Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
  • G03G15/00—Apparatus for electrographic processes using a charge pattern
  • G03G15/22—Apparatus for electrographic processes using a charge pattern involving the combination of more than one step according to groups G03G13/02 - G03G13/20
  • G03G15/32—Apparatus for electrographic processes using a charge pattern involving the combination of more than one step according to groups G03G13/02 - G03G13/20 in which the charge pattern is formed dotwise, e.g. by a thermal head
  • G03G15/321—Apparatus for electrographic processes using a charge pattern involving the combination of more than one step according to groups G03G13/02 - G03G13/20 in which the charge pattern is formed dotwise, e.g. by a thermal head by charge transfer onto the recording material in accordance with the image
  • G03G15/323—Apparatus for electrographic processes using a charge pattern involving the combination of more than one step according to groups G03G13/02 - G03G13/20 in which the charge pattern is formed dotwise, e.g. by a thermal head by charge transfer onto the recording material in accordance with the image by modulating charged particles through holes or a slit

Definitions

• the present invention relates to electrographic imaging devices, and more particularly to electrographic printing devices using ion projection technology.
• Ions can be generated to form electrostatic Images in a wide variety of ways. Common techniques include the use of air gap breakdown, corona discharges and spark discharges. A successful ion projection printing technique Is disclosed in U.S. Patent Nos. 4,155,093, 4,160,257, and 4,409,604, all are commonly assigned with the present application. These prior art patents disclose various ion generating devices which incorporate a pair of electrodes separated by a solid dielectric member. A high voltage time-varying potential between the two electrodes causes the formation of a pool of ions In an air region adjacent one of the electrodes (the "control" electrode) and the solid dielectric member.
• Ions may be extracted from this pool to form electrostatic images on a remote, dielectric surface, typically by means of a -DC extraction potential interposed between one of the electrodes and a backing electrode for the Imaging surface.
• the control electrode has an edge surface which forms a junction with the solid dielectric member, partially or completely defining the air region.
• the '093 apparatus is modified by including an additional, "screen" electrode which is separated from the control electrode by a dielectric spacer layer; i.e. is incorporated in an integral structure. The screen electrode is apentured to modulate the flow of ions from the control electrode, providing an electrostatic lensing action.
• U.S. Patent No. 4,409,604 produces ions using an elongate conductor having a dielectric sheath, such as a glass-coated wire, with a transversely oriented conductor contacting or closely spaced from the sheathed conductor.
• the transverse conductor serves as the control electrode with ions produced at the crossover region with the sheathed conductor.
• This patent also discloses the possibility of incorporating a screen electrode as part of an integral structure, similar to that of U.S. Patent No. 4,l6 ⁇ ,257»
• the above imaging devices all employ a self-limiting discharge ion production principle wherein the time- varying potential causes the charging of a surface portion of the solid dielectric member, leading to an atmospheric discharge upon achieving an "inception potential".
• This characteristic provides a number of operating advantages. These include reliable output currents, which are typically higher than those obtained from prior art devices such as corona ion sources. Such devices also enjoy an extended service life. There are, however, certain operating disadvantages associated with such structures. The unitary construction of these devices complicates their manufacture and testing. Furthermore, the close spacing of the various components increases the tendency towards fouling with dust, chemical byproducts of Ion generation, and other contaminants.
• One particular design of electrographic imaging apparatus commonly employed in the art uses a corona ion source in combination with means for creating an accelerating field to extract ions from the corona to a remote, dielectric imaging surface. Typically this is done by Imposing a direct current potential difference between the corona electrode and a backing electrode for the imaging surface.
• the ion flow Is modulated using an apertured mask which is interposed between the corona ion source and the Image receptor surface.
• Representative patents include U.S. Patent Nos.
• Such apparatus may be used to form a visible image either by subsequently toning a latent electrostatic image formed by the above method, or " by using such a device to control the flow of an ink mist or other visible medium.
• corona discharges typically a maximum discharge current on the order of 10 microamperes per centimeter.
• limited currents in turn Impose printing speed limitations.
• coronas can cause significant maintenance problems. Corona wires are small and fragile and easily broken. Because of their high separating potentials they collect dirt and dust and must be frequently cleaned or replaced.
• a primary object of the invention to provide an improved electrographic printing head.
• a related object is to utilize ion projection imaging technology in the design of such a device.
• One criterion of concern is the magnitude of available ion output currents.
• a related object is the achievement of high operating speeds.
• Another object of the invention is to facilitate - -manufacture and testing of such devices.
• a further object of the invention is the design of a printing head which enjoys prolonged, relatively maintenance-free operation. It is desirable that such . device be easily cleanable, to remove built up contaminants, or be self-cleaning.
• Still another aspect of the invention is the achievement of uniform, reliable output currents over various imaging elements in a matrix-defined printing head. It is furthermore desirable that such a device permit multiplexing of the control electronics.
• the invention provides an Ion projection printing head for electrographic imaging, incorporating a self- limiting discharge ion source, with a discrete ion modulating screen member. Ions are generated using two electrodes, a "bias electrode” and a “driver electrode”, separated by a solid dielectric member. A high voltage time-varying potential between these electrodes Induces self-limiting electrical discharges, creating a pool of positive and negative ions in an air region adjacent the control electrode and the solid dielectric member. Ions are extracted from this region using an extraction potential between the bias electrode and a counterelectrode, advantageously a backing electrode for a dielectric imaging surface to which the ions are attracted.
• Suitable ion generating devices of these types are disclosed in commonly assigned U.S. Patent No. 4,379,969, and U.S. Application Serial No. 381,600, filed May 24, 1982. It is desirable to employ an ion generator which has a relatively high transconductance, which is a quantitative measurement of ion output efficiency.
• Another aspect of the invention relates to a preferred design of the ion-modulating plate, which consists of two apertured electrodes mounted onto opposite faces of a dielectric support. These layers are perforated by one or more screen apertures to permit passage of ions.
• the dielectric layer is recessed from the aperture edges to reduce wall charging effects.
• a further aspect of the invention relates to the electrical actuating parameters.
• the drive voltage, between the bias and driver electrodes advantageously comprises a high voltage R.F. alternating potential.
• the extraction potential preferably comprises a D.C. bias, while the screen electrode or electrodes receive a user- variable potential to control the ion-gating characteristics of the screen member.
• the electrode nearer the ion source receives an extraction potential ⁇ , with a second extraction potential V2 applied to the electrode nearer the imaging surface.
• these screen potentials take the form of pulsed voltages, which may be supplied by pulse generators in combination with D.C. bias potentials to the respective screen electrodes.
• the second screen electrode provides an Ion-accelerating field for a range of values of V 2 , with a maximum ion current in the center of this range.
• the maximum attainable ion current in turn varies with ⁇ .
• the device provides peak ion current densities on the order of 200 microamperes per square centimeter.
• the electrical actuation parameters are designed to provide "on" and "off” states for individual screen apertures. The "on" or “print” state corresponds to a given nominal ion output current for the. screen aperture.
• the print current may be empirically determined by the user in view of various parameters of the printing system.
• switching between on and off states may be accomplished by switching either or both of V ⁇ and v 2 .
• an electrostatic imaging device is incorporated in a printing system of the type described in U.S. Patent No. 4,365,549.
• the device forms a latent electrostatic image on the dielectric surface of a rotatable imaging cylinder.
• the image is developed as a visible toner image, which may then be transferred and simultaneously fused to a receptor sheet using high pressure.
• the imaging devices of the invention characterized by high Ion current densities, provide high quality developed images at extremely high imaging speeds in apparatus of this type.
• FIGURE 1 is a sectional schematic view of a simple form of electrostatic imaging device embodying the invention, as used for charging a dielectric surface
• FIGURE 2 is a sectional schematic view of an alternative form of imaging device incorporating a dual electrode ion-modulating screen
• FIGURE 3 plots ion output current as a function of control bias potential for the ion source device of Figure 1, for various spacings from an ion receptor surface;
• FIGURE 4 plots ion output current from the apertured plate of Figure 2, for various actuating potentials V ⁇ and V 2 to the screen electrodes;
• FIGURE 5 Is a plan view of a dot matrix ion- modulating screen of the type shown in Figure 2;
• FIGURE 6 is a partially sectioned schematic end view of electrographic printing apparatus incorporating an electrostatic imaging device in accordance with the invention;
• FIGURE 7 is a sectional schematic view of an alternative ion source assembly for use in the imaging devices of the Invention.
• FIGURE 8A is a sectional schematic view of a further alternative ion source assembly
• corona electrode 11 mounted on a dielectric support 12, covered with a conductive mesh electrode 18.
• the corona electrode 11 consists of an elongate conductor 14 (seen here in an end view) covered with a dielectric jacket or sheath 16; the conductor 14 functions as the "driver electrode”.
• Ion generator 10 Is electrically actuated to form a pool of positive and negative ions in an air region adjacent the mesh electrode 18 by means of a time-varying potential 17 Imposed between the elongate conductor 14 and mesh electrode 18. Ions of a given polarity (in the illustrated embodiment, negative ions) are attracted from this air region to the dielectric imaging surface 155 by means of a D.C. extraction potential 19 imposed between the mesh electrode (or more generally, "bias electrode”) and a counterelectrode 160 backing the dielectric surface 155-
• FIG. 2 is a schematic sectional view of a further embodiment of electrostatic imaging device 110 Incorporating a two-electrode ion-modulating plate 30.
• the ion-modulating plate 30 consists of planar electrodes
• the transconductance g is a measure of the current output efficiency of these devices, and will vary according to the ion generator design; choice of materials and dimensions for the dielectric 16; magnitude and frequency of the drive potential 17; and gap widths Z 1 , Z 2 , and Z ⁇ between the ion generator 10, screens 32 and 34, and surface 15 •
• a primary advantage of these electrostatic imaging devices Is their characteristically high ion output currents, due to the use of a self-limiting discharge ion source.
• transconductance will increase linearly with the magnitude of drive voltage 17, and when using continuous waveform drive potentials will increase with the frequency of this voltage.
• the magnitude of this potential is limited by the dielectric strength of the dielectric 16, while an excessive high drive frequency will cause undesirably high temperatures in the discharge region.
• the Ion output current densities are also observed to vary roughly in proportion to the bias electrode potential V Q .
• V 0 should be maintained below a value which would cause arcing to the screen 30; this is dependent on the separation of electrodes 18, l6 ⁇ .
• Transconductance g Is also observed to vary hyperbolically (i.e. as 1/Z) with increasing separation from the ion receptor surface (such as dielectric surface 155).
• Figure 3 gives a family of i-V plots — i.e. output current per centimeter length of generator 10, plotted against the bias V Q . These are measured for the ion generator 10 of Figure 1 for different values of Z, where Z is the distance from a current-measuring probe.
• Ion generator 10 and surface 155 to keep these tolerances within reasonable limits. Additionally, it is beneficial to provide a separation Z ⁇ between ion generator 10 and screen 30 to facilitate cleaning of these structures, for example using high pressure air flow through this region.
• the separation between screens 32 and 34 (in Figure 2), and Z 2 from screen 34 to imaging surface 155, is desirably kept at a minimum to reduce spreading of the ion stream projected from the screen aperture.
• An illustrative ratio of "Z- i '- Z ?, is on the order of 10:1.
• the ion generator provides an Ion current density of- ' at least 50 microamperes per cm ⁇ , most advantageously greater than 200 microamperes per cm .
• the ion output characteristics of these imaging devices may be analyzed not only as respects the parameters of the ion generator 10, but also In the transfer characteristic of the ion-modulating screen.
• the output current I may be controlled by variation of either or both of the screen electrode potentials V- ⁇ and V 2 .
• Figure 4 plot ' s the ion output current density I for a given plate aperture 36 as a function of the screen potentials V- j _ and V 2 .
• the ion output current is plotted as a family of isopotential curves, wherein each curve is mapped for a given potential V ] _ over a range of values of V 2 .
• i may be seen that for a given value of Vi, the output current I' is a parabolic function of V 2 reaching a maximum at the midpoint of the range.
• the maximum output current is observed to Increase with greater absolute values of _.
• the imaging devices of the preferred embodiment are advantageously characterized by a peak output current density for the screen 30 greater than 200 microamperes per cm ⁇ .
• the above ion transfer characteristics are attributable to the effect of the respective electrode potentials on the local electrostatic fields- in the screen aperture regions.
• These local fields are a combination of the propulsion field E p provided by the extraction potential V Q , and the superimposed fields due to the screen potentials V ⁇ and V 2 .
• the superimposed fields can be directed to oppose and overcome the propulsion field E p within the screen.
• the field within each screen apert,ure Is the vector sum of the propulsion field E p and the fringing field from the screen electrode.
• the fringing field from the screen eiectrode must be of sufficient magnitude to create a net zero or negative field in the center of the aperture (considering E p as positive).
• a positive net propulsion field must exist within both successive screen apertures (as well as in the regions downstream of these apertures) in order to achieve a "print" condition.
• This apparatus may be used to provide an aperture modulation effect — i.e. an apparent aperture diameter which may vary in accordance with the screen potentials V and V 2 «
• an aperture modulation effect i.e. an apparent aperture diameter which may vary in accordance with the screen potentials V and V 2 «
• a nominal "on" or “print” value of output current density Is determined for the aperture 36 i.e. a value sufficient to provide a suitable charge density of the electrostatic images on dielectric 155, for an electrostatic image of a predetermined diameter.
• This value Is generally a function of imaging speed, the physical properties of dielectric surface 155, toning thresholds, and other parameters.
• An approximately zero output current corresponds to the "no print” state; effectively this should be less than 3 percent of the "print” value.
• V- j _ may be set to a preselected value according to various criteria discussed below, and V 2 may be switched between appropriate values according to the given screen transmission characteristics such as those plotted In Figure 4. Alternatively, V- j _ may be switched between a value providing the requisite "print" current, and a negligible value. With reference to the printing system 200 shown in Figure 6, discussed in detail below, one may wish to provide electrode 34 with a slightly positive bias where employing a low toning threshold developing system 220.
• Figure 5 shows in a plan view a dot matrix aperture plate 40 of the general type shown in section in Figure 2.
• Aperture plate 40 consists of a dielectric sheet 41 having on one face a series of parallel electrodes 42 and on the opposite face a crossing series of electrodes 44. Screen apertures 46 are located at the crossover regions of pairs of electrodes 42 and 44.
• the apparatus of Figure 5 is well suited to a dot matrix multiplexing scheme such as that employing the electrical actuation apparatus of
• FIG. 6 shows somewhat schematically an electrostatic printer 200 Incorporating an ion projection printing head in accordance with the invention.
• a cylinder 230 is mounted for rotation about an axis 238 and has an electrically conductive core 237 coated in a dielectric layer 235.
• Cylinder 230 receives an electrostatic image from a printing head 210 of the type described herein, driven by an electronic control system 215 and connected by mechanical connectors 217 * As the cylinder rotates In the direction shown, an electrostatic image is formed by the cartridge 210 on the outer surface of the dielectric layer 235 and comes into contact with toner supplied from a hopper 223 by a feeder mechanism
• the resulting toned image is carried by the cylinder t 230 towards a nip formed with a pressure roller 240 having a compliant outer layer 245 positioned in the path of a receptor such as paper 270.
• the receptor sheet 270 enters between a pair of feed rollers 280, is driven by the cylinder 230 and roller 240, and leaves between a pair of output rollers 282.
• the pressure in the nip is sufficient to cause the toner to transfer to the receptor 270, and with sufficient pressure, the toner will be fused to the receptor.
• the axes of rotation of the cylinder 230 and roller 240 are skewed relative to one another.
• any toner remaining on the surface of the dielectric layer 235 is removed by a scraper blade assembly 250, and any residual electrostatic charge remaining on the surface is neutralized by a discharge head 260 positioned between the scraper assembly 250 and the printing head 210, to prepare the cylinder for reimaging.
• EXAMPLE 1 An electrostatic imaging device of the type illustrated in Figure 1 was constructed as follows.
• the ion generator 10 included an Insulating substrate 12 fabricated of glass epoxy G10 laminate.
• the corona electrode 11 consisted of a 7 mil diameter tungsten wire 14 coated with a 2 mil thick glass coating 16, providing an outer diameter of .011 inch.
• a fine woven electrode screen 18 was stretched ovfer the wire and bonded with a thermoset adhesive to the sides of the substrate.
• the screen consisted of a plain woven 1 mil tungsten wire, 17
• the apertured plate 20 consisted of a copper-clad glass epoxy insulating sheet having a circular etched aperture 27 with a nominal diameter of 6 mils.
• the copper-clad surface 26 of apertured plate 20 was located • ⁇ 6 ⁇ inch from the ion source 10.
• the excitation source 17 consisted of a 200 kilohertz, 2000 volt peak-to-peak continuous wave alternating potential.
• a bias potential of 2000 volts D.C. was imposed between the woven screen electrode 18 and an aluminum plate counterelectrode, which was treated in accordance with the method of U.S. Patent No. 4,413,049 to provide a dielectric Imaging surface 155.
• the dielectric imaging surface 155 was separated from the insulating face 24 of the apertured plate by 10 mils.
• the ion source 10 was operated continuously and a circular electrostatic ion image formed on the dielectric surface 155 when a negative pulse potential was supplied by a pulse generator 22 between the copper cladding 26 and the aluminum substrate 160.
• the electrode 26 was normally maintained at a bias of 50 volts, and a negative pulse of 300 volts having a duration of 60 microseconds was employed.
• Example 2 The electrostatic imaging apparatus of Example 1 was modified as follows to provide a double-screen system of the type Illustrated in Figure 2.
• the Ion-modulating screen 30 was comprised of a 6 mil thick Kapton layer 35 having 1 mil stainless steel foil laminated to each face. The foil was photoetched to provide a row of 6 mil diameter screen apertures 36, and the Kapton was then etched from the screen apertures to provide clearance regions several aperture diameters In width.
• a probe micrometer spindle was employed to measure the output current .from ion-modulating plate 30.
• the probe was connected to an electrometer to measure a portion of the screen current, which could be used to calculate current output per aperture.
• EXAMPLE 3 The electrostatic Imaging apparatus of Example 2 was Incorporated In a high-speed electrographic printing system of the type illustrated In Figure 6, using a dot- matrix printing head 70 of the type shown in Figure 5 * High velocity air was blown Into the gap between the ion generator 10 and screen 30 In order to remove chemical byproducts of the atmospheric electrical discharges and to assist in rapidly removing the ions from their region of generation.
• the electrode 32 was biased at a level of -200 volts, whil t e electrode 34 was biased at 50 volts.
• the dielectric cylinder 230 and pressure roller 240 were fabricated and mounted as described in Example 1 of U.S. Patent No.
• Example 6 The electrostatic imaging apparatus of Example 2 was modified by substituting the alternative Ion generator illustrated in Figure 9 «
• the ion generator 70 consisted of a glass epoxy plate 76 to which was bonded a 7 mil diameter tungsten wire coated with a 2 mil thick glass layer. Two 11 mil diameter tungsten wires 75 were bonded to the substrate, closely fitted on either side of the glass-coated wire-72. It was found with this apparatus that high quality developed Images could be obtained at pulse repetition rates as short as 30 microseconds.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Printers Or Recording Devices Using Electromagnetic And Radiation Means (AREA)
• Electrophotography Using Other Than Carlson'S Method (AREA)

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• 1985
  • 1985-10-15 WO PCT/US1985/001989 patent/WO1987002451A1/en not_active Application Discontinuation
  • 1985-10-15 EP EP19850905352 patent/EP0241459A4/de not_active Withdrawn
  • 1985-10-15 JP JP50474885A patent/JPS63501555A/ja active Pending

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