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Charging - Discharging

Aufladung - Enladung

Uncontrolled electrostatic charges are always undesirable and impede the efficiency of manufacturing processes. On the other hand, controlled electrostatic charges – purposely applied – improve manufacturing efficiency by using controlled electrostatic properties to accomplish something beneficial in the process.

Charging – invisible, but present
The most frequent form of undesirable charging is contact charging. It mainly involves two material bodies whose boundary layers are in contact with each other – in our example, the surfaces of a film web. High-speed unwinding causes the film surfaces to separate at very high speed, with the process described on the previous page setting in. The result: the plastic film charges up. 
The currents developed during this process may be almost negligible, but the voltages may be several million high. Even if there is only a split-second between contact and separation, relaxation (i.e. charge decay) may take days. One thing is certain, however: discharging is necessary.

Discharging: the wrong way – and the right way
A static high voltage is essentially “trapped” on an insulator (such as plastic film), because there is no electrically conductive path to relieve it. Discharging the wrong way means that something “physical” spontaneously becomes that conductive path – and current flows through it to neutralize the charge. A worker accidentally becomes the discharge path for static on a plastic film web and “gets zapped.” Or, the film discharges against a part of the machinery.
Discharging the right way eliminates charges immediately after they develop. The charged material is exposed to antistatic bars that fill the air with ions carrying positive or negative charges. These ions safely neutralize the static charge on the material, preventing hazardous spontaneous discharges.



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