Page 32 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

1.18 CHAPTER 1

into better contact with barrel wall, thereby promoting better melting. It also compresses
the molten polymer in the screw channels. The root diameter becomes constant again in
the metering zone, but the channel depth is very small. This geometry facilitates pressure
generation and helps maintain the temperature of the polymer melt (i.e., polymers are poor
conductors of thermal energy, and so thin melt layers have more uniform temperatures).
The compression ratio (i.e., ratio of the channel depths in the feed and metering zones) and
length of the transition zone significantly affect the melting in the single-screw extruders.
Typically, extruder screws have length to diameter (L/D) ratios of about 30:1, with each
zone requiring about one-third of the screw length. Barrier screws are used to improve
melting performance while an assortment of mixing elements incorporated into the meter-
ing zone enhance mixing and the melt temperature uniformity of the melt. These include
the addition of mixing pins on the barrel of the screw, ring barriers, and modified designs
that involve very large screw diameters so as to force molten polymer through a small
clearance between the mixing head and the inside of the barrel wall. Two stage-screws
permit devolatilization of polymer melts, thereby eliminating entrapped moisture, air, and
other volatiles from the melt.
Typical extruders have diameters of 25 to 150 mm, but this can vary from 20 to 600
mm (6 to 24 in). They typically operate at 1 to 2 rev/s (60 to 120 rpm) for large extruders
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and 1 to 5 rev/s (60 to 300 rpm) for small extruders. Output varies as a function of pro-
cessing parameters (particularly screw speed and pressure), the thermal and mechanical
properties of the polymer, and the design and geometry of the screw. A 600-mm dia sin-
gle-screw extruder is capable of delivering 29 metric tons of product an hour, whereas the
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smallest 20-mm dia single-screw extruders have a throughput capacity of 5 kg/h. Oper-
ating pressures are typically 1 to 35 MPa (200 to 5000 psi).
Single-screw extruders account for 90 percent of all extruders, with the three types of
twin-screw extruders making up the bulk of the remaining 10 percent. In nonintermeshing
(tangential) extruders, the counter-rotating screws do not interlock with each other and
convey the polymer using drag flow (i.e., like a single-screw extruder). These extruders
permit tight control of heating and shear and so have been used for devolatilization, coag-
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ulation, reactive extrusion, and halogenation of polyolefins. With intermeshing twin-
screw extruders (Fig. 1.15), the flights of one screw fit into the channels of the other, and
polymer is transferred from the channels of one screw to those of the other, thereby pro-
viding positive conveyance of the polymer and increased mixing. In counter-rotating, in-
termeshing twin-screw extruders, some material flows between the screws and the barrel
wall, and the remainder is forced between the two screws. Polymer in co-rotating twin
screws moves in a figure-eight pattern around the two screws, with little material flowing
between the screws. The longer flow path produces longer extruder residence times than
observed with counter-rotating, intermeshing twin-screw extruders but increases the de-
gree of elongational flow and enhances mixing. Intermeshing twin-screw extruders are
typically used in applications where mixing and compounding need to be accomplished,
because the screws’ elements can be rearranged (programmed) to suit a specific applica-
tion. They are highly capable of dispersing small agglomerates such as carbon black and
can be used, for example, to blend the components of duct tape adhesive as well as coat
the finished adhesive onto the tape backing. Counter-rotating, intermeshing twin-screw ex-
truders, which permit tight control of shear and residence time, are also employed for the
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extrusion of PVC pipes and profiles. Although twin-screw extruders have relatively low
pressure-generating capabilities, some materials can be compounded and formed directly
if a gear pump is added to the end of the extruder.
Die designs depend on the product that will be formed. Typically, spiral flow and spi-
der arm dies are used for blown film, tubing, and pipes. Crosshead dies are employed for
tubing and wire coating. Wide dies with tee, coat hanger, and exponential are employed

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