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

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of the extruder. The purpose of the extruder is to feed a die with a homogeneous

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

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determined by the melt forming that occurs in the die.

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.

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Figure 5 Blown film extruder and die

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

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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.

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

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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.

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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.

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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)

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