Development device and image forming apparatus including the same

In a development device, a developer carrier arranged in a housing containing a one-component developer has a circumferential surface carrying the developer. A developer conveying path includes one conveying path along the developer carrier in a first direction and another conveying path in a second direction opposite to the first direction. The developer is circulated and conveyed between the conveying paths. A developer conveying member is arranged in the one conveying path and rotationally driven to convey the developer along the first direction and to supply the developer to the developer carrier. A layer thickness adjusting member is arranged at a distance from the circumferential surface to adjust a layer thickness of the developer supplied to the developer carrier. The developer carrier has a circumferential speed Vd and the developer conveying member has a circumferential speed Vs so that a circumferential speed ratio Vd/Vs satisfies a relationship represented by 1.3≦Vd/Vs≦5.0.

INCORPORATION BY REFERENCE

This application is based on and claims the benefit of priority from Japanese Patent application No. 2012-256076 filed on Nov. 22, 2012, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to a development device configured to develop an electrostatic latent image formed on an image carrier by using a one-component developer and to an image forming apparatus including the development device.

As a development device used in an image forming apparatus, such as printer, and configured to develop an electrostatic latent image formed on an image carrier by using magnetic one-component developer, a development device having a technique as mentioned below is known. Such a development device includes a developer carrier including a fixed magnet, a layer thickness adjusting member configured to adjust the thickness of a developer layer formed on the developer carrier, and a developer conveying member configured to supply the developer to the developer carrier.

According to the above-mentioned technique, the circumferential speed of the developer carrier is set to be higher than five times the circumferential speed of the developer conveying member in a position where the developer carrier and the developer conveying member faces to each other.

As the above-mentioned technique, in a case where the circumferential speed of the developer carrier is set to be higher than five times the circumferential speed of the developer conveying member, in other words, when the circumferential speed of the developer conveying member is set to be less than ⅕ of the circumferential speed of the developer carrier, the stress subjected to the developer is decreased. In this case, however, the supply of developer from the developer conveying member to the developer carrier was occasionally deficient.

In particular, in a case where the developer conveying member has a screw shape having a helical blade arranged around a rotation shaft, the supply amount of the developer is large in the helical blade portion and the supply amount of the developer is deficient in the rotation shaft portion. As a result, a partial difference in the amount of the developer occasionally often may occurs on the back side of the layer adjusting member, thereby causing a difference in density in the shape of a longitudinal streak on the image.

On the other hand, when the circumferential speed of the developer conveying member is significantly increased to supply a lot of the developer from the developer conveying member to the developer carrier, many developer is retained on the back side of the layer adjusting member. In this case, a problem arose in that deterioration of the developer is accelerated and the driving torque of the development device increases.

SUMMARY

In accordance with an embodiment of the present disclosure, a development device includes a housing, a developer carrier, a developer conveying path, a developer conveying member and a layer thickness adjusting member. The housing contains a one-component developer. The developer carrier is arranged in the housing and configured to be rotationally driven and to have a circumferential surface carrying the developer. The developer conveying path includes one conveying path through which the developer is conveyed along the developer carrier in a first direction in the housing and another conveying path arranged along the one conveying path through which the developer is conveyed in a second direction opposite to the first direction, wherein the developer is circulated and conveyed between the one conveying path and the other conveying path. The developer conveying member is arranged in the one conveying path and rotationally driven to convey the developer along the first direction and to supply the developer to the developer carrier. The layer thickness adjusting member is arranged at a distance from the circumferential surface of the developer carrier and configured to adjust a layer thickness of the developer supplied to the developer carrier. In addition, the developer carrier has a circumferential speed Vd and the developer conveying member has a circumferential speed Vs so that a circumferential speed ratio Vd/Vs satisfies a relationship represented by 1.3≦Vd/Vs≦5.0.

In accordance with another embodiment of the present disclosure, an image forming apparatus includes a development device, an image carrier and a transferring device. The development device includes a housing, a developer carrier, a developer conveying path, a developer conveying member and a layer thickness adjusting member. The housing contains a one-component developer. The developer carrier is arranged in the housing and configured to be rotationally driven and to have a circumferential surface carrying the developer. The developer conveying path includes one conveying path through which the developer is conveyed along developer carrier in a first direction in the housing and another conveying path arranged along the one conveying path and through which the developer is conveyed in a second direction opposite to the first direction, wherein the developer is circulated and conveyed between the one conveying path and the other conveying path. The developer conveying member is arranged in the one conveying path and rotationally driven to convey the developer along the first direction and to supply the developer to the developer carrier. The layer thickness adjusting member is arranged at a distance from the circumferential surface of the developer carrier and configured to adjust a layer thickness of the developer supplied to the developer carrier. In addition, the developer carrier has a circumferential speed Vd and the developer conveying member has a circumferential speed Vs so that a circumferential speed ratio Vd/Vs satisfies a relationship represented by 1.3≦Vd/Vs≦5.0. The image carrier has a surface on which an electrostatic latent image is formed, to which the developer is supplied from the developer carrier. The transferring device is configured to transfer an image onto a sheet from the image carrier.

DETAILED DESCRIPTION

Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings.FIG. 1is a perspective view showing an exterior of an image forming apparatus1according to the embodiment of the present disclosure.FIG. 2is a side sectional view showing an interior structure of the image forming apparatus1according to the embodiment of the present disclosure. While a monochrome printer is illustrated herein as the image forming apparatus1, the image forming apparatus may be any one of a copier, a facsimile device, and a multifunction peripheral having such functions, or may be another image forming apparatus configured to form color images.

The image forming apparatus1includes a main body housing10having a housing structure with a substantially rectangular parallelepiped shape and an image forming part30, a fixing part40, a toner container50, and a sheet feeding part90that are housed in the main body housing10.

At a front face side of the main body housing10, a front cover11is provided and, at a rear face side of the main body housing10, a rear cover12is provided. When the front cover11is opened, the toner container50is exposed to the front face side. This enables a user to take out the toner container50from the front face side of the main body housing10when a toner (a developer) runs out. The rear cover12is a cover that is opened in an event of a sheet jam, maintenance, or the like. The respective image forming part30and fixing part40can be taken out from the rear face side of the main body housing10by opening the rear cover12. In addition, in side faces of the main body housing10, a left cover12L (FIG. 1) and a right cover (not shown inFIG. 1) being opposite to the left cover12L are individually arranged to extend along the vertical direction. In a front part of the left cover12L, an intake port12La configured to take air into the main body housing10is arranged. Further, in an upper face of the main body housing10, a sheet ejecting unit13to which an image-formed sheet is ejected is provided. Various components configured to perform image-forming are housed in an inner space S (FIG. 2) defined by the front cover11, rear cover12, left cover12L, right cover and sheet ejecting unit13.

The image forming part30performs an image forming process to form a toner image on a sheet fed from the sheet feeding part90. The image forming part30includes a photosensitive drum31(image carrier) and a charging device32, an exposing device (not shown inFIG. 2), a development device33, a transfer roller34(transferring device), and a cleaning device35that are arranged around the photosensitive drum31. The image forming part30is arranged between the left cover12L and right cover.

The photosensitive drum31includes a rotation shaft and a cylindrical face rotating about the rotation shaft. An electrostatic latent image is formed on the cylindrical face, and a toner image according to the electrostatic latent image is carried on the cylindrical face. A photosensitive drum using an amorphous silicon (a-Si) based material may be used as the photosensitive drum31.

The charging device32is configured to uniformly charge the surface of the photosensitive drum31and includes a charging roller coming into contact with the photosensitive drum31.

The cleaning device35includes a cleaning blade, cleans the toner adhered to the circumferential face of the photosensitive drum31after transfer of the toner image, and conveys the toner to a collection device.

The exposure device includes optical devices, such as a laser light source, mirrors, and lenses, and forms an electrostatic latent image by emitting light modulated in accordance with image data provided from an external apparatus, such as a personal computer, to the circumferential face of the photosensitive drum31. The development device20supplies the toner to the circumferential face of the photosensitive drum31in order to develop the electrostatic latent image on the photosensitive drum31and to form a toner image. The development device20includes a developing roller21carrying the toner supplied to the photosensitive drum31and a first conveying screw24and a second conveying screw23that are configured to circulate and convey the developer while agitating the developer in a development housing210(FIG. 3). The development device20according to the embodiment will be described in detail below.

The transfer roller34is a roller configured to transfer the toner image, that is formed on the circumferential face of the photosensitive drum31, onto a sheet. The transfer roller34comes into contact with the cylindrical face of the photosensitive drum31, thereby forming a transfer nip part. To the transfer roller34A, transfer bias having a polarity opposite to the toner is applied.

The fixing part40performs a fixing process fixing a transferred toner image onto the sheet. The fixing part40includes a fixing roller41having a heat source provided inside and a pressure roller42pressed in contact with the fixing roller41to form a fixing nip part with the fixing roller41. When the sheet having the transferred toner image is passed through the fixing nip part, the toner image is heated by the fixing roller41and pressed by the pressure roller42, and then, the image is fixed onto the sheet.

The toner container50stores the toner supplied to the development device20. The toner container50includes a container main body51serving as a main toner storage section, a cylindrical part52projecting from a lower part of one side face of the container main body51, a covering member53covering another side face of the container main body51and a rotating member54housed in the container and configured to convey the toner. When the cylindrical part52is rotationally driven, the toner stored in the toner container50is thereby supplied into the development device20through a toner discharge port521provided on a lower face of a top end of the cylindrical part52. A container top plate50H covering an upper side of the toner container50is located below the sheet ejecting unit13(seeFIG. 2).

The sheet feeding part90includes a sheet feeding cartridge91configured to store the sheets to which the image forming process is applied (FIG. 2). The sheet feeding cartridge91has a part projecting further forward from the front face of the main body housing10. In the sheet feeding cartridge91, an upper face of a part housed in the main body housing10is covered by a feeding cartridge top plate91U. The sheet feeding cartridge91includes a sheet storage space storing a stack of the sheets, a lift plate lifting up the stack of sheets to feed the sheet and others. In an upper part of a rear end side of the sheet feeding cartridge91, a sheet pickup part91A is provided. In the sheet pickup part91A, a sheet feeding roller91B is arranged to feed an uppermost sheet of the sheet stack one by one from the sheet feeding cartridge91.

In the main body housing10to convey the sheet, a main conveying path92F and a reverse conveying path92B are provided. The main conveying path92F extends from the sheet pickup part91A of the sheet feeding part90via the image forming part30and fixing part40to a sheet ejecting port14provided facing to the sheet ejecting unit13in the upper face of the main body housing10. The reverse conveying path92B is a conveying path configured to return a single-side printed sheet to an upstream side from the image forming part30in the main conveying path92F when duplex printing is performed on the sheet.

The main conveying path92F extends to pass through the transfer nip part formed with the photosensitive drum31and transfer roller34from a downward side to an upward side. In addition, a registration roller pair93is arranged at an upstream side from the transfer nip part in the main conveying path92F. The sheet is temporarily stopped at the registration roller pair93and, after skew correction is made, the sheet is fed to the transfer nip part at a predetermined timing for image transfer. Several conveying rollers configured to convey the sheet are arranged at suitable positions of the main conveying path92F and reverse conveying path92B. For example, in the vicinity of the sheet ejecting port14, a sheet ejecting roller pair94is arranged.

The reverse conveying path92B is formed between an outside face of a reversing unit95and an inside face of the rear cover12of the main body housing10. In an inside face of the reversing unit95, the transfer roller34and one roller of the registration roller pair93are mounted. The rear cover12and reversing unit95individually are turnable about a shaft of a supporting pivot121provided on lower ends thereof. If a sheet jam occurs in the reverse conveying path92B, the rear cover12is opened. If a sheet jam occurs in the main conveying path92F, or if a unit including the photosensitive drum or the development device20is taken out to the outside, the reversing unit95also is opened in addition to the rear cover12.

Next, the development device20according to the present embodiment will now be described in detail.FIG. 3is a sectional view showing an interior structure of the development device20.FIG. 4is a plan view showing the interior structure of the development device20. The development device20includes the development housing210(a housing) having an elongated box-liked shape along one direction (an axial direction of the developing roller21). The development housing210includes an inner space220. In the inner space220, there are arranged the developing roller21(a developer carrier), a first agitating screw23(another developer conveying member) a second agitating screw24(a developer conveying member) and a toner replenishment port25. In the present embodiment, as a one-component developing system, in the inner space220the toner containing a magnetic material is filled as the developer (a magnetic developer). The toner is agitated and conveyed in the inner space220and consecutively supplied from the developing roller21to the photosensitive drum31in order to develop the electrostatic latent image.

The developing roller21includes a cylindrical shape extending along a longitudinal direction of the development housing210. The developing roller21includes a cylindrical-shaped sleeve21S rotationally driven and a round columnar-shaped magnet21M fixedly arranged along the axial direction in the sleeve21S. The sleeve21S is rotationally driven in a direction indicated by an arrow D31 shown inFIG. 3by a driving means (not shown), thereby carrying the magnetic toner on the circumferential face. The magnet21M is a fixed magnet having several magnetic poles along a circumferential direction of the sleeve21S in the sleeve21S. The magnet21M has four magnetic poles, namely, an S1 pole, an N1 pole, an S2 pole and an N2 pole arranged along the circumferential direction. InFIG. 3, a curved line MC surrounding the developing roller21represents a radial magnetic force of the developing roller21which is brought about by the individual magnetic poles as a distribution of in the circumferential direction on the sleeve21S. The S1 pole is arranged at a front upper position in the magnet21M. The S1 pole is used as an adjusting pole in order to adjust a toner layer. The N1 pole is arranged at a rear upper position in the magnet21M. The N1 pole has a function of supplying the toner to the photosensitive drum31as a developing pole. The N2 pole is arranged at a front lower position in the magnet21M. N2 pole serves as a magnetic catch pole and has a function of scooping up the toner to the developing roller21. The S2 pole is arranged at a position in the magnet21M at a downstream side from the N1 pole in a rotation direction of the sleeve21S and at an upstream side from the N2 pole in the rotation direction of the sleeve21S. The S2 pole is arranged mainly at a rear lower position in the magnet21M. The S2 pole has a function as a conveying pole collecting the toner not moved to the side of the photosensitive drum31at the N1 pole into the development housing210. The toner carried on the sleeve21S is conveyed to an opening arranged in the development housing210and is supplied to a photosensitive drum31arranged on the facing side.

The inner space220of the development housing210is covered by a top plate and partitioned into a first conveying path221(another conveying path) and a second conveying path222(one conveying path) elongated in left and right directions by a partition plate22extending along the left and right directions. The partition plate22has the width shorter than the development housing210in the left and right directions and, in a left end and a right end of the partition plate22, a first communication route223and a second communication route224respectively allowing communication between the first conveying path221and the second conveying path222are provided. According to this, in the development housing210, a circulation path (a developer conveying path) running through the first conveying path221, first communication route223, second conveying path222and second communication route224is formed. The toner is conveyed clockwise in the circulation path inFIG. 4.

The toner replenishment port25is an opening bored in the top plate and arranged above the vicinity of the left end of the first conveying path221(FIG. 4). The toner replenishment port25is arranged facing to the circulation path and has a function of receiving replenishing toner replenished from the toner container50and intruding it into the inner space220. In the present embodiment, the toner replenishment port25is formed of an opening with dimensions of a 14 mm×8 mm in a plan view, for example.

The first agitating screw23is arranged in the first conveying path221. The first agitating screw23includes a first rotation shaft23aand a first helical blade23bspirally projecting on the periphery of the first rotation shaft23a. The first agitating screw23is rotationally driven by a driving means (not shown) about the first rotation shaft23a(an arrow D33 inFIG. 3, an arrow R2 inFIG. 4), thereby conveying the toner in a direction indicated by an arrow D1 inFIG. 4(an arrow D1 direction). The first agitating screw23conveys the developer so as to pass a position facing to the toner replenishment port25in the first conveying path221. Thus, the first agitating screw23has a function mixing new toner flowing in from the toner replenishment port25with other toner conveyed through the first conveying path221and delivering the mixed toner to the second conveying path222's side. In the present embodiment, the outside diameter of the first helical blade23bis 14 mm and a pitch in the axial direction is set to 20 mm, for example. While the pitch is changeable in proportion to the conveying performance of the first agitating screw23, it is preferable to determine the pitch to 15 mm as a lower limit to maintain the toner conveying ability. In a downstream side of the first agitating screw23in the toner conveying direction (the arrow D1 direction), a first paddle23cis arranged. The first paddle23cis a plate-like member arranged on the first rotation shaft23a.

The first paddle23cis rotated together with the first rotation shaft23ato deliver the toner from the first conveying path221to the second conveying path222in a direction indicated by an arrow D3 shown inFIG. 4. In the present embodiment, the axial length of the first paddle23cis set to 20 mm, for example.

The second agitating screw24is arranged in the second conveying path222. The second agitating screw24includes a second rotation shaft24a(a rotation shaft) and a second helical blade24b(a screw blade) spirally projecting on the periphery of the second rotation shaft24a. The second agitating screw24is rotationally driven by a driving means (not shown) about the second rotation shaft24a(an arrow D32 inFIG. 3, an arrow R1 inFIG. 4), thereby conveying the toner in a direction of indicated by an arrow D2 inFIG. 4(an arrow D2 direction). The second agitating screw24conveys and supplies the toner to the developing roller21in the second conveying path222. In the present embodiment, the outside diameter of the second helical blade24bis 14 mm and the pitch in the axial direction is set to 20 mm, for example. While the pitch is changeable in proportion to the conveying performance of the second agitating screw24, it is preferable to determine the pitch to 15 mm as a lower limit to maintain the toner conveying ability.

The second agitating screw24is arranged at a front upper position of the developing roller21. More specifically, the second agitating screw24is arranged facing to the N2 pole of the magnet21M. In conjunction with the rotation of the second agitating screw24(the arrow D32 inFIG. 3), the tonner is supplied from the second agitating screw24to the sleeve21S. The second rotation shaft24aof the second agitating screw24is located below the rotation shaft of the sleeve21S. In addition, the rotation shaft24aof the second agitating screw24is located below the lower end part of the circumferential face of the sleeve21S. In the present embodiment, the supply path of toner to the developing roller21is formed by the path supplying the toner from the second agitating screw24. The second agitating screw24supplies the toner to the sleeve21S by scooping up from a downward side to an upward side for the developing roller21.

in a downstream side from the second agitating screw24in the toner conveying direction (the arrow D2 direction), a second paddle24cis arranged. The second paddle24cis a plate-like member arranged on the second rotation shaft24a. The second paddle24cis rotated together with the second rotation shaft24a, and delivers the toner from the second conveying path222to the first conveying path221in a direction indicated by an arrow D4 shown inFIG. 4. In the present embodiment, the axial length of the second paddle24cis set to 20 mm, for example.

The development device20further includes a layer adjusting member60(a layer thickness adjusting member) and a magnetic plate70(a facing magnet).

The layer adjusting member60is located at a front upper position of the developing roller21. The layer adjusting member60is arranged along the axial direction of the developing roller21so as to face to the circumferential face of the developing roller21(the sleeve21S). More specifically, the layer adjusting member60is arranged facing to the S1 pole of the magnet21M in the developing roller21. The layer adjusting member60is a plate-like member made of a magnetic material. The layer adjusting member60has a rectangular shape with longer sides extending toward the developing roller21in a cross section perpendicular to the rotation shaft of the developing roller21. A top end part of the layer adjusting member60is located at a distance from the sleeve21S of the developing roller21. As a result, between the top end part and sleeve21S, a layer adjusting gap G is formed. The layer adjusting member60adjusts the layer thickness of the toner scooped up onto the sleeve21S from the second agitating screw24.

The magnet plate70is arranged along the layer adjusting member60at a front side of the layer adjusting member60. In other words, the magnet plate70is arranged at an upstream side from the layer adjusting member60in the rotation direction of the sleeve21S of the developing roller21(an arrow D31 inFIG. 3). In the present embodiment, the magnetic plate70is composed of a permanent magnet having a plate-like shape. The magnet plate70has a substantially rectangular shape extending along the layer adjusting member60in a cross section perpendicular to the rotation shaft of the developing roller21. The magnet plate70is fixed to a lower part of the layer adjusting member60. The magnet plate70has a magnetic force of an S pole having the same polarity as the S1 pole at a position facing to the S1 pole of the magnet21M. In addition, the magnet plate70has an N pole at a position more distant than the S pole to the S1 pole of the magnet21M.

Thus, in the present embodiment, the magnet plate70is located at an upstream side from the layer adjusting member60in the rotation direction of the developing roller21(the sleeve21S). In other words, from the upstream side toward the downstream side in the rotation direction of the developing roller21, the magnet plate70and the layer adjusting member60are arranged in order so as to face to the circumferential face of the developing roller21.

Thus, in the present embodiment, the second agitating screw24supplies the toner to the sleeve21S toward a first position P1 facing to a downward side of the circumferential face of the sleeve21S. In addition, the layer adjusting member60adjusts the thickness of the toner on the sleeve21S in a second position P2 facing to an upward side of the circumferential face of the sleeve21S being located above the first position P1. At this time, because the S1 pole of the magnet21M and the S pole of the magnetic plate70have magnetic forces of the same polarity, a repulsive magnetic field acts between the sleeve21S and magnetic plate70. The repulsion magnetic field is classified into a magnetic field propagating toward the upstream side in the rotation direction of the sleeve21S and a magnetic field propagating toward the downstream side (the layer adjusting member60's side) in the rotation direction of the sleeve21S. As a result, to the toner conveyed on the sleeve21S and entered into the lower part of the magnetic plate70, a force for moving to the circumferential face of the sleeve21S is applied. Consequently, toner layer adjustment is implemented in a state where the toner is formed into a thin layer. In addition, the toner having not been entered into the layer adjusting gap G of the layer adjusting member60is promoted by the repulsive magnetic field to flow toward an upstream side in the rotation direction of the sleeve21S.

Referring toFIG. 4, the toner container50mentioned above is located above the toner replenishment port25of the development housing210. The toner container50includes a toner conveying path50athrough which the toner is conveyed inside, the rotating member54and the toner discharge port521. The toner container50is assembled with the development device20so that a longitudinal direction of the toner container50(the direction along which the toner container50is formed) is positioned in a direction perpendicular to the longitudinal direction of the development device20(the developer conveying direction of the first agitating screw23, the arrow D1 direction).

The toner discharge port521is arranged at a bottom part of the toner container50to correspond to the toner replenishment port25of the development device20. The rotating member54includes a shaft part and a blade part rotating about the shaft part (seeFIG. 2), and conveys the replenishing toner in the toner conveying path50atoward the toner discharge port521. The toner dropped from the toner container50is replenished to the development device20via the toner replenishment port25.

Next, the flow of the toner newly replenished for replenishment from the toner replenishment port25to the development housing210will be described.FIG. 5is sectional view showing the toner replenishment port25arranged to the development device20and the vicinity of the toner discharge port521arranged in the toner container50. Note that, inFIG. 5, the arrangement of the toner container50is shown by being rotated 90 degrees in the horizontal direction for explanation. Actually, the rotating member54in the toner container50is configured to extend to the front side (a viewer side) with respect to the drawing paper and the first agitating screw23and the rotating member54in the toner container50have a positional relationship perpendicular to each other.FIG. 6is a partially enlarged perspective view of the first agitating screw23.

Replenishing toner T2 supplied from a toner discharge port521of the toner container50drops into the first conveying path221, and is mixed with existing toner T1 and conveyed by the first agitating screw23along the arrow D1 direction. At this time, the toners T1 and T2 are agitated and charged.

The first agitating screw23includes a conveyance capacity suppressing part26arranged at a downstream side from the toner replenishment port25in the toner conveying direction and configured to partially suppress developer conveyance capacity. In the present embodiment, the conveyance capacity suppressing part26is formed by omitting the first helical blade23bof the first agitating screw23(seeFIG. 6). More specifically, the first agitating screw23includes the first helical blade23bextending from the upstream side to the toner replenishment port25in the toner conveyance direction D1. In the present embodiment, the axial length of the conveyance capacity suppressing part26is set to 12 mm, for example. In other words, the conveyance capacity suppressing part26is correspondent to the portion where the first rotation shaft23ais partially arranged. In this case, the conveyance capacity suppressing part26does not have the developer conveying performance in the axial direction of the first rotation shaft23a. Therefore, in the first conveying path221, the toner conveyed from the upstream side of the conveyance capacity suppressing part26begins to be retained in the conveyance capacity suppressing part26in the conveyance capacity suppressing part26. The retained toner is progressively accumulated to a position at an immediately upstream side of the conveyance capacity suppressing part26and in which the toner replenishment port25faces to the first conveying path221. As a result, a retention part27of the developer is formed in the vicinity of the inlet port of the retention part27.

When the replenishing toner T2 is supplied from the toner replenishment port25and the amount of the toner in the inner space220increases, the toner retained in the retention part27blocks (or closes) the toner replenishment port25, thereby suppressing any further toner supply. Thereafter, when the toner in the inner space220is consumed from the developing roller21, and then, the toner retained in the retention part27decreases, the toner having blocked the toner replenishment port25decreases. Thereby, spacing between the retention part27and toner replenishment port25occurs. As a result, the replenishing toner T2 flows in again from the toner replenishment port25to the inner space220. Thus, the present embodiment adopts a volume-replenishment type toner replenishment manner in which the reception amount of the replenishing toner is adjusted corresponding to the decrease in the amount of the toner retained in the retention part27.

In a development device using one-component developer, agitation of different particles (toner and carrier) as carried out in a case using a two-component developer is not needed. According to this, a conveying member (hereafter, called as a “third agitating screw”) excepted for the present disclosure corresponding to the second agitating screw24was rotated at a lowest possible peripheral speed in order to reduce the stress subjected to the toner. In particular, the circumferential speed of the developing roller21was often set to be higher than five times the circumferential speed of the third agitating screw. In other words, the stress subjected to the developer was reduced by setting the circumferential speed of the third agitating screw to be less than ⅕ of the circumferential speed of the developing roller21. In this case, however, the supply of the developer from the third agitating screw to the developing roller21was occasionally deficient.

In particular, similarly to the second agitating screw24, in the case that the third agitating screw has a screw shape having the helical blade arranged about the rotation shaft, the supply amount of the developer is large in the helical blade portion and the supply amount of the developer is deficient in the rotation shaft portion. As a result, a partial difference in the amount of the developer occasionally may occurs on the back side of the layer adjusting member60, thereby causing a difference in density in the shape of a longitudinal streak on the image. In addition, in the case where, as in the present embodiment, the magnetic plate70is arranged at the upstream side of the layer adjusting member60, due to the magnetic force of the magnetic plate70, the developer in the periphery cannot be moved, and accordingly, the difference in the amount of the developer becomes further significant.

On the other hand, when the circumferential speed of the third agitating screw may be significantly increased to supply a lot of the developer from the third agitating screw to the developing roller21, many developer is retained on the back side of the layer adjusting member60. In such a case, a problem may arise in that deterioration of the developer is accelerated and the driving torque of the development device20increases.

In the present embodiment, in order to solve the problem mentioned above, the ratio between a circumferential speed Vd of the developing roller21and a circumferential speed Vs of the second agitating screw24is suitably determined. In particular, in the present embodiment, the circumferential speed Vd of the developer carrier and the circumferential speed Vs of the developer conveying member are determined so that the ratio between the circumferential speeds satisfies a relationship represented by 1.3≦Vd/Vs≦5.0. As a result, a sufficient amount of the toner is supplied from the second agitating screw24to the developing roller21, thereby suppressing the occurrence of nonuniformity in the amount of the developer adjusted by the layer adjusting member60. In addition, because an excessive amount of the toner is supplied to the developing roller21from the second agitating screw24, it is suppressed that the toner is overloaded in the periphery of the layer adjusting member60to the extent of accelerating the deterioration of the toner.

Next, the present embodiment will be described on the basis of examples, but note that the embodiment is not limited to the examples. The examples described herebelow were each conducted under the following experimental conditions:Photosensitive drum31: OPC drum;Circumferential speed of photosensitive drum31: 146 mm/sec;Layer adjusting gap G: 0.3 mm;Developing bias AC component: rectangular wave amplitude of 1.7 kV, duty of 50%;Developing bias DC component: 270 V;Surface potential of photosensitive drum31(background part/image part): 430 V/30 V;Diameter of developing roller21: 16 mmDiameter of photosensitive drum31: 24 mm; andAverage particle size of magnetic toner: 6.8 μm (D50).

Table 1 shows evaluation a result of Experiment 1 regarding the circumferential speed ratio Vd/Vs between the circumferential speed Vd of the developing roller21and the circumferential speed Vs of the second agitating screw24.

In Experiment 1, with adjustment of the circumferential speed Vs of the second agitating screw24, the circumferential speed ratio Vd/Vs was varied, and the longitudinal streak level and the image density deterioration under each condition were evaluated. The longitudinal streak level was evaluated in a halftone image formed on the sheet. In the line of the longitudinal streak level in Table 1, a double-circle mark represents a state where no longitudinal streak occurred. A single-circle mark represents a state where almost no longitudinal streak occurred, and a triangle mark represents a state of quite unquestionable in the practical use while some very minor longitudinal streaks occurred. A cross mark represents a state where the longitudinal streaks occurred.

In addition, the image density deterioration depends upon the decrease in electrostatic property in accordance with the deterioration of the developer and was evaluated by measuring the transition of the density on an image sample. In the line of the image density deterioration in Table 1, the single-circle mark represents a state where no density deterioration occurred, and the cross mark represents a state where density deterioration occurred. The image density deterioration was evaluated in a case where the images each having the image density of 1% on the sheet are printed multiple times one by one (1% 1-sheet intermittence) and in a case where the similar images are printed multiple times in units of 3 sheets (1% 3-sheet intermittence). In addition, the evaluation was conducted also in a case where the images each having the image density of 3.8% are printed multiple times in units of 3 sheets (3.8% 3-sheet intermittence). A practical use condition is correspondent to the “3.8% 3-sheet intermittence” condition, and the “1% 1-sheet intermittence” condition is correspondent to a stress condition exceeding the practical use condition.

As shown in Table 1, when the circumferential speed ratio Vd/Vs is within the range of 1.3≦Vd/Vs≦5.0, the longitudinal streak level and the image density deterioration were led to excellent results. More specifically, in the above-mentioned range, the longitudinal streak level became the triangle mark's level or more, and no image density deterioration occurred under the “3.8% 3-sheet intermittence” condition being the practical use condition, and the “1% 3-sheet intermittence” condition.

In addition, when the circumferential speed ratio is within the range of 2.0<Vd/Vs, no image density deterioration occurred even under the “1% 1-sheet intermittence” condition being the stress condition, and then, stable image quality was maintained. Further, when the circumferential speed ratio is within the range of Vd/Vs≦4.0, the longitudinal streak level became the single-circle mark's level or more, and then, as the halftone image, an image without nonuniformity was obtained.

Table 2 shows a result of Experiment 2 where the diameter and circumferential speed Vd of the developing roller21were varied in comparison to Experiment 1. Evaluation of the longitudinal streak level and the image density deterioration was carried out in the same way as described above.

As shown in Table 2, even in a case where the diameter and the circumferential speed Vd of the developing roller21is different from those in Experiment 1, when the circumferential speed ratio Vd/Vs is within the range of 1.3≦Vd/Vs≦5.0, the longitudinal streak level and the image density deterioration became excellent results. More specifically, in the above-mentioned range, the longitudinal streak level became the triangle mark's level or more, and no image density deterioration occurred under every conditions including the “3.8% 3-sheet intermittence” condition being the practical use condition.

Thus, according to the embodiment described above, the second agitating screw24conveys the tonner in a first direction in the second conveying path222and supplies the toner to the developing roller21. In addition, the circumferential speed Vd of the developing roller21and the circumferential speed Vs of the second agitating screw24are determined so that the circumferential speed ratio satisfies the relationship represented by 1.3≦Vd/Vs≦5.0. According to this, the toner is sufficiently supplied from the second agitating screw24to the developing roller21, thereby suppressing the occurrence of nonuniformity in the amount of the toner adjusted by the layer adjusting member60. Moreover, because an excessive amount of the toner is supplied to the developing roller21from the second agitating screw24, it is suppressed that the toner is overloaded in the periphery of the layer adjusting member60to the extent of accelerating the deterioration of the toner.

Further, according to the embodiment described above, the circumferential speed ratio satisfies the relationship represented by 2.0<Vd/Vs, thereby further suppressing the overloading occurred in the toner in the periphery of the layer adjusting member60. Further, since the circumferential speed ratio satisfies the relationship represented by Vd/Vs≦4.0, a sufficient amount of the toner is stably supplied to the developing roller from the second agitating screw24, thereby further suppressing the occurrence of nonuniformity in the amount of the developer adjusted by the layer adjusting member60.

Further, according to the present embodiment, the second agitating screw24supplies the developer to the developing roller21while being rotated from the downward side to the upward side in the region facing to the developing roller21. According to this, relative to a two-component developer,

the one-component developer in no need of the agitation of the different particles relative to a two-component developer can be more stably supplied from the second agitating screw24to the developing roller21.

Further, according to present embodiment described above, even when the force exerted to supply the toner from the second agitating screw24to the developing roller21is partially varies while depending on the pitch of the helical blade of the second agitating screw24, the circumferential speed Vd of the developing roller21and the circumferential speed Vs of the second agitating screw24are determined so that the circumferential speed ratio satisfies the relationship represented by 1.3≦Vd/Vs≦5.0, whereby a sufficient amount of the developer is supplied from the second agitating screw24to the developing roller21.

Further, according to the present embodiment, even when the toner on the developing roller21is difficultly moved in the axial direction (the direction intersecting with the rotation direction) of the developing roller21by the magnetic plate70arranged at the upstream side of the layer adjusting member60, a sufficient amount of the developer is supplied from the second agitating screw24to the developing roller21. Therefore, the occurrence of nonuniformity in the amount of the toner adjusted by the layer adjusting member60is suppressed.

Further, according to the image forming apparatus1including the development device20of the embodiment described above, occurrence of longitudinal-streak liked image quality defects in the image formed on the sheet are suppressed. Further, stable images are formed on the sheets for a long term.

Although the development device20and the image forming apparatus1using the same have been described as above, the present disclosure is not limited thereto, modified embodiments such as described below may be adopted, for example.

In the above-mentioned embodiment, a situation in which the toner replenishment from the toner container50to the development device20was adjusted by the conveyance capacity suppressing part26(the retention part27) was described. However, the present disclosure is not limited thereto. Another situation may be applied so that the toner is supplied for replenishment from the toner container50to the development housing210in response to a detection result of, a density sensor (not shown) detecting the image density or a toner sensor (not known) detecting the amount of the toner.

In the development device20in which, as described above, the toner is supplied for replenishment from the toner container50to the development housing210according to the amount of the toner in the retention part27, the circumferential speed Vd of the developing roller and the circumferential speed Vs of the second agitating screw24are determined so that the circumferential speed ratio satisfies the relationship represented by 1.3≦Vd/Vs≦5.0, whereby the amount of the distribution in the axial direction of the toner retained in the back side of the layer adjusting member60is stabilized. More specifically, the toner distribution according to the pitch of the second helical blade24bof the second agitating screw24is not easily formed on the back side of the layer adjusting member60. In such case, because the distribution of the toner in the development housing210is stabilized, the amount of the toner in the retention part27is accurately changed in proportion to the amount of the toner consumed by the developing roller21. Consequently, the toner can suitably be supplied for replenishment from the toner container50to the development housing210in proportion to the amount of toner consumption.

While the present disclosure has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present disclosure.