Abstract: A method and apparatus for the continuous manufacture of plastic bags from an extruder which includes a differential tachometer which senses the speed of the film in two locations so that the film drive can be adjusted to compensate for the shrinkage of the film and to eliminate tension in the film. Welding and perforating blades are provided which contact the travelling film and travel with the film to complete their weld and perforations before returning to their initial positions. One type of welding and perforating device uses a pair of opposed blocks which come together around the film and travel with the film while the perforating blade pierces through the film and the weld is being completed. A timing cam coupled to the film drive determines the spacing of the welds and perforations and thus the length of each bag produced.