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14 Q U I C K P R I N T I N G / N o v e m b e r 2 0 1 4 w w w. M y P R I N T R e s o u r c e . c o m used in inkjet printing "because it's going to absorb so much of that ink. In essence, we might as well be taking a hose and spray- ing water on the paper." Coated papers can have the opposite problem. "In coated papers, the coating is hydrophobic [water-repel- lent]," said Ross Allen, Senior Technology Specialist in HP's Imaging and Printing Group. HP's offerings in this space include the T200, T300, and T400 series inkjet web presses. "Put a drop of water-based ink on it and it tends to bead on the surface and not penetrate it. That creates all sorts of problems for feeding and runnability in a press because you can be running a web that is wet as it goes through rollers and other parts of the press." As a result, offset paper grades must often first be pretreated, typically using a roller coater. The pretreatment comprises a bonding agent, which is essentially a water-based colorless ink that contains chemical compounds that help quickly immo- bilize the pigments on the surface of the paper and, at the same time, also help the ink penetrate the surface. The specific mechanism and/or formulation will vary from press to press and, ergo, from ink set to ink set. "The whole big thing about printing on a web press using inkjet is matching the ink to the media," said Allen. "The printheads, the press, the press's color management, every- thing about it—drying in particular—is optimized for a par- ticular set of inks." "It's the trifecta of inkjet," said Schilling: "machine, paper, and ink." When working with inkjet systems, operators have to pay close attention to saturation—"saturation" in two senses. Col- orimetrically, saturation refers to a given color's intensity, but in inkjet printing, high saturation in the colorimetric sense can also lead to saturation in the sense of soaking the paper. This leads to paper defects like cockling and curling as well as to inconsistent drying. The advent of digital printing changed the relationship of ink and media, and early toner-based devices required that more attention be paid to substrate choice and pre-treatments, although they, too, have become more forgiving over the years. Today's high-speed production inkjet presses require a greater symbiosis of ink, paper, and press—and even other parts of the production process, such as finishing. The big technologi- cal hurdle has been getting aqueous inkjet inks to perform on desired substrates at very high production speeds, with high image quality and accurate and consistent color reproduction. Working with inkjet inks in a production environment is a seachange from offset, and while many shops are proving that the technology may very well be the future of commercial print- ing, at present, working effectively with production inkjet inks can be a challenge. Water, Water Everywhere… The textbook definition of offset lithography is that it is based on the principle that "oil and water don't mix." (That's not entirely true; they mix a little.) As a result, the vast majority of the paper available for commercial printing has been designed and optimized for oil-based inks. Production inkjet, however, uses water-based inks. "Inkjet inks are 75 percent to 95 percent water, depending on whose ink and machine you're using," said Mary Schilling, principal of Schilling Inkjet Consulting, a strong inkjet advo- cate and troubleshooter of inkjet printing workflows. "That's a lot of water, and that water has to go somewhere. Paper is fiber, and what's going to happen to that fiber when that col- ored water hits it?" "Take a spray bottle, spray water on a piece of paper, and it's going to go wavy on you," said Brian Dollard, director of prod- uct marketing, BISG, Canon Solutions America (CSA). CSA's ColorStream and JetStream series comprise high-speed inkjet web presses that have found niches in transactional and book printing, among other markets. The same is largely true of paper By Richard Romano High-speed production inkjet presses require a greater symbiosis of ink, paper, and press— and even other parts of the production process, such as finishing. O ver the course of more than 100 years of offset printing, we've become accustomed to the fact that virtually any ink will work with any press. While media choice does often need to be made with care, offset lithography is a very forgiving process when it comes to putting ink on a substrate. Drops On Demand: Making Water Work With Inkjet Inks