What Causes Jetting to Change Over Time?

Inkjet nozzles that look fine in a short jetting test can drift or fail in subtle ways once they’ve been jetting continuously for hours. The mechanisms behind that drift — fluid formulation changes, thermal limits, wetting buildup, air entrainment — only surface under extended uninterrupted operation, which makes them hard to catch with normal print tests. A custom basin module on the JetXpert Print Station gives you a way to study these effects systematically.

Short-duration jetting tests are good for verifying that a fluid jets at all, course parameter adjustment, and producing printed samples. What remains unanswered from that testing is whether that fluid will keep jetting cleanly for the duration of a real production run. Many sneaky inkjet failures only show up after minutes or hours of continuous operation, which can be challenging to reproduce outside of the production machine itself. However, the costs of sustainability tests on a production machine are extremely high. First you need to allocate a machine to be used for testing or bring a production machine offline. You need to produce liters and liters of test fluids and spend hours priming the system with them. Then you need to watch meters and meters of expensive substrate go into the trash. In this post, we discuss what causes these failures that only show up after hours, and showcase a solution that we’ve developed for long term testing more efficiently.

The reasons jetting changes over time fall into a few broad categories: things happening to the fluid, things happening to the printhead, and things happening in the path between them. Most are slow enough to be invisible in a short test, and most are time-dependent in different ways — which is why long uninterrupted runs are the only place they show up.

How fluids change during continuous jetting

Fluids that look stable in the bottle can become something different by the time they reach the nozzle, especially when pigments or other particles are involved. Fluids are subject to a lot of forces through a fluid delivery system on their way to being a droplet, and they don’t always tolerate that gracefully over a long run. Pumps and piezo walls within the inkjet printhead are applying shear forces continuously to the fluid, especially in recirculating systems.

One of the more dramatic examples we’ve worked on involved a silver nanoparticle ink. After enough time circulating through a diaphragm pump, the mechanical action inside the pump will cause the particles to separate and clump, forming a sludge inside the diaphragm itself. Once that starts, clogs and erratic jetting will quickly follow. Using peristaltic or gear pumps has become more and more common for this reason.

That kind of drift is more common with pigmented and particle-laden fluids, but it isn’t limited to them. Even without a pump, fluid sitting in a bottle while being drawn from can stratify over the course of a long run, so by Hour 2 you’re effectively pulling from a different fluid than you started with.

Thermal limits that don’t show up in a short test

Another customer’s fluid jetted cleanly for the first few minutes and then you could watch the nozzles drop out in waves. The fluid was fine. The waveform was tuned. Nothing obvious explained it.

After more testing, the answer was the fluid heater. It had enough capacity to deliver well-heated fluid out of the reservoir for a short test, but it couldn’t keep up with the demands during continuous jetting. Once the preheated buffer in the reservoir was consumed, incoming fluid was being drawn through the heater faster than the heater could bring it up to temperature. Viscosity rose over time and the jetting eventually would fail. That failure mode is invisible to any test that doesn’t run long enough to drain the preheated buffer — which is most short tests. It’s also one of the easier problems to fix once it’s identified, but identifying it requires running long enough to see it.

Printhead self-heating works in the opposite direction. Continuous piezo actuation generates energy into the head over time. Depending on how the head is built and how aggressive the drive waveform is, the nozzle plate can warm to the point that meniscus behavior shifts and jetting characteristics drift with it.

Wetting, air, and what’s happening around the nozzle

Continuous jetting also produces fluid and airflow conditions around the nozzle plate that you simply don’t get in short bursts. Mist and overspray accumulate over time and at some point are enough to impact the jetting. Airflow currents from the moving drops develop slowly and once formed can start sweeping mist back onto the nozzle plate, where it wets the area around individual nozzles and pulls jets off-axis or blocks them entirely.

Air entrainment is another slow-build failure: foaming inside delivery pumps, microbubbles working their way through the supply line, or fluid starvation at the meniscus when supply can’t quite keep up. All of these are time-dependent, and may take a while to manifest in a way that disrupts jetting.

How the basin module works

The basin module on our JetXpert Print Station is an available customization that makes efficient sustainability testing available when needed. It sits on the print station deck and acts as a target the printhead can jet into between print events. During the soak portion of the test, the head jets into the basin continuously, just as it would during a real print run.

At programmed intervals, the head moves out of the basin and prints a nozzle check pattern onto substrate, then returns to the basin and continues. Each nozzle check gives you a clean evaluation of every nozzle at that point in time. The basin contains the rest of the test, so it doesn’t consume substrate. The basin fluid is collected by vacuum and routed either to a waste container or — if the fluid is suitable for recirculation — back into the delivery system, so you can run extended tests without burning through expensive fluid.

There isn’t a universal answer for how long a continuous jetting test should run, but as you’d expect, you only get one pass across the substrate to evaluate the nozzle pattern. Additional passes beyond that are depositing on top of substrate that’s already been printed. If you want multiple checkpoints over the course of your test, one solution is to have the user replace the substrate between each swathe. A more automated solution is to have a larger substrate and do a serpentine pattern over it.

The video below walks through the hardware and the test sequence in action, with the automated serpentine program.

Evaluating if the basin is right for your testing

The basin module is perfect for certain types of testing, but it does have limitations. You don’t have continuous monitoring of all the nozzles, so you may miss intermittent effects that happen between the print checkpoints. If the airflow caused by the continuously moving substrate impacts your jetting, this won’t be replicated when jetting into a stationary bucket. The basin module is only one of our approaches to long-duration jetting testing. In a follow-up post, we’ll compare several different approaches to this kind of testing — including the basin and other tools — with the trade-offs of each. (Link to come once that post is published.)

In the meantime, if you’re running into nozzle dropout that only appears during long runs, or you’re trying to qualify a new fluid for an application with extended duty cycles, contact us — there’s usually a good test setup for the question you’re trying to answer.