The inkjet printing industry has evolved rapidly over a very short period since the refinement of piezoelectric printhead technology began in the early 1990s. Looking back to that time—only about 15 years ago—Spectra’s (now Dimetrics) piezoelectric printhead technology’s primary application was molten wax-like inks. They would freeze on the surface of the substrate being printed. The ink image was quite fragile and it did not laminate well. One wide format printer which was introduced using this technology failed to attract many buyers and was discontinued. Hewlett Packard’s first color thermal inkjet offering, the DesignJet Model 650, used high glycol color inks similar to those used in the PaintJet desktop printer. These inks were the only wide format color water-based inks available at the time and required specially coated paper to achieve reasonable image saturation and resolution. The colors were based on very unstable dye chemistry which faded quickly if used in a signage application.

Vutek was still selling airbrush printers which spewed a directed mist of ink giving a fuzzy image at low resolution for billboard printing using highly flammable solvent inks. The residual solvent smell made them largely unsuitable for indoor use even weeks after printing.

Nur’s Wideboard and Blueboard printers used continuous inkjet with highly flammable solvent inks. These printers produced an ink mist which caused a potentially explosive fog in the print room if ventilation was inadequate. Metromedia was using a valve jet technology similar to that developed for industrial marking technology and very large drum printers large enough to hold one piece of vinyl the size of a billboard. These nozzles ejected “paint-like” ink onto the vinyl at only a few spots per inch.

HOW FAR WE HAVE COME!
Today we have piezoelectric printheads with hundreds of nozzles ejecting droplets as small as a few picoliters. A single 2-picoliter droplet makes a spot which can only be seen under magnification. Whether water based, solvent based or UV curable, most inks are now pigment based, using much more stable, finely dispersed pigments.

The refinement of piezoelectric printhead technology has resulted in today’s printheads with hundreds of nozzles ejecting droplets as small as a few picoliters.

Both water based inks and solvent based inks have come a long way in stability giving the end user a far more reliable system. In its liquid ink form UV curable inks are solvent inks until they react on exposure to UV light. UV light causes the components to react with one another forming a durable solid polymer image. Dimetrics, now with sales estimated at $100 million, provides printheads for wide and grand format printers manufactured all over the world for signage and other industrial applications. Few if any use molten inks today. Now they have moved largely to solvent and UV curable inks. Hewlett Packard now offers thermal inkjet printers which produce very small drops of pigmented inks from thousands of nozzles. With its acquisition of Scitex Vision, HP now offers a full line of solvent ink printers for production printers based on piezoelectric printhead technology. Today Vutek offers a full line of both solvent and UV curable printers for the sign market utilizing modern piezoelectric printheads—no more airbrushes!

Metromedia has modified its large drum printers with up-to-date printheads. New inks which still resemble solvent based paint are used in the low resolution printers for grand format. The company has incorporated higher resolution printheads and appropriate solvent inks on some of its large drum printers and has expanded into P.O.P. and vehicle wrap applications.

WHAT CAN WE EXPECT NEXT?
In the last few years there has been a big push to move from water based inks to solvent inks in the signage market. The water based ink prints require coated media and expensive water based inks. They nearly always need to be laminated—or both mounted and laminated—making the materials and labor required for a finished graphic expensive. Water based systems compare unfavorably to the new “mild” or “eco” solvent inks in cost. Initially these eco-solvent inks required specially coated media. That initiative was a failure and the inks were reformulated to be more aggressive and perform better on the most important substrate, uncoated vinyl.

These inks as they have evolved, as a class, are generally not as durable as strong solvent inks, generally do not give as good color saturation and gloss and may need to be top coated. They are in general less hazardous both in flash point and toxicity. For many large format printers they have become the choice over strong solvent inks.

For grand format the strong solvent inks reign supreme with their very low cost and superior performance on uncoated media. Since the grand format market is highly price competitive it is unlikely that the other technologies will make much headway against the low cost, strong solvent inks on flexible substrates even with the “green” initiatives offered by some in the marketplace. I predict reactors or other VOC abatement technology will be the approach used to deal with the solvents for these end-users instead of other more expensive “green” solutions.

While strong solvent inks will probably maintain their position as the lowest cost inks in the industry and thus a strangle hold on grand format flexible printing, the much more costly UV curable inks may actually provide a lower cost solution on rigid substrates because they can print directly without mounting or laminating.

The early UV curable offerings have however been deficient in performance in several areas. Since they are touted to be able to print on anything, many people have found that on some substrates adhesion is inadequate, full curing may not be attained and in some cases the ink shrinks and pops off of the substrate on curing. Some of these deficiencies have been corrected by improvements in the design of the inks. Flexible UV curable inks are an example. Some ink manufacturers offer at least two sets of inks, one for flexible substrates and one for rigid substrates.

As with screen printing, for optimum performance the ink must be designed to work with the substrate. No large screen printer could survive with only one or two formulations of UV curable screen printing ink. The chemistry and sometimes the curing must be compatible with the substrate being printed. I predict that you will see specialized UV ink sets and substrate pretreatments to overcome these performance deficiencies.

For fabrics or porous media the substrates must be prepared to accept the UV inks in a way that “holds out” the ink so it can be cured. If the UV light does not reach the interface between the ink and the substrate the curing will be incomplete. A “hold out” coating along with proper UV exposure assures this.

THE “GREEN” SOLVENT INK AND MEDIA FRENZY!
With the concerns about “Global Warming” and the stampede to buy the Toyota Prius, some customers of inkjet service providers demand a “Green” solution. Inkware, part of the EFI/Vutek family, has developed ink sets for use in the Vutek 3360 line of grand format printers, as well as for use in Mutoh’s ValueJet 1608 Bio-Hybrid printer, which have the enviable attribute of improved “Greenness”. Branded as EFI BioVu (Vutek) and MUBIO INK (Mutoh), the solvents and binders used to produce the ink are made from natural products like corn and not from petroleum.

Second and truly important is the fact that the main solvent, called ethyl lactate is truly biodegradable. The inks are formulated to have a low enough flash point so they may be shipped and stored without the precautions required of strong solvent inks.

Another important attribute is that the ethyl lactate is not very toxic to humans. This means the expensive ventilation systems are likely not needed. The drawback is that with this “Greener” ink, even more compromises in performance have been made. Ethyl lactate is a very important solvent. It holds a lot of promise in enabling the ink designers to produce “Greener” and less hazardous products. I predict that “Greener” solvents, which are similar to ethyl lactate will be incorporated in inks for many new printers over the next couple of years. Remember however that the pigments used in the inks are not “Green” and no “Greener” alternatives appear on the horizon. In addition scrim vinyl and cast vinyl used extensively in billboards, banners, vehicle wraps, signs and the like contain chlorine. Chlorine-containing films are not “Green” either. This includes chlorine-containing degradable films. They may fall apart in a landfill but they are still bad chemicals. Work is underway to move away from vinyl chloride (“Vinyl”) containing substrates as alternatives can be developed. UV curable inks provide a “Greener” solution as well. Since they involve very little if any VOCs and encapsulate the pigments when cured they offer another “Greener” digital printing solution. The combination of “Greener” ink and media will go a long way toward a cleaner digital printing environment.

SUMMARY
The future outlook for ink development for the sign market can be summarized this way. The market will further fragment into Mild Solvent, Strong Solvent and UV Curable inkjet systems with many specialized solutions and some “Greener” ones as well. The performance of ink and media solutions designed to be application specific will continue to improve and we will continue to see inkjet printed signage as very vital and healthy business for everyone.