advantage to using identical plastics for the two parts consists not only in that the long term behavior determined by changes in the material characteristics is the same but also in particular that both parts then have the same melting temperature in a second embodiment example shown in fig2 the bushing 2 that is fitted into the bellows 1 has a different inner contour than that in the first embodiment example the functionally determined geometry is limited in this case to an area of the inner surface smaller than the height h wherein the edge 4 defines a collar 7 having a constant thickness d over its entire circumference ideally thickness d is equal to the minimum thickness d 1 this results in more favorable conditions for coupling in the laser beam the angle may be greater by up to 90 u00b0 so that the laser beam impinges perpendicularly on the surface of the bushing 2 and on the contact surface between the bushing 2 and the bellows 1 in a third embodiment example shown in fig3 a bevel 8 is provided adjoining the edge 4 instead of the collar 7 in contrast to the first embodiment example in which the laser beam 6 impinges at an angle on the surface of the bushing 2 and on the contact surface between the bushing 2 and the bellows 1 the laser beam 6 can be coupled into the bushing 2 perpendicularly by means of the bevel 8 so that in particular radiation losses due to reflection are prevented in a fourth embodiment example the bushing 2 is made of a material that absorbs the laser radiation and the bellows 1 is transparent for the laser beam.