JP2016109787A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent ABARA wrinkles from being generated in a sheet during continuous print processing.SOLUTION: An image formation device includes: an image formation unit for forming an image based on job data; a fixation unit having a heating member and a compression member to fix an image formed on a sheet to be transported onto the sheet; a temperature measurement unit for measuring the temperature of a passage-of-sheets region and a non passage-of-sheets region of the heating member or the compression member; and a controller for controlling productivity. The controller discriminates an image to be fixed on the sheet and calculates difference in temperature between the passage-of-sheets region and the non passage-of-sheets region of the heating member or the compression member based on the temperature measured by the temperature measurement unit to switch productivity according to the discriminated image and the difference in temperature.SELECTED DRAWING: Figure 6

Description

  The present invention relates to an image forming apparatus including a fixing device.

  An electrophotographic image forming apparatus includes a fixing device that melts and fixes a toner image onto a sheet (transfer material). For example, a heat roller type fixing device has a heating roller as a heating member and a pressure roller as a pressure member, and sandwiches and conveys a sheet at a pressure contact portion (fixing nip portion) between the heating roller and the pressure roller. As a result, the unfixed toner image on the paper is heated to fix the toner image on the paper.

  Inside the heating roller, a heat source such as a halogen lamp is provided along the axial direction, and the heating roller is heated from the inside by radiant heat from the heat source. In recent years, the core of the heating roller has been thinned in order to shorten the warming time for heating until the temperature of the heating roller reaches a predetermined temperature suitable for fixing and to save energy.

  As a result of the thinning of the core of the heating roller, the amount of heat transferred in the axial direction decreases, making it difficult to make the temperature distribution in the axial direction of the heating roller uniform. In other words, heat is consumed by heating the paper in the area of the heating roller where the paper passes (paper passing area), but in the non-paper passing area outside the paper passing area, the paper is deprived of heat. There is no heat accumulated. As a result, the temperature of the non-sheet passing region of the heating roller and the pressure roller in pressure contact with the heating roller becomes higher than a predetermined temperature.

  As a countermeasure against this, Patent Document 1 discloses a heating member, a pressure member disposed in pressure contact with the heating member, and at least one of the heating member and the pressure member, and the heat of the contacted portion. There is disclosed a fixing device having a heat uniformizing member for homogenizing. The fixing device is provided with a cleaning member for cleaning the outer peripheral surface of the heat uniformizing member.

  Further, Patent Document 2 discloses a heating rotator, a pressurizing rotator, an auxiliary heating rotator that has a heat source disposed therein and is in contact with the surface of the heating rotator or the pressurizing rotator, and an auxiliary heating rotator. An image recording apparatus having a fixing device that includes a scraping member for scraping off foreign matter adhering to the outer surface of the auxiliary heating rotator in contact with the surface of the recording medium and fixing a recording material on which a toner image is mounted by heating and pressing. Has been.

JP 2004-163719 A JP 2004-020803 A

  However, in the fixing device as described above, when images are continuously formed on a plurality of sheets (hereinafter referred to as “continuous printing process”), the temperature of the non-sheet passing region of the heating member and the pressure member rises. . As a result, the end portion of the pressure member having the elastic layer formed on the surface layer expands, and the sheet conveyance speed (linear velocity) increases at the end portion of the pressure member. That is, there is a difference in the sheet conveyance speed between the central portion and the end portion of the heating member and the pressure member, so that the end portion of the sheet is fed faster than the central portion. In particular, distortion of the sheet conveyance speed accumulates on the rear end side (hereinafter referred to as “second half portion”) of large size paper (eg, A3 size or more) whose stiffness in the sheet conveyance direction is weak, and the latter half of the sheet in the sheet conveyance direction. I am prone to wrinkles. This wrinkle is formed at the both end sides of the sheet in the latter half of the sheet by rapidly feeding and extending both ends to deform the sheet in a wave shape, and the deformed portion is pushed by the fixing nip portion. Such wrinkles are called “Avala wrinkles”. Depending on the state of the image formed on the paper, the occurrence of wrinkles becomes more significant.

FIG. 10 shows an example of image patterns and loose wrinkles that tend to cause loose wrinkles.
As shown in FIG. 10, the loose wrinkles 202 and 203 are generated in the latter half of the large-size paper 200 having a low rigidity in the paper conveyance direction. The applicant of the present application is an image 201 in which the toner gradually decreases from the first half portion to the second half portion in the paper conveyance direction of the paper, and the toner at the front and back end portions decreases with respect to the central portion in the paper width direction. It has been discovered that when such an image is formed on a sheet, the wrinkles 202 and 203 are more likely to occur.

  In the fixing device described in Patent Document 1, by disposing a cleaning member that cleans the outer peripheral surface of the heat uniformizing member, a decrease in the ability to uniformize the temperature distribution of the heating member of the heat uniformizing member is prevented. However, it is insufficient as a countermeasure against loose wrinkles. In the image forming apparatus described in Patent Document 2, the fixing property is improved by the auxiliary heating rotator, but no countermeasure against wrinkles is taken.

  From the above situation, there has been a demand for a method for preventing the occurrence of wrinkles on the paper during the continuous printing process.

An image forming apparatus according to one embodiment of the present invention includes an image forming unit that forms an image based on job data, a heating member, and a pressure member, and fixes the image formed on the conveyed sheet to the sheet. A fixing unit that controls the temperature, a temperature measuring unit that measures the temperature of the sheet passing area and the non-sheet passing area of the heating member or the pressure member, and the productivity expressed as the number of pages or the number of sheets output in a certain time. And a control unit.
The control unit determines an image fixed on the paper and calculates a temperature difference between the paper passing region and the non-paper passing region of the heating member or the pressure member based on the temperature measured by the temperature measuring unit, Productivity is switched according to the determined image and the temperature difference.

  According to at least the image forming apparatus of one embodiment of the present invention, the determination result regarding the image formed on the sheet and the sheet passing condition (temperature difference between the sheet passing region and the non-sheet passing region of the heating member or the pressure member) are used. Accordingly, the occurrence frequency of the loose wrinkles can be predicted in advance. By feeding back the information on the occurrence frequency of the loose wrinkles to the productivity control, it is possible to suppress the occurrence of defects when the fixing unit nipping and conveying the sheet.

1 is an overall configuration diagram illustrating an example of an image forming apparatus to which the present invention is applied. FIG. 2 is a plan view illustrating a configuration example of a fixing unit in FIG. 1. It is a figure with which it uses for description of the mechanism in which a loose wrinkle occurs. It is the graph which showed the temperature difference of the center part and edge part of a heating member for every productivity. 1 is a block diagram illustrating a hardware configuration example of each unit of an image forming apparatus to which the present invention is applied. FIG. 2 is a block diagram illustrating a function example of a control unit provided in the image forming apparatus of FIG. 1. 3 is a plan view illustrating an example in which an image forming surface of a sheet is divided into a plurality of regions. FIG. It is an example of the condition table which shows the combination of an image pattern and productivity. It is explanatory drawing which shows the other example of the image pattern which a wrinkle tends to generate | occur | produce. It is explanatory drawing of an image pattern and a loose wrinkle which a wrinkle tends to generate | occur | produce.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the following description and each drawing, the same elements or elements having the same functions are denoted by the same reference numerals, and redundant descriptions are omitted.

<1. First Embodiment>
[Configuration example of image forming apparatus]
First, an outline of an image forming apparatus to which the present invention is applied will be described with reference to FIG.
FIG. 1 is an overall configuration diagram showing an example of an image forming apparatus to which the present invention is applied.

  As shown in FIG. 1, the image forming apparatus 1 forms an image on a sheet by an electrophotographic method, and has four colors of yellow (Y), magenta (M), cyan (C), and black (Bk). This is a tandem color image forming apparatus that superimposes toner. The image forming apparatus 1 includes a document transport unit 10, a paper storage unit 20, an image reading unit 30, an image forming unit 40, an intermediate transfer belt 50, a secondary transfer unit 70, and a fixing unit 80. .

  The document transport unit 10 includes a document feed table 11 on which a document G is set, a plurality of rollers 12, a transport drum 13, a transport guide 14, a document discharge roller 15, and a document discharge tray 16. Yes. The documents G set on the document feeder 11 are conveyed one by one to the reading position of the image reading unit 30 by the plurality of rollers 12 and the conveying drum 13. The conveyance guide 14 and the document discharge roller 15 discharge the document G conveyed by the plurality of rollers 12 and the conveyance drum 13 to the document discharge tray 16.

  The image reading unit 30 reads an image of the document G transported by the document transport unit 10 or the document placed on the document table 31 and generates image data. Specifically, the image of the original G is irradiated by the lamp L. The reflected light from the document G is guided in the order of the first mirror unit 32, the second mirror unit 33, and the lens unit 34 and forms an image on the light receiving surface of the image sensor 35. The image sensor 35 photoelectrically converts incident light and outputs a predetermined image signal. The output image signal is created as image data by A / D conversion.

  The image reading unit 30 has an image reading control unit 36. The image reading control unit 36 performs processing such as shading correction, dither processing, and compression on the image data created by the A / D conversion, and stores it in the RAM 103 (see FIG. 5) of the control unit 100. The image data is not limited to data output from the image reading unit 30 and may be data received from an external device such as a personal computer connected to the image forming apparatus 1 or another image forming apparatus.

  The paper storage unit 20 is disposed at the lower part of the apparatus main body, and a plurality of paper storage units 20 are provided according to the size and type of paper. The sheet is fed by the sheet feeding unit 21 and sent to the transport unit 23, and is transported by the transport unit 23 to the secondary transfer unit 70 having a transfer position. That is, the transport unit 23 functions to transport the paper fed from the paper feed unit 21 to the secondary transfer unit 70 and forms a transport path for transporting the paper. Further, a manual feed portion 22 is provided in the vicinity of the paper storage portion 20. From the manual feed section 22, paper of a size not stored in the paper storage section 20, tag paper having a tag, special paper such as an OHP sheet is sent to the transfer position. In FIG. 1, a sheet S fed by the sheet feeding unit 21 is denoted by a symbol “S”.

  An image forming unit 40 and an intermediate transfer belt 50 are disposed between the image reading unit 30 and the paper storage unit 20. The image forming unit 40 includes four image forming units 40Y, 40M, 40C, and 40K in order to form toner images of respective colors of yellow (Y), magenta (M), cyan (C), and black (Bk). .

  The first image forming unit 40Y forms a yellow toner image, and the second image forming unit 40M forms a magenta toner image. The third image forming unit 40C forms a cyan toner image, and the fourth image forming unit 40K forms a black toner image. Since these four image forming units 40Y, 40M, 40C, and 40K have the same configuration, only the first image forming unit 40Y will be described here.

  The first image forming unit 40Y includes a drum-shaped photoconductor 41, a charging unit 42 arranged around the photoconductor 41, an exposure unit 43, a developing unit 44, and a cleaning unit 45. The photoreceptor 41 is rotated by a drive motor (not shown). The charging unit 42 applies a charge to the photoconductor 41 and uniformly charges the surface of the photoconductor 41. The exposure unit 43 performs exposure scanning on the surface of the photoconductor 41 based on the image data generated by the image reading unit 30 or the image data transmitted from the external device, thereby electrostatic latent images on the photoconductor 41. Form an image.

  The developing unit 44 attaches yellow toner to the electrostatic latent image formed on the photoconductor 41 using, for example, a two-component developer composed of toner and carrier. As a result, a yellow toner image is formed on the surface of the photoreceptor 41.

  The developing unit 44 of the second image forming unit 40M attaches magenta toner to the photoconductor 41, and the developing unit 44 of the third image forming unit 40C attaches cyan toner to the photoconductor 41. Then, the developing unit 44 of the fourth image forming unit 40K adheres black toner to the photoconductor 41.

The toner image formed on the photoreceptor 41 is transferred to an intermediate transfer belt 50 that is an example of an image carrier. The intermediate transfer belt 50 is formed in an endless shape and is stretched around a plurality of rollers. The intermediate transfer belt 50 is rotationally driven in a direction opposite to the rotation (movement) direction of the photoconductor 41 by a drive motor (not shown).
The cleaning unit 45 removes the toner remaining on the surface of the photoreceptor 41 after the toner image is transferred to the intermediate transfer belt 50.

  A primary transfer portion 51 is provided at a position on the intermediate transfer belt 50 that faces the photoreceptor 41 of each of the image forming units 40Y, 40M, 40C, and 40K. The primary transfer unit 51 primarily transfers the toner image formed on the photoreceptor 41 to the intermediate transfer belt 50 by applying a voltage having a polarity opposite to that of the toner to the intermediate transfer belt 50.

  When the intermediate transfer belt 50 is driven to rotate, the toner images formed by the four image forming units 40Y, 40M, 40C, and 40K are sequentially transferred onto the surface of the intermediate transfer belt 50. As a result, yellow, magenta, cyan, and black toner images are superimposed on the intermediate transfer belt 50 to form a color toner image.

  A belt cleaning device 53 faces the intermediate transfer belt 50. The belt cleaning device 53 cleans the surface of the intermediate transfer belt 50 that has finished transferring the toner image onto the paper.

  A secondary transfer unit 70 is disposed in the vicinity of the intermediate transfer belt 50 and downstream of the transport unit 23 in the sheet transport direction. The secondary transfer unit 70 brings the conveyed paper into contact with the intermediate transfer belt 50 and secondarily transfers the toner image formed on the outer peripheral surface of the intermediate transfer belt 50 onto the paper. The secondary transfer unit 70 has a secondary transfer roller 61. The secondary transfer roller 61 is pressed against the opposing roller 52 with the intermediate transfer belt 50 interposed therebetween. A portion where the secondary transfer roller 61 and the intermediate transfer belt 50 come into contact becomes a secondary transfer nip portion. The secondary transfer nip portion is a transfer position at which the toner image formed on the outer peripheral surface of the intermediate transfer belt 50 is transferred to the paper S.

  A fixing unit 80 is provided on the paper discharge side of the secondary transfer unit 70. The fixing unit 80 pressurizes and heats the paper to fix the transferred toner image on the paper. The fixing unit 80 includes, for example, a heating roller 81 (an example of a heating member) and a pressure roller 82 (an example of a pressure member) that are a pair of fixing members. The heating roller 81 and the pressure roller 82 are arranged in pressure contact with each other, and a fixing nip portion N is formed as a pressure contact portion at a position where the heating roller 81 and pressure roller 82 are in contact with each other. Further, an external heating roller 83 (an example of an external heating member) for heating the pressure roller is provided at a position in contact with the outer periphery of the pressure roller 82.

  A heating unit is provided inside the heating roller 81.

  The sheet is conveyed so that the surface (fixed target surface) onto which the toner image is transferred by the secondary transfer unit 70 faces the heating roller 81, and passes through the fixing nip N. Therefore, the sheet passing through the fixing nip N is pressed by the heating roller 81 and the pressure roller 82 and heated by the heat of the heating roller 81.

  A non-contact type temperature sensor 111 (an example of a temperature measuring unit) is disposed in the vicinity of the central portion (paper passing region) in the axial direction of the heating roller 81. Based on the temperature detected by the temperature sensor 111, the temperature of the heating roller 81 is adjusted by the control unit 100 (see FIG. 5). In addition, a non-contact temperature sensor 112 (an example of a temperature measurement unit) is disposed in the vicinity of the end of the heating roller 81 on the back side in the axial direction (outside the paper passing area). The temperatures detected by the temperature sensors 111 and 112 are used for controlling the productivity of the fixing unit 80 by the control unit 100. The temperature sensor 112 may be a contact type temperature sensor.

  A switching gate 24 is disposed downstream of the fixing unit 80 in the sheet conveyance direction. The switching gate 24 switches the conveyance path of the sheet that has passed through the fixing unit 80. That is, the switching gate 24 moves the paper straight when performing face-up paper discharge in which the image forming surface is discharged upward in single-sided image formation. As a result, the paper is discharged by the pair of paper discharge rollers 25. Further, the switching gate 24 guides the sheet downward when performing face-down paper discharge and double-sided image formation in which the image forming surface in single-sided image formation is discharged downward.

When face-down paper discharge is performed, after the sheet is guided downward by the switching gate 24, the sheet reverse transport unit 26 reverses the front and back and transports the sheet upward. As a result, the sheet whose front and back sides are reversed and whose image forming surface faces downward is discharged by the pair of discharge rollers 25.
When performing double-sided image formation, after the sheet is guided downward by the switching gate 24, the front and back are reversed by the sheet reversing conveyance unit 26, and sent again to the transfer position of the secondary transfer unit 70 by the refeed path 27.

  A post-processing device that folds the paper or performs stapling processing or the like on the paper may be disposed on the downstream side of the pair of paper discharge rollers 25.

[Configuration of fixing unit]
FIG. 2 shows a configuration example of the fixing unit 80 of the image forming apparatus 1.
As the fixing unit 80 shown in FIG. 2, for example, a fixing device described in Japanese Patent Application Laid-Open No. 2004-20803 (FIG. 2) can be used.

  The fixing unit 80 includes a heating roller 81 that includes a heat source Ha such as a halogen lamp and is a heating rotator. The rotation shaft of the heating roller 81 is rotatably held in a housing (not shown) of the fixing unit 80. The outer peripheral portion of the heating roller 81 is warmed by the radiant heat from the heat source Ha. Then, the heat of the heating roller 81 is transmitted to the paper S, so that the toner image on the paper S is thermally fixed.

  The fixing unit 80 includes a pressure roller 82 that is a pressure rotator facing the heating roller 81. The pressure roller 82 is driven to rotate along with the rotation of the heating roller 81 while pressing the heating roller 81 by urging an elastic member such as a spring (not shown).

  The heating roller 81 sandwiches the paper S between the pressure roller 82 and presses the paper S, conveys the paper S, heats and presses the unfixed toner image transferred onto the paper S, and forms a toner image on the paper S. Let it settle.

  A web W for cleaning the surface of the heating roller 81 is in contact with the outer periphery of the heating roller 81. A back-up roller 84 that presses the heating roller 81 through the web W is disposed on the back surface of the surface of the web W that contacts the surface of the heating roller 81. In the vicinity of the backup roller 84, a web take-up roller 85 and a web feed roller 86 are disposed.

  An external heating roller 83 that has a heat source Hb inside and is an auxiliary heating rotator is disposed at a position in contact with the outer periphery of the pressure roller 82. The external heating roller 83 is rotatably held by a housing (not shown) of the fixing unit 80 and is urged in a direction in which it is pressed against the outer periphery of the pressure roller 82 by an elastic member such as a spring (not shown). Followed by rotation. The external heating roller 83 is formed using a metal such as aluminum, and has a function of making the temperature distribution of the pressure roller 82 uniform in addition to the function of heating the pressure roller 82.

  Further, a scraping unit KB and a temperature detection sensor ST are provided in the vicinity of the outer periphery of the external heating roller 83 disposed so as to be in contact with the outer periphery of the pressure roller 82. The scraping unit KB is arranged so that the tip of the scraping member KB1 bites into the outer periphery of the external heating roller 83. Further, the temperature detection sensor ST is disposed in the housing of the fixing unit 80. The scraping unit KB includes a scraping member KB1 and a scraping member holder KB2.

  When the sheet S on which the unfixed toner image is transferred is sandwiched between the fixing nip N where the heating roller 81 and the pressure roller 82 are in pressure contact, the toner on the sheet S is melted and fixed by heat and pressure. At this time, not all toner is fixed on the paper S, and some toner may adhere to the heating roller 81.

  Most of the toner adhering to the heating roller 81 is wiped off by the web W, but some of the toner passes through the web W and reaches the fixing nip portion N. Some of the toner that has reached the fixing nip N is transferred to the external heating roller 83 via the pressure roller 82 and adheres.

  The scraping member KB1 included in the scraping unit KB disposed on the outer periphery of the external heating roller 83 scrapes off foreign matters such as toner and paper dust attached to the outer periphery of the external heating roller 83, and the outer periphery of the external heating roller 83 and the pressure roller. 82 is removed from the outer periphery.

  On the other hand, if the external heating roller 83 is rotated when the temperature of the external heating roller 83 is low, the solidified toner is sandwiched between the external heating roller 83 and the scraping member KB1, and the surface of the external heating roller 83 is damaged. There is a fear.

  In order to prevent this, a predetermined temperature when the temperature of the external heating roller 83 decreases and the toner solidifies is input in advance to the memory of the control unit 100 (see FIG. 5) of the image forming apparatus 1. When the temperature detected by the temperature detection sensor ST reaches the predetermined temperature, a command is issued from the control unit 100 to control (block) the pressure roller 82 or the heating roller 81 so as not to rotate.

  Further, when the temperature of the external heating roller 83 detected by the temperature detection sensor ST rises and reaches a predetermined temperature at which the toner is softened, the rotation of the pressure roller 82 or the heating roller 81 is blocked by a command from the control unit 100. To release.

  That is, the temperature of the external heating roller 83 is detected by the temperature detection sensor ST provided near the outer periphery of the external heating roller 83, and the rotation of the heating roller 81 or the pressure roller 82 is stopped at a temperature at which the toner is solidified. Accordingly, the external heating roller 83 is not driven to rotate, and the surface of the external heating roller 83 can be prevented from being damaged by the solidified toner.

  An example of the specifications of the heating roller 81, the pressure roller 82, and the external heating roller 83 constituting the fixing unit 80 is shown below.

Heating roller 81: diameter 70 mm, aluminum core metal thickness 5 mm,
Surface layer PTFE (Poly Tetra Fluoro Etylene) coating / pressure roller 82: diameter 70 mm, elastic layer 6.5 mm,
Surface layer PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (fluorinated resin copolymer)) tube thickness 0.07mm
-Nip width: 11mm, load 1400N
-Paper transport speed, PPM: 460 mm / s, 100 ppm
Heater Ha: 2 (inside heating roller 81)
External heating roller 83: Diameter 30 mm, aluminum core metal thickness 2 mm,
Surface layer PTFE coating

  In the present embodiment, the fixing unit 80 includes the external heating roller 83, but the external heating roller 83 is not the essence of the present invention.

[Mechanism of generating loose wrinkles and examples of image patterns in which loose wrinkles are likely to occur]
Next, a mechanism for generating the loose wrinkles as shown in FIG. 10 will be described in detail.
FIG. 3 is a diagram for explaining the mechanism of occurrence of loose wrinkles.

  When the sheet S is continuously passed through the fixing unit 80, the temperature of the heating roller 81 outside the sheet passing area (non-sheet passing area) starts to gradually increase. When the temperature of the non-sheet passing area rises, heat is diffused in the axial direction of the heating roller 81, and the temperature of the boundary areas A1 and A2 between the sheet passing area and the non-sheet passing area starts to gradually increase.

  When the temperature of the boundary areas A1 and A2 of the heating roller 81 increases, the temperature of the boundary areas A1 and A2 of the pressure roller 82 also increases at the same time. When the temperature of the boundary areas A1 and A2 of the pressure roller 82 rises, an elastic layer (for example, a rubber material layer) formed on the surface layer of the pressure roller 82 thermally expands.

  When the elastic layer is thermally expanded, the sheet conveyance speed (linear speed) at both ends of the pressure roller 82 is increased, and finally a difference occurs between the feeding speed Vc at the center of the sheet S and the feeding speed at the end Ve. The length of the arrow indicating the feed rate represents the speed of the feed rate, with the center portion being slow and the end portion being fast. Due to this difference in feeding speed, distortion (distortion such that the end is pulled in the traveling direction with respect to the central portion) occurs on the end side of the second half portion of the sheet S.

  Further, the heating roller 81 is more slidable in the portion of the paper where the toner is formed (hereinafter referred to as “toner portion”) than the white background where no toner is formed, and the paper S is fed in the toner portion. The speed is slow. An image 201 in which the toner gradually decreases from the first half portion to the second half portion in the paper transport direction (paper passing direction), and the toner at the front and back end portions decreases with respect to the central portion in the paper width direction. In the case of a simple image pattern, since the toner at the edge of the paper is small, the difference in the feeding speed between the center and the edge of the paper S is further promoted. Further, since the toner in the latter half of the paper is less than that in the first half of the paper, the feeding speed of the latter half is faster than that of the first half of the paper, and the distortion is further increased.

  Further, the elastic layer of the pressure roller 82 expands greatly as the temperature increases. Therefore, the larger the temperature difference between the paper passing area and the non-paper passing area of the heating roller 81, the more the thermal expansion of the elastic layer of the pressure roller 82 is caused, and the difference in the feeding speed between the central portion and the edge portion of the paper S is increased. growing.

  As described above, the toner gradually decreases from the first half portion to the second half portion of the sheet S, the toner on the front side and the rear side is less than the central portion in the width direction of the sheet S, and the sheet passing through the heating roller 81 is passed. As the temperature difference between the area and the non-sheet passing area increases, distortion is accumulated in the latter half of the sheet S. As a result, loose wrinkles 202 and 203 as shown in FIG. 10 are likely to occur.

  In the present embodiment, the temperature sensors 111 and 112 are disposed in the vicinity of the central portion (paper passing region) and the end portion (non-paper passing region) of the heating roller 81 in the axial direction. You may arrange | position in the vicinity of the center part and edge part of the pressure roller 82, respectively.

[Abala wrinkle prevention measures]
Next, measures for preventing wrinkles will be described with reference to FIG.
FIG. 4 is a graph showing the characteristics of the temperature difference between the central portion and the end portion of the heating member (heating roller 81) for each productivity. In FIG. 4, the horizontal axis represents time (seconds), and the vertical axis represents the temperature difference (° C.) between the central portion and the end portion of the heating roller 81. In the low temperature and low humidity environment (LL environment), after the printing of the image forming apparatus has been warmed up, until the end of 1000 printing (forming and fixing an image on 1000 sheets), The temperature difference from the end was measured.

  Productivity is expressed as the number of pages or the number of sheets output in a certain time. PPM (Page Per Minute) is one of indexes representing productivity, and is the number of pages (or the number of sheets) output per minute. When the value of PPM is decreased, productivity is lowered. The characteristic at the time of PPM down of FIG. 4 is data when productivity is reduced to 75% of the normal.

  As shown in FIG. 4, the temperature difference between the central portion and the end portion of the heating roller 81 when the PPM is down is about 6 ° C. lower than that in the normal case. That is, by reducing the productivity, the temperature difference between the central portion (sheet passing region) and the end portion (non-sheet passing region) of the heating roller 81 during the continuous printing process is reduced. When the productivity is lowered, it is considered that the temperature difference is reduced because the amount of heat taken by the paper per unit time in the central portion is reduced.

  When the temperature difference between the paper passing area and the non-paper passing area is reduced, the temperature rise of the elastic layer in the boundary areas A1 and A2 of the pressure roller 82 is suppressed, and the central portion and the end portion of the pressure roller 82 are reduced. The difference in paper conveyance speed is reduced. Thereby, generation | occurrence | production of a loose wrinkle can be suppressed. Through such experiments and considerations, the applicant of the present invention has produced an image pattern that is likely to cause wrinkles or a temperature difference between the sheet passing area and the non-sheet passing area that exceeds a predetermined value. The inventors have come up with a configuration that reduces the property and prevents the occurrence of loose wrinkles.

  Hereinafter, a method for preventing occurrence of loose wrinkles by switching productivity according to the first embodiment will be described.

[Configuration of control system of image forming apparatus]
A control system of the image forming apparatus 1 will be described with reference to FIG.
FIG. 5 is a block diagram illustrating a hardware configuration example of each unit of the image forming apparatus 1. In this block diagram, elements necessary for explaining the present invention or related elements are described, and the image forming apparatus 1 is not limited to this example.

  As illustrated in FIG. 5, the image forming apparatus 1 includes a control unit 100. The control unit 100 is connected to the communication unit 108, the image reading unit 30, the image processing unit 110, the image forming unit 40, the paper feeding unit 21, the paper transport mechanism 109, and the fixing unit 80 via the system bus 107. . Further, the control unit 100 is connected to an HDD (Hard Disk Drive) 104, an operation display unit 105, and temperature sensors 111 and 112 via a system bus 107.

  The control unit 100 includes, for example, a CPU (Central Processing Unit) 101, a ROM (Read Only Memory) 102 for storing programs executed by the CPU 101, various data, and the like, and a RAM (Random Access) used as a work area of the CPU 101. Memory) 103. As the ROM 102, for example, an electrically erasable programmable ROM is used. The control unit 100 controls each block, that is, controls the entire apparatus.

  The HDD 104 stores image data of an original image obtained by reading by the image reading unit 30, or stores output image data and the like. The operation display unit 105 is a touch panel including a display such as a liquid crystal display (LCD) or an organic ELD (Electro Luminescence Display). The operation display unit 105 displays an instruction menu for the user, information about the acquired image data, and the like. Further, the operation display unit 105 includes a plurality of keys, receives input of various instructions, data such as characters and numbers, and outputs an input signal to the control unit 100.

  Image data generated by the image reading unit 30 and image data transmitted from a PC (personal computer) 120 which is an example of an external device connected to the image forming apparatus 1 are sent to the image processing unit 110 for image processing. Is done. The image processing unit 110 performs image processing such as shading correction, image density adjustment, and image compression on the received image data as necessary.

  The image forming unit 40 receives the image data processed by the image processing unit 110 and forms an image on the paper S based on the image data.

  The paper feeding unit 21 is driven and controlled by the control unit 100 and sends out the paper stored in the paper storage unit 20 to the transport path.

  The fixing unit 80 is driven and controlled by the control unit 100 and heats and pressurizes the sheet S while nipping and conveying the sheet S conveyed to the fixing nip N, thereby fixing the toner image on the sheet S.

  The paper transport mechanism 109 is driven and controlled by the control unit 100 and transports the paper S on which an image is formed. The transport unit 23 is a part of the paper transport mechanism 109. The sheet transport mechanism 109 includes a roller disposed between the sheet feeding unit 21 and the fixing unit 80 of the image forming apparatus 1 and a rotation driving unit (not shown) that rotates each roller. As the rotation driving unit, a combination of a motor and various mechanisms and various actuators can be employed. The control unit 100 controls the rotation of each roller of the paper transport mechanism 109 to adjust the interval (inter-paper distance) of the paper S that is continuously transported from the paper feed unit 21 to the fixing unit 80.

  The temperature sensor 111 measures the temperature of the paper passing area (see FIG. 3) of the heating roller 81, and supplies the measurement result to the control unit 100 (CPU 101) as temperature data. Further, the temperature sensor 112 measures the temperature of the non-sheet passing region (see FIG. 3) of the heating roller 81 and supplies the measurement result as temperature data to the control unit 100 (CPU 101).

  For example, the communication unit 108 receives job data transmitted from the PC 120 which is an external information processing apparatus via a communication line. The received job data is sent to the CPU 101 via the system bus 107.

  In this embodiment, an example in which a personal computer is applied as an external device has been described. However, the present invention is not limited to this, and various other devices such as a facsimile device can be applied as the external device. .

[Function of control unit]
FIG. 6 is a block diagram illustrating a functional example of the control unit 100 included in the image forming apparatus 1.
Each function is realized by the CPU 101 of the control unit 100 executing a program recorded in the ROM 102.

  As illustrated in FIG. 6, the control unit 100 includes an image determination unit 100a, a temperature difference calculation unit 100b, and a productivity determination unit 100c.

  The image determining unit 100a analyzes image data included in job data supplied from an external device such as the PC 120, and determines an image pattern. The determination result is supplied to the productivity determining unit 100c. The image data may be image data of a document image read by the image reading unit 30 or image data stored in the HDD 104. As shown in FIG. 7 to be described later, the image determination unit 100a divides the image forming surface of the paper S into at least two or more regions in the paper direction, and based on the area of the image pattern included in each region. To determine the image pattern. Further, the image determination unit 100a divides the image forming surface of the paper S into at least three regions with respect to the direction perpendicular to the paper direction, and the image pattern is based on the area of the image pattern included in each region. To determine.

  The temperature difference calculation unit 100b calculates the temperature difference between the central portion (sheet passing region) and the end portion (non-sheet passing region) of the heating roller 81 based on the temperature data supplied from the temperature sensor 111 and the temperature sensor 112. To do. Information on the calculated temperature difference is supplied to the productivity determining unit 100c.

  The productivity determining unit 100c is based on the determination result of the image pattern supplied from the image determining unit 100a and the temperature difference between the sheet passing area and the non-sheet passing area of the heating roller 81 supplied from the temperature difference calculating unit 100b. Thus, the productivity of the image forming apparatus 1 is switched. That is, the productivity determining unit 100c determines the productivity command value and outputs the productivity command value to each unit such as the paper transport mechanism 109 and the fixing unit 80. More specifically, the productivity determining unit 100c determines that the combination of the image pattern and the temperature difference between the paper passing area and the non-paper passing area of the heating roller 81 or the pressure roller 82 satisfies a predetermined condition. Control to reduce productivity.

  Further, the productivity determining unit 100c may acquire sheet information such as a sheet size from the job data, and switch the productivity by reflecting the sheet information on the predetermined condition. That is, when the combination of the image pattern, the temperature difference between the paper passing area and the non-paper passing area of the heating roller 81 or the pressure roller 82, the paper size, and the like satisfies a predetermined condition, the productivity determining unit 100c Control to reduce productivity.

  Each unit such as the paper transport mechanism 109 and the fixing unit 80 switches productivity based on the productivity command value. That is, the interval between the sheets S that are continuously passed through the fixing unit 80 is adjusted (PPM control). For example, when the productivity is reduced due to the PPM down, the paper feeding unit 21 feeds the paper S from the paper storage unit 20 with a large distance between the papers. Each unit of the image forming apparatus 1 operates in accordance with the distance between the sheets S to be conveyed. When the continuous printing process ends, the control unit 100 cancels the PPM down and restores the productivity.

[Image pattern division example]
FIG. 7 shows an example in which the image forming surface of the paper S is divided into a plurality of regions.
In the example shown in FIG. 7, the image forming surface of the paper S is divided into two regions with respect to the paper transport direction. A first half portion of the paper S in the paper conveyance direction is an area A, and a second half portion of the paper S in the paper conveyance direction is an area B. The area ratio between the region A and the region B is 1: 1. Further, the image forming surface of the paper S is divided into three regions with respect to a direction perpendicular to the paper transport direction. A central portion of the sheet S in a direction perpendicular to the sheet conveyance direction is defined as a region C, an end portion on the front side is defined as a region D, and an end portion on the heel side is defined as a region E. The area ratio of the region C, the region D, and the region E is 2: 1: 1.

  The image pattern of the image 201 will be described using regions A to E. In the image 201, the area of the image included in the region A (the integrated area of the print portion) is larger than the area of the image included in the region B. Further, the area of the image 201 included in the region C is larger than the area obtained by adding the images included in the region D and the region E. That is, as described with reference to FIG. 10, the image 201 has toner gradually decreasing from the first half portion to the second half portion in the paper conveyance direction, and the front and back end portions with respect to the central portion in the paper width direction. This means that the image pattern has less toner. An image having such an image pattern has one of the conditions for causing loose wrinkles.

  In the present embodiment, image discrimination is performed based on the difference in image area included in each region obtained by dividing the image forming surface of the paper S with respect to the paper transport direction and / or the direction perpendicular to the paper transport direction. Used for productivity control. Hereinafter, image discrimination will be described using the printing rate. In general, the printing rate is expressed as a percentage of the integrated area of the printed portion with respect to the entire area of the printed paper. The integrated area of the print portion is obtained as the print portion area (or the number of dots of the ON signal of the image data of the page) converted from the number of dots of the ON signal of the image data output to a certain page.

FIG. 8 is an example of a condition table showing combinations of image patterns and productivity.
In the condition table shown in FIG. 8, “A−B difference” is a print rate [%] calculated based on the area of the image included in the area A and a print calculated based on the image included in the area B. It represents the difference from the rate [%]. That is, “A−B difference” is a difference between the printing rate of the region A and the printing rate of the region B. Further, “C− (D + E) difference” represents the difference between the printing rate of the region C and the printing rate of the region D and the region E.

  For example, when the printing rate of the portion (solid image) related to the region C in the region A of FIG. 7 is 100%, the portion related to the region D and the region E of the region A (the area is half of the portion related to the region C). ) Will be 50% respectively. If these are recalculated based on the area A, that is, half the area of the paper S, the printing ratios of the areas A, D, and E in the area A are 50%, 12.5%, and 12.5%, respectively. It becomes.

  On the other hand, the printing rate of the portion related to the region C in the region B is 50%, and the printing rate of the portion related to the region D and the region E (area is half of the portion related to the region C) of the region B is 0%. It is. When these are recalculated based on the area B, that is, half the area of the paper S, the print ratios of the areas B, C, D, and E are 25%, 0%, and 0%, respectively. Therefore, the “A−B difference” in this case is 75−25 = 50 [%], and the “C− (D + E) difference” is 75− (12.5 + 12.5) = 50 [%]. Become.

  The “temperature difference between the central portion and the end portion” is a temperature difference between the paper passing area and the non-paper passing area of the heating roller 81 calculated from the temperature data of the temperature sensors 111 and 112. “B4” is the paper size, and the size is 257 × 364 mm.

  Furthermore, “PPM control” is to change the productivity by adjusting the distance between sheets of continuously passing paper. When the normal productivity is 100%, 75% means that the productivity is reduced to 75%. In this case, assuming that the distance from the front edge of the normal paper to the front edge of the next continuous paper is 100%, in the case of 75% productivity, the distance from the front edge of the preceding paper to the front edge of the next paper is 75. %. In the condition table of FIG. 8, the down width when the PPM is down is set to three stages of large, medium, and small.

  The condition table shown in FIG. 8 is stored in advance in the ROM 102 or the HDD 104. When the combination of the image pattern of the image formed on the sheet and the temperature difference between the sheet passing area and the non-sheet passing area of the heating roller 81 satisfies the condition registered in the condition table, the control unit 100 Control to switch to the productivity set for.

  In the example of FIG. 8, when (1) “paper size” is less than B4, PPM control, that is, control for switching productivity is not performed (maintaining productivity). (2) Even if the “paper size” is B4 or more, the “difference between AB” and the “difference between C− (D + E)” is less than 30% and the “temperature difference between the central portion and the end portion”. When the temperature is lower than 15 ° C., the PPM control is not performed. Furthermore, (3) even if the “paper size” is B4 or more and the “A−B difference” is 30% or more, the “C− (D + E) difference” is less than 30% and When the “temperature difference” is less than 15 ° C., the PPM control is not performed.

  Further, according to the condition table of FIG. 8, PPM control is performed even when the “temperature difference between the central portion and the end portion” is less than 15 ° C. That is, (4) When “paper size” is B4 or more and “temperature difference between center and edge” is less than 15 ° C., “difference between AB” and “difference between C− (D + E)” is 30. If it is equal to or greater than%, control is performed with the PPM down width as medium (decrease in productivity). (5) When the “paper size” is B4 or more and the “temperature difference between the central portion and the end portion” is less than 15 ° C., the “A−B difference” is less than 30% and “C− (D + E)” When the “difference” is 30% or more, control for reducing the PPM down width is performed.

  Further, according to the condition table of FIG. 8, when the “temperature difference between the central portion and the end portion” is 15 ° C. or more, the PPM control is always performed. That is, (6) When the “paper size” is B4 or more and the “temperature difference between the center and the end” is 15 ° C. or more, the “difference between AB” and the “difference between C− (D + E)” are 30 If it is greater than or equal to%, control is performed to increase the PPM down width. (7) When the “paper size” is B4 or more and the “temperature difference between the central portion and the end portion” is 15 ° C. or more, the “A−B difference” is less than 30% and “C− (D + E)” When the “difference” is 30% or more, the control with the PPM down width set as medium is performed. Further, (8) When “paper size” is B4 or more and “temperature difference between the central portion and the end portion” is 15 ° C. or more, “A−B difference” and “C− (D + E) difference” are 30. Even if it is less than%, control for reducing the PPM down width is performed.

  In the example of FIG. 7, the image forming surface of the paper S is divided into six (two divisions in the paper conveyance direction and three divisions in the direction perpendicular to the paper conveyance direction), but may be further subdivided. Even when the image forming surface of the paper S is subdivided, the concept of the difference in the printing ratio calculated based on the area of the image included in each region (the integrated area of the printing portion) is the same as in the case of FIGS. It is. In other words, the plurality of areas subdivided with respect to the paper transport direction are divided into a first half portion and a second half portion of the paper S in the paper transport direction. At this time, the ratio of the area corresponding to the first half of the sheet S to the area corresponding to the second half is 1: 1. Then, the difference between the print rate of the areas corresponding to the first half portion of the paper S and the print ratio of the areas corresponding to the second half portion is calculated.

  The same applies to the case where the image forming surface of the paper S is subdivided in the direction perpendicular to the paper transport direction. A plurality of regions are divided into a central portion and an end portion in the width direction of the paper S. At this time, the ratio of the area corresponding to the central portion and the area corresponding to the end portion is 1: 1. Then, the difference between the print rate of the areas corresponding to the central portion of the paper S and the print ratio of the areas corresponding to the end portions is calculated.

  Further, in the condition table of FIG. 8, the PPM down width in the PPM control is three steps, but may be one step or more than three steps. For example, when the PPM down width in the PPM control is one step, for example, the PPM down width is set to 75% as shown in FIG. It is desirable that the PPM down width is appropriately set according to the actual occurrence frequency of the loose wrinkles.

  Further, in the condition table of FIG. 8, the threshold value for the difference in the area (printing rate) of the image included in each region is set to 30%, but is not limited to this example. Moreover, although the threshold value with respect to the temperature difference of the center part and the edge part of the heating roller 81 is 15 degreeC, it is not limited to this example. Furthermore, although the paper size threshold is set to B4, it is not limited to this example. These threshold values are desirably changed as appropriate according to the actual occurrence frequency of the loose wrinkles.

  The data used by the image discrimination unit 100a to discriminate the image is the area of the image included in each area obtained by dividing the image forming surface of the paper, and therefore there is no distinction between photographs and characters. Further, in addition to the image area (printing rate), the toner density may be taken into account to determine the image. An index that takes into account the printing rate and toner density is called coverage. In general, coverage refers to the amount of toner used [%] per sheet (the cumulative total when there are a plurality of toner colors). In this embodiment, the coverage is used as the amount of toner used per fixed area. That is, a difference in coverage of each area obtained by dividing the image forming surface of the paper is calculated, and an image is determined based on the calculation result. For example, when the entire image forming surface is filled with black solid, the black toner coverage is 100%.

(Another example of an image pattern that tends to cause loose wrinkles)
FIG. 9 shows another example of an image pattern in which loose wrinkles are likely to occur.
Regions A to E on the image forming surface of the paper S shown in FIG. 9 are the same as the regions A to E in FIG. In FIG. 9, elliptic images 210a and 210b are formed on the paper S in the paper transport direction. The image 210 a is formed over the areas C, D, and E in the area A that is the first half of the paper S. Further, the image 210b is formed in a region B which is the latter half of the sheet S and a region C which is the central portion, and the area thereof is much smaller than the area of the image 210a. Therefore, in the image pattern composed of the image 210a and the image 210b, the toner gradually decreases from the first half portion to the second half portion in the paper conveyance direction, and at the front and back end portions with respect to the central portion in the paper width direction. This corresponds to an image pattern with less toner.

  According to the first embodiment configured as described above, according to the image determination result by the image determination unit 100a and the sheet passing condition (temperature difference between the sheet passing area and the non-sheet passing area of the heating roller 81). The frequency of occurrence of loose wrinkles (fixing defects) can be predicted in advance. By feeding back the information on the occurrence frequency of the loose wrinkles to the productivity control, it is possible to suppress the occurrence of defects when the fixing unit 80 holds and conveys the sheet.

  In addition, by reflecting the paper size on the productivity control as one of the conditions that the wrinkle is likely to occur, when the large size paper that is likely to generate the wrinkle is passed, Occurrence is suppressed.

<2. Second Embodiment>
Next, a second embodiment of the image forming apparatus to which the present invention is applied will be described focusing on differences from the first embodiment.

  As mentioned above. The fixing unit 80 includes an external heating roller 83 (see FIG. 2). When the external heating roller 83 is in the off state, the temperature difference between the heating roller 81 and the pressure roller 82 is widened, and the temperature distribution of the pressure roller 82 is less uniform. Therefore, as compared with the case where the external heating roller 83 is in the ON state, the wrinkles of the paper S are more likely to occur. Therefore, when the external heating roller 83 is in the OFF state, the threshold value for the difference in image area included in each region of the condition table in FIG. 8 is lowered from 30% to 20%. By setting the threshold value strictly in this way, it is possible to prevent occurrence of loose wrinkles in an environment where loose wrinkles are likely to occur.

  The threshold value may be changed by the control unit 100 detecting that the external heating roller 83 is in an off state and automatically lowering the threshold value in the condition table. The threshold value can be arbitrarily set, and changing from 30% to 20% is an example.

  Further, the control unit 100 may turn on the external heating roller 83 in parallel with lowering the threshold value for the difference in image area included in each region of the condition table. When the temperature of the external heating roller 83 measured using the temperature detection sensor ST (FIG. 2) reaches the set temperature, the control unit 100 sets a threshold value for the difference in image area included in each region of the condition table. You may make it perform control which returns to the original value (for example, 30%).

  The embodiment to which the invention made by the present inventor is applied has been described above. However, the present invention is not limited by the description and drawings which form part of the disclosure of the invention according to the above-described embodiments, and various modifications may be made without departing from the spirit of the invention described in the claims. Is possible.

  In the first and second embodiments described above, the configuration is shown in which the productivity is controlled in consideration of the paper size included in the paper information as shown in FIG. 8, but the present invention is not limited to the paper size. For example, the control unit 100 may control productivity based on information such as the paper type, eye direction, paper passing direction, and paper thickness of the paper. The eye direction is the direction of the flow of fibers constituting the paper, and the paper type is the paper type. There is a difference in ease of occurrence of wrinkles depending on the paper type, eye direction, paper passing direction, paper thickness, and the like. Since the image forming apparatus 1 cannot determine the paper type, eye direction, paper thickness, and the like, the user operates the operation display unit 105 to input the information. Reflecting these pieces of paper information in the productivity control enables more accurate control to be executed, and the occurrence of defects in the fixing unit 80 is further suppressed.

  In the first and second embodiments described above, the productivity is changed by the PPM control. However, the productivity may be changed by controlling the sheet conveyance speed.

  In the first and second embodiments described above, an example in which the present invention is applied to an image forming apparatus that forms a color image has been described. However, the present invention may be applied to an image forming apparatus that forms a monochrome image.

  In the above-described embodiment, the heat roller type fixing unit has been described. However, the present invention is also applicable to an induction heating type fixing unit. The material of the heating roller used in the induction heating method is made of a ferromagnetic material such as carbon steel or nickel alloy so that a large eddy current (magnetic flux density) is generated by the exciting coil. These have lower thermal conductivity than an aluminum alloy that has been generally used as a material for a heating roller including a halogen lamp or the like. For this reason, when a sheet having a small width is passed, the temperature rise in the non-sheet passing portion region becomes more significant than when the aluminum alloy heating roller is used.

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 21 ... Paper feed part 40 ... Image forming part 80 ... Fixing part 81 ... Heating roller 82 ... Pressure roller 83 ... External heating roller 100 ... Control part 100a ... Image discrimination | determination part 100b ... temperature difference calculation unit, 100c ... productivity determination unit, 101 ... CPU, 102 ... ROM, 103 ... RAM, 109 ... paper transport mechanism, 106 ... image processing unit, 111, 112 ... temperature sensor, S ... paper

Claims (8)

An image forming unit that forms an image based on job data;
A fixing unit that includes a heating member and a pressure member, and fixes the image formed on the conveyed paper to the paper;
A temperature measuring unit that measures the temperature of the sheet passing region of the heating member or the pressure member and the non-sheet passing region where the sheet does not pass;
A control unit that controls productivity expressed as the number of pages or the number of sheets output in a certain period of time,
The control unit includes an image determination unit that determines the image fixed on the sheet;
A temperature difference calculation unit that calculates a temperature difference between a paper passing region and a non-paper passing region of the heating member or the pressure member based on the temperature measured by the temperature measuring unit;
An image forming apparatus, comprising: a productivity determining unit that determines the productivity according to a determination result of the image by the image determining unit and the temperature difference calculated by the temperature difference calculating unit. The image discriminating unit divides the image forming surface of the paper into at least two regions with respect to the paper conveyance direction, and discriminates the image based on the area of the image included in each of the divided regions. The image forming apparatus according to 1. The image determination unit divides the image forming surface of the paper into at least three regions with respect to a direction perpendicular to the paper conveyance direction, and the image is determined based on the area of the image included in each of the divided regions. The image forming apparatus according to claim 1. The image discriminating unit divides the image forming surface of the paper into at least two regions with respect to the paper transport direction, and the image determining surface of the paper is at least 3 with respect to a direction perpendicular to the paper transport direction. The image forming apparatus according to claim 1, wherein the image is discriminated based on an area of an image included in each of the divided areas. The image discriminating unit is based on a difference in area of the image included in each region when the image forming surface of the paper is divided with respect to the paper transport direction and / or a direction perpendicular to the paper transport direction. The image forming apparatus according to claim 2, wherein the image is discriminated. The productivity determining unit reduces the productivity when a combination of the image and a temperature difference between the paper passing area and the non-paper passing area of the heating member or the pressure member satisfies a predetermined condition. The image forming apparatus according to claim 1. The image forming apparatus according to claim 6, wherein the productivity determining unit further includes that the size of the sheet is equal to or larger than a predetermined size as the predetermined condition. An external heating member that contacts the outer periphery of the pressure member and heats the pressure member;
The control unit is included in each region obtained by dividing the image forming surface of the sheet, which is used when the productivity determining unit determines whether a combination of the image and the temperature difference satisfies the predetermined condition. The threshold value with respect to the difference of the area of the said image to be set is set, and when the said pressurizing member is not heated by the said external heating member, the said threshold value is lowered | hung. Image forming apparatus.

Images