Abstract:
A thermoelastic actuator to be arranged in an inkjet printer comprises an active heater layer and at least one passive thermal, conductor layer. A thermal insulator is located between the heater layer and said at least one thermal conductor layer. The at least one thermal conductor layer is also embedded in the thermal insulator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a Continuation Application of U.S. application Ser. No. 10/728,791 filed on Dec. 8, 2003, now issued U.S. Pat. No. 7,066,580 B2, which is a Continuation-In-Part of U.S. application Ser. No. 10/120,359 filed on Apr. 12, 2002, now issued U.S. Pat. No. 6,688,719 all of which are herein incorporated by reference. 

   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 in  FIG. 1 .  FIG. 1  illustrates a side perspective view of the nozzle arrangement and  FIG. 2  is an exploded perspective view of the nozzle arrangement of  FIG. 1 . The single nozzle arrangement  1  includes two arms  4 ,  5  which operate in air and are constructed from a thin 0.3 micrometer layer of titanium diboride  6  on top of a much thicker 5.8 micron layer of glass  7 . The two arms  4 ,  5  are joined together and pivot around a point  9  which is a thin membrane forming an enclosure which in turn forms part of the nozzle chamber  10 . The arms  4  and  5  are affixed by posts  11 ,  12  to lower aluminium conductive layers  14 , 15  which can form part of the CMOS layer  3 . The outer surfaces of the nozzle chamber  18  can be formed from glass or nitride and provide an enclosure to be filled with ink. The outer chamber  18  includes a number of etchant holes e.g.  19  which are provided for the rapid sacrificial etchant of internal cavities during construction by MEM processing techniques. 
   The paddle surface  24  is bent downwards as a result of the release of the structure during fabrication. A current is passed through the titanium boride layer  6  to cause heating of this layer along arms  4  and  5 . The heating generally expands the T 1 B 2  layer of arms  4  and  5  which have a high Young&#39;s modulus. This expansion acts to bend the arms generally downwards, which are in turn pivoted around the membrane  9 . The pivoting results in a rapid upward movement of the paddle surface  24 . The upward movement of the paddle surface  24  causes the ejection of ink from the nozzle chamber  21 . The increase in pressure is insufficient to overcome the surface tension characteristics of the smaller etchant holes  19  with the result being that ink is ejected from the nozzle chamber hole  21 . 
   As noted previously the thin titanium diboride strip  6  has a sufficiently high young&#39;s modulus so as to cause the glass layer  7  to be bent upon heating of the titanium diboride layer  6 . Hence, the operation of the inkjet device is as illustrated in  FIGS. 3-5 . In its quiescent state, the inkjet nozzle is as illustrated in  FIG. 3 , generally in the bent down position with the ink meniscus  30  forming a slight bulge and the paddle being pivoted around the membrane wall  9 . The hearing of the titanium diboride layer  6  causes it to expand. Subsequently, it is bent by the glass layer  7  so as to cause the pivoting of the paddle  24  around the membrane wall  9  as indicated in  FIG. 4 . This causes the rapid expansion of the meniscus  30  resulting in a positive pressure pulse and the general ejection of ink from the nozzle chamber  10 . Next the current to the titanium diboride is switched off and the paddle  24  returns to its quiescent state resulting in a negative pressure pulse causing a general sucking back of ink via the meniscus  30  which in turn results in the ejection of a drop  31  on demand from the nozzle chamber  10 . 
   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 the present invention there is provided a thermoelastic actuator assembly including: 
   a heat conduction mean s 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 related aspect of the present invention provides 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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a prior art thermoelastic actuator. 
       FIG. 2  is an exploded view of the thermoelastic actuator of  FIG. 1 . 
       FIG. 3  is a cross sectional view of the thermoelastic actuator of  FIG. 1  during a first operational phase. 
       FIG. 4  is a cross section view of the thermoelastic actuator of  FIG. 1  during a second operational phase. 
       FIG. 5  is a cross sectional view of the thermoelastic actuator of  FIG. 1  during a further operational phase. 
       FIG. 6  is a cross sectional view of a portion of a prior art thermoelastic actuator assembly. 
       FIG. 7  is a cross sectional view of a portion of a thermoelastic actuator assembly according to a first embodiment of the present invention. 
       FIG. 8  is a cross sectional view of a portion of a thermoelastic actuator assembly according to a second embodiment of the present invention. 
       FIG. 9  is a cross sectional view of a portion of a thermoelastic actuator assembly according to a further embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 6 , there is depicted a simplified side profile of a portion of a prior art thermoelastic actuator  40 . Actuator  40  includes a heating element in the form of a heater layer  42  and a passive bend layer  44 . Typically the passive bend layer comprises an insulator of low thermal conductivity such as Silicon Dioxide. A fluid such as ink fills reservoir  46 . The direction of heat flow from heater layer  42  is indicated by arrows  50  and  52 . 
   A preferred embodiment of a thermoelastic actuator according to the present invention will now be described with reference to  FIG. 7 . The actuator includes a thin layer  54  of very high thermally conductive material located in the middle of the non-heat conductive passive bend layer  56 . Thus as heat energy is conducted away from the heater layer it ultimately encounters the conductive layer and is conducted away as indicated by arrows  59 . The heat is conducted away from the actuator by heat conductive layer  54  to the large relatively cold thermal mass of the supporting structure (not shown) as opposed to further conduction through the thickness of the actuator itself. 
   In the particular embodiment shown, the thermally conductive layer  54  is aluminium, or more particularly, an aluminium/silicon alloy (2% silicon). However, the heat conductor  54  can be formed from other suitable materials such as copper, diamond-like carbon (DLC), silicon nitride or even silicon itself can function as a heat sink if designed appropriately. Skilled workers in this field will appreciate that there are many materials with high thermal conductivity and good compatibility with CMOS chips. 
   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 layer  54  to heater layer  58 . Locating the heat conductive layer closer to the heater layer results in an actuator that cools down more quickly. 
   The heat conductive layer  54  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. 8  depicts a thermoelastic actuator according to a further embodiment of the invention wherein the conductive pathway comprises a laminate  60  of 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&#39;s Modulus might be used so as not to interfere with the mechanical characteristics of the actuator. 
   In the embodiments of  FIGS. 7 and 8  the heating layer  58  is directly and continuously bonded to the passive bend layer  56 . 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. In  FIG. 9  there is shown a further embodiment of the invention applied to an isolated type actuator wherein a heating element  64  is partly separated from passive substrate  56  by an air space  62 . Once again heat conductive layer  54  acts 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. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.