Color fidelity is essential to the quality of a printed piece. Print service providers (PSPs) sometimes struggle with achieving true black. Ink sets are evolving to address this issue.
Cyan (C), magenta (M), yellow (Y), and black (K) are the four primary elements to color formulation within the inkjet printing space. Realizing true color requires a bit of science, art, and experience.
CMY + K
Within the CMYK color space, several formulas are used to achieve true black. The densest and blackest color is achieved by creating four color blacks, says Steve Igoe, sales manager, North America, Bordeaux Digital Printink. He explains that this is important when rich solids are needed.
Bordeaux ink formulations are formulated to give a true neutral black, which is defined as not having a predominance of blue, purple, green, or brown. “The best way to get a solid black is to use a combination of C, M, and Y—roughly about 70 percent of each—with an additional amount of pure black,” he says. This is important for backlit signage as well as graphics placed outdoors and that tend to get washed out by a large amount of light.
“In the perfect world, mixing CMY should produce true black,” notes Julie Gederos, product manager, ecommerce, supplies and accessories, Roland DGA Corporation. However, variables such as impurities in the CMYK ink may cause the CMY mix, or even K ink by itself, to produce a color similar to dark brown as opposed to black. Thus, true black is often difficult to achieve without adding additional colored inks.
For Eco-Sol Max inks, Roland offers the Roland Color Library in Roland VersaWorks RIP software. Within the color library, saturated black is achieved by using the RVW_BK22 swatch color, which is Roland’s version of a dark black spot color.
Tom Sheehan, wide format ink specialist, Van Son Ink Corporation, says that it is important to start with an ink that matches the OEM printer’s black. From there, greater depth and richness can be obtained by adding CMY to the black ink. “Once you decide, for example, that you need to add 40 percent CMY to the 100 percent black, it is important that your colors are consistent to attain the same result each time.”
John Kaiser, product marketing manager, inkjet inks, graphic systems division – wide format and specialty ink systems, Fujifilm North America Corporation, suggests that true black is best produced using only the black channel; however, achieving color density with black alone is challenging. He notes that UV inks are better at this than solvent as higher initial saturation levels are achievable. “It is at times necessary to use a traditional four-color rich black build to achieve the desired effect. This high ink build will sometimes result in bi-directional gloss banding or uneven gloss,” he adds.
Fujifilm’s Uvijet and Color+ inks were built from the ground up to feature extremely high density and accurate hues, which allows for less ink being put down to achieve a desired black and less banding or gloss issues.
Reaching true black is subject to the same limitations that reaching pure tones for all primaries including C, Y, and M.
Dispersion is essential to reaching true black. “Theoretically, for pigment K, the color of the black is a function of pigment particle size. The bigger the pigment particle size, the richer the black it is,” notes Joseph Terramagra, technical service manager, Western region, Sensient Technologies Corporation. “It is an optical phenomenon. Small particle size black tends to be brownish. Typically, you can use some C and M pigment in K to turn it to the color desired,” he adds.
“In order to extend durability in terms of color fastness, most ink utilizes pigment dispersions as active colorants,” says Jeff Edwards, international product marketing manager, display graphics systems, Océ North America.
The available range of synthetic pigments and the dispersion techniques available to ink manufacturers is limited. Pigment selection is further stunted by the curing technology used, base chemistry of the ink, and photo initiator choice, explains Edwards.
An ICC profile producing a rich black can be used printing the other primaries in specific values under the overprinted black ink. The biggest challenge is staying with reasonable ink restrictions for the printer/ink/media combination to ensure a pleasing, uniform result.
Absorption and Adhesion
The absorption of the inks into the inkjet coating also creates challenges to achieving true black. Cindy Laramee, digital imaging specialist, InteliCoat Technologies, explains that this can also create the issue of incorrect profiles.
The objective of InteliCoat engaging the ink business was to provide a complete solution to the marketplace, notes Laramee. The company’s strong background in testing and qualification of inkjet solutions is applied to its Magic Inks offering, guaranteeing high quality, repeatable optimized output. With matched performance to OEM inks, existing profiles are used without compromise to quality.
Adhesion to the substrate can be an issue when attempting true black. “The stronger the adhesion to the substrate, the higher the ink load,” notes Jeff Olson, national sales manager, Seiko I Infotech Inc.
Seiko’s ColorPainter inks do not rely on extreme temperature baking to adhere to the surface of a substrate; rather, they dig right in and fuse at a molecular level. “Stronger adhesion allows higher ink loads without running or uncontrolled dot gain. Higher ink loads allow deeper, richer colors,” states Olson.
In addition to black, Olson suggests true gray is also a problem as the color is often created using a combination of other colors. “Often, this results in a color cast of magenta or green in the gray print,” he explains. ColorPainter H series printers address this by using two gray inks—gray and light gray—and black to achieve better grays in any print and perfect monochrome prints using three ink colors with absolutely no color casts.
Mastering true black is important to color fidelity. A variety of methods are used to achieve this. A combination of science and experience offer the best results.
Click here to read part one of this exclusive online series, Ink by Application.