A sealing bar is the heated metal bar on a bag-making machine that joins layers of film with heat and pressure. The film is held under the bar for a short time; the layers soften and fuse together, forming the bag’s seal seam.
For a good seal, three variables are set together: temperature, pressure and dwell time. If the temperature is too low, the layers do not fuse fully and the seam opens easily. Too much heat, on the other hand, thins or burns the film, leaving a weak line right next to the seam. On some machines, sealing and cutting are done with the same bar. The surface of the bar is protected with a coating or tape that stops the film from sticking.