Abstract:
A developing device for electrophotography includes a developer supporter faced to an image bearing member with a constant space maintained therebetween, a magnetic field generator for limiting the thickness of developer on the developer supporter. An element is provided within the magnetic field generated by the generator and in the vicinity of the developer supporter to limit the thickness of developer which is rendered erect by the magnetic field. The element is effective to limit the thickness of developer supported on the supporter to a value not causing contact of the developer with the non-imaged area on the image bearing member.

Description:
This is a continuation of application Ser. No. 938,494, filed Aug. 31, 1978, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for developing an electrostatic image, and more particularly to a developing apparatus wherein a magnetic field is formed for using a magnetic developer (hereinafter also called magnetic toner) for the image development and the thickness of layer of said toner on a toner carrying member can be limited. 
     2. Description of the Prior Art 
     The conventional methods employed in the developing apparatus in electrophotography and electrostatic recording can be classified into dry developing methods and wet developing methods. The former can further be classified into those utilizing a two-component developer and those utilizing a single-component developer. The two-component methods include, according to the classification by the species of carrier employed in combination with the toner, a magnetic brush method utilizing iron power carrier, a cascade developing method utilizing bead carrier, a fur brush method utilizing a fur brush etc. Also the single-component methods include a powder cloud method in which the toner particles are used in a state of sprayed cloud, a contact development or toner development method in which toner particles are brought into direct contact with a surface carrying electrostatic image, a jumping development method in which the toner particles are not brought into direct contact with said surface but are electrically charged and made to fly toward said surface by the electric field of the electrostatic image, a magnetic dry developing method in which magnetic electroconductive toner particles are brought into contact with the surface carrying the electrostatic image etc. 
     The two-component developing methods employing a mixed developer consisting of carrier particles and toner particles of which the latter are consumed in much larger proportion than the former to alter the mixing ratio of said particles with the progress of development, is fundamentally associated with the drawbacks of the fluctuation of image density resulting from said change in the mixing ratio and the deterioration of image quality resulting from the deterioration of carrier particles which remain unconsumed for a prolonged period. 
     On the other hand, among the single-component developing methods, the magnetic dry developing method utilizing magnetic toner and the contact developing method not utilizing magnetic toner both involve indiscriminate contact of toner particles with the entire surface to be developed, image area and non-image area inclusive, which tends to cause toner deposition even in the non-image area, thus resulting in so-called background fog. Such background fog is also unavoidable in the two-component developing methods. Also the powder cloud method is associated with said background fog resulting from deposition of toner particles in powder cloud state onto the non-image area. 
     As a single-component developing method there is already known so-called jumping development method as disclosed in the U.S. Pat. Nos. 2,839,400 and 3,232,190 wherein a toner carrying member such as a sheet member uniformly coated with toner particles is maintained at a small distance from a surface carrying an electrostatic image, and the toner particles are attracted from said carrying member toward said image carrying surface by means of the electric charge of electrostatic image thereby performing the development of said image. The above-mentioned method is advantageous in that it is almost free from the above-mentioned background fog as the toner particles is not attracted in the non-image area having no electrostatic charge nor brought into contact with the non-image area, and also in that it is free from the above-mentioned change of mixing ratio and also from the deterioration of carrier particles since there are no carrier particles involved. 
     However, the above-mentioned method have been associated with other drawbacks which are enumerated in the following: 
     (1) Difficulty in obtaining practically uniform toner coating. Uniform toner deposition is difficult to obtain although the toner carrying sheet is previously provided with an electric field to facilitate toner deposition. Different from liquid coating, a thin and uniform coating of particles is difficult to achieve for example with a known rigid blade. Uneven coating is not suitable for practical image reproduction as the unevenness is directly reproduced on the developed image. As an improvement it has been proposed to use a cloth or paper as the toner carrying sheet and embedding the toner particles into the fibers thereof, but it is still difficult to obtain a uniform coating as the toner particles finer than the fiber size is difficult to prepare. Also the toner deposition onto the toner carrying sheet by cascade developing method is not practical as it requires a large-sized apparatus. 
     (2) Difficulty in uniform toner removal from the toner carrying member. The coated toner layer, when brought into facing relationship with the electrostatic image, should cause uniform removal and transfer of toner as otherwise uniform image development cannot be expected. Such uniform removal of toner particles depends on the surface characteristics of sheet carrying the toner particles, the coating condition thereon and the characteristics of toner particles, and has never reached the practically acceptable level. 
     (3) Low image resolution. 
     In the known jumping development method the toner particles are electrostatically deposited on the toner carrying member, and, even if a relatively thin toner layer is formed on said carrying member, the toner particles are considered to fly toward the surface carrying the electrostatic image by the mutually repulsive charges of said toner particles when the distance to said surface is reduced to approximately 3 mm. However the flight of toner particles over such a wide distance from the toner carrying surface to the image holding surface requires a long time and tends to be influenced by the air stream flowing through the gap, the toner gravity, and the eventual vibration of image carrying surface or toner carrying member, giving rise to deterioration of developed image. Also the electric field of fine lines or fine characters in the electrostatic image does not exactly reach the toner carrying surface, so that there may result thinning of fine lines or fine characters, or significant deterioration of resolution due to the lack of flight of toner particles. On the other hand if said distance is rendered too small, it is again difficult to obtain exact reproduction as the fine lines or fine characters tend to become thicker. 
     SUMMARY OF THE INVENTION 
     The object of the present invention, therefore, is to provide an apparatus for developing electrostatic image free from the above-mentioned drawbacks and capable of providing stable image quality with high fidelity. More specifically the object of the present invention is to provide a developing apparatus for developing an electrostatic image constructed: 
     (a) to form a uniform toner layer by means of a simple device; 
     (b) to maintain an extremely thin toner layer of a uniform thickness in the developing station; and 
     (c) to form a toner layer allowing uniform release of toner particles toward the surface carrying electrostatic latent image, whereby the toner layer is maintained separate from the non-image area to completely eliminate the background fog, and the toner particles are released from the carrying member thereof so as to form a uniform toner image of an elevated resolution in the image area. The image area on the surface carrying an electrostatic image thereon herein used shall mean an area on which the developer is to be deposited in the developing step, while the non-image area shall mean an area which should be free from toner deposition. 
     An another object of the present invention is to provide a developing apparatus for electrostatic image comprising a developer supporting means positioned in facing relationship to an electrostatic image carrying means carrying an electrostatic image thereon, a means for maintaining a constant distance between said image carrying means and said developer supporting means, a means for supplying a magnetic developer onto said developer supporting means, a magnetic field generating means for limiting the thickness of the developer on said developer supporting means to a thickness not causing contact of developer with the non-image area on said image carrying means at least in the developing station, and a means provided in the vicinity of said developer supporting means and within the magnetic field generated by said magnetic field generating means for limiting the thickness of developer increased by the magnetic field. 
     A still another object of the present invention is to provide a developing apparatus for electrostatic image wherein said magnetic field generating means being a magnet provided behind said developer supporting means, and the poles of said magnet being positioned so as to face said means for limiting the developer thickness which is provided in the vicinity of said developer supporting means. 
     A still another object of the present invention is to provide a developing apparatus for electrostatic image wherein said magnetic field generating means being a magnet with rotating poles provided behind said developer supporting means, and said means for limiting the developer thickness provided in the vicinity of said developer supporting means being a magnetic substance. 
     A still another object of the present invention is to provide a developing apparatus for electrostatic image wherein said means for limiting the developer thickness provided in the vicinity of said developer supporting means being a magnet. 
     A still another object of the present invention is to provide a developing apparatus for electrostatic image comprising a movable developer supporting member maintained in the developing station in facing relationship to an electrostatic image carrying surface and with a distance therefrom not causing contact of developer supported on said member with the non-image area, a developer supplying means for supplying developer to said movable developer supporting member, and a developer layer limiting member composed of an elastic material maintained in pressure contact with said developer supporting member in a position between said developer supplying means and the developing station, said elastic limiting member being adapted to form a developer layer to be transported to the developing station. 
     A still another object of the present invention is to provide a developing apparatus for electrostatic image capable of developing an electrostatic image formed on a moving electrostatic image carrying member by maintaining a developer supporting member carrying developer thereon in facing relationship to said image carrying member with such a distance therebetween as not to cause contact of developer with the non-image area, and constructed to cause the displacement of developer layer supported on said developer supporting member, in the developing station, in a substantially same direction and at a substantially same speed with those of said electrostatic image. 
     Still other objects and advantages of the present invention will be made apparent from the detailed description thereof to be made in the following with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic explanatory view of an electrophotographic apparatus utilizing the developing apparatus embodying the present invention; 
     FIG. 2 is a cross-sectional view of an embodiment of the developing apparatus of the present invention; 
     FIGS. 3 and 4 are explanatory views showing the working principles of the developing apparatus of the present invention; 
     FIGS. 5A, 5B and 6 are cross-sectional views of other embodiments of the developing apparatus of the present invention; 
     FIGS. 7A and 7B are explanatory views showing the position of an elastic limiting plate shown in FIG. 6; 
     FIG. 8 is a perspective view of a mechanism for moving the elastic limiting plate shown in FIG. 6; and 
     FIGS. 9 and 10 are explanatory views showing the state of transfer of developer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically shows an example of a copying apparatus or recording apparatus in which the developing apparatus of the present invention is applicable, though the application thereof is naturally not limited thereto. 
     In FIG. 1 there are shown a photosensitive drum 1 comprising a photoconductive layer which may be provided or not with an insulating layer on the surface thereof and which may be formed in a sheet or a belt, a known sensitizing charger 2, and an image projecting device 3 for projecting an original image, a light image or a light beam modulated by an image signal. By means of the above-mentioned components there is formed an electrostatic image on said photosensitive member 1 through so-called Carlson process, processes disclosed in the U.S. Pat. Nos. 3,666,363 and 4,071,361 or other suitable processes. There is also shown a developing apparatus 4 of the present invention, which forms a visible toner image corresponding to said electrostatic image on said photosensitive member 1. The thus developed visible image is transferred, in a transfer station 5, onto a transfer sheet supplied from a paper feed station 6, wherein 7 indicates a transfer charger. After said transfer, the transfer sheet is subjected to charging for separation by a charger 8, then to heat fixing by a fixing lamp 9 during transportion on a conveyor belt, and ejected by a paper eject roller 10 onto a tray 11 provided outside the apparatus. On the other hand the photosensitive drum 1 is subjected to charge elimination by flush exposure to a lamp 12 and to removal of remaining developer in a cleaning station, thereby being prepared for the next copying cycle. Also in the above-explained apparatus it is possible to employ a so-called electrostatic image transfer process wherein the electrostatic image formed on the photosensitive member 1 is transferred onto an another image carrying member and then is rendered visible by the developing apparatus 4. 
     FIG. 2 shows a first embodiment of the developing apparatus of the present invention, wherein there are shown a photosensitive drum 1 functioning as an electrostatic image carrying means which may also be realized in a form of a sheet or belt, a developer supporting means 14 positioned in facing relationship to said carrying means and formed, in the present embodiment, of a non-magnetic cylinder, a magnet 15 positioned rotatably in said cylinder and provided at least with magnetic poles capable of scooping the developer onto said cylinder, further provided preferably with developing magnetic poles functionable at the developing position and further suitably provided with developer carrying poles between the above-mentioned magnetic poles, and a doctor blade 16 for defining the thickness of magnetic toner 18 supplied onto said cylinder. The developer supporting means 14, on which the toner 18 is deposited, is rotated in the direction of arrow while supporting thereon a thin layer of toner thereby developing the latent image on the image carrying means 1 without contacting the non-image area thereof. The thickness of toner layer 17 is limited by the magnet roll 15 and the doctor blade 16, preferably to a value within a range of 30 to 100μ. The magnetic toner particles, in the magnetic field, are mutually linked in a fiber-like state, so that the density thereof is significantly lower than that in ordinary state. Therefore, by limiting the thickness of toner layer inside the magnetic field, it is rendered possible to obtain a thickness much smaller than that obtainable by limiting outside the magnetic field. The limiting with a doctor blade outside the magnetic field has been mechanically difficult as the distance between the blade and the toner supporting means 14 has to be made very small. Also such narrow gap is apt to be clogged with coagulated toner particles and is therefore unable to ensure stable operation. The effect of magnet 15 is recognizable as long as the blade 16 is positioned within the reach of magnetic field of said magnet, but a thinnest limiting is obtainable when the blade faces a magnetic pole as shown in FIG. 3. Also if the blade 16 is formed of a magnetic material, the magnetic field will be concentrated toward said blade 16 to form a curtain of toner particles linked in a brush-like state between the toner supporting member 14 and the blade 16 which blocks the passage of toner 18 except for a small amount dragged by the toner supporting means 14 along the surface thereof, thus forming an extremely thin toner layer 17 as explained in the foregoing. The use of a magnet as the doctor blade 16 further enhances the magnetic field and is therefore more effective, but it is not usable when the magnetic poles of magnet 15 are rotated since the blade made of a magnet, when used in combination with a rotating magnet 15, generates a noise by vibration of blade and increases the rotating torque of said rotating magnet. However a doctor blade 16 made of a magnetic material can be employed in combination with a rotating magnet 15. In this case the blade 16 is alternately approached by different magnet poles to create a strong magnetic field between the blade and the toner supporting means, and the vigorously changing magnetic field thus formed has an effect of decomposing the coagulated toner particles. 
     In the following there will be explained other embodiments of the developing apparatus according to the present invention shown in FIGS. 5A and 5B, wherein the components common with those shown in FIG. 2 are represented by common numbers. In FIG. 5A the electrostatic image carrying surface 1 is displaced in the direction of arrow, and the non-magnetic cylinder 14 constituting the toner supporting means is driven in the same direction while the multi-pole permanent magnet 15 is rotated in the same or opposite direction whereby the single-component insulating ferromagnetic toner 18 supplied from a toner container 19 is coated on the surface of cylinder 14 and is given an electrostatic charge of a polarity opposite to that of the electrostatic image by means of the friction between said cylinder surface and the toner particles. Also by means of an iron doctor blade 16a positioned close to the surface of cylinder (with a distance of 50 to 500μ), the toner layer is limited uniformly and to a small thickness (in a range of 30 to 500μ, preferably 30 to 100μ). The rotating speeds of said non-magnetic cylinder and multi-pole permanent magnet are adjusted in such a manner that the surface speed (and preferablyinternal speed) of the toner layer becomes substantially equal or close to the speed of said electrostatic image carrying surface 1. To achieve this result, the cylinder may be rotated at substantially the same speed as or a speed slightly lower than that of said image carrying member. In the above-mentioned arrangement the surface of toner layer is separated by a small gap (20 to 400μ) from the image carrying surface 1 whereby a satisfactory image can be obtained by the flight of toner particles across said gap. 
     In a variation of the foregoing embodiment the image carrying surface 1 is moved in the direction of arrow while the multi-pole permanent magnet 15 is maintained stationary. In this case the non-magnetic cylinder 14 constituting the toner supporting means is rotated in the same direction as said surface 1 whereby the single-component insulating ferromagnetic toner 18 supplied from a toner container 19 is coated on the surface of said cylinder and is given a charge of a polarity opposite to that of the electrostatic image by means of the friction between the surface of cylinder and the toner particles. Furthermore an iron doctor blade 16a is provided close to the surface of cylinder (with a gap of 50 to 500μ) so as to face one of the magnetic poles of the permanent magnet 15 thereby limiting the toner layer uniformly and to a small thickness (30 to 300μ, preferably 30 to 100μ). The speed of said cylinder is adjusted in such a manner that the surfacial speed, and preferably the internal speed, of the toner layer becomes substantially same or close to the speed of the image carrying surface. The doctor blade 16a may also be composed of a permanent magnet, instead of iron, to constitute a counter pole. 
     In an embodiment shown in FIG. 5B there are provided two iron doctor blades 16b, 16c on both sides of the toner container 19. In this case the non-magnetic cylinder 14 constituting the toner supporting means and the magnet 15 are rotated in the same direction as the moving direction of the image carrying surface 1. If the rotating speed of the magnet 15 is sufficiently high, there may result a case wherein the moving direction of the surfacial toner layer is opposite to that of the internal layer. In such case it is found effective, in obtaining a thin toner layer as explained in the foregoing, to use two doctor blades 16b, 16c for limiting both the surfacial flow and internal flow of toner. 
     In the foregoing embodiments the doctor blade may be composed integrally with the toner container, and also may be structured to incline along the surface of cylinder 14. Also the toner supporting means is not necessarily of a cylindrical shape but can be of a belt shape or any other suitable shape. 
     FIG. 6 shows an another embodiment of the developing apparatus of the present invention, wherein 1 is the abovementioned photosensitive drum rotated in the direction of arrow, and 20 is a sleeve or cylinder made of a non-magnetic material such as aluminum rotated in the direction of arrow at a constant speed by means of an unrepresented motor. Thus, in the developing position D said sleeve or cylinder 20 is displaced in substantially the same direction as the drum 1, with a peripheral speed substantially identical with that of said drum 1 in order to prevent eventual inertial effect resulting from the movement of the sleeve 20 on the toner particles to be transferred onto said drum, thereby obtaining a toner image without unacceptable unevenness in the density. 21 is a multi-pole magnet provided inside said toner supporting sleeve 20. In the illustrated embodiment the magnet 21 is fixed in such a manner that an N-pole forms a magnetic field in the developing position D substantially perpendicular to the surface of toner supporting means and also to the electrostatic image carrying member, but it may also be rotated if desired. In the developing position the sleeve 20 and the drum 1 are maintained in a facing relationship with such a distance therebetween that the outermost portion, facing the non-image area, of the toner layer formed into a brush-like state under the influence of magnetic field is still separated from the drum periphery, or, stated differently, the toner layer is unable to come into contact with the non-image area, and the particles in said toner layer is still capable of transferring to the image area under the effect of electrostatic attractive force. Namely in the developing position D where the toner transfer takes place, the periphery of sleeve 20 is separated from the periphery of drum 1 by such a distance that the above-mentioned toner layer does not come into contact with the drum periphery not having electrostatic image thereon. In order to maintain said constant distance, the sleeve 20 is for example provided with a coaxial roller which is pressed, for example by a spring, against said drum to perform frictional rotation, wherein the diameter of said roller being larger than that of said sleeve by said distance. In this manner the sleeve 20 is rotated with a peripheral speed substantially identical to that of the drum 1, the direction of rotation being same in the developing position. 22 is a toner container holding magnetic toner 23, with which the sleeve 20 is maintained in contact in the lower portion thereof to scoop up said magnetic toner 23 along the upwardly moving periphery. Thus the magnetic toner is attracted to the sleeve 20 by the magnetic force of said magnet 21 and transported with the rotation of sleeve 20 by the friction therewith. 
     24 is a rigid plate provided to face a magnetic pole for removing a part of the thick toner layer 26 transported from the container 22 thereby forming a toner layer 27 of which thickness is approximately the same as or smaller than the distance between the drum 1 and sleeve 20 at the developing position D. Thus said rigid toner limiting plate 24 is positioned so as to be separated from the sleeve 20 by a distance approximately the same as or smaller than the distance between the sleeve 20 and drum 1 at said developing position. Also said rigid plate may be replaced by a rotated rigid roller separated from said sleeve by the above-mentioned distance. In either form, said rigid limiting member functions to limit the thickness of toner layer to a value approximately the same as or preferably smaller than the distance between the drum surface 1 and the sleeve surface 20. The above-mentioned rigid limiting member is provided to prevent that a coagulated toner particle of a size larger than said instance eventually formed in the toner layer, passing through the succeeding limiting member 25, is crushed in the gap between the drum 1 and sleeve 20 and grows therein thereby giving undesirable effect to the developed image. The above-mentioned rigid limiting member functions to crush such coagulated toner particle to a size capable of passing through said gap. 
     25 is a flexible toner layer limiting plate made of an elastic material such as rubber and provided downstream of said rigid limiting member 24 and upstream of the developing position D along the rotational movement of the sleeve 20. Said elastic plate 25 is supported at an end thereof by a support member 29 and maintained at the other end thereof in pressure contact with the periphery of toner supporting sleeve 20, forming a contact portion therebetween. Said elastic plate 25 functions to limit the thickness of the thin toner layer 27 formed by said rigid limiting member 24 thereby forming a thinner toner layer 28. Said plate 25, when made of rubber of a hardness of 70 degrees, is in pressure contact under a pressure of 0.4 to 40 gr/cm in the axial direction of the sleeve 20. The toner layer becomes too thick or too thin respectively when the pressure is under or above said range. As an example, a limiting member 25 made of urethane rubber or silicon rubber and maintained in contact with the sleeve 20 under a pressure of ca. 8 gr/cm allows the obtaining of a uniform toner layer of a thickness of approximately 50 μ. 
     The above-mentioned use, as the second limiting member 25, of an elastic member in pressure contact with the sleeve 20 is intended to reduce the quantity of passing toner by the contact portion formed with the sleeve 20 thereby forming a thin toner layer preferably of a thickness smaller than 100μ, and also for rendering the thickness of the toner layer satisfactorily uniform by means of the elastic recovering force resulting from the deformation caused by the passage of toner particles. More specifically, although the member 25 is maintained at an end thereof in pressure contact with the periphery of sleeve 20, an extremely thin toner layer can pass therethrough because of the elasticity of said member 25. When a locally somewhat thickner toner layer is going to pass through the contact portion between the elastic member 25 and the sleeve 20, the elastic recovering force of said member 25 increases to reduce the thickness of toner layer, and vice versa. In this manner it is therefore possible to maintain the toner layer at a satisfactorily uniform thickness. The member 25 is maintained on a lateral surface thereof with the sleeve 20 not only for forming a thin and uniform toner layer but also for extending the contact portion along the periphery of sleeve 20 to prolongate the contact region of toner and sleeve 20 thereby ensuring frictional charging to be caused between the insulating toner particles and the non-magnetic sleeve 20. Also in order to improve the frictional charging efficiency, the elastic member 25 is preferably made of a material suitably selected from the triboelectric series for providing a charge of a desired polarity to the toner particles upon friction therewith. For example in case of positive charging of toner particles composed of polystyrene, magnetite, carbon etc., the charging efficiency can be improved by employing an elastic limiting member 25 composed of ethylene-propylene rubber, fluorinated rubber, natural rubber, polychlorobutadiene, polyisoprene, N.B.R. etc., and in case of negative charging there can be employed an elastic limiting member composed of silicon rubber, polyurethane, styrene-butadiene rubber etc. Also the use of a conductive rubber suitably selected in the triboelectric series prevents excessive charging of toner, thereby preventing or destructing electrostatic coagulation or solidification of toner particles. 
     Naturally the effect of forming a thin and uniform toner layer and the effect of causing friction between the toner and sleeve are expectable also when the elastic member is maintained in pressure contact at a ridge of front end thereof with the sleeve 20, but these effects are enhanced when the contact is achieved on a lateral face of said elastic member. Now with regards to the relationship between the contact position of elastic member 25 with sleeve 20 and the position of magnetic pole of the magnet provided inside non-magnetic sleeve 20, a toner layer limiting performed in front of a magnetic pole as illustrated, namely in the presence of a magnetic field (preferably perpendicular to the surface of sleeve 20) in the contact region between the member 25 and sleeve 20, provides a uniform but somewhat thicker toner layer, while a limiting performed between adjacent magnetic poles provides a thinner but somewhat uneven toner layer. However, the extent of said unevenness is naturally very small and much smaller than that observed in the conventional apparatus. 
     The pressure contact of the elastic limiting plate 25 to the sleeve 20 can be achieved, with respect to the rotating direction thereof, in a forward manner or in a reverse manner as respectively shown in FIGS. 7A and 7B. The forward contact, as shown in FIG. 7A, means an arrangement in which the distance between the plate 25 and the periphery of sleeve 20 is decreasing in the rotating direction thereof, while the reverse contact, as shown in FIG. 7B, means an arrangement of plate 25 wherein the distance between said plate and periphery of sleeve 20 is increasing in the rotating direction thereof. The arrangement shown in FIG. 7A is advantageous in that it provides a relatively thick toner layer to increase the developed toner density, whereas the arrangement shown in FIG. 7B is advantageous in that it provides a toner image of extremely fine granularity as coarse toner particles in the toner layer are blocked by the slit formed between the front ridge of plate 25 and the periphery of sleeve 20. Although the elastic plate is maintained in contact with the sleeve 20 on a lateral face, the contact may also be achieved at a ridge of the front end of said plate 25. 
     An eventual trapping of toner coagulate etc. between the elastic limiting member 25 and the periphery of sleeve 20 may result in a slight unevenness on the toner layer to be formed, but this trouble can be solved by oscillating the elastic limiting member, while being maintained in pressure contact with the toner supporting surface, in a direction orthogonal to or parallel to the advancing direction of said surface or in an another direction. FIG. 8 shows a mechanism for this purpose, in which the support member 29 for the elastic plate 25 is connected to a rack 30 engaging with a pinion gear 31 which is oscillatingly rotated by a forward-reverse motor 32 with a short period thereby oscillating the elastic plate 25 in the axial direction of the sleeve 20. Naturally the mechanism as shown in FIG. 8 is not necessary in case the toner coagulates are not formed or the effect thereof is negligible even if they are formed. 
     Further, it is also effective to employ an elastic limiting plate 25 composed of two or more laminated plates of different elastic moduli, and to select the material of the plate maintained in pressure contact with the sleeve 20 and coming into contact with the toner and the material of other plates supporting the contacting force so as to obtain a thin toner layer of an improved evenness. 
     Furthermore, the elastic limiting member is not necessarily limited to a plate member but also can be composed of a rubber roller, a felt plate, a roller or an elastic metal plate, but a rubber plate is advantageous in that it provides a stable function of forming a uniform thin toner layer with a simple structure. Whether in plate shape or in roller shape, the rubber to be employed is preferably of a hardness not exceeding 70 degrees, since a higher hardness may result in a slight unevenness in the toner layer as such limiting member, when a giant toner particles is trapped thereunder, may form a gap allowing free passage of smaller toner particles. 
     Furthermore, in case of a limiting member made of rubber, there should be employed a winterized rubber in order to prevent a deteriorated function resulting from hardening in a cold season. 
     In the embodiment shown in FIG. 6, the advantages of employing a magnetic toner and rotating the toner supporting sleeve 20 around the magnet 21 lie, in addition to those explained in the foregoing, in that the toner can be continuously supplied to the limiting member 25 and that the toner particles gathered in a brush-shape perform the movements of standing up and tumbling when the sleeve 20 supporting the magnetic toner thereon passes through the magnetic field thereby achieving an improved uniformity of the toner distribution. However the present invention is naturally applicable to the developing apparatus utilizing non-magnetic toner. 
     Although the toner supporting member in the foregoing embodiments is cylindrical, it may also be formed as an endless belt having an endless toner supporting surface provided between plural rollers. In such case there will be provided along the path of said endless belt, similar to FIG. 6, a toner supply station (container) 22, a rigid limiting member 24 spaced from said belt by a small distance and an elastic limiting member 25 maintained in pressure contact with said belt, said belt being maintained in the developing position separate from the image carrying member by such a small distance as not to cause contact of toner with the non-image area but to allow transfer of toner to the image area, thereby performing development of the electrostatic image by means of a toner layer formed on said belt by the above-mentioned various means. It is also possible to utilize a magnetic toner and to provide magnets corresponding to the developing position or along the entire path of said endless belt. 
     As explained in the foregoing, the present invention is featured by the use of a single-component ferromagnetic powdered material as the toner, by the use of a non-magnetic cylinder provided therein with a multi-pole permanent magnet as a toner supporting member for realizing easily controllable toner supporting, and also by the use of a doctor blade composed of a thin plate of a magnetic material of a permanent magnet provided in the vicinity of the surface of said cylinder for achieving a thin and uniform toner layer. It is made clear that such magnetic supporting of toner layer on the surface of supporting member allows toner transfer to the latent image in a far superior manner with respect to uniformity, stability and easiness of control in comparison with the case of toner supporting by Van der Waals force or by electrostatic attractive force. Also the use of a doctor blade made of a magnetic material functioning to form a counter magnetic pole against the magnetic pole of permanent magnet provided inside the toner supporting member forces the toner particle chains into erect state between said doctor blade and said toner supporting member, and is effective for forming a thin toner layer in other portions on said toner supporting member, for example in the portion facing the electrostatic image. Besides such forced movement of toner particles renders the toner layer more uniform and achieves the formation of a thin and uniform toner layer not realizable with the conventional non-magnetic doctor blade. Now there will be given an explanation on the composition and material of the components commonly used in the foregoing embodiments. 
     The magnetic toner is for example composed of a mixture of 50 parts of polystyrene, 40 parts of magnetite, 3 parts of a charge controlling agent and 6 parts of carbon and formed into particles of an average particle size of 5 to 10μ by a known process, but any other known magnetic toners are naturally usable also for this purpose. The toner supporting member is made of an aluminum cylinder. The magnet is provided with a surface flux density within a range of 600 to 1300 gauss, for example 800 gauss, when a magnetic pole is positioned where the toner supporting member is closest to the image carrying member holding an electrostatic image of a potential contrast of ca. 600 V. 
     The image development according to the present invention is achieved by the formation of such a toner layer as to be separate from the non-image area of the image carrying surface and as to allow toner transfer in the image area thereof. At said toner transfer, the toner layer facing the image area increases the thickness thereof under the attractive force of electric field and the toner particles develop erect and extending brush-like chains (a phenomenon hereinafter called &#34;toner extention&#34;) under the influence of magnetic field in a position corresponding to a magnetic pole, whereby thus extended toner comes into contact with the large area of image carrying surface when it approaches the surface of toner layer and a part of toner remains on said image carrying surface when it is separated from the toner layer, thereby completing the image development. This method, different from so-called contact development or jumping development, is considered to achieve image development by toner contact with the image area through said toner extention while toner is maintained contactfree in the non-image area. 
     When the distance between the surface of toner layer and the image carrying surface is larger, the image development is considered to be achieved, in addition to the above-explained development by the toner contact through the toner extension, by a phenomenon in which the toner chains extending but not reaching the image carrying surface are also maintained erect in the electric field and the end portion of said chains are torn and fly toward the image carrying surface. 
     Thus the present invention allows to achieve the image development by the above-mentioned toner extention phenomenon and also the image development by the co-existing toner flight phenomenon according to the distance between the image carrying surface and the toner supporting member. The utilization of said toner extension phenomenon wherein the toner layer is rendered erect and extending to directly contact the image carrying surface in the image area thereof allows to reduce the amount of toner flying across the developing gap, and it is possible to obtain an excellent image quality completely free from background fog when the dimensions of gaps are selected suitably so as to minimize the effect of air stream in said gap, weight of toner, and vibrations of image carrying surface and toner supporting member. In order to ensure satisfactory toner extension, the distance between the image carrying surface and the surface of toner layer in non-extended state in the non-image area should not exceed three times the toner layer thickness. Also in order to achieve a development principally by said toner extension but also by the co-existing toner flight phenomenon, said distance should not exceed ten times of the toner layer thickness. 
     Based on the experimental analysis and theoretical analysis including the foregoing explanation, the distance D between the toner supporting member and the electrostatic image carrying surface is preferably within a range from 50 to 500μ, wherein the upper limit is determined from a requirement of reproducing with a satisfactory resolution a character printed with the smallest commercial type-face (100μ), while the lower limit is determined in relation to the thickness of toner layer. Also experimentally the thickness t of toner layer to be supported on the toner supporting member is preferably within a range from 30 to 300μ. At the image development said toner layer is extended under the influence of a magnetic field to a height which is considered to be in the order of three times of said thickness as explained in the foregoing. In order to allow the surface of toner layer to reach the image carrying surface, therefore, the distance s between the surface of toner layer and the image carrying surface should not exceed 300μ. In general satisfactory results are obtained when s≧t/5. A predetermined distance between the image carrying surface and the toner supporting member can be maintained by a positioning member such as a spacer, a roller and a spring which is in abutment with the image carrying surface or a counter electrode provided therebehind and is engaging with the toner supporting surface. 
     In a process disclosed in the U.S. Pat. No. 3,232,190 a web supporting a toner layer thereon is transported, in the developing station, in a direction opposite to the moving direction of the photosensitive drum. However, in such counter movement, there may result an anisotrophy in the distribution of toner deposited on the image area of image carrying surface if the relative speed of toner layer with respect to the electrostatic image is large. This phenomenon will be explained in the following with reference to FIGS. 9 and 10. 
     Referring to FIG. 9, an electrostatic image carrying member 1 holds thereon an electrostatic image consisting of an image area d (composed of uniformly distributed positive charge) and a non-image area 1, said image area d generating lines of electric force E as shown in the illustration. A toner supporting member 62 supports a thin toner layer 63 provided with negative charge which is opposite to the polarity of charge in said image area d. Said image carrying member 1 is maintained in facing relationship to said toner supporting member 62 with such a small gap therebetween as not to cause toner transfer to the non-image area 1 but to allow toner transfer to the image area d by the electrostatic attractive force. 
     It is now assumed that the toner supporting member 62 performs a relative movement in the direction of arrow A with respect to the image carrying member 1. In such case each toner particle in toner layer 63 is considered to have a inertia in the direction of arrow and of a magnitude corresponding to the relative speed of said toner supporting member 62. Thus the toner particle transferring from the toner supporting member to the image area d by the electrostatic attraction of said image area d moves in a direction of the force resultant from said inertial force and the electrostatic attractive force along said line of electric force E. FIG. 10 shows the movement paths of toner by the arrows B. As can be seen in FIG. 10, the density of arrows B is higher at the front end side of arrow A than at the rear end side thereof. State differently, even if the image area d is provided with a uniform charge distribution, the image area receives more toner deposition at the front end than at the rear end, thus resulting in a density difference between said two portions and also resulting in blurred image reproduction at the rear end side. This density difference becomes naturally larger as said relative speed becomes larger or the toner particles become heavier (for example particles of magnetic toner containing magnetite or r-hematite). Also it is to be noted that the above-mentioned phenomenon appears even when the image carrying member and the toner supporting member are moved in a same direction, as long as the speed difference therebetween is not negligible. 
     Thus, in the large development according to the foregoing embodiments of the present invention wherein an electrostatic image formed on a moving image carrying member is maintained in facing relationship to a developer supporting member holding a developer thereon with such a distance therebetween as not to cause transfer of developer to the non-image area of said image carrying member, it is possible to obtain a satisfactory image quality without background fog and also without the above-mentioned drawback if the developer layer supported on said developer supporting member is displaced in the developing station in the same direction as and at a substantially same speed as the electrostatic image. 
     In addition to the foregoing advantages, the developing apparatus of the present invention, when applied to the copying or recording apparatus utilizing particularly a transfer process, exhibits an extremely excellent transfer effect, thereby enabling to reproduce an image of an extremely high quality without background fog on a plane paper etc. 
     Naturally the present invention is not limited to the foregoing embodiments but includes the modifications and variations within the scope of the present invention.