Uncontrolled electrostatic charges are always undesirable and can impede the efficiency of manufacturing processes. On the other hand, controlled electrostatic charges – purposely applied – improve manufacturing efficiency by using electrostatic properties to accomplish something beneficial in the process.
Charging – invisible, but present
The most common 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 plastic film. High-speed unwinding causes the film surfaces to separate suddenly at a very high speed, which triggers the process described on the previous page. The result: the plastic film is charged. The currents developed during this process may be almost negligible, but the voltage levels may be several million volts. 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, and this requires charge equalization.
Discharging: the wrong way – and the right way
If there is no electrostatic monitoring, the following scenario can occur: A static high voltage is essentially “trapped” on an insulator (such as plastic film) because there is no electrically conductive path to remove 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. Both of these discharge processes must be avoided. Electrostatic systems eliminate all risks. 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 spontaneous and uncontrolled discharges.