Abstract:
A method, and apparatus for performing the method, are intended to prevent all of the ink discharged from a defective one of multiple nozzles in a continuous inkjet printhead from being used for printing on a print medium. This can be done by periodically heating the defective nozzle at a frequency that is greater than frequencies other nozzles which are not defective are periodically heated, to cause the defective nozzle to only discharge ink droplets that have a smaller volume than ink droplets discharged from the nozzles that are not defective. Then, the smaller volume droplets discharged from the defective nozzle are prevented from reaching a print medium, but the larger volume ink droplets discharged from the nozzles that are not defective are allowed to reach the print medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    Reference is made to commonly assigned co-pending application Ser. No. 09/751,232, entitled CONTINUOUS INKJET PRINTING METHOD AND APPARATUS and filed Dec. 28, 2000 in the names of David L. Jeanmaire and James M. Chwalek.  
         [0002]    The cross-referenced application published Dec. 14, 2001 as European Patent Application No. EP 1219429A2 and is incorporated in this patent application. 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The invention relates generally to continuous inkjet printing, and in particular to preventing a defective nozzle ink discharge in a continuous inkjet printhead from being used for printing.  
         BACKGROUND OF THE INVENTION  
         [0004]    Typically in continuous inkjet printers, a pressurized ink is formed into continuous inkjet filaments projecting from multiple ink discharge nozzles in a printhead. Filament stimulation sources such as ink heaters or transducers operate as ink droplet generators each time they are activated, by causing filament end-lengths to be broken off at the respective nozzles to provide discrete ink droplets which, in turn, are deposited on a print medium moving relative to the printhead. The interval between successive droplet break-offs at any one nozzle matches the interval between successive activations of the filament stimulation source for that nozzle. The longer the interval between successive activations of the filament stimulation source for the nozzle, the longer the opportunity for the continuous inkjet filament to increase lengthwise at the nozzle and the larger the ink droplet. Conversely, the shorter the interval between successive activations of the filament stimulation source for the nozzle, the shorter the opportunity for the continuous inkjet filament to increase lengthwise at the nozzle and the smaller the ink droplet. Thus, the volume of the ink droplet, when a droplet break-off occurs at the nozzle, corresponds to the frequency of activation of the filament stimulation source for the nozzle.  
           [0005]    Successive ink droplets can be altered between printing and non-printing trajectories or paths. Those ink droplets that are in a printing trajectory are allowed to reach the print medium. Those ink droplets that are in a non-printing trajectory can be collected in a ink gutter or catcher and then recycled.  
           [0006]    A problem that exists is that dirt or dried ink can accumulate on a nozzle, particularly in the region where the continuous inkjet filament projects from the nozzle. When this occurs, the nozzle must be considered defective because the ink droplets that result from filament end-lengths being broken off at the nozzle may be misdirected with respect to the printing trajectory that the ink droplets should take. Consequently, the printed image may be of a lesser quality.  
           [0007]    The problem of misdirected ink droplets is particularly acute in continuous inkjet printers because ink flow to form a continuous inkjet filament at a nozzle that is defective cannot be stopped.  
         SUMMARY OF THE INVENTION  
         [0008]    According to one aspect of the invention, there is provided a method of preventing all of the ink discharged from a defective one of multiple nozzles in a continuous inkjet printhead from being used for printing on a print medium. Generally speaking, the method comprises:  
           [0009]    diverting all of the ink discharged from a defective nozzle from reaching a print medium, and allowing at least some of the ink discharged from other nozzles which are not defective to reach the print medium.  
           [0010]    More specifically, the method comprises:  
           [0011]    causing the defective nozzle to discharge only non-printing ink droplets, and allowing other nozzles which are not defective to discharge printing ink droplets which are volume-differentiated from non-printing droplets; and  
           [0012]    preventing non-printing droplets discharged from the defective nozzle from reaching a print medium, and allowing printing ink droplets discharged from the nozzles that are not defective to reach the print medium.  
           [0013]    Further specifically, the method comprises:  
           [0014]    periodically heating the defective nozzle at a frequency that is greater than frequencies other nozzles which are not defective are periodically heated, to cause the defective nozzle to only discharge ink droplets that have a smaller volume than ink droplets discharged from the nozzles that are not defective; and  
           [0015]    preventing smaller volume droplets discharged from the defective nozzle from reaching a print medium, and allowing larger volume ink droplets discharged from the nozzles that are not defective to reach the print medium.  
           [0016]    According to another aspect of the invention, there is provided apparatus for performing the foregoing method. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 depicts in schematic block form an ink droplet forming assemblage to be included in a continuous inkjet printer;  
         [0018]    [0018]FIG. 2 is depicts in cross-section an ink discharge nozzle, an ink heater, and a continuous ink filament projecting from the nozzle;  
         [0019]    [0019]FIGS. 3A depicts a multi-burst heater-activating pulse waveform for activating ink heaters at non-defective nozzles;  
         [0020]    [0020]FIG. 3B depicts ink droplets resulting from the pulse waveform in FIG. 3A;  
         [0021]    [0021]FIG. 4A depicts a multi-burst heater-activating pulse waveform for activating ink heaters at a nozzle;  
         [0022]    [0022]FIG. 4B depicts ink droplets resulting from the pulse waveform in FIG. 4A; and  
         [0023]    [0023]FIG. 5 shows an air blower mechanism for separating ink droplets into printing and non-printing trajectories or paths. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    The invention is intended to be embodied in a continuous inkjet printer. Because the features of such a printer are generally known, the description which follows is directed in particular only to those elements forming part of or cooperating with the disclosed embodiment of the invention. It is to be understood, however, that other elements not disclosed may take various forms known to a person of ordinary skill in the art.  
         [0025]    [0025]FIG. 1 shows an ink droplet forming assemblage  10  that is to be included in a continuous inkjet printer such as the one disclosed in prior art U.S. Pat. No. 6,079,821 issued Jun. 27, 2000. The &#39;821 patent is incorporated in this patent application.  
         [0026]    Coincident with a description of the ink droplet forming mechanism  10  which follows, there is provided a method of preventing all of the ink discharged from a defective one of multiple nozzles in the mechanism from being used for printing on a print medium.  
         [0027]    The ink droplet forming assemblage  10  shown in FIG. 1 generally comprises a printhead  12 , at least one ink supply  14  and a controller  16 . It is depicted in a schematic block form, which is not to scale for the sake of clarity. The controller  16  may, for example, be a known type logic control device or a suitably programmed microprocessor as in the incorporated &#39;821 patent.  
         [0028]    Multiple ink discharge nozzles or outlets  18  (only five shown in FIG. 1) are provided in a nozzle plate  19  on the printhead  12 . Each one of the nozzles  18  is in continuous pressurized ink-receiving communication with the ink supply  14  via an ink passage  20 , for example to provide black and white or single-color printing. Alternatively, the nozzles  18  may be in continuous pressurized ink-receiving communication with multiple continuous ink supplies, for example to provide multi-color printing using three or more ink colors such as yellow, cyan and magenta. A known pump, not shown, can serve as a continuous ink-pressurizing means.  
         [0029]    Respective known ink droplet generators, i.e. filament stimulation sources, which preferably are ink heaters  22 , are positioned on the printhead  12  around the ink discharge nozzles  18  as shown in FIG. 1. Each one of the ink heaters  22  is formed in a circular or ring shape and has a similar shape resistive heating element  24  electrically connected to a conductive contact pad  26  via a conductor  28 . See FIGS. 1 and 2. The conductors  28  and contact pads  26  in FIG. 1 are at least partially formed or positioned on the printhead  12 , and they provide an electrical connection between the controller  16  and the ink heaters  22 .  
         [0030]    Typically, as shown in FIG. 2, a pressurized ink  30  is formed into continuous inkjet filaments  32  (only one shown in FIG. 2) projecting from the ink discharge nozzles  18 . Each time the ink heaters  22  are activated they operate (when heat-producing) as ink droplet generators, by causing respective filament end-lengths  34  to be broken off from the continuous inkjet filaments  32  at the nozzles  18  to provide discrete ink droplets (not shown in FIG. 2). The interval between successive droplet break-offs at any one nozzle  18  matches (corresponds to) the interval between successive activations of the ink heater  22  for that nozzle. The longer the interval between successive activations of the ink heater  22  for the nozzle  18 , the longer the opportunity for the continuous inkjet filament  32  to increase lengthwise at the nozzle and the larger the ink droplet. Conversely, the shorter the interval between successive activations of the ink heater for the nozzle, the shorter the opportunity for the continuous inkjet filament to increase lengthwise at the nozzle and the smaller the ink droplet. Thus, the volume of the ink droplet, when a droplet break-off occurs at the nozzle, corresponds to the frequency of activation of the ink heater for the nozzle.  
         [0031]    [0031]FIG. 3A shows an example of a multi-burst heater-activating pulse waveform  36  that can be provided by the controller  16  to one of the ink heaters  18  to activate the ink heater successive times to generate successive ink droplets. The pulse waveform  36  depicts a repeating series of heater-activating pulses  38 ,  40 ,  42  and  44 . Each sequence of the four pulses  38 ,  40 ,  42  and  44  constitutes a single pulse burst. The intervals or delays  46  between the pulses  38  and  40 ,  40  and  42 , and  44  and  38  are the same. Consequently, the ink droplets  48  resulting from the respective pulses  38 ,  40  and  42  have the same volume. See FIG. 3B. The interval or delay  50  between the pulses  42  and  44  is shorter than the intervals  46  between the pulses  38  and  40 ,  40  and  42 , and  44  and  38 . Consequently, the ink droplets  52  resulting from the pulses  44  have a similar volume that is less than the volume of the ink droplets  48 .  
         [0032]    The ink droplets  46  that have the larger volume are intended to be used as printing ink droplets. Conversely, the ink droplets  52  that have the smaller volume are non-printing ink droplets.  
         [0033]    As shown in FIG. 5, the printing or larger volume ink droplets  46  are intended to take a printing trajectory or path  54  from the nozzles  18  to a print medium  56  such as a paper sheet which may be supported on a known rotating drum (not shown). Conversely, the non-printing or smaller volume ink droplets  52  are intended to take a non-printing trajectory or path  58  from the nozzles  18  to an ink gutter or catcher  60 , in order to prevent the non-printing or smaller volume ink droplets  52  from reaching the print medium  56 . Then, the non-printing or smaller volume ink droplets  52  are recycled back to the ink supply  14  via an appropriate conduit (not shown). A known air blower  62  blows air at a sufficient velocity to divert or deflect the non-printing or smaller volume ink droplets  52  into their non-printing trajectory  58  to the ink catcher  60 . The air velocity is insufficient to remove the printing or larger volume ink droplets  46  from the printing trajectory  54 .  
         [0034]    A problem that exists is that dirt or dried ink can accumulate on at least one of the nozzles  18 , particularly in the region where the continuous inkjet filament  32  projects from the nozzle, and also possibly in the vicinity of the heating elements  24 . When this occurs, the nozzle  18  must be considered defective because the ink droplets that result from the filament end-lengths  34  being broken off at the nozzle may be misdirected with respect to the printing trajectory  54  that the ink droplets should take. Consequently, the printed image may be of a lesser quality.  
         [0035]    The solution to the problem is as follows. As shown in FIGS. 1 and 2, respective annular detectors  64  line the nozzles  18 , particularly in the region where the continuous inkjet filaments  32  project from the nozzles, and also in the vicinity of the heating elements  24 , to detect any accumulation of dirt or dried ink at each nozzle, in order to determine whether a nozzle is defective. Alternatively, the detectors  64  can be positioned to detect any ink droplets that are misdirected with respect to the printing trajectory  54  because of the accumulation of dirt or dried ink, in order to determine whether a nozzle is defective. The detectors  64  are connected to the controller  16  to enable the controller to provide a multi-burst heater-activating pulse waveform  66  to the ink heater  22  of a defective one of the nozzles  18  to activate the ink heater successive times to generate successive ink droplets as shown in FIGS. 4A and 4B. The pulse waveform  66  in FIG. 4A depicts a repeating series of heater-activating pulses  68 . A twelve-pulse sequence constitutes a single pulse burst. The intervals or delays  70  between the pulses  68  for the defective nozzle are the same, and they are shorter than the intervals  46  between the pulses  38  and  40 ,  40  and  42 , and  44  and  38  and the interval  50  between the pulses  42  and  44  for the non-defective nozzles. Consequently, in FIG. 4B, the ink droplets  72  resulting from the pulses  68  have the smallest volume, i.e. they have a smaller volume than the ink droplets  48  resulting from the respective pulses  38 ,  40  and  42  (which in turn have a smaller volume than the ink droplets  52  resulting from the pulses  44 ). Compare FIGS. 3A and 3B with FIGS. 4A and 4B.  
         [0036]    Like the non-printing ink droplets  52  from a non-defective one of the nozzles  18 , the smallest volume ink droplets  72  from a defective nozzle are non-printing ink droplets. Of course, this methodology can be reversed or modified. That is to say, the non-printing ink droplets  52  and  68  might have different volumes that are each larger than the volume of the printing ink droplets  48 . Alternatively, the non-printing ink droplets  52  and  68  might have the same volume (but different than the volume of the printing ink droplets  48 ).  
         [0037]    As shown in FIG. 5, the non-printing or smallest volume ink droplets  72  from a defective one of the nozzles  18  are intended to take a non-printing trajectory  74  to the ink gutter or catcher  60 , in order to prevent the non-printing or smallest volume ink droplets from reaching the print medium  56 . Then, the non-printing or smallest volume ink droplets  72  are recycled back to the ink supply  14  via the appropriate conduit (not shown). The non-printing trajectory  74  of the non-printing ink droplets  72  from a defective nozzle is substantially parallel to (and in the same direction as) the non-printing trajectory  58  of the non-printing ink droplets  52  from a non-defective nozzle. A known air blower  76 , similar to the air blower  62 , blows air at a higher velocity than the velocity of air blown by the latter blower to divert or deflect the non-printing or smallest volume ink droplets  72  into their non-printing trajectory  74  to the ink catcher  60 . The higher air velocity is insufficient to remove the printing or larger volume ink droplets  46  from the printing trajectory  54 .  
         [0038]    Instead of one or both of the air blowers  76  and  62  which divert the non-printing ink droplets  72  and  52  from defective and non-defective nozzles  18  to the non-printing trajectories  74  and  58 , a vacuum source can be used to attract the non-printing ink droplets  72  and/or  52  to the respective trajectories. Moreover, instead of the non-printing trajectory  74  being in the same direction as the non-printing trajectory  58 , the two non-printing trajectories can be in opposite directions—in which case a second ink gutter, in addition to the ink gutter  60 , would be used.  
         [0039]    If the non-printing ink droplets  52  and  68  had the same volume (but different than the volume of the printing ink droplets  48 ), only a single air blower or vacuum source wold be sufficient since the non-printing ink droplets could be diverted to the same non-printing trajectory.  
         [0040]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.  
       Parts List  
       [0041]    [0041] 10 . ink droplet forming assemblage  
         [0042]    [0042] 12 . printhead  
         [0043]    [0043] 14 . ink supply  
         [0044]    [0044] 16 . controller  
         [0045]    [0045] 18 . ink discharge nozzle  
         [0046]    [0046] 19 . nozzle plate  
         [0047]    [0047] 20 . ink passage  
         [0048]    [0048] 22 . ink heater  
         [0049]    [0049] 24 . heating element  
         [0050]    [0050] 26 . contact pad  
         [0051]    [0051] 28 . conductor  
         [0052]    [0052] 30 . pressurized ink  
         [0053]    [0053] 32 . continuous inkjet filament  
         [0054]    [0054] 34 . filament end-length  
         [0055]    [0055] 36 . pulse waveform  
         [0056]    [0056] 36 . heater-activating pulse  
         [0057]    [0057] 40 . heater-activating pulse  
         [0058]    [0058] 42 . heater-activating pulse  
         [0059]    [0059] 44 . heater-activating pulse  
         [0060]    [0060] 46 . pulse interval  
         [0061]    [0061] 48 . larger volume printing ink droplet  
         [0062]    [0062] 50 . pulse interval  
         [0063]    [0063] 52 . smaller volume non-printing ink droplet  
         [0064]    [0064] 54 . printing trajectory or path  
         [0065]    [0065] 56 . print medium  
         [0066]    [0066] 58 . non-printing trajectory or path  
         [0067]    [0067] 60 . ink gutter or catcher  
         [0068]    [0068] 62 . air blower  
         [0069]    [0069] 64 . defective nozzle detector  
         [0070]    [0070] 66 . pulse waveform  
         [0071]    [0071] 68 . heater-activating pulse  
         [0072]    [0072] 70 . pulse interval  
         [0073]    [0073] 72 . smallest volume non-printing ink droplet  
         [0074]    [0074] 74 . non-printing trajectory or path  
         [0075]    [0075] 76 . air blower