An article having a flange is molded between a mold core and a unitary mold cavity, the flange being formed during the process as a consequence of the existence of an undercut existing in a first surface of the cavity. A ridge is provided on a second surface of the mold cavity adjacent the undercut. A fluid, i.e., air, is injected between the cavity and the molded article as the core and cavity separate in such a volume and at such a pressure as will maintain the article in substantially complete and constant contact with the core as the core and article are withdrawn from the cavity. The flange conforms to the cavity wall until the article is free from the cavity, and subsequently the molded article is separated from the mold core.