by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
FLEXO Magazine : October 2012
Part of the “magic” of the original incarnation of HD flexo was the use of variable screen dot sizes in the highlights close to zero. They are not all the same size, so they progressively disappear before others. The eyes are tricked to view the dots in a smooth way, seeing a homogenous gray. Some disappear before others, and eliminate the problem of a hard edge. This is in strong contrast to stochastic models, which use fewer dots when the tonal value is reduced. The penalty for this lower dot frequency is much more visible graininess in the highlight areas, especially when several colors are printed together. This is why AM screening is the only really viable answer. HD screens allow only the small dots to fade away at the extreme end of a vignette to zero. The reason for this is quite inventive. Over the 1.5 to 2.5 percent dot range, these dots disappear, but 0.5, 1 .0 and 1.5 percent dots still are present, and still visible. Thus, it provides a nice, smooth vignette. It is even more attractive the wider (in area) the gradation is. Along with the use of dots of different sizes in the highlights are a few dots that are even larger than the typical minimum dot size in standard screening. These dots are so stable, that they easily can withstand the force of the press during the printing process. The other dots in the grid are smaller, as little as 10 microns in size. Figure 3 depicts how small dots are supported by the larger dots and, therefore, do not bend on the press. So, they can deliver tiny printed dots that stay stable even during long runs, bringing the tonal values of highlights even nearer to zero! HD BOOSTS SID Adding more ink does not necessarily mean that the solids will improve. Flexible packaging printing often suffers from inhomogeneous ink laydown; particularly on foils with solvent inks. This is caused by pinholes in solid areas printed with digital flexo plates. Ink lay down is influenced by the self-organization effect of ink due to surface tension. It causes the ink film to reticulate into cells of about 100 micron in size, and the press direc- tion and speed transforms these cells into lines. The result is visible defects on the solids (pinholes) that reduce the flat appearance of solids, overprints—and even Pantone simula- tions. It also leads to reduced solid ink density (SID) in gen- eral, making it necessary to separate linework and process work into two plates. Higher volume anilox rollers can be used to increase the solid appearance, but this concurrently reduces the highlight quality, especially for underprints or overprints, solid white is often just printed twice. Other than the lower quality appear- ance to the human eye, all these technology options really result in higher costs to the printer. There is a fix. Second generation HD flexo added very tight, organized screen patterns. The self-organization of the ink is destroyed and the ink laydown becomes more homogeneous. Figure 4 shows the difference in solids between standard screens and micro screens. The micro screens allow ink to stay in place, rather than pooling wherever it wants to go. Ink is encouraged to lay down even in patterning by using cells across solid areas. Figure 6: Image of the surface structure with a 75 percent tint. Small cell structures do not come close to the edge of the elements by the specific RIPping, so that edges of objects and screening dots still stay sharp. Figure 7: Second generation high definition plates offer improvements throughout the entire tonal range —from extreme highlights to clean mid-tones to high-contrast shadows and even denser, smoother solids, color overprints and more Pantone emulations. Figure 4: Difference in solids between standard screens and micro screens. Micro screens allow ink to stay in place, rather than pooling. Ink is encouraged to lay down even in patterning by using cells across solid areas. Figure 5. 62 FLEXO octobeR 2012 www.flexography.org