JPS59109372A - Heating element for thermal ink jet printing head - Google Patents

Abstract

PURPOSE:To reduce as much as possible the damage due to cavitation and prolong the useful life of a heating resistor layer provided on the surface of a head, by convering the surface of the heating resistor layer with a heat-accumulating layer. CONSTITUTION:In a thermal ink jet printer wherein a pulse current is impressed on a heating resistor to generate heat and thereby an ink droplet is discharged, the heat-accumulating layer is provided on the surface of the heating resistor layer 37 provided on the substrate 39 of the printing head, and preferably, an inactivated layer is provided thereon. The heat-accumulating layer 33 is prefreferably comprised of, for example, an aluminum oxide layer of about 1mum in thickness. The inactivated layer 35 is preferably a thin film of about 0.5mum in thickness of silicon carbide, silicon dioxide, aluminum oxide or the like.

Description

TECHNICAL FIELD OF THE INVENTION This invention relates to a heating element for a thermal ink jet print head having a structure suitable for improving durability.

[Technical background of the invention and its problems]

Thermal ink jet printers are known that apply hiss to a thermal ink jet print head. In this printer, resistive heating creates air bubbles in the ink that increase pressure to force the desired ink droplet from the printing head.

The lifespan of a thermal ink jet print head depends on the lifespan of this heating element. The majority of heating element failures are caused by cavity damage, which occurs during the collapse of air bubbles.
tation damage). To make multiple print heads (e.g., page-width arrays) economically viable, thermal ink jet printheads should have a lifetime of more than 100 million ink droplets. SUMMARY OF THE INVENTION The present invention provides a heating element for a thermal ink jet print head that minimizes cavitation damage and extends life. The heating element of the thermal ink jet print head of the present invention is provided on the substrate, and its composition includes a resistive layer, a heat storage layer provided thereon, and a passivat 1onlayer.
) to supervise. The heat generated by the pulsed current applied to the heating element serves two purposes: a portion of this heat is used to form bubbles in the ink due to the primary evaporation of the ink. The remaining heat is then used for σ) to reevaporate the ink. To explain in more detail,
When a pulsed current is applied to the heating element, the temperature of the three layers mentioned above increases, creating vapor bubbles due to the primary evaporation of the ink. When the pulse current force reaches tli, the resistive layer cools quickly due to the heat conduction path through the substrate. However, since the heat storage layer is sandwiched between the resistive layer and the ink vapor and is thermally insulated, it cools down considerably slowly. The total amplitude of the pulse current is set to be sufficiently large. Thereby, even when the ink that has drifted away due to the collapse of bubbles due to the primary evaporation of the ink comes into contact with the heating element, the temperature of the heating element is maintained at a high enough temperature to cause the ink to evaporate in the work area.

This secondary evaporation of the ink alleviates the acoustic shock generated when the bubbles of secondary evaporation collapse. The passivation layer and the heat storage layer 1-, which are on top of the resistance layer, can also be used to chemically
It is also used for mechanical protection.

[Embodiments of the invention]

FIG. 1 is a schematic block diagram of a thermal ink jet printer in which a heating element of a thermal ink jet print head according to the present invention is used. The thermal ink jet printer itself shown in Figure 1 is described in Japanese Patent Application Laid-open No. 5, cited earlier.
This is explained in more detail in Japanese Patent No. 8-36465, and since it is not directly related to the gist of the present invention, no further explanation will be given here. The thermal ink jet shown in Figure 1.
Ink droplets in printers? print head 3
When it is desired to emit heat, a pulsed current generator 1 is used to generate a pulsed current and applied to the heating element 5 having a resistance.

This pulsed current generates Joule heat in the heating element 5 to generate bubbles in the ink, which cause ink droplets to be ejected from the print head 3. Note that ink is supplied from the ink reservoir 7 to the print head 3 through a tube 15.

FIG. 2 is a more detailed cutaway view of print head 30. Ink is supplied to capillary region 11 via tube 15 . When a pulsed current is supplied to the heating element 5 through a conductor (not shown), a bubble of ink vapor is created in the ink above the heating element 5. As a result, the pressure within the ink increases and causes the required ink droplet to be ejected from the nozzle 9.

When multiple heating elements 5 are used in print head 3, barriers 13 are provided to prevent crosstalk with adjacent nozzles 9.

FIG. 3 is a diagram showing the cross-sectional structure of the heating element 5. The heating element is formed on the surface of a silicon substrate 390 and is provided on a silicon oxide layer 31 with a thickness of 5 μm, and includes a resistance layer 37 provided on the silicon oxide layer 31, a heat storage layer 33 covering the resistance layer 37, and a heat storage layer 33 covering the resistance layer 37. It is composed of a passivation layer 35 that covers a heat storage layer 33. Resistor 1-37 is half made of aluminum alloy and half aluminum alloy1), 225 cl/s
It is. Resistive layer 37 can be made using well-known thin film techniques. The function of the heat storage layer 330 on the resistance layer 37 is to first transfer the hot ink while a pulsed current is applied to the resistance layer 37, and then to collapse the bubbles formed by the primary evaporation of the ink when the pulsed current is removed. Until then, it acts as a heat storage element. Therefore, it is necessary to make the thermal diffusivity of the heat storage layer 33 smaller than that of the resistive layer 37.In the thermal energy e-jet printer of FIG. 1, the heat storage layer 33 has a thickness of micrometers. It is an aluminum layer, and the thermal diffusivity is l), 065 tn/s.

In addition, the passivation layer 35 covers the heat storage layer 33 and the resistance layer 37 and serves as a chemical and mechanical protection during use.

It is important that the thermal diffusivity of the passivation layer 35 is approximately equal to or greater than the thermal diffusivity of the heat storage layer 33, so that heat can be quickly conducted through the passivation layer 35. . As the passivation layer 35, silicon carbide,
Thin films such as silicon oxide or aluminum oxide can be used.

In the thermal ink jet printer of FIG. 1, the passivation layer 35 is an aluminum oxide layer with a thickness C) of 5 μm.

FIG. 4 shows the temperature distribution within the heating element 5 immediately after the trailing edge of the pulse current and before the vapor bubbles due to the primary evaporation of the ink collapse. The point to be noted here is that the resistance layer 37 has a high thermal diffusivity, so the resistance layer 37
The temperature gradient within the resistor layer 37b is flat, and the cooling of the resistance layer 37b by diffusion of heat is achieved by heat conduction through the silicon oxide layer 31 and the silicon substrate 39. Both the heat storage layer 33 and the passivation layer 35 have a thermal diffusivity equal to that of the resistance layer 3.
7, the temperature gradient within these layers 2' is steep.

In addition, heat flows from the resistance layer 37 to the ink vapor bubbles, and since the ink vapor bubbles are thermal insulators, the temperature drop in the heat storage layer 33 after the pulse current is turned off is much slower than in the resistance layer 37. . Therefore, if the total amplitude of the pulse current is sufficiently large, the temperature of the four thermal layers 33 (and the passivation layer 35) will decrease as the incoming ink contacts the heating element 5 due to the collapse of bubbles caused by the primary evaporation of the ink. Even if the temperature is 2 at the time of the drop, it is still higher than the boiling point of the ink. In such a case, secondary evaporation of the ink occurs, and cavitation loss becomes minimal.

FIG. 5 is a block diagram of an array of test equipment that can be used to select the pulsed current that will cause the desired secondary evaporation of ink. The heating element 5 of the thermal ink jet print head to be tested is placed at the bottom of the ink container 51. Also, P manufactured by Matchlett Co.
A high frequency pressure transducer 53 such as a VF-2 type device υ) is installed directly above the heating element 5 at a distance of about IC'rrL. The output of the high frequency pressure transducer 53 is compared to that of Tektronix Corporation.
) is monitored with a digital storage type oscilloscope 55 such as 468m. The amplitude of the output of the pulsed acid flow generator 57 is varied in various ways and applied to the heating element 5, and the output of the high frequency pressure transducer 53 when stiff is recorded.

Figure 6 is a graph plotting typical pressure transducer outputs observed with the test equipment in Figure 5.The first pressure drop (near the center of the time axis) is a bubble caused by primary evaporation of ink. represents the acoustic shock generated by the collapse of

Even if the amplitude of the pulse stream increases beyond the amplitude required to create ink vapor bubbles due to selective evaporation of ink, the maximum pressure that rises will continue until this amplitude reaches a certain threshold. Kasbaik has remained relatively constant.

As mentioned above, when the pulse current amplitude reaches this threshold, secondary evaporation of ink occurs, and the amplitude of the last spike becomes approximately 1.
/2. As the amplitude of the pulse current increases further, the amplitude of the second spike gradually increases, indicating that the ink is undergoing unnecessary tertiary evaporation. When the amplitude of the pulse current reaches its limit, the heating element is destroyed due to thermal stress.

The heating element of the thermal ink jet print head resistor described above was analyzed in the test apparatus of FIG. 5 using water and a pulsed current of 6 μs width. As a result, we found that the minimum pulse current amplitude required to eject an ink droplet is 0.42 amps and that secondary evaporation of ink occurs at a pulse current amplitude of +1.62 amps. Rina. When the amplitude of the pulse current is 0.
62 to 1), 64 a/bear.

[Brief explanation of drawings]

FIG. 1 is a schematic block diagram of a thermal ink jet printer in which the heating element of the thermal ink jet print head according to the present invention is used, and FIG. 2 is a cutaway view of the print head shown in Figure g1. Figure 3 is a diagram showing the surface structure of the heating element in Figures 1 and 2. Figure 4 shows the inside of the heating element at the point immediately after the trailing edge of the pulse current and before the bubble collapses. Figure 5 is a diagram showing the configuration of an example of a test device for determining the pulse current to be supplied to the heating body, and Figure 6 is an example of the waveform of the test device shown in Figure 5. It is a graph showing an example. l: Pulse current generator 3 Niblint φ Hand 5: Heating element 7: Ink reservoir 9: Nozzle 11: Capillary head 13: Barrier 15: Tube 31 Noricon dioxide layer 33: Heat storage layer 35: Inactivation 1 Bioactivation no 37: Resistance Layer 39: silicon substrate

Claims (1)

[Claims] (11) A thermal ink having a resistor layer provided on a substrate.
A heating element for a thermal ink jet print head, characterized in that the resistor is covered with a heat storage layer. (2. A heating element for a thermal ink jet print head according to claim 1, characterized in that the heat storage layer is covered with a passivation layer. heating element.