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FLEXO Magazine : October 2012
define the shape of the dot. While this is not quite as pronounced with larger dots, the difference in small screen dots is very pronounced. Experience shows that just going to 4000 ppi will create a minimum dot percentage in print that is about 40 percent lower than what is possible with 2540 ppi. Figure 1 shows how dots are formed on files at the bitmap pixel level for a 175 lpi screen for both 2540 ppi and 4000 ppi imaging resolutions. You can see that the 2 percent dots at 2540 ppi consist of only a few pixels, creating a jagged dot shape. Although oxygen inhibition can smooth out the top of the dot during plate processing, the jagged structure still exists. Consequently, small dots tend to bend on the press more easily—or some of them might even tear off due to wear, leading to dirty printing. At 4000 ppi, all dots are nearly perfectly round and, thus, fully equal on the plate. For the mid tones and the dot closure area, 4000 ppi imaging leads to much better defined structures. The distance between the dots is exactly the same, producing a much more standardized dot closure area and less problems on the press. At 2540 ppi, “bridging” can occur, when pixels stick out from two dots, allowing ink to join together to dramatically increase the dot gain in that area. While at 2540 ppi special screening dot shapes are often needed to better print the dot closure area, at 4000 ppi the standard circular screen already performs the job. NO HARD EDGE A common problem with flexo is the hard edge in the highlight—the sudden finish of the tonal range. There is not the progressive gradation down to zero as with offset and gravure. With standard CTP flexo, there is low tonal contrast, and a visible vignette edge due to large minimum highlight tonal values. A lot of this is caused be- cause in flexo, minimum dots bend on the press, which can result in consider- able dot gain in those highlights. This is not the case with traditional gravure and offset. You can see it in Figure 2. Figure 2: Hard edges and highlights are evident in a comparison of offset and gravure print results to standard CTP flexo. Figure 3: Small dots are supported by larger dots, do not bend on the press and deliver tiny printed dots that stay stable even during long runs. The result, tonal values of highlights get nearer to zero. www.flexography.org oCTober 2012 FLEXO 61