EP0484034A1 - Thermal ink jet print device having phase change cooling - Google Patents

Abstract

A thermal ink jet print device having a phase change material, a solid or fluid, disposed in heat exchange proximity to the thermal ink jet printhead to absorb printhead heat energy by changing physical state.

Description

• This invention relates generally to thermal ink jet print devices having printhead cooling systems.
• Thermal ink jet print devices such as a print cartridge, for example, are used to print text and images on a media such as paper. Such devices include thermal ink jet printheads, which comprise nozzle or orifice plates mounted on substrates secured to the body of the print device in communication with a supply of ink in an ink chamber or bladder within the body. Small electric heaters, each in the form of a small resistor in the ink passage at each nozzle, when electrically pulsed, heat the ink which is then expelled as a droplet from the nozzle thereat.
• Typical nozzle plate structures are described in U. S. Patent 4,694,308, to C. S. Chan et al, filed November 22, 1985, entitled "Barrier Layer and Orifice Plate for Thermal Ink Jet Printhead Assembly", and U. S. Patent 4,812,859, to C. S. Chan et al, filed March 14, 1989, entitled "Multi-Chamber Ink Jet Recording Head for Color Use", particularly Fig. 6. Both patents are assigned to the assignee of this invention and their teachings are incorporated herein by reference.
• A typical thermal ink jet print device comprises a printhead having a silicon substrate structure of glass or monocrystalline silicon on which a silicon dioxide barrier is deposited. The individual heater resistors are each deposited on the silicon barrier in an ink passage or priming cavity at each nozzle, individual circuit traces for each resistor provide communication with discrete supplies of electrical energy, for firing the resistors in varying sequences which are orchestrated to print selected characters and images, as is well known. Transfer of resistor heat to the ink boils the ink. The expanding bubble ejects an ink droplet from the nozzle thereat. Resistor heat also heats the silicon substrate structure. During high density printing, such as increasing the number of nozzles being fired and/or resolution, say going from 300 dots per inch to 600 dots per inch, or increasing the firing frequency, the printhead tends to get too hot. Thermal ink jet printhead performance is degraded when the printhead temperature is too high. Temperatures at which print quality degrades vary widely, depending upon the ink jet printhead design.
• Thermal ink jet print devices frequently employ a plastic body on which the printhead is mounted. Without the provision of a heat sink, to avoid print quality degradation, a print rate limit has to be determined and not exceeded. Other attempts to solve this overheating problem have included an all metal print device body to conduct the heat away, or a metal fin coupled with air convection cooling. The metal acted like a capacitor or bucket, and once the metal had heated sufficiently, print quality degraded. Convection cooling helped to dissipate the heat, but was expensive and required air velocities that adversely affected ink droplet trajectories which degraded print quality. Reducing the drop ejection frequency lowers the heat flux. This keeps the head cooler. It is also possible to employ various print modes in which the pen scans multiple times over a line to create the desired output. For example, if every other nozzle fired, it would take 2 passes to complete a line, etc. This reduces hard copy throughout.
• Improvement over prior art devices and practices is realized according to this invention in the provision of a print device having a heat sink employing a phase change material for absorbing print head thermal energy. The heat sink may comprise a heat pipe containing a circulating phase change material or, in a presently preferred embodiment and best mode for practicing the invention, a solid material disposed in heat exchange relation with the printhead, for example, in proximity to or in contact with the substrate of the printhead. Such a solid phase change material is preferably solid in a temperature environment for the printhead in which acceptable print quality is achieved and melts at a temperature below that at which print degradation takes place. Such a heat sink takes advantage of the heat of fusion of the solid phase change material. A heat pipe is a heat-transfer device comprising a sealed container which contains a small amount of fluid in a partial vacuum. Heat is absorbed at one end in heat exchange relationship with the printhead by vaporization of the fluid and is released at another location on said container, removed from said one end, by condensation of the vapor. The condensate returns along the sides of the sealed container to the original reservoir. Here the heat of vaporization absorbs printhead heat energy.
• Heat energy from the printhead is used to change the physical state of the phase change material which by this means is absorbed or given up in thermal energy used in changing the physical state of the material and thereby removed from the substrate and adjacent print body.
• When changing the physical state of a material, the thermal energy input to the material, liquid or solid, is used to break down the molecular bonds, and does not appreciably heat up the material. For solids, once the last bit of solid material is melted, of course, the temperature of the melt will begin to rise if the print rate is maintained. With a heat pipe, if the thermal capacity is not exceeded the change in physical state is continuous. Using this phase change principle, the heat generated by the thermal ink jet printhead is used or put to work to change the physical state of a material. The printhead is maintained at a constant acceptable temperature as long as a change in physical state of the material takes place.
• It is apparent that the principles of this invention, while explained in connection with a printhead, can be extended and used in cooling integrated circuits and other electrical components.
• Additionally, the glass transition of a material is usable for cooling purposes in these applications.
• This invention will be better understood by reference to the following specification when considered in conjunction with the accompanying drawings in which:
• Fig. 1 is a plot of Temperature v Time, typically indicating the thermal energy required in changing the physical state of a solid material to a liquid.
• Fig. 2 is an exploded perspective view of a thermal ink jet print device embodying the principles of this invention.
• Fig. 3 is an enlarged perspective view showing the assembly of the printhead of print device of Fig. 2 and the attachment of the flexible circuit thereto.
• Fig. 4 is an enlarged sectional view taken in the section plane IV-IV of Fig. 3.
• Fig. 5 is an enlarged sectional view taken in the section plane V-V of Fig. 2.
• Fig. 6 is a top plan view of a modified printhead body with the printhead and flexible circuit removed for clarity.
• Fig. 7 is a sectional view taken on the section line VII-VII of Fig. 6.
• Fig. 8 is sectional view taken on the section line VIII-VIII of Fig. 6.
• Fig. 9 illustrates plots of printhead temperatures derived from identical printhead tests without and with heat sinks using different solid materials.
• Fig. 10 is an isometric view of a different print device utilizing a printhead of the type of Figs. 3 and 4, and embodying a heat pipe.
• Fig. 11 is a fragmentary side elevational view of the print device of Fig. 10.
• Fig. 12 is a perspective view of the heat pipe of Figs. 10 and 11.
• Fig. 13 is a side elevational view of the heat pipe of Fig. 12, and
• Fig. 14 is a plan view of the back side of a printhead substrate.
• The hard copy throughput potential of many thermal ink jet printers cannot be realized because of overheating of the printhead and consequent degradation of print quality. Print rate can be increased, according to this invention, by employing a heat sink for the printhead in which materials are employed which undergo a change in physical state when subject to printhead upper limit operating temperatures. Fig. 1, which is a plot of Temperature v Time, typically depicting the thermal energy input in changing the physical state of a material, e.g. melting a solid or vaporizing a fluid, plots the relatively constant temperature which exists over a period of time during which the material continuously undergoes a change in physical state. If the material is a solid, once the phase change is complete, if the thermal energy input is not reduced, the temperature of the liquid will rise. During the phase change interval the thermal energy is used to change the physical state.
• The use of a heat sink requires space in the print device for receiving the heat sink and placing the phase change material in heat exchange proximity to the printhead. A solid heat sink material is preferably of a thermally conductive material that melts at a printhead temperature at or below that temperature beyond which print quality is unacceptably degraded. Temperature control is provided by the heat sink during that period of time required to achieve the complete change in physical state. The printhead storage capacity for phase change materials provides about four minutes of blackout printing for printheads which have been tested, as will be explained at a later point. Thus many variable print density printing projects can be accommodated which have periodic high print density demands without reducing the print rate in use. This can be further optimized for much longer times.
• The use of a finite volume of a solid phase change material within a cavity in the print body, offers a convenient solution to the problem of overheating of the printhead. High density printing intervals however, can be further extended or made continuous by the use of an arrangement in which the phase change material is circulated, as in a heat pipe, a part of which may comprise the body cavity, or, alternatively, the heat pipe may be a self contained unit having hot and cold junctions for heat input and heat output for receiving heat from the printhead at the hot junction and removing heat at the cold junction. The general requirements of materials in this instance, insofar as temperatures at which changes in phase or physical state take place, are the same as for solid material. The advantage of the heat pipe is that the period of temperature control for the printhead is continuous.
• One type of print device in which the invention has been practiced is illustrated, without limitation, in the exploded perspective view of Fig. 2. Theprint device 1 comprises aprint body 3 sealed to anink chamber 5 by means of agasket 7 or other suitable seal. A thermalink jet printhead 8, see also Fig. 3, comprising aresistor substrate 9 and an orifice ornozzle plate 11 are laminated together in a liquid tight relationship and fitted in arecess 3a, in which the resistor substrate is seated and sealed, in the upper face of theprint body 3. Aslot 3b in therecess 3a of theprint body 3, communicates with ink in theink chamber 5. When theprinthead assembly 8 is sealed in therecess 3a, theslot 9a in theresistor plate 9 is aligned with theslot 3b, which admits ink from theink chamber 5 to the back face of thenozzle plate 11. As will be seen by reference to Fig. 4, to be described,passages 11b in the printhead structure behind the back face of thenozzle plate 11, communicate with the ink channel orslot 9a in theresistor substrate 9 and admit ink into the individual ink cavities or primingcavities 11c at eachnozzle 11a.Individual resistors 9b on theresistor substrate 9 are disposed oppositerespective nozzles 11a. Ink directly over a resistor is vaporized and a vapor bubble is formed when the resistor is excited. As the vapor bubble grows, momentum is transferred to the ink above the bubble which expels ink from thenozzle 11a thereat. Theresistors 9b are individually coupled to any of well known systems which orchestrate their firing, by means offlexible circuits 13 having individual circuit traces 13a, which are only fragmentarily shown, connected to theindividual resistors 9b. As seen in Fig. 2, the flexible circuits are shaped to fit over the sloping sides of theprint body 3.
• Theprint device 1 of Fig. 2 and theprinthead 8 of Fig. 3 are illustrated in positions of convenience for purposes of illustration. In some applications, the print device occupies a position, such as illustrated in Fig. 8, in which theprinthead body 3 and theprinthead 8 are disposed substantially in a vertical plane. This position of theprint device 1 provides a gravity induced flow of ink to theprinthead 8. Of course other print device positions are possible.
• Provision for temperature control of theprinthead 8 by means of a phasechange heat sink 15 is generally illustrated in Figs. 2 and 5. Theheat sink cavities 15a are defined within the walls of theprint body 3. The open back side of theprint body 3 is closed by thegasket 7 backed by anend face 5a of theink chamber 5, as seen in Fig. 5. Asolid material 15b, which has a melting point at or below the maximum acceptable printhead temperature, fills thecavity 15a in heat exchange relation with the back face of thesubstrate 9 by contact therewith.
• The ink path between theink chamber 5 and thepriming cavity 11c is evident in the sectional view of Fig. 5, in which theprint device 1 is shown assembled. The ink path comprises anopening 5b in theend face 5a of theink chamber 5 and in anopening 7a in thegasket 7. Both of these openings are aligned with theslot 3b in theprint body 3 which communicates with thepriming cavities 11c behind thenozzle plate 11 through theslot 9a in theresistor substrate 9.
• Further details of this ink distribution system to theindividual nozzles 11a are evident in Fig. 4. This is a fragmentary sectional view taken in the section plane IV-IV of Fig. 3 and typically shows, at only onenozzle 11a, the attachment of thesubstrate 9, of theprinthead 8, to the upper end of theprint body 3, in thecavity 3a, at theslot 3b. Theprint body 3 is sealed to theink chamber 5 by the gasket 7 (see Fig. 2). Theprinthead 8 comprises amonocrystalline silicon substrate 9c, sealed in therecess 3a, on which a silicon dioxide (SiO₂)layer 9d, functioning as a thermal capacitor barrier, is deposited.Individual resistors 9b of tantalum aluminum TaAl), one being shown, are deposited on the silicon dioxide layer 9d. Circuit traces or conductors 9bb for theindividual resistors 9b are deposited on theresistors 9b in positions leaving the resistor portion at, or opposite, thenozzle 11a exposed. Passivation, resistor protection layers, 9p and 9q, are successively deposited on theresistor 9b. The layer 9p is of silicon carbide SiC or silicon nitride SiN. The layer 9q is tantalum Ta. The passivation layers permit heat transfer from the resistor to the ink in thepriming cavity 11c while providing physical, chemical and electrical isolation from the ink.
• Abarrier layer 11e of a photo imageable polymer defines the ink cavities, which include thepriming cavity 11c for each nozzle and a manifold passage orcavity 11b. Thenozzle plate 11, usually electroformed of nickel, overlays and is sealed to the barrier layer.Individual nozzles 11a communicate with each primingcavity 11c. The approximate ink meniscus line is shown bridging the opening of thenozzle 11a. Thepriming cavity 11c for eachnozzle 11a is joined with the others by themanifold cavity 11b. Thismanifold cavity 11b communicates with theslot 9a in theresistor substrate 9 which, as seen, extends through all of the substrate layers. Asealant 9e seals theresistor substrate 9 about the edge of theslot 3b and in and about therecess 3a.
• Figs. 6, 7 and 8 illustrate aplastic print body 3 of the type employed in reducing this invention to practice, using a solid phase change material, from which test data depicted in the temperature plots of Fig. 9 was developed. A 300 dot perinch printhead 8 was employed. In this embodiment, theprinthead recess 3a in theprint body 3 is sealed from theheat sink cavities 15a in theprinthead 3 by an integralend plate section 3d which closes therecess 3a except for the opening of theslot 3b. In Fig. 6, to clearly show theend plate 3d, theprinthead 8 and the flexible circuits have been removed; however, in FIg. 7, the sectional view taken on the section line VII-VII of Fig. 6, these features are included. Theintegral end plates 3d obviate seal failures between theheat sink 15 and theprinthead 8. Direct heat exchange between the resistor substrate and thephase change material 15 no longer takes place, requiring that the printhead operate at a slightly higher temperature using the same heat sink material, but this can be compensated for by selection of a heat sink material which melts at a lower temperature to compensate the thermal drop across theend plate 3d if necessary.
• Fig. 8 is a sectional view taken on the section line VIII-VIII of Fig. 6. The section plane includes the longitudinal axis of theslot 3b and outlines the interior structure of theslot 3b defining the passage 3bb between the opposite sides or openings of theslot 3b. In the position of theprint device 1 seen in Fig. 8, it is apparent that there is a gravity induced flow of ink to the printhead at the outer opening of theslot 3b. In addition, expelling ink from the nozzles acts as a pump to draw ink into the priming cavities of the printhead.
• Solid materials which have been found to be suitable for heat sink applications include gallium and polyethylene glycol. Low temperature solder is also acceptable. The melting point of the solid phase change material which is used depends upon the specific printhead with respect to the upper limit of temperature at which the printhead may operate without unacceptable degradation of print quality. Experiments withplastic body 300 dpi printheads indicates that the upper acceptable limit of thermal ink jet printhead temperatures varies widely. Thus a solid phase change material selected for this application should in any case have a melting temperature compatible with the known upper temperature limit of a particular printhead at which acceptable print quality still exists. Experiments with the plastic body printhead indicate a requirement that the materials change physical state at a temperature below the temperature limit of the printhead and have a moderate thermal conductivity, which for solids tested are of about 10 watts per meter per degree Kelvin. Material selection, solid or liquid, depends only upon known upper limits of print head temperature. Thermal conductivity is a factor in the rate at which the change in physical state must take place to absorb the rate of delivery of heat energy.
• In an experiment conducted with a 300 dpi thermal ink jet pen or printhead having a plastic case, without a provision for conducting the heat away, the printhead temperature continued to rise during printing. In a further experiment conducted with the same type of thermal ink jet pen or printhead, having a plastic case and provided with a heat sink, using gallium as the phase change material, the printhead temperature was constant at an acceptable level during printing throughout the melting period of the phase change material. The application of a heat sink employing a solid phase change material shows a remarkable improvement in thermal management based upon these experiments.
• Using gallium, for example, as the phase change material in a heat sink in the same type of printhead assembly, it has been found that the printhead could be used to continuously fire in a high density print mode for 3 to 4 minutes, without exceeding the printhead's maximum operating temperature. One specific successful test was to print a 100% optical print density, A-size plot, without slowing down. A second specific successful test was to print ten (10) 50% dense A-size plots, in a row without a decrease in print quality. Again, the number of nozzles, the size of silicon substrate, the firing frequency, and the resolution (300 dpi), make a difference in performance.
• The results of tests referred to above are shown in Fig. 9 which plots test data derived from tests of a 300 dot perinch print device 1, having aprint body 3 of the type of FIgs. 6, 7 and 8, which is fabricated of a plastic material. The four tests were conducted without a heat sink in one case and with different heat sink materials in the other three (3) cases. All tests were conducted with thisplastic print body 3, which in Fig. 9 is referred to as a manifold. Theprinthead 8, was fired in a high density mode for about 260 seconds as indicated and then shut down. Without a heat sink, the printhead temperature exceeded 100°C at the end of the test interval. With a heat sink employing polyethylene glycol as the phase change material, the upper temperature reached by the printhead was lessened by about 16°C, but had an upper limit, following a gentle rise throughout the test period, which while proving the inventive concept worked, prompted the use of other materials having phase change temperatures and thermal conductivity properties better suited to the instant application. The addition of copper fibers to polyethylene glycol as indicated in the third test, improved thermal conductivity and slightly lessened the upper temperature at the end of the test interval. The final test recorded in Fig. 9 employed gallium as the heat sink material and showed a remarkable improvement in the thermal management compared with the control case which used only theplastic print body 3 or manifold. In the last test, once the gallium began to melt, the printhead temperature was constant during the test interval.
• Figs. 10-14 illustrate a heat pipe and its application to aprint device 10. In these figures, parts corresponding to those of Figs. 2-8, bear like reference characters. Theprint device 10 comprises abody 30 which contains an ink bladder 31, Fig. 11. Aprinthead 8 having asubstrate 9 is sealed in a recess in thebody 30. The ink bladder 31 has aneck portion 31a, Fig. 11, the outlet of which is sealed marginally about theslot 9b, see Fig. 14, on the back face of thesubstrate 9 of theprinthead 8. In this position ink in the bladder 31 communicates with theslot 9b to supply ink to thepriming cavities 11c and to thenozzles 11a of thenozzle plate 11, see Fig. 4.
• Aheat pipe 25 is disposed within thebody 30 of theprint device 10. The heat pipe comprises a pair oftubes 25a and 25b, which may be joined together abovebladder neck 31a, each of which has a lower end, respectively, 25c and 25d, and respective open, enlarged upper ends 25e and 25f. The lower ends, 25c and 25d, are also open and are adhesively bonded and sealed at their extremities, by an epoxy type of sealant, for example, to the back face of thesubstrate 9, in positions denoted by the dot dash outlines, 25g and 25h, on opposite sides of theink feed slot 9a and theneck 31a of the bladder 31, shown in Fig. 14. The open upper ends, 25e and 25f, are similarly bonded and sealed to a cold junction comprising aplate 33 of high thermal conductivity, such as aluminum or copper, here shown projecting from an upper sloping face of thebody 30 of theprint device 10, to reject heat to the ambient environment or to a cold junction metal clamp on theplate 33, such as a highly conductive thermal mass on thebody 30. Aheat pipe fluid 25j in the bottom end of each heat pipe tube, 25a, 25b, is in contact with the back face of thesubstrate 9, the wetted area being defined within the dot-dash outlines 25g and 25h. These areas are as large as substrate space permits to maximize area exposure of the heat pipe fluid to thesubstrate 9.
• As in the case of the solid phase change materials, heat energy generated in thesubstrate 9 by the firing of theresistors 9b, produces a physical change. In this case the heat pipe fluid is vaporized. The warm vapor rises upwardly in theheat pipe tubes 25a and 25b, as indicated by the dotted arrows of Fig. 13. In the enlarged upper ends, 25e and 25f, of the heat pipes, 25a and 25b, the vapor contacts the inner face of the cold junction plate 31 where it is cooled and changes phase state, returning to a fluid, which as shown by the solid arrows flows down the walls of the heat pipe tubes to thefluid supply 25j.
• In one embodiment, the heat pipes were pressurized and each contained about 1cc of fluid, 25j. Pressure ranges, P, in atmospheres for 0°C to 70°C are given for each fluid listed in the table below.

  
• The heat rate = ṁ h_fg
  where ṁ = mass flow rate in grams/sec. and
     h_fg = heat of evaporation in Joules/gram.
• For Freon, the heat of evaporation h_fg is 180$$\frac{\text{J}}{\text{G}}\text{.}$$

  
• These teachings herein indicate that the selection of a phase change material is simply based upon the upper limit of printhead temperature together with a thermal conductivity of the material compatible with the heat rateto provide a change in physical state of the phase change material at a rate commensurate with the rate at which heat energy is developed.
• Although the invention has been described in its application to print devices having plastic print bodies, the principles taught herein are, at least, equally advantageously applied where metallic print bodies are employed. Although specific phase change materials, solids and fluids, have been named and data presented with respect thereto, other materials for known printhead temperature limits, and rates at which heat energy is generated, are easily selected from available tables of physical properties for materials. Additionally, changes in physical state, such as the glass transition of a material, within the temperature ranges of acceptable print quality, are contemplated and usable for cooling purposes.

Claims (12)

1. A thermal ink jet print device, comprising: a body containing ink storage means; a printhead having a plurality of nozzles through which ink is ejected, disposed on said body in communication with said ink storage means; electrical heating means for each nozzle of the plurality of nozzles for heating and ejecting ink through the nozzle thereat; characterised by heat sink means comprising a phase change material which changes physical state in the presence of printhead heat energy, disposed on said body with said phase change material in heat exchange relationship with said printhead to be exposed to and to absorb printhead heat energy in changing physical state.
2. An ink jet print device according to claim 1 in which said phase change material changes physical state at printhead temperatures below temperatures which cause degradation of printing.
3. An ink jet print device according to claim 1 or 2 in which: said phase change material is a solid which has a melting point below printhead temperatures at which printing degrades.
4. An ink jet print device according to claim 1,2 or 3 in which said phase change material has a thermal conductivity commensurate with the rate at which heat energy is generated by said printhead.
5. An ink jet print device according to any preceding claim in which said phase change material is a solid which has a melting point in the range of about 35° to 85°C and has a thermal conductivity of about ten (10) watts per meter per degree kelvin.
6. An ink jet print device according to any preceding claim in which: said heat sink means includes at least one cavity in said body adjacent said printhead, and said phase change material is disposed in said at least one cavity in heat exchange relationship with said printhead.
7. An ink jet print device according to claim 6 in which: said heat sink means includes two cavities in side-by-side relationship in said body; and said printhead comprises a substantially rectangular substrate bridging said cavities.
8. An ink jet print device according to claim 3 in which said phase change material is gallium or polyethylene glycol, or low temperature solder.
9. The invention according to any preceding claim in which said body is plastic, or metal.
10. An ink jet print device according to claim 1, in which: said heat sink means comprises heat pipe means supported by said body, having a hot junction disposed in heat exchange relationship with said print head and having a cold junction for rejecting heat energy, from said body, and a fluid within said heat pipe at said hot junction in heat exchange relationship with said printhead, which vaporizes at a temperature below that at which print degradation occurs.
11. An ink jet print device according to claim 10, in which, said print head is connected to said heat pipe at said hot junction, and said fluid is in contact with said print head.
12. The invention according to claim 11, in which said fluid is freon, or methanol, or ethanol, or I.P alcohol, or pentane.

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