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Figure 2: Generation of Fractional Geometry
M
F
F
T
T
M
Figure 1: Fractional Four-lobed Element Geom-
etry 1.4.50
continued...
The end profile of a fractional element formed as a result of transforming a one-lobe to
a four-lobe profile as shown in Figure 1. This shows the profile at a location that is
halfway through the transformation. At every step of the transformation the profile
obtained has to be intermeshing and fully wiping. Figure 2 clearly shows the basis for the
classification. The last two digits provides the location of the transformation from one-
lobe to two-lobe profile. If ‘n’ is the number of lobes on one end with less lobes and ‘N’
is the number of lobes on the other end with more lobes, then the condition for perfect
transformation is that ‘N/n’ should be a whole number. Figure 3 shows the configura-
tion of the extruder screw where the regular kneading blocks are formed with fractional
lobed geometry. The screw speed and configuration are the two key variables. The zone
temperatures are shown on the barrel segments. The kneading length in the mixing zone
of both the configuration are maintained at 180mm and also increase in pressure
matches both configuration. The fractional lobed geometry shown in this configuration
is selected based on certain observations made on the changes in cross-section during
rotation of the corresponding geometry. The motivation was to avoid high pressure
points during the full 360 degree turn of the flights.
Processing on various materials with fractional-lobed elements have already been carried
out. This involved processing of polymers with a high filling of carbon black, organic
pigments, glass fibre, etc. Fractional elements were also used on PVC compounding.
Using fractional elements, it is possible to finely control the compounding process to a
greater extent than what is considered possible with standard elements.
The
EXTRUDER
TIMES
FRACTIONAL ELEMENT GEOMETRY
IN
HIGH-SPEED DEEP-FLIGHTED TWIN-SCREW EXTRUDER
'Elements having different tip angles
continue to work as conjugate pairs,
while suiting the needs of twin-screw
extruder'
'Using fractional elements, it is possible
to finely control the compounding
process to a great extent'
Figure 3: Configuration with Fractional Kneading Blocks
This article is contributed by Dr. Babu Padmanabhan, Managing Director and
Founder of Steer Engineering. He is a PhD in Mechanical Engineering from
Virginia Polytechnic Institute and State University Blacksburg VA