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FLEXO Magazine : May 2012
printers often generate the white background by printing white ink. It is not uncommon for these printers to print 100 percent white coverage and then colors on top—or bottom if reverse printing. With such a high coverage of white ink it is easy to see how important white ink can be from a cost standpoint. But white ink can be equally important from a performance standpoint. White ink is often used to hide the contents of the package, and to provide a bright background for the bril- liant colored graphics, which we see on the retail shelf. Both attributes require opaque white ink. From a graphics point of view, the opacity cannot be too high. Since opacity is generated by the TiO2 pigment content of the white ink, one might infer to simply add more pigment to the formula. But, flexo inks must be fluid, so they can transfer through the ink train. There is a range of viscosity at which the ink will print the best. Simply adding a drier pigment increases the viscosity. The increased viscosity requires a solvent to get back to the printing viscosity. Now the pigment content is reduced to a lower level and nothing has been gained. So, there are formulating constraints that must be worked through. If higher opacity is the goal, it is not always possible to just apply a thicker ink film. A thicker ink film would increase opacity, but there are limits to how much wet ink can be applied and still be dried. Insufficiently dried ink can cause tracking of other colors, offsetting on the backside of the web, roll blocking, high-retained solvents, and odor issues. So, again, there are processing constraints to how much opacity can be built through ink film thickness. Opac- ity and ink coating weight are not linear. At lower levels of opacity a small amount of additional ink can make a signifi- cant difference in opacity, but as opacity increases more ink is needed to provide the same impact. From the chart (see page 32), one can see the coating weight difference to move opacity one point from 52 to 53 versus the coating weight difference from 62 to 63 opacity. The additional ink required is nearly three times as much. Increasing opacity can be very expensive particularly at the high end of the curve. Large or small flexible packaging printers must closely monitor opacity, since it can dramatically alter the amount of white ink used and consequently impact costs. It is not un- usual to see opacity specifications with a range of ±3 points. Actual print samples can have variation within printruns that can be even larger. Low opacity may generate a complaint from a customer but high opacity will never come back as a complaint. Therefore, pressmen may run at the top of a specification range to ensure the print is not rejected. The cost impact this would have can be understood by looking at the chart on page 32. At higher opacities, the impact is substantially more pronounced. This can add up to thousands of dollars of extra cost each year for even a relatively small printer, and much more for a large printer. www.flexography.org may 2012 FLEXO 33