|
(11) | EP 0 000 671 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||||||
| (54) | Improved electrophotographic apparatus and method for use with electrophotosensitive toner |
| (57) Improved electrophotographic apparatus and method for use with electrophotosensitive
toner are disclosed for electrophotographic imaging. Improved image density, color
separation and toner background depositation levels are achieved by triboelectrically
charging a dry, powdered, electrophotosensitive loner by interaction with an applicator,
e.g., a magnetic brush 12 Figure 1, and then moving such toner into contact with an
oppositely-charged electrode member 2. Upon exposure of a light image of an original
at the interface between the toner and image electrode, the unexposed toner particles
transfer to the tmage electrode and the exposed particles are discharged and not deposited
on that member. A positive reproduction of the original is thus formed. |
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to electrophotography and, in particular, to improved electrophotographic methods and apparatus for producing images with electrophotosensitive toner.
DESCRIPTION OF THE PRIOR ART
[0002] Early development work in imaging with electrophotosensitive toners involved establishing an electric field across a layer of such toner, comprising colored particles in a powdered, i.e. substantially dry, particulate form, and made of or including a photoconductive material, and exposing the toner layer to a light image of an original to be reproduced.
[0003] In one prior art apparatus disclosed in U.S. Patent No. 3,100,426 to Kaprelian, multi-colored photoconductive toner particles are directed by an air blast through a screen grid and against a transparent conductive layer carried on a transparent plate which is spaced away, from and parallel to the screen grid. The conductive layer and screen grid are maintained at a suitable potential difference by connection to a source of high voltage while a multi-color light image pattern is projected onto the transparent plate. The appropriately colored toner particles are charged by their passage through the screen grid and adhere to the conductive layer forming a color image thereon. Toner particles'which remain inactive because of their non-response to the wavelengths of the exposing light are drawn into ducts and returned to a receiving chamber for reuse. The colored toner particles on the conductive layer are then transferred to a black paper or plastic base and fixed thereon by heat or other means.
[0004] However, certain difficulties have arisen during efforts to commercialize dry electrophotographic imaging systems using photoconductive toner mixtures. High density images are difficult to attain in such systems, for example, because of the limited quantity of uniformly charged toner particles which can be transported through the screen grid of the Kaprelian apparatus. Furthermore, the images produced by the prior art systems have an undesirably high level of toner deposition in the background areas..
SUMMARY OF THE INVENTION
[0005] In accordance with the present invention the problem of producing images with high density, and lower levels of background deposition is solved by providing a method and apparatus wherein an electric field is provided across a layer of electrophotosensitive toner particles in powdered form and the particles within such field are exposed to a light image of an original to be reproduced characterized in that the toner particles are triboelectrically charged and moved by an applicator into contact at an image zone with successive portions of an oppositely charged image electrode while said particles are simultaneously exposed to corresponding successive portions of the light image of the original. In one preferred embodiment the toner particles are in mixture with magnetic carrier particles and a magnetic brush is utilized for applying the toner particles to the image electrode at the image zone. In another preferred embodiment a multicolor mixture of first, second and third colored toner particles which are primarily electrically photosensitive to light of first, second and third wavelength bands, respectively, is used in connection with a multicolor original to obtain a positive color reproduction.
IMPROVED ELECTROPHOTOGRAPHIC APPARATUS AND METHOD
FOR USE WITH ELECTROPHOTOSENSITIVE TONER
TECHNICAL FIELD OF THE INVENTION
[0006] The present invention relates to electrophotography and, in particular, to improved electrophotographic methods and apparatus for producing images with electrophotosensitive toner.
DESCRIPTION OF THE PRIOR ART
[0007] Early development work in imaging with electrophotosensitive toners involved establishing an electric field across a layer of such toner, comprising colored particles in a powdered, i.e. substantially dry, particulate form, and made of or including a photoconductive material, and exposing the toner layer to a light image of an original to be reproduced.
[0008] In one prior art apparatus disclosed in U.S. Patent No. 3,100,426 to Kaprelian, multi-colored photoconductive toner particles are directed by an air blast through a screen grid and against a transparent conductive layer carried on a transparent plate which is spaced away, from and parallel to the screen grid. The conductive layer and screen grid are maintained at a suitable potential difference by connection to a source of high voltage while a multi-color light image pattern is projected onto the transparent plate. The appropriately colored toner particles are charged by their passage through the screen grid and adhere to the conductive layer forming a color image thereon. Toner particles-which remain inactive . because of their non-response to the wavelengths of the exposing light are drawn into ducts and returned to a receiving chamber for reuse. The colored toner particles on the conductive layer are then transferred to a black paper or plastic base and fixed thereon by heat or other means.
[0009] However, certain difficulties have arisen during efforts to commercialize dry electrophotographic imaging systems using photoconductive toner mixtures. High density images are difficult to attain in such systems, for example, because of the limited quantity of uniformly charged toner particles which can be transported through the screen grid of the Kaprelian apparatus. Furthermore, the images produced by the prior art systems have an undesirably high level of toner deposition in the background areas..
SUMMARY OF THE INVENTION
[0010] In accordance with the present invention the problem of producing images with high density, and lower levels of background deposition is solved by providing a method and apparatus wherein an electric field is provided across a layer of electrophotosensitive toner particles in powdered form and the particles within such field are exposed to a light image of an original to be reproduced characterized in that the toner particles are triboelectrically charged and moved by an applicator into contact at an image zone with successive portions of an oppositely charged image electrode while said particles are simulta- neausly exposed to corresponding successive portions of the light image of the original. In one preferred embodiment the toner particles are in mixture with magnetic carrier particles and a magnetic brush is utilized for applying the toner particles to the image electrode at the image zone. In another preferred embodiment a multicolor mixture of first, second and third colored toner particles which are primarily electrically photosensitive to light of first, second and third wavelength bands, respectively, is used in Connection with a multicolor original to obtain a positive color reproduction. modulate light in accordance with their image pattern; and the toner which has been moved proximate the opposite surface of the image electrode is exposed in accordance with these light patterns. Upon such exposure, the illuminated toner particles become conductive and lose, or reverse, their charge. Toner particles which are not subjected to light retain their triboelectric charge and are attracted to the oppositely biased surface of the image electrode, thus forming a positive image sense reproduction of the original on the contacted surface of the image electrode 2. The neutral or reversely charged toner particles are not attracted to the image electrode and are recirculated back into the reservoir for renewed triboelectric charging,
[0011] As mentioned above, other applicator arrangements can be utilized in accordance with the present invention. For example fur brush applicators of the type having animal or synthetic bristles, properly triboelectrically related to the toner, can be used. It is desirable, however, that the brush/developer combination of any particular implementation of the present invention allow for a high field in the exposure station while also insuring that carrier particles themselves are not attracted to the image electrode. This is accomplished in the Fig. 1 embodiment by utilizing electrically insulative carrier material. Electrically insulative bristles on a fur brush applicator can be utilized. Also, an electrically insulative surface layer on the outer periphery of the transport drum such as shown in Fig. 1 can be utilized. Similarly, an electrically insulative layer could be provided on the surface of image electrode 2, so long as the attractive force provided behind it is sufficient to attract unexposed particles from their triboelectric bond with the applicator.
[0012] As will be understood, there are many alternative configurations and approaches which can be used to provide a flowing light pattern of successive portions of the original at the successive applicator-image electrode interfaces. Scanning mirror arrangements, moving the original at a separate exposure station or a scanning laser beam trolled in accordance with signals representative of the original are examples of such alternative approaches, but there are many others known in the art.
[0013] As mentioned, the developer mixture can comprise either a single type of colored electrophotosensitive toner particles or a mixture.of differently colored toner particle types. In the first instance it is necessary only that the particles be sensitive to one or more wavelength ranges over the panchromatic exposure source output. However, when using multicolor particle types, it is desirable that each toner particle type have a sensitivity to a particular color light. One desirable embodiment utilizes cyan, magenta and yellow toner particles which are sensitive substantialy only to red, green and blue light respectively.
[0014] Figs. 2a-2c schematically indicate the effects involved in creating a multicolor copy in accordance with the present invention using a toner mixture such as described above. Figs. 2a and 2b indicate successive stages of cooperation between the applicator and image electrode, with the light color components which comprise different projected image portions indicated by arrows R, G and B (red, green and blue) directed at the zones of the image electrode through which such image portions are directed. Fig. 2c illustrates the color of toner deposited on the image electrode at the zones illustrated in Figs. 2a and 2b, after cooperation with the applicator. As can be seen at A at the right side of the image electrode, where R, G and B (i.e., white) light had been projected on the toner, all toner types were discharged and no toner attracted to the image electrode. Where red light only had been projected, cyan particles C-were reversed or neutralized in charge and magenta M and yellow Y particles attracted. Where red and blue light had been projected, only magenta M particles were deposited, the cyan C and yellow Y particles being discharged by the projected light. Similarly G and B exposure produced a deposition of only cyan C particles, and absence of projected light produced deposition of C, M and Y particles, forming black. It is also to be noted that the amount of discharge of toner of a particular color is proportional to the intensity of light of its activating type. For example, high-intensity red light discharges or reverses the charge of a larger quantity of cyan particles than is discharged by low-intensity red-light image, which would allow a partial deposition of cyan toner particles present at the interface. Thus, a tone scale for all colors can be attained.
[0015] The toner image, which continues to bear an electrostatic charge can be transferred readily to a copy sheet, e.g., plain paper, in known manner by moving portions of the copy sheet sequentially into transfer relation with portions of the image electrode, at a transfer station. Conventional electrophotographic transfer apparatus and techniques can be utilized, e.g., a corona charging device of polarity opposite the toner and magnitude greater than the bias on the image electrode.
[0016] The toner bearing copy sheet can then be moved past a conventional fusing station where the toner is fixed to the surface of the paper. Radiant or roller fusers or other types of toner fixing devices used in electrography are suitable.
[0017] The image electrode is then cleaned of residual toner and ready for another image cycle. Cleaning can be performed by conventional techniques or by recycling the image electrode past the applicator with the bias field between applicator and image electrode reversed to drive the toner back to the brush.
[0018] In one embodiment and mode of operation it is possible to provide an ultraviolet radiation sensitive photoconductor as the image electrode and provide the bias to that image electrode as a uniform electrostatic pre- ' charge on its imaged surface. In such embodiment, cleaning and elimination of residual charge would be facilitated by an ultraviolet exposure during cleaning.
[0019] Fig. 3 illustrates one embodiment of the present invention for continuous operation, i.e., repetitive cycling of the image electrode along an operative path for successive imaging and trans.fer of the same or different originals. The "continuous" apparatus 50 comprises a cylindrical image member 51 which can be formed, e.g., of conductive glass. A source of high voltage potential 52 provides a positive polarity bias for the drum, in effect at least during the portion of its cycle of rotation between applicator 53 and transfer station 54. The cylinder 51 is mounted for rotation on its axis by means not shown, but which provide an access opening for projection of the original image, via lens 55 and mirror 56, at the exposure zone 58. The image projected at station 58 is a flowing one, e.g., provided by scanning successive portions of an original moving past an object plane of lens 55. Magnetic brush 60, at the applicator station 53 is of the type described with respect to Fig. 1 and is mounted for rotation in developer reservoir 61 to provide triboelectrically-charged toner at the image station 58 in the manner previously described. The brush can be connected to ground electrically.
[0020] Spaced downstream from the applicator station 53, along the operative path of the image member 51, is transfer station 54 which can comprise a conductive roller 70 coupled to a source of positive potential 72 of magnitude sufficient to attract toner from the image member 51 and onto copy sheet 71. The operative path of the image member 51 then leads to a cleaning station 80 which can comprise, e.g., a soft brush 81 and vacuum source.
[0021] During an imaging operation, light patterns corresponding to portions of the original passing the object plane of lens 55 are directed through the successive transverse portions of the image member 51 which are opposite the applicator 60. Toner between the applicator 60 and the image member 51 is imagewise discharged in the manner previously described to provide a positive sense image on the outer surface of image member 51. The toner image is next transferred to the copy sheet 71 during its movement past roller 7Q and the drum is cleaned for the next image cycle at cleaning station 80.
[0022] While the theory of image formation is not completely understood, it is believed that the present invention provides the following advantages to dry electrophotographic imaging systems using photoconductive toner particles. The triboelectric charging of photoconductive toner particles provides a mechanism for delivering a large quantity of such particles to the imaging zone located between the applicator and the image electrode. This increases image density. Another advantage of triboelectric charging is that it provides the toner particles with uniform potential of correct.polarity. This, of course, is necessary to obtain proper control of the particle migration. Further, triboelectric charging of the toner particles in combination with the exposure of such toner particles to successive portions of the light image is believed to aid in restricting the random motion of the toner particles during their migration from the applicator to the image electrode. This improves the level of particle deposition in the background image areas.
[0023] The following detailed Examples will provide a further understanding of the practice of the present invention.
Example 1
[0024] A magnetic brush was located in an image arrangement as shown in Fig. 1. The developer mixture comprised 500 grams of Xerox brand 5R54 carrier beads and 6 grams of photoconductive toner particles comprised of:
5 parts - Lexan 145 polycarbonate polymer (General Electric Co.);
1 part - poly(vinyltoluene-co-lauryl methacrylate-co-lithium methacrylate-co-methacrylic acid) (56%-40%-3.6%-0.4%) referred to hereinafter as PVT; and ,
1 part - Cyan Blue GTNF pigment (American Cyanamid Co.).
[0025] The toner developed a negative triboelectric charge. A Nesatron plate was used as the transparent electrode and a Kodak Carousel 110 size slide projector, spaced seven (7) inches from the plate, was used to project a transparency image co-moving with the transparent electrode. Voltages in the range of 900-1500 volts were provided between the brush and plate (plate positivel. Positive images were formed on the transparent electrode as it passed over the rotating magnetic brush. These images could be completely erased by passing the plate back over the brush with a negative voltage applied to the plate.
Example 2
[0026] Instead of using a Nesatron electrode, a 10- micron layer of Lexan 145 polymer, supported on conductive, nickelized plastic film, was mounted against a glass plate bearing an image, and an image was formed as in Example 1, using a voltage of 1350. volts (plate positive). This example demonstrates that the toner may lose or reverse charge by interaction with the brush or carrier, because the insulative coating of Lexan polymer prevented intimate contact of the toner particles with a metallized electrode.
Example 3
[0027] Particles composed of 60% Watchung Red B pigment (E. I. duPont deNemours & Co.), 20% 2,4,7-trinitrofluore- none, 10% Lexan 145 polymer and 10% PVT were mixed with carrier (Xerox 5R54) in the ratio of 6 g. toner to 500 g. carrier. Good positive images were obtained on Nesatron plates (biased positive) at 1100V. A grey scale was obtained.
Example 4
[0028] Composite particles consisting of 50% Cyan Blue GTNF pigment and 50% poly[4-di-p-tolylamino)styrene] were added to IBM Brand carrier (sold for IBM II copiers) in the ratio of 5g. toner to 500 g. carrier. Images were obtained at 1KV on the image electrode. Toning was excellent, indicative of a high triboelectric charge level (particles positively chargedl.
INDICATION OF INDUSTRIAL USE
1. A method of recording an image with electrophotosensitive toner particles in powdered
form comprising the steps of providing an electric field across a layer of photoconductive
toner particles and exposing said particles within such field to the light image of
an original to be reproduced, characterized in that said method includes triboelectrically
charging and moving said particles into contact at an image zone with successive portions
of a charged image electrode (2) while simultaneously exposing said particles to corresponding
successive portions of the light image of the original.
2. A method according to claim 1, characterized in that said photoconductive toner
particles comprise a mixture of first, second and third colored particles which are
primarily electrically photosensitive to light of first, second and third wavelength
bands, respectively.
3. A method according to claim 1, characterized in that said photoconductive toner
particles are charged by interaction with triboelectrically related, magnetic carrier
particles and that said carrier and toner particles are moved into contact with said
image electrode by a magnetic brush.
4. Electrographic recording apparatus for use with powdered electrophotosensitive
toner particles, said apparatus comprising a first image electrode, a second electrode
located proximate said first electrode, means for establishing a potential difference
between said image electrode and said second electrode and means for projecting a
light image into the space between said first image electrode and said second electrode,
characterized.in that said second electrode is an applicator for applying. successive
quantities of triboelectrically charged electrophotosensitive toner particles at an
image zone to successive portions of saido image electrode and in that said projecting
means projects corresponding successive portions of said light image to be reproduced
onto said toner particles located in said image zone.
5. The apparatus defined in claim 4, charac≈ terized in that substantially zero relative
velocity exists between said image electrode and said light image portions projected
at said image zone.
6. The apparatus defined in claim 4, wherein said image electrode is transparent to
said light image and said projecting means directs light onto said toner particles
in said image zone through said image electrode.
7. The apparatus defined in claim 4, characterized in that said toner particles are
contained in a reservoir with triboelectrically related magnetic carrier particles
and in that said applicator includes. a magnetic rotary brush adapted to attract and
move quantities of said triboelectrically charged toner particles into contact with
said successive portions of said image electrode at said image zone.
8. The apparatus defined in claim 7, characterized in that the potential difference
established between said image electrode and said brush is such that said image electrode
attracts unexposed toner particles thereto with a force exceeding the triboelectric
bond between said toner particles and said carrier particles.
9. The apparatus defined in claim 8, characterized in that said carrier particles
have an electrical resistivity sufficient to prevent attraction thereof to said image
electrode.