Apparatus for blow molding a container having an upward bulged bottom

Claim 1

Apparatus for forming a container from a cylindrical preform having a side wall and a bottom closure, said apparatus comprising an internal mandrel on which the preform is fitted, a mold including two mold halves, and a bottom part in said mold in axial opposition to said mandrel, said mandrel and bottom part being movable axially of the mold relative to each other and relative to the mold halves for shaping the preform into a container, said mandrel having a concave end surface, said bottom part having a convex end surface facing said concave surface for enclosing and reshaping the bottom closure of the preform, said mold halves together defining a mold inner surface for engaging the wall of the preform at the outer surface thereof proximate said end closure, said mandrel having an outer surface engaging the wall of the preform at the inner surface thereof, the engagement of the inner and outer surfaces of the preform in region of said end closure being without clearance to maintain the wall thickness of the preform during reshaping of the bottom closure of the preform, said end surfaces of said mandrel and said bottom part including respective central regions which a opposite one another and define a substantially uniform gap for engaging the inner and outer surfaces of the bottom closure of the preform to turn the bottom closure inwardly, said end surfaces further including peripheral regions which are opposite one another and of differing curvature to form an annular ring-shaped region in which the material of the preform in a ring-shaped transition region between the inwardly bulged bottom closure and the side wall is stretched and becomes oriented so that the inward bulged shaped of the bottom closure is stabilized and prevented from turning inside-out said bottom part being positionable within said mold inner surface such that said mold inner surface and said convex end surface define an annular molding recess therebetween.