Transfer Apparatus

Abstract

An apparatus and method for efficiently transferring a toner image from a rotatable electrostatic drum to a copy sheet while the copy sheet remains in a substantially flat position without degrading the quality of the latent image on the drum by imposing an electrical field between the copy sheet and drum as the sheet moves away from the drum surface after the toner image has been transferred to the copy sheet. In one embodiment of the invention, a movable, endless, two-layer belt having an inner layer of conductive material and an outer layer of relatively nonconductive material carries the copy sheet through the transfer station in a plane substantially tangent to the surface of the drum. An electrical bias is applied to the inner layer of the belt which attracts toner from the electrostatic drum to the copy sheet, but charge is prevented from reaching the copy sheet and drum by the relatively nonconductive layer of the belt. Small holes can be made in the belt and a vacuum placed on the copy sheet through the holes to insure there is no relative motion between the copy sheet and belt during the transfer step. In a second embodiment of the invention, a movable, endless, relatively nonconducting belt carries the copy sheet through the transfer station in a substantially flat position. Directly under the belt, adjacent the transfer area and that area through which the copy sheet travels as it leaves the drum surface after transfer has taken place, is a stationary conductive means having an electrical bias thereon. In a third embodiment, the stationary conductive means is constructed so that the electrical bias placed on it can be increased as a given portion of the copy sheet increases its distance from the surface of the drum.

Description

BACKGROUND OF THE INVENTION

This invention relates to electrostatic transfer apparatus and, more particularly, to a transfer apparatus having a movable belt which carries a copy sheet through the transfer station adjacent an electrostatic drum and means to maintain an electrical field between the drum and copy sheet in the transfer area and that area adjacent the transfer area through which the copy sheet passes as it moves away from the drum surface.

In reproduction processes, such as in the process of xerography, wherein a latent image is first formed on an electrostatic member and then made visible, or developed, with a powderous material, the electrostatic member can be used repeatedly to form additional powder images in successive cycles if the developed image is transferred from it to another substrate such as a copy sheet during each cycle. When the xerographic process is employed, the electrostatic member can take the form of a photoconductive layer over a conductive backing material. The photoconductive layer is given a uniform electrostatic charge and then exposed to a light image conforming to the information to be reproduced to form a latent electrostatic image on the member. The member is then developed with a finely divided, pigmented, electroscopic, resinous powder called toner and the toner image is then electrostatically transferred to a copy sheet. After transfer is complete, the surface of the electrostatic member can be cleaned and then used for another latent image which is developed with toner and the toner image transferred in a similar manner.

If more than one copy of given information is to be made by the process described above, the latent image formation steps can be eliminated by using an electrostatic member having good latent image preservation characteristics. In this mode of operation, a latent image is formed on the electrostatic member by placing a uniform charge on it and then exposing it to a light image of the information to be reproduced during the formation of the first copy. The latent image is then developed, the toner image transferred from the electrostatic member to a copy sheet and the surface of the electrostatic member cleaned. Then, when the second copy and all succeeding copies are to be made, the charging and exposing steps can be eliminated since the latent image retention qualities of the electrostatic member maintain the latent image intact. As a result, in the image preservation mode of operation, the second and all subsequent copy cycles include only the steps of developing the latent image, transferring the toner image from the electrostatic member to a copy sheet and cleaning the electrostatic member. It is obvious that this type of reproduction mode is faster than the more conventional xerographic process described beforehand since two steps, the steps of charging and exposing, are eliminated during making of all copies after the first copy.

When a reproducing system is used in the image preservation mode, special attention must be given to the technique and apparatus utilized in transferring the toner image from the electrostatic member to the copy sheet to assure that the latent image on the member does not deteriorate. A common apparatus used for the transfer step is a corona charging device. This device is widely used in electrostatic copiers, but has serious disadvantages when a copier functions in the image preservation mode. The non-image device tends to electrostatically tack the copy sheet to the electrostatic member during transfer since the copy sheet acquires a charge during the transfer step. In addition, when the copy sheet is stripped from the electrostatic member after transfer has taken place, charges on the copy sheet tend to be conducted to nonimage areas of the member due to air breakdown between it and the copy sheet. Both types of charging either add or subtract charge from the electrostatic member leaving a distorted latent image thereon which is reflected in subsequent copies. When such charges are placed in the non-image areas of the electrostatic member, they are consequently developed with toner in succeeding cycles and result in background in all subsequent copies. This situation is undesirable since distorted toner images and background toner cause noticeable degradation to copy quality.

Corona charging devices are also sensitive to humidity and altitude and under certain conditions transfer efficiency; i.e., the amount of toner transferred to the copy sheet, is considerably lessened. In some cases a large part of the toner image is prevented from transferring to the copy sheet and the resulting reproduction becomes unintelligible. Corona devices normally require a high-electrical potential; e.g., in the order of 4,000 volts, to effect good transfer even under the most favorable operating conditions, and this necessitates the use of a large power supply in the copier. In addition, since a copy sheet is tacked to the electrostatic member with the aid of a corona device when transfer takes place, sheet removal apparatus is necessary to strip the copy sheet from the member.

Another transfer device used with better success than the corona device in the image preservation mode is a biased roller. This device consists of a rotatable conductive core having a relatively nonconductive surface layer. Although adequate transfer occurs when approximately 2,000 volts is placed on the roller, the roller does not efficiently effect transfer of the toner image at significantly lower voltages due to back transfer. At low voltages toner which is transferred to the copy sheet at the transfer area is retransferred back to the photosensitive member when the copy sheet is separated from the member.

Transfer efficiency can be increased by imposing a higher electrical bias on the roller, however, a higher bias will cause air breakdown as the copy sheet is removed from the electrostatic member. This condition results in charging the nonimage areas of the electrostatic member which distorts the quality of the latent electrostatic image on the member and renders it undesirable for use in the image preservation mode of operation. There has been no value found for the electrical bias placed on a transfer roller at which both poor transfer efficiency and background charging are eliminated.

The present invention is a transfer apparatus having a movable belt which carries the copy sheet through the transfer station in a substantially flat position while an electrical field is maintained between the copy sheet and electrostatic member in the transfer area and the adjacent area through which the copy sheet passes as it is separated from the surface of the member. Reliable, efficient transfer takes place with the present device when a potential in the vicinity of 1,000 volts is placed between the copy sheet and electrostatic member, and the problems of charging of the electrostatic member and stripping the copy sheet from the member encountered in the prior art are avoided. In addition, the copy sheet is removed from the transfer area in a plane tangent to the surface of the electrostatic member which eliminates the undesirable condition of back transfer. The present apparatus operates at relatively low transfer voltages in a wide range of environmental conditions and is particularly adaptable to copiers operating in the image preservation mode.

Accordingly, it is an object of the invention to improve apparatus for transferring toner images from an electrostatic member to a copy sheet.

It is a further object of the invention to improve transfer apparatus so that degradation of the latent image on an electrostatic member is prevented during the transfer step and the latent image remains suitable to be reused to form another toner image.

It is a further object of the invention to improve transfer apparatus so that back transfer of the toner from the copy sheet to the electrostatic member is prevented.

It is a further object of the invention to improve transfer apparatus so that transfer of toner images from an electrostatic member to a copy sheet can be carried out efficiently at relatively lower voltages than known previously.

It is a further object of the invention to improve transfer apparatus so that transfer of a toner image is carried out in a uniform, efficient manner regardless of relative humidity and altitude.

It is a further object of the invention to improve transfer apparatus by utilizing a belt to transport the copy sheet adjacent an electrostatic member in the shape of a drum in a plane substantially tangent to the surface of the drum.

SUMMARY

The present invention is a transfer apparatus comprising a movable belt which carries a copy sheet, or the like, through a transfer station of an electrostatic copier having an electrostatic drum in a substantially flat position and a means to impose an electrical field between the copy sheet and drum in the transfer area and that area adjacent the transfer area through which the copy sheet passes as it is removed from the drum surface. In one embodiment of the invention, a movable, endless belt having an inner layer of conductive material which is electrically biased and an outer layer of relatively nonconductive material is used to carry out transfer of a toner image from an electrostatic drum to a copy sheet. The belt, which is supported and driven by rollers, carrier the copy sheet in a plane that is tangent to the surface of the drum while a toner image on the drum is transferred under the electrical potential placed on the conductive layer of the belt. The belt contains a plurality of holes through which a vacuum can be placed on the copy sheet to prevent relative movement between the copy sheet and belt while being transported through the transfer station.

In a second embodiment, a belt of relatively nonconducting material is used to carry the belt through the transfer station while an electrical field is created by a stationary conducting means adjacent the belt extending from the transfer area to the area adjacent the transfer area through which the copy sheet passes as it is being removed from the drum surface. In a third embodiment, the stationary conducting means is constructed so that an increasing electrical potential influences the copy sheet as it increases its distance from the drum surface after transfer of the toner image has occurred.

BRIEF DESCRIPTION OF THE DRAWINGS

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 used in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of the invention.

FIG. 2 is a detailed view of a first embodiment of the invention.

FIG. 3 is a detailed view of a second embodiment of the invention.

FIG. 4 is a detailed view of a third embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention can be adapted to any reproduction apparatus wherein an electroscopic, pigmented powder used to develop an electrostatic latent image on an electrostatic member is transferred from the member to a copy sheet. For the purpose of this disclosure, however, the invention will be described within the environment of a xerographic reproduction apparatus.

Referring to FIG. 1, there is shown a continuous xerographic copier having a photosensitive member in the shape of drum 13 on which a latent electrostatic image of the information to be reproduced is formed. The rotatable drum 13 is driven by shaft 14 by any suitable drive means (not shown). The peripheral surface of the drum is covered by layer 12, an electrically conductive material, which, in turn, is covered on its outer surface with layer 11, a conducting material such as vitreous selenium. The drum has five processing stations located about its periphery which carry out the steps of the xerographic process. These stations include charging station A, exposing station B, developing station C, transfer station D, and cleaning station E.

A latent electrostatic image is formed on the drum by passing its surface through charging station A and exposing station B. The charging station includes any suitable means for placing a uniform charge on layer 11 such as a corona charging device. Exposing station B can include any suitable device which projects and focuses a light pattern on the drum conforming to the image to be reproduced by the xerographic system. The light image projected onto the charge conductive layer of the drum is synchronized with the movement of the drum and causes selective charge dissipation on elemental areas of layer 11 to form a latent electrostatic image thereon.

After the formation of the latent electrostatic image by passing the drum through stations A and B, the drum carrier the latent image to developing station a pigmented, resinous, electroscopic powder called toner is deposited on the drum in imagewise configuration in any suitable manner to develop, or make visible, the latent image. Following the development step, the drum carrier the toner image through a transfer station D where the toner image is transferred from the drum surface to any suitable support material such as copy sheet 15 which can be made of paper. In addition to sheets, a continuous web or any other form of substrate may be used to receive the toner image.

Transfer of the toner image onto the copy sheet is carried out by belt 20. The copy sheet 15 is fed onto the surface of the belt by any suitable device such as feeding device 30 which may include tray 31 to hold a supply of copy sheets and feed roller 32 which feeds the sheets one at a time as needed onto the belt. The belt 20 has an inner layer 22 of any suitable conductive material and an outer layer 21 of any suitable relatively nonconductive material. The belt is supported by rollers 23 and 24 and driven in the clockwise direction by motor M. An electrical potential is placed on layer 22 through roller 24 which is also made of a conductive material. The belt may contain a plurality of holes, if desired, shown only in FIG. 2, through which a vacuum can be imposed on the copy sheet 15 by vacuum chamber 14.

A sheet is fed onto the belt by feeding device 30 and is carried through transfer station D adjacent photosensitive drum 10. As it passes through the transfer station, a toner image on the drum surface adjacent the copy sheet is transferred to the copy sheet due to the electrical field created between the copy sheet and drum by the bias on layer 22 of the belt.

After the toner image have been transferred to the copy sheet and the copy sheet has passed through the transfer station, the copy sheet passes onto belt 40 which is supported by rollers 41 and 42 and driven by motor M. Any suitable fixing device 42 makes the toner image permanent on the copy sheet. The copy sheet then moves off the belt 40 as the belt turns about roller 41 and falls into collection device 50 where it is stored in tray 31.

The final station shown in the drawing is cleaning station E which can include any suitable cleaning device such as a fur brush which contacts the photoconductive surface of the drum. The cleaning station is utilized to remove any residue toner particles from the photosensitive surface after transfer occurs and before another cycle is begun. It is intended that the various moving elements mentioned above be driven by any suitable means to allow the copier to function as described; for instance, a single motor such as motor M can drive drum 10 and the other moving elements in the copier as well as the belts 20 and 40.

It is within the confines of the transfer station D that the present invention is utilized. FIG. 2 showns a more detailed view of the transfer station, but it should be kept in mind that the various elements shown in FIG. 2 are not necessarily to scale. Belt 20 has an inner layer 22 of any suitable conductive material; e.g., a flexible steel band, and an outer layer 21 of any suitable relatively nonconductive material; e.g., a dielectric material. The belt moves in a clockwise direction around rollers 23 and 24 while the drum rotates in a counterclockwise direction bringing successive portions of the developed image, shown by grossly enlarged toner particles on the surface of the drum, through the transfer station, at the same speed as a copy sheet carried by the belt.

The belt contains a plurality of holes 16 through which a vacuum can be imposed on the copy sheet to prevent it from slipping. The holes in the belt and the chamber 14 which supplies the vacuum are optional to the transfer belt assembly, especially if the transfer station is located as shown in FIG. 2 and the copy sheet is conveyed in a substantially horizontal plane. However, if the transfer station were so that the copy sheet had to be conveyed in a plane other than a horizontal plane, the vacuum force of the copy sheet would be desirable to prevent relative movement between the copy sheet and belt.

The belt is driven at a speed which is substantially equal to the surface speed of the drum and a suitable electrical potential is placed on layer 22 to effect transfer of the toner image from the drum to the copy sheet. The dielectric material actually used in testing the invention was a 0.002 inch thick polyester resin material and the conductive material was a rubber which was impregnated with graphite. Using these materials, it was found that transfer was carried out efficiently when the potential on the belt was between 500 volts and 1,500 volts. The polarity of the potential placed on the inner layer of the belt is determined by the polarity of the toner particles to be transferred. For instance, if the toner particles bear a negative charge, as indicated in FIG. 2, a positive potential would be placed on the belt. If, on the other hand, the toner particles were to bear a positive charge, a negative potential would be placed on the layer 22 of the belt.

The proximity of the belt relative to the surface of the drum is depended upon the thickness of the copy sheet. In FIG. 2, the belt is located so that the top surface of the copy sheet is adjacent the surface of the belt, however, the copy sheet can be placed into intimate contact with the drum surface during the transfer step also. Although the copy sheet is shown as an individual sheet, it can take any other suitable form such as a continuous web and can be made of any suitable material such as paper, plastic, fabric, etc.

The path of the copy sheet through the transfer station is in a plane which is substantially tangent to the surface of the drum. This arrangement has been found to be the most preferrable to prevent back transfer of toner to the drum as the copy sheet is separated from the drum. Since the potential placed on the inner layer of the belt is relatively low and the copy sheet is conveyed in a path that is tangent to the surface of the drum, uncontrolled charging of the drum due to conduction through the belt and copy sheet is prevented. Both of these aspects, low potential and tangentral path, enhance the value of the present system when it is utilized in a reproduction system which is operated in the image preservation mode, and the quality of succeeding copies is always maintained at a high level.

Another embodiment of the invention is seen in FIG. 3. In this apparatus the belt 31 which carries the copy sheet 15 through the transfer station is made of a relatively, nonconductive material and the electrical field imposed in the transfer area to effect transfer of the toner image from the electrostatic member to the copy sheet and the area adjacent the transfer area through which the copy sheet travels as it is separated from the drum is generated by a stationary plate 30. Plate 30 is located adjacent the path of belt 31 and, in addition to being next to the transfer area; i.e., where the belt is closest to the surface of the drum and the toner image is transferred to the copy sheet, extends to the right of the transfer area.

As the copy sheet exits from the transfer area, it is separated from the photoconductive layer 11 of the drum and continues in the same plane as belt 15. During the time that the copy sheet is being separated from the drum, a field is maintained between the copy sheet and drum to prevent back transfer of toner on the copy sheet. Plate 30 is made of a conducting material and is biased by a suitable power supply shown schematically in FIG. 3. The plate is separated at all times from the copy sheet 15 by belt 31 to prevent any conduction of charge to the copy sheet and/or drum during the time that the transfer of the toner image is being carried out or the time the copy sheet is being separated from the drum. The plate 30 also serves to support the belt 31 and hence the copy sheet in a plane tangent to the drum while the two are within the transfer station.

FIG. 4 shows another embodiment of the invention. When the electrical field between the copy sheet and drum is stepped up as a given portion of the copy sheet is increasingly separated from the drum, back transfer is further eliminated. The apparatus shown in FIG. 4 accomplishes this result. Segmented plate 30 has both conducting portions 35a-35f and relatively nonconducting portions 34a-34e which extend across the belt 31 parallel to the axis of the drum.

If an appropriate bias is placed on conductive section 35a of the plate 30 to carry out transfer of the toner image from the drum to the copy sheet, then the potential placed on conductive sections 35b-35f can be stepped up in successive degree to intensify the fields as the copy sheet is increasingly separated from the drum to eliminate back transfer. Of course, relatively nonconductive sections 34a-34e will remain electrically inert. Any suitable control apparatus 36 can be used to step the voltages to the various conductive sections, the apparatus being joined to each conductive section by wire 37.

As an example of the stepping technique, section 35a can have a voltage of 1,000 volts to carry out the transfer of the toner image to the copy sheet in the transfer area. Then, conductive sections 35b, 35c, 35d, 35e, and 35f can have voltages of 1,800 volts, 2,600 volts, 3,200 volts, 4,000 volts and 4,800 volts, respectively. It is pointed out at this juncture, that the voltages mentioned immediately above are illustrative only, and it is not intended that the stepping technique be limited to these values or the difference between the values. Ultimately, the actual values of the various sections will depend on the diameter of the drum, width of the conductive sections, width of the copy sheet, relative humidity and other characteristics of the system.

In addition to the apparatus outlined above, many other modifications and/or additions to this invention will be readily apparent to those skilled in the art upon reading this disclosure, and these are intended to be encompassed within the invention disclosed and claimed herein.

Claims

What is claimed is:

1. In a reproduction system having a rotatable electrostatic drum, means to rotate the drum at a predetermined speed, means to form a toner image on the surface of the drum with toner particles having a predetermined polarity, and means to transfer the toner image from the surface of the drum to a copy sheet, or the like, at a transfer station, a transfer apparatus comprising:

a. an endless, flexible, movable belt of relatively nonconducting material;

b. means to support the belt in a substantially flat plane in the transfer station;

c. means to move successive portions of the belt through the transfer station at a predetermined speed;

d. means to load the copy sheet on the belt so that the copy sheet is carried through the transfer station by the belt in registration with a toner image on the drum;

e. first conducting means located in the transfer station adjacent the belt on the side thereof opposite the drum;

f. means to impose a first electrical potential on the first conducting means having a polarity opposite to the predetermined polarity on the toner particles adapted to transfer a toner image from the surface of the drum to the copy sheet;

g. second conducting means located adjacent the belt on the side thereof opposite the drum in the area where the copy sheet is separated from the surface of the drum after a toner image is transferred to the copy sheet; and

h. means to concurrently impose a plurality of different potentials on the conducting means having a polarity opposite to the predetermined polarity on the toner particles, each different potential in said plurality being of greater value than first potential and increasing in value in the direction towards which the belt moves, whereby the retransfer of toner from the copy sheet to the drum is prevented as the copy sheet is separated from the surface of the drum.

2. The apparatus in claim 1 wherein the second conducting means is a plate having sections of conducting and nonconducting material alternately arranged and each conducting section has one of the different potentials in said plurality of potentials imposed thereon.

3. In an automatic reproducing apparatus having a movable image retaining member, means to move said member at a predetermined rate, means to formulate a charged toner image upon said member, and means to transfer the toner image from said member to a sheet of final support material within a transfer zone, the transfer apparatus including

an endless belt having at least an outer coating thereon formulated of a relatively nonconductive material,

means to support a portion of said belt in a substantially flat position adjacent to said moving image retaining member within the transfer zone,

means to move said belt over an endless path of travel at a rate substantially equal to said predetermined rate,

means to secure a sheet of final support material upon the belt whereby the sheet is transported through the transfer zone in registration with a toner image on said image retaining member,

electrical means positioned within the transfer zone to electrically transfer the toner image from said image retaining member onto said support sheet, and

control means to create a regulated electrical force field within a sheet separating region located subsequent to said transfer zone, in the direction of sheet movement, said force field being of a strength and polarity to hold the toner image to said sheet as said sheet is separated from said member thereby preventing the toner image from being repelled away from said sheet back toward said image retaining member.

4. The apparatus of claim 3 wherein said means to secure a sheet to said belt is adapted to place said sheet in moving contact with said image retaining member within said transfer zone.

5. An automatic xerographic reproducing device of the type having a rotatable xerographic plate arranged to move at a predetermined rate through a circular path of travel, means to formulate a charged toner powder image of an original upon said plate, means to transfer the toner image to a sheet of final support material within a transfer zone, the transfer apparatus further including

a movable belt formulated of a relatively nonconductive material being arranged to move at said predetermined rate over an endless path of travel through said transfer zone, that portion of said belt moving through the transfer zone being supported in a substantially flat plane that is adjacent to and substantially tangential with said xerographic plate within the transfer zone,

means to secure a sheet of final support material to at least the flat portion of said belt wherein the sheet is transported through the transfer zone in registered synchronous moving contact with a toner image on said xerographic plate,

electrical means positioned within the transfer zone to electrically transfer the toner image from said plate onto said support sheet, and

control means to create a regulated electrical force field within a separation region located subsequent to said transfer zone in the direction of belt movement said force field being of a strength and polarity to retain the toner image on said sheet as said sheet is separated from said plate.

6. The apparatus of claim 5 wherein said regulated force field increases as the separation distance between the sheet and the plate increases.

7. The apparatus of claim 5 wherein a means to secure said sheet to said belt comprises a vacuum means operatively associated with said belt and being arranged to hold said sheet to the flat portion of said belt wherein the sheet is maintained in a relatively flat condition as it is moved through the transfer zone.