JP5968055B2 - Image forming apparatus - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine using an electrophotographic system, an electrostatic recording system, etc., and particularly for adjustment on an image carrier and an intermediate transfer body. The present invention relates to a structure for detecting a toner image.

  2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, for example, an electrostatic image (latent image) formed on an electrophotographic photosensitive member (photosensitive member) that is an image carrier is developed with toner to form a toner image. Thereafter, the toner image formed on the photoconductor is finally transferred to a recording material (recording paper, OHP sheet, etc.), fixed, and then output to the outside of the image forming apparatus. As a method for transferring the toner image on the photosensitive member to the recording material, the toner image formed on the photosensitive member is once transferred onto the intermediate transfer member, and then the toner image on the intermediate transferring member is transferred onto the recording material. There is an intermediate transfer method. In addition, as an intermediate transfer method, a plurality of image forming units are arranged in the rotation direction of the intermediate transfer member, and toner images of respective colors formed by the respective image forming units are sequentially transferred onto the intermediate transfer member. The structure of is known.

  Further, an adjustment toner image (hereinafter referred to as a patch) having a predetermined density is formed on the photosensitive member or the intermediate transfer member, and the density of the patch is detected and fed back to the image forming condition according to the density of the patch density. A density correction technique for performing the above is known (see, for example, Patent Document 1). Furthermore, a technique for correcting the registration error of each color component by detecting the relative position of the patch of each color component is also known.

JP 2003-202711 A

  By the way, with respect to detection of patch density and position (patch detection) as described above, in an image forming apparatus that forms a multicolor image, a multicolor image is formed on the intermediate transfer body at once, and the patch is detected on the intermediate transfer body. It is possible to read all at once. However, the intermediate transfer member has a possibility that the background signal of the intermediate transfer member may fluctuate greatly due to changes in surface roughness and surface color due to long-term use. For example, the surface roughness may increase due to adhesion of paper filler, and the surface may become white due to adhesion of an external additive of toner. If the background signal of the intermediate transfer body is not stable, the toner density is calculated based on the contrast with the background when detecting patches. For this reason, the detection accuracy decreases for colors that are difficult to contrast with the background (for example, black). End up.

  On the other hand, it is also conceivable to perform patch detection on a photoreceptor that does not directly contact the recording material. In this case, since there is no transfer material from the recording material, the durability fluctuation in the surface property is small as compared with the intermediate transfer member, which is more preferable from the viewpoint of the stability of the ground signal. Therefore, black patch detection that is particularly low in brightness and easily affected by background fluctuations is performed on the photoconductor, and patch detection of color components with high lightness such as magenta, cyan, and yellow is performed on the intermediate transfer member. It is possible.

  In addition, when such a configuration is applied to a tandem type image forming apparatus, the black image forming unit (station) is moved to the most downstream side in response to a request for shortening an FCOT (first copy out time) when forming a black monochrome image. It is preferable to arrange.

  However, when configured in this manner, the patch formed at the upstream station passes through the primary transfer portion that transfers the toner image from the photosensitive member to the intermediate transfer member at the downstream station, and then reappears on the photosensitive member. There is a possibility of transcription. If retransfer occurs, the accuracy of the patch detection on the intermediate transfer member may be reduced.

  For example, as shown in FIG. 9, when a normal transfer bias is applied when the toner image is transferred from the photosensitive drum 1 as a photosensitive member to the intermediate transfer belt 51 as an intermediate transfer member in the primary transfer portion T1, the primary transfer portion A discharge phenomenon occurs downstream of T1. Here, when the toner charging polarity is negative (negative), the primary transfer roller 6 as a primary transfer member is positive (positive) with respect to the intermediate transfer belt 51 in order to electrostatically transfer the negatively charged toner to the intermediate transfer belt. Inject charge. Then, the negatively charged toner is transferred to the intermediate transfer belt 51 by charging the intermediate transfer belt 51 to a positive polarity. At this time, since a large potential difference is generated between the photosensitive drum 1 downstream of the primary transfer portion T1 and the intermediate transfer belt 51 (gap), a discharge phenomenon occurs in the gap downstream of the primary transfer portion T1 after toner transfer. To do.

  When such a discharge phenomenon occurs, charge movement occurs so as to reduce the potential difference between the intermediate transfer belt 51 and the photosensitive drum 1, so that positive charges move from the intermediate transfer belt 51 toward the photosensitive drum 1. . At that time, the positive charge jumps into the toner held on the intermediate transfer belt 51, so that the toner may be reversed to the positive polarity. The positively reversed toner moves in the direction of the photosensitive drum 1. The above is the generation mechanism of retransfer.

  On the other hand, in order to pass the toner image on the photosensitive drum 1 through the primary transfer portion T1 without being transferred to the intermediate transfer belt 51, it is conceivable that the applied bias is a negative polarity that is the same polarity as the toner. In this case, however, toner retransfer may occur as follows. That is, if negative charge is supplied from the primary transfer roller 6 to the intermediate transfer belt 51, electrostatic repulsion occurs with the negatively charged toner supplied to the primary transfer portion T1. Therefore, the toner acts to electrostatically blow off the toner from the intermediate transfer belt 51 to the photosensitive drum 1.

  In this way, retransfer can occur regardless of whether the bias applied to the primary transfer portion is positive or negative. Here, as described above, black patch detection is performed on the photosensitive member, and patch detection of color components with high brightness such as magenta, cyan, yellow, etc. is performed on the intermediate transfer member. It is necessary to appropriately set the bias applied to the transfer portion.

  For example, when the yellow patch passes through the primary transfer portion of the downstream magenta station, it is necessary to transfer the magenta patch to the intermediate transfer member while suppressing the yellow patch from being transferred again. On the other hand, since the black patch is detected on the drum, it is not necessary to transfer it to the intermediate transfer member. Therefore, when patches of each color such as yellow pass through the primary transfer portion of the downstream black station, it is required to suppress retransfer of the patches of each color without transferring the black patch.

  In view of such circumstances, the present invention is configured to detect the adjustment toner image on the intermediate transfer member and the image carrier, respectively, and perform retransfer when the adjustment toner image passes through the primary transfer portion. It was invented to realize a structure that can be suppressed.

The present invention includes a rotating intermediate transfer member, a first image forming unit, a second image forming unit, and a third image forming unit arranged side by side in the rotation direction of the intermediate transfer member, and the first and first image forming units. 2, a first adjustment toner detecting unit that is disposed downstream of the third image forming unit in the rotation direction of the intermediate transfer member and detects an adjustment toner image formed on the surface of the intermediate transfer member; A rotating image carrier provided in each of the first, second, and third image forming units, a charging unit that charges the surface of the image carrier, and the charged image carrier are exposed and statically exposed. An exposure unit that forms an electrostatic latent image, a developing unit that develops the electrostatic latent image formed on the surface of the image carrier with toner, and a toner image formed on the surface of the image carrier include the intermediate transfer member. Formed on the surface of the image bearing member of the third image forming unit. Second adjustment toner detecting means for detecting the adjusted toner image, and the second image forming unit disposed downstream of the first image forming unit with respect to the rotation direction of the intermediate transfer member. When transferring the toner image formed on the surface of the image carrier of the second image forming unit to the intermediate transfer member, the transfer bias is applied to the image forming unit upstream of the second image forming unit. The adjustment toner image formed and transferred to the intermediate transfer member passes through the primary transfer portion of the second image forming portion and is formed on the surface of the image carrier of the second image forming portion. A first bias applying means for applying a first passing bias to the primary transfer portion of the second image forming portion when transferring the toner image to the intermediate transfer member, and the intermediate transfer member; respect of the rotational direction, the third fraction which is disposed furthest downstream When the toner image formed on the surface of the image carrier of the third image forming unit is transferred to the intermediate transfer member, the forming unit applies a transfer bias to the upstream side of the third image forming unit. And the adjustment toner image formed on the surface of the image carrier of the third image forming unit is formed in the third image forming unit. A second bias applying means for applying a second pass bias to the primary transfer portion of the third image forming portion when passing through the primary transfer portion, and is applied to the primary transfer portion. If the reverse polarity is larger than the same polarity as the charging polarity of the toner, the bias is charged by the transfer bias, the first passing bias, the second passing bias, and the charging means in descending order. Table of image carrier not exposed The image forming apparatus is characterized in that the potential is set to be a non-exposed portion potential of the surface.

  According to the present invention, the bias applied to the primary transfer portion is set to be the transfer bias, the first passing bias, the second passing bias, and the non-exposed portion potential in descending order. It is possible to suppress retransfer when the adjustment toner image passes through the primary transfer portion.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. The schematic block diagram of a patch sensor. FIG. 3 is a diagram illustrating a black patch formed on a photosensitive drum. FIG. 4 is a diagram showing yellow, magenta, and cyan patches formed on an intermediate transfer belt. The figure which shows the relationship between a patch density | concentration and the signal of a patch sensor. FIG. 6 is a diagram showing a relationship between primary transfer contrast and retransfer toner amount. FIG. 6 is a diagram illustrating a relationship between primary transfer contrast and drum residual toner amount at a magenta station. FIG. 4 is a diagram illustrating a relationship between a surface potential of a photosensitive drum and a bias applied to a primary transfer unit during normal image formation and patch formation in the present embodiment. The schematic diagram of the primary transfer part vicinity for demonstrating the mechanism of retransfer.

  An embodiment of the present invention will be described with reference to FIGS. First, a schematic configuration of the image forming apparatus of the present embodiment will be described with reference to FIG.

[Image forming apparatus]
The image forming apparatus 100 of the present embodiment is a laser beam printer that can form a full color image on a recording material (recording paper, OHP sheet, cloth, etc.) using an electrophotographic system. The image forming apparatus 100 includes image forming units (stations) Py, Pm, Pc, and Pk for forming yellow, magenta, cyan, and black images, respectively, as image forming units that form toner images. These image forming portions Py, Pm, Pc, and Pk are arranged side by side in the rotation direction of the intermediate transfer belt 51 as an intermediate transfer member. Therefore, the image forming apparatus of the present embodiment is a tandem type intermediate transfer type image forming apparatus. In this embodiment, the yellow image forming unit Py is the first image forming unit, the magenta and cyan image forming units Pm and Pc are the second image forming unit, and the black image forming unit Pk is the third. Respectively corresponding to the image forming units. In the present embodiment, the black image forming portion Pk is disposed on the most downstream side in the rotation direction of the intermediate transfer belt 51.

  The basic configuration of each image forming unit is substantially the same except for the diameter of the photosensitive drum, the color of toner to be used, and the configuration of patch detection. Therefore, in the following, when there is no particular need to distinguish, subscripts y, m, c, and k attached to the reference numerals to indicate that the element is provided for one of the colors will be omitted, and a general description will be given. To do.

  The image forming unit (station) P is provided with a cylindrical photosensitive member, that is, a photosensitive drum 1 as an image carrier. Around the photosensitive drum 1, a charging roller 2 as a charging unit and a laser beam scanner 3 as an exposure unit are arranged. Further, around the photosensitive drum 1, a developing device 4 as a developing unit and a cleaning device 7 as a cleaning unit are arranged.

  The intermediate transfer belt 51 is wound around a driving roller 92, a secondary transfer inner roller (inner roller) 71, and two idler rollers 912 and 913 as a plurality of support members. When the driving force is transmitted to the driving roller 92, the intermediate transfer belt 51 rotates (rotates) in the direction of the arrow R2 in the drawing. A primary transfer roller (electrode member) 6 as a primary transfer member is disposed on the inner peripheral surface side of the intermediate transfer belt 51 at a position facing the photosensitive drums 1 y to 1 k of each image forming unit. The primary transfer rollers 6y to 6k press the intermediate transfer belt 51 toward the photosensitive drums 1y to 1k, so that the primary transfer portions (primary transfer nips) T1y to T1k where the intermediate transfer belt 51 contacts the photosensitive drum 1 are respectively provided. It is formed.

  Further, a secondary transfer outer roller (outer roller) 72 as a contact member is disposed at a position facing the inner roller 71 via the intermediate transfer belt 51. The intermediate transfer belt 51 is sandwiched between an inner roller 71 and an outer roller 72 constituting a secondary transfer unit. The inner roller (electrode member) 71 is in contact with the inner periphery of the intermediate transfer belt 51, and the outer roller 72 is It contacts the outer periphery of the intermediate transfer belt 51. Then, the secondary transfer portion T2 is formed.

  The secondary transfer outer roller 72 is disposed so as to be able to contact and separate from the intermediate transfer belt 51. For this purpose, a contact / separation device 80 as contact / separation means is provided. The contact / separation device 80 has a cam 81 that contacts the frame 721 that supports the secondary transfer outer roller 72 and swings the frame 721 relative to the swing shaft 722, and a motor 82 that rotationally drives the cam 81. The secondary transfer outer roller 72 contacts or separates from the intermediate transfer belt 51 when the cam 81 rotates and the frame 721 swings. The motor 82 is controlled by the control unit 200 which is also a contact / separation control unit. In addition, as such contact / separation means, other structures, such as a solenoid, can also be used, for example.

  The photosensitive drum 1 is rotationally driven at a predetermined peripheral speed (process speed) in a direction (counterclockwise direction) indicated by an arrow R1 in the drawing. The surface of the photosensitive drum 1 is charged (primarily charged) to a predetermined polarity and potential by a charging roller 2 that is a contact charging member.

  The laser beam scanner 3 outputs laser light that is on / off modulated in accordance with image information input from an external device such as an image scanner or a computer. The laser beam scanner 3 scans and exposes the charged surface on the photosensitive drum 1 with the laser beam. An electrostatic latent image corresponding to the target image information is formed on the photosensitive drum 1 by scanning exposure by the laser beam scanner 3. Such a laser beam scanner 3 is controlled by a control unit 200 as control means.

  The electrostatic latent image formed on the photosensitive drum 1 is developed as a toner image by the developing device 4. In the present embodiment, the developing device 4 contains a two-component developer including nonmagnetic resin toner particles (toner) and magnetic carrier particles (carrier) as developers. The developing device 4 has a developing sleeve as a developer carrying member disposed to face the photosensitive drum 1. The electrostatic latent image on the photosensitive drum 1 is developed by supplying toner from the developer carried on the developing sleeve to the photosensitive drum 1.

  The toner image formed on the surface of the photosensitive drum 1 is transferred to the intermediate transfer belt 51 by applying a primary transfer bias to the primary transfer portions T1y, T1m, T1c, and T1k. In this embodiment, the image is electrostatically transferred (primary transfer) onto the intermediate transfer belt 51 by the primary transfer rollers 6y, 6m, 6c, and 6k. At this time, an appropriate primary transfer bias is output from the power supplies 61y to 61k as the bias applying means, and applied to the primary transfer rollers 6y, 6m, 6c, and 6k as the primary transfer members. In the present embodiment, the power sources 61m and 61c correspond to the first bias applying unit, and the power source 61k corresponds to the second bias applying unit.

  Such power supplies 61y to 61k are controlled by the control unit 200 which is also a control unit, and the bias applied to the primary transfer rollers 6y, 6m, 6c and 6k can be appropriately switched. As an example, the primary transfer bias is + 900V.

  The primary transfer residual toner remaining on the photosensitive drum 1 after the primary transfer process is scraped off and collected by the cleaning device 7. As a result, the surface of the photosensitive drum 1 is cleaned and repeatedly used for image formation.

  For example, when a full-color image is formed, the above-described operations are sequentially performed in the first to fourth image forming units. As a result, the toner images of the respective colors are transferred on the intermediate transfer belt 51 while being superimposed on each primary transfer portion T1.

  On the other hand, in synchronization with the toner image on the intermediate transfer belt 51, the recording material is conveyed from a recording material supply unit (not shown) to the secondary transfer unit T2. The recording material is supplied to the secondary transfer portion T2 in timing with the toner image on the intermediate transfer belt 51.

  The toner image on the intermediate transfer belt 51 is electrostatically transferred to the recording material by the electric field between the inner roller 71 and the outer roller 72 in the secondary transfer portion T2. Here, by applying a secondary transfer bias to either the inner roller 71 or the outer roller 72, an electric field can be formed between these rollers. As an example, the secondary transfer bias is 2.3 kV.

  The recording material onto which the toner image has been transferred in the secondary transfer portion T2 is then conveyed and introduced into a fixing device (not shown) through a conveyance path (not shown). The toner image is fixed on the recording material by being pressurized and heated by a fixing device. The secondary transfer residual toner remaining on the intermediate transfer belt 51 after the secondary transfer process is scraped off and collected by the cleaning blade 52. As a result, the surface of the intermediate transfer belt 51 is cleaned and repeatedly used for image formation.

In this embodiment, as the intermediate transfer belt 51, a semiconductive polyimide resin having a relative dielectric constant ε = 3 to 5, a volume resistivity ρv = 10 ⁶ to 10 ¹¹ Ω · m, and a thickness of 85 μm was used.

As the primary transfer rollers 6y, 6m, 6c, and 6k, it is possible to use semiconductive ones having a resistance value of 10 ² to 10 ⁸ Ω when 2000V is applied. In this embodiment, as the primary transfer roller 6, an ion conductive sponge roller formed by blending nitrile rubber and ethylene-epichlorohydrin copolymer and having an outer diameter of φ18 mm and a core metal diameter of φ8 mm was used. The resistance values of the primary transfer rollers 6y, 6m, 6c, and 6k were about 10 ⁶ to 10 ⁸ Ω (applied voltage 2 kV) at a temperature of 23 ° C. and a humidity of 50%.

As the inner roller 71, a semiconductive roller having an outer diameter of 20 mm and a core metal diameter of 16 mm, in which conductive carbon is dispersed in EPDM rubber, was used. The resistance value of the inner roller 71 was about 10 ¹ to 10 ⁵ Ω (applied voltage 10 V) at a temperature of 23 ° C. and a humidity of 50% by the same measuring method as described above.

As the outer roller 72, an ion conductive sponge roller formed by blending nitrile rubber and ethylene-epichlorohydrin copolymer and having an outer diameter of 24 mm and a core metal diameter of 12 mm was used. The resistance value of the outer roller 56 was about 10 ⁶ to 10 ⁸ Ω (applied voltage 2 kV) at a temperature of 23 ° C. and a humidity of 50% by the same measuring method as described above.

  In the present embodiment, an on-drum patch sensor Sd is disposed facing the photosensitive drum 1k of the black image forming unit Pk, and an on-belt patch sensor Sb is disposed facing the intermediate transfer belt 51. That is, the on-belt patch sensor Sb serving as the first adjustment toner detecting means is downstream in the rotation direction of the intermediate transfer belt 51 from the image forming units Py to Pk which are the first, second, and third image forming units. Placed in. Then, a patch that is an adjustment toner image formed on the surface of the intermediate transfer belt 51 is detected. The on-drum patch sensor Sd as the second adjustment toner detection unit detects a patch that is an adjustment toner image formed on the surface of the photosensitive drum 1k.

  In this embodiment, the polyimide resin is used as the intermediate transfer belt 51 as described above, but it has been found that the surface properties of the belt fluctuate when a wide variety of recording materials are used. When filler such as talc is transferred onto the belt from the recording material, the surface roughness of the belt does not change, but the glossiness may be lowered. Alternatively, when paper dust is transferred from the recording material for a long period of time, the paper is used for a long period of time with the paper powder sandwiched in the nip portion between the cleaning blade 52 and the intermediate transfer belt 51. As a result, when an excessive amount of paper dust is supplied, circumferential scratches are generated in the rotational direction of the belt, which may increase the surface roughness of the belt.

  As the glossiness of the belt is reduced or the surface roughness of the belt is increased, there is a possibility that the fluctuation range of the belt surface detection becomes large for a sensor that performs optical detection.

  In the present embodiment, the on-belt patch sensor Sb is configured as shown in FIG. That is, the light emitted from the LED light source 101 detects the reflected light amount of the light component reflected on the surface of the intermediate transfer belt 51 by the photosensor 102 for the regular reflection component and the photosensor 103 for the irregular reflection component. The on-belt patch sensor Sb detects patches of three color components of yellow, magenta, and cyan. In particular, when detecting the density of the patch, the toner density is calculated by the difference between the reflected light amount component on the belt surface and the reflected light amount component of the patch portion.

  As the LED light source 101 of the on-belt patch sensor Sb, an LED (Light Emitting Diode) having a wavelength of 940 nm was used. Since all of yellow, magenta, and cyan detect the density of patches in the visible region, the light source preferably has a wavelength region that does not absorb any of the above color components.

  The configuration of the on-drum patch sensor Sd is the same as that of the on-belt patch sensor Sb. On the other hand, for the on-drum patch sensor Sd, the toner density is detected by the irregular reflection component in order to detect the patch of the black color component. An LED having a wavelength of 880 nm was used as the LED light source of the on-drum patch sensor Sd. Although exposure is performed only when a patch is read from the photosensitive drum 1k, it is preferable that the drum is in a wavelength region where the sensitivity of the drum is low when the photosensitive drum 1 is exposed. A wavelength region is preferable.

  The on-drum patch sensor Sd is disposed in the vicinity of the photosensitive drum 1k, and is disposed between the downstream of the developing sleeve facing position and the upstream of the primary transfer portion T1k with respect to the rotation direction of the photosensitive drum 1k. Thus, detection can be performed without disturbing the patch at the primary transfer portion T1k.

  In the present embodiment, patches of three color components of yellow, magenta, and cyan are read by the on-belt patch sensor Sb. As described above, it is possible to further reduce the size of the main body by collectively detecting patches of three color components on the intermediate transfer belt 51. On the other hand, the patch of the black color component can be detected stably over a long period of time by reading it with the on-drum patch sensor Sd. As a result, it is possible to stabilize the toner density and the registration error over a long period of time while reducing the size of the apparatus.

[Operation of contact / separation device in patch formation sequence]
Next, the operation of the contact / separation apparatus 80 in the patch formation sequence will be described. When shifting from the normal image formation to the patch formation sequence, the bias applied to the primary transfer portion is switched and the electrostatic latent image corresponding to the patch image is exposed. Further, when the yellow, magenta, and cyan color component patches formed on the intermediate transfer belt 51 are supplied to the secondary transfer portion T2, the secondary transfer outer roller 72 is released from pressure by the contact / separation device 80. The operation of separating from the intermediate transfer belt is also performed. Then, the secondary transfer outer roller 72 is prevented from being soiled by the patch.

  Here, it is necessary to allow a time of 170 msec as the time until the secondary transfer outer roller 72 is released from the pressure and separated from the intermediate transfer belt 51. Note that this time also allows for the time for the vibration of the intermediate transfer belt 51 to be settled when the pressure is released. Therefore, the separation itself is achieved in a time earlier than 170 msec.

  In addition, exposure of electrostatic latent image data corresponding to the black color component is started after 170 msec from the end of exposure of the final image before the transition from the normal image formation to the patch formation sequence. At this time, even when the exposure of the final image is completed, the rotation driving of the charging roller and the developing sleeve of the black station and the high voltage application are not stopped but are continuously performed. On the other hand, the secondary transfer outer roller 72 starts the pressure release and separation operation 30 msec after the completion of the secondary transfer of the final image of normal image formation to a recording material (not shown). By providing a margin of 30 msec, it is possible to reliably transfer the recording material from the secondary transfer portion T2 to a fixing device (not shown) when the final image is formed.

  On the other hand, after 170 msec from the timing when the final image formation is completed, the patch is adhered to the intermediate transfer belt 51 and conveyed to the secondary transfer portion T2. At this time, the secondary transfer outer roller 72 is intermediate. It is separated from the transfer belt 51. That is, the patch is conveyed to the secondary transfer portion T2 while the secondary transfer outer roller 72 is separated from the intermediate transfer belt 51 and is waiting for the vibration of the intermediate transfer belt 51 to settle. For this reason, contamination of the secondary transfer outer roller 72 due to the patch can be avoided.

  As described above, in this embodiment, the control unit 200 contacts the secondary transfer outer roller 72 away from the intermediate transfer belt 51 at the timing when the patch transferred to the intermediate transfer belt 51 reaches the secondary transfer unit T2. The separation device 80 is controlled. For this reason, contamination of the secondary transfer outer roller 72 due to the patch can be avoided.

[Black patch]
The configuration of the black patch formed on the photosensitive drum 1k of the black image forming portion Pk is as shown in FIG. In the present embodiment, patches having density levels of 0.4, 0.8, and 1.2 are formed in series in the rotational direction of the photosensitive drum 1k, and each density level is detected by the irregular reflection component of the on-drum patch sensor Sd. Detect the density of the patch. In the present embodiment, the control unit 200 determines that the laser beam scanner is based on a comparison between signal values detected for patches of density levels of 0.4, 0.8, and 1.2 and a signal level corresponding to a predetermined density. The density correction is performed by correcting the exposure light quantity by controlling 3k. The exposure light quantity is corrected by controlling the laser power and the PWM value, for example.

  Here, in the present embodiment, the length of each patch in the circumferential direction of the photosensitive drum 1k is 25 mm. Further, an interval of 5 mm is provided between patches of each density level. Since the spot diameter of the on-drum patch sensor Sd used in the present embodiment is 5 mm, the patches of each density level are spaced apart from each other by a distance equal to or greater than the spot diameter so as not to read adjacent density level patches simultaneously. It is necessary. In addition, it is desirable to read and average a plurality of patches in each density level in the patch in order to improve the detection accuracy of the patch density. For this reason, in the present embodiment, the patch length is set to 25 mm as described above, the density is detected at four locations in the patch, and the accuracy is improved by averaging.

  Here, in this embodiment, the black photosensitive drum 1k has a size of an outer diameter of 30 mm. Accordingly, since the circumferential direction of the photosensitive drum 1k is about 94 mm in one turn, the toner patch described above has a total density of 25 mm × 3 + 5 mm × 2 = 85 mm, including the three density levels and the interval between adjacent patches. It is possible to fit in one drum.

[Yellow, magenta, cyan patches]
On the other hand, the yellow, magenta, and cyan color component patches formed on the intermediate transfer belt 51 are as shown in FIG. When optically detecting the patch density, the amount of specular reflection decreases as the patch density increases. FIG. 5 shows the relationship between the patch density and the signal of the on-belt patch sensor Sb (same for the on-drum patch sensor Sd). In FIG. 5, the horizontal axis indicates the patch density, and the vertical axis indicates the difference between the regular reflection light amount on the surface of the intermediate transfer belt and the regular reflection light amount at the time of patch detection as a signal. FIG. 5 shows a saturation behavior in which the signal does not increase linearly even if the patch concentration is increased in a high concentration region exceeding a certain concentration region. The reason is considered as follows. That is, in order to form a toner image, an image is formed by laminating toner particles corresponding to the electrostatic latent image on a drum or a belt. It is considered that when the layers are stacked as described above, the reflected light amount does not change greatly.

  On the other hand, even in very low concentration patches, the signal linearity with respect to the patch concentration is low. The reason is considered as follows. That is, when forming a very low density patch image, when toner corresponding to an electrostatic latent image is stacked, the toner particles are only arranged sparsely, while the detection range of the patch sensor. Detects the amount of reflected light in the range of several millimeters. For this reason, in the case of the on-belt patch sensor Sb, it is considered to be affected by the belt surface.

  For the above reason, it is considered that the accuracy of density detection with respect to the patch density is lowered in a high density area where the signal value with respect to the patch density is saturated and the linearity is impaired and in a very low density area. Therefore, in the present embodiment, the patch density is targeted at three levels of 0.4, 0.8, and 1.2. As shown in FIG. 4, patches with different densities of three levels of yellow, magenta, and cyan are collectively detected at different positions in the width direction crossing the belt rotation direction. The length and interval of the patches are the same as in the case of black. Such yellow, magenta, and cyan patches are formed at positions shifted in the width direction of the photosensitive drum 1k and the intermediate transfer belt 51 with respect to the black patch. The timing when the yellow, magenta, and cyan patches pass through the primary transfer portion T1k is the same as the timing when the black patch passes through the primary transfer portion T1k. Accordingly, the yellow, magenta, and cyan patches pass through the non-exposed portion of the black photosensitive drum 1k.

  Further, regarding the on-belt patch sensor Sb, it is possible to simultaneously detect patches of three colors by providing an independent detection unit for each color component of yellow, magenta, and cyan (not shown). By detecting each color component at the same time, it is possible to reduce downtime of the main body as much as possible for density correction control for four colors by combining black.

  In the present embodiment, the control unit 200 determines whether the laser 200 is a laser based on a comparison between the signal value detected for each patch having a density level of 0.4, 0.8, and 1.2 and a signal level corresponding to a predetermined density. The exposure light quantity is corrected by controlling the beam scanners 3y, 3m, and 3c. Then, the patch density correction is performed as described above. The exposure light quantity is corrected by controlling the laser power and the PWM value, for example.

  In this embodiment, the yellow, magenta, and cyan stations are not provided with patch sensors in the vicinity of the drum, so the configuration around the photosensitive drum can be simplified, the diameter of the photosensitive drum can be reduced, and the size of the main body can be reduced. It is possible to realize.

[Switching the bias applied to the black primary transfer area]
Next, switching of the bias applied to the black primary transfer portion T1k in the patch formation sequence will be described. In the present embodiment, when a patch of three color components, yellow, magenta, and cyan, passes through the primary transfer portion T1k of the black image forming portion Pk, a bias applied to the primary transfer portion T1k is a normal transfer. Switching from bias. At this time, the black color component patch also passes through the primary transfer portion T1k. However, since the black patch is detected by the on-drum patch sensor Sd, it is not necessary to transfer it to the intermediate transfer belt 51. Then, the influence on the density due to the toner retransfer of the patch formed by the upstream image forming portions Py, Pm, and Pc is suppressed. Hereinafter, in a normal image forming sequence, the bias applied to the primary transfer portion T1k is a normal bias (transfer bias), and when the patch passes through the primary transfer portion T1k in order to suppress the retransfer of the patch, The bias to be applied is a second passing bias.

  That is, in the present embodiment, the power source 61k switches the bias applied to the primary transfer portion T1k between the normal bias and the second passing bias that is different from the normal bias. The normal bias is applied when the toner image formed on the surface of the photosensitive drum 1k is transferred to the intermediate transfer belt 51. The second pass bias is formed by the upstream image forming portions Py, Pm, and Pc, and the patch transferred to the intermediate transfer belt 51 and the patch formed on the surface of the photosensitive drum 1k are the primary transfer portion T1k. Applied when passing through. When only the black patch is formed, the second passing bias is applied when the patch formed on the surface of the photosensitive drum 1k passes through the primary transfer portion T1k.

  In this embodiment, the non-exposed portion potential of the photosensitive drum 1k (the potential of the surface of the photosensitive drum 1k that is charged by the charging roller 2k and not exposed) is set to −700 V in both the normal image forming sequence and the patch formation. . The development bias is a bias in which an AC component is superimposed on a DC component for the purpose of improving development efficiency, but the DC component is set to -600V. In this way, by setting the DC component at the developing portion to a positive side potential of about 100 V with respect to the non-exposed portion potential of the photosensitive drum 1k, the adhesion (fogging) of the developer to the non-exposed portion is effectively suppressed. ing. In addition, by avoiding an excessively large potential difference, it is possible to suppress carrier adhesion in the non-exposed portion.

  Here, as shown in FIG. 6, the primary transfer contrast potential, which is the potential difference between the bias applied to the primary transfer roller and the non-exposed portion potential of the photosensitive drum, does not fall below 0, and the primary transfer roller side is on the positive polarity side. Thus, retransfer can be effectively suppressed by not causing an excessively large potential difference. In the present embodiment, the bias applied to the primary transfer portion T1k of the black image forming portion Pk is +900 V as described above in the normal image forming sequence, but the second passing bias is the same as the DC component of the developing bias. , Switch to -600V.

  Since the non-exposed portion potential of the photosensitive drum 1k is −700 V, the primary transfer contrast is +1600 V during normal image formation, whereas the second passing bias has a small potential difference of +100 V. Thereby, it is possible to avoid discharge at the primary transfer portion T1k and to effectively suppress retransfer due to positive reversal of toner of a patch such as yellow passing therethrough. On the other hand, retransfer due to electrostatic repulsion can be effectively avoided by not making the primary transfer contrast on the negative polarity side. This makes it possible to effectively suppress retransfer of yellow, magenta, and cyan patches when they pass through the black primary transfer portion T1k. Here, as described above, since the black patch does not need to be transferred to the intermediate transfer belt 51 at the primary transfer portion T1k, the second passing bias can be reduced to −600V as described above.

  In the present embodiment, the second pass bias has a non-exposed portion potential (−700 V) when the reverse polarity (positive polarity) is larger than the same polarity (negative polarity) as the toner charging polarity. ) And smaller than the normal bias (+900 V). In other words, the second pass bias is such that the primary transfer contrast, which is the potential difference from the non-exposed portion potential, is on the positive polarity side and sufficiently smaller than the primary transfer contrast with the normal bias. Specifically, as described above, the non-exposed portion potential is set to −700 V, the second pass bias applied to the primary transfer roller 6 k is set to −600 V, and the normal bias is set to +900 V.

[Switching the bias applied to the magenta and cyan primary transfer sections]
Next, switching of the bias applied to the primary transfer portions T1m and T1c of magenta and cyan in the patch formation sequence will be described. In the case of the present embodiment, there is a risk that retransfer occurs at each of the magenta and cyan stations. Therefore, when the yellow color component patch passes through the primary transfer portion T1m of the magenta image forming portion Pm, the bias applied to the primary transfer portion T1m is switched from the normal transfer bias. At the same time, the magenta color component patch is transferred to the intermediate transfer belt 51 by the primary transfer portion T1m. Similarly, when the yellow and magenta color component patches pass through the primary transfer portion T1c of the cyan image forming portion Pm, the bias applied to the primary transfer portion T1c is switched from the normal transfer bias. At the same time, the cyan color component patch is transferred to the intermediate transfer belt 51 by the primary transfer portion T1c.

  As described above, the bias is switched when the patch formed in the upstream image forming unit passes to suppress the influence on the density due to the toner retransfer. Hereinafter, the bias applied to the primary transfer portions T1m and T1c in the normal image forming sequence is referred to as a normal bias (transfer bias). Further, in order to suppress retransfer of the patch, a bias applied to the primary transfer portions T1m and T1c when the patch passes through the primary transfer portions T1m and T1c is set as a first passing bias.

  That is, in the present embodiment, the power supplies 61m and 61c switch the bias applied to the primary transfer portions T1m and T1c between the normal bias and the first passing bias different from the normal bias. The normal bias is applied when the toner images formed on the surfaces of the photosensitive drums 1m and 1c are transferred to the intermediate transfer belt 51. The first pass bias is formed in the upstream image forming unit, and the patch transferred to the intermediate transfer belt 51 passes through the primary transfer units T1m and T1c and is formed on the surface of the photosensitive drums 1m and 1c. Applied when the patch is transferred to the intermediate transfer belt 51.

  Here, the amount of toner remaining on the magenta photosensitive drum 1m with respect to the primary transfer contrast (remaining drum toner amount) will be described using FIG. 7 as an example of a magenta image forming unit (magenta station) Pm. If the yellow toner formed at the upstream station is retransferred, the amount becomes the residual toner amount of the drum, and if the magenta toner is not transferred to the intermediate transfer belt 51 and remains on the drum, the amount is reduced. This is the drum residual toner amount.

  In the magenta station, in order to transfer the magenta patch to the intermediate transfer belt 51, it is necessary to set the primary transfer bias to a potential on the positive side with respect to the exposure portion potential of the photosensitive drum 1m. Therefore, the primary transfer contrast needs to be set to a potential difference larger on the positive side than the latent image contrast as a potential difference between the non-exposed portion potential and the exposed portion potential. In the present embodiment, the exposure portion potential is about −200 to −250 V, and +900 V is applied as the normal bias as described above during normal image formation so as to be equal to or less than a predetermined toner remaining amount. .

  However, when the yellow patch passes, there is a risk that the yellow toner may be retransferred although the amount is small. That is, in order to transfer the magenta patch, it is necessary to provide a predetermined positive potential as the primary transfer contrast. At this time, retransfer due to toner polarity reversal occurs for the yellow patch that passes simultaneously. . In particular, since the yellow patch passes through the cyan station in addition to the magenta station, it undergoes retransfer twice, so that the toner density of the patch may be greatly reduced. Therefore, in the present embodiment, the bias applied when the patch passes at each of the magenta and cyan stations is switched to the first passing bias that is different from the normal bias at the time of normal image formation as follows. Yes.

  In the present embodiment, for the three color components of yellow, magenta, and cyan, the maximum density is about 1.45, and the maximum density of the patch is 1.2. For this reason, the maximum amount of toner to be transferred to the intermediate transfer belt 51 is smaller during patch formation than during normal image formation that may be formed. For this reason, when transferring the patch, +700 V, which is lower than the normal bias +900 V at the time of normal image formation, is used as the first passing bias.

  By switching the first passing bias to a bias lower than the normal bias in this way, for example, when a yellow patch passes through a magenta station, the first passing bias is re-applied to the magenta photosensitive drum 1m by reversing the polarity of yellow toner. Transfer can be reduced. Similarly, when yellow and magenta patches pass through the cyan station, retransfer due to polarity reversal of yellow and magenta toner on the cyan photosensitive drum 1c can be reduced. The bias applied to the primary transfer portion T1y at the time of patch formation at the yellow station may be a normal bias, but the first passing bias may be applied as in the magenta and cyan stations.

[Relationship between bias applied to primary transfer area]
As described above, in the case of the present embodiment, the bias applied to the primary transfer portion has a reverse polarity (positive polarity) larger than the same polarity (negative polarity) as the charging polarity of the toner. It is set as shown. That is, the normal bias, the first pass bias, the second pass bias, and the non-exposed portion potential of the photosensitive drum are set in descending order. Further, the second passing bias is set to be smaller than the exposure portion potential because the toner is not transferred. Further, the second pass bias is set so that the potential difference from the non-exposed portion potential is smaller than the discharge potential at which discharge occurs at the primary transfer portion in order to suppress retransfer. That is, the second bias for passing is set to the positive side with respect to the non-exposed portion potential of the photosensitive drum in order to reliably prevent retransfer at the black station. Further, in the primary transfer portion, a relationship that is equal to or lower than a discharge start voltage at which a positive polarity discharge that is opposite to the charging polarity of the toner is generated is satisfied.

  Specifically, the non-exposed portion potential is set to −700V. Further, the second pass bias applied to the black station primary transfer portion T1k is -600V, the first pass bias applied to the magenta and cyan station primary transfer portions T1m and T1c is + 700V, and the normal bias is applied. + 900V. The exposed portion potential is about -200 to -250V. If the above conditions are satisfied, the above-described biases can be set as appropriate. However, the first pass bias has the same polarity as the normal bias, and the maximum amount is smaller than that of the normal toner image. It is desirable to ensure transfer.

  In the present embodiment, as described above, the bias applied to the primary transfer portion is set so as to become the normal bias, the first passing bias, the second passing bias, and the non-exposed portion potential in descending order. ing. Therefore, as described above, it is possible to suppress retransfer when the yellow, magenta, and cyan patches pass through the primary transfer portion of the downstream station.

  In other words, in the configuration in which patches of color components other than black are collectively detected on the belt, as shown in FIG. 8, the primary transfer portion T1k of the black station uses the normal bias for the second passage when the patch is formed. Switch to bias. On the other hand, in the primary transfer portions T1m and T1c of the magenta and cyan stations, switching from the normal bias to the first passing bias is performed at the time of patch formation. Then, in order of increasing polarity toward the positive side, the normal bias, the first passing bias, the second passing bias, and the non-exposure portion potential are set. Thereby, retransfer of the patch to the drum can be effectively suppressed, and the accuracy of patch density detection can be improved.

  Note that the first passing bias of the cyan station downstream from the magenta station may be set to be smaller than the first passing bias of the magenta station. As a result, retransfer of yellow toner that has passed through the magenta station at the cyan station can be further suppressed. That is, the patch formed at the upstream station needs to pass through the primary transfer portions of a plurality of stations, and a charge is supplied each time it passes. Therefore, retransfer is more likely to proceed downstream. For this reason, retransfer of the passing patch can be further suppressed by making the first passing bias smaller in the downstream station. In the present embodiment, since the second transfer bias smaller than the first pass bias is applied to the primary transfer portion of the black station arranged on the most downstream side, it is advantageous for suppressing retransfer. .

  1 (1y, 1m, 1c, 1k) ... photosensitive drum (image carrier), 2 (2y, 2m, 2c, 2k) ... charging roller (charging means), 3 (3y, 3m, 3c, 3k) ) ... Laser beam scanner (exposure means), 4 (4y, 4m, 4c, 4k) ... Developer (developing means), 51 ... Intermediate transfer belt, 6 (6y, 6m, 6c, 6k) ... Primary transfer roller (primary transfer member), 61m, 61c ... Power source (first bias applying means), 61k ... Power source (second bias applying means), 100 ... Image forming apparatus , 200... Control unit (control means), Py... Image forming unit (first image forming unit), Pm, Pc... Image forming unit (second image forming unit), Pk. Image forming unit (third image forming unit), Sb... Belt patch sensor (first adjustment toner) Detecting means), Sd · · · drum patch sensor (second adjusting toner detecting means), T1y, T1m, T1c, T1k ··· primary transfer portion, T2 · · · secondary transfer portion

Claims (3)

A rotating intermediate transfer member;
A first image forming unit, a second image forming unit, and a third image forming unit arranged side by side in the rotation direction of the intermediate transfer member;
A first adjustment toner that is disposed downstream of the first, second, and third image forming units in the rotation direction of the intermediate transfer member and detects an adjustment toner image formed on the surface of the intermediate transfer member. Detection means;
Provided in each of the first, second, and third image forming units,
A rotating image carrier;
Charging means for charging the surface of the image carrier;
Exposure means for exposing the charged image carrier to form an electrostatic latent image; and
Developing means for developing the electrostatic latent image formed on the surface of the image carrier with toner;
A primary transfer portion to which a toner image formed on the surface of the image carrier is transferred to the intermediate transfer body;
Second adjustment toner detecting means for detecting an adjustment toner image formed on the surface of the image carrier of the third image forming unit,
With respect to the rotation direction of the intermediate transfer member, the second image forming unit disposed downstream of the first image forming unit includes:
When the toner image formed on the surface of the image carrier of the second image forming unit is transferred to the intermediate transfer member, a transfer bias is formed in the image forming unit upstream of the second image forming unit. The adjustment toner image transferred to the intermediate transfer member passes through the primary transfer portion of the second image forming portion and is formed on the surface of the image carrier of the second image forming portion. A first bias applying means for applying a first pass bias to the primary transfer portion of the second image forming portion when transferring the image to the intermediate transfer member;
Regarding the rotation direction of the intermediate transfer member, the third image forming unit disposed on the most downstream side is:
When the toner image formed on the surface of the image carrier of the third image forming unit is transferred to the intermediate transfer member, a transfer bias is formed in the image forming unit upstream of the third image forming unit. The adjustment toner image transferred to the intermediate transfer member and the adjustment toner image formed on the surface of the image carrier of the third image forming unit are used as the primary transfer unit of the third image forming unit. A second bias applying means for applying a second passing bias to the primary transfer portion of the third image forming portion when passing,
The bias applied to the primary transfer portion has a transfer bias, a first passing bias, a second passing bias in descending order when the opposite polarity is larger than the same polarity as the charging polarity of the toner. It is set to be a non-exposed portion potential on the surface of the image carrier that is charged by the charging means and not exposed,
An image forming apparatus. When the opposite polarity is larger than the same polarity as the charging polarity of the toner, the second image forming unit exposed by the exposure unit of the third image forming unit is used as the second passing bias. Is set to be smaller than the exposed portion potential of the surface of the image carrier,
The image forming apparatus according to claim 1, wherein: In the second pass bias, the potential difference from the non-exposed portion potential of the image carrier of the third image forming portion is smaller than the discharge potential at which discharge occurs in the primary transfer portion of the third image forming portion. Is set to be
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

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