Process for the manufacturing of a pressure vessel and a pressure vessel, especially for a water heater, made by the process

A process for the manufacturing of a pressure vessel (1) made from a thermoplastic material or a thermosetting resin, which pressure vessel (1) comprises a first and a second vessel end part manufactured through injection moulding or press moulding and possibly an intermediate tube part which preferably is manufactured through extrusion, wherein the vessel end parts include a circumfering shell (4) which forms the sides of the pressure vessel (1) and each one dome (2 or 3) which form the upper and lower ends respectively of the pressure vessel (1) when joined, wherein a number of interconnecting and thereby load distributing walls (5) stretch from the domes (2, 3) along the circumfering shell (4) towards the open part of each vessel end part, wherein a corresponding number of matching walls (5) are stretching from one open end of the possible intermediate tube part along the circumfering shell of the tube part to the other end of the same so that when two vessel end parts are joined, possibly with an intermediate tube part placed in between, the edges of each of the walls (5) in the different parts meet, between which walls (5) a number of cells (6) are formed, and wherein the walls (5) are placed so that a number of crossing areas (7) are formed between intersecting or connecting walls (5), characterised in that at least one preferably all crossing areas (7) are provided with a termination by achieving a hole (14) from the outside of the dome, which hole (14) is bigger than the crossing area (7) and has a depth greater than the goods thickness of the dome part (2, 3) wherein communicating openings (8) between the cells (6) in the pressure vessel (1) are prepared.