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FLEXO Magazine : May 2012
ance can be chosen. The banded roll testing shows 10 mea- surements. The first through fourth are outside the desired tolerance of ∆e 3.0. Measurements five through 10 bring us very close or below the desired tolerance of ∆e 3.0. These are the actual volumes on the banded roll. The volumes were measured using the Troika Systems Limited AniCam. The ink set was water based and each color was proofed with a mechanically engraved hand proofer 360 volume 5.0 , using a Parmarco precision proofer. This test indicates that a 5.5 volume is needed to print with the water-based ink set. Now that consistent ink is coming out of the inkroom at ∆e 1.5 or better, a minimum of volume 5 anilox roll is required. If a roll is measuring at a lower volume, add base prior to the ink going to press. It has been found that about 10 pounds of base is necessary for every 1.0 volume. So, a .05 volume dif- ference between what was initially thought and what is reality, could require five to six pounds of toner. Next, to determine the print tolerance, the print must be measured. VARIATION & REPEATABILITY International Standards Organization (ISO) 126471 1996 defines variation and deviation as two separate aspects of color repeatability: • Variation is a measure of differences among samples taken at random from production. Further, variation is divided into spatial (side to side) uniformity and temporal (front to back) consistency • Deviation, or accuracy, is the difference between the average of production and the actual standard In order to understand variation and repeatability in print, the print should be measured and compared to the standard. Deviation is a function of tolerance. These can be measured with a spectrophotometer. Remember, a spectrophotometer has a resolution as tight as 10 nanometers and some models have an even lower resolu- tion. Measuring one color in the exact same spot 10 times with a 10-nm spectro can have a deviation in a single spectrum of ∆e -E .03. That may not seem like much, but it adds up. Spatial deviation (color across the web) will have the largest impact. Measuring color across a 45-in. web will have deviation outside of ∆e .80. Uneven camber pressures, variation in the volume of the anilox rolls, and dyne level variation of the substrate cause this. The anilox roll supplier will designate the variation across the roll. Typically it is 3 percent; meaning a 5.0 volume could possibly have a 4.85 volume variation across the web. Once these measurements have been made, add inkroom tolerance to the print vari- ance to arrive at the print tolerance. Example: Inkroom tolerance = ∆e-E 1.5 Printing variance = ∆e-E .80 The process is printing at a ∆e-E 2 .3 from standard CUSTOMER EXPECTATIONS Are tolerances known? Does the customer even know what the tolerances are? Does the customer agree with said toler- ances? How accurately can the process reproduce spot color? Agree on what the tolerances are—make sure customers understand and educate them if necessary. Some printers may even improve tolerances. The FIRST manual is a good start, but the FIRST manual is not a set of standards. It is sim- ply a guide to assist you in determining your own tolerances. FINAL THOUGHTS There is always variation in color whether perceived or actual. Determine tolerances through testing and statistical analysis. Manage tolerances and improve where possible. Make an agreement with customers on specification and tolerances to avoid any color mismanagement. n About the Author: Rick Rosenberger is director of corporate paper printing for Group 360 Worldwide. He is a past FFTA Forum chairman and a frequent speaker and session mod- erator at flexographic trade industry symposiums. 60 FLEXO may 2012 www.flexography.org 2.00 3.00 4.00 4.50 5.00 5.50 6.00 6.50 7.00 8.00 2.83 3.51 2.61 2.93 5.33 5.82 6.84 7.21 6.62 6.85 2.83 3.51 2.61 2.93 5.33 5.82 6.84 7.21 6.62 6.85