What is Kogation? Causes, Prevention, and Thermal Inkjet Ink Formulation
- Terry Clayton
- 3 days ago
- 6 min read
Inkjet Chemist | 30+ Years of Inkjet Ink Development Experience
Ink companies targeting thermal printheads have been reaching out due to kogation problems in their development programs. More and more printers and ink companies want to offer water-based inkjet inks and printing solutions for industrial digital printing so I thought I would put together my experience on kogation and how to minimize it.
Kogation is a phenomenon that reduces the lifespan of printheads and degrades printed output quality, making it a critical topic for anyone working with inkjet technology.
Let me explain kogation in inkjet printing, what it is, why it happens, and how to manage it effectively. Whether you are a printer operator, ink formulator, engineer, or enthusiast, understanding kogation will help you push the inkjet technology further and make world class output.
What Is Kogation in Inkjet Printing?
Kogation refers to the buildup of unwanted material on the firing resistor. A secondary effect is build up in nozzles or chambers of an inkjet printhead. This accumulation usually consists of colorant, ink residues, and other contaminants. Over time, kogation will cause:
Reduced drop weight
Missing nozzles and nozzle failure
Print defects (banding, streaking, density loss)
Reduced drop velocity and poor jetting accuracy
Shortened printhead life and increased maintenance
Kogation is a common issue in thermal printheads due to the heater resistor that is repeatedly heated to extreme temperatures (typically >300°C for a few microseconds) to generate a vapor bubble.
How Kogation Develops
Kogation forms through a combination of factors related to ink chemistry. You can also dive into printhead design and how the printhead is being driven but I am going to focus on ink chemistry here.
The five leading causes of kogation are:
Thermally unstable ingredients
Adsorption of ink components onto the firing resistor
Poor pigment or dye stability
Metal ions and other impurities
Insufficient anti-kogation additives
So if I am building an inkjet ink for thermal printheads and I want to eliminate or minimize kogation risk, here is what I do in the lab:
I begin by selecting thermally stable dyes, pigments, polymers, humectants, and surfactants that can withstand millions of rapid heating cycles without degrading. Purity is key here. The colorant system is chosen to ensure dyes remain fully soluble or pigments remain well dispersed under the extreme temperature and concentration changes that occur during bubble formation. Raw materials are carefully selected and purity confirmed to minimize dissolved metals, salts, and other contaminants that can promote deposits on the firing resistor. The formulation's pH, conductivity, and ionic balance are adjusted to maintain long-term chemical stability while minimizing interactions with the printhead. Finally, proven anti-kogation fighting additives are incorporated into the formulation. The inkjet ink must be validated through extended printhead life testing, ensuring consistent drop formation, reliable jetting, and minimal deposit buildup over the life of the cartridge. Yes, printhead testing is a mandatory step!
Effects of Kogation on Print Quality and Printhead Performance
As deposits accumulate on the firing resistor, they interfere with efficient heat transfer and vapor bubble formation, resulting in a gradual decline in printhead performance. The most common effects include:
Reduced Drop Weight: Deposits insulate the firing resistor, producing smaller ink droplets and lower optical density.
Print Quality Defects: Inconsistent droplet formation leads to banding, streaking, missing lines, color variation, and reduced image sharpness.
Poor Jetting Performance: Changes in bubble formation can reduce droplet velocity and alter droplet trajectory, causing inaccurate dot placement and degraded print resolution.
Nozzle Failure: As kogation progresses, droplets become intermittent or stop ejecting altogether, resulting in missing nozzles and permanent print defects.
Reduced Printhead Life: Progressive deposit buildup increases the energy required to eject droplets, accelerates printhead wear, and ultimately shortens cartridge or printhead life. Frequent cleaning cycles may temporarily restore performance but increase ink consumption and printer downtime.
Strategies to Minimize Kogation
Developing a low-kogation thermal inkjet ink requires careful selection and optimization of every component in the formulation. The most effective strategies include:
1. Select Thermally Stable Raw Materials
Choose dyes, pigments, polymers, humectants, surfactants, and other additives that remain chemically stable during millions of rapid heating cycles. Avoid ingredients that readily decompose or form deposits on the firing resistor.
2. Maximize Colorant and Dispersion Stability
Maintain complete dye solubility or robust pigment dispersion under the extreme temperature and concentration changes that occur during bubble formation. Stable colorant systems are far less likely to form deposits on the firing resistor.
3. Minimize Contaminants and Inorganic Impurities
Use high-purity raw materials and deionized water to reduce dissolved metals, residual salts, and other impurities that can promote deposit formation. Proper purification and fine filtration further improve long-term printhead reliability.
4. Optimize Ink Chemistry
Carefully balance pH, conductivity, ionic composition, and solvent selection to maintain formulation stability while minimizing interactions with the firing resistor. These parameters influence both printhead reliability and kogation resistance.
5. Incorporate Effective Anti-Kogation Additives
When appropriate, incorporate proven anti-kogation additives, such as selected phosphates or phosphonates, that inhibit deposit formation on the firing resistor. These additives should be optimized as part of the overall formulation rather than relied upon as a standalone solution.
Validate Through Printhead Life Testing
Ultimately, the only reliable way to evaluate kogation performance is by extended firing in the intended thermal printhead. Monitoring drop weight, drop velocity, nozzle health, and deposit formation over millions of firing cycles provides the best measure of long-term ink performance.
Scanning electron microscope (SEM) image showing severe kogation deposits accumulated on the tantalum-protected firing resistor of an HP45 thermal inkjet printhead after prolonged inkjet operation. The insulating deposit layer reduces heat transfer from the firing resistor, degrading bubble formation, drop ejection, and overall printhead performance. Reproduced from Lejeune, M., Evaluation of the Direct Inkjet Printing Method for the Fabrication of Three-Dimensional Ceramic Components, RWTH Aachen University, 2009, Fig. 6.3.
Real-World Examples of Kogation
Kogation has been a major focus of thermal inkjet research since the commercialization of desktop inkjet printers. Early thermal inkjet systems frequently experienced deposit buildup on the firing resistor, leading to reduced drop weight, poor print quality, and shortened printhead life. This challenge prompted extensive research by HP and other manufacturers into the chemistry of deposit formation and the development of specialized resistor coatings and anti-kogation additives.
More recently, the growing use of thermal inkjet technology for industrial coding, marking, pharmaceutical printing, and additive manufacturing has renewed the importance of low-kogation ink formulations. Modern thermal inkjet inks are carefully engineered using thermally stable raw materials, highly purified ingredients, optimized colorant systems, and anti-kogation additives to ensure reliable operation over millions of firing cycles while maintaining consistent print quality.
Monitoring and Diagnosing Kogation
Because kogation develops on the firing resistor inside the printhead, it is difficult to observe directly during normal printer operation. Instead, ink formulators and printhead engineers monitor its effects on print performance and verify deposit formation through laboratory analysis.
Printhead Performance Testing
Extended life testing is the primary method for evaluating kogation resistance. Parameters such as drop weight, drop velocity, nozzle health, and print quality are monitored over millions of firing cycles. A gradual decline in these measurements often indicates deposit buildup on the firing resistor.
Print Quality Evaluation
Kogation typically manifests as reduced optical density, missing nozzles, banding, inconsistent drop placement, and other print quality defects as deposits interfere with bubble formation and ink ejection.
Electrical and Firing Performance
As deposits accumulate, the insulating layer on the firing resistor reduces heat transfer efficiency. This can increase the energy required to generate a vapor bubble and eventually lead to intermittent or complete nozzle failure.
Microscopic and Surface Analysis
Following life testing, printheads can be sectioned and examined using optical microscopy or scanning electron microscopy (SEM) to visualize deposits on the firing resistor. Surface analytical techniques such as energy-dispersive X-ray spectroscopy (EDS/EDX) are often used to characterize the chemical composition of the deposits and assists the ink formulator in identifying their source.
Summary
Kogation is the accumulation of deposits on the firing resistor of a thermal inkjet printhead. As these deposits build up, they reduce heat transfer, interfere with vapor bubble formation, and ultimately degrade printhead performance. The result can be reduced drop weight, poor print quality, missing nozzles, and a shortened printhead life.
Developing a low-kogation ink begins with sound formulation. Ink chemists carefully select thermally stable raw materials, optimize colorant stability, minimize inorganic impurities, balance the overall ink chemistry, and incorporate proven anti-kogation additives when appropriate. These formulations are then validated through extended printhead life testing to ensure reliable performance over millions of firing cycles.
Understanding the mechanisms of kogation enables ink formulators to design more robust thermal inkjet inks that deliver consistent print quality, maximize printhead reliability, and reduce the total cost of ownership for end users. As thermal inkjet technology continues to expand into industrial printing applications, controlling kogation remains one of the most important aspects of successful ink development.
Need an Inkjet Ink Expert?
Whether you're developing a new ink or troubleshooting an existing one, I can help solve formulation and printhead challenges from kogation and jetting to stability and adhesion.
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