US3306198A

US3306198A - Electrostatic printing process

Info

Publication number: US3306198A
Application number: US328086A
Authority: US
Inventors: Kenneth W Rarey
Original assignee: Continental Can Co Inc
Current assignee: Continental Can Co Inc
Priority date: 1963-12-04
Filing date: 1963-12-04
Publication date: 1967-02-28
Anticipated expiration: 1984-02-28
Also published as: GB1030383A

Description

Feb. 28, 1967 K. w. RAREY 3,305,198

ELECTROSTATIC PRINTING PROCESS Filed Dec. 4, 1963 5 Sheets-Sheet l STAGE & lo

INVENTOR.

KEMN ETH W EAEEY BY bu 2L, v JAM ATTORNEYS Feb. 28, 1967 K. w. RAREY 3,306,198

ELECTROSTATIC PRINTING PROCESS Filed D80. 4, 1965 5 Sheets-Sheet 2 INVENTOR KENNETH w. RQREY BY W ,7, L r ATTORNEY) K. W.RAREY ELECTROSTATIC PRINTING PROCESS Feb. 28, 1967 3 Sheet -Sheet 5 Filed Dec. 4, 1963 INVENTOR KENNETH LU. R AREY ,o l t L CL ATTORNEYS O. .mv M

United States Fatent Ofiice 3,306,198 Patented Feb. 28, 1967 3,306,198 ELECTROSTATIC PRINTING PROCESS Kenneth W. Rarey, South Holland, 111., assignor to Continental Can Company, Inc., New York, N.Y., a corporation of New York Filed Dec. 4 1963, Ser. No. 328,036 13 Claims. (Cl. 101-426) This is a continuationin-part of application Serial No. 234,426, filed October 31, 1962, now abandoned.

This invention relates to electrostatic printing and more particularly relates to a new and useful electrostatic printing process utilizing reusable printing plates whereby copies having improved image resolution and contrast and comparative freedom from background marking are readily obtained.

In one present-day form of electrostatic printing, printing is done from a plate consisting of an electrically grounded conductive layer, the image areas of which are covered with a thin dielectric layer or film. To print from the plate, the insulating layer surface is electrically charged either positively or negatively by subjecting it to a suitable electric discharge, such as a corona discharge or the like, and a powder containing microscopically sized particles of a dielectric material, having an electrical charge of opposite polarity to that of the plate insulating layer surface, is deposited on the plate. Generally, the powder consists of particles, called the toner, of a finely ground dielectric resin containing a pigment, the particles of which are carried on coarser powder particles, called the carrier, which latter particles serve both to carry the toner particles and to maintain the correct electrical charge on the toner.

The toner particles of the powder, when flowed onto the printing plate, remain on the raised insulating layer of the plate by electrostatic attraction to form a powder image in those areas. The carrier particles, by virtue of their individual masses, simply flow along and 01? the plate. Toner particles likewise flow from and leave the conductive areas of the printing plate.

Printed copies of the powder image are made by contacting the raised powder formed image with the surface of paper or other receiving substrate to be printed, much as in letter press printing. Transfer of the powder image to the receiving substrate to be printed is accomplished by establishing an electrical charge on the receiving sub strate of opposite polarity to that of the charged powder on the plate and superior to that of the insulating layer whereby the powdered image is transferred to the receiving sheet. Once on the receiving substrate sheet the image is fixed by fusing, as by heating or by solvent softening. Another transfer technique is to roll the receiving sheet into contact with the plate with a conductive-rubber transfer roll while applying a voltage to the roll on the order of about 1000 volts, thereby effecting transfer of the image from the plate to the sheet.

To reuse the plate, it is simply necessary to recharge the insulating layer and apply more image forming powder.

While this type of printing has many advantages in that it is fast, economical, and dry, there is a tendency for the charged toner particles to cling to the exposed conductive areas of the printing plate, that is, the non-image portions of the plate; and transfer of these mis-positioned particles to the receiving substrate sheet results in undesirable background marking on the receiving sheet around the developed image. While methods have been devised to minimize this effect, these methods also reduce the amount of image forming powder that collects on the image areas of the plate, and pale prints result.

It is a principal object of this invention to provide an electrostatic process which is not only economical, fast and a completely dry process but which furthermore produces printed copies having better contrast and sharpness with less false background markings than heretofore obtained from dry electrostatic printing plate processes.

It is a specific object of this invention to provide a dry electrostatic printing process wherein the non-image areas of the printing plate actively repel the toner particles therefrom and thus reduce the possibility of the appearance of undesirable background markings from transfer of toner from the non-image areas of the plate to the receiving sheet.

It is an important object of this invention to provide an electrostatic printing process wherein printing may be done from conductive areas of the printing plate, as in rotogravure printing, thereby facilitating cleaning of the non-conductive or background areas of the plate and enabling the production of electrostatic printed copy having good resolution.

Another important object is to provide an electrostatic printing process which enables printing without necessity for contact of the printing plate with the surface to be printed, thereby facilitating the production of distortionfree printing on corrugated and other uneven surfaces.

A further object of the invention is to provide an electrostatic printing process whereby the transfer of the image forming material or toner to the receiving substrate sheet is accomplished by electrically attracting the toner to the receiving sheet while eliminating the primary mechanism of attraction of the powdered image to the printing plate.

A still further object of the invention is to provide an electrostatic printing process capable of effecting adhesion of relatively larger amounts of toner particles on the image pattern of the printing plate than possible heretofore, to thereby provide printed images having better contrast on the receiving sheet.

Other and further objects and advantages will appear as the description proceeds.

In accordance with one practice of this invention, a composite printing plate is prepared by applying a coating of insualting material to a metal plate. For example, a sheet of commercial tin plate is given an all-over coating of a resinous lacquer having an insulating solids content, and then baked to fix and cure the resin: normal coating materials and procedures may be employed, with the end purpose of having an all-over insulating coating of uniform thickness and characteristics. The coating is then removed from the areas from which printing is to be done. The printing is done from an electrostatic printing plate, the non-image areas of which are electrically charged and the image areas of which are electrically conductive. The plate comprises a conductive layer, bared surfaces of which form the image areas of the plate, and an insulating layer over-lying the areas of the conductive surface which it is not desired to print; these insulator covered areas constitute the non-image areas of the plate. Formation of the printable image on the plate is carried out by estab lishing an electrical charge on the non-image insulating layer. This charge induces a charge of opposite polarity on the surface of the conductive image areas of the plate. Then a suitable image forming material containing image forming toner particles having a charge of the same polarity as that of said non-image insulating areas is applied in such manner as to facilitate flow of unused image material from the plate.

It has been discovered that a toner having particles of the same polarity as that of the insulated areas of an electrostatic printing plate when applied to a plate prepared in this manner adheres excellently to the conductive surface of the plate and will collect thereon in relatively large amounts to form dense, opaque images which provide prints having excellent contrast. It appears that the charge on the insulating areas of the plate creates an electrical field which induces an opposite charge in the conductive areas of the plate. This charge is enhanced by the oppositely charged toner particles whereupon these particles strongly adhere to the conductive areas of the plate. Since the non-image areas have a surface influencing charge of the same polarity as that of the powder toner particles, the particles are actively repelled therefrom and little or no deposit thereof has been found to adhere to the insulated surface areas of the plate. Consequently, upon transferring, to the receiving substrate, the image formed by the toner a print having good contrast and comparatively free from objectionable background marking results.

The toner may be applied to the plate by a variety of techniques. A preferred procedure, from the standpoint of simplicity, is to simply cascade the dry toner particles over the plate and position the plate at an angle to the horizontal so that unused powder particles gravitationally flow off the plate.

A two-component developer powder is preferably employed in such dry process, comprising carrier and toner particles. The carrier particles are preferably spheroidal to promote flow and reduce abrasive tendencies and are quite large as compared to the toner particles, e.g., 1 mm. compared to microns. The carrier particles and the toner particles are oppositely charged so that the toner particles collect on and are carried by the carrier particles and clumping or agglomeration of masses of the toner particles is restricted. The toner particles are in a size range of from 10 to microns; the carrier particles should be sufficiently massive to flow off all areas of the plate by gravity but sufiiciently small to assure deposition of the charged toner particles therefrom onto the image areas of the plate while maintaining the toner particles free from undue agglomeration. Grains of sand have been found to be excellent carrier particles.

Alternately, the metal plate may have the insulating coating applied only at the areas from which printing is not to be done.

The invention may also be practiced by etching a metal plate at the areas from which no printing is to be done: and then providing an insulating filling for the etched areas only, e.g. by applying a resinous lacquer to the plate, and removing the parts of the coating which overlie unetched portions of the plate thereby to provide a smooth exposed surface at a common level with portions presented by the bare metal where printing is to be done and by the insulating filling of the etched areas where no printing is to occur.

The invention will be further illustrated with reference to the accompanying drawing wherein:

FIGURE 1 illustrates method steps and a suitable apparatus by means of which the plate can be charged and the transfer on the developed image effected; stage a shows a printing plate including a conductive lower layer overlaid with an insulating layer; stage b shows the printing,

plate after a portion of the insulating layer has been removed to expose conductive surface areas constituting the image areas of the plate; stage c illustrates the printing plate being treated by a corona source; stage d is a diagrammatic view showing the printing plate being developed by a developer mixture which includes large carrier particles and smaller toner particles; stage e illustrates the developed printing plate with toner particles adhering to the conductive image areas; stage 1 illustrates one procedure for effecting transfer of the powdered image from the printing plate to a receiving substrate; stage g shows a substrate having toner particles adhering thereto to form an image corresponding to the pattern determined by the conductive areas of the printing plate; stage h shows the substrate and powdered image after the toner particles have been fused upon the substrate;

FIGURE 2 is a schematic illustration of a suitable apparatus by means of which the plate can be charged and transfer of the powder image effected;

FIGURES 3 to 5 are upright sections through a section of metal plate being prepared for a modified practice of the invention;

FIGURE 6 is a perspective view of a composite plate thus prepared;

FIGURE 7 is a conventional elevation of an apparatus for charging the plates of FIGURES lb and 6;

FIGURE 8 corresponds to FIGURE 1d, and shows the development of a composite plate after pre-charging',

FIGURE 9 is a conventional elevation of an apparatus for printing a substrate from the pre-charged and developed plates of FIGURES 1c and 8;

FIGURES 1013 are diagrammatic views showing electrical field effects.

With reference to the accompanying drawing, it is to be understood that the illustrations therein are primarily schematic to facilitate understanding of the principles of the invention and are in nowise drawn to scale, particularly with reference to layer thicknesses, particle sizes, etc.

Referring to the sequential steps illustrated in FIGURE 1 the procedures involved from the preparation of the plate to obtaining printed copies from the plate are depicted in step-by-step relation in FIGURES la-lh.

FIGURE 1a discloses a printing plate 10 before preparation thereof for reception of the printing pattern thereon. The plate comprises a conductive layer 12 overlaid with an insulating layer 14 of suitable insulator material which may be coated onto or otherwise applied over the conductive substrate.

FIGURE 112 discloses the preparation of the plate 10 for printing by removing portions, e.g., by mechanical or chemical etching of the insulating layer 14 to expose surface areas 16 of the conductive layer, these exposed conductive surface areas constituting the image areas of the plate with the remaining surface areas 18 of the insulating layer 14 constituting the non-image areas of the plate.

Thereafter, as shown at FIGURE 10 an electrical charge is established on the insulating layer 14, e.g., by employing a conductor such as corona bar 20 charged to a potential at which corona effects appear, the charge being shown as positive in the illustrated embodiment. The charged insulating areas induce an opposite charge on the conductive layer 12, the strength of which is most pronounced on the edges of conductive image areas 16 of the plate.

Next, as shown at FIGURE 1d, the plate 10 is tilted at an angle to facilitate gravity fiow and a powder 22 is dusted, by cascading or otherwise, on to the plate surface. The powder 22 is composed of carrier particles 24 having negative charges which hold smaller particles 26 having positive charges, which latter particles comprise the image forming or toner particles. As the powder 22 strikes the plate any toner particles 26 released from the carrier particles 24 are repelled by the non-image surface areas 18 of the plate and thus do not remain thereon to cause false printing later. However, these toner particles 26 are electrically attracted to the conductive surface area 16 from which printed copy is to be made and consequently collect in these recessed image areas. While the carrier particles 24 are attracted to the charged non-image areas 18 of the plate, they are of such size and mass that they move along the plate by gravity (as shown by directional arrows) without being held thereon and flow into a suitable collection trough 27 or the like. The attraction of toner particles to the conductive areas 16 is greater than their attraction to the carrier particles 24. Thus, toner particles 26 jarred loose from the carrier particles 24 as these particles strike and roll over the insulating layer, are repelled by the insulating layer and strongly attracted by the conductive surface areas 16 and consequently collect thereon. Of course, the collection of toner particles 26 on the image areas 16 is also brought about by the greater attraction of these image areas for the toner particles than the carrier particles.

As shown at FIGURE 1e; by disposing the plate at an angle, the toner particles 26 adhere well to the conductive image areas 16 and form a powdered image thereon while the remainder of the plate remains clean. The result is a plate from which prints can be made electrostatically which are exceptionally clean, i.e., free from background marking.

The procedure for effecting transfer of the powdered image from the plate 10 to a receiving substrate or sheet 28 may be effected in various ways. One of these is illustrated at FIGURE 1 wherein the plate It) is reversed relative to the corona bar from the position shown in FIGURE 10, in that the insulating layer 14 is downward and the conductive layer 12 on top and adjacent to corona bar 20; the substrate 28 is below the insulating layer 14 so the particles held electrostatically on the image areas 16 have free paths available toward the substrate. The substrate can be disposed on a suitable conductive support 30 or back electrode. The charging from the corona bar 20 then establishes a potential difference from the image areas 16 to the substrate. The electric field established between the conductive layer 12 of the printing plate and the conductive support 30 serves two functions: (1) the attraction of the conductive image areas for the toner particles is changed to a repulsion; (2) the capability of the charged insulating regions of the plate for inducing charges which are attractive to the powder particles is reduced; and therefore the powder particles are attracted toward the substrate 28 to be printed. Thereafter, the powdered image designated generally by the numeral 32 is fixed and made permanent on the substrate as shown at FIGURE 1h by fusing or other means known to the art.

The schematic representation of FIGURE 2 shows a suitable high voltage source 34 capable of supplying from about 5 to 15 thousand volts connected at one terminal to a backing 38 and from the other terminal of which one or more long thin corona discharge wires or bars 36 extend in overlying relation to the printing plate 10 carried on backing 38. The high voltage in the wire creates an intense electrical field about the wire from which a corona discharge is emitted to electrically charge the insulating surface 18 of the plate 10 as the plate passes thereunder in the direction of the arrow. The backing 38 is of a conductive nature and may constitute the shell of a drum or roller to which the plate is attached. As the printing plate 10 is passed under the wire, the relative movement being in the direction of the arrow, the non-image insulating areas thereof are electrically charged, thereby inducing an opposite charge on the conductive image areas 16. After charging the plate, the image powder 22 may be cascaded over the plate to form the powdered image 32 on the conductive surface 16 and the image transferred and fixed in the manner described with reference to FIG- URE 1.

In the practice shown in FIGURES 39, a composite printing plate is prepared by etching (FIGURE 3) a metal plate 41 to a depth of about 0.005 to 0.030 inch, for example by methods employed in making electrotypes, the recesses and depressions such as 42 being formed at the background or non-image areas and the unetched portions 43 at the original plate surfaces providing printing areas. The recesses and depressions are then filled (FIGURE 4) with a solid insulating material 44 such as a casting plastic or epoxy resin, noting that the mass can extend above the original metal surface and overlap onto image areas from which printing is to be done. The composite plate is then ground and polished to a smooth surface (FIGURE 5) with the metal image areas as at 45 and the insulating non-image areas as at 46 at the same level. The composite plate 40 is illustrated in FIGURE 6, with the metal areas 45 forming the image to be printed, and the surrounding areas 46 being the non-image areas for background and provided. by the insulating material: it being understood that the thicknesses are exaggerated for clearness of illustration.

Before printing, the plate is precharged electrically, e.g. as shown in FIGURE 7. The composite plate 40 is placed on a conductive support 50, with the upper surface of the plate 40 having the image pattern of bare metal and the non-image pattern of insulating material, and with the metal base 41 in conductive contact with the support 51). A conductor 51 is moved relatively along and in spaced relation to the upper plate surface, for example by supporting it on an insulating traveller 52. Several bars 51 can be employed. A source 53 establishes a potential difference between the conductor 51 and the meal plate 41 so that a corona discharge forms at the conductor 51.

Arrays of fine wires and of sharp metal points have been employed for the conductor 51. Platinum and stainless steel wires of around three mils diameter have been employed: the material should be non-corroding to avoid atmospheric deterioration over a course of time. Sewing needles and pins have been employed in points arrays. Spacing of wires and points by a half inch from one another has been found satisfactory with like spacing from the

metal base

12, 41 during the precharging. High voltage direct current supplies of 5 to 10 kilovolts have been used in eifecting a precharging of insulation portions to a condition of active repulsion of toner particles. Currents of 10 to microamperes have been drawn during charging.

In FIGURE 7, the electrical charging is opposite to that of FIGURES 12, to indicate that the relative polarities of the

plate

12, 41 and the

insulation

10, 46 is not critical: noting that a like change of relative polarities of carrier and toner particles is then employed.

The conductor 51 is illustratively negative relative to the support 50, and the corona effect then causes negative charges to become established on the exposed surfaces of the insulation or non-image areas 45, while the source acts to maintain the metal body 41 of plate 40 and the exposed top surfaces 45 thereof relatively positive: this effect being assisted by the induction of positive charges on the metal plate 41 from and. to balance the negative charges at the surfaces of the insulation material 46.

The development of the electrically charged composite plate 40 can now be effected as in FIGURE 8. The corona charging source 53 is disconnected. The negative charges on the surfaces 46 of the insulation material remain, with corresponding positive charges in and on the metal plate 41 and its bared top portions 45. The plate 40 can be supported in slanting position, with the charged insulation portions 46 upward, and a developing powder 55 is caused to cascade along the plate, as by delivery from a hopper 56. This powder 55 can be the mixture of larger and more massive carrier particles with smaller toner particles electrostatically carried thereby, the toner particles in FIGURE 8 having a negative charge. The attractive forces between the negative toner particles, and the positive charges at the bare metal areas 45 of the plate 40, cause toner particles to collect on and cover the exposed metal as shown at 57. Conversely, there are repulsive forces between the negative toner particles and the charges resident on the insulation areas 46, and toner does not cling thereto. The carrier particles have such mass that they do not come to rest on the sloping plate 40, but roll therealong and fall into a receiving trough 58.

The plate 40, with the toner accretions 57 on bare metal portions 45 and essentially absent from the insulation portions 46 can now be placed on a support or base electrode 60, FIGURE 9, for conductive contact therewith. Insulating spacers 61 are placed upon plate 40. The substrate 62, to be printed, is placed on the spacers and thus held away from the plate by a distance of, say, inch. The charge on the metal plate 41 is now reversed in polarity, so that the bare metal portions 44 thereon now become repulsive to the toner particles. This can be done by the relative movement of a conductor 63 along the substrate but spaced therefrom, e.g. by employment of the insulating traveller 64 for the conductor 63. A high voltage source 65 is connected from its negative terminal to the base electrode 60 and from its positive terminal to the conductor 63 so that a corona is developed at the conductor 63 and positive charge effects are established at the surface of the substate 62 opposite to that at which the printing is to be done, such positive charges being attractive to the negative toner particles on the plate 40. These particles leave the plate 40 and proceed to the substrate 62 and collect thereon in a pattern determined by the shapes of the exposed metal portions 45 from which they came. Intimate contact of the printing plate 40 and the substrate 62 is not required and the surface of the substrate need not be smooth.

In practice, the same source and traveller can be employed for the precharging of FIGURE 7, and the printing transfer of FIGURE 9, by reversing the connections from source to the base and the travelling conductor.

When the source 71 is disconnected, the forces which hold the toner particles on the metal regions are those induced by the negative toner particles themselves. the electrostatic field generated from source 65 for effecting transfer to the substrate acts on the one hand to cause repulsion of toner particles from the metal areas 45 employing for this purpose the charges on the particles which have been holding them to the metal at the accretions 57; and on the other hand to attract the particles toward the substrate 62.

In practice, it has been found that prints so made are not of uniform density over extended areas. The exposed surfaces of insulation regions 46 are negatively charged, and there are positive charges flowing from the source to the metal surfaces at the bottom of the respective recesses. Thus each mass of insulation acts as the dielectric in a capacitor, with the several masses acting and being charged like a parallel plate capacitor. Charges of equal magnitude but opposite polarity appear at opposite faces of the dielectric bodies, which here are the smoothed individual masses of insulation 46; with the charging at the exposed surface being from the corona discharge, and at the bottom surface of the respective recess by conduction from the source. Capacitors are known to exhibit relatively large electric fields in the spaces between the plates and around the edges; the field outside such a capacitoir is negligible except for this edge effect. Similarly, for the charged printing plate, the external field is small except at the edges of non-image areas. Only at such edges is the field large enough to significantly affect the motions of toner particles.

It has been found that deficiencies from this cause can be eliminated by use of an auxiliary development electrode. This is shown in FIGURE 8 as a conductive plate 70 coextensive in area with the regions of plate 40 having bare metal and insulation portions by which a pattern is to be printed, preferably being parallel thereto and spaced therefrom so that the flow of the cascading particles 55 is not impeded. This spacing may be established by local insulating supporting devices 72 at non-printing regions. A source 71 is employed to maintain the electrode 70 at the same polarity as the charged insulation regions 46 of Hence the composite pattern plate 40, but at slightly less magnitude than the potential difference between the charges on insulation regions 46 and the metal plate 41. Thus, in the illustrated practice of FIGURES 7-9, where the development powder comprises positively charged carrier particles and negatively charged toner particles, and the corona bars 51 of FIGURE 7 have caused negative charges to be located on the insulation regions 46, the source 71 acts to make the auxiliary electrode 70 negative relative to the metal plate 41. Therewith, at the regions between the negatively charged insulation portions 46 and the development electrode 70, an electric field is established which acts to prevent toner from adhering to the portions 46; and conversely the electric field between the development electrode 70 at the positively charged exposed metal portions 45 acts to effect toner deposition on the portions 45 regardless of their area or the induced field effects at the edges thereof. When the source 71 is disconnected, and the plate 40 with toner 57 on the metal portions 45 thereof is withdrawn from the electrode 70, the plate 40 can then be employed as in FIGURE 9 for transfer of the toner to the substrate 62.

In lieu of employing a source 71 as in FIGURE 8 to establish a potential difference between the conductive development electrode 70 and the composite plate 40, the electrode 70 may have a coating of dielectric and this coating be subjected to a corona discharge, e.g., as shown for the insulating regions 46 of plate 40 in FIGURE 7, and then employed with this charged dielectric area at the lower surface of electrode 70 in FIGURE 8 without employment of the battery 71 but preferably with the

conductive elements

70 and 41 electrically connected.

The effects of an auxiliary development electrode are shown by the comparative diagrams of FIGURES 10 to 13. In these diagrams plots or graphs of the field effect values are superimposed above a composite plate 40 to exemplify the field effects above exposed metal portions 45 and insulation portions 46 thereof, with lines 103 showing the boundaries or edges between such portions. Each plot or graph has a zero value or reference base line marked 0.

The electric field intensity in the air just outside the surface of an isolated charged plate, in the direction indicated by the vector E in FIGURE 10, is essentially zero at all locations removed from the edge of the coating and also immediately at the edge. The field intensity is indicated by the plot or graph line -101-102-106 102-101-100 which represents a zero value at points remote from the insulation area 46, and is non-zero only in

narrow regions

101, 102 near and on either side of a boundary between metal and insulation areas of the composite plate 40, this boundary being projected from the plate to the graph line 100 by the upright dotted line 103. A like but symmetrical condition exists at the opposite edge of the insulation area 46. The horizontal

dotted lines

104, 105 in FIGURE 10 are spaced from the horizontal or zero positions of the graph line 100 and indicate a range of field intensities too small in magnitude to produce a significant interaction with a charged toner particle. The field intensity 106 above the insulation region 46 is due to ions deposited there by exposure to corona discharge. The non-zero field intensity 100 above the exposed metal portions 45 of the printing plate adjacent to the coating is due to the charge that appears near the interface, due to conduction, when the printing plate is charged. A charged particle would experience a significant force due to the charge on the printing plate if brought sufficiently near the plate in those regions where the plot of field intensity in FIGURE 10 falls outside the limits indicated by the horizontal dotted

lines

104, 105. If the charge of the particle is the same as the precharge on the composite plate 40, that is, the precharging effect resident on the insulation regions 46 of the composite plate 40, it will be repelled from the plate by the field effect on the surface of the coating 46 and attracted toward the plate 40 by the field effect at the exposed metal portions 45 adjacent to the coating 46. The forces of repulsion are those represented by the parts of regions 102 above the non-activity reference line 104; and the forces of attraction are those represented by the parts of region 101 below the non-activity reference line 105. At the immediate edge of the coating, that is, at a point 107 where the boundary line 103 crosses the zero reference line of the graph, there would be no interaction, nor will there be interaction at regions 45 of the bare metal, such as regions 108, which are substantially removed from the boundary line 103, and the field effect at such points can be represented by the points 109 on the zero reference line. If the charge on the particle is opposite to the precharge on the composite plate, it will be repelled from the metal regions 45 and attracted by the insulation regions 46. In each case, the field effect at points of the insulation region 45, which points are remote from the boundary edge 103, has a value 106 in FIGURE 10, which is not zero but is less than the edge field effects 102.

When the auxiliary electrode 70 is employed and electrically connected to the metal plate 41 by a conductor 90, the plot of field effects is as shown in FIGURE 11. The spacing S between the composite plate 40 and the electrode 70 must be sufficient for the development powder to be introduced and to move for effecting the development. Assuming that the dimensions of area of the composite plate 40 with metal image regions 45 and insulation non-image regions 46 are very large compared to the separation S between the

electrodes

41, 70, the electric field intensities may be considered as those of an infinite parallel plate capacitor. On computing and vectorially adding the field intensity values, the plot or graph in FIGURE 11 is obtained.

The electric field intensity between the exposed conductive metal portions of the printing plate and the development electrode is of course equal to zero except near the edge of the coated portion. A particle with a charge of the same polarity as that on the insulation region 46 would experience repulsion as represented by the graph portion 113, when near the region 46, even when removed from the coating edge. This particle would experience little, if any, attractive force due to the composite plate surface charge, when near the exposed conductive portions of the plate, as represented by the graph portion 110. When the particle is close to an exposed metal region and near a boundary line 103, the zero field effect 110 thereon is modified by the presence of the charges on the insulation region 46, and a minor attraction 111 toward the plate 40 occurs.

If the composite plate 40 has not been precharged and the charge effects on exposed metal regions 45 and on insulation regions 46 are the same, then the application of a potential difference between the

electrodes

41, 70, causes field effects upon a particle in the space s between the electrodes as shown by the plot or graph lines in FIGURE 12. Assuming the auxiliary electrode 70 negatively charged relative to the metal plate 41, the field effect on a negatively charged particle between the electrodes is an attraction represented by graph portions 121 opposite metal regions 45 and a lesser attraction represented by graph portion 122 opposite the insulation portion 46 due to negative charges induced on the portions 46 by the positive charges in the underlying plate 41.

When the composite plate 40 has been precharged, and the electrode 70 is at a potential V relative to the relative to the metal plate 41, the field effects represented in FIGURES 11 and 12 are combined, as represented by the plot or graph lines in FIGURE 13. With the plate 41 connected to the positive pole of source 71, FIGURE 8, and the auxiliary development electrode 70 connected to the negative pole, graph portions 130 represent a Zero electric field effect remote from the space between the

electrodes

41, 70. A negative toner particle between the electrodes is subjected to a field effect 131 when above an exposed metal region 45 by which the particle is attracted to the metal, and to a field effect 132 when above an insulation region 46 by which it is repelled therefrom by a uniform force over essentially the entire area of the region 46 and with no greater force effect at the edges or boundaries 103 as shown at 102 in FIGURE 10. Conversely, the edge effects 133 of greater attraction, adjacent the boundaries 103, toward the metal regions 45 are greatly reduced in result, compared to the effects 101 in FIGURE 10, noting that the field effects above most of the metal regions 45 were zero in FIGURES 10 and 11, but are dominant at 131 in FIGURE 13.

The potential drop V between the

electrodes

41, 70 must be less than the precharging effect resident on the insulation regions 46.

The effect of employing the auxiliary development electrode 70, With a charge of the same polarity as the precharging upon the insulation regions and that upon the toner particles being employed for printing, is to provide toner accretions 57 of uniform density upon the exposed metal pattern regions 45 and to maintain the insulation pattern regions 46 free of toner, and to sharply delimit the boundaries between such regions. Therewith prints are produced in which the image area of toner are sharply fined and of uniform depth of tone, while the background or non-image areas are clear of toner. The printed areas may be of large extent without loss of depth.

The process is ideally suited for the reproduction of line copy, area copy and half tone copy and since the powdered image is formed on the conductive arears of the printing plate rather than on the insulator covered areas thereof, the powdered image is formed in those areas where the electrical fields of force are strongest.

The process is a relatively rapid one, each of the charging, dusting and printing steps requiring less than about one second. By carrying the steps out in rapid sequence and by beginning to copy the second print before the first is completed, the processing time per print can be reduced to but a few seconds to give printed images having excellent contrast and sharpness of definition on almost any kind of receiving substrate, whether the receiving substrate be smooth surfaced or irregularly surfaced as by wafiling, corrugating, etc. Since printing can be accomplished without requiring contact of the receiving substrate to be printed with the image areas of the printing plate, excellent and undistorted prints can be obtained on such uneven surfaced material as corrugated board.

The practice of the invention is further illustrated by the examples following:

Example 1 A printing plate was prepared as in FIGURES 1a and 112 from a sheet of tinplate coated with a protective lacquer by removing portions of the lacquer to expose the tinplate on the area to be printed by scraping off the lacquer with a sharp metal stylus. The lacquer coating was an insulator having an average thickness on the plate of less than about 1 mil and comprising an oleoresin of about equal parts of a meleic acid resin ester and China-wood oil. The lacquer remaining on the tinplate was provided with an electrical charge as in FIGURE 10 by passing the plate under a plurality of corona discharge wires from an apparatus such as that illustrated in FIGURE 2, the wire diameter being 2 mils and the plate to Wire distance being about /2 to A1 of an inch. The high voltage source generated from 5000 to about 15,000 volts and produced a corona effect around the wire, thereby establishing the charge on the remaining insulating layer. The plate was retained on a conductive support, the electrical connections being those illustrated in FIGURE 2.

After charging, the plate and support were tilted as in FIGURE 1d at an angle to the horizontal and an image powder cascaded thereover. The image powder consisted of carrier particles of an Ottawa-type sand coated with ethyl cellulose, which particles averaged about seventenths of a millimeter in diameter and were generally spherioidal. Carried on these carrier particles were toner particles of about microns in diameter, the toner particles comprising a styrene-methyl methacrylate polymer in which were suspended fine particles of carbon black, the styrene and ethylacrylate were present in about a 1:1 weight ratio.

After excess powder had cascaded off the plate so that only toner particles remained to form the powdered image on the conductive image surface areas of the plate, a sheet of corrugated board was placed on the support and the printing plate turned up-side-down as in FIGURE 1 so that the powdered image thereon faced the corrugated board receiving sheet. This was again run under the corona discharge wires whereupon the powdered image was transferred to the face of the corrugated board as shown in FIGURE 1g. The powdered image deposited on the corrugated board surface was fixed and made permanent by heating the printed area of the board under a photo-flood lamp.

The resultant printed corrugated board as in FIGURE 111 displayed an image which had excellent contrast and sharpness of detail with no distortion. While a few false-printing spots occured on the face of the board, they were widely scattered and appeared to be the result of defects in the insulating surface of the printing plate.

Example 2 Following generally the procedures in Example 1, printed copies were made from a printing plate of anodized aluminum having a photographic coating thereover. The photographic coating was exposed and the unexposed areas of the coating overlying the anodizing coating of the aluminum and the anodized coating in these areas were removed to expose the aluminum surface by treatment with a solution of lye.

Following the procedure of the preceding example excellent printed copies were made.

Example 3 In order to prevent contact of a copy to be printed with the printing area of the printing plate, a plate was prepared as in Example 1 and the copy to be printed maintained about of an inch away from the plate while the image transferred thereto. It was found that copies could be readily made in this manner.

In the printing methods shown, the

composite plate

10 or 40 with toner particles on the exposed metal portions thereof, does not have the development field of FIGURES 1d and 8 present during the printing. The only field effect to be overcome in transferring the toner from the

plate

10 or 40 to the

substrate

28 or 62 is that due to the induction from the charged toner particles and which has been retaining them on the exposed metal regions of the composite plate: as distinguished from developing and printing by use of electric forces which cause the toner particles to form deposits upon insulation portions of the composite plate, and wherein the pre-charge upon such portions must be overcome before the particles can be detached therefrom and caused to migrate to the substrate.

Furthermore, the effects of differing thicknesses or non-homogeneities in the insulation regions are minimized, noting that such regions acquire different potentials during the precharging. Thus when such insulation regions are employed to hold the toner for printing, the amount so held is not uniform over the area, and the print correspondingly has different depths of tone. With the instant procedure, the toner is repelled from such insulation regions, and the exposed metal, with its uniform conductivity and smooth surface, collects the toner during development.

Like examples of practice have been performed as described for FIGURES 3 to 13.

Following these procedures, such diverse receiving substrates as glass, corrugated board, plastic and metal have been successfully printed. The process is equally useful for printing on insulators or conductors since the practice of the process relies on the establishment of differences in electrical potentials between the plate and the image powder.

While coated sand granules were used as the carrier particles in the specific examples noted hereinbefore, it is to be understood that the invention is not limited to the utilization of any particular carrier or toner particles in the image powder: it is only necessary that the carrier and the toner particles be sufficiently far apart in the triboelectric series so that the particles will assure a sufiicient charge, the electrical attraction between them being such that the toner particles will be carried by the carrier without tending to agglomerate too greatly.

As noted hereinbefore, the size range of the toner particles is preferably between about 10 and about 30 microns for the production of good printed copy. The pigment in the toner may be carbon black: and other coloring ingredients, such as vegetable dyes or other colorants which do not adversely affect the dielectric properties of the toner particles, can be used to provide a visible image.

The size of the carrier particles should not be so large as to prevent deposition of toner particles on the desired printing areas or so small as to permit agglomeration of toner particles or retention of carrier particles on the plate. A preferred size range is from about 0.1 millimeter to about one millimeter.

For the insulator layer on the printing plate, any of the materials normally used for insulating can be made which maintain an electrical charge effect for a sufficient time to permit the development at the image areas.

While the invention has been described with respect to the polarity of the electrical charges, polarity differences are not critical. Thus, so long as a difference in electrical potential exists between the conductive image areas of the plate and the non-image areas of the plate such that the charged toner particles are sufficiently attracted to the former to adhere thereto and repelled from the latter, the invention is operative.

I claim:

1. A method of printing from a printing plate upon a substrate, said method comprising the steps of:

(a) providing a printing plate having image and nonimage areas of different electrical potentials, the image areas being electrically conductive surfaces and the non-image areas being electrically insulative surfaces;

(b) charging said non-image areas for providing said non-image areas with a predetermined polarity;

(c) developing the conductive image areas with an image-forming material which is comprised of electrically charged image particles which have a polarity the same as the polarity of the charged nonirnage areas;

(d) establishing a potential difference between the conductive surfaces of said printing plate and said substrate for transferring the image-forming material from said conductive surfaces to said substrate, the potential difference being established so that the polarity of the substrate is opposite to the polarity of the charged image particles whereby the charged image particles are attracted from said conductive surfaces to said substrate.

2. The method of claim 1 in which the image forming material also comprises carrier particles electrically 13 attracted by said non-image area and repelled by said image area, the electrical forces between the carrier "and image particles being less than the attractive forces between the image particles and the image area when the carrier and image particles are adjacent the image area.

3. The method of claim 2 in which the carrier particles have masses greater than the masses of the image particles, and in which the said image and non-image areas are positioned so that the image forming powder moves therealong under the action of gravity whereby image particles are captured by the image area and the carrier particles move over the non-image areas and are competent to separate from the non-image areas, image particles adhering thereto.

4. The method of claim 2 in which the image area is provided by an exposed portion of a surface of a metal body and the non-image area is provided by the exposed surface of an insulating layer overlying other portions of said body surface.

5. The method of claim 1, including the step of presenting a development electrode in spaced relation to the surface of said printing plate which has the image and non-image areas, and in which the image forming material is brought between the printing plate and the development electrode, and comprising the step of establishing a potential difference between the printing plate and the development electrode during the application of the image forming material for maintaining the development electrode at the polarity of the insulative non-image areas and at a lesser electrical potential.

6. A method of electrostatic printing which comprises providing a printing plate having a conductive underlayer and an insulating overlayer, portions of said conductive underlayer being exposed and forming the image area of said plate, utilizing the remaining overlying portions of said insulating layer to form insulative non-image areas of said plate, providing said insulative non-image areas with an electrical charge of one polarity and thereby inducing an electrical charge of different polarity on said conductive image areas, then applying to said plate an image forming powder containing charged image forming particles having a polarity opposite to the polarity of the electrical charge on the conductive image areas so as to be attracted to said conductive image area and repelled from said insulative non-image area, transferring the powdered image formed by said particles on said conductive image areas to a substrate to be printed by creating an electrical potential difference between said image area and said substrate whereby said particles are repelled by said image area and attracted to said substrate, and thereafter fixing said powdered image on said substrate.

7. A method of electrostatic printing which comprises providing a printing plate having a conductive image surface area and an electrically insulative non-image surface area, establishing an electrical charge of one polarity on said insulative non-image area and thereby inducing a charge of opposite polarity on said image area, then applying to said plate an image powder comprising carrier particles having a charge of the same polarity as said image area and to which are electrostatically adhered toner particles having a charge of pposite polarity to that of said image area and thereby causing toner particles to be repelled from said insulative non-image area and to separate from carrier particles and to collect and form a powder image on said image area, re moving carrier particles and excess toner particles from said plate, and then transferring said powder image to a substrate to be printed therewith by establishing a charge on said substrate of the polarity opposite that of said powder image while establishing a charge of the same polarity as said powder image on said conductive surface area of said plate, and thereafter fusing said powder image to said substrate.

8. The method of claim 7 wherein said electrical charge is established in said non-image area by subjecting said plate to a corona discharge.

9. The method of claim 7 wherein said electrical charge on said non-image area and on said substrate are established by corona discharge.

10. A method of printing from a metal printing plate upon a substrate, which comprises providing a printing plate having conductive image and insulative non-image areas and applying a potential therebetween for providing said image and non-image areas with charges of opposite electrical polarities, the image areas being provided by exposed metal surfaces of the plate and the non-image areas having their surfaces insulated from the metal of the plate, positioning a development electrode parallel to and spaced from the said image and non-image areas of the printing plate, applying an electrical potential to the development electrode which is of the same polarity as and of lesser potential than the charged non-image areas of said printing plate, bringing between the development electrode and the printing plate an image forming material comprising electrically charged image particles having a predetermined polarity and attracted by said image areas and repelled by said non-image areas whereby a powder image is formed upon said image areas, and thereafter transferring said image forming material to the substrate, by establishing a potential difference between said image areas and the surface of the substrate so that the polarity of said image areas is the same as the polarity of said particles and so that the polarity of the substrate is opposite to the predetermined polarity of said image particles and said particles of the image-forming material are repelled by the image areas and received by the substrate.

11. The method of electrostatic printing, which comprises preparing a printing plate of conductive material having recesses in its surface at non-image areas, depositing non-conductive insulation material in said recesses to fill the same and establishing a common exposed surface on said plate comprising conductive image areas of exposed metal and non-conductive insulation regions constituting exposed non-image areas, electrically precharging the insulation regions at the exposed areas thereof, applying a charged non-conductive image-forming powder to said common exposed surface with the particles having an electrical charge of the same polarity to that of the pre-charge upon the insulation regions whereby the particles are repelled from the insulation areas and are attracted to and collect on said exposed metal areas, and thereafter transferring the image forming particles from said plate to the substrate to be printed by establishing a potential difference between the metal plate and the surface of the substrate such that the polarity of the metal plate is the same as the polarity of said particles.

12. The method of printing from a composite printing plate upon a substrate comprising the steps of providing a printing plate having a conductive base layer constituting that part of the printing plate from which printing is to be done and an exposed patterned insulation layer thereon with the conductive base layer exposed between the parts of the insulation pattern, providing said base layer and insulation layer with charges of differing electrical polarities, positioning a development electrode parallel to and spaced from said composite plate, applying particles of a printing toner powder to the surface of the composite plate, said particles being of opposite charge to the parts of said composite plate from which printing is to be done, applying an electrical potential to the development electrode which is of the same polarity as the image particles whereby to facilitate the collection of a uniform layer of the toner particles at the parts of said composite plate from which printing is to be done and thereafter transferring the toner particles from the composite plate to the substrate to be printed by establishing a potential difierence between the composite plate and the substrate with the substrate at a polarity opposite to that of the toner particles.

13. The method of claim 12, in which the toner particles'are applied as a component of a development powder having carrier particles of larger mass and toner particles of lesser mass, the carrier and toner particles having opposite electrical charges, and in which the composite plate and development electrode are positioned at an angle to the horizontal so that the carrier particles are caused by gravity forces to move across the face of the composite plate.

References Cited by the Examiner UNITED STATES PATENTS DAVID KLEIN, Primary Examiner.

Claims

1. A METHOD OF PRINTING FROM A PRINTING PLATE UPON A SUBSTRATE, SAID METHOD COMPRISING THE STEPS OF: (A) PROVIDING A PRINTING PLATE HAVING IMAGE AND NONIMAGE AREAS OF DIFFERENT ELECTRICAL POTENTIALS, THE IMAGE AREAS BEING ELECTRICALLY CONDUCTIVE SURFACES AND THE NON-IMAGE AREAS BEING ELECTRICALLY INSULATIVE SURFACES; (B) CHARGING SAID NON-IMAGE AREAS FOR PROVIDING SAID NON-IMAGE AREAS WITH A PREDETERMINED POLARITY; (C) DEVELOPING THE CONDUCTIVE IMAGE AREAS WITH AN IMAGE-FORMING MATERIAL WHICH IS COMPRISED OF ELECTRICALLY CHARGED IMAGE PARTICLES WHICH HAVE A POLARITY THE SAME AS THE POLARITY OF THE CHARGED NONIMAGE AREAS;