heat is applied to a heat sealable material which may be in the form of a bag , for example , to seal a portion of the bag . this may be the top , bottom or side of the bag , and one form of heat sealable material is a plastic film such as polyethylene , either as a coating on the inside of a paper kraft bag or as a separate ply . the two portions to be heat sealed are juxtaposed and passed between endless metal bands which move in synchronism with the heat sealable material as a part of the conveyor means for the heat sealable material . heat is imparted to the heat sealable material from the metal bands and it is imparted to the metal bands by electrical current conducted into the bands by collector rings on the pulleys over which the metal band is entrained . the heat is conducted through the kraft ply to the heat sealable material to cause the two portions to be sealed together . the foregoing abstract is merely a resume of one general application , is not a complete discussion of all principles of operation or applications , and is not to be construed as a limitation on the scope of the claimed subject matter .