US3537427A - Electrostatic latent image developing device - Google Patents

Description

0 United States Patent 11113,537,427

[72] Inventor MaslmlchlSato [51] lnt.Cl B05b5/00 Saltama,.lapan [50] Field ofSearch 118/637, 21] Appl. N0. 768,267 637(Misc); 117/175; 346/74(ES) 221 Filed 061. 17, 1968 4s Patented Nov. 3, 1970 1 Referencesclted [73] Assignee FujllhotoFllm Co.,Ltd. UNITED STATES PATENTS s mhm 3,357,402 12/1967 Bhagat 118/637 Priority Oct-17,1967 3,375,806 4/1968 Nost 118 637 137 2 Primary Exam iner-Morris Kaplan [54] ELECTROSTATIC LATENT IMAGE DEVELOPING Attorney-Sughrue, Rothwell, Mion, Zinn and MacPeak ABSTRACT: A cascade developing unit of an electrostatic latent image developing device is provided with a plurality of closely spaced rollers disposed adjacent the electrostatic image carrying layer for transferring the toner from the surface of carrier particles which are cascaded over said rollers to said layer without having said particles contact said layer.

Patented Nov. 3, 1970 Sheet 1 of 2 FIG. I

INVENTOR SATQ H l M A S A M ATTORNEYS.

Patented Nov. 3, 1970 Sheet INVENTOR ATTORNEYS.

ELECTROSTATIC LATENT IMAGE DEVELOPING DEVICE BACKGROUND OF THE INVENTION This invention relates to an improvement of the electrostatic latent image developing device of a dry type.

In a typical example of a dry type developing process, cascade development is well known. For instance, as described in U.S. Pat. No. 2,618,552, uniform electric charges are provided on the surface of a xerographic plate composed of a photoconductive insulating layer on a conductive substrate and then the charged plate may be exposed to light through a photographic positive or negative so that portions of the charge on the light receiving areas leak off leaving a latent electrostatic image on the plate.

The latent electrostatic image thus obtained may be developed by applying a mixture (hereinafter called cascade developer") comprising a finely divided, pigmented, electroscopic powder (hereinafter called toner") in combination with a granular carrier particle to the surface of a xerographic plate. The electroscopic powder deposits in conformity with the charge pattern on the surface of a xerographic plate.

The carrier material is triboelectrically charged with a polarity opposite to that of the electroscopic powder upon frictional engagement therewith and acts to retain the electroscopic powder which is attracted to and surrounds the particle of carrier material. Thereafter, the toner image developed in this way is transferred to a sheet of paper or any other material and fixed by any suitable method. For the purpose of bringing the toners into contact with the surface of a photoconductive insulating layer, particles are cascaded across the surface of the xerographic plate. In the usual live-copy form of development, such as black letters on a white background, the developer powder in cascading over the plate surface adheres to the electrostatic latent image with sharpness and uniform density. However, where the copy to be reproduced is possessed of images having at least some areas of other than livecopy form such as completely dark large areas termed solid areas, the cascade development technique is by itself known to be inherently unable to effect a faithful development thereof.

The use of development electrodes in order to at least partially alleviate this difficulty is known in the art and apparatus therefore are described, for example, in U.S. Pat. Nos. 2,777,418 and 3,011,474.

Despite the improvements afforded by these prior art electrode devices, it has not been heretofore possible to exploit the benefits thereof to the maximum. The effect of an electrode is known to decrease as its spacing from the image surface increases and contra, minimum spacing affords maximum benefit. Because the carrier particles employed in the cascade development system are of a hard, granular material, as compared to a relatively soft, easily destructible photoconductive material comprising the xerographic plate, it has always been necessary with prior art apparatus to support the electrode a sufficient distance from the plate surface to permit freedom of developer flow without binding. Accordingly, because of a practical limitation it has not heretofore been possible to maintain the electrode at the minimum spacing at which optimurn results can be obtained. That is, in a continuously operative system with the developer composition cascading over the photoconductive surface of a xerographic plate moving relative to a stationary electrode, it has been necessary to compromise the electrode to drum spacing in order to prevent any binding of the carrier particles therebetween which might likely score or otherwise deleteriously affect the relatively soft surface of the photoconductor. Spacing therefore, has been critical and difficult to maintain, particularly as associated with rotating drum type xerographic plates with which electrode curvature had to be accurately maintained in order to prevent localized binding. As a result these prior art devices, while improving development characteristics-as compared to development with an absence hereof, have never fully realized the maximum benefits that could be obtained with much closer spacings. I

Now in accordance with the present invention, there has been discovered a novel developing apparatus by which the benefits afforded by the electrode field'correction can be obtained to a maximum. This is achieved in accordance herewith by means of a plurality of rotatable conductive rollers controllably and closely spaced from the surface of the xerographic plate. Dry type developer may be applied to the side of the plurality of rotatable rollers not facing the photoconductive surface whereby toners are held ttiboelectrically to the surface of the rollers and the electrostatic latent image is developed byattracting these toners electrostatically towards the electrostatic latent image.

Some embodiments of the invention are illustrated in the following drawings, and advantages of the present invention will become apparent upon consideration of the following detailed disclosure of specific embodiments of the invention.-

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation view, partly in section, of the device according to this invention;

FIG. 2 is a partial side elevation view showing the principal parts in greater detail thereof;

FIG. 3a, 3b and 3:: show cross sections of various types of rollers which may be employed;

FIG. 4 is a perspective view showing the arrangement and mounting of the rollers;

FIG. 5a and 5b show the longitudinal side elevations of the principal parts of the device illustrated in FIG. I after modification of the roller arrangement; and

FIG. 6 shows the longitudinal side elevation of another example of device according to this invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a side elevation view of a xerographic copying apparatus according to this invention. In FIG. I, the apparatus includes a xerographic drum 1 comprising a conductive cylinder coated with a photoconductive insulating layer as shown and described in U.S. Pat. No. 2,965,756, a set of conductive rotating rollers 21 each having the axis thereof disposed parallel to the xerographic drum 1 facing the surface of the xerographic drum 1 and maintaining, with respect thereto, a close proximity slightly larger than the thickness of a toner image to be formed on the surface of the xerographic drum and a developing unit 3 to feed cascade developer 31 over the surface of the rotating rollers 2 remote from the surface of the xerographic drum.

The diameter of each rotating roller 21 is usually considerably less than the diameter of the xerographic drum and the rollers rotate at nearly the same speed and in the same direction as the drum. The spacing of rollers 21 from each other is smaller than the diameters of carriers of the cascade developer 31. Inside the developing unit 3, the cascade developer 31 is picked up from the bottom of the developing unit 3 and cascaded over the surface of the rotating rollers 21 by a number of buckets 32 on an endless driven conveyer belt 33 and fall from above the rotating rollers 21. When the carrier beads with toner particles clinging to them are cascaded over the surface of the rotating rollers 21 remote from the surface of the xerographic drum 1, the toner particles are pulled off the carrier beads and held to the surfaces of the rotating rollers. The electrostatic fields from the charge pattern on the xerographic drum 1 then pull the toner particles off the surfaces of the rotating rollers. The carrier beads not used to cover the surfaces of the rotating rollers then fall back into the bottom of the developing unit 3.

The surface of each rotating roller is preferably made of a material having a triboelectric relationship of opposite polarity to the toners. This requirement will be satisfied, if, for instance it is made of a material the same as the surface of carrier beads.

The xerographic drum when in operation is generally rotated at a uniform velocity in the direction indicated by the arrow in FIG. 1 so that after portions of the drum periphery pass the charging unit (not shown) and have been uniformly charged they come beneath a projector or other means (not shown) for exposing the charged surface to the image to be reproduced. Subsequent to charging the exposure sections of the drum surface move past a developing unit 3. The rotating rollers 21 act as the development electrode, and a solid area may be developed uniformly.

The rotating rollers which act as the development electrode will now be described with reference also to FIG. 2. p

The spacing of rollers 21 must be smaller than the sizes of the carrier beads and the rollers rotate preferably in the same direction, preventing the carrier beads from jamming and crushing between the rotating rollers.

As previously explained, the charges on the surfaces of the rollers 21 have a polarity opposite the polarity of electrostatic image on the surface ofthe xerographic drum 1.

The charge on the surface of rollers 21 tends to hold the layer of toner on the surface of rollers 21. Within the small air gap between the rollers 21 and the xerographic drum 1, the areas of the photoconductor which are at a relatively high potential create an electrostatic field which is sufficient to cause particles of the layer of toner on the surface of the rollers 21 to jump across the gap and become adhered to those areas of the photoconductor. However, the remaining areas of the photoconductor do not cause an electrostatic field sufficient to overcome the electrostatic forces holding the toner to the surface of the rollers 21. Toner is attracted to and held on the areas of the photoconductor which are at a relatively high potential and represent the visual information on the xerographic drum 1. The toner never comes in contact with the remaining areas of the photoconductor so that background clutter never appears in the final copy.

It is important to note that only a small portion of the available toner on the rollers 21 is transferred to the photoconductor at any one time. The arrangement of plural rollers 21 in the present invention increases the quantity of toner transferred to the photoconductor.

FIG. 3 illustrates the sectional construction of the rotating rollers 21. in FIG. 3a the roller 21a is a solid metallic roller and in FIG. 3b roller 21b is comprised of a conductive core 22 coated with a thin layer 23 of material such as resin which is triboelectrically the same as the carrier or material covering the surface of carrier particles. For instance, the same material as that of the carrier may be employed. In FIG. 3c roller 21:: is composed of an insulating core 24 provided with a conductive layer 25 and an additional thin layer 26, similar to layer 23, covers said conductive layer.

As inferred in these embodiments, the rotating rollers 21 possess conductive parts and these conductive parts act as a developing electrode. Accordingly, these conductive parts 21 should preferably be at a same electric potential. The rotating roller 21 may be theoretically the same in diameter as the xerographic drum 1, but in the case of this invention plural rollers of a small diameter are provided to make the apparatus compact. Between the rotating rollers 21 and the xerographic drum 1, a bias voltage may be impressed in such direction as to drive the toner towards the electrostatic latent image.

FIG. 4 shows an arrangement for mounting the roller for rotation in a pair of end plates 34. The end plates 34 are curved to conform to the drum surface and axes of the rollers are disposed parallel to the axis of the drum. Suitable drive means such as gearing interconnecting the rollers and the drum or other portions of the drive train of the machine may be provided. In accordance with the embodiment the rollers will be geared together for rotation in the same direction at the same speed.

In above mentioned embodiments, the rotating rollers 21 rotate in the same direction and the spaces therebetween are smaller than the diameters of the carriers of the cascade developer 31. FIG. illustrates other embodiments, where the intervals between the rotating rollers are larger than the diameters of carriers. In FIG. 5a and b the screening plates 1, 4' may be placed in between the respective rotating rollers 21 so that the interval between rotating roller 21 and the screening plate 4 or 4' is smaller than the diameter of carrier. The rotating rollers 21 need not always rotate in the same direction in this apparatus. Even if the interval between rotating rollers is larger than the diameter of the carriers, the entanglement and crushing of the carriers between the rotating rollers 21, the stoppage of the rotating rollers and the contact of carriers with the vacuum-evaporated selenium layer can be prevented.

FIG. 6 illustrates an example of a device for feeding toner to the rotating rollers 21 by a magnetic brush method. In this device the developer feeding unit 3' is provided with a helical rotary powder lifting mechanism 5. By means of this mechanism 5, the developer, comprising iron powders as the carriers, is supplied from above to the rotary magnets 6 to produce magnetic brushes" 61 whereby the toners are caused to adhere to the nonmagnetic rotating rollers 21. The rotary magnets 6 are provided with doctor blades 62 to scrape off the magnetic brushes 61, so as to provide new brushes" on the rotary magnets 6 at all times. In other respects, the device is similar to the device illustrated in FIG. 1.

In the device according to this invention, as described above, toners are supplied to an insulating layer bearing an electrostatic latent image through the medium of the conductive rollers facing the said layer with a very narrow interval and therefore the rotating rollers, having conductive portions at least near their surface, serve as the developing electrodes and a satisfactory image not involving any edge effect can be obtained despite the fact that the device is a dry type developing device. Accordingly, excellent development, even with respect to an image with continuous gradation, can be accom plished.

Further, the toners do not stick mechanically to an insulating layer and therefore the possibility of fogging is minimized. Also, since the carriers won't come in touch with an insulating layer as in the case of the conventional dry type developing devices, the insulating layer will in no way be damaged. Consequently, image reproduction involving little scratch can be achieved. Especially in the case of a device provided with such an insulating layer as a zerographic plate to be used repeatedly, the life of the insulating layer is extended and satisfactory images can be obtained. Moreover, as plural rotating rollers are provided, clogging of dry type developer never takes place and further, inasmuch as the toners fully face an insulating layer and the facing time is long enough, the developing process can be carried out at high speed, without involving any uneven development. As above, the device of this invention is a device suitable for the reproduction of continuous gradation involving neither fogging nor edge effect and without causing the clogging of dry type developer, while maintaining, on the other hand, the simplicity and reliability of the conventional dry type developing equipment. The device itself can be constructed quite compactly and in addition, it is suitable for automation.

lclaim:

1. In a dry electrostatic latent image developing apparatus of the type having a photoconductive layer of material disposed over a conductive base layer and bearing an electrostatic charge pattern; a cascade developing unit comprising a plurality of rollers mounted parallel to and in close proximity to each other and said photoconductive layer and means for cascading a plurality of toner carrying particles over the portion of said rollers remote from said photoconductive layer and said rollers being in an arrangement to effect transfer of toner material to said photoconductive layer but to inhibit the transfer of carrier particles thereto.

2. In an apparatus as set forth in claim ll wherein the spacing between the rollers is smaller than the diameters of the toner carrying particles to inhibit the particles from contacting said photoconductive layer.

3. In an apparatus as set forth in claim 2 further comprising means for rotating each of the rollers in the same direction.

a resin coating on said conductive material having a triboelectric charge adapted to attract toner material thereto.

7. ln an apparatus as set forth in claim 5 wherein said conductive base layer is connected to said conductive material of said rollers.

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