GB2073057A - Developing Electrostatic Images - Google Patents

Abstract

Electrostatic latent images are developed using one-component toner by employing a magnetic brush (either a roll (12) or belt (22)) to supply toner to a developer member (either roll (11) or belt (21)). At the transfer region between the brush and member, opposed magnetic poles may be provided. <IMAGE>

Description

SPECIFICATION Image Developing Method and Device Therefor Background of the Invention Field of the Invention The present invention relates to developing method and a device therefor for developing an electrostatic latent image with one-component magnetic developer.

Description of the Prior Art There are already known various developing methods utilizing one-component developer.

Among these particularly noteworthy is so-called toner transition development, in which onecomponent developer applied as a thin uniform layer on developer bearing means is brought into facing relationship to an electrostatic latent image bearing surface with a small clearance therebetween whereby the developer is caused to fly from said developer bearing means to the image bearing surface by the electrostatic attractive force to perform image development, as disclosed in the Japanese Patent Publication Sho 41-9475 and in the U.S. Patent No. 2,839,400. Such developing method assures satisfactory development completely without background fog as the developer is not attracted nor brought into contact with the non-image area having no image potential.Besides the absence oF carrier particles eliminates the troubles arising from the change in mixing ratio thereof in the developer or from the time-dependent deterioration of the carrier particles.

Furthermore, the present applicant proposed novel developing methods as disclosed in the U.K.

Patent Applications 7835339, 35338/78 and Serial Nos. 7925725 and 7925724.

The former method is featured in positioning one-component magnetic developer, a developer bearing non-magnetic means and a magnetic field generating means in this order to form a thin uniform layer of said developer on said developer bearing means under the effect of the magnetic field generated by said magnetic-field generating means, and maintaining said thin developer layer in facing relation to an electrostatic latent image bearing surface with a small clearance therebetween thereby extending the developer corresponding to the image area by the electrostatic attractive force of said latent image to perform the image development. This method also is capable of providing a completely fog-free visible image as the development is carried out without contact of the developer with the nonimage area.

The latter method is featured in positioning one-component magnetic developer, a developer bearing non-magnetic means and a magnetic field generating means in this order to form a thin uniform layer of said developer on said developer bearing means under the effect of the magnetic field generated by said magnetic-field generating means, and maintaining said thin developer layer in facing relation to an electrostatic latent image bearing surface with a small clearance therebetween, wherein the image development is effected by applying an AC developing bias voltage across said clearance between the image bearing surface and developer bearing means and changing said clearance in time.In this method the developer reaches also the non-image area in the initial stage to effect the development of halftone image but reaches only the image area in the later stage, thereby providing the image without background fog and with improved halftone reproducibility in comparison with the above-explained method.

As explained in the foregoing, the developing methods in which a thin layer of one-component developer is faced to the latent image bearing surface are far superior to other conventionally known methods in terms of the developing performance, image reproducibility, service life of the developer etc.

These developing methods, different from the brush development in which brush-shaped developer is maintained in sweeping contact with the latent image bearing surface to obtain a visible image, utilize a thin layer of developer which is spaced from the latent image bearing surface in the absence of externally applied electric field. Fig.1 shows an example of developing device employed in such developing method, wherein shown are a latent image bearing member 1, a sleeve roller 2 internally provided with fixed magnetic poles 2a-2d, a fixed magnetic doctor blade 3, a one-component magnetic developer 4, a container 5 therefor, a scraper 6 for removing the developer from the surface of said sleeve roller, and an AC bias voltage source 7 as described in the aforementioned U.K.Patent Applications Ser. Nos. 7925725 and 7925724.

In such device the magnetic pole 2b positioned inside the sleeve 2 and said magnetic blade 3 provide a strong magnetic field for obtaining a practical thin developer layer of 50-100 u in thickness, and the insulating one-component developer is subjected to frictional charging through contact with the rotating sleeve 2 and is defined in thickness by the magnetic field present between the sleeve 2 and the blade 3 before being supplied to the developing area D. The clearance between the sleeve 2 and the blade 3 is an important factor in forming a strong magnetic field and has to be as narrow as 200 to 400 u, for example for forming a flux density of 800 gauss on the sleeve, corresponding to the above mentioned layer thickness.For this reason the magnetic blade 2 has to be finished with a high preceision at the pointed end thereof. Also the magnetic thickness defining in said narrow clearance causes a strong distortive force in the developer 4 leading to developer caking and thus giving rise to a streak in the developer layer. Such streak is also caused by a foreign matter stuck in said clearance. Also certain developers cannot satisfactorily fill the already consumed portions of the developer layer, thus leading to so-called sleeve ghost phenomenon. Furthermore, incidentally aggregated developer, if deposited on the sleeve 2, creates a relatively thick developer layer around such aggregate, which can hardly be restored to the thin state even after many turns of the sleeve.Also the amount of charge on the developer, which is generally determined by the frictional charging between the sleeve 2 and the insulating developer becomes reduced after prolonged use as the sleeve surface is covered by minute particles of resin employed in the developer, eventually leading to so-called sleeve deterioration involving insufficient developer application on the sleeve. For these reasons there is often needed scraping means, as the scraper 6 in Figure 1, for removing the developer from the sleeve 2 before developer application.

The present invention aims to provide a developing method and a device therefor capable of alleviating at least partly the aforementioned drawbacks, particularly the troubles arising from the use of narrow clearance of strong magnetic field mentioned above.

One aspect of the present invention provides a developing device with relatively loose requirements for the precisions of component parts and assembly thereof.

Another aspect of the present invention provides a developing method and a device therefor capable of removing the aggregated developer and foreign matter deposited on the surface of developer bearing means such as a developing sleeve in the course of rotation thereof without particular developer scraping means coming into direct contact with said developer bearing means and or restoring a thin developer layer thereon.

A further aspect of the present invention provides a developing method and a device therefor capable of preventing the aggregation of developer while the developing sleeve is stopped.

Thus, according to the present invention there is provided a developing method in which one-component magnetic developer is transported on a developer bearing member to a developing position for image development, featured in supplying said developer onto a developer-feeding bearing member to form thereon a magnetic brush of developer under the effect of a magnetic field, then bringing said magnetic brush into contact with the surface of an image-developing developer bearing member to form thereon a thin uniform layer of the developer, and bringing said thin layer to a developing position to develop an electrostatic latent image formed on a latent image bearing surface.

There is also provided: a developing method as described above wherein said thin layer of the developer is so spaced in the developing position from the latent image bearing surface as to form a clearance therebetween, across which an AC bias electric field is applied; There is further provided:: a developing device adapted for transporting one-component magnetic developer on a developer bearing member to a developing position for image development, comprising a developer-feeding bearing member for supporting said magnetic developer thereon, a magnetic-field generating means for forming a magnetic brush of said developer on said bearing member, an image-developing developer bearing member so positioned as to be in contact with said magnetic brush, means for positioning said developerfeeding bearing member in the proximity of said image-developing bearing member, and means for displacing said image-developing bearing member to transport said thin developer layer to the developing position.

There is also provided a developing device as described above wherein said image-developing bearing member is provided with magnetic members; a developing device as described above wherein said magnetic developer is insulating developer; a developing device as a preferred embodiment of the present invention, adapted for forming a magnetic brush of one-component developer on a first sleeve roller internally provided with fixed magnetic poles, and bringing said magnetic brush into contact with the surface of a second developing roller for transporting the developer to a developing position, thereby causing frictional charging of insulating developer and thus forming a thin developer layer on said developing roller; There is still further provided:: a developing method and a device therefor featured in forming a magnetic brush of onecomponent magnetic developer on a developerfeeding bearing member by means of a magnetic field, bringing said magnetic brush into contact with the surface of an image-developing bearing member positioned in the proximity of a surface to be developed thereby forming a thin developer layer on said image-developing bearing member, transporting said thin developer layer to a developing position to develop said surface, and regulating said magnetic brush present in a portion of said developer-feeding bearing member facing said image-developing bearing member in such a manner as to reduce the developer density in said facing portion after the completion of image development but prior to the termination of displacement of said image-developing bearing member.

The foregoing and still other objects and advantages of the present invention will be made apparent from the following description to be taken in conjunction with the attached drawings.

Brief Description of the Drawings Fig. 1 is a cross-sectional view of an example of conventionally known developing devices; Fig. 2 is a cross-sectional view of an embodiment of the basic structure of the developing device of the present invention; Fig. 3 is a magnified view of the developer application in the developing device shown in Fig.

2; Fig. 4 is a cross-sectional view showing another embodiment of the developing device of the present invention; Fig. 5 is a magnified view of the developer application in the developing device shown in Fig.

4; Figs. 6 and 7 are schematic cross-sectional view showing other embodiments of the developing device of the present invention; Figs..8A,8B,9A,9B, 1 OA, 1 OB, 11A, 11B, 11 C, 1 2A and 1 2B are schematic cross-sectional views showing still other embodiments of the developing device of the present invention; Fig. 13 is a view showing the magnetic force and the frictional resistance working on the developer at the application of a thin developer layer on the developing sleeve; Fig. 14 is a view showing an inappropriate magnetic force arrangement incapable of providing a thin developer layer; Fig. 15 is a schematic view of another example providing a stable thin developer layer; Fig. 16 is a schematic view showing an example of inappropriate magnetic pole arrangement;; Fig. 1 7 is a schematic laterial view of an embodiment; Fig. 18 is a magnified view of the developer retaining portion in the device shown in Fig. 17; Fig. 19 is a chart showing the relation between the number of turns of developing sleeve and the density of applied developer; Fig. 20 is a chart showing the state of developer application as a function of the clearance a between the magnetic brush defining blade and the magnetic brush sleeve roller and of the clearance b between said sleeve roller and the developing sleeve; Fig. 21 is a lateral view showing an inappropriate application state; and Fig. 22 is a lateral view showing another embodiment.

Detailed Description of the Preferred Embodiment Now the present invention will be clarified in detail by the preferred embodiments thereof shown in Fig. 2 and other ensuing drawings.

Fig. 2 shows a first embodiment of the present invention, wherein a latent image bearing member 1 and one-component magnetic developer are same as those shown in Fig. 1. An image-developing bearing member 11 or a developing sleeve, is provided in the proximity of said image bearing member 1 at such distance that a clearance is formed between said image bearing member and a thin developer layer to be formed on said developing sleeve 11 as will be explained in the following.A non-magnetic sleeve roller 12 is rotatabiy provided in facing relationship to and parailel to said developing sleeve with such a clearance thereto as will be explained in the following and as will be achieved by spacing means for example spacing rollers, and constitutes developer-feeding bearing member internally provided with a fixed magnet roll 13 having plural magnetic poles 1 3a-1 3d as illustrated. Suitable drive means are provided to drive said developing sleeve 11 anticlockwise for causing a peripheral movement in the same direction as that of the latent image bearing member 1 and to drive said sleeve roller 12 clockwise for causing a peripheral movement in the same direction as that of said developing sleeve 11.

A doctor blade 1 6 is provided in the magnetic field generated by the magnetic pole 1 3d and with a small clearance from said sleeve roller 12, for limiting the height of the magnetic developer brush formed on said sleeve roller 12 under the effect of said magnet roll 13. Said developing sleeve 11 and develop-feeding sleeve roller 12 are made of such materials capable, in the frictional charging series, of furnishing a charge of a polarity opposite to that of the latent image to the one-component magnetic insulating developer 4 to be explained further in the following, and can for example be composed of metals such as stainless steel, aluminum, iron, etc. or insulating materials such as synthetic resins satisfying the above-mentioned requirement.

As an example, said one-component magnetic insulating developer or toner can be of the following compcsition: Styrene-maleic 70 wt.% acid resin Ferrite 25 wt.% Carbon black 3 wt.% Charge controlling 2 wt.% agent Colloidal silica 0.2 wt.% (added externally) A voltage source 17 for applying an AC bias field across the developing clearance is provided, thereby causing the thin developer layer on the developing sleeve to deposit in the image area of the latent image bearing member with improved tonal rendition, as described in the U.K. Patent Applications Ser. Nos. 7925725 and 7925724.

The one-component magnetic developer supplied from a container 1 5 is applied onto the surface of the sleeve roller 12 along with the rotation thereof to form magnetic brushes thereon under the effect of magnetic poles. Said magnetic brushes are formed by the magnetic poles 13b, 1 3a and magnetically transported in succession by the rotation of the sleeve roller 12, and limited in height by the blade 1 6 in a position facing the magnetic pole 13d.Subsequently in a position facing the developing sleeve 11, the magnetic brush is extended into vertical spikes under the effect of magnetic field from the pole 13c, and said spikes are agitated in contact with the surface of said developing sleeve 11 whereby said magnetic insulating developer is charged by friction with said surface to form a thin layer converting said surface Said thin layer is transported with the rotation of the developing sleeve 11 to a developing position D for performing the image development as already explained in relation to Fig. 1.

Now there will be shown a preferred example of the above-explained embodiment.

Example I The developing device shown in Fig. 2 was prepared with an aluminum blade 16 spaced by 1 mm from a stainless-steel sleeve roller 12 with a surface flux density of 800 gauss, which was in turn spaced by 1 mm from a stainless-steel developing roller 11. The device provided a thin layer, approximately 80 u in thickness, of a onecomponent developer showing a sufficient density as high as 1.60 in reflection density and also provided a sufficiently high density in the developed image.

As will be apparent from the foregoing example, the developing device of the present invention allows to employ a clearance between the developer defining member or blade 16 and the sleeve roller 12 more than twice larger than that in the conventional devices, and this fact reduces the frequency of caking even in easily aggragating developer such as microcapsule toner and the trouble of uneven developer application caused by the aggregate developer clogging the clearance between the blade 16 and sleeve roller 12. Also such wider clearance alleviates the precision requirements for the component parts and assembly thereof.

Fig. 4 shows another embodiment of the developing device of the present invention, wherein there are shown a sleeve roller 12 for forming magnetic developer brushes along the periphery thereof, magnetic poles 1 3a-1 3d fixed inside said sleeve roller 12, a developing sleeve 11, an applying station 19, a magnetic pole 18 positioned inside said developing sleeve 11, a blade 16, one-component insulating magnetic developer 4, and a thin developer layer 4' formed on said developing sleeve 11.The developer 4 in the form of magnetic brushes with a height defined by the blade 16 is transported along the periphery of the sleeve roller 12 and strongly collides with the surface of the developing sleeve 11 under the effect of a magnetic field generated by the magnetic pole 18 in the developing sleeve 11 and the magnetic pole 1 3c positioned inside the sleeve roller 12, whereby the developer is frictionally charged in the applying station 19 and thus attracted to the surface of developing sleeve 11 to form a thin layer 4' showing a satisfactory developing performance.In this manner the blade 16 and the sleeve roller 12 can be spaced by a clearance of 0.5 to 1.0 mm while said roller 12 and the developing sleeve 11 can be spaced by a clearance of 1.0 to 2.0 mm, thus alleviating the load applied to the developer in comparison with the conventional magnetic applying method utilizing a sleeve roller having internal fixed magnetic poles and a magnetic blade spaced therefrom by a narrow clearance in the order of 300 y.

Fig. 5 is a partial magnified view of the embodiment shown in Fig. 4 wherein a magnetic pole 18 is positioned inside the developing sleeve 11 corresponding to the applying station 1 9. Said magnetic pole 18 may be replaced by a magnetic substance for achieving a same effect, though the following description will be concentrated on the case of using such magnetic pole.

The embodiment shown in Fig. 5 provides more enhanced spikes of the magnetic brush in comparison with the embodiment shown in Fig. 3 not utilizing the internal magnetic pole in the developing sleeve 11 in the applying station 19, thus increasing the friction between the developing sleeve 11 and the developer and leading to additional advantages as will be explained in the following.

Firstly the amount of developer applied is increased to provide a thicker or visually denser developer layer due to the stronger friction between the developer and the developing sleeve 11.

Secondly a stable application of a thin developer layer is rendered possible even when the sleeve roller 12 is spaced from the developing sleeve 11 by a widened clearance that would not bring the magnetic brushes into contact with the developing sleeve or roller 11 if in the absence of the magnetic pole therein. This fact is extremely effective in preventing the aggregation of easily aggregating developer for pressure fixation.

Thirdly, the ghost image eventually formed on the developing sleeve in certain developers is completely prevented by the use of internal magnetic pole in the developing sleeve. This is due to the enhanced spike formation of the magnetic brushes B, which exhibit an effect of cleaning the surface of developing sleeve 11 and forming a developer layer anew by frictional charging. Also the magnetic brushes function to remove the aggregated developer eventually deposited on the developing sleeve 11, thus restoring a uniform thin iayer, and constantly clean the surface of the developing sleeve 11, thus avoiding the time-dependent deterioration thereof. In this manner it is rendered possible to dispense with the developer scraping means which has often been necessitated for removing the developer from the developing sleeve.

Fourthly eventual streaks and uneveness present on the magnetic brushes on the sleeve roller 12 are erased by the enhanced spike formation in the applying station 1 9 and are therefore not reproduced on the developing sleeve 11.

In this manner the developing device of the present invention enables the developer application in a thin layer without the drawbacks associated with the conventional developing devices.

Fig. 6 shows another embodiment of the present invention wherein the same components as those in the foregoing embodiments are represented by the same numbers. In this embodiment provided is a non-magnetic bearing belt 21 so positioned and movable supported as to face a latent image bearing member with such a small clearance therebetween that a thin developer layer formed on said belt is slightly separated from said latent image bearing member in the absence of bias field therebetween. Pulleys 23a, 23b are provided for driving said belt 21 in such a manner that said developer layer moves in the same direction and at a substantially same speed with said latent image bearing member at the mutually facing position.

A non-magnetic belt 22 for bearing magnetic brushes is provided with magnets 25 therein and is driven by pulleys 24a, 24b in the direction of arrow. It is also possible to employ a magnetic belt 22 for magnetically transporting the magnetic developer 4. Magnets 26 are positioned inside said non-magnetic belt 22 and in opposed relation to said belt 22 to perform the same function as already explained in relation to Figs. 4 and 5, whereby the magnetic developer brushes supported on said belt 22 are brought into contact with the belt 21 to form thereon a thin layer of magnetic developer which is then transported to the developing station D to perform the image development by the aforementioned principle.

There may be additionally provided an unrepresented blade for defining the height of magnetic brushes formed on the belt 22. Also preferred is the development with an external AC bias field as proposed in the U.S. Patent Application Ser. No. 58,435.

Fig. 7 shows a variation of the embodiment shown in Fig. 6, wherein the same components as those in Fig. 6 are represented by the same numbers. A non-magnetic rotary sleeve 31 is provided therein with a magnet roll 32 and functions to transport insulating magnetic developer 4 by the effect of magnetic field, said developer being supplied from a container 33.

The magnetic developer brushes formed on said sleeve 31 are brought into contact with a nonmagnetic belt 21 to form a thin developer layer by frictional charging on said belt, said developer layer being transported to the developing station for performing the image development according the aforementioned principle.

The present invention is not limited to the foregoing embodiments but provides in general manner a developing method in which onecomponent magnetic developer is transported on a developer bearing member to a developing position for image development, featured in supplying said developer onto a developerfeeding non-magnetic bearing member to form thereon a magnetic brush of developer under the effect of a magnetic field, and bringing said magnetic brush into contact with the surface of an image-developing developer bearing member to form thereon a thin uniform layer of the developer for effecting the image development, and a developing device for conducting image development with one-component magnetic developer transported to a developing position on a developer bearing member, comprising a developer-feeding non-magnetic bearing member for supporting said magnetic developer, magnetic-field generating means for forming a magnetic brush of said developer on said bearing member, an image-developing bearing member so postitioned as to come into contact with said magnetic brush, means for positioning said developer-feeding bearing member in the proximity of said image-developing bearing member, and means for driving said imagedeveloping bearing member to transport said developer layer to the developing position, and therefore provides the following particular advantages.

(1) The image development is achieved with uniform density and with a minimum necessary amount of developer for image formation, avoiding all the drawbacks associated with the conventional developing methods and devices.

(2) A thin uniform developer layer can be formed on the image-developing bearing member without the use of a blade directly contacting said layer, thus avoiding the danger of aggregate formation at such blade.

(3) The developer is charged by friction between the magnetic developer brush and the image-developing bearing member, thus alleviating the load applied to the developer and enabling the use of easily coagulable developer for pressure fixation.

The pressure-fixable developer, though advantageous in power economy by dispensing with the heating means required for the conventional heat-fixable developer, easily aggregates under pressure or an elevated load and has therefore to be maintained free of excessive pressure in the process steps prior to the image fixation. Such pressure-fixable developer is represented for example by microcapsule toner composed of an easily fixable core material covered with a shell resin, as disclosed in the Japanese Patent Publications Sho49-1588 and Sho51-35867.The developing device of the present invention allows the use of such toner without any trouble and provides a thin uniform developer layer, thus eliminating the eventual uneveness in the developed image resulting from the uncoated streaks or eneveness in said developer layer.

(4) The developing device of the present invention does not require additional means, such as a scraper, for removing the remaining developer from the bearing member, thus enabling a simpler structure of the device and avoiding the danger of developer caking in the vicinity of such scraper.

In the foregoing embodiments there however remains a drawback requiring further consideration. In the embodiment shown in Fig. 4, in contrast to the afore-mentioned advantages achievable in the course of developing process in which the magnetic brush sleeve roller 12 and the developing sleeve 11 are both rotated, the developing sleeve 11 in a stopped state may develop a thick aggregated developer layer corresponding to the applying station 19, which may remain for a considerable period after the start of developing process despite the cleaning effect of the magnetic brushes. Such abnormality in the developer layer becomes visible in the developed image and should therefore be eliminated.

Such difficulty can be eliminated by regulating the density of the magnetic brush, before the movement of the developer-feeding bearing member is terminated, in such a manner as to decrease the length or intensity of said brush thereby controlling the load applied to the developer in the applying station 19.

In the following explained are embodiments capable of such density regulation of the magnetic brush, wherein the same components as those shown in Fig. 2 or Fig. 4 are represented by same numbers and are omitted from the explanation. The AC bias source 17 shown in Figs.

2 and 4 is preferably incorporated though it will not be particularly explained in the following embodiments.

Fig. 8 shows an embodiment in which the blade 16 is displaceable between a position (A) during the developing process and a position (B) after the composition thereof. After the completion of developing process, the blade 16 is positioned closer to the magnetic brush sleeve roller 12 as shown in Fig. 8B, whereby the magnetic brush becomes shorter in length and is no longer in contact with the developing roller 11.

The blade 1 6 is returned to the original position shown in Fig. 8A at the re-start of the developing process whereby it is rendered possible to constantly obtain a thin uniform layer without developer aggregation. Though Fig. 8B shows a state in which the the magnetic brush is completed separated from the developing sleeve 11, sufficient prevention of aggregation can be achieved if the length of brush is reduced to an extent that It only lightly touches the developing sleeve 11.

Such approaching and retraction motion of the blade 16 to and from the sleeve roller 12 can be achieved by an already known mechanism, for example a cam mechanism as shown in Fig. 8B and provided with a cam 34 and a cam follower arm 35 suporting said blade 16 and constantly biased toward the sleeve roller 12 by means of a compression spring 36, wherein the cam 34 is activated to the state shown in Fig. 8B by means of a suitable signal obtained after the completion of developing process but prior to the termination of the copying operation.

Figs. 9A and 9B show an embodiment in which the magnetic brush is formed on the sleeve roller 12 by a blade 16 composed of a magnetic material, wherein the length of said magnetic brush is shortened by the displacement of a counter magnetic pole 13' for said blade 1 6 from a position during the developing process as shown in Fig. 9A to a position after the developing process as shown in Fig. 98. During the developing process (Fig. 9A) the magnetic pole 13' is displaced in the transporting direction of the developer from the magnetic blade 1 6 to provide magnetic brush of a length enough for forming a thin developer layer on the developing sleeve 11, but after the developing process the magnetic pole 13' is positioned almost vertically below the blade 16 as shown in Fig. 9B to form only a much thinner developer layer on the sleeve roller 12, thus avoiding the developer aggregation in the applying station B.

Such displacement of the magnetic pole 13' can be achieved by a known mechanism, for example by displacing a magnet 13', movably mounted on the magnet roll 13, from the position (A) to (B) after the termination of the developing process. In case of absence of such magnet roll 13, the magnetic pole 13' has to be made displaceable with respect to the device.

Figs. 1 OA and 1 OB show an embodiment in which the magnetic pole 18 positioned inside the developing sleeve as shown in Fig. 1 OA is displaced from the applying station B after the completion of the developing process as shown in Fig. 108. In this case the length of magnetic brush after the blade 16 remains unchanged, but the developer aggregation during the standstill state of the developing sleeve 11 is prevented since the spike formation in the applying station B is no longer present.

Such displacement of the magnetic pole 18 can be achieved by rotating a magnet roll 20 for supporting said magnetic pole 18 by a determined amclum after the completion of the developing process to the position shown in Fig.

103, or in the absence of such magnet roll 20, by displacing said magnetic pole 1 8 itself.

Figs.11 A and 11 B show another embodiment in which the developer aggregation is prevented by changing the mutually attracting magnetic field in the applying station during the developing process to a mutually repulsive magnetic field after the termination of the developing process.

Such repulsive magnetic field can be obtained in the applying station B by shifting a magnetic pole - 37, provided inside the developing sleeve 11 for this purpose, to said applying station B after the termination of the developing process, as shown in Fig. 118.

Similarly a suitable magnetic pole inside the magnetic brush sleeve roller 12 may be rotated to a position shown in Fig. 11 C for achieving the same purpose of aggregation prevention. Such displacement of the magnetic pole 18, 37 or 13 is also achievable by a known mechanism.

Figs. 1 2A and 128 show an embodiment in which the magnetic poles inside the magnetic brush sleeve roller 12 are displaced from the applying station B after the completion of the developing process, thereby reducing the spike formation at said station B and thus preventing the aggregation of developer on the developing sleeve 11 during the stopped state thereof.

For this purpose the magnetic poles 1 3a-1 3d are mounted or magnetized on the magnet roll 13, which, as explained in the foregoing embodiments shown in Figs. 9, 10 and 11, is rotated by a known drive means.

In the foregoing embodiments, the displacement of the blade 16 of magnetic poles has to be achieved after the completion of developing process but before the developing sleeve 11 comes to a standstill, and the effect of such displacement is almost effectless if it is conducted after the developing sleeve 11 is already stopped.

As explained in the foregoing, the developing device of the present invention is capable of stably providing a thin uniform developer layer without the aggregation of developer onto the developing sleeve, which has not been avoidable in the conventional devices.

There is observed a general trend for the increased use of one-component insulating magnetic developer and pressure-fixable developer in place of conventional heat-fixable developer for power economy in the future, but such new developers are susceptible to pressure and load, easily leading to the aggregation on the developing sleeve. As an example of such developer there is known so-called microcapsule toner composed of an easily fixable core material covered with a shell resin, as disclosed in the Japanese Patent Publications Sho49-1 588 and Sho51-35861, but the developing method and device of the present invention allows uniform image development with such developer without the danger of coagulation thereof.

The present invention if not limited to the foregoing embodiments, and the image-developing cylindrical bearing member and/or the developerfeeding bearing member can be replaced by endless belts or other sutable means.

As explained in the foregoing, the developing device structured as shown in Fig. 4 is capable of performing the toner transition development, or "jumping" development as it will hereinafter be called, in a better manner than in the conventional devices, but the absence of a doctoring blade or the like for directly defining the thickness of the developer layer 4' on the developing sleeve 11 tends to result in a slight instability of the layer thickness.Particularly the magnetic pole 18 in the developing sleeve if selected inappropriately in strength or in position thereof, will lead to an uneven thickness of the developer layer 4' on the developing sleeve 11 or a developer layer 4' partially excessively thick in consideration of the clearance a, in the order of 300 , for jumping development between the latent image bearing surface or photosensitive drum 1 and the developing sleeve 11, eventually resulting in a developer layer thicker than said clearance, causing a damage to the photosensitive drum by developer aggregation in the developing station.

The present invention therefore provides also an improvement on the developing device comprising the magnetic brush sleeve roller 12 constituting a first developer bearing member and the developing sleeve 11 constituting a second developer bearing member, for ensuring constant formation of a stable developer layer 4' without the drawbacks of unevenness in said developer layer 4' or of excessively large thickness thereof leading to the developer aggregation between the photosensitive drum 1 and the developing sleeve 11, said improvement being featured in that the developer-transporting magnetic force of said developing sleeve 11 at a portion thereof facing the magnetic brush sleeve roller 12 is selected smaller than that of said sleeve roller 12.

Now reference is made to Fig. 13. In general, the developer in the magnetic brush 4a is subjected to a magnetic force generated by the magnetic pole 1 3c in the sleeve roller 12 and the magnetic pole 18 in the developing sleeve 11, a frictional drag force resulting from the rotation of the sleeves 11 and 12, and an electrical force resulting from frictional charging between the developer particle and the sleeve surface or between the developer particles, among which the magnetic force and frictional drag force are major factors governing the behaviour of the magnetic brush.

Fig. 13 shows the magnetic force, frictional drag force and resultant force thereof applied to the developer present in the magnetic developer brush 4a, in which there are shown an attractive force f1 exerted by the magnetic pole 1 3c in the sleeve roller 12, an attractive force t2 exerted by the magnetic pole 18 in the developing sleeve 11, a frictional drag force f1 ' exerted by the rotation of said sleeve roller 12, and a frictional drag force f2' exerted by the rotation of the developing sleeve 11.T1 represents a developer particle present in the vicinity of the base portion of magnetic brush, or namely in the vicinity of the sleeve roller 12, while T2 represents a developer particle present in the vicinity of the outer end of magnetic brush, or namely in the vicinity of the developing sleeve 11, and FT1 and FT2 respectively represent the resultant forces of magnetic attraction and frictional drag applied to said developer particles.

The schematic view in Fig. 13 shows a normal formation of a thin developer layer 4', wherein the developer particle T1 is principally governed by the attractive force f1 exerted by the magnetic pole 1 3c in the sleeve roller 12 and is therefore displaced in the same direction as said sleeve roller 12, while said particle is pressed against said sleeve roller 12. It is important to note that the force FT2 working on the developer particle T2 positioned close to the developing sleeve 11 does not have a frictional force for attracting said particle T2 toward the developing sleeve 11.

Consequently the fundamental force for forming the developer layer 4' on the developing sleeve 11 is a mirror force or the like resulting from frictional charging generated upon contact of the developer particle with the surface of the developing sleeve 11, and the developer deposition onto the developing sleeve 11 is not achieved by the attractive force of the magnetic pole 18 in the developing sleeve 11 but by the above-mentioned electrical forces against the aforementioned magnetic attraction and frictional drag.

The instability in the formation of thin developer layer in the conventional devices results from a resultant force FT of the magnetic attractions fl, f2 and the frictional drags f1', f2' directed, as shown in Fig. 14, to press the developer particle T toward the developing sleeve 11. In such case the magnetic brush itself is transferred in bulk to the developing sleeve 11 to provide a developer layer unsuitable for jumping development.

Thus, in a jumping developing device utilizing the magnetic brush sleeve roller 12 and the developing sleeve 11, it is essential that the developer constituting the magnetic brush is attracted and transported by the magnetic brush sleeve roller, except a small portion thereof forming the thin developer layer in the mutually facing area of said sleeves. Stated differently, the developer-transporting magnetic attraction of the developing sleeve 11 in said mutually facing area should be smaller than that of the magnetic brush sleeve roller 12.

This requirement is achieved by selecting the perpendicular component of the magnetic flux density generated by the magnetic pole 18 at the surface of the developing sleeve 11 smaller than that of the magnetic flux density generated by the magnetic pole 1 3c positioned inside the sleeve roller 12.

Said requirement is also achievable by replacing said magnetic pole 18 in the developing sleeve 11 with a magnetic substance or simply by eliminating said magnetic pole 18, and, in these cases, the perpendicular component of the flux density associated with the developing sleeve 11 is rendered assuredly smaller, at the surface of the developing sleeve 11, than that of the flux density associated with the magnetic brush sleeve roller 12.

In contrast to the above-explained case in which the magnetic pole 1 3c in the sleeve roller 12 and the magnetic pole 18 in the developing sleeve 11 both contributing to the formation of thin developer layer are arranged in a mutually opposed relation at the mutually facing portion of said sleeves, Fig. 15 shows another arrangement of said magnetic poles 1 3c, 18 for achieving stable formation of the thin developer layer 4', wherein the components other than the sleeves 11, 12 and magnetic poles 1 3c, 18 are omitted for the purpose of clarity.In this arrangement the magnetic pole 18 in the developing sleeve 11 is positioned upstream, with respect to the moving direction thereof, to the magnetic pole 18 in the sleeve roller 12, whereby the magnetic field formed by said magnetic poles 1 3c and 18 has a component H1' directed opposite to the moving direction of the developing sleeve 11. It is thus rendered possible to securely prevent the excessive application of the developer onto the developing sleeve 11 by the magnetic fractional force exerting on the developer in a direction opposite to the moving direction of the developing sleeve 11 at the mutually facing area of said sleeves 11 and 12.In this case the maximum flux density generated by the magnetic pole 18 in the developing sleeve 11 may exceed the flux density on the magnetic brush sleeve roller 12, as long as the flux density on the developing sleeve 11 is smaller than that on the sleeve roller 12 at said mutually facing area.

On the other hand Fig. 1 6 shows an arrangement in which the magnetic pole 18 in the developing sleeve 11 is positioned downstream, with respect to the moving direction thereof, to the mutually facing position of said sleeves 11 and 12, whereby the magnetic field formed by the magnetic poles 1 3c, 18 has a component H1" direction in the same direction as the moving direction of the developing sleeve 11. As the result a part of the magnetic brush is transferred in bulk to the developing sleeve 11 to form uncontinuous developer blocks thereon thus leading to the developer aggregation between the photosensitive drum and the developing sleeve, eventually damaging the surface of said photosensitive drum.Also the magnetic pole 18, if positioned between point A on the sleeve 11 corresponding to the mutually facing point of said sleeves 11 and 12 and a tangential point B, in the down-stream side, to a tangential line C passing through the center 0 of the magnetic brush sleeve roller 12, attracts the developer particles eventually scattered from the magnetic brush to result in random unevenness in the thin developer layer4'.

It is therefore concluded necessary to avoid the presence of magnetic poles in the area between the points A and B in order not to have a magnetic maximum affecting the movement or magnetic brush or the scattered developer particles.

As explained in the foregoing, the present invention allows to obtain a developing device capable of constantly providing the developing sleeve with a stable thin developer layer suitable for jumping development.

Example 2 The developing device shown in Fig. 4 or 13 was prepared with a clearance P of 1.00 mm between the magnetic brush sleeve roller 12 and the developing sleeve 11, a perpendicular magnetic flux component of 400 gauss generated by the magnetic pole 18 at the surface of the developing sleeve 11 and a perpendicular magnetic flux component of 700 gauss generated by the magnetic pole 1 3c inside the sleeve roller 12 and transmitted through the magnetic brush, whereby obtained on the developing sleeve 11 was a satisfactory thin layer of the onecomponent magnetic developer with a reflective density of 1.60.

Example 3 A developing device as shown in Fig. 1 5 was prepared with magnetic poles 1 3c, 18 respectively showing flux densities of 800 gauss on the magnetic brush sleeve roller 12 and 1,000 gauss on the developing sleeve 11, the latter being deviated by 100 upstream in the rotating direction of the developing sleeve 11. The clearance p between the sleeves 11 and 12 was selected as 1.00 mm to obtain a stable and satifactory thin layer of the one-component magnetic developer with a reflective density of 1.8.

Example 4 The embodiment shown in Fig. 4 was tried with an aluminum blade 1 6 spaced by 0.8 mm from a stainless-steel sleeve roller 12 with a surface flux density of 800 gauss, which was in turn spaced by 1.5 mm from a stainless-steel developing sleeve 11 and with a magnetic pole 18 positioned inside said developing sleeve 11 to provide a surface flux density of 600 gauss in the applying station 19. The device provided a denser thin layer of one-component developer of reflective density of 1.8 on the developing sleeve 11, and the developed image showed sufficient density as high as 1.6.

Though the device shown in Fig. 4 is improved from the conventional devices as explained in the foregoing, it is still associated with certain difficulties te be resolved as explained in the following.

Firstly the developer layer formation on the developing sleeve 11, being almost entirely dependent on the frictional charging of the developer, tends to result in a slightly deficient density, thus leading to an unstable image quality.

Secondly a foreign matter eventually trapped between the blade 1 6 and the magnetic brush sleeve roller 12 may form a lacking portion in the toner layer 4a which correspondingly forms a white streak or a insufficiently applied portion with deficient density on the developer layer 4' on the developing sleeve 11, and such irregularity in the developer layer 4' directly appears, in the jumping development, as a defect in the image.

Thirdly, the absence of the doctoring blade or the like for directly defining the thickness of the developer layer 4' on the developing sleeve 11 tends to result in a slight instability in the layer thickness of said layer eventually leading to an unevenness therein whereby the layer thickness may partially become excessively large for the clearance c between the photosensitive drum 1 and the developing sleeve 11 for the jumping development or even exceed said clearance to cause damage to the photo-sensitive drum 1 by developer aggregation in the developing position.

Fourthly, the developer layer on the developing sleeve 11 may not be restored within one turn thereof after consumption in the development, leading to an insufficient image quality involving a ghost image of the preceding development.

In consideration of the foregoing, the present invention provides a developing device utilizing the first developer bearing member 12 and the second developer bearing member 11 and capable of preventing the above-mentioned difficulties and providing a stable thin developer layer.

Thus a first object of the present invention is to provide a developing device capable of forming a thin developer layer 4' of a sufficient density on the developing sleeve 11.

A second object of the present invention is to provide a developing device in which the thin developer layer 4' formed on the second developer bearing member 11 is not affected by the irregularity in the magnetic brush formation.

A third object of the present invention is to provide a developing device not causing uneven thickness in the developer layer 4' nor excessively large thickness leading to the developer aggregation between the photo-sensitive drum 1 and the second developer bearing member 11.

A fourth object of the present invention is to provide a jumping developing device not associated with the ghost image resulting from the development.

The foregoing objects are satisfactorily achieved by rotating the magnetic brush sleeve roller 12 constituting the first developer bearing member and the developing sleeve 11 constituting the second developer bearing member in a same direction so that the surfaces thereof are mutually displaced in the opposite directions in the mutually facing area to cause vigorous agitation of the developer in said mutually facing area, by maintaining the clearance a between the blade 16 and the magnetic brush sleeve roller 12 in a certain relationship to the clearance b between the sleeves 11 and 12, and by establishing a certain relationship between the magnetic flux densities at the surfaces of said sleeves in the mutually facing portion thereof.

Fig. 17 shows an embodiment wherein the magnetic brush sleeve roller 12 and the developing sleeve 11 are both rotated anticlockwise to form a developer retained area 40 at the mutually facing area of the sleeves 11 and 12 during the course of transportation of the layer 4a of the one-component developer, said developer retained area being magnifiedly shown in Fig. 18.

The developer in the layer 4a, particularly in the vicinity of the sleeve roller 12, is transported in the same direction as the rotation of said sleeve roller 12, but the developer present in the end portion of the magnetic brush and coming into contact with the developing sleeve 11 is prevented, by the opposite movement of the surface of said developing sleeve 11, from the displacement with said developer layer 4a, thereby forming a retained developer portion 40 rotating clockwise, namely in a direction opposite to the rotation of either sleeve.Such retained developer portion, of which size is variable according to various conditions to be explained later, is unique to a structure in which the magnetic brush sleeve roller 12 and the developing sleeve 11 are both rotated in the same direction, and a stable developer application on the developing sleeve 11 can be achieved, because of the following reasons, by selecting such rotating direction of the developing sleeve as to form the above-mentioned retained developer portion 40 in the mutually facing area of the sleeves 11, 12, or, namely as to displace the surface of the developing sleeve 11 in a direction opposite to the displacing direction of the magnetic brush in said mutually facing area of the sleeves 11 and 12.

Firstly such arrangement allows to obtain a sufficiently high density even if the revolution of the magnetic brush sleeve roller 1 2 is equal to or less than that of the developing sleeve 11, since, in this case, the developer performs random movement in the retained portion 40 as represented by the arrows in Fig. 18 to drastically increase the agitation, thus increasing the probability of contact of developer with the surface of developing sleeve 11 or peeling therefrom and accelerating the frictional charging, in comparison with the foregoing case wherein the surface of the developing sleeve 11 is displaced in the same direction as the displacement of the developer layer 4a.

The above-mentioned fact is verified from the change in the density of the applied developer 4' as a function of number of turns of the developing sleeve 11. Fig. 19 shows the change in density of the developer layer 4' on the developing sleeve 11 up to the saturation as a function of the number of turns on the developing sleeve 11, wherein the full line indicates a case in which the developing sleeve 11 and the magnetic brush sleeve roller 12 are rotated in a same direction so that the surface of said developing sleeve 11 is displaced in a direction opposite to the displacement of the developer layer 4a, while the broken line indicates a case in which the developing sleeve 11 and the magnetic brush sleeve roller 12 are rotated in mutually opposite directions so that the surface of said developing sleeve 11 is displaced in a same direction as the displacement of the developer layer 4a. The density is measured by reflection from the applied developer layer and normalized to unity at the saturation. While the opposite rotations of the sleeves 11, 12 only provide unstable frictional charging of the developer and requires four turns of the developing sleeve to reach the saturation density of the applied developer as shown by the broken line, the rotations in the same direction allows an enhanced frictional charging to reach the saturation density after one turn of the developing sleeve as shown by the full line.

In addition to the foregoing, the increased density is also assisted by the enlarged contact area between the developing sleeve 11 and the developer due to the presence of said retained portion 40, leading to an elongated application time.

Secondly the arrangement shown in Figs. 17 and 18 is practically extremely advantageous in that the applied developer layer 4' is scarcely affected by the partially insufficient length of magnetic brush in the developer layer 4a caused by a foreign matter eventually trapped between the blade 16 and the sleeve roller 12, since the retained portion 40 functions as a buffer before the application of the developer onto the developing sleeve 11 to attenuate the difference between the normal area and abnormal area of the developer layer 4a.

An important factor in the arrangements shown in Figs. 1 7 and 1 8 is the relationship of the clearance a between the blade 16 and the sleeve roller 12 to the clearance b between the sleeves 11 and 12. Fig. 20 summarizes the result of application of developer onto the developing sleeve 11 in various combinations of said clearances a and b, respectively adjusted as 0.5, 1.0, 1.5 or 2.0 mm, wherein the result is classifiec as a sufficient density almost without ghost (o), a sufficient density but associated with certain ghost (o), a slightly insufficient density with evident ghost (A), an insufficient density with evident ghost (x) and an uneven developer layer due to overflowing developer supply (-).

The results shown in Fig. 20 indicate that the density of the developer layer on the developing sleeve 11 increases as the clearance a between the blade 1 6 and the sleeve roller 12 approaches the clearance b between the sleeves 11 and 12 within an extent said clearance a is smaller than the clearance b, but the developer layer 4' becomes irregular once the clearance a exceeds the clearance b, as shown in Fig. 21.This due to a fact that said retained developer portion 40 becomes large as said clearance a approaches said clearance b thereby increasing the density of the applied developer layer by the afore mentioned enlarging of the applying area of the retained portion 40, while, if the clearance a exceeds the clearance b, the amount of developer flowing into the retained portion 40 is constantly in excess of the amount flowing out therefrom so that the retained portion 40 is no longer in equilibrium and repeats the steps of growing to saturation and discharging the excessive developer as an overflow 4" periodically onto the developing sleeve 11.

In consideration of the foregoing and also of the reduced load to the developer inherent to the present jumping developing device, the condition for stable developer application in relation to the clearance a between the blade 1 6 and the sleeve roller 12 and the clearance b between the sleeves 11 and 12 can be represented by: c < a < b wherein c is the clearance between the developing sleeve 11 and the photosenstive drum 1.

In the above-mentioned relation particularly noticeable, in relation to the development ghost image, is a condition: a=b located at the boundary between the stable application and the overflowing application. As will be understood from Fig. 20, the ghost becomes reduced with the increase in density of the developer layer 4' as the clearance a approaches the clearance be, and almost vanishes at a=b. This phenomenon is also understood as an effect of the retained portion 40.More specifically the developer layer 4' formed by the collision of the developer layer 4a with the developing sleeve 11 is thereafter smoothed and subjected to additional developer application in said retained portion 40, thus reducing the remaining ghost Consequently a larger retained portion 40 reduces the ghost image more completely, thus leading to a satisfactory condition a=b.

Fig. 22 shows a modification of the device shown in Fig 17, further comprising a magnetic pole 18 inside the developing sleeve. In this arrangement it is essential that the magnetic flux density on the sleeve roller 12 in the applying area is larger than that on the developing sleeve 11, as otherwise a thin developer layer cannot obtained by the transition of the developer layer 4a itself in bulk onto the developing sleeve 11.

The magnetic pole 18 positioned inside the developing sleeve 11 ensures a further stabilized developer layer 4', since the retained developer portion 40 is stabilized and prevented from overflowing by the attractive force of said magnetic pole 18, particularly when the relationship of the clearance a between the blade 16 and the sleeve roller 12 to the clearance b between the sleeves 11 and 12 is located close to the boundary condition between a stable application and an overflowing application. Also the magnetic pole 1 8 enhances the collision of the developer layer 4a with the developing sleeve 11 to increase the density of the applied developer layer 4', thus reducing the ghost image.

Example 5 The arrangement shown in Fig. 17 was experimented with the following parameters: peripheral speed of sleeve roller 12: 200 mm/sec peripheral speed of developing sleeve 11: 350 mm/sec rotation of said sleeves: in same direction, anticlockwise in Fig. 1 7 clearance a between the blade 16 and sleeve roller 12: 1.00 mm clearanc b between the sleeves 11 and 12: 1.00 mm magnetic flux densities in the applying area: 800 gauss on the sleeve roller 12 300 gauss on the developing sleeve 11 and was capable of constantly and stably providing a thin layer, approximately 50 u in thickness, of one-component magnetic developer of a high resistivity in the order of 10169cm, on the developing sleeve 11. The ghost on the developing sleeve 11 was not observable on the developed image.

It is also preferable, in the developing devices shown in Figs. 1 3 to 22, to provide a clearance between the developer layer 4' on the developing sleeve and the photosensitive drum 1, and to apply an AC bias field across said clearance by means of power source as shown by 17 in Figs. 2 and 4.

Claims (32)

Claims

1. A method for developing a latent image on a latent image bearing member, comprising the steps of: supplying one-component magnetic developer onto a developer-feeding bearing member to form a magnetic developer brush under the effect of a magnetic field; bringing said magnetic developer brush into contact with the surface of an image-developing bearing member to form a thin uniform developer layer thereon; and transporting said developer layer into a developing position to develop the latent image formed on the latent image bearing member.

2. A method for developing a latent image on a latent image bearing member, comprising the steps of: forming a magnetic brush of one-component magnetic developer on a developer-feeding bearing member under the effect of a magnetic field; bringing said magnetic brush into contact with the surface of an image-developing bearing member positioned in the proximity of the surface to be developed thereby forming a thin developer layer on said bearing member; transporting said thin developer layer to a developing position for performing image development on said surface; and regulating the magnetic brush present in the mutually opposed portions of said developerfeeding bearing member and said imagedeveloping bearing member to reduce the density of developer present in said mutually opposed portions after the completion of developing process but prior to the terminal of displacement of said image-developing bearing member.

3. A developing method according to the claim 1 or 2, wherein the image development is conducted by said thin developer layer spaced in the developing position from the latent image bearing surface by a clearance therebetween.

4. A developing method according to the Claim 3, wherein an AC bias electric field is applied across said clearance.

5. A developing device for developing a latent image on a latent image bearing member, comprising: an image-developing bearing member for supporting one-component magnetic developer on the periphery thereof; means for displacing said image-developing bearing member for transporting a layer of said developer to a developing position; a developer-feeding bearing member positioned in the proximity of said imagedeveloping bearing member; magnetic-field generating means positioned opposite to said image-developing bearing member across said developer-feeding bearing member; and a magnetic pole positioned on said magneticfield generating means in facing relationship to said image-developing bearing member for forming a magnetic brush on said developerfeeding bearing member in such a manner that said magnetic brush comes into contact with said image-developing bearing member to form a thin developer layer thereon.

6. A developing device according to the Claim 5, further comprising a magnetic member positioned inside said image-developing bearing member so as to be in facing relationship to said magnetic pole for formlng magnetic brush.

7. A developing device for developing a latent image on a latent image bearing member, comprising: an image-developing bearing member for supporting one-component magnetic developer on the periphery thereof; means for displacing said image-developing bearing member for transporting a layer of said developer to a developing position; a developer-feeding bearing member positioned in the proximity of said imagedeveloping bearing member; magnetic-field generating means positioned opposite to said image-developing bearing member across said developer-feeding bearing member;; a magnetic pole positioned on said magneticfield generating means in facing relationship to said image-developing bearing member for forming a magnetic brush on said developerfeeding bearing member in such a manner that said magnetic brush comes into contact with said image-developing bearing member to form a thin developer layer thereon; and a magnet positioned inside said imagedeveloping bearing member in facing relationship to said magnetic pole for forming magnetic brush.

8. A developing device according to the Claim 5, wherein said image-developing bearing member and said developer-feeding bearing member are formed as belts.

9. A developing device according to the Claim 6, wherein said image-developing bearing member is formed as a belt while sald developerfeeding bearing member is formed as a cylindrical sleeve.

10. A developing device for developing a latent image on a latent image bearing member, comprising: an image-developing bearing member for supporting one-component magnetic developer on the periphery thereof; means for displacing said image-developing bearing member for transporting a layer of said developer to a developing position; a developer-feeding bearing member positioned in the proximity of said imagedeveloping bearing member; magnetic-field generating means positioned opposite to said image-developing bearing member across said developer-feeding bearing member;; a magnetic pole positioned on said magneticfield generating means in facing relationship to said image-developing bearing member for forming a magnetic brush on said developerfeeding bearing member in such a manner that said magnetic brush comes into contact with said image-developing bearing member to form a thin developer layer thereon; and regulating means for regulating the magnetic brush in the mutually facing portions of said developer-feeding bearing member and said image-developing bearing member to control the developer density in the developing position after the completion of developing process but prior to the termination of displacement of the image-developing bearing member.

11. A developing device according to either one of the Claims 5 to 10, further comprising a member for defining the thickness of developer positioned in facing relation to said developerfeeding bearing member with a clearance therebetween.

12. A developing device according to either one of the Claims 5 to 10, wherein the surface of said image-developing bearing member is composed of a material capable, in the triboelectric series, of charging said developer in a polarity opposite to that of the latent image potential.

13. A developing device according to either one of the Claims 5 to 10, wherein said onecomponent magnetic developer is electrically insulating.

14. A developing device according to either one of the Claims 5 to 10, wherein said imagedeveloping bearing member and said developerfeeding bearing member are so mutually spaced that the thin developer layer on said imagedeveloping bearing member is spaced by a clearance from the latent image bearing surface.

15. A developing device according to the Claim 14, wherein an AC bias electric field is applied across said clearance.

16. A developing device according to the claim 10, wherein said regulating means for magnetic brush comprises a doctoring blade which is displaced from a position during the developing process to another position closer to the developer-feeding bearing member after the completion of the developing process.

17. A developing device according to the Claim 10, wherein said regulating means for magnetic brush comprises a magnetic pole positioned inside said developer-feeding bearing means in facing relation to a magnetic doctoring blade, said magnetic pole being displaced from a position during the developing process to another position for reducing the height of said magnetic brush after the completion of the developing process.

18. A developing device according to the Claim 10, wherein said regulating means for magnetic brush comprises a magnetic pole or a magnetic member positioned inside said image-developing bearing member in facing relation to the magnetic pole for forming magnetic brush, said magnetic pole or magnetic member being displaced from a position during the developing process to another position more distant from said developer-feeding bearing member after the completion of the developing process.

19. A developing device according to the Claim 10, wherein said regulating means for magnetic brush comprises a magnetic pole positioned inside said image-developing bearing member in facing relation to said magnetic pole for forming said magnetic brush and another magnetic pole opposite polarity also positioned inside said image-developing bearing member, said magnetic poles being so shifted after the completion of the developing process as to generate a repulsive magnetic field.

20. A developing device according to the Claim 10, wherein said regulating means for magnetic brush comprises a magnetic brush-forming pole positioned inside said developer-feeding bearing member in facing relation to said image developing bearing member and another magnetic pole of the opposite polarity also positioned in said developer-feeding bearing member, said magnetic poles in said developerfeeding bearing member being so shifted after the completion of the developing process as to form a repulsive magnetic'field.

21. A developing device according to the Claim 10, wherein said regulating means for magnetic brush comprises a magnetic brush-forming pole positioned inside said developer-feeding bearing member in facing relation to said imagedeveloping bearing member, said magnetic pole being displaced from a position during the developing process to another position more distant frdm said image-developing bearing member after the completion of the developing process thereby removing the magnetic pole from the portion of said developer-feeding bearing member facing said image-developing bearing member.

22. A developing device according to the Claim 5 or 7, wherein the magnetic transportion force of said image-developing bearing member is selected smaller than that of said developerfeeding bearing member at the mutually facing area of said members.

23. A developing device according to the Claim 22, wherein the perpendicular component of the magnetic flux.density associated with said imagedeveloping bearing member is selected not larger than that of the magnetic flux density associated with the developer-feeding bearing member in the vicinity of the surface of said image-developing bearing member in the mutually closest portions of said members.

24. A developing device according to the Claim 22, wherein the image-developing bearing member is not provided therein with a magnetic pole in a position corresponding to the mutually facing area of said bearingmembers.

25. A developing device according to the Claim 22, wherein a magnetic pole contributing to the formation of the thin developer layer is provided inside the image-developing bearing member at a position upstream, with respect to the moving direction of the image-developing bearing member, to the facing position thereof to the developer-feeding bearing member.

26. A developing device according to the Claim 22, wherein the image-developing bearing member is not provided therein with a magnetic pole within a surface area extending from the closest point to the developer-feeding bearing member to a tangential point at the downstream side, with respect to the moving direction of the image-developing bearing member, of a tangential line passing through the center of the developer-feeding bearing member.

27. A developing device according to the Claims 5, 6, 7 or 10, wherein the displacing direction of the magnetic brush in the mutually facing area of said image-developing bearing member and developer-feeding bearing member is opposite to the displacing direction of the surface of said image-developing bearing member.

28. A developing device according to the Claim 27, wherein the clearance a between the magnetic brush defining blade and the developerfeeding bearing member, the clearance b between both bearing members and the clearance c between the image-developing bearing member and the latent image bearing member are so selected as to satisfy the following relation: c < aSb.

29. A developing device according to the Claim 28, wherein the clearance a between the magnetic brush defining blade and the developerfeeding bearing member is substantially equal to the clearance b between both bearing members.

30. A developing device according to the Claim 27, wherein the magnetic transporting force of the image-developing bearing member is selected smaller than that of the developer-feeding bearing member in the mutually facing area of said image-developing bearing member and said developer-feeding bearing member.

31. A developing method substantially as hereinbefore described with reference to any of Figures 2 to 27 of the accompanying drawings.

32. A developing device substantially as hereinbefore described with reference to any of Figures 2 to 22 of the accompanying drawings.