Abstract:
A printhead includes a droplet-forming heater operable in a first state to form droplets from a fluid stream having a first volume traveling along a path direction and in a second state to form droplets from the fluid stream having a second volume traveling along the path direction. A droplet deflector system is positioned relative to the droplet-forming heater, which applies a force to the droplets traveling along the path direction, whereby the droplets having the first volume diverge from the path direction by a greater extent than do the droplets having the second volume. A droplet-steering heater is adapted to selectively asymmetrically apply heat to the stream such that the path direction is changed.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to the field of digitally controlled printing devices, and in particular to continuous ink jet printers in which a liquid ink stream breaks into droplets, some of which are selectively deflected. Either the deflected droplets or the non-deflected droplets can be printed on a print medium with the droplets having corrected print locations. 
   BACKGROUND OF THE INVENTION 
   Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. The first technology, commonly referred to as “drop-on-demand” ink jet printing, 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 printhead and the print media and strikes the print media. The second technology, commonly referred to as “continuous stream” or “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices and deflector plates. Examples of conventional continuous ink jet printers include U.S. Pat. No. 1,941,001 issued to Hansell, on Dec. 26, 1933; U.S. Pat. No. 3,373,437 issued to Sweet et al., on Mar. 12, 1968; U.S. Pat. No. 3,416,153 issued to Hertz et al., on Dec. 10, 1968; U.S. Pat. No. 3,878,519 issued to Eaton, on Apr. 15, 1975; and U.S. Pat. No. 4,346,387 issued to Hertz, on Aug. 24, 1982. 
   U.S. Pat. No. 3,709,432, which issued to Robertson on Jan. 9, 1973, discloses stimulation of an ink filament to cause the ink to break up into uniformly spaced droplets. Before they break up into droplets, the lengths of the filaments are regulated by controlling the stimulation energy supplied to transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air across their paths affects the trajectories of the filaments before they break up into droplets. 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 receiver. 
   U.S. Pat. No. 6,079,821, which issued to Chwalek et al. on Jun. 27, 2000, discloses a continuous ink jet printer. A printhead includes a plurality of nozzles, each of which uses actuation of a single asymmetric heater to both create individual ink droplets from a filament of working fluid and deflect those ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a receiver, 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. 
   The paths of drops ejected from a row of equally spaced nozzles should be parallel. Continuous inkjet printheads often require fine adjustments in jet direction and drop placement to counteract flight path errors due, for example by manufacturing defects in the printhead, differences in the resistances of the drop-formation heaters, particles and other debris near the nozzle bores, air turbulence and splay, stitching defects, etc. It has been suggested that such adjustments can be effected by segmenting the drop formation heater in much the way suggested by Chwalek et al. in above-mentioned U.S. Pat. No. 6,079,821. Different power levels can then be applied to the heater segments in order to steer the jet in a desired direction to compensate for flight path errors. However, use of the drop formation heater to also adjust jet direction and drop placement convolutes the two processes, potentially requiring trade-offs in the optimization of drop formation and drop placement. 
   U.S. Pat. No. 6,517,197, which issued to Hawkins et al. on Feb. 11, 2003, recognized that, while the ink droplet-forming mechanism and the ink droplet-steering mechanism may be the same mechanism, it is also possible to make the droplet-forming mechanism and the droplet-steering mechanism separate distinct mechanisms. The examples provided by Hawkins et al. included a piezoelectric actuator droplet-forming mechanism with a segmented heater droplet-steering mechanism. While such a system overcomes the need for trade-offs in the optimization of drop formation and drop placement that would exist in the Chwalek et al. device, the use of a segmented heater droplet-steering mechanism would add a little extra energy to a jet. This would undesirably increase the velocity of corrected jets and cause the corrected jet to be out of sync with the non-corrected jets. It is feature of the present invention to compensate for the additional energy added by the segmented heater by providing a heater as the droplet-forming mechanism and to adjust the total amount of energy applied to corrected jets so as to keep the velocity of the corrected jets the same as the velocity of the non-corrected jets by reducing the energy from the droplet-forming mechanism by an amount substantially equal to the additional energy added by the segmented heater. 
   It is an object of the present invention is to simplify construction of a continuous ink jet printhead and printer having improved placement accuracy of individual ink drops in order to render images of high quality. 
   It is another object of the present invention to provide a continuous ink jet printhead and printer capable of rendering high-resolution images with reduced image artifacts using large volumes of ink. 
   It is yet another object of the present invention is to improve the reliability of a continuous ink jet printhead. 
   It is still another object of the present invention to simplify construction and operation of a continuous ink jet printer suitable for printing high quality images having reduced artifacts due to systematic errors of drop placement. 
   SUMMARY OF THE INVENTION 
   According to a feature of the present invention, a continuous ink jet printer in which a continuous stream of ink is emitted from a nozzle bore includes an ink delivery channel and a source of pressurized ink to establish a continuous stream of ink. A droplet-forming heater causes the stream to break up into a plurality of droplets. Actuation of a droplet-steering heater, having at least one selectively actuated section associated with less than the entire perimeter of the nozzle bore, produces an asymmetric application of heat to the stream to control the direction of the stream. The droplet-steering heater is preferably formed of a plurality of heater sections that, in the aggregate, substantially surround the nozzle bore so that selective actuation of the heater sections steers the stream in any of a plurality of directions away from the actuated heated sections. 
   According to another feature of the present invention, a printhead includes a droplet-forming heater operable in a first state to form droplets from a fluid stream having a first volume traveling along a path direction and in a second state to form droplets from the fluid stream having a second volume traveling along the path direction. A droplet deflector system is positioned relative to the droplet-forming heater, which applies a force to the droplets traveling along the path direction, whereby the droplets having the first volume diverge from the path direction by a greater extent than do the droplets having the second volume. A droplet-steering heater is adapted to selectively asymmetrically apply heat to the stream such that the path direction is changed. 
   According to still another feature of the present invention, a method of correcting droplet placement error in a printhead includes the steps of forming droplets from fluid ejected through a first nozzle using a first, droplet-forming heater, the droplets traveling in an ejection direction; determining when the ejection direction is other than in a desired direction; and using a second, droplet-steering heater to change the ejection direction of the droplets to the desired direction by asymmetrically applying heat to the fluid. 
   According to yet another feature of the present invention, a method of printing an image includes the steps of forming, by means of a droplet-forming heater, droplets having a first volume traveling along a path direction and droplets having a second volume traveling along the path direction; applying a force to the droplets traveling along the path direction such that the droplets having the first volume diverge from the path direction by a greater extent than do the droplets having the second volume; and using a droplet-steering heater to selectively asymmetrically apply heat to the stream such that the path direction is changed. 

   
     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; 
       FIG. 2  is a schematic plan view of an ink droplet-forming heater used in the printhead of  FIG. 1 ; 
       FIG. 3  is a schematic plan view of an ink droplet-steering heater used in the printhead of  FIG. 1 ; 
       FIG. 4  is a top plan view of the assembled ink droplet-forming heater of  FIG. 2  and the ink droplet-steering heater of  FIG. 3 ; 
       FIG. 5  is a side sectional view of the printhead of  FIG. 1  taken along line  5 - 5  of  FIG. 4 ; 
       FIG. 6  is a schematic plan view of a printhead made in accordance with another preferred embodiment of the present invention; 
       FIG. 7  is a diagram illustrating a frequency control of a droplet-forming heater and the resulting ink droplets; 
       FIG. 8  is a schematic view of an ink jet printer made in accordance with the preferred embodiment of the present invention; and 
       FIG. 9  is a side sectional view of a printhead wherein droplets emitted with a crooked trajectory have not been corrected; 
       FIG. 10  is a side sectional view of a printhead of  FIG. 9  wherein droplets, which would have been emitted with a crooked trajectory, have been corrected; 
       FIG. 11  is a top plan view of an alternative embodiment of the assembled ink droplet-forming heater of  FIG. 2  and the ink droplet-steering heater of  FIG. 3 ; 
       FIG. 12  is a top plan view of an alternative embodiment of the assembled ink droplet-forming heater of  FIG. 2  and an alternative embodiment of the ink droplet-steering heater of  FIG. 3 ; 
       FIG. 13  the ink droplet-forming heater the ink droplet-steering heater of  FIG. 12  with the stacking order of the heaters reversed from that of  FIG. 12 ; 
       FIG. 14  shows that droplet-forming heater can also be split for controlling the trajectory of the droplets in a direction normal to the control offered by the droplet-steering heater; 
       FIG. 15  shows the two heaters one outside of the other and lying in the same plane; and 
       FIGS. 16 and 17  are alternative side sectional views taken along line  16 - 17  of  FIG. 15 . 
   

   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 , an integrated printhead  10  is associated with at least one ink supply  12  and a controller  14 . Controller  14  can be of any type, including a microprocessor-based device having a predetermined program, etc. Although integrated printhead  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 the preferred. 
   At least one nozzle bore  16  is formed on printhead  10 . Nozzle bore  16  is in fluid communication with ink supply  12  through an ink passage  17  also formed in or connected to printhead  10 . Black and white or single color printing may be accomplished using a single ink supply  12  and one set of nozzle bores  16 . Printhead  10  may incorporate additional ink supplies and corresponding nozzle bore sets in order to provide color printing using three or more primary ink colors. 
   Integrated printhead  10  can be manufactured using known techniques, such as CMOS and MEMS techniques. There can be any number of nozzle bores  16  and the distance between the nozzle bores can be adjusted in accordance with the particular application to avoid ink coalescence, and deliver the desired resolution. Integrated printhead  10  can be formed using a silicon substrate, etc. Also, integrated printhead  10  can be of any size and components thereof can have various relative dimensions. 
   An ink droplet-forming heater  18  and an ink droplet-steering split heater  19  are formed or positioned on printhead  10  around a corresponding nozzle bore  16 .  FIG. 2  is a detailed view of droplet-forming heater  18 ,  FIG. 3  is a detailed view of droplet-steering heater  19 , and  FIG. 4  is an assembled view of heaters  18  and  19 .  FIG. 5  is a sectional view taken through printhead  10  along section line  5 - 5  of  FIG. 4 . Ink droplet-steering heater  19  comprises a first side  20   a  and a second side  20   b  formed or positioned on printhead  10  around a corresponding nozzle bore  16 . Although droplet-steering heater  19  may be disposed radially away from an edge of corresponding nozzle bore  16 , the split heater is preferably disposed close to the corresponding nozzle in a concentric manner. In a preferred embodiment, the split heater is formed in a substantially circular or ring shape. In an alternative preferred embodiment, shown in  FIG. 6 , droplet-steering heater  19  has a rectangular first side  20   a  and rectangular second side  20   b . Droplet-steering heater  19  may be formed in a partial segmented ring, square, etc. Droplet-forming heater  18  and droplet-steering heater  19  are made of an electric resistive material, for example a thin film material such as titanium nitride or polysilicon, and are connected to electrical contact pads  22  and  23 , respectively, via conductors  25 . The heaters may be deposited by well-known techniques of thin film deposition and patterning, such as evaporation, sputtering, chemical vapor deposition, photolithography, and etching. 
   Conductors  25  and their associated electrical contact pads  22  and  23  may be at least partially formed or positioned on printhead  10  and provide an electrical connection between controller  14  and the heaters. Alternatively, the electrical connection between controller  14  and the heaters may be accomplished in any well-known manner. Droplet-forming heaters  18 , droplet-steering heaters  19 , electrical contact pads  22  and  23  and conductors  25  can be formed and patterned through vapor deposition and lithography techniques, etc. Droplet-forming heaters  18  and droplet-steering heaters  19  can include heating elements of any shape and type, such as resistive heaters, radiation heaters, convection heaters, chemical reaction heaters (endothermic or exothermic), etc. 
   An example of the electrical activation waveform provided by controller  14  to droplet-forming heater  18  is shown generally in  FIG. 7  as a function of time (horizontal axis). Individual ink droplets  30 ,  31 , and  32 , resulting from the ejection of ink from nozzle  16  and actuation of droplet-forming heater  18 , are shown schematically at the bottom of  FIG. 7 . A high frequency of activation of heater  18  results in small volume droplets  31 ,  32 , while a low frequency of activation of heater  18  results in large volume droplets  30 . The drops are spaced in time as they are ejected from nozzle  16  according to the same time axes of the electrical waveforms. 
   In a preferred implementation, which allows for the printing of multiple droplets per image pixel, a time  39  associated with printing of an image pixel includes time sub-intervals reserved for the creation of small printing droplets  31 ,  32  plus time sub-intervals for creating one larger non-printing droplet  30 . In  FIG. 7 , only time for the creation of two small droplets  31 ,  32  is shown for simplicity of illustration, however, it should be understood that the reservation of more time for a larger number of small droplets is clearly within the scope of this invention. 
   When printing each image pixel, large droplet  30  is created through the activation of droplet-forming heater  18  with electrical pulse time  33 , typically from 0.1 microseconds to 10 microseconds in duration, and more preferentially 0.5 to 1.5 microseconds. The additional (optional) activation of droplet-forming heater  18 , after delay time  36 , with an electrical pulse  34  is conducted in accordance with image data wherein at least one small droplet is required. When image data requires another small droplet be created, droplet-forming heater  18  is again activated after delay  37 , with a pulse  35 . In this example, small droplets  31 ,  32  are printed while large droplet  30  is guttered. 
   Heater activation electrical pulse times  33 ,  34 , and  35  are substantially similar, as are delay times  36  and  37 . Delay times  36  and  37  are typically 1 microsecond to 100 microseconds, and more preferentially, from 3 microseconds to 6 microseconds. Delay time  38  is the remaining time after the maximum number of small droplets have been formed and the start of electrical pulse time  33 , concomitant with the beginning of the next image pixel with each image pixel time being shown generally at  39 . The sum of droplet-forming heater  18  electrical pulse time  33  and delay time  38  is chosen to be significantly larger than the sum of a heater activation time  34  or  35  and delay time  36  or  37 , so that the volume ratio of large droplets to small droplets is preferentially a factor of four or greater. It is apparent that droplet-forming heater  18  activation may be controlled independently based on the ink color required and ejected through corresponding nozzle  16 , movement of printhead  10  relative to a print medium, and an image to be printed. The absolute volume of the small droplets  31  and  32  and the large droplets  30  may be adjusted based upon specific printing requirements such as ink and media type or image format and size. As such, reference below to large volume non-printed droplets  30  and small volume printed droplets  31  and  32  is relative in context for example purposes only and should not be interpreted as being limiting in any manner. 
     FIG. 8  illustrates one embodiment of a printing apparatus  42  (typically, an ink jet printer or printhead) wherein large volume ink droplets  30  and small volume ink droplets  31  and  32  are ejected from integrated printhead  10  substantially along a path X in a stream. A droplet deflector system  40  applies a force (shown generally at  46 ) to ink droplets  30 ,  31 , and  32  as ink droplets  30 ,  31 , and  32  travel along path X. Force  46  interacts with ink droplets  30 ,  31 , and  32  along path X, causing the ink droplets  31  and  32  to alter course. As ink droplets  30  have different volumes and masses from ink droplets  31  and  32 , force  46  causes small droplets  31  and  32  to separate from large droplets  30  with small droplets  31  and  32  diverging from path X along small droplet or printed path Y. While large droplets  30  can be slightly affected by force  46 , large droplets  30  remain traveling substantially along path X. 
   Droplet deflector system  40  can include a gas source that provides force  46 . Typically, force  46  is positioned at an angle with respect to the stream of ink droplets operable to selectively deflect ink droplets depending on ink droplet volume. Ink droplets having a smaller volume are deflected more than ink droplets having a larger volume. 
   Droplet deflector system  40  facilitates laminar flow of gas through a plenum  44 . An end  48  of the droplet deflector system  40  is positioned proximate path X. An ink recovery conduit  70  is disposed opposite a recirculation plenum  50  of droplet deflector system  40  and promotes laminar gas flow while protecting the droplet stream moving along path X from air external air disturbances. Ink recovery conduit  70  contains a ink guttering structure  60  whose purpose is to intercept the path of large droplets  30 , while allowing small ink droplets  31 ,  32 , traveling along small droplet path Y, to continue on to a receiver W carried by a print drum  80 . 
   In a preferred implementation, the gas pressure in droplet deflector system  40  and in ink recovery conduit  70  are adjusted in combination with the design of ink recovery conduit  70  and recirculation 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 media W is transported in a direction transverse to path X by print drum  80  in a known manner. Transport of recording media W is coordinated with movement of integrated printhead  10 . This can be accomplished using controller  16  in a known manner. 
   Ink recovery conduit  70  communicates with an ink recovery reservoir  90  to facilitate recovery of non-printed ink droplets by an ink return line  100  for subsequent reuse. Ink recovery reservoir  90  can include an open-cell sponge or foam  130 , which prevents ink sloshing in applications where the integrated printhead  10  is rapidly scanned. A vacuum conduit  110 , coupled to a negative pressure source  112  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 small droplet path Y. Additionally, gas recirculation plenum  50  diverts a small fraction of the gas flow crossing ink droplet path X to provide a source for the gas that is drawn into ink recovery conduit  70 . 
   Droplet deflector system  40  can be of any type and can include any number of appropriate plenums, conduits, blowers, fans, etc. Additionally, droplet deflector system  40  can include a positive pressure source, a negative pressure source, or both, and can include any elements for creating a pressure gradient or gas flow. Ink recovery conduit  70  can be of any configuration for catching deflected droplets and can be ventilated if necessary. 
   In the illustrated embodiment, small droplets form printed droplets that impinge on the receiver while large droplets are collected by an ink guttering structure. However, large droplets can form the printed droplets while small droplets are collected by the ink guttering structure. This can be accomplished by positioning the ink guttering structure, in any known manner, such that the ink guttering structure collects the small droplets. Printing in this manner provides printed droplets having varying sizes and volumes. 
   Large volume droplets  30  and small volume droplets  31  and  32  can be of any appropriate relative size. However, the droplet size is primarily determined by ink flow rate through nozzle bore  16  and the frequency at which droplet-forming heater  18  is cycled. The flow rate is primarily determined by the geometric properties of nozzle bore  19  such as nozzle diameter and length, pressure applied to the ink, and the fluidic properties of the ink such as ink viscosity, density, and surface tension. As such, typical ink droplet sizes may range from, but are not limited to, 1 to 10,000 Pico liters. 
   Although a wide range of droplet sizes are possible, at typical ink flow rates, for a 10 micron diameter nozzle, large volume droplets  30  can be formed by cycling heaters at a frequency of 50 kHz producing droplets of 20 Pico liter in volume and small volume droplets  31  and  32  can be formed by cycling heaters at a frequency of 200 kHz producing droplets that are 5 Pico liter in volume. These droplets typically travel at an initial velocity of 10 m/s to 20 m/s. Even with the above droplet velocity and sizes, a wide range of separation distances S between large volume and small volume droplets is possible depending on the physical properties of the gas used, the velocity of the gas and the interaction distance L, as stated previously. For example, when using air as the gas, typical air velocities may range from, but are not limited to 100 to 1000 cm/s while interaction distances L may range from, but are not limited to, 0.1 to 10 mm. 
   Receiver W can be of any type and in any form. For example, the receiver can be in the form of a web or a sheet. Additionally, receiver W can be composed from a wide variety of materials including paper, vinyl, cloth, other large fibrous materials, etc. Any mechanism can be used for moving the printhead relative to the receiver, such as a conventional raster scan mechanism, etc. 
   In the embodiments discussed above, controller  14  is provided to control the trajectory of ink drops  30 ,  31 ,  32  ejected from nozzle bore  16  in the slow scan direction which controls the placement of ink drops on a receiver in the slow scan. As such, a simplified printhead and printer having reduced image artifacts due to ink drop misalignment in the slow scan direction is provided. It is also contemplated that if the printed ink drop position, in the slow scan direction, differs from the desired printed position, ink drop misplacement is corrected by controlling or modifying the electrical activation waveforms provided to integrated printhead  10 . In order to accomplish this, the extent of ink drop misplacement in the slow scan direction of ink drops ejected from one or more printhead nozzle bores is ascertained. This can be accomplished using any device and/or method known in the art. In the event that correction is needed, voltage waveforms from controller  14  provide electrical activation waveforms so as to correct misplacement. To this extent, it is understood that the slow scan direction is generally perpendicular to the direction of motion of the recording medium and integrated printhead  10  during a fast scan printing of one or more image swaths. 
   As is well known in the art of inkjet printing, misplacement errors may be determined by observing, for example with a digital imager, etc., the placement of ink drops intended to be printed at particular locations. Then, using a look-up table to determine the appropriate electrical activation waveforms to be provided to integrated printed  10 . Alternatively, determination procedures, for example, the procedure of using an optical sensor including a quad photodiode detector whose outputs are indicative of the positions of vertical test lines; projecting light upon a flying ink drop and detecting misalignment by the amount of light reflected; using an optical technique for detecting droplet position; and using a piezoelectric detector for drop position determination, can be used. It is contemplated that determining the extent of ink drop misplacement can be made repeatedly, correcting as necessary, thereby reducing subsequent errors in ink drop placement during each printing iteration as look-up tables are refined. 
   While the drop volumes, spacings, velocities etc. are provided by droplet-forming heater  18 , droplet steering is controlled by heater  19 . Droplets ejected using different electrical activation of first and second sides  20   a  and  20   b , respectively, differ in their printed positions in a direction substantially parallel to the direction defined by the row of nozzle bores on integrated printhead  10 . By controlling the electrical activation waveforms, for example by using controller  14 , the printed positions of droplets can be controlled. More generally stated, in accordance with the present invention, the drops provided by integrated printhead  10  can be printed in different positions in a direction parallel to a steering direction of droplet-steering heater  19 . These positions depend on the electrical activation waveforms. 
   The ability to print droplets in different positions comes from the action of droplet-steering heater  19 , which causes angulation of the droplet path or trajectory along the steering direction. Thereby, in conjunction with controller  14 , the paths of drops ejected from nozzle bores  16  can be controlled. For example, the paths of drops ejected from nozzle bores  16  can be controlled to be parallel when viewed along the fast scan direction. 
   The droplet-steering mechanism of  FIGS. 1 and 2  steers the jetted drops in a left and right direction as viewed in  FIGS. 1 and 2 . Hence the positions of droplets on the recording medium are controlled in a line parallel to the row of nozzles, that is, in the slow scan direction. The steering direction of droplet-steering heater  19  is perpendicular to its axis of symmetry, and thus the steering direction would change if, for example, droplet-steering heater  19  were rotated in  FIG. 1 . More generally stated, the steering direction of droplets and thus the direction in which droplets can be controllable positioned by the steering mechanism on the receiver is parallel to a line between corresponding sides  20   a  and  20   b  of droplet-steering heater  19 . 
     FIGS. 9 and 10  illustrate a pair of nozzle bores on a printhead. Ink droplet-forming heaters have been omitted from these schematic drawings for clarity. In  FIG. 9 , droplet-steering heaters  19  have not been activated. The ink droplets from left nozzle bore  16   a  follow a vertical trajectory, but the trajectory of the ink droplets from right nozzle bore  16   b  is crooked. Such crooked trajectory may be due to misalignment of the bore and ink channel. If the angle of deviation is severe enough and not corrected, the crooked trajectory will cause image artifacts. It will be understood by those skilled in the art that the present invention is not limited to the correction of crooked trajectories, but may be applied to purposely change the direction of straight trajectory jets to improve drop placement accuracy, to mask streak artifacts, to dither the jets, and to hide stitching artifacts. 
   If drops from one or more nozzle bores  16  are found to be systematically misaligned due to a nozzle defect, controller  14  can control the electrical activation waveforms applied to either of the first and second sides  20   a  and  20   b  of the associated droplet-steering heater  19  of the misaligned nozzles so that for each misaligned nozzle, the drop trajectory is caused to be the desired trajectory and the misalignment is corrected. 
   Correction of misalignment is illustrated in  FIG. 10 , wherein an electrical activation waveform has been applied to first side  20   a  of droplet-steering heater  19  to restore a proper trajectory of the ejected droplets. The misalignment of nozzle  16   b  has been corrected by altering the electrical activation waveform applied to the first side  20   a  of the split droplet-steering heater  19 . 
   It should be understood that the energy applied to the droplet by steering heater  19  to restore a proper trajectory of the ejected droplet, if not compensated for, will increase the velocity of the drop formed by droplet-forming heater  18  and result in a misplaced drop on the receiver. Because the droplet-forming mechanism and droplet-steering mechanism are both heaters and are separate one from the other, the extra energy added to a droplet by droplet-steering heater  19  can easily be compensated for by programming controller  14  to reduce the energy supplied by droplet-forming heater  18 . 
   While the embodiment of the invention illustrated in  FIGS. 2-4  have droplet-forming heater  18  below droplet-steering heater  19  in the orientation of the drawings, an embodiment illustrated in  FIG. 11  reverses the order of the heaters. In another embodiment illustrated in  FIG. 12 , droplet-steering heater  19  is split into four quadrants  20   c ,  20   d ,  20   e  and  20   f  for additional control of the droplet trajectory.  FIG. 13  shows this feature with the stacking order of the heaters reversed from that of  FIG. 12 .  FIG. 14  shows that droplet-forming heater  18  can also be split into two segments  18   a  and  18   b  for controlling the trajectory of the droplets in a direction normal to the control offered by droplet-steering heater  19 . Of course any amount of angular rotation of the split heaters can be used for trajectory control. 
   The embodiments of the present invention described above provide for droplet-forming heater  18  and droplet-steering heater  19  to be stacked one above the other. This is not a requirement, and other orientations are contemplated within the scope of the invention. For example,  FIG. 15  shows the two heaters one outside of the other and lying in the same plane, as indicated in the alternative views of  FIGS. 16 and 17 . 
   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 scope of the invention, as is intended to be encompassed by the following claims and their legal equivalents. 
   PARTS LIST 
   
       
       
         
             10 . integrated printhead 
             12 . ink supply 
             14 . controller 
             16 . nozzle bore 
             17 . passage 
             18 . ink droplet-forming heater 
             19 . ink droplet-steering heater 
             20   a . first side of steering heater 
             20   b . second side of steering heater 
             20   c . heater quadrant 
             20   d . heater quadrant 
             20   e . heater quadrant 
             20   f . heater quadrant 
             22 . contact pad 
             23 . contact pad 
             25 . conductor 
             30 . large volume ink droplet 
             31 . small volume ink droplet 
             32 . small volume ink droplet 
             40 . droplet deflector system 
             42 . printing apparatus 
             44 . plenum 
             46 . deflection force 
             48 . end 
             50 . recircualtion plenum 
             60 . ink guttering structure 
             70  ink recovery conduit 
             80 . print drum 
             90 . ink recovery reservoir 
             100 . ink return line 
             110 . vacuum conduit 
             112 . negative pressure source 
             130 . sponge or foam