An apparatus for welding a first plastic sheet to a second plastic sheet is provided. The apparatus includes an elongate die including a channel extending longitudinally along the die's welding face. In combination with a plate and a high frequency alternating current generator, the die and the plate are operable to compress the first and second plastic sheets in an overlapping arrangement and generate a high frequency alternating electromagnetic field between the die and the plate and weld the first and second plastic sheets to one another. The depth of the die channel prevents the formation of an electromagnetic field that is strong enough to weld the first and second plastic sheets in an area between the die channel and the plate. A die for use in such an apparatus and a method of use thereof are further provided.