Ed #9 Understanding Ink

Hundreds, if not thousands, of different inks are available today. By far, the single largest category, accounting for close to half of all ink sales, are those used in offset printing.

Web offset printing inks are used with high-speed presses that can run at the rate of several thousand feet per minute. To work at those speeds, the inks are formulated with low viscosity and tack. Heat-set web inks contain special solvents that help the ink dry when it passes though the dryer units of the press. Inks used with web newspaper cold-set presses, which typically do not have drying tunnels, are formulated to be absorbed into the fibers of uncoated paper—and to leave ink on your fingers.

Most sheetfed presses use non–heat-set inks that dry by oxidation. The inks typically have a higher tack than web inks to provide good results at slower speeds.

Waterless inks—and waterless printing—eliminate the water or dampening solution that repels ink from the non-image areas of the plate. Instead, a silicone rubber-coated printing plate draws ink to the image area through temperature, which is controlled by chiller rollers on the press.

Several types of ink tackle the environmental issues associated with conventional, petroleum-based inks. As conventional inks dry, alcohol, petroleum evaporate, and volatile organic compounds (VOCs) are released, adding to air pollution. To control VOC emissions, printers must install elaborate ventilation systems. Or they can switch to different ink technologies.

Vegetable-based inks replace petroleum with soy, corn, walnut, linseed, or coconut oils. In addition to practically eliminating emissions of VOCs, vegetable inks allow presses to be cleaned with water-based cleaners, which further reduces VOC emissions and the printed product is easier to de-ink during recycling. Vegetable oils also tend to be clearer than their petroleum-based counterparts, so the color can be brighter.

In the past, vegetable inks had poorer rub resistance than conventional inks, but the difference is not as great as it once was. Drying times have also been improved. Aside from somewhat higher costs, vegetable inks now compare well with petroleum inks, and they even allow for laser printing, foil stamping, UV coating, and other finishing processes.

Ultraviolet, or UV inks, and other energy-cure inks, which are dried by a beam of ultraviolet energy, emit no VOCs. The chemical curing process which the inks undergo incorporates all of their components, so none are released into the air. Because they sit on the top of the paper, UV inks score high in ink holdout, which makes them especially well suited for use with uncoated stocks, as well as with substrates such as glass and plastic films. Applying a white UV ink, followed by process or spot colors, allows you to print on dark stocks in one pass through the press. While UV inks offer a number of advantages, they are roughly twice the cost of conventional inks, and not all printers have the equipment or experience to using them.

Six-color process inks like Pantone Inc.’s Hexachrome® and other “high fidelity” inks expand the gamut of colors produced by conventional four-color printing.

Adding extra colors—which often include fluorescent colors—to CMYK inks captures a wider slice of the spectrum, which allows designers to create special effects and match custom colors. Depending on the two additional colors that are chosen, six-color printing can be especially useful in reproducing oranges, greens and purples.

Six-color printing is not for every project, however. It may not be worthwhile when it comes to blues, which can be reproduced well using the four process colors. There is the time and expense of preparing additional plates, and specialized separating and proofing systems also are required. To gain some of the benefits of six-color without those drawbacks, many printers use highly refined four-color inks, such as Toyo’s Kaleido inks, which offer much of the range of six-color inks, without requiring additional printing units. If six-color inks are used, some experts recommend printing them using stochastic rather than conventional halftone screens.

Used mainly in flexography and gravure printing, water-based inks virtually eliminate ink-derived VOC emissions along with solvent-based cleaners. To use them successfully, however, presses must be calibrated especially carefully, and older presses may not be able to achieve the precision required.

There also is a wide variety of specialty inks.

Metallic inks contain metallic particles such as bronze or aluminum. The metallic particles make the inks opaque rather than transparent, and because the particles cannot be ground as fine as conventional pigments, they can often cause problems on press. Metallic inks require a different ink-water balance than conventional inks, and it may be necessary to apply two hits of the inks to gain the desired coverage. The inks also may need a relatively long time to dry, and the dried ink is often susceptible to rub-off and tarnishing, which can necessitate the use of varnishes or coatings. Unfortunately, coatings will tend to soften the metallic effect.

Available in a wide variety of colors, pearlescent inks create a deep, pearl-like shimmer thanks to special pigments that reflect light at varying angles. The pigments combine titanium dioxide that has been coated with mica and then colored with varying organic pigments. Pearlescent inks are commonly used in car sales brochures.

Fluorescent pigments yield bright colors by absorbing visible or ultraviolet light and emitting it again at a longer wavelength. Fluorescent inks are often used to attract attention in such applications as posters and displays and are sometimes added to CMYK inks to increase the brilliance of the process colors. The inks tend to fade quickly in direct sunlight, however, and two hits of ink are often required to achieve adequate coverage.

Edible inks are not intended as a meal in themselves, but used in packaging and wrappers that come into contact with food. They are formulated with ingredients that are proven to be nontoxic.

Thermochromatic inks are heat activated, changing color when they reach a specified temperature.

Photochromatic inks are activated by ultraviolet light, which changes them from a clear
varnish to bright color.

Phosphorescent inks glow in the dark after they have been exposed to light. They are often used in road signs, toys and novelties, and safety products.

Scratch-and-sniff ink is made up of tiny capsules of fragrance, each about half a thousandth of an inch in diameter. The fragrance is released when the microcapsules are broken by rubbing or tearing the paper. While scratch-and-sniff inks are used most widely in perfume advertising, the inks are available in a wide range of scents, including cookies and leather.

Scratch-off ink is used in instant lottery tickets and game cards to hide the numbers or symbols that are printed beneath. The information is printed on the card and then covered by a coating that can be scratched off.