

One of the common failures in inkjet heads is ink non-ejection. The causes of non-ejection are generally attributed to nozzle clogging due to ink drying and bubble formation inside the head. However, since it is difficult to directly observe the internal state of the head from the outside, the detailed process leading to this failure has not been fully clarified. To analyze this process, we fabricated a transparent direct-mode head that allows external observation of the nozzle, pressure chamber, and supply channel, and employed a high-speed camera to visualize the ink flow and bubble formation inside the head. The results revealed that bubbles inside the head can be generated through multiple modes: one is bubble ingestion from the nozzle, and the other is vapor bubble formation caused by significant negative pressure in the channel. These bubbles, subjected to pressure oscillations within the ink flow path, repeatedly undergo cycles of growth and collapse, eventually developing into resident bubbles. The process leading to non-ejection is strongly influenced by ink quality. By evaluating different inks, valuable insights can be obtained for the development of high-quality inks.

The generation of ink droplets in piezoelectric drop-on-demand printheads is governed by a tightly coupled multiphysics process involving electromechanical actuation, fluid dynamics and free-surface interactions. Conventional simulation approaches often treat these subsystems separately, limiting predictive fidelity for design optimization. In this work, we present a fully coupled three-dimensional fluid structure interaction (FSI) framework that integrates computational structural mechanics (CSM) that incorporates the piezoelectric effect with compressible multiphase computational fluid dynamics (CFD). The framework employs a partitioned solution strategy with strong interface coupling, enabling accurate capture of the dynamic interplay between actuator deformation and droplet ejection. The method is applied to recirculating inkjet printhead designs, utilizing volume-of-fluid (VOF) based free-surface modelling to resolve ink-air interactions and validate performance against benchmark structural and fluidic cases of the Seiko RC1536 printhead. Results demonstrate accurate prediction of droplet formation dynamics, capturing the influence of actuation waveforms, chamber geometry and surface tension effects. By combining high-fidelity electromechanical and fluidic models, this work advances predictive capabilities for inkjet printhead design and operation. The methodology lays the foundation for optimizing printhead architectures, tailoring actuation strategies and accelerating innovation in industrial and functional inkjet applications.

RISO Technologies, formerly Toshiba Tec’s Inkjet Business Unit, continues to provide high-quality shear-mode piezoelectric printhead products to the industrial inkjet printing market since 2001. A non-destructive method has been developed to evaluate piezoelectric actuators by simple LCR measurement. With empirical equivalent circuit modeling, we demonstrate that the approach may provide an insight into the actuator’s physical thickness in a finished printhead product and can be used as an alternative method to assess the effectiveness of the precision manufacturing processes by determining dimensional variance.

Modern piezoelectric drop-on-demand inkjet printing relies on precisely tuned drive waveforms to control droplet formation. We present a simple simulation tool that visualizes the pressure wave dynamics inside an inkjet nozzle in response to multi-pulse drive waveforms. Unlike computational fluid dynamics (CFD) models, which require significant computational resources and complex boundary conditions, this simulation is intentionally lightweight. By modeling each voltage pulse as a decaying sinusoidal pressure response, the simulation intuitively demonstrates key waveform design concepts. For example, a primary pulse timed to the nozzle’s acoustic resonance produces maximum pressure, while a well-timed secondary “cancel pulse” effectively dampens residual oscillations. This simplified model qualitatively replicates real-world behaviors observed in high-frequency printing, where without waveform optimization, leftover pressure oscillations from one droplet can interfere with the next. With an appropriately tuned cancel pulse, the simulation shows stabilized pressure and predicts more consistent jetting performance. Simulated high-frequency resonance effects and corresponding measurements are qualitatively consistent across multiple printhead models. This work, though based on known principles, provides an interactive visual approach to waveform design, valuable for both inkjet engineers and educators.

The wood-graining effect is a print defect arising from complex aerodynamic interactions within the narrow gap between printhead and substrate. Unsteady vortical structures, substrate-induced Couette flows and drop-induced entrainment destabilize jetting trajectories, leading to wavering and consolidation of printed tracks. Conventional RANS methods fail to capture these transient dynamics, necessitating high-fidelity turbulence modelling. In this study, large-eddy and detached-eddy simulations are combined with a two-way Lagrangian particle tracking framework to resolve the mutual interactions between jetted drops and surrounding gas flows. This coupling accounts for shielding, wake effects and drop-induced airflow modifications that drive cross-stream displacement. To connect droplet flight dynamics with final print quality, a wall-film model simulates spreading, coalescence and redistribution upon impact, directly linking flow-driven instabilities to visible wood-grain artifacts. The resulting CFD-Lagrangian-film framework provides a predictive description of wood-graining under realistic conditions, enabling systematic exploration of nozzle spacing, print gap and substrate speed. Demonstrated in 600 npi printheads, the study highlights how high nozzle density magnifies the impact of small aerodynamic disturbances. By establishing a physics-based understanding of wood-graining onset and evolution, the methodology supports the design of next-generation printheads and airflow control strategies to mitigate aerodynamic instabilities.

The paper presents a method on how to predict the fading of particular printing colours based on action spectra. The action spectra are measured in a custom-built exposure chamber equipped with narrow band LED covering the visible light range. The action spectra are used to predict broadband exposure to ‘white light’ of a particular application. As a first example, the predicted fading curves are compared with actual fading data obtained using white-light LED. In the next step, the action spectra are applied to predict fading for the same sample set when exposed to a commercial, LC37-filtered Xe-arc light source, as specified in ISO 18937-2. The action spectra of colour print materials reveal that the main degradation reactions are caused by radiation below 420 nm, some degradation happens in the wavelength range 420 nm to 550 nm and very little degradation is caused by light above 550 nm. A severity factor to estimate the degrading potential of light sources for the material class of colour photographic and printing materials is developed.

Inkjet printing suffers from a number of quality issues caused by unstable air flow between the print head and the substrate. A comprehensive understanding of the mechanisms underlying these defects is imperative. Computational Fluid Dynamics (CFD) facilitates rapid scanning of various configurations, enabling efficient execution of parametric studies. Yet, the validity of the CFD simulation must be confirmed with experimental measurements of the flow. In this work, a numerical simulation framework was developed and experimentally validated. Good quantitative and qualitative agreement was obtained. The numerical simulations were also used to successfully reproduce typical print quality issues induced by unstable air flows.

A two-dimensional microplasma array operating under atmospheric conditions has been developed and optimized for direct surface metallization. Unlike conventional plasma arrays where all cells ignite simultaneously, this system enables independent ignition and quenching of individual plasma cells “on demand,” a unique feature protected by a granted patent. The plasma head integrates ink injection capillaries and an aerosol generator, enabling one-step deposition of metallic layers, with silver selected as a representative material for decoration and functional coatings. The project focused on improving plasma head stability, deposition resolution, and sustainability through enhanced electronics, optimized gas flow, and advanced manufacturing methods such as laser cutting, 3D ceramic printing, and plasma copper deposition. Results demonstrate stable operation up to 30 minutes, improved plume length (1.5 mm at 2 slm), and reproducible ignition during ink injection without clogging. Surface metallization was achieved in one step, though further optimization is required to eliminate ink residues and leakage. This technology offers scalable, high-resolution solutions for functional printing, selective coatings, and advanced manufacturing.

Modern displays utilize color conversion layers to convert blue backlight into colored sub-pixels. Perovskite quantum dots (PQDs) are a very efficient material for this conversion and are currently applied as one continuous film combined with a color filter layer. Inkjet printing could enable the application of PQDs directly into colored sub-pixels, eliminating the filter layer and improving efficiency of the display by design. In this work, PQDs were modified for application by inkjet. UV-curable resin and PQD dispersion were modified to reduce viscoelasticity down into inkjetable range, which was characterized using the TriPAV high frequency rheometer. Printability of PQDs was further shown by dropwatching and manufacturing an inkjet-printed pixel-array demonstrator. The highly loaded ink is UV-curable and solvent-free. It can be applied by inkjet into precise sub-pixel arrays with a pixel thickness of 10µm in one pass. At that thickness, color conversion efficiency and optical density of the PQD sub-pixels fulfill specifications needed for application in modern displays.