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FLEXO Magazine : November 2009
www.flexography.org NOVEMBER 2009 FLEXO 25 tained. G7 does this by specifying ISO 12647-2 (Grades 1 and 2). Remember, this is the offset version! Since G7 found its start in com- mercial and publication offset print- ing, it made sense to specify the offset version of the ISO 12647 standard. But as G7 has grown in popularity and acceptance, it is starting to be used and specified in the other printing processes, including flexography. The concept, which is a good one, is that if various printing systems can all print to the same target condition (the NPDC), the same graphics can be repurposed and printed with different processes on different substrates with the result being an acceptable match between them. The problem, however, isn't so much with hitting the NPDC condition but more with the color of the solids (both process colors and overprints). Having to shoot for offset-based colorimetric values with flexographic inks can cer- tainly present a problem. Whether it's pigment availability and compatibility or end-use functional requirements of the product (fade resistance, chemical resistance, etc.), the simple fact is that it can be darn near impossible to hit the target values as listed in ISO 12647-2. Now, it's not all bad news. Many of the off-the-shelf flexographic inks man- ufactured today, in mono-pigmented form, either already meet or are close to the specifications in ISO 12647-2. Also, the stated tolerances, or devia- tions, as listed in ISO 12647 are quite large---a Delta E(ab) of 8.0. In fact, in comparing the values between the offset and flexo versions of ISO 12647, it's interesting to note the similarities, or differences, between the two, as detailed in the table below. As you can see, with an ISO-specified deviation tolerance of 8.0, all but the black are within tolerance between the two standards. So, it's fair to reason that many of the "typical" available flexo pigments and inks may very well be within specification to the offset targets. However, an 8.0 Delta E(ab) is quite large, much larger than is typically allowed for spot color matches in our industry. Also, the fact that the Delta E equation specified is the non-weighted version from 1976 does not help. With this version, lightness, chroma, and hue are weighted equally, which has proven to lead to problems with aligning to human color difference perception. In other words, you can have a mismatch between the numbers and the visual color difference. CURRENT DIRECTIONS FTA has been hearing from its mem- bership that printers are being required by brand owners to hit the ISO 12647-2 offset ink target values as a condition of doing business. Because of the issues detailed in this article, it is creating some problems for printers. To help address this issue, FTA is involved in a number of efforts. One effort involves FTA's Flexo Quality Consortium (FQC), the Association's re- search arm. We have launched a project Ink Color ISO Ink Standard L* a* b* Delta E(ab) Cyan ISO 12647-2 (Offset)* 55 -37 -50 1.41 ISO 12647-6 (Flexo)** 54 -36 -50 Magenta ISO 12647-2 (Offset)* 48 74 -3 3.74 ISO 12647-6 (Flexo)** 50 71 -2 Yellow ISO 12647-2 (Offset)* 89 -5 93 6.48 ISO 12647-6 (Flexo)** 88 -9 88 Black ISO 12647-2 (Offset)* 16 0 0 10.20 ISO 12647-6 (Flexo)** 26 0 2 *ISO 12647-2 (Grades 1&2 Coated Papers) **ISO 12647-6 (Coated Paper)
Sustainable Fall 2009