ADVANCED DROP WATCHER OPTICS – DE-MYSTIFYING SATELLITE AND INK GENERATION

Inkjet printing has emerged as the leading solution for the contactless deposition of inks and functional fluids. Advances in printhead technology are pushing the boundaries of what is possible. This ranges from depositing nL-sized drops of high-viscosity coatings and adhesives to using 1,200 nozzles per inch (npi) printheads to print low-viscosity inks at speeds close to that of analogue processes. Drop watchers are often used to tailor a fluid for jetting and meet specific application requirements for volume and productivity. They are also used to refine the fluid’s formulation and pair it with novel waveforms.

Meteor Inkjet builds and supplies drop-watching equipment to both original equipment manufacturer (OEM) builders of inkjet-print systems and ink/material developers and researchers. These stroboscopic camera systems use Meteor’s PrintEngine software, market-leading drive electronic components and optics expertise. Together, they precisely co-ordinate image capture and analysis with a printhead’s jetting outputs. The optics and software are designed to integrate seamlessly with print-test stations or production presses. They work agnostically across more than 100 models of printheads from Meteor’s partners.

In hybrid presses, inkjet printheads now operate at speeds up to 300m/min, augmenting and even replacing traditional analogue print processes. To achieve this, the printheads jet ink at a frequency of up to 100kHz – enabling 100,000 drops fired from each nozzle every second! The physics behind this high-speed operation requires nozzles to be smaller to reach a resolution of 1,200 npi across the web. In turn, this pushes the viscosity ranges lower to sustain drop ejection and nozzle refill.

Lowering an ink’s viscosity by using co-solvents and low molecular weight additives can often reduce the ink’s dynamic surface tension and elasticity. This directly impacts the ink’s jettability, making it more prone to issues such as persistent ligament formation, satellite drop formation or mist generation.

With higher print speeds, the turbulent airflow around the printhead also increases. This creates a need for higher drop velocities. This helps the drop travel the distance to the substrate and maintain the direction of the drop in flight. Ultimately, this aids the maintainence of accurate colour-to-colour registration, ensuring the final image is reproduced with high fidelity.

“By altering the strobe timing, it is possible to observe the formation of an inkjet drop and elements of its creation”

Previous generations of Meteor Drop Watchers used a technique to observe ink drops in flight, bypassing the need for high-speed cameras. They relied on the consistency of inkjet drop generation and a synchronised stroboscope. By firing the stroboscope at the same frequency as the printhead – and using a long exposure (15-20 FPS) – they could capture a high-contrast image of the average position, speed and volume of drops. However, any variation (jitter) in the velocity or direction of the drops would blur at the edge of the droplet. This blurring made it difficult to calculate the droplet’s volume. It also made it impossible to resolve more dynamic events, such as ligament breakdown, meniscus oscillations or mist generation. Fundamentally, the success of the system was entirely dependent on the precise, consistent behaviour of the drops it was trying to measure.

The development of a new strobe with an addressable, high-intensity pulse output has significantly improved imaging quality. 
By synchronising the strobe to the camera’s exposure trigger at 30Hz, it provides constant illumination regardless of the print frequency. This has greatly enhanced the contrast at the edges of drops and improved analysis potential. 

“The use of drop watchers for process and material development is long established”

By altering the strobe timing – relative to the droplet ejection – it is possible to observe the formation of an inkjet drop and elements of its creation that affect print quality. Single-event frame capture shows how each nozzle and each firing cycle can result in dynamic differences in drop and satellite formation. These differences depend on the print frequency, nozzle duty and chemical composition of the formulation.

This process allows for the visualisation of droplet formation – from persistent satellites to fine mist. In this case, mist is defined as droplets with a diameter of less than one micrometer and an fL volume. 

A satellite drop has enough mass and momentum to travel away from the printhead, potentially appearing as ‘noise’ on the printed image. Conversely, fine mist typically lacks both mass and velocity, causing it to remain near the nozzle. This can lead to nozzle plate wetting and nozzle obscuration, which may result in print deviations or missing nozzle artefacts. Observing the creation of this mist can be reduced through waveform optimisation or reformulation. 

For the increasing adoption of inkjets in new applications, it is crucial that when data is sent to a nozzle, it accurately and repeatedly prints the desired volume to resolve the image feature. To quantify this, observations can be performed where print data and idle periods are combined with precise measurement timing. This allows the analysis of the first ejected pixels after inactive periods ranging from microseconds to minutes or even hours.

By co-ordinating a fire-on-line-based trigger to the stroboscope and camera precisely with the print data, it is possible to isolate these specific frames and evaluate the nozzle’s response. This feedback is then used to refine the efficacy of embedded ‘tickle’ pulses within the waveform or develop maintenance modes to manage open time and latency. 

These ‘tickle’ activations in the ‘white space’ help maintain the high sheer dynamic viscosity and surface tension of the ink within the nozzle. It also reduces ink drying, skinning or curing. Without these techniques, prints at high frequency, but with low active nozzle duty – such as fine lines in printed electronics – would be challenging to achieve with precision. 

The use of drop watchers for process and material development is long established. Partnering with a supplier who can provide consistent output from lab to production ensures efficiency and reduces time-to-market. Considered use of novel print modes allows lab evaluation of deposition strategies new to inkjet. Additionally, they go beyond the traditional fixed arrays of binary droplets.