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Ultrasonic Welding

Ultrasonic welding uses friction to heat the parts being welding (not to be confused with friction welding).  In ultrasonic welding, the parts being welded are clamped together between a stationary anvil and a vibrating horn.  The horn vibrates at ultrasonic frequencies and hence the name Ultrasonic welding.  This process is commonly used for welding certain thermoplastics and soft metals.  The inner workings of the power supply used for welding is the same for both plastics and metals.  The difference is in the way which the energy is delivered to the parts.  To weld metals, the horn vibrates parallel the weld interface; for plastics, the horn vibrates perpendicular to the weld interface.

The fundamentals of the processes are often confused with friction welding, since it appears that the two metal parts are being rubbed against each other.  In reality the parts do not have any relative motion between the two at the weld interface.  The heat, and subsequent softening, is produced by internal friction, i.e., the atoms of the parts vibrating against each other at high frequency and amplitude.  Once the parts are softened, the atoms across the interface are brought in intimate contact by the action of the welding force and are able to form a solid-state bond.  Ultrasonic welding is not a cold welding processes.  The parts do get quite hot, though not as hot as in other fusion welding processes.  Factors that will affect weld quality include ductility of metals being welded, coatings/platings, and part cleanliness.

UW welding of plastics

Ultrasonic welding is perhaps the most common welding technique for plastics.  Possible reasons include speed, relatively simple fixture design, modest capital costs, and applicability to a wide range of thermoplastics.  In UW, the weld energy is provided in terms of a horn that vibrates at high frequencies ranging from 15-35 kHz; the vibrations are transmitted through the parts being welded by suitable part and horn design.  When the energy travels across the weld interface, it produces localized softening of the plastics which then bond under the action of the applied force.  Other applications of UW apparatus to create a bond include Ultrasonic Swaging and Ultrasonic Staking.

Once the horn and fixture are properly aligned, the process has few variables to control including energy, power, pressure, and/or time.  Proper design of the horn is critical and becomes more important as the weld size increases.  Frequent aligning checks and tuning checks should be conducted to ensure proper process control.  One good aspect of the welding process is that with experience, one can hear a good or bad weld based on how the long the weld is on and what kind of sound it makes.  Common applications include assembling enclosures such as transformer housings, telephone handsets, and battery pack casings.  UW is also used for sealing food cartons and also finds applications in "sewing" garments that have greater than 50% synthetic content such as lab gowns.