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FLEXO Magazine : September 2010
Technologies & Techniques Often times, pinholing is caused by the substrates. A coarse paper or board surface will absorb much more ink than coated papers. As the ink gets absorbed, the color or density will be lighter and we see more pinholing. Assuming water-based ink is being used, if enough attention is paid to pH, generally a higher pH (9.2-9.4) will assure good ink coverage. When pinholing happens on printing on film substrates, the surface tension must be identified. Was the film properly treat- ed? Is there any coating or wax residue on the film surface? When printing wide web film, the condition and cleanliness of the CI drum must also be known. On older presses, the sur- face of the drum may be slightly pitted. The uneven surface of the CI drum will show through the thin film many printers use today. All too often I see old ink on the drum surface, dust, or even powder, if anti-block oxidry powder is in use in the plant. This will cause pinholing or truly fisheye-like circles to appear. Determine the Cause As it is true with any press troubleshooting techniques, we are expecting our operators to be well informed about all fac- ets of the press conditions. So, if pinholing appears unexpect- edly, during setup or in the middle of a pressrun, there must be a clear next step to determine what has changed. Assume that the same or similar job that ran before did not exhibit the same kind of pinholing. • Is the ink of the same viscosity? • Is the anilox roller the same, screen count and volume? • When was the last time the anilox roller was cleaned? • When was the last anilox roller audit done? • Is the doctor blade adjusted properly? • Is the plate used of the same thickness, same durometer, or even the same surface texture? • Has the solvent blend changed? • Is the press speed the same? • Is the tape too soft? • Is the same press model in use for the same job? In other words, the operator needs to be able to look up all previous press conditions, as well as the previous press sheets, and compare the conditions with the printed results. Too many times there is a lack of information available that would make troubleshooting pinholing, or any other press is- sues, much easier to identify. Is there enough verbal or written communication between departments? Prepress, platemak- ing, press department or shift to shift? It is not uncommon for me to hear a press operator say: “T hey don’t tell me anything.” Now let’s assume that all press conditions are the same as from a previous successful run. The image is still pinholing. At this point, most press operators will resort to the most logical step, and that is to add printing pressure. Maybe 1mil or 2mil more pressure anilox to plate, or 1mil or 2mil more plate-to- substrate pressure? In most cases the pinholing will improve. While the pinholing has improved and the desired solid ink density number has been reached, the edges of the image may be showing a halo. If the image includes relatively fine reverse type, and the added impression “fixed”one issue, but created another, a closer look is needed. how Can it be FixeD? How can we achieve good ink coverage and still maintain a light impression setting? The operator needs to revisit all the aforementioned conditions. If no previous job information is available to rely upon, the operator must go back to basics and use published information and experience to increase ink laydown without creating poor press conditions. In order to avoid excess print- ing pressure, the choices will come down to ink volume and plate/tape package. A higher volume anilox (from 3.5bcm to 4.5bcm) may do the trick. The upper limit of the anilox volume obviously depends upon the size of the reverse type involved. If there is still a small amount of pinholing noticeable, a firmer density tape will be needed. A very common adjustment is to slow down the ink drying. For water-based inks and paper substrates, the ink suppliers are offering surfactants to be added to the ink. It lowers the surface tension of the ink, which helps the ink to flow better as it is transferred to the substrate—thus, less or no pinhol- ing. It is advisable to use only small amounts (1 to 2 percent). Too much of a good thing can be dangerous and lead to ink blocking or poor ink trapping. The same holds true for solvent inks. A typical wide web solvent blend may be 80/20 NP alcohol/NP acetate. Again, a small amount (1 to 2 percent) of butyl alcohol will help with spreading the ink. However, caution is advisable because of possible ink blocking. In terms of plate choice, most printers are using relatively high durometer plates for process printing (upper 60s to low 70s). It assures clean process printing with less dot gain. If that is the experience, then it can be said that a lower durometer plate will transfer more ink and will help to combat pinholing. Another popular fix for pinholing has been to employ various screening technologies. Many times, a 95 percent shadow dot will print better than a 100 percent. A plate test may be necessary to determine the specific percentage for optimum results. There are other aspects and conditions that need to be looked at. For instance: what is the plate relief? We know that a higher relief choice (26-30), particularly on smaller print- ing cylinders, will lead to plate cupping. The edges will print, while the rest of the image prints too light. Plate cupping is a function of plate distortion, and is directly related to plate relief. The higher the relief the more plate cupping will occur. So, if we can lower the relief, we can effectively lower plate cupping and thus lessen or eliminate the high amount of pinholing we see at times. A relief height of .020in. to .022in. is very manageable on press and lets us achieve good press settings without pinholing. For quite some time, printers have been reluctant to accept less plate relief. But the benefits are clear and proven. n ABOUT THE AUTHOR: Peter Menzian started his career in flexography in Hamburg, Germany. He came to the U.S . in 1965 and continued working as a press operator in the NY/ NJ area. He held several positions as shift supervisor and pressroom manager at Print Pack, Parquet Oneida and Gravureflex before joining DuPont and the Cyrel group in 1981. Menzian travels the U.S. and Canada to assist Cyrel customers in troubleshooting, training and in all aspects of flexo printing. 62 FLeXO september 2010 www.flexography.org FLX_Sept2010_mech.indd 62 8/31/10 5:41 PM