JP2015118347A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device capable of suppressing occurrence of positive ghosts without providing an additional member such as a pre-exposure device.SOLUTION: An image formation device of a tandem method which is equipped with a plurality of image formation parts includes a first image formation part, and a second image formation part provided on a lower stream than the first formation part. At the first image formation part, a patch for detecting ghost is formed and is transferred on an intermediate transfer belt. After the patch for detecting ghost passes the second image formation part, a detection mode is executed which detects concentration of a ghost image formed by the second image formation part. Based on a result of the detection mode, a fogging removing voltage or a transfer current at the second image formation part is controlled so that no ghost occurs.

Description

  The present invention relates to an image forming apparatus using an electrophotographic process such as a copying machine, a printer, a facsimile machine, and a multifunction machine having a plurality of these functions.

  In recent years, with the increase in speed of image forming apparatuses, a configuration in which a plurality of image forming units are arranged in parallel and image formation in each image forming unit is processed in parallel has become mainstream. For example, an electrophotographic image forming apparatus includes a tandem method using an intermediate transfer belt as a transfer medium.

  In the tandem method, a plurality of image forming units are arranged in parallel along the intermediate transfer belt. This is an image forming method in which toner images primarily transferred from each image forming unit onto an intermediate transfer belt are collectively transferred to a recording medium to obtain an image.

  Incidentally, in an electrophotographic image forming apparatus, an image defect in which a history of an image output immediately before appears on the next image is generally called a ghost. In particular, an image defect called a positive ghost may occur in an image forming apparatus in which a plurality of image forming units are arranged in parallel as in the tandem method. Positive ghost depends on the toner image formed on the upstream side in the moving direction of the intermediate transfer belt as the transfer medium and depends on the toner image formed on the downstream side image forming unit. This is a phenomenon in which toner adheres to the white background. In positive ghost, toner is placed on a white background, so even a very small density change is easily recognized by human eyes.

  Various proposals have heretofore been made in order not to generate a ghost. For example, Patent Document 1 proposes a configuration (pre-exposure) in which charge removal light is irradiated on the surface of a photosensitive drum after transfer and before charging in order to eliminate unevenness of the surface potential of the photosensitive drum after transfer.

JP-A-5-265307

  However, when the pre-exposure is configured as in Patent Document 1, there is a problem that the cost increases.

  An object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of positive ghost without providing an additional member as in the pre-exposure apparatus.

  Therefore, an image forming apparatus according to the present invention includes a first image forming unit that has a rotatable first photoconductor and forms a first toner image on the surface of the first photoconductor, and a rotation. A second photosensitive member capable of charging, a charging unit for charging the second photosensitive member, and exposing the second photosensitive member charged by the charging unit to form an electrostatic latent image on the second photosensitive member. An exposure unit for forming, and a developing unit for developing the electrostatic latent image formed on the second photoconductor by the exposure unit with toner to form a second toner image on the surface of the second photoconductor A second image forming unit including: a charging application unit that applies a charging bias of a DC voltage to the charging unit; a developing application unit that applies a developing bias to the developing unit; and the first toner image. A first transfer means for transferring to a transfer medium at a first transfer portion; A second transfer means for transferring an image to a transfer medium at a second transfer portion, a transfer applying means for applying a transfer current to the second transfer means, and the density of the toner image transferred to the transfer medium. An image forming apparatus including: a density detecting unit configured to detect the first transfer unit, wherein the second transfer unit is located downstream of the first transfer unit in the moving direction of the transfer medium; The detection toner image is formed on the second photosensitive member by the detection toner image after the detection toner image transferred to the transfer medium passes through the second transfer portion. Based on the density of the ghost image detected by execution of the detection mode, execution means for executing a detection mode for detecting the density of the ghost image transferred to the transfer medium by the density detection means, The above Charging potential of the second photosensitive member and a control means for controlling the potential difference between the DC voltage value of the developing bias, and having a.

  According to the present invention, it is possible to suppress the occurrence of positive ghost without providing an additional member as in the pre-exposure device.

It is an example of a positive ghost image. It is explanatory drawing of the generation | occurrence | production mechanism of positive ghost. 1 is a schematic configuration diagram of an image forming apparatus in the present embodiment. It is a test image used in the ghost detection mode in a present Example. It is an example of the reflectance profile detected in the ghost detection mode in a present Example. 3 is a block diagram of a ghost control sequence in Embodiment 1. FIG. It is a flowchart figure of the ghost control sequence in Example 1. FIG. It is a flowchart figure of the ghost control sequence in Example 2. FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, what attached | subjected the same code | symbol in each drawing has the same structure or effect | action, The duplication description about these is abbreviate | omitted suitably. Note that the dimensions, materials, shapes, relative positions, and the like of the component parts are not intended to limit the scope of application of this technical idea only to those unless otherwise specified.

  First, the generation mechanism of positive ghost will be described by taking a document image as shown in FIG. 1A as an example.

  An image including a patch region as shown in FIG. 1A is formed by an image forming unit (forming a yellow image) as the most upstream image forming unit, and the solid white having a print rate of zero in other image forming units. Consider the case of outputting the image. At this time, the state of the potential on the surface of the photosensitive drum as the photosensitive member in the image forming unit downstream of the image forming unit for forming the yellow image will be described with reference to FIG.

  FIG. 2A is a diagram schematically showing an image forming unit downstream of the yellow image forming unit. Each member for forming an image is provided along the peripheral surface of the photosensitive drum 2. The charging roller 3 charges the photosensitive drum 2. The developing device 4 develops the electrostatic latent image on the photosensitive drum 2 to form a toner image. The transfer roller 6 transfers the toner image formed on the photosensitive drum 2 onto the intermediate transfer belt 1. The cleaner blade 5 removes transfer residual toner remaining on the photosensitive member 2 after transfer. Further, the change in potential on the surface of the photosensitive drum due to the passage of the yellow patch is divided into four stages: (B) before passing the patch, (C) after passing the patch, (D) after charging, and (E) during development. Showed.

  First, at a position before passing through the patch (before transfer), the image forming unit downstream of the yellow image forming unit outputs a solid white image, so the potential of the photosensitive drum 2 is as shown in FIG. It is flat. When the patch passes, the discharge between the photosensitive drum 2 and the intermediate transfer belt 1 is performed through the toner layer of the patch portion, but the transfer current applied to the transfer roller 6 is applied to the portion where the toner exists. Since it does not flow easily, the transfer current concentrates on a portion where no toner exists. For this reason, the potential of the non-patch portion is pulled more strongly by the transfer bias than the potential of the patch portion, and a potential difference is generated between the patch portion and the non-patch portion on the surface of the photosensitive drum after transfer. Further, since the discharge occurs in the gaps between the toner particles, the potential of the patch portion is not uniform, and there is micro potential disturbance due to the discharge from the gaps of the toner. This is shown in FIG.

  When the next charging step in the rotation of the photosensitive drum is performed in a state in which such potential disturbance exists, the potential disturbance remains after charging, and the photosensitive drum as shown in FIG. The surface of 2 may not be uniformly charged. In particular, at locations where the amount of charge is insufficient, the fog removal potential difference, which is the potential difference between the dark portion potential that is the charging potential on the surface of the photosensitive drum 2 and the DC voltage of the developing bias, becomes small. This is shown in FIG. Where the fog removal potential difference is small, the force to move the toner away from the photosensitive drum is small, and the phenomenon of fogging that the toner adheres to the white background portion is likely to occur.

  That is, the toner image formed by the upstream image forming unit causes disturbance in the surface potential of the photosensitive drum of the downstream image forming unit, and the toner adheres to the part where the fogging potential difference is locally reduced. This is the mechanism by which positive ghosts are generated.

  With respect to the original image in FIG. 1A, an image having the same shape as the image formed by the upstream image forming unit as shown in FIG. 1B is downstream at a position shifted by the circumferential length of the photosensitive drum. A color fog image (ghost image) of the image forming unit appears on the image as a positive ghost.

  In general, such a ghost image tends to appear more noticeably in the DC charging method using only a DC voltage as the charging bias than in the AC charging method using a charging bias in which an AC voltage is superimposed on the DC voltage. This is because the AC charging method has better potential convergence than the DC charging method, so the surface of the photosensitive drum is uniform even if there is some non-uniformity in the surface potential of the photosensitive drum before charging. This is because it can be charged.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 3 is a schematic configuration diagram of the image forming apparatus according to the present embodiment. The image forming apparatus in this embodiment is an electrophotographic printer, and forms an image on a recording medium in accordance with an image signal sent from a computer (not shown). In addition, the image forming unit of the four colors (Y, M, C, Bk) is a tandem type image forming apparatus in which the image forming units are arranged side by side on an intermediate transfer belt as an intermediate transfer member as a transfer medium.

<Image formation process>
First, a process for forming an image will be described. The image forming apparatus according to this embodiment includes four image forming units of YMCK. The image forming unit 109Y forms an image with yellow (Y) toner. The image forming unit 109M forms an image with magenta (M) toner. The image forming unit 109C forms an image with cyan (C) toner. The image forming unit 109Bk forms an image with black (Bk) toner. Since the image forming unit 109Y, the image forming unit 109M, the image forming unit 109C, and the image forming unit 109Bk have the same configuration except for the toner color, the image forming unit 109Y will be described as a representative.

  The image forming unit 109 </ b> Y that is a toner image forming unit includes a photosensitive drum 103 that is a photosensitive member and each unit that is disposed around the photosensitive drum 103. The image forming unit 109Y includes a charger 104 as a charging unit, an exposure device 105 as an exposure unit, and a developing device 106 as a developing unit. Further, a primary transfer device 107 as a transfer unit and a photoreceptor cleaner 108 are also included. The surface of the photosensitive drum 103 rotating in the direction of the arrow in the figure is uniformly charged by the charger 104. The exposure device 105 is driven based on the input image information signal, and the charged photosensitive drum 103 is exposed to form an electrostatic latent image. The electrostatic latent image formed on the photosensitive drum 103 is developed by the developing device 106, and a toner image is formed on the photosensitive drum 103. Thereafter, the primary transfer device 107 transfers a yellow toner image onto an intermediate transfer belt 101 as an intermediate transfer member, which is a belt member, with a predetermined pressure and an electrostatic load bias. Thereafter, the transfer residual toner remaining on the photoconductive drum 103 is collected by the photoconductive cleaner 108 to prepare for the next image formation again.

  In the case of FIG. 3, the image forming unit 109 described above has four sets of yellow (Y), magenta (M), cyan (C), and black (Bk). Therefore, the magenta toner image formed by the image forming unit 109M is transferred to the intermediate transfer belt 101 with respect to the yellow toner image formed on the intermediate transfer belt 101. Further, the cyan toner image formed by the image forming unit 109 </ b> C is transferred to the intermediate transfer belt 101 with respect to the formed magenta toner image. Further, the black toner image formed by the image forming unit 109Bk is transferred to the intermediate transfer belt 101 with respect to the cyan toner image. In this way, toner images of different colors are formed on the intermediate transfer belt 101 so that a full color toner image is formed on the intermediate transfer belt 101.

  The full-color toner image formed on the intermediate transfer belt 101 is conveyed to the secondary transfer unit. The secondary transfer portion is a transfer formed by a driving roller (also serving as a secondary transfer inner roller) 110 as a first secondary transfer member and a secondary transfer outer roller 111 as a second secondary transfer member, which are opposed to each other. It is a nip part. A full color toner image formed on the intermediate transfer belt 101 is secondarily transferred onto the recording material P by applying a predetermined pressing force and an electrostatic load bias.

  Thereafter, the recording material P is conveyed to the fixing device 112. The fixing device 112 has various configurations and methods. In FIG. 3, a toner image is formed on the recording material P by applying a predetermined pressure and heat in a fixing nip formed by the opposing fixing roller 112 a and pressure roller 112 b. Is melt-fixed. In this way, a full color image fixed on the recording material P can be obtained.

  Further, the transfer residual toner remaining on the intermediate transfer belt 101 after the secondary transfer is collected by the intermediate transfer belt cleaner 102 to prepare for the next image formation.

<Details of image forming unit>
Next, details of each component of the image forming unit 109 will be described.
The photosensitive drum 103 as a photosensitive member is a negatively chargeable organic photosensitive member (OPC) having an outer diameter of 30 mm in the present embodiment, and is rotationally driven in a direction indicated by an arrow by a driving device (not shown). The photosensitive drum 103 has three layers of an undercoating layer that suppresses light interference and improves adhesion of an upper layer, a photocharge generation layer, and a charge transport layer on the surface of an aluminum cylinder (conductive drum base). It is constituted by applying in order.

  The charger 104 as a charging means is a contact charging type charging roller using a conductive rubber roller, and is composed of a cored bar and a conductive rubber layer on the outer periphery thereof. Both ends of the cored bar are rotatably held by a bearing member (not shown), and are urged toward the center of the photosensitive drum 103 by a pressing spring and pressed against the surface of the photosensitive drum 103 with a predetermined pressing force. Has been. The charger 104 is driven to rotate as the photosensitive drum 103 rotates. A charging bias composed of a DC voltage is applied to the core of the charger 104 by a power source (not shown) as a charging application unit, and the surface of the photosensitive drum 103 is contact-charged to a predetermined polarity and potential. For example, in this embodiment, when a DC voltage of −1350 V is applied to the charger 104, the charging potential on the surface of the photosensitive drum 103 is charged to −750V.

  In the present embodiment, a laser beam scanner using a semiconductor laser is used as the exposure device 105 which is an exposure means. The laser beam scanner outputs a laser beam modulated in accordance with an image signal input from an image reading device (not shown). Then, the uniformly charged surface of the photosensitive drum 103 is subjected to scanning exposure (image exposure). By this scanning exposure, the potential of the region irradiated with the laser light on the surface of the photosensitive drum 103 is lowered, and electrostatic latent images corresponding to the image information subjected to the scanning exposure are sequentially formed on the surface of the photosensitive drum 103. .

  The developing device 106 as the developing means is a reversal developing device of the two-component magnetic brush developing system in this embodiment, and the electrostatic latent image is reversed by the toner adhering to the exposed portion (bright portion) of the surface of the photosensitive drum 103. Developed. The developing device 106 includes a developing sleeve, a magnet roller, a developer regulating blade, and a developer stirring member. A developing sleeve as a developer carrying member is a rotatable nonmagnetic element tube, and a magnet roller as a magnetic field generating means is fixedly disposed inside the developing sleeve. The developer regulating blade is a metal plate, and is fixedly arranged at a predetermined distance from the developing sleeve.

  The developer in this embodiment is a mixture of a non-magnetic toner and a magnetic carrier, is conveyed to the developing sleeve while being uniformly stirred by the developer stirring member, and is carried on the developing sleeve by the magnetic force of the magnet roller. The developer on the developing sleeve is regulated to a predetermined value by a developer regulating blade, and is conveyed to a developing area facing the photosensitive drum 103 as the developing sleeve rotates. A predetermined developing bias is applied to the developing sleeve from a power source as a developing application unit (not shown), whereby a toner image corresponding to the electrostatic latent image is formed on the surface of the photosensitive drum 103. In this embodiment, the developing bias applied to the developing sleeve is an oscillating voltage obtained by superimposing a DC voltage and an AC voltage. Specifically, a DC voltage: −600 V, an AC voltage; a frequency of 6.0 kHz, and a peak-to-peak voltage of 1 It is an oscillating voltage superimposed with a rectangular wave of 4 kV.

  In this embodiment, a toner having an average particle diameter of about 6 μm obtained by pulverizing and classifying a resin binder mainly containing polyester and kneading a pigment is used. Further, the resistance of the magnetic carrier in this embodiment is about 1013 Ω · cm and the particle diameter is 40 μm, and the toner is triboelectrically charged to the negative polarity by sliding with the magnetic carrier.

  The primary transfer device 107 as a transfer means is a conductive sponge roller, and both ends of the cored bar are rotatably held by a bearing member (not shown) and are pressed toward the photosensitive drum 103 by a pressing pressure spring. It is pressed by pressure. At this time, the primary transfer device 107 and the photosensitive drum 103 are pressed against each other so as to sandwich the intermediate transfer belt 101, thereby forming a primary transfer portion as a transfer portion. When the toner image reaches the primary transfer portion as the photosensitive drum 103 rotates, the toner image is transferred to the intermediate transfer belt 101 by a primary transfer bias as a transfer current applied from a transfer power source (not shown) as a transfer application unit. Is done.

  The photoreceptor cleaner 108 is a cleaning blade using urethane rubber. After the primary transfer, transfer residual toner remaining on the photosensitive drum 103 is collected by the photosensitive cleaner 108. In this embodiment, the setting of the photoconductor cleaner 108 is a setting angle of 25 °, a contact pressure of 30 gf / cm, the hardness of the urethane rubber is 75 degrees in terms of JIS-A hardness, and the thickness of the urethane rubber is 2 mm.

  Next, the intermediate transfer belt 101 as an intermediate transfer member will be described. The intermediate transfer belt 101 is a belt member that is conveyed and driven in the direction of the arrow in FIG. 3, and is driven by a driving roller 110 that is a driving member, a steering roller 115 that is a steering member, and stretching rollers 113 and 114 that are stretching members. It is stretched.

  In this embodiment, the drive roller 110 has the function of the secondary transfer inner roller which is the secondary transfer inner member. The steering roller 115 also has the function of a tension roller that applies a predetermined tension to the intermediate transfer belt 101, but the number of rollers that stretch the intermediate transfer belt 101 is not limited to the configuration shown in FIG. .

The material of the intermediate transfer belt 101 is preferably a material for preventing the belt from wrinkling during rotational driving. For example, it is desirable to use a highly rigid resin such as PVDF (polyvinylidene fluoride), polyamide, polyimide, PET (polyethylene terephthalate), and polycarbonate. If the intermediate transfer belt 101 is too thin, sufficient durability due to wear cannot be obtained. On the other hand, if the thickness is too thick, the driving roller 110, the steering roller 1, and the stretching rollers 113 and 114 may not be bent properly, and a dent or break may occur. Therefore, a range of 0.02 mm to 0.50 mm is desirable. In this embodiment, the intermediate transfer belt 101 is a resin belt having polyimide as a base layer, and has a tensile elastic modulus E = 18000 N / cm ² and a film thickness of 0.08 mm.

<Ghost detection mode>
In the image forming apparatus according to the present exemplary embodiment, positive ghost is detected on the intermediate transfer belt 101, and feedback is performed on the image forming conditions. First, a method for detecting a positive ghost on the intermediate transfer belt 101 (hereinafter referred to as a ghost detection mode) will be described.
In the ghost detection mode as the detection mode, the amount of toner (toner image density) on the intermediate transfer belt 101 as a transfer medium is detected by a patch sensor 116 as a density detection means. The patch sensor 116 includes a light emitting element and a light receiving element. The light receiving element detects regular reflection or diffuse reflection of light irradiated on the intermediate transfer belt 101 by the light emitting element. When toner is present on the intermediate transfer belt 101, the amount of reflected light varies depending on the amount of toner, so the toner on the intermediate transfer belt 101 is detected by the amount of light detected by the light receiving element. You can know the amount of loading. Further, the patch sensor 116 is disposed with the tension roller 114 as a counter.

  However, even when positive ghost is generated, the amount of toner loaded in the positive ghost portion is very small, and it is difficult to detect the positive ghost with the patch sensor 116 as it is.

  Therefore, in the ghost detection mode in this embodiment, the fog removal potential difference (hereinafter referred to as Vback) is changed by changing the DC voltage value of the developing bias applied to the developing sleeve. At this time, as the value of Vback is smaller, the phenomenon of fogging in which toner adheres to the white background of the image is more likely to occur, and the reflectance of the intermediate transfer belt measured by the patch sensor decreases due to the fogging. Hereinafter, the change in reflectivity on the surface of the intermediate transfer belt due to the occurrence of fog is referred to as fog reflectivity. That is, as the fogging amount increases, the output of the fogging reflectance increases.

  As shown in FIG. 4A, the smaller the value of Vback, the greater the fogging amount, and the change becomes sharper. As already described, positive ghost is generated by micro disturbance of the surface potential of the photosensitive drum 103. That is, if Vback is large, the change in the fogging amount due to micro disturbance of the surface potential is small. However, if Vback is small, the change in the fogging amount due to micro disturbance of the surface potential becomes large, and the patch sensor 116 can detect the change in the fogging reflectance (change in the density of the toner image). As an example in this embodiment, an example of the profile of the fog reflectance when Vback is 150V is shown in FIG. 4B, and an example of the profile when Vback is 50V is shown in FIG. 4C. In this profile, the tip of the positive ghost generator is set to 0 μm on the horizontal axis.

  Details of the ghost detection mode will be described below with reference to FIG. In the ghost detection mode, a test image as shown in FIG. The test image has a yellow patch formed in the most upstream image forming unit as the first image forming unit on the leading end side of the image, and this yellow patch as the detection toner image in this embodiment is a normal image forming condition. The toner is formed so as to have a maximum toner loading amount. At this time, the Vback of the downstream image forming unit as the second image forming unit that is desired to detect the presence or absence of the occurrence of positive ghost is set to 50V. Note that the Vback value is not limited to 50 V as long as the patch sensor 116 can detect a positive ghost.

  Here, as shown in FIG. 5B, the position P0 near the rear end of the image is centered by the circumference of the photosensitive drum as the second photosensitive member of the image forming portion downstream of the leading edge of the yellow patch of the test image. The patch sensor 116 detects the reflectance of the region of the predetermined length L. That is, when the position coordinate x is defined along the moving direction of the intermediate transfer belt 101 with P0 as the origin, and the direction from the front end to the rear end of the image as positive, reflection in the range of −L / 2 ≦ x ≦ L / 2. A rate profile is recorded. At this time, if the change ΔR in the fog reflectance before and after the origin is larger than a predetermined value R0, that is, if the change in density before and after the origin is larger than a predetermined value, it is determined as a positive ghost.

  In the above description, the yellow patch is used as the toner image for detection of the test image. However, if it is upstream from the image forming unit where it is desired to detect the presence or absence of positive ghost, the patch is applied to the image forming unit of another color. It may be formed.

  Further, during execution of the ghost detection mode, if a bias having a polarity opposite to that at the time of normal image formation is applied to the secondary transfer portion, toner contamination of the secondary transfer outer roller 111 can be prevented. The toner that has passed through the secondary transfer portion is collected by the intermediate transfer belt cleaner 102.

<Ghost control sequence>
As can be seen from the above description, if the value of Vback is set to be large, the positive ghost does not become apparent and can be made indistinguishable on the image. However, when developing with a two-component developer as in this embodiment, problems such as adhesion of the magnetic carrier to the photosensitive drum occur, and it is not desirable to increase the value of Vback. Therefore, it is desirable to suppress the occurrence of positive ghosts by controlling image forming conditions other than Vback.

  According to the study by the present inventors, it has been clarified that positive ghost has a large relationship with a primary transfer current as a transfer current applied to a primary transfer device as a transfer means. Table 1 shows the change in the value of the primary transfer current as the transfer current applied to the primary transfer device as the second transfer unit when the Vback is 150 V in the image forming unit 109M as the second image forming unit. This is an example of the result of confirming the presence or absence of occurrence of positive ghost. As shown in Table 1, positive ghost was generated when the primary transfer current was smaller than 35 μA, but no positive ghost was generated when the primary transfer current was 35 μA or more. However, transfer failure occurred when the primary transfer current was 50 μA or more.

  In the example of Table 1, the occurrence of positive ghost can be suppressed if the primary transfer current is set to 35 μA or more. However, the threshold of the primary transfer current depends on the deterioration state of the photosensitive drum and the developer and the temperature and humidity environment. It is also different. Therefore, it is preferable to detect the occurrence of positive ghost at a predetermined timing and control the value of the primary transfer current so that positive ghost does not occur. At this time, it is preferable to set an upper limit on the value of the primary transfer current so that transfer defects do not occur. For example, in the case of this embodiment, the initial setting value +15 μA in the environment is set as the upper limit of the primary transfer current. Further, when changing the value of the primary transfer current, it is preferable to change the value stepwise. In this embodiment, the value is increased every 5 μA. However, the primary transfer current control method is not limited to these.

  Hereinafter, a sequence (hereinafter referred to as a ghost control sequence) for controlling the value of the primary transfer current as a transfer current so as not to generate a positive ghost based on the detection result in the ghost detection mode will be described. At this time, the patch as the toner image for detection of the test image is formed in the upstream image forming unit of the downstream image forming unit that executes the ghost control sequence. In addition, the ghost control sequence is performed in order from the most downstream image forming unit. By adopting such a configuration, it is possible to exclude the influence when a positive ghost occurs in another image forming unit.

  The details of the ghost control sequence will be described by taking as an example the case where the ghost control sequence is executed for the image forming unit 109M as the second image forming unit. In this case, the test image patch is formed by the image forming unit 109Y as the first image forming unit. FIG. 6 is a block diagram of a ghost control sequence, and FIG. 7 is a flowchart.

  After image formation is started (S101), the controller 201 compares the cumulative number N of formed images with a predetermined number N0 (S102). If N is less than N0, the process proceeds to step S107 after the end of image formation (S108). If all jobs have been completed, the image forming apparatus stops. If there are jobs remaining, the process returns to step S101.

  If N is equal to or greater than N0 in step S102, after the end of image formation (S103), the controller 201 as the execution unit executes a ghost detection mode for the image forming unit 109M as the second image forming unit (S104). . At this time, the controller 201 as the execution unit causes the image forming unit 109Y as the first image forming unit to form a test image patch as a detection toner image. Next, the controller 201 as the execution means compares the change ΔR in the fog reflectance detected by the patch sensor 116 as the density detection means with a predetermined value R0 (S105). If ΔR is smaller than R0, the cumulative number of formed images is cleared (S106), and the process proceeds to step S107.

  If ΔR is equal to or greater than R0 in step S105, the controller 201 as the control unit confirms whether or not the primary transfer current of the image forming unit 109M as the second image forming unit has reached the upper limit (S109). . If the upper limit has not yet been reached, the value of the primary transfer current as the transfer current is increased (S110). Thereafter, returning to step S104, the ghost detection mode for the image forming unit 109M is executed again. In step S109, if the primary transfer current of the image forming unit 109M has already reached the upper limit, a message for prompting replacement of the image forming unit 109M is displayed on a display (not shown) as a display unit (S111).

  As described above, when the density (density difference ΔR) of the ghost image detected in the ghost detection mode is equal to or higher than a predetermined density, the control of the primary transfer current as the transfer current is continuously performed a plurality of times. Generation of positive ghost can be suppressed.

In this embodiment, the ghost control sequence was executed with N0 being 200 sheets and R0 being 3%. However, it is not limited to these.
When the ghost control sequence is executed for the image forming unit 109C and the image forming unit 109Bk, the same flow as described above may be used.

  As described above, according to the present embodiment, a positive ghost is detected by reducing the value of Vback, and the value of the primary transfer current as the transfer current is controlled based on the detection result, so that Generation of positive ghost can be suppressed without providing an additional member.

  In the first embodiment, the method of controlling the transfer current value to form an image under the condition that no positive ghost is generated has been described. However, when a one-component developer that does not use a carrier is used as the developer, the Vback It is also possible to suppress the occurrence of positive ghost by controlling the value. In the present embodiment, a method for controlling the value of Vback to form an image under conditions where no positive ghost is generated will be described.

<Image forming apparatus>
Since the image forming apparatus according to the present embodiment is the same as the image forming apparatus according to the first embodiment except for the developing device 106 as a developing unit, only the developing device 106 according to the present embodiment will be described below.

  In this embodiment, the developing device 106 is a reversal development of a non-magnetic one-component contact development system, and toner that is a non-magnetic one-component developer adheres to the exposed portion (bright portion) of the surface of the photosensitive drum 103 and electrostatically develops. The latent image is reversely developed. The developing device 106 serving as a developing unit includes a developing roller, a developer regulating blade, and a developer stirring member. The developing roller is a conductive rubber roller, and is composed of a cored bar and a conductive rubber layer on the outer periphery thereof. The developing roller is in contact with the photosensitive drum 103 with a predetermined contact pressure, and is driven to rotate by a driving device (not shown). The developer regulating blade is a sheet spring-like metal thin plate coated with a thin film such as urethane resin as an insulating layer, and is in contact with the developing roller with a predetermined contact pressure.

  The toner as the developer is conveyed to the developing roller while being uniformly stirred by the developer stirring member. The toner on the developing roller is applied to the developing roller while the layer thickness is regulated to a predetermined value by the developer regulating blade. Further, at this time, the toner is given sufficient electric charge for development by friction with the respective surfaces of the developing roller and the developing blade. With the subsequent rotation of the developing roller, the toner thin layer on the developing roller is conveyed to a developing area which is a contact portion between the photosensitive drum 103 and the developing roller. A predetermined developing bias is applied to the developing roller from a developing power source (not shown) as a developing application unit, whereby a toner image corresponding to the electrostatic latent image is formed on the surface of the photosensitive drum 103. In this embodiment, when a DC voltage is applied to the developing roller as the developing bias, the photosensitive drum 103 is charged to −750V, and for example, when the developing bias is −600V, Vback is set to 150V.

<Ghost control sequence>
In this embodiment, the occurrence of positive ghost is suppressed by increasing the value of Vback. However, when Vback is too large, a toner having a polarity opposite to that of a normal toner adheres to a white background, and a phenomenon called reversal fogging occurs. Is more likely to occur. Table 2 shows an example of the result of confirming the presence or absence of positive ghost by changing the value of Vback when the primary transfer current is 25 μA in the image forming unit 109M as the second image forming unit. As shown in Table 2, positive ghost was generated when Vback was small, but no positive ghost was generated when Vback was 200 V or higher. However, when Vback was 350 V or higher, a reversal fog occurred.

  In the example of Table 2, the occurrence of positive ghost could be suppressed if Vback was set to 200 V or more. However, the threshold value of Vback varies depending on the deterioration state of the photosensitive drum and the developer and the temperature and humidity environment. . Therefore, it is necessary to detect the occurrence of positive ghost at a predetermined timing and control the value of Vback so that positive ghost does not occur. At this time, it is preferable to set an upper limit on the value of Vback so that the reverse fogging does not occur. For example, in the case of the present embodiment, the initial setting value +100 V in the environment is set as the upper limit of Vback. However, the present invention is not limited to these.

  In the ghost control sequence in the present embodiment, the controller 201 as an execution unit executes the same ghost detection mode as in the first embodiment, and controls the value of Vback based on the value of the fogging reflectance ΔR. It is desirable that there are a plurality of Vback values after the change. In this embodiment, three types of R′0, R′1, and R′2 are provided as ΔR threshold values. In this embodiment, the Vback value is changed to the initial setting value + 50V when R′0 ≦ ΔR <R′1 and the initial setting value +100 V when R′1 ≦ ΔR <R′2. .

  Hereinafter, the details of the ghost control sequence will be described by taking as an example the case where the ghost control sequence is executed for the image forming unit 109M as the second image forming unit. FIG. 8 shows a flowchart of a ghost control sequence in the present embodiment. The block diagram is the same as in the first embodiment.

  After image formation is started (S201), the controller 201 compares the cumulative number N ′ of formed images with a predetermined number N′0 (S202). If N ′ is smaller than N′0, after the end of image formation (S208), the process proceeds to step S207. If all jobs have been completed, the image forming apparatus is stopped. If there are jobs remaining, the process returns to step S201.

  When N ′ is equal to or greater than N′0 in step S202, after the image formation is completed (S203), the controller 201 as the execution unit executes the ghost detection mode for the image forming unit 109M as the second image forming unit. (S204). At this time, the image forming unit 109Y as the first image forming unit forms a test image patch as a detection toner image. Next, the controller 201 as execution means compares the change ΔR in the fogging reflectance detected by the patch sensor 116 with a predetermined value R′0 (S205). If ΔR is smaller than R｀0, the cumulative number of formed images is cleared (S206), and the process proceeds to step S207.

  If ΔR is equal to or greater than R′0 in step S205, the process proceeds to step S209, and ΔR is compared with a predetermined value R′1. If ΔR is smaller than R｀1, the controller 201 as the control unit changes the value of Vback of the image forming unit 109M to the initial setting value + 50V (S210), and proceeds to step S206. If ΔR is equal to or greater than R′1 in step S209, the process proceeds to step S211 and ΔR is compared with a predetermined value R′2. When ΔR is smaller than R′2, the controller 201 as the control unit changes the Vback value of the image forming unit 109M to the initial setting value + 100 V (S212), and proceeds to step S206. If ΔR is equal to or greater than R′2 in step S211, a message prompting replacement of the image forming unit 109M is displayed on a display (not shown) (S213).

  Note that the Vback in this embodiment is specifically changed by changing the DC voltage value of the developing bias applied to the developing means. However, Vback may be changed by changing the charging potential on the surface of the photosensitive drum by changing the DC voltage value applied to the charging means.

  In this embodiment, the ghost control sequence is executed with N′0 of 200 sheets, R′0 of 3%, R′1 of 5%, and R′2 of 7%. However, it is not limited to these.

  As described above, in the present embodiment, as in the first embodiment, it is possible to suppress the occurrence of positive ghost without providing an additional member as in the pre-exposure apparatus.

  In the first and second embodiments, the configuration having the intermediate transfer belt as the transfer medium for transferring the toner image from the photosensitive member has been described. However, the present invention is not limited thereto, and the density of the ghost image transferred onto the recording material may be detected by performing a direct transfer to the recording material as a transfer medium. Alternatively, the density of the ghost image may be detected by transferring the recording material onto a conveying belt that conveys and conveys the recording material.

101 Intermediate transfer belt (intermediate transfer member)
103 photoconductor drum (first photoconductor)
104 Charging roller 105 Laser scanner 106 Developer 107 Transfer roller (first transfer means)
109 Image forming unit 116 Density sensor (density detection means)
201 controller (execution means, control means)

Claims (4)

A first image forming unit having a rotatable first photoconductor and forming a first toner image on the surface of the first photoconductor;
A rotatable second photoconductor, a charging unit for charging the second photoconductor, and exposing the second photoconductor charged by the charging unit to form an electrostatic latent image on the second photoconductor And developing the electrostatic latent image formed on the second photoconductor by the exposure unit with toner to form a second toner image on the surface of the second photoconductor A second image forming unit having means;
A charging application means for applying a charging bias comprising a DC voltage to the charging means;
Developing application means for applying a developing bias to the developing means;
First transfer means for transferring the first toner image to a transfer medium at a first transfer portion;
Second transfer means for transferring the second toner image to a transfer medium at a second transfer portion;
Transfer application means for applying a transfer current to the second transfer means;
An image forming apparatus having density detecting means for detecting the density of the toner image transferred to the transfer medium,
The second transfer portion is located downstream of the first transfer portion in the moving direction of the transfer medium;
A detection toner image is formed in the first image forming unit, and after the detection toner image transferred to the transfer medium passes through the second transfer unit, the second toner is detected by the detection toner image. Execution means for executing a detection mode in which the density detection means detects the density of the ghost image formed on the photosensitive member and transferred to the transfer medium;
Control means for controlling a potential difference between the transfer current or a charging potential of the second photosensitive member and a DC voltage value of the developing bias based on the density of the ghost image detected by executing the detection mode; An image forming apparatus comprising:   The transfer medium conveys the first toner image and the second toner image transferred from the first image forming unit and the second image forming unit and transfers them to a recording material in a secondary transfer unit. The image forming apparatus according to claim 1, wherein the image forming apparatus is an intermediate transfer body.   In the detection mode, the execution means includes a charging potential on the surface of the second photoconductor on the next circumference in rotation of the second photoconductor in which the detection toner image has passed through the second transfer portion. 3. The image forming apparatus according to claim 1, wherein a potential difference from the DC voltage of the developing bias is set smaller than that when the second toner image is formed. 4. The method according to claim 1, wherein the execution means and the control means continuously execute and control the detection mode when the density of the ghost image is equal to or higher than a predetermined density. The image forming apparatus according to claim 1.

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