Plastic Component Design

By Paul Campbell

This book is for the industrial designer interested in the applications of plastics in products and industry. It explains how different plastics are processed, and it contains extensive examples of common and unusual plastic components and products with an explanation of how they are manufactured. Every year, more products are being replaced or augmented by the same product made from plastic, and this trend has resulted in much debate about the effectiveness of plastic replacements. Today's plastics can be designed to operate in all weather conditions and chemical surroundings. They can be economically produced for short run part production or readily adapted to high quantity production, and they can be cut, glued, tapped, or machined by traditional methods to suit design needs. Explains how to choose the best processing method, what fastening or joining methods can be used, and how to use the characteristics of a plastic to judge its suitability for an application. Covers all major contemporary molding processes. Discusses, in detail, important topics such as surface finish and special effects.  

• Language: English
• Publisher: Industrial Press, Inc.
• Release date: Sep 8, 2022
• ISBN: 9780831196257

Book preview

Chapter One

The Plastics Industry

THE PLASTICS INDUSTRY IS A BROAD CONCERN, overlapping several other industries. This is typical of all materials industries. For example, the steel industry overlaps the mining industry, the milling industry, and the various manufacturing industries. The plastics industry overlaps the petroleum industry, the refining industry, and many manufacturing industries.

However, plastic production differs from steel production in more ways than the actual production process. For example, companies that smelt iron and steel usually do nothing but smelt iron and steel. Companies that refine plastics for commercial use are typically involved in other businesses first. I.E. DuPont, for example, is a chemical manufacturer; General Electric is predominantly an electronics company; and Monsanto is another chemical manufacturer. Yet all of these companies, and many others, are also producers of commercial plastic materials.

The reason this is important is because the first concern for the plastic component designer is matching the appropriate type of plastic to the application. In the search for the appropriate plastic material, the source for that material may not always be so obvious; that is, if the designer is not previously aware that plastics are typically found at companies they might not expect to be manufacturers of plastics.

The solution to this dilemma is multifaceted. First, the plastic component designer will need to find the names and addresses of those companies manufacturing plastics. The local library is the best place to start. There, you will find in the reference section a number of different books listing companies that manufacture plastics. The books are often cross referenced with both a listing of manufacturers and their respective products, and conversely a listing of products and their respective manufacturers.

Second, plastic materials are constantly improving and therefore changing. The plastic component designer will need to start collecting manufacturers’ catalogs of the various products currently available on the market. These catalogs should be dominated by those materials the designer feels will be the most frequently used, but they should also include catalogs of materials not so frequently used. The object is to be constantly looking for superior material replacements.

In this chapter, we will examine the general uses of plastics, and give an overview of common plastic component manufacturing processes. In several chapters of the book, we will cover individual plastic materials in detail, with an emphasis on their respective applications. We will then study the detailed manufacturing processes. Finally we will cover design and drafting considerations.

It is nearly impossible to look around the room — any room — without finding some object made of plastic. Probably even the clothes you have on contain some parts of plastic, such as the buttons. Plastics are continually finding new applications, and are frequently offering better utility than their counterparts made of traditional materials.

Plastics have also made possible a whole new group of materials know as composite materials. These materials are a blend of plastic and traditional materials into a composite form that would be impossible without the existence of both component materials.

Some applications of these blended materials, or composite materials, are simply steel components molded inside of a plastic component. This process is called potting. Electrical components are frequently potted to give them certain protection. The central processing unit of a computer, for example, is made of silicon, but is always potted inside plastic to protect it.

More complex composite materials are made for construction. The floor boards of some commercial aircraft are made from a composite material of plastic molded over an aluminum honey-comb. Figure 1-1 illustrates this construction. This material is both very strong and very light, allowing for the construction of larger aircraft with lower fuel consumption. There can be many reasons for the use of plastics; and many of these uses are not immediately obvious.

Figure 1-1. Shown here is an illustration of a composite material construction board. This board is a composite of metal and plastic to produce a material used in further products, it is not a finished product itself. This material is both strong and lightweight — a combination that would be nearly impossible using traditional materials alone.

Why We Use Plastics

There are two groups of reasons for the employment of plastics in product design: design reasons and business reasons. Separating these two groups is not always clearcut. The designer must always be mindful of the business ramifications of the materials chosen, and the business must always be mindful of the fitness of their products.

In other words, the designer does not want to specify materials that make the manufacturer’s products prohibitively expensive so that no one can afford to purchase them. Conversely, the manufacturer does not want to produce inferior products that likewise will not sell. The designer and manufacturer must work in cooperation to use materials with both a fitness of purpose and an economic return.

Design concerns will center around a material’s fitness of purpose. The designer will want to specify a material for a product that gives the best possible results. The manufacturer will want to use the least expensive material. Manufacturing is a function of business, and business, in most of the free world, is concerned chiefly with profit.

Consider, for a moment, an example: the evolution of the milk container. This vessel has been glass, then waxed cardboard, and now plastic. Each step of this evolution has been a matter of economy, that is, to reduce the cost of containing the milk. Each had an equal fitness of purpose, but each was progressively less expensive than its predecessor. Yet it could be argued that none was the absolute best choice for the purpose. Stainless steel might possibly be the very best material for the purpose. The manufacturing equipment is stainless steel, and the bulk milk truck tank is stainless steel. However, this would make the cost of milk twenty or more times as expensive as it would be if packaged in plastic. The choice therefore became a tradeoff between cost and fitness of purpose of the material.

Granted, this is a blatantly obvious choice, but the designer must be careful to watch for solutions of a similar nature which are not always so blatantly obvious. The plastic components of an artificial heart, the plastic automobile bumper, and the plastic Sousaphone all took more creative thinking and a greater leap of faith than did the milk bottle.

Advantages of Plastics

For the most part, the advantages of plastics almost always outweigh the disadvantages. This stems from the fact that plastic is not a single all-encompassing material; rather it is a generic term designating a whole group of similar materials. These materials differ in their physical and chemical properties, and each will therefore be suited to a particular application.

Some plastic materials will have superior heat resistance, some superior chemical resistance, some better electrical resistance, and better durability. Choosing a plastic material will demand that the designer be aware of the variety of materials available. Once aware of the properties of the materials and the methods of processing them, the application will begin to become self-apparent.

Cost Advantages

Plastic has the common reputation of being cheap. To a certain degree, this is true — at least in the respect that it is less expensive than alternative materials. However, the cost reduction is not necessarily a simple matter of volume unit of plastic-to-volume unit of other material comparison. Often, plastic will actually be more expensive in terms of physical volume than the cost of alternate materials.

Why, then, should plastic be considered so much cheaper? The reason is basically labor reduction. The methods of processing plastic are such that the labor, or the number of processing steps, is greatly reduced. Since labor is one of the inherent costs of producing anything, this labor reduction is an obvious cost savings. It is, for example, faster and less laborious to mold a toy, such as a toy car, from plastic in a single operation than it is to stamp it from metal and assemble it in many operations, as was done in the past.

Let us consider this example further in terms of cost reduction to the manufacturer. One of the sad sociological trends in recent years is that we live in a litigation-happy community. Lawsuits, whether won or lost, are a serious cost to the manufacturer. If our toy car were made from metal in such a way to allow sharp edges to be exposed or parts to be removed, which could injure a child, the manufacturer would get sued because the plaintiff would claim that the product was unsafe; and courts typically do not question the parent for not examining the product before its purchase.

This manufacturer would be in a tenuous situation. Even if the suit was frivolous, the manufacturer would spend thousands of dollars defending its product. Therefore, the product designer must always consider the possibility of making the product safer. To this end, there can be considerations of large unitized construction, protection from electrical current (as with an electrical outlet cover plate), and any number of other ways of protecting consumers from themselves. Again, I would like to point out that the reasons for using plastics are not always so obvious.

Chemical Advantages of Plastics

Chemical advantages of plastics are where we start to consider the advantages of plastics predominantly from a design standpoint. This is also where we begin to become concerned with the fact that different plastic materials have different properties. For example, some plastics are soluble in gasoline, while others are very resistant to its effects. These latter materials are excellent choices to reduce the weight of machines by making the fuel tank from plastic. Chain saws and weed trimmers are often made with a plastic fuel tank. The obvious resistance to water is another of the chief chemical properties of plastic that make it so advantageous. It is both noncorrosive and fairly sterile. This makes it an excellent candidate for application in food packaging and preparation.

If we go back to our example of the milk container, we will consider that the milk will not corrode the container, and the container will not add any harmful materials to the pasteurized milk. Other common examples include: soda pop, which is under pressure and contains various acidic compounds; mustard and ketchup, both of which are highly acidic; plates, cups, tumblers, and tableware, all of which come in direct contact with the food we eat.

Chemical resistance to the climate is another of the advantages to the use of plastics. Aside from the moisture resistance, plastic, with the addition of certain chemicals, is resistant to the effects of ultraviolet light. Ultraviolet light will discolor and decompose the coatings on some materials, such as paint or varnish. It will discolor certain materials such as wood. Plastic is finding its way into applications such as fencing and house siding which requires no painting, and is virtually immune to the effects of weather.

Plastic’s chemical resistance to the weather is also useful for resisting the effect of associated phenomenon. Salt used to clear roadways of snow and ice in northern climates is highly corrosive to metal, but has no serious effect on plastic, making plastic ideal for automotive applications.

Manufacturing Advantages of Plastics

Perhaps the greatest advantage of plastics is their ease of manufacture. Plastic molding eliminates the need for many secondary processes. With a little foresight and investment in molds, even operations such as tapping can be eliminated; the threads are simply cast into the product and the core of the mold is screwed out. Most plastic products come out of the mold ready for use with no secondary processing at all.

Plastic molding is also by its very nature a very fast process. Having a hardening time much shorter than most metals, plastic molding is typically done in very rapid cycles. It is also common to make several components, either identical or not, in each cycle. This process of molding several components at once in a single cycle is done in what is called a family die.

Electrical Advantages of Plastics

Another of the engineering considerations of the use of plastic materials is the electrical conductivity of the materials. Some plastics are useful because they do not conduct electricity, others are useful because they do conduct electricity.

For electrical and electronics applications, plastics that do not conduct electricity are useful as insulators, coatings, enclosures and other components that will contact electrically active items. Some items with this concern would be the encasement of a computer chip, an appliance plug, an automobile distributor cap, or an electrical outlet in a building.

Electrically conductive plastic is used for applications such as automotive trim, household appliance components, and water faucets. This material has good resistance to moisture and chemicals, but it lends itself well to chrome electroplating for aesthetic appearances.

Disadvantages of Plastics

The disadvantages of using plastics are few, and most of those are some oversight concerning exposure conditions on the part of the product designer. If plastic becomes the material of choice, it is almost certain that there is a correct type to meet all of the criteria. For this reason, it is imperative that the designer consider as many of the potential conditions the product will be subjected to as possible.

Heat Resistance

Heat resistance can be a disadvantage with some plastics. First, however, it is time we established that there are two main classifications of plastics: thermoplastics and thermosetting plastics.

Thermoplastics are shipped to the product manufacturer as granules or pellets, which are in turn melted down and pressed into a mold. When they cool, they are removed from the mold and put into use. However, these plastics can be remelted at the same temperature at which they were originally cast. Obviously, these plastics will have a problem when inadvertently subjected to excessive heat.

Thermosetting plastics are usually poured into a mold as a liquid or pressed in as granules. Liquid types are hardened with the addition of a second material, a reagent, and then removed from the mold. Granular types are heated to allow the particles to fuse. The particles soften and fuse in a chemical reaction called polymerization. Initial heating softens the material to allow this process, but additional heat only hardens the material more. These plastics are ideal for application where high temperatures are anticipated.

Durability

Durability can be something of a real design problem for the plastic component designer; although this can also be mitigated by using the correct material for the application. Durability problems are exacerbated when accompanied by extremes in temperature, as is typical in outdoor applications.

However, the resiliency of plastic allows it to withstand impact deflection which would bend sheet metal. If the plastic is destroyed because of an impact, chances are that metal components would have been destroyed as well.

The problem of durability is mitigated by the addition of filler materials such as glass fibers (fiber glass) or mineral particles such as mica. New high impact plastics are constantly being developed to overcome this problem.

Social Acceptance

Now we have a real problem on our hands. When the personal computer began to become popular, it was already made with plastic housings and components. Since no one had ever seen a PC before, it seemed perfectly normal — that is just the way they are.

On the other hand, a chair, for example, is something we are all very familiar with; and we are also accustomed to having them made from traditional materials. When the plastic chair made its appearance on the market, it was largely perceived as cheap and inferior. Gradually, it has found its place. It is obviously more likely to find a plastic chair on a porch, in a yard, or at the beach than it is to find an oak Queen Anne captain’s chair. However, it is equally unlikely to find a plastic chair at a dining room table. This is gradually changing, and in the future it is perceived that furniture will be more austere, simple, utilitarian— and largely made from plastic.

The point here is that society’s acceptance of plastic products is dependent on the product’s familiarity. If the product is something completely new and foreign to the consumer, they have nothing to compare it to, and therefore no preconceptions about how it should look. Conversely, if the product is either very common, like the automobile body, or has been around a long time, like our chair, then the mental set of society will be more resistant to accepting the product being made from an alternate material, namely plastic.

When Do We Use Plastics?

We have covered the why of plastic use, now it’s time to cover the second question: when to use plastics? It is not always easy to differentiate between these two reasons because they are so closely intertwined. For example, cost advantages might be considered a why reason, but the production volume will also make it a when reason.

All of the reasons for plastic use must be considered simultaneously. In the last section, the considerations were largely design questions. In this section, the question is largely one of manufacturing. In other words, when is it practical or advantageous to use plastics in lieu of other traditional materials?

Production Volume and Economies of Scale

Plastic components lend themselves particularly well to mass production. However, the expense involved in the tooling required to produce plastic products is substantial. The production costs of any item will always include more than the material to build it and the labor involved. Tooling is one of the biggest costs in producing any item.

Tooling is a single, large investment, but the total cost of the tooling is spread over all of the items made with it. This is called amortization. It is the same financial process used to spread the payments of a house over a number of years. Each item produced absorbs part of the overall cost of the tooling required to build it.

It therefore becomes obvious that if the tooling costs are great and the