Method for blow molding a container having a concave bottom

Claim 1

In a method of blow molding a plastic container having one concave wall from a thermoplastic material, by the inflation of a thermoplastic parison interiorly of a blow mold into a blown pre-form having the overall configuration and size of the final container with the exception of the configuration of said one wall, the one wall of the pre-form being generally convex and protruding from the remainder of the pre-form, and said one wall being at a formable temperature, the steps of, in the blow mold and while the pre-form remains inflated, (1) contacting the exterior surface of said one convex pre-form wall with a plurality of independently axially movable, concentric sleeves surrounding a central rod which is also independently axially movable, said rod and said sleeves each having end faces provided with stepped, angularly related surfaces interrupted by radial grooves, 2. moving said rod and said sleeves with the end faces thereof in contact with said convex wall and while retaining the remainder of said pre-form stationary, the rod being moved a greater distance than any of said sleeves and the outer of said sleeves being moved a lesser distance than the inner of said sleeves, (3) the performance of step (2) deflecting the one convex wall to a concave configuration, reducing the amount of protuberance of said one wall from the remainder of the container, and forming an annular support ring having an interior surface communicating with the interior of the container, (4) the performance of step (2) while the pre-form remains inflated also forcing the exterior surface of the concave wall inwardly of said support ring against the end faces of said rod and said sleeves to form on said wall annular reinforcing ribs and additional radial ribs intersecting said annular ribs, and (5) chilling the one wall in its deflected, concave configuration.