Cut Sheet Length Control in a Corrugator Dry End

In a system for cutting sheets from a running web that is slit along its length into output webs directed to respective upper and lower knife assemblies, the unslit web upstream of the slit line joining the output webs to move together, each output web utilizing a separate driven infeed pull roll nip imposing a first tension on the output web and directing the output web into a. driven cutoff knife for cutting the sheets, and a driven outfeed nip downstream of the cutoff knife, the outfeed nip engaging the lead edge of the output web and imposing a second tension on the output web to control the sheets after they are cut and to pull a gap between each cut sheet and the leading edge of the output web moving through the knife, the output web being pulled by the sum of the first and second tensions until the sheet is cut, the output web being pulled only by the first tension until the lead edge of the output web reaches the outfeed nip,

• a method for reducing sheet length variations in output webs as a result of changes in tensions in the output webs during a sheet cutting cycle and for providing sheets cut to a consistent length, the method comprising the steps of:
• (1) utilizing a torque controlled drive for the infeed pull roll to provide a high level of first web tension;
• (2) utilizing an infeed pull roll torque command to step down the torque to provide a lower level of first web tension and utilizing a signal from a web length measuring device to determine when the leading edge of the output web reaches the outfeed nip;
• and,
• (3) using a cutoff knife position signal to indicate completion of the cut and to step up the pull roll torque to the high level of first web tension.

In a system for cutting sheets from a running web that is slit along, its length into output webs directed to respective upper and lower knife levels, the unslit web upstream of the slit line joining the output webs to move together, each output web utilizing a separate driven infeed pull roll nip imposing a first tension on the output web and directing the output web into a driven cutoff knife for cutting the sheets, and a driven outfeed nip downstream of the cutoff knife, the outfeed nip engaging the lead edge of the output web and imposing a second tension on the output web to control the sheets after they are cut and to pull a gap between each cut sheet and the leading edge of the output web moving through the knife, the output web being pulled by the sum of the first and second tensions until the sheet is cut, the output web being pulled only by the first tension until the lead edge of the output web reaches the outfeed nip,

• a method for controlling cut sheet length changes resulting from changes in tension in the output webs through a sheet cutting cycle in which the respective output webs are cut to provide sheets of different lengths, the method comprising the steps of:
• (1) maintaining a higher level of first web tension in the output webs to both knife levels as the output webs travel through their respective infeed pull roll nip and the cutoff knife;
• (2) adjusting the higher level of first web tension to a lower level of first web tension when the leading edge of the output web at one knife level reaches the outfeed nip;
• and
• (3) operating the cutoff knife to cut the shed and adjusting the first web tension to the higher level;
• whereby the sum of the first and second web tensions at both knife levels is substantially uniform through the cutting cycles and the sheets are cut to a consistent length.

In a system for cutting sheets from a running web that is slit along its length into output webs directed to respective upper and lower knife assemblies, the unslit web upstream of the slit line joining the output webs to move together, each output web utilizing a separate driven infeed pull roll nip imposing a first tension on the output web and directing the output web into a driven cutoff knife for cutting the sheets, and a driven outfeed nip downstream of the cutoff knife, the outfeed nip engaging the lead edge of the output web and imposing a second tension on the output web to control the sheets after they are cut and to pull a gap between each cut sheet and the leading edge of the output web moving through the knife, the output web being pulled by the sum of the first and second tensions until the sheet is cut, the output web being pulled only by the first tension until the lead edge of the output web reaches the outfeed nip;

• an apparatus for reducing sheet length variations in output webs as a result of changes in tensions in the output webs during a sheet cutting cycle and for providing sheets cut to a consistent length, comprising:
• (1) a torque controlled drive for the infeed pull roll operative to provide a high level of first web tension;
• (2) the drive being operative to provide an inked pull roll torque command to step down the torque to provide a lower level of first web tension, and a web length measuring device operative to determine when the leading edge of the output web reaches the outfeed nip;
• and
• (3) a cutoff knife position device operative to generate a signal to indicate completion of the cut and to step up the pull roll torque to the high level of first web tension.