Xerographic Image Transfer Apparatus

Abstract

A shield for preventing premature transfer of xerographic toner images from a photoconductive plate to a sheet of final support material, both of which are being moved at synchronous speeds through a transfer zone beneath a corona generating device. The shield, which is made up of a grounded plate having a thin dielectric coating thereover, is positioned within the generator ion stream to prevent the charged ions from reaching the support material prior to its being placed in contact with the photoconductive plate. This device is particularly well suited to prevent premature transfer of toner images in a machine environment wherein a xerographic drum having a relatively small diameter is utilized.

Description

This invention relates to apparatus for electrically transferring a xerographic toner image from an intermediate support surface onto a sheet of final support material.

More specifically, this invention relates to apparatus for electrically transferring the xerographic toner images from a xerographic drum having a relatively small diameter onto a substantially flat sheet of final support material. In the process of xerography, a latent electrostatic image of an original to be reproduced is created upon a photoconductive plate and the latent image then developed or made visible by applying thereto a specially prepared, oppositely charged finely divided toner material. The fine toner particles, when brought into close proximity with the latent image force field, are captured by the force field and held on the plate in image configuration thus rendering the latent image visible. Conventionally, a sheet of plain copy paper, or any other similar suitable support material having insulating properties, is placed in overlying contact with the image bearing plate surface and the charged toner particles are electrically transferred from the plate to the copy sheet. Normally, image transfer is accomplished by spraying the back side of the copy sheet with a corona discharge having a polarity opposite that of the charged toner while the sheet is in contact with the plate surface. Sufficient corona is deposited upon the sheet to electrically couple the sheet to the grounded xerographic plate. A force field acting normal to the sheet is thus created between the two contacting bodies which causes the toner particles to be electrically transferred from the plate to the copy sheet.

In order to preserve the integrity of the toner image created upon the plate surface, image transfer must be accomplished in a manner wherein little or no agitation of the images is produced. It has been found that it is extremely important, therefore, to avoid charging the final support material prior to its being moved into contact with the image recording plate. Bringing a prematurely charged support sheet towards an image bearing plate causes the toner to be momentarily lifted from the plate prior to contact. The airborne toner particles, being of a like charge, tend to move away from each other thus expanding the size of the image which eventually comes in contact with the support material. As a consequence, the image that is finally transferred to the sheet is enlarged and generally has an undesirable haloed appearance.

In most automatic xerographic copying machines, this type of image distress can be avoided by properly positioning the corona generator in respect to the contact region whereby the corona stream is applied to the sheet well after contact is established. This particular arrangement, however, calls for a relatively long contact zone being greater than the width of the corona stream, or extremely accurate positioning of the generator. In some cases, however, particularly in the case of compact copiers utilizing a xerographic drum having a relatively small radius, the transfer or contact zone is relatively small; smaller in fact than the corona stream generated by most conventional corona generating devices known and used in the art. As can be seen, in this situation, the corona stream extends beyond the boundaries of the contact region and premature charging of the support material can result.

It is therefore an object of the present invention to improve apparatus for electrically transferring xerographic toner images from a plate surface to a final support sheet.

It is a further object of this invention to provide an electrical image transfer device for use in a compact copier having a relatively restricted image transfer zone.

A still further object of the present invention is to prevent premature charging of a copy sheet as the sheet is entering a xerographic transfer station.

Yet another object of the present invention is to efficiently transfer xerographic toner images in a machine environment wherein the contact region between a receiving copy sheet and an image bearing xerographic plate is more confined than the corona stream utilized to effect transfer.

These and other objects of the present invention are attained by means of an image transfer device comprised of a sheet feeding apparatus arranged to bring a sheet of final support material into overlying contact with a moving xerographic plate, a corona generator mounted in close proximity to the plate surface adjacent to the region where the sheet of final support material contacts the plate, the generator being adapted to spray the back side of the sheet with a corona discharge to effect an electrical transfer of the toner images from the plate onto the sheet, and a grounded conductive baffle having a dielectric coating thereon mounted between the corona generator and the plate surface with the baffle extending into the corona stream and being positioned to prevent the stream from reaching the copy sheet prior to the sheet contacting the image bearing plate.

For a better understanding of the invention, as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings wherein:

FIG. 1 illustrates the image transfer apparatus of the present invention embodied in an automatic xerographic machine employing a reusable photoreceptor in the form of a relatively small diameter drum;

FIG. 2 is an enlarged partial view of the baffle used to prevent premature charging of the copy sheet.

As illustrated in FIG. 1, a relatively small diameter image recording drum 11 suitable for use in an automatic xerographic copier is arranged to rotate in the direction indicated about a horizontal support shaft 12 to transport the drum through a series of xerographic processing stations. The drum is comprised of a grounded substrate 13 over which is coated a photoconductive layer 14, such as selenium or the like. The photoconductive surface is conventionally charged initially to a given positive potential and then exposed to a light image of the original to be copied thereby recording the image information on the photoreceptor in the form of a latent electrostatic image. The latent image is next passed through a xerographic development station 15 wherein the latent image information is rendered visible by applying oppositely or negatively charged toner particles to the image areas. The toner image bearing drum surface is then transported through an image transfer station 10 wherein the image is transferred to a sheet of final support material in the manner described in greater detail below.

In operation, cut sheets of final support material 17 are forwarded through the transfer station in synchronous timed relation with the developed images transported therethrough upon xerographic drum surface. To accomplish the advancement of the final support sheets, there is herein provided a sheet forwarding mechanism 19 made up of a pair of cooperating advancing rolls 20, 21. In practice, the sheet of support material is normally separated from a supply stack (not shown) and properly registered with the moving drum surface prior to its being brought into the nip of the advancing roll assembly. The sheet is brought into the nip in engagement between the rolls and advanced at drum speed towards the transfer station. A guide plate 24 is positioned at the outlet side of the advancing roll assembly and serves to direct the moving sheet into the image transfer station.

Mounted within the transfer station are a pair of corona generators 30, 31 which are mounted in a side by side relationship as shown in FIG. 1. The first generator 30, in the direction of drum rotation, is a transfer generator which is herein employed to electrically transfer the toner particles from the drum surface to the copy sheet. The second generator, 31, is a sheet detacking generator which serves to reduce or neutralize any electrical forces generated during transfer which may tend to hold copy sheets onto the drum surface and thus effect the release of the sheets from the photoreceptor. For further information concerning the structure and operation of this type of generator, reference is had to copending U.S. application Ser. No. 200,253 filed Nov. 18, 1971.

Referring now more specifically to the transfer generator 30, this discharge device is formed of a U-shaped conductive shield 31 which extends longitudinally along the entire photoconductive drum surface. The open end of the U-shaped grounded shield 32 is arranged to face the drum surface adjacent to the region wherein the copy sheet is advanced into contact with the photoreceptor. Mounted within the shield is a corona generating wire 33 which is electrically connected to a DC source of power 34. In operation, an electrical potential is placed upon the wire which causes the wire to emit a positive stream of corona through the shield opening onto the back side of the copy sheet.

The grounded generator shield 32 acts as an electrical control in the generating system and also serves to focus a uniform stream of corona towards the copy sheet. The wire to shield spacing and the generator shield opening are related, among other things, to the power available to excite the wire. As disclosed by Vyverberg in U.S. Pat. No. 2,836,725, a corona device having a wire of about 0.0035 inches in diameter which has an input of about 8000 volts DC requires a shield opening of about 0.500 inches when operating under optimum conditions. However, as the generator opening is made more restrictive the power requirements of the generating system will increase drastically. In fact, a point is soon reached where the power requirements become prohibited in that the electrical demands will exceed the power available in most copying machines. In a machine environment, such as herein disclosed, where the diameter of the xerographic drum is small and, as a consequence, the contact zone between the drum and the flat copy sheet is limited, it is quite possible that the corona stream that can be produced to accomplish image transfer will be relatively larger, in comparison, to the contact region. As a result, there will exist an ever present danger that the corona stream will act outside the boundaries of the contact region and thus produce an unwanted premature charging of the copy sheet.

The apparatus of the present invention is provided with a control baffle 40 which is capable of both physically and electrically shielding the copy sheet from the corona stream until such time as the copy sheet is well seated in contact with the xerographic plate surface. In the present embodiment, the baffle forms a guide for directing the copy sheet into contact with the plate surface prior to its being exposed to the corona stream emitted by the generator 30. The baffle 40 is positioned adjacent to the drum surface in close proximity therewith so that the sheet of final support material, which is forwarded into the image transfer station by the advancing roll members, is guided into tangential moving contact with the moving photoreceptor surface prior to the sheets entering the corona stream.

As illustrated in FIG. 1, the baffle is comprised of an elongated body 41, pivotably mounted at one end upon pivot pin 42, and an extended arm 43 which projects horizontally between the generator shield and the photoreceptor surface. The location of the baffle is controlled by means of a locating pin 45 against which boss 46 on the baffle rests. When the baffle is seated against the locating pin, the extended arm 43 is physically positioned in the corona stream emitted by generator 30 and physically shields a portion of the plate from the corona stream.

As more clearly illustrated in FIG. 2, the upper surface of the baffle is provided with an upwardly protruding surface 48 which is arranged to engage the bottom surface of a copy sheet forwarded into the transfer station. The surface is adapted to direct the copy sheet moved thereover upwardly into contact with the photoconductive plate with the region shielded from the corona stream by arm 43 thus assuring that the sheet is well seated against the drum prior to being exposed to the transfer stream.

The baffle is made up of a conductive metal substrate 49 that is coated with a thin layer of dielectric material 50. The dielectric coating is sufficiently thin whereby the electrostatic charge directed thereupon by the transfer generator is felt internally by the substrate. The substrate is grounded and thus is capable of acting as a control device, in respect to the generator, to limit the amount of charge that will be accepted upon the dielectric coating. At the beginning of the corona generating operation, a predetermined amount of corona is accepted upon the baffle. Depending upon the thickness of the dielectric coating, any further corona directed at the baffle is repelled therefrom by the electrostatic field established thereon by the previously deposited corona.

The apparatus of the present invention was employed in an automatic xerographic system which included a xerographic drum having a diameter of about 3.3 inches. The contact zone between the copy sheet and the photoconductive surface varied from between one-eighth of an inch and three-sixteenths of an inch depending upon the weight of the support sheet in process. A corona generator similar to that disclosed by Vyverberg was positioned adjacent to the contact zone to accomplish a conventional electrical image transfer of flow images from the plate to the copy sheet. Initially the plate was charged to a positive potential of approximately 800 volts and the initial potential reduced in the background areas during exposure to a value of about 300 volts. The image was developed with a negatively charged toner material and transfer accomplished by means of a single wire generator consisting of a 3 mil. tungsten wire positioned about 0.250 inches above the photoreceptor surface adjusted to draw approximately +40 micro-amps DC bare plate current. The generator was positioned within a grounded shield having a one-half inch opening opposite the contact zone. A baffle similar to the one herein described was positioned at about 0.080 inches from the drum surface and brought under the shield so as to extend into the corona stream. The baffle was fabricated of a conductive metal substrate having a 0.003 to 0.005 inch coating of a dielectric material which was sprayed over the metal substrate. The baffle acted both as a physical and an electrical barrier to the corona stream to prevent premature charging of the copy sheet and eliminated image distress.

Although the baffle substrate is placed at a ground potential in the present embodiment, it is within the scope of the present invention to bias the substrate to some potential other than ground without departing from the teachings of the present invention. By biasing the substrate to some voltage between ground potential and the generator wire potential, the initial surge current that flows from the corona generator to the baffle can be minimized and the dielectric strength requirements of the baffle coating material can be considerably relaxed. It should be clear, however, that this bias voltage should be insufficient, by itself, to initiate premature transfer of the toner material carried on the xerographic plate surface.

While this invention has been described with reference to the structure disclosed herein, it is not necessarily confined to the details as set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims

What is claimed is:

1. In an image transfer apparatus including:

a photoconductive plate;

means for bringing a sheet of final support material into contact with said plate at a transfer region;

means for transferring a toner image from said plate to said sheet, said transferring means including a corona generator for generating a corona stream comprising corona discharge means extending across said plate and a shield partially enclosing said discharge means and having an opening facing said plate;

the improvement comprising;

a conductive baffle positioned between said plate and said generator, said baffle including means for guiding said sheet into contact with said plate prior to said sheet being exposed to said corona stream, a portion of said baffle extending partially between said opening and said plate.

2. The apparatus of claim 1 wherein said baffle is comprised of a conductive material which is placed at a ground potential.

3. The apparatus of claim 2 wherein the grounded conductive baffle is coated with a dielectric material to minimize current flow between the generator and said baffle.

4. The apparatus of claim 3 wherein the plate and the copy sheet are both moved through the transfer region at synchronous speeds.

5. The apparatus of claim 4 wherein the xerographic plate is a drum having a relatively small diameter.

6. The apparatus of claim 3 wherein the dielectric coating is between 0.003 and 0.005 inches thick.

7. The apparatus of claim 3 wherein said generator is adapted to produce a corona stream that is wider than the contact region between said sheet and said plate at said transfer region.

8. The apparatus of claim 1 wherein the baffle includes a conductive substrate biased to a potential between ground potential and the operating potential of the generator to minimize current flow therebetween.

9. The apparatus of claim 8 wherein the substrate is coated with a thin dielectric layer to further minimize current flow where the generator is in operation.

10. The apparatus of claim 1 wherein said shield has a U-shaped cross-section and said discharge means comprises a wire.

11. The apparatus of claim 10 wherein said baffle comprises an elongated body which is pivotably mounted, said baffle including an extended arm which projects between said shield and said plate, said baffle resting against a locating means.

12. The apparatus of claim 11 wherein said guiding means comprises an upwardly protruding surface of said baffle which is arranged to engage the bottom surface of said sheet, said surface being adapted to direct said sheet as it is moved thereover upwardly into contact with said plate at a region shielded from said corona stream by said arm.