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Vents are required to continuously remove air and moisture (or volatiles) during or after the melting stage. Removal of moisture to prevent
hydrolysis of condensation polymers such as Nylon and PET is one of the most important requirements. Additionally, down-stream to the
venting zone, entrapped air may be pumped into the extruder from a side-feeder that need to be removed in order to increase the capacity of
the extruder.
The most important controlling factor affecting the functioning of this zone is the degree of fill (Dof). Dof is the ratio of actual space (volume)
occupied by material to the actual free space in that particular section of the extruder. When the Dof is high, material can escape through the
vent port. The Dof in an extruder is controlled by the factors such as extruder speed, feed rate, element design and element configuration.. In
the case of under-cut elements (SK type elements) actual free volume is increased by 20% compared to the regular constant clearance
elements. Lead of the screw elements do not change the free volume but can still reduce the Dof. This is because velocity of the melt in that
zone is increased.
Zone # 3 The VENTING ZONE
THE
HEART OF A
TWIN SCREW EXTRUDER IS
-5-
P R O J E C T S A N D P R O G R AM S : A n U P D AT E
Some of the completed projects executed for our
clients are:
1. Continuous compounding of up to 80% filled
masterbatches with specially designed Twin-
Screw Extruders.
2. Compounding of flame retardant
Engineering Plastics like PC, PBT, Pa6.
3. Compounding of biodegradable polymers.
SCS
STEER CONSULTANCY SERVICES
E-mail: scsinfo@steer.in
www.steer.in
Ongoing activities and developments:
1. Product positioning and new advertising
campaigns for its client SEPL.
2. Customized services for Masterbatch
industries, Plant Engineering etc.
3. PET Recycling.
The behavior of the material at the processing temperature
can be slippery, Viscous, Watery, and Clumpy. A different
flow characteristics is expected at an open vent depending
on the material. The handling of these different materials
involves specialized vent inserts in the barrel vent opening.
Generally, there are three types of inserts used. The open
type is used for watery material. The fully closed type is
used for viscous material. A partially closed type is used for
clumpy material. Slippery material and materials that show
a transition in viscosity due to addition of additives and
fillers may also be run with partially closed type inserts.