A cynical flexo printer once told me, “We use the flexographic printing process because we have to, not because we want to.” He went on to explain that with the substrates he printed on, the inks he was required to use, and the economic necessity to print roll to roll with variable repeat and inline die cutting, he simply could not do the work with any other process.
He reflected that, while flexo has limitations, his company has learned to use tools, techniques and science (much of which was learned through FTA) to overcome those limitations. As a result, he stated, “The 4-color process we print via flexo is every bit as good as 4-color process printed offset.” One has only to view the Excellence in Flexography Award winners on display every year at Forum to confirm his point.
Then, almost as an afterthought, he added, “But we sure do spend a lot of time and money manually retouching each separation to achieve those results.” Indeed, the next breakthroughs in flexo color separation are as much about simplicity and productivity as about quality.
“Flexo-izing” the Color Separation
The process of manually retouching each separation to enable it to print cleanly on press is sometimes called “flexo-izing” the separation. There are many techniques for “flexo-izing” a color separation, but they all, in some way, are an attempt to cover up the large amount of highlight dot gain in the print process, the hard edge that becomes visible if you drop the highlight dot, or the scum that results when interspersed areas of dropouts and min dots create partial or isolated dots on plate.
Figure 1: The scales on the left were printed on an offset press (the November issue of FLEXO Magazine); the scales on the right simulate flexo highlight gain.
All artwork courtesy of Mark Samworth
Figure 1 attempts to simulate full-range vignette scales printed offset (top) and flexo (bottom). As seen, these scales are identical from about 25 percent to 100 percent, but because of highlight dot gain, the flexo scale is much darker in the highlight areas.
Additionally, a hard edge can be seen at the very beginning of the flexo vignettes. While a zero percent dot on plate will always print as a zero percent dot on press, the smallest “holdable” dot on plate (say 0.5 percent) will often print to 15 percent or higher on press. This dramatic change from zero percent to 15 percent is known as the “flexo highlight break.” It can ruin the appearance of process color images, as well as gradients and drop shadows, on the flexo-printed job.
The most common technique for “flexo-izing” a separation is to create an extreme skeleton black. The logic behind this approach is that the hard edge is more objectionable than a dark highlight. The strategy is to set cyan, magenta and yellow to have no dropout at all, then—to avoid a 4-color highlight—set black to have extreme dropouts. As a normal black plate will have hard edges (the upper pair of images in Figure 2), the trick is to use separation settings and manual retouching to “hide” the black hard edge in darker areas of the image (the lower pair of images in Figure 2).
Figure 2: These images attempt to simulate flexo print. Typical “offset” black separation (left); extreme “flexo” skeleton black separation (right)
This technique eliminates the visually objectionable hard edge, but compromises detail and results in dark highlights and desaturated colors associated with holding dots through the entire range of cyan, magenta and yellow.
In order for flexo color separation to advance in terms of both quality and simplicity, we must be able to print color separations that have dropouts everywhere—with the confidence they will print cleanly on press with no highlight break. We need to move beyond manual retouching of dropout areas and into automated separation based on profile conversion. This can, and is, being done today. There are numerous tools to assist in this advancement. The two big tools are: moving source LED (MS LED) plate exposure and minimalist HD hybrid screening.
MS LED Plate Exposure & Color Separation
While legacy bank light plate exposure technology allows for the optimization of main and back exposure time, MS LED plate exposure technology enables the optimization of intensity (technically “irradiance”) and time and allows the option to deliver energy in many short bursts (fast moving source with many passes) or a fewer number of long bursts (slow moving source with fewer passes). Add to this the ability to control irradiance—a parameter that is fixed in bank light exposure units—and to separately optimize main and back exposure, and we have the potential to advance flexo plate quality to a whole new level.
While these images are not an exact comparison (a top view image of a bank light plate on the left; an angled view image of an MS LED-exposed plate on the right), they do show the ability of MS LED exposure to hold small and isolated dots with extreme shoulder support.
The benefits of MS LED exposure technology have been the subject of magazine articles, conference presentations and industry awards. Since this article is focused on color separation, we will examine what many people believe to be MS LED exposure’s single biggest benefit: the ability to hold small and isolated dots on a flexo plate. This, in turn, allows the very best type of hybrid screening to be used: minimalist HD hybrid. It is the minimalist hybrid screen on a high-end flexo plate—potentially combined with two other tools to be discussed—that allows for the complete disregard of dropouts in flexo color separations. It also eliminates the flexo highlight break in drop shadows and gradients.
To be sure, with the recent introduction of premium “flat top” plates, the quality of plates exposed on bank light units has also dramatically improved. These plates—which feature a flat top dot built into the plate—truly make the flexo process “easier.” But while the ability of bank light exposure to hold small, isolated dots is better than ever, MS LED has still proven to hold the smallest and most isolated dots across a range of plate materials.