Mechanical Fastening of Plastics

An Engineering Handbook

Kenneth J. Gomes, Brayton Lincoln, James F. Braden

• Editorial: Routledge
• Año: 2021
• ISBN:
  9781351432641
• Edición: 1
• Categoría: Tecnologia e ingegneria
• Categoría: Scienza dei materiali

This text provides a unique, practical and comprehensive 'how to' introduction to plastic-to-plastic, non-permanent assemblies. Covering a full range of information in an easy to understand, nontechnical format, this outstanding work affords the confident understanding needed to keep pace with advances in plastic technology.

1

INTRODUCTION TO PLASTICS

The use of plastics has become widespread throughout industry, replacing aluminum and zinc die castings and other metal parts in virtually every category of product design. In 1981, it was estimated that 20% of the diecast market had transferred to plastics (1). This trend is expected to continue for several reasons, in particular the cost differential between molding plastics and forming metals, even though the newer replacements may be closer to the functional limitations of plastics.

Although the material cost of the raw plastic material may be higher than that of metals, the processing cost is generally lower, so that, overall, parts can be made more economically. In some cases, perhaps because of volume considerations, the cost of a plastic part may exceed the cost of metal. Nevertheless, other considerations, such as weight differential or corrosion resistance, may be deciding factors in the selection of plastics over metals.

A favorite example of lecturers and writers on the expanded use of plastics has been the automobile. In 1971, it was estimated that around 100 pounds of plastics were being used per car. A couple of years ago it was projected that by 1990 300 pounds of plastics per car would be used. There is now more reticence in citing estimates on autos because of the rapid increase in the actual and experimental usages of plastics in automobile manufacturing. It is not entirely facetious to remark that soon there will be nothing left to convert but the passengers, and we are not sure how much plastic they will contain.

Also pertinent to the increased usage of plastics is the continuing development of new and stronger combinations of materials and fillers and the development of a broader understanding of how to work with plastics. The increased usage of structurally strong plastics has presented more opportunities to use mechanical fasteners that meet the demands of the new materials.

In the past, an understanding of fastening in plastics was difficult to obtain. Part of the difficulty was due to the proliferation of grades, types, combinations, and possible quantity variations of fillers. In our talks on the subject of fastening in plastics we have been using the figure of 4500 variations. Perhaps this figure is somewhat exaggerated. In any case, because of the large number of materials and filler combinations, each with different fastening characteristics, care must be taken to match the correct fastener to the individual application.

Of course, design engineers rarely enjoy the luxury of selecting a fastening method at the planning or drawing board stage. Functional and environmental requirements take top priority, and chemical, mechanical, and electrical characteristics, as well as manufacturing, material costs, and cosmetic considerations will undoubtedly have to be considered in material selection—thus, design of the fastening site and selection of the fastener are usually last to be considered. Furthermore, in the past there has been little information on fastening in plastics, and what was available was sometimes misleading.

1.1 ORGANIZATION OF THE PLASTIC GOODS MANUFACTURING PROCESS

Because of the wide variety of plastic materials on the market today and the range of operations involved, the manufacturing process for plastic goods has become fairly complex (2). It may seem to the outsider that there is no clear chain of command. For those of us who are newly interested in the subject, and at the risk of oversimplification we will attempt to show how the manufacturing process works (Fig. 1.1).

1.1.1 Resin Producers

Resin producers are the chemical organizations that supply the basic plastics materials or polymers from which the finished goods will be manufactured. They are generally rather large companies, including Du Pont, Union Carbide, General Electric, and Monsanto with large technical staffs that are usually enthusiastic about promoting their resins by providing technical assistance and advice. The polymers that they produce can be in liquid, pellet, granule, powder, or other form. This material is supplied to the compounder.

1.1.2 Compounders

Compounders may be part of the resin producer’s organization (captive compounders) or they may be independent compounders. In either case, the compounder may further modify or enhance the basic resin by adding or blending other materials with it. Such modifications could be as simple as mixing in filler such as minerals or glass, in powdered or fiber form.

FIG. 1.1 Organization of the plastics goods manufacturing process. (Adapted from “The Plastics Production Process,” Plastics Design Forum ©1980 Industry Media, Inc., Denver, CO)

1.1.3 Processors

The processor’s job is to form the compounded material into a finished shape or part by molding, extruding, laminating, or other method. The processor can also be a captive organization or an independent contractor. They may or may not be doing the work for their own use. They work closely with the designer, the finished goods manufacturer, and the compounder.

1.1.4 Finished Goods Manufacturers

The manufacturer of the finished product produces the completed product for sale. As part of the operation, the manufacturer may also decorate and/or machine the product and may also do the processing. The designer of the product may or may not be a direct employee. In either case, the designer, or design consultant, is at the heart of the process. He or she will probably be working with processing engineers who can help in the material manufacturing process and the material selection, production methods, finishing, and so forth.

Correlation of finished product design with the assembly and processing techniques to be employed is an obvious requirement in order to produce the product at the optimum cost level. Less obvious to the newcomer, but of equal importance, is the correlation of material with the processing technique to be used in the molding. However, it has been conclusively demonstrated that the finished material’s physical and mechanical characteristics are heavily influenced by the molding techniques employed.

1.1.5 Suppliers

Also involved in the manufacturing process are mold and die makers, process machinery manufacturers, chemical additive suppliers, and many other suppliers (2), including the fastener manufacturer, who will be working with the designer and later with the assembly engineers.

1.2 PLASTICS VERSUS METALS

The decision to use plastic in place of metal is not always clear-cut and should be made only after a detailed comparison of the many accounting and physical requirements involved (3). For instance, if disassembly is involved, then consideration should be given to fastening-site design at this point. This factor would differ with the use of metals.

It is not the purpose of this volume to discuss material selection in detail, but for the benefit of some readers it should be noted that if the necessary physical requirements can be met with plastic materials, then cost factors regarding secondary operations required for metal parts may well favor selection of plastics (3) (Fig. 1.2). It will be noted from this illustration that manufacturing operations are far fewer with plastics then with metals.

When making the choice between metals and plastics, the designer will have to consider required physical conditions including environment (temperature, chemical atmosphere, humidity, etc.), longevity, electrical conductivity or resistivity, weight, appearance, and tolerances. With plastics, the designer will have to consider required strength, creep resistance, dimensional stability, stress and strain curves, and loadings (4), perhaps even coefficient of friction. Considerably more care is required in selecting plastics for design use than metals as the structural properties of plastics are more sensitive to environmental factors. On the other hand, raw material considerations involving factors of future availability, energy requirements, and inflationary trends probably favor selection of plastic.

FIG. 1.2 Why die cast aluminum costs so much. (Courtesy of General Electric Company, Specialty Plastics Division, Pittsfield, MA. Reprinted from Plastics Design Forum, January/February, 1981, p. 35; copyright ©Industry Media, Inc., Denver, CO, by permission of the publisher.)

The long-term trends regarding strength-to-weight comparisons also appear to favor selection of plastics in many instances, even though die castings have better dimensional stability, have closer tolerances, and are stiffer and stronger. It is becoming increasingly clear, as plastics engineers work with glass and other fiber fillers (including carbon), as well as mineral reinforcements, that composite-filled materials can be engineered or “tailored” to meet high-performance requirements (1), and that this can be done on a selected location basis.

In addition, energy requirements for plastic processing are also generally lower than those necessary for processing metals. For example, aluminum, which requires electrical energy for processing, may need three times as much energy for initial manufacturing than the comparative strength equivalent in plastic. Of course, energy requirements for reprocessing aluminum are considerably lower than for initial manufacture.

1.3 A BRIEF EXPLANATION OF PLASTICS FOR THE FASTENER ENGINEER

Plastics are synthetic materials composed of a series or chain of molecules which, when heat or...