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FLEXO Magazine : June 2014
much more likely. It is also easier to measure volumetrically though standard microscopy and a mathematical formula or more modern automated measurement technology. However, these positive attributes only exist if this pattern is created within certain parameters or a relationship between the depth of the particular engraving and the size of the cell opening related to that engraving is established. The concept is more commonly known as the Depth to Opening (D/O) ratio (Image 4). Through early industry testing and on press production printing, it was learned that anywhere within the “ideal” ratio of 23 percent to 33 percent, these benefits and a cell cavity shape conducive to good ink release characteristics could be had. Hence, it has been long known that a consistent cell geometry at the surface, combined with the prop- er cavity shape, are the two keys to anilox roll and print performance. If a 60 degree hex engraving is produced outside this ratio, all of the positive attri- butes no longer exist. Image 5 is of a 1,000 cells per inch (cpi) 60 degree hex engraving at 1.0 bcm volume, which equates to an 18 percent D/O. Note the inconsistency of the cell size, cell shape and cavity shape. InImage6wehavea250cpiat8.6bcm, for a 35 percent ratio. The surface is rough, and the cells are irregular in size and shape. Engravings at this high a ratio also exhibit deep conical cell cavities, which are ineffi- cient in transferring ink, difficult to consis- tently reproduce, very difficult to accurately measure and most importantly very much a challenge to clean. The effect of this “ideal” 23 percent to 33 percent ratio is that for a given cell count, there exists a narrow window of volumes available. There are certain flexo printing applications where a higher cell count and higher volume are desirable. A vignette with a solid fading down to a small tone value is a good example, where sufficient volume is needed to get a color match at the solid, while a smaller cell or higher cpi is needed to sufficiently support the small dots at the bottom of the gradation. Printing an opaque white on clear film with a high opacity level is another limitation area. Uniform, pinhole free coverage in large solids on rough paper and corrugated substrates is another. Granted, there is prepress screening and plate imaging technology available that helps in these situations and other areas of flexo, but the reality is that not all printers have the in house capability and financial wherewithal to take advantage of it. MODERN IMPROVEMENTS The latest iteration of laser engraving technology that addresses some the limitations of the 60 degree hex is that of extending or “stretching” the hexagonal pattern in the north to south direction, perpendicular to the axis of a roll (Image 7). By extending the hexagonal shape, the cell takes advantage of several things. First, stretching the shape in effect removes some of the angular walls around the circumference of the roll. Removal of these walls adds volumetric capacity to the cell, enabling higher volumes to be achieved for a particular cell count. Secondarily, there is more open area at the surface of the roll, resulting in more efficient transfer and better, more uniform ink film delivered to the plate. In addition, a higher volume can be achieved at a shallower cell depth, which in addition to also improving transfer efficiency makes for easier and less frequent roll cleaning. The most important benefit of an extended hexagon engraving is the two keys of consistent cell size and shape, combined with an efficient cell cavity, can be achieved at ratios higher and lower than the 33 percent and 23 percent. Image 7 is of a 550 cpi 2.0 bcm engraving at a 19 percent D/O ratio. In contrast, Image 8 is of an 800 cpi, 4.0 bcm engraving at 50 percent ratio. Both of these exhibit the desirable geomet- ric characteristics required for high quality flexo printing. A higher achievable volume at a higher cell count with more surface area of ink transferrable to the plate ad- dresses some existing limitations with traditional engraving patterns. Truth be told, extended hex engravings are not the “silver bullet” either; there are some applications where this technology is not a good fit. Just as with printing presses, inks, mounting tapes and plates, there are choices out there. Engravings are no different. As a printer, you should seek out the options that best suit your applications and needs. n About the Author: Tom Cassano is the technical service manager for ARC International in Charlotte, NC. He has been in the flexo industry for 27 years in various manufacturing, technical and management ca- pacities. He is currently a member of FTA’s Excel- lence in Flexography awards committee. He can be reached at email@example.com Image 4 Image 5 Image 6 Image 7 Image 8 68 FLEXO | JUNE 2014