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I can assure you that this functionality is often critical to prospective customers. But why is it so important? Are there meaningful applications? Are current implementations up to the task? Let's consider some of the technical characteristics of piezoelectric inkjet white ink capability and the possibilities of its use.

White Ink Challenges
Embedding white ink capability in a piezoelectric inkjet printer is tough, but implementing it in a high-quality, credible way is even tougher. Formulating a successful white ink solution presents several unique engineering challenges. To begin with, the most appropriate pigment material for white is titanium dioxide, which is very heavy compared to the pigments used in colored inks, and therefore doesn't like to stay suspended in a fluid. As a result, most white ink-equipped printers go to great lengths to circulate, agitate or otherwise "excite" the ink in some way during idle times to keep it well mixed.

Titanium dioxide presents other challenges as well. Unlike the pigments used in colored inks, it must be used in a mass/volume proportion of pigment/ink at least three to four times the normal rate in order to achieve sufficient optical density. Partly as a result of this high pigment loading, the mass/volume proportion acts as a thickening agent in the ink.

Both these characteristics make it particularly difficult to jet white ink through the tiny nozzles in piezoelectric inkjet print heads, especially if it is not very carefully formulated, consistently manufactured, stored and distributed. These characteristics also usually result in a shorter shelf life than colored inks. Assuming your printer manufacturer of choice has surmounted these considerable technical hurdles, there are other issues to consider.

A white ink solution must offer sufficient opacity to completely cover the media (no color show-through) with a bright, white base when printing on non-white media or objects. It also must have sufficient translucence and smoothness of tone to act as an even light diffuser when printed on top of the color in a backlit application on transparent media. Some wide-format solutions available today offer mediocre white options that simply are not bright enough, white enough, opaque enough or smooth enough to use with much commercial credibility.

Understanding the exact image quality limitations of the white ink implementation is critical, but there are other questions to consider:

• Can white be printed as a layer underneath and/or on top of the colored inks?
• Can white be printed as a layer between two layers of colored inks, for day/night backlit applications?
• Can white/colored layered prints be made on both rigid and flexible media?
• Does white print with the same resolution and/or droplet size as the other colors for fine detail printing, or is it suitable only as a fill (flood) coating?
• Can white ink be printed at full speed when printing a non-interfering spot color area, or are layered print modes the only ones available?

There are myriad questions to consider, but the answers to these questions will determine the total applications capability of the printer. The most important point to take from this is that putting a "check" in the box next to "white ink capable" is simply not enough when it comes to this important feature. Understanding the image quality and technical capabilities of a printer is critical to successfully implementing this service in your offerings.

Endless Application Possibilities
Assuming that you have purchased a printer with a credible white ink solution that works, what do you do with it? The answer, of course, is anything you want! White ink capability dramatically expands possibilities, especially for flatbed-style printers.

When considering transparent media applications, there are two possibilities. Rigid media such as polycarbonate, acrylic or PET-G can be used for second surface backlit applications where the image is viewed from the unprinted side of the media. This is most commonly used for backlit point-of-purchase and retail advertising displays. The quality of some printers is such that they almost rival the quality of laser-based photographic printing solutions such as the Océ LightJet® printer. A more interesting twist on this technique is available on some printers that actually enables the white layer to be printed between two colored layers on the same (second) surface of the media. This facilitates the use of the print in a day/night mode where it is actively backlit at night but not during daylight hours. This has great appeal to the ecologically sensitive end user by requiring less than half the electricity of standard backlit prints. Of course the ability to print in two or three independent layers on flexible (roll based) media is also important. Flexible transparent or translucent media deliver all the same application options as rigid, but for lower cost, short-term display purposes. Flexible media also expands use to such applications as static-cling window decoration, where the white-between-color method of printing makes the prints even more interesting by enabling them to be viewed from both sides - doubling the number of potential views in the retail environment.

When considering non-white media, things get even more interesting. Imagine being able to print on any reasonably flat object or media, regardless of base color. Doors, wood, glass, tiles, carpet, stone, cardboard, metals, foils for packaging, fabrics - the list is endless. And if the system doesn't move the media/object during printing, such as a stationery flatbed, the choice of substrate is not limited to media but includes almost anything you can imagine.

White ink capability offers print producers the ability to sell high-value, backlit graphics on rigid or flexible media. White ink also gives print providers with flatbed printers the ability to create specialty applications on almost any media or object. Adding high value, high margin propositions that enable printer providers to differentiate their business is a great defense against the commoditization of wide format printing.

White ink printing - it's real and it's profitable, and that makes it a really big deal.

Jeff Edwards is the International Product Marketing Manager at Océ Display Graphics Systems and is responsible for the Océ Arizona® Series UV curable flatbed printer line. He is based at the company's headquarters near Vancouver, British Columbia. jeff.edwards@oce.com

"Océ LightJet" and "Océ Arizona" are registered trademarks of Océ Display Graphics Systems. © 2009, Océ Display Graphics Systems. All rights reserved.

This article appeared in the SGIA Journal, 1st Quarter 2009 Issue and is reprinted with permission. Copyright 2009 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.