Inkjet Technology - Drop-on-Demand (Piezoelectric DOD) Droplet Formation | Thermal Inkjet (TIJ) Drop-on-Demand (DOD)
Inkjet Technology - Drop-on-Demand(Piezoelectric DOD) Droplet Formation
Droplet Formation
Drop-on-Demand (DOD) printheads can contain hundreds to thousands of integrated miniature nozzles, which can be individually activated to generate droplets as needed. Each nozzle is supplied by a jetting chamber, and various methods are used to create a raised pressure pulse inside the chamber to eject droplets. Adjusting the pressure pulse generates droplets with well-defined volumes and speeds. Piezoelectric DOD and Thermal Inkjet (TIJ) are the most common inkjet technologies, although electrostatic and acoustic DOD printheads have also been developed.
Droplet Generation Cycle
In DOD systems, the droplet generation cycle can be considered in three distinct stages:
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Droplet Ejection: A raised pressure pulse is generated inside the chamber to eject a substantial amount of ink through the nozzle.
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Droplet Formation: The ejected ink column deforms and is pinched off to form individual droplets.
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Refilling: Ink flows back into the jetting chamber from the reservoir to refill it in preparation for the next droplet ejection.
The entire droplet generation cycle can be simulated using Computational Fluid Dynamics (CFD) analysis.
CFD Modeling
CFD modeling refers to the process of numerically simulating physical phenomena like fluid flow, heat and mass transfer, and chemical reactions using computational fluid dynamics. It is a multi-step, iterative process that involves integrating physical knowledge, mathematical methods, and computational tools.
Piezoelectric DOD
In Piezoelectric DOD, voltage-driven piezoelectric actuators are used to mechanically deform the walls of the jetting chamber to eject droplets. Each nozzle is driven by an individual sensor, which generates an electric field on the piezoelectric material layer when voltage is applied, causing the sensor to deform. This deformation causes the volume of the chamber to either shrink or expand depending on the polarity of the applied voltage. This controls the pressure within the chamber: high pressure occurs when the chamber shrinks, and low pressure occurs when the chamber expands. Custom voltage waveforms are applied to generate the desired time-dependent pressure inside the chamber, optimizing droplet ejection and chamber refilling.
Simulating piezoelectric DOD involves both Fluid-Structure Interaction (FSI)—which accounts for the coupling motion between the actuator and the fluid—and Free-Surface Analysis (FSA), which tracks the fluid surface during the entire jetting cycle. Specifically, FSA is needed to track the movement of the meniscus in the nozzle during the initial phase of ejection, the surface of the ejected ink mass when it turns into a droplet, and the meniscus during refilling.
FSI (Fluid-Structure Interaction): The interaction of fluids and structures.
FSA (Free-Surface Analysis): The analysis of the free surface of a fluid.
A major challenge in simulating piezoelectric DOD involves the accurate prediction of FSI, i.e., the bidirectional coupling between the actuator deformation and changes in the fluid flow field inside the ink. When the actuator deforms, it displaces the ink, and the flowing ink provides pressure loads that affect the actuator deformation. A rigorous analysis of this process requires coupled and self-consistent calculations for both ink dynamics and structural mechanics. However, most piezoelectric DOD models do not fully account for coupled FSI. Instead, models simplify the process by assuming limited one-way coupling (where the actuator deformation excites the ink, but the ink’s reaction on the actuator is ignored) or by entirely neglecting the mechanical actuation and substituting time-dependent pressure conditions inside the chamber. In the latter case, the applied pressure completely eliminates the need for FSI analysis.
In practice, given a time-dependent model, one assumes limited one-way coupling, where the actuator deformation excites the ink, but the reaction of the ink on the actuator is ignored. Alternatively, mechanical actuation is completely omitted, and time-dependent pressure conditions inside the jetting chamber are used instead.
CFD/FSA Execution
This process involves CFD/FSA simulations to predict the mass of the ink being ejected, the droplet formation, and the capillary refilling of the jetting chamber. These models provide insights into the jetting process and predict droplet velocity and volume, which generally align with experimental observations.
During jetting and refilling, tracking the fluid-structure interface and the associated fluid flow and pressure is difficult, especially because incremental structural displacements occur during each simulation time step. Various numerical methods have been developed to address this challenge. One such method uses the Volume of Fluid (VOF) technique in CFD analysis to predict unstable fluid flow and related pressure at the fluid-structure interface, while different numerical techniques, such as Finite Element Analysis (FEA), are used to compute structural deformation. When the interface deforms, it defines a new boundary for the fluid dynamics calculations, producing a new pressure load.
TIPS:
VOF (Volume of Fluid) is a key technology in CFD used to simulate the interface capturing and tracking of multiphase flows, especially immiscible fluids such as gas-liquid or liquid-liquid.
Inkjet Process — Thermal Inkjet (TIJ) Drop-on-Demand (DOD)
Thermal Inkjet (TIJ)
A thermal inkjet printhead contains hundreds to thousands of integrated micro-nozzles, each with its own firing chamber. A thin-film resistive heater is integrated into the wall of each firing chamber. When a droplet is required, the heater is activated by a short voltage pulse with a duration on the order of microseconds.
The magnitude of the applied voltage is sufficient to raise the temperature of a thin layer of ink in contact with the chamber wall (directly above the heater) to an extremely high level, typically around 300 °C. Once this temperature is reached, the ink undergoes explosive vaporization, forming a uniform vapor bubble with a very high initial pressure (greater than 100 bar). Under this pressure, the bubble rapidly expands within the ink chamber, imparting sufficient momentum to eject ink through the nozzle, forming a droplet with the desired volume and velocity.
The vapor bubble then collapses, and due to capillary pressure generated by surface tension, the firing chamber is refilled with ink drawn from the reservoir.
A rigorous analysis of TIJ droplet ejection is highly complex. It involves predicting the voltage-driven excitation of the resistive heater, heat transfer from the heater to the ink, phase transition from superheated ink to vapor, vapor bubble dynamics, droplet ejection, refilling of the firing chamber, and the formation of the ejected droplet.
Key Knowledge Summary
Principle
Thermal inkjet is a core inkjet printing mechanism. Inside the printhead, microscopic heating resistors (typically thin-film resistors) are heated to extremely high temperatures (approximately 300–400 °C) within a very short time (microseconds) by an electrical pulse.
Bubble Formation
The high temperature causes a thin layer of ink in contact with the heater to vaporize instantaneously, forming a microscopic vapor bubble.
Droplet Ejection
The rapid expansion of the vapor bubble generates a pressure wave that forces ink above it to be expelled through the nozzle (orifice) at high speed, forming a single ink droplet.
Bubble Collapse
Once the heating pulse ends, the heater cools rapidly, and the vapor bubble condenses and collapses.
Ink Refill
The negative pressure generated by bubble collapse, together with capillary action, draws fresh ink from the ink manifold into the nozzle region, preparing it for the next firing cycle.
Characteristics
TIJ printheads have a relatively simple structure (primarily heaters and nozzles), lower manufacturing cost, and can operate at very high firing frequencies (kHz range). However, the high operating temperature places stringent requirements on ink formulation (high thermal stability, resistance to kogation), and heater lifetime is a critical consideration.
Extended Knowledge
★ DOD (Drop-On-Demand)
Definition
“Drop-On-Demand” refers to an inkjet method in which droplets are ejected only when required, in contrast to Continuous Inkjet (CIJ).
Control Method
Each nozzle is precisely controlled by electronic signals. A droplet is generated only when a signal is applied (in TIJ, this means energizing the thermal resistor).
Advantages
High ink utilization (minimal waste), broader ink compatibility, high printing accuracy, and compact system architecture.
Applications
DOD is the dominant inkjet technology today and is widely used in desktop printers, wide-format printers, industrial printing (packaging, textiles, decoration, electronics), and 3D printing.
★ PCA (Printhead Control ASIC / Printhead Controller ASIC / Printhead Driver ASIC)
Definition
A PCA is a dedicated integrated circuit for printhead control.
Function
It acts as the “electronic brain” and “neural center” of the printhead, and is directly responsible for:
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Data Reception
Receiving image data (typically serialized) from the printer’s main controller. -
Data Conversion and Distribution
Converting image data into precise voltage and current drive pulses and distributing them accurately to the driving circuits of hundreds or thousands of nozzles (heaters). -
Precise Timing Control
Controlling firing timing, pulse width (affecting droplet size and velocity), and energy (affecting heating temperature) with nanosecond-level precision. -
Thermal Management
Monitoring printhead temperature and dynamically adjusting drive parameters to maintain consistent droplet size and velocity through temperature compensation. -
Nozzle Management
Supporting advanced functions such as nozzle failure compensation and nozzle health monitoring. -
Communication
Exchanging status information and error reports with the printer’s main controller.
Importance
The PCA is a critical component for achieving high resolution, high speed, and high stability in inkjet printing. Its performance directly affects print quality, productivity, and reliability. Modern high-performance printheads often integrate multiple PCAs.
★ FSA (Fluidic Self-Assembly)
Definition
Fluidic Self-Assembly is an advanced microfabrication technology used to manufacture ultra-high-density, high-precision silicon-based microfluidic structures, particularly for printhead chips with extremely large numbers of micro-nozzles.
Principle
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Microscopic cavities or channels of specific shapes are etched into a silicon substrate.
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Corresponding micro-components (typically nozzle plates) are fabricated on another substrate.
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These micro-components are suspended in a liquid.
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As the liquid flows over the patterned silicon substrate, precisely designed fluid dynamics (such as surface tension and shear forces), together with cavity geometry, cause the micro-components to automatically align and settle into their matching cavities—much like assembling a puzzle.
Applications in Printheads
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Enables precise, large-scale assembly (thousands of nozzles at once) of micro-nozzle plates onto silicon ink chamber and channel structures.
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Overcomes the resolution limits, high cost, and low yield associated with traditional lithography and etching when producing ultra-high-density nozzle arrays (e.g., 1200 dpi and above).
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Allows fabrication of nozzle arrays with extremely small pitch (on the order of a few micrometers) and exceptionally high positional accuracy.
Advantages
Enables unprecedented nozzle density and precision, reduces manufacturing cost, and improves production yield. It is one of the core manufacturing technologies behind modern ultra-high-resolution industrial inkjet printheads.