Abstract: It is disclosed a process comprising the steps of making the first layer of a membrane with a first plastic material chemically compatible with the fluid to be treated, making a second layer of a second plastic material defining the mechanical performance of the membrane and coupling said layers, the process consisting in: inserting a first plastic material selected from polyolefines, polyamides, polyesters, vinyl resins into a moulding cavity (2) and injection-moulding at least one second plastic material defined by a thermoplastic elastomer (TPE) in said moulding cavity (2) to form at least one second layer (8) directly joined to the first layer (7).