- September 01, 1998, Mykytiuk, Andrew
Static electricity is described as a nonflowing electrical charge that builds up on nonconductive or ungrounded conductive materials. The most common causes of static are friction, pressure, and separation. Simply put, static is the exchange of electrons between two materials. When a nonconductive material rubs against another, it deposits electrons from its surface onto the other surface. The material that loses electrons will become negatively charged, and the surface that gains electrons will become positively charged.
Converters who work with roll materials have a first-hand familiarity with static electricity, primarily from the occasional shock. These shocks, though disconcerting, are not dangerous, because the current in the static shock is too low. But the uncontrollable "jerk" away from the shock has been known to cause an operator to move a hand into a moving roller, fall off a scaffold, or drop a tool into the machine.
Less well known but equally or more troublesome are a host of other static-induced problems: poor sheeting and stacking; the attraction of dust and debris; disruptive effects on sensitive electronic equipment; webs that slip and slide out of register; webs that cling to rolls or jam the machine; the reduction of line speeds in an effort to reduce static buildup; and the possibility of explosions caused by static electricity in certain hazardous environments. All of these can be very serious, bottom-line-busting problems.
Scott Kessler, sales engineer for Pillar Technologies, reports that static electricity can turn a roll of film into a "dust magnet," literally pulling dust particles out of the air. "Converters today want to run higher quality products at higher speeds, and dust will lower the quality of laminated products, coated products, and printed products," says Kessler. "The bottom line is that static costs the converter money."
Fortunately, there are several effective ways to control static. The first, and least expensive, is to ground all machinery. Another effective passive solution is to string copper tinsel across the width of the web. Tinsel-significantly reduces static charge, but it's often not sufficient to completely eliminate the entire static charge, especially if it's less than 1,500 v.
"Passive is good for knocking the charge down," explains Andy Grzesik, applications engineer for Tantec Inc. "The problem is that all materials have the ability to store a minute electrical charge. That small charge may not give an operator a shock, but it may be strong enough to attract dust and debris to your web.
To completely eliminate static, converters often resort to what is called an "active" approach. The most common means is through the use of ionized air. Static bars use high voltages to ionize air. When used in conjunction with tinsel, static bars will completely eliminate static.
A cloud of ionized air created by a static bar literally makes air conductive and carries away the charge. The effective range is limited to only 1.5 in. requiring the bar be mounted as close as 0.5 in. If the bar is not mounted close enough to the web, the ions will recombine with themselves before they've had any effect on the static. If for some reason the bar cannot be mounted close enough, an ingenious invention called the air-knife uses compressed air to blow the ionized air wherever it's needed.
Ionizing systems work; the main problem is that customers often don't know where to mount them. For example, the static charge can be eliminated at the unwind, but, due to friction downstream, static can rebuild. Where the ionizing system is placed is critical for efficient operation, as is the maintenance of the system.
So, how does a converter know where to mount the ion bar he ordered out of a catalog? Most companies that sell static control equipment offer what they call a "static audit." Company representatives will help you locate where the charge is being generated, recommend the proper system, and give complete instructions on how to mount and maintain the static bar. This is an extremely important service, since no two applications are the same.
On page 236 of our June '96 Buyers Guide, we list 46 different suppliers of static elimination equipment. Another excellent source of information is the Electrostatic Discharge Assn. This group is developing industry standards and can be reached at 815/394-0741.