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;
• adjusting the diameter of the drum in response to the nominal bag length;
• determining a phase of the registration mark relative to a seal on the plastic bag;
• temporarily adjusting the diameter of the drum in response to the phase of the registration mark relative to the seal on the plastic bag;
• and
• processing the plastic film downstream from the drum and adjusting the speed of the process in response 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;
• a motor responsive to the first signal and connected to the drum, wherein the diameter of the drum is adjusted in response to the nominal bag length;
• further wherein the controller has a second signal indicative of a phase of the registration mark relative to the seal on the plastic bag;
• the motor being responsive to the second signal, wherein the diameter of the drum is temporarily adjusted in response to the second signal;
• and
• a processing device located downstream from the drum, and a second motor, responsive to the first signal, connected to the processing device, wherein the speed of the processing device is adjusted in response to the nominal bag length.