Moldflow Design Guide

Foreword

Preface

Acknowledgements

Contents

1 Polymer Flow Behavior in Injection Molds

1.1 Phases of Injection Molding

1.1.1 How Plastic Fills a Mold

1.1.2 The Filling Phase

1.1.3 The Pressurization Phase

1.1.4 The Compensation Phase

1.2 How Do Plastics Flow?

1.2.1 Material Behavior

1.2.2 Deformation

1.2.3 Viscoelastic Behavior

1.2.4 Melt Shear Viscosity

1.2.5 Newtonian Fluid vs. Non-Newtonian Fluid

1.2.6 Shear-thinning Behavior

1.2.7 Shear Rate Distribution

1.2.8 Pressure-driven Flow

1.2.9 Pressure Gradient and Injection Times

1.2.10 Melt Flow Length

1.2.11 Injection Pressure vs. Fill Time

1.2.12 Flow Instability

2 Molding Conditions and Injection Pressure

2.1 Injection-pressure Overview

2.1.1 Pressure Drives the Flow Front

2.2 Factors Influencing Injection-pressure Requirements

2.3 Equations

2.3.1 Circular Channel Flow

2.3.2 Strip Channel Flow

2.4 Effect of Molding Conditions

2.4.1 Part Quality

2.4.2 Melt Temperature

2.4.3 Mold Temperature

2.4.4 Fill Time

2.4.5 Shear Stress Variation

2.4.6 Packing Pressure and Time

2.4.7 Summary

2.4.8 Back Flow

2.5 Using Moldflow to Determine Optimum Molding Conditions

2.5.1 Part

2.5.2 Molding Window Size

2.5.3 Injection Pressure

2.5.4 Flow Front Temperature

2.5.5 Cooling Time

2.5.6 Summary

3 Filling Pattern

3.1 Filling Pattern Overview

3.1.1 What Is the Filling Pattern?

3.2 Flow in Complex Molds

3.2.1 Overpack

3.2.2 Racetrack Effect

3.2.3 Varying Injection Rate

3.2.4 Underflow Effect

3.2.5 Hesitation Effect

3.2.6 Weld Lines

3.2.7 Meld Lines

3.2.8 Sink Marks

3.2.9 Multidirectional Flow

3.2.10 Unstable Flow

3.2.11 Simple Flow Pattern

3.3 Flow-front Velocity and Flow-front Area

3.3.1 What are FFV and FFA?

3.3.2 Flow-front Velocity Influences Filling Pattern

3.3.3 Equation

3.4 Using Moldflow to Determine the Filling Pattern

3.4.1 Computer Simulation Can Eliminate Molding Trials

3.4.2 Using a Flow Analysis

3.5 Using Moldflow to Achieve Constant FFV

3.5.1 Controlling the FFV Through Ram Speed

4 Moldflow Design Principles

4.1 Product Design and Moldflow

4.2 Sequence of Analysis

4.2.1 Part Filling Optimization

4.2.2 Molding Conditions

4.2.3 Runner Design

4.2.4 Cooling Optimization

4.2.5 Packing Optimization

4.2.6 Warpage Optimization

4.3 Moldflow Flow Concepts

4.3.1 Unidirectional and Controlled Flow Pattern

4.3.2 Flow Balancing

4.3.3 Constant Pressure Gradient

4.3.4 Maximum Shear Stress

4.3.5 Uniform Cooling

4.3.6 Positioning Weld and Meld Lines

4.3.7 Avoid Hesitation Effects

4.3.8 Avoid Underflow

4.3.9 Balancing with Flow Leaders and Flow Deflectors

4.3.10 Controlled Frictional Heat

4.3.11 Thermal Shutoff of Runners

4.3.12 Acceptable Runner/Cavity Ratio

5 Meshes Used In Moldflow Analyses

5.1 Mesh Types Used by Moldflow

5.1.1 Finite Elements Used in Moldflow

5.1.2 Mesh Types

5.1.3 Solver Assumptions

5.2 Mesh Requirements

5.2.1 Mesh Density Considerations

5.2.2 Part Details

5.3 Geometry Creation

5.4 Importing Geometry

5.5 Using Different Mesh Types

5.5.1 Door Panel

5.5.2 Manifold

6 Product Design

6.1 Material Properties for Product Design

6.1.1 Plastics Are Sensitive to Operating Conditions

6.1.2 Stress-Strain Behavior

6.1.3 Creep and Stress Relaxation

6.1.4 Fatigue

6.1.5 Impact strength

6.1.6 Thermal Mechanical Behavior

6.2 Design for Strength

6.2.1 Predicting Part Strength

6.2.2 Loading/Operating Conditions

6.3 Part Thickness

6.3.1 Part Thickness Drives Quality and Cost

6.3.2 Cycle Time Increases with Thickness

6.3.3 Thick Parts Tend to Warp

6.3.4 Thin, Uniform Parts Improve Surface Quality

6.3.5 Reducing Part Thickness

6.4 Boosting Structural Integrity with Ribs

6.4.1 Structural Integrity: the Goal of Every Design

6.4.2 Designing Ribs

6.5 Design for Assembly

6.5.1 Molding One Part vs. Separate Components

6.5.2 Tolerances: Fit between Parts

6.5.3 Press-fit Joints

6.5.4 Snap-fit Joints

6.5.5 Cantilever Snap Joints

6.5.6 Torsion Snap-fit Joints

6.5.7 Fasteners

6.5.8 Inserts

6.5.9 Welding Processes

7 Gate Design

7.1 Gate Design Overview

7.1.1 What Is a Gate?

7.1.2 Single vs. Multiple Gates

7.1.3 Gate Dimensions

7.1.4 Gate Location

7.2 Gate Types

7.2.1 Manually Trimmed Gates

7.2.2 Automatically Trimmed Gates

7.3 Design Rules

7.3.1 Determining the Number of Gates

7.3.2 Flow Patterns

7.3.3 Gate Position

7.3.4 Avoiding Common Problems

7.3.5 Gate Length

7.3.6 Gate Thickness

7.3.7 Freeze-off Time

7.4 Using Moldflow for Gate Design

7.4.1 Gate Location

7.4.2 Molding Window Size for the Three Gate Locations

7.4.3 Filling Pattern

7.4.4 Gate Size Based on Shear Rate

8 Runner System Design

8.1 Definitions

8.1.1 Feed System

8.1.2 Runner System

8.1.3 Cold Runner

8.1.4 Hot Runner

8.1.5 Hot Manifold

8.1.6 Hot Drop

8.1.7 Sprue

8.2 Runner System Design Principles

8.2.1 Benefits of Good Runner Design

8.2.2 Runner Design Philosophy

8.2.3 Flow Balancing

8.2.4 Flow Control

8.2.5 Frictional Heating in Runners

8.2.6 Thermal Shutoff

8.2.7 System and Runner Pressures

8.2.8 Constant Pressure Gradient

8.2.9 Cold Slug Wells

8.2.10 Easy Ejection

8.3 Runner Types

8.3.1 Cold Runners

8.3.2 Hot Runner Systems

8.4 Runner Layout

8.4.1 Determining the Number of Cavities

8.4.2 Planning the Runner System Layout

8.4.3 Partially Balanced Runners

8.4.4 Geometrically Balanced Runners

8.5 Initial Runner Sizing

8.5.1 Determining Sprue Dimensions

8.5.2 Designing Runner Cross Sections

8.5.3 Determining Runner Diameters

8.6 Runner Balancing

8.6.1 How Runner Balancing Works

8.6.2 When Are the Runner Sizes Optimized?

8.6.3 Validating the Balance

8.6.4 Processing Window

8.7 Using Moldflow for Runner Balancing

8.7.1 Runner Balancing a 48-cavity Tool

8.7.2 Runner Balancing for a Family Mold

8.7.3 Runner Balancing for a Multigated Part

9 Cooling System Design

9.1 Mold Cooling System Overview

9.1.1 Importance of Cooling System Design

9.1.2 Mold Cooling System Components

9.2 Cooling-channel Configuration

9.2.1 Types of Cooling Channels

9.3 Alternative Cooling Devices

9.3.1 What Do They Do?

9.3.2 Baffles

9.3.3 Bubblers

9.3.4 Thermal Pins

9.3.5 Cooling Slender Cores

9.3.6 Cooling Large Cores

9.3.7 Cooling Cylinder Cores

9.4 Cooling System Equations

9.4.1 Cooling Time

9.4.2 Reynolds Number and Coolant Flow

9.5 Design Rules

9.5.1 Mold Cooling Design Considerations

9.5.2 Location and Size of Channels

9.5.3 Flow Rate and Heat Transfer

9.6 Using Moldflow for Cooling System Design

9.6.1 Example Setup

9.6.2 Cycle Time Determined by Design and Processing Parameters

10 Shrinkage and Warpage

10.1 Injection Molding and Shrinkage

10.1.1 What Are Shrinkage and Warpage?

10.1.2 Shrinkage and Machine Settings

10.1.3 Mold Filling and Packing

10.1.4 How Pressure and Time Affect Shrinkage

10.1.5 Thermally Unstable Flow

10.2 Basic Causes of Shrinkage and Warpage

10.2.1 Causes of Shrinkage

10.2.2 Causes of Warpage

10.2.3 Relating Orientation and Area Shrinkage to Warpage

10.3 Designing Accurate Parts Considering Warpage

10.3.1 Material Selection

10.3.2 Wall Thickness Variation

10.3.3 Gate Position and Runner Dimensions

10.3.4 Molding Conditions

10.3.5 Cooling Line Layout

11 Moldflow Design Procedure

11.1 Determine Analysis Objectives

11.2 Moldflow Analysis Steps Framework

11.2.1 The Whole Process

11.2.2 Optimize Fill

11.2.3 Balance and Size the Runners

11.2.4 Optimize Cooling

11.2.5 Optimize the Packing Profile

11.2.6 Optimize Warpage

11.3 Using Moldflow to Evaluate an Initial Design

11.3.1 Description of this Example

11.3.2 Molding Window

11.3.3 Filling Analysis

11.3.4 Gate and Runner Design

11.3.5 Cooling System Design

11.3.6 Packing Analysis

11.3.7 Warpage Analysis

11.4 Using Moldflow to Optimize the Design

11.4.1 Determine the Cause of Warpage

11.4.2 Investigating Different Gate Locations

11.4.3 Validating the Best Gate Location

12 Part Defects

12.1 Air Traps

12.1.1 What Is an Air Trap?

12.1.2 Problems Caused by Air Traps

12.1.3 Remedies

12.2 Black Specks and Black Streaks

12.2.1 What Are Black Specks and Black Streaks?

12.2.2 Causes of Black Specks and Black Streaks

12.2.3 Remedies

12.3 Brittleness

12.3.1 What Is Brittleness?

12.3.2 Causes of Brittleness

12.3.3 Remedies

12.4 Burn Marks

12.4.1 What Is a Burn Mark?

12.4.2 Causes of Burn Marks

12.4.3 Remedies

12.5 Delamination

12.5.1 What Is Delamination?

12.5.2 Causes of Delamination

12.5.3 Remedies

12.6 Dimensional Variation

12.6.1 What Is Dimensional Variation?

12.6.2 Causes of Dimensional Variation

12.6.3 Remedies

12.7 Discoloration

12.7.1 What Is Discoloration?

12.7.2 Causes of Discoloration

12.7.3 Remedies

12.8 Fish Eyes

12.8.1 What Are Fish Eyes?

12.8.2 Causes of Fish Eyes

12.8.3 Remedies

12.9 Flash

12.9.1 What Is Flash?

12.9.2 Causes of Flash

12.9.3 Remedies

12.10 Flow Marks

12.10.1 What Is A Flow Mark?

12.10.2 Causes of Flow Marks

12.10.3 Remedies

12.11 Hesitation

12.11.1 What Is Hesitation?

12.11.2 Problems Caused by Hesitation

12.11.3 Remedies

12.12 Jetting

12.12.1 What Is Jetting?

12.12.2 Problems Caused by Jetting

12.12.3 Remedies

12.13 Ripples

12.13.1 What Are Ripples?

12.13.2 Cause of Ripples

12.13.3 Remedies

12.14 Short Shots

12.14.1 What Is a Short Shot?

12.14.2 Causes of Short Shots

12.14.3 Remedies

12.15 Silver Streaks

12.15.1 What Are Silver Streaks?

12.15.2 Causes of Silver Streaks

12.15.3 Remedies

12.16 Sink Marks and Voids

12.16.1 What Are Sink Marks and Voids?

12.16.2 Causes of Sink Marks and Voids

12.16.3 Remedies

12.17 Weld Lines and Meld Lines

12.17.1 What Are Weld Lines and Meld Lines?

12.17.2 Problems Caused by Weld Lines

12.17.3 Strength of Weld Lines

12.17.4 Remedies

Appendix A: Injection Molding

A.1 Injection-molding Overview

A.1.1 Process

A.1.2 Applications

A.2 Development of the Injection-molding Machine

A.2.1 Benefits of the Reciprocating Screw

A.3 Development of the Injection-molding Process

A.4 Alternative Injection-molding Processes

A.4.1 Co-injection (Sandwich) Molding

A.4.2 Fusible Core Injection Molding

A.4.3 Gas-assisted Injection Molding

A.4.4 Injection-compression Molding

A.4.5 Lamellar (Microlayer) Injection Molding

A.4.6 Live-feed Injection Molding

A.4.7 Low-pressure Injection Molding

A.4.8 Push-pull Injection Molding

A.4.9 Reactive Molding

A.4.10 Structural Foam Injection Molding

A.4.11 Thin-wall Molding

Appendix B: Injection-molding Machine: System and Operations

B.1 Injection-molding Machine

B.1.1 Components

B.1.2 Machine Specification

B.1.3 Machine Function

B.1.4 Auxiliary Equipment

B.2 Machine Components

B.2.1 Injection System

B.2.2 Mold System

B.2.3 Hydraulic System

B.2.4 Control System

B.2.5 Clamping System

B.3 Molded System

B.3.1 The Delivery System

B.4 Machine Operating Sequence

B.4.1 Process Cycle

B.5 Screw Operation

B.5.1 Back Pressure

B.5.2 Injection Speed

B.5.3 Screw Rotation Speed

B.5.4 Cushion

B.6 Secondary Operations

B.6.1 Assembly

B.6.2 Decoration

B.6.3 Other Secondary Operations

Appendix C: Injection-molding Process Control

C.1 Importance of Process Conditions

C.1.1 Setting Machine Process Conditions

Appendix D: Plastic Materials

D.1 What Are Plastics?

D.1.1 Polymerization Process

D.1.2 Structure of Polymers

D.1.3 Polymer Alloys and Blends

D.1.4 Polymer Composites

D.2 Classification of Plastics

D.2.1 Classes of Plastics

D.2.2 Structures and Properties of Plastics

D.3 Thermoplastics

D.3.1 Market Share Distribution of Thermoplastics

D.3.2 Structures and Properties of Thermoplastics

D.3.3 Amorphous Polymers

D.3.4 Semicrystalline Polymers

D.4 Thermosets

D.5 Properties and Applications of Thermoplastics

D.5.1 ABS

D.5.2 PA 12

D.5.3 PA 6

D.5.4 PA 66

D.5.5 PBT

D.5.6 PC

D.5.7 PC/ABS

D.5.8 PC/PBT

D.5.9 HDPE

D.5.10 LDPE

D.5.11 PEI

D.5.12 PET

D.5.13 PETG

D.5.14 PMMA

D.5.15 POM

D.5.16 PP

D.5.17 PPE/PPO

D.5.18 PS

D.5.19 PVC

D.5.20 SAN

D.5.21 Additives, Fillers, and Reinforcements

D.5.22 Modifying Polymer Properties

D.5.23 Low-aspect Fillers

D.5.24 High-aspect Fillers: Fibers

Index