Maintaining perforation phasing

In an apparatus for making plastic bags from a continuous film of material comprising a sealing drum having at least one seal bar for imparting transverse seals to the film at regularly spaced intervals and a perforator having a rotatable perforator blade for imparting transverse perforations to the film at regularly spaced intervals, the improvement comprising:

• means for applying registration marks to the film at regularly spaced intervals;
• means for generating a signal representative of the position of each registration mark;
• means for generating a signal representative of the position of each perforation;
• means for generating a positional reference signal against which each registration mark signal can be compared with each perforation signal;
• means responsive to the positional reference signal generating means, the registration mark signal means and the perforation signal means for determining a positional difference between each registration mark and each perforation;
• means for rotating the perforator blade, the rotating means having a predetermined cycle length;
• means for computing a perforation offset value as a function of a desired bag length, the cycle length and a distance between the registration mark signal means and the perforation signal means;
• means for comparing the positional difference with the perforation offset value and applying a difference therebetween to the cycle length of the rotating means to thereby maintain a desired spacing between each perforation and each registration mark.