Digital technologies have made possible the production of low-volume print jobs at prices that screenprinting could not approach. Chris Lynn, VP sales & marketing, Xaar Americas, discusses the use of digital printing in the display graphics industry.
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 screenprinting 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. Screenprinting 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 solventbased inks, from paper to vinyl, and from 100dpi to 360dpi or more. But the market is now starting to look almost mature, and new points of differentiation are being found: UV inks for better durability without lamination, flatbed printers for direct printing onto rigid materials, and 6-colour ink sets. I am referring to the use of additional colours like violet, orange and green, that actually make for a more colour-accurate reproduction.
As this illustration shows, the spectrum of colours that can be reproduced with a six-colour ink set is significantly broader than with conventional CMYK alone, resulting in punchier graphics.
At up to six times the price of solvent inks, the cost of UV inkjet inks has been much-criticized, and they are certainly expensive on a per-litre basis. But there are a lot of positives to UV-cured inks: the fact that the ink sits on the surface of the substrate instead of being absorbed means that less ink is used to achieve full coverage; the potential to avoid the entire process of lamination to ensure durability helps to provide a cost advantage; the fact that – unlike solvent inks – UV inks cannot cure in the printhead, improving printhead life and reducing the need for maintenance; and the avoidance of VOC-handling, with its attendant OSHA- and EPA-compliance requirements. Taken together, these benefits lead many printers to conclude that the use of UV makes a lot of sense.
Another development that drive quality expectations to yet higher levels: the use of grayscale printheads. ‘Grayscale’ refers to the ability of an inkjet printhead to deliver drops of ink of varying sizes. Conventional inkjet printheads are ‘binary’ – they either eject a drop of ink or they do not – and the drop size is typically between 20 and 80 picolitres.
Grayscale heads eject even smaller drops – typically in the range of 3-8 picolitres – but they do so in groups at a much faster rate. A group of small drops join together to make a larger single drop by the time it lands on the substrate, a millimeter or so from the nozzle. The volume of the resulting larger drop is a multiple of the volume of each of the smaller drops, up to the number of ‘gray levels’ supported by the printhead. e.g. 8, 16, 24, 32, or 40 picolitres for a 6-level head with 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 printhead has a native resolution of 360dpi, and is capable of printing variable drops from 8 to 40 picolitres in size i.e. 6 gray levels. So the apparent resolution of the head is 360 x 6 = 882dpi. This is approaching the quality of a photographic print made on your desktop photo-printer – and exceeds the 720 x 720dpi mode on a typical wide-format printer such as Epson’s SP9800 – while providing the ability to print several hundred square feet per hour.
Do you need photo-quality on a billboard? Clearly not, if the viewing distance is too great to tell the difference from conventional 100- 200dpi prints. But with a wideformat 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 needing to buy separate machines for smaller format, photo-quality work and for wider format, highproductivity use. Alternatively, you can simply use the extra resolution to buy productivity: scale back the resolution in the print direction to 180dpi, knowing that the variable dot size will make it look like 440dpi, and print at twice the speed.
Who makes grayscale printheads for solvent and UV inks and where are they used? Xaar has its OmniDot range, and has licensed its technology to Toshiba TEC, which offers two 8-level grayscale models, and to Konica-Minolta which has two 4-level grayscale models. Spectra too has discussed a grayscale head, though this has not yet appeared in the product line-up. Because the technology is relatively new, few printer manufacturers have gone public with machines based on grayscale to date. Agfa announced the introduction of its UPH-head Annapurna 100, a 100 inch UV rollto- roll/flatbed machine capable of printing up to 1,000 sqft/hour. The UPH head is Agfa’s version of Xaar’s OmniDot 760, with up to 16 gray levels. The same heads – 64 of them – feature in Agfa’s MPress, a digital press that integrates with Thieme’s 5000XL series screenprinting modules to form a hybrid press.
The use of digital printing in the display graphics industry is well enough established to encourage talk of market maturity, but the rate of innovation 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.
(For more info, contact: Neeraj Thappa, field applications engineer, Xaar India, B&B1, Enkay Towers, Udyog Vihar – V, Gurgaon – 122016, Ph: 0124 4109227, 0124 4109223, fax: 0124 4109225, mobile: 9810249227.)