Method for forming container having re-entrant flange

Claim 1

A short-stroke method for forming thermoformed unitary container members having an undercut flange, comprising the steps of: longitudinally moving a web of thermoformable material over an undercut flange-forming member along the periphery of a thermoforming molding cavity, the undercut flange-forming member having a movable leading end that is at the upstream end of the molding cavity, the upstream end being defined by the direction of said longitudinal movement of the web, the undercut flange-forming member having a stationary trailing end that is downstream of the leading end of the flange-forming member; thermoforming a unitary container member within the molding cavity by thermoforming a portion of the web over the flange-forming member and into a body-forming member of the molding cavity, said unitary container member having a body, said container member body having a flange along the periphery of the walls of the container member body; said thermoforming step including imparting a re-entrant condition to the flange so as to form an undercut portion while thermoforming said body and said flange from the web of thermoformable material, said step of imparting a re-entrant condition including drawing the thermoformable material over the undercut flange-forming member; opening the leading end of the molding cavity flange-forming member by moving same through a short-stroke in said upstream direction away from the molding cavity and by moving same away from the trailing portion of the flange-forming member; separating the body-forming member and the leading end of the flange-forming member from the rest of the molding cavity until the body-forming member of the molding cavity and the leading end of the flange-forming member clear the container member body; slidingly moving the web and the container member formed therein in the upstream direction and away from the molding cavity until the container member is completely removed from the molding cavity; and closing the leading end of the molding cavity flange-forming member through a short-stroke in a downstream direction until the leading end operatively engages the trailing portion of the flange-forming member.