Keycontributions to the development of the co-rotating twin-screw were made by employees ofthe chemical industry: a basic patent for Meskat and Erdmenger’s “threaded screws” of 1944was g
Trang 1Fundamentals, Technology, and Applications
Co-Rotating
Twin-Screw
Extruders
Trang 4Dr.-Ing Michael Bierdel
Dr.-Ing Jens Hepperle
Dr.-Ing Jörg Kirchhoff
Dr.-Ing Thomas König
Dr.-Ing Klemens Kohlgrüber
Dipl.-Ing Ulrich Liesenfelder
Dr.-Ing Reiner Rudolf
Dipl.-Ing Martin Ullrich
Coperion Werner & Pfleiderer GmbH & Co KG, Stuttgart, Germany
Dipl.-Ing Herbert Christ, Dipl.-Ing Ralf Davids Dr.-Ing Peter Heidemeyer, Dipl-Ing Frank Lechner Dipl.-Ing Hans-Joachim Sämann Dipl.-Ing Ulrich Weller
Dr.-Ing Werner Wiedmann Dipl.-Ing Reinhard Wuttke Fundamentals, Technology, and Applications
Trang 5SDL Multilingual Services GmbH & Co KG, D-70563 Stuttgart, Germany
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Library of Congress Cataloging-in-Publication Data
Kohlgrüber, Klemens.
[Gleichlaufige Doppelschneckenextruder English]
Co-rotating twin-screw extruder / Klemens Kohlgrüber.
Bibliografische Information Der Deutschen Bibliothek
Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie;
detaillierte bibliografische Daten sind im Internet über <http://dnb.d-nb.de> abrufbar.
ISBN 978-3-446-41372-6
All rights reserved No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying or by any information storage and retrieval system, without permission in wirting from the publisher.
© Carl Hanser Verlag, Munich 2008
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Coverdesign: MCP • Susanne Kraus GbR, Holzkirchen, Germany
Printed and bound by Kösel GmbH & Co KG, Altusried-Krugzell, Germany
Trang 6The co-rotating twin-screw is used in many of today’s industries, particularly in polymerprocessing However, the development of this type of machine goes back a long way Keycontributions to the development of the co-rotating twin-screw were made by employees ofthe chemical industry: a basic patent for Meskat and Erdmenger’s “threaded screws” of 1944was granted in 1953 and in the same year, Bayer awarded an exclusive worldwide license forthe patent utility rights to Werner & Pfleiderer.
The first twin-screw compounder (ZSK) went into production at Werner & Pfleiderer in
1957, marking the beginning of a success story for this type of machine The first majorapplications were in the chemical industry Today, the machine is predominantly used in theplastics industry, e g., in extrusion and compounding These screw machines are thereforealso known as extruders and the twin-screw is known as the twin-screw extruder
The 2007 international plastics trade fair and the 50th anniversary of the ZSK have inspiredBayer (Bayer Technology Services) and Werner & Pfleiderer (Coperion Werner & Pfleiderer)
to publish a book covering the history, principles and applications, and current art of this technology The book is based on a seminar regularly held by the editor andorganized by the Association of German Engineers (VDI) entitled “The co-rotating twin-screw extruder”
state-of-the-As the book contains contributions from several authors, readers are also offered a variety
of viewpoints I would like to take this opportunity to offer heartfelt thanks to all authors fortheir contributions I would particularly like to thank Mrs M Stüve of Carl HanserPublishers and my colleague Mr J Hepperle for their invaluable assistance in the layout andediting I would also like to thank Mr W Wiedmann of Coperion Werner & Pfleiderer forthe organisation of the CWP contributions and for being a major driving force behind theproject
Trang 81 Introduction 1
2 Historical Development of the Co-Rotating Twin Screw 9
2.1 Introduction 9
2.2 Early Developments 9
2.2.1 Basic Geometry 10
2.2.2 Basic Patents 13
2.2.2.1 Basic Patent of the Threaded Screw 13
2.2.2.2 Basic Patent for Kneading Discs, DBP [16], USP [17], DBP [20] 15
2.2.2.3 Basic Patent for Modular Design 18
2.3 Pioneering Period 20
2.3.1 Machine Development 20
2.3.2 Use in Chemical Processes 20
2.3.3 Licensing 22
2.3.4 Recognition for R Erdmenger 22
2.4 New High Viscosity Technology with Co-Rotating Extruders 23
2.4.1 Screw Machines in Process Engineering 23
2.4.2 Similarity Theory for Screw Machinery 23
2.4.3 Versatile High Viscosity Processes 25
2.5 Special Developments from Bayer-Hochviskostechnik (High Viscosity Technology Group) 27
2.5.1 Extended Kinematics, Profile Geometries 27
2.5.2 Clearance Strategies 28
2.6 Developments after Licensing 29
2.7 Developments after Expiration of the Primary Patents 32
3 Rheological Properties of Polymer Melts 35
3.1 Introduction and Motivation 35
3.2 Classification of Rheological Behavior of Solids and Fluids 36
3.3 Comparison of Viscous and Viscoelastic Fluids 40
3.3.1 Viscous Fluids 41
3.3.2 Viscoelastic Fluids 42
Trang 93.4 Temperature Dependence of Shear Viscosity 44
3.4.1 Temperature Dependence for Semi-Crystalline Polymers 45
3.4.2 Temperature Dependence for Amorphous Polymers 46
3.5 Influence of Molecular Parameters on Rheological Properties of Polymer Melts 47
3.6 Shear Flows 49
3.6.1 Flow Profiles of Pressure-Driven Pipe Flow 50
3.6.2 Flow Profiles of the Simple Drag Flow 51
3.7 Extensional Flows 52
4 General Overview of the Compounding Process: Tasks, Selected Applications, and Process Zones 57
4.1 Compounding Tasks and Requirements 57
4.2 Tasks and Design of the Processing Zones of a Compounding Extruder 59
4.2.1 Intake Zone 60
4.2.2 Plastification Zone 61
4.2.3 Melt Conveying Zone 65
4.2.4 Distributive Mixing Zone 65
4.2.5 Dispersive Mixing Zone 67
4.2.6 Devolatilization Zone 69
4.2.7 Pressure Build-Up Zone 70
4.3 Characteristic Process Parameters 72
4.3.1 Specific Energy Input 72
4.3.2 Residence Time Characteristics 74
4.4 Process Examples 76
4.4.1 Incorporation of Glass Fibers 76
4.4.2 Incorporation of Fillers 78
4.4.3 Production of Masterbatches 80
4.4.3.1 Premix Process 80
4.4.3.2 Split Feed Process 81
4.4.3.3 Color Matching 82
4.4.4 Coloring 83
4.5 Technical Trends in Compounding 84
4.5.1 Gear Element 84
4.5.2 Ring Extruder 85
4.5.3 TPE Production 85
4.5.4 ZSK-NT Technology 86
4.5.5 Injection Molding Compounder 87
4.6 Symbols and Abbreviations 87
5 Geometry of the Co-Rotating Extruders: Conveying, and Kneading Elements 91
5.1 Introduction 91
5.2 The Fully Wiped Profile from Arcs 92
5.3 Geometric Design of Closely Intermeshing Profiles 94
5.4 Dimensions of Screw Elements with Clearances 95
5.5 Transition between Different Numbers of Threads 98
Trang 105.6 Calculation of a Screw Profile for Production According to Longitudinal
Offset 99
5.7 Conveying Characteristics of Different Geometries 101
5.8 Kneading Elements 102
6 Modeling: Possibilities and Limitations 105
6.1 The Motivation for Modeling 105
6.2 Screw Design 106
6.3 Modeling Approaches 107
6.4 Model Dimensions 108
6.5 Extruder: 0-Dimensional 110
6.5.1 Whole Extruder 110
6.5.2 Pumping Efficiency 112
6.5.3 Extruder Section 112
6.6 Extruder: 2-Dimensional 113
6.7 Extruder: 1-Dimensional 114
6.7.1 Extruder: 1-Dimensional, Extruder Section 114
6.7.2 Extruder: 1-Dimensional, Whole Extruder 116
6.8 Extruder: 3-Dimensional 117
6.8.1 Model Depths and Results 117
6.8.2 Extruder: 3-Dimensional, Fields 119
6.8.3 Extruder: 3-Dimensional, Scalar Values 119
6.9 Simulation: Possibilities and Limitations 120
7 Pressure Generation and Energy Input in the Melt 121
7.1 Operating States of Conveying Screw Elements 121
7.2 Dimensionless Representation with Descriptive Impact 123
7.3 Calculation of the Back-Pressure Length 128
7.4 Efficiency during Pressure Generation 129
7.5 Example for the Design of a Pressure Build-Up Zone 130
7.6 Feed Behavior with Shear Thinning 131
8 Computational Fluid Dynamics 139
8.1 Why Computational Fluid Dynamics? 139
8.2 Workflow of a Computational Fluid Dynamics Process 140
8.2.1 Pre-Processing 140
8.2.2 Flow Computation and Post-Processing 142
8.3 Computational Examples 142
8.3.1 Example 1 142
8.3.2 Example 2 153
8.4 Conclusion and Outlook 156
9 Mixing and Dispersing: Principles 159
9.1 Introduction 159
9.2 Distributive Mixing 159
9.2.1 Mixing in Laminar Flow 160
Trang 119.2.2 Axial Mixing and Residence Time Distribution 164
9.3 Dispersive Mixing 167
9.3.1 Dispersion of Solid Particles 167
9.3.2 Dispersion of Melts, Liquid Droplets, and Gas Bubbles 169
9.3.3 Types of Loads and Frequency of Loads in an Extruder 171
9.4 Determining the Mixing Quality 173
9.4.1 Parameters 173
9.4.2 Experimental Methods for Determining Mixing Quality and Residence Time Distribution 175
10 Degassing Polymer Melts with Co-Rotating Twin Screw Extruders 181
10.1 Requirements for Degassing 181
10.2 Function-Specific Design 183
10.2.1 Flash Vaporization 184
10.2.2 Multi-Stage Vacuum 185
10.2.3 Residual Degassing and Use of Stripping Agents 186
10.2.4 Process Set-Up and Design of Degassing Zones 190
10.3 Process Limits 193
10.4 Scale-Up 194
10.5 Process Examples 194
10.5.1 Devolatilization of Solvents from LLDPE Melt Solutions 194
10.5.2 Degassing Solvents from Synthetic Rubber (Styrene-Butadiene Compounds) 195
10.5.3 Degassing Vinyl Acetate from LDPE/EVA Copolymer 195
10.5.4 Degassing POM 196
10.5.5 Degassing PC 197
10.5.6 Degassing PES and PSU 197
10.5.7 Degassing ABS 198
10.5.8 Degassing Un-Dried PET 199
10.6 Conclusion 201
11 Simulation or Scale-Up – Alternatives for Extruder Layout? 203
11.1 Process Sections of the Compounding Extruder 203
11.1.1 Feed and Solid Conveying Section 203
11.1.2 Plastification and Homogenizing Sections 207
11.1.3 Devolatization and Discharge Sections 208
11.1.4 Computation Possibilities in the Melt Phase 208
11.2 Computation Possibilities for Discharge Parts 210
11.3 Scale-Up 211
12 Screw Elements for Co-Rotating, Closely Intermeshing, Twin-Screw Extruders 215
12.1 Design of the Screw Element 215
12.2 Combining Screw Elements 218
12.3 Screw Elements and How They Work 220
12.3.1 Conveying Elements 220
12.3.2 Kneading Elements 225
Trang 1212.3.3 Backward-Pumping Elements 227
12.3.4 Mixing Elements 228
12.3.5 Special Elements 232
13 Overview of Patented Screw Elements 237
13.1 DE 813154, US 2670188 A 238
13.2 DE 19947967A1, EP 1121238 B1, WO 2000020188 A1 239
13.3 US 1868671 A 240
13.4 DE 10207145 B4, EP 1476290 A1, US 20050152214 A1 240
13.5 DE 940109 B, US 2814472 A 241
13.6 US 3717330 A, DE 2128468 A1 241
13.7 DE 4118530 A1, EP 516936 B1, US 5338112 A 242
13.8 US 4131371 A 243
13.9 DE 3412258 A1, US 4824256 A 243
13.10 DE 1180718 B, US 3254367 A 244
13.11 US 3900187 A 245
13.12 US 3216706 A 246
13.13 EP 2131 A1 B2, JP 54072265 AA, US 4300839 A 247
13.14 DE 19718292 A1, EP 875356 A1, US 6048088 A 248
13.15 DE 4239220 A1 248
13.16 DE 1529919 A, US 3288077 A 249
13.17 EP 330308 A1, US 5048971 A 250
13.18 US 6783270 B1, WO 2002009919 A2 251
13.19 DE 10114727 B4, US 6974243 B2, WO 2002076707 A1 251
13.20 DE 4329612 A1, EP 641640 B1, US 5573332 A 252
13.21 DE 19860256 A1, EP 1013402 A2, US 6179460 B1 253
13.22 DE 4134026 A1, EP 537450 B1, US 5318358 A 254
13.23 DE 19706134 A1 254
13.24 WO 1998013189 A1, US 6022133 A, EP 934151 A1 255
13.25 WO 1999025537 A1, EP 1032492 A1 255
13.26 US 6116770 A, EP 1035960 A1, WO 2000020189 A1 256
13.27 DE 29901899 U1 256
13.28 US 6170975 B1, WO 2000047393 A1 257
13.29 DE 10150006 A1, EP 1434679 A1, US 7080935 B2 257
13.30 DE 4202821 C2, US 5267788 A, WO 9314921 A1 258
13.31 DE 3014643 A1, EP 37984 A1, US 4352568 A 258
13.32 DE 2611908 A1, US 4162854 A 259
13.33 WO 1995033608 A1, US 5487602 A, EP 764074 A1 259
13.34 DE 102004010553 A1 260
13.35 DE 4115591 A1, EP 513431 B1 260
14 The ZSK Series and Applications in the Chemical Industry and for Renewable Raw Materials 261
14.1 Development of High Torques, Volumes and Screw Speeds 261
14.2 Torque-Limited and Volume-Limited Throughputs 266
14.3 Process-Dependent Energy Requirement 268
Trang 1314.3.1 Throughput-Energy Diagram 268
14.3.2 High Torque for Glass Fiber Reinforcement of Plastics 270
14.3.3 High Torque for Film Extrusion of Non-Dried PET or PLA 271
14.3.4 Applications of Low Torques and High Volume Requirement 271
14.4 Chemical and Pharmaceutical Applications 272
14.4.1 Silicone Sealants 272
14.4.2 Pressure-Sensitive Adhesives 272
14.4.3 Ceramic Catalyst Carriers 273
14.4.4 Insulating Films 276
14.4.5 Battery Separator Films 277
14.4.6 Metal and Ceramic Mixtures 278
14.4.7 Pharmaceutical Mixtures 280
14.5 Applications for Renewable Raw Materials in the Plastic and Food Sectors 281
14.5.1 Composite Materials Made from Wood Fibers in Polyolefins 281
14.5.2 Biodegradable Materials Made from Thermoplastic Starch and Polylactic Acid 282
14.5.3 Extrusion Cooking of Cereals for Foodstuffs and Animal Feed 285
14.5.4 Applications in the Confectionery Industry 286
15 ZSK-NT the New Two-Stage Processing System for High Throughputs 289
15.1 Current Requirements for the Processing of Polyolefins 289
15.2 Two-stage Large-Scale Plants for the Processing of Bimodal Polyethylene 290
15.3 Quality Assessments for Bimodal Pipes 291
15.4 ZSK-NT Compared with the Standard Technology 292
15.5 Design of Pressure Build-Up Zones 295
15.7 Outlook 300
15.8 Notation 300
16 Material Selection for Twin Screw Extruder Components in Contact with Resin 303
16.1 Introduction 303
16.2 What is Wear? 303
16.3 Wear in Operating Experience 304
16.4 Choice of Materials for Extruder Barrel and Screw Elements 307
16.4.1 Materials for Extruder Barrel 307
16.4.1.1 Barrel Designs 307
16.4.1.2 Material Variations 308
16.4.2 Materials for Screw Elements 311
16.4.2.1 Designs 311
16.4.2.2 Material Variants 311
17 Drive Units for Co-Rotating Twin-Screw Extruders 315
17.1 Introduction 315
17.2 Drive Units for Small- to Medium-Size Co-Rotating Twin-Screw Extruders 315
Trang 1417.2.1 Electric Motors 316
17.2.2 Drive Configuration 317
17.2.3 DC Drives 319
17.2.3.1 Power Converters 320
17.2.3.2 DC Motors 321
17.2.4 Asynchronous Drives 322
17.2.4.1 Frequency Converters 322
17.2.4.2 Asynchronous Motors 324
17.2.5 Network Feedback and EMC 326
17.2.6 Motor Monitoring 326
17.2.7 Torque Measurement 326
17.2.7.1 Torque Measurement for Extruder Protection 326
17.2.7.2 Torque Measurement for Scale-Up 327
17.2.8 Bearing Currents 327
17.2.8.1 Bearing Currents Caused by Asymmetry 327
17.2.8.2 Bearing Currents Caused by Common Mode Voltage 329
17.2.8.3 Bearing Currents Caused by Circular Flux 329
17.2.8.4 Avoidable Sources of Error 330
17.2.8.5 Corrective Measures 331
17.3 Drive Units for Large Co-Rotating Extruders 332
17.3.1 Drive Types 332
17.3.2 Medium Voltage Asynchronous Motor 333
17.3.3 Medium Voltage Synchronous Motors 335
17.3.4 Fixed-Speed Drives 335
17.3.4.1 Starting Aids 336
17.3.4.2 Direct on-Line Starting 336
17.3.5 Variable Speed Drives 337
17.3.5.1 Frequency Converters 337
17.3.5.2 Converter-Transformers 339
17.3.7 Emergency Running Properties 340
17.4 Safety Clutches 340
17.4.1 Slip Clutches 341
17.4.2 Mechanically Disengaging Clutches 342
17.5 Gearbox 342
17.5.1 Design 343
17.5.2 Gear Teeth 344
17.5.3 Bearing 345
17.5.4 Lubrication 346
Trang 15Survey Extruders
Single screw extruderg
Single screw extruder Twin screw extruder Multiple screw extruderMultiple screw extruderp
static center shaft
rotating counter-rotating
co-non intermeshing intermeshing
C
o-K ne
a de r
+
+
+ + + + + + + +
P
ne ta rol
le r R
ing-e tr ude r
Mul
ti pl
e S
c re w
E x
tr ud
e r TS
E
FC
M TS
E S
Single-shaft (single-screw) machines with a smooth-bore housing (barrel) as well as thosewith grooves and/or pins in the housing are employed in plastics processing primarily formelting and pressure build-up Since the mixing ability of single screw extruders is limited,co-rotating twin screws (with two shafts) are often employed for compounding tasks Other
Trang 16multi-shaft (multi-screw) extruders that mimic in some ways the geometry of the screw arrangement have also been developed.
twin-Co-rotating twin screws are built using a modular design and can thus be adapted easily tohandle a variety of processing requirements and product characteristics The optimumdesign of a co-rotating twin-screw arrangement for a specific task and product requires in-depth knowledge of the machine (what are its performance characteristics?) and of theprocess (how does the product behave in the machine?) This is exactly where this bookbegins There is no emphasis on special processes, rather the objective is to present the basicprinciples, which can then be employed for screw design and scale-up Accordingly, thisintroduction also touches on some of the concepts discussed in the following chapters
In contrast to single-screw machines, an essential aspect of closely intermeshing co-rotatingtwin screws is that the flights mesh tightly except for the necessary clearance The screws,and thus the machine, are designated as kinematically “self-cleaning” Compared to a normalsingle-screw machine, where the flights scrape the inside of the housing (while maintaining
a certain clearance between the screw and housing), the flights in a closely intermeshingtwin-screw arrangement also clean each other Conceptually, the twin-screw arrangementcan thus be understood as a primary screw and a “cleaning screw”
The first closely intermeshing twin screw extruders were built by Bayer using their owndesign (Section 2.1) and featured a vertical arrangement of the screws Figures 1.2 and 1.3show such a screw arrangement built by Bayer for chemical reactions
The extruder shown in Figs 1.2 and 1.3 has been restored by Coperion Werner & Pfleidererand is on display
As described in the preface, the first co-rotating twin screw extruders in the ZSK series wereput into operation in 1957 According to Herrmann [1], ZSK is the abbreviation for theGerman expression “Two-shaft kneading disc extruder” (Zweiwellige Knetscheiben-Schneckenpresse) The expression “twin-screw compounder” used in the preface is the term
Trang 17Figure 1.3: Screw shown in Fig 1.2, view from above
usually encountered today Other companies have introduced different abbreviations for thistype of screw arrangement
As a laboratory and trial machine, the machine shown in Figs 1.2 and 1.3 has no guards inorder to provide easy access and be readily convertible In contrast, the machines sold by WP(then known as Werner & Pfleiderer, but today called CWP, Coperion Werner & Pfleiderer)were fully enclosed in the “fashion of the day” see Fig 1.4 which shows two ZSK machinesfrom the 1950s Initially, WP also built the ZSK machines with a vertical screw arrangement,
as the sectional drawing in Fig 1.5 shows The figure also shows that the actual processingsection is very small in relation to the drive unit At that time, the machine brochurepromoted the “oversized drive” (with reliable operation as the benefit)
Truth be told, the available torque was high for machines built then, but over the course oftime it was possible to increase it even further Chapter 14 presents further developmentincluding today’s Megacompounder PLUS In addition to the torque, screw speeds alsoincreased to over 1000 rpm
Chapter 2 presents a detailed summary of additional historical developments relating to rotating twin screw extruders Here, the modular approach to screw configuration will bespecifically detailed The shaft with screw flights is no longer manufactured “in one piece”,but rather consists of a core shaft with slipped-on screw elements and kneading elements.Chapter 12 provides an overview of the many screw elements employed and their principle
co-of operation An extensive review co-of patents relating to screw elements and screw geometriescan be found in Chapter 13
Chapter 5 presents a very detailed description of how to create the basic geometries forconveying and kneading elements For closely intermeshing profiles, the geometric cross-section of these screw elements depends on only three characteristics: the number of flights,the diameter and the distance between shaft centers (Chapter 5, Fig 5.4) The aspect offlight pitch as it relates to the screw profile comes into play only in terms of necessaryclearance Thus, six elements are needed to establish the basic geometry: the number of
Trang 18Figure 1.4: Werner & Pfleiderer’s ZSK machine from the 1950s Original figure caption: “In the 1950s
WP, together with major chemical companies, made a significant contribution to the burgeoning age of plastics The top figure shows a plasticizing unit for producing flexible PVC; the bottom the first twin-screw extruder for compounding plastic pellets.”
Trang 19Compounding systems delivered
Total 6,890
More than 25% of all compounding systems installed worldwide were manufactured by Coperion Werner &
Pfleiderer
flights, the housing diameter, the distance between shaft centers, the flight pitch, theclearance between the screws themselves, and the clearance between the screw and housing.For the clearance between the screws themselves, a “clearance strategy” must be developed,see Section 4.2 The so-called planar offset represents a good compromise
The technical concept of co-rotating twin screw extruders was quickly adopted verysuccessfully for the processing and compounding of plastics, see Fig 1.4 Figure 1.6 showsthe compounding machines sold by CWP alone for various products until mid 2006.Co-rotating twin screws are very important when it comes to compounding, because duringcompounding, several processing steps must be performed in a single screw machine; eg.,melting the pellets, incorporating fillers, and building up pressure to discharge the product.Chapter 4 deals with compounding in great detail In addition to describing the settings andprocess variables, practical information about the design of compounding machines isprovided
It would be desirable, of course, to be able to design screw machines without the need foraccompanying trials In recent years both 2- and 3-dimensional models were developed,
with which the flow, temperature, and pressure fields in the machine can be described.Chapter 5 provides an introduction to this subject, while Chapter 6 presents the results of3-dimensional modeling The boundary conditions and the material characteristics that areincorporated into the calculation are crucial for 3-dimensional calculations Viscosity, whichcan vary over several orders of magnitude as the result of the influence of shear andtemperature, must be mentioned in particular Chapter 3 presents a detailed overview of therheological characteristics of polymer melts The behavior of viscoelastic fluids should bementioned here especially
One objective of presenting screw models in this book is to proceed from the simple tothe more difficult With this in mind, the descriptions have focused on models based onreliable principles The contributors from Bayer Technology Services have intentionally notincluded descriptions of some models for processing steps, as these models require further
Trang 203 1
1
D n
V A
11
1
2 3
D p A D n
V A
A1 and A2: intersection points on axes
Kinematic parameter of flow Pressure characteristic
B1 and B2: intersection points on axes
11
1
2 2 2 3
P B D n
V B
Power characteristic
3 1
1
D n V A
3
D n V
L D p
Back
conveying
screw
Forward conveying screw
Overrun screw
refinement in order to ensure reliable application The various models for melting of plasticpellets are noted here as an example
Very important among the screw models are the 1-dimensional models applying thedimensionless parameters introduced by Pawlowski [2] for highly viscous fluids with aconstant viscosity In this case, there are linear relationships for the pressure and powercharacteristics as a function of throughput The dimensionless representation, see Fig 1.7,often used in this book is thus especially important
The approach taken by Pawlowski for single screws was applied to twin screws by Ulrich.Böhme has proven the general theoretical relationship for creeping flow [3]
The dimensionless constants of a specific screw, the “intersection points on the axes”, A1, A2,
B1 and B2 as shown in Fig 1.7 which depend on the actual screw geometry, have a
“fundamental value”, just like the pressure constant 128/p for flow in a pipe, see Chapter 6,Fig 6.13
In principle it is possible to use the screw models for scale-up However, with the modelsavailable today, it is not yet possible to calculate all of the process steps such as melting,mixing of components, and flow processes with mass and heat transport very accurately.There are also limitations when it comes to partially filled screw segments Details of theseefforts can be found in Chapter 6, Fig 6.17 Chapter 11 demonstrates how an approximatescale-up is possible without models using calculations for the screw segments that arerelevant to real-world practice
Chapter 9 presents the basic principles of mixing and dispersion In addition, this book alsocontains numerous applications based on these principles The concept shown in Chapter
15 for processing bimodal polyolefins is an example in this respect Here, two co-rotatingtwin screws arranged in series, with the first being used for melting and the second fordispersing and mixing Additional applications in the chemical and pharmaceuticalindustries as well as applications in food processing are presented in Chapter 14
Trang 21Chapter 17 presents an overview of the drive units for co-rotating twin screw extruders,along with a discussion of the need for high torque at low speeds.
References
[1] Werner, H.: Schneckenmaschinen in der Verfahrenstechnik, Springer-Verlag Berlin, Heidelberg,
New York: 1972
[2] Pawlowski, J.: Die Ähnlichkeitstheorie in der physikalisch-technischen Forschung, Springer-Verlag
Berlin, Heidelberg, New York: 1971
[3] Böhme, G.: Theoretische Betrachtungen über schleichende Strömungen In: Festschrift,
Universität-GH Essen 1995, S 27–40
Trang 22M ARTIN U LLRICH
This chapter covers the engineering-related history of the twin-screw, or more precisely theco-rotating twin screw extruder, where both screw shafts rotate in the same direction In thefollowing, we will simply refer to them as co-rotating extruders (instead of co-rotating,twin-screw extruders)
The best place to start is with a brief account of the origin of all multiple screws, namely thesingle screw extruder Its inventor, Archimedes [1] (approximately 2250 years ago), used it
to transport water overcoming differences in elevation The same principle is still used today
in Egypt, Holland, and in many water purification plants
The single screw as an “extrusion apparatus” was developed in the second half of the 19thcentury and was used intensively in industrial and heavy engineering applications It wasused in three major industries:
Pottery industry: ceramic compounds
– extrusion, shaping
Rubber industry: natural rubber, gum
– plastification, extrusion,
– profile production
Food industry: oily fruits, oil seeds
– extracting biological oils,
– separation of material using “strainer screws”
– meat processing by meat grinder
Product feed in a single screw initially appears somewhat strange While every metalmolecule remains in the same cross-sectional plane, the product is nevertheless conveyedaxially Following is an attempt at an explanation: when considering the screw and theproduct, the screw rotates without changing position, although the product does not rotatebut slides axially, in other words, it is axially conveyed This so-called “theoretical” conveyingdoes not exist in practice, however, since the product is not a solid body but a highly viscousfluid with a rheological character
Trang 23The adhesion and friction characteristics of the plastic material determine the intensity ofthe flow In the case of Newtonian fluids this is half of the theoretical conveying (at constantpressure) and even less with counterpressure (extrusion), even down to zero In the lattercase, the product rotates with the shaft and throughput ceases.
This weakness of the single screw, particularly the fact that there is no cleaning of the shaftand the strong dependence of conveying on rheological properties, motivated inventors toseek solutions to these problems The co-rotating extruder was therefore initially proposed
as a self-cleaning mechanism Six patent citations [2 to 7] over a 70-year period (1869 –1939) show that the co-rotating extruder remained very much at the forefront of engineers’minds J L White [44] provides an extensive, thorough description of the developments andpatent situation in this field
In the early 1940s, a systematic investigation into the co-rotating extruder system began atthe IG plant in Wolfen, Saxony-Anhalt It involved the combined physical, mathematical,engineering, and mechanical expertise of a team composed of W Meskat, A Geberg, R.Erdmenger, and their staff The team was commissioned to develop a reliable “mechanicalapparatus” for chemical processes with highly viscous products
The work was continued at Bayer AG in Leverkusen with strongly process-orientated groups[8] under the new “applied physics” (AP) organizational structure introduced by K Riessand implemented by K Sigwart in 1948 R Erdmenger founded and led one of theseengineering groups, which was composed of 10 to 15 specialists and employees, until 1976.The group was later given the name “High Viscosity Technology”
Naturally this team, part of the chemical industry, was primarily involved with solvingproblems in the area of high-viscosity engineering, particularly in developing chemicalprocesses for the Bayer AG The mechanical aspect was developed as required and to varyingdegrees of intensity
With respect to screw geometry, we turn back the clock to Wolfen, where the team wassearching for the perfect mechanical apparatus for high-viscosity technology It had tofunction despite the adhesive, frictional, and antifrictional properties of the product, copewith various material consistencies, and overcome rheological changes caused, for instance,
by reactions in the machine
The desired self-cleaning function led to the development of the intermeshing twin screw.The counter-rotating screw was discarded, because it tended to get blocked by solids andwas a poor mixer; attention focused instead on the intermeshing, co-rotating extruder
A Geberg addressed the geometric kinematic problem with a mathematical equivalent view
He discovered the fact that the co-rotation of two shafts around their fixed axes is thekinematic equivalent of the “movement without rotation” of one shaft around another fixedshaft (Fig 2.1) In the case of this so-called “movement without rotation”, which happenswhen the profiles are touching, all mass points of the moved screw move in circles with radiiequivalent to the centerline distance (Fig 2.1)
Trang 24Screw 1 idle
Screw 2 pushed Position A Position B Position C
48
Centerline distance A
A
x
x y x
r =A
Since the – mathematically precise – system is intended to be fully wiping, the central shaftcan be a wax blank that is shaped to its corresponding contour by the metal moved screw.The moved screw (Fig 2.1) with its metal tip × then forms the flank arc y (bold) in the fixedwax shaft As all mass points of the moved screw describe circles with a radius equal to thecenterline distance, including the tip x, the flank arc y of the wax screw must also be an arcwith a radius equal to the centerline distance of the two screw shafts: an astonishingly simplesolution
Trang 25Calculation of angle in twin screw system
1 c 1 2c
δ δ 2
1 2c
1 2c 2
β 3 = 60 - δ = f3(c)
δ = Flank angle
β 1 Tip angle of single-flighted screw
β 2 Tip angle of double-flighted screw
β 3 Tip angle of triple-flighted screw
Tip width = Incline xβ
360 Flank width = Incline xδ
360
Real screws do not have points in position x They have specific tip widths (Fig 2.2), whichhave previously been omitted in order to clarify the kinematics (Fig 2.1) It helps here todetermine the kinematics in cross-section, then to advance the resulting cross-sectionprofiles axially, and finally to apply a twist to obtain the longitudinal section contour andthe desired three-dimensional screw (Fig 2.3)
A Geberg supplemented his investigations by determining the basic geometries of screws inpractical applications with varied parameters: number of threads and channel depth andtheir dependent variables, tip angle (Fig 2.4), and free cross-sectional area that can be filledwith product (Fig 2.5)
Trang 26Appendix 2
Calcualtion of free cross-section surface of
single-, double- and triple-flighted twin screws
(plus single-flighted twin screws assembled
from eccentric discs)
With double-flighted screw,
c must not be greater than 2.41
With triple-flighted screw,
c must not be greater
The following applies to single-flighted screws:
c < 1.5 and F1= F circular ring:
In 1978, during his retirement, R Erdmenger published a paper [9] looking back over hiscareer and the significant role he played in the development of the co-rotating extruder Inthis paper he cites the work of W Meskat, A Geberg, R Erdmenger (1944) on the basicgeometry in Wolfen [10], extracts from R Erdmenger, S Neidhardt (1948) in Leverkusen[11], and his own tests on laboratory screws in Leverkusen in 1948/51 [12]
The bibliography includes 11 patents [13 to 23] with inventor, year of application, patentnumber, and one of the characteristic abstracts describing the fundamental subject of thepatent
They concern co-rotating extruders with a self cleaning profile, all relevant types of flighted, double-flighted and triple-flighted) mixing and kneading discs, modular designs,mixed-technology, backwards-pumping screw principle, and special designs for dewateringand evaporating
(single-It is interesting to note that
the 20 years in which these 11 patents were issued is far shorter than the preliminaryperiod mentioned above, and
the material of the co-rotating extruder system is now precisely mathematically founded,
is used in process engineering, and is being advanced by engineers
2.2.2.1 Basic Patent of Threaded Screws
In view of its significance, an extract from the basic patent for the co-rotating threadedscrew [13] is reproduced here (Figs 2.6 and 2.7):
Trang 27Figure 2.6: Basic patent for threaded screws – title page and patent claims
Trang 28Figure 2.7: Basic geometry (illustration 3 according to patent)
The following items are of interest and worthy of mention:
After the application in 1944, this patent was issued in West Germany in 1953 via a
“bridging act” and was given a five-year post-war extension of validity (23 instead of 18
years, i e., valid until 1967)
The rapidity with which the language of technology changes is evident from the titlegiven in 1944: “Vorrichtung zum Verkneten, Gelatinieren und Verpressen von plastischenMassen” (Apparatus for kneading, gelatinizing and compressing of plastic masses).Just 20 years later, in the 1960s, the wording was as follows: “Vorrichtung zum Kneten,Plastifizieren und Extrudieren von Polymeren” (Apparatus for kneading, plastificationand extrusion of polymers)
The first and the primary claims are extremely telling: Double-or multiple-shaft rotating extruders in single or multiple-flighted versions with the sealing profile, i e.,defined with the flank profiles in cross-section by circles with radius equal to thecenterline distance (see Section 2.2.1)
co-The second and final claim with the varied tip width along the axis with correspondinglyvaried groove widths of the neighboring shaft (for the purpose of increasing the mixingeffect) has lost its significance with the subsequent invention of simpler kneading discs(see below)
In his paper [9], R Erdmenger added material movements with explanations to Fig 1 of thepatent [13] (Fig 2.8): a is the purely axial movement (= theoretical conveying, see Section2.2 above), b is the drag flow of a Newtonian fluid (b a), and c is the minimal flow of amaterial adhering to the shafts (c b)
2.2.2.2 Basic Patents for Kneading Discs, DBP [16], USP [17], DBP [20]
Kneading discs are prismatic bodies formed by axial displacement of the cross sectioncontour discussed above but without rotation common with screw threads (see Section2.2.1)
Trang 29Figure 2.8: Multiple shaft co-rotating extruder system with full intermeshing and mutual cleaning of
the shafts There is a continuous connection between A, B, C, and D etc The possible material movements in principle are as follows:
a) Purely axial shift of a rigid sliding body (e g., wax)
b) Dragging of deforming substances (e g., stringy liquids) in the direction of the arrow (angle h), shearing in the gap, section G H.
c) Purely tangential conveyance of materials adhered to the screws with smaller axial components (x) at transition from shaft I to shaft II.
Several kneading discs arranged on a core shaft create kneading elements (or kneadingblocks) with special effects The geometric variables of these kneading elements are thethickness of the individual discs and their offset angles with respect to one another, viewed
in cross-section, and the resulting rotation of the individual kneading discs with the effect
of a spiral staircase This results in kneading, crushing, and shearing effects combined withactive conveying (right-hand rotation), neutral, or reversing effects (left-hand rotation).The same principles apply for the cross-sections of kneading discs as for the cross sections
of their corresponding screw threads (see Section 2.2.1) The geometrical form isdetermined by number and depth of flights
Trang 30Figure 2.10: Operating method of eccentric, synchronous, co-rotating, circular discs:
a, b, c three operating positions: mass M is crushed and sheared in the key during the quarter turn around their rotation axis described by each disc (I and II).
Figure 2.9 represents the normal case of a single-flighted kneading disc Figure 2.10 showsthe interesting special case of the single-flight, self-cleaning circular discs
Figure 2.11 also shows the normal double-flighted kneading disc, also with Erdmenger’scommentary The practical limit for double-flighted elements is shown in Fig 2.12 with asmall tip angle and large product volume (see Section 2.2.1)
a Commencement of kneading of deformable material (M)
b End of the kneading phase
The mass M is crushed twice per revolution due to the co-rotation of discs I and II (approach of points 1–1) and escapes vertically to the indicated plane.
Trang 31Figure 2.13 explains the triple-flighted kneading disc and its effect according to R.Erdmenger [9].
a, b, c three operating positions.
The mass M is forced to flow around the sharp barrel ridge G three times per revolution
by the synchronous rotation of the co-rotating discs I and II wherein it is sheared to an extreme degree.
2.2.2.3 Basic Patents for Modular Design
Further patents describe the modular principle for different screw and kneading elementsthreaded and clamped to a core shaft The benefits are striking:
Formation of different functional zones along the machine with optimized technicalprocesses
Variation of the screw geometry for tests and at start up
Adjustment to subsequent process changes
Typifying and standardization of screw and kneading element production
Simplification and cost savings in spare parts acquisition
These benefits resulted in the USP [21] with priority 1959 for R Erdmenger The modulardesign is illustrated in Fig 2.14 below with a combination of different consecutive screw andkneading zones and in Fig 2.15 as a modular assembly diagram
Trang 32Figure 2.15: Modular assembly diagram
W Meskat und J Pawlowski obtained a German patent DBP [18] by 1950 for a co-rotatingextruder with sections featuring localized backward-feed with respect to the primary feeddirection This was achieved by backward pumping screws, i e., by right-to-left handrotation combinations in the screw geometry whereby specific mixing effects are achieved.Figure 2.16 shows this apparatus in detail
These three groups of patents, namely, the widely used threaded screws, the kneading elements, and the modular design principle paved the way for the development of a highly
effective new machine type for process engineering and plastics technology
At this point it is also worth mentioning the (modified) comment made by H Herrmann[36]:
“W Meskat, A Geberg and R Erdmenger’s idea of a co-rotating and closely intermeshingtwin screw of 1944 was readdressed and turned into a complete and well founded
geometrical solution Since this time it has been possible to talk of close meshing and cleaning co-rotating extruder machines.”
Trang 33self-2.3 Pioneering Period
R Erdmenger had worked together with A Geberg since 1943 in W Meskat’s twin-screwcore team in Wolfen, and he pursued this work intensively after 1945 for Bayer AG inLeverkusen, achieving success and acclaim with his team His work comprised threefundamental components:
1.) The necessary in-house machine development for co-rotating extruders.
2.) The use of these machines in chemical processes and/or the realization of innovative
chemical processes using co-rotating extruders
3.) The issue of the license for this machinery system in the face of increasing demand.
With respect to the first point it is obvious that laboratory screws, developmentalexperiments, and prototype product machines were developed with the workshoptechnology available in-house Here in particular, Erdmenger’s university training inmechanical engineering and his leadership were an essential driving force on the road tosuccess The team produced the DA/DK = 32/24 mm laboratory screw as a Bayer housemodel around 1948
Figure 2.17 illustrates another example of a highly specialized machine from the pioneeringperiod Produced around 1955, it has a degressive pitch in the transport direction, which,surprisingly, was cut from the solid feedstock
Trang 34Solution with metering pump
Kneading screw
Eulan, dry
Feed Duo gear
Böehringer C 68 variable speed gear 10.5 kW
n = 1,500
3 kW Hutt pelletizing device
Pellets Paste
Figure 3: Schematic plant of pasting and pelletizing of Eulan
The application of co-rotating extruders in chemical processes only stood a chance, if thenew components of the co-rotating extruder made the product
– more economically viable, or
– to a higher quality, or
– more environmentally friendly.
This is taken into account in an engineering sketch drawn by R Erdmenger himself in 1953
of a chemical screw process (Fig 2.18) From today’s point of view, the last two illustrationsdisplay the historical patina and archaic simplicity of engineering times long past The samealso applies for Fig 2.19 which shows a somewhat rustic experimental center from 1957
Trang 352.3.3 Licensing
Licensing of the new technology came to fruition in 1953 After intensive preliminary workand negotiations with several mechanical engineering firms, the noted company Wernerund Pfleiderer in Stuttgart obtained the exclusive worldwide license (as is so often annoyingfor the inventor, nine years had passed since the initial invention in 1944) Although thelicensee today operates under the name “Coperion Werner & Pfleiderer GmbH & Co KG,Stuttgart”, we will refer to them as “Werner & Pfleiderer” in the following
A key reason for the issue of the license was that in-house production in chemicalworkshops was no longer possible for screw diameters beyond 120 mm At the same time,there was a demand for higher quality machinery
The development period of the co-rotating extruders, which was shaped by R Erdmenger(he is portrayed in Fig 2.21), is considered the pioneering period for this technology today
Award” for R Erdmenger in 1986
After his retirement, Erdmenger received two awards in the US:
the “Distinguished Service Award” of the Society of Plastics Engineers “SPE” (Fig 2.20),and
introduction into the University of Akron/Ohio’s “Hall of Fame”
Trang 362.4 New High Viscosity Technology with Co-Rotating
Extruders
Soon after the patents cited above were issued, publications about the new types of rotating extruders and their applications began to appear in increasing numbers Thesecondary literature also cites further publications [24 to 35] from the period between 1951and 1974, many of which were authored by the licensors: Bayer, i e., predominantly R.Erdmenger and the licensee: Werner und Pfleiderer, i e., H Herrmann These publicationsincreasingly pushed process engineering and the various uses of the co-rotating extruder
It is to the great credit of H Herrmann that, after the licensing process and alongside hisprimary developmental role at Werner & Pfleiderer, he edited the standard work on thistopic, published in 1972 [36] In this work, two further important points are worthemphasizing: on the one hand, the mechanical engineering description and historicalclassification of all known screw types and on the other hand, the appreciation of the co-rotating extruder along with their process engineering application areas and operatingmethods
J Pawlowski extended the knowledge base and deepened understanding of the physicalfunctions and operating methods of screw machinery considerably through the
development of the similarity theory and its application to these machines [37 to 39] His
work significant consequences:
– Seven independent, dimension-specific values for filled screws were reduced to just threedimensionless parameters for throughput, pressure, and power (Fig 2.22)
– The dependencies of these dimensionless parameters for Newtonian fluids can besimplified to two linear equations (Fig 2.23) for the target values (measured on a co-rotating extruder) of pressure (Fig 2.24) and power (Fig 2.25) These straight lines arefixed by their axial sections A1, A2for pressure and B1, B2for power The value of these
“profile parameters” depends on the “internal geometry” of the screw, i e., on the shape
of the profile (see Section 2.5.1) and on the screw type (single shaft, multiple shaft, rotating, counter-rotating) The “external geometry” (d, L) is already incorporated intothe three dimensionless parameters (Fig 2.22)
co-The four profile parameters are determined experimentally in a far less time consuming
process The measurements for co-rotating extruders from [40] in Figs 2.24 and 2.25 show
that Pawlowski’s similarity method is valid in terms of flow engineering [37] even for thiscomplex screw type
Trang 38Figure 2.25: Power measurements on a co-rotating extruder
The same data, determined by not too complex means, can also be used to answer furtherquestions, e g., relating to the efficiency of the pressure build-up Figure 2.26 illustrates the
“product stress” (the reciprocal value of the pump efficiency) comparing counter-rotatingand co-rotating extruders [40], i e., two substantially different machines
The high-viscosity processes developed during and after the pioneering period and theapplications discovered for high-viscosity machines [8] are as striking as they are varied
Trang 39Better quality and higher throughput
Mixing, compounding, kneading, pasting,
dissolving
Plastics, dyes, fibers, silicon polymer
Polymerization, polycondensation, polyaddition
and other reactions
Thermoplastic polyurethranes, silicone polymers
The process engineering tasks are classified according to the product groups for which theyare used There are four main areas: material mixing, material separation, reaction, andphase transformation The common factor of all the processes in Table 2.1 is of course thehigh-viscosity phase and consequently the specific “difficulties” of the machinery (technicaldesign, dimensioning with respect to forces, torques, drive power, pressures)
Along with the work within the parameters of the basic operations, the process engineer has
to investigate important processes in-depth to develop and optimize those processes (Fig.2.27) [8] This involves meeting the constantly increasing market demand for higher qualitywhile maintaining economic viability, in other words increasing throughput From aphysical point of view, however, the residual content of volatile materials increasesdramatically as the throughput increases Therefore, specific process engineering measuresare required to reach the goal of increasing throughput by a factor of six while reducing theresidual content by a factor of five (Fig 2.27)
Trang 402.5 Special Developments from Bayer-Hochviskostechnik
(High Viscosity Technology Group)
During the pioneering time of co-rotating extruders, predominantly graphical methodswere applied for the basic geometry illustrated in Section 2.2.1 and the technical design
of new machines, but the advances in automated computing technology soon proved to
be ideal for the design of co-rotating extruders Here, the internal geometry of the screw(Fig 2.28) is determined by six independent geometrical factors:
1 Internal barrel diameter d
The clearances d and S are not required for the fully-wiping screw (see Section 2.5.2) Thisleaves four independent factors Other six-fold combinations of independent geometricalvalues can also be used
The application of the basic geometry by computer for a screw described in this way (seeSection 2.2.1) results in the calculation of the profile curves in cross-section and inlongitudinal section and, for example output for manufacture Process engineering targetvalues, such as the usable product volume and all surfaces, are also determined