Thermoelastic inkjet actuator with heat conductive pathways

A thermal inkjet actuator for use in an inkjet printer assembly includes heat conduction means arranged to realize a predetermined negative pressure profile to facilitate droplet formation. In a preferred embodiment the heat conduction means comprises a thin layer (54) of very high thermally conductive material such as Aluminium located in the middle of a non-heat conductive passive bend layer (56). The overall cool-down speed of the actuator, and hence the speed with which the passive bend layer returns to its quiescent position can be controlled by controlling the proximity of the heat conductive layer to the actuator's heater during fabrication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of inkjet printing and, in particular, discloses an improved thermoelastic inkjet actuator.

2. Description of Related Art

Thermoelastic actutator inkjet nozzle arrangements are described in U.S. patent applications Ser. Nos. 09/798,757 and 09/425,195 which are both co-owned by the present applicant and herein incorporated by cross reference in their entireties.

A first nozzle according to an embodiment of the invention described in that document is depicted inFIG. 1.FIG. 1illustrates a side perspective view of the nozzle arrangement andFIG. 2is an exploded perspective view of the nozzle arrangement ofFIG. 1. The single nozzle arrangement1includes two arms4,5which operate in air and are constructed from a thin 0.3 micrometer layer of titanium diboride6on top of a much thicker 5.8 micron layer of glass7. The two arms4,5are joined together and pivot around a point9which is a thin membrane forming an enclosure which in turn forms part of the nozzle chamber10. The arms4and5are affixed by posts11,12to lower aluminium conductive layers14,15which can form part of the CMOS layer3. The outer surfaces of the nozzle chamber18can be formed from glass or nitride and provide an enclosure to be filled with ink. The outer chamber18includes a number of etchant holes e.g.19which are provided for the rapid sacrificial etchant of internal cavities during construction by MEM processing techniques.

The paddle surface24is bent downwards as a result of the release of the structure during fabrication. A current is passed through the titanium boride layer6to cause heating of this layer along arms4and5. The heating generally expands the T1B2 layer of arms4and5which have a high Young's modulus.

This expansion acts to bend the arms generally downwards, which are in turn pivoted around the membrane9. The pivoting results in a rapid upward movement of the paddle surface24. The upward movement of the paddle surface24causes the ejection of ink from the nozzle chamber21. The increase in pressure is insufficient to overcome the surface tension characteristics of the smaller etchant holes19with the result being that ink is ejected from the nozzle chamber hole21.

As noted previously the thin titanium diboride strip6has a sufficiently high young's modulus so as to cause the glass layer7to be bent upon heating of the titanium diboride layer6. Hence, the operation of the inkjet device is as illustrated inFIGS. 3-5. In its quiescent state, the inkjet nozzle is as illustrated inFIG. 3, generally in the bent down position with the ink meniscus30forming a slight bulge and the paddle being pivoted around the membrane wall9. The hearing of the titanium diboride layer6causes it to expand. Subsequently, it is bent by the glass layer7so as to cause the pivoting of the paddle24around the membrane wall9as indicated inFIG. 4. This causes the rapid expansion of the meniscus30resulting in a positive pressure pulse and the general ejection of ink from the nozzle chamber10. Next the current to the titanium diboride is switched off and the paddle24returns to its quiescent state resulting in a negative pressure pulse causing a general sucking back of ink via the meniscus30which in turn results in the ejection of a drop31on demand from the nozzle chamber10.

By shaping the electrical heating pulse the magnitude and time constants of the positive pressure pulse of the thermoelastic actuator may be controlled. However, the negative pressure pulse remains uncontrolled. The characteristics of the negative pressure pulse becomes more influential for fluids of high viscosity and high surface. Accordingly it would be desirable if theromelastic inkjet nozzles with tailored negative pressure pulse characteristics were available.

A further difficulty with some types of thermoelastic actuators is that it is not unusual for very high temperature actuators to induce temperatures above the boiling point of any given liquid on the bottom surface of the non-conductive layer. It is an object of the present invention to provide a thermoelastic actuator with a tailored negative pressure pulse characteristic.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a thermoelastic actuator assembly including:

a heat conduction means positioned to conduct heat generated by a heating element away from said actuator assembly thereby facilitating the return of the actuator to a quiescent state subsequent to operation.

Preferably the heating element comprises a heating layer which is bonded to a passive bend layer wherein the heat conduction means is located within the passive bend layer.

The heat conduction means may comprise one or more layers of a metallic heat conductive material located within the passive bend layer.

Preferably the one or more layers of metallic heat conductive material is sufficient to prevent overheating of ink in contact with said actuator.

Typically the one or more layers of metallic heat conductive material comprise a laminate of heat conductive material, for example Aluminium, and passive bend layer substrate.

It is envisaged that the thermoelastic actuator be incorporated into an ink jet printer.

A method of producing a thermoelastic actuator assembly having desired operating characteristics including the steps of:

determining a desired negative pressure pulse characteristic for the actuator;

determining a heat dissipation profile corresponding to the desired negative pressure pulse characteristic; and

forming the thermoelastic actuator with a heat conduction means arranged to realize said profile.

Preferably the step of determining a desired negative pressure pulse characteristic includes a step of determining the physical qualities of a fluid to be used with the thermoelastic actuator.

The step of forming the thermoelastic actuator with a heat conduction means arranged to realize said profile may include forming one or more heat conductive layers in a passive bend layer of the actuator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 6, there is depicted a simplified side profile of a portion of a prior art thermoelastic actuator40. Actuator40includes a heating element in the form of a heater layer42and a passive bend layer44. Typically the passive bend layer comprises an insulator of low thermal conductivity such as Silicon Dioxide. A fluid such as ink fills reservoir46. The direction of heat flow from heater layer42is indicated by arrows50and52.

A preferred embodiment of a thermoelastic actuator according to the present invention will now be described with reference toFIG. 7. The actuator includes a thin layer54of very high thermally conductive material, such as Aluminium located in the middle of the non-heat conductive passive bend layer56. Thus as heat energy is conducted away from the heater layer it ultimately encounters the conductive layer and is conducted away as indicated by arrows58. The heat is conducted away from the actuator by heat conductive layer54to the large relatively cold thermal mass of the supporting structure (not shown) as opposed to further conduction through the thickness of the actuator itself.

The overall cool-down speed of the actuator, and hence the speed with which the passive bend layer returns to its quiescent position, and so the shape of the negative pressure pulse, can be controlled by the proximity of heat conductive layer54to heater layer58. Locating the heat conductive layer closer to the heater layer results in an actuator that cools down more quickly.

The heat conductive layer may be positioned to prevent the bottom surface of the bonded actuator from getting excessively hot, thus the actuator can be in direct contact with any given fluid without causing boiling or overheating.

FIG. 8depicts a thermoelastic actuator according to a further embodiment of the invention wherein the conductive pathway comprises a laminate60of three Aluminium layers and passive bend material. By alternating Aluminium layers with the passive bend material the effect of the heat conductive layers on the mechanical characteristics of the actuator may be minimized. Alternatively a single layer of another heat conductive material having a relatively low Young's Modulus might be used so as not to interfere with the mechanical characteristics of the actuator.

In the embodiments ofFIGS. 7 and 8the heating layer58is directly and continuously bonded to the passive bend layer56. In so called “isolated” type thermoelastic actuators a heating element is not continuous with a passive substrate but is partly separated from it by an air space. InFIG. 9there is shown a further embodiment of the invention applied to an isolated type actuator wherein a heating element64is partly separated from passive substrate56by an air space62. Once again heat conductive layer54acts to conduct heat away towards the actuator support assembly (not shown).

The present invention provides an actuator with a tailored negative pulse characteristic. This has been done by providing a heat conduction means in the form of a layer of a good heat conductor such as Aluminium. By varying the heat conduction properties of the actuator the cool down time may be increased so that the actuator will return more quickly to its quiescent position. Accordingly the present invention also encompasses a method for designing actuators to have desired characteristics.

The method involves firstly determining a desired negative pressure pulse characteristic for the actuator. The pressure pulse characteristic will be due to the speed with which the actuator returns to its quiescent position. Typically the negative pressure pulse will be designed to cause necking of ink droplets for ink of a particular viscosity.

Once the pressure pulse characteristic has been decided upon a heat dissipation profile corresponding to the desired negative pressure pulse characteristic is determined. The determination may be made by means of a trial and error process if necessary or alternatively mathematical modeling techniques may be utilized. The thermoelastic actuator is then fabricated with a heat conduction layer arranged to realize said profile.

It may be simplest to form the actuator with a number of heat conductive layers in order to preserve the mechanical characteristics of the passive bend layer thereby reducing the number of variables involved in realizing the heat dissipation profile.

It will be realized that the actuator will find application in inkjet printer assemblies and ink jet printers.