Patent ID: 7278439

Claim:
A method of closing off an opening in a tank, said method comprising: providing a tank defining said opening and having an outer layer of plastic; providing a plastic sheet of multilayer structure having an outer layer with a composition compatible with that of said plastic of said outer layer of said tank; slimming down a peripheral region of said plastic sheet according to the following relationship: l/e ≧5 in which 1 represents a width of the slimmed-down peripheral region and e a thickness of a layer of the plastic sheet configured to contact the wall of the tank, both expressed in a same length unit; and after said slimming down, welding said plastic sheet to said tank, wherein said welding comprises: hot-plate welding a non-slimmed-down portion of the plastic sheet to said tank, and laser beam welding said slimmed-down peripheral region to said tank.