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					4. Extrusions
 Brief introduction
    Compression forming process in which the work
     metal is forced to flow through a die opening to
     produce a desired cross-sectional shape
    Process is similar to squeezing toothpaste out of a
     toothpaste tube
    In general, extrusion is used to produce long parts of
     uniform cross-sections
    Can be hot or cold
       Hot – steels
       Cold –soft metals
Extrusions, and examples of
products made by sectioning
off extrusions.
 Extrusion Process
   Direct –
   Indirect –
   Hydrostatic – billet is smaller in diameter than
    chamber and fluid pressure forces billet by a ram
   Side (lateral) – extrusion forced at right angle to
    billet
Schematic illustration of the direct extrusion process.
Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral.
 Metal Flow in Extrusion
   Elongated grain structure
   Can test with grid test
Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low
friction, or in indirect extrusion. (b) Pattern obtained with high friction at the billet-
chamber interfaces. (c) Pattern obtained at high friction, or with cooling of the
outer regions of the billet in the chamber. This type of pattern, observed in metals
whose strength increases rapidly with decreasing temperature, leads to a defect
known as pipe, or extrusion defect.
• Extrusion-Die Configurations
(a)                      (c)




(b)




                               Typical          extrusion-die
                               configurations: (a) die for
                               nonferrous metals; (b) die for
                               ferrous metals; (c) die for T-
                               shaped extrusion
• Hot Extrusion

   Dies can be preheated to reduce wear and to reduce
    cooling of the billet
   Oxide film can form so a dummy block smaller in
    diameter is placed ahead of ram
   Square dies used for non-ferrous metals
   Work piece has high plasticity and low deformation
    resistance
   Used for parts with big sizes and high strength
 Cold Extrusion
   Improved mechanical properties by work hardening
   Good dimensionals
   Improved surface finish
   Elimination of billet heating
   Competitive production rates
   High stresses on tooling
Two examples of cold extrusion. Thin arrows
indicate the direction of metal flow during
extrusion.
Production steps for a cold extruded spark plug.




            A cross-section of the metal part above,
            showing the grain flow pattern.
(a) An extruded 6063-T6 aluminum ladder lock for aluminum extension ladders.
This part is 8 mm (5/16 in.) thick and is sawed from the extrusion. (b)-(d)
Components of various dies for extruding intricate hollow shapes.
Poor and good examples of
cross-sections    to     be
extruded.       Note the
importance of eliminating
sharp corners and of
keeping section thicknesses
uniform.
 Impact Extrusion
   Punch descends rapidly on the blank and extruded
    backward
   Collapsible tubes
   Thin walled parts
• Impact Extrusion

                     Schematic illustration
                     of     the    impact-
                     extrusion    process.
                     The extruded parts
                     are stripped by the
                     use of a stripper
                     plate, because they
                     tend to stick to the
                     punch.
(a) Two examples of products made by impact extrusion. These parts may also be
made by casting, by forging, or by machining; the choice of process depends on the
dimensions and the materials involved and on the properties desired. Economic
considerations are also important in final process selection. (b) and (c) Impact
extrusion of a collapsible tube by the Hooker process.
 Hydrostatic Extrusion
   No container wall friction
   High pressure
   Defects can be reduced by fluid-to-fluid extrusion
   Vegetable oils used at room temperature
   Waxes, polymers, glass used for high temp
   Limited use due to complex tooling, high pressures,
    special equipment, and long cycle times
 Extrusion Defects
   Surface cracking
      Temperature, friction, or speed too high
      Sticking along die land
   Pipe
      Impurities drawn toward center of billet
      Can be minimized by controlling flow and by eliminating
       impurities
   Internal cracking
       Tensile stresses
       Increases with increased impurities
       Decreases with increased extrusion ratio and friction
                    (a)




(a) Chevron cracking (central burst) in extruded round steel bars. Unless the products are
inspected, such internal defects may remain undetected, and later cause failure of the part
in service. This defect can also develop in the drawing of rod, of wire, and of tubes.
 Extrusion Equipment
   Horizontal hydraulic press
      Can control stroke and speed
   Vertical can be used for cold extrusion
General view of a
9-MN (1000-ton)
hydraulic-
extrusion press.
5. Drawing

 Brief introduction
   Cross-section of wire or rod reduced by pulling through
    die
   Limit is 63% reduction in cross section
   Shapes such as flat strips can be drawn
Process variables in wire drawing. The die angle, the reduction in
cross-sectional area per pass, the speed of drawing, the temperature,
and the lubrication all affect the drawing force, F.
Examples of tube-
drawing operations,
with and without an
internal mandrel.
Note that a variety
of diameters and
wall     thicknesses
can be produced
from the same
initial tube stock
(which has been
made by other
processes).
 Drawing Practice
   Speeds of 200-500 ft/min for heavy to 10,00 ft/min for
    thin
   Reduction of 0-45%
   Bundling – drawing many wires simultaneously
   Material is usually tool steel or carbide
   Wet drawing – completely immersed in lube
   Dry drawing – surface of rod coated
• Die for Round Drawing


                          Terminology of a
                          typical die used for
                          drawing round rod or
                          wire.
Schematic illustration of roll straightening of a drawn round rod
Cold drawing of an extruded channel on a draw bench, to reduce its cross-section.
Individual lengths of straight rod or of cross-sections are drawn by this method.
Two views of a
multistage      wire-
drawing machine
that is typically
used in the making
of copper wire for
electrical wiring
Questions
1. What are some of the attractive features
   of the cold extrusion process?
2. What is the unique capabilities and
   special limitations of hydrostatic
   extrusion?
6. Sheet metalworking
 Brief introduction
  Cold-working
  The starting stock is sheet metal
  Higher deformation forces
  Closer dimensional tolerances
  Increased strength
  High production rate
  High die cost for small quantities
 Sheet metal process
  Shearing
  Bending
  Drawing
  Sheet metal forming
 Shearing
  Mechanical cutting of materials without the
   formation of chips or the use of burning or
   melting.
  Shearing process
  Sheared edges
     Burr
     Break
     Burnished land
     Roll over
Sheared edge improvement
   Clamping the starting stock firmly against the die
   Maintaining proper clearance and alignment
    between the punch and the die
   Restraining the movement of the workpiece
Simple shearing
   Sheets of metal are sheared along a straight line
   Shear angle is often used to reduce shear force on
    larger shear
Piercing and blanking
   Shearing operations where the shear blades are
    closed
   In piercing, the punch-out piece is the scrap and
    the remaining strip is the workpiece.
   In blanking, the piece being punched out becomes
    the workpiece and the remaining strip is the scrap.
Die design
   The punch should fit within the die with a clearance
    between 5% to 7% of the stock thickness.
   Punches and dies should be in proper alignment
    so that a uniform clearance is maintained.
 Bending
  The plastic deformation of metals about a
   linear axis with little or no change in the
   surface area.
  Bending process
      The metal on the outside is stretched while that on
       the inside is compressed.
      The location that is neither stretched nor
       compressed is neutral axis.
      The outside surface undertakes the maximum
       tensile stress
Bend radius
   The smaller the bend radius, the larger the
    deformation
   Should be as large as possible to permit easier
    forming and wider variety of engineering materials
Springback
   Elastic recovery occurs when the bending load is
    removed
   The yield strength of the metal and the bending
    radius are involved in the amount of springback
   The metal must be overbent by an amount equal to
    the subsequent springback
Design of bending
   The smallest bend radius
      The metal is formed without cracking
      The value is dependent on both the ductility and the
        thickness of the metal sheet
    The process must provide springback
     compensation when the bend radius is greater
     than four times of the material thickness
    The bend axis should perpendicular to the
     direction of grain flow
 Drawing (sheet metal drawing)
  The forming of closed-bottom cylindrical or
   rectangular containers from metal sheet
  Drawing process
  Shallow drawing and deep drawing
     Shallow drawing   The depth of the product is less
      than its diameter
     Deep drawing       The depth of the product is
      greater than the diameter
Drawing defects
   Wrinkle and tear
   Can be limited by multiple operations when deep
    drawing parts are made
Trimming of drawn parts
   Establish both the size and uniformity of the final
    part
 Sheet metal forming
  Embossing
     A pressworking process in which raised lettering or
      other designs are impressed in sheet material
     Very shallow drawing operation where the depth of
      the draw is limited to one to three times the
      thickness of the metal
  Bulging
     Locally expand a metal blank or tube outward
      against a split die
  Flanging
     Flanges are rolled on sheet metal
 Superplastic sheet forming
  Metals achieve tensile elongations as high as
   2000 to 3000%
  Ultrafine-grain-size, low strain rates and
   elevated temperatures are required
  Can be used to form material into large,
   complex-shaped products with compound
   curves by single operation
  Can be extrusion, drawing, forging, etc.
  The major limitation is the long cycle times
Questions
1. What is springback? What determines
     the amount of springback in bending?
2.   What factors determine the smallest
     bend radius for materials?
3.   What measures can be employed to
     improve the quality of a sheared edge?
4.   What is the purpose of having a shear
     angle on a punch?
5.   What is the major limitation of the
     superplastic forming of sheet metal?
     What are some of the attractive features?

				
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