Abstract: An apparatus and method for forming flat cut carton blanks into erected cartons with debowed carton side panels is disclosed. The apparatus includes a forming head receiving and directing a carton blank through forming guides, side walls engaging and supporting the carton side panels that bend into upright position relative to the carton bottom panel during descending movement through the forming guides. Presser fingers pivotally secured to the forming guides project inwardly into the forming head path. The presser fingers bend the side panels into cutout openings provided in the side walls to overbreak the side panels at fold lines between the carton side and bottom panels. An inclined surface with a sharp creasing edge abutting the fold lines enables the presser fingers to initially crease the fold lines and thereafter bend the carton side panels inwardly onto the inclined surface for controlled overbreaking movement.