Apparatus for developing a latent image

This invention relates generally to a development apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to an apparatus and method for loading dry Xerographic toner onto a donor member and developing a latent electrostatic image.

BACKGROUND OF THE INVENTION

Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two component and single component developer materials are commonly used for development. A typical two component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface, the toner powder image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.

The electrophotographic marking process given above can be modified to produce color images. One color electrophotographic marking process, called image on image processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While image on image process is beneficial, it has several problems. For example, when recharging the photoreceptor in preparation for creating another color toner powder image, it is important to level the voltages between the previously toned and the untoned areas of the photoreceptor.

In the application of the toner to the latent electrostatic images contained on the charge-retentive surface, it is necessary to transport the toner from a developer housing to the surface. A basic limitation of conventional xerographic development systems, including both magnetic brush and single component, is the inability to deliver toner (i.e. charged pigment) to the latent images without creating large adhesive forces between the toner and the conveyor, which transport the toner to latent images. As will be appreciated, large fluctuation (i.e. noise) in the adhesive forces that cause the pigment to tenaciously adhere to the carrier, severely limit the sensitivity of the developer system, thereby necessitating higher contrast voltages forming the images. Accordingly, it is desirable to reduce such noise, particularly in connection with latent images formed by contrasting voltages.

Fluidized beds have been used to provide a means for storing, mixing and transporting toner in certain single component development systems and loading onto developer rolls. Efficient means for fluidizing toner and charging the particles within the fluidized bed are disclosed in U.S. Pat. No. 4,777,106 and U.S. Pat. No. 5,532,100, which are hereby incorporated by reference. In these disclosures, corona devices are embedded in the fluidized toner for simultaneous toner charging and deposition onto a receiver roll. While the development system as described has been found satisfactory in some development applications, it leaves something to be desired in the way applications requiring the blending of two or more dry powder toners to achieve custom color development. Also, it has been found in the above systems that there are frequently disturbances to the flow in the fluidized bed associated with charged particles in the high electric fields surrounding corona devices immersed in the reservoir. Also, wire contamination presents a reliability issue.

Triboelectric charging (contact electrification) of dry toners is a standard method used to electrically charge toner particles for development of latent electrostatic images. An alternate method to charge toners is via ion bombardment (Ion Charging) which offers many advantages, especially in applications to custom color where “in-situ” toner mixing is advantageous. Triboelectric charging of colored toners requires different additives dependent on toner color to achieve stable charging, whereas ion charging of toners offers the advantage of charging toner particles based mainly on their size, independent of their intrinsic composition and surface structure. Triboelectric charging of toners also can create localized patches of charge on the toner particles which can lead to strong adhesion of these toners to various surfaces requiring special measures to remove them in the development, transfer and cleaning steps in the xerographic process. In the ion charging process, charged ions bombarding the toner particles are driven by the net field around the particles which tends to uniformly charge the toner, helping to decrease adhesion of these toners to donor or photoreceptor surfaces. One method to charge toner via ion bombardment involves fluidizing the toner and charging it using corona generation in close proximity to this fluidized bed.

However, noting the issues above the achievement of high reliability and simple, economic manufacturability of the system continue to present problems.

SUMMARY OF THE INVENTION

Briefly, the present invention obviates the problems noted above by utilizing an apparatus for loading and charging toner and developing an image. An apparatus for developing a latent image recorded on an imaging surface, including a housing, defining a chamber, storing a supply of toner; a donor member, spaced from the imaging surface, for transporting developer material on the surface thereof, to a region opposed from the imaging surface, a transport member for transporting toner from said housing and loading toner onto said donor member, said transport member further includes a surface with uniformly distributed, high density fringe fields, to pickup and transport a toner layer and a corona device to ionicly charge the toner on said fringe field surface.

In accordance with another aspect of this invention, the transport member further includes a mask layer adjacent to a photoconductive layer, said mask layer having a screen pattern; a charging device for charging said photoconductive layer and an exposure device for discharging unmasked portions of said transport member thereby generating said fringe field on the surface of the transport member to attract toner from said housing to the surface of the transport member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a schematic elevational view of an illustrative electrophotographic printing or imaging machine or apparatus, incorporating a development apparatus having the features of the present invention therein;

FIG. 2

shows a typical voltage profile of an image area in the electrophotographic printing machines illustrated in

FIG. 1

after that image area has been charged.

FIG. 3

shows a typical voltage profile of the image area after being exposed;

FIG. 4

shows a typical voltage profile of the image area after being developed;

FIG. 5

shows a typical voltage profile of the image area after being recharged by a first recharging device;

FIG. 6

shows a typical voltage profile of the image area after being recharged by a second recharging device;

FIG. 7

shows a typical voltage profile of the image area after being exposed for a second time;

FIG. 8

is a schematic elevational view showing the development apparatus used in the

FIG. 1

printing machine;

Inasmuch as the art of electrophotographic printing is well known, the various processing stations employed in the printing machine will be shown hereinafter schematically and their operation described briefly with reference thereto.

DETAILED DESCRIPTION OF THE INVENTION

Referring initially to

FIG. 1

, there is shown an illustrative electrophotographic machine having incorporated therein the development apparatus of the present invention. An electrophotographic printing machine creates a color image in a single pass through the machine and incorporates the features of the present invention. The printing machine uses a charge retentive surface in the form of an Active Matrix (AMAT) photoreceptor belt

10

which travels sequentially through various process stations in the direction indicated by the arrow

12

. Belt travel is brought about by mounting the belt about a drive roller

14

and two tension rollers

16

and

18

and then rotating the drive roller

14

via a drive motor

20

.

As the photoreceptor belt moves, each part of it passes through each of the subsequently described process stations. For convenience, a single section of the photoreceptor belt, referred to as the image area, is identified. The image area is that part of the photoreceptor belt which is to receive the toner powder images that, after being transferred to a substrate, produce the final image. While the photoreceptor belt may have numerous image areas, since each image area is processed in the same way, a description of the typical processing of one image area suffices to fully explain the operation of the printing machine.

As the photoreceptor belt

10

moves, the image area passes through a charging station A. At charging station A, a corona generating device, indicated generally by the reference numeral

22

, charges the image area to a relatively high and substantially uniform potential.

FIG. 2

illustrates a typical voltage profile

68

of an image area after that image area has left the charging station A. As shown, the image area has a uniform potential of about −500 volts. In practice, this is accomplished by charging the image area slightly more negative than −500 volts so that any resulting dark decay reduces the voltage to the desired −500 volts. While

FIG. 2

shows the image area as being negatively charged, it could be positively charged if the charge levels and polarities of the toners, recharging devices, photoreceptor, and other relevant regions or devices are appropriately charged.

After passing through the charging station A, the now charged image area passes through a first exposure station B. At exposure station B, the charged image area is exposed to light which illuminates the image area with a light representation of a first color (say black) image. That light representation discharges some parts of the image area so as to create an electrostatic latent image. While the illustrated embodiment uses a laser-based output scanning device

24

as a light source, it is to be understood that other light sources, for example an LED printbar, can also be used with the principles of the present invention.

FIG. 3

shows typical voltage levels, the levels

72

and

74

, which might exist on the image area after exposure. The voltage level

72

, about −500 volts, exists on those parts of the image area which were not illuminated, while the voltage level

74

, about −50 volts, exists on those parts which were illuminated. Thus after exposure, the image area has a voltage profile comprised of relative high and low voltages.

After passing through the first exposure station B, the now exposed image area passes through a first development station C which is identical in structure with development station E, G, and I. The first development station C deposits a first color, say black, of negatively charged toner

31

onto the image area. That toner is attracted to the less negative sections of the image area and repelled by the more negative sections. The result is a first toner powder image on the image area. It should be understood that one could also use positively charged toner if the exposed and unexposed areas of the photoreceptor are interchanged, or if the charging polarity of the photoreceptor is made positive.

The first development station C includes a donor roll. As illustrated in

FIG. 8

, electrode grid

42

is electrically biased with an AC voltage relative to donor roll

40

for the purpose of detaching toner therefrom. This detached toner forms a toner powder cloud in the gap between the donor roll and photoconductive surface. Both electrode grid

42

and donor roll

40

are biased with DC sources

102

and

92

respectively for discharge area development (DAD). The discharged photoreceptor image attracts toner particles from the toner powder cloud to form a toner powder image thereon.

FIG. 4

shows the voltages on the image area after the image area passes through the first development station C. Toner

76

(which generally represents any color of toner) adheres to the illuminated image area. This causes the voltage in the illuminated area to increase to, for example, about −200 volts, as represented by the solid line

78

. The unilluminated parts of the image area remain at about the level −500.

Referring back to

FIG. 1

, after passing through the first development station C, the now exposed and toned image area passes to a first recharging station D. The recharging station D is comprised of two corona recharging devices, a first recharging device

36

and a second recharging device

37

. These devices act together to recharge the voltage levels of both the toned and untoned parts of the image area to a substantially uniform level. It is to be understood that power supplies are coupled to the first and second recharging devices

36

and

37

, and to any grid or other voltage control surface associated therewith, so that the necessary electrical inputs are available for the recharging devices to accomplish their task.

FIG. 5

shows the voltages on the image area after it passes through the first recharging device

36

. The first recharging device overcharges the image area to more negative levels than that which the image area is to have when it leaves the recharging station D. For example, as shown in

FIG. 5

, the toned and the untoned parts of the image area reach a voltage level

80

of about −700 volts. The first recharging device

36

is preferably a DC scorotron.

After being recharged by the first recharging device

36

, the image area passes to the second recharging device

37

. Referring now to

FIG. 6

, the second recharging device

37

reduces the voltage of the image area, both the untoned parts and the toned parts (represented by toner

76

) to a level

84

which is the desired potential of −500 volts.

After being recharged at the first recharging station D, the now substantially uniformly charged image area with its first toner powder image passes to a second exposure station

38

. Except for the fact that the second exposure station illuminates the image area with a light representation of a second color image (say yellow) to create a second electrostatic latent image, the second exposure station

38

is the same as the first exposure station B.

FIG. 7

illustrates the potentials on the image area after it passes through the second exposure station. As shown, the non-illuminated areas have a potential about −500 as denoted by the level

84

. However, illuminated areas, both the previously toned areas denoted by the toner

76

and the untoned areas are discharged to about −50 volts as denoted by the level

88

.

The image area then passes to a second development station E. Except for the fact that the second development station E contains a toner

40

which is of a different color (yellow) than the toner

31

(black) in the first development station C, the second development station is substantially the same as the first development station. Since the toner

40

is attracted to the less negative parts of the image area and repelled by the more negative parts, after passing through the second development station E, the image area has first and second toner powder images which may overlap.

The image area then passes to a second recharging station F. The second recharging station F has first and second recharging devices, the devices

51

and

52

, respectively, which operate similar to the recharging devices

36

and

37

. Briefly, the first corona recharge device

51

overcharges the image areas to a greater absolute potential than that ultimately desired (say −700 volts) and the second corona recharging device, comprised of coronodes having AC potentials, neutralizes that potential to that ultimately desired.

The now recharged image area then passes through a third exposure station

53

. Except for the fact that the third exposure station illuminates the image area with a light representation of a third color image (say magenta) so as to create a third electrostatic latent image, the third exposure station

38

is the same as the first and second exposure stations B and

38

. The third electrostatic latent image is then developed using a third color of toner

55

(magenta) contained in a third development station G.

The now recharged image area then passes through a third recharging station H. The third recharging station includes a pair of corona recharge devices

61

and

62

which adjust the voltage level of both the toned and untoned parts of the image area to a substantially uniform level in a manner similar to the corona recharging devices

36

and

37

and recharging devices

51

and

52

.

After passing through the third recharging station the now recharged image area then passes through a fourth exposure station

63

. Except for the fact that the fourth exposure station illuminates the image area with a light representation of a fourth color image (say cyan) so as to create a fourth electrostatic latent image, the fourth exposure station

63

is the same as the first, second, and third exposure stations, the exposure stations B,

38

, and

53

, respectively. The fourth electrostatic latent image is then developed using a fourth color toner

65

(cyan) contained in a fourth development station I.

To condition the toner for effective transfer to a substrate, the image area then passes to a pretransfer corotron member

50

which delivers corona charge to ensure that the toner particles are of the required charge level so as to ensure proper subsequent transfer.

After passing the corotron member

50

, the four toner powder images are transferred from the image area onto a support sheet

57

at transfer station J. It is to be understood that the support sheet is advanced to the transfer station in the direction

58

by a conventional sheet feeding apparatus which is not shown. The transfer station J includes a transfer corona device

54

which sprays positive ions onto the backside of sheet

57

. This causes the negatively charged toner powder images to move onto the support sheet

57

. The transfer station J also includes a detack corona device

56

which facilitates the removal of the support sheet

57

from the printing machine.

After transfer, the support sheet

57

moves onto a conveyor (not shown) which advances that sheet to a fusing station K. The fusing station K includes a fuser assembly, indicated generally by the reference numeral

60

, which permanently affixes the transferred powder image to the support sheet

57

. Preferably, the fuser assembly

60

includes a heated fuser roller

67

and a backup or pressure roller

64

. When the support sheet

57

passes between the fuser roller

67

and the backup roller

64

the toner powder is permanently affixed to the support sheet

57

. After fusing, a chute, not shown, guides the support sheet

57

to a catch tray, also not shown, for removal by an operator.

After the support sheet

57

has separated from the photoreceptor belt

10

, residual toner particles on the image area are removed at cleaning station L via a cleaning brush contained in a housing

66

. The image area is then ready to begin a new marking cycle.

The various machine functions described above are generally managed and regulated by a controller which provides electrical command signals for controlling the operations described above.

Referring now to

FIG. 8

in greater detail, development station

38

includes a donor roll

40

. A development apparatus advances developer materials into development zones. The development station

38

is scavengeless. By scavengeless is meant that the developer or toner of station

38

must not interact with an image already formed on the image receiver. Thus, the station

38

is also known as a non-interactive development station. The development station

38

comprises a donor structure in the form of a roll

40

. The donor structure

40

conveys a toner layer to the development zone which is the area between the photoreceptor belt

10

and the donor structure

40

. The toner layer

82

can be formed on the donor structure

40

by either a two-component developer (i.e. toner and carrier), as shown in

FIG. 8

, or a single-component developer deposited on donor structure

40

via a combination single-component toner metering and charging device. The development zone contains an AC biased electrode grid

90

self-spaced from the donor roll

40

by the toner layer. The single-component toner may comprise positively or negatively charged toner. The electrode grid

42

may be coated with TEFLON-S (trademark of E. I. DuPont De Nemours) loaded with carbon black.

For donor roll, loading with developer material is accomplished by the present invention. Transport member

200

in

FIG. 8

is shown as a belt or can take the form of a drum as shown in FIG.

9

. The transport member is charged by charging device

204

. Transport member

200

as shown in

FIG. 10

is a schematic Active Matrix (AMAT) photoreceptor belt having a mask screen pattern

203

therein. Mask screen pattern

203

can be a fine line pattern or a random pattern. The transport member can take form as a belt entrained about roller

212

and

214

as shown in

FIG. 8

or a drum as shown in FIG.

9

.

Transport member

200

is then exposed by a flood exposure device

215

which creates voltage differences on the surface of the transport member resulting in a fringe electric field pattern being generated on the surface of the transport member

200

. The flood exposure can be from the inside of the belt or outside the belt. Preferably, the resultant fringe electric field pattern has a contrast greater than 100 Volts. Transport member

200

travels through a toner bed

300

. The toner bed can be fluidized if desired by an air plenum (not shown). Toner is attracted to the surface of transport member

200

by the fringe field and a toner layer is formed thereon. Preferably, the toner layer has a thickness between 10 to 200 □m. Thereafter, the toner is ion charged by charging device

210

. Preferably, the toner layer is charged to about 10 to 100 □C/g. Next, the layer of toner is transferred to donor roll

40

.

Let's focus on the toner pickup and toner layer formation process. Fringe lines are the boundaries between areas with high and low voltages. Fringe fields exist along the fringe lines, The fields originate from the high voltage side of the fringe line and end on the low voltage side. The fringe fields are typically very strong due to a high voltage drop across a short distance and is spatially localized only in the neighborhood of the fringe lines. Because of its bi-directional nature (fields are in opposite directions on the high/low voltage sides), it can pickup toner of both polarities. Also due to its strong spatial gradient (fields decay away from the fringe area quickly), it can also pickup neutral toner due to polarization effect. According to the present invention, a surface with uniformly distributed high density fringe lines is used to load toner materials. The density of the boundary lines should be greater than 5 mm/mm

2

for uniform and effective toner loading. In a further preferred embodiment, the density is higher than 20 mm/mm

2

. The pattern can be regular patterns such as lines and dots or randomly but uniformly distributed fringe lines.

An advantageous feature of the present invention is that the fringe fields can pick up both charge toner and neutral toner. Further, the charging characteristics of the transport member can be modified so that the desired rate (i.e. rotation speed of transport member) of loading of donor roll

40

and discharging rate of transport member

200

can be matched so that the fringe fields are minimized or zero at reload zone

220

.

Other embodiments of transport member

200

could take the form wherein the fringe field patterns are burned in the fringe field surface structure. As one preferred embodiment, the surface layer of the transport member has a fixed spatial conductivity pattern which consists of more conductive and less conductive regions so that when the transport member is charged, the more conductive regions discharge soon thereafter thereby causing a fringe field pattern to be generated on the surface of the transport member. The more conductive region can be conductive or semiconductive and the less conductive region can be semiconductive or insulative. Alternatively, the transport member can be made by doping of a photoconductive layer to create a conductivity pattern so that when the transport member is charged, the more conductive pattern discharges to a lower potential thereby causing fringe fields to be generated on the surface of the transport member.

Other embodiments and modifications of the present invention may occur to those skilled in the art subsequent to a review of the information presented herein; these embodiments and modifications, as well as equivalents thereof, are also included within the scope of this invention.

Number	Name	Date
4777106	Fotland et al.	Oct 1988
5532100	Christy et al.	Jul 1996
5734955	Gruber et al.	Mar 1998
5742885	Wayman	Apr 1998
5950057	Erhardt et al.	Sep 1999

Apparatus for developing a latent image

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Abstract

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US Referenced Citations (5)