Practical Guide to Rotational Moulding

Practical Guide to Rotational Moulding
Second Edition
Roy J. Crawford and Mark P. Kearns
Contents
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
Contents iv
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 . 79v
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 111
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 . 125vi
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 – Grinding/Pulverising . 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 . 168vii
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 207viii
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
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