Film Extrusion Guide.pmd - LyondellBasell

Blown Film Dies
In blown film extrusion, the melt
enters the round die either through
the bottom or the side (Figure 27).
The melt is forced through spiral
grooves around the surface of a
mandrel inside the die and
extruded through the circular die
opening in the form of a
thick-walled tube. Melt distribution
can be improved by lengthening
and/or increasing the number of the
spiral grooves.
The tube, while still in the
molten state, is expanded into a
long “bubble” of desired diameter
and correspondingly decreased
thickness. This expansion results
from the volume of air inside the
bubble, which is introduced into the
tube through the center of the
mandrel.
Blown film dies have a number
of circular heating zones. Die
temperature should be the same as
melt temperature.
Most blown film dies for
polyethylenes are positioned
vertically to push the tube upward.
However, there are dies which
extrude downward.
The gap between the mandrel
and the die ring ranges from 25 to
120 mil (0.5 to 3 mm). In some
dies, this opening can be changed
by moving the mandrel lengthwise
in the die. Most dies require a
change in mandrels in order to
adjust the die gap opening. This
adjustment changes the extruded
tube thickness. A wider ring
opening increases output slightly,
but due to uneven flow, it may
make gauge and frost line control
more difficult. It also tends to
promote film snap-off, particularly
when the film is drawn down to a
gauge of less than 0.5 mils (13
microns). Blown film dies can be as
large as 80 inches in diameter,
producing film with 125 inches or
more in lay flat width.
23
Courtesy of W&H Corp.
Figure 28. Automatic gauge adjustment can involve air jets around the die
which selectively cool segments of the die lip.
Figure 29. Schematic of a typical seven-layer blown film die assembly.