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How to handle electrostatic paperchargesProblemElectrostatically charged paper can cause any number of problems during print runs, from poor performancein general to blocking and misfeeding. With some basic understanding of the phenomenon, however, and ofthe circumstances under which electrostatic charging is likely to occur, most of the common issues becomefairly manageable.Definition – What is static electricity ? • Identical charges reject, opposite charges attract. Paper and press infeed have different charges. • Static electricity is the result of charge separation. When the air, or the paper itself, is too dry, the discharging rate of the paper is diminished, causing sheets to misfeed. • Paper is an insulator, which makes it a non-conducting material (a condensator). A natural papers surface resistance is around 1010 Ohm. • At sheet separation, charges are separated as well, resulting in high field strength charging. • There are essentialy different types of charges – parallel or opposite. What to do in a specific case, in other words, the choice of remedies, depends very much on the type of charge. • How and to what extent paper becomes charged, is influenced by a number of variables – paper moisture, climate conditions and run speed for instance. Each paper also has specific electric characteristics, which play a part as well. • The dryer the paper, the better it insulates. What this means, is that the surface resistance is increased and that, as a result, the papers ability to discharge is decreased. • Statically charged paper has a better chance to discharge at modest running speeds. It doesnt additionally charge as fast as it would at high speed, so the existing charge has more time to dissipate. • Another important factor is the papers smoothness, in combination with its stiffness. Smoothness determines the number of contact points with other materials, in the infeed, or with the following sheet. In general, a relatively smooth paper will have more contact points. • The other thing is stiffness. Papers of relatively low grammages are less stiff than "heavy" papers, which means that they are also less resistant to electrostatic forces. In general, higher grammages mean more stiffness. The digital universe of paper and printing knowledge 05/03/13 ● www.sappi.com/houston
Charge or no charge?When the paper is cut into sheets, vertical pressure of 2 to 3 tons is applied to the stack in order to removeair. This also causes the sheets to slightly stick together, which is often, but mistakenly, attributed to staticcharge. The only reason the sheets stick together, in this case, is the absence of buffering air layers.Solutions, remedies and precautions 1. Always carefully measure and manage moisture levels. The moisture percentage of all papers should be in the range of 48 to 55%. 2. Infrared drying is used as part of many printing procedures. IR heat however, though drying is obviously the purpose of its use, can cause excessive loss of moisture in an extremely short period of time. 3. Temperature in the print room should be at approx. 23 °C, with a relative humidity of around 50% to 55%. 4. In case of very dry air in the print room, atomizers can be used for moisturization. 5. Powder can be appled to the paperstack in order to keep the sheets from sticking. This powder, which is blown between the sheets by the front blowers of the feeder, acts as a separation layer. 6. Before use, always allow the paper time to adapt to the conditions in the print room. It should be left in its packaging for at least 24 hours. 7. To increase the air humidity in the direct environment of the press, water can be applied to the floor around it. The digital universe of paper and printing knowledge 05/03/13 ● www.sappi.com/houston