JP4871578B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a printer, a copying machine, and a facsimile machine, which includes a process unit having a drum cleaner and a process unit that does not have a drum cleaner and performs simultaneous development cleaning.

  2. Description of the Related Art Conventionally, image forming apparatuses such as printers, copiers, and facsimiles using an electrophotographic system form a toner image on a transfer material such as paper by each process of charging, exposure, development, transfer, cleaning, and fixing.

  In the above-described cleaning process, the toner remaining on the photosensitive drum (transfer residual toner) in the transfer process is wiped from the photosensitive drum by sliding the cleaning member (for example, a cleaning blade) on the photosensitive drum. For this reason, there has been a problem that the surface of the photosensitive drum is worn to shorten the life of the photosensitive drum.

  As one measure for solving this problem, a cleaner type image forming apparatus has been proposed. In this method, the transfer residual toner on the photosensitive drum is removed and collected from the photosensitive drum by “development simultaneous cleaning” in the developing device and reused. The simultaneous development cleaning is a method in which a transfer residual toner present in a non-image portion on the photosensitive drum that should not be developed with toner is collected in a developing device by a fog removal bias during a subsequent development step. Here, the fog removal bias means a fog removal potential difference Vback which is a potential difference between a DC voltage applied to the developing device and the photosensitive drum surface potential. According to this method, the cleaning device does not rub the surface of the photosensitive drum, which is advantageous for extending the life of the photosensitive drum. An image forming unit using such a simultaneous development cleaning system is provided for four colors of Y (yellow), M (magenta), C (cyan), and K (black). An image forming apparatus that performs full-color image formation has also been proposed.

  However, since there are many K single-color text originals currently used in offices, the photosensitive drum for forming K is particularly quickly consumed compared to other Y, M, and C photosensitive drums. There is a problem that the replacement frequency of the system becomes high.

  Patent Document 1 describes an image forming apparatus including a plurality of photosensitive drums for forming each color of Y, M, C, and K using a simultaneous development cleaning configuration. In particular, in order to extend the life of the K photosensitive drum more than that of the other Y, M, and C photosensitive drums, the peripheral speed difference (speed difference) at the transfer portion between the K photosensitive drum and the transfer medium (intermediate transfer member). A configuration is proposed in which the color is different from other colors. According to this, it is said that when the peripheral speed difference between the photosensitive drum and the transfer medium is provided, the transfer efficiency is improved, but the image carrier is worn and scratched by the relative speed difference. For this reason, for Y, M, and C having a large amount of transfer toner because a plurality of colors are superimposed and transferred, a setting is made to ensure transfer efficiency by providing a peripheral speed difference between the photosensitive drum and the intermediate transfer member. It is said. This sacrifices the life of the photosensitive drum, but the transfer efficiency is guaranteed. On the other hand, K, which is used only for inking when forming a color image, is said to be able to sufficiently achieve transfer efficiency because the amount of transfer toner is small. For this reason, in order to suppress wear of the photosensitive drum due to the peripheral speed difference between the photosensitive drum and the intermediate transfer member, the peripheral speed is made the same to extend the life of the photosensitive drum. With the above configuration, the life of a frequently used photosensitive drum of K is extended, and the replacement frequency equivalent to other Y, M, and C photosensitive drums is achieved.

  Further, Patent Document 2 discloses a configuration in which the cleaner of the K image forming station is arranged to reduce the wear of the photosensitive drum as compared with the cleaners of the other three color image forming stations. Yes.

JP 2002-162804 A JP 2003-162107 A

  However, in recent years, printing with a large amount of K transfer toner, such as photographs, has begun to be required at the same time as a K original document. When such an image is printed, in the configuration of Patent Document 1, sufficient transfer efficiency is not ensured in the transfer portion of K that does not provide a peripheral speed difference. In addition, the Y, M, and C transfer portions having a difference in peripheral speed also employ the simultaneous development cleaning method, which may cause a problem of color change due to color mixing.

  In the configuration of Patent Document 2, although wear of the photosensitive drum of K is reduced, rubbing by the cleaner occurs, so wear still occurs.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus in which an image forming unit in which the image carrier is less worn and an image forming unit in which color mixing is difficult to occur while ensuring sufficient transfer efficiency is provided. To do.

The present invention provides a first developer for developing an electrostatic image on a first image carrier with a first toner to form a first toner image, and collecting the first toner on the first image carrier. A removing unit that contacts the second image carrier and removes the second toner on the second image carrier; and an electrostatic image on the second image carrier from which the second toner has been removed by the removing unit. and a second developing device to form a second toner image by developing the second toner, the first transfer means for transferring the first toner image on said first image bearing member by applying a transfer bias to the transfer medium And a second transfer unit that applies a transfer bias to transfer the second toner image on the second image carrier to the transfer medium, and transfers the image from the second image carrier. The first image bearing member superimposes the first toner image transferred onto the medium to the first toner image. The difference between the moving speed of the first image carrier and the moving speed of the transfer medium is the difference between the moving speed of the second image carrier and the moving speed of the transfer medium. The transfer bias applied to the first transfer unit is smaller than the transfer bias applied to the second transfer unit .

  According to the present invention, since the second image carrier removes the first toner by the removing means, color mixing is unlikely to occur, but the life tends to be reduced due to abrasion caused by rubbing with the removing means. Further, since the first image carrier has no removing means and is not rubbed by the removing means, color mixing tends to occur, but the life can be extended.

  Here, as a method of increasing the transfer efficiency of the toner image from the image carrier to the transfer medium, there are a method of increasing the transfer bias and a method of increasing the speed difference between the two. By the way, when the transfer bias is increased, the toner to be retransferred tends to increase accordingly.

  In view of this, the first image bearing member without the rubbing of the removing unit can reduce the transfer speed difference with the transfer medium by the rubbing of the removing unit in order to suppress an increase in retransfer toner due to an increase in the transfer bias. The transfer efficiency is increased by making it larger than a certain second image carrier.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, what attached | subjected the same code | symbol in the same drawing or a different drawing performs the same structure or the same effect | action, The duplication description is abbreviate | omitted suitably about these.

<Embodiment 1>
FIG. 1 shows an image forming apparatus to which the present invention can be applied. The image forming apparatus shown in the figure is a four-color full-color printer of an electrophotographic system, an intermediate transfer body system, and a tandem system. FIG. 1 is a schematic view corresponding to a longitudinal sectional view of the printer (hereinafter referred to as “image forming apparatus”) viewed from the front side, that is, the side where the user is located when operating the image forming apparatus.

  The configuration and operation of the image forming apparatus will be described with reference to FIG.

  The image forming apparatus shown in FIG. 1 includes four process units (image forming units) Pa, Pb, Pc, and Pd that form images (toner images) of different colors. In this order, the process units Pa, Pb, Pc, and Pd form toner images of yellow (Y), magenta (M), cyan (C), and black (K). The Y, M, C, and K toner images formed on the respective photosensitive drums 1 of the process units Pa, Pb, Pc, and Pd of the respective colors are primary formed with the intermediate transfer belt (transfer medium) 51, respectively. In the transfer portion d (see FIGS. 2 and 4), the images are sequentially transferred onto the intermediate transfer belt 51 and superimposed. The four-color toner images formed on the intermediate transfer belt 51 in this way are collectively secondary-transferred onto the recording material P such as paper at the secondary transfer portion. The recording material P after the secondary transfer of the toner image is fixed on the recording material P by a fixing device (fixing means) 7. Thus, the four-color full-color image formation on one side of one recording material P is completed.

  As shown in FIG. 1, in this embodiment, a K process unit Pd (first image forming unit group) includes a photosensitive drum (first image carrier) 1, a charging roller (charging means) 2, an exposure unit. An apparatus (exposure means) 3 and a developing device (first developing device) 4 are disposed. However, the K process unit Pd is not provided with a dedicated cleaning device (removing means) for cleaning the photosensitive drum 1. That is, the K process unit Pd employs the simultaneous development cleaning method. Note that the toner image formed on the photosensitive drum 1 as the first image carrier is the first toner image.

  In contrast, the Y, M, and C process units (second image forming unit groups) Pa, Pb, and Pc include a photosensitive drum (second image carrier) 1, a charging roller 2, an exposure device 3, and a developing device. A (second developing device) 4 is provided. However, a dedicated cleaning device (removing means) 6 not provided in the K process unit Pd is provided. Note that the toner image formed on the photosensitive drum 1 as the second image carrier is the second toner image. Note that the above-described image forming unit group includes a case where there is one image forming unit.

  The process units for each color will be described in detail with reference to FIGS. FIG. 2 shows the K process unit Pd. FIG. 3 is a longitudinal sectional view showing the layer structure of the photosensitive drum 1 and the charging roller 2 of each process unit Pa, Pb, Pc, Pd. FIG. 4 shows Y, M, and C process units Pa, Pb, and Pc.

(1) Photosensitive drum In this embodiment, a rotating drum type electrophotographic photosensitive member (photosensitive drum) 1 is used as an image carrier. The photosensitive drum 1 has an organic photoconductor (OPC) having a negative charging characteristic as a photosensitive layer. As shown in the layer configuration model diagram of FIG. 3, the photosensitive drum 1 has an undercoat layer 1b in order from the inner side (lower side in FIG. 3) on the surface of a conductive drum base 1a formed by an aluminum cylinder. The photocharge generation layer 1c and the charge transport layer 1d are overlaid on top of each other. Here, the undercoat layer 1b is for suppressing light interference and improving the adhesion of the upper layer. The OPC is composed of a photocharge generation layer 1c and a charge transport layer 1d. The photosensitive drum 1 has an outer diameter of 30 mm, and is driven to rotate in the direction of the arrow (counterclockwise) in FIGS. 2 and 4 with a process speed (circumferential speed) of 100 mm / sec around the center support shaft.

(2) Charging roller The charging means uniformly charges the surface of the photosensitive drum 1 with a predetermined polarity and potential. In this embodiment, the charging roller 2 is used.

  The charging roller 2 has a length in the longitudinal direction of 320 mm. The charging roller 2 has a three-layer structure in which a lower layer 2b, an intermediate layer 2c, and a surface layer 2d are laminated in order from the inner side on the outer periphery of a core metal (support member) 2a, as shown in the layer configuration model diagram of FIG. is there. The lower layer 2b is a foamed sponge layer for reducing charging noise, and the intermediate layer 2c is a conductive layer for obtaining a uniform resistance as the entire charging roller. The surface layer 2d is a protective layer provided to prevent leakage when the photosensitive drum 1 has a defect such as a pinhole.

  As shown in FIG. 3, the charging roller 2 holds both ends of the cored bar 2a rotatably by bearing members (not shown). The bearing members are urged toward the photosensitive drum by the pressing springs 2e. Accordingly, the charging roller 2 is brought into pressure contact with the surface of the photosensitive drum 1 with a predetermined pressing force, and is rotated by the rotation of the photosensitive drum 1. A pressure contact portion between the photosensitive drum 1 and the charging roller 2 is a charging portion (charging nip portion) a.

  A charging bias voltage of a predetermined condition is applied to the cored bar 2a of the charging roller 2 from the power source S1. As a result, the peripheral surface of the photosensitive drum 1 is contact-charged to a predetermined polarity / potential. In the present embodiment, the charging bias voltage for the charging roller 2 is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac).

  Specifically, the DC voltage is -600V. The AC voltage is a sine wave having a frequency f of 1000 Hz and a peak-to-peak voltage Vpp of 1400V. An oscillating voltage in which the DC voltage and the AC voltage are superimposed is applied. As a result, the surface of the photosensitive drum 1 is contact-charged uniformly at −600 V (dark potential Vd).

  More specifically, the specification of the charging roller 2 of the present embodiment is as follows.

Core 2a: Stainless steel round bar with a diameter of 6 mm Lower layer 2b: Carbon dispersion foamed EPDM, specific gravity 0.5 g / cm ³ , volume resistivity 10 ² to 10 ⁹ Ω · cm, layer thickness 3.0 mm, length 320 mm
Intermediate layer 2c: carbon-dispersed NBR rubber, volume resistance value 10 ² to 10 ⁶ Ω · cm, layer thickness 700 μm,
Surface layer 2d: tin oxide and carbon dispersed in resin resin of fluorine compound, volume resistance value 10 ⁷ to 10 ¹⁰ Ω · cm, surface roughness (JIS standard 10-point average surface roughness Ra) 1.5 μm, layer thickness 10 μm

  As shown in FIG. 3, a charging roller cleaning member 2 f formed of a flexible cleaning film is in contact with the surface of the charging roller 2. The charging roller cleaning member 2f is arranged in parallel to the longitudinal direction of the charging roller 2 and is fixed at one end by a support member 2g that reciprocates a certain amount in the longitudinal direction. The charging roller cleaning member 2f is disposed so as to form a contact nip with the charging roller 2 on the surface near the free end side. The support member 2g is reciprocated by a certain amount in the longitudinal direction through a gear train by a drive motor (not shown) of the printer, and the surface of the charging roller 2 is slid by the charging roller cleaning member 2f. As a result, contaminants on the surface of the charging roller 2 (fine toner, external additives, etc.) are removed.

(3) Exposure Apparatus As shown in FIGS. 2 and 4, the exposure apparatus 3 is used as information writing means. In the present embodiment, a laser beam scanner using a semiconductor laser is used as the exposure apparatus 3. The exposure device 3 outputs a laser beam L that is modulated in accordance with an image signal sent from a host device such as an image reading device (not shown) to the image forming apparatus side. As a result, the surface of the photosensitive drum 1 that has been charged while rotating is subjected to laser scanning exposure (image exposure) at the exposure position b. An electrostatic latent image corresponding to image information is sequentially formed on the surface of the photosensitive drum 1 as the potential of the laser light irradiation portion is lowered by this laser scanning exposure.

(4) Developer As shown in FIGS. 2 and 4, a developer 4 as a developing unit is disposed on the downstream side of the exposure position b along the rotation direction of the photosensitive drum 1. The developing device 4 supplies a developer (toner) to the electrostatic latent image on the photosensitive drum 1 to visualize the electrostatic latent image. In the present embodiment, a two-component magnetic brush developing system is used. Reversal development.

  The developing device 4 includes a developing container 4a and a nonmagnetic developing sleeve 4b. The developing sleeve 4b is rotatably disposed in the developing container 4a with a part of its outer peripheral surface exposed to the outside. The developing device 4 includes a fixed magnet roller 4c inserted in the developing sleeve 4b, and a developer coating that regulates the layer thickness of the two-component developer 4e restrained on the surface of the developing sleeve 4b by the magnet roller 4c. It has a blade 4d. Further, a developer agitating member 4f disposed at the bottom of the developing container 4a and a toner hopper 4g containing replenishing toner for replenishing the developing container 4a are provided.

The two-component developer 4e in the developing container 4a is a mixture of toner and a magnetic carrier and is stirred by the developer stirring member 4f. In this embodiment, the magnetic carrier has a resistance of about 10 ¹³ Ω · cm and a particle size of about 40 μm. The toner is triboelectrically charged to a negative polarity by rubbing against the magnetic carrier.

  The developing sleeve 4b is disposed opposite to the photosensitive drum 1 so that the closest distance (S-Dgap) to the photosensitive drum 1 is 350 μm. A facing portion between the photosensitive drum 1 and the developing sleeve 4a is a developing portion c. The developing sleeve 4b is rotationally driven so that the moving direction of the surface thereof is opposite to the moving direction of the surface of the photosensitive drum 1 in the developing unit c. A part of the two-component developer 4e in the developing container 4a is attracted and held on the surface of the developing sleeve 4b as a magnetic brush layer by the magnetic force of the magnet roller 4c. Then, it is conveyed along with the rotation of the developing sleeve 4b, and is layered into a predetermined thin layer by the developer coating blade 4d. The thin two-component developer 4e is rubbed against the surface of the photosensitive drum 1 at the developing portion c. At this time, a predetermined developing bias is applied to the developing sleeve 4b from the power source S2. In the present embodiment, the developing bias voltage for the developing sleeve 4b is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, it is an oscillating voltage in which a DC voltage of −450 V and an AC voltage of a peak-to-peak voltage of 1600 V are superimposed.

  As described above, the toner in the developer coated as a thin layer on the surface of the rotating developing sleeve 4b and conveyed to the developing unit c corresponds to the electrostatic latent image on the surface of the photosensitive drum 1 by the electric field due to the developing bias. Selectively adhere. As a result, the electrostatic latent image becomes a toner image developed with toner. In the present embodiment, the toner adheres to the exposed bright portion on the surface of the photosensitive drum 1 and the electrostatic latent image is reversely developed.

  The thin layer of developer on the developing sleeve 4b that has passed through the developing section c is returned to the developer reservoir in the developing container 4a as the developing sleeve 4b continues to rotate.

  In order to maintain the toner concentration of the two-component developer 4e in the developing container 4a within a predetermined substantially constant range, the toner concentration of the two-component developer 4e in the developing container 4a is, for example, an optical toner concentration sensor (not shown). Detected by. The toner hopper 4g is driven and controlled in accordance with the detected information, and the toner in the toner hopper 4g is supplied to the two-component developer 4e in the developing container 4a. The toner supplied to the two-component developer 4e is stirred by the stirring member 4f.

(5) The image forming apparatus shown in the intermediate transfer unit Figure 1, an intermediate transfer unit 5. The intermediate transfer body unit 5 is disposed below each photosensitive drum 1. The intermediate transfer unit 5 includes an intermediate transfer belt 51 and four primary transfer rollers 53 (first transfer means and second transfer means) , an intermediate transfer belt drive roller 55, a secondary transfer counter roller 56 , a secondary transfer roller 57, A tension roller 59 and an intermediate transfer belt cleaner 60 are provided.

The intermediate transfer belt 51 is made of a dielectric made of synthetic resin. In this embodiment, a PI resin having a volume resistivity of 10 ⁹ Ω · cm (using a probe conforming to the JIS-K6911 method, applied voltage of 100 V, application time of 60 sec, 23 ° C., 60% RH) and thickness t = 90 μm is employed. The material and thickness of the intermediate transfer belt are not limited to those described above as long as the intermediate transfer belt has an intrinsic volume resistance of 10 ⁸ to 10 ¹² Ω · cm when 100 V is applied.

  The primary transfer roller 53 is formed by covering the outer peripheral surface of a core metal having a diameter of 8 mm with a cylindrical conductive urethane sponge layer having a thickness of 4 mm. The resistance value was measured by applying a voltage of 100 V to the metal core while rotating the primary transfer roller 53 to a grounded metal roller with a load of 500 g and rotating at a peripheral speed of 50 mm / sec. The value is about 105Ω (23 ° C., 60% RH).

  The toner images of the respective colors formed on the photosensitive drums 1 of the process units Pa, Pb, Pc, and Pd are transferred by the transfer bias applied from the power source S3 to the transfer roller 53 in the respective transfer portions (primary transfer portions) d. Then, the images are sequentially primary transferred onto the intermediate transfer belt 51.

(6) Fixing Device The fixing device 7 includes a fixing roller 71 that is rotatably arranged and a pressure roller 72 that rotates while being pressed against the fixing roller 71. A heater 73 such as a halogen lamp is disposed inside the fixing roller 71, and the surface temperature of the fixing roller 71 is adjusted by controlling the voltage to the heater 73.

(7) Toner Charge Amount Control Unit As shown in FIG. 2, in this embodiment, the K (black) process unit Pd includes a cleaning device for removing the transfer residual toner on the surface of the photosensitive drum 1. Not. A so-called cleanerless system is adopted. For this reason, in this embodiment, the first toner (developer) charge amount control means is used in order to make the charging polarity of the transfer residual toner equal to the negative polarity which is the normal polarity and efficiently collect in the developing device 4. 7 and second toner charge amount control means 8 are provided. These are disposed downstream of the primary transfer portion d along the rotational direction of the photosensitive drum 1 and upstream of the charging portion a, and the first toner charge amount control means 7 includes a second toner charge amount. It is arranged upstream of the control means 8.

In the present embodiment, the first toner charge amount control means 7 and the second toner charge amount control means 8 are brush-like members having an actual resistance of 10 ⁵ to 10 ⁷ Ω and are brushes in the rotation direction of the photosensitive drum 1. The width is set to 5 mm, the brush length is set to 4 mm, and they are arranged in contact with the surface of the photosensitive drum 1. The first toner charge amount control means 7 is applied with a positive voltage from the power source S4, and the second toner charge amount control means 8 is applied with a negative voltage from the power source S5.

  As shown in FIG. 2, the first toner charge amount control means 7 and the second toner charge amount control means 8 are in contact with the surface of the photosensitive drum 1 at contact portions e and f, respectively. The transfer residual toner remaining on the surface of the photosensitive drum 1 after the transfer of the toner image includes those having both positive and negative polarities. Among these, the first toner charge amount control means 1 temporarily collects the toner charged mainly in the negative polarity by the positive voltage applied from the power source S4. The collected negative toner is charged to a positive polarity by a positive voltage applied to the toner charge amount control means 7, and then gradually separated and attached to the surface of the photosensitive drum 1 and conveyed. The On the other hand, the toner having the positive polarity immediately after passing through the primary transfer portion d passes through without being collected by the first toner charge amount control means 7. The positively charged toner conveyed downstream from the contact portion e of the first toner charge amount control means 7 is temporarily caused by the negative voltage applied to the second toner charge amount control means 8. To be collected. Then, the toner charge amount control means 7 becomes negative due to the negative voltage applied from the power source 5, and then gradually separates and adheres to the surface of the photosensitive drum 1 and is conveyed. In this way, the transfer residual toner whose charging polarity is set to the negative polarity that is the normal polarity is conveyed to the developing unit b without being attached to the charging roller 2 applied to the negative polarity located downstream, and in the developing unit 4 It is collected at the time of development and reused.

(8) Cleaning Device On the other hand, the process units Pa, Pb, and Pc that form toner images of three colors Y, M, and C are provided with a cleaning device 6 as shown in FIG. Adhered matters such as transfer residual toner remaining on the surface of the photosensitive drum 1 after the transfer are removed. The cleaning device 6 has a cleaning blade 61 and a conveying screw 62. The cleaning blade 61 is in contact with the photosensitive drum 1 by a pressurizing unit (not shown) at a predetermined angle and pressure, and collects toner and the like remaining on the surface of the photosensitive drum 1. The collected transfer residual toner and the like are transported by the transport screw 62 and discharged.

  In the above-described image forming apparatus shown in FIG. 1, the four color toner images superimposed on the intermediate transfer belt 51 are conveyed to the secondary transfer portion h (see FIG. 1) as the intermediate transfer belt 51 rotates. On the other hand, by this time, the recording material P taken out from the primary paper feed cassette 80 is transported by the transport rollers 82 and 83 through the pickup roller 81 and further transported along the pre-transfer transport guide 84 to be subjected to the secondary transfer. Supplied to part h. At this time, a secondary transfer bias is applied between the secondary transfer counter roller 56 and the secondary transfer roller 57. As a result, the four color toner images on the intermediate transfer belt 51 are secondarily transferred onto the recording material P all at once. During the secondary transfer, the transfer residual toner that is not transferred onto the recording material P and remains on the intermediate transfer belt 51 is removed and collected by the intermediate transfer belt cleaner 60.

  The recording material P onto which the four-color toner images have been secondarily transferred is separated from the secondary transfer portion h, and is conveyed along the post-transfer conveyance guide 85 to the fixing device 7. When the recording material P passes between the fixing roller 71 and the pressure roller 72, the recording material P is pressurized and heated from both the front and back surfaces at a substantially constant pressure and temperature, whereby an unfixed toner image on the surface of the recording material P is obtained. Is melted and fixed. Thereby, the four-color full-color image formation on one side of one recording material P is completed.

  In the following, the image forming apparatus of the present embodiment uses a process unit Pd that employs simultaneous development cleaning and a process unit Pa, Pb, and Pc that includes a cleaning device 6 for cleaning the photosensitive drum 1. Describe the reason.

(1) Reason for providing the cleaning device 6 in the process units Pa, Pb, Pc of Y, M, C In the multiple transfer image forming apparatus as in the present embodiment, a phenomenon called “retransfer” occurs. There is. FIG. 5 is a schematic diagram for explaining the retransfer phenomenon. Here, the photosensitive drum 1 of the Y process unit Pa is the photosensitive drum 1a, and the transfer roller 53 is the transfer roller 53a. The photosensitive drum 1 of the M process unit Pb is referred to as a photosensitive drum 1b, and the transfer roller 53 is referred to as a transfer roller 53b.

  The toner image formed on the photosensitive drum 1 of the Y process unit Pa is transferred onto the intermediate transfer belt 51 by a transfer bias applied to the transfer roller 53. A part of the toner image is transferred again from the intermediate transfer belt 51 onto the photosensitive drum 1 when passing through the primary transfer portion d of the downstream M process unit Pb. This is retransfer. FIG. 6 shows the relationship between the transfer voltage (transfer bias) V applied to the transfer roller 53 of the M process unit Pb and the transfer efficiency of the M toner image and the retransfer efficiency of the Y toner image at that time. Here, for the M toner image, the transfer efficiency when the M toner image formed on the photosensitive drum 1 is transferred to the intermediate transfer belt 51 is shown. The Y toner image shows the retransfer rate when the Y toner image on the intermediate transfer belt 51 is retransferred to the photosensitive drum 1 without the toner image. According to FIG. 6, as the transfer voltage V is increased, the transfer efficiency of the M toner image increases. However, the retransfer of the Y toner image starts from a voltage at which the transfer efficiency of M is not maximized. End up. In general, the transfer voltage is set to a voltage at which transfer efficiency is sufficiently ensured. Therefore, the transfer voltage is set to a voltage indicated by a broken line in FIG. 6, and as a result, retransfer occurs. If this phenomenon occurs in a process unit that employs simultaneous development cleaning, color mixing will occur. That is, the Y toner is collected in the M or C developing container, and the M or C color changes. This color mixing phenomenon is more affected by Y, M, and C having higher lightness than K having low lightness.

  Therefore, in the present embodiment, the cleaning device 6 for the photosensitive drum 1 is provided in the process units Pa, Pb, and Pc of Y, M, and C where color mixture is not allowed. The Y process unit Pa, which is the first color, does not necessarily require the cleaning device 6 because there is no color mixing due to retransfer. Here, the same configuration is employed for the purpose of sharing the process unit with the other M and C process units Pb and Pc. Thus, since the retransfer toner generated in the process units Pa, Pb, and Pc of Y, M, and C is collected and discharged by the cleaning device 6, the mixed color in the process units of these process units Pa, Pb, and Pc The problem is solved.

(3) Reason for Adopting Simultaneous Development Cleaning in K Process Unit Pd According to the study of the present inventors, the amount of wear of the photosensitive drum 1 by the cleaning device 6 for the photosensitive drum 1 is the travel distance of the surface of the photosensitive drum 1. In addition, when (AC + DC) charging by the charging roller 2 is performed, the amount of wear deteriorates to about 10 times. If the four colors Y, M, C, and K are provided with the cleaning device 6 for the photosensitive drum 1, and when forming an image only with the K process unit Pd, the photosensitive drums for all Y, M, C, and K are provided. It is assumed that the Y, M, and C photosensitive drums 1 are not charged by the charging roller 2 while rotating 1. At this time, the wear amount of the Y, M, and C photosensitive drums 1 is about 1/10 that of the K photosensitive drum 1, and only the K photosensitive drum 1 is rapidly worn. For example, if the wear amount of the photosensitive drum 1 for K when a predetermined number of sheets are printed is 10 μm and the color image ratio is 10%, the photosensitive drum 1 for Y, M, and C is The amount of wear is approximately (10 μm × 10%) + (10 μm × 1/10 × 90%) = 1.9 μm. That is, the replacement frequency of the K photosensitive drum 1 is about five times that of Y, M, and C.

  Therefore, the K process unit Pd in the present embodiment employs the simultaneous development cleaning method without providing a cleaning device. This is for preventing the photosensitive drum 1 from being worn by the cleaning device and extending the life of the photosensitive drum 1. With this configuration, the wear amount of the photosensitive drum 1 of K is suppressed to 1/3 to 1/10 or less when the cleaning device for the photosensitive drum 1 is provided. Therefore, even when K single-color image formation is performed about 10 times as often as full-color image formation, the replacement frequency of the K photosensitive drum 1 is almost the same as that of the other Y, M, and C photosensitive drums 1. The frequency of replacement can be reduced.

  However, according to this simultaneous development cleaning method, there is a problem that the retransfer toner is mixed into the developing device 4. As described above, K is less susceptible to color mixing than Y, M, and C, but according to the inventor's study, this was not in an acceptable range. Therefore, in the present embodiment, in the primary transfer portion d of the K process unit Pd, a transfer speed is improved by providing a peripheral speed difference (speed difference) between the photosensitive drum 1 and the intermediate transfer belt 51, thereby transferring the transfer voltage. By reducing the setting, a configuration that minimizes retransfer is adopted.

  FIG. 7 shows the transfer efficiency and retransfer rate in the primary transfer portion d of the K process unit Pd of the present embodiment. The horizontal axis indicates the transfer voltage applied to the transfer roller 53 of the K process unit Pd. The transfer efficiency when the K toner image formed on the K photosensitive drum 1 is transferred to the intermediate transfer belt 51 at each transfer voltage is shown on the left vertical axis. In addition, the vertical axis on the right indicates the rate at which images of other colors on the intermediate transfer belt 51 are retransferred to the photosensitive drum 1 of the K process unit Pd. In the figure, the transfer efficiency indicated by the solid line indicates that there is no difference in peripheral speed between the photosensitive drum 1 and the intermediate transfer belt 51 (in reality, the peripheral speed difference is about ± 0. 5% or less). On the other hand, the transfer efficiency indicated by the alternate long and short dash line is a value when a difference in peripheral speed of 1.5% is provided between the photosensitive drum 1 and the intermediate transfer belt 51 (here, the intermediate transfer belt is faster). According to this, the transfer efficiency is improved by providing the peripheral speed difference, and the transfer voltage can be set lower, so that retransfer can be reduced. On the other hand, there is no change in the retransfer rate due to the peripheral speed difference. Therefore, in the K process unit Pd of the present embodiment, a peripheral speed difference of 1.5% is provided between the photosensitive drum 1 and the intermediate transfer belt 51, and retransfer is suppressed while maintaining transfer efficiency. As a result, the effect of color mixing has been improved to a level where there is no problem.

  With the above configuration, in the K process unit Pd, the photosensitive drum 1 can achieve a longer life than the Y, M, and C photosensitive drums 1, and color mixing in the developing device 4 can be minimized. .

  On the other hand, the process units Pa, Pb, and Pc of Y, M, and C can ensure transferability by setting a high transfer bias because there is no concern of color mixing due to retransfer. For this reason, the setting is made such that there is no difference in peripheral speed at the primary transfer portion d, and the setting is made such that the driving torque of the intermediate transfer belt 51 is minimized.

  In the Y, M, and C process units Pa, Pb, and Pc of the present embodiment, the configuration provided with the cleaning device 6 for the photosensitive drum 1 is compared to the configuration for simultaneous cleaning of all-color development in Patent Document 1 described above. Therefore, the life of the photosensitive drum tends to be shortened. Further, in the K process unit Pd of the present embodiment, a configuration in which a peripheral speed difference is provided instead of providing the cleaning device for the photosensitive drum 1 is the same as that of the K process unit of Patent Document 1 with the simultaneous development cleaning and peripheral speed. The life of the photosensitive drum 1 tends to be shorter than the configuration in which no difference is provided. For this reason, the replacement frequency of the photosensitive drums 1 for each color increases.

  However, the configuration of the present embodiment does not cause poor transfer or color mixing due to retransfer even when an image with a large amount of toner such as a photograph is printed, and the frequently used K and other Y, M, and C photosensitivities. The drum 1 is made to have the same replacement frequency.

  In the present embodiment, the image forming apparatus using the intermediate transfer method has been described. However, the present invention can be applied to an image forming apparatus using a direct transfer method using a recording material conveyance belt or the like. is there. Further, the physical property value such as the absolute value of the peripheral speed difference, the resistance value of each member, the bias setting value, and the like are not limited to the above-described numerical values.

  In the above description, in the process units Pa, Pb, and Pc of Y, M, and C, the example in which the peripheral speed difference is not provided between the photosensitive drum 1 and the intermediate transfer belt 51 has been described, but the peripheral speed difference is provided. It may be. By providing the peripheral speed difference, if the transfer voltage is the same, the transfer efficiency can be increased as compared with the case where the peripheral speed difference is not provided. As a result, the toner remaining after transfer is reduced, so that there is no risk of color mixing even when the contact pressure of the cleaning blade 61 of the cleaning device 6 with respect to the photosensitive drum 1 is lowered. In addition, since the transfer efficiency can be increased by providing the peripheral speed difference, the transfer voltage can be reduced to that extent, and the damage to the photosensitive drum due to the application of the transfer voltage can be reduced.

  Here, as described above, the Y, M, and C process units Pa, Pb, and Pc are provided with the cleaning device 6 unlike the K process unit Pd. For this reason, retransfer increases as the transfer voltage is increased for the purpose of increasing transfer efficiency. However, since the retransferred toner is removed by the cleaning device 6, there is no possibility of color mixing. Therefore, as described above, also in the Y, M, and C process units Pa, Pb, and Pc, when a peripheral speed difference is provided between the photosensitive drum 1 and the intermediate transfer belt 51, the peripheral speed difference is K. It may be set smaller than the peripheral speed difference in the process unit Pd. In other words, the peripheral speed difference in the Y, M, and C process units Pa, Pb, and Pc need not be as large as the peripheral speed difference in the K process unit Pd.

1 is a longitudinal sectional view schematically showing a schematic configuration of an image forming apparatus according to a first embodiment. It is a figure explaining the structure of the process unit without a cleaning apparatus. It is a figure explaining the layer structure of a photosensitive drum and a charging roller. It is a figure explaining the structure of the process unit which has a cleaning apparatus. FIG. 6 is a diagram illustrating toner retransfer. FIG. 6 is a diagram illustrating a relationship between a transfer voltage, a transfer efficiency, and a retransfer rate when there is no peripheral speed difference between the photosensitive drum and the intermediate transfer belt. FIG. 6 is a diagram illustrating a relationship between a transfer voltage, a transfer efficiency, and a retransfer rate when a peripheral speed difference is provided between the photosensitive drum and the intermediate transfer belt.

Explanation of symbols

1 Black photosensitive drum (first image carrier)
1 Yellow, magenta and cyan photosensitive drums (second image carrier)
4 Black developer (first developer)
4 Yellow, magenta, and cyan developers (second developer)
5 Intermediate transfer unit 6 Cleaning device (removing means)
51 Intermediate transfer belt (transfer medium, intermediate transfer member)
53 Black transfer roller (first transfer means)
53 Yellow, magenta and cyan transfer rollers (second transfer means)
P Recording material (transfer medium)
Pa yellow process unit (second image forming unit group)
Pb Magenta process unit (second image forming unit group)
Pc cyan process unit (second image forming unit group)
Pd black process unit (first image forming unit group)

Claims (3)

An electrostatic image on the first image carrier is developed with a first toner to form a first toner image, and a first developer for collecting the first toner on the first image carrier, and a second image A removing unit that contacts the carrier and removes the second toner on the second image carrier, and an electrostatic image on the second image carrier from which the second toner has been removed by the removing unit is a second toner. A second developing unit that forms a second toner image by developing the first toner image, a first transfer unit that applies a transfer bias to transfer the first toner image on the first image carrier to a transfer medium, and a transfer bias. An image forming apparatus comprising: a second transfer unit configured to transfer the second toner image on the second image carrier to the transfer medium by applying
The first toner image can be transferred from the first image carrier in a superimposed manner on the second toner image transferred from the second image carrier to the transfer medium;
The difference between the moving speed of the first image carrier and the moving speed of the transfer medium is larger than the difference between the moving speed of the second image carrier and the moving speed of the transfer medium ,
A transfer bias applied to the first transfer unit is smaller than a transfer bias applied to the second transfer unit;
An image forming apparatus. The brightness of the first toner is lower than the brightness of the second toner;
The image forming apparatus according to claim 1. The moving speed of the first image carrier and the moving speed of the transfer medium are different from each other.
The image forming apparatus according to claim 1, wherein:

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