Abstract:
A continuous stream ink jet printer is provided having an ink droplet forming mechanism for ejecting a stream of ink droplets having a selected one of at least two different volumes toward a print medium, a droplet deflector for producing a flow of gas that interacts with the ink droplet stream to separate droplets having different volumes, and a gas flow conditioner for preconditioning the gas flow produced by the deflector with a solvent vapor. The provision of a solvent vapor in the gas flow prevents or at least reduces changes in ink viscosity which could otherwise interfere with the recycling and filtering of ink droplets captured by the gutter of the printer.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of digitally controlled continuous ink jet printing devices, and in particular to continuous ink jet printers in which the into droplets are selectively deflected by a transverse flow of gas that has been preconditioned with a solvent to minimize ink drying on the printhead. In both technologies, droplets of ink are ejected from nozzles in a printhead toward a print medium. 
     BACKGROUND OF THE INVENTION 
     Traditionally, color ink jet printing is accomplished by one of two technologies, referred to as drop-on-demand and continuous stream printing. Both technologies require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce up to several million perceived color combinations. 
     In drop-on-demand ink jet printing, ink droplets are generated for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print medium and strikes the print medium. The formation of printed images is achieved by controlling the individual formation of ink droplets as the medium is moved relative to the printhead. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean. 
     Conventional drop-on-demand ink jet printers utilize a pressurization actuator to produce the ink jet droplet from the nozzles 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. This causes 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 pulse of mechanical movement stress in the material, thereby causing an ink droplet to be expelled by a pumping action. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate. 
     The second technology, commonly referred to as continuous stream or continuous ink jet printing, uses a pressurized ink source for producing a continuous stream of ink droplets. Conventional 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 print 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. Typically, continuous ink jet printing devices are faster than drop on demand devices and produce higher quality printed images and graphics. 
     Other methods of continuous ink jet printing employ air flow in the vicinity of ink streams for various purposes. For example, U.S. Pat. No. 3,596,275 issued to Sweet in 1978 discloses the use of both collinear and perpendicular air flow to the droplet flow path to remove the effect of the wake turbulence on the path of succeeding droplets. This work was expanded upon in U.S. Pat. No. 3,972,051 to Lundquist et al., U.S. Pat. No. 4,097,872 to Hendriks et al. and U.S. Pat. No. 4,297,712 to Sturm in regards to the design of aspirators for use in droplet wake minimization. U.S. Pat. No. 4,106,032, to Miura and U.S. Pat. No. 4,728,969 to Le et al. employ a coaxial air flow to assist jetting from a drop-on-demand type head. 
     One problem associated with ink jet printers in general and such printers employing gas or air flows in particular, is the drying of the ink. Ink drying in the vicinity of the printhead nozzles can lead to spurious droplet trajectories and nozzle clogging. Additionally, the evaporation of the ink solvent from the droplets as they fly through the air can increase the viscosity of the ink captured by the gutter, thereby causing difficulties during the ink recycling operation when the recycled ink is passed through a filter. This last problem becomes particularly difficult if the loss of solvent in the ink is large enough to cause the pigments in the ink to coagulate. 
     Solvents have been introduced into the regions surrounding nozzles to prevent ink drying. For example, U.S. Pat. No. 4,228,442 to Krull teaches the use of absorbent or wick-like material disposed partly in a liquid ink solvent to evaporate solvent in front of or around the nozzles prevent drying or thickening of the ink at the nozzles. Miura et al discloses the use of humidified air to minimize nozzle clogging in an air assisted, drop on demand, ink jet printhead. However, none of the inventions described are sufficient to address the problems of solvent evaporation due to high-velocity air streams which interact with droplet streams in printers which employ the air streams to direct droplets along different trajectories according to drop volume. 
     Clearly, there is a need for a means of mitigating the drying effect that a gas flow has on the ink droplet streams in printers which involve gas flow interaction with ink droplets during printer operation. The primary problem is not the drying of ink at the nozzles, since the air flow in such printers is principally removed from the immediate vicinity of the nozzles. Rather, the difficulty is that the drying of droplets along their trajectory toward the ink catcher increases the viscosity to a point that impedes ink recycling and filtration. 
     SUMMARY OF THE INVENTION 
     The invention is an ink jet printing apparatus that solves or at least ameliorates all of the aforementioned problems associated with the prior art. To this end, the ink jet printing apparatus of the invention comprises an ink droplet forming mechanism for ejecting a stream of ink droplets having a selected one of at least two different volumes, a droplet deflector for producing a flow of gas that interacts with the ink droplet stream to separate ink droplets having different volumes from one another, and a gas flow conditioner for preconditioning with solvent vapor the gas flow produced by the droplet deflector. 
     Preferably, the ink jet printing apparatus is a continuous stream ink jet printer, and the flow of gas produced by the droplet deflector is oriented transversely to the stream of ink droplets and functions to deflect smaller volume droplets from larger volume droplets. The solvent used in the gas flow conditioner may be water, and the gas flow is preferably a flow of air. 
     The gas flow conditioner may include a sensor responsive to a solvent concentration level in the gas flow. The conditioner may also include a control circuit connected to the sensor for adjusting a solvent addition rate to the gas flow in order to maintain a selected solvent concentration in the gas flow. 
     In operation, the solvent concentration in the gas flow is set at a point that substantially prevents an increase in the viscosity of the ink in the droplets. Consequently, the droplets recaptured by the gutter of the printer may be filtered through the recycling mechanism of the printer without clogging the filter or interfering with the recycling operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view of a printhead made in accordance with a preferred embodiment of the present invention; 
     FIGS. 2A-F illustrate the relationship between the switching frequency of the heaters of the printhead and the volume of ink droplets produced by the nozzles adjacent to the heaters, 
     FIG. 3 is a schematic side view of the operation of an ink jet printhead made in accordance with the preferred embodiment of the present invention illustrating how the droplet deflector deflects smaller volume droplets from larger volume droplets, and 
     FIG. 4 is schematic side view of an ink jet printer made in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     With reference to FIGS. 1 and 4, wherein like reference numerals designate like components throughout all of the several figures, the continuous stream printer of the invention generally comprises an ink droplet forming mechanism in the form of a printhead  2 . 
     In a preferred embodiment of the present invention, printhead  2  is formed from a semiconductor material (silicon, etc.) using known semiconductor fabrication techniques (CMOS circuit fabrication techniques, micro-electro mechanical structure (MEMS) fabrication techniques, etc.). However, it is specifically contemplated and therefore within the scope of this disclosure that printhead  2  may be formed from any materials using any fabrication techniques conventionally known in the art. 
     Referring in particular to FIG. 1, a plurality of annular heaters  3  are at least partially formed or positioned on the silicon substrate  6  of the printhead  2  around corresponding nozzles  7 . Although each heater  3  may be disposed radially away from an edge of a corresponding nozzles  7 , the heaters  3  are preferably disposed close to corresponding nozzles  7  in a concentric manner. In a preferred embodiment, heaters  3  are formed in a substantially circular or ring shape. However, it is specifically contemplated that heaters  3  may be formed in a partial ring, square, or other shape adjacent to the nozzles  7 . Each heater  3  in a preferred embodiment is principally comprised of a resistive heating element electrically connected to contact pads  11  via conductors  18 . Each nozzle  7  is in fluid communication with ink supply  14  through an ink passage (not shown) also formed in printhead  2 . It is specifically contemplated that printhead  2  may incorporate additional ink supplies in the same manner as supply  14  as well as additional corresponding nozzles  7  in order to provide color printing using three or more ink colors. Additionally, black and white or single color printing may be accomplished using a single ink supply  14  and nozzle  7 . 
     Conductors  18  and electrical contact pads  11  may be at least partially formed or positioned on the printhead  2  and provide an electrical connection between a controller  13  and the heaters  3 . Alternatively, the electrical connection between the controller  13  and heater  3  may be accomplished in any well known manner. Controller  13  may be a relatively simple device (a switchable power supply for heater  3 , etc.) or a relatively complex device (a logic controller or programmable microprocessor in combination with a power supply) operable to control many other components of the printer in a desired manner. 
     In FIGS. 2A-F, examples of the electrical activation waveforms provided by controller  13  to the heaters  3  are shown. Generally, a high frequency of activation of heater  3  results in small volume droplets  23  as shown in FIGS. 2C and 2D, while a low frequency of activation results in large volume droplets  21  as illustrated in FIGS. 2A and 2B. In the preferred embodiment, large ink droplets are to be used for marking the print medium, while smaller droplets are captured for ink recycling. It must be understood, however, that this could be reversed in operation (depending on imaging requirements), where the smaller droplets are used for printing, and the larger drops recycled. Also in this example, only one printing droplet is provided for per image pixel, thus there are two states of heater actuation, printing or non-printing. The electrical waveform of heater  3  actuation for large ink droplets  21  is presented schematically as FIG.  2 ( a ). The individual large ink drops  21  produced from the jetting of ink from nozzle  7  as a result of low frequency heater actuation are shown schematically in  2 B. Heater actuation time  25  is typically 0.1 to 5 microseconds in duration, and in this example is 1.0 microsecond. The delay time  28  between subsequent heater actuation is 42 microseconds. The electrical waveform of heater  3  actuation for the non-printing case is given schematically as FIG.  2 ( c ). Electrical pulse  25  is 1.0 microsecond in duration, and the time delay  32  between activation pulses is 6.0 microseconds. The small droplets  23 , as illustrated in FIG. 2D, are the result of the activation of heater  3  with this non-printing waveform. 
     FIG.  2 ( e ) is a schematic representation of an electrical waveform of heater activation for mixed image data where a transition is shown from the non-printing state to the printing state, and back to the non-printing state. Schematic representation FIG. 2F is the resultant droplet stream formed. It is apparent that heater activation may be controlled independently based on the ink color required and ejected through corresponding nozzle  7 , the movement of printhead  17  relative to a print media W, and an image to be printed. It is specifically contemplated that the absolute volume of the small droplets  23  and the large droplets  21  may be adjusted based upon specific printing requirements such as ink and media type or image format and size. 
     With reference now to FIG. 3, the operation of printhead  2  in a manner such as to provide an image-wise modulation of droplets, as described above, is coupled with a droplet deflector  45  which separates droplets into printing or non-printing paths according to drop volume by means of a transversely disposed gas flow  47 . Ink is ejected through nozzle  7  in printhead  2 , creating a filament of working fluid  96  moving substantially perpendicular to printhead  2  along axis X. The physical region over which the filament of working fluid is intact is designated as r 1 . Heater  3  is selectively actuated at various frequencies according to image data, causing filament of working fluid  96  to break up into a stream of individual ink droplets. Some coalescence of droplets often occurs in forming non-printing drops  21 . 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 , such that at the distance from the printhead  2  that the gas flow from the deflector  45  is applied, droplets are substantially in two size classes: small, printing drops  23  and large, non-printing drops  21 . In the preferred implementation, the force  46  provided by the gas flow  47  is perpendicular to axis X. The force  46  acts across distance L, which is less than or equal to distance r 3 . Because area increases with the square of the radius of a sphere while mass increases with the cube of the radius, large, non-printing droplets  21  have a greater mass and more momentum than small volume droplets  23  which more than offsets the greater force applied to them by the gas flow as a result of their layer area. As gas force  46  interacts with the stream of ink droplets, the individual ink droplets separate depending on each droplets volume and mass. Accordingly, the gas flow rate can be adjusted to create a sufficient differentiation angle D in the small droplet path S from the large droplet path K, permitting large droplets  21  to strike print media W while small, non-printing droplets  23  are captured by a ink guttering structure  60  described in more detail in the apparatus below. 
     An amount of separation D between the large, non-printing droplets  21  and the small, printing droplets  23  will not only depend on their relative size but also the velocity, density, and viscosity of the gas flow producing force  46 ; the velocity and density of the large printing droplets  21  and small, non-printing droplets  23 ; and the interaction distance (shown as L in FIG. 3) over which the large printing droplet  21  and the small, non-printing droplets  23  interact with the gas flow  47 . Gases, including air, nitrogen, etc., having different densities and viscosities can also be used with similar results. 
     Referring to FIGS. 3 and 4, a printing apparatus (typically, an ink jet printer or printhead) used in a preferred implementation of the current invention is shown schematically. Large volume ink droplets  21  and small volume ink droplets  23  are formed from ink ejected from printhead  17  substantially along ejection path X in a stream. The droplet deflector  45  contains lower plenum  40  which facilitates a laminar flow of gas. Vacuum pump  150  communicates with plenum  40  and provides a sink for the gas flow  47 . In the center of the droplet deflector  45  is positioned proximate path X. The application of force  46  due to gas flow  47  separates the ink droplets into small-drop path S and large-drop path K. An upper plenum  50  is disposed opposite the plenum  40  and promotes laminar gas flow while protecting the droplet stream moving along path X from external air disturbances. Pump  220  draws in air, while filter  210  removes dust and dirt particles. 
     The printing apparatus further includes a gas flow conditioner  55  for providing a selected concentration of solvent into the gas flow  47  generated by the droplet deflector  45 . Gas flow conditioner  55  includes a conditioning chamber  190  that contains a supply of liquid solvent, which may be water in a case where aqueous inks are used in the printhead  2 , and a heater  200  for evaporating the solvent and for compensating for the cooling effect of solvent evaporation. Pressurized air from pump  220  enters conditioning chamber  190  where vaporized solvent and is mixed with the air. Separator filter  190  prevents any solvent droplets from entering upper plenum  50 . Differential pressure sensor  180  is used to determine the air flow rate through plenum  50  and a control signal is fed to pump  220  so that constant air flow rate is maintained. Air conditioned with solvent which has been used in droplet separator  45  and drawn into vacuum pump  150  is recirculated back into pump  220  in order to minimize solvent consumption. Sensor  160  senses solvent concentration in the air flow, and in a preferred implementation where aqueous inks are employed, is a capacitive-type humidity sensor as is well known in the art. A signal from sensor  160  is used to control heater  200 , thereby adjusting the solvent evaporation rate, and hence, the solvent concentration in the air flow in droplet separator  45 . 
     An ink recovery conduit  70  contains a ink guttering structure  60  whose purpose is to intercept the path of small droplets  23 , while allowing large ink droplets  21  traveling along small droplet path K to continue on to the recording media W carried by print drum  80 . Ink recovery conduit  70  communicates with ink recovery reservoir  90  to facilitate recovery of non-printed ink droplets by an ink return line  100  for subsequent reuse. Ink recovery reservoir contains open-cell sponge or foam  135  which prevents ink sloshing in applications where the printhead  17  is rapidly scanned. A vacuum conduit  110 , coupled to a negative pressure source can communicate with ink recovery reservoir  90  to create a negative pressure in ink recovery conduit  70  improving ink droplet separation and ink droplet removal. The gas flow rate in ink recovery conduit  70 , however, is chosen so as to not significantly perturb large droplet path K. Lower plenum  40  is fitted with filter  140  and drain  130  to capture any ink fluid resulting from ink misting, or misdirected jets which has been captured by the air flow in plenum  40 . Captured ink is then returned to recovery reservoir  90 . 
     Ink recovery reservoir  90  is fitted with a sensor  120  which measures the electrical conductivity of the ink in reservoir  90 . Generally, as solvent is lost from the ink due to interaction with the gas flow, the concentration of an ionic colorant will increase, and consequently cause a rise in electrical conductivity of the recovered ink. A control signal from sensor  120 , in combination with the control signal from solvent sensor  160  in a cascade loop configuration, is applied to heater  200 , so that the ink may have a solvent concentration in the range suitable for re-use without further need for make-up solvent additions in recycling. 
     Additionally, a portion of plenum  50  diverts a small fraction of the gas flow from pump  220  and conditioning chamber  190  to provide a source for the gas which is drawn into ink recovery conduit  70 . The gas pressure in droplet deflector  45  and in ink recovery conduit  70  are adjusted in combination with the design of ink recovery conduit  70  and plenum  50  so that the gas pressure in the print head assembly near ink guttering structure  60  is positive with respect to the ambient air pressure near print drum  80 . Environmental dust and paper fibers are thusly discouraged from approaching and adhering to ink guttering structure  60  and are additionally excluded from entering ink recovery conduit  70   
     In operation, a recording medium W is transported in a direction transverse to axis x by print drum  80  in a known manner. Transport of recording medium W is coordinated with movement of print mechanism  10  and/or movement of printhead  17 . This can be accomplished using controller  13  in a known manner. Recording media W may be selected from a wide variety of materials including paper, vinyl, cloth, other fibrous materials, etc. 
     While the foregoing description includes many details and specificities, it is to be understood that these have been included for purposes of explanation only, and are not to be interpreted as limitations of the present invention. Many modifications to the embodiments described above can be made without departing from the spirit and scope of the invention, as is intended to be encompassed by the following claims and their legal equivalents. 
     Parts List 
       1  continuous stream printer 
       2  printhead 
       3  heater 
       6  silicon substrate 
       7  nozzle 
       11  electrical contact pad 
       12  printing apparatus 
       13  controller 
       14  ink supply 
       21  large drop 
       23  small drop 
       25  electrical pulse time 
       28  delay time 
       31  pixel time 
       32  delay time 
       40  lower plenum 
       45  droplet deflector 
       46  force 
       47  gas flow 
       50  upper plenum 
       60  ink guttering structure 
       70  ink recovery conduit 
       80  print drum 
       90  ink recovery reservoir 
       96  working fluid 
       100  ink return line 
       110  vacuum conduit 
       120  ink conductivity sensor 
       130  ink return line 
       135  foam 
       140  filter 
       150  vacuum pump 
       160  solvent sensor 
       170  gas recycling line 
       180  differential pressure sensor 
       190  separation filter 
       200  heater 
       210  intake filter 
       220  pressure pump 
     W print media 
     L interaction distance 
     D separation distance 
     X ejection path 
     S small droplet path 
     K large droplet path