How Winding and Web Handling Are Affected by Roll Hardness, Tension, Materials
There are so many variables to consider when running a press that it may be easy to overlook one of the most basic assumptions: That the substrate will get from the front of the press to the rear with nothing changing except for the image printed on it. Every printer knows that is something to not take for granted, and the proof lies in the need for winding and web handling control.
As explained by Flexography: Principles & Practices (FP&P) 6.0, “Web handling involves the physical mechanics related to the running and control of a continuous roll of substrates,” and a primary goal of web handling is “to transport the substrate through press without incurring defects and losses.”
Material Conditions
All manufactured goods have some level of variance to them, and as a result, it is not especially uncommon to find materials with manufacturing faults. Specific to working in rewinding, there are several conditions to expect to encounter:
- Gauge variation: Gauge variation is a change in a web’s thickness, either from edge to edge or along the length of the material
- Gauge bands: A sudden change in web thickness over a small width is a gauge band. A gauge band runs parallel to the web’s primary axis. They can exist for a small portion of a web or as much as the web’s entire length
- Slack zone: A slack zone stems from uneven production of the original material. It can be visualized by laying the web on a flat surface, cutting the web into strips parallel to the main axis and comparing the resulting pieces. Strips cut from the slack zone would be longer than those from the normal/tight zone
Fortunately, when material is supplied, it should include a document which notes the gauge variation and gauge bands.
Roll Hardness
A roll’s hardness is largely dictated by the amount of air trapped between the wound layers of material—less air means a harder roll. That amount is determined by several factors, namely the material’s permeability (more permeable material will allow air to escape during and after winding), how rough the material is (a coarser option will better retain air) and its residual tension (a tighter wound roll will force air out).
When a material is wound, a layer of air comes with it, and that layer travels with the roll when the web is slit and rewound. It is important during the rewinding to control the amount of air that seeps into the roll; if the layer of air becomes too large, it will act as a hindrance to the material’s layers laying properly on each other. In general, increasing the speed of the rewind will suck in more air between layers, causing quality to deteriorate.
The ideal wound roll takes a Goldilocks approach: It contains neither too much air (as a soft roll is difficult to transport) nor too little air (the absence of the air layer can cause damage to the material). A soft roll becomes especially problematic when it is stored vertically and an operator attempts to lift it by the core—A roll with too much air trapped between layers will unwind.
Contact Force
To control the amount of air that slides between a roll’s layers, a layon roll is used. The layon roll applies contact pressure to the material, forcing out air as it is wound. This can work in both directions; the layon roll, by reducing pressure, can “add” air to the roll as well. Two variables are considered when adjusting contact force of the layon roll:
- Rewind roll diameter: As the material continues to be wound, the diameter of the roll will naturally increase. As that diameter increases, the force applied by the layon roll is spread over a larger area, rendering it less effective
- Machine speed: A faster machine delivers more air to the surface of the material, while also decreasing the window of time for a layon roll to remove that air. As a result, the contact force must be increased as the machine’s speed increases