Bias Roll Transfer

Abstract

A biasable transfer member is herein disclosed suitable for use in transferring xerographic images from a photoconductor to a final support sheet. The member is adapted to electrically cooperate with the photoconductor to establish a directional force field therebetween capable of attracting toner from the photoconductor toward the member and features a structure which provides for a more efficient transfer operation while at the same time accurately matching the speed of the support sheet to the photoconductor.

Description

This invention relates to xerography and, in particular, to apparatus for transferring xerographic toner images from one support surface to another.

In conventional xerography, a photosensitive plate, which consists of a photoconductive coating placed over a conductive backing, is charged uniformly and the charge plate then exposed to a light image of an original. Under the influence of the light image, the charge on the plate is selectively dissipated to record the original input scene information on the plate in the form of a latent electrostatic image. The latent image is developed, or made visible, by applying oppositely charged toner particles to the plate surface in a manner so that the toner particles are attracted into the imaged areas. The developed images are generally transferred from the photoconductor to a final support material, such as paper or the like, and affixed thereto to form a permanent record of the original.

Heretofore, image transfer was generally accomplished by means of corona induction using a corona generator similar to that disclosed by Vyverberg in U. S. Pat. No. 2,836,725. In corona induced transfer, the final support sheet is placed in direct contact with the toner image while the image is supported on the photoconductive surface. The back of the sheet, that is, the side away from the image, is sprayed with a corona discharge having a polarity opposite to that carried by the toner particle causing the toner to be electrostatically transferred to the sheet.

The Vyverberg generator, as attested to by its wide commercial acceptance, has proven to be an extremely reliable device for transferring a single toner image to a final support sheet. The term "single toner image" as herein used, is employed in the broad sense to define an image that is created by means of a single exposure and developing step and the image may include many separate and distinct pieces of information. However corona induced transfer does not lend itself readily for use in systems where a multiplicity of toner images must be sequentially transferred to a single support sheet as exemplified by many xerographic color and duplexing processes.

Biased roll transfer has been tried with some limited success as a means of controlling the forces acting on the toner during transfer. This type of transfer was first disclosed by Fitch in U. S. Pat. No. 2,807,233 and involved the use of a metal roll coated with a resilient coating having a resistivity of about 10.sup.6 to 10.sup.8 cm. Because of the resistivity of the coating, the amount of bias that can be applied to the roll is limited to relatively low operating values because, at the higher ranges, the air in and about the transfer zone begins to break down, i.e. ionizes, causing the image to be degradated during transfer. Shelffo in U. S. Pat. No. 3,520,604, suggests that the resilient coating have a resistivity of between 10.sup.11 -10.sup.16 ohms cm. Here, in order to give the roll the needed resiliency required in most practical applications, the coating must be relatively thick. A thick coating of high resistivity acts to build up a surface charge on the roll resulting in air breakdown in the transfer region and eventually copy degradation.

It is therefore an object of this invention to improve apparatus for electrically transferring a toner image from a photoconductive surface to a final support sheet.

A further object of this invention is to provide a device to better match the speed of the support sheet to that of the photoconductive surface on which is supported to toned image to be transferred.

A further object of this invention is to provide a device which would accurately register the support sheet to the toned image.

A further object of this invention is to provide a highly responsive device for transferring xerographic toner images under controlled conditions.

A further object of this invention is to improve apparatus by which a multiplicity of toner images can be efficiently transferred from a photoconductive surface to a single sheet of final support material.

These and other objects of the present invention are attained by means of a biased transfer member being capable of electrically cooperating with a conductive support surface to attract charge toner particles from the support surface towards the member, the member having a conductive substrate for supporting a biased potential thereon, an intermediate blanket placed in contact with the substrate having an electrical resistivity capable of readily transmitting the bias potential on the substrate to the outer periphery of the blanket and a relatively thin outer coating placed over the blanket having an electrical resistivity to minimize ionization of the atmosphere when the transferred member is placed in electrical cooperation with the image support surface and provides a good toner release property enabling the device to be cleaned of said toner.

For a better understanding 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 is a schematic view in partial section illustrating an automatic xerographic reproducing apparatus embodying the present invention;

FIG. 2 is a perspective view in partial section showing the construction of a transfer roll embodying the teachings of the present invention that is suitable for use in the apparatus illustrated in FIG. 1;

FIG. 3 is a partial sectional view showing a paper gripping mechanism associated with the transfer roll illustrated in FIG. 2 with the paper gripper extended in a paper stripping position.

Although the apparatus of the present invention has application in any number of xerographic devices in which a plurality of images are to be applied to a single sheet of support material, it will nevertheless be disclosed, for explanatory reasons, with reference to an automatic machine having a duplex capability and it should be clear that this particular machine environment is in no way intended to limit the present invention. Referring now specifically to FIG. 1, the apparatus of the present invention is shown embodied in a drum type automatic xerographic reproducing machine. The central element of the machine is a drum 10 which is mounted for rotation in the machine frame upon shaft 11 and the drum driven in the direction indicated by means of a motor (not shown). The drum basically comprises an outer surface 13 of a photoconductive insulating material such as vitreous selenium or the like that is placed upon a grounded conductive substrate 14.

A uniform electrostatic charge is placed on the photoconductive surface by means of a conventional corona charging device 15 similar to that disclosed by Vyverberg in the above noted U. S. patent. The uniformly charged surface is then moved past an exposure means, generally referenced 17, capable of exposing the charged surface to a flowing light image of the original input scene information to be reproduced thus forming a latent electrostatic image on the photoconductor. The optical system herein utilized is similar to that disclosed in U. S. Pat. No. 2,940,358 and is of the type wherein the original input scene information to be reproduced is stored as minified data upon a film input 18. A movable mirror system 19 is positioned in the optical light path and is arranged to redirect the flowing light image of the original onto the bottom portion of the drum surface. The mirror system comprises a plane mirror surface 20 and a roof mirror 21 which, in operation, are alternately interposed into the light path of the optical system so that each successive image presented to the drum surface is optically reversed. In operation, the optical means exposes the photoconductive drum to a first input wherein a right reading latent image is formulated thereon. Following the formation of the first right reading latent image, the image is moved past a conventional xerographic developing device 25 wherein the latent image is brought in contact with oppositely charged toner particles and the particles are attracted into the imaged areas thus making the image visible.

After development, the now visible first right reading image is transported on the drum to a transfer station 26 where the image is temporarily transferred to and stored in image configuration upon the surface of an intermediate biased transfer roll 30. The transfer roll is arranged to extend transversely across the photoconductive drum surface and to move in intimate contact therewith as shown in FIG. 1. In practice, the roll, which is initially placed at a relatively high bias potential, is arranged to coact electrically with the grounded photoconductive drum to establish an electrostatic force field in and about the contact region. The force field is of sufficient strength to attract the charged toner particles moving through this region from the photoconductive surface towards the transfer member.

Referring now more specifically to FIG. 2, there is shown a cut-away view of the transfer roll 30 clearly illustrating the internal construction thereof. The roll is basically formed upon a rigid hollow cylinder 31 that is fabricated of a conductive metal, such as aluminum or the like, capable of readily responding to a biasing potential placed thereon. Over the core is placed a relatively thick intermediate blanket 32 of elastomeric material having a hardness of between 15- 25 durometers. The intermediate blanket is preferably formed of a polyurethane rubber approximately 0.25 in thickness having sufficient resiliency to allow the roll to deform when brought into moving contact with the photoconductive drum surface to provide an extended contact region in which the toner particles can be transferred between the contacting bodies. The intermediate blanket should be capable of responding rapidly to the biasing potential to electrically impart the charge potential on the core to the outer extremities of the roll surface. The blanket therefore should have a resistivity of between 10.sup.9 and 10.sup.10 ohms cm. Over the intermediate blanket is placed a relatively thin outer coating 33 which is also formed of an elastomeric material being approximately 0.0025 in thickness and having a hardness in the 65-75 D durometer range. However, in order to minimize ionization of the atmosphere in and about the contact region, it is preferred that the outer coating have a resistivity of about 3.2 .times. 10.sup.14 ohms cm or, alternatively, in a range between 10.sup.13 and 10.sup.15 ohm cm. It is further preferred that the outer coating of the roll should be formulated of a material capable of providing a relatively smooth surface exhibiting relatively good mechanical release properties in respect to the toner materials employed. A polyurethane material manufactured by the duPont Company under the tradename "Adiprene" has been found to possess the heretofore mentioned desired properties and shows extremely good release characteristics in respect to most commercially available toners.

The transfer roll member is closed at both ends by means of a pair of dielectric end caps 35, 36 which serve to electrically isolate the transfer roll member from the supporting machine frame. Segmented shafts 37 are secured in both end caps and are mounted in coaxial alignment with the cylindrical core 31. The shafts, in turn, are journaled for rotation in the machine frame in bearing means provided (not shown) so that the outer surface of the roll continually moves through the transfer zone in contact with the photoconductive surface 13. A pulley 38, operatively connected to the machine's main drive system, is secured to one end of the shaft and causes the transfer roll to be rotated in predetermined timed relation with the moving photoconductive drum.

A commutator ring 40 (FIG. 1) is embedded in the end cap 35 and is arranged to pass through the cap and communicates electrically with the metal core 31. As shown in FIG. 1, a commutating brush 41, which is electrically connected to a suitable source of DC power 42 via electrical connector 43, is arranged to ride in contact with the outer surface of the commutator ring and provides a moving contact by which the conductive core is electrically connected to the biasing source. The transfer force field associated with the electrically isolated roll is dependent upon the establishment of a voltage contrast between the roll and the photoconductor. The strength of the field is proportional to the initial charge placed on the roll and inversely proportional to the distance between the two coacting bodies. Initially, the roll is biased to about 3,500 volts DC, the polarity of which is, of course, opposite to that of the charged toner particles, whereby the first image delivered into the transfer zone is transferred, in image configuration, from the photoconductor to the surface of the transfer roll.

Subsequent to the formation of the first right reading image upon the drum, a second area on the photoconductive surface thereof is again uniformly charged by means of the charging corotron 15. This second uniformly charged area is then moved past exposure means 17 causing the photoconductor to be exposed to a flowing light image composed of additional input scene information in a manner wherein a wrong reading latent electrostatic image is formulated on the drum surface. To produce this wrong reading latent electrostatic image, roof mirror 21 is interposed into the optical light path of the system replacing mirror 20. This wrong reading latent electrostatic image is then transported on the moving drum surface to the development station 23 and the second image is made visible in a manner similar to that herein described in reference to the first right reading image.

The time sequence of charging, exposing and developing the two oppositely reading images on the drum surface is controlled in predetermined timed relation by means of the machine control logic circuitry (not shown). Of course, the particular selection of the times sequence of operation may be dependent on many factors, such as the location of the various processing stations around the drum surface, the length and velocity of the image storage member relative to the size of the drum surface, and the like. For convenience of illustration, however, the circumference of the transfer roll is herein deemed to be one half that of the photoconductive drum. By rotating the two members at the same peripheral speed, the first image, which has been transferred to the transfer roll, is transported around the circular path of travel transcribed by the drum and will arrive at the transfer station at approximately the same time as the second developed image carried on the photoreceptor. Accordingly, the leading edges of the two images lying respectively on the storage drum and the xerographic drum surface, confront each other at approximately the same moment in time.

Prior to bringing the two toner images together within the contact region, a sheet of final support material, such as paper or the like, is secured to the transfer roll member in a manner wherein the sheet overlies the first image supported thereon. A sheet gripping device is provided within the roll to properly align and secure individual sheets of material to the roll surface. Basically, two sheet aligning and gripping assemblies 60 (FIG. 3) are supported in spaced parallel alignment within the roll to act upon the two leading edge corners of a support sheet that is brought into operative communication therewith. Each assembly is made up of two components, a lever arm component 63 and a lifting component 64 that are supported upon a common base plate 65 by means of a mounting bracket 66. The base plate is rigidly affixed to the interior walls of the roll and is arranged to move in unison with the roll member.

Mounting bracket 66 also serves to rotatably support a control shaft 67 which runs longitudinally across the interior of the roll and extends through the end cap 36 (FIG. 2). Each of the lever arm components are rotatably secured to bracket 66 by means of a pin 68 and the arms arranged to rotate freely in openings 69 provided in the roll surface. Lifting elements 64 are secured to the control shaft 67 and move in accordance therewith. A cam face 70 is machined on the lifting element and is forced to ride in contact with the bottom surface of the lever arm by means of a spring 71 so that the lifting arm is raised and lowered in response to the movement of the control shaft. The portion of the control shaft that extends beyond end cap 36 is provided with a cam followed 78 arranged to ride in contact with a profile surface 79 to impart a predetermined motion to the lever arm.

In operation, sheets of individual support material 82, preferably paper, are forwarded from a supply bin or the like into a sheet receiving station 83 (FIG. 1) by means of a feed roll assembly 27. At this time, tabs 84, (FIG. 3) on the lifting arm are elevated slightly above the roll surface and the leading edge of the sheet is driven into alignment against stop face 85. Once aligned, the tab element is caused to move downwardly thus locking the sheet against recess surface 87 on the roll. When in the sheet locking position, the entire gripping mechanism is sufficiently retracted below the roll surface to allow the roll to move freely through the contact or transfer zone.

With the support sheet 82 secured to the roll surface over the first image the first image is recirculated once again through the transfer or contact region in synchronization with the second image carried on the photoconductive drum. The introduction of an added dielectric, such as a paper sheet, into the contact region usually requires that the transfer bias be reduced in order to prevent air breakdown from occurring in and about the transfer region. To this end, the bias on the roll is reduced from about 3,500 to about 2,500 volts DC, the voltage being of the same polarity. Now, as the second image passes through the directional force field within the contact region, it is electrically transferred from the photoconductor to the outside surface of the sheet.

After transfer, the support sheet is moved on the roll surface to a region of low voltage contrast wherein there is positioned a corona generator 55 similar to that described in the previously noted Vyverberg patent. A region of low voltage contrast is a region wherein the bias roll surface does not electrically communicate with another voltage source. It should be understood, that the term "voltage source" as herein used is broad enough to include a grounded body or the like. The corona generator is adapted to apply corona to the top side of the support sheet of a polarity to cause the first image, that is, the image in contact with the roll surface, to be transferred from the roll to the bottom side of the support sheet. Alternately, the bias potential on the roll can be removed prior to the corona induced transfer step.

With the two toner images thus electrostatically adhering to opposite sides of the support sheet, the sheet is once again moved on the roll surface into the sheet receiving station 83. Here, the lever arm 63 of the gripping mechanism is moved to a fully extended position, as shown in FIG. 3, causing the sheet to be elevated well above the roll surface. As the roll continues to move in the direction indicated, the elevated sheet is carried over a stripping bar 89 which guides the sheet into a fuser assembly 90. The sheet is advanced through the fuser assembly by means of a transport 91 and the images are permanently fixed to both sides of the support sheet. The now duplexed copies are taken from the fuser and stored in a collecting tray 83.

Finally, the rotating photoconductive surface as it moves out of transfer station is brought into operative communication with a fibrous brush member 50 that is moving at a speed sufficient to dislodge any residual toner remaining on the drum surface after the transfer operation. The residual toner is collected and removed from the machine environment by means of conventional apparatus known and used in the art.

While this invention has been disclosed 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. A transfer member for electrically cooperating with a conductive support surface to electrically attract charge particles from the support surface towards the member including

a conductive substrate for supporting a uniform bias potential thereon,

an intermediate resilient blanket placed in contact with said substrate having an electrical resistivity such that the blanket is capable of transmitting said bias potential on said substrate to the outer periphery of said blanket, and

an outer coating placed over said blanket having an electrical resistivity to minimize ionization of the surrounding atmosphere when said transfer member is placed in electrical cooperation with said support surface.

2. The transfer member of claim 1 wherein said intermediate blanket is formed of an elastomeric material having a resistivity of between 10.sup.9 and 10.sup.10 ohms cm.

3. The transfer member of claim 2 wherein said outer coating is formed of an elastomeric material having a resistivity of between 10.sup.13 and 10.sup.15 ohms cm.

4. The transfer member of claim 3 wherein said substrate is formed of a conductive metal in the shape of an endless belt.

5. A transfer roll for electrically cooperating with a photoconductive plate when brought into contact therewith to attract charged toner particles from the plate toward the roll including

a rigid cylindrical core or electrically conductive material,

means to connect said core to a biasing source whereby a uniform biasing potential is placed upon said core,

a resilient intermediate blanket placed over said core, in contact therewith, having an electrical resistivity such that the blanket is capable of transmitting said bias potential from said conductive core to the outer periphery of said blanket,

an outer resilient coating placed over said blanket having a resistivity to minimize ionization of the surrounding atmosphere in and about the zone of contact between the transfer roll and the photoconductive plate.

6. The transfer roll of claim 5 wherein said intermediate blanket is formed of a polyurethane material having a resistivity of between 10.sup.9 and 10.sup.10 ohms cm.

7. The transfer roll of claim 6 wherein said outer coating is formed of a polyurethane material having a resistivity of between 10.sup.13 and 10.sup.15 ohms cm.

8. The transfer roll of claim 7 further including dielectric end caps for rotatably supporting and electrically isolating said transfer roll.

9. The apparatus of claim 8 further including sheet gripping means to secure a sheet of final support material to the outer surface of said transfer roll.

10. The apparatus of claim 7 wherein said intermediate blanket has a hardness of between 15 and 25 durometers, Shore A.

11. The apparatus of claim 10 wherein said outer coating has a hardness of between 65 and 75 durometers, Shore D.