Lab 12: Film Blowing
1. Introduction
Blown film extrusion is one of the most important polymer processing methods.
Several billion pounds of polymer, mostly polyethylene, are processed annually by
this technique. While some applications for blown film are quite complex, such as
scientific balloons, the majority of products manufactured on blown film equipment
are used in commodity applications with low profit margins: grocery sacks, garbage
bags, and flexible packaging. Although polymer chemistry and molecular structure are
vital in establishing film properties, bubble geometry resulting from processing
conditions is also significant. There are many process variables – screw speed, nip
roller speed, internal bubble air volume, and cooling rate (frost-line height) – that
influence bubble geometry and, as a result, film properties.
Figure 1 A blown film extrusion line
1.1 Extruder Hardware systems
Final product quality and production efficiency are highly dependent on the operation
of the extruder. The purpose of the extruder is to feed a die with a homogeneous
1
material at constant temperature and pressure. Components of the extruder hardware
include drive system, feed system, screw/barrel system, head/die system, and
instrumentation and control system.
Figure 2 The five extruder hardware systems
The drive system supplies the mechanical energy to the polymeric material by rotating
the screw. The feed system holds the solid material and delivers it to the extruder. The
screw/barrel system has been called the “heart” of the operation. Not only does it melt
the solids and pump the polymer through the die, it also prepares the melt to be
homogeneous and of constant temperature and pressure.
Figure 3 An extruder screw
The screw is a long shaft with a flight wrapped helically around it. The barrel is a
hollow cylinder extending from the end of the feed throat to the tip of the screw.
The head/die system receives the melt stream as it exits the barrel. The die has been
called the “brains” of the operation because the product’s final shape is most
determined by the melt forming that occurs in the die.
2
Figure 4 A hopper and the extruder screw inside
Last but not least, the purpose of the instrumentation and control system is to measure
and control important processing parameters. Without the data provided by this
system, it would be very difficult to maintain a safe and efficient process and to
troubleshoot extrusion problems. Three most important parameters to measure are
temperature, head pressure, and motor current. It is common for an extrusion line to
be separated into several temperature control zones. The number of zones depends on
the length of the barrel, the type of adapter or transfer line to the die, and the size and
complexity of the die. While the measurement of extruder head pressure is very
important for product quality purposes, it is also the most important measurement
from a safety standpoint. Excessive pressure can cause rupture of the barrel, damage
to head and die components, and injury to personnel from projected hardware and hot
polymer.
1.2 Hardware for Blown Film
One of the most important components of the hardware for blown film is the die
which is designed to receive polymer melt from the extruder and deliver it to the die
exit as a thin annular film, generally exiting the die gap vertically upward.
Figure 5 Blown film extruder and die
3
Before talking about other blown film hardware, it is necessary to introduce the
bubble geometry because the hardware directly affects the bubble’s geometry.
Figure 6 Bubble geometry characteristics
The specific shape of the bubble depends on the combined influence of several
process parameters. 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
moves upward toward solidification. Above some point, the geometry is frozen and
remains virtually constant. Several parameters used to describe the geometry of the
bubble include die diameter, die gap, frost-line height, stalk, bubble diameter, film
thickness, and layflat width. 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 stalk or neck, particularly when it is relatively
long. Above the frost line, where geometry is effectively frozen, the terms bubble
diameter and film thickness are simply used for those characteristics. Once the film is
collapsed flat and passes through the nip rollers, the two layer web is characterized by
a flat width.
4
Figure 7 Nip roller and collapsing frame
Moreover, there are several process variables work together to determine the bubble
geometry, which are melt speed, nip roller speed, internal bubble volume, and 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 roller speed, also called film 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. The internal bubble volume is the amount of air
contained inside the bubble between the die face and the nip rollers. The cooling rate
is determined by the speed at which the cooling air impinges on the bubble and the
temperature of that air. Bubble cooling is generally accomplished by blowing a large
volume of air on the film as it exits the die. This may take place on only the outside of
the bubble or on both the inside and the outside.
Additionally, the bubble is kept inflated to remove more heat from the film as it
travels up through ambient air in the cooling tower. As the bubble moves upward and
approaches the nip rollers, it is “preflattened” by the collapsing frame. This device
provides a smooth transition from a round tube shape to a flattened tube shape. The
last part is the nip roller. A pair of nip rollers is located at the top of the cooling tower.
Their purpose is to pull the film up from the die. Also, the nip servers as an air seal for
the top end of the bubble, so, at least one of the rolls is usually rubber covered.
5
2. Experiment
All the processing parameters can be controlled by a computer connected to the blown
film extrusion machine.
Figure 8 Brabender extruder program
2.1 Experimental procedure
1) Click TEMP to setup the temperature for different control zone.
2) Click EXTR to setup the torque to 5 rpm. A thin annular film will come out of the
die slowly.
3) Open the air control valve under the die. Set the pressure of inner air to 0.5 psi and
the outer to 5-10 psi.
Figure 9 Air cooling valve.
4) Increase the torque gradually but no more than 20-25 rpm.
5) Manually help the extruded film pass through the nip roller. Adjust the speed of
nip roller and pressure of cooling air to get uniform film.
6
3. Assignment
1) Investigate the influence of air flow rate and nip roller speed on the thickness of
the bag and discuss the relationships between them.
2) Take the film with you, and, outside of lab, identify ways you can test the
properties of the films (such as relative elongation and puncture resistance).
3) Compare these properties with a commercially available polymer film.
What are
the differences between the film you made and the commercially available
polymer films (such as garbage bags)?
4. Reference
1) K. Osborn and W. Jenkins, Plastic Films, Technomic, Lancaster, PA (1992)
2) C. Rauwendaal, Understanding Extrusion, Carl Hanser, Munich (1998)
3) Kirk Cantor, Blown Film Extrusion, Hanser Publishers, Munich (2006)
7