Sign & Digital Graphics

August '18

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38 • August 2018 • S I G N & D I G I T A L G R A P H I C S DIGITAL PRINTING AND FINISHING DIGITAL GRAPHICS Ink Choices for Printing on Rigid Substrates UV-curable inks have a new challenger when it comes to printing onto rigid boards 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, busi- ness development and business manage- ment with DuPont. Dr. Work holds a Ph.D. in physical inorganic chemistry from the University of New Orleans. He can be reached via email at workassociates@ comcast.net, or visit his website www.workassoc.com. B Y R A Y W O R K , P H . D . Think Ink ever, may be viewed negatively. The relatively thick ink layer can provide relief and may not result in a perfectly smooth surface since the drops become 100 percent polymer when cured. Also, an objectionable smell when used indoors, and marginal adhe- sion to some untreated surfaces may be challenging. UV inks use ingredients that are expensive compared to non-UV inks. UV inks are 100 percent solids vs. water-based inks which are only about 10 percent solids. Water-Based Pigmented Inks For porous substrates, like corrugated paper board, water- based pigmented or dye-based inks in thermal ink jet printers can provide a good alternative since the ink vehicle, mostly water, can soak into the paper and dry quickly. If the paper sur- face is white the result can be more cost effective than UV-cure inks since the system is simpler and the inks' ingredients are less expensive. However, these inks cannot perform on non- porous substrates so they are generally dedicated to corrugated board, and if a hybrid printer, paper or coated roll media. Some substrates may require a pre-coat prior to printing to optimize hold out of the pigment and improve image quality. HP and Durst offer high-volume water-based flatbed printers aimed at corrugated packaging markets. Other advantages of water-based inks in this application include ease of de-inking compared to UV-curable inks, and a lack of odor for indoor applications. Latex Inks The newest entrance into the rigid printer market is the HP R2000 Latex hybrid printer, first introduced this year at FESPA in Europe. It provides water-based latex ink printing on porous and non-porous rigid media. In addition they offer white ink, a first for water-based inks from HP. The inks used in the R2000 are different from other HP Latex inks since they print two layers in addition to their preprint optimizer. This offers a very thin pigment layer which improves their ability to manage drying and maintain high image quality. The optional top clear layer provides durability. If the print is to be laminated the top layer is omitted to provide better adhesion of the laminate to the media. The inks are film formers not chemically crosslinked like UV inks and provide a less brittle image. The result is that ink cracking when cutting the substrate is reduced or eliminated. The white ink is whiter and more opaque than most UV white inks and does not yellow with age as some UV inks will, accord- ing to the company. Like water-based pigment inks there is no smell commonly associated with UV inks. F or several years there was basically one choice for printing graphics on non-porous, rigid substrates using inkjet tech- nology—UV-curable inks using piezo inkjet printheads. This has now changed. Today there are several additional choices, using both piezo and thermal inkjet technologies. Here we will discuss these alternatives and comment on their differ- ences, advantages, applications and how they compare with UV-curable inks. UV-Curable Inks Clearly, unlike flexible roll-to-roll substrates, rigid sub- strates must be printed in sheets requiring feeding them one at a time. Because UV curable inks require protection from pre-exposure to light and they must be immediately pinned as the printing takes place to avoid coalescence of the drops. The implementation today is often LED-based UV light sources attached to the shuttle containing the piezo printheads. For additional printing speed with good curing, an oxygen-free environment is necessary since oxygen inhibits the curing pro- cess. This technology has been the mainstay of companies like EFI for rigid substrates. Several aspects of this process, how- HP's new HP R2000 Latex hybrid unit, although water-based, prints onto porous and non-porous materials, includes white ink and prints in two layers. (Image courtesy of HP Inc.)

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