Apparatus For Transferring A Developed Image From A Photosensitive Member To A Receiver

Abstract

Apparatus is provided for transferring a developed toner image from a photoconductor to a receiver by a method which includes applying to the receiver a bias voltage opposite in polarity to the charge on the developed toner image. The toned photoconductor is precharged to the same polarity as the developed toner image to assure a maximum charge on the toner particles forming the image prior to their contact with the receiver and then flooded with light prior to and/or during the transfer to reduce the forces of attraction between the toner particles and the photoconductor.

Description

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a method and apparatus for transferring a toned electrostatic image from a photoconductive member to a receiver and, more particularly, to an improved transfer method and apparatus utilizing electrical bias means and illuminating means.

2. Description of the Prior Art

In a conventional electrophotographic process, an electrostatic image is formed on a photoconductive surface as by placing a uniform electrostatic charge on the photoconductive surface which surface is subsequently exposed to a radiation pattern to dissipate the charge in the exposed areas. This electrostatic image is then toned by a suitable developer having toner particles which are attracted by the charged areas of the image. The toner particles comprising the developed image are then transferred to a suitable receiver by contacting the receiver to the image bearing photoconductive surface and applying a potential to the receiver of opposite polarity to the charge on the toner particles to bring about image transfer. To reproduce copies of excellent quality and with a high contrast image, it is desirable to transfer to the receiver as much of the developed image as possible. Any residual toner particles which remain on the photoconductive surface after transfer must be removed therefrom prior to the forming and transfer of subsequent images to prevent ghost images from being formed during subsequent transfers. It is difficult to clean off excess toner particles without scratching or otherwise injuring the delicate photoconductive surface. As a result, the life of the photoconductive surface tends to be limited by mechanical and electrical failure resulting from toner abrasion.

To improve the quality of the copies reproduced and/or to reduce the deleterious effects of cleaning, various attempts have been made to increase the efficiency of the transfer process. U.S. Pat. No. 3,414,409 to Gallo discloses a transfer method in which image transfer is accomplished without charging the receiver. In accordance with the teachings of this patent the image bearing photoconductive layer is contacted with the receiver and the photoconductive layer is uniformly illuminated at the point of transfer. U.S. Pat. No. 3,520,604 to Shelffo discloses a transfer roller which is charged by a DC voltage supply having a polarity which is the same as the polarity of the electrostatic charges in the latent image portions. U.S. Pat. No. 3,444,369 to Malinaric discloses subjecting a developed toner image on a photoconductor to a low level corona discharge of a polarity opposite the charge on the toner particles overlying the image area. In some environments, such transfer devices have not been found entirely satisfactory. Incomplete transfer produces a poor quality print of low image contrast and the residual toner particles remaining on the photoconductive surface have resulted in scumming and scratching of the photoconductive surface during the cleaning operation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved apparatus for transferring toner particles from a photoconductive surface to a receiver.

Another object of the invention is to provide an improved method for transferring toner particles from a photoconductive surface to a receiver.

Still another object of the invention is to provide a more complete transfer of a toned image from a photoconductive surface to a receiver.

Yet another object of the invention is to reduce the scumming and scratching of a photoconductive surface resulting from toner particle abrasion.

These and other objects are accomplished, in accordance with the present invention, by applying an electrical charge to a previously charged, exposed and developed photoconductor of the same polarity as the charge on the toner particles forming the developed image. The photoconductor is then brought into face to face relationship with the receiver along a line or area of contact. A bias potential opposite in polarity to the charge on the toned image is applied to the receiver to attract the toner particles forming the image to the receiver and the photoconductor is exposed to radiation along the line or area across which transfer occurs to reduce the forces of attraction between the photoconductor and the toner particles.

In a preferred embodiment of the invention, a transfer apparatus is provided with a corona charger which applies to the photoconductor an electrostatic charge of the same polarity as the charge on the toner particles forming the toner image. The photoconductor and the receiver are subsequently brought into a face to face relationship along at least a line of contact. Along the line of contact, an electrical potential opposite in polarity to the charge on the toned image is applied to the receiver by a source of potential and a source of radiation illuminates the photoconductor. The precharging of the photoconductor increases the attraction between the toner image and the receiver and its illumination at the time of transfer reduces the attraction between the photoconductor and the toner image thereby achieving a more complete transfer of the toner particles forming the image.

More specifically, the preferred embodiment of the invention is adapted for use with a flexible photoconductor comprising a photoconductive insulating layer overlying a transparent base. The photoconductor carrying a developed toner image thereon is moved past a corona which applies an electrostatic charge to the photoconductive surface of the photoconductor of the same polarity as the charge on the toner particles forming the image. The toner image and the receiver are then brought into face to face contact by movement of the photoconductor and the receiver through the nip formed by a transparent cylinder and an opposed electrically conductive backing roller. As the receiver and the photoconductor pass together through the nip, a source of direct current potential connected to the backing roller applies to the receiver a bias voltage opposite in polarity to the charge on the toned image and a lamp mounted inside the transparent roller uniformly illuminates the photoconductive surface of the photoconductor through the transparent conductive base.

BRIEF DESCRIPTION OF THE DRAWING

In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawing wherein corresponding parts are identified by like numerals and in which:

FIG. 1 is a diagrammatic side elevational view of a conventional electrophotographic mechanism embodying the transfer station of the present invention.

FIG. 2 is an enlarged, fragmentary view of a portion of FIG. 1, showing the transfer station in greater detail.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now the the drawing there is shown an electrophotographic mechanism M, which includes an endless, flexible photoconductive element such as a web or belt 1 comprising a photoconductive insulating surface 2 overlying a conductive base 3 and extending around spaced rollers 4 and 6. The belt 1 is moved through an endless path and is illustrated as being driven by motor 8 through a drive belt 10 connected to roller 6. The roller 4 is mounted as shown so that it is freely rotatable. The belt 1 is driven in the direction of arrows 11 past a charging station 12 which places a uniform electrical charge (either positive or negative) over its photoconductive surface; an exposure station 14 whereat the belt is exposed to a radiation pattern, such as a light image from an original, to discharge the photoconductive surface in accordance with the radiation pattern to form an electrostatic latent image; a developing station 16 whereat the latent image is developed by depositing toner thereon; a novel transfer station T, described in detail below, whereat the developed image is transferred to a receiver 18; and finally a cleaning station 20 whereat the photoconductive surface of the belt 2 is cleaned of residual toner particles. The charging, exposure, developing and cleaning stations are mentioned herein without detailed description and may be understood to be selectable as required by one skilled in the art from any suitable form known in the prior art.

At the transfer station T, the developed toner image on the photoconductive surface 2 of the belt 1 is brought into contact with the receiver 18 which is fed from a supply (not shown) by a pair of opposed feed rollers 21 through a nip 22 formed between rollers 4 and 24. The roller 24 has a soft, compliant, electrically conductive surface and can be set to apply a controlled pressure at the nip 22 to provide firm contact between the receiver 18 and the toned image as they pass together between the rollers 4 and 24. The roller 24 is insulated from ground so that an electrical potential from a DC source 26 opposite in polarity to the charge on the toner particles forming the image, may be applied to it to effect image transfer as the receiver passes through the nip. To assure immediate separation of the receiver from the photoconductive surface after transfer, it is furthermore desirable that the guide path for the exiting receiver 18 formed by the opposed rollers 23 is arranged so that the clockwise angle A from the plane passing through the axes of the rollers 4 and 24 to the plane of the exiting receiver be 90.degree. or less. In some embodiments of the invention, the single conductive pressure or backing roller 24 shown in FIG. 2 can be replaced by two or more such rollers to increase contact time between the receiver 18 and photoconductive belt 1 to enhance image transfer. In this case, the angle A would be the clockwise angle from the planes passing through the axis of the exit roller and the axis of the roller 4 to the plane of the exiting receiver 18. After separation, the receiver 18, in accordance with known practice, can be fed to a fusing station 25 whereat the toner image is permanently fixed to the receiver, as by opposed feed rollers 23.

To increase the charge on the toner particles forming the image and therefore their attraction to the receiver 18, which is oppositively biased by the source 26, means are provided, such as a corona discharge device 28 upstream of nip 22. The corona 28 applies a uniform electrostatic charge to the photoconductive surface 2 of the belt 1 and the developed image of a polarity opposite to the potential applied to the roller 24 by the source 26.

To reduce the charge on the photoconductive surface 2 of the belt 1 in the toned areas and thereby decrease the attraction between the photoconductive belt and the developed toner image, the photoconductive surface of the belt is exposed along the line or area of image transfer to radiation, such as actinic radiation, by means of, for example, an incandescent lamp 30. The lamp 30 also prevents charge buildup on the photoconductive surface in the untoned areas as a result of charging with the corona 28. As best seen in FIG. 2, the tubular light source 30 is mounted inside of roller 4. Accordingly, both the roller 4 and the conductive base 3 of the belt 1 must be translucent, and preferably transparent. Alternatively, the illumination source 30 can be mounted inside roller 24 so that it can directly illuminate the photoconductive surface 2 of the belt 1. In this instance, of course, roller 24 and the receiver 18 must be formed from a properly selected translucent or transparent material. If desired, a reflector 32, such as an eliptical reflector, may be provided around a portion of the light source 30 as shown in FIG. 2, to direct the radiation to the area of transfer. The use of reflector 32 is optional and illumination of the photoconductive surface may occur before and/or during transfer. Illumination of the image bearing photoconductive surface 2 of the belt may be by means of a very brief flash of high intensity light or by a relatively longer exposure to comparatively low level illumination. The former has the advantage of high speed and is, therefore, more suitable for a continuous process apparatus.

Exposure of the photoconductive surface to radiation, such as actinic radiation, substantially neutralizes the electrostatic charge pattern. The toner particles forming the image are attracted toward the oppositively biased receiver 18 with which they are in contact.

From the foregoing, the advantages of the present invention are readily apparent. Novel transfer apparatus has been disclosed which achieves a more complete transfer of the toned image than previously known apparatus. This is accomplished by precharging the toned image and by flooding the photoconductor with light and applying a bias voltage to the receiver at the time of transfer to facilitate the transfer.

The invention has been described in detail with reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

I claim:

1. Apparatus for transferring a toner image from an element, comprising a photoconductive insulating layer overlying a translucent conductive base, to a receiver, the toner image being formed of toner particles having a charge of a first polarity thereon which create forces of attraction between the toner particles and the photoconductive insulating layer, said apparatus comprising:

a. a corona discharge electrode for applying an electrostatic charge of said first polarity to the toner image on said photoconductive insulating layer;

b. means, including a rotatable cylinder and an opposed backing roller forming a nip there between, for bringing the toner image on said photoconductive insulating layer into face to face relationship with the receiver along at least a line of contact, said line of contact and the respective central axes of said cylinder and said roller lying in a common plane;

c. a source of direct current potential for applying an electrical potential of a second and opposite polarity to the receiver at said line of contact to attract the toner image to the receiver;

d. a source of illumination for exposing said photoconductive insulating layer to actinic radiation through said translucent conductive base to reduce the forces of attraction between the toner particles and said element ; and

e. means for separating said element from the receiver as the receiver passes from said nip along an exit plane, wherein the included angle between said common plane and said exit plane is less than 90.degree..

2. Apparatus for transferring a toner image as claimed in claim 1 wherein said cylinder is translucent and said source of illumination comprises a tubular light source mounted inside said cylinder.

3. In an electrophotographic machine, apparatus for transferring a toner image from a flexible element, comprising a photoconductive insulating layer overlying a transparent conductive base, to a receiver, the toner image being formed of toner particles having a charge of a first polarity thereon which create forces of attraction between the toner particles and the photoconductive insulating layer, said apparatus comprising:

a. a transparent rotatable roller;

b. an electrically conductive roller mounted in opposed relationship with said transparent roller to form a nip there between;

c. means for moving the receiver through said nip , in face to face contact with the toner image on said photoconductive insulating layer;

d. a corona discharge electrode for applying an electrostatic charge of said first polarity to the toner image on said photoconductive insulating layer prior to the entry of the toner image into said nip;

e. a source of direct current potential connected to said electrically conductive roller for applying an electrical potential of a second and opposite polarity to said receiver as said receiver passes through said nip to attract the toner image to said receiver; and

f. a lamp mounted inside said transparent roller for uniformly illuminating said photoconductive insulating layer through said conductive base as said element carrying the toner image thereon passes through said nip to reduce the forces of attraction between the toner particles and said element.

4. Apparatus for transferring a toner image from a photoconductive element to a receiver, the toner image being formed of toner particles having an electrostatic charge of a first polarity thereon which create forces of attraction between the toner particles and the photoconductive element, said apparatus comprising:

a. charging means for applying an electrostatic charge of said first polarity to the toner image on said photoconductive element;

b. means for bringing the charged toner image on said photoconductive element into face to face relationship with the receiver along at least a line of contact, said means comprising:

1. a rotatable cylinder, and

2. a translucent backing roller disposed in opposed relationship to said cylinder, said backing roller being formed of an electrically conductive material, said cylinder and said backing roller forming a nip between which said photoconductive element and the receiver pass;

c. potential applying means connected to said backing roller for applying an electrical potential of a second and opposite polarity to the receiver to attract the toner image to the receiver; and

d. a source of radiation mounted within said backing roller for exposing said photoconductive element to actinic radiation during contact between the toner image and the receiver to reduce the forces of attraction between the toner particles and said photoconductive element.

5. Apparatus for transferring a toner image from a photoconductive element of the type having a photoconductive insulating layer overlying a substantially transparent conductive layer to a receiver, the toner image being formed of toner particles having an electrostatic charge of a first polarity thereon which create forces of attraction between the toner particles and the photoconductive element, said apparatus comprising:

a. charging means for applying an electrostatic charge of said first polarity to the toner image on said photoconductive element;

b. means for bringing the charged toner image on said photoconductive element into face to face relationship with the receiver along at least a line of contact, said means comprising:

1. a substantially transparent rotatable cylinder located to contact the side of said photoconductive element opposite the toner image, and

2. at least one electrically conductive backing roller disposed in opposed relationship to said cylinder for contacting the receiver and said cylinder, said cylinder and said backing roller forming a nip between which said photoconductive element and the receiver pass;

c. potential applying means connected to said backing roller for applying an electrical potential of a second and opposite polarity to the receiver during contact with the toner image to attract the toner image to the receiver; and

d. means for exposing said photoconductive element to actinic radiation during contact between the toner image and said receiver to reduce the forces of attraction between the toner particles and said photoconductive element, said means for exposing being located inside said cylinder.