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Extrudate Swell
Whenever a polymer melt emerges from a die,
the diameter or thickness is larger than the
diameter (or gap) of the die. At usual produc-
tion throughputs, diameter or thickness ratios
range from 1.20—1.40 for PVC to 1.50—2.00
for commercial grade polyethylenes and are
much more for some polymers containing a
high-molecular-weight tail. It is an indication
of the polymer elasticity. The more elastic
polymers give larger swell. Pulling the extru-
date reduces the swell, and extrudates can be
drawn down to diameters (or thicknesses)
much smaller than the die diameter or gap.
source: SPE
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ET enables recapitulating few
definitions and technical terms
with respect to extruders and
compounds
MAY-JUNE 2006
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VOL .01 / ISSUE-03
This article is contributed by Dr. Pradeep Bakshi. Pradeep is an Independent
Consultant in the field of Plastics Compounding, based in Vadodara, India.
He is a PhD from Plastics Division, University Department of Chemical
Technology, Mumbai and I. Eng., AMIM from Institute of Materials, UK.
Fellow of the Indian Plastics Institute.
Living with contradictions is, in fact, an inherent part of the art of Compounding. The
very basic case of MIXING, is a process of Compounding two or more different materials.
There are two types of mixing processes namely, Dispersive Mixing and Distributive
Mixing.
Dispersive mixing is the reduction in the size of the agglomerate such as pigments, ideally
to the size of the primary particles and get uniformly distributed.
Distributive mixing is the uniform distribution of the agglomerated pigment particle,
distribution does not reduce the size of the additional component, it increases the
interfacial area between the polymer matrix and the dispersed phase.
Distribution may occur without causing dispersion and dispersion usually causes distribu-
tion. The perfect example of compounding is the preparation of bread dough, used in
everyday life. It has all the essentials of a good compounding process. This process serves
as a model for all the compounding processes.
For that matter, quite a few compounding extruders, very similar to the ones used for
plastics are being used in the food processing industry, too.
The following equations define torque, shear rate and shear stress
Effective channel depth is the actual depth of the material that fills the screw flight. It is
a variable that is dependent upon other parameters like feed, RPM, etc. In Eq (2) this
parameter is the most important in deciding shear rate of the material. Most importantly,
the feed rate and screw RPM are set (and hence torque) to optimum levels. This case is
seen while processing fragile materials like glass fibers.
The
conclusions
drawn from the above equations are as follows:
• Effective channel depth is directly proportional to the feed rate and inversely propor
tional to the screw RPM
• Shear rate is directly proportional to screw RPM and inversely proportional to feed rate
• Shear is inversely proportional to torque
• Torque is directly proportional to effective channel depth
=
3.1415 × Screw Ø × Screw RPM
Effective Channel Depth × 60
Shear Rate (sec
-1
)
EQUATION 2
Shear Stress = Viscosity × Shear Rate
EQUATION 3
FUNDAMENTAL CONCEPTS OF COMPOUNDING
Equation (2) forms the heart of the process.
Substituting Equations (1) and (2) in (3), we get
R E V I E W - 2
The first part of this article “The Art of Compounding Thermoplastics” was carried out in ET2
DISPERSIVE AND DISTRIBUTIVE MIXING
DISPERSIVE
MIXING
DISTRIBUTIVE
MIXING
DISTRIBUTIVE
MIXING
HOMOGENOUS
MIXTURE
"Not everything that counts can be counted,
and not everything that can be counted counts"
- Albert Einstein
Torque ∞ (Power) ÷ (Screw RPM)
EQUATION 1
Shear Stress ∞
Screw Ø × Power × Viscosity
Effective Channel Depth × Torque
EQUATION 4