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FLEXO Magazine : November 2009
www.flexography.org NOVEMBER 2009 FLEXO 23 an effort to drive consistency and repeat- ability along with accuracy in all parts of the process, and inks are no exception. ISO, the International Standards Orga- nization, has had standards in place for the color of process inks for many years. There is good news and bad news here. The good news is that by having an internationally adopted and approved standard available, we can all shoot for the same target---an excellent concept. The bad news is that we really don't have one target to shoot for. Alternatively, the target that is being chosen by CPCs and brand owners is for a different printing process, the pigments for which we can't necessarily use in flexography. ISO & FIRST There are two ink standards that ap- ply to the color of process inks: ISO 2846 and ISO 12647. Both offer versions for each printing process. The two stan- dards should not be confused with each other---they have different purposes. ISO 2846 is strictly to verify the color and transparency of the four process inks, independent of the substrate. It is well understood that the substrate greatly influences the resulting color of the ink/ substrate combination. The way ISO 2846 deals with this is to isolate the ink by standardizing the substrate used for the proof prints. It specifies Apco II coated paper from Scheufelen in Ger- many. By using the same substrate for each proof and color analysis, one can effectively evaluate the ink by itself. ISO 12647, on the other hand, builds upon ISO 2846, but specifies colorimet- ric values for the process inks and over- prints (red, green, and blue) for different categories of production substrates. It provides colorimetric aim-points for the pressroom, taking into account various classes of substrates. For example, for the flexo version (ISO 12647-6), val- ues are provided for uncoated paper, coated paper, and film/foil substrates. Accordingly, the values are different for each category of substrate. In the case of the offset version (ISO 12647-2), val- ues are provided for five categories, or grades of paper from Grade 1 to Grade 5, again with different values the various grades. The premise is that inks that meet the specifications within ISO 2846- 5 should yield values within specifica- tions found in ISO 12647-6 when run on the listed categories of substrates. To add to the mix, FIRST (Flexograph- ic Image Reproduction Specifications and Tolerances) 3rd Edition and version 4.0 both specify colorimetric values for the process colors, but list different values for each of the three main ink systems: water-based, solvent-based, and UV. This is due to the fact that the various ink systems cannot all use the same pigments, and therefore, can- not attain the same colors in a mono- pigmented formulation. The values are based on the ISO 2846 test procedure (utilizing the Apco II standard sub- strate), and are the foundation for the values found in ISO 2846-5, which in fact are derived from averaging the values from the three ink systems. G7 & OTHER DEVELOPMENTS Even though the ISO ink standards have been in existence for many years, it wasn't until G7 came along that more of a focus and emphasis was put on them. One of the main premises of G7 is to nail down, or anchor, the color of the process inks. This is so that, when one achieves the specified target printing condition in G7 known as the Neutral Print Density Curve, or NPDC (defining both gray balance requirements and tone reproduction), the overall color match between two output devices (presses, proofers) can be very good. The reason for this is that just about any set of process color inks can be printed to achieve gray balance as specified in G7, but if the colors of the inks are different between devices, as image content approaches the solids (and overprints), the color of the images will deviate unacceptably. Therefore, by specifying the color of the process color inks, it levels the playing field so that a much better color match can be ob- • Investigate pigment availability for process inks. • Benchmark colormetric values from multiple ink suppliers. • Test all inks following ISO 2846 protocol. • Collect data. • Issue report of findings. • Update current standards.
Sustainable Fall 2009