Chasing the Holy Grail of Inkjet Printing: The Second of Three Parts

Episode 2: To Prime or not to Prime?

by Noel Ward, Editor@Large

The story so far: Inkjet printing on glossy and coated stocks is a Holy Grail of inkjet printing. But like in human relationships, there’s chemistry involved, only here it is the chemistry of inks and the surface of the paper. The technologies are advancing all the time, but for the moment it comes to one question:

To Prime or Not to Prime?
The surfaces of glossy or coated stocks are resistant to ink penetration. Inks must sit on the surface instead of being absorbed as happens on an uncoated paper. As noted in Episode 1, this results in two problems: the inks cannot spread into the paper fibers and can have trouble drying quickly, which can result in offsetting or smudging when the paper is rewound onto a take-up reel or stacked after printing. Complicating these issues is that inkjet presses use more ink to create an image than does an offset press.

Inkjet press vendors already offer treatments that help inks bond to uncoated offset stocks, and are upping the ante with “priming fluids” said to improve ink adhesion and color vibrancy on glossy or coated stocks. The ultimate goal is enabling printing on enough glossy or coated stocks so that printers will be able to shift more jobs from offset presses to inkjet systems. Or so the theory goes. But what is really happening?

The printers I talked with have found they can successfully print on many coated and some glossy stocks even when using the standard inks developed by press manufacturers. All had evaluated a range of potential papers using the latest standard inks their respective press vendors provided, so they had a good sense of what worked and what didn’t. More importantly, they did not work in a vacuum. Each printer also asked customers about inkjet image quality and substrate selection. They found that while the priming fluids add some visual appeal, they are not necessarily essential when using standard inks on glossy or coated stocks. The people I spoke with agreed that obtaining the best possible results on a coated stock would require a priming fluid, but that usage might come with the expense of print head upgrades and more costly inks. This made them reluctant to change, believing they could achieve satisfactory results with standard inks, thoughtful substrate selection, and in some cases slowing down the press.

Remember, these printers are all seeing customers shifting from cut-sheet, toner-based machines to continuous-feed inkjet, due in part to the significantly faster speeds of the inkjet presses. They said their customers were happy with the print quality, which combined with speed and the comparatively lower cost of the inkjet pages, provided a compelling rationale compared to the use of toner presses.

But technology marches on, with machines entering the market already designed to handle glossy and coated papers. A Belgian firm I spoke with was in a beta test that simultaneously evaluated a new press and its inks while running a wide range of coated, uncoated and glossy stocks. The owner says he has encountered few issues of any note when using the press supplier’s latest ink sets. Drying the ink was his (and the vendor’s) greatest concern but he has so far been able to run any stock his customers require at the press’s full rated speed.

But there's more to this than just laying down a fluid to aid ink adhesion. The right inks and the substrates are essential factors.

Coming tomorrow: Ink and Substrates Matter
You’ve always known that ink and substrates --especially the papers-- are critical elements. Yet the papers are not all created equal. So how do you decide?
 

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