“Controlling tension and wander on any web process is difficult, especially where the feed reel has variations in winding accuracy or tension. However, where the web material is non-conductive these difficulties are compounded by the forces induced by electrostatic charges in the web," explains Graham Dawson of Arrowquint. This company is the UK agent for Eltex Elektrostatik, Germany, a supplier of static control systems either as a new installation or retrofit. "Increase the speed, and demand a more accurate end product and web control can become very expensive to achieve," he says.
"Moving non-conductive surfaces such as plastics, dry paper and boards against each other produces electrostatic charges which can reach exceedingly high energy levels as speeds increase. These energy levels cause materials to behave as though magnetized, making them stick together or float apart. If the energy levels are allowed to rise too high they will discharge to ground through the most direct path, like lightning, often causing marking or pin-holing of the web."
He explains: "The relationship between static charge effects and web tension are dependent to a large extent on speed and overall tension. The effects of static induced forces of attraction or repulsion on higher tension webs moving between steel rollers are seldom seen, because the forces of tension in the web are far higher than the electrostatic forces trying to stick the web to the rollers. However, if web tension is allowed to reduce or is removed, the situation reverses and the electrostatic charges become the dominating factor.
"These static effects are also more noticeable where the web passes over insulating rollers, including anodised aluminium, where like polarity charges can build up and cause the web to float over a roller without actually making contact. This effect increases any tendency of the web to wander as well as possibly affecting the tension in the problem area. Similarly, opposite charges, or charged webs passing over grounded, conductive rollers can make the web stick to the rollers, increasing tension or causing snatching."
Just where does this happen? "Prime candidates for tension problems related to static charges are slitter-rewinders, where different materials run at different speeds and where speed is often increased and decreased progressively at the beginning and end of a wound reel. Printing presses also suffer from web wander, which can affect both print register and fold quality, whilst slit and merge systems also suffer to a lesser degree."
So what is the answer? “Static charges must be properly controlled at critical points in the web path; as well as discharging the web, this may mean using static charges to pin the web to a selected roller co prevent wander or float. Having successfully brought static levels under control, higher web speeds will be within reach without compromising quality."