Web Tension

Web tension is almost invariably a critical parameter when laminating soft and flexible materials.

Any type of processing that involves unwinding a material from a roll, performing some operation on that material (printing, perforating, laminating. etc.) and then rewinding it again is called web converting. The “web” is that portion of the material that has

been unwound but not yet rewound. In order to perform the planned operation on the web, it must be controlled – it must be guided for proper alignment to the operation, so that the printing, perforating, slitting occurs in the correct location of the web. This guidance relies heavily on proper web tension. With too much tension, the web can stretch or “rope” (the edges of the material roll inward) or even break. With too little, wrinkling occurs, or the material can move side to side.

When lamination is the planned operation, the issue of tension takes on additional importance. When laminating two or more substrates together, the web for each substrate must exhibit the same tension. Otherwise, the finished laminate may exhibit a linear curl resulting from feeding more of one material than the other.

Most web converting equipment has one or more features designed to control guidance and tension. One common element is adjustable braking on the unwind shaft(s) to impart drag on the web. Another common feature is a photo-electric or hydraulic edge guide, which senses the edge of the material and moves it left or right to align it (side to side) in the operation. A third common feature is a clutched (and overdriven) rewind shaft that attempts, within an acceptable range, to pull the material through the operation at a slightly higher speed than the operation itself – the clutch allowing the material to slip to match the operation speed.

There are other factors that can be important for web converting, and there are many (often custom) equipment features that are designed to meet the particular need, but we can, perhaps, address some of those at another time. In the meantime, be aware of web tension requirements for your lamination job. Does the material easily stretch or rack or rope, or does it pose particular challenges that must be met before lamination can takeplace.