Extrusion Molding Methods

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Transcript of Extrusion Molding Methods

  • 3/17/2004Extrusion Molding Methods, Tech 140

    Credit: A. Lopez1

    Extrusion Processes

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

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    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

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    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

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    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 pointFor 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

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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

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    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

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    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

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    Material Flow Around Twin Screws

    Plastic extrusion technology handbook pg57

  • 11

    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

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    Effects of Die Swell on Profile Shapes

    Text pg 172

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    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

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    Extrusion Shapes

    For melt extrusion the extrusion machine will be similar for all cases, the die and application being differentiating facetsThe 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

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    Sheet Extrusion

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

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    T-Shaped Sheet Die

    Plastics extrusion technology handbook

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    Coat Hanger Sheet Die

    Plastics extrusion technology handbook

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    Cut Away Coat Hanger Sheet Die View

    (Shown is a Flex-Lip adjustable preland replacing the restrictor-bar)

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    Film Extrusion

    ASTM has defined film as being equal to or less than 0.25 mm in thicknessFilm 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 balloonClick 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

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    Blow MoldingThe 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

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    Injection Blow Moldingparison

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    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

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    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

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