Take control of tension

Tension control plays a vital role in composites manufacturing to achieve automated and continuous processing, reduced scrap, increased product quality and more, says a white paper released by the Montalvo Corporation. The following excerpt provides an expert view into developments and technical needs surrounding tension control.

Web tension is defined as the amount of stress or strain put on a web of material as it is moves through processing. Different materials have different properties and thus different tension requirements to ensure they maintain their quality. Too little or too much tension can create a variety of problems and process defects; stretching, wrinkling, breaking, wandering, delamination and more.
There are three primary types of tension control: manual, closed-loop and open-loop. The first, as the name suggests, is the process of visually or physically determining if the tension is correct, and then making manual adjustments to maintain the desired tension level. Controlling tension in this way means that a web is subject to constant up and down spikes as an operator constantly corrects the tension, and will fluctuate depending on the operator.
Open-loop systems use products such as ultrasonic sensors or proximity sensors to measure or calculate roll diameter. The controller uses any detected changes in roll diameter to regulate torque and maintain tension, as opposed to closed-loop systems, which receive tension feedback directly from the web of material.
Although not as precise as closed-loop, open-loop systems still provide automated, consistent tension control and outperform manual control. They are generally less expensive because no additional products such as load cells are needed.
Finally, closed-loop tension control refers to a continuous flow of communication from a web tension measurement device to a torque controller, then to a torque device. Web tension measurement devices are constantly relaying tension changes directly from the web to the tension controller, which may be a brake, clutch, or drive, to maintain the desired tension level.
It is often the case that, because of their manufacturing process or poor tension control, manufacturers are not able to run their material rolls all the way down to the core, which affects productivity and profitability. Web breaks and inconsistency in the end product also create more scrap, which equates to lost revenue and wasted material, as well as lost productivity. Quality tension control helps to eliminate these scrap products by creating a uniform and consistent process.

Multitow tension control

As the image shows, some applications may have several tension zones and require various types of tension control. By selecting a desired tension to be applied to each tow, for example 0.25pli, you can determine the tension setpoint required to run the tows exiting the NIP control by multiplying the 0.25pli with the width of all of the tows. If there are 240 tows coming from this bank of creels, and the width of 240 tows is 60in, you multiply 0.25(pli) with the 60in of width, or 15lb. When you run the tows through the NIP, the control system will monitor the tension and make minor adjustments to the torque device (the brake) to maintain tension at the selected setpoint.
The next tension zone is the top paper carrier rewind. Since this material is not that important -basically waste - the control does not need to be that precise. You only need to be able to wind it without breaking the web. An open-loop control system might be used in this application. An ultrasonic diameter sensor or proximity sensors can be used to monitor the diameter of the roll and increase the torque as it builds.
The next tension zone is downstream from where the top carrier sheet is pulled from the extruded product. The poly layer is extensible, so the tension applied to this layer needs to be precisely controlled to a fraction of that required for the paper. If the poly is pulled too tight, the web necks down. When the roll is rewound, even though the roll may look fine, the poly will, during storage or curing, try to get back to its relaxed state. This can cause wrinkles or starring of rolls. If the poly is not tight enough, again you may find wrinkles in the web after curing.
The last tension zone is the rewind zone - an open-loop control that again acts according to the increase in diameter as the roll is built. For this, you can use the less expensive open-loop systems to provide automated, consistent tension control, or closed-loop control - either would improve roll quality.

Laminator tension control


When laminating two or more materials together, it is important to control the tension applied to each web. The image above depicts three different materials in a process that will laminate into one web. The bottom layer is a 60lb basis weight paper that will probably run best at about 1.5pli - this means it is tight enough to withstand wrinkling or bagging but not so tight that it will break. If the web width in this application is 50in, we again multiply the width in inches by the assumed pli, or 75lb of total tension.
The final layer in this application is a 2mm poly film. The expected tension on this layer would be about 0.50pli. If closed-loop tension control is not used on these three different materials, there is a risk of damaging one or all layers of the laminated end product. If the film layer is run too tight, it will neck down and will not completely cover the other layers. The film may also try to relax after being wound into the finished product, creating wrinkles or delamination.

Controlling tension in rovings applications
As with tow control, where the tows are pulled from creels, a NIP station can be used to control the tension of each tow being pulled from the centre of the roll of material in a roving. A NIP station is the best option in this case, because no torque can be developed as the tow is pulled from the center of the rovings. It does not matter if any tension is applied to the tow as it enters the NIP; the tension on each tow will be equal and controlled by trimming the torque developed at the holdback brake. The operator will select a total tension setpoint based on the width and type of material running through the NIP.

Hand layup

Open or closed-loop tension control, or a combination of both, can be useful in hand lay-up applications. By using a brake on the paper carrier unwind roll and adding a drive motor to the rewind, it is possible to automate a hand lay application. By adding a drive motor to the wind up and a brake at the unwind roll, you can run slowly enough (1-2fpm) along the table for the pieces to be properly positioned.
By adding tension control to the rewind, it becomes possible to build the rolls with a consistent web tension. If a poly web layer is required, a closed-loop control system can be applied, and the insulating layer of poly can be added without wrinkles and will not neck down as the rolls cure.
Even if the web is not constantly moving, you can automatically run the carrier sheet the length of the table, stop the web and lay out the pattern, then wind up the web until the carrier is clear the length of the table, stop the web, place the pattern again and wind up that portion. Since everything is under control, the tension throughout the roll will be consistent. By controlling the rewind tension, either open or closed-loop, it is possible to build consistent rolls, which will ensure the roll goes to its core without wasted material or lost product.