JP2011141345A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which forms a superior image over a long term, without causing residual-image phenomenon of abnormal image, even if the image area ratio of an image to be formed and the fatigue conditions of a photoreceptor are varied. <P>SOLUTION: The image forming apparatus includes a photoreceptor 11; a charging device 12 for charging the surface of the photoreceptor 11; a surface-potential measuring means 111 for measuring the surface potential of the photoreceptor 11; and a transfer device 20 for transferring toner image, by applying a charge having reverse polarity, to a charging polarity of toner to a recording medium 6, wherein a transfer voltage control means performs a constant-voltage control, and applies prescribed potential and forms an image. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, and a printer, and a process cartridge employed in the image forming apparatus.

Conventionally, as an image forming apparatus, the surface of a moving photosensitive member is charged to a negative polarity, then exposed to form an electrostatic latent image, and a developing device supplies negative polarity toner to the electrostatic latent image. One that forms a toner image on the surface of a photoreceptor is known.
Then, the toner image formed on the surface of the photosensitive member is transferred to a transfer portion where the transfer member such as an intermediate transfer member or a recording member faces the recording member such as a recording paper with the transfer member and the photosensitive member facing each other. Transfer to the surface.
After the toner image is transferred to the recording medium, the toner remaining on the surface of the photoreceptor is removed by a cleaning device, and is prepared for the next image formation.
A transfer device including a transfer member that forms a transfer portion includes transfer voltage control means for applying a positive polarity transfer voltage to the transfer member. Then, by applying a positive polarity transfer voltage to the transfer member, a transfer electric field is formed between the photoconductor and the transfer member in the transfer portion, and the negative polarity toner moves from the surface of the photoconductor to the recording medium, and the toner Image transfer is established.

However, in an image forming apparatus that forms an image by an electrophotographic method, it is difficult to completely transfer all of the toner that forms the toner image on the photoconductor, and an untransferred toner image is formed on the photoconductor. Afterimage phenomenon appears.
FIG. 1 is a diagram showing an example of a document pattern in which an afterimage phenomenon is likely to appear. For example, as shown in FIG. 1, an original pattern is a solid image with a clear contrast (here, represented by “R”), followed by a halftone image of a certain size (here, represented by a rectangle). Have).
FIG. 2 is a schematic diagram illustrating an example of an image pattern in which an afterimage phenomenon appears. As shown in FIG. 2, an image pattern “R” printed before the halftone image may appear in the image that the halftone image should originally be a uniform and uniform image. An abnormal image with such an afterimage phenomenon that has deteriorated is called “positive afterimage” or “positive ghost”. In particular, in an image forming apparatus that forms a full-color image that requires high image quality, it is necessary to suppress such image deterioration. Otherwise, in a full-color image in which a solid image and a halftone image appear repeatedly, such image degradation appears at a high frequency. FIG. 3 is a schematic diagram showing another example of an image pattern in which an afterimage phenomenon appears. is there. On the other hand, an abnormal image of image degradation in which an image pattern previously formed in the halftone image portion is identified with a low density is called “negative afterimage” or “negative ghost”. Similarly, in an image forming apparatus that forms a full-color image that requires high image quality, it is necessary to suppress such image deterioration.

Several mechanisms can be considered for the afterimage phenomenon. For example, Patent Document 1 (paragraph numbers: [0002] to [0006]) can be interpreted as being caused by nonuniformity of the photoreceptor surface potential.
FIG. 4 is a diagram schematically showing changes in the photoreceptor surface potential in each step after latent image formation, development, transfer, and recharging. In this case, when the latent image shown in FIG. 4A is formed, the surface of the photosensitive member is uniformly charged to −700 V, and then image information is exposed (the arrow indicates the exposed portion). The potential of the exposed portion is approximately 0V.
Then, at the time of development in FIG. 4B, toner is attached to the surface of the photoconductor and developed according to the potential difference between the development potential and the surface of the photoconductor. Next, at the time of transfer, the recording medium side is charged positively to transfer the toner image from the photoreceptor to the recording medium.
As shown in FIG. 4C or FIG. 4D, when a reverse bias is applied to the photosensitive member by the transfer unit, the surface potential of the photosensitive band after the transfer changes in the positive direction as a whole, but the toner adheres. The photosensitive belt surface potential of the image area and the non-image area where no toner is present is not uniform. This is because the surface potential of the photoconductor before transfer varies depending on the presence or absence of exposure, the ease of transfer current flow varies depending on the presence or absence of toner, and charge carriers are generated inside the exposed portion.

As a result, when the image portion is close to positive as shown in FIG. 4C, even if the photosensitive belt surface is uniformly negatively charged by the charging means, the surface potential of the portion close to positive is charged. After that, as shown in FIG. Since the difference in development potential is larger in the portion transitioned in the positive direction than in the other portions, apparent sensitization occurs and a dark toner image is formed. This portion is identified as a positive afterimage.
On the other hand, when the image portion is negative as shown in FIG. 4D, the surface potential of the negative portion is negative as shown in FIG. The difference in development potential is reduced, and a toner image thinner than other portions is formed. This part is identified as a negative afterimage. Even if the charging polarity is reversed, the principle is the same because only positive and negative are switched.

  There are many factors in the afterimage phenomenon, but the biggest factor is the transfer bias. Therefore, attempts have been made to suppress afterimages by controlling the transfer current. (For example, Patent Documents 2 to 5). However, even if the transfer current is controlled to a good value of the afterimage by these methods, the amount of toner intervening between the photoconductor and the recording medium changes at the time of transfer when the image area ratio to be formed changes, The amount of the transfer current flowing into the image area and the non-image area may also fluctuate to change the potential of the image area and the non-image area, resulting in an afterimage. In addition, since the afterimage generation conditions vary depending on the temperature and humidity environment and the fatigue condition of the photoreceptor, the afterimage cannot be sufficiently suppressed.

  In view of the above problems, an object of the present invention is to provide an image that forms a good image over a long period of time without causing an afterimage phenomenon of an abnormal image even if the image area ratio of the image to be imaged or the fatigue condition of the photoreceptor changes. A forming apparatus is provided.

The features of the present invention, which is a means for solving the above problems, are listed below.
The image forming apparatus according to the present invention includes an image carrier, a charging unit that charges the surface of the image carrier, a surface potential measuring unit that measures the surface potential of the image carrier, and the electrostatic latent surface by exposing the surface of the image carrier. An image exposure means for forming an image; a developing means for visualizing the electrostatic latent image with a toner charged to the same polarity as the charge polarity of the image carrier; and a toner image on the recording medium; In the image forming apparatus including a transfer unit that applies a charge of reverse polarity to transfer a toner image and a transfer voltage control unit that controls a transfer voltage applied to a transfer member of the transfer unit, the transfer voltage control unit is This is voltage control, and while increasing or decreasing the transfer voltage by a predetermined value, the charging potential VD1 after recharging of the surface area of the image carrier on which the black solid image is formed and the image are not formed at each transfer voltage. Of the image carrier surface area The charge potential VD2 of the charged, measured by the surface potential measuring means, if the positive and negative values of the potential difference ΔVD of the formula (I) is changed,
If there are two transfer voltages VTi and VTj before and after the change, and the potential difference ΔVD at that time is ΔVDi and ΔVDj, the transfer voltage VT is controlled to a value represented by the formula (II),
When there is no positive / negative change in the potential difference ΔVD, the transfer voltage VT is controlled so as to minimize the absolute value of the potential difference ΔVD.
(I) ΔVD = VD1−VD2
(II) VT = (VTi × ΔVDj−ΔVDi × VTj) / (ΔVDj−ΔVDi)
The image forming apparatus of the present invention is further characterized in that the transfer member is a transfer roller.
In the image forming apparatus of the present invention, the transfer unit further primarily transfers the toner image developed on the image carrier onto the intermediate transfer member, and then transfers the toner image on the intermediate transfer member onto the recording medium. Secondary transfer.
In the image forming apparatus of the present invention, the image forming apparatus further includes a plurality of image carriers, and the developed toner images are sequentially superimposed on each image carrier to form a color image. It is characterized by.

  The image forming apparatus of the present invention further includes a process cartridge that integrally supports at least the image carrier and a surface potential measuring unit that measures the surface potential of the image carrier and is detachable from the image forming apparatus. Features.

The present invention, which is a means for solving the above problems, has the following specific effects.
According to the image forming apparatus of the present invention, it is possible to provide a high-quality image forming apparatus that does not generate an afterimage over a long period of time.

FIG. 5 is a diagram illustrating an example of a document pattern in which an afterimage phenomenon is likely to appear. It is a schematic diagram which shows an example of the image pattern in which the afterimage phenomenon appeared. It is a schematic diagram which shows the other example of the image pattern in which the afterimage phenomenon appeared. It is a figure which shows typically the change of the photoreceptor surface potential in each process after latent image formation, image development, transfer, and recharging. 1 is a schematic diagram for explaining an image forming apparatus of the present invention, and is a diagram showing a main part of the image forming apparatus. It is a figure which shows the relationship between the transfer voltage VT and memory potential (DELTA) VD. It is the schematic which shows the structure of the process cartridge of this invention. It is the schematic which shows another structure of the image forming apparatus of this invention. It is the schematic which shows another structure of the image forming apparatus of this invention. It is the schematic which shows another structure of the image forming apparatus of this invention. 1 is a schematic diagram illustrating an overall configuration of an image forming apparatus of the present invention.

  The best mode for carrying out the present invention will be described below with reference to the drawings. Note that it is easy for a person skilled in the art to make other embodiments by changing or correcting the present invention within the scope of the claims, and these changes and modifications are included in the scope of the claims. The following description is an example of the best mode of the present invention, and does not limit the scope of the claims.

The image forming apparatus of the present invention will be described in detail below with reference to the drawings.
FIG. 5 is a schematic diagram for explaining the image forming apparatus of the present invention, and is a diagram showing a main part of the image forming apparatus.
As shown in FIG. 5, in the image forming apparatus of the present invention, a charging device 12 that is a charging unit for charging the photoconductor 11 and a charged photoconductor 11 are statically placed around the photoconductor 11 that is an image carrier. A laser beam from an exposure device 19 that forms an electrostatic latent image, a developing device 13 that visualizes the electrostatic latent image on the photoreceptor 11 with toner, and a toner image visualized on the photoreceptor 11 as a recording medium 6 or an intermediate transfer member. The transfer roller 23 to be transferred to the intermediate transfer belt 21, the cleaning device 14 for cleaning untransferred toner remaining on the photoconductor 11, and the remaining charge on the photoconductor 11 cleaned for the next image forming step. Alternatively, a static elimination device 15 that erases the potential is provided. In addition, the image forming apparatus of the present invention is provided with a surface potential measuring unit 111 that measures the surface potential of the charged photoconductor 11.

More specifically, although the photoconductor 11 has a drum shape, it may have a sheet shape or an endless belt shape.
For the charging device 12, known means such as a corotron, a scorotron solid state charger, and a charging roller are used.
Light sources used for the exposure means 19, the charge removal means 15 and the like include fluorescent lamps, tungsten lamps, halogen lamps, mercury lamps, sodium lamps, light emitting diodes (LEDs), semiconductor lasers (LDs), electroluminescences (ELs) and the like. General. Various types of filters such as a sharp cut filter, a band pass filter, a near infrared cut filter, a dichroic filter, an interference filter, and a color temperature conversion filter can be used to irradiate only light in a desired wavelength range. In particular, a semiconductor laser (LD) is preferable as a light source for forming a latent image. Control of light emission and the like is easy, and a latent image with high light directivity and high accuracy can be formed. Further, electroluminescence (EL) is preferable for the static elimination means 15. Low power consumption and durability.
The toner 131 developed on the photosensitive member 11 by the developing device 13 which is a developing means is transferred to the recording medium 6 such as recording paper, but not all is transferred, but the toner remaining on the photosensitive member 11. Also occurs. Such toner is removed from the photoreceptor 11 by a cleaning device 14 which is a cleaning means. As the cleaning device 14, a rubber cleaning blade, a brush such as a fur brush, a mag fur brush, or the like can be used.

A known charging member such as a corotron, a scorotron, a solid state charger (solid state charger), or a charging roller can be used for the transfer device 20 as a transfer unit. In particular, a charging roller is preferable as the transfer member. Generation of ozone is small, durability is long, and the photoreceptor 11 can be charged stably over a long period of time.
A transfer power source 26 is connected to these. As a transfer bias application method in the transfer device 20, there are constant voltage control for making the voltage value constant and constant current control for making the current value constant. However, in the image forming apparatus 1 according to the present invention, the constant voltage control unit 261 performs constant voltage control on the transfer power supply 26 based on a signal from a control unit inside the image forming apparatus 1 (not shown), thereby reducing the image area to be imaged. Regardless, it is preferable because the surface potential of the photoreceptor can be made constant. In addition, there is a merit that the device to be used can be reduced in cost compared to constant current control. The constant voltage control unit 261 is not provided independently, but may be included in the control unit of the image forming apparatus 1 and may be provided on the same substrate as a part of the control unit.
Further, it has a surface potential measuring means 111, and a control unit (not shown) controls the transfer voltage of the transfer power supply 26 by the constant voltage control means 261 based on the information obtained from the surface potential measuring means 111. . This control is performed at times other than the normal printing operation.

In this control, first, as in a normal printing operation, the photoconductor 11 is developed in the charging process, the exposure process, and the development process, and a solid portion (hereinafter referred to as a “patch”) having a certain black solid area is referred to as the photoconductor. 11 on the recording medium 6 formed directly on the paper transfer means in the case of the direct transfer method, or an endless belt or a rotating cylindrical drum in the case of the intermediate transfer method. Transfer to an intermediate transfer member composed of
By this operation, the surface potential measuring means 111 obtains a potential as shown in FIG. 4 (e) or FIG. 4 (f). From this, the potential difference (memory potential) ΔVD between the surface potential VD1 of the portion where the patch is formed and the surface potential VD2 of the portion where the patch is not formed is measured.
(I) ΔVD = VD1−VD2
The portion where the patch is formed and the portion where the patch is not formed are the time from the exposure timing depending on the position of the photoconductor 11 irradiated by the laser beam of the exposure device 19, the angle of the surface potential measuring means 111 and the rotation speed of the photoconductor 11. Can be derived and the surface potential of a predetermined portion on the photoconductor 11 can be measured.

FIG. 6 is a graph showing the relationship between the transfer voltage VT and the potential difference ΔVD.
By performing this operation with a plurality of transfer voltages within a predetermined range, a graph of potential difference ΔVD with respect to transfer voltage VT as shown in FIG. 6 can be obtained. Therefore, in this case, an exposure unit that forms a patch with a small area and a non-exposure unit that does not expose the laser beam are repeatedly provided, and further, the positional relationship between the charging voltage of the charging device 12 and the exposure unit / non-exposure unit is captured. However, the transfer voltage is changed by the constant voltage control means 261. Therefore, the graph shown in FIG. 6 can be created by rotating the photoconductor 11 at least twice.
As shown in FIG. 6, there is a point where the value of the potential difference ΔVD changes from positive to negative (or from negative to positive) as the transfer voltage VT is increased. In this way, when the transfer voltages at two points before and after the sign of the potential difference ΔVD is inverted are VTi and VTj, the potential difference ΔVD at the transfer voltage VTi is ΔVDi, and the potential difference ΔVD at the transfer voltage VTj is ΔVDj. Then, the transfer voltage VT is controlled to a value represented by the formula (II).
(II) VT = (VTi × ΔVDj−ΔVDi × VTj) / (ΔVDj−ΔVDi)
When there is no positive / negative change in the potential difference ΔVD, the transfer voltage VT is set so that the absolute value of the potential difference ΔVD is minimized.
The transfer voltage VT represented by the formula (II) indicates a transfer voltage that brings the potential difference ΔVD to near zero. By controlling the transfer voltage VT, the surface potential VD1 after recharging of the exposed portion and the non-exposure are determined. The surface potential VD2 after recharging of the portion is made uniform, and a good image can be obtained without generating any positive afterimage or negative afterimage.
Further, when there is no positive / negative change in the potential difference ΔVD represented by the formula (I), the transfer voltage VT is set to the minimum absolute value of the potential difference ΔVD. By controlling to this transfer voltage VT, it is possible to minimize potential unevenness after recharging of the exposed portion and the non-exposed portion, and to suppress the occurrence of negative afterimage and positive afterimage.
In this way, by controlling the transfer voltage VT to an optimum value by the constant voltage control means 261, neither negative afterimages nor positive afterimages are generated, and the situation changes due to changes in the temperature and humidity environment and fatigue of the photoreceptor. Even so, it is possible to minimize the occurrence of afterimages.
Such control is desirably executed when the image forming apparatus 1 is started or when an instruction is given from the outside of the image forming apparatus 1, and may be performed at predetermined intervals (for example, every predetermined number of printed sheets). . As a result, an abnormal image such as a positive afterimage / negative afterimage can be prevented, and a high-quality image can be obtained. .

The image forming means as described above may be fixedly incorporated in the copying machine, facsimile, or printer, but may be incorporated in the image forming apparatus in the form of the process cartridge 10. The process cartridge 10 includes a photoconductor 11 and is detachable from the image forming apparatus 1. Further, the process cartridge 10 of the present invention integrally supports a surface potential measuring means 111 for measuring the image carrier surface potential. By integrating the photoconductor 11 and the surface potential measuring unit 111, the surface potential can be measured with high accuracy.
In addition, the process cartridge 10 of the present invention may include a charging device 12, an exposure device 19, a developing device 13, a transfer device 20, a cleaning device 14, and a charge removal device 15.
FIG. 7 is a schematic view showing the configuration of the process cartridge of the present invention. Although many shapes and the like of the process cartridge 10 can be mentioned, a general example is shown in FIG. The photoconductor 11 has a drum shape, but may have a sheet shape or an endless belt shape. The process cartridge of FIG. 7 also has a surface potential measuring unit 111, and the constant voltage control unit 261 controls the transfer voltage of the transfer power source 26.

  FIG. 8 is a schematic view showing another configuration of the image forming apparatus of the present invention. In the image forming apparatus 1, a charging device 12, an exposure device 19, a developing device 13 Bk for each color toner of black (Bk), cyan (C), magenta (M), and yellow (Y) are disposed around the photoreceptor 11. 13C, 13M, and 13Y, an intermediate transfer belt 21 that is an intermediate transfer member, and a cleaning device 14 are sequentially arranged. Here, the subscripts Bk, C, M, and Y shown in the figure correspond to the color of the toner, and are added or omitted as appropriate. The surface potential measuring unit 111 is provided after being exposed, and performs the above-described transfer voltage control.

  The developing devices 13Bk, 13C, 13M, and 13Y for the respective colors can be controlled independently, and only the developing devices 13Bk, 13C, 13M, and 13Y for forming the image are driven. The toner image formed on the photoconductor 11 is transferred onto the intermediate transfer belt 21 by a primary transfer roller 23 that is a first transfer unit disposed inside the intermediate transfer belt 21. The primary transfer roller 23 is disposed so as to be able to come into contact with and separate from the photosensitive member 11, and causes the intermediate transfer belt 21 to contact the photosensitive member 11 only during a transfer operation. A toner image is sequentially formed for each color, and the toner images superimposed on the intermediate transfer belt 21 are collectively transferred to the recording medium 6 by the secondary transfer roller 25 as the second transfer means, and then become a fixing means. The image is fixed by 50 and an image is formed. The secondary transfer roller 25 is also arranged so as to be able to come into contact with and separate from the intermediate transfer belt 21 and is in contact with the intermediate transfer belt 21 only during the transfer operation.

  In the transfer drum type image forming apparatus 1, the toner image of each color is sequentially transferred to the recording medium 6 electrostatically attracted to the transfer drum 21a. In the intermediate transfer type image forming apparatus 1 provided with the intermediate transfer belt 21, the toner images of the respective colors are superimposed on the intermediate transfer body 21, and thus the recording medium 6 is not limited. Such an intermediate transfer method can be applied to an image forming apparatus of any image method.

FIG. 9 is a schematic view showing another configuration of the image forming apparatus of the present invention.
This image forming apparatus is a so-called tandem type, and is a type that uses four colors of yellow (Y), magenta (M), cyan (C), and black (Bk) as toner, and an image forming unit is provided for each color. They are arranged in parallel.
In addition, photoconductors 11Y, 11M, 11C, and 11Bk for each color are provided. Around each of the photoreceptors 11Y, 11M, 11C, and 11Bk, charging devices 12Y, 12M, 12C, and 12Bk, exposure devices 19Y, 19M, 19C, and 19Bk, developing devices 13Y, 13M, 13C, and 13Bk, and cleaning devices 14Y and 14M , 14C, 14Bk, etc. are arranged. Further, a driving roller 211 having a conveying transfer belt 24 as a transfer material carrier that is in contact with and separating from each transfer position of each photoconductor 11Y, 11M, 11C, and 11Bk arranged on a straight line and a drive roller 211 is provided. It is stretched by a supporting roller 212 to be supported. Primary transfer rollers 23Y, 23M, 23C, and 23Bk are disposed at transfer positions that face the photoreceptors 11Y, 11M, 11C, and 11Bk with the conveyance transfer belt 24 interposed therebetween.

FIG. 10 is a schematic view showing another configuration of the image forming apparatus of the present invention.
This image forming apparatus is called a so-called tandem type, and is an intermediate transfer system having an intermediate transfer belt 21 as an intermediate transfer member. Photoconductors 11M, 11C, 11Y, and 11Bk for each color are provided. The toner images formed on the photoconductors 11M, 11C, 11Y, and 11Bk are transferred onto the intermediate transfer belt 21 by the primary transfer roller 23 disposed inside the intermediate transfer belt 21. The primary transfer rollers 23 arranged on the respective photoconductors 11M, 11C, 11Y, and 11Bk are arranged so as to be able to come into contact with and separate from the photoconductors 11M, 11C, 11Y, and 11Bk. 21 is brought into contact with the photoconductors 11M, 11C, 11Y, and 11Bk. Each color image is sequentially formed, and the toner images superimposed on the intermediate transfer belt 21 are collectively transferred to the recording medium 6 by the secondary transfer roller 25 and then fixed by the fixing device 50 to form an image. The secondary transfer roller 25 is also arranged so as to be able to come into contact with and separate from the intermediate transfer belt 21 and is in contact with the intermediate transfer belt 21 only during the transfer operation.
9 and 10 also has a surface potential measuring means 111, and performs the above-described transfer voltage control for each of the photoconductors 11M, 11C, 11Y, and 11Bk.

FIG. 11 is a schematic diagram showing the configuration of the image forming apparatus of the present invention.
The image forming operation of the image forming apparatus 1 of the present invention will be described. The main part of image formation is shown in FIG. It should be noted that the lowermost portion includes a paper feeding unit 2 for storing a recording medium 6 for forming an image, an image forming unit 3 for forming an image with an image forming unit 10 and the like, a scanner unit 4 for reading a document, and automatically conveying the document. An automatic document feeder (ADF) 5 is provided. The image forming unit 3 includes a transfer device 20 provided with an intermediate transfer belt 21 as an intermediate transfer member at the center. The intermediate transfer belt 21 is wound around a driving roller 211 that drives the intermediate transfer belt 21 by being driven by a motor (not shown) and support rollers 212 and 213 that support the intermediate transfer belt 21 among the three rollers. It can be rotated and conveyed clockwise in the figure.
In this illustrated example, a belt cleaning device 22 that removes residual toner remaining on the intermediate transfer belt 21 after image transfer is provided to the left of the support roller 213 among the three.
Among the three images, four images of yellow, magenta, cyan, and black are arranged on the intermediate transfer belt 21 stretched between the second support roller 212 and the third support roller 213 along the conveyance direction. The tandem type image forming unit 3 is configured by arranging the forming units 10Y, 10M, 10C, and 10Bk side by side. However, these four color orders are merely examples, and the present invention is not limited to these.
The individual image forming units 10Y, 10M, 10C, and 10Bk are detachably attached to the image forming apparatus 10 as process cartridges 10, and charging devices 12Y, 12M are disposed around the photoreceptors 11Y, 11M, 11C, and 11Bk. , 12C, 12Bk, developing devices 13Y, 13M, 13C, 13Bk, primary transfer roller 23 as primary transfer means, photoconductor cleaning devices 14Y, 14M, 14C, 14Bk, static eliminators 15Y, 15M, 15C, 15Bk, etc. It is prepared.

When a full-color image is copied using the color copying machine having the above configuration, a document is set on the document tray 31 of the automatic document feeder 5.
Alternatively, the automatic document feeder 5 is opened, a document is set on the contact glass 32 of the scanner 4, and the automatic document feeder 5 is closed and pressed by it.
When a start switch (not shown) is pressed, when a document is set on the automatic document feeder 5, the document is transported and moved onto the contact glass 32, and then the document is set on the other contact glass 32. Immediately, the scanner 4 is driven and the first traveling body 33 and the second traveling body 34 travel.
Then, the first traveling body 33 emits light from the light source and further reflects the reflected light from the document surface toward the second traveling body 34, and is reflected by the mirror of the second traveling body 34 and passes through the imaging lens 35. The document is placed in the reading sensor 36 and the original content is read.
Then, the photoconductors 11Y, 11M, 11C, and 11Bk are rotated by the process cartridges 10Y, 10M, 10C, and 10Bk serving as individual image forming units, and first, the charging device 12Y is rotated along with the rotation of the photoconductors 11Y, 11M, 11C, and 11Bk. , 12M, 12C, and 12Bk uniformly charge the surfaces of the photoreceptors 11Y, 11M, 11C, and 11Bk, and form a patch image for determining the transfer voltage VT.

Here, as shown in FIG. 10, the photosensitive members 11Y, 11M, 11C, and 11Bk in the process cartridges 10Y, 10M, 10C, and 10Bk are rotated, and the surface potential measuring unit 111 of each process cartridge 10 has a small area. An exposure unit that forms a patch and a non-exposure unit that is not exposed to laser light are repeatedly provided, and the transfer voltage VT is changed while capturing the positional relationship between the charging voltage of the charging device 12 and the exposure unit / non-exposure unit. Then, the graph shown in FIG. 6 is created by rotating the photoconductor 11 at least twice.
Then, while increasing or decreasing the transfer voltage VT by a predetermined value, at each transfer voltage VT, the charging potential VD1 after recharging the patch portion of the photoreceptor 11 on which the black solid image is formed, and no image is formed. The charged potential VD2 of the photoreceptor 11 after recharging is measured by the surface potential measuring means 111, and the value of the potential difference ΔVD potential difference ΔVD represented by the formula (I) is changed from positive to negative (or from negative to positive). The changing point is detected, and the optimum transfer voltage VT is determined by the equation (II), with the transfer voltages at two points before and after the detected point being VTi and VTj. Thereafter, the image forming apparatus 1 performs image formation while applying the determined transfer voltage VT from the constant voltage power supply 26 by a signal from a control unit (not shown).

Next, the photoconductors 11Y, 11M, 11C, and 11Bk are charged again in the same process, and then the laser or the exposure device 19 described above is used in accordance with the read contents of the scanner 4 to perform actual image formation. An electrostatic latent image is formed on the photoconductors 11Y, 11M, 11C, and 11Bk by irradiating writing light L from an LED or the like. Each of the photoconductors 11Y, 11M, 11C, and 11Bk on which the electrostatic latent images are formed is visualized by making the electrostatic latent images visible with toners attached thereto by the developing devices 13Y, 13M, 13C, and 13Bk. Monochromatic toner images of cyan, magenta, yellow, and black are formed on 11Y, 11M, 11C, and 11Bk, respectively. One of the support rollers 211, 212, and 213 is rotationally driven by a drive motor (not shown), the other two support rollers are driven to rotate, and the intermediate transfer belt 21 is rotated and conveyed. The toner images on the photoconductors 11Y, 11M, 11C, and 11Bk are sequentially transferred onto the intermediate transfer belt 21 by applying a constant voltage from the transfer power source 26 to the primary transfer roller 23. As a result, a toner image having no abnormal image is formed on the intermediate transfer belt 21, and a full-color image in which the toner images of the respective colors are superimposed is formed.
The surfaces of the photoconductors 11Y, 11M, 11C, and 11Bk after the image transfer are cleaned by removing residual toner with photoconductor cleaning devices 14Y, 14M, 14C, and 14Bk, and are neutralized with the static eliminators 15Y, 15M, 15C, and 15Bk. To prepare for another image formation.
On the other hand, when a start switch (not shown) is pressed, one of the paper feeding cassettes 41 in the paper feeding device 40 provided in the paper feeding unit 2 is selectively rotated, the recording medium 6 is fed out by the paper feeding roller 42, and the separation roller 45 Then, the sheets are separated one by one, put into the paper feed path 46, conveyed by the conveying roller 47 and abutted against the registration roller 49 and stopped.
Alternatively, the recording roller 6 is rotated to feed out the recording medium 6 on the manual feed tray 411, separated one by one by the separation roller 45, put into the manual feed path 46, and abutted against the registration roller 44 and stopped.
Then, the registration roller 44 is rotated in synchronization with the composite color image on the intermediate transfer belt 21, and the recording medium 6 is fed between the intermediate transfer belt 21 and the secondary transfer roller 25, and transferred by the secondary transfer roller 25. Thus, a color image is recorded on the recording medium 6.
The recording medium 6 after the image transfer is conveyed by the secondary transfer roller 25 and sent to the fixing device 50. The fixing device 50 applies heat and pressure to fix the transferred image, and then the switching medium is switched by the switching claw 491. The paper is discharged by the discharge roller 47 and stacked on the paper discharge tray 48.
Alternatively, it is switched by the switching claw 491 and put into the reverse conveying device 49, where it is reversed and guided again to the transfer position, and the image is recorded also on the back surface, and then discharged onto the discharge tray 48 by the discharge roller 47.

(Evaluation example)
Using a tandem type and intermediate transfer type full-color electrophotographic apparatus (Ricoh's Imagio MPC5000), black solid patches were image-formed under various conditions of transfer voltage, and the relationship between the transfer voltage and the potential difference ΔVD was measured. Further, evaluation images as shown in FIG. 1 were output at the respective transfer voltages, and afterimages were evaluated. The applied voltage of the charging roller was set so that the charging potential of the photosensitive member was −600 V, and the developing bias was −450 V. Moreover, static elimination is not performed.
The test environment is 23 ° C. and 55% RH.

Afterimage evaluation: An evaluation image having a solid black portion “R” and a halftone portion as shown in FIG. 1 was output to evaluate the afterimage. Evaluation performed rank evaluation.
The evaluation rank is as shown in Table 1.

Table 2 shows the results of the potential difference ΔVD and the rank evaluation.

(Example)
Using the apparatus used in the evaluation example, a printing durability test that prints 10,000 sheets in total using a 5% writing rate chart (characters equivalent to 5% as the image area are written on the entire A4 surface) Went. The voltage applied to the charging roller was set so that the charging potential of the photosensitive member was −600 V at the start of the test, and the developing bias was −450 V. In Table 2, the transfer voltage was set to 1000 V at which the absolute value of ΔVD was minimized. These conditions were tested without change until the end of the printing durability test and the afterimage evaluation after completion.
The test environment is 23 ° C. and 55% RH.
After the printing durability test, the relationship between the transfer voltage and the memory potential ΔVD and the afterimage evaluation were performed as in the evaluation example. At the same time, the measurement of the memory potential ΔVD and the afterimage evaluation were performed at the optimum transfer voltage value of 1029 V calculated by applying the formula (II) based on the results in Table 2.

Table 3 shows the results of ΔVD and rank evaluation.
Due to the change in the memory potential accompanying the electrostatic fatigue of the photoconductor during the printing durability test, a positive afterimage is generated at a transfer voltage of 1000 V, and the afterimage rank deteriorates by 2 points. However, if the transfer voltage is controlled to 1029 V, 1 Can be suppressed by worsening points. Thus, by setting the transfer voltage to an optimum value, even if the memory potential changes due to electrostatic fatigue of the photoreceptor, the occurrence of afterimage can be suppressed to a minimum. Further, afterimages can be further reduced by appropriately controlling the transfer voltage thereafter.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Paper feed part 3 Image forming part 4 Scanner part 5 Automatic document feeder (ADF)
6 Recording medium 10 Image forming means (process cartridge)
DESCRIPTION OF SYMBOLS 11 Photoconductor 111 Surface potential measurement means 12 Charging apparatus 13 Developing apparatus 14 Cleaning apparatus 15 Static elimination apparatus (organic EL)
DESCRIPTION OF SYMBOLS 19 Exposure apparatus 20 Transfer apparatus 21 Intermediate transfer belt 21a Transfer drum 211 Drive roller 212, 213 Support roller 22 Belt cleaning apparatus 23 Primary transfer roller 24 Conveyor belt 25 Secondary transfer roller 26 Transfer power supply 261 Transfer voltage control means 31 Document tray 32 Contact Glass 33 Lamp 34 First traveling body 35 Second traveling body 36 Lens 37 CCD
DESCRIPTION OF SYMBOLS 40 Paper feeder 41 Paper feed cassette 411 Manual feed tray 42 Pickup roller 43 Conveyance roller 44 Registration roller 45 Separation roller 46 Paper feed path 47 Discharge roller 48 Paper discharge tray 49 Reverse conveyance device 491 Switching claw 50 Fixing device

Japanese Patent Laid-Open No. 11-133825 JP 2006-251204 A JP 2006-78678 A JP 2003-295527 A JP 2008-224785 A

Claims (5)

An image carrier;
Charging means for charging the surface of the image carrier;
Surface potential measuring means for measuring the surface potential of the image carrier;
Image exposure means for exposing the surface of the image carrier to form an electrostatic latent image;
Developing means for visualizing the electrostatic latent image with a toner charged to the same polarity as the charged polarity of the image carrier and forming a toner image;
Transfer means for transferring a toner image by applying a charge opposite to the charge polarity of the toner to the recording medium;
In an image forming apparatus having a transfer voltage control means for controlling a transfer voltage applied to a transfer member of the transfer means,
The transfer voltage control means is a constant voltage control,
While increasing or decreasing the transfer voltage by a predetermined value, at each transfer voltage, the charging potential VD1 after recharging the surface area of the image carrier on which the black solid image is formed;
The charged potential VD2 after recharging of the surface area of the image carrier that did not form an image,
Measured by the surface potential measuring means,
When the sign of the potential difference ΔVD represented by the formula (I) changes,
If there are two transfer voltages VTi and VTj before and after the change, and the potential difference ΔVD at that time is ΔVDi and ΔVDj, the transfer voltage VT is controlled to a value represented by the formula (II),
An image forming apparatus, wherein when there is no positive / negative change in the potential difference ΔVD, the transfer voltage VT is controlled such that the absolute value of the potential difference ΔVD is minimized.
(I) ΔVD = VD1−VD2
(II) VT = (VTi × ΔVDj−ΔVDi × VTj) / (ΔVDj−ΔVDi) The image forming apparatus according to claim 1, wherein the transfer member is a transfer roller. The transfer means performs primary transfer of a toner image developed on an image carrier onto an intermediate transfer member, and then secondary transfer of the toner image on the intermediate transfer member onto a recording medium. The image forming apparatus according to 1 or 2. 4. The image forming apparatus according to claim 1, wherein the image forming apparatus includes a plurality of image carriers, and the toner images developed on the image carriers are sequentially superimposed to form a color image. The image forming apparatus described in 1. The image forming apparatus includes a process cartridge that integrally supports at least an image carrier and a surface potential measuring unit that measures a surface potential of the image carrier, and is detachable from the image forming apparatus. 5. The image forming apparatus according to any one of 4 to 4.