Developing Apparatus for Image Forming Equipment Using Developer Carrier for Forming Microfields

1. Field of the Invention

The present invention relates to a developing apparatus incorporated in an electrophotographic copier, printer, facsimile transceiver or similar image forming equipment. More particularly, the present invention is concerned with a developing apparatus of the type depositing a developer on a developer carrier and transporting it to a developing position where the developer carrier faces an image carrier to thereby develop a latent image electrostatically formed on the image carrier.

2. Discussion of the Background

A developing apparatus of the type described is disclosed in, for example, Japanese Patent Publication No. 32375/1983 and includes an image carrier and a developer carrier located face-to-face at a developing position. An alternating electric field is generated in the developing position to repetitively transfer a developer from the developer carrier to the image carrier and from the image carrier to the developer carrier, thereby developing an electrostatic latent image formed on the image carrier. In this type of apparatus, the developer carrier is implemented as a cylindrical nonmagnetic sleeve accommodating a permanent magnet in the form of a roll. A magnetic toner contacts such a developer carrier due to the force of the magnet and gravity. The toner is charged to a predetermined polarity by friction thereof with the surface of the nonmagnetic sleeve. As the toner retained on the sleeve by the force of the magnet reaches a position where a magnetic blade faces the sleeve at a predetermined spacing, it is regulated by the blade to form a layer which is about 70 .mu.m.

The problem with the conventional apparatus described above is that the magnetic toner cannot be sufficiently charged since the toner contacting the nonmagnetic sleeve due to the force of the magnet and gravity is simply charged by friction in contact with the sleeve being rotated. For example, the amount of charge on the toner forming the second layer and successive layers as counted from the surface of the sleeve is extremely small. The toner with a comparatively small amount of charge easily flies toward the image carrier due to the alternating electric field and, therefore, enhances the tonality of an image. However, such a toner is apt to contaminate the background of the image carrier to thereby thicken lines of an image and lower the resolution.

It is, therefore, an object of the present invention to provide a developing apparatus for image forming equipment capable of producing desirable images with high resolution while preserving tonality.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:

FIG. 1 is a section showing the general construction of a developing apparatus embodying the present invention;

FIG. 2 is a perspective view of a developing roller included in the embodiment;

FIG. 3 is an enlarged section of the developing roller shown in FIG. 2;

FIG. 4 shows electric lines of force representative of microfields developed in the vicinity of insulative portions appearing on the surface of a sleeve which forms part of the developing roller;

FIGS. 5A-5C are enlarged views each showing a specific configuration of the surface of the sleeve;

FIGS. 6A and 6B plot respectively the variation of potential on insulative portions included in the sleeve with respect to time and the variation of potential on conductive portions also included in the sleeve;

FIGS. 7A and 7B indicate respectively the variation of electric field in the conductive portions with respect to time occurring when the conductive portions face an image portion of a photoconductive drum, and the variation of the same occurring when the conductive portions face a non-image portion;

FIGS. 8A and 8B indicate respectively the variation of electric field in the insulative portions occurring when the insulative portions face the image portion on the drum, and the variation of the same occurring when the insulative portions face the non-image portion;

FIG. 9 is an enlarged section schematically showing dielectric members and toner particles representative of a modified form of the sleeve;

FIG. 10 is an enlarged plan view schematically showing the dielectric members of the sleeve shown in FIG. 9;

FIG. 11 is a section along line IX--IX of FIG. 10;

FIG. 12 shows electric lines of force representative of microfields generated in the vicinity of the surface of the sleeve shown in FIG. 9; and

FIG. 13 is a fragmentary section showing another modified form of the sleeve.

Referring to FIG. 1 of the drawings, a developing apparatus embodying the present invention is shown and generally designated by the reference numeral 2. As shown, the developing device 2 has a casing formed with an opening in a portion thereof which faces a photoconductive drum 3. A developing roller 1 is disposed in the casing and partly exposed to the outside through the opening. The developing roller 1 is implemented as a sleeve 1a made of aluminum or similar nonmagnetic and conductive material. A magnetic roller 1b is accommodated in the sleeve 1a and provided with magnetic poles, as illustrated. While the magnetic roller 1b is fixed in place, the sleeve 1a is rotated counterclockwise, as indicated by an arrow in the figure, by a drive mechanism, not shown. The developing roller 1 is supported in the casing at a predetermined spacing from the drum 3. The distance between the roller 1 and the drum 3 ranges from 30 .mu.m to 500 .mu.m, preferably 50 .mu.m to 250 .mu.m, so that the sleeve 1a may substantially not contact the drum 3. In this configuration, an excessive load as would be required to develop an electrostatic latent image by holding the sleeve 1a in contact with the drum 3 is not needed, allowing a miniature drive motor to suffice.

A toner tank 5 is defined in the casing and provided with an agitator 6 therein. As the agitator 6 is rotated clockwise, as indicated by an arrow in the figure, it moves a magnetic toner, or simply toner, toward the sleeve 1a while agitating the toner due to the resistance of the edge thereof.

A toner supply roller 8 is located at the right-hand side of and in contact with the sleeve 1a. This roller 8 is made of sponge produced by causing urethane rubber to foam, or implemented as a brush of polyester or tetraethylene fluoride resin fibers. The roller 8 supplies the toner driven by the agitator 6 to the sleeve 1a by rubbing it against the surface of the sleeve 1a in the forward or reverse direction. At the same time, the roller 8 scrapes off the toner remaining on the sleeve 1a after development.

A doctor blade 4 is disposed above the sleeve 1a and spaced apart from the latter by a predetermined distance. Made of a magnetic material, the doctor blade 4 regulates the toner layer carried on and transported by the sleeve 1a to a predetermined thickness. The doctor blade 4 and the magnetic poles of the magnetic roller 1b generate magnetic fields therebetween. As the toner supply roller 8 is rotated, the doctor blade 4 regulates the thickness of the toner layer on the sleeve 1a at the position where it faces the sleeve 1a. If desired, the doctor blade 4 may be replaced with a roller or a belt made of a magnetic material.

The sleeve 1a and doctor blade 4 are held in electrical conduction. Bias applying means 9 applies a bias for development to the conductive support member of the drum 3, as will be described specifically later.

A bias may be applied to the toner supply roller 8 in such a manner as to generate electric fields between the roller 8 and the sleeve 1a which tend to urge the toner of predetermined polarity toward the sleeve 1a.

In the above construction, the toner in the toner tank 5 is driven toward the toner supply roller 8 by the agitator 6. Then, the toner is electrostatically retained on the surface of the sleeve 1a by being charged due to the friction of the roller 8 and sleeve 1a. As the sleeve 1a is rotated, the toner is transported to a developing position where the drum 3 and sleeve 1a face each other, while being regulated in thickness by the doctor blade 4. At the developing position, the toner is transferred from the sleeve 1a to a latent image electrostatically formed on the drum 3 in an amount matching the latent image and under the application of a predetermined bias voltage.

In the illustrative embodiment, the surface of the sleeve 1a is configured such that two different kinds of portions each having a particular resistance or a particular dielectric constant appear in a regularly or irregular distribution. FIG. 2 shows a specific configuration of the sleeve 1a while FIG. 3 shows the surface of the sleeve 1a in an enlarged section. As shown, the sleeve 1a is produced by knurling the surface of a base in a lattice configuration, and then filling the resulting grooves with polycarbonate, acryl, polyester, tetraethylene fluoride or a similar dielectric resin belonging to a charge sequence whose polarity is opposite to the polarity of the toner. As a result, the sleeve 1a has on the surface thereof insulative portions 22 arranged in a lattice and conductive portions 21 constituted by the base.

FIGS. 5A-5C each show a particular surface configuration of the sleeve 1a in which grooves inclined 45 degrees to the direction of movement of the sleeve surface are formed by knurling, and the insulative portions 22 and conductive portions 21 are formed by the above-stated step. In FIGS. 5A-5C, the knurling pitch P is 0.3 mm, each insulative portion 22 has a width W1 of 0.075 mm, a width W2 of 0.015 mm or a width W3 of 0.225 mm, and the insulative portions 22 and conductive portions 21 exist together at a pattern pitch of 0.3 mm on the sleeve surface.

It is to be noted that the above-described method of forming the two different kinds of portions 21 and 22 is only illustrative and may be replaced with any other suitable method. Further, the inclination of the insulative portions 22 to the circumferential direction is not limited to 45 degrees and may preferably be selected in a range of from 30 degrees to 60 degrees.

The insulative portions 22 are 30 .mu.m to 2000 .mu.m, preferably 50 .mu.m to 1000 .mu.m, in terms of mean diameter. Assuming that the insulative portions 22 each have a circular shape, the diameter D1 thereof, FIG. 4, is selected to be 30 .mu.m to 2000 .mu.m, preferably 100 .mu.m to 400 .mu.m, and the distance P1, FIG. 4, between the centers of nearby portions 22 is selected to set up a desirable balance. When the insulative portions 22 are rectangular, the shortest side of each portion 22 is selected to be about 30 .mu.m to about 2000 .mu.m. Likewise, when the insulative portions 22 are oval or oblong, the width of the shorter axis is selected to be about 30 .mu.m to 2000 .mu.m. This is also true with other possible shapes of the insulative portions 22. The insulative portions 22 may occupy 50% to 80%, desirably 65% to 75%, of the entire surface of the sleeve 1a. When the sleeve 1a is provided with such a structure, the toner can be frictionally charged when rubbed against the sleeve 1a by the toner supply roller 8 and then deposited in a sufficient amount on the surface of the sleeve 1a.

Specifically, the insulative portions 22 of the sleeve 1a are charged to a positive polarity opposite to the polarity of the toner by the friction thereof with the toner supply roller 8. On the other hand, the toner being conveyed toward the sleeve 1a in contact with the surface of the toner supply roller 8 is charged to a negative polarity by friction. On reaching the sleeve 1a, the toner of negative charge is further negatively charged due to friction thereof with the sleeve 1a, particularly the insulative portions 22. As a result, the toner is electrostatically deposited on the surface of the sleeve 1a.

At this instant, the insulative portions 22 of the sleeve 1a are charged to a positive polarity. This, coupled with the fact that the conductive portions 21 adjoin the insulative portions 22, causes positive polarity to deposit only on the number of insulative portions 22. As a result, as shown in FIG. 4, closed electric fields are generated between the insulative portions 22 and the conductive portions 21, whereby a number of microfields are developed in the vicinity of the surface of the sleeve 1a. Specifically, as indicated by a number of arcuate lines in FIG. 4, electric lines of force extending from and returning to the sleeve 1a are formed in the space adjoining the surface of the sleeve 1a, generating microfields between the insulative portions 22 and the conductive portions 21.

Since each insulative portion 22 has an extremely small area, as stated earlier, each closed electric field is noticeably intensified by a fringing effect or peripheral field effect. Such closed electric fields cause the negatively charged toner to be strongly attracted by the insulative portions 22 and firmly retained on the sleeve 1a in a great amount.

A magnetic field is developed between the doctor blade 4 and the pole of the magnetic roller 1b to generate a magnetic force, while the microfields on the surface of the sleeve 1a exert electrostatic attraction. When the toner retained on the sleeve 1a is regulated by the doctor blade 4, only part of the toner having been sufficiently charged is retained on and transported by the sleeve 1a due to the balance between the above-mentioned magnetic force and the electrostatic attraction. The rest of the toner cannot pass through the gap between the doctor blade 4 and the sleeve 1a and is, therefore, removed by the blade 4 due to the short charge thereof. Consequently, toner particles with an intense charge, e.g., about 5 .mu.C/g to 20 .mu.C/g (preferably 7 .mu.C/g to 15 .mu.C/g) are allowed to reach the developing position.

Presumably, at the developing position, the bias from the bias applying means 9 acts on the microfields existing between the conductive portions 21 and the insulative portions 22 on the surface of the sleeve 1a and on the charged toner, exerting dynamic energy suitable for the development of an electrostatic latent image.

Specifically, the surface potential of the sleeve 1a differs from the insulative portions 22 to the conductive portions 21 since the former holds the above-stated charge while the latter does not hold it. More specifically, the surface potential of the insulative portions 22 is biased by a predetermined amount by the charge ascribable to the voltage from the bias applying means 9, while the surface potential of the conductive portions 21 is identical with the voltage from the bias applying means 9. It follows that the electric fields between the surface of the sleeve 1a and the drum 3 depend not only on to which of the image portions and non-image portions of the drum 3 they correspond, but also on to which of the insulative portions 22 and conductive portions 21 of the sleeve 1a they correspond. The toner existing on the insulative portions 22 is subjected to the charge deposited on the insulative portions 22 and, therefore, is prevented from depositing in an excessive amount. On the other hand, the toner existing on the conductive portions 21 tends to comparatively easily move to the drum 3. In addition, the conductive portions 21 serve to uniformize the image density by suppressing an edge effect.

The sleeve 1a, therefore, attains both of the characteristic of a developing roller having an insulative surface and the characteristic of a developing roller having a conductive surface. Specifically, a developing roller with an insulative surface is desirable in the reproducibility of lines and in tonality although the image density available therewith is low, but the reproducibility of lines and tonality decrease if the density is increased. A developing roller with a conductive surface produces a dense image whose solid portions are highly uniform due to the electrode effect thereof, but the reproducibility of lines and tonality are low.

The conductive portions 21 and insulative portions 22 existing together on the surface of the sleeve 1a eliminate the charge-up of the sleeve 1a and toner supply roller 8. This is presumably because the portions 22 charge the toner while the portions 21 discharge the toner supply roller 8, setting up a well-balanced charge distribution as a whole.

A more specific example of the illustrative embodiment will be described hereinafter.

In the specific example, the drum 3 was made of OPC. The drum 3 was applied with a surface potential of -900 V in the background and a potential of -100 V in the exposed portion. The sleeve 1a having the surface configuration shown in FIG. 5B was spaced apart from the drum 3 by a distance of 100 .mu.m. The drum 3 and sleeve 1a were each rotated in the direction indicated by an arrow so as to effect reversal development. The insulative portions 22 of the sleeve 1a held a charge which set up a potential of +200 V with ground as a reference by being rubbed by the toner supply roller 8. In this condition, the insulative portions 22 caused a negatively charged toner to deposit thereon in an amount of about 1.0 mg/cm.sup.2 to 1.2 mg/cm.sup.2. The bias applying means 9 applied to the sleeve 1a a pulse voltage having a peak-to-peak (p-p) voltage of 1000 V, a maximum voltage of 0 V, a frequency of 500 Hz, and a duty ratio of 30% (T.sub.2 /T.sub.1).

FIGS. 6A and 6B show the variations of the surface potential of the sleeve 1a with respect to time and using ground as a reference and are associated with the insulative portions 22 and the conductive portions 21, respectively. In these figures, the level of the surface potential of the background (-900 V) and that of the surface potential of the exposed portion (-100 V) of the drum 3 are indicated by horizontal lines. As the rectangular continuous line in FIG. 6A indicates, the surface potential of the insulative portions 22 is biased by +200 V by the charge ascribable to the voltage from the bias applying means 9. On the other hand, as FIG. 6B indicates, the surface potential of the conductive portions 21 is identical with the voltage from the bias applying means 9.

How the electric field between the sleeve 1a and the drum 3 is effected by such variations of the surface potential of the sleeve 1a will be described. This electric field differs from the insulative portions 22 to the conductive portions 21 of the sleeve 1a and from the image portions to the background of the drum 3.

The electric field on the conductive portions 21 whose surface potential changes as plotted in FIG. 6B is shown in FIGS. 7A and 7B. When any of the conductive portions 21 faces the image portion (exposed portion) of the drum 3, the difference in potential between the two portions varies as plotted in FIG. 7A. On the other hand, when the conductive portion 21 faces the non-image portion (unexposed portion) of the drum 3, the difference in potential between the two portions varies as plotted in FIG. 7B. FIGS. 8A and 8B show electric fields on the insulative portions 22 whose surface varies as shown in FIG. 6A. Specifically, when any of the insulative portions 22 faces the image portion of the drum 3, the difference in potential between the two portions varies as plotted in FIG. 8A. When the insulative portion 22 faces the non-image portion of the drum 3, the potential difference varies as plotted in FIG. 8B.

The electric field of interest exerts an electrostatic force on the toner deposited on the surface of the sleeve 1a or on the surface of the drum 3. For this reason, the potential difference associated with the electric field of one direction in which the toner moves toward the drum 3 and the potential difference associated with the electric field of the other direction in which the toner moves toward the sleeve 1a are respectively represented by positive and negative in order to distinguish the directions of the electrostatic force. Further, the threshold level of +100 V of the potential difference causing the toner to move from the sleeve 1a to the drum 3 and the threshold level of -100 V of the electric field causing the toner to move from the drum 3 to the sleeve 1a, which were determined by experiments, are indicated by horizontal lines. The hatching indicates portions corresponding to the electric fields contributing to the transfer of the toner beyond the thresholds.

Presumably, when the toner on the conductive portion 21 of the sleeve 1a faces the image portion of the drum 3, it is moved toward the drum 3 when an electric field corresponding to the potential difference of +900 V is reached, as indicated by hatching in FIG. 7A. When the toner on the conductive portion 21 faces the non-image portion of the drum 3, it is presumed to move toward the sleeve 1a when the electric field of -900 V is reached, as indicated by hatching in FIG. 7B.

The insulative portion 22 of the sleeve 1a is originally charged to +200 V. Hence, when the toner on the insulative portion 22 faces the image portion of the drum 3, a negative electric field of -300 V and a positive electric field of +700 V appear alternately with each other, as indicated by hatching in FIG. 8A; presumably the toner moves from the sleeve 1a toward the drum 3 when the field is positive or moves from the drum 3 to the sleeve 1a when the field is negative. When the toner on the insulative portion 22 faces the non-image portion of the drum 3, it presumably moves from the drum 3 to the sleeve 1a when the electric field is -1100 V and does not move in a reciprocating motion, as indicated by hatching in FIG. 8B.

As stated above, in the specific example, the toner on the insulative portion 22 is subjected to positive and negative electric fields exceeding the respective thresholds, as shown in FIG. 8A. This is successful in preventing an excessive amount of toner from depositing on the portion 22. On the other hand, the toner on the conductive portion 21 exhibits a higher developing ability than the toner on the insulative portion 22, as represented by the electric field of FIG. 7A. In addition, the conductive portion 21 serves to uniformize the image density by suppressing the edge effect.

Experiments showed that images formed by the above conditions are free from irregular image densities and have high density and desirable tonality and line reproducibility.

It is noteworthy that the surface configurations of the sleeve 1a shown in FIGS. 5A and 5C were also found to realize images free from irregular image density distributions and having high density and desirable tonality and line reproducibility under the same conditions as the configuration shown in FIG. 5B.

While the embodiment has been shown and described as charging the insulative portions 22 to a polarity opposite to that of the toner, the insulative portions 22 may be charged to the same polarity as the toner by friction by suitable selecting the material constituting the surface of, for example, the toner supply roller 8. This is also successful in generating microfields due to the difference in potential between the insulative portions and the conductive portions. In this case, the toner will mainly deposit on the conductive portions.

A reference will be made to FIGS. 9-12 for describing a developing apparatus using a modified form of the sleeve 1a. The apparatus to be described is essentially identical with the above embodiment except that the toner is charged to a positive polarity. As shown in FIG. 9, the sleeve 1a is made up of a base made of aluminum or similar nonmagnetic and conductive material, and medium resistance members 12 and high resistance members 11 affixed to the periphery of the base. FIG. 10 shows the sleeve with the dielectric members 11 and 12 in an enlarged view. FIG. 11 is a section along line IX--IX of FIG. 10. FIG. 12 shows electric lines of force representative of microfields developed in the vicinity of the surface of the sleeve 1a.

The resistivity of the medium resistance members 12 is selected to be higher than that of the conductive base surface (conductive roller 10 in the embodiment) and about 10.sup.3 .OMEGA.cm to 10.sup.8 .OMEGA.cm by way of example. The resistivity of the high resistance members 11 is selected to be even higher than that of the medium resistance members 12 and about 10.sup.3 .OMEGA.cm to 10.sup.15 .OMEGA.cm by way of example. Specifically, the resistance members 11 and 12 are each made of a dielectric substance having such a resistivity.

In FIG. 10, the high resistance members 11 are indicated by horizontal lines to be readily distinguished from the medium resistance members 12. As shown in FIGS. 9, 10 and 11, the two kinds of resistance members 11 and 12 are arranged in a regular pattern (or possibly in an irregular pattern) and appear on the surface of the sleeve 1a together.

The resistance members 12 and 11 may each be provided with any suitable shape. When the resistance members 12 and 11 are provided with a rectangular shape, as shown in FIG. 10, they may have one sides D1 and D2 dimensioned, for example, about 10 .mu.m to 500 .mu.m. The gist is that the sizes and resistivities of the resistance members 11 and 12 be suitably selected in such a manner as to intensify microfields, which will be described, to thereby deposit an optimum amount of toner on the sleeve 1a.

In the illustrative embodiment, the resistance members 11 and 12 are made of materials which will be charged to an opposite polarity to the toner, i.e., to a negative polarity by friction.

When the toner carrier is implemented as a belt, the resistance members 11 and 12 will be affixed to the surface of the belt in the above-described configuration.

On the other hand, the toner supply roller 8 contacting the sleeve 1a is made of a material which will charge the resistance members 11 and 12 to a opposite polarity to the toner, i.e., to a negative polarity on contacting them. In the arrangement shown in FIG. 9, the roller 8 is constituted by a conductive core member 14, and a cylindrical foam body (e.g. polyurethane foam) 15 surrounding the core member 14. The foam body 15 is pressed against the sleeve 1a by being elastically deformed. When use is made of such a roller 8, the foam body 15 may be made of a material which will charge the resistance members 11 and 12 to a negative polarity by friction. If desired, the foam body 15 is replaced with a fur brush or similar conventional implementation.

In the above construction, as the resistance members 11 and 12 contact the toner supply roller 8, they are charged to a negative polarity by friction. Even when an electrostatic residual image is left on the resistance members 11 and 12 moved away from the developing position, it is erased since the resistance members 11 and 12 are charged substantially to a saturation level due to friction thereof with the roller 8. As a result, the sleeve 1a is initialized.

As shown in FIG. 9, the toner being conveyed toward the sleeve 1a in contact with the surface of the toner supply roller 8 is charged to a positive polarity due to friction thereof with the roller 8. When such a toner is fed to the sleeve 1a, it is further charged to a positive polarity by the sleeve 1a and electrostatically deposited on the sleeve 1a. At this instant, although both of the resistance members 11 and 12 are negatively charged, a greater amount of charge is deposited on the resistance members 11 than on the resistance members 12 due to the difference in resistivity, as shown in FIG. 12. As a result, the surface potentials of the resistance members 11 and 12 are different from each other, generating microfields.

Since the number of resistance members 11 and 12 is almost infinite, an almost infinite number of microfields are developed on the surface of the sleeve 1a in a uniform distribution. Specifically, as indicated by a number of arcuate lines in FIG. 12, electric lines of force E extending from and returning to the sleeve 1a are formed in the space adjoining the surface of the sleeve 1a, generating microfields which are different in field gradient.

Since the surfaces of the resistance members 11 and 12 each has an extremely small area, as stated earlier, each microfield is also extremely small and noticeably intensified by the fringing effect or peripheral field effect. Such microfields cause the positively charged toner to be strongly attracted by the resistance members 11 and firmly retained on the sleeve 1a in a great amount. Specifically, the charged toner is firmly restrained by the microfields and held on the sleeve 1a along the electric lines of force E.

Again, the doctor blade 4, FIG. 1, selects part of the toner having been sufficiently charged and regulates the thickness thereof.

As shown in FIG. 12, the microfields are sometimes developed over the entire surface of the sleeve 1a and sometimes developed together with electric fields which are not closed. In any case, microfields are present and intensified to deposit a great amount of toner on the sleeve 1a.

If desired, the resistance members 11 and 12 may be charged to the same polarity as the toner so as to deposit a great amount of toner especially on the surfaces of the resistance members 12.

Furthermore, an arrangement may be made such that the medium resistance members 12 are substantially not charged while the high resistance members 11 are charged to a predetermined polarity, generating microfields therebetween. The gist is that at least the high resistance members 11 be charged to deposit the toner by the above-described principle.

When the sleeve 1a of this embodiment was located to face the drum 3 and applied with an alternating voltage as in the previous embodiment, it was also found to improve the negative characteristic and insure an image with high density and desirable tonality and line reproducibility. In addition, in the illustrative embodiment, the resistance members 11 and 12 appear on the surface of the sleeve 1a, but the conductive surface of the conductive roller does not appear. This surely suppresses the leak of charge between the drum 3 and the sleeve 1a which would disturb the latent image formed on the drum 3.

FIG. 13 shows a developing apparatus using another modified form of the sleeve 1a. As shown, the sleeve 1a has a conductive base, and a surface layer surrounding the base and made of a conductive material 21a in which insulative particles 22a are dispersed. On the surface of the sleeve 1a, the insulative particles 22a appear together with the conductive portions constituted by the conductive material 21a.

How the toner deposits on the sleeve 1a is as follows. As shown in FIG. 1, part of the surface of the sleeve 1a moved away from the developing position is brought into contact with the toner supply roller 8. The toner supply roller 8 scrapes off the toner remaining on the non-image portions of the sleeve 1a mechanically and electrically. At this instant, the insulative portions are charged to a opposite polarity to the toner by friction. The charges deposited on the sleeve 1a and toner by the previous development are made constant and initialized by friction. The toner conveyed by the toner supply roller 8 is charged by friction and electrostatically deposited mainly on the insulative portions of the sleeve 1a. As shown in FIG. 2, the electric fields in the form of microfields are developed on the sleeve 1a, as shown in FIG. 2. Such electric fields with great field gradient cause the toner to deposit on the sleeve 1a in multiple layers. Then, the toner is firmly retained on the sleeve 1a since the microfields are closed.

While the embodiment has been shown and described as charging the insulative portions to a polarity opposite to that of the toner, the insulative portions may be charged to the same polarity as the toner by friction by suitably selecting the material constituting the surface of the toner supply roller 8. This is also successful in generating microfields due to the difference in potential between the insulative portions and the conductive portions. In this case, the toner will mainly deposit on the conductive portions.

The toner layer on the sleeve 1a is regulated in thickness by the doctor blade 4, FIG. 1, and then reaches the developing position. In the developing position, the electric fields between the sleeve 1a and the drum 3, FIG. 1, exert a greater electrode effect. As a result, the toner on the sleeve 1a is easily transferred to the drum 3 to develop a latent image.

The sleeve 1a of this embodiment will be described more specifically. The conductive material with the insulative particles dispersed therein may have a resistivity of less than 10.sup.12 .OMEGA.cm, preferably less than 10.sup.8 .OMEGA.cm. In practice, use may be made of an organic polymer to which an agent providing it with conductivity is added. The organic polymer may be resin (plastomer) or rubber (elastomer). The agent providing the polymer with conductivity may be metal powder, carbon black, conductive oxide, graphite, metal fibers or carbon fibers, by way of example.

When the conductive material is implemented by, among the above-mentioned organic polymers, elastomer, the surface layer of the sleeve 1a will have elasticity and easily contact the rigid drum 3. This will enhance easy contact development.

On the other hand, insulative particles are implemented by a material whose resistivity is higher than 10.sup.13 .mu.cm, preferably higher than 10.sup.14 .mu.cm. The mean particle size of such a material should preferably be greater than 30 .mu.m. Particle sizes smaller than 30 .mu.m would be difficult to generate microfields and, therefore, to maintain the toner and the charge stable. It is to be noted that the insulative particles may even be amorphous. For example, use may be made of alumina or similar inorganic particles or epoxy resin or similar organic particles. When the conductive material is implemented by the conductive elastomer, it is preferable to use elastomer as the insulative particles in order to enhance the low hardness. The insulative elastomer particles may be produced by any conventional method, e.g., one consisting of freezing elastomer by, for example, Dry ice and then pulverizing it, or one consisting of preparing an aqueous emulsion by use of, for example, a surface active agent and then hardening it.

Regarding the concentration, the insulative particles are added in an amount ranging from 10 Wt % to 200 Wt % to 100 Wt % of conductive material. The area of each insulative portion as measured on the surface of the sleeve 1a should preferably be 20% to 60%. The amount of insulative particles is adequately adjusted to set up such a range after the fabrication of the sleeve 1a.

The sleeve 1a of the embodiment is fabricated by, for example, adding the insulative particles to the conductive material by an ordinary dispersing method using a ball mill or the like, molding the resulting mixture on an aluminum or similar base by injection molding, extrusion molding, spray coating or dipping, and then polishing the surface of the sleeve. To enhance the bond between the conductive material and the conductive base, plastomer may be used, in which case the plastomer should preferably be conductive.

Specifically, 100 Wt % of conductive paint Electrodag 440 (available from Nihon Attison; containing 70% of solid and Ni particles), 50 Wt % of acryl resin (mean particle size of 80 .mu.m), and 200 Wt % of diluent SB-1 (available from Nihon Attison) were mixed and applied to a SUS metallic roller by spray coating, dried at 80 degrees centigrade for 1 hour, and then polished to produce a sleeve having a 100 .mu.m thick surface layer. When such a sleeve 1a was located to face the drum 3 and applied with the previously stated pulse voltage for development, the sleeve 1a was found to improve the negative characteristic and produce an image with high density and desirable tonality and line reproducibility.

In the embodiments shown and described, the developer carrier may be implemented as a belt in place of the sleeve. Further, the magnetic roller 1b which is a specific form of magnetic field generating means may be replaced with, for example, a permanent magnet accommodated in the sleeve 1a in such a manner as to form an electric field only around the doctor blade 4.

In summary, in accordance with the present invention, a magnetic developer is sufficiently charged by a charging means and then deposited on a developer carrier concentratedly and in a great amount by numerous microfields developed on the surface of the image carrier. Further, when the surface of the developer carrier reaches a regulating member, only a desired part of the developer is caused to form a layer having a predetermined thickness due to the balance between electrostatic attraction ascribable to the microfields and a magnetic force ascribable to magnetism generating means. Hence, a layer of sufficiently charged magnetic developer can be formed stably. The developer is transported to a developing position where the developer carrier faces an image carrier. At the developing position, the movement of the developer is controlled by electric fields determined by a relation of the potential of the image carrier, the potential of the developer carrier, and the voltage applying means. As a result, an adequate amount of developer is deposited on the image carrier in matching relation to an electrostatic latent image formed on the image carrier. This is successful in enhancing image density while preserving tonality and in preventing lines included in an image from thickening, whereby high quality images with desirable resolution are insured.

Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.