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As a result of this, peak shear values are experienced by less than 2% of the entire filled
volume during one revolution, at a shear rate exceeding 1000 1/s at even moderate speeds
of 300 RPM in the intermeshing zone. Typically, the material stays in the extruder for about
500 to 1000 turns of the screw. During this period, generally, all the material may have
passed through this space and experienced the shear in fully filled zones. Certain conditions
have to be met, for example a pressure of 10 Mpa or higher (however this article will not go
into that level of detail).
It may be noted from comparing the equations that to obtain the mean shear value and the
peak shear value, the difference in magnitude is of the order of the flight-depth (h in mm)
itself. This is a non-scalable effect that has been a challenge to face since experiments
conducted in smaller extruders could not be repeated in larger extruders. The peak shear
rate grows geometrically with diameter of the extruder and in large extruders (90mm) is
16 to 30 times (depending on c value) that of the mean shear value.
When the screw to screw gap is reduced considerably as shown in the figure above, it is
possible to eliminate, to a significant extent, the ill effects of this peak shear. This allows
extruders to be operated at high speeds of rotation which leads to an increase in the mean
shear rates.
Reduction of the number of turns required for mixing by the use of Fractional
Kneading Blocks andMixing Elements
Apart from the reduction in the effect of peak shear rates, the more significant attention has
to be paid to shear not in the intermeshing zone but the shear in the transverse direction of
the extruder. In a 1963 patent, Erdmenger said that, “One disadvantage that was hitherto
encountered in apparatuses of this type (twin-screw extruders) was that it was only possible
to vary the dimension lying in the axial direction but not the dimension lying transversely to
the axis, e.g. the thickness of the layer of material used, which often has an important effect
on the transfer of heat or the transfer of material or the course of the reaction, which in
practice is the most important alteration.” Fractional lobe technology allows altering the
thickness of the lobes in the transverse direction while maintaining the conjugal
requirements of the twin-screw extruder design.
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Figure 3
Tighter Gap in Axial Directions leads to
elimination of Peak shear
ME LT I NG
The melting step is a critical step in any
compounding process that involves
polymers. Melting occurs when a solid
polymer turns fluid as its temperature
increases. Other industrially relevant
terms used for this step are massing and
fluxing.
Enough energy has to be imparted to the
solid feed to raise its temperature to a
point where it becomes fluid. Energy
required for melting comes from the
mechanical energy from the kneader’s
drive motor, and heat energy from metal
surfaces such as barrels and screws.
Courtesy: Society of Plastic Engineers