UV Gel

Latest Inkjet Technology: UV Gel

Ray Work, Ph.D., heads Work Associates, a consultant firm specializing in inkjet printing technologies, applications and markets. He worked for more than 28 years in research, research management, business development and business management with DuPont. Dr. Work holds a Ph.D. in physical inorganic chemistry from the University of New Orleans. He can be reached via e-mail at workassociates@comcast.net, or visit his Web site www.workassoc.com.

In the August issue of Sign & Digital Graphics I wrote about the new hybrid water-based UV ink technology and the potential benefits over traditional UV-cure or Latex ink technology. Now there is another new ink technology on the horizon showing a lot of promise. The "UV gel" inkjet ink technology currently under development purports to offer similar improvements in performance over traditional ink technology. 

Here I will explain this technology, its pros and cons as well as the challenges faced by the ink developers and printer manufacturers. Remember, all of these technologies are targeting traditional analog printing technology performance not currently being met by traditional UV-cure, solvent/eco solvent and Latex inkjet inks.

So far only two companies have come forward and publicly announced this technology: Xerox Research Centre in Canada; and Canon/Océ headquartered in Holland. They provide us some insight into the future of this new technology. Xerox indicates that their technology is an outgrowth of the phase-change technology currently used in the Xerox 600 solid ink wide-format printer. Although they do not claim  it, Canon/Océ is in my opinion likely developing their technology from that used in their Océ ColorWave 500 and 700 solid inkjet printers.

What is UV Gel Ink?

UV gel ink is unique. The ink formulators have designed the ink to be in the form of a gel when it is at room temperature. At an elevated temperature it is present as a very thin watery liquid so that it can be jetted by piezo inkjet printheads.  After jetting the droplet returns to its gel state. The biggest advantage in this technology over conventional UV ink is that no curing is needed to keep the droplet in place on the substrate after it arrives. The entire droplet remains on the surface.  The ink behaves a lot like phase change ink which changes from a liquid to a solid after jetting.

This ink is more like a paste so it can still conform and spread somewhat on the surface of the substrate. Xerox compares the consistency of the gel to toothpaste and Canon/Océ says it is closer to peanut butter. As the droplet arrives on the substrate it immediately gels, remaining where it lands. Multiple nozzles in the printheads can be used to place un-crosslinked droplets on top of one another to build up the color density for applications such as backlit prints. 

Since backlit applications provide only one pass of the light through the color layer to achieve adequate color density more ink is required than on reflective media. Regular UV ink is cured as it is printed the second and subsequent passes are printed on cured ink. This can add to the height of the ink layer and may cause printing artifacts and adhesion issues. UV gel white ink may be printed either before or after the colored inks allowing the printing to be complete before curing. This could be a big advantage in image quality and uniformity.

What About the Curing?

Another advantage of this technology is the curing. The Canon/Océ technology is designed for UV LED curing after printing. The curing is separate from the printing process.  This approach is only viable with gel or hybrid UV inks.  With traditional UV inkjet a UV lamp is mounted on the shuttle along with the printheads. Reflected UV light can cause premature crosslinking so unused nozzles must be purged frequently to avoid plugging.  This is not the case with UV gel ink.  In an animated printing presentation by Canon at the recent SGIA tradeshow the curing was accomplished in a separate part of the printer system. They announced in September that their first product will be a 64” wide roll to roll printer to be released in Spring 2017.

Xerox indicates that their technology will first be applied to the packaging market.  They have been awarded a patent on a smoothing roller system to smooth the ink prior to curing.  It is not known if this will be incorporated in the design of their printing systems but it could result in an even thinner more uniform ink layer in the final print.

Safety and Stability

There is a clear safety advantage with UV gel ink. Since it cannot splash when it is spilled the hazard to the eyes and skin are much reduced. Its safety profile is similar to water-based inks. One of the difficulties in developing a stable pigmented inkjet ink is keeping the dispersions properly dispersed.  Settling is unacceptable.  Gel inks cannot settle prior to being heated.  They are only heated just before use so good shelf life should be easier to obtain. 

All UV inks contain reactive ingredients that are susceptible to attack by oxygen from the air.  However oxygen diffusion through a gel should be far slower than through a low viscosity liquid traditional UV ink. All of these characteristics should bode well for the success of the technology.

Conclusions

Both hybrid UV and gel UV inkjet inks are welcomed advances in ink technology and, when well executed in a printing system, should offer the graphics printer and packaging printer a superior level of performance.  Time will tell if these technologies can be perfected.  The inherent characteristics offer us a truly exciting potential we all hope can be realized.