2. In order to make a good product using the
extrusion process it is of utmost
importance to have a well homogenised
melt delivered to the die (blown, cast, pipe,
profile etc.)
The homogenous melt is the key to
dimensional stability, process stability,
uniform texture, and for good mechanical
properties of any extruded product like
film, sheet, pipe, profile, etc.
3. For extrusion the polymer mixing is a two step process:
Dry Mixing: mixing of different polymers (in granular
or powder form), master batches and additives before
the mix is introduced to the extruder. This can be done
either offline or online with the help of sophisticated
dosing systems (volumetric or gravimetric type).
Wet Mixing: The final mixing takes place inside the
extruder when the polymer blend is melted inside the
barrel with the help of external (heater bands) and
frictional heat (screw rotation). Various types of
mixing elements or components are used for the
homogenous mixing of the polymer melt inside the
extruder.
4. The most common dry mixing methods are volumetric mixing and
gravimetric mixing. This can be batch mixing or continuous mixing
(loss in weight).
In batch mixing various polymers and additives are stored in
different hoppers and a pre set amount of each component is filled
in a mixing hopper, once the batch is completed the mixer motor
started and the blend is prepared.
In continuous/loss in weight mixing all the components are feeded
simultaneously in to the feed throat of the barrel as per the set
percentage.
Another good example of dry mixing is the preparation of PVC
compound. This is a batch mixing process where all the ingredients
(PVC resin in powder form and liquids like plasticizers, stabilizers
etc.) are poured in a high speed mixer using a certain method and
then this compound (almost dry) is cool down in a cooling mixer
prior to be introduced to the extruder.
5. Wet mixing takes place inside the extruder
when all the ingredients start melting.
In the smooth bore extruders, it starts when
the polymer is melted.
In grooved feed extruder, it starts right at the
grooved feed section where pellets are
chopped by the grooves.
Various mixing techniques, components are
used to get the homogenous mix.
We will discuss wet mixing in detail but before
that let’s talk about the fundamentals of
extrusion process and the equipment involved.
6. Extrusion: is a continuous process in
which the plastic is melted, mixed,
homogenized and pushed out of a die.
Co-Extrusion: When two or more extruders
are used to feed the die than it’s called Co-
extrusion process. It can be used in Blown,
Cast, Pipe, profile or Sheet applications.
7. Single Screw Extruder: Only one screw is used to convey, melt and
plasticize the polymer.
Twin Screw Extruder: In this extruder the barrel is in the design of
horizontal No.8 in which two screws are either inserted to be in co-
rotation or counter rotation arrangement.
Vented Barrel Extruder: In this extruder , the barrel has an
opening where the vacuum is applied to extract the fumes, gases
etc. This arrangement can be applied in single screw or twin screw
extruders.
Planetary Extruder: This is a very special design where two
extruders are connected to complete the job of homogenizing the
polymer melt. The first extruder is a type of conveyor with pre
melting and the second extruder is the real mixer where mostly
shear heat is used to melt and homogenize the polymer. It contains
a center screw with 6, 8 or 10 counter rotating satellite screws
embedded in the core screw. This is commonly used for PVC where
both shear heat and short residence time are crucial factors.
8. The typical components of an extruder:
Electric Motor
Reduction Gear Box with thrust bearing
Flange
Coupling
Cooling Jacket for Feed throat
Smooth/Grooved Feed Section
Barrel
Screw
Heating and Cooling options – Band Heater and Cooling
Blowers, Water cooled heaters
Screen Changer
Breaker Plate and Screens
Adaptor
Melt Pressure Transducer
Rupture disc
Melt Temperature Probe
9. Barrel is a strong hollow pipe made out
of steel the bore of which is hardened
either by Nitriding or a bi-metallic liner
is used.
On one end Barrel is attached to the
gear box whereas screen changer is
usually connected to the other end. The
barrel is levelled and supported using
the brackets to prevent bending.
Bi-metallic Barrel: Is made out of micro-
alloy (Nickel-rich iron-boron alloy) steel
with a wear and corrosion resistant
alloy lining. This is a straight seamless
pipe in which the screw rotates. The
barrel provides the controlled
temperature to heat the polymer inside.
The heaters can be air cooled or water
cooled depending upon the resin being
processed. Water cooling is preferred
for heat sensitive polymers.
10. Screw is the most critical component of the
extruder. It is the screw which provides the
homogenized melt to the die. Screws can be
general purpose or specific.
General purpose screws are mostly used in case
of co-extrusion where various polymers are
processed using the same extruder depending
upon the film structure.
The screw can be of :-
Conventional design:
Feed,
Compression and
Mixing/Metering Section
Barrier design:
Feed-it can be single flight or double
flight,
Barrier flight with or without
compression,
Metering section with one or multiple
Mixers.
Feed section
Beginning of barrier flight
End of barrier flight and
beginning of mixing section
11. Feed Throat: This is the
opening where dry mixed
polymers are introduced to
the extruder. Feed throat
can be directly cut in to the
barrel (built in) or it can be
provided in an additional
part (feed box and feed
sleeve) connected to the
barrel. Feed throat is
normally kept cold (except
in the case of PA where it is
kept warm) to prevent the
resin from melting in the
feeding area, otherwise a
solid polymer ring (also
called bridging) will form
around the screw and will
not allow the resin to move
forward.
In built feed throat
Add on feed throat
12. The feed area can be smooth finish or
with linear grooves. In North America
smooth bore is commonly used whereas
in Europe most of the extruders are
designed with grooved feed section. The
linear slots can be rectangular or V-
shape. Also the number of slots can vary
with different machine manufacturers.
The main advantage of using grooved
feed section is the improvement in
throughput.
Polymer slippage at the feed throat is a
big problem small diameter (1”-1.5”)
smooth feed extruders that can be
corrected by either using shallow
grooves in the barrel or by using a
proper grooved feed section.
Feeder sleeve
Grooved feed
13. Smooth bore extruders have positive pressure gradient
towards the tip of the screw. The pressure inside the
barrel builds gradually and is maximum at the tip of
the screw. The screw acts as a pump and the pre-mix
dry blend is conveyed like a displacement pump. All
the ingredients starts melting one at a time (depending
upon their melting temperature) using shear heat as
well as external heat provided by the barrel heaters.
The polymers/additives with the lowest melt
temperature will melt first followed by the higher melt
temperature polymers. Most of the mixing takes place
after the compression section when all the polymers
are fully melted, the pressure is slightly dropped once
the melt passed through the mixing sections of the
screw. Up to certain extent the throughput of these
extruders is directly affected by the melt pressure.
14. Old design grooved bore extruders are negative pressure extruders
where the pressure is maximum at the feed section and gradually
drops till the tip of the screw. The above condition was a draw
back (high torque required, high wear and tear, poor mixing) for
the grooved feed extruders which has been taken care in the newer
extruders with the help of screw design.
In newer grooved feed extruders , even the pressure is still
maximum at the feed section but it is only for a short period (2-3 D
only) after that the screw has a decompression section which helps
in reducing the pressure. After this decompression section the
screw is similar to the smooth bore screw but slightly flat (less
C/R). The screw works as a positive pressure screw with positive
pressure at the tip of the screw. The new generation screws for the
grooved feed extruders have taken care of the torque. The mixing
has also been improved but still it’s not as good as in the smooth
bore extruders so various mixing sections are added in the screw
design.
16. This mixer is one of the most commonly used
mixers in North America. This mixer was originally
developed by Union Carbide lab in New Jersey. Over
a period many new versions specially twisted
Maddock were developed. Twisted Maddock mixers
have a very complex design with many variables so
they can not be generalised for all the polymers.
Therefore, straight Maddock is still the most
preferable design and is commonly used. This mixer
is dispersive type but some distributive mixing also
takes place. The most critical dimensions for this
mixer are:
Clearance of the barrier flight: lower the
clearance, high shear, more frictional heat,
better mixing, high pressure drop.
Barrier flight width: wider flight means high
shear, high frictional heat, high pressure drop
and better mixing.
Total length of the mixing section: Long mixing
section will lead in high pressure drop, more
frictional heat, (common length is 2-2.5D),
possible degradation.
Channel depth: must be calculated on the basis
of throughput. Too shallow will reduce the
throughput, if too deep than polymer can hang
and degrade, long purging time.
17. This mixer is considered
quite aggressive and
provides both distributive
as well as dispersive
mixing. The melt is
divided in to several melt
streams as it passes
through the criss-cross
holes of the mixer rings.
Though this mixer
provides good mixing but
at the same time the
pressure drop is quite
high along with high
frictional heat. The wear
and tear is quite high.
18. Pin mixers have been used
in the extrusion screws
since long time. Pins can be
mounted in various
patterns. They always
restrict the flow and the
shear heat is generated
along with the pressure
drop which depends upon
the number of pins and the
covered area. There is a
draw back in this design as
dead spots occur behind the
pins. These kind of mixers
works fairly well and
provide good distributive
mixing but the performance
varies by the type of
polymer being processed.
19. These mixers are
usually in-built almost
at the tip of the screw
and are commonly
used along with the
Maddock mixer or
another high shear
mixers for improved
dispersive/distributive
mixing. If designed
properly they work
fairly well.
20. Screen Changer: The function of
the screen changer is to remove
the foreign particles from the melt
with the help of breaker plate and
screens/mesh.
Breaker Plate and Screens/mesh:
Breaker plate is a cylindrical plate
with holes and is a removable part
of the screen changer. Breaker
plate supports the screen/mesh.
The screens/mesh are made with
woven wires and comes in various
sizes.
Apart from preventing the
foreign particles passing
through the die the breaker
plate and screen pack assembly
also serves to create back
pressure in the barrel. Back
pressure is required for uniform
melting and proper mixing of
the polymer.
21. The pressure transducer is
normally mounted at the end of
the barrel and measures the melt
pressure exerted on the barrel
wall. The pressure is a good
controlling factor in the extrusion
process and the probe must
always be in working order.
Melt Pressure is a good diagnostic
factor in extrusion process, a
sudden increase in pressure
indicates blockage due to foreign
particles, un-melted polymer etc.,
a sudden drop in pressure
indicates empty hopper or some
blockage at the feed throat
(bridging) etc. A gradual increase
in pressure over a period of time is
a good indicator for the operator
that screens are choked and must
be replaced with clean ones.
22. This is the safety plug for
the extruder. It is mounted
at the exit side of the barrel.
There are different pressure
rated rupture discs
available. The selection of
rupture disc depends upon
the downstream hot parts
(die) pressure bearing
capacity. If for any reason
the pressure goes higher
than the disc pressure the
disc membrane will rupture
and the polymer will start
leaking.
23. The melt temperature probe
comes in direct contact with
the polymer melt and
measures the actual melt
temperature. It can be installed
in the barrel or in the adaptor.
Variable depth melt probes are
used to check the temperature
difference, ∆T of the melt
stream inside the adaptor. The
probe tip can be moved in and
out to measure the melt
temperature at various depths
in the melt stream. The
uniformity (within +/- 5 °C ) of
the melt temperature close to
the adaptor wall and in the
center of the melt stream is an
indicator of good mixing. If the
difference in melt temperature
is over +/- 5 °C than efforts
must be made to resolve this
issue.
Variable Depth Melt Probe
Fixed and variable depth melt probes
24. Static Mixers: If for some reason the melt is
not homogenized properly by the screw
than the melt quality can be improved
inside the adaptor by using certain static
mixers. There are various designs available
of which Sulzer mixer is quite common. The
mixer is installed inside the adaptor bore
and helps in bisecting/mixing the melt.
If the temperature difference (∆T) inside
the adaptor is too much (measured using
variable depth thermocouple then these
mixers help in bringing the melt at a
uniform temperature before it enters the
die. Even though these mixers do not fix
the root cause of the problem (mostly poor
screw design) still they are helpful in
improving the melt quality
Note: Static mixers should not
be used for thermal sensitive
polymers like PVC, EVOH,
PVdC etc.
25. Feed section Beginning of barrier flight
Melt separation End of barrier flight
Fully melted , just
before the mixing
section
26. With over 20 yrs. Of experience in the field of plastic extrusion
machinery, Mr. Ajay Beniwal has gained knowledge through hands on
practical experience. What ever information is shared in this
presentation or any other previous presentations is the collection of
hard work and dedication towards his work in the field of plastic
extrusion.
Extru-tech Solutions is a consulting and service providing company in
the field of plastic extrusion with specialization in Blown film, Cast
film, Sheet, Double/triple-Bubble process, PVC cling film process, EVA
sheet (PV application), PVB Sheet process for windshield and
architectural application, and various Coating/Lamination processes.
We also provide consulting services to the equipment manufacturers
interested in developing any of the above mentioned technologies.
We do provide training to Supervisors and Operators
at very reasonable cost.
