Abstract:
An improved printer and method of printing uses an electrostatic field at the point of ink contact with the print medium to improve ink application. This improved printer and printing method makes possible a higher resolution final image; provides for greater control of the application of ink on the print medium; permits the use of a wider range of inks having variable viscosities, surface tensions, and elasticities; and permits the use of a variety of ink application speeds.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/019,783, filed Jan. 8, 2008, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     One known printer technology includes a print head that (1) immerses a wire in a reservoir of pigmented liquid material (e.g., ink) for a period of time sufficient to allow the wire to be coated with ink, (2) places the coated wire in close proximity to a print medium, and (3) directs a stream of air to contact the coated wire and thereby causes at least some of the ink on the wire to be deposited onto the print medium. The speed of the wire, the proximity of the wire to the print medium, and the force of the air stream may be digitally controlled by a processor, controller, microprocessor, or other computing device to ensure that a desired image resolution is achieved. By forming a print head with multiple wires; multiple, differently colored ink reservoirs; and multiple air streams, and by controlling and coordinating the metering of the ink and the position of the print head in relation to the print medium, a digital image can be created on a large-sized print medium. U.S. Pat. Nos. 5,944,893; 5,972,111; 6,089,160; 6,090,445; 6,190,454; 6,319,555; 6,398,869; and 6,786,971 describe this printer technology in greater detail and are hereby incorporated by reference. 
       FIG. 1  is a perspective view of one embodiment of the above-identified prior art single color ink injector, generally indicated at  10 , for depositing paint, ink, dye, or other liquid pigmented material that could be used for painting or printing onto a print medium. A pulley  13  having a circumscribing groove  38  defined therein is secured to a shaft  15  of a motor  14 . An elongate frame member  32  depends from and is secured to print medium  12  and extends into a reservoir of ink  24 . A rotatable or stationary guide  34  is attached to a distal end  37  of elongate frame member  32 . Guide  34  is illustrated as a cylindrical, non-rotatable member having a groove  40  circumscribing guide  34  in which a wire cable  36 , can slide during rotation of wheel  13 . Wire cable  36  is described in greater detail in the above-identified patents. Wire cable  36  is disposed in groove  38  circumscribing the wheel  13  and in groove  40  circumscribing guide  34 . 
     An elongate reservoir retaining member  16  is attached to plate  12  and includes a flange  18  defining a notch  20  between the flange  18  and elongate reservoir retaining member  16 . Notch  20  is configured to receive a top lip  22  of ink reservoir  24 . A bottom plate  26  is secured to a distal end  28  of elongate reservoir retaining member  16  with a threaded nut  31  that is threaded onto a threaded shaft  33 . Threaded shaft  33  is secured to distal end  28  of elongate reservoir retaining member  16 . Bottom plate  26  abuts against the bottom  30  of the ink reservoir  24  and holds it between flange  18  and bottom plate  26 . 
     An air supply hose  42  is secured to a nozzle body  44  and supplies air through a nozzle orifice  46  that is aimed at a portion of cable  36 . A cable guide  48  defining a longitudinal slot  50  is positioned proximate nozzle orifice  46 . Cable  36  rides within slot  50  and is thus held in relative position to nozzle orifice  46  so that air passing therethrough does not substantially move cable  36  from in front of nozzle orifice  46  or cause cable  36  to substantially vibrate. 
     Rotation of shaft  15  is controlled by a controller, generally indicated at  57 , comprising circuitry  54  in a module  56  that receives signals from a signal generating device  52 , such as a personal computer employing a microprocessor or other devices that can supply discrete signals to instruct selective rotation of the shaft  15  of the motor. Circuitry  54  receives a signal(s) from generating device  52  and rotates shaft  15  of the motor according to the signal(s). 
     In operation, ink contained in reservoir  24  is picked up by cable  36  and advanced by rotation of wheel  13 , indicated by the arrow, in front of nozzle orifice  46 . Air that is blown through nozzle orifice  46  disperses or pulls ink from cable  36  toward the print medium. Depending on the viscosity of the ink, the cross-sectional diameter of cable  36 , and the diameter of wheel  13 , a relatively precise amount of ink can be deposited on print medium. Such an apparatus may produce images having a resolution of approximately 50 dpi or better. 
       FIG. 2  is a side view of the prior art single color ink injector of  FIG. 1 . 
     SUMMARY 
     The present application is directed to a method of increasing the accuracy with which the ink is applied to the print medium. 
     The present application is directed to a method of forming a high resolution image on the print medium. 
     An improved printer and method of printing uses an electrostatic field at the point of ink contact with the print medium to improve ink application. This improved printer and printing method makes possible a higher resolution final image; provides for greater control of the application of ink on the print medium; permits the use of a wider range of inks having variable viscosities, surface tensions, and elasticities; and permits the use of a variety of ink application speeds. 
     One embodiment of a method involves coating at least a portion of an exterior surface of a cable with an electrostatically charged, pigmented material and directing an air stream at the portion of the cable coated with the pigmented material. The force of air in the air stream causes a metered amount of the electrostatically charged, pigmented material to be removed from the exterior surface of the cable and to be deposited onto a print medium that is placed in close proximity to the cable. Advancement of the cable through the air stream is electronically controlled. The electrostatically charged print medium and the electrostatically charged, pigmented material have opposite charges such that they are attracted to one another at a location where the electrostatically charged, pigmented material is removed from the exterior surface of the cable and is deposited onto the print medium. 
     One embodiment of an apparatus for digitally printing a high resolution image on a print medium includes a support structure, a carriage associated with and movable in at least one direction relative to the support structure, and a plurality of paint injectors secured to the carriage. Each of the paint injectors includes a motor having a rotatable shaft, a wheel rotatable by the shaft, an idler, and an elongate segment (or cable) disposed around at least a portion of the wheel and a portion of the idler. The elongate segment is advanceable by the wheel and has a quantity of charged, pigmented material coated onto at least a portion of it. The paint injectors also each include at least one fluid nozzle positioned and oriented for directing a jet of fluid toward at least a portion of the elongate segment to remove an amount of electrostatically charged, pigmented material from the elongate segment and direct the amount toward a surface of an electrostatically charged print medium. A controller that is electronically connected to each motor controls the rotation of each wheel and controls the position of the carriage relative to the support structure. The electrostatically charged print medium and the electrostatically charged, pigmented material have opposite charges such that they are attracted to one another at a location where the electrostatically charged, pigmented material is directed toward the surface of the electrostatically charged print medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of one embodiment of a prior art single color ink injector. 
         FIG. 2  is a side view of the prior art single color ink injector of  FIG. 1 . 
         FIG. 3  is a schematic view of a printer including one embodiment of an electrostatic coating apparatus. 
         FIG. 4  is a schematic view of a printer including another embodiment of an electrostatic coating apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     Electrostatic coating applications involve applying an electrostatic charge to at least one of the ink and/or the print medium such that the ink and print medium are electrostatically attracted to one another due to their charged states. The ink is thus pulled toward the print medium with added force, facilitating greater control and/or improvement of certain variables relating to the application of the ink to the print medium, such as, for example, ink droplet diameter, resolution of the final image, and consistency of ink droplet size across the image. A wide variety of electrostatic coating methods and systems are known in the art and may be used in connection with the printer described above. A list of exemplary patents that describe some of these exemplary methods and systems are as follows: U.S. Pat. Nos. 3,052,131; 2,952,559; 3,206,323; 4,837,045; 5,609,923 and EP 390774 B1. 
     Some specific exemplary electrostatic coating methods and assemblies are described below. However, the invention is not meant to be limited to the specific methods and embodiments described below. 
     One method of using an electrostatic field at the point of ink contact with the print medium to improve ink application involves precharging the print medium (applying a charge to the print medium before the ink is applied). As is known to those of skill in art, precharging can be accomplished in a variety of ways, such as, for example, use of a corona wire and use of charged brushes. 
       FIG. 3  shows one exemplary method of and apparatus for an electrostatic coating application in connection with the prior art single color ink injector of  FIGS. 1 and 2 . In  FIG. 3 , electrostatic charges are applied to a first side  126  of a print medium  120  at a charge application station  136  spaced longitudinally upstream from a print head  124  of a type described above. The charges could alternatively be applied to a second side  128  of print medium  120 . At charge application station  136 , a laterally disposed corona discharge wire  138  applies positive (or negative) electrical charges  139  to print medium  120 . Wire  138  can be on either of first or second side  126  or  128  of print medium  120 . The ink  132  that is applied to print medium  120  is grounded (such as by print head  124 ), and is electrostatically attracted to the charged print medium  120 . Optionally, a laterally disposed air dam  140  can be disposed adjacent and upstream of print head  124  to reduce print medium  120  boundary layer air interference at a print head-print medium interface  141 . Wire  138  could be aligned in free space along print medium  120  (as is shown in  FIG. 3 ) or, alternatively, could be aligned adjacent first side  126  of print medium  120  while the print medium is in contact with a backing roll at interface  141 . 
     Another method of using an electrostatic field at the point of ink contact with the print medium to improve ink application involves energizing a support structure adjacent to the print medium. As is known to those of skill in art, energizing the support structure can be accomplished in a variety of ways, such as, for example, using conductive elevated electrical potential rolls, using nonconductive roll surfaces that are precharged, and using powered semiconductive rolls. 
       FIG. 4  shows this alternative exemplary method of and apparatus for an electrostatic coating application in connection with the prior art single color ink injector of  FIGS. 1 and 2 . In  FIG. 4 , a relatively large diameter backing roll  142  supports second side  28  of print medium  120  at print head-print medium interface  141 . Backing roll  142  can be, for example, a charged dielectric roll, a powered semiconductive roll, or a conductive roll. The conductive and semiconductive rolls can be charged, for example, by a high voltage power supply. With a dielectric roll, the roll can be provided with electrical charges by suitable means, such as a corona charging assembly  143 . Regardless of the type of backing roll  142  or its means of being charged, its outer cylindrical surface  144  is adapted to deliver electrical charges  139  to second side  128  of print medium  120 . As is shown in  FIG. 4 , electrical charges  139  from backing roll  142  are positive charges, and ink  132  is grounded by grounding print head  124 . Accordingly, ink  132  is electrostatically attracted to charges residing at the interface between print medium  120  and the outer cylindrical surface  144  of roll  142 . Optional air dam  140  reduces web boundary layer air interference at the print head-print medium interface  141 . 
     As shown above, the ink stream is aligned with the gravitational vector. Those of skill in the art will recognize that it can be aligned at other angles that are not specifically shown. Also, the printer and printing method can be used to form a continuous or discontinuous coating. Further, although the examples show a print medium moving past a stationary print head, the print medium can be stationary while the print heads moves, or both the print medium and print head can move relative to one another or to a fixed point. 
     As used herein, the term “cable” is meant to include the use of a wire, a cable formed of multiple wires, a rod, a saw tooth wheel, or variations thereof. 
     As used herein, the term “ink” is meant to include any pigmented material, including, but not limited to, inks, dyes, paints, or other similarly pigmented liquids. Inks that can be printed in the printer of the present invention include inks having a viscosity between about 200 and about 2000. This is a still significantly higher viscosity than can be jetted though conventional ink jet printers. A list of exemplary commercially available inks that fall within this range includes: 3M Process Color Series 700, 3M Process Color Series 880-00, 3M Process Color Series 880i, 3M Process Color Series 990, 3M Scotchlite Transparent Screen Printing Ink Series 2900, 3M Screen Printing Ink Series 1900, 3M Screen Printing Ink Series 9700UV, Nazdar 3500 Series UV Vinex Screen Ink, Avery Dennison Series 4930 Series Inks (10 year-1 Component Solvent Ink*), Sericol UVTS Series Ink, Nazdar UVTS Series Ink, Avery Dennison® UVTS-Sericol Ultraviolet Curable Printing Inks, Avery Dennison® UVTS-NazDar Ultraviolet Curable Printing Inks, Kiwalite KT Series Screen Process Ink, Avery Dennison® 10TS SeriesTwo-Component Printing Inks For Traffic Sign Products, Sericol Sinvacure UV Curable Screen Ink, Avery Dennison® 7TS Series Inks One Component Solvent Ink System For Traffic Sign Products, and Ink Dezyne VP-000 Series Vinyl Plus Screen Ink. 
     As used herein, the term “print medium” is meant to include any print medium known in the art, including but not limited to paper, plastic, synthetic paper, metal foil, vinyl, and films, and variations thereof. 
     Various modifications and alterations of the present invention will become apparent to those skilled in the art without departing from the spirit and scope of the invention.