Offset printers know very well what this problem represents: printing sheets stick together because they are electro-statically charged. This condition may result in feeder trip-offs, print voids from surface contamination, ink offset, or poor sheet jog in the delivery. Static electricity arises from either an electron deficit (positive charge) or an excess of electrons (negative charge) on the surface of the paper.
- Improper Grounding – This results in the inability of the metal parts of the printing or converting equipment to effectively neutralize charges. It will not eliminate static electricity, but properly grounded machinery will help drain-off and dissipate large charges, making the problem much more manageable.
- Low Humidity – Exceptionally dry ambient and/or material moisture levels can aggravate static build. Adequate moisture is necessary to help dissipate static by raising conductivity. Without moisture, air and coated paper are basically non-conductive. Low moisture paper, either by design or due to adverse exposure to a dry environment, can be especially problematic.
- Temperature – Winter months and colder temperatures bring dryer climates and higher potential for static build. Thinner air at higher elevations also tends to be dry and does not consistently hold temperature resulting in humidity fluctuations. In addition, colder paper tends to be less conductive and more prone to static build than warmer paper properly acclimated to recommended pressroom temperature.
Options and Solutions
- The moisture content of paper should range between 45 % and 55 %. The electrical conductivity of paper, and consequently electrical run off, increases with increasing moisture content.
- Working spaces should be air-conditioned or humidified at a 50 % to 55 % level of relative humidity. Particularly during colder times of the year in heated rooms, the atmospheric humidity can sink dramatically.
- Antistatic equipment such as discharge electrodes and ionic blowers reduce electrostatic charge; ionization equipment raises the surrounding air’s electronic conductivity, thereby preventing the build up of static charge.