Developing device and image forming apparatus

A developing device includes a static latent image supporting member having a photosensitive layer and being arranged to be rotatable; a developer supporting member arranged to be rotatable for developing a static latent image formed on the static latent image supporting member using developer to form an image; a developer supplying member arranged to contact with the developer supporting member and be rotatable for supplying developer; and a drive transmission unit disposed on a side of same end portions of the developer supporting member and the developer supplying member for rotating the developer supporting member and the developer supplying member in a same rotational direction. The developer supplying member is formed so that an outer diameter thereof on a side of the drive transmission unit becomes smaller than an outer diameter thereof on an opposite side.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a developing device and an image forming apparatus for forming an image.

In an electro-photography type image forming apparatus as a conventional image forming apparatus, a charging roller as a charging member is arranged to uniformly charge a surface of a photosensitive member as an image supporting member. An exposure unit is arranged to form a static latent image on the photosensitive drum, and a developing device is arranged to form a toner image on the static latent image on the photosensitive drum. The developing device includes a developing roller as a developer supporting member; a supplying roller as a developer supplying member for supplying toner as developer to the developing roller; and a regulating blade as a developer layer forming member for forming a toner thin layer on the developing roller.

In the conventional image forming apparatus, after the toner image is transferred to a sheet, a cleaning blade formed of a rubber plate member is arranged to collect toner remaining on the photosensitive drum. Further, a fixing device is arranged to fix the toner image on the sheet, and the sheet is discharged outside the conventional image forming apparatus. The conventional image forming apparatus may include a plurality of developing devices arranged in series for forming toner images in four colors, namely, black, cyan, magenta, and yellow, so that it is possible to form a color image.

In the conventional image forming apparatus described above, it may configured such that a toner supply voltage applied to the supplying roller is controlled according to a detection result of toner attached to a non-exposed region of the photosensitive drum or an area of a transfer member corresponding to the non-exposed region of the photosensitive drum. Accordingly, it is possible to prevent a stain from generating on the sheet (refer to Patent Reference).Patent Reference: Japanese Patent Publication No. 2007-093775

In the conventional image forming apparatus disclosed in Patent Reference, it is configured such that the toner supply voltage applied to the supplying roller is controlled, so that an amount of toner supplied to the supplying roller is adjusted. However, when an outer circumferential portion (a rubber portion) of the supplying roller is worn out, the amount of toner supplied to the supplying roller tends to be excessive, thereby causing a stain on the sheet.

In view of the problems described above, an object of the present invention is to provide a developing device and an image forming apparatus capable of solving the problems of the conventional developing device and the conventional image forming apparatus. In the present invention, it is possible to reduce a stain generated on a sheet.

SUMMARY OF THE INVENTION

In order to attain the objects described above, according to an aspect of the present invention, a developing device includes a static latent image supporting member having a photosensitive layer and being arranged to be rotatable; a developer supporting member arranged to be rotatable for developing a static latent image formed on the static latent image supporting member using developer to form an image; a developer supplying member arranged to contact with the developer supporting member and be rotatable for supplying developer; and a drive transmission unit disposed on a side of same end portions of the developer supporting member and the developer supplying member for rotating the developer supporting member and the developer supplying member in a same rotational direction. The developer supplying member is formed so that an outer diameter thereof on a side of the drive transmission unit becomes smaller than an outer diameter thereof on an opposite side.

In the developing device and the image forming apparatus of the present invention, it is possible to reduce a stain generated on a sheet.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, embodiments of the present invention will be explained with reference to the accompanying drawings. It should be noted that the present invention is not limited to the following description, and the embodiments can be modified within a scope of the present invention.

First Embodiment

A first embodiment of the present invention will be explained.FIG. 2is a schematic sectional view showing a configuration of a printer1as an image forming apparatus according to the first embodiment of the present invention.

As shown inFIG. 2, the printer1includes developing devices2(2K,2C,2M, and2Y) corresponding to toner30(30K,30C,30M, and30Y) as developer in four colors of black (B), cyan (C), magenta (M), and yellow (Y); toner cartridges3(3K,3C,3M, and3Y) for retaining the toner30(30K,30C,30M, and30Y); and transfer units4(4K,4C,4M, and4Y) for transferring toner images developed on photosensitive drums21(21K,21C,21M, and21Y; described later) as static latent image supporting member to a sheet P as a transfer medium.

In the first embodiment, the printer1further includes exposure units5(5K,5C,5M, and5Y) for irradiating surfaces of the photosensitive drums21to form the static latent images thereon; a sheet supply cassette6for retaining the sheet P and supplying the sheet P in an arrow direction X; a fixing unit7for fixing the toner images transferred to the sheet P; and a sheet transportation path8formed in an S character shape relative to a lower frame of the printer1.

In the first embodiment, the developing devices2K,2C,2M, and2Y are sequentially disposed along the sheet transportation path8from a sheet supply side to a sheet discharge side of the sheet P in a sheet transportation direction Y from an upstream side to a downstream side. Further, the developing devices2K,2C,2M, and2Y are integrated as an image forming unit20, and arranged to be freely detachable relative to the printer1. The developing devices2K,2C,2M, and2Y have an identical configuration except colors of the toner30K,30C,30M, and30Y to be developed. Accordingly, in the following description, only the configuration of the developing device2K for developing the toner30K in black, and explanations of the configurations of the other developing devices2C,2M, and2Y are omitted.

In the first embodiment, the developing device2K includes the photosensitive drum21K as the static latent image supporting member arranged to be rotatable and having a photosensitive layer; a charging roller22K as a charging member for uniformly charging the surface of the photosensitive drum21K; a developing roller23K arranged to be rotatable for developing the toner30K to the static latent image formed on the photosensitive drum21K to form an image; and a developing blade24K as a toner layer regulating member for regulating a layer thickness of the toner30K supplied to the developing roller23K.

In the first embodiment, the developing device2K further includes a supplying roller25K as a developer supplying member arranged to be rotatable and contact with the developing roller23K for supplying the toner30K to the developing roller23K; a cleaning blade26K as a toner removing member or a cleaning member for removing the remaining toner30K remaining on the photosensitive drum21K and not transferred to the sheet P; and a first transportation unit27K as a transportation member for transporting the remaining toner30K removed with the cleaning blade26K as the waste toner30K.

In the first embodiment, the photosensitive drum21K is formed of a conductive supporting member and a photoconductive layer. More specifically, the photosensitive drum21K is formed of a metal pipe such as aluminum and the like as the conductive supporting member. Further, a blocking layer and the photoconductive layer of an electron charge generation and an electron charge transportation layer are sequentially laminated on the metal pipe, thereby constituting an organic photosensitive member. The charging roller22K is formed of a metal shaft and a semi-conductive rubber layer such as an epichlorohydrin rubber. It should be noted that an outer circumferential surface of the charging roller22K is arranged to abut against an outer circumferential surface of the photosensitive drum21K with a specific pressing force, so that the charging roller22K follows and rotates when the photosensitive drum21K rotates.

In the first embodiment, the developing roller23K is formed of a metal shaft and a semi-conductive urethane rubber layer. It should be noted that an outer circumferential surface of the developing roller23K is arranged to abut against the outer circumferential surface of the photosensitive drum21K with a specific pressing force, so that the charging roller22K follows and rotates while maintaining a specific circumferential speed ratio in a direction that the photosensitive drum21K rotates.

In the first embodiment, the developing blade24K is formed of a metal thin plate member for regulating the layer thickness of the toner30. The developing blade24K has a thickness of, for example, 0.08 mm and a length substantially equal to a length of the developing roller23K in a longitudinal direction thereof. The developing blade24K has one end portion fixed to a frame (not shown), and an inner side surface slightly inside a distal end portion of the other end portion is arranged to abut against the developing roller23K.

In the first embodiment, the supplying roller25K is formed of a metal shaft and a semi-conductive foamed silicone sponge layer. It should be noted that an outer circumferential surface of the supplying roller25K is arranged to abut against the outer circumferential surface of the photosensitive drum21K with a specific pressing force, so that the charging roller22K rotates while maintaining a specific circumferential speed ratio in a direction opposite to the direction that the developing roller23K is rotated.

In the first embodiment, the cleaning blade26K is formed of a urethane rubber member. The cleaning blade26K has a length substantially equal to a length of the photosensitive drum21K in a longitudinal direction thereof. The developing blade24K has one end portion extending in a longitudinal direction thereof and fixed to the frame (not shown), and the other end portion arranged to abut against the outer circumferential surface of the photosensitive drum21K with a specific pressing force.

In the first embodiment, the first transportation unit27K is arranged to transport the remaining toner30K and an attached substance removed with the cleaning blade26K as the waste toner30K toward a front side in a rotational axis direction of the photosensitive drum21K. A second transportation unit28is arranged to collectively transport the waste toner30K,30C,30M, and30Y transported from the first transportation units27K,27C,27M, and27K disposed in the developing devices2K,2C,2M, and2Y in an arrow direction Z.

In the first embodiment, the toner cartridges3K,3C,3M, and3Y respectively include supply toner storage portions31K,31C,31M, and31Y having a hollow structure for retaining the unused toner30K,30C,30M, and30Y in the four colors of black (K), cyan (C), magenta (M) and yellow (Y). Among the toner cartridges3K,3C,3M, and3Y, only the toner cartridge3K of black (K), which is disposed at the most upstream position of the sheet transportation path8in the sheet transportation direction, includes a waste toner storage portion32disposed adjacent to the supply toner storage portion31K. The waste toner storage portion32has an independent space disposed adjacent to the supply toner storage portion31K for retaining the waste toner30K,30C,30M, and30Y transported with the second transportation unit28.

In the first embodiment, each of the image forming unit20and the toner cartridges3K,3C,3M, and3Y is configured to be a replacement unit of the printer1, so that the replacement unit is detachable relative to the printer1. Accordingly, when the toner30K,30C,30M, and30Y retained therein is consumed, or a component thereof is worn, it is possible to replace each of the image forming unit20and the toner cartridges3K,3C,3M, and3Y.

In the first embodiment, the transfer unit4includes a transfer belt9for statically attaching and transporting the sheet P; a drive roller (not shown) driven with a drive unit (not shown) to rotate for driving the transfer belt9; a tension roller (not shown) to be paired with the drive roller for extending the transfer belt9; and transfer rollers4K,4C,4M, and4Y arranged to face the photosensitive drum21K,21C,21M, and21Y with the transfer belt9in between for applying a voltage, so that the toner images formed on the photosensitive drum21K,21C,21M, and21Y are transferred to the sheet P. The exposure units5K,5C,5M, and5Y are formed of LED (Light Emitting Diode) heads including a light emitting element such as an LED and the like and a lens array.

In the first embodiment, the sheet supply cassette6is configured to retain the sheet P therein in a stacked state, and is detachably attached to a lower portion of the printer1. A sheet transportation unit (not shown) is disposed at an upper portion of the sheet supply cassette6, and the sheet transportation unit includes a hopping roller and the like for picking up and feeding the sheet P one by one. The fixing unit7is disposed on a downstream side of the image forming unit20in the sheet transportation direction of the sheet transportation path8. The fixing unit7includes a heating roller7a, a pressing roller7b, a thermistor (not shown), and a heating heater.

In the first embodiment, the heating roller7aincludes a metal core formed of aluminum and the like and having a hollow cylindrical shape; a heat resistance elastic layer formed of a silicone rubber and covering the metal core; and a PFA (tetrafluoroethylene-perfluoroalkylvinylether copolymer) tube covering the heat resistance elastic layer. The heating heater such as a halogen lamp is disposed in the metal core of the heating roller7a. Similarly, the pressing roller7bincludes a metal core formed of aluminum; a heat resistance elastic layer formed of a silicone rubber and covering the metal core; and a PFA tube covering the heat resistance elastic layer. The pressing roller7bis arranged to form a pressing portion (an abutting portion) with respect to the heating roller7a. The thermistor is provided as a surface temperature detection member of the heating roller7a, and is arranged at a close proximity of the heating roller7ain a non-contact state.

FIG. 3is a schematic sectional view showing the image forming unit20of the printer1as the image forming apparatus according to the first embodiment of the present invention.

As shown inFIG. 3, in the image forming unit20, the developing devices2K,2C,2M, and2Y in each color are arranged at a constant interval. A first side frame42with high rigidity and a second side frame43with high rigidity are provided for supporting both side portions of each of the developing devices2K,2C,2M, and2Y in a medium transportation direction (an arrow direction X inFIG. 3). Further, a front frame44and a back frame45are provided for supporting both end portions of each of the developing devices2K,2C,2M, and2Y in the medium transportation direction.

In the first embodiment, photosensitive drum shafts41K,41C,41M, and41Y with specific rigidity are provided as a photosensitive drum rotation supporting member, and are formed of a metal with sufficient conductivity. When the image forming unit20is attached or detached, the photosensitive drum shafts41K,41C,41M, and41Y are moved along a guide (not shown) in the printer1. Further, it is configured such that the photosensitive drum shafts41K,41C,41M, and41Y are capable of moving in an arrow direction W inFIG. 3with a developing device lifting up mechanism (not shown), so that the developing devices2K,2C,2M, and2Y are capable of moving away from the transfer belt9shown inFIG. 2.

FIGS. 1(a) and1(b) are schematic views showing the supplying roller25of the printer1as the image forming apparatus according to a first embodiment of the present invention. More specifically,FIG. 1(a) is a schematic front view showing the supplying roller25of the printer1, andFIG. 1(b) is a schematic enlarged view showing a conductive foamed layer200of the supplying roller25of the printer1.

As shown inFIG. 1(a), the supplying roller25includes a shaft202as a metal core and the conductive foamed layer200disposed on a circumference (a surface layer) of the shaft202. As shown inFIG. 1(b), the conductive foamed layer200includes a large number of cells (foamed cells)201.

In the first embodiment, the conductive foamed layer200is formed of a rubber material including a rubber material such as a silicone rubber, a silicone-modified rubber, a natural rubber, a nitrile rubber, an ethylene-propylene rubber, an EPDM, a styrene-butadiene rubber, an acrylonitrile-butadiene rubber, a butadiene rubber, an isoprene rubber, an acryl rubber, a chloroprene rubber, a butyl rubber, an epichlorohydrin rubber, a urethane rubber, a fluorine rubber, a polyether rubber, and the like; an elastomer such as polyurethane, polystyrene, a polybutadiene block copolymer, polyolefin, polyethylene, a chlorinated polyethylene, an ethylene-vinyl acetate copolymer, and the like; or a mixture rubber or a modified rubber of one or more than two of the materials described above. It should be noted that the rubber materials described above can be arbitrarily selected from a millable type or a liquid type. In particular, it is preferred that the rubber materials are the millable type.

In the first embodiment, the shaft202may be formed of a metal material with specific rigidity and sufficient conductivity including steel, capper, stainless, aluminum, nickel, and the like. Further, the shaft202may be formed of a material other than the metal material as far as the material possesses conductivity and sufficient rigidity. For example, the material includes a resin molded component and a ceramic material in which conductive particles are dispersed. Further, the shaft may be formed in a hollow pipe shape other than the roller shape.

In the first embodiment, gear attachment step portions203and204are formed on both end portions of the shaft202. A pin hole may be formed in the both end portions of the shaft202. Further, a bearing portion is formed on the both end portions of the shaft202, and the bearing portion may be generally formed to have a diameter smaller than that of the shaft202where the conductive foamed layer200is disposed.

In the first embodiment, when the supplying roller25is produced, first, a reinforcement filler, a vulcanization agent necessary for vulcanization, a foaming agent, and a conductivity imparting agent are added to the rubber material described above to obtain a mixture. In the next step, after the mixture is thoroughly mixed with a pressure kneader, a mixing roll, and the like to obtain a rubber compound, the rubber compound is formed on the shaft202in an un-vulcanized state with an extruding method and the like. Then, the rubber compound is heated for vulcanization and foaming.

Alternatively, the rubber compound may be extruded in a tube shape in advance, and then is heated for vulcanization and foaming to form a sponge rubber tube. Then, the sponge rubber tune is placed on the shaft202to produce the supplying roller25. In this method, as necessary, an adhesive may be applied between the shaft202and the conductive foamed layer200for fixing. Afterward, the supplying roller25thus produced is machined and ground to have a specific outer diameter.

In the first embodiment, the conductive foamed layer200of the supplying roller25has a length of 220.0 mm in a rotational axis direction. As shown inFIG. 1(a), three points D1, D2, and D3 are defined. The point D1 is located at a position 5.0 mm away from a left end portion (an end portion on a non-drive transmission side) of the conductive foamed layer200of the supplying roller25toward a right end portion where a drive gear is disposed (an end portion on a driven side). Similarly, the point D2 is located at a position away from the left end portion by 110.0 mm, and the point D3 is located at a position away from the left end portion by 215.0 mm. Further, a distance from the left end portion (the end portion on the non-drive transmission side) of the conductive foamed layer200of the supplying roller25is defined as X (mm).

FIGS. 4(a) and4(b) are schematic views showing the supplying roller25, the developing roller24, the photosensitive drum21, and drive gears of the printer1as the image forming apparatus according to the first embodiment of the present invention. More specifically,FIG. 4(a) is a schematic perspective view showing the supplying roller25, the developing roller24, the photosensitive drum21, and the drive gears of the printer1as the image forming apparatus, andFIG. 4(b) is a schematic side view showing the drive gears of the printer1as the image forming apparatus.

As shown inFIG. 4(a), the drive gears as a drive transmission unit are disposed on a same side of the end portions of the supplying roller25, the developing roller23and the photosensitive drum21. Accordingly, it is configured such that the developing roller23is rotated in a rotational direction the same as that of the supplying roller25.

As shown inFIG. 4(a), the drive gears includes a developing roller gear303as a first gear, a supplying roller gear301as a second gear, and a drive transmission gear302as a third gear. The supplying roller gear301is disposed at the one end portion of the rotational axis of the supplying roller25, and is connected to the drive transmission gear302. The developing roller gear303is disposed at the one end portion of the rotational axis of the developing roller23, so that the developing roller gear303transmits a drive force to the drive transmission gear302. A photosensitive drum gear304is disposed at the one end portion of the rotational axis of the photosensitive drum21, so that the photosensitive drum gear304transmits the drive force to the developing roller gear303. The photosensitive drum gear304is configured to receive the drive force from a drive motor gear (not shown).

FIG. 6is a schematic side view showing the supplying roller25of the printer1as the image forming apparatus according to the first embodiment of the present invention.

As shown inFIG. 6, the supplying roller25has a largest outer diameter φD1 at the point D1, and an outer diameter φD2 at the point D2 and an outer diameter φD3 at the point D3 are gradually decreased in this order. In other words, the supplying roller25is configured such that the outer diameter thereof is decreased from the side of the drive transmission unit toward the non-drive transmission side. Further, the supplying roller25is configured such that the outer circumferential surface thereof becomes smooth along a continuous straight line or a curved line between the points D1, D2, and D3 obtained through a conventional polishing method. Alternatively, the supplying roller25is configured such that the outer circumferential surface thereof becomes stepwise.

An experiment of evaluating a continuous durability of the supplying roller25will be explained next. In the experiment, a continuous durability print test was conducted using Sample 1 to Sample 4 having different shapes. More specifically, Sample 1 had a straight cylindrical shape having an identical outer diameter of φD1, φD2, and φD3. Sample 2 had the outer diameter φD3 smaller than the outer diameter φD1 by about 0.2 mm. Sample 3 had the outer diameter φD3 smaller than the outer diameter φD1 by about 0.4 mm. Sample 4 had the outer diameter φD3 smaller than the outer diameter φD1 by about 0.6 mm. The outer diameter profiles of Sample 1 to Sample 4 in an initial state are shown in Table 1.

An effect of the configuration described above will be explained. First, an operation of the printer1as the image forming apparatus will be explained next with reference toFIG. 2.

After the printer1receives print data, the developing devices2K,2C,2M, and2Y are driven, so that the toner30K,30C,30M, and30Y are supplied from the toner cartridges3K,3C,3M, and3Y. Further, after the printer1receives the print data, the printer1feeds the sheet P in the sheet supply cassette6in the arrow direction X, so that the sheet P is transported along the sheet transportation path8in the arrow direction Y. When the sheet P is transported, the sheet P sequentially passes below the developing devices2K,2C,2M, and2Y. At this moment, the exposure units (the LED heads)5K,5C,5M, and5Y respectively expose the photosensitive drums21K,21C,21M, and21Y to form the toner images thereon, and the transfer unit4transfers the toner images to the sheet P. Afterward, the fixing unit7fixes the toner images to the sheet P, and the sheet P is discharged outside the printer1.

In the first embodiment, the developing devices2K,2C,2M, and2Y basically perform an identical operation. Accordingly, in the following description, an operation of the developing device2K for developing the toner30K in black (K) will be explained, and explanation of the operation of the developing devices2C,2M, and2Y is omitted.

In the first embodiment, the charging roller22K is configured to uniformly charge the surface of the photosensitive drum21K, and the exposure unit5K exposes the photosensitive drum21K to form the static latent image thereon. A charge roller power source (not shown) is connected to the charging roller22K for applying a bias voltage having a polarity the same as that of the toner30K. When the charge roller power source applies the bias voltage to the charging roller22K, the charging roller22K uniformly charges the surface of the photosensitive drum21K. A developing roller power source (not shown) is connected to the developing roller23K for applying a bias voltage having a polarity the same as or opposite to that of the toner30K. When the developing roller power source applies the bias voltage to the developing roller23K, the developing roller23K is configured to attach the toner30K thus charged to the static latent image on the photosensitive drum21K to form the toner image.

In the first embodiment, the developing roller power source (not shown) or the charge roller power source (not shown) is connected to the developing blade24K for applying a bias voltage having a polarity the same as or opposite to that of the toner30K. When the developing roller power source or the charge roller power source applies the bias voltage to the developing blade24K, the developing blade24K charges the toner30K and regulates the layer thickness of the toner30K on the developing roller23K along with the abutting force thereof. A supplying roller power source (not shown) is connected to the supplying roller23K for applying a bias voltage having a polarity the same as or opposite to that of the toner30K. When the supplying roller power source applies the bias voltage to the supplying roller23K, the supplying roller23K is configured to supply the toner30K supplied from the supply toner storage portion31K as the developer storage portion of the toner cartridge3K to the developing roller23K. Further, the supplying roller25K is arranged to abut against the developing roller23K, so that the supplying roller25K charges the toner30K with a contact frictional force relative to the developing roller23K.

In the first embodiment, the cleaning blade26K is arranged to scrape off the toner30K remaining on the surface of the photosensitive drum21K after the toner image is transferred to the sheet P. Further, the cleaning blade26K is also arranged to scrape off a small amount of a foreign substance attached to the surface of the photosensitive drum21K from the transfer belt9.

In the first embodiment, the first transportation unit27K is arranged to transport the remaining toner30K and the attached substance removed with the cleaning blade26K as the waste toner30K toward the front side inFIG. 2in the rotational axis direction of the photosensitive drum21K. After the first transportation unit27K transports the waste toner30K, the second transportation unit28transports the waste toner30K to the waste toner storage portion (the waste substance storage portion)32. The second transportation unit28as the transportation unit is connected to the first transportation unit27K to form the transportation path of the waste toner30K.

In the first embodiment, the second transportation unit28is arranged to collectively transport the waste toner30K,30C,30M, and30Y transported from the first transportation units27K,27C,27M, and27K disposed in the developing devices2K,2C,2M, and2Y in the arrow direction Z inFIG. 2. A stirring supply member (not shown) is disposed in each of the toner storage portions31K,31C,31M, and31Y of the toner cartridges3K,3C,3M, and3Y for supplying the toner30K,30C,30M, and30Y in the unused state to the developing devices2K,2C,2M, and2Y, respectively.

In the first embodiment, in the transfer unit4, a transfer roller power source (not shown) or the charge roller power source (not shown) is connected to the transfer rollers4K,4C,4M, and4Y for applying a bias voltage having a polarity the same as or opposite to that of the toner30K. When the transfer roller power source applies the bias voltage to the transfer rollers4K,4C,4M, and4Y, the transfer rollers4K,4C,4M, and4Y are arranged to transfer the toner images formed on the photosensitive drums21K,21C,21M, and21Y to the sheet P transported from the sheet supply cassette6. It should be noted that the exposure units5K,5C,5M, and5Y are configured to irradiate light on the photosensitive drums21K,21C,21M, and21Y, respectively, according to the print data thus input, so that a potential of a light irradiated are is optically decreased to form the static latent image.

In the first embodiment, after the sheet P stored in the sheet supply cassette6is transported to a sheet supply portion in the arrow direction X, a transportation roller (not shown) is arranged to transport the sheet P to the image forming unit20. In the fixing unit7, the heating heater is controlled according to the surface temperature of the heating roller7adetected with the thermistor. Accordingly, it is possible to maintain the surface temperature of the heating roller7aat a specific level. While the surface temperature of the heating roller7ais maintained at a specific level, after the toner images are transferred to the sheet P, the sheet P passes through the pressing portion between the heating roller7aand the pressing roller7b, so that heat and pressure are applied to the sheet P and the toner30K,30C,30M, and30Y, thereby fixing the toner images to the sheet P.

An operation of the image forming unit20will be explained next with reference toFIG. 3. As shown inFIG. 3, the image forming unit20includes the developing devices2K,2C,2M, and2Y integrated in one unit, so that the image forming unit20, in which the developing devices2K,2C,2M, and2Y are integrated, is detachably attached to the printer1. When the printer1performs a color printing operation, the photosensitive drum shafts41K,41C,41M, and41Y are arranged in an image forming position with own weight along a guide disposed in the printer1, so that the developing devices2K,2C,2M, and2Y perform the printing operation. When the printer1performs a monochrome printing operation, a developing device lifting mechanism (not shown) is configured to lift up the photosensitive drum shafts41C,41M, and41Y in an arrow direction W inFIG. 3. Accordingly, the developing devices2C,2M, and2Y are moved to a non-image forming position, so that only the developing device2K is situated at the image forming position to perform the printing operation.

The configuration of the supplying roller25will be explained in more detail next with reference toFIGS. 1 and 4. The supplying roller25includes a base member formed of a foamed silicone rubber compound. The cells201of the conductive foamed layer200are separate foamed cells individually independent. The conductive foamed layer200of the supplying roller25generally has hardness of 45° to 65° measured with an Asker F hard meter (a product of KOBUNSHI KEIKI CO., LTD.). In the first embodiment, the conductive foamed layer200of the supplying roller25has hardness of 47°.

The cells201of the conductive foamed layer200generally have a size (a diameter) of 100 μm to 1,000 μm. In the first embodiment, the cells201have a size (a diameter) of 200 μm to 200 μm at a surface of the conductive foamed layer200. A resistivity value of the supplying roller25is measured when a voltage of 300V is applied through the shaft202while the supplying roller25is rotating in a state that the supplying roller25contacts with a ball barring made of an SUS material having a width of 2.0 mm and a diameter of 6.0 mm with a force of 20 gf. It is preferred to adjust the resistivity value of the supplying roller25between 1 MΩ and 100 MΩ. In the first embodiment, the resistivity value of the supplying roller25is adjusted to be 10 MΩ.

In the first embodiment, the conductive foamed layer200of the supplying roller25has a total length of 220 mm in a rotational axis direction thereof. Outer diameters of the conductive foamed layer200are measured at five locations, where the distance X is 5.0 mm, 57.5 mm, 110.0 mm, 162.5 mm, and 215.0 mm. As described above, the distance X (mm) is defined as the length from a reference position S shown inFIG. 1(a) (the left end portion (the end portion on the non-drive transmission side) of the conductive foamed layer200of the supplying roller25) toward the right end portion where the drive gears are disposed (the end portion on the driven side). Further, as described above the three points D1, D2, and D3 are defined such that the point D1 is located at the distance X of 5.0 mm, the point D2 is located at the distance X of 110.0 mm, and the point D3 is located at the distance X of 215.0 mm. The outer diameters of the conductive foamed layer200at the points D1, D2, and D3 are defined as φD1, φD2, and φD3.

In the first embodiment, the supplying roller25is formed in a shape such that the outer diameters φD1, φD2, and φD3 of the conductive foamed layer200at the points D1, D2, and D3 are satisfied a condition of φD1>φD2>φD3 under a condition in which a contact pressure between the developing roller23and the supplying roller25is less than 0.10 kgf/cm2. It should be noted that the contact pressure is measured with a pressure sensor (a film type pressure distribution measurement system EH-2-0317, a product of Nitta Corporation).

FIG. 5is a graph showing a relationship between the contact pressure and a pressed amount NIP between the developing roller23and the supplying roller25of the printer1as the image forming apparatus according to the first embodiment of the present invention.

As shown inFIG. 5, the contact pressure increases in proportional to the pressed amount NIP between the developing roller23and the supplying roller25in an inter-axial direction. InFIG. 5, the developing roller23and the supplying roller25are arranged such that the inter-axial distance in between becomes 12.0 mm. Further, the supplying roller25has a straight shape having the outer diameter φ of 13.0 mm, and the outer diameter of the developing roller23is changed while the pressed amount NIP is being adjusted. The contact pressure is measured as an average value of pressure applied to an entire portion of the developing roller23and the supplying roller25in the axial direction in which the developing roller23contacts with the supplying roller25.

As described above, in the experiment, the continuous durability print test was conducted using the color printer having one of Sample 1 to Sample 4 of the supplying roller25having different shapes. More specifically, Sample 1 had the straight cylindrical shape having the identical outer diameter of φD1, φD2, and φD3. Sample 2 had the outer diameter φD3 smaller than the outer diameter φD1 by about 0.2 mm. Sample 3 had the outer diameter φD3 smaller than the outer diameter φD1 by about 0.4 mm. Sample 4 had the outer diameter φD3 smaller than the outer diameter φD1 by about 0.6 mm.

FIGS. 7(a) to7(c) are graphs showing the outer diameter profile of the supplying roller25of the printer1as the image forming apparatus according to the first embodiment of the present invention. More specifically,FIG. 7(a) is a graph showing the outer diameter profile of the supplying roller25of the color printer as the image forming apparatus in the initial state.

In the continuous durability print test, the color printer had the image forming unit with a life of 20,000 drum count. The color printer was configured such that the drum count increased by one every time the photosensitive drum made on rotation. Further, a letter sheet Hammarmill Laser Print LT241b (a product of International Paper) as an evaluation medium (a sheet).

FIG. 12is a schematic side view showing a printed pattern of the image forming apparatus according to the first embodiment of the present invention. As shown inFIG. 12, in the continuous durability print test, the printed pattern having a print density of 0.3% duty was printed on every other sheet in a printable area of the evaluation sheet having the letter size in an alternate printing operation until the drum count became 30,000.

FIG. 7(b) is a graph showing the outer diameter profile of Sample 1 to Sample 4 of the supplying roller25of the color printer as the image forming apparatus after the continuous durability test. Also, the outer diameter profiles of Sample 1 to Sample 4 of the supplying roller25after the continuous durability test are shown in Table 2.

FIG. 7(c) is a graph showing a wear amount of Sample 1 to Sample 4 of the supplying roller25of the color printer as the image forming apparatus after the continuous durability test. Also, the wear amounts of Sample 1 to Sample 4 of the supplying roller25after the continuous durability test are shown in Table 3. The wear amount (yielding of sponge) is defined as a difference in the outer diameters Sample 1 to Sample 4 of the supplying roller25after the continuous durability test. The wear amount (yielding of sponge) will be explained in more detail later.

In the continuous durability test, the outer diameters and the wear amount were measured with the contact pressure of 0.08 kgf/cm2.

In the experiment, a print quality test was evaluated after the continuous durability test while the contact pressure was being changed. The results of the print quality test are shown in Table 4. In Table 4, when there was no print quality problem, the result is represented as good. When there was a print quality problem due to a poor initial scraping off, the result is represented as fair. When there was a print quality problem due to the wear (yielding) during the continuous durability test, the result is represented as poor.

As shown in Table 4, in Sample 1, a stain (the print quality problem) occurred at the point D3 on the driven side of the supplying roller25within the entire range of the contact pressure between 0.08 kgf/cm2and 0.24 kgf/cm2. As described above, Sample 1 had the straight shape with the outer diameter φ of 12.3 mm over the entire length thereof in the rotational axis direction. Accordingly, it was supposed that the outer diameter should uniformly be worn over the entire length thereof in the rotational axis direction after the continuous durability test. However, according to the results of the experiment, the wear amount (yielding) became most excessive at the point D3 on the driven side of the supplying roller25, and the wear amount (yielding) was less excessive at the point D1 on the non-driven side of the supplying roller25. Accordingly, it is concluded that the contact pressure between the developing roller23and the supplying roller25on the driven side of the supplying roller25was greater than that on the non-drive side of the supplying roller25for the reasons explained below.

As described above with reference toFIG. 4, the developing roller23is arranged to rotate while abutting against the supplying roller25. Further, bearing receiving portions at the both end portions of the developing roller23and the supplying roller25are tightly fitted in a developing device bearing portion (not shown), so that the shafts of the developing roller23and the supplying roller25are fixed. However, due to a general dimensional variance of the bearing receiving portions and the device bearing portion, there is a gap between the bearing receiving portions and the device bearing portion in a direction that an external force is alleviated. In the printer1, when the drive force is transmitted through the photosensitive drum gear304, the toner305, the drive transmission gear302, and the supplying roller gear301engaged with each other, a reaction force proportional to a load torque is applied in a direction perpendicular to a pressing angle direction of a teeth engagement portion of the gears on the drive side and the gears on the driven side. Further, the load torque necessary for rotating each roller tends to decrease with being away from the photosensitive drum21.

In the gear arrangement shown inFIG. 4(b), a sum of the reaction forces applied to the developing roller gear303and the supplying roller gear301is applied in a direction of pressing the supplying roller25against the developing roller23. Accordingly, the force of pressing the supplying roller25against the developing roller23tends to be greater on the drive side than the driven side. Further, a wobble of the drive gear or a twist of the developing device2in the loading direction on the driven side enlarges the force of pressing the supplying roller25against the developing roller23.

Further, at this moment, the shaft202of the supplying roller25is deformed in the direction opposite to the contacting direction of the developing roller23, so that the wear amount (yielding) became small at the point D2 at the center of the supplying roller25. In other words, the supplying roller25is pressed against the developing roller23with a relatively small force, so that the wear amount (yielding) became small.

In the experiment, with regard to Sample 1, the wear amount (yielding) exhibited the maximum level of 0.38 mm at the point D3 on the driven side, so that the stain occurred at the point D3, thereby lowering the print quality. It should be noted that the stain due to the wear (yielding) tends to occur when the wear amount (yielding) exceeds 0.30 mm. Accordingly, the stain did not occur at the point D1 or the pint D2.

In the experiment, with regard to Sample 2, the load on the supplying roller25on the driven side was reduced, so that it was possible to perform the printing operation without the print quality problem at the contact pressure of 0.08 kg/cm2until the drum count became 30,000. Further, the wear amount (yielding) exhibited less than 0.30 mm at all of the points D1 to D4. Accordingly, as compared to Sample 1, Sample 2 of the supplying roller25worn uniformly. However, when the contact pressure became greater than 0.10 kg/cm2, the wear amount (yielding) increased, thereby causing the stain.

In the experiment, with regard to Sample 3, similar to Sample 2, it was possible to perform the printing operation without the print quality problem at the contact pressure of 0.08 kg/cm2until the drum count became 30,000. Further, the wear amount (yielding) exhibited less than 0.30 mm at all of the points D1 to D4. Accordingly, as compared to Sample 1, Sample 3 of the supplying roller25worn uniformly. However, when the contact pressure became greater than 0.10 kg/cm2, the wear amount (yielding) increased on the non-driven side, thereby causing the stain.

In the experiment, with regard to Sample 4, it was not possible to perform the printing operation without the print quality problem even at the contact pressure of 0.08 kg/cm2. In Sample 4, the diameter difference was large on the pint D1 and the point D3. Accordingly, the pressure on the point D1 on the non-driven side became excessive, so that the wear amount became 0.34 mm, thereby causing the stain. On the other hand, the pressure on the point D3 on the driven side became insufficient, so that the scraping off did not appear to be sufficient from the initial state, thereby causing the stain.

In the first embodiment, as described above, the supplying roller25is formed in the shape such that the outer diameters φD1, φD2, and φD3 of the conductive foamed layer200at the points D1, D2, and D3 satisfy the condition of φD1>φD2>φD3 under the condition in which the contact pressure between the developing roller23and the supplying roller25is less than 0.10 kgf/cm2. Accordingly, it is possible to reduce the wear amount (yielding) due to the excessive pressure on the driven side. As a result, it is possible to reduce the stain due to the wear (yielding) of the supplying roller25up to 1.5 times of the life of the image forming unit20, and to obtain an image with good quality. It should be noted that the wear amount (yielding) of the supplying roller25increases in proportional to the print sheet number in the continuous durability print test. Accordingly, when the supplying roller25is formed in the shape as defined in the first embodiment, it is possible to perform the continuous printing operation longer than the case that the supplying roller25is formed in the straight shape.

In the first embodiment, when the condition of φD1>φD2>φD3 is satisfied, it is preferred that the difference between φD1 and φD2 is between 0.1 mm and 0.2 mm, and the difference between φD2 and φD3 is between 0.1 mm and 0.2 mm. It should be noted that the difference between φD1 and φD2 and the difference between φD2 and φD3 are variable according to the arrangement of the photosensitive drum gear304, the developing roller gear303, the drive transmission gear302, and the supplying roller gear301, and the load torque of each roller. Accordingly, it is preferable that the difference between φD1 and φD2 and the difference between φD2 and φD3 are determined through an experiment.

Next, the wear (yielding) of the supplying roller25will be explained in more detail with reference toFIGS. 13(a) and13(b).FIGS. 13(a) and13(b) are schematic sectional views showing the conductive foamed layer200of the supplying roller25of the printer1as the image forming apparatus according to the first embodiment of the present invention. More specifically,FIG. 13(a) is a schematic sectional view showing the conductive foamed layer200of the supplying roller25of the printer1as the image forming apparatus in the initial state, andFIG. 13(b) is a schematic sectional view showing the conductive foamed layer200of the supplying roller25of the printer1as the image forming apparatus after the continuous durability test.

As shown inFIG. 13(a), in the conductive foamed layer200of the supplying roller25in the initial state, all walls of the cells201stand straight. Accordingly, opening portions are formed in the surface of the conductive foamed layer200, so that it is possible to stably supply toner retained in the cells201to the developing roller23. Further, it is possible to stably scrape off excess toner on the developing roller23.

On the other hand, as shown inFIG. 13(b), in the conductive foamed layer200of the supplying roller25after the continuous durability test, the conductive foamed layer200is worn out and the walls of the cells201are deformed in an arrow direction A toward an upstream side in the rotation direction of the supplying roller25, thereby causing the yielding. When the walls of the cells201are deformed, it is difficult to supply a sufficient amount of toner to the developing roller23. Further, it is difficult to securely scrape off excess toner on the developing roller23. Generally speaking, it is possible to increase an amount of toner supplied to the developing roller23through increasing the bias voltage applied to the supplying roller25. However, in this case, an amount of excess toner on the developing roller23also increases, thereby causing the stain due to insufficient scraping off.

As explained above, in the first embodiment, the supplying roller25is formed in the shape such that the outer diameters φD1, φD2, and φD3 of the conductive foamed layer200at the points D1, D2, and D3 satisfy the condition of φD1>φD2>φD3 under the condition in which the contact pressure between the developing roller23and the supplying roller25is less than 0.10 kgf/cm2. The point D1 is located at the position 5.0 mm away from the end portion on the non-drive transmission side of the conductive foamed layer200of the supplying roller25toward the opposite end portion on the driven side. Similarly, the point D2 is located at the position away from the end portion by 110.0 mm, and the point D3 is located at the position away from the end portion by 215.0 mm. Further, it is preferred that the difference between φD1 and φD2 is between 0.1 mm and 0.2 mm, and the difference between φD2 and φD3 is between 0.1 mm and 0.2 mm. Accordingly, it is possible to prolong the life of the image forming unit20, and to obtain an image with good quality.

Second Embodiment

A second embodiment of the present invention will be explained next. In the second embodiment, the printer1as the image forming apparatus includes a supplying roller25bhaving a shape different from that of the supplying roller25in the first embodiment. In the second embodiment, the printer1as the image forming apparatus, the image forming unit20, and the exposure unit5have configurations similar to those in the first embodiment except the supplying roller25. Accordingly, a similar component is designated with the same reference numeral, and an explanation thereof is omitted.

FIG. 8is a schematic side view showing the supplying roller25bof the printer1as the image forming apparatus according to the second embodiment of the present invention.

As shown inFIG. 8, the supplying roller25bhas an outer diameter φD1 at the point D1 is substantially the same as an outer diameter φD2 at the point D2, and an outer diameter φD3 at the point D3 is the smallest. Further, the supplying roller25bis configured such that the outer circumferential surface thereof becomes smooth along a continuous straight line or a curved line between the points D1, D2, and D3 obtained through a conventional polishing method. Alternatively, the supplying roller25is configured such that the outer circumferential surface thereof becomes stepwise.

An experiment of evaluating the continuous durability of the supplying roller25bwill be explained next. In the experiment, the continuous durability print test was conducted using Sample 1 and Sample 5 to Sample 7 having different shapes. More specifically, Sample 1 had a straight cylindrical shape having an identical outer diameter of φD1, φD2, and φD3. Sample 5 had the outer diameter φD3 smaller than the outer diameter φD1 and the outer diameter φD2 by about 0.2 mm. Sample 6 had the outer diameter φD3 smaller than the outer diameter φD1 and the outer diameter φD2 by about 0.4 mm. Sample 7 had the outer diameter φD3 smaller than the outer diameter φD1 and the outer diameter φD2 by about 0.6 mm. The outer diameter profiles of Sample 1 and Sample 5 to Sample 7 of the supplying roller25bin the initial state are shown in Table 5.

An effect of the configuration described above will be explained with reference toFIGS. 1,4,8, and9(a) to9(c). It should be noted that an operation of the printer1as the image forming apparatus, an operation of the image forming unit20, and an operation of the exposure unit5are similar to those in the first embodiment, and explanations thereof are omitted. Further, the configuration of the supplying roller25bexcept the shape thereof, and the continuous durability print test are similar to those in the first embodiment, and explanations thereof are omitted.

In the second embodiment, the supplying roller25bis formed in the shape such that the outer diameters φD1, φD2, and φD3 at the points D1, D2, and D3 satisfy the condition of φD1≈φD2>φD3 under the condition in which the contact pressure between the developing roller23and the supplying roller25bis equal to or greater than 0.10 kgf/cm2, or equal to or smaller than 0.15 kgf/cm2. The outer diameters φD1 and φD2 may have a manufacturing variance of about ±0.05 mm. Further, it is preferred that the difference between the outer diameters φD2 and φD3 is between 0.4 mm and 0.6 mm.

FIGS. 9(a) to9(c) are graphs showing an outer diameter profile of the supplying roller25bof the printer1as the image forming apparatus according to the second embodiment of the present invention. More specifically,FIG. 9(a) is a graph showing the outer diameter profile of the supplying roller25bof the printer1as the image forming apparatus in the initial state. Also, the outer diameter profiles of Sample 1 and Sample 5 to Sample 7 of the supplying roller25bin the initial state are shown in Table 5.

Further,FIG. 9(b) is a graph showing the outer diameter profile of the supplying roller25bof the printer1as the image forming apparatus after the continuous durability test. Also, the outer diameter profiles of Sample 1 and Sample 5 to Sample 7 of the supplying roller25bafter the continuous durability test are shown in Table 6.

Further,FIG. 9(c) is a graph showing the wear amount of the supplying roller25bof the printer1as the image forming apparatus after the continuous durability test. Also, the wear amounts of Sample 1 and Sample 5 to Sample 7 of the supplying roller25bafter the continuous durability test are shown in Table 7. Similar to the first embodiment, the wear amount (yielding of sponge) is defined as a difference in the outer diameters Sample 1 and Sample 5 to Sample 7 of the supplying roller25bafter the continuous durability test.

In the continuous durability test, the outer diameters and the wear amount were measured with the contact pressure of 0.15 kgf/cm2.

In the experiment, the print quality test was evaluated after the continuous durability test while the contact pressure was being changed. The results of the print quality test are shown in Table 8. In Table 8, when there was no print quality problem, the result is represented as good. When there was a print quality problem due to the poor initial scraping off, the result is represented as fair. When there was a print quality problem due to the wear (yielding) during the continuous durability test, the result is represented as poor.

As shown in Table 8, with regard to Sample 1, the stain (the print quality problem) occurred due to the yielding at the point D3 on the driven side of the supplying roller25bover the entire range of the contact pressure between 0.08 kgf/cm2and 0.24 kgf/cm2. As shown in Table 7, as compared with the first embodiment, the wear amount (yielding) became worse at the point D3 by about 25% due to the greater contact pressure than that in the first embodiment.

In the experiment, when the supplying roller25bhas the shape of Sample 5, and the load on the supplying roller25bon the driven side was reduced, it was possible to perform the printing operation without the print quality problem at the contact pressure of 0.10 kg/cm2until the drum count became 30,000. However, when the contact pressure became greater than 0.10 kg/cm2, the stain occurred on the driven side due to the yielding.

Further, with regard to Sample 5, when the contact pressure became 0.15 kg/cm2, the wear amount (yielding) became 0.32 mm exceeding the threshold value of 0.30 mm at the point D3. Accordingly, as compared with Sample 6 and Sample 7, the wear amount (yielding) became greater. Further, when the supplying roller25bhas the shape of Sample 5, due to the large difference between the outer diameter φD2 and the outer diameter φD3, the stain occurred due to the insufficient scraping off from the initial state at the contact pressure less than 0.15 kg/cm2.

In the experiment, when the supplying roller25bhas the shape of Sample 6, and the load on the supplying roller25bon the driven side was reduced, it was possible to perform the printing operation without the print quality problem at the contact pressure between 0.10 kg/cm2and 0.15 kg/cm2until the drum count became 30,000. However, when the contact pressure became greater than 0.15 kg/cm2, the stain occurred on the driven side due to the yielding. When the contact pressure became 0.15 kg/cm2, the wear amount (yielding) at the point D3 became smaller than that of Sample 1 and Sample 5, and the wear amount (yielding) became less than 0.30 mm at all of the points D1 to D3, thereby achieving the good result. When the contact pressure became less than 0.10 kg/cm2, similar to Sample 5, the stain occurred due to the insufficient scraping off from the initial state.

In the experiment, with regard to Sample 7, the results were similar to Sample 6. Further, due to the large difference between the outer diameter φD2 and the outer diameter φD3, the wear amount (yielding) became relatively large on the non-driven side. That is, when the contact pressure became 0.15 kg/cm2, the shape of Sample 6 exhibited most favorable results.

In the second embodiment, as described above, the supplying roller25bis formed in the shape such that the outer diameters φD1, φD2, and φD3 of the conductive foamed layer200at the points D1, D2, and D3 satisfy the condition of φD1≈φD2>φD3 under the condition in which the contact pressure between the developing roller23and the supplying roller25bis equal to or greater than 0.10 kgf/cm2, or equal to or smaller than 0.15 kgf/cm2. Accordingly, it is possible to reduce the wear amount (yielding) due to the excessive pressure on the driven side. As a result, it is possible to reduce the stain due to the yielding of the supplying roller25bup to 1.5 times of the life of the image forming unit20, and to obtain an image with good quality. It should be noted that the difference between φD1 and φD2 and the difference between φD2 and φD3 are variable according to the arrangement of the photosensitive drum gear304, the developing roller gear303, the drive transmission gear302, and the supplying roller gear301, and the load torque of each roller. Accordingly, it is preferable that the difference between φD1 and φD2 and the difference between φD2 and φD3 are determined through an experiment.

As explained above, in the second embodiment, the supplying roller25bis formed in the shape such that the outer diameters φD1, φD2, and φD3 of the conductive foamed layer200at the points D1, D2, and D3 satisfy the condition of φD1≈φD2>φD3 under the condition in which the contact pressure between the developing roller23and the supplying roller25bis equal to or greater than 0.10 kgf/cm2, or equal to or smaller than 0.15 kgf/cm2. The point D1 is located at the position 5.0 mm away from the end portion on the non-drive transmission side of the conductive foamed layer200of the supplying roller25btoward the opposite end portion on the driven side. Similarly, the point D2 is located at the position away from the end portion by 110.0 mm, and the point D3 is located at the position away from the end portion by 215.0 mm. Further, it is preferred that the difference between φD1 and φD2 is ±0.05 mm. Accordingly, it is possible to prolong the life of the image forming unit20, and to obtain an image with good quality.

Third Embodiment

A third embodiment of the present invention will be explained next. In the third embodiment, the printer1as the image forming apparatus includes a supplying roller25chaving a shape different from that of the supplying roller25in the first embodiment and that of the supplying roller25bin the second embodiment. In the third embodiment, the printer1as the image forming apparatus, the image forming unit20, and the exposure unit5have configurations similar to those in the first embodiment except the supplying roller25and those in the second embodiment except the supplying roller25b. Accordingly, a similar component is designated with the same reference numeral, and an explanation thereof is omitted.

FIG. 10is a schematic side view showing the supplying roller25cof the printer1as the image forming apparatus according to the third embodiment of the present invention.

As shown inFIG. 10, the supplying roller25chas a largest outer diameter φD2 at the point D2. Further, the supplying roller25chas an outer diameter φD1 at the point D1 smaller than the outer diameter φD2, and an outer diameter φD3 thereof at the point D3 is the smallest. Further, the supplying roller25bis configured such that the outer circumferential surface thereof becomes smooth along a continuous straight line or a curved line between the points D1, D2, and D3 obtained through a conventional polishing method. Alternatively, the supplying roller25is configured such that the outer circumferential surface thereof becomes stepwise.

An experiment of evaluating the continuous durability of the supplying roller25cwill be explained next. In the experiment, the continuous durability print test was conducted using Sample 1 and Sample 8 to Sample 10 having different shapes. More specifically, Sample 1 had a straight cylindrical shape having an identical outer diameter of φD1, φD2, and φD3. Sample 8 had the outer diameter φD1 smaller than the outer diameter φD2 by about 0.1 mm, and the outer diameter φD3 smaller than the outer diameter φD2 by about 0.4 mm. Sample 9 had the outer diameter φD1 smaller than the outer diameter φD2 by about 0.2 mm, and the outer diameter φD3 smaller than the outer diameter φD2 by about 0.4 mm. Sample 10 had the outer diameter φD1 smaller than the outer diameter φD2 by about 0.3 mm, and the outer diameter φD3 smaller than the outer diameter φD2 by about 0.5 mm. The outer diameter profiles of Sample 1 and Sample 8 to Sample 10 of the supplying roller25cin the initial state are shown in Table 9.

An effect of the configuration described above will be explained with reference toFIGS. 1,4,10, and11(a) to11(c). It should be noted that an operation of the printer1as the image forming apparatus, an operation of the image forming unit20, and an operation of the exposure unit5are similar to those in the first embodiment and the second embodiment, and explanations thereof are omitted. Further, the configuration of the supplying roller25cexcept the shape thereof, and the continuous durability print test are similar to those in the first embodiment and the second embodiment, and explanations thereof are omitted.

In the third embodiment, the supplying roller25cis formed in the shape such that the outer diameters φD1, φD2, and φD3 at the points D1, D2, and D3 satisfy the condition of φD2>φD1>φD3 under the condition in which the contact pressure between the developing roller23and the supplying roller25bis equal to or greater than 0.15 kgf/cm2, or equal to or smaller than 0.20 kgf/cm2. It is preferable that the difference between the outer diameters φD1 and φD2 between 0.1 mm and 0.3 mm, and the difference between the outer diameters φD2 and φD3 is between 0.4 mm and 0.6 mm.

FIGS. 11(a) to11(c) are graphs showing an outer diameter profile of the supplying roller25cof the printer1as the image forming apparatus according to the third embodiment of the present invention. More specifically,FIG. 11(a) is a graph showing the outer diameter profile of the supplying roller25cof the printer1as the image forming apparatus in the initial state. Also, the outer diameter profiles of Sample 1 and Sample 8 to Sample 10 of the supplying roller25cin the initial state are shown in Table 9.

Further,FIG. 11(b) is a graph showing the outer diameter profile of the supplying roller25cof the printer1as the image forming apparatus after the continuous durability test. Also, the outer diameter profiles of Sample 1 and Sample 8 to Sample 10 of the supplying roller25cafter the continuous durability test are shown in Table 10.

Further,FIG. 11(c) is a graph showing the wear amount of the supplying roller25cof the printer1as the image forming apparatus after the continuous durability test. Also, the wear amounts of Sample 1 and Sample 8 to Sample 10 of the supplying roller25cafter the continuous durability test are shown in Table 11. Similar to the first embodiment and the second embodiment, the wear amount (yielding of sponge) is defined as a difference in the outer diameters Sample 1 and Sample 8 to Sample 10 of the supplying roller25cafter the continuous durability test.

In the continuous durability test, the outer diameters and the wear amount were measured with the contact pressure of 0.20 kgf/cm2.

In the experiment, the print quality test was evaluated after the continuous durability test while the contact pressure was being changed. The results of the print quality test are shown in Table 12. In Table 12, when there was no print quality problem, the result is represented as good. When there was a print quality problem due to the poor initial scraping off, the result is represented as fair. When there was a print quality problem due to the wear (yielding) during the continuous durability test, the result is represented as poor.

As shown in Table 12, with regard to Sample 1, the stain occurred due to the yielding at the point D3 on the driven side of the supplying roller25cover the entire range of the contact pressure between 0.08 kgf/cm2and 0.24 kgf/cm2. Further, the stain occurred due to the yielding at the point D1 on the non-driven side of the supplying roller25c. As compared with the first embodiment and the second embodiment, the wear amount (yielding) became worse by about 35% due to the greater contact pressure than that in the first embodiment and the second embodiment.

In the experiment, when the supplying roller25chas the shape of Sample 8, and the load on the supplying roller25con the driven side was reduced, it was possible to perform the printing operation without the print quality problem at the contact pressure in the range between 0.15 kg/cm2and 0.24 kg/cm2until the drum count became 30,000. However, when the contact pressure became smaller than 0.15 kg/cm2, the stain occurred on the driven side due to the insufficient scraping off in the initial state.

In the experiment, when the supplying roller25chas the shape of Sample 9, the results were similar to those of Sample 8. More specifically, the wear amount (yielding) of the supplying roller25cas a whole was uniform, thereby exhibiting the most balanced performance.

In the experiment, when the supplying roller25chas the shape of Sample 10, it was possible to perform the printing operation without the print quality problem at the contact pressure equal to or greater than 0.15 kgf/cm2, or equal to or smaller than 0.20 kgf/cm2until the drum count became 30,000. However, when the contact pressure became 0.24 kg/cm2, the stain occurred on the driven side due to the yielding.

As shown in Table 11 andFIG. 11(c), when the contact pressure became 0.20 kg/cm2, the wear amount (yielding) at the point D3 became excessive. This is because there was the large difference in the outer diameters at the point D1 on the non-driven side and the point D2 at the middle, thereby decreasing the contact pressure on the non-driven side. As a result, the contact pressure on the driven side became excessive with the middle as a pivot, thereby increasing the wear amount (yielding) on the driven side.

As described above, in the third embodiment, the supplying roller25cis formed in the shape such that the outer diameters φD1, φD2, and φD3 thereof at the points D1, D2, and D3 satisfy the condition of φD2>φD1>φD3 under the condition in which the contact pressure between the developing roller23and the supplying roller25cis equal to or greater than 0.15 kgf/cm2, or equal to or smaller than 0.20 kgf/cm2. Further, it is preferred that the difference between φD1 and φD2 is between 0.1 mm and 0.3 mm, and the difference between φD2 and φD3 is between 0.4 mm and 0.6 mm. Accordingly, it is possible to reduce the wear amount (yielding) due to the excessive contact pressure on the driven side, to reduce the stain due to the yielding up to 1.5 times of the life of the image forming unit20, and to obtain an image with good quality.

It should be noted that the difference between φD1 and φD2 and the difference between φD2 and φD3 are variable according to the arrangement of the photosensitive drum gear304, the developing roller gear303, the drive transmission gear302, and the supplying roller gear301, and the load torque of each roller. Accordingly, it is preferable that the difference between φD1 and φD2 and the difference between φD2 and φD3 are determined through an experiment.

As described above, in the third embodiment, the supplying roller25cis formed in the shape such that the outer diameters φD1, φD2, and φD3 thereof at the points D1, D2, and D3 satisfy the condition of φD2>φD1>φD3 under the condition in which the contact pressure between the developing roller23and the supplying roller25cis equal to or greater than 0.15 kgf/cm2, or equal to or smaller than 0.20 kgf/cm2. The point D1 is located at the position 5.0 mm away from the end portion on the non-drive transmission side of the conductive foamed layer200of the supplying roller25ctoward the opposite end portion on the driven side. Similarly, the point D2 is located at the position away from the end portion by 110.0 mm, and the point D3 is located at the position away from the end portion by 215.0 mm. Further, it is preferred that the difference between φD1 and φD2 is between 0.1 mm and 0.3 mm, and the difference between φD2 and φD3 is between 0.4 mm and 0.6 mm. Accordingly, it is possible to prolong the life of the image forming unit20, and to obtain an image with good quality.

In the first to third embodiments, the printer1is explained as the image forming apparatus. The present invention is not limited thereto, and may be applicable to a copier, a facsimile, a multi-function product (MFP), and the like.

The disclosure of Japanese Patent Application No. 2012-123691, filed on May 30, 2012, is incorporated in the application.