Plastics joining vibration

Ultrasonic head forming and welding is a fast assembly technique. It is a very rapid operation of about 2 seconds or less and lends itself to full automation. In this process high-frequency vibrations and pressure are applied to the products to be joined, heat is generated at the plastic causing it to flow, and, when the vibrations cease, the melt solidifies. The heart of the ultrasonic system is the horn, which is made of a metal that can be carefully tuned to the frequency of the system. The manufacture of the horn and its shape is normally developed by the manufacturer of the equipment. The results from this operation are not only economical, but also most satisfactory from a quality control standpoint. 

Vibration welding A joining method in which two plastic parts are pressed together and one is vibrated through a small angular displacement in the plane of the joint. The frictional heat so generated melts the plastic at the interface. Vibration is stopped and pressure and alignment are maintained until the joint freezes. 

The part assembly design addresses the ability to join/assemble the component parts. Where the components are assembled with adhesives, it is important to know the compatibility and strength of adhesion to dissimilar substrates, in addition to the chemical compatibility of the plastic with the specific adhesive and its constituents. If melt bonding methods, like ultrasonic, vibration, or spin-welding processes, are employed, thermal compatibility aspects have to be taken into account. The broad possibilities of mechanical assembly methods include snap fits, press fits, bolts, and threads. The material properties needed for each of these design calculations are listed in Table 11.9. 

Joining of plastic pipes is shown to be done more effectively, economically and in a much shorter time on-site by linear vibration welding , rather than the conventional hot plate welding or electrofusion welding techniques. 

Structural adhesives, which have found applications in the aircraft, automobile, construction, and electronics industries, can alleviate the problems caused by stress concentration and provide additional advantages to the bonded system (see the introductory chapter of this book). Many adhesive applications involve the joining of high-modulus substrates such as metals, wood, composites, and plastics where the adhesive provides a medium for stress transfer between the substrates. The adhesive layer improves the vibrational damping capabilities of the system. Although the bonded system is susceptible to property degradation by environmental exposure, the adhesive layer increases the corrosion resistance when dissimilar metals are bonded. 

Figure 8.11 Ultrasonic joining operating, (a) Swaging the plastic ridge is melted and reshaped (left) by ultrasonic vibrations to lock another part into place, (b) Staking ultrasonic vibrations melt and reform a plastic stud (left) to lock a dissimilar component into place (right), (c) Insertion a metal insert (left) is embedded in a preformed hole in a plastic part by ultrasonic vibration (right), (d) Spot welding two plastic components (left) are joined at localized points (right).

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