Abstract: A method and apparatus for registration of a seal and perforation on a plastic bag include a drum for forming a seal on a continuous plastic film with respect to a registration mark on the film and a perforator/cutting station for forming a perforation on the film with respect to the location of the seal. This method and apparatus is used in a plastic bag making machine to produce bags of different lengths. A detector locates the registration mark printed on the film and the drum diameter may be adjusted to form the seal at a desired location on the film with respect to the registration mark. A seal detecting station locates the seal and a controller adjusts the speed of the perforator/cutting station to properly locate the perforation with respect to the seal and to maintain a desired skirt length.

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. The bags are made from a plastic film with registration marks that are detected by a detector disposed near the film. A detector also detects each revolution of the drum and an encoder provides signals indicative of the position of the film. A controller determines the distance phase of the registration marks and the rum size error. The drum diameter is adjusted in response to an output of the controller in such a way as to correct for both the initial drum size error, the initial phase error, and the phase error accumulated while the drum diameter is being corrected.