JP2010160236A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of keeping print density uniform in a first half and a second half of one page print. <P>SOLUTION: At the beginning of development, a supply bias voltage Vb is made lower than a development bias voltage Va. As development starts, the voltage value of a supply roller 55 is increased, and while a development roller 54 makes approximately one revolution, the magnitude relationship of the voltages is reversed so that the supply bias voltage Vb is higher than the development bias voltage Va in the second half of the development. Thereby, the amount of the positively-charged toner that migrates from the supply roller 55 to the development roller 54 can be controlled in the first half, and the density unevenness caused by the toner supply capacity of the supply roller 55 being unable to keep up with the development can be prevented in the second half of the printing. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus capable of satisfactorily forming an image.

  2. Description of the Related Art Conventionally, as a developing device provided in an image forming apparatus such as a laser printer, toner stored in a toner storage chamber is supplied to a developing roller via a supply roller, and the toner is attached to an electrostatic latent image on the surface of a photoreceptor. There was something that formed. Specifically, as shown in Patent Document 1, for example, toner accommodated in the toner accommodating chamber first adheres to the surface of the rotating supply roller. The developing roller disposed in contact with the supply roller is supplied with toner attached to the surface of the supply roller at a frictional position with the supply roller while rotating. The developing roller is applied with a predetermined bias from a bias applying circuit provided in the image forming apparatus main body.

  The toner supplied to the surface of the developing roller moves from the developing roller to the photosensitive member surface based on the potential difference between the photosensitive member surface and the developing roller. In this way, a toner image is formed on the surface of the photoreceptor.

Japanese Patent Laid-Open No. 1-252979

  However, in the conventional configuration, since a sufficient amount of toner is carried on the developing roller at the initial stage of image formation, an excessive amount of toner is formed at the leading end of the printing result, particularly when a black solid image is formed on one page of paper. It was sometimes used. That is, as shown in FIG. 7, an image is formed on the leading edge of the paper P with a sufficient amount of toner.

  On the other hand, as the paper P is moved downstream in the printing direction, the color becomes light despite the black solid printing. This is because, when development is performed using a large amount of toner, the amount of toner supplied to the developing roller by the supply roller gradually cannot catch up with the consumption during development. For this reason, the printing density at the rear end portion is lower than that at the front end portion in one page, resulting in a so-called density unevenness. As described above, in the printing of one page, there is a problem that the density is nonuniform at the leading end and the trailing end of the printing result.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of maintaining a uniform printing density in one page.

  To achieve this object, the present invention provides an image carrier on which an electrostatic latent image is carried, and a developer carrier that carries a developer for developing the electrostatic latent image into a developer image. A developer supply member that supplies the developer to the developer carrier, a transfer member that transfers the developer image to a sheet, a development bias voltage applied to the developer carrier, and the developer supply And a bias applying unit that applies a supply bias voltage to the member. In the first half of the development for one page of the paper, the bias applying unit has an absolute value of the supply bias voltage that is the development bias. And control so that the absolute value of the supply bias voltage is larger than the absolute value of the development bias voltage in the latter half of the development for one page of the paper. Features and That.

  The developer carrying member is a developing roller, and the first half of the development for one page of the paper is from the start of development until the developing roller makes approximately one rotation.

  Further, the bias applying means controls the supply bias voltage to gradually change in one page of the paper.

  In addition, the bias applying unit gradually increases the absolute value of the developing bias voltage within a range in which the first half is constant and the magnitude relationship does not change in the second half.

  The bias applying means is applied to the developing bias voltage applied to the developing roller carrying positively charged toner and the supply roller disposed in contact with the developing roller during the development of one page of the paper. Regarding the relationship of the supply bias voltage, the supply bias voltage is lower in the first half than the development bias voltage in the first half, and the supply bias voltage is set higher than the development bias voltage in the second half after the developing roller has rotated approximately once. The value of the supply bias voltage is switched.

  According to the first aspect of the present invention, since the absolute value of the supply bias voltage is controlled to be smaller than the absolute value of the development bias voltage in the first half of the development for one page of the sheet, the developer carrier The developer carried on the sheet is easily returned to the developer supply member. That is, excessive developer is not supplied to the image carrier in the initial stage of development, a part of the developer is returned to the developer supply member, and the amount of developer returned to the developer supply member is thereafter Preserved for development.

  On the other hand, in the latter half of the development for one page of paper, since the absolute value of the supply bias voltage is controlled to be larger than the absolute value of the development bias voltage, the developer carried on the developer supply member Is easily supplied to the developer carrying member. That is, the developer necessary for development in the latter half of the development can be sufficiently supplied, and the developer returned to the developer supply member in the first half of the development is also supplied to the developer carrier.

  Therefore, it is possible to control the amount of developer on the developer carrier so that it does not change significantly between the first half and the second half of development, so that it is possible to keep the printing density uniform during printing one page of paper. It becomes.

  According to a second aspect of the present invention, the developer carrying member is a developing roller. The first half of the development for one page of paper corresponds to the time from the start of development until the developing roller makes approximately one rotation.

  A sufficient amount of developer is carried on the developing roller when preparing for printing. Therefore, development with a sufficient density is possible while the developing roller rotates substantially once. However, when the developing roller rotates approximately once, almost all of the developer carried on the peripheral surface is supplied to the image carrier, and development of the electrostatic latent image is delayed when the developer supply after the second rotation is delayed. There is a risk of impact. Therefore, density unevenness can be satisfactorily suppressed by controlling the absolute value of the supply bias voltage to be smaller than the absolute value of the development bias voltage from the start of development until the developing roller makes approximately one rotation.

  According to the third aspect of the present invention, the absolute value of the supply bias voltage is controlled to gradually increase during development for one page of paper. As a result, the developer is developed from the direction in which the developer returns from the developer carrier to the developer supply member at a position where the magnitudes of the absolute values of the supply bias voltage and the development bias voltage are reversed (that is, the boundary between the first half and the second half). A sudden change in the direction of supply to the agent carrier can be prevented. As a result, it is possible to satisfactorily suppress density unevenness at a position where the absolute values of the supply bias voltage and the development bias voltage are reversed. Furthermore, by increasing the difference between the development bias voltage and the supply bias voltage, the developer supply capability of the developer supply member to the developer carrying member does not drop when the development proceeds. Therefore, it is possible to make the supply amount of the developer uniform.

  According to the fourth aspect of the present invention, it is possible to satisfactorily develop the electrostatic latent image by making the development bias voltage constant in the first half of the development for one page of paper. Further, by increasing the second half, it is possible to prevent a shortage of the amount of developer developed on the image carrier in the second half of the development.

  According to the fifth aspect of the present invention, the positively charged toner by the contact between the developing roller and the supply roller is more easily supplied to the developer carrying member in the latter half of the development. Therefore, density unevenness is less likely to occur in the printing result of the positively charged toner during one page printing.

1 is a cross-sectional view illustrating a configuration of a laser printer 1. FIG. FIG. 2 is a schematic diagram showing a configuration in the vicinity of a developing roller 54. 6 is a graph showing the relationship between the values of development bias and supply bias during one-page printing. The horizontal axis represents time, and the vertical axis represents the bias value applied from the bias supply unit 70. It is the graph which expanded the part enclosed with the dotted line of FIG. 6 is a graph showing how a deviation between a print density and a target value occurs with time in one page for each magnitude relationship between a developing bias and a supply bias. 6 is a graph showing the relationship between the values of the developing bias and the supply bias during one-page printing as another embodiment. It is explanatory drawing which showed the fall of the printing density in 1 page in a prior art.

[Laser printer configuration]
Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

  Here, in the following description, the direction will be described with reference to the user who uses the laser printer 1. That is, in FIG. 1, the left side toward the paper surface is the “front” side, the right side toward the paper surface is the “rear” side, the front side toward the paper surface is the “right” side, and the rear side toward the paper surface is “left”. ”Side. In addition, the vertical direction toward the page is the “vertical” direction.

  As shown in FIG. 1, the laser printer 1 forms a toner image in a main body housing 2 for feeding paper P, a scanner unit 4 and a toner image, and transfers the toner image onto the paper P. It mainly includes a process cartridge 5 and a fixing device 60 for thermally fixing the toner image transferred to the paper P. A front cover 21 that can be freely opened and closed is provided on the front side of the main body housing 2, and the process cartridge 5 is detachably mounted through an opening that is formed when the front cover 21 is opened. Further, on the upper surface of the main body housing 2, a paper discharge tray 22 for storing the paper P discharged to the outside of the main body housing 2 is provided.

  The paper feed unit 3 is provided in the lower part of the main body housing 2, and the paper feed tray 31 is detachably attached to the main body housing 2. A pickup roller 34 that conveys the paper P from the tray 31, a paper feed roller 35, a paper feed pad 36, a pinch roller 37, and a registration roller 38 are provided.

  The paper P in the paper feed tray 31 is brought close to the pickup roller 34 and is sent between the paper feed roller 35 and the paper feed pad 36 by the rotation of the pickup roller 34 to be separated one by one, and the pinch roller 37 and the registration roller After passing through 38, it is conveyed to the process cartridge 5.

  The scanner unit 4 is provided at an upper portion in the main body housing 2 and mainly includes a laser light emitting unit (not shown), a polygon mirror 41 that is rotationally driven, lenses 42 and 43, and reflecting mirrors 44 and 45. . The laser light based on the image data emitted from the laser light emitting part is reflected or passed through the polygon mirror 41, the lens 42, the reflecting mirror 44, the lens 43, and the reflecting mirror 45 in this order, as indicated by a chain line, and the process cartridge 5 The surface of the photosensitive drum 52 is irradiated with high-speed scanning.

  The process cartridge 5 is provided below the scanner unit 4 and is detachably attached to the main body housing 2. The process cartridge 5 includes a photosensitive drum 52 as an example of an image carrier of the present invention, a charger 53, and a developer as an example of a developer carrier of the present invention in a hollow casing 51 constituting an outer frame. The apparatus mainly includes a roller 54, a supply roller 55 as an example of a developer supply member of the present invention, a layer thickness regulating blade 56, a toner accommodating portion 57, and a transfer roller 58 as an example of a transfer member of the present invention. ing. As shown in FIG. 2, the toner storage portion 57 stores a non-magnetic one-component positively chargeable toner as an example of the developer of the present invention.

  The developing roller 54 has a surface formed of rubber and is disposed in contact with the photosensitive drum 52. The developing roller 54 rotates so that the peripheral surface thereof moves in the same direction (clockwise in FIG. 2) at the contact position with the photosensitive drum 52 during image formation to be described later. A circumferential speed difference is provided between the developing roller 54 and the photosensitive drum 52, and the speed of the photosensitive drum 52 is slower than that of the developing roller 54. By moving the toner on the developing roller 54 by providing a peripheral speed difference between them, it is possible to reduce the deterioration of the adhesion force of the toner due to the influence of intermolecular force acting on the toner.

  Further, the supply roller 55 has a diameter smaller than that of the developing roller 54 and is formed of a rotating shaft and a conductive foaming sponge roller. The supply roller 55 is always in contact with the developing roller 54 and rotates so that the peripheral surface moves in the opposite direction (clockwise) at the contact position with the developing roller 54 during image formation.

  Further, the developing roller 54 and the supply roller 55 are connected to bias supply units 70A and 70B, respectively, as an example of bias applying means provided in the main body housing 2. The bias supply units 70A and 70B are configured by power supplies 72A and 70B for applying a DC voltage, variable resistors 74A and 74B, a control device 76 using a known CPU, and the like. Then, by controlling the bias supply units 70A and 70B, the development bias voltage Va to the developing roller 54 and the supply bias voltage Vb to the supply roller 55 are applied from the respective power sources 72A and 72B. Further, the value of the bias voltage applied to the developing roller 55 and the supply roller 55 is changed by changing the resistance value of the variable resistors 74A and 74B by the control device 76. Control of the bias voltage applied to the developing roller 54 and the supply roller 55 will be described later.

  In the process cartridge 5, the surface of the photosensitive drum 52 is uniformly charged by the charger 53 and then exposed by high-speed scanning of the laser light from the scanner unit 4. Thereby, the potential of the exposed part is lowered, and an electrostatic latent image based on the image data is formed.

  At this time, as shown in FIG. 2, the toner in the toner container 57 is supplied to the supply roller 55 by the rotation of the agitator 57A. Then, the supply roller 55 and the developing roller 54 are brought into sliding contact with each other by the rotation of the supplying roller 55 and the developing roller 54, whereby the toner is supplied onto the developing roller 54. The toner supplied onto the developing roller 54 enters between the layer thickness regulating blade 56 and the developing roller 54 by the rotation of the developing roller 54 and is positively charged while being thinly formed on the developing roller 54 with a constant thickness. Supported.

  The toner carried on the developing roller 54 is supplied to an electrostatic latent image formed on the photosensitive drum 52 when the developing roller 54 and the photosensitive drum 52 are in contact with each other. As a result, the electrostatic latent image is visualized and a toner image is formed on the photosensitive drum 52. Then, the paper P is conveyed between the photosensitive drum 52 and the transfer roller 58, whereby the toner image on the photosensitive drum 52 is transferred to the paper P.

  The toner image transferred to the paper P is transported to the fixing device 60 provided behind the process cartridge 5 (on the downstream side in the transport direction of the paper P) and thermally fixed. The paper P on which the toner image has been thermally fixed is conveyed from the clamping unit to the discharge path 23, discharged from the discharge path 23 to the outside of the main body housing 2 by the discharge roller 24, and accumulated on the discharge tray 22.

In the laser printer 1, the residual toner remaining on the surface of the photosensitive drum 52 after being transferred onto the paper P by the transfer roller 58 is recovered by the developing roller 54, so that the residual toner is recovered by a so-called cleanerless system. I have to. If the residual toner on the surface of the photosensitive drum 52 is collected by such a cleaner-less method, there is no need to provide a cleaning device such as a blade or a waste toner storage means, so that the device configuration is simplified, downsized, and cost is reduced. Reduction can be achieved.
[Control of supply bias during printing]
Next, supply bias control will be described with reference to FIGS. Here, a case where black solid printing is performed on the paper P will be described as an example.

  First, when a print command is given to the control device 76, the photosensitive drum 52, the developing roller 54, and the supply roller 55 in the process cartridge 5 are rotated as a print preparation operation as shown in FIG. At this time, the toner stored in the toner storage chamber 57 is carried on the surface of the rotating supply roller 55. At this time, no bias voltage is applied to the supply roller 55 and the developing roller 54.

  The toner carried on the supply roller 55 moves from the upstream side of the contact position with the developing roller 54 in the rotational direction of the supply roller 55 to the downstream side of the contact position with the supply roller 55 in the rotational direction of the development roller 54. (See FIG. 2.) At this time, the toner carried on the supply roller 55 is carried on the surface of the developing roller 54 while being positively charged by the friction at the contact position. In addition, the toner is triboelectrically charged between the developing roller 54 and the layer thickness regulating blade 56.

  When the developing roller 54 and the like are rotated several times and toner is carried on the peripheral surface of the developing roller 54, development of the electrostatic latent image is started according to the above-described process. When development is started, 200 V is applied to the supply roller 55 as the initial supply bias voltage Vb as shown in FIG. Further, 400 V is applied to the developing roller 54 as a constant developing bias voltage Va. When an electrostatic latent image is formed on the photosensitive drum 52, the positively charged toner carried on the developing roller 54 moves to the electrostatic latent image on the photosensitive drum 52 and is developed.

  In the process in which the toner moves from the supply roller 55 to the development roller 54, the development bias voltage Va is applied between the supply roller 55 and the development roller 54 so that the development bias voltage Va is higher than the supply bias voltage Vb. This means that a bias is applied in the direction in which the positively charged toner moves from the developing roller 54 to the supply roller 55. Therefore, a part of the toner moved from the supply roller 55 to the developing roller 54 moves to the downstream side of the contact position with the developing roller 54 in the rotation direction of the supply roller 55. Accordingly, it is possible to prevent the positively charged toner from being excessively carried on the developing roller 54. Further, the positively charged toner moves from the developing roller 54 to the supply roller 55, so that the positively charged toner can be preserved for supplying the toner to the developing roller 54 by the supply roller 55 in the second and subsequent rotations.

  Then, the bias supply unit 70B increases the supply bias voltage Vb applied to the supply roller 55 as shown in FIG. Specifically, the value of the supply bias voltage Vb increases stepwise as shown in FIG. The timing at which the supply bias voltage Vb is increased is, for example, every time the laser light emitted from the scanner unit 4 forms an electrostatic latent image for one line on the photosensitive drum 52. The bias supply unit 70B increases the supply bias voltage Vb applied to the supply roller 55 by several volts by changing the resistance value of the variable resistor 74B at the timing when the electrostatic latent image for one line is formed.

  On the other hand, the development bias voltage Va is uniform in one page for stable development. Then, as the supply bias voltage increases, the potential difference between the supply roller 55 and the developing roller 54 becomes smaller, so that the positively charged toner carried on the surface of the developing roller 54 is less likely to move toward the supply roller 55.

  As described above, the bias supply unit 70B gradually increases the supply bias voltage Vb applied to the supply roller 55 with the start of development. Here, the bias supply unit 70B supplies the supply bias voltage Vb so that the potential difference between the development bias voltage Va and the supply bias voltage Vb becomes zero during the rotation of the developing roller 54 approximately once after the development starts. To do. Here, the timing at which the potential difference between the two becomes 0 is most preferable when the developing roller 54 rotates exactly once, but may be between 0.90 and 1.1 rotations of the developing roller 54. Various methods can be taken, such as whether or not the developing roller 54 has made one rotation based on the time from the start of rotation of the developing roller 54 by the control device 76. Hereinafter, the period from the start of development until the developing roller 54 makes approximately one rotation is referred to as “first half part”, and the subsequent part is referred to as “second half part”.

  In the preparatory period before the start of development, the developing roller 54 carries positively charged toner having a density sufficient for normal development. The positively charged toner carried in the preparation period by one rotation of the developing roller 54 from the start of development moves to the photosensitive drum 52, but in order to ensure sufficient toner supply capability in the second rotation as well as the first rotation, 2 At the rotation stage, the toner must be positively moved toward the developing roller 54. For this purpose, it is necessary to apply the supply bias voltage Vb so that the supply bias voltage Vb is larger than the development bias voltage Va after the second rotation of the developing roller 54.

  Accordingly, as described above, the bias supply unit 70B supplies the supply bias so that the development bias voltage Va and the supply bias voltage Vb become the same potential during approximately one rotation (first half) after the development roller 54 starts development. A voltage Vb is supplied. Further, even after the magnitude relationship between the supply bias voltage Vb and the development bias voltage Va is reversed (second half), the value of the supply bias voltage Vb is increased stepwise, and the supply bias voltage Vb of, for example, 600 V is reached at the end of printing for one page. Is applied to the supply roller 55.

  That is, in the latter half, the potential difference between the two increases while maintaining the relationship that the supply bias voltage Vb is larger than the development bias voltage Va. As a result, the positively charged toner easily moves to the surface of the developing roller 54 having a lower potential, and as a result, even if the development proceeds, a decrease in the amount of toner supplied from the supply roller 55 to the developing roller 54 is prevented.

  As described above, the development of the electrostatic latent image can be stably performed by making the development bias voltage Va constant throughout the development of one page of paper. Further, the supply bias voltage Vb is controlled to be smaller than the developing bias voltage Va in the first half, and the supply bias voltage Vb is controlled to be larger than the developing bias voltage Va in the second half. The toner supply capability of the supply roller 55 can be maintained after the second rotation of the developing roller 54, which tends to be unable to catch up with the toner supply capability. Further, by increasing the supply bias voltage Vb in the second and subsequent rotations, it is possible to prevent blur due to toner shortage in the latter half of the development in one page.

  The above process can be conceptually explained as follows. As shown in FIG. 5, it is required that the density for solid black printing be kept uniform throughout the entire page. However, if the supply bias voltage Vb and the development bias voltage Va are kept at the same potential, the printing density in one page decreases as the second half of printing as shown by the dotted line in the figure due to the delay in toner supply from the supply roller 55. As a result, density unevenness and blurring occur.

  As a countermeasure against blurring, when the supply bias voltage Vb is set larger than the development bias voltage Va in order to prevent the density drop in the latter half, the change in the printing density over time in one page is as shown by the one-dot chain line in the figure. Become. In this case, it is considered that the latter half is settled to the target density, but when the first half and the second half are compared, a print density difference occurs. In addition, as described above, since the toner of sufficient density is carried on the developing roller 54 immediately after the start of development, the toner is excessively supplied at the time of development, and the toner is wasted.

  Therefore, if the control is performed so that the change in the print density shown in FIG. 5 is changed in the direction of the arrow, printing can be performed while maintaining the target density in one page. Specifically, in the first half of the development, the supply bias voltage Vb is made smaller than the development bias voltage Va, so that the printing density in the first half in FIG. suppress.

Then, control is performed so that the chain line in the latter half of FIG. 5 approaches the target value by reversing the magnitude relationship in the latter half. By controlling in this way, the print density does not decrease even in the latter half. Therefore, it is possible to keep the printing density in one page of paper uniform.
[Modification]
As mentioned above, although embodiment of this invention was described, a various deformation | transformation is possible for this invention in the range which does not deviate from the meaning. An example is described below.

  First, in the embodiment, the image forming process is performed with the developing bias voltage Va being constant, but there is a possibility that blurring may occur in the latter half of the development in one page. Therefore, the developing bias voltage Va may be controlled to slightly increase within a range where the magnitude relationship between the supply bias voltage Vb and the developing bias voltage Va does not change. Specifically, as shown in FIG. 6, the development bias voltage Va is increased in the latter half of the development, and the development bias voltage Va of 500 V is applied when printing for one page is completed. As a result, it is possible to more reliably prevent a shortage of toner developed on the photosensitive drum 52 in the second half of printing in one page.

  In the embodiment, the value of the supply bias voltage Vb is changed in a curve with time. For example, the supply bias voltage Vb is linearly set with the horizontal axis as the printing time in one page and the vertical axis as the value of the supply bias voltage Vb. It may be changed linearly like a function. If the magnitude relationship between the developing bias voltage Va and the supply bias voltage Vb is suddenly reversed simultaneously with the end of one rotation of the developing roller 54, the supply bias voltage Vb changes rapidly from 200V to 600V in this embodiment. Become. Then, the toner supply is disturbed at the moment of reverse rotation, and as a result, the formation of a uniform density image is hindered. Furthermore, there is a risk of damage to the electric circuit constituting the control device 76.

  Therefore, for example, if the supply bias voltage Vb is changed step by step by 0.1 V for each line, the developer bias voltage Va and the supply bias voltage Vb are increased or decreased while improving the developer supply force of the supply roller to the developing roller. Avoid sudden changes to reverse the relationship. As a result, density unevenness during reverse rotation can be suppressed satisfactorily.

  In the embodiment, the black solid is taken as an example, but the present invention can also be applied to normal document printing. In that case, even if the printing starts from the middle of the paper P, the same effect as in the above embodiment can be obtained if the supply bias voltage Vb is reversed within approximately one rotation of the developing roller from the printing start line.

  In the embodiments, positively charged toner has been described as an example, but it goes without saying that negatively charged toner can also be applied. That is, regardless of the charging polarity of the toner, the absolute value of the supply bias voltage Vb is smaller than the absolute value of the development bias voltage Va in the first half of development, and the supply bias is smaller than the absolute value of the development bias voltage Va in the second half. The supply bias voltage Vb may be supplied so that the absolute value of the voltage Vb is larger.

  In the embodiment, contact development is performed. However, the present invention can be applied to a non-contact development method in which development is performed with a distance between the photosensitive drum and the development roller being increased. Furthermore, in the embodiments, the monochrome printer has been described as an example, but the present invention can also be applied to a color laser printer.

DESCRIPTION OF SYMBOLS 1 Laser printer 2 Main body housing | casing 5 Process cartridge 52 Photosensitive drum 54 Developing roller 55 Supply roller 70A, 70B Bias supply unit

Claims (5)

An image carrier on which an electrostatic latent image is carried;
A developer carrying member carrying a developer for developing the electrostatic latent image into a developer image;
A developer supply member for supplying the developer to the developer carrier;
A transfer member for transferring the developer image to a sheet;
An image forming apparatus comprising: a bias applying unit that applies a developing bias voltage to the developer carrying member and applies a supply bias voltage to the developer supplying member;
The bias applying unit controls the absolute value of the supply bias voltage to be smaller than the absolute value of the development bias voltage in the first half of the development for one page of the paper, In the latter half of the image forming apparatus, the absolute value of the supply bias voltage is controlled to be larger than the absolute value of the developing bias voltage. The developer carrier is a developing roller;
The image forming apparatus according to claim 1, wherein the first half of the development for one page of the paper is from the start of development until the developing roller makes approximately one rotation.   The image forming apparatus according to claim 1, wherein the bias applying unit controls the supply bias voltage to gradually change in one page of the sheet.   4. The bias applying unit gradually increases the absolute value of the developing bias voltage within a range in which the first half is constant and the magnitude relationship does not change in the second half. The image forming apparatus according to any one of the above. The bias applying means applies the developing bias voltage applied to a developing roller carrying positively charged toner during the developing for one page of the paper and the supplying bias applied to a supplying roller disposed in contact with the developing roller. Voltage relationship,
In the first half, the supply bias voltage is smaller than the development bias voltage, and the value of the supply bias voltage is switched to make the supply bias voltage larger than the development bias voltage in the second half after the developing roller has rotated approximately once. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.