Jay Shoemaker
Moldflow Design Guide
A Resource for Plastics Engineers
Foreword
6
Preface
8
Acknowledgements
10
Contents
12
1 Polymer Flow Behavior in Injection Molds
22
1.1 Phases of Injection Molding
22
1.1.1 How Plastic Fills a Mold
22
1.1.2 The Filling Phase
24
1.1.3 The Pressurization Phase
27
1.1.4 The Compensation Phase
27
1.2 How Do Plastics Flow?
28
1.2.1 Material Behavior
28
1.2.2 Deformation
29
1.2.3 Viscoelastic Behavior
29
1.2.4 Melt Shear Viscosity
30
1.2.5 Newtonian Fluid vs. Non-Newtonian Fluid
31
1.2.6 Shear-thinning Behavior
31
1.2.7 Shear Rate Distribution
32
1.2.8 Pressure-driven Flow
32
1.2.9 Pressure Gradient and Injection Times
34
1.2.10 Melt Flow Length
34
1.2.11 Injection Pressure vs. Fill Time
34
1.2.12 Flow Instability
36
2 Molding Conditions and Injection Pressure
38
2.1 Injection-pressure Overview
38
2.1.1 Pressure Drives the Flow Front
39
2.2 Factors Influencing Injection-pressure Requirements
39
2.3 Equations
41
2.3.1 Circular Channel Flow
41
2.3.2 Strip Channel Flow
41
2.4 Effect of Molding Conditions
42
2.4.1 Part Quality
42
2.4.2 Melt Temperature
43
2.4.3 Mold Temperature
43
2.4.4 Fill Time
44
2.4.5 Shear Stress Variation
44
2.4.6 Packing Pressure and Time
45
2.4.7 Summary
46
2.4.8 Back Flow
47
2.5 Using Moldflow to Determine Optimum Molding Conditions
47
2.5.1 Part
48
2.5.2 Molding Window Size
48
2.5.3 Injection Pressure
49
2.5.4 Flow Front Temperature
50
2.5.5 Cooling Time
51
2.5.6 Summary
52
3 Filling Pattern
54
3.1 Filling Pattern Overview
54
3.1.1 What Is the Filling Pattern?
54
3.2 Flow in Complex Molds
55
3.2.1 Overpack
55
3.2.2 Racetrack Effect
56
3.2.3 Varying Injection Rate
56
3.2.4 Underflow Effect
57
3.2.5 Hesitation Effect
58
3.2.6 Weld Lines
59
3.2.7 Meld Lines
59
3.2.8 Sink Marks
59
3.2.9 Multidirectional Flow
60
3.2.10 Unstable Flow
61
3.2.11 Simple Flow Pattern
62
3.3 Flow-front Velocity and Flow-front Area
62
3.3.1 What are FFV and FFA?
62
3.3.2 Flow-front Velocity Influences Filling Pattern
62
3.3.3 Equation
63
3.4 Using Moldflow to Determine the Filling Pattern
64
3.4.1 Computer Simulation Can Eliminate Molding Trials
64
3.4.2 Using a Flow Analysis
64
3.5 Using Moldflow to Achieve Constant FFV
65
3.5.1 Controlling the FFV Through Ram Speed
65
4 Moldflow Design Principles
68
4.1 Product Design and Moldflow
68
4.2 Sequence of Analysis
68
4.2.1 Part Filling Optimization
69
4.2.2 Molding Conditions
69
4.2.3 Runner Design
69
4.2.4 Cooling Optimization
70
4.2.5 Packing Optimization
70
4.2.6 Warpage Optimization
70
4.3 Moldflow Flow Concepts
70
4.3.1 Unidirectional and Controlled Flow Pattern
71
4.3.2 Flow Balancing
72
4.3.3 Constant Pressure Gradient
73
4.3.4 Maximum Shear Stress
74
4.3.5 Uniform Cooling
75
4.3.6 Positioning Weld and Meld Lines
76
4.3.7 Avoid Hesitation Effects
76
4.3.8 Avoid Underflow
76
4.3.9 Balancing with Flow Leaders and Flow Deflectors
78
4.3.10 Controlled Frictional Heat
79
4.3.11 Thermal Shutoff of Runners
79
4.3.12 Acceptable Runner/Cavity Ratio
80
5 Meshes Used In Moldflow Analyses
82
5.1 Mesh Types Used by Moldflow
82
5.1.1 Finite Elements Used in Moldflow
82
5.1.2 Mesh Types
83
5.1.3 Solver Assumptions
84
5.2 Mesh Requirements
85
5.2.1 Mesh Density Considerations
85
5.2.2 Part Details
88
5.3 Geometry Creation
89
5.4 Importing Geometry
90
5.5 Using Different Mesh Types
90
5.5.1 Door Panel
90
5.5.2 Manifold
91
6 Product Design
92
6.1 Material Properties for Product Design
92
6.1.1 Plastics Are Sensitive to Operating Conditions
92
6.1.2 Stress-Strain Behavior
93
6.1.3 Creep and Stress Relaxation
98
6.1.4 Fatigue
100
6.1.5 Impact strength
101
6.1.6 Thermal Mechanical Behavior
101
6.2 Design for Strength
103
6.2.1 Predicting Part Strength
103
6.2.2 Loading/Operating Conditions
103
6.3 Part Thickness
107
6.3.1 Part Thickness Drives Quality and Cost
107
6.3.2 Cycle Time Increases with Thickness
107
6.3.3 Thick Parts Tend to Warp
107
6.3.4 Thin, Uniform Parts Improve Surface Quality
108
6.3.5 Reducing Part Thickness
108
6.4 Boosting Structural Integrity with Ribs
109
6.4.1 Structural Integrity: the Goal of Every Design
109
6.4.2 Designing Ribs
109
6.5 Design for Assembly
111
6.5.1 Molding One Part vs. Separate Components
111
6.5.2 Tolerances: Fit between Parts
111
6.5.3 Press-fit Joints
112
6.5.4 Snap-fit Joints
114
6.5.5 Cantilever Snap Joints
116
6.5.6 Torsion Snap-fit Joints
117
6.5.7 Fasteners
119
6.5.8 Inserts
122
6.5.9 Welding Processes
122
7 Gate Design
124
7.1 Gate Design Overview
124
7.1.1 What Is a Gate?
124
7.1.2 Single vs. Multiple Gates
124
7.1.3 Gate Dimensions
124
7.1.4 Gate Location
125
7.2 Gate Types
125
7.2.1 Manually Trimmed Gates
125
7.2.2 Automatically Trimmed Gates
131
7.3 Design Rules
134
7.3.1 Determining the Number of Gates
134
7.3.2 Flow Patterns
136
7.3.3 Gate Position
137
7.3.4 Avoiding Common Problems
143
7.3.5 Gate Length
143
7.3.6 Gate Thickness
143
7.3.7 Freeze-off Time
144
7.4 Using Moldflow for Gate Design
144
7.4.1 Gate Location
144
7.4.2 Molding Window Size for the Three Gate Locations
145
7.4.3 Filling Pattern
146
7.4.4 Gate Size Based on Shear Rate
146
8 Runner System Design
148
8.1 Definitions
148
8.1.1 Feed System
148
8.1.2 Runner System
148
8.1.3 Cold Runner
148
8.1.4 Hot Runner
149
8.1.5 Hot Manifold
149
8.1.6 Hot Drop
149
8.1.7 Sprue
149
8.2 Runner System Design Principles
150
8.2.1 Benefits of Good Runner Design
150
8.2.2 Runner Design Philosophy
150
8.2.3 Flow Balancing
151
8.2.4 Flow Control
151
8.2.5 Frictional Heating in Runners
153
8.2.6 Thermal Shutoff
153
8.2.7 System and Runner Pressures
153
8.2.8 Constant Pressure Gradient
154
8.2.9 Cold Slug Wells
154
8.2.10 Easy Ejection
155
8.3 Runner Types
155
8.3.1 Cold Runners
155
8.3.2 Hot Runner Systems
156
8.4 Runner Layout
157
8.4.1 Determining the Number of Cavities
157
8.4.2 Planning the Runner System Layout
159
8.4.3 Partially Balanced Runners
160
8.4.4 Geometrically Balanced Runners
162
8.5 Initial Runner Sizing
162
8.5.1 Determining Sprue Dimensions
162
8.5.2 Designing Runner Cross Sections
163
8.5.3 Determining Runner Diameters
166
8.6 Runner Balancing
168
8.6.1 How Runner Balancing Works
168
8.6.2 When Are the Runner Sizes Optimized?
168
8.6.3 Validating the Balance
168
8.6.4 Processing Window
169
8.7 Using Moldflow for Runner Balancing
170
8.7.1 Runner Balancing a 48-cavity Tool
170
8.7.2 Runner Balancing for a Family Mold
172
8.7.3 Runner Balancing for a Multigated Part
173
9 Cooling System Design
174
9.1 Mold Cooling System Overview
174
9.1.1 Importance of Cooling System Design
174
9.1.2 Mold Cooling System Components
175
9.2 Cooling-channel Configuration
177
9.2.1 Types of Cooling Channels
177
9.3 Alternative Cooling Devices
178
9.3.1 What Do They Do?
178
9.3.2 Baffles
179
9.3.3 Bubblers
180
9.3.4 Thermal Pins
180
9.3.5 Cooling Slender Cores
181
9.3.6 Cooling Large Cores
182
9.3.7 Cooling Cylinder Cores
183
9.4 Cooling System Equations
183
9.4.1 Cooling Time
183
9.4.2 Reynolds Number and Coolant Flow
184
9.5 Design Rules
184
9.5.1 Mold Cooling Design Considerations
184
9.5.2 Location and Size of Channels
185
9.5.3 Flow Rate and Heat Transfer
187
9.6 Using Moldflow for Cooling System Design
188
9.6.1 Example Setup
189
9.6.2 Cycle Time Determined by Design and Processing Parameters
190
10 Shrinkage and Warpage
194
10.1 Injection Molding and Shrinkage
194
10.1.1 What Are Shrinkage and Warpage?
194
10.1.2 Shrinkage and Machine Settings
194
10.1.3 Mold Filling and Packing
195
10.1.4 How Pressure and Time Affect Shrinkage
196
10.1.5 Thermally Unstable Flow
197
10.2 Basic Causes of Shrinkage and Warpage
198
10.2.1 Causes of Shrinkage
198
10.2.2 Causes of Warpage
202
10.2.3 Relating Orientation and Area Shrinkage to Warpage
203
10.3 Designing Accurate Parts Considering Warpage
204
10.3.1 Material Selection
205
10.3.2 Wall Thickness Variation
205
10.3.3 Gate Position and Runner Dimensions
206
10.3.4 Molding Conditions
208
10.3.5 Cooling Line Layout
211
11 Moldflow Design Procedure
212
11.1 Determine Analysis Objectives
212
11.2 Moldflow Analysis Steps Framework
213
11.2.1 The Whole Process
213
11.2.2 Optimize Fill
213
11.2.3 Balance and Size the Runners
218
11.2.4 Optimize Cooling
220
11.2.5 Optimize the Packing Profile
221
11.2.6 Optimize Warpage
222
11.3 Using Moldflow to Evaluate an Initial Design
226
11.3.1 Description of this Example
226
11.3.2 Molding Window
226
11.3.3 Filling Analysis
227
11.3.4 Gate and Runner Design
227
11.3.5 Cooling System Design
228
11.3.6 Packing Analysis
230
11.3.7 Warpage Analysis
230
11.4 Using Moldflow to Optimize the Design
231
11.4.1 Determine the Cause of Warpage
231
11.4.2 Investigating Different Gate Locations
231
11.4.3 Validating the Best Gate Location
232
12 Part Defects
236
12.1 Air Traps
236
12.1.1 What Is an Air Trap?
236
12.1.2 Problems Caused by Air Traps
237
12.1.3 Remedies
237
12.2 Black Specks and Black Streaks
238
12.2.1 What Are Black Specks and Black Streaks?
238
12.2.2 Causes of Black Specks and Black Streaks
238
12.2.3 Remedies
239
12.3 Brittleness
240
12.3.1 What Is Brittleness?
240
12.3.2 Causes of Brittleness
240
12.3.3 Remedies
240
12.4 Burn Marks
241
12.4.1 What Is a Burn Mark?
241
12.4.2 Causes of Burn Marks
242
12.4.3 Remedies
243
12.5 Delamination
243
12.5.1 What Is Delamination?
243
12.5.2 Causes of Delamination
244
12.5.3 Remedies
244
12.6 Dimensional Variation
245
12.6.1 What Is Dimensional Variation?
245
12.6.2 Causes of Dimensional Variation
245
12.6.3 Remedies
245
12.7 Discoloration
246
12.7.1 What Is Discoloration?
246
12.7.2 Causes of Discoloration
246
12.7.3 Remedies
247
12.8 Fish Eyes
248
12.8.1 What Are Fish Eyes?
248
12.8.2 Causes of Fish Eyes
248
12.8.3 Remedies
248
12.9 Flash
249
12.9.1 What Is Flash?
249
12.9.2 Causes of Flash
249
12.9.3 Remedies
250
12.10 Flow Marks
251
12.10.1 What Is A Flow Mark?
251
12.10.2 Causes of Flow Marks
251
12.10.3 Remedies
251
12.11 Hesitation
252
12.11.1 What Is Hesitation?
252
12.11.2 Problems Caused by Hesitation
252
12.11.3 Remedies
253
12.12 Jetting
253
12.12.1 What Is Jetting?
253
12.12.2 Problems Caused by Jetting
254
12.12.3 Remedies
254
12.13 Ripples
256
12.13.1 What Are Ripples?
256
12.13.2 Cause of Ripples
256
12.13.3 Remedies
257
12.14 Short Shots
257
12.14.1 What Is a Short Shot?
257
12.14.2 Causes of Short Shots
258
12.14.3 Remedies
258
12.15 Silver Streaks
259
12.15.1 What Are Silver Streaks?
259
12.15.2 Causes of Silver Streaks
260
12.15.3 Remedies
260
12.16 Sink Marks and Voids
261
12.16.1 What Are Sink Marks and Voids?
261
12.16.2 Causes of Sink Marks and Voids
261
12.16.3 Remedies
262
12.17 Weld Lines and Meld Lines
264
12.17.1 What Are Weld Lines and Meld Lines?
264
12.17.2 Problems Caused by Weld Lines
264
12.17.3 Strength of Weld Lines
265
12.17.4 Remedies
266
Appendix A: Injection Molding
268
A.1 Injection-molding Overview
268
A.1.1 Process
268
A.1.2 Applications
269
A.2 Development of the Injection-molding Machine
269
A.2.1 Benefits of the Reciprocating Screw
269
A.3 Development of the Injection-molding Process
270
A.4 Alternative Injection-molding Processes
270
A.4.1 Co-injection (Sandwich) Molding
270
A.4.2 Fusible Core Injection Molding
271
A.4.3 Gas-assisted Injection Molding
272
A.4.4 Injection-compression Molding
273
A.4.5 Lamellar (Microlayer) Injection Molding
274
A.4.6 Live-feed Injection Molding
274
A.4.7 Low-pressure Injection Molding
275
A.4.8 Push-pull Injection Molding
275
A.4.9 Reactive Molding
276
A.4.10 Structural Foam Injection Molding
277
A.4.11 Thin-wall Molding
277
Appendix B: Injection-molding Machine: System and Operations
280
B.1 Injection-molding Machine
280
B.1.1 Components
280
B.1.2 Machine Specification
281
B.1.3 Machine Function
281
B.1.4 Auxiliary Equipment
281
B.2 Machine Components
282
B.2.1 Injection System
282
B.2.2 Mold System
284
B.2.3 Hydraulic System
285
B.2.4 Control System
285
B.2.5 Clamping System
285
B.3 Molded System
286
B.3.1 The Delivery System
286
B.4 Machine Operating Sequence
287
B.4.1 Process Cycle
287
B.5 Screw Operation
289
B.5.1 Back Pressure
289
B.5.2 Injection Speed
289
B.5.3 Screw Rotation Speed
289
B.5.4 Cushion
290
B.6 Secondary Operations
290
B.6.1 Assembly
290
B.6.2 Decoration
290
B.6.3 Other Secondary Operations
291
Appendix C: Injection-molding Process Control
292
C.1 Importance of Process Conditions
292
C.1.1 Setting Machine Process Conditions
292
Appendix D: Plastic Materials
302
D.1 What Are Plastics?
302
D.1.1 Polymerization Process
302
D.1.2 Structure of Polymers
302
D.1.3 Polymer Alloys and Blends
304
D.1.4 Polymer Composites
304
D.2 Classification of Plastics
304
D.2.1 Classes of Plastics
305
D.2.2 Structures and Properties of Plastics
305
D.3 Thermoplastics
306
D.3.1 Market Share Distribution of Thermoplastics
306
D.3.2 Structures and Properties of Thermoplastics
307
D.3.3 Amorphous Polymers
307
D.3.4 Semicrystalline Polymers
308
D.4 Thermosets
309
D.5 Properties and Applications of Thermoplastics
310
D.5.1 ABS
311
D.5.2 PA 12
312
D.5.3 PA 6
314
D.5.4 PA 66
315
D.5.5 PBT
317
D.5.6 PC
318
D.5.7 PC/ABS
320
D.5.8 PC/PBT
321
D.5.9 HDPE
322
D.5.10 LDPE
324
D.5.11 PEI
325
D.5.12 PET
326
D.5.13 PETG
327
D.5.14 PMMA
328
D.5.15 POM
330
D.5.16 PP
331
D.5.17 PPE/PPO
333
D.5.18 PS
334
D.5.19 PVC
335
D.5.20 SAN
337
D.5.21 Additives, Fillers, and Reinforcements
338
D.5.22 Modifying Polymer Properties
339
D.5.23 Low-aspect Fillers
340
D.5.24 High-aspect Fillers: Fibers
340
Index
342
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