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JULY-AUGUST 2006
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VOL .01 / ISSUE-04
STEER’s Fractional-lobed Kneading
elements (patented) provide a more
uniform shear action vis-à-vis standard
kneading elements. For high speed
extruders the melt mixing is greatly
enhanced with these elements having
unequal tip width and multi lobed
geometry.
This article is contributed by Dr. Babu Padmanabhan, Chief Executive Officer
and Founder of Steer Engineering. He is a PhD in Mechanical Engineering
from Virginia Polytechnic Institute and State University Blacksburg VA
TABLE OF MELTING ZONE ELEMENTS
POTENTIAL USE
ELEMENTS
NOMENCLATURE GEOMETRY &
PROFILES
AMORPHOUS
MELTING ABILITY
CRYSTALLINE DISPERSIVE
MIXING ABILITY
SHEAR
UNIFORMITY
CHARACTERISTICS
Fractional
Kneading
Element
Highest
Highest
Highest
High
Right Handed
Kneading
Element
Low
Medium Low
Medium
Left Handed
Kneading
Element
Medium High
Medium Medium
3KB Kneading
Elements
High
Highest
High
Medium
Neutral
Handed
Kneading
Elements
Medium High
High
Low
All types of Melting
and Dispersive Mixing
Easy to Melt
Crystalline Material
Crystalline Material
Amorphous
Not usually recommended
for Melting
Used for Mixing
Compaction is the removal of air trapped in the material. For example, knocking or
vibrating a vessel containing a powder compacts the material (reduces the volume) while
stirring does not. Breaking up results in reduction is particle size or changes in shape of
particle.
Melting Zone
It is common to imagine melting as a phase transformation from Solid to Liquid. In case
of polymers, melting is generally associated with reaching the required melt viscosity or
melt temperature. It is really that the crystalline polymers undergo some amount of
melting while amorphous polymers undergo glass transition. The work that can be
carried out depends on the resistance offered to it. As the Polymer melts, the resistance
drops as the viscosity reduces. Strong compression aids in the melting of Crystalline
Polymers like Polyamides and Polyolefins. Frictional heat generation by shearing forces is
sufficient for increasing the temperature of amorphous Polymers like Atactic Polystyrene
and Polycarbonate to the required process temperature which is usually above the Glass
Transition Temperature.
Mixing
3
that accompanies melting is dispersive in nature. This is due to the high viscosity
levels of the melt at the time of melting and as a result of which the high shear stresses
opens an opportunity for elongation and break-up.
This melt mixing is achieved by Kneading Elements by the amount of shearing between
the tip of the kneading block and the barrel. The intensity of shear experienced by the
material will vary depending on the gap it passes through as the Kneading blocks
complete a full rotation. In many cases, a hot zone is created due to intense action at
certain points.
1
Choice of material of construction (MOC) generally has no
effect on the function of the zones. However, wear (or
increased clearances) can have serious effect. It is to avoid
wear that the right material has to be chosen keeping in
mind the ‘specific wear mechanism’.
2
SFE and RSE screws follow the Erdmenger design and are
fully wiping profiles. SKE, DSK and SFV are modified screw
profiles that have incomplete wiping profiles.
3
Mixing at the time of melting has some difficulty. Certain
materials that have the tendency to agglomerate will do so
before it is mixed into the polymer. Therefore, pre-mix
preparations become vital.
(Please note: In the subsequent articles, we shall cover the
other zones (Venting Zone, Mixing Zone, Vacuum (Venting)
Zone and Metering Zone.)