a method for manufacturing a blow - molded body employs a two - part blow - mold . a tubular preform is introduced , with the material in a thermoplastic state , into the opened blow - mold halves , and a reinforcing element is supported inside the preform by a displaceable holding device . during closing of the blow - mold halves , pressure is exerted on the wall regions of the preform that are adjacent to the reinforcing element . this pressure results in a reduced cross section of the preform wall in the region of the reinforcing element , and thermoplastic material is displaced . the interior of the preform is subjected to a blowing - pressure no later than when the pressure is applied from the exterior , and the blowing pressure causes the preform to inflate inside the mold . the displaced material anchors the reinforcing element inside the blow - molded body , and preferably forms bridges which extend across the blow - molded body and which cover the sides of the reinforcing element . the reinforcing element may also be provided with passages into which displaced material enters to form further bridges .