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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.
Zone # 2
MELTING ZONE
The
TABLE OF MELTING ZONE ELEMENTS
CHARACTERISTICS
POTENTIAL USE
ELEMENTS
GEOMETRY &
PROFILES
NOMENCLATURE
SHEAR
UNIFORMITY
DISPERSIVE
MIXING ABILITY
CRYSTALLINE
AMORPHOUS
MELTING ABILITY
Fractional
Kneading
Element
Right Handed
Kneading
Element
Left Handed
Kneading
Element
3KB Kneading
Elements
Neutral
Handed
Kneading
Elements
Highest Highest Highest High All types of Melting
and Dispersive Mixing
Low Medium Low Medium
Crystalline Materia
l
Easy to Melt
Medium High Medium Medium Crystalline Material
High Highest High Medium Amorphous
Medium High High Low
for Melting
Used for Mixing
Not usually recommended
3
Mixing
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.
F E E D I N G
Nearly all twin-screw extruders are starve-fed,
although the degree of fi l l can vary,
depending on the requirements of the process.
M E LT I N G
All extruders melt plastic by heating it, but the
melting process is different in co-rotating and
counter rotating extruders. In a co-rotating
extruder, the plastic is subjected to intense
shearing and deformation in the kneading
Blocks.
Melting (also called fusion) should be nearly, but
not fully, complete before the material leaves
the compression section and enters the vent
section. Insufficient fusion will allow powder to
enter the vacuum system, while complete fusion
will interfere with the removal of gas and vapor
at the vent.
Source: Handbook-TSE by SPE.
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