JP2015230407A - Control method of transfer current and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control method of a transfer current, when controlling the transfer current according to an image area ratio, capable of suppressing an occurrence of image density variation due to response delay of a transfer electric field.SOLUTION: An image forming apparatus includes a primary transfer roller 25 for transferring a toner image carried on a photoreceptor 2 to an intermediate transfer belt 21. A target control value of a primary transfer current to be applied from a primary transfer power supply 81 to the primary transfer roller 25 is set as follows, where a C toner is cited as an example. A target control value ICt of the primary transfer current, when the primary transfer current is changed, is set by taking account of an image area ratio x1c of an area B as a prescribed area for passing through an outlet of a primary transfer nip and an image area ratio x0c of an area A as a downstream-side area to which the C toner image is transferred.

Description

  The present invention relates to a transfer current control method used in an electrophotographic image forming apparatus such as a copying machine, a facsimile machine, and a printer, and an image forming apparatus using the transfer current control method.

  Conventionally, for a transfer means for transferring a toner image carried on an image carrier to a transfer target, a target control value of a transfer current applied from a power source is set to an image area of a predetermined area passing through an exit of the transfer area. An image forming apparatus that performs control according to a rate is known.

For example, Patent Document 1 describes the following image forming apparatus.
A photoconductor as four image carriers, an intermediate transfer belt as a transfer medium forming a primary transfer nip as a transfer area between the photoconductors, and a toner image carried on each photoconductor as an intermediate transfer belt And four primary transfer rollers as transfer means for transferring the toner to the ink. Then, a target control value (target value) for controlling the primary transfer current supplied from the power source to the primary transfer roller is set to a predetermined area (for 10 pixels in the sub-scanning direction) that passes through the exit of the primary transfer nip that is the transfer area. The area is controlled (changed) in accordance with the image area ratio (average image area ratio).

Further, the reason for the above control is described as follows.
Most of the current flowing between the photoconductor and the intermediate transfer belt is due to peeling discharge between the two at the primary transfer nip exit where the photoconductor and the intermediate transfer belt are separated. If the image area ratio of the photosensitive member is relatively low at the primary transfer nip exit, even though the amount of current supplied from the primary transfer power source is relatively small, most of the current supplied from the primary transfer power source It will be used for peeling discharge between the image area and the belt. Then, almost no current flows through the image portion of the photosensitive member, thereby causing a transfer failure.

By passing a transfer current according to the image area ratio in the vicinity of the primary transfer nip exit, an appropriate current is passed through the image area of the photoconductor, and the potential difference between the image area of the photoconductor and the intermediate transfer belt is determined from the discharge start voltage. Can also be reduced.
Further, it is described that by changing the transfer current according to the image area ratio of the photoconductor, the occurrence of image density fluctuation (image density unevenness) according to the image area ratio can be suppressed.
In other words, the image forming apparatus described in Patent Document 1 controls the transfer unit with a target control value of the transfer current supplied from the power source based on the image area ratio of a predetermined region that passes through the exit of the transfer region. Thus, it is possible to suppress the occurrence of image density fluctuations according to the image area ratio.

However, as a result of detailed investigations by the present inventors, even if the target control value of the transfer current supplied from the power source to the transfer unit is set according to the image area ratio as in Patent Document 1, the target control value is set. It has been found that image density fluctuation may occur in the area immediately after the change. This is considered to occur for the following reason.
If the target control value of the transfer current is changed according to the image area ratio for each predetermined area that simply passes through the transfer area, the transfer is delayed due to delay in feedback control of the power supply or charge / discharge due to transfer nip resistance / capacitor components. The electric field response cannot follow the control of the target control value. For this reason, it is considered that the effect of suppressing the occurrence of image density fluctuation by controlling the transfer current in accordance with the image area ratio is reduced in the region immediately after the target control value is changed.

  The present invention has been made in view of the above problems, and its purpose is to control the transfer current that can suppress the occurrence of image density fluctuations due to a response delay of the transfer electric field when the transfer current is controlled according to the image area ratio. It is to provide a control method.

  In order to achieve the above object, the invention described in claim 1 is provided with transfer means for transferring a toner image carried on an image carrier to a transfer target, and is supplied from a power source to the transfer means. In a transfer current control method used in an image forming apparatus that controls a transfer current based on an image area ratio of a predetermined region that passes through an exit of a transfer region formed between the image carrier and the transfer target. The target control value for changing the transfer current is set in consideration of the image area ratio of the predetermined area and the image area ratio of the area other than the predetermined area.

  The present invention can provide a transfer current control method capable of suppressing the occurrence of fluctuations in image density due to a response delay of a transfer electric field when the transfer current is controlled according to the image area ratio.

1 is a schematic configuration diagram showing a printer according to an embodiment. FIG. 3 is a block diagram showing a part of an electric circuit of the printer. FIG. 3 is an enlarged schematic diagram showing a chevron patch formed on an intermediate transfer belt. Explanatory drawing of the predetermined area | region and downstream area which pass a primary transfer nip. Explanatory drawing of the definition of an image area ratio. The graph which showed the acquisition result of the image area ratio of the image of FIG. 5 by the pixel position of a subscanning direction. The graph which showed the target control value of the primary transfer current of Example 1 set with respect to the acquisition result of the image area ratio in FIG. 6 by the pixel position in the sub-scanning direction. The graph which showed the target control value of the primary transfer current by the conventional structure by the pixel position of a subscanning direction set with respect to the acquisition result of the image area ratio of FIG. The graph which showed the image density fluctuation | variation which arises by the control based on the target control value set by the conventional structure by the pixel position of a subscanning direction. The graph which showed the intensity | strength of the primary transfer electric field obtained by control based on the target control value set by the conventional structure by the pixel position of the subscanning direction. The evaluation chart used when evaluating image density fluctuation. 10 is a graph showing the result of obtaining the image area ratio according to Example 2 in terms of pixel positions in the sub-scanning direction. FIG. 13 is a graph showing the target control value of the primary transfer current of Example 2 as a pixel position in the sub-scanning direction set for the image area ratio acquisition result of FIG. FIG. 13 is a graph showing another example of the target control value of the primary transfer current of Example 2 set for the image area ratio acquisition result of FIG. 12 in the pixel position in the sub-scanning direction. 10 is a graph showing the result of acquiring the image area ratio according to Example 3 in terms of pixel positions in the sub-scanning direction. The graph which showed the target control value of the primary transfer current of Example 3 by the pixel position of a subscanning direction set with respect to the acquisition result of the image area ratio of FIG. When the image area ratio corresponding to each color is the same for all colors, the target control value of the secondary transfer current of Example 4 set for the image area ratio acquisition result in FIG. 15 is set in the sub-scanning direction. The graph shown by the pixel position. FIG. 9 is a schematic configuration diagram illustrating a printer according to a first modification. FIG. 9 is a schematic configuration diagram illustrating a printer according to a second modification. FIG. 9 is a schematic configuration diagram illustrating a printer according to a third modification.

Hereinafter, an embodiment in which the present invention is applied to a color printer (hereinafter simply referred to as “printer”) that forms a color image by a tandem type image forming unit as an image forming apparatus will be described.
Note that this embodiment shows an example, and it has been confirmed in a plurality of image forming apparatuses and various image forming environments that the effect of the present invention does not change even if the configuration and process conditions change.

First, a basic configuration of the printer according to the embodiment will be described.
FIG. 1 is a schematic configuration diagram illustrating a printer according to the present embodiment.
This printer has four process units 1Y, 1M, 1C, and 1K for yellow (Y), magenta (M), cyan (C), and black (K) as means for forming toner images (hereinafter referred to as toner images). It has. The process units 1Y, M, C, and K use Y, M, C, and K toners of different colors as image forming substances for forming an image, but are otherwise configured in the same manner.

  Here, taking the process unit 1Y for forming a Y toner image as an example, the process unit 1Y holds the photosensitive member 2Y, the developing device 3Y, the charging device 4Y, the photosensitive member cleaning device 5Y, etc. as one unit. Hold on the body. Then, it is integrally attached to and detached from the printer main body.

  The charging device 4Y has a charging roller disposed so as to be in contact with or close to the photoreceptor 2Y, and the charging roller is rotationally driven by a driving unit (not shown). A predetermined charging bias is applied to the charging roller by a charging power source (not shown). Then, by generating a discharge between the charging roller and the photoreceptor 2Y, the surface of the photoreceptor 2Y is uniformly charged to the same polarity (for example, about −500 [V]) as the normal charging polarity of the toner. A scorotron charger or the like may be employed instead of such a charging device.

The photoreceptor 2Y is a drum-shaped latent image carrier having a diameter of 30 mm with an organic photosensitive layer coated on the surface, and the electrostatic capacity is adjusted to 9.5 × 10 ⁻⁷ [F / m ² ]. Yes. And it is rotationally driven in the clockwise direction in the figure by a driving means (not shown). The surface of the photoreceptor 2Y that is uniformly charged by the charging device 4Y is exposed and scanned by a laser beam emitted from an optical writing unit 90, which will be described later, and carries an electrostatic latent image for Y.

  The developing device 3Y contains a developer (not shown) containing Y toner, which is a polyester-based polymer toner, and a magnetic carrier. An opening is formed in the casing of the developing device 3Y, and a part of the peripheral surface of the cylindrical developing sleeve provided in the developing device 3 is exposed from the opening and faces the surface of the photoreceptor 2Y. The developing sleeve carries the developer in the casing by a magnetic force generated by a magnet roller (not shown) that is fixed to the developing sleeve so as not to rotate with itself that rotates counterclockwise in the drawing by a driving motor (not shown). Then, along with its own rotational drive, a necessary amount of developer is transported to a development area where the photoconductor 2Y faces itself.

In the development area, the developer held on the surface of the development sleeve is spiked by the magnetic force of the magnet roller in the development area, and the magnetic spike on the surface of the development sleeve comes into contact with the surface of the photoreceptor 2Y.
Then, a developing potential for moving negatively charged Y toner from the developing sleeve side to the photosensitive member 2Y side acts between the developing bias applied to the developing sleeve and the electrostatic latent image on the photosensitive member 2Y. The toner moves from the agent to the surface of the photoreceptor 2Y and is developed. In addition, a non-image potential that moves Y toner of negative polarity from the photosensitive member side to the sleeve side acts between the developing sleeve and the background portion of the photosensitive member 2Y. The Y toner in the developer is transferred to the electrostatic latent image on the photoreceptor 2Y in the development area by the action of the development potential described above. As a result, the electrostatic latent image on the photoreceptor 2Y is developed into a Y toner image.

The developing device 3Y has a toner density sensor (not shown) that measures the toner density of the internal developer. The detection result by the toner density sensor is sent to a control unit (not shown) as a voltage signal. The control unit includes a RAM, in which the target value of the output voltage from the toner density sensor is stored. Then, the value of the output voltage from the toner density sensor is compared with the target value, and the Y toner supply device (not shown) is driven for a time corresponding to the comparison result. By this driving, an appropriate amount of Y toner is supplied to the developer whose Y toner concentration has been reduced by consumption of Y toner accompanying development, and the toner concentration of the developer in the developing device 3Y is maintained within a predetermined range. .
Further, Y toner is supplied from a toner bottle (not shown) to the toner supply device for Y, and the same toner supply control is performed for the developers in the developing devices 3M, C, and K for other colors. The

Although the Y process unit 1Y has been described in detail, the process units 1M, 1C, and 1K for other colors have the same configuration, and M, C, and K toner images are formed on the photoreceptors 2M, C, and K, respectively. It is formed. Further, the image forming operation of each color by each process unit 1 is performed from the upstream side in the endless movement direction of the intermediate transfer belt 21 so that the formed toner images are transferred to the same position on the intermediate transfer belt 21. It is executed with the timing shifted toward the side.
Note that the toner adhesion amount per unit area when a full-color image is formed on the photoreceptor is about 0.45 [mg / cm ² ].

  An optical writing unit 90 is disposed below the process units 1Y, M, C, and K. The optical writing unit 90 as a latent image forming unit includes four laser diode (LD) light sources prepared for each color, and a set of polygon scanners composed of six polygon mirrors and a polygon motor. I have. In addition, a lens such as an fθ lens and a long cylindrical lens arranged in the optical path of each light source and a mirror are also provided. The optical writing unit 90 scans the uniformly charged surfaces of the photoconductors 2Y, 2M, 2C, and 2K by irradiating laser light L emitted from each light source from each light source and performing deflection scanning with a polygon scanner. To do.

The potential of the image portion which is a laser exposure portion in the photoreceptors 2Y, 2M, 2C, and 2K is attenuated, for example, the residual potential at the time of overall exposure is attenuated to about −30 [V], and the non-image that is the surrounding background portion. The potential is lower than that of the portion. A portion in such a state becomes an electrostatic latent image of Y, M, C, and K.
The optical writing unit 90 applies the laser light L emitted from the light source to the photoreceptors 2Y, M, C, and K via a plurality of optical lenses and mirrors while deflecting the laser light L by a polygon mirror that is driven to rotate by a motor. Irradiation. Instead of such a configuration, it is also possible to employ one that performs optical scanning with an LED array.

Above the process units 1Y, 1M, 1C, and 1K, the intermediate transfer belt 21 is moved endlessly in the counterclockwise direction in the drawing while the lower stretched surface is brought into contact with the photoreceptors 2Y, M, C, and K. A transfer unit 20 that forms a primary transfer nip for Y, M, C, and K is disposed. The Y, M, C, and K toner images formed on the photoreceptors 2Y, 2M, 2C, and 2K are primarily transferred onto the intermediate transfer belt 21 in a primary transfer nip for each color.
The transfer residual toner adhering to the surface of the photoreceptors 2Y, M, C, and K after passing through the primary transfer nips for Y, M, C, and K is removed by the photoreceptor cleaning devices 5Y, M, C, and K. It is removed from the surface of the photoreceptor.

  A paper feed cassette 95 is disposed below the optical writing unit 90. In the paper feed cassette 95, a plurality of paper sheets P as recording media are stored in a bundle of paper sheets, and a paper feed roller 95a is in contact with the uppermost paper sheet P. When the paper feed roller 95a is driven to rotate counterclockwise in the figure by a driving means (not shown), the uppermost paper P in the paper feed cassette 95 extends in the vertical direction on the right side of the cassette in the figure. The paper is discharged toward the disposed paper feed path. The paper P sent to the paper feed path is conveyed from the lower side to the upper side in the figure. The process linear velocity, which is the linear velocity of the photoreceptors 2Y, M, C, K and the intermediate transfer belt 21, is set to about 350 [mm / s].

  A registration roller pair 32 is disposed at the end of the paper feed path. The registration roller pair 32 temporarily stops the rotation of both rollers as soon as the sheet P is sandwiched between the rollers. Then, the paper P is sent out toward a secondary transfer nip described later at an appropriate timing. Note that the conveyance timing of the paper P is detected by a sensor (not shown), and the registration roller pair 32 sends the paper P toward the secondary transfer nip according to an instruction from the sensor.

  The transfer unit 20 disposed above the process units 1Y, 1M, 1C, and 1K includes, in addition to the intermediate transfer belt 21, primary transfer rollers 25Y, M, C, and K, and a driven roller 23 disposed in the belt loop. And a secondary transfer counter roller 24. Further, it also has a secondary transfer roller 26 and a belt cleaning device 28 disposed outside the belt loop.

The intermediate transfer belt 21 is an endless belt having a thickness of 80 [μm] having a belt base made of conductive polyamideimide resin in which carbon is dispersed, and its volume resistivity is 1 × 10 ⁹ [Ω · cm]. Has been adjusted. The volume resistivity is a value measured under a voltage application condition of 100 V with a Hirester UP MCP HT450 manufactured by Mitsubishi Chemical.
Then, in a state of being stretched around each of the rollers disposed in the belt loop, it is endlessly moved in the counterclockwise direction in the figure by the rotational drive of at least one of the rollers.

  The primary transfer rollers 25 </ b> Y, M, C, and K are for primary transfer of the toner images formed on the photoreceptors 2 </ b> Y, M, C, and K onto the intermediate transfer belt 21. Then, the primary transfer rollers 25Y, 25M, 25C, 25C, and 25K are placed at positions facing the respective photoreceptors 2 with the intermediate transfer belt 21 that moves endlessly interposed therebetween. It is arranged to press. As a result, primary transfer nips for Y, M, C, and K where the intermediate transfer belt 21 and the photoreceptors 2Y, 2M, 2C, and 2K abut are formed.

The primary transfer rollers 25Y, 25M, 25C, and 25K are formed by providing a conductive sponge roller portion made of a resin in which an ionic conductive agent is dispersed on the peripheral surface of a metal rotating shaft member. The volume resistivity of the conductive sponge roller portion is about 5 × 10 ⁸ [Ω · cm]. Then, the axis of the metal rotating shaft member is arranged 3 mm downstream from the rotating shaft of the photoreceptors 2Y, 2M, 2C, and 2K in the belt moving direction.

To the primary transfer rollers 25Y, 25M, 25C, and 25K, a primary transfer bias having a polarity opposite to the charging polarity of the toner is applied by primary transfer power supplies 81Y, 81M, 81C, and 81K. As a result, a transfer electric field is formed in the primary transfer nip to draw the toner image on each photoconductor 2 from the photoconductor 2 side to the intermediate transfer belt 21 side.
The intermediate transfer belt 21 passes through the primary transfer nips for Y, M, C, and K sequentially along with the endless movement thereof, and the photoreceptors 2Y, M, C, The Y, M, C, and K toner images on K are superimposed and primarily transferred. As a result, a toner image in which four colors are superimposed on the intermediate transfer belt 21 (hereinafter referred to as a four-color toner image) is formed.

  The secondary transfer counter roller 24 disposed on the inner side of the belt loop is disposed so that the intermediate transfer belt 21 is sandwiched between the secondary transfer roller 26 disposed on the outer side of the belt loop. Thus, a secondary transfer nip where the front surface of the intermediate transfer belt 21 and the secondary transfer roller 26 abut is formed on the right side of the intermediate transfer belt 21 in the drawing. The registration roller pair 32 described above feeds the paper P sandwiched in the secondary transfer nip toward the secondary transfer nip at a timing at which the paper P can be synchronized with the four-color toner image on the intermediate transfer belt 21.

  A secondary transfer bias having a polarity opposite to that of the toner is applied to the secondary transfer roller 26. The four-color toner images on the intermediate transfer belt 21 are secondarily transferred collectively onto the paper P in the secondary transfer nip by the action of the secondary transfer bias and nip pressure. Then, combined with the white color of the paper P, a full color toner image is obtained.

  Untransferred toner that has not been transferred to the paper P adheres to the intermediate transfer belt 21 after passing through the secondary transfer nip. This is cleaned by the belt cleaning device 28. The belt cleaning device 28 has a cleaning roller in contact with the front surface of the intermediate transfer belt 21, and removes transfer residual toner on the intermediate transfer belt 21 by electrostatic transfer to the cleaning roller. .

A fixing device 40 is disposed above the secondary transfer nip. The fixing device 40 forms a fixing nip by abutting a fixing roller 41 containing a heat source such as a halogen lamp and a pressure roller 42 pressed toward the fixing roller 41. The paper P that has passed through the secondary transfer nip is separated from the intermediate transfer belt 21 and then sent into the fixing device 40. Then, in the process of being conveyed from the lower side to the upper side in the figure while being sandwiched by the fixing nip in the fixing device 40, the full-color toner image is fixed by being heated or pressed by the fixing roller 41. .
The paper P thus subjected to the fixing process exits the fixing device 40 and is then discharged out of the apparatus through a pair of paper discharge rollers (not shown).

The printer also includes a control unit 200 (not shown in FIG. 1) as a control unit for each unit in the apparatus main body.
FIG. 2 is a block diagram showing a part of the electric circuit of the printer.
As shown in FIG. 2, the control unit 200 includes a CPU (Central Processing Unit) 200 a that is a calculation unit. Further, it also includes a ROM (Read Only Memory) 200b which is a non-volatile memory, a RAM (Random Access Memory) 200c which is a temporary storage unit, and the like.
The control unit 200 controls the entire apparatus, and various devices and sensors are connected. In FIG. 2, only a part of the devices is shown.

Based on a control program stored in the ROM 200b, the control unit 200 reads data input from each sensor or external device into the RAM 200c, performs arithmetic processing by the CPU 200a, and controls the operation of each unit in the apparatus main body. .
Further, based on a writing signal at the time of exposure generated from image information sent from a personal computer or the like which is an external device, the image area ratio of each color toner image (image) on each photoconductor 2 is determined as a predetermined pixel. It has a function of calculating in units of areas.
That is, the image area ratio of the toner image to be transferred from each photoconductor 2 to the intermediate transfer belt 21 and from the intermediate transfer belt 21 to the paper P is calculated in units of a predetermined pixel area. .

Based on the calculated image area ratio, a target control value of the primary transfer current applied to each primary transfer roller 25 is determined, and the primary transfer power supply 81Y is set so that the applied primary transfer current becomes the determined target control value. , M, C, K also functions as a primary transfer current control means. Similarly, a target control value of the secondary transfer current applied to the secondary transfer roller 26 is determined based on the calculated image area ratio, and the applied secondary transfer current becomes the determined target control value. Also, it functions as a secondary transfer current control means for controlling the secondary transfer power source 82.
Note that the target control value of the primary transfer current output and the target control value of the secondary transfer current output are output from the control unit 200 as PWM signals, and the primary transfer power supplies 81Y, 81M, 81C, 81K, and the secondary transfer current are output. Input to the power source 82.

Further, the control unit 200 performs a positional deviation amount correction process immediately after a main power switch (not shown) is turned on or whenever a predetermined number of prints are performed. In this misregistration amount correction process, misregistration detection images made up of a plurality of toner images called chevron patches PV as shown in FIG. 3 are formed on the intermediate transfer belt 21.
The optical sensor unit 86 passes the light emitted from the light emitting means through the condensing lens, reflects the light on the surface of the intermediate transfer belt 21, and receives the reflected light by its own light receiving means according to the amount of light received. Output voltage.

When the toner image in the chevron patch PV formed on the intermediate transfer belt 21 passes directly below the optical sensor unit 86, the amount of light received by the light receiving means of the optical sensor unit 86 changes greatly. Accordingly, the control unit 200 detects each toner image in the chevron patch PV formed on the intermediate transfer belt 21, thereby detecting the position of each toner image in the sub-scanning direction (moving direction of the intermediate transfer belt 21). Can do. As described above, the optical sensor unit 86 functions as an image detection unit in combination with the control unit 200.
As the light emitting means, an LED or the like having an amount of light that can generate reflected light necessary for detecting a toner image is used. As the light receiving means, a CCD in which a large number of light receiving elements are arranged in a straight line is used.

As shown in FIG. 3, the chevron patch PV tilts the toner image of each color of Y, M, C, and K by about 45 [°] from the main scanning direction (the direction in which the laser beam scans on the surface of each photoreceptor 2). A line pattern group arranged at a predetermined pitch in the belt moving direction, which is the sub-scanning direction, with a predetermined posture. For such Y, M, and C toner images in the chevron patch PV, the optical sensor unit 86 reads the detection time difference from the K toner image.
In FIG. 3, the vertical direction corresponds to the main scanning direction in the drawing, and after the Y, M, C, and K toner images are arranged in order from the left in the drawing, the posture differs by 90 [°]. K, C, M, and Y toner images are further arranged.
Based on the difference between the actual measurement value and the theoretical value of the detection time differences tky, tkm, and tkc from the reference color K, the amount of deviation in the sub-scanning direction of each color toner image, that is, the amount of positional deviation is obtained. Then, the optical writing start timing for each photoconductor 2 of the optical writing unit 90 is corrected based on each positional deviation amount, and the toner image of each color caused by the speed fluctuation of each photoconductor 2 or intermediate transfer belt 21 is corrected. Reduces misalignment.

  Next, a method for determining a target control value for a primary transfer current and a secondary transfer current based on image area ratio information of a plurality of regions on each photoconductor 2, which is a feature of the printer of the present embodiment, Several examples will be described.

(Example 1)
First, Example 1 of this embodiment will be described with reference to the drawings, taking as an example the control of the primary transfer current when the C toner image formed by the process unit 1C is transferred to the intermediate transfer belt 21.
FIG. 4 is an explanatory diagram of a predetermined region and a downstream region that pass through the primary transfer nip, FIG. 5 is an explanatory diagram of the definition of the image area ratio, and FIG. 6 is a sub-scan of the acquisition result of the image area ratio of the image of FIG. It is the graph shown by the pixel position of a direction. FIG. 7 is a graph showing the target control value of the primary transfer current of the present embodiment, which is set for the image area ratio acquisition result of FIG. 6, in the pixel position in the sub-scanning direction.

  FIG. 8 is a graph showing the target control value of the primary transfer current in the conventional configuration (Comparative Example 1) set for the image area ratio acquisition result of FIG. FIG. 9 is a graph showing the image density variation caused by the control based on the target control value set in the conventional configuration as the pixel position in the sub-scanning direction. FIG. 10 is a graph showing the intensity of the primary transfer electric field obtained by the control based on the target control value set in the conventional configuration in terms of the pixel position in the sub-scanning direction, and FIG. 11 is used when evaluating the image density fluctuation. It is an evaluation chart.

The target control value of the primary transfer current for transferring the C toner image is determined using a plurality of pieces of image area ratio information with respect to the main scanning direction of the C toner image.
Specifically, the image area ratio is calculated as an average image area ratio that is an average value of 300 pixel units (600 dpi) in the sub-scanning direction (moving direction of the intermediate transfer belt 21). Then, the target control value of the primary transfer current set while the image area on the intermediate transfer belt 21, which is an individual predetermined area for each 300 pixels for which the average image area ratio is calculated, passes through the outlet of the primary transfer nip. Is determined as follows.
As shown in FIG. 4, the target control value of the predetermined area passing through the exit of the primary transfer unit is adjacent to the average image area ratio of 300 pixels of itself and the downstream side in the moving direction of the intermediate transfer belt 21 that has already passed through the exit. It is determined from two pieces of information, that is, the average image area ratio of the downstream area that is the area. The downstream area is also an area on the intermediate transfer belt 21 where the C toner image has already been primarily transferred through the outlet as described above.

Here, the average image area ratio will be described with reference to FIG.
The average image area ratio is a numerical value indicating the ratio of the image area in the pixels in each sub-scanning direction to the width: W (= 297 [mm]) of the image forming region of the intermediate transfer belt 21 in the main scanning direction. In the embodiment, an average value for every 300 pixels is defined as an average image area ratio.
For example, in the area A shown in FIG. 5, there is only a belt-like solid pattern having a width of 14.9 [mm], and the width in the main scanning direction is the width of the image forming area: W × 0.05 [ mm].
This corresponds to 0.05 (5%) of the image forming area width W of the intermediate transfer belt 21, and the average image area ratio is 5%.

On the other hand, since the entire area of the regions B and C is a solid image, the average image area ratio is 100%.
The average image area ratio can be calculated by using an image input signal obtained from a personal computer or a scanner or a laser writing signal when forming a latent image on the photosensitive member 2C. As described above, the latter method is adopted.

Next, a method for determining the target control value of the primary transfer current will be described.
The average image area ratio of an arbitrary predetermined area passing through the primary transfer nip of the C toner image on the photoreceptor 2C is x1c, and the predetermined area adjacent to the arbitrary predetermined area passing through the primary transfer nip has already been primary transferred. Let x0c be the average image area ratio of the downstream region. However, 10 ≦ x0c ≦ 1 and 0 ≦ x1c ≦ 1.
Then, the target control value: ICt of the primary transfer current supplied (applied) from the primary transfer power supply 81C to the primary transfer roller 25C is determined using the following equation (1).
(Formula 1):
ICt (x0c, x1c) = 24 × (x0c−2 × 1c) +84 [μA]
Note that the coefficient “84” in Equation 1 is a base current value, and the coefficient “24” is a value derived from an experiment or the like.

As can be seen from Equation 1, an arbitrary target control value: ICt that passes through the primary transfer nip is determined as follows.
When the average image area ratio x0c of the downstream area adjacent to the downstream side is larger than the average image area ratio x1c of the predetermined area, it is determined so as to increase as the difference increases. On the other hand, when the average image area ratio x0c in the downstream region is small, it is determined so as to decrease as the difference increases.

Here, the above equation 1 is obtained when the average image area ratio: x1c of an arbitrary predetermined region passing through the primary transfer nip is the same as the average image area ratio: x0c of the downstream region adjacent to the downstream side (x0c = x1c), the variable can be transformed into the following formula 2 with only the average image area ratio: x1c.
(Formula 2):
ICt1 (x1c) = − 24 × x1c + 84 [μA]
This predetermined formula 2 is an arbitrary predetermined area that passes through the primary transfer nip without considering the average image area ratio x0c in the downstream area that passes through the primary transfer nip of the conventional configuration (hereinafter referred to as the conventional example) described later. The average image area ratio is equivalent to an equation for obtaining a target control value according to x1c alone.
Hereinafter, a target control value: ICt1 corresponding to only an average image area ratio: x1c of an arbitrary predetermined region passing through the primary transfer nip is referred to as a reference target set value.
When the average image area ratio: x1c of an arbitrary predetermined area passing through the primary transfer nip and the average image area ratio: x0c of the downstream area are the same, the target control value: ICt of the predetermined area is the reference of the downstream area. The target control value: ICt1, which is the same as the target control value: ICt1, is determined.

For example, when the image transferred from the photoreceptor 2C to the intermediate transfer belt 21 is the image shown in FIG. 5 and the average image area ratio calculated in units of 300 pixels is as shown in FIG. The target control value of the primary transfer current when passing through the outlet is set as shown in FIG.
That is, when the intermediate transfer belt 21 and the photoconductor 2C move by 300 pixels in the sub-scanning direction, the primary transfer current supplied from the primary transfer power supply 81C to the primary transfer roller 25 is set to the target control value: ICt obtained by Expression 1. It is controlled to change.
Here, both the area B and the area C have an average image area ratio of 100%, but in the case of the area B, the average image area ratio of the area A serving as the downstream area is 5%. A target control value: ICt lower than that in the region C is set.
In this way, by setting the target control value: ICt for the region B and controlling the primary transfer current, an intermediate from the photoreceptor 2C that occurs near the beginning of the region B, which is the boundary between the region A and the region B, is performed. Occurrence of fluctuations in the image density of the C image transferred to the transfer belt 21 can be suppressed. Further, in the case of full color, it is possible to suppress the occurrence of fluctuations in image density due to reverse transfer of Y and M toner images that have already been primarily transferred onto the intermediate transfer belt 21.

  Here, as in the conventional example, without considering the average image area ratio of the downstream area, only the average image area ratio of the areas A, B, and C is substituted into the above equation 2 and is simply shown in FIG. As described above, it is assumed that the reference target set value: ICt1 is determined and the primary transfer power supply 81C is controlled. As a result, the image density of the C toner image in the region B on the photoreceptor 2C and the Y and M toner images already existing on the intermediate transfer belt 21 in the case of full color are affected by the delay of the feedback control of the power source, the transfer nip resistance, The charging / discharging delay due to the capacitor component generally varies as shown in FIG. This is considered to be due to the following reason.

In the conventional example set as shown in FIG. 8, only the average image area ratio: x1c of an arbitrary predetermined region passing through the primary transfer nip so that the primary transfer electric field becomes constant at the reference target setting value: ICt1. The value of the reference target set value ICt1 obtained by the above equation 2 is set. However, as described above, when the reference target set value: ICt1 is simply set due to the delay in feedback control of the primary transfer power supply 81 or the delay in charging / discharging due to the transfer nip resistance / capacitor component, the response of the primary transfer current or the primary transfer electric field may be caused. I can not follow. For this reason, it is considered that the primary transfer electric field changes as shown in FIG.
For this reason, the effect of controlling the primary transfer current according to the average image area ratio is weakened, resulting in image density fluctuations and reverse transfer in the case of full color. In particular, when an image in which the average image area ratio changes drastically at a pitch of about 300 pixels is output, an image resulting from a decrease in the primary transfer ratio in an area immediately after changing the reference target setting value: ICt1. Phenomena in which density fluctuation occurs or reverse transfer occurs more remarkably appear.

On the other hand, as shown in FIG. 7, the target control value: ICt obtained by the equation 1 in consideration of the average image area ratio of the downstream region: x0c is set as shown in FIG. If the primary transfer current is controlled so as to be ICt, the above fluctuation can be suppressed.
In other words, the target control value: ICt is determined in consideration of the delay of the primary transfer power supply 81C and the like according to Equation 1 using the average image area ratio: x1c of the downstream area in addition to the average image area ratio: x1c of the predetermined area. Can be requested. Then, the target control value ICt obtained in this way is set as shown in FIG. 7, and the primary transfer current supplied to the primary transfer roller 25C is controlled to improve the primary transfer rate and reverse transfer. Reduction can be realized reliably.

The reason why the fluctuation of the primary transfer electric field can be suppressed as described above is as follows.
That is, when the average image area ratio x1c of the predetermined area is different from that of the downstream area, the target control value ICt of the predetermined area is lowered or raised below the reference target control value ICt1 according to the difference. By doing so, the amount of change in the target control value: ICt of the primary transfer current when the average image area ratio is switched from the average image area ratio of the downstream region: x0c to the average image area ratio of the predetermined region: x1c. Enlarge. By increasing the amount of change in this way, the amount of change per unit time when changing the target control value: ICt of the primary transfer current can be increased. Then, it is possible to shorten the time until the primary transfer current actually supplied from the primary transfer power supply 81C to the primary transfer roller 25C reaches the reference target control value: ICt1.

Therefore, the time until the primary transfer electric field is generated according to the average image area ratio: x1c is shortened at least in the vicinity of the most downstream portion of the predetermined area where the average image area ratio is switched, and the response characteristic of the primary transfer electric field is higher than in the past. Can be improved. Then, by repeating the control for changing the target control value: ICt of the primary transfer current in this manner for each predetermined region that sequentially passes through the exit of the transfer region, the response characteristic of the primary transfer electric field can be set to the abbreviation of the image forming region. It is possible to improve the conventional configuration over the entire area.
Therefore, when controlling the primary transfer current according to the average image area ratio: x1c, it is possible to provide a transfer current control method capable of suppressing the occurrence of image density fluctuation due to the response delay of the primary transfer electric field.

Similarly, target control values of the primary transfer currents of the Y, M, and C stations: IYt (x0y, x1y), IMt (x0m, x1m), and IKt (x0k, x1k) are expressed by using the following Expressions 3 to 5. decide. However, 10 ≦ x0y, m, k ≦ 1, 0 ≦ x1y, m, k ≦ 0.
(Formula 3):
IYt (x0y, x1y) = 24 × (x0y−2x1y) +84 [μA]
(Formula 4):
IMt (x0m, x1m) = 24 × (x0m−2 × 1m) +84 [μA]
(Formula 5):
IKt (x0k, x1k) = 22 × (x0k−2 × 1k) +73 [μA]

In the conventional example shown in FIG. 8, the following equations 6 to 8 are used.
(Formula 6):
IYt1 (x1y) = − 24 × x1y + 84 [μA]
(Formula 7):
IMt1 (x1m) = − 24 × x1m + 84 [μA]
(Formula 8):
IKt1 (x1k) = − 22 × x1k + 73 [μA]

The inventors conducted the following evaluation test in order to confirm the effect of the primary transfer current control method of this example. In addition, the same evaluation test was conducted as the comparative example with respect to the conventional example.
As shown in FIG. 11, the average image area ratio is 300 pixels in the sub-scanning direction with respect to the green solid vertical band (image area is 10% constant) formed in the approximate center of the image forming area along the sub-scanning direction. An image in which 90% K solid horizontal bands were repeatedly arranged every 300 pixels was used as an evaluation chart.
Then, when the control is performed using the target control value IKt obtained from Expression 5 and when the control is performed using the reference target control value IKt1 obtained from Expression 8, The color difference ΔE with the green at the position of the horizontal band of the color was measured. That is, in the case of the present embodiment using the equation 5 for obtaining the target control value: ICt in consideration of the delay such as the followability of the primary transfer power supply 81C and the reference obtained simply from the average image area ratio of each region: x1k. The color difference ΔE was measured in the case of Comparative Example 1 using Equation 8 for obtaining the target control value: IKt1.
Note that an X-Rite 938 Spectrodensitometer was used to measure the color difference ΔE.

Since the evaluation of the image density fluctuation itself varies depending on the paper type to be used and the image to be formed, it is difficult to evaluate with one criterion. It is difficult for those skilled in the art to evaluate using the color fluctuation (color difference ΔE). This is the method used in the past. That is, the occurrence of the image density fluctuation can be reduced by reducing the color fluctuation (color difference ΔE).
Note that when the color difference ΔE is 2 or more, the image density fluctuation can be seen with the naked eye, and when the color difference ΔE is 3 or more, it can be clearly confirmed. The ideal value of the color difference ΔE is “0”.
The reason why the solid color and the K (black) solid are evaluated without evaluating the single color is that green is most prone to color variation (color difference ΔE).

上記表１に示すように、比較例の色差：ΔＥが１．０であるのに対し、実施例１では、色差：ΔＥが０．５であり、本実施例の制御を行うことによって、ΔＥは０．５改善されており、色変動が低減できていることがわかる。すなわち、本実施例の制御を行うことで、画像濃度変動の発生を低減できていることを確認できた。 And the measurement result as shown in following Table 1 was obtained.

As shown in Table 1 above, the color difference: ΔE of the comparative example is 1.0, whereas in Example 1, the color difference: ΔE is 0.5. By performing the control of this embodiment, ΔE Is improved by 0.5, and it can be seen that the color variation can be reduced. That is, it was confirmed that the occurrence of image density fluctuations can be reduced by performing the control of this embodiment.

  In this embodiment, the average image area ratio is calculated every 600 dpi and 300 pixels. However, the interval for calculating the average image area ratio is not limited to this, and may be larger, for example, 1000 pixels. It is also possible to set a smaller value. As a matter of course, the resolution is not limited to 600 dpi.

(Example 2)
Example 2 of this embodiment will be described with reference to the drawings, taking as an example the control of the primary transfer current when the C toner image formed by the process unit 1C is transferred to the intermediate transfer belt 21, as in Example 1. To do.
This embodiment differs from the first embodiment described above only in that the target control value of the primary transfer current is changed within a predetermined region in the present embodiment. Since the points relating to the other configurations are the same, in the following description, the same configurations as in the first embodiment, and the operations and effects thereof will be omitted as appropriate.

  FIG. 12 is a graph showing the image area ratio acquisition result according to the pixel position in the sub-scanning direction according to the present embodiment, and FIG. 13 is set for the image area ratio acquisition result of FIG. 5 is a graph showing the target control value of the primary transfer current in the pixel position in the sub-scanning direction. FIG. 14 is a graph showing another example of the target control value of the primary transfer current of the present embodiment set for the image area ratio acquisition result of FIG. 12 in the pixel position in the sub-scanning direction.

In the first embodiment described above, the average image area ratio: x1c of the predetermined area to be primarily transferred and the average image area ratio: x0c of the downstream area downstream thereof are not changed (with respect to time) within the predetermined area. The target control value ICt of a fixed primary transfer current was determined.
However, the target control value ICt of the primary transfer current is not necessarily determined to be constant within a predetermined area.
Therefore, in this embodiment, unlike the first embodiment in which the target control value of the primary transfer current that is constant in the predetermined area is set, the target control value: ICt of the primary transfer current is changed in the predetermined area. .

  For example, as shown in FIG. 12, when the average image area ratio acquisition interval is large, such as 1000 pixels, a method of changing the target control value: ICt within a predetermined area is also effective as shown in FIGS. is there. That is, in the example shown in FIG. 13, the average image area ratio: x1c of the predetermined area is acquired every 1000 pixels, but the target control value: ICt of the primary transfer current is the average image area ratio of the downstream area: x0c. In consideration of the difference between the two, every 500 pixels are changed in steps. By changing in this way, it is possible to improve the followability of the transfer electric field at least in the vicinity of the most downstream end of the predetermined region.

In the control method of the target control value: ICt of the primary transfer current shown in FIG. 13, the following equations 9 and 10 are set.
(Formula 9):
ICt (x0c, x1c) = 24 × (x0c-2x1c) +84 [μA] (region of 500 pixels downstream)
(Formula 10):
ICt (x1c) = − 24 × 1 + 84 [μA] (upstream 500 pixel region)
Here, x0c represents the image area ratio of the downstream area, and x1c represents the image area ratio of the predetermined area.
In other words, the target control value: ICt of the primary transfer current in the portion corresponding to 500 pixels downstream of the predetermined region is obtained from Equation 9 in consideration of the average image area ratio: x0c of the downstream region, and is calculated for 500 pixels upstream of the predetermined region. Was obtained from Equation 10 for deriving the reference target control value: ICt1.

Further, in the control method of the target control value: ICt of the primary transfer current shown in FIG. 14, a control function is used in which the target control value: ICt of the 500 pixels downstream of the image changes with a linear function. In other words, the target control value: ICt at the leading end portion (the most downstream end) of the image is set to the value obtained by Equation 9 above, and is obtained from Equation 10 at a position 500 pixels upstream from the leading end while being changed by a linear function. It is controlled so that it becomes the value.
Thus, when changing the target control value: ICt of the primary transfer current within a predetermined area, the change timing, the number of steps, the function, etc. are determined in consideration of the followability of the electric field of each actual machine.

Actually, the switching timing, the number of steps, and the function are determined so that ΔE when the chart as shown in FIG. 11 is printed becomes small. However, if the primary transfer current is frequently switched within a predetermined area, the load on the CPU and the memory of the control unit 200 increases. Therefore, the area should be determined in consideration of the CPU performance and other control loads. .
Therefore, as in this embodiment, by changing the target control value: ICt of the primary transfer current within a predetermined area, the CPU performance of the control unit 200 mounted on the printer and other control loads are taken into consideration. Therefore, it is possible to set the optimum number of pixels in the predetermined area.

(Example 3)
In Example 3 of this embodiment, as in Examples 1 and 2, the control of the primary transfer current when the C toner image formed by the process unit 1C is transferred to the intermediate transfer belt 21 is taken as an example, and the figure is used. I will explain.
In the present embodiment and the first and second embodiments described above, when the present embodiment determines the target control value of the primary transfer current, in addition to the image area ratio of the predetermined region and the downstream region, the upstream side of the predetermined region The only difference is that it also considers the image area ratio of the upstream region adjacent to. Since the points relating to the other configurations are the same, in the following description, the configurations similar to those of the first and second embodiments, and the operations and effects thereof will be omitted as appropriate.
15 is a graph showing the image area ratio acquisition result according to the pixel position in the sub-scanning direction according to the present embodiment, and FIG. 16 is set for the image area ratio acquisition result of FIG. 5 is a graph showing the target control value of the primary transfer current in the pixel position in the sub-scanning direction.

In Examples 1 and 2 described above, the target control value: ICt of the primary transfer current is obtained in consideration of the average image area ratio: x1c of the predetermined area and the average image area ratio: x0c of the downstream area.
On the other hand, in this embodiment, in addition to the average image area ratio of the predetermined area: x1c and the average image area ratio of the downstream area: x0c, the average image area ratio of the upstream area: x2c is also taken into consideration. A target control value of the primary transfer current: ICt is obtained.

For example, as shown in FIG. 16 for the acquisition information of the average image area ratio shown in FIG. 15, not only the average image area ratio of the downstream area D: x0c but also the average image area ratio of the upstream area F: x2c is also taken into consideration to determine the target control value: ICt of the primary transfer current of the area E that is the predetermined area.
Specifically, in the example illustrated in FIG. 16, when viewed from the tip (downstream side) in 1000 pixels in the region E, 0 to 250 pixels, 250 to 750 pixels, and 750 to 1000 pixels, The target control value obtained from Expressions 12 and 13 is changed.
By controlling in this way, the response delay of the primary transfer electric field of the primary transfer current in the upstream region can be corrected at an earlier timing as compared with the case where only the average image area ratio of the downstream region: x0c is considered. Therefore, it is effective in reducing density unevenness in which the image density changes greatly.

(Formula 11):
ICt0 (x0c, x1c) = 24 × (x0c−2x1c) +84 [μA] (0 to 250 pixels: 250 pixels at the tip)
(Formula 12):
ICt1 (x1c) = − 24 × 1c + 84 [μA] (250 to 750 pixels: 500 pixels in the center)
(Formula 13):
ICt2 (x1c, x2c) = 24 × (x2c−2x1c) +84 [μA] (750 to 1000 pixels: 250 pixels at the rear end)
Here, x0c represents the image area ratio of the downstream area: D, x1c represents the area of the predetermined area: E, and x2c represents the image area ratio of the upstream area: F.

As described above, the regions other than the predetermined region considered when determining the target control value of the primary transfer current of the region: E are the region D that is the downstream region adjacent to the region: E and the region F that is the upstream region. As a result, the following effects can be obtained.
By considering the image area ratio of the region D adjacent to the region E and setting the target control value of the primary transfer current: ICt0 to control the transfer current, the vicinity of the most downstream portion of the predetermined region in the first and second embodiments The effect similar to the effect of improving the response characteristic of the primary transfer electric field can be obtained.
In addition, the control for changing the target control value of the most upstream area of the region E, that is, the target control value of the most downstream area of the adjacent area F can be started at an early timing. By repeating such control sequentially for each predetermined area passing through the primary transfer area, the response characteristics of the primary transfer electric field can be further improved as compared with the first and second embodiments.
Therefore, when the target control value of the primary transfer current is controlled according to the image area ratio, it is possible to further suppress the occurrence of image density fluctuation due to the response delay of the primary transfer electric field.

The specific control of the primary transfer current in this embodiment is performed as follows.
When the average image area ratio x0c of the region D is larger than the average image area ratio x1c of the region E that is the predetermined region, the tip portion in the region E that passes through at least the primary transfer nip increases as the difference increases. The target control value: IC0t is set so that the transfer current increases. On the other hand, when the average image area ratio x0c of the region D is small, the target control value IC0t is set so that the transfer current at the tip end portion in the region E passing through the primary transfer nip becomes small as the difference increases. To do.
Further, when the average image area ratio: x2c in the region F is large, the target control value: IC2t is set so that the primary transfer current at least at the rear end in the region E passing through the primary transfer nip decreases as the difference increases. Set. On the other hand, when the average image area ratio of the region F: x2c is small, the larger the difference is, the larger the primary transfer current at the rear end portion in the region E that passes through the primary transfer nip: the target control value: IC2t is set.

Then, as shown in FIG. 15, when the average image area ratio (image area ratio) changes, that is, the area D for every 1000 pixels is 5 [%], the area E is 100 [%], and the area F is 5 [%]. ], The target control value of the primary transfer current in each region is controlled as follows.
As shown in FIG. 16, in the rear end portion (750 to 1000 pixels) of the region D that is the downstream region, the response characteristic of the primary transfer electric field at the front end portion of the region E that is the predetermined region is improved. The target control value of the primary transfer current of the tip portion of the area E obtained from the above is set to ICt0. In the area E, the target control value: ICt0 of the tip (0 to 250 pixels) is maintained, and thereafter, in the central part (250 to 750 pixels), the reference target control value of the area E obtained from Expression 12: ICt1 Change to
Thereafter, in the rear end portion (750 to 1000 pixels) of the region E, the rear end portion of the region E obtained from Expression 13 is used to improve the response characteristic of the primary transfer electric field at the front end portion of the region F that is the upstream region. Primary transfer current target control value: ICt2.

Then, in the region F, the target control value: ICt2 of the tip portion (0 to 250 pixels) is maintained, and thereafter, in the central portion (250 to 750 pixels), the reference target control value of the region F obtained from Expression 12: ICt1. change.
As described above, the primary transfer current target control values: ICt0, ICt1, and ICt2 are determined and the primary transfer current is controlled to be repeated for each predetermined region that sequentially passes through the primary transfer nip. By repeating in this way, when the target control value of the primary transfer current is controlled according to the image area ratio, it is possible to further suppress the occurrence of image density fluctuation due to the response delay of the primary transfer electric field.

Example 4
Regarding Example 4 of the present embodiment, control of the secondary transfer current when the color toner image formed by the process units 1Y, 1M, 1C, and 2K and secondarily transferred to the intermediate transfer belt 21 is transferred to the paper P is transferred. Will be described with reference to the drawings.
This embodiment differs from the first to third embodiments described above only in that the target control value of the transfer current is determined in this embodiment is the target control value of the secondary transfer current. Since the points relating to the other configurations are the same, in the following description, the configurations similar to those of the first and second embodiments, and the operations and effects thereof will be omitted as appropriate.
FIG. 17 shows the present embodiment, which is set with respect to the image area ratio acquisition result of FIG. 15 used in the description of the above-described third embodiment when the image area ratio corresponding to each color is the same for all colors. 10 is a graph showing the target control value of the secondary transfer current: I2t in the pixel position in the sub-scanning direction.

The method for determining the primary transfer current target control values: ICt0, ICt1, and ICt2 described in the first to third embodiments is not limited to the application to the primary transfer current. Application is also possible.
In this embodiment, the target control value I2t of the secondary transfer current is set as shown in Equation 13, Equation 14, and Equation 15. In the following embodiment, the sum of the image area ratios of the respective colors in the region for every 1000 pixels (600 dpi) is calculated as the average image area ratio. That is, the average image area ratio of the secondary transfer can take a value of 0 to 4 (= 400%).
Then, the target control value set for a predetermined area that passes through the secondary transfer nip, as shown in the following expressions 13, 14, and 15, the downstream area that has already passed through the secondary transfer nip, and the subsequent area Is set in consideration of the image area ratio of the predetermined region in the upstream region. Expression 14 corresponds to the reference target control value of the secondary transfer current when the average image area ratio of the downstream area and the predetermined area or the predetermined area and the upstream area is the same.

(Formula 13):
I2t0 (x0, x1) = − 84 × (2 × 1−x0) −45 [μA] (0 to 250 pixels: 250 pixels at the tip)
(Formula 14):
I2t1 (x1) = − 84 × x1-45 [μA] (250 to 750 pixels: 500 pixels in the central portion)
(Formula 15):
I2t2 (x1, x2) = − 84 × (2 × 1-x2) −45 [μA] (750 to 1000 pixels: 250 pixels at the rear end)
However, I2t0, 1, 2 ≦ 0, 0 ≦ x0 ≦ 1, 0 ≦ x1 ≦ 1, 0 ≦ x2 ≦ 1. In addition, x0, x1, and x2 are defined as an image area ratio of all solid images of Y, M, C, and K colors being 0.25 (4 color overlapping solid images are 1).

As described above, the regions other than the predetermined region considered when determining the target control value of the primary transfer current of the region: E are the region D that is the downstream region adjacent to the region: E and the region F that is the upstream region. As a result, the following effects can be obtained.
Area: Considering the image area ratio of the areas D and F adjacent to E, the target control value of the secondary transfer current at the front end of the predetermined area: I2t0 and the target of the secondary transfer current at the rear end of the predetermined area Control value: I2t2 is set to control the transfer current. By controlling in this way, it is possible to obtain the same effect as the effect of improving the response characteristic of the primary transfer electric field in the vicinity of the most downstream portion of the predetermined region in the third embodiment.

That is, in the secondary transfer in the intermediate transfer system, the same effect as the improvement effect of the response characteristic of the primary transfer electric field in the predetermined area described in the third embodiment can be obtained.
Therefore, in the secondary transfer in the intermediate transfer method, when the target control value of the secondary transfer current is controlled according to the image area ratio, it is possible to suppress the occurrence of image density fluctuation due to the response delay of the secondary transfer electric field.
In this embodiment, the example in which the target control value of the secondary transfer current is determined in consideration of the average image area ratio of the region D that is the downstream region and the region F that is the upstream region has been described. The invention is not limited to such a configuration.
For example, similarly to the method for determining the target control value of the primary transfer current described in the first and second embodiments, the region E is based on the average image area ratio of the region D that is the downstream region and the region E that is the predetermined region. Alternatively, the target control value for the tip of the region E may be determined. However, as in this embodiment, based on the average image area ratio of the region D, the region E, and the region F, the target control values of the front end portion, the central portion, and the rear end portion of the region E are determined, and two By controlling the next transfer current, it is possible to further suppress the occurrence of image density fluctuation.

The specific control of the secondary transfer current in this embodiment is performed as follows.
When the average image area ratio x0 of the area D is larger than the average image area ratio x1 of the area E that is the predetermined area, the tip in the area E that passes through at least the secondary transfer nip increases as the difference increases. The target control value: I2t0 is set so that the absolute value of the secondary transfer current of the part becomes small. On the other hand, when the average image area ratio x0 of the region D is small, the absolute value of the secondary transfer current at the front end in the region E passing through the secondary transfer nip increases as the difference increases. Control value: I2t0 is set.
Further, when the average image area ratio x2 in the region F is large, the absolute value of the secondary transfer current at the rear end portion in the region E passing through the secondary transfer nip is decreased as the difference increases. Target control value: IC2t is set. On the other hand, when the average image area ratio: x2c in the region F is small, the absolute value of the secondary transfer current at the rear end portion in the region E passing through the secondary transfer nip increases as the difference increases. Target control value: I2t2 is set.

Then, as shown in FIG. 15, when the average image area ratio (image area ratio) changes, that is, the area D for every 1000 pixels is 5 [%], the area E is 100 [%], and the area F is 5 [%]. ], The target control value of the primary transfer current in each region is controlled as follows.
As shown in FIG. 17, in the rear end portion (750 to 1000 pixels) of the region D that is the downstream region, in order to improve the response characteristic of the secondary transfer electric field at the front end portion of the region E that is the predetermined region, 13 is set to the target control value of the secondary transfer current at the tip of the area E obtained from 13: I2t0. In the region E, the target control value: I2t0 of the tip (0 to 250 pixels) is maintained, and thereafter, in the center (250 to 750 pixels), the reference target control value of the region E obtained from Expression 14: I2t1 Change to
Thereafter, at the rear end portion (750 to 1000 pixels) of the region E, the rear end portion of the region E obtained from Expression 15 is used to improve the response characteristic of the secondary transfer electric field at the front end portion of the region F that is the upstream region. The target control value of the secondary transfer current of the part is set to I2t2.

In the region F, the target control value: I2t2 at the front end (0 to 250 pixels) is maintained, and thereafter, in the central portion (250 to 750 pixels), the reference target control value of the region F obtained from Expression 14 is set to I2t1. change.
As described above, the secondary transfer current target control values: I2t0, I2t1, and I2t2 are determined and the secondary transfer current is controlled to be repeated for each predetermined region that sequentially passes through the secondary transfer nip. By repeating in this way, when the target control value of the secondary transfer current is controlled according to the image area ratio, it is possible to further suppress the occurrence of image density fluctuation due to the response delay of the secondary transfer electric field.

In this embodiment, an example in which the present invention is applied to an intermediate transfer type printer has been described. However, the present invention is not limited to such a configuration. For example, the present invention can be applied to an image forming apparatus using a direct transfer method, an image forming apparatus including only one photoconductor, and the like.
Next, an example of a printer according to an embodiment to which the present invention is applicable will be described with a plurality of modifications.
Unless otherwise specified, the configuration of the printer according to each modification is substantially the same as that of the present embodiment.

(Modification 1)
FIG. 18 is a schematic configuration diagram illustrating a printer according to the first modification.
This printer is different from the printer according to the embodiment that conveys the paper P in the vertical direction while conveying the paper P in the apparatus in the horizontal direction and forms an image on the paper P.

  The tandem toner image forming unit 10 includes four process units 1Y, 1M, 1C, and 1K for forming Y, M, C, and K color toner images. The transfer unit 20 serving as transfer means includes an endless intermediate transfer belt 21 as a nip forming member, a driving roller 22, a driven roller 23, a secondary transfer counter roller 24, four primary transfer rollers 25Y, M, C, and K. A next transfer roller 26 and the like are included.

  The endless intermediate transfer belt 21 is wound around the drive roller 22, the driven roller 23, and the secondary transfer counter roller 24 in such a posture that the side view is an inverted triangular shape. Then, the driving roller 22 is driven to rotate by a driving device (not shown), and the intermediate transfer belt 21 is moved endlessly in the clockwise direction in the drawing. In addition to the driving roller 22, the driven roller 23, and the secondary transfer counter roller 24, four primary transfer rollers 25 Y, M, C, and K are also disposed inside the loop of the intermediate transfer belt 21.

  The tandem toner image forming unit 10 is disposed above the transfer unit 20 in a posture in which the four process units 1Y, 1M, 1C, and 1K are arranged in a horizontal direction along the overlaid surface of the intermediate transfer belt 21. . The process units 1Y, 1M, 1C, and 1K, which are image forming units, include drum-shaped photoreceptors 2Y, 2M, 2C, and 2K that are driven to rotate counterclockwise in the drawing, and developing devices 3Y, 3M, 3C, and 3K. And charging devices 4Y, 4M, 4C, and 4K. The photosensitive members 2Y, 2M, 2C, and 2K, which are latent image carriers, are in contact with the overlaid surface of the intermediate transfer belt 21 to form primary transfer nips for Y, M, C, and K, respectively, and are not shown. It is rotationally driven in the counterclockwise direction in the figure by the driving means.

  Below the primary transfer nips for Y, M, C, and K, in the loop of the intermediate transfer belt 21, the primary transfer rollers 25Y, 25M, 25C, and 25K move the intermediate transfer belt 21 to the photoreceptors 2Y, M, C, and K. It is pushing toward. To these primary transfer rollers 25Y, 25M, 25C, 25K, a primary transfer current having a polarity opposite to the toner charging polarity is applied by primary transfer power supplies 81Y, 81M, 81C, 81K.

  The electrostatic latent images formed on the photoreceptors 2Y, 2M, 2C, and 2K are developed with toner by the developing devices 3Y, 3M, 3C, and 3K, and become Y, M, C, and K toner images. These Y, M, C, and K toner images are primarily transferred to the Y, M, C, and K primary transfer nips, sequentially superimposed on the front surface of the intermediate transfer belt 21. As a result, a four-color superimposed full-color toner image is formed on the front surface of the intermediate transfer belt 21. Then, secondary transfer is performed on the paper P at the same time at the secondary transfer nip.

(Modification 2)
FIG. 19 is a schematic configuration diagram illustrating a printer according to the second modification.
This printer is an image forming apparatus of a direct transfer system in which an endless paper conveying belt 121 is brought into contact with the photosensitive members 2Y, 2M, 2C, and 2K of each color instead of an intermediate transfer belt as a nip forming member. This is different from the printer according to this embodiment. The paper transport belt 121 sequentially passes the recording paper, which is a transfer target, held on the surface thereof, to the transfer nips for Y, M, C, and K along with its endless movement. In this process, the Y, M, C, and K toner images on the photoconductors 2Y, 2M, 2C, and 2K are sequentially superimposed and transferred onto the surface of the paper P.

(Modification 3)
FIG. 20 is a schematic configuration diagram illustrating a printer according to the third modification.
This printer has Y, M, C, and K developing devices 3Y, M, C, and K around one photosensitive member 2. When image formation is performed, first, the surface of the photoreceptor 2 is uniformly charged by the charging device 4, and then the surface of the photoreceptor 2 is irradiated with laser light L modulated based on Y image data. Then, an electrostatic latent image for Y is formed on the surface of the photoreceptor 2. The Y electrostatic latent image is developed by the developing device 3Y to obtain a Y toner image, which is then primarily transferred onto the intermediate transfer belt 21.

  Thereafter, the transfer residual toner on the surface of the photoconductor 2 is removed by the photoconductor cleaning device 5, and then the surface of the photoconductor 2 is uniformly charged again by the charging device 4. Next, the surface of the photoconductor 2 is irradiated with laser light L modulated based on the image data for M to form an electrostatic latent image for M on the surface of the photoconductor 2. Is developed by the developing device 3M to obtain an M toner image. The M toner image is primary-transferred superimposed on the Y toner image on the intermediate transfer belt 21. Thereafter, in the same manner, the C toner image and the K toner image are sequentially developed on the photosensitive member 2, and are sequentially superimposed and superimposed on the Y and M toner images on the belt.

  As a result, a four-color toner image is formed on the intermediate transfer belt 21. Thereafter, the four-color toner image on the intermediate transfer belt 21 is secondarily transferred collectively onto the surface of the paper P at the secondary transfer nip to form a full-color image on the paper P. After the full color image is fixed on the paper P by the fixing device 40, the paper P is discharged out of the apparatus.

By applying the present invention to the printer according to each of the above-described modifications, the time until the transfer electric field is generated according to the image area ratio is determined at least in the vicinity of the most downstream part or the most upstream part of the predetermined area where the image area ratio is switched. The response characteristics of each transfer electric field can be improved as compared with the conventional case. Then, by repeating the control for changing the target control value of the transfer current in this manner for each predetermined region that sequentially passes through the exit of the transfer region, the response characteristic of the transfer electric field can be achieved over almost the entire image forming region. It is possible to improve more than the configuration.
Therefore, when controlling the transfer current in accordance with the image area ratio, it is possible to provide a transfer current control method capable of suppressing the occurrence of image density fluctuation due to the response delay of the transfer electric field.

What has been described above is merely an example, and the present invention has a specific effect for each of the following modes.
(Aspect A)
A transfer unit such as a primary transfer roller 25C for transferring a toner image such as a C toner image carried on an image carrier such as the photosensitive member 2C to a transfer target such as an intermediate transfer belt 21 is provided. A region B through which a transfer current such as a primary transfer current supplied from a power source such as a primary transfer power source 81C passes through an exit of a transfer region such as a primary transfer nip formed between the image carrier and the transfer target. Average image area ratio of a predetermined area such as: a transfer current control method used in an image forming apparatus such as a printer that is controlled based on an image area ratio such as x1c, and the image area ratio of the predetermined area and a downstream area In consideration of the average image area ratio of the area other than the predetermined area such as the area A: image area ratio such as x0c, the target control value when changing the transfer current: the target control value such as ICt It is characterized in that constant.

According to this, as explained in the above embodiment, the following effects can be obtained.
Considering the image area ratio of the area other than the predetermined area, that is, the downstream area where the toner image has already been transferred to the transfer target, adjacent to the area to be transferred, and the upstream area to be transferred later, By setting a target control value, it is possible to control the transfer current when the predetermined area passes through the transfer area.
In the predetermined area passing through the exit of the primary transfer area, for example, when a target control value of the primary transfer current supplied from the power source to the transfer means is set as in the first embodiment, and the predetermined area passes through the primary transfer area. To control the primary transfer current.

That is, when the image area ratio of the downstream area is larger than the image area ratio of the predetermined area and the reference target control value of the predetermined area is larger than the reference target control value of the downstream area, the difference in the image area ratio is large. The target control value in the predetermined area is set to be larger than the reference target control value.
On the other hand, when the image area ratio of the downstream area is smaller than the image area ratio of the predetermined area and the reference target control value of the predetermined area is smaller than the reference target control value of the downstream area, the difference in the image area ratio is large. The target control value in the predetermined area is set to be smaller than the reference target control value.
When the image area ratios of the downstream area and the predetermined area are the same and these reference target control values do not change, the target control value of the predetermined area is set to be constant at the reference target control value.

As described above, when the image area ratio of the predetermined region is different from the region other than the predetermined region, the target control value of the transfer current of the predetermined region is lowered or increased from the reference target control value according to the difference. Thus, the amount of change in the target control value of the transfer current when the image area ratio is switched is increased. By increasing the amount of change in this way, the amount of change per unit time when changing the target control value of the transfer current is increased, and the transfer current actually supplied from the power source to the transfer means is the reference target control value. Time to reach can be shortened.
Therefore, it is possible to shorten the time until the transfer electric field corresponding to the image area ratio is generated at least in the vicinity of the most downstream portion of the predetermined region where the image area ratio is switched, thereby improving the response characteristic of the transfer electric field as compared with the conventional case. Then, by repeating the control for changing the target control value of the transfer current in this manner for each predetermined region that sequentially passes through the exit of the transfer region, the response characteristic of the transfer electric field can be achieved over almost the entire image forming region. It is possible to improve more than the configuration.
Therefore, when controlling the transfer current in accordance with the image area ratio, it is possible to provide a transfer current control method capable of suppressing the occurrence of image density fluctuation due to the response delay of the transfer electric field.

(Aspect B)
In (Aspect A), the region other than the predetermined region is a downstream region such as the region A adjacent to the predetermined region such as the region B.

According to this, as explained in the above embodiment, the following effects can be obtained.
By taking into account the image area ratio of the downstream area adjacent to the predetermined area and setting the target control value to control the transfer current, the transfer electric field response characteristics can be improved in the same way as the improvement effect described in aspect A above. An effect can be obtained.

(Aspect C)
In (Aspect B), the image carrier is a latent image carrier such as the photosensitive member 2, the transfer target is an intermediate transfer member such as the intermediate transfer belt 21 or a recording medium such as paper P, and the region B or the like When the average image area ratio of the predetermined area, such as x1c, is larger than the average image area ratio of the downstream area, such as the area A: x0c, the larger the difference, When a target control value such as a target control value: ICt is set so that the transfer current at the most downstream portion such as the tip in the predetermined region becomes large, and the image area ratio of the downstream region is small, the difference is The target control value is set such that the transfer current at the most downstream portion in the predetermined area passing through the transfer area becomes smaller as the value increases.

According to this, as explained in the above embodiment, the following effects can be obtained.
In the primary transfer by the intermediate transfer method or the transfer by the direct transfer method, the same effect as the effect of improving the response characteristic of the transfer electric field at the most downstream portion of the predetermined region described in the above aspect A can be obtained.
Therefore, in the primary transfer in the intermediate transfer method or the transfer in the direct transfer method, when the target control value of the transfer current is controlled according to the image area ratio, the image density fluctuation due to the response delay of the transfer electric field is generated. Can be suppressed.

(Aspect D)
(Aspect B) In addition, in (Aspect C), the image forming apparatus such as a printer includes an intermediate transfer body such as an intermediate transfer belt 21, and the area E such as an area E passing through a secondary transfer area such as a secondary transfer nip. When the average image area ratio of the predetermined area, such as x1, is larger than the average image area ratio of the downstream area, such as the area D, such as x0, the larger the difference is, A target control value such as a target control value: I2t0 is set so that the absolute value of the secondary transfer current at the most downstream portion such as the tip in the predetermined region passing through the secondary transfer region is small, and the downstream side When the image area ratio of the area is small, the target control value is set so that the absolute value of the secondary transfer current in the most downstream portion in the predetermined area passing through the secondary transfer area increases as the difference increases. Setting It is characterized in being.

According to this, as explained in the above embodiment, the following effects can be obtained.
In the secondary transfer in the intermediate transfer method, or in the primary transfer and the secondary transfer transfer in the intermediate transfer method, the same effect as the improvement effect of the response characteristic of the transfer electric field at the most downstream portion of the predetermined area described in the above aspect A Can be obtained.
Accordingly, when the target control value of the secondary transfer current or the target control value of the primary transfer current: the target control value such as ICt and the target control value of the secondary transfer current are controlled according to the image area ratio, The occurrence of fluctuations in image density due to the response delay can be suppressed.

(Aspect E)
In (Aspect A), the region other than the predetermined region is a downstream region such as region D adjacent to the predetermined region such as region: E and an upstream region such as region F. .

According to this, as explained in the above embodiment, the following effects can be obtained.
In consideration of the image area ratio of the downstream area and the upstream area adjacent to the predetermined area, by setting the target control value and controlling the transfer current, at least near the most downstream part of the predetermined area described in the above aspect A The effect similar to the effect of improving the response characteristics of the transfer electric field can be obtained.
In addition, the control for changing the target control value of the most upstream part of the predetermined region, that is, the target control value of the most downstream part of the adjacent upstream region can be started at an early timing. By repeating such control sequentially for each predetermined region that passes through the transfer region, the response characteristic of the transfer electric field can be further improved as compared with the above-described aspect A.
Therefore, when the target control value of the transfer current is controlled according to the image area ratio, it is possible to further suppress the occurrence of image density fluctuation due to the response delay of the transfer electric field.

(Aspect F)
In (Embodiment E), the image carrier is a latent image carrier such as the photosensitive member 2, and the transfer target is an intermediate transfer member such as the intermediate transfer belt 21 or a recording medium such as paper P. When the average image area ratio of the predetermined area, such as x1c, is larger than the average image area ratio of the downstream area, such as the area D, such as x0c, the larger the difference, A target control value such as a target control value: IC0t is set so that the transfer current in the most downstream portion such as the tip in the predetermined region passing through the transfer region such as a primary transfer nip increases. When the image area ratio is small, the target control value is set so that the transfer current at the most downstream portion in the predetermined area passing through the transfer area decreases as the difference increases. When the image area ratio such as the average image area ratio: x2c in the upstream region is large, the transfer current at the most upstream portion such as the rear end of the predetermined region passing through the transfer region increases as the difference increases. When a target control value such as IC2t is set such that the image area ratio of the upstream region is small, the larger the difference is, the at least in the predetermined region passing through the transfer region. The target control value is set so that the transfer current in the most upstream area is increased.

According to this, as explained in the above embodiment, the following effects can be obtained.
In the primary transfer by the intermediate transfer method or the transfer by the direct transfer method, the same effect as the effect of improving the response characteristic of the transfer electric field at the most downstream portion of the predetermined region described in the above aspect A can be obtained.
In addition, the control for changing the target control value of the most upstream part of the predetermined region, that is, the target control value of the most downstream part of the adjacent upstream region can be started at an early timing. By repeating such control sequentially for each predetermined region that passes through the transfer region, the response characteristic of the transfer electric field can be further improved as compared with the above-described aspect A.
Therefore, in the primary transfer in the intermediate transfer method or the transfer in the direct transfer method, when the target control value of the transfer current is controlled according to the image area ratio, the image density fluctuation due to the response delay of the transfer electric field is generated. Further, it can be further suppressed than in the above-described aspect C.

(Aspect G)
In (Aspect E) or (Aspect F), the image forming apparatus such as a printer includes an intermediate transfer body such as an intermediate transfer belt 21, and the predetermined area such as an area E passing through a secondary transfer area such as a secondary transfer nip. When the average image area ratio of the region: the average image area ratio of the downstream region such as the region D: x0 or the like is larger than the average image area ratio of the region: x1 or the like, the larger the difference, A target control value such as a target control value: I2t0 is set so that the absolute value of the secondary transfer current in the most downstream portion such as the tip in the predetermined region passing through the secondary transfer region is small, and the downstream region When the image area ratio is small, the absolute value of the secondary transfer current at the most downstream portion such as the tip in the predetermined region passing through the secondary transfer region increases as the difference increases. When the control value is set and the image area ratio of the upstream area such as the area F is large, such as x2, the larger the difference is, the larger the difference is within the predetermined area passing through the secondary transfer area. If the target control value such as the target control value: I2t2 is set so that the absolute value of the secondary transfer current at the most upstream portion such as the rear end portion is large and the image area ratio of the upstream region is small, The target control value is set such that the absolute value of the secondary transfer current at the most upstream portion in the predetermined area passing through the secondary transfer area becomes smaller as the difference increases. .

According to this, as explained in the above embodiment, the following effects can be obtained.
In the secondary transfer in the intermediate transfer method, or in the primary transfer and the secondary transfer transfer in the intermediate transfer method, the same effect as the improvement effect of the response characteristic of the transfer electric field at the most downstream portion of the predetermined area described in the above aspect A Can be obtained.
In addition, the control for changing the target control value of the most upstream part of the predetermined region, that is, the target control value of the most downstream part of the adjacent upstream region can be started at an early timing. By repeating such control sequentially for each predetermined region that passes through the transfer region, the response characteristic of the transfer electric field can be further improved as compared with the above-described aspect A.
Therefore, in the secondary transfer in the intermediate transfer method, or in the primary transfer and the secondary transfer transfer in the intermediate transfer method, when the target control value of the secondary transfer current is controlled according to the image area ratio, the transfer electric field response Occurrence of the image density fluctuation due to the delay can be further suppressed as compared with the aspect D.

(Aspect H)
In any one of (Aspect A) to (Aspect G), target control values of transfer currents such as a primary transfer current and a secondary transfer current set for the predetermined region such as region B and region E: ICt and target A control value: a target control value such as I2t changes within the predetermined area.
According to this, as explained in the above embodiment, the following effects can be obtained.
An effect similar to the effect of improving the response characteristic of the transfer electric field at the most downstream portion of the predetermined region described in the aspect A can be obtained. Moreover, the control which changes the target control value of the most downstream part of an adjacent upstream area can also be started at an early timing.
In addition, by changing the target control value of the primary transfer current: ICt and the target control value of the secondary transfer current: I2t within a predetermined region, the performance of the CPU of the control unit 200 mounted on the printer and other control In consideration of the load, the optimum number of pixels in the predetermined area can be set.

(Aspect I)
Transfer means such as a primary transfer roller 25 and a secondary transfer roller 26 for transferring a toner image carried on an image carrier such as the photoreceptor 2 or the intermediate transfer belt 21 to a transfer body such as the intermediate transfer belt 21 or paper P A transfer current such as a primary transfer current and a secondary transfer current supplied from a power source such as a primary transfer power source 81 and a secondary transfer power source 82 to the transfer unit, between the image carrier and the transfer unit. Average image area ratio: x1c and average image area ratio: x1 in a predetermined area such as area B or area E passing through the exit of the transfer area such as the primary transfer nip or secondary transfer nip formed between them In an image forming apparatus such as a printer controlled according to a rate, the transfer current control method of any one of (Aspect A) to (Aspect H) is used as a transfer current control method for controlling the transfer current. It is an feature.

  According to this, as described in the above-described embodiment, it is possible to provide an image forming apparatus that can achieve the same effect as the transfer current control method of any one of (Aspect A) to (Aspect H).

DESCRIPTION OF SYMBOLS 1 Process unit 2 Photoconductor 3 Developing device 4 Charging device 5 Photoconductor cleaning device 10 Tandem toner image formation part 20 Transfer unit 21 Intermediate transfer belt 22 Drive roller 23 Follower roller 24 Secondary transfer counter roller 25 Primary transfer roller 26 Secondary transfer Roller 28 Belt cleaning device 32 Registration roller pair 40 Fixing device 41 Fixing roller 42 Pressure roller 81 Primary transfer power source 82 Secondary transfer power source 86 Optical sensor unit 90 Optical writing unit 95 Paper feed cassette 95a Paper feed roller 121 Paper transport belt 200 Control unit

JP 2011-164533 A

Claims (9)

A transfer means for transferring the toner image carried on the image carrier to the transfer medium;
A transfer current supplied from a power source to the transfer unit is controlled based on an image area ratio of a predetermined region passing through an exit of a transfer region formed between the image carrier and the transfer target. In a control method of a transfer current used in an image forming apparatus,
A method for controlling a transfer current, wherein a target control value for changing the transfer current is set in consideration of an image area ratio of the predetermined area and an image area ratio of an area other than the predetermined area. The transfer current control method according to claim 1,
The transfer current control method, wherein the region other than the predetermined region is a downstream region adjacent to the predetermined region. The transfer current control method according to claim 2,
The image carrier is a latent image carrier, and the transfer target is an intermediate transfer member or a recording medium,
For the image area ratio of the predetermined region,
When the image area ratio of the downstream area is large, the target control value is set so that the transfer current at least in the most downstream area in the predetermined area increases as the difference increases.
When the image area ratio in the downstream area is small, the target control value is set so that the transfer current at least in the most downstream area in the predetermined area passing through the transfer area decreases as the difference increases. A method for controlling a transfer current, characterized by: In the transfer current control method according to claim 2 or 3,
The image forming apparatus includes an intermediate transfer member,
For the image area ratio of the predetermined area that passes through the secondary transfer area,
When the image area ratio of the downstream area is large, the target value is set such that the larger the difference is, the smaller the absolute value of the secondary transfer current in the most downstream area in the predetermined area passing through the secondary transfer area is at least. Control value is set,
When the image area ratio of the downstream region is small, the target value is set so that the absolute value of the secondary transfer current at least in the most downstream area in the predetermined region passing through the secondary transfer region increases as the difference increases. A transfer current control method, wherein a control value is set. The transfer current control method according to claim 1,
The transfer current control method, wherein the regions other than the predetermined region are a downstream region and an upstream region adjacent to the predetermined region. In the transfer current control method according to claim 5,
The image carrier is a latent image carrier, and the transfer target is an intermediate transfer member or a recording medium,
For the image area ratio of the predetermined region,
When the image area ratio of the downstream region is large, the target control value is set so that the transfer current at the most downstream portion in the predetermined region passing through the transfer region increases as the difference increases.
When the image area ratio of the downstream region is small, the target control value is set so that the transfer current at the most downstream portion in the predetermined region passing through the transfer region decreases as the difference increases.
When the image area ratio of the upstream region is large, the target control value is set so that the transfer current at the most upstream portion in the predetermined region passing through the transfer region is reduced as the difference increases.
When the image area ratio of the upstream region is small, the target control value is set so that the transfer current of at least the most upstream portion in the predetermined region passing through the transfer region increases as the difference increases. A method for controlling a transfer current, characterized by: In the transfer current control method according to claim 5 or 6,
The image forming apparatus includes an intermediate transfer member,
For the image area ratio of the predetermined area that passes through the secondary transfer area,
When the image area ratio of the downstream area is large, the target value is set such that the larger the difference is, the smaller the absolute value of the secondary transfer current in the most downstream area in the predetermined area passing through the secondary transfer area is at least. Control value is set,
When the image area ratio of the downstream region is small, the target value is set so that the absolute value of the secondary transfer current at least in the most downstream area in the predetermined region passing through the secondary transfer region increases as the difference increases. Control value is set,
When the image area ratio of the upstream area is large, the target value is set so that the absolute value of the secondary transfer current in the most upstream area in the predetermined area passing through the secondary transfer area increases as the difference increases. Control value is set,
When the image area ratio of the upstream region is small, the target value is set such that the larger the difference is, the smaller the absolute value of the secondary transfer current in the most upstream area in the predetermined region passing through the secondary transfer region is at least. A transfer current control method, wherein a control value is set. The transfer current control method according to any one of claims 1 to 7,
A transfer current control method, wherein a target control value of a transfer current set for the predetermined region changes within the predetermined region. A transfer means for transferring the toner image carried on the image carrier to the transfer medium;
A transfer current supplied from a power source to the transfer unit is controlled in accordance with an image area ratio in a predetermined region passing through an exit of a transfer region formed between the image carrier and the transfer unit. In the image forming apparatus,
As a transfer current control method for controlling the transfer current,
An image forming apparatus using the transfer current control method according to claim 1.

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