Building on the basics of web tension control, real-world applications are complex and subject to many variables. However, achieving proper web tension control is important in preventing roll winding defects that can compromise the integrity of your operation. As a matter of course, many roll winding defects come down to having the correct web tension control at the rewind or unwind stations. Also, high-quality and automated tension control of the web is dependent on the traction provided by the machine idler rolls and driven rolls.
Troubleshooting and problem-solving information is important for operators and winders to aid in uncovering reasons behind production problems such as core issues, telescoping, poor starts, baggy or sagging rolls, curling edges, dished rolls and so on. That said, many of the roll winding defects occur at such low rates, sometimes less than 5%, that troubleshooting is essential to alert users to problems in the web. Here are a few common issues to watch out for:
Problem: Core issues can stem from loose, crushed or offset rolls. Loose cores cause rotational displacement with the web tearing free from the core, causing tension and control problems with subsequent processes. Crushed rolls are usually from stacking rolls on end too high with the bottom rolls failing in an axial direction. An offset core is a defect that results in an abrupt shift in positioning along the edge of the roll.
Solution: Core issues can result from winding the web too softly. The web should be wound evenly, but harder at the start of the roll. Do not stack rolls too high, and to avoid offset, make any necessary tension adjustments gradually and avoid abrupt changes in tension, rider roll loading, or web draw control at winder.
Problem: This roll winding defect exhibits the classic concave or convex roll caused by progressive roll edge misalignment. It is only noticed after the roll has started to be unwound.
Solution: Controlling the hardness of the wind, reducing web tension, or reducing cross-deckle caliper variation can resolve telescoping.
Problem: When the appearance between the web near the core and the remainder of the roll show obvious differences, you’re off to a poor start.
Solution: The web needs to be tightened before fastening to core, make sure to use good quality, properly stored cores, or begin with proper tension, nip and/or torque.
Slitter edge curl
Problem: The telltale wavy or shaggy edge at the slit causes edge curl.
Solution: Ensuring slitters are set to the proper depth or reducing web tension through the slitter section can help prevent slitter edge curl.
Baggy or slack ends
Problem: When web width does not draw uniformly, slack and tight sections across the width of the sheet will occur. This rolling defect can cause web tension difficulty in the roll and in subsequent operations.
Solution: Slack or baggy ends or areas are a result of non-uniform cross-deckle web thickness and the cross-deckle caliper variation needs to be reduced to a minimum. It could also be from winding too tightly or a combination of the cross-deckle caliper and a tight wind. Wind as softly as practical and adjust the caliper accordingly.
Machine tension burst
Problem: Cross-machine tension burst, full machine-direction tension burst, or partial machine-direction burst are defects relating to fiber separation. In the case of cross-machine the defect is not visible at the edge of the roll, whereas full machine-direction it is. Partial occurs at the edge of the roll appearing as an offset.
Solution: In each instance, winding too tightly can exceed the strength of the paper and the roll needs to be wound softer. Also, it may mean the cross-machine sheet caliper variation needs to be minimized. Partial machine-direction tension burst may be caused by non-uniform cross-deckle caliper variation, which needs to be minimized.
Problem: Dished rolls are those rolls that are wound with a forward edge misalignment. The rolls may be convex or concave and it happens while the rolls are winding.
Solution: To avoid, check for machine misalignments, make sure cores are firmly retained during winding and the hardness does not increase during winding, and have a good, hard start at the core.
Trim wound in rolls
Problem: When winder trim is not collected properly into the trim removal system, it will follow the web into the winding roll.
Solution: Be sure the air velocity at intake of the system is greater than winding speed, check the edge guide unwinding roll, or make sure that offsets are not greater than trim width.
Problem: When rolls have a star pattern at the ends of the rolls it is due to the shifting of the layers of web, usually beginning at or near the core and continuing outward toward the outer wraps.
Solution: To remedy, start wind tight then steadily soften roll hardness as diameter increases. Also, make sure the cross-caliper variation is at a minimum, and oscillate before slitting.
Further Tension Control Troubleshooting
The above is by no means an exhaustive list of all the issues that can arise from improper tension control. If you are still having issues troubleshooting your roll winding defects, contact the web tension control experts at Montalvo today!