U.S. patent number 3,713,734 [Application Number 05/197,877] was granted by the patent office on 1973-01-30 for apparatus for forming a positive electrostatic image.
This patent grant is currently assigned to Electroprint, Inc.. Invention is credited to Hewitt D. Crane, George J. Eilers, Gerald L. Pressman.
United States Patent |
3,713,734 |
Crane , et al. |
January 30, 1973 |
APPARATUS FOR FORMING A POSITIVE ELECTROSTATIC IMAGE
Abstract
A machine for making a copy on a medium such as paper of a
visual image. Particles of toner material are accelerated toward
the paper through an apertured screen. Formed on the screen is a
pattern of electrostatic charge regions that corresponds to the
image so that the arrangement of the toner particles impinging on
the paper corresponds to the image. A composite apertured screen
composed of two conductive layers separated by an insulative layer
and having an insulative layer on one outer surface. Means for
establishing oppositely polarized fields across the inner
insulative layer and across the outer insulative layer. One of the
fields is selectively discharged to control the passage of toner
particles through the apertures in the screen.
Inventors: |
Crane; Hewitt D. (Portola,
CA), Pressman; Gerald L. (Cupertino, CA), Eilers; George
J. (Redwood City, CA) |
Assignee: |
Electroprint, Inc. (Palo Alto,
CA)
|
Family
ID: |
22731103 |
Appl.
No.: |
05/197,877 |
Filed: |
November 11, 1971 |
Current U.S.
Class: |
399/135; 430/53;
430/68 |
Current CPC
Class: |
G03G
15/346 (20130101) |
Current International
Class: |
G03G
15/00 (20060101); G03G 15/34 (20060101); G03g
015/00 () |
Field of
Search: |
;355/3,4,16,17
;96/1,1.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
553,355 |
|
Feb 1958 |
|
CA |
|
1,156,308 |
|
Oct 1963 |
|
DT |
|
832 |
|
Apr 1966 |
|
JA |
|
Primary Examiner: Greiner; Robert P.
Claims
What is claimed is:
1. Apparatus for forming an electrostatic image that corresponds to
an optical image comprising a composite screen defining a plurality
of spaced apart apertures and having an outer conductive layer
forming the obverse face of said screen, an insulative layer
superposed on said outer conductive layer, an inner conductive
layer superposed on said insulative layer, and a photoconductive
layer superposed on said inner conductive layer and forming the
reverse face of said screen, means connected between said outer
conductive layer and said inner conductive layer for establishing a
voltage gradient across said insulative layer, said voltage
gradient forming a first electrostatic field that extends into said
apertures to form blocking fields within said apertures polarized
in a first direction, means for establishing a charge on said
reverse face that forms in cooperation with said inner and outer
conductive layers a second field in said apertures that has a
polarity and magnitude sufficient to counteract said first field
when said photoconductive layer is in a dark condition, and means
for impressing the optical image on said reverse face so that
relatively light regions of said image cause corresponding regions
of said photoconductive layer to become conductive and discharge
corresponding portions of said second field.
2. Apparatus for forming a positive image of an object on a surface
comprising a plurality of particles, means for forming an electric
field between said particles and a particle receiving surface for
accelerating the particles on a path toward said surface, a
particle pervious screen disposed in said path in substantial
parallelism with said surface, said screen having a reverse face
facing said surface, an obverse face opposite said reverse face,
and a plurality of apertures extending therethrough between said
obverse face and said reverse face, said apertures being of
sufficient size to admit said particles therethrough, said screen
including an electrically conductive outer layer on said obverse
face, an electrically insulative layer underlying said outer
conductive layer, an electrically conductive inner layer underlying
said insulative layer and a photoconductive layer underlying said
inner conductive layer and defining said reverse face, means for
establishing a first electric field between said inner and outer
conductive layer so polarized as to repel particles approaching the
apertures from the direction of said obverse face, means for
establishing a second electric field between said inner conductive
layer and said photoconductive layer that has a polarity and
magnitude sufficient to counteract said first field, and means for
projecting an image of the object on said reverse face so that
portions of said photoconductive layer that are illuminated will
establish a conductive path to discharge said second field in the
illuminated portions, whereby particles are repelled by said first
field at apertures in such illuminated portions of the screen.
3. A system for controlling the flow of a stream of charged
particles comprising:
means for establishing a coplanar array of a first plurality of
bipolar electrostatic fields uniformly oriented and having a
strength sufficient to prevent a flow of a stream of charged
particles directed through the array;
means for establishing a coplanar array of a second plurality of
bipolar electrostatic fields, said second plurality of fields being
uniformly oriented in a direction opposite said first plurality of
fields, said second plurality of electrostatic fields being
superimposed over said first plurality of fields and having a
strength sufficient to counteract said first plurality of
fields;
and means for selectively dissipating said second plurality of
bipolar fields thereby to modulate the flow of a stream of charged
particles directed through said arrays in accordance with an image
to be reproduced.
4. Apparatus for forming a positive image of an object on a surface
with a plurality of particles comprising means for forming an
electric field between the particles and a particle receiving
surface for accelerating the particles on a path toward said
surface, a particle pervious screen disposed in said path in
substantial parallelism with said surface, said screen having a
reverse face facing said surface, an obverse face opposite said
reverse face, and a plurality of apertures extending therethrough
between said obverse face and said reverse face, said apertures
being of sufficient size to admit said particles therethrough, said
screen including an electrically conductive outer layer on said
obverse face, an electrically insulative layer underlying said
outer conductive layer, an electrically conductive inner layer
underlying said insulative layer and a photoconductive layer
underlying said inner conductive layer and defining said reverse
face, means for establishing a first electric field between said
inner and outer conductive layer so polarized as to repel particles
approaching the apertures from the direction of said obverse face,
means for establishing a second electric field between said inner
conductive layer and said photoconductive layer that has a polarity
and magnitude sufficient to counteract said first field, and means
for projecting an image of the object on said reverse face so that
portions of said photoconductive layer that are illuminated will
establish a conductive path to discharge said second field in the
illuminated portions, whereby particles are repelled by said first
field at apertures in such illuminated portions of the screen.
Description
This invention relates to a method and apparatus for forming a
planar pattern of electrostatic charge regions that are arranged to
correspond to an image; the invention has particular application in
a system for reproducing on a suitable medium, such as paper, a
copy of a visible image.
Systems for reproducing images as mentioned above are exemplified
in U.S. Pat. applications Ser. No. 673,499, filed Oct. 6, 1967,
Ser. No. 776,146, filed Nov. 15, 1968, and Ser. No. 85,070, filed
Oct. 29, 1970, which are incorporated hereinto by this reference.
Such systems can include a support for a medium such as paper on
which it is desired to form a visible image. The system also can
include a source of particles that have a color contrasting with
that of the medium. (Such particles are typically referred to as
"toner", and such usage will be employed hereinafter). In one type
of system, there is interposed between the toner supply and the
medium an apertured screen. An electrostatic field between the
source of the toner particles and the medium is established to
propel the particles toward the medium through the apertures in the
screen. A plurality of electrostatic charge regions is formed by
the screen in a pattern that corresponds to the image so that
electrostatic fields within the apertures selectively control
passage of toner particles. Toner particles egressing from the
apertured screen therefore conform to the pattern of the image.
After the toner particles impinge on the medium they can be fixed
or fused thereon, if required, in accordance with known technology
to provide a permanent image.
In another type of system, e.g., the systems described in U.S. Pat.
applications Ser. No. 709,578, filed Mar. 1, 1968; Ser. No.
864,022, filed Oct. 6, 1969; and Ser. No. 101,681, filed Dec. 28,
1970, ions or like charged particles are formed into a suitable
pattern which pattern can be directed through a cloud of toner
particles. The screen according to the present invention is useful
in both types of system or in any other system in which formation
of a pattern of charged particles is required.
In the systems described above, the present invention provides an
improved screen that offer certain advantages over various prior
screens. The screen construction according to the present invention
operates at a relatively low voltage thereby simplifying
fabrication and operation of the device.
The embodiment of the invention described in more detail
hereinafter includes a composite apertured screen formed by four
layers. On the upstream or obverse face of the screen, the side
facing the source of toner particles, the composite screen has a
conductive layer underlying which is an insulative layer. On the
downstream face of the insulative layer is a second conductive
layer underlying which is an outer insulative layer; the exposed
surface of the outer insulative layer constitutes the downstream or
reverse face of the screen.
The improved screen is incorporated into the system by so biasing
the two conductive layers that within the apertures is established
an electrostatic blocking field that has a polarity and magnitude
sufficient to block the passage of toner particles through the
apertures. The outer insulative layer is selectively
electrostatically charged to a polarity and magnitude sufficient to
form a field that counteracts and overrides portions of the
blocking field to afford passage of toner particles in a pattern
corresponding to an image.
In one satisfactory embodiment of the invention, the outer
insulative layer is formed of photoconductive material, a material
that has a high resistance when in a dark state and a low
resistance when in an illuminated state. While in the dark state,
the photoconductive layer is charged to a polarity and magnitude
sufficient to create fields within the apertures that override or
counteract the abovementioned blocking field. Consequently, when
the screen is in a dark state, all apertures are biased to permit
passage of toner particles traveling from the toner particle source
to the medium. The image to be copied on the medium is then
projected on the photoconductive layer so that portions of that
layer that correspond to light parts of the image assume a
conductive state. In such state, the charge previously formed on
the photoconductive layer is discharged thereby correspondingly
discharging the counteracting field to permit the blocking field to
become effective. Consequently, apertures residing in such
illuminated portions block the flow of toner particles whereas
apertures associated with dark portions of the screen continue to
permit passage of toner particles. With the screen in such
condition a supply of charged toner particles is directed through
the screen to the medium as a consequence of which the particles
egressing from the screen are arranged in a pattern directly or
positively corresponding to the image.
The objects, features and advantages of the present invention will
be more apparent after referring to the following specification and
accompanying drawings in which:
FIG. 1 is a diagrammatic view of a system employing the screen of
the present invention;
FIG. 2 is an enlarged fragmentary view of a screen according to the
present invention showing the position of the electrostatic fields
within an aperture;
FIG. 3 is a fragmentary view of a larger portion of a screen
constructed according to the present invention;
FIG. 4 is a fragmentary view similar to FIG. 3 showing the charged
condition of the screen prior to receiving an image thereon;
FIG. 5 is a view similar to FIGS. 3 and 4 showing the charged
condition of the screen after a visible image has been impressed on
the photoconductive layer thereon;
FIG. 6 is a diagrammatic view similar to FIG. 1 showing an
alternate embodiment of the invention; and
FIG. 7 is a diagrammatic view of another alternate embodiment of
the invention.
Referring more particularly to the drawings and specifically to
FIG. 1, reference numeral 12 schematically indicates an air and
light impervious enclosure in which a system employing the improved
screen of the invention is enclosed. Mounted within enclosure 12 is
a source of toner particles 14, a composite apertured screen 16, an
image receiving medium 18 and a conductive support 20 for the
medium. A power source 22 is provided for establishing a field
between toner source 14 and conductive plate 20 so that toner
particles are accelerated from the toner source to the conductive
plate. Screen 16 defines a large plurality of apertures 24 in which
apertures appropriate fields are established either to block or to
pass toner particles in accordance with the shape of an image I to
be formed on medium 18.
Within enclosure 12 is a charged particle source, such as a corona
source 26, which is employed to form a uniform charge on the
reverse surface 28 of screen 24. After such uniform charge is
formed, an image source 30 is projected onto the reverse surface of
screen 24 through an optical system that includes a reflector 32
and a shutter mechanism 34. Illumination of reverse face 28 of
screen 24 alters the charge pattern within the apertures of screen
24 so that toner particles egress from the screen apertures in a
pattern corresponding to image source 30, whereby the image I is
formed on medium 18.
A satisfactory embodiment of screen 16 is fragmentarily shown at
greatly enlarged size in FIG. 2. The screen includes an obverse
face 36 that faces toward the source of toner particles. Obverse
face 36 is defined by a conductive layer 38 underlying which is an
insulative layer 40. On the surface of insulative layer 40 opposite
from conductive layer 36 is a second or inner conductive layer 42;
screen 16 is structurally completed by a layer 44 of
photoconductive material the outer surface of which defines reverse
surface 28 of screen 16. In one screen designed according to the
present invention insulative layer 40 and photoconductive layer 44
have thicknesses of approximately 0.001-inch and conductive layers
38 and 42 are thin films deposited on such insulative layers. In
such exemplary structure apertures 24 have a diameter of
approximately 0.006-inch and as shown in the figure, the apertures
extend through all layers so that each layer terminates in the wall
bounding the apertures.
Blocking fields are formed within apertures 24 by establishing a
bias or potential between conductive layers 38 and 42 that has a
magnitude and polarity sufficient to form a charge across
insulative 40 that creates the blocking field. For this purpose,
and assuming that toner particle source 14 is biased to charge
toner particles with a negative charge, conductive layer 38 is
biased positively with respect to conductive layer 42. Such bias is
achieved by connecting a tap 46 in power source 22 to conductive
layer 38 and by connecting a tap 48 of the power source to
conductive layer 42. The effect of such bias is to form at the
surface of insulative layer 40 that abuts conductive layer 38 a
plurality of positive charges and at the surface of the insulator
that abuts conductive layer 42 a plurality of negative charges; the
fringe of such charges within each aperture 24 form an
electrostatic field within the aperture identified by field lines
50 which have a magnitude and polarization sufficient to block
passage of toner particles through the aperture. In the exemplary
structure referred to above, a potential between conductive layers
38 and 42 of about 200-300 volts is considered adequate to form a
field within aperture 24 that totally blocks passage of toner
particles through the aperture.
In order to permit passage of toner particles through aperture 24,
the present invention provides for creation of an electrostatic
field that counteracts or overrides the field designated by field
lines 50. In FIG. 2 the field lines of the counteracting field are
identified by reference numeral 52. Such counteracting field is
formed by impressing on surface 28 of photoconductive layer 44,
while such layer is in the dark or nonconductive state, charges of
a suitable polarity, positive charges in the case exemplified in
the drawings. Such charges can be formed by any suitable expedient,
for example, by bombardment of surface 28 with positive ions from a
corona wand, radioactive source, or the like. In any event, the
charges impressed on reverse surface 28 are of a magnitude and
polarity such that the counteracting field indicated by field lines
52 is formed by the cooperation of the charges on surface 28 and
the charges existing on the surfaces of inner insulative layer 40.
In the example depicted in the drawings, the charges formed on
reverse face 28 are more positive than the charges arising from the
bias supplies connected to conductive layers 38 and 42.
The directional arrows associated with field lines 50 and 52 in the
drawings merely depict that the fields are oppositely polarized.
Obviously, the direction of the field or the force resulting
therefrom depends upon the polarity of the charge on the toner
particles and not on any convention adopted for the purpose of
illustrating the invention. Similarly, the blocking and
counteracting fields may be viewed in the singular or in the plural
for a given screen.
The field formed between the charges on reverse face 28 and the
charges on the surfaces of inner insulator 40 cause fringe fields
within aperture 24 that have a magnitude and direction sufficient
to counteract blocking field 50 to the end that the toner particles
can pass through aperture 24, a condition that subsists so long as
conductive layer 44 is retained in a dark state.
Projection of image I onto reverse surface 28 illuminates all
portions of the conductive layer except those areas corresponding
to the location of dark lines and/ or areas of the image. Those
portions of photoconductive layer 44 that are illuminated become
locally conductive so that current flow through the layer is
permitted. Such current flow discharges counteracting field 52 in
all apertures 24 that are associated with light portions of image I
so that only blocking field 50 exists in such apertures. In
apertures corresponding to dark areas of the image, however, no
current flow occurs through photoconductive layer 44 as a
consequence of which counteracting fields 52 subsist in such
apertures. Thus, even after removal of the image from reverse face
28, an electrostatic image remains on screen 16 so that when an
accelerating or projection field is activated to move toner
particles from source 14 to conductive plate 20, only apertures
corresponding to dark portions of image I will admit and pass toner
particles to medium 18. All other apertures, i.e., apertures
corresponding to regions of screen 16 that have been illuminated,
will block passage of toner particles since in such apertures only
blocking field 50 is effective. Consequently, a positive image is
formed on medium 18 by the toner particles and such image can be
made permanent by subsequent fixing or fusing of the toner
particles on medium 18.
A typical image projected on the reverse face of screen 16 includes
regions that have no contrasting information, regions that are
completely black (or other contrasting color), and regions that are
intermediate the two extremes (e.g., varying degrees of gray). The
present invention permits accurate reproduction of all regions of
the image. When the image is projected on photoconductive layer 44,
regions of the layer become conductive in proportion to the
intensity of light impinging thereon. Regions of the
photoconductive layer corresponding to bright or highlight portions
of the image are maximally illuminated and therefore substantially
totally discharged. Regions of the photoconductive layer
corresponding to gray portions of the image are only partially
illuminated and therefore only partially discharged. Regions of the
photoconductive layer, corresponding to black portions of the image
are not illuminated and therefore not discharged. Accordingly,
passage of toner particles through various apertures 24 in screen
16 occurs in direct proportion to the position and relative
intensity in the image.
The control functions referred to above are carried out in
accordance with conventional techniques and circuitry, and such
conventional circuitry is indicated schematically in FIG. 1 by
reference numeral 53. To recapitulate the operation of the present
invention, reference is now made to FIGS. 3-5. FIG. 3 depicts
screen 16 as it exists after power source 22 has been activated. In
such condition, a potential difference between inner electrode 42
and outer electrode 38 establishes blocking fields 50 within each
aperture 24 so that any negatively charged toner particles
approaching the obverse face, the lower face as viewed in FIGS. 3,
4 and 5, will be blocked and, therefore, will not pass through the
apertures in the screen. FIG. 4 depicts the screen after a charge
from particle source 26 has been impressed on photoconductive layer
44 while the screen is in a dark condition. In such state,
counteracting fields 52 are established within apertures 24 which
fields fully or partially override blocking fields 50 so that the
net effect within each aperture so affected is that toner particles
approaching the screen will pass through the screen.
In FIG. 5, the region to the left labeled "illuminated area" and
indicated by reference numeral 54 corresponds with portions of an
image that are clear, or bright, whereas the remainder of the
screen corresponds with portions of the image that are black or of
some other contrasting color. In illuminated area 54,
photoconductive layer 44 becomes locally conductive and the charges
thereon that sustain counteracting field 52 are dissipated.
Consequently, as to the apertures within the illuminated area,
e.g., aperture 24a, blocking field 50 is solely effective so that
toner particles e.g., 14a, are repelled by the field and do not
pass through aperture 24a. As to the apertures that were not
illuminated by the image, however, the counteracting field remains
effective to override the blocking field so that the toner
particles e.g., 14b, can pass through such apertures. Those
apertures in regions of the screen that are partially illuminated
will pass toner particles in reduced quantities. Accordingly, an
image is formed downstream, i.e., above, as viewed in FIG. 5, of
the plate 16 by toner particles identified by reference number
14c.
Several alternate techniques for establishing counteracting fields
are shown in FIGS. 6 and 7. In such figures, a modified screen 16'
is shown. In such modified screen, conductive layers 38 and 42 and
inner insulative layer 40 are identical to the structure described
above and therefore bear identical reference numerals; in the
modified screen, however, photoconductive layer 44 is replaced by
an insulative or dielectric layer 60 that is insensitive to light.
Because layer 60 is formed of insulative material, it can store a
charge in the dark or in the light in the same manner that
photoconductive layer 44 stores a charge when in a dark
condition.
In the embodiment shown in FIG. 6, it is assumed that counteracting
fields 52 have been established by impression of a charge on
conductive layer 60 in a manner equivalent to that described
hereinabove; i.e., by bombardment from a corona source or the like.
A multilayered discharge plate 61 on which has been formed an
electrostatic image corresponding to the visible image desired, is
moved into contact with insulative layer 60 for selectively
discharging counteracting fields 52.
Plate 61 includes a transparent support layer 62 on one surface in
which is placed a thin, transparent conductive layer 64. Overlying
transparent conductive layer 64 is a photoconductive layer 66
adapted for contact with insulative layer 60 of screen 16'. A
suitable bias potential is applied to the plate by connection to
conductive layer 64. In a dark condition, plate 61 is moved into
contact with insulative layer 60 after which the visible image is
projected through transparent layers 62 and 64 onto photoconductive
layer 66. The regions of the photoconductive layer that are
illuminated become conductive and therefore discharge fields 52 on
corresponding regions of plate 16'. Consequently, the charge
distribution on insulative layer 60 is selectively modified in
accordance with the image projected onto plate 61 so that upon
removal of the plate and acceleration of toner particles through
screen 16', the toner particles will be arranged in accordance with
the visible image.
FIG. 7 shows still another modification wherein screen 16' is
identical to that described above in connection with FIG. 6. The
embodiment of FIG. 7 includes a plate 68 on which has been formed a
latent electrostatic image in accordance with the procedures
disclosed in U.S. Pat. applications Ser. No. 673,499 and Ser. No.
776,146. The charges on plate 68 that define an electrostatic
latent image thereon are transferred to the surface of insulative
layer 60 of screen 16' when the plate is brought into physical
contact with the screen. Such charges form counteracting fields 52
in only those apertures through which it is desired to admit toner
particles. Thus, by employing the modification of FIG. 7, the
intermediate step of impressing counteracting field 52 throughout
the entire area of the screen is eliminated.
Thus, it will be seen that the present invention provides an
improved screen for forming an electrostatic image which is
positive in reference to a visual image, to the end that positive
images on a suitable medium can be formed by a screen according to
this invention. Moreover, the screen is capable of operation at
relatively low voltages and is of relatively uncomplex
construction.
Although several embodiments of the invention have been shown and
described, it will be obvious that other adaptations and
modifications can be made without departing from the true spirit
and scope of the invention.
* * * * *