Patent Grant
10401756
The present invention relates to an image forming apparatus of, for example, an electrophotographic system or an electrostatic recording system, and particularly to an image forming apparatus that uses two-component developer which is a mixture of nonmagnetic toner and magnetic carrier.
In an image forming apparatus such as a copier employing an electrophotographic system, a visible image is formed by attaching developer to an electrostatic latent image formed on an image bearing member. Hereinafter, the image bearing member will be also referred to as a photosensitive drum. Conventionally, a developing unit that uses two-component developer including toner and magnetic carrier is known. As such a developing unit, a unit that conveys two-component developer to the vicinity of a photosensitive drum while magnetically attaching the two-component developer to a rotating developer bearing member and thus develops an electrostatic latent image on the photosensitive drum into a visible image by toner in the developer is widely used. Hereinafter, the developer bearing member will be also referred to as a developing sleeve.
In this method, the developer is magnetically held on the developing sleeve by providing a magnet fixedly disposed inside the rotating developing sleeve. It is general that the two-component developer is further conveyed to the vicinity of a photosensitive member while regulating the amount of the two-component developer by disposing a regulation blade so as to oppose the developing sleeve in a certain distance from the developing sleeve. In order to stably convey the developer at this time, conventionally, a developing sleeve whose surface is roughened by sand blasting using abrasive grains or a developing sleeve whose surface is provided with a plurality of grooves extending parallel to the rotation axis of the developing sleeve is generally used.
The developing sleeve roughened by sand blasting has a problem that the developer conveyance performance thereof decreases if the degree of roughness is low. Meanwhile, there is also a problem that, in the case where the degree of roughness is increased to improve the developer conveyance performance, the developing sleeve needs to be blasted by strongly hitting the abrasive grains and thus the developing sleeve is deformed. Therefore, the developing sleeve that has been subjected to sand blasting is usually used with a low degree of roughness. This is likely to cause a problem that the recesses and projections on the developing sleeve are worn out after use of a long period and the performance of conveying developer decreases. This can be a cause of shortening the lifetime of the developing unit.
In recent years, the demand for high quality, high reliability, and high stability of image forming apparatus has been growing. In satisfying the demand for these, stability over time of the amount of developer on the developing sleeve is important. Therefore, in Japanese Patent Laid-Open No. 2003-208027, a developing sleeve in which a plurality of grooves extending parallel to the rotation axis of the developing sleeve is proposed. Since the grooves are defined by, for example, a drawing process using a die, according to this developing sleeve, the degree of roughness can be increased without deforming the developing sleeve as in sand blasting. Therefore, this developing sleeve is less likely to be influenced by wear from long-time use compared with a developing sleeve subjected to sand blasting, and thus the performance of conveying developer can be stabilized.
However, although the developing sleeve provided with grooves has a stable performance of conveying developer, there is a risk that periodical density unevenness appears in the conveyance state of the developer on the developing sleeve due to the pitch of periodical recesses of the grooves. There is a possibility that the periodical density unevenness appearing in the conveyance state is made visible as density unevenness in an image corresponding to the period thereof. For example, in the case where periodical density unevenness occurs, the visual sensitivity of a person visually recognizing the density unevenness differs depending on the pitch, and it is said that the visual sensitivity drops drastically in the case where the spatial frequency of the density unevenness is larger than 1 line/mm as shown in FIG. 5 of Japanese Patent Laid-Open No. 62-278522. Therefore, the periodical density unevenness can be made less visible by reducing the pitch of the grooves on the developing sleeve as much as possible.
Meanwhile, since durability such as wear resistance against mechanical external force applied by, for example, charging and cleaning, is required for the surface of a photosensitive drum, a technique of using a resin having a high wear resistance as a surface layer of the photosensitive drum is known. Examples of the resin include curable resin. Examples of a problem that arises as a result of increasing the wear resistance of the surface of the photosensitive drum includes decrease in cleanability caused by increase in the rotational torque due to increase in the dynamic friction coefficient of the surface of the photosensitive drum. To solve such a problem, Japanese Patent Laid-Open No. 2007-233355 proposes a technique of providing periodical recess portions on the circumferential surface of a photosensitive drum. However, such periodical recess portions on the photosensitive drum can be also made visible as density unevenness in an image corresponding to the period thereof. Similarly to the case of the grooves of the developing sleeve described above, the periodical density unevenness can be made less visible by reducing the pitch of the recess portions of the photosensitive drum as much as possible, specifically, by setting the pitch to 1 line/mm or smaller.
However, in the developing sleeve of Japanese Patent Laid-Open No. 2003-208027 or the photosensitive drum of Japanese Patent Laid-Open No. 2007-233355, even if the grooves or the recess portions are provided to have a pitch of 1 line/mm or smaller, an opposing pattern that opposes the grooves or the recess portions in a developing region is not considered. Therefore, in the case where development is performed on the photosensitive drum provided with the periodical recess portions by using the developing sleeve provided with the periodical grooves, there is a risk that a large periodical density unevenness occurs due to the occurrence of beat between the frequencies of the two and the density unevenness becomes more visible. This beat is density unevenness periodically occurring at a larger pitch than the pitch of the recess portions of the developing sleeve as a result of recess portions of the developing sleeve not opposing the recess portions of the photosensitive drum and not increasing the density being interposed between recess portions of the developing sleeve that oppose the recess portions of the photosensitive drum and increase the density. Therefore, even if the period of the recess portions of each of the developing sleeve and the photosensitive drum is set to 1 mm or smaller, there is a risk that density unevenness of a pitch larger than 1 mm occurs due to the beat and the density unevenness becomes more visible.
According to one aspect of the present invention, an image forming apparatus includes a rotatable image bearing member, configured to carry a toner image, with first recess portions provided at intervals substantially equal to a first pitch Pdr (mm) in a rotation direction of the image bearing member, and a rotatable developer bearing member, configured to carry developer comprising toner and carrier and to form the toner image on the image bearing member, with second recess portions provided at intervals substantially equal to a second pitch Ps (mm) in a rotation direction of the developer bearing member, wherein the following relationship is satisfied
Vs/Vdr<Ws/Pdr
where Vdr represents a movement speed (mm/sec) of the first recess portions, Vs represents a movement speed (mm/sec) of the second recess portions, and Ws represents a length (mm) of each of the second recess portions in the rotation direction of the developer bearing member.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
An image forming apparatus 1 of an exemplary embodiment of the present invention will be described in detail below with reference to
Toner
Toner having a weight average particle diameter of 4 μm to 10 μm is preferable. In the present exemplary embodiment, toner for color copier having a weight average particle diameter of 6 μm is used. In the case where the weight average particle diameter of toner is represented by M and a particle diameter of toner is represented by r, it is preferable that particle diameters of toner particles of 90% by weight or more are in the range of M/2<r<2M/3 and particle diameters of toner particles of 99% by weight or more are in the range of 0<r<2M to form a clearer color image. Examples of binder resin used for the toner include polyester resin and styrene-based copolymer resin such as styrene-acrylic acid ester resin and styrene-methacrylic acid ester resin. Considering color blendability of color toner at the time of fixation, polyester resin is preferable because the polyester resin has a sharp melting property.
Carrier
Carrier preferably has an average particle diameter of 25 μm to 50 μm based on volume distribution. In the present exemplary embodiment, carrier having a volume average particle diameter of 35 μm is used. The volume average particle diameter is also referred to as a 50% particle diameter D50. In the description below, a particle diameter of carrier refers to a volume average particle diameter unless otherwise described explicitly. As such carrier particles, ferrite particles with or without thin resin coating thereon can be preferably used. Examples of the ferrite particles include Cu—Zn ferrite particles having a maximum magnetization of about 230 emu/cm3. The average particle diameter based on volume distribution, that is, the 50% particle diameter D50 of the carrier is measured by using, for example, a multi-image analyzer manufactured by Beckman Coulter, Inc.
As the carrier, magnetic resin carrier constituted by binder resin and magnetic metal oxide, nonmagnetic metal oxide, or the like may be used. The magnetic resin carrier is characterized by having a maximum magnetization of about 190 emu/cm3, which is smaller than the maximum magnetization of the ferrite particles. Therefore, magnetic interaction between adjacent magnetic brushes is smaller, and, as a result, naps of the magnetic brushes are more densely packed and shorter. Thus, an image of a higher resolution with less uneven brushing marks or the like can be provided.
As illustrated in
The image forming portion 40 includes image forming units 80, toner hoppers 41, toner containers 42, a laser scanner 43, an intermediate transfer unit 44, a secondary transfer portion 45, and a fixing unit 46. The image forming portion 40 is capable of forming an image on a sheet S on the basis of image information. To be noted, the image forming apparatus 1 of the present exemplary embodiment is a full-color printer, and image forming units 80y, 80m, 80c, and 80k are respectively provided for four colors of yellow, magenta, cyan, and black. Here, y, m, c, and k respectively correspond to yellow, magenta, cyan, and black. The image forming units 80y, 80m, 80c, and 80k are separate image forming units that have similar configurations. Similarly, toner hoppers 41y, 41m, 41c, and 41k, and toner containers 42y, 42m, 42c, and 42k are respectively provided for the four colors of, yellow, magenta, cyan, and black. The toner hoppers 41y, 41m, 41c, and 41k are separate hoppers that have similar configurations, and the toner containers 42y, 42m, 42c, and 42k are separate containers that have similar configurations. Therefore, in
The toner containers 42 are, for example, bottles of cylindrical shapes, accommodate toner, and are disposed above the respective image forming units 80 so as to be coupled to the image forming units 80 via the toner hoppers 41. The laser scanner 43 exposes the surfaces of photosensitive drums 81 charged by charging rollers 82, and thus forms electrostatic latent images on the surfaces of the photosensitive drums 81.
The image forming units 80 include the four image forming units 80y, 80m, 80c, and 80k for forming toner images of four colors. The image forming units 80 each include a photosensitive drum 81, a charging roller 82, a developing unit 20, and a cleaning blade 84. The photosensitive drum 81 moves while carrying a toner image, and serves as an image bearing member. The photosensitive drums 81, the charging rollers 82, the developing units 20, the cleaning blades 84, and developing sleeves 24 that will be described later are also respectively provided separately in similar configurations for the four colors of yellow, magenta, cyan, and black.
The photosensitive drum 81 includes a photosensitive layer formed to have a negative charging polarity on the outer circumferential surface of an aluminum cylinder, and rotates at a predetermined process speed that is a peripheral speed. The charging roller 82 comes into contact with the surface of the photosensitive drum 81 and charges the surface of the photosensitive drum 81 to, for example, a uniform dark potential of a negative polarity. After charging the surface of the photosensitive drum 81, an electrostatic latent image is formed on the surface of the photosensitive drum 81 by the laser scanner 43 on the basis of image information. The photosensitive drum 81 rotationally moves while carrying the electrostatic latent image that has been formed, and the electrostatic latent image is developed with toner by the developing unit 20. The detailed configurations of the photosensitive drum 81 and the developing unit 20 will be described later.
The toner image that has been formed is transferred onto the intermediate transfer belt 44b that will be described later through primary transfer. After the primary transfer, the charge on the surface of the photosensitive drum 81 is removed by a pre-exposing portion that is not illustrated. The cleaning blade 84 is disposed in contact with the surface of the photosensitive drum 81, and cleans residual matter such as transfer residual toner remaining on the surface of the photosensitive drum 81 after primary transfer.
The intermediate transfer unit 44 is disposed above the image forming units 80y, 80m, 80c, and 80k. The intermediate transfer unit 44 includes a plurality of rollers and the intermediate transfer belt 44b looped over these rollers. The plurality of rollers include a driving roller 44a, a driven roller, primary transfer rollers 44y, 44m, 44c, and 44k, and so forth. The primary transfer rollers 44y, 44m, 44c, and 44k are respectively disposed to oppose the photosensitive drums 81y, 81m, 81c, and 81k, and are in contact with the intermediate transfer belt 44b.
By applying a positively polarized transfer bias to the intermediate transfer belt 44b by the primary transfer rollers 44y, 44m, 44c, and 44k, negatively polarized toner images on the photosensitive drums 81y, 81m, 81c, and 81k are sequentially transferred onto the intermediate transfer belt 44b so as to be superimposed on one another. In this manner, the toner images obtained by developing the electrostatic latent images on the surfaces of the photosensitive drums 81y, 81m, 81c, and 81k are transferred onto and moved by the intermediate transfer belt 44b.
The secondary transfer portion 45 includes a secondary transfer inner roller 45a and a secondary transfer outer roller 45b. By applying a positively polarized secondary transfer bias to the secondary transfer outer roller 45b, a full-color image formed on the intermediate transfer belt 44b is transferred onto the sheet S. The fixing unit 46 includes a fixing roller 46a and a pressurizing roller 46b. The sheet S is nipped and conveyed between the fixing roller 46a and the pressurizing roller 46b, and thus the toner image transferred onto the sheet S is heated and pressurized to be fixed on the sheet S.
The sheet conveyance portion 11 conveys the sheet S fed from the sheet feeding portion 30 to the sheet discharge portion 12 from the image forming portion 40. The sheet discharge portion 12 supports the sheet S discharged from the sheet conveyance portion 11 through a discharge port 10a in an arrow X direction.
The controller 13 is constituted by a computer, and includes, for example, a central processing unit: CPU, a read-only memory: ROM, a random access memory: RAM, and an input/output circuit. The ROM stores a program for controlling each portion, the RAM temporarily stores data, and a signal is input and output from and to the outside by the input/output circuit. The CPU is a microprocessor that manages overall control of the image forming apparatus 1, and is a main component of a system controller. The CPU is connected to the image reading portion, the sheet feeding portion, the image forming portion 40, the sheet conveyance portion 11, and an operation portion via the input/output circuit, and controls an operation by communicating signals with each portion.
Next, an image forming operation in the image forming apparatus 1 configured as described above will be described below.
In the case where the image forming operation is started, first, the photosensitive drums 81 rotate and the surfaces thereof are charged by the charging rollers 82. Then, laser light is emitted to the photosensitive drums 81 by the laser scanner 43 on the basis of image information, and thus electrostatic latent images are formed on the surfaces of the photosensitive drum 81. Toner attaches to these electrostatic latent images, and thus the electrostatic latent images are developed and visualized as toner images. Then, the toner images are transferred onto the intermediate transfer belt 44b.
Meanwhile, in parallel with this toner image forming operation, the sheet feeding portion operates and the sheet S is conveyed to the secondary transfer portion 45 at a timing matching conveyance of the toner images on the intermediate transfer belt 44b. Then, the toner images are transferred from the intermediate transfer belt 44b onto the sheet S, the sheet S is conveyed to the fixing unit 46, and the unfixed toner images are heated, pressurized, and thus fixed onto the surface of the sheet S. Thereafter, the sheet S is discharged through the discharge port 10a and supported on the sheet discharge portion 12.
Developing Unit
Next, the developing units 20 will be described in detail with reference to
Toner is supplied to the developing container 21 from the toner container 42 filled with toner and illustrated in
The first conveyance screw 22 is disposed, in the developing chamber 21b, approximately parallel to the developing sleeve 24 along the axial direction of the developing sleeve 24, and agitates and conveys the developer in the developing chamber 21b. The second conveyance screw 23 is disposed, in the agitation chamber 21c, approximately parallel to the shaft of the first conveyance screw 22, and conveys the developer in the agitation chamber 21c in a direction opposite to the direction in which the first conveyance screw 22 conveys the developer. That is, the developing chamber 21b and the agitation chamber 21c constitute a circulation path of developer in which the developer is agitated and conveyed. The toner is agitated by the screws 22 and 23, and is thus charged to a negative polarity as a result of being rubbed against the carrier.
The developing sleeve 24 serving as a developer bearing member is rotatably provided to be capable of moving while carrying developer including nonmagnetic toner and magnetic carrier, and thus conveying the developer to a developing region Da opposing the photosensitive drum 81 to develop the electrostatic latent image formed on the photosensitive drum 81. Here, a region where magnetic brushes formed by carrier on the surface of the developing sleeve 24 come into contact with the photosensitive drum 81 is a contact nip, and this contact nip is regarded as the developing region Da in the present exemplary embodiment. That is, the developing region Da is a region where magnetic brushes carried on the developing sleeve 24 come into contact with the photosensitive drum 81.
For example, the developing sleeve 24 has a cylindrical shape with a diameter of 18 mm, and is formed from a nonmagnetic material such as aluminum, nonmagnetic stainless steel, or the like. In the present exemplary embodiment, the developing sleeve 24 is formed from aluminum. In addition, in the present exemplary embodiment, the smallest gap in the developing region Da is about 260 μm. In the present exemplary embodiment, in the developing region Da, the developing sleeve 24 rotates in a rotation direction R1, which is a same rotation direction R2 of the photosensitive drum 81, at a peripheral speed of Vs=491.4 mm/sec. The movement speed of the developing sleeve 24 is faster than the movement speed of the photosensitive drum 81, and the peripheral speed rate thereof with respect to the photosensitive drum 81 is 1.8. That is, Vs/Vdr=1.8 is satisfied. In a developing system in which two-component developer is used, at the time of development, magnetic carrier is bound by a magnetic flux of a magnet roller 24m and carried on the surface of the developing sleeve 24.
The developing sleeve 24 is connected to a direct current power source 28 that applies a direct current voltage as a developing bias voltage. On the surface of the developing sleeve 24, toner charged to a negative polarity is electrostatically bound on the surface of carrier charged to a positive polarity, and thus magnetic brushes are formed. By providing a potential difference between the direct current voltage applied to the developing sleeve 24 and the electrostatic latent image on the photosensitive drum 81, toner is caused to fly toward the photosensitive drum 81, and thus the electrostatic latent image is turned into a visible image.
A developing process on the photosensitive drum 81 with toner in the developing region Da is as follows. First, the photosensitive drum 81 is uniformly charged to a charging potential Vd [V] by the charging roller 82, and then the potential of an image portion is changed to an exposure potential Vl [V] as a result of being exposed by the laser scanner 43. A developing bias in which a direct current voltage and an alternate current voltage are superimposed is normally applied to the developing sleeve 24 to improve the attachment rate of toner to the electrostatic latent image. However, in the present exemplary embodiment, a DC development system in which an alternate current voltage is not applied and only a direct current voltage from a direct current power source is applied is employed. That is, the developing unit 20 applies a direct current voltage to the developing sleeve 24 as a developing bias without using an alternate current voltage, and thus develops the electrostatic latent image formed on the photosensitive drum 81 with toner. In the case where Vdc represents voltage of a direct current component of the developing sleeve 24, an absolute value |Vdc−Vl| of difference from the exposure potential is referred to as Vcont, and this forms an electric field for conveying the toner to an image portion. To be noted, an absolute value |Vdc−Vd| of difference between the direct current voltage Vdc and the charging potential Vd is referred to as Vback, and this forms an electric field for drawing toner back from the photosensitive drum 81 toward the developing sleeve 24. This is provided to suppress a so-called fogging phenomenon in which toner attaches to a non-image portion.
The regulation member 25 is provided in the developing container 21 to oppose a regulation pole N1 of the magnet roller 24m. The regulation member 25 is fixed to the developing container 21 in a state where a predetermined gap is provided between a distal end portion of the regulation member 25 and the developing sleeve 24, and regulates the thickness of a layer of the developer carried on the surface of the developing sleeve 24 by cutting the magnetic brushes. The regulation member 25 is constituted by a nonmagnetic metal plate disposed in an axial direction W of the developing sleeve 24, and the developer passes between the distal end portion of the regulation member 25 and the developing sleeve 24 and is delivered to the developing region Da. The regulation member 25 is constituted by, for example, an aluminum plate. In the present exemplary embodiment, the thickness of the regulation member 25 is set to 1.2 mm.
By adjusting the gap between the distal end portion of the regulation member 25 and the surface of the developing sleeve 24, the amount of developer carried by the developing sleeve 24 and conveyed to the developing region Da is adjusted. In the present exemplary embodiment, the amount of developer coating the developing sleeve 24 per unit area is adjusted to 0.3 mg/mm2=30 mg/cm2. From the viewpoint of graininess of an image, it is preferable that the amount of developer per unit area after passing by the regulation member 25 is set to be within the range of 0.3±0.2 mg/mm2=30±20 mg/cm2. In addition, it is preferable that the gap between the regulation member 25 and the developing sleeve 24 at this time is 0.2 mm or larger. This is because, in the case where the gap between the regulation member 25 and the developing sleeve 24 is small, the gap is likely to be clogged by foreign matter or the like, and this can affect the image.
The magnet roller 24m including a plurality of magnet poles on the surface thereof and unrotatably supported in the developing container 21 is disposed inside the developing sleeve 24. In the present exemplary embodiment, the magnet roller 24m includes a developing pole S2, a regulation pole N1, a conveyance pole N2, a peeling pole S3, and a draw-up pole S1. The developing pole S2 is disposed to oppose the photosensitive drum 81 in the developing region Da. The regulation pole N1 is disposed to oppose the regulation member 25. The draw-up pole S1 is disposed upstream of the regulation pole N1 in the rotation direction R1, and draws up the developer from the developing chamber 21b. The peeling pole S3 is disposed upstream of the draw-up pole S1 in the rotation direction R1, generates a repulsive magnetic field between the peeling pole S3 and the draw-up pole S1, and peels off the developer between the peeling pole S3 and the draw-up pole S1. The conveyance pole N2 is disposed between the developing pole S2 and the peeling pole S3. The magnetic flux densities of the magnetic poles are set in the range of 40 mT to 100 mT.
Groove portions 24a having groove shapes recessed with respect to the surface of the developing sleeve 24 along the axial direction W of the developing sleeve 24 are defined on the surface of the developing sleeve 24 as illustrated in
Therefore, in the present exemplary embodiment, the following configuration is employed. Generally, in the case where periodical density unevenness occurs, the visual sensitivity of a person visually recognizing the density unevenness differs depending on the pitch, and the visual sensitivity tends to drop when the spatial frequency is set to be larger than 1 line/mm. Therefore, in the configuration of the present exemplary embodiment, the pitch of the density unevenness in the visible image to be formed on the photosensitive drum 81 is set to be larger than 1 line/mm. Therefore, the pitch of the groove portions 24a in the movement direction, that is, the rotation direction R1, is set to be within a first range.
In the present exemplary embodiment, the diameter of the developing sleeve 24 is 18 mm. In addition, 72 lines of the groove portions 24a are arranged on the surface of the developing sleeve 24 at substantially equal intervals. The number of these lines is the number of lines present on the same circumference. At this time, a second pitch between adjacent groove portions 24a is Ps=18×3.14/72=0.785 mm. The pitch is calculated by dividing the circumferential length by the number of lines. The developing sleeve 24 rotates at a peripheral speed 1.8 times as fast as the peripheral speed of the photosensitive drum 81. Therefore, in the case where the density unevenness of the period of the adjacent groove portions 24a appears as density unevenness on the photosensitive drum 81, the pitch of the density unevenness is Ps/(Vs/Vdr)=0.785/1.8=0.436 mm. This corresponds to a spatial frequency Fs=1/Ps=about 2.3 lines/mm, which is sufficiently higher than 1 line/mm. That is, the pitch is preferably set such that Ps/(Vs/Vdr)<2 mm is satisfied, that is, Ps<2×(Vs/Vdr) mm is satisfied. More preferably, the pitch is set such that Ps/(Vs/Vdr)<1 mm is satisfied, that is, Ps<Vs/Vdr mm is satisfied. That is, the upper limit of the first range in which the pitch of the groove portions 24a is set is Vs/Vdr mm. Meanwhile, in the case where the pitch is too small, the recesses of the surface become finer and steeper, and thus the groove portions 24a become more likely to be clogged by toner, and thus there is a risk that toner or the like is fused and adheres thereto. Therefore, the lower limit of the first range in which the pitch of the groove portions 24a is set is preferably set to be 0.01 mm or larger, that is, such that Ps≥0.01×(Vs/Vdr) mm is satisfied. As described above, the first range is 0.01×(Vs/Vdr) mm≤Ps<Vs/Vdr mm.
As illustrated in
To be noted, the shape and dimensions of the groove portion 24a of the developing sleeve 24 are of course not limited to the shape and dimensions described above. For example, although the sectional shape of the groove portion 24a has been described as a trapezoidal shape in the present exemplary embodiment, the sectional shape may be another shape such as a V shape. In addition, the width Ws of the groove portion 24a of the present exemplary embodiment refers to the width, or length, in the rotation direction of the developing sleeve 24 of a portion of the developing sleeve 24 where the groove portion 24a is defined. To be noted, there is a case where an edge portion of a groove portion becomes dull to some extent and the width of the groove portion 24a becomes unclear compared with a case where the edge portion has an angle. In this case, considering the gist of the present invention, the dull portion does not have to be included in the width of the groove portion 24a.
Photosensitive Drum
Next, the photosensitive drum 81 will be described in detail. In the present exemplary embodiment, the photosensitive drum 81 is an organic photosensitive member including an organic photosensitive layer including a plurality of layers having separate functions. The photosensitive drum 81 has a layer structure in which a conductive layer, an undercoat layer, a charge generation layer, a charge transfer layer, and a protective layer are laminated in this order from the bottom to the top on the circumferential surface of a support body constituted by an aluminum tube or the like. To be noted, among the layers described above, the layers other than the conductive layer are collectively referred to as a photosensitive layer. The photosensitive drum 81 has a diameter of 30 mm, and rotates in the rotation direction R2 at a process speed, which is a peripheral speed, of Vdr=273 mm/sec.
As illustrated in
There is a possibility that these periodical recess portions 81a of the photosensitive drum 81 also cause the periodical density unevenness to be visible in the image as in the case of the developing sleeve 24 described above. Some can be considered as causes for the occurrence of the density unevenness, and specific one of these is change in the charging potential caused by difference in the way in which the charging roller 82 is in contact with the photosensitive drum 81 between the recess portions 81a and the smooth portion. In addition, there is also a case where the density unevenness occurs due to change in capacitance caused by the difference in the layer thickness of the photosensitive layer of the photosensitive drum 81 between the recess portions 81a and the smooth portion. In the case of the photosensitive drum 81, similarly to the case of the groove portions 24a of the developing sleeve 24 described above, the periodical density unevenness can be made less visible by reducing the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 as much as possible.
The pitch of the recess portions 81a present on the same circumference in the rotation direction R2 that is the movement direction is set to be within a second range. Here, by setting a frequency Fdr=1/Pdr of the recess portions 81a to be higher than 1 line/mm, that is, by setting Pdr to be smaller than 1 mm, the periodical density unevenness can be made less visible, and thus the upper limit of the second range in which the pitch of the recess portions 81a is set is 1 mm. More preferably, the upper limit is 0.2 mm. Meanwhile, in the case where the pitch is too small, the recesses of the surface become finer and steeper, and thus the recesses become more likely to be clogged by toner, and thus there is a risk that toner or the like is fused and adheres thereto. Therefore, the lower limit of the second range in which the pitch of the recess portions 81a is set is preferably set to be 0.01 mm or larger, that is, such that Pdr≥0.01 mm is satisfied. That is, the second range is 0.01 mm≤Pdr<1 mm. More preferably, the second range is 0.01 mm≤Pdr<0.2 mm. In the present exemplary embodiment, for example, Pdr is set to 0.03 mm. In addition, the recess portions 81a are arranged in a grid pattern with respectively predetermined pitches in the rotation direction R1 and the axial direction W. However, the arrangement is not limited to this, and, for example, a recess portion 81a may be further disposed at the center of each square of the arrangement illustrated in
In the case where development is performed on the photosensitive drum 81 provided with the periodical recess portions 81a by using the developing sleeve 24 provided with the periodical groove portions 24a, there is a possibility that beat occurs between the periods of these two, thus a large periodical density unevenness occurs, and the visibility of the density unevenness increases. First, the developing region Da in the case where the pitch Pdr of the recess portions 81a of the photosensitive drum 81 is larger than Ws/(Vs/Vdr) where Ws/(Vs/Vdr) represents the width of a groove portion 24a of the developing sleeve 24 as illustrated in
The present inventors have conducted intensive study on this problem, and have conceived the dimensions and shapes of the groove portions 24a of the developing sleeve 24 and the recess portions 81a of the photosensitive drum 81 to solve the problem described above. First, beat occurs as a result of the print density of the visible image on the photosensitive drum 81 being increased and decreased at certain intervals in the developing region Da due to the groove portions 24a of the developing sleeve 24. In the case where beat occurs, since the frequency of the beat is lower than the frequency of the groove portions 24a of the developing sleeve 24, the frequency of the density unevenness becomes lower than 1 line/mm and the density unevenness becomes more visible. Therefore, in order not to cause the beat, a configuration in which the groove portions 24a of the developing sleeve 24 always contribute to constructive interference is needed. The periodical density unevenness caused by the groove portions 24a of the developing sleeve 24 occurs as a result of the developer concentrating at the groove portions 24a and the density of the part corresponding to the groove portions 24a increasing at the time of release of developer on the downstream side of the developing nip. Beat occurs in the case where the recess portions 81a of the photosensitive drum 81 oppose and overlap this part corresponding to the groove portions 24a. Therefore, in order not to cause the beat, a configuration in which the part corresponding to the groove portions 24a of the developing sleeve 24 always overlap the recess portions 81a of the photosensitive drum 81 may be employed. That is, the groove portions 24a of the developing sleeve 24 may be caused to always oppose the recess portions 81a of the photosensitive drum 81 in the developing region Da while moving the groove portions 24a and the recess portions 81a.
The developing sleeve 24 rotates at a peripheral speed rate of Vs/Vdr=1.8 with respect to the photosensitive drum 81. Therefore, a part on the photosensitive drum 81 corresponding to the groove portions 24a of the developing sleeve 24 is contracted to be 1/1.8 in size. Since the width of a groove portion 24a on the developing sleeve 24 is Ws=135 μm, the width corresponding to the groove portion 24a on the photosensitive drum 81 is Ws/(Vs/Vdr)=(135 μm)/(1.8)=75 μm. For a configuration in which the groove portions 24a always oppose the whole of a recess portion 81a of the photosensitive drum 81, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 may be 75 μm or smaller. That is, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 may be smaller than (width Ws of the groove portion 24a of the developing sleeve 24)/(peripheral speed rate of the developing sleeve 24). By satisfying Pdr<Ws/(Vs/Vdr), that is, Vs/Vdr<Ws/Pdr, the beat can be suppressed.
To be noted, there is a possibility that, depending on the shape of the recess portions 81a, overlapping becomes insufficient in the case where the recess portions 81a and the groove portions 24a oppose one another at end portions thereof. Therefore, in order to make the groove portions 24a always oppose the recess portions 81a not at the end portions thereof, the pitch Pdr of the recess portions 81a on the photosensitive drum 81 in the rotation direction may be smaller than Ws/(VsNdr)/1.5. That is, the pitch Pdr may be set such that Pdr<Ws/(VsNdr)/1.5 is satisfied, in other words, such that (Vs/Vdr)<(Ws/Pdr)/1.5 is satisfied. Further, in the case where the pitch Pdr is set such that Pdr<Ws/(Vs/Vdr)/2 is satisfied, in other words, such that (Vs/Vdr)<(Ws/Pdr)/2 is satisfied, one or more of the recess portions 81a of the photosensitive drum 81 can be made to fully overlap a part corresponding to each of the groove portions 24a of the developing sleeve 24. In this case, the occurrence of the low-frequency beat is more effectively suppressed, and thus the visibility of the periodical density unevenness can be suppressed.
By reducing the peripheral speed rate Vs/Vdr with respect to the photosensitive drum, the relationship represented by the expressions described above are more likely to be satisfied. However, if the peripheral speed rate is reduced too much, there is a possibility that the amount of developer supplied to the photosensitive drum 81 decreases and the development efficiency also decreases. Therefore, the peripheral speed rate Vs/Vdr with respect to the photosensitive drum is preferably set to 1.0 or greater, and more preferably to 1.35 or greater.
The pitch Ps of the groove portions 24a of the developing sleeve 24 is set such that the pitch on the photosensitive drum 81 corresponding to the pitch Ps of the groove portions 24a satisfies the relationship of Ps/(Vs/Vdr)<1 mm, and the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 satisfies Pdr<1 mm. This is a condition that guarantees that the visibility of the density unevenness is low in the first place in a state in which beat does not occur. In addition, in the case where Ps/(Vs/Vdr) is set to be a multiple of Pdr, the strength of constructive interference at all the groove portions 24a becomes substantially equal, and there is a possibility that the constructive interference becomes too strong in some cases. Therefore, the value of Ps/(Vs/Vdr) is preferably set not to be a multiple of Pdr, specifically not an integer multiple of Pdr.
Next, an operation of performing development on the photosensitive drum 81 by the developing unit 20 described above will be described.
As illustrated in
As described above, according to the image forming apparatus 1 of the present exemplary embodiment, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 is set to be smaller than the length Ws/(Vs/Vdr) on the visible image corresponding to the groove portions 24a of the developing sleeve 24. Therefore, the groove portions 24a always oppose the recess portions 81a and contribute to constructive interference to increase the print density, and therefore beat having a pitch equal to or larger than the pitch of the groove portions 24a of the developing sleeve 24 does not occur. As a result of this, in the case of performing development on the photosensitive drum 81 provided with the periodical recess portions 81a by using the developing sleeve 24 provided with the periodical groove portions 24a, the occurrence of density unevenness caused by the periodical beat between the groove portions 24a and the recess portions 81a can be suppressed.
In addition, according to the image forming apparatus 1 of the present exemplary embodiment, the value of Ps/(Vs/Vdr) is set not to be a multiple of Pdr. Therefore, a phenomenon of the constructive interference becoming too strong can be suppressed as compared with a case where the strength of constructive interference is substantially equal at all the groove portions 24a and the constructive interference becomes too strong as in the case where the value of Ps/(Vs/Vdr) is a multiple of Pdr.
In addition, according to the image forming apparatus 1 of the present exemplary embodiment, the DC development system in which only the direct current voltage from the direct current power source 28 is applied to the developing sleeve 24 is employed. Therefore, compared with the case where a developing bias in which an alternate current voltage is superimposed on a direct current voltage is applied, the fly of toner from the developing sleeve 24 to the photosensitive drum 81 can be suppressed, and the density unevenness caused by the periodical beat between the groove portions 24a and the recess portions 81a can be made less visible.
To be noted, although a case where the first recess portions provided on the developing sleeve 24 are the groove portions 24a having groove shapes has been described with regard to the image forming apparatus 1 of the present exemplary embodiment described above, the shape of the first recess portions is not limited to this. For example, recess portions of other shapes such as columnar shapes may be applied as the first recess portions of the developing sleeve 24. In addition, although a case where the second recess portions provided on the photosensitive drum 81 are the recess portions 81a having columnar shapes has been described with regard to the image forming apparatus 1 of the present exemplary embodiment described above, the shape of the second recess portions is not limited to this. For example, recess portions of shapes such as prismatic shapes or other shapes, or groove portions may be applied as the second recess portions of the photosensitive drum 81.
In addition, although the peripheral speed rate with respect to the photosensitive drum is set to 1.8 in the image forming apparatus 1 of the present exemplary embodiment, the peripheral speed rate with respect to the photosensitive drum may be changed as appropriate. Also in this case, the occurrence of the density unevenness caused by the periodical beat between the groove portions 24a and the recess portions 81a can be suppressed by setting the length Ws/(Vs/Vdr) on the visible image corresponding to the groove portions 24a to be smaller than the pitch Pdr of the recess portions 81a.
Next, by using the image forming apparatus 1 of the exemplary embodiment described above, the peripheral speed rate Vs/Vdr with respect to the photosensitive drum and the pitch Pdr of the recess portions 81a of the photosensitive drum 81 were changed. The width Ws and the pitch Ps of the groove portions 24a of the developing sleeve 24 were each set to be constant. In the image forming apparatus 1 satisfying the conditions, formation of an image having an optical density of about 0.6 was performed, and the occurrence of the density unevenness caused by the pitch of the groove portions was evaluated. Here, a state where no density unevenness was present is represented by A, a state where hardly any density unevenness occurred except subtle density unevenness is represented by B, and a state where obvious density unevenness occurred is represented by F. The results are shown in Table 1. As shown in Table 1, the pitch Ps/(Vs/Vdr) on the photosensitive drum 81 corresponding to the pitch Ps of the groove portions 24a of the developing sleeve 24 and the pitch Pdr of the recess portions 81a of the photosensitive drum 81 were both smaller than 1 mm.
In a photosensitive drum 81 of Comparative Example, the recess portions 81a were periodically arranged on the photosensitive drum 81 in the rotation direction R2 with the pitch Pdr set to 0.095 mm. The pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was larger than the width Ws/(Vs/Vdr) on the photosensitive drum 81 corresponding to the width Ws of the groove portions 24a of the developing sleeve 24. Therefore, beat occurred, and a large density unevenness occurred in the circumferential direction and the visibility was degraded.
In Example 1, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was set to 0.06 mm, which was smaller than in Comparative Example. Therefore, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was smaller than the width Ws/(Vs/Vdr) on the photosensitive drum 81 corresponding to the width Ws of the groove portions 24a of the developing sleeve 24. Therefore, the occurrence of beat was suppressed, and an effect better than in Comparative Example was obtained. However, since the pitch Pdr of the recess portions 81a was larger than Ps/(Vs/Vdr)/1.5, overlapping of some of the groove portions 24a and the recess portions 81a was insufficient, and slight density unevenness occurred in some cases.
In Example 2, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was set to 0.049 mm, which was smaller than in Example 1. Therefore, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was smaller than 1/1.5 of the width Ws/(Vs/Vdr) on the photosensitive drum 81 corresponding to the width Ws of the groove portions 24a of the developing sleeve 24, that is, smaller than 0.05 mm. Therefore, the overlapping became more sufficient than in Example 1, and thus the occurrence of beat was more highly suppressed.
In Example 3, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was set to 0.03 mm, which was further smaller than in Example 2. Therefore, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was smaller than ½ of the width Ws/(Vs/Vdr) on the photosensitive drum 81 corresponding to the width Ws of the groove portions 24a of the developing sleeve 24, that is, smaller than 0.0375 mm. Therefore, the overlapping became more sufficient, and thus the occurrence of beat was more highly suppressed.
In Example 4, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was set to 0.045 mm, which was further smaller than in Example 2. Therefore, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was smaller than 1/1.5 of the width Ws/(Vs/Vdr) on the photosensitive drum 81 corresponding to the width Ws of the groove portions 24a of the developing sleeve 24, that is, smaller than 0.05 mm, and thus an effect better than Comparative Example was obtained. However, in contrast with the case of Example 2, slight density unevenness occurred in Example 4. The reason for this is considered to be because Ws/(Vs/Vdr) was a multiple of Pdr in the configuration of Example 4. This is because, in such a case, the strength of constructive interference become substantially equal at all the groove portions 24a, and the constructive interference may become too strong in some cases.
In Example 5, the configuration of the photosensitive drum 81 and the developing sleeve 24 is the same as in Example 1. However, the peripheral speed rate Vs/Vdr of the developing sleeve 24 with respect to the photosensitive drum was lowered to 1.35. Therefore, the pitch Pdr of the recess portions 81a of the photosensitive drum 81 in the rotation direction R2 was smaller than the width Ws/(Vs/Vdr) on the photosensitive drum 81 corresponding to the width Ws of the groove portions 24a of the developing sleeve 24. Therefore, the occurrence of beat was suppressed. In this way, a configuration that can achieve the effect of the present invention can be achieved by not only changing the shape and dimensions of the photosensitive drum 81 and the developing sleeve 24.
According to Examples described above, it has been confirmed that the occurrence of the density unevenness caused by the periodical beat between the groove portions 24a and the recess portion 81a can be suppressed by the image forming apparatus 1 according to the present exemplary embodiment. To be noted, although a case where the same developing sleeve 24 was used in all of Examples described above has been described, the configuration that can achieve the effect of the present invention can be of course also achieved by changing the shape or pitch Ps of the groove portions 24a of the developing sleeve 24.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2017-091716, filed May 2, 2017, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-091716 | May 2017 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 7551878 | Ogaki et al. | Jun 2009 | B2 |
| 7556901 | Anezaki et al. | Jul 2009 | B2 |
| 7622238 | Uematsu et al. | Nov 2009 | B2 |
| 7718331 | Uematsu et al. | May 2010 | B2 |
| 7749667 | Kawahara et al. | Jul 2010 | B2 |
| 9052634 | Furukawa | Jun 2015 | B2 |
| 20080096123 | Shimada et al. | Apr 2008 | A1 |
| 20090170023 | Shimada et al. | Jul 2009 | A1 |
| 20150338780 | Matsumoto | Nov 2015 | A1 |
| Number | Date | Country |
|---|---|---|
| 62-278522 | Dec 1987 | JP |
| 2003-208027 | Jul 2003 | JP |
| 2007-233355 | Sep 2007 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20180321612 A1 | Nov 2018 | US |
