Plastics Product Design E-Book

Paul F. Mastro

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 399

Veröffentlichungsjahr: 2016

Beschreibung

Plastics Product Design

Provides the reader with access to lessons learned in the author’s 40 years of plastics product design experience

Part 1 of the book provides the reader with an introduction to plastics as a design material and a discussion of materials commonly in use today. There is a discussion of a variety of processes available to the designer to make a part along with the design considerations each process will entail. This section also includes a discussion of useful prototyping processes, including advantages and disadvantages of each. Next, the book discusses general design considerations applicable to most plastics product designs.

In Part 2 of the book the author discusses elements of design of a number of generic plastic product types based on his more than 40 years of experience of product design and development for several companies with a variety of products. This section includes discussions of structural components, gears, bearings, hinges, snap fits, packaging, pressure vessels, and optical components. This section also discusses the general considerations that apply to these applications as well as specific insights about each particular application. There is also a chapter on the most important joining techniques. Part 2 ends with a discussion of the general design process.

The book concludes with 3 appendices on thermal properties of selected generic materials; properties of selected structural components; and common abbreviations for plastic materials.

Audience

This book is aimed at product designers and plastics application engineers who have had limited or no experience with plastics materials as well as a more experienced designer who is designing a part for a use, process or an application that they are not familiar with.

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Preface

Part 1: Plastics as a Design Material

Chapter 1: Introduction to Plastics Materials

1.1 History of Plastics

1.2 Definition of Plastics

1.3 Thermoplastics and Thermosets

1.4 How Plastics are Made

1.5 General Plastics Properties

1.6 Plastics Feedstocks and Volumes

Chapter 2: Properties of Plastics

2.1 Molecular Weight and Molecular Weight Distribution

2.2 Melt Flow Index

2.3 Molecular Structure of Polymers

2.4 Thermal Properties of Plastics

2.5 Physical Properties of Plastics

2.6 Electrical Properties

2.7 Flammability

Chapter 3: Overview of Plastics Materials

3.1 Polyethylene

3.2 Polypropylene

3.3 Polystyrene

3.4 Polyvinyl Chloride

3.5 Engineering Plastics

3.6 Thermoplastic Elastomers

3.7 Biopolymers

3.8 Thermosets

3.9 Fillers and Reinforcements

Chapter 4: Process Overviews, Advantages and Constraints

4.1 Extrusion

4.2 Injection Molding

4.3 Extrusion Blow Molding

4.4 Injection Blow Molding and Stretch Blow Molding

4.5 Compression Molding

4.6 Transfer Molding

4.7 Rotational Molding

4.8 Reaction Injection Molding

4.9 Thermoforming

4.10 Filament Winding

4.11 Pultrusion

4.12 Additive Manufacturing (3D Printing)

4.13 Other Prototyping Processes

Chapter 5: General Design Considerations

5.1 Shrinkage

5.2 Dimensional Tolerances

5.3 Draft

5.4 Gating

5.5 Coring and Holes

5.6 Rib Design

5.7 Color and Appearance

5.8 Chemical Resistance

5.9 Weathering and Environmental Effects

5.10 Recycling and Recycling Codes

Part 2: Plastics Product Design

Chapter 6: Structural Components

6.1 Rigidity and Strength

6.2 Creep

6.3 Fatigue

6.4 Torsion

6.5 Impact

6.6 Other Elevated Temperature Considerations

Chapter 7: Enclosures

7.1 Cosmetics

7.2 Structural Support

7.3 Ventilation

7.4 Flammability

7.5 Electrical Considerations

Chapter 8: Packaging and Containers

8.1 Impact and Tear Resistance

8.2 Strength and Rigidity

8.3 Barrier Properties

8.4 Packaging Processes

8.5 Printing and Decorating

Chapter 9: Snap Fits and Hinges

9.1 Snap Fit Designs

9.2 Design of Cantilever Snaps Using Classical Beam Theory

9.3 Assembly and Disassembly

9.4 Non-Rectangular Cantilevered Beams

9.5 Effects of Stress Concentration

9.6 Annular Snap Fits

9.7 Manufacturability

9.8 Plastic Hinges

Chapter 10: Plastic Gears

10.1 How Gears Work

10.2 Types of Gears

10.3 Terminology

10.4 Gear Tooth Loading

10.5 Contact Stress

10.6 Gear Tolerances

10.7 Gear Tooth Design

10.8 Gear Mesh Conditions and Operating Distances

10.9 Software

10.10 Prototyping

10.11 Gear Manufacturability

10.12 Gear Materials

Chapter 11: Bearings

11.1 Wear

11.2 Bearing Life and Performance

11.3 Bearing Design

11.4 Bearing Materials

Chapter 12: Pressure Vessels and Pipes

12.1 Pipe

12.2 Miner’s Rule

12.3 Other Pressure Vessels

12.4 Other Types of Pressure Vessels

12.5 Material and Manufacturing Considerations

Chapter 13: Plastic Optics

13.1 Optical Fundamentals

13.2 Mirrors

13.3 Light Pipes

13.4 Lenses

13.5 Manufacturing Processes for Optical Components

13.6 Measuring Techniques

Chapter 14: Joining Techniques

14.1 Threads and Threading

14.2 Self-Tapping Screws

14.3 Metal Inserts

14.4 Ultrasonic Welding

14.5 Vibration and Hot Plate Welding

14.6 Spin Welding

14.7 Solvent and Adhesive Bonding

14.8 Bolt and Screw Assembly

Chapter 15: Product Design Process

15.1 Design Process

15.2 Material Selection

15.3 Design Services

Appendix A

Thermal Properties of Selected Generic Materials

Appendix B

Properties of Selected Structural Components

Appendix C

Common Abbreviations for Plastic Materials

References

Index

Plastics Product Design

Scrivener Publishing 100 Cummings Center, Suite 541JBeverly, MA 01915-6106

Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Copyright © 2016 by Scrivener Publishing LLC. All rights reserved.

Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts.Published simultaneously in Canada.

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Library of Congress Cataloging-in-Publication Data:

ISBN 978-1-118-84271-3

Preface

My first job as a plastics engineer fresh out of college was working for a large custom molder. That gave me a wonderful opportunity to be involved in the design and manufacture of a wide array of different products spanning a number of different industries. What that experience taught me was that in the myriad of different products that I got to work on, the basic engineering principles were the same, but each different industry or product group had constraints and requirements unique to them and often used very specialized engineering techniques that one would not necessarily be aware of if they did have experience in that industry. Working on different products every day gave me an appreciation for how important this knowledge was and how difficult it could be to quickly acquire it. This was reinforced throughout my career as I worked in engineering and product design in a number of different industries. The main purpose of this book is to convey the knowledge I obtained in all of these experiences to help facilitate the design process for people involved in designing or manufacturing similar kinds of parts.

Plastic materials provide the design engineer a wide variety and ever increasing number of possible solutions to some of the most difficult design problems for an ever widening scope of applications. The purpose of this book is to provide the reader with a basic understanding of the range of plastics materials, properties, and processes available to them and an understanding of how to design a variety of basic components from plastic materials.

While the basic design principles that will be discussed in the first section of the book are applicable to the design of any plastic part, each product type or industry has specific or unique requirements, many of which will be discussed in detail in the chapters in part two that should help someone unfamiliar with them to begin to develop a part design.

This book is intended for use by designers who have had limited or no experience with plastics materials as well as a more experienced designer who is designing a part for a use, process or an application that they are not familiar with. Also, the book has an extensive discussion of materials and processes that will provide a solid introduction to plastics for anyone.

In Section 1, Plastics as a Design Material, the reader is provided with an introduction to plastics as a design material. The section is introduced with a brief history of the plastics industry and a discussion of general plastics material properties. The next two chapters will give an overview of the plastics materials commonly in use today and a discussion of a variety of processes available to the designer to make a part along with the design considerations each process will entail. This section also includes a discussion of useful prototyping processes, including advantages and disadvantages of each. Finally, chapter 5 will discuss general design considerations that are applicable to most plastics product designs.

In Section 2, Plastics Product Design, the discussion will turn to the specific design and manufacturing requirements for a number of different product types and components. This section starts with an introduction to plastic materials being used as structural components, where many basic mechanical engineering principles are reviewed as well as a discussion of how they need to be adapted to account for the viscoeleastic behaviour of plastics materials. Chapters follow on enclosures, packaging, gears, hinges, bearings, snap fits, pressure vessels, including pipe, and optical components. These sections will discuss the general considerations that are relevant to these applications as well as specific insights about each particular application. Discussions of plastic joining techniques that are applicable across all product groups will layout the design choices available for putting these components together. The book concludes with a discussion of the product development process and role of the past design and designer in this process.

This book is the result of my experiences as a design and manufacturing engineer and is intended to provide the reader with a basic understanding of plastics materials and processes and to provide a resource to assist them with the design of a number of different components.

I would like to thank my wife Lois for her help and support throughout my career and in this endeavour.

PART 1

PLASTICS AS A DESIGN MATERIAL

Plastics materials have a number of unique properties that allow a wide variety of solutions to many design problems. The nature of some of these properties requires the designer to approach the application of these materials to a product design a little differently than many traditional design materials. This section will review common materials and processes and look at some of the general engineering approaches that need to be taken in developing a plastic product design.

Chapter 1

Introduction to Plastics Materials

In this chapter we will briefly discuss the history of plastics, examine what plastics are, how they are made and some of the general properties of plastics materials. We will also look at the overall size of the plastics industry today.

1.1 History of Plastics

It is hard to imagine a world without plastics, but plastics are a family of relatively new materials and have been around for a little more than 100 years. The start of the plastics industry dates back to 1868 when John Wesley Hyatt, in search of an alternate material to ivory for billiard balls, discovered celluloid, the first commercially successful plastic material. Celluloid also found application in photographic still and movie film and shirt collars and buttons. It is still in use today to make ping pong balls.

Celluloid was a modified naturally occurring polymer, cellulose. In 1907 Dr. Leo Baekeland, through a condensation reaction of phenol and formaldehyde, invented phenolic, the first plastic produced entirely from synthetic materials. This was an easily moldable, cost effective material that became widely used in electrical components and general moldings. Its major limitation was that it was only available in dark colors. This problem was solved in 1929 when American Cyanamid Company introduced urea formaldehyde thermoset molding compounds which could be produced in a wide array of colors.

In 1934 Dr. Wallace Carothers, working for DuPont, invented nylon. This is notable because he was hired to develop a synthetic material to replace silk and he developed a polymer to meet this specific need, a first for polymer chemists.

The first inorganic polymer, polytetrafluoroethylene, more commonly known as Teflon®, was discovered by another DuPont chemist, Dr. Roy Plunkett, in 1938.

Throughout the 1940s thermoset materials dominated the plastics market, but starting in the 1950s new thermoplastic materials and processes began to take over. The first commercial reciprocating screw injection molding machine appeared in Germany in the mid-1950s from Ankerwerk. Due to its ability to produce significantly improved thermoplastic melts, numerous manufacturers around the world soon offered their own versions. Injection-molded thermoplastics started to replace many thermoset applications and many new opportunities for growth were found.

In 1953, the first reinforced plastic car bodies appeared in the Chevrolet Corvette [1], and plastics continue to make inroads in the auto industry as their low costs and high strength-to-weight ratios help engineers meet ever-increasing fuel economy requirements. Use of plastics materials to reduce the weight of cars is a major strategy of the automobile industry as they strive to meet the US National Highway Traffic Safety Administration 2025 CAFE (Corporate Average Fuel Economy) standards of 54.5 miles per gallon by 2025.

Advances in polymer chemistry and catalysts now allow polymer chemists to scientifically develop plastic materials with specific properties to meet the needs of specific applications. Stereospecific catalysts like Zeigler Natta catalysts and metallocene catalysts can help control how and where the molecules attach to one another. Ruthenium catalysts enable ring opening metathesis polymerization, which has opened the possibilities of new families of high performance polymers.

This is allowing plastics to move into areas of much more demanding functional requirements and to be used in a wide array of engineering applications. Plastic materials are not only used in housewares, toys and packaging, but also aerospace, construction, electronics, transportation and industrial applications.

1.2 Definition of Plastics

What actually is a plastic material? There are many similar definitions used, but for our purposes, plastics are materials that are composed of large molecules that are synthetically made and, under the proper conditions, can be readily formed or molded into the desired shape. The large plastic molecules are called polymers from the Greek words poly, which means many, and meros which means units. The polymer is made up of many smaller molecules called monomers which are joined together through chemical bonding, generally through either a condensation or an addition polymerization reaction. The chemical properties of the monomer will determine if and how it can form into a polymer, as well as what properties the finished polymer might have.

1.3 Thermoplastics and Thermosets

Plastics are divided into two basic families, thermoplastics and thermosets. Thermoplastics are materials that when heated will soften and flow, allowing the polymer chains to slide over one another, and when cooled, they will harden. This process can be repeated many times. This allows thermoplastic materials to be easily recycled and reused. A thermoset material will soften and flow when it is heated, but additional heat will cause a chemical reaction called crosslinking to occur. In crosslinking, chemical bonds form between the polymer chains. This crosslinking reaction locks the polymer chains together and prevents them from sliding over one another, causing the polymer to harden. This process is irreversible. As a result, parts made from thermosets cannot be easily recycled.

Figure 1.1 shows the differences in how thermoplastics and thermosets respond to changes in temperature. At lower temperatures (upper left on the chart) both thermoplastics and thermosets are solids (usually – although a thermoset resin can start out as a liquid). As the temperature is increased, the viscosity (resistance to flow) of both materials will lower until they go from a solid state to a viscous (thick) liquid. As the temperatures continue to increase, the viscosity of both materials continues to decrease. With thermoplastics, this drop in viscosity continues as temperatures increase (up until the chemical bonds start to disassociate and the polymer begins to degrade). With thermosets, the material will rapidly drop in viscosity with increasing temperature until chemical crosslinks start to form between the chains. At that point, viscosity will increase as the crosslinks restrict the polymer chains from sliding past one another. With thermoplastics, we can drop the temperature and reverse the process. With thermosets, once the crosslinks start to form, we cannot reverse the process and the material is said to be cured.

Figure 1.1 Temperature vs Viscosity.

1.4 How Plastics are Made

Polymers are generally formed by two different mechanisms, addition polymerization and condensation polymerization. In addition polymerization the monomer is introduced into the reactor and a chemical initiator is added to create a free radical. The free radical will bond with another monomer, creating a new free radical at the same time. This process will repeat, causing the polymer chain to continue to grow. The growth process will continue until the monomer supply is exhausted, two free radicals meet, or a quenching molecule(s) is added to stop the reaction. Addition polymerization processes can be scaled up to a very large size, has no byproducts (the initiator is consumed by the reaction) and are very economical. Addition polymerization is also called chain-growth polymerization. The common high volume commodity plastics (see Chapter 3) are manufactured by this method. In condensation polymerization two different monomers are introduced into the reactor vessel. One end of molecule 1 will react with one end of molecule 2 and usually as part of this process a water molecule is also created (hence the name “condensation polymerization”). Newly formed molecule 1–2 reacts with either a molecule 1 or a molecule 2, extending the polymer chain. The reaction is ended by either adding a quenching molecule or cooling the process down to stop the reaction. Usually, catalysts are added to help drive and control the reaction. Unlike initiators, catalysts are not consumed by the reaction and must be removed from the finished polymer, along with the water that was produced in the reaction. Many of the engineering polymers are made by condensation polymerization. This is generally a more expensive process and will increase the cost of plastics made by this method. Condensation polymerization is also called stepwise polymerization.

1.5 General Plastics Properties

There are a large number of commercial polymers available, offering a wide range of properties that can be used in a very diverse set of applications. In general, plastics offer a low density resulting in a low specific weight (weight per unit volume) giving a high strength to weight ratio over competing materials. This characteristic is finding value for weight reduction in transportation applications, such as automobiles, aircraft and high-speed trains, to improve fuel economy. Plastics typically have low melting points which help processing but limit applications in high temperature environments. Although plastics do have lower melting temperatures than many competing materials, there are plastics that can withstand some surprisingly high temperatures, allowing them to be used in under the hood automobile applications and for sterilizable parts. They have a low thermal conductivity which makes them ideal for many thermal insulation applications. They also have good electrical insulative characteristics which make them key materials in a wide range of electrical applications, including wire and cable insulation, tool housings, circuit boards and components and household appliances. Plastics are available with optical clarity and can be made in an infinite array of colors. Lastly, one of the most significant general properties of plastics is that of relatively low cost, which has led plastics to be used in a number of high volume, low cost applications such as packaging materials, containers and insulation. Although these low cost polymers can be obtained for less than a dollar per pound, many higher performance polymers will cost from more than $2.00 per pound up to over $100 per pound.

1.6 Plastics Feedstocks and Volumes

The raw material feedstock for today’s commercial plastics come principally from petroleum (currently about 2% of petroleum production goes into the manufacture of plastic materials) and natural gas, although there is considerable movement into renewable feedstock materials such as soybeans, castor oil beans, corn, wood fibers and algae. The cost of the polymerized plastic is driven by the costs of the basic feedstocks, the cost of operating the polymerization process, and the volume of the material being produced. Plastics production is a globalized industry and this leads to local feedstock economics influencing how a specific polymer might be made. An example of this would be one of the highest volume plastics produced in the world, polyvinyl chloride (PVC). PVC is made by the addition polymerization of vinyl chloride monomer. In the Middle East and North America, because of the abundance of cheap natural gas, the first step in the process is to make ethylene from natural gas, which is then used to make ethylene dichloride and then VCM. In Europe, where natural gas is more expensive, the ethylene is made from petroleum. In China, where neither petroleum nor natural gas is cheap, they use the carbide process, where cheap coal is the feedstock that leads to the VCM.

We extend the range of properties available in plastics through a compounding process where we take the basic polymer and mix in a variety of additives to enhance and extend properties. We can add ultraviolet absorbers to improve performance in outdoor exposures, flame retardants to improve flammability, thermal stabilizers to improve heat stability and a variety if colorants, including pearlescents and fluorescents, to give a tremendous array of cosmetic effects. By adding in one or more of a wide variety of available reinforcements we can create a class of plastics called composites, which can yield substantial property enhancements, making plastics one of the primary materials used in advanced aircraft such as the Boeing Dreamliner and the BMW electric car.

Plastic volumes worldwide are approximately 650 billion pounds a year. The largest user is China followed by Europe and North America. The United States uses over 100 billion lbs of plastics per year. The single largest used plastics material is polyethylene. Worldwide consumption is over 180 billion pounds in many diverse application from packaging films and bags to milk bottles to gas and water service pipe. The annual volume of plastic surpassed that of steel in 1979 and continues to grow.

Chapter 2

Properties of Plastics

Each plastic material exhibits a unique array of properties. These properties are determined by the basic chemical makeup and structure of the monomer, the size and variations in size of the polymer chains that make up the polymer and the interactions of one polymer chain with another.

It is assumed the reader has a basic understanding of chemistry, but to facilitate our following discussion of plastic materials, let’s first review some polymer chemistry basics. As was discussed in Chapter 1, polymers or plastics are made up of long chains of molecules chemically bonded together. The smallest building block molecule for any polymer is called the monomer. We can represent a polymer with the general formula -(X)-, where X is the molecule or building block of the polymer (the monomer) and n represents the average number of monomer units in a polymer chain. The bar “–” represents a pair of electrons which bond the two adjacent molecules together. Most plastics are primarily bonded together by covalent chemical bonds. This means that the adjacent atoms share electrons to fill their orbitals.