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FLEXO Magazine : October 2010
Technologies & Techniques The TesT ParameTers To evaluate this even further, two technical teams got together to gain an understanding of the effect that variations within sol- vent ink systems have on printing smooth solids. The variables of this designed experiment includ- ed: ink resin, drying speed, and viscosity. Each ink is used to print on film for lamination, surface or reverse print applications. Plate type and cushion tape modulus were not varied. The plate type chosen was a higher durometer digital plate commonly used for high quality work, and the cushion tape selected was a me- dium modulus that is well suited for this plate type. We asked several printers to select the substrate that would be most difficult for them to achieve a smooth solid and polypropylene was selected. The print test was completed at Packaging Specialties in Gainesville, GA using a PCMC wide web press. This was a single-color test using a high line screen/low volume anilox roll running at 800fpm. Since the goal of the trial was to see which of the ink vari- ables contributed to printing smooth, the concentration of the analysis was solely on the solid coverage and not on screens or even the end use properties of the inks. The analysis was mainly completed through a visual analysis using photomicro- graphs taken of solids on both sides and the top and bottom of the web. The ink types used for each condition were not identi- fied on the photos used for the visual analysis. These photos were sent out to individuals within both the Flexographic Prod- ucts and Packaging Inks Divisions of Flint Group. Each sample was given a grade of one to five, with five being the highest. The resulTs The rankings were somewhat varied, with the exception of one ink type, with this ink being the one best suited to print on this substrate. The samples printed with the lowest quantity of pinholes and with trail edge void were only detectable at high magnification. Unfortunately, there was not a great discovery made through these tests. However, by using the most suitable ink you could improve both trail edging and pinholing. But to really improve the print results, this is not the only parameter that needs to be adjusted. Also, this does take ink out the picture as a contribu- tor of these printing issues (as someone involved in the flexo plate business for 25 years, this is hard to admit). Since a clear solution was not met, we continued to look into other areas that contribute or provide a solution to these concerns. The criteria for this round of tests were to see what could be done to improve the solid coverage without chang- ing the plate type, ink or anilox. The variables were plate thickness (67mil and 30mil) and alternating the plate’s surface texture using surface screening. The conclusions drawn from these tests provided evidence that reducing the thickness of the plate helped to reduce the trail edge printing void with no significant difference found with the quantity of pinholes at either plate thickness. As far as surface screening, this variable had the most dramatic ef- fect with both reduction of pinholes and trail edge void within all rounds of testing. Not only were the pinholes and trail edge void reduced, but, under some of the conditions, the solid ink den- sity level was significantly increased between 0.2 and 0.4 . The repeatabil- ity of surface screening results was replicated several times on different presses and substrates finding that all the benefits stated above were repeatable and with strong surface screening parameters being defined. In conclusion, to print smoother solids, make sure you are using the ideal ink for your substrate, use the highest modu- lus cushion tape possible without increasing the size of the printed dot, reduce the plate thickness and, lastly, gain an understanding of the benefits of surface screening. Selecting the proper screening parameters will provide great improve- ments in printing smooth solids with an added benefit of an increase in the solid ink density. A great deal of data was collected during these tests and, due to the article size limitation, we are only able to share a small quantity of the results. Please feel free to contact us for more of the details. n ABOUT THE AUTHOR: Rich Emmerling is the North American technical manager for Flint Group Flexographic Products. He has been in the printing industry for more than 30 years with 25 years being involved in the flexographic plate busi- ness. Emmerling has held a number of positions that include product management, sales, technical specialist and techni- cal support manager. He is a frequent speaker at various industry functions. He lives in Peachtree City, GA with his wife, Carol, and his son, Matthew. As senior applications specialist for Flint Group, Daniel Reilly has more than 35 years experience in flexo and offset print operations and management. He has and is currently working in a technical support capacity on the supplier side. Microscopic images showing how surface screening eliminates the trail edge void issue. solids areas screened (left) and without surface screening (right) printed with identical press conditions. www.flexography.org october 2010 FLeXO 13 FLX_Oct10_mech.indd 13 10/15/10 12:31 AM
Sustainable Fall 2010