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 explained: 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.
Many print finishing companies use Eltex static nailing systems to improve both running speeds and product quality on their Corona binders by preventing signature slip during the gathering and binding processes.
Renmar is able to provide specialist static pinning heads and customised brackets as well as generators and distribution systems to enable the static pinning systems on individual feeders to be connected or disconnected at will, giving the flexibility necessary for a modern, high quality, reactive bindery.
Renmar static pinning systems enable print finishers to increase capacity as well as improving both throughput and quality to meet the needs of a highly competitive market.
Discharging explained: 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.
Discharging of static electrical charges, or Static Elimination as it is often called is an essential component in many production processes in the printing, plastics and packaging sectors.
Static Discharging (Static Elimination) can be applied in 2 forms: Passive or Active.
Passive discharging generally uses grounded discharging points or conductive brushes (Carbon Brushes) to neutralise static charges on moving webs or sheets of paper or plastics. This method is unlikely to completely discharge or eliminate static charges, but will reduce by 50 – 70%, depending on the magnitude of the charge. However, contrary to common belief, carbon brushes should not contact the web or sheet but should be positioned 2-3mm away from the surface to allow an ionising cloud to be created in the air gap. It is this ionised cloud which neutralises the static charge.
Active discharging is used where a more complete discharge or elimination of static charges is required, or where discharging is needed in an area with an explosion risk. Active discharging uses a series of points charged to a high voltage (Typically 5-8kV) which create a stream of ions flowing towards the charged surface to neutralise the static charge. Most active systems use a mix of positive and negative ions to ensure that charges of both polarities can be neutralised with same electrode or static bar. In some applications, active static discharging is be used to create a larger cloud of ionised air inside an enclosure or tunnel to discharge irregular shaped objects such as injection moulded products. In others, the range of the ion stream is extended by mixing it with an air stream to discharge more distant objects such as FIBCs or unwind and rewind reels.
Renmar is able to supply a large range of both active and passive static elimination, or static discharging systems. We also supply bespoke systems for specialised or difficult applications.