the invention concerns a process for the production of hollow bodies of thermoplastic material by extrusion blow molding . in the process according to the invention continuous extrusion of at least two preforms in web form of plasticized material is effected by means of a plurality of extrusion devices . the preforms are jointly shaped to form a hollow body in the still plastic condition , that is to say in the first heat , within a multi - part tool forming a mold cavity . in that case a wall thickness profile is imparted to at least one preform during the extrusion operation independently of the respective other preform by continuous nozzle gap adjustment during the extrusion operation .