Blown Film Basics

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Blown Film Basics

7 June 2018

The process of producing film by extruding molten resin into a continuous tube is,
at first glance, extremely simple. The elements of the process (Figure 1) include
the resin pellets which are fed through a hopper into an extruder. Here,
heat and friction convert the pellets to a melt which is forced through an annular
or ring-shaped die to form a tube. The tube is inflated to increase its
diameter and decrease the film gauge. At the same time, the tube is drawn away
from the die, also to decrease its gauge. The tube, also called a “bubble,” is then
flattened by collapsing frames and drawn through nip rolls and over idler rolls to a
winder which produces the finished rolls of film.
However, anyone familiar with blown film extrusion knows this simplified explanation
is less than half the story.The blown film extrusion system is, in fact,
one of the most complex and sensitive of all plastics processing technologies. The
tubular blown film process is efficient and economical, and can produce a
magnificent array of products from a light gauge, clear converter film to heavy
gauge construction film, which when slit and opened, may measure 40 feet or more
in width.

Main Arena of Action
More of the problems in blown film extrusion take place in the section of the
tube illustrated in Figure 2 from within the die to the far side of the nip rolls
than in any other portion of the line. Elements
in this section are labeled and are referred to again in this booklet.
Even though practice does not always follow theory, theory can help explain
many of the problems encountered in extruding polyethylene into blown film. For
example, blow-up ratio (BUR) used alone as a film-making parameter is meaningless.
BUR must be related to draw-down ratio and die gap. In Figure 3, all three of
these parameters are used to illustrate a theory of melt orientation, an important
factor in extruding the high quality film required by customers.

To illustrate melt orientation, it is
necessary to separate the blow-up and
drawdown functions. In reality, however,
these take place simultaneously in the melt
below the frost line. In this area almost all
of the important characteristics of the film
are fixed-orientation, shrink properties,
clarity, gloss, strength, etc.
The formula to obtain the BUR and
drawdown ratios and their meanings are as
follows:

 

Blow Up Ratio (BUR) = Bubble Diameter
Die Diameter

BUR indicates the increase in the bubble diameter over the die diameter. The
die gap divided by the BUR indicates the theoretical thickness of the melt
after reduction by blowing. Since it is difficult to use calipers on the bubble
to measure its thickness unless you knock it down, a more practical formula is:
BUR = 0.637 x Layflat Width
Die Diameter
The final thickness reduction in the melt after blowing is indicated by a
drawdown ratio.
Drawdown Ratio (DDR) = Width of Die Gap
Film Thickness x BUR
A third ratio, called the blow ratio (BR), is the increase of layflat width
over die diameter. BR is used less frequently, but can easily be confused in
conversation with the more common BUR. A blow-up ratio greater than 1
indicates the bubble has been blown to a diameter greater than that of
the die orifice. The film has been thinned and possesses an orientation in
the transverse direction (TD). A drawdown ratio greater than 1 indicates
that the melt has been pulled away from the die faster than it issued from
the die. The film has been thinned and possesses an orientation in the
machine direction (MD). In practice these numbers are only approximate
because the melt swells as it leaves the die gap. The above calculations
are made using the die gap dimension because the degree of swell varies
with the resin used and processing conditions.
Collapsing the Bubble
Although these ratios provide general parameters, some incompatibility
exists between the configuration of the bubble and that of the film after it
has been collapsed over the various rolls. After film is wound, its size is
called the layflat width. Brief study of Figure 4 shows the reason for this
incompatibility. The sketches show front and side views of a bubble 16
inches in diameter collapsed to a layflat width of 25 inches (some
numbers here are rounded off for ease of comparison).

 

In Figure 4, on the front, a right
triangle is formed (shaded area) with the
length of the vertical side equal to D, the
distance between the nip rolls and the
bottom of collapsing frame; the length of
the base side is equal to half the layflat
width minus the radius of the bubble, or 4½
inches.

 

 

On the side view, a right triangle is formed (shaded area) with the vertical side
equal to D as before, but the base side is equal to the radius of the bubble, or 8
inches (½ of the diameter). Since the two triangles have vertical sides of equal
length, D, but different base lengths, 4½ inches vs 8 inches, the third sides of
the two triangles (E vs C) must also have different lengths. In other words,
the length of film that forms the edge of the layflat (E) is not equal to the length
that forms the center of the layflat (C). Yet these unequal lengths must travel from
the plane of their point of contact with the collapsing frame to the nip rolls in the
same amount of time. Tabulated data at the bottom of Figure 4 show the
magnitude of this discrepancy in length. If the angle A, formed by the center line
of the bubble and the edge of the collapsing frame is 22°, then distance D must be
20 inches for a collapsing frame long enough to accommodate the full bubble
width. By calculation, the edge E is found to be 20½ inches long, while the center
C is 21½ inches. The center of this section of film is one inch, or about 5%, longer
than the edge. To bring a layflat out of the nips that actually lays flat, the edge
of the film should theoretically travel faster than the center. In other words, the
velocity of the film should gradually increase from the center until, at the edge,
it is 5% greater than that of the center. With a line speed of 120 feet per minute
(fpm) at the center, the edge must travel at about 126 fpm.
Fortunately, film made from low density polyethylene can stretch. The edge must
stretch to permit the center to remain taut as it goes through the nips. If the edge
does not speed up (stretch), the center will be baggy and broad “smile” wrinkles
will appear across the web. Less extensible film — stiff overwrap from resin with
a density of 0.935 g/cm3, or a high density, paper-like film — does not have the
ability to stretch. The broad “smile” wrinkles appear if no attempt is made to
increase the edge velocity. However, if the edge velocity is too great, edge
wrinkles occur. Normal procedure at this point is to close down the collapsing
frame. This procedure decreases the angle A (see Figure 4) and reduces the
difference between the lengths of the center and edge. Decreasing the angle
from 22° to 11° narrows the difference in length between the center and edge to
1¼ %. At a 5½° angle, the difference is a mere 5/8%, essentially solving the
problem, although not completely.

Conclusion
Many problems occur in blown film extrusion in the hot melt between the die
and the frost line and where the tube is collapsed at the main nip. Other sections
in this booklet deal more specifically with problems such as uneven rolls, gauge
bands, wrinkles, maintainng output, physical and optical problems and solutions.
Prevention Checklist
To prevent problems in extruding blown film, purchase the proper equipment
to do the job. Then, make sure the equipment is installed properly, checked and
maintained regularly and scheduled efficiently so that high quality film can be
produced at high output ates with minimum scrap. Checklists are excellent
memory joggers for both production and maintenance personnel. An operator
should check each film line against a checklist at least one time per shift. Lines
should also be checked when there is an order change, and at start-up or at
shutdown, since some equipment can be inspected only at those
times. All or part of the following checklist usually is incorporated in plant process
descriptions, plant operating standards or the plant maintenance department’s
schedule. The checklist cannot include everything because each blown film
extrusion shop is different. However, regular use of the checklist should minimize
many potential problems.
Resin, Additives and Regrind
1. Are the polyolefin resin and additives the right grades for the film being
extruded?
2. Are the quantities sufficient to complete the run?
3. Are the resin loading system filters clean or have they recently been
replaced?
4. Were the additive feeders emptied and cleaned when out of service to
prevent dribble from contaminating the next resin to be run?
5. Were the recycle/regrind systems for salvaging edge trim and roll scrap
recently checked to make sure that the proportion of scrap to virgin resin
is accurate?
6. Are the scrap rolls of film compatible with the virgin pelletized resin being
used?
7. Are all rolls of scrap clean and labeled by resin type?
8. Was the equipment cleaned between resin changes to prevent contamina-
tion?
9. Are all hoppers and boxes of resin covered, not only to prevent
contamination of the resin, but also to prevent possible damage to the
extruder by tramp metal or other materials?
Extruder Drives
1. Most extruders are equipped with some type of variable speed drive for
consistent output control. Are all fluctuations in revolutions per minute
or power consumption, as indicated by the screw tachometer or drive
ammeter, checked along with other causes for extruder surging?