mixing in single screw extrusion

Mixing in Single Screw Extrusion Martin Gale i Preface 1 1 The Need for Good Mixing in Single Screw Extrusion 3 1.2 Examples of Mixing Problems 9 1.2.1 Polyethylene Pipes and Cables 9 1.2.2 Blow Moulded Bottles 11 1.2.3 Chalk Filled Polypropylene Pipe 12 1.2.4 Blown Film 13 1.2.5 Industrial Blow Mouldings 13 1.2.6 Production Scrap Re-use 13 1.2.7 Agglomerates and Gels in Thin Extrusions 14 1.2.8 Transparent Polycaprolactone/SAN Blends 15 1.2.9 Decorative Wood Grain Effects 15 References 16 2 Dispersive and Distributive Mixing 17 2.1 Definitions and Illustrations 17 2.2 Dispersive Mixing 20 2.2.1 Dispersive Mixing Mechanisms 20 2.2.2 Dispersive Mixing of Additive Powders Such as Pigments 24 2.3 Distributive Mixing 29 2.3.1 Laminar Shear Flow Mixing 29 2.3.2 Measurement of Distributive Mixing Achieved by Laminar Shearing 32 C ontents ii Mixing in Single Screw Extrusion 2.3.3 Limitations of Lamina Flow Mixing 34 2.3.4 Eliminating Laminar Striations 36 References 55 3 Measurement of Mixing 59 3.1 The Need for Measurement of Mixing 59 3.2 Striation Thickness Measurement 60 3.3 Agglomerate Measurement 61 3.3.1 Microscopy Examination of Thin Samples 61 3.3.2 Agglomerate Count for Blown Film 62 3.3.3 Screen Pack Filtration Test 62 3.4 Influences of Mixing on Product Properties 68 3.5 Preparation of Thin Sections for Optical Microscopy Assessment 69 References 69 4 Single Screw Extruder Stages: Effects on Mixing 71 References 75 5 Pellet Handling: A Source of Variable Composition 77 5.1 Introduction 77 5.2 Hopper Design 78 5.2.1 Mass Flow Hopper 78 5.2.2 Non-mass Flow Hopper 79 5.2.3 Round Hoppers 80 5.2.4 Square and Rectangular Hoppers 81 5.2.5 Ledges and Corners 81 5.3 Composition Variations 82 5.3.1 Example 1 82 5.3.2 Example 2 82 5.3.3 Example 3 82 iii Contents 5.3.4 Other Systems 83 5.4 Measurement of Particulate Properties 84 5.4.1 Hopper Flow Tests 84 References 85 6 Solids Conveying in the Feed/Transport Zone 87 6.1 Smooth Feed Zones 87 6.2 Grooved Feed Zones 90 6.3 Particulate Friction Measurements 96 6.4 Friction in the Feed Zone 99 References 100 7 Melting 101 7.1 Melting Mechanism 101 7.2 Variations in Melting Rate 103 7.3 Solids Bed Break-up 105 7.4 Melting Devices 107 7.5 Barrier Flight Melting Screws 115 7.5.1 The Barrier Screw Concept 115 7.5.2 Maillefer Barrier Screw 117 7.5.3 North American Barrier Screws 118 7.5.4 Combined Barrier Screws and Grooved Feed Zones 123 7.5.5 Barrier Screw Developments 124 7.6 Other Melting Screws 125 7.6.1 Double Wave Screw 125 7.6.2 Barr Energy Transfer Screws 126 7.6.3 Stratablend Mixing Screw 126 7.6.4 Shear-Ring Screw 127 iv Mixing in Single Screw Extrusion 7.7 Barrier Flight Screws versus Conventional Screws 127 References 131 8 Screw Channel Mixing and the Application of Mixing Sections 135 8.1 Striations: Their Formation and Mixing in the Screw Channel 135 8.2 Mixing During Melting 137 8.3 Mixing in the Melt Filled Screw Channel 137 8.4 Residence Time Distribution (RTD) 144 8.4.1 Concentration Smoothing 147 8.4.2 Variation of Residence Time with Channel Position 147 8.4.3 Implications of Pressure/Drag Flow Effects 147 8.5 Mixing Sections 148 8.5.1 Maddock Mixer 148 8.5.2 Pins and Slots 149 8.5.3 Mixer Evaluation Using an Independent Drive 152 References 164 9 Interacting Rotor/Stator Mixers 167 9.1 Overview 167 9.2 Turbine Mixing Heads 168 9.2.1 Stanley (ICI) Mixer 168 9.2.2 Other Turbine Mixers 170 9.3 Woodroffe Key Slot Mixers 171 9.3.1 Gerber (Metal Box) Mixer 171 9.3.2 Renk (Barmag) Mixer 172 9.4 Rounded Cavity Mixers 176 9.4.1 Rapra Cavity Transfer Mixer 176 9.4.2 Reifenhauser Staromix 184 References 186 v 10 Floating Ring Mixing Devices 189 10.1 Introduction 189 10.2 Injection Moulding Check-ring Mixers 189 10.3 Adaption of the Check Ring Mixer to Extrusion 193 References 196 11 Static (or Motionless) Mixers 197 11.1 Mixing Mechanism 197 11.2 Static Mixers Used in plastics extrusion 197 11.2.1 Helical Mixers 198 11.2.2 Honeycomb Mixers 199 11.3 Application in Heat Exchangers 200 11.4 Disadvantages 200 References 202 12 Incorporation of Liquid Additives and Dispersions by Direct Addition 203 12.1 Viscosity Differences 204 12.2 Incorporating Liquid Additives 204 12.3 Some Examples of Liquid Injection Processes 208 12.3.1 Polybutene in Pallet-wrap and Silage-wrap Film 208 12.3.2 Injection of Liquid Colours (General) 208 12.3.3 Wire Insulation Colouring 209 12.3.4 Fibre Extrusion 213 12.3.5 Skin Colouring Pipes and Profiles 214 12.3.6 Crosslinking Polyethylenes 216 12.3.7 Silicone Lubricant Injection 220 12.3.8 Extrusion Foaming 220 References 227 Contents vi Mixing in Single Screw Extrusion 13 Dispersive Mixing of Fillers and Pigments 229 13.1 Formation of Agglomerates 229 13.2 Formation of Filler Agglomerates in a Single Screw Extruder 230 13.3 Starved Feeding to Avoid Agglomerate Formation 234 13.4 Dispersive Mixing Using Polymer Powders 239 13.5 Dispersive Mixing Using Polymeric Waxes 239 References 242 14 Dispersive Mixing Applied to Polymer Blending 243 14.1 Polymer Blends 243 14.2 Polymer Scrap 246 14.3 Polymer Waste 246 14.4 Blending Immiscible Viscous Fluids 246 14.5 Polymer Blending Mechanisms in a Single Screw Extruder 250 14.6 Break-up of Fibrils into Droplets 252 14.7 Polymer Blending in Single Screw Extrusion: Overall Mechanism 254 14.8 Mixing by Controlled Continuous Chaotic Advection 257 14.9 Blending Mixed Polymer Waste: Comparison of Twin Screw and Single Screw Extruders 259 14.10 Elongational Flow Mixing 261 14.11 Elimination of Gels 262 References 263 15 Compounding with Single Screw Extruders 269 References 270 Appendix – Preparation of Microtome Sections for Assessment of Dispersive and Distributive Mixing 273 Flattening Sections 273 vii Trimming the Block 274 Flattening the Rolled Sections 276 Holey Sections 276 Brushing Flat 276 Distortion 276 Washing and Mounting 277 Abbreviations 279 Index 281 Contents viii Mixing in Single Screw Extrusion 1 P reface Most extruded plastics products contain additives and therefore mixing is involved at some stage in their production. Mixing is normally associated with twin screw extruders, and conversion to products associated with single screw extruders. Consequently, the latter’s potential mixing performance and economic gains tend to be overlooked. During the many years I was involved with the Smithers Rapra training course: Exploring Extrusion, the subject of mixing in single screw extruders always generated a lot of interest. It seemed, therefore, logical to treat this subject in more detail, particularly with regard to present day economics. The attendees of these training courses came from a very wide spectrum of expertise and experience. These included engineers, chemists, supervisors, plant operators, quality controllers, technical service and sales people. I decided to write this book with this readership in mind. As a consequence, I have paid little attention to mathematical derivations and instead concentrated on the results. In any case, extrusion theory is very well covered by a number of books on extrusion to which I have referred. Most of these books, which cover specific topics in depth have individual authors for each chapter, each one an expert in their field. By writing a book completely on one’s own, this advantage is denied. On the other hand, it gives the author complete freedom to decide what to include and what to omit, to link the chapters together and to make them as long or short as appears justified by each individual topic. I have been very fortunate in having access to the Smithers Rapra Polymer Library – a very comprehensive library which has a number of reports which I produced some years ago. Although some topics may read like a technical review, I have selected only sufficient information to make a point and not exhaustively included every reference. I am very grateful to many people for assisting me with this book: Frances Gardiner (iSmithers) for commissioning and co-ordinating the production. Steve Barnfield (iSmithers) for help, advice and preparation of figures and for typesetting the book and designing the cover. Elaine Cooper (iSmithers) for all her assistance in tracking down old reports. [...]... concerned with single screw extruders, most of the mixing which takes place is ‘distributive’ and accomplished by laminar shearing In comparatively recent developments, dispersive mixing in single screw extrusion has been investigated for polymer blending (Chapter 14) As droplet mixing has traditionally been considered a dispersive mixing process, methods of mixing incompatible polymers using (dispersive)... standards providing their number and size are within specified limits) As described in Chapter 7, the prime function of barrier screws is to control melting Mixing will often be improved as a result of this function since it is dependant on melting as explained in later chapters 19 Mixing in Single Screw Extrusion There are a confusing number of terms used to describe mixing ‘Distributive mixing may be... described as ‘simple mixing, ’ ‘extensive mixing, ’ or ‘blending’ whilst ‘dispersive mixing may be described as ‘intensive mixing and ‘elongational mixing Furthermore, dispersive mixing is usually accompanied by distributive mixing but the opposite does not apply Many of the terms describe the mechanism involved, such as elongation, (or stretching), squeezing (or kneading), laminar shearing, etc As this... Need for Good Mixing in Single Screw Extrusion To meet these requirements, final products often need to contain additives such as colour, antioxidants, slip, antiblock, flame retardants, tackifiers, fillers and so on, which have to be efficiently mixed into the polymer for the product to perform satisfactorily in service Before examining the technology involved in mixing during single screw extrusion of... compounding However, these machines are complex and costly They also tend to be limited to pellet production unless a gear pump or in- line 8 The Need for Good Mixing in Single Screw Extrusion single screw extruder is added downstream to generate the required die pressure to make, for example, sheet containing fillers or fibres In comparison, single screw extruders are simple, rugged, low cost, low maintenance... Having achieved this, the particles must be further mixed to achieve a uniform concentration throughout the polymer These two steps, which are described as ‘Dispersive Mixing ‘ and ‘Distributive Mixing are covered in more detail in Chapter 2, but are introduced here to fit the single screw extruder into the overall mixing picture In general, single screw extruders are unsuitable for dispersive mixing. .. cost, low maintenance machines capable of developing whatever die pressures are required to make a very diverse range of extrusions Consequently, there are ongoing and very diverse approaches into introducing ways of reducing the inherent dispersive mixing limitations of single screw extruders such that single pass extrusion might be used 1.2 Examples of Mixing Problems The following examples illustrate... problem 13 Mixing in Single Screw Extrusion 1.2.7 Agglomerates and Gels in Thin Extrusions The presence of undispersed particle clusters in thin sheet and film can result in holes or thin lines prone to splitting in use Normally an agglomerate will be found at the edge of the hole and at, or near, the start of the thin section line These particles may be additive agglomerates, gels, or contaminants The... using (dispersive) elongational mixing are usually adopted However, it has been found that although the mixing mechanisms which occur in polymer blending can be surprisingly complex, they are achievable using single screw extruders (see Chapter 14) 2.2 Dispersive Mixing 2.2.1 Dispersive Mixing Mechanisms Dispersive mixing is required for the mixing of solid additives into polymers Most additives are... rotating all the sprockets in the same direction, a laminar shear flow field was generated By having two surfaces moving in the opposite direction, the agglomerate remained in the field of view Under these conditions the agglomerate rotated, subjecting the particle interfaces to alternating tension and compression 23 Mixing in Single Screw Extrusion Figure 2.6 Model experiments illustrating laminar . Mixing in Single Screw Extrusion Martin Gale i Preface 1 1 The Need for Good Mixing in Single Screw Extrusion 3 1.2 Examples of Mixing Problems 9 1.2.1 Polyethylene. ‘Dispersive Mixing ‘ and ‘Distributive Mixing are covered in more detail in Chapter 2, but are introduced here to fit the single screw extruder into the overall mixing picture. In general, single screw. Mixing 29 2.3.1 Laminar Shear Flow Mixing 29 2.3.2 Measurement of Distributive Mixing Achieved by Laminar Shearing 32 C ontents ii Mixing in Single Screw Extrusion 2.3.3 Limitations of Lamina