a heat sealing jaw assembly for effecting a heat seal of at least two juxtaposed heat sealable sheet materials is described . preferably , but not exclusively , the heat sealing jaw assembly is adapted for use in a form , fill and seal machine forming thermoplastic bags . the heat sealing jaw assembly comprises a first and a second metal jaw member , each having a heating element for heating the jaw members . each of the jaw members has an end face disposed in facial alignment with one another . a ribbon member is secured to each of the end faces and disposed in planar alignment with one another . the ribbon member is made of soft material having rubber - like properties and capable of conducting heat from the sealing jaw to a fusing temperature to melt the heat sealable sheet materials . the first and second jaw members are reciprocatable towards and away from one another to press the aligned heated ribbon members against opposed sides of at least two juxtaposed heat sealable sheet materials disposed between the jaw members to effect a heat seal .