Image forming apparatus

Abstract

To suppress image defects in a simple constitution in which a common transfer bias is applied to transfer portions of a plurality of image forming portions.SOLUTION: An image forming apparatus 100 includes, in storage means 112 storing information correlating with a charge amount of toner accommodated in each of first and second accommodating portions 45Y and 45M, when the information on the toner accommodated in the first accommodating portion 45Y of a first image forming portion SY is changed from first information to second information and the information on the toner accommodated in the second accommodating portion 45M of a second image forming portion SM is not changed, control means 101 configured to control to change a transfer bias of supply applied to first and second transfer portions N1Y and N1M from a first transfer bias to a second transfer bias and change a potential of an image portion formed on a second photoreceptor 1M of the second image forming portion SM from a first potential to a second potential.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, such as a printer, a copying machine, or a facsimile machine, that uses an electrophotographic method. [Background technology]

[0002] In an electrophotographic image forming apparatus, the surface of a photoconductor is uniformly charged by a charging means, and then exposed by an exposure means to form an electrostatic latent image on the photoconductor, which is then developed by a developing means with toner as a developer to form an image. In the case of a tandem-type image forming apparatus such as a full-color image forming apparatus using an intermediate transfer method, the toner images formed on the photoconductors of the image forming units of each color are primarily transferred to an intermediate transfer body by a primary transfer means, superimposed, and then secondarily transferred to a recording material by a secondary transfer means. As the primary transfer means, for example, a primary transfer member disposed in the vicinity of a position facing the photoconductor across the intermediate transfer body is used, and a predetermined primary transfer bias is applied to this primary transfer member to primarily transfer the toner image onto the intermediate transfer body. For simplicity, the term "primary transfer" may be simply referred to as "transfer".

[0003] In the transfer process, if the potential difference (hereinafter also referred to as "transfer contrast") between the photoconductor carrying the toner and the transfer member is insufficient, the transfer efficiency decreases, and image defects such as missing images may occur. Here, the transfer contrast required to avoid a decrease in transfer efficiency varies depending on the charge amount of the toner, etc. For example, when the charge amount of the toner is high, if the transfer bias is not set high so as to increase the transfer contrast, the transfer efficiency decreases, and image defects such as missing images may occur.

[0004] Patent Documents 1 and 2 propose a method of detecting the charge amount of toner and controlling the transfer bias so that the transfer efficiency does not decrease even if the charge amount of toner changes. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 5-134561 [Patent Document 2] Japanese Patent Application Publication No. 6-130768 Summary of the Invention [Problem to be solved by the invention]

[0006] However, a tandem type image forming apparatus having a plurality of image forming units has the following problems.

[0007] In a tandem type image forming apparatus having multiple image forming units, a common (same) voltage power supply is sometimes used for the transfer bias of each color in order to reduce the size and cost of the apparatus. In such a configuration, the same transfer bias is applied to all colors.

[0008] Here, the minimum value of transfer contrast required to avoid reducing transfer efficiency differs for each color due to the difference in the charge amount of each color toner caused by the difference in the usage conditions of each color toner. Therefore, it is conceivable to set a large transfer bias common to all colors so that the transfer contrast of each color is equal to or greater than the minimum value of the required transfer contrast. However, if the transfer contrast is large, "transfer scattering," a phenomenon in which the toner image scatters during transfer and the image becomes blurred, may worsen, resulting in a decrease in image quality. Conversely, if the transfer bias common to all colors is lowered to improve transfer scattering, image defects may occur due to "decrease in transfer efficiency" in colors whose transfer contrast is smaller than the required transfer contrast.

[0009] As described above, in a tandem type image forming apparatus having multiple image forming units, when a transfer power supply is shared among the multiple image forming units, it is difficult to simultaneously suppress image defects without reducing the transfer efficiency of each color and improve image quality by improving transfer scattering.

[0010] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to suppress image defects in a simple configuration in which a common transfer bias is applied to the transfer sections of a plurality of image forming stations. [Means for solving the problem]

[0011] The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides a first image forming section including a rotatable first photoconductor, a first charging means for charging the surface of the first photoconductor, and a first developing means having a first container for containing toner and supplying the toner to the surface of the first photoconductor to form a toner image, a second image forming section including a rotatable second photoconductor, a second charging means for charging the surface of the second photoconductor, and a second developing means having a second container for containing toner and supplying the toner to the surface of the second photoconductor to form a toner image, an exposure means for exposing the charged surfaces of the first and second photoconductors, respectively, to light, thereby forming an image section for forming the toner image on the surfaces of the first and second photoconductors, respectively, and a light exposure means for exposing the charged surfaces of the first and second photoconductors to light, thereby forming an image section for forming the toner image on the surfaces of the first and second photoconductors, respectively. The image forming apparatus comprises an application means for applying a common transfer bias to each transfer section where the toner image is transferred from the first storage section to a transfer recipient, a memory means for storing information correlating with the charge amount of the toner contained in the first and second storage sections, respectively, and a control means for controlling the application means and the exposure means, wherein the control means controls the transfer bias from the first transfer bias to the second transfer bias and changes the potential of the image section formed on the second photosensitive body from the first potential to the second potential when the information of the toner contained in the first storage section is changed from first information to second information in the memory means and the information of the toner contained in the second storage section remains unchanged.

[0012] According to another aspect of the present invention, there is provided a first process cartridge including an apparatus main body, a rotatable first photoconductor, a first charging means for charging the surface of the first photoconductor, a first developing means having a first container for containing toner and supplying the toner to the surface of the first photoconductor to form a toner image, and a memory for storing information correlating with the charge amount of the toner contained in the first container, a second process cartridge including a rotatable second photoconductor, a second charging means for charging the surface of the second photoconductor, and a second developing means having a second container for containing toner and supplying the toner to the surface of the second photoconductor to form a toner image, and an image forming unit for forming the toner image on the surfaces of the first and second photoconductors by exposing the charged surfaces of the first and second photoconductors to light. The image forming apparatus includes an exposure means, an application means for applying a common transfer bias to each transfer section where the toner image is transferred from the first and second photosensitive bodies to a transfer medium, and a control means for controlling the application means and the exposure means, wherein the first process cartridge is configured to be detachably attached to the apparatus main body, and the control means controls the transfer bias from a first transfer bias to a second transfer bias and changes the potential of the image section formed on the second photosensitive body from a first potential to a second potential when the first process cartridge attached to the apparatus main body is replaced with a first process cartridge different from the first process cartridge and the information of the toner contained in the first storage section is changed from first information to second information. Effect of the Invention

[0013] According to the present invention, image defects can be suppressed in a simple configuration in which a common transfer bias is applied to the transfer sections of a plurality of image forming stations. [Brief description of the drawings]

[0014] [Figure 1] 1 is a schematic cross-sectional view showing a schematic configuration of an image forming apparatus. [Diagram 2]FIG. 2 is a schematic block diagram illustrating a control configuration of the image forming apparatus. [Diagram 3] FIG. 4 is a flow chart showing a procedure of potential control in the embodiment. [Figure 4] FIG. 11 is a graph showing the relationship between transfer contrast (Vt-Vl) and transfer efficiency. [Diagram 5] 1 is a graph showing the relationship between the charge amount of toner and the minimum value T of the required transfer contrast. [Figure 6] FIG. 4 is a graph showing a change in charge amount depending on a toner usage state. [Figure 7] 5A to 5C are schematic diagrams showing examples of potential control results of each image forming station in the embodiment. [Figure 8] FIG. 2 is a schematic diagram for explaining the positions of members around the photosensitive drum. [Figure 9] FIG. 11 is a schematic cross-sectional view showing a schematic configuration of another example of an image forming apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0016] {Example 1} <Outline of image forming device> The overall configuration and image forming operation of the image forming apparatus 100 of this embodiment will be described. Fig. 1 is a schematic cross-sectional view showing the general configuration of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is a tandem type (in-line type) printer that employs an intermediate transfer method and is capable of forming full-color images using an electrophotographic method.

[0017] The image forming apparatus 100 has imaging stations SY, SM, SC, and SK, which are image forming units that form images of the colors yellow (Y), magenta (M), cyan (C), and black (K). These four image forming units SY, SM, SC, and SK are arranged in a row along the moving direction of the image transfer surface of the intermediate transfer belt 53 described later. Elements having the same or corresponding functions or configurations provided for each color may be generally described by omitting the Y, M, C, and K at the end of the reference numerals indicating that the elements are for any one of the colors. The configurations of the imaging stations S are substantially the same except that the colors of the toner 90 contained in the respective developing devices 4 are different. In this embodiment, the imaging stations S mainly include a photosensitive drum 1, a charging roller 2, an exposure device 3, a developing device 4, and a primary transfer roller 51. The exposure device 3 is configured as one unit that exposes the four photosensitive drums 1. In this embodiment, in each image forming station S, the photosensitive drum 1 and the charging roller 2 and developing device 4 acting as process means are integrated into a process cartridge 8. The process cartridge 8 is detachably mounted to the main body 10 of the image forming apparatus 100 (a portion of the image forming apparatus 100 excluding the process cartridge 8).

[0018] The photosensitive drum 1, which is a drum-shaped (cylindrical) photosensitive body (electrophotographic photosensitive body) serving as an image carrier, is rotated around its axis in the direction of the arrow R1 in the figure (counterclockwise direction). In this embodiment, the photosensitive drum 1 is rotated at a peripheral speed (process speed) of 100 mm / sec. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative polarity in this embodiment) by a charging roller 2, which is a roller-shaped charging member serving as a charging means. In this embodiment, the charging roller 2 is a conductive roller having a conductive elastic layer on a core metal, and is arranged in contact with the surface of the photosensitive drum 1 with a predetermined pressure. The charging roller 2 is rotated in accordance with the rotation of the photosensitive drum 1. During charging, a predetermined charging bias (charging voltage), which is a DC voltage of the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment), is applied to the charging roller 2 by a charging power source Ec (FIG. 2) serving as a charging voltage application means. As a result, a discharge occurs between the charging roller 2 and the photosensitive drum 1, and the surface of the photosensitive drum 1 is charged to a predetermined dark area potential (charging potential) Vd. In this embodiment, a charging bias of DC -1200V is applied to the charging roller 2. In this embodiment, the dark area potential Vd of the surface of the photosensitive drum 1 after charging is set to -600V. The surface of the charged photosensitive drum 1 is scanned and exposed to a laser beam modulated according to an image signal by an exposure device (scanner unit) 3 as an exposure means, and the potential of the exposed area of ​​the surface of the photosensitive drum 1 is attenuated to a light area potential Vl. As a result, an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1.

[0019] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the developing device 4 as a developing means by supplying toner 90 as a developer, and a toner image (developer image, toner image) is formed on the photosensitive drum 1. In this embodiment, the developing device 4 is a contact development type reversal developing device that uses a non-magnetic one-component developer (toner 90) with a normal charging polarity (charging polarity for developing the electrostatic latent image) of negative polarity as the developer. The developing device 4 has a developer storage section (developing container) 45 that stores the toner 90, a developing roller 42 as a developing member (developer carrier), a toner supply roller 43 as a supply member, and a regulating blade 44 as a regulating member. The developing roller 42 is configured by providing an elastic rubber layer on a core metal, and is arranged in contact with or close to the photosensitive drum 1. The developing roller 42 is driven to rotate in the direction of the arrow in the figure (clockwise direction). The toner 90 contained in the developer container 45 is supplied to the developing roller 42 by the toner supply roller 43, and is held on the developing roller 42 in a thin layer state by the regulating blade 44. The toner 90 carried by the rotating developing roller 42 and conveyed to the opposing portion (contact portion) with the photosensitive drum 1 adheres to the image portion of the electrostatic latent image on the photosensitive drum 1. During development, a predetermined developing bias (developing voltage) which is a DC voltage of the same polarity (negative polarity in this embodiment) as the normal charging polarity of the toner 90 is applied to the developing roller 42 by a developing power source Ed (FIG. 2) as a developing voltage application means. As a result, an electrostatic force acts on the toner 90 due to the potential difference between the developing bias and the potential (light area potential) Vl of the exposed portion of the surface of the photosensitive drum 1, and the electrostatic latent image is developed. In this embodiment, DC-350V is applied to the developing roller 42 as the developing bias. Thus, in this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 1 (negative polarity in this embodiment) adheres to the exposed portion (image portion) on the photosensitive drum 1, the absolute value of the potential of which has been reduced by exposure after being uniformly charged. Note that the potential of the image portion on the surface of the photosensitive drum 1 is the potential from the exposure position to before it first reaches the transfer position.

[0020] An intermediate transfer belt 53, which is an endless belt serving as an intermediate transfer body, is disposed so as to face the four photosensitive drums 1. The intermediate transfer belt 53 is stretched by a drive roller 54, a tension roller 55, and a secondary transfer counter roller 56 serving as a plurality of tension rollers (support rollers). The intermediate transfer belt 53 rotates (moves orbitally, moves circulatingly) in the direction of arrow R2 (clockwise direction) in the figure at a circumferential speed corresponding to the circumferential speed of the photosensitive drums 1, by transmitting a driving force as the drive roller 54 is driven to rotate. The tension roller 55 applies a predetermined tension to the intermediate transfer belt 53. The secondary transfer counter roller (secondary transfer inner roller) 56 functions as a counter member (counter electrode) of the secondary transfer roller 52 described later. On the inner peripheral surface side of the intermediate transfer belt 53, primary transfer rollers 51Y, 51M, 51C, and 51K, which are roller-shaped primary transfer members serving as primary transfer means, are disposed corresponding to the four photosensitive drums 1Y, 1M, 1C, and 1K, respectively. In this embodiment, the primary transfer roller 51 is disposed facing the photosensitive drum 1 via the intermediate transfer belt 53. The primary transfer roller 51 is pressed toward the photosensitive drum 1 and comes into contact with the photosensitive drum 1 via the intermediate transfer belt 53 to form a primary transfer portion (primary transfer nip portion) N1 which is the contact portion between the photosensitive drum 1 and the intermediate transfer belt 53. The tension rollers other than the drive roller 54 and each primary transfer roller 51 are rotated in accordance with the rotation of the intermediate transfer belt 53.

[0021] The toner image formed on the photosensitive drum 1 is electrostatically transferred onto the rotating intermediate transfer belt 53 as a transfer target by the action of the primary transfer roller 51 at the primary transfer portion N1. During the primary transfer, a predetermined primary transfer bias (primary transfer voltage) Vt, which is a DC voltage of the polarity opposite to the normal charging polarity of the toner 90 (positive polarity in this embodiment), is applied to the primary transfer roller 51 by a primary transfer power source Ep (FIG. 2) as a primary transfer voltage application means. At this time, the transfer contrast (Vt-Vl), which is the potential difference between the primary transfer bias and the potential (light potential) Vl of the exposed portion of the surface of the photosensitive drum 1, is set to a transfer bias Vt that is a value on the side of the polarity opposite to the normal charging polarity of the toner 90 (positive polarity in this embodiment). As a result, an electric field proportional to the transfer contrast (Vt-Vl) is formed between the photosensitive drum 1 and the intermediate transfer belt 53, and the toner image is transferred from the photosensitive drum 1 to the intermediate transfer belt 53 by the electrostatic force acting on the toner 90. In this embodiment, in order to reduce the size and cost of the device, a common (same) voltage power supply is used as a primary transfer power supply Ep that applies a primary transfer bias Vt to the primary transfer roller 51 of each image forming station S. Therefore, the same transfer bias Vt is applied to the primary transfer roller 51 of each image forming station S. For example, when a full-color image is formed, the toner images of the colors Y, M, C, and K formed on each photosensitive drum 1 are transferred onto the intermediate transfer belt 53 in succession, superimposed on each other.

[0022] A secondary transfer roller (secondary transfer outer roller) 52, which is a roller-shaped secondary transfer member serving as a secondary transfer means, is disposed at a position facing the secondary transfer opposing roller 56 across the intermediate transfer belt 53 on the outer peripheral surface side of the intermediate transfer belt 53. The secondary transfer roller 52 is pressed toward the secondary transfer opposing roller 56 and contacts the secondary transfer opposing roller 56 across the intermediate transfer belt 53, forming a secondary transfer portion (secondary transfer nip portion) N2, which is a contact portion between the intermediate transfer belt 53 and the secondary transfer roller 52. The secondary transfer roller 52 may be driven to rotate, or may be driven to rotate in accordance with the rotation of the intermediate transfer belt 53. The toner image formed on the intermediate transfer belt 53 is transferred, by the action of the secondary transfer roller 52, onto a recording material P, which is a transfer target material being conveyed while being sandwiched between the intermediate transfer belt 53 and the secondary transfer roller 52, in the secondary transfer portion N2. During the secondary transfer, a predetermined secondary transfer bias (secondary transfer voltage), which is a DC voltage of the polarity opposite to the normal charging polarity of the toner 90 (positive polarity in this embodiment), is applied to the secondary transfer roller 52 by a secondary transfer power source Es (FIG. 2) as a secondary transfer voltage application means. The secondary transfer opposing roller 56 is electrically grounded. It is also possible to apply a secondary transfer bias of the same polarity as the normal charging polarity of the toner 90 to an inner roller corresponding to the secondary transfer opposing roller 56 in this embodiment, and to electrically ground an outer roller corresponding to the secondary transfer roller 52 in this embodiment. A recording material (transfer material, recording medium, sheet) P such as paper is stored in a recording material cassette 11 as a recording material storage section, and is fed from the recording material cassette 11 one sheet at a time by a feed roller 12 as a feeding member. This recording material P is conveyed to the secondary transfer section N2 by a conveying roller 13 as a conveying member in synchronization with the toner image on the intermediate transfer belt 53. Then, as the recording material P passes through the secondary transfer portion N2, the toner image is transferred from the intermediate transfer belt 53 to the recording material P. The recording material P is not limited to paper, and may be made of a material other than paper or a material containing a material other than paper, such as synthetic paper or film made of a material mainly containing synthetic resin, or metal-deposited paper (specialty paper) having a metal layer.

[0023] The recording material P onto which the toner image has been transferred is transported to a fixing device 6 serving as a fixing means. The fixing device 6 applies pressure and heat to the recording material P carrying the unfixed toner image while transporting it with a heating roller and a pressure roller, thereby fixing (melting and bonding) the toner image onto the recording material P. The recording material P onto which the toner image has been fixed is discharged (output) as an image-formed product onto a discharge tray 14 serving as a discharge section provided at the top of the device body 10 of the image forming apparatus 100 in the drawing.

[0024] Further, a belt cleaning device 7 as an intermediate transfer body cleaning means is disposed on the outer peripheral surface side of the intermediate transfer belt 53. The belt cleaning device 7 is disposed downstream of the secondary transfer portion N2 and upstream of the primary transfer portion N1 (the most upstream primary transfer portion N1Y) in the rotation direction (the direction of movement of the surface) of the intermediate transfer belt 53. Adherents such as toner 90 remaining on the intermediate transfer belt 53 after the secondary transfer are removed from the intermediate transfer belt 53 and collected by the belt cleaning device 7.

[0025] In this embodiment, the image forming apparatus 100 employs an image carrier cleanerless system in which each imaging station S is not provided with a dedicated cleaning device for cleaning the photosensitive drum 1. Here, as shown in FIG. 8(a), the position where the charging roller 2 charges the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 is defined as a charging position (charging portion) I1. The charging roller 2 charges the photosensitive drum 1 by discharging generated in at least one of minute gaps between the photosensitive drum 1 and the charging roller 2 formed on the upstream side and downstream side of the contact portion between the photosensitive drum 1 and the charging roller 2 in the rotation direction of the photosensitive drum 1. However, the contact portion between the photosensitive drum 1 and the charging roller 2 may be regarded as the charging position I1. Also, the position where the exposure device 3 exposes the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 is defined as an exposure position I2. Also, the position where the developing device 4 supplies toner to the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 (in this embodiment, the opposing position (contact position) between the photosensitive drum 1 and the developing roller 42) is defined as the developing position (developing portion) I3. Also, the position where the toner image is transferred from the photosensitive drum 1 to the intermediate transfer belt 53 in the rotation direction of the photosensitive drum 1 (in this embodiment, the opposing position between the photosensitive drum 1 and the primary transfer roller 51 corresponding to the above-mentioned primary transfer portion N1) is defined as the transfer position I4. In this embodiment, there is no member that contacts the surface of the photosensitive drum 1 before the surface of the photosensitive drum 1 that has passed the transfer position I4 reaches the charging position I1. That is, in the rotation direction of the photosensitive drum 1, the charging position I1 is located downstream of the transfer position I4 and upstream of the exposure position I2 and the developing position I3. As a result, it is possible to collect the toner 90 that remains on the photosensitive drum 1 without being transferred to the intermediate transfer belt 53 at the developing position I3 by the developing roller 42. That is, at the development position I3, if residual toner 90 is present in a non-image area on the photosensitive drum 1, due to the electrostatic force between the photosensitive drum 1 and the development roller 42, this toner 90 adheres to the development roller 42 from the photosensitive drum 1 and is collected and returned to the developer storage section 45 of the development device 4.

[0026] In this embodiment, the image forming apparatus 100 is described as having a cleanerless configuration, but the present invention is not limited to this embodiment. As shown in Fig. 8(b), the image forming apparatus 100 may be provided with a cleaning device 30 having a cleaning member 31 that removes toner 90 from the surface of the photosensitive drum 1. The cleaning member 31 may be a cleaning blade or the like that is disposed in contact with the photosensitive drum 1 so as to remove the toner 90 from the surface of the photosensitive drum 1 at a cleaning position I5 between the transfer position I4 and the charging position I1 in the rotation direction of the photosensitive drum 1.

[0027] FIG. 2 is a schematic block diagram showing the control configuration of the image forming apparatus 100 of this embodiment. Only one process cartridge 8 and one primary transfer roller 51 are shown as representatives. The control unit 101 as a control unit has a CPU 111 as an arithmetic control unit, a memory 112 such as a ROM, RAM, or nonvolatile memory as a storage unit, and an input / output unit (not shown) that controls the exchange of information (signals) between the control unit 101 and an external device. The CPU 111 executes a predetermined arithmetic process. The ROM of the memory 112 stores a predetermined control program, table data, threshold values, and the like. The RAM of the memory 112 temporarily stores data used for control. The nonvolatile memory of the memory 112 stores the usage history of each unit, and the like. The control unit 101 controls each unit of the image forming apparatus 100 using the information stored in the RAM or nonvolatile memory as appropriate according to the control program stored in the ROM. The control unit 101 receives image information and print instructions transmitted from an external device (not shown) such as a host computer, and controls the image forming operation of the image forming apparatus 100. That is, various operations of the image forming operation described here are controlled by the control unit 101.

[0028] To the control unit 101, for example, a conveying roller drive motor 15 for driving the conveying roller 13 for conveying the recording material P, a photosensitive member drive motor 16 for driving the photosensitive drum 1, a developing unit motor 17 for driving the developing roller 42 and the toner supply roller 43 of the developing device 4, an intermediate transfer member drive motor 18 for driving the drive roller 54 of the intermediate transfer belt 53, a secondary transfer roller drive motor 19 for driving the secondary transfer roller 52, a fixing drive motor 20 for driving the pressure roller of the fixing device 6, and the like are connected. In addition, when the primary transfer roller 51 is driven to rotate, the primary transfer roller drive motor may be provided. In addition, when the secondary transfer roller 52 is driven to rotate, the secondary transfer roller drive motor 19 may not be provided. The above-mentioned various rollers and the like are rotated by the driving force transmitted from the above-mentioned various drive motors (power sources) provided in the device main body 10. In addition, all or part of the above-mentioned drive motors may be shared. The control unit 101 is also connected to a charging power supply Ec, a developing power supply Ed, a primary transfer power supply Ep, a secondary transfer power supply Es, and the like. The charging roller 2, the developing roller 42, the primary transfer roller 51, the secondary transfer roller 52, and the like are each applied with a predetermined voltage from the various power supplies provided in the apparatus main body 10. As described above, in this embodiment, the primary transfer power supply Ep that applies a primary transfer bias Vt to the primary transfer roller 51 of each image forming station S is shared. The charging power supply Ec and the developing power supply Ed may each be shared by a plurality of image forming stations S, or may be provided for each image forming station S.

[0029] <How to set the transfer bias> In this embodiment, the transfer contrast (Vt-Vl) is set so as not to reduce the transfer efficiency. This is because a reduction in the transfer efficiency leads to image defects such as missing images. Furthermore, in a cleanerless configuration, a reduction in the transfer efficiency leads to a ghost image due to poor charging, an image with uneven shading in the image area (printed area) due to the adhesion of residual toner after transfer to the charging roller 2, and background fogging in the non-image area (non-printed area). Below, a basic method for setting the transfer contrast (Vt-Vl) and the transfer bias Vt will be described.

[0030] FIG. 4 is a graph showing the relationship between the transfer contrast (Vt-Vl) and the transfer efficiency when the toner 90 is used under the same conditions. The transfer efficiency is expressed as a ratio (percentage) of the weight of the toner 90 transferred onto the intermediate transfer belt 53 divided by the weight of the toner 90 on the photosensitive drum 1 before the transfer. The use state of the toner 90 is a state that correlates with the charge amount of the toner 90 in the developer container 45, which changes in correlation with the amount (number of times, etc.) of the image forming operation performed using the toner 90 contained in the developer container 45. For simplicity, the change in the use state of the toner 90 that correlates with the charge amount of the toner 90 in the developer container 45 with an increase in the amount of the image forming operation is expressed as "the use state progresses." The charge amount of the toner 90 is expressed as the charge amount μC / g (which may be an average value) per unit weight of the toner 90 in the developer container 45. The charge amount of the toner can generally be calculated by measuring the charge per unit weight (mass) by a suction method. This is a method of measuring the weight (g) and charge (μC) of the sucked toner to measure the charge (μC / g), and it is possible to grasp the average charge of the toner. For convenience, the level of charge of the toner refers to the level when compared in absolute values, unless otherwise specified.

[0031] 4, when the transfer contrast (Vt-Vl) falls below a predetermined value, the electrostatic force acting on the toner 90 decreases, and the transfer efficiency decreases. Therefore, in order to prevent the transfer efficiency from decreasing, the transfer contrast (Vt-Vl) is increased from a predetermined minimum value T toward the opposite polarity (positive polarity) from the normal charging polarity of the toner 90.

[0032] Also, the transfer efficiency relative to the transfer contrast (Vt-Vl) may differ depending on the type of image. Figure 4 shows the relationship between the transfer contrast (Vt-Vl) and the transfer efficiency when a solid black image is formed and when an image in which 1 mm wide lines are arranged at 2 mm intervals in the longitudinal direction of the photosensitive drum 1 is formed. Line images are more difficult to transfer than solid black images. This is because the amount of toner 90 developed at the edge parts of the image increases due to the electric field formed by the electrostatic latent image at the edge parts of the image, which is called sweeping.

[0033] In this embodiment, for example, in the case of using toner 90 showing the relationship between transfer contrast (Vt-Vl) and transfer efficiency in Fig. 4, the transfer contrast (Vt-Vl) is set as follows: In other words, in order to make the transfer efficiency 99% or more regardless of the type of image, the transfer contrast (Vt-Vl) is set to a value that is greater than 300V, which is the minimum value T required for a line image, and is on the opposite polarity (positive polarity) side to the normal charging polarity of the toner 90.

[0034] The minimum value T of the transfer contrast (Vt-Vl) required to avoid such a decrease in transfer efficiency varies depending on the charge amount of the toner 90. Fig. 5 is a graph showing the relationship between the charge amount of the toner 90 and the minimum value T of the transfer contrast required to avoid a decrease in transfer efficiency. When the charge amount of the toner 90 is high, the toner 90 has a high electrostatic adhesion to the photosensitive drum 1 and is therefore difficult to transfer, so the transfer contrast (Vt-Vl) needs to be increased.

[0035] Here, as the toner 90 is used more and more by repeating image forming operations, the charge amount of the toner 90 decreases. One of the reasons for this is that the external additives added to the toner 90 may be embedded in the surface of the toner 90 or peeled off due to friction between members such as the developing roller 42 and the regulating blade 44. As an index representing the use state of the toner 90, for example, the cumulative number of images formed counted from the beginning of use of the developing device 4 of each image forming station S (the toner 90 is in the early stage of use). As another index, for example, the cumulative rotation distance of the developing roller 42 may be used to eliminate the influence of the length of one image. FIG. 6 is a graph showing the relationship between the cumulative number of images formed and the charge amount (-μC / g) of the toner 90 in this embodiment. It can be seen that the charge amount of the toner 90 decreases as the cumulative number of images formed increases between 0 and 1000 sheets.

[0036] 5 and 6, when the cumulative number of image formation sheets is small and the toner 90 is not used much, the charge amount of the toner 90 is large, and therefore the minimum value T of the transfer contrast required to prevent a decrease in transfer efficiency is large. Therefore, it is necessary to set the potential Vl of the exposed portion of the surface of the photosensitive drum 1 to a larger polarity (negative polarity) that is the same as the normal charge polarity of the toner 90, or to set the transfer bias Vt to a larger polarity (positive polarity) opposite to the normal charge polarity of the toner 90.

[0037] However, if the potential Vl is set to a large negative polarity, the potential difference between the potential Vl and the developing bias becomes small, and this may result in a decrease in the development efficiency and a decrease in the image density. This is particularly likely to occur when the toner 90 is not in a state of use yet and the toner has a large charge amount. Therefore, in this embodiment, the potential Vl is set not to be larger than the potential Vlm=-100V on the negative polarity side so that the image density does not decrease even when the toner 90 is in an early state of use. Therefore, in this embodiment, when the toner 90 is not in a state of use yet, the transfer bias Vt is set to a large positive polarity so as not to decrease the transfer efficiency. A specific setting method will be described later.

[0038] <Electric potential control method> Next, a method for controlling the transfer bias Vt and the potential Vl in this embodiment will be described. Fig. 3 is a flow chart showing an outline of the procedure for controlling the transfer bias Vt and the potential Vl (potential control) in this embodiment. In this embodiment, the control unit 101 controls the transfer bias Vt common to each image forming station S and the potential Vl of each image forming station S according to the flow of Fig. 3.

[0039] First, the control unit 101 checks the usage state of the toner 90 in each image forming station S before performing an image forming operation (S101). The usage state of the toner 90 can be checked based on the cumulative number of images formed by the developing device 4 and the cumulative rotation distance of the developing roller 42. In this embodiment, the control unit 101 checks the usage state of the toner 90 based on the cumulative number of images formed by the developing device 4. The greater the cumulative number of images formed by the developing device 4, the more advanced the usage state of the toner 90 can be determined. In this embodiment, the control unit 101 accumulates the cumulative number of images formed by the developing device 4 in each image forming operation, and updates and stores information related to the cumulative number of images formed in the non-volatile memory of the memory 112 as a usage state storage unit. In addition, when the developing device 4 is replaced with a new one by replacing the process cartridge 8, or when the developing device 4 is replenished with toner 90, the control unit 101 resets the cumulative number of images formed (usage state of the toner 90) related to the developing device 4 to a predetermined state. In this embodiment, the developing device 4 is replaced with a new one by replacing the process cartridge 8, and when the replacement is performed, the cumulative number of images formed (the usage status of the toner 90) is reset to zero as a specified state (initial value).

[0040] Here, the control unit 101 can detect that the process cartridge 8 has been replaced based on the detection result of a sensor (not shown) provided in the apparatus main body 10. Also, as shown in FIG. 9, the process cartridge 8 can be provided with a memory (non-volatile memory) 113, and the apparatus main body 10 can be provided with a reading unit (not shown) for reading information from the memory 113. In this case, the control unit 101 can detect that the process cartridge 8 has been replaced based on the result of reading by the reading unit information indicating whether the process cartridge 8 is new or not, which is stored in the memory 113. For example, the control unit 101 can detect that the process cartridge 8 has been replaced based on the fact that the memory 113 stores information indicating that the process cartridge 8 is new (unused). Alternatively, the control unit 101 can detect that the process cartridge 8 has been replaced based on the fact that the memory 113 does not store information indicating that the process cartridge 8 has already been used. In addition, the control unit 101 may be configured to detect that the process cartridge 8 has been replaced based on a signal input from an operation unit 21 (Figure 2) provided in the apparatus main body 102 or an external device in response to an operation by an operator.

[0041] As shown in FIG. 9, when the process cartridge 8 is provided with a memory 113, the memory 113 may store information regarding the cumulative number of images formed for the process cartridge 8. In this case, the control unit 101 can recognize the cumulative number of images formed based on the information regarding the cumulative number of images formed, and can determine whether the process cartridge 8 is new or not. In addition, the usage state of the toner 90 when the toner 90 is replenished may be considered to be the same as replacement if it is assumed that there is no toner 90 remaining in the developing device 4 at the time of replenishment. In addition, if there is toner 90 remaining in the developing device 4 at the time of replenishment, for example, the usage state of the toner 90 can be updated so as to return (subtract the cumulative number of images formed) based on the ratio of the newly replenished toner 90 to the toner 90 in the developing device 4 after replenishment. The amount of the toner 90 usage state to be returned can be preset based on the change in the charge amount of the toner 90 in the developing device 4 due to the replenishment of the toner 90.

[0042] Next, the control unit 101 compares the usage state of the toner 90 in each imaging station S, and determines the "imaging station Sm" in which the toner 90 is least used among all the imaging stations S (S102). In this embodiment, the imaging station S in which the developing device 4 has formed the smallest cumulative number of images is determined to be the imaging station Sm in which the toner 90 is least used. Note that there may be multiple imaging stations Sm in which the toner 90 usage state is the same (within a predetermined range) as the imaging station Sm in which the toner 90 usage state is least used.

[0043] Further, the control unit 101 estimates the charge amount of the toner 90 of each image forming station S from the cumulative number of images formed at each image forming station S based on the relationship between the cumulative number of images formed at each image forming station S and the charge amount of the toner 90 (FIG. 6) obtained in advance. Then, the control unit 101 estimates the minimum value T of the required transfer contrast (Vt-Vl) at each image forming station S from the estimated charge amount of the toner 90 at each image forming station S based on the relationship between the charge amount of the toner 90 and the minimum value T of the required transfer contrast obtained in advance (FIG. 5) (S106). In this embodiment, the processes of S105 and S106 are performed in parallel with the process of S102. Here, the processes of S105 and S106 may be performed before the process of S102, or the processes of S105 and S106 may be performed after the process of S102. The information on the relationship between the cumulative number of image formation sheets and the charge amount of the toner 90, and the information on the relationship between the charge amount of the toner 90 and the minimum value T of the required transfer contrast are stored in advance in the ROM of the memory 112 as table data or the like.

[0044] Next, the control unit 101 determines the transfer bias Vt to be applied during image formation (S103, S104).

[0045] Here, the transfer bias Vt is set so that the transfer contrast (Vt-Vl) is equal to or greater than the minimum value T of the transfer contrast required to prevent a decrease in transfer efficiency in all image forming stations S. As described above, in this embodiment, when the toner 90 is not yet in use, the transfer bias Vt is set to a large positive polarity.

[0046] However, if the transfer contrast (Vt-Vl) is large, the transfer scattering, a phenomenon in which the toner image scatters during transfer and the image becomes blurred, may worsen, resulting in a decrease in image quality. The transfer scattering is aggravated by discharge that occurs in a minute gap between the photosensitive drum 1 and the intermediate transfer belt 53, before the position where the photosensitive drum 1 and the intermediate transfer belt 53 come into contact with each other and the toner 90 is transferred. Therefore, if the transfer contrast (Vt-Vl) is large, discharge is likely to occur, and the transfer scattering may worsen. Therefore, it is desirable not to set the transfer bias Vt to a value that is greater than necessary on the positive polarity side.

[0047] Taking these factors into consideration, in this embodiment, the following settings are made for the image forming station Sm, which has the least amount of toner 90 used among the image forming stations S. First, the control unit 101 sets the potential Vl to Vlm=-100V, which is the minimum value required to avoid reducing the development efficiency described above (S103). Then, the control unit 101 sets the transfer bias Vt based on the potential Vlm and the minimum value Tm of the required transfer contrast (Vt-Vl) for that image forming station Sm calculated in S106 (S104).

[0048] The calculation method of the set value of the transfer bias Vt is as follows. First, based on the previously obtained relationship between the cumulative number of images formed and the charge amount of the toner 90 (FIG. 6), the charge amount of the toner 90 is estimated from the usage state of the toner 90 in which the usage state of the toner 90 is the least advanced. Then, as expressed by the following formula 1, the transfer bias Vt is determined to a value that matches the minimum value Tm of the required transfer contrast (Vt-Vl) estimated from the previously obtained relationship between the charge amount of the toner 90 and the minimum value T of the required transfer contrast (FIG. 5). That is, based on the previously obtained relationship between the charge amount of the toner 90 and the minimum value T of the required transfer contrast (FIG. 5), the minimum value Tm of the required transfer contrast (Vt-Vl) of the image forming station Sm in which the usage state of the toner 90 is the least advanced is estimated. Then, based on the following formula 1, the transfer bias Vt is determined from the potential Vlm (-100V) and the estimated minimum value Tm of the required transfer contrast (Vt-Vl). Vt=Tm+Vlm...Equation 1

[0049] For example, if there is even one image forming station S where the toner 90 is in an early state of use, the transfer contrast (Vt-Vl) required at that image forming station S is estimated to be 320 V or more from the relationship in Figures 5 and 6. In this case, the transfer bias Vt is set to 220 V (= 320 V + (-100 V)) without being made larger than necessary.

[0050] Also, for example, when the usage state of the toner 90 in all the image forming stations S has progressed to a state where the cumulative number of image formation sheets is 1000 or more, it is estimated from the relationship in Figures 5 and 6 that the transfer contrast (Vt-Vl) required in all the image forming stations S is 300 V or more. In this case, the transfer bias Vt is set to 200 V (= 300 V + (-100 V)) without being made larger than necessary.

[0051] By doing this, in all image-forming stations S, the transfer contrast (Vt-Vl) required to avoid reducing the transfer efficiency can be set, while the transfer bias Vt can be set not to be larger than necessary so as to prevent the transfer scattering from worsening.

[0052] Next, the control unit 101 determines the potential Vl of the exposed portion of the surface of the photosensitive drum 1 during image formation in each of the imaging stations S (S107, S108). The potential Vl of the imaging station Sm in which the toner 90 is least used is determined to be Vlm as described above, so here, the potential Vl of the other imaging stations S is determined. The potential Vl is adjusted by adjusting the light emission intensity of the exposure device 3. In this embodiment, the light emission intensity of the exposure device 3 is set based on the relationship between the light emission intensity of the exposure device 3 and the potential Vl obtained in advance. When the potential Vl is set to the negative polarity side, the light emission intensity of the exposure device 3 is weakened, and when the potential Vl is set to the positive polarity side, the light emission intensity of the exposure device 3 is strengthened. The light emission intensity of the exposure device 3 can be specifically controlled by adjusting the current supplied to the light source, for example. By controlling the light emission intensity of the exposure device 3, the amount of exposure by the exposure device 3, which is represented by the energy value irradiated per unit time per predetermined area of ​​the surface of the photosensitive drum 1, can be controlled, and the potential Vl of the exposed portion of the surface of the photosensitive drum 1 can be controlled. In this embodiment, the charging bias and the developing bias are not changed. However, in order to change the potential Vl of the exposed portion of the surface of the photosensitive drum 1, the charging bias may be changed instead of or in addition to the exposure amount by the exposure device 3.

[0053] Here, as described above, if the transfer contrast (Vt-Vl) is large, the transfer scattering may worsen, and the image quality may deteriorate. Therefore, in this embodiment, in the image forming stations S other than the image forming station Sm in which the toner 90 is least used, the potential Vl is set as follows. That is, the potential Vl is set to the opposite polarity (positive polarity) to the normal charging polarity of the toner 90 from the minimum value Vlm (-100V) required to prevent the development efficiency from decreasing. At this time, the potential Vl is set within a range in which the transfer contrast (Vt-Vl) is equal to or greater than the minimum value T of the transfer contrast required to prevent the transfer efficiency from decreasing. For the following explanation, the potential Vl that matches the minimum value T of the required transfer contrast, expressed by the following formula 2, is set to potential Vln. Vln = Vt - T Equation 2

[0054] That is, the control unit 101 determines the potential Vln for each of the imaging stations S other than the imaging station Sm that has the least amount of toner 90 used, based on the minimum value T of the required transfer contrast (Vt-Vl) and the transfer bias Vt (S107).Then, the control unit 101 sets the potential Vl of the imaging station S to a value on the opposite polarity (positive polarity) side of the normal charging polarity of the toner than the potential Vlm, and to the same value as the potential Vln or on the normal charging polarity (negative polarity) side of the potential Vln (S108).

[0055] The direction in which the potential Vl is made more positive than the potential Vlm (-100V) is the direction in which the potential difference with the developing bias is made easier to develop. In this embodiment, as described above, even when the potential Vl is the potential Vlm (-100V), the toner 90 on the developing roller 42 can be sufficiently developed. Therefore, the effect on the image density caused by making the potential Vl more positive than the potential Vlm (-100V) is small. However, as will be described in detail later, the potential Vl of the imaging stations S other than the imaging station Sm is preferably set to a value between the potentials Vlm and Vln.

[0056] The control unit 101 controls the image forming operation to be performed using the transfer bias Vt common to each image forming station S and the respective potentials Vl of each image forming station S determined as described above. That is, the control unit 101 determines the light emission intensity of the exposure device 3 from the respective potentials Vl of each image forming station S determined as described above (S109). As described above, the control unit 101 can set the light emission intensity of the exposure device 3 from the relationship between the light emission intensity of the exposure device 3 and the potential Vl obtained in advance. Then, the control unit 101 controls the image forming operation to be performed using the determined transfer bias Vt and light emission intensity of the exposure device 3 (S110). After the image forming operation, the control unit 101 updates the cumulative number of images formed at each image forming station S to the number of images formed (S111).

[0057] 3 again when forming an image. That is, the control unit 101 checks the usage state of the toner 90 in each image forming station S. Then, when the usage state of the toner 90 is updated, the control unit 101 recalculates and changes the transfer bias Vt common to each image forming station S and the potential Vl of each image forming station S.

[0058] Here, in the above-described control of the potential Vl, a case is considered in which the usage state of the toner 90 in a certain imaging station (first imaging station) Sa is updated from a first usage state U1 to a second usage state U2 that is less advanced than the first usage state U1. In another imaging station (second imaging station) Sb in which the usage state of the toner 90 has not been updated, if the usage state of the toner 90 is more advanced than the second usage state U2 of the first imaging station Sa, the following control is performed. The amount of change (Vt2-Vt1) in the setting between the setting Vt1 before the change of the transfer bias Vt and the setting Vt2 after the change, and the amount of change (Vl2-Vl1) in the setting between the setting Vl1 before the change of the potential Vl in the second imaging station Sb are on the opposite polarity (positive polarity) side to the normal charging polarity of the toner 90. By setting the transfer bias Vt and potential Vl in this way, the optimum transfer contrast (Vt-Vl) can be set for all image forming stations S, and the transfer scattering can be improved while suppressing the decrease in transfer efficiency.

[0059] Next, a method for determining the potential Vl of the image forming station S other than the image forming station Sm having the least amount of toner 90 used in S108 of the flow in Fig. 3 will be described in further detail. As described above, the potential Vl of the image forming station S other than the image forming station Sm is preferably set to a value between the potentials Vlm and Vln. In this embodiment, in S108 of the flow in Fig. 3, the potential Vl of the image forming station S other than the image forming station Sm is set as follows.

[0060] If the potential Vl is too positive, the transfer efficiency of the line image may decrease due to the worsening of the concentration of the development. Therefore, in this embodiment, the potential Vl is not set to the opposite polarity (positive polarity) to the normal charging polarity of the toner 90 up to the potential Vln, which is the potential Vl that matches the minimum value T of the required transfer contrast described above. In other words, in this embodiment, the potential Vl is set to the same polarity (negative polarity) as the normal charging polarity of the toner 90 rather than the potential Vln. For example, in this embodiment, the potential Vl is set to the intermediate value between the potentials Vlm and Vln, as expressed by the following formula 3. Vl = (Vlm + Vln) / 2 Equation 3

[0061] For example, when the potential Vln is -80V, the potential Vl is set to -90V, which is the intermediate value between the potential Vln and the potential Vlm (-100V). In the control for preventing the potential Vl from becoming positive up to the potential Vln, the potential Vl is not limited to the intermediate value between the potentials Vlm and Vln. It can be set to any value between the potentials Vlm and Vln depending on the ease of occurrence of sweep concentration, etc.

[0062] Here, in the above-mentioned control in which the potential Vl is not changed to the positive polarity side up to the potential Vln, a case is considered in which the usage state of the toner 90 of a certain imaging station (first imaging station) Sa is updated from the first usage state U1 to a second usage state U2 that is less advanced than the first usage state U1. In the other imaging station (second imaging station) Sb in which the usage state of the toner 90 has not been updated, if the usage state of the toner 90 is more advanced than the second usage state U2 of the first imaging station Sa, the following control is performed. The amount of change (Vl2-Vl1) in the setting between the setting Vl1 before the change and the setting Vl2 after the change in the potential Vl in the second imaging station Sb becomes smaller than the amount of change (Vt2-Vt1) in the setting between the setting Vt1 before the change and the setting Vt2 after the change in the transfer bias Vt. By setting the potential Vl in this way, it is possible to suppress the concentration of the development and prevent the transfer efficiency of the line image from decreasing.

[0063] The likelihood of sweeping concentration varies depending on the configuration and operating conditions of the image forming apparatus 100, such as the circumferential speed of the developing roller 42. For example, sweeping concentration may occur easily when the circumferential speed of the developing roller 42 is high. Therefore, depending on the configuration and operating conditions of the image forming apparatus 100, such as the circumferential speed of the developing roller 42, the potential Vl of the imaging stations S other than the imaging station Sm may be set to the positive polarity side up to the potential Vln.

[0064] As described above, in this embodiment, in order to suppress the decrease in transfer efficiency and improve transfer scattering, not only the transfer bias Vt but also the potential Vl of each image forming station S is adjusted. This makes it possible to set the optimal transfer contrast (Vt-Vl) for all image forming stations S.

[0065] <Example of potential control results> In the image forming apparatus 100 of this embodiment, process cartridges 8 with different cumulative image formation numbers as the usage state of the toner 90 were created for each imaging station, and the potential control results were evaluated. Paper feed (image forming operation) for creating process cartridges 8 with different cumulative image formation numbers and the evaluation of the potential control results were performed in an environment with a temperature of 23°C and a relative humidity of 50%. Table 1 shows the cumulative image formation numbers, the minimum value T of the required transfer contrast, the set transfer bias Vt, and the potential Vln in the four usage states created, UA, UB, UC, and UD. In this embodiment, it is determined that the usage state of the toner 90 has changed when the cumulative image formation number has changed by 10 or more, and the cumulative image formation number is expressed as the number of sheets with the first digit rounded down.

[0066] [Table 1]

[0067] As an example of a case where the usage state of the toner 90 is updated, consider a case where the usage state is updated from a first usage state U1 to a second usage state U2 by replacing a process cartridge 8 in one of the usage states UA, UB, UC, and UD. As shown in Table 2, update patterns 1 to 8 are defined, which are different update patterns of combinations of the first usage state U1 and the second usage state U2. Table 2 also shows which of the following each imaging station S corresponds to in each of update patterns 1 to 8: imaging station S1 whose usage state has been updated to an unadvanced usage state, imaging station S2 whose usage state has been updated to an advanced usage state, or imaging station S3 whose usage state has not been updated.

[0068] [Table 2]

[0069] Table 3 shows in this embodiment the transfer bias Vt1 set in the first usage state U1 of each of the update patterns 1 to 8, the potential Vl1 of each image forming station S, and the transfer bias Vt2 set in the second usage state U2, and the potential Vl2 of each image forming station S. Table 3 also shows the image forming stations S whose potential Vl has changed among the image forming stations S3 whose usage state has not been updated.

[0070] [Table 3]

[0071] 7 is a schematic diagram showing an example of the potential control result of each imaging station S (Y, M, C, K) in this embodiment. FIG. 7(a) is a diagram showing the potential control result in the case of the usage state UA. FIG. 7(b) is a diagram showing the potential control result in the case of the usage state UB. In the case of update pattern 1, the potential control result of FIG. 7(a) changes to the potential control result of FIG. 7(b). As shown in FIGS. 7(a) and 7(b), the potential control of this embodiment changes the potential Vl not only of the imaging station SY of Y, in which the usage state of the toner 90 has been updated, but also of the imaging stations SM, SC, SK of M, C, K, in which the usage state of the toner 90 has not been updated.

[0072] <Image evaluation> [Evaluation method] In Example 1 (this embodiment) and Comparative Examples 1 and 2 described later, charging failure ghosts and transfer scattering occurring due to a decrease in transfer efficiency were evaluated according to the following evaluation criteria.

[0073] (Evaluation criteria for charging failure ghost) The charging failure ghost was evaluated by outputting a 1 mm wide line image and visually observing it according to the following criteria. ○: No charging failure or ghost image occurrence ×: Poor charging and ghost images occurring

[0074] (Evaluation criteria for transfer scattering) The transfer scattering was evaluated by outputting a 1-dot thin line image and visually observing it based on the following criteria. ○: No blurring of fine lines △: Fine lines are blurred, but this does not affect actual use. ×: Fine lines are blurred, which is problematic in practical use.

[0075] [Comparative Example 1] As Comparative Example 1, in the configuration of Example 1, the following constant potential control was performed in all the image forming stations S, and the comparison was performed in the same manner as in Example 1. Transfer bias Vt=200V ·Potential Vl=-100V Transfer contrast (Vt-Vl) = 300V

[0076] [Comparative Example 2] As a comparative example 2, in the configuration of the first embodiment, the following constant potential control was performed in all the image forming stations S, and the comparison was performed in the same manner as the first embodiment. Transfer bias Vt=220V ·Potential Vl=-100V Transfer contrast (Vt-Vl) = 320V

[0077] [Comparison between Example 1 and Comparative Examples 1 and 2] Table 4 shows the evaluation results of charging failure ghost and transfer scattering in Example 1 and Comparative Examples 1 and 2, when the first usage state is defined as usage state UA and the second usage state U2 is defined as usage state UB.

[0078] [Table 4]

[0079] As shown in Table 4, a charging defect ghost image occurred in Comparative Example 1. This is because the transfer contrast (Vt-Vl) was below the minimum value T required to avoid reducing the transfer efficiency.

[0080] As shown in Table 4, in Comparative Example 2, the transfer contrast (Vt-Vl) of all the imaging stations S was equal to or greater than the minimum value T required for preventing a decrease in transfer efficiency, so no charging defect ghost images occurred. On the other hand, in Comparative Example 2, the transfer contrast (Vt-Vl) was greater than in Comparative Example 1, so transfer scattering worsened.

[0081] In this way, in Comparative Examples 1 and 2, since the potential Vl of the image forming station S3 whose usage state has not been updated is not adjusted, it is difficult to simultaneously suppress the charging failure ghost and improve the transfer scattering.

[0082] In contrast, in Example 1, as shown in Table 3, it is possible to adjust the potential Vl of the image forming station S3 whose usage status has not been updated. In other words, when the usage status of the toner 90 of a certain image forming station Sa is updated, it is possible to adjust the potential Vl of the image forming stations Sb other than that image forming station Sa. This makes it possible to lower the transfer contrast (Vt-Vl) of all the image forming stations S within a range in which the transfer efficiency does not decrease. As a result, as shown in Table 4, in Example 1, no charging-incomplete ghost images occurred, and the transfer scattering was better than in Comparative Example 2.

[0083] Table 5 also shows the evaluation results of charging failure ghosts and transfer scattering in Example 1 and Comparative Examples 1 and 2, where the first usage state is usage state UA, and the second usage state U2 is usage state UA, usage state UC, and usage state UD.

[0084] [Table 5]

[0085] As shown in Table 5, in Example 1, no charging defect ghost images occurred under any of the conditions of use. In addition, the transfer scattering was improved compared to Comparative Example 2.

[0086] As described above, according to this embodiment, it is possible to improve the transfer scattering while suppressing the decrease in the transfer efficiency with a simple configuration.

[0087] In addition, in the cleaner-less configuration, the control of this embodiment makes it possible to improve transfer scattering while suppressing poorly charged ghost images due to a decrease in transfer efficiency, uneven shading images in image areas (printed areas) due to adhesion of residual toner after transfer to the charging roller 2, and background fogging in non-image areas (non-printed areas). As shown in FIG. 8(b), in a configuration in which a cleaning member 31 is provided to remove toner 90 from the photosensitive drum 1 at a cleaning position I5 between the transfer position I4 and the charging position I1 in the rotation direction of the photosensitive drum 1, the following effects can be obtained. In other words, in such a configuration, the control of this embodiment makes it possible to reduce the amount of residual toner after transfer collected by the cleaning member 31, thereby saving resources, extending the life of the device, and making the device more compact, and also improving transfer scattering.

[0088] Thus, in this embodiment, the image forming apparatus 100 includes a first image forming unit S (e.g., SY) including a rotatable first photoconductor 1, a first charging means 2 for charging the surface of the first photoconductor 1, and a first developing means 4 having a first container 45 for containing toner and supplying the toner to the surface of the first photoconductor 1 to form a toner image, a second image forming unit (e.g., SM) including a rotatable second photoconductor 1, a second charging means 2 for charging the surface of the second photoconductor 1, and a second developing means 4 having a second container 45 for containing toner and supplying the toner to the surface of the second photoconductor 1 to form a toner image, and an exposure unit 3 for exposing the charged surfaces of the first and second photoconductors 1Y and 1M to light to form image units for forming toner images on the surfaces of the first and second photoconductors 1Y and 1M, respectively. The image forming apparatus further comprises: a transfer means 3; an application means Ep for applying a common transfer bias to each transfer section N1Y, N1M where a toner image is transferred from the first and second photosensitive bodies 1Y, 1M to a transferee 53; a memory means 112 for storing information correlating with the charge amount of the toner contained in the first and second storage sections 45Y, 45M, respectively; and a control means 101 for controlling the application means Ep and the exposure means 3, and when the information of the toner contained in the first storage section 45Y is changed from first information to second information in the memory means 112 and the information of the toner contained in the second storage section 45M remains unchanged, the control means 101 controls to change the transfer bias from the first transfer bias to the second transfer bias and to change the potential of the image portion formed on the second photosensitive body 1M from the first potential to the second potential. In this embodiment, when the unit (process cartridge) 8Y including the first storage portion 45Y is replaced, the information on the toner stored in the first storage portion 45Y is changed from the first information to the second information in the storage means 112. In this embodiment, the units 8Y and 8M including the developing means 4Y and 4M are detachable in the first and second image forming portions SY and SM, respectively. However, when the first storage portion 45Y is replenished with toner, the information on the toner stored in the first storage portion 45Y may be changed from the first information to the second information in the storage means 112.In this case, the first and second image forming units SY and SM are configured to be able to replenish toner to the first and second storage units 45Y and 45M, respectively. In this embodiment, the storage unit 112 stores information on an index value that correlates with the amount of image forming operation performed using the toner stored in the first and second storage units 45Y and 45M. In this embodiment, the control unit 101 changes the emission intensity of the exposure unit 3 to change the above potential.

[0089] In this embodiment, when the change from the first information to the second information is a change in the direction in which the absolute value of the charge amount of the toner contained in the first container 45Y increases, and the absolute value of the charge amount of the toner contained in the second container 45M after the change of the potential is smaller than the absolute value of the charge amount of the toner indicated by the second information, the control unit 101 changes the transfer bias so that the second transfer bias is opposite to the normal charge polarity of the toner compared to the first transfer bias, and changes the potential so that the second potential is opposite to the normal charge polarity of the toner compared to the first potential. In this embodiment, the difference (Vl2-Vl1) between the second potential Vl2 and the first potential Vl1 is smaller than the difference (Vt2-Vt1) between the second transfer bias Vt2 and the first transfer bias Vt2. In this embodiment, the control means 101 changes the transfer bias from the first transfer bias to the second transfer bias, sets the potential of the image portion formed on the first photoconductor 1Y to a limit value Vlm on the normal charge polarity side of the toner, and sets the second potential to a value between the limit value Vlm and a potential Vln corresponding to a lower limit of the absolute value of the potential difference between the transfer bias and the potential of the image portion according to the charge amount of the toner contained in the second container 45M after the potential is changed. In this embodiment, the transferred body 53 is a circulative intermediate transfer body that comes into contact with the first and second photoconductors 1Y and 1M to form transfer portions N1Y and N1M, respectively.

[0090] According to one aspect of the present embodiment, the image forming apparatus 100 includes an apparatus main body 10, a first process cartridge 8Y including a first photoconductor 1Y, a first charging means 2Y, a first developing means 4Y including a first container 45Y, and a memory 113 that stores information correlated with the charge amount of the toner contained in the first container 45Y, a second process cartridge 8M including a second photoconductor 1M, a second charging means 2M, and a second developing means 4M including a second container 45M, an exposure means 3, an application means Ep that applies a common transfer bias to transfer portions N1Y and N1M, and a control means 50. and the first process cartridge 8Y is configured to be detachably attached to the apparatus main body 10, and when the first process cartridge 8Y attached to the apparatus main body 10 is replaced with a first process cartridge 8Y different from the first process cartridge 8Y, and the information of the toner contained in the first container 45Y is changed from first information to second information, the control means 50 controls to change the transfer bias from the first transfer bias to the second transfer bias and to change the potential of the image portion formed on the second photosensitive member 1M from the first potential to the second potential.

[0091] According to this embodiment, image defects can be suppressed in a simple configuration in which a common transfer bias is applied to the transfer sections of a plurality of image forming stations.

[0092] {Other Examples} Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-mentioned embodiments.

[0093] In the above embodiment, an intermediate transfer type image forming apparatus in which a toner image is transferred from a photosensitive drum to an intermediate transfer belt, which is an intermediate transfer body, as a transferee, is exemplified. However, the present invention is not limited to an image forming apparatus having such a configuration. The present invention can also be applied to a direct transfer type image forming apparatus in which a toner image is transferred directly from a photosensitive drum to a recording material, which is a transferee. The direct transfer type image forming apparatus has a recording material carrier made of an endless belt or the like, instead of the intermediate transfer body in the image forming apparatus of the above embodiment. Then, the toner image is transferred directly from the photosensitive body to the recording material carried and conveyed on the recording material carrier, in the same manner as the primary transfer of the toner image to the intermediate transfer body in the image forming apparatus of the above embodiment. This transfer is performed, for example, by applying a transfer bias to the transfer member in the same manner as the application of the primary transfer bias to the primary transfer member in the image forming apparatus of the above embodiment. In a tandem type image forming apparatus employing a direct transfer type, when a transfer bias is applied from a common power source to the transfer members of each image forming unit, problems similar to those of a tandem type image forming apparatus employing an intermediate transfer type may occur. Therefore, in such a direct transfer type image forming apparatus, by applying the potential control according to the present invention in the same manner as in the above-mentioned embodiment, the same effect as in the above-mentioned embodiment can be obtained. In this manner, the transferred body may be a recording material carried on a circulatively movable recording material carrier that contacts the first and second photoconductors to form transfer portions, respectively.

[0094] Furthermore, the present invention is not limited to the case where a transfer power supply is shared for all of the transfer members of the multiple image forming units of a tandem image forming apparatus.Even if a transfer power supply is shared for the transfer biases of some and multiple image forming units among all of the image forming units of a tandem image forming apparatus, the present invention can be applied to the multiple image forming units that share a transfer power supply. [Explanation of symbols]

[0095] 1 Photosensitive drum 2. Charge roller 3. Exposure equipment 4. Developing device 51 Primary transfer roller 53 Intermediate transfer belt 90 Toner

Claims

1. a first image forming unit including a rotatable first photosensitive member, a first charging means for charging the surface of the first photosensitive member, and a first developing means having a first container for containing toner and supplying the toner to the surface of the first photosensitive member to form a toner image; a second image forming unit including a rotatable second photosensitive member, a second charging means for charging the surface of the second photosensitive member, and a second developing means having a second container for containing toner and supplying the toner to the surface of the second photosensitive member to form a toner image; an exposure unit that exposes the charged surfaces of the first and second photosensitive drums to light to form image areas on the surfaces of the first and second photosensitive drums, respectively, for forming the toner images; an application means for applying a common transfer bias to each transfer section where the toner image is transferred from the first and second photosensitive bodies to a transfer receiving body; a storage unit for storing information correlating with the charge amount of the toner contained in each of the first and second containers; a control means for controlling the application means and the exposure means; and The image forming apparatus is characterized in that the control means controls the transfer bias to be changed from a first transfer bias to a second transfer bias and the potential of the image area formed on the second photosensitive member to be changed from a first potential to a second potential when the information of the toner contained in the first storage unit is changed from first information to second information in the memory means and the information of the toner contained in the second storage unit remains unchanged.

2. 2. The image forming apparatus according to claim 1, wherein when a unit including the first storage section is replaced, the information about the toner stored in the first storage section is changed from the first information to the second information in the memory means.

3. 3. The image forming apparatus according to claim 2, wherein units including the first and second developing means are detachable in the first and second image forming sections, respectively.

4. 2. The image forming apparatus according to claim 1, wherein when the toner is replenished in the first storage unit, the memory means detects that the information of the toner stored in the first storage unit is changed from the first information to the second information.

5. 5. The image forming apparatus according to claim 4, wherein the toner can be replenished to the first and second containers in the first and second image forming units, respectively.

6. 2. The image forming apparatus according to claim 1, wherein the storage means stores information on an index value correlated with the amount of toner stored in the first and second storage units used in image forming operations.

7. 2. The image forming apparatus according to claim 1, wherein the control means changes the potential by changing the light emission intensity of the exposure means.

8. When the change from the first information to the second information is a change in the direction in which the absolute value of the charge amount of the toner contained in the first storage unit increases, and the absolute value of the charge amount of the toner contained in the second storage unit after the change of the potential is smaller than the absolute value of the charge amount of the toner indicated by the second information, 2. The image forming apparatus according to claim 1, wherein the control means changes the transfer bias so that the second transfer bias is opposite in polarity to the normal charging polarity of the toner than the first transfer bias, and changes the potential so that the second potential is opposite in polarity to the normal charging polarity of the toner than the first potential.

9. 9. The image forming apparatus according to claim 8, wherein a difference (Vl2-Vl1) between the second potential Vl2 and the first potential Vl1 is smaller than a difference (Vt2-Vt1) between the second transfer bias Vt2 and the first transfer bias Vt2.

10. 10. The image forming apparatus according to claim 9, wherein the control means changes the transfer bias from the first transfer bias to the second transfer bias, sets the potential of the image portion formed on the first photosensitive member to a limit value Vlm on the normal charging polarity side of the toner, and sets the second potential to a value between the limit value Vlm and a potential Vln corresponding to a lower limit value of the absolute value of the potential difference between the transfer bias and the potential of the image portion, depending on the charge amount of the toner contained in the second container after the potential change.

11. 11. The image forming apparatus according to claim 1, wherein the transfer medium is a circulatively movable intermediate transfer medium that contacts the first and second photosensitive bodies to form the transfer portions.

12. 11. An image forming apparatus according to claim 1, wherein the transfer object is a recording material carried on a circulatively movable recording material carrier that contacts the first and second photosensitive bodies to form the transfer portions, respectively.

13. A device body, a first process cartridge including a rotatable first photosensitive member, a first charging means for charging the surface of the first photosensitive member, a first developing means having a first container for containing toner and supplying the toner to the surface of the first photosensitive member to form a toner image, and a memory for storing information correlating with the charge amount of the toner contained in the first container; a second process cartridge including a rotatable second photosensitive member, a second charging means for charging the surface of the second photosensitive member, and a second developing means having a second container for containing toner and supplying the toner to the surface of the second photosensitive member to form a toner image; an exposure unit that exposes the charged surfaces of the first and second photosensitive drums to light to form image areas on the surfaces of the first and second photosensitive drums, respectively, for forming the toner images; an application means for applying a common transfer bias to each transfer section where the toner image is transferred from the first and second photosensitive bodies to a transfer receiving body; a control means for controlling the application means and the exposure means; and the first process cartridge is configured to be detachably mountable to the apparatus main body, an image forming apparatus characterized in that, when the first process cartridge mounted in the apparatus main body is replaced with a different first process cartridge and the information of the toner contained in the first container is changed from first information to second information, the control means controls the transfer bias to be changed from a first transfer bias to a second transfer bias and the potential of the image portion formed on the second photosensitive element to be changed from a first potential to a second potential.

Images