JP2008180790A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus, using existent constitution where transfer voltage is assumed, to reduce variance of potential by electrification on the surface of an image carrier caused by transfer preventing voltage so as not to influence image quality. <P>SOLUTION: The image forming apparatus forms a discharged toner image which is not transferred to recording material between paper and paper in ordinary toner image formation and transfer, and applies high voltage having a reverse polarity to that in ordinary transfer to a transfer roller 23a so as not to transfer the discharged toner image to an intermediate transfer belt 28. After forming the discharged toner image, a photoreceptor drum 21a is idly rotated by two or more times in such a state that a solid white image is formed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that forms a discharged toner image that is not transferred onto a recording material on an image carrier for the purpose of maintaining image quality. More specifically, the image forming apparatus adheres to a recording material conveyance body or an intermediate transfer body as the discharged toner image is formed. The present invention relates to control for efficiently removing toner by an existing mechanism.

  2. Description of the Related Art Image forming apparatuses that transfer a toner image formed on a photosensitive drum using charged toner to a recording material carried on a recording material conveyance belt or an intermediate transfer belt have been put into practical use. In such an image forming apparatus, for the purpose of image quality control, a discharged toner image that is not transferred onto a recording material may be formed on a photosensitive drum.

  Patent Document 1 discloses an image forming apparatus that forms a discharged toner image on a photosensitive drum during pre-rotation prior to normal image formation to be transferred to a recording material. Here, the discharged toner image is supplied to the cleaning blade of the cleaning device and the photosensitive drum is rubbed to remove the contaminated layer adhering to the surface of the photosensitive drum to prevent image deterioration such as image flow.

  Patent Document 2 discloses an image forming apparatus that forms a toner image discharged on a photosensitive drum at intervals of normal image formation transferred to a recording material. Here, when the continuous use time of the developing device reaches a predetermined value, a discharged toner image is formed on the photosensitive drum, and the developed toner is wiped away for a long time by the developing roller and replaced with new toner.

JP 2000-172027 A JP 2002-244512 A

  Since the image forming apparatuses of Patent Document 1 and Patent Document 2 do not use a recording material conveyance body or an intermediate transfer body, consideration is given to the toner that adheres to the recording material conveyance body or the intermediate transfer body when a discharged toner image is formed. It has not been.

  When forming a discharged toner image with an image forming apparatus having a recording material conveyance body or an intermediate transfer body, it is not desirable that the discharged toner image not transferred to the recording material is transferred to the recording material conveyance body or the intermediate transfer body. This is because the amount of toner reaching the cleaning blade is reduced, and each part of the mechanism including the recording material conveyance body or the intermediate transfer body is stained with toner.

  Therefore, when a discharge toner image is formed, a transfer inhibition voltage (reverse bias voltage) having a polarity opposite to that of the normal transfer voltage is applied to the recording material conveyance body or intermediate transfer body that passes through the transfer portion. It is desirable to pass the toner image through the transfer portion.

  However, as will be described later, in order to reduce the toner adhering to the recording material conveyance body or the intermediate transfer body in contact with the discharged toner image to a practical level, the absolute value is an order of magnitude from 1 kV to 2 kV. Requires a high voltage.

  Then, a large charged potential difference is generated between the surface of the image carrier that is in contact with such a high transfer blocking voltage at the transfer portion and a portion that is not in contact with the transfer blocking voltage.

  Then, the surface potential difference of the image carrier due to the transfer blocking voltage cannot be eliminated by a single charging process performed during normal image formation. When a normal image exposure is performed and the electrostatic latent image is written in a state where the charged potential of the surface varies after one charging process, a step in potential is formed in the electrostatic latent image having the same exposure intensity. The step of the potential of the electrostatic latent image becomes a step of the toner adhesion amount after development, and becomes an image density step corresponding to the boundary of the region to which the transfer blocking voltage is applied.

  If a pre-exposure device is installed upstream of the charging device to irradiate the surface of the image carrier to eliminate the electrostatic latent image, the surface potential difference of the image carrier due to the transfer blocking voltage until the charging device is reached. Is alleviated. However, if the pre-exposure device is operated under the same conditions as in normal image formation, the density difference in the image cannot be effectively eliminated, and density differences and density unevenness are formed in the image formed immediately after the discharge toner image is formed. Turned out to be.

  In other words, the charged potential on the surface of the image carrier that is opposite in polarity to the transfer voltage and exposed to a high transfer inhibition voltage is an existing mechanism for eliminating the charged potential on the surface of the image carrier exposed to a low transfer voltage. Then it cannot be recovered to a practical level. With existing pre-exposure devices and charging devices that assume a transfer voltage, it is impossible to uniformly charge a region exposed to a transfer blocking voltage and a region not exposed to a level that does not affect image density. When the process speed is increased, the time for passing through the pre-exposure device and the charging device is also shortened, so that it becomes more difficult to perform the charging process evenly.

  Here, it is impractical to increase the charging capacity of the charging device to an unnecessary level during normal image formation by increasing the capacity of the charging device. It is not practical to provide a new charging device or to introduce complicated charging voltage control for forming the discharged toner image. Increasing the amount of light in the pre-exposure device may reduce the life of the image bearing member.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that uses an existing configuration that assumes a transfer voltage, reduces the variation in charging potential on the surface of an image carrier caused by a transfer blocking voltage, and does not affect image quality. It is said.

  An image forming apparatus of the present invention forms an electrostatic latent image by exposing an image carrier, a recording material conveyance body or an intermediate transfer member, a charging unit for charging the surface of the image carrier, and the charged surface. An exposure means for developing, a developing means for developing the formed electrostatic latent image with a charged toner to form a toner image, and a recording material carried on the recording material conveyance body passing through a transfer portion or the intermediate transfer Transfer means for transferring a toner image from the image carrier to a body, power supply means for applying a second voltage having a polarity opposite to the first voltage when the toner image is transferred by the transfer portion to the transfer means, A cleaning unit that removes the toner on the surface that has passed through the transfer unit, and a discharged toner image formed by adhering the charged toner to the surface is passed through the transfer unit by the second voltage by the cleaning unit. Removed And it has a discharging control and that. Charging adjustment in which the surface to which the second voltage is applied during the discharge control is charged by the charging unit a number of times more than after normal transfer using the first voltage, prior to normal exposure by the exposure unit. Have control.

  In the image forming apparatus of the present invention, when the discharged toner image that is not transferred onto the recording material is formed, a second voltage having a polarity opposite to that during normal toner image transfer is applied to the charging means to the intermediate transfer member or the recording material carrier. Block transcription. For this reason, potential variations that cannot be eliminated by the charging means assuming the first voltage can be formed between the surface area of the image carrier exposed to the second voltage and the surface area not exposed to the second voltage in the discharge control. There is sex.

  Therefore, the charge adjustment control is executed subsequent to the discharge control, and the charging means charges the surface of the image carrier more frequently than when the first voltage is used. Even if the charging means cannot obtain sufficient charging potential uniformity by a single charging process, the charging potential is reduced to a practical level by performing the charging process a plurality of times. As a result, there is no density difference or density unevenness in the image formed immediately after the discharge toner image is formed.

  Hereinafter, some embodiments of the present invention will be described in detail with reference to the drawings. The image forming apparatus of the present invention has a part or all of the configuration of each embodiment as long as the discharged toner image that is not transferred to the recording material is formed in the image forming apparatus including the intermediate transfer member or the recording material transport member. Other embodiments with alternative configurations are possible.

  The present invention includes not only a tandem type full-color image forming apparatus, but also an image forming apparatus in which a plurality of developing devices are attached to one image carrier, an intermediate transfer body, or an image carrier that is attached to a recording material conveyance body. It can also be implemented in an image forming apparatus.

  In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  In addition, about general matters, such as a structural member of an image forming apparatus, a power supply, material, and process control shown by patent document 1, 2, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted.

<First Embodiment>
FIG. 1 is an explanatory diagram of a configuration of the image forming apparatus according to the first embodiment. The image forming apparatus 100 according to the first embodiment is a tandem full-color copying machine in which yellow, magenta, cyan, and black image forming portions Pa, Pb, Pc, and Pd are arranged in a linear section of the intermediate transfer belt 28.

  As shown in FIG. 1, the intermediate transfer belt 28, which is an example of an intermediate transfer member, is supported around a driving roller 51, a driven roller 52, and a secondary transfer inner roller 53. The drive roller 51 is rotationally driven by a drive motor (not shown) (for example, a stepping motor), and circulates the intermediate transfer belt 28 in the clockwise direction in the drawing. The intermediate transfer belt 28 is formed of a dielectric resin film such as polycarbonate, polyethylene terephthalate, or polyvinylidene fluoride.

  Image forming portions Pa, Pb, Pc, and Pd are arranged in a straight section of the intermediate transfer belt 28, and transfer portions T1 are formed between the photosensitive drums 21a, 21b, 21c, and 21d. The photosensitive drums 21a, 21b, 21c, and 21d each have a diameter of 30 mm, and the intermediate transfer belt 28 has a peripheral length of 700 mm.

  The image forming portions Pa, Pb, Pc, and Pd are configured similarly except that the color of the toner used in the developing devices 23a, 23b, 23c, and 23d is different from yellow, magenta, cyan, and black. Therefore, in the following description, the image forming unit Pa will be described in detail, and for the image forming units Pb, Pc, and Pd, the reference numerals a of the constituent members of the image forming unit Pa are read as b, c, and d. Shall be understood.

  The image forming unit Pa includes a photosensitive drum 21 a that rotates at the same peripheral speed as the intermediate transfer belt 28. A charging device 22a, an exposure device 26a, a developing device 23a, a transfer roller 24a, and a cleaning device 25a are disposed around a photosensitive drum 21a that is an example of an image carrier.

  The charging device 22a, which is an example of charging means, is a charging roller that is formed of a conductive sponge-like rubber material and is urged by a biasing spring (not shown) to roll on the surface of the photosensitive drum 21a. The charging device 22a is charged with a charging voltage obtained by superimposing an AC voltage on a DC voltage from a charging power source 31a included in the charging unit, and charges the surface of the photosensitive drum 21a to a uniform potential.

  An exposure device 26a, which is an example of an exposure device, scans and exposes the surface of the photosensitive drum 21a with a laser beam obtained by pulse-modulating a yellow component color image signal of a document to form a yellow component color electrostatic latent image.

  The developing device 23a, which is an example of a developing unit, mixes toner supplied from a toner bottle 30a, which is an example of a developer supply unit, with a magnetic carrier, and is charged by stirring with a supply roller R1 and a developing roller R2. The charged toner is carried on the developing sleeve Sa in a thin layer state together with the magnetic carrier, and is carried to the surface facing the photosensitive drum 21a as the developing sleeve Sa rotates. By applying a developing voltage in which an alternating voltage is superimposed on a direct current voltage to the developing sleeve Sa, the toner moves from the developing sleeve Sa to the photosensitive drum 21a, adheres to the electrostatic latent image, and the toner image is developed.

  The transfer roller 24a, which is an example of a transfer unit, is always in pressure contact with the photosensitive drum 21a via the intermediate transfer belt 28, and a transfer portion T1, which is an example of a transfer portion, is interposed between the photosensitive drum 21a and the intermediate transfer belt 28. Form.

  The transfer power supply 29a, which is an example of power supply means, outputs a transfer voltage (first voltage) having a polarity opposite to the normal charging polarity of the toner to the transfer roller 24a, and electrostatically transfers the toner image from the photosensitive drum 21a to the intermediate transfer belt 28. Move. However, when a discharged toner image that is not transferred to the recording material is formed, a transfer inhibition voltage (second voltage) having the same polarity as the normal charging polarity of the toner is output to the transfer roller 24a, and the discharged toner image is passed through the transfer portion T1. Let

  In the first embodiment, the reversal development method is adopted by charging the toner negatively. For example, after charging the surface of the photosensitive drum 21a to -500V having a negative polarity by the charging device 21a, the portion exposed by the exposure device 26a is decharged to -150V. A developing voltage of 350 V is applied to the developing sleeve Sa, and the negatively charged toner is attached to the part of the photosensitive drum 21a that has been decharged. Then, the transfer power source 29a outputs 300V to the transfer roller 24a to transfer the negatively charged toner image to the intermediate transfer belt 28.

  The cleaning device 25a, which is an example of a cleaning unit, slides a cleaning blade on the surface of the photosensitive drum 21a to remove the transfer residual toner that has passed through the transfer portion T1 without being transferred to the intermediate transfer belt 28.

  The pre-exposure device 27a, which is an example of an erasing unit, uniformly exposes the surface of the photosensitive drum 21a using a linear fluorescent lamp, and the surface of the photosensitive drum 21a from which the toner image is removed by the cleaning device 25a. Erase the electrostatic latent image.

  The yellow component color toner image transferred to the intermediate transfer belt 28 at the transfer portion T1 of the photosensitive drum 21a is conveyed to the transfer portion T1 of the photosensitive drum 21b as the intermediate transfer belt 28 moves. By this time, a magenta component color toner image has been formed on the photosensitive drum 21b in the same procedure, and is transferred to the yellow component color toner image in a superimposed manner at the transfer portion T1.

  Similarly, the cyan component color toner image is transferred with overlapping positions at the transfer portion T1 of the photosensitive drum 21c, and the black component color toner image is transferred with overlapping positions at the transfer portion T1 of the photosensitive drum 21d. The four color toner images transferred by the image forming units 21a, 21b, 21c, and 21d are transported to the secondary transfer unit T2 as the intermediate transfer belt 28 is moved, and are collectively secondary transferred to the recording material 8. The recording materials 8 taken out one by one from a sheet feeding cassette (not shown) stand by the registration roller 32 and are synchronized with the four color toner images on the intermediate transfer belt 28 from the registration roller 32 to the secondary transfer portion T2. Sent out.

  The secondary transfer outer roller 54 is pressed against the secondary transfer inner roller 53 via the intermediate transfer belt 28 to form a secondary transfer portion T2 between the intermediate transfer belt 28 and the secondary transfer outer roller 54. The secondary transfer inner roller 53 is connected to the ground potential, and a transfer voltage is applied to the secondary transfer outer roller 54 from the transfer power supply 55. As a result, a transfer electric field for electrostatically moving the four color toner images on the intermediate transfer belt 28 to the recording material 8 is formed between the secondary transfer inner roller 53 and the secondary transfer outer roller 54. In the first embodiment, for example, the transfer power supply 55 outputs a transfer voltage of 300 V to the secondary transfer outer roller 54 to secondary transfer the negatively charged toner in the four color toner images to the recording material 8. .

  The recording material 8 onto which the four color toner images have been transferred is separated from the intermediate transfer belt 28 and then conveyed to the fixing device 9 by the conveyance belt 56. In the fixing device 9, the recording material 8 is nipped and conveyed by a fixing nip T3 formed by a fixing roller 9t heated by a heater 9h and a pressure roller 9k. Thus, heat and pressure are applied to the four color toner images to fix the toner images on the surface of the recording material 8.

  The toner on the intermediate transfer belt 28 that has passed through the secondary transfer portion T 2 without being transferred to the recording material 8 is conveyed to the cleaning device 11 by the intermediate transfer belt 28. The cleaning device 11 employs a blade cleaning method, and a urethane rubber cleaning blade 11 e is spring-pressed with a contact pressure and slid against the intermediate transfer belt 28.

<Discharge control>
FIG. 2 is a time chart of the discharge control in the first embodiment. In the image forming apparatus 100, discharge control is performed in order to maintain the image quality by controlling the charging state of the toner in the developing devices 23a, 23b, 23c, and 23d. By charging the toner bottles 30a, 30b, 30c, and 30d with unused toner corresponding to the amount of toner discharged from the developing devices 23a, 23b, 23c, and 23d, the charged state of the toner in the developing devices 23a, 23b, 23c, and 23d Is adjusted.

  The discharge control is executed by temporarily waiting for image formation executed at normal intervals and inserting the image formation intervals. In the discharge control, charging, exposure, and development are performed under the same conditions as in image formation, and discharge toner images that are not transferred to the recording material 8 are formed on the photosensitive drums 21a, 21b, 21c, and 21d. However, the transfer rollers 24a, 24b, 24c, and 24d are applied with a transfer blocking voltage (reverse bias voltage) having a polarity opposite to that of the normal transfer voltage, and the toner images discharged from the transfer portion T1 are passed through to clean the cleaning devices 25a and 25b. , 25c, 25d.

  Further, in order to shorten the discharge control time and resume normal image formation at an early stage, a solid image having a uniform density pattern on the entire surface is placed on the photosensitive drums 21a, 21b, 21c, and 21d, and the overlapping position on the intermediate transfer belt 28 is placed. To execute discharge control of each color.

  The discharged toner image can also be formed without exposure by controlling the charging potential and the developing bias. However, the method of this embodiment is desirable in that the discharge amount can be precisely controlled according to the exposure conditions while minimizing the change in the control conditions between the transfer toner image and the discharge toner image.

  Next, the discharge sequence will be described.

  In the first embodiment, the integrated value of the toner usage amount is obtained for each component color and integrated for each image. When the difference from the predetermined reference consumption integration amount reaches a predetermined value, The discharge control corresponding to the difference is performed. When the difference from the reference consumption integrated amount for one color reaches a predetermined value, the other three colors simultaneously perform discharge control for eliminating the difference between the colors up to that point.

  Specifically, when the number of pixels is integrated for each image, the difference from the constant value is integrated for an image in which the number of pixels per image is less than or equal to a certain value, and when the integrated value of the difference reaches a certain amount A toner amount corresponding to the integrated value of the difference is discharged from the developing device.

  For example, assuming that a total image of 297 × 210 mm (A4 size) in horizontal feed is 100%, when an image of 5% or less comes, the difference with 5% of the number of pixels of the image is integrated. When the integrated value of the differences reaches 100%, discharge control between the sheets is executed in a form in which the next image formation is waited and the interval between the sheets is extended. That is, when 4% images of A4 size are continuously passed, 1% of image information is insufficient per sheet, and thus the integrated value of the difference reaches 100% when 100 sheets are passed.

  Therefore, as shown in FIG. 3 with reference to FIG. 1, a toner amount corresponding to the A4 solid image is set as the discharged toner image between the 100th image and the 101st image. , 21c, 21d. As a result, when the discharged toner image is formed, the downtime occurs only for the time corresponding to “A4 length + drum white rotation”, and the productivity of the image forming apparatus 100 is not greatly affected.

  Also, normally, the frequency at which the integrated value of the differences reaches 100% at each component color is low. Therefore, when any one of yellow, magenta, cyan, and black reaches 100%, a sequence for discharging all the component colors as a solid image by a length corresponding to each integrated value is set. This eliminates the need for having a separate discharge timing for each component color, thereby further reducing downtime.

  In the transfer portion T1 during discharge control, a transfer inhibition voltage (reverse bias voltage) having a reverse polarity to that of a normal toner image transfer is applied to the transfer rollers 24a, 24b, 24c, and 24d. Thus, the discharged toner image charged to the negative polarity reaches the cleaning devices 25a, 25b, 25c, and 25d and is removed without being transferred to the intermediate transfer belt 28.

  Thereafter, exposure is not performed by the exposure devices 26a, 26b, 26c, and 26d while the charging device 22 and the pre-exposure device 27a are turned on at least for one rotation of the photosensitive drums 21a, 21b, 21c, and 21d. That is, after “control for idly rotating the photosensitive drum at least once in a state where a solid white image is formed” which is an example of the charge adjustment control, the toner image forming operation of the next 101st image is started. As a result, the negative ghost potential generated on the surfaces of the photosensitive drums 21a, 21b, 21c, and 21d to which the discharge toner image is formed and the reverse bias voltage is applied is erased, and the image quality of the next 101st and subsequent sheets is affected. Never give.

<Example 1>
Next, an experimental result of the process leading to the discharge control of the first embodiment as described above will be described as a first example. FIG. 3 is a diagram showing the relationship between the reverse bias voltage and the amount of toner adhering to the intermediate transfer belt, and FIG. 4 is a time chart of the discharge control in the first conventional example.

  As shown in FIG. 1, in the image forming apparatus 100, discharge control experiments were performed by varying the reverse bias voltages applied to the transfer rollers 24a, 24b, 24c, and 24d. As a result, as shown in FIG. 3, it was confirmed that as the reverse bias voltage was increased to the negative polarity side, the amount of toner transferred onto the intermediate transfer belt 28 at the transfer portion T1 gradually decreased. In the image forming apparatus 100, it has been found that, when the reverse bias voltage is −1 KV or higher, the contamination of the intermediate transfer belt 28 does not become a problem with only one cleaning by the cleaning device 11.

  Therefore, the discharge control of Conventional Example 1 in which the reverse bias voltage applied to the transfer rollers 24a, 24b, 24c, and 24d when the discharged toner image passes through the transfer portion T1 was set to -1 KV was tested. As shown in FIG. 4 with reference to FIG. 1, the discharge control of Conventional Example 1 is performed by applying a reverse bias voltage of −1 KV and passing the discharged toner image at the transfer portion T <b> 1, and then the pre-exposure device 27 a and the charging device. After 22a, an electrostatic latent image of the next image is immediately written by the exposure device 26a. That is, the “control for idly rotating the photosensitive drum in a state of forming a solid white image” as shown in FIG. 2 is not included.

  As a result, although the cleaning failure of the intermediate transfer belt 28 does not occur, a negative ghost is generated in a portion to which the reverse bias voltage is applied, and an abnormal density step is formed in the next image.

  This phenomenon is explained as follows. In the reversal development type image forming apparatus 100, the exposure unit exposed by the exposure unit 26 a to form an electrostatic latent image has a lower potential than the non-exposure unit, and the pre-exposure unit 27 a has a potential reset function. Receive pre-exposure irradiation. At this time, a difference occurs in the carrier generation amount of the photosensitive layer of the photosensitive drum 21a between the exposed portion and the non-exposed portion, and the difference is reflected in the potential attenuation amount after charging by the charging device 22a. As a result, even if exposure is performed equally by the exposure device 26a, the potential at the image portion where the amount of carrier generation is relatively low becomes high at the subsequent development position of the development device 23a, causing a negative ghost. is there.

  For the same reason, if the reverse bias voltage applied to the transfer roller 24a during discharge control is increased, the potential step formed on the surface of the photosensitive layer in the inter-paper portion due to the reverse bias voltage passes through the charging device 22a once. Doing so makes it difficult to erase. As a result, the charged potential in the region that has received the reverse bias voltage in accordance with the discharge control on the photosensitive drum 21a is lower than the charged potential during normal image formation. As a result, in the image formed immediately after the discharge control, the surface potential difference in the inter-paper portion becomes a density step and is generated in the image.

  On the other hand, if the pre-exposure dose by the pre-exposure device 27a is increased, the surface potential difference between the region that has received the reverse bias voltage and the region that has not received it can be reduced. However, it is not desirable to increase the output of the pre-exposure device 27a only for the discharge control. Further, when the pre-exposure irradiation amount is increased, deterioration of the charging performance of the photosensitive drum is promoted and the durability life is shortened. Therefore, it is not appropriate to increase the exposure amount more than necessary.

  Therefore, as described above, the discharge control according to the first embodiment in which “control for idly rotating the photosensitive drum in a state where a solid white image is formed” is added to the discharge control is employed. In the first embodiment, the rotational speed of the photosensitive drums 21a, 21b, 21c, and 21d after the discharged toner image has passed through the transfer portion T1 is set to one round. The experimental results are shown in Table 1.

  In Table 1, in the conventional example 1, as described above, the reverse bias voltage during discharge control is set to −1 KV, and the rotational speeds of the photosensitive drums 21a, 21b, 21c, and 21d after the discharged toner image passes through the transfer portion T1 are as follows. 0 laps. In Comparative Example 1, the reverse bias voltage during discharge control is set to −100 V, and the rotational speeds of the photosensitive drums 21a, 21b, 21c, and 21d after the discharged toner image passes through the transfer portion T1 are set to zero.

  As shown in Table 1, in Comparative Example 1, no negative ghost is generated. However, the reverse bias voltage is too low, and the discharged toner image is transferred to the intermediate transfer belt 28 in excess of the capability of the cleaning device 11, resulting in a defective cleaning and disturbing the next image.

  Therefore, in order to prevent this, it is necessary to clean the intermediate transfer belt 28 by circulating the intermediate transfer belt 28 twice by the cleaning device 11 with the toner transferred to the intermediate transfer belt 28. However, in this case, since the intermediate transfer belt 28 is long, the downtime is longer than when the photosensitive drum 21a is idled once, and the productivity of the image forming apparatus 100 is reduced.

  In Conventional Example 1, as described above, since the discharged toner image is difficult to be transferred to the intermediate transfer belt 28, a cleaning failure does not occur, but a negative ghost is generated by setting the reverse bias voltage to -1 KV.

  On the other hand, as shown in FIG. 2, the reverse bias voltage at the discharge control is set to −1 KV, and “control for idling the photosensitive drum in a state of forming a solid white image” is added. Does not occur. There was no poor cleaning due to the transfer of the discharged toner image, no negative ghost, and good results were obtained that did not affect the next image. In the image forming apparatus 100 that performs discharge control between sheets during normal image formation, it is possible to achieve both reduction in downtime and reduction in load on the cleaning device 11.

<Example 2>
FIG. 5 is a time chart of discharge control according to the second embodiment. The discharge control of Example 2 is performed in the image forming apparatus 100 of the first embodiment, replacing the discharge control of Example 1. Therefore, redundant description will be omitted unless necessary, and will be described below with reference to FIGS.

  As shown in FIG. 5 with reference to FIG. 1, in the discharge control of the second embodiment, the reverse bias voltage applied to the transfer roller 24a when the discharged toner image passes through the transfer portion T1 is equal to that of the first embodiment. 1 kV. However, after the toner image passes through the transfer portion T1, it does not return to the transfer voltage when the normal toner image is transferred, and the reverse bias voltage is one step lower for the “region where the solid white image is formed”. −100 V was applied to the transfer roller 24a.

  In other words, the reverse bias voltage applied to the transfer roller 24a is set to −100 V while the toner discharged to the photosensitive drum 21a through the discharge control passes through the transfer portion T1 and rotates in a solid white image forming state.

  The transfer power supply 29a, which is a general high-voltage power supply used in the electrophotographic process, may cause overshoot and undershoot in the output voltage when the output voltage is greatly changed. When the voltage applied to the transfer roller 24a is reversed from the negative polarity side to the positive polarity side while the photosensitive drum 21a is charged to the negative polarity, if the overshoot is greatly increased to the positive polarity side, the overshoot is received. A positive ghost is generated in the part. If a positive ghost is generated, an extra idle rotation of the photosensitive drum 21a is required to eliminate this.

  On the other hand, overshoot can be prevented by changing the reverse bias voltage to low -100V once before reversing it to the positive polarity side as in Example 2 and then reversing it to the positive polarity side. .

  As described above, when the discharged toner image passes and then idles in a solid white image formation state, the reverse of the transfer voltage between the case of preventing the transfer of the discharged toner image and the transfer voltage at the time of normal toner image transfer. Set the bias voltage. Thereby, in the image forming apparatus 100 that executes the discharge control to prevent the deterioration of the developer, it is possible to prevent the occurrence of an excessive overshoot and to reduce the downtime during the discharge control.

Second Embodiment
FIG. 6 is an explanatory diagram of a configuration of the image forming apparatus according to the second embodiment. The image forming apparatus 200 of the second embodiment is configured by replacing the cleaning device 11 of the image forming apparatus 100 of the first embodiment with an electrostatic fur brush type cleaning device 12. Other than this, the configuration is the same as that of the first embodiment. Therefore, in FIG. 6, the same reference numerals are given to the same configurations as those of the first embodiment, and redundant description is omitted. In the second embodiment, since the amount of toner that can be cleaned at one time decreases with the change to the cleaning device 12, the reverse bias voltage during discharge control is increased from 1 kV to 2 kV in the first embodiment.

  As shown in FIG. 6, the untransferred toner that has passed through the secondary transfer portion T2 without being secondarily transferred to the recording material 8 is transported to the cleaning device 12 and removed as the intermediate transfer belt 28 moves. .

  In the housing of the cleaning device 12, a pair of conductive fur brushes 121 and 122 are disposed in contact with the intermediate transfer belt 28. On the opposite side of the intermediate transfer belt 28, aluminum metal rollers 123 and 124, which are in contact with the conductive fur brushes 121 and 122 and subjected to hard alumite treatment on the surface, are disposed. On the opposite side of the conductive fur brushes 121 and 122, cleaning blades 125 and 126 are disposed in contact with the metal rollers 123 and 124, respectively.

  The conductive fur brush 121, the metal roller 123, and the cleaning blade 125 constitute an upstream cleaning unit 12a, and the conductive fur brush 122, the metal roller 124, and the cleaning blade 126 constitute a downstream cleaning unit 12b. The upstream cleaning unit 12 a and the downstream cleaning unit 12 b are arranged in parallel to the intermediate transfer belt 28.

The conductive fur brushes 121 and 122 are configured by flocking carbon-dispersed nylon fibers having a resistance value of 10 MΩ and a fiber thickness of 6 denier on a metal roller at a rate of flocking density of 500,000 / inch ² . The conductive fur brushes 121 and 122 are arranged in sliding contact with the intermediate transfer belt 28 while maintaining an intrusion amount of about 1.0 mm. The conductive fur brushes 121 and 122 are driven by a drive motor (not shown) and have a peripheral speed (about 125% relative speed) of about ¼ of the peripheral speed of the intermediate transfer belt 28 with respect to the intermediate transfer belt 28. Rotate counter.

  The metal rollers 123 and 124 are arranged in sliding contact with the conductive fur brush while maintaining an intrusion amount of about 1.0 mm so that the surface movement direction is the same as that of the conductive fur brush 121 and 122. It is rotationally driven. The cleaning blades 125 and 126 that are in contact with the metal rollers 123 and 124 are made of urethane rubber, and are disposed with an intrusion amount of 1.0 mm with respect to the metal rollers 123 and 124.

  A DC voltage of −500 V (with respect to ground, the same shall apply hereinafter) is applied to the metal roller 123 of the upstream cleaning unit 12 a by a DC power supply 127. As a result, an electric field for moving the (+) toner in the transfer residual toner on the intermediate transfer belt 28 to the conductive fur brush 121 side between the driven roller 52 connected to the ground potential and the conductive fur brush 121. It is formed. Then, the (+) toner electrostatically transferred to the conductive fur brush 121 contacts the metal roller 123 as the conductive fur brush 121 rotates, and is electrostatically transferred from the conductive fur brush 121 to the metal roller 123. To do. The (+) toner adsorbed to the metal roller 123 is scraped off by the cleaning blade 125 as the metal roller 123 rotates.

  The (−) toner in the transfer residual toner on the intermediate transfer belt 28 passes through the upstream cleaning unit 12a and reaches the conductive fur brush 122 of the downstream cleaning unit 12b. The toner with a small charge amount in the transfer residual toner on the intermediate transfer belt 28 is negatively charged in the process of passing through the conductive fur brush 121 and reaches the conductive fur brush 122 of the downstream cleaning unit 12b. This is considered to be charged by charge injection or discharge.

  A DC voltage of +500 V is applied from the DC power supply 128 to the metal roller 124 of the downstream cleaning unit 12b. As a result, an electric field that moves the (−) toner on the intermediate transfer belt 28 toward the conductive fur brush 122 is formed between the driven roller 52 connected to the ground potential and the conductive fur brush 122. Then, the (−) toner electrostatically transferred to the conductive fur brush 122 is transferred to the metal roller 124 along with the rotation of the conductive fur brush 122 and then scraped off by the cleaning blade 126. In this way, all the transfer residual toner that has passed through the secondary transfer portion T2 and remained on the intermediate transfer belt 28 can be removed.

  The electrostatic fur brush method is often used for a member to be cleaned whose surface hardness is relatively difficult to completely remove the transfer residual toner by blade cleaning. In recent years, an elastic intermediate transfer belt having a relatively low surface hardness is often used for the intermediate transfer belt 28 in order to cope with high image quality and various types of recording materials.

  The electrostatic fur brush type cleaning device 12 is in no way inferior to the blade cleaning type cleaning device (11: FIG. 1) in the amount of secondary transfer residual toner in normal image formation. However, in general, the electrostatic fur brush method has a smaller amount of toner that can be cleaned at a time than the blade cleaning method.

  Therefore, in the image forming apparatus 200 of the second embodiment, the reverse bias voltage at the time of discharge control is increased from 1 kV to 2 kV in Example 1 to reduce the load on the cleaning device 12. Since the reverse bias voltage during discharge control was increased from 1 kV to 2 kV in the first embodiment, the “control for idly rotating the photosensitive drum in a state where a solid white image is formed” was set to two rotations. The experimental results are shown in Table 2 as Example 3.

  Table 2 shows the relationship between the reverse bias voltage applied to the transfer roller 24a during discharge control, the cleaning performance of the cleaning device 12, the rotational speed of the photosensitive drum 21a after the discharged toner image passes through the transfer portion T1, and the negative ghost. Is shown.

  In the second embodiment, since the electrostatic fur brush type cleaning device 12 is employed, the amount of toner that can be cleaned has decreased. Therefore, if the reverse bias voltage during discharge control is set to −1 KV and “control for idly rotating the photosensitive drum in a state where a solid white image is formed” is set to one rotation as in Example 1 in Table 1, intermediate transfer is performed. A defective cleaning of the belt 28 occurs. In order to eliminate the cleaning failure of the intermediate transfer belt 28, as shown in Table 2 as Conventional Example 2, it is necessary to circulate the intermediate transfer belt 28 twice and continue cleaning by the cleaning device 12. .

  As shown in Comparative Example 2, even if the reverse bias voltage is set to 2 kV, if the “control for idly rotating the photosensitive drum in a state where a solid white image is formed” is one rotation, a negative ghost is generated. It was.

  On the other hand, as shown in the third embodiment, when the reverse bias voltage during discharge control is set to −2 KV and “control for idly rotating the photosensitive drum in a state of forming a solid white image” is set to two rotations, The problem was solved. After the discharged toner image has passed through the transfer portion T1, the photosensitive drum is idled twice with the pre-exposure device 27a and the charging device 22a turned on in the same manner as in the normal toner image formation, and passes the charging device 22a on the third round. The exposure device 26a writes the electrostatic latent image of the next image on the portion thus processed.

  As a result, the toner transferred from the discharged toner image between the sheets to the intermediate transfer belt 28 can be sufficiently cleaned, and at the same time, the occurrence of negative ghost can be prevented.

  As described above, since the diameter of the photosensitive drum 21 is 30 mm, even when the photosensitive drum 21a is idled twice as in the third embodiment, the distance between sheets consumed for discharging control is only 188.5 mm. On the other hand, since the peripheral length of the intermediate transfer belt 28 is 700 mm, when the intermediate transfer belt 28 is circulated twice in a circle as in the conventional example 2, the distance between sheets consumed for discharging control is 1400 mm. That is, in the discharge control according to the third embodiment, the time for waiting for the next image formation for the discharge control is shortened to 1/7 or less as compared with the discharge control according to the second conventional example. Accordingly, the discharge sequence is performed in a sufficiently shorter time than the conventional example 2 in which the intermediate transfer belt 28 is idled twice after passing the discharged toner image to the transfer portion T1 using a reverse bias voltage 1 kV that does not generate a negative ghost. Can be terminated.

  That is, the circumferential length of the photosensitive drum 21a is Rd, the circumferential length of the intermediate transfer belt 28 is Rt, and the total circumferential length when the photosensitive drum 21a is idly rotated n times in the non-exposed state by charging with the charging device 22a is r. (= N · Rd). At this time, by setting the rotational speed n of the idling rotation of the photosensitive drum 21a to a value satisfying Rd ≦ r ≦ Rt, the discharge control can be completed in a sufficiently short time compared to the discharge control of the conventional example 2.

<Third Embodiment>
FIG. 7 is an explanatory diagram of a configuration of the image forming apparatus according to the third embodiment. The image forming apparatus 300 according to the third embodiment is configured by replacing the intermediate transfer belt 28 in the first embodiment with a recording material conveyance belt 38. The image forming portions Pa, Pb, Pc, Pd, the cleaning device 11, the fixing device 9 and the like are the same as those in the first embodiment. Therefore, the corresponding members in FIG. 7 are assigned the same reference numerals as in FIG.

  The present invention can be implemented not only in the image forming apparatus using the intermediate transfer belt (28: FIG. 1) but also in the image forming apparatus 300 using the recording material conveying belt 38 as shown in FIG. The image forming apparatus 300 executes discharge control for discharging the toner in the developing devices 23a, 23b, 23c, and 23d between the sheets of the photosensitive drums 21a, 21b, 21c, and 21d in order to prevent the deterioration of the developer. At this time, a reverse bias voltage is applied to the transfer roller 24a and the reverse bias voltage is set to 1000 V in order to prevent transfer of the discharged toner image to the recording material conveyance belt 38, thereby preventing a cleaning failure of the cleaning device 11. . As a result, the downtime can be reduced without imposing a burden on the cleaning device 11 provided on the recording material conveyance belt 38.

  As shown in FIG. 7, the recording material 8 taken out from a paper feeding cassette (not shown) is adsorbed and supported on a recording material conveyance belt 38 which is an example of a recording material conveyance body through a registration roller 32. The recording material conveyance belt 38 is supported by being supported between a driving roller 51 and a driven roller 52, and above the recording material conveyance belt 38, four image forming portions Pa, Pb, Pc, and Pd are arranged in series. ing. The recording material transport belt 38 is driven to rotate by a drive roller 51 by a drive motor (not shown) (for example, a stepping motor), and this rotational driving force is transmitted from the drive roller 51 to the recording material transport belt 38.

  The surface moving speeds of the photosensitive drums 21a, 21b, 21c, and 21d and the recording material conveyance belt 38 in each transfer portion T1 are substantially the same. Since the image forming processes other than the transfer portion T1 in the image forming portions Pa, Pb, Pc, and pd are the same as those in the first embodiment, description thereof is omitted.

  The image formation to transfer process is similarly repeated at the image forming portions Pa, Pb, Pc, and Pd. The recording material 8 carried on the recording material conveyance belt 38 in synchronization with the toner images of the respective component colors formed on the photosensitive drums 21a, 21b, 21c, and 21d passes through the respective transfer portions T1.

  At this time, transfer power supplies 29a, 29b, 29c, and 29d, which are examples of power supply means, output transfer voltages to transfer rollers 24a, 24b, 24c, and 24d that are examples of transfer means. As a result, the yellow toner image is first transferred from the photosensitive drum 21a onto the recording material 8, and then the magenta toner image is transferred from the photosensitive drum 21b. Further, a cyan toner image is transferred from the photosensitive drum 21c, and finally a black toner image is transferred from the photosensitive drum 21d. In this way, the recording material 8 on which the four color toner images are directly transferred in a superimposed manner is separated from the recording material conveying belt 38 and delivered to the fixing device 9. The fixing device 9 heats and presses the recording material 8 to fix the four color toner images, and the fixed recording material 8 is discharged out of the image forming apparatus 300, and a series of image forming steps is completed.

  A cleaning device 11 for cleaning the fog toner and the like on the recording material conveyance belt 38 is attached to the recording material conveyance belt 38 so as to face the driven roller 52. As in the first embodiment, the cleaning device 11 employs a blade cleaning method.

  In the image forming apparatus 300 according to the third embodiment, when a discharged toner image is formed between sheets, a desired toner amount can be discharged first by placing a so-called solid image on the photosensitive drum 21a. Further, the discharge control for each component color at the same time is performed so that the discharge toner images of the respective component colors overlap on the recording material conveyance belt 38, so that the downtime until returning to the normal toner image formation is minimized. Can be shortened.

  However, the cleaning device 11 is for cleaning the fog toner on the photosensitive drum 21a and the toner erroneously transferred to the recording material conveying belt 38 when the recording material 8 is jammed. For this reason, it does not have the ability to clean the toner with four colors superimposed on the recording material conveyance belt 38. Further, when the contact pressure of the blade is increased in order to clean the four-color solid image, the recording material conveying belt 38 is scraped or the blade is turned up. Further, when the electrostatic fur brush type cleaning device described in the second embodiment is used to clean four color solid images at a time, the cleaning device becomes large and complicated.

  Therefore, in the image forming apparatus 300 of the third embodiment, as in the first embodiment, a reverse bias voltage is applied to the transfer roller 24a during discharge control so that the discharged toner image is not transferred to the recording material conveyance belt 38. As shown in FIG. 4, a reverse bias voltage having a reverse polarity to the normal transfer voltage is applied to the transfer roller 24a while the toner image passes through the transfer portion T1 so that the toner image does not transfer to the recording material conveyance belt 38. Apply to. As shown in FIG. 3, as the reverse bias voltage increases, the amount of toner transferred to the recording material conveyance belt 38 decreases, and the cleaning load of the cleaning device 11 is reduced. Further, by setting the reverse bias voltage to −1 KV, the cleaning failure of the recording material transport belt 38 can be eliminated even by one cleaning by the cleaning device 11.

  When the discharged toner image passes through the transfer portion T1, as shown in FIG. 2, the reverse bias voltage is inverted to the normal transfer voltage, and the charging device 22a and the pre-exposure device 27a are turned on to output the exposure device 26a. In the stopped state, the photosensitive drum 21a was idled once. That is, the discharge control is performed to return to normal toner image formation and transfer to the recording material 8 after the photosensitive drum 21a is rotated once in a solid white image formation state.

  By providing such discharge control, the same effects as those of the first embodiment and example 1 could be obtained in the third embodiment. In order to prevent the deterioration of the developer, in the image forming apparatus 300 that discharges the toner in the developing device 23a to the photosensitive drum 21a, the downtime can be shortened without imposing a burden on the cleaning device 11 provided on the recording material conveyance belt 38. It was.

<Correspondence with Invention>
The image forming apparatus 100 exposes the photosensitive drum 21a, the intermediate transfer belt 28, the charging device 22a for charging the surface of the photosensitive drum 21a, the charging power source 31a, and the charged surface to form an electrostatic latent image. The toner image is transferred from the photosensitive drum 21a to the device 26a, the developing device 23a that develops the formed electrostatic latent image with charged toner to form a toner image, and the intermediate transfer belt 28 that passes through the transfer portion T1. The transfer roller 24a, a transfer power source 29a that applies a second voltage having a polarity opposite to the first voltage when the toner image is transferred by the transfer unit T1, and the toner on the surface that has passed through the transfer unit T1. And a cleaning device 25a to be removed. The control unit 50 performs discharge control that causes the discharged toner image formed by attaching the charged toner to the surface to pass through the transfer unit by the second voltage and is removed by the cleaning device 25a. Following the discharge control, the control unit 50 causes the surface to which the second voltage is applied during the discharge control to precede the normal exposure by the exposure device 26a, and after the normal transfer using the first voltage. Charge adjustment control for charging by the charging device 22a and the charging power source 31a is performed many times.

  The image forming apparatus 100 includes a pre-exposure device 27a that erases the electrostatic latent image by irradiating the surface from which the toner image has been removed by a cleaning device 25a, a charging device 22a, a charging power source 31a, and a pre-exposure device 27a. And a control unit 50 that operates the developing device 23a under the same conditions as normal image formation and executes the discharge control at regular image formation intervals. The controller 50 performs the charge adjustment control by operating the charging device 22a, the charging power source 31a, and the pre-exposure device 27a under the same conditions as normal image formation in a state where exposure by the exposure device 26a is prohibited.

  When the first voltage is Vt, the second voltage is Vh, and the third voltage applied to the transfer roller 24a by the charge adjustment control is Vi, in the first embodiment, the first voltage is positive. Vh ≦ Vi ≦ Vt.

  In this embodiment, when the circumferential length of the intermediate transfer belt 28 is Rt, the circumferential length of the photosensitive drum 21a is Rd, and the moving distance of the intermediate transfer belt 28 passing through the transfer portion T1 through the charge adjustment control is r. Rd ≦ r ≦ Rt.

It is explanatory drawing of a structure of the image forming apparatus of 1st Embodiment. It is a time chart of the discharge control in 1st Embodiment. FIG. 6 is a diagram illustrating a relationship between a reverse bias voltage and an amount of toner attached to an intermediate transfer belt. 10 is a time chart of discharge control of Conventional Example 1. 6 is a time chart of discharge control according to the second embodiment. It is explanatory drawing of a structure of the image forming apparatus of 2nd Embodiment. It is explanatory drawing of a structure of the image forming apparatus of 3rd Embodiment.

Explanation of symbols

8 Recording material 9 Fixing device 11, 12 Cleaning device 21a, 21b, 21c, 21d Image carrier (photosensitive drum)
22a, 22b, 22c, 22d Charging means (charging device)
31a, 31b, 31c, 31d Charging means (charging power source)
23a, 23b, 23c, 23d Developing means (developing device)
24a, 24b, 24c, 24d Transfer means (transfer roller)
25a, 25b, 25c, 25d Cleaning means (cleaning device)
27a, 27b, 27c, 27d Erasing means (pre-exposure device)
28 Intermediate transfer member (intermediate transfer belt)
29a, 29b, 29c, 29d Power supply means (transfer power supply)
30a, 30b, 30c, 30d Developer supply means (toner bottle)
38 Recording material transport body (Recording material transport belt)
50 Discharge control, charge adjustment control, control means (control unit)
T1 transcription section

Claims (4)

An image carrier;
A recording material carrier or an intermediate transfer member;
Charging means for charging the surface of the image carrier;
Exposure means for exposing the charged surface to form an electrostatic latent image;
Developing means for developing the formed electrostatic latent image with charged toner to form a toner image;
Transfer means for transferring a toner image from the image carrier to the recording material carried on the recording material carrier passing through the transfer portion or the intermediate transfer member;
Power supply means for applying to the transfer means a second voltage having a polarity opposite to the first voltage when the toner image is transferred at the transfer section;
Cleaning means for removing toner on the surface that has passed through the transfer portion;
In the image forming apparatus, the discharge toner image formed by adhering the charged toner to the surface is discharged through the transfer unit by the second voltage and removed by the cleaning unit.
Charging adjustment in which the surface to which the second voltage is applied during the discharge control is charged by the charging unit a number of times more than after normal transfer using the first voltage, prior to normal exposure by the exposure unit. An image forming apparatus having control. Erasing means for irradiating the surface from which the toner image has been removed by the cleaning means to erase the electrostatic latent image;
Control means for operating the charging means, the erasing means, and the developing means under the same conditions as normal image formation, and executing the discharge control at intervals of normal image formation,
2. The control unit according to claim 1, wherein the control unit performs the charge adjustment control by operating the charging unit and the erasing unit under the same conditions as normal image formation in a state where exposure by the exposure unit is prohibited. Image forming apparatus. When the first voltage is Vt, the second voltage is Vh, and the third voltage applied to the transfer means in the charge adjustment control is Vi,
If the first voltage is positive, Vh ≦ Vi ≦ Vt,
The image forming apparatus according to claim 2, wherein if the first voltage is negative, Vh ≧ Vi ≧ Vt. The recording material conveyance body or the intermediate transfer body has a circumferential length Rt, the image carrier has a circumferential length Rd, and the recording material conveyance body or the intermediate transfer body passes through the transfer section through the charge adjustment control. When the moving distance is r,
4. The image forming apparatus according to claim 2, wherein Rd ≦ r ≦ Rt.

Images