Blown Film Extrusion

173 Pages·2011·6.359 MB·Russian

by  Cantor K.

Cantor
Blown Film Extrusion
Kirk Cantor
Blown Film Extrusion
2nd Edition
Hanser Publishers, Munich Hanser Publications, Cincinnati
The Author:
Prof. Kirk Cantor, Pennsylvania College of Technology, Williamsport, PA, USA
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Library of Congress Cataloging-in-Publication Data
Cantor, Kirk.
Blown film extrusion / Kirk Cantor. -- 2nd ed.
p. cm.
1. Plastic films. I. Title.
TP1183.F5C36 2011
668.4’95--dc22
2011008677
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ISBN 978-3-446-41705-2
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Acknowledgements
I am grateful for the many people that have supported my efforts to write this book
and develop the software included. Funding for these projects was provided through
a grant to the Plastics Resources for Educators Program (PREP) by supporters at the
National Science Foundation. My colleagues at Pennsylvania College of Technology
provided administrative support.
I am very thankful for my good friend and co-worker, Tim Weston, for his vision and
leadership through PREP. Other PREP colleagues for whom I am thankful are Alex
Bierly, who masterfully created all of the graphics for the simulator, and our other very
talented artists, Mike Fleck, Matt Byers, and Craig Reitz.
Many thanks go to another close friend and mentor, Chris Rauwendaal, for not only
helping with the text of this book, but for teaching me so much about extrusion over
the years. I am thankful for the text review and insight given to me by Robert Krycki.
My friends at Hanser have been very helpful with the creation of the manuscript and
artwork. Thanks especially to Christine Strohm for her years of encouragement and
assistance.
Finally, I am most thankful for the support of my devoted family and the opportunity
given to me by God. My lovely wife, Patsy, and my four beautiful daughters, Kristen,
Caylee, Kelsey, and Shannon, have patiently endured my hours away and have even
cheered me on to completion.
January 2011 Kirk Cantor
Contents
Acknowledgements ......................................................... V
Introduction ................................................................. 1
1 Materials for Blown Film ................................................ 5
1.1 Polymers .......................................................... 5
1.1.1 Polyethylene (PE) ........................................... 6
1.1.2 Low Density Polyethylene (LDPE) ............................ 6
1.1.3 High Density Polyethylene (HDPE) .......................... 8
1.1.4 Linear Low Density Polyethylene (LLDPE) .................... 9
1.1.5 Metallocene Polyethylene (mPE) ............................ 10
1.1.6 Polypropylene (PP) ........................................ 10
1.1.7 Polystyrene (PS) ........................................... 11
1.1.8 Ethylene Vinyl Acetate (EVA) ............................... 12
1.1.9 Ethylene Vinyl Alcohol (EVOH) ............................ 13
1.1.10 Polyvinyl Chloride (PVC) .................................. 13
1.1.11 Polyvinylidene Chloride (PVDC) ........................... 14
1.1.12 Polyamide (PA) ............................................ 14
1.1.13 Polyurethane (PU) ......................................... 15
1.2 Additives ......................................................... 15
1.2.1 Antiblocking Agents ....................................... 16
1.2.2 Antioxidants .............................................. 16
1.2.3 Antistatic Agents .......................................... 16
1.2.4 Colorants ................................................. 17
1.2.5 Lubricants ................................................. 18
1.2.6 Reinforcements and Fillers ................................. 18
1.2.7 Stabilizers ................................................. 20
1.2.8 Tackifiers ................................................. 20
2 Extrusion Overview .................................................... 23
2.1 Extruder Hardware Systems ....................................... 23
2.1.1 Drive System .............................................. 25
2.1.1.1 Motor ............................................ 25
VIII Contents
2.1.1.2 Speed Reducer .................................... 25
2.1.1.3 Thrust Bearing .................................... 26
2.1.2 Feed System ............................................... 28
2.1.3 Screw/Barrel System ....................................... 30
2.1.3.1 Screw ............................................ 30
2.1.3.2 Barrel ............................................ 32
2.1.4 Head/Die System .......................................... 33
2.1.4.1 Head Assembly.................................... 34
2.1.4.2 Adapter .......................................... 34
2.1.4.3 Breaker Plate ...................................... 34
2.1.4.4 Melt Filter ........................................ 35
2.1.4.5 Die ............................................... 37
2.1.5 Instrumentation and Control System ........................ 38
2.1.5.1 Temperature Control .............................. 39
2.1.5.2 Head Pressure ..................................... 42
2.1.5.3 Motor Current .................................... 43
2.2 Extrusion Functional Zones ....................................... 44
2.2.1 Solids Conveying .......................................... 45
2.2.1.1 Gravity-Induced Region ........................... 45
2.2.1.2 Drag-Induced Region .............................. 46
2.2.2 Melting ................................................... 47
2.2.3 Melt Pumping ............................................. 49
2.2.4 Mixing .................................................... 51
2.2.4.1 Distributive Mixing ................................ 52
2.2.4.2 Dispersive Mixing ................................. 52
2.2.4.3 Mixing Devices ................................... 53
2.2.5 Degassing ................................................. 55
2.2.6 Die Forming ............................................... 56
3 Hardware for Blown Film ............................................... 61
3.1 Upstream Components ............................................ 62
3.2 Grooved Feed Throat ............................................. 64
3.3 Screws for Blown Film Extrusion ................................... 66
3.4 Blown Film Dies .................................................. 67
3.5 Bubble Geometry ................................................. 72
3.6 Bubble Cooling ................................................... 75
3.7 Bubble Stabilization ............................................... 80
3.8 Collapsing Frames ................................................ 80
3.9 Haul-off .......................................................... 81
Contents IX
3.10 Winders ......................................................... 83
3.11 Film Treatment ................................................... 84
3.12 Line Control ..................................................... 85
4 Processing ............................................................. 91
4.1 Process Variables vs. Bubble Geometry ............................. 92
4.2 Characteristic Bubble Ratios ....................................... 94
4.3 Process/Structure/Property Relationships ........................... 97
4.4 Double Bubble Processing ......................................... 99
5 Coextrusion ...........................................................101
5.1 Dies ............................................................102
5.2 Interfacial Instabilities ............................................104
5.3 Product Applications .............................................105
5.3.1 Breathable Packaging ......................................105
5.3.2 Shrink Film ..............................................106
5.3.3 High Barrier Film .........................................107
6 Film Properties .......................................................109
6.1 Tensile Strength (ASTM D882) ...................................111
6.2 Elongation (ASTM D882) ........................................113
6.3 Tear Strength (ASTM D1004, ASTM D1922, and D1938) ............113
6.4 Impact Resistance (ASTM D1709, D3420, and D4272) ..............115
6.5 Blocking Load (ASTM D3354) and Coefficient of Friction
(ASTM D1894) ..................................................116
6.6 Gel (Fisheye) Count (ASTM D3351 and D3596) ....................117
6.7 Low Temperature Brittleness (ASTM D1790) .......................117
6.8 Gloss (ASTM D2457) ............................................118
6.9 Transparency (ASTM D1746) .....................................118
6.10 Haze (ASTM D1003) .............................................119
6.11 Density (ASTM D1505) ..........................................119
6.12 Melt Index (ASTM D1238) .......................................120
6.13 Viscosity by Capillary Rheometry (ASTM D3835) ..................122
7 Troubleshooting .......................................................125
7.1 Extruder Problems ...............................................126
7.1.1 Surging ..................................................126
7.1.2 High Melt Temperature ....................................127
7.1.3 Excessive Cooling .........................................129
X Contents
7.1.4 Low Output ..............................................130
7.2 Film Problems ...................................................131
7.2.1 Melt Fracture .............................................131
7.2.2 Thickness Variation .......................................132
7.2.3 Die Lines .................................................136
7.2.4 Gels .....................................................137
7.2.5 Low Mechanical Properties ................................138
7.2.6 Poor Optical Properties ...................................139
7.2.7 Wrinkles .................................................140
Appendix A: The Blown Film Extrusion Simulator ...........................141
A.1 Introduction ....................................................141
A.2 Installation ......................................................142
A.3 Running the Program ............................................143
A.4 Worksheet ......................................................150
Appendix B: Useful Equations ..............................................153
References .................................................................161
Subject Index ..............................................................165
Introduction
Blown film extrusion is one of the most significant 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 (Fig. 1), 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 (Fig. 2). Consequently, sophisticated hardware, materials, and
processing methods have been developed to yield film at very high output rates exhibit-
ing both low dimensional variation and consistent solid-state properties.
Figure 1 A high altitude, scientific balloon being prepared for launch
(National Aeronautics and Space Administration)
2 Introduction
Figure 2 Blown film extrusion is used to produce very high volumes of commodity products such
as grocery and produce bags
Polymer chemistry and molecular structure are vital in establishing film properties, but
bubble geometry resulting from processing conditions is also significant. Molecular
orientation and crystalline structure – controlled by bubble dimensions – affect proper-
ties such as tensile strength, impact toughness, and clarity.
As a manufacturing process, blown film is somewhat unique, even compared with other
extrusion processes. Molten polymer generally exits the die vertically in the form of a
freely extruded bubble reaching heights of 50 feet (15 meters) or more (Fig. 3). Guides
surrounding the bubble may limit its mobility, but it is still quite exposed to dimensional
variation compared to the fixed extrudate in most other extrusion processes, which
use vacuum sizers, calibrators, rollers, or other techniques. Depending on processing
conditions, the blown film bubble has a shape freedom that allows almost any number
of profiles within a designed range. Operators must have a relatively high skill level to
accurately obtain the required bubble geometry (i.e., the shape resulting in specified
product dimensions and properties).
The strong interdependence of process variables is another aspect of the process that
requires a high level of operator skill and has led to extensive advancements in mea-
surement and control techniques. There are many process variables – screw speed, nip
speed, internal bubble air volume, and cooling rate (frost line height) – that influence
bubble geometry and, as a result, film properties. An adjustment to any one of these