Plastics joining fasteners

Chapter 14, Mechanical Fastening, Handbook of Plastics Joining, Plastics Design Library, Norwich, NY, 1997. 

Figure S.l General calculations of interference fit between a shaft and a hub. (Ref Mechanical Fastening , Handbook of Plastics Joining, Product Design Laboratory, Norwich, NY, 1997)...

Different methods are used for joining or fastening and assembling plastic products and plastic to other materials. It is im-... 

A variety of techniques is available for joining plastics to themselves or to other materials—mechanical fastening, welding, and bonding with adhesives—and all are used extensively. Each method has both advantages and drawbacks for plastics, bonding might be said to be preferable when they are to be joined with dissimilar materials. 

However, even with these potential difficulties, adhesive bonding can be an easy and reliable method of fastening one type of plastic to itself, to another plastic, or to a nonplastic substrate. Pocius et al. provides an excellent treatise on the use of adhesives in joining plastics.51... 

The reactive hotmelt adhesives retain a high level of flexibility and dynamic load-bearing capacities even after crossUnking and are used to join plastics to one another as well as to wood (furniture industry), glass (construction industry), aluminum (sandwich elements), and painted surfaces. Typical applications include window casement sheathing, foil laminations, clip fasteners, bookbinding, textile... 

Joints are potential failure sites. This applies whether they are adhesively bonded or mechanically fastened, and whether they join two reinforced plastics sections, or one reinforced plastics component and one constructed from another material. 

The relative differences in thermal expansion coefficient and elastic modulus also make joining of plastics to nonplastic materials difficult. They may also cause very high loads or loose-fitting fasteners in parts assembled with mechanical fasteners. 

With plastic materials, the designer also has a greater choice of bonding techniques than with many other materials. Thermosets must be adhesively bonded or mechanically joined, but most thermoplastics can also be joined by solvent or heat welding. Additionally, plastic parts can be designed for assembly by means of molded-in, snap-fit, press-fit, pop-on, and threaded fasteners so that no additional adhesives, solvents, or special equipment is required. 

Typical mechanical fasteners are listed on the left. Devices are made of metal or plastic. Type selected will depend on how strong the end product must be, appearance factors. Often used to join dissimilar plastics or plastics to nonplastics. 

A large variety of mechanical fasteners can be used for joining plastic parts to themselves and to other materials. These include machine screws, self-tapping screws, rivets, spring clips, and nuts. In general, when repeated disassembly of the product is anticipated, mechanical fasteners are used. 

Mechanical fasteners provide an economical and reliable means of joining plastic parts to themselves and to other materials. However, mechanical fasteners are generally not employed in high-volume, fast-production operations because of the added inventory requirements. Also, the time required to join parts with mechanical fasteners relative to other processes is substantial, especially for larger parts that require multiple fasteners. 

Adhesive bonding presents several distinct advantages over other methods of fastening plastic substrates. These are summarized in Table 7.5. Adhesive bonding is often preferred when different types of substrates (e.g., metals to plastics) need to be joined, when high-volume production is necessary, or when the design of the finished part prohibits the use of mechanical fasteners. 

How to Fasten and Join Plastics, Materials Engineer, March, 1971. 

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