The parts to be welded are inserted between anvil and sonotrode and overlap each other. The anvil keeps the workpiece in position and secures it against any uncontrollable movement (figures 27 and 28).

Figure 27: Welding device

Although the oscillating part of the workpiece is subject to certain restrictions with respect to its mass and its geometrical shape, no restrictions apply to the mass of the workpiece on the side of the anvil. This means that the workpiece on that side can be of any size, however, it still has to fulfill the criteria for ultrasonic weldability.

The anvil absorbs the pressure and oscillation forces introduced by the sonotrode. Its sturdy design must insure that a constant pressure is applied to the welding area during the whole welding process. The construction must be rigid and solid to avoid interferences superimposed with the transducer's vibration.

Figure 28: Welding device

Interferences must be avoided

If an accompanying oscillation of the workpiece on the anvil were to take place, it would no longer be possible to achieve a high welding standard. And if the anvil or the clamping parts were to oscillate in conjunction with the workpiece, a large portion of the oscillating energy would be lost and could not be used for welding. The proper design of the anvil and the fixturing to suit the requirements of ultrasonic welding therefore requires a lot of experience from the manufacturer.

The immediate welding area of the anvil, as well as the work surface of the sonotrode, is roughened to enable a secure gripping of the parts to be welded. This prevents slippage of the workpieces and thus an efficient oscillating energy is delivered. An accommodating fixture, such as pneumatically operated jaws or other machine parts, should enable easy insertion and removal of the parts to be welded (figure 29). When used in conjunction with automatic feeding mechanisms or installed in rotary tables, transfer lines, etc., they are very often suited for use in fully automatic production lines.