Method and apparatus for registration of a seal on a plastic bag

A method for making plastic bags from plastic film having a plurality of registration marks using a bag making machine having a cylindrical drum with a variable diameter, a circumference, an axis and at least one seal bar substantially parallel to the axis and defining the circumference, comprising,

• detecting the rotation of the drum;
• detecting the registration marks on the film;
• determining the nominal length of the bags from the detection of the registration marks and the rotation of the drum;
• determining a phase of the registration mark relative to a seal on the plastic bag;
• adjusting the diameter of the drum in a first direction in response to the nominal bag length and the phase of the registration mark relative to the seal on the plastic bag such that a circumferential distance between subsequent seal bars is changed in a first direction to be substantially equal to the nominal bag length;
• and
• continued adjusting of the diameter of the drum in the first direction such that the circumferential distance between subsequent seal bars is not substantially equal to the nominal bag length.

An apparatus for making plastic bags from plastic film having a plurality of registration marks comprising:

• a cylindrical drum with a variable diameter, a circumference, an axis and at least one seal bar substantially parallel to the axis and defining the circumference, wherein the at least one seal bar seals the plastic bags;
• a sensor disposed to detect the rotation of the drum;
• a registration mark detector disposed to detect the registration marks;
• a controller, connected to receive a first input from the sensor and a second input from the detector, wherein the controller has a first signal indicative of the nominal length of the bags;
• further wherein the controller has a second signal indicative of a phase of the registration mark relative to the seal on the plastic bag;
• further wherein the controller has at least one output having a magnitude responsive to the first and second signals;
• and
• a motor connected to the drum and responsive to the at least one output, wherein the diameter of the drum is adjusted in response to the at least one output such that a circumferential distance between subsequent seal bars is changed in a first direction to be substantially equal to the nominal bag length, and further such that the diameter of the drum is continued to be adjusted in the first direction such that the circumferential distance between subsequent seal bars is not substantially equal to the nominal bag length.