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FLEXO Magazine : February 2010
42 FLEXO FEBRUARY 2010 www.flexography.org Last month, my article focused on the necessities of good information gathering, how to obtain the requirements (see FLEXO January, page 35). It also described the application components in detail. In this second installment, we will put to the test sample coating data by evaluating examples of how to refine calculated volume when all the in- formation is available. We are also going to look at strategies like banded rolls and sample evaluation when the target coat weight is not already defined for your calculation. We learned in the first article that good data allows exact computation of what volume will be needed to achieve coat weight target. This is the preferred mathematical way to deter- mine what you need without physically going to press for test- ing. The following lists required data for the calculation itself: • Coat Weight Target • Percent Solids • Ream Size • Deposit Efficiency • Weight per Gallon (Coating) Let's assume for the following examples that we have all the information we need. Note that having a numerical value for each category without confirming it will lead to errant calculations. Example 1: Printing Method: Flexo, water-based Coat Weight Target (1.0-1.2 lbs/ream) Percent Solids: 50 percent Ream Size: 3,000sq.ft. (432,000sq.in.) Deposit Efficiency: 20 percent Weight Per Gallon: 9.2lbs REVIEW DATA The first thing we notice is a range for the coat weight tar- get. If a more specific target is not identified, then use 1.1 lb/ ream as the default weight. The range allows for a plus/minus difference of 10 percent in required volume, so even though it does not seem like a large difference, it affects the engraved volume by a lot. The solids are at 50 percent, and in most cases with a water-based coating, a reduction is not made in viscosity and is used as formulated. Ream size is 3,000sq. ft., or 432,000sq.in. The deposit efficiency checks out as flexo, Weighing Your Coating Options By Sean Teufler Make sure you stir a coating so it is one component at press. Confirm printable viscosity. • 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 differ- ence, you need a scale that allows for smaller weight differences. • 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 you can reasonably weigh. • 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. • Test other target weight samples to confirm it is what you want and to make sure no errors were made. Average the net weights if necessary. TECHNOLOGIES & TECHNIQUES