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SCS
STEER CONSULTANCY SERVICES
Reader’s Queries on Extruders, Elements,
Polymer-Compounding etc are answered
by ET’s Expert Group
Q1. Does clearance between the elements affect the processing capability?
Q 2. What should be the ideal clearance between the elements and why ?
The clearance between the screws and the barrels mainly depends up on following aspects in
order to achieve lowest clearance.
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Alignment of the gearbox output shaft with respect to the axial movement under
load condition.
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Alignment of the barrels.
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Manufacturing capability of the machine to maintain constant clearance between the
screws.
If the above said conditions are satisfied, it is possible to maintain close clearance between
the screws and the barrel. Current typical clearance in an extruder is around 0.5 mm
clearance between screws and 0.15 mm clearance between the screws and barrel.
Yes, clearance between the screws and screw to barrel clearance play a vital role in improving
processing condition inside the extruder. Lower the clearances, better the self wiping action
and it also improves dispersion and homogenous mixing.
There are several elements geometry used in twin screw extruder and the best known
geometry is the Erdmenger profile. Even today screw elements in use are based on
traditional, self-cleaning Erdmenger profiles.
Some of the basic profiles in use are as follows:
Eccentric screw profile: In this design, the screws profile is arranged eccentrically on
the shaft. These profiles consume less energy because of unequal clearance between
the screws and the barrel.
Transition elements: These profiles are very difficult to make as the screw profiles alter
along the axis from one number of flights to another and maintain complete self
cleaning action.
Non self cleaning profiles: These include screw elements that are used for melting &
mixing. Kneading discs benefit from reduced energy input and product stress.
Fractional screw profile: These screw profiles are developed by Dr. Babu Padmanabhan.
These elements have multi-lobe geometry that gives greater flexibility for processes
such as melting, mixing, dispersing, alloying, blending & kneading. This geometry
maintains constant minimum clearance between each other when rotating in the same
direction. The exact nature of the geometry involved is truly complex compared to
simple known geometry in twin-screw extruders. These elements have perfect
transition at each cross section of the element, thus aids in maintaining self wiping
action.
Answer provided by: Mr. Prasanna Kumar, STEER, India
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Answer for ET 9
QUESTION YOU
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Printed and Published by: Mr. K.S. Ram, Director and Principal Consultant SCS, on behalf of STEER CONSULTANCY SERVICES, #298/299, 5th Main, 4th Phase,
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Q 1.
?
The best answer will be published. Readers
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to
The side feeders location do play a
crucial role in any co-rotating twin-
screwextruder.
The question is what is the ideal
location (as viewed from the die-head
end) andwhy?
How it would affect the intake
efficiency
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