Machinery's Handbook, 31st Edition
DESIGNING PLASTIC PARTS FOR ASSEMBLY 599 alignment provided by the welding fixture. Typical frequencies used are 120 to 240 Hz, and amplitudes range between 0.10 and 0.20 inch (2.54–5.08 mm). When the geometry or assembly design prevents linear movement, vibration-welding equipment can be designed to produce angular displacement of parts. Like ultrasonic welding, vibration welding produces high-strength joints and is better suited to large parts and irregular joint faces. Spin Welding: Like vibration welding, spin welding uses frictional heat to melt the faces of circular joint surfaces. It is a rapid and economical method of joining melt-compatible parts. The process is completed in about three seconds and is easily automated. Fric tional heat is generated by rotating one part against its mate (usually fixed) with a con trolled pressure. When rotation is stopped, pressure is maintained during cooling and solidification of the joint. Simple equipment such as a drill press can serve for this process. Hot-Gas Welding: Hot-gas welding is a process resembling acetylene brazing and welding of metals. Used mostly with polyolefins, which do not accept adhesives, the hot-gas welding equipment consists of an electric hot-gas “gun” that provides, through any of several interchangeable nozzles, a stream of hot air or preferably hot nitrogen at a temperature above the melting range of the plastic to be worked. A welding rod of the same material provides the melt that forms the joint to be made between two parts. The parts to be joined are set up in their final orientation in a jig. A groove will usually be machined in one or both to accept the molten plastic. The operator directs the hot gas stream along the joint surfaces while feeding and melting the welding rod into the joint. The welded structure is then allowed to cool several hours. Some huge tanks have been fabricated from thick polyethylene sheets by this technique. It is also useful for repairing broken polyolefin parts. Hot-gas welding can produce joints with 85 percent of the strength of the parent material. Hot-gas welding is not recommended for use with substrates less than 1 ⁄ 16 inch (1.6 mm) thick. Dielectric Welding: Dielectric welding is also known as radio-frequency welding . This process utilizes the dielectric loss that occurs in many plastics when they are placed in a rapidly reversing electric field. It is widely used with flexible thermoplastics films and sheets of materials—typically plasticized PVC and polyurethane—and for joining some injection-molded parts. The parts to be joined are brought together between two electrodes that also supply joining pressure. The field passes through the material, generating heat throughout the volume between the electrodes until it reaches melting range. The field is then turned off and pressure is maintained until the joined parts freeze. Polyolefins and some other plastics are so nonpolar that their dielectric loss is too small to permit welding them by this method. Electromagnetic or Induction Welding: This form of welding uses inductive heating to generate fusion temperatures in thermoplastics as shown at the top in Fig. 24. Fine iron particles embedded in a gasket, preform, filament, ribbon, adhesive, or molded part are excited by the rapidly alternating magnetic field, generating eddy currents that, through resistive loss, cause distributed local heating. The heated parts are pressed together, and, as the temperature rises, the material of the particle carrier flows under pressure through the joint interface, filling voids and cavities and becoming an integral part of the weld. Ideally, the melted material should be contained and subjected to internal pressure by the surround ing component surfaces. Proper joint design is essential to successful induction welding. Requirements of the preform often add cost to this welding method, but the cost is off- set by low reject rates resulting from good reliability of the welds. Structural, hermetic welds can be produced in most thermoplastics materials, and automation can be used for large-volume production. The process also offers great latitude in joint size, configura - tion, tolerance requirements, and ability to bond some dissimilar materials. A limitation is that no ferrous metal can be near the joint line during the energizing of the inductor coil. All components of an assembly (except the weld medium) to be induction-welded must therefore be nonferrous, or ferrous components must be placed where they will not be subjected to the high-frequency field from the inductor.
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