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The difference between normal extrusion and compounding extrusion is that in the
latter, there is special emphasis on Mixing whereas in the former, emphasis is on Melting
and Metering. (See figure 1)
In “normal” extrusion, either the mixing section is absent or if present, it is quite
insignificant. This also suggests that compounding extruders, by virtue of the extra
mixing section, tend to have a higher L/D as compared to normal extruders.
The 5-Step Process
In practice, there is much more to compounding than just the three ‘M’ processes.
Actually, it is at least a five-step process, namely,
Feeding, Melting, Mixing, Venting and
Metering.
More often than not, it becomes a six-step process too, when there is an
additional feeding operation - downstream of the melt section, especially when using
glass fibers. The importance of “process” occurring in Feeding or Venting is a subject for
debate – one just feeds the stuff at the feeding port or applies vacuum at the vent port.
In reality, things are not so simple – indeed, the entire process of compounding seems to
be deceptively simple.
Modern Twin-screw Extruders are STARVE-FED devices. Which means that you may
NEVER fill-up the feed hopper and control the output just by setting the Screw RPM.
When run with full hoppers, a modern co-rotating, parallel, fully intermeshing twin-
screw extruder will surely be overloaded (few exceptions, though). In practice, these
extruders run with their flights 100% full only in specified areas of the extruder. Theo-
retically, “higher the percentage of filling of flights, better it is for the material that is
being compounded”.
PS: Why do these contradictions exist? How to live with them? The author would elaborate them
in the next issues of Extruder Times.
The
EXTRUDER
TIMES
Modern Twin-screw Extruders are
STARVE-FED devices. Practically, these
extruders are rarely (if ever) run with
their flights 100% full
Figure 1: The THREE ‘M’ s of Compounding; Melt, Mix, Meter
This article is contributed by Dr. Pradeep Bakshi. Pradeep is an Indepen-
dent 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. Visiting Professor in
UDCT, Mumbai.
THE
‘ART OF COMPOUNDING THERMOPLASTICS’
USING TWIN-SCREW EXTRUDERS
The word “extrusion” is derived from the Latin words
ex
and
trudere
, which mean, respectively, “out” and
“to thrust.” In an extrusion process, the polymer is
melted and then pumped through a die to form a
product of the desired shape. Plastic film, pipe, rod,
fiber, profiles, coated wire and coated paper are a
few examples of commercially important products
Applications of twin-screw extruders in plastics were
started in the 1930s and 1940s by Colombo,
Pasquetti, Meskat, Erdmenger, Leistritz and others.
Twin-screw extruders come in a great variety of
configurations, depending upon the sense of
rotation and degree of intermeshing. When the
screws rotate in the same direction, the extruder is
called co-rotating, and when in the opposite
direction, it is called counter-rotating. Frequently,
the screws are made in modular design with
elements that slide over a common shaft. Twin-
screw extruders are much more expensive than
single-screw machines. However, because of their
versatility, flexibility and superior mixing capabilities,
they find increased use in various polymer process-
ing operations.
NPE Society of the Plastics Industry Inc
June 19 - 23, 2006
McCormick Place, Chicago, IL, USA
INDOPLAST
June 1 - 3, 2006
Jakarta International Expo
Kemayoran, Indonesia,
Indonesia International Plastics Exhibition
Plastivision India 2007
7th National Exhibition and Seminar,
Mumbai, conducted by AIPMA.
MELTING
SECTION
MIXING
SECTION
METERING
SECTION