Abstract:
A print head provides for multi-level printing with colorants of different densities without print head replication. A continuous ink jet printer includes a plurality of ink sources; a print head connected to multiple ink sources; and apparatus adapted to selectively transfer ink from each of the connected sources to the print head or to block such transfer. The nozzles selectively create a streams of ink droplets having a plurality of volumes. The apparatus also includes a droplet deflector having a gas source to interact with the stream of ink droplets, thereby separating ink droplets into printing and non-printing paths. The apparatus includes a print heads which can be switched between “light” and “dark” ink sources. This allows multi-level printing, thus achieving higher print quality at the same resolution without incurring the costs associated with additional dedicated print heads.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of digitally controlled printing devices, and in particular to continuous ink jet printers in which a liquid ink stream breaks into droplets, some of which are selectively deflected.  
         BACKGROUND OF THE INVENTION  
         [0002]    Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. Both require independent ink supplies for each of the colors of ink provided. Ink is fed through channels to a nozzle set from which droplets of ink are selectively ejected. Typically, each technology requires separate ink delivery systems for each ink color used in printing.  
           [0003]    Conventional “drop-on-demand” ink jet printers utilize a pressurization actuator to produce the ink jet droplet at orifices of a print head. Typically, one of two types of actuators are used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink droplet to be expelled.  
           [0004]    The second technology, commonly referred to as “continuous stream” or simply as “continuous” ink jet printing, uses a pressurized ink source which produces a continuous stream of ink droplets. Some continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no printing is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism.  
           [0005]    U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member.  
           [0006]    U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and deflect thoses ink droplets. A print head includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a print media, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher. While this device is capable of high quality printing, it is limited to ink fluids which have a large viscosity change with temperature.  
           [0007]    U.S. Pat. No. 6,554,410, which issued to Jeanmaire et al. on Apr. 29, 2003, and U.S. patent application Ser. No. 09/751,232, filed Dec. 28, 2000, disclose continuous-jet printing methods wherein nozzles with annular heaters are selectively actuated at a plurality of frequencies to create the stream of ink droplets having the plurality of volumes. A gas stream then separates droplets into printing and non-printing paths according to drop volume. Larger droplets are directed to a recording media, whereas smaller droplets are captured in a plenum and recycled.  
           [0008]    For traditional color printing applications, three or four print heads are required (i.e., CMY or CMYK). The use of additional inks, for example, multiple concentrations of a colorant, can provide superior photographic reproduction as presented in U.S. Pat. No. 4,672,432 to Sakurada et al. in 1987. Six print heads were required, one for each of high density black, high density yellow, high density cyan, high density magenta, low density cyan and low density magenta. While this approach can improve the image quality for photographic printing, additional print heads significantly increase the cost of the apparatus.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to provide an improvement to continuous ink jet printers of type described by Jeanmaire and Chwalek. The features of low-power and low-voltage print head operation are desirable to retain, while providing for multi-level printing with colorants of different densities without the complexity of print head replication.  
           [0010]    In accordance with the present invention, a continuous ink jet printer includes a plurality of ink sources; a print head fluidly connected to multiple ink sources; and apparatus adapted to selectively transfer ink from each of the connected ink source to the print head or block such transfer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments of the invention and the accompanying drawings, wherein:  
         [0012]    [0012]FIG. 1 is a schematic representation of an ink jet print head made in accordance with a preferred embodiment of the present invention and showing fluidic connections;  
         [0013]    [0013]FIG. 2 is a side view of an ink jet print head and illustrating droplet separation;  
         [0014]    [0014]FIG. 3 is a cross-sectional view of an ink jet print head assembly made in accordance with a preferred embodiment of the present invention and highlighting droplet deflector and ink catcher assemblies; and  
         [0015]    [0015]FIG. 4 is a schematic view of an ink jet printer made in accordance with a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring to FIG. 1, an ink droplet forming mechanism  10  includes a print head  12  and associated fluidic connections. The print head consists of a row of nozzles  14  fabricated in a silicon die  16 . Die  16  is bonded to manifold  18  which has an integral ink manifold to provide fluid communication to the nozzles. In this example, provision is made for switching between two inks having differing concentrations of colorant. The principle of this invention is not limited to two inks, and that switching between larger numbers of ink sources is clearly within the scope of this invention. The ink sources are reservoirs  20  and  22 . Reservoir  20  contains a “dark” density ink, and reservoir  22  a “light” density ink. For printing with the dark ink, reservoir  20  is coupled to both ends of manifold  18  through fluid lines  24  and  26 . Electro-mechanical solenoid valves  28  and  30  either permit or block pressurized ink from flowing into manifold  18 . For printing with light ink, reservoir  22  is coupled to both ends of manifold  18  through fluid lines  32  and  34 . Solenoid valves  36  and  38  control ink flow into manifold  18 .  
         [0017]    When manifold  18  is supplied with pressurized ink, a fraction of the ink flowing into manifold  18  is jetted from the nozzles in die  16 . The balance of the ink flow is recirculated by exiting from the middle of manifold  18  into a recirculation line  40 . A four-way valve  42  directs the ink back to the active ink source through either line  44  or  46 . Dark ink flows into a circulation pump  48  which communicates with reservoir  20 , and light ink flows into a circulation pump  50  which communicates with ink reservoir  22 . Following each switching event between ink sources, valve  42  briefly connects recirculation line  40  to a line  52 . This permits several seconds of rapid purging to occur in manifold  18  to shorten the conversion time between inks. Ink collected during the purging time flows via line  52  to a container  54 , and is then periodically reprocessed for reuse. The ink in container  54  will be intermediate in colorant concentration between the light and dark ink in reservoirs  20  and  22 . Generally, it is most convenient in the printer system to combine this ink with the light ink recovered from the ink catcher assembly (discussed in more detail later), whereby make-up solvent can be added to this ink to re-condition the ink to the light ink colorant concentration.  
         [0018]    [0018]FIG. 2 illustrates one form of continuous ink jet technology, and is included as background material. Drop volume can be controlled in a known manner by controlling the electrical waveform to a heater  60 . In general, a rapid pulsing of heater  60  forms small ink droplets  62 , while slower pulsing creates larger droplets  64 . In the example presented here, small ink droplets  62  are to be used for marking on the image receiver, while larger droplets  64  are captured for ink recycling.  
         [0019]    In the drop formation for each image pixel, a non-printing large drop  64  is always created, in addition to a variable number of small, printing droplets  62 . All small, printing droplets  62  are the same volume, however the volume of the larger, non-printing droplets  64  varies depending on the number of small droplets  62  created in the pixel time interval, because the creation of small droplets takes mass away from the large drop during the pixel time interval P.  
         [0020]    The operation of print head  20  in a manner such as to provide an image-wise modulation of drop volumes, as described above, is coupled with an gas-flow discriminator which separates droplets into printing or non-printing paths according to drop volume. Ink is ejected through nozzle  14  in print head  12 , creating a filament  66  of working fluid moving substantially perpendicular to print head  12  along axis X. The physical region over which the filament of working fluid is intact is designated as r 1 . Heater  60  is selectively activated at various frequencies according to image data, causing filament  66  of working fluid to break up into a stream of individual ink droplets. Coalescence of droplets often occurs in forming non-printing droplets  64 . This region of jet break-up and drop coalescence is designated as r 2 . Following region r 2 , drop formation is complete in region r 3  and small, printing droplets and large, non-printing droplets are spatially separated. Beyond this region in r 4 , aerodynamic effects can cause merging of adjacent small and large droplets, with concomitant loss of imaging information. A discrimination force  68  is provided by a gas flow perpendicular to an axis X. The force acts over a distance L, which is less than or equal to distance r 3 . Large, non-printing droplets  64  have greater masses and more momentum than small volume droplets  62 . As gas force  68  interacts with the stream of ink droplets, the individual ink droplets separate depending on individual volume and mass. Accordingly, the gas flow rate can be adjusted to produce a sufficient differentiation angle D in a small droplet path S from a large droplet path K, permitting small droplets  62  to strike print media while large, non-printing droplets  64  are captured by a ink guttering structure described below.  
         [0021]    A preferred embodiment of a print head assembly is shown in cross-sectional view in FIG. 3, where the droplet deflector and ink catcher elements are emphasized. Large volume ink droplets  64  and small volume ink droplets  62  are formed from ink ejected from print head  12  substantially along ejection paths K and S, respectively. A droplet deflector  70  contains an upper plenum  72  and a lower plenum  74  which facilitate a laminar flow of gas in droplet deflector  70 . Pressurized air enters lower plenum  74  which is disposed opposite plenum  72  and promotes laminar gas flow while protecting the droplet stream moving along path X (FIG. 2) from external air disturbances. The application of force  68  due to gas flow separates the ink droplets into small-drop path S and large-drop path K.  
         [0022]    An ink collection structure  76 , disposed adjacent to lower plenum  74  near path X, intercepts path K of large droplets  64 , while allowing small ink droplets  62  traveling along small droplet paths S to continue on to a recording media. Large, non-printing ink droplets  64  strike an ink catcher  78  in ink collection structure  76 . Ink recovery conduits  80  and  82  return ink to separate recovery reservoirs (not shown). Negative pressure in conduits  80  and  82  facilitate the motion of recovered ink to the recovery reservoirs. The pressure reduction in conduits  80  and  82  is sufficient to draw in recovered ink, but is not large enough to cause significant air flow to substantially alter drop paths S. A valve  84  directs the flow of recovered ink into either conduit  80  or  82 , depending upon the source ink jetted from print head  20 .  
         [0023]    A small portion of the gas flowing through upper plenum  72  is re-directed by a plenum  86  to the entrance of ink collection structure  76 . The gas pressure in droplet deflector  70  is adjusted in combination with the design of plenums  74  and  72  so that the gas pressure in the print head assembly near ink catcher  78  is positive with respect to the ambient air pressure external to the print head assembly. Environmental dust and paper fibers are thusly inhibited from approaching and adhering to ink catcher  78  and are also excluded from entering ink recovery conduits  80  and  82 .  
         [0024]    An “O” ring  88  and a spill channel  90  provide a means to capture and recycle ink that comes from misdirected nozzles in print head  20  which fail to properly enter droplet deflector  70 .  
         [0025]    [0025]FIG. 4 is a schematic diagram illustrating a preferred embodiment of the ink fluidic system in a six-color printer. In this example, “light” and “dark” magenta inks and “light” and “dark” cyan inks are formulated with different concentrations of colorant. These inks are supplemented with a single yellow ink and a single black ink. Magenta inks are supplied to a print head assembly  100  from either a source reservoir  102  of “light” magenta ink or a source reservoir  104  of “dark” magenta ink, cyan inks are supplied to a print head assembly  106  from either a source reservoir  108  of “light” cyan ink or a source reservoir  110  of “dark” cyan ink, yellow ink is supplied from a source reservoir  114  to a print head assembly  112 , and black ink is supplied to a print head assembly  116  from a source reservoir  118 . Pressurized ink circulates through the print heads and back to appropriate ink mixing units  120 ,  122 ,  124 ,  126 ,  128  and  130  associated with the ink source reservoirs. Non-printing ink recovered from the ink catchers in the print head assemblies is directed into six circulation pumps  132 ,  134 ,  136 ,  138 ,  140  and  142 . The function of the ink recycling pumping units is to filter out particulates and re-adjust the colorant concentrations to match that in the source reservoirs  102 ,  104 ,  108 ,  110 ,  114  and  118  respectively.  
         [0026]    In operation, a recording media W is transported in a direction transverse to axis X by a print drum  144  in a known manner. Transport of recording media W is coordinated with movement of print mechanism  10  and the switching between “light” and “dark” inks in a known manner. Recording media W may be selected from a wide variety of materials including paper, vinyl, cloth, other fibrous materials, etc.  
       Parts List  
       [0027]    [0027] 10  ink droplet forming mechanism  
         [0028]    [0028] 12  print head  
         [0029]    [0029] 14  nozzles  
         [0030]    [0030] 16  silicon die  
         [0031]    [0031] 18  manifold  
         [0032]    [0032] 20  dark ink reservoir  
         [0033]    [0033] 22  light ink reservoir  
         [0034]    [0034] 24  “dark” ink supply line  
         [0035]    [0035] 26  “dark” ink supply line  
         [0036]    [0036] 28  solenoid valve  
         [0037]    [0037] 30  solenoid valve  
         [0038]    [0038] 32  “light” ink supply line  
         [0039]    [0039] 34  “light” ink supply line  
         [0040]    [0040] 36  solenoid valve  
         [0041]    [0041] 38  solenoid valve  
         [0042]    [0042] 40  Recirculation line  
         [0043]    [0043] 42  four-way valve  
         [0044]    [0044] 44  line  
         [0045]    [0045] 46  line  
         [0046]    [0046] 48  circulation pump  
         [0047]    [0047] 50  circulation pump  
         [0048]    [0048] 52  line  
         [0049]    [0049] 54  container  
         [0050]    [0050] 60  heater  
         [0051]    [0051] 62  small drop  
         [0052]    [0052] 64  large drop  
         [0053]    [0053] 66  filament  
         [0054]    [0054] 68  discrimination force  
         [0055]    [0055] 70  deflector  
         [0056]    [0056] 72  upper plenum  
         [0057]    [0057] 74  lower plenum  
         [0058]    [0058] 76  collection structure  
         [0059]    [0059] 78  catcher  
         [0060]    [0060] 80  conduit  
         [0061]    [0061] 82  conduit  
         [0062]    [0062] 84  valve  
         [0063]    plenum  
         [0064]    [0064] 88  O ring  
         [0065]    [0065] 90  spill channel  
         [0066]    [0066] 100  magenta print head assembly  
         [0067]    [0067] 102  “light” magenta ink source reservoir  
         [0068]    [0068] 104  “dark” magenta ink source reservoir  
         [0069]    [0069] 106  cyan print head assembly  
         [0070]    [0070] 108  “light” cyan ink source reservoir  
         [0071]    [0071] 110  “dark” cyan ink source reservoir  
         [0072]    [0072] 112  yellow print head assembly  
         [0073]    [0073] 114  yellow ink source reservoir  
         [0074]    [0074] 116  black print head assembly  
         [0075]    [0075] 118  black ink source reservoir  
         [0076]    [0076] 120  “light” magenta ink mixing unit  
         [0077]    [0077] 122  “dark” magenta ink mixing unit  
         [0078]    [0078] 124  “light” cyan ink mixing unit  
         [0079]    [0079] 126  “dark” cyan ink mixing unit  
         [0080]    [0080] 128  yellow ink mixing unit  
         [0081]    [0081] 130  black ink mixing unit  
         [0082]    [0082] 132  “light” magenta ink circulation pump  
         [0083]    [0083] 134  “dark” magenta ink circulation pump  
         [0084]    [0084] 136  “light” cyan ink circulation pump  
         [0085]    [0085] 138  “dark” cyan ink circulation pump  
         [0086]    [0086] 140  yellow ink circulation pump  
         [0087]    [0087] 142  black ink circulating pump  
         [0088]    [0088] 144  print drum