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FLEXO Magazine : February 2010
44 FLEXO FEBRUARY 2010 www.flexography.org and we have already confirmed that with the printer. The weight per gallon was confirmed by the technical data sheet and should not change because the coating has not been reduced with any liquids. Let's plug in some numbers: Material Type Water Material Type Water Material Type Water Coating Method Flexo Coating Method Flexo Coating Method Flexo Desired Dry Weight 1.00 Desired Dry Weight 1.10 Desired Dry Weight 1.20 Deposit Efficiency 20% Deposit Efficiency 20% Deposit Efficiency 20% Ream Size 432,000 Ream Size 432,000 Ream Size 432,000 % Solids 50.0% % Solids 50.0% % Solids 50.0% Weight Per Gallon 9.2 Weight Per Gallon 9.2 Weight Per Gallon 9.2 Required Volume: 9.52 Required Volume: 10.48 Required Volume: 11.43 Notice how the seemingly small change in the coat weight requirement (1.0, 1.1, 1.2) makes approximately a plus/minus 1bcm adjustment in the resulting volume. This is a good re- minder to be specific in target weight, or you will end up with a volume range as well. Example 2: Printing Method: Gravure, two-part solvent-based coating Coat Weight Target (1.2 lbs/ream) Percent Solids: 70 percent (or is it?) Ream Size: 3,000sq.ft. (432,000sq.in.) Deposit Efficiency: 40 percent Weight Per Gallon: 9.5 lbs (or is it?) REVIEW DATA The first thing we notice is solvent coating that has at least two parts. Ideally, coatings like these come with a recipe for how much of each component to mix together and a dilution schedule for the addition of a third unspoken-for compo- nent---the solvent. We have been given a specific target for the coat weight, 1.2 lbs/ream. The solids is at 70 percent per the MSDS/technical sheet, and in most cases with a solvent- based coating, a reduction is made in viscosity and is not used as packaged without alteration. This coating requires mixing the two components to the proper ratio and adding solvent for reduction. The formula states that the resulting solids content will be 35 percent, but you want to make sure by recording all the weighing information to confirm the batch is correct. Describing the solids content and the weight per gallon accurately requires knowing the weight per gallon of all three components and how much is used of each in ratio. Once that is determined the true solids content and weight per gallon will be known. There is no issue with the ream size on this application. The deposit efficiency checks out as gravure, meaning it could still be a flexographic-type deck but the actual application goes from anilox to substrate without the use of a plate or sleeve. Once again we have confirmed that information. What is our formula? Our formula for this application: 9:1 ratio of the base and cross-linker. Reduce in half by solvent (identified as normal propyl acetate). Since the base and cross-linker happen to have the same solids content, once the mixture of the two is reduced by solvent then the resulting solids content is 35 percent. • Base: 70 percent solids, 9.5 lbs/gallon • Cross-linker: 70 percent solids, 10.0 lbs/gallon • Solvent: Normal Propyl Acetate: 0 percent solids, 7.28/ lbs gallon We take the ratio of 9:1 of the components and multiply by their weight per gallon: 9 (9.5) + 1 (10.0), add the dilution of solvent 10 (7.28) and divide by 20 for the real weight per gal- lon you will be using. Where did the 20 come from? This is the addition of all the parts, 9 + 1 +10. So, (9(9.5) + 1(10) + 10 (7.28))/20 parts equals 8.415 lbs/ gallon, the true weight per gallon. Use that with the true solids of 35 percent and we're in business! Let's plug in some numbers: Material Type Solvent Material Type Solvent Material Type Solvent Coating Method Gravure Coating Method Gravure Coating Method Gravure Desired Dry Weight 1.20 Desired Dry Weight 1.20 Desired Dry Weight 1.20 Deposit Efficiency 40% Deposit Efficiency 40% Deposit Efficiency 40% Ream Size 432,000 Ream Size 432,000 Ream Size 432,000 % Solids 70.0% % Solids 35.0% % Solids 46.6% Weight Per Gallon 9 Weight Per Gallon 8.415 Weight Per Gallon 8.79 Required Volume: 4.17 Required Volume: 8.93 Required Volume: 6.42 What the tech sheet told us before factoring in ratios and reduction/dilution. Using correct ratio, solids & wpg. 9:1 components and 10 parts Solvent. If you change the reduction, you change the solids and weight/gallon. 9:1 components and 5 parts Solvent. BANDED ANILOX TEST The lesson is to always establish accurate data for the formula and final solids/weight per gallon of the coating actu- ally used at press. If a coating requires significant additions of components and/or reducer while you test, you will need to document and double check the numbers so you are calcu- lating with data from the final mixture you are using at press. What happens if you don't have conclusive data to work with, especially the target coat weight? You are going to need physical samples to weigh. There are a number of ways to get the samples. You can take existing anilox inventory and print TECHNOLOGIES & TECHNIQUES