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Thermal Ink Jet Research

This research is a partnership of:
Santa Clara University
Hewlett Packard-Aguadilla (HP-Aguadilla)
University of Puerto Rico at Mayaguez (UPRM)
with support from the National Science Foundation (NSF)

Objective:

Characterization of the thermal ink jet process through modeling and experimentation.

Description:

Good printing quality on ink based printers depend on the characteristics of the droplet being ejected by the cartridges. A study of the droplet characteristics and its relation to dynamic variables such as velocity, frequency and others, was achieved.

Experience in Thermal Ink-Jet Cartridges

Thermal ink-jet printing (TIJ) emerged in the mid 1980’s as an innovative alternative to the dot matrix technology providing printing of much greater quality at reasonable speeds and producing very little noise. This technology was initially conceived at Hewlett-Packard Laboratories in 1979 and made its debut in early 1984. It eventually became available at competitive prices through competition and established itself as the peripheral of choice for new computer buyers. The incorporation of color printing in these devices gave consumers yet another reason to prefer TIJ printers.

In TIJ printing, ink droplets are ejected through a small nozzle, of about 50 mm in diameter or less, by a sudden change in pressure within the firing chamber caused by the formation of a micro-bubble. The heat needed to generate this bubble is generated by a micro-heater when a brief pulse of relatively high current is applied. The micro-heater is typically of rectangular cross section of 50 mm by 50 mm. Boiling of liquids can occur by heterogeneous nucleation, homogeneous nucleation or a combination of both. Heterogeneous nucleation takes place at pre-existing surface cavities or entrained gas bubbles on the liquid container. Heterogeneous nucleation can happen at liquid temperatures as low as 10ºC above the saturation temperature. On the other hand, homogeneous nucleation takes place when the liquid reaches a temperature close to its superheat limit. TIJ printers use extremely high heat flux pulses to quickly raise the temperature of the liquid to its superheat limit and thus force spontaneous (homogeneous) nucleation as the boiling mechanism. Spontaneous nucleation is employed mainly because: (1) The boiling process is more explosive due to the initial large bubble pressure. This characteristic is necessary to impart enough momentum to the liquid to defeat surface tension and viscous forces and break free from the nozzle. (2) The homogenous boiling process is more reproducible.

After ejection of the droplet, the firing chamber is given enough time to stabilize both dynamically and thermally before another bubble-generation cycle begins. One such cycle spans only a few hundred microseconds yielding a droplet ejection rate in the range of a few thousands per second depending on the particular printer model. The liquid leaves the orifice initially as a jet from which, eventually, one, two or three droplets of successively smaller size detach. The ink is sprayed onto the paper by a moving print head in a programmed sequence of movements generating the characters and/or graphics to be printed.

A collaborative effort HP/SCU/UPRM aimed to characterize the thermo and hydrodynamics processes of in-flight droplets coming from TIJ was initiated four years prior. Water was used as the working fluid in this case. The initial experimental set-up used for these studies is shown in Figure 1. An electrical circuit was designed and built that allowed full control of the firing conditions of a commercial ink-jet cartridge. The circuit allowed variations in the frequency, intensity and width of the electrical pulse needed to initiate the micro nucleate boiling process and eject the water droplets. The schematic in Figure 1 shows the set-up used to apply the LIFT method to measure the temperature of the in-flight droplets coming from the TIJ. Figure 2 shows photographs of the liquid jet fired from the TIJ and of the main droplet formed after breakup. Figure 2A presents the jet with the tip already exhibiting a spherical shape as it grows into the main droplet. In Figure 2B, the main droplet and two satellites have detached from the jet (also shown, lagging behind). The elongated shape in the second satellite is typical on the smallest satellites upon breakup. It then follows a short period of oscillations elongating in the horizontal direction and vertical directions until it stabilizes into spherical shape.

A series of experiments were conducted to determine the impact of frequency, pulse width and pulse intensity in the droplet size, velocities and temperature. Results indicate that the most influential parameter was the frequency and that the most affected parameters were the diameter and the velocity. The temperature of the in-flight droplet was found to be closed to the ambient temperature an indication that enough cooling of the liquid has occurred at the nozzle tip after the initial heating.

An interesting finding of these studies was the long-term effects that sequential activation of the electrical resistance may have in the reliability of the cartridge. It was noticed a degradation of the velocity of the droplets with time as shown in Figure 3. This velocity decrease is attributed to the accumulation of ink deposits on the surface of the micro-resistor, or the so-called Kogation effect.

 

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Figure 2. (A) Water jet, (B) droplet break-up and (C) fluorescence emission

 

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Figure 3. Results from long term diameter and velocity Tests