Blown Film Extrusion

This is the most important process for the

production of plastic films, ranging in thickness
from 0.5 mm to as thin as 5 μm. The polymer is
melted in an extruder and the hot melt is
pumped through a die to form a thin-walled
tube, which is simultaneously axially drawn and
radially expanded. In most installations the
extruder are horizontal and the blown film
bubble is formed vertically upward.
The purpose of an extrusion die is to impart the
desired shape to the polymer melt stream produced
continuously by the extruder. In blown film
extrusion a thin tubular film is formed as the melt
flows through the die lips. The die lip gap usually
ranges from 0.76 mm to 3 mm and the die diameter
from a few centimeters for laboratory lines to more
than one meter for industrial installations producing
more than one ton of film per hour. The annular
flow is formed in the gap between the inner
mandrel and the outer die body.
Another fundamental function of the die is to
distribute the polymer melt evenly so that the
thickness measured around the circumference of the
tube being produced is uniform. Due to variations in
distribution in the die and uneven cooling after the
melt exits the die lips, there is always some film
thickness nonuniformity. If the film thickness variation
is left unaddressed, when the film is wound up into a
roll, thickness non-uniformity will be evident because
the thicker or thinner spots (gauge bands) in the film
will cause the roll to be of an uneven diameter.
Different types of dies used for blown film extrusion
Sometimes, side-fed as opposed to bottom-
fed dies are used. The problem with these
designs is that they result in the formation of
weld lines in the machine direction. These are
formed when two polymer streams merge
together. Along these merging lines (or weld
lines), the polymer is poorly bonded due to
very low diffusion coefficients of the large
polymer molecules within the highly viscous
melt. By far the most common die geometry
for blown film production is the spiral mandrel
geometry
The polymer melt
flows from extruder
through a melt pipe
at the bottom into
the runners which
guide it to the ports
from where the
spirals originate.
The polymer is fed by a number of melt tubes ending with
a port at the start of each spiral. The melt flows both along
the spirals and in the gap between the mandrel and outer
body of the die. The flow rate becomes progressively more
uniform around the circumference towards the die exit.
After the end of the spirals the melt may pass through a
low shear relaxation chamber for the purpose of reducing
the memory of its complex strain history.

Relaxation chambers
Each spiral is machined into the outer diameter of
the mandrel and travels a certain distance around
the mandrel. Typically, a spiral starts deep and
becomes progressively shallower the further
down its length. The area which separates the
spiral channels is known as the land. The gap over
the land formed by the outer body that encloses
the mandrel is what determines how much
polymer flows over the land and how much
travels down the spiral. As the spiral becomes
shallower, the gap above the land becomes
greater, allowing more polymer to travel over the
land as opposed to in the spiral channel.
 In general, the bubble
usually has a small
diameter and large
thickness at the die
exit and transitions to
a large diameter and
small thickness as it
Bubble moves upward toward
Geometry solidification.
Above some point, the
geometry is “frozen-in”
and remains virtually
constant.
There are several parameters used to describe the
geometry of the bubble:
 Die diameter (D)
 Die gap
 Frost line height
 Neck
 Bubble diameter (BD)
 Film thickness
 Layflat width (LF)
The die diameter represents the initial bubble
diameter as it leaves the die, and the die gap
determines the initial bubble wall thickness. As the
bubble travels upward from the die face in the
molten state, it is cooled and eventually reaches a
temperature where it becomes a solid. The distance
from the die face to where this solidification takes
place is called the frost line height. Conventionally,
the frost line is defined as the lowest point where the
bubble is at its maximum diameter because there is
effectively no further stretching above this point.
The bubble region below
the frost line is known as
the neck. Once the film is
collapsed flat and passes
through the nip rollers,
the two-layer web is
characterized by a lay flat
width. Twice the lay flat
width is equivalent to the
circumference of the
bubble (BD = 2 LF).
Several process variables work together to
determine the bubble geometry:
 Melt speed
 Nip speed
 Internal bubble volume
 Cooling rate
The melt speed is the upward velocity of the
polymer as it exits the die gap. It is controlled by
the screw speed. The nip speed (also called film
speed, line speed, and take-off speed) is the
velocity of the polymer as it travels through the nip
rollers.
The film travels essentially at the nip speed at all
points above the frost line. In all cases, the film
increases in velocity from the die face, where it
travels at the melt speed, to the frost line, where it
travels at the nip speed. This acceleration leads to
thinning of the melt curtain to obtain a thin film.
The internal bubble volume is the amount of air
contained inside the bubble between the die face
and the nip rollers. A similar variable that can be
used alternatively is the internal bubble pressure.
The hot melt is cooled externally (and
sometimes internally) by annular streams
of high velocity air from film cooling
devices called air rings. These air rings are
situated close to the die lips outside and
inside the film bubble and cooling air is
blown onto the film as it is being extruded
from the die. The cooling air helps to cool
the hot melt as it exits the die and
stabilizes the shape of the molten tubular
film.
One of the goals of blown film extrusion is to cool the
film being produced so that it solidifies at the highest
possible rate. The maximum achievable output rate of
the production line is often limited by the air-cooling
capability of the air rings.

Usually, high cooling towers are necessary to remove

enough heat in the film to make sure the two sides
will not adhere together while passing through the
nip rollers.
Bubble with external and internal cooling
Three primary process variables are responsible for the
efficiency of cooling: air speed, air temperature, and air
humidity. At higher air speed, more heat is removed
from the film per unit time. Cooler air will remove heat
more quickly, but using chilled air increases processing
costs, therefore a balance must be reached. Ambient
air temperature around the extrusion line also has a
large effect on bubble cooling, even when chilled air is
used in the process. This is why frost line height may
change significantly from day shift to night shift in
plants that are not air-conditioned.
A third cooling variable is air humidity. If a chiller is
used, then the air impinging on the bubble is
typically quite dry. However, if ambient air is used,
the humidity will vary seasonally and may affect
cooling efficiency. Most plant personnel report that
cooling efficiency goes down when the air humidity
is higher. However, humidity is generally higher at
the same time that air temperature is higher, so
separating the effects simply by observation is
difficult.
The principal component responsible for
cooling is the air ring. This device is located
just on top of the die with a layer of insulating
material (or air) between it and the hot die
face.
The air ring surrounds the bubble and delivers
cooling air directly onto the bubble. It receives air
from the blower through, typically, a number of
hoses that attach around the circumference of
the device. Inside the air ring, a series of baffled
flow channels distribute the air in such a way as
to produce a uniform airflow (volume and
velocity) at all points around the circumference
of the bubble.
Many blown film
extrusion lines today,
particularly larger ones,
employ an internal
bubble cooling (IBC)
system to increase
cooling efficiency, and
with that production
rates.
An IBC device is a continuous heat exchanger,
bringing cool air into the bubble while
removing heated air from inside the bubble.
An additional benefit of IBC is the ability to
perform closed-loop control of bubble
diameter through adjustment of the airflow
rates.
A fifty-foot high, six-foot diameter bubble with a
wall thickness of one thousandth of an inch is
very susceptible to lateral movement from
environmental effects such as drafts. When
movement of the bubble occurs, often called
“dancing”, the result is non-uniform wall
thicknesses, originating at the die lips. For this
reason, bubbles are usually stabilized externally
using devices such as cages and irises. In some
cases, internal stabilization can be performed as
well.
While a benefit is gained
using stabilizers, care must
also be taken to ensure
that the stabilizers are not
the cause of defects. Film
scratching and marring can
occur when these devices
do not perform properly.
Generally, cage rollers are
Teflon-coated and should
be checked regularly to
make certain they rotate
freely. Noncontact (air-
bearing) cages are
available as well.
As the bubble moves upward and approaches the nip rollers,
it is “pre-flattened” by the collapsing frame. This device
provides a smooth transition from a round tube shape to a
flattened tube shape. Collapsing frames utilize wooden slats,
metal rollers, Teflon-coated rollers, or an air cushion to
perform the shape transition.
In addition to flattening the tube, the collapsing
frame also helps eliminate wrinkles in the final
product. Wrinkles may be caused by bubble
movement or film thickness variations, but an
inherent asymmetry in the collapsing process is one
of the major contributors. While the bubble is
collapsing, film traveling along the center of the
frame has a longer distance to the nip point than film
traveling along the edge of the frame. The presence
of these unequal path lengths has a tendency to
make the film wrinkle.
The fixed-position roll is motor-driven to
establish the line speed. Because the line
speed (nip speed) is a primary control
determining film thickness, bubble
diameter, and frost line height, fluctuations
in the motor speed should be minimized,
generally less than ±1% full scale. The size of
the nip rolls and all downstream idle rollers
determines the maximum lay flat width that
the system is capable of producing.