Abstract:
Disclosed is an inkjet print cartridge including an ink supply, a substrate having a plurality of individual ink ejection chambers defined by a barrier layer formed on a first surface of the substrate and having an ink ejection element in each of the ink ejection chambers, for ejecting drops of ink having a predetermined drop volume and drop velocity. The ink ejection chambers each have the same inlet channel length and are arranged in an array spaced so as to provide a predetermined resolution. A nozzle member having a plurality of ink orifices formed therein is positioned to overlie the barrier layer with the orifices aligned with the ink ejection chambers. An ink channel connects the reservoir with the ink ejection chambers. The inkjet print cartridge has several advantages of over previous printing systems in creating high quality images by using very small individual ink drops of low volume and high velocity. Highlight regions may be formed by using single low volume drops to form a dot. The individual drops are nearly invisible and can be used to form highlights with low graininess. As the density of the image increases, multi-drop dots are formed from two or more drops merging on the media to form a composite drop.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 08/960,927 filed concurrently herewith, entitled “Multi-Drop Merge on Media Printing System”; U.S. patent application Ser. No. 08/960,945 filed concurrently herewith, entitled “Apparatus and Method for Generating High Frequency Ink Ejection and Ink Chamber Refill” and U.S. patent application Ser. No. 08/796,835, filed Feb. 6, 1997, entitled “Fractional Dot Column Correction for Scan Axis Alignment During Printing.” The foregoing commonly assigned patent applications are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to inkjet printers and more particularly to apparatus and methods for generating photographic quality images on a color inkjet printer. 
     BACKGROUND OF THE INVENTION 
     Thermal inkjet hardcopy devices such as printers, large format plotters/printers, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in “Ink Jet Devices,” Chapter 13 of  Output Hardcopy Devices  (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. The basics of this technology are further disclosed in various articles in several editions of the  Hewlett - Packard Journal  [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper. 
     An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes termed “dot locations”, “dot positions”, or “pixels”. Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink. 
     Inkjet hardcopy devices print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed. 
     The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the ink ejection element. When electric printing pulses activate the ink ejection element, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper. 
     The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied by the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium. 
     In an inkjet printhead ink is fed from an ink reservoir integral to the printhead or an “off-axis” ink reservoir which feeds ink to the printhead via tubes connecting the printhead and reservoir. Ink is then fed to the various ink ejection chambers either through an elongated hole formed in the center of the bottom of the substrate, “center feed,” or around the outer edges of the substrate, “edge feed.” In center feed the ink then flows through a central slot in the substrate into a central manifold area formed in a barrier layer between the substrate and a nozzle member, then into a plurality of ink channels, and finally into the various ink ejection chambers. In edge feed ink from the ink reservoir flows around the outer edges of the substrate into the ink channels and finally into the ink ejection chambers. In either center feed or edge feed, the flow path from the ink reservoir and the manifold inherently provides restrictions on ink flow to the ink ejection chambers. 
     Color inkjet hardcopy devices commonly employ a plurality of print cartridges, usually two to four, mounted in the printer carriage to produce a full spectrum of colors. In a printer with four cartridges, each print cartridge can contain a different color ink, with the commonly used base colors being cyan, magenta, yellow, and black. In a printer with two cartridges, one cartridge can contain black ink with the other cartridge being a tri-compartment cartridge containing the base color cyan, magenta and yellow inks, or alternatively, two dual-compartment cartridges may be used to contain the four color inks. In addition, two tri-compartment cartridges may be used to contain six base color inks, for example, black, cyan, magenta, yellow, light cyan and light magenta. Further, other combinations can be employed depending on the number of different base color inks to be used. 
     The base colors are produced on the media by depositing a drop of the required color onto a dot location, while secondary or shaded colors are formed by depositing multiple drops of different base color inks onto the same or an adjacent dot location, with the overprinting of two or more base colors producing the secondary colors according to well established optical principles. 
     In color printing, the various colored dots produced by each of the print cartridges are selectively overlapped to create crisp images composed of virtually any color of the visible spectrum. To create a single dot on paper having a color which requires a blend of two or more of the colors provided by different print cartridges, the nozzle plates on each of the cartridges must be precisely aligned so that a dot ejected from a selected nozzle in one cartridge overlaps a dot ejected from a corresponding nozzle in another cartridge. 
     The print quality produced from an inkjet device is dependent upon the reliability of its ink ejection elements. The ability to achieve good tone scale is crucial to achieving photographic image quality. In the highlight region of the tone scale, nearly invisible dots and lack of graininess are required. Areas of solid fill require saturated colors, high optical density and no white space. Also, the ability to place more than one nearly imperceptible drop from a given printhead into a pixel is essential to achieving this photographic image quality. 
     Another solution for achieving good tone scales is to use a six-ink printing system. This approach uses black ink, yellow ink, light cyan ink, dark cyan ink, light magenta ink and dark magenta ink. Good image quality is achieved in highlight regions by using only the yellow, light cyan and light magenta inks. The black, dark cyan and dark magenta inks are used in more saturated areas of the image. The disadvantages of this system are (1) the complexity of having a six-ink system (more inks, more complicated color maps and product cost and size, and (2) transitions that degrade image quality are observed in the tone scale when the dark cyan and dark magenta, which are highly visible, are first used. 
     Another approach to form different dot sizes is to use multiple drop volumes on the same printhead (See, U.S. Pat. No. 4,746,935). The primary disadvantage of this approach is the need for multiple drop generators which increases cost and complexity. 
     Even when using the above described methods and apparatus, the creation of crisp and vibrant images with accurate tone equal to those produced by conventional silver halide photography has not been achieved. 
     Due to the increasing use of digital cameras to produce digital images and the use of scanners to input conventional photographs into personal computers, the demand has rapidly increased for printers which can produce photographic quality prints from these images. Accordingly, there is a need for printers which can produce photographic quality prints. 
     SUMMARY OF THE INVENTION 
     The present invention is an inkjet print cartridge including an ink supply, a substrate having a plurality of individual ink ejection chambers defined by a barrier layer formed on a first surface of the substrate and having an ink ejection element in each of the ink ejection chambers, for ejecting drops of ink having a predetermined drop volume and drop velocity. The ink ejection chambers each have the same inlet channel length and are arranged in an array spaced so as to provide a predetermined resolution. A nozzle member having a plurality of ink orifices formed therein is positioned to overlie the barrier layer with the orifices aligned with the ink ejection chambers. An ink channel connects the reservoir with the ink ejection chambers. The present invention also includes a printer wherein the print cartridge is mounted in a scanning carriage. 
     The present invention has several advantages of over previous printing systems in creating high quality images by using very small individual ink drops of low volume. Highlight regions may be formed by using single low volume drops to form a dot. The individual drops are nearly invisible and can be used to form highlights with low graininess. As the density of the image increases, multi-drop dots are formed from two or more drops merging on the media to form a composite drop. Another advantage of the present invention is that drop velocity and volume are much less sensitive to ink viscosity and surface tension. Previous architectures required higher viscosity inks with higher surface tension which also required media which is not acceptable for high quality photographic imaging. The present invention can utilize inks having much lower viscosities and surface tensions and allows the use of media that closely resembles the paper used in silver halide photographic prints. The present invention&#39;s less sensitivity to ink properties permits flexibility in designing an ink that will dry relatively quickly, while not compromising overall ink reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of one embodiment of an inkjet printer incorporating the present invention. 
     FIG. 2 is a top perspective view of a single print. 
     FIG. 3 is a bottom perspective view a single print cartridge. 
     FIG. 4 is a schematic perspective view of the back side of a simplified printhead assembly. 
     FIG. 5 is a top perspective view, partially cut away, of a portion of the TAB head assembly showing the relationship of an orifice with respect to a ink ejection chamber, a heater ink ejection element, and an edge of the substrate. 
     FIG. 6 is a cross-sectional view of the printhead assembly showing the flow of ink to the ink ejection chambers in the printhead. 
     FIG. 7 is a top plan view of a magnified portion of a printhead showing two ink ejection chambers and the associated barrier structure and ink ejection elements. 
     FIG. 8 is an elevational cross-sectional view of the printhead assembly of FIG. 7 showing the thickness of the barrier layer and the nozzle member. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention will be described below in the context of an off-axis printer having an external ink source, it should be apparent that the present invention is also useful in an inkjet printer which uses inkjet print cartridges having an ink reservoir integral with the print cartridge. 
     FIG. 1 is a perspective view of one embodiment of an inkjet printer  10  suitable for utilizing the present invention, with its cover removed. Generally, printer  10  includes a tray  12  for holding virgin paper. When a printing operation is initiated, a sheet of paper from tray  12 A is fed into printer  10  using a sheet feeder, then brought around in a U direction to now travel in the opposite direction toward tray  12 B. The sheet is stopped in a print zone  14 , and a scanning carriage  16 , supporting one or more print cartridges  18 , is then scanned across the sheet for printing a swath of ink thereon. After a single scan or multiple scans, the sheet is then incrementally shifted using a conventional stepper motor and feed rollers to a next position within the print zone  14 , and carriage  16  again scans across the sheet for printing a next swath of ink. When the printing on the sheet is complete, the sheet is forwarded to a position above tray  12 B, held in that position to ensure the ink is dry, and then released. 
     The carriage  16  scanning mechanism may be conventional and generally includes a slide rod  22 , along which carriage  16  slides, a flexible circuit (not shown in FIG. 1) for transmitting electrical signals from the printer&#39;s microprocessor to the carriage  16  and print cartridges  18  and a coded strip  24  which is optically detected by a photodetector in carriage  16  for precisely positioning carriage  16 . A stepper motor (not shown), connected to carriage  16  using a conventional drive belt and pulley arrangement, is used for transporting carriage  16  across print zone  14 . 
     The features of inkjet printer  10  include an ink delivery system for providing ink to the print cartridges  18  and ultimately to the ink ejection chambers in the printheads from an off-axis ink supply station  30  containing replaceable ink supply cartridges  31 ,  32 ,  33 , and  34 , which may be pressurized or at atmospheric pressure. For color printers, there will typically be a separate ink supply cartridge for black ink, yellow ink, magenta ink, and cyan ink. Four tubes  36  carry ink from the four replaceable ink supply cartridges  31 - 34  to the print cartridges  18 . 
     Referring to FIGS. 2 and 3, a flexible tape  80  containing contact pads  86  leading to electrodes  87  (not shown) on printhead substrate  88  is secured to print cartridge  18 . These contact pads  86  align with and electrically contact electrodes (not shown) on carriage  16 . An integrated circuit chip or memory element  78  provides feedback to the printer regarding certain parameters such as nozzle trajectories and drop volumes of print cartridge  18 . Tape  80  has a nozzle array, or nozzle member,  79  consisting of two rows of nozzles  82  which are laser ablated through tape  80 . An ink fill hole  81  is used to initially fill print cartridge  18  with ink. A stopper (not shown) is intended to permanently seal hole  81  after the initial filling. 
     A regulator valve (not shown) within print cartridges  18  regulates pressure by opening and closing an inlet hole to an ink chamber internal to print cartridges  18 . When the regulator valve is opened, hollow needle  60  is in fluid communication with an ink chamber (not shown) internal to the cartridge  18  and the off-axis ink supply. When in use in the printer  10 , the print cartridges  18  are in fluid communication with an off-carriage ink supply  31 - 34  that is releasably mounted in an ink supply station  30 . 
     Referring to FIGS. 3 and 4, printhead assembly  83  is preferably a flexible polymer tape  80  having a nozzle member array  79  containing nozzles  82  formed therein by laser ablation. Conductors  84  are formed on the back of tape  80  and terminate in contact pads  86  for contacting electrical contacts on carriage  16 . The other ends of conductors  84  are bonded to electrodes  87  of substrate  88  on which are formed the various ink ejection chambers and ink ejection elements. The ink ejection elements may be heater ink ejection elements or piezoelectric elements. 
     A demultiplexer (not shown) may be formed on substrate  88  for demultiplexing the incoming multiplexed signals applied to the electrodes  87  and distributing the address and primitive signals to the various ink ejection elements  96  to reduce the number of contact pads  86  required. The incoming multiplexed signals include address line and primitive firing signals. The demultiplexer enables the use of fewer contact pads  86 , and thus electrodes  87  than, ink ejection elements  96 . The demultiplexer may be any decoder for decoding encoded signals applied to the electrodes  87 . The demultiplexer has input leads (not shown for simplicity) connected to the electrodes  87  and has output leads (not shown) connected to the various ink ejection elements  96 . The demultiplexer demultiplexes the incoming electrical signals applied to contact pads  86  and selectively energizes the various ink ejection elements  96  to eject droplets of ink from nozzles  82  as nozzle array  79  scans across the print zone. Further details regarding multiplexing are provided in U.S. Pat. No. 5,541,269, issued Jul. 30, 1996, entitled “Printhead with Reduced Interconnections to a Printer,” which is herein incorporated by reference. 
     Preferably, an integrated circuit logic using CMOS technology should be placed on substrate  88  in place of the demultiplexer in order to decode more complex incoming data signals than just multiplexed address signals and primitive signals, thus further reduce the number of contact pads  86  required. The incoming data signals are decoded into address line and primitive firing signals and increase the speed of the signal processing. 
     Also formed on the surface of the substrate  88  using conventional photolithographic techniques is the barrier layer  104 , which may be a layer of photoresist or some other polymer, in which is formed the ink ejection chambers  94  and ink channels  132 . 
     FIG. 5 is an enlarged view of a single ink ejection chamber  94 , ink ejection elements  96 , and frustum shaped orifice  82  after the substrate structure is secured to the back of the flexible circuit  80  via the thin adhesive layer  106 . A side edge of the substrate  88  is shown as edge  114 . In operation, ink flows from the ink reservoir  12  around the side edge  114  of the substrate  88 , and into the ink channel  132  and associated ink ejection chamber  94 , as shown by the arrow  92 . Upon energization of the ink ejection element  96 , a thin layer of the adjacent ink is superheated, causing ink ejection and, consequently, causing a droplet of ink to be ejected through the orifice  82 . The ink ejection chamber  94  is then refilled by capillary action. 
     FIG. 6 illustrates the flow of ink  92  from the ink chamber  61  within print cartridge  18  to ink ejection chambers  94 . Energization of the ink ejection elements  96  cause a droplet of ink  101 ,  102  to be ejected through the associated nozzles  82 . A photoresist barrier layer  104  defines the ink channels and chambers, and an adhesive layer  106  affixes the flexible tape  80  to barrier layer  104 . Another adhesive  108  provides a seal between tape  80  and the plastic print cartridge body  110 . 
     The assembly of the printhead may be similar to that described in U.S. Pat. No. 5,278,584, by Brian Keefe, et al., entitled “Ink Delivery System for an Inkjet printhead,” assigned to the present assignee and incorporated herein by reference. 
     The frequency limit of a thermal inkjet pen is limited by resistance in the flow of ink to the nozzle. However, some resistance in ink flow is necessary to damp meniscus oscillation, but too much resistance limits the upper frequency at which a print cartridge can operate. The inlet channel geometry, barrier thickness, shelf length or inlet channel length which is the distance between the ink ejection elements and the edge of the substrate, must be properly sized to enable fast refill of ink into the ink chamber  94  while also minimizing sensitivity to manufacturing variations. As a consequence, the fluid impedance is reduced, resulting in a more uniform frequency response for all nozzles. An additional component to the fluid impedance is the entrance to the ink ejection chamber  94 . The entrance comprises a thin region between the nozzle  82  and the substrate  88  and its height is essentially a function of the thickness of the barrier layer  104 . This region has high fluid impedance, since its height is small. 
     To increase resolution and print quality, the printhead nozzles must be placed closer together. This requires that both heater ink ejection elements and the associated orifices be placed closer together. To increase printer throughput, the firing frequency of the ink ejection elements must be increased. When firing the ink ejection elements at high frequencies, conventional ink channel barrier designs either do not allow the ink ejection chambers to adequately refill or allow extreme blowback or catastrophic overshoot and puddling on the exterior of the nozzle member. Also, the closer spacing of the ink ejection elements create space problems and restricted possible barrier solutions due to manufacturing concerns. 
     FIGS. 7 an  8  show a printhead architecture that is advantageous when the printing of very high dot density, low drop volume, high drop velocity and high frequency ink ejection is required. However, at high dot densities and at high ink ejection rates cross-talk between neighboring ejection chambers becomes a serious problem. During the ejection of a single drop, initiated by an ink ejection element displaces ink out of nozzle  82  in the form of a drop. At the same time, ink is also displaced back into the ink channel  132 . The quantity of ink so displaced is often described as “blowback volume.” The ratio of ejected volume to blowback volume is an indication of ejection efficiency. In addition to representing an inertial impediment to refill, blowback volume causes displacements in the menisci of neighboring nozzles. When these neighboring nozzles are fired, such displacements of their menisci cause deviations in drop volume from the nominally equilibrated situation resulting in non-uniform dots being printed. An embodiment of the present invention shown in the printhead assembly architecture of FIG. 7 is designed to minimize such cross-talk effects. 
     The ink ejection chambers  94  and ink channels  132  are shown formed in barrier layer  104 . Ink channels  132  provide an ink path between the source of ink and the ink ejection chambers  94 . The flow of ink into the ink channels  132  and into the ink ejection chambers  94  is via ink flow around the side edges  114  of the substrate  88  and into the ink channels  132 . The ink ejection chambers  94  and ink channels  132  may be formed in the barrier layer  104  using conventional photolithographic techniques. The barrier layer  104  may comprise any high quality photoresist, such as Vacrel™ or Parad™. 
     Ink ejection elements  96  are formed on the surface of the silicon substrate  88 . As previously mentioned, ink ejection elements  96  may be well-known piezoelectric pump-type ink ejection elements or any other conventional ink ejection elements. Peninsulas  149  extending out to the edge of the substrate provide fluidic isolation of the ink ejection chambers  94  from each other to prevent cross-talk. The pitch D of the ink ejection chambers  94 , shown below in Table II, provides for 600 dots per inch (dpi) printing using two rows of ink ejection chambers  94 . 
     While the ink ejection elements and ink ejection chambers are shown as essentially being square in FIG. 7, it will be appreciated that they can be retangular or circular in shape. 
     The definition of the dimensions of the various elements shown in FIGS. 7 and 8 are provided in Table I. 
     
       
         
               
             
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 DEFINITIONS FOR DIMENSIONS OF PRINTHEAD ARCHITECTURE 
               
             
          
           
               
                 Dimension 
                 Definition 
               
               
                   
               
               
                 B 
                 Barrier Thickness 
               
               
                 C 
                 Nozzle Member Thickness 
               
               
                 D 
                 Orifice/Ink Ejection Element Pitch 
               
               
                 F 
                 Ink Ejection Element Length 
               
               
                 G 
                 Ink Ejection Element Width 
               
               
                 H 
                 Nozzle Entrance Diameter 
               
               
                 I 
                 Nozzle Exit Diameter 
               
               
                 J 
                 Chamber Length 
               
               
                 K 
                 Chamber Width 
               
               
                 M 
                 Channel Length 
               
               
                 N 
                 Channel Width 
               
               
                 O 
                 Barrier Peninsula Width 
               
               
                 P 
                 Entrance Chamber Gap 
               
               
                 Q 
                 Back Wall Chamber Gap 
               
               
                 R 
                 Side Chamber Gap 
               
               
                 S 
                 Side Chamber Gap 
               
               
                 U 
                 Inlet Channel Length 
               
               
                   
               
             
          
         
       
     
     Table II lists the nominal values, as well as their preferred ranges, of some of the dimensions of the printhead assembly structure of FIGS. 7 and 8. It should be understood that the preferred ranges and nominal values of an actual embodiment will depend upon the intended operating environment of the printhead assembly, including the type of ink used, the operating temperature, the printing speed, and the dot density. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 INK CHAMBER DIMENSIONS IN MICRONS 
               
             
          
           
               
                   
                 Dimension 
                 Minimum 
                 Nominal 
                 Maximum 
               
               
                   
                   
               
             
          
           
               
                   
                 B 
                 8 
                 14 
                 20 
               
               
                   
                 C 
                 15 
                 25.4 
                 39 
               
               
                   
                 D 
                   
                 84.7 
               
               
                   
                 F 
                 11 
                 17 
                 23 
               
               
                   
                 G 
                 11 
                 17 
                 23 
               
               
                   
                 H 
                 24 
                 34 
                 44 
               
               
                   
                 I 
                 8 
                 12 
                 14 
               
               
                   
                 J 
                 20 
                 27 
                 38 
               
               
                   
                 K 
                 20 
                 27 
                 38 
               
               
                   
                 M 
                 15 
                 30 
                 45 
               
               
                   
                 N 
                 12 
                 20 
                 30 
               
               
                   
                 O 
                 10 
                 23 
                 40 
               
               
                   
                 P 
                 2 
                 6 
                 12 
               
               
                   
                 Q 
                 2 
                 6 
                 9 
               
               
                   
                 R 
                 2 
                 5 
                 9 
               
               
                   
                 S 
                 2 
                 5 
                 9 
               
               
                   
                 U 
                 70 
                 160 
                 220 
               
               
                   
                   
               
             
          
         
       
     
     FIGS. 7 and 8 and Table II show the design features and dimensions characteristics of printheads which can be used to successfully print photographic quality images at a high drop velocities and a constant small drop volume of less than 10 picoliters. The printhead architecture design is a key factor of the present invention. Flex circuit  80  thickness has to be matched to the dimensions of the ink channel  132 , ejection chamber  94 , ink ejection element  96 , barrier  104  thickness and design, as well as the ink formulation. Simply reducing the horizontal dimensions F, G, H, I, J and K of the ink chamber  94  reduces the volume of the ejected drops, but creates a low drop ejection velocity. Referring to Table III, a standard 2-mil. (50.8 micron) flex circuit  80  and a nozzle outlet diameter of 14 microns creates a long nozzle with a C/I of approximately 4.0. Consequently, drops are ejected at a velocity of approximately 3.5-7.5 meters/second which is too low. These low velocity drops can lead to nozzle plugs, mis-direction, and thermal inefficiency. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                 Nozzle 
                 Barrier 
                 Orifice 
                 Resistor 
                   
                 Drop 
                 Drop 
               
               
                 Thickness 
                 Thickness 
                 Diameter 
                 Size 
                   
                 Volume 
                 Velocity 
               
               
                 C 
                 B 
                 I 
                 F, G 
                 C/I 
                 Picoliters 
                 meters/sec 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 50.8 
                 14 
                 14 
                 17 
                 3.6 
                 3.5 
                 3.0 
               
               
                 50.8 
                 14 
                 14 
                 21 
                 3.6 
                 5.9 
                 7.5 
               
               
                 25.4 
                 14 
                 12 
                 17 
                 2.1 
                 5.3 
                 14.0 
               
               
                   
               
             
          
         
       
     
     Referring to again to Table III, the ink ejection chamber  94  can eject small drops in high frequency bursts when the nozzle  82  thickness is matched to ink ejection element  96  size, barrier  104  thickness, and nozzle  82  exit diameter. As shown in Table III, drop velocity is nearly doubled when the nozzle  82  or flex circuit  80  thickness is reduced from 50.8 microns to 25.4 micron. The surprising result of using a 25.4-micron flex circuit  80  or nozzle  82  leads to a robust, reliable design that is thermally efficient. 
     The present invention has several advantages over previous printing systems and methods. The drop volume and velocity of the individual drops in high frequency bursts in the range of 15 to 60 kHz remain nearly constant at approximately 3-5 picoliters (pl) and velocities greater than 10 meters per second (m/s), respectively. In previous printhead architectures the first drop ejected from the ink ejection chamber  94  was the largest and slowest drop. Successive drops after the first ejected drop were significantly lower in volume. However, to create a smooth gray level ramp, it is desirable to have precisely the opposite effect, i.e., a smaller, nearly imperceptible first drop, followed by successive drops of larger cummulative volume. In addition, drops with low velocity are undesirable because they cannot clear mild nozzle plugs and are easily misdirected by puddles on the nozzle member surface. 
     Another advantage of the present invention is that the design of the ink ejection chamber and ink inlet channel allows for high frequency ink refill of the ink ejection chamber. The ink ejection chamber refill frequency must at least equal to the ink ejection frequencies of 15 to 60 kHz. 
     Another advantage of the present invention is that drop velocity and volume are much less sensitive to ink viscosity and surface tension. Previous multi-drop architectures required higher viscosity ink (approximately 10 centipoise) and higher surface tension (approximately 50 dynes/cm), e.g., a 70% diethylene glycol/30% H 2 O mix. Such inks also required the use of paper which is not acceptable for photographic quality imaging. The present invention can use inks which have a viscosity of approximately 1.5 centipoise and a surface tension of approximately 25 dynes/cm. This allows the use of a gelatin or voided media that closely resembles the paper used in the 35 mm film/photo industry. Less sensitivity to ink properties also permits flexibility in designing an ink that will dry relatively quickly, but does not compromise overall reliability. 
     Other advantages of the present invention are: (1) individual drops remain nearly constant in volume for bursts of one to eight drops at high frequencies. This allows smooth gray level ramps, which is a fundamental requirement in high quality imaging, and (2) does not require ink viscosity and dynamic surface tension that are incompatible with imaging media, lightfastness, waterfastness, and dry time goals. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made within departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.