Abstract: A mold is formed the interior volume of which corresponds to the exterior envelope required for the container (2). A core corresponding to the shape required for the cavity (14) to be formed inside the container (2) is prepared. Said core is placed inside the interior volume of the mold and an appropriate plastics material is injected under pressure into said interior volume to constitute an envelope comprising two substantially parallel walls (5, 6) extending at a predetermined small distance from each other between two common lateral edges (9, 10), with a common transverse edge (16) at the same end as the neck (14b). The product to be packaged is injected into the interior of the envelope. The transverse edge (15) of the body (14a) is closed by welding the plastics material.