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Vacuum vents in the barrels are provided in an extruder for removal of gases effectively in a
continuous manner at lower than atmospheric pressure. This degassing helps to conserve
the quality of the melt that is affected by the presence of monomers, solvents, moisture and
other volatiles. These generally separate from the melt only at those low to high vacuum
conditions. Further, the elements used in this zone should expose the material by
continuously thinning the melt for effective degassing. This task of smearing the melt on the
barrel surface by thinning for degassing is efficiently exhibited by the long forwarding
screw elements or SK type Elements.
Applications involve
Ÿ
Removal of monomers and oligomers in the production of polymers (PS, HDPE, PP etc).
Ÿ
Removal of residual carried fluid in Emulsion and Suspension polymerization (PS, PVC)
Ÿ
Removal of Solvent and Un-reacted monomers in Solution Polymerization (HDPE)
Ÿ
Removal of Volatile bonding agents particularly with Glass Fiber reinforced polymers.
Ÿ
Removal of Reaction products like Water and Methanol etc from condensation
Polymerization.
Ÿ
Removal of water from hygroscopic polymers (Ex ABS, PMMA, PA, PC, SAN, PU,
Polysulfone, CA, PPO, etc)
Ÿ
Removal of Volatile Components in compounding of polymers with additives and other
ingredients.
*
A melt seal is the most vital requirement for removal of moisture and entrapped air from the
melt during compounding at lower than atmospheric pressure. Adequate lengths of Vent
opening and Vent hood design that maximizes the pressure gradient inside the extruder
further facilitate this process. Melt seal is achieved by simply ensuring that prior to starting
of the vent zone and just after its completion the melt completely fills up (degree of fill = 1).
Usually a reverse lead element will ensure build up of a sufficient wall of material behind it.
However, depending on the nature of work, adequate melt seal may have to be built up to
avoid frequent breaking of seal resulting in poor devolatilization. At least “1D” length of the
melt seal may be required while running high vacuumwith thin melts.
Apart from this, the same issues encountered in the Venting zone to prevent melt from
raising and filling the vent opening is even more acute here. The best remedy for this
situation is the use of a Side-feeder (a.k.a side-stuffer) with proper seal in the gearbox
end of the screw to allow a vacuum pump to be connected to it.
*
Reference - Principles of Extrusion, Dr. Chris Rauwendaal
Zone # 5 The VACUUM ZONE
THE
HEART OF A
TWIN SCREW EXTRUDER IS
-9-
THE EXTRUDER TIMES / ISSUE 05 / JANUARY-MARCH 2008
New Ideas can be threatening, and they often
provoke a negative reaction. For example, when
Stravinsky first presented his ‘Rite of Spring’
ballet with its unusual harmonies and primitive
rhythms, he was met with a rioting audience.
When Kepler correctly solved the orbital
problem of the planets by using ellipses rather
than circles, he was denounced. Be prepared for
such a reaction and don't let it prevent you from
acting on your idea. As German statesman
Konrad Adenauer put it, "A thick skin is a gift
fromGod."
What negative reaction do you expect? How can
you deflect it?
Source: Roger von Oech’s Creative Whack Pack.
USE YOUR SHIELD