Abstract:
A method for producing an apparatus for ejecting liquid droplets includes the steps of providing a porous medium having a plurality of pores therein, forming a plurality of holes through the porous medium and assembling with the porous medium a plurality of transducers and nozzles. At least some of the plurality of pores are in the porous medium, and enable a liquid to flow into a liquid cavity prior to the liquid droplet ejection. Each transducer corresponds to a corresponding liquid cavity, a corresponding nozzle and a corresponding hole, generally disposed along the same axis. Each transducer is operative to eject liquid droplets through the corresponding nozzle via the corresponding hole.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 08/276,572, filed on Jul. 18, 1994, now U.S. Pat. No. 5,940,099, and incorporated by reference herein. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to liquid droplet ejection systems and, more particularly, ink jet system and, even more particularly, to drop-on-demand ink jet systems. 
     Ink jet systems generally fall into two categories—continuous systems and drop-on-demand systems. Continuous ink jet systems operate by continuously ejecting droplets of ink, some of which are deflected by some suitable means prior to reaching the substrate being imprinted, allowing the undeflected drops to form the desired imprinting pattern. In drop-on-demand systems, drops are produced only when and where needed to help form the desired image on the substrate. 
     Drop-on-demand ink jet systems can, in turn, be divided into two major categories on the basis of the type of ink driver used. Most systems in use today are of the thermal bubble type wherein the ejection of ink droplets is effected through the boiling of the ink. Other drop-on-demand ink jet systems use piezoelectric crystals which change their planar dimensions in response to an applied voltage and thereby cause the ejection of a drop of ink from an adjoining ink chamber. 
     Typically, a piezoelectric crystal is bonded to a thin diaphragm which bounds a small chamber or cavity full of ink or the piezoelectric crystal directly forms the cavity walls. Ink is fed to the chamber through an inlet opening and leaves the chamber through an outlet, typically a nozzle. When a voltage is applied to the piezoelectric crystal, the crystal attempts to change its planar dimensions and, because the crystal is securely connected to the diaphragm, the result is the bending of the diaphragm into the chamber. The bending of the diaphragm effectively reduces the volume of the chamber and causes ink to flow out of the chamber through both the inlet opening and the outlet nozzle. The fluid impedances of the inlet and outlet openings are such that a suitable amount of ink exits the outlet nozzle during the bending of the diaphragm. When the diaphragm returns to its rest position ink is drawn into the chamber so as to refill it so that it is ready to eject the next drop. 
     Thermal bubble systems, although highly desirable for a variety of applications, suffer from a number of disadvantages relative to piezoelectric crystal systems. For example, the useful life of a thermal bubble system print head is considerably shortened, primarily because of the stresses which are imposed on the resistor protecting layer by the collapsing of bubbles. In addition, because of the inherent nature of the boiling process, it is relatively difficult to precisely control the volume of the drop and its directionality. As a result, the produced dot quality on a substrate may be less than optimal. 
     Still another drawback of thermal bubble systems is related to the fact that the boiling of the ink is achieved at high temperatures, which calls for the use of inks which can tolerate such elevated temperatures without undergoing either mechanical or chemical degradation. As a result of this limitation, only a relatively small number of ink formulations, generally aqueous inks, can be used in thermal bubble systems. 
     These disadvantages are not present in piezoelectric crystal drivers, primarily because piezoelectric crystal drivers are not required to operate at elevated temperatures. Thus, piezoelectric crystal drivers are not subjected to large heat-induced stresses. For the same reason, piezoelectric crystal drivers can accommodate a much wider selection of inks. Furthermore, the shape, timing and duration of the ink driving pulse is more easily controlled. Finally, the operational life of a piezoelectric crystal driver, and hence of the print head, is much longer. The increased useful life of the piezoelectric crystal print head, as compared to the corresponding thermal bubble device, makes it more suitable for large, stationary and heavily used print heads. 
     Piezoelectric crystal drop-on-demand print heads have been the subject of much technological development. Some illustrative examples of such developments include U.S. Pat. Nos. 5,087,930 and 4,730,197, which are incorporated by reference in their entirety as if fully set forth herein and which disclose a construction having a series of stainless steel layers. The layers are of various thicknesses and include various openings and channels. The various layers are stacked and bonded together to form a suitable fluid inlet channel, pressure cavity, fluid outlet channel and orifice plate. 
     The systems disclosed in the above-referenced patents illustrate the use of a fluid inlet channel having a very small aperture, typically, 100 microns or less. The use of a very small aperture is dictated by the desirability of limiting the backflow from the ink cavity during ejection of a drop but is problematic in that the small aperture is susceptible to clogging during the bonding of layers as well as during normal operation of the print head. 
     The construction disclosed in the above-referenced patents requires the very accurate alignment of the various layers during manufacture, especially in the vicinity of the small apertures which form portions of the fluid path. Furthermore, the openings in the orifice plate which form the outlets of the various flow channels have sharp edges which could have adverse effects on the fluid mechanics of the system. 
     Additionally, the techniques used in forming the openings in the orifice plate, which typically include punching, chemical etching or laser drilling, require that the thickness of the orifice plate be equal to, or less than, the orifice diameter which is itself limited by resolution considerations to about 50 microns. 
     Finally, any air bubbles trapped inside the flow channel cannot easily be purged and, because the bubbles are compressible, their presence in the system can have detrimental effects on system performance. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a liquid droplet ejection device, comprising: (a) a plurality of liquid ejection nozzles; (b) a liquid supply layer including porous material, the liquid supply layer featuring holes related to the nozzles; and (c) a plurality of transducers related to the holes for ejecting liquid droplets out through the nozzles. 
     In preferred embodiments of devices according to the present invention, the porous material includes sintered material, most preferably, sintered stainless steel. 
     According to one embodiment of the present invention, the transducers are piezoelectric elements, the nozzles are the outlets of capillaries and the device further comprises: (d) a deflection plate, the piezoelectric elements being connected to the deflection plate; and (e) a liquid cavity layer formed with cutouts therethrough, the cutouts being related to the piezoelectric elements, the liquid cavity layer adjoining the deflection plate, the liquid cavity layer adjoining the liquid supply layer, the holes of the liquid supply layer being related to the cutouts, the capillaries located in the holes, the liquid supply layer being configured so that liquid is able to flow from the porous material into the cutouts. 
     According to another embodiment of the present invention, the liquid cavity layer is omitted and the deflection layer directly adjoins the liquid supply layer. 
     According to yet other embodiments of the present invention, the nozzles are formed by an orifice plate which adjoins the liquid supply layer, which may, in turn, adjoins the deflection plate or the liquid cavity layer, when present. 
     According to other embodiments of the present invention, the transducers are heat elements and droplet ejection is effected by the thermal bubble method, rather than through the use of piezoelectric elements. 
     The ejection of ink drops using a device according to one embodiment of the present invention is accomplished as follows: A pressure pulse is imparted to a volume of ink in an ink cavity through the deflection of a thin deflection plate, or diaphragm, located on top of the ink cavity. The plate is deflected downward by the action of a piezoceramic crystal whenever a voltage is applied across its electrodes, one of which is in electrical contact with the usually metallic deflection plate. 
     The pressure pulse created by the downward bending of the deflection plate drives the ink towards and through an outlet, preferably a glass capillary having a convergent nozzle at its outlet end, causing the ejection of a drop of a specific size. 
     When the piezoelectric crystal is de-energized, it returns to its equilibrium position, reducing the pressure in the ink cavity and causing the meniscus at the outlet end of the glass capillary to retract. 
     The retracted meniscus generates a capillary force in the glass capillary which acts to pull ink from an ink reservoir into the ink cavity and into the glass capillary. The refilling process ends when the meniscus regains its equilibrium position. 
     In alternative embodiments of devices of the present invention there are provided systems similar to those presented above but which, instead of relying on piezoelectric elements and a deflecting plate, features heating elements which serve to boil the ink, thereby causing its ejection. 
     A key element in print heads according to the present invention is the presence of porous material which is in hydraulic communication with both the ink reservoir and the individual ink cavities. Preferably, the glass capillaries are embedded in openings in the porous material. The porous material preferably also defines part of the walls of the ink cavities. 
     Proper selection of the porous material makes it useful as a filter, serving to prevent any foreign particles which may be present in the ink from reaching the nozzles and possibly blocking them. 
     It will be readily appreciated that in order to achieve high drop ejection rates, the time required to refill the ink cavity following ejection of a drop must be as short as possible. The refilling time can be reduced by reducing the restriction to flow into the ink cavity. However, reduction of the restriction to inflow tends to increase the adverse effects of cross talk, i.e., the undesired interactions between separate ink cavities. 
     The optimization of the system in terms of the conflicting requirements of low cross talk and high refill rate can be effected through the judicious selection of a porous material having optimal characteristics for the intended application, taking into account, in addition, the viscosity of the ink and the nozzle geometry. The important characteristics of the porous material include the pore size and the permeability to flow (together referred to as “micron grade”), as well as the macro and micro geometries of the porous material. 
     As stated above, the optimal balance between the in-flow of ink into the ink cavity and its out-flow from the cavity is also affected by the ink viscosity and nozzle dimensions. The lower the viscosity of the ink, the faster is the refilling rate of the ink cavity but the more pronounced is the cross talk between separate cavities. Also, the smaller the outlet nozzle diameter, the more pronounced is the capillary action of the nozzle and hence, the higher is the refilling rate. 
     Ink jet print heads are generally designed so that the dimensions of the ink channels into and out of the ink cavity are such that the channels have acoustic impedances which are optimal for a specific ink of a given viscosity and for a specific nozzle diameter. If it is desired to use a print head with a different nozzle diameter and/or with a different viscosity ink, the print head channels must be redesigned to accommodate the new nozzle diameter and/or different viscosity ink. 
     By contrast, use of a porous material according to the present invention, makes it possible to preserve the same print head geometry and structure even when ink of a different viscosity and/or when a different nozzle geometry are to be used. The optimization of the acoustic impedances of the channels can be effected merely through the proper selection of a suitable porous material having suitable characteristics, such as a suitable micron grade. 
     Apart from the ability to optimize the print head without the need to redesign the flow channels, use of porous materials according to the present invention eliminates the small, and easily clogged, ink inlet apertures leading to the ink cavities. 
     Still another advantage offered by the use of the porous material according to the present invention is the materials ability to act as a filter, thereby reducing, or even completely obviating, the need for special filtration of the in-flowing ink. 
     Finally, the fabrication of print heads including porous material according to the present invention can be effected using simple production techniques without the need for complex and expensive micro-machining. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1 is an exploded perspective view of an ink jet print head of the piezoelectric element type according to a preferred embodiment of the present invention; 
     FIG. 2 is an assembled side cross-sectional view of the print head of FIG. 1; 
     FIG. 2A is an assembled side cross-sectional view of an alternative print head similar to the embodiment of FIG. 1 but using the thermal bubble type featuring heating elements connected to the lower surface of the top plate; 
     FIG. 3 is an assembled side cross-sectional view of another embodiment of an ink jet print head similar to the embodiment of FIG. 1 but without the ink cavity layer; 
     FIG. 4 is an assembled side cross-sectional view of yet another embodiment of an ink jet print head according to the present invention similar to the embodiment of FIG. 1 but using an orifice plate instead of glass capillaries; 
     FIG. 4A is an assembled side cross-sectional view of an embodiment as in FIG. 4 but without an ink cavity layer; 
     FIG. 5 is a schematic depiction of a skewed arrangement of nozzles in a multi-nozzle print head; 
     FIG. 6 is a partial plan view of a number of print heads according to the present invention assembled on a frame; 
     FIG. 7 is a schematic depiction of a printer with two-dimensional motion wherein both the print head and the substrate move. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is of an ink jet print head which can replace conventional print heads and which has improved properties as described herein. 
     Although the description throughout is largely related to systems for ejecting drops of ink for purposes of printing, it will readily be appreciated that systems and methods according to the present invention are not limited to the ejection of ink and that such systems and methods are also suitable for the ejection of a large variety of incompressible fluids, or liquids. It is intended that the applications systems according to the present invention to all of these liquids be included within the scope of the present invention. The description of the present invention, which is largely confined to ink jet printing applications is illustrative only, and is not intended to limit the scope of the present invention. It is believed that systems according to the present invention can be usefully applied to eject droplets of a variety of incompressible fluids having a surface tension greater than about 40 dynes/cm and a viscosity lower than about 50 cps. 
     The principles and operation of a print head according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Referring now to the drawings, FIGS. 1 and 2 illustrate the structure of a preferred embodiment of a print head according to the present invention in exploded perspective view and in assembled side cross-sectional view, respectively. 
     The structure of the preferred embodiment of the print head includes three layers—an activation layer  10 , an ink cavity layer  16  and an ink supply layer  20 . 
     Activation layer  10  includes a diaphragm, or deflection plate  12 , which may be made of any suitable material, including, but not limited to, stainless steel. Connected to the upper surface of deflection plate  12  are transducers, which are preferably piezoceramic elements, most preferably disk-shaped. The term ‘transducer’ is used herein to designate any mechanism which uses force or energy to cause a drop to eject, including, but not limited to piezoelectric elements and heating elements, as in the thermal bubble method described below, among others. For illustrative purposes, four piezoelectric elements  14  are shown in FIG. 1 but any convenient number may be used. 
     Deflection plate  12  is preferably made of stainless steel and is approximately 50 microns in thickness. Other materials, such as glass or alumina can be used, provided that the surface of deflection plate  12  to which the piezoelectric elements are bonded is an electrical conductor. This can be achieved by metallizing the surface, for example, through the use of nickel, gold or silver electrodes on both faces of piezoelectric elements  14 , which can then be readily bonded to the upper surface of deflection plate  12  by means of a thin layer of electrically conductive epoxy. 
     The range of suitable plate thicknesses is believed to be from about 30 to about 100 microns, depending on the specific material selected for the plate and its modulus of elasticity. 
     While piezoceramic elements  14 , typically made of PZT material, are, preferably, disk-shaped, they may be of other shapes, including, but not limited to, square, rectangular or octagonal. Disk-shaped piezoelectric elements are believed to be superior to their square or rectangular equivalents with regard to the efficiency of the transducer. The manufacturing cost of disk-shaped piezoelectric elements is, however, relatively high and requires the positioning of discrete elements on the deflection plate. The thickness of the piezoelectric elements is preferably from about 2 to about 2.5 times the thickness of deflection plate  12 . 
     The cost of the piezoelectric elements can be reduced without significant adverse effect on performance by first bonding a large piezoelectric sheet to deflection plate  12  and subsequently cutting the sheet into, for example, octagons by means of a diamond saw, a laser or selective chemical etching. 
     The diameter, or effective diameter, of the circular, or octagonal, piezoelectric element is preferably approximately 2 mm. Larger diameters can be used, subject to the limitation imposed by the maximum distance between adjacent ejection nozzles in the overall design of the print head. 
     Ink cavity layer  16 , preferably made of stainless steel sheet or of a polymer, such as polyimide, is located below activation layer  10 . Ink cavity layer  16  is formed with cutouts  18 , preferably circular, which are each aligned with a corresponding piezoelectric element  14  and each of which forms a separate ink cavity when the top surface of ink cavity layer  16  is bonded (FIG. 2) to the bottom surface of activation layer  10  and to the top surface of ink supply layer  20 . 
     Ink cavity layer  16  is preferably fabricated of stainless steel plate and preferably has a thickness of approximately 200 microns. The cross sectional area of cutouts  18 , is preferably about 10% larger than the cross sectional area of piezoelectric elements  14 , such as the PZT elements. A typical diameter of cutouts  18  might be approximately 2.2 mm. 
     Cutouts  18 , can be formed by various means, including, but not limited to, punching, laser cutting, EDM, chemical etching and drilling. 
     The ink cavities formed by cutouts  18  can be of any shape, such as, for example, square or circular, but should preferably be of the same shape as piezoelectric element  14  while having a cross sectional area which is about 10% larger than that of piezoelectric element  14 , as described above. 
     Ink cavity layer  16  may be bonded to deflection plate  12  in any suitable manner including, but not limited to, by means of epoxy adhesive or by brazing. 
     The thickness of ink cavity layer  16  defines the height of the ink cavities and, along with the size and shape of cutouts  18 , determines the volume of the ink cavities. Preferably, the volume of the ink cavities should be kept small in order to achieve significant pressure rises in the ink inside the cavity whenever deflection plate  12  bends downwards into the ink cavity. 
     The thickness of ink cavity layer  16  should preferably range from about 100 to about 200 microns. 
     Ink cavity layer  16  may alternatively be formed from an adhesive film or plate having a thickness as described above and having cutouts  18  which have been created in the layer through drilling or photoforming. 
     Ink cavity layer  16  is bonded on its lower surface to ink supply layer  20  which includes suitable porous material. Any suitable porous material may be used. Preferably, the porous material is a sintered material, most preferably, stainless steel porous plate of suitable characteristics. Sintered stainless steel is available from a number of suppliers, for example, from Mott Metallurgical Corp. of Connecticut, U.S.A., and comes in a variety of sheet sizes, thicknesses and micron grades. 
     Ink supply layer  20  is formed with holes  22  which extend continuously between the top and bottom surfaces of ink supply layer  20 , each hole  22  of ink supply layer  20  being associated with a particular circular cutout of ink cavity layer  16 . Holes  22  are smaller than cutouts  18 , allowing ink which enters porous ink supply layer  20  from an ink reservoir (not shown), for example, through its face  24 , to flow through the top surface of ink supply layer  20  into the ink cavities, as indicated by an arrow  26  (FIG.  2 ). 
     The centerlines of holes  22  in ink supply layer  20  and cutouts  18  in ink cavity layer  16  are preferably aligned. 
     Ink supply layer  20  has a thickness which preferably ranges from about 0.5 mm to several mm. 
     Holes  22 , which are preferably approximately 800 microns in diameter, are used to hold the glass capillaries, which are described below. Holes  22  can be made by any suitable technique including, but not limited to, machining by EDM, drilling by conventional means or drilling by laser. 
     In the preferred embodiment of the present invention, the porous material provides the structure which holds the glass capillaries  28  in place. As a result, the spacing of holes  22  and their diameters should be machined using close tolerances. EDM machining can provide tolerances as small as 0.005 mm while conventional drilling techniques give tolerances which can be as low as 0.01 mm. 
     The upper surface of porous ink supply layer  20  is preferably bonded to the lower surface of ink cavity layer  16  using epoxy of high viscosity or using dry epoxy film adhesive having suitably located holes. In the latter case, the holes in the dry epoxy film adhesive should be somewhat larger than cutouts  18  so as to prevent any adhesive from covering the open pores of the porous material in the cavity, e.g., in the region of arrow  26  (FIG.  2 ). Other methods such as, for example, brazing or diffusion bonding can be used provided that the bonding material does not penetrate the porous material, for example, by wicking action. 
     The porous material which makes up ink supply layer  20  preferably serves multiple functions: 
     (a) The porous material allows ink to flow from an ink reservoir, preferably through one or more of the side, top or bottom faces of the porous material, to the various separate ink cavities, preferably through the top faces of the ink cavities, as indicated by arrow  26  (FIG.  2 ), but the actual flow patterns will depend on the precise configuration; 
     (b) The porous material filters the ink throughout the ink&#39;s travel from the inlet portion of the porous medium at the ink reservoir and until the ink leaves the porous medium to enter an ink cavity; 
     (c) The porous material provides optimized acoustic impedances to optimize system performance, as discussed above; 
     (d) The porous medium provides a structure or a substrate in which the capillaries are properly mounted or held. 
     As will be readily appreciated, the micron grade and the surface area the porous material which is open for flow into the ink cavity has a crucial impact on the refill time of the ink cavities and hence on the maximum drop ejection rate, or frequency. 
     For example, for an open area of 4.2 mm 2  and a porous material of 0.5 micron grade, the maximum ejection frequency was found experimentally to be about 2 kHz for 100 picoliter drops of a fluid having viscosity of 1 cps. Using a 0.8 micron grade porous material and the same fluid and drop volume, the maximum ejection frequency was found to be about 4 kHz. 
     Connected to each hole  22  in ink supply layer  20  in some suitable fashion is an appropriate capillary  28 , preferably a glass capillary, which includes a straight capillary tube having a capillary inlet  30 , and a capillary outlet, or nozzle  32 . Preferably, capillary  28  is a converging capillary having a diameter of approximately 50 microns near its outlet, or nozzle  32  where drops are ejected. 
     Preferably, glass capillaries  28  are inserted into holes  22  of the porous ink supply layer  20 , in such a way that capillary inlet  30  is flush with the upper surface of ink supply layer  20  while capillary outlet  32  protrudes beyond the lower surface of ink supply layer  20 . An epoxy adhesive layer  34 , or similar material, may be used to fill in the space below ink supply layer  20  and between capillaries  28  and serves to hold glass capillaries  28  in place and to seal the lower surface of ink supply layer  20 . 
     Capillaries  28  are preferably glass capillaries made of quartz or borosilicate capillary tubes. The tubes in the preferred embodiment have an outer diameter of about 800±5 μm and an inner diameter of about 500±5 microns. A converging nozzle  32  is formed at end of capillary  28 . The fabrication of capillary  28  can be effected in various suitable ways. Preferably, the fabrication is accomplished by rotating the capillary while simultaneously heating it using, for example, a discharge arc or a laser beam targeted at a suitable location on the capillary. The heating serves to lower the viscosity of the glass. As the viscosity of the glass falls below a certain lower limit, the inner walls of the capillary at the location of heating begin to flow and converge radially inward, forming a narrow throat. The diameter of the throat of capillary  28 , as well as the geometry of the converging section, can be precisely controlled through control of the glass temperature and the duration of the heating. For applications in a print head having a resolution of 300 dots per inch (dpi), the throat diameter is preferably about 50 microns. Much smaller diameters can be achieved with the above method and may be desirable for certain applications. 
     Cutting the glass at the throat can be achieved using a high power laser beam which yields a clean polished surface. It is also possible to cut the capillary at the throat by a diamond saw and then polish the cut surface. The inlet end of the capillary may be cut in a similar manner. 
     To complete the fabrication, glass capillaries  28  are inserted into holes  22 , with their inlets  30  being flush with the upper surface of porous ink supply layer  20 . 
     In an alternative embodiment, shown in FIG. 2A, the device is similar to that shown in FIGS. 1 and 2, except for the elimination of piezoelectric elements  14  and their replacement by a plurality of heating elements  114 , which are used to boil the ink in the ink cavities producing the high pressure which causes its ejection, i.e., using the thermal bubble technique described above. Heating elements  114  are situated so as to be able to heat the ink located in the ink cavity, preferably connected to the lower surface of a top plate  112 , which is no longer flexible as was the case with deflection plate  12  (FIGS.  1  and  2 ). Preferably, heating elements  114  are suitably coated so as to eliminate the adverse effects of chemical and physical attack by the hot ink. Having illustrated the possibility of applying systems according to the present invention in the context of a thermal bubble system, the rest of the description will be confined, for purposes of illustration, to descriptions of additional embodiments of piezoelectric element systems, it being understood, that corresponding thermal bubble systems are also possible and are intended to fall within the scope of the present invention. 
     Shown in FIG. 3 is another embodiment of the present invention similar to that of FIGS. 1 and 2 but wherein ink cavity layer  16  (FIGS.  1  and  2 ) has been eliminated and ink cavities have been provided in an alternative manner, as described below. 
     In the embodiment of FIG. 3, ink supply layer  20 , includes porous material and features holes  22  of a diameter which is about 10% larger than the diameter of piezoelectric elements  14  and is typically in the range of from about 2 to about 2.5 mm. The centerlines of holes  22  are preferably aligned with those of piezoelectric elements  14 . Glass capillaries  28  have an outer diameter which is slightly smaller than the diameter of holes  22  with their centerlines being aligned with the centerlines of piezoelectric elements  14  and holes  22 . 
     Holes  22  are machined in such a way as to keep open the pores at the circumference of porous ink supply layer  20  which border on the upper portion of holes  22 . This allows ink to flow from the porous material into the ink cavities, as is described below. 
     Glass capillaries  28 , with outer diameter slightly smaller than the diameter of holes  22 , are inserted into holes  22 . Unlike the embodiment of FIGS. 1 and 2, wherein inlets  30  of capillaries  28  are placed so as to be flush with the upper surface of ink supply layer  20 , in the embodiment of FIG. 3 inlets  30  of capillaries  28  are positioned so as to be somewhat below the plane of the top surface of ink supply layer  20 , thereby forming ink cavities which are bounded by deflection plate  12  on top, by capillary  28  at the bottom and by inner walls of holes  22  in porous ink supply layer  20  on the sides. 
     The ink moves from porous ink supply layer  20  and enters the ink cavity as shown by the dashed arrow  36  (FIG.  3 ). The total area available for flow of ink during the refilling of the ink cavity following drop ejection can be calculated by multiplying the circumference of the ink cavity by its height. Again, as described in the preferred embodiment, the open area and the micron grade of the porous material is selected to provide optimal fluid impedances and system performance. 
     A third embodiment of the present invention is depicted in FIG.  4 . Here the structure of the print head is similar to that described in the preferred embodiment (FIGS.  1  and  2 ). However, glass capillaries  28  of FIGS. 1 and 2 have been replaced by an orifice plate  38  having a series of orifices  40 . 
     Orifice plate  38  with orifices  40  can be formed using any suitable material, preferably it is made of a thin sheet of glass, such as a fused silica sheet having a thickness in the range of from about 0.1 to about 1 mm. Each of orifices  40  can be formed by using a short pulse of a properly directed laser beam of an appropriate type. Through proper selection of beam intensity, diameter and pulse duration, an opening of approximately 50 microns can be formed with a bell mouth shape with the larger diameter opening on the side of the glass nearer the laser source. Preferably, the glass sheet is first bonded to the lower surface of ink supply layer  20  with orifices  40  being created after the bonding. Since the holes in ink supply layer  20  are much larger than the diameter of the laser beam, the formation of orifices  40  can readily be performed after the bonding of the glass sheet to ink supply layer  20  without adversely affecting the holes of ink supply layer  20 . Creating orifices  40  after the bonding of the glass sheet to ink supply layer  20  allows for the very precise location and spacing of orifices  40 . 
     Orifice plate  38  with orifices  40 , which are typically approximately 50 microns in diameter, can alternatively be formed by various other techniques including, but not limited to, electroplating. 
     Orifice plate  38  is bonded to the porous ink supply layer  20  in such a way that the centerlines of orifices  40  are aligned with corresponding holes  22  in porous ink supply layer  20 . 
     A fourth embodiment of the present invention is shown in FIG.  4 A. Here, as in the embodiment of FIG. 4, orifice plate  38  is used but, unlike the embodiment of FIG.  4  and similar to the embodiment of FIG. 3, ink cavity layer  16  has been eliminated and ink cavities have been provided in an alternative manner, as described above in the context of the embodiment of FIG.  3 . 
     Reference is now made to FIG. 5, which is a partial view from the paper side of a multi-nozzle print head. Shown in FIG. 5 is an arrangement of nozzles  32  laid out as an array made up of horizontal rows which are horizontally staggered, or skewed, with respect to one another. The print head preferably extends the full width of the paper. Writing over the full area of the paper is achieved by effecting relative vertical motion between the head and the paper  50 . For example, the print head may be stationary while the paper moves vertically. 
     The timing of the ejection of drops from any one row relative to any other row is made to be equal to the time of paper travel between such rows. Thus, for example, in order to write a solid horizontal line at a given vertical position on the paper, each row of nozzles is made to eject an ink drop when the given paper position passes opposite that row. 
     The extent of stagger between the various rows is such that, as the paper moves, the traces of ink drops from the various nozzles define non-overlapping, essentially equally spaced parallel lines. The spacing of these lines determines the effective horizontal resolution of the head. 
     The minimal distance between adjacent nozzles is determined by the maximum dimensions of the ink cavity of the transducer. This distance is typically ⅛ of an inch. Thus, the nozzles may be horizontally spaced, for example, 7.5 per inch. In order to achieve an effective horizontal resolution of 300 dots per inch, which is typical for a high quality printer, the total number of nozzles must, in this example, be 40 times that in a single row. Therefore, 40 mutually staggered rows are required in the complete head. 
     For reasons of efficient manufacturing and servicing, it is preferable to divide the print head horizontally or vertically into several identical sections, or modules  42 . FIG. 6 schematically shows an example of a head constructed out of such vertically adjacent modules  42 . A rigid frame  46  has along its sides a pair of registration pins  48  for each module. Pins  48  engage a hole  43  and a slot  44  at corresponding ends of module  42 . The horizontal positions of pins  48  are such as to locate each module  42  at its proper staggered position. 
     It will be appreciated that with a head, such as described above, printing at full resolution simultaneously across the full width of the paper, the achievable printing rate, in terms of pages per minute, can be relatively high—much higher than state-of-the-art drop-on-demand printers and comparable to presently available commercial laser printers. If a lower printing rate is sufficient, then a proportionately smaller head (i.e., one with fewer nozzles) may be utilized, but then two-dimensional motion between the head and the paper is necessary. 
     An embodiment of a printer with a two-dimensional motion is shown schematically in FIG.  7 . The head extends the full height of paper  50  and includes an array of a few, say, four, vertical rows which are vertically staggered so as to define equally spaced horizontal lines. The head moves repeatedly across the paper, ejecting ink drops along the horizontal lines. After each such crossing the paper moves vertically one resolution unit, so that the next set of horizontal ink traces is immediately adjacent the previous one. This process continues until the full interline space has been covered with traces. If, for example, each row has 7.5 nozzles per inch, the four rows define 30 lines per inch, spaced {fraction (1/30)} inch apart. It then takes ten passes of the head, with the paper moving {fraction (1/300)} inch at a time, to cover the entire page area. Such a printer may still be faster than the state-of-the-art drop-on-demand printers. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made, all of which are intended to fall within the scope of the present invention.