Abstract:
Method and apparatus ( 10, 102 ) for continuous inkjet printing wherein a first continuous stream of ink droplets ( 66 ) traveling above a first flow path ( 48 ) is used as a mask for colliding with a second continuous stream of ink droplets ( 70, 72 ) traveling along an second, intersecting flow path ( 56 ) en route to a receiver ( 12 ) on which an image is to be printed. Selective droplets ( 72 ) of the second droplet stream are timed and of a size to pass between and avoid the masking droplets ( 66 ) of the first droplet stream so as to travel on and impinge the receiver ( 12 ) for forming the image thereon. The colliding masking and masked droplets ( 66, 70 ) are larger than the selected printing droplets ( 72 ) to facilitate collision. The smaller printing droplets ( 72 ) facilitate sharp pixel formation. The apparatus is compatible with low voltage CMOS print head systems and provides reliable operation, yet is relatively inexpensive to manufacture compared to other continuous ink jet print head constructions.

Description:
BACKGROUND OF THE INVENTION 
     This invention generally relates to a method and apparatus for continuous inkjet printing, and more particularly to a continuous inkjet printing method wherein a first stream of ink droplets traveling along a first flow path is used as a mask by colliding with a second stream of ink droplets traveling along a second, intersecting flow path in route to a receiver on which an image is to be printed, selected droplets of the second droplet stream being timed to pass between and avoid the masking droplets so as to travel on and impinge the receiver for forming the image thereon. 
     An inkjet printer produces images on a receiver by ejecting ink droplets onto the receiver in an image-wise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of inkjet printers in the marketplace. 
     Inkjet printing mechanisms can be categorized as either Drop-on-Demand or continuous inkjet. Continuous inkjet printing dates back to at least 1929. See U.S. Pat. No. 1,941,001 to Hansell. 
     The term “Drop-on-Demand” characterizes inkjet printers, wherein at every orifice a pressurization actuator is used to produce the inkjet droplet. In this regard, either one of two types of actuators may be used. These two types of actuators are heat actuators and piezoelectric actuators. With respect to heat actuators, a heater placed at a convenient location heats the ink and a quantity of the ink will phase change into a gaseous steam bubble and raise the internal ink pressure sufficiently for an ink droplet to be expelled to the recording medium. A feature of the heat-type actuators is the ability to incorporate them easily into modern known print head constructions, particularly those using silicon substrates with CMOS electrical circuitry. One disadvantage, however, is that the overall electrical power consumption is large, especially in “page-width” arrays. With respect to piezoelectric actuators, a piezoelectric material is used, which piezoelectric material possesses piezoelectric properties such that a mechanical stress is produced when an electric field is applied. 
     The most common of the “continuous” inkjet printers utilize electrostatic charging tunnels that are placed close to the point where ink droplets are being ejected in the form of a stream. Selected ones of the droplets are electrically charged by the charging tunnels. The charged droplets are deflected downstream by the presence of deflector plates that have a predetermined electric potential difference between them. A gutter may be used to intercept the charged droplets, while the uncharged droplets are free to strike the recording medium. A disadvantage of the known continuous inkjet printers, however, is that the charging apparatus is complex and costly to incorporate into the print head. In addition, the interaction between charged drops can adversely affect image quality. 
     A novel continuous inkjet printer is described and claimed in U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, and assigned to the Eastman Kodak Company. Such printers use asymmetric heating in lieu of electrostatic charging tunnels to deflect ink droplets toward desired locations on the recording medium. In this device, a droplet generator formed from a heater having a selectively-actuated section associated with only a portion of the nozzle bore perimeter is provided for each of the ink nozzle bores. Periodic actuation of the heater element via a train of uniform electrical power pulses creates an asymmetric application of heat to the stream of droplets to control the direction of the stream between a print direction and a non-print direction. 
     While such continuous inkjet printers have demonstrated many proven advantages over conventional inkjet printers using electrostatic charging tunnels, there are still some areas in which such printers can be improved, particularly in the area of the ability to operate reliably on a wide range of different ink fluids, and in lower-temperature operation of heaters. 
     For example, the use of two fluid jets in droplet formation, has been disclosed by Sangiovanni et al. in U.S. Pat. No. 4,341,310 issued on Jul. 27, 1982, for a method called “masking”. In this “masking” method, separate streams of “polar” and “non-polar” monodispersed liquid droplets are coordinated to intersect at an intersection point to “mask” or prevent passage of the “nonpolar” liquid droplets. This technique, however, does not involve colliding jet streams of ink in an image-wise manner for printing purposes. But rather, it requires a complex charging apparatus for altering the path of the “polar” droplets. This is costly and requires a relatively high voltage, not easily compatible with known low voltage CMOS print head systems, typically operating at from two to six volts. 
     Therefore, there is a need to provide an inkjet printing method that provides the respective advantages of continuous inkjet printing, and Drop-on-Demand inkjet printing, with low voltage operation and low power consumption. To accomplish this by the use of a inkjet-masking concept, which avoids the complexity and cost disadvantages of the known “masking” methods would be a surprising but welcomed advancement in the art. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a continuous inkjet printing method and apparatus which utilizes desirable aspects of “on-demand” printing and “masking” concepts without including the undesirable aspects of their respective printing apparatus. 
     With this object in view, the present invention resides in an inkjet printing method comprising the steps of (1) generating a first stream of ink droplets traveling along a first flow path, and (2) generating a second stream of ink droplets traveling along a second flow path which intersects the first flow path at a predetermined location. The second stream of ink droplets includes ink droplets traveling in timed relation to the droplets of the first stream so as to collide with the droplets of the first stream at the predetermined location and be diverted to an ink receptacle. The second stream of ink droplets also includes selected droplets traveling in timed relation to the droplets of the first stream so as to pass between the droplets of the first stream at the predetermined location and continue along the second flow path so as to impinge a receiver at a down stream location along the second flow path for forming an image on the receiver. 
     According to an exemplary embodiment of the present invention, an inkjet printer is provided comprising an element for emitting a first ink stream along a first flow path; an element located along the first flow path upstream of the predetermined location for controllably breaking the first ink stream into successive ink droplets traveling along the first flow path; an element for emitting a second ink stream along a second flow path which intersects the first flow path at a predetermined location; an element located along the second flow path upstream of the predetermined location for controllably breaking the second ink stream into successive ink droplets traveling along the second flow path; and an element for controlling the time relationship of droplet formation between the ink streams such that selected ink droplets of the first stream will pass between or collide with the ink droplets of the second stream at the predetermined intersection location in an image-wise manner. In the absence of a collision between droplets, the droplets moving along the first path impinge on an image receiver located beyond the predetermined jet-crossing location. 
     Another feature of the present invention is the provision of an element for controllably generating a stream of ink droplets by intermittently effecting surface tension and viscosity changes in a continuous stream of ink. 
     Another feature of the present invention is the provision of two streams of ink droplets traveling along intersecting flow paths, wherein one of the streams of ink droplets includes selected droplets timed to pass between the droplets of another of the streams so as to travel on and impinge a receiver for forming an image thereon. 
     Another feature of the present invention is the provision of an element for controllably breaking a stream of ink into a succession of ink droplets traveling in timed relations to one another along a flow path. 
     Another feature of the present invention is the provision of streams of ink droplets generated by transiently heating continuous streams of ink to break the streams into the droplets, wherein larger ink droplets are generated by longer time intervals between the heat pulses and smaller ink droplets are generated by shorter intervals between the heat pulses. 
     An advantage of the present invention is the capability to selectively mask a stream of ink droplets without requiring droplet electrical polarization. 
     Another advantage of the present invention is the capability to generate different size ink droplets from a single continuous ink stream. 
     Still another advantage of the present invention is the ability to provide a drop-masking continuous ink jet printing method that is compatible with a low voltage print head system. 
     These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings which show and describe illustrative embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following detailed description when taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a simplified schematic representation illustrating a method and apparatus for drop-masking continuous inkjet printing according to the present invention. 
     FIG. 2 is a simplified schematic sectional representation of one embodiment of a print head of the invention shown emitting intersecting streams of ink droplets for illustrating a masking aspect of the invention. 
     FIG. 3 is a graphical representation of electrical drive signal traces for the apparatus of FIG. 1 in a non-printing mode. 
     FIG. 3 a  is a graphical representation of electrical drive signal traces for the apparatus of FIG. 1 in a printing mode. 
     FIG. 4 is a simplified schematic sectional representation of another embodiment of a print head according to the invention shown emitting intersecting streams of ink droplets for illustrating the masking aspect of the 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. 
     Referring to FIG. 1, there is shown apparatus  10  for drop-masking continuous inkjet printing constructed and operable according to the teachings of the present invention. Apparatus  10  is shown in association with a receiver  12  onto which an image is to be formed by apparatus  10 , which receiver  12  can comprise any suitable conventional recording medium, such as a sheet of paper, a transparent film or the like. Apparatus  10  includes a print head  14 , an ink supply reservoir  16  connected to print head  14  by an ink supply channel  18  for supplying ink thereto, a print head electrical drive  20  connected to print head  14  by a conductive path  22  for communicating electrical drive signals to print head  14  for controllably operating print head  14 , an ink gutter  24  disposed between receiver  12  and print head  14  connected to an ink return reservoir  26  via an ink return conduit  28 , and a rotatable drum  30  for holding and moving receiver  12  relative to print head  14  during the printing operation. 
     Referring also to FIG. 2, print head  14  includes a nozzle plate  32  including a plurality of pairs of ink ejecting nozzles  34  and  36  having orifices  38  and  40 , respectively, communicating with at least one ink chamber  42  connected in fluid communication with ink supply reservoir  16  via an ink supply channel  18  in a conventional and well known manner. Ink within ink chamber  42  is emitted from print head  14  through orifices  38  and  40  of ink ejecting nozzles  34  and  36  in continuous ink streams  44  and  46 , respectively, under pressure generated using a suitable conventional device such as a pump or the like (not shown). Ink stream  44  is emitted along a flow path  48 , and has a cross-sectional extent as denoted at  50  and an angular orientation as denoted at  52  relative to a front surface  54  of nozzle plate  32  which are determined by the size of orifice  38  and angle thereof relative to front surface  54 . Similarly, ink stream  46  is emitted from orifice  40  along a flow path  56 , and has a cross-sectional extent  58  and an angular orientation  60  relative to front surface  54  which are determined by the cross-sectionals extent of orifice  40  and angular orientation thereof relative to front surface  54 . Flow path  48  and flow path  56  are oriented with respect to one another so as to intersect at a predetermined location  62  spaced from front surface  54  of nozzle plate  32 . Print head  14  includes an element  64  operable for controllably breaking ink stream  44  into successive ink droplets flowing along flow path  48 , represented by ink droplet  66 , upstream of predetermined location  62 . Similarly, print head  14  includes an element  68  operable for controllably breaking ink stream  46  into ink droplets flowing along flow path  56 , represented by ink droplets  70  and  72 , upstream of location  62 . 
     As a result of the size and timing of the respective ink droplets  66 ,  70  and  72 , ink droplets  66  traveling along flow path  48  collide with ink droplets  70  traveling along flow path  56  at location  62 , to thereby “mask” the affected ink drops  70 , that is, prevent their continued passage along flow path  56  past location  62  while permitting ink droplets  72  to proceed along flow path  56 . Referring briefly again to FIG. 1, drum  30  is positioned in spaced relation to flow path  56  such that ink droplets  72  that travel pass location  62  can impinge receiver  12 . Ink gutter  24  is positioned to receive any ink droplets  66  traveling along flow path  48  which do not collide with ink droplets  70 , and also ink droplets  74  which are formed by the collisions of ink droplets  66  and ink droplets  70 , the collision causing ink droplets  74  to be directed along a new flow path  76  disposed between flow paths  48  and  56 . To facilitate the masking function of ink droplets  70 , it has been found to be advantageous for those individual droplets  66  to be larger than droplets  70  and  72  for several reasons. Namely, the larger that ink droplets  66  are, the more momentum they will have to cause combined droplets  74  to travel along new flow path  76  divergent from flow path  56 . Also, the larger that ink droplets  66  are, the easier it is to coordinate the collision thereof with ink droplets  70 . In droplets  66  larger than ink droplets  70  and  72  can be achieved by using a variety of techniques. Here, orifice  38  of ink ejecting nozzle  34  has a larger cross-sectional extent than the cross-sectional extent of orifice  40  of ink ejecting nozzle  36 , such that ink stream  44  has a correspondingly larger cross-sectional extent  50  than the cross-sectional extent  58  of ink stream  46 . Additionally, elements  64  and  68  operable for controllably breaking ink streams  44  and  46  into ink droplets  66  and ink droplets  70  and  72 , respectively, include annular shaped heaters  78  and  80  disposed on front surface  54  of nozzle plate  32  around respective ink ejecting nozzles  34  and  36 , heaters  78  and  80  being selectively operable to heat ink streams  44  and  46  as they pass from nozzles  34  and  36 , to reduce the surface tension of the ink which results in sufficient widening of the ink streams, as denoted at regions or zones  82 , such that the resulting pressure differences in the stream cause ink droplets to form. Here, it should be noted that ink droplets  66 ,  70 ,  72  and  74  are depicted as circles in two dimension so as to represent spheres in three dimension, although in practice, the droplets may have somewhat different shapes. It should also be noted that ink droplets  70  are substantially larger than ink droplets  72 , and that ink droplets  70  are intended to be masked, that is collide with ink droplet  66 , whereas ink droplet  72  are intended to pass between ink droplets  66  so as to continue along flow path  56  and impinge receiver  12  for forming the image thereon. In this regard, the larger ink droplets facilitate collision, whereas sequences of one to several successive small ink droplets are preferred to form correspondingly small pixels on a receiver such as receiver  12  to produce a sharper image thereon. As noted above, another advantage is that the small ink droplets  72  are able to pass more readily between the successive ink droplet  66 . 
     Referring to FIG. 3, an electrical signal trace representing drive signals generated by print head electrical drive  20  communicated to heater  78  for energizing that heater to produce ink droplets  66  versus time is shown, above a signal trace  84  representing electrical signals generated by drive  20  for energizing heater  80 . Traces  82  and  84  represent a nonprinting mode, that is, wherein the ink droplets generated from ink stream  46  collide with ink droplets  66  so that no droplets of ink stream  46  pass location  62  intact. In traces  82  and  84 , signal intervals  86  and  88  represent time periods wherein heaters  78  and  80  are not energized, such that ink streams  44  and  46  are unaffected by the heaters, whereas elevated signal amplitude intervals  90  and  92  between intervals  86  and  88  represent time periods wherein heaters  78  and  80  are energized, which results in the synchronous breaking of ink streams  44  and  46  into ink droplets. Here, signal intervals  90  and  92  are timed so as to be simultaneous such that ink streams  44  and  46  will be broken into droplets timed to collide with one another thereby providing the desired masking effect. 
     Referring to FIG. 3 a , electrical signal traces  94  and  96  representing electrical drive signals provided to heaters  78  and  80 , respectively, in a printing mode are shown. Trace  94  includes the same signal intervals  86  and  90  as trace  82 , corresponding to the regular breaking of ink streams  44  into uniformly spaced and sized ink droplets such as ink droplets  66  of FIG.  2 . Trace  96 , however, is significantly different from non-printing mode trace  84 . In a preferred implementation, which allows for the printing of multiple drops per image pixel, the time P associated with the printing of an image pixel consists of a burst of short-duration elevated-amplitude signal intervals  93  separated by low-amplitude signal intervals  98 . The signal intervals  93  are center-weighted in time during the time P as indicated in FIG. 3 a , and are separated from the next pixel data by lower-amplitude signal intervals  100 . The number of elevated-amplitude signal intervals  93  to be used in the activation of heater  80  is the number of drops to be printed per pixel plus one. An example is given here for the printing of 3 drops per pixel, although it must be realized that this is for illustrative purposes only, and that the number of drops to be printed is intended to be varied according to image data. Additionally, this invention is not limited to a particular maximum number of drops per image pixel. Again, the elevated-amplitude signal intervals  93  result in the breaking of ink steam  46  of FIG. 2 into ink droplets. The intervening low signal amplitude intervals  98  are proportional to the volume of ink droplets  72 , and the longer low amplitude signal intervals  100  are proportional to the volume of ink droplets  70 . The relative timing of higher amplitude signal intervals  90  and  93  of traces  94  and  96  are selected such that ink droplets  66  and  70  will collide at location  62 , whereas ink droplets  72  will pass between ink droplets  66  so as to continue along flow path  56  to impinge the receiver. Here, it should be recognized and understood that the size and spacing parameters of the ink droplets broken from ink streams  44  and  46  are controlled by operation of respective heaters  78  and  80 , and thus such parameter can be altered as desired to provide desired image characteristics. Additionally, it is contemplated that any desired number of ink droplets can be utilized for forming the pixels of an image. Still further, it should be recognized and understood that elements  64  and  68  can additionally and alternatively include other elements for breaking ink streams  44  and/or  46  into the desired ink droplets, including, but not limited to, other thermoelectric heater constructions, heaters located at different locations, mechanical devices, and electromechanical devices. It should also be understood that ink ejecting nozzles  34  and  36  can include orifices that differ from orifices  38  and  40  (FIG. 2) including orifices oriented so as to be perpendicular to front surface  54  of nozzle plate  32 , as long as at least one element is provided for directing the ink streams emitted therefrom along the required intersecting flow paths. 
     Turning to FIG. 4, alternative apparatus  102  for drop masking continuous ink jet printing constructed and operable according to the teachings of the present invention is shown. Like elements of apparatus  102  and apparatus  10  are identified by like numbers. Apparatus  102  includes a print head  104  including an ink chamber  42  adapted for connection in fluid communication with an ink supply reservoir such as reservoir  16  (FIG.  1 ), and a nozzle plate  32  including a plurality of pairs of ink ejecting nozzles  106  and  108  having respective orifices  110  and  112  therethrough in communication with ink chamber  42  for emitting ink streams  44  and  46  therefrom. Orifices  110  and  112  differ from previously disclosed and discussed orifices  38  and  40  of apparatus  10  in that orifices  110  and  112  are perpendicular to front surface  32  of print head  104 . In order to direct ink streams  44  and  46  emitted from orifices  110  and  112  along flow paths  48  and  56  so as to intersect at predetermined location  62 , nozzles  106  and  108  include raised structures  114  and  116  formed of or coated with a suitable conventional hydrophilic material (for use with aqueous inks). Bead structures  114  and  116  function by attracting the ink of the ink streams  44  and  46  so as to effect a change in the meniscus  118  at the juncture of ink stream  44  and nozzle  106 , and in the meniscus  120  at the juncture of ink stream  46  and nozzle  108 , sufficiently so as to skew or direct flow paths  48  and  56  toward location  62 . 
     Apparatus  102  includes elements  64  and  68  adapted for operative connection to a print head electrical drive such as drive  20  (FIG. 1) for breaking ink streams  44  and  46  into ink droplets such as ink droplets  66 ,  70  and  72 , here including piezoelectric devices  122  and  124  energizable for deforming thinner membrane portions  126  and  128  of nozzle plate  32  sufficiently to cause the desired intermittent breaking of ink streams  44  and  46 . 
     Therefore, what is provided is a continuous inkjet printing method and apparatus which utilizes desirable aspects of on-demand and masking concepts, while eliminating more complex and costly aspects of the above, namely, charging apparatus with associated high voltage circuitry. 
     The apparatus and methods described herein are preferred as they facilitate simplified, lower cost print head manufacture. 
     The foregoing describes a number of preferred embodiments of the present invention. Modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the invention. 
     PARTS LIST 
       10  apparatus for drop-masking 
     continuous inkjet printing 
       12  receiver 
       14  print head 
       16  ink supply reservoir 
       18  ink supply channel 
       20  print head electrical drive 
       22  conductive path 
       24  ink gutter 
       26  ink return reservoir 
       28  ink return conduit 
       30  drum 
       32  nozzle plate 
       34  ink ejecting nozzle 
       36  ink ejecting nozzle 
       38  orifice 
       40  orifice 
       42  ink chamber 
       44  inkstream 
       46  ink stream 
       48  flow path 
       50  cross-sectional extent 
       52  angular orientation 
       54  front surface 
       56  flow path 
       58  cross-sectional extent 
       60  angular orientation 
       62  location 
       64  element 
       66  ink droplet 
       68  element 
       70  ink droplet 
       72  ink droplet 
       74  ink droplet 
       76  flow path 
       78  heater 
       80  heater 
       82  trace 
       84  trace 
       86  signal interval 
       88  signal interval 
       90  signal interval 
       92  signal interval 
       93  signal interval 
       94  trace 
       96  trace 
       98  signal interval 
       100  signal interval 
       102  apparatus for drop-masking 
     continuous inkjet printing 
       104  print head 
       106  ink ejecting nozzle 
       108  ink ejecting nozzle 
       110  orifice 
       112  orifice 
       114  bead structures 
       116  bead structures 
       118  meniscus 
       120  meniscus 
       122  piezoelectric device 
       124  piezoelectric device 
       126  membrane position 
       128  membrane position