The invention concerns a method which consists in moulding a plastic cap (1) and steps which consist in: providing, proximate to an inner surface of base web (7) of the cap (1), a first layer (6) of material impermeable to a predetermined gas or gases and depositing, on said first layer (6) set in the cap (1), a second layer (8) made of an elastic material different from that of the first layer (6), said second layer (8) being designed to form the joint, by isolating the first layer from ambient air.