Back in the “old days”—when printed graphics were less complex, when quality and consistency standards were less stringent, and Delta E reminded people more of an airline and a concourse rather than a color difference—specifying an anilox roll was relatively simple.
Whether it be from empirical testing or experience through the school of hard knocks, an appropriate linescreen and volume could be chosen for a particular application with a reasonable expectation of success. At the very least, they would be “close enough” to where a pressman could adjust inks and settings to achieve a “pass” on quality.
Anilox rolls used to be specified by linescreen, then volume. A line work print job would be specified with 360 linescreen rolls, while screen work would use 440s, and process print would be run with 600s at volumes of 5 bcm, 4 bcm, and 3 bcm, respectively. If correct color could not be achieved, or the screen gradient wasn’t consistent throughout the tone range after pressmen tried to make adjustments, then new rolls were specified at the same linescreen, but a different volume, to overcome the particular print problem. This was a time consuming, costly and inefficient way to produce a print job.
The “bar” of quality, consistency and color has been raised significantly since then and any inefficiency in the printing process makes it more difficult—if not even impossible—to maintain those three attributes for customer satisfaction.
Old & New
I reference the old, inefficient way of specifying rolls by linescreen first as a method to challenge readers to now think in terms of volume (bcm) initially and then linescreen, and then the engraving design to most efficiently deliver that volume.
We know it’s most advantageous to print with the least amount of ink (lowest volume or bcm) possible and still achieve color and coverage. Once that minimum volume is determined, then the appropriate linescreen can be chosen.
Herein lies a dilemma: If one was to look at the engraving charts/ anilox selection guides from any roll supplier, you’d see there are many linescreen offerings for a particular volume. As an example, if a 2.2 bcm volume was required, the linescreen selection can be anywhere from a 600 to an 800. Which is best depends on the graphic elements to be printed, in order to provide the best support for them on the plate. Choosing the highest linescreen is typically the path taken, but it may not be the wisest choice.
If the graphic elements allow it, choosing the 600 at 2.2 bcm could be a better option for several reasons. Even though the 2.2 bcm is within the optimum range for both the 600 and 800 linescreen engravings, the 600 yields a larger cell opening and the 2.2 bcm falls into the lower end of the volume range, as compared to the 800, where the 2.2 bcm is at the higher end of that range. In this case, the 600 linescreen will be more efficient in delivering ink to the plate, without the need to apply more inking impression to maximize transfer. You might need that with the 800 linescreen engraving, since 100 percent of the ink in the cells is not transferred to the plate.
This is premised on the industry “standard” of a 60 degree hex engraving, with a surface view and the typical parabolic cavity profile seen in Figure 1a & Figure 1b.
Expanded Volume Engravings
However, as I noted earlier, the third piece of the specification and application puzzle is that of engraving design.
Laser engraving technology has progressed to a point where the suppliers of anilox rolls can and do provide different cell cavity profiles and shapes to enhance printers’ graphics and quality capabilities. The ability to alter the cavity shape brought about what I’ll generically call “expanded volume” engravings, still in the traditional 60 degree hex pattern as shown in Figure 2a & Figure 2b.
The expanded volume engravings keep the traditional hexagonal cell pattern, but the “U” shaped cavity increases the volume compared to the parabolic cavity found with the standard 60 degree hex engraving. These can be—and are—used in low volume process print applications with water, solvent and curable inks. However, they are best suited for use in the medium volume ranges for line work applications with difficult to match colors like many pastels, purples, reds and reflex blue, where a bit more cell volume is beneficial. Since the additional volume is not a result of increasing the depth of the engraving, they are also efficient in the transfer of ink to the plate.
There is a secondary advantage in utilizing this type of engraving: less volume loss over time as the engraving wears down. With a parabolic cavity, both the cell depth and opening are reduced by wear, hence the reduction in volume is greater. The more vertical walls of the U shaped cavity of expanded volume engravings result in only the cell depth changing as a function of wear, not the opening. Ergo, there is less volume reduction over the same run time compared to conventional engraving. This is important for printing the pastels, purples, reds and others as significant volume changes in the anilox roll can yield large color shifts on press.