Chapter 16
Bulk Forming Processes
(Part 2)
Extrusion & Drawing
EIN 3390
Manufacturing Processes
Summer A, 2011
16.6 Extrusion


Metal is
compressed and
forced to flow
through a shaped
die to form a
product with a
constant cross
section
A ram advances
from one end of the
die and causes the
metal to flow
plastically through
the die
Pressing ram
Figure 16-25 Direct extrusion schematic showing
the various equipment components. (Courtesy of
Danieli Wean United, Cranberry Township, PA.)
Extrusion

Definition:
◦ Process of forcing a billet through a die above its
elastic limit, taking shape of the opening.

Purpose:
◦ To reduce its cross-section or to produce a
solid or hollow cross section.

Analogy:
a tube”.
“Like squeezing toothpaste out of
Extrusion

Extruded products always have a constant crosssection.

It can be a semi-continuous or a batch process.

Extrusions can be cut into lengths to become
discrete parts like gears, brackets, etc.

A billet can also extruded individually in a chamber,
and produces discrete parts.

Typical products: railings, tubing, structural
shapes, etc.
Typical Extruded Products
Figure 16-26 Typical shapes produced by extrusion. (Left) Aluminum products. (Courtesy of
Aluminum Company of America, Pittsburgh, PA.) (Right) Steel products. (Courtesy of Allegheny
Ludlum Steel Corporation, Pittsburgh, PA.)
Extrusion


Can be performed at elevated temperatures or
room temperatures, depending on material
ductility.
Commonly extruded materials include aluminum,
magnesium (low yield strength materials),
copper, and lead.

Steels and nickel based alloys are far more
difficult to extrude (high yield strength materials).

Lubricants are essential to extrude high
strength alloys to avoid tendency of material to
weld to die walls.
Advantages of Extrusion





Many shapes can be produced that are
not possible with rolling
No draft is required
Amount of reduction in a single step is
only limited by the equipment, not the
material or the design
Dies are relatively inexpensive
Small quantities of a desired shape can
be produced economically
Extrusion Methods
Methods of extrusion:
◦ Hot extrusion is usually done by either the direct or
indirect methods.
◦ Direct extrusion
 Solid ram drives the entire billet to and through a
stationary die
 Must provide additional power to overcome friction
between billet surface and die walls
Extrusion Methods
◦ Indirect extrusion
 A hollow ram pushes the die back through a
stationary, confined billet
 No relative motion and no friction
between billet and die walls.
 Lower forces required, can extrude longer
billets.
 More complex process, more expensive
equipment required.
Extrusion Methods
Figure 16-27 Direct and indirect extrusion. In direct extrusion, the ram and billet both
move and friction between the billet and the chamber opposes forward motion. For
indirect extrusion, the billet is stationary. There is no billet-chamber friction, since
there is no relative motion.
Variables in Extrusion

Die Angle

Extrusion Ratio R (A0 / Af): where A0 and
Af are billet and extruded product areas.

Billet Temperature

Ram Velocity

Type of Lubricant used.
Variables in Extrusion

Extrusion

Parameters defining the extruded
shape:
◦ CCD (Circumscribing Diameter):
 Diameter of the smallest circle into which the
extruded cross section can fit.
◦ Shape Factor = Perimeter / Cross-Area:
 the larger the shape factor, the more complex
the part.
Variables in Direct Extrusion
Fig : Process variables in
direct extrusion. The
die angle, reduction in
cross-section,
extrusion speed, billet
temperature, and
lubrication all affect
the extrusion
pressure.
Fig : Method of determining
the circumscribing-circle
diameter (CCD) of an
extruded cross-section.
Extrusion

Parameters defining the extruded
shape
Example of Shape Factors between circle and square
shapes:
Shape factor of a circle = (p .D)/ (0.25p .D2) = 4/D, and
shape factor of square = 4 a/a2 =4/a .
If areas of the circle and the square are the same: Ac = As ,
then
so
a2 = (p .D2)/4 ,
a = 0.8862D, or D = 1.1284a
shape factor of square = 4/a = 4/(0.8862D) =
= 1.1884 of shape factor of circle
Extrusion

Parameters defining the extruded
shape:
◦ Reduction Ratio R = Ab/Ap
◦ Where, Ab – cross section area of starting billet stock
Ap – across section area of extruded product
For a cylinder-to-cylinder extrusion,
the area of starting cylinder: Ab = (p .Db2)/4 , and
the area of extruded cylinder: Ap = (p .Dp2)/4 .
R = Ab/Ap = (0.25p .Db2)/ (0.25p .Dp2) = (Db/Dp)2
if (Db/Dp) = 4, then R = 16.
Considering the volume of billet and the volume of extruted
product is the same: Vb = Vp, we have AbLb = ApLp , and
Lp/Lp = Ab/Ap = R
Extrusion Practices

Usually billets less than 25’ in length.

CCD ranges from ¼” to 40”.

Typical values for R range between 10 and 100.

Ram speeds up to 100 ft/min, with lower
speeds for the most common extruded alloys.

Dimensional tolerances (+/- 0.01” to +/- 0.1”)
increase with cross section.
Extrusion Force


Factors for determining extrusion force:
billet strength, extrusion ratio, friction
between billet and die surfaces,
temperature, and extrusion speed.
Estimation of Force required:
 F = A0 k ln (A0/Af)
◦ k = extrusion constant
◦ Ao, Af billet and extruded product cross section areas
Extrusion Constant K:
Fig : Extrusion
constant k for
various
metals at
different
temperatures
Example for calculation
Extrusion Force

Given:
Find:
 Assumptions:
 Solution:

F = p (2.5)
2
a 70-30 brass round billet is
extruded at 1,250 deg. F. Billet
diam. = 5”. Extrusion Diam. = 2”.
Required force.
friction is negligible.
Find k from Fig. 15.6 for 70-30
brass : 30,000 psi at 1,250 deg. F.
(30,000) ln [(p (2.5) 2) / (p (1.0) 2)]
= 1.08 x 106 lb = 490 tons.
Forces in Extrusion


Lubrication is
important to
reduce friction
and act as a heat
barrier
Metal flow in
extrusion
◦ Flow can be complex
◦ Surface cracks,
interior cracks and
flow-related cracks
need to be monitored
◦ Process control is
important
Figure 16-28 Diagram of the ram force versus ram
position for both direct and indirect extrusion of the
same product. The area under the curve corresponds
to the amount of work (force x distance) performed.
The difference between the two curves is attributed to
billet-chamber friction.
Lubrication

An acceptable lubricant is expected to
1) reduce friction, and
2) act as a barrier to heat transfer at all
stages of the process.
Metal Flow
Quite complex.
 Impact quality and mechanical properties of
product: must not overlook to prevent defects.
 Extruded products have elongated grain
structure.
◦ Metal at center passes through die w/little
distortion
◦ Metal near surface undergoes considerable
shearing.
◦ Friction between moving billet and stationary
chamber walls impedes surface flow.
◦ Result is deformation pattern

Extrusion of Hollow Shapes

Mandrels may
be used to
produce hollow
shapes or
shapes with
multiple
longitudinal
cavities
Figure 16-30 Two methods of extruding hollow shapes using internal mandrels. In part (a) the
mandrel and ram have independent motions; in part (b) they move as a single unit.
Extrusion Methods

In Hydrostatic Extrusion:
◦ The chamber, which is larger than the billet, is
filled with a fluid.
◦ The fluid is compressed with the ram and
pushes the billet forward.
◦ Benefit: no friction to overcome along sides of
chamber.
Hydrostatic Extrusion
High-pressure fluid
surrounds the
workpiece and applies
the force to execute
extrusion
◦ Billet-chamber
friction is
eliminated
 High efficiency
process
 Temperatures are
limited because the
fluid acts as a heat
sink
 Seals must be
designed to keep the
fluid from leaking

Figure 16-32 Comparison of conventional (left)
and hydrostatic (right) extrusion. Note the
addition of the pressurizing fluid and the O-ring
and miter-ring seals on both the die and ram.
Extrusion can be Hot or Cold

Hot Extrusion
◦ Takes place at elevated temperatures.
◦ Used in metals that have low ductility at room
temperature.
◦ Need to pre-heat dies to prolong die life and
reduce billet cooling.
◦ Hot working tends to develop an oxide film on
the outside of the work unless done in an inert
environment.
◦ Solution:
 place smaller-diameter dummy block ahead of ram before
the billet. A layer of oxidized material is then left in the
chamber, and is later removed and final part is free of
oxides.
Extrusion can be Hot or Cold

Cold Extrusion (also know as Impact
Extrusion)
◦ Designated as cold when combined with other
forging operations.
◦ Slugs having less than 1.5” diam. are sheared /
ends ground; larger slugs are machined.
◦ Punch descends on a blank, which is extruded
backward.
◦ Slug dimensions and material, as well as lubrication
are key variables.
◦ Diameters up to 6” and thin walls can be made.
◦ Collapsible tubes can be made this way (toothpaste
tubes).
Advantages Cold vs. Hot
Extrusion

Cold:
◦ Better mechanical properties due to workhardening.
◦ Good dimensional tolerances & surface finish.
◦ No need to heat billet.
◦ Competitive production rates & costs.

Hot:
◦ Larger variety of materials.
◦ Less forces required.
◦ Better material flow.
Guidelines for Die Design

Avoid sharp corners

Have similarly sized voids if possible.

Have even thickness in walls if possible.

Have even flow.
Defects in Extrusions

Surface Cracking / Tearing
◦ Occurs with high friction or speed.
◦ Can also occur with sticking of billet material on die
land.
◦ Material sticks, pressure increases, product stops
and starts to move again.
◦ This produces circumferential cracks on surface,
similar to a bamboo stem.
Defects in Extrusions

Internal Cracking
◦ Center of extrusion tends to develop cracks of
various shapes.
◦ Center, center-burst, and arrowhead
◦ Center cracking:
◦ Increases with increasing die angle.
◦ Increases with impurities.
◦ Decreases with increasing R and friction.
16.7 Wire, Rod, and Tube Drawing
Reduce the cross section of a material
by pulling it through a die
 Similar to extrusion, but the force is
tensile

Figure 16-36 Cold-drawing smaller tubing
Figure 16-34 Schematic drawing of the rod-or bar- from larger tubing. The die sets the outer
drawing process.
dimension while the stationary mandrel sizes
the inner diameter.
Drawing

Definition
◦ Cross section of a round rod / wire is
reduced by pulling it through a die.


Variables:
◦ Die Angle, Extrusion Ratio R (A0 / Af) ,
Friction between die and workpiece,
drawing speed.
“There is an optimum angle at which the
drawing force is minimum” for a given
diameter reduction and friction parameter.
Drawing

Estimation of Drawing Force required:
F = Yavg Af ln (A0/Af)

Yavg = average true stress of material in
the die gap.

Assumptions: no friction.
Drawing
Work has to be done to overcome friction.
 Force increases with increasing friction.
 Cannot
increase force too much, or
material will reach yield stress.
 Maximum reduction in cross-sectional area
per pass = 63%. In general, the reduction in
area is between - 50%.
 To produce a desired size or shape, multiple
draws may be required through a series of
progressively
smaller
dies.
Intermediate
annealing may also be required to restore
ductility and enable further deformation.

Drawing Die Design
Die angles range from 6 to 15
degrees.
 Two angles are typically present in a
die:

◦ Entering angle
◦ Approach angle
Bearing Surface
diameter.
 Back relief angle

(land):
sets
final
Figure 16-39 Cross section
through a typical carbide
wire-drawing die showing the
characteristic regions of the
contour.
Figure 16-40 Schematic of a multistation synchronized wire-drawing machine. To prevent
accumulation or breakage, it is necessary to ensure that the same volume of material passes through
each station in a given time. The loops around the sheaves between the stations use wire tensions
and feedback electronics to provide the necessary speed control.
Figure 16-39 Cross section
through a typical carbide
wire-drawing die showing the
characteristic regions of the
contour.
Figure 16-40 Schematic of a multistation synchronized wire-drawing machine. To prevent
accumulation or breakage, it is necessary to ensure that the same volume of material passes through
each station in a given time. The loops around the sheaves between the stations use wire tensions
and feedback electronics to provide the necessary speed control.
Defects in Drawing
Center cracking.
 Seams (folds in the material)
 Residual stresses in cold-drawn
products.
 If % reduction is small:

◦ (Compressive at surface / Tensile at Center)

If % reduction is larger, opposite
occurs:
◦ (not desirable- can cause stress corrosion
cracking.)
Tube and Wire Drawing

Tube sinking does
not use a mandrel
◦ Internal diameter
precision is sacrificed
for cost and a floating
plug is used
Figure 16-38 Schematic of wire drawing with a
rotating draw block. The rotating motor on the
draw block provides a continuous pull on the
incoming wire.
Figure 16-37 (Above) Tube drawing with
a floating plug.
16.8 Cold Forming and Impact
Extrusion

Slugs of material are
squeezed into or
extruded from shaped
die cavities to produce
finished parts of
precise shape and size
Figure 16-46 Cold-forming sequence involving
cutoff, squaring, two extrusions, an upset, and a
trimming operation. Also shown are the finished
part and the trimmed scrap. (Courtesy of National
Machinery Co., Tiffin, OH.)
16.8 Cold Forming and Impact
Extrusion

Slugs of material are
squeezed into or
extruded from shaped
die cavities to produce
finished parts of
precise shape and size
Figure 16-46 Cold-forming sequence involving
cutoff, squaring, two extrusions, an upset, and a
trimming operation. Also shown are the finished
part and the trimmed scrap. (Courtesy of National
Machinery Co., Tiffin, OH.)
Impact Extrusion



A metal slug is
positioned in a die
cavity where it is
struck by a single
blow
Metal may flow
forward, backward
or some combination
The punch controls
the inside shape
while the die controls
the exterior shape
Figure 16-43 Backward and forward extrusion
with open and closed dies.
Cold Extrusion
Figure 16-44
(a) Reverse
(b) forward
(c) combined
forms of cold
extrusion.
(Courtesy the
Aluminum
Association,
Arlington, VA.)
Figure 16-45
(Right) Steps in
the forming of a
bolt by cold
extrusion, cold
heading, and
thread rolling.
(Courtesy of
National
Machinery Co.
Tiffin, OH.)
Summary
There are a variety of bulk deformation
processes
 The main processes are rolling, forging,
extrusion, and drawing
 Each has limits and advantages as to its
capabilities
 The correct process depends on the
desired shape, surface finish,
quantity, etc.

HW
(Not required to turn in)
Review questions (page 418):
40, 42, 45, 51
Problems (page 418 – 419):
2: a, b, c, d