a method and apparatus are disclosed for sealing flexible plastic sheets together . the method is performed by use of a die wherein a pair of longitudinal grooves are provided at the edge of the sealing area . the die is connected to a source of radio frequency energy and the sheets are placed between the die faces to effect the seal . due to the provision of the grooves the sealing is effected without the extrusion of material into the space between the sheets thereby to produce a stronger and more satisfactory bond .