a method for using an expandable forming plug in forming apparatus such as plug assisted vacuum or pressure differential forming machines and the like wherein the plug cooperates with a mold to form hollow articles such as containers , closures , trays , and the like , from thermoplastic synthetic resinous film or sheeting . the expandable plug provides hollow articles which during forming can be programed or controlled , as desired , with respect to thickness distribution in the walls thereof and , more particularly , provides hollow articles with side and bottom walls of desired thickness , such as of substantially uniform or of a varying thickness .