JP5699740B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms an image on a conveyed recording sheet.

  In an electrophotographic image forming apparatus such as a laser printer, development in which a developer is supplied from a developing roller to which a predetermined developing bias is applied to an electrostatic latent image formed on a photosensitive member (photosensitive drum). As a result, a developer image is formed on the photosensitive member, and this developer image is transferred to a transported recording sheet (paper) and thermally fixed to form an image on the recording sheet ( For example, see Patent Document 1).

JP 2010-128152 A

  By the way, in an electrophotographic image forming apparatus, an image with a large amount of dots (an image with a large amount of developer supplied from a developing roller to a photoconductor) is formed on a recording sheet as in so-called black solid printing. In some cases, density unevenness occurs in which the density of the image gradually decreases from the front end to the rear end in the recording sheet conveyance direction. This is due to the fact that the amount of developer supplied to the developing roller gradually cannot catch up with the amount of developer supplied from the developing roller to the photoreceptor.

  Conventionally, in order to suppress such density unevenness, the absolute value is larger than the developing bias with respect to the supply roller that supplies the developer to the developing roller for the purpose of increasing the amount of the developer supplied to the developing roller. There was a configuration in which a bias was applied. However, in such a configuration, since a current flows between the developing roller and the supply roller, there is a possibility that deterioration of the developer (such as a decrease in charging performance) may proceed due to this current.

  The present invention has been made in view of the background as described above, and provides an image forming apparatus capable of suppressing density unevenness of an image even when the amount of dots of an image formed on a recording sheet is large. For the purpose.

  The present invention for achieving the above-described object is an image forming apparatus for forming an image on a conveyed recording sheet, which carries a photosensitive member on which an electrostatic latent image is formed, a developer, and has a developing bias. A developing roller that performs development to supply developer to the electrostatic latent image when applied; and a control device that controls a developing bias applied to the developing roller, the control device forming on the recording sheet When the dot amount of the image is less than a predetermined reference, the absolute value of the developing bias applied to the developing roller is maintained at the first value in correspondence with the leading edge to the trailing edge of the conveyed recording sheet. When the dot amount of the image formed on the sheet is equal to or larger than a predetermined reference, the absolute value of the developing bias applied to the developing roller is set to correspond to the first value by associating the leading value with the trailing edge of the conveyed recording sheet. 2nd smaller than From the value, and characterized by increasing towards the value greater than the first value or the first value.

  According to the image forming apparatus configured as described above, when an image with a large amount of dots is formed on a recording sheet, when developing an image corresponding to the vicinity of the leading end of the recording sheet (initial stage of development), the developing bias is reduced. Since the absolute value is reduced, the amount of developer that moves from the developing roller to the photosensitive member can be suppressed, and the developer can be preserved on the developing roller. Thereafter, the absolute value of the developing bias is increased toward the final stage of development where the supply of the developer to the developing roller gradually cannot catch up, so the amount of developer that moves from the developing roller to the photosensitive member is increased. be able to. As a result, the amount of the developer supplied from the developing roller to the photosensitive member can be kept substantially uniform from the start to the end of the development of the image formed on one recording sheet. It is possible to suppress the density unevenness of the image such that the density gradually decreases toward the edge.

  According to the image forming apparatus of the present invention, it is not necessary to apply a bias having an absolute value larger than the developing bias to a supply roller that supplies developer to the developing roller. Therefore, it is sufficient to apply a bias having the same polarity as the developing bias and having the same potential or less (including 0) to the member that supplies the developer to the developing roller. With such a configuration, it is possible to suppress the current flowing between the developing roller and the member that supplies the developer to the developing roller, and thus it is possible to suppress the deterioration of the developer.

  According to the present invention, when the dot amount of the image formed on the recording sheet is equal to or larger than the predetermined reference, the absolute value of the developing bias applied to the developing roller is made to correspond to the leading end and the trailing end of the conveyed recording sheet. Since the value is increased from a small value to a large value, the density unevenness of the image can be suppressed even when the dot amount of the image formed on the recording sheet is large.

1 is a diagram illustrating a schematic configuration of a laser printer as an example of an image forming apparatus according to a first embodiment. It is a figure which shows the structure relevant to control of a development bias. 3 is time charts (a) and (b) showing control of developing bias. It is a flowchart which shows control of the developing bias by a control apparatus. It is a time chart which shows control of development bias concerning a 2nd embodiment. It is a time chart which shows control of development bias concerning a modification.

[First Embodiment]
Next, a first embodiment of the present invention will be described in detail with reference to the drawings as appropriate. In the following description, first, after briefly explaining the schematic configuration of the laser printer 1 as an example of the image forming apparatus according to the embodiment of the present invention, the development bias control according to the characteristic part of the present invention will be described in detail. explain.

  In the following description, the direction will be described with reference to the user who uses the laser printer 1. That is, the right side in FIG. 1 is “front”, the left side is “rear”, the front side is “left”, and the back side is “right”. Also, the vertical direction in FIG.

<Schematic configuration of laser printer>
As shown in FIG. 1, a laser printer 1 includes a main body housing 2 that feeds a sheet S as an example of a recording sheet, an exposure device 4, and a toner image (developer) on the sheet S. A process cartridge 5 for transferring the image) and a fixing device 8 for thermally fixing the toner image on the paper S.

  The paper feeding unit 3 is provided in the lower part of the main body housing 2 and mainly includes a paper feeding tray 31, a paper pressing plate 32, a paper feeding mechanism 33, and a registration roller 34. The paper S stored in the paper feed tray 31 is moved upward by the paper pressing plate 32, supplied toward the process cartridge 5 by the paper feed mechanism 33, passes through the registration rollers 34, and the photosensitive drum 61 and the transfer roller 63. It is conveyed between.

  The exposure apparatus 4 is disposed in the upper part of the main body housing 2 and includes a laser light emitting unit (not shown), a polygon mirror, a lens, a reflecting mirror, and the like that are not shown. In this exposure apparatus 4, the surface of the photosensitive drum 61 after charging is exposed by scanning the surface of the photosensitive drum 61 at high speed with laser light (see the chain line) based on the image data emitted from the laser light emitting unit. To do.

  The process cartridge 5 is disposed below the exposure apparatus 4 and is detachably mounted on the main body housing 2 through an opening formed when the front cover 21 provided on the main body housing 2 is opened. Yes. The process cartridge 5 includes a drum unit 6 and a developing unit 7.

  The drum unit 6 mainly includes a photosensitive drum 61 as an example of a photosensitive member, a charger 62, and a transfer roller 63. The developing unit 7 is configured to be detachably attached to the drum unit 6. The developing roller 71, the supply roller 72, the layer thickness regulating blade 73, and positive chargeability as an example of a developer. The toner container 74 mainly stores a toner container 74 and an agitator 75.

  In the process cartridge 5, first, the surface of the photosensitive drum 61 is uniformly charged by the charger 62 to a positive potential larger than the developing bias. Thereafter, the surface of the photosensitive drum 61 is exposed by laser light based on the image data from the exposure device 4, so that the surface potential on the photosensitive drum 61 becomes smaller than the developing bias (electrostatic latent image). Is formed.

  The toner in the toner container 74 is supplied to the supply roller 72 while being agitated by the agitator 75, and then supplied from the supply roller 72 to the development roller 71 as the supply roller 72 and the development roller 71 rotate. . The toner supplied onto the developing roller 71 enters between the developing roller 71 and the layer thickness regulating blade 73 as the developing roller rotates, and is carried on the developing roller 71 as a thin layer having a constant thickness. In the above process, the toner is positively frictionally charged between the developing roller 71, the supply roller 72, the layer thickness regulating blade 73, and the like.

  Then, the toner carried on the developing roller 71 is developed to be supplied to the electrostatic latent image formed on the photosensitive drum 61 from the developing roller 71 to which a positive developing bias is applied. The electrostatic latent image is visualized and a toner image is formed on the photosensitive drum 61. Thereafter, the sheet S is conveyed between the photosensitive drum 61 and the transfer roller 63 so that the toner image on the photosensitive drum 61 is transferred onto the sheet S.

  The fixing device 8 is disposed behind the process cartridge 5, and mainly includes a heating roller 81 and a pressure roller 82 disposed so as to face the heating roller 81 and press the heating roller 81. The sheet S on which the toner image has been transferred by the process cartridge 5 is conveyed between the heating roller 81 and the pressure roller 82, whereby the toner image is thermally fixed, and is discharged onto the discharge tray 22 by the discharge roller 23. The As a result, an image is formed on the paper S.

<Development bias control>
As shown in FIG. 2, the laser printer 1 further includes a developing bias application circuit 90 and a control device 100 in the main body housing 2.

  The developing bias application circuit 90 is a circuit that applies a developing bias to the developing roller 71, and is configured to be able to adjust the developing bias applied to the developing roller 71. Since such a development bias application circuit 90 can employ a widely known configuration, detailed description thereof will be omitted in this specification. In the present embodiment, no bias (supply bias) is applied to the supply roller 72.

  The control device 100 is a device that controls the developing bias applied to the developing roller 71 via the developing bias applying circuit 90, and includes a CPU, a RAM, a ROM, an input / output interface, and the like (not shown). The control device 100 mainly includes an image determination unit 110 and a development bias control unit 120 as functional units related to the present invention.

  The image determination means 110 is a dot amount of an image to be formed on the sheet S (one sheet S) based on image data included in a print job input from a device external to the laser printer 1, such as a personal computer (not shown). Is a functional unit that determines whether or not is equal to or greater than a predetermined reference.

  Specifically, the image determination unit 110 counts the number of dots of the image formed on each sheet S based on, for example, information on the number of dots (pixels) included in the image data, and for each sheet S. It can be configured to determine whether or not the count value is equal to or greater than a preset reference value (predetermined reference). When the count value is equal to or greater than the reference value, the image determination unit 110 determines that the dot amount of the image formed on the paper S is equal to or greater than the predetermined reference (the image dot amount is large), and the count value is If it is less than the reference value, it is determined that the dot amount of the image formed on the paper S is less than the predetermined reference (the image dot amount is small).

  As another example, the image determination unit 110 is actually formed based on the image data and occupies an area where the image of each sheet S is formed (the maximum area where the laser image can be transferred by the laser printer 1). It is also possible to calculate the area ratio of the image to be determined and determine for each sheet S whether or not the area ratio is equal to or greater than a preset reference value (predetermined reference). Then, the image determination unit 110 determines that the image dot ratio is large when the image area ratio is equal to or larger than the reference value, and the image dot ratio is small when the image area ratio is less than the reference value. Is determined.

  As another example, the image determination unit 110 may be configured to determine whether the amount of dots in the image is large or small based on the type of input image data. For example, if the image data is so-called document data mainly composed of characters, it is determined that the image dot amount is small, and if the image data is photo data, it is determined that the image dot amount is large. May be.

Incidentally, the image determining unit 110, until the print job is input, if it contains image data for forming an image on two or more sheets S are developed image is completed to form immediately before the sheet S _n Then, it is determined whether or not the dot amount of the image to be formed on the next sheet _{Sn + 1} is greater than or equal to a predetermined reference.
The determination result by the image determination unit 110 is output to the development bias control unit 120.

Developing bias control means 120, when the dot of the image formed on the paper S is small, as shown on the left side of FIG. 3 (b), the developing bias applied to the developing roller 71, the sheet S _n to be conveyed the tip so as to correspond to (the right end from the left end in FIG. 3) from the rear end in the transport direction (direction indicated by a hollow arrow), until development of the image formed on the sheet S _n is completed, the first value V _b1 To maintain.

On the other hand, when the dot amount of the image formed on the paper S is large, the development bias control means 120 applies the development bias applied to the developing roller 71 to the conveyed paper S as shown on the left side of FIG. _n tip in correspondence with the rear end from, from less than the first value V _b1 second value V _b2, gradually increased toward the first value V _b1. More specifically, a developing bias control unit 120, when dot density of the image formed on the paper S is large, from the start of development of the image to be formed on at least the sheet S _n (time t1), the developing roller 71 rotates one The developing bias is maintained at the second value V _b2 until the time T _D elapses, and thereafter (after time t2), the developing bias is increased toward the first value V _b1 .

  As a precaution, as shown in FIG. 2, development is performed by the development roller 71 to which the development bias as described above is applied at the development position DP where the photosensitive drum 61 and the development roller 71 face each other. The toner image on the photosensitive drum 61 is moved as the photosensitive drum 61 rotates, and is transferred to the conveyed paper S at the transfer position TP where the photosensitive drum 61 and the transfer roller 63 face each other. It becomes.

Therefore, specifically, the development bias is increased from the second value V _b2 toward the first value V _b1 in correspondence with the leading edge to the trailing edge of the sheet S being conveyed. A second value V _b2 is applied as a development bias at the development position DP where the image transferred to the leading edge is developed, and then to the development position DP where the image transferred to the trailing edge of the same sheet S is developed. This means that the developing bias is increased toward the first value V _b1 .

  When the input print job includes image data for forming an image on two or more sheets S, the developing bias control unit 120 is based on the determination result for each sheet S in the image determination unit 110. The developing bias applied to the developing roller 71 is controlled every time the image formed on each sheet S is developed.

For example, as shown by the solid line in FIG. 3 (a), (b) , if dot density of the image formed on the following sheet S _{n + 1} is large, after the completion of the development of the image formed on the sheet S _n, the developing bias applied to the developing roller 71, from a first value V _b1 changed to a second value V _b2 (time t3), then, as in the case of the sheet S _n in FIG. 3 (a), the sheet to time T _D from the start of development of the image formed on the S n _{+ 1} has elapsed to keep the developing bias to a second value V _b2, thereafter to increase toward the developing bias to a first value V _b1 .

Further, FIG. 3 (a), as shown by a chain line (b), the case dot amount of the image formed on the following sheet S _{n + 1} is small, even after completion of the development of the image formed on the sheet S _n The first value V _b1 is continuously applied to the developing roller 71 as a developing bias. In this case, in order to suppress the deterioration of the toner due to the current flowing between the photosensitive drum 61 and the developing roller 71, the time corresponding to the interval between the sheet _Sn and the sheet _{Sn + 1} (the interval between the sheets) is Once the developing bias is made smaller than the first value V _b1 , the developing bias may be stopped.

  In the present embodiment, the timing for starting the application of the development bias after the print job is input and the timing for stopping the application of the development bias after the completion of all development are not particularly limited. For example, when the leading edge of the paper S reaches a paper sensor (not shown) provided on the upstream side of the registration roller 34 (see FIG. 1), the application of the developing bias is started, and the rear edge of the paper S uses the paper sensor described above. It is possible to configure so that the application of the developing bias is stopped after a predetermined time (a time during which it can be estimated that the sheet S has passed the fixing device 8) has passed.

The flow of developing bias control will be described with reference to the flowchart of FIG. 4 (see also FIG. 3B as appropriate).
As shown in FIG. 4, when a print job (image data) is input, the control device 100 first determines whether or not the dot amount of the image formed on the paper S is greater than or equal to a predetermined reference (image dot amount). (S10).

When the dot amount of the image to be formed on the sheet S (S _n ) is small (S10, No), the control device 100 applies the developing bias of the first value V _b1 to the developing roller 71 via the developing bias applying circuit 90. Apply (see S20, until time t3 in FIG. 3B). Thus, the process cartridge 5, the developing dot small amount of the image formed on the sheet S _n is performed, an image developed on the sheet S _n (toner image) is transferred.

In this process, the control device 100 determines whether the input print job includes image data to be formed on the second sheet S (S _{n + 1} ) (there is an image to be formed on the next sheet S). It determines whether (S60), if there is an image to be formed on the next sheet S (S60, Yes), the control unit 100 after completion of the development of the image formed on the sheet S _n, the process proceeds to step S10.

In step S10, when the dot amount of the image formed on the sheet S (S _{n + 1} ) is large (S10, Yes), the control device 100 applies the second value to the developing roller 71 via the developing bias applying circuit 90. A developing bias of V _b2 is applied (S30, see time t3 in FIG. 3B).

Thereafter, the controller 100 maintains the developing bias to a second value V _b2 to the time T _D which the developing roller 71 rotates one has elapsed (S40, No → S30). Then, when the time T _D which the developing roller 71 rotates one has elapsed (S40, Yes), the control device 100, a developing bias from the second value V _b2 is increased toward the first value V _b1 (S50, (See times t4 to t5 in FIG. 3B).

In this series of processes, the process cartridge 5, the developing dot intensive image formed on the sheet S _{n + 1} is performed, the developed image is transferred to the sheet S _{n + 1.} At this time, the control device 100 determines whether there is an image to be formed on the next (third) sheet S (S60).

When there is an image to be formed on the next sheet S (S60, Yes), the control device 100 proceeds to step S10 after the development of the image to be formed on the sheet S _{n + 1} is completed, and the image to be formed on all the sheets S. Steps S10 and after are repeated until this development is completed.

On the other hand, when there is no image to be formed on the next sheet S (S60, No), the control device 100 stops applying the developing bias after the development of the image formed on the sheet S _{n + 1} is finished (S70). The process is terminated.

According to the above, the following effects can be obtained in the present embodiment.
When an image with a large amount of dots is formed on the paper S, when the image corresponding to the vicinity of the leading edge of the paper S is developed (initial stage of development), the development bias is set to the second value V _b2 (decrease). In addition, the amount of toner that moves from the developing roller 71 to the photosensitive drum 61 can be suppressed, and the toner can be retained on the developing roller 71. Thereafter, the supply of toner to the developing roller 71 gradually becomes unable to catch up, and the developing bias is increased (increased) toward the first value V _b1 toward the end of development. The amount of toner that moves to the drum 61 can be increased.

As a result, the amount of toner supplied from the developing roller 71 to the photosensitive drum 61 can be kept substantially uniform from the start to the end of the development of the image formed on one sheet of paper S. In the image on the paper S to which the toner image is transferred, the occurrence of density unevenness in which the density gradually decreases from the front end to the rear end is suppressed.
As described above, according to the laser printer 1 of the present embodiment, the density unevenness of the image can be suppressed even when the dot amount of the image formed on the paper S is large.

Further, in the present embodiment, when the dot of the image is large, from the beginning of the development of the image formed on the sheet S, a small value developing bias until the time T _D which the developing roller 71 rotates one has elapsed (second (The value V _b2 ) and the developing bias is increased thereafter, so that the toner supply amount can be kept uniform during one rotation of the developing roller 71. As a result, the occurrence of density unevenness in the toner image (image) developed while the developing roller 71 rotates once can be suppressed, and the density unevenness of the entire image can be more reliably suppressed.

  Further, in this embodiment, since no bias is applied to the supply roller 72, no current flows between the developing roller 71 and the supply roller 72. Thereby, it is possible to prevent toner deterioration (such as a decrease in charging performance) between the developing roller 71 and the supply roller 72.

[Second Embodiment]
Next, a second embodiment of the present invention will be described in detail with reference to the drawings as appropriate. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.

Also in the present embodiment, the control device 100 (see FIG. 2), when the dot amount of the image formed on the paper S is small, the developing bias applied to the developing roller 71 is changed from the leading edge of the conveyed paper S to the trailing edge. in correspondence, and maintained at a first value V _b1, corresponding often dot amount of the image formed on the sheet S, the developing bias applied to the developing roller 71, from the front end to the rear end of the sheet S _n to be conveyed Thus, the second value V _b2 is increased toward the first value V _b1 .

Then, as shown in FIG. 5, the control device 100 according to the present embodiment develops at least from the start (time t1) of the development of the image formed on the paper S when the amount of dots of the image formed on the paper S is large. The developing bias is maintained at the second value V _b2 until the total time (T _D + T _S ) of the time T _{D for} one rotation of the roller 71 and the time T _S for one rotation of the supply roller 72 elapses. The development bias is increased toward the first value V _b1 .

  Here, the toner on the developing roller 71 is supplied to the photosensitive drum 61 and decreases while the developing roller 71 makes one rotation after the development of an image with a large amount of dots is started. However, when there is a sufficient amount of toner on the surface of the supply roller 72, a sufficient amount of toner is supplied from the supply roller 72 to the portion on the developing roller 71 where the toner is reduced. The phenomenon that the supply of toner cannot catch up with the developing roller 71 is less likely to occur while the supply roller 72 rotates once after the toner-reduced surface of the roller 71 faces the supply roller 72.

Therefore, in the present embodiment, when the dot amount of the image formed on the paper S is large, the developing bias is maintained at the second value V _b2 until the total time (T _D + T _S ) elapses. When a sufficient amount of toner is carried on the developing roller 71 and further supplied, the amount of toner moving from the developing roller 71 to the photosensitive drum 61 is suppressed, and the toner is kept on the developing roller 71. ing.

After the total time (T _D + T _S ) elapses, the development bias is increased toward the first value V _b1 , so that the supply of toner to the development roller 71 gradually becomes unable to catch up. The amount of toner that moves from the developing roller 71 to the photosensitive drum 61 is increased.

  As a result, the amount of toner supplied from the developing roller 71 to the photosensitive drum 61 can be kept substantially uniform from the start to the end of development of an image formed on one sheet of paper S. Even when the amount of dots of an image to be printed is large, density unevenness of the image can be suppressed. Further, since the time during which the developing bias is small is lengthened, it is possible to suppress the deterioration of the toner due to the current flowing between the photosensitive drum 61 and the developing roller 71.

  In particular, before the image formed on the first sheet S is developed, there is a sufficient amount of toner on the surface of the supply roller 72, so an image having a large amount of dots formed on the first sheet S. When developing an image with a large amount of dots formed on the second and subsequent sheets S, the development bias is controlled according to the first embodiment. You may comprise as follows.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. About a concrete structure, it can change suitably in the range which does not deviate from the meaning of this invention.

In the above-described embodiment, when the dot amount of the image formed on the paper S is large, a certain time (such as time T _D when the developing roller 71 rotates once) elapses from the start of development of the image formed on the paper S. Up to this point, the development bias is maintained at the second value V _b2 and then the development bias is increased. However, the present invention is not limited to this. For example, as indicated by a solid line in FIG. 6, the development bias may be increased toward the first value V _b1 from the start of development (start of application of the second value V _b2 ).

In the above embodiment, when the dot of the image formed on the paper S is large, the developing roller 71 to the developing bias applied to the second value V _b2 in correspondence with the rear end from the front end of the sheet S _n to be conveyed from configured to increase toward the first value V _b1, but the present invention is not limited thereto. For example, as indicated by a chain line in FIG. 6, the developing bias applied to the developing roller 71 is changed from the second value V _b2 to a value larger than the first value V _b1 (for example, the third value V _b3 ). You may comprise so that it may increase toward.

In the above embodiment, as shown in FIG. 3 and the like, the developing bias is linearly increased from the second value V _b2 to the first value V _b1 , but the present invention is limited to this. It is not a thing. That is, in the present invention, the tendency of increase in the developing bias is not particularly limited, and it is desirable to set according to the specific configuration of the image forming apparatus, the charging characteristics of the developer, and the like. For example, the development bias may be increased stepwise, in a parabolic manner, or increased so that it suddenly rises from the start to the end of the development of the image formed on one sheet. May be.

  In the embodiment, the supply bias is not applied to the supply roller 72, but the present invention is not limited to this. For example, a supply bias having the same polarity and the same potential as the development bias is applied to the supply roller. May be. This also prevents toner deterioration due to current flowing between the developing roller 71 and the supply roller 72.

  In the above embodiment, the photosensitive drum 61 is exemplified as the photosensitive member, but the present invention is not limited to this, and may be, for example, a photosensitive belt.

  In the embodiment, the laser printer 1 that forms a monochrome image is exemplified as the image forming apparatus. However, the present invention is not limited to this, and may be a printer that forms a color image, for example. The image forming apparatus is not limited to a printer, and may be, for example, a copier or a multi-function machine including a document reading apparatus such as a flat bed scanner. In an image forming apparatus having a copying function, image data is also input by reading with a document reading apparatus.

  In the above-described embodiment, the image forming apparatus that performs development using the positively chargeable toner (developer) is exemplified. However, the present invention is an image forming apparatus that performs development using the negatively chargeable developer. Can also be applied. In this case, the first value and the second value as the developing bias are negative values, and the second value has a smaller absolute value than the absolute value of the first value.

  In the embodiment, the paper S such as so-called plain paper or postcard is shown as an example of the recording sheet. However, the present invention is not limited to this, and may be, for example, an OHP sheet.

DESCRIPTION OF SYMBOLS 1 Laser printer 61 Photosensitive drum 71 Developing roller 72 Supply roller 100 Control device S Paper

Claims (4)

An image forming apparatus for forming an image on a transported recording sheet,
A photoreceptor on which an electrostatic latent image is formed;
A developing roller for carrying the developer and performing development for supplying the developer to the electrostatic latent image by applying a developing bias;
A control device for controlling a developing bias applied to the developing roller,
The controller is
When the dot amount of the image formed on the recording sheet is less than a predetermined reference, the absolute value of the developing bias applied to the developing roller is set to a first value corresponding to the leading edge to the trailing edge of the conveyed recording sheet. To maintain
When the dot amount of the image formed on the recording sheet is equal to or greater than a predetermined reference, the absolute value of the developing bias applied to the developing roller is made to correspond to the first end to the rear end of the conveyed recording sheet, and the first An image forming apparatus, wherein the second value smaller than the value is increased toward the first value or a value larger than the first value.   When the dot amount of the image formed on the recording sheet is equal to or greater than a predetermined reference, the control device is at least from the start of the development of the image formed on the recording sheet until the time for one rotation of the developing roller elapses. The image forming apparatus according to claim 1, wherein the developing bias is maintained at the second value, and thereafter the developing bias is increased. A supply roller for supplying a developer to the developing roller;
When the dot amount of the image formed on the recording sheet is equal to or greater than a predetermined reference, the control device determines that the time when the developing roller makes one rotation from the start of the development of the image formed on the recording sheet and the supply roller 2. The image forming apparatus according to claim 1, wherein the developing bias is maintained at the second value until a total time of one rotation elapses, and thereafter the developing bias is increased. A supply roller for supplying a developer to the developing roller;
4. The supply roller according to claim 1, wherein a supply bias is not applied to the supply roller, or a supply bias having the same polarity and the same potential as the development bias is applied. The image forming apparatus described.

Images