JP2006047486A - 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 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 having an image carrier, an electrifying means, an exposing means, a developing means, the intermediate transfer body in multi-layered structure having the conductive layer, and a transfer means is characterized in having a control means of controlling a transfer potential and an electrification potential so that the difference between the transfer potential and a non-image part potential on an 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 referred to as reverse contrast potential Vr. Assuming Vr = | Vb−Vd |, when Vr = small, toner scattering is large and fogging increases. On the other hand, when Vr = large, both the toner scattering amount and the fogging amount decrease, but the toner adheres 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. 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 reverse contrast potential Vr, 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 affects the image density of the toner image. A halftone toner image is formed while changing the image density control factor that gives the toner image in multiple stages, and the image density control factor is optimized based on the detection result of the image density of the toner image. An image forming method for optimizing image density has been proposed. According to this image forming method, the reverse contrast potential Vr, 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, so that Image density can be optimized while preventing toner scattering.
JP-A-11-153910 JP 2003-215862 A

  However, in an image forming apparatus using an intermediate transfer belt having a multilayer structure including a conductive layer and a resistance layer, assuming that the potential difference between the non-image portion potential Vd on the image carrier and the primary transfer bias potential Vt1 is Vdt, Vdt When || 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 remarkable 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.

  The present invention solves the above-described problems, suppresses toner scattering in the image forming apparatus, and further suppresses the occurrence of abnormal images due to abnormal discharge in the primary transfer unit, and can secure a proper image density and resistance. An object is to provide an image forming apparatus and an image forming method using an intermediate transfer belt having a multilayer structure including layers.

  The first aspect of the present invention is an image carrier having an image bearing member, a charging unit, an exposing unit, a developing unit, an intermediate transfer member having a multi-layer structure having a conductive layer, and a transfer unit. Control means for controlling the transfer potential and the charging potential so that the difference from the upper non-image portion potential is 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 present invention, the control means sets the development bias potential Vb and the non-image portion potential Vd on the image carrier within the variable range of the development bias potential Vb. A normal mode in which the reverse contrast potential Vr (Vr = | Vd−Vb |), which is the absolute value of the difference, is controlled to be constant, and a control region in which the reverse contrast potential Vr is constant within the variable range of the developing bias potential Vb. It has an abnormal discharge response mode for controlling ΔVb1 and a control region ΔVb2 for decreasing the reverse contrast potential Vr.

  According to a third aspect of the present invention, in the image forming apparatus according to the second aspect of the invention, the control means sets the non-image portion potential Vd on the image carrier in the control region ΔVb2 where the reverse contrast potential Vr decreases in the abnormal discharge response mode. The charging potential is controlled so as to be fixed.

  The fourth invention is characterized in that, in the image forming apparatus of the second or third invention, the control means can switch between the normal mode and the abnormal discharge handling mode.

  According to a fifth aspect of the present invention, in the image forming apparatus according to the fourth aspect of the present invention, 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.

  According to a sixth aspect of the present invention, in the image forming apparatus according to the fourth or fifth aspect of the present invention, the switching between the normal mode and the abnormal discharge response mode is performed by changing the life information such as the number of sheets used in the apparatus main body, the atmospheric pressure, the temperature and the humidity. It is possible to switch based on environmental information such as

  According to the seventh aspect of the present invention, a charging bias is applied to the charging unit to charge the surface of the image carrier, and then an electrostatic latent image is formed on the surface of the image carrier by an exposure unit, and a developing bias is applied to the developing unit. The electrostatic latent image is visualized with toner to form a toner image, and the toner image on the surface of the image carrier is primarily transferred to a multi-layer intermediate transfer body having a conductive layer by applying a transfer bias. In the forming method, the transfer potential and the charging potential 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.

In the middle of the multilayer structure having a conductive layer, the control means controls the transfer potential and the charging potential 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 transfer body.
In the abnormal discharge handling mode, the control means has a reverse contrast potential Vr that is an absolute value of a difference between the developing bias potential Vb and the non-image portion potential Vd on the image carrier within a variable range of the developing bias potential Vb. There is a possibility that an abnormal image is generated due to abnormal discharge of the primary transfer portion by controlling the control region ΔVb1 in which (Vr = | Vd−Vb |) is constant and the control region ΔVb2 in which the reverse contrast potential Vr is decreased. In a certain control region, by reducing the reverse contrast potential Vr, the non-image potential Vd on the image carrier is changed to a potential difference Vdt = | Vd− between the primary transfer bias potential Vt1 and the non-image portion potential on the image carrier. Since Vt1 | can be kept below the threshold Vth of occurrence of abnormal discharge, abnormal discharge peculiar to an image forming apparatus using an intermediate transfer belt having a multilayer structure including a conductive layer. Therefore, the amount of toner scattered into the apparatus can be minimized by limiting the control area in which the reverse contrast potential Vr is reduced.
The control means controls the charging potential so as to fix the non-image portion potential Vd on the image carrier in the control region ΔVb2 where the reverse contrast potential Vr decreases in the abnormal discharge handling mode. Since the non-image portion potential Vd is fixed even if the developing bias potential Vb that affects the image characteristics is set high, the potential difference Vdt between the primary transfer bias potential Vt1 and the non-image portion potential on the image carrier V | Since it can be kept below the threshold Vth of occurrence of abnormal discharge of Vd−Vt1 |, it is possible to prevent the occurrence of abnormal images due to abnormal discharge unique to an image forming apparatus using a multi-layer intermediate transfer belt including a conductive layer.
The control means is configured to enable switching between the normal mode and the abnormal discharge response mode, so that the control region in which the reverse contrast potential Vr is reduced has an abnormality peculiar to an image forming apparatus using a multi-layer intermediate transfer belt including a conductive layer. Since it can be limited only to the occurrence of an abnormal image due to discharge, the influence of decreasing the reverse contrast potential 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. Quick response when abnormal images occur.
Switching between the normal mode and the abnormal discharge response mode can be switched based on the life information such as the number of sheets used in the main body of the device and the environmental information such as atmospheric pressure, temperature and humidity, etc. It can respond quickly as the probability of occurrence increases.
A charging bias is applied to the charging unit to charge the surface of the image carrier, and then an electrostatic latent image is formed on the surface of the image carrier by an exposure unit, and a developing bias is applied to the developing unit to apply the electrostatic latent image. In the image forming method, a toner image is formed with toner and a toner image is formed, and the toner image on the surface of the image carrier is primarily transferred to a multi-layer intermediate transfer member having a conductive layer by applying a transfer bias. By transferring the charging potential and the charging potential so that the difference from the non-image portion potential on the image carrier at the transfer position falls within a predetermined range, the intermediate transfer of a multilayer having a conductive layer as in the image forming apparatus is performed. It is possible to prevent the occurrence of an abnormal image due to the abnormal discharge peculiar to the primary transfer portion using the body.

  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.

  Around the image carrier 10, along the rotation direction, a charging roller 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 are provided. 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 charging roller 11 contacts the outer peripheral surface of the image carrier 10 and uniformly charges the outer peripheral surface. 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 is charged to a negative surface potential Vo by the charging roller 11, and the surface is irradiated with the exposure L from the exposure means. Part of the charge of the part 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, and thus the electrostatic latent image corresponding to the image signal Are 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. 3, the potential of 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, the image carrier 10 in the non-image portion. 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 Vr. That is, the reverse contrast potential Vr = | Vb−Vd |.

  Here, in order to examine the influence of the potential relationship, first, consider a case where the reverse contrast potential Vr is reduced by bringing the developing bias potential Vb close to the level of the non-image portion potential Vd. 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, when the absolute value of the non-image portion potential Vd is increased and the reverse contrast potential Vr is increased while the developing bias potential Vb is left as it is, the amount of toner scattered into the apparatus can be reduced, but the non-image of the image carrier 10 can be reduced. Since the negative charge held in the area increases the power to spread negatively charged toner, the toner is less likely to adhere to the image area of the electrostatic latent image, particularly in a narrow area sandwiched between non-image areas. In other words, the quality of a low-density image having a relatively low dot area ratio is deteriorated, such as an isolated dot or fine line being faded or the uniformity of the line width being impaired.

  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.

  For this reason, the potential difference between the developing bias potential Vb and the non-image portion potential Vd on the image carrier 10, that is, the reverse contrast potential Vr is kept constant, and many image density control factors that affect the image density of the toner image are provided. The developing means is formed by forming a halftone toner image as a patch image while changing the setting in stages, and optimizing the image density control factor based on the detection result of the image density of the patch image by the density detecting means And a control means for controlling the image density of the toner image formed by. 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 non-image portion potential Vd, the reverse contrast potential Vr = | Vb−Vd | is maximized at any developing bias potential Vb, and toner scattering in the apparatus 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, that is, the reverse contrast potential Vr is kept 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.

  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. 4, the non-image portion potential Vd on the image carrier (photoconductor) 10 is used. 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. 5A 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. 5B shows a potential relationship in a state where abnormal discharge occurs. In this case, if 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, the photosensitive layer of the image carrier is printed after printing a large number of sheets. As a result, the developing bias potential Vb needs to be set to a large value due to a decrease in developability (flying performance). 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.

  As 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 has an abnormal discharge response mode. In the abnormal discharge handling mode, 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, as shown in FIG. A control region ΔVb1 that keeps the reverse contrast potential Vr that is the difference between the potential Vb and the non-image portion potential Vd constant, and the control that reduces the reverse contrast potential Vr that is the difference between the development bias potential Vb and the non-image portion potential Vd. A region ΔVb2 is set. 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 non-image portion potential Vd is controlled to be within the threshold value Vth of occurrence of abnormal discharge, and the occurrence of abnormal images due to abnormal discharge can be prevented. . FIG. 7 illustrates a control region ΔVb2 in which the reverse contrast potential Vr in the abnormal discharge response mode is decreased. The non-image portion potential Vd is controlled to be fixed, and the potential difference Vdt between the non-image portion potential Vd and the primary transfer bias potential Vt1 is set. An embodiment in which an abnormal discharge occurs within a threshold value Vth is shown.

  FIG. 8 shows the potential relationship in the control region ΔVb2 in which the reverse contrast potential Vr in the abnormal discharge response mode is decreased. Even if the primary transfer bias potential Vt1 is kept constant and the non-image portion potential Vd is kept constant, and the development bias potential Vb is set high to maintain developability, the reverse contrast potential Vr is Although it decreases, the potential difference Vdt between the non-image portion potential Vd and the primary transfer bias potential Vt1 can be maintained within the threshold Vth for occurrence of abnormal discharge, and therefore, an abnormal image due to abnormal discharge can be prevented.

  Since it is possible to switch between the normal mode and the abnormal discharge handling mode by operating a control panel provided in the apparatus main body, the control region ΔVb2 for decreasing the reverse contrast potential Vr is provided 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.

Table 1 shows the experimental results of the image forming apparatus of the present invention.

  From the experimental results shown in Table 1, the non-image portion potential Vd and the primary transfer are set by setting a control region ΔVb2 for decreasing the reverse contrast potential Vr in the region where the abnormal image is generated due to the abnormal discharge in the abnormal discharge support mode. It was found that the potential difference Vdt with respect to the bias potential Vt1 can be within the threshold value Vth for occurrence of abnormal discharge, and the occurrence of abnormal images due to abnormal discharge can be prevented.

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. 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. It is explanatory drawing of the two control area | regions of the abnormal discharge response mode of this invention. It is explanatory drawing of the two control area | regions of the abnormal discharge response 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

10: Image carrier, 10a: Conductive substrate, 10b: Photosensitive layer, 11: Charging roller, 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, 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)

In an image forming apparatus having an image carrier, a charging unit, an exposure unit, a developing unit, a multi-layer intermediate transfer body having a conductive layer, and a transfer unit, the transfer potential and the non-image portion potential on the image carrier at the transfer position An image forming apparatus comprising control means for controlling a transfer potential and a charging potential so that a difference between the transfer potential and the charge potential is within a predetermined range. The control means has a reverse contrast potential Vr (Vr = | Vd−) which is an absolute value of a difference between the developing bias potential Vb and the non-image portion potential Vd on the image carrier within a variable range of the developing bias potential Vb. Vb |) at a constant level, a control region ΔVb1 for keeping the reverse contrast potential Vr constant within a variable range of the developing bias potential Vb, and a control region ΔVb2 for reducing the reverse contrast potential Vr. The image forming apparatus according to claim 1, further comprising an abnormal discharge response mode to be controlled. The control means controls the charging potential so as to fix the non-image portion potential Vd on the image carrier in a control region ΔVb2 where the reverse contrast potential Vr decreases in the abnormal discharge handling mode. The image forming apparatus according to 2. The image forming apparatus according to claim 2, wherein the control unit is capable of switching between the normal mode and the abnormal discharge response 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 handling mode can be switched based on life information such as the number of sheets used provided in the apparatus main body and environmental information such as atmospheric pressure, temperature and humidity. The image forming apparatus according to 4 or 5. A charging bias is applied to the charging unit to charge the surface of the image carrier, and then an electrostatic latent image is formed on the surface of the image carrier by an exposure unit, and a developing bias is applied to the developing unit to apply the electrostatic latent image. In an image forming method of forming a toner image by manifesting the toner with a toner, and transferring the toner image on the surface of the image carrier to a multi-layer intermediate transfer body having a conductive layer by applying a transfer bias, An image forming method comprising: controlling a transfer potential and a charging potential so that a difference from a non-image portion potential on an image carrier at a transfer position falls within a predetermined range.

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