A system for joining flat portions of sheets with a joint blank formed of thermoplastic material. The joint blank initially defines first and second flat sides extending substantially perpendicularly one from the other. One flat side is disposed in overlying relationship with the flat edge portions of the sheets and the other side in overhanging relationship with respect to the flat edge portions. A rotating roller die is provided for engagement with the overhanging portion of the joint blank, and the roller die is gradually displaced for deforming the overhanging portion for molding this portion into engagement with the flat edge portions of the sheets.