Developing apparatus and image forming apparatus that incorporates the developing apparatus

A developing apparatus includes a developing roller and a sealing member. The developing roller applies toner to an electrostatic latent image to develop the electrostatic latent image. The sealing member presses the developing roller to seal the developer. The apparatus includes a developing roller with a shaft and a resilient layer formed on the shaft. The developing roller rotates about the shaft. The resilient layer covers the outer circumferential surface of the developing roller and is in contact with the sealing member. The resilient layer has a diameter that becomes larger nearer longitudinal ends of the developing roller. The diameter has a maximum value and a minimum value. The difference between the maximum value and the minimum value is such that 10 μm<ΔΦ<300 μm, where ΔΦ is a difference between the maximum value and the minimum value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an electrophotographic image-forming apparatus such as a printer and a facsimile machine.

2. Description of the Related Art

A conventional electrophotographic image-forming apparatus is of the configuration as shown inFIG. 1. A charging roller2charges the surface of a photoconductive drum1. A light source3illuminates the charged surface of the photoconductive drum1in accordance with print data to form an electrostatic latent image. A developing roller4applies, for example, toner to the electrostatic latent image to develop the electrostatic latent image into a visible image. A toner-supplying roller5charges the toner and supplies the charged toner to the developing roller. A developing blade6forms a toner layer having a uniform thickness on the developing roller4. A transfer roller7transfers the toner image from the photoconductive drum1onto a recording medium. A cleaning blade8collects residual toner remaining on the photoconductive drum that failed to be transferred onto the recording medium. Sealing members9are pressed against the circumferential surface of longitudinal end portions of the developing roller4to seal against the toner from leaking through the gaps in the vicinity of the longitudinal ends of the developing roller4. The sealing member9effectively reduces the surface roughness of the developing roller4so that the toner is difficult to leak. Alternatively, a cylindrical film is attached to the longitudinal end portions of the developing roller4to prevent toner leakage.

However, the sealing members9pressed against the longitudinal end portions of the developing roller4are not sufficient and some toner still leaks. If the sealing member9is firmly pressed against the developing roller4prevents the toner leakage but the friction between the developing roller4and sealing members9increases. Thus, the force that presses the sealing members9should be limited.

SUMMARY OF THE INVENTION

An object of the invention is to prevent toner from leaking from a developing unit.

Another object of the invention is to provide a configuration of a developing apparatus in which a developing roller is in contact with a photoconductive drum to apply a substantially uniform pressure across the length of the photoconductive drum.

Still another object of the invention is to provide a configuration of a developing apparatus in which the toner is pushed back toward the middle portion of the developing roller, thereby preventing toner leakage.

Yet another object of the invention is to provide a configuration of a developing apparatus in which a sealing member is pressed against the surface of the end portion of the resilient layer of the developing roller, thereby preventing toner leakage.

A developing apparatus includes a developing roller that rotates on a shaft and applies developer to an electrostatic latent image formed on a photoconductive body to develop the electrostatic latent image into a visible image. A resilient layer covers an outer circumferential surface of the developing roller. The resilient layer being in contact with the sealing member, wherein said resilient layer has a diameter that becomes larger nearer a longitudinal end of the developing roller.

The diameter has a maximum value and a minimum value. The difference between the maximum value and the minimum value is such that 10 μm<ΔΦ<300 μm, where <ΔΦ is the difference between the maximum value and the minimum value.

The average diameter of particles of the developer is such that ΔΦ>(50/Ψ)+5 μm, where ΔΦ is an average diameter of the particles of the developer.

The resilient layer has a resin coating formed on it.

The resin coating has a thickness such that 2 μm<t<100 μm where t is the thickness.

The resilient layer has an outer diameter that becomes larger nearer a longitudinal end of the resilient layer.

The resilient layer has a first diameter at a longitudinal middle portion of said resilient layer and a second diameter at a longitudinal end of said resilient layer, the first diameter and the second diameter are related such that Φend<Φmid where Φmid is the first diameter and Φend is the second diameter.

The resilient layer has a first diameter at a longitudinal middle portion and a second diameter at a point between the longitudinal middle portion and a longitudinal end, the second diameter being a smallest diameter across a length of said resilient layer, wherein the first diameter and the second diameter are such that 10 μm<(Φmid−ΦA)<500 μm, where Φmid is the first diameter in μm and ΦA is the second diameter in μm.

The longitudinal end has a surface that lies in a plane at an angle other than 90 degrees with an axis of rotation of the developing roller.

The resilient layer has an outer surface that extends to continuously become further away from an axis of rotation of the developing roller with decreasing distance from a longitudinal end of the developing roller.

The resilient layer has an outer surface that extends to become further away from a rotational axis of the developing roller the axis nearer a longitudinal end of said resilient layer, the outer surface becoming further away from the rotational axis stepwise.

The resilient layer has an outer surface that becomes continuously further away from a rotational axis of the developing roller nearer a longitudinal end of said resilient layer.

The developing apparatus further includes a sealing member disposed to press a circumferential surface of the developing roller to seal the developer.

The developing apparatus further includes a sealing member (9A,9A1) disposed to press a surface of a longitudinal end of the developing roller to seal the developer.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

FIG. 1is an electrophotographic image forming apparatus according to a first embodiment.

Referring toFIG. 1, a photoconductive drum1, a charging roller2, a light source3, a developing roller4, a toner-supplying roller5, transfer roller7that extend longitudinally and rotate. The cleaning blade8, a developing6, and a sealing member9extend in their longitudinal directions. The charging roller2, developing roller4, transfer roller7, cleaning blade8are in contact with the photoconductive drum1. The developing blade8and toner-supplying roller5are adjacent to and parallel to the developing roller4. The sealing member9is disposed in the vicinity of the longitudinal ends of the developing roller4, being pressed against the circumferential surface of the developing roller4.

FIG. 2illustrates the positional relation among the photoconductive drum1, developing roller4, toner-supplying roller5, and sealing member9. The sealing members9are pressed against the developing roller4in such a way that an inner circumferential surface9aof the sealing member9is in intimate contact with an outer circumferential surface of the developing roller4. The sealing member9is disposed so that a corner9bof the sealing member9does not press the developing roller4. As shown inFIG. 1, the developing roller4rotates counterclockwise so that the developing blade6slides on the developing roller4clockwise relative to the developing roller4. Alternatively, the developing blade6and developing roller4may be configured such that the developing roller4rotates clockwise and the developing blade6slides on the developing roller4counterclockwise relative to the developing roller4. The developing roller4has a metal shaft42covered with a resilient layer42. In the first embodiment, the resilient layer42is a single layer but may also be of multi-layer structure, e.g., dual-layer structure. For example, an inner layer is made of a material having a low hardness and an outer layer is made of a material having a high hardness. Such a dual-layer structure offers a soft developing roller in which the developing roller1will not be dented at an area in contact with the photoconductive drum1even if the dual-layer structure is left inoperative for a long time. The resilient layer42is preferably formed of a resilient material such as urethane rubber or semiconductive silicone rubber.

The longitudinal end portions of the developing roller4have a diameter that becomes larger nearer the longitudinal ends. It is to be noted that the diameter does not change linearly but exponentially. The sealing members9are in contact with the circumferential surface of the longitudinal end portions. The geometry of the longitudinal end portions may be shaped using a metal mold and is then subjected to polishing for a final shape. Alternatively, the developing roller may be molded or extruded, and the developing roller4may then be polished into a desired final shape.

FIGS. 3A–5Bshow various shapes of the developer roller4.FIG. 3Ais a cross-sectional view of a developing roller4A andFIG. 3Billustrates the pressure that is exerted on the sealing member9in the vicinity of longitudinal end portions of the developing roller4A.FIG. 4Ais a cross sectional view of a developing roller4B andFIG. 4Billustrates pressure that is exerted on the sealing member in the vicinity of longitudinal end portions of the developing roller4B.FIG. 5Ais a cross sectional view of a developing roller4C andFIG. 5Billustrates pressure that is exerted on the sealing member9in the vicinity of longitudinal end portions of the developing roller4C. The longitudinal end portions of the developing roller4may be made at will in any shapes during polishing, by adjusting the speed and pressure with which the polishing is performed in an axial direction of the developing roller. For example, the developing roller4A inFIG. 3Ahas a diameter that becomes continuously progressively larger nearer the longitudinal ends of the developing roller4A. The developing roller4B inFIG. 4Ais tapered linearly toward the longitudinal ends. The developing rollers4A and4B may be shaped in appropriate known methods. The diameter of the developing roller4C inFIG. 5Abecomes continuously progressively larger nearer the longitudinal ends of the developing roller4C. Such a shape needs to be formed by molding. The developing rollers4A–4C preferably have a circumferential surface in contact with the sealing member9, the circumferential surface having a maximum diameter Φmax and a minimum diameter Φmin such that 10 μm<ΔΦ<300 μm (ΔΦ=Φmax−Φmin). When toner particles have an average diameter of Ψ, it is desirable that Ψ<ΔΦ.

FIG. 6illustrates the configuration of the sealing member9. The sealing member9includes a felt91and a foamed resilient body92. The felt91has a surface in direct contact with the developer roller4. The surface has a shape that is configured to mate with the circumferential surface of the developing roller4. The width L2of an area of the sealing member9in contact with the developing roller4is preferably in the range of 3 to 15 mm. The resilient body92has a hole92athrough which a shaft of the toner-supplying roller5extends.

When the developing roller4is driven to rotate counterclockwise, the developing blade6forms a layer of toner having a uniform thickness on the developing roller4. The layer of toner is strongly attracted by the Coulomb force to the developing roller4and will not leak to the outside environment. The toner-supplying roller5scrapes the toner off the developing roller4as well as supplying fresh toner to the developing roller4. The electrostatic force no longer affects residual toner scraped by the toner-supplying roller5. Thus, the sealing member9is effective in preventing the scraped toner from leaking to the outside environment. The diameter of the developing roller4near its longitudinal end portions becomes larger nearer the longitudinal ends of the developing roller4. Thus, the sealing member9does not exert a pressing force that is uniformly distributed across the length of the developing roller4but a pressing force that becomes larger nearer the longitudinal ends. This profile of distribution of the pressing force is effective in preventing the toner from leaking toward the longitudinal ends of the developing roller4.

The pressing force exerted by the sealing member9on the developing roller4is proportional to the ΔΦ. For ΔΦ smaller than 10 μm, the pressing force is distributed relatively uniformly and is therefore not effective in blocking the leakage of the toner. For ΔΦ larger than 300 μm, a gap is created between the developing roller4and the sealing member9, so that the toner accumulates between the developing roller4and the sealing member9to cause “filming.” The filming of toner scratches the developing roller4to cause grooves in the developing roller4through which the toner leaks. The smaller the particle diameter of toner is, the larger the ΔΦ should be, so that the particle diameter and the ΔΦ are inversely proportional. In other words, there is a relation that ΔΦ>(a/Ψ)+b (a and b are fixed values). The values of a and b can be calculated from the data listed in Table 1. The values of Ψ and ΔΦ that provide the advantages listed in Table 1 are put into Equation ΔΦ>(a/Ψ)+b. In other words, (Ψ, ΔΦ)=(4, 20), (6, 15), (8, 15), and (10, 10) are put into Equation ΔΦ>(a/Ψ)+b, thereby calculating the values of a and b. The thus calculated values of a and b are 50 and 5, respectively. The data in Table 1 reveal that toner will not leak if ΔΦ>(50/Ψ)+5 μm.

TABLE 1Δ Φ (μm)Ψ (μm)1015204xx∘6x∘∘8x∘∘10Δ∘∘Δ: leakage is serious toward the end of lifetime of the developing roller.

Because the longitudinal end of the developing roller4B has not the largest diameter, the pressing force exerted by the sealing member9is concentrated on the largest diameter portion, so that the developing roller4B is subjected to serious wear. If the sealing member9is disposed in such a way that the largest diameter portion is closer to the longitudinally middle portion of the developing roller4B than the mid point in the width of the sealing member9, the effective areas for sealing become smaller. If the diameter of the developing roller4is formed to have a diameter that rapidly changes, then a gap will be created between the developing roller4B and the sealing member9. This gap allows the toner to be deposited on the developing roller4B to cause filming, the filming of toner scratching the developing roller4B. Thus, the diameter of the developing roller should not change rapidly. For developing rollers4A and4C, the diameter is a maximum at the longitudinal ends and therefore the pressing force will decrease. The pressing force acts toward the longitudinal ends of the developing roller, so that the wear of the developing rollers inFIGS. 3A and 4Awill be reduced. The diameter of the developing roller4C inFIG. 5Aincreases stepwise toward the longitudinal ends, thus the step size should be as small as possible so as not to create gaps at the corner portions.

The developing rollers4A–4C allow the sealing members9to be pressed against the developing roller in a limited area without increasing the pressing force. In addition, the shape of the end portions of the developing rollers4A–4C creates a force tending to push the toner toward the longitudinally middle portions of the developing rollers. Thus, the toner will not migrate toward the longitudinal end portions, firmly sealing the toner which would otherwise adhere to the drive gears to cause an increase in drive torque. Further, the toner is prevented from falling on the recording medium and therefore does not cause soiling of the recording medium.

Second Embodiment

An electrophotographic image forming apparatus according to a second embodiment has the same configuration as the first embodiment.FIG. 7is a cross-sectional view of a developing roller4D. The developing roller4D is provided with a resilient layer43that covers the outer circumference of a meal shaft40. The resilient layer43is formed of a semiconductive material such as silicone rubber or urethane. The developing roller4D is polished at an area in contact with a sealing member9in such a way that the diameter becomes larger nearer the longitudinal ends. It is to be noted that the diameter does not change linearly but exponentially. In other words, the developing roller4D is shaped such that the diameter of the resilient layer43becomes larger nearer the longitudinal ends. The resilient layer43may be of dual-layer structure as in the first embodiment, in which case, a resin coating42formed of, e.g., urethane resin, is formed as a second layer on the resilient layer43.

The resin coating42has a resin coating having a uniform thickness in the range of 2 to 100 μm. A resin coating42having a thickness smaller than 2 μm causes a poor durability. The resin coating42is harder than the resilient layer43and therefore the change in shape at the longitudinal ends is smaller than the resin coating42. If the thickness of the resin coating42is greater than 100 μm, the change in the diameter of the developing roller4D is too small near the longitudinal ends, not being effective in preventing toner leakage.

Resin coating is small in surface roughness and viscosity as compared to semiconductive rubber, and therefore resin coating has good sliding ability. Because the resin coating42covers the surface of the developing roller4D, the toner is easy to slip on the resin coating42. For this reason, the toner-supplying roller5can effectively scrape the toner on the developing roller4D, preventing an “after-image” from occurring. This also prevents filming on the developing roller4D due to adhesion of toner that would otherwise fail to be scraped by the toner-supplying roller5. However, the good ability of resin coating to slide reduces the friction between the developing roller4D and the toner-supplying roller5. As a result, the toner entered between the developing roller4D and the sealing member9tends to move in an axial direction. Thus, if the pressing force between the developing roller4D and sealing member9is distributed uniformly, the toner is apt to leak from the inside of the developing unit to the outside of the developing unit. However, the diameter of the resilient layer43, which becomes larger nearer the longitudinal end of the developing roller4D, exerts a force on the coating42, the force causing the resin coating42to widen outwardly. Therefore, despite the fact that the resin coating42is made of a material having a uniform thickness, the developing roller4D has an overall diameter that becomes large nearer the longitudinal ends. Even if the toner enters between the developing roller4D and the sealing member9, the toner is pushed back toward the inner space of the developing unit.

Resin coating is harder than semiconductive rubber, so that the resin coating is difficult to be polished to into a shape such that the diameter becomes larger nearer the longitudinal ends. In the second embodiment, the resilient layer43of the developing roller4D is formed such that the diameter becomes larger nearer the longitudinal ends. Therefore, despite the fact that the resilient layer43is covered with the resin coating42, an area on the developing roller4D in contact with the sealing member9also becomes larger in diameter nearer the longitudinal ends. This provides the developing roller4D that prevents the toner from leaking to the outside environment and allows the toner to slide thereon.

Third Embodiment

An electrophotographic image forming apparatus according to a third embodiment is of the same configuration as the first embodiment.FIG. 8Ais a perspective view illustrating the overall shape of a developing roller4E.FIG. 8Bis a front view of the developing roller4E. The developing roller4E has a metal shaft40covered with a resilient layer44. The resilient layer44may be of dual-layer structure. The developing roller4E is formed such that the diameter is a maximum (Φmid) at the longitudinally middle portion and becomes continuously progressively smaller nearer the longitudinal end portions to reach a minimum (ΦA) nearer the ends, and then again becomes larger nearer the longitudinal ends, reaching Φend at the longitudinal ends. A sealing member9is in contact with the outer surface of the developing roller4E that extends from ΦA to Φend. The diameters Φend and Φmid are related such that Φend<Φmid. The diameters Φmid and ΦA are related such that 10 μm<(Φmid−ΦA)<200 μm.

In the first embodiment, the developing rollers4A,4B, and4C are pressed against the photoconductive drum1with a certain pressing force. Therefore, the pressing force exerted on the surface of the photoconductive drum1by the developing roller is a maximum at the longitudinal ends and a minimum at the longitudinal middle portion. Thus, the efficiency of the toner to be deposited on the photoconductive drum1is not uniform along the length of the photoconductive drum1.

In contrast, the developing roller4E according to the third embodiment provides a substantially uniform profile of the distribution of pressing force across the entire length of the photoconductive drum1. For Φmid<Φend, there is no advantage of having a diameter that becomes larger nearer the longitudinal middle. If the developing roller4E has too large a diameter at the longitudinally middle portion, the pressing force tends to be concentrated to the longitudinal middle. Thus, for the developing roller4E, when the minimum diameter is ΦA, the diameters Φmid and φA are related preferably such that 10 μm<(Φmid−ΦA)<500 μm.

When the developing roller4E and the photoconductive drum1are in pressure contact with each other, because the pressing force is uniform along the length of the developing roller4E, the movability of the toner from the developing roller4E to the photoconductive drum1can be uniform across the length of the developing roller4E. Thus, the density of an image becomes substantially the same across the width of an image formed on the recording medium. Moreover, the pressure acting between the photoconductive drum1and the developing roller4E is not concentrated on the longitudinal end portions, and therefore deteriorated toner will not accumulate in the vicinity of the longitudinal end portions of the developing roller4E.

Fourth Embodiment

An electrophotographic image forming apparatus according to a fourth embodiment is of the same configuration as the first embodiment.FIG. 9Ais a cross-sectional view of a developing roller4F andFIG. 9Billustrates the pressure exerted on a sealing member9in the vicinity of longitudinal end portions of the developing roller4F. The developing roller4F has a metal shaft40covered with a resilient layer41. The resilient layer may be of multi-layer structure. The developing roller4F is formed such that the longitudinal end surface4alies in a plane at an angle θ other than 90 degrees with a rotational axis of the developing roller4F. The outline of the geometry of the developing roller4F is preferably formed by using a metal mold and the thus formed developing roller4F is polished into precise dimensions. The surfaces of the longitudinal end portions of the developing roller4F are formed during the polishing stage such that the diameter becomes larger nearer the longitudinal ends. It is to be noted that the diameter does not change linearly but exponentially. The developing roller4F may also be shaped by other methods such as extrusion molding, injection molding, and cutting. The difference in the longitudinal dimension of the surface of the developing roller4F is preferably in the range of ½ to ⅔ of the width of the sealing member9.

When the developing roller4F rotates, the developing roller4F rubs the sealing member9. The sealing member9exerts a frictional force on the developing roller4F so that the developing roller4F is twisted by a torsional force exerted at its longitudinal ends. Because the resilient layer41of the developing roller4F tends to correct the torsion, a force acts on the toner in such a direction as to push the toner toward the longitudinally middle portion of the developing roller4F. The force that acts on the toner is a maximum at the longitudinal end of the developing roller4F. This longitudinal end lies in a plane at an angle other than 90 degrees with the rotational axis of the developing roller4F. Thus, when the developing roller4F rotates about its rotational axis, the longitudinal ends repeat its reciprocating motion along the rotational axis. This increases the force tending to push back the toner toward the longitudinally middle portion of the developing roller4F. In other words, a force acting in a direction toward the longitudinally middle portion of the developing roller4F is greater than a force acting in a direction toward the longitudinal end. Thus, leakage of the toner is effectively prevented.

Fifth Embodiment

In the first to fourth embodiments, the developing roller is shaped to have a diameter that becomes larger nearer the longitudinal ends and the Coulomb force attracts the toner to the developing roller, thereby preventing the toner from leaking. However, some of the toner may still leak. In a fifth embodiment, a sealing member9is pressed against a resilient layer45of a developing roller4G. Even when the toner layer is formed on the full length of the developing roller4G, the toner is prevented from leaking to the outside environment.

FIG. 10illustrates an electrophotographic image forming apparatus according to the fifth embodiment. The image forming apparatus includes a photoconductive drum1, a charging roller2, a light source3, a developing roller4G, a toner-supplying roller5, a developing blade6, a transfer roller7, a cleaning blade8, and a sealing member9A.

FIG. 11illustrates the positional relation among the photoconductive drum1, developing roller4G, toner-supplying roller5, and sealing member9A.FIG. 12is a cross-sectional view of the developing roller4G.FIG. 13illustrates the sealing member9A. The sealing member9A is preferably made of a foamed resilient material but may be formed of other materials such as a foamed material, plastics, or metal. The sealing member9A is formed with holes9A2and9A3therein through which a shaft40of the developing roller4G and a shaft of the toner-supplying roller5extend, respectively. The developing roller4G has the shaft40covered with the resilient layer45. The resilient layer45may be of multi-layer structure.

If the sealing member9A is to be in direct contact with the photoconductive drum1, the sealing member9A is formed to have an area9A1that is configured to accommodate the circumferential surface of the photoconductive drum1. The sealing member9A has a flat surface that is in contact with the longitudinal end of the resilient layer45. In order to prevent wear of the sealing member9A and the resilient layer45, a thin ring-shaped film10may be inserted between the sealing member9A and the developing roller4G. The thin ring-shaped film10has a thickness of about 10 μm and an outer diameter smaller than that of the longitudinal end of the developing roller4G.

The resilient layer45is formed such that the diameter becomes larger nearer the longitudinal end of the resilient layer45, starting 15 mm from the longitudinal end. The diameter of the resilient layer45is related such that 10 μm<(Φ1−Φ0)<300 μm. The surface of the longitudinal end of the resilient layer45of the developing roller4G lies in a plane substantially perpendicular to the shaft40. The sealing member9A may also be applied to the developing roller4D according to the second embodiment and the developing roller4E according to the third embodiment.

A frictional force exists between the sealing member9and the developing roller G. When the developing roller4G rotates, the frictional force causes the end portions of the resilient layer45to twist. The torsional force acts toward the rotational axis of the developing roller, so that a gap tends to be created between the developing roller4G and the sealing member9A. The geometry of the resilient layer45is such that the diameter of the resilient layer45extends to become away from the rotational axis of the resilient layer toward the longitudinal ends of the resilient layer45. It is to be noted that the diameter does not change linearly but exponentially. The resilient force acts radially outwardly of the developing roller and is greater than the torsional force acting toward the rotational axis of the developing roller. Thus, a gap is difficult to be created between the developing roller4G and the sealing member9A. Because the outer diameter of the thin ring-shaped film10is smaller than that Φ1 of the longitudinal end of the developing roller4G, the thin ring-shaped film10is intimate contact with the resilient layer45. Thus, a gap is not created between the resilient layer45of the developing roller4G and the sealing member9A and therefore the toner will not leak between the sealing member9and the thin ring-shaped film10.