Method And Apparatus For Electrostatically Charging Particles For Printing Or Coating

Abstract

Provisions for electrostatically charging particles, as in an electrostatic printing operation, include a corona source for providing ionization in a predetermined region, a plate electrode spaced from the corona source, a grid electrode adjacent the corona source and spaced from the plate electrode and a nozzle for directing a flow of the finely divided particles intermediate the plate and grid electrodes. The grid electrode draws ions from the corona source into the region intermediate the plate and grid electrodes. A rotatable drum may be provided proximate the region of electrostatic charging of the particles to receive the particles on the surface thereof for transportation to a printing or application zone. Alternatively, a stencil screen may be provided proximate the region of charging of the particles to direct passage of the particles therethrough to define a desired image upon a substrate proximate the stencil screen.

Parent Case Text

This is a division of Ser. No. 787,977, filed Dec. 30, 1968 and now U.S. Pat. No. 3,635,157.

Description

This invention relates generally to methods and apparatus for electrostatically charging particles and more specifically to methods and apparatus for electrostatically charging particles of printing medium for employment in an electrostatic printing or coating operation.

The employment of a corona source for electrostatically charging toner or printing medium particles in an electrostatic printing or coating operation has previously been known. However, previously known methods and apparatus for charging particles through the employment of a corona source have often been characterized by a failure to apply a consistent charge upon a predetermined quantity of particles, resulting in the production of an insufficient charge upon a portion of the quantity of particles intended for use in a printing or coating operation. Similarly, previously known methods and apparatus have failed to provide a precisely determined region of ionization, allowing the charging of particles in that region to provide a known, controlled and uniform electrostatic charge upon particles passing through such region.

In view of the foregoing, it is a primary object of this invention to provide improved methods and apparatus for electrostatically charging finely divided particles through the employment of a corona source.

Another object of this invention is to provide apparatus for electrostatically charging finely divided particles including a corona source, a plate electrode and a grid electrode, wherein the plate and grid electrodes define an a region therebetween into which ions are drawn and maintained.

A further object of this invention is to provide apparatus in accordance with the immediately foregoing object of this invention wherein the grid electrode has applied thereto a potential chosen to draw produced ions into the region between the plate and grid electrodes and wherein a potential is applied to the plate electrode preventing passage of ions therepast.

Additionally, it is an object of this invention to provide methods and apparatus for consistently charging a quantity of finely divided particles of printing medium for application to aprinting drum having an image-producing intaglio surface thereon and for withdrawing charge particles from the drum to effect production of a predetermined image upon a substrate.

Yet another object of this invention is to provide methods and apparatus for uniformly charging a quantity of finely divided particles of printing medium and for directing passage of such particles through a stencil screen and onto a substrate proximate the screen, the stencil screen limiting passage of such particles to define a predetermined image upon the substrate.

These and further objects of this invention will be more clearly understood by reference to the following written description, the attached drawings referred to therein and the appended claims.

IN THE DRAWINGS:

FIG. 1 is a diagrammatic illustration of printing or coating apparatus formed in accordance with this invention and illustrates provisions for the application of an electrostatic charge to toner particles in transit therethrough, a rotatable printing drum for receiving charged toner particles and for transporting the charged particles to an application or printing area.

FIG. 2 is an enlarged diagrammatic illustration of the provisions of FIG. 1 for electrostatically charging toner particles and shows a corona source proximate a grid electrode for drawing produced ions therepast and a plate electrode, cooperating with the grid electrode to maintain ions intermediate the plate and grid electrode during passage of toner particles intermediate such electrodes.

FIG. 3 is a diagrammatic illustration of a further embodiment of this invention and shows a corona source, a grid electrode and a plate electrode similar to those shown in FIG. 2 and a stencil screen electrode masked to define a desired image to be produced upon a proximate substrate .

Referring to the drawings in detail, there is illustrated in FIG. 1 an electrostatic printing or coating apparatus, generally referred to by the numeral 10, including a rotatable printing wheel or drum 11, mounted for rotation by a centrally provided shaft 12. Rotation of the drum 11 may be provided by the employment of a number of suitable or conventional and known driving provisions (not shown). The drum 11 includes a metallic peripheral surface 13 which may be provided by a relatively thin printing plate 14, relieved in the fashion of an intaglio printing plate to provide etched or engraved areas defining a desired image.

Mounted adjacent the peripheral surface 13 of the drum 11 is a nozzle 15 terminating a supply line 16 communicating with a source of toner or printing medium particles, not separately shown. The supply line 16 and nozzle 15 provide a flow of fluidized or aerated toner particles toward the metallic peripheral surface 13 of the drum 11. The nozzle 15, terminating the line 16, is spaced slightly from the peripheral surface 13 of the drum 11 allowing free passage of a flow of toner 17 through an a region 18 intermediate the nozzle 16 and surface 13.

Particle charging provisions, generally referred to by the numeral 20 are provided adjacent the peripheral surace 13 of the drum 11 for the production of an electrostatic charge upon the toner or printing medium particles in transit through the region 18. The particle charging provisions 20 include a corona source 21 defined by a conductive plate 22 having affixed thereto a pluraltiy of pin electrodes 23. Additionally, a plate electrode 24 and a grid electrode 25, best illustrated in FIG. 2, form a part of the particle charging provisions 20 and the plate electrode 24 is spaced from the corona source 21 on the opposite side of the flow of toner 17 in the area 18. The grid electrode 25 is located adjacent the corona source 21 and spaced from the plate electrode 24 to define between the plate electrode 24 and the grid electrode 25 the region 18 in which charging of the printing medium or toner particles is effected.

Application of an electrostatic charge to the toner particles in transit to the peripheral surface 13 of the wheel or drum 11 is effected by application of a potential to the corona source 21 from a high-voltage D.C. power supply 26, via a conductor 27 and current limiting resistor 28 to produce ionization in the area adjacent the corona pin electrodes 23. The plate and grid electrodes 24 and 25, respectively, are energized from a further high-voltage D.C. power supply 30 via a pair of conductors 31 and 32. The potential at which the electrodes 24 and 25 are energized is of the same polarity but less than the potential at which the corona source 21 is energized. Accordingly, an electric field is produced intermediate the corona source 21 and the grid electrode 25, drawing produced ions toward and through the grid electrode 25. The grid electrode 25 preferably is formed of metallic screen or mesh as illustrated in FIG. 2 and allows the free passage of ions therethrough into the area 18 intermediate the plate electrode 24 and the grid 25. Energization of the plate electrode 24 at a potential equal to the potential applied to the grid electrode prevents passage of a substantial portion of the produced ions therepast, thus maintaining substantial ionization in the region 18 intermediate the two spaced electrodes 24 and 25.

The metallic peripheral surface 13 of the drum 11 is energized from a further D.C. power supply 33 via a conductor 34 and a brush or other suitably selected connector 35 as best seen in FIG. 1. The potential applied to the drum 11 is of the same polarity as the potentials applied to the corona source 21 and the electrodes 24 and 25. The potential applied to the drum 11 is, however, less than the potential applied to the electrodes 24 and 25, thus developing a further electric field between the electrodes 24 and 25 and the peripheral surface 13 of the drum 11. The polarity of the charge electrostatically applied to the toner particles in the region 18 will, of course, correspond to the polarity of the ions produced by the corona source 21 and the polarity of energization of that source. Accordingly, energization of the peripheral surface 13 of the drum 11 at a potential lower than the potential applied to the electrodes 24 and 25 urges movement of the relatively highly charged toner particles into engagement with the intaglio printing surface provided upon the drum 11.

Rotation of the drum 11 in the direction of the unnumbered arrow shown in FIG. 1 effects movement of the applied charged particles toward a printing zone or area 36. A metallic doctor blade 37 engages the peripheral surface 13, scraping the uppermost surfaces of the intaglio surface to remove charged toner particles therefrom. The doctor blade 37 is energized from the power supply 33 at a potential equal to the potential applied to the drum 11, thus minimizing decay of the charge on the toner particles upon the drum 11 through contact of the drum 11 and the doctor blade 37.

An auxiliary corona source 38 is mounted proximate the surface 13 of the drum 11 and is circumferentially spaced from the doctor blade 37 in the direction of rotation of the drum. The auxiliary corona source may include a wire 40, energized at a relatively high potential from an additional D.C. power supply 41 via a conductor 42. The auxiliary corona source 38 replenishes the charge on the toner particles deposited upon the peripheral surface 13 of the drum 11 prior to movement of the charged particles into the printing or application area 36.

Spaced from the drum 11 in the printing area 36 is a space grounding corona source generally indicated by the numeral 43. The space grounding corona source 43 includes a mesh or screen electrode 44 which is grounded with respect to the potential applied to the surface 13 of the drum 11 as indicated at 45. In the printing area 36, charged particles carried on the surface of the drum 11 will be drawn toward the grounded screen electrode 44 to produce a desired image upon a substrate 46 located intermediate the screen electrode 44 and the surface of the drum 11.

The space grounding corona source 43 includes a plurality of corona pin electrodes 47 energized from a D.C. supply 48 via a conductor 50. The potential applied to the pin electrodes 47 is opposite in polarity to the charge upon the toner particles applied to the substrate 46 and upon deposition of charged toner particles upon the undersurface of the substrate 46, ions produced adjacent the pin electrodes 47 will be drawn through the screen electrode 44 to the upper surface of the substrate 46, effectively neutralizing the charge upon the substrate 46. This, then, prevents repulsion of charged particles from the substrate 46 once the substrate is moved from within the field extending between the grounded screen electrode 44 and the positively energized surface 13 of the drum 11.

In an operative embodiment of the apparatus illustrated in FIG. 1, a positive potential of 13 to 14 KV may be applied to the pin electrodes 23 of the corona source 21 while a positive potential of 8 to 10 KV may be applied to each of the plate and grid electrodes 24 and 25, respectively. The metallic surface 13 of the printing drum 11 may be energized at from 4 to 6 KV which is sufficiently low to allow attraction of the charged particles in the region 18 while allowing withdrawal of the particles in the printing area 36, under the influence of the grounded screen electrode 44. Attention should be given to the potential at which the auxiliary corona source 38 is energized as well as to the spacing of the source 38 from the periphery 13 of the printing drum 11 in order to assure corona production without discharge to the surface of the drum, while preventing attraction of charged particles to the wire 40 of the auxiliary source 38. In the preferred arrangement discussed herein a positive potential of 6 to 12 KV may be applied to the wire 40 to effect replenishment of the toner particle charge. Effective neutralization of the substrate 46 is effected by the application of a negative potential of from 7.5 to 8.5 KV to the pin electrodes 47 for the production of negative ions adjacent thereto.

As illustrated in FIG. 1, complete cleansing of the peripheral surface 13 of the drum 11 following a printing operation may be provided by a rotary brush 51 driven in opposition to the movement of the drum as indicated by the unnumbered arrow associated therewith in FIG. 1. The brush 51 preferably contacts the peripheral surface 13 at a point circumferentially spaced from the printing area 36 to allow application of newly charged toner particles to the surface 13 for further printing operations.

Whereas employment of the power supplies 26, 30, 33, 41 and 48 with the polarities indicated in FIG. 1 provides successful charging and printing with non-conductive toner or printing medium particles, preferably the polarities illustrated are employed with powders of materials having a known naturally positive charge balance for consistent charging of the particles in the region 18 and where powders known to have a naturally negative charge balance or negative charging propensity are employed, complete reversal of the polarities illustrated in FIG. 1 and discussed hereinabove is preferred.

FIG. 3 illustrates a further embodiment of the invention employing the charging provisions 20 discussed hereinabove with respect to FIGS. 1 and 2 and including the corona source 21, the plate electrode 24 and the grid electrode 25 in substantially the same relationship as discussed hereinabove. Again the nozzle 15 and supply line 16 open adjacent the area 18 intermediate the electrodes 24 and 25 to provide a cloud-like flow 17 of finely divided particles in the region 18. The corona source 21 is energized from a D.C. supply 52, applying a positive potential to the corona pin electrodes 23 and the electrodes 24 and 25 are energized from a D.C. supply 53, applying a lesser positive potential thereto. A stencil screen electrode 54 is negatively energized from a D.C. supply 55, drawing postiively charged toner or printing medium particles thereto and masking provisions 56, defining a desired image are provided upon the stencil screen electrode 54, limiting the passage of toner particles therethrough for application to a substrate 57 to print thereon the desired predetermined image. It will, of course, be apparent that the establishment of an electric field in the region of the stencil screen 54 and the substrate 57 to draw the charged particles toward the substrate 57 may be effected in any convenient fashion and, of course, space grounding corona provisions similar to those provisions 43 illustrated in FIG. 1 may be provided adjacent the substrate 57 to provide drawing of charged toner into contact with the substrate 57 and to effectively neutralize the substrate 57 prior to its removal from the printing area in the embodiment illustrated in FIG. 3.

While preferred methods and apparatus have been described hereinabove, it will be readily apparent that variations may be made therein without departure from the scope and spirit of the invention embodied in such methods and apparatus.

Claims

We claim:

1. Electrostatic apparatus for charging a plurality of finely divided particles including corona source means for the production of ions, a plate electrode having a first potential and spaced from said corona source means, a grid electrode having the same potential of like polarity with respect to said plate electrode and adjacent said corona source means and spaced from said plate electrode for drawing ions from said corona source means for passage to the region between said grid electrode and said plate electrode and for maintaining ions in said region, said corona source means being a direct current source, and means directing finely divided particles between said grid electrode and plate electrode for charging said particles in said region therebetween.

2. Apparatus according to claim 1 wherein said means for directing finely divided particles between said grid electrode and plate electrode comprises a nozzle opening into said area for directing a cloud of particles therethrough.

3. Apparatus according to claim 1 wherein said corona source means comprises ionizing electrode means having a relatively high potential of the same polarity and higher than the potential of the grid electrode and the plate electrode, said grid electrode comprising a metallic screen, whereby ions are drawn through said screen and into the region between said grid electrode and said plate electrode.

4. The method of applying a charge to finely divided particles including the steps of providing a direct current corona source, locating a plate electrode spaced from the corona source, locating a grid electrode between the corona source and the plate electrode and spaced from the plate electrode, energizing the corona source for producing ions of a predetermined polarity, applying eual potentials of the predetermined polarity to the grid and plate electrodes for maintaining the produced ions therebetween, and directing a flow of particles intermediate the plate electrode and grid electrode for charging said particles by the produced ions.

5. The method according to claim 4 wherein the step of providing a corona source includes providing at least one corona needle and said step of energizing the corona source includes applying a potential to the at least one needle higher than and of the same polarity as the potentials applied to the grid and plate electrodes for drawing the produced ions through the grid electrodes into the region intermediate the grid and plate electrodes.

6. The method according to claim 4 further including the steps of locating a further member in the path of flow of particles and outside the region between the grid and plate electrodes, and applying to said further member a potential to aid withdrawal of charged particles from the region intermediate the grid and plate electrodes.