Abstract: A blow molding method including a clamp for clamping two molds in back-to-back relationship, the clamp having first and second movable, inner platens supporting interior sections of two molds; first and second movable outer platens supporting exterior mold sections facing the interior sections; a positioner to move the inner and outer platens between opened and closed positions; an extrusion means to extrude parisons between mold sections, injectors to inject air into the interior of the parisons; and a platen retainer to resiliently position the inner platens between the outer platens to equalize pressure in the two molds.