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Two important design parameters are associated with this zone are: Different
compounds or operating pressures require different compression ratios & The length
of the compression zone affects the rate of compression. These two parameters will
be different for different compounds.
3. The metering zone has a constant channel depth (less than feed zone) and
primarily exists to further mix molten resin. The end result is a smooth consistent melt
with uniform temperature.
4. In some processes, a de-gassing or devolatizing section is required. This is a
shorter zone that immediately follows the compression zone. Channel depth is
suddenly increased, and the resulting pressure drop causes a release of any gas,
which can be vented or drawn off via vacuum pump. The remaining melt is re-
compressed and metered.
The mechanical friction in compression zone produces about 85% of the required
heat, so it is critical that the drive equipment have sufficient Horse power capabilities
to overcome friction and turn the feed screw at a steady and controlled rate. Some
extruders can continue to plasticate materials long after their external heat sources
are shut down.
BARREL:
The extruder barrels are smooth bore pipe that houses the feed screws. The barrel is
wrapped with heater bands over it to provide uniform heat distribution and the feed
screw rotates within. Barrels are manufactured out of extremely hard-wearing and
corrosion-resistant materials which are often bimetallic and lined
internally with material to increase wear and corrosion
resistance. Minimum 25 mm thickness is preferred to
ensure that the barrel is able to withstand pressures
generated in the extrusion process. The barrel cooling is
achieved by fans or blowers over it. The feed throat section of the
barrel is equipped with water cooling channels so that feed throat is cool
enough to prevent premature pellet melting and bridging. Small clearances
are essential between screw flights and the barrel wall for preventing back flow of
molten resin and a possible surge in the output of the extruder.
How do we describe a Screw?
The screws are generally described by their L/D ratio and Compression ratio

Example: the L/D ratio of a screw is the flighted length of screw divided by the outside
screw diameter, so for example, on a 40mm screw, if the flighted length was 640mm
long then the screw L/D would have been a 16:1 L/D screw.
The compression ratio is measured as the channel depth at feed zone divided by the
channel depth in the metering zone. C/R is used to select the proper feed depth of the
screw, but it significantly impacts the melting rate as well..

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