Originally published on Inkjet Insight, October 2017
Inkjet printing is action at a distance. A convulsion of the piezo or a belch in the silicon chamber causes a droplet the size of a millionth of a teardrop[1] to fly from a nozzle. Though ‘fly’ might be the wrong word, conveying as it does the impression of great speed, since most droplets leave the nozzle at a leisurely 18mph or so – Usain Bolt could outrun them. Still, they don’t have far to go: the result of this pulse in the print-head is a mark on a substrate. The substrate (paper, plastic, carton-board or whatever) is positioned a millimeter or two away from the print-head, and the droplet lands a hundred microseconds or so after it left the nozzle. There may be thousands of such nozzles in a typical inkjet printing array, each ejecting their individual droplets thousands of times per second. The micro-teardrops become tears – cry me a river – and in a few seconds, an image is built up as the substrate moves under the array of nozzles. Action happens at a distance, albeit a small one.
This ability to create permanent images at a distance is both inkjet’s unique advantage and its original sin[2]. Why? Because inkjet is the only printing technology that does not rely on squishing the ink onto the substrate. Think of the rubber pad transferring the image from cliché to plastic part in a pad printer; the squeegee pressing the image through a silkscreen; the web pressed in contact with the gravure cylinder; the sheet nipped to the blanket cylinder in an offset press, or to a flexographic plate – all of them rely on pressure to transfer ink to substrate. (The clue is in the name: a printing press!) Even the original digital printing technology –electro-photography, otherwise known as laser printing – uses pressure to transfer toner particles from the photo-conductive drum (or a transfer belt) to the substrate, which is usually paper.
Inkjet printing stands apart from these technologies, both literally and figuratively: without the need to be in intimate contact with the substrate, inkjet printers can be agnostic about both the type of material they are printing on, and the form factor of that material. Rigid or flexible, paper or plastic, rough or smooth, woven or non-woven, metal or ceramic – inkjet is indifferent, because all it needs is a transport mechanism that moves the print-heads over the substrate. The indifference is not complete, of course – curved surfaces, lips, nooks and crannies all present problems, as they do for all printing techniques.
Without the need for pressure, inkjet is ideal for fragile surfaces. For example, inkjet has transformed the ceramic tile printing industry over the past few years, replacing decades of investment in screen printing lines, in part because inkjet printing cracks fewer tiles than screen printing and allows tiles to be made thinner and lighter.
Another inkjet benefit: without the need to wrap the substrate around a cylinder, or to constrain an image to the maximum size of a rectangular sheet, inkjet allows images of ‘infinite’ length to be printed there is no ‘repeat length’, opening up new possibilities in textile printing, signage and décor applications like wallpaper and laminate flooring.
For document printers, inkjet’s attraction lies mainly in its cost model rather than the technology per se; the higher capital cost is offset by lower cost per sheet and greater uptime than toner-based systems. But this is a topic for another article.
This is the good news – so where is the original sin? That lies in the nature of inkjet inks, and the downside of not squishing the ink into the substrate. I said earlier that all inkjet needs is a transport mechanism, but this is a ‘necessary, but not sufficient’ condition for success. Inkjet also needs extra carefully-designed inks.
All conventional print service providers are concerned with the interaction between ink and substrate – surface tension versus surface energy, film thickness, drying, absorption and so on. But inkjet inks are different – the burp or the squeeze in the ink chamber behind the nozzle can’t impart much energy, so the ink droplets have to be small and watery. Flexo, offset, or screen inks are hundreds of times too viscous to be jetted. The inks must function in a narrow ‘operating window’ of viscosity, temperature and surface tension in order for make well-formed drops that emerge from the nozzles reliably, and at a consistent velocity. And because the nozzles are a fraction of the diameter of a human hair, the pigments[3] must be ground more finely than in conventional inks, and mustn’t stick together, or ‘agglomerate’. All of these factors make inkjet inks more challenging (and more expensive!) to manufacture than conventional printing inks.
The problems of inkjet action at a distance don’t end there. An inkjet print-head is a micro-fluidic pump that can quickly place thousands of precisely-metered drops of fluid with great accuracy onto any surface. But when they hit the surface, anything can happen. They might get absorbed into the substrate where they land, causing dot gain, or conversely, washed-out colors; they might be well-behaved, sitting docilely until they are dried or cured; or they might roll around and, under the influence of surface tension, join up with an adjacent drop. This can give rise to the well-known ‘corduroy effect’ of alternating light and dark lines on the print if the drops do not spread out enough to hide the density variations. This ’reticulation’[4] is not necessarily confined to one color – if you want to get your inkjet printer salesman to break into a sweat, ask him to print some flat tints of colors that need at least 3 inks – muddy browns and dark greens, for instance. The ability to avoid banding in such patches is a good test of both the mechanical engineering of the printer, and the compatibility of the ink and substrate.
Conventional presses have these problems too, but to a much lesser degree. Because they press the dots into the substrate – stay there, dammit – they can tolerate variations in inks and substrate characteristics that would cause unacceptable print defects on an inkjet printer. So that millimetric stand-off between print-head and substrate is both the source of inkjet’s main advantages, and its original sin. But the technology and the chemistry are continuously improving. The machine designer’s reach exceeds his grasp (else what’s a heaven for?), and perhaps inkjet engineering, like golf, is the last bastion of mankind’s belief in its perfectibility…
[1] “The mean tear volume in normals was found to be 6.5±0.3μl (S.E.M.) with a range of 3.4 to 10.7 μl.” https://link.springer.com/article/10.1007/BF00410700
[2] “By analogy the term is used in fields other than religion to indicate a pervading inherent flaw.” http://christianity.wikia.com/wiki/Original_sin
[3] Dye-based inks, of course, do not have this particular problem – instead, they suffer from different drawbacks – mainly related to longevity.
[4] From the Latin, ‘reticulum’: a mesh, or forming a network
