Patent Application
20150293470
The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2014-082588, filed Apr. 14, 2014. The contents of this application are incorporated herein by reference in their entirety.
The present disclosure relates to a developing device and an image forming apparatus including the developing device.
Electrographic image forming apparatuses, such as copiers, printers, and facsimile machines, include a developing device that supplies toner to an electrostatic latent image formed on a photosensitive drum thereby to develop the electrostatic latent image. This forms a toner image on the photosensitive drum. The developing device includes a development roller (toner bearing member) rotatably disposed in a housing of the developing device. The development roller is spaced a predetermined gap away from the photosensitive drum and has a circumferential surface for bearing a developer, which at least contains toner. In one disclosure, a development roller is disposed opposite to the photosensitive drum. In another disclosure, a development roller is provided with a resin layer covering the surface of the development roller. In a yet another disclosure, a development roller is formed through a dipping process (dip method, dipping method) of dipping an element tube into a liquid resin in which a resin material has been dissolved.
One aspect of the present disclosure provides a developing device that includes a housing, a toner bearing member, and a drive transmission section. The drive transmission section is disposed at one axial end of the toner bearing member and configured to transmit a rotational drive force to the toner bearing member. The toner bearing member has a circumferential surface for carrying toner thereon. The toner bearing member is axially rotatable in the housing and disposed a predetermined gap away from an image bearing member. The image bearing member has a circumferential surface on which an electrostatic latent image is formed. The toner bearing member includes a cylindrical base and a surface layer disposed over the base. The surface layer is formed through a dipping process of dipping the base into a dipping bath with the base directed axially vertically. The toner bearing member is mounted to the housing such that a lower axial end of the toner bearing member during the dipping process is an opposite axial end to the one axial end at which the drive transmission section is disposed.
Another aspect of the present disclosure provides an image forming apparatus that includes: the developing device according to the one aspect of the present disclosure described above; and the image bearing member having a circumferential surface on which an electrostatic latent image is formed and configured to receive supply of the toner from the toner bearing member.
The following explains an embodiment of the present disclosure with reference to the accompanying drawings. The present disclosure is applicable to electrographic image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction peripherals combining such functions.
The main body 11 is composed of a lower body 111, an upper body 112, and a connecting portion 113. The upper body 112 is disposed above the lower body 111. The connecting portion 113 is disposed between the upper body 112 and the lower body 111, connecting the lower body 111 and the upper body 112 with the paper discharging section 15 secured therebetween. In
The image forming section 12, the fixing device 13, and the paper feed section 14 are disposed in the lower body 111.
The image forming section 12 forms a toner image on a sheet of paper P fed from the paper feed section 14. The image forming section 12 includes an unit 12Y for yellow toner, a unit 12M for magenta toner, a unit 12C for cyan toner, a unit 12Bk for black toner, an intermediate transfer belt 125, a secondary transfer roller 196, and a belt cleaner 198. The units 12Y, 12M, 12C, and 12Bk are disposed in the stated order horizontally from the upstream to downstream in the moving direction of the intermediate transfer belt 125 (from the right to left in
The units 12Y, 12M, 12C, and 12Bk of the respective colors each include a photosensitive drum 121 (image bearing member), a developing device 122, a toner cartridge (not shown), a charger 123, and a drum cleaner 127. Each developing device 122 supplies toner (developer) to the corresponding photosensitive drum 121. Each toner cartridge contains toner of a corresponding color. Below the developing devices 122 adjacent to one another, an exposure device 124 is horizontally disposed for light exposure to the respective photosensitive drums 121.
Each photosensitive drum 121 has a cylindrical shape and is rotated on its axis. The photosensitive drum 121 has a circumferential surface on which an electrostatic latent image is formed and a toner image developed with toner from the electrostatic latent image is carried. The photosensitive drum 121 according to the present embodiment is a known organic photoconductor (OPC). The photosensitive drum 121 has layers, such as a charge generating layer and a charge transport layer, on the surface. These layers are formed through a dipping process, in a manner similar to a development roller 83, which will be described later.
Each developing device 122 supplies toner to an electrostatic latent image formed on the circumferential surface of the corresponding photosensitive drum 121 that is rotating in the direction of the arrow shown in
Each charger 123 is disposed immediately under the corresponding photosensitive drum 121 and uniformly charges the circumferential surface of the photosensitive drum 121.
The exposure device 124 is disposed below the chargers 123. The exposure device 124 irradiates the charged circumferential surface of each photosensitive drum 121 with a laser beam in accordance with image data of the corresponding color, thereby forming an electrostatic latent image on the circumferential surface of the photosensitive drum 121. The image data may be input from a computer or the like or acquired by the document reading section 16. The exposure device 124 emits a laser beam to provide a predetermined amount of exposure so as to form a latent image at a predetermined potential on each photosensitive drum 121. Each drum cleaner 127 is disposed on the left of the corresponding photosensitive drum 121 and removes residual toner from the circumferential surface of the photosensitive drum 121.
The intermediate transfer belt 125 is an endless belt. More specifically, the intermediate transfer belt 125 is a conductive soft belt having a multilayered structure with a base layer, an elastic layer, and a coating layer. The intermediate transfer belt 125 is entrained around a plurality of rollers that are aligned substantially horizontally above the image forming section 12. The rollers around which the intermediate transfer belt 125 is entrained include a drive roller 125A and a driven roller 125E. The drive roller 125A is disposed near the fixing device 13 and drives the intermediate transfer belt 125 to rotate. The driven roller 125E is horizontally spaced a predetermined distance away from the drive roller 125A and is rotated by following the rotation of the intermediate transfer belt 125. By a rotational drive force applied to the drive roller 125A, the intermediate transfer belt 125 is driven to circulate clockwise in
The secondary transfer roller 196 is electrically connected to a section for applying a secondary transfer bias (not shown). A secondary transfer bias is applied between the secondary transfer roller 196 and the drive roller 125A. The transfer bias causes transfer of the toner image formed on the intermediate transfer belt 125 to a sheet P conveyed from a pair of conveyance rollers 192, which is disposed below. The belt cleaner 198 is disposed opposite to the driven roller 125E across the intermediate transfer belt 125.
The fixing device 13 includes a heating roller 132 and a pressure roller 134. In the interior of the heating roller 132, a conductive heating element, such as a halogen lamp, is provided as a heat source. The pressure roller 134 is disposed opposite to the heating roller 132. The fixing device 13 applies heat from the heating roller 132 to a toner image that is transferred to a sheet P by the image forming section 12, carrying out a fixing process of the toner image. The fixing process by the fixing device 13 is conducted while the sheet P passes through the fixing nip formed between the heating roller 132 and the pressure roller 134. After the fixing process, the sheet P having a color image formed thereon is conveyed through a discharge conveyance path 194 extending from the upper portion of the fixing device 13 and discharged to an exit tray 151 disposed on the top of the main body 11.
The paper feed section 14 includes a manual feed tray 141 and a paper feed cassette 142. The paper feed cassette 142 is detachably disposed in the main body 11 at a position below the exposure device 124. The paper feed cassette 142 contains a sheet stack P1, which is a stack of a plurality of sheets P. A pickup roller 143 is disposed above the paper feed cassette 142. The pickup roller 143 feeds a topmost sheet P from the sheet stack P1 stored in the paper feed cassette 142 into a paper conveyance path 190. In
The paper conveyance path 190 is disposed to extend vertically on the left of the image forming section 12. The pair of conveyance rollers 192 is disposed at appropriate positions on the paper conveyance path 190. The pair of conveyance roller 192 conveys a sheet P fed from the paper feed section 14 to a secondary transfer nip N formed between the secondary transfer roller 196 and the drive roller 125A.
The paper discharging section 15 is provided between the lower body 111 and the upper body 112. The paper discharging section 15 includes the exit tray 151 formed in the top surface of the lower body 111. The exit tray 151 is for receiving a sheet P discharged after the fixing process of the sheet P by the fixing device 13.
The document reading section 16 is disposed in the upper body 112. The document reading section 16 includes contact glass 161, a document holding cover 162, and a scanning mechanism 163. The contact glass 161 is for placing a document thereon. The document holding cover 162 is freely opened and closed to hold a document placed on the contact glass 161. The scanning mechanism 163 scans the document placed on the contact glass 161 to read an image of the document. The scanning mechanism 163 includes an image sensor, such as charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS), to optically read an image of the document and generates image data representing the image. The main body 11 includes an image processing section (not shown) for creating an image for printing based on the image data.
The following explains the developing device 122 in detail.
As shown in
As shown in
By the magnetic force of the pump pole 821, the magnetic roller 82 magnetically pumps up (attracts) the developer from the developer reservoir 81 onto its circumferential surface 82A. The magnetic roller 82 magnetically holds a layer of the attracted developer (magnetic brush layer) on the circumferential surface 82A. The magnetic roller 82 then supplies toner to the development roller 83. As the magnetic roller 82 rotates, the developer is conveyed toward the developer limiting blade 84.
The developer limiting blade 84 is disposed opposite to the magnetic roller 82 at a position upstream from the development roller 83 in the rotation direction of the magnetic roller 82. The developer limiting blade 84 limits the thickness of the developer accumulated on the circumferential surface 82A of the magnetic roller 82. The developer limiting blade 84 defines a limiting gap G of a predetermined size with the circumferential surface 82A of the magnetic roller 82. The arrangement described above ensures that the developer layer formed on the circumferential surface 82A to have a uniform predetermined thickness.
The development roller 83 is disposed to extend in parallel to the magnetic roller 82 and driven to rotate clockwise shown in
As shown in
As shown in
The developing bias applying section 88 applies a developing bias, which is generated by superimposing an alternating-current (AC) voltage on a direct-current (DC) voltage, to the magnetic roller 82 and the development roller 83. An AC voltage is applied between the photosensitive drum 121 and the development roller 83 as well as between the development roller 83 and the magnetic roller 82. As a consequence, toner is supplied from the magnetic roller 82 to the development roller 83 and subsequently from the development roller 83 to the photosensitive drum 121. The development roller 83 therefore receives a higher AC voltage for causing the toner transfer, as compared with a known one-component or two-component developing device.
As shown in
The developer discharging section 87 is in communication with the second chamber 81b at a position forward of the reverse conveyance section 86A. The developer discharging section 87 includes a cylindrical wall defining an interior space and a discharge screw 87A rotatable in the interior space. The discharge screw 87A is a screw impeller coaxially fixed to the second screw feeder 86. The discharge screw 87A is disposed to have the same feeding direction as the screw impeller of the second screw feeder 86. Some of the developer once retained by the reverse conveyance section 86A passes over the reverse conveyance section 86A to flow into the developer discharging section 87. The developer flown into the developer discharging section 87 is conveyed forward by the discharge screw 87A and discharged from an exit port not shown in the figures. As has been described above, the present embodiment employs a trickle technique for causing some of the developer to be discharged from the developing device 122. To replenish the developing device 122 with carrier, the toner cartridge (not shown) may contain carrier in addition to toner or the developing device 122 may be provided with a carrier replenishing tank.
As shown in
As show in
Yet, forming the coating layer 83C through the dipping process as described above involves that the liquid mixture adhering on the surface of the sleeve 830 tends to flow down by gravity when the sleeve 830 is lifted up from the liquid mixture. As a consequence, the coating layer 83C formed on the surface of the sleeve 830 is thicker at a portion closer to an axial end of the sleeve 830 that was the lower axial end during the dipping than at a portion corresponding to the axial center of the sleeve 830. In particular, the coating layer 83C tends to have a thicker portion 83C1 at a position corresponding to the lower axial end (the front end in
The development roller 83 is pressed at the rear end toward the photosensitive drum 121 due to the meshing of the gear teeth between the input gear 82G and the roller gear 83G that is caused upon transmission of the rotational drive force from the input gear 82G to the roller gear 83G. Thus, at the rear end portion of the development roller 83, the gap between the development roller 83 and the photosensitive drum 121 is stably maintained by the pair of tracking rollers TR described above. At the front end portion of the development roller 83, on the other hand, the development roller 83 may not be reliably positioned due to the absence of the pressing force produced by the input gear 82G and the roller gear 83G meshing with each other in a manner described above. The development roller 83 may wobble or may be off centered within predetermined tolerances. Under the influence by these factors, the gap between the development roller 83 and the photosensitive drum 121 tends to fluctuate at the front end portion of the development roller 83. When the gap between the development roller 83 and the photosensitive drum 121 is larger at the front end portion of the development roller 83 than at the rear end portion, toner images formed on the photosensitive drum 121 suffer from reduction in image density or inconsistency in image density appearing at intervals corresponding to the rotation pitch of the development roller 83.
According to the present embodiment, the coating layer 83C is relatively thicker at a portion closer to an end that was the lower end of the development roller 83 during the dripping process, and the development roller 83 is mounted to the developing device 122 such that the relatively thicker portion of the coating layer 83C is positioned toward the front of the developing device 122. Therefore, by the difference in the thickness of the coating layer 83C, the gap between the development roller 83 and the photosensitive drum photosensitive drum 121 is narrower in part, and the developing electric field at such a portion is maintained relatively strong. This can restrict the reduction or inconsistency in image density at the front end portion (opposite to the driving end) of the developing device 122.
Note in addition that the present embodiment employs a touchdown developing method as described above. In the developing device 122, a magnetic brush is formed from toner and carrier on the circumferential surface of the magnetic roller 82. The strong abrasive force of the magnetic brush results in wear of the coating layer 83C of the development roller 83. The abrasive force of the magnetic brush fluctuates according to the concentration of the toner in the magnetic brush. When the concentration of the toner is low and thus the carrier surfaces tend to be exposed, the abrasive force of the magnetic brush increases to accelerate wear of the coating layer 83C. As shown in
According to the present embodiment, the coating layer 83C is relatively thicker at a portion closer to an end of the development roller 83 that was the lower end during the dipping process, and the development roller 83 is mounted to the developing device 122 such that the lower end of the coating layer 83C is positioned toward the front of the developing device 122. As a consequence, despite the strong abrasive force of the magnetic brush, the portion of the coating layer 83C closer toward the front is restricted from becoming thinner than the portion closer toward the rear. In addition, the arrangement described above is effective to prevent detachment of the coating layer 83C by a mechanical force applied by the magnetic brush. Note that, at the front end portion of the magnetic roller 82, the concentration of the toner tends to be lower and thus the chargeability of the toner tends to be higher. As a result, the toner carried on the front end portion of the development roller 83 may be in sufficient to appropriately develop a latent image on the photosensitive drum 121, which tends to cause reduction in the resulting image density. Yet, as described above, the gap between the photosensitive drum 121 and the development roller 83 is set to be partially narrower at a position closer to the front end of the development roller 83 which promotes the developing action and thus prevents the image density reduction.
The developing device 122 according to the present embodiment has the developer discharging section 87. Components, carrier in particular, of the developer are gradually replaced while the carrier is held in the developer reservoir 81, which increases the longevity of the developer. Consequently, the present embodiment ensures stable image formation over a long period of time. With reference to
The accumulation K formed in the downstream end portion of the second chamber 81b leads to an increase in the amount of the developer carried on the front end portion of the circumferential surface of the magnetic roller 82. Consequently, the amount of the developer that is carried beyond the developer limiting blade 84 is greater at the front end portion than at the rear end portion. This leads to a further increase of the abrasive force of the magnetic brush. As has been described above, the development roller 83 is mounted such that the lower end during the dipping process is disposed toward the front of the developing device 122. This restricts the coating layer 83C from becoming thin even in the structure that the developer discharging section 87 is disposed at the downstream of the second chamber 81b.
The explanation given above is directed to the developing device 122 and the image forming apparatus 1 according to the embodiment of the present disclosure. However, the present disclosure is not limited to the specific embodiment, and various alterations including the following may be made.
(1) In the embodiment given above, the image forming apparatus 1 is explained as being a full color image forming apparatus, which should not be construed as a limitation. The image forming apparatus 1 may be a monochrome image forming apparatus that prints black and white images.
(2) In the embodiment above, the second screw feeder 86 conveys developer from the side closer to the roller gear 83G, which should not be construed as a limitation. The second screw feeder 86 may convey the developer toward the side closer to the roller gear 83G Alternatively, the second screw feeder 86 may convey the developer in a direction toward the thicker portion 83C1 (the lower end of the development roller 83 at the time of the dipping) irrespective of the disposition of the roller gear 83G Similarly, the developer discharging section 87 may be disposed in accordance with the lower end of the development roller 83 at the time of the dipping, irrespective of the disposition of the roller gear 83G
Now, the following explains a preferred manner of the development roller 83 of the developing device by way of example. Examples given below were subjected to experiments in the following conditions.
Printing rate: 30 sheets/min
Photosensitive drum 121: OPC drum
Peripheral speed of the photosensitive drum 121: 180 mm/sec
Development roller 83: anodized surface treatment+nylon resin coating
Peripheral speed of the development roller 83: Ratio of 1.5 relative to the peripheral speed of the photosensitive drum 121 (the same rotation direction as the photosensitive drum 121)
Peripheral speed of magnetic roller: Ratio of 1.1 relative to the peripheral speed of the development roller 83 (the opposite rotation direction from the photosensitive drum 121)
Gap between the photosensitive drum 121 and the development roller 83: 0.12 mm
Gap between the magnetic roller 82 and the development roller 83 0.3 mm
Surface potential of the photosensitive drum 31: +430 V (at background portion) and +100 V (at image portion)
Developing bias applied to the development roller 83: Frequency of AC voltage=3.7 kHz, Duty=27%, Vpp=1,500 V, and DC voltage=190 V
Developing bias applied to the magnetic roller 82: Frequency of AC voltage=3.7 kHz, Duty=73%, Vpp=650 V, and DC voltage=490 V
Average size of toner particles: 6.8 μm (positively chargeable)
In Example 1, the development roller 83 was disposed such that the lower axial end during the dipping process was positioned toward the front of the developing device 122 (at a position away from the roller gear 83G) as in the embodiment described above. In Comparative Example 1, the development roller 83 was disposed such that the lower axial end during the dipping process was positioned toward the rear of the developing device (at a position toward the roller gear 83G). Example 1 and Comparative Example 1 were each subjected to a process of continuously producing 500K (500×1,000) prints of an image at a coverage rate of 3.8%. Table 1 shows changes in the thickness of the coating layer 83C.
As shown in Table 1, the development roller 83 of Example 1 was disposed such that the lower axial end (initial layer thickness of 10 μm) was positioned away from the driving side where the roller gear 83G was disposed. As a result, the thickness of the coating layer 83C was not below 3 μm upon completion of the process of producing 500K prints. Therefore, stable image forming operation was maintained. On the other hand, the development roller 83 of Comparative Example 1 was disposed such that the upper axial end (initial thickness of 4 μm) was positioned away from the driving side where the roller gear 83G was disposed. As a result, at the time of producing 200K prints and onward, density inconsistency appeared at the intervals corresponding to the rotation pitch of the development roller 83.
In Example 2, in the same manner as the embodiment described above, the aluminum sleeve 830 (base) having a diameter of 20 mm was anodized and then the coating layer 83C was formed to an average thickness of 6 μm on the sleeve 830. The thickness of the coating layer 83C was 10 μm at a portion corresponding to the lower axial end of the development roller 83 during the dipping process. The coating layer 83C was formed from a nylon resin containing 100 parts by mass of titanium oxide dispersed therein. In Comparative Example 2, the aluminum sleeve 830 (base) having a diameter of 20 mm was anodized and then a coating layer was formed by spraying to an average thickness of 6 μm on the sleeve 830. The coating layer of Comparative Example 2 was formed from a urethane resin containing 100 parts by mass of titanium oxide and 5 parts by mass of carbon black dispersed therein. Example 2 and Comparative Example 2 were both subjected to a process of continuously producing 100K (100×1,000) prints of an image at coverage rate of 50%. Table 2 shows changes in the thickness of the respective coating layers.
As shown in Table 2, the thickness of the coating layer 83C of Example 2 was not below 3 μm upon completion of continuous 100K prints of an image at a high coverage rate of 50%. Consequently, favorable images were stably formed. On the other hand, the thickness of the coating layer of Comparative Example 2 formed by spraying was reduced at the end portion away from the driving side and worn out by the time of completion of 80K prints. Different from Comparative Example 2, in addition, the coating layer of Example 2 contained titanium oxide as the sole conducting material, which improved the strength of the resulting coating layer and the abrasion amount of the coating layer.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-082588 | Apr 2014 | JP | national |
