How Ultrasonic Welding Works

Rub your hands together rapidly. Notice anything? They warmed up, right? If you take a hammer and pound a metal surface rapidly and repeatedly, you will find that the place where the hammer strikes the metal warms up, too. In both these examples, the heat is due to friction. Now imagine rubbing your hands or pounding that hammer thousands of times per second.

The frictional heat generated can raise the temperature significantly in a very short time. Basically, high-frequency sound (ultrasound) causes rapid vibrations within the materials to be welded. The vibrations cause the materials to rub against each other and the friction raises the temperature at the surfaces in contact. This rapid frictional heat is what sets the conditions for the materials to bind together.

Ultrasonic welding equipment has four main parts. A power supply converts low-frequency electricity (50-60 Hz) to high-frequency electricity (20 - 40 kHz; 1 kHz = 1000 Hz). Next, a transducer or converter changes the high-frequency electricity into high-frequency sound (ultrasound). A booster makes the ultrasound vibrations bigger. Finally, a horn or sonotrode focuses the ultrasound vibrations and delivers them to the materials to be welded. Besides these pieces, there is an anvil upon which the welded materials are stacked and held. There is also some method to apply force (usually air pressure supplied by a pneumatic piston) to hold the materials together during welding.

So what materials and industries take advantage of this clever process? Ultrasonic welding of plastics is used widely in making electronics, medical devices and car parts. For example, ultrasonic plastic welding is used to make electrical connections on computer circuit boards, and assemble electrical components such as transformers, electric motors and capacitors.

Medical devices, such as catheters, valves, filters and face masks are rely on ultrasonic welding processes. The packaging industry uses this technique to make films, assemble tubes and blister packs. Even Ford Motor Company has explored using ultrasonic welding to make aluminum chassis in cars.

Now that you know the basics behind ultrasonic welding, let's look at the welding process itself.

The basic process of ultrasonic welding can be described by the following steps:

The welding times, applied pressures and temperatures are controlled by a computer or microprocessor within the welding apparatus. And what actually happens during the welding process depends on the nature of the materials. In metals, 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.

In the case of 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 are welded together. Welding times can vary, but the welds can form in as little as 0.25 seconds.

The factors that vary in ultrasonic welding are the frequency of the sound waves (usually 20, 30 or 40 kHz), the pressure applied to hold the materials together, and the time over which the ultrasound is applied (fractions of a second to more than one second).

The ultrasonic welding technique described are good for materials (metals, plastics) that are similar. But what about materials that are not similar. Let's address this question by looking at how New Balance has used ultrasonic welding to assemble athletic shoes.

Look at a pair of athletic shoes. While traditional shoes may be made of a single material such as canvas or suede leather, many athletic shoes have several materials such as lightweight plastic polymers, suede or synthetic suede, and mesh combined. These composite materials make the shoes light, flexible, durable and breathable. For example, one style of New Balance athletic shoe has an upper portion which consists of three parts.

But how do you put these materials together? Most commonly, shoe companies sew the materials together. About two to three years ago, however, New Balance set out to make the upper part of a shoe without stitching. After experimenting with polymer adhesive films and irons, they came up with a way of making this portion of the shoe using ultrasonic welding.

To assemble the upper part of the shoe, workers start with a piece of synthetic suede material. They use an iron press to bond a thin sheet of hot melt film to the back of the material. Next, an ultrasonic welding assembly presses a pattern vamp into a suede material. Likewise, a similar ultrasonic welding machine presses the saddle eye row from another piece of synthetic suede. The vamp shape gets cut out of the suede. The saddle eye row and mesh material get welded ultrasonically to the vamp. In the processes, the frictional heat from the ultrasonic welder melts the hot melt film, which bonds the saddle eye row and the mesh material to the vamp. The finished vamp then gets shaped and bonded to the sole and heel pieces using water-based solvents.

According to Kenneth Straka, Senior Product Developer for New Balance, the ultrasonic welding methods have increased productivity by saving time. Not only do the ultrasonic welders distribute heat more evenly than iron presses, they also heat up and cool down faster. So, the assembly process requires fewer steps and is faster than traditional sewing methods.

Now that we have seen how ultrasonic welding is used to bond various materials, let's look at the advantages and disadvantages of this technique.

Ultrasonic welding has many advantages over traditional methods. For one, welding occurs at low temperatures relative to other methods. So, the manufacturer does not need to expend vast amounts of fuel or other energy to reach high temperatures. This makes the process cheaper. It's also faster and safer.

The process occurs in fractions of a second to seconds. So, it can be done more quickly than other methods. In fact, it can bond plastics better and faster than glues. For example, the new smart keys in cars have a transponder chip in them. The car can only start when it senses the chip. To make the key, one end of the metal key blank and the chip get placed into one half of the plastic top. The other half gets placed over them and bonded to the base half. This bonding would usually be done with glue, which takes time to cure. The same task can be done with ultrasonic welding in less than a second.

Ultrasonic welding does not require flammable fuels and open flames, so compared to other welding methods, it's a safer process. Workers are not exposed to flammable gases or noxious solvents. In electronics, copper wires are usually bonded to electrical contacts on circuit boards with solder. The same task can be done using ultrasonic welding in a fraction of the time and without exposing workers to fumes from smoldering lead solder. Although workers' hearing may be damaged by exposure to high-frequency sound, this potential danger is easily reduced by enclosing the ultrasonic welding machine in a safety box or cage and/or using ear protection.

Finally, ultrasonic welds are as strong and durable as conventional welds of the same materials -- which is just one of the reasons the method is being used in car manufacturing. To make cars lighter and more fuel efficient, auto makers are turning to aluminum as the main metal in car bodies. Ultrasonic welding can be used to bond the metal in less time and at lower temperatures than traditional welding.

Ultrasonic welding does have its limitations, though. First, the depths of the welds are less than a millimeter, so the process works best on thin materials like plastics, wires or thin sheets of metal. Ultrasonically welding a steel girder for a building would not be practical. Second, it does work best when welding similar materials like similar plastics or similar metals. As you saw with New Balance shoes, ultrasonically welding dissimilar materials requires an additional material -- in the case of the New Balance shoes, it's a film that can be bonded between the synthetic suede and the mesh.

Despite these limitations, the popularity and potential of ultrasonic welding continues to grow.