JP6015697B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that performs a process of fixing a toner image transferred onto a sheet onto the sheet by heating. More specifically, the present invention relates to an image forming apparatus capable of reducing the power consumption by reducing the temperature of the fixing process as much as possible.

  A general image forming apparatus using an electrophotographic system such as a printer or a copier has a fixing processing unit that performs processing for fixing a toner image transferred onto a sheet. The fixing processing unit performs the fixing process by pressing the paper carrying the toner image while heating. The paper that has passed through the fixing processing unit may be curled and curved. The paper that has been curled becomes a cause of jamming in subsequent conveyance. Furthermore, troubles may occur in the accumulation on the paper discharge tray. In addition, even when binding a plurality of sheets, there is a possibility that the bookbinding quality may be deteriorated. Therefore, it is preferable that curl generated on the paper is reduced as much as possible.

  Therefore, in order to reduce the curling of the sheet, the heating temperature of the sheet in the fixing processing unit is controlled. For example, in Patent Document 1, in double-sided printing in which image formation is continuously performed on both the first surface and the second surface that is the back surface thereof, the leading edge in the transport direction on the second surface during fixing processing on the first surface. An image forming apparatus is described in which a fixing process to a part to be performed is performed at a lower temperature than other parts. Thereby, it is said that curl at the front end side on the second surface can be reduced, and conveyance failure at the time of image formation on the second surface can be reduced.

  Patent Document 2 describes an image forming apparatus that performs fixing processing at different temperatures at both ends and the center in the width direction orthogonal to the sheet conveyance direction. Specifically, at the time of image formation on the first surface, fixing processing is performed with both ends set to a temperature lower than the center. Further, when an image is formed on the second surface, the fixing process is performed with both ends being higher than the center and higher than the fixing process temperature at the end part when forming the image on the first surface. As a result, it is said that curling can be suppressed and paper jams can be prevented.

Japanese Patent Laid-Open No. 10-10915 JP 2012-32654 A

  In recent years, in order to reduce power consumption, there is an image forming apparatus that performs smart fixing control that keeps the temperature of a fixing processing unit low according to an image to be formed. Specifically, the smart fixing control performs fixing processing at a lower temperature as the toner adhesion amount in the toner image for image formation is smaller. And with smart fixing control, heating can be minimized and power consumption can be reduced. The toner adhesion amount of the toner image can be determined based on information such as density, character width, character size, color, or monochrome for the image to be formed.

  In such a smart fixing control, when the first sheet for fixing processing at a low temperature and the second sheet for fixing processing at a high temperature pass through the fixing processing section in this order, the second sheet is fixed. Before reaching the processing section, the fixing processing section must be heated to a high temperature. In this case, even if the first sheet is passing through the fixing processing unit, the temperature of the fixing processing unit must be increased. However, there is a problem that curling of the first sheet is likely to occur when the first sheet starts to rise in temperature while passing through the fixing processing unit. In addition, none of the above-described conventional techniques consider such smart fixing control, and thus cannot solve the problems caused in such smart fixing control.

  The present invention has been made for the purpose of solving the problems of the prior art described above. That is, an object of the present invention is to provide an image forming apparatus capable of reducing power consumption in fixing processing and suppressing curling.

The image forming apparatus of the present invention, which has been made for the purpose of solving this problem, includes an image forming unit that forms a toner image and transfers the toner image to the sheet to form the toner image on the sheet, and heating by a heat source. A heating unit that receives the pressure unit that forms a nip together with the heating unit, and a fixing processing unit that performs a fixing process by passing a sheet onto which the toner image has been transferred through the nip, and controls each part of the apparatus to form an image. An image forming apparatus having an apparatus control unit for performing a heating-side temperature output unit that outputs a heating-side temperature value that indicates a temperature at a nip position of a heating unit, and a temperature at a nip position of a pressure unit. A pressure-side temperature output unit that outputs a pressure-side temperature value to be output, and the apparatus control unit has a larger amount of heat required to fix the toner image transferred to the sheet by the image forming unit than when the amount of heat is small. The fixing temperature, which is the target of the heating side temperature value when passing the sheet through the nip, is set low, and the heating side temperature value is equal to or higher than the fixing temperature determined for the sheet before the sheet reaches the nip. In this way, the smart fixing control for controlling the heating by the heat source is performed, and the fixing temperature of the second sheet is set to the first sheet for the first sheet and the second sheet next to the first sheet that are continuously passed through the nip. When the temperature is higher than the fixing temperature of the sheet, the temperature difference that is the difference between the heating-side temperature value and the pressure-side temperature value when the first sheet passes through the nip does not exceed a predetermined upper limit temperature difference. The temperature- side limiting control for performing image formation is executed . The heating-side temperature output unit and the pressure-side temperature output unit are device control units, and the device control unit is related to the first sheet by the image forming unit. Toner image Before starting the composition, the heating side temperature value and the pressure side temperature value when the heating side temperature value is raised to the fixing temperature of the second sheet by heating the heat source while passing the first sheet through the nip are simulated. The estimated temperature difference, which is an estimated value of the temperature difference when the trailing edge of the first sheet reaches the nip, is obtained from the heating side temperature value and the pressure side temperature value obtained by simulation, and the estimated temperature difference is the upper limit. When the temperature difference is exceeded, it is decided to execute the temperature difference restriction control, and when it is decided to execute the temperature difference restriction control, the temperature difference restriction control is executed at the time of image formation of the first sheet. There is provided an image forming apparatus.

In the image forming apparatus of the present invention, power consumption in the fixing processing unit is reduced by performing heating by a heat source to a minimum by smart fixing control. Also, when performing smart fixing control, when the fixing temperature of the second sheet is higher than the fixing temperature of the first sheet, the heating unit and the pressurization in the nip are pressed while the first sheet passes through the nip. There is a possibility that the temperature difference from the portion becomes large and a large curl is generated in the first sheet. Therefore, curling generated in the first sheet can be reduced and jamming can be suppressed by temperature difference limiting control for forming an image so that the temperature difference does not exceed the upper limit temperature difference. Furthermore, the image forming apparatus according to the present invention performs temperature difference restriction control when the actual temperature difference between the heating unit and the pressurizing unit in the nip is estimated to exceed the upper limit temperature difference by simulation. This is because the temperature difference can be suppressed below the upper limit temperature difference. Thereby, the curl which generate | occur | produces in a 1st sheet | seat can be reduced and jam can be suppressed.

  In the above-described image forming apparatus, the apparatus control unit, in the simulation, determines that the temperature difference during the passage of the first sheet through the nip from the obtained heating side pressure value and pressure side temperature value is the upper limit temperature. In the temperature difference limiting control, the heating by the heat source is stopped before the exceeding timing obtained by the simulation, and the heating by the stopped heat source is performed at the rear end of the first sheet at the nip. It can be resumed after passing. This is because curling generated in the first sheet can be reduced and jamming can be suppressed.

  In the image forming apparatus described above, in the temperature difference limiting control, the apparatus control unit limits heating by the heat source so that the heating side temperature value increases more slowly than the degree of increase in the simulation. can do. This is because curling generated in the first sheet can be reduced and jamming can be suppressed.

  In the image forming apparatus described above, in the temperature difference limiting control, the apparatus control unit may advance the start timing of starting the increase of the heating side temperature value by heating the heat source earlier than the start timing of increase in the simulation. It is preferable that This is because productivity can be maintained high while reducing the first curl by suppressing the temperature difference to be equal to or less than the upper limit temperature difference.

  In the image forming apparatus described above, in the temperature difference restriction control, the apparatus control unit is configured so that the front end of the first sheet reaches the image forming unit and the rear end of the first sheet is the image forming unit. The conveyance speed of at least one of the first sheets after passing through the first sheet reaches the nip is slower than the conveyance speed of the first sheet in the simulation. Is preferred. This is because jamming can be suppressed by reducing the curl generated in the first sheet while maintaining high quality of the formed image. Further, to slow down the conveyance speed includes temporarily stopping conveyance of the first sheet.

  In the image forming apparatus described above, in the temperature difference limiting control, the apparatus control unit delays the rise start timing for starting the rise of the heating-side temperature value by heating the heat source from the rise start timing in the simulation. Can be. This is because curling generated in the first sheet can be reduced and jamming can be suppressed.

  Further, in the image forming apparatus described above, the reversing unit that reverses the front and back of the sheet on which the image is formed on the first surface through the nip, and the sheet that has passed the reversing unit are disposed on the back of the first surface. In order to form an image on the second surface, a double-sided conveyance path that conveys the image again to a position upstream from the image forming unit is provided, and the apparatus control unit applies this to the first and second surfaces of the first sheet. It is preferable to perform temperature difference restriction control during the fixing process to the first side in double-sided printing in which image formation is performed in order. The rear end of the sheet when the image is formed on the first surface becomes the front end when the image is formed on the second surface after the front and back surfaces are reversed by the reversing unit. Further, when an image is formed on the first surface, a large curl is likely to occur on the rear end side of the sheet by passing through the nip. That is, it is possible to prevent jamming at the time of image formation on the second surface by suppressing the curl at the tip portion at the time of image formation on the second surface. In addition, when image formation is performed only on the first surface, and when image formation is performed on the second surface, the front and back surfaces are not reversed thereafter. It is low.

  Further, in the image forming apparatus described above, the reversing unit that reverses the front and back of the sheet on which the image is formed on the first surface through the nip, and the sheet that has passed the reversing unit are disposed on the back of the first surface. In order to form an image on the second surface, the apparatus has a double-sided conveyance path for conveying the image to a position upstream from the image forming unit. The fixing temperature of the sheet passing through the nip before the first sheet is lower than the fixing temperature of the first sheet, and an image is formed on the first surface of the first sheet. In addition, when the second sheet passes through the duplex conveyance path and an image is formed on the second surface, the image formation on the second sheet is performed before the image formation on the first sheet. While the second sheet image is being formed. It may be configured to run. This is because curling generated in the first sheet can be reduced and jamming can be suppressed.

  In the image forming apparatus described above, at least one of the group consisting of the thickness of the first sheet, the length in the conveyance direction of the first sheet, and the speed when passing through the nip of the first sheet. A storage unit that stores an upper limit temperature difference table that is a relationship between two conditions and an upper limit temperature difference. In the temperature difference limit control, the device control unit refers to the upper limit temperature difference table as the upper limit temperature difference. A predetermined value may be used. This is because curling can be suppressed even if a different value is used as the upper limit temperature difference based on these conditions.

  According to the present invention, there is provided an image forming apparatus capable of reducing power consumption in fixing processing and suppressing curling.

1 is a diagram illustrating an image forming apparatus. FIG. 3 is a diagram illustrating a fixing processing unit of the image forming apparatus. 2 is a schematic diagram of a control configuration of the image forming apparatus. FIG. It is a figure for demonstrating smart fixing control. FIG. 6 is a graph showing the temperature of a fixing processing unit when a sheet is passed through a fixing nip in a low temperature mode. FIG. 6 is a graph showing the temperature of a fixing processing unit when a sheet is passed through a fixing nip that is being heated. It is a graph for demonstrating the temperature difference restriction | limiting control in a 1st form. It is a graph for demonstrating the temperature difference restriction | limiting control in a 2nd form. It is a graph for demonstrating the temperature difference restriction | limiting control in a 3rd form. It is a graph for demonstrating the temperature difference restriction | limiting control in a 4th form. It is a graph for demonstrating the temperature difference restriction | limiting control in a 5th form. It is a graph for demonstrating the temperature difference restriction | limiting control in a 6th form.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below in detail with reference to the accompanying drawings.

[First embodiment]
FIG. 1 shows a schematic configuration of an image forming apparatus 1 of the present embodiment. The image forming apparatus 1 is a so-called tandem type color printer having an intermediate transfer belt 30. The intermediate transfer belt 30 is an endless belt member having conductivity, and both end portions in FIG. 1 are supported by rollers 31 and 32. At the time of image formation, the roller 31 on the right side in FIG. 1 is driven to rotate counterclockwise as indicated by an arrow in the figure. As a result, the intermediate transfer belt 30 and the roller 32 on the left side in FIG.

  A secondary transfer roller 40 is provided on the outer peripheral surface of the portion of the intermediate transfer belt 30 supported by the right roller 31 in FIG. The secondary transfer roller 40 is pressed against the intermediate transfer belt 30 in a direction perpendicular to the axis (leftward in FIG. 1). A secondary transfer nip N1 for transferring the toner image on the intermediate transfer belt 30 onto the paper P is formed at a portion where the intermediate transfer belt 30 and the secondary transfer roller 40 are in contact with each other. At the time of image formation, the secondary transfer roller 40 is driven to rotate by a frictional force generated by pressure contact with the rotating intermediate transfer belt 30.

  Also, a belt cleaner 41 is provided on the outer peripheral surface of the portion of the intermediate transfer belt 30 supported by the left roller 32 in FIG. The belt cleaner 41 is for collecting toner adhering to the surface of the intermediate transfer belt 30. In other words, the belt cleaner 41 is for collecting untransferred toner or the like that has not been transferred to the paper P in the secondary transfer nip N1.

  In the lower part of the intermediate transfer belt 30 in FIG. 1, the image forming units 10Y, 10M, 10C for yellow (Y), magenta (M), cyan (C), and black (K) are sequentially arranged from left to right. 10K is arranged. Each of the image forming units 10Y, 10M, 10C, and 10K is for forming a toner image of the corresponding color and transferring it onto the intermediate transfer belt 30. The image forming units 10Y, 10M, 10C, and 10K all have the same configuration. For this reason, in FIG. 1, the image forming unit 10 </ b> Y is represented by a reference numeral.

  The image forming units 10Y, 10M, 10C, and 10K include a photosensitive member 11 that is a cylindrical electrostatic latent image carrier, a charging device 12, an exposure device 13, a developing device 14, and a primary transfer disposed around the photosensitive member 11. A roller 15 is provided. The charging device 12 is for charging the surface of the photoreceptor 11 uniformly. The exposure device 13 is for irradiating the surface of the photoreceptor 11 with laser light based on the image data of the corresponding color to form an electrostatic latent image. The developing device 14 is for applying the contained toner to the surface of the photoreceptor 11.

  The primary transfer roller 15 is disposed at a position facing the photoconductor 11 with the intermediate transfer belt 30 interposed therebetween. The primary transfer roller 15 is pressed against the intermediate transfer belt 30 in a direction perpendicular to the axis (downward in FIG. 1). Due to this pressure contact, primary transfer nips for transferring the toner images on the photosensitive drums 11 of the respective colors onto the intermediate transfer belt 30 are formed at the portions where the intermediate transfer belt 30 and the photosensitive drum 11 are in contact with each other. .

  The image forming units 10Y, 10M, 10C, and 10K according to the present embodiment are of a cleanerless type that collects untransferred toner on the photoconductor 11 by the developing device 14. However, a photoreceptor cleaner for collecting toner that has not been transferred from the photoreceptor 11 to the intermediate transfer belt 30 may be provided. Examples of the photoconductor cleaner include a plate-like one whose one end is in contact with the outer peripheral surface of the photoconductor 11. Alternatively, other cleaning members such as a fixed brush, a rotating brush, a roller, or a combination of a plurality of these members can be used.

  Further, toner bottles 20Y, 20M, 20C, and 20K that store toners of the respective colors are disposed above the intermediate transfer belt 30 in FIG. The toners of the respective colors accommodated therein are appropriately supplied to the developing devices 14 of the respective colors.

  A paper feed cassette 51 is mounted at the lower part of the image forming apparatus 1. A conveyance path 50 is provided above the right side in FIG. Then, the sheets P stored in the sheet feeding cassette 51 by stacking are sent out one by one from the uppermost one to the conveying path 50 by the sheet feeding roller 52.

  A pair of registration rollers 53, a secondary transfer nip N <b> 1, a fixing nip N <b> 2, and a paper discharge roller 54 are arranged in this order on the conveyance path 50 of the paper P delivered from the paper feed cassette 51. On the further downstream side of the conveyance path 50, a paper discharge unit 55 for discharging the paper P on which image formation has been completed is provided. The registration roller 53 is for finely adjusting the timing of feeding the paper P to the secondary transfer nip N1. The fixing nip N <b> 2 is for performing a fixing process for fixing the toner image transferred onto the paper P, and is formed by the fixing processing unit 70.

  With reference to FIG. 2, the fixing processing unit 70 of this embodiment will be described. As shown in FIG. 2, the fixing processing unit 70 includes a pressure roller 71, a fixing roller 72, a heating roller 73, a heater 74, a fixing belt 75, and a temperature sensor 76. Among these, the fixing roller 72 other than the pressure roller 71, the heating roller 73, the heater 74, the fixing belt 75, and the temperature sensor 76 are disposed on the side of the surface carrying the toner image of the paper P. The fixing processing unit 70 performs a fixing process for fixing the toner image on the paper P transported through the transport path 50.

  As shown in FIG. 2, a fixing belt 75 is wound around the fixing roller 72 and the heating roller 73. The heater 74 is a heat source provided inside the heating roller 73. The temperature sensor 76 is for detecting the temperature of the heating roller 73. Therefore, the fixing roller 72, the heating roller 73, the heater 74, the fixing belt 75, and the temperature sensor 76 are configured on the heating side. On the other hand, the pressure roller 71 is disposed at a position facing the fixing roller 72 with the fixing belt 75 interposed therebetween. The pressure roller 71 is in pressure contact with the outer peripheral surface of the fixing belt 75, and a fixing nip N2 is formed in the contact portion. For this reason, the pressure roller 71 has a configuration on the pressure side.

  The fixing roller 72 is driven counterclockwise as indicated by an arrow in FIG. 2 when performing the fixing process. By the rotation of the fixing roller 72, the pressure roller 71, the heating roller 73, and the fixing belt 75 are driven to rotate. The heating roller 73 is heated by the heater 74 so that the temperature detected by the temperature sensor 76 becomes a predetermined temperature. As a result, the temperature of the fixing belt 75 and the fixing roller 72 also increases. The paper P is pressurized while being heated when it passes through the fixing nip N2. The toner image is fixed on the paper P by this fixing process.

  Further, the image forming apparatus 1 of the present embodiment has a double-sided conveyance path 60 as shown in FIG. The double-sided conveyance path 60 is for conveying the paper P that has once passed through the conveyance path 50 to the conveyance path 50 again. As a result, the image forming apparatus 1 can form images on both sides of the paper P. Sensors for detecting the arrival and passage of the paper P are provided at various locations on the transport path 50 and the duplex transport path 60.

  FIG. 2 shows an outline of the control configuration of the image forming apparatus 1. The image forming apparatus 1 includes an engine unit 2 and a controller unit 3 in order to control each unit. The engine unit 2 includes a CPU 4 that performs overall control processing and a nonvolatile memory 5 attached to the main body.

  The nonvolatile memory 5 stores values such as the rotation speed of the roller 31, the rotation speed of each photoconductor 11, and the conveyance speed of the paper P. Further, an upper limit temperature difference TDlim, which is an upper limit value of an allowable temperature difference between the fixing belt 75 and the pressure roller 71 in the fixing nip N2, is also stored. The upper limit temperature difference TDlim will be described in detail later.

  The CPU 4 executes an image forming job by controlling each unit of the image forming apparatus 1 based on the values stored in the nonvolatile memory 5. That is, at the time of execution of the job, the rotation of the photoreceptor 11, the roller 31, each roller related to the conveyance of the paper P is controlled. In addition, heating by the heater 74 is controlled so that the temperature detected by the temperature sensor 76 is maintained at a predetermined temperature for performing the fixing process. Further, the CPU 4 acquires the actual position of the paper P being transported from sensors installed at various locations on the transport path 50 and the duplex transport path 60. Therefore, the CPU 4 can detect the occurrence of the jam of the paper P by comparing the target position of the paper P by controlling the conveyance with the actual position acquired from a sensor such as the conveyance path 50. . Furthermore, the CPU 4 of this embodiment can calculate the temperatures of the fixing belt 75 and the pressure roller 71 at the position of the fixing nip N2 from the input image forming job information by simulation described in detail later.

  The engine unit 2 controls various loads 6 of the image forming apparatus 1 and writes / reads data to / from the nonvolatile memory 7 attached to each unit. The various loads 6 include, for example, a toner bottle and an imaging unit. For the nonvolatile memory 7 attached to each unit, for example, the remaining amount of toner is stored in the memory attached to the toner bottle, and the number of prints is stored in the memory attached to the imaging unit.

  The controller unit 3 is connected to an external personal computer or the like and receives an instruction input. For example, an image forming job is generated in the image forming apparatus 1 by receiving an image forming command from a personal computer. Further, various information such as a dot counter value is exchanged between the engine unit 2 and the controller unit 3.

  Next, an example of a normal image forming operation by the image forming apparatus 1 of this embodiment will be briefly described. The following description is an example of an image forming operation in a color mode in which a color image is formed on the paper P stored in the paper feed cassette 51 using four color toners. At the time of forming a color image, the intermediate transfer belt 30 and the photoconductors 11 of the respective colors are rotated at a predetermined peripheral speed in a direction indicated by an arrow in FIG.

  The outer peripheral surface of the photoconductor 11 is charged almost uniformly by the charging device 12. Light corresponding to the image data is projected onto the outer peripheral surface of the charged photoconductor 11 by the exposure device 13 to form an electrostatic latent image. The start of exposure by the exposure device 13 is the start of image formation. Subsequently, the electrostatic latent image is developed by the developing device 14 and a toner image is formed on the photoreceptor 11. The toner images of the respective colors are transferred (primary transfer) onto the intermediate transfer belt 30 at the primary transfer nip formed by the pressure contact of the primary transfer roller 15. Accordingly, yellow (Y), magenta (M), cyan (C), and black (K) toner images are superimposed on the intermediate transfer belt 30 in this order. The superimposed four color toner images are conveyed to the secondary transfer nip N1 by the rotation of the intermediate transfer belt 30.

  On the other hand, the paper P stored in the paper feed cassette 51 is pulled out one by one from the uppermost one to the transport path 50. The drawn paper P is transported along the transport path 50 to the secondary transfer nip N1. The entry timing of the sheet P into the secondary transfer nip N1 is finely adjusted by the registration roller 53 so as to coincide with the entry timing of the toner image on the intermediate transfer belt 30 into the secondary transfer nip N1. As a result, the superimposed four color toner images are transferred (secondary transfer) onto the paper P in the secondary transfer nip N1.

  The paper P on which the toner image has been transferred is further conveyed downstream of the conveyance path 50. In other words, the paper P passes through the fixing nip N <b> 2 and is subjected to toner image fixing processing, and then is discharged to the paper discharge unit 55 by the paper discharge roller 54. Note that the transfer residual toner remaining on the intermediate transfer belt 30 even after passing through the secondary transfer nip N1 is collected by the belt cleaner 41. As a result, the intermediate transfer belt 30 is removed.

  When double-sided printing is performed on the paper P, the paper P on which the image is formed on the first surface by passing through the transport path 50 is transported to the double-side transport path 60 while being reversed by the paper discharge roller 54. Is done. That is, in the image forming apparatus 1 of this embodiment, the paper discharge roller 54 also serves as a reversing unit that reverses the front and back of the paper P. Then, the sheet P that has passed through the duplex conveyance path 60 is conveyed again to the conveyance path 50. Thereby, an image can be formed on the second surface opposite to the first surface of the paper P. The paper P on which images are formed on both sides in this way is then discharged to the paper discharge unit 55.

  Here, the image forming apparatus 1 can perform smart fixing control in which the temperature of the fixing processing unit 70 varies depending on the image to be formed. Specifically, in the smart fixing control of the present embodiment, heating by the heater 74 is controlled according to the toner adhesion amount of the toner image in the image to be formed. That is, when the paper P carrying a toner image with a small amount of toner passes through the fixing nip N2, the paper P carrying a toner image with a large amount of toner attached passes through the fixing nip N2. Also, the heater 74 is controlled to heat at a low temperature. This is because when the toner adhesion amount is small, the toner image can be fixed on the paper P by heating at a lower temperature than when the toner adhesion amount is large.

  When the fixing process is performed on the paper P carrying the toner image with a small amount of toner attached by the smart fixing control, the power consumption can be suppressed. In the image forming apparatus 1 of the present embodiment, when a color image with a large amount of toner adhesion is fixed, the heating temperature of the paper P in the fixing nip N2 is set in a normal mode with a predetermined fixing temperature T1. Fixing process is performed. On the other hand, when performing a fixing process for a monochrome image with a small amount of toner, the fixing process is performed in a low-temperature mode in which the heating temperature of the paper P is set to a fixing temperature T2 lower than the fixing temperature T1 in the normal mode. That is, the image forming apparatus 1 performs the fixing process while switching the temperature control mode of the fixing processing unit 70 between the normal mode and the low temperature mode having different temperatures.

  In the present embodiment, when the color image is formed, the temperature adjustment mode is switched on the assumption that the amount of toner attached to the toner image is larger than when the monochrome image is formed. However, whether the fixing process is performed in the normal mode or the low temperature mode may be determined based on other information. For the determination, in addition to whether the image to be formed is a color image or a monochrome image, information such as the character width, character size, and image density of the image to be formed can be used. Further, whether the fixing process is performed in the normal mode or the low-temperature mode may be comprehensively determined based on the information on the toner adhesion amount of the formed toner image. For example, when forming a glossy image, the fixing temperature may be set high even when the amount of toner adhesion is small. Therefore, whether the fixing process is performed in the normal mode or the low temperature mode can be determined in consideration of information other than information on the toner adhesion amount.

  An example of smart fixing control will be described with reference to FIG. FIG. 4 is a diagram for explaining a change in temperature of the fixing processing unit 70 when a plurality of images A, B, and C are formed in this order. FIG. 4 shows the passage of time from left to right. Further, the images A and C are monochrome images, and the image B is a color image. Therefore, the fixing process is performed in the low temperature mode for the images A and C, and the fixing process is performed in the normal mode for the image B.

  First, as shown in FIG. 4, the CPU 4 issues print commands for images A, B, and C in this order when an image forming job is executed. The print command for each image is generated based on information related to image formation such as an image related to the print command and map data for the next image. In other words, the print command for image A is generated based on the information for image A and image B, and the print command for image B is generated based on the information for image B and image C. The print command for image C is based only on the information of image C. When the next image is formed after the image C, a print command for the image C may be generated in consideration of the image information.

  Next, exposure on the photosensitive member 11 is started by the exposure device 13 of the image forming unit 10 based on the print command for each image. Thereafter, each developed image is transferred to the paper P in the secondary transfer nip N1, and passes through the fixing nip N2. In FIG. 4, the time from when the exposure for the image A is started until the leading edge of the image A reaches the fixing nip N2 is indicated by t1. Further, the time from when the leading edge of the image A reaches a certain position by conveyance until the leading edge of the image B reaches the same position is indicated by t2. Both times t1 and t2 are known values when the print command for image A is issued. This is because the print command for image A is based on the information of image A and image B.

  Here, the fixing process is performed for the image A in the low-temperature mode and for the image B in the normal mode. However, the image quality of the image A, which is a monochrome image with a small amount of toner adhesion, does not deteriorate even if the fixing process is performed at a temperature higher than the fixing temperature T2 in the low temperature mode. On the other hand, for the image B, when the fixing process is performed at a temperature lower than the fixing temperature T1 in the normal mode, the image quality may be deteriorated. This is because in order to satisfactorily fix the toner image on the paper P, it is necessary to appropriately heat the toner image according to the amount of toner adhering to the toner image.

  Therefore, in the image forming apparatus 1, as shown in FIG. 4, when the leading edge of the image B reaches the fixing nip N2, the heater 74 is controlled so that the heating temperature in the fixing nip N2 becomes the fixing temperature T1 in the normal mode. The Here, as shown in FIG. 4, time t3 is required to raise the heating temperature at the fixing nip N2 to the fixing temperature T1 until the leading edge of the image B reaches. The time t3 is the shortest time required to raise the heating temperature at the fixing nip N2 from the fixing temperature T2 to the fixing temperature T1. Therefore, hereinafter, t3 is the shortest temperature rise time. For this reason, the temperature adjustment mode is switched from the low temperature mode to the normal mode earlier than the time when the leading edge of the image B reaches the fixing nip N2 by the shortest temperature increase time t3.

  Note that the shortest temperature rise time t3 is also a known value when the image A print command is issued. Further, since the times t1, t2, and t3 are known, the time from the start of exposure of the image A indicated by Δt in FIG. 4 to the time when the temperature adjustment mode is switched from the low temperature mode to the normal mode is also as follows. This is a known value when the image A print command is issued. Also, the times t1, t2, t3 and the time Δt in FIG. 4 are cases where the quality of the formed image is set to a normal level and the productivity is made as high as possible.

  Then, as shown in FIG. 4, the sheet P on which the image A has been transferred is changed to the normal mode while the heating temperature in the fixing nip N2 rises from the fixing temperature T2 to the fixing temperature T1. It passes through the fixing nip N2.

Then, the heating temperature in the fixing nip N2 of NoboriAtsushichu is fixing temperature T 2 or more at a low temperature mode. For this reason, the image quality of the image A does not deteriorate due to passing through the fixing nip N2 during the temperature rise. However, the paper P related to the image A that has passed through the fixing nip N2 that is being heated may be curled to the extent that a jam occurs during subsequent transport.

  Next, the occurrence of curling on the paper P that has passed through the fixing nip N2 during temperature rise will be described. FIG. 5 is a graph showing the temperature of the fixing processing unit 70 when the paper P is passed through the fixing nip N2 in the low temperature mode. The horizontal axis in FIG. 5 represents time, and the vertical axis represents temperature. Further, FIG. 5 shows the temperature of the fixing processing unit 70, the temperature detected by the temperature sensor 76 is indicated by a two-dot chain line, the temperature near the fixing nip N2 of the fixing belt 75 is indicated by a solid line, and the vicinity of the fixing nip N2 of the pressure roller 71. The temperature is indicated by a broken line.

  The temperature in the vicinity of the fixing nip N2 of the fixing belt 75 and the pressure roller 71 is detected by a sensor for detecting the temperature at each position. FIG. 5 shows the temperature detected by each sensor when the sheet P is passed through the fixing nip N2 in the low temperature mode fixing temperature T2.

  As shown in FIG. 5, the temperature near the fixing nip N2 of the fixing belt 75 is maintained at the fixing temperature T2. As described above, the fixing belt 75 has a configuration on the side of the surface carrying the toner image of the paper P. Since the temperature in the vicinity of the fixing nip N2 of the fixing belt 75 is the fixing temperature T2, the fixing process can be performed on the toner image carried on the paper P in the low temperature mode.

  Note that the temperature of the fixing belt 75 decreases as heat is taken by the paper P that has passed through the fixing nip N2. However, the temperature of the fixing belt 75 in the vicinity of the fixing nip N2 is maintained at the fixing temperature T2 while the paper P passes through the fixing nip N2 due to the reduced temperature. Yes. Further, the temperature detected by the temperature sensor 76 is maintained at a temperature slightly higher than the fixing temperature T2. This is because the detection position of the temperature sensor 76 is closer to the heater 74 than the fixing nip N2.

  The temperature near the fixing nip N2 of the pressure roller 71 is maintained at a temperature slightly lower than the fixing temperature T2 before the paper P reaches the fixing nip N2. This is because the pressure roller 71 does not have a heat source and is heated by heat transfer from the fixing belt 75. In FIG. 5, the difference between the temperature near the fixing nip N2 of the fixing belt 75 and the temperature near the fixing nip N2 of the pressure roller 71 before the sheet P reaches the fixing nip N2 is shown as a temperature difference TD1. .

  Further, as shown in FIG. 5, the temperature in the vicinity of the fixing nip N2 of the pressure roller 71 is lowered by the sheet P passing through the fixing nip N2. This is because the pressure roller 71 is deprived of heat by the paper P that has passed through the fixing nip N2. Further, while the sheet P passes through the fixing nip N2, heat transfer from the fixing belt 75 is blocked by the sheet P.

  The temperature of the pressure roller 71 continues to decrease while the paper P passes through the fixing nip N2, and is lowest when the trailing edge of the paper P reaches the fixing nip N2. A difference between the temperature of the fixing belt 75 near the fixing nip N2 and the temperature of the pressure roller 71 when the trailing edge of the sheet P reaches the fixing nip N2 is shown as a temperature difference TD2.

  The temperature difference TD2 when the trailing edge of the paper P reaches the fixing nip N2 is larger than the temperature difference TD1 before the paper P reaches the fixing nip N2. Further, the temperature difference between the fixing belt 75 and the pressure roller 71 in the vicinity of the fixing nip N2 increases from the front end to the rear end of the paper P passing through the fixing nip N2. In the paper P, the larger the temperature difference between the fixing belt 75 and the pressure roller 71 in the fixing nip N2, the larger the curl generated after passing through the fixing nip N2. That is, in FIG. 5, a larger curl is generated on the rear end side of the paper P that has passed through the fixing nip N2 than on the front end side. However, the temperature difference TD2 shown in FIG. 5 is not so large, and the paper P that has passed through the fixing nip N2 does not have such a large curl as to cause a jam in the subsequent conveyance.

  Next, FIG. 6 shows the temperature of the fixing processing unit 70 when the sheet P is passed through the fixing nip N2 whose temperature is being increased. That is, FIG. 6 shows a state in which the sheet P passes through the fixing nip N2 while the heating temperature of the fixing nip N2 rises from the fixing temperature T2 to the fixing temperature T1 after switching from the low temperature mode to the normal mode. belongs to. In the case of FIG. 6, unlike the case of FIG. 5, as described in FIG. 4, the paper P may be curled to the extent that a jam occurs in the subsequent transport.

  As shown in FIG. 6, the temperature near the fixing nip N2 of the fixing belt 75 starts to increase from the fixing temperature T2 before the paper P reaches the fixing nip N2. The sheet P passes through the fixing nip N2 while the temperature in the vicinity of the fixing nip N2 of the fixing belt 75 rises from the fixing temperature T2 to the fixing temperature T1.

  On the other hand, the temperature in the vicinity of the fixing nip N2 of the pressure roller 71 is lowered when the paper P passes through the fixing nip N2, and is lowest when the rear end of the paper P reaches the fixing nip N2. In FIG. 6, the difference between the temperature of the fixing belt 75 and the temperature of the pressure roller 71 near the fixing nip N2 when the trailing edge of the paper P reaches the fixing nip N2 is shown as a temperature difference TD3.

  As shown in FIG. 6, the temperature difference TD3 when the trailing edge of the paper P reaches the fixing nip N2 is larger than the temperature difference TD1 before the paper P reaches the fixing nip N2. Further, the temperature difference between the fixing belt 75 and the pressure roller 71 in the vicinity of the fixing nip N2 increases from the front end to the rear end of the paper P passing through the fixing nip N2. For this reason, also in FIG. 6, a larger curl is generated on the rear end side of the paper P that has passed through the fixing nip N2 than on the front end side.

  In FIG. 5, as described above, the temperature near the fixing nip N2 of the pressure roller 71 decreases while the temperature near the fixing nip N2 of the fixing belt 75 is kept constant at the fixing temperature T2. On the other hand, in FIG. 6, the temperature of the fixing nip N2 of the fixing belt 75 is increased from the fixing temperature T2, whereas the temperature near the fixing nip N2 of the pressure roller 71 is decreased. Therefore, in FIG. 6, the temperature difference TD3 when the trailing edge of the paper P reaches the fixing nip N2 is larger than the temperature difference TD2 in FIG. That is, in the case shown in FIG. 6, a larger curl is generated than in the case shown in FIG. In the case of FIG. 6, the paper P that has passed through the fixing nip N <b> 2 may have such a large curl that a jam occurs during subsequent conveyance.

  In view of this, the image forming apparatus 1 according to the present embodiment performs a fixing process so that the temperature difference between the fixing belt 75 and the pressure roller 71 does not increase in order to suppress the occurrence of a large curl on the paper P. That is, while the sheet P is passing through the fixing nip N2, the temperature difference between the fixing belt 75 and the pressure roller 71 at the position of the fixing nip N2 is the upper limit temperature difference TDlim stored in the nonvolatile memory 5. Temperature difference restriction control is performed to control each part so as not to exceed. As described above, the upper limit temperature difference TDlim is the maximum allowable temperature difference between the fixing belt 75 and the pressure roller 71 at the fixing nip N2, and the curl of the paper P is conveyed in the conveyance after passing through the fixing nip N2. This temperature can be suppressed to such an extent that jam does not occur. The upper limit temperature difference TDlim can be obtained by conducting an experiment in advance.

  In this embodiment, first, prior to the temperature difference limiting control, the CPU 4 performs a simulation for estimating the temperature difference between the fixing belt 75 and the pressure roller 71 in the fixing nip N2 while the paper P is passing through the fixing nip N2. Do. Similar to FIG. 4, this simulation is based on the time t1, t2, t3 and the time Δt based on known information in the job relating to image formation, such as the thickness and size of the paper P, the conveyance speed of the paper P, and the degree of heating by the heater 74. Can be done while seeking. Specifically, before the exposure relating to image formation is started, the temperatures of the fixing belt 75 and the pressure roller 71 in the fixing nip N2 while the sheet P is passing through the fixing nip N2 are obtained. The temperature difference can be estimated from the obtained temperature.

  If the estimated temperature difference exceeds the upper limit temperature difference TDlim, the fixing process of the paper P is performed while performing the temperature difference restriction control. In the temperature difference restriction control of this embodiment, heating by the heater 74 is stopped before the temperature difference between the fixing belt 75 and the pressure roller 71 in the fixing nip N2 exceeds the upper limit temperature difference TDlim. The timing at which heating of the heater 74 is stopped can be obtained by the CPU 4 in the above simulation. For example, the heating by the heater 74 may be stopped a predetermined time before the timing at which the temperature difference between the fixing belt 75 and the pressure roller 71 in the simulation exceeds the upper limit temperature difference TDlim. Alternatively, the upper limit temperature difference TDlim is added to the temperature of the pressure roller 71 when the trailing edge of the paper P in the simulation reaches the fixing nip N2. Furthermore, it is conceivable that heating of the heater 74 is stopped before the timing at which the temperature of the fixing belt 75 reaches the temperature obtained by the addition.

  The temperature in the vicinity of the fixing nip N2 of the fixing belt 75 when the temperature difference limiting control of this embodiment is performed is shown by a thick solid line in FIG. As shown in FIG. 7, after the sheet P enters the fixing nip N2, the heating of the heater 74 is stopped by the temperature difference restriction control before the trailing end of the sheet P passes through the fixing nip N2. While the heating is stopped, the temperature rise of the fixing belt 75 is suppressed. FIG. 7 shows a temperature difference TD4 when the trailing edge of the paper P reaches the fixing nip N2 when heating by the heater 74 is stopped.

  As shown in FIG. 7, the temperature difference TD4 is suppressed to be lower than the temperature difference TD3 when the temperature difference restriction control is not performed and the heating by the heater 74 is not stopped. Further, as shown in FIG. 7, the temperature difference TD4 is suppressed to be lower than the upper limit temperature difference TDlim. Therefore, in this embodiment, the fixing process can be performed without causing a large curl on the rear end side of the paper P. Thereby, it is possible to suppress the occurrence of a jam in the conveyance of the paper P after passing through the fixing nip N2.

  In the example of FIG. 7, when the trailing edge of the paper P passes through the fixing nip N2, heating by the heater 74 that has been stopped is resumed. Then, after the heating temperature in the fixing nip N2 reaches the fixing temperature T1 in the normal mode, the next sheet P can be passed through the fixing nip N2. That is, in the example of FIG. 4, the paper P related to the image B is fixed after the paper P related to the image A passes through the fixing nip N2 and the heating temperature by the fixing nip N2 reaches the fixing temperature T1 in the normal mode. It can be passed through the nip N2.

  In addition, the above simulation and temperature difference limitation control are performed only when it is determined that the fixing temperature for the image related to the print command is lower than the fixing temperature for the next image after the print command is generated. It may be done. In other words, when the fixing temperature in the smart fixing control is the same for the image related to the print command and the next image, or when the fixing temperature of the next image is lower, the simulation and the temperature difference limiting control are performed. It is good also as not performing.

  In this embodiment, as described above, the temperature difference between the fixing belt 75 and the pressure roller 71 in the fixing nip N2 while the sheet P is passing through the fixing nip N2 is estimated, and based on the estimated temperature difference. Temperature difference limit control is performed. However, for example, in the configuration in which the fixing belt 75 and the pressure roller 71 are provided with temperature sensors that detect the temperature near the fixing nip N2, the sheet P is added to the fixing belt 75 while passing through the fixing nip N2. The actual temperature with the pressure roller 71 can be detected. Further, the actual temperature difference can be obtained from the detected actual temperature between the fixing belt 75 and the pressure roller 71 in the vicinity of the fixing nip N2.

  In order to detect the temperature in the vicinity of the fixing nip N2 of the fixing belt 75, for example, a hollow fixing roller 72 as shown in FIG. 2 is used, and two points near the fixing nip N2 inside the fixing roller 72 are used. A temperature sensor 77 indicated by a chain line may be provided. Similarly, the pressure roller 71 may be provided with a temperature sensor 78 indicated by a two-dot chain line at a position close to the fixing nip N <b> 2 inside the hollow pressure roller 71.

  Then, the temperature difference is calculated based on the difference between the detected temperatures of the temperature sensors 77 and 78, and the temperature difference limiting control is performed to control the heating by the heater 74 so that the calculated temperature difference is suppressed to be lower than the upper limit temperature difference TDlim. Can do. Specifically, the heating by the heater 74 is stopped when the temperature difference when the paper P passes through the fixing nip N2 becomes high to a heating stop temperature difference which is a threshold value set to the upper limit temperature difference TDlim or less. That's fine. In this case, it is not necessary to perform a simulation for estimating the temperature difference. Strictly speaking, the detected temperatures of the temperature sensors 77 and 78 are slightly different from the temperature of the fixing belt 75 and the pressure roller 71 in the actual fixing nip N2. Therefore, the temperature obtained by correcting the temperature detected by the temperature sensors 77 and 78 may be set as the temperature of the fixing belt 75 and the temperature of the pressure roller 71 in the fixing nip N2.

  Further, by temporarily stopping the heating by the heater 74 while the paper P relating to the image A passes through the fixing nip N2, it takes some time to reach the fixing temperature T1 in the normal mode. Become. However, since the heating stop time by the heater 74 is not so long, the productivity of image formation is not greatly reduced. That is, curling of the paper P can be suppressed while maintaining high image forming productivity.

  Further, it is preferable that the temperature difference restriction control is performed when an image is formed on the first surface in double-sided printing. As described above, in duplex printing, the sheet P that has passed through the fixing nip N2 is then conveyed to the duplex conveyance path 60 with the front and back reversed. By reversing the front and back, when the image is formed on the second surface of the paper P, the leading edge and the trailing edge are opposite to those when the image is formed on the first surface. That is, the end that was the rear end when the image is formed on the first surface of the paper P becomes the front end when the image is formed on the second surface with the front and back sides reversed.

  If a large curl is generated on the leading end side of the sheet P, the leading end may not be able to pass through, for example, the nip of the transport roller disposed along the double-side transport path 60. Accordingly, the curl on the rear end side when forming an image on the first surface of the paper P may cause a jam when forming an image on the second surface after the front and back sides are reversed. In double-sided printing, curling of the rear end side during image formation on the first surface is controlled by temperature difference restriction control, so that the image formation on the second surface of the paper P with the front and back sides reversed is jammed. It is because it can carry out satisfactorily without generating.

  On the other hand, when only image formation is performed on the first surface, and at the time of image formation on the second surface after image formation on the first surface, after the conveyance direction of the paper P at the fixing nip N2. Even if curling occurs on the end side, the frequency of jamming in subsequent conveyance is not so high. It should be noted that temperature difference restriction control may be performed when only image formation is performed on the first surface, and also during image formation on the second surface after image formation on the first surface. This is because the curling of the paper P can be suppressed. That is, it is possible to satisfactorily stack the paper P on which the image formation has been completed on the paper discharge unit 55. This is also because high-quality bookbinding can be performed using a plurality of sheets P on which image formation has been completed.

  The upper limit temperature difference TDlim is preferably set to a different value depending on, for example, the type of paper P used for image formation. For example, the thicker type of paper P is stronger and more likely to jam when curled. For this reason, it is preferable that the thicker the paper P is, the more curl is reduced. Table 1 shows the upper limit temperature difference TDlim that can suppress the curling of the paper P to the extent that no jamming occurs in the conveyance in the image forming apparatus 1 for each type of the paper P.

  As shown in Table 1, the thicker the paper P, the lower the upper limit temperature difference TDlim, and it is preferable to suppress curling. Table 1 below is an example of the image forming apparatus 1 according to the present embodiment. For example, depending on different image forming apparatuses, different temperature difference values can be used as the upper limit temperature difference TDlim.

  Further, the upper limit temperature difference TDlim shown in Table 1 can be corrected according to the conditions when the paper P passes through the fixing nip N2. This is because the temperature difference between the fixing belt 75 and the pressure roller 71 when the trailing edge of the paper P reaches the fixing nip N2 varies depending on the conditions when the paper P passes through the fixing nip N2. For example, the temperature difference between the fixing belt 75 and the pressure roller 71 when the rear end of the paper P reaches the fixing nip N2 becomes smaller as the length in the transport direction of the paper P is shorter. Become. This is because as the length of the paper P in the transport direction is shorter, the amount of heat taken by the pressure roller 71 by the entire paper P is reduced. Therefore, the upper limit temperature difference TDlim can be corrected to be lower as the paper P having a shorter length in the transport direction is used. Table 2 shows examples of correction values for correcting the upper limit temperature difference TDlim according to the length of the paper P in the conveyance direction.

  Further, the temperature difference between the fixing belt 75 and the pressure roller 71 when the rear end of the paper P reaches the fixing nip N2 becomes smaller as the speed when the paper P passes through the fixing nip N2 is higher. Become. This is because the amount of heat taken by the pressure roller 71 by the entire sheet P decreases as the conveying speed of the sheet P passes through the fixing nip N2 increases. Therefore, it is possible to correct the upper limit temperature difference TDlim to be lower as the transport speed of the paper P is higher. Table 3 shows examples of correction values for correcting the upper limit temperature difference TDlim according to the conveyance speed of the paper P.

  Further, the temperature difference between the fixing belt 75 and the pressure roller 71 when the trailing edge of the paper P reaches the fixing nip N2 is as much as the rate of increase in the heating temperature while the paper P is passing through the fixing nip N2 is slower. It will be small. This is because the difference between the temperature of the fixing belt 75 when the leading end of the sheet P reaches the fixing nip N2 and the temperature of the fixing belt 75 when the trailing end of the sheet P reaches the fixing nip N2 is small. In addition, the temperature difference between the fixing belt 75 and the pressure roller 71 when the trailing edge of the paper P reaches the fixing nip N2 is smaller as the difference between the fixing temperature T2 in the low temperature mode and the fixing temperature T1 in the normal mode is smaller. It tends to be small. Therefore, the upper limit temperature difference TDlim can be corrected by the rate of increase of the heating temperature of the paper P in the fixing nip N2 and the difference in heating temperature between the low temperature mode and the normal mode.

  Further, the degree of curling of the paper P is naturally affected by the environmental temperature and humidity. Therefore, the image forming apparatus 1 can be provided with a sensor for detecting the temperature and humidity of the environment, and the upper limit temperature difference TDlim can be corrected by the detection value of the sensor. Accordingly, the relationship between the type of the sheet P, the length in the conveyance direction, the conveyance speed when the sheet P passes through the fixing nip N2, and the upper limit temperature difference TDlim is acquired in advance through experiments and the like. May be stored in the nonvolatile memory 5 as an upper limit temperature difference table. At the time of image formation, the temperature difference restriction control may be executed using a value determined by referring to the upper limit temperature difference table of the nonvolatile memory 5 according to the conditions corresponding to the image formation as the upper limit temperature difference TDlim.

[Second form]
The second embodiment will be described. Also in this embodiment, the configuration of the image forming apparatus 1 is the same as that in the first embodiment. Also in the second embodiment, smart fixing control is performed in the fixing processing by the fixing processing unit 70. As a result, when the paper P passes through the fixing nip N2, the temperature difference between the fixing belt 75 and the pressure roller 71 at the position of the fixing nip N2 may increase. Therefore, there is a possibility that a curl large enough to cause a jam in subsequent conveyance may occur on the rear end side of the sheet P that has passed through the fixing nip N2.

  For this reason, also in the second embodiment, the temperature difference for suppressing the temperature difference between the fixing belt 75 and the pressure roller 71 at the fixing nip N2 so as not to cause a large curl on the paper P that has passed through the fixing nip N2. Perform limit control. In the second embodiment, temperature difference restriction control different from that in the first embodiment is performed. Specifically, in the temperature difference limiting control of the present embodiment, it is performed by adjusting the rate of increase in the heating temperature of the paper P in the fixing nip N2.

  Also in this embodiment, the CPU 4 first presses the fixing belt 75 and the pressure in the fixing nip N2 while the sheet P is passing through the fixing nip N2 based on the job information before the execution of the image forming job. A temperature difference with the roller 71 is estimated by simulation. If the estimated temperature difference exceeds the upper limit temperature difference TDlim, the fixing process of the paper P is performed while performing the temperature difference restriction control. In the temperature difference restriction control of this embodiment, the heating temperature of the paper P in the fixing nip N2 is raised while restricting the heating by the heater 74.

  The temperature difference limiting control of this embodiment will be described in detail with reference to FIG. FIG. 8 also shows the temperature of the fixing processing unit 70 when the paper P related to the image A which is a monochrome image and the paper P related to the image B which is a color image pass through the fixing nip N2 in this order. It is. In FIG. 8, when the temperature difference restriction control according to the second embodiment is performed, the temperature near the fixing nip N2 of the fixing belt 75 is indicated by a solid line, and the temperature near the fixing nip N2 of the pressure roller 71 is indicated by a broken line. ing.

  In addition, the temperature in the vicinity of the fixing nip N2 of the fixing belt 75 when the temperature difference restriction control is not performed is indicated by a two-dot chain line. The temperature in the vicinity of the fixing nip N2 of the fixing belt 75 indicated by the two-dot chain line does not limit the heating by the heater 74, and the heating temperature of the sheet P in the fixing nip N2 is the shortest temperature increase time t3 from the fixing temperature T2 to the fixing temperature T1 This is when it is raised in FIG. That is, the temperature in the vicinity of the fixing nip 75 of the fixing belt 75 indicated by a two-dot chain line is based on the simulation.

  Also in this embodiment, the temperature adjustment mode is switched from the low temperature mode to the normal mode when the time Δt has elapsed since the exposure of the image A was started. This is because the image B next to the image A which is a monochrome image is a color image. Further, since the temperature control mode is switched from the low temperature mode to the normal mode, the temperature in the vicinity of the fixing nip N2 of the fixing belt 75 rises while the paper P related to the image A passes through the fixing nip N2.

  In this embodiment, as shown in FIG. 8, the temperature in the vicinity of the fixing nip N2 of the fixing belt 75 when the temperature difference restriction control is performed is that when the temperature difference restriction control indicated by the two-dot chain line is not performed. Compared to gradual increase. This is because the heating by the heater 74 is limited by the temperature difference limiting control as compared with the simulation.

  Therefore, the temperature difference TD5 between the fixing belt 75 and the pressure roller 71 in the vicinity of the fixing nip N2 when the temperature difference restriction control is performed can be suppressed to be lower than the temperature difference TD3 when the temperature difference restriction control is not performed. ing. Further, as shown in FIG. 8, the temperature difference TD5 is suppressed to be lower than the upper limit temperature difference TDlim. Therefore, also in this embodiment, the fixing process can be performed without causing a large curl on the rear end side of the paper P. Thereby, it is possible to suppress the occurrence of a jam in the conveyance of the paper P after passing through the fixing nip N2.

[Third embodiment]
A third embodiment will be described. Also in this embodiment, the configuration of the image forming apparatus 1 is the same as that in the above embodiment. Also in the third embodiment, smart fixing control is performed in the fixing processing by the fixing processing unit 70. As a result, when the paper P passes through the fixing nip N2, the temperature difference between the fixing belt 75 and the pressure roller 71 at the position of the fixing nip N2 may increase. Therefore, there is a possibility that a curl large enough to cause a jam in subsequent conveyance may occur on the rear end side of the sheet P that has passed through the fixing nip N2.

  For this reason, also in the third embodiment, the temperature difference for suppressing the temperature difference between the fixing belt 75 and the pressure roller 71 at the fixing nip N2 is low so that a large curl is not generated on the paper P that has passed through the fixing nip N2. Perform limit control. In the third embodiment, temperature difference restriction control different from that in the above embodiment is performed. Specifically, in the temperature difference limiting control of this embodiment, the switching timing from the low temperature mode to the normal mode is made earlier than in the above embodiment.

  Also in this embodiment, the CPU 4 first presses the fixing belt 75 and the pressure in the fixing nip N2 while the sheet P is passing through the fixing nip N2 based on the job information before the execution of the image forming job. A temperature difference with the roller 71 is estimated by simulation. If the estimated temperature difference exceeds the upper limit temperature difference TDlim, the fixing process of the paper P is performed while performing the temperature difference restriction control. The temperature difference limiting control of the present embodiment increases the heating temperature of the paper P in the fixing nip N2 while advancing the switching timing from the low temperature mode to the normal mode.

  The temperature difference limiting control of this embodiment will be described in detail with reference to FIG. FIG. 9 also shows the temperature of the fixing processing unit 70 when the sheet P related to the image A which is a monochrome image and the sheet P related to the image B which is a color image pass through the fixing nip N2 in this order. It is. In FIG. 9, when the temperature difference restriction control according to the third embodiment is performed, the temperature near the fixing nip N2 of the fixing belt 75 is indicated by a solid line, and the temperature near the fixing nip N2 of the pressure roller 71 is indicated by a broken line. ing.

  As shown in FIG. 9, in the third embodiment, the temperature adjustment mode is switched from the low temperature mode to the normal mode when the time Δt1 has elapsed since the exposure of the image A was started. That is, in this embodiment, the temperature adjustment mode is switched from the low temperature mode to the normal mode earlier than the time Δt described with reference to FIG. The time α can be set to a time less than the time Δt. The time Δt is the temperature control mode switching timing in the simulation.

  That is, in the temperature difference limiting control of this embodiment, the timing for switching the temperature adjustment mode from the low temperature mode to the normal mode is earlier than the timing in the simulation. As a result, the temperature at the fixing nip N2 of the fixing belt 75 can be raised to the fixing temperature T1 earlier than in the above embodiment. Therefore, it is possible to pass the paper P related to the image B next to the image A to the fixing nip N2 at a timing earlier than that in the above embodiment.

  Further, since the temperature control mode is switched from the low temperature mode to the normal mode, the temperature in the vicinity of the fixing nip N2 of the fixing belt 75 rises while the paper P related to the image A passes through the fixing nip N2. In the temperature difference limiting control of this embodiment, as shown in FIG. 9, after the paper P enters the fixing nip N2, the heating of the heater 74 is stopped before the trailing edge of the paper P passes through the fixing nip N2. doing. The stopping of heating by the heater 74 is the same as that described with reference to FIG. 7 of the first embodiment. That is, by temporarily stopping the heating by the heater 74, the temperature difference TD6 when the trailing edge of the paper P reaches the fixing nip N2 is suppressed to be lower than the upper limit temperature difference TDlim. Accordingly, the fixing process can be performed without causing a large curl on the rear end side of the paper P, and the occurrence of a jam in the conveyance of the paper P after passing through the fixing nip N2 can be suppressed.

  Therefore, in the temperature difference restriction control of the present embodiment, the temperature difference TD6 between the fixing belt 75 and the pressure roller 71 in the vicinity of the fixing nip N2 can be suppressed lower than the upper limit temperature difference TDlim, and the image forming productivity can be increased. . In the above description, similarly to the first embodiment, the temperature difference TD6 is kept lower than the upper limit temperature difference TDlim by temporarily stopping heating by the heater 74 (FIG. 7). However, by limiting the heating by the heater 74 as in the second embodiment (FIG. 8), it is possible to keep the temperature difference TD6 lower than the upper limit temperature difference TDlim.

[Fourth form]
A fourth embodiment will be described. Also in this embodiment, the configuration of the image forming apparatus 1 is the same as that in the above embodiment. Also in the fourth embodiment, smart fixing control is performed in the fixing processing by the fixing processing unit 70. As a result, when the paper P passes through the fixing nip N2, the temperature difference between the fixing belt 75 and the pressure roller 71 at the position of the fixing nip N2 may increase. Therefore, there is a possibility that a curl large enough to cause a jam in subsequent conveyance may occur on the rear end side of the sheet P that has passed through the fixing nip N2.

  For this reason, also in the fourth embodiment, the temperature difference for suppressing the temperature difference between the fixing belt 75 and the pressure roller 71 in the fixing nip N2 so as not to cause a large curl on the paper P that has passed through the fixing nip N2. Perform limit control. And in the 4th form, temperature difference restriction control different from the above-mentioned form is performed. Specifically, the temperature difference restriction control of this embodiment is performed by adjusting the conveyance speed of the paper P.

  Also in this embodiment, the CPU 4 first presses the fixing belt 75 and the pressure in the fixing nip N2 while the sheet P is passing through the fixing nip N2 based on the job information before the execution of the image forming job. A temperature difference with the roller 71 is estimated by simulation. If the estimated temperature difference exceeds the upper limit temperature difference TDlim, the fixing process of the paper P is performed while performing the temperature difference restriction control. In the temperature difference limiting control of this embodiment, the conveyance speed of the paper P before reaching the fixing nip N2 is made slower than the normal conveyance speed.

  The temperature difference limiting control of this embodiment will be described in detail with reference to FIG. FIG. 10 also shows the temperature of the fixing processing unit 70 when the paper P related to the image A which is a monochrome image and the paper P related to the image B which is a color image pass through the fixing nip N2 in this order. It is. The solid line indicates the temperature near the fixing nip N2 of the fixing belt 75, and the broken line indicates the temperature near the fixing nip N2 of the pressure roller 71.

  As shown in FIG. 10, in the fourth embodiment, the time from the start of exposure of the image A described with reference to FIG. 4 until the leading edge of the image A reaches the fixing nip N2 is later than the time t1. In addition, the leading edge of the image A has reached the fixing nip N2. The time t1 is the time from the start of exposure of the image A in the simulation until the leading edge of the image A reaches the fixing nip N2. That is, the timing at which the paper P related to the image A passes through the fixing nip N2 is set later than the timing in the simulation.

  Specifically, in the fourth embodiment, the temperature in the vicinity of the fixing nip N2 of the fixing belt 75 reaches the fixing temperature T1 before the trailing edge of the paper P related to the image A passes through the fixing nip N2. Yes. While the sheet P relating to the image A passes through the fixing nip N2, the temperature of the fixing belt 75 that has reached the fixing temperature T1 does not rise any further, so that the fixing belt 75, the pressure roller 71, The temperature difference TD7 is small. Thereby, in this embodiment, the temperature difference TD7 is suppressed to be lower than the upper limit temperature difference TDlim.

  In addition, in order to make the timing of passing the paper P related to the image A to the fixing nip N2 slower than the time t1 in the simulation, in this embodiment, the conveyance speed of the paper P reaching the fixing nip N2 is slower than in the simulation. doing. Specifically, the conveyance speed of the paper P related to the image A is set so that the leading edge reaches the fixing nip N2 before the leading edge reaches the secondary transfer nip N1 or after the trailing edge passes through the secondary transfer nip N1. Before arriving, it is switched to a transport speed that is slower than the transport speed during simulation. This is because if the conveyance speed is changed slowly while the paper P passes through the secondary transfer nip N1, noise may occur in the image A transferred to the paper P. That is, the temperature difference TD7 can be suppressed to be lower than the upper limit temperature difference TDlim while appropriately transferring the image A onto the paper P in the secondary transfer nip N1. Therefore, in the present embodiment, it is possible to reduce the curl of the paper P while keeping the quality of the formed image high, and to suppress the occurrence of a jam in the transport of the paper P after passing through the fixing nip N2. Note that the conveyance speed of the paper P related to the image A may be temporarily reduced from that during the simulation and then returned to the conveyance speed during the simulation before the leading edge of the paper P reaches the fixing nip N2. Alternatively, before the leading edge of the paper P related to the image A reaches the secondary transfer nip N1, or before the leading edge reaches the fixing nip N2 after the trailing edge passes through the secondary transfer nip N1, the paper P It is good also as stopping conveyance of this temporarily.

[Fifth embodiment]
A fifth embodiment will be described. Also in this embodiment, the configuration of the image forming apparatus 1 is the same as that in the above embodiment. Also in the fifth embodiment, smart fixing control is performed in the fixing processing by the fixing processing unit 70. As a result, when the paper P passes through the fixing nip N2, the temperature difference between the fixing belt 75 and the pressure roller 71 at the position of the fixing nip N2 may increase. Therefore, there is a possibility that a curl large enough to cause a jam in subsequent conveyance may occur on the rear end side of the sheet P that has passed through the fixing nip N2.

  For this reason, also in the fifth embodiment, the temperature difference for suppressing the temperature difference between the fixing belt 75 and the pressure roller 71 at the fixing nip N2 so as not to cause a large curl on the paper P that has passed through the fixing nip N2. Perform limit control. And in a 5th form, temperature difference restriction | limiting control different from said form is performed. Specifically, the temperature difference limiting control of this embodiment is performed by delaying the switching timing from the low temperature mode to the normal mode as compared with the above embodiment.

  Also in this embodiment, the CPU 4 first presses the fixing belt 75 and the pressure in the fixing nip N2 while the sheet P is passing through the fixing nip N2 based on the job information before the execution of the image forming job. A temperature difference with the roller 71 is estimated by simulation. If the estimated temperature difference exceeds the upper limit temperature difference TDlim, the fixing process of the paper P is performed while performing the temperature difference restriction control. The temperature difference limiting control of this embodiment is performed by delaying the switching timing from the low temperature mode to the normal mode.

  The temperature difference limiting control of this embodiment will be described in detail with reference to FIG. FIG. 11 also shows the temperature of the fixing processing unit 70 when the sheet P related to the image A which is a monochrome image and the sheet P related to the image B which is a color image pass through the fixing nip N2 in this order. It is. The solid line indicates the temperature near the fixing nip N2 of the fixing belt 75, and the broken line indicates the temperature near the fixing nip N2 of the pressure roller 71.

  As shown in FIG. 11, in the fifth embodiment, the temperature adjustment mode is switched from the low temperature mode to the normal mode when the time Δt2 has elapsed since the exposure of the image A was started. In other words, in the present embodiment, the temperature adjustment mode is switched from the low temperature mode to the normal mode by a time β later than the time Δt described with reference to FIG. Time Δt is the temperature control mode switching timing in the simulation. That is, in the temperature difference limiting control of this embodiment, the timing for switching the temperature adjustment mode from the low temperature mode to the normal mode is set later than the timing in the simulation.

  In the fifth embodiment, as shown in FIG. 11, while the paper P relating to the image A passes through the fixing nip N2, the temperature adjustment mode is set to the low temperature mode. For this reason, while the paper P relating to the image A passes through the fixing nip N2, the temperature near the fixing nip N2 of the fixing belt 75 is kept constant at the fixing temperature T2.

  Then, since the temperature of the fixing belt 75 is maintained at the fixing temperature T2 while the paper P relating to the image A passes through the fixing nip N2, the fixing belt 75 and the pressure roller 71 in the fixing nip N2 are maintained. The temperature difference TD8 is small. Therefore, in this embodiment, the temperature difference TD8 is suppressed to be lower than the upper limit temperature difference TDlim. In addition, after the rear end of the sheet P related to the image A passes through the fixing nip N2, the temperature of the fixing belt 75 rises to the fixing temperature T1 because the mode is switched to the normal mode. Note that the temperature adjustment mode may be switched before the trailing edge of the sheet P related to the image A reaches the fixing nip N2. In this case, the time Δt2 may be obtained on condition that the temperature difference TD8 does not exceed the upper limit temperature difference TDlim.

[Sixth embodiment]
A sixth embodiment will be described. Also in this embodiment, the configuration of the image forming apparatus 1 is the same as that in the above embodiment. Also in the sixth embodiment, smart fixing control is performed in the fixing processing by the fixing processing unit 70. As a result, when the paper P passes through the fixing nip N2, the temperature difference between the fixing belt 75 and the pressure roller 71 at the position of the fixing nip N2 may increase. Therefore, there is a possibility that a curl large enough to cause a jam in subsequent conveyance may occur on the rear end side of the sheet P that has passed through the fixing nip N2.

  For this reason, also in the sixth embodiment, the temperature difference for suppressing the temperature difference between the fixing belt 75 and the pressure roller 71 in the fixing nip N2 so as not to cause a large curl on the paper P that has passed through the fixing nip N2. Perform limit control. In the sixth embodiment, unlike the above embodiment, the temperature difference restriction control is performed while changing the image formation order when a plurality of images are continuously formed.

  In this embodiment, as shown in FIG. 4, a case where three image formations of an image A, an image B, and an image C are performed will be described. Also in this embodiment, the image A and the image C are monochrome images, and the image B is a color image. Therefore, by the smart fixing control, the fixing process is performed at the fixing temperature T2 in the low temperature mode for the paper P related to the images A and C, and the fixing process is performed at the fixing temperature T1 in the normal mode for the paper P related to the image B.

  Even in such a case, the print command is normally issued in the order of image A, image B, and image C as shown in FIG. That is, normally, the image A is formed on the first surface of the paper P1, and the image B is formed on the first surface of the paper P while the paper P1 passes through the duplex conveyance path 60. Further, after the image B is formed, the image C is formed on the second surface of the paper P1.

  However, in this embodiment, when the print command for the image is generated, the temperature control mode information for the image before the image related to the print command is also taken into consideration. Then, it is determined whether or not to perform image formation by switching the order in which the print command is generated between the image for generating the print command and the next image.

  This will be described when the print command for the image B is generated. First, when generating a print command for image B, is the fixing temperature in the temperature control mode of image A preceding image B and image C following image B both lower than the fixing temperature in the temperature control mode of image B? Judge whether or not. When both the fixing temperatures for the images A and C are lower than the fixing temperature for the image B, it is determined whether or not the image B is formed on the first surface of the paper P. Further, when the image B is formed on the first surface of the paper P, whether or not the image C is formed on the second surface of the paper P that has already been subjected to image formation on the first surface. Determine whether. If the image C is to be formed on the second side of the paper P, the order of the image B and the image C is changed, and after the image C is formed, the image B is formed. In the fixing process of the image C, since the fixing temperature in the temperature control mode of the image B is higher than the fixing temperature in the temperature control mode of the image C, the temperature difference restriction control is executed.

  Next, in the case where the image A and the image C are formed on the first surface and the second surface of the same sheet P, respectively, and the image B is formed on the first surface of the sheet P different from the images A and C, FIG. 12 will be described in detail. Here, the paper P on which the image A and the image C are formed is referred to as paper P1. That is, the image A is formed on the first surface of the paper P1, and the image C is formed on the second surface of the paper P1. Further, the image B is formed on the first surface of the paper P2 different from the paper P1.

  First, since the image A is the first image to be formed, a print command is generated based on the information about the images A and B. In the print command for image A, image B is in the normal mode and image A is in the low temperature mode. Therefore, as shown in FIG. 12, the temperature adjustment mode is switched from the low temperature mode to the normal mode when the time Δt has elapsed since the exposure of the image A was started. When the sheet P1 carrying the image A on the first surface passes through the fixing nip N2, the temperature difference limiting control for stopping the heating by the heater 74 is performed as in FIG. 7 according to the first embodiment. It has been broken. Therefore, the temperature difference TD9 between the fixing belt 75 and the pressure roller 71 in the fixing nip N2 at the rear end of the sheet P1 carrying the image A on the first surface is suppressed to be lower than the upper limit temperature difference TDlim. Further, the sheet P1 on which the image A is formed on the first surface by passing through the fixing nip N2 is conveyed to the duplex conveyance path 60 after the front and back are reversed. Further, after the sheet P1 with the image A formed on the first surface passes through the fixing nip N2, heating by the heater 74 that has been stopped is resumed.

  Next, the print command for the image B is generated based on the information of the image A, the image B, and the image C. Here, since the temperature control mode of the image A before the image B and the image C after the image B are both the low temperature mode, the fixing temperature is lower than the fixing temperature T1 of the image B which is the normal mode. It is temperature T2. The image B is formed on the first surface of the paper P2. Further, the image C is formed on the second surface of the paper P1 on which the image A has already been formed on the first surface. For this reason, it is determined that the order of the images B and C is changed and the image C is formed before the image B.

  In the print command of the image C, the image B after the replacement is in the normal mode, and the fixing temperature T1 is higher than the fixing temperature T2 of the image C in the low temperature mode. For this reason, as shown in FIG. 12, when the sheet P1 carrying the image C on the second surface passes through the fixing nip N2, the heating by the heater 74 is performed as in FIG. 7 according to the first embodiment. Temperature difference limit control to stop is performed. Therefore, the temperature difference TD10 between the fixing belt 75 and the pressure roller 71 in the fixing nip N2 at the rear end of the sheet P1 carrying the image C on the second surface is suppressed to be lower than the upper limit temperature difference TDlim. Further, after the sheet P1 on which the image C is formed on the second surface passes through the fixing nip N2, the heating by the heater 74 that has been stopped is resumed. Then, after the fixing belt 75 at the fixing nip N2 reaches the fixing temperature T1 in the normal mode, the sheet P2 carrying the image B on the first surface passes through the fixing nip N2.

  Therefore, also in this embodiment, curling of the paper P1 on which the images A and C are formed on both sides is reduced to suppress the occurrence of jamming, and the fixing process of the image B to the first surface of the paper P2 is improved. It can be carried out. In this embodiment, it is described that the temperature difference restriction control is performed in the same manner as in the first embodiment (FIG. 7) while changing the image formation order. However, the temperature difference restriction control similar to any one of the second to fifth embodiments (FIGS. 8 to 11) is not limited to the first embodiment.

  As described in detail above, the image forming apparatus 1 of the above embodiment performs smart fixing control. Further, while performing smart fixing control, image formation is performed so that the temperature difference between the fixing belt 75 and the pressure roller 71 in the fixing nip N2 while the paper P passes through the fixing nip N2 does not exceed the upper limit temperature difference TDlim. The temperature difference limiting control is performed. As a result, power consumption in the fixing process is reduced and curling is suppressed.

  Note that this embodiment is merely an example, and does not limit the present invention. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof. For example, the present invention is applicable not only to a color printer but also to a copying machine. In addition, for example, the image forming apparatus 1 has a preferable temperature difference from the viewpoint of productivity of image formation and power consumption in the fixing processing unit 70 among the different temperature difference limiting controls described in the first to fifth embodiments. You may select and perform restriction | limiting control. For example, in the smart fixing control, the temperature adjustment mode is not limited to the normal mode and the low temperature mode, and a mode having a higher fixing temperature than the normal mode and a mode having a fixing temperature lower than the low temperature mode may be provided.

1 ... image forming apparatus 4 ... CPU
DESCRIPTION OF SYMBOLS 10 ... Image forming part 70 ... Fixing process part 71 ... Pressure roller 72 ... Fixing roller 73 ... Heating roller 74 ... Heater 75 ... Fixing belt N2 ... Fixing nip P Paper

Claims (9)

An image forming unit that forms a toner image on the sheet by forming the toner image and transferring the toner image to the sheet;
A heating unit that is heated by a heat source, and a pressing unit that forms a nip together with the heating unit, and a fixing processing unit that performs a fixing process by passing a sheet on which a toner image is transferred through the nip;
An image forming apparatus having an apparatus control unit that controls each part of the apparatus to form an image.
A heating side temperature output unit that outputs a heating side temperature value indicating the temperature at the position of the nip of the heating unit;
A pressure side temperature output unit that outputs a pressure side temperature value indicating the temperature at the nip position of the pressure unit;
The device controller is
The fixing temperature, which is a target of the heating side temperature value when the sheet is passed through the nip portion, is set lower than when the amount of heat required for fixing the toner image transferred to the sheet by the image forming unit is small,
Before the sheet reaches the nip, smart heating control is performed to control heating by the heat source so that the heating side temperature value is equal to or higher than the fixing temperature determined for the sheet,
When the fixing temperature of the second sheet is higher than the fixing temperature of the first sheet with respect to the first sheet and the second sheet next to the first sheet continuously passed through the nip,
The temperature at which image formation is performed so that the temperature difference that is the difference between the heating-side temperature value and the pressing-side temperature value when the first sheet passes through the nip does not exceed a predetermined upper-limit temperature difference. The difference limit control is executed .
The heating side temperature output unit and the pressure side temperature output unit are the device control unit,
The device controller is
Before starting the formation of the toner image related to the first sheet by the image forming unit,
A simulation is performed on the heating side temperature value and the pressure side temperature value when the heating side temperature value is raised to the fixing temperature of the second sheet by heating the heat source while passing the first sheet through the nip,
Obtaining an estimated temperature difference that is an estimated value of the temperature difference when the trailing edge of the first sheet reaches the nip from the heating side temperature value and the pressure side temperature value obtained by the simulation;
When the estimated temperature difference obtained exceeds the upper limit temperature difference, it is determined that the temperature difference restriction control is executed,
An image forming apparatus characterized in that, when it is determined to execute the temperature difference restriction control, the temperature difference restriction control is executed at the time of image formation of the first sheet . The image forming apparatus according to claim 1 ,
The device controller is
In the simulation,
Obtaining an excess timing at which the temperature difference while the first sheet passes through the nip exceeds the upper limit temperature difference from the obtained heating side temperature value and the pressure side temperature value;
In the temperature difference limiting control,
Stop heating by the heat source before the excess timing determined in the simulation,
An image forming apparatus, wherein heating by the stopped heat source is resumed after the trailing edge of the first sheet has passed through the nip. The image forming apparatus according to claim 1 ,
In the temperature difference limiting control, the device control unit
An image forming apparatus, wherein heating by the heat source is limited so that the heating side temperature value rises more slowly than the degree of rise in the simulation. The image forming apparatus according to claim 2 or 3 ,
In the temperature difference limiting control, the device control unit
An image forming apparatus characterized in that an increase start timing for starting an increase in the heating side temperature value by heating the heat source is made earlier than an increase start timing in the simulation. The image forming apparatus according to claim 1 ,
In the temperature difference limiting control, the device control unit
Before the leading edge of the first sheet reaches the image forming portion, and after the trailing edge of the first sheet passes through the image forming portion, until the leading edge of the first sheet reaches the nip. An image forming apparatus characterized in that at least one of the first sheet conveyance speeds is slower than the first sheet conveyance speed in the simulation. The image forming apparatus according to claim 1 ,
In the temperature difference limiting control, the device control unit
An image forming apparatus characterized in that an increase start timing for starting an increase in the heating side temperature value by heating the heat source is delayed from an increase start timing in the simulation. The image forming apparatus according to any one of claims 1 to 6 ,
A reversing portion for reversing the front and back of the sheet on which the image is formed on the first surface through the nip;
In order to form an image on the second surface behind the first surface of the sheet that has passed through the reversing unit, the sheet has a double-sided conveyance path that conveys the sheet again to a position upstream of the image forming unit,
The device controller is
An image characterized in that the temperature difference restriction control is performed at the time of fixing processing to the first surface in duplex printing in which images are formed in this order on the first surface and the second surface of the first sheet. Forming equipment. The image forming apparatus according to any one of claims 1 to 6 ,
A reversing portion for reversing the front and back of the sheet on which the image is formed on the first surface through the nip;
In order to form an image on the second surface behind the first surface of the sheet that has passed through the reversing unit, the sheet has a double-sided conveyance path that conveys the sheet again to a position upstream of the image forming unit,
The device controller is
When the fixing temperature of the second sheet is lower than the fixing temperature of the first sheet,
The fixing temperature of the sheet passing through the nip before the first sheet is lower than the fixing temperature of the first sheet;
When the first sheet has an image formed on the first side and the second sheet has passed the double-sided conveyance path and the image is formed on the second side,
An image forming apparatus, wherein the temperature difference restriction control is executed at the time of image formation on the second sheet while image formation on the second sheet is performed before image formation on the first sheet. The image forming apparatus according to any one of claims 1 to 8,
Relationship between at least one condition selected from the group consisting of the thickness of the first sheet, the length of the first sheet in the conveyance direction, and the speed of the first sheet passing through the nip, and the upper limit temperature difference A storage unit storing an upper limit temperature difference table,
In the temperature difference limiting control, the apparatus control unit uses a value determined with reference to the upper limit temperature difference table as the upper limit temperature difference.

Images