Traditional flexography has been applied mainly to packaging printing. In this application, the printing process incorporates both standard 4-color process color printing for images and special spot color printing for brand identities, logos and graphics.
With the advent and wide spread use of digital color management, it has become possible to extend process printing beyond the three subtractive primaries of cyan, magenta and yellow inks, supplemented with a black ink; to include additional process primaries. Some of the early attempts at this were found in commercial offset printing, where image fidelity was of major importance. This approach was initially known as high fidelity printing. Examples include the “Big Top” ink set from Hallmark1 and Hexachrome from Pantone2. In both of these applications, the standard process color inks were supplemented with fluorescent inks, including a fluorescent magenta and orange.
But fluorescent inks have several disadvantages. They are not very stable to exposure to light and ultraviolet radiation, they are more expensive than conventional inks and they are more difficult to control in the ink kitchen. So, this approach was not widely used in packaging. It did, however, see some application in personal care products, as increased fidelity was important in the reproduction of faces and hair.
Reports given at the annual IS&T Color Imaging Conference3 documented the potential increase in color gamut achieved when using a fifth, sixth and seventh process color primary. Running simulations using a simple Neugebauer additive mixing model, comparisons between the various combinations of primaries were reported. The largest gamuts were obtained by adding an orange, a green and a violet primary to the cyan, magenta and yellow primaries. The second largest gamut was obtained using a neutral red, one that was neither yellowish nor bluish, a true green and a red shade blue primary. However, the studies focused mainly on publication and commercial printing.
Some approaches, such as that by MY Cartons4 in Europe attempted to utilize a hybrid, where standard halftone images were printed by conventional 4-color process inks using analog plates and the spot color graphics were printed using three additional primaries that were printed using frequency modulation (FM) screening.
Other approaches, such as that of the Australian Opaltone5 system focused on using standard Pantone primaries, adding a Pantone conventional spot color yellow shade red, a green made by mixing a yellow primary and a blue primary and a red shade blue primary.
Following this logic, some brand owners in the U.K. and in the U.S. attempted to substitute a 6- or 7-color process set in place of their traditional 4-color process set and three brand colors. The thinking was that by using only process colors, the ink wells would be kept active and full and there would be an operating cost reduction due to not having to stop the press to change inks. Additionally, different products could be printed on a wide web at the same time, since all products, no matter what brand colors were needed, would be able to utilize the same ink sets.
Flexo Quality Consortium (FQC) was charged with identifying a series of inks that could be combined with the Flexographic Image Reproduction Specifications & Tolerances (FIRST) process inks to give an extended color gamut process set. The exact combination of primaries would be left up to the converter or determined jointly between the converter and the print buyer.
A survey was distributed to Flexographic Technical Association membership and a review of the concepts of Expanded Color Gamut (ECG) printing was undertaken. The survey showed that the majority of FTA members would choose an orange and a violet as the first two extended gamut primaries. Then there was an almost equal distribution for green, blue and red.
So, the plan attempted to define carefully an orange primary and a violet primary and document the gamut gains. Then it added the green primary and documented the gamut gain, followed by substituting the red for the orange and the blue for violet.
The color science of ECG gives guidelines on how to choose primaries. Figure1 shows a CIELAB a*, b* diagram with the positions of the standard ISO 12647-5 primaries shown. The theory is that one would want a secondary primary that falls in the preferred location of an ideal two-color overprint, which never happens since the transfer of wet ink onto a previously printed ink (wet or dry) is never the same as the transfer of ink onto unprinted substrate.
In Figure 1, we see the usual dilemma facing an ink supplier trying to select a set of two or three additional process primaries. The orange is a good complement to the cyan and will give a very large extension to the CMYK gamut. Similarly, the magenta is a good complement to the green primary and would be even better if the magenta were slightly more yellow, like most inks for use in light and solvent fast applications. Both of these new primaries show much better complements than they do for the two-color overprint colors. Unfortunately, neither blue-1 nor the violet is a good complement for the typical yellow, though the violet could be a good complement to the two-color overprint of a green primary and the yellow primary. Blue-2, on the other hand, seems to provide a good aim for the yellow. So, the optimum set of primaries from the colorimetric point would be to add a bright orange, a bright green and a strong red-shade blue.
FTA sponsored a student at Clemson University who surveyed four of the largest ink suppliers in North America6. The list of 24 pigments and inks that might be used as extended gamut primaries were tabulated and then documented in a manner similar to that used to document the four process primaries in the FIRST guide7. Table I shows the results of the Clemson study.
The student evaluated the impact of each new colorant using Esko Color Engine Pilot8 software by substituting each colorant, one at a time into an ICC profile for an ANSI CGATS9 IT8/7-4 chart for sheetfed commercial offset printing (GRACoL10). Thus, the addition of red was evaluated by replacing magenta with a true red or by replacing yellow with an orange, etc. The report indicated that adding one pigment could result in a gain in gamut of up to 40 percent over CMYK alone. Figure 2 shows the expected gamut increases in the region of CIELAB space near to the added primary, as reported by the Esko software.
The report from Clemson recommends adding an ink containing pigment Green-7, an ink containing pigment Violet-23 and an ink containing an orange pigment. But the report was unable to make a specific recommendation on a single orange pigment. Of the five submitted orange inks, the study was able to reduce the number to three colorants but was not able to distinguish between, pigments Orange-5, Orange-34 and Orange-64. However, Orange-5 is a difficult pigment to manufacture and its shade and undertone vary dramatically from batch-to-batch and would thus not be a suitable candidate for a fixed process set. Orange-64 is a high performance reddish orange with better light and chemical fastness than Orange-34. Due to its improved stability O-64 is most often used in high performance inks and coatings, as a result it generally has a higher cost per pound than O-34.