WHAT IS STATIC?
Static electricity is the result of an imbalance between negative and positive charges in an object. These charges can build up on the surface of an object until they find a way to be released or discharged. … The rubbing of certain materials against one another can transfer negative charges, or electrons.
Static build-up is the phenomenon wherein electric charges are exchanged between the surfaces of two objects that come into contact with each other.
In this process, one object takes on a positive charge and the other a negative charge. It is because of this that static electricity builds up on the surface of objects.
TYPES OF STATIC BUILD–UP
Static build-up consists of the following three major types.
Contact Static Build-up
This is the static build-up that occurs when two objects come into contact with each other and electrons are transferred from one object to the other.
Frictional Static Build-up
This is the static build-up created when friction occurs between two objects..
Detachment Static Build-up
This is the static build-up that occurs when items like adhesive tape and protective film are removed.
As with frictional static build-up, the principle behind this kind of static electricity is the same as that for contact static build-up.
The tape or film is in very close contact with the object so the effective contact surface is large. The amount of static electricity generated is overwhelmingly greater than that of contact static build-up.
CAUSES and CONTROL of STATIC Electricity
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.
For example, coated web papers, manufactured to lower moisture content, may be more susceptible to static charge when converted to run sheet fed.
Pressure and Contact
The greater the contact area between paper and other surfaces, the higher the potential for electron exchange and static build. Friction, itself, does not create a charge, but processes that increase friction also increase contact. Thus, large format, lightweight, lower stiffness coated papers are more susceptible to static charge. Exceptionally smooth, gloss-coated papers with greater contact area may be especially difficult to separate, not only when a static charge is present, but also when tightly pressed together.
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.
Combination effect
When combined, different materials or objects with variable levels of static charge can create a cumulative effect causing very high static charge.
Electro-static locator meters can measure the amount of static electricity Present and identify the polarity as either positive or negative. Without a meter, the presence of static within a load of paper can be determined by
removing approximately 30 sheets from the top of the pile and then slowly sliding the next sheet across the surface without lifting it. If significant resistance is felt, the presence of a static charge is causing a material attraction.
The potential for static charge can be determined by rubbing the top sheet against the pile surface. This increase in surface contact can simulate the potential for building a static charge during the paper converting or feeding
process.
OPTIONS AND SOLUTIONS
Both the presence and potential for static electricity in paper can be effectively managed by a combination of effective grounding, ionization, and proper humidity / temperature control.
Grounding
Plant machinery should be grounded to a steel rod driven into the ground, Ultimately as deep as 8 feet, depending upon voltage currents, local electrical codes, and manufacturer specifications. A properly grounded machine will
help dissipate high charges of static electricity from semi-conductors such as Coated paper.
When properly grounded, metal core tinsel can dissipate static by induction. The tinsel must be tightly stretched approximately 1/8″ from the paper surface without contact. It is best placed across the pile at the feeder head at
the point of sheet separation and across the feed board where adequate air space exists under the sheet. The tinsel should then be attached to a properly grounded press or other conductive object.
Ionization
Electronic static eliminators, or neutralizers, use a high-voltage discharge to ionize the surrounding air; the ionized air acts as an electron source which, in turn, neutralizes the static charge on the paper.
In-line air ionizers are simple to install in the feeder blower line and very effective if installed properly. As the air blows between the sheets, the paper surfaces build up similar, or like, charges that now repel each other, enabling sheet separation. It is recommended that these in-line ionizers be replaced annually. Be sure that air blower filters and blower heads are clean with adequate air pressure and distribution to insure optimum sheet separation.
Ionizing string and/or elastic cord dissipates static electricity with tiny conductive micro-fibers that cause the static charge to ionize and flow to the ground. It is easily placed in multiple locations both over and under the sheet, but is most effective in removing static as it emerges from the stack or roll.
Anti-static or silicone sprays increase conductivity and minimize excessive contact from friction by applying a thin silicone or soap-based coating to the machine components that transport the material. Application to contact
points such as the feed board, transport tapes, and wheels can be helpful.
Humidity / Temperature
Adequate moisture in a conditioned pressroom increases conductivity and helps dissipate static charge. A properly climate-controlled facility includes humidification that maintains 45% (+/-5%) Relative humidity (Rh) @ 72° (+/-5°) F. ( Approx. 23 degree C) Ambient relative humidity below 35% Rh can adversely affect paper and decrease conductivity, which results in greater potential for static.
Paper, properly acclimated to recommended pressroom temperature, has a better chance of running static-free as compared to cold paper.
To maintain the relative humidity of paper consistent with original moisture content, do not unwrap paper any sooner than necessary before going to press. Paper can be properly acclimated to pressroom temperature without unwrapping.
Avoid storing paper in proximity to extreme hot or cold sources and minimize exposure to high heat in the drying process which may adversely decrease moisture content. Keep press IR dryers to a minimum with pile temperatures
not to exceed 95° F. / 35° C.
If static and smooth-surfaced sheet cling make feeding difficult in a properly controlled pressroom environment of 40% Rh or greater, it might be helpful to gently hand-wind the paper and rebuild feeder loads just prior to running on press. This consideration may improve sheet acclimation and separation but be sure to minimize any potential for surface friction during the process.
Static is a common problem in web heatset sheeting and post-press converting due to the extremely low moisture content of the paper after drying at high temperatures.
Moisturizing the web after the chill rolls with a mix of water, silicone, and liquid fabric softener can increase conductivity and reduce surface contact from friction, ultimately minimizing static while helping the paper re-acclimate to ambient environment.