This is the third article in a series on printhead latency effects and the tricks that allow OEMs to overcome the limitations of ink performance when there is competition between the drying process and keeping the inkjet nozzles happy. Last time we showed how recirculation can be a real benefit, so now it is time to look at the other main methods used in both production inkjet and other industrial inkjet applications to get perfect results.
Don’t Spit, it’s rude right?
Well, they forget to tell that to soccer players over here in Europe; but anyway, I digress… When left stationary, the material in the nozzles can still dry out, even in the presence of recirculation. To understand why, it helps to think about the dimensions involved.
Nozzles are often pictured as small holes in a thin layer, often called the “nozzle plate” – for obvious reasons – or sometime the “faceplate”. However, to give the print head sufficient mechanical rigidity it is not possible to have this layer too thin or it will act to absorb the pressure generated by the piezo (or bubble). This means real nozzles, are better thought of as tall cylinders, or sometimes long funnels, depending on the manufacturing methods which vary between electroforming, laser drilling, wet/dry etching of silicon and even mechanical punching. As a result, even where there is recirculation, the flow generated at the back of the nozzle is not always enough to keep everything well mixed and thus achieve predictable printing, as shown in our previous post in the series.
In our closing remarks, we promised to shed a little more light on different ink recirculation types so below we give some examples, taken mostly from previous presentations from the European Inkjet Conference (now IP&I), as attributed. Each design has a slightly different geometry and flow rate combination, with the most similar being those from Toshiba Tec (now part of Riso) and Xaar, with the latter suggesting that higher flow in the channel creates some nozzle “turnover”.
To reinforce our comments made in the inaugural article on this topic, the best way to keep nozzles fresh with ink, is simply to keep pumping like in a CIJ head. What’s continuous inkjet? Well, in production printing, CIJ generally means Kodak technology – search the Knowledge Base to find webinars and articles on this topic. For drop-on-demand (DoD) technologies like thermal and piezo inkjet, something else is needed.
That’s why some OEMs use “spitting”. This means continuing to eject ink, usually at a low frequency to avoid waste and keep the nozzle fresh. As a maintenance activity this can be done into the capping station, but in printing, and especially production printing with fixed paper/image width it can be done in-process. That means putting ink droplets randomly into the print data for the sole purpose of making the intended print patterns print perfectly when needed.
Happier without “scum” Dots
That phrase “scum dots” is personal tribute to Inkjet Insight co-founder Mary Schilling, who was the first person I ever heard use it. Mary was talking at the time (~2008) about something irritating RIPs sometimes do when managing CMYK blends, but it applies equally well to the type of spitting I was just talked about. You get the idea if you look at the photo of a print sample below.
The alternative is to use non-ejecting pulses, also known “meniscus-shaking” pulses or as implied by our title, the less literal vernacular “tickle” pulses. The idea is to activate the volume of fluid in the nozzle sufficiently to keep it mixed up and it can work on both non-recirculating and recirculating print heads. The images below show a schematic of different types of “tickling” and a dropwatcher example of a pulse that fails to eject a drop but cause a positive meniscus “bump” at the nozzle.
The image data bit that generates the tickle is often “zero” which is correlated to the white areas of the print pattern, and so at DoDxAct we called this a “whitespace” tickle. In this case the pulse applies for every single pixel in the print and the tickle frequency thus = the print frequency. In high-speed printers this can be 60kHz or more. As you might imagine, the action of the “tickle” actuation can cause heat generated to build up in nozzle that are not firing very often and this cause the local ink viscosity to drift, thus resulting in worse results when it comes to generate the next required droplet. One way to deal with this is to have a special data channel (or bit level) just for the tickle, so it can be actuated less often, but heating issue still occurs over longer times not printing.
In more industrial applications, if the ink is highly volatile (e.g. a solvent), or highly particle laden, or even worse – both these things – then the effect of the tickle can be bad even when there is recirculation. Complicated huh? That’s why I have a lab!
To be consistent with our prior examples, we again look at our lab “staircase” prints to assess the effect. The example below compared a water-based ink a medium flow recirculating head to a solvent ink in fast-recirculating printhead, although for practical reasons the flow rate is ~50% of the printhead manufacturer’s recommendation, although it is ~20x the actual usage when printing
Effect of ink type, head design and droplet size in influencing predictable printing at the pixel level.
The tickle pulses can also be used during no printing periods, such as between print job, if the driver hardware allows it. This means that there needs to be a separate drive signal is used to trigger a special non-printing waveform. We called this an “idle” waveform and it can improve the first ever lines in a print, which is good for sensitive applications, like printed electronics where every single drop location matters.
Volatility is not the only effect that can benefit from a tickle-pulse. We have jetted UV inks that are high viscosity in several different printhead type and noted that using tickling can be helpful in achieving predictable firing even when there are no solvents or no known troublesome monomers. Our last lab example shows the effect on a small part of nozzle test pattern using a recirculating printhead with a waveform designed to produce 160pL drops from 8 sub-pulses.
In this particular printhead, pulses equivalent to “whitespace” tickle are already in the waveform, even for the tests labelled no tickle and this is simply due to how the head works. Adding an “idle” tickle to the waveform improves the droplet predictability. This is presumed due to continuous input of energy helping to overcome the initial resistance of the ink to being pushed out the nozzle something we call the viscoelasticity of the ink.
So, as it turns out, I got a little bit nerdier than I planned to do, especially with all that talk about waveforms but that is why lab nerds need good editors. The great news though is that waveform was indeed our planned topic of our next article, so the many mentions can whet your appetite for even more talk of bit levels. In the meantime, as always, let us know if you have any questions that come up or any requests to include next time.