US3783827A

US3783827A - Liquid development apparatus for electrophotography

Info

Publication number: US3783827A
Application number: US00214252A
Authority: US
Inventors: O Fukushima; M Sato; S Honjo
Original assignee: Fuji Photo Film Co Ltd
Current assignee: Fujifilm Holdings Corp
Priority date: 1970-12-30
Filing date: 1971-12-30
Publication date: 1974-01-08
Anticipated expiration: 1991-01-08
Also published as: DE2165450B2; BE777528A; GB1362074A; DE2165450C3; AU466983B2; AU3746471A; CA949309A; IT945720B; DE2165450A1; FR2121085A5; JPS5124254B1; NL7118114A

Abstract

An electrophotographic developing apparatus comprising: multiple carrier rollers, held in supporting means, conveying an electrophotographic sheet material to be processed and driven at a common speed by driving means; two endless belts hung at the side ends of said carrier rollers to form a loop including at least some of said carrier rollers; multiple development electrodes arranged facing to carrier rollers; and a means of supplying liquid developer onto the electrophotographic sheet which is conveyed between said carrier rollers and said development electrodes, whereby a developer pool is formed on the electrophotographic sheet, said pool being limited by the sheet as bottom plane, and the adjacent developing electrodes and the supporting means as side borders, said sheet being conveyed with its rear surface in contact with said endless belts.

Description

United States Patent [191.

Fukushima et a1.

[ Jan. 8, 1974 1 LIQUID DEVELOPMENT APPARATUS FOR ELECTROPHOTOGRAPHY [73] Assignee: Fuji Photo Film Co., Ltd.,

Kanagawa, Japan 221 Filed: Dec. 30, 1971 211 App]. No.: 214,252

[30] Foreign Application Priority Data Dec. 30, 1970 Japan 45/122799 [52] US. Cl 118/637, 95/89A, 117/37 LE, 11'8/407, l18/DlG. 23, 355/10 [51] Int. Cl .f. G03g 13/10 [58] Field of Search 118/410, 419, 637,

ll8/DIG. 23, 625, 407, 424; 117/37 LE, 93.4 A; 355/10; 95/89 R 3,643,628 2/1972 Sugarman 118/637 3,299,791 l/1967 Fukuda 95/89 3,650,622 3/1972 Morse 355/10 3,672,329 Yamanoi 118/637 Primary ExaminerMervin Stein Assistant Examiner-Leo Millstein Attorney-Gerald J. Ferguson, Jr. et a1.

[57] ABSTRACT An electrophotographic developing apparatus comprising: multiple carrier rollers, held in supporting means, conveying an electrophotog raphic sheet material to be processed and driven at a common speed by driving means; two endless belts hung at the side ends of said carrier rollers to form a loop including at least some of said carrier rollers; multiple development electrodes arranged facing to carrier rollers; and a means of supplying liquid developer onto the electrophotographic sheet which is conveyed between said carrier rollers and said development electrodes, whereby a developer pool is formed on the electrophotographic sheet, said pool being limited by the sheetas bottom plane, and the adjacent developing electrodes and the supporting means as side borders, said sheet being conveyed with its rear surface in contact with said endless belts.

2 Claims, 7 Drawing Figures PAIENTEIJJAII 8:914

SHEEI 1 [If 5 PAIENTED 8 SHEET 2 0F 5 FIG. 2

FIG. 4

PATENTED 81974 3. 783 827 SHiU 3 OF 5 FIG.3

PATENTEB JAN 8 i974 SHEET [1F 5 PAIENTEDJ 81974 3,783,827

sum 5 0r 5 FIG. 60

LIQUID DEVELOPMENT APPARATUS FOR ELECTROPIIOTOGRAPI-IY BACKGROUND OF THE INVENTION x 1. Field of the Invention This invention relates to a development apparatus for use in electrophotography and, inparticular, to one suited to process flexible electrophotographic sheet materials to obtain visible images.

2. Description of the Prior Art A flexible electrophotographic sheet material may be produced by treating a flexible support such as paper or a plastic film to impart electroconductivity thereto and providing thereon a photoconductive layer by vacuum deposition of selenium or by coating thereon a mixture comprising an inorganic photoconductor such as zinc oxide or titanium oxide and a resinous binder. The sheet material thus prepared is uniformly charged under subdued light to sensitize the photoconductive layer, and then image-exposed whereby the surface charge decays in proportion to the intensity of impinged light and a corresponding electrostatic latent image results. Then the sheet is brought into contact lines of force, however, above the photoconductive layer do not distribute in proportion to the charge density of the latent image, which gives rise to the so-called edge effect in the developed image-Hence the resulting image takes an appearance quite different from the original one. To resolve this problem it is a common practice to employ grounded conductor, which is referred to as a development electrode, which is located in parallel relation-to and close to the latent image bearing surface and functions to reduce the edge development. ln this case the resulting toner image becomes a far more exact reproduction of the charge distribution of the latent image. This is attributed to the fact that the conductive development electrode increases the number of lines of force directed above the surface from the latent image and effectively absorbs them. Such an effect is referred to as the development electrode effect. This effect is more marked with a smaller specific resistance electrode and with a smaller gap between the electrode and the surface to be developed.

In the case of electrophoretic development, however, too small an electrode gap disturbs the supply of developerliquid onto the latent image, sometimes causing a final image with a low saturation density or one with mottles or non-uniformity. Hence, a'liquid development apparatus is required which exhibits an intense electrode effect and at the same time permits a sufficient amount of developer to reach the developed surface.

In addition to the edge effect, streaks and nonuniformity caused by an insufficient developer supply are the main defects to be'considered. When high and low density areas exist contiguously, streaks are observed which flow out of the former to the latter area like the tail of a comet. This phenomenon has been explained as follows. When a relative movement exists between the developer and the latent image bearing surface, toner particles coming upon a high charge density area are subject to an electrostatic attraction as well as inertia due to developer flow, which leads to toner deposition diffusing out of the high density area towards the downstream side into the low density area. As is empirically known, a larger velocity of the relative movement and a greater electrode effect for a constant relative velocity gives more streaks.

Another serious problem attending liquid development is the difficulty with which the developer is supplied uniformly on the whole surface of the sheet to be developed. Even when a uniform supply is realized, uniform development cannot be expected if the amount of the developer is not abundant. Thus the developer has to be supplied uniformly and sufficiently in a relatively brief period, since prolonged development will bring about a decay of the latent image charge and at the same time an increase of fog due to the residual potential. I Y

The decay of the latent image charge is effected, besides through the neutralization of the charge by toner deposition, by the neutralization thereof .by ionic species other than the toner, and also by leakage through the photoconductive coating. Such charge decay provides a developed image with a lower saturation toner density than what would be expected for perfect neutralization of the charge solely by toner deposition.

Residual charge is observed at thehighlight regions in an exposed coating where a sufficient amount of light has been irradiated and no toner deposition is expected. At the early stage of development, toner deposition proceeds at high charge density areas, and then progressively at lower areas. When the development is extended, which is also not preferred from the viewpoint of processing efficiency, the residual charge begins to be detected, thus forming an undesirably high level of background.

From the above descriptions, one will understand, especially in the case of developingcontinuous-tone images, the significance of a development method which can provide, in a brief developing'period, toner images free of edge effect, streaks, and mottles due to non-uniform developer supply, and which does .not have significant residual charge, thus giving a low back ground. As is obvious from .the above explanations, conditions conflicting each other-must be fulfilled in order to solve all of these problems. To minimizethe edge effect leads to an extremely small'electrode gap between the electrode and the surface to be developed. Such a small gap hinders the ample supply and exchange of developer, resulting in lack of uniformity in the extent of development throughout the whole surface. If one resorts to a swift stream of the developer, streaks tend to appear due to a large relative velocity between the latent image bearing surface and the de- SUMMARY OF THE INVENTION The principal object of the present invention is to disclose developing apparatus fulfilling the abovedescribed mutually conflicting requirements as much as possible with simple and compact structures.

Another object of the invention is to disclose apparatus suited for developing continuous-tone images on flexible electrophotographic sheet materials.

Apparatus in accordance with the present invention can develop images showing a high degree of uniformity in the extent of development, and free of streaks which are frequently observed at the boundaries between very highand low density areas. Moreover, it can easily be incorporated in automatic processing machines because of its simple and very compact structure.

The essential feature of the invention lies in providing a temporary pool of developer on the surface to be developed only when an electrophotographic sheet is fed into the apparatus. Such a developer pool enables refreshment and supply of developer without causing relative movement of developer to the image surface. The word pool in the present specification means a state wherein a developer layer not thinner than two mms is stationarily formed on the surface to be developed and disturbance occurring near the top of said layer is not directly propagated onto the image surface.

Care must be taken, as a matter of course, in the design of the apparatus so that there exists between the liquid layer and the image surface no relative large difference of the flowing rates of 3 cm/sec.

Developer supply by means of pool formation is proved to be quite even and thus provides uniformly developed images. Further, the developer spraying from supply nozzles does not directly run against the image surface but onto the upper surface of the pool, so that the relative velocity of the developer to the image surface is small enough to avoid the occurrence of streaks.

A flat conductive plate may be used as the development electrode, which is immersed in the developer pool facing the image surface. However, such an arrangement tends to cause uneven development due to insufficient supply of developer, since the electrode covers the surface with a small spacing. According to our experiments, as a more preferred arrangement, an electrode comprising multiple conductive rollers of small diameter is employed, in which case the developer liquid is provided through the spacing between the rollers onto the image surface. As the rollers are rotated at the same circumferential speed as the transport speed of the sheet, the relative movement between the developer and the image surface is minimized, thus preventing streaks.

In the apparatus in accordance with the present invention, the developer pool is formed only when an electrophotographic sheet is fed into the apparatus, and after the sheet is exhausted the liquid falls down and circulates freely. Therefore, refreshment of the developer is completed for every newlycoming sheet.

In a development method in which a developer pool is always formed in a vessel through which sheet materials are passed, the circulation or exchange of developer is inferior to the present method.

BRIEF DESCRIPTION or THE DRAWINGS FIG. 1 illustrates a cross-sectional side view of one embodiment of the invention, while FIG. 2 shows the crosssectional view of the same embodiment cut along the line A-A' in FIG. 1;

FIG. 3 illustrates another embodiment of the invention by the cross-sectional side view, which differs from the foregoing embodiment in the use of another pair of endless. belts of narrow width 25 suspended between the electrode rollers at the side ends. These belts hold the side edges of the sheetwhich are outside the imaging area whereby the electrode rollers are lifted up by the thickness of said belts 25 so as to leave an electrode gap of the same magnitude.

As is clearly seen from FIG. 4, which is the crosssectional longitudinal view of the apparatus shown in FIG. 3, there remains a uniform gap between the sheet surface and the electrode rollers since the endless belts 25 hold only the side edges of the sheet.

FIG. 5 illustrates still another preferable embodiment of the invention in the form of schematic crosssectional side view. This embodiment differs from that shown in FIG. 3 in that the endless belt 15 is not hung over the whole carrier rollers. Instead, the first and terminal ones are excluded of the loop.

Other types of auxiliary electrode are shown in FIG. 6; in FIG. 6a,a flat plate is used as electrode, while in FIG. 6b, one having a cross section like that of a trough is employed into which the developer is fed. This electrode proved quite effective to reduce the nonuniformity of developer supply.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention will now be explained in more detail by describing some practical embodiments with reference to the drawings.

Referring now to FIG. 1, an electrophotographic sheet is coming into the developing unit with the surface upward on which is formed an electrostatic latent image. Carrier rollers 1-7 have their axes fixed at the bearings provided in side supporting boards (not shown) and are driven by an external motor (not shown) through gears or chains (not shown) at a constant and common circumferential speed. Electrode rollers 8-14 are made of a conductive material, such as metal. On both side tips of the carrier rollers are suspended two endless belts 15. Developer is fed through supply nozzles 16-21 and is kept in a reservoir tank 22, circulated through an exit 23 provided in the bottom of the tank to the nozzles by means of pump (not shown). When the toner is consumed and its concentration is reduced after processing of a large number of sheets, a separately kept replenishing concentrate is added to the developer automatically or manually.

Developer is sprayed from nozzles 16-21 through fine slits or lines of small openings. Guide plates 22-27 are provided for each of the nozzles so as to ensure a uniform flow of developer. The guide plates and the rollers form a wedge-shaped space, in which the liquid is briefly held up instead of directly running against the sheet; this stagnation is effective to reduce the occurrence of streaks and provide an even supply of developer.

The essential feature of the invention, however, lies in the use of the endless belts 15. As will now be explained referring to FIG. 2, the belts are hung at the side tips of the carrier rollers and rotated in contact engagement with both

side support boards

28 and 29. The belts prevent the developer from falling freely through the gap between the sheet and the

side boards

28 and 29. Since the sheet to be processed 100 places its side ends on the belts 15, the bottom of the temporary pool comprises the sheet 100 and the two endless belts 15. With such an arrangement, the developer liquid is kept on the running sheetduring the passage through the developing unit.

As is illustrated in FIG. 2, the axis of each carrier roller may protrude at one end out of the side board to provide a driving mechanism such as gear or chain which is driven by a motor.

To prevent developer leakage along the surface of the protruded axis, a shielding disk 30 is set on the axis.

Since the electrode rollers are held in such a manner that they can move freely in the vertical direction, the side boards are provided with apertures larger than the size of the roller axis. To prevent the liquid developer.

from leaking through these apertures, covers 31 and 32 are installed parallel to the direction of travel of the sheet. The electrode roller has a uniform diameter to form the side wall transverse to the direction of travel of the sheet for thetemporary pool, preventing developer leakage. Since the rollers are rotated by the friction with the image surface of the sheet, the friction imposed onthe rollerby the side boards must be as light as possible. Experiments have proven that a clearance of about one-fifth mm is permissible between the side board and the end of the roller to effect pool formation. Such clearance is also desirable in view of frictional load for roller rotation. As is shown in FIG. 1, when a sheet is fed into the developing unit, the development electrode rollers are lifted up by the thickness of the sheet and convey the sheet, together with the carrier rollers, at the same time forming a developer pool between adjacent roller pairs.

Suitable materials for the endless belt include electrically conductive belts, insulating plastics, fabrics, etc. When the belt is insulating, the sheet may be connected to ground through the carrier rollers. In the case where grounding is required through the endless belt, an insulating plastic belt coiled with a fine metal wire may be employed, far easier to make than a metallic belt.

The image surface of an electrophotographic sheet.

has microscopic roughness in the surface. Where the degree of roughness is high, one may bring the electrode rollers into direct contact with the image surface during development without causing deterioration of the developed image clue to directly contacting the roller, since the roller surface is only in virtual contact with the minutely raised portions of the sheet surface. In such case, as a matter of course, the directly contacted raised portions are soiledby toner. Such soiling, however, is hardly recognizable by the naked eye as long as the dimensions of the raised portions are less than that which is resolvable by human eyesight (0.1 mm or less).

The apparatus as shown in FIGS. 1 and 2 is used only for roughened sheets. If, on the contrary, the roughness is small, that is, the image surface is quite smooth, the

FIG. 5 shows a schematic sectional side view of the I other embodiment. In this apparatus, the endless belts 15 are suspended on both side tips of carrier rollers 2-6. The difference between the apparatus of FIGS. 3 and 5 is that in the latter apparatus the endless belts 15 are not suspended on the end rollers 1 and 7. This apparatus is divided by seven pairs of rollers into six divisions.

The leakage of developer through the gap becomes sufficiently small to form a developer pool when this gap is adjusted smaller than 0.5 mm. More strict limitation is required for the first roller, 8, and the terminal rollers 14 shown in FIG. 3 to prevent counterflow and leakage of developer. For these rollers a gap about 0.1 mm is permissible, whereby the pool can be formed as quickly as in the case where no gap exists between the electrode rollers and the sheet surface. Providing grooves on the electrode roller enables ready adjustment of the electrode gap as well as prevention of undesirable shift of the endless belts.

The endless belt may be made of conductive or insulating materials. Through the use of wire-coiled belts, grounding of the conductive coating of the sheet material or an application of bias voltage may be performed.

The sheet material can be conveyed under the grasp by the upper and lower belts at its edges along the pass with certainty even if the sheet tends to curl of is in a curled condition.

In the FIG. 3 embodiment which shows a schematic sectional side view of another embodiment, the differene from that shown in FIG. 1 is the provision of two endless belts 25 of narrow width. at both ends of the electrode rollers. These endless belts ride on the edge portions of the electrophotographic sheet and the electrode rollers are kept away from the image surface of the sheet by the thickness of the endless belt.

In this arrangement no pool is formed for the first and last divisions, since the developer is not fed to these divisions from the nozzles and streams therein from the adjacent divisions, falls down through the clearance between the side boards, whereby rushing out or counterflow of developer into another unit can be eliminated. Further, smaller conductive rollers can be provided between the larger rollers to enhance the effect of electrode development. These smaller rollers are also held by the side boards so that they are movable along the vertical direction. These smaller'rollers work also as development electrodes, and may be conductive plates or A-shaped electrodes as shown in FIGS. 6a and 6b.

The pass of the sheet is most preferably straight, as is the case with the illustrated embodiments, in view'of the ease with which the electrode gap can be maintained uniform. However, to improve the draining off of developer and prevent the flowing out of developer, the pass may be made arc-shaped.

The main electrodes may be made of flat metal plate, or a large number of fine rollers may be closely arranged. The number of the carrier rollers may also be more or less than seven which was the number of the carrier rollers for the illustrated embodiments.

In the present invention, the upper rollers and the lower rollers may be electrically connected or insulated from each other, or a bias voltage may be applied therebetween according to the choice of normal and reversal'development.

What we claim'is:

1. An electrophotographic developing apparatus comprising:

i. multiple carrier rollers, held in and between fixed side plate supporting meansyconveying an electrotween said carrier rollers and said development electrodes, whereby a developer pool is formed on the electrophotographic sheet, said pool being limited by the sheet and the endless belts as bottom plane, and the adjacent development electrodes and the fixed side plate supporting means as side borders, said sheet being conveyed with its rear surface in contact with said endless belts.

2. Apparatus as in claim 1 wherein the development electrodes are rollers and including a further pair of endless belts disposed at the ends of said development rollers for spacing said development rollers from said electrophotographic sheet material.

Claims

1. An electrophotographic developing apparatus comprising: i. multiple carrier rollers, held in and between fixed side plate supporting means, conveying an electrophotographic sheet material to be processed and driven at a common speed by driving means; ii. two endless belts hung at the side ends of said carrier rollers to form a loop including at least some of said carrier rollers, said endless belts being contiguous to said fixed side plates; iii. multiple development electrodes arranged facing to carrier rollers; and iv. a means of supplying liquid developer onto the electrophotographic sheet which is conveyed between said carrier rollers and said development electrodes, whereby a developer pool is formed on the electrophotographic sheet, said pool being limited by the sheet and the endless belts as bottom plane, and the adjacent development electrodes and the fixed side plate supporting means as side borders, said sheet being conveyed with its rear surface in contact with said endless belts.