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FLEXO Magazine : August 2010
Technologies & Techniques INSeRT AD that may take anywhere from weeks to months, depending on the skill of the engraver and what you are looking to achieve. Now all that said, let us assume that you are new to flexo or are simply wanting to move from solid and line work to pro- cess screen work, or want a selection of screen counts to suit a variety of work; what would be a reasonable selection? As a rule of thumb the lower the screen count, the higher the volume and the higher the screen count, the lower the volume. So, for most flexo printers wanting to print solid and line work, a screen count below 360lpi is a good selection. Please be aware, there can be a significant volume range of anywhere from 5.5bcm to 12.5bcm for even a 200lpi engrav- ing. The reason for this generally has to do with the degree of absorbency or lack of absorption of the substrate being printed on. Liner paper used in corrugated and tissue are going to absorb a much greater amount of ink than is put onto them, particularly when compared to a film or plastic that has absolutely no absorbency. For those designs that have a background or other solid area with some line and text, the choice usually falls to a lower line count/higher volume engraving, which will vary depending on the choice of substrate. There are some very lower line counts, but these are generally reserved for var- nishes, cold and heat seals and other very viscous coatings and adhesives. Those designs that have finer text, bar codes and screen tints dictate a higher screen count and lower volume engrav- ing, usually ranging between 400lpi and 700lpi with volumes of 4.5bcm to 3.00bcm. You will note that while there is a much tighter volume range, the line count now begins to have a much greater impact of the level of print sharpness and overall quality. Generally, the greater percentage of fine detail or screen tints, the higher the line count choice and lower the volume will be. Next, if you have process screens of 85lpi or above in your design on your plate, you will now move into the realm of 900lpi and above engravings, where volumes tend to be around 2.5bcm or less. At this point, even subtle changes of engraving or volume can have a dramatic affect on the fin- ished result and even greater care must be taken during this portion of the selection process. Clearly, far higher line counts are offered, and many are in daily use. But once you get above 1,000lpi, the degree of care, handling and best possible cleaning practices become critical if you are to have any longevity and success with these very fine line counts and shallow engravings. So, unless you are prepared to ensure that the very best housekeeping and roll maintenance programs are in place, these very high line counts may not be the route for you. Again, the ink system will have some effect on the engrav- ing/volume choice. In particular, UV inks tend to use the highest line counts and lowest volume combinations because of the decreased likelihood of drying in and thus plugging the cells. Water-based inks, if left to dry on the anilox surface can be very difficult to remove. These inks are very resistant to rewetting and can stain, not just plug, the cells. When rinsing the anilox with clean water, you should dry the roll and use an air line to blow the dirty water residue from the cells if pos- sible. If the water is left in the cells and left to drip and bead along the base of the roll, it can lead to circular stain marks and run lines that will show up in print later on. 96 FLeXO august 2010 www.flexography.org Doctor BlaDe Q&a (con’t.) How mucH sHoulD I pay for a Doctor BlaDe? after all, It’s a consumaBle anD a commoDIty, Isn’t It? As with most things, you get what you pay for. You have to remember what job the blade is doing and understand what impact it is having on the mileage you get from your ink, the life of your anilox, the quality of graphics that you will be able to print, the amount of scrap that you will incur if you fail to meter the ink correctly. Yes, blades are a consumable and to many they are seen as a commod- ity. While buying on price may save you a few dollars at the outset, in the long run it could completely undermine your final bottom line. In addition, you need to ensure that your vendor is willing and capable of giving you any support as and when you need it. It is unlikely that the lowest-priced supplier will be capable, or even willing to give any advice, so you need to ask this question before you choose to change vendors, just because you might be able to save a few dollars per box. tHere appears to Be a lot of BlaDe tIp confIguratIons, wHIcH one Is Best? The most basic blade tip shape is the square edge, but, if used, it is usually only recommended for low anilox line counts, as it can be prone to digging into the face of the roll, if not correctly set up. Blades with a rounded edge have become very popular. They are more forgiving when being set up. They have also been found to help reduce blade chatter and scoring of the anilox. Stepped or lamella blades have a thinner step, yet retain a thicker edge that is held in the blade holder. This generally gives a more uniform blade edge thickness throughout its run life. Beveled blades which were the most used blade profile because they initially presented a thinner tip to the anilox surface are losing ground, as they unavoidably increase in thickness as they wear, which will change the ink film thickness being metered. In an encloseD cHamBer system, wHat materIal sHoulD Be useD as tHe contaIn- ment BlaDe? Opinions vary, but the general logic is that as the con- tainment blade is doing nothing other than holding the ink in to form the bottom of the chamber, then expensive steel blades are not necessary and a plastic or thicker mylar blade material is normally the way to go. are tHere any safety or HanDlIng Issues we sHoulD consIDer wItH BlaDes? When handling any doctor blade, the potential of the operator getting cut is very real. Regardless of the blade material or tip configuration, cut-resistant gloves should be worn by the operator at all times when handling blades. For many printers, plastic blades and round-edged blade profiles have become very popular to further minimize the handling issue, in particular during installation. It should be noted, however, that any blade material will become sharp after running its edge against a roll, and thus should always be handled carefully. August2010_mech.indd 96 8/13/10 7:47 AM
Sustainable Summer 2010