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FLEXO Magazine : September 2010
Technologies & Techniques ing point, which restricts the ability to print a vignette down to zero. Because of dot gain, photopolymer plates tend to start printing the first dots at 5 to 8 percent, in the best cases. This means that gradations in vignettes suddenly break at that level, generating a visual contrast with the white of substrate. This contrast is quite visible, even if the smallest dots are printed in the region of 5 to 6 percent. This phenomenon generates a visible breaking point or line, which is well known by anyone practicing flexo. Because of smaller dot size, low dot gain and high dot stability of the new plate material, this particular trouble is virtually eliminated, especially if one uses the latest generation of screening technologies. The second difficulty that is quite common for most of flexo impressions is often referred to as the V-shape effect, mainly found with digital plates. The dilemma is typically due to instability of the smallest dots. It is quite common to observe dots about 10 to 20 microns in size on finished plates. If these dots are not well formed on the plate itself, their behavior on press is quite erratic and uncontrollable, caused by collaps- ing—which usually ends up with the smallest highlights of 1 or 2 percent printing far larger than they should, generating more dot gain than 3 or 4 percent dots, which hold up to the pressure of the flexo press. This is often quite visible on the gray scale of control targets. When establishing the impression curve based on the values captured in densitometry or spectrophotometry, one can clearly see the typical “V ” shape of the curve in the highlight area. Here again, the stability of the dots on the new plate—even the smallest ones—overcomes this problem. The new technology has been in use for more than two years at customer sites in full production environments. Ex- tensive trials and live production jobs have proven the ability of the plate technology to print down to 1 or 2 percent, using purely classical screening, such as circular dots. With the lat- est available screening technologies (Figure 3), the plate does much more, and is able to reproduce a vignette fading down to zero. In that respect, results can be compared to those Figure 2. in a typical solvent-wash plate, too small highlights on plate produce irregular and oval shaped dots on print, leading to higher optical density. SYS TEC NORTH AMERICA, LLC. 108 State Street Suite 103 Greensboro NC 27408 Phone 336-273-4866 www.systecamerica.com firstname.lastname@example.org INNOVATIVE FLEXO MOUNTING SOLUTIONS WITH VIRTUAL IMAGE SYSTEM www.flexography.org september 2010 FLeXO 71 FLX_Sept2010_mech.indd 71 8/31/10 5:42 PM