When quality counts size isn't everything


No one could ever accuse printing equipment manufacturers of lacking ambition. Just when printers think their kit is absolutely the very latest thing and represents the pinnacle of speed, quality and user-friendliness, another iteration promising even more spectacular performance is brought to the market.


And the printer is left wondering whether the machine they’ve invested in, perhaps quite recently, is still up to the job of serving ever-more demanding customers.

This is certainly a question wide-format printers have been left asking by the development of machines touting ever-tinier droplet sizes and so ever-greater print quality. Specifications for most engines now often include minimum and maximum drop size and references to picolitres (pl) have become almost as common a benchmark as dpi when it comes to explanations about improved print quality.

A pertinent question, though, is just how good, in terms of resolution, the majority of wide-format print needs to be. Those tempted to go for the highest quality possible in all situations, regardless of application, need to be mindful that too much attention to drop size can compromise other, potentially more important, variables, such as ink usage and speed. And it must also be realised that the relationship between drop size and quality of output is not as straightforward as it might seem.

 

Smaller is better

The theory has it that the smaller the size of the ink droplets, the higher the resolution and the better the quality of the print. Drop sizes are measured in picolitres, or trillionths of a litre. To put that number into some kind of perspective, a 5ml teaspoon can hold 5bn picolitres of liquid.

But printers need to remember that the droplet size isn’t the only, or even the most important, factor determining the final quality of the print. Critical is, of course, the quality of the artwork supplied, something those in the photographic sector would do well to bear in mind.

Here, the emphasis rests very much on the belief that the smaller the droplet size, the more accurate the final print will be. Manufacturers of digital cameras are accustomed to squashing increasing numbers of pixels onto their sensors, and this ethos tends to rub off on end-users who believe that the higher the numbers of megapixels and the smaller the size of their inkjet’s droplet size, then the better the finished prints will be.

What tends to get forgotten, however, is that the result is dependent on the standard of the original image and the selected resolution chosen in the output device, a principle that is pertinent to all areas of wide-format printing.

So long as the resolution of the image and output device are correctly managed, photographic printers will benefit from smaller droplet sizes as they are better for reproducing continuous tones and smooth colour graduations.

Elsewhere, however, common-sense is needed – after all, there’s no point in printing a billboard or scaffolding wrap at photographic quality as nobody will be close enough to appreciate it.

But why, you may ask, shouldn’t wide-format printers go for higher quality even if it isn’t strictly necessary? The answer is that this can quite seriously compromise the speed and cost-effectiveness of the typical wide-format system.

Opting for a smaller droplet size means more dots will be needed to generate the final image and that means the job will have to be processed more slowly or with more passes. You can get around this by buying a machine with more, or faster, printheads, but that will represent a significant investment. So the printer must decide whether this cost will be worthwhile for the types of jobs they run.

"The first question is what are you gaining with a small drop – greyscale, finer text and line features, and colour gradients? I believe that, if you’re not using the fine droplets to achieve greyscale then you’re gaining the other features often at the expense of speed," says Ken Hanulec, vice-president of inkjet marketing at EFI. "The droplet size is irrelevant if you have to add more cost to the printer by compensating with more heads or if you have to slow down the machine to achieve the right coverage and colour balance," he adds.

Also, smaller printhead nozzles can cause problems using some inks, including blocking and clogging, and the whole machine can behave less robustly through requiring extreme fine-tuning in the ink feed and print areas. So as a general guide, the smaller the droplet size, the more overall care the print engine is likely to require.

So how do you know what droplet size will be best for a particular application? The bad news is there are no hard and fast rules, but some general guidelines can be established using a little common sense. For example, banners and posters can be output on a machine with a droplet size of 30pl or greater and the results are unlikely to be perceivably worse than a machine with a head firing droplets of, say, 12pl. Meanwhile, larger jobs that will be viewed from some distance can quite happily be printed using an 80pl droplet size.

But before wide-format printers decide to limit how small they’re willing to go on droplet size, they need to consider printers that offer the option of smaller droplet sizes without compromising the speed of every job.

EFI’s Hanulec explains that greyscale printheads are capable of offering the best of both worlds. Mark Alexander, director of marketing at Xaar, agrees. "Greyscale printheads fire variable-sized drops made up of smaller sub drops – typically between 6-20pl," he says. "By using drop sizes of different sizes, the human eye is tricked into seeing a smooth image. Small drops provide the fine details in an image and larger drops give coverage and colour density. The ability to print multiple drop sizes means that you can get higher productivity combined with high quality out of a smaller number of nozzles."

New opportunities

Meanwhile Durst’s marketing and sales manager Michael Lackner advocates his company’s Variodrop technology. "There is indeed an increasing demand for smaller drops and also for high-speed machines," says Lackner. "We have reflected this with the introduction of the 12pl Rho 1012 machine, which prints solid and uniform areas with large drops and fine text and colour graduations with smaller drops. The advantage is a productivity gain of up to 25% on our Rho P10 series."

One factor that might give printers reason to consider such a machine, or perhaps even one offering exclusively small droplet sizes, is the growing demand for promotional materials. The growth of smaller UV-curable machines for industrial and promotional work promotes the argument for variable sizes, states Duncan Jefferies, sales and marketing manager for Mimaki distributor Hybrid Services.

"Both application and print environment need to be taken into account when considering drop size," he says. "The required quality for a watch face is understandably going to be different to that for a lorry curtain side. Ultimately, if your output device doesn’t have that high-end print capability built in, it can’t be generated; so the flexibility of variable dot printing gives print providers the benefit of choice."

Of course, the fact remains that for the large-scale jobs, such as lorry curtain sides, high-quality output just isn’t needed. And for those deciding whether to invest in a machine that offers a small droplet size, and those deciding what droplet size to run a certain job at, it’s key to remember that droplet size isn’t the be all and end all when it comes to quality anyway.

In fact, as part of the overall design and construction of the printhead and nozzles, and the ink and media being used, droplet size is a single element in achieving good quality. Droplet sizes are relevant inasmuch as they determine the overall quality standards that can be achieved by a particular print engine. They also provide a yardstick relating to expectations where smaller drop volumes might be desirable. Nonetheless, it’s the overall versatility of RIPs and print engines to be able to work with variations to suit the end application, that determines the effectiveness and true value of different droplet dimensions.

SMALL DROPLET KIT ROUNDUP

Epson SureColor SC-70600
One of Epson’s latest family of wide-format machines, this has white and metallic ink options and utilises the company’s Micro Piezo TFP printhead with a 4.2picolitre (pl)droplet size. It uses Epson’s Ultrachrome GSX inkset and prints at resolutions of up to 1,440dpi.

Durst Rho 1012
A member of Durst’s flagship Rho 1000 series, this is a 12pl flatbed printer with the company’s Variodrop refined greyscale technology. It claims 1,000dpi resolution and throughput speeds of 490m2/hour and has three-quarters automation and a roll-to-roll/roll-to-sheet option.

EFI Vutek HS100 Pro
This is the latest 3.2m high-end hybrid machine from Vutek which features greyscale printheads (0-24pl drop sizes), and the company’s own ‘Pin & Cure’ technology for accurate ink laydown and gloss control. It is integration-ready for EFI’s web-to-print and MIS/ERP solutions via native JDF connectivity.

Mimaki UJF-6042
The larger verson of the desktop UJF-3042, this is an A2-size UV-curable flatbed printer with a minimum 4pl droplet size. There is a choice of ink types and an automatic primer feature. Small jobs can be nested making it suitable for decals, gifts, industrial products and promotional items.

 


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