Metodos De Moldeo Por Extrusion

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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Extrusion Molding Methods, Tech 140 Credit: A. Lopez

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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 extruderis 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 Credit: A. Lopez 2

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Single Screw Extruder(Plastics Extrusion Technology, 2nd ed pg 23)

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Extrusion Molding Methods, Tech 140 Credit: A. Lopez

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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 theapplication

Text pg 169

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Extrusion Molding Methods, Tech 140 Credit: A. Lopez

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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 point For hard to melt materials or to increase melt homogeneity, a second or third flightmay be added to a screw in order to prevent unmelted material from reaching the die

3/17/2004

Extrusion Molding Methods, Tech 140 Credit: A. Lopez

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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 mixingPins 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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Extrusion Molding Methods, Tech 140 Credit: A. Lopez

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Pin Arrangements and Fluted Sections

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Extrusion Molding Methods, Tech 140 Credit: A. Lopez

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Twin Screw Extruders
Twin screw extrudersare 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 oppositedirections

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 Credit: A. Lopez 8

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Twin Screw Extruder Melt Flow
Co-rotating intermeshing extrudersare 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 screwsand 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

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