successive increments of a thermoplastic tube are extruded in vertical direction from an orifice , and a first partible mold is closed about the increment immediately downstream of the orifice whereupon the increment is expanded to the shape of a first semi - finished container , while simultaneously completing the expansion of the neck region of an already filled second semi - finished container accommodated in a second partible mold downstream of the first mold and communicating with the interior of the first container , is completed . the containers are then sealed and separated from one another and the first container in the first mold is filled . the second container is discharged from the second mold and the relative positions of the molds are reversed in preparation for repetition of the preceding steps .