PURPOSE: To prevent adhesion of penetration liquid or other contaminant until a weld process is completed.

CONSTITUTION: Membrane parts M1 and M2 of a slitted wall continued to a lock half part immersed in suspension liquid are intercoupled. The other lock half part of a next membrane part M1 consisting of an inner profile element 1 is inserted in a manner to make surface contact with one lock half part of one membrane part M2 consisting of an inner profile element 1. An inner groove 4 extending along approximately the overall length of a lock is formed in a wall of an inner profile element. This groove separates the inner profile element 1 from each other and reaches a wall of an outer profile film element 2. Before the profile elements 1 and 2 are welded together, the wall range of a profile element having a groove is sealed from slitted wall suspension by a sealant.