A device (10) for welding thin-walled work pieces (52) such as e.g. food
containers of foil, fibrous web or foil-fibrous web composites using
ultrasound has at least two components, i.e. a sonotrode (12) and a
counter tool (14) for the sonotrode (12). One component (12) has a working
region (16) which faces a working region (18) of the other component (14).
The working region (18) of one component (14) comprises at least one
projection (41) and the working region (16) of the other component (12)
comprises at least one recess (46) into which the projection (41) can
engage during the welding process. In this manner the work piece is
securely held without slippage.
