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FLEXO Magazine : February 2010
46 FLEXO FEBRUARY 2010 www.flexography.org Page 5 A1 Special Press 13.34 440 60 53.0000 5.0000 3.0000 Cells are slightly plugged.Clean thoroughly. 2.91 A2 Special Press 16.86 500 60 47.0000 4.0000 4.6400 Continue Use. 4.58 up test samples or you can also have a banded anilox made or borrowed that will have many volumes provide different coat weights for testing. Check your anilox inventory first to see if you already have a roll that will work for your situation. You should have the volume checked to confirm what you will actually be using instead of believing what the roll has been stamped. If you do not have an anilox that will work, then contact your anilox supplier to see if it has a banded roll already made that you can borrow for testing. If you choose to go the route of having a banded roll cre- ated, seek assistance from the technical staff of the coating and anilox suppliers to help narrow your choices to the most likely successful engravings based on the information. All you'll need to get started is a target volume or volumes. Once you've ran a test, you can then evaluate the samples for fit/ form/function and determine which volume and cell geometry works best for you. In the example I will use, the banded roll is testing a range of volumes (8.0 to 11.0bcm) in 0.5bcm increments. There is enough volume separation to distinguish each band. Often it is recommended to add dead band areas between engrav- ings to simplify identification of each band. Once you have samples printed, you will need to prepare them for weighing. Evaluation of samples is simple if you keep a few things in mind. You need to determine the net weight value while making sure you don't corrupt the sample. The net weight value can be determined by weighing printed and un- printed samples to get the net difference (coat weight). Make sure the only difference is in the coat weight. Interference in the net coat weight can come from other print on the sample that is not consistent, the size of the coated area measured, contamination, etc. The coat weight difference is very slight in small samples so any disturbance in the measurement leads to chaotic results. Getting dis- similar size samples for comparison frequently leads to area and net weight errors. Remember the anilox is indifferent to the area coated but sensitive to the coat weight required in a given area. The means there is a direct relation between the deposit and the volume of the anilox. STEPS FOR EVALUATION A. Use a calibrated scale that measures the weight range of the samples down to thousandths to ten-thousandths of a gram. If you don't capture a difference, you need a scale that allows for smaller weight differences. B. Weigh a substrate sample with and without coating to determine exactly how much adhesive/coating is there. Don't change anything else or the data will be false. Try to get as large a sample as possible that you can reasonably weigh. C. Know the current volume is correct. It may not be what is stamped on the side of the anilox due to cell plugging or wear, so have the volume checked to make sure what the volume is at the time just before testing and that the samples were generated from a clean anilox. D. Test other target weight samples to confirm it is what you want and to make sure no errors were made. Aver- age the net weights if necessary. CROSS MULTIPLY There is one other possibility in evaluating samples and it has the simplest solution for determining the required volume. In the case where you have known coat weight and volume and know what coat weight you would like to target, you can simply cross-multiply to determine the target volume. Current weight/Current volume = Target weight/x. Solve for x, which is the target volume and the only unknown value. Just cross-multiply and you will get your answer. For example: • Current weight: 1.5lbs/ream • Current volume: 15bcm • Target weight: 1.0lbs/ream • Target volume: x Current Weight / Current Volume = Target Weight / Target Volume. That means 1.5lbs/ream / 15bcm = 1.0lbs/ream / x. Solve for x. 1.5x = (1.0)(15). Therefore, x = 10, target is 10bcm. Your target volume is 10bcm in order to get the target coat weight of 1.0lbs/ream. The task of determining the proper coat weight for product application, cost savings and/or performance characteristics without strong analysis techniques can certainly lead to frus- tration and failure. The dynamics of a coating application re- quire reliable data acquisition and refinement to give you the correct results the first time. Careful and deliberate analysis prevents many of the errors that can occur and the resulting frustration. While the majority of applications net straight-for- ward calculations, you may come across situations where you must refine the data set to get your true volume. You may also discover that with limited information you will need to perform sample weight calculations or banded roll trials. In the final analysis, always remember that no matter what direction you take for a coating application, seek assistance and guidance from both coating and anilox suppliers to help you determine the right anilox for your application. ABOUT THE AUTHOR: Sean Teufler proudly serves the cen- tral U.S. as a technical graphics advisor for Harper Graphic- Solutions, a division of Harper Corporation of America. Confirm anilox volume before testing at press. TECHNOLOGIES & TECHNIQUES