JP2006047491A - Image forming apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that uses an intermediate transfer belt in multi-layered structure including a conductive layer and a resistance layer capable of securing proper image density while preventing an image from deteriorating owing to the frequency of use of a corona charger etc., and suppressing toner scatter in the image forming apparatus and formation of an abnormal image due to abnormal discharge at a primary transfer part, and an image forming method. <P>SOLUTION: The image forming apparatus is equipped with an image carrier, the corona charging means of charging the image carrier, the corona charging means being equipped with a discharge electrode, a back plate, and a grid electrode, an exposure means of forming an electrostatic latent image on the charged image carrier, a developing means of forming a toner image by visualizing the electrostatic latent image formed on the image carrier with toner, the intermediate transfer body of multi-layered structure including the conductive layer, and a control means of controlling an image density control factor including a developing bias potential applied to the developing means, and the control means controls a transfer potential, a grid bias potential, and a discharging current so that the difference between the transfer potential and a non-image part potential on the image carrier at a transfer position lies within a specified range. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a printer, a facsimile machine, and a copier that forms an image using electrophotographic technology, and an image forming method.

  An image forming apparatus using an electrophotographic technique includes an image carrier having a photosensitive layer on an outer peripheral surface, a charging unit that uniformly charges the outer peripheral surface of the image carrier, and an image carrier that is uniformly charged by the charging unit. An exposure means for selectively exposing the outer peripheral surface of the body to form an electrostatic latent image, a developing means for applying toner to the electrostatic latent image formed by the exposure means to form a toner image, and development by the developing means Transfer means for transferring the toner image to a transfer medium such as paper. In order to transfer the toner image developed on the image carrier to the transfer medium, the toner image on the image carrier is primarily transferred to the intermediate transfer belt, and the toner image on the intermediate transfer belt is secondarily transferred to the transfer medium. Forming devices are known.

  An intermediate transfer belt includes a single layer structure made of a dielectric. This type of intermediate transfer belt is pressed against the image carrier by two conductive transfer rollers, and a voltage having a polarity opposite to that of the toner on the image carrier is applied. In an intermediate transfer belt having a single-layer structure made of a dielectric, a large potential difference exists between two transfer rollers and a pressure contact portion (transfer portion) between the image carrier that is an intermediate portion thereof. For this reason, if an electric field that can provide sufficient transfer efficiency is formed at the pressure contact portion (transfer portion) with the image carrier, the contact portion between the two transfer rollers on the intermediate transfer belt and the image carrier. There is a problem in that the potential difference between the two becomes too large and discharge occurs in this portion, and toner scattering occurs due to this discharge, which affects the image quality. In addition, there is a certain distance between the two transfer rollers forming the electrode section and the pressure contact section (transfer section) between the image carrier and the transfer due to the influence of uneven surface resistance of the intermediate transfer belt itself. There is a problem that unevenness is easily generated in the electric field at the portion, and as a result, transfer unevenness is likely to occur.

  As a countermeasure, an intermediate transfer belt having a multi-layer structure, which includes a conductive layer and a resistance layer formed on the conductive layer and presses the resistance layer against an image carrier, has been developed. Since the intermediate transfer belt having a multilayer structure including the conductive layer and the resistance layer can apply a uniform potential over the entire area of the pressure contact portion between the image carrier and the intermediate transfer belt, the single layer structure is made of a dielectric. Therefore, it is possible to suppress toner scattering due to electric discharge and transfer unevenness due to uneven surface resistance, which are problems of the image forming apparatus using the intermediate transfer belt.

In addition, in an image forming apparatus using an electrophotographic technique, an image density control factor (exposure energy, non-image part potential, image part potential) is set so that the image density is optimized even in various use environments (temperature, humidity, etc.). It is known to have a process control means for appropriately adjusting the development bias potential and the like. However, in an actual image forming process, a plurality of these factors are associated with each other to form a toner image. Therefore, these factors cannot always be controlled independently and arbitrarily. Among these factors, the absolute value of the potential difference between the developing bias potential Vb and the non-image portion potential Vd on the image carrier is called a reverse contrast potential. When the reverse contrast potential is small, toner scattering is large and fogging occurs. Will also increase. On the other hand, when the reverse contrast potential is large, both the toner scattering amount and the fogging amount are reduced, but the toner is applied to the image portion of the electrostatic latent image on the image carrier, particularly in a narrow region sandwiched between non-image portions. It becomes difficult to adhere. As a result, it is known that the quality of a low-density image with a relatively low dot area ratio is deteriorated, such as blurring of isolated dots and thin lines, and loss of line width uniformity. As a countermeasure, the inverse contrast potential, which is the absolute value of the potential difference between the developing bias potential Vb applied to the developing means and the non-image portion potential Vd on the image carrier, is kept constant, and the image density of the toner image is affected. By changing the setting of the image density control factor to be given in multiple stages, a halftone toner image is formed, and the image density control factor is optimized based on the detection result of the image density of the toner image, thereby the image of the toner image An image forming method for optimizing the density has been proposed. According to this image forming method, the reverse contrast potential, which is the absolute value of the potential difference between the developing bias potential Vb and the non-image portion potential Vd on the image carrier, is maintained at an appropriate value, whereby toner into the image forming apparatus is stored. The image density can be optimized while preventing scattering.
A corona charger is generally used as means for charging the image carrier. As a corona charger, a discharge electrode is installed in a back plate which is a casing, a high voltage Va is applied to the discharge electrode to generate a corona discharge, and the image carrier surface is uniformly charged. It is generally known that a grid electrode is disposed between and a grid bias potential Vg is applied to the grid electrode. The grid bias potential Vg and the non-image portion potential Vd at the transfer portion of the image carrier have a functional relationship.
Also, in the corona charger, the charging current is increased stepwise based on the life information such as the number of times of use in order to prevent contamination of the discharge electrode, grid electrode, or back plate with toner or image deterioration due to aging. What has been developed has been developed.
JP-A-11-153910 JP 2003-215862 A Japanese Patent Publication No. 7-21671

  However, in an image forming apparatus using an intermediate transfer belt having a multilayer structure including a conductive layer and a resistance layer, the potential difference between the non-image portion potential Vd and the primary transfer bias potential Vt1 at the transfer portion on the image carrier is Vdt. Then, when Vdt = | Vd−Vt1 | becomes large, an abnormal image that is not partially or wholly transferred may occur in the primary transfer portion, and this phenomenon is particularly noticeable in a half image.

  This phenomenon includes the image carrier, the conductive layer, and the resistance layer before the transfer nip locally and instantaneously when the potential difference Vdt between the non-image portion potential Vd and the primary transfer bias potential Vt1 becomes larger than a certain threshold value Vth. This is because abnormal discharge occurs between the intermediate transfer belts having a multilayer structure, and a necessary transfer potential cannot be obtained. This can be confirmed from the fact that the toner that should transfer from the surface of the image carrier to the intermediate transfer belt is hardly transferred and remains on the image carrier at the primary transfer nip when an abnormal image occurs.

  Further, the phenomenon of abnormal image generation due to this abnormal discharge is not confirmed at all in an image forming apparatus using a single-layer intermediate transfer belt made of a dielectric, and therefore has a multilayer structure including a conductive layer and a resistance layer. This is a phenomenon peculiar to an image forming apparatus using an intermediate transfer belt. For this reason, the abnormal discharge in the present invention is completely different from the discharge before the transfer nip that causes toner scattering that occurs in an image forming apparatus using a single-layer intermediate transfer belt made of a dielectric. The influence on the image quality is so large that it cannot be compared with a decrease in image quality due to toner scattering due to discharge generated in an image forming apparatus using an intermediate transfer belt having a single layer structure made of a dielectric.

  This abnormal discharge is caused by a change in the threshold value Vth of the potential difference Vdt between the non-image portion potential Vd and the primary transfer bias potential Vt1 at which an abnormal image starts to occur due to abnormal discharge due to a change in the film thickness of the photosensitive layer of the image carrier. It turns out that the threshold value Vth decreases as the film thickness decreases. For example, if the threshold value Vth of Vdt at which abnormal image due to abnormal discharge starts when the film thickness of the photosensitive layer of the image carrier is 25 μm is 1000 V, and the film thickness of the photosensitive layer of the image carrier is reduced to 20 μm, The threshold value Vth decreases to 950V.

  In addition, the phenomenon of the occurrence of an abnormal image due to the abnormal discharge has been found to change the threshold Vth of Vdt at which an abnormal image due to the abnormal discharge begins to occur due to a change in atmospheric pressure, and the threshold Vth decreases as the atmospheric pressure decreases. For example, when the threshold Vth of Vdt at which an abnormal image starts to be generated due to abnormal discharge when the atmospheric pressure is 760 mmHg (equivalent to altitude 0 m) is 1000 V, an abnormal image is generated due to abnormal discharge when the atmospheric pressure is 560 mmHg (equivalent to 2500 m altitude). The threshold value Vth of Vdt that starts to decrease is reduced to 950V.

  Furthermore, the phenomenon of occurrence of abnormal images due to abnormal discharge has been found to change the threshold Vth of Vdt at which abnormal images due to abnormal discharge start due to changes in temperature and humidity, and the threshold Vth becomes smaller under high temperature and humidity. . For example, when the threshold Vth of Vdt at which abnormal images start to occur due to abnormal discharge at a temperature of 15 ° C. and a humidity of 35% is 1000 V, an abnormal image starts to occur due to abnormal discharge at a temperature of 30 ° C. and a humidity of 85%. The threshold Vth of Vdt decreases to 950V.

  In a corona charger, if the charging current is increased stepwise based on the number of times of use, etc., to prevent image deterioration due to contamination of the discharge electrode, grid electrode, and back plate after endurance, etc., a functional relationship with the grid bias potential Vg The non-image portion potential Vd on the surface of the image carrier increases and exceeds the threshold Vth of Vdt at which abnormal images are generated due to abnormal discharge, causing abnormal discharge.

  The present invention solves the above problems, prevents image deterioration due to the number of times the corona charger is used, suppresses toner scattering in the image forming apparatus, and further suppresses occurrence of abnormal images due to abnormal discharge in the primary transfer unit. An object of the present invention is to provide an image forming apparatus and an image forming method using an intermediate transfer belt having a multilayer structure including a conductive layer and a resistance layer capable of ensuring an appropriate image density.

  In order to solve the above problems, according to the first aspect of the present invention, in an image forming apparatus, an image carrier, corona charging means for charging the image carrier, and the corona charging means include a discharge electrode, a back plate, and a grit electrode. An exposure unit that forms an electrostatic latent image on a charged image carrier, a developing unit that visualizes the electrostatic latent image formed on the image carrier with toner and forms a toner image, and a conductive layer. An image forming apparatus comprising: an intermediate transfer member having a multilayer structure; and a control unit that controls an image density control factor including a developing bias potential applied to the developing unit. The transfer potential, the grid bias potential, and the discharge current are controlled so that the difference from the non-image portion potential on the carrier falls within a predetermined range.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect of the invention, the control unit increases the discharge current value and the discharge time stepwise based on the life information such as the number of times of use and the grid bias potential stepwise. It is characterized by controlling so that it may decrease to.

  According to a third aspect of the present invention, in the image forming apparatus of the first or second aspect of the invention, the control means has an absolute value Vr of a difference between the development bias potential Vb and the grit bias potential Vg within a variable range of the development bias potential. The absolute value Vr (= | Vb−Vg |) of the difference between the development bias potential Vb and the grit bias potential Vg within the normal mode in which (= | Vb−Vg |) is constant and the variable range of the development bias potential. It is characterized by having a control region ΔVb1 that keeps constant, and an abnormal discharge response mode that controls to a control region ΔVb2 that reduces Vr.

  According to a fourth aspect of the present invention, in the image forming apparatus according to the third aspect of the invention, the control unit can switch between the normal mode and the abnormal discharge response mode.

  According to a fifth aspect of the present invention, in the image forming apparatus according to the third or fourth aspect, the normal mode and the abnormal discharge response mode can be switched by operating a control panel provided in the apparatus main body. To do.

  According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the second to fifth aspects of the present invention, switching between the normal mode and the abnormal discharge handling mode is performed by changing the life information such as the number of used sheets provided in the apparatus body, the atmospheric pressure, The switching is enabled based on at least one of environmental information such as temperature and humidity and a change in transfer current detected by the transfer current detecting means.

  In the seventh invention, a discharge current is applied to the discharge electrode of the corona charging unit and a grid bias potential is applied to the grid electrode to charge the surface of the image carrier, and then an electrostatic latent image is formed on the surface of the image carrier by the exposure unit. Forming a developing bias to the developing means to reveal the electrostatic latent image with toner to form a toner image, and applying a transfer bias to the toner image on the surface of the image bearing member to form a composite layer having a conductive layer. In an image forming method for transferring to an intermediate transfer member having a layer structure, the transfer potential, grid bias potential, and discharge current are set so that the difference between the transfer potential and the non-image portion potential on the image carrier at the transfer position is within a predetermined range. It is characterized by controlling.

A multi-layer structure having a conductive layer is configured by controlling the transfer potential, the grid bias potential, and the discharge current so that the difference between the transfer potential and the non-image portion potential on the image carrier at the transfer position is within a predetermined range. It is possible to prevent the occurrence of an abnormal image due to the abnormal discharge peculiar to the primary transfer portion using the intermediate transfer member.
The control means is configured to increase the discharge current value and the discharge time stepwise based on the life information such as the number of times of use, and to control the grid bias potential to decrease stepwise. It is possible to prevent the occurrence of an abnormal image due to the abnormal discharge peculiar to the primary transfer portion using the intermediate transfer body having a multilayer structure having a conductive layer.
A normal mode in which the control means makes the absolute value Vr (= | Vb−Vg |) of the difference between the development bias potential Vb and the grit bias potential Vg constant within the variable range of the development bias potential; Are controlled in a control region ΔVb1 in which the absolute value Vr (= | Vb−Vg |) of the difference between the developing bias potential Vb and the grit bias potential Vg is constant, and a control region ΔVb2 in which Vr is decreased. In a control region where an abnormal image due to abnormal discharge in the primary transfer portion may occur due to the configuration having the abnormal discharge response mode, the non-image potential Vd on the image carrier is reduced to the primary by reducing the potential Vr. The potential difference Vdt = | Vd−Vt1 | between the transfer bias potential Vt1 and the non-image portion potential on the image carrier can be kept below the threshold Vth for occurrence of abnormal discharge. It is possible to prevent the occurrence of abnormal images due to abnormal discharge peculiar to an image forming apparatus using an intermediate transfer belt having a multilayer structure including an electric layer, and to limit the amount of toner scattered into the machine by limiting the control area for reducing Vr. Can be stopped.
The control means controls the grid bias so as to fix the grid bias potential Vg that is functionally related to the non-image portion potential Vd on the image carrier in the control region ΔVb2 where the abnormal discharge response mode Vr decreases. Therefore, even if the developing bias potential Vb that affects the developing property (flying property) is set high, the grid bias potential Vg that is functionally related to the non-image portion potential Vd is fixed, so that the primary transfer bias potential Vt1 Since the potential difference Vdt = | Vd−Vt1 | with the non-image portion potential on the image carrier can be kept below the threshold Vth of occurrence of abnormal discharge, an image forming apparatus using an intermediate transfer belt having a multilayer structure including a conductive layer Generation of abnormal images due to specific abnormal discharge can be prevented.
With the configuration in which the control unit can switch between the normal mode and the abnormal discharge response mode, the control region for reducing Vr is caused by abnormal discharge peculiar to an image forming apparatus using an intermediate transfer belt having a multilayer structure including a conductive layer. Since it can be limited only to the occurrence of an abnormal image, the influence of decreasing Vr can be minimized.
By the configuration that enables switching between the normal mode and the abnormal discharge handling mode by operating a control panel provided in the apparatus main body, the abnormal discharge peculiar to an image forming apparatus using a multi-layer intermediate transfer belt including a conductive layer is used. The control area can be quickly converted when an abnormal image occurs.
Switching between the normal mode and the abnormal discharge response mode is at least one of life information such as the number of used sheets provided in the main body of the apparatus, environmental information such as atmospheric pressure, temperature and humidity, and a change in the transfer current detected by the transfer current detecting means. Therefore, it is possible to quickly respond to an increase in the probability of occurrence of an abnormal image due to abnormal discharge.
After applying a discharge current to the discharge electrode of the corona charging means and a grid bias potential to the grid electrode to charge the surface of the image carrier, an electrostatic latent image is formed on the surface of the image carrier by the exposure means, An intermediate transfer member having a multilayer structure in which a developing bias is applied to make the electrostatic latent image visible with toner to form a toner image, and the toner image on the surface of the image bearing member is provided with a transfer bias and has a conductive layer. In the image forming method to be transferred to the image forming method, the transfer potential, the grid bias potential, and the discharge current are controlled so that the difference between the transfer potential and the non-image portion potential on the image carrier at the transfer position is within a predetermined range. Similarly to the image forming apparatus, it is possible to prevent the occurrence of an abnormal image due to an abnormal discharge peculiar to the primary transfer portion using a multilayer intermediate transfer member having a conductive layer.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic view showing an embodiment of an image forming apparatus according to the present invention, and FIG. 2 is an enlarged partial end view taken along line II-II in FIG. FIG. 3 is a partially enlarged view of an embodiment of the corona charging means.

    Around the image carrier 10, along the rotation direction, a corona charger 11 as a charging unit, a developing roller 20 (Y, M, C, K) as a developing unit, an intermediate transfer unit 30, and a cleaning unit 12 is arranged. The image carrier 10 has a cylindrical conductive substrate 10a (see FIG. 2) and a photosensitive layer 10b formed on the surface thereof. The corona charger 11 includes a discharge electrode 23 and a grid electrode 24 disposed between the image carrier 10 and the discharge electrode 23 in a back plate 22 as a casing (see FIG. 3). The outer peripheral surface is charged uniformly. The exposure unit selectively exposes L corresponding to desired image information on the outer peripheral surface of the uniformly charged image carrier 10, and an electrostatic latent image is formed on the image carrier 10 by the exposure L.

  In this embodiment, a developing roller 20Y for yellow, a developing roller 20C for cyan, a developing roller 20M for magenta, and a developing roller 20K for black are provided as the developing roller 20. These developing rollers 20Y, 20C, 20M, and 20K can selectively come into contact with the image carrier 10, and when contacted, toner of any one of yellow, cyan, magenta, and black is imaged. The electrostatic latent image on the image carrier 10 is developed by being applied to the surface of the carrier 10. The developed toner image is transferred onto the intermediate transfer belt 36 of the intermediate transfer device 30. The cleaning unit 12 includes a cleaner blade 13 that scrapes off toner adhering to the outer peripheral surface of the image carrier 10 after transfer, and a receiving portion 14 that receives the scraped toner.

  The intermediate transfer device 30 includes a driving roller 31, four driven rollers 32, 33, 34, and 35, and an endless intermediate transfer belt 36 stretched around each roller. As shown in FIG. 2, the intermediate transfer belt 36 has a multilayer structure including a conductive layer 36a and a resistance layer 36b formed on the conductive layer 36a and pressed against the image carrier 10. . In this embodiment, the conductive layer 36a is formed on an insulating substrate 36c made of a synthetic resin, and a primary transfer voltage Vt1 is applied to the conductive layer 36a via an electrode roller 37. Note that the conductive layer 36a is exposed in a strip shape by removing the resistive layer 36b in a strip shape at the belt 36 side edge, and the electrode roller 37 is in contact with the exposed portion. In the process in which the intermediate transfer belt 36 is circulated, the toner image on the image carrier 10 is transferred onto the intermediate transfer belt 36 at the primary transfer portion T1, and the toner image transferred onto the intermediate transfer belt 36 is In the next transfer portion T2, the image is transferred by applying the secondary transfer voltage V2 to the recording medium S such as paper supplied between the secondary transfer roller 38 and the like. The recording medium S is fed from a sheet feeding device (not shown) and is supplied to the secondary transfer portion T2 by the gate roller pair 40 at a predetermined timing. The cleaner blade 39a of the belt cleaner 39 contacts the intermediate transfer belt 36, removes the residual toner on the intermediate transfer belt 36 after the secondary transfer, and drops it to the receiving portion 39b.

  According to the image forming apparatus as described above, the intermediate transfer belt 36 stretched between the rollers and pressed against the image carrier 10 between the rollers is formed on the conductive layer 36a and the conductive layer 36a. Since it has a multilayer structure having a resistance layer 36b pressed against the image carrier 10, a voltage having a polarity opposite to that of the toner carried on the image carrier 10 is applied to the conductive layer 36a. The toner on the carrier 10 can be transferred onto the intermediate transfer belt 36. Since the intermediate transfer belt 36 has a multi-layer structure including a conductive layer 36a and a resistance layer 36b formed on the conductive layer 36a and pressed against the photoreceptor 10, the image carrier 10 And the intermediate transfer belt 36 over the entire area of the pressure contact portion (that is, the primary transfer portion) T1, the potential on the back side of the resistance layer 36b of the intermediate transfer belt 36 becomes uniform, and as a result, transfer with less toner scattering is obtained. Will be.

  Further, it is difficult to be affected by the surface resistance unevenness of the intermediate transfer belt 36, and transfer unevenness is less likely to occur. In addition, since the potential on the back side of the resistance layer 36b of the intermediate transfer belt 36 is uniform over the entire area of the pressure contact portion (that is, the primary transfer portion) T1 between the image carrier 10 and the intermediate transfer belt 36, the necessary minimum Transfer with a limited voltage is possible.

  An image forming mechanism of such an image forming apparatus will be described with reference to FIG. In the image forming apparatus of this embodiment, the outer surface of the image carrier 10 immediately below the corona charger 11 is charged to a negative surface potential Vo by the grid bias potential Vg of the grid electrode 24 of the corona charger 11. When the surface of the image carrier 10 is irradiated with the exposure L from the exposure means, a part of the charge of the irradiated portion is neutralized and the surface potential changes to Von. In this way, by scanning and exposing the image carrier 10 while turning on and off the exposure L corresponding to the image signal, the potential of the surface region corresponding to the image portion corresponding to the image signal becomes Von (≠ Vo On the other hand, the potential of the surface region corresponding to the non-image portion attenuates from the surface potential Vo immediately after charging to Vd (| Vd | ≦ | Vo |) due to dark decay. That is, | Vo | = α | Vg |, | Vd | <| Vo |, and the relationship between the grid bias potential Vg and the surface potential Vo of the image carrier just below the grid electrode 24 is a functional relationship, and its inclination α Is mainly determined by the distance between the grid electrode 24 and the surface of the image carrier 10, the opening width of the back plate 22, and the charging ability of the image carrier 10. The relationship between Vo and the non-image portion potential Vd at the primary transfer portion is determined by the characteristic of dark decay that is peculiar to the material of the image carrier 10, and Vd is the time from the dark decay to the primary transfer portion from the charging position. It depends on. Therefore, the relationship between the grid bias potential Vg and the non-image portion potential Vd at the primary transfer portion is a functional relationship. Thus, an electrostatic latent image corresponding to the image signal is formed on the image carrier 10. The electrostatic latent image formed in this way is conveyed to a developing position facing the developing roller 20 constituting the developing means by the rotation of the image carrier 10. A negatively charged toner is carried on the developing roller 20 and a developing bias potential Vb is applied to promote toner adhesion to the image portion of the image carrier 10. As shown in FIG. 4, the developing bias potential Vb is set to a value between the non-image portion potential Vd and the image portion potential Von. Therefore, at the developing position, in the non-image portion, the image carrier 10 is set. The surface of the image carrier 10 has a lower potential than the developing roller 20, whereas the surface of the image carrier 10 has a higher potential than the developing roller 20 in the image portion. Therefore, of the negatively charged toner carried on the developing roller 20, the toner at the position facing the image portion is moved to the image carrier 10 side by electrostatic force, while the toner at the position facing the non-image portion is compared with the toner at the position facing the non-image portion. Accordingly, a force in the direction of drawing toward the developing roller 20 is applied. As described above, by attaching the toner only to the image portion, the electrostatic latent image on the image carrier 10 is visualized by the toner.

  In the image forming process for forming a toner image in this way, parameters such as exposure energy of exposure L, non-image portion potential Vd, image portion potential Von, and developing bias potential Vb are adjusted to the final image density of the toner image. It is known to have a great influence, and many techniques for optimizing the image density by appropriately adjusting some of these parameters as image density control factors have been proposed. However, in an actual image forming process, these parameters are related to each other so that a toner image is formed. Therefore, they cannot always be controlled independently and arbitrarily. In particular, the relative potential relationship between the development bias potential Vb and the non-image portion potential Vd is large not only in the density of the image, but also in the image quality of the obtained toner image and the amount of toner scattered inside the apparatus. Therefore, it is important to set these values appropriately in order to set the image density control factor with higher accuracy and form a toner image with good image quality. The absolute value of the potential difference between the developing bias potential Vb and the non-image portion potential Vd is referred to as a reverse contrast potential. That is, the reverse contrast potential is | Vb−Vd |. Since the grid bias potential Vg and the non-image portion potential Vd at the primary transfer portion have a functional relationship as described above, Vr = | Vb−Vg | can be used as the reverse contrast potential.

  Here, in order to examine the influence of the potential relationship, first, when the developing bias potential Vb is brought close to the level of the grid bias potential Vg having a functional relationship with the non-image portion potential Vd, the reverse contrast potential Vr is reduced. think of. At this time, the potential difference between the image portion potential Von and the developing bias potential Vb, that is, the contrast potential (= | Vb−Von |) increases, and toner transfer from the developing roller 20 to the image carrier 10 occurs in the image portion. As a result, a high image density can be obtained. However, on the other hand, since the potential difference with the developing roller 20 is small in the non-image portion of the image carrier 10, the action of pulling excess toner back to the developing roller 20 side is weak. Therefore, the amount of toner that is separated from the developing roller 20 and scatters inside the apparatus increases. On the other hand, if the absolute value of the grid bias potential Vg having a functional relationship with the non-image portion potential Vd is increased and the reverse contrast potential Vr is increased with the developing bias potential Vb as it is, the amount of toner scattered into the apparatus can be reduced. However, since the negative charge held in the non-image portion of the image carrier 10 has a strong power to spread negatively charged toner, the image portion of the electrostatic latent image, particularly in a narrow region sandwiched between the non-image portions, As a result, it becomes difficult for toner to adhere, resulting in deterioration of image quality in a low-density image with a relatively low dot area ratio, such as fading of isolated dots and fine lines, and loss of uniformity of line width. .

  As described above, it is preferable to increase the reverse contrast potential Vr in order to suppress toner scattering, but it is contrary to the desire to decrease the reverse contrast potential Vr in order to ensure image quality such as uniformity of fine lines. In order to perform image formation with good image quality while suppressing toner scattering inside the apparatus, it is necessary to set parameters such as the developing bias potential Vb so that the reverse contrast potential Vr is always an appropriate value. There is. In particular, when optimizing the density control factor for forming a halftone toner image, the accuracy of the reproducibility of micro dots and fine lines greatly affects the accuracy. It is important to form a patch image in a state where the reverse contrast potential Vr is kept at an appropriate value.

  Therefore, the potential difference between the developing bias potential Vb and the grid bias potential Vg having a functional relationship with the non-image portion potential Vd on the image carrier 10, that is, the reverse contrast potential Vr is kept constant, and the image density of the toner image is maintained. A halftone toner image is formed as a patch image while changing the setting of the image density control factor that affects the image density control factor, and the image density control factor is determined based on the detection result of the image density of the patch image by the density detection unit. Control means for controlling the image density of the toner image formed by the developing means by optimizing the image quality. In the development bias optimization process in the optimization of the image density control factor, for example, the development bias potential Vb is changed in multiple stages in a state where the absolute values of the exposure energy and the non-image portion potential are fixed to the maximum values in the variable range. Set and form a patch image. By maximizing the absolute value of the grid bias potential Vg that is functionally related to the non-image portion potential Vd, the reverse contrast potential Vr = | Vb−Vg | is maximized at any development bias potential Vb, and the toner in the apparatus Spattering can be minimized. With this configuration, when a halftone toner image is formed as a patch image, the potential difference between the developing bias potential Vb and the non-image portion potential Vd and the grid bias potential Vg having a functional relationship, that is, the reverse contrast potential Vr is set. Hold constant. Therefore, since such a patch image can be formed under good reproducibility conditions for the fine dots and fine lines used to obtain the halftone, the optimum image density control factor to be performed based on the image density of this patch image Conversion processing can be performed with high accuracy, and as a result, a toner image with good image quality can be stably formed. Furthermore, by maintaining the reverse contrast potential Vr at an appropriate value, it is possible to effectively suppress toner scattering inside the apparatus as described above. In particular, in order to stably form a toner image on the low density side, it is desirable to use a patch image having a dot area ratio of 20% or less with respect to the entire patch image.

  Further, in the corona charger 11 including the back plate 22, the discharge electrode 23, and the grid electrode 24, in order to prevent image deterioration due to toner contamination or a change with time, the corona charger is based on the life information such as the number of uses. By increasing the charging current stepwise according to the contamination of the charger 11, the life of the corona charger 11 is increased while preventing image deterioration.

  In the image forming apparatus using the intermediate transfer belt 36 having a multilayer structure including the conductive layer 36a and the resistance layer 36b, as shown in FIG. 5, the non-image portion potential Vd on the image carrier (photoconductor) 10 is obtained. When the potential difference Vdt = | Vd−Vt1 | with the primary transfer bias potential Vt1 becomes large, an abnormal image may be generated partially or entirely in the primary transfer portion due to abnormal discharge. This phenomenon is particularly noticeable in half images.

  The mechanism by which this phenomenon occurs will be described with reference to the potential relationship shown in FIGS. FIG. 6A shows the potential relationship in the normal mode, and primary transfer is performed while keeping the reverse contrast potential Vr, which is the difference between the developing bias potential Vb and the non-image portion potential Vd, constant. Since the potential difference Vdt = | Vd−Vt1 | between the non-image portion potential Vd and the primary transfer bias potential Vt1 is within the threshold Vth of occurrence of abnormal discharge, no abnormal image is generated due to abnormal discharge. FIG. 6B shows a potential relationship in a state where abnormal discharge occurs. In this case, if primary transfer is performed while maintaining a constant reverse contrast potential Vr, which is the difference between the development bias potential Vb and the non-image portion potential Vd and the grid bias potential Vg that has a functional relationship, after printing a large number of sheets, etc. In this case, the film thickness of the photosensitive layer of the image carrier is reduced, the threshold value for occurrence of abnormal discharge is lowered, and the development bias potential Vb needs to be set to be large due to the decrease in developability (flight property). Since the development bias potential Vb is set high, the reverse bias potential Vr needs to be kept constant, so the non-image portion potential Vd is also set high, and the potential difference Vdt between the non-image portion potential Vd and the primary transfer bias potential Vt1. = | Vd-Vt1 | exceeds the threshold value Vth of occurrence of abnormal discharge, and an abnormal image is generated due to abnormal discharge.

  Further, FIGS. 7A and 7B show the change in the non-image portion potential Vd and the state of occurrence of abnormal discharge when the charging current is fixed by the corona charger 11 and when the charging current is increased stepwise. Is. As shown in FIG. 7A, in the case where the charging current is fixed, as the number of printed sheets increases, the corona charger 11 also decreases the non-image portion potential Vd due to contamination of toner and the like, and the non-image portion potential Vd Since the potential difference Vdt = | Vd−Vt1 | with the transfer bias potential Vt1 is within the threshold value Vth of the abnormal discharge occurrence, no abnormal image is generated due to the abnormal discharge. However, image degradation occurs because the non-image portion potential Vd decreases. In order to prevent this image deterioration, as shown in FIG. 7B, when the charging current is increased stepwise according to the number of printed sheets and the non-image portion potential Vd is increased stepwise, the non-image portion potential Vd. Since the potential difference Vdt = | Vd−Vt1 | between the primary transfer bias potential Vt1 exceeds the threshold value Vth for occurrence of abnormal discharge at a certain stage, an abnormal image is generated due to abnormal discharge.

  As a means for preventing the occurrence of an abnormal image due to this abnormal discharge, the control means of the image forming apparatus of the present invention applies to the discharge electrode 23 according to the number of printed sheets of the corona charger 11, as shown in FIG. Although the discharge current is increased stepwise, the non-image portion potential Vd, which should increase as the discharge current increases stepwise, is reduced as indicated by a dotted line, so that the grid bias potential that is functionally related to the non-image portion potential Vd. Vg is decreased so that the potential difference Vdt = | Vd−Vt1 | between the non-image portion potential Vd and the primary transfer bias potential Vt1 is within the threshold Vth of occurrence of abnormal discharge. Since the decrease in the non-image portion potential Vd leads to image degradation, as a countermeasure, image degradation is prevented by extending the discharge time in accordance with the decrease in the grid bias potential Vg.

  Further, the control means of the image forming apparatus according to the present invention sets the potential difference Vdt = | Vd−Vt1 | between the non-image portion potential Vd and the primary transfer bias potential Vt1 within the threshold Vth of occurrence of abnormal discharge. As shown in FIG. In the abnormal discharge handling mode, the control region ΔVb1 that keeps the reverse contrast potential Vr, which is the difference between the development bias potential Vb and the grid bias potential Vg that is functionally related to the non-image portion potential Vd, and the development bias potential Vb non- A control region ΔVb2 is set in which the reverse contrast potential Vr, which is the difference between the image portion potential Vd and the grid bias potential Vg having a functional relationship, is reduced. That is, in the control region ΔVb1 in a normal state where no abnormal image is generated due to abnormal discharge, the reverse contrast potential Vr is kept constant, a toner image with good image quality is stably formed, and toner scattering inside the apparatus is prevented. Is also effectively suppressed. In the control region ΔVb2 when an abnormal image due to abnormal discharge may occur or when an abnormal image due to abnormal discharge occurs, the potential difference Vdt between the non-image portion potential Vd and the primary transfer bias potential Vt1 is set as a threshold for occurrence of abnormal discharge. In order to keep it within Vth, the primary transfer bias potential Vt1 is left as it is, and the reverse contrast potential Vr is controlled to decrease. That is, even if the developing bias potential Vb is set high in order to maintain developability, the grid bias potential Vg that is functionally related to the non-image portion potential Vd is controlled to be within the threshold value Vth for occurrence of abnormal discharge. The occurrence of abnormal images due to discharge can be prevented.

  FIG. 10 shows the initial use of the two control regions ΔVb1 and ΔVb2 in the abnormal discharge response mode and the state after the endurance of the device. In the initial stage of use of the apparatus of FIG. 8A, assuming that the grid bias potential Vg = non-image portion potential Vd, the film thickness of the photosensitive layer of the image carrier decreases after the apparatus endurance of FIG. The starting voltage decreases and | Vd | also decreases.

  FIG. 11 shows the potential relationship in the two control regions ΔVb1 and ΔVb2 in the abnormal discharge response mode. In the control region of ΔVb2, the primary transfer bias potential Vt1 is kept constant, and further, the grid bias potential Vg having a functional relationship with the non-image portion potential Vd is controlled, and even if the development bias potential Vb is set high, the reverse contrast Although the potential Vr decreases, the potential difference Vdt between the non-image portion potential Vd and the primary transfer bias potential Vt1 can be held within the threshold value Vth for occurrence of abnormal discharge, so that the occurrence of abnormal images due to abnormal discharge can be prevented.

  Switching between the normal mode and the abnormal discharge handling mode can be performed by operating a control panel provided in the apparatus main body, so that the control region ΔVb2 for reducing the reverse contrast potential Vr is switched only when an abnormal image is generated due to abnormal discharge. Thus, the influence of reducing the reverse contrast potential Vr can be minimized.

Further, as described above, the life information such as the number of used sheets and the environment such as the temperature and humidity, the atmospheric pressure, etc., which are the fluctuation factors of the abnormal discharge occurrence threshold Vth of the potential difference Vdt between the non-image portion potential Vd and the primary transfer bias potential Vt1 A sensor for detecting information may be provided in the apparatus main body, and switching between the normal mode and the abnormal discharge response mode may be performed based on data from each sensor.
Further, it has been found that a large amount of transfer current flows when an abnormal discharge occurs at the primary transfer site between the intermediate transfer belt 36 having a multilayer structure including a conductive layer and the image carrier 10. Accordingly, since the occurrence of abnormal discharge can be detected by detecting the change in the transfer current value, the change in the transfer current value may be detected to switch between the normal mode and the abnormal discharge response mode.

1 is a schematic diagram illustrating a main part of an embodiment of an image forming apparatus of the present invention. It is the II-II enlarged partial end view of FIG. It is a partial enlarged view of a corona charger. 1 is a principle diagram of an image forming apparatus of the present invention. It is a figure explaining the state of abnormal discharge. (A) (b) It is a figure of the electric potential relationship when an abnormal discharge generate | occur | produces at the time of normal. (A) (b) It is a figure which shows the change of a non-image part electric potential in the case of the charging current fixation by a corona charger, and the case of the stepwise increase in charging current. It is a figure which shows the relationship between the charging current stepwise increase of a corona charger, and a non-image part electric potential. It is explanatory drawing of the two control area | regions of the abnormal discharge response mode of this invention. (A) (b) It is explanatory drawing of two control area | regions of the abnormal discharge corresponding | compatible mode of this invention. It is a figure which shows the electric potential relationship of the two control area | regions of abnormal discharge response mode.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10: Image carrier, 10a: Conductive base material, 10b: Photosensitive layer, 11: Corona charger, 12: Cleaning means, 13: Cleaner blade, 14: Receiving part, 20: Developing roller, 20Y: Developing roller for yellow 20C: developing roller for cyan, 20M: developing roller for magenta, 20K: developing roller for black, 22: back plate, 23: discharge electrode, 24: grid electrode, 30: intermediate transfer device, 31: driving roller, 32, 33, 34, 35: driven roller, 36: intermediate transfer belt, 36a: conductive layer, 36b: resistance layer, 36c: insulating substrate, 37: electrode roller, 38: secondary transfer roller, 39: belt cleaner, 39a: Cleaner blade, 39b: receiving portion, 40: gate roller

Claims (7)

An image carrier, a corona charging unit for charging the image carrier, and a corona charging unit including a discharge electrode, a back plate, and a grit electrode, and an exposure unit for forming an electrostatic latent image on the charged image carrier; An electrostatic latent image formed on the image bearing member is visualized with toner to form a toner image, a multi-layer intermediate transfer member including a conductive layer, and a developing bias potential applied to the developing unit. In the image forming apparatus including the control means for controlling the image density control factor, the control means makes the difference between the transfer potential and the non-image portion potential on the image carrier at the transfer position fall within a predetermined range. An image forming apparatus that controls a transfer potential, a grid bias potential, and a discharge current. 2. The control unit according to claim 1, wherein the control means controls to increase the discharge current value and the discharge time stepwise and to decrease the grid bias potential stepwise based on life information such as the number of uses. The image forming apparatus described. A normal mode in which the control means makes the absolute value Vr (= | Vb−Vg |) of the difference between the development bias potential Vb and the grit bias potential Vg constant within the variable range of the development bias potential; Are controlled in a control region ΔVb1 in which the absolute value Vr (= | Vb−Vg |) of the difference between the developing bias potential Vb and the grit bias potential Vg is constant, and a control region ΔVb2 in which Vr is decreased. The image forming apparatus according to claim 1, wherein the image forming apparatus has an abnormal discharge handling mode. The image forming apparatus according to claim 3, wherein the control unit is capable of switching between a normal mode and an abnormal discharge handling mode. The image forming apparatus according to claim 4, wherein switching between the normal mode and the abnormal discharge handling mode is enabled by an operation of a control panel provided in the apparatus main body. The switching between the normal mode and the abnormal discharge response mode is performed by at least one of life information such as the number of used sheets provided in the apparatus main body, environmental information such as atmospheric pressure and temperature and humidity, and a change in transfer current detected by the transfer current detecting means. The image forming apparatus according to claim 2, wherein the image forming apparatus can be switched based on the above. After applying a discharge current to the discharge electrode of the corona charging means and a grid bias potential to the grid electrode to charge the surface of the image carrier, an electrostatic latent image is formed on the surface of the image carrier by the exposure means, An intermediate transfer member having a multilayer structure in which a developing bias is applied to make the electrostatic latent image visible with toner to form a toner image, and the toner image on the surface of the image bearing member is provided with a transfer bias and has a conductive layer. In the image forming method, the transfer potential, the grid bias potential, and the discharge current are controlled so that the difference between the transfer potential and the non-image portion potential on the image carrier at the transfer position falls within a predetermined range. An image forming method.

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