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Inadequate exhaust ductwork design. A faulty exhaust system design has the tendency to provide far more static pressure on the exhaust fan than desired. The result of this is reduced air volume exhausted from the treater station itself. This in turn does not trap and capture as much of the ozone generated in the corona treating process as needed, but merely allows it to escape out and around the station itself; thus not removing it from the workplace. Also, assure that the final exhaust point of the ozone is exhausted at least 10’ above any roofline and is not exhausted near any fresh air or make-up air units.
Continue with the scheduled maintenance as instructed within each equipment manual. Inspect and maintain cleanliness of both the ground roll(s) and electrode(s), as well as eliminate broken web or loose substrate near the exhaust capture ports of the station. Inspect all high voltage connections. Assure proper exhaust/cooling air to eliminate issues associated with fugitive ozone emissions.
Back-side treat is caused by air getting in between the web and the ground roll. This in turn is caused by many things, some of which are: Dirty treater ground rolls, nicks or deterioration of the ground roll dielectric material, out of round rolls, and misaligned treater stations when installed, increase in line speeds where a nip is not present. We have also seen this to be a problem due to static charges with thin gauge films. Keep in mind, when back-side treat occurs most problems are generally do to web handling issues.
In general, tension is controlled via line speed analog reference signals (4-20ma or 0-10VDC) that assure the treater ground rolls are moving at the same speed (actually a fraction faster) as that of the product off the extrusion line or printing press. The web tension part is still with the same signal but is monitored additionally with a load cell on an idler roll or such. This load cell device is designed to sense a change in tension of the web via up and down movement of the web. This slight web movement is sensed with a small percentage of the corresponding analog signal sent back to the drive to adjust its speed up or down accordingly.
Although there are numerous theories explaining the principles behind the process of corona treating, the most commonly accepted theory appears to be the theory of high speed oxidation. Basically, this theory states that the energy of the corona breaks the molecular bonds on the surface of the non-polar substrate. The broken bonds then recombine with the free radicals in the corona environment to form additional polar groups on the film surface. These polar groups have a strong chemical affinity to the polar inks and adhesives, which results in improved adhesion. Similarly, the polar surface results in an increased surface energy which correlates with improved wettability For more information, read out article on Corona Treatment Theory.
Without the proper tension wrinkles could develop allowing air onto the back-side of the web. Anytime air gets up anywhere behind the web (e.g.; in between the web and the ground roll of the corona treater) back-side treat can occur. With this phenomena occurring, treatment would be seen at a lower dyne level on the primary side and some minor treat on the back-side. This obviously leads to inconsistent ink adhesion and poor printing capability.
Establish a continuous quality product by assuring assure that the resins are added in proper proportions prior to extrusion. Purchase quality resins and train your plant personnel to note the need for a change in the power levels of the corona treater power supply. Begin your process by establish a beginning point for the power level and work your way up on power till the anticipated dyne level is achieved; this being done through QA dyne checks of the film. Once the power level is established for the given product at the given speed, note the power level, re-use it next time, and you are assured of desired dyne repeatability. Read more about covered and uncovered roll treatment.
Over treating is not as common as under treating, but can occur as well. Mostly PE’s can be over treated quite easily, especially when treating in-line with the extrusion process. When this occurs the film can reach a dyne plateau. This plateau can actually be less than the actual treat that is achievable when not overpowering with the power supply. With this said, over treating is indicated by “blocking” or after the initial treat (at too high a power level) a dyne level reading that cannot achieve what is expected for the given film. This ultimate dyne level can vary as well based on the additives in the PE resin. The additives (slip, TIO2, carbon black, etc.) can make the film much more difficult to treat.
Under treating also gives you readings of lower than as anticipated dyne levels for the given film and additives. So you see they could look identical. Production personnel need to work with their plants Quality Assurance to check what the applied power levels were at the given treat speed to determine which problem is occurring.
Blocking, poor adhesion, low dyne levels, not adequate wettability are all problems and indications of either over or under-treated film.
Corona treatment is a common method used to increase the surface energy of plastic by means of a high voltage electrical discharge, thus improving its wettability and adhesion characteristics for printing and laminating.
Dyne checks with proper uncontaminated dyne solution via their plant QA.
Different inks require different acceptable dyne levels for different types of substrates. A very “general” rule of thumb is that the substrate should be no less than 10 Dyne higher in surface energy than that of the surface tension of the ink or adhesive. Certainly also printing personnel should pre-check the dyne levels of the substrate via dyne solution. Application is with a single soft cotton tip applicator (Q-Tip), used only once. Dyne pens are not reliable as they could become contaminated.
We have charts, based upon the past 40 years of experience, that will help estimate the right watt density for your application. If your material is rare, or if you wish to test the watt density, contact your Pillar Sales Representative to schedule a sample run on our line. This way, you will know the exact watt density for your application and ensure your system is neither over sized nor under sized.
Proper Size Charts