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FLEXO Magazine : June 2012
improve ink transfer and print clarity. As we all know, if you use a CPI that is too high for the volume being applied, you restrict the amount of ink that you transfer, so we want to use an engraving specification that will comfortably allow the ink to transfer to the substrate when doing coatings. Engraving specification requirements can change depend- ing on substrate type and application type, like flexo vs. gra- vure. Uncoated materials with a higher surface tension will pull ink out of the cell much better than a coated paper or film material. Volume is what dictates the amount of ink or coating that is going to get supplied to the substrate, therefore it may be common to use lower cell count on uncoated substrate types to allow ink to wetout thoroughly and provide substan- tial ink coverage to the uncoated materials. Choosing the proper depth to opening ratio is key, but using a geometry that may allow the ink or coating to transfer out of the cell more efficiently will be beneficial and this will prob- ably come into play at volumes that are higher than typical flexo graphic volumes. Higher viscosity inks and coatings may struggle as they transfer out of the cell, so an alternative geom- etry can help these heavier coatings transfer more efficiently. Lower depth to opening ratios will let coatings transfer bet- ter as well. When you combine both of these ways of thinking, you will improve the ink deposit efficiency of the application. To achieve specific coat weights, it is suggested to conduct banded roller testing. You want to make sure you apply what is needed for the product to pass any requirements, but at the same time, only apply the amount required to meet your customer’s needs. Over specifying the coat weight can lead to excessive cost. Calculating coat weights or coating thickness should be done to all the geometries discussed here. Pounds per ream, pounds per gallon, transfer percentage and application type (flexo or gravure) should all be calculated when doing coat- ing applications. Banded roll testing is a great alternative for finetuning any coating application. When the target coat weight is achieved, one will be able to calculate how much coating is being used, as well as develop a window for acceptability for all applications. Even a simple gloss coating should be measured and validated, so you do not waste ink or coatings. n About the Author: Bill Poulson is the technical graphic advisor for Harper Corporation of America. He has been in the flexo industry for more than 30 years and has specialized in coat- ing and multi-web applications. Since August 2011, he has been a FIRST Certified Implementation Specialist and is also an NCCO certified multi-color press operator. www.flexography.org june 2012 FLEXO 77 A picture is worth 1,000 words. Leading Cleaning Technology 888.493.5396 • www.flexowashus.com FLEXO WASH CLEAN THEIR CLEAN A picture is worth 1,000 words. FLEXO WASH CLEAN THEIR CLEAN