Fixed array printing sounds easy enough. Just put a row of printheads together and pass the media to be printed underneath it, right? Well, not so fast. As with all ink jet printing you can print on anything since the ink flies through the air to the media being printed. The key to success in employing this approach is to print without noticeable defects in the resulting print, print after print. That is quite a different problem.

In my January 2013 article for SDG—“Full-Width Array Print Technology”—I wrote about the Hewlett-Packard desktop array printer and Memjet’s “Waterfall Technology” and how both have the potential to be developed into successful wide-format high-speed single-pass printing solutions. Here we will discuss how that development is progressing and some of the issues that must be overcome to enable this fixed array printhead design to displace scanning head printers as the choice for print service providers.

Printheads

A number of companies including HP, Memjet and Canon/Océ have employed printheads capable of jetting at more than 1,000 drops per inch at high speed onto a moving web. Web presses, label presses and wide-format printers have been introduced featuring these printheads. Printheads driven at high drop rates don’t seem to be a limitation. Stitching them together so that these very tiny drops all land where they are supposed to is a challenge; keeping all the nozzles healthy while printing at high speed continues to create serious issues for the equipment builders.

One reason for this is the fact that nozzles on the end of an array behave differently than those in the center. Overlapping the adjacent printheads is one approach to avoiding sticking artifacts, and the idea has been implemented with varying degrees of success. The human eye is very effective at detecting a line of drops which are different in color density so if the drops vary in drop volume or velocity across the array, land off-center, or are missing, a density streak will appear in a solid-colored area of the print.

With scanning-head printers the drops are not printed in a single pass. Nozzles which are misbehaving and causing a defect have the missing or misdirected drop randomized in the print so they are not generally observed unless they are severe. Nozzles which are not used may become plugged due to increased ink viscosity as the water evaporates from the open nozzles.

To solve this problem in a fixed array system, random drops are ejected from unused nozzles to keep the nozzles functioning. This adds a background of spots to the print that are too small to see as individual spots.

Inks    

Two types of inks have been demonstrated in array printing so far: water-based dye inks and water-based pigmented inks. These present additional issues that must be addressed in fixed array printing.

First, the drops are printed “wet on wet,” essentially simultaneously, making the potential for bleeding and coalescing of the drops a problem that must be dealt with. With scanning-head printers the drops can be printed next to one another in separate scans, allowing time for drying and absorption before the next drop is placed beside it. Therefore, the receptive media for fixed array printing must be very absorbent. For plastic films or photo papers, care must be taken to balance the printing speed and the absorbance since the water and co-solvent in the ink cannot penetrate below the surface coating.

In Memjet-equipped label presses, print speeds of up to 225 feet per minute have been demonstrated. This requires an instant dry coating on the film or paper to achieve drying. Label stock generally used for analog printing cannot achieve this so special media or media coatings must be used. For wide-format fixed arrays, a similar problem exists so media and ink optimization are also required.

Overcoming Artifacts

In the balancing act of resolution verses artifacts several approaches have been taken. With the Memjet label presses and wide-format printers the drops are slightly more than 1 picoliter, creating spots too small to even see with the naked eye.

IPT Digital offers the JFLEX 870 digital label conversion system, powered by Memjet, which is a drop-in inkjet printing unit that the company installs on its clients’ existing analog label presses. The 8.64”-wide JFLEX 870 uses redundant printheads in order to achieve speeds of 225 feet per minute and freedom from printing artifacts due to poor-preforming nozzles. And since this label printing system has a print width of just one Memjet printhead, stitching is not a problem.

With wide-format applications and Memjet-powered products like the Canon/Océ ColorWave 900, stitching is much more of a problem. These printers are marketed into the CAD and P.O.P. markets primarily because they struggle to achieve the high-quality print expected of the photo market—as delivered by Epson, Canon and HP scanning printers.

HP also addresses the CAD/P.O.P. market with its first introductions into the fixed-array wide-format market—the HP PageWide XL series. Drops in these printers are at least three times larger than those delivered in Memjet systems, and they deliver pigmented inks rather then dye inks. In my opinion this presents both an advantage and a disadvantage.

Pigments on ColorLoc paper or water-resistant matte coated plastic films tend to stay on the surface while the ink vehicle is absorbed making control of the bleed and penetration easier. Dot spread can be controlled as well, which “heals” some printing artifacts. As with the Canon/Océ ColorWave 900 printer, lack of redundancy makes the HP PageWide XL susceptible to stitching artifacts and misdirected nozzles, and they may not be suitable for high-quality photo printing.

Summary

Good progress is being made in developing cost-effective high-speed wide-format printers using fixed-array printheads. Certainly the future will be great for these products, especially when redundancy is incorporated as in the IPT Digital JFLEX 870 label press configuration (which is suitable for high-quality photo printing).

With printing speeds of more than 50 feet per minute these wide-format printers will further erode the analog printing market share. And as often happens with new technologies, disputes may arise between companies about new technologies being employed—evidenced by the patent infringement lawsuit that Memjet brought against HP on August 11. The outcome of that dispute may shift the dynamics of the marketplace somewhat, but still, we will look forward with anticipation for the continued development of this powerful technology.