JP2009098452A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of forming excellent images at a point of image density by allowing the density of formed toner images to be in a preset allowable range. <P>SOLUTION: The image forming apparatus P sequentially arranges a plurality of image forming parts Y, M, C, K including an image carrier 1, a charger 2, an image exposure device 3, and a developing unit 4 along a recording medium conveying belt 7 and provided with a transfer unit 5 for transferring toner images on the image carrier in each image forming part to a recording medium S. Transfer bias is applied on two or more transfer units 7 from a common transfer power source P3. Each of the image forming parts corresponding to the transfer unit 5 applying the transfer bias from the common transfer power source P3 controls previous charging potential by the charger 2 so that the density of the toner images formed therein is in the allowable range on the surface part of the image carrier 1 in which transfer charge flows in from the transfer unit 5 without interposing the recording medium S. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine that combines two or more of these.

  As image forming apparatuses such as a copying machine, a printer, a facsimile machine, and a combination machine combining two or more of these, there are color image forming apparatuses capable of forming two or more color images in addition to a monochrome image forming apparatus.

  Various types of color image forming apparatuses are known. Among them, an image carrier such as a photoconductor that is rotationally driven, a charger that charges the surface of the image carrier, and a charger. A plurality of image forming units including an image exposure apparatus that forms an electrostatic latent image by performing image exposure on a charged area on the surface of the image carrier, and a developing unit that develops the electrostatic latent image with a color toner to form a toner image Are arranged sequentially along the recording medium conveying belt, and each image forming unit faces the image carrier of the image forming unit via the belt and is conveyed as the belt travels There is an image forming apparatus provided with a transfer device for transferring a toner image on the image carrier under application of a transfer bias.

  This image forming apparatus is commonly referred to as a tandem color image forming apparatus because of the sequential arrangement of a plurality of image forming units, and a toner image formed on an image carrier in each image forming unit is recorded on a recording medium conveying belt. Since it is directly transferred to a recording medium conveyed along with the travel, it is also called a direct transfer type or direct rotation type color image forming apparatus. The toner image or the multiple toner image transferred in this way is usually fixed to a recording medium by heating and pressing with a fixing device.

  In this type of image forming apparatus, each transfer device can be provided with a transfer power source, and a transfer bias can be applied from each transfer power source to the corresponding transfer device. However, the configuration of the image forming device is simplified and the cost is reduced. In order to meet this requirement, a common transfer power source is provided for two or more transfer devices, and a transfer bias is applied to each of the transfer devices from the common transfer power source.

  For example, in Japanese Patent Application Laid-Open Nos. 11-184270 and 2001-255561, in a direct transfer type tandem color image forming apparatus, each of a plurality of image carriers forming a toner image is provided on the image carrier. In transferring a toner image to a recording medium, it is described that a transfer bias is applied to each of a plurality of transfer units provided corresponding to each image carrier from one DC power source common to the plurality of transfer units. Has been.

  In the image forming apparatus described in Japanese Patent Laid-Open No. 11-184270, the same transfer bias is applied to each transfer unit from a common transfer power source. In the image forming apparatus described in Japanese Patent Laid-Open No. 2001-255661, each transfer is performed from a common transfer power source. A transfer bias corresponding to each transfer means is applied to the means using voltage adjustment by a Zener diode.

Japanese Patent Laid-Open No. 11-184270 JP 2001-255761 A

  However, the image carrier to be rotationally driven, a charger for charging the surface of the image carrier, an image exposure apparatus that forms an electrostatic latent image by performing image exposure on a charged area of the surface of the image carrier by the charger, and A plurality of image forming units including a developing unit that develops an electrostatic latent image with toner of charge color to form a toner image are sequentially arranged along the recording medium conveyance belt, and the image forming unit is formed for each image forming unit. In the image forming apparatus provided with a transfer unit that faces the image carrier of the portion through the belt and transfers the toner image on the image carrier to a recording medium conveyed as the belt travels. There are the following problems.

In each image forming portion, an image carrier surface portion where transfer charges flow from the transfer device without the recording medium is generated on the image carrier surface.
For example, in the case where images are sequentially formed on a plurality of recording media (recording paper or the like), prior to the first recording medium arriving at each image forming unit and its corresponding transfer device as the recording medium transport belt travels. Each of the image forming units and the transfer unit is normally operated before the recording medium arrives for image formation. Therefore, transfer charges are transferred from the transfer unit to the image carrier via the belt without the recording medium. In this way, an image carrier surface portion where transfer charges flow from the transfer device without any recording medium is generated in each image forming unit without any recording medium.

  In addition, since a plurality of recording media are sequentially conveyed at a predetermined interval, a state occurs in which transfer charges flow from the transfer device to the image carrier via the belt without any recording media between the recording media. Even when the last recording medium of the plurality of recording media leaves each image forming unit and the transfer unit, the last recording medium passes through the last image forming unit and the corresponding transfer unit. In general, each image forming unit and the transfer device continue to operate, and at this time, the transfer charge flows from the transfer device to the image carrier via the belt without any recording medium. Thus, in each image forming portion, an image carrier surface portion in which transfer charges flow from the transfer device without any recording medium is generated on the surface of the image carrier.

  At this time, a common transfer power source is provided for two or more of the plurality of transfer devices, and a predetermined transfer bias is applied to each of the transfer devices from the common transfer power source for transferring the toner image. With such a configuration, an excessive amount of transfer charge flows into the surface portion of the image carrier from which the transfer charge flows from the transfer device without any recording medium, because the recording medium does not exist. Since a common transfer power source is employed, the transfer bias cannot be adjusted for each transfer unit so as to prevent an excessive transfer charge from flowing in.

  For the surface portion of the image carrier where excessive transfer charge flows in this way, the regular (normal) charge potential by the charger, exposure by the image exposure device, development bias in the developer, etc. are set for image formation. If it remains as it is, the electrostatic latent image obtained by image exposure under the normal image exposure setting occurs due to the excessive transfer charge, so-called “deepening” phenomenon occurs, and the deepened electrostatic latent image A larger amount of toner adheres to the image than the intended amount of toner adhesion, resulting in a higher image density than the intended one (in other words, "image carrier memory noise" due to excessive transfer charge occurs. ), The problem that the image quality deteriorates arises.

Here, the electrostatic latent image “becomes deeper” will be described with an example.
For example, in normal image formation, the normal charging potential of the image carrier by the charger is −600 V, the exposure portion potential by image exposure under the normal image exposure setting is about −50 V, and this exposure portion potential is −50 V. In the case where a negatively chargeable toner is attached to the portion with a normal developing bias of −400 V, development is performed, a toner image is formed, and this toner image is transferred to a recording medium with a transfer device to which a transfer bias of +3000 V is applied. The surface of the image carrier where the transfer charge flows from the transfer device to the image carrier without the recording medium under the transfer bias of +3000 V is charged by the charger under the same conditions as those for normal image formation. When the image exposure is performed by the image exposure apparatus, the potential of the exposed portion is reduced to the plus side of the surface potential of the image carrier due to excessive transfer charge flow. Decrease to the positive side than about 50 V (for example, a -20 V). Such a phenomenon is an example of an electrostatic latent image “deepening”. In this way, when the negatively chargeable toner is attached under the normal development bias of −400 V to the exposed portion where the potential is lowered to the positive side from the normal one, more toner is attached and the image density is increased. Such a phenomenon becomes remarkable particularly in an isolated dot image in a half image or the like.

  Therefore, the present invention provides an image exposure body that forms an electrostatic latent image by performing image exposure on a charged area of the surface of the image carrier by the charger. A plurality of image forming units including an apparatus and a developing unit that develops the electrostatic latent image with the assigned color toner to form a toner image are sequentially disposed along the recording medium conveyance belt, and each image forming unit There is provided a transfer device that faces the image carrier of the image forming unit through the belt and transfers the toner image on the image carrier to a recording medium conveyed as the belt travels. At least two of the image forming apparatuses are image forming apparatuses to which a transfer bias for transferring a toner image is applied from a common transfer power source provided in common to the transfer devices, and the transfer bias is applied from the common transfer power source. Compatible with transfer devices In the image forming section, the toner density formed on the surface of the image carrier where the transfer charge flows from the transfer device without the intervention of the recording medium is also within the predetermined allowable range, and the image density is accordingly increased. An object of the present invention is to provide an image forming apparatus capable of forming a good image.

In order to solve the above problems, the present invention
An image carrier that is driven to rotate, a charger that charges the surface of the image carrier, an image exposure device that forms an electrostatic latent image by performing image exposure on a charged area of the surface of the image carrier by the charger, and the electrostatic A plurality of image forming units including a developing unit that develops the latent image with the assigned color toner to form a toner image are sequentially arranged along the recording medium conveyance belt, and the image forming unit is provided for each image forming unit. There is provided a transfer unit that faces the image carrier through the belt and transfers a toner image on the image carrier to a recording medium that is conveyed as the belt travels. At least two of the transfer units are provided. One is an image forming apparatus to which a transfer bias for transferring a toner image is applied from a common transfer power source provided in common to the transfer units,
In each of the image forming portions corresponding to the transfer device to which the transfer bias is applied from the common transfer power source, the surface portion of the image carrier on which the transfer charge flows from the transfer device without the intervention of the recording medium. When the transfer bias is such that the density of the toner image formed on the surface portion of the image carrier is higher than a predetermined allowable range, the density of the toner image formed on the surface portion of the image carrier The charging potential of the surface portion of the image carrier is controlled by the charger so as to be within a predetermined allowable range. In the control of the charging potential, the charging portion of the surface portion of the image carrier is controlled by the charger. The charging potential is set to a potential having the same polarity as the normal charging potential for image formation and an absolute value larger than the absolute value of the normal charging potential, and the normal charging potential having the same polarity as the normal charging potential. When the transfer charge flows from the transfer device to the image carrier through the recording medium as an absolute value greater than the absolute value, the excess transfer charge is neutralized with respect to the transfer charge flowing into the image carrier. An image forming apparatus in which a potential is set is provided.

Here, the “toner image density within a predetermined allowable range” means that the transfer device under a normal transfer bias in a state where the recording medium conveyance belt and the recording medium are interposed between the transfer device and the image carrier. A regular (ordinary) one in which the charging potential by the charger, the exposure by the image exposure device, the developing bias in the developing device, etc. are set for image formation on the surface portion of the image carrier on which the transfer charge flows. The image density is such that the density of the toner image formed in the state can be regarded as the same or practically the same.
Whether or not the density of the toner image formed on the surface of the image carrier where the transfer charge flows from the transfer device without the intervention of the recording medium is higher than a predetermined allowable range. For example, it is possible to grasp in advance through experiments or the like.

  According to the image forming apparatus of the present invention, in each of the image forming units corresponding to the transfer device to which the transfer bias is applied from the common transfer power source, the transfer charge from the transfer device without the intervention of the recording medium on the surface of the image carrier. When the transfer bias is such that the density of the toner image formed on the surface of the image carrier is higher than a predetermined allowable range, the surface of the image carrier The charging potential of the partial charger is set to a potential having the same polarity as the normal charging potential for image formation and an absolute value larger than the absolute value of the normal charging potential, and the same polarity as the normal charging potential. When the transfer charge flows from the transfer unit to the image carrier through the recording medium as an absolute value larger than the absolute value of the normal charging potential, an excessive flow with respect to the transfer charge flowing into the image carrier is obtained. Therefore, the electrostatic latent image formed on the surface of the image carrier where the transfer charge flows from the transfer device without the intervention of the recording medium becomes deeper than expected. Thus, the density of the toner image formed on the surface of the image carrier on which the transfer charge flows from the transfer device without any recording medium on the surface of the image carrier is within a predetermined allowable range. Therefore, a good image is formed in terms of image density.

  The above-mentioned “image carrier surface portion of the image carrier surface into which transfer charge flows from the transfer device without the intervention of the recording medium, the charging potential of the image carrier surface portion by the charger is the regular charge for image formation. The potential is set to an absolute value greater than the absolute value of the normal charging potential with the same polarity as that of the electric potential, and the potential of the absolute value larger than the absolute value of the normal charging potential is the same polarity as the normal charging potential. If the transfer charge flows from the container to the image carrier through the recording medium, the potential for neutralizing the transfer charge excessively with respect to the transfer charge flowing into the image carrier is set. It may be performed in advance on the upstream side of the transfer unit in the surface moving direction, or may be performed on the surface portion of the image carrier after passing through the transfer unit on the downstream side of the transfer unit.

  Incidentally, the transfer charge required for transferring the toner image formed on the image carrier to the recording medium differs depending on the surrounding temperature and humidity environment, the type of the recording medium (for example, its thickness), and the like. Therefore, the transfer bias applied from the common transfer power source to the transfer device may be selected from a plurality of transfer biases depending on the situation. Depending on the selected transfer bias, a charging potential by the charger is set to be large for the surface portion of the image carrier on which the transfer charge flows from the transfer device without any recording medium, and the transfer is performed. In some cases, it may be desirable to vary the magnitude of the charging potential in accordance with the bias voltage value.

  Therefore, in each of the image forming portions corresponding to the transfer device to which the transfer bias is applied from the common transfer power source, the image carrier surface portion in which the transfer charge flows from the transfer device without the recording medium among the image carrier surface, The charging potential control of the image carrier surface portion by the charger, which is performed so that the density of the toner image formed on the image carrier surface portion is within a predetermined allowable range, is transferred from the common transfer power source. You may perform according to the transfer bias output to a device.

  As for grasping the transfer bias in performing the charging potential control in this way, for example, the transfer bias applied from the common transfer power source to the transfer device is the transfer bias adjustment element including the recording medium to be used and the ambient temperature and humidity environment. May be determined based on the transfer bias adjustment element.

  When the image carrier is a photoconductor, the amount of transfer charge flowing into the surface of the image carrier (transfer current) into which the transfer charge flows from the transfer device without the recording medium is reduced. There is a tendency to increase.

Therefore, when the image carrier in each of the image forming units corresponding to the transfer device to which a transfer bias is applied from the common transfer power source is a photosensitive member that is rotationally driven,
In each of the image forming portions corresponding to the transfer device to which the transfer bias is applied from the common transfer power source, the image carrier surface portion of the image carrier surface into which the transfer charge flows from the transfer device without the intervention of the recording medium. The charging potential control of the surface portion of the image carrier by the charger performed so that the density of the toner image formed on the surface portion of the carrier is within a predetermined allowable range is the thickness of the photosensitive layer of the photoreceptor. May be performed when is less than or equal to a predetermined thickness.
Here, the “thickness equal to or less than a predetermined thickness” of the photosensitive layer means that the density of the toner image formed on the surface of the image carrier where the transfer charge flows from the transfer unit without any recording medium is within the predetermined allowable range. The thickness becomes higher than the concentration of.

  Since the thickness of the photosensitive layer of the photoconductor decreases with the use of the photoconductor, the thickness of the photoconductive layer may be grasped by counting the cumulative number of rotations of the photoconductor. Such a count may be performed by, for example, a control unit that controls the operation of the image forming apparatus, and the control unit can also determine whether the thickness of the photosensitive layer is equal to or less than a predetermined thickness.

  With respect to the first recording medium in the case where images are sequentially formed on one recording medium or a plurality of recording media when an image is formed on one recording medium, the recording medium receives a transfer bias from the common transfer power source. The time required to arrive at each of the image forming units corresponding to the applied transfer device is longer in the downstream image forming unit in the recording medium conveyance direction, and accordingly, in the downstream image forming unit in the recording medium conveyance direction. If the transfer charge flows from the transfer device to the image carrier without the recording medium, the charging device on the surface of the image carrier where the transfer charge flows from the transfer device without the recording medium. It is desirable that the charging potential is controlled as follows.

  That is, a transfer medium to which a transfer bias is applied from the common transfer power supply is applied to the one recording medium when an image is formed on one recording medium or the first recording medium when an image is sequentially formed on a plurality of recording media. Prior to arriving at each of the image forming units corresponding to each of the image forming units, in each of the image forming units, the transfer charge flows from the transfer unit (via the recording medium transport belt) without the recording medium to the surface portion of the image carrier. In the charging potential control performed by the charger, it is desirable that the absolute value of the charging potential is set to be larger in the downstream image forming unit than in the upstream image forming unit in the recording medium conveyance direction.

  Further, with respect to the last recording medium in the case where the image is sequentially formed on one recording medium or a plurality of recording media when the image is formed on one recording medium, the recording medium receives a transfer bias from the common transfer power source. The timing of moving away from each of the image forming units corresponding to the applied transfer device is earlier in the upstream side image forming unit in the recording medium conveyance direction. If the transfer charge flows from the transfer device to the image carrier without the media, the charging potential of the surface portion of the image carrier from which the transfer charge flows from the transfer device without the recording medium is charged. It is desirable to perform the control as follows.

  That is, the last recording medium in the case of sequentially forming an image on one recording medium or a plurality of recording media when an image is formed on one recording medium is a transfer device to which a transfer bias is applied from the common transfer power source. When moving away from each of the corresponding image forming units, in each of the image forming units, the transfer charge flows from the transfer device (via the recording medium conveying belt) without the intervention of the recording medium. In the charging potential control performed by the charger, it is desirable that the absolute value of the charging potential is set to be larger in the upstream image forming unit than in the downstream image forming unit in the recording medium conveyance direction.

As explained above, according to the present invention,
An image carrier that is driven to rotate, a charger that charges the surface of the image carrier, an image exposure device that forms an electrostatic latent image by performing image exposure on a charged area of the surface of the image carrier by the charger, and the electrostatic A plurality of image forming units including a developing unit that develops the latent image with the assigned color toner to form a toner image are sequentially arranged along the recording medium conveyance belt, and the image forming unit is provided for each image forming unit. There is provided a transfer unit that faces the image carrier through the belt and transfers a toner image on the image carrier to a recording medium that is conveyed as the belt travels. At least two of the transfer units are provided. And an image forming apparatus to which a transfer bias for transferring a toner image is applied from a common transfer power supply that is provided in common to the transfer devices, and is applied to the transfer device to which the transfer bias is applied from the common transfer power supply. In the corresponding image forming unit In addition, the toner image formed on the surface of the image carrier where the transfer charge flows from the transfer unit without the intervention of the recording medium is also within the predetermined allowable range, and the image density is good. Can be provided.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows an example of an image forming apparatus according to the present invention. The image forming apparatus shown in FIG. 1 is a tandem color image forming apparatus (here, a printer).

The printer P has a drive roller 71 and an endless recording medium conveyance belt 7 wound around a roller 72 facing the drive roller 71. The belt 7 is rotated counterclockwise in the figure (arrow direction α in the figure) by a driving roller 71 driven by a belt driving unit (not shown).
The belt can electrostatically attract the recording medium S when a voltage is applied from a power supply (not shown), and can be transported as the belt travels. The recording medium S will be described later.

  A cleaning device 73 that removes and cleans the toner on the belt 7 with a cleaning blade faces the facing roller 72.

  The fixing device 9 is disposed above the drive roller 71, the timing roller pair 8 is disposed below the opposing roller 72, and the recording medium storage cassette 10 that stores the recording medium S is further disposed below the fixing roller 9. Is arranged. The recording medium S is recording paper in this example.

The fixing device 9 includes a fixing heating roller incorporating a heat source such as a halogen lamp heater and a pressure roller pressed against the fixing heating roller.
The recording paper S stored in the recording medium storage cassette 10 can be pulled out one by one by the medium supply roller 101 and supplied to the timing roller pair 8.

  Between the rollers 71 and 72 around which the recording medium conveyance belt 7 is wound, the yellow image forming unit Y, the magenta image forming unit M, the cyan image forming unit C, and the rollers 72 to 71 along the belt 7. Black image forming portions K are arranged in this order.

  Each of the image forming units Y, M, C, and K includes a drum-type photoreceptor 1 as an electrostatic latent image carrier, and a charger 2, an image exposure device 3, and a developer 4 around the photoreceptor. And the cleaning device 6 are arranged in this order. A transfer device 5 is disposed on the photoreceptor 1 with a belt 7 therebetween.

Here, the photosensitive member 1 in each image forming unit is a negatively chargeable photosensitive member, and is driven to rotate clockwise in the drawing by a photosensitive member driving motor M (see FIG. 2).
The charger 2 in each image forming unit is not limited to this, but is a charging roller in this example, and a charging voltage is applied from a charging power source P1 (see FIG. 2) at a predetermined timing.

  The exposure device 3 can perform dot exposure on the photosensitive member 1 using a laser beam in accordance with image information provided from a personal computer (not shown) or the like to form an electrostatic latent image.

  The developing device 4 in each image forming unit is not limited thereto, but in this example, an electrostatic latent image formed on the photosensitive member 1 using a negatively chargeable toner is developed into a developing bias power source P2 (see FIG. 2). Thus, reversal development can be performed by the developing roller 41 to which a negative developing bias is applied.

  The transfer unit 5 corresponding to each image forming unit is a transfer roller in this example, and can be driven to rotate as the belt 7 travels. Further, the same transfer bias can be applied to these transfer rollers 5 from a common transfer power supply (transfer bias power supply) P3 (see FIGS. 1 and 2). The transfer bias is a bias having a polarity opposite to the charging polarity of the toner (in this example, a positive polarity bias).

  Each unit in the printer P operates at a predetermined timing under the instruction of a control unit Cont that controls the operation of the image forming apparatus as shown in FIG. Information on an image to be formed by the printer P is input to the control unit Cont from a personal computer, an image scanner, a facsimile machine, or the like (not shown).

According to the printer P, an image can be formed using one or more of Y, M, C, and K image forming units under the instruction of the control unit Cont.
A case where a full-color image is formed using all of the image forming portions Y, M, C, and K will be described below as an example.

A yellow toner image is formed in the yellow image forming portion Y. That is, in the yellow image forming portion Y, the photosensitive member 1 is rotated in the clockwise direction in the drawing, and the surface is uniformly charged to a predetermined potential by the charging roller 2. The yellow electrostatic latent image is formed by the above-described image exposure, which is developed by the developing device 4 under a developing bias, and thus a yellow toner image is formed on the photoreceptor 1.
After a predetermined time delay, the magenta image forming unit M also starts forming a magenta toner image on the photosensitive member 1, the cyan image forming unit C starts forming a cyan toner image on the photosensitive member 1, and the black image forming unit K. Then, formation of a black toner image on the photosensitive member 1 is started.

  On the other hand, the recording paper S is pulled out from the recording medium storage cassette 10 by the medium supply roller 101 and supplied to the timing roller pair 8. The timing roller pair 8 conveys the recording paper S to the recording medium so that the color toner images that are started to be formed at sequential time differences in the image forming units Y, M, C, and K are transferred onto the recording paper S in an overlapping manner. Supply to belt 7.

  The recording medium S is electrostatically attracted to the belt 7 and is conveyed while being sandwiched with the belt 7 between the photosensitive member 1 of each image forming unit and the transfer roller 5 facing the recording medium as the belt travels. Yellow, magenta, cyan, and black toner images are sequentially transferred onto the recording paper S, and the toner images are transferred in an overlapping manner.

  The transfer of the toner image on the photoreceptor 1 to the recording paper S in each image forming unit is performed by applying the same transfer bias from the common transfer power supply P3 to each transfer roller 5.

  The recording paper S on which the multiple toner images are formed in this way is passed through the fixing device 9, where the toner images are fixed under heat and pressure, and then discharged onto the discharge tray DT by the medium discharge roller pair DR.

  In the transfer of the toner image onto the recording paper S, residual toner or the like remaining on the photoreceptor 1 is cleaned by the cleaning device 6, and toner or the like that may remain on the belt 7 is cleaned by the cleaning device 73.

As described above, an image is formed. In this printer P, the same transfer bias is applied from the common transfer power supply P3 to each transfer roller 5 in order to simplify the structure and reduce the price. ing.
Therefore, in each image forming unit, the surface of the photoreceptor 1 where the transfer charge flows from the transfer roller 5 without the recording paper S on the surface of the photoreceptor 1 (more precisely, the transfer roller without the recording paper S interposed). 5 to the photosensitive member surface portion into which transfer charge flows only through the belt 7 occurs. As an example, FIG. 3 shows a state where such a photoreceptor surface portion 11 is generated in the magenta image forming unit M.

At this time, excess transfer charge flows into the photosensitive member surface portion (surface portion 11 in FIG. 3) where transfer charge flows without the recording paper S, because the recording paper does not intervene.
As for the surface portion of the photoreceptor where excessive transfer charge flows in this way, the charging potential by the charging roller 2, the exposure by the image exposure device 3, the developing bias in the developing device 4 and the like are set to normal (normally). If this is the case, the electrostatic latent image obtained by image exposure under the normal image exposure setting will cause a so-called “deepening” phenomenon due to excessive transfer charge, and the deep static A larger amount of toner adheres to the electrostatic latent image than the intended amount of toner adhesion, resulting in a higher image density than the intended one (in other words, photoreceptor memory noise due to excessive transfer charge occurs). This causes a problem that the image quality is degraded.

The transfer bias to be applied to the transfer roller 5 in order to transfer the toner image on the photoreceptor 1 in each image forming unit to the recording paper S in each image forming unit is the electric resistance value of the recording medium conveyance belt 7. Depending on the ambient temperature and humidity environment and the type (for example, thickness) of the recording medium, it is about 2 kV to 3.5 kV. The belt 7 has a high electrical resistance (for example, a surface resistivity of about 5 × 10 ¹⁰ Ω / □), a low temperature and low humidity environment (10 ° C. or less, about 15% RH or less), and the recording medium is a cardboard such as a postcard. In some cases, a transfer bias of about 3.5 kV is required. In general, when transfer charge flows from the transfer roller 5 to the photosensitive member 1 without a recording medium in a state where the transfer bias exceeds 3.0 kV, excessive transfer charge flows. Even if the surface portion of the photoreceptor is charged by the charging roller 2 to which a normal charging voltage is applied from the power supply P1, the potential of the surface portion of the photoreceptor is lowered, and the density of the toner image formed on the portion where the surface potential is lowered. Becomes darker and photoreceptor memory noise occurs.

  Photoconductor memory noise is likely to occur in a photoconductor (in particular, a photoconductor having a photoconductive layer of 15 μm or less) having a thin photoconductive layer as a result of repeated image formation. In particular, a half image (particularly a half image of an isolated point). It is easy to occur in.

  Therefore, in the printer P, in each image forming unit, with respect to the photosensitive member surface portion where the transfer charge flows from the transfer roller 5 without the recording paper S, the transfer bias from the power source P3 is formed on the photosensitive member surface portion. When the density of the image is higher than a predetermined allowable range, the charging roller 2 precharges the surface portion of the photoconductor 1 on the upstream side of the transfer roller 5 in the photoconductor rotation direction. The charging potential has the same polarity as that of a normal (normal) charging potential for image formation, and a charging potential having a larger absolute value than that. The output of the charging power source P1 that applies a charging voltage to the charging roller 2 is set to a control unit Con so that the charging roller 2 has a potential to neutralize the excessive transfer charge flowing from the transfer roller 5 to the surface of the photoreceptor. It is controlled by.

  The timing chart of FIG. 4 shows an example of controlling the photoreceptor surface potential. As shown in FIG. 4, when the belt 7 faces the photosensitive member 1 without the recording paper S, the surface portion of the photosensitive member where the transfer charge flows from the transfer roller 5 without the recording paper is charged. The charged potential (negative polarity) of the portion by the roller 2 is set large in advance to the negative side so as to neutralize the excess transfer charge.

  Thus, the density of the toner image formed on the surface of the photoreceptor where transfer charge flows from the transfer roller 5 without the recording paper S is also transferred from the transfer roller 5 with the recording paper S interposed as well as the belt 7. The density of the surface of the photoconductor on which the toner flows is considered to be the same or practically the same as the density of the toner image formed under normal charging potential, image exposure and development bias, etc. Thus, a good image is formed in terms of image density.

Such output control of the charging power supply P1 is performed according to the magnitude of the transfer bias applied from the common transfer power supply P3 to the transfer roller 5. In the printer P, it is also performed when the thickness of the photosensitive layer of the photoreceptor 1 is a predetermined thickness (for example, 15 μm or less). This is because as the thickness of the photosensitive layer decreases, the capacitance increases, and it becomes easier to generate photoconductor memory noise.
Since the thickness of the photosensitive layer of the photosensitive member 1 is determined according to the cumulative number of revolutions of the photosensitive member 1, it is detected by grasping the cumulative number of revolutions of the photosensitive member 1 in the control unit Cont, and whether it is a predetermined thickness. It is also judged by the control part Cont.

  As for the transfer bias, for example, when the charged potential on the surface of the photosensitive member 1 when the regular charging voltage −1 kV is applied from the charging power supply P1 to the charging roller 2 is about −600 V, the photosensitive member with the same charged potential. When the surface portion is in direct contact with the belt 7, when the transfer bias applied to the transfer roller 5 from the common transfer power supply P3 is +3200 V or more, “photoconductor memory noise” occurs due to excessive transfer charge flow.

  As shown in the following table, assuming that the photosensitive member surface potential Vo is further shifted by −200 V (200 V in the minus direction) only at the photosensitive member surface portion corresponding to the space between the sheets and Vo = −800 V, the transfer bias becomes +3200 V until “3200 V”. “Photoreceptor memory noise” does not occur (“◯” in the table. “X” in the table indicates occurrence). Further, if the photosensitive member surface potential Vo is shifted by −500 V only in the photosensitive member surface portion corresponding to the interval between sheets, “photosensitive memory noise” does not occur until the transfer bias reaches + 3600V.

Transfer bias (V) 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000
Paper spacing Vo regular ○ ○ ○ ○ ○ ○ × × × × ×
Between papers Vo-200V ○ ○ ○ ○ ○ ○ ○ × × × ×
Paper interval Vo-500V ○ ○ ○ ○ ○ ○ ○ ○ ○ × ×

  As can be seen from the above table, the photoconductor memory noise is more likely to occur as the transfer bias voltage becomes higher. Therefore, it is desirable to change the photoconductor surface potential setting in accordance with the transfer bias voltage value. When the transfer bias voltage is +3200 V, the charging potential by the charging roller 2 on the surface of the photoreceptor where the transfer charge flows without recording paper is changed by −200 V, and when the transfer bias voltage is +3600 V, the surface of the photoreceptor is charged. It is desirable to change the charging potential by the charging roller 2 on the surface of the photoreceptor where the transfer charge flows without the recording paper, according to the magnitude of the voltage value of the transfer bias, such as by changing the potential by −500V.

In the printer P, the control unit Cont does not change the charging potential by the charging roller 2 on the surface of the photosensitive member where the transfer charge flows without the recording paper when the transfer bias is +3000 V or less from the normal (normal) according to the above table. . For example, in a high humidity environment (60% RH or more) or when using thin recording paper, the required transfer bias does not exceed +3000 V, and the control unit Cont controls the common transfer power supply P3. The transfer bias is set to a potential determined in advance by an experiment or the like of + 3000V or less. In such a case, the control unit Cont does not change the charging potential by the charging roller 2 on the surface of the photoreceptor where the transfer charge flows without the recording paper from the normal one.
The control unit Cont is grasped by a temperature / humidity detection sensor (not shown), and is set in a computer (not shown) that receives ambient temperature / humidity information and image information input to the control unit Cont. Based on the type of recording medium to be used (for example, whether it is thick paper or thin paper), the transfer bias determined in advance corresponding to the conditions and stored in the control unit Cont is grasped, and the photoconductor is in accordance with the grasped transfer bias. The charging potential can be controlled.

  Further, as shown in FIG. 5, the control unit Cont determines whether the first recording sheet when the image is sequentially formed on the one recording sheet or the plurality of recording sheets when the image is formed on one recording sheet. Prior to arrival at each of the image forming portions corresponding to the transfer roller 5 to which the transfer bias is applied from the common transfer power supply P3, the belt 7 is removed from the transfer roller 5 without any recording paper in each image forming portion. In the control of the charging potential by the charging roller 2 performed on the surface portion of the photoreceptor 1 through which the transfer charge flows through, as illustrated in FIG. 6, the image on the downstream side from the upstream image forming unit in the recording paper conveyance direction. The absolute value of the charging potential is set to be larger at the formation portion.

  When forming an image on one sheet of recording paper, the last recording sheet when sequentially forming images on one sheet or a plurality of sheets corresponds to the transfer roller 5 to which a transfer bias is applied from the common transfer power source P3. In each of the image forming units, the charging roller is applied to the surface portion of the photoreceptor 1 where the transfer charge flows from the transfer roller 5 through the belt 7 without the recording paper S in each image forming unit. In the control of the charging potential by 2, the absolute value of the charging potential may be set larger in the upstream side image forming unit than in the downstream side image forming unit in the recording medium conveyance direction.

  In the printer P described above, the common transfer power supply P3 is provided for all the transfer rollers 5 of the four image forming units Y, M, C, and K. However, as illustrated in FIG. 7, yellow, magenta, and cyan are used. A common transfer power supply P3 ′ is provided for the transfer rollers 5 in the image forming portions Y, M, and C. In these image forming portions, the transfer rollers are not provided with no recording paper as in the case of the image forming apparatus in FIG. 5, the charge potential control is performed in advance on the surface of the photosensitive member into which the transfer charge flows, and for the black image forming unit, the control unit Cont controls the output of the power supply P4 that applies the transfer bias to the transfer roller 5. The generation of the photoconductor memory noise in the image forming unit K may be suppressed.

  The present invention includes a plurality of image forming units as in a tandem type color image forming apparatus, and further forms an image of a type in which a toner image formed in each image forming unit is directly transferred to a recording medium by a transfer device. In an image forming apparatus in which a transfer bias is applied from a common transfer power source to a transfer unit corresponding to each image forming unit, the transfer unit is formed on a surface portion of an image carrier on which transfer charges flow from the transfer unit without intervention of a recording medium. The image density of the toner image is also within a predetermined allowable range, and can be used to provide an image forming apparatus capable of forming a good image in terms of image density.

1 is a diagram schematically illustrating a configuration of an example of an image forming apparatus according to the present invention. FIG. 2 is a block diagram illustrating an outline of a control circuit of the image forming apparatus illustrated in FIG. 1. FIG. 4 is a diagram illustrating that transfer charge may flow from a transfer roller to a photoconductor without the intervention of a recording medium. An example of output control from a charging power source to a charging roller (in other words, photosensitive member charging potential control) that precharges the surface portion of the photosensitive member surface where transfer charge flows from the transfer roller without the intervention of a recording medium. It is a timing chart which shows. When an image is formed on one recording medium, the image is formed from the transfer roller without the intervention of the recording medium before the arrival of the first recording medium in the case of forming an image on the one recording medium or a plurality of recording media. It is a figure which shows that a transcription | transfer charge may flow into a body. As shown in FIG. 5, from the charging power source to the charging roller that precharges the surface portion of the surface of the photoreceptor where the transfer charge flows from the transfer roller without the intervention of one or the first recording medium. 6 is a timing chart showing an example of output control (in other words, photosensitive member charging potential control). FIG. 3 is a diagram illustrating an outline of an example of an image forming apparatus that employs a common transfer power supply for three transfer rollers.

Explanation of symbols

P Printer Y Yellow image forming unit M Magenta image forming unit C Cyan image forming unit K Black image forming unit 1 Photoconductor P1 Photoconductor charging power source 2 Charging roller 3 Image exposing device 4 Developer 41 Developing roller P2 Developing bias power source 5 Transfer roller P3 Common transfer power supply 6 Cleaning device 7 Recording medium conveying belt 71 Driving roller 72 Opposing roller 73 Cleaning device 8 Timing roller pair 9 Fixing device 10 Recording medium supply cassette 101 Medium supply roller DR Recording medium discharge roller pair DT Recording medium discharge tray S Recording medium

Claims (7)

An image carrier that is driven to rotate, a charger that charges the surface of the image carrier, an image exposure device that forms an electrostatic latent image by performing image exposure on a charged area of the surface of the image carrier by the charger, and the electrostatic A plurality of image forming units including a developing unit that develops the latent image with the assigned color toner to form a toner image are sequentially arranged along the recording medium conveyance belt, and the image forming unit is provided for each image forming unit. There is provided a transfer unit that faces the image carrier through the belt and transfers a toner image on the image carrier to a recording medium that is conveyed as the belt travels. At least two of the transfer units are provided. One is an image forming apparatus to which a transfer bias for transferring a toner image is applied from a common transfer power source provided in common to the transfer units,
In each of the image forming portions corresponding to the transfer device to which the transfer bias is applied from the common transfer power source, the surface portion of the image carrier on which the transfer charge flows from the transfer device without the intervention of the recording medium. When the transfer bias is such that the density of the toner image formed on the surface portion of the image carrier is higher than a density within a predetermined allowable range, the toner image formed on the surface portion of the image carrier The charging potential of the surface portion of the image carrier is controlled by the charger so that the density of the surface of the image carrier is within a predetermined allowable range. The charging potential by the charger is set to the same polarity as the normal charging potential for image formation and to an absolute value greater than the absolute value of the normal charging potential, and the normal charging is set to the same polarity as the normal charging potential. If the transfer charge flows from the transfer device to the image carrier through the recording medium as an absolute potential greater than the absolute value of the position, the transfer charge that is excessively flowing relative to the transfer charge that flows into the image carrier is An image forming apparatus in which a potential to be summed is set.   In each of the image forming portions corresponding to the transfer device to which a transfer bias is applied from the common transfer power source, the image carrier surface portion of the image carrier surface into which the transfer charge flows from the transfer device without intervention of the recording medium. The charging potential control of the surface portion of the image carrier by the charger, which is performed so that the density of the toner image formed on the surface portion of the carrier is within a predetermined allowable range, is performed from the common transfer power source to the transfer device. The image forming apparatus according to claim 1, wherein the image forming apparatus is performed in accordance with an output transfer bias.   A transfer bias applied from the common transfer power source to the transfer device is set in advance according to a transfer bias adjusting element including a recording medium to be used and an ambient temperature and humidity environment, and the transfer bias is set to the transfer bias adjusting element. The image forming apparatus according to claim 2, wherein the image forming apparatus is grasped based on the image. The image carrier in each of the image forming units corresponding to a transfer device to which a transfer bias is applied from the common transfer power supply is a photoconductor that is driven to rotate.
In each of the image forming portions corresponding to the transfer device to which the transfer bias is applied from the common transfer power source, the image carrier surface portion of the image carrier surface into which the transfer charge flows from the transfer device without the intervention of the recording medium. The charging potential control of the surface portion of the image carrier by the charger performed so that the density of the toner image formed on the surface portion of the carrier is within a predetermined allowable range is the thickness of the photosensitive layer of the photoreceptor. The image forming apparatus according to claim 1, wherein the image forming apparatus is performed when is less than or equal to a predetermined thickness.   The image forming apparatus according to claim 4, wherein the thickness of the photosensitive layer of the photosensitive member is grasped by counting the cumulative number of rotations of the photosensitive member.   In the case of forming an image on one recording medium, the one recording medium in the case of sequentially forming an image on one recording medium or a plurality of recording media is transferred to a transfer device to which a transfer bias is applied from the common transfer power source. Prior to arrival at each of the corresponding image forming units, in each of the image forming units, the charging potential control by the charger is performed on the surface portion of the image carrier where the transfer charge flows from the transfer unit without the intervention of the recording medium. 6. The image forming apparatus according to claim 1, wherein the absolute value of the charging potential is set to be larger in the downstream side image forming unit than in the upstream side image forming unit in the recording medium conveyance direction.   The last recording medium in the case where images are sequentially formed on one recording medium or a plurality of recording media when an image is formed on one recording medium corresponds to a transfer device to which a transfer bias is applied from the common transfer power source. In the control of the charging potential by the charger, which is performed on the surface portion of the image carrier in which the transfer charge flows from the transfer device without the intervention of the recording medium in each image forming portion when moving away from each of the image forming portions, The image forming apparatus according to claim 1, wherein the absolute value of the charging potential is set to be larger in the upstream side image forming unit than in the downstream side image forming unit in the medium transport direction.

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