Q: What’s new with tension control?
A: The market push toward conveying thinner, wider webs at lighter tensions and higher speeds has placed emphasis on web handling components on the web processing line. These web handling components, including proper tension control systems, proper idler roll design, good web spreading and proper web guiding, are required to ensure consistent production of defect-free web materials.
Tension control systems need to be properly sized to control the requirements of each tension zone and web tension isolation needs between each zone. All unwinds need to have regenerative drives for tension control of the webs being introduced into the process. Then high-quality load cells sized for the low unwinding web tension with optimized web wrap and measurement of the resulting web tension force vector need to be provided. These properly sized and mounted load cells are needed on both sides of a lightweight, precision balanced tension roll in each tension section.
New idler roll designs enable more delicate web materials to be processed at lighter tensions while avoiding distortion, wrinkles and other issues. Consideration of the roll’s shell material, diameter, bearing type (live shafted or dead shafted designs) and the roll’s surface texture are all essential design elements based on application. Idler roll must be supported by high-quality, low-friction bearings. The roller spacing and alignment also need to be optimized to assure successful conveyance of the web.
Using the right type of web spreader can eliminate problems in areas prone to generating wrinkles and produce significant cost savings. Spreading devices should be used before all nips, before critical processes such as coating and laminating, and just before winding. Common web spreaders used on web forming and converting lines include: reverse crown spreader roll; flex spreader roll; herringbone grooved roll (aluminum foil); bowed spreader roll; and expanding surface spreader roll. It’s important to seek guidance on your best option based on application.
Edge guiding is needed on almost every web processing line. Consideration of the application is important as the guiding system needs to accommodate the maximum lateral web displacement. For OPG and 90-degree steering guides, the lead-in web length needs to be as long as possible because the steering action takes place upstream of the first shifting roller. On all web guides, the sensor needs to align to the web and be located as close to the shifting roll as possible. Finally, the shifting idler just before the sensor needs to have surface traction for steering without slippage.
Q: When purchasing tension control equipment, what major factors should be considered?
A: It’s important to consider several variables, including characteristics of the web, web handling roller design and process parameters such as speed, width and environmental conditions. Each must work in concert with the others. Winder drives and tension control systems are a key part of this. The ability of the winder to properly control slitting, spreading, and winding of properly tensioned webs depends completely on the winder drive (or drives) and the tension control system. It is imperative that electric motors and drive-control systems provide precise speed and torque control throughout the buildup of the winding roll. This control must be programmable to consistently reproduce the speed and tension profiles for all products and conditions to which the winder is exposed. An AC Vector digital drive is an absolute must as winding drives need to have a wide torque and speed range. Winding motors must also have sufficient cooling and good filter maintenance for high-torque requirements at low motor RPMs when winding larger diameter rolls of high-quality webs.
In terms of the tension control system, programmable winder tension for tapered winding tension as the roll builds is a must. This system needs to have dual load cells properly designed and mounted as close to the winding roll as possible.
Q: Where is tension control important in the process?
A: Tension control is essential throughout the conveying and winding processes in the polymer film, flexible packaging, paper and paperboard, and extrusion coating industries. No web is perfectly flat, which is why we need to convey and wind imperfect webs. This requires a certain amount of web tension to avoid defects such as wrinkles. The suggested amount of tension is typically between 10-25% of the web material’s tensile strength or the elastic limit for stretchy materials. The important thing to remember in tension control is that more web tension is usually not better. The goal is to convey and wind a web with the least amount of web tension as possible. Both TAPPI and SPE have done studies and gathered a great deal of empirical data on the suggested amount of tension. They have also published charts for suggested tension ranges for various materials.
Q: What are the factors that cause misalignment and how can it be avoided?
A: All web handling rolls need to be optically aligned to the equipment supplier’s recommended tolerances. If you have the right conveying, winding and web handling equipment for the application and follow amount of web tension recommendations, you can avoid wrinkles from misalignment and other issues. In terms of conveying and winding the web flat and straight through the process, the cardinal rule is to provide good web tension control and isolation between the unwinding web tension, the process web tensions and the winding tension.
Q: Is there anything else you want to share about web guiding and tension control or Davis-Standard in general?
A: Proper tension control and web guiding are both critical on high speed web production, coating and converting lines. The answers presented here only skim the surface of this in-depth topic. That’s why it’s important for converters to take advantage of available resources in order to achieve best practices regarding tension control. I would recommend the bestselling book from TAPPI Press entitled The Ultimate Roll and Web Defect Troubleshooting Guide. The technical paper, “Thinner/Wider/Faster Challenges in Web Handling,” is available from Davis-Standard. This technical paper as well as many more are posted on the Davis-Standard website and are available free of charge. Click on the “Tech Tips/Papers” button on the top right, register as a new user and request technical papers or Tech Tip that are listed.