US3921571A

US3921571A - Multiple development method and apparatus for electrophotographic copiers

Info

Publication number: US3921571A
Application number: US482040A
Authority: US
Inventors: Robert A Moore; Daniel J Sullivan
Original assignee: Itek Corp
Current assignee: Ricoh Co Ltd
Priority date: 1974-06-24
Filing date: 1974-06-24
Publication date: 1975-11-25
Anticipated expiration: 1992-11-25
Also published as: SE407118B; DE2528109A1; GB1495429A; FR2276617A1; CA1058980A; AU8234875A; BE830585A; JPS5118548A; DE2528109C2; SE7507188L; FR2276617B1; NL7507130A; IT1039178B

Abstract

Electrophotographic copying apparatus having a multiple development capability for more completely developing and, hence, improving the quality of images which contain extended solid areas. This multiple development capability is attained by designing the development station in such a manner that the latent electrostatic image to be developed and the developer mixture will move through the development station at a substantially identical velocity as distinguished from conventional systems in which the developer passes through the development station much faster than the image. By matching the velocities of the image and the developer, degradation of the developed, or partially developed, image by the rubbing or sweeping of the carrier beads across the image will be substantially eliminated, and, accordingly, multiple development becomes possible. The substantial matching of velocities is preferably accomplished by significantly reducing the width of the gap between the latent image bearing medium and the development plate through which the developer falls. This increases the density of developer in the gap and the degree of developer agitation and, hence, causes it to fall much slower while still ensuring an adequate supply for effective development of all areas of the image. With a system designed in this way, copies having a quality equal or superior to those obtainable with conventional systems can be achieved with a single development step and at speeds comparable to those available in todays high speed machines. When, however, the image to be developed contains large extended solid areas, any needed number of additional development cycles can be selectively carried out to better develop such images and, hence, provide superior quality copies.

Description

[ Nov. 25, 1975 1 MULTIPLE DEVELOPMENT METHOD AND APPARATUS FOR which contain extended solid areas. This multiple development capability is attained by designing the de- ELECTROPHQTOGRAPHIC COPIERS velopment station in such a manner that the latent [75] Inventors: Robert A. Moore, Amherst, NH;

Daniel J. Sullivan, Hyde Park, Mass.

[73] Assignee: Itek Corporation, Lexington, Mass.

[22] Filed:

June 24, 1974 image. By matching the velocities of the image and the developer, degradation of the developed, or partially 21 Appl. No.: 482,040

developed, image by the rubbing or sweeping of the carrier beads across the image will be substantially eliminated, and

, accordingly, multiple development 76 H m5 001 no ;3 50 G 1. 1. m m 8n 1" m 1! 0 Sn um 11. 21 55 ll becomes possible. The substantial matching of veloci- [58] Field of Search 118/7, DIG. 24, 636, 637;

ties is preferably accomplished by significantly reducing the width of the gap between the latent image bearing medium and the development plate through which the developer falls. This increases the density of References Cited UNITED STATES PATENTS developer in the gap and the degree of developer agi- 118/637 tation and, hence, causes it to fall much slower while 118/6 still ensuring an adequate supply for effective develop- H3/637 ment of all areas of the image. With a system designed in this way copies having a quality equal or Superior to those obtainable with conventional systems can be 3,592,642 3,736,055 Davidge et al..

3,754,531 Donalies...........

Primary ExaminerW. C. Reynolds achieved with a single development step and at speeds comparable to those available in todays high speed machines. When, however, the image to be developed Assistant Examiner-Douglas Salser Attorney, Agent, or Firm-Homer 0. Blair; Robert L. Nathans; Gerald I-l. Glanzman contains large extended solid areas, any needed number of additional development cycles can be selec- ABSTRACT Electrophotographic copying apparatus having a multively carried out to better develop such images and, hence, provide superior quality copies.

tiple development capability for more completely developing and, hence, improving the quality of images 13 Claims, 1 Drawing Figure 051 51 OPME/VT CYCLE CONTROL C/RCU/TZ] U.S. Patent Nov. 25, 1975 momDOm MULTIPLE DEVELOPMENT METHOD AND APPARATUS FOR ELECTROPHOTOGRAPHIC COPIERS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to the field of electrophotographic copiers, and, more particularly, to an electrophotographic copier having a multiple development capability for improving the quality of images containing extended solid and half-tone areas.

2. Description of the Prior Art In the typical cascade development station of an electrophotographic copier, the latent electrostatic image to be developed and a developer mixture are brought together such that toner powder can be attracted to and developed on appropriate areas of the latent image. In such systems, to produce high quality copies it is essential that the latent image to be developed, an adequate supply of developer, and a sufficient degree of developer agitation be maintained in the development station of a sufficient period of time to ensure that substantially complete development of the entire image can take place.

Although many systems presently available are able to accomplish this goal in a generally satisfactory manner when the document to be copied contains ordinary line copy or the like, they are not fully acceptable when the image to be developed contains large solid or halftone areas such as pictures, graphs or the like. In such situations, the latent image must receive especially large amounts of toner to more fully neutralize the charges on the image and to produce a good quality high density black image that approaches the quality obtainable by conventional printing techniques, and most commercial electrostatic systems are simply not able to accomplish this in an effective manner, and, as a result, generally provide copies in which solid areas are of an inferior quality.

Most of the suggestions that have been made to solve this problem entail the act of keeping the latent image to be developed in the development station for a longer period of time than normal such that larger amounts of toner are able to be deposited on the image. For example, one suggested solution is to increase the size of the development station and, hence, increase the length of time that the latent image takes in passing through it. This, however, is not a very good solution because, for one thing, it tends to make the entire copying system larger, bulkier, and more expensive.

Another approach is to leave the size of the developing station alone, but instead cause the latent image to move through it at a slower speed. This is also unsatisfactory because it requires that other operations of the machine be accelerated or adjusted to maintain high overall operating speed and this makes the machine more complex and less reliable.

In general, there is no system presently available in the marketplace which can provide high quality copies of those originals which contain large extended solid areas in a satisfactory manner without sacrificing machine compactness and/or efficiency.

SUMMARY OF A PREFERRED EMBODIMENT OF THE INVENTION In accordance with a presently preferred embodiment of the invention, the above inadequacies have been significantly reduced by providing an electrophotographic copying system having a multiple development capability which may be utilized in a selective manner to more fully develop those latent images 5 which contain extended solid areas. More particularly,

the present invention permits the latent image to be directed through the development station a plurality of times as needed or desired to ensure that high density black images of pictures, graphs or the like can be produced.

This multiple development capability is attained by the provision of a development station of novel construction which avoids any significant image degradation that would normally be expected to occur as a result of the image passing through the development station more than one time. In the typical system, multiple development would actually tend to damage the developed (or partially developed) image more than enhance it, because as the image is passed through the development station for a second or subsequent time, there would be a substantial tendency for the carrier beads to physically rub against and noticeably damage background areas by producing a high density of black spots thereon, and, also, to cause edges of the image to become less well defined. This would happen because in the conventional system, the carrier beads upon which the toner particles are electrostatically bound are passed through the development station at a speed substantially in excess of the speed of the moving latent image to ensure that adequate amounts of toner will be supplied to all areas of the image and it is this velocity differential which creates the rubbing or sweeping activity.

In the present invention, however, the developing station has been designed such that the developer and the latent electrostatic image can be moved through it at substantially identical speeds such that there will not be any significant scrubbing or wiping action between them, and because there is no significant scrubbing, there will be no significant image degradation during second and subsequent development operations.

Reduction in the rate of travel of the developer relative to the latent image can be accomplished in several ways while still ensuring that an adequate supply of toner will be available to all areas of the image and without sacrificing machine speed for a single development cycle. In the presently most preferred embodiment, for example, the spacing or gap between the drum that carries the latent electrostatic image and the development plate or electrode is reduced significantly. This reduced spacing results in a higher particle density and a higher frequency of developer collisions in the gap and, hence, in a slower movement of the developer through the gap. Also, this increased density will ensure that a large amount of developer be present adjacent each area of the image to be developed to ensure full development. In addition, the development electrode is preferably designed with a knurled surface in the manner disclosed in copending U.S. Patent Application Ser. No. 429,616 filed on Jan. 2, 1974 to James E. Genthe and William R. Buchan and entitled CASCADE DE- VELOPMENT STATION HAVING A ROUGHENED DEVELOPMENT PLATE FOR ENHANCING DE- VELOPER MIXTURE TURBULENCE, now abandoned in favor of continuation-in-part application Ser. No. 547,851, filed on Feb. 6, I975. Such an electrode design permits a better and more uniform spreading of the developer to be effected to further ensure that sufficient toner will be available to all areas of the image.

In general, the system provided permits multiple development to be selectively utilized in those situations when needed or desired to completely develop an image such as when the original contains extended solid areas. When multiple development is not needed, such as when the original contains only line copy or when improved image quality is not important, only a single development need be performed without any loss in copy speed. Yet further features of the invention will be set out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS The FIGURE schematically illustrates the development station of an electrophotographic copier in accordance with a presently preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The FIGURE illustrates, in somewhat schematic form, the development area of an electrophotographic copier. Other conventional portions of the copier such as the imaging station, the corona charging station, the exposure station and the drum cleaning station have not been shown in the interest of clarity and because they do not form a part of the present invention.

In the FIGURE, reference number 1 identifies the photoconductive drum upon the surface of which a latent electrostatic image has been formed in the conventional manner. Reference number 2 identifies the developer supply hopper while reference numbeer 3 refers to the development plate or electrode which is spaced from the surface of drum 1 to define a gap or space 4 therebetween as understood by those skilled in the art.

In operation of the system, developer is dispensed from developer hopper 2 and cascades downwardly through the gap 4 such that toner will be attracted to the latent electrostatic image on the surface of drum 1 being carried through the development station in the direction indicated by arrow 5. The developer thereafter falls to the bottom of the machine where it is usually replenished with fresh toner and is picked up by conveyer 6 controlled by a motor 10 which carries it back to the top of the machine to be recirculated therethrough.

After the latent image has been suitably developed on the drum 1, it is then transferred to a suitable transfer belt 8 of the type described in copending US Patent Application Ser. No. 403,696 by William R. Buchan and Robert A. Moore, filed on Oct. 4, 1973, and entitled ELECTROPHOTOGRAPHIC TONER TRANSFER AND FUSING APPARATUS AND METHOD, or to some other medium in a manner rec ognized by those skilled in the art. In particular, belt 8 is adapted to be carried across the drum in the direction indicated by arrow 7 by a suitable roller system 9, 11 and can be selectively brought into or out of contact with the drum by means of a spring roller 12 or the like as will be explained in greater detail hereinafter. Belt 8 and its associated structure does not form part of the present invention, and is illustrated schematically only to help explain the invention. It should be clearly understood, however, that the present invention could also be employed in those systems wherein an image is transferred directly from the photoconductive drum to paper or to other mediums using electrostatic transfer.

Development plate 3 functions as a metallic electrode and is thus electrically connected to a conventional bias source 13 as is well recognized by those skilled in the art. Preferably also, development electrode 3 is provided with a knurled surface of the type described in the earlier-mentioned copending US. Patent Application Ser. No. 429,616 which assists in distributing the developer more uniformly over the entire image to be developed.

An important feature of the invention is that the gap 4 formed between the surface of drum 1 and plate 3 is designed to be substantially narrower than those employed in conventional systems. Specifically, in the presently constructed machine, gap 4 has a spacing of approximately 175 mils in region 16, a spacing of about mils in region 17, a spacing of about 85 mils in re gion 18 and a spacing of about mils in region 19. This variable spacing has been provided to ensure a rel atively smooth flow of developer into and out of gap 4 without creating excessive pile-ups while maintaining the very narrow spacing desired in the longer central region of the gap. Other spacings, both variable and uniform may also be employed if desired, however, and, as a general rule, a spacing of from about 50 mils to about 300 mils with central areas of the gap preferably being less than about lOO mils would satisfy the requirements of the present invention and permit multiple development.

The toner mixture itself is of conventional type consisting of carrier beads having toner powder adhered thereto and having an average diameter of about 15 mils. Obviously, for different sized beads, somewhat different gap widths might be employed.

With the development station constructed as described above, the electrophotographic copier will, in only a single development cycle, produce copies of a quality equal to or better than those obtainable in present systems, At the same time, it will also permit images to be developed a plurality of times so that those images which contain large or extended solid areas can be more completely developed than is possible with conventional systems. The reason why the present inven tion is able to do this can perhaps be better understood by briefly describing the operation of conventional copiers.

Basically, in conventional development stations the width of gap 4 is kept at a relatively large size, for example, 250 mils. As a result, the carrier beads tend to fall through the gap quite freely at a velocity which is significantly higher than the velocity of the drum surface (which is preferably about lO inches/sec. for ap proximately 5 inch diameter drum) e.g. twice as fast or more. This high developer velocity is provided intentionally and generally believed to be necessary so as to ensure that an adequate supply of fresh toner will be supplied to all areas of the latent image as it is passing through the development station. Because of this velocity difference between the developer and the drum surface, however, there will be a substantial amount of physical contact between the carrier beads and the latent image on the drum in the form of a tangential rubbing or wiping action of the beads against the image. Because of this rubbing action, second or subsequent development cycles would cause unacceptably high background density, and hence make multiple development procedures unacceptable. Furthermore, because gap 4 is relatively wide, conventional systems require relatively large amounts of developer and this tends to compound the problem because with more developer there will be more rubbing. Thus, in these conventional systems, in order to obtain high densityblack areas of good quality it has been though to be necessary to either sacrifice machine speed or'machine compactness by utilizing one of the techniques described previously.

These deficiencies in prior art, however, have been overcome in the present invention by making the width of gap 4 substantially narrower than in the prior art. Because thegap 4 is quite narrow, the developer falling through it will be squeezed together into a higher collision frequency than in the prior'artpFor this reason, they will continuously be contacting and interacting with one another and thus be forced to fall at a much lower velocity thanin the prior art and, in fact, will fall at a velocity substantially equal to or only slightly greater than that of the surface of the rotating drum. Because the carrier beads and the drum surface are moving at substantially the same velocity, the amount of scrubbing or wiping between them will be reduced to a minimum and there will not be any significant image degradation during second or subsequent development cycles and, hence, multiple development becomes possible. In fact, it has actually been found that the quality of even background portions is somewhat improved by the multiple development procedure. This is believed to be because in later development cycles, any background toner will tend to be scavenged by the depleted or partially depleted carrier beads and this is noticeable in improving the quality of background areas of the image.

It should also be emphasized that notwithstanding the fact that the developer and the drum surface move through the development station at substantially the same velocity, there is no reduction in the degree of development that takes place with only a single development cycle'over that found in the prior art. This is because in because in the present invention, although the developer is moving slower than in the prior art, it is more densely packed in the gap and, hence, there is still an adequate supply of toner available to develop the image. Also, because the developer is moving slower, there is more time for thelatent electrostatic image to attract powder to it than in the prior art. Finally, by also providing the knurled development electrode as described in the above-referred to US. Patent Application Ser. No.' 429,616, there is substantial assurance that all areas of the image will receive an adequate supply of toner. For these reasons, when the image to be copied contains only line copy or when time is of the essence, a single development cycle at speeds at least comparable to those found in the fastest present-day machines will produce a copy as good as or better than those presently obtainable, and it is only when the original contains large solid areas, that multiple development need be used in order to significantly improve the quality of such images.

The number of development cycles desired without image transfer to belt 8 can be selected as desired by means of control 21 mounted in the machine console. If the image contains primarily line copy, a single development is generally all that is needed. If, however, there are extended dark areas on the image, additional cycles, as required, can be provided merely by turning the dial on control 21. Although essentially as many development cycles as desired can be carried out, (the present invention has actually been practiced with more than 10 cycles without image degradation) the image is usually fully developed after about three cycles and this is all that shouldnormally be necessary. After the desired inumber .of development cycles has been carried out, control 21 willautomatically actuate roller 12 and belt driving rollers 9 11 to move the belt 8 into contact with the drum for transfer of the image thereto.

Thus, with the present invention a development station for electrophotographic copiers has been provided which can selectively be controlled to develop an image as many times as desired or as needed for the particular type of image being copied. Thesystem is highly efficient because all that is required-is that a single control be adjusted. If there are no extended dark areas on the image, a single development is all that is required and the system will work as fast and as efficiently as any other. If there are extended dark areas on the image, it will, of course, take longer to produce copies but this will be more than compensated for by the better quality image.

It should be understood that the above-described em.- bodiment is a presently preferred embodiment only, For example, the multiple development capability described above can also be used to more completely develop images on zinc oxide sheets or the like to be used in plate making operations. In such applications, the developed image would not be transferred to a belt or paper as described above, but, instead, would be used directly for printing. As used herein, the term electrophotographic copier is also intended to include such an application. Because many additions, alterations and omissions may be made from the invention without departing from the scope thereof, it should be recognized that it should be limited only insofar as required by the scope of the following claims.

What is claimed is: I

1. In an electrophotographic copying apparatus including:

a first support medium carrying a latent electrostatic image to be developed thereon; a second support medium for receiving the developed image from said first support medium;

transfer means for transferring said developed image from said first support medium to said second support medium; and,

developing means for developing said latent electrostatic image on said first support medium prior to the transfer thereof to said second support medium by said transfer means, the improvement comprismg:

wherein said developing means includes control means for selectively controlling the number of times that said latent electrostatic image is developed prior to the transfer thereof from said first support medium to said second support medium by said transfer means.

2. Apparatus as recited in claim 1 wherein said control means further includes selection means for selecting the number of times that said latent electrostatic image is to be developed prior to the transfer thereof from said first support medium to said second support medium, said number of times being from one to at least three.

3. Apparatus as recited in claim 1 wherein said developing means further includes:

a. a development station;

b. drive means coupled to said first support medium for passing said latent electrostatic image through said development station;

c. means for supplying developer to said development station as said latent electrostatic image is passed therethrough for developing said latent electrostatic image; and,

d. wherein said control means includes means coupled to said drive means for passing said latent electrostatic image through said development station said selected number of times prior to the transfer thereof from said first support medium to said second support medium.

4. Apparatus as recited in claim 3 wherein said devel oper supplying means includes means for passing said developer through said development station at a velocity substantially equal to the velocity that said latent electrostatic image is passed through said development station by said drive means.

5. Apparatus as recited in claim 4 wherein said development station further includes a development plate spaced from said first support medium to define a narrow gap therebetween, at least a portion of said gap having a width of less than about 100 mils. and wherein said means for passing said developer through said development station comprises means for dropping said developer through said gap, the narrow width of said gap causing said developer to fall at a velocity substantially equal to the velocity of said latent electrostatic image.

6. Apparatus as recited in claim 5 wherein said first support medium comprises a rotatable drum having a surface upon which said latent electrostatic image is formed.

7. Apparatus as recited in claim 6 wherein said development plate includes a knurled surface.

8. Apparatus as recited in claim 1 wherein said transfer means includes means coupled to said control means for bringing said second support medium into contact with said first support medium after said selected number of developing steps for transferring said developed image from said first medium to said second medium.

9. Electrophotographic copying apparatus comprismg:

a. a first support medium carrying a latent electrostatic image to be developed;

b. a second support medium for receiving the developed image from said first support medium;

c. means for transferring said developed image from said first support medium to said second support medium;

d. developing means for developing said latent electrostatic image on said first support medium prior to the transfer thereof to said second support medium by said transfer means; and,

e. control means for controlling the number of times that said latent electrostatic image is developed prior to the transfer thereof from said first support medium to said second support medium by said transfer means.

10. Apparatus as recited in claim 9 wherein said control means includes selection means for selecting the number of times that said latent electrostatic image is to be developed prior to the transfer thereof from said first support medium to said second support medium, said number of times being from one to at least two.

11. Apparatus as recited in claim 9 wherein said developing means further includes:

a. a development station;

b. drive means coupled to said support medium for passing said latent electrostatic image through said development station;

0. means for supplying developer to said development station as said latent electrostatic image is passed therethrough for developing said latent electrostatic image; and,

d. wherein said control means includes means coupled to said drive means for passing said latent electrostatic image through said development station a selected number of times prior to the transfer thereof from said first support medium to said second support medium.

12. Apparatus as recited in claim 11 wherein said developer supplying means includes means for passing said developer through said development station at a velocity substantially equal to the velocity that said latent electrostatic image is passed through said development station by said drive means.

13. Apparatus as recited in claim 12 wherein said development station further includes a development plate spaced from said support medium to define a narrow gap therebetween, at least a portion of said gap having a width of less than about mils. and wherein said means for passing said developer through said development station comprises means for dropping said developer through said gap by gravity, the narrow width of said gap causing said developer to fall at velocity substantially equal to the velocity of said latent electro-

Claims

1. In an electrophotographic copying apparatus including: a first support medium carrying a latent electrostatic image to be developed thereon; a second support medium for receiving the developed image from said first support medium; transfer means for transferring said developed image from said first support medium to said second support medium; and, developing means for developing said latent electrostatic image on said first support medium prior to the transfer thereof to said second support medium by said transfer means, the improvement comprising: wherein said developing means includes control means for selectively controlling the number of times that said latent electrostatic image is developed prior to the transfer thereof from said first support medium to said second support medium by said transfer means.

3. Apparatus as recited in claim 1 wherein said developing means further includes: a. a development station; b. drive means coupled to said first support medium for passing said latent electrostatic image through said development station; c. means for supplying developer to said development station as said latent electrostatic image is passed therethrough for developing said latent electrostatic image; and, d. wherein said control means includes means coupled to said drive means for passing said latent electrostatic image through said development station said selected number of times prior to the transfer thereof from said first support medium to said second support medium.

4. Apparatus as recited in claim 3 wherein said developer supplying means includes means for passing said developer through said development station at a velocity substantially equal to the velocity that said latent electrostatic image is passed through said development station by said drive means.

9. Electrophotographic copying apparatus comprising: a. a first support medium carrying a latent electrostatic image to be developed; b. a second support medium for receiving the developed image from said first support medium; c. means for transferring said developed image from said first support medium to said second support medium; d. developing means for developing said latent electrostatic image on said first support medium prior to the transfer thereof to said second support medium by said transfer means; and, e. control means for controlling the number of times that said latent electrostatic image is developed prior to the transfer thereof from said first support medium to said second support medium by said transfer means.

11. Apparatus as recited in claim 9 wherein said developing means further includes: a. a development station; b. drive means coupled to said support medium for passing said latent electrostatic image through said development station; c. means for supplying developer to said development station as said latent electrostatic image is passed therethrough for developing said latent electrostatic image; and, d. wherein said control means includes means coupled to said drive means for passing said latent electrostatic image through said development station a selected number of times prior to the transfer thereof from said first support medium to said second support medium.

13. Apparatus as recited in claim 12 wherein said development station further includes a development plate spaced from said support medium to define a narrow gap therebetween, at least a portion of said gap having a width of less than about 100 mils. and wherein said means for passing said developer through said development station comprises means for dropping said developer through said gap by gravity, the narrow width of said gap causing said developer to fall at velocity substantially equal to the velocity of said latent electrostatic image.