an apparatus for producing blow - molded containers having thickened flanges . the apparatus utilizes movable mold halves having separable side and end components and a stationary central mold element disposed between the movable mold halves to form containers having thickened radial flanges . the movable mold halves and stationary central mold elements together enclose a mold chamber . after the parison of thermoplastic material has been blow - molded to conform to the mold chamber but while the material is still moldable , the movable mold halves simultaneously move together along the axis of the mold chamber to press the material against the stationary central mold element and thereby fold and bond the material onto itself , forming a thickened radial flange on each container . the flanges are each substantially twice as thick as a single layer of blow - molded material . the thickened flanges may also be formed to each include a thickened axial lip of material extending down from the respective radial flanges . after the thermoplastic material has cooled sufficiently to retain its shape , the side components of the mold halves are axially moved apart so that the mold chamber can be opened along its longitudinal axis without damaging the thickened radial flanges or axial lips . by keeping the end components of the mold halves substantially still during the retraction of the side components of the mold halves , the tail of material formed at the junction of the halves of the molding apparatus opposite to the feed end of the blow - molding chamber is preserved , thereby facilitating post - processing handling of the containers .