Best Practices: Tackling Tension Control

Load cells, or transducers, like these from Dover Flexo Electronics, measure web tension.

It’s logical for printers and converters to expect that as a web moves through the entire printing process that it does not get wrinkled or stretched along the way, resulting in out of registration printing or unusable material.

But to avoid these pitfalls, the web must maintain an optimum tension as it passes through various points in the process. And because processes vary, multiple tension control options exist to fit the needs of any application.

Starting Points

According to Gavin Rittmeyer, VP of sales and marketing for Martin Automatic, a material undergoing a web fed process should be run at the lowest tension possible, so long as it is able to track through the machine and can take on all required functions.

For example, whether it’s flexographic printing or offset, the goal would be to have the web move through the process with the minimum amount of tension while still being able to be printed on.

“By doing so, you are not inducing energy into the web that can stretch the web or otherwise damage the web,” he says.

Tension controllers, like this one from Montalvo, adjust web tension to a set point.

In the printing process however, there are more essential functions than just printing, and each one typically requires a different tension. For example, the tension level needed in unwinding a web will usually differ from the tension level needed as the web passes through the press. And in many instances, there could be multiple tension levels required in different parts within the press.

Each individual area where a new tension is needed is commonly referred to as a zone, and to ensure the proper tension can be implemented in each zone, they need to be distinctly isolated from each other with nip points.

“If I have 100 lbs. of tension at my unwind zone, but my process zone requires 80 lbs. of tension or 120 lbs., I have to be able to isolate and manage that,” explains Doug Brockelbank, technical manager for the Montalvo Corp., a web tension control product and service provider.

Understanding Zones

Because the goal for each zone can be different, the method of applying and controlling tension can differ between each one. For example, starting with the unwinding phase, Brockelbank explains that a constant tension level is desired as the web exits the unwind stand and enters the process phase.

But, because the roll diameter is continually decreasing, tension will tend to increase and will need to be compensated for. As the web passes through the process zone, such as a printing press, it is easier to maintain a consistent tension, as it is a less dynamic zone, without changing diameters or greatly varying torque loads.

However, where an operator will want to have varying tension within a zone is during the rewind process. Glyn Green, technical support manager for Dover Flexo Electronics, explains that as a roll of material is wound, it will slide on itself. If this happens during the rewind process, unwanted effects like starring or telescoping could occur on the final roll, rendering it useless.

“As the roll grows, the torque increases,” Green says. “But if you taper that tension, you have less increase in the torque applied to the material as it winds up.”

Keeping Control

Because running a web at an improper level of tension can cause severe problems in the printing and finishing processes, tools are available for printers to ensure the tension requirements for any specific job are met and maintained.

One way tension control can be implemented is through the use of load cells or transducers, which can provide a signal to a controller. Prior to the job, the controller is set to a desired tension level. If the tension in a certain zone is not at that level, the controller provides an output either increasing or decreasing the tension to that set point.

“The real idea in tension control is to be able to give the machine operator one less thing to worry about,” Green says. “When he tells the thing what he wants for running tension, it does the job for him. He doesn’t have to keep running back and adjusting something.”

Brockelbank explains that controllers can be particularly helpful in a dynamic tension zone such as an unwind, as they help ensure constant tension as the reducing size of the roll will naturally cause tension fluctuation.

But in process zones, where there will be minimal disruption to a web, small tweaks from an operator can be all that’s necessary to control tension. But even in situations where a controller may not be necessary, Brockelbank says using a tension indicator to display the actual tension in a zone can help a converter better understand what is happening in the press.

“In the process zone, there may be several points within that zone where I want to at least indicate, if not control, what the tension is,” Brockelbank says. “Tension indication is very important. It’s information about what the web is doing. Without a good way of doing that, it can be difficult to pinpoint problems or issues that are going on within the machine.”

An infeed tension control device from Martin Automatic.

Rittmeyer explains that Martin Automatic believes the best way of controlling tension is with an inertia compensated dancer system. Rittmeyer says that with an inertia compensated dancer, which is free to move, disturbances in the web can be absorbed and feedback can be provided to a drive or brake system, which can then compensate for increased or decreased tension in real time.

“It’s an integral real-time device that filters any tension disturbance and prevents that from passing into the next phase of the printing press,” Rittmeyer explains.
The major difference, Rittmeyer adds, is that with a load cell and tension controller system, a correction is made after a tension change is detected. With an inertia compensated dancer system, any tension change gets filtered out in advance, before the web moves into the next zone.

Whether a printer or converter prefers a dancer system or a load cell-based control system, Brockelbank explains tension indicators are always beneficial. He says that by understanding tension levels and maintaining consistency, printers will likely see better results.

“Shift to shift, if you have three operators on the same machine, you can get different results on the same product because you don’t have tension indication on there,” he says. “It really helps to get process consistency. The most important thing you need to know is what your tension is and most people don’t know it. Well, how do you find out? You need to indicate it.”