How Ultrasonic Welding Works

We’ve all seen scenes in movies where a singer hits a ridiculously high note that shatters a champagne glass. It’s proof that high-pitched and high-frequency sounds can break materials apart. And it’s the basis of Lithotripsy, a treatment that uses ultrasound waves to break up large kidney stones.

Traditionally, materials have been bonded together by fasteners such as nails, screws or thread, which are appropriate for metals, wood, fabrics and plastics. For many plastics, glues have been used. Glue forms a chemical bond between itself and the surface of the plastic material to be conjoined.

Metals have typically been held together by heating other metals as a bonding agent, such as lead solder in electrical connections. Alternatively, the metals can be directly melted together (welded). After the melted metal surfaces cool, the metals bond together. Welding usually requires an open flame or torch to achieve temperatures high enough to melt the metal surfaces together. As a result, it can be an expensive process for certain manufacturers. But did you know that high-frequency sound can also be used to bond things? It’s called ultrasonic welding, and it was first introduced in the 1940s and further developed through the rest of the century as electronics in the equipment became more sophisticated and computer controlled. The technique, which uses high-frequency sound waves and pressure to bond metals together, provides a more cost-effective way of conjoining metals and plastics and is the fastest welding technique, with a weld time in the range of 0.1 to 1 s.

Ultrasonic welding is fast, economical, easily automated, and well suited for mass production, with production rates up to 60 parts per minute being possible. In particular, it is becoming an important method for welding polymeric composites, as it conjoins materials (metals and plastics) without the need for melting. Ultrasonic welding of plastics is used widely in the making of electronics, medical devices, and car parts. For example, ultrasonic welding is used to make electrical connections on computer circuit boards, and assemble electronic components such as transformers, electric motors, and capacitors. Medical devices, such as catheters, valves, filters and face masks are also assembled using ultrasonic welding. The packaging industry uses this technique to make blister packs. Ford Motor Company has even explored the use of ultrasonic welding to make aluminum car chassis. Ultrasonic welding is even used to make shoes. New Balance has been at the forefront of this cutting-edge shoe assembly, which formerly used stitching.

The Ultrasonic Welding Process

If you rub your hands together rapidly, they warm right up. If you take a hammer and pound a metal surface rapidly and repeatedly, the point of impact will warm up, too. In both cases, heat is produced by friction. Now, imagine rubbing your hands or pounding the hammer thousands of times per second. The frictional heat generated can raise the temperature significantly in a very short time. In essence, high-frequency sound (ultrasound) uses rapid vibrations that cause materials to rub against each other, and the resulting friction raises the temperature at the contact point. This rapid, frictional heat is what sets in motion the conditions for the materials to bind together. The ultrasonic welding process is broken down into a number of steps:

1. 1. 1. The parts to be welded are placed in the anvil or fixture.
      2. The horn contacts the parts to be welded.
      3. Pressure is applied to keep the horn in contact with the welded materials and to hold them together.
      4. The horn delivers ultrasonic vibrations to heat up the materials. The vibrations move less than a millimeter either up-and-down or side-to-side.
      5. The materials are welded together.
      6. The horn gets retracted and the welded materials can be removed from the anvil.

   The welding times, applied pressures and temperatures are controlled by a computer or microprocessor within the welding apparatus. What actually happens during the welding process depends upon the nature of the materials. The welding parameters depend on the material and the design of the molding, and good results are achieved with the following settings:

   • frequency 20–40 kHz
   • amplitude 20–60 μm
   • welding time 0.1–2.0 seconds
   • specific welding pressure 1–5 MPa (145–725 psi)

   When metals are welded, the ultrasonic vibrations are delivered parallel to the plane of the materials. The frictional heat increases the temperature of the metal surfaces to about one third of the melting temperature, but does not melt the metals. Instead, the heat removes metal oxides and films from the surfaces. This allows the metal atoms to move between the two surfaces and form bonds that hold the metals together. When ultrasound is used to conjoin plastics, the vibrations are perpendicular to the plane of the materials and the frictional heat increases the temperature enough to melt the plastics. The plastic molecules mix together and form bonds. Upon cooling, the plastic surfaces become welded together. Welding times can vary, but the welds can form in as little as 0.25 seconds.