Types of control devices.
Given that static is generated when two surfaces separate, it can be created at a number of points during the manufacturing or converting process. Even if you correct the cause of static being generated at one point, it can be generated at the next point where two surfaces separate. That is why if the material cannot be made conductive enough through the use of an anti-static ingredient, you must control static at any point where it is being generated. The field meter will help you identify these points so you can install a control device.
When it comes to control devices, there are two basic types: passive and active. Passive devices include; tinsel and conductive brushes, which are relatively inexpensive. Active devices include electronically powered bars and blowers, which cost more. FLEXcon uses a combination of these approaches to reduce the likelihood of static.
In terms of passive devices, the material generates a charge that ionizes air, making a conductive pathway to the ground. As the device drains the charge, the static is dissipated. When the film’s charge is no longer strong enough to ionize the air, the device is no longer effective. This is the main limitation of passive devices: it will reduce the charge, but it will not eliminate it.
Passive devices work best when they do not touch the web. If they do touch the web, static dissipation is limited to the contact point. The tinsel or brushes should be placed between a ¼ inch to a ½ inch from the film’s surface, and about three to six inches from the separation point. They should be on the same side as the separation to dissipate the surface charge rather than directing it through the web.
Most active devices are powered by electricity. They have a series of charged points surrounded by a grounded grid. The voltage between the points and the grid is enough to break down the air into both positive and negative ions. These ions float to the web and combine with the positive and negative charges on the surface, canceling out the surface charge. These devices can reduce the surface charge to zero. More expensive models can be made explosion proof as well.
Power bars are placed between one inch and three inches from an unsupported web. Blowers use air flow to extend the range of ions to between two feet and four feet. These are particularly effective with unwinds and reels where the web changes position as time passes. The closer the blowers can be placed to the web, the more efficient they are. That’s because the ions will recombine and cancel each other’s charge as they travel toward the web.