apparatus for continuously producing seals in tube - shaped plastics film material , comprising means for supplying the web of material , at least one heating beam travelling along with the web and subsequently returning , means for keeping the web mechanically tension - free at the location where it is warm , means for keeping the heated film layers one against the other up to a pressing station , said pressing station comprising two rollers at least one of which is provided with cooling means , and means for discharging the web . a plurality of heating beams being provided which are interconnected by flexible and longitudinally adjustable means such as belts so as to form an endless loop which is guided over and driven by appropriate adjustable tensioning rollers .