JP2010276976A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain the occurrence of a ghost image due to reverse transfer of a developer to a photoreceptor drum. <P>SOLUTION: A photoreceptor drum 51K for monochrome is provided upstream of a plurality of photoreceptor drums 51Y, 51M, 51C other than the photoreceptor drum for monochrome in the paper carrying direction. A control device 100 controls the absolute value of developing bias voltage applied to a developing roller corresponding to the photoreceptor drum 51K for monochrome to be smaller in monochromatic printing than in color printing, and controls a charger so that the absolute value of surface potential of the photoreceptor drum 51K for monochrome is smaller in monochromatic printing than in color printing. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus having monochrome printing and color printing modes.

  In an electrophotographic color image forming apparatus, toners of a plurality of colors are prepared, and a photosensitive drum, a developing roller, a transfer roller, and the like corresponding to each color are provided, and an intermediate transfer belt or a recording sheet (in this specification, an intermediate transfer belt is used). The recording sheets are collectively referred to as a “transfer medium”.) A toner image of each color is placed on the recording sheet.

  When performing monochrome printing in a color image forming apparatus, it is sufficient to use only monochrome (usually black) toner, and therefore, for other color developing apparatuses, the photosensitive drum and the developing roller are separated from each other. Therefore, there is known a configuration for suppressing the deterioration of the toner of these colors (Patent Document 1).

  On the other hand, since all the toner on the surface of the photosensitive drum cannot be transferred from the photosensitive drum to the transfer medium, a small amount of toner remains on the surface of the photosensitive drum. There is a need. Cleaning of the remaining toner is often performed by providing a cleaning blade or roller, but a method of collecting the toner by the developing roller and dispersing it in the toner in the developing device may be employed. Such a cleaning method is called a cleanerless method (see Patent Document 2).

JP 2009-3377 A JP-A-6-75484

  However, if a cleanerless system is used in an image forming apparatus configured to separate the photosensitive drum from a non-monochrome developing roller during monochrome printing, the monochrome toner placed on the transfer medium during monochrome printing is disposed downstream of the image forming apparatus. When the photosensitive drum is attached to the photosensitive drum (this is referred to as “reverse transfer”), the photosensitive drum is not collected by the developing roller due to the separation of the photosensitive drum and the developing roller. The toner reversely transferred to the drum may be transferred to the transfer medium again to generate a ghost image.

  The present invention has been made in view of the above background, and is configured to separate a developing roller other than a monochrome developing roller from a photosensitive drum during monochrome printing, and in a cleanerless type image forming apparatus, An object is to suppress the generation of a ghost image due to reverse transfer to a body drum.

  In order to achieve the above object, an image forming apparatus according to the present invention is an image forming apparatus having printing modes of monochrome printing and color printing, and includes at least a first photosensitive drum used for monochrome printing, and color printing. A plurality of second photosensitive drums used for the first and second photosensitive drums are provided corresponding to each of the first and second photosensitive drums, and in contact with the photosensitive drum during printing, a developer is supplied to each photosensitive drum. And a plurality of developing rollers for collecting the developer remaining on the surface of the photosensitive drum, and a charger for charging the photosensitive drum provided corresponding to each of the first and second photosensitive drums. A contact / separation mechanism for contacting or separating a developing roller corresponding to the plurality of second photosensitive drums with respect to the second photosensitive drum; and each of the first and second photosensitive drums. And a plurality of the second photoconductors for monochrome printing by controlling a transfer means for transferring a developer image on the photoconductor drum onto a transfer medium by a transfer bias and the contact / separation mechanism. Control means for separating the plurality of developing rollers corresponding to the drums from the second photosensitive drum, wherein the first photosensitive drum is the plurality of second photosensitive drums in the transfer direction of the transfer medium. The control means controls the absolute value of the developing bias voltage applied to the developing roller corresponding to the first photosensitive drum so that the absolute value during monochrome printing is smaller than during color printing. In addition, the charger is controlled so that the absolute value of the surface potential of the first photosensitive drum is smaller during monochrome printing than during color printing.

  As the amount of the developer transferred from the first photosensitive drum to the transfer medium increases, the second photosensitive drum (second and subsequent colors) disposed downstream of the first photosensitive drum becomes larger. However, according to the configuration of the present invention, the absolute value of the developing bias voltage applied to the developing roller is smaller in monochrome printing than in color printing. Therefore, the amount of developer loaded on the first photosensitive drum from the developing roller is smaller in monochrome printing than in color printing. For this reason, the amount of developer placed on the transfer medium is also smaller during monochrome printing than during color printing, and the developer is less likely to be reversely transferred to the second and subsequent photosensitive drums. Also, simply reducing the absolute value of the developing bias voltage applied to the developing roller during monochrome printing reduces the amount of developer supplied to fine parts such as fine lines, making it difficult to see fine lines. In the invention, the charger is controlled so that the absolute value of the surface potential of the first photosensitive drum is smaller during monochrome printing than during color printing, so that the amount of developer supplied to the thin line is increased. The change in is not much different from that during color printing, and fine lines can be printed easily. That is, at the time of monochrome printing, the developer concentration can be lowered in a dark portion close to solid printing, and the developer concentration can be made the same as that in color printing in a portion having a low gradation such as fine fine lines. It is possible to achieve both suppression of reverse transfer and ease of viewing the image.

  According to the present invention, at the time of monochrome printing, in a dark portion close to solid printing, by reducing the developer concentration, reverse transfer of the developer can be suppressed, thereby suppressing the occurrence of a ghost image.

1 is a side sectional view of an image forming apparatus according to an embodiment of the present invention. FIG. 6 is a diagram illustrating the separation between a photosensitive drum and a developing roller. It is a figure explaining application of the voltage from a control apparatus to a developing roller, a charger, and a transfer roller. 6 is a table showing (a) a transfer current in a color mode and (b) a transfer current in a monochrome mode. 6 is a table showing a developing bias voltage and a surface potential of a photosensitive drum in a color mode and a monochrome mode. A graph (a) for explaining the formation width of the toner image on the electrostatic latent image of the fine line on the photosensitive drum when only the developing bias voltage is reduced, and the surface potential of the photosensitive drum while reducing the developing bias. 6 is a graph (b) for explaining the formation width of a toner image on an electrostatic latent image of a fine line on the photosensitive drum in a case where the value is lowered. It is a graph which shows the output image density for every input image density in this embodiment.

  Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. In the following description, the direction will be described with reference to the user when using the color printer. That is, in FIG. 1, the left side toward the paper surface is “front side”, the right side toward the paper surface is “rear side”, the rear side toward the paper surface is “left side”, and the front side toward the paper surface is “right side”. To do. In addition, the vertical direction toward the page is defined as the “vertical direction”.

<Overall configuration of color printer>
As shown in FIG. 1, the color printer 1 includes, in the apparatus main body 2, a paper feeding unit 20 that supplies paper P as an example of a recording sheet, and an image forming unit 30 that forms an image on the fed paper P. A paper discharge unit 90 that discharges the paper P on which an image is formed, and a control device 100 as an example of a control unit.

  An opening 2 </ b> A is formed in the upper part of the apparatus main body 2. The opening 2A is opened and closed by an upper cover 3 that is rotatably supported by the apparatus main body 2. The upper surface of the upper cover 3 is a paper discharge tray 4 that accumulates the paper P discharged from the apparatus main body 2, and a plurality of LED attachment members 5 that hold the LED units 40 are provided on the lower surface.

  The paper feeding unit 20 is provided in the lower part of the apparatus main body 2, and a paper feeding tray 21 that is detachably attached to the apparatus main body 2, and a paper supply mechanism that conveys the paper P from the paper feeding tray 21 to the image forming unit 30. 22 is mainly provided. The paper supply mechanism 22 is provided on the front side of the paper feed tray 21 and mainly includes a paper feed roller 23, a separation roller 24, and a separation pad 25.

  In the paper feed unit 20 configured in this way, the paper P in the paper feed tray 21 is separated one by one and sent upward, and passes through between the paper dust removal roller 26 and the pinch roller 27 in the process. After the powder is removed, the direction is changed backward through the conveyance path 28 and supplied to the image forming unit 30.

  The image forming unit 30 mainly includes four LED units 40, four process cartridges 50, a transfer unit 70, a cleaning unit 10, and a fixing unit 80.

  The LED unit 40 is swingably connected to the LED mounting member 5, and is appropriately positioned and supported by a positioning member provided in the apparatus main body 2.

The process cartridge 50 is arranged side by side between the upper cover 3 and the paper feeding unit 20 in the front-rear direction, and a photosensitive drum 51 on which an electrostatic latent image is formed, a charger 52, a developing roller 53, and a developer. As an example, a toner storage chamber 54 that stores toner is provided.
In the process cartridge 50, those indicated by reference numerals 50K, 50Y, 50M, and 50C containing toners for black, yellow, magenta, and cyan are arranged in this order from the upstream side in the conveyance direction of the paper P. ing. In the present specification and drawings, when the photosensitive drum 51, the charger 52, the developing roller 53, and the transfer roller 74 corresponding to the color of the toner are specified, they are associated with black, yellow, magenta, and cyan, respectively. The symbols K, Y, M, and C are attached.

  As shown in FIGS. 2 and 3, the photosensitive drum 51 includes a photosensitive drum 51 </ b> K as an example of a first photosensitive drum and a photosensitive drum as an example of a second photosensitive drum in accordance with the corresponding colors. There are body drums 51Y, 51M, 51C.

  As shown in FIGS. 2 and 3, the developing roller 53 includes developing rollers 53K, 53Y, 53M, and 53C corresponding to the corresponding colors. As shown in FIG. 2, the developing roller 53 controls a known contact / separation mechanism 110 (similar to the switching mechanism described in Patent Document 1) by the control device 100, thereby performing color printing (color mode). In FIG. 5, all the developing rollers 53K, 53Y, 53M, and 53C come into contact with the corresponding photosensitive drums 51K, 51Y, 51M, and 51C, and supply toner to the respective photosensitive drums 51K, 51Y, 51M, and 51C. However, during monochrome printing (monochrome mode), only the black developing roller 53K is in contact with the photosensitive drum 51K, and the other three-color developing rollers 53Y, 53M, and 53C are associated with the corresponding photosensitive drum 51Y. , 51M, 51C.

  The transfer unit 70 is provided between the paper feeding unit 20 and each process cartridge 50, and mainly includes a drive roller 71, a driven roller 72, a conveyance belt 73, and a transfer roller 74 as an example of a transfer unit.

  The driving roller 71 and the driven roller 72 are arranged in parallel with a space in the front-rear direction, and a conveyance belt 73 formed of an endless belt is stretched between them. The outer surface of the conveyance belt 73 is in contact with each photosensitive drum 51. In addition, four transfer rollers 74 that sandwich the conveyor belt 73 with the respective photosensitive drums 51 are arranged inside the conveyor belt 73 so as to face the respective photosensitive drums 51. A transfer bias is applied to the transfer roller 74 by constant current control during transfer.

  The fixing unit 80 is disposed on the rear side of each process cartridge 50 and the transfer unit 70, and includes a heating roller 81 and a pressure roller 82 that is disposed to face the heating roller 81 and presses the heating roller 81.

  In the image forming unit 30 configured as described above, in the color mode, first, the surface of each photosensitive drum 51 is uniformly charged by the charger 52 and then exposed by each LED unit 40. As a result, the potential of the exposed portion is lowered, and an electrostatic latent image based on the image data is formed on each photosensitive drum 51. Thereafter, toner is supplied to the electrostatic latent image from the developing roller 53, so that the toner image is carried on the photosensitive drum 51.

  The paper P supplied onto the conveyor belt 73 passes between the photosensitive drums 51 and the transfer rollers 74 disposed inside the conveyor belt 73, whereby the toner formed on the photosensitive drums 51. The image is transferred onto the paper P. Then, as the paper P passes between the heating roller 81 and the pressure roller 82, the toner image transferred onto the paper P is thermally fixed.

  The paper discharge unit 90 mainly includes a paper discharge side transport path 91 formed so as to extend upward from the exit of the fixing unit 80 and to be reversed forward, and a plurality of pairs of transport rollers 92 that transport the paper P. ing. The paper P on which the toner image has been transferred and heat-fixed is transported along a paper discharge side transport path 91 by a transport roller 92, discharged outside the apparatus main body 2, and stored in the paper discharge tray 4.

  The control device 100 includes a CPU, a ROM, a RAM, and the like, and receives print data and controls the paper feed unit 20, the image forming unit 30, the paper discharge unit 90, and the contact / separation mechanism 110 according to a program prepared in advance. It is configured as follows.

<Control of transfer bias>
Next, control of the transfer bias applied to the transfer roller 74 by the control device 100 will be described.
In this embodiment, a positively charged toner is described as an example, but the present invention can be similarly applied to a negatively charged toner. The polarity of the transfer bias is appropriately set according to the charging polarity of the toner.

  The transfer bias is a voltage applied between the photosensitive drum 51 and the transfer roller 74 in order to transfer the toner image on the photosensitive drum 51 onto the paper P conveyed on the conveying belt 73. In this embodiment, the transfer bias is controlled at a constant current so that the transfer current (current flowing through the photosensitive drum 51 and the transfer roller 74) is constant. However, the transfer bias control method is not limited to constant current control, and may be performed by constant voltage control.

  When printing in the color mode, as shown in FIG. 4A, black (K) is controlled at a constant current of 10 μA, yellow (Y) is 8 μA, magenta (M) is 10 μA, and cyan (C) is 10 μA. . These target current values are optimally set in consideration of the ease with which the toner of each color is charged and the coloring power after printing. Even in the constant current control, normally, in order to maintain a high current, it is necessary to increase the potential difference between the photosensitive drum 51 and the transfer roller 74. That is, it can be understood that the potential difference between the photosensitive drum 51 of each color and the transfer roller 74 is approximately a value corresponding to the target current value shown in FIG.

  When printing in the monochrome mode, as shown in FIG. 4B, constant current control is performed so that black (K) is 8 μA, yellow (Y) is 20 μA, magenta (M) is 5 μA, and cyan (C) is 5 μA. . That is, compared with the color mode, yellow, which is the next color after black, has a large transfer bias, and magenta and cyan, which are the third and fourth colors in order from black, have a small transfer bias.

  The reason for this will be explained. First, the reason that the transfer bias of the photosensitive drum 51Y of the next color of the black toner, which is a monochrome toner, is large is that toner particles with low chargeability are easily transferred back to the photosensitive drum of the second color. Therefore, it is necessary to suppress reverse transfer by increasing the electric field from the photosensitive drum 51Y toward the transfer roller 74Y.

  On the other hand, if all the transfer biases for the second and subsequent colors are increased, the toner is gradually and strongly charged each time it passes between the photosensitive drum 51 and the transfer roller 74 (this is referred to as “charge-up”). ), Discharge between toner particles and between toner and paper due to overcharging may occur, and negatively charged toner may be generated. When negatively charged toner is generated in this way, reverse transfer of toner occurs in the third and subsequent colors. Therefore, in the third and subsequent colors, the transfer bias is set lower than in the color mode to charge up the toner. Is suppressed.

  Note that black has a smaller transfer current in the monochrome mode than in the color mode. In the black photosensitive drum 51K of the present embodiment, the potential of the non-exposed portion in the monochrome mode is lower than the potential of the non-exposed portion in the color mode, but the same potential is applied to the exposed portion. is there. For this reason, in order to transfer the developer image to the transfer medium, the transfer bias potential having the same potential (for example, −1000 V) may be applied when the exposed portion is used as a reference. In that case, the potential difference between the non-exposed portion potential and the transfer bias potential in the monochrome mode is smaller than the potential difference between the non-exposed portion potential and the transfer bias potential in the color mode, and the flowing current is reduced. Can do.

  There are also the following reasons. When the leading edge of the sheet P enters the nip position between the photosensitive drum 51K and the transfer roller 74K, the transfer current becomes uneven between the portion where the sheet P is present and the portion where the sheet P is not present. As a result, the surface potential of the photosensitive drum 51K is increased. May be non-uniform. This non-uniformity of the surface potential also affects the surface potential of the photosensitive drum 51K during the subsequent development, but this effect increases as the transfer current increases. In particular, in this embodiment, the surface potential after charging of the photosensitive drum 51K in the monochrome mode is set to be smaller than that in the color mode. Therefore, the potential difference between the exposed portion and the non-exposed portion is small, and it is easily affected by the leading edge of the paper P. Therefore, in such a case, it is preferable to make the transfer current in the monochrome mode smaller than the transfer current in the color mode in order to suppress the nonuniformity of the surface potential of the photosensitive drum 51K.

<Development bias voltage control>
Next, the control of the developing bias voltage applied to the developing roller 53 by the control device 100 will be described.
The developing bias voltage is a voltage applied to the developing roller 53, and is controlled to a voltage having an absolute value smaller in the monochrome mode than in the color mode. For example, as shown in FIG. 5, 450 V is applied in the color mode, and 300 V is applied in the monochrome mode. Thereby, in the monochrome mode, the amount of toner transferred from the developing roller 53 to the photosensitive drum 51 is smaller than that in the color mode. The reason for setting the developing bias voltage low in the monochrome mode in this way is that when a large amount of toner is loaded on the paper P, the toner is easily reversely transferred from the paper P to the photosensitive drums 51Y, 51M, and 51C for the second and subsequent colors. Because.

<Control of surface potential of photosensitive drum>
Next, control of the surface potential of the photosensitive drum 51 by the charger 52 by the control device 100 will be described.
The surface potential of the photosensitive drum 51 is controlled by a voltage applied to the charger 52. In the present embodiment, as shown in FIG. 5, the surface potential of the photosensitive drum 51 is controlled to 700 V in the color mode and to 550 V in the monochrome mode. This is because, as will be understood in the operation of the color printer 1 to be described later, it is possible to express a thin line or fine point with toner having a sufficient width. In the column of the surface potential of the photosensitive drum in FIG. 5, the value in parentheses indicates the surface potential after exposure by the LED unit 40.

<Description of operation>
[In color mode]
The operation of the color printer 1 having the above configuration will be described.
When the print data is a color image, the color printer 1 that has received the print data has developing rollers 53K, 53Y, 53M, and 53C corresponding to the colors black, yellow, magenta, and cyan as shown in FIG. Are brought into contact with the photosensitive drums 51K, 51Y, 51M, and 51C, respectively. The surface of each photosensitive drum 51 is charged to 700 V by the charger 52, and light corresponding to the print data is emitted from the LED unit 40, so that the surface of the photosensitive drum 51 has a relatively low potential of 150V. An electrostatic latent image is formed. Then, each developing roller 53 faces each photoconductor drum 51K, 51Y, 51M, 51C, so that each color toner is supplied to the electrostatic latent image to form each color toner image. When this toner image is formed, a developing bias voltage of 450 V is applied to the developing roller 53, and a sufficient amount of toner moves from the developing roller 53 to the photosensitive drum 51 to form a toner image. Then, the toner images on the photosensitive drums 51K, 51Y, 51M, and 51C are transferred onto the paper P. The transfer current at this time is controlled at a constant current so as to be the value shown in FIG.

  Residual toner that could not be transferred from the photosensitive drum 51 of each color to the paper P passes through the front of the charger 52 while remaining on the surface of the photosensitive drum 51. When passing in front of the charger 52, the surface potential of the photosensitive drum 51 becomes as high as 700V. In the present embodiment, since the toner is positively charged and easily moves from the higher potential to the lower potential, the remaining toner on the surface of the photosensitive drum 51 faces the developing roller 53 and then the developing roller 53. It is recovered.

  The toner image transferred onto the paper P is fixed on the paper P by the fixing unit 80, and is discharged onto the paper discharge tray 4 through the paper discharge unit 90.

[In monochrome mode]
When the print data received by the color printer 1 is a monochrome image, the control device 100 controls the contact / separation mechanism 110 to develop the developing rollers 53Y, corresponding to each of yellow, magenta, and cyan, as shown in FIG. 53M and 53C are separated from the photosensitive drums 51Y, 51M and 51C, respectively, and only the developing roller 53K corresponding to black is brought into contact with the corresponding photosensitive drum 51.

  In the black process cartridge 50K, unlike the color mode, the surface of the photosensitive drum 51K is charged to 550 V by the charger 52 and then exposed by the LED unit 40. Then, a developing bias voltage of 300 V lower than that in the color mode is applied to the developing roller 53K, and toner is supplied from the developing roller 53K to the electrostatic latent image on the photosensitive drum 51K.

  At this time, the amount of toner supplied to the photosensitive drum 51K is reduced by lowering the developing bias voltage of the developing roller 53 to 300 V. As a result, the sheet P is reversed to the photosensitive drums 51Y, 51M, and 51C for the second and subsequent colors. The amount of toner to be copied is reduced. However, if the developing bias voltage of the developing roller 53K is simply lowered while keeping the surface potential of the photosensitive drum 51K as high as 700V, sufficient toner cannot be supplied to thin lines or fine points. This will be described with reference to FIG.

  FIG. 6 shows the surface potential of the photosensitive drum 51K when, for example, a portion corresponding to a thin line is exposed when exposure is performed for an extremely short time. As shown in FIG. 6A, when the surface potential of the photosensitive drum 51 is 700V and the developing bias voltage of the developing roller 51 is 450V (the same as in the color mode), the developing roller 53K and the photosensitive drum 51K face each other. Since the toner moves to a range where the potential drops below 450V, a thin line having a width B1 in the range of the thick line shown at the position of 450V is formed. On the other hand, when the developing bias voltage is 300V, the toner moves to a range where the potential is lowered to less than 300V, so that a thin line having a width B2 in the thick line range indicated by the position of 300V is formed. That is, when the developing bias voltage is lowered to 300 V, the thickness of the thin line is reduced.

  However, as shown in FIG. 6B, if the surface potential of the photosensitive drum 51K is set to 550 V, which is lower than that in the color mode, as the developing bias voltage is lowered to 300 V, the position of 300 V in FIG. As indicated by the thick line, the toner can be transferred to the range of the width B3 similar to the thick line at the position of 450V in FIG.

  The output image density in the color printer 1 of this embodiment will be described with reference to FIG. In FIG. 7, the broken line indicates the output image density in the color mode, and the solid line indicates the output image density in the monochrome mode. As shown in FIG. 7, when the input image density (exposure area ratio) is high (close to solid printing), the color printer 1 of the present embodiment realizes an output image density lower than that of the color mode in the monochrome mode. For an image with a low input image density (thin lines or fine dots), a sufficient output image density is realized as in the color mode. As a result, in the monochrome mode, the amount of toner reversely transferred from the paper P to the photosensitive drums 51Y, 51M, and 51C for the second and subsequent colors by reducing the amount of toner in a portion where the input image density is high is reduced. In addition, it is possible to obtain an image that is easy to view by ensuring a sufficient output image density in a portion where the input image density is low, such as a fine point. The reason why it is easy to see such a density pattern is that the apparent density greatly contributes to the exposure area ratio at a low density, and the toner amount greatly contributes to a high density.

  Next, the operation of the process cartridges 50 other than black will be described. In the process cartridges 50Y, 50M, and 50C other than black, the three-color developing rollers 53Y, 53M, and 53C are associated with the corresponding photosensitive drums 51Y and 51M. 51C, the photosensitive drums 51Y, 51M, and 51C are different from the color mode in that the remaining toner on the surface of the photosensitive drums 51Y, 51M, and 51C becomes the developing rollers 53Y, 53M, and 51C. It is not collected by 53C. For this reason, when reverse transfer of toner from the paper P to the photosensitive drums 51Y, 51M, and 51C occurs, a ghost image may be generated on the paper P because the reversely transferred toner is placed on the paper P again.

In the color printer 1 of the present embodiment, the transfer bias applied between the photosensitive drum 51 and the transfer roller 74 is controlled at a constant current so as to be the transfer current shown in FIG.
That is, the transfer current of yellow arranged next to black in the transport direction of the paper P is 8 μA in the color mode, but is controlled to 20 μA in the monochrome mode. For this reason, even if there are toner particles on the paper P that are difficult to be charged due to variations in charging performance, they are applied between the photosensitive drum 51Y corresponding to the second color yellow and the transfer roller 74Y. Due to the strong transfer bias, the sheet P is strongly moved. Thereby, reverse transfer of toner from the paper P to the photosensitive drum 51Y corresponding to yellow is suppressed.

  In the toner cartridges for the third and subsequent colors, since the transfer current is set to a small value of 5 μA, excessive toner charge-up is suppressed, and reverse transfer of negatively charged toner due to discharge between toner particles is suppressed. The

  As described above, according to the color printer 1 of the present embodiment, since the transfer bias for the second color in the monochrome mode is made larger than that in the color mode, the toner is attracted to the paper P in the process cartridge 50Y for the second color. The reverse transcription of is suppressed. Further, since the developing bias voltage applied to the developing roller 53K corresponding to black is controlled so that the absolute value is smaller in the monochrome mode than in the color mode, the amount of toner supplied to the portion where the input image density is high is reduced. Thus, the reverse transfer of the toner in the second and subsequent colors can be suppressed. By suppressing the reverse transfer, it is possible to suppress the generation of a ghost image. Further, since the surface potential of the photosensitive drum 51K corresponding to black is controlled to be smaller in the monochrome mode than in the color mode, sufficient toner is supplied in a portion where the input image density is low. In addition, it is possible to maintain the visibility of fine images such as fine lines.

  Further, in the process cartridges 50M and 50C for the third and subsequent colors counted from black, the transfer bias in the monochrome mode is made smaller than that in the color mode, thereby preventing excessive charge-up of the toner and the toner to be originally used. It is possible to suppress the generation of toner that is charged in reverse to the charged polarity (the generation of negatively charged toner in this embodiment), and to suppress reverse transfer of the toner.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and can be modified as appropriate. For example, five or more developers may be prepared and five or more photosensitive drums 51 may be provided.

  In the above embodiment, a color printer is exemplified as the image forming apparatus, but the present invention can also be applied to a multifunction machine or a copier.

  In the above embodiment, only the case where the transfer bias for the third and subsequent colors is lower in the monochrome mode than in the color mode has been described, but the transfer bias in the color mode and the monochrome mode is the same, or the monochrome mode is higher. The case of transfer bias is also included in the present invention.

  In the embodiment, the paper P is exemplified as the transfer medium. However, the transfer medium may be an intermediate transfer belt.

DESCRIPTION OF SYMBOLS 1 Color printer 20 Paper feed part 30 Image formation part 40 LED unit 50 Process cartridge 51 Photosensitive drum 52 Charger 53 Developing roller 70 Transfer unit 73 Conveyor belt 74 Transfer roller 80 Fixing unit 90 Paper discharge part 100 Controller 110 Contact / separation mechanism P paper

Claims (2)

An image forming apparatus having printing modes for monochrome printing and color printing,
A first photosensitive drum used for at least monochrome printing;
A plurality of second photosensitive drums used for color printing;
A developer provided corresponding to each of the first and second photosensitive drums, contacting the photosensitive drum at the time of printing, supplying the developer to each photosensitive drum, and the developer remaining on the surface of the photosensitive drum A plurality of developing rollers for collecting,
A charger for charging a photosensitive drum provided corresponding to each of the first and second photosensitive drums;
A contact / separation mechanism for contacting or separating a developing roller corresponding to the plurality of second photosensitive drums with respect to the second photosensitive drum;
Transfer means provided corresponding to each of the first and second photosensitive drums, and transferring a developer image on the photosensitive drum onto a transfer medium by a transfer bias;
Control means for controlling the contact / separation mechanism to separate the plurality of developing rollers corresponding to the plurality of second photosensitive drums from the second photosensitive drum during monochrome printing;
The first photosensitive drum is provided upstream of the plurality of second photosensitive drums in the transport direction of the transfer medium,
The control unit controls the absolute value of the developing bias voltage applied to the developing roller corresponding to the first photosensitive drum so that the absolute value during monochrome printing is smaller than that during color printing. An image forming apparatus, wherein the charger is controlled so that the absolute value of the surface potential of the photosensitive drum is smaller in monochrome printing than in color printing.   2. The control unit according to claim 1, wherein the control unit controls the transfer bias corresponding to the first photosensitive drum so that the transfer bias is smaller during monochrome printing than during color printing. Image forming apparatus.

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