Practical Guide to Rotational Moulding Second Edition Roy J. Crawford Mark P. Kearns 4CC%SMITHERS %))J R A P R A A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.polymer-books.com
(T,s Preface ix 1. Introduction to the Rotational Moulding Process 1 1.1 Introduction 1 1.2 The Rotational Moulding Process 5 1.3 Overview of Rotational Moulding 11 1.4 Special Nature of Rotational Moulding 13 1.5 Advantages of Rotational Moulding 15 1.6 Disadvantages of Rotational Moulding 17 1.7 Common Applications for Rotomoulded Products 17 1.7.1 Material Handling Products 18 1.7.2 Industrial Products 18 1.7.3 Environmental Products 19 1.7.4 Leisure Products 19 1.7.5 Marine Products 20 1.7.6 Road Signage 20 2. Moulds 21 2.1 Introduction 21 2.2 Mould Materials 22 2.3 Sheet Steel 23 2.4 Aluminium 23 2.4.1 The Aluminium Casting Process 24 2.5 Electroformed Nickel 25 2.6 Comparison between Mould Materials 26 in
Practical Guide to Rotational Moulding 2.7 Mould Design 27 2.7.1 Mould Frame 29 2.7.2 Moulded-in Inserts 30 2.8 Moulded-in Structural Components 34 2.9 Temporary Inserts 36 2.10 Threads 37 2.11 Mould Clamps 38 2.12 Mould Guides and Hinges 41 2.13 Automatic Mould Movement 44 2.14 Mould Venting 46 2.15 Automatic Venting 47 2.16 Mould Surface Finish 50 2.17 Mould Outer Surface - Heat Transfer Enhancement 51 2.18 Mould Shielding (Internal) 53 2.19 Mould Shielding (External) 54 2.20 Mould Release 55 2.20.1 Mould Preparation for Release Agent 56 2.20.2 Reactive Systems 57 2.20.2.1 Spray-on Zinc Stearates 57 2.20.2.2 Silicones 57 2.20.2.3 Disiloxanes 57 2.20.3 Conventional Systems 58 2.20.4 Permanent Systems 58 2.20.5 Hybrid Release Systems 60 2.21 Moulding Aids 60 2.22 Airflow Amplification 69 3. Rotational Moulding Machinery 73 3.1 Introduction 73 3.2 Types of Rotational Moulding Machines 73 3.2.1 Carousel Machines 74 3.2.2 Shuttle Machines 76 3.2.3 Clamshell Machines 77 3.2.4 Rock and Roll Machines 79 IV
Contents 3.2.5 Other Types of Machines 81 3.2.6 Hot Oil Rotational Moulding machines 81 3.3 Mould Swing 87 3.4 Mould Speed 90 3.5 Speed Ratio 90 3.6 Oven Air Flow Amplification 91 3.7 Cooling 93 3.8 Material Feed Systems 94 3.9 Developments in Machine Control 95 3.10 Internal Air Temperature Measurement in Rotational Moulding 96 3.11 Preparation of Rotolog for Moulding Trials 100 3.12 Monitoring Pressure inside a Mould 104 4. Materials for Rotational Moulding 111 4.1 Introduction Ill 4.2 Typical Characteristics of Rotationally Moulded Plastics 113 4.3 Materials Used in Rotational Moulding 113 4.4 Polyethylene 116 4.4.1 Low-density Polyethylene 119 4.4.2 High-density Polyethylene 119 4.4.3 Medium-density Polyethylene 120 4.4.4 Linear Low-density Polyethylene 120 4.4.5 Metallocene Polyethylene 121 4.4.6 Ethylene-vinyl Acetate 122 4.4.7 Ethylene-butyl Acrylate 122 4.5 Polypropylene 122 4.6 Polyamides (Nylons) 123 4.6.1 Nylon 6 123 4.6.2 Nylon 11 and Nylon 12 123 4.6.3 Reaction Injection Moulding Nylon 124 4.7 Amorphous Materials 125 4.7.1 Polyvinyl Chloride 125 4.7.2 Fluoropolymers 125 v
Grinding/Pulverising Practical Guide to Rotational Moulding 4.8 Other Plastics 126 4.9 Additives Used in Rotational Moulding Materials 127 4.9.1 Fillers 127 4.9.2 Plasticisers 128 4.9.3 Lubricants 128 4.9.4 Stabilisers 128 4.9.5 Anti-oxidants 129 4.9.6 Ultraviolet Stabilisers 129 4.9.7 Flame Retardants 129 4.9.8 Crosslinking Agents 130 4.9.9 Foaming or Blowing Agents 130 4.9.10 Chemical Blowing Agent Technology 133 4.9.11 Pigments 137 4.10 Powders for Rotational Moulding - 138 4.10.1 Introduction 138 4.11 Particle Size Distribution 142 4.12 Dry Flow 145 4.13 Bulk Density 146 4.14 Factors Affecting Powder Quality 147 4.14.1 Gap Size 148 4.14.2 Number of Mill Teeth 148 4.14.3 Grinding Temperature 149 4.15 Other Techniques for Analysing Powders for Rotational Moulding. 150 4.15.1 Powder Shape and Size 151 4.15.2 Melting Parameters 154 4.16 Micropelletising 156 4.17 Colouring of Plastics for Rotational Moulding 159 4.18 Types of Pigments 162 4.18.1 Pigment Selection Criteria 164 5. Design for Functionality 167 5.1 Introduction 167 5.2 Preliminary Design Considerations 168 5.3 Material 168 VI
Contents 5.4 Number of Parts 169 5.5 Size of Moulds 170 5.6 Part Complexity 171 5.7 Wall Thickness Distribution 173 5.8 Tolerances and Shrinkage 177 5.8.1 Shrinkage Guidelines 178 5.8.2 Control of Shrinkage 178 5.8.3 Effect of Release Point on Shrinkage 179 5.8.4 Other Factors Affecting Shrinkage 180 5.9 Warpage 181 5.9.1 Control of Warpage 183 5.10 Residual Stress 185 5.10.1 Short-term Effects of Residual Stresses 188 5.10.2 Long-term Effects of Residual Stresses 188 5.10.3 Cures for Residual Stress Problems 189 5.11 Stiffening of Parts 190 5.12 Double Walled Parts 191 5.13 Corner Radii 192 5.14 Draft Angles 192 5.15 Surface Quality 192 5.16 Surface Texture 194 5.17 Graphics and Surface Decoration 195 5.17.1 Painting 195 5.17.2 Hot Stamping 197 5.17.3 Adhesives 197 5.17.4 In-mould Decoration 197 5.17.5 Post Moulding Decoration 200 5.18 Design of Foamed Sections 202 5.18.1 Solid/Foam Cross-sections 203 5.18.2 Solid/Foam/Solid Cross-sections 204 6. The Future for Rotational Moulding 205 6.1 Materials 205 6.2 Moulds 207 VII
Practical Guide to Rotational Moulding 6.3 Moulding Equipment 210 6.4 The Challenges 212 6.5 The Role that the Moulder Must Play 213 6.6 The Role that the Suppliers Must Play 213 Abbreviations and Acronyms 215 Index 217 viii