US3592678A - Liquid donor development with electrophoretic cleaning - Google Patents

Abstract

AN ELECTROSTATOGRAPHIC IMAGING MEMBER BEARING AN ELECTROSTATIC LATENT IMAGE ON A RECORDING SURFACE IS BROUGHT INTO CONTACT WITH A TONER LAYER ON THE SURFACE OF A DONOR MEMBER IN A LIQUID DEVELOPER BATRH, SEPARATED SLIGHTLY FROM THE DONOR MEMBER AND THEREAFTER SUBJECTED TO AN ELECTRIC FIELD WHICH CAUSES TONER PARTICLES IN THE BACKGROUND AREAS OF THE RECORDING SURFACE AS WELL AS TONER PARTICLES SUSPENDED IN THE SPACE BETWEEN THE RECORDING SURFACE AND THE DONOR MEMBER TO ELECTROPHORETICALLY MIGRATE TO THE DONOR MEMBER. THE DEVELOPED IMAGE ON THE RECORDING SURFACE PASSES THROUGH THE ZONE SUBJECTED TO THE ELECTRIC FIELD TO FURTHER CLEAN THE RECORDING SURFACE. SUBSEQUENTLY A UNIFORM LAYER OF TONER IS ELECTROPHORETICALLY DEPOSITED ON THE DONOR.

D R A W I N G

Description

July 13, 1971 SATORU HQNJO EI'AL 3,592,678

LIQUID DONOR DEVELOPMENT WITH ELECTROPHORETIC CLEANING Filed March 14, 1969 INVEN'IURS SATORU HONJO YASUO TAMAI BY MASAMICHI SATO mat X4 ATTORNEY United States Patent Office 3,592,678 Patented July 13, 1971 U.S. Cl. 117-37 Claims ABSTRACT OF THE DISCLOSURE An electrostatographic imaging member bearing an electrostatic latent image on a recording surface is brought into contact with a toner layer on the surface of a donor member in a liquid developer bath, separated slightly from the donor member and thereafter subjected to an electric field which causes toner particles in the background areas of the recording surface as well as toner particles suspended in the space between the recording surface and the donor member to electrophoretically migrate to the donor member. The developed image on the recording surface passes through the zone subjected to the electric field to further clean the recording surface. Subsequently a uniform layer of toner is electrophoretically deposited on the donor.

BACKGROUND OF THE INVENTION This invention relates to imaging systems, and more particularly, to an improved method and apparatus for developing electrostatic latent images with a liquid developer.

Processes for the formation and development of images on the surface of photoconductive materials by electrostatic means is well known. These processes include dry techniques such as cascade, powder cloud and magnetic brush processes and wet techniques such as the liquid development process. One conventional liquid development process involves placing a uniform electrostatic charge on a photoconductive insulating layer comprising zinc oxide powder and a resinous binder carried on a conductive paper substrate, exposing the layer to a light-and-shadow image ot dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a charged toner which is dispersed in an insulating liquid. The charged toner may be suitably colored and may have a polarity of charge identical or opposite to that of the latent image to be developed. If the polarity or charge of the toner is identical to that of the latent image, reversal de velopment will occur whereas a toner having a charge opposite to that of the latent image will be attracted to the latent image.

Although considered a highly desirable technique for the formation of images, difficulties are encountered with attempts to form high quality images with the liquid development process. Generally, liquid development is effected by either immersing the electrostatic latent image bearing surface into the liquid developer or contacting the image bearing surface with a uniform film of liquid developer carried on an applicator surface. The liquid developer adhering to the imaging surface is thereafter dried by mere exposure to ambient air at room temperature, circulating heated air, infrared heating, pressure from squeegee rollers or the like. Unfortunately, the liquid developer adhering to the background areas of the imaging surface contain toner particles which remain on the imaging surface after drying. These undesirable background deposits are particularly acute in high speed continuous tone development systems employing liquid developers containing high concentrations of toner particles.

It has been found that background toner deposits can be reduced by rinsing imaging surfaces immediately after development with a liquid a liquid which contains little or no toner particles. However, a rinsing step presents additional problems in high speed development systems because treatment time and equipment complexity is increased. In addition, as a result of increasing toner concentration in the rinsing liquid, additional equipment is required to remove the accumulated toner particles. Since most liquid development techniques are deficient in one or more of the above areas, there is a continuing need for improved imaging processes.

SUMMARY OF THE INVENTION It is therefore, an object of this invention to provide an imaging system overcoming the above noted deficiencies.

It is another object of this invention to provide an imaging technique which improves electrostatographic image quality.

It is a further object of this invention to provide an imaging technique which forms images more rapidly.

It is still another object of this invention to provide a more compact imaging apparatus.

It is another object of this invention to provide images having reduced background deposits.

It is a further object of this invention to provide an imaging system superior to those of known systems.

The above objects and others are accomplished by providing an imaging system in which an electrostatographic member bearing an electrostatic latent image on its recording surface is brought into contact with a toner layer on the surface of a donor member in a liquid developer bath, then separated slightly from the donor member and thereafter subjected to an electric field which causes toner particles in the background areas of the recording surface as well as toner particles suspended in the space between the recording surface and the donor member to electrophoretically migrate to the donor member, The migration of toner particles to the donor member replenishes toner material consumed during development. Since toner material is also removed from the liquid developer trapped in the space between the recording surface and the donor member, background deposits on the recording surface is further reduced by the rinsing effect of the substantially toner free trapped insulating carrier liquid. Surprisingly, the electric field does not cause destruction of the toner image by removal of toner particles from the toner image.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of the improved electrostatographic imaging system of this invention will become even further apparent upon consideration of the following disclosure of the invention, particularly when taken in conjunction with the accompanying drawing illustrating a schematic sectional view of an electrophotographic imaging apparatus employing the donor member and electric field applying means of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, reference character 1 designates a developer container containing a liquid developer 2 comprising toner particles suspended in an insulating liquid. An endless donor belt 3' supported by drive rollers 32 and 33 is suspended in the liquid developer '2. For purposes of illustration only, a positively charged toner layer is carried on the surface of donor belt 3. A photoreceptor web 4 carrying a negatively charged electrostatic latent image is brought into contact with donor belt 3 by means of rollers 41 and 42. The photoreceptor web 4 is separated from endless donor belt 3 with the aid of a split roller 43 which contacts only the edges of photoreceptor web 4. Photoreceptor web 4 is withdrawn from the bath of liquid developer 2 by means of roller 44 and squeeze rollers 45 and 46. The surface of squeeze roller 46 is imparted with a mirror finish to promote the appearance of the developed recording sur face of photoreceptor web 4. An insulated block is disposed between the upper and lower portions of endless donor belt 3 as well as between dry rollers 32 and 33. An electrically grounded development electrode 51 is secured to the insulated block 5 and positioned in sliding contact with most of that portion of the rear surface of donor belt 3 which is in contact with photoreceptor web 4 during development. Electrodes '52 and 53 are positioned on the rear surfaces of photoreceptor web 4 and donor belt 3, respectively, in the zone where photoreceptor 4 and donor web 3 are maintained in a slightly spaced apart relationship. Electrode 52 is electrically grounded and electrode 53 is supplied with a negative electric potential. A'fter separation of the endless donor belt 3 from photoreceptor web 4, endless donor belt 3 is transported around roller 33 and between an electrode 54 secured to the lower surface of insulated block 5 and grid electrode 55 positioned substantially parallel to both the endless donor belt 3 and electrode 54. A negative potential is applied to electrode 54 and a positive potential is applied to grid electrode 55. A pump 6 driven by a motor 7 is employed to withdraw highly concentrated liquid developer through conduit 61 from the bottom of developer container 1 and deliver the concentrated liquid developer through grid electrode 55 by means of perforated supply unit 62.

When the above described apparatus is in operation, all of the rollers are rotated at appropriate speeds to drive the endless donor belt 3 and photoreceptor web 4 at an equal rate of speed. The toner layer 31 carried on the surface of donor belt 3 is brought into contact with an electrostatic latent image bearing surface of photoreceptor web 4 by means of rollers 32 and 41, respectively. Since the driving means for photoreceptor web 4 and donor belt 3 are synchronized, there is no relative movement between the surface of photoreceptor web 4 and donor belt 3 while they are in contact with each other. During the period of contact, the toner in toner layer 31 is electrostatically adsorbed on the electrostatic latent image bearing surface of photoreceptor web 4. The development electrode 51 positioned on the rear side of donor belt 3 promotes rapid development and avoids the edge effect problem. Upon completion of development, the photoreceptor web 4 is passed between rollers 42 and 43 and is separated from the donor belt 3. Immediately after separation, photoreceptor web 4 is maintained in adjacent spaced relationship with donor belt 3 by means of rollers 43 and 44. An electric field between endless donor belt 3 and photoreceptor web 4 is established by means of electrodes 52 and 53. This field causes toner mechanically adhering to the background areas of the developed surface of photoreceptor web 4 and toner particles suspended in the developer liquid trapped between electrodes 52 and 53 to migrate in the direction of electrodes 53 and deposit on donor belt 3 to form a toner layer 31 thereon. The potential difference between electrodes 52 and 53 depends upon several factors including the potential difference between the electrodes and the speed at which the photoreceptor web 4 is transported. Satisfactory results are obtained with a potential difference of about 100 volts and a distance between the electrodes of about 2 millimeters. Since substantially all of the toner particles suspended in the liquid developer 2. trapped in the area between electrodes 52 and 53 are removed, the toner free liquid in this area promotes removal of toner particles from the background areas of the developed surface of 4 photoreceptor web 4. The photoreceptor web 4 is thereafter conveyed between squeeze rollers 45 and 46. Since squeeze roller 46 has a mirror finished surface, the possibility of photoreceptor web 4 breakage is eliminated.

Upon separation from the photoreceptor web 4, donor belt 3 is conveyed around roller 33 and into sliding contact along the lower surface of electrode 54. During the period in which donor belt 3 is passed over electrode 54, an electric field is established between electrode 54 and grid electrode to promote deposition of toner particles from liquid developer 2 onto endless donor belt 3 to form a uniform toner layer 31. To promote rapid deposition of toner particles on donor belt 3, liquid developer 2 is delivered by pump 6 through a perforated supply unit 62 into the space between grid electrode 55 and donor belt 3. After a sufficient quantity of toner particles is deposited on donor belt 3 to form toner layer 31, the donor belt 3 is again brought into contact with photoreceptor web 4 to develop additional electrostatic latent images. The development electrode 51 is extended within insulating block 5 between electrodes 53 and 54 to shield these latter electrodes thereby preventing toner from being deposited thereon.

Although the embodiment depicted in the drawing illustrates a system in which the donor belt 3 retains a negative toner image upon separation from photoreceptor web 4, this toner layer may be removed if desired. Thus, a cleaning brush or other suitable means may be positioned adjacent roller 33 to remove the toner layer from the surface of donor belt 3 for redispersal into liquid developer 2.

Any suitable material may be employed in the donor belt and the photoreceptor web of this invention. The material should resist dissolving or swelling when contacted with the liquid developer. Preferably, the donor belt material should possess a high electric resistance and a low dielectric constant. Typical donor belt materials include cellulose, triacetate, Teflon, polyethylene terephthalate and the like.

Any conventional insulating liquid developer may be employed in the system of this invention. For optimum results, toner particles having an average particle diameter of less than about 1 micron are preferred because removal of toner particles from the developed image areas is substantially eliminated during the cleaning operation in which toner particles are cleaned from the background areas. This peculiar characteristic of toner particles having an average particle size of less than 1 micron renders the system of this invention highly suitable for forming multicolored images on the same surface of a photoreceptor web because disruption of the toner image does not occur.

Although positive images are formed in the specific example described above, it is apparent that reversal images may be obtained by reversing the polarity of toner material or by reversing the polarity of the electric potential applied to the electrodes. Obviously, one may apply a potential difference to the development electrode if desired. As described above the apparatus of this invention includes a developing unit and a cleaning unit incorporated within a single housing. Further, the technique of this invention is adapted to rapidly form a toner layer on a donor member for reuse. Thus, the compact developing system of this invention forms low background toner images at high processing speeds with very little toner consumption.

Although specific materials and conditions are set forth in the foregoing examples, these are merely intended as illustrations of the present invention. Various other suitable photoreceptor materials, liquid developers and voltage potentials including those listed above may be substituted for those in the specific example with similar results. Other materials may also be added to the liquid developer or photoreceptor to sensitize, synergize or otherwise improve the imaging properties or other desirable properties of the system.

Other modifications of the present invention will occur to those skilled in the art upon the reading of the present disclosure. These are intended to be included Within the scope of this invention.

What is claimed is:

1. A method of forming images comprising providing an electrostatic latent image on a recording surface of an electrostatographic imaging member, contacting the said recording surface With a uniform toner layer carried on the surface of a donor member immersed in an insulating liquid developer bath comprising an insulating carrier liquid and suspended toner particles, separating said recording surface from said toner layer thereby forming an imaged recording surface having image areas and background areas, positioning said recording sur face adjacent to and spaced from said toner layer thereby forming an open cleaning zone in said insulating liquid developer bath between said recording surface and said toner layer and providing an electric field across said cleaning zone, said field having a direction and intensity suflicient to cause toner particles in said background areas of said recording surface as Well as toner particles suspended in said zone to electrophoretically migrate to said toner layer.

2. A method according to claim 1 including minimizing relative movement between said recording surface and said toner layer While said recording surface and said toner layer are maintained in contact with each other.

3. A method according to claim 1 wherein said toner particles have an average particle size of less than about 1 micron.

4. A method according to claim 1 including removing said surface of said donor member from said cleaning zone, depositing a uniform layer of toner particles on said surface of said doner member and contacting said uniform layer of toner particles with an undeveloped electrostatic image on a recording surface of an electrostatographic recording member.

5. A method according to claim 4 including removing substantially all toner particles from said donor surface prior to depositing said uniform layer of toner particles.

6. A method according to claim 1 including providing an externally applied electric field through said recording surface and said toner layer while said recording surface and said toner layer are maintained in contact with each other.

7. An imaging apparatus comprising a liquid developer container, a donor member having a donor surface positioned within said container, means to provide a uniform toner layer on said donor surface, means to contact an electrostatic latent image bearing recording surface of an imaging member with said donor surface While said donor surface is immersed in a liquid developer, means to separate said recording surface from said donor surface, means to maintain said recording surface adjacent to and spaced from said donor surface to form a cleaning zone in said liquid developer therebetween, means to supply an electric field across said cleaning zone from said recording surface to said donor surface and means to remove said recording surface from said cleaning zone.

8. An imaging apparatus according to claim 7 wherein said donor member has a continuous donor surface.

9. An imaging apparatus according to claim 8 wherein said donor member is a continuous belt.

10. An imaging apparatus according to claim 7 wherein the said means to provide a uniform toner layer on said doner surface comprises electrodes positioned on opposite sides of a portion of said donor member.

References Cited UNITED STATES PATENTS 3,096,198 7/1963 Schaffert 117-37 3,129,115 4/1964 Clark et al. 117-37X 3,276,896 lO/1966 Fisher 11737 3,346,475 10/1967 Matkan et a1. 11737X 3,376,852 4/1968 Weiler 118--637 WILLIAM D. MARTIN, Primary Examiner E. I. CAB-1C, Assistant Examiner U.S. Cl. X.R.

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