a method of making a concave bottom on a blown thermoplastic container . a blowable pre - form is initially expanded against a composite mold surface defined by the end faces of a plurality of concentric tubes surrounding a central actuating rod . the rod and the tubes are initially telescopically positioned to define a composite concave surface , so that a first convex bottom is blown . subsequently , the rod and tubes are actuated telescopically to progressively invert the convex bottom to a concave shape . the end faces of the tubes may be grooved to define reinforcing ribs in the concave bottom wall , if desired .