Handbook of Adhesives and Sealants

538 Chapter Fifteencesses in industries such as consumer products, automotive, and packaging.Speed, simplicity, and reliability are key concerns in most ofthese high volume assembly processes. Speed, simplicity, and reliabilityare also often the order of priority in selecting a joining method.Because of the nature of the substrate and the application, exceedinglyhigh strengths and durability in exotic environments are not generallynecessary, and these properties are gladly sacrificed for faster, low costproduction methods. Often, there is not enough time for critical surfacepreparation or non-destructive testing of every part. In certainindustries such as the automotive industry, plastic materials may bechosen because of their fast joining ability. Thus, thermoplastics areoften preferred over thermosets because they can be joined via thermalwelding processes in a few seconds, compared to possibly severalhours for adhesive bonding.Although adhesive bonding often proves to be effective, there arevarious other ways of joining plastics to themselves or to other materials.The principal methods of joining plastics are adhesive bonding,thermal welding, solvent cementing, and mechanical fastening. Thenon-adhesive methods of joining will be the subject of this chapter.The sections that follow will describe these various methods of joining.Information will be provided regarding how to choose the most appropriateprocess for a specific substrate and application. The plastic materialsbest suited for each will be identified. Important process parametersand test results are reviewed. Recommendations regardingjoining methods for specific types of plastic materials (e.g., polyethylene,glass reinforced epoxy, polysulfones) will be given in Chapter 16.15.2 Plastic MaterialsThere are many types of plastic materials with a wide range of propertiesdepending on the base polymer and the additives used. Thereare excellent sources of information available regarding plastics materials,design practices, manufacturing methods, and applications.References 1–4 provide broad tutorials on these materials. Plastics areused routinely in many commodity items such as packaging, pipe,clothing, appliances, and electronics. They are also increasingly beingused in structural and engineering applications such as in the aerospace,building, and automotive sectors.All plastics can be classified into two categories: thermoplastics andthermosets. Thermoplastics are not crosslinked, and the polymericmolecules making up the thermoplastic can easily slip by one another.This slip or flow can be caused by thermal energy, by solvents or otherchemicals, and by the application of continuous stress. Thermoplasticscan be repeatedly softened by heating and hardened on cooling. Hence,