Abstract:
An apparatus for printing an image is provided. In this apparatus, each nozzle is operable to selectively create a stream of ink droplets having a plurality of velocities. The apparatus also includes a droplet deflector having a gas source. The gas source is positioned at an angle with respect to the stream of ink droplets and is operable to interact with the stream of ink droplets thereby separating ink droplets into printing and non-printing paths.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    Reference is made to commonly assigned, co-pending U.S. patent applications Ser. No. 09/750,946, filed in the names of David L. Jeanmaire et al. on Dec. 28, 2000; Ser. No. 09/861,692 filed in the name of David L. Jeanmaire on May 21, 2001; Ser. No. 09/892,831 filed in the name of David L. Jeanmaire on Jun. 27, 2001; and Ser. No. 09/910,405 filed in the name of David L. Jeanmaire on Jul. 20, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to the field of digitally controlled printing devices, and in particular to continuous inkjet printers wherein a liquid ink stream breaks into droplets, some of which are selectively deflected.  
         BACKGROUND OF THE INVENTION  
         [0003]    Traditionally, digitally controlled color ink jet 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 formed in the print head. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a receiving 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, in general, up to several million perceived color combinations.  
           [0004]    The first technology, commonly referred to as “drop-on-demand” ink jet printing, typically provides ink droplets for impact upon a recording surface 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 print head and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. 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.  
           [0005]    With thermal actuators, a heater, located at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble. This increases the internal ink pressure sufficiently for an ink droplet to be expelled. The bubble then collapses as the heating element cools, and the resulting vacuum draws fluid from a reservoir to replace ink that was ejected from the nozzle.  
           [0006]    Piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to vanLintel, on Jul. 6, 1993, have a piezoelectric crystal in an ink fluid channel that flexes when an electric current flows through it forcing an ink droplet out of a nozzle. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.  
           [0007]    In U.S. Pat. No. 4,914,522, which issued to Duffield et al. on Apr. 3, 1990, a drop-on-demand inkjet printer utilizes air pressure to produce a desired color density in a printed image. Ink in a reservoir travels through a conduit and forms a meniscus at an end of an ink nozzle. An air nozzle, positioned so that a stream of air flows across the meniscus at the end of the nozzle, causes the ink to be extracted from the nozzle and atomized into a fine spray. The stream of air is applied for controllable time periods at a constant pressure through a conduit to a control valve. The ink dot size on the image remains constant while the desired color density of the ink dot is varied depending on the pulse width of the air stream.  
           [0008]    The second technology, commonly referred to as “continuous stream” or “continuous” inkjet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no print is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as catcher, interceptor, or gutter). When print is desired, the ink droplets are directed to strike a print media.  
           [0009]    Typically, continuous ink jet printing devices are faster than drop-on-demand devices and produce higher quality printed images and graphics. However, each color printed requires an individual droplet formation, deflection, and capturing system.  
           [0010]    U.S. Pat. No. 1,941,001, issued to Hansell on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al. on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous ink jet.  
           [0011]    U.S. Pat. No. 3,416,153, issued to Hertz et al. on Oct. 6, 1963, discloses a method of achieving variable optical density of printed spots in continuous ink jet printing using the electrostatic dispersion of a charged droplet stream to modulate the number of droplets which pass through a small aperture.  
           [0012]    U.S. Pat. No. 3,878,519, issued to Eaton on Apr. 15, 1975, discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.  
           [0013]    U.S. Pat. No. 4,346,387, issued to Hertz on Aug. 24, 1982, discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a droplet formation point located within the electric field having an electric potential gradient. Droplet formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect droplets.  
           [0014]    U.S. Pat. No. 4,638,382, issued to Drake et al. on Jan. 20, 1987, discloses a continuous ink jet print head that utilizes constant thermal pulses to agitate ink streams admitted through a plurality of nozzles in order to break up the ink streams into droplets at a fixed distance from the nozzles. At this point, the droplets are individually charged by a charging electrode and then deflected using deflection plates positioned the droplet path.  
           [0015]    As conventional continuous ink jet printers utilize electrostatic charging devices and deflector plates, they require many components and large spatial volumes in which to operate. This results in continuous inkjet print heads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control.  
           [0016]    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 amplitude stimulations resulting in longer 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.  
           [0017]    While this method does not rely on electrostatic means to affect the trajectory of droplets, it does rely on the precise control of the break up points of the filaments and the placement of the air flow intermediate to these break up points. Such a system is difficult to control and to manufacture. Furthermore, the physical separation or amount of discrimination between the two droplet paths is small, further adding to the difficulty of control and manufacture.  
           [0018]    U.S. Pat. No. 4,190,844, issued to Taylor on Feb. 26, 1980, discloses a continuous inkjet printer having a first pneumatic deflector for deflecting non-printed ink droplets to a catcher and a second pneumatic deflector for oscillating printed ink droplets. A print head supplies a filament of working fluid that breaks into individual ink droplets. The ink droplets are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector is an “on/off” type having a diaphragm that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the ink droplet is to be printed or non-printed. The second pneumatic deflector is a continuous type having a diaphragm that varies the amount that a nozzle is open, depending on a varying electrical signal received the central control unit. This oscillates printed ink droplets so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at a time, being built up by repeated traverses of the print head.  
           [0019]    While this method does not rely on electrostatic means to affect the trajectory of droplets, it does rely on the precise control and timing of the first (“ON/OFF”) pneumatic deflector to create printed and non-printed ink droplets. Such a system is difficult to manufacture and accurately control, resulting in at least the ink droplet build up discussed above. Furthermore, the physical separation or amount of discrimination between the two droplet paths is erratic due to the precise timing requirements, increasing the difficulty of controlling printed and non-printed ink droplets and resulting in poor ink droplet trajectory control.  
           [0020]    Additionally, using two pneumatic deflectors complicates construction of the print head and requires more components. The additional components and complicated structure require large spatial volumes between the print head and the media, increasing the ink droplet trajectory distance. Increasing the distance of the droplet trajectory decreases droplet placement accuracy and affects the print image quality. Again, there is a need to minimize the distance that the droplet must travel before striking the print media in order to insure high quality images.  
           [0021]    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 to deflect those 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 receiving medium, 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 the ink jet printer disclosed in Chwalek et al. works extremely well for its intended purpose, it is best adapted for use with inks that have a large viscosity change with temperature.  
           [0022]    Each of the above-described ink jet printing systems has advantages and disadvantages. However, print heads which are low-power and low-voltage in operation will be advantaged in the marketplace, especially in page-width arrays. U.S. Pat. No. 3,709,432, issued to J. Robertson on Jan. 9, 1973, discloses continuous-jet printing wherein nozzle transducers are selectively actuated at a plurality of activation powers to vary the breakup length of ink filaments extruded from the nozzles. A gas stream provides a force that displaces the filaments more before they breakup into droplets, than the droplets themselves. Thus ink droplets can be separated into printing and non-printing paths according to transducer power. While this process consumes only moderate power, and is compatible with a wide range of inks, the gas flow, when directed in the region of droplet breakoff, interferes with droplet formation in such a way that ink droplets of varying volumes are created along both printing and non-printing paths. In particular, the droplets selected for printing then are deflected along somewhat different paths according to variations in volume, thus resulting in poor droplet placement on the print media, and consequently low image quality results.  
           [0023]    Therefore, it can be seen that there is an opportunity to provide an improvement to continuous inkjet printers that use a gas flow for droplet separation, by providing a mechanism to generate droplets of constant volume. Low-power and low-voltage print head operation are achieved, while providing for quality consistent with the printing of photographic images.  
         SUMMARY OF THE INVENTION  
         [0024]    An object of the present invention is to provide for improved droplet placement in printers with print heads in which heat pulses are used to break up fluid into droplets, and which use a gas flow to separate the droplets along printing and non-printing paths. The improved registration of printed droplets improves the quality of the image on the receiver media.  
           [0025]    According to a feature of the present invention, print head includes one or more nozzles from which a stream of ink droplets is emitted. A mechanism, associated with each nozzle, is adapted to independently adjust the velocity of the ink droplets emitted by the associated nozzle. The mechanism has a first state wherein the velocities of the droplets emitted from the nozzles are within a first range of velocities, and a second state wherein the velocities of the droplets emitted from the nozzles are within a second range of velocities, wherein velocities within the second range are greater than velocities within the first range. Droplet selection apparatus is provided adapted to cause ink droplets within the first range of velocities to move along a first path, and ink droplets within the second range of velocities to move along a second path.  
           [0026]    According to a feature of the present invention, print head includes one or more nozzles from which a stream of ink droplets is emitted. A mechanism, associated with each nozzle, is adapted to independently adjust the velocity of the ink droplets emitted by the associated nozzle. The mechanism has a first state wherein the velocities of the droplets emitted from the nozzles are within a first range of velocities, and a second state wherein the velocities of the droplets emitted from the nozzles are within a second range of velocities, wherein velocities within the second range are greater than velocities within the first range. Droplet selection apparatus is provided adapted to cause ink droplets within the first range of velocities to move along a first path, and ink droplets within the second range of velocities to move along a second path. An ink catcher positioned to allow droplets moving along said first path to move unobstructed past the catcher, while intercepting droplets moving along said second path. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    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:  
         [0028]    [0028]FIG. 1 is a schematic plan view of a print head made in accordance with a preferred embodiment of the present invention;  
         [0029]    [0029]FIG. 2 is a diagram illustrating a frequency control of a heater as described in said embodiment of the present invention;  
         [0030]    [0030]FIG. 3 is a cross-sectional view of an ink jet print head made in accordance with said embodiment of the present invention; and  
         [0031]    [0031]FIG. 4 is a schematic view of an ink jet printer made in accordance with said embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    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.  
         [0033]    With reference to FIGS.  1  through FIG. 4, like reference numerals designate like components throughout all of the figures.  
         [0034]    [0034]FIG. 1 shows an ink droplet forming mechanism  10  of a preferred embodiment of the present invention, including a print head  20 , at least one ink supply  30 , and a controller  40 . Although ink droplet forming mechanism  10  is illustrated schematically and not to scale for the sake of clarity, one of ordinary skill in the art will be able to readily determine the specific size and interconnections of the elements of a practical apparatus according to a specific desired application.  
         [0035]    In a preferred embodiment of the present invention, print head  20  is formed from a semiconductor material, such as for example silicon, using known semiconductor fabrication techniques (CMOS circuit fabrication techniques, micro-electro mechanical structure (MEMS) fabrication techniques, etc.). However, print head  20  may be formed from any materials using any fabrication techniques conventionally known in the art.  
         [0036]    As illustrated in FIG. 1, a row of nozzles is formed on print head  20 . Nozzles  25  are in fluid communication with ink supply  30  through ink passage  50 , also formed in print head  20 . Single color printing, such as socalled black and white, may be accomplished using a single ink supply  30  and a single set of nozzles  25 . In order to provide color printing using two or more ink colors, print head  20  may incorporate additional ink supplies in the manner of supply  30  and corresponding sets of nozzles  25 .  
         [0037]    A set of heaters  60  is at least partially formed or positioned on print head  20  around corresponding nozzles  25 . Although heaters  60  may be disposed radially away from the edge of corresponding nozzles  25 , they are preferably disposed close to corresponding nozzles  25  in a concentric manner. In a preferred embodiment, heaters  60  are formed in a substantially circular or ring shape. However, heaters  60  may be formed in a partial ring, square, etc. Heaters  60  in a preferred embodiment consist principally of an electric resistive heating element electrically connected to electrical contact pads  55  via conductors  45 .  
         [0038]    Conductors  45  and electrical contact pads  55  may be at least partially formed or positioned on print head  20  to provide an electrical connection between controller  40  and heaters  60 . Alternatively, the electrical connection between controller  40  and heaters  60  may be accomplished in any well-known manner. Controller  40  is typically a logic controller, programmable microprocessor, etc. operable to control many components (heaters  60 , ink droplet forming mechanism  10 , etc.) in a desired manner.  
         [0039]    [0039]FIG. 2 is a schematic example of the electrical activation waveform provided by controller  40  to heaters  60 . In general, pulsing of heaters  60  at high power levels forms ink droplets moving at higher velocity, while pulsing at lower powers creates droplets moving at slower velocity. In the first example presented here, the faster moving ink droplets are to be used for marking the image receiver, while slower, non-printing droplets are captured for ink recycling.  
         [0040]    In this example, a single droplet per nozzle per image pixel is created. Period P is the time associated with the printing of an associated image pixel. The schematic illustration shows that droplets of constant volume are created continuously as a result of the application of the waveforms of heater activation, and essentially independently of pulse amplitude.  
         [0041]    In the droplet formation for a non-printing image pixel, a droplet  95  is created using a lower power electrical pulse  65  and a delay time  80 . In the case of a printing image pixel, droplet  100  is created with a higher power pulse  70  and a delay time  80 . As a result of the higher power of heater activation, printing droplets  100  have a higher velocity than non-printing droplets  95 .  
         [0042]    Referring to FIG. 3, print head  20 , which is adapted to provide printing droplets of a first velocity and non-printing droplets of a second velocity, is coupled with a droplet deflector adapted to produce a force on the droplets. In the illustrated embodiment, a gas-flow discrimination means separates droplets into printing or non-printing paths according to droplet velocity. Ink is ejected through nozzles  25  in print head  20 , creating a filament of working fluid  120  moving substantially perpendicular to print head  20  along axis X. The physical region over which the filament of working fluid is intact is designated as r 1 . Heaters  60  are selectively activated at various powers according to image data, causing filaments of working fluid  120  to break up into streams of individual ink droplets. Coalescence of initial droplets  110  occurs in forming both printing droplets  100  and non-printing droplets  95 . This region of jet break-up and droplet coalescence is designated as r 2 .  
         [0043]    Following region r 2 , droplet formation is complete in a region r 3 , and faster moving printing droplets and slower moving, non-printing droplets are spatially separated. A discrimination force  130  is provided by a gas flow at a non-zero angle with respect to axis X. For example, the gas flow may be perpendicular to axis X. Discrimination force  130  acts over distance L, which is less than or equal to distance r 3 . Lower velocity, non-printing droplets  95  have a greater interaction time with force  130  than do faster moving droplets  100 . As a result, droplets  95  and droplets  100  separate into two paths with gas force  130  deflecting droplets  95  more than droplets  100 . The gas flow rate can be adjusted to provide sufficient deviation D 1  between the fast droplet path K 1  and the slower droplet path K 2 . This permits faster moving droplets  100  to strike print media W while slower moving, non-printing droplets  95  are captured by a ink guttering structure  240  described below.  
         [0044]    As an example, an aqueous ink is formulated to contain 40% by weight of dipropylene glycol monomethyl ether (DOW Chemical). This results in an ink fluid that exhibits a significant reduction in viscosity with temperature. In the waveform of FIG. 2, pulse  65  is 1 microsecond in duration and dissipates 10 microjoules of power in heater  60 , while pulse  70  is 1 microsecond in duration and dissipates 50 microjoules of power in heater  60 . Alternatively, the amplitudes of pulse  95  and pulse  100  could be held constant and the width varied to give an equivalent result amplitudes of Delay time  80  is 50 microseconds. The ink pressure in supply  30  is adjusted to give droplets  95  a velocity of 6.5 m/sec. As a result of the heat generated from pulse  70  droplets  100  have a 5% higher velocity than droplets  95 . Consequently, deviation D 1  and deviation D 2  differ by the square of the velocity ratio, or by 10% in this example.  
         [0045]    Delay time  80  can be adjusted to create droplets  95  and droplets  100  of different volumes, however, shorter times will decrease the overall separation of droplets  95  and droplets  100 . If this separation is too small, the velocity increase of droplets  100  relative to droplets  95  will cause droplets  100  to overtake and merge with droplets  95  before separation force  130  directs droplets  95  and droplets  100  along different paths, and proper printing operation will be lost.  
         [0046]    Referring to FIG. 4, a printing apparatus (typically, an ink jet printer or print head) includes a print head here containing a row of nozzles  25 . Greater velocity ink droplets  100  and lower velocity ink droplets  95  are formed from ink ejected in streams from print head  20  substantially along ejection path X. A droplet deflector  140  contains upper plenum  230  and lower plenum  220 , which facilitate a laminar flow of gas in droplet deflector  140 . Pressurized air from pump  150  enters upper plenum  230  which is disposed opposite plenum  220  and promotes laminar gas flow while protecting the droplet stream moving along path X from external air disturbances. The application of force  130  due to gas flow separates the ink droplets into fast-droplet path K 1  and slow-droplet path K 2 .  
         [0047]    An ink collection structure  165 , disposed adjacent to plenum  220  near path X, intercepts path K 2  of lower velocity droplets  95 , while allowing higher velocity ink droplets  100 , traveling along path K 2  to continue on to the recording media W carried by print drum  200 .  
         [0048]    Slower, non-printing ink droplets  95  strike ink catcher  240  in ink collection structure  165 . Ink recovery conduit  210  communicates with recovery reservoir  160  to facilitate recovery of non-printed ink droplets by an ink return line  170  for subsequent reuse. A vacuum conduit  175 , coupled to negative pressure source  180  can communicate with ink recovery reservoir  160  to create a negative pressure in ink recovery conduit  210  improving ink droplet separation and ink droplet removal as discussed above. The pressure reduction in conduit  210  is sufficient to draw in recovered ink, however it is not large enough to cause significant air flow to substantially alter droplet path K 1 . Ink recovery reservoir contains open-cell sponge or foam  155 , which prevents ink sloshing in applications where the print head  20  is rapidly scanned.  
         [0049]    A small portion of the gas flowing through upper plenum  230  is re-directed by plenum  190  to the entrance of ink recovery conduit  210 . The gas pressure in droplet deflector  140  is adjusted in combination with the design of plenum  220  and  230  so that the gas pressure in the print head assembly near ink catcher  240  is positive with respect to the ambient air pressure near print drum  200 . Environmental dust and paper fibers are thusly discouraged from approaching and adhering to ink catcher  240  and are additionally excluded from entering ink recovery conduit  210 .  
         [0050]    In operation, a recording media W is transported in a direction transverse to axis X by print drum  200  in a known manner. Transport of recording media W is coordinated with movement of print mechanism  10  and/or movement of print head  20 . In addition, this can be accomplished using controller  40  in a known manner. Recording media W may be selected from a wide variety of materials including paper, vinyl, cloth, other fibrous materials, etc.  
         [0051]    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.