In an ultrasonic welding apparatus comprising an ultrasonic sonotrode and
a backing plate, flat workpieces to be welded are pressed between the
sonotrode and the backing plate. Projections are provided on a portion of
the surface of the sonotrode and on a portion of the surface of the
backing plate, each projection-bearing surface portion being adjoined by
a smooth portion. The flat workpieces are for example ends of a strapping
tape formed in a loop around a package in a strapping machine and the aim
is to produce a weld of increased strength.