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FLEXO Magazine : January 2010
36 FLEXO JANUARY 2010 www.flexography.org REAM SIZE This defines the area where the coat weight target is applied. Common imperial unit choices: 1,000sq.ft. (144,000sq.in.) 3,000 sq ft (432,000sq.in.) Metric choice: Square meters The area must be clearly defined because the difference between the two ream size choices, shown above, is a factor of three, which in a calculation can triple or cut to one-third the volume required. If someone tells you a certain weight must be applied, ask over what area and how they define the units. The ream size or area of coat weight can get complicated when printing spot coverage instead of a flood or 100 percent cover- age, so keep actual printed/coated area in mind. COATING TYPE Is this a water, solvent or UV application? Knowledge of this helps to identify issues within the data of the coating ap- plication such as how many actual components are involved and any complications the coating may present to the anilox, namely corrosion potential. COATING MATERIAL What you are trying to apply, be it lamination, glue, varnish, silicone, etc. in general terms. COATING SPECIFICATIONS This refers to data related to coating specifications and must be verified and agreed upon because the calculation significantly depends on their values: Percent Solids. Very simply, the term "percent solids" is the amount of solids left over in the dried coating or adhe- sive from the original wet printed film, ideally applied at a printable viscosity. It is very common that the amount on a specification sheet will not take into account the addition of reducers to water and solvent based applications. Reduction in viscosity will change the concentration of solids by dilution in the wet film, and thereby lower the percentage of solids, and the corresponding yield, once dried. Weight per gallon. How much does the coating or adhe- sive weigh in pounds per gallon of material. WPG is affected by reducer additions, which tend to lower the actual weight per gallon. Be sure you discuss with the coating supplier whether the product will need any reduction, or if it can be used without alteration. pH Range. For water-based applications, low pH (less than 5.0) and very high pH (more than 11.8) will indicate a poten- tial for corrosion. Steps must be taken to apply a corrosion barrier to the anilox base cylinder, which will help increase corrosion resistance. Viscosity. Viscosity specifications give you a better under- standing of potential flow resistance and will indicate, in many cases, the proper cell geometry to promote release. Higher viscosities tend to fare better on 30 channels, quads and trihelicals, depending on run speed. It is important to note any differences between printable and incoming viscosity because, if a dilution/reduction was made, it lowers the percent solids. CURRENT WEIGHT APPLIED If this is a situation where you are already applying a par- ticular product, record what are you currently targeting and achieving. The data is useful in verifying/projecting required anilox volume for the new coat weight target. CURRENT ANILOX TYPE Most aniloxes are ceramic, but chrome cylinders are still available and have a different release characteristic. It is im- portant to distinguish which type is in use when determining volume in comparison to current use. CURRENT LINE-SCREEN/VOLUME This information is frequently a great guideline when pro- jecting/verifying data based on history and samples. GEOMETRY The most common choices are 60 degree, 30 degree chan- nel, 45 quad or trihelical. Anilox suppliers are well-versed in determining the correct geometry depending on viscosity/flow issues, press speed, etc. TYPE OF COATER (FLEXO OR GRAVURE) If a plate or tint sleeve is used, then consider this a flexo ap- plication and the deposit efficiency runs 19 to 23 percent. If the transfer from the anilox is direct to the substrate, this constitutes a gravure-style application (even on a flexographic press) and the deposit efficiency jumps to 39 to 43 percent. Failure to dis- tinguish between the two styles can affect an error by doubling or halving the volume calculated. Be sure to verify. DRIVE SYSTEM There are two types: direct and variable. Direct, or 1:1 ratio, does not effect the basic calculation. This means in essence the anilox surface matches the pace of the plate surface for flexo and matches the nip surface and web for gravure. Variable Drive can change the deposit through adjust- ment. Know how much adjustment you have and whether or not the data was taken with the rotation maximized or mini- mized. Optimally, have the rotation somewhat neutral to allow for freedom of adjustment as necessary at press. General concepts for variable speeds, assuming the anilox is clean: • If the anilox rotation is with the flow of the web and the rotation is maximized (faster than web), that is the most coating you can apply. Diligent documentation starts with a coating application worksheet. TECHNOLOGIES & TECHNIQUES
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