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EXCLUSIVE FEATURE Web Cleaning & Static Control Taming the ‘invisible force’

Controlling electrostatic energy in paper, film or board processing has long been looked at as a ‘black art’. Here, Graham Dawson, of electrostatic control specialist, Arrowquint, looks at the role of electrostatic discharging in manufacturing

Edge pinning on film cast unit to minimise necking and reduce waste

Rarely considered by equipment manufacturers, and often neglected by process industries,

electrostatic discharging technology can be the key to faster, more economic production, says Graham Dawson, sales manager at Arrowquint. The company supplies a whole range of static control and elimination equipment, such as static elimination bars, static charging and discharging systems, ionising guns, blowers and nozzles. There are hundreds of applications for electrostatic discharging technology in manufacturing and other equipment, Dawson continues. Moving non-conductive surfaces on materials such as paper and board against each other produces electrostatic charges that can reach alarmingly high energy levels as speeds increase. These cause materials to stick together and, when high enough, will discharge to ground through the most direct path. Electrostatic energy is justifiably called ‘the invisible force’, as it cannot generally be seen or detected until it causes a problem. By this time, production is lost, costs are mounting, or an operator has been hurt. But how can electrostatic charges create problems and how can these be solved? Sheeters are a prime example of the

reversed situation where the forces produced by the electrostatic charge are more dominant than the forces of tension in the

22 November 2015

web itself. The moment a sheet is cut from the web, it becomes fair game for all the forces acting on it, including those applied by the machine and the electrostatic forces in the paper; the sheet may cling to the delivery roller or grippers, giving a poor stack or even jamming the machine. The solution is generally to fit a pair of ionising discharge bars above and below the sheet as soon as possible after the knife or last delivery rollers. In some instances, one of the bars may need to incorporate an air blower to help separate the sheet from the roller. Another process problem relates to lockups or noise in electronic control circuits due to electrostatic discharges from the web. Sparks are usually accompanied by difficult-to-

detect electromagnetic pulses which can crash a computer or microprocessor.

Operator safety Two common places where an operator may come into contact with a strong electrostatic field are near a rewinder reel and under a span of moving film. In the most severe cases, a discharge may happen between the operator and the charged film, leading to lengthy investigations for potential electrical safety problems. The stronger the field, the higher the induced voltage and the stronger and more dangerous the electric shock is. Fortunately, the probability of an operator being injured by an electrostatic shock is statistically minute, but this could cause secondary accidents as operators jump away from the source of the shock. The most effective way to minimise this hazard is to neutralise the electrostatic fields in the film at the rewinder and before any of the webs long free spans. This can be done using electrostatic discharging bars or ion blowers. A common mistake is to position ion blowers or discharging bars from 0.5 to one metre away from the web in the belief that it will discharge the whole of it. An ionising blower one metre away from the web is much less effective that one 50mm away, and will not have a significant effect on the levels of energy in a fast moving field.

Fire prevention

Solvent-based and solvent borne chemicals are still extensively used in printing, laminating, converting and coating processes. Solvents evaporate quickly and permit high processing speeds, and an electrostatic discharge from the web in the atmosphere could cause ignition and a fire or explosion. Fortunately, modern suppression technology generally reduces the effects of such fires to an inconvenience rather than a disaster, but the loss of production can still cause problems. Neutralising this static energy in an ‘Ex’

area requires highly specialised discharging equipment, which needs to be fully ‘ATEX’ approved and must give a positive and continuous indication of the effectiveness of the discharging system. System failures must be readily identified, whilst interlocks should be used where practicable.

Surface cleanliness In the simplest terms, electrostatic forces contribute to attracting and retaining debris on the surfaces of sheets and webs. Many web cleaners use electrostatic discharging before vacuum or blow-off systems, but forget to discharge the web after, meaning its new electrostatic charge will attract dirt. Electrostatic discharging systems represent

a relatively small part of the capital cost of most web cleaners and are often overlooked. However, their function in neutralising the force holding the contaminants to the web and in preventing re-attraction after cleaning is vital to the cleaning process.

... Continued on page 24.

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