The requirement to nip or to drive the Corona treater roll is more often dictated by machine or substrate characteristics than by technical treatment demands. For example, treatment of substrates, especially ultra light weight substrates running under light tension, is seriously complicated by the tendency for these substrates to wrinkle under the corona discharge. The wrinkling not only causes a problem with the quality of the wound roll, but also produces backside treatment which can be detrimental to the final product.
Wrinkling and the resultant back-side treatment can be reduced by increasing the amount of wrap and tension of the substrate on the treater roll. For minor wrinkling problems this may be sufficient. For more serious wrinkling, the addition of a nip roll on the treater roll where the film enters the station can eliminate or mitigate the problem. The addition of a nip on the treater roll will require that the treater roll be driven, which will require a drive shaft on the roll. This may necessitate a complete rebuild or replacement of the station.
If there is any possibility that wrinkling may be a problem in your application, it is best to plan ahead and provide space for the inclusion of a nip roll, even if you are not having the nip roll installed immediately. In this case a drive shaft should be provided on the treater roll.
The majority of corona treatment stations do not require the treater roll to be driven. Therefore, most stations are not provided with a drive shaft. Even light substrates will provide sufficient friction and tension to rotate the roll at line speed. Even if the roll does not rotate when the substrate is moving and treater power is off, the roll will rotate when treater power is applied. This phenomenon is referred to as electrical pinning and is used by companies manufacturing what are called electrostatic pinning bars. These devices are sometimes touted as electrical nips, but they are not nearly as effective as a mechanical nip roll. Their real benefits are that they are small and can be added easily after the machine has been installed; but they are not replacements for nip rolls.
Since the corona discharge provides electrical pinning of the film to the treater roll, one might ask, “Why doesn’t it prevent wrinkling?” The answer is that even though the corona discharge will provide pinning power, in may cases, that pinning is accompanied by static electrical forces that cause the film to wrinkle. This effect is enhanced by the fact that the roll is not rotating at a rate exactly matching the line speed. Driving the roll will therefore reduce the possibility of wrinkling.
Driving the treater roll and, in some cases, adding a nip roll also can also be required when working with substrates that are highly extensible. These substrates require highly uniform and tightly controlled tensions throughout the process. Using the substrate to drive the treater roll can cause an unbalanced tension from one side of the station to the other and this imbalance may be sufficient to stretch the substrate. In some cases the machine design will require that the treater station be constructed as a ‘pull-roll.’ This requires that the treater roll be nipped and driven and provides the machine with a tension control point.