Page 16 - ET10

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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.
Alignment of the gearbox output shaft with respect to the axial movement under
load condition.
Alignment of the barrels.
Manufacturing capability of the machine to maintain constant clearance between the
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
Answer provided by: Mr. Prasanna Kumar, STEER, India
Answer for ET 9
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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,
Peenya Indl. Area, Bangalore 560 058, India.
Q 1.
The best answer will be published. Readers
to send-in their answers with
appropriate drawings / pictures / illustrations
The side feeders location do play a
crucial role in any co-rotating twin-
The question is what is the ideal
location (as viewed from the die-head
end) andwhy?
How it would affect the intake
A mechanic was removing a cylinder head from
the motor of a Harley motorcycle when he
spotted a well-known heart surgeon in his
The surgeon was there, waiting for the service
manager to come and take a look at his bike.
The mechanic shouted across the garage, "Hey,
Doc, can I ask you a question?"
The surgeon a bit surprised, walked over to the
mechanic working on the motorcycle. The
mechanic straightened up, wiped his hands on
a rag and asked, "So Doc, look at this engine. I
open its heart, take valves out, fix 'em, put 'em
back in, and when I finish, it works just like
new. So how come I get such a small salary and
you get the really big bucks, when you and I are
doing basically the same work?"
The surgeon paused, smiled and leaned over,
and whispered to the mechanic...
"Try doing it with the engine running.”