Overview of Ultrasonic Welding

Ultrasonic welding uses high-frequency ultrasonic acoustic vibrations which are applied to materials that are being held together under pressure to create a solid-state weld. This method is most commonly used to join thermoplastics and in joining materials that aren’t similar and does not require any bolts, nails, soldering material or adhesives that are needed to join the materials together.

Ultrasonic-welding-1Image Credit: How Stuff Works

Although ultrasonic welding uses vibration to create a weld, ultrasonic welding is a different method from “vibration welding” (also known as friction welding). In vibration welding one material is held in place, while the other is moved back and forth by either an alternating electromagnetic or hydraulic linear motion generator. In ultrasonic welding, both materials are held in place and a high frequency noise is applied to produce friction and then create a weld.

During the welding process, the materials to be welded are placed between a fixed shaped nest and a sonotrode which is connected to a transducer which produces a low amplitude acoustic vibration.  The acoustic energy creates friction, produces heat, and then the parts are welded together in less than a second—making  it one of the fastest welding methods in use today.

While vibration welding has the capacity for larger components, ultrasonic welding offers a far less complex process and presents less limitations for which materials can be joined together. These advantages for ultrasonic welding generally apply when it’s compared to other plastic welding processes such as adhesive welding, spin bonding, and hot plate welding. Having said that, ultrasonic welding has its limitations when joining together certain materials (see this chart).

History of Ultrasonic Welding

Although ultrasonic welding was first developed in the 1940’s, Assembly Magazine writes that it was first used industrially in the early 1960’s for fine wire binding in the growing electronics industry. However, in 1963 ultrasonic welding was introduced into the plastic welding process where it has proven one of the most popular welding methods. It has since been used for welding aluminum and thin sheet metal for the automotive industry in addition to welding copper to aluminum for ignition modules, wire to terminals, and wire-to-wire for harnesses.

Advantages of Ultrasonic Welding

Because ultrasonic welding requires no connective bolts or soldering materials to bind the two pieces together it saves on manufacturing and production costs.  The welds created using this process are visually appealing since there are no visible seams which can detract from the finished piece.  The process of ultrasonic welding is primarily an automated process that only requires the welder to pull a lever or push a button to produce the ultrasonic weld. 

Since ultrasonic welding can be used to join wires and circuitry, it is perfect for use in industries that produce computer and electrical components.  It is ideal because it creates reliable bonds without introducing any impurities or thermal distortion to the components being joined. The weld also dries rapidly because there are no adhesives or solvents to dry and, there is no need for the materials to cure.

Limitations of Ultrasonic Welding

Perhaps the greatest limitation of ultrasonic welding is that the largest piece it can effectively weld is 250 mm. According to the technology company TWI, “This is due to limitations in the power output capability of a single transducer, the inability of the horns to transmit very high power, and amplitude control difficulties due to the fact that joints of this length are comparable to the wavelength of the ultrasound.”

Ultrasonic welding also requires a lower moisture content in the materials that it welds. Vibration welding is preferable for materials with a higher moisture content. In addition, if the materials are too thick, ultrasonic welding is not an effective way to join them.

Resources for Ultrasonic Welding

Written by Ed Cyzewski