Thermal inkjet

Thermal inkjet printheads produce ink droplets from the thermal vaporization of the ink solvent (25). In the inkjet process, a resistor is rapidly heated to produce a vapor bubble, which subsequently ejects a droplet from the orifice. 

This process is extremely efficient and reproducible. Modem thermal inkjet printheads for industrial graphics applications are capable of generating uniform drops of 4 p / or smaller in volume at frequencies of 36 k Hz or greater. Typical commercial thermal inkjet devices are specifically designed to vaporize water or solvents that 

Nearly all of the commercial inks available for thermal iniqet systems are water-based, so they contain more than 50% water. Such aqueous inks have one or more drawbacks such as long ink dry times or poor adhesion to semiporous or nonporous substrates. 

Inks with attractive performance characteristics, such as short dry times, long decap times and good adhesion when using a thermal inkjet system, have been developed (25). 

These compositions contain volatile organic solvents, humec-tants, binder resins, and dyes. The solvents are low molecular alcohols, e.g., ethanol or methanol, and ketones, e.g., methyl ethyl ketone. 

One of the most commercially successful MEMS devices has been the thermal inkjet. In this device a small resistor heats an ink to create a vapor bubble. As the vapor bubble expands it displaces a drop of ink out of a nozzle. We will consider the design of a thermal inkjet device in Chapter 6 on microfluidics, but here we consider the heater that generates the vapor bubble. The typical propellant in inkjet inks is water, which is superheated to approximately 330°C in a few microseconds. If we consider a polysilicon heater that is 25 pm on a side on top of oxide that is 2 pm thick, the thermal conductance of the oxide would be 

---

Generally, ink dyes for ink jet applications and writing, drawing, or marking materials are selected from food, acid, direct, sulfur, and reactive dyes. The choice of dye depends on the application and the ink used, whether it is aqueous, solvent based, or hot melt, and on the printer type continuous ink jet or drop-on-demand, piezo or thermal inkjet. 

Recently, the structurally related disazo dye 2 was developed for commercial thermal inkjet printers [3],... 

The ejection process of an ink drop from a thermal inkjet (TIJ) print head begins as an electrical pulse is apphed to a resistor. Within 2-5 microseconds, the resistor surface reaches a temperature of... 

Fig. 1. Ejection cycle of a thermal inkjet (TIJ) print head.

A further distinction, when referring to inkjet inks, is the printing mechanism. Solvent inks are used widely in drop-on-demand piezo inkjet printing (DOD PIJ) and in continuous inkjet (CIJ). To a lesser extent there is an effort to introduce solvents into thermal inkjet drop-on-demand (DOD TIJ). 

Several types of fluids may be jetted via inkjet heads, which can be of the continuous inkjet variety (CIJ, primarily aqueous or solvent-based), thermal inkjet (TIJ, primarily aqueous, but some solvent and UV/aqueous hybrids exist), or piezoelectric drop-on-demand (DOD, aqueous, solvent, oil, UV curable). While there are advantages and disadvantages for each type of hardware and fluid, a clear leader in growth has emerged from the list. UV curable inkjet... 

Wickramanayake P. (1994) Black-to-color bleed control in thermal inkjet printing, US5342440. 

Wickramanayake P, Moffatt JR. (1993) Solubilization of water-insoluble dyes via microemulsions for bleedless, non-threading, high print quality inks for thermal inkjet printers, US5226957. 

L. F. De La Torre-Quintana, S. P. Hemandez-Rivera, Characterization of thermal inkjet technology TNT deposits by fiber optic-grazing angle probe FTIR spectroscopy. Sensors, and Command, Control, Communications, and Intelhgence (C3I) Technologies for Homeland Security and Homeland Defense IV Edward M. Carapezza Ed., Proc. SPIE, 5778, 543-552 (2005). 

Ejected drop printing (e.g. through piezo or thermal inkjet)... 

IS. Aden, XH. Bohorquez, D.M. Collins, M.D. Crook, A. Garda, and UE. Hess, The third generation hp thermal inkjet printhead. Hewlett-Packard Journal 45(1) (1994) 41 5. 

The theory of bubble nucleation in a superheated liquid was first applied to the concept of thermal inkjet by Allen et al. [7]. They were able to determine the minimum cmiditions for the first bubble nucleation by applying Hsu s theory [10]. Time dependent temperature profiles above a heater surface were obtained. By superimposing the activation curve with the thermal boimdary layer, the initial bubble size and the minimum temperature for nucleation were determined. Based on a one-dimensional model and by assuming the nucleation temperature to be the superheat limit of the liquid at 330°C transient temperature profiles for the heater structure and the bubble surface after nucleation were obtained. It was noticed that the decay time to ambient temperature from its initial state was only several microseconds after 6 ps heating pulse. The thermal effects of the passivation (protective coating) layer on the heater surface were also analyzed. The results showed that the effective pulse energy required for bubble nucleation increases with the thickness of the passivation layer. 

J. R. Andrews and M. P. O Horo Initial stages of vapor bubble nucleation in thermal inkjet processes. Proceedings of SPIE - The Intemation Society for Optical Engineering, 2413, 182-188 (1995). 

C. T. Avedisian, W. S. Osbourne, F. D. McLeod, and C. M. Curley Measuring bubble nucleation temperature on the surface of a rapidly heated thermal inkjet heater immersed in a pool of water. Proceedings of the Royal Society of London, Series A, 455, 3875-3899... 

Ballarin, B., A. Fraleoni-Morgera, D. Frascaro, S. Marazzita, C. Piana, and L. Setti. 2004. Thermal inkjet microdeposition of PEDOT PSS on ITO-coated glass and characterization of the obtained film. Synthetic Met 146 201-205. 

The later developed thermal inkjet technology has the advantage of being able to position a drop on demand. Ink drops are not emitted continuously but only when needed for printing. This property eliminates the need for additional systems to capture and recirculate... 

A thermal inlqet ink composition has one or more attractive features, such as short unassisted dry times of printed alphanumeric or graphic images, long decap times, good adhesion to semiporous and nonporous substrates, and safety or material compatibility with one or more components of a thermal inkjet printer (25). The decap time is the time that printer nozzles can be imcovered and idle before they will become ineffective and need to be cleaned. 

K.B. Gundlach and R.L. Colt, Thermal inkjet composition, US Patent 5 531815, assigned to Xerox Corporation (Stamford, CT), July 2,1996. 

Some encapsulation methods of polymer-encapsulated pigments often do not perform well for thermal inkjet inks. A common method involves capsules formed with dissolved polymers that are adsorbed onto the pigment surface. 

---