Electrostatic printing system and method using ions and toner particles

[57] ABSTRACT

A system for electrostatic printing including a corona source and electrode for generating a substantially uniform ion stream in the direction of a print receiving medium and a multilayered apertured grid modulator interposed in the ion stream for modulating the cross sectional flow density of ions in the stream in accordance with a pattern to be reproduced. A toner supply introduces a cloud of substantially uncharged toner marking particles adjacent the print receiving medium whereby the modulated ion stream selectively impinges upon and charges toner particles in the cloud which are thereby accelerated and deposited on the print receiving medium in accordance with the pattern to be reproduced.

Claims, 19 Drawing Figures

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PATENTED DEC 181973

3,779,166

SHEET 1 OF 4

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<img file="US3779166A_D0003.tif" />

INVENTORS \ 33

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GERALD L. PRESSMAN HUGH E FROHBACH DAVID E. BLAKE

FIG—6

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ATTORNEYS

PATENTED DEC 181973

SHEET SOFA

3,779,166

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ATTORNEYS

PATENTED DEC 18 1973

3.779.166

SHEET 3 OF 4

FIG.8A

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<img file="US3779166A_D0009.tif" />

INVENTORS

GERALD L. PRESSMAN

HUGH E FROHBACH

DAVID E. BLAKE

PATENTED 0EC181973

3,779,166

SHEET U OF 4

-| ^82

DRUM

FIG-13

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100

FIG_ 14

103

104

105

106

112

SCREEN DRIVE .'90'y

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101

102

96___

SCREEN DRIVE

113

114

114

115

FIG-15

121

120

138

AIR

SCREEN DRIVE

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132

133

TONE

135 137

116

115

INVENTORS

GERALD L. PRESSMAN

HUGH F. FROHBACH

DAVID E. BLAKE

ATTORNEYS

3,779,166

ELECTROSTATIC PRINTING SYSTEM AND METHOD USING IONS AND TONER PARTICLES

This invention relates to a new and improved system for electrostatic reproduction and in particular to an improvement in modulated aperture non-contact electrostatic printing, useful in a wide range of applications including document copiers, computer printouts, silent typewriters, etc.

The system of electrostatic printing referred to herein as modulated aperture non-contact electrostatic reproduction is generally set forth in U.S. Pat. applications Ser. Nos. 673,499, now U. S. Pat. No. 3,625,604 and 776,146 now U. S. Pat. No. 3,647,291 assigned to the assignee of the present case. According to these disclosures, there is provided in a preferred embodiment a multilayered apertured screen including at least a conductive layer and an adjacent insulative layer on which an electrostatic latent image is formed for modulating a flow of charged toner particles, ions or other printing particles directed through the apertures of the screen by an accelerating field. The minimum of two layers for the screen construction permits establishing a double layer charge on the opposite sides of the insulative layer for selectively producing overlapping lines of force or “fringing” fields within the apertures of the screen. Thus, lines of force generated by bi-polar electrostatic fields extend within the apertures of the screen and can be oriented to oppose the passage of charged particles, enhance the flow of charged particles, or be ³⁰neutralized to have no effect on charged particles directed through the apertures of the screen. Thus, the double layer charge can be selectively established across the face of the screen to substantially completely block the passage of charged particles through certain apertures, enhance and accelerate the passage of charged particles through other apertures, and control the width and density of the particle stream through other apertures over a continuous spectrum. A stream or flow of charged particles directed through the screen by an overall applied accelerating field is therefore modulated to provide a cross sectional density variation at least substantially corresponding with the image or pattern to be reproduced. The modulated stream of toner particles or other charged particles passing through the apertured screen is transferred by the overall applied electrostatic projection field across a gap or space to the print receiving medium. The pattern image formed by this non-contact printing arrangement is thereafter fixed according to known techniques if dry particles are used. If the toner particles consist of liquid aerosols the fixing step may not be necessary. Furthermore, if the charged particle stream consists of ions the ions impinge on the paper forming an electrostatic latent image which is thereafter dusted and fixed. In this latter embddiment, treated dielectric paper must be used in order to retain the electrostatic latent image formed by the ion stream.

In one approach for establishing the double layer charge or bi-polar electrostatic latent image across the screen, a substantially uniform charged distribution is initially established across both sides of the insulative layer with opposite charges on the respective sides to provide fringing fields within the apertures. These fringing fields are oriented to produce blocking fields to block or partially block particles of a predetermined charge sign. By use of photosensitive insulating materi2 als the charge distribution and therefore the blocking fields are dissipated according to a light pattern projected on the screen. The fringing fields also may be oriented to include enhancing fields which are established by additional techniques. The screen must generally be charged initially with blocking fields which block the passage of charged toner particles through the screen however. The blocking fields are thereafter selectively neutralized according to the light pattern or 10 reversed to enhancing fields according to additional procedures, and the resulting modulation of the flow of toner particles through the screen results in certain embodiments in negative printing. In order to achieve positive printing special contact charging techniques for 15 charging the screen and other special expedients for non-contact or field charging are used.

Modulated aperture electrostatic printing systems permitting direct positive printing without the necessity for contact charging the screen modulator or other spe20 cial expedients are described in other patent applications assigned to the present assignee. In the U.S. Pat. application Ser. No. 85,070, now U.S. Pat. No. 3,694,200 entitled Electrostatic Modulator for Controlling Flow of Charged Particles, filed on Oct. 29, 1970, 25 there is described an apertured screen having substantially the entire surfaces formed of a dielectric material with the dielectric preferably thicker on one face of the screen than on the other. The screen is charged with like charges over substantially its entire surfaces to develop fringing fields in the apertures and is preferably charged to a higher level on the thicker side so that the screen initially carried a higher potential on one side providing a uniform charge inequality or potential difference through the screen apertures. The charge inequality or potential difference is oriented to provide enhancing fields within the apertures of the screen to enhance the flow of charged particles passing through the screen from the direction of the side of higher charge density. The charge distributed across the side ⁰ of the screen having a higher charge density is selectively dissipated in accordance with a pattern to be reproduced thereby establishing a bi-polar electrostatic latent image as a modification of the fringing fields for density control of a flow of charged particles directed ⁵ through the screen. Full modulation control of the particle flow including blocking fields, enhancing fields, and a spectrum of fringing fields over a range in between, is therefore possible with a single selective ₅θ charge dissipation. This arrangement also permits direct positive printing when the charge is selectively dissipated by optical light image addressing. For optical addressing the insulating material on the thicker side is a photoconductive insulating material. Thus, the pas₅₅ sage of charged particles is enhanced or unimpeded in the dark areas while the passage of charged toner particles is blocked or impeded in the light areas. A different modulated aperture non-contact printing screen and system permitting direct positive printing is also set ₆₀ forth in the U.S. Pat. application Ser. No. 197,877, entitled Method and Apparatus for Forming A Positive Electrostatic Printing, filed on Nov. 11, 1971.

Another approach to modulated aperture noncontact electrostatic printing is set forth in U.S. Pat. ap₆₅ plication Ser. No. 864,022, now U.S. Pat. No. 3,713,734 entitled Electrostatic Line Printer, filed on Oct. 6, 1969 and assigned to the assignee of the present case, now U. S. Pat. No. 3,689,935. This arrangement

3,779,166 is particularly applicable in electronically addressed electrostatic printing and computer printout. The Electrostatic Line Printer incorporates in one embodiment, a multilayered particle modulator comprising a layer of insulating material, a continuous layer of conducting 5 material on one side of the insulating layer, and a segmented layer of conducting material on the other side of the insulating layer. Both conductive layers can be segmented however. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of conductive material is formed adjacent or around at least one aperture and is insulatively isolated from every other segment of the segmented conductive layer. More than one aperture may be formed through each segment. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer. Charged layers are thereby established on either side of the insulating layer permitting fringing fields to be established within the modulator apertures oriented to block or enhance a flow of charged particles directed through the screen or to control the flow over a continuous range in between. The bi-polar fringing fields established within the apertures are separately controllable according to the electric potentials applied to the insulatively isolated segments along the row of apertures. An overall applied field projects charged particles through the row of apertures of the particle modulator and the density of the particle stream is modulated according to the ³⁰pattern of potentials applied to the segments of the segmented conductive layer. A modulated stream of charged particles impinge upon a print receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide ³⁵line by line or line scan printing.

In each of the above arrangements for modulated aperture non-contact electrostatic printing the twodimensional screen or line grid modulator can be used to modulate a flow of charged toner particles or ions. When the screen or line grid modulator is used to control a flow of toner particles either dry or liquid, a toner cloud is produced from a toner source on the side of the modulator opposite the print receiving medium and an overall applied accelerating field generates a stream ⁴⁵of toner particles toward the print receiving medium which is modulated by the interposed screen or line grid. This arrangement enables direct deposition of toner particles on any surface, regular or irregular, dielectric or conductive. However, a percentage of the ³toner particles charged by the toner source on the other side of the screen or line grid do not pass through the apertures but land on solid portions of the screen or line grid, or on walls of the toner supply ducts.

The use of ions in the particle flow, instead of toner marking material itself, avoids any problem of toner buildup and further permits the use of lower potentials for gating the particle stream. However, the use of an ion stream generated from a corona source for establishing an electrostatic latent image on the print receiving medium requires the use of high dielectric paper or treated paper in order to permit retention of sufficient charge magnitude in the electrostatic latent image.

It is therefore an object of the present invention to ₆₅provide a modulated aperture electrostatic printing system incorporating the advantages of both modulated toner particle flow and modulated ion stream flow non4 contact printing while suffering the disadvantages of neither. In order to accomplish this result the present invention generally contemplates the method of generating an ion stream in the direction of a print receiving medium, modulating the cross sectional flow density of ions in the stream in accordance with a pattern to be reproduced, and introducing a cloud of substantially uncharged toner marking particles adjacent the print receiving medium whereby the modulated ion stream 10 selectively impinges upon and charges toner particles in the cloud which are thereby deposited on the print receiving medium in accordance with a pattern to be reproduced.

Thus, the invention generally contemplates providing 15 in one embodiment a corona source and spaced electrode for generating a substantially uniform ion stream, and a support for positioning a print receiving medium in the path of the ion stream. A multilayered apertured two-dimensional screen or line grid modulator is inter20 posed in the ion stream between the source and print receiving medium for modulating the cross sectional flow density of ions in the stream in accordance with a pattern to be reproduced. Finally, a toner source is provided and arranged for introducing a cloud of substan25 tially uncharged toner marking particles adjacent the print receiving medium whereby the modulated ion stream selectively impinges upon and charges toner particles in the cloud. The selectively charged toner particles adjacent the print receiving medium are thereafter accelerated and deposited on the medium in accordance with the pattern to be reproduced.

A feature and advantage of this system is that charged toner particles need not be ducted or transported through channels in the machinery or through the screen or line grid modulator thereby avoiding toner loss and toner contamination while at the same time permitting direct toner deposition printing on the print receiving medium so that electrostatic printing can take place on any surface.

Another feature and advantage of this arrangement is that the ion stream alone passes through the screen or line grid modulator permitting the use of lower gating potentials, high frequency gate switching, and high velocity particle transport for high speed printing.

In a preferred form of the invention, modulation of the ion flow is accomplished using a multilayered apertured element spaced between the ion source and the accelerating electrode having at least a conductive layer and an insulative layer capable of supporting charged potentials of differing magnitude on different layers of the element for establishing the electrostatic lines of force within the apertures of the element for controlling passage of ions in accordance with a pattern ₅₅ to be reproduced. Thus, the preferred embodiments generally include all of the two-dimensional screen and one-dimensional line grid modulators described in the patent applications referred to above and assigned to the assignee of the present case.

In order to further eliminate toner contamination in the printing machinery the invention contemplates establishing an air pressure differential from one side of the modulating element to the other with the low pressure on the side adjacent the print receiving medium whereby a small air flow is established through the apertures of the modulating element to prevent the toner particles from entering the apertures. According to another aspect of the invention, air streams can be in3,779,166 jected along the surfaces of the modulating element and positioned print receiving medium in order to provide air boundaries around the introduced toner cloud.

In the preferred forms, whether using the twodimensional screen or the one-dimensional line grid modulator, the invention contemplates line by line or line scan printing to provide optimum control over the uniformity of toner distribution. A feature and advantage of this approach is that toner cloud uniformity is necessary only along one major dimension namely the printing line. This result, when printing with the twodimensional screen, is accomplished as follows.

A row corona source and spaced accelerating electrode generate a substantially uniform stream of ions having a line or linear cross sectional configuration. A multilayered apertured screen formed with a twodimensional array of apertures for modulating the ion flow is interposed in the space between the row corona source and the accelerating electrode. The multilayered apertured screen is formed with at least a conductive layer and an insulative layer as heretofore described capable of supporting charge potentials of different magnitude on different layers of the screen for establishing electrostatic lines of force within the apertures of the screen for controlling passage of ions. Means are provided for translating the screen across the linear cross-section stream of ions for variably modulating the line stream in accordance with the pattern of electrostatic lines of force within the apertures across the screen. At the same time a print receiving medium is supported and positioned between the modulating screen and the accelerating electrode in the path of the linear ion stream and the print receiving medium is transported across the line cross-section of the ion stream at a rate synchronized with the modulating screen. It is, of course, the relative motion of the ion stream to the modulator and print receiver which is useful, and thus, the latter two may be fixed and the ion stream translated. Finally a channel is provided for introducing a cloud of substantially uncharged toner marking particles into the path of the linear crosssection ion stream in the space between the translating modulating screen and the translating print receiving medium whereby the modulated linear cross-section ion stream selectively impinges upon and charges toner particles in the cloud which are accelerated and deposited on the print receiving medium in accordance with the pattern to be reproduced. Preferably, a velocity component is imparted to the toner cloud substantially equal to and in the direction of motion of the modulating screen and print receiving medium.

The system also incorporates elements for establishing an electrostatic latent image on the insulative layer of the two-dimensional modulating screen and for applying a potential to the conductive layer of the screen. In one example optical addressing is utilized to establish the electrostatic latent image and the insulative layer comprises a layer of photoconductive insulating material. The photoconductive layer is first uniformly charged and the pattern to be reproduced is optically imaged onto the photoconductive layer for selectively dissipating the charge to form an electrostatic latent image. A potential is applied to the conductive layer to provide the bi-polar electrostatic latent image of fringing fields within the apertures. The screen can be formed in the configuration of an elongate web trans ported along a plurality of printing stations for multiple copy electrostatic printing.

According to one embodiment of this aspect of the invention the screen is formed in the configuration of a hollow cylinder and a row corona source is positioned inside the cylindrical screen with the accelerating electrode spaced from the row corona source outside the cylindrical screen. The cylindrical screen is rotated relative to the row corona source and the accelerating electrode to variably modulate the line cross-section ion flow in accordance with the pattern of electrostatic lines of force within the apertures across the screen. A printing receiving niedium is spaced from the cylindrical screen between the cylindrical screen modulator and accelerating electrodes and is transported across the path of the line cross-section ion flow at a rate synchronized with the rotation of the cylinder. As in the previous embodiment a channel is provided for introducing a cloud of substantially uncharged toner marking particles in the path of the linear ion stream and the space between the cylindrical screen surface and the print receiving medium. The cylindrical screen may also be optically addressed for establishing the double layer charge electrostatic latent image.

The invention is also applied in a novel method for erasure in electrostatic printing with dry toner particles comprising depositing a pattern of dry toner particles on a print receiving medium and selectively blowing the deposited dry toner particles off the print receiving medium prior to fixing. Thus, an electrostatic printing head with erasing capability is provided with a first channel arranged between the modulating element and print receiving medium for introducing a cloud of substantially uncharged dry toner marking particles in the space between the modulating element and a positioned print receiving medium. A modulated ion stream passing through the modulating elements selectively impinges upon and charges toner particles in the cloud which are accelerated and deposited upon the print receiving medium in accordance with the pattern to be reproduced. The printing head, however, is also provided with a second channel arranged adjacent the surface of the positioned print receiving medium for delivering pulses of air against the print receiving medium at the location of dry toner particles desposited on the medium in order to disperse a pattern of toner particles deposited on the paper prior to fixing.

The invention thus generally contemplates the method of electrostatic printing comprising generating an ion stream in the direction of a print receiving medium, modulating the cross sectional density flow of ions in the stream in accordance with a pattern to be reproduced, and introducing a cloud of substantially uncharged toner marking particles adjacent the print receiving medium whereby the modulated ion stream selectively impinges upon and charges toner particles in the cloud which are deposited on the print receiving medium in accordance with a pattern to be reproduced. In the event dry toner marking particles are used, the invention also contemplates the additional step of selectively blowing the so deposited toner particles off the print receiving medium for erasure.

The invention also contemplates a novel character sequential printing method using the electrostatic printing head according to which the printing head is returned to its original position after the character has been printed by a small deflection of the paper carrier

3,779,166 and return, a small deflection of the printing head and return, or electrostatic deflection of the stream of charged particles.

Other objects, features and advantages of the present invention will become apparent in the following specification and accompanying drawings.

FIG. 1 is a fragmentary plan view and FIG. 1A a fragmentary side cross-sectional view of a multilayered apertured screen for modulating the flow of charged particles.

FIG. 2 is a detailed cross-sectional view of one aperture of the screen showing a portion of the double layer charge electrostatic latent image supported by the screen.

FIG. 3 is a fragmentary side cross-sectional view of another multilayered screen.

FIG. 4 is a sequence of diagrammatic views showing the steps in modulated aperture electrostatic printing.

FIG. 5 is a fragmentary plan view and FIG. 5A a fragmentary side cross-sectional view of a line or bar modulating element for modulated aperture electrostatic line printing.

FIG. 6 is a diagrammatic view of a system for modulated aperture electrostatic line printing.

FIG. 7 is a fragmentary side cross-sectional view of a system for modulated aperture electrostatic printing in which a flow of charged toner particles is modulated.

FIG. 8 is a fragmentary side cross-sectional view of a system for modulated aperture electrostatic printing according to the present invention in which a flow of ions is modulated to impinge on an introduced cloud of toner marking particles.

FIG. 8A is a fragmentary perspective view of the system of FIG. 8.

FIGS. 9, 10 and 11 are fragmentary side crosssectional views of variations of the modulated aperture electrostatic printing system illustrated in FIG. 8.

FIG. 12 is a diagrammatic side cross-sectional view of a system for modulated aperture electrostatic printing according to the present invention utilizing a twodimensional modulated aperture screen.

FIG. 13 is a diagrammatic view of another system for modulated aperture electrostatic printing in which the screen is formed in the configuration of a drum or cylinder.

FIG. 14 is a diagrammatic side view of a system with multiple printing stations of the type shown in FIG. 13.

FIG. 15 is a fragmentary diagrammatic view of a system for optically addressing the cylindrical screen to establish the double layer charge electrostatic latent image.

FIG. 16 is a diagrammatic side cross-sectional view of an electrostatic printing head with erasing capability.

In order to more fully understand the invention, a general description of modulated aperture electrostatic printing as heretofore developed is first presented with reference to FIGS. 1-7. In FIGS. 1 and 1A is illustrated a multilayered apertured screen suitable for use in the present invention. The screen 10 generally comprises a conductive layer 11 and an insulative layer 12 with rows of holes 13 formed therethrough. As shown in more detail in FIG. 2, the construction of the screen is such that a layer of charges can be supported on either side of the insulative layer 12. Thus, a potential can be applied to the conductive layer 11 in order to supply charges 14 of a particular sign such as, for example, positive, along the side of the insulative layer 12 adja cent the conductive layer 11. A second layer of charges 15 can be established along the outer side of the insulative layer of 12 or for example opposite or negative charge around the apertures 13 of the screen. The double layer charge establishes lines of force within the apertures 13 of the screen which can be oriented to either block or enhance a flow of charged particles directed through the apertures or to control the density of particles passing through the apertures over a continuous range in between. The pattern of electrostatic lines of force established within the apertures 13 and the screen is also referred to herein as a bi-polar electrostatic latent image.

The pattern of charges 15 on the outside of the insulative layer can be established by a variety of contact charging techniques or by optical addressing as set forth in the patent applications referred to above. By way of example the insulative layer 12 can be formed of a photoconduCtive insulating material. As shown in FIG. 4, the double layer screen 10 is first sprayed from a corona source 20, the photoconductive insulative layer facing the corona source, and the conductive layer maintained at some fixed potential such as ground so that a bi-polar electrostatic latent image is formed across the insulative layer of the screen. The screen 10 is then exposed to light from a source 21 in a pattern in accordance with an image to be reproduced thereby selectively to dissipate the charge on the photoconductive insulative layer. This step of course occurs in a dark environment. Finally, the screen 10 is positioned adjacent a print receiving medium carried on an accelerating electrode 22 with a toner source 23 positioned on the opposite side of the screen. A potential derived from source 24 establishes an accelerating field between the toner supply 23 and the accelerating electrode 22 so that a stream of charged toner particles is directed through the apertures of the screen 10. At the same time, a selected potential from source 25 is applied to the conductive layer of the screen to maintain the electrostatic field between the screen and source and between the screen and accelerating electrode. With blocking fields initially established across the screen, toner is permitted to pass through those apertures from which the charge on the insulative layer is dissipated by light exposure in the previous step, while the unexposed apertures continue to block the passage of charged toner particles forming a deposit of toner particles on the print receiving medium at electrode 22 in accordance with the pattern to be reproduced.

A variety of other modulating screen configurations are described in the patent applications referred to above. By way of example in FIG. 3 there is shown a three-layered apertured screen comprising a conductive layer 26 and a photoconductive insulating layer 27 separated by a high dielectric insulating material 28. A bi-polar or double layer charged electrostatic latent image is established across the insulative layers of this screen according to a variety of charging techniques set forth in the patent applications referred to above.

Another type of modulating element for modulated aperture electrostatic line printing is set forth in FIGS. 5 and 5A. The modulating element 30 consists of an elongate length or bar of insulating material 31 having formed along one side a continuous layer of conducting material 32 and having formed along the opposite side a segmented conductive layer 33 consisting of individual insulatively isolated segments 33A, 33B, 33C, etc.

3,779,166

The so formed elongate bar is formed with a row of apertures therethrough, each aperture surrounded by a conductive segment 33A, 33B, 33C etc. An electrical lead is provided to the continuous conductive layer 32 for applying a uniform potential across one face of the 5 insulative layer 31 and around one side of each aperture. A plurality of electrical leads are provided one for each of the conductive segments 33A, 33B, 33C, etc., so that a different potential can be applied to each of the segments for building up differing selective charge 10 layers around the other side of each of the apertures 35 in accordance with a pattern to be reproduced. The double layer charge along each side of the insulative layer 31 permits development of electrostatic lines of force or fringing fields within the apertures for control- 15 ling the flow of a linear cross-sectional stream of ions directed through the modulating element. A system for electrostatic line printing using the line or bar modulator of FIGS. 5 and 5A is shown in the diagrammatic view of FIG. 6. A toner supply 40 and an accelerating 20 electrode 41 are spaced apart and interconnected with a potential source for charging and accelerating toner particles from the toner source 40 in the direction of the electrode 41. The outlet from the toner source is shaped to provide an elongate or linear cross-sectional 25 stream of toner particles. The modulating element 30 is positioned in the path of the toner flow with a biasing potential applied to the conductive layer 32 via electrical lead 41 and with selected electrical potentials applied to the segmented elements to the segmented layer ³⁰33 via the bank of electrical leads 42 which may be addressed for example by appropriate logic circuitry to provide a changing pattern of charges across the segmented layer in accordance with a pattern to be reproduced. The flow of toner particles directed through the ³⁵apertures of the modulating element is appropriately shaped or modulated in accordance with the pattern to be reproduced and accelerated toward the back electrode 41. A print receiving medium 43 is transported across the modulated toner stream in a space between ⁴⁰the modulating element 30 and electrode 41 and the speed of transport of the paper or other print receiving medium 43 is synchronized with the frequency or rate of change of potentials applied to the segmented portions of the segmented layer 33. As an alternative to the continuous line printing described above, other modes of modulated aperture electrostatic line printing are set forth in the patent application Ser. No. 864,022 now U.S. Pat. No. 3,689,935 referred to above.

Representative screens and systems for modulated aperture electrostatic printing have been described in the foregoing discussion. Other examples of modulated aperture printing screens and systems are set forth in detail in the patent applications referred to above. One example of a printing operation of these systems is shown in FIG. 7 with reference to a line modulator of the type shown in FIGS. 5 and 5A. As illustrated in FIG. 7, a corona source 45, particle modulator 54, print receiving medium 47, and accelerating electrode 48 are provided in sequence. The accelerating electrode 48 can itself form the support for the print receiving medium 47. The corona source 45 generates a stream of charged particles 50 which are accelerated toward the back electrode 48 by a suitable potential difference applied between the corona source 45 and electrode 48. Toner marking particles 51 are delivered by a source 52 into the vicinity of the corona emissions and acquire charges. The charged toner particles 53 are thereafter accelerated toward the electrode 48 through the apertures of the line modulator 46. As heretofore described, the modulating element 46 consists of an insulative layer 54 with conductive layers 55 and 56 formed on either side thereof, at least one of which is segmented into insulatively isolated segments surrounding the apertures formed in a row along the modulating element. One or more rows of apertures can be used as set forth in U.S. Pat. application Ser. No. 864,022 referred to above. Potentials applied to the conductive layers over electrical lines 57 and 58 one of which comprises a bank of electrical leads to the segments of the segmented layer selectively develop lines of force within the apertures in accordance with a pattern to be reproduced selectively modulating the flow of toner particles so that the particles come to rest on the print receiving medium 47 in accordance with the pattern to be reproduced.

Alternatively, as described in the above referenced patent application, the ion stream itself can be modulated by the modulating element so that the modulated stream of ions impinges upon the print receiving medium 47 in accordance with the pattern to be reproduced. In this arrangement high dielectric paper, for example, is used for the print receiving medium 47 in order to retain the electrostatic latent image deposited by the ions. The paper is thereafter dusted and fixed in accordance with known techniques of electrostatic printing.

According to the present invention, a novel system and method for modulated aperture electrostatic printing using screens of the type set forth above is provided. The basic system and method are depicted in FIGS. 8 and 8A in contrast to the arrangement of FIG. 7 with corresponding elements numbered accordingly. Thus, according to the embodiment of the present invention illustrated in FIG. 8 there is provided a corona source 45 modulating element 46 print receiving medium 47 and accelerating electrode 48 in sequence as described with reference to FIG. 7. Corona source 45 generates a stream of ions 50 which are accelerating toward the back electrode 38 through the apertures of modulating element 46. In the example of FIG. 8A, a ³ linear corona source in the form of a wire is shown, but a row of point source or other corona source arrangement may be used. In contrast to the previously developed modulated aperture printing systems and methods ₅θ described above, and in the patent applications referred to, the toner particles 51 are introduced by a source 60 into the space between the modulated element 46 arid positioned print receiving medium 47. The toner particles 51 are neutral and form a cloud in ₅₅ the interspace between the modulating element 46 and print receiving medium 47. The ion stream passing through the apertures of the modulating element 46 and modulated according to the array of fields established within the apertures, impinges on the cloud of ₆₀ uncharged toner particles selectively charging the particles in accordance with the pattern to be reproduced so that the charged particles are accelerated by the electrode 48 to deposit on the print receiving medium 47 in accordance with the pattern to be reproduced. ₆₅ This novel arrangement and method incorporates the advantages of both the toner modulating and ion modulating systems described in the patent applications referred to above. Thus, because only the ion stream is

3,779,166 modulated by the modulating element 46 lower potentials can be used on electrical leads 57 and 58 with higher switching frequencies. Because of the higher transit speed of the ions printing can be conducted at higher speeds. Toner loss at the screen or modulated element is avoided as are any cleaning problems. After the ions pass through the apertures of the modulating element they intercept and charge the toner particles which are then propelled by the accelerating electrostatic field toward the electrode to deposit on the print receiving medium 47. Thus, printing with the toner particles is accomplished directly, rather than first creating an electrostatic latent image on the print receiving medium. Printing can therefore take place on any surface without the necessity of treated or high dielectric paper. Furthermore, because the injected toner particles 51 either in solid or aerosol form are neutral, there is little tendency for toner to drift and deposit on the printing medium or other apparatus prior to charging by the ion stream. The toner particles become charged toner particles 53 generally only upon impingement of or close proximity to the ion stream 50.

A feature and advantage of this arrangement is that either liquid aerosol or solid particles can be used with equal facility. When the toner stream itself is modulated with the modulating element or screen, the use of solid particles causes a percentage of particles to deposit on the screen or modulating element thereby making cleaning steps desirable in the printing process. Therefore, when the toner stream itself is modulated liquid aerosols have been used to more easily facilitate removal of aerosol particles deposited on the printing screen or modulating element. Liquid droplets coalescing on the face of the bar or screen modulator would be drawn off at the low edge by means of a wick. In the configuration of the present invention as shown in FIGS. 8 and 8A, however, toner is not deposited on the bar or screen modulator and is kept out of the apertures. Therefore dry toner can be used with equal facility and fused by heat after printing is completed with the fuser located beyond the printing station.

In order to further prevent any drift of toner particles toward the print receiving medium prior to charging and to further prevent any drift of toner particles toward the modulating element or screen, protective “air boundaries” can be injected over the surface of the modulating element and paper as shown in FIG. 9. As shown in that example, the toner source 62 positioned for delivering toner in the space between the modulating element 63 and print receiving medium 64 is shaped to define air passageways or channels 65 and 66 along the surface of the modulating element 63 and print receiving medium 64 respectively. Air under slightly higher pressure is delivered via channel 65 and 66 to effectively contain the neutral toner particles within air boundaries until they become charged by impingement from the ion stream and are accelerated toward electrode 67.

Another approach for preventing drift of uncharged toner particles into the apertures of the modulating element is illustrated in FIG. 10. According to this arrangement a slight pressure differential is established on either side of the modulating element with the slightly higher pressure on the corona side. As a result, a very small air flow 70 is established through the apertures of the modulating element 71 thereby preventing the toner cloud from entering the apertures.

By means of the expedients illustrated in FIGS. 9 and 10, a directional velocity parallel to the plane of the printing surface can be imparted to the tomer cloud in a lateral direction between the print receiving medium and modulating element. The velocity component imparted to the toner cloud can be correlated with the speed of transport of the print receiving medium in order to prevent any distortion in the reproduced pattern.

Another arrangement for delivering and picking up toner and imparting a controlled lateral velocity component to the toner cloud is shown in FIG. 11. In that arrangement the toner source 75 tapers to a narrow delivery outlet while a toner pick up channel 76 is positioned across from the toner source on the other side of the apertures of the screen or modulating element. The pick up channel 76 is provided with an inlet having a larger diameter than the outlet from the toner source 75. By means of differential pressures established within the channels 75 and 76, the toner can be delivered with a controlled lateral component of motion synchronized with the velocity of transport of the print receiving medium 77.

In one example of the electrostatic line printing embodiment of the present invention described above, charged toner particles were accelerated to a velocity of 15 centimeters per second through a line modulating element having a thickness of 0.25 millimeters. The resulting transit time of 1.6 milliseconds theoretically permits a highest addressing frequency or switching frequency of the potentials applied to the conductive layers of approximately 300 cycles per second. A practical addressing frequency of approximately 100 cycles per second was achieved. The switching rate for controlling ions is greater by a factor of more than one hundred.

Embodiments of the present invention for modulated aperture electrostatic printing using a screen formed with a two-dimensional array of apertures spaced between an ion source and accelerating electrode are illustrated in FIGS. 12 through 15. As shown diagrammatically in FIG. 12, the multilayered apertured screen 80 is interposed between a corona source 81, and accelerating electrode 82 haying positioned across its surface a print receiving medium 83. The screen 80 is formed with a two-dimensional array of apertures and at least a conductive layer and an insulative layer capable of supporting charge potentials of differing magnitude on different layers of the screen for establishing electrostatic lines of force within the apertures of the screen for controlling passage of ions in accordance with a pattern to be reproduced as heretofore described. Uncharged toner particles 84 in the form of an aerosol or solid toner cloud is introduced into the space between the screen 80 and the print receiving medium 83. A stream of ions originating from corona source 81 in the direction of accelerating electrode 82 intercepts the cloud of neutral toner particles and the particles become charged and are transported in the direction of the accelerating field. Because the ion flow is modulated as it passes through the apparatus of screen 80 as heretofore described, the toner cloud is selectively charged and the charged particles are attracted to the printing surface 83 by the accelerating electrostatic field in accordance with the pattern to be reproduced. Because the printing takes place over a twodimensional area, however, problems are encountered

3,779,166 in the embodiment illustrated in FIG. 12 in uniformly distributing a cloud of toner particles 84 throughout the space over the surface area to be printed. Nonuniform printing density may result.

In order to overcome this difficulty the invention is 5 embodied in a form so that printing occurs at any instance only over a linear cross-section of the printing surface area even though a two-dimensional screen is utilized. This is accomplished according to one example as shown in FIG. 13. In this arrangement, the print- 10 ing or toning occurs along a narrow slit defined by baffles 90 while the screen 92 and print receiving medium 93 are transported synchronously across the slit. The paper or other printing surface 93 is transported across the slit by means of a drum support 94, which also 15 serves as the accelerating electrode, and appropriate rollers 95. A stream of ions generated by corona source 96 is accelerated in the direction of the drum passing through the apertures of screen 92. Uncharged toner is delivered to the space over the slit defined by baffles 90 20 in the path of the modulated ion stream through a channel defined by the baffle walls 90 and walls 91 so that the line cross-section ion stream passing through the apertures of screen 92 impinge upon the neutral cloud selectively charging particles in the cloud which 25 are accelerated toward the drum 94 to deposit on the print receiving medium 93 in accordance with the pattern to be reproduced. The linear cross-section stream of ions originating from row corona source 96 is variably modulated as the screen 92 is transported across ³⁰the ion stream path. The toner cloud is therefore variably charged and comes to rest on the printing surface 93 in a continuously changing pattern as the paper or other printing surface is transported across the path of charged particles. Continuous line printing can there- ³⁵fore be achieved.

In a preferred embodiment the cloud of toner particles 98 is delivered through the channels defined by walls 90 and 91 with a lateral component of motion in the direction of transport of the screen and printing ⁴⁰surface. By this expedient, distortion at the printing surface due to differential velocity between the printing particles and printing surface is avoided. Thus, the velocity component imparted to the cloud in the direction of transport is synchronized with the velocity of trans- ⁴⁵port of the screen and paper or other printing surface.

The paper or other print receiving medium can be either sheet-fed or web-fed. For simultaneous copies on web-fed paper, a system such as that shown in FIG. 14 is provided. In this arrangement multiple printing stations 100, 101 and 102 of the type shown in FIG. 13 produce essentially simultaneous identical copies of the same charge image formed across the two-dimensional multilayered apertured screen 103, except that the ₅₅continuous line printing occurs simultaneously across ^Jdifferent linear cross-sectional portions of the screen 103. The amount of ion current used from the sources 104, 105 and 106 is adjusted to a level so that the screen charge pattern is not discharged and therefore ₆θ so that three or more images can be printed from the same double layer charge bi-polar electrostatic latent image formed on the two-dimensional screen 103. The embodiment of FIG. 14 is particularly suited to computer output applications where web-fed paper is often used.

Another embodiment for the two-dimensional modulated aperture screen system is shown in FIG. 15 in which the screen 110 is formed in the configuration of a drum 111. In this configuration the back electrode 112 which also serves as the carrying element for the paper or other print receiving medium 13 is either flat as illustrated in FIG. 15 or curved as shown in FIG. 13. As in each of the previous embodiments, the substantially neutral toner cloud may consist either of solid or liquid toner particles introduced into the channel defined by baffles 114 and walls 115. The toner is again transported across the slit defined by the two sections of the channel to be exposed to the linear cross-section ion stream originating from the corona source 116 positioned inside the drum 111. The ion stream is modulated as it passes through the apertures of the screen 110, which forms the surface of drum 111. The toner cloud is selectively charged and accelerated toward the electrode 112 so that toner particles deposit on the print receiving medium 113 in the pattern to be reproduced as the paper or other medium 113 is transported across the path of the charged particles. Continuous line printing is thereby achieved as the ion stream is variably modulated by the rotating drum. Again, a velocity component is preferably imparted to the toner cloud in the direction of transport, synchronized with the velocity of the transport of the drum surface and printing surface so that printing distortions resulting from differential velocities are avoided.

Problems of printing “smear” are also eliminated by “narrow slit” printing. In this embodiment the width of the toner aperture slit formed by the two sides of the toner delivery channel formed by baffles 114 and walls 115 has an effective width electrostatically as wide as a row of screen apertures formed in the twodimensional screen. Because the narrow slit is reduced in width to that of one aperture, the relative velocity of paper and screen and toner during printing is not critical in obtaining high resolutions. This permits mechanical simplicity to the design of the machine. However, the best performance is obtained when the screen velocity, toner cloud velocity, and paper or other printing medium velocity are all nearly equal. The quantity of toner in the form of liquid aerosol or solid particles in the space between the screen and print receiving medium will then determine the density of the print for a given toner material, and the most uniform toning is then achieved.

In the embodiment of FIG. 15, the double layer charge bi-polar electrostatic latent image is established across the screen by optical addressing in the manner heretofore described. In this example, the screen is initially charged uniformly over the surface of a photoconductive insulative layer by means of corona source 120. The charge on the photoconductive insulating layer is thereafter selectively dissipated through the conductive layer which may be, for example, grounded. This is accomplished by imaging, by means of a lens system 121, the light pattern to be reproduced, onto the surface of the photoconductive insulating layer while the conductive layer is for example grounded. A uniform potential is thereafter applied to the conductive layer so that selective fringing fields are established within the apertures of the screen for modulating the ion stream.

A feature and advantage of the modulated aperture electrostatic non-contact printing system contemplated by the present invention is that non-contact printing with dry toner particles is greatly facilitated as hereto

3,779,166 fore described. Thus, the invention enables an electrostatic printing head with erasure capability for application in, for example, typewriters and consoles. According to this aspect, the invention generally contemplates the electrostatic printing method of forming a pattern of dry toner particles on a print receiving medium in accordance with a pattern to be reproduced and selectively blowing the dry toner particles off the print receiving medium prior to fixing.

A representative printing head for accomplishing this function is shown in FIG. 16. The printing head incorporates a modulating element 130 for modulated aperture electrostatic line printing of the type illustrated in FIG. 5 and 6. Thus, the modulating element 130 includes a central insulative layer with conductive layers coated on either side thereof at least one of which consists of segments insulatively isolated and surrounding apertures of a row of apertures formed across the modulating element for line printing. The conductive segments are electrically addressed for modulating a stream of ions in accordance with the letters or other symbols to be reproduced by the printing head. The ion stream originates from corona source 131 in the direction of a back accelerating electrode 132. A print receiving medium 133 is positioned in the path of ions originating from corona source 131 and passing through the apertures of screen 130. A first channel 134 is provided defined by walls 135 and 136 for delivering or introducing a cloud of substantially uncharged dry toner marking particles in the space between the modulating element 130 and a positioned print receiving medium 133 and adjacent the modulating element 130. The modulated ion stream passing through the apertures of the modulating element selectively impinge upon and charge toner particles in the cloud which are accelerated and deposited on the print receiving medium 133 in accordance with the pattern to be reproduced. A second channel 137 defined by walls 135 and 138 is arranged between the modulating element and print receiving medium adjacent the surface of the print receiving medium 133 for delivering pulses of air against the print receiving medium at the location of dry toner particles deposited on the medium in the pattern determined by the modulating element 130. Air pulses are initiated in channel 137 in order to erase a pattern of toner particles erroneously deposited on the paper and prior to fixing.

From another point of view, the novel electrostatic typewriter printing head consists of a first housing 138 having an opening for positioning adjacent a print receiving medium 133. A second housing 135 is supported within the first housing 138 so that the first and second housings 138 and 135 define a channel for delivering air pulses to the surface of the print receiving medium through the opening. The second housing is also formed with an opening in alignment with the opening in the first housing. A third housing 136 is positioned within the second housing 135, the second and third housing 135 and 136 defining a channel 134 for introducing a cloud of substantially uncharged dry toner marking particles in the space adjacent the opening in the second housing. An apertured modulated grid element 130 for controlling the flow of ions through the apertures of the modulating grid element is constructed with the housing 136 so that the apertures are in substantial alignment with the openings with the first and second housings. Finally, a corona discharge source 130 is positioned within the third housing 136 for delivering a stream of ions in alignment with the apertures of the modulating element 130 and the openings within the first and second housings 138 and 135.

In a typewriter or console environment, the printing head is adapted for printing individual symbols each in a continuous line printing mode and thereafter permitting erasure of any symbol erroneously deposited in the form of dry toner particles on the print receiving medium, by blowing off and dispersing the deposited toner particles via the toner conduit 134. The desired symbol can then be retoned or redeposited on the printing surface in the same location.

This invention contemplates a variety of methods for this type of character sequential printing suitable for use in a continuous writing printer such as typewriter or console. This type of operation is particularly important when the printer is hand or keyboard addressed and where it is important for the operator to observe the printing as it is entered. To perform this function, where one character at a time is being printed, the writing head, as described with reference to FIG. 16, scans the distance of one character height, and during this scan forms the addressed character in the appropriately indexed position. After the character has been printed the position of the writing head is returned to the original position, shifted one lateral position for printing the next character. Or, the writing head is returned to the original position for erasure as heretofore described. The scanning action of this character sequential mode can be achieved in either of several ways. According to one method a small deflection of the paper carriage and return is used. According to a second method a small deflection of the printing head and return is used. And according to a third method, electrostatic deflection of the writing toner particles charged by the ion stream accomplishes this scanning action. Indexing and character forming and control operations are performed electronically with the use of shift registers.