Ink Isn’t "Just Ink"
Ink isn’t "just ink" any more than food is "just food." In fact, to be a printer these days you pretty much need to be a chemist as well.
How ink is produced depends on the results you want to achieve, and this depends on a number of factors, such as the brilliance of the colors, the absorbancy of the substrate on which it is printed, the need for static (i.e., duplicate or "all the same") copies vs. the need to print variable data, the flexibility of the substrate, and the durability of the ink in the face of moisture and UV rays (sunlight).
When I checked back through the PIE Blog and PIE Quick Tips archives in composing this article, I saw that I had written similar articles in 2005 and 2012. Now, almost a decade later, the information is not only relevant, but inks have also gradually improved over this time period.
If you are a designer or print buyer, the best way to approach ink choices is to consider your goals. Here are a few scenarios.
Great Inks for Interior Signage
If you are producing signs or art prints for interior use, you don’t have to think about sunlight (probably) or rain. Therefore, you have a few options.
1. You can choose a dye-based ink in water. The ink will seep into the fibers of the paper (if it is uncoated) and reflect brilliant colors, because it provides more surface area than the color particles within pigment ink. This is because the dye has actually been mixed with the vehicle (water) in a solution (as opposed to being held between water molecules in a suspension). However, if you get the printed art wet, the ink will flow again. And sunlight streaming through your windows and even (over time) interior light will degrade the colors. Plus, in many cases you will need to pre-treat the substrate to accept the ink.
2. Or you can choose water-based pigment inks. These are somewhat more durable than dye-based inks, but they are still meant for indoor use. Due to the shape of the suspended particles, as noted above, the colors are less vibrant than in dye-based ink solutions.
Great Inks for Outdoor Signage
Once you decide to display your banner outside, it needs to resist fading (sunlight) and moisture. If it’s a sign rather than a banner, it may also need to be printed on a plastic surface, which will not be porous (as canvas and fine art paper for indoor use would be).
Therefore, replacing the water in the aqueous inks with a solvent (volatile organic compound, or VOC) will ensure the longevity of your printed product during exterior use. The banner or sign will tolerate sun and water. This might be useful if you’re printing lawn signs on Coroplast (corrugated plastic). Solvent ink actually sticks to this non-porous surface better than it adheres to natural products like paper or canvas. So it’s ideal for outdoor signage (billboards, vinyl banners, decals, etc.).
Granted, the solvent in solvent inks is toxic, which necessitates ventilation and other protective measures in the printing of solvent ink products. So a nice alternative to this is eco-solvent inks (mild-solvent inks). These require less ventilation and protective gear in their application. They also take a little longer than regular solvent inks to dry, and they are not quite as durable, so it is important to consider the length of time your outdoor printed product must last. (One source I found noted that solvent inks resist fading for five to seven years.)
Still another option for outside signage is latex ink. This is water-based ink (no solvents, no VOCs) containing latex polymer dyes. Its additional benefit (beyond the reduced toxicity) is its flexibility. You can print latex ink on fabric banners, and unlike solvent ink it will not crack when folded. Rather it will stretch and flex with the substrate. Latex inks are applied and then heated within the digital press. Therefore, when they exit the equipment, they are immediately dry (i.e., they can be used right away, since there’s no "outgassing period" in which solvents are still being released). They are also odorless since they include no solvents.
Still another ink option for both interior and exterior signage is UV ink. This ink cures instantly when exposed to UV light. For this reason, the ink sits up on the surface of the substrate (even on uncoated substrates), yielding brilliant colors. In addition, the ink can be applied in an extra-thick film (also increasing the brilliance of the colors). Due to its drying properties, you can print on glass, metal, and wood. Moreover, the advent of flatbed inkjet printers (as opposed to the prior generation of roll-fed inkjet printers) makes printing on rigid substrates an option. Unfortunately, though, due to the thickness of the ink film, UV inks can crack if bent, folded, or twisted. Other than that, the color is long-lasting, and the ink is durable.
Still More Inks
Indoor/outdoor and short-term/long-term use are among the determining factors for the inks noted above. However, there are still many more inks.
1. Dye sublimation inks: You print (backwards) on a carrier sheet, ostensibly using inkjet technology. You then place the transfer sheet against polyester fabric (or even aluminum or ceramic) and apply intense heat. The ink sublimates (changes from a solid state to a gaseous state, skipping the liquid state) and permeates the substrate. This is an aqueous process (no solvents are needed).
2. Indigo inks: These are composed of minuscule toner particles suspended in oil. The particles are electrically charged (via laser printing technology) and fused to the substrate with heat and pressure. What makes this unique is the fineness of the particles, allowing for 2400 dpi imaging (in contrast to the usual 600 dpi resolution of conventional toner-based printing using a mixture of carbon powder, iron oxide, and polymer).
3. Food grade inks: These are used when product packaging will come in contact with food. (This would include not only ink for food and beverage cartons but also ink for printing on the top of sheetcakes and such.) The FDA is especially strict about their guidelines to prevent contamination.
4. Magnetic inks: These are used for MICR equipment that reads the magnetic numbering on bank checks.
5. Temperature-specific inks: Let’s say you’re printing on labels for a blood bank. Since the blood will be refrigerated, the labels (and the ink) must tolerate the low temperatures.
6. Low-heat inks for plastic film: Let’s say you’re printing on shrink-sleeve material or the plastic used for bags in which bread is sold. (Usually you would use aqueous inks and flexographic technology.) In such cases the temperature during ink application would be important so as not to vaporize the plastic packaging material.
I’m sure there are many more inks (such as the ceramic inks used to make floor tiles, which must withstand the high-firing temperatures of the kilns). But this is a good primer to get you started on your own research. As the years progress, I’m sure there will be many more of these developed for specialty use. And some may be retired. (For instance, it has been a while since I last saw phase-change inkjet inks, which were solid ink blocks like colored crayons. Heat would sublimate the ink into a gas, which would migrate to the substrate and then harden instantly as it cooled.)
[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]