Issue #0825/1 – As indicated in the article Epson’s inkjet photo printers have incorporated print heads capable of producing multiple sizes of ink drops for many years - since the Stylus Colour 740 and Stylus Photo 750 were launched in 1998. The technology is known as VSDT (Variable Size Drop Technology), capable of firing drops of three different sizes, and is now operating at an ‘Advanced’ (AVSDT) level (only in 6-colour printers), capable of firing drops of five different sizes – all from the same nozzles in the same print head.
It is in the ability of the system to fire variable drops from one nozzle that Epson’s VSDT technology differs from Hewlett-Packard’s dual drop size technology () and where we find certain print quality implications between the two technologies. It is also where we see the major practical implication of micro piezo inkjet technology against thermal inkjet technology.
Thermal inkjet technology, as used by Brother, Canon, Hewlett-Packard and Lexmark is constrained by the necessary physical structure of the firing chamber and the ability of the resistor to heat a quantity of ink very fast to expel a drop of ink and then cool again in time to reheat for the next drop. It is these physical constraints that limit a nozzle to firing only one size of drop, determined by the size of the firing chamber and the heating resistor.
This video from Hewlett-Packard shows a graphical representation of the technology in action and real life video of drops being fired.
Hewlett-Packard Inkjet Technology
By contrast, Piezo technology uses a mechanical diaphragm that is pulled and pushed by an electric current to fire an ink drop from the nozzle. In this instance there are other parameters that influence the drop size in addition to the size of the firing chamber. These obviously include the size of the diaphragm but also include the thickness of the diaphragm and the flexibility of the material that that diaphragm is made of.
When Epson introduced its new, faster print head a year ago article ("Epson announces new, faster inkjet"), the new manufacturing tolerances and materials used allowed the diaphragm much more flexibility. The accompanying diagram indicates the extent to which the new head allows more flexing over a smaller area than the old head.
Epson’s new micro piezo head utilises a much more flexible diaphragmThis has two major benefits.
Firstly, a smaller firing chamber can be built, thus reducing the amount of ink involved and, thereby, the size of droplet that can be fired. Epson is now capable of firing a droplet that is as small as 1.5 picolitres. While this is not an industry best, it represents continuing improvement on the 2pl droplets fired from the previous generation of print head and the 4pl best drop size achieved only half a decade ago.
And, remember – until only a couple of months ago, Hewlett-Packard’s best was 5pl!
Secondly, because the diaphragm is flexible, and electromechanically activated, it can be flexed by different amounts according to how much electrical power is applied and how much ink is required in the droplet being fired. Hence, a large amount of flexing results in a large droplet while a small amount of flexing results in a small droplet.
Taking this principle one stage further, two droplets can be fired in rapid succession so that they combine into one supersize dot on the paper. In this way, Epson’s VSDT is able to place drops of 2pl, 6pl and 10pl. As indicated, the smallest droplet available from the Advanced VSDT print head is 1.5pl and the nozzles will fire five sizes of droplet.
Three sizes of ink drop can be fired from each nozzle of a VSDT print headEpson’s video below shows a graphical representation of a micro piezo nozzle firing three sizes of ink droplet.
Epson Micro Piezo Technology
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It is important here to re-emphasise the point made in article that photo print quality is dependent on drop size – the smaller the better. Large dots can be used in areas of solid fill to ensure dense colouration with a minimum number of drops but the overall image quality is dependent on the small dots not the large dots.
Epson AMC Pull-Push-Pull action
Regular, round dots fromEpson micro piezo inkjet printer
Pulling the diaphragm back initially, before firing the nozzle, removes any danger of there being any excess ink around the mouth of the nozzle and primes the nozzle for firing. Pushing the diaphragm then ejects the droplet from the nozzle.
Epson applies an additional Pull action just as the drop leaves the nozzle, thus cutting off the fired droplet, pulling the unused ink firmly back into the firing chamber and stabilising the meniscus. This has the effect of cutting off the flow of ink very sharply, the benefit of which is twofold: the production of round droplets, resulting in round dots on the page; and the virtual elimination of ink spray.
Take a look at the two following videos supplied by Epson. These illustrate how the surface of ink in the nozzle (the meniscus) and the ink drop behave with a piezo element that both pushes and pulls, creating the Active Meniscus Control effect, compared to those that have no control and eject the ink with a push motion only.
Piezo technology with AMC
Piezo technology without AMC
Ink spray from a thermal print head is a result of the shape of the ink droplet as it explodes out of a nozzle. As shown in the Hewlett-Packard video above, thermally produced ink droplets tend to build a long tail of ink as they are fired. The tail is liable to break up into several droplets that find their way to the paper in addition to the main droplet.
Tail dots and spray fromtypical thermal inkjet printer
Ink mess inside thermal inkjet printerafter printing only a few hundred pages
This is not the only consequence of running a print head that is liable to produce stray droplets however. By their very nature, stray droplets are uncontrolled and uncontrollable, meaning that they could land anywhere on the paper or even fly around inside the printer itself, spreading themselves around the inside casing of the printer, causing significant mess – as this photo from recent testing shows.
Piezo technology is actually able to fire at a higher frequency than thermal technology (more firings per second from each nozzle). Epson’s latest heads can fire at 56 KHz compared to a typical 36 KHz from thermal technology.
Higher firing frequencies mean faster print speeds because the print head can be moved across the paper faster. In practice, this benefit is slightly nullified by the low nozzle density of piezo print heads. However, the combination of high frequency firing and Variable Size Drop Technology means that Epson is achieving print speeds that compete very favourable with their thermal inkjet competition.
In fact, our testing showed that, when printing colour and mixed mono/colour office documents, the Stylus DX7400 proves to be the fastest in its class. Even printing in mono only, the device performs respectably and is never the slowest in the class regardless of the nature of the target print.
So, with VSDT, and AVSDT in particular, users benefit in two major respects. Firstly, print speeds can compete with the speeds achieved from the higher nozzle-density found in a thermal inkjet print head and, secondly, print quality is maximised because of the regularity of the dots placed on the page and the fact that both small and large dots are used to produce smooth and vivid images.
Thus, the value to user of the latest Epson inkjet printers is enhanced considerably beyond the value experienced not so long ago when Epson printers were inclined to be painfully slow (pretty much the slowest in class) and a little pre-purchase research would very likely have moved a user to thermal technology rather than piezo.
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