Abstract: A plastic bag making machine includes a cylindrical drum with a variable diameter. A plurality of seal bars define the circumference of the drum, and are substantially parallel to the drum's rotational axis. A controller determines the distance between registration marks on the film using the output of an encoder as a position signal, and adjusts the diameter of the drum in response to the determined distance. The controller also determines the position of the seal bars relative to the registration marks and adjusts the diameter of the drum in response to the relative position. A downstream device includes a perforation to seal registration controller with a registration control input. The controller provides a control signal to the registration control input in response to changes in the diameter of the drum.