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![]() Upcoming Developments in Industrial Inkjet for Display Graphics
By Chris Lynn, VP Sales & Marketing, Xaar Americas
The wide-format graphics industry has been transformed through the emergence over the past dozen years of digital inkjet printing technology. Digital technologies have made possible the production of low-volume print jobs at prices that screen printing could not approach — but this has not so much eaten into the screen market as taken away its growth, according to researchers at Web Consulting
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Screen printing still rules where high volumes or quality requirements allow the cost of pre-press to be recovered in the total price. Digital printing for outdoor graphics has moved from aqueous to solvent-based inks, from paper to vinyl, and from 100 dpi to 360 dpi or more. But the digital market is now starting to look almost mature, and new points of differentiation are emerging: UV inks for better durability without lamination, flatbed printers for direct printing onto rigid materials, and six-color ink sets. I am not talking about the use of light cyan and light magenta in addition to conventional CMYK inks — those are now commonplace. They do a useful service in improving tonal range, reducing contouring, but do not do anything to broaden the color gamut. I am referring to the use of additional colors, such as violet, orange and green, which actually make for a more color-accurate reproduction. As Figure 1 shows, the color spectrum that can be reproduced with a six-color ink set is significantly broader than with conventional CMYK alone, resulting in punchier graphics.
Grayscale heads eject even smaller drops — typically in the range of 3 to 8 picolitres — but they do so in groups at a much faster rate. A group of small drops joins together to make a larger single drop that lands on the substrate, which is a millimeter or so from the nozzle (Figure 2). The volume of the resulting larger drop is a volume multiple for each of the smaller drops, up to the number of ‘gray levels’ supported by the print head, i.e. 8, 16, 24, 32 or 40 picolitres for a six-level head, which has an 8 picolitre sub-drop volume. (Zero counts as one level.) What’s the point of this? Image quality. By varying the size of the dots made on the substrate, grayscale heads simulate a much higher resolution than their physical nozzle spacing and firing frequency allow. The apparent resolution is roughly equal to the real resolution multiplied by the square root of the number of gray levels. For example, Xaar’s OmniDot 760GS8 print head has a native resolution of 360 dpi, and is capable of printing variable drops from 8 to 40 picolitres in size (i.e. six gray levels). So the apparent resolution of the head is 360 times the square root of six, which equals 882 dpi (360 x √6 = 882 dpi). This is approaching the quality of a photographic print made on a desktop photo-printer. It exceeds the 720-by-720-dpi mode on a typical wide-format printer, such as Epson’s SP9800, while providing the ability to print several hundred square feet per hour. (The Epson prints only up to 40 square feet per hour in a bi-directional print mode. But it sho uld be said that this represents a good value for a very modest investment.)
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Do you need photo-quality on a billboard? Certainly not if the viewing distance is too great to tell the difference from conventional 100 to 200 dpi prints. But with a wide-format or grand-format machine that prints grayscale, you can offer clients the ability to print the same quality of image for all their display graphics — and do it without having to buy separate machines for smaller format, photo-quality work and wider format, high-productivity use. Alternatively, you can simply use the extra resolution to buy productivity: Scale back the resolution in the print direction to 180 dpi, knowing that the variable dot size will make it look like 440 dpi, and print at twice the speed.
The use of digital printing in the display graphics industry is well-established enough to encourage talk of market maturity. But the innovation rate in the field has not diminished. Continued developments in inks and substrates, print quality and machine productivity and reliability will drive digital printing deeper into analog territory. Like all disruptive technologies, digital printing will initially fail to meet all of the requirements that customers expect. Its adoption has not been and will not be painless. But the economics of inkjet printing ultimately will prove to be compelling, and the result will be better value for technology users and their customers.
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