An apparatus for processing workpieces containing plastics material by welding, stamping, cutting and/or bonding is described. The apparatus includes a loading and unloading station for loading and unloading the workpieces (35), a processing station with a free space for processing the workpieces (35) and at least one interchangeable module (15) which can be inserted and fixed in the free space and which comprises at least one workpiece receptacle (34) and at least one processing tool (37) which are both adapted to a selected processing task. In accordance with the invention the interchangeable module (15) is supported on running wheels (17) and can be inserted in the free space therewith, and the relative position between the receptacle (34) and the processing tool (37) required in the processing is determined by the interchangeable module (15) alone.