INK JET PRINTING APPARATUS

Abstract: An ink jet printing apparatus comprising a nozzle plate (09,1) having a plurality of nozzles (08,2) formed therein for ink ejection; an ink chamber (012,9) which communicates with said nozzles (08,2); heating means (013,10) to ensure that ink supplied to the nozzles (08,2) has been changed from a solid to a liquid phase ink or has had its viscosity reduced; and electro-mechanical transducer means (010,011,3,4,5) for forcing the ink through the nozzles (08,2) so as to effect printing characterised in that the electro-mechanical transducer means (010,011,3,4,5) is spaced from the nozzle plate (09,1) by a gap (021,G) which permits the escape of air bubbles therethrough.

Description: This invention relates to an ink jet printing apparatus in which nozzles eject ink to effect printing. An ink jet printer having nozzles to jet ink into characters and images is disclosed in U.S. Patent No. 4,072,959. The system comprises a plurality of nozzles which are provided in a nozzle plate, and, at the back of the piezo nozzles, electric transducers which are in direct contact with ink to apply piezo-electric impact to the ink for ink ejection. The piezo-electric transducers consist of vibrators that are arranged to produce oscillatory movement in a direction perpendicular to the plane of the nozzle plate. In addition, the ink flow passages leading to the respective nozzles in the plate are short. Because of these features, the system can ensure enhanced efficiency and stability in ejecting ink drops from the nozzles. Only a minimum amount of electrical power is required to cause a vibrator to produce the necessary potential displacement to expel ink so as to effect printing. However, ink jet printers of this type have their own limitations. For example, because of having to have the piezo-electric transducers submerged in ink, the ink used must be a non-conductive oil-based ink having an organic solvent as its main ingredient. This poses a serious problem when the printer prints on woody paper, since the ink is liable to bleed in the paper, making the print blur or locally thin out. Printing quality is therefore liable to be seriously impaired. To eliminate the above mentioned problem, efforts have been made as disclosed in laid-open Japanese patent specification no. 61-98,547, which discloses an ink jet recording apparatus using hot melt ink in order to maintain enhance printing quality. A critical problem with this type of printing apparatus, however, is that when the ink changes phase from the solid to the liquid state, air bubbles come up in the liquid ink. In an extreme case, such air bubbles can increase so as to absorb the oscillatory impacts from the piezo-electric transducers thus impairing proper ink discharge. The ink jet printer as disclosed in U.S. Patent No. 4072 959 could be used with such hot melt ink as disclosed in JP patent publication no. 61 98547 or EP patent publication no. 178 883. In the former the nozzle plate is spaced from the transducer means which in practice may allow bubbles to escape. However the optimum distance to ensure bubble release as well as accurate printing is not suggested. According, therefore, to the present invention, there is provided an ink jet printing apparatus comprising a nozzle plate having a plurality of nozzles formed therein for ink ejection; an ink chamber which communicates with said nozzles; heating means to ensure that ink supplied to the nozzles has been changed from a solid to a liquid phase ink or has had its viscosity reduced; and electro-mechanical transducer means spaced from the nozzle plate for forcing the ink through the nozzles so as to effect printing characterised in that the electro-mechanical transducer means is spaced from the nozzle plate by a gap in the range of 20 to 50 µm which permits the escape of air bubbles therethrough. The apparatus of the present invention can therefore offer the full advantage of the hot melt ink to provide enhanced printing quality. In the apparatus of the present invention, the bubbles which are generated as the ink changes from the solid to the liquid phase can escape through said gap and are therefore prevented from disturbing the effective operation of the transducer means, thereby ensuring a constantly stable discharge of ink drops. In its preferred form, the ink jet printing apparatus of the present invention ensures a uniform distribution of temperature in the ink and reduces to a possible minimum the thermal energy used to heat the ink. In its preferred form, moreover, the apparatus of the present invention can be brought into operation after a pause in a very short period of time. The transducer is preferably housed in the ink chamber. The ink chamber is preferably made of electrically insulating material. The transducer preferably comprises a piezo-electric vibrator. The latter may comprise a piezo-electric element and a metal film which is integral with said element, the metal film having a different linear thermal expansion coefficient than that of said element. The piezo-electric element is preferably made of a material whose piezo-electric modulus increases with temperature. The piezo-electric vibrator may be divided into a plurality of vibrators which are respectively disposed opposite to the nozzles. A member is preferably mounted in the ink chamber so as to define with the transducer a capillary space which communicates with the said gap and in which, in operation, the ink rises above the level of the nozzles due to the capillary effect, whereby the said air bubbles may pass through the gap and the space to an upper portion of the ink chamber. The ink chamber may comprise an ink reservoir portion and a further portion in which the transducer is mounted, the ink reservoir portion and further portion being separated from each other by means comprising the said member, ink overflowing from the space returning to the ink reservoir portion by way of a capillary return passage in which the ink rises by capillarity to a level above that of the nozzles. The said upper portion of the ink chamber may be provided with a vent hole. The capillary space may have a width in the range 0.3mm to 1.5mm. The plurality of vibrators may be coupled together by a support base portion of the transducer, the support base portion being arranged to be disposed above the level of the ink. Preferably, the ink reservoir portion and further portion are separated from each other both by the said member and by a filter. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 is a perspective view of a part of a first embodiment of an ink jet printing apparatus according to the present invention, Figure 2 is an exploded perspective view of components forming part of the structure shown in Figure 1; Figures 3(a) and 3(b) shows diagrammatically different operational states of the structure shown in Figure 2; Figure 4 shows curves plotted to indicate the piezo-electric constant-temperature relationships of materials; Figures 5(a) and 5(b) are views similar to Figures 3(a) and 3(b) but illustrating a second embodiment of the present invention; Figure 6 is a cross-sectional view of a solid-phase ink supply device for supplying ink to an apparatus of the present invention; Figure 7 is a detailed cross-sectional view of a third embodiment of the present invention; Figure 8 illustrates on an enlarged scale a portion of the third embodiment shown in Figure 7; Figure 9 is a cross-sectional view of a fourth embodiment of the present invention; and Figure 10 is a perspective view of the piezo-electric transducer assembly for a fourth embodiment shown in Figure 10. Referring first to Figure 1, a printer comprises an ink jet head 03, a pair of guide shafts 01 on which the ink jet head 03 is mounted to travel across a sheet 07 of printing paper in the printer, a timing belt 02 which drives the head 03 along the shafts 01, an ink cartridge 04, an ink supply system 05 to deliver the ink from the cartridge to the head 03, and a paper feed system 06 to feed the sheet 07 of paper past the head 03 for printing. Referring then to Figure 2, which shows the basic structure of the ink jet head constructed in accordance with the present invention, the head 03 comprises a base plate or nozzle plate 09, a piezo-electric transducer, a frame or ink chamber 012 of receptacle shape, and a heater 013 to heat ink in the frame 012 so as to change it from a solid to a liquid phase ink or so as to reduce its viscosity. The heater 013 may be a posistor (POSISTOR being a Registered Trade Mark of Murata Manufacturing Co.). A posistor is a heating resistor element whose electrical resistance increases with temperature such that there is a sharp increase in the said resistance at a predetermined temperature. This enables a posistor to be used to maintain the heating temperature constant. The nozzle plate 09 is provided with a plurality of nozzle orifices 08 which communicate with the ink chamber 012 to receive ink therefrom. The piezo-electric transducer, which is provided to force the ink through the nozzle orifices 08 so as to effect printing, comrpises a metal plate 011 and a number of piezo-electric elements 010 made integral with the metal plate 011. The metal plate may preferably have a thickness ranging from 20 to 50 µm. The frame 012 is preferably made of electrically insualting material such as a ceramic material of high thermal insulation property. As can best be shown in Figure 3(a) and 3(b), the nozzle orifices 08 in the nozzle plate 09 are shaped to progressively expand in diameter toward the inside of the plate 09. The piezo-electric transducer is disposed inwardly of the nozzle plate 09, is parallel thereto and is slightly spaced apart therefrom. The metal plate 011, which forms the piezo-electric transducer, also comprises an electrode common for all the piezo-electric elements 010, which is connected to one pole of an external power source, not shown, through wiring. The piezo-electric elements 010 are all connected to the opposite pole of the power source. Generally, a piezo-electric element possesses a piezo-electric constant as a parameter which indicates its particular property. The piezo-electric constant d of a piezo-electric element is expressed by the following equation:- d = k √ εt/Y (m/v) where k is the electromechanical coupling factor ε is the dielectric constant, and Y is Young's modulus. Generally, for a piezo-electric element, the dielectric constant εt increases as its temperature rises, and peaks when the element reaches the Curie point. Piezo-electric elements differ from one another in their electro-mechanical coupling factor depending on the material they are made of. Thus, the piezo-electric constant as computed by the above equation differs with different piezo-electric materials. The curves plotted in Figure 4 compare the relationships between the piezo-electric constant and the temperature for different piezo-electric elements, one of which is made of material of type "a" and the other of which is made of material of type "b". The curve for the material of type "a" shows that the piezo-electric constant increases as temperature rises. On the other hand, the curve for the material of type "b" indicates an opposite trend, the piezo-electric constant declining in this case with the rise of temperature. The piezo-electric elements 010 used in this particular embodiment have the temperature characteristic of piezo-electric constant shown by the curve for the material of the type "a". Since the ink in the ink head is heated to 120 degrees Celcius, the piezo-electric elements must be made of a material that is over 300 degrees Celsius at the Curie point. In addition, the maximum possible temperature under which these piezo-electric elements 010 are used must be below half their Curie point. Thus, using a piezo-electric element 010 made of material of type "a" as described above, the piezo-electric constant would be considerable, producing sufficiently effective displacement when the piezo-electric transducer is exposed to the high temperature of the ink in which the transducer is submerged. Furthermore, the linear thermal expansion factor for both the piezo-electric elements 010 and the metal plate 011, which together make up the piezo-electric transducer, range from 1.0 x 10⁻⁶ to 3.0 x 10⁻⁶ and 1.0 x 10⁻⁶ to 2.0 x 10⁻⁶, respectively. When the piezo-electric transducer in operation is heated from room temperature to about 100 degrees Celcius, they will be caused to bend because of their differences in their linear thermal expansion factors ; the metal plate 011 would expand more than the piezo-electric elements 010. The use of constantly maintained bending in the same direction of a piezo-electric transducer can produce a stable and considerable displacement. A spacer 014 is mounted to hold the piezo-electric transducer 010, 011 properly spaced from the nozzle plate 09 so as to provide a gap 021 therebetween which permits the escape of air bubbles therethrough. The spacer 014, which is shown in Figure 2 as a pair of bars, is provided to offer an important role in relation to the length of time that the ink is supplied to the nozzle orifices 08 and the transfer of piezo-electric pressure in the ink from the piezo-electric transducer to the nozzle orifices 08 and the prevention of an air bubble staying between the piezo-electric transducer and the nozzle orifices 08. These factors are determined by the spacing that the spacer 014 establishes between the piezo-electric transducer and the plate 09. Experiment conducted by the inventors of the present invention has proved that favourable results have come from spacers 014 having a thickness in the range from 20 to 50µm. This spacing enables the ink to be drawn through the said spacing by capillary action and ensures that the pressure exerted by the piezo-electric transducer 010,011 is properly transmitted to the ink in the nozzle orifices 08. Referring to Figure 3, the heating element 013 is provided to cause the ink 015 in the ink head to melt into a liquid phase. In this particular embodiment, the heating element 013 is mounted inside the frame 012, which constitutes a receptacle or ink chamber. It is so designed that air bubbles,when generated as the ink 015 is fused into a liquid state by the heating element 013, would be allowed to rise easily towards the piezo-electric transducer, escape through the gap 021, and pass and up into a space 016 defined on top of the ink 015 in the frame 012. In this embodiment, the ink used may be a non-conductive solid-phase type ink with a melting point of 100 degrees Celsius, a composition comprising stearone (diheptadecyl ketone), amide stearate (Stearic acid amide), and a dye. Also, the ink may have a viscosity of 3.5 mPas at 120 degrees Celsius, with a surface tension of 30 dyn/cm. In operation, with the ink kept at a constant temperature of 120 degrees Celsius, and hence in liquid state, the application of voltage across a piezo-electric element 010 causes the associated piezo-electric transducer to displace as depicted in Figure 3(b), exerting piezo-electric movement in the ink filling the space between the transducer and nozzle plate 09 until a drop of ink 019 comes out from an orifice 08. The ink drop, upon contact with the surface of the record paper sheet, will immediately turn into a solid phase creating a clear, sufficiently thick printed image. Figure 5 shows a second embodiment of the apparatus according to the present invention. The apparatus employs a piezo-electric transducer different in structure from the one having a bimorph structure shown in Figures 3(a) and 3(b). The piezo-electric transducer in the second embodiment comprises a metal strip 011 and a pair of piezo-electric elements 010 coupled to opposite ends of the strip 011. It is so designed that flexing up and down of the metal strip 011 at its middle creates a pressure on the ink disposed beneath the strip 011 expelling a drop of ink through the nozzle orifice 08 that is formed below the metal strip 011. As shown in Figure 5(a), the metal strip 011 is, in the rest state, in an upwardly bent position. With this arrangement, when a voltage is applied across the piezo-electric elements 010 fixed to the opposite ends of the metal strip 011, the elements 010 shrink due to the piezo-electric effect, so causing the metal stirp 011 to straighten up to a shape as shown in Figure 5(b). As a result, a drop of ink is forced out from the nozzle orifice 08 at the bottom of the ink head. Although not shown in Figure 5, heating elements corresponding to the elements 013 of Figures 3(a) and 3(b) are mounted in the ink chamber 012. Figure 6 shows a solid phase ink supply device for supplying ink to an apparatus of the present invention in which ink 023 is supplied to an ink jet head 03. The ink 023 used is in the solid-state at room temperature. The ink supply device includes an ink cartridge 04 which contains ink in the granular state. A push rod 029 having a helical screw is mounted in a cylinder 026 which communicates with the ink cartridge 04. The rod 029 moves the ink from the cartridge 04 into the ink head 03 through the cylinder 026. A spring 025, which is mounted in the cartridge 04, exerts downward pressure on the ink 023 in the cartridge 04 forcing the granular ink into the cylinder 026. A heater 027 is provided in the outside wall of the cylinder 026 to heat the ink present in the cylinder 026. A small motor 028 is drivingly connected to the rod 029 to rotate the latter and so move the ink 023 in the cylinder 026 forwardly into the ink head 03. When the rod 029 rotates in the ink 023, the temperature of the ink will rise due to friction and it will begin to fuse. By taking advantage of such frictional heating, it is possible to save on the energy used by the heater 027. A continuous supply of ink 023 of sufficiently fused phase in the minimum required amount can be provided for the ink head 03 in an easy manner. Means are provided to keep the outlet of the ink supply system normally closed. The outlet is opened when the ink head 03 is replenished with fresh ink. A filter is preferably installed at the outlet of the ink supply system to prevent entrance of dust and dirt into the ink head 03. Figure 7 is a detailed cross-section view of a third embodiment of the present invention. A nozzle plate 1 is provided, which has a plurality of nozzle orifices 2 bored therein.The plate 1 is preferably made of nickel produced by electro-forming. A plurality of piezo-electric elements 3 are provided, each of which is mounted to face one of the nozzle orifices 2. The piezo-electric elements 3 are preferably made of PZT with a thickness of 100 µm, PZT being Pb (Zr, Ti) 0₃. Each of the elements 3 is laminated on its side facing the orifices 2 with an electrode 4. The electrodes 4 are preferably made of nickel and have a thickness ranging from 0.5 to 1.5 µm. Each of the elements 3 is plated with a vibration plate 5, which is preferably of nickel of a thickness in the range of from 10 to 30 µm. A piezo-electric element 3, an electrode 4 and a vibration plate 5 in combination constitute a piezo-electric transducer. A spacer 6 made of thin metal is secured to the inside of the plate 1 to hold the elements 3 in positions in which they are spaced apart from the plate 1 so as to define a space or gap G therebetween through which air bubbles can escape. An FPC (flexible printed circuit) conductor 7 is provided, which is electrically connected to each of the electrodes 4. The FPC conductor 7 is also electrically connected to a common electrode 8 which serves for the vibration plates 5 to which the electrode 8 is electrically connected. A flexible printed circuit may, for example, be constituted by a printed circuit, e.g. of a copper foil, formed on a flexible plastics film, optionally with an insulating film disposed on the copper foil. A frame or ink chamber 9 is provided, which is preferably made of an aluminium die casting, and which communicates with the nozzles 2. The frame 9 carries at a bottom side thereof a heater (posistor) 10. A vent hole 11 is provided at an upper end of the frame 9. An ink supply/hold member 12 is provided in the frame 9, the member 12 preferably being made of an aluminium die casting. The member 12 extends parallel to the plate 1, with the piezo-electric elements 3 interposed therebetween. The member 12 is spaced 0.3 to 1.5 mm apart from the plate 1 to define a gap 13 between them. The member 12 is also spaced from the elements 3 by an elongated space 26 of capillary size so that ink rises in the space 26 to form a meniscus 24 which is disposed above the orifices 2. The space 26 communicates with the gap G so that air bubbles may pass through the gap G and the space 26 to an upper portion of the frame 9. A filter 14 is mounted below the member 12 to divide the inside of the frame 9 into an ink jet head section 15 and an ink reservoir section 16. The filter 14 is preferably made of stainless steel mesh. An ink return passage 17 is defined between the member 12 and the frame 9. The passage 17 is preferably below 1 mm in dimension across, so that the ink in the liquid phase forms a meniscus in the passage 17 due to capillarity. A lid 18 is provided to cap the reservoir section 16 of the frame 9. The lid 18 has therein a vent hole 20 provided to ventilate the frame 9. The lid 18 is pivotally attached to the frame 9 such that the lid can be opened by rotating about a pivot 19 in the frame 9. With the above arrangement, when a power switch, not shown, on the printing apparatus is turned on, the heater 10 is energized and starts warming the bottom of the frame 9. By heat transfer,the nozzle plate 1, the filter 14 and then the ink supply/hold member 12 is heated. When their temperature reaches 120 degrees Celsius, all the ink 21 filling the space between the plate 1 and the member 12, along with that part of the ink 21-S in the ink reservoir section 16 which stays near the bottom of the frame 9 and along the filter 14, begins to melt into a liquid phase ink 21-L. With the fusion of the solid-phase ink 21-S, air bubbles 23 develop from what were voids 22 in the ink. When a required amount of liquid-phase ink 21-S is obtained within 10 to 30 seconds of the start, a control circuit, not shown, is actuated to apply a voltage to the piezo-electric transducer. As a result, the piezo-electric elements 3 affected begin to be displaced in the direction of an arrow B in Figure 7 by the piezo-electric effect. When a piezo-electric element 3 bends sufficiently, pressure is exerted on the ink 21 until a drop of ink is expelled through the respective nozzle orifice 2 in the plate 1. The ink drop, upon contact with the surface of the record paper, not shown, is immediately solidified so forming a print on the paper. As a result of successive oscillations of the piezo-electric elements 3, the liquid-phase ink 21-L present between the elements 3 and the ink supply/hold member 12 is violently stirred allowing air bubbles 23 c that might be generated in the ink 21-L to move up in the ink to be released at the meniscus 24 that the ink forms between the elements 3 and the member 12. The air bubbles 23 c may escape into the atmosphere through the vent hole 11 in the frame 9. Of the air bubbles 23 that might be generated in the space defined between the elements 3 and the nozzle plate 1, those 23 a hovering sufficiently far from the nozzle orifice 2 would probably remain unmoved in various places, due partly to their being exposed to the smaller piezo-electric movement of the elements 3 and partly to the narrowness of the surrounding space in which the bubbles 23 a are confined. However, it is unlikely that the presence of these bubbles 23 a has any significant effect on the ejections of the ink through the nozzle orifices 2 because of their distance from the point of ink ejection. On the other hand, those bubbles 23 b that might happen to be present in the vicinity of the nozzle orifices 2, and hence where the elements 3 would produce a maximum piezo-electric impact, will be caused to move away from such places, as the surrounding ink would be violently agitated by the flexing of the elements 3. Each time ink is expelled from a nozzle orifice 2, the ink head section 15 is replenished with a new supply of liquid-phase ink from the ink reservoir section 16 through the filter 14. With the supply of ink to the ink head section 15, the reservoir section 16 comes to have less and less ink, with the ink level 25 lowering. However, the liquid-phase ink 21-L both in the space between the elements 3 and member 12 and in the return passage 17 would be able to keep its ink level sufficiently high by the effect of capillarity. The said ink level is above that of the nozzles 2. As the ink reservoir section 16 transfers more of its liquid-phase ink 21-L to the ink head section 15, the solid-phase ink 21-S, which has so far remained unfused and floating in the upper part of the reservoir section 16, would come down under its own weight near to the bottom of the frame 9 and would be heated by the heater 10 into a liquid-phase ink 21-L. In the process of printing, as the ink jet head and hence the plate 1, is moved over the surface of the record paper sheet set on the apparatus, the impacts generated by this movement can result In the meniscus 24 breaking up, allowing part of the liquid ink 21-L to spill over the top brim of the member 12 into the return passage 17. However, the ink in the return passage 17 would absorb any amount of split ink into itself due to the ability of the ink in the return passage 17 to maintain the capillary effect, so that the ink in the return passage 17 would be able to keep its meniscus at a correct level. When all the ink in the ink reservoir section 16 is used up, with the height of the meniscus 24 falling to the level of the orifice 2, the operation of the ink jet head comes to an end. If the operation has to be resumed, the ink reservoir section 16 must be replenished with an additional supply of solid-phase ink. The replenishment is done by opening the lid 18. When the new supply of solid phase ink is fused by the heater 10 into a liquid phase ink 21-L, it will flow into the head section 15 filling the return passage 17 and the spaces between the elements 3 and plate 1 and between the elements 3 and the member 12 to a required level for operation, by capillarity. When the power switch, not shown, is turned off to end the operation, the heater 10 is de-energised and the ink head section 15 is allowed to fall in temperature. As a result, the liquid-phase ink 21-L in the head section 15 begins to cool off so as to turn into a solid-state ink 21-S. In this process of solidification, the ink reduces its volume by about 20%. Voids 22 would be formed in the solid ink about the head section 15 and the reservoir section 16. These voids 22 originally come from the air contained in the ink 21 itself and, for a while following the solidification of the ink, would be maintained at a low pressure. However, the voids 22 will begin to be gradually filled with external air until they come to be at atmospheric pressure. Thus, these voids 22 would develop into air bubbles 23 when the ink in the head section 15 is warmed up by the heater 10 in the course of the restarted operation. The behavior of air bubbles will be explained in more detail with respect to Figure 8. When a piezo-electric element 3 in operation bends in the direction of the arrow B, the resultant piezo-electric movement exerts force on the liquid-phase ink 21-L causing a drop of ink to jet through the nozzle orifice 2 in the plate 1 in the direction of the arrow C. The rest of the liquid ink 21-L would be forced back in the direction of the arrow D. The greater is the portion of the ink which is allowed to move in the direction of the arrow D than the portion which is ejected through the nozzle orifice 2 in the direction of the arrow C, the further the bubbles that might be developed in the vicinity of the element 3 would be kept away from the nozzle orifice 2. Thus, the effect that these bubbles 23 b could have on ink ejection through the orifice 2 can be minimized by designing the head section 15 such that the ratio of the amount of ink which would be forced back to that which would be ejected is set to a level that produces the required result. If the amount of ink which would be forced back into the ink head section 15 is Qb, and the amount of ink which would be ejected is Qn, the ratio K of these amounts is Qb/Qn. When the ink jet head is designed so that the value of K exceeds 3, the bubbles 23 that might be generated would be efficiently kept away from the vicinity of the nozzle orifice 2. However, if the value of K is in the range from 3 to 10, some bubbles 23, entrained in the ink drop being ejected, may find a way through the orifice 2 resulting in defective printing. If K is equal to or greater than 10, the head section 15 would be able to keep all bubbles 23 that might be present away from the nozzle orifices 2 and increase printing stability. In theory, the ratio K is inversely proportional to the ratio of fluid impedance for the mass of the ink moving away from the nozzle orifices 2 within the head section 15 to that for the mass being ejected through the nozzle orifice 2. The magnitude of the fluid impedance for the ink is determined by its inertia and viscous resistance. Accordingly, in practice, the desired results can be obtained by arranging that the ratio of the inertia and the viscous resistance for the two ink amounts, respectively (i.e. the ratio of the amount of ink moved back within the head section 15 to that which is being ejected) is of a predetermined value or more. Experiment mostly has proved that, for the higher frequencies of piezo-electric transducer vibration, the effect of the ink inertia is greater than the viscous resistance. On the other hand, for the lower frequencies, the viscous resistance has the greater effect than the ink inertia. In addition, viscous resistance and inertia can be defined as follows: 〈viscous resistance〉 α 〈length of flow passage〉 ÷ 〈cross-sectional area of the flow passage〉 ; and 〈inertia〉 α 〈length of flow passage〉 ÷ 〈cross-sectional area of the flow passage〉 . As a result of experiment which the inventors of the present invention conducted with reference to the above-mentioned relationships, it has been proved that the ratio of the viscous resistance Kr to that of the inertia Ki can be computed by the use of the following formulas: where W is the width of a piezo-electric element; g is the distance between the piezo-electric elements and the nozzle plate; t is the total thickness of the piezo-electric element; s is the spacing between piezo-electric elements, ℓ is the axial length of the orifice; and r is the orifice radius. If the value of K has to be equal to or greater than than 3, both Kr and Ki must be equal to or greater than 3. If ℓ is 50 µm; d (orifice diameter) is 60µm; Wp (piezo-electric element width) is 100µm; t is 50µm; and g is 50µm, the value of s being 50µm, the values for Kr and Ki computed by the above formulas 1 and 2 are 6.2 and 3.5, respectively. Both results can produce desired results. A series of tests subsequently carried out by the inventors have proved that there was no effect of the air bubbles on printing performance. In addition, another series of tests using different sets of parameters which produced values of Kr and Ki smaller than 3 have proved that the bubbles 23 have an objectionable influence on printing. For example, in one such test, d was 100 µm; g was 10 µm and s was 50µm. From these parameters, the above formulas 1 and 2 gave the values of Kr and Ki as 0.06 and 0.36, respectively. However, although a greater value of K can increase the extent to which the possible bubbles 23 would be moved away from the vicinity of the nozzle orifices 2, it would have an adverse effect on ink ejection through the nozzle orifices 2. For example, the voltage must be increased to drive the piezo-electric transducer effectively. The inventors have discovered experimentally that desired results were obtained when the value for K is below 100, and preferably below 50 for a practical application. Although the above descriptions of the preferred embodiments are made with respect to hot melt ink, they are also applicable to types of ink that are of high viscosity at room temperature. Printing performance with ordinary paper would be good as well, without causing ink bleeding. However, with these types of high viscosity ink, when the temperature of the apparatus is increased to around 100 degrees Celsius, the air dissolved in the ink will begin to develop into air bubbles because the solubility of the ink is decreased at high temperature. In this case, there would be no problem of possible air bubbles generating from the voids, as in the case of solid-phase ink. The present invention, therefore, is also applicable to apparatus in which high-viscosity ink is used at high temperature. In the embodiment of Figure 7, the head section 15 is designed to have its vent hole 11 kept normally open. However, this is a matter of choice. In an alternative version,, it may be designed that the vent hole 11 is kept normally closed and is opened to the atmosphere only when the need arises. In a still further alternative modification, the vent hole 11 may be entirely eliminated so that the ink head section 15 has the meniscus 24 directly exposed to the atmosphere. Furthermore, in Figure 7, the ink in the head section 15 forms its meniscus 24 between the piezo-electric elements 3 and the ink supply/hold member 12. In an alternative construction, however, the member 12 may be longer than shown in Figure 7 so that it extends into an upper portion of the frame 9 until all the piezo-electric elements 3 are submerged in the ink 21. In this case, the ink would form a meniscus 24 between the member 12 and the opposite inner wall of the frame 9. In the Figure 7 embodiment, because of the fact that the member 12 is spaced no more than 1.5mm from the nozzle plate 1, the stresses which arise in the ink 21 upon solidification are prevented from concentrating on the elements 3 due to the difference in thermal expansion. In other words, the ink 21 has a 10 to 100 times greater thermal expansion coefficient than the elements 3, while the thermal expansion coefficient of both the plate 1 and the member 12 is almost equal to that of the elements 3. Thus, due to the selected spacing between the member 12 and the plate 1, the stress concentration of the ink 21 on the elements 3 is reduced. If this spacing were equal to or greater than 3mm, the stress which would be exerted by the ink 21 on the elements 3, when the ink solidifies, would be considerably greater, so that the elements 3 would be liable to break or have a shorter service life. Figure 9 shows a fourth embodiment of the present invention in which parts similar to those used in the Figure 7 embodiment are given similar reference numerals. A casing or ink chamber 9 is provided which is preferably made of a metallic material of high heat conductivity and which is shaped as a receptacle. A heater (posistor) 10 is attached to the bottom of the casing 9. In a face 9 a of the casing 9 opposite a platen 30 there are provided a desired number of ink drop jetting orifices 9 b that are formed in alignment with the axis of the platen 30. Behind the face 9 a ,a nozzle plate 1 is provided unitarily therewith, with the interposition of a spacer 9 c therebetween. The plate 1 has a number of nozzle orifices 2 bored therein, each located just opposite one of the orifices 9 b . A piezo-electric transducer is provided on the back side of the plate 1, with the interposition of a second spacer 6. As can best be shown in Figure 10, the piezo-electric transducer comprises a piezo-electric element 3 made of PZT, a common electrode 5 laminated on one side of the element 3 and a patterned electrode 4 plated on the opposite side of the element 3. Also, the element 3 is provided with a plurality of cutouts 3 c along one side thereof to thereby form a plurality of vibrators 3 b . The cutouts 3 c are spaced apart from one another by the same spacing as the nozzle orifices 2. The piezo-electric transducer includes a support base portion 3 a that extends along one side thereof. The support base portion 3 a is thus provided along one side thereof with a plurality of the vibrators 3 b . The piezo-electric transducer, thus arranged, is installed in the casing 9, with the ends of the vibrators 3 b placed opposite the nozzle orifices 2 and the support base portion 3 a located above the nozzle orifices 2. Therefore, most of the piezo-electric transducer including the support base portion 3 a is exposed above the level of the ink. Behind the piezo-electric transducer, there is provided a divider 112 that defines an ink reservoir section 16. The divider 112 is provided with a projection 112 a which abuts against and supports the plate 1 from behind. The projection 112 a has a designed width D to hold the plate 1 in a position in which it is spaced by about 0.2 to 2.0 mm from the divider 112. The dimension of the spacing D is arranged to be such that the ink in an ink chamber 113 rises by capillarity to a level above the centre of the nozzle orifice 2. Also, the spacing D must be small enough not to cause the ink level to fluctuate due to vibrations caused when the carriage, not shown, and hence the ink jet head is moved in operation. In addition, the spacing D is provided to allow air bubbles that might be generated in the ink to easily move away from the nozzle orifices 2. Furthermore, the spacing D must be of a sufficient dimenion as to ensure a constant supply of fresh ink to the ink chamber 113 at a desired high frequency during operation. A lid 18 is provided on the casing 9 to close the ink reservoir section 16. The lid 18 is located at a level below the nozzle orifices 2 so as to keep the liquefied ink level L constantly below the nozzle orifices 2. A sensor, not shown, may be provided in the reservoir section 16 to detect when the ink level L drops below a lower permissible height. Means may also be provided to ensure a supply of fresh ink from a source of solid-phase ink, not shown, into the ink reservoir section 16, in an amount that would not cause the liquefied ink to rise above the orifices 2 in the plate 1. A filter 14 is provided in the passage interconnecting the ink reservoir section 16 and the ink chamber 113 to prevent entrance of dust and dirt into the chamber. In operation, when the heater 10, which is mounted below the casing 9, is energized, the casing 9 is uniformly heated since it is made of a material of high heat transfer property. As a result, the ink in the solid state which fills the narrow space of the ink chamber 113 turns into a liquid-state ink to a sufficient extent to be ejected through the nozzle orifices 2 for printing. When a selected group of electrodes 4 that forms a pattern or image to be printed are then energized, the corresponding vibrators 3 b in the piezo-electric transducer whose greater part is exposed above the level of the ink, are moved towards the plate 1 effectively. As a consequence, the ink filling the space between the plate 1 and divider 112 is pressurized when the vibrators 3 b oscillate until drops of ink are expelled through the nozzle orifices 2 that lie just opposite the oscillating vibrators so as to jet ink onto the paper sheet on the platen 30. In the meantime, the solid-phase ink in the reservoir section 16 is liquefied by exposure to the high temperature generated by the heater 10 at the bottom of the casing 9. Convection in the now molten ink in the reservoir section 16 helps warm the entire casing 9. As the casing 9 heats up to a sufficiently high temperature, the flow of liquefied ink through the passage to the ink chamber 113 is facilitated. When the power switch is turned off at the end of the printing operation, the casing 9 is allowed to cool off and the liquid ink in the ink chamber 113 will quickly solidify, because of its small volume, thereby holding the vibrators 3 b stationary in positions in the solidified ink. Thus, the vibrators 3 b made stationary in the solidified ink in the earlier stages immediately following the stopping of the apparatus would be protected from physical deformation that might occur due to pressures exerted by the remaining part of the ink starting to solidify later in time.

Claims: An ink jet printing apparatus comprising a nozzle plate (09,1) having a plurality of nozzles (08,2) formed therein for ink ejection; an ink chamber (012,9) which communicates with said nozzles (08,2); heating means (013,10) to ensure that ink supplied to the nozzles (08,2) has been changed from a solid to a liquid phase ink or has had its viscosity reduced; and electro-mechanical transducer means (010,011,3,4,5) spaced from the nozzle plate for forcing the ink through the nozzles (08,2) so as to effect printing characterised in that the electro-mechanical transducer means (010, 011,3,4,5) is spaced from the nozzle plate (09,1) by a gap (021,G) in the range of 20 to 50 µm which permits the escape of air bubbles therethrough. Apparatus as claimed in claim 1 in which the transducer (010,011,3,4,5) is housed in the ink chamber (012,9). Apparatus as claimed in claim 1 or 2 in which the ink chamber (012) is made of electrically insulating material. Apparatus as claimed in any preceding claim in which the transducer means (010,011,3,4,5) comprises a piezo-electric vibrator (3,4,5). Apparatus as claimed in claim 4 in which the piezo-electric vibrator (3,4,5) comprises a piezo-electric element and a metal film which is integral with said element, the metal film having a different linear thermal expansion coefficient than that of said element. Apparatus as claimed in claim 5 in which the piezo-electric is made of a material whose piezo-electric modulus increases with temperature. Apparatus as claimed in claim 4 in which the piezo-electric vibrator (3,4,5) is divided into a plurality of vibrators (3 b ) which are respectively disposed opposite to the nozzles (2). Apparatus as claimed in claim 2 or any claim dependent thereon in which a member (12) is mounted in the ink chamber (9) so as to define with the transducer (3,4,5,) a capillary space (26) which communicates with the said gap (G) and in which, in operation, the ink rises above the level of the nozzles (2) due to the capillary effect, whereby the said air bubbles (23) may pass through the gap (G) and the space (26) to an upper portion of the ink chamber (9). Apparatus as claimed in claim 8 in which the ink chamber (9) comprises an ink reservoir portion (16) and a further portion (15) in which the transducer (3,4,5) is mounted, the ink reservoir portion (16) and further portion (15) being separated from each other by means comprising the said member (12), ink overflowing from the space (26) returning to the ink reservoir portion (16) by way of a capillary return passage (17) in which the ink rises by capillarity to a level above that of the nozzles (2). Apparatus as claimed in claim 8 or 9 in which the said upper portion of the ink chamber is provided with a vent hole (11). Apparatus as claimed in any of claims 8 to 10 in which the capillary space (26) has a width in the range 0.3mm to 1.5mm. Apparatus as claimed in claim 7 or in any claim dependent thereon in which the plurality of vibrators (3 b ) are coupled together by a support base portion (3 a ) of the transducer (3,4,5), the support base portion (3 a ) being arranged to be disposed above the level of the ink. Apparatus as claimed in claim 9 or in any claim dependent thereon in which the ink reservoir portion (16) and further portion (15) are separated from each other both by the said member (12) and by a filter (14). Apparatus as claimed in 8 or in any claim dependent thereon in which the said ink chamber (9) and an ink supply/hold plate means (12) are made of a material having a high heat transfer property.

Inventors: KOTO, HARUHIKO, KURASHIMA, NORIHIKO, MIYAZAWA,  YOSHINORI, NAKAMURA, OSAMU, MIYAZAWA, YOSHINORI
Applicants: SEIKO EPSON CORPORATION
Application year: 1988
Main classification: B41J2
IPC codes: B41J2