Print-heads and picolitres: the relevance of droplet sizes
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By Sophie Matthews-Paul
11 April 2011
Useful reading: it's available free from Xaar's website
In the specifications for most of today's wide-format ink-jet printers, there is normally a reference or two made to picolitres. In specific terminology, this represents the size of the ink droplet which is ejected from the print-head and, as a general rule of thumb, the smaller this droplet the greater the detail will be in the final print.
Of course, these generalisations don't work in quite such a straightforward manner when applied to wide-format principles. The picolitre value is a fraction of a litre (imagine a tablespoon of ink being 15ml and this equals 15bn picolitres); because it denotes the measure of ink volume which is dispensed, its role could be considered quite insignificant in the general scheme of things where quality is concerned. The bit that's important is how print-head developers control picolitres.
A typical photo-quality desktop A4 ink-jet printer might boast a picolitre drop size of being just 1.5. But, for the typical user of an everyday unit which can be purchased for remarkably little money, the type of software being used and how the ink droplets are controlled are probably of greater importance to the overall standards being produced by the machine.
In wide-format ink-jet there are numerous criteria which go to create the perceived end quality of an application. Whilst picolitre size goes some way in controlling the droplet's dimensions, certainly not every job demands the smallest; while photo-quality printers such as those from Epson, Canon and HP might boast droplet sizes of four picolitres or even less, in larger printers the quality and placement of larger droplets are more than adequate when printing posters, billboards and other wide-format jobs. As an example, typically, to print an application at a resolution of 300dpi, a picolitre size might range from 30 to 80. The variation in end result through this range might not be discernible to the human eye, and the choice affects the print speed. The larger the droplet volume the faster the machine can run.
Print-head developers take picolitre requirements seriously, of course, and Xaar's Proton head, developed as a wide-swathe binary option, was steered towards the Chinese market for high-performance display graphics. Originally introduced in a 35-picolitre version, the more recent 60-picolitre addition is also proving its flexibility through being suitable for solvent-based and UV-curable inks.
Likewise, Fujifilm Dimatix introduced its Q-Class Emerald print-heads, again directed at the performance market, with two models offering 30 and 80 picolitres. These combine high-speed multi-pulse binary jetting and greyscale functionality. So the Emerald QE-256/30 can be adjusted for 30- to 80-picolitre droplets in binary mode (see Talking heads) or a 30-picolitre droplet size in greyscale mode. Following a similar principle, the Emerald QE-256/80 can be used at an 80-picolitre drop size in greyscale mode but adjusted up to 200 picolitres in binary mode.
But although it doesn't take rocket science to calculate that the larger the picolitre size the bigger the droplet, it is also important to ascertain the type of ink being used within the print-heads and whether they are binary or greyscale. Everyone knows that solvent-based formulations tend to be more finicky to deal with in maintenance terms, and that's because the ink can clog in the heads.
So it's logical that the smaller the picolitre size, the more nurturing the machine will need. In hot and humid countries, running a wide-format printer with, say, a 14-picolitre droplet using solvent-based inks is likely to prove more problematic than using the same technology with UV-curable ink which stays in its 'liquid' state until it is cured.
In summary, as part of the overall design and construction of the print-head and its nozzles, the picolitre size is only one element to achieving a successful printed result. The larger the droplet size, the lower the end resolution but, in wide-format applications there are the benefits that include the necessity for fewer passes and the fact that overall ink coverage will tend to conceal any blemishes. Smaller droplet sizes need a greater number of passes so that full coverage can be accomplished and, for many wide-format jobs, this is really overkill as the chances are the application is going to be viewed from a distance.
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