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Chapter 16 Bulk Forming Processes (Part 2) Extrusion

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Title: Chapter 16 Bulk Forming Processes (Part 2) Extrusion


1
Chapter 16Bulk Forming Processes(Part
2)Extrusion DrawingEIN 3390 Manufacturing
ProcessesFall 2011
2
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.)
3
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.)
4
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.

5
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.

6
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.)
7
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 metal-to-metal contact through
    the process.

8
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

9
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

10
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.

11
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.
12
Variables in Extrusion
  1. Die Angle
  2. Extrusion Ratio R (Ab / Af) where Ab and Af are
    billet and extruded product cross sections
    areas.
  3. Billet Temperature
  4. Ram Velocity
  5. Type of Lubricant used.

13
Variables in Extrusion

14
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.

15
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.
16
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

17
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

18
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.

19
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 Ab k ln (Ab/Af)
  • k extrusion constant
  • Ab, Af billet and extruded product cross section
    areas

20
  • Extrusion Constant K

Fig Extrusion constant k for various metals at
different temperatures
21
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.

22
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.
23
Lubrication
  • Essential in extrusion to improve die life,
    reduce extrusion forces/temperature, improve
    surface finish, particularly in hot extrusion.
  • An acceptable lubricant is expected to reduce
    friction and act as a barrier to heat transfer at
    all stages of the process.

24
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 form direct
    extrusion.
  • Result is deformation pattern

25
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.
26
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.

27
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.
28
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

29
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.
30
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.

31
Extrusion can be Hot or Cold
  • Cold Extrusion (also know as Impact Extrusion)
  • Designated as cold when combined with other
    forging operations.
  • 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).

32
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.

33
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.

34
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).

35
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.

36
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.
37
Drawing
  • Definition
  • Cross section of a round rod / wire is reduced by
    pulling it through a die.
  • Variables
  • Die Angle, Area Deduction Ratio R (Ab / 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.

38
Drawing
  • Estimation of Drawing Force required
  • F Yavg Af ln (A0/Af)
  • Yavg average true stress of material in the die
    gap.
  • Assumptions for the formular no friction.

39
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.

40
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.
  • Back relief angle

41
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.
42
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.
43
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.)

44
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.
45
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.
46
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.
47
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.)
48
Figure 16-47 Typical parts made by upsetting and
related operations. (Courtesy of National
Machinery Co., Tiffin, OH.)
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.)
49
Figure 16-47 Typical parts made by upsetting and
related operations. (Courtesy of National
Machinery Co., Tiffin, OH.)
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.)
50
16.9 Piercing
  • Thick-walled seamless tubing can be made by
    rotary piercing
  • Heated billet is fed into the gap between two
    large, convex-tapered rolls
  • Forces the billet to deform into a rotating
    ellipse

Figure 16-50 (Left) Principle of the Mannesmann
process of producing seamless tubing. (Courtesy
of American Brass Company, Cleveland, OH.)
(Right) Mechanism of crack formation in the
Mannesmann process.
51
16.10 Other Squeezing Processes
  • Roll extrusion- thin walled cylinders are
    produced from thicker-wall cylinders
  • Sizing-involves squeezing all or select regions
    of products to achieve a thickness or enhance
    dimensional precision
  • Riveting- permanently joins sheets or plates of
    material by forming an expanded head on the shank
    end of a fastener
  • Staking-permanently joins parts together when a
    segment of one part protrudes through a hole in
    the other

52
Other Squeezing Processes
Figure 16-51 The roll-extrusion process (a) with
internal rollers expanding the inner diameter
(b) with external rollers reducing the outer
diameter.
Figure 16-52 Joining components by riveting.
Figure 16-54 Permanently attaching a shaft to a
plate by staking.
53
Other Squeezing Operations
  • Coining- cold squeezing of metal while all of the
    surfaces are confined within a set of dies
  • Hubbing- plastically forms recessed cavities in a
    workpiece

Figure 16-55 The coining process.
Figure 16-56 Hubbing a die block in a hydraulic
press. Inset shows close-up of the hardened hub
and the impression in the die block. The die
block is contained in a reinforcing ring. The
upper surface of the die block is then machined
flat to remove the bulged metal.
54
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.

55
Homework
Review questions (page 418) 40, 42, 45,
48 Problems (page 418 419) 2 a, b, c, d
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