Digital Glass Decoration

Sign painters solved the problem by using the gold leaf process, but it was time-consuming and the cost of materials was very high. So most glass decorating was done with sign paint. It was inexpensive and looked good (for a while anyway). The advent of digitally-cut vinyl graphics, and then full-color digital imaging onto relatively permanent adhesive substrates, changed the glass decorating world.

Printing on glass has typically been done using screen-printing techniques with ceramic-frit inks. However, new inks based on sol-gel technology are now available for use with UV-curing print inkjet technology. (Image courtesy of Durst U.S.)

Glass is non-porous and hard—very hard. Since its inception in the Bronze Age, adding decoration to glass has been a major problem. Well, let me rephrase that: getting anything to stick durably to glass has been the challenge.

Early adhesive materials had opaque white substrates that worked well for covering windows with delightful images, but didn’t work as well at letting light into the interiors (the main purpose of the windows in the first place). Two new products changed that. One was the invention of perforated film that allowed one-sided viewing of the graphic, while it appeared to disappear from the other side. The other was clear printable films with optically clear adhesives. Imaging onto these materials is done with standard solvent or light solvent inks, making the process easy.

Durst now offers a glass decoration system for use with its Rho flatbed printers. The system features Durst’s Organic Glass Ink, which employs sol-gel ink chemistry and offers good adhesion. The components of the system include the special organic UV-curable inks, the printing system (typically the Durst Rho 700 or Rho 800), and a drying system. These can then be incorporated into a complete manufacturing production line. (Image courtesy of Durst U.S.)

PRINTING DIRECTLY ONTO GLASS

But there are times when a durable image needs to be printed directly onto the glass. This has typically been done using screen-printing techniques with ceramic-frit inks. These inks need to be fired in an oven, then cured in order to become stable. Ceramic inks are inorganic pigments and are very durable for long-term outdoor use. They can resist fading, weathering, scratching and chemical cleaners. We all know how cost prohibitive screen printing is on short runs though, so the world has been waiting for digital on glass.

Applications for printed glass include bar decorations, trade shows, furniture glass, partitions, gaming machines, mirrors and custom jobs like this interesting installation on a dry sauna. (Image courtesy of Durst U.S.)

Well, digital is here. UV-cured inks in a flatbed were the first on the block. UV inks contain an adhesive that helps the pigment bond to a variety of surfaces. Standard UV inks will adhere to glass, but not very durably. Several systems have been developed to aid the adhesion.

First the glass has to be cleaned with a special chemical, then coated with a clear primer. After printing, the ink needs to be cured in an oven. UV inks are only suitable for short-term use outdoors, but are fine for indoor use. UV inks are convenient- the same printing equipment can be used for glass or regular substrates. However the colors are not translucent on glass, but also not completely opaque, leaving them looking a little dull compared to other ink types.

GLASS-FRIENDLY INKS

A new family of inks, called sol-gel—originally developed by researchers at the University of Illinois—are proving to be excellent for printing onto glass. This has a good side and a bad side. It means there is a better glass-printing solution than UV inks, but it also means a dedicated printer would be required. One big advantage is that sol-gel inks do not require a primer coat first. The glass does have to be cleaned, typically with acetone, before printing. And after printing, it requires baking at 200 degrees for 10 minutes. This results in a chemical bond of the ink to the glass for excellent resistance to weathering, scratching and chemicals. Since they are organic inks, however, fading is still a problem for long-term outdoor use.

Hi-end retail settings are ideal for printed glass applications such as this. (Image courtesy of Durst U.S.)

Ceramic-frit inks—traditionally used with screen printing systems—are now available digitally. This requires not only special inks, but also a special printer, ink dryer and glass kiln. The results are worth the cost if the market for the product is there. No special cleaners or primer are needed.

After printing with ceramic-frit, the ink is cured in a dryer and then fired in a glass kiln. After that it, is quenched and cooled—a fairly large process. Printers typically can vary the amount of translucence and print density, allowing for brilliant stained-glass effect colors. Spot colors are available, as are opaque backing inks and etch appearance inks. These are inorganic inks that are fired right into the glass, they will last a long time outdoors or indoors and withstand the maximum abuse.

Is there a “best” way to print on glass? Of course not. The right process is going to be different for every circumstance. Glass is heavy, glass is fragile, glass is hard, glass is non-porous. All of these factors create their own problems, and there is no perfect answer. But if it was easy, everybody would be doing it.

Ken Mergentime

February 21, 2011 5:26 pm

Stephen, I've contacted the author on your behalf, and the following is his response:

"There is of course no one good answer to that question. Ink manufacturers typically do not warrant their products for long periods of time because so much depends on the preparation for printing (especially on glass) and the conditions it will be used under.
"There are many variables. What is the UV rating of the glass? How many lumens of light will be hitting the ink for how many hours a day? No one has done tests that include all variables for all circumstances. And then the question remains- what is acceptable fading? At what point does it become noticeable, at what point does it become objectionable?
"Sorry if my answer sounds evasive, but there is just no one answer. If it is a big project where this is a serious issue, we always go to a testing lab to have accelerated testing done. If it is not feasible, the bottom line is the manufacturers warranty (which will be very conservative). Then you can add "rule of thumb" to it for enhancements that you have added (like UV glass).
"To find out what the warranty is for your brand ink, check with the mfg. They should also be able to help you with some rough calculation guidelines."
--Duane

galloway

February 15, 2011 3:33 pm

You say that "UV inks are only suitable for short-term use outdoors, but are fine for indoor use." When you say they are fine, how long of a life are you talking about? How long will UV inks last in an interior use, with only exposure to flourescent light and no daylight?

I'm planning an application of UV-cured inks encapsulated in a laminated glass. The laminating material provides additional UV filtration.

stephen@stephengalloway.com

Sign & Digital Graphics

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