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Shear action in a Co-rotating Twin-ScrewExtruder
During the early stage of development of the EPZ of the co-rotating twin-screw extruder, an
untested hypothesis was that a uniform gap should be found between the intermeshing
elements as shown in Figure 1. This leads to a low shear rate in the regular screw channels
and a very high shear rate in the intermeshing zone of the screws. The magnitude of these
two shear rates can be expressed as follows.
Mean shear rate = 2
p
a N / 60 h
It may be noted that a/h is a constant for a particular Do/Di ratio. The mean shear rate is
diameter independent. Generally, this value lies between 200 to 500 1/s for screw speeds of
300 to 1200 RPM.
When the flank A crosses behind flank B, the two surfaces move in opposite directions and in
this space, material is trapped between the two surfaces and experiences shear. The
intensity of this shear is provided below
Peak Shear Rate = 2
p
a N / 60 c
Where a is the Center Distance in mm
N is the screw RPM
and c is the distance of separation of the two flank surfaces
-3-
THE EXTRUDER TIMES / ISSUE 10 / AUGUST 2010
Game Changing
Technological Innovations in
MELT COMPOUNDING
Figure 1
Uniform Gap in Axial and Radial Directions
leads to Peak shear
Figure 2
Peak Shear in the intermeshing zone for
conveying screw and kneading blocks