US3914460A

US3914460A - Development utilizing electric fields

Info

Publication number: US3914460A
Application number: US322254A
Authority: US
Inventors: John Maksymiak
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1973-01-09
Filing date: 1973-01-09
Publication date: 1975-10-21
Anticipated expiration: 1992-10-21
Also published as: NL7400317A; DE2400716A1; JPS5815790B2; CA1027427A; JPS49104639A

Abstract

To control toner used in developing a latent electrostatic image carried by, say, a photoconductive surface, there is a shaped electric field acting on a toner laden donor member for separating toner from the donor member as it enters a development zone and for redepositing any excess toner on the donor member as it leaves the development zone.

Description

I United States Patent 1191 1111 3,914,460 Maksymiak Oct. 21, 1975 1 1 DEVELOPMENT UTILIZING ELECTRIC 3,232,190 2/1966 Willmott 117/175 FIELDS 3,257,223 6/1966 King l17/l7.5 3,306,193 2/1967 Rarey et al. 117/175 e o J y a P -Y- 3,331,355 7/1967 Donalies et al. 118/637 3,592,166 7/1971 Wu 118/637 [73] Ass'gnee' Stamford 3,645,770 2/1972 Flint 96/1 s1) Com 3,696,783 10 1972 Fantuzzo 118/637 22 Fil d; J 9 1973 3,696,785 10/1972 Andrus 355/3 R 3,707,389 12/1972 Maksymiak et a1 1 18/637 [21] Appl. No.: 322,254 3,739,748 6/1973 Rittler et a1 118/637 52 U.S. c1. 427/14; 96/1 SD; 118/637; pimary sofoclews 355/3 R 51 Int. Cl. G03g 13/08; G03g 15/08 [57] ABSTRACT [58] Field of Search 117/175; 118/637; To Control toner used in developmg a latent electro- 355/3 R 3 119 SD static image carried by, say, a photoconductive surface, there is a shaped electric field acting on a toner 5 References Cited laden donor member for separating toner from the UNITED STATES PATENTS donor member as it enters a development zone and for 2 901 374 8/1959 G (H h l 17 17 5 redepositing any excess toner on the donor member as 310401704 6/1962 118 /63 7 it leaves the development Zone 3,203,394 8/1965 Hope et al. 117/175 9 Claims, 6 Drawing Figures US. Patent Oct.21, 1975 Sheetlof2 "3,914,460

U.S. Patent Oct. 21, 1975 Shet 2 of 2 DEVELOPMENT UTILIZING ELECTRIC FIELDS BACKGROUND OF THE INVENTION This invention relates to developing latent images with toner and, more particularly, to a development method and apparatus utilizing a toner-laden donor member to apply toner particles to a surface bearing an electrostatic latent image thereon.

In reproduction processes, such as in xerography, wherein a latent image is first formed on a surface and then made visible, or developed, with a powderous material commonly called toner, it is necessary to provide a means for bringing the toner material adjacent the surface so that it can be attracted to the surface in a pattern conforming to the latent image thereon. One manner in which this is carried out is by the employment of a donor member. The donor member, which can take the shape of a roller, endless belt or other convenient form, is first loaded with toner and then brought into close proximity to the surface being developed. The toner is attracted from the donor member onto the surface in conformance with the electrostatic latent image thereby creating a visible image.

A number of measures can be taken to improve the operation of a donor member used in this development mode. For instance, if the toner layer is spaced from the surface being developed so that it does not touch it, there is a less likelihood that toner supplied by the donor member will deposit in the non-image areas. This is because the toner on the donor member does not touch the surface and the non-image areas will not attract the toner.

This technique is described in US. Pat. No. 3,232,190 wherein a thin layer of toner is positively held and maintained in a spaced relation to the surface being developed. The high potential areas of the charge pattern exert electrostatic forces of sufficient intensity to overcome the forces holding the toner on the donor member and these toner particles are attracted to image areas. However, the electrostatic forces exerted by the remaining areas of the surface, the non-image areas, are insufficient to overcome forces attracting the adjacent toner to the donor member and, consequently, the non-image areas remain relatively free of toner.

The density of the final copy can be further controlled by selecting the gap distance between the donor member and surface. Since the strength of the electrostatic fields terminating on the surface being developed decreases as a function of the distance from that surface, the gap distance between the toner layer on the donor member and the surface is very critical. Even at relatively modest gap distances it has been found that less than complete development takes place. Development has been enhanced by placing a potential on the donor member itself which tends to aid the toner in overcoming the forces attracting it to the donor member, but, when such a potential is applied, the toner tends to be driven to the surface in random fashion and deposited in non-image areas. This condition is known as background development and degrades the quality of the final copy.

Another known method for releasing the toner from the donor member is described in US. Pat. No. 3,257,223. This patent describes the creation of a cloud of toner particles between the donor member and the surface being developed. The donor member is a three-layer structure including a conductive base, a dielectric layer over the base and a conductive grid on top of the dielectric layer.

During loading of the donor member, a potential opposite in polarity to a charge on the toner particles is placed on the conductive base thereby attracting toner particles to the surface areas between the grid elements. Then, when development is to take place, the potential is removed from the conductive base, or, alternatively, a potential of the same polarity as the charge on the toner particles is placed on the conductive base. Both create a condition which tends to blow the toner off the donor member in a cloud and into contact with the surface being developed. The overlying conductive grid serves generally to breakup the field lines emanating from the conductive backing of the donor during loading and blow-off so as to create the characteristically strong fringe fields which exist over a surface having variable electrical potential. Because the toner is in the form of a cloud, the possibility of random deposition of toner, particularly in nonimage areas, is increased.

A third type of donor member also involves creating fringe fields and this is described in US. Pat. No. 3,203,394. The donor member has a conductive base with a poorly insulating layer thereon and then a pattern of good insulating posts as a top layer. This donor configuration is used basically to make it easier to load toner on the periphery of the donor prior to bringing it into the development zone. In operation, the structure is electrostatically charged and then brought into the loading area. Between the charging station and the loading station, the poorly insulating layer loses its charge so that the fringe fields are created on the surface between the insulating posts and the poorly insulating intermediate layer.

As a result of the fringe fields, toner is placed only between the insulating posts thereby creating a discontinuous layer of toner on the donor. This is not totally desirable because it limits the amount of toner that can be brought to the development zone. In addition, the position of the piles of toner on the donor relative to the image areas on the surface may not always be in synchronism. If this is the case, certain of the image areas on the surface would not be fully developed for lack of toner.

The present invention relates to a development method and apparatus which imposes electrical fields on the toner layer and donor member within the development zone so that toner is made readily available to image areas of the surface being developed yet is not randomly deposited on the non-image areas. In one embodiment, the donor member takes the form of an endless rotatable belt passing through a toner loading area and then adjacent the surface being developed. Toner loaded onto the belt is subjected to an electrical field which activates the toner on the donor member as it passes through the development zone. Those toner particles are attracted to image areas of the surface during activation follow the field back to the donor surface where they are removed from the development zone. In the development zone, although toner is loosened and- /or separated from the donor due to the electrical field, it continues to be under the control of the electrical field. This arrangement prevents random deposition of toner in the non-image areas thereby reducing background. This arrangement also allows greater flexibility in establishing gap distance between donor and surface without the usual drop off in development quality.

Accordingly, it is an object of the invention to provide an improved method and apparatus for developing electrostatic latent images.

It is a further object of the invention to provide an improved method and apparatus for providing toner to a surface being developed by a donor member.

It is a further object of the invention to provide an improved method and apparatus for providing toner to the surface being developed when the toner donor member is spaced from the surface.

It is a further object of the invention to provide an improved method and apparatus for the control of toner particles in the development zone.

It is a further object of the invention to provide a development system wherein little or no background is produced.

SUMMARY The present invention relates to a method and apparatus for developing electrostatic latent images by bringing a toner-laden donor member adjacent a surface bearing a latent image and thereupon transferring the toner to the surface in conformance with the image. The toner is exposed to an electric field in the development zone which activates it as it passes into the development zone thereby making it readily-attractable by the electrostatic image on the surface. The electric field is shaped so that the activated toner which is not attracted to image areas of the surface is redeposited on the donor member as it leaves the development zone.

In one embodiment, the toner layer on the donor is separated from the surface by a gap and the electric field is sufficient to reduce the attraction of the toner to the donor or, in the alternative, separate the toner from the donor. In a second embodiment, the toner layer can be in contact with the surface being developed and the affect of the field is to control the action of the toner in the development zone so that little or no toner is deposited in non-image areas of the surface.

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, which is to be read in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic illustration of the invention in a xerographic system.

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

FIGS. 3 and 4 are detailed views of the electric field in the development zone.

FIG. 5 is a detailed view of another embodiment of .the invention.

FIG. 6 is another detailed view of the electric field in the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention can be adapted to any reproduction apparatus wherein toner is used to develop an electrostatic latent image. For the purpose of this disclosure, however, the invention will be described principally within the environment of a xerographic reproduction apparatus.

Referring to FIG. 1, there is shown a schematic view of a continuous xerographic copier having a photosensitive member in the shape of a drum 1] on which a latent electrostatic image of the information to be reproduced is formed. The rotatable drum 1] is driven about shaft 2 by any suitable means so that the drum surface passes through stations AE in a continuous manner. The peripheral surface of the drum is covered by an electrically conductive material which, in turn, is covered on its outer surface with a photoconductive 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.

An electrostatic latent image is formed on the drum 1] by passing its surface through charging station A and exposure station B. Charging station A can include any suitable means for placing a uniform charge on the photoconductive layer 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 charged photoconductive layer of the drum is synchronized with the movement of the drum and causes selective charge dissipation on elemental areas of the drum to form an electrostatic latent image thereon.

After the formation of the electrostatic latent image by passing the drum through stations A and B, the drum carries the latent image through developing station C. Within the developing station, a suitable developer material (hereinafter referred to a toner), such as a pigmented resinous electroscopic powder, is deposited on the drum in imagewise configuration according to the teachings of this invention. A more detailed description of the development station follows.

After the development step, the drum carries the toner image through transfer station D where the toner image is transferred from the drum surface to any suitable support material such as a sheet of paper, continuous web, or any other form of substrate which can be used to receive toner images. Transfer of the toner image onto the copy sheet is carried out in any suitable manner such as by electrostatically attracting the toner image from the drum onto a sheet of paper with a corona discharge device.

After the toner image has been transferred to the copy sheet and the copy sheet has passed through the transfer station, a fixing device or other suitable processing station fixes the toner image. Any suitable fixing device which makes the toner permanent on the copy sheet is suitable.

The final station shown is cleaning station E. Station E can include any suitable cleaning device, such as 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 FIG. 1, toner 18 picked up'from reservoir 17 by donor member 12 is brought adjacent the drum 11 at station C. The donor member is shown in the form of an endless belt and rotates in the clockwise direction. The toner particles adhere to the belt and pass around roller 13 and field producing device 3 where they are transferred to the drum to develop the image thereon. As the belt transports the toner towards the development zone, the toner can be sprayed electrostatically by corona charging device 16 to assure that all toner particles have the same charge.

FIG. 2 provides a more detailed view of the donor member. The belt 12 may be insulating and may be made of a material which when rubbed with toner produces a triboelectric action making the toner attractive to it. However, loading of the belt may be carried out without regard to its triboelectric relationship to the toner. In such a case, loading may be aided by grounding roller 13 and vibrating reservoir 17 by any suitable means.

A number of different loading systems could be carried to load toner on the donor member. For instance, the donor member could be made of a material having a suitable triboelectric relationship to toner particles so that toner could be cascaded along with carrier beads over the surface of the donor, the toner adhering to the donor member in traditional cascade development fashion. Another known way of placing toner on a donor member would be to expose the member to a charged cloud of toner particles which would be attracted to the donor member surface.

After loading toner on the belt for transport to the development station the toner layer 19 may optionally be sprayed with a uniform charge such as by corona device 16. This provides good attraction between the toner and image areas of the drum during development. The charge sprayed on the toner layer will be selected based on the development system used. For instance, it is common to charge a xerographic to a high positive potential and then expose the non-image areas to light which reduces the potential on those portions of the drum. In this case, if the high potential areas are to be developed, the toner would be charged negatively. However, if the low potential areas were to be developed, the toner would be charged positively.

Field producing apparatus 3 includes ground plate and biased plate 14. Ground plate 15 is placed behind belt 12 and forms the surface upon which belt 12 rides. Plate 15 may extend from the development zone through the charging area as shown. The other portion of field-producing apparatus 3 is bias plate 14 which can also form the surface upon which the belt rides as it exits the development zone. Plates l4 and 15 are not in electrical contact because it is the combination of these two members which creates the electrical field through the development zone to be described in more detail in FIGS. 3 and 4. In FIG. 1 there is provided an air space 30 between

members

14 and 15 for electric insulation while in FIG. 2 insulating plate 31 provides proper insulation between the plates.

The potential applied to plate 14 will depend on the charge of the toner particles and is opposite thereto in polarity. As described in FIG. 2, if a negative charge is sprayed on to the toner layer by corona device 16, the bias on plate 14 will be positive. On the other hand, if the positive toner is used, plate 14 will be biased to a negative polarity. The combination of biased plate 14 and grounded plate 15 generate an electric field between the two plates and toner in the development zone will be attracted towards plate 14.

FIGS. 3 and 4 show a more detailed view of the development zone. Due to the fact that member 15 is grounded and member 14 is at a positive potential, a fringe field effect will be created between these two members in the vicinity of the development zone. This electric field extends through donor member 12 and toner layer 19. The lines of force in the fringe field is indicated by broken lines passing between

members

14 and 15.

The purpose of the field is to activate the toner while it is in the development zone so that it is readily attractable by the electrostatic image on the drum. The term activation as used herein includes, in the alternative, reducing the forces that attract the toner to the donor or reversing the forces to separate the toner from the donor. In out-of-contact systems wherein the toner layer is separated from the drum by a gap, as shown in FIGS. 3 and 4, the electric field can be of such force as to actually dislodge and separate toner particles from the belt in the development zone. In other systems, such as wherein the toner layer is in contact with the drum, the attraction of the toner to the donor is merely lessened or reduced to zero so that the image areas of the drum can attract it to the drum.

In out-of-contact systems, it is desirable to separate the toner particles from the donor sufficiently so that the particles are easily attractable to the image areas on the surface being developed. However, it is equally important not to lose control of these activated toner particles since this encourages spurious deposition on the non-image areas of the drum.

In FIG. 3, the pluses indicate the image areas of the drum, the balance of the drum being the non-image areas. Toner which is activated by following the electric field created by

plates

14 and 15 is free to be attracted to the image areas. However, since an image area is not adjacent the donor at the moment shown, the activated toner does not cross the gap to the surface being developed. Instead, the toner particles complete their journey in conformance with the electric field generated by

plates

14 and 15 and are reattracted onto the donor memberdue to the electrical field produced by member 14.

Referring to FIG. 4, there is shown the condition when an image area is more clearly in the development zone and is available for the receipt of toner from the donor member. In this case, the fields created by the image areas of the drum attract activated toner as needed across the gap and onto the drum surface to develop the image. Any activated toner not so attracted completes its journey through the development zone in conformance with the electric field created by the donor member and is redeposited onto the surface of member 15.

As can be seen from the above description, although all toner on the development member may be activated, there is complete control over the movement of this toner at all times. This control operates to prevent deposition of toner on the non-image areas which is quite usual in those systems wherein blow-off fields are used to drive the toner across the gap onto the surface being developed. In addition, all toner on the donor is subject to activation since it must pass the field producing means. This affords the fullest opportunity for complete development of the drum even in out-of-contact systems.

In addition to preventing the random deposition of toner in background areas, the present invention also allows a good deal of flexibility in the spacing of the donor member from the surface being developed. One problem in previously known apparatus has been that it was not alwayspossible to have sufficiently strong fields from the image areas of the drum to reliably remove toner from the donor member at larger gap distances. In the present invention, gap spacing is not as critical and larger spacing is readily permitted. This is because the electric field which activates the toner frees the toner from its adhesion to the donor member, thereby reducing the electrostatic force required to attract toner to the drum.

FIG. discloses another embodiment of the invention. Here an endless belt such as insulating belt 20 is supported and driven about

rollers

21 and 22 in a counterclockwise manner. The external surface of the belt picks up toner particles 7 from vibrating toner reservoir 26. To aid the deposition of toner on the external surface of belt 20, a grounded electrode 23 is placed over the hopper 26 and extends toward the development zone. Toner particles 28 picked up by the belt are brought into the development zone of drum 29 by the moving belt. In the development zone and behind the donor belt 20, a structure similar to that of the embodiment in FIG. 2 is shown. Plate 23 which is electrically grounded is also electrically insulated from plate 25 by insulating material 24. Plate 25 has a polarity opposite that of the toner particles.

As can be seen from FIG. 6, the electric field generated in the development zone across plates 23 and 25 is similar to that described in conjunction with FIGS. 3 and 4. When image areas in FIG. 6 pass through the development zone, activated toner particles which are following the electric field between plates 23 and 25 are free to be attracted to the image areas on drum 11. Any activated toner particles which are not attracted to the drum are deposited onto the belt as it exits the development zone, and are not left in an uncontrolled situation within the development zone.

It should be understood that a number of alternatives to the system are available which would fall within the scope of the invention. For instance, the charge on the toner particles delivered by the donor member can be positive or negative depending upon the nature of the charge in the image areas. The configuration of the donor member is not necessarily limited to an endless belt. For instance, the donor could take to the shape of a roller or a continuous web. Also, in an endless belt arrangement, it may be desirable to remove the residue toner from the belt before it is reloaded with toner to avoid history effects. This can be carried out by any suitable cleaning device such as brush 4 shown in FIG. 2.

The developing method and apparatus disclosed can be used as part of any larger reproduction apparatus where it is desired to develop an electrostatic latent image. Thus, in addition to xerographic systems, other electrostatographic imaging systems can utilize this process including other electrophotographic and electrography systems.

In addition to the apparatus outlined, many 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.

What is claimed is: l. A method of developing an electrostatic latent image carried by a surface, said method comprising the steps of:

bringing a toner-laden donor member adjacent said surface in a development zone, and

subjecting the toner to an electric field shaped to first separate'toner' from the donor member as the toner enters the development zone and to then redeposit toner not attracted to the surface onto the donor member as the toner leaves the development zone, whereby the toner in said development zone is controlled but still readily'attractable to said surface in conformance with the image thereon.

2. The method according to claim 1 further including a preliminary step of loading toner on the donor member.

3. The method according to claim 1 further including a preliminary step of charging the toner to a given polarity prior to its passing through the development zone.

4. The method according to claim 1 wherein all the toner on the donor member is subjected to the electric field as it passes through the development zone.

5. A method of developing an electrostatic latent image carried by a surface as said surface movesthrough a development zone, said method comprising the steps of:

transporting toner to the development zone by a web,

and

subjecting the toner and web to an electric field shaped to separate toner from the web as the toner enters the development zone and to redeposit the toner not attracted to the surface onto the web as the toner leaves the development zone, whereby toner is available for development of said image and excess toner is removed by said web from said development zone.

6. The method according to claim 5 wherein the web is an endless belt adapted to continuously pass through the development zone and further including the preliminary step of loading toner onto the belt.

7. The method according to claim 6 further including the additional step of .cleaning unused toner from the belt.

8. In a method of developing an electrostatic latent image carried by a surface, said method including the step of bringing a toner-laden donor member adjacent said surface in a development zone; the improvement comprising the additional step of:

subjecting the toner to an electric field shaped to reduce the attraction of the toner to the donor member as the toner enters the development zone whereby the toner is readily-attractable to the surface in conformance with the image thereon and to increase the attraction of the toner to the donor member as said donor member leaves the development zone whereby toner not attracted to said surface is removed from said development zone.

9. In a method of developing an electrostatic latent ima'ge carried by a surface, said method including the step of transporting toner to the development zone by a web; the improvement comprising the additional step of subjecting the toner and web to an electric field which activates the toner as it enters the development zone whereby the toner is readily-attractable to the surface in conformance with the image thereon and which reattracts the activated toner not attracted to the surface onto the web as it leaves the development zone.

Claims

1. IN METHOD OF DEVELOPING AN ELECTROSTATIC LATENT IMAGE CARRIED BY A SURFACE, SAID METHOD COMPRISING THE STEPS OF: BRINGING A TONER-LADEN DONOR MEMBER ADJACENT SAID SURFACE IN A DEVELOPMENT ZONE, AND SUBJECTING THE TONER TO AN ELECTRIC FIELD SHAPED TO FIRST SEPARATE TONER THE DONOR MEMBER AS THE TONER ENTERS THE DEVELOPMENT ZONE AND TO THEN REDEPOSIT TONER NOT ATTRACTED TO THE SURFACE ONTO THE DONOR MEMBER AS THE TONER LEAVES THE DEVELOPMENT ZONE, WHEREBY THE TONER IN SAID DEVELOPMENT ZONE IS CONTROLLED BUT STILL READILY ATTRACTABLE TO SAID SURFACE IN CONFORMANCE WITH THE IMAGE THEREON.

5. A method of developing an electrostatic latent image carried by a surface as said surface moves through a development zone, said method comprising the steps of: transporting toner to the development zone by a web, and subjecting the toner and web to an electric field shaped to separate toner from the web as the toner enters the development zone and to redeposit the toner not attracted to the surface onto the web as the toner leaves the development zone, whereby toner is available for development of said image and excess toner is removed by said web from said development zone.

7. The method according to claim 6 further including the additional step of cleaning unused toner from the belt.

8. In a method of developing an electrostatic latent image carried by a surface, said method including the step of bringing a toner-laden donor member adjacent said surface in a development zone; the improvement comprising the additional step of: subjecting the toner to an electric field shaped to reduce the attraction of the toner to the donor member as the toner enters the development zone whereby the toner is readily-attractable to the surface in conformance with the image thereon and to increase the attraction of the toner to the donor member as said donor member leaves the development zone whereby toner not attracted to said surface is removed from said development zone.

9. In a method of developing an electrostatic latent image carried by a surface, said method including the step of transporting toner to the development zone by a web; the improvement comprising the additional step of subjecting the toner and web to an electric field which activates the toner as it enters the development zone whereby the toner is readily-attractable to the surface In conformance with the image thereon and which reattracts the activated toner not attracted to the surface onto the web as it leaves the development zone.