The Mould Design Guide Peter Jones The Mould Design Guide Peter Jones Smithers Rapra Technology Limited A wholly owned subsidiary of The Smithers Group Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net First Published in 2008 by Smithers Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK ©2008, Smithers Rapra Technology Limited All rights reserved Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder A catalogue record for this book is available from the British Library Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked Soft-backed ISBN: 978-1-84735-088-6 Hard-backed ISBN: 978-1-84735-087-9 Typeset by documen.co.uk Cover designed by Smithers Rapra Technology Limited Printed and bound by Lightning Source Contents Introduction The Injection Moulding Process 2.1 Background 2.2 Machine Design 2.2.1 Machine Base Unit 2.2.2 Clamp Unit 2.2.3 Mould Height 2.2.4 Daylight 2.2.5 Distance Between Tie Bars 2.2.6 Clamping Mechanisms 2.2.7 The Injection Unit 13 2.3 Theoretical Mould Locking Force 19 2.4 The Moulding Cycle 20 2.4.1 Mould Closing Phase 20 2.4.2 Mould Protection Phase 20 2.4.3 Injection (Mould Filling) Phase 20 2.4.4 Holding Time and Pressurising Phase 20 2.4.5 Cooling and Refill Phase 21 2.4.6 Screw Back Phase 21 2.4.7 Mould Open Phase 21 2.4.8 Ejection Phase 22 i Mould Design Guide ii Plastics Materials 23 3.1 Types of Plastics Materials 23 3.2 Definition of Plastics 23 3.3 The Nature of Plastics Materials 24 3.4 Monomers, Polymerisation and Polymers 25 3.5 Classification of Plastics 26 3.5.1 Thermosets and Thermoplastics 26 3.5.2 Homopolymers, Copolymers and Polymer Blends (Alloys) 27 3.5.3 Amorphous and Semicrystalline Thermoplastics 29 3.6 Melting and Solidification 30 3.7 Shrinkage 31 3.8 Engineering and Commodity Plastics 32 3.8.1 Engineering Plastics 32 3.8.2 Commodity Plastics 32 3.9 Material Additives 33 3.10 Flow Properties of Thermoplastic Materials 35 3.11 Variable Molecular Weight 35 3.12 Melt Flow Index (MFI) 36 3.13 Reprocessed Material 37 3.14 Polymer Molecules 37 3.15 Material Names and Abbreviations 37 3.16 Material Applications 40 Contents 3.17 The Behaviour of Thermoplastics During the Injection Moulding Process 41 3.17.1 Pretreatment of Materials Before Injection Moulding 41 3.17.2 Reprocessed Materials 41 3.17.3 Colouring Materials 41 3.17.4 Additives 41 3.17.5 Material Drying 42 3.17.6 Plasticising or Melting 43 3.17.7 Measurement of Melt Temperature 44 3.17.8 Degradation of Materials During Plasticising 44 3.17.9 Selecting the Optimum Melt Temperature 45 3.17.10 The Effect of Screw Rotational Speed and Back Pressure 45 3.17.11 Flow Characteristics of the Melt During the Injection Phase 46 3.17.12 Selection of Injection Speed 46 3.18 Initial Cavity Filling Phase 48 3.19 Cavity Holding Pressure Phase 49 3.20 Gate Freeze-off Phase 49 3.21 Melt Compressibility and Shrinkage 49 3.22 Sinks and Voids 50 3.23 Weld Lines and Meld Lines 53 3.24 Cooling and Solidification of the Melt 54 Good Design Practice 55 4.1 Predesign Analysis 56 4.2 Reading General Arrangement Diagrams (GA) 56 4.3 Understanding Toolmaking Concepts 57 iii Mould Design Guide iv 4.4 Observing Mould Tools 58 4.5 Summary of Good Design Practice 58 Design Checklist 59 5.1 Predesign Checklist 59 5.2 Original Estimate Details 60 5.3 Component Drawing 60 5.4 Component Geometry 60 5.5 Component Material 61 5.6 Quantity Required 61 5.7 Component Function 61 5.8 Component Tolerances 62 5.9 Number of Impressions 62 5.10 Gating Method 62 5.11 Ejection Method 63 5.12 Component Aesthetics 63 Determining the Right Number of Impressions 65 6.1 Quality Versus Quantity 66 6.2 Appearance 66 6.3 Part Geometry 67 6.4 Drawing Tolerances 67 6.5 Discussion 67 6.6 More Cavities = Less Control 68 6.7 Summary 70 Mould Design Guide Milling 86 8.3.4 Grinding 87 8.3.5 Fabrication 88 8.3.6 Standard Electrodischarge Machining (EDM) 89 8.3.7 Wire Electrodischarge Machining 91 8.3.8 Cold Hobbing 92 8.3.9 Beryllium-Copper 93 8.3.10 Electroforming 93 8.3.11 Cavity Corrosion and Erosion 95 8.3.12 Gassing and Burning 95 8.4 Differential Shrinkage 96 8.5 Maximum Metal Conditions 97 8.6 Example 97 Two-Plate Mould Tools 99 9.1 vi 8.3.3 Design Details 99 9.1.1 Locating or Register Ring 101 9.1.2 Top Plate .101 9.1.3 Split Line 102 9.1.4 Cavity Insert 102 9.1.5 Front Cavity Plate 102 9.1.6 Rear Cavity Plate .102 9.1.7 Cavity Support Plate 103 9.1.8 Ejection System 103 9.1.9 Ejection Gap 104 Contents 10 9.1.10 Support Blocks .104 9.1.11 Guide Pillar 104 9.1.12 Return Pins 104 9.1.13 Fine Tuning the Mould Tool 104 9.1.14 Clearances .106 9.1.15 Bushes .106 9.1.16 Screws .106 9.1.17 Support Pillars 106 9.1.18 Taper Threads 107 9.1.19 Stand-off Buttons .107 9.1.20 Chamfers and Radii 107 9.1.21 Guide Bushes 107 Ejection Systems 109 10.1 Requirements .109 10.1.1 Part Geometry 109 10.1.2 Draft Angles 109 10.1.3 Tolerances .110 10.1.4 Material 110 10.1.5 Gating .111 10.1.6 Ejection Balance .112 10.1.7 Machine Specifications 113 10.1.8 Mould Opening Stroke 113 10.1.9 Machine Ejection Features 113 10.1.10 Movement Control Features .114 10.1.11 Component Finish Requirements 114 vii Mould Design Guide Figure 24.17 Rotating cavity design with axially fixed cavity 482 Mould Design Guide 484 25 Mathematical and Reference Tables 485 Mould Design Guide 25.1 Logarithms Continued 486 Mathematical and Reference Tables Logarithms continued 487 Mould Design Guide 25.2 Anti-logarithms Continued 488 Mathematical and Reference Tables Anti-logarithms continued 489 Mould Design Guide 25.3 Natural Sines Continued 490 Mathematical and Reference Tables Natural Sines continued 491 Mould Design Guide 25.4 Natural Cosines Continued 492 Mathematical and Reference Tables Natural Cosines continued 493 Mould Design Guide 25.5 Natural Tangents Continued… 494 Mathematical and Reference Tables Natural Tangents continued 495 Mould Design Guide 25.6 Square Roots Continued… 496 Mathematical and Reference Tables Square Roots continued Continued… 497 ... into consideration at the earliest stages of the design Similarly, mould bases and the mould cavities must be designed in such a manner that they can actually be made The moulds must also be economic... determining the optimum mass size of the mould tool and its components for maximum economy of production and part quality The theme throughout the book is based on design simplicity The simpler the design. .. The Mould Design Guide Peter Jones Smithers Rapra Technology Limited A wholly owned subsidiary of The Smithers Group Shawbury, Shrewsbury, Shropshire,