Viscosity is the top line of any print head specification, but have you ever wondered why? Well, viscosity is defined as a resistance to fluid flow so unsurprisingly it has a lot to do with squeezing ink out those tiny jets, requiring much lower viscosities than almost all other industrial print/deposition methods. But when printing in single pass the repercussions of that lower viscosity start to have impact.
The most obvious printed side-effect in graphics applications is color bleed and is a well-known effect in production inkjet printers. It is caused by the readiness of the lower viscosity inkjet drops to mix once they impact the surface, as shown in the images below. The reason this is worse for single-pass across all applications is that the volume is put down in one pass, compared to multi-pass printers where a finite amount of absorption or evaporation can help reduced the mixing.
Ink Control with Primers
As well all know from production print, one of the most common ways on controlling this is to use special primers to control the droplets, either by absorption, (like in photopaper), or by pinning the drops in place by controlling the chemistry relative to the ink. The photos below compare the printing of water-based ink onto an un-treated polyethylene film with two different primer coats with different properties.
Untreated media using different primer coatings
The challenge that is more specific to single-pass for industrial printing/manufacturing is that the volume of ink can often be quite high, and the need to control the ink interaction quite important. One such example that has become quite common in recent years is decorative UV varnish finishing for analog or other digital prints, like book covers for example; where very thick ink layers are applied but edge accuracy is important to the quality look of the final product.
Curing in Small Doses
Over the years UV inks especially have been very popular in different markets due to the combination of ease-of-use in print heads and the fact that UV-LEDs can be used to control bleeds effects using small “pinning” doses. In water-based applications, the analogous use of IR lamps has
been attempted too, but requires careful control of the energy input, depending on colour. This has led to the development and use of different ink types that have higher viscosity as a function of temperature, like hotmelt or gel inks.
Viscosity and Firing Pulse Works Together
When it comes to print heads, the viscosity influence can be critical to the reliability of operation. To understand this, it helps to have a picture in mind of the principle of head firing. For our example we use piezoelectric heads again, as shown below. The firing pulse is a voltage that creates a movement of a piezo to induce a pressure difference. Once a drop is fired there is still an alternating oscillation of pressure that reduces over time (i.e. it is damped). The way the damping occurs is dependent on the head design and critically, the ink viscosity, so that the next fire can be made quickly, i.e. at higher frequency / print speed.
If viscosity is too low then ink does not damp out the pressure pulses created by firing, resulting in jet failures. Too high and the refill of the nozzle is too slow, and the nozzle “starves” of ink. In the worst case the ink absorbs all the energy of the firing pulse and does not eject at all.
The trouble with industrial inkjet formulations is that it is sometime difficult to adjust the viscosity to an ideal point for firing, and this is where the waveform tuning, we discussed in the article Single Pass Part 1 – More than a One-Hit Wonder comes in use again.
If the ink viscosity is too low, then the pressure waves can be intentionally damped by the waveform. Likewise, if the ink is hard to push out, then extra energy can be added by applying multiple pulses making use harmonic reinforcement of the pressure. UK Printhead manufacturer Xaar have demonstrate printing of UV-curable 3D print materials with viscosities > 50cP using this kind of approach, which they call high-laydown technology.
The Importance of Shear
Another important aspect to viscosity, is shear. This is the rate at which different regions of the same fluid volume is moved relative to itself and is expressed in unit per second (S-1). Why it is important to inkjet is that the jetting process itself is very high shear (105-106 s-1), whilst the flow of ink down tubes to the head occurs at much lower shear rate. This means that inkjet fluids need to be understood in terms of their shear behaviour. If inks get thicker or thinner with increasing shear than the effects can be detrimental to the jetting and the inks supply system.
Shear rate has direct impact on an ink’s viscosity
If controlled on purpose, however the viscosity under different conditions can be engineered as part of the print process, which is something Canon make use of ink their Colorado printer (although that example is a multi-pass system).
How the ink moves throughout the head is critical to the jetting properties we discussed in article Single Pass – More than a One-Hit Wonder requiring precision chemistry and compatibility with the print head.
In the next post in our single-pass series we will look at the effects of the surface tension of inks and the contradictory requirements that arise from the ability the jet the ink and wet the substrate.