Basic HTML Version
The
EXTRUDER
TIMES
JANUARY-FEBRUARY 2006
VOL.01 / ISSUE-01
I N T H I S I S S U E :
Review:
Fractional Element Geometry
Catalyst:
Conveying Elements
ReCap:
Multiflow Index
Brainstorm:
Drastic Temperature Decrease
FRACTIONAL ELEMENT GEOMETRY
IN
HIGH-SPEED DEEP-FLIGHTED TWIN-SCREW EXTRUDER
R E V I E W
Co-rotating twin-screw extruders have evolved through the years. Deeper flights lead to
increased efficiencies and reduced shear rates in a twin-screw extruder. This feature is
advantageous in increasing the screw speed thereby maintaining mixing rates and
increasing the volumetric capacity of the machine. An improvement in process efficiency
is realized due to the reduction in viscous dissipation per unit mass of material. The
ability to process material has benefited from improvements made in diameter ratio and
specific torque. The advantages with deeper screw flights are higher intake capacity
(especially in starve feed), lower melt temperature due to decreased shear stress, and
greater devolatisation capacity. Further, the ability to process certain shear and tempera-
ture sensitive materials is greatly enhanced in an extruder with deeper flights. The
increase in melt temperature due to the kneading elements at high speeds is solved by
fractional lobed element geometry with unequal tip angles. This geometry can easily
replace standard kneading elements.
Forty years ago, Erdmenger determined the need for fractional elements. Erdmenger
said, “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”.
The standard twin-screw elements have equal tip angles for all lobes. These limit the
types of elements that operate in a twin-screw extruder if the geometry is based solely
on the design of Erdmenger.
For example, in an eccentric trilobed kneading element, the small tip angle leads to
higher wear rate. Increasing the tip-angle gives a circular shape to the element and
reduces the free volume available in the extruder. Elements having different tip-angles
have been invented by Dr. Babu Padmanabhan. These elements continue to work as
conjugate pairs while suiting the needs of twin-screw extrusion. The new element
geometry provides greater flexibility in design of individual elements.
Continued...
A special initiative of
CONSULTANC Y SERV I CES , I ND I A
The Generation Next Co-rotating
Twin-screw Extruder
This issue of ET is sponsored by STEER's
• To share breakthroughs and happenings in the
Extruder Industry with special emphasis on
Twin/Single Screw Extruders.
• To serve as an information sharing platform for
STUDENTS, ENGINEERS and COMPOUNDERS.
• To generate triggers and encourage contributions
from scientists, technocrats and teaching
community for new innovations.
A IMS AND OB JECT IVES