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.