Top Banner
THIRD EDITION EDITED BY DOMINICK V. ROSATO, P. E. DONALD V. ROSATO, PH.D. MARLENE G. ROSATO, P. E. Kluwer Academic Publishers Boston/Dordrecht/London
1481

[D.v. Rosato, Marlene G. Rosato] Injection Molding(BookZZ.org)

Nov 22, 2015

Download

Documents

josa12
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
DONALD V. ROSATO, PH.D.
Kluwer Academic Publishers Boston/Dordrecht/London
Distributors for North, Central and South America: Kluwer Academic Publishers 101 Philip Drive Assinippi Park Norwell, Massachusetts 02061 USA Telephone (781) 871-6600 Fax (781) 871-6528 E-Mail < kluwer@wkap.com>
Distributors for all other countries: Kluwer Academic Publishers Group Distribution Centre Post Office Box 322 3300 A H Dordrecht, THE NETHERLANDS Telephone 31 78 6392 392 Fax 31 78 6546 474 E-Mail <orderdept@wkap.nl>
\A Electronic Services < http://www.wkap.nl> c,r Library of Congress Cataloging-in-Publication Data
Injection molding handbook / Dominick V. Rosato, Donald V. Rosato, Marlene G. Rosato. - 3rd ed.
p. cm. ISBN 0-7923-8619-1 1. Injection molding of plastics-Handbooks, manuals, etc. I. Rosato, Dominick
Rosato, Donald V. 111. Rosato, Marlene G. 7. I
TP1150.155 2000 668.4' 12-dc2 1
99-049946
Copyright 0 2000 by Kluwer Academic Publishers.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, mechanical, photo-copying, recording, or otherwise, without the prior written permission of the publisher, Kluwer Academic Publishers, 101 Philip Drive, Assinippi Park, Norwell, Massachusetts 02061
Printed on acid-free paper.
Contents
Preface Chapter 1 The Complete Injection Molding Process
Introduction Machine Characteristics Molding Plastics Molding Basics and Overview
People and Productivity 6; Plastic Materials 6; Morphology and Performance 9; Melt Flow and Rheology 11; Plasticating 12; Screw Designs 14; Molds 15; Processing 16; Process Controls 18; Control Guides 20; Art of Processing 21; Fine Tuning 21
Automatic 22; Semiautomatic 22; Manual 22; Pri- mary 23; Secondary 23
Molding Operations
Training Programs Processor Certifications Plastics Machinery Industry Summary
Captive 23; Custom 24; Proprietary 24
Chapter 2 Injection Molding Machines Introduction Reciprocating (Single-Stage) Screw Machines Two-Stage Machines
Reciprocating vs. Two-Stage Machines Other Machine Types Machine Operating Systems Hydraulic Operations
Injection Hydraulic Accumulator 32
Reservoirs 40; Hydraulic Controls 42: Propor- tional Valves 42; Servovalves 43: Digital Hydraulic Control 43: Hydraulic Fluids and Influence of
xxix 1 1 4 4 4
22
vi Contents
Heat 44; Pumps 44; Directional Valves 45; Servo and Proportional Valves 46
Electric Motors 47; Adjustable-Speed Drive Mo- tors 47; Servo Drives 47; Microtechnology Mold- ings 47; Injection Molding: A Technology in Tran- sition to Electrical Power 48
Electrical Operation
Hybrid Operations Clamping Systems
Clamping Pressures 60; Hydraulic Clamps 61; Toggle Clamps 62; Hydromechanical Clamps 62; Hydroelectric Clamps 63; Comparison of Clamp Designs 64; Tie-bars 64; Tie-barless Systems 69; Platen Systems 71
Barrel Borescoping 72; Barrel and Feed Unit 72; Barrel Heaters 73; Barrel Cooling 74; Barrel Char- acteristics 75
Barrels
Screw Operations Machine Sizes and Design Variations Rebuilding and Repairs
Stripping, Polishing, and Plating 79; Machine Downsizing and Upsizing 79
Machine Lockout 80; Machine Safety 81; Identi- fication of Hazards 82; Safety Built into the Ma- chines 82; Current and Former Installations 88; IMM Safety Checklist 88; Safety Rules for Mold- ing Department 88; American National Standard 92; Safety Standards 92; Plasticator Safety 93; Barrel-Cover Safety 93; Plant Safety 93; Safety Information 93
Designing Facilities Upgrading 93; Clean Room 94; Clean Machines 94
Noise Generation Startup and Shutdown Operations Molding Operation Training Program
Safety
First Stage: Running an IMM 99; The Sequence in a Cycle 102; Second Stage: Parameter Setting and Starting a Job 105
Factors to Consider 113; Operating the Ma- chine 127; Final Stage: Optimizing Molding Pro- duction 128; Specification Information, General 130; Specification Information, Details 131; Pro- ductivity and People 134; Training Information 136
Shear-Rate-Sensitive and -Insensitive Materials
46
Plastics Melt Flow 154; Barrel Temperature Over- ride 157
Feed Section 157; Transition Section 161; Meter- ing Section 162
Screw Rotation 163; Soak Phenomena 164; In- jection Stroke 165; Injection Pressure Required 166
Screw Design Basics 170; Sequence of Oper- ations 172; Advantages of Screw Plasticizing 173; Length-to-Diameter Ratios 173; Compres- sion Ratios 174; Rotation Speeds 175
Screw Sections
Screw Plasticizing
Mechanical Requirements 177; Torque 177; Torque vs. Speed 177
Injection Rates Back Pressures Melt Performance Melt Pumping Melt Temperature
Temperature Sensitivity 179; Temperature Con- trols Required 179; Barrel Heating 180; Cooling 180
Melt Performance Residence Time Melt Cushions Melt Shear Rate Melt Displacement Rate Shot Size
Screw-Barrel Bridging Vented Barrels
Overview 182; Basic Operations 184; Barrel- Venting Safety 188
Design Basics 189; Design Performance 189; Mix- ing and Melting Devices 189; Screw Barriers 193; Specialized Screw Designs 196; Screw Tips 197; Influence of Screw Processing Plastics 201; Melt Quality 202; Materials of Construction 204
Screw Designs
Screw Outputs Influence of Screw and Barrel Wear on Output
Influence of the Material on Wear 205; Screw Wear 205; Production Variations 205; Screw Wear Inspections 207; Output Loss Due to Screw Wear
151 151 151
181 181 181 181 181 181
182 182
Chapter 4
Purging Patents Influence Screw Designs Terminology
Molds to Products Overview
Interrelation of Plastic, Process, and Product 221; Molding Process Windows 221; Cycle Times 223; Molding Pressure Required 224; Products 224
Basics of Melt Flow 225; Mold Filling Hesitation 225; Melt Cushioning 225; Mold Filling Monitor- ing 225; Sink Marks 226
Processing Plastics
Mold Types Computer Systems 235
Molds For Thermosets 238; Mold Classifications 241
Cold-Slug Well 243; Melt Orientation 244;
Fill Rates 250; Melt Temperature 250; Mold Tem- perature 250; Packing Pressure 251; Mold Geom- etry 251; Flash Guide 251
Plastic Melt Behaviors
Cavity Melt Flow
Molding Variables vs. Performance Shot-To-Shot Variation Cavities
Cavity Melt Flow Analyses 254; Cavity Melt Foun- tain Flow 254
Machine Size 258; Plasticizing Capacity 258; Eco- nomics 258; Cavity Draft 259; Cavity Packing 259; Cavity Surface 259
Contact Area at Parting Line 262
Sprues 263; Runner Systems 264; Gates 277; Gate Summary 287
Correcting Mold Filling Imbalances in Geometrically Balanced Runner Systems
Isolating Mold Variations in Multicavity Molds 291
Ejector Systems 293; Ejector Pin Strength 296; Sprue Pullers 300; Side Actions 300; Angle Pins 301; Cam Blocks 302; Stripper-Plate Ejection 302;
Cavity Evaluation
Clamping Forces
Sprue-Runner-Gate Systems
Mold Components
Contents ix
External-Positive-Return Systems 302; Cam Ac- tuation 303; Sprue Bushing and Locating Ring 303; Ring and Bar Ejection 303; Top-and-Bottom Ejection 304; Inserts 305; Side Guide Slides 307; Ejector Blades 307
Mold Venting Molds for Thermoset Plastics
Mold Construction 313; Cold-Runner Systems 314; Injection-Compression Moldings 314
Overview 314; Design Considerations 315; Ba- sic Principles of Heat Flow 317; Heat Transfer by Heat Pipes 321; Heat Balance of Halves 321; Mold Connection for Fluid 321; Cooling Time 321; Cooling with Melt Pulses 322; Flood Cooling 322; Spiral Cooling 322; Cooling Rates 322; Cooling Temperatures 322; Cooling Flow Meters 323
Mold Cooling
Undercuts Mold Shrinkages and Tolerances
Ejection of Molded Products Mold Release Agents Mold Materials of Construction
Shrinkage vs. Cycle Time 329
Steels 334; Heat Treating 342; Requirements to be Met by Mold Steel 342; Aluminum 343; Beryllium-Copper 343; Kirksite 343; Brass 343
Etching Cavity Surfaces Machining Safety Moldmaker Directory Mold Material Selection Software Fabrication of Components
Hobbing 346; Cast Cavities 346; Electroforming 346; Electric-Discharge Machining 346
Tooling Polishing
SPI Finish Numbers 348; Hand Benching 349; Direction of Benching 350; Ultrasonic Tools 351; Textured Cavities 351; Patterns of Different Tex- tures 351; Mold Steels 352; Conditions Required for Polishing 352
Nickel 355; Chrome 355; Nitriding and Carbur- izing 356; Other Plating Treatments 357; Coating Treatments 357; Heat Treatments 358
Overview 359; Manual Cleaning 362; Oven Clean- ing 362; Solvent Cleaning 362; Triethylene Gly- col Cleaning 363; Postcleaning 363; Salt Bath Cleaning 363; Ultrasonic Solvent Cleaning 363; Fluidized-Bed Cleaning 363; Vacuum Pyrolysis Cleaning 363
Platings, Coatings, and Heat Treatments
Cleaning Molds and Machine Parts
307 313
347 347
X Contents
Chapter 5
Strength Requirements for Molds Stress Level in Steel 364; Pillar Supports 365; Steel and Size of Mold Base 366
Mold Filling 367; Deflection of Mold Side Walls 368
Eyebolt Holes Quick Mold Change Mold Protection
Preengineered Molds Standardized Mold Base Assemblies Specialty Mold Components Collapsible and Expandable Core Molds Prototyping
Deformation of Mold
Overview 387; Stereolithography 387; Rapid Tooling 388
Introduction 389; Industry Guide 389; Purchase Order 390; Mold Design 390; Production of Molds 392
Buying Molds
Mold Storage Computer-Aided Mold and Product Design Production Control Systems Computer Monitoring of Information Productivity and People Value Analyses Zero Defects Terminology
Fundamentals of Designing Products Overview Molding Influences Product Performance Design Optimization
Material Optimization Material Characteristics Behavior of Plastics
Molding Tolerances
Thermal Stresses 437; Viscoelastic Behavior 437
Tolerances and Designs 443; Tolerance Allow- ances 443; Tolerances and Shrinkages 444; Tole- rances and Warpages 444; Thin-Wall Tolerances 444; Micron Tolerances 444; Tolerance Damage 444; Full Indicator Movements (FIMs) 444; Tole- rance Selection 444; Tolerance Stack-Ups 445; Standard Tolerances 445
Tolerance Measurement and Quenching Dimensional Properties Dimensional Tolerances
Product Specifications 449; Using Geometric Tol- erancing 450
364
367
389
415 415 417 421
423 423 43 1
Accidental Orientation 453; Orientation and Chemical Properties 453; Orientation and Me- chanical Properties 454; Orientation and Optical Properties 454; Orientation Processing Character- istics 454; Orientation and Cost 454
Molecular Orientation: Design of Integral Hinges Interrelation of Material and Process with Design Design Shapes Shapes and Stiffness Stress Relaxation Predicting Performance Choosing Materials and Design
Design Concept 458; Engineering Considerations 458
Design Parameters 460; Types of Plastics 460
Designing with Creep Data 463; Allowable Work- ing Stress 465; Creep Behavior Guidelines 466
Stapler 466; Snap-Fits 467; Springs 467
Design Considerations
Design Examples
Design Approach Example Design Accuracy Risks and the Products
Acceptable Risks 472; Acceptable Goals 473; Ac- ceptable Packaging Risks 473; Risk Assessments 473; Fire Risks 473; Risk Management 473; Risk Retention 473
Perfection Cost Modeling Innovative Designs Protect Designs Summary
Terminology Molders’ Contributions 476
Molding Materials Overview
Definition of Plastics 484; Heat Profiles 488; Costs 489; Behavior of Plastics 490; Checking Materials Received 491
Neat Plastics Polymer Synthesis and Compositions
Copolymers Interpenetrating Networks Graftings
455 455 455 456 457 458 458
459
461
466
477
Alloys and Blends Thermoplastic and Thermoset Plastics
Thermoplastics 511; Thermoset Plastics 511; Cross-Linking 512; Cross-Linking Thermoplastics 512; Thermoplastic Vulcanizates (TPVs) 512; Cur- ing 512; Heat Profiles 513
Liquid Crystal Plastics (LCPs) Elastomers, Thermoplastic, and Thermoset
Thermoplastic Elastomers 515; Thermoset Elas- tomers 515; Natural Rubbers 515; Rubber Elas- ticity 515; Rubber Market 515
Commodity and Engineering Plastics Injection Molding Thermoplastics and Thermosets High Performance Reinforced Moldings
Injection Moldings 518; Bulk Molding Com- pounds (BMCs) 518; Characterizations 519; Di- rectional Properties 521
Newtonian Flow 522; Non-Newtonian Flow 523 Viscosities
Viscoelasticities Plastic Structures and Morphology
Chemical and Physical Characteristics 524; Crys- talline and Amorphous Plastics 524; Catalysts and Metallocenes 526; Plastic Green Strength 527
Average Molecular Weight 527; Molecular Weight Distribution 529; Additives 529; Molecular Weight and Melt Flow 530; Molecular Weight and Aging 530
Flow 531; Viscosity 531; Viscoelasticity 532; In- trinsic Viscosity 533; Shear Rate 533; Laminar and Nonlaminar Melt Flows 535; Melt Flow Analyses 535; Melt Flow Analysis Programs 535; Analyzing Melt Flow Results 536; Melt Flow Defects 536; Hindering Melt Flow with Additives 536; Melt Fractures 536
Molecular Weight ( M W )
Rheology and Melt Flow
Weld Line Strengths and Materials Material Selections
Colorants 548; Concentrates 549; Barrier Plastics 549
Melt Shear Behaviors 537
498 498
507 510
513 514
Thermal Properties and Processability Melt Temperatures 554; Glass Transition Tem- peratures 555; Dimensional Stabilities 555; Ther- mal Conductivities and Thermal Insulation 556; Heat Capacities 556; Thermal Diffusivities 556; Coefficients of Thermal Expansion 556; Thermal Stresses 556
Shrinkages Drying Material Handling Annealing Recycling
Recycled Plastic Definitions 559; Recycled Plas- tic Identified 560; Recycled Plastic Properties 560; Recycling Size Reductions 560; Recycling Mixed Plastics 560; Integrated Recycling 560; Re- cycling Methods and Economic Evaluations 560; Recycling and Lifecycle Analysis 561; Recycling Commingled Plastics 561; Recycling Automati- cally Sorting Plastics 561; Recycling and Common Sense 561; Recycling Limitations 561
Recycling Facts and Myths Warehousing
Storage and Condensation 562; Material Storage 562; Silo Storage 562
Processing Different Plastics Polyethylenes
Molding Conditions 570
Molding Conditions 573; Purging 574; Shutdown and Start-up 574; Thermal and Rheological Prop- erties 574; Drying 574; Mechanical Properties 575; Chemical Resistance 575; Weatherability 575; Color 575
Formulations 576; Molding Conditions 576; Screw Design 577; Material Handling Equipment 578; Processing Parameters 579; Problem Solving 579; Splay 579
Molding Conditions 581; Performance Parame- ters 585; Design Parameters 586; Molding Perfor- mance Parameters 591; Mold Release 593; Close Tolerance: Fast Cycles 595; Recycling Plastics 596
Molding Variables and Cause-and-Effect Links 597; Molding Variables and Property Responses 599; Appearance Properties 599; Warping 600;
Polypropylenes
561 562
563 563
xiv Contents
Chapter 7
Mechanical Properties and Molding Variables 601; Izod impact 602; Molding for Electroplating 605; Property Variation with Position Mold Ge- ometry 605; Summary 606
Drying 606; Recycle and Virgin Proportions 607; Processing 608; Hydrolysis 609; Rheology 609; Heat Transfer 609; Residual Stress 610; Annealing 61 1
Process 613; Hot- and Cold-Runner Molding 614; Material Stuffer 615
Polycarbonates
Process Control Process Control Basics
Developing Melt and Flow Control 630; Inspec- tion 630; Computer Process Data Acquisition 630; Control Flow Diagrams 632; Fishbone Diagram 632
Technology 636; Fast Response Controls 638; Control Approaches 639; Process Control Meth- ods 640; Production Monitoring 640; On-Machine Monitoring 641
Overview
Temperature Control of Barrel and Melt Electronic Controls Fuzzy Logic Control Process Control Techniques Process Control Approaches
What Are the Variables? 652; Why Have Process Control? 654; Control of Which Parameters Can Best Eliminate Variability? 654; What Enables Parameter Controllability? 657; Where Does the Process Controller Go? 661; Basic Features a Pro- cess Controller Should Have 662; Applications 664; Summary 666
Process Control Problems Cavity Melt Flow Analyses
Problem 669; Melt Viscosities versus Fill and Pack 669; Test Methodology 670; Analyzing Results 673; Example Test 673; Using Empirical Test Data to Optimize Fill Rates 674; Melt Vibrations dur- ing Filling 675; Stabilizing via Screw Return Time 675
Sensor Requirements 676; Molding Parameters 676; Display of Monitored Molding Parameters
Relating Process Control to Product Performances
606
611
667 668
Functions 680; Rotary and Linear Motion 680
Optimization via PVT 681; PMT Concept 683
Designs 684
Controllers
Transputer Controllers Temperature Controllers
Temperature Variations 688; Melt Temperature Profiles 690; Automatic Tuning 691; Temperature Sensors 691; Fuzzy Logic Controls 692; Fuzzy-PID Controls 692
Temperature Timing and Sequencing Pressure Controls
Pressure PID Controls Screw Tips 692; Cavity Fillings 692
PID Tuning: What It Means 693; The Need for Rate Control on High-speed Machines 694
Fuzzy-Pressure Controls Injection Molding Holding Pressures Process Control Fill and Pack Process Control Parameter Variables
Injection Molding Boost Cutoff or Two-Stage Control Injection Molding Controller Three-Stage Systems
Mold Cavity Pressure Variables Programmed Molding
Adaptive Ram Programmers 696
Parting Line Controls 702; Computer Micropro- cessor Controls 703; Computer Processing Con- trol Automation 703
Molding Thin Walls Control System Reliabilities Operations Optimized Control Tradeoffs Process Control Limitations and Troubleshooting
Control 705; Tie-Bar Growth 706; Tie-Bar Elon- gation 706; Thermal Mold Growth 706; Shot-to- Shot Variation 706
Intelligent Communications 709; Systematic In- telligent Processing 710
Intelligent Processing
709
Automatic Detections Terminology
Chapter 8 Design Features That Influence Product Performance Overview
Audits 717; Computer Approaches 717; Design Feature That Influence Performance 718
Plastic Product Failures Design Failure Theory Basic Detractors and Constraints
Tolerance and Shrinkage 721; Residual Stress 725; Stress Concentration 726; Sink Mark 727
Design Concept Terminology Sharp Corners Uniform Wall Thickness Wall Thickness Tolerance Flow Pattern Parting Lines Gate Size and Location Taper or Draft Angle Weld Lines
Vent, Trapped Air, and Ejector Undercuts Blind Holes Bosses Coring Press Fits Internal Plastic Threads External Plastic Threads Molded-In Inserts Screws for Mechanical Assembly Gears Ribs Geometric Structural Reinforcement Snap Joints Integral Hinges Mold Action
Meld Lines 740
Communication Benefits 773; Computerized Databases of Plastics 775; CAD/CAM/CAE Methods 775; Computer-Integrated Manufactur- ing 775
710 710 710 71 1
712 713
716 716
727 730 730 732 732 733 733 733 735 738
740 740 740 747 750 751 752 752 753 754 759 760 763 764 765 766
770 770
Benefits of CAD/CAM/CAE for Mold Design Productivity 776; Quality 777; Turnaround Time 778; Resource Utilization 778
Mechanical Design 779; Computer-Aided Engi- neering 780
Product Designers 783; Mold Designers and Moldmakers 784; Injection Molders 785
Multisections 789; Finite Element Techniques 790; Shrinkage and Warpage 791; Benefit Appraisal 795; Moldflow Basic Technology 795
Introduction 796; Fundamentals 799; Mold Cool Analysis 801
Modeling Methods Applied to Part and Mold Design Wire Frame Modeling 824; Surface Modeling 826; Solids Modeling 828
Group Technology 829; Finite Element Model- ing 830; Digitizing 831; Layering 832; Groups 833; Patterns 833; Large-Scale Geometry Manipu- lation 833; Local Coordinates or Construction Planes 834; Model and Drawing Modes and Associativity 834; Verification of Geometric Rela- tionships 835; Automatic Dimensioning and Au- tomatic Tolerance Analysis 836; Online Calcula- tion Capabilities and Electronic Storage Areas 836
Illustration of Mold Design Process The Manual (Paper) Method 837
The CAD/CAM/CAE Method Online Databases
Basics in CAD/CAM/CAE Modeling
Computer Capabilities for Part and Mold Design
The Database Concept 843; Graphics Databases 844; Defining the Library Database 845
Tolerances and Dimensional Controls Computer Controllers CAD/CAM/CAE and CIM Numerical Control Process Programmable Controller Safety Devices Computer Optical Data Storage Artificial Intelligence Computers and People Computer-Based Training Myths and Facts Capability and Training Computer Software Molding Simulation Programs RAPRA Free Internet Search Engine Software and Database Programs
xvii
776
778
781
786
796
823
829
836
840 843
846 846 847 849 849 850 850 850 850 850 85 1 852 854 854 854
xviii Contents
Injection Moldings and Molds 856: Materials 857; Shrinkage 858; Materials and Designs 859; Design Products 860: Engineering 861; Graphics 861; Management 862: General Information 862; Training 862
Plastics, Toys, and Computer Limitations Computers Not Designed for Home Summary Terminology
Chapter 10 Auxiliary Equipment and Secondary Operations Introduction
Energy Conservation 870; Planning Ahead, Sup- port Systems 871
Hoppers 871; Material Handling, Feeding, and Blending 872; Material Handling Methods 872: Sensors 874
Bulk Density 875; Basic Principles of Pneumatic Conveying 876; Air Movers 883; Pneumatic Ven- turi Conveying 886; Powder Pumps 886; Pip- ing 888; Hoppers 889; Filters 889; Bulk Storage 891; Blenders 891; Unloading Railcars and Tank Trucks 894
Nonhygroscopic Plastics 895; Hygroscopic Plas- tics 895; Drying Overview 895; Dryers 896
Overview 904; Heat-Transfer Calculations 905; Requirements Vary with Materials 905; Water Re- covery 907; General considerations 908; Calcula- tion of the Cooling Load 911; Determining Water Loads 913
Overview
Energy-Saving Heat Pump Chillers Granulators
Safety 916; Basics 917; Hoppers 917; Cutting Chambers 918; Cutting Chamber Assembly 921; Hard Face Welding 921; Screen Chambers 922; Auger Granulators 922; Granulating and Perfor- mance 924
Dewpoints 929; Mold Surface Temperatures 929; Effect of Change in Air Properties 930; Air Condi- tioning and Desiccant Dehumidification 931; De- humidification System 932
Controlled Motions 933; People and PHE 935; Different Types 935; Value in Use 937; Detriments 938: Robots Performance 938; Safety Measures 938
Mold Dehumidification
Parts-Handling Equipment
Adhesives 941; Solvents 946; Welding Techniques 948; Welding Process Economic Guide 953
Abrasives 953; Carbon Dioxide 953; Cryogenic Deflashing 954; Brass 954; Hot Salts 954; Solvents 954; Ultrasonics 954; Vacuum Pyrolysis 954; Coat- ings 955
Potential Preparation Problems 955; Pretreat- ments 959; Removing Mold Release Residues 959
Robot Terms 966
Joining and Assembling
Definitions
Troubleshooting Guides Flashes Injection Structural Foams Hot-Runners Hot-Stamp Decorating Paint-Lines Granulator Rotors Auxiliary Equipment Screw Wear Guide
Plastic Material and Equipment Variables 970
Defects 972
Inspection Rollers 1010; Diameters 1010; Depths 1011; Concentricity and Straightness 1011; Hard- ness 1011; Finish and Coating Thickness 1012; Screw Manufacturing Tolerances 1012
Inside Diameters 1012; Straightness and Concen- tricity 1012; Barrel Hardness 1012; Barrel Speci- fications 1012
Cleaning the Plasticator Screw 1014; Oil Changes and Oil Leaks 1015; Checking Band Heaters, Thermocouples, and Instruments 1015; Align- ment, Level, and Parallelism 1015; Hydraulic,
Barrel Inspection Guide
1001 1001 1001
Pneumatic, and Cooling-Water Systems 1015; Hydraulic Hose 1016
Keep the Shop Clean Keep Spare Parts in Stock Return on Investment Maintenance
Hydraulic Fluid Maintenance Procedures 1020; Problems and Solutions 1020; Downtime Mainte- nance 1021: Preventative Maintenance 1021; Ser- vices 1022
Safety Maintenance Software Summary Terminology
Testing, Inspection, and Quality Control Testing Design and Quality Basic versus Complex Tests Sampling
Acceptable Quality Level 1032; Sampling Plan 1032; Sampling Size 1033
Orientation and Weld Lines 1033; Density and Specific Gravity 1035; Morphology: Amorphous and Crystalline Plastics 1036; Molecular Struc- tures 1037
Mechanical Test Equipment 1042; Tensile Test 1042; Deflection Temperature under Load 1045; Creep Data 1045
Characterizing Properties and Tests
Liquid Chromatography 1049; Gel Permeation Chromatography 1049: Gas Chromatography 1050; Ion Chromatography 1050; Thermoanalyt- ical Method 1051; Thermogravimetric Analysis 1051; Differential Scanning Calorimetry 1052; Thermomechanical Analysis 1053; Dynamic Me- chanical Analysis 1054; Infrared Spectroscopy 1054: X-Ray Spectroscopy 1055; Nuclear Mag- netic Resonance Spectroscopy 1055; Atomic Ab- sorption Spectroscopy 1055; Raman Spectroscopy 1055; Transmission Electron Microscopy 1056; Optical Emission Spectroscopy 1056; Summary of Characterizing Properties 1056
Selected ASTM Tests 1062; Viscoelastic Proper- ties 1079; Rheology, Viscosity, and Flow 1080;
Types of Tests
1033
1041
Optical Analysis via Microtoming Thermal Properties
Useful Temperature Range 1084; Glass Transition and Melt Temperatures 1084; Thermal Conduc- tivity 1086; Heat Capacity 1086; Coefficient of Linear Thermal Expansion 1086; Temperature Dependence of Mechanical Properties 1089; Dif- fusion and Transport Properties 1091; Permeabil- ity 1091; Migration 1092
Overview of Plastic Properties Melt Tests
Melt Flow Tests 1095; Melt Index Test 1095; Melt Index Fractional Tests 1098; Molding Index Tests 1098; Measurements 1098
Types of Scales 1099
Radiography 1099; Ultrasonics 1100; Liquid Pen- etrants 1100; Acoustics 1100; Photoelastic Stress Analysis 1100; Infrared Systems 1101; Vision Sys- tem Inspections 1101; Computer Image Proces- sors 1102
Temperature Scales
Nondestructive Tests
Laboratory Organizations Worldwide Determining Moisture Content 1103
American Society for Testing and Materials 1105; International Organization for Standardization 1105; Underwriters’ Laboratory Classifications 1106
International System of Units Inspections Identification of Plastics Estimating Plastic Lifetimes Quality Control
Quality Control Defined 1110; Quality Control Variables 1110
No More ABCs 1112; Need for Dependability 1112; Quality Auditing 1112
QC Begins When Plastics Are Received
Reliability and Quality Control Failure Analysis Quality Control Methods
Quality Control and Quality Assurances Auditing by Variables Analysis Acceptable Quality Levels Quality Optimization Goals Quality System Regulation
Image Quality Indicators 1114
1111
xxii Contents
Total Quality Management Training and People Training and Quality Emerging Trends in Training Training versus Education Economic Significance of Quality
Terminology Cost of Quality 1119
Chapter 13 Statistical Process Control and Quality Control Overview
Combining Online SPC and Offline SQC 1127; Improve Quality and Increase Profits 1128; Statis- tical Material Selections: Reliabilities 1128; Sta- tistical Material Selections: Uncertainties That Are Nonstatistical 1129; Statistical Probabilities and Quality Control 1129; Statistics and Commit- ments 1129; Statistics and Injection Molding 1129 Computers and Statistics 1131; Statistical Tools 1134
Online Monitoring of Process Variables Gathering and Analyzing Data Process Control and Process Capability
Defect Prevention Understanding Modern Methods of Control
Control Charts 1138
Standard Deviations 1142; Frequency Distribu- tion 1143; Control Chart 1145
Standard Deviation versus Range Basic Statistical Concepts
Mean Value, Range, and Standard Deviation 1148; Distribution 1149; Process Control Chart 1150; Machine Capability 1150; Process Capabil- ity 1150
Importance of Control Charts Practical Example
Machine Capability 1153; Process Capability 1153; Control Limits for the Process Control Chart 1154
Production Controls 115.5; SPC Step One: Raw Material 1156; SPC Step Two: Materials Han- dling 1156; SPC Step Three: Injection Molding 11.56; SPC Implementation: Summary of Experi- ence 1156
How to Succeed with SPC Outlook Terminology
A Successful SPC System
1119
Chapter 14 Costing, Economics, and Management Overview
Machine Sales 1163; Formulas for Business Fail- ures 1164; Managing 1164
Estimating Part Cost 1167; Automation of Data Gathering 1169; Machinery Financing 1169; En- ergy Savings 1170
Costing
Technical Cost Modeling Cost Analysis Methods
Material Times Two 1171; Material Cost plus Shop Time 1172; Material Cost plus Loaded Shop Time 1172; Quotes 1172
Variable Cost Elements 1173; Fixed Costs 1174; Summary of Fixed and Variable Costs 1177; Pro- cess Parameters 1178; Technical Cost Modeling 1178; Summary of Technical Cost Analysis 1179
Technical Cost Analysis
Financial Plant Management Cost Management
Information Necessary for Product Costing and Cost Control 1182; Reporting from the Produc- tion Floor and Management Control Reports 1183
Gathering the Data for Profit Planning and Bud- geting 1186; Establishing Profit, Goals, and Sales Forecasts 1186; Developing the Detailed Plans and Budgets 1187; Flexible Budgeting 1187
Order Processing 1188; Inventory Control 1189; Production Scheduling and Control 1189; Scheduling Approaches 1190; Purchasing 1191
Profit Planning and Budgeting
Injection Blow Moldings 1201; Stretched Blow Moldings 1204; Stretched Blow Moldings with Handle 1206; Stretched Blow Molding Operation Specialties 1207; Blow Molding Shrinkages 1209; Troubleshooting 1211; Blow Molding versus In- jection Molding 1215
Coinjection Molding Injection Molding Sandwich Structures Gas-Assist Injection Molding
Advantages and Disadvantages 1220; Basic Pro- cesses and Procedures 1220; Molding Aspects 1223; Shrinkage 1224; Summary 1224
1163 1163
Gas Counterflow Molding Melt Counterflow Molding Structural Foam Molding
Overview 1225; Performance 1226; Plastic Mate- rials 1226; Characteristics of Foam 1226; Design Analysis 1227; Blowing Agents 1229; Methods of Processing SF with Chemical Blowing Agents 1230; Processing SF with Gas Blowing Agents 1232; Tooling 1234; Start-up for Molding 1234
Injection-Compression Molding (Coining) Multiline Molding Counterflow Molding Oscillatory Molding of Optical Compact Disks
Continuous Injection Molding Digital Video Disk Moldings 1238
Velcro Strips 1239; Electrically Insulated Buttons for Coaxial Cables 1242; Railtrack Molding 1243
The Mold 1248; Process Controls 1249 Reaction Injection Molding
Liquid Injection Molding Soluble Core Molding Insert Molding Inmolding
Two-Color Molding 1253; Decoration 1253; Paint Coating 1254; Back Molding 1254; Two-Shot Molding 1254; Inmold Assemblies 1254; Double- Daylight Process 1255
Overmolding Compatible Plastics with No Welding Closure Moldings
Unscrewing Closures 1256; Conventional Un- screwing Molds 1256; Unscrewing System Mold- ings 1256; Collapsible and Expandable Core Molds 1257; Split-Cavity Molds 1258; Strippable Thread Molds 1258
Vacuum Molding Tandem Injection Molding Molding Melt Flow Oscillations Ram Injection Molding Golf Ball Moldings Micro Injection Molding Aircraft Canopies Injection Molding Nonplastics
Introduction 1266; Metal Injection Molding 1266; Ceramic Injection Molding 1268
Terminology
1225 1225 1225
1268
Contents xxv
Cold Forming Cold Draw Forming Dip Forming Pressure Forming Rubber Pad Forming Compression-Stretched Moldings Solid-Phase Scrapless Forming Solid-Phase Pressure Forming Slip Forming Castings Foam Molding Expandable Plastics
Compression Molding
Transfer Molding
Reinforced Plastics
Molds 1291
Directional Properties 1301; Processes and Prod- ucts 1301
Stampable Reinforced Plastics Machining Plastics Processor Competition Legal Matters
Accident Reports 1304; Acknowledgments 1304; Chapter 11 Act 1304; Conflicts of Interest 1304; Consumer Product Safety Act 1304; Copy- right 1305; Defendant 1305; Employee Inven- tion Assignment 1305; Expert Witness 1305; In- surance Risk Retention Act 1305; Invention 1305; Mold Contractional Obligation 1305; Patent 1305; Patentability 1306; Patent Information 1306; Patent Infringement 1306; Patent Pooling with Competitors 1306; Patent Search 1306; Patent Term Extension 1306; Patent Terminology 1306; Plaintiff 1306; Processor, Contract 1307; Product Liability Law 1307; Protection Strategies 1307; Quotations 1307; Right-To-Know 1307; Shop- Right 1307; Software and Patents 1307; Tariff 1307; Term 1307; Tort Liability 1308; Trademark 1308; Trade Name 1308; Warranty 1308
17 Summary The Most Important Forming Technique Processing Trends Productivity
Machine Aging 1315; Response to Change 1316
1284 1288 1289
1291 1292 1292 1292 1292 1293 1293 1293 1293 1293 1294 1294
1295
1298
1298
Process and Material Selections Plastics and Equipment Consumption Machinery Sales
Trends in Machinery 1318; Computers and Injec- tion Molding 1320; Interfacing Machine Perfor- mance 1320
Molding in an Industrialized Country Compromises Must Frequently Be Made Standard Industrial Classification Plastic Industry Size Energy and Plastics Plastic Data: Theoretical Versus Actual Values Markets
Packaging 1325; Velcro for Flexible Packaging 1327; Building and Construction 1327; Lumber 1327; Pallets 1327; Automotive Parts 1329; Printed Circuit Boards and Surface Mounted Technology 1330; U.S. Postal Service 1330; Medical Applica- tions 1330; Toilets and Water Conservation 1330; Bearings 1330; Blow Molding Innovations 1330; Beer Bottles 1331; Collapsible Squeeze Tubes 1331; Asthma Inhalers 1331
Automated Production 1334; Energy Savings 1335
Discipline 1337; Productivity 1338; Experience 1338; Plant Controls 1338
Example 1 1339; Example 2 1339; Example 3 1340
Myths and Facts 1341; Limited Oil Resources 1342; Limited US. Steel Resources 1342; Plastic Advocates 1342
Statistics: Fact and Fiction 1344; Landfill 1345; Re- cycling 1345; Incineration 1345; Degradable 1346
Economic Control of Equipment
Correcting Misperceptions about Plastics
Analyze Failures Creativity
Innovations and the Markets 1348; Industrial De- signers 1348; Da Vinci’s Creativity 1348
Target for Zero Defects 1349 Design Successes
Excess Information: So What’s New? Fabricating Employment History
Barrel History 1351; Hopper Magnet 1352; Blow Molding 1352; Coca-Cola Bottle 1353; Coor’s Beer Bottle 1353; Recycling History 1353; Squeeze Tube 1353; Zipper 1353; Waste Contain- ers 1354; Shotgun Shells 1354; Water Treatment 1354
1318 1318 1318
1331
1337
1339
1341
1342
Profits and Time 1354
Appendices 1. Abbreviations 2. Conversions 3. Symbols and Signs 4. Web Sites on Plastics
References
1411
Preface
This third edition has been written to thoroughly update the coverage of injection molding in the World of Plastics. There have been changes, including extensive additions, to over 50% of the content of the second edition. Many examples are provided of processing different plastics and relating the results to critical factors, which range from product design to meeting performance requirements to reducing costs to zero-defect targets. Changes have not been made that concern what is basic to injection molding. However, more basic information has been added concerning present and future developments, resulting in the book being more useful for a long time to come. Detailed explanations and interpretation of individual subjects (more than 1500) are provided, using a total of 914 figures and 209 tables. Throughout the book there is extensive information on problems and solutions as well as extensive cross- referencing on its many different subjects.
This book represents the ENCYCLOPEDIA on IM, as is evident from its extensive and detailed text that follows from its lengthy Table of CONTENTS and INDEX with over 5200 entries. The worldwide industry encompasses many hundreds of useful plastic-related computer programs. This book lists these programs (ranging from operational training to product design to molding to marketing) and explains them briefly, but no program or series of programs can provide the details obtained and the extent of information contained in this single sourcebook.
In the manufacture of molded products there is always a challenge to utilize advanced techniques, such as understanding the different plastic melt flow behaviors, operational moni- toring and control systems, testing and quality control, and statistical analysis. However, these techniques are only helpful if the basic operations of molding are understood and characterized, to ensure the elimination or significant reduction of potential problems.
The book provides an understanding that is concise, practical, and comprehensive and that goes from A to Z on the complete subject of injection molding. It provides concise information for either the technical or the nontechnical reader, interrelating and understanding basic factors starting with the plastic’s melt flow behavior during processing. It should be useful to the fabricator, moldmaker, designer, engineer, maintenance person, accountant, plant manager, testing and quality control worker, cost estimator, sales and marketing person, venture capitalist, buyer, vendor, educator/trainer, workshop leader, librarian/information provider, lawyer, consultant, and others. People with different interests can focus on and interrelate across subjects that they have limited or no familiarity with in the World of Plastics. As explained throughout this book, this type of understanding is required in order to be successful in the design, prototyping, and manufacture of the many different marketable molded products worldwide.
xxix
xxx Preface
The reader will have a useful reference for pertinent information readily available in the table of contents and the index. As past book reviewers have commented, the information contained in this book is of value to even the most experienced designers and engineers, and provides a firm basis for the beginner. The intent is to provide a complete review of all aspects of the injection molding process that goes from the practical to the theoretical, and from the elementary to the advanced.
This book can provide people not familiar with injection molding an understanding of how to fabricate products in order to obtain its benefits and advantages. It also provides information on the most common and costly pitfalls or problems that can develop, resulting in poor product performance or failures. Accompanying the problems are solutions. This book will enhance the intuitive skills of those people who are already working in plastics. Its emphasis is on providing a guide to understanding the worldwide technology and business of injection-molded products.
From a pragmatic standpoint, every theoretical aspect that is presented has been expressed so that it is comprehensive and useful. The theorist, for example, will gain insight into the limitations of plastics relative to other materials such as steel and wood. After over a century of worldwide production of all kinds of injection-molded products, they can be processed successfully, yielding high quality, consistency, and profitability. As described in this book, one can apply the correct performance factors based on an intelligent understanding of the subject.
This book has been prepared with the awareness that its usefulness will depend on its simplicity and its ability to provide essential information. With the experience gained in working in the injection molding industry worldwide and in preparing the first and second editions as well as other books, we believe that we have succeeded in that purpose and have provided a useful, practical reference work.
The injection molding industry consumes about 32 wt% of all plastics. The plastics industry as a whole is ranked as the fourth largest industry in the United States. With plastics, to a greater extent than other materials, opportunity for improvement will always exist, since new and useful developments in materials and processing continually are on the horizon. Examples of these developments are given in this book, providing guides to future trends in the world of plastics.
The limited data presented on the properties of plastics are provided as comparative guides; readers can obtain the latest information from material suppliers, industry software, and/or sources mentioned in this book’s reference section. Our focus in the book is to present, interpret, analyze, and interrelate the basic elements of injection molding for processing plastic products. As explained in this book, there are over 17,000 plastic materials worldwide, and selecting the right one requires specifying all product performance requirements, properly setting up and controlling the injection molding process to be used, and intelligently preparing a material specification purchase document and work order to produce the product.
The many properties of different plastics are important for different purposes. Some meet high performance requirements such as long-time creep resistance, fatigue endurance, or toughness. On the other hand, for some plastics, ready supply and low cost are the main ad- vantages. As explained in this book, each of the different materials requires specific injection molding operating procedures.
Patents or trademarks may cover some of the information presented. No authorization to utilize these patents or trademarks is given or implied; they are discussed for informa- tion purposes only. The use of general descriptive names, proprietary names, trade names, commercial designations or the like does not in any way imply that they may be used as common nouns. While the information presented is believed to be true and accurate, neither the authors nor the publisher can accept any legal responsibility for any errors, omissions, inaccuracies, or other factors.
Preface xxxi
In preparing this book and ensuring its completeness and the correctness of the subjects reviewed, use was made of the authors’ worldwide personal, industrial, and teaching expe- rience that totals over a century, as well as worldwide information from industry (personal contacts, conferences, books, articles, etc.) and trade associations.
THE ROSATOS
Introduction
This chapter provides an introduction and overview of the injection molding machine (IMM) process. It provides text with pictorial reviews. Details on the important informa- tion pertaining to IMM and reviewed in this chapter are provided in the other chapters. Figure 1-1 provides an overview that basi- cally summarizes what should be considered to ensure that the molded product meets per- formance requirements and provides a good return on investment to produce all types and shapes of products for all types of markets.
Injection molding is a major part of the plastics industry and is a big business world- wide, consuming approximately 32 wt% of all plastics. It is in second place to extrusion, which consumes approximately 36 wt% (1, 3, 7). 11: the United States alone there are about 80,000 IMMs and about 18,000 extrud- ers operating to process all the many differ- ent types of plastics. In the industry an IMM is not regarded as an extruder; however, it is basically a noncontinuous extruder and in some operations is even operated continu- ously (Chap. 15). IMMs have a screw plas- ticator, also called a screw extruder, that pre- pares the melt (3).
As summarized in Fig. 1-2, injection mold- ing is an important plastic processing method. The figure shows the necessary components for the injection molder to be successful and profitable. Recognize that the first to market with a new product captures 80% of mar- ket share. The young tree cannot grow if it is in the shadow of another tree or if it does not keep up with competition. You need to be at the top of the tree looking over the other trees. Factors such as good engineer- ing and process control are very important but only represent pieces of the pie. Without proper marketinghales you are literally out of business. This diagram is basically a philo- sophical approach to the overall industry in that it provides examples of all aspects of the technology and business that range from local to global competition. The old adage about the better mousetrap is no longer com- pletely true, since you need factors such as the support services from the “tree” to achieve commercial success and meet product design requirements (Chap. 5) (1,499).
There are many different types of IMMs that permit molding many different prod- ucts, based on factors such as quantities, sizes, shapes, product performance, or eco- nomics. These different types of IMMs are
1
~
I Individual CONTROL for each aperatton, from sorimre to hardware
-m SOFTWARE --- ANALYSIS approach to meet psiiormanoo
FALLO Follow ALL Opportunities
I I Y SOFTWARE
MOLDED PART
Set UP TESTINQ I QUALITY CONTROL !
Immediately afier part is in production--next step IMPORTANT STEP -- is to produce part to meet same requirements but produced at a lower cost
Use FALLO approach. Reevaluate all parameters used from part design (use less plastic), use lower cost plastic with similiar process- ing cost (or plastic with higher cost, but faster process, results in lower total cost), check hardware performance, & other para- meters described in the IM HANDBOOK.
A Characterize properties, mechanical, physical, chemical, thermal, etc
I t- Set up practical i useful TROUBLESHOOTING
potential faults. C/ GUIDE based on causes & remedies of
I I I ! D.V.1
Fig. 1-1 The FALLO approach: Follow ALL Opportunities.
reviewed throughout this book, particularly in Chap. 15. Small- and large-size IMMs both have their advantages. For example, if sev- eral small machines are used rather than one large one, a machine breakdown or shutdown for routine maintenance will have less effect on production rates. However, the larger ma- chine is usually much more profitable while it is running. Because there are fewer cavities in molds for the small machines, they may per- mit closer control of the molding variables in the individual cavities.
The two most popular kinds of IMM are the single-stage and the two-stage; there are also molding units with three or more stages. The single-stage IMM is also known as the reciprocating-screw IMM. The two-stage IMM also has other names, such as the piggy- back IMM. It is comparable in some ways to a continuous extruder.
The IMM has three basic components: the injection unit, the mold, and the clamping system. The injection unit, also called the plasticator, prepares the proper plastic melt
and via the injection unit transfers the melt into the next component that is the mold. The clamping system closes and opens the mold.
These machines all perform certain essen- tial functions: (1) plasticizing: heating and melting of the plastic in the plasticator, (2) injection: injecting from the plasticator under pressure a controlled-volume shot of melt into a closed mold, with solidification of the plastics beginning on the mold's cavity wall, (3) afterjiilling: maintaining the injected material under pressure for a specified time to prevent back flow of melt and to compen- sate for the decrease in volume of melt during solidification, (4) cooling: cooling the ther- moplastic (TP) molded part in the mold until it is sufficiently rigid to be ejected, or heat- ing: heating the thermoset (TS) molded part in the mold until it is sufficiently rigid to be ejected, and (5) molded-part release: opening the mold, ejecting the part, and closing the mold so it is ready to start the next cycle with a shot of melt.
1 The Complete Injection Molding Process 3
Selecting Material Proce
Compression Reinforced Plastics
Cost Analysis
r Rheology
t LEADS
I Etc.
First to market with a new product captures 80% of market share.The young tree cannot grow if it is in the shadow of another tree or if it does not keep up with the competition.You need to be at the top of the tree looking over the other trees.
DVR
Fig. 1-2 Plastic product growth compared to tree growth.
This cycle is more complex than that other processes such as extrusion in that it involves moving the melt into the mold and stopping it, rather than having a continuous flow of melt. The injection molding process is, how- ever, extremely useful, since it permits the manufacture of a great variety of shapes, from
simple ones to intricate three-dimensional (3-D) ones, and from extremely small to large ones. When required, these products can be molded to extremely very tight tolerances, very thin, and in weights down to fractions of a gram. The process needs to be thor- oughly understood in order to maximize its
4 1 The Complete Injection Molding Process
performance and mold products at the least cost, meeting performance requirements, and with ease (see the section on Molding Toler- ances in Chap. 5).
Machine Characteristics
IMMs are characterized by their shot ca- pacity. A shot represents the maximum vol- ume of melt that is injected into the mold. It is usually about 30 to 70% of the actual available volume in the plasticator. The dif- ference basically relates to the plastic mate- rial’s melt behavior, and provides a safety factor to meet different mold packing con- ditions. Shot size capacity may be given in terms of the maximum weight that can be in- jected into one or more mold cavities, usu- ally quoted in ounces or grams of general- purpose polystyrene (GPPS). Since plastics have different densities, a better way to ex- press shot size is in terms of the volume of melt that can be injected into a mold at a spe- cific pressure. The rate of injecting the shot is related to the IMM’s speed and also the process control capability for cycling the melt into the mold cavity or cavities (fast-slow- fast, slow-fast, etc.).
The injection pressure in the barrel can range from 2,000 to at least 30,000 psi (14 to 205 MPa). The characteristics of the plastic being processed determine what pressure is required in the mold to obtain good products. Given a required cavity pressure, the barrel pressure has to be high enough to meet pres- sure flow restrictions going from the plastica- tor into the mold cavity or cavities.
The clamping force on the mold halves re- quired in the IMM also depends on the plastic being processed. A specified clamping force is required to retain the pressure in the mold cavity or cavities. It also depends on the cross- sectional area of any melt located on the part- ing line of the mold, including any cavities and mold runner(s) that are located on the parting line. (If a TP hot-melt runner is lo- cated within the mold half, its cross-sectional area is not included in the parting-line area.) By multiplying the pressure required on the melt and the melt cross-sectional area, the
clamping force required is determined. To provide a safety factor, 10 to 20% should be added.
Molding Plastics
Most of the literature on injection mold- ing processing refers entirely or primarily to TPs; very little, if any at all, refers to ther- moset TS plastics. At least 90 wt% of all injection-molded plastics are TPs. Injection- molded parts can, however, include combi- nations of TPs and TSs as well as rigid and flexible TPs, reinforced plastics, TP and TS elastomers, etc. (Chap. 6). During injection molding the TPs reach maximum tempera- ture during plastication before entering the mold. The TS plastics reach maximum tem- perature in the heated molds.
Molding Basics and Overview
The following information provides a com- plete overview of the process of IM (Figs. 1-3 to 1-10). Continually required is better under- standing and improving the relationship of process-plastic-product and controlling the complete process.
Injection molding is a repetitive process in which melted (plasticized) plastic is injected (forced) into a mold cavity or cavities, where it is held under pressure until it is removed in a solid state, basically duplicating the cavity of the mold (Fig. 1-11). The mold may con- sist of a single cavity or a number of similar or dissimilar cavities, each connected to flow channels, or runners, which direct the flow of the melt to the individual cavities (Fig. 1-12). Three basic operations take place: (1) heat- ing the plastic in the injection or plasticizing unit so that it will flow under pressure, (2) al- lowing the plastic melt to solidify in the mold, and (3) opening the mold to eject the molded product.
These three steps are the operations in which the mechanical and thermal inputs of the injection equipment must be co- ordinated with the fundamental properties and behavior of the plastic being processed; different plastics tend to have different
1 The Complete Injection Molding Process 5
Fig. 1-3 View of an injection molding machine.
-Clamping cylinder Injection unit
Fig. 1-4 Basic elements of injection molding.
melting characteristics, with some being ex- tremely different. They are also the prime de- terminants of the productivity of the process, since the manufacturing speed or cycle time (Fig. 1-13) will depend on how fast the ma- terial can be heated, injected, solidified, and ejected. Depending on shot size and/or wall thicknesses, cycle times range from fractions of a second to many minutes. Other impor- tant operations in the injection process in- clude feeding the IMM, usually gravimetri- cally through a hopper, and controlling the plasticator barrel’s thermal profile to ensure high product quality (Fig. 1-14).
An example of complete injection molding operation is shown in Fig. 1-1. This block di- agram basically summarizes what should be considered to ensure a good return on in-
Heating Injecting Molding Fig. 1-5 The basic cycle.
vestment to produce all types and shapes of molded products. The block diagram meets the objective in bringing you up to date on today’s technology as well as what is ahead. These important steps must come together properly to produce products consistently meeting performance requirements at the lowest cost. Basically, the approach is to: (1) design a mold around the product to be molded, (2) put the proper auxiliary equip- ment around the mold, and (3) set up the necessary fabricating process such as qual- ity controls, troubleshooting guides, preven- tative maintenance, and operational safety procedures. To be effective, the evaluation of a product should proceed according to a logical step-by-step process (Fig. 1-15). The result is to target for zero defects.
6 1 The Complete Injection Molding Process
Fig. 1-6 Schematic of plastic material flow through hopper and screw to the mold cavity.
People and Productivity
The recipe for productivity includes a list of ingredients such as R&D, new technologies, updated equipment, computer automation systems, and adequate modern facilities. But the one ingredient that ties the recipe to- gether is people. None of the ingredients have much use without the right people. As an example, computer software (CAD, CAM, CIIM, etc.) have their place together with the systems hardware. However, while the software and hardware all provide impor- tant resources for automating the manufac- turing line, to have the line run efficiently re- quires people to use these resources properly. Equipment and plastic materials are not per- fect, so that they require the human touch to ensure their repeatability, etc. (see the sub- section on Plastic Material and Equipment Variables in Chap. 11 .).
Achievable processing plans begin with the recognition that smooth does not mean perfect. Perfection basically is an unrealis- tic ideal, however one strives to approach it. The expectation of perfection can block gen- uine communication between workers, de- partments, management, customers and ven- dors (see the section on Perfection in Chap. 5 ) . A smooth run program can be defined as one that creates a product meeting fac- tors such as performance specification and
delivery time and that falls within budget. It can be said that perfection is never reached; there is always room for more development and/or improvement. As has been stated throughout history, to live is to change, and to approach perfection is to have changed often (in the right direction).
Plastic Materials
Many thousands of different plastics (also called polymers, resins, reinforced plastics, elastomers, etc.) are processed (Chap. 6). Each of the plastics has different melt be- havior, product performance (Figs. 1-16 and 1-17), and cost.
To ensure that the quality of the different plastics meets requirements, tests are con- ducted on melts as well as molded products. There are many different tests to provide all kinds of information. Important tests on molded products are mechanical tests such as those shown in Fig. 1-18, the main one being the tensile test (Chap. 12).
There are basically two types of plastic ma- terials molded. Thermoplastics (TPs), which are predominantly used, can go through repeated cycles of heating/melting [usu- ally at least to 260°C (500"F)I and cool- ing/solidification. The different TPs have dif- ferent practical limitations on the number
1 The Complete Injection Molding Process 7
CLAMP OPEN
INJECTION MOLD INJECT ION
C L A M P HYDRAULIC C Y L l NDE R
T I M E R S ELECTRIC MOTOR
CLAMP OPEN
INJECTION MOLD INJECT ION
C L A M P HYDRAULIC C Y L l NDE R
T I M E R S ELECTRIC MOTOR
ELECTRICAL
START CYCLE
SWITCH DIRECTING O I L TO
INJECT
P U M P PRESSURIZES SYSTEM
OIL FLOWS TO CLAMP CYLINDER C L A M P CLOSES
FROM HYDRAULIC MANIFOLD
4 . INJECTION T I M E S OUT
5 . C L A M P COOLING T I M E S OUT
AND SCREW TRIPS SHOT S I Z E
L I M I T SWITCH
OIL FLOWS TO SCREW DRIVE SCREW PUMPS ITSELF
MOTOR BACK A S PARTS COOL IN
MOLD
OIL FLOWS TO CLAMP CYLINDER SCREW STOPS ROTATING
ROD AND C L A M P OPENS
6. EJECTION L I M I T SWITCH I S OIL FLOWS TO EJECTOR CYLINDER PART I S EJECTED FROM
TRIPPED MOLD
7. RECYCLE T I M E R T I M E S OUT S T A R T CYCLE ETC.
Fig. 1-7 Molding-machine functions.
Density
Process
Fig. 1-8 Interrelation of product, resin, and process.
of heating-cooling cycles before appear- ance and/or properties are affected. Ther- mosets (TSs), upon their final heating [usually at least to 120°C (248"F)], be- come permanently insoluble and infusible. During heating they undergo a chemical (cross-linking) change. Certain plastics re- quire higher melt temperatures, some as high as 400°C (752°F) (see section on Recycling in Chap. 6).
Extensive compounding of different amounts and combinations of additives (colorants, flame retardants, heat and light stabilizers, etc.), fillers (calcium carbonate, etc.), and reinforcements (glass fibers, glass flakes, graphite fibers, whiskers, etc.) are used
with plastics. Compounding also embraces the mixing (alloying, blending, etc.) of two or more plastics that may be miscible or immiscible, with or without additives.
With TPs, the mold initially is kept at as low a temperature as possible, below the melting point of the plastic melt. This approach causes the injected hot melt to initiate surface freez- ing on the cavity wall, followed by formation of the solid product. After a sufficient cool- ing time, the mold opens and the part(s) are ejected. When processing TSs [from the in- jection unit (plasticizer)], the hot melt enter- ing the heated mold initially remains below the temperature that would cause premature solidification due to its exothermic reaction.
P R O C E S S P R O D U C T
Fig. 1-9 Simplified processing steps.
1 The Complete Injection Molding Process 9
1 Performance Requirements I
I Material Selection I
.;........;l=r;’ Ideal choiceiCompromise
Fig. 1-10 Flow diagram for setting up the selec- tion procedure.
After properly filling the cavity or cavities, the mold’s higher temperature causes the melt to undergo its final chemical cross- linking action resulting in solidification.
Morphology and Performance
The processability and performance of TPs, such as meeting product tolerance re- quirements and mechanical properties, are influenced by factors such as molecule size and weight, molecular distribution, and shapes or structures of individual molecules. TPs are formed by combining into long chains of molecules, or molecules with branches (lat- eral connections) to form complex molecular shapes. All these forms exist in either two or three dimensions, Because of their geome- try (morphology), some of these molecules can come closer together than others. These are identified as crystalline (such as PE, PP, and PA); the others are amorphous (such as PMMA, PS, SAN, and ABS). Morphology pertains to TPs but not TSs. When TSs are processed, their individual chain segments
scmw tJOzzle 2.25 in. dla.
16.000 wl
Mold injector -\
CUREINMOLDCAW
10 I The Complete Injection Molding Process
MATERIAL
INJECTION
TEMPERATURE PRESSURE
SHRINKAGE CCCCRS
TIME
are strongly bonded together during a chem- ical reaction that is irreversible.
Plastics are either truly homogeneous, amorphous solids or heterogeneous, semi- crystalline solids. There are no purely crys- talline plastics; so-called crystalline materials also contain different amounts of amorphous material. The term semicrystalline is techni- cally more accurate, but seldom used. Vari- ous methods of characterizing and evaluating
HIGH-QUALITY I MOLDED PARTS I 1 GOODMOLD 1 ADEQUATE MOLD
CLAMPING FORCE
Complete preliminary appraisal
Assign deaign prlorlty
1 The Complete Injection Molding Process 11
TOUGH
Fig. 1-16 Range of properties.
plastics are used, such as their molecu- lar weight distribution (MWD). A narrow MWD enhances the performance of plastic products. MWD affects melt flow behavior (Chap. 6).
Melt Flow and Rheology
Rheology is the science that deals with the deformation and flow of matter under various conditions. An example is plastic melt flow.
The rheology of plastics, particularly TPs, is complex but manageable. These materials combine the properties of an ideal viscous liquid (pure shear deformations) with those of an ideal elastic solid (pure elastic defor- mation). Plastics are therefore said to be vis- coelastic. The mechanical behavior of plastics is dominated by the viscoelastic parameters such as tensile strength, elongation at break, and rupture energy. The viscous attributes of melt flows are very important considerations during any processing system (see section on Molding Thin Walls in Chap. 7).
Viscosity is a material's resistance to vis- cous deformation (flow). Quantitatively it is expressed by the modulus of elasticity E (Chap. 12).
Plastics undergo non-Newtonian flow: the curve of pressure vs. flow rate for the melt is not a straight line. By contrast, the flow of water is nearly Newtonian.
Not only are there these two classes of deformation; there are also two modes in which deformation can be produced: simple shear and simple tension. The actual beha- vior during melting, as in a screw plasticator (injection unit), is extremely complex,
NATURAL GAS PETROLEUM COAL AGRICULTURE v
I 1
\I 1
7 ETHYLENE STYRENE FORMALDEHYDE POLYOL ADIPATE PROPYLENE VINYL CHLORIDE CUMENE ACRYLIC 77 v 1
T? \I 1
77 v 1
TK? \I 1
ELECTRICAL CONSUMER INDUSTRIAL 7
PRODUCTS SIDING COMMUNICATION ELECTRICAL MEDICAL AUTO TOOL - 1 I
Fig. 1-17 Raw materials to products.
12 1 The Complete Injection Molding Process
Tensile loud %+4
*--\
Fig. 1-18 Examples of mechanical tests.
displaying many types of shear-tension re- lationships. Together with the screw design, the deformation determines the pumping ef- ficiency of the plasticator and controls the re- lationship between output rate and pressure drop through the melt flow to solidification in the mold cavity(s).
Plasticating
Plasticating is the process that melts the plastics. Different methods are used. The most common are the single-stage (recip-
rocating screw) and the two-stage. In Fig. 1-19, (a) and (b) show the ram (also called plunger) systems used in the original IMMs since the 1870s, and now used mainly to pro- cess plastics with very little melt flow, such as ultrahigh-molecular-weight polyethylene. They use a piston, with or without a torpedo, for plastication. Part (c) shows the single- stage reciprocating screw plasticator, and (d) the two-stage screw plasticator.
There are different IMM operating de- signs in use: all-hydraulic, all-electrical, and hybrid (combination of hydraulic and electrical). Each design provides different
1 The Complete Injection Molding Process 13
- I I MOLD 4 E , i E , SINGLE-STAGE RAM INJECTION
MOLD
SCREW PLASTICATOR
Fig. 1-19 Examples of different plasticating systems.
advantages such as reducing product weight (reducing plastic consumption), eliminat- ing or minimizing molded-in stresses, mold- ing extremely small to very large products, and/or improving performance. There are also IMMs that perform specialty molding operations. An example is the gas-injection molding machine (GIMM) systems. They ba- sically involve the injection of an inert gas, usually nitrogen, into the melt as it enters
the mold. The gas forms a series of inter- connecting hollow channels within the melt. The gas pressure at about 4,300 psi (30 MPa) is maintained through the cooling cycle. In effect the gas packs the plastic against the cavity (Chap. 15).
Another design is injection-compression molding, also called injection stamping or more often coining. It uses a compression type mold having a male plug that fits into
I 4 1 The Complete Injection Molding Process
Fig. 1-20 Sections of a screw.
a female cavity. After a short shot enters the mold (which has been previously opened and closed so that it is unpressurized), the stress-free melt is compressed to mold the finished product. Other systems include coin- jection, two-color injection molding, coun- terflow injection molding, multi-live injec- tion molding, oscillatory injection molding, reaction injection molding, liquid injection molding, foam injection molding, fusible- and soluble-core injection molding, tandem in- jection molding, injection blow molding, in- jection molding with rotation, continuous injection molding (Velcro strips, etc.), metal- plastic injection molding, and vacuum injec- tion molding (Chap. 15)
Screw Designs
The primary purpose for using a screw located in the plasticator barrel is to take advantage of its mixing action. The motion of the screw is controlled to keep the IMM’s process controls operating at their set points. The usual variation in melt temperature, melt uniformity, and melt output is kept to a mini-
mum prior to entering the mold. Heat is sup- plied by heater bands around the barrel and by the mixing action that occurs when the plastic is moved by the screw. Both conduc- tion heating and mechanical friction heating of the plastic occur during screw rotation. The different controls used during injection mold- ing, such as back pressure and screw rota- tional speed, influence the melt characteris- tics (Chap. 3).
Most IMMs use a single constant-pitch, metering-type screw for handling the plastics. The screw has three sections, for feed, melt- ing (transition), and metering (Fig. 1-20). The feed section, which is at the back end of the screw (where plastic first enters), can occupy from very little to 75% of the screw length, usually 50 to 75%. Its length essentially de- pends upon how much heat has to be added to the plastic that enters the hopper, where it may be preheated.
The melting (transition) section is where the softening of the plastic occurs; the plastic is transformed into a continuous melt. It can occupy from 5 to 50% of the screw length. This section, usually called the compression zone, has to be sufficiently long to make
1 The Complete Injection Molding Process 15
sure that the plastic is melted. A straight compression-type screw is one having no feed or metering section. For certain plastics, par- ticularly TSs, there tends to be no compres- sion zone, since overheating and solidifica- tion of the melt could occur between the screw and barrel.
In the metering section, the plastic is smeared and sheared to give the melt its fi- nal uniform composition and temperature for delivery to the mold. As high shear action will tend to increase the melt’s temperature, the length of the metering section is depen- dent upon the plastic’s heat sensitivity and whether any additional mixing is required. For certain heat-sensitive plastics very little or no metering action can be tolerated. For other plastics it averages about 20 to 25% of the screw length. Both the feed and meter- ing sections usually have a constant cross sec- tion (zero compression ratio). However, the depth of flight in the feed section is greater than that in the metering section. The screw’s compression ratio can be determined by di- viding the flight depth in the feed section by that in the metering section. Depending on the plastic processed, ratios usually range from 0 to 4.
Molds
The mold is the most important part of the IMM. It is a controllable, complex, and ex- pensive device. If not properly designed, op- erated, handled, and maintained, its opera- tion will be a costly and inefficient.
Under pressure, hot melt moves rapidly through the mold. During the injection into the mold, air in the cavity or cavities is re-
leased to prevent melt burning and the for- mation of voids in the product. With TPs, temperature-controlled water (with ethylene glycol if the water has to operate below its freezing point) circulates in the mold to re- move heat; with TSs, electrical heaters are usually used within the mold to provide the additional heat required to solidify the plastic melt in the cavity.
The mold basically consists of a sprue, a runner, a cavity gate, and a cavity. The sprue is the channel located in the stationary platen that transports the melt from the plastica- tor nozzle to the runner. In turn, melt flows through the runner and gate and into the cav- ity. With a single-cavity mold, usually no run- ner is used, so melt goes from the sprue to the gate.
Different runner systems are in use to meet different processing requirements. The most popular are cold and hot runners. With a TP cold runner, the melt flowing from the sprue to the gate solidifies by the cooling action of the mold as the melt in the cavity or cavities solidifies. With a TP hot runner the sprue to the gate is insulated from the chilled cavity or cavities and remains hot, so that the melt never cools; the next shot starts from the gate, rather than from the nozzle as in a cold run- ner. With a TS hot runner, the melt in the runner solidifies. The TS cold runner keeps the plastic melted by using a cooled insulated manifold; its next shot starts from the gate, rather than from the nozzle as in a TP hot runner.
Molds are provided with different means, such as sliders, unscrewing devices, undercuts (Fig. 1-21), and knockout systems, to eject products as well as solidified runners at the proper time. These basic operations in turn
Nominal thickness should be maintained throughout part
deeper hole intersecting side walls
Fig. 1-21 Methods of molding holes or openings in side walls without undercutting mold movements.
16 1 The Complete Injection Molding Process
DEPTH OF DRAW !-L DIMENSION DIFFERENCE
Fig. 1-22 Example of mold-cavity draft angle re- quired to ensure removal of molded product dur- ing its mold ejection action.
require control of various parameters such as fill time and hold pressure (Chap. 4).
To simplify molding, whenever possible one should design the product with fea- tures that simplify the mold-cavity melt filling operation. Many such features can improve the product’s performance and/or reduce cost. An example is choosing the mold-cavity draft angle according to the plastic being processed, tolerance requirements, etc. (Fig. 1-22). Figure 1-23 shows a situation where it is possible to eliminate or significantly re- duce shrinkage, sink marks, and other defects (Chap. 8).
Processing
Processing steps are summarized in Figs. 1-9, 1-10, and 1-24 to 1-27. Different ma- chine requirements and material conditions are considered in choosing the most efficient injection molding process. It is important to understand and properly operate the basic IMM as well as its auxiliary equipment. In particular, in practically all operations the screws must not be damaged or worn and the plastic must be properly dried. Special dryers and/or vented barrels are required for drying hygroscopic TP materials such as PC, PMMA, PUR, and PET (Chap. 10).
Use of TP regrind may have little effect on product performance (appearance, color, strength, etc.). However, reduction in perfor- mance can occur with certain TPs after even one passage through the IMM. Granulated TSs cannot be remelted but can be used as additives or fillers in plastics.
Many TPs can be recycled indefinitely by granulating scrap, defective products, and so on. During these cycles, however, the plas- tic develops a “time-to-heat” history or res- idence time. This phenomenon can signifi- cantly compromise processing advantages
POOR DESIGN SINK MARKS
v2 t
MATCH OUTSIDE CONFIGURATION TO INSIDE CORES
POOR RECTANGULAR PART WITH ROUND HOLES
Fig. 1-23 Example of coring in molds to eliminate or reduce shrinkage and sink marks.
1 The Complete Injection Molding Process
Fig. 1-24 Relationship between manufacturing process and properties of products.
Bulk density
Shrinkage characteristic Water content Stickiness (adhesion)
Feeding ease I Feeding accurcy
I ~~ ~
Pre-heating
pre-heating (temperature, time) mold filling (time, pressure) curing (temperature, time)
Dimensional stability Demoldlng behavior
Mold life Machine wear
Fig. 1-25 Processing behavior.
OPERATING RANGES MACHINE PARAMETER
Fig. 1-26 Process control model.
17
Preimpregnated
and properties, requiring compensation in the product design or process setup, and/or material modification by incorporating addi- tives, fillers, and/or reinforcements.
For all types of plastics, injection molding troubleshooting guides have been written to allow fast corrective action when products do not meet their performance requirements. Examples of errors in the mold and product design with possible negative consequences during processing and/or product perfor- mance are presented throughout this book. Troubleshooting guides can be incorporated in process control systems (Chap. 11). An ex- ample is checking dryer performance as sum- marized in Table 1.1.
Process Controls
Proper injection of plastic melt into the mold is influenced by several process control conditions (Chap. 7). Any one or combina- tion of these can affect various performance parameters, such as the rate of which the raw
material is fed into the IMM (Fig. 1-28), flow of melt, packing of mold cavity or cavities and cycle time, which in turn affect product performance (Chap. 8). As an example, pa- rameters that influence product tolerances in- volve (1) product design, (2) plastics used, (3) mold design, (4) IMM capability, and ( 5 ) molding cycle time.
Different types of machine process con- trols (PCs) can be used to meet different re- quirements based on the molder’s needs. PC systems range from simple monitors (alarm buzzers, flashing lights, etc.) to very sophisti- cated program controllers [personal comput- ers (PCs) interrelate different IMM functions and melt process variables]. (Note that PC has two meanings; see Appendix 1, Abbrevi- ations.)
Knowledge of the machine and plastic ca- pabilities is needed before an intelligent PC program can be developed (Chap. 9). The use of PC or SPC (statistical PC) software requires continual study of the endless new computer technology as it applies to basically melting plastic (Chap. 13).
1 The Complete Injection Molding Process
Table 1-1 Trouble shooting dehumidifier dryer performance
19
Heater failure.
Line, hopper, or filter blockage.
2. Dewpoint as measured at air inlet to the hopper is unacceptable. line fuses.
Loss of regeneration heaters in one or both beds or
Loss of timer or clock motor ability to switch from one head to the other, Le., continuous operation on only one desiccant bed.
Desiccant has deteriorated or been contaminated.
Loss of power to one or both desiccant beds.
3. Airflow low or nonexistent.
Fan motor burned out. Loose fan on motor shaft. Clogged filter(s). Restricted or collapsed
Blower motor is reversed. air lines.
Check process air or afterheaters- regeneration heaters play no part in this aspect of operation.
Locate and repair-if the hose is old and brittle, replace. Shorten all hose to minimum lengths.
Check for collapsed or pinched lines, valves that are closed (some makes have airflow valves located on the air inlet side of the hopper). Filters should be changed or cleaned frequently- a good trial period is every four weeks until experience dictates a shorter or longer period.
These can be checked with a voltmeter at the control panel.
Check clock motor for movement by observing either function indicators or valve-shifting mechanisms. Note that loss of regeneration heaters may occur if the clock motor or shifting mechanism malfunctions.
Most manufacturers suggest checking the desiccant annually and replacing when it does not meet test criteria. Typically two to three years is a reasonable interval, depending upon the severity of service.
During regeneration cycle, exterior of the desiccant bed should be hot to touch. Check contacts on relays or printed circuit board for flaws; check line fuses if so equipped.
Replace. Tighten. Change. Correct and relieve restrictions.
Use of a pressure gauge or flowmeter is suggested. Proper rotation is that at which the highest flow is indicated.
20 I The Complete Injection Molding Process
4 \
Control Guides
Adequate PC and its associated instrumen- tation are essential for product quality con- trol (QC). The goal in some cases is precise adherence to a control point. In other cases, maintaining the temperature within compar- atively small range is all that is necessary for effortless control (of temperature, time, pressure, melt flow, rate, etc.) that will pro- duce the desired results (Chaps. 7,9, and 13).
Regardless of the type of controls available, the processor setting up a machine uses a sys- tematic approach that should be outlined in the machine and/or control operating man- uals. Once the machine is operating, the op- erator methodically targets one change at a time to achieve maximum injection molding efficiency.
With injection molding, as with all types of plastics processing, troubleshooting guides are established to take fast corrective action
1 The Complete Injection Molding Process 21
when parts do not meet their performance re- quirements (Chap. 11). This problem-solving approach fits into the overall PC and fabri- cating interface.
Control systems for units with complex processes such as injection molding are be- coming increasingly common. Such systems consist mostly of control chains and circuitry that are often coupled in their functions, as well as the corresponding exchange of data. In a broad sense, the control systems serve the purpose of cost reduction by monitoring quality and establishing high line efficiency, in addition to the reduction of raw mate- rial consumption and labor costs. A control system contributes in different ways, partic- ularly in controlling the flow of plastic melt. It can function by itself and fulfill the duties assigned to it, often resulting in product im- provement.
Since the 1960s, a procedure to influence important properties of the final product has been developed. The solutions, when intro- duced into practice, served first of all to improve the product line in different manu- facturing plants. However, initially these sys- tems established themselves in only relatively small niches of the commercial market. Later many more came aboard.
The use of flexibly automated injection molding controls and systems definitely de- pends on the tasks the machine has to perform and the production sequences re- quired. Automation is one possibility for putting in-house aims into practice and/or meeting market-dictated demands such as (1) production-cost reduction, (2) short job processing time, (3) low expenditure on setup, (4) greatest possible preparedness for meeting delivery dates, (5) large product range, and (6) improved delivery consistency.
In order to utilize the advantages of flexi- bly automated injection molding cells, a con- siderably larger capital investment is nec- essary than with other choices of systems, which are less automated and flexible. This increases the investment risk, so that the question of the profitability of such systems becomes more urgent. The following are examples of productivity-increasing effects: (1) an increase in the annual utilization time,
(2) an increase in annual production vol- ume, (3) a reduction of demolding time, and (4) a shortening of transit time if additional activities can be carried out within the pro- grammed cycle time.
The profitability of a flexibly automated injection molding plant is influenced by (1) increased capital cost, (2) reduced per- sonnel costs due to fewer personnel required, and (3) changes in energy costs and the mold-cost structure. With automation, new goals can be met through plant flexibility, such as (1) improved delivery consistency, (2) greatest possible preparedness for meet- ing delivery dates, (3) large range of products, and (4) short job processing time. There are also quality-related effects that result in im- proved quality assurance and a reduced num- ber of rejects. Work environment changes oc- cur in (1) psychological and physical stresses on staff, (2) qualification requirements from staff, (3) social welfare of staff employed on the injection molding machine, and (4) the ac- cident risk situation. An evaluation of the uti- lization efficiency serves for assessing the cri- teria that cannot be quantified in monetary terms. An established utilization efficiency value can be taken as a decision aid, which in conjunction with the investment calculation will allow a better selection of alternatives under consideration.
Art of Processing
Processing of plastic is an art of detail. The more you pay attention to details, the fewer hassles you will get from the process. If a pro- cess has been running well, it will continue running well unless a change occurs. Correct the problem; do not compensate. That may not be an easy task, but understanding your equipment, material, environment, and peo- ple can make it possible.
Fine Tuning
A computer-integrated injection molding (CIIM) system makes it possible to target for: (1) approaching a completely automated
22 1 The Complete Injection Molding Process
injection molding system, (2) simultaneously achieving high quality (zero defects), ( 3 ) increasing productivity, and (4) minimizing cost. It does this in several ways, basically by enabling the molder to fine-tune all the re- lationships that exist among the many ma- chine settings and properties of the plastic melt. These systems, when properly used, readily adapt to enhanced processing capa- bilities.
Once processing variables (machine and plastic) are optimized through computer sim- ulation (rather than the usual trial-and-error method), these values are entered in com- puter programs in the form of a rather large number of machine settings. Establishing the initial settings during startup can be inher- ently complex and time-consuming. Regard- less, the many benefits of these systems are well recognized and accepted. However, it is evident that self-regulation of injection molding can be effective only when the de- sign of the product and the mold are opti- mized with the correct processing conditions. Otherwise, a self-regulating IMM is confused and can issue conflicting instructions. The re- sults can be disastrous, including damage to the machine and/or the mold as well as safety hazards. Therefore, the efficient utilization of microprocessor control systems depends on the success of utilizing correct and opti- mum programs with knowledgeable people (Chap. 9). On the horizon is the potential for fuzzy control to provide an important aid to optimizing process control performance. As reviewed in Chap. 7, fuzzy logic, since its in- ception in 1981, has striven with increasing success to mimic the control actions of a hu- man operator.
Molding Operations
The following modes of operations typify injection molding operations.
functions repeat. The IMM stops only in the event of a malfunction or if it is man- ually interrupted. Machinery and mecha- nisms are self-controlled so that manual in- put is not necessary during operation. The continuing development of more sophisti- cated processing equipment in turn allows the development of more integrated process- ing equipment. This action results in many improvements, such as (1) increased operat- ing efficiency through reducing scrap and/or rejects, (2) improved quality through uni- form, repeatable manufacturing procedures, (3) decision making and record keeping by converting data to information, (4) access to manufacturing information by supervisors and management, and ( 5 ) process control and process management.
Automation level The automation level is the degree to which a process operates au- tomatically. The choice of level must take into account the ability of the system to di- agnose problems in operation, the ability of the system to recover from error or fault, the ability of a system to start up and shut down without human intervention, and the like.
Automated vision Vision automation provides a means to achieve automatic equipment operation by adaptive part re- moval. It provides the capability of detecting a variety of part problems or defects by critical part inspection.
Semiautomatic
A semiautomatic machine will perform a complete cycle of programmed molding func- tions automatically and then stop. It will then require an operator to start another cycle manually.
Manual Automatic
A machine operating automatically will perform a molding cycle where programmed
It is an operation in which each function and the timing of each function is controlled manually by an operator.
1 The Complete Injection Molding Process 23
Primary
Identifies the main molding operation equipment to fabricate products namely the injection molding machine (Chap. 2).
Secondary
After fabricating (primary) molded prod- ucts, secondary operations may be required to produce the final finished product. These operations can occur online or offline. They include any one or a combination of opera- tions such as the following: annealing (to re- lieve or remove residual stresses and strains), postcuring (to improve performance); plat- ing; joining and a