Image-forming device

Abstract

To provide an image-forming device capable of suppressing a decrease in image quality generated when the end of a paper sheet passes a fixing nip.SOLUTION: An image-forming device includes a transfer section provided so as to transfer a toner image of an image carrier to a paper sheet, a fixing section provided so as to fix a toner image on the paper sheet, and a control section provided so as to apply a transfer voltage for transferring a toner image to the paper sheet, the transfer section is provided so as to form a transfer nip in which a paper sheet passes between the image carrier and the transfer section, the fixing section includes a heating section and a pressurizing section and is provided such that a paper sheet after passing the transfer nip passes the fixing nip between the heating section and the pressurizing section, and the control section is provided so as to control a transfer voltage such that a transfer current flowing through the transfer nip becomes a setting value and is fixed, and is provided so as to increase a setting value of the transfer current after the end of the paper sheet passes the transfer nip and before the end of the paper sheet passes the fixing nip.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] It is known that if the impact of the paper entering the fixing section is transmitted through the paper and reaches the transfer section, the toner image being transferred becomes distorted and image quality deteriorates (see, for example, Patent Document 1). The propagation of the impact that causes such deterioration in image quality can be suppressed by bending the paper between the transfer section and the fixing section. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-055678 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, when printing on thick paper, the propagation of the shock may not be suppressed even if the paper is bent because the thick paper is hard. Also, the shock generated when the thick paper enters the fixing unit is larger than that generated when the normal paper enters the fixing unit. The present invention has been made in consideration of the above circumstances, and provides an image forming apparatus capable of suppressing deterioration in image quality that occurs when the leading edge of a sheet of paper passes through the fixing nip. [Means for solving the problem]

[0005] The image forming apparatus of the present invention comprises a transfer unit facing the image carrier and arranged to transfer a toner image of the image carrier to paper, a fixing unit arranged to fix the toner image to the paper, and a control unit arranged to apply a transfer voltage to transfer the toner image to the paper, wherein the transfer unit is arranged to form a transfer nip through which the paper passes between the image carrier and the transfer unit, the fixing unit comprises a rotatable heating unit and a rotatable pressure unit, and is arranged so that the paper after passing through the transfer nip passes through the fixing nip between the heating unit and the pressure unit, and the control unit is arranged to control the transfer voltage so that the transfer current flowing through the transfer nip is constant at a set value, and is arranged to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper passes through the fixing nip. Effect of the Invention

[0006] According to the present invention, it is possible to suppress the deterioration of image quality that occurs when the leading edge of the paper passes through the fixing nip. [Brief description of the drawings]

[0007] [Figure 1] 1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. [Diagram 2] FIG. 2 is an enlarged view of the area A enclosed by the dashed line in FIG. [Diagram 3] 11 is a graph showing changes in the set value of the transfer current. [Figure 4] 11 is a graph showing changes in the set value of the transfer current. [Diagram 5] 11 is a graph showing changes in the set value of the transfer current. [Figure 6] 11 is a graph showing changes in the set value of the transfer current. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The image forming apparatus of the present invention comprises a transfer unit facing the image carrier and arranged to transfer a toner image of the image carrier to paper, a fixing unit arranged to fix the toner image to the paper, and a control unit arranged to apply a transfer voltage to transfer the toner image to the paper, wherein the transfer unit is arranged to form a transfer nip through which the paper passes between the image carrier and the transfer unit, the fixing unit comprises a rotatable heating unit and a rotatable pressure unit, and is arranged so that the paper after passing through the transfer nip passes through the fixing nip between the heating unit and the pressure unit, and the control unit is arranged to control the transfer voltage so that the transfer current flowing through the transfer nip is constant at a set value, and is arranged to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper passes through the fixing nip.

[0009] It is preferable that the control unit is configured to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip, and to decrease the set value of the transfer current after the leading edge of the paper, which is passing through the transfer nip, has passed through the fixing nip. It is preferable that the transport distance of the paper from when the set value of the transfer current is increased to when the set value of the transfer current is decreased be 13 mm or more and 30 mm or less.

[0010] It is preferable that the control unit is configured to determine whether to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip, based on the type of paper passing through the transfer nip. When the paper is plain paper, it is preferable that the control unit is configured to keep the set value of the transfer current constant while the paper is passing through the transfer nip, and when the paper is thick paper, the control unit is configured to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip.

[0011] It is preferable that the control unit is configured to determine whether to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip, based on the electrical resistance value between the front and back surfaces of the paper. It is preferable that the image forming apparatus further includes a sensor unit configured to measure the electrical resistance value between the front and back surfaces of the paper, and it is preferable that the control unit is configured to determine whether or not to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip based on the electrical resistance value measured by the sensor unit. It is preferable that the control unit is configured to keep the set value of the transfer current constant while the paper is passing through the transfer nip when a toner image of text is transferred to the paper at the transfer nip at the timing when the leading edge of the paper passes through the fixing nip, and to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip when a toner image of an image is transferred to the paper at the transfer nip at the timing when the leading edge of the paper passes through the fixing nip.

[0012] It is preferable that the control unit is configured to increase the set value of the transfer current after the leading edge of the paper passes through the transfer nip and before the leading edge of the paper passes through the fixing nip when the amount of cyan toner or yellow toner contained in the toner image to be transferred to the paper at the transfer nip is greater than a predetermined value at the time the leading edge of the paper passes through the fixing nip. It is preferable that the control unit is configured to adjust the amount of increase in the set value of the transfer current based on the type of paper passing through the transfer nip, the electrical resistance value between the front and back sides of the paper, or the amount of cyan toner or yellow toner contained in the toner image transferred to the paper at the transfer nip at the time the leading edge of the paper passes through the fixing nip.

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

[0014] FIG. 1 is a schematic diagram showing the configuration of an image forming apparatus according to this embodiment, and FIG. 2 is an enlarged view of an area A enclosed by a dashed line in FIG. The image forming apparatus 50 of this embodiment includes a transfer unit 8 facing the image carrier 20 and arranged to transfer the toner image of the image carrier 20 to the paper 3, a fixing unit 28 arranged to fix the toner image to the paper 3, and a control unit 7 arranged to apply a transfer voltage for transferring the toner image to the paper 3, the transfer unit 8 being arranged to form a transfer nip 24 through which the paper 3 passes between the image carrier 20 and the transfer unit 8, the fixing unit 28 being arranged to include a rotatable heating unit 25 and a rotatable pressure unit 26, and being arranged so that the paper 3 after passing through the transfer nip 24 passes through a fixing nip 27 between the heating unit 25 and the pressure unit 26, the control unit 7 being arranged to control the transfer voltage so that the transfer current flowing through the transfer nip 24 is constant at a set value, and being arranged to increase the set value of the transfer current after the leading edge of the paper 3 has passed the transfer nip 24 and before the leading edge of the paper 3 has passed the fixing nip 27.

[0015] The image forming apparatus 50 is an electrophotographic image forming apparatus that forms an image using electrophotographic technology. The image forming apparatus 50 may be a monochrome image forming apparatus capable of forming a monochrome image, or may be an intermediate transfer type color image forming apparatus capable of forming a color image as shown in FIG. 1. The image forming apparatus 50 is a so-called tandem type full-color image forming apparatus having a configuration in which, for example, a toner image forming section 18a in which a toner image of black toner is formed, a toner image forming section 18b in which a toner image of cyan toner is formed, a toner image forming section 18c in which a toner image of magenta toner is formed, and a toner image forming section 18d in which a toner image of yellow toner is formed are arranged in a predetermined direction (for example, a horizontal direction or a vertical direction). The image forming apparatus 50 may also be another color image forming apparatus, a copier, a multifunction machine, or a facsimile machine. The image carrier 20 is a member on whose surface a toner image is formed, and is a member that forms a transfer nip 24 through which the paper 3 passes between the image carrier 20 and the transfer section 8. When the image forming apparatus 50 is a full-color image forming apparatus, the image carrier 20 is an intermediate transfer belt 20. When the image forming apparatus 50 is a monochrome image forming apparatus, the image carrier is a photoconductor.

[0016] The control unit 7 is a part that controls the image forming apparatus 50. The control unit 7 may have, for example, an arithmetic processing unit (e.g., a CPU), a RAM, a storage device (e.g., a HDD), a network controller, a video controller, a power supply unit, and the like. The control unit 7 is also provided to apply a transfer voltage between the intermediate transfer belt roller 13a and the transfer unit 8. By applying this transfer voltage, a transfer current can be passed through the transfer nip 24, and the toner image on the intermediate transfer belt 20 (image carrier) can be transferred to the paper 3. The control unit 7 is also provided to control the transfer voltage so that the transfer current (the direction in which the current flows is constant) flowing through the transfer nip 24 is constant at a set value (constant current control). The control unit 7 is also provided so that the set value of the transfer current can be changed while the paper 3 is passing through the transfer nip 24. In addition, the control unit 7 can control the rotation of the transport rollers 4a to 4k, the registration roller 5, the pressure unit 26, the rollers 13a to 13h for the intermediate transfer belt, the rollers 12a to 12f for the transfer unit, etc., thereby controlling the transport of the paper 3 in the paper transport path 2, the transfer timing, etc.

[0017] The toner image forming units 18a to 18d each include photoconductors 19a to 19d, a charger, an exposure unit 33, a developing unit, a transfer means, and a cleaning unit. The photoconductors 19a to 19d are members on whose surfaces latent images and toner images are formed. As the photoconductors 19a to 19d rotate, a toner image of black toner, a toner image of cyan toner, a toner image of magenta toner, or a toner image of yellow toner is continuously formed based on image data. The photoconductors 19a to 19d are, for example, photoconductor drums. The charger, the exposure unit 33, the developing unit, the transfer means, and the cleaning unit are provided in this order along the outer circumferential surfaces of the photoconductors 19a to 19d from the upstream side to the downstream side in the rotation direction of the photoconductors 19a to 19d. The toner may be negatively or positively charged.

[0018] The toner images of black toner, cyan toner, magenta toner, and yellow toner formed on the photoconductors 19a-19d are transferred to the intermediate transfer belt 20 by Coulomb force due to the primary transfer electric field (primary transfer) and are superimposed. As a result, color toner images are formed on the intermediate transfer belt 20. The color toner images on the intermediate transfer belt 20 driven by the intermediate transfer belt rollers 13a-13h are transferred to the paper 3 conveyed through the paper conveying path 2 by Coulomb force due to the secondary transfer electric field generated by the transfer voltage in the transfer unit 8 (secondary transfer).

[0019] The transfer unit 8 is a portion provided to transfer the toner image on the intermediate transfer belt 20 (image carrier) to the paper 3. The transfer unit 8 may be a transfer roller, and may have a configuration in which the transfer belt 11 is driven and rotated by the transfer unit rollers 12a to 12f as shown in FIG. 1 and FIG. 2. The transfer unit 8 is provided so that the paper 3 passes through the transfer nip 24 between the intermediate transfer belt 20 (image carrier) and the transfer unit 8. The transfer unit 8 is also electrically connected to the control unit 7 so that a secondary transfer voltage (a voltage that causes a current to flow in one direction) can be applied between the intermediate transfer belt roller 13a and the transfer unit 8. This secondary transfer voltage forms a secondary transfer electric field, and a transfer current flows in the transfer nip 24. For example, the secondary transfer voltage can be applied by connecting the intermediate transfer belt roller 13a to ground and controlling the potential of the transfer unit 8 by the control unit 7. The secondary transfer voltage is also controlled by the control unit 7 so that the transfer current flowing in the transfer nip 24 is constant at a set value (constant current control).

[0020] The paper transport device is a device that transports paper along the paper transport path 2. The paper transport device can include the paper transport path 2, transport rollers 4a to 4k, registration rollers 5, transfer unit 8, fixing unit 28, paper feed tray 21, manual feed tray 22, and paper discharge tray 23. The paper transport device can also have a plurality of paper sensors. Paper transport by the paper transport device is controlled by a control signal sent from the control unit 7 to a motor that rotates each roller. The paper transport device can also include a sensor unit that is provided to measure the electrical resistance value (through electrical resistance value) between the front and back surfaces of the paper 3.

[0021] The paper transport device is configured to pick up paper 3 from the paper feed tray 21 or the manual feed tray 22, transfer the color toner image from the intermediate transfer belt 20 to the paper 3 in the transfer section 8, fix the color toner image to the paper 3 in the fixing section 28, and discharge the paper 3 with the color toner image onto the paper output tray.

[0022] The fixing unit 28 includes a rotatable heating unit 25, a rotatable pressure unit 26, and a fixing unit motor 29 arranged to rotate the heating unit 25 and the pressure unit 26, and is arranged so that the paper 3 after passing through the transfer nip 24 passes through the fixing nip 27 between the heating unit 25 and the pressure unit 26.

[0023] The fixing unit motor 29 is a motor provided to drive either the heating unit 25 or the pressure unit 26. In FIG. 2, the motor 29 is provided to drive the pressure unit 26. Since the pressure unit 26 and the heating unit 25 are in contact with each other at the fixing nip 27 (when the paper 3 is not passing through), when the motor 29 rotates to drive the pressure unit 26, the heating unit 25 also rotates in response. Therefore, the rotation of the heating unit 25 is reverse to the rotation of the pressure unit 26, and the fixing unit 28 can transport the paper 3 that has entered the fixing nip 27 through the fixing nip 27. The control unit 7 controls the rotation of the motor 29 using a control signal, thereby controlling the paper transport speed of the fixing unit 28.

[0024] The heating section 25 is a section for heating the toner image on the paper 3 passing through the fixing nip 27, and the pressure section 26 is a section for applying pressure to the paper 3 passing through the fixing nip 27. The fixing section 28 has the heating section 25 and the pressure section 26, so that the toner image can be melted and pressed to be fixed to the paper 3. The heating section 25 may be a heating roller, a heating belt, or may have a configuration in which a roller and a belt are combined. The pressure unit 26 may be a pressure roller or a pressure belt.

[0025] The control unit 7 is provided so as to increase the set value of the transfer current from the normal value after the leading edge of the paper 3 has passed through the transfer nip 24 and before the leading edge of the paper 3 has passed through the fixing nip 27. The control unit 7 can also maintain the set value of the transfer current constant after the set value of the transfer current has been increased from the normal value while the leading edge of the paper 3 is passing through the fixing nip 27. This can suppress the deterioration of image quality that occurs when the leading edge of the paper 3 passes through the fixing nip 27. Although the reason for this is unclear, it is believed that the increase in the transfer current increases the electric field strength of the secondary transfer electric field, and thus suppresses the occurrence of disturbances in the transfer of the toner image to the paper 3 even if the impact caused by the leading edge of the paper 3 entering the fixing nip 27 is transmitted through the paper 3 and reaches the transfer nip 24. The normal setting value of the transfer current is, for example, 50 μA or more and 120 μA or less. The normal value is the setting value of the transfer current that is set when starting the transfer of the toner image to the paper 3. The normal setting value of the transfer current can be changed depending on the type of paper 3 passing through the transfer nip 24. The increase in the set value of the transfer current is, for example, 20 μA or more and 120 μA or less.

[0026] The control unit 7 may be configured to increase the set value of the transfer current after the leading edge of the paper 3 has passed through the transfer nip 24 and before the leading edge of the paper 3 has passed through the fixing nip 27, and to decrease the set value of the transfer current after the leading edge of the paper 3 passing through the transfer nip 24 has passed through the fixing nip 27. This makes it possible to prevent white spots or white spots from appearing in the printed image.

[0027] 3 to 6 are graphs showing changes in the set value of the transfer current. The horizontal axis of these graphs is the movement distance [mm] of the paper 3 when the position of the paper 3 when the leading edge of the paper 3 enters the transfer nip 24 is set to 0 mm, and the vertical axis is the set value of the transfer current flowing through the transfer nip 24. In this example, the width of the transfer nip 24 is approximately 20 mm, and the width of the fixing nip 27 is approximately 13 mm.

[0028] Here, the control of the transfer current by the control unit 7 when printing on thick paper (paper 3) will be described with reference to FIG. The control unit 7 starts applying the transfer voltage when the leading edge of the paper 3 (cardboard) enters the transfer nip 24, and controls the transfer current to a constant current setting of 80 μA (normal value). Then, just before the leading edge of the paper 3 enters the fixing nip 27, the control unit 7 sets the transfer current setting to 150 μA, and maintains the transfer current setting at 150 μA until the leading edge of the paper 3 leaves the fixing nip 27. After the leading edge of the paper 3 leaves the fixing nip 27, the control unit 7 returns the transfer current setting to 80 μA (normal value). Also, the application of the transfer voltage is stopped when the trailing edge of the paper 3 leaves the transfer nip 24. In this way, by changing the setting of the transfer current, it is possible to suppress the deterioration of image quality that occurs when the leading edge of the paper 3 passes through the fixing nip 27, and to suppress the occurrence of white spots and white spots in the printed image.

[0029] There is no particular limitation on the timing at which the control unit 7 returns the set value of the transfer current to the normal value after the leading edge of the paper 3 has left the fixing nip 27, so long as it is after the leading edge of the paper 3 has left the fixing nip 27. For example, in the graphs shown in Figures 4 to 6, the control unit 7 returns the set value of the transfer current to the normal value when the travel distance of the paper 3 reaches 200 mm. The transport distance of the paper 3 from when the set value of the transfer current is increased above the normal value until the set value of the transfer current is returned to the normal value is, for example, 13 mm or more and 30 mm or less. Alternatively, the set value of the transfer current, which is increased before the leading edge of the paper 3 enters the fixing nip 27 , may be maintained until the trailing edge of the paper 3 leaves the transfer nip 24 .

[0030] The control unit 7 may be configured to determine whether to increase the set value of the transfer current after the leading edge of the paper 3 has passed through the transfer nip 24 and before the leading edge of the paper 3 has passed through the fixing nip 27, based on the type of paper 3 passing through the transfer nip 24. The control unit 7 can determine the type of paper 3, for example, based on the print settings set by the user or the paper feed tray used. If the control unit 7 determines that the paper 3 is plain paper, the control unit 7 can keep the set value of the transfer current constant at a normal value while the paper 3 is passing through the transfer nip 24. If the control unit 7 determines that the paper 3 is thick paper, the control unit 7 can increase the set value of the transfer current from the normal value after the leading edge of the paper 3 has passed through the transfer nip 24 and before the leading edge of the paper 3 has passed through the fixing nip 27.

[0031] The control unit 7 may be provided to determine whether or not to increase the set value of the transfer current from the normal value after the leading edge of the paper 3 passes through the transfer nip 24 and before the leading edge of the paper 3 passes through the fixing nip 27, based on the electric resistance value (through electric resistance value) between the front and back surfaces of the paper 3. The control unit 7 may determine the through electric resistance value of the paper 3, for example, based on the print settings set by the user or the paper feed tray to be used. In this case, the control unit 7 may store a previously measured through electric resistance value of the paper 3. For example, when the through electric resistance value of the paper is smaller than a predetermined threshold value, the control unit 7 may keep the set value of the transfer current constant at the normal value while the paper 3 passes through the transfer nip 24. Also, when the through electric resistance value of the paper is larger than a predetermined threshold value, the control unit 7 may increase the set value of the transfer current from the normal value after the leading edge of the paper 3 passes through the transfer nip 24 and before the leading edge of the paper 3 passes through the fixing nip 27.

[0032] If the through-hole electrical resistance value of the paper 3 is large, there is a risk that the electric field strength of the secondary transfer electric field formed in the transfer nip 24 when the paper passes through will be small. Therefore, by increasing the setting value of the transfer current while the leading edge of the paper 3 is passing through the fixing nip 27, the deterioration of image quality can be suppressed.

[0033] The image forming apparatus 50 may include a sensor unit arranged to measure the electrical resistance value between the front and back sides of the paper 3. The sensor unit may have, for example, two conductive rollers arranged to sandwich the paper transport path 2, and a measuring unit arranged to measure the electrical resistance value of the paper 3 passing between the two conductive rollers. The control unit 7 may be configured to determine whether or not to increase the set value of the transfer current from the normal value after the leading edge of the paper 3 has passed through the transfer nip 24 and before the leading edge of the paper 3 has passed through the fixing nip 27, based on the electrical resistance value measured by the sensor unit. For example, when the electrical resistance value measured by the sensor unit is smaller than a predetermined threshold value, the control unit 7 can keep the set value of the transfer current constant at the normal value while the paper 3 is passing through the transfer nip 24. Furthermore, when the electrical resistance value measured by the sensor unit is larger than a predetermined threshold value, the control unit 7 can increase the set value of the transfer current from the normal value after the leading edge of the paper 3 has passed through the transfer nip 24 and before the leading edge of the paper 3 has passed through the fixing nip 27.

[0034] The control unit 7 may be provided so as to keep the set value of the transfer current constant at a normal value while the paper 3 is passing through the transfer nip 24 when a toner image of text is transferred to the paper 3 in the transfer nip 24 at the timing when the leading edge of the paper 3 passes through the fixing nip 27. The control unit 7 may also be provided so as to increase the set value of the transfer current from the normal value after the leading edge of the paper 3 has passed through the transfer nip 24 and before the leading edge of the paper 3 has passed through the fixing nip 27 when a toner image of an image (e.g., a solid image) is transferred to the paper 3 in the transfer nip 24 at the timing when the leading edge of the paper 3 passes through the fixing nip 27. Since the image quality is more likely to deteriorate than text due to the impact when the leading edge of the paper 3 enters the fixing nip 27, the deterioration of image quality can be suppressed by increasing the set value of the transfer current from the normal value when printing an image.

[0035] The control unit 7 may be configured to increase the set value of the transfer current from the normal value after the leading edge of the paper 3 has passed through the transfer nip 24 and before the leading edge of the paper 3 has passed through the fixing nip 27 when the amount of cyan toner or yellow toner contained in the toner image to be transferred to the paper 3 at the transfer nip 24 is greater than a predetermined value at the timing when the leading edge of the paper 3 passes through the fixing nip 27, and to keep the set value of the transfer current constant at the normal value while the paper 3 is passing through the transfer nip 24 when the amount of cyan toner or yellow toner is less than a predetermined value. Since an image containing a large amount of cyan toner or yellow toner is more likely to deteriorate in image quality due to the impact when the leading edge of the paper 3 enters the fixing nip 27, deterioration in image quality can be suppressed by increasing the set value of the transfer current from the normal value when printing an image with a large amount of cyan toner or yellow toner.

[0036] The control unit 7 may be provided to adjust the amount of increase from the normal value of the set value of the transfer current based on the type of paper 3 passing through the transfer nip 24, the electrical resistance value (through electrical resistance value) between the front and back sides of the paper 3, or the amount of cyan toner or yellow toner contained in the toner image transferred to the paper 3 at the transfer nip 24 at the timing when the leading edge of the paper 3 passes through the fixing nip 27. For example, when printing on paper 3 on which image quality is likely to deteriorate, when printing on paper 3 with a particularly large through electrical resistance value, when printing an image with a large amount of cyan toner, etc., the amount of increase from the normal value of the set value of the transfer current can be made larger (100 μA) than normal (FIG. 4 (70 μA)) as shown in the graph in FIG. 5. Also, for example, when printing on paper 3 on which image quality is relatively unlikely to deteriorate, when printing on paper 3 with a relatively small through electrical resistance value, when printing an image with a large amount of yellow toner, etc., the amount of increase from the normal value of the set value of the transfer current can be made smaller (40 μA) than normal (FIG. 4 (70 μA)) as shown in the graph in FIG. 6.

[0037] Printing a solid blue image A digital full-color multifunction printer manufactured by Sharp Corporation was used to print a sheet with a basis weight of 360 g / m 2A blue image was printed on an A4 size sheet of paper. In the printing of the example, the set value of the transfer current was changed by the control unit as shown in the graph in Figure 3. In the printing of the comparative example, the set value of the transfer current was kept constant at 80 μA.

[0038] In the printed image of the comparative example, toner banding appeared in the area where transfer was performed in the transfer nip at the timing when the leading edge of the paper passed through the fixing section. In the printed image of the embodiment, the toner banding that appeared in the comparative example did not appear. Therefore, it was found that the deterioration of image quality can be suppressed by increasing the setting value of the transfer current when the leading edge of the paper passes through the fixing unit. [Explanation of symbols]

[0039] 2: Paper transport path 3: Paper 4a-4k: Transport rollers 5: Registration roller 7: Control unit 8: Transfer unit 11: Transfer belt 12a-12f: Rollers for transfer unit 13a-13h: Rollers for intermediate transfer belt 18a-18d: Toner image forming unit 19a-19d: Photoconductor 20: Intermediate transfer belt (image carrier) 21: Paper feed tray 22: Manual feed tray 23: Paper discharge tray 24: Transfer nip 25: Heating unit 26: Pressure unit 27: Fixing nip 28: Fixing unit 29: Motor for fixing unit 33: Exposure unit 50: Image forming device

Claims

1. a transfer section disposed opposite the image carrier and arranged to transfer a toner image from the image carrier to a sheet of paper, a fixing section disposed to fix the toner image to the sheet of paper, and a control section disposed to apply a transfer voltage for transferring the toner image to the sheet of paper, the transfer unit is provided so as to form a transfer nip between the image carrier and the transfer unit, through which a sheet of paper passes; the fixing unit includes a rotatable heating unit and a rotatable pressure unit, and is disposed so that the paper after passing through the transfer nip passes through a fixing nip between the heating unit and the pressure unit; The control unit is configured to control the transfer voltage so that the transfer current flowing through the transfer nip is constant at a set value, and is configured to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip.

2. 2. The image forming apparatus of claim 1, wherein the control unit is configured to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip, and to decrease the set value of the transfer current after the leading edge of the paper passing through the transfer nip has passed through the fixing nip.

3. 3. The image forming apparatus according to claim 2, wherein a transport distance of the paper from when the set value of the transfer current is increased to when the set value of the transfer current is decreased is 13 mm or more and 30 mm or less.

4. The image forming apparatus of any one of claims 1 to 3, wherein the control unit is configured to determine whether to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip, based on the type of paper passing through the transfer nip.

5. An image forming apparatus as described in any one of claims 1 to 3, wherein, when the paper is plain paper, the control unit is configured to keep the set value of the transfer current constant while the paper is passing through the transfer nip, and when the paper is thick paper, the control unit is configured to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip.

6. The image forming apparatus of any one of claims 1 to 3, wherein the control unit is configured to determine whether to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip, based on the electrical resistance value between the front and back surfaces of the paper.

7. A sensor unit is provided to measure an electrical resistance between the front surface and the back surface of the paper, An image forming apparatus as described in any one of claims 1 to 3, wherein the control unit is configured to determine whether to increase the set value of the transfer current after the leading edge of the paper passes through the transfer nip and before the leading edge of the paper passes through the fixing nip, based on the electrical resistance value measured by the sensor unit.

8. The control unit is configured to keep the set value of the transfer current constant while the paper is passing through the transfer nip when a toner image of text is transferred to the paper at the transfer nip at the timing when the leading edge of the paper passes through the fixing nip, and to increase the set value of the transfer current after the leading edge of the paper has passed through the transfer nip and before the leading edge of the paper has passed through the fixing nip when a toner image of an image is transferred to the paper at the transfer nip at the timing when the leading edge of the paper passes through the fixing nip.

9. The image forming apparatus of any one of claims 1 to 3, wherein the control unit is configured to increase the set value of the transfer current after the leading edge of the paper passes through the transfer nip and before the leading edge of the paper passes through the fixing nip when the amount of cyan toner or yellow toner contained in the toner image to be transferred to the paper at the transfer nip is greater than a predetermined value at the time the leading edge of the paper passes through the fixing nip.

10. The image forming apparatus of any one of claims 1 to 3, wherein the control unit is configured to adjust the amount of increase in the set value of the transfer current based on the type of paper passing through the transfer nip, the electrical resistance value between the front and back sides of the paper, or the amount of cyan toner or yellow toner contained in the toner image transferred to the paper at the transfer nip at the timing when the leading edge of the paper passes through the fixing nip.

Images