JP2011113007A - Image forming apparatus - Google Patents

Abstract

Monochrome printing is possible even when there is a non-mounted color photoconductor drum during monochrome printing, and the problem caused by the transfer bias applied corresponding to the non-mounted color photoconductor drum is solved. To do.
Sensors SY and SM for detecting mounting or non-mounting of a photosensitive drum 51K used for monochrome printing, color photosensitive drums 51Y, 51M, and 51C used for color printing, and a color photosensitive drum, respectively. , SC, transfer rollers 74K, 74Y, 74M, and 74C that transfer the developer image on the photosensitive drum onto the paper P by a transfer bias, and a control device 100 that controls a transfer current flowing through the transfer roller. 1, the control device 100 controls the flow of a transfer current only to the transfer roller corresponding to the photosensitive drum 50K for monochrome printing and the transfer roller corresponding to the color photosensitive drum detected by the sensor during monochrome printing. I do.
[Selection] Figure 1

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.

In such a color image forming apparatus, in order to enable monochrome printing even when the color toner is used up, a configuration that can perform monochrome printing even when the color drum unit is not attached may be employed ( For example, see Patent Document 1).
In such an image forming apparatus, a transfer bias is also applied to a transfer roller corresponding to a non-mounted color drum unit to suppress current leakage that occurs when the transfer medium passes through the image forming unit.

JP 2004-45698 A

However, when a transfer bias is applied to the transfer roller corresponding to the non-mounted color drum unit, there is no photosensitive drum to which the transfer current flows, so that if the resistance value of the transfer medium is low, the transfer medium is transmitted to the other transfer drum. An electric current may flow through the photosensitive drum. Then, there is a possibility that the photosensitive drum is damaged due to a large current flowing.
Further, when a current flows from a transfer roller corresponding to a non-mounted color drum unit during monochrome printing to a black photosensitive drum, there is a possibility that transfer efficiency is lowered and image defects are caused.

  The present invention has been made in view of the above-described background. In an image forming apparatus capable of performing monochrome printing even when there is a non-mounted color photosensitive drum during monochrome printing, the non-mounted color photosensitive drum is used. The object is to solve the problems caused by the correspondingly applied transfer bias.

  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 above, a detecting means for detecting whether each of the plurality of second photosensitive drums is mounted or not, and each of the first and second photosensitive drums. A transfer unit provided correspondingly and configured to transfer a developer image on the photosensitive drum onto a transfer medium by a transfer bias; and a control unit configured to control a transfer current flowing through the transfer unit. During monochrome printing, a transfer current is applied to the transfer means corresponding to the first photosensitive drum and the transfer means corresponding to the second photosensitive drum whose attachment is detected by the detection means. And, non-wearing the transfer means corresponding to the second photosensitive drum is detected and performing a control that does not apply a transfer bias by said detecting means.

  Note that as a characteristic of the image forming apparatus, a small amount of current of about 1 μA flows through the transfer unit corresponding to the second photosensitive drum that is not mounted, even though the control unit performs control without applying the transfer bias. There is. The present invention includes such an image forming apparatus.

  According to the configuration of the present invention, the control unit transfers to the transfer unit corresponding to the first photoconductive drum and the transfer unit corresponding to the second photoconductive drum detected by the detection unit during monochrome printing. Since a control is performed so as not to apply a transfer bias to the transfer means corresponding to the second photosensitive drum that has been detected to be non-mounted by the detection means by applying an electric current, the transfer bias is applied corresponding to the non-mounted color photosensitive drum. It is possible to solve various problems caused by the transfer bias, such as damage to other mounted photoconductive drums, reduction in transfer efficiency during monochrome printing, and image defects.

  According to the present invention, there is no occurrence of damage to other photoconductor drums due to a transfer bias applied corresponding to a non-mounted color photoconductor drum, a decrease in transfer efficiency during monochrome printing, or an image defect. An image forming apparatus can be provided.

1 is a side sectional view of an image forming apparatus according to an embodiment of the present invention. It is a figure explaining application of the voltage from a control apparatus to a developing roller, a charger, and a transfer roller. It is a figure explaining the structure which detects mounting | wearing or non-mounting of each process unit. It is a table | surface which shows the transfer current in color mode. (A) Transfer current arrangement table in monochrome mode, (b) Transfer current during monochrome printing with all process units mounted, and (c) During monochrome printing with yellow process units not mounted It is a table | surface explaining the transfer current. It is a flowchart which shows the printing process 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 has monochrome printing and color printing modes, and a paper feeding unit 20 that supplies paper P as an example of a recording sheet into the apparatus main body 2. 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 are provided.

  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 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 by the paper supply mechanism 22 and sent upward, and passes between the paper dust removing roller 26 and the pinch roller 27. After the paper dust is removed in the process of passing, the paper is redirected 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 coupled to the LED mounting member 5 and is appropriately positioned and supported by a positioning member (not shown) provided in the apparatus main body 2.

The process cartridge 50 is arranged side by side in the front-rear direction between the upper cover 3 and the paper feeding unit 20, and the photosensitive drum 51, the charger 52, the developing roller 53, and the developer each forming an electrostatic latent image. 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, in particular, the process cartridges 50Y, 50M, and 50C corresponding to yellow, magenta, and cyan may be referred to as the color process cartridge 50. 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 FIG. 3, the color printer 1 detects whether or not each process cartridge 50K, 50Y, 50M, or 50C is attached or not as a detection means, and transmits a signal indicating attachment or non-attachment to the control device 100. Sensors SK, SY, SM, and SC are provided corresponding to each process cartridge 50.
As such a sensor, a known toner remaining amount detection sensor may be used to detect whether each process cartridge 50 is attached or not, or a known sensor may be provided separately.

  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.

  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 from the developing roller 53 corresponding to the electrostatic latent image, so that the toner image is carried on each photosensitive drum 51.

  When the paper P supplied on the conveyor belt 73 passes between each photosensitive drum 51 and each transfer roller 74 disposed inside the conveyor belt 73, a transfer bias is applied to each photosensitive drum 51. The formed toner 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 is configured to receive print data and control the paper feed unit 20, the image forming unit 30, and the paper discharge unit 90 according to a program prepared in advance. Yes.

<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.

[1. Color printing]
When printing in the color mode, as shown in FIG. 4, 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.

[2. Monochrome printing]
When printing in the monochrome mode, constant current control is performed as in the color mode. However, in the monochrome mode, the target current value for each color is set according to the arrangement table shown in FIG.
In the arrangement table of FIG. 5A, the reference number “0” corresponds to the non-mounted process cartridge 50, and in this case, a target current value of 0 μA is set.
On the other hand, reference numerals “1” to “4” correspond to the process cartridge 50 mounted on the apparatus main body 2 and indicate the order from the most upstream side with reference to the transport direction of the paper P.
Accordingly, when all the process cartridges 50K, 50Y, 50M, and 50C are mounted, the target current values corresponding to the reference numbers “1” to “4” are set, respectively. If some or all of the color process cartridges 50 are not mounted, a target current value of 0 μA is set for the unmounted process cartridges 50 and the paper P is loaded on each mounted process cartridge 50. A predetermined target current value is set in order from the most upstream side in the transport direction.

[3. Monochrome printing with all process cartridges installed]
When process cartridges 50 of all colors are mounted, as shown in FIG. 5B, black (K) is 8 μA (first current), yellow (Y) is 15 μA (second current), Constant current control is performed so that magenta (M) is 5 μA (third current) and cyan (C) is 5 μA (third current). 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 called “charge-up”). .) After passing through the transfer of the second color, the amount of low-charged toner is reduced by charge-up. However, discharge between toner particles and between toner and paper due to excessive charging 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 to be smaller than that in the second color, thereby charging up the toner. Is suppressed.

[4. Black and white printing when the color process cartridge is not installed]
For example, when the yellow process cartridge 50Y is not attached, as shown in FIG. 5C, the target current value corresponding to yellow is set to 0 μA. Then, counting from the most upstream side in the transport direction of the paper P, the first color black (K) is 8 μA (first current), the second color magenta (M) is 15 μA (second current), 3 The cyan (C) of the color is controlled to a constant current of 5 μA (third current).

The reason for this will be described. When a transfer bias is applied to the transfer roller 74 corresponding to the unmounted process cartridge 50, if the resistance value of the sheet P is low, the sheet P having a low resistance value is transmitted from the transfer roller 74. This is because current may flow through the other photosensitive drums 51, and as a result, the photosensitive drums 51 may be damaged. Further, if a current flows from the transfer roller 74 corresponding to the non-attached process cartridge 50 to the black photosensitive drum 51K, there is a possibility that the transfer efficiency may be reduced or an image defect may be caused.
In view of this, the transfer roller 74 corresponding to the non-mounted process cartridge 50 is controlled so as not to apply a transfer bias, thereby preventing damage to the other photosensitive drums 51, deterioration in transfer efficiency during monochrome printing, and image defects. ing.

On the other hand, as described above, in order to prevent toner particles having low chargeability from being reversely transferred to the photosensitive drum 51 of the second color, the transfer bias of the photosensitive drum 51M of the next color of the black toner is set large. The
Further, in order to suppress toner charge-up, the transfer bias is set to be smaller than that of the second color in the photosensitive drum 51C for the third and subsequent colors counted from the most upstream side in the transport direction of the paper P.

  When all the color process cartridges 50 are not attached, a target current value of 8 μA is set at the transfer position of black (K), and a target current value of 0 μA is set at the transfer positions of other colors. The

<Description of operation>
The operation of the color printer 1 configured as described above will be described with reference to FIG.

[Color printing mode]
When the color printer 1 receives the print data, the control device 100 determines whether the print data is a monochrome image or a color image (S1). If the image is a color image (S1: No), it is determined whether all of the process cartridges 50K, 50Y, 50M, and 50C corresponding to the respective colors of black, yellow, magenta, and cyan are mounted (S2). Specifically, signals indicating whether the corresponding process cartridges 50K, 50Y, 50M, and 50C are attached or not are transmitted from the sensors SK, SY, SM, and SC to the control device 100, and any one of the process cartridges 50 is detected. If it is not attached (S2: No), the color printer 1 displays an error by the control device 100 (S7).

  When mounting of all process cartridges 50 is detected (S2: Yes), color printing is performed under constant current control so that the transfer current corresponding to the transfer position of each color becomes the value shown in FIG. 4 (S3). ).

  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.

[Monochrome printing mode (when all process cartridges are installed)]
When the print data received by the color printer 1 is a monochrome image (S1: Yes), the control device 100 sets the variable A indicating the transfer position counted from the most upstream side in the transport direction of the paper P to 1 which is an initial value. (S4).
In the color printer 1 of this embodiment, when the transfer position A is 1, black (K), when the transfer position A is 2, yellow (Y), when the transfer position A is 3, magenta (M), transfer When the position A is 4, cyan (C) is shown.
Next, the control device 100 sets the variable B representing the reference number of the array table shown in FIG. 5A to 1 that is an initial value (S5).

Then, the control device 100 determines whether or not the process cartridge 50K is mounted at the transfer position A (= 1, that is, the black (K) position) (S6).
When the black process cartridge 50K is not attached (S6: No), monochrome printing cannot be performed, and the color printer 1 displays an error by the control device 100 (S7).
When the black process cartridge 50K is mounted (S6: Yes), the control device 100 sets the value of the variable B in the memory (RAM) corresponding to the black (K) transfer position A (= 1). 1 is input (S8). As a result, 8 μA (first current) is set as a target current value to be passed to the black (K) transfer position.
Thus, in the table shown in FIG. 5B, the transfer position, table reference number, and current value (μA) fields corresponding to black (K) are input.

Next, the control device 100 increments the variable A and the variable B (S9, S10).
Then, the control device 100 determines whether or not the process cartridge 50Y is mounted at the transfer position A (= 2, that is, the yellow (Y) position) (S11).
Here, since the yellow process cartridge 50Y is mounted (S11: Yes), the control device 100 stores the value of the variable B in the memory (RAM) corresponding to the transfer position A (= 2) of yellow (Y). Is input (S12). As a result, 15 μA (second current) is set as a target current value to be passed to the yellow (Y) transfer position.
In this way, in the table shown in FIG. 5B, the transfer position, table reference number, and current value (μA) fields corresponding to yellow (Y) are input.

Next, the control apparatus 100 increments the variable A and the variable B (S13, S14), and repeats S11 to S14 until the variable A becomes 5. As a result, 5 μA (third current) is set as a target current value to be passed to the magenta (M) and cyan (C) transfer positions.
In this way, in the table shown in FIG. 5B, the columns of transfer position, table reference number, and current value (μA) corresponding to magenta (M) and cyan (C) are input.

When the variable A becomes 5 (S17: Yes), the control device 100 performs monochrome printing by performing constant current control so that the transfer current corresponding to the transfer position of each color becomes the value shown in FIG. 5B. (S18).
Specifically, 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 15 μA in the monochrome mode. For this reason, even if there are toner particles having a low charge amount due to variations in charging performance among the toner particles placed on the paper P, there is a gap between the photosensitive drum 51Y corresponding to the second color yellow and the transfer roller 74Y. The sheet P is strongly brought close to the sheet P by the applied strong transfer bias. Thereby, reverse transfer of toner from the paper P to the photosensitive drum 51Y corresponding to yellow is suppressed. In addition, the charge amount of toner with a low charge amount can be increased.

  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

[Monochrome printing mode (when no color process cartridge is installed)]
As an example, a case where monochrome printing is performed in a state where the yellow process cartridge 50Y is not mounted will be described below.
Even when the color process cartridges of other colors are not attached or when two or more color process cartridges are not attached, monochrome printing can be performed by the same procedure.

  When the print data received by the color printer 1 is a monochrome image (S1: Yes), the control device 100 initially sets a variable A representing the transfer position and a variable B representing the reference number of the arrangement table shown in FIG. The value is 1 (S4, S5).

Then, the control device 100 determines whether or not the process cartridge 50K is mounted at the transfer position A (= 1, that is, the black (K) position) (S6).
Here, since the black process cartridge 50K is mounted (S6: Yes), the control device 100 stores the value of the variable B in the memory (RAM) corresponding to the black (K) transfer position A (= 1). Is input (S8). As a result, 8 μA (first current) is set as a target current value to be passed to the black (K) transfer position.

Next, the control device 100 increments the variable A and the variable B (S9, S10).
Then, the control device 100 determines whether or not the process cartridge 50Y is mounted at the transfer position A (= 2, that is, the yellow (Y) position) (S11).
Here, since the yellow process cartridge 50Y is not mounted (S11: No), the control device 100 uses the value of the variable B in the memory (RAM) corresponding to the yellow (Y) transfer position A (= 2). A certain 0 is input (S15). As a result, 0 μA is set as a target current value to be passed to the yellow (Y) transfer position.

Next, the control device 100 increments the variable A (S16), and determines whether or not the variable A is 5 (S17).
Since the variable A is not 5 (S17: No), the control device 100 returns to S11 and determines whether or not the process cartridge 50M is mounted at the transfer position A (= 3, that is, the magenta (M) position) (S11). ).
Here, since the magenta process cartridge 50M is mounted (S11: Yes), the control device 100 stores the value of the variable B in the memory (RAM) corresponding to the transfer position A (= 3) of magenta (M). Is input (S12). As a result, 15 μA (second current) is set as a target current value to be passed to the magenta (M) transfer position.

  Next, the control device 100 increments the variable A and the variable B (S13, S14), and repeats S11 to S16 until the variable A becomes 5. Accordingly, 5 μA (third current) is set as a target current value to be passed to the cyan (C) transfer position.

When the variable A becomes 5 (S17: Yes), the control device 100 performs monochrome printing by performing constant current control so that the transfer current corresponding to the transfer position of each color becomes the value shown in FIG. 5C. (S18).
More specifically, by performing control not to apply a transfer bias to the transfer roller 74Y corresponding to the yellow process cartridge 50Y that is not attached to the apparatus main body 2, the photosensitive member is caused by a large current flowing through the other photosensitive drums 51. It is possible to suppress damage to the body drum 51, a decrease in transfer efficiency during monochrome printing, and occurrence of image defects.
Further, among the color process cartridges 50 mounted in the apparatus main body 2, the transfer current value to be supplied corresponding to the magenta process cartridge 50M located on the most upstream side in the conveyance direction of the paper P is set to 15 μA, and the black transfer current is set. By controlling so that a transfer current larger than the value of 8 μA flows, reverse transfer of toner from the sheet P to the photosensitive drum 51M corresponding to magenta is suppressed.
Further, among the color process cartridges 50 mounted on the apparatus main body 2, the cyan process cartridge 50C for the second color counted from the most upstream side in the transport direction of the paper P has a transfer current value of 5 μA and a magenta transfer current. By controlling the transfer current to be smaller than 15 μA, which is a value, excessive charge-up of toner is suppressed, and reverse transfer of negatively charged toner due to discharge between toner particles is suppressed.

<Effect of color printer>
(1) As described above, according to the color printer 1 of the present embodiment, the control device 100 mounts the transfer roller 74K corresponding to the black process cartridge 50K and the color process cartridge 50 during monochrome printing. A transfer current is supplied to the transfer roller 74 corresponding to the color process cartridge 50 that has been detected to be mounted by each of the sensors SY, SM, and SC that detect non-mounting, and the transfer roller 74 corresponding to the color process cartridge 50 that has been detected to be unmounted. By performing control without applying a transfer bias, even when there is a non-attached process cartridge 50, damage to the photosensitive drum 51 of another attached process cartridge 50, reduction in transfer efficiency during monochrome printing, and image The occurrence of defects can be suppressed.

  (2) The black process cartridge 50K is provided upstream of the color process cartridge 50 in the conveyance direction of the paper P, and the control device 100 applies a first current to the transfer roller 74K corresponding to the black process cartridge 50. The first current is supplied to the transfer roller 74 corresponding to the process cartridge 50 located on the most upstream side in the conveyance direction of the paper P among the color process cartridges 50 that are detected to be mounted. By controlling so that a current of 15 μA flows as a second current larger than the second current (second current> first current), the reverse transfer of toner from the paper P to the most upstream photosensitive drum 51 is suppressed. can do.

  (3) The control device 100 applies the first current to the transfer roller 74 corresponding to the second process cartridge 50 counted from the most upstream side in the transport direction of the paper P among the color process cartridges 50 detected to be mounted. Is controlled so that a current of 5 μA flows as a small third current (third current <first current), thereby preventing excessive charge-up of the toner and the charge polarity of the toner originally intended to be used. Inversely, generation of toner that is charged (in this embodiment, generation of negatively charged toner) can be suppressed, and reverse transfer of toner can be suppressed.

  (4) The control device 100 applies a third current to the transfer roller 74 corresponding to the third and subsequent process cartridges 50 counted from the most upstream side in the transport direction of the paper P among the color process cartridges 50 detected to be mounted. As a result, the toner is prevented from being excessively charged up on the downstream side in the conveyance direction of the paper P, and the generation of toner that is charged in reverse to the charging polarity of the toner to be originally used ( In this embodiment, generation of negatively charged toner) can be suppressed, and reverse transfer of toner can be suppressed.

  (5) The control device 100 performs constant current control to apply a transfer bias so that a transfer current having a constant current value flows to each transfer roller 74, so that the resistance values of the transport belt 73, each roller, and the paper P are reduced. Even if it fluctuates, a constant transfer current can be stably supplied.

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, in the color printer 1 according to the above-described embodiment, each sensor SK, SY, SM, SC is configured to detect whether the process cartridge 50 is mounted or not, but is used in combination with a toner cartridge or a developer cartridge. It can be configured to detect the mounting or non-mounting of the drum cartridge, or to detect the mounting or non-mounting of the photosensitive drums 51K, 51Y, 51M, and 51C.

Further, a color printer in which five or more colors of developers are prepared and five or more photosensitive drums 51 are provided can be provided. In this case, the transfer current flowing through the transfer roller 74 corresponding to the fifth and subsequent photosensitive drums 51 in monochrome printing is set to a current smaller than 8 μA as the third current for the purpose of preventing excessive charge-up of the toner. can do.
Further, in the arrangement table shown in FIG. 5A, the current values (third current) corresponding to the reference numbers “3” and “4” are set to the same value, but the third current <the first current. Different values can be used as long as the relationship is satisfied. However, for the purpose of preventing toner charge-up, it is desirable that the third current value be a constant value or set to become smaller toward the downstream side in the transport direction of the paper P.

Furthermore, in the color printer 1 according to the above-described embodiment, the current value passed through each transfer roller 74 is controlled by constant current control. However, as long as the relationship of third current <first current <second current is satisfied, It is also possible to perform other controls such as constant voltage control.
In the embodiment, the transfer roller 74 is described as an example of the transfer unit. However, a transfer corotron or a transfer film may be used as the transfer unit.

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

  In the above-described 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. However, the transfer bias in the color mode and the monochrome mode may be set to the same setting value, Can be set to a high transfer bias.

  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 P Paper S sensor

Claims (5)

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;
Detecting means for detecting mounting or non-mounting of each of the plurality of second photosensitive drums;
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 a transfer current flowing through the transfer means,
The control means applies a transfer current to the transfer means corresponding to the first photoconductive drum and the transfer means corresponding to the second photoconductive drum detected by the detection means during monochrome printing, and An image forming apparatus, wherein a transfer bias corresponding to the second photosensitive drum whose non-mounting is detected by the detection unit is controlled not to apply a transfer bias. The first photosensitive drum is provided upstream of the plurality of second photosensitive drums in the transport direction of the transfer medium,
The control means controls the first current to flow through the transfer means corresponding to the first photoconductive drum, and among the second photoconductive drums detected to be mounted, the control means is the most in the transport direction. 2. The image forming apparatus according to claim 1, wherein control is performed so that a second current larger than the first current flows through a transfer unit corresponding to the photosensitive drum positioned on the upstream side.   The control means is smaller than the first current in the transfer means corresponding to the second photosensitive drum counted from the most upstream side in the transport direction among the second photosensitive drums detected to be mounted. The image forming apparatus according to claim 2, wherein the third current is controlled to flow.   The control means causes the third current to flow through a transfer means corresponding to the third and subsequent photosensitive drums counted from the most upstream side in the transport direction among the second photosensitive drums whose attachment is detected. The image forming apparatus according to claim 3, wherein the image forming apparatus is controlled as follows. The said control means performs constant current control which applies a transfer bias so that the transfer current of a fixed electric current value may flow through the said transfer means. Image forming apparatus.

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