a first drive apparatus for a thermoplastic web operates at an automatically adjusted , substantially constant , speed to deliver the thermoplastic web to a welding tool which rotates with a second pair of rollers . the second pair of rollers are driven at a cyclically variable speed in which the minimum speed is attained at the time when the welding tool is in contact with the thermoplastic web . an automatic control system adjusts the speed of the feed rollers to correspond to the speed of the welding tool at the instant during which welding is performed . adjustment of the ratio of the maximum to the minimum speed of the cyclically variable speed of the second pair of rollers controls the length of thermoplastic web between welds without changing the speed of the driving input to the second pair of rollers .