By Pillar Technologies
A typical corona treatment system is designed to produce a corona in the air gap between the high voltage electrode and the ground roller. Corona will be produced anywhere there is air within this gap.
The occurrence known as “backside treat” is the result of a corona being produced on both sides of the web even though the treating system is only intended to treat one side. The resulting undesired treatment on the backside of the web can result in blocking and picking. It may also cause an insufficient treat level on the side of the web requiring treatment, since some the power intended for treating the web was delivered to the opposite side, thus, reducing the watt density provided to the side to be treated.
There are a number of causes for backside treat. All of which are the result of air being entrapped under the web being treated. One of the most common causes for backside treat is dirt buildup on the ground roll. Regardless if the system is a bare roll or a covered roll, dirt build-up gets deposited on the rolls surface. These uneven deposits lift the web off the rolls surface, entrapping air underneath it. The only way to eliminate this is with regularly scheduled cleaning. The frequency of the cleaning will be dependent upon the use of the treater on the amount of slip and by products that are produced from the substrates.
Another cause of backside treat is due to the increase in line speeds. As webs exceed the 600 FPM processing speeds, boundary layers of air travelling with the web develop. These boundary layers of air can cause the web to float over the ground roll, resulting in air underneath the web. This air ionizes in the gap, producing a corona and backside treat. This problem is most often resolved by adding a nip roll to the treater. The use of nip rolls tends to breakup any boundary layer of air that may be travelling with the web. Nip rolls can also squeeze out entrapped air, although treater nip rolls usually don’t involve high nip tension.
The use of treater roll coverings made from organic materials such as epoxy or rubber may also cause backside treat as the covering begins to deteriorate from the heat, ozone and UV exposure produced in the corona process. The deterioration is not very controlled and therefore, can produce an uneven surface. This is most obvious when running different web widths, and visible pattern of the different webs begins to appear on the roller. These uneven surfaces allow for air to become entrapped underneath the web, resulting in backside treat.
The surface finish of the ground roll may also be capable of causing backside treat. This becomes more common with harder roller coverings. Softer materials are usually more forgiving and will conform to the web as it passes over. However, the harder roll coverings are less compliant so the web can ride on the peaks of a rough surface. Lighter web tensions compound this problem.
Wrinkles are another cause of backside treat. There are a variety of causes for wrinkles and a number of possible solutions. The most common method of eliminating wrinkles is with spreader or stretcher rolls. These rollers can be in bowed or banana rolls; grooved rubber or idler rolls; or pinch or guide rolls. Regardless of the type selected, each is intended to eliminate the wrinkle. Another approach is to try and eliminate the cause for the wrinkle, which could involve roller realignment, readjusting web tension, improving gauge control, static elimination, etc.
Static build-up can cause backside treat without causing wrinkles. Thin gauge films, with a static charge on the surface, can be pulled off the rolls surface if the charge on the web is attracted to the charge on the high voltage electrode. The result is for air to become trapped underneath, causing backside treat. This is most common with thin gauged, light tension films treated on bare roll treater systems. Applying a roll covering to the ground roll can redistribute the voltage in the air gap and help keep the web on the ground roll.
Backside treatment can be the cause of expensive scrap. There are a variety of causes and an equal number of solutions. Most causes are the direct or indirect result of web handling problems.