number of teeth on the sprockets could be varied to adjust the ratio of the rate of movement of the upper bar and lower bar rate a ratio of 2 1 has been found to be preferable for positioning the seal at the best height up the side of typical trays but a ratio of from 1 1 to 3 1 is suitable to accommodate a wide variation in size height of trays of course for given sprockets the ratio is fixed to position the seal before shrinking so that after shrinking the seal line is formed at a vertical position close to the top of the tray the top of the film is best moved about twice as far down as the bottom of the film is moved up an additional advantage of a 2 1 ratio is that generally there is little space below the top of the conveyor belt but plenty of space above it so the lower seal bar is somewhat constricted the movements of the trays along the conveyor the movement of the seal head horizontally and the movement of the sealing bars vertically are all connected interactively to provide constant dwell time during sealing and constant positioning of the seal with respect.