Printing and Design Tips: May 2017, Issue #190

UV Coating Digital Print Jobs

I recently spoke to a local printer with an HP Indigo digital press about UV coating a book cover. I was told it couldn’t be done. Ironically, the same printer had laminated press sheets from the same digital press for a color book I had brokered to a “fashionista.” (It had been a small, PMS-like swatch book of colors relevant to one’s complexion, make-up, and fashion choices.)

What’s the difference, I asked myself. How can one coating work and another coating not work. So I went to school on the subject and did some research in the online chat rooms, where various printers and (more specifically) operators of digital print equipment and various coating equipment expressed their views and told stories about their experiences.

I’m condensing the gist of what I learned for a few reasons:

1. Because I know that with the explosive growth of digital printing, this may be an important subject for many PIE readers.

2. Because I learned that the key issue is adhesion (coating sticking to the digital colorant).

3. Because the different answers pointed to the fact that different technologies and materials are used in different digital printing workflows.

4. And because many of the people whose views I read had different experiences from one another.

Therefore, rather than providing answers, I’m encouraging you to ask questions of your print providers. Moreover, I’m encouraging you to ask for samples as well, since nothing speaks more clearly than a physical product. If the UV coating, aqueous coating, or varnish won’t adhere to the digitally printed book cover or other product, you’ll know it quickly if you have a physical sample in your hands.

Here are some things to ask about:

1. Some digital inks include fuser oils in their mixture. Apparently these need to dissipate before you do any finishing operations. This includes binding as well as coating. Ask your provider if their printing process involves fuser oils and ask which cover coatings (laminates, UV, aqueous, and such) work with their process.

2. The chat rooms mentioned digital ink (used with the HP Indigo). Since the vehicle that carries the toner particles is an oil, ask the same question as noted above. It may be the same process.

3. Other digital equipment uses dry toner. According to the chat rooms, this includes some Xerox and Ricoh machines. The brand names are not important. Whether your particular cover coating adheres to dry toners is important, so this is what you need to ask.

4. Still other digital print equipment uses wax-based toner. (Articles referenced Konica and Canon equipment.) Again, the brand names are less relevant than the process. So you want to ask your print provider if the wax-based toner repels the cover coating you have chosen.

5. The chat rooms mentioned a test you may want to ask about: the tape pull test. If tape can pull the coating off several sheets of your print run, it’s time to stop and discuss matters with the printer’s coating supplier. In this case, you have an incompatibility between the coating and the toner.

6. A similar issue is “glue-able” coating. I had always been taught to knock out the coating on any areas that will accept glue. (Think of the various folds and attachments in a pocket folder.) I had always been taught to never glue anything that has a coating on it. Apparently technology has moved on, and this is now possible in some cases. But it would behoove you to ensure compatibility before you proceed. You don’t want all the glue flaps on a pocket folder to come apart.

7. The final thing I learned from my research is that flaws based on incompatibility between the digital print and the cover coating come in a number of varieties. One of these is the lifting of the coating away from the toner particles (as noted in the tape-pull test). But a second flaw I read about is cracking of the cover coating. So you may want to use a high-power printer’s loupe to carefully check the printed samples you get from your printer.

Overall, the take-away from this discussion is that there usually is a work-around. It may be as simple as heating the toner before coating the press sheet. Choosing the proper cover coating for the particular digital printing process is the goal, and it requires experience on the part of your print provider. Make sure he has this experience before you commit to working with him.

Thermochromatic Inks

Books could be written on this subject, so this is just a taste to whet your appetite. Thermochromatic inks change color based on temperature. If you were around in the 1970s, as I was, you may remember the “Mood Ring.” It changed color based on your mood. Or so they said. Really it was changing based on the temperature of your skin.

I did some research on the subject and came upon an article by Nathan Chandler entitled “How Thermochromatic Ink Works.” It discussed the science behind thermochromatic liquid crystals and leuco dyes. With the former, in a low-temperature state, the liquid crystals are crystalline and do not reflect light. Hence they appear black.

As you raise the temperature, the spacing between the liquid crystals changes, and the ability to reflect light changes, so the colors can shift through almost all colors of the rainbow.

Thermochromatic light crystals are very precise in the temperature at which they change color, and this makes them ideal for scientific purposes (such as thermometers). However, they are sensitive to UV light, water, and chemicals, and the equipment needed to microencapsulate them (to protect them) is expensive. You probably won’t see these used in marketing initiatives. They seem to be more appropriate for scientific work.

In contrast, leuco dyes are not as precise in terms of the temperature at which they change color. However they are much easier to use. Therefore, they are also cheaper to use. You can even include these microencapsulated leuco dyes in paints and inks.

On a chemical level, leuco dyes reflect color when they are cold. As you apply heat they become translucent, revealing any color or design printed underneath.

Chandler’s article describes a Coors Beer initiative with a graphic image of mountains on the can. The graphic is white when at room temperature, but as you cool the can the mountains turn blue. As the can warms up again, the graphic returns to the initial white.

This really touches more on the “wow” factor than on the practical application of the technology. Its goal is to attract attention. However, on a more functional level, leuco dyes and thermochromatic inks can be used to show that a particular food has not warmed up to the point that it has become unsafe to eat. Or they can be used to indicate that a coffee cup or baby bottle is hot.

According to Chandler’s article, in microencapsulated form leuco inks are more durable than thermochromatic inks. They can be added to water-based, solvent-based, and epoxy-based inks, and they can be used in offset, gravure, and screen printing. You may have even seen them on fabric. Some t-shirts, for instance, are screen printed with leuco dyes in such a way that when you touch them, the heat of your hand will change the color for a short time, leaving a hand-print and adding to the “wow” factor.

So expand your horizons. Think outside the box, or (in the case of this standee) outside the clear plastic shell.
