Electric-field manipulation of ejected ink drops in printing

Abstract: A method and apparatus is provided which compensates for environmental factors which cause misdirection of ink drops (10) ejected from an ink jet printhead (18). Ink drops (10) are electrostatically accelerated in a direction perpendicular to a print substrate (15), decreasing the ink drop flight time from the printhead (18) to print substrate (15). The decrease in flight time decreases the misdirecting effect of the environmental factors on the ink drops (10) since the environmental factors act on the ink drops for a shorter amount of time. Accelerating the ink drops also increases the spot size created when the drop impacts the print substrate (15), decreasing the amount of ink needed to create an image on the print substrate (15). The decrease in ink use results in less cockle and curl in the print substrate (15). The device also provides electrostatic deflection of the ink drops (10) in directions parallel to the print substrate (15), increasing the resolution of the printhead (18).

Description: This invention generally relates to electric-field manipulation of ink drops in printing. Conventional ink drop printing systems use various different methods to produce ink drops directed toward a print substrate. Well-known devices for ink drop printing include thermal ink jet printheads, piezoelectric transducer-type ink jet printheads, and acoustic ink jet printheads. All of these technologies produce roughly spherical ink drops having a 15-100 micron diameter directed toward a print substrate at approximately 4 m/sec. The actuators in the printheads which produce the ink drops are controlled by a printer controller. The printer controller activates the actuators in conjunction with movement of the print substrate relative to the printhead. By controlling the activation of the actuators and the print substrate movement, the print controller directs the ink drops to impact the print substrate in a specific pattern, thus forming an image on the print substrate. Ideally, all of the actuators in a printhead produce ink drops directed toward the print substrate in a direction perpendicular to the print substrate. In practice, however, some ink drops are not directed exactly perpendicular to the print substrate. The ink drops which deviate from the desired trajectory are undesirable since the misdirected drops impact the print substrate at a point not anticipated by the print controller. Therefore, misdirected drops affect the quality of the printed image by impacting the print substrate in unwanted positions. US-A-4,386,358 and 4,379,301 to Fischbeck disclose a method for electrostatically deflecting electrically charged ink drops ejected from an ink jet printhead. Charges placed on electrodes on the printhead disclosed by Fischbeck are controlled to steer the charged ink drops in desired directions to compensate for known printhead movement. By electrostatically steering the charged ink drops, the method disclosed in Fischbeck compensates for ink drop misdirection caused by the known printhead movement when the ink drop is ejected. However, the electrostatic deflection method disclosed by Fischbeck does not compensate for unpredictable environmental factors which can affect ink drop trajectories. Such environmental factors include air currents and temperature gradients between the printhead and the print substrate. In acoustic ink jet printheads, unpredictable variations in the dynamics of ink drop creation also detrimentally affect ink drop trajectories. Some of the variations in ink drop creation are caused by aberrations in the lithography of the Fresnel lens which focusses the acoustic wave used to create the ink drops. JP-A-62267146 discloses an ink jet printer according to the precharacterizing portion of claim 1. In accordance with the present invention, we provide an ink jet printer which is characterized in that the drop accelerating means comprises a charged print substrate that induces a charge on an ink surface such that the expelled drops are charged and that accelerates the drops in a direction perpendicular to the print substrate. This invention provides a device which compensates for unpredictable environmental factors which cause ink drops to have a trajectory other than the desired trajectory. The invention also provides a device which accelerates drops in a direction perpendicular to the print substrate so that less ink is needed to produce an image and therefore paper cockle and curl are decreased or diminished. This invention compensates for deviations in the desired trajectory of each ink drop ejected from the printhead by accelerating the ink drops in a direction perpendicular to the print substrate. Each ink drop ejected from the printhead is accelerated toward the print substrate by electrostatic attraction. Accelerating each ink drop toward the print substrate compensates for the various environmental factors affecting ink drop trajectory by decreasing the flight time of each ink drop. By decreasing the flight time of each ink drop, the environmental factors tending to force the ink drop from a desired trajectory have less time to act upon the ink drop. Therefore, the environmental factors misdirect each ink drop to a lesser extent than if the ink drop moved more slowly toward the print substrate. By accelerating the ink drops in the direction perpendicular to the print substrate, the invention also increases the size of the spot created when the ink drop impacts the print substrate. The larger spot size is due to the increased spreading upon impact resulting from the higher ink drop velocity and means that less ink is needed to produce an image on the print substrate. Cockle and curl in a print substrate are generally caused by ink saturation of the substrate. Therefore, since the amount of ink needed to produce an image is lessened, cockle and curl of the print substrate is lessened or eliminated. The ink drops can be steered by electrostatically deflecting the ink drops in directions parallel to the print substrate. By appropriately controlling the electrostatic deflection, the ink drops created by each column of actuators in the printhead are selectively directed to impact the print substrate at positions both left of a center position and right of the center position. The ink drops not deflected impact the print substrate at the center position. This means that each actuator can create at least two vertical print columns of spots on the print substrate. Therefore, the number of differently positioned spots created by each actuator is increased. The present invention will be described further, by way of examples, and with reference to the following figures, wherein like reference numerals refer to like elements, and: Fig. 1 is a block diagram of the general preferred embodiments of the invention; Fig. 2 is a comparative embodiment in which ink drops are accelerated toward a print substrate and steered by electrodes formed on the face of the printhead; Fig. 3 shows a set of interdigitated electrodes used to electrostatically steer ink drops; Fig. 4 shows the spot pattern created by a conventional printhead; Fig. 5 shows the spot pattern created by the preferred embodiments of the invention; Fig. 6 is a flow chart for controlling the acceleration and steering of ink drops in the first embodiment of the invention; and, Fig. 7 is an embodiment of the invention where a static charge on the print substrate serves to charge and accelerate ink drops toward the print substrate. Fig. 1 shows the communication between a print controller 1, a paper feed mechanism 2, a plurality of ink jet actuators 11, and the electrodes 3 in the general preferred embodiments of the invention. The print controller 1 directly communicates with and controls the paper feed mechanism 2, which moves the print substrate relative to the printhead. The print substrate is generally a sheet of paper, but can be formed of other materials. In the following preferred embodiments of the invention, the ink jet printhead is a page-width printhead and the print substrate is moved relative to the printhead. However, other embodiments are possible, including moving an ink jet printhead cartridge relative to the print substrate or moving both the ink jet printhead cartridge and the print substrate simultaneously. The print controller 1 also controls a set of ink drop actuators 11 formed in the printhead. In the following preferred embodiments of the invention, an acoustic ink drop printhead is used, although other types of ink drop actuators are possible, including thermal ink jet and piezoelectric transducer-type ink jet actuators. Finally, the print controller 1 directly communicates with and controls one or more sets of electrodes 3 which accelerate ink drops in directions perpendicular and parallel to the print substrate. Fig. 2 shows a comparative embodiment. A printhead 18 ejects ink drops 10 through apertures 13 directed toward a print substrate 15 using acoustic actuators 11. Each acoustic actuator 11 has a piezoelectric transducer which creates a sound wave in the ink. A lens, such as a Fresnel lens, focuses the wave at the ink surface 12. Acoustic pressure at the ink surface 12 causes an ink drop 10 to form which is directed toward the print substrate 15 at an ejection velocity of approximately 4 m/sec. Wave effects at the ink surface 12 and other physical effects cause variations in the velocity and the trajectory of the ink drops 10. Thus, although all of the ink drops 10 are ideally directed in a direction perpendicular to the print substrate 15, in practice some of the ink drops 10 are misdirected and have velocity components parallel to the print substrate 15. Positive ions in the ink congregate at the ink surface 12 in response to a high negative potential, approximately -1000V, placed on the charging plate 14, which is positioned behind the print substrate 15. This effect is enhanced by the protrusion of the ink during ink drop 10 formation. Therefore, when each ink drop 10 separates from the ink surface 12, the ink drop 10 is positively charged. The positively charged ink drop 10 carries a charge on the order of 2x10 -14 C and is strongly attracted toward the charging plate 14. As the ink drop 10 travels the 1mm distance separating the printhead 18 and the print substrate 15, the ink drop 10 is accelerated to approximately 3 or 4 times its original ejection velocity, or approximately 12-16 m/sec. The acceleration of the ink drop 10 decreases the amount of time, the flight time, the ink drop 10 takes to travel the 1mm distance to the print substrate 15. Therefore, the environmental factors, such as air currents, temperature gradients, ink drop formation variations, and the like, which cause misdirection of the ink drop 10 have a shorter period of time to act upon the ink drop 10. Accordingly, the ink drops 10 tend to impact the print substrate 15 at points closer to the desired position (directly opposite the aperture 13) than if the ink drops 10 were not accelerated toward the print substrate 15. For example, assume the ink drop 10 has a velocity component of 4 m/sec in a direction perpendicular to the print substrate 15. Thus, it takes the ink drop 10 0.25 milliseconds to travel the 1mm distance separating the printhead 18 and the print substrate 15. Assume also that the ink drop 10 has a velocity component in a direction parallel to the print substrate 15 due to an instability effect when the drop 10 was created equal to 0.01 m/sec. Therefore, the ink drop 10 will impact the print substrate 15 at a point approximately 2.5 microns from the desired position. If the ink drop 10 were accelerated toward the print substrate 15 such that the flight time of the ink drop 10 was decreased by half, or 0.125 milliseconds, the ink drop 10 would impact the print substrate 15 at a point approximately 1.25 microns from the desired position. Also shown in Fig. 2 are the steering electrodes 16 and 17, which are formed on the face of the printhead 18. An insulating layer 20 separates the steering electrodes 16 and 17 from the printhead 18 and also covers the steering electrodes 16 and 17. The steering electrodes 16 and 17 are encased in the insulating layer 20 to avoid short circuits and corrosion of the steering electrodes 16 and 17 due to stray ink droplets or other foreign matter on the steering electrodes 16 and 17. The steering electrodes 16 and 17 can be formed on the printhead 18 in a variety of different ways, including screen printing, sputter deposition using a shadow mask, photolithographic patterning or other standard lithography techniques. The steering electrodes 16 and 17 are preferably formed of a conductive metal, such as aluminum, gold, nickel or the like. The steering electrodes 16 and 17 communicate with the print controller 1, which selectively charges the steering electrodes 16 and 17 to steer the charged ink drops 10 in a desired direction. For example, an ink drop 10, which is ejected from an aperture 13 positioned to the right of a first steering electrode 16 having a potential of -100V and to the left of a second steering electrode 17 having a potential of +100V, will be deflected to the left toward the first steering electrode 16 in accordance with well-known electrostatic principles. Likewise, if the potentials on the steering electrodes 16 and 17 are reversed, the ink drop 10 will be deflected to the right. If the steering electrodes 16 and 17 are both set to a 0V potential, the ink drop 10 will travel in a center trajectory and not be directed toward either the left or the right. Other voltage potentials can be used as will be appreciated by those skilled in the art. Fig. 3 shows a possible configuration for the steering electrodes 16 and 17 on the printhead 18. The steering electrodes 16 and 17 are interdigitated and one portion of the steering electrodes 16 or 17 lies between each column 19 of the apertures 13. Therefore, the print controller 1 can set the voltage potentials on the steering electrodes 16 and 17 such that an entire column 19 of apertures 13 will eject a series of ink drops 10 directed either toward the right, left or center position. Fig. 4 shows the spot pattern created by a conventional acoustic ink jet printhead having a 600 spot per inch (spi) resolution capacity. Apertures within a column 19 of apertures 13 in the conventional ink jet printhead are offset at a center-to-center distance of approximately 43 microns in the direction perpendicular to the columns 19. Therefore, the spots created by the apertures 13 are spaced approximately 43 microns apart, thus giving a 600 spi resolution. Fig. 5 shows the spot pattern produced by the Figure 2 embodiment. As in the conventional ink jet printhead, the apertures 13 in the preferred embodiments are also spaced at the center-to-center distance of approximately 43 microns. However, since the steering electrodes 16 and 17 are controlled by the print controller 1 to deflect the ink drops 10 to both left and right positions, the resolution of the printhead 18 is increased. The steering electrodes 16 and 17 are controlled such that the left and right spots are deflected approximately 14 microns from the center spot position. This places 3 dots within each 43 micron "pixel" centered on each column 19 of apertures 13, resulting in an overall center-to-center spacing for the dots of approximately 14-15 microns. A spot spacing of approximately 14 microns gives a resolution of approximately 1,800 spi in the horizontal direction. Since the conventional ink jet printhead creates the spot pattern shown in Fig. 4 and has a relatively lower resolution, the conventional printhead uses more ink (i.e. more ink drops per unit area) to produce an image on the print substrate than a printhead of higher resolution. Higher ink use saturates the print substrate with the ink and results in cockle and curl of the print substrate. Also, higher resolution printheads exhibit greater greytone control, i.e. the ability to produce varying shades of grey in a printed image. Fig. 6 is a flowchart outlining the method for controlling the comparative embodiment. In step S10, the print controller 1 charges the charging plate 14 to -1000 V. Next, in step S20, the print controller 1 moves the print substrate 15 relative to the printhead 18. In step S30, the print controller 1 grounds the steering electrodes 16 and 17 to 0V and the ink drops 10 are ejected from the desired apertures 13 in step S40. This series of steps creates the center spots produced by the columns 19 of apertures 13 as shown in Fig. 5. In step S50, the print controller 1 charges the steering electrodes 16 and 17 to +100 V and - 100 V, respectively. In step S60, the ink drops 10 are ejected from the desired apertures 13 to create a series of left or right deflected spots depending on which sides the steering electrodes 16 and 17 are on relative to the columns 19 of apertures 13. In step S70, the print controller 1 charges the steering electrodes 16 and 17 to -100V and +100V, respectively. That is, in step S70, the steering electrodes 16 and 17 are charged oppositely to the charges used in step S50. The ink drops 10 are then ejected from the desired apertures 13 in step S80, to create another set of left and right deflected ink drops 10 which are oppositely deflected from those ejected in step S60. In step S90, the print controller 1 determines if there is more printing to be done. If so, control jumps back to step S30. Otherwise, the print controller 1 stops printing. Fig. 7 shows a preferred embodiment of the invention. The print head 18 is configured in the same manner as in the comparative embodiment and operates similarly to eject the ink drops 10. However, a ground plate 30 is positioned behind the print substrate 15 and is connected to ground. A corona discharge device 31 or similar apparatus places a negative static charge on the surface of the print substrate 15. The negative surface charge on the print substrate 15 acts identically to the charging plate 14 of the comparative embodiment. Control of the preferred embodiment of the invention is the same as that shown in Fig. 6, except that in step S10 the print controller 1 directs the corona discharge device 31 to place the negative surface charge on the print substrate 15. Another difference between the comparative and preferred embodiments is the voltage potential created by the surface charge placed on the print substrate 15 in the second embodiment must be somewhat higher, possibly as high as -2000V, to maintain the proper charging and accelerating of the ink drops 10. The reason is that as the positively charged ink drops 10 impact the print substrate 15, some of the negative surface charge placed on the print substrate 15 is neutralized. The relatively higher static charge on the print substrate 15 compensates for the neutralizing effect of the positively charged ink drops 10 impacting the print substrate 15.

Claims: An ink jet printer for forming an image on a print substrate (15), comprising: a printhead (18) comprising, a face nearest the print substrate, a plurality of apertures (13) formed in the face, and drop expelling means (11) for expelling a drop (10), the drop having a velocity directed toward the print substrate; drop accelerating means that accelerates the drops in a direction perpendicular to the print substrate; and a controller (1) controlling the drop expelling means and the drop accelerating means, characterized in that the drop accelerating means comprises a charged print substrate (15) that induces a charge on an ink surface (12) such that the expelled drops are charged and that accelerates the drops in a direction perpendicular to the print substrate. The printer of claim 1, wherein the drop accelerating means comprises: corona discharge means (31) for placing a charge on the print substrate.

Inventors: SMITH DONALD LEONARD, STEARNS RICHARD GREGORY, SMITH, DONALD LEONARD, STEARNS, RICHARD GREGORY
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1996
Main classification: B41J2
IPC codes: B41J2