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Article Excerpt
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Adhesive and solvent bonding, mechanical fastening, and a variety of welding techniques are used to join plastic parts and subcomponents that are too large or too complex to be produced as single assemblies. Adhesive bonding systems are versatile, and they produce consistent, predictable results in joints requiring strength and durability. Welding, suitable only for thermoplastics, not thermosets, melts the materials at the joint interface to form strong molecular bonds. The packaging industry makes extensive use of welding techniques to seal packages. Plastics welding and adhesives are both used extensively in the automotive industry.
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Expert Technical Support
Resin manufacturers offer recommendations and technical support for joining and assembling parts made of their materials. Lanxess, in a design guide for joining its engineering plastics, observes that designers need to consider early in the development stage how they will join mating components into functional units. Mechanical fasteners, including screws, bolts, and rivets, offer one of the least expensive and most commonly used joining methods for assemblies that must be taken apart. For permanent bonds, solvents and adhesives are among the least expensive joining methods, notes Lanxess.
For adhesive bonding, two-part epoxy and urethane adhesives impart excellent bond strength. Cyanoacrylate-based adhesives can produce quick bonds, but they can be damaging to polycarbonate resins, especially if parts have high levels of molded-in and/or applied stresses, the design guide says. Two-part acrylic adhesives usually show high bond strength, but some of their accelerators can be damaging to PC blends. All parts should be prototyped and tested to determine a given adhesive's suitability, Lanxess advises.
Plastics can be welded using either mechanical movement such as vibration or the external application of heat to produce melting in the area of the joint interface. Ultrasonic assembly, one of the most widely used joining techniques for thermoplastics, produces permanent, aesthetically pleasing joints. High-frequency mechanical vibration is used to melt mating surfaces in the most common ultrasonic techniques: welding, staking, spot welding, and ultrasonic inserts.
According to Lanxess, small amounts of fillers, such as fiberglass, will not inhibit welding. If glass content exceeds 30%, a poor bond can result and can produce wear on the welding device. Some external mold-release agents, lubricants, and flame retardants can also negatively affect weld quality.
Sabic Innovative Plastics, in a handbook on plastics welding, notes that vibration welding, also called linear welding and linear friction welding, is ideally suited to the welding of thermoplastic parts along flat seams. In the process, parts to be joined are rubbed against each other under pressure. In commercial vibration-welding machines, the moving half of the part to be welded is vibrated by exciting a tuned, stiff spring and mass system by means of an externally imposed oscillatory force.
Other types of friction welding include spin welding, angular vibration, and orbital welding. Sabic observes that plastics and plastics composites are increasingly being used in complex structural assemblies in which joining considerations and cost are important. Weldable filled and unfilled thermoplastics resins are called for in many demanding structural applications that require joints that can withstand static and fatigue loads.
Sabic cites as an example the first all-plastic 8 km/h (5 mph) automotive bumper, which was made of Sabic's Xenoy@ 1102 resin, an unfilled thermoplastic blend especially developed for the application. The bumper was fabricated by vibration-welding of two injection-molded parts. Plastics-welding technology is becoming more important because of the advent of high-performance thermoplastic composites, which promise to revolutionize assembly techniques m aerospace applications, says the company. Recently, recyclability considerations have made welding even more attractive because, unlike adhesive joining, additional materials are not introduced into the assembly, the handbook notes.
Other types of welding used with thermoplastics include laser welding and resistance and induction welding. Laser welding passes light or laser radiation through one plastic part while the second component absorbs it, causing heat and melting of the interface. Resistance welding utilizes an electrically resistive implant between the bonding surfaces, providing the heat needed for the weld joint. Induction welding uses a coil to...
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