Abstract: An apparatus for forming articles such as paperboard cartons from a single piece blank, comprising a first article holding member containing a cut-out and a second article holding member adapted to mate with the cut out. A portion of the workpiece is grasped by the mating surfaces of the holding members. Article forming members are provided in which the ungrasped portions of the workpiece are cradled. The forming members are capable of mating in close fitting relationship about the holding members. As the forming members move towards the holding members, they move the ungrasped cradled portions of the workpiece against the lateral surfaces of the mated holding members, sandwiching them between the mated holding members and mated forming members. The mated holding members serve as a forming mandrel to impart the desired shape to the workpiece. By properly sizing the forming members, corner seals on items such as paperboard cartons can be formed as the forming members mate around the holding members.