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(11) | EP 0 845 358 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Ink-jet system |
(57) Ink-jet system comprising:
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[0001] The invention relates to an ink-jet system comprising: a nozzle head defining a cavity and a nozzle in communication with said cavity, the cavity containing liquid ink, and pressurizing means for pressurizing the ink so that an ink droplet is expelled from the nozzle, said pressurizing means comprising heating means and utilizing thermal expansion or evaporation of a medium as pressure source.
[0002] A conventional ink-jet system of this type has been described for example in US-A-4 463 359. The pressurizing means used in this system are formed by a heating electrode which is disposed directly on the internal wall of the cavity so that it is exposed to the ink. When the heating electrode is energized in accordance with the printing signal, a current flows through the heating electrode and the ink liquid in the vicinity of the electrode is abruptly heated and evaporated, thereby creating a rapidly expanding bubble in the portion of the cavity where the heating electrode is disposed. As a result, the pressure of the ink liquid raises abruptly and an ink droplet is expelled from the nozzle.
[0003] A system of this type which is frequently called a "bubble-jet" system has the advantage that the pressurizing means have a simple and inexpensive construction and can be driven with moderate voltages and currents. However, this conventional system has the disadvantage that the quality of the ink may be degraded due to electrolytic or thermal effects occurring in the cavity in which the ink is directly exposed to the heating electrode. In addition, the variety of inks that can be used with this system is limited by the constraint that the ink must have a suitable boiling point.
[0004] In other known types of ink-jet systems the cavity is limited at least on one side by a flexible membrane, and the pressurizing means are arranged to cause a deflexion of the membrane by means of electric forces (EP-A-0 701 899), magnetic forces (US-A-4 057 807) or by means of piezoelectric actuators (EP-B-0 402 172). These systems have the disadvantage that the pressurizing means are comparatively bulky and expensive and/or require rather high energizing voltages or currents. Manufacturing problems become increasingly intricate, when a plurality of nozzle units are to be closely packed in a multiple-nozzle head for achieving a high printing resolution.
[0005] It is accordingly an object of the invention to provide an ink-jet system which can avoid degradation of the ink and in which the pressurizing means nevertheless can have a simple construction.
[0006] According to the invention, this object is achieved by a system according to the preamble of claim 1 which is characterized by a flexible membrane separating the cavity from the medium exposed to the heating means.
[0007] Similarly as the bubble-jet system, the system according to the invention utilizes the effect of the thermal expansion or evaporation of a medium as the pressure source. However, in contrast to the bubble-jet system the medium that is heated by means of the heating means is not the ink itself but another medium that is separated from the ink in the cavity by a flexible membrane. The pressure generated by thermal expansion or evaporation of this medium is transmitted to the ink via the flexible membrane. As a result, the system takes advantage of many of the beneficial properties of the pressurizing means used in the bubble-jet system but avoids the problems of electrolytic or thermal degradation of the ink, since the ink is separated from the heating means by the flexible membrane.
[0008] The system according the invention has the further advantage that the composition of the ink on the one hand and the composition of the pressure generating medium on the other hand can be selected and optimized independently from one another. Thus, it is possible for example to use a so-called hot-melt ink which is solid at room temperature and is melted in the nozzle head only when the printer is operative. Then, the boiling point of the pressurizing medium can be set to be only slightly higher than the melting point of the ink, so that only a little amount of heat energy is needed for generating the pressure, and a quick response of the pressurizing means can be assured.
[0009] The cavity and the nozzle can be designed to concentrate the acoustic energy conferred to the ink by the deflexion of the flexible membrane onto the ink volume in the nozzle, so that the available energy is used for droplet formation with high efficiency, while, on the other hand, the acoustic properties of the pressurizing medium and the geometry of a reservoir containing this medium can be designed to attenuate acoustic waves in the medium within a rather short time, so that droplets can be generated at a high frequency without interference between successive droplet generation processes.
[0010] In case of a multiple-nozzle head, the properties of the medium and of the reservoir can also be designed to minimize cross-talk between neighbouring channels.
[0012] In a preferred embodiment of the invention the heating means are formed by a heating electrode disposed directly on the flexible membrane on the side opposite to the ink cavity. As a result, when the medium, e. g. a liquid fluid, is evaporated, a bubble is created directly in the vicinity of the ink cavity, and the high local pressure caused by expansion of the bubble is directly conferred to the ink, whereas the reaction pressure in the liquid is dissipated into a comparatively large volume so that it can readily be absorbed by the mechanical structure defining the fluid reservoir.
[0013] The nozzle head can easily be manufactured by sandwiching a first block member defining the cavity and the nozzle, and a second block member defining the fluid reservoir with the flexible membrane sealingly interposed therebetween. In case of a multiple-nozzle head the ink cavities can closely be arranged side-by-side, and the heating electrodes can be formed by providing a suitable conductive pattern on the flexible membrane by means of known photolithographic techniques or the like. Preferably, the heating electrodes for the individual ink cavities are connected to a common ground electrode interconnecting the ends of the heating electrodes adjacent to the nozzles. The opposite ends of the heating electrodes can then easily be connected to an electronic system for selectively energizing the individual electrodes. The fluid reservoir may be common to a plurality of ink cavities.
[0014] A preferred embodiment of the invention will now be described in conjunction with the drawings, wherein:
Fig. 1 is a partly broken-away perspective view of a section of multiple-nozzle head embodying the invention;
Fig. 2 is a sectional view of a nozzle head, the viewing direction corresponding to arrows II-II in Fig. 1;
Fig. 3 is a longitudinal section along the line III-III in Fig. 2; and
Figs. 4 and 5 are sectional views corresponding to Figs. 2 and 3 and illustrating the process of droplet generation.
[0015] Fig. 1 shows a portion of an elongate ink-jet nozzle head forming a linear array of nozzles 12. The nozzle head 10 has a sandwich structure with a lower block member 14, an upper block member 16 and a flexible plate 18 interposed therebetween. The block members 14, 16 and the flexible plate 18 are firmly bonded together by means of adhesive or the like or are clamped together by clamping means (not shown).
[0016] The upper block member 16 defines a plurality of channel-like cavities 20 arranged in parallel to one another, one end of each cavity being smoothly converged towards a respective one of the nozzles 12. The opposite ends of the cavities 20 are connected to ink supply means (not shown). The cavities 20 and the nozzles 12 are formed by recesses in the bottom surface of the upper block member 16, i.e. the surface of the block member facing the flexible plate 18. Thus, the bottom of each cavity 20 is formed by a corresponding portion of the flexible plate 18. The upper surface of the lower block member 14 is recessed to form a trough-shaped reservoir 22 extending across the plurality of cavities 20. The reservoir 22 and each of the cavities 20 are liquid-tightly sealed by the flexible plate 18, and each cavity 20 is separated from the reservoir 22 by a flexible membrane 24 which is formed by a portion of the flexible plate 18.
[0017] As is shown in Fig. 2, the cavity 22 is closed at both ends by separating walls or end walls 26 formed by portions of the lower block member 14. For illustration purposes, Fig. 2 shows an embodiment in which the reservoir 22 extends only over three of the cavities 20. In practice, however, the number of channels associated with the same reservoir 22 may be significantly larger, and the cavity 22 may even extend over the whole length of the nozzle head 10. The cavity 22 is entirely filled with a fluid 28, preferably a liquid. The liquid 28 may permanently be sealed in the nozzle head 10. As an alternative, the reservoir 22 may be connected to a fluid tank via a fluid supply system (not shown), so that any possible diffusion or evaporation losses of the liquid can be compensated. The plate 18 forming the flexible membranes 24 is impermeable for the liquid 28 and for the ink contained in the cavities 20 but may be gas-permeable in order to facilitate the venting of the reservoir 22 when the same is filled with the liquid 28.
[0018] A heating electrode 30 is provided on the lower surface of each of the flexible membranes 24, i.e. on the surface of the membrane exposed to the liquid 28.
[0019] As is shown in Fig. 3, the ends of the various heating electrodes 30 adjacent to the nozzles 12 are interconnected by an electrically conductive lead 32 (e. g. a ground electrode) which is also formed on the lower surface of the plate 18, preferably outside of the area of the reservoir 22. The opposite ends of the electrodes 30 are connected to an electronic circuit (not shown) by which the electrodes 30 can be energized selectively. When an individual heating electrode 30 is energized, a current flows through this heating electrode and through the lead 32, and the heating electrode 30 is rapidly heated due to its electrical resistance. As a result, the liquid 28 in contact with the heating electrode 30 is heated and evaporated, so that a gas bubble 34 is formed in the vicinity of the flexible membrane 24, as is shown in figures 4 and 5. The rapid expansion of the gas bubble 34 causes a sharp local pressure rise and a corresponding deflexion of the flexible membrane 24. As a result, the liquid ink contained in the associated cavity 20 is compressed, and an ink droplet 36 is expelled from the nozzle 12.
[0020] In order to achieve a rapid pressure rise upon actuation of an individual heating electrode 30 the liquid 28 should have a comparatively small heat capacity and a boiling point only slightly higher than the maximal operating temperature of the nozzle head. This can be achieved by appropriately selecting the composition of the liquid 28. When the bubble 34 is formed, an acoustic pressure wave will propagate in the liquid 28. This might lead to a small but nevertheless undesirable deflexion of the flexible membranes 24 of the other cavities (cross-talk). If necessary, this phenomenon can be mitigated by one or more of the following measures: adjusting the compressibility and/or viscosity of the liquid 28 in order to achieve a rapid attenuation of the propagating wave, providing baffle plates (not shown) or separating walls (such as 26) in the reservoir 28 in order to shield the neighboring membranes 24 against the propagating wave, or providing corrugations or damping members on the internal walls of the reservoir 22 in order to absorb or divert reflected acoustic waves.
[0021] If the flexible membranes 24 are separated by baffles or separating walls in the reservoir 22, it is preferable that the various compartments of the reservoir 22 associated with the individual membranes 24 are nevertheless in fluid communication with each other, so that all membranes 24 are subject to the same static pressure of the liquid 28.
[0022] While only specific embodiments of the invention have been described above, it will be understood that the invention is not limited to these embodiments and that various modifications are possible within the scope of the invention.
[0023] For example, while the heating means is formed by a resistance-heating electrode 30 in the embodiment described above, it is possible to use any other suitable heating means, e.g. a spark discharge device, a radiation source (laser) or the like. Further, instead of disposing the heating means directly on the flexible membrane, the heating means may be arranged at another location within the reservoir 22. By way of example, the heating means may be associated with reflectors such that the acoustic pressure wave propagating from the heating means is focussed onto the flexible membrane 24.
[0024] Moreover, while a liquid 28 has been described as a thermally expansible medium, this medium may also be formed by a gas, a gel, a paste or even a solid material.
[0025] Further instead of using a nozzle on the end side of the ink chamber as shown in Fig. 5 the invention is also very good applicable to an ink-jet system wherein the nozzle is situated perpendicular to the ink chamber (a top-shooter) in block member 16. In this way a two-dimensional array of nozzles in block member 16 can easily be achieved.
1. Ink-jet system comprising:
- a nozzle head (10) defining a cavity (20) and a nozzle (12) in communication with said cavity, the cavity containing liquid ink, and
- pressurizing means for pressurizing the ink so that an ink droplet (36) is expelled from the nozzle, said pressurizing means comprising heating means (30) and utilizing thermal expansion or evaporation of a medium as pressure source,
characterized by a flexible membrane (24) separating the cavity (20) from the medium (28) exposed to the heating means.2. Ink-jet system according to claim 1, wherein the medium (28) is a liquid and the heating
means (30) are arranged to heat the liquid above its boiling point.
3. Ink-jet system according to claim 1 or 2, wherein the heating means are formed by
an electrically powered heating electrode (30).
4. Ink-jet system according to claim 3, wherein the heating electrode (30) is disposed
on the surface of the flexible membrane (24) facing away from the cavity (20).
5. Ink-jet system according to any of the claims 1 - 4, wherein the nozzle head (10)
comprises a first block member (14) accommodating said medium (28), a second block
member (16) in which the cavity (20) is formed as a recess open to a surface of the
second block member (16) facing the first block member (14), and a flexible plate
(18) sandwiched between the first and second block members, the flexible membrane
(24) being formed by a portion of the flexible plate (18) separating the cavity (20)
from the medium (28).
6. Ink-jet system according to claim 5, wherein the nozzle head (10) has a plurality
of nozzles (12) and a plurality of channel-like cavities (20) arranged side-by-side,
and wherein the medium (28) is a fluid contained in a reservoir (22) defined in the
first block member (14) and extending across a plurality of cavities (20).
7. Ink-jet system according to claim 6, wherein heating electrodes (30) associated with
each of the cavities (20) and extending in longitudinal direction of the channel-like
cavities are formed by a conductive pattern on the flexible plate (18), the ends of
the heating electrodes (30) adjacent to the nozzles (12) being interconnected by another
conductor (32) of said conductive pattern.