Film Extrusion Guide.pmd - LyondellBasell

Figure 12: Process diagram for PP production in a vertical, stirred fluidized
bed, gas-phase reactor.
The Film Extrusion Process
There are three basic stages in
the overall film extrusion process:
1. Materials Conditioning/Handling
2. Film Extrusion
3. Film Takeoff
Materials Conditioning/Handling
At its resin manufacturing
plants, Equistar has extensive
systems, such as filters, cyclones,
elutriators, etc. to prevent resin
contamination during production,
storage, loading and shipment.
Since polyolefin resins are
non-hygroscopic (they absorb
virtually no water), they do not
require drying prior to extrusion.
However, precautions should be
taken to ensure the cleanliness of
the polyolefin pellets as they are
handled at the processor’s
facilities.
One way to decrease problems
in processing polyolefin resin is to
prevent contaminants from entering
transfer systems. Whenever a
polyolefin resin is moved by a
current of air through transfer
piping, the possibility of
contamination exists. Dust, fines,
streamers and angel hair can be
generated and plug filters or other
components of the transfer system
that results in starvation of the
extruder. Occasionally, large
clumps of angel hair or streamers
may accumulate in a silo and plug
the exit port. All of these problems
can result in extruder downtime,
excessive scrap and the time and
work force costs of cleaning silos,
transfer lines and filters.
Many transfer systems consist
of smooth bore piping that conveys
the resin from hopper cars to
storage silos or holding bins. Some
transfer systems may include long
radius bends. A polyolefin pellet
conveyed by forced air through
such a transfer line travels at a very
high velocity. As the pellet comes in
contact with the smooth pipe wall, it
slides and slows down due to
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friction. The friction, in turn, creates
sufficient heat to raise the pellet’s
surface temperature to the resin’s
melting point. As this happens, a
small deposit of molten polyolefin is
left on the pipe wall. This deposit
solidifies almost instantly. Overtime,
these deposits buildup and are
called “angel hair” when they
slough off and are found mixed with
the pellets.
As the pellets continuously
come in contact with the pipe wall,
such as along the curved outside
surface of a long radius bend, the
deposits of polyolefin become
almost continuous and long
streamers are formed. Eventually,
the angel hair and streamers are
dislodged from the pipe wall and
find their way into the extrusion
process, the storage silo or the
transfer filters. The size and
number of streamers formed
increase with increases in
conveying air temperature and
velocity and are also greater with
smooth bore piping than with other
kinds of handling systems.
Materials Handling Equipment
Design
Since smooth piping is a
leading contributor to angel hair
and streamers, the logical solution
is to roughen the interior wall of the
piping. This roughness will cause
the pellets to tumble instead of
slide along the pipe, thus
decreasing streamer formation.
However, as a rapidly moving
polyolefin pellet comes in contact
with an extremely rough surface,
small particles break oft, and fines
or dust are created.
Two specific finishes for
materials handling piping have
proven to be the best performers
and give the longest life; the first is
a sand blasted finish of 600 to 700
RMS roughness. This finish is
probably the easiest to obtain;
however, until they become
rounded with use, the initial sharp
edges of the finish will result in dust
and fines.
The other finish is achieved by