in the production of hollow bodies from oriented thermoplastic material , a cylindrical portion of a heated hollow preform is stretched , with a simultaneous reduction in the wall thickness thereof , at a starting temperature below the vitreous transition temperature of the material . a mandrel is positioned in the preform to support it during stretching , the outside diameter of the mandrel corresponding to the inside diameter of the preform . directly before the mandrel is inserted into the preform the preform is subjected to re - alignment to adapt at least the cross - sectional shape of the mouth opening of the preform to the cross - section of the mandrel . the re - aligning operation is effected by inserting the preform into a sleeve adjoining a drawing ring for carrying out the stretching operation .