If the clearance between the punch and die is correct, the two fracture
lines meet, resulting in a clean separation of the work into two pieces.
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Cont…
 At the top of the cut surface is a region called the rollover. This
corresponds to the depression made by the punch in the work prior to
cutting. It is where initial plastic deformation occurred in the work.
 Just below the rollover is a relatively smooth region called the burnish.
This results from penetration of the punch into the work before fracture
began. Beneath the burnish is the fractured zone, a relatively rough
surface of the cut edge where continued downward movement of the
punch caused fracture of the metal.
 Finally, at the bottom of the edge is a burr, a sharp corner on the edge
caused by elongation of the metal during final separation of the two
pieces. If the clearance between the punch and die is correct, the two
fracture lines meet, resulting in a clean separation of the work into two
pieces.
Fig. Characteristic sheared edges of the work. 27

SHEARING, BLANKING, AND PUNCHING
 The three most important operations in press working that cut metal by the
shearing mechanism just described are 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 press working operations. It is performed on a
machine called a power shears, or squaring shears.
 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 is similar to blanking except that it produces a hole, and the
separated piece is scrap, called the slug. The remaining stock is the desired
part.
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Cont…
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Fig. Shearing operation: (a) side view of the shearing operation; (b) front view of power shears
equipped with inclined upper cutting blade.
Fig. (a) Blanking and (b) punching.

ENGINEERING ANALYSIS OF SHEET-METAL CUTTING
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Cont…
Fig x. Effect of clearance: (a) clearance Too small causes less-than optimal
fracture and excessive forces; and (b) clearance too large causes oversized burr.
Symbols v and F indicate motion and applied force, respectively.
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Punch and die sizes for a round blank of diameter Db are determined as:
 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 diameter = 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 (see below Figure) of 0.25 to 1.5 on each side.
 Cutting force, F in sheet metalworking can be determined by: F =StL
where S = shear strength of the sheet metal, MPa ; t = stock thickness, mm , and L =
length of the cut edge, mm .
Table x. Clearance allowance value for three sheet-metal groups.
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Cont…
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Fig. Die size determines blank size Db; punch size determines hole size Dh.; c = clearance.
 If shear strength is unknown, an alternative way of estimating the cutting
force is to use the tensile strength:
F = 0.7(TS)t L,
where TS = ultimate tensile strength MPa

OTHER SHEET-METAL-CUTTING OPERATIONS
 Cutoff and Parting
 Slotting, Perforating, and Notching
 Trimming, Shaving, and Fine Blanking
Cutoff is a shearing operation in which blanks are separated from a sheet-metal
strip by cutting the opposite sides of the part in sequence.
Parting involves cutting a sheet-metal strip by a punch with two cutting edges
that match the opposite sides of the blank,
Fig (a) Cutoff and (b) parting. 34

Slotting, Perforating, and Notching
 Slotting is the term sometimes used for a punching operation that cuts out an
elongated or rectangular hole,
 Perforating involves the simultaneous punching of a pattern of holes in sheet
metal
 Notching involves cutting out a portion of metal from the side of the sheet or
strip.
 Seminotching removes a portion of metal from the interior of the sheet.
Fig (a) Slotting, (b) perforating, (c) notching and seminotching. Symbol v indicates motion
of strip. 35

Problem
#1. Around disk of 150-mm diameter is to be blanked from a
strip of 3.2-mm, half-hard cold-rolled steel whose shear strength
= 310 MPa. Determine (a) the appropriate punch and die
diameters, and (b) blanking force.
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OPERATIONS PERFORMED WITH METAL TOOLING
Include (1) ironing, (2) coining and embossing, (3) lancing, and (4) twisting.
 If the thickness of this stock is greater than the clearance between the
punch and die, it will be squeezed to the size of the clearance, a process
known as ironing.
Fig Ironing to achieve a more uniform wall thickness in a drawn cup: (1) start of
process; (2) during process. Note thinning and elongation of walls. Symbols v and F
indicate motion and applied force, respectively.
 Coining It is frequently used in sheet-metal work to form indentations and
raised sections in the part.
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Cont…
 Embossing is a forming operation used to create indentations in the sheet,
such as raised (or indented) lettering or strengthening ribs,
Fig Embossing: (a) cross section of punch and die configuration during pressing; (b)
finished part with embossed ribs.
 Lancing is a combined cutting and bending or cutting and forming operation
performed in one step to partially separate the metal from the sheet.
Fig Lancing in several forms: (a) cutting and bending; (b) and (c) two types of cutting
and forming.
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Cont…
 Twisting: subjects the sheet metal to a torsion loading rather than a
bending load, thus causing a twist in the sheet over its length.
 This type of operation has limited applications. It is used to make such
products as fan and propeller blades. It can be performed in a
conventional punch and die which has been designed to deform the
part in the required twist shape.
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Extrusion, Drawing, Forging and Sheetmetal working processes

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