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- Herausgeber: Carl Hanser Verlag GmbH & Co. KG
- Kategorie: Fachliteratur
- Sprache: Englisch
This practical introductory guide to injection molding simulation is aimed at both practicing engineers and students. It will help the reader to innovate and improve part design and molding processes, essential for efficient manufacturing. A user-friendly, case-study-based approach is applied, enhanced by many illustrations in full color. The book is conceptually divided into three parts: Chapters 1–5 introduce the fundamentals of injection molding, focusing the factors governing molding quality and how molding simulation methodology is developed. As they are essential to molding quality, the rheological, thermodynamic, thermal, mechanical, kinetic properties of plastics are fully elaborated in this part, as well as curing kinetics for thermoset plastics. Chapters 6–11 introduce CAE verification of design, a valuable tool for both part and mold designers toward avoiding molding problems in the design stage and to solve issues encountered in injection molding. This part covers design guidelines of part, gating, runner, and cooling channel systems. Temperature control in hot runner systems, prediction and control of warpage, and fiber orientation are also discussed. Chapters 12–17 introduce research and development in innovative molding, illustrating how CAE is applied to advanced molding techniques, including co-/bi-Injection molding, gas-/water-assisted injection molding, foam injection molding, powder injection molding, resin transfer molding, and integrated circuit packaging. The authors come from the creative simulation team at CoreTech System (Moldex3D), winner of the PPS James L. White Innovation Award 2015. Several CAE case study exercises for execution in the Moldex3D software are included to allow readers to practice what they have learned and test their understanding.
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Veröffentlichungsjahr: 2018
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Maw-Ling Wang Rong-Yeu Chang Chia-Hsiang (David) Hsu
Molding Simulation: Theory and Practice
The authors:
Maw-Ling Wang, Chupei City, Hsinchu County 302, TaiwanRong-Yeu Chang, Chupei City, Hsinchu County 302, TaiwanChia-Hsiang (David) Hsu, Chupei City, Hsinchu County 302, Taiwan
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Print-ISBN 978-1-56990-619-4 E-Book-ISBN 978-1-56990-620-0
Table of Contents
Title page
Copyright page
Table of Contents
Acknowledgments
Preface
1 Overview of Plastics Molding
1.1 Introduction to Injection Molding
1.1.1 The Systems of Injection Molding
1.1.2 Defects of Injection Molded Products
1.2 Core Values of Molding Simulation
1.2.1 Application of CAE Technology in Injection Molding
2 Material Properties of Plastics
2.1 Overview
2.2 Rheological Properties
2.2.1 Viscosity
2.2.2 Viscoelastic Fluids
2.3 Thermodynamic and Thermal Properties
2.3.1 Specific Heat Capacity
2.3.2 Melting Point and Glass Transition Temperatures
2.3.3 PVT Equation of State
2.3.4 Thermal Conductivity and Heat Transfer Coefficient
2.4 Mechanical Properties
2.4.1 Stress and Strain of Plastics
2.4.2 Solid Viscoelasticity
2.4.3 Theoretical Model
2.5 Kinetic Properties
2.5.1 Crystalline
2.5.2 Theoretical Models
2.5.3 Effects of Cooling Rate on Crystallization
2.6 Curing Kinetics
2.6.1 Curing Phenomenon
2.6.2 Theoretical Models
2.6.3 Curing Effect on Viscosity
3 Part and Mold Design
3.1 Part Design
3.1.1 Golden Rule: Uniform Wall Thickness
3.1.2 Wall Thickness versus Flow Length
3.1.3 Radius/Fillets and Chamfer Angle
3.1.4 Rib and Boss
3.1.5 Draft Angle
3.1.6 Design for Manufacturing (DFM)
3.1.7 Summary
3.2 Mold Design
3.2.1 Basics
3.2.2 Gate Design
3.2.3 Runner Design
3.2.4 Cooling Design
3.2.5 Others
4 Process Conditions
4.1 Introduction of Injection Molding Cycle
4.1.1 Brief Introduction to Injection Molding Machine Units
4.1.2 Injection Molding Cycle
4.1.3 Molding Window
4.1.4 PVT Variations during Injection Stages
4.2 Plasticizing Conditions
4.2.1 Nozzle Temperature and Cylinder Temperatures
4.2.2 Back Pressure, Screw rpm, Suck Back, and Metering Stroke
4.3 Filling Conditions
4.3.1 Filling Time versus Injection Velocity
4.3.2 Injection Pressure
4.3.3 VP Switch
4.4 Packing Conditions
4.4.1 Packing Time and Packing Pressure
4.5 Cooling Conditions
4.5.1 Cooling Time
4.5.2 Coolant Flow Rate
4.5.3 Mold Temperature
5 Molding Simulation Methodology
5.1 The Goal of Molding Simulation
5.1.1 Design Verification and Optimization
5.1.2 Process Conditions Optimization
5.2 Basics of Simulation Equations
5.2.1 Governing Equations
5.2.2 Numerical Approximation
5.3 What is Molding Simulation?
5.3.1 Brief History of Molding Simulation
5.3.2 Simulation Workflow
6 Flow Consideration versus Part Features
6.1 Basics
6.1.1 Flow Behavior of Plastic Melt in the Cavity
6.1.2 Effects of Filling Time
6.1.3 Flow Rate versus Injection Pressure
6.1.4 VP Switch and Cavity Pressure
6.1.5 Effects of Part Thickness
6.1.6 Material Viscosity and Flow Behavior
6.1.7 Summary
6.2 Practical Applications
6.2.1 CAE Solution to Stress Mark in a Phone Shell
6.2.2 Flow Rate Effect on Injection Pressure of Laptop Product
6.3 CAE Case Study
7 Runner and Gate Design
7.1 Basics
7.1.1 General Design Guide of Runners
7.1.2 General Design Guide of Gates
7.1.3 Gate Sealing
7.1.4 Flow Balance
7.2 Practical Applications
7.2.1 CAE Verification on MeltFlipper® Design
7.2.2 CAE Verification of Multi-Cavity Systems
7.3 CAE Case Study
8 Cooling Optimization
8.1 Basics
8.1.1 Heat Transfer Mechanism
8.1.2 Design Golden Rule: Uniform Mold Temperature
8.1.3 General Design Guide of Cooling Channel
8.1.4 Cooling Efficiency: Coolant Flow Consideration
8.1.5 Cooling Time Estimate
8.1.6 Use CAE Cooling Analysis
8.1.7 Conformal Cooling Application
8.2 Practical Applications
8.2.1 Digital Camera Cover
8.2.2 Cartridge
8.3 CAE Case Study
9 Warpage Control
9.1 Basics
9.1.1 The Causes of Warpage
9.1.2 Material Effects
9.1.3 Geometrical Effects
9.1.4 Process Condition Effects
9.1.5 Criteria of CAE Warp Analysis
9.1.6 Methods to Minimize Warpage
9.2 Practical Applications
9.2.1 CAE Solution on Warpage of Connector
9.3 CAE Case Study
10 Fiber Orientation Control
10.1 Basics
10.1.1 Process Principle
10.1.2 Theory Models
10.1.3 Advantages and Challenges
10.2 Practical Applications
10.3 CAE Case Study
11 Hot Runner Optimization
11.1 Basics
11.1.1 Process Principle
11.1.2 Temperature Control in a Hot Runner System
11.1.3 Advantages and Challenges
11.2 Practical Applications
11.2.1 CAE Verification on a Single-Gate Hot Runner System
11.2.2 CAE Pin Movement Control of Valve Gate
11.3 CAE Case Study
12 Co-/Bi-Injection Molding
12.1 Basics
12.1.1 Process Principle
12.1.2 Advantages and Challenges
12.1.3 Theory Models
12.2 Practical Applications
12.2.1 Co-Injection Molding of Fork Model
12.2.2 Co-Injection Molding: Core Breakthrough and Flow Imbalance
12.2.3 CAE Case of Bi-Injection Molding
12.3 CAE Case Study
13 Gas-/Water-Assisted Injection Molding
13.1 Basics
13.1.1 Process Principle
13.1.2 Advantages and Challenges
13.2 Practical Applications
13.2.1 CAE Verification on GAIM
13.2.2 CAE Verification on WAIM
13.2.3 CAE Verification on GAIM: Fingering Effect
13.3 CAE Case Study
14 Foam Injection Molding
14.1 Basics
14.1.1 Microcellular Process Principle
14.1.2 Advantages and Challenges
14.1.3 Theory Models
14.2 Practical Applications
14.2.1 CAE Verification on Microcellular Injection Molding
14.2.2 CAE Verification on Chemical Foaming Injection Molding
14.2.3 Summary
14.3 CAE Case Study
15 Powder Injection Molding
15.1 Basics
15.1.1 Process Principle
15.1.2 Advantages and Challenges
15.1.3 Theory Models
15.2 Practical Applications
15.2.1 CAE Verification on an Electronic Device
15.3 CAE Case Study
16 Resin Transfer Molding
16.1 Basics
16.1.1 Process Principle
16.1.2 Advantages and Challenges
16.2 Theoretical Models
16.2.1 2.5D Analysis
16.2.2 3D Analysis
16.2.3 Measurement of Permeability
16.2.4 Porosity
16.2.5 Measurement of Chemorheological Properties
16.2.6 Simulation Parameters
16.3 Practical Applications
16.3.1 CAE Verification on Edge Effects
16.3.2 CAE Verification on Thickness-Direction Flow
16.3.3 CAE Verification on a Wind Turbine Blade
16.3.4 CAE Verification on Mat Effects
16.3.5 CAE Verification on Flybridge
16.4 CAE Case Study
17 Integrated Circuit Packaging
17.1 Basics
17.1.1 Process Principle
17.1.2 Advantages and Challenges
17.1.3 Theoretical Models
17.2 Practical Applications
17.2.1 CAE Verification on Void Prediction
17.2.2 Fluid-Structure Interactions: Wire Sweep Analysis
17.2.3 Fluid-Structure Interactions: Paddle Shift and Chip Deformation Analysis
17.2.4 Warpage Prediction of In-Mold/Post-Mold Process
17.3 CAE Case Study
We would like to thank and acknowledge Beaumont Technologies, Inc. for the flow imbalance case in Section 7.1.4 and Section 7.2.1, OPM Laboratory Co., Ltd. for the conventional and conformal cooling design cases in Section 8.2.1, Ann Tong Industrial Co., Ltd. for the single-gate hot runner system case in Section 11.2.1, Prof. Shi-Chang Tseng and Prof. Shia-Chung Chen for the gas-assisted injection molding cases in Section 13.2.1 and Section 13.2.3, respectively, Prof. Shih-Jung Liu for the water-assisted injection molding case in Section 13.2.2, Trexel Inc. for the MuCell® case in Section 14.2.1, Prof. Shyh-Shin Hwang for the chemical foaming injection molding case in Section 14.2.2, Prof. Shun-Tian Lin for the metal injection molding case in Section 15.2.1, Atech Composites Co., Ltd. for resin transfer molding cases in Section 16.3.3 and Section 16.3.5, Associate Prof. Yuan Yao for the resin transfer molding case in Section 16.3.4, and Amkor Technology Korea Inc. for the IC packaging cases in Section 17.2.1 and Section 17.2.2. These practical cases are quite valuable and helpful to illustrate how to co-develop innovative molding technologies and solve molding issues with the CAE tool.
We would also like to thank the following for their contributions: Dr. Che-Ping (Barton) Lin on Chapter 1, Dr. Chen-Chieh (Jye) Wang and Dr. Chih-Wei (Joe) Wang on Chapter 2, Tsai-Hsin (Sam) Hsieh, Tsai-Heng (Paul) Tsai, and Dr. Ying-Mei (May) Tsai on Chapter 3, Tsai-Hsin (Sam) Hsieh, Wen-Bing (Webin) Liu, and Dr. Ying-Mei (May) Tsai on Chapter 4, Hsien-Sen (Ethan) Chiu and Dr. Ying-Mei (May) Tsai on Chapter 5, Yu-Chih (Goran) Liu and Wen-Bing (Webin) Liu on Chapter 6, Dr. Che-Ping (Barton) Lin and Dr. Sung-Wei (Franz) Huang on Chapter 7, Dr. Chih-Wei (Joe) Wang and Dr. Sung-Wei (Franz) Huang on Chapter 8, Dr. Shih-Po (Tober) Sun on Chapter 9, Dr. Huan-Chang (Ivor) Tseng on Chapter 10 and Chapter 15, Tsai-Hsin (Sam) Hsieh on Chapter 11, Dr. Chih-Chung (Jim) Hsu on Chapter 12, Chapter 13, and Chapter 17, Yuan-Jung (Dan) Chang on Chapter 14, and Hsun (Fred) Yang on Chapter 16. They dedicated their wisdom and skills, and a great deal of time, to complete this wonderful book.
Moreover, a very special thanks to Chia-Lin (Carol) Li for redrawing figures, Pao-Hui (Ryan) Wan for his assistive editing, and Dr. Ying-Mei (May) Tsai and Dr. Che-Ping (Barton) Lin for their executive editing.
Injection molding techniques have been developed over decades and well-applied in automotive, 3C(Computer, Communication, and Consumer electronics), optics, medical products, and in daily necessities, among other areas. Due to this long-term development and widely ranging applications, the individual molding criteria have been specialized in several industries to fit various product specifications and innovative materials.
Many industries are producing novel plastic products, such as FRPs (fiber reinforced plastics) replacing metal to reduce weight while maintaining the structural strength of automotive parts. The unique appearance achievable from techniques such as multi-component design of 3Cproductsis more attractive than that from the common design by conventional injection molding. The high turnover rate in mobile products has raised the demand for plastic lenses so that the capacity and profits of the optics industry are ensured as long as production stability of high-precision-shaped lenses can be achieved in multi-cavity molding. Usually, medical plastic products have a high added value, but they especially must pass severe material certification standards at the primary stage, and dust-free or sterile production may be necessary. As for daily necessities, although part dimensional precision is not demanded as much as in other industries, there are still molding issues to consider; for example, conformal cooling method might be evaluated in order to reduce the cycle time.
The increasing requirements and diversity of plastic products demand a shorter time to market. However, much time can be spent in developing the procedures for some products, from concept generation, design drawing, mold tooling and assembling, trial-molding to mass production. “How can the procedures be shortened using CAE (Computer Aided Engineering) tools?” then becomes a key question for industry. The idea is to predict potential molding problems and defects by CAE during the design stage, modify the design according to these results, and then re-analyze until the best design is obtained. Since the 1970s, virtual trial moldings have been implemented by computer using injection molding simulation CAE tools to check whether the molding parameters are good enough for manufacture. These parameters are part design, gate design, runner layout, cooling layout, molding materials, process conditions, and so on. From CAE, the optimized parameters can be estimated efficiently and provided as the initial-guess settings for the real molding to cost down in time, manpower, material, and energy. To summarize, CAE is a decades-proven design-verification tool for real applications of the injection molding process.
In addition to conventional injection molding, there are many innovative molding processes that have appeared. Characteristics of lightweight parts, low clamping force, and low shrinkage are noted in the G/WAIM (Gas-/Water-Assisted Injection Molding) and MuCell® processes. With co-/bi-injection, multi-component or multi-functional parts can be produced by one-shot molding. The compression operation of ICM (Injection Compression Molding) provides a uniform packing mechanism on a plastic melt that compensates the non-homogeneous packing of injection molding. Metal or ceramic PIM (Powder Injection Molding) is especially adapted to manufacture the green part of highly precise and complicated geometry products. When using a hot runner system, the most important thing is accurate temperature control. Moreover, improvements in plastics molding techniques are not only exhibited in injection processes, but also, for example, in consideration of the resin curing reaction within the multi-substrate molding of an IC package, a process that has evolved greatly.
Molding issues become more challenging and complicated with innovations in processes and materials, which can lead to a longer time and higher cost in conditions optimization. In particular, when fiber composites are involved, obtaining favorable fiber orientations and maintaining longer fiber lengths after processing are extremely important goals, because the microstructures of the materials dominate the product quality. In RTM (Resin Transfer Molding), the microstructures are related to the resin impregnation degree inside the continuous fiber mat or woven roving. The involvement of the effects of surface tension, fiber deformation, and resin curing reaction all make RTM process control more difficult. Fortunately, CAE is nowadays available to simulate these new procedures and as a process innovation tool.
From decades of experience in CAE assistance in molding troubleshooting, we have found that processing knowledge is as important as software operation to CAE users. To make a high-quality molded product, the total effects of part design, mold design and manufacture, machine capability, and material properties must all be taken into account and then integrated into the CAE tool to implement design verification and conditions optimization wisely. Each of these definitely involves a deep knowledge, whether in theory and/or empirical formula. When talking about molding issues, plastics rheology and the designs of part and mold are especially the key criteria since their interactions will dominate the material property variations inside the mold.
At Moldex3D, as worldwide leaders in molding simulation software, we are not just continuously enhancing CAE capability but also intend to help industry people improve their molding-related abilities. The importance of training and instruction has become strongly apparent to us. As a result, this book consists of plastics molding theory, practical applications, and case studies intended to elaborate the molding system and melt flowing behaviors in an easy-to-understand way. The practical examples show how to use CAE to achieve design verification and process innovation in conventional injection molding, G/WAIM, co-/bi-injection, foam injection molding, PIM, RTM, and IC packaging. With this book, readers can effectively learn molding simulation applications and its importance in molding industries.
The CAE case study exercises found in the book for execution in the Moldex3D software can be downloaded from the Website: https://moldex3d.box.com/s/zr6fvc1vlhbi4ocx111jwd3wmxt4ooif, for which the QR code is as follows:
Maw-Ling Wang
Rong-Yeu Chang
Chia-Hsiang (David) Hsu
The context of plastics molding will be briefly introduced in this chapter using the most popular method of injection molding. Two major topics are included in this chapter:
Introduction to Injection Molding: The systems of injection molding and the defects of injection molded products are described.
Core Values of Molding Simulation: The core values of simulation in injection molding will be introduced at the end of this chapter.
Plastics can be casted because of their ductility and plasticity. Therefore, plastics have been widely used in daily life and become a necessary part of the current world. The source of plastic products comes from customers' “needs”, which are then developed to “design concepts”. Such design concepts will be delivered to product designers for product design before being handed over to mold designers for mold design and development, as shown in Figure 1.1.
There are four stages from product development to mass production:
Design product drawings according to its functions, appearance, material, and processes, and hand over to mold factories for the design and manufacturing of molds.
Mold designers undertake discussion, drawing design, machining, mold-closing, and other procedures upon receiving product drawings, samples, or relevant specifications regarding material, weight, color, etc., which are used to manufacture the molds based on the conclusions made in the mold manufacturability meeting. The manufactured molds will be delivered to molding factories for mold test, modification, and detection.
Molding engineers execute tests to obtain the molding conditions for smooth production during the mold test stage, and provide feedback comments for mold modification regarding the difficult points for molding. The mold test will be executed repeatedly after the mold modification until the product quality achieves the specification of the mold test.
The production yield is improved via small-scale production and quality certification before the mass production stage begins.
Figure 1.1Development of workflow of mold products
First, what is injection molding? Simply speaking, it is a process of making a product by injecting plastic material of liquid state into a mold cavity via the help of injection molding machines. When the plastic material enters the injection molding machine through a hopper, it is turned into a melted state after being squeezed by the screw from which a large amount of heat is generated through friction. The melted plastic accumulates in the front of the cylinder and is constantly heated in order to maintain the temperature for injection. The process mentioned above is called plastification, as shown in Figure 1.2.
Figure 1.2Cycle of injection molding
Then, the melted plastic will be pushed forward into the closed mold cavity by the screw, a process that is called injection. After the initial injection is completed, when the high molecular weight melted plastic has fully filled the mold cavity, more melted plastic is injected under high pressure in order to compensate for the decrease in the volume of the plastic due to cooling as well as to make sure the mold cavity is perfectly filled until the sprue is solidified, a process that is called packing. Finally, the movable side moves back until the ejection pin reaches the rear plate to eject the molded product, runner system, and waste. This cycle is known as the molding cycle of injection molding.
Figure 1.3 shows a basic injection molding machine (injection machine), which is suitable for manufacturing products of different shapes from thermoplastic or thermosetting plastics. There are only two basic functions: 1) heating of the plastics to a melted state, and 2) application of high pressure to inject the melted plastic to fill the mold cavity completely.
Figure 1.3Injection machine
The injection molding machine can be divided into the mold-closing and injection devices. The mold-closing device is mainly for mold opening/closing and product ejection, and has itself two types: one is the linkage type, and the other is the direct pressing type using oil pressure for mold closing. The injection device is for heating of the resin material to a melted state and injecting it into the mold. As shown in Figure 1.4, the resin is squeezed into the cylinder and moved to the front side of the cylinder through rotation of the screw.
In industrial plastics processing methods, whether extrusion, injection, calendering, blow molding, film blowing, or spinning, a huge quantity of additional auxiliary equipment is always required to complete each processing step. The optimization, automation, and rationalization of auxiliary equipment for plastics processing play a role in determining product quality and the economic viability of the process.
Next, we will introduce the equipment required for injection molding in detail. This includes the feed, control, plasticating, injection, and mold systems.
Figure 1.4(a) Hopper and (b) plastics feed
The hopper (Figure 1.4(a)) installed on an injection machine usually has the shape of a spinning top or cone, and its capacity supplies material for the operation of the injection machine for 1 to 2 hours. Plastic feeds (Figure 1.4(b)) in many injection machines have meters for material feeding of constant amount or weight, and some come with heating or drying devices. For a mixed operation of adding dyeing powder, master batches, and foaming agent, the screw design should include a proper mixing unit. If using only an ordinary screw, the backside pressure is increased; however, proper control is required as plastic tends to flow into the mold under a large backside pressure. In the case of adding dyeing powder, the injection temperature is raised by 10°C to 20°C, since the dyeing power cannot be mixed with plastics at a low temperature.
One of the biggest problems encountered in common development and quality control of molded products during the production process is ineffective quality control due to using a one-way, indirect method for operating injection machines and system control devices. When the mold system is installed in the injection machine, usually the mold test table is converted into the operating conditions for the manufacturing process and entered into the machine via the operator panel (Figure 1.5).
Figure 1.5Operator panel
According to the type of plastic raw material, each parameter is properly controlled for injection molding, such as injection temperature, injection speed, injection pressure, mold clamping force, etc. The heating temperature of a general material cylinder is usually divided into 3 to 5 levels. The main specifications of power control units are motor power, oil tank volume, response speed, and various program control functions. Theoretically, the larger the volume the oil tank has, with a larger amount of oil, the lower the air volume has to be carried along with the extracting oil of the pump. In addition, a larger amount of oil also improves heat dissipation and impurity sedimentation, but it also means a higher cost in oil purchase.
The injection cylinder is similar to that of the extruder, but the inner wall is as smooth as possible, and streamlined to avoid gaps, dead angles, or uneven surfaces. The combination of each machine part shall be precisely working with each other. The cylinder size determines the largest injection amount of the injection machine.
Plasticating is a process that uses the mechanical energy of the screw and the thermal energy of the heater to melt the incoming solid plastic, which is then applied with high pressure to be ready for injection. The plastic is turned into a melted state after being squeezed by the screw from which a large amount of heat is generated through friction. The melted plastic accumulates in the front of the cylinder and is constantly heated in order to maintain the temperature.
As shown in Figure 1.6 and Figure 1.7, the solid plastic enters the screw channel via the inlet hopper. With the high rotation speed of the screw that generates a shear stress effect with the barrel, the plastic is mixed and transferred along the screw channel. As the solid plastic is heated by the electric heaters outside the barrel and due to the shear stress effect, it turns into a melted state as the temperature rises inside the barrel.
Figure 1.6Barrel and plastification
Figure 1.7Screw
The screw is divided into three zones (Figure 1.7):
Feed zone: The fixed feed depth of the screw channel is for pre-heating, transferring, and pushing the granulate plastic, which starts to melt at the end of the feed zone.
Transition zone: The thread depth is gradually decreased, which is for plastics melting, mixing, shearing, compressing, and pressurized venting. The plastic melts completely as wthe volume decreases.
Metering zone: The fixed metering depth of the screw channel is for mixing, transfer, and metering of the melted plastic, as well as providing sufficient pressure to maintain a uniform temperature and stabilize the flow of the melted plastic.
The injection system is mainly responsible for filling and packing (Figure 1.8). For the filling stage, the screw moves forward to inject the melted plastics into the closed mold cavity through the nozzle to finish the filling process. When the melted plastic enters the cavity, the air is expelled from the ejection pin, parting line, and vent holes. Underinjection would occur if the liquidity is poor or the injection pressure is insufficient; in contrast, if the liquidity is too high, flash (see Section 1.1.2.3) would occur on the parting facet of the plastic part.
Figure 1.8Injection system
The core side goes back until the ejection pin reaches the rear plate to eject the molded product, runner system, and waste. The mold cavity is opened and the molded product, runner system, and waste are ejected (Figure 1.9).
Figure 1.9Open mold
Molds are important in injection molding. The basic structure of a mold is generally divided into three types: two-plate mold, three-plate mold, and hot runner. The decision for a particular mold structure is generally made by customers or according to products. A plastic mold consists of seven major systems: guiding, support, molding component, pouring, cooling, ejection, and venting systems. Using a sliding block is a way to handle undercuts, but normally the mechanism of mold opening and closing is sufficient for lateral parting, core extracting, and position reset.
A complete cycle time of plastics injection molding consists of filling, packing, cooling, and mold opening times, among which the cooling time has the highest proportion at about 70–80%. Therefore, the design of cooling system is a critical step which directly affects the length of cycle time, production efficiency, and cost. The following chapters will introduce the characteristics of plastic processing and the key points of mold design in detail. In addition, the injection machine system is also fairly important.
Product defects present another concern when plastic is turned into the final product through the foregoing processes. The plastic takes its shape gradually through the cooling during the process of injection molding, and usually it has the form of a near-finished product when it leaves the mold. If there are defects in the plastic product, it is necessary to analyze and understand the factors that are the cause.
The common injection molding defects are briefly described as follows:
The phenomenon called the “short shot”, shown in Figure 1.10, gives a defective appearance to the final plastic product, and is caused by under-filling of the mold cavity. It is most apparent at in thinner zones or at the end of runners, and is mainly caused by insufficient plastic supply or poor liquidity of the plastic itself, so that the liquid state halts prematurely during the filling process. Therefore, any factor that affects the smooth flowing of the melted plastic is likely to cause short shot defects, such as insufficient amount of plastic injected, too high flow resistance, or insufficient liquidity.
Figure 1.10Short shot
Poor liquidity: In addition to a low temperature of melted plastic and mold wall, a thin part geometry or improper sprue location or length can also generate short shot defects as the mold cavity cannot be filled up. Improper configuration of the vent hole is also likely to cause a short shot.
Injection process issue: It should be confirmed that the hopper has enough plastic if a short shot is observed. Then, the cylinder should be checked for blockage and the back-pressure valve checked for failure, which can result in a low injection pressure or material leakage. However, a long injection time can also cause a short shot.
Warp denotes the distortion or deformation of a product after injection molding. It is the defect type most commonly seen in injection molded products. Figure 1.11 shows a warped finished product which has two parts that cannot be assembled together. However, even if the product is not a combinative part but a single product, warp can also cause customer complaint and product return.
Figure 1.11Warpage resulting in a molded product that cannot be fitted
Thermal expansion and contraction are also seen in plastics. The melted plastic starts to cool down and solidify as it enters the mold cavity, and it contracts during the process of cooling and solidification. If the contraction rate is evenly distributed across the product, warp would not be seen and only shrinkage would result. However, with the interaction between the external factors, e.g. molding conditions, mold cooling design, product appearance design, and the plastics characteristics, e.g. molecular chain and fiber orientation, it is very difficult for plastic finished products to contract evenly or with low contraction rate.
Flash is generated as there is a gap existing at the split plane via which the melted plastics spills outside the mold cavity, as shown in Figure 1.12.
Figure 1.12Flash
The main causes of such formation are as follows:
Mold clamping force is too small: A pushing force is applied to the mold by the melted plastics during the injection molding process, especially if the central area of the mold cavity is subjected to excessive high pressure the mold will separate from the parting plane.
Mold gap: The causes that the moving and stationary side of the mold cannot contact completely consist of a) a parting plane that is defective and not parallel between each side, and b) impurities on the parting plane that create gaps on the parting plane.
Improper molding conditions: Wrong selection of molding machine, over-temperature of melted plastics, and excessive injection pressure are all causes of flash generation.
Improper venting system: Flashes will be generated if the venting is insufficient or the venting groove is too deep.
The formation of sink marks and voids is a phenomenon observed in thick areas where there is not enough plastic supplied during the cooling process, as shown in Figure 1.13.
Figure 1.13Sink mark/void
Sink marks are generated as the plastic contacting the mold walls cools and hardens before the inner plastic starts to cool down, and hence the surface is pulled inward by contraction. If the surface strength is sufficient, voids are generated instead of sink marks. Therefore, sink marks and voids are often seen at the rib parts or the backside of a convex surface. In conclusion, sink marks and voids are generated easily if the contraction is uneven between the inner and outer part in some areas.
Air trap denotes a condition in which the melt front of the melted plastic traps the air inside the mold cavity, so that the air cannot escape from the venting holes or the gaps in the mounting parts. A possible consequence is shown in Figure 1.14.
Figure 1.14Air trap
Generally speaking, air trap mostly occurs at the area filled at last, where there is no venting hole or the venting holes are too small, so that voids, bubbles, short shot, or surface defects are generated inside the plastic part.
The causes of burn marks are very similar to those of air traps. The major cause is that the air trapped inside the mold cavity is overheated by compression and creates dark marks on the plastic surface as shown in Figure 1.15. When the air inside the mold cavity is compressed, the pressure and temperature rise so rapidly that the surface of the plastic part at the end of the flowing path is decomposed and thereby burn marks are generated.
Figure 1.15Burn mark
The main cause for delamination (layer separation) is due to the mixing of two incompatible materials or material types used in the molding process that are too dissimilar, as shown in Figure 1.16. In addition, delamination is possible to occur if the temperature of the melted plastic is too low, the humidity of the material is too high, or the runner and sprue are not smooth.
Figure 1.16Delamination
Fish eyes, as shown in Figure 1.17, are usually caused by unmelted plastic because of insufficient cylinder temperature and screw rotating speed, and low backside pressure. The phenomenon can also result from using too much recycled material or contaminated plastic.
Figure 1.17Fish eye
Flow marks are generated mainly if the temperature of the melted plastic is not evenly distributed or the melted plastic is of excessive high viscosity. An excessively low temperature causes friction and pushing between the plastics and the mold cavity, and results in the plastic hardening too quickly leaving the flow mark, as shown in Figure 1.18.
Figure 1.18Flow mark
If the thickness of a plastic part varies a lot, the cooling speed at a thinner area differs significantly from that of other areas. Then, the uncooled melted plastic will apply stress on the cooled plastics, which generates a stress mark due to inner stress, as shown in Figure 1.19.
Figure 1.19Stress mark
When the mold cavity is being filled, the melted plastic tends to move to a thicker and low-flow-resistance area, which it fills up first before filling the thinner area. Therefore, the melted plastic usually hardens at a stagnation point as the flow stops at the thinner area. It is is highly possible for the solidified plastic to be pushed to the surface of the plastic part and create a hesitation mark when the following melted plastic starts to move to such thinner area, as shown in Figure 1.20.
Figure 1.20Hesitation
Jetting is usually generated when the melted plastic passes through a narrow sprue or runner into the mold cavity with a high speed, as shown in Figure 1.21. Jetting usually causes inter-contact between cold materials as the temperature of plastic strips injected into the mold cavity drops and the strips contact with each other afterward. Jetting should be avoided as much as possible in order to assure the production quality.
Figure 1.21Jetting
Splays, as shown in Figure 1.22, are caused by using plastics under humidity or other volatile gases, or by gases generated from slight decomposition of the plastic due to an excessively high pre-heating temperature.
Figure 1.22Splays
If the plastic is not properly dried before manufacturing, the moisture inside will evaporate into steam during injection filling, which causes bubbles to flow along with the melted plastics inside the mold cavity and thus silver stripes (splays) will occur along the flowing direction. If the bubbles cannot be expelled entirely upon completion of filling, splays will also occur on the surface of the plastic part.
If there are two or more plastic flow fronts that merge together during the injection process, an incomplete fusion can occur as the melt front is of lower temperature and hardens first. Hence, weld lines, as shown in Figure 1.23, are generated. Such defects are usually seen around the holes or the merging boundaries of the finished products. Therefore, when race-tracking effects take place, they are usually accompanied by weld lines. To avoid the generation of weld lines, extreme care should be taken regarding conditions such as significant thickness changes or multiple sprues in the mold.
Figure 1.23Weld line
From the foregoing introduction to plastics injection molding, it can be seen that the types of defects are of various kinds, which can be categorized as appearance or dimensional issues, as shown in Figure 1.24.
Figure 1.24Molding challenges: design quality
When facing these defect issues, the traditional solution is the trial-and-error method, i.e. to repeat mold test and mold modification until the product specifications are met, which usually takes gigantic amount of time, labor, and other costs. However, with the assistance of CAE (Computer Aided Engineering) tools, the production process becomes more efficient and the product quality gets better, while waste is further reduced, with environmental benefits. An introduction to CAE, including its application possibilities in injection molding, is given below.
CAE is a kind of computer aided engineering software and technology that uses computer simulation and analysis to assist the diagnosis and development of complicated injection molding processes. CAE is able to integrate the complicated rheological, thermal, and mechanical properties of a material, and enables designers and developers to do qualitative and quantitative analysis and diagnosis for mold design as well as analysis and diagnosis for existing molds and operating conditions.
From the CAE analysis result, developers can explore the causes of problems that occur and test different design changes to find out the most appropriate solution, which is not achievable by the traditional trial-and-error method. Furthermore, if the design change involves modifications of products or molds, the cost of time, labor, machine, material, and energy by repeated mold tests and modifications is even beyond estimation. Hence, it is common to implement CAE for design verification during the development process.
We know that the injection process is a major factor that determines product quality. Going through solid, melted, and back to solid states in a short time involves rearrangement of plastic molecules. If we can effectively control the transition of plastic properties in the process, the structural strength of the product can be assured.
Where is CAE used?
CAE is not always suitable to provide assistance for injection molding, as shown in Figure 1.25.
Figure 1.25New concepts in product development of injection molding processes
The best opportunities for using CAE in the injection molding process are:
At the product design stage and before mold construction: Predict and amend possible defects in product design and reduce the cost for mold opening.
After mold construction but before mass production: If the product is defective, but the causes are difficult to solve via onsite mold tests, this can be analyzed to determine the causes for defect generation through regenerating the actual defects and issues via simulation.
After mold construction and during mass production: Find out any possibility for yield improvement and molding cycle time shortening via simulation to further increase the productivity.
Create an in-house database: Summarize the problem-solving knowledge and create problem-determination SOPs (standard operation procedures) by constantly accumulating project counts.
Hereunder is an example of a cap for spraying deodorant in mass production. How do we utilize CAE to help the manufacture? This situation meets the above-mentioned CAE application opportunity item 3. For the product in mass production, using trial-and-error to further improve productivity or yield is time- and labor-consuming, which makes CAE a pretty good choice in this application.
As shown in Figure 1.26(a), with CAE analysis we can see that the temperature difference between inside and outside of the cap is 45°C in EOC (end of cooling), which leads to thermal stress and causes quality issues. The high product temperature also affects the ejection time (problem discovery).
As shown in Figure 1.26(b), originally, the cooling channel design was a big pipe; however, by considering abnormal cooling channel design and comparing three different models, the one in the middle shows the best result. This is a good example of using CAE to save time and labor without redesigning the mold.
As shown in Figure 1.26(c), the new design reduces the temperature difference by 15°C.
Figure 1.26CAE example: spray deodorant cap
Final result: The productivity is increased by 25% as the cycle time is shortened. For an annual production of 4 million pieces, 4 million seconds (46 days) are saved therefrom. In addition, the quality is improved simultaneously as the temperature difference is reduced.
This example clearly tells us that with proper use of CAE tools, we can improve existing processes without affecting current production lines and with only little cost.
