Under this method, an ultrasonic head (referred to as a horn) transmits vibratory energy via two separate pieces of plastic film kept in contact with each other. Since the vibratory energy gets converted into heat at the film interface, it creates a near immediate weld. This conversion is attributed to the fact that the two film surfaces create a strong friction at the frequencies employed, thereby generating heat.
Generally, this technique can only join similar plastics; however, this is not regarded as a significant disadvantage. Since only the interface is heated, it is especially helpful for low melting rigid plastics or for oriented film.
This method is also helpful in cases where there is a need to make the seals between surfaces that are contaminated by oils and fats. It is also useful in situations where thick layers of paper are sealed in unison by placing thin plastic coatings on their surfaces.
However, one major drawback of this method is its tendency to be far slower as compared to the previously mentioned radiant heat methods.