JP2019061070A - Image forming apparatus - Google Patents

Abstract

To inhibit, during transfer of a toner image from an image holding body to an intermediate transfer belt, a toner in the toner image transferred onto the intermediate transfer belt from being re-transferred to the image holding body, compared with a case where the pressing force (nip pressure) of the intermediate transfer belt against the image holding body is maintained constant.SOLUTION: When a toner image is not present between a primary transfer roll 15 and an image holding body 131, an image forming apparatus measures a transfer resistance that is determined by a voltage applied to the primary transfer roll 15 and a current flowing in the primary transfer roll 15, and according to the transfer resistance, when the transfer resistance is in a relatively high resistance region, increases a nip pressure compared with that when the transfer resistance is in a relatively low resistance region.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an image forming apparatus.

  Patent Document 1 describes that the uniformity of the transfer electric field is detected, and a force is applied to the transfer nip according to the detection result to further enhance the uniformity of the nip.

  Patent Document 2 describes switching of the transfer nip pressure according to the type of recording material.

JP, 2013-222199, A JP, 2010-152008, A

  In an image forming apparatus having a plurality of image carriers for holding toner images and configured to sequentially transfer a toner image onto an intermediate transfer belt sequentially, the toner on the uppermost layer on the intermediate transfer belt is the image carrier on the downstream side Image transfer may occur by reverse transfer to the

  According to the present invention, at the time of transferring a toner image from the image carrier to the intermediate transfer belt, the toner image transferred onto the intermediate transfer belt as compared with the case where the pressing force of the intermediate transfer belt against the image carrier is kept constant. An object of the present invention is to provide an image forming apparatus in which retransfer of toner on the image carrier is suppressed.

Claim 1 is
An image holding unit that holds a toner image according to image data;
An intermediate transfer unit to which the toner image formed on the image holding unit is transferred;
A primary transfer unit is disposed between the image holding unit and the intermediate transfer unit, and the primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. Primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the means;
Resistance measurement means for measuring a transfer resistance which is determined by the voltage applied to the primary transfer device and the current flowing to the primary transfer device;
According to the transfer resistance between the primary transfer unit and the image holding means, when the transfer resistance is relatively high, the primary transfer unit compared to when the transfer resistance is relatively low And control means for performing control to increase the pressing force of the intermediate transfer means to the image holding means.

  According to a second aspect of the present invention, in the case where the amount of toner of the toner image to be transferred onto the recording material, which is estimated based on the image data, is smaller than a predetermined threshold value, the control means The image forming apparatus according to claim 1, wherein the pressing force of the intermediate transfer unit by the transfer unit is maintained at a pressing force independent of the transfer resistance.

Claim 3 is
An image holding unit that holds a toner image according to image data;
An intermediate transfer unit to which the toner image formed on the image holding unit is transferred;
A primary transfer unit is disposed between the image holding unit and the intermediate transfer unit, and the primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. Primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the means;
Environmental measurement means for measuring at least one of the environmental temperature and the environmental humidity;
When the at least one is relatively low according to at least one of the environmental temperature and the environmental humidity measured by the environmental measurement means, the primary transfer is compared to when the at least one is relatively high. A control unit for performing control to increase the pressing force of the intermediate transfer unit to the image holding unit by the image forming unit.

  According to a fourth aspect of the invention, in the case where the amount of toner of the toner image to be transferred onto the recording material, which is estimated based on the image data, is smaller than a predetermined threshold value, the control means The image forming apparatus according to claim 3, wherein the pressing force of the intermediate transfer unit by the transfer unit is maintained at a pressing force independent of the at least one.

Claim 5 is:
An image holding unit that holds a toner image according to image data;
An intermediate transfer unit to which the toner image formed on the image holding unit is transferred;
A primary transfer unit is disposed between the image holding unit and the intermediate transfer unit, and the primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. Primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the means;
Recording means for recording usage history;
When the use history recorded by the recording means represents a relatively long use period, the image by the primary transfer device is compared to when the use history represents a relatively short use period. And a control unit configured to perform control to increase the pressing force of the intermediate transfer unit to the holding unit.

  According to a sixth aspect of the present invention, in the case where the amount of the toner of the toner image transferred onto the recording material, which is estimated based on the image data, is smaller than a predetermined threshold value, the control means 6. The image forming apparatus according to claim 5, wherein the pressing force of the intermediate transfer unit by the transfer unit is maintained at a pressing force independent of the period of use.

Claim 7 is:
An image holding unit that holds a toner image according to image data;
An intermediate transfer unit to which the toner image formed on the image holding unit is transferred;
A primary transfer unit is disposed between the image holding unit and the intermediate transfer unit, and the primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. Primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the means;
When the amount of toner is relatively large according to the amount of toner of the toner image transferred onto the recording material estimated based on the image data, the amount of toner is relatively small The image forming apparatus further comprises control means for performing control to increase the pressing force of the intermediate transfer means to the image holding means by the primary transfer device.

The eighth aspect is
A resistance measuring unit that measures a transfer resistance determined by the voltage applied to the primary transfer unit and the current flowing to the primary transfer unit;
When the control unit is in a predetermined resistance region between the primary transfer unit and the image holding unit, which is measured by the resistance measurement unit, the control unit performs the primary transfer unit using the primary transfer unit. 8. The image forming apparatus according to claim 7, wherein the pressing force of the intermediate transfer unit to the image holding unit is maintained at a pressing force which does not depend on the amount of toner.

The ninth aspect is
Environmental measurement means for measuring at least one of the environmental temperature and the environmental humidity;
When the at least one measured by the environment measuring unit is within a predetermined range, the pressing force of the intermediate transfer unit to the image holding unit by the primary transfer unit does not depend on the amount of toner. The image forming apparatus according to claim 7, wherein the pressing force is maintained.

Claim 10 is:
It has recording means to record usage history,
When the control means indicates that the use history recorded by the recording means is within a predetermined use period, the pressing force of the intermediate transfer means to the image holding means by the primary transfer device is 8. The image forming apparatus according to claim 7, wherein the pressing force does not depend on the amount of toner.

Claim 11 is:
A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer device sandwiching the intermediate transfer means with the image holding means is disposed corresponding to the image holding means, and a primary transfer for transferring the toner image on the image holding means onto the intermediate transfer means A plurality of primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the image holding means;
The plurality of image holding means includes, in addition to an image holding means for forming a toner image with process color toner, a spot color corresponding image holding means for forming a toner image with spot color toner other than the process color toner,
In the case where the image data is image data in which the formation of a toner image transferred onto the recording material is taken over or shared by the spot color corresponding image holding means, the formation of a toner image transferred onto the recording material is The image processing apparatus further comprises control means for performing control to increase the pressing force of the intermediate transfer means to the image holding means by the primary transfer device as compared to the case where the image data for the special color corresponding image is not shared. An image forming apparatus characterized by

Claim 12 is:
The primary transfer means is a constant voltage power supply means for supplying a transfer bias to the primary transfer device, wherein the constant voltage power supply means is capable of adjusting the voltage of the transfer bias supplied to the primary transfer device. Equipped
When the control unit increases the pressing force according to the pressing force of the intermediate transfer unit to the image holding unit by the primary transfer unit, the constant transfer voltage supply unit performs the primary transfer. The image forming apparatus according to any one of claims 1 to 11, wherein a voltage of a transfer bias supplied to the image forming apparatus is lowered.

Claim 13 is:
A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and a primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. When,
A resistance measuring unit that measures a transfer resistance determined by a voltage applied to a measurement target primary transfer unit among the plurality of primary transfer units and a current flowing to the measurement target primary transfer unit;
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
The transfer resistance is relatively high when the transfer resistance is relatively high according to the transfer resistance between the primary transfer unit to be measured and the image holding means corresponding to the primary transfer unit to be measured. Control means for performing control to increase the pressing force of the intermediate transfer means to the corresponding downstream side image holding means by the downstream side primary transfer devices as compared with the case where the pressure is low. It is an image forming apparatus.

  According to a fourteenth aspect of the present invention, in the case where the amount of toner of the toner image transferred onto the recording material, which is estimated based on the image data, is smaller than a predetermined threshold value, the control means 14. The image formation according to claim 13, wherein the pressing force of the intermediate transfer unit to the corresponding downstream image holding unit by the primary transfer unit is maintained at a pressing force independent of the transfer resistance. It is an apparatus.

Claim 15 is:
A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and is a primary transfer unit transferring the toner image on the image holding unit onto the intermediate transfer unit When,
Environmental measurement means for measuring at least one of the temperature environment and the humidity environment;
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
According to the at least one measured by the environment measuring means, when the at least one is relatively low, compared with the case where the transfer resistance is relatively high, the respective corresponding downstream primary transfer devices The image forming apparatus is further provided with a control unit that performs control to increase the pressing force of the intermediate transfer unit to each of the downstream side image holding units.

  According to a sixteenth aspect of the present invention, in the case where the control means estimates the amount of toner of the toner image transferred onto the recording material, which is estimated based on the image data, each of the downstream sides does not satisfy a predetermined threshold. The image formation according to claim 15, characterized in that the pressing force of the intermediate transfer unit to the corresponding downstream image holding unit by the primary transfer unit is maintained at a pressure independent of the at least one. It is an apparatus.

Claim 17 is:
A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and is a primary transfer unit transferring the toner image on the image holding unit onto the intermediate transfer unit When,
And recording means for recording the usage history,
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
When the use history recorded by the recording means represents a relatively long use period, compared to when the use history represents a relatively short use period, the respective downstream primary transfer devices The image forming apparatus is further provided with control means for performing control to increase the pressing force of the intermediate transfer means to the corresponding downstream side image holding means.

  According to an eighteenth aspect of the present invention, in the case where the control means estimates the amount of toner of the toner image transferred onto the recording material, which is estimated based on the image data, each of the downstream sides does not satisfy a predetermined threshold. The image formation according to claim 17, characterized in that the pressing force of the intermediate transfer unit to the corresponding downstream image holding unit by the primary transfer unit is maintained at a pressure independent of the period of use. It is an apparatus.

Claim 19 is:
A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and is a primary transfer unit transferring the toner image on the image holding unit onto the intermediate transfer unit With,
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
When the amount of toner is relatively large according to the amount of toner of the toner image transferred onto the recording material estimated based on the image data, the amount of toner is relatively small The image forming apparatus further comprises control means for performing control to increase the pressing force of the intermediate transfer means to the corresponding downstream image holding means by the respective downstream primary transfer devices. is there.

Claim 20 is:
A resistance measuring unit configured to measure a transfer resistance determined by a voltage applied to a measurement target primary transfer unit among the plurality of primary transfer units and a current flowing to the measurement target primary transfer unit;
When the control resistance is in a predetermined resistance region between the measurement target primary transfer unit and the image support unit corresponding to the measurement target primary transfer unit by the resistance measurement unit, 20. The image forming apparatus according to claim 19, wherein the pressing force of the intermediate transfer unit to the corresponding downstream image holding unit by each of the downstream primary transfer devices is maintained at a pressure independent of the amount of toner. The image forming apparatus according to

Claim 21 is:
An environmental measurement means for measuring at least one of a temperature environment and a humidity environment;
When the at least one measured by the environment measuring means is within a predetermined range, the pressing force of the intermediate transferring means to the corresponding downstream image holding means by the respective downstream primary transfer devices 16. The image forming apparatus according to claim 15, wherein the pressing force does not depend on the amount of toner.

Claim 22 is:
It has recording means to record usage history,
When the use history recorded by the recording means represents within a predetermined use period, the pressing of the intermediate transfer means onto the corresponding downstream image holding means by the respective downstream primary transfer devices 22. The image forming apparatus according to claim 21, wherein the pressure is maintained at a pressure independent of the amount of toner.

Claim 23 is:
A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and is a primary transfer unit transferring the toner image on the image holding unit onto the intermediate transfer unit Equipped with
The plurality of image holding means includes, in addition to an image holding means for forming a toner image with process color toner, a spot color corresponding image holding means for forming a toner image with spot color toner other than the process color toner,
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
In the case where the image data is image data in which the formation of a toner image transferred onto the recording material is taken over or shared by the spot color corresponding image holding means, the formation of a toner image transferred onto the recording material is The control for increasing the pressing force of the intermediate transfer means to the corresponding downstream image holding means by the respective downstream primary transfer devices as compared to the case where the image data not shared by the spot color corresponding image holding means is shared. An image forming apparatus further comprising control means for performing the process.

Claim 24 is:
The primary transfer means is a constant voltage power supply means for supplying a transfer bias to the primary transfer device, and at least a constant voltage capable of adjusting the voltage of the transfer bias supplied to the respective downstream primary transfer devices. Equipped with power supply means,
The constant voltage power supply when the control means increases the pressing force according to the pressing force of the intermediate transfer means to the corresponding downstream image holding means by the respective downstream primary transfer devices. The image forming apparatus according to any one of claims 13 to 23, wherein the voltage of the transfer bias supplied to each of the downstream primary transfer devices is lowered by the supply means.

  According to the image forming apparatus of claim 1, 3, 5, 7, 11, 13, 15, 17, 19 and 23, when the toner image is transferred from the image carrier to the intermediate transfer belt, the image carrier is transferred to the image carrier Compared with the case where the pressing force of the intermediate transfer belt is maintained constant, the toner in the toner image transferred onto the intermediate transfer belt is suppressed from being re-transferred onto the image carrier.

  According to the image forming apparatus of claims 2, 4, 6, 8 to 10, 14, 16, 18, and 20 to 22, when the transfer resistance is high, the pressing force is uniformly increased by being measured or estimated. In comparison, the pressure is increased only when necessary.

  According to the image forming apparatus of claims 8 to 10 and 20 to 22, when the thickness of the toner of the toner image transferred to the recording material exceeds the threshold, the pressing force is uniformly increased. The pressing force is increased only when necessary.

  According to the twelfth and twenty-fourth aspects, in the case of the constant voltage power source, better transfer is performed as compared with the case where the voltage of the transfer bias is not changed by the pressing force.

FIG. 1 is a schematic configuration diagram of an image forming apparatus as an embodiment of the present invention. FIG. 2 is a schematic view showing a configuration around one image forming unit. FIG. 2 is a schematic view showing six image forming units and an intermediate transfer belt. It is explanatory drawing of a nip. It is the figure which illustrated the control contents of the 1st example and the 6th example. It is the figure which illustrated the control contents of the 2nd example and the 7th example. It is the figure which illustrated the control contents of the 3rd example and the 8th example. In the third and eighth examples, the usage time of the primary transfer roll in one day, the vertical axis is the increase width of the nip pressure. It is the figure which illustrated the control contents of the 4th example and the 9th example. It is the figure which illustrated the control contents of the 5th example and the 10th example.

  Hereinafter, embodiments of the present invention will be described.

  FIG. 1 is a schematic view of an image forming apparatus according to an embodiment of the present invention.

  The image forming apparatus 10 has two housings of a first housing 10a and a second housing 10b connected to each other, and the members constituting the image forming apparatus 10 are divided into the two housings. Is incorporated.

  The image forming apparatus 10 is configured to form an image using toners of up to six colors, and six toner cartridges 11CT, 11Y, 11M, 11C for storing toners of respective colors on the top of the first housing 10a. , 11K and 11W are arranged.

  Here, the alphabet in the code represents the color of the toner contained in the toner cartridge, among them, Y represents yellow, M represents magenta, C represents cyan, and K represents black. Further, CT and W are special features other than Y, M, C and K, and in this embodiment, a toner cartridge containing transparent toner as the toner cartridge 11CT and a toner cartridge containing white toner as the toner cartridge 11W It is used.

  In the following, as a code indicating a toner cartridge, when it is not necessary to distinguish colors, the alphabet representing the color is omitted and simply referred to as toner cartridge 11 and when it is necessary to distinguish colors, the above It shall be indicated with an alphabet representing that color. The same applies to each element other than the toner cartridge 11.

  The toner in each toner cartridge 11 is supplied to an image forming unit 13 described below. Each toner cartridge 11 is replaceable, and when it is emptied, it is replaced with a new toner cartridge 11 containing a toner of the same color.

  Below the toner cartridge 11 in the first housing 10a, six exposure units 12 and six image forming units 13 corresponding to the six toner cartridges 11 described above are provided.

  FIG. 2 is a schematic view showing a configuration around one image forming unit.

  The image forming unit 13 includes a drum-type image carrier 131 which rotates in the direction of arrow A, around which a charger 132, a developer 133, a cleaning blade 134, and a static eliminator 135 are disposed. . The above-described exposure unit 12 is disposed above the image carrier 131, and the primary transfer roller 15 is disposed at a position where an intermediate transfer belt 14 described later is sandwiched between the image carrier 131 and the image carrier 131. It is arranged.

  The primary transfer roll 15 moves up and down by the rotation of the cam mechanism 152 which rotates in response to the rotational driving force from the motor 151, and adjusts the pressing force of the intermediate transfer belt 14 against the image carrier 131. There is. Further, a transfer bias is applied to the primary transfer roll 15 from a bias power supply 153, and the primary transfer roll 15 applies the toner image on the image carrier 131 onto the intermediate transfer belt 14 by the applied transfer bias. Transfer. In the present embodiment, a constant current power supply is used as the transfer bias power supply 153. The transfer bias power supply 153 also measures the transfer resistance which is determined by the voltage of the transfer bias applied to the primary transfer roll 15 and the current flowing through the primary transfer roll 15. The transfer bias power supply 153 corresponds to an example of the resistance measurement means in the present invention.

  The operation of the image forming unit 13 will be described.

  The image carrier 131 is charged by the charger 132 while rotating in the direction of arrow A, and is exposed to the exposure beam L from the exposure device 12. The exposure unit 12 repeatedly scans the image carrier 131 in the direction perpendicular to the sheet of FIG. 2 with the exposure beam L modulated according to the image data, and repeatedly scans the image carrier 131 with the exposure beam L. Form an electrostatic latent image. The electrostatic latent image is developed with toner in the developer by a developing device 133 containing a developer containing toner and carrier, and a toner image is formed on the image carrier 131. The toner is supplied from the corresponding toner cartridge 11 (see FIG. 1) so that a predetermined amount of toner is stored in the developing device 133. The toner image formed on the image carrier 131 by the action of the developing device 133 is moved in the direction of arrow B by the action of the primary transfer roll 15 rotating in the direction of arrow C while receiving the application of the transfer bias. 14 is transferred onto.

  After transfer, the toner remaining on the image carrier 131 is scraped off and collected from the image carrier 131 by the cleaning blade 134, and the charge carrier 135 is neutralized by the charge eliminator 135, and the latent image remaining there is It is erased and newly charged by the charger 132.

  Returning to FIG. 1, the description will be continued.

  An endless intermediate transfer belt 14 is provided under the six image forming units 13. The intermediate transfer belt 14 is supported by a plurality of rolls 16 including a drive roll 16a and a backup roll 16b, and circulates in the direction of arrow B while in contact with the respective image carriers 131 constituting the image forming unit 13.

  A secondary transfer roll 17 is provided at a position facing the backup roll 16 b with the intermediate transfer belt 14 interposed therebetween. The toner images transferred so as to sequentially overlap on the intermediate transfer belt 14 by the action of the primary transfer rolls 15 disposed corresponding to the respective image forming units 13 are further conveyed by the intermediate transfer belt 14 in the arrow B direction. . The toner image on the intermediate transfer belt 14 is secondarily transferred onto the recording material conveyed to a position sandwiched between the intermediate transfer belt 14 and the secondary transfer roll 17 by the action of the secondary transfer roll 17. Ru. As a result, an unfixed toner image is formed on the recording material.

  Two recording material accommodating portions 18a and 18b are provided in the lower part of the first housing 10a, and a large number of recording materials P are accommodated in each of the recording material accommodating portions 18a and 18b. It is done. At the time of image formation, the recording material P is taken out from the recording material accommodating portions 18a and 18b, but the bottom plates 181a and 181b are configured to rise as the recording material accommodated in the recording material accommodating portions 18a and 18b decreases. .

  When forming an image, one of the uppermost layers of the recording materials P accommodated in one of the recording material accommodating portions 18a and 18b designated by the operator either manually or automatically When the recording material P is taken out by the pickup roll 19a and a plurality of recording materials are taken out at one time, the recording material P is surely separated into one sheet by the separation roll 19b, and the one recording material is conveyed by the transport roll 19. The leading end of the recording material P conveyed on the conveyance paths 20a, 20b, 20c and conveyed reaches the positioning roll 19c. Further, the first case 10a is provided with a take-in port 111 for taking in the recording material from the outside of the first case 10a, and when the recording material is taken in from the take-in port 111, it is taken in. The recording material is conveyed on the conveyance paths 20d and 20c, and the leading end thereof reaches the positioning roll 19c. The positioning roll 19c plays the role of correcting the posture of the conveyed recording material, adjusting the timing of the subsequent feeding of the recording material, and feeding the recording material further toward the downstream side in the conveyance direction.

The positioning roll 19 c is configured such that the toner image on the intermediate transfer belt 14 is a secondary transfer roll 17.
The recording material is fed so that the recording material is conveyed to the position of the secondary transfer roll 17 at the same timing as the conveyance to the position of.

  The recording material which has received the transfer of the toner image by the action of the secondary transfer roll 17 is conveyed by the conveyance belt 21, enters the second housing 10 b, and reaches the fixing device 22. The fixing device 22 includes a heating belt 221 and a pressure roller 222, and the recording material conveyed to the fixing device 22 is sandwiched between the heating belt 221 and the pressure roller 222 and is subjected to heat and pressure. The toner image on the recording material is fixed to the recording material. The recording material passing through the fixing unit 22 is cooled by the cooler 24. The cooler 24 is of a type in which the recording material is cooled by being sandwiched between two endless belts 241 and 242. The recording material having exited the cooler 24 has its warp corrected by the decurler 25 and the optical measuring device 26 measures an image consisting of a fixed toner image on the recording material. The optical measuring instrument 26 monitors, for example, that an image is correctly formed on a recording material at the time of general image formation. In addition, at the time of adjustment, various charts, for example, various color patches are arranged on the recording material by the image forming apparatus 10, and color patches of those color patches are measured to perform color tone correction. The optical measuring instrument 26 also plays a role in performing measurement for various adjustments, such as forming an image for magnification adjustment and measuring the image to adjust an image forming position and an image magnification. Furthermore, an image of uniform color and uniform density is formed on the recording material by the image forming apparatus 10, and measurement by the optical measuring instrument 26 is carried out so that there is no generation of scratches or shading on the image. Confirmation is also performed.

  The recording material having passed the optical measuring instrument 26 is discharged onto the paper discharge tray 28 by the paper discharge roller 27.

  On the other hand, the intermediate transfer belt 14 moves in the direction of arrow B to reach the cleaner 41 even after the secondary transfer of the toner image is performed on the recording material by the action of the secondary transfer roll 17 and reaches the cleaner 41. The toner remaining on the transfer belt 17 is removed from the intermediate transfer belt 17.

  The above is the process for forming an image only on the first surface of the recording material, but when forming an image on both sides of the recording material, the following process is performed. In this case, an image is formed on the first surface of the recording material in the same process as described above, and then passes by the optical measuring instrument 26. The recording material having passed the optical measuring instrument 26 enters the conveyance path 20e before reaching the paper discharge roll 27, is conveyed on the conveyance path 20e, and further enters the conveyance path 20f. When the recording material enters the conveyance path 20f, the rotation direction of the conveyance roll constituting the conveyance path 20f reverses, and the recording material is fed out from the conveyance path 20f in the reverse direction and returns to the first housing 10a. It is conveyed on the paths 20b and 20c to reach the positioning roll 19c. The recording material at this time has a posture in which the second surface on which the image is not formed is directed to the intermediate transfer belt 14. While the recording material passes through the transport path to reach the positioning roll 19c, on the image forming unit 13 side, a toner image of the image to be formed on the second surface of the recording material is formed, and the intermediate transfer belt 14 is formed. Transcribed on top. Thereafter, in the same manner as image formation on the first surface of the recording material, the recording material is delivered from the positioning roll 19 c and the toner image is transferred to the second surface of the recording material by the action of the secondary transfer roll 17. Further, the sheet passes through the fixing unit 22, the cooler 24, the decurler 25, and the optical measuring unit 26, and is discharged onto the sheet discharge tray 28 by the sheet discharge roll 27 this time.

  Further, an image processing / control unit 30 is provided on the upper portion of the second housing 10 b of the image forming apparatus 10. The image processing / control unit 30 includes a memory for storing image data and the like sent from the outside, an arithmetic circuit for performing various processes such as image processing on the image data, and the image forming apparatus 10. A control circuit responsible for overall control is provided. The image processing / control unit 30 records the usage history of each member of the image forming apparatus 10. The image processing / control unit 30 also controls the pressing force of the intermediate transfer belt 14 on the image carrier 131 by controlling the rotation of the motor 151 shown in FIG. 2. The image processing / control unit 30 corresponds to an example of the recording unit and the control unit in the present invention.

  Further, a power supply unit 33 is provided below the image processing / control unit 30. The power supply unit 33 also includes the power supply bias 153 shown in FIG. 2, and the power supply unit 33 supplies necessary power to each part of the image forming apparatus 10. Further, the transfer resistance measured by the transfer bias power supply 153 (see FIG. 2) constituting the power supply unit 33 is reported to the image processing / control unit 30.

  Furthermore, the image forming apparatus 10 is provided with an environment sensor 34 that measures the environmental temperature and humidity in the image forming apparatus 10. The temperature and humidity measured by the environment sensor 34 are also reported to the image processing / control unit 30.

  Further, a monitor 31 for displaying various states of the image forming apparatus 10 and an operation panel 32 for receiving an operation by an operator are disposed on the lower portion of the second housing 10b.

  FIG. 3 is a schematic view showing an intermediate transfer belt and six image forming units (represented by six image carriers in this case) constituting the image forming apparatus shown in FIG. 1 and FIG. Here, with reference to FIG. 3, problems to be solved by the image forming apparatus 10 of the present embodiment will be described.

  As described above, in the image forming apparatus 10 according to the present embodiment, the six image carriers 131CT, 131Y, 131M, 131C, 131K, and 131W are sequentially transferred to the intermediate transfer belt 14 in the order of the first image carrier 131CT. The second image carrier 131Y, the third image carrier 131M, the fourth image carrier 131C, the fifth image carrier 131K, and the sixth image carrier 131W may be referred to. Similarly to this, the first image carrier 131CT, the second image carrier 131Y, the third image carrier 131M, the fourth image carrier 131C, the fifth image carrier 131K, and the sixth image carrier 131W. The primary transfer rolls 15CT, 15Y, 15M, 15C, 15K, and 15W respectively provided for the first primary transfer roll 15CT, the second primary transfer roll 15Y, and the third primary It may be referred to as transfer roll 15M, fourth primary transfer roll 15C, fifth primary transfer roll 15K, and sixth primary transfer roll 15W.

  Here, FIG. 3A is a schematic side view, and FIG. 3B is a plan view of the intermediate transfer belt 14.

  Here, a toner image of transparent toner is formed on the first image carrier 131CT, and the toner image of the transparent toner is transferred onto the intermediate transfer belt 14 moving in the direction of arrow B by the action of the primary transfer roll 15CT. Be done. Further, a toner image of yellow (Y) toner is formed on the second image carrier 131 Y, and the toner image of transparent toner on the intermediate transfer belt 14 is superimposed on the toner image of the intermediate transfer belt 14 by the action of the primary transfer roll 15 Y. Is transferred to Further, a toner image of magenta (M) toner is formed on the third image carrier 131M, and the toner image of yellow (Y) toner on the intermediate transfer belt 14 is formed by the action of the primary transfer roll 15M. It is transcribed so that it overlaps with.

  Here, no toner image is formed on the fourth image carrier 131C, the fifth image carrier 131K, and the sixth image carrier 131W. However, in the present embodiment, the same transfer bias is simultaneously applied to the primary transfer rolls 15CT, 15Y, 15M, 15C, 15K, and 15W. For this reason, although no toner image is formed on the fourth image carrier 131C, the fifth image carrier 131K, and the sixth image carrier 131W, the transfer operation to the intermediate transfer belt 14 is performed. Ru.

  Here, when the transfer bias voltage to the primary transfer roll 15 is high, a discharge due to the high transfer bias occurs, a toner to be charged to the opposite polarity is generated by the discharge, and the toner is charged on the intermediate transfer belt 14. A phenomenon may occur in which part of the toner M once transferred is re-transferred to the image carrier 131. In FIG. 3, the toner re-transferred onto the image carrier 131 is shown as a re-transfer toner M '. When this retransfer occurs, an image defect Y 'resulting from the removal of the retransfer toner M' is generated on the intermediate transfer belt 14.

  This retransfer phenomenon can be suppressed by lowering the transfer bias voltage applied to the primary transfer roll 15. However, if the transfer bias voltage is simply lowered, the normal operation from the image carrier 131 to the intermediate transfer belt 14 is possible. There is a possibility that the transfer may be defective.

  As a method of suppressing the phenomenon of retransfer without causing a transfer failure, pressing force of the primary transfer roll 15 against the image carrier 131 via the intermediate transfer belt 14 by pushing up the primary transfer roll 15 (nip It is conceivable to raise the pressure to increase the nip area.

  FIG. 4 is an explanatory view of the nip. Here, in FIG. 4 (A), in FIGS. 4 (B) and 4 (C), the nip when the image carrier 131 is viewed in the longitudinal direction (direction perpendicular to the sheet of FIG. 4 (A)). It is the figure which showed the shape. An arrow B indicates the traveling direction of the intermediate transfer belt 14.

  In FIG. 4B, the pressing force (nip pressure) of the primary transfer roll 15 is low, and as a result, the width (nip width d) of the nip area NIP is narrow, and the nip area NIP is a narrow nip area. There is. On the other hand, in FIG. 4C, the pressing force (nip pressure) of the primary transfer roll 15 is high, and as a result, the nip width d is wide, and the nipping area NIP has a wide nipping area. In the present embodiment, the adjustment of the nip pressure is performed by the adjustment of the rotation angle of the cam mechanism 152 by the rotation of the motor 151 shown in FIG.

  When the pressing force (nip pressure) of the primary transfer roll 15 is increased, the area of the nip region NIP is expanded, and the transfer resistance determined by the transfer bias voltage applied to the primary transfer roll 14 and the current flowing there is reduced. When the transfer resistance decreases, normal transfer with a low transfer bias voltage is possible, discharge is suppressed, and retransfer is therefore suppressed. However, when the nip pressure is increased, there is a possibility that adverse effects such as accelerated deterioration of the image carrier 131 and the transfer roller 15 may occur.

  Therefore, in the present embodiment, the nip pressure is lowered when the risk of retransfer is low based on any of the determination criteria described below, and the nip pressure is increased when the risk of retransfer is increased. Control is performed.

(First example)
FIG. 5 is a diagram illustrating the control content of the first example. The horizontal axis of FIG. 5 is the transfer resistance, and the vertical axis is the nip pressure.

  In the case of the first example, as an example, when a toner image is not present between the primary transfer roll 15 and the image carrier 131, the voltage applied to the primary transfer roll 15 and the current flowing to the primary transfer roll 15 Control to increase the nip pressure when the transfer resistance is in a relatively high resistance region according to the transfer resistance as compared to when the transfer resistance is in a relatively low resistance region. Is done.

  In the example shown in FIG. 5, the transfer resistance is divided into a first region D1 having a low transfer resistance, a second region D2 having a slightly higher transfer resistance, and a third region D3 having a high transfer resistance. When inside, a low nip pressure is maintained, and when the transfer resistance is in the third region D1, it is adjusted to a high nip pressure, and when the transfer resistance is in the second region D1, the transfer resistance is adjusted. The higher the transfer resistance, the higher the nip pressure.

  According to the first example, re-transfer of the toner in the toner image transferred onto the intermediate transfer belt 14 to the image carrier 131 is suppressed as compared with the case where the nip pressure is always kept constant.

  In the first example and each example to be described later which measures the transfer resistance, the time of the resistance measurement is not limited to the time when the toner image is not present between the primary transfer roll 15 and the image carrier 131 The resistance may be measured between the primary transfer roll 15 and the image carrier 131, with the toner image as a test pattern and the paper, or the toner image to be printed and the paper being sandwiched.

  Here, in the image processing / control unit 30 (see FIG. 1), the image data is transferred onto the recording material P based on the image data input from the outside, that is, the image data that is the source of the image to be formed. The amount of toner in the toner image is estimated. Then, when the estimated toner amount does not reach a predetermined threshold (for example, the halftone dot area ratio of each color total is 260%), the nip pressure is used as the transfer resistance without measuring the transfer resistance. Is maintained at a nip pressure which is independent of, in the example shown in this FIG. 5, the same low nip pressure as the nip pressure in the first region.

  In the present embodiment, as described above, a constant current power supply is employed as the transfer bias power supply 153 (see FIG. 2). Therefore, the transfer bias voltage is adjusted to a voltage corresponding to the transfer resistance. When the toner amount is equal to or less than the threshold value, it is known that the transfer resistance is low. Therefore, the transfer bias voltage is also low, and control to increase the nip pressure is unnecessary. If the toner amount exceeds the threshold value, the transfer resistance is measured as described above, and the nip pressure is controlled according to the transfer resistance.

(Second example)
FIG. 6 is a diagram illustrating the control content of the second example. The horizontal axis in FIG. 6 is the temperature and humidity, and the vertical axis is the nip pressure.

  In the case of the second example, when the transfer resistance is in a relatively high resistance region according to the transfer resistance estimated based on the environmental temperature and humidity measured by the environment sensor 34 (see FIG. 1), Control is performed to increase the nip pressure, as compared to the case where is in a relatively low resistance region.

  In the example shown in FIG. 6, the temperature and humidity are divided into a first area D1 having low temperature and humidity, a second area D2 having slightly higher temperature and humidity, and a third area D3 having high temperature and humidity. When in the first area D1, it is estimated that the transfer resistance is high, so it is adjusted to a high nip pressure, and when the temperature and humidity are in the third area D3, it is estimated that the transfer resistance is low Therefore, low nip pressure is maintained. Then, when the temperature and humidity are within the second region D2, the nip pressure is adjusted to an intermediate value between the nip pressure in the first region D1 and the nip pressure in the third region D3.

  Even when the second example is employed, retransfer of toner in the toner image transferred onto the intermediate transfer belt 14 to the image holding member 131 is suppressed as compared with the case where the nip pressure is always kept constant. Be done.

  Here, as in the first example, when the toner amount estimated based on the image data does not satisfy the predetermined threshold value, it is known that the transfer resistance is low, and the nip pressure is the temperature and humidity. In the example shown in FIG. 6, the same nip pressure as the nip pressure in the third region D3 is maintained. When the toner amount exceeds the threshold value, the nip pressure is controlled according to the temperature and humidity as described above.

  Here, although the nip pressure is adjusted based on the temperature and the humidity, the nip pressure may be adjusted based on only the temperature among the temperature and the humidity or only the humidity.

(Third example)
FIG. 7 is a diagram illustrating the control content of the third example. The horizontal axis in FIG. 7 is the cumulative use time of the primary transfer roll 15 from when the image forming apparatus 10 is new, and the vertical axis is the nip pressure.

  Further, FIG. 8 shows the usage time of the primary transfer roll 15 in one day in the third example, and the vertical axis is the increase width of the nip pressure.

  In the image processing / control unit 30 shown in FIG. 1, usage histories of respective members constituting the image forming apparatus 10 are recorded. Here, according to the transfer resistance estimated based on the cumulative use time of the primary transfer roll 15 in the use history recorded in the image processing / control unit 30, the transfer resistance is relatively high in the resistance region. At certain times, control is performed to increase the nip pressure as compared to when the transfer resistance is in a relatively low resistance region.

  In the example shown in FIG. 7, the first region D1 in which the cumulative use time of the primary transfer roll 15 is short, the second region D2 in which the cumulative use time is longer than the first region D1, the third region D3 in which the cumulative use time is longer, It is divided into a long fourth area D4, and the area where the cumulative use time is long is adjusted to a higher nip pressure.

  If the cumulative use time of the primary transfer roll 14 is long, the surface of the primary transfer roll 14 becomes rough or foreign matter adheres to that extent, and the transfer resistance tends to increase. Therefore, in this case, the nip pressure is controlled to be higher as the cumulative use time is longer.

  Further, in the case of the image forming apparatus 10 of the present embodiment, when the power is turned on and used one day on that day, the transfer resistance is low for a while after the start of use, and the transfer resistance is used when used for a while. It tends to be higher.

  Therefore, here, as shown in FIG. 8, the first area D1 from when the power is turned on until reaching a predetermined use time and the second area D2 thereafter are divided into the first area D1 in FIG. In some cases, the nip pressure is adjusted as shown in FIG. 7, and when moving to the second area D2 of FIG. 8, the nip pressure shown in FIG. Adjusted to the width. Here, although the additional width Δp is constant regardless of the respective regions shown in FIG. 7, the additional width Δp may be changed for each of the regions shown in FIG.

  Even when the third example is employed, retransfer of toner in the toner image transferred onto the intermediate transfer belt 14 to the image holding member 131 is suppressed as compared with the case where the nip pressure is always kept constant. Be done.

  Here, as in the first and second examples, it is known that the transfer resistance is low when the toner amount estimated based on the image data does not meet the predetermined threshold value, and the nip pressure However, the nip pressure does not depend on the cumulative use time, and in the example shown in FIG. 7, is maintained at the same low nip pressure as the nip pressure in the first region D1. When the toner amount exceeds the threshold value, the nip pressure is controlled according to the temperature and humidity as described above.

(4th example)
FIG. 9 is a diagram illustrating the control content of the fourth example. The horizontal axis of FIG. 9 is the toner amount of the toner image to be formed, and the vertical axis is the nip pressure.

  In the image processing / control unit 30 (see FIG. 1), the amount of toner of the toner image to be transferred onto the recording material P based on the image data that is input from the outside and is the origin of the image to be formed Is estimated. When the amount of toner is large, the transfer resistance is increased, and when the nip pressure is kept constant, a high transfer bias voltage is applied to the primary transfer roll 14 by the transfer bias power supply 153 (see FIG. 2) as a constant current power supply. Ru. Therefore, here, when the amount of toner is relatively large according to the amount of toner of the toner image transferred onto the recording material P estimated based on the image data, the amount of toner is relatively small As compared with the case, control is performed to increase the nip pressure.

  In the example shown in FIG. 9, the toner amount is divided into a first region D1 having a small toner amount, a second region D2 having a slightly larger toner amount, and a third region D3 having a large toner amount. When it is inside, a low nip pressure is maintained, and when the amount of toner is in the third area D1, it is adjusted to a high nip pressure, and when the amount of toner is in the second area D1, according to the amount of toner The higher the toner amount, the higher the nip pressure.

  The amount of toner in the toner image may be calculated for each of the image carriers 131CT, 131Y, 131M, 131C, 131K, 131W instead of the amount of toner in the toner image transferred onto the recording material P. For example, for the third image carrier 131M, the toner amounts of three toner images formed on the three image carriers of the first image carrier 131CT, the second image carrier 131Y, and the third image carrier 131M. It becomes the total value of Such calculations are performed, and the nip pressure is set for each image carrier 131CT, 131Y, 131M, 131C, 131K, 131W according to the toner amount for each image carrier 131CT, 131Y, 131M, 131C, 131K, 131W. You may control.

  According to the fourth example, as in the first to third examples, the toner in the toner image transferred onto the intermediate transfer belt 14 is an image compared to the case where the nip pressure is always kept constant. Retransfer to the support 131 is suppressed.

  Here, a modification described below may be adopted in which the first to third examples described above are added to the fourth example.

(First modification)
The transfer resistance at the time when there is no toner image between the primary transfer roll 15 and the image carrier 131 is measured, and when the transfer resistance is in a predetermined low resistance area, the nip pressure is Maintain at low nip pressure independent of.

(2nd modification)
The environmental temperature and humidity are measured by the environmental sensor 34 (see FIG. 1). Then, when the transfer resistance estimated based on the measured environmental temperature and humidity is in a predetermined low resistance region, the nip pressure is maintained at a low nip pressure independent of the amount of toner.

(Third modification)
When the transfer resistance estimated based on the accumulated use time of the primary transfer roll 15 in the use history recorded in the image processing / control unit 30 is in a predetermined low resistance area, the nip pressure is Maintain a low nip pressure independent of the amount of toner. The transfer resistance may be estimated based on either temperature or humidity.

  Even when any of the first to third modifications is employed, the nip pressure can be increased only when necessary, as compared to the case where the nip pressure is uniformly increased with a large amount of toner.

(5th example)
FIG. 10 is a diagram illustrating the control content of the fifth example. The horizontal axis of FIG. 10 represents that an image is being formed, and the vertical axis represents a nip pressure.

  The image forming apparatus 10 according to the present embodiment shown in FIG. 1 adds special color toners such as transparent toner (CT) and white toner (W) in addition to process color toners consisting of four colors of Y, M, C and K toners. It is configured to be able to form the used image.

  Therefore, here, the image processing / control unit 30 analyzes the image data, and the image to be formed this time is an image to be assigned to or shared by the first image holder 131 CT or the sixth image holder 131 W using spot color toner. It is determined whether there is any. Then, when the image to be formed this time is an image using a special color, control is performed to increase the nip pressure as compared with the case where the image is not using a special color.

  For example, white toner is spread on the lowermost layer of the image on the recording material to form a white background, transparent toner is spread on the uppermost layer on the recording material to form a glossy image, or the like, and special color toner is used. And the amount of toner at the time of transfer onto the recording material tends to increase greatly. Therefore, in the fifth example, the nip pressure is increased when using the special color toner, depending on whether or not the special color toner is used.

  Even in the case where the fifth example is employed, retransfer of the toner in the toner image transferred onto the intermediate transfer belt 14 to the image holding member 131 is suppressed as compared with the case where the nip pressure is always kept constant. Be done.

  In the case of the first to fifth examples described above, the nip pressure adjusting mechanism and the transfer resistance measuring means shown in FIG. 2 are provided corresponding to each of the six image carriers 131, including the modification. It has been described as a configuration. However, as can be seen from FIG. 3, the amount of toner increases toward the downstream side. Therefore, thereafter, a predetermined image carrier (here, as an example, the third image carrier 131M is predetermined) located downstream of the uppermost first image carrier 131CT located on the most upstream side. A nip pressure adjustment mechanism is provided only for each image carrier 131M, 131C, 131K, 131W located further downstream than the image carrier (third image carrier 131M) including an image carrier). It shall be. In this case, the third image carrier 131M corresponds to an example of the second image carrier in the present invention. Also, after that, when it is necessary to measure the transfer resistance, the transfer resistance is measured only for the first primary transfer roll 15CT provided corresponding to the first image carrier 131CT located on the most upstream side. It shall be. The most upstream primary transfer roll 15CT for measuring transfer resistance corresponds to an example of the primary transfer object to be measured in the present invention.

  In the following, only the image carriers 131M, 131C, 131K, and 131W are provided with the nip pressure adjusting mechanism, and the first primary transfer roller positioned on the most upstream side when it is necessary to measure the transfer resistance. Reference is made to the sixth to tenth examples, which correspond to a configuration in which the transfer resistance is measured only for 15 CT. Although the sixth to tenth examples correspond to the first to fifth examples described above, and can be analogized from the first to fifth examples described above, the sixth example is just in case. The tenth to tenth examples will be described including overlapping portions with the first to fifth examples described above. In the description of the sixth to tenth examples, the drawings (FIGS. 5 to 10) used in the description of the first to fifth examples will be used as they are.

(6th example)
FIG. 5 is a diagram illustrating the control content of the sixth example. The horizontal axis of FIG. 6 is the transfer resistance, and the vertical axis is the nip pressure.

  In the case of the sixth example, the toner image is applied to the first primary transfer roll 15CT when there is no toner image between the first primary transfer roll 15CT positioned on the most upstream side and the first image carrier 131CT. Transfer resistance, which is determined by the voltage applied to the first primary transfer roller 15CT and the current flowing to the first primary transfer roller 15CT, and the transfer Control is performed to increase the nip pressure for the image carriers 131M, 131C, 131K, and 131W, as compared to when in the low resistance region.

  In the example shown in FIG. 5, the transfer resistance is divided into a first region D1 having a low transfer resistance, a second region D2 having a slightly higher transfer resistance, and a third region D3 having a high transfer resistance. When inside, a low nip pressure is maintained, and when the transfer resistance is in the third region D1, it is adjusted to a high nip pressure, and when the transfer resistance is in the second region D1, the transfer resistance is adjusted. The higher the transfer resistance, the higher the nip pressure.

  According to the first example, re-transfer of the toner in the toner image transferred onto the intermediate transfer belt 14 to the image carrier 131 is suppressed as compared with the case where the nip pressure is always kept constant.

  Here, in the image processing / control unit 30 (see FIG. 1), the image data is transferred onto the recording material P based on the image data input from the outside, that is, the image data that is the source of the image to be formed. The amount of toner in the toner image is estimated. Then, when the estimated toner amount does not reach a predetermined threshold value (for example, the halftone dot area ratio of each color total is 260%), the image holding members 131M, 131C, and 131C are not measured without measuring the transfer resistance. The nip pressure for 131 K, 131 W is maintained at a nip pressure that is not dependent on the transfer resistance, in the example shown in FIG. 5 the same low nip pressure as the nip pressure in the first region.

  In the present embodiment, as described above, a constant current power supply is employed as the transfer bias power supply 153 (see FIG. 2). Therefore, the transfer bias voltage is adjusted to a voltage corresponding to the transfer resistance. When the toner amount is equal to or less than the threshold value, it is known that the transfer resistance is low. Therefore, the transfer bias voltage is also low, and control to increase the nip pressure is unnecessary. If the toner amount exceeds the threshold value, the transfer resistance is measured as described above, and the nip pressure is controlled according to the transfer resistance.

(7th example)
FIG. 6 is a diagram illustrating the control content of the seventh example. The horizontal axis in FIG. 7 is the temperature and humidity, and the vertical axis is the nip pressure.

  In the case of the seventh example, when the transfer resistance is in a relatively high resistance region according to the transfer resistance estimated based on the environmental temperature and humidity measured by the environment sensor 34 (see FIG. 1), The control is performed to increase the nip pressure for the image carriers 131M, 131C, 131K, and 131W, as compared with the case where R is in a relatively low resistance region.

  In the example shown in FIG. 6, the temperature and humidity are divided into a first area D1 having low temperature and humidity, a second area D2 having slightly higher temperature and humidity, and a third area D3 having high temperature and humidity. When in the first area D1, it is estimated that the transfer resistance is high, so it is adjusted to a high nip pressure, and when the temperature and humidity are in the third area D3, it is estimated that the transfer resistance is low Therefore, low nip pressure is maintained. Then, when the temperature and humidity are within the second region D2, the nip pressure is adjusted to an intermediate value between the nip pressure in the first region D1 and the nip pressure in the third region D3.

  Even when the seventh example is employed, retransfer of the toner in the toner image transferred onto the intermediate transfer belt 14 to the image holding member 131 is suppressed as compared with the case where the nip pressure is always kept constant. Be done.

  Here, as in the sixth example, when the toner amount estimated based on the image data does not satisfy the predetermined threshold value, it is known that the transfer resistance is low, and the nip pressure is the temperature and humidity. In the example shown in FIG. 6, the same nip pressure as the nip pressure in the third region D3 is maintained. When the toner amount exceeds the threshold value, the nip pressure is controlled according to the temperature and humidity as described above.

  Here, although the nip pressure is adjusted based on the temperature and the humidity, the nip pressure may be adjusted based on only the temperature among the temperature and the humidity or only the humidity.

(Eighth example)
FIG. 7 is a diagram illustrating the control content of the eighth example. The horizontal axis in FIG. 7 is the cumulative use time of the primary transfer roll 15 from when the image forming apparatus 10 is new, and the vertical axis is the nip pressure.

  Further, FIG. 8 shows the usage time of the primary transfer roll 15 in one day in the eighth example, and the vertical axis is the increase width of the nip pressure.

  In the image processing / control unit 30 shown in FIG. 1, usage histories of respective members constituting the image forming apparatus 10 are recorded. Here, according to the transfer resistance estimated based on the cumulative use time of the primary transfer roll 15 in the use history recorded in the image processing / control unit 30, the transfer resistance is relatively high in the resistance region. At certain times, control is performed to increase the nip pressure as compared to when the transfer resistance is in a relatively low resistance region.

  In the example shown in FIG. 7, the first region D1 in which the cumulative use time of the primary transfer roll 15 is short, the second region D2 in which the cumulative use time is longer than the first region D1, the third region D3 in which the cumulative use time is longer, It is divided into a long fourth area D4, and the area where the cumulative use time is long is adjusted to a higher nip pressure.

  If the cumulative use time of the primary transfer roll 14 is long, the surface of the primary transfer roll 14 becomes rough or foreign matter adheres to that extent, and the transfer resistance tends to increase. Therefore, here, the nip pressure is controlled to be higher for the image carriers 131M, 131C, 131K, and 131W as the cumulative use time is longer.

  Further, in the case of the image forming apparatus 10 of the present embodiment, when the power is turned on and used one day on that day, the transfer resistance is low for a while after the start of use, and the transfer resistance is used when used for a while. It tends to be higher.

  Therefore, here, as shown in FIG. 8, the first area D1 from when the power is turned on until reaching a predetermined use time and the second area D2 thereafter are divided into the first area D1 in FIG. In some cases, the nip pressure is adjusted as shown in FIG. 7, and when moving to the second area D2 of FIG. 8, the nip pressure shown in FIG. Adjusted to the width. Here, although the additional width Δp is constant regardless of the respective regions shown in FIG. 7, the additional width Δp may be changed for each of the regions shown in FIG.

  Even in the case where this eighth example is adopted, retransfer of the toner in the toner image transferred onto the intermediate transfer belt 14 to the image holding member 131 is suppressed as compared with the case where the nip pressure is always kept constant. Be done.

  Here, as in the sixth and seventh examples, it is known that the transfer resistance is low when the toner amount estimated based on the image data does not meet the predetermined threshold value, and the image holding The nip pressure for the bodies 131M, 131C, 131K, and 131W is maintained at a nip pressure which does not depend on the cumulative use time, and in the example shown in FIG. 7, the same low nip pressure as the nip pressure in the first region D1. When the toner amount exceeds the threshold value, the nip pressure is controlled according to the temperature and humidity as described above.

(The 9th example)
FIG. 9 is a diagram illustrating the control content of the ninth example. The horizontal axis of FIG. 9 is the toner amount of the toner image to be formed, and the vertical axis is the nip pressure.

  In the image processing / control unit 30 (see FIG. 1), the amount of toner of the toner image to be transferred onto the recording material P based on the image data that is input from the outside and is the origin of the image to be formed Is estimated. When the amount of toner is large, the transfer resistance is increased, and when the nip pressure is kept constant, a high transfer bias voltage is applied to the primary transfer roll 14 by the transfer bias power supply 153 (see FIG. 2) as a constant current power supply. Ru. Therefore, here, when the amount of toner is relatively large according to the amount of toner of the toner image transferred onto the recording material P estimated based on the image data, the amount of toner is relatively small As compared with the case, control is performed to increase the nip pressure for the image carriers 131M, 131C, 131K, and 131W.

  In the example shown in FIG. 9, the toner amount is divided into a first region D1 having a small toner amount, a second region D2 having a slightly larger toner amount, and a third region D3 having a large toner amount. When it is inside, a low nip pressure is maintained, and when the amount of toner is in the third area D1, it is adjusted to a high nip pressure, and when the amount of toner is in the second area D1, according to the amount of toner The higher the toner amount, the higher the nip pressure.

  The amount of toner in the toner image may be calculated for each of the image holding members 131M, 131C, 131K, 131W instead of the amount of toner in the toner image transferred onto the recording material P. For example, for the third image carrier 131M, the toner amounts of three toner images formed on the three image carriers of the first image carrier 131CT, the second image carrier 131Y, and the third image carrier 131M. It becomes the total value of Such calculation may be performed to control the nip pressure for each of the image carriers 131M, 131C, 131K, and 131W in accordance with the toner amount for each of the image carriers 131M, 131C, 131K, and 131W.

  According to the fourth example, as in the first to third examples, the toner in the toner image transferred onto the intermediate transfer belt 14 is an image compared to the case where the nip pressure is always kept constant. Retransfer to the support 131 is suppressed.

  Here, a modification described below may be adopted in which the above-described sixth to eighth examples are added to the ninth example.

(First modification)
The transfer resistance relating to the first primary transfer roll 15CT is measured at the time when there is no toner image between the first primary transfer roll 15CT and the first image carrier 131CT, and the transfer resistance is determined in advance. When in the low resistance region, the nip pressure for the image carriers 131M, 131C, 131K, 131W is maintained at a low nip pressure independent of the amount of toner.

(2nd modification)
The environmental temperature and humidity are measured by the environmental sensor 34 (see FIG. 1). When the transfer resistance estimated based on the measured environmental temperature and humidity is in a predetermined low resistance area, the nip pressure for the image carriers 131M, 131C, 131K, and 131W depends on the amount of toner. Do not maintain low nip pressure. The transfer resistance may be estimated based on either temperature or humidity.

(Third modification)
When the transfer resistance estimated based on the cumulative use time of the primary transfer roll 15 in the use history recorded in the image processing / control unit 30 is in a predetermined low resistance area, the image carrier 131 M , 131 C, 131 K, and 131 W are maintained at a low nip pressure independent of the amount of toner.

  Even when any of the first to third modifications is employed, the nip pressure can be increased only when necessary, as compared to the case where the nip pressure is uniformly increased with a large amount of toner.

(Tenth example)
FIG. 10 is a diagram illustrating the control content of the tenth example. The horizontal axis of FIG. 10 represents that an image is being formed, and the vertical axis represents a nip pressure.

  The image forming apparatus 10 according to the present embodiment shown in FIG. 1 adds special color toners such as transparent toner (CT) and white toner (W) in addition to process color toners consisting of four colors of Y, M, C and K toners. It is configured to be able to form the used image.

  Therefore, here, the image processing / control unit 30 analyzes the image data, and the image to be formed this time is an image to be assigned to or shared by the first image holder 131 CT or the sixth image holder 131 W using spot color toner. It is determined whether there is any. Then, when the image to be formed this time is an image using a spot color, control is performed to increase the nip pressure for the image carriers 131M, 131C, 131K, and 131W, as compared to the case where the image is not using spot color. .

  For example, white toner is spread on the lowermost layer of the image on the recording material to form a white background, transparent toner is spread on the uppermost layer on the recording material to form a glossy image, or the like, and special color toner is used. And the amount of toner at the time of transfer onto the recording material tends to increase greatly. Therefore, in the tenth example, the nip pressure is increased when using the special color toner, depending on whether or not the special color toner is used.

  Even in the case where this tenth example is adopted, retransfer of toner in the toner image transferred onto the intermediate transfer belt 14 to the image holding member 131 is suppressed as compared with the case where the nip pressure is always kept constant. Be done.

  In the first to tenth examples described so far, a constant current power source is used as the transfer bias power source 153 (see FIG. 2), including the modification. Therefore, the case where a constant voltage power source is adopted as the transfer bias power source 153 will be mentioned here. When a constant voltage power source is employed as the transfer bias power source 153, it is necessary to adjust the voltage value so that an appropriate current flows. As part of the adjustment, when the nip pressure is increased according to the nip pressure, control is performed to lower the transfer bias voltage by the transfer bias power supply 153 as the constant voltage power supply.

  According to each of the above examples, when transferring the toner image from the image carrier to the intermediate transfer belt, the intermediate transfer is performed as compared with the case where the pressing force (nip pressure) of the intermediate transfer belt to the image carrier is kept constant. Retransfer of the toner in the toner image transferred onto the belt to the image carrier is suppressed.

DESCRIPTION OF SYMBOLS 10 image forming apparatus 13 image forming unit 14 intermediate transfer belt 22 fixing device 24 cooler 25 decurler 26 optical measuring device 30 image processing / control unit 32 operation panel 33 power supply unit 34 environment sensor 131 image carrier 151 motor 152 cam mechanism 153 bias Power supply

Claims (24)

An image holding unit that holds a toner image according to image data;
An intermediate transfer unit to which the toner image formed on the image holding unit is transferred;
A primary transfer unit is disposed between the image holding unit and the intermediate transfer unit, and the primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. Primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the means;
Resistance measurement means for measuring a transfer resistance which is determined by the voltage applied to the primary transfer device and the current flowing to the primary transfer device;
According to the transfer resistance between the primary transfer unit and the image holding means, when the transfer resistance is relatively high, the primary transfer unit compared to when the transfer resistance is relatively low An image forming apparatus comprising: a control unit configured to perform control to increase a pressing force of the intermediate transfer unit to the image holding unit;   When the amount of toner of the toner image transferred onto the recording material, which is estimated based on the image data, by the control means is less than a predetermined threshold value, the intermediate transfer by the primary transfer device is performed. The image forming apparatus according to claim 1, wherein the pressing force of the transfer unit is maintained at a pressing force independent of the transfer resistance. An image holding unit that holds a toner image according to image data;
An intermediate transfer unit to which the toner image formed on the image holding unit is transferred;
A primary transfer unit is disposed between the image holding unit and the intermediate transfer unit, and the primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. Primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the means;
Environmental measurement means for measuring at least one of the environmental temperature and the environmental humidity;
When the at least one is relatively low according to at least one of the environmental temperature and the environmental humidity measured by the environmental measurement means, the primary transfer is compared to when the at least one is relatively high. An image forming apparatus comprising: control means for performing control to increase the pressing force of the intermediate transfer means to the image holding means by the   When the amount of toner of the toner image transferred onto the recording material, which is estimated based on the image data, by the control means is less than a predetermined threshold value, the intermediate transfer by the primary transfer device is performed. The image forming apparatus according to claim 3, wherein the pressing force of the transfer unit is maintained at a pressing force independent of the at least one. An image holding unit that holds a toner image according to image data;
An intermediate transfer unit to which the toner image formed on the image holding unit is transferred;
A primary transfer unit is disposed between the image holding unit and the intermediate transfer unit, and the primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. Primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the means;
Recording means for recording usage history;
When the use history recorded by the recording means represents a relatively long use period, the image by the primary transfer device is compared to when the use history represents a relatively short use period. An image forming apparatus comprising: control means for performing control to increase the pressing force of the intermediate transfer means to a holding means.   When the amount of toner of the toner image transferred onto the recording material, which is estimated based on the image data, by the control means is less than a predetermined threshold value, the intermediate transfer by the primary transfer device is performed. The image forming apparatus according to claim 5, wherein the pressing force of the transfer unit is maintained at a pressing force independent of the period of use. An image holding unit that holds a toner image according to image data;
An intermediate transfer unit to which the toner image formed on the image holding unit is transferred;
A primary transfer unit is disposed between the image holding unit and the intermediate transfer unit, and the primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. Primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the means;
When the amount of toner is relatively large according to the amount of toner of the toner image transferred onto the recording material estimated based on the image data, the amount of toner is relatively small And a control unit configured to perform control to increase the pressing force of the intermediate transfer unit to the image holding unit by the primary transfer unit. A resistance measuring unit that measures a transfer resistance determined by the voltage applied to the primary transfer unit and the current flowing to the primary transfer unit;
When the control unit is in a predetermined resistance region between the primary transfer unit and the image holding unit, which is measured by the resistance measurement unit, the control unit performs the primary transfer unit using the primary transfer unit. The image forming apparatus according to claim 7, wherein the pressing force of the intermediate transfer unit to the image holding unit is maintained at a pressing force independent of the amount of toner. Environmental measurement means for measuring at least one of the environmental temperature and the environmental humidity;
When the at least one measured by the environment measuring unit is within a predetermined range, the pressing force of the intermediate transfer unit to the image holding unit by the primary transfer unit does not depend on the amount of toner. The image forming apparatus according to claim 7, wherein the pressing force is maintained. It has recording means to record usage history,
When the control means indicates that the use history recorded by the recording means is within a predetermined use period, the pressing force of the intermediate transfer means to the image holding means by the primary transfer device is 8. The image forming apparatus according to claim 7, wherein the pressing force does not depend on the amount of toner. A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer device sandwiching the intermediate transfer means with the image holding means is disposed corresponding to the image holding means, and a primary transfer for transferring the toner image on the image holding means onto the intermediate transfer means A plurality of primary transfer means capable of adjusting the pressing force of the intermediate transfer means to the image holding means;
The plurality of image holding means includes, in addition to an image holding means for forming a toner image with process color toner, a spot color corresponding image holding means for forming a toner image with spot color toner other than the process color toner,
In the case where the image data is image data in which the formation of a toner image transferred onto the recording material is taken over or shared by the spot color corresponding image holding means, the formation of a toner image transferred onto the recording material is The image processing apparatus further comprises control means for performing control to increase the pressing force of the intermediate transfer means to the image holding means by the primary transfer device as compared to the case where the image data for the special color corresponding image is not shared. An image forming apparatus characterized by The primary transfer means is a constant voltage power supply means for supplying a transfer bias to the primary transfer device, wherein the constant voltage power supply means is capable of adjusting the voltage of the transfer bias supplied to the primary transfer device. Equipped
When the control unit increases the pressing force according to the pressing force of the intermediate transfer unit to the image holding unit by the primary transfer unit, the constant transfer voltage supply unit performs the primary transfer. The image forming apparatus according to any one of claims 1 to 11, wherein a voltage of a transfer bias supplied to the image forming apparatus is lowered. A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and a primary transfer unit transfers the toner image on the image holding unit onto the intermediate transfer unit. When,
A resistance measuring unit that measures a transfer resistance determined by a voltage applied to a measurement target primary transfer unit among the plurality of primary transfer units and a current flowing to the measurement target primary transfer unit;
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
The transfer resistance is relatively high when the transfer resistance is relatively high according to the transfer resistance between the primary transfer unit to be measured and the image holding means corresponding to the primary transfer unit to be measured. Control means for performing control to increase the pressing force of the intermediate transfer means to the corresponding downstream side image holding means by the downstream side primary transfer devices as compared with the case where the pressure is low. Image forming apparatus.   When the amount of toner of the toner image transferred onto the recording material, which is estimated based on the image data, by the control means is less than a predetermined threshold value, the respective downstream primary transfer devices The image forming apparatus according to claim 13, wherein the pressing force of the intermediate transfer unit to the corresponding downstream image holding unit is maintained at a pressing force that does not depend on the transfer resistance. A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and is a primary transfer unit transferring the toner image on the image holding unit onto the intermediate transfer unit When,
Environmental measurement means for measuring at least one of the temperature environment and the humidity environment;
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
According to the at least one measured by the environment measuring means, when the at least one is relatively low, compared with the case where the transfer resistance is relatively high, the respective corresponding downstream primary transfer devices An image forming apparatus, further comprising: control means for performing control to increase the pressing force of the intermediate transfer means to each of the downstream side image holding means.   When the amount of toner of the toner image transferred onto the recording material, which is estimated based on the image data, by the control means is less than a predetermined threshold value, the respective downstream primary transfer devices 16. The image forming apparatus according to claim 15, wherein the pressing force of the intermediate transfer unit to the corresponding downstream image holding unit is maintained at a pressing force independent of the at least one. A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and is a primary transfer unit transferring the toner image on the image holding unit onto the intermediate transfer unit When,
And recording means for recording the usage history,
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
When the use history recorded by the recording means represents a relatively long use period, compared to when the use history represents a relatively short use period, the respective downstream primary transfer devices An image forming apparatus further comprising: control means for performing control to increase the pressing force of the intermediate transfer means to the corresponding downstream image holding means;   When the amount of toner of the toner image transferred onto the recording material, which is estimated based on the image data, by the control means is less than a predetermined threshold value, the respective downstream primary transfer devices The image forming apparatus according to claim 17, wherein the pressing force of the intermediate transfer unit to the corresponding downstream image holding unit is maintained at a pressing force independent of the period of use. A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and is a primary transfer unit transferring the toner image on the image holding unit onto the intermediate transfer unit With,
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
When the amount of toner is relatively large according to the amount of toner of the toner image transferred onto the recording material estimated based on the image data, the amount of toner is relatively small The image forming apparatus according to claim 1, further comprising: control means for increasing the pressing force of the intermediate transfer means to the corresponding downstream image holding means by the respective downstream primary transfer devices. A resistance measuring unit configured to measure a transfer resistance determined by a voltage applied to a measurement target primary transfer unit among the plurality of primary transfer units and a current flowing to the measurement target primary transfer unit;
When the control resistance is in a predetermined resistance region between the measurement target primary transfer unit and the image support unit corresponding to the measurement target primary transfer unit by the resistance measurement unit, 20. The image forming apparatus according to claim 19, wherein the pressing force of the intermediate transfer unit to the corresponding downstream image holding unit by each of the downstream primary transfer devices is maintained at a pressure independent of the amount of toner. The image forming apparatus according to claim 1. An environmental measurement means for measuring at least one of a temperature environment and a humidity environment;
When the at least one measured by the environment measuring means is within a predetermined range, the pressing force of the intermediate transferring means to the corresponding downstream image holding means by the respective downstream primary transfer devices The image forming apparatus according to claim 15, wherein the pressing force does not depend on the amount of toner. It has recording means to record usage history,
When the use history recorded by the recording means represents within a predetermined use period, the pressing of the intermediate transfer means onto the corresponding downstream image holding means by the respective downstream primary transfer devices 22. The image forming apparatus according to claim 21, wherein the pressure is maintained at a pressing force independent of the amount of toner. A plurality of image holding means for holding a toner image according to image data;
An intermediate transfer unit that moves in a circular path following a path along each of the plurality of image holding units, and sequentially transfers toner images formed on each of the plurality of image holding units;
A primary transfer unit sandwiching the intermediate transfer unit with the image holding unit is disposed corresponding to the image holding unit, and is a primary transfer unit transferring the toner image on the image holding unit onto the intermediate transfer unit Equipped with
The plurality of image holding means includes, in addition to an image holding means for forming a toner image with process color toner, a spot color corresponding image holding means for forming a toner image with spot color toner other than the process color toner,
Among the plurality of primary transfer devices, the arrangement positions of the image holding means for holding the toner image transferred to the front and back respectively on the intermediate transfer means with respect to the circulating movement direction of the intermediate transfer means The second image including a predetermined second image holding means located downstream of the first image holding means disposed on the most upstream side of the image holding means at the downstream side Each downstream primary transfer device provided corresponding to each downstream image holding means further downstream than the holding means adjusts the pressing force of the intermediate transfer means to the corresponding downstream image holding means. Possible primary transfer devices,
In the case where the image data is image data in which the formation of a toner image transferred onto the recording material is taken over or shared by the spot color corresponding image holding means, the formation of a toner image transferred onto the recording material is The control for increasing the pressing force of the intermediate transfer means to the corresponding downstream image holding means by the respective downstream primary transfer devices as compared to the case where the image data not shared by the spot color corresponding image holding means is shared. An image forming apparatus further comprising control means for performing the process. The primary transfer means is a constant voltage power supply means for supplying a transfer bias to the primary transfer device, and at least a constant voltage capable of adjusting the voltage of the transfer bias supplied to the respective downstream primary transfer devices. Equipped with power supply means,
The constant voltage power supply when the control means increases the pressing force according to the pressing force of the intermediate transfer means to the corresponding downstream image holding means by the respective downstream primary transfer devices. The image forming apparatus according to any one of claims 13 to 23, wherein the voltage of the transfer bias supplied to each of the downstream primary transfer devices is lowered by the supply means.

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