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FLEXO Magazine : June2010
Technologies & Techniques 50 FLeXO june 2010 www.flexography.org Though the dot surface and the base of the dot remain exactly the same, the side walls become steeper and more severe as the ratio changes. As relief depth is increased, elements become more susceptible to impression damage, instability and the inability to fully release ink to the substrate. Digital Diligence The same basic principle applies to the digital imaging por- tion of our workflow. If an insufficient amount of the laser ablative material (LAMS) layer is removed during imaging, then insuf- ficient UV energy is available to fully polymerize the printing ele- ments. It must be verified that the imaging laser is fully removing the LAMS layer and that the energy is properly focused. If the LAMS layer is not properly ablated, we are leaving a residue behind on the plate surface. Two essential quality control tests for imaging are a focus search test and a “stain” test to evaluate the removal of the LAMS layer. The focus search test, as described by EskoArtwork, is a determination of optimal distance between laser optics and digital plate surface. This is a high value, minimally invasive quality control step in ensuring consistency of your imager. After loading a plate onto the imager, the laser will ablate 20 parallel lines while slightly altering the focal depth between optics and plate. The size of the test patch is approximately 5mm by 50mm and only takes a few seconds to complete. By advancing the optics and changing the focal depth, lines of varying width are imaged. Choosing the thinnest line tells the system what adjustment is necessary to achieve the most precise focus. Please note that this test is specific to only some imagers. Other systems may use an internal autofocus function. The stain test verifies proper removal of the LAMS during imaging. Using a transmission densitometer, we can mea- sure the level of ablation at certain points in the tonal scale to ensure that our digital imagers are stable and repeatable. Normally, a 100 percent cleared area and a 50 percent patch are used as our values. Any amount of value points in the tonal scale can be used for reference. First, physically remove a patch of the mask by peeling away with tape. This will be our area to zero the densitometer. Then, measure the 100 percent cleared patch from the test target. If the laser is fully removing the LAMS layer, a num- ber lower than .07 (typically around .03 is the goal) will be achieved. Next, zero the densitometer again, this time on the 50 per- cent imaged patch. This will allow us a comparison between the two imaged patches. A value of 0.30 +/-.02 is normal for the 50 percent value. If the laser is underpowered, it can not fully ablate the LAMS layer and will leave a portion behind. This remaining residue will act as a filter to the UV light during main expo- sure. As we detailed earlier in this article, insufficient polym- erization can lead to unsupported dots and contribute to dirty highlight printing. With the demands on flexography continu- ally increasing (i.e. higher line screens, faster press speeds, new screening technologies, etc.), precise digital imaging is key to a successful run. Finally, selection of the minimum printable dot on a flexo plate is critical to the elimination of dirty printing. Different photopolymers, whether inside the DuPont Cyrel® plate port- folio or from our competitors, have different characteristics and need to be considered independently. Different durom- eter, gauges, relief depths, line screens, imaging resolutions, etc, can all impact the minimum printable dot. To select a minimum dot that will be successful on press, image sets of linear scales at the various line screens used in a particular workflow. Once the plate is processed, evaluate the scales to find at what level a minimum printable dot is formed. It is important to note that the smallest dot held on a test plate isn’t always the minimum printable dot. The aim is to select the smallest dot that will survive the rigors of the pressrun. By providing a mask opening that is clean and sufficient in size, the proper amount of UV energy is allowed to pass through and fully polymerize the element underneath. This test pinpoints what the proper bump curve should be for each polymer type, gauge and line screen. The correct printable dot allows the correct amount of ink to be drawn from the anilox roller and delivered to the substrate without any build up, slurring or deformation. When ink is fully transferred to the substrate with every revolution, dirty printing is avoided. Many different quality control tools can be utilized throughout the plate making workflow. One that is part of the FTA FIRST (Flexographic Image Reproduction Specifications and Toler- ances) tool box is the DFTA CTP control strip (v1.2 or higher). This control strip has several valuable tests included in one small test. When the relief depth is too severe (right), isolated elements and fine dots can be compromised. FLX_June2010_mech.indd 50 6/10/10 9:39 AM