Chapter 16 Bulk Forming Processes (Part 2: Extrusion

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Chapter 16Bulk Forming Processes(Part 2
Extrusion Drawing)(Review)EIN 3390
Manufacturing ProcessesSpring, 2011
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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

Figure 16-25 Direct extrusion schematic showing
the various equipment components. (Courtesy of
Danieli Wean United, Cranberry Township, PA.)
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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 Like squeezing toothpaste out of a
  tube.

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Extrusion

• Extruded products always have a constant
  cross-section.
• 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.

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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.

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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

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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

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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.

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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.
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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.

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Variables in Extrusion

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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.

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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 a2 (p .D2)/4 ,
• so a 0.8862D, or D 1.1284a
• shape factor of square 4/a 4/(0.8862D)
  1.1884 of shape factor of circle

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Extrusion

• Parameters defining the extruded shape
• Reduction Ratio R Ab/Ap
• Where, Ab cross section area of starting 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

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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.

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Extrusion Force

• Factors for determining extrusion force
• billet strength, extrusion ratio, friction
  between billet and die surfaces, temperature,
  extrusion speed).
• Estimation of Force required
• F A0 k ln (A0/Af)
• k extrusion constant
• Ao, Af billet and extruded product cross section
  areas

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• Extrusion Constant K

Fig Extrusion constant k for various metals at
different temperatures
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Example for calculation Extrusion Force

• Given a 70-30 brass round billet is extruded at
  1250 deg. F. Billet diam. 5. Extrusion
  Diam. 2.
• Find Required force.
• Assumptions friction is negligible.
• Solution Find k from Fig. 15.6 for 70-30
  brass 30,000 psi at 1250 deg. F.
• F p (2.5) 2 (30,000) ln (p (2.5) 2) / (p
  (1.0) 2)
• 1.08 x 106 lb 490 tons.

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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.
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Lubrication

• Essential in drawing to improve die life, reduce
  drawing forces/temperature, improve surface
  finish, particularly in hot extrusion.
• Difficult to maintain a constant lubricant film
  constant between the die and the workpiece.
• Can coat with zirconia to add life to die.

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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

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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.

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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.
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Extrusion Methods

• Conform process
• Continuous feedstock is fed into a grooved wheel
  and is driven by surface friction into a chamber
  created by a mating die segment
• The material upsets to conform to the chamber
• Feedstock can be solid, metal powder, punchouts,
  or chips
• Metallic and nonmetallic powders can be
  intimately mixed

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Continuous Extrusion
Figure 16-33 Cross-sectional schematic of the
Conform continuous extrusion process. The
material upsets at the abutment and extrudes.
Section x-x shows the material in the shoe.
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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.

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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).

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Advantages Cold vs. Hot Extrusion

• Cold
• Better mechanical properties due to
  work-hardening.
• 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.

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Guidelines for Die Design

• Avoid sharp corners
• Have similarly sized voids if possible.
• Have even thickness in walls if possible.
• General idea is to favor even flow.

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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. (referred to as
  bambooing).

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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 from
larger tubing. The die sets the outer dimension
while the stationary mandrel sizes the inner
diameter.
Figure 16-34 Schematic drawing of the rod-or
bar-drawing process.
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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.

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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.

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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.

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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 (land) sets final diameter of
  the drawn stock.
• Back relief angle exit zone

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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.
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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.)

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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 16-37 (Above) Tube drawing with a floating
plug.
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.
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16.8 Cold Forming, Cold Forging, and Impact
Extrusion

• Slugs of material are squeezed into or extruded
  from shaped die cavities to produce finished
  parts of precise shape and size
• Cold heading is a form of upset forging
• Used to make the enlarged sections on the ends of
  rod or wire (i.e. heads of nails, bolts, etc.)

Figure 16-41 Typical steps in a shearing and
cold-heading operation.
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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.
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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.)
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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.)
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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.