JP2019074601A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that can reduce an image defect that may occur due to a change in surface resistance of an intermediate transfer belt even if the intermediate transfer belt is replaced at an early stage.SOLUTION: When a predetermined toner image on an intermediate transfer belt formed in an upstream station passes through a transfer unit in a downstream station, an image forming apparatus sets a primary transfer bias during image formation on the basis of a charging current that is detected when an area of a photoconductor drum in the downstream station passing through the transfer unit passes through a charging area in the downstream station.SELECTED DRAWING: Figure 9

Description

  The present invention relates to an image forming apparatus using an electrophotographic method.

  In an image forming apparatus using an electrophotographic technique, an intermediate transfer belt is formed of a heat resistant high strength resin such as polyester resin, and a conductive powder containing layer containing conductive powder is provided on the surface to adjust surface resistance. There is. In such an intermediate transfer belt, the discharge due to the transfer bias may change the chain or dispersion state of the conductive agent on the surface of the intermediate transfer member, and the surface resistance may decrease.

  When the surface resistance is largely changed as described above, in the full color image forming apparatus, transfer efficiency and retransfer efficiency at the primary transfer portion are changed. As a result, an image failure may occur in a halftone image. The mechanism of the occurrence of the image defect will be described with reference to FIGS. 16 (a) and 16 (b).

  16A and 16B show C, Bk stations of a tandem type image forming apparatus in which Y, M, C, Bk four-color stations (st) are arranged from the upstream side in the rotational direction of the intermediate transfer belt. FIG. 6 is an enlarged schematic view of a primary transfer region in FIG.

  An image defect that occurs when the surface resistance of the intermediate transfer belt is high will be described with reference to FIG. As shown in FIG. 16A, when the surface resistance of the intermediate transfer belt is high, the current flowing from the primary transfer roller hardly flows in the in-plane direction of the intermediate transfer belt but flows to the photosensitive drum. As described above, when the current flowing from the primary transfer roller to the photosensitive drum is high, when the C toner formed by Cst reaches the Bkst primary transfer portion, a portion of the C toner is in the direction of the Bk photosensitive drum. The phenomenon of so-called "re-transfer", which is transferred to The toner to be re-transferred is, in particular, a toner having a low charge amount or a large amount of reverse polarity toner.

  On the other hand, when using a two-component development method in which development is performed using a two-component developer consisting of toner and carrier, carrier particles may be slightly transferred from the development portion to the photosensitive drum during the development process. For example, consider the case where a cyan halftone image passes through the primary transfer portion at the timing when carrier particles transferred onto the Bk photosensitive drum pass through the Bkst primary transfer portion. In this case, the retransfer amount differs between the region where the carrier particles are in the primary transfer portion and the region where the carrier particle is not. This is because in the portion of the Bk photosensitive drum to which the carrier particles are attached, the retransfer becomes weak due to the carrier particles, and in the other portions, the retransfer becomes relatively strong. As a result, an image defect (hereinafter, also referred to as a black spot) occurs in which a cyan halftone image appears dark only in the portion of the photosensitive drum on which the carrier particles are present. Such image defects are likely to occur at an early stage when the surface resistance of the intermediate transfer belt is high.

  Next, image defects that occur when the surface resistance of the intermediate transfer belt is low will be described using FIG. As shown in FIG. 16B, when the surface resistance of the intermediate transfer belt is low, the transfer current flows from the primary transfer roller to the photosensitive drum so that some of the current escapes in the in-plane direction through the surface of the intermediate transfer belt. It will That is, the transfer current flowing from the primary transfer roller to the photosensitive drum is reduced.

  If the transfer current flowing from the primary transfer roller to the photosensitive drum is low, the electric field to be transferred from the photosensitive drum to the intermediate transfer belt weakens at the time of primary transfer. That is, the "transfer efficiency" is low. As described above, it is assumed that carrier particles adhere from the developing unit to the photosensitive drum under the situation where the transfer efficiency is lowered. Under the situation where the transfer efficiency is lowered, the transfer efficiency becomes extremely low where the carrier particles are attached. Therefore, for example, when carrier particles adhere to the cyan photosensitive drum when primary transfer of a cyan halftone image is performed, transfer efficiency of the portion where the carrier particles are present in the primary transfer portion is higher than other portions. Differences occur. As a result, an image defect (hereinafter, also referred to as white spots) in which the density of a part of the cyan halftone image is reduced occurs. Such an image defect occurs at a later stage of the durability of the intermediate transfer belt where the surface resistance of the intermediate transfer belt is reduced.

  As described above, white spots and black spots caused by the carrier attached to the photosensitive drum change the frequency of occurrence on the image due to the resistance of the surface of the intermediate transfer belt.

  Conventionally, the transfer bias is set in a range in which image defects such as white spots and black spots do not occur, and belt replacement is performed before image defects occur due to a decrease in resistance of the intermediate transfer belt. For example, Patent Document 1 discloses that the life of the transfer belt is detected from the counter of the number of revolutions of the intermediate transfer belt, the application time of bias, and the intensity.

JP-A-11-338271

  However, the configuration of Patent Document 1 has the following problems. The occurrence of the image defects such as black spots and white spots described above can be suppressed by changing the setting of the transfer bias. However, in the configuration of Patent Document 1, the transfer bias is not changed according to the surface resistance value of the intermediate transfer belt. Therefore, it has not been possible to sufficiently cope with an image defect generated according to the surface resistance of the intermediate transfer belt. As a result, there is a problem that the intermediate transfer belt is replaced early.

  Therefore, an object of the present invention is to provide an image forming apparatus capable of suppressing an image defect which may occur due to a change in surface resistance of the intermediate transfer belt even if the usable period of the intermediate transfer belt is extended.

  The image forming apparatus according to the present invention is rotatably provided with a first image forming unit for forming an image, a second image forming unit for forming an image, and the first image forming unit and the second image. An intermediate transfer belt to which a toner image formed by a forming unit is transferred, and a control unit configured to control the operation of the first image forming unit and the second image forming unit; The forming unit includes an image carrier, a charging device for charging the image carrier in a charging region, a current detection unit for detecting a current flowing through the charging device, and a latent image formed on the image carrier. And a developing device for developing with a developer containing a carrier, and a transfer device for transferring a toner image formed on the image bearing member to the intermediate transfer belt at a transfer region, to which a transfer bias is applied. The first image forming unit is the intermediate transfer bell In the image forming apparatus provided downstream of the second image forming unit with respect to the rotational direction of the image forming unit, the control unit charges the surface of the image carrier with the charging device during non-image formation. When the surface of the image carrier charged by the charging device passes through the transfer area, a predetermined transfer bias is applied to the transfer device, and the predetermined transfer bias is applied. When a predetermined toner image formed on the intermediate transfer belt by the image forming unit 2 is passed to the transfer area of the first image forming unit, and the predetermined toner image is passing the transfer area Moving the first region of the image carrier passing through the transfer region toward the charging region, and charging the current flowing to the charging device when the first region passes the charging region. When the Ic, and detects the charging current Ic at the current detection unit, configured to set the transfer bias said applied to the transfer device during image formation based on the charging current Ic, characterized in that.

  According to the present invention, it is possible to provide an image forming apparatus capable of suppressing an image defect which may occur due to a surface resistance change of the intermediate transfer belt even if the usable period of the intermediate transfer belt is extended.

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment. FIG. 6 is a view for explaining the layer configuration of the charging roller and the layer configuration of the photosensitive drum of the image forming apparatus according to Embodiments 1 and 2. FIG. 6 is an operation sequence diagram of the image forming apparatus according to the embodiment. It is the graph which described transition of the surface resistance of the intermediate transfer belt concerning an example. FIG. 5 is a schematic configuration view showing a relationship between a charging current Ic and a surface resistance of an intermediate transfer belt according to an embodiment. FIG. 6 is a diagram describing the relationship between the post-transfer potential and the post-charge potential, which is generated by the surface resistance difference of the intermediate transfer belt according to the embodiment. FIG. 6 is a view for explaining a mechanism of causing a difference in potential after transfer due to a difference in surface resistance of the intermediate transfer belt according to the embodiment. 6 is a graph showing the relationship between the charging current and Ic according to Example 1, and the set value of the primary transfer current to be set. 5 is a flowchart for explaining the operation of the image forming apparatus according to the first embodiment. FIG. 16 is a graph showing the relationship between the charging current and Ic according to Example 2 and the set value of the primary transfer current to be set. FIG. 6 is a flowchart for explaining the operation of the image forming apparatus according to the second embodiment. FIG. 7 is a view for explaining the layer configuration of a charging roller and the layer configuration of a photosensitive drum of an image forming apparatus according to a third embodiment. FIG. 16 is a schematic configuration view showing a relationship between a ratio of a charging current Ic to a charging current Iw according to a third embodiment and a surface resistance of an intermediate transfer belt. FIG. 16 is a graph describing the relationship between the charging current, Ic / Iw, and the set value of the primary transfer current to be set according to Example 2. FIG. FIG. 14 is a flowchart for explaining the operation of the image forming apparatus according to the third embodiment. It is a mechanism figure of generation | occurrence | production of a white spot and a black spot in the primary transfer part which concerns on an Example. It is a control block diagram which performs control operation concerning an example.

Example 1
<Image forming apparatus>
FIG. 1 is a schematic cross-sectional view of the image forming apparatus 100. In the image forming apparatus 100, four image forming stations for forming toner images of respective color components are arranged in one line. Such an image forming apparatus 100 is called a tandem type image forming apparatus. In each image forming station, the image forming unit Y forms a yellow toner image, the image forming unit M forms a magenta toner image, the image forming unit C forms a cyan toner image, and the image forming unit Bk Form a black toner image. Since the image forming units Y, M, C, and Bk of each image forming station have the same configuration, the image forming unit Y that forms a yellow toner image will be described below, and the other image forming units M, C, and Bk Some of the explanations for

  The image forming unit Y includes a photosensitive drum 1a, a charging roller 2a, an exposure device 3a, a developing device 4a, a primary transfer roller 5a, and a drum cleaner 6a.

  The photosensitive drum 1a as an image carrier is a negatively chargeable organic photosensitive member (OPC) with a diameter of 30 mm, and rotates in the direction of the arrow at a process speed (circumferential speed) of 200 mm / s by driving of a driving device (not shown). It is driven. The photosensitive drum 1a is, as shown in FIG. 2, formed of a photogenerating layer 1r, a charge transport layer 1s, and a surface layer 1t containing a curable material on the surface of an aluminum cylinder (conductive drum base 1p). A photosensitive layer is provided.

  A high voltage power supply circuit (charging bias power supply) 20 is electrically connected to the charging roller 2a as the charging device. The high voltage power supply circuit (charging bias power supply) 20 includes a DC voltage generation circuit 21 and a DC voltage amplification circuit 22, and the surface of the photosensitive drum 1a is charged by a charging bias generated by a combination of the DC voltage generation circuit 21 and the DC voltage amplification circuit 22. It uniformly charges to a predetermined potential. In the present embodiment, the charging bias is −1300 V, and the potential (Vd) on the photosensitive drum 1 a is set to −700 V at the opposing position of the developing device 4 a. In FIG. 1, the DC voltage applied to the charging roller 2 a of the image forming unit Y is applied by the DC voltage generation circuit 21 a in the DC voltage generation circuit 21. Further, the magnitude of the DC voltage value is adjusted by the DC voltage amplification circuit 22 a in the DC voltage circuit 22. The direct current flowing through the direct current voltage generation circuit 21a is detected by the charging current detection circuit 101a as a current detection unit, and the detected charging current is recorded and controlled by the controller 12 as a control unit. The controller 12 controls the image forming operation of the apparatus main body.

  The longitudinal length of the charging roller 2a is 320 mm, and as shown in FIG. 2, the lower layer 2q, the intermediate layer 2r, and the surface layer 2s are sequentially laminated from the bottom around the core metal (supporting member) 2p. It is a layer structure. The lower layer 2 q is a foam sponge layer for reducing charging noise, and the surface layer 2 s is a protective layer provided to prevent the occurrence of a leak even if there is a defect such as a pinhole on the photosensitive drum. .

More specifically, the specifications of the charging rollers 2a to 2d in the present embodiment are as follows.
Core metal 2p; 6 mm diameter stainless steel round bar lower layer 2 q; carbon dispersed foam EPDM, specific gravity 0.5 g / cm ³ , volume resistivity 10 ² to 10 ⁹ Ω, layer thickness 3.0 mm
Middle layer 2r; NBR-based rubber with carbon dispersion, volume resistivity 10 ² to 10 ⁵ Ω, layer thickness 700 μm
Surface layer 2s; dispersed tin oxide and carbon in resin of fluorine compound, volume resistivity 10 ⁷ to 10 ¹⁰ Ω, surface roughness (JIS standard 10 points average surface roughness Ra) 1.5 μm, layer thickness 10 μm

  The charging rollers 2a to 2d are urged toward the center of the photosensitive drum by a pressing spring 2t and pressed against the charging nip a with a predetermined pressing force against the surface of the photosensitive drum, and are driven by the rotational driving of the photosensitive drum And rotate in the direction of R2.

In the present embodiment, the charging roller having a total volume resistance of 1.0 × 10 ⁵ Ω was adopted.

  The exposure apparatus 3a is a laser beam scanner using a semiconductor laser, and has a light source for emitting a laser beam and an ASIC for controlling the light source based on an input image signal. The laser beam scans the surface of the photosensitive drum 1a uniformly charged to a predetermined potential, and forms an electrostatic latent image corresponding to the image signal input on the photosensitive drum 1a. In the present embodiment, the light portion potential (VL) of the photosensitive drum 1a irradiated with the laser light was set to -200V.

  The developing device 4a contains a two-component developer including toner and a carrier. Further, a developing bias in which a DC voltage (Vdc) and an AC voltage (Vac) are superimposed is applied to the developing device 4a. Specifically, the frequency of the AC voltage is 8 kHz, the DC voltage is -550 V, and the peak-to-peak voltage Vpp of the AC voltage is 1800 V.

  The primary transfer roller 5a is electrically connected to a power supply unit (not shown). The transfer power supply circuit 51 connected to the primary transfer roller 5a continuously applies several levels of transfer biases of different voltage values to the photosensitive drum 1a charged to a desired charging potential by the charging roller 2a. And each current value is detected and the current-voltage characteristic is stored. From the obtained current-voltage characteristics, a voltage necessary to obtain a predetermined transfer current is calculated, and a transfer bias to be applied at the time of image formation is determined. The control for determining the primary transfer bias is referred to herein as primary transfer ATVC control, which is performed at the time of non-image formation.

  The drum cleaner 6a includes a plate-like elastic member for collecting the toner on the photosensitive drum 1a.

  The intermediate transfer belt 7 is wound around a plurality of rollers and rotatably provided. The intermediate transfer belt 7 is rotationally driven in the arrow direction by a drive roller. The intermediate transfer belt 7 is made of polyimide resin. In this embodiment, a polyimide resin and a predetermined amount of a resistance adjuster such as carbon are dispersed and mixed, and the center resistance of the surface resistance at 100 V application is adjusted to about 10 ^ 12 Ω. Be done. Further, as the intermediate transfer belt 7, in addition to the polyimide resin, a heat resistant high strength resin such as a polyester resin, a polyether ketone resin, and a polysulfide resin can be used.

  Around the intermediate transfer belt 7, a secondary transfer roller 8 as a secondary transfer device, an intermediate transfer belt cleaner 30, and a density detection sensor 50 are provided. An inner roller is provided on the opposite side of the secondary transfer roller 8 via the intermediate transfer belt 7. The secondary transfer roller 8 presses the intermediate transfer belt 7 against the inner roller, and the intermediate transfer belt 7 and the secondary transfer roller 8 form a nip. The secondary transfer roller 8 and the inner roller are electrically connected to a power supply unit (not shown), and the secondary transfer bias is determined by the secondary transfer ATVC control which is the same control as the primary transfer ATVC control.

<Image forming apparatus process>
Next, an image forming operation in which the image forming apparatus 100 forms an image on the recording material P based on the image data subjected to image processing will be described.

  In the image forming unit Y, first, the charging roller 2a uniformly charges the photosensitive drum 1a, and the exposure device 3a controls the laser light based on the image data. The laser beam of the exposure device 3a scans the photosensitive drum 1a, and an electrostatic latent image is formed on the photosensitive drum 1a. The developing device 4a develops the electrostatic latent image on the photosensitive drum 1a using yellow toner. As a result, a yellow toner image is formed on the photosensitive drum 1a. Then, the yellow toner image is transferred from the photosensitive drum 1 a to the intermediate transfer belt 7 at a primary transfer portion as a transfer area formed by the photosensitive drum 1 c and the transfer roller 5 c. At this time, the primary transfer bias determined by the primary transfer ATVC control is applied to the primary transfer roller 5a. In this embodiment, the primary transfer current is set to 20 μA by ATVC control. The toner remaining on the photosensitive drum 1a is removed by the drum cleaner 6a.

  The toner image transferred to the intermediate transfer belt 7 is conveyed to the transfer nip portion between the intermediate transfer belt 7 and the secondary transfer roller 8. At this time, the recording material P is conveyed by the registration roller 11 to the transfer nip portion so as to contact the toner image on the intermediate transfer belt 7. While the toner image on the intermediate transfer belt 7 and the recording material P pass through the transfer nip portion, the power source unit performs a secondary transfer bias determined by the secondary transfer ATVC control between the secondary transfer roller 8 and the inner roller. Supply. Thus, the toner image on the intermediate transfer belt 7 is transferred onto the recording material P. The toner remaining on the intermediate transfer belt 7 without being transferred to the recording material P is removed by the belt cleaner 30.

  The recording material P carrying the toner image is conveyed to the fixing device 9, and the heat roller 9a applies heat to the recording material P carrying the unfixed toner image at the fixing nip portion, thereby applying the toner image onto the recording material P Melt and fix. Then, the recording material P on which the toner image is fixed is discharged from the image forming apparatus 100 by a discharge roller (not shown).

<Control unit>
As shown in FIG. 1, the image forming apparatus 100 of the present embodiment includes a controller 12. The controller 12 will be described using FIG. 17 with reference to FIG. Although various devices such as a motor and a power source for operating the image forming apparatus 100 are connected to the controller 12 in addition to those illustrated, the illustration and the description thereof are omitted because they are not the gist of the invention.

  The controller 12 performs various controls of the image forming apparatus 100 such as an image forming operation, and includes a CPU (Central Processing Unit) whose illustration is omitted. The controller 12 is connected to a memory 13 such as a ROM or RAM as a storage means or a hard disk drive. The memory 13 stores various programs, data, and the like for controlling the image forming apparatus 100. The controller 12 can operate the image forming apparatus 100 to form an image based on the information stored in the memory 13.

  The controller 12 is connected to a charging current detection circuit 101 that detects the current flowing to the charging rollers 2a to 2d. Further, a high voltage power supply circuit 20 for applying a bias to the charging rollers 2a to 2d and a transfer power supply circuit 51 for applying a primary transfer bias to the transfer rollers 5a to 5d are connected.

<Operation Process of Image Forming Apparatus>
FIG. 3 is an operation sequence diagram of the image forming apparatus.

a. Initial rotation operation (pre-multi-rotation process)
It is a start operation period (start operation period, warming period) when the printer is started. When the power switch is turned on, the photosensitive drum is rotationally driven, and a preparation operation of predetermined process equipment such as start-up of the fixing device to a predetermined temperature is performed.

b. Print preparation rotation operation (pre-rotation process)
Print signal-This is the preparatory rotation operation period before image formation from the time when the image formation (printing) process operation is actually performed until the print operation is performed during the initial rotation operation, and is performed following the initial rotation operation when the print signal is input. Be done. When the print signal is not input, the drive of the main motor is temporarily stopped after the end of the initial rotation operation to stop the rotational drive of the photosensitive drum, and the printer is kept in the standby state until the print signal is input. When the print signal is input, the print preparation rotation operation is performed.

c. Printing process (image forming process, image forming process)
When the predetermined printing preparation rotating operation is completed, the image forming process is subsequently executed on the rotating photosensitive drum, and the toner image formed on the surface of the rotating photosensitive drum is transferred to the transfer material, and the fixing process of the toner image is performed by the fixing device. The image formation is printed out.
In the case of the continuous printing (continuous printing) mode, the above-described printing process is repeatedly performed for a predetermined set number of printing n.

d. Inter-paper process In the continuous printing mode, after the rear end of the transfer material passes the transfer position, the recording paper does not pass at the transfer position until the front end of the next transfer material reaches the transfer position. It is a period.

e. The post-rotation operation is a period in which the driving of the main motor is continued for a while after the printing process of the last transfer material is finished, the photosensitive drum is rotationally driven, and the predetermined post-operation is performed.

f. When the predetermined post-rotation operation is completed, the drive of the main motor is stopped and the rotational drive of the photosensitive drum is stopped, and the printer is kept in the standby state until the next print start signal is input.

  In the case of printing on only one sheet, after the printing is completed, the printer goes through a post-rotation operation and goes into a standby state.

  In the standby state, when the print start signal is input, the printer shifts to the pre-rotation step.

  The printing process of c is the time of image formation, and the initial rotation operation of a, the pre-rotation operation of b, the sheet interval process of d, and the post-rotation operation of e are non-image formation. The above-described ATVC control of primary transfer and secondary transfer operates at the time of the front multi-rotation operation of a or the pre-rotation operation of b according to a predetermined frequency and the timing of detection of a change in the environment, and is necessary. Determine primary and secondary transfer biases.

<Change Characteristics of Surface Resistance of Intermediate Transfer Belt>
FIG. 4 is a graph showing the surface resistance of the intermediate transfer belt 7 as it passes through in each of the usage conditions in the tandem type image forming apparatus as described above.

  In (a) of FIG. 4, the number of single-sided sheets of plain paper in three environments (NL: temperature 23 ° C., humidity 5%, NN: temperature 23 ° C., humidity 50% HH: temperature 30 ° C., humidity 80%) 6 is a graph showing a reduction in surface resistance of the intermediate transfer belt. As the environment is NL, that is, the environment in which the amount of water in the environment is lower, the resistance of the paper is increased, and the secondary transfer roller 8 needs a higher secondary transfer bias necessary to discharge the toner onto the paper. As the secondary transfer bias increases, the discharge at the secondary transfer portion also increases, thereby increasing the change in the chain or dispersed state of the conductive agent on the surface of the intermediate transfer member, and as a result, the surface resistance of the intermediate transfer belt 7 decreases. Promoted.

  (B) of FIG. 4 is a graph showing plain paper, single-sided sheet passing, double-sided sheet count, and reduction in surface resistance of the intermediate transfer belt. The number of sheets passed on both sides means that two sheets have passed per sheet. In the double-sided sheet passing, the paper passes through the fixing device 9, and the paper again reaches the secondary transfer roller 8, and the second side is transferred. The paper passing through the fixing device 9 evaporates the moisture in the paper and the resistance of the paper itself is increased, and the second side of the paper is a secondary transfer roller necessary for secondary transfer to discharge toner onto the paper The bias is higher. That is, the surface resistance of the intermediate transfer belt 7 is more likely to decrease when passing on both sides.

  (C) of FIG. 4 is a graph showing the number of sheets of paper on which one side of plain paper and recycled paper is passed and the decrease in surface resistance of the intermediate transfer belt. The recycled paper has lower smoothness than plain paper, and the secondary transfer roller 8 is likely to generate uneven discharge with respect to the intermediate transfer belt 7 and tends to generate strong discharge. Therefore, it is known that the surface resistance of the intermediate transfer belt 7 is more likely to be reduced with recycled paper than with plain paper.

  In the present embodiment, when the surface resistance of the intermediate transfer belt 7 is 10 11 Ω or more, the above-described black spots are easily generated, and when the surface resistance of the intermediate transfer belt 7 is 10 7 Ω or less, the above-described white spots are generated. It turned out that it is easy to occur.

  The inventors of the present invention have found that the surface resistance of the intermediate transfer belt 7 is correlated with the charging current measured under certain conditions as a result of the sincerity examination. That is, it is assumed that a resistor such as a solid image formed at the upstream station is transported together with the intermediate transfer belt 7 to the transfer portion of the downstream station. In this case, it has been found that the post-transfer potential of the photosensitive drum subjected to the primary transfer has a characteristic depending on the surface resistance of the intermediate transfer belt 7 at the transfer portion of the downstream station.

  Therefore, in the present embodiment, the surface resistance of the intermediate transfer belt 7 is detected using this phenomenon, and the setting of the primary transfer bias is changed according to the surface resistance of the intermediate transfer belt to generate white spots and black spots. Suppress.

<Measurement of charging current correlated to surface resistance of intermediate transfer belt>
Hereinafter, measurement of the surface resistance of the intermediate transfer belt 7 will be described. First, in the above-described tandem type image forming apparatus, 100% printing (solid) is formed at the image forming stations of yellow (Y) and magenta (M) which are two image forming stations on the upstream side. These Y and M solid images are superimposed on the intermediate transfer belt 7 to form a red solid image which is a secondary color solid image. These solid images are formed as so-called horizontal band images formed in the printable area maximum width orthogonal to the image conveyance direction. That is, a red solid (R solid) horizontal band toner image in which the Y solid horizontal band toner image and the M solid horizontal band toner image are superimposed is formed on the intermediate transfer belt 7.

  Further, the image forming station of cyan (C), which is an image forming station one downstream, does not form the above-mentioned solid lateral band toner image, but applies the primary transfer bias in the same manner as in the normal image formation. Then, the R solid horizontal band toner image passes through the primary transfer portion in the cyan image forming station. At this time, the R-solid-lateral-band toner image contact portion of the photosensitive drum 1c is moved to the charging region which is the contact portion with the charging roller 2c by the drive rotation of the photosensitive drum 1c. At this time, assuming that the charging current flowing to the charging roller 2c is charging current Ic (hereinafter also referred to as charging current Ic for R solid image), the charging current Ic is detected by the charging current detection circuit 101c.

  FIG. 5 is a graph showing the relationship between the charging current Ic of the R solid image and the surface resistance of the intermediate transfer belt 7. The amount of applied toner on the R solid lateral band toner image was 1.0 (mg / cm 2). Further, the charging potential of the photosensitive drum 1c of Cst was -700V.

  According to the result of FIG. 5, the charging current Ic of the R solid image has a correlation with the surface resistance value of the intermediate transfer belt 7, and the charging roller 2c of the C image forming station is decreased as the surface resistance value of the intermediate transfer belt 7 decreases. It was found that the flowing charging current decreased.

  This phenomenon will be described in detail with reference to FIG.

  FIG. 6 is a schematic cross-sectional view when the above-mentioned R solid horizontal band toner image reaches the transfer nip portion of the cyan image forming station, and shows the drum potential of the photosensitive drum 1c. On the left side of the graph showing the drum potential shown in FIG. 6, the surface potential of the photosensitive drum 1c (transfer) at the Vp position on the photosensitive drum 1c after the R solid horizontal band toner image contact portion contacts the photosensitive drum 1c The afterpotential Vp is shown. The graph shown on the right side of FIG. 6 shows the surface potential (charging potential) Vq of the photosensitive drum 1c at the position Vq after being recharged by the charging roller 2c.

  6 (a) shows the case of the intermediate transfer belt 7 in the initial state where the surface resistance value is high, and FIG. 6 (b) shows the intermediate transfer in the durable state where the surface resistance value is lowered by the sheet passing. The case of the belt 7 is shown.

  The vertical axis of each graph represents the surface potential of the photosensitive drum 1c, and in the present embodiment, the arrow direction indicates a negative potential.

  Reference is made to FIG. 6A showing the case of the intermediate transfer belt 7 in the initial stage. In this case, the post-transfer potential Vp on the photosensitive drum 1c of the portion in contact with the R solid horizontal band toner image has an absolute value larger than the post-transfer potential Vp on the photosensitive drum 1c in the contact portion with the non-toner image portion. . The post-transfer potential of the photosensitive drum 1c is higher on the part where the R solid horizontal band toner image contacts, than on the part where the toner image is not present. In this embodiment, as shown in FIG. 6A, the post-transfer potential Vp of the photosensitive drum 1c is -200 V at the contact portion with the toner image free portion (also referred to as solid white portion or solid white region). The contact portion with the horizontal band toner image bearing portion was -300V. This is because the R solid horizontal band toner image serves as a resistor at the R solid horizontal band toner image contact portion. Therefore, the inflow current to the photosensitive drum 1c when the primary transfer bias is applied decreases, and the post-transfer potential Vp of the photosensitive drum 1c does not drop compared to the solid white contact portion.

  Thereafter, the photosensitive drum 1c is recharged to the target charging potential Vq at the time of image formation by the charging roller 2c. However, since the post-transfer potential Vp is higher at the R-solid horizontal band toner image contact portion than at the solid white contact portion, the potential difference with the charging potential Vq decreases. As a result, compared to the solid white contact portion, the charging current at the solid red toner image contact portion decreases. In FIG. 6A, the post-transfer potential Vp of the photosensitive drum 1c at the R solid horizontal band toner image contact portion is -300 V, the charging potential Vq is -700 V, and the potential difference thereof is 400 V. It is stated that the charging current was 10 .mu.A.

  FIG. 6 (b) similarly shows the case of the intermediate transfer belt 7 in which the surface resistance value after passing paper has been lowered. When the surface resistance value of the intermediate transfer belt 7 is low, the post-transfer potential Vp of the photosensitive drum 1c at the R solid area toner image contact portion is higher than when the surface resistance value is high. In the case of the intermediate transfer belt 7 in which the surface resistance value is lowered, the post-transfer potential Vp at the solid white contact portion is -200 V, and the post-transfer potential Vp at the R solid lateral band toner image contact portion is -500 V.

  Thereafter, similarly, the photosensitive drum 1c is recharged to the target charging potential Vq at the time of image formation by the charging roller 2c. The potential difference between the post-transfer potential Vp and the charging potential Vq of the photosensitive drum 1c at the R solid horizontal band toner image contact portion is 200 V, which is smaller than 500 V in the case of the initial intermediate transfer belt having a high surface resistance value. Therefore, the charging current detected by the charging current detection circuit 101c is also reduced by 5 μA.

  As described above, when the surface resistance value of the intermediate transfer belt 7 is lowered due to the sheet feeding durability, the post-transfer potential Vp of the R solid horizontal band toner image contact portion is increased and the potential difference with the charging potential Vq is decreased. As a result, the charging current flowing to the charging roller 2c is reduced.

  FIG. 7 is a mechanism diagram for explaining the above-mentioned phenomenon. In the case of the intermediate transfer belt 7 having a high surface resistance as in the initial stage, as shown in FIG. 7, the toner of the R solid horizontal band toner image becomes a resistor, and the current flowing from the transfer roller 5c to the photosensitive drum 1c Decrease. However, in the case of the intermediate transfer belt 7 whose surface resistance value has become low due to the sheet feeding durability, as shown in FIG. The current flowing in the 1c direction is further reduced. This is divided into the current flowing from the transfer roller 5c in the direction of the photosensitive drum 1c when the surface resistance value of the intermediate transfer belt 7 decreases, and the current flowing laterally across the surface of the intermediate transfer belt 7 having a low surface resistance value. It is because it As a result, the transfer current corresponding to the current flowing to the photosensitive drum 1c is decreased, so that the post-transfer potential Vp of the photosensitive drum 1c does not easily decrease, and as a result, the charging current is decreased.

  Therefore, in the present embodiment, the surface of the intermediate transfer belt is utilized by utilizing the characteristic that the current flowing from the transfer roller toward the photosensitive drum decreases when the resistance value of the intermediate transfer belt comes, when the resistor such as toner comes. Measure the resistance value. Then, based on the measurement results, control was performed to set the primary transfer current so that black spots and white spots are less likely to occur.

  FIG. 8 is a graph showing the relationship between the charging current Ic measured as described above and the setting value of the current supplied to the primary transfer portion in this embodiment. This relationship is stored in advance in the memory 13 and is configured to be readable by the controller 12. The primary transfer current is normally set to 20 μA in this embodiment.

  The controller 12 determines that the resistance value of the intermediate transfer belt is high if the charging current Ic is 21 μA or more, and sets the primary transfer current to be reduced in order to suppress the occurrence of black spots. The controller 12 determines that the resistance value of the intermediate transfer belt is low if the charging current Ic is 6 μA or less, and sets the primary transfer current to be low in order to suppress the occurrence of white spots. That is, when the charging current Ic has a first predetermined value, the controller 12 sets the primary transfer bias so that the first transfer current flows to the transfer roller 5a at the time of image formation. Further, when the charging current Ic is a second predetermined value smaller than the first predetermined value, the controller 12 performs the primary operation so that a second transfer current larger than the first transfer current flows to the transfer roller 5a at the time of image formation. Set the transfer bias.

<Measurement of surface resistance of intermediate transfer belt and setting flow of primary transfer bias>
The process of measuring the surface resistance of the intermediate transfer belt 7 and the setting flow of the primary transfer bias in this embodiment will be described.

  FIG. 9 is a control operation flowchart used in this embodiment. FIG. 17 is a control block diagram for executing this control operation flow. By this control, the surface resistance of the intermediate transfer belt 7 is measured, and the primary transfer bias is set. In the present embodiment, the timing for executing this control is once for every 1000 sheets. Hereinafter, the control flow for measuring the surface resistance of the intermediate transfer belt 7 is also referred to as a detection mode.

  First, the controller 12 executes the detection mode of the present embodiment after performing the pre-rotation operation in the pre-rotation process of FIG. 3. The pre-rotation operation rotates the photosensitive drums 1a to 1d and the intermediate transfer belt 7 respectively. Then, in each of the image forming sections, the photosensitive drums 1a to 1d are charged by the charging rollers 2a to 2d as in the case of the normal image formation. Then, the primary transfer bias is applied to the transfer rollers 5a to 5d when the respective regions of the charged photosensitive drums 1a to 1d pass through the portions facing the transfer rollers 5a to 5d. During the detection mode, charging and transfer bias are continuously applied.

  Next, the controller 12 forms an R solid horizontal band toner image on the intermediate transfer belt 7 by the yellow (Y) image forming portion and the magenta (M) image forming portion. (S101) At this time, the exposure device of the cyan image forming unit is turned off so as not to perform image exposure. That is, solid white is formed on the cyan image forming portion. The loading amount of the R solid horizontal belt toner image is 1.0 (mg / cm 2), the width in the longitudinal direction is the maximum image formable width (maximum developable width), and the width in the rotational direction of the intermediate transfer belt is 10 mm.

  The R solid horizontal band toner image formed on the intermediate transfer belt 7 reaches the primary transfer portion of the cyan image forming portion (S102). The surface of the photosensitive drum 1c in contact with the R-solid horizontal band toner image at the primary transfer portion passes through the charging region in contact with the charging roller 2c. At this time, the charging current Ic flowing to the charging roller 2c is detected by the charging current detection circuit 101c (S103).

  Next, the controller 12 acquires information of the charging current Ic detected by the charging current detection circuit 101c, acquires information of the charging current Ic, and determines whether the charging current Ic is 21 μA or more (S104).

  If the charging current Ic is 21 μA or more, the controller 12 changes the setting to lower the primary transfer current based on the relationship of FIG. 8 stored in the memory 13 (S105).

  If the charging current Ic is less than 21 μA, then the controller 12 determines whether the charging current Ic is 6 μA or less (S106).

  If the charging current Ic is 6 μA or less, the controller 12 changes the setting to lower the primary transfer current based on the relationship of FIG. 8 stored in the memory 13 (S107).

  The controller 12 sets the primary transfer current to 20 μA if the charging current Ic is greater than 6 μA and less than 21 μA.

  That is, the controller 12 sets the primary transfer bias based on the relationship shown in FIG. That is, when the charging current Ic is a first predetermined value (for example, 23 μA), the primary transfer bias is set so that the first transfer current (15 μA) flows to the transfer roller 5a at the time of image formation. Further, when the charging current Ic is a second predetermined value (for example, 15 μA) smaller than the first predetermined value, the controller 12 generates a second transfer current larger than the first transfer current to the transfer roller 5a at the time of image formation. The primary transfer bias is set so that (20 μA) flows.

  With the above configuration, the surface resistance value of the intermediate transfer belt can be measured timely, and the generation of black spots and white spots is suppressed by controlling the primary transfer current according to the surface resistance value. We were able to.

Example 2
The configuration of the second embodiment is the same as that of the first embodiment, so the description will be omitted, and parts different from the first embodiment will be described. In addition, the parts not described in particular are the same as in Example 1 described above. In the first embodiment, the primary transfer current is controlled to be changed only when the charging current Ic becomes equal to or larger than a predetermined value. In this embodiment, the primary transfer current is controlled to be changed also in the surface resistance value of the intermediate transfer belt in which the generation of black spots and white spots is not generated.

  As described with reference to FIGS. 16A and 16B, when the surface resistance of the intermediate transfer belt is high, re-transfer is frequent. Therefore, the primary transfer current is set in a range where black spots do not occur. In addition to this, it is desirable to set the primary transfer current to be lower as the surface resistance of the intermediate transfer belt becomes higher, since the retransfer toner is reduced and the utilization efficiency of the toner is increased.

  When the surface resistance of the intermediate transfer belt is low, the transfer efficiency is lowered. Therefore, the primary transfer current is set in the range where white spots do not occur. In addition, it is desirable to set the primary transfer current to be higher as the surface resistance of the intermediate transfer belt is lower, in order to increase the transfer efficiency and to increase the utilization efficiency of the toner.

  Therefore, in the present embodiment, as shown in FIG. 10, the primary transfer current is set for the charging current Ic.

<Measurement of surface resistance of intermediate transfer belt and setting flow of primary transfer bias>
FIG. 11 is a control operation flowchart used in this embodiment. The surface resistance of the intermediate transfer belt 7 is measured by this control. The timing to execute this control is once for every 1000 sheets.

  First, the controller 12 executes the detection mode of the present embodiment after performing the pre-rotation operation in the pre-rotation process of FIG. 3. The pre-rotation operation rotates the photosensitive drums 1a to 1d and the intermediate transfer belt 7 respectively. Then, in each of the image forming sections, the photosensitive drums 1a to 1d are charged by the charging rollers 2a to 2d as in the case of the normal image formation. Then, the primary transfer bias is applied to the transfer rollers 5a to 5d when the respective regions of the charged photosensitive drums 1a to 1d pass through the portions facing the transfer rollers 5a to 5d.

  Next, the controller 12 forms an R solid horizontal band toner image on the intermediate transfer belt 7 by the yellow (Y) image forming portion and the magenta (M) image forming portion. (S201) At this time, the exposure device of the cyan image forming unit is turned off so as not to perform image exposure. That is, solid white is formed on the cyan image forming portion. The loading amount of the R solid horizontal belt toner image is 1.0 (mg / cm 2), the width in the longitudinal direction is the maximum image formable width (maximum developable width), and the width in the rotational direction of the intermediate transfer belt is 10 mm.

  The R solid horizontal band toner image formed on the intermediate transfer belt 7 reaches the primary transfer portion of the cyan image forming portion (S202). The surface of the photosensitive drum 1c in contact with the R-solid horizontal band toner image at the primary transfer portion passes through the charging region in contact with the charging roller 2c. At this time, the charging current Ic flowing to the charging roller 2c is detected by the charging current detection circuit 101c (S203).

  Next, the controller 12 obtains information of the charging current Ic detected by the charging current detection circuit 101c. The controller 12 also reads FIG. 10 stored in the memory 13. Then, based on FIG. 10, the primary transfer current is set (S204). With the above configuration, the surface resistance value of the intermediate transfer belt can be measured timely, and by controlling the primary transfer current according to the surface resistance value, the utilization efficiency of the toner can be improved. Also, the occurrence of black spots and white spots could be suppressed.

[Example 3]
The third embodiment has the same configuration as that of the first embodiment except that the configuration of the photosensitive drum is different. Therefore, the description will be omitted, and the portions different from the first embodiment will be described. In addition, the parts not described in particular are the same as in Example 1 described above. Since the image forming units Y, M, C, and Bk of each image forming station have the same configuration, the image forming unit Y that forms a yellow toner image will be described below, and the other image forming units M, C, and Bk Some of the explanations for

  In Examples 1 and 2, the photosensitive drum 1a adopts a long-lived photosensitive drum in which the surface layer 1t contains a curable material and the scraping amount is extremely low, that is, the capacity fluctuation of the photosensitive drum hardly occurs.

  Therefore, if the charging potential and the transfer current are constant, the pre-charging potential is constant, and the current flowing to the charging roller when solid white passes is constant.

  In the present embodiment, the photosensitive drum 1a is, for example, the photosensitive drum without the surface layer 1t shown in FIG.

  As shown in FIG. 12, the photosensitive drum 1a includes an undercoat layer 1q for suppressing light interference and improving adhesion of the upper layer, and a photogenerating layer 1r on the surface of an aluminum cylinder (conductive drum base) 1p. The charge transport layer 1s is formed by applying three layers in order from the bottom.

  In such a configuration, the charge transport layer 1s is worn out along with the paper passage durability. That is, even if the potential before charging is constant, the absolute value of the charging current flowing to the charging roller when solid white passes increases as the photosensitive drum is scraped, even though the capacity of the photosensitive drum fluctuates.

  In addition, even if the transfer current is constant, the post-transfer potential also changes as the capacity of the photosensitive drum fluctuates. As the photosensitive drum is scraped off, the amount of decrease in potential after transfer decreases (the charge elimination effect in transfer decreases).

  That is, in a system in which the capacity of the photosensitive drum fluctuates, the intermediate transfer is the absolute value of the charging current in the cyan image forming portion when the R solid is formed on the intermediate transfer belt 7 as in the first and second embodiments. It is difficult to measure the surface resistance of the belt 7.

  Therefore, in the present embodiment, in addition to the charging current Ic of the R solid image as in the first embodiment, the measurement is performed in a state in which the R solid image described in the first embodiment is replaced with a white solid image (white solid area). Use the charging current Iw. That is, an area (solid white area) in which a toner image is not formed is formed on the intermediate transfer belt 7, and the white solid area is allowed to pass through the transfer portion of the cyan station. At this time, when the area of the photosensitive drum 1c of the cyan station passing the transfer portion of the cyan station reaches the charging area in contact with the charging roller 2c, the charging current flowing to the charging roller 2c is taken as the charging current Iw. The charging current Iw is detected by the charging current detection circuit 101c. Then, based on the charging current Ic of the R solid image and the charging current Iw of the white solid image, the decrease in the surface resistance of the intermediate transfer belt 7 was determined. In this embodiment, the ratio (Ic / Iw) of the charging current Ic of the R solid image and the charging current Iw of the white solid image is used as information correlated with the surface resistance of the intermediate transfer belt 7.

  As described above, by using (Ic / Iw), it is possible to cancel the influence of the fluctuation of the capacity of the photosensitive drum and to detect the surface resistance of the intermediate transfer belt 7.

  FIG. 13 is a graph in which the ratio of the charging current Ic of the R solid image to the charging current Iw of the white solid image is the vertical axis, and the surface resistance of the intermediate transfer belt 7 is the horizontal axis. The loading amount of R solid was 1.0 (mg / cm 2). Further, the charging potential of the photosensitive drum 1c of Cst was -700V.

  From FIG. 13, (R solid image charging current Ic) / (white solid image charging current Iw) has a correlation with the surface resistance of the intermediate transfer belt 7. It was found that (R solid image charging current Ic) / (white solid image charging current Iw) decreases as the surface resistance of the intermediate transfer belt 7 decreases.

  Therefore, in the present embodiment, the surface resistance of the intermediate transfer belt 7 is detected from ((R solid image charging current Ic) / (white solid image charging current Iw) in FIG. 13 to determine the primary transfer current. I made it.

  FIG. 14 is a graph showing the relationship between the above-described (R solid image charging current Ic) / (white solid image charging current Iw) and the setting value of the current supplied to the primary transfer portion in this embodiment. This relationship is stored in advance in the memory 13 and is configured to be readable by the controller 12. The primary transfer current is normally set to 20 μA in this embodiment.

  The controller 12 determines that the resistance value of the intermediate transfer belt is high when (R solid image charging current Ic) / (white solid image charging current Iw) is 0.68 or more. Then, in order to suppress the occurrence of the black spots, the primary transfer current is set to be reduced. Further, the controller 12 determines that the resistance value of the intermediate transfer belt is low when (R solid image charging current Ic) / (white solid image charging current Iw) (0.32 or less), and white spots. In order to suppress the occurrence of dots, the primary transfer current is set to be reduced.

<Measurement of surface resistance of intermediate transfer belt and setting flow of primary transfer bias>
15 shows an operation flowchart of the present embodiment. The surface resistance of the intermediate transfer belt 7 is measured by this control. The timing of execution of this control is once for every 1000 sheets.

  First, the controller 12 rotates the photosensitive drums 1 a to 1 d and the intermediate transfer belt 7 in the pre-rotation step of FIG. 3. Then, in each of the image forming sections, the photosensitive drums 1a to 1d are charged by the charging rollers 2a to 2d as in the case of the normal image formation. Then, the primary transfer bias is applied to the transfer rollers 5a to 5d when the respective regions of the charged photosensitive drums 1a to 1d pass through the portions facing the transfer rollers 5a to 5d.

  Next, the controller 12 forms an R solid horizontal band toner image on the intermediate transfer belt 7 by the yellow (Y) image forming portion and the magenta (M) image forming portion. (S301) At this time, the exposure device of the cyan image forming unit is turned off so as not to perform image exposure. That is, solid white is formed on the cyan image forming portion.

  The loading amount of the R solid horizontal belt toner image is 1.0 (mg / cm 2), the width in the longitudinal direction is the maximum image forming width (maximum developable area width), and the width in the rotational direction of the intermediate transfer belt is 10 mm. Further, on the intermediate transfer belt 7, a white solid area on which no toner image is formed is formed at a position adjacent to the R solid horizontal band toner image in the rotation direction of the intermediate transfer belt 7. The width in the longitudinal direction of the white solid area is the entire width direction of the intermediate transfer belt 7. Further, the width of the white solid area in the rotational direction of the intermediate transfer belt 7 was 10 mm. Next, in (S301), the R solid horizontal band toner image formed on the intermediate transfer belt 7 and the primary transfer portion of the image forming portion where the white solid area is cyan are reached. (S302)

  Next, the controller 12 controls the charging current Ic flowing to the charging roller 2c by the charging current detection circuit 101c when the portion of the photosensitive drum 1c in contact with the R solid horizontal band toner image comes to the charging area contacting the charging roller 2c. To detect. (S303)

  Similarly, the controller 12 detects the charging current Iw flowing to the charging roller 2c by the charging current detection circuit 101c when the portion of the photosensitive drum 1c in contact with the white solid area comes in contact with the charging roller 2c. (S303)

  Next, the controller 12 calculates the ratio of charging current, Ic / Iw (S304).

  Next, the controller 12 determines whether Ic / Iw is 0.68 or more (S305).

  If Ic / Iw is 0.68 or more, the controller 12 changes the setting to lower the primary transfer current based on the relationship of FIG. 14 stored in the memory 13 (S306).

  If the charging current Ic is less than 0.68, then the controller 12 determines whether Ic / Iw is 0.32 or less in the controller 12 (S307).

  If Ic / Iw is 0.32 or less, the controller 12 changes the setting to increase the primary transfer current based on the relationship of FIG. 14 stored in the memory 13 (S308).

  If Ic / Iw was greater than 0.32 and less than 0.68, the primary transfer current was kept at 20 μA.

  With the above configuration, even when the photosensitive drum is easily scraped and the capacity fluctuation is large, the surface resistance value of the intermediate transfer belt can be measured timely, and the primary transfer current is controlled according to the surface resistance value. By doing this, it was possible to suppress the occurrence of black spots and white spots.

(Others)
In Example 3, the method of detecting the life of the intermediate transfer belt from the ratio of the charging current Ic at the time of R solid of Cst and the charging current Iw at the time of solid white uses the curable material for the surface layer of Example 1 The present invention is also effective in a photosensitive drum having almost no capacity fluctuation.

  In the present embodiment, a four-color tandem system of Y, M, C, and Bk is exemplified, but, for example, three colors of Y, M, and C may be used. Further, it may have an image forming portion other than Y, M, C, Bk.

  In the first and second embodiments, the tandem system of Y, M, C, and Bk in order is taken as an example, R which is a secondary color of Y and M is transferred, and the value of the charging current of Cst is measured. An example is to detect the life of the transfer belt. However, it is equally effective to transfer R and measure the value of Bkst charging current.

  It is also obvious that measuring the charging current with Bkst has the same effect on G (green), which is the secondary color of Y and C, and B (blue), which is the secondary color of M and C. It is.

  In this embodiment, the secondary color solid formed in the upstream image forming unit is placed on the intermediate transfer belt, and the charging current of the downstream image forming unit is detected as a representative example. The reason is that the larger the amount of toner as the resistor, the more the change of the charging current in the downstream st, and the higher the accuracy. Also, the secondary color solid is usually the upper limit of the amount of toner formed by the image forming apparatus. Therefore, the maximum amount of toner loaded on the intermediate transfer belt can use density control of a normal image forming apparatus. Therefore, it is recommended to detect the surface resistance of the intermediate transfer belt 7 of this embodiment with a solid color secondary color image. However, although the accuracy is reduced, it may be a primary color solid image. In addition, for example, since the purpose is not to transfer onto paper, a tertiary color may be placed to measure the charging current at the final st.

  In the present embodiment, a so-called DC charging method in which a direct current voltage is applied to the charging roller and a potential is applied to the photosensitive drum is taken as an example. However, it is also possible to adopt an AC charging method in which an AC voltage is superimposed on a DC voltage on the charging roller. In this case, since the difference between the potential before charging and the potential after charging is detected as a direct current, it is apparent that the AC charging method can also be employed.

  In the present embodiment, the control for transferring the horizontal band of the secondary color solid is performed at a timing of once per 1000 sheets, but this horizontal band is not only transferred for the purpose of the present embodiment, but other control is also performed. It may be used in combination with the control of For example, toner band supplied as a lubricant to the cleaning portion of the photosensitive drum, lateral band formation for maintaining the surface property of the transfer belt CLN, control for discharging toner for maintenance of the developer life in the developing device, etc. Can be used in combination with By doing so, it is also possible to reduce the amount of toner used.

Claims (5)

A first image forming unit for forming an image;
A second image forming unit for forming an image;
An intermediate transfer belt provided rotatably and to which the toner images formed by the first image forming unit and the second image forming unit are respectively transferred;
A control unit that controls operations of the first image forming unit and the second image forming unit;
The first image forming unit includes an image carrier, a charging device for charging the image carrier in a charging region, a current detection unit for detecting a current flowing to the charging device, and the image carrier. A developing device for developing the latent image with a developer containing toner and carrier, and a transfer device for applying a transfer bias and transferring the toner image formed on the image carrier to the intermediate transfer belt in a transfer region; In the image forming apparatus, the first image forming unit is provided downstream of the second image forming unit with respect to the rotation direction of the intermediate transfer belt.
The controller charges the surface of the image carrier with the charging device during non-image formation, and the surface of the image carrier charged by the charging device passes through the transfer area. A predetermined transfer bias is applied to the transfer device, and when the predetermined transfer bias is applied, the predetermined toner image formed on the intermediate transfer belt by the second image forming unit is used as the first toner image. The first area of the image carrier which passes through the transfer area of the image forming unit and passes the transfer area when the predetermined toner image passes through the transfer area is moved toward the charging area The charging current Ic is detected by the current detection unit when the charging current Ic is a current flowing through the charging device when the first area passes through the charging area, and the charging current Ic is detected based on the charging current Ic. Picture Wherein setting the transfer bias applied to the transfer device at the time of forming, the image forming apparatus characterized by.   The control unit sets the transfer bias so that a first transfer current flows to the transfer device at the time of image formation when the charging current Ic is a first predetermined value, and the charging current Ic is the first predetermined. In the case of a second predetermined value smaller than the value, the transfer bias is set so that a second transfer current larger than the first transfer current flows in the transfer device at the time of image formation. An image forming apparatus according to claim 1.   The image forming apparatus further includes a third image forming unit provided upstream of the second image forming unit with respect to the rotational direction of the intermediate transfer belt and forming a toner image on the intermediate transfer belt, wherein the predetermined toner image is 3. The image formation according to claim 1, wherein the solid image formed by the second image forming unit and the solid image formed by the third image forming unit are superimposed. apparatus.   In the detection mode for detecting the charging current Ic, when the predetermined transfer bias is applied, the controller controls the white solid area where the toner image is not formed in the area of the intermediate transfer belt. A second area of the first image carrier that passes through the transfer area of the first image forming unit and passes the transfer area when the white solid area passes through the transfer area; The charging current Iw is detected by the current detection unit, and the charging current Iw is moved when the current flowing through the charging device is set to the charging current Iw when the second region passes the charging region. The image forming apparatus according to any one of claims 1 to 3, wherein the transfer bias applied to the transfer device at the time of image formation is controlled based on the charging current Ic.   When the value of (Ic / Iw), which is the ratio of the charging current Iw to the charging current Ic, is a first value, the control unit causes the first transfer current to flow to the transfer device during image formation. Setting the transfer bias to a value of (Ic / Iw) is a second value smaller than the first value, the first transfer current larger than the first transfer current to the transfer device at the time of image formation; The image forming apparatus according to claim 4, wherein the transfer bias is set so that a transfer current of 2 flows.

Images