JP7590691B2 - Image forming device - Google Patents

Description

This invention relates to image forming devices such as copiers, printers, facsimiles, or combination machines thereof.

In image forming devices such as copiers and printers, a technique has been known for adjusting the transfer bias applied to a transfer member such as a transfer roller or a secondary transfer roller in order to transfer a toner image formed on the surface of an image carrier such as a photosensitive drum or an intermediate transfer belt onto a sheet satisfactorily (see, for example, Patent Document 1).

On the other hand, Patent Document 1 discloses a technology that calculates the printing rate and adjusts the primary transfer bias applied to the primary transfer roller based on the printing rate, with the aim of suppressing deterioration of transferability.

In conventional image forming devices, when a toner image formed on the surface of an image carrier is transferred to a sheet, the transfer bias applied to the transfer member can deviate significantly from the target value, resulting in abnormalities such as unevenness in the image at the leading edge.
Such problems could not be solved even by adjusting the primary transfer bias applied to the primary transfer roller based on the printing rate by applying the technique disclosed in Japanese Patent Laid-Open No. 2003-233666.

This invention was made to solve the above-mentioned problems, and aims to provide an image forming device that is less likely to cause problems such as the transfer bias applied to the transfer member significantly deviating from the target value when a toner image formed on the surface of an image carrier is transferred to a sheet.

The image forming apparatus of the present invention includes a transfer member for transferring a toner image formed on the surface of an image carrier to a sheet, a power supply unit for outputting a transfer bias to be applied to the transfer member, and a calculation unit for calculating the number of pixels of a toner image formed on the surface of the image carrier, and is configured to be switchable between a constant current mode in which the transfer bias is output by constant current control by the power supply unit and a constant voltage mode in which the transfer bias is output by constant voltage control by the power supply unit, the constant current mode being executed during a transfer step in which the toner image formed on the surface of the image carrier is transferred to a sheet by the transfer member, and the constant voltage mode being executed during a non-transfer step, and during the non-transfer step, before the constant voltage mode is executed a transfer bias of a different polarity from the polarity of the transfer bias applied to the transfer member when the constant current mode is executed is applied to the transfer member, and the constant voltage mode monotonically changes the transfer bias from the transfer bias of the different polarity toward the transfer bias at the start of the subsequent transfer process following the non-transfer process, and adjusts the output time of the transfer bias when the constant voltage mode is executed to be shorter when the number of pixels calculated by the calculation means for the toner image formed on the surface of the image carrier in the subsequent transfer process is small compared to when the number of pixels is large, and adjusts the absolute value of the transfer bias at the start of the subsequent transfer process to be smaller .

The present invention provides an image forming apparatus that is less susceptible to problems such as the transfer bias applied to the transfer member significantly deviating from the target value when a toner image formed on the surface of an image carrier is transferred to a sheet.

1 is an overall configuration diagram showing an image forming apparatus according to an embodiment of the present invention; FIG. 2 is an enlarged view showing a part of an image forming unit; FIG. 2 is a schematic diagram showing an intermediate transfer belt and its vicinity. 6 is a timing chart showing an example of adjustment control of a secondary transfer bias. 10 is a flowchart illustrating an example of adjustment control of a secondary transfer bias. FIG. 2 is a schematic diagram showing a toner image formed on the surface of an intermediate transfer belt. 11 is a timing chart showing adjustment control of a secondary transfer bias in Modification 1. 8 is a flowchart showing adjustment control of the secondary transfer bias in FIG. 7 . 10 is a timing chart showing adjustment control of the secondary transfer bias in Modification 2. 10 is a flowchart showing adjustment control of the secondary transfer bias in FIG. 9 .

The following describes in detail the embodiments of the present invention with reference to the drawings. Note that in each drawing, the same or corresponding parts are given the same reference numerals, and the repeated explanations will be appropriately simplified or omitted.

First, the overall configuration and operation of an image forming apparatus 100 will be described with reference to FIGS.
FIG. 1 is a diagram showing the configuration of a printer as an image forming apparatus, and FIG. 2 is an enlarged view showing a part of the image forming section.
1, an intermediate transfer belt 8 (intermediate transfer body) serving as an image carrier is provided in the center of the image forming apparatus main body 100. Also, imaging units 6Y, 6M, 6C, and 6K corresponding to the respective colors (yellow, magenta, cyan, and black) are provided in parallel so as to face the intermediate transfer belt 8.

Referring to FIG. 2, the imaging unit 6Y corresponding to yellow is composed of the photosensitive drum 1Y as a photosensitive member, a charging device 4Y, a developing device 5Y, a cleaning device 2Y, a lubricant supplying device 3, a static electricity removing device (not shown), etc., which are arranged around the photosensitive drum 1Y. Then, an image creation process (charging process, exposure process, developing process, transfer process, cleaning process, static electricity removing process) is performed on the photosensitive drum 1Y, and a yellow image is formed on the photosensitive drum 1Y.

The other three imaging units 6M, 6C, and 6K are configured in a similar manner to the imaging unit 6Y corresponding to yellow, except that they use different toner colors, and form images corresponding to their respective toner colors. Below, we will omit the explanation of the other three imaging units 6M, 6C, and 6K as appropriate, and only explain the imaging unit 6Y corresponding to yellow.

2, the photoconductor drum 1Y is rotated counterclockwise by a motor, and the surface of the photoconductor drum 1Y is uniformly charged at the position of the charging device 4Y (charging step).
Thereafter, the surface of the photosensitive drum 1Y reaches a position irradiated with the laser light L emitted from the exposure device 7, and an electrostatic latent image corresponding to yellow is formed by exposure scanning in the width direction (the direction perpendicular to the paper surface in Figures 1 and 2, which is the main scanning direction) at this position (this is the exposure process).

Thereafter, the surface of the photoconductor drum 1Y reaches a position facing the developing device 5Y, where the electrostatic latent image is developed to form a yellow toner image (developing process).
Thereafter, the surface of the photoconductor drum 1Y reaches a position facing the intermediate transfer belt 8 (image carrier) as an intermediate transfer body and the primary transfer roller 9Y, and at this position the toner image formed on the surface of the photoconductor drum 1Y is primarily transferred onto the surface of the intermediate transfer belt 8 (the primary transfer process). At this time, a small amount of untransferred toner remains on the photoconductor drum 1Y.

Thereafter, the surface of the photoreceptor drum 1Y reaches a position facing the cleaning device 2Y, and the untransferred toner remaining on the photoreceptor drum 1Y at this position is collected into the cleaning device 2Y by the cleaning blade 2a (cleaning process).
Here, a lubricant supplying device 3 (photoconductor lubricant supplying device) consisting of a lubricant supplying roller 3a, a solid lubricant 3b, a compression spring 3c, etc. is installed inside the cleaning device 2Y. The lubricant supplying roller 3a, which rotates clockwise in Fig. 2, scrapes off the lubricant little by little from the solid lubricant 3b, and the lubricant is supplied to the surface of the photoconductor drum 1Y by the lubricant supplying roller 3a.
Finally, the surface of the photosensitive drum 1Y reaches a position facing the charge eliminating device, where the residual potential on the photosensitive drum 1Y is eliminated.
Thus, a series of image forming processes carried out on the photosensitive drum 1Y is completed.

The above-mentioned image forming process is performed in the other image forming units 6M, 6C, and 6K in the same manner as in the yellow image forming unit 6Y. That is, laser light L based on image information is irradiated from an exposure device 7 disposed above the image forming units onto the photoconductor drums 1M, 1C, and 1K of the image forming units 6M, 6C, and 6K.
Thereafter, the toner images of each color formed on the photoconductor drums 1M, 1C, and 1K through the developing process by the developing devices 5M, 5C, and 5K are primarily transferred onto the intermediate transfer belt 8 in a superimposed manner. In this manner, a color image is formed on the intermediate transfer belt 8.

Referring to FIG. 3, the intermediate transfer belt device is composed of an intermediate transfer belt 8 (intermediate transfer body), four primary transfer rollers 9Y, 9M, 9C, and 9K, a drive roller 16, a driven roller 17, a pre-transfer roller 18, a tension roller 19, a cleaning counter roller 20, a lubricant counter roller 21, a backup roller 22, an intermediate transfer cleaning device 10, a lubricant supply device 30 (intermediate transfer lubricant supply device), a secondary transfer counter roller 80, and secondary transfer devices 70 to 73. The intermediate transfer belt 8 is stretched and supported by a number of roller members 16 to 22 and 80, and is moved endlessly in the direction of the arrow in FIG. 3 by the rotational drive of one roller member (drive roller 16) by a drive motor.

The four primary transfer rollers 9Y, 9M, 9C, and 9K sandwich the intermediate transfer belt 8 between the photoconductor drums 1Y, 1M, 1C, and 1K, respectively, to form primary transfer nips. A transfer voltage (primary transfer bias) having a polarity opposite to that of the toner is applied to the primary transfer rollers 9Y, 9M, 9C, and 9K.
Then, the intermediate transfer belt 8 travels in the direction of the arrow and passes through the primary transfer nips of the primary transfer rollers 9Y, 9M, 9C, and 9K in sequence. In this way, the toner images of each color on the photoconductor drums 1Y, 1M, 1C, and 1K are primarily transferred onto the surface of the intermediate transfer belt 8 in a superimposed manner (primary transfer process).

Then, the intermediate transfer belt 8, on which the toner images of each color have been superimposed and primarily transferred, reaches a position facing the secondary transfer belt 72, which serves as a transfer rotating body. At this position, the secondary transfer opposing roller 80 sandwiches the intermediate transfer belt 8 and the secondary transfer belt 72 between itself and the secondary transfer roller 70, forming a secondary transfer nip. The four-color toner images formed on the intermediate transfer belt 8 are then secondarily transferred onto a sheet P, such as paper, that has been transported to the position of this secondary transfer nip (this is the secondary transfer process). At this time, untransferred toner that has not been transferred to the sheet P remains on the intermediate transfer belt 8.

Thereafter, the intermediate transfer belt 8 reaches the position of the intermediate transfer cleaning device 10. At this position, the untransferred toner and other deposits adhering to the surface of the intermediate transfer belt 8 are removed.
Furthermore, the intermediate transfer belt 8 reaches the position of a lubricant supplying device 30 serving as an intermediate transfer lubricant supplying device. At this position, a lubricant is supplied onto the surface of the intermediate transfer belt 8.
Thus, a series of transfer processes carried out on the intermediate transfer belt 8 is completed.

Referring now to FIG. 1, the sheet P transported to the position of the secondary transfer nip is transported from a paper feed device 26 arranged below the main body 100 of the apparatus via a paper feed roller 27, a pair of registration rollers 28, etc.
More specifically, a plurality of sheets P such as paper are stored in a stack in the paper feed device 26. When the paper feed roller 27 is rotated counterclockwise in FIG. 1, the topmost sheet P is fed toward between the rollers of the pair of registration rollers 28 via the first transport path K1.

The sheet P transported to the registration roller pair 28 (timing roller pair) stops temporarily at the roller nip of the registration roller pair 28, which has stopped rotating. Then, in time with the color image on the intermediate transfer belt 8, the registration roller pair 28 is rotated and the sheet P is transported toward the secondary transfer nip. In this way, the desired color image is transferred onto the sheet P.

Thereafter, the sheet P onto which the color image has been transferred at the secondary transfer nip position is transported by the secondary transfer belt 72, and after being separated from the secondary transfer belt 72, is transported by the transport belt 60 to the position of the fixing device 50. Then, at this position, the color image transferred onto the surface is fixed onto the sheet P by heat and pressure from the fixing belt and pressure roller (fixing process).
Thereafter, the sheet P passes through a second transport path K2 and is discharged to the outside of the apparatus by a pair of discharge rollers. The sheets P discharged to the outside of the apparatus by the pair of discharge rollers are sequentially stacked on a stack unit as output images.
In this manner, a series of image forming processes in the image forming apparatus is completed.

When the "double-sided print mode" is selected, which prints on both sides of the sheet P (the front side and the back side), the sheet P after the fixing process on the front side is not discharged as it is when the above-mentioned "single-sided print mode" is selected, but is guided to the third transport path K3, where its transport direction is reversed and it is transported again to the position of the secondary transfer nip (secondary transfer devices 70-73) via the fourth transport path K4. Then, at the position of the secondary transfer nip, an image is formed on the back side of the sheet P by the image formation process (image forming operation) similar to that described above, and then the sheet P is subjected to the fixing process in the fixing device 50 and discharged from the image forming apparatus main body 100 via the second transport path K2.

Next, the configuration and operation of the developing device 5Y (developing device) in the image forming section will be described in more detail with reference to FIG.
The developing device 5Y is composed of a developing roller 51Y facing the photosensitive drum 1Y, a doctor blade 52Y facing the developing roller 51Y, two transport screws 55Y disposed in a developer container, and a concentration detection sensor 56Y for detecting the toner concentration in the developer. The developing roller 51Y is composed of a magnet fixed inside and a sleeve rotating around the magnet. A two-component developer G consisting of a carrier and a toner is contained in the developer container.

The developing device 5Y thus configured operates as follows.
The sleeve of the developing roller 51Y rotates in the direction of the arrow in Fig. 2. The developer G carried on the developing roller 51Y by the magnetic field formed by the magnet moves on the developing roller 51Y as the sleeve rotates. The developer G in the developing device 5Y is adjusted so that the ratio of toner in the developer G (toner concentration) falls within a predetermined range. Specifically, when a toner concentration sensor installed in the developing device 5Y detects a low toner concentration, new toner is replenished from the toner container 58 into the developing device 5Y so that the toner concentration falls within the predetermined range.
Thereafter, the toner replenished from the toner container 58 into the developer storage section is circulated between the two separated developer storage sections (movement in the direction perpendicular to the plane of the paper in FIG. 2) while being mixed and stirred by the two transport screws 55Y together with the developer G. The toner in the developer G is attracted to the carrier due to frictional charging with the carrier, and is carried on the developing roller 51Y together with the carrier by the magnetic force formed on the developing roller 51Y.

The developer G carried on the developing roller 51Y is transported in the direction of the arrow in FIG. 2 and reaches the position of the doctor blade 52Y. The developer G on the developing roller 51Y is then adjusted to an appropriate amount at this position, and then transported to a position facing the photoconductor drum 1Y (the developing area). The toner is then attracted to the latent image formed on the photoconductor drum 1Y by the electric field formed in the developing area. The developer G remaining on the developing roller 51Y then reaches the top of the developer container as the sleeve rotates, and is separated from the developing roller 51Y at this position.
The toner container 58 is detachably (replaceably) installed in the developing device 5Y (image forming device 100). When the new toner contained therein becomes empty, the toner container 58 is removed from the developing device 5Y (image forming device 100) and replaced with a new toner.

Next, the intermediate transfer belt device according to the present embodiment will be described in detail with reference to FIG.
3, the intermediate transfer belt device is made up of an intermediate transfer belt 8 as an image carrier, four primary transfer rollers 9Y, 9M, 9C, 9K, a drive roller 16, a driven roller 17, a pre-transfer roller 18, a tension roller 19, a cleaning opposed roller 20, a lubricant opposed roller 21, a backup roller 22, an intermediate transfer cleaning device 10, a lubricant supplying device 30 as an intermediate transfer lubricant supplying device, a secondary transfer opposed roller 80 as a transfer member (secondary transfer member), secondary transfer devices 70 to 73, and the like.

The intermediate transfer belt 8, which serves as an image carrier, contacts the four photosensitive drums 1Y, 1M, 1C, and 1K, which carry the toner images of each color, forming a primary transfer nip. The intermediate transfer belt 8 is stretched and supported mainly by eight roller members (a drive roller 16, a driven roller 17, a pre-transfer roller 18, a tension roller 19, a cleaning counter roller 20, a lubricant counter roller 21, a backup roller 22, and a secondary transfer counter roller 80).

The primary transfer rollers 9Y, 9M, 9C, and 9K are in contact with the corresponding photoconductor drums 1Y, 1M, 1C, and 1K, respectively, via the intermediate transfer belt 8. More specifically, the yellow transfer roller 9Y is in contact with the yellow photoconductor drum 1Y via the intermediate transfer belt 8, the magenta transfer roller 9M is in contact with the magenta photoconductor drum 1M via the intermediate transfer belt 8, the cyan transfer roller 9C is in contact with the cyan photoconductor drum 1C via the intermediate transfer belt 8, and the black transfer roller 9K is in contact with the black photoconductor drum 1K via the intermediate transfer belt 8. Each of the primary transfer rollers 9Y, 9M, 9C, and 9K is an elastic roller in which a conductive sponge layer with an outer diameter of about 16 mm is formed on a core metal with a diameter of about 10 mm, and the volume resistance is adjusted to be in the range of 10 ⁶ to 10 ¹² Ω (preferably, 10 ⁷ to 10 ⁹ Ω).

The drive roller 16 is positioned downstream of the four photosensitive drums in the running direction of the intermediate transfer belt, so as to abut against the inner peripheral surface of the intermediate transfer belt 8 with the intermediate transfer belt 8 wrapped around it at a wrap angle of about 120 degrees. The drive roller 16 is driven to rotate in the clockwise direction in FIG. 3 by the drive motor Mt1 controlled by the control unit 90. This causes the intermediate transfer belt 8 to run in a predetermined running direction (clockwise in FIG. 3).

The driven roller 17 is disposed upstream of the four photosensitive drums in the running direction of the intermediate transfer belt 8, and is disposed so as to abut against the inner peripheral surface of the intermediate transfer belt 8 with the intermediate transfer belt 8 wrapped around it at a wrapping angle of about 180 degrees. The portion of the intermediate transfer belt 8 from the driven roller 17 to the drive roller 16 is set to be substantially horizontal. The driven roller 17 rotates clockwise in FIG. 3 as the intermediate transfer belt 8 runs.

The tension roller 19 contacts the outer circumferential surface of the intermediate transfer belt 8. The pre-transfer roller 18, the cleaning opposed roller 20, the lubricant opposed roller 21, the backup roller 22, and the secondary transfer opposed roller 80 contact the inner circumferential surface of the intermediate transfer belt 8.
Between the secondary transfer opposing roller 80 and the lubricant opposing roller 21, an intermediate transfer cleaning device 10 (cleaning blade) is installed so as to come into contact with the cleaning opposing roller 20 via the intermediate transfer belt 8.
A lubricant supplying device 30 (intermediate transfer lubricant supplying device) is installed between the cleaning facing roller 20 and the tension roller 19 so as to contact the lubricant facing roller 21 via the intermediate transfer belt 8. The lubricant supplying device 30 is composed of a lubricant supplying roller, a solid lubricant, a compression spring, etc., similar to the lubricant supplying device 3 for the photosensitive drum. The lubricant supplying roller rotates counterclockwise in FIG. 3 to scrape off a small amount of lubricant from the solid lubricant, and the lubricant is supplied to the surface of the intermediate transfer belt 8 by the lubricant supplying roller.
All of the roller members 17 to 22 and 80 except for the driving roller 16 are rotated as the intermediate transfer belt 8 travels.

3, the secondary transfer opposing roller 80 is in contact with the secondary transfer roller 70 via the intermediate transfer belt 8 and the secondary transfer belt 72. The secondary transfer opposing roller 80 is formed by forming an elastic layer 83 (layer thickness is about 5 mm) made of NBR rubber having a volume resistance of about 10 ⁷ to 10 ⁸ Ω and a hardness (JIS-A hardness) of about 48 to 58 degrees on the outer circumferential surface of a cylindrical core metal made of stainless steel or the like.

In this embodiment, the secondary transfer opposing roller 80 is electrically connected to a power supply unit 95 (bias output means), and a secondary transfer bias, which is a high voltage, is applied from the power supply unit 95. The secondary transfer bias applied to the secondary transfer opposing roller 80 is for secondarily transferring the toner image, which has been primarily transferred onto the surface of the intermediate transfer belt 8, onto the sheet P conveyed to the secondary transfer nip, and is a secondary transfer bias (DC voltage) of the same polarity as that of the toner (negative polarity in this embodiment). As a result, the toner carried on the toner carrying surface (outer peripheral surface) of the intermediate transfer belt 8 is electrostatically moved from the secondary transfer opposing roller 80 side toward the secondary transfer devices 70 to 73 side by the secondary transfer electric field.
The control of applying the secondary transfer bias to the secondary transfer opposing roller 80 will be described in detail later with reference to FIGS.

3, the secondary transfer device is made up of a secondary transfer belt 72, a secondary transfer roller 70, a separation roller 71, a secondary transfer blade 73 (cleaning blade), and the like.
The secondary transfer belt 72 (transfer rotating body) is an endless belt stretched and supported by two rollers (secondary transfer roller 70 and separation roller 71), and is made of substantially the same material as the intermediate transfer belt 8. The secondary transfer belt 72 comes into contact with the intermediate transfer belt 8 (intermediate transfer body) to form a secondary transfer nip, and conveys the sheet P sent out from the secondary transfer nip.

The secondary transfer roller 70 forms a secondary transfer nip between the intermediate transfer belt 8 and the secondary transfer belt 72 and the secondary transfer opposing roller 80. The secondary transfer roller 70 is a hollow core metal made of stainless steel, aluminum, or the like, on which an elastic layer having a hardness (Asker C hardness) of about 40 to 50 degrees is formed (covered). The elastic layer of the secondary transfer roller 70 can be formed in a solid or foamed sponge shape by dispersing conductive filler such as carbon or by incorporating ionic conductive material in rubber material such as polyurethane, EPDM, or silicone. In this embodiment, the volume resistance of the elastic layer is set to about 10 ^6.5 to 10 ^7.5 Ω in order to suppress the concentration of transfer current. In this embodiment, the secondary transfer roller 70 is grounded (earthed).
In addition, the secondary transfer roller 70 is rotated in the counterclockwise direction in Figure 3 by a drive motor Mt2 controlled by the control unit 90, causing the secondary transfer belt 72 to rotate (run) in the counterclockwise direction in Figure 3 and causing the separation roller 71 to rotate counterclockwise in Figure 3.

The separation roller 71 is disposed at a position downstream of the secondary transfer nip in the transport direction of the sheet P. The sheet P sent out from the secondary transfer nip is transported along the secondary transfer belt 72 traveling in the counterclockwise direction in FIG. 3, and then separated from the secondary transfer belt 72 (curvature separation) at the position of the separation roller 71 by the secondary transfer belt 72 having a curved surface formed along the outer periphery of the separation roller 71.
The secondary transfer blade 73 comes into contact with the surface of the secondary transfer belt 72 to remove foreign matter such as toner and paper powder adhering to the surface of the secondary transfer belt 72. The secondary transfer blade 73 is in pressure contact with the secondary transfer roller 70 via the secondary transfer belt 72 so as to come into contact in a counter direction with respect to the running direction of the secondary transfer belt 72.

The configuration and operation of image forming apparatus 100, which is characteristic of this embodiment, will be described in detail below with reference to FIGS.
3 and the like, the image forming apparatus 100 in this embodiment is provided with a secondary transfer opposing roller 80 as a transfer member for transferring a toner image (image) formed on the surface of the intermediate transfer belt 8 as an image carrier to a sheet P. The secondary transfer opposing roller 80 as a transfer member is a secondary transfer member for secondarily transferring the toner image, which has been primarily transferred onto the surface of the intermediate transfer belt 8 as an intermediate transfer body, to the sheet P.
The image forming apparatus 100 is also provided with a power supply unit 95 that outputs a transfer bias (secondary transfer bias) to be applied to the secondary transfer opposing roller 80 as a transfer member.

Here, the image forming apparatus 100 in this embodiment is configured to be switchable between a constant current mode in which the power supply unit 95 outputs a secondary transfer bias (transfer bias) under constant current control, and a constant voltage mode in which the power supply unit 95 outputs a secondary transfer bias (transfer bias) under constant voltage control.
That is, a secondary transfer bias that is constant current controlled or constant voltage controlled is applied to the secondary transfer opposing roller 80 by the power supply unit 95 .
4, in the constant current mode, the constant current control signal is ON and the constant voltage control signal is OFF. In contrast, in the constant voltage mode, the constant voltage control signal is ON and the constant current control signal is OFF.

4, the constant current mode is executed during a transfer process (secondary transfer process) in which a toner image formed on the surface of the intermediate transfer belt 8 (image carrier) is transferred to a sheet P by a secondary transfer opposing roller 80 (transfer member). On the other hand, the constant voltage mode is executed during a non-transfer process (when the secondary transfer process is not performed, such as between sheets of paper).
6 etc., when an image portion (a position corresponding to each of the surfaces of a plurality of sheets P1 to P3, which is an area where an image can be formed) on the intermediate transfer belt 8 passes through the secondary transfer nip, a secondary transfer bias controlled at a constant current is applied to the secondary transfer opposing roller 80 by the power supply unit 95. In contrast, when a non-image portion (an area other than the image portion, such as before and after the image portion or between sheets) on the intermediate transfer belt 8 passes through the secondary transfer nip, a reverse bias, which will be described later, is applied, and then a secondary transfer bias controlled at a constant voltage is applied to the secondary transfer opposing roller 80 by the power supply unit 95. Therefore, during continuous paper passing, the constant current mode and the constant voltage mode are alternately switched.
The reason for this type of control is that the secondary transfer bias is more likely to be stable if it is controlled to a constant current during the transfer process, and is more likely to be stable if it is controlled to a constant voltage during the non-transfer process.

Also, referring to Figure 4, during the non-transfer process (between sheets), before the constant voltage mode (when the constant voltage control signal is on) is executed, a secondary transfer bias of a polarity different from the polarity of the secondary transfer bias applied to the secondary transfer opposing roller 80 (transfer member) when the constant current mode (when the constant current control signal is on) is executed is controlled to be applied to the secondary transfer opposing roller 80.
Specifically, during the transfer step, a constant current controlled negative polarity secondary transfer bias is applied to the secondary transfer opposing roller 80 by the power supply unit 95. Then, when switching to the non-transfer step (between sheets), a positive polarity secondary transfer bias (reverse bias) is applied to the secondary transfer opposing roller 80 by the power supply unit 95 before performing constant voltage control.
As a result, during the transfer process, the negative polarity toner (toner image) carried on the intermediate transfer belt 8 is actively moved onto the sheet P by repulsive force, making it possible to obtain good transferability. Also, during the non-transfer process, the negative polarity toner (toner image) carried on the intermediate transfer belt 8 is actively held on the intermediate transfer belt 8 by attractive force, making it possible to reduce toner stains on the secondary transfer belt 72 (toner stains caused by the adhesion of background stain toner adhered to the intermediate transfer belt 8).
In FIG. 4, the polarity of the secondary transfer voltage value immediately after the application of the reverse bias in the paper gap (non-transfer step) is negative due to switching from the negative polarity during the transfer step.

Here, the image forming apparatus 100 in this embodiment is provided with a calculation unit 91 as a calculation means for calculating the number of pixels of a toner image formed on the surface of the intermediate transfer belt 8 as an image carrier.
This calculation unit 91 is a part of the control unit 90, and calculates the number of pixels of the toner image formed on the intermediate transfer belt 8 (or the toner image transferred onto the sheet P) based on the image information acquired by the exposure device 7. Note that since the image forming apparatus 100 in this embodiment is a printer, the above-mentioned image information is input from an external terminal such as a personal computer, but if the image forming apparatus is a copier, the above-mentioned image information is read from an original by a scanner.
Here, the "number of pixels" of the toner image mentioned above is the ratio (image area ratio) that the image occupies per unit area (for example, per surface area of sheet P) and is also called a "pixel counter."

In this embodiment, at least one of the output time, output start time, and output voltage of the secondary transfer bias when the constant voltage mode is executed is adjusted based on the number of pixels (pixel counter) calculated by the calculation unit 91 (calculation means).
More specifically, in this embodiment, the output time of the secondary transfer bias when the constant voltage mode is executed is adjusted based on the number of pixels (pixel counter) calculated by the calculation unit 91 .
Specifically, when the number of pixels (pixel counter) calculated by the calculation unit 91 (calculation means) is small, the output time of the secondary transfer bias when the constant voltage mode is executed is adjusted to be shorter than when the number of pixels (pixel counter) is large. In other words, when the number of pixels is small, the time for performing constant voltage control before switching to constant current control is controlled to be shorter than when the number of pixels is large.

The reason for this control is that, depending on the number of pixels (pixel counter) of the toner image (image) to be subjected to the secondary transfer process, the secondary transfer bias applied to the secondary transfer opposing roller 80 (transfer member) during the secondary transfer process may deviate significantly from the target value (target value), resulting in abnormal images such as uneven leading edge images and reduced leading edge image density. Specifically, when the number of pixels is small, the secondary transfer bias during the secondary transfer process deviates significantly from the target value compared to when the number of pixels is large, and it takes time for it to stabilize at the target value.
Therefore, in this embodiment, the output time of the secondary transfer bias when the constant voltage mode is executed is optimized according to the number of pixels (pixel counter) so that the secondary transfer bias during the secondary transfer process quickly approaches the target value. Therefore, regardless of the number of pixels of the output image, the occurrence of abnormal images such as leading edge image unevenness and leading edge image density reduction can be reduced.

In particular, in this embodiment, the calculation unit 91 (calculation means) calculates the number of pixels (pixel counter) of a toner image formed within a predetermined range X1 from the leading edge in the sub-scanning direction range of the intermediate transfer belt 8 that can be transferred to one sheet P (the left-right range of the ``image area'' shown in Figure 6).
This is because the above-mentioned problem of the secondary transfer bias deviating from the target value is particularly related to the number of pixels of the image formed at the leading edge of the sheet P in the transport direction.
The above-mentioned "sub-scanning direction" is a direction perpendicular to the main scanning direction, and is the same as the running direction of the intermediate transfer belt 8 and the rotating direction of the photoconductor drum 1Y. In addition, in Fig. 6, the intermediate transfer belt 8 runs (rotates) in the direction of the white arrow.

An example of the adjustment control of the secondary transfer bias described above will be described below with reference to the control flow of FIG. 5 and the timing chart of FIG. 4 as appropriate.
As shown in FIG. 5, first, when image information based on a print command is input to the control unit 90, the pixel counter for each of the divided areas X1 to X3 (see FIGS. 4 and 6) is calculated by the calculation unit 91 based on the image information (steps S1 and S2).
Then, based on the pixel counter of the leading edge area X1, the output voltage target value (target value) of the secondary transfer bias is determined (step S3). With reference to the example shown in FIG. 4, it takes 11 ms to complete the constant voltage output of the bias output -10 kV from the reverse bias voltage value of 1 kV. When the printing area of the first area X1 is narrow, it is set to -2 kV, and when the printing area is wide, it is set to -4 kV. When printing in the entire area such as a solid image, it is set to -7 kV. When the image density is low, it is set to -2 kV, and when the image density is high, it is set to -6 kV. The default value is set to -10 kV, and in the example of FIG. 4, it is set to -2 kV based on the pixel counter of area X1.
Then, the output time of the secondary transfer bias in the constant voltage mode is calculated and determined from the output voltage target value determined in step S3 (step S4). With reference to the example shown in FIG. 4, it takes 11 ms to complete the constant voltage output of the bias output -10 kV from the reverse bias voltage value of 1 kV depending on the sheet load, the current value, the target voltage default setting value, etc. At this time, the output time (arrival time) of the secondary transfer bias is calculated to be 3 ms. In detail, it takes 11 ms to change by 11 kV from 1 kV to -10 kV, so that 1 kV/ms (=11 kV/11 ms) per ms is obtained, and if the image tip is reached when it reaches -2 kV, it changes by 3 kV from 1 kV to -2 kV, so that the time to reach the image tip is 3 ms (=3 kV÷1 kV/ms).
Thereafter, when the paper gap is reached, application of a reverse bias (secondary transfer bias with a polarity different from that during the secondary transfer process) is started (step S5). Then, measurement by the timer of the control unit 90 is started, and after the time for starting output of the secondary transfer bias under constant voltage control (in the example of FIG. 4, the arrival time is 40 ms) has elapsed, output of the secondary transfer bias under constant voltage control is started (steps S6 and S7). Then, after the output time determined in step S4 has elapsed (in the example of FIG. 4, the output time is 3 ms), control of the secondary transfer bias is switched from constant voltage control to constant current control (step S9), and this flow ends.

<Modification 1>
The image forming apparatus 100 in the first modified example adjusts the output start time (output start timing) of the secondary transfer bias when the constant voltage mode is executed based on the number of pixels (pixel counter) calculated by the calculation unit 91 (calculation means).
Specifically, when the number of pixels (pixel counter) calculated by the calculation unit 91 (calculation means) is small, the output start time of the secondary transfer bias when the constant voltage mode is executed is adjusted to be earlier than when the number of pixels (pixel counter) is large. In other words, when the number of pixels is small, the timing of switching from constant current control to constant voltage control is controlled to be earlier than when the number of pixels is large.
In this way, by optimizing the output start time of the secondary transfer bias when the constant voltage mode is executed according to the number of pixels (particularly the number of pixels of the toner image formed within a predetermined range X1 from the leading edge), the secondary transfer bias during the secondary transfer process approaches a target value early. Therefore, regardless of the number of pixels of the output image, the occurrence of abnormal images such as leading edge image unevenness and leading edge image density reduction can be reduced.

An example of the adjustment control of the secondary transfer bias in the above-described first modified example will be described below with reference to the control flow of FIG. 8 and the timing chart of FIG. 7 as appropriate.
8, first, when image information based on a print command is input to the control unit 90, the pixel counters for the divided areas X1 to X3 (see FIGS. 6 and 7) are calculated by the calculation unit 91 based on the image information (steps S1 and S2). Then, based on the pixel counter for the leading area X1, the output voltage target value (target value) of the secondary transfer bias is determined (step S3).
Then, the time required to reach the output voltage target value is calculated (step S10), and the time to advance the output of the secondary transfer bias when switching from constant current control to constant voltage control is calculated (step S11). That is, in the first modification, unlike the control described in Fig. 4 and Fig. 5, the output time of the secondary transfer bias in the constant voltage mode is fixed, and the output start time of the secondary transfer bias in the constant voltage mode is adjusted based on the pixel counter calculated in step S2.
With reference to the example shown in FIG. 7, it takes 11 ms to complete the constant voltage output of the bias output -10 kV from the reverse bias voltage value of 1 kV. In a control example in which the image tip is reached when the voltage reaches -4 kV, the output start time of the secondary transfer bias in the constant voltage mode is 24 ms. More specifically, in the example of FIG. 7, the time from when the output of the secondary transfer bias reaches -10 kV by constant voltage control to when it is changed to constant current control and switched to -4 kV is 5 ms, and 16 ms (=11 ms+5 ms) are required to reach the image tip. This 16 ms is the value calculated in step S10. And, if the default value is 40 ms, the time to advance the output of the secondary transfer bias during constant voltage control is 24 ms (=40 ms-16 ms). This 24 ms is the value calculated in step S11.
Thereafter, when the paper gap is reached, application of a reverse bias (secondary transfer bias with a polarity different from that during the secondary transfer process) is started (step S5). Then, measurement by the timer of the control unit 90 is started, and after the time for starting output of the secondary transfer bias under constant voltage control (24 ms in the example of FIG. 7 , found in step S11) has elapsed, output of the secondary transfer bias under constant voltage control is started (steps S6 and S7). Then, after reaching the output target value determined in step S3 (step S12), control of the secondary transfer bias is switched from constant voltage control to constant current control (step S9), and this flow ends.

<Modification 2>
The image forming apparatus 100 in the second modified example adjusts the output voltage of the secondary transfer bias when the constant voltage mode is executed, based on the number of pixels (pixel counter) calculated by the calculation unit 91 (calculation means).
Specifically, when the number of pixels (pixel counter) calculated by the calculation unit 91 (calculation means) is small, the absolute value of the output voltage of the secondary transfer bias when the constant voltage mode is executed is adjusted to be smaller than when the number of pixels (pixel counter) is large. In other words, when the number of pixels is small, the absolute value of the secondary transfer bias in the constant voltage mode is controlled to be smaller than when the number of pixels is large.
In this way, by optimizing the output voltage of the secondary transfer bias when the constant voltage mode is executed according to the number of pixels (particularly the number of pixels of the toner image formed within a predetermined range X1 from the leading edge), the secondary transfer bias during the secondary transfer process quickly approaches a target value. Therefore, regardless of the number of pixels of the output image, the occurrence of abnormal images such as uneven leading edge images and reduced leading edge image density can be reduced.

Hereinafter, an example of adjustment control of the secondary transfer bias in the above-described modified example 2 will be described with reference to the control flow of FIG. 10 and the timing chart of FIG. 9 as appropriate.
As shown in FIG. 10, first, when image information based on a print command is input to the control unit 90, a pixel counter for each of the divided areas X1 to X3 (see FIGS. 6 and 9) is calculated by a calculation unit 91 based on the image information (steps S1 and S2).
Then, based on the pixel counter of the leading edge area X1, the output voltage target value (target value) of the secondary transfer bias is determined (step S3). That is, in the second modification, unlike the control described in Fig. 4 and Fig. 5, the output time of the secondary transfer bias in the constant voltage mode is fixed, and the output voltage (magnitude) of the secondary transfer bias in the constant voltage mode is adjusted based on the pixel counter calculated in step S2.
Thereafter, when the sheet gap is reached, application of a reverse bias (secondary transfer bias with a polarity different from that during the secondary transfer process) is started (step S5). Then, measurement by the timer of the control unit 90 is started, and after the time for starting output of the secondary transfer bias under constant voltage control (40 ms in the example of FIG. 9) has elapsed, output of the secondary transfer bias under constant voltage control is started (steps S6 and S7). Then, since the output target value is reached at the leading edge of the sheet in calculations, when the arrival time (3 ms in the example of FIG. 9) has elapsed, the control of the secondary transfer bias is switched from constant voltage control to constant current control (step S9), and this flow is ended.
In addition, in the second modification, when the number of pixels is small, the absolute value of the secondary transfer bias in the constant voltage mode is controlled to be smaller than when the number of pixels is large. On the other hand, when the number of pixels is small, the absolute value of the secondary transfer bias in the constant voltage mode can also be controlled to be larger than when the number of pixels is large.

As described above, the image forming apparatus 100 in this embodiment includes the secondary transfer opposing roller 80 (transfer member) for transferring the toner image formed on the surface of the intermediate transfer belt 8 (image carrier) to the sheet P, the power supply unit 95 for outputting the secondary transfer bias (transfer bias) to be applied to the secondary transfer opposing roller 80, and the calculation unit 91 (calculation means) for calculating the number of pixels of the toner image formed on the surface of the intermediate transfer belt 8. The image forming apparatus 100 is configured to be switchable between a constant current mode in which the secondary transfer bias is output by the power supply unit 95 under constant current control, and a constant voltage mode in which the secondary transfer bias is output by the power supply unit 95 under constant voltage control. Based on the number of pixels calculated by the calculation unit 91, at least one of the output time, output start time, and output voltage of the secondary transfer bias when the constant voltage mode is executed is adjusted.
This makes it possible to prevent the occurrence of a problem in which the secondary transfer bias applied to the secondary transfer opposing roller 80 deviates significantly from the target value when the toner image formed on the surface of the intermediate transfer belt 8 is secondarily transferred to the sheet P.

In this embodiment, the present invention is applied to a repulsive transfer type image forming apparatus 100 in which a power supply unit 95 is configured to apply a secondary transfer bias to the secondary transfer opposing roller 80. In contrast, the present invention can also be applied to an attractive transfer type image forming apparatus in which a power supply unit is configured to apply a secondary transfer bias to the secondary transfer roller 70. In this case, the secondary transfer roller 70 becomes a transfer member (secondary transfer member) for transferring a toner image formed on the surface of the intermediate transfer belt 8 (image carrier) to a sheet. In addition, the secondary transfer bias has a polarity opposite to that of the repulsive transfer type. In addition, the present invention can also be applied to an image forming apparatus in which the repulsive transfer type and the attractive transfer type are used in combination.
In the present embodiment, the present invention is applied to the image forming apparatus 100 that uses the secondary transfer belt 72 as the transfer rotator. However, the present invention can also be applied to an image forming apparatus that uses a secondary transfer roller as the transfer rotator.
In this embodiment, the present invention is applied to an image forming apparatus 100 that uses an intermediate transfer belt 8 (intermediate transfer body) as an image carrier and a secondary transfer belt 72 as a transfer rotator. In contrast, the present invention can also be applied to a device that does not include an intermediate transfer body such as an intermediate transfer belt or intermediate transfer drum, but includes a developing device that develops toner, a photosensitive drum (photosensitive body) as an image carrier on which a toner image developed by the developing device is formed, and a transfer member that contacts the photosensitive drum to form a transfer nip and transfers the toner image on the photosensitive drum to a sheet transported to the transfer nip, that is, a so-called direct transfer type image forming apparatus.
In the present embodiment, the present invention is applied to the image forming apparatus 100 that forms color images. However, the present invention can also be applied to an image forming apparatus that forms only monochrome images.
Even in such cases, the same effects as those of this embodiment can be obtained.

In the present embodiment, the output time of the transfer bias (secondary transfer bias) when the constant voltage mode is executed is adjusted based on the number of pixels calculated by the calculation means (calculation unit 91). In the first modification, the output start time of the transfer bias (secondary transfer bias) when the constant voltage mode is executed is adjusted based on the number of pixels calculated by the calculation means (calculation unit 91). In the second modification, the output voltage of the transfer bias (secondary transfer bias) when the constant voltage mode is executed is adjusted based on the number of pixels calculated by the calculation means (calculation unit 91).
In contrast to this, based on the number of pixels calculated by the calculation means (operation unit 91), it is also possible to adjust two or all three of the output time, output start time, and output voltage of the transfer bias (secondary transfer bias) when the constant voltage mode is executed by combining them.
Even in such a case, the same effects as those of this embodiment can be obtained.

It is clear that the present invention is not limited to this embodiment, and that within the scope of the technical concept of the present invention, this embodiment may be modified as appropriate in ways other than those suggested in this embodiment. Furthermore, the number, position, shape, etc. of the components are not limited to this embodiment, and may be any number, position, shape, etc. that is suitable for implementing the present invention.

1Y, 1M, 1C, 1K photoconductor drum (photoconductor),
8 intermediate transfer belt (image carrier, intermediate transfer body),
70 Secondary transfer roller,
80 Secondary transfer opposing roller (transfer member, secondary transfer member),
90 control unit,
91 calculation unit (calculation means),
95 Power supply unit (secondary transfer power supply),
100 Image forming apparatus (image forming apparatus main body),
P, P1 to P3 sheets (recording media).

JP 2018-146740 A

Claims (5)

a transfer member for transferring a toner image formed on the surface of the image carrier onto a sheet;
a power supply unit that outputs a transfer bias to be applied to the transfer member;
a calculation unit for calculating the number of pixels of a toner image formed on the surface of the image carrier;
Equipped with
A constant current mode in which the transfer bias is output by constant current control using the power supply unit and a constant voltage mode in which the transfer bias is output by constant voltage control using the power supply unit are switchable,
the constant current mode is executed during a transfer step in which the toner image formed on the surface of the image carrier is transferred to a sheet by the transfer member,
The constant voltage mode is performed during a non-transfer step,
During the non-transfer step, before the constant voltage mode is executed, a transfer bias having a polarity different from a polarity of a transfer bias applied to the transfer member when the constant current mode is executed is applied to the transfer member;
the constant voltage mode monotonically changes the transfer bias from the transfer bias of the different polarity toward a transfer bias at the start of a subsequent transfer step following the non-transfer step;
an image forming apparatus characterized in that, when the number of pixels calculated by the calculation means for the toner image formed on the surface of the image carrier in the subsequent transfer process is small, the output time of the transfer bias when the constant voltage mode is executed is adjusted to be shorter than when the number of pixels is large, and the absolute value of the transfer bias at the start of the subsequent transfer process is adjusted to be smaller. a transfer member for transferring a toner image formed on the surface of the image carrier onto a sheet;
a power supply unit that outputs a transfer bias to be applied to the transfer member;
a calculation unit for calculating the number of pixels of a toner image formed on the surface of the image carrier;
Equipped with
A constant current mode in which the transfer bias is output by constant current control using the power supply unit and a constant voltage mode in which the transfer bias is output by constant voltage control using the power supply unit are switchable,
the constant current mode is executed during a transfer step in which the toner image formed on the surface of the image carrier is transferred to a sheet by the transfer member,
The constant voltage mode is performed during a non-transfer step,
During the non-transfer step, before the constant voltage mode is executed, a transfer bias having a polarity different from a polarity of a transfer bias applied to the transfer member when the constant current mode is executed is applied to the transfer member;
the constant voltage mode monotonically changes the transfer bias from the transfer bias of the different polarity toward a default value having an absolute value greater than a transfer bias at the start of a subsequent transfer step following the non-transfer step;
an image forming apparatus characterized in that, when the number of pixels calculated by the calculation means for the toner image formed on the surface of the image carrier in the subsequent transfer process is small, the output start time of the transfer bias when the constant voltage mode is executed is adjusted to be earlier than when the number of pixels is large, and the absolute value of the transfer bias at the start of the subsequent transfer process is adjusted to be smaller. a transfer member for transferring a toner image formed on the surface of the image carrier onto a sheet;
a power supply unit that outputs a transfer bias to be applied to the transfer member;
a calculation unit for calculating the number of pixels of a toner image formed on the surface of the image carrier;
Equipped with
A constant current mode in which the transfer bias is output by constant current control using the power supply unit and a constant voltage mode in which the transfer bias is output by constant voltage control using the power supply unit are switchable,
the constant current mode is executed during a transfer step in which the toner image formed on the surface of the image carrier is transferred to a sheet by the transfer member,
The constant voltage mode is performed during a non-transfer step,
During the non-transfer step, before the constant voltage mode is executed, a transfer bias having a polarity different from a polarity of a transfer bias applied to the transfer member when the constant current mode is executed is applied to the transfer member;
the constant voltage mode monotonically changes the transfer bias from the transfer bias of the different polarity toward a transfer bias at the start of a subsequent transfer step following the non-transfer step;
an image forming apparatus characterized in that, when the number of pixels calculated by the calculation means for the toner image formed on the surface of the image carrier in the subsequent transfer process is small, the absolute value of the output voltage of the transfer bias after the application of the transfer bias of the different polarity to the transfer member is completed when the constant voltage mode is executed is adjusted to be smaller than when the number of pixels is large, and the absolute value of the transfer bias at the start of the subsequent transfer process is adjusted to be smaller. The image forming apparatus according to any one of claims 1 to 3, characterized in that the calculation means calculates the number of pixels of a toner image formed within a predetermined range from the leading edge in the sub-scanning direction range of the image carrier that can be transferred to one sheet. The image forming apparatus according to any one of claims 1 to 4, characterized in that the transfer member is a secondary transfer member for secondarily transferring the toner image that has been primarily transferred onto the surface of the intermediate transfer body onto a sheet.

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