shows a ring 23 with a plurality of spring elements formed as tongues 24 the ring 23 consists of a material with mechanical characteristics similar to spring steel the ring 23 is inserted into the opening 13 of the body 12 fig3 on inserting the shell mold the tongues 24 are deflected perpendicular to their longitudinal direction or perpendicular to the centering axis 6 the dimensions of the ring 23 and the opening 13 in the body 12 are adapted to each other so that the tongues 24 do not touch the wall 14 or only touch it when the deflection of the tongues 24 reaches a certain degree one advantage of the solution with the ring 23 in comparison with the solution with the o ring 16 exists in the lower frictional forces on inserting the shell mold into the centering station 3 fig1 and in reduced wear of the ring the two grippers 1 and 2 are constructed the same the construction of the grippers is described in more detail based on fig9 the gripper contains a support 25 on which the passive side 26 of the shell mold 27 comes to rest as well as a circular deformable sealing element 28 arranged within the support 25 in order to seal a cavity 29 formed between the shell mold 27 and the gripper to which vacuum can be applied the support 25 is located in a plane running perpendicular.