Plastics Extrusion Technology - PDF

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					               Extrusion Processes

    Extrusion of plastics, like injection molding, is a
    relatively simple concept, but the design and
    application of extruders is a complex field




                    Extrusion Molding Methods, Tech 140   1
3/17/2004                     Credit: A. Lopez
 Melt Extrusion, Single Screw Extruders

    The work horse of melt extrusion is the single
    screw extruder
   In injection molding the purpose
   of the screw extruder is simply
   to obtain a melt, a dedicated
   extrusion machine works on the
   same principle but also must
   mix, homogenize and melt the
   material
   Higher back pressures may be
   generated in single screw
   extrusion machines compared to
   injection molding machines and
   the screws may be longer for
   better mixing

                           Extrusion Molding Methods, Tech 140   2
3/17/2004                            Credit: A. Lopez
                     Single Screw Extruder
     (Plastics Extrusion Technology, 2nd ed pg 23)




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                           The Extrusion Screw
          In combination with the barrel, the purpose of the screw is
          to convey solid material to the melt zone, melt, mix and
          pump material to the die in an efficient manner
                 The screw design and length of screw will depend on the polymer
                 being processed as well as the application




Text pg 169




                                  Extrusion Molding Methods, Tech 140         4
     3/17/2004                              Credit: A. Lopez
     Melt Considerations in Extrusion Screws

   Increasing the capacity by “pumping” more material
   through the screw can be done by increasing the length of
   the screw and or flight dept, but this only helps to a point
   For hard to melt materials or to increase melt homogeneity,
   a second or third flight may be added to a screw in order to
   prevent unmelted material from reaching the die




                      Extrusion Molding Methods, Tech 140   5
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  Mixing Considerations in Extrusion Screws


    While melting and “pumping” as much polymer to
    the die as possible is important, mixing material in
    compounding applications is equally important;
    there are many ways of achieving good mixing
            Pins may be placed in strategic placed in the metering
            section of a screw
            Fluted sections at the end of the screw help to both mix
            and melt left over solid polymer




                           Extrusion Molding Methods, Tech 140   6
3/17/2004                            Credit: A. Lopez
   Pin Arrangements and Fluted Sections




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                      Twin Screw Extruders
    Twin screw extruders are used to process heat sensitive
    and shear sensitive materials and may provide intensive
    mixing by kneading
            PVC is the highest volume material processed by twin screw extruders


    There are two types of screw extruders, co-rotating and
    counter-rotating screw extruders
            The screws in co-rotating extruders turn in the same direction
            The screws in counter rotating extruders turn in opposite directions


    For either class of extruder, the flights may or may not be
    intermeshing
            Intermeshing extruders work on a different principle than single screw
            extruders; material is “pumped” by forward action of the root of one screw
            against the flights of the other

                                Extrusion Molding Methods, Tech 140                8
3/17/2004                                 Credit: A. Lopez
            Twin Screw Extruder Melt Flow
    Co-rotating intermeshing extruders are the most common
    twin screw extruder type
            In this case the root of one screw is wiped by the crests of the adjacent
            screw flights
            The material follows a figure 8 patterns
            Residence time is short and shearing is low
            Provides very accurate process control
    Counter rotating intermeshing extruders are the next most
    common type
            Material in this case is moved as a unit volume between the screws
            and barrel
            Spacers, disk rings and other flow interrupters may be used to
            disrupt the characteristic encapsulated flow
            Higher material volumes may be extruded in this configuration
                This type of extruder evolved from pumping machines

                                Extrusion Molding Methods, Tech 140                     9
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          Material Flow Around Twin Screws




Plastic extrusion
technology handbook pg57




                           Extrusion Molding Methods, Tech 140   10
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                         Die Swell
When polymer exits the die of the extruder it will swell,
this action is referred to as die swell
   Die swell occurs because the sudden release of pressure causes the polymer
   chains to relax; remember, polymers are viscoelastic and display time
   dependent stress relaxation
   Go to http://www.pct.edu/prep/ex.htm and click on the die swell animation
   to view a cartoon of the concept
High shear rates (and thus compression at the die) followed by
sudden release of pressure is the main factor responsible for
die swell; shear rate and pressure can be influenced by…
   Increasing the output rate
   Decreasing die orifice dimension
   Reducing the melt temperature
   Increasing molar mass (molecular weight), chain branching
   Decreasing the die L/D ratio



                                                                        11
       Effects of Die Swell on Profile Shapes



Text pg 172




                   Extrusion Molding Methods, Tech 140   12
   3/17/2004                 Credit: A. Lopez
                Post Forming of Profiles
    To avoid the necessity of
    having a complex die orifice for
    complex shapes, post forming
    may be accomplished on basic
    shapes

    This is essentially a
    thermoforming process where
    the still warm extrudate is
    shaped and drawn through a
    series of plates, shoes, rollers or
    dies to form the final shape




                            Extrusion Molding Methods, Tech 140   13
3/17/2004                             Credit: A. Lopez
                          Extrusion Shapes
    For melt extrusion the extrusion machine will be similar
    for all cases, the die and application being differentiating
    facets
    The shape of the extruded section will depend on the die
    orifice shape (and die swell, of course); the type of shape
    produced is divided into several categories
            Sheet (more than 0.25 mm thick)
            Film (less or equal to 0.25 mm thick)
                Blown, film casting, calendering
            Filament
            Blow molding
            Wire insulation


                              Extrusion Molding Methods, Tech 140   14
3/17/2004                               Credit: A. Lopez
                 Sheet Extrusion

    ASTM has defined sheet as being more than 0.25
    mm thick
    Sheet is extruded by cast extrusion, which simply
    refers to sheets being extruded through a
    horizontal die




                   Extrusion Molding Methods, Tech 140   15
3/17/2004                    Credit: A. Lopez
                      T-Shaped Sheet Die


Plastics extrusion
technology handbook




                         Extrusion Molding Methods, Tech 140   16
    3/17/2004                      Credit: A. Lopez
                      Coat Hanger Sheet Die


Plastics extrusion
technology handbook




                          Extrusion Molding Methods, Tech 140   17
    3/17/2004                       Credit: A. Lopez
 Cut Away Coat Hanger Sheet Die View
(Shown is a Flex-Lip® adjustable preland replacing the restrictor-bar)




                         Extrusion Molding Methods, Tech 140             18
3/17/2004                          Credit: A. Lopez
                               Film Extrusion
    ASTM has defined film as being equal to or less than 0.25
    mm in thickness
    Film can be extruded in a similar manner as sheet in a
    process referred to as slot casting
            In this case, the film is drawn from the die onto chilled rollers that quickly
            quench the material
    The blown film process extrudes a tube of material and
    pinches of the end while simultaneously blowing the tube
    into a thin walled “balloon”
    Click on the download button at
    http://www.glad.com.au/student_main.asp to get product
    propaganda but also some interesting info. on how Glad
    Wrap® and garbage bags are made at Glad
                                 Extrusion Molding Methods, Tech 140                  19
3/17/2004                                  Credit: A. Lopez
                              Blow Molding
    The basic principle of blow molding is simple; a hollow molten
    preform or tube is placed in a female mold and inflated to
    conform to the shape of the mold
            Two different varieties of blow molding
              Injection blow molding – is a two step process where a blank
              preform is first molded, it is then transferred to the mold cavity
              where it is blown into shape
                    The advantages of injection molding are good control of
                    wall thickness, no bottom pinch off line, no scrap; the
                    main disadvantage is requiring two molds and transfer
                    time to the mold after performing
              Extrusion blow molding – a molten tube (parison) is
              extruded, without pre-forming, the bottom of the parison is
              pinched of by the mold and air is used to blow it into shape
                    The main disadvantage of this method is the less
                    controllable wall thickness, though some basic control is
                    possible

                              Extrusion Molding Methods, Tech 140           20
3/17/2004                               Credit: A. Lopez
             Injection Blow Molding
   parison




                  Extrusion Molding Methods, Tech 140   21
3/17/2004                   Credit: A. Lopez
             Extrusion Blow Molding
    The wall thickness of
    extruded parisons may be
    controlled by the use of a
    variable orifice die

    This type of die can
    continuously change the
    orifice tube width and is
    typically computer
    controlled



                       Extrusion Molding Methods, Tech 140   22
3/17/2004                        Credit: A. Lopez
                Variations on Blow Molding
    Injection and extrusion blow molding are the basic
    methods by which blow molded products are made, there
    are many variations on these concepts to make the process
    faster, and more efficient
            Merry go round like mold arrays may be used to continuously
            mold without having to waiting for cooling before molding again
            Multilayered containers can be made by co-extrusion
                Used extensively in food storage containers, high strength bags
            Sheet blow molding can be accomplished by heating two separate
            sheets, clamping them together in a mold and blowing to shape
                Different color sides can be produced but high scrap rates are
                prevalent



                              Extrusion Molding Methods, Tech 140          23
3/17/2004                               Credit: A. Lopez
                   Resources and References
    As with injection molding there is a great quantity of literature on
    extrusion and extrusion processes, following are just some references
            Levy and Carley, eds.. Plastics Extrusion Technology Handbook, 2nd Ed.. New
            York, First Printing, 1989.
            Rosato, Domic and Donald, et al, eds.. Plastics Engineering Manufacturing and
            Data Book, Vol. 1. Boston, Kluwer Academic Pub., 2001.
            Birley, Arthur; Haworth, Barry; Batchelor, Jim. Physics of Plastics, Processing,
            Properties and Materials Engineering. New York, Oxford Press, 1992. (Presents
            more of a materials engineering view of processing)
            SPE extrusion division on line at http://www.extdiv4spe.org/
    The Plastics Resources for Educators Program.
    http://www.pct.edu/prep/core.htm - great descriptions and animations of
    processes. Review animations for extrusion and injection.




                                  Extrusion Molding Methods, Tech 140                     24
3/17/2004                                   Credit: A. Lopez

				
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