JP6454903B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an improvement in sheet separation at a toner fixing unit in an image forming apparatus, particularly an electrophotographic image forming apparatus such as a printer or a copying machine.

  In an electrophotographic image forming apparatus such as a copying machine or a printer, an electrostatic latent image corresponding to an original is formed on a photosensitive member, and the electrostatic latent image is visualized by applying toner. The imaged toner image is transferred and fixed on the recording paper and then discharged. For monochrome images, each process is executed by a single image forming unit. For color images, Y, M, C, and K toner images respectively formed on four photoconductors arranged side by side are intermediate. After the primary transfer (synthesis) on the transfer body (belt), the secondary transfer is performed on the recording paper and fixed.

  As a fixing device for fixing a toner image on a recording paper, a fixing roller having an endlessly stretched fixing belt between a heating roller having a built-in heat source and a fixing roller, and a pressure roller for pressing the fixing roller through the fixing belt. It is known (hereinafter referred to as a fixing belt tension type). In this fixing device, the toner image is heated and fixed by nipping and conveying the recording paper onto which the toner image has been transferred at the nip formed by the fixing belt and the pressure roller, while being heated and pressed. .

  In the fixing device, from the viewpoint of energy saving, it is desirable to lower the fixing temperature. For this reason, it is conceivable to increase the width of the nip portion in the paper direction to increase the heating time. In order to increase the nip width, at least one of increasing the thickness of the rubber fixing roller body or pressure roller body, decreasing the rubber hardness, or increasing the diameter is adopted for the fixing belt stretcher type. Has been.

  However, when the thickness of the roller body is increased or the rubber hardness is lowered, the durability of the rubber is lowered, so that the life of the fixing device is shortened. Further, when the thickness of the roller main body is increased, the heat capacity increases, so that the warm-up time becomes longer. Further, increasing the diameter of the roller leads to an increase in parts cost and an increase in the size of the fixing device.

  On the other hand, in a fixing device with a fixing belt stretched, a fixing pad is used instead of the fixing roller, and the pressure roller is pressed against the fixing pad via the fixing belt. In such a fixing device, since the fixing nip portion is determined by the shape of the fixing pad, by devising the shape of the fixing pad, the nip width can be reduced without significantly changing the thickness, diameter and rubber hardness of the pressure roller. Can be increased. As a result, the fixing temperature can be set low, and energy saving is realized.

  By the way, in the fixing device, since the unfixed toner on the recording paper is heated when passing through the fixing nip portion, the heated toner acts as an adhesive and the fixing belt even after the recording material passes through the nip portion. Therefore, the belt is wound around the fixing belt without being separated from the fixing belt, and jamming occurs (separation failure). In particular, when a small amount of paper (thin paper) is used as the recording paper, when the usage environment is high humidity, when the image coverage (the ratio of the image to the entire paper surface) is large, the fixing device itself is close to a new one. When the smoothness of the surface of the fixing belt is high, the separation performance is further deteriorated.

  Conventionally, in order to solve the separation failure, a plurality of claw members are arranged on the exit side of the fixing nip portion, and the tips of the claw members are inserted into the gap between the fixing belt and the recording paper, thereby mechanically. A method of separating the recording paper from the fixing belt has been adopted. However, such a separation claw mechanism adds cost because it adds parts. In addition, since it is desirable that there is a slight gap between the tip of the separation claw and the surface of the fixing belt or the separation claw is in light contact with the fixing belt, the position management of the separation claw at the time of manufacturing the fixing device is desirable. Adjustment is difficult, and the fixing belt may be damaged by the separation claw.

  Patent Document 1 describes an image forming apparatus including a mechanism for reducing the radius of curvature on the exit side of the fixing nip portion by switching the tension of the fixing belt. However, in this mechanism, since the nip portion is formed by holding the fixing belt with a roller, there is a problem that when the radius of curvature is reduced, the nip width is narrowed so that the fixing performance is deteriorated. Yes.

  On the other hand, Patent Document 2 describes a fixing belt tension type fixing device using a fixing pad. Here, the radius of curvature of the fixing belt is reduced by forming the fixing pad so as to protrude toward the pressure roller on the exit side of the nip portion. However, this fixing device has a problem in that the fixing belt is largely bent at the exit side of the nip portion, so that the load on the fixing belt is increased and the durability of the belt is lowered. Further, the fixing belt is configured to slide with respect to the fixing pad via a lubricant. From the viewpoint of durability, it is desirable that the pressure acting on the fixing pad is small. Therefore, it is desirable to set the tension of the fixing belt small. However, if the tension is set to be small, the radius of curvature of the fixing belt becomes larger due to the elasticity of the belt itself further downstream in the sheet passing direction from the exit of the nip portion, and the effect of improving the separation performance is reduced. On the other hand, when the tension of the fixing belt is increased, a state in which the radius of curvature is small is maintained, but the contact pressure between the fixing belt and the fixing pad is increased, which impairs durability. That is, when the tension of the fixing belt is increased, the separation performance is improved, but the durability is lowered. When the tension is lowered, the durability is improved, but the separation performance is lowered.

JP 2006-235573 A JP 2014-6317 A

  An object of the present invention is to provide an image forming apparatus provided with a fixing belt stretcher type fixing device capable of improving the separation performance of a recording material without reducing the durability of the fixing belt or the like as much as possible.

An image forming apparatus according to an aspect of the present invention is
And at least one holding roller, a pad member, a fixing belt stretched endlessly on the holding roller and the pad member, and a pressure roller that presses the pad member via the fixing belt. The nip portion formed between the fixing belt held by the pad member and the pressure roller is heated to a predetermined fixing temperature, and the recording material is passed through the nip portion so that the recording material is placed on the recording material. It has a fixing device that heats and fixes unfixed toner,
The fixing device is provided on the pad member, and is capable of projecting to a contact surface side of the fixing belt with respect to the pressure roller on the downstream side of the nip portion in the recording material passing direction, and a projecting amount of the projecting portion A protrusion amount switching means for switching the tension, and a tension switching means for switching the tension of the fixing belt,
Furthermore, a separation property detecting means for detecting information relating to the separation property from the nip portion of the recording material at the time of image printing, and a control means,
The control means controls the protrusion amount switching means and the tension switching means to switch the protrusion amount and the tension based on the information on the separation property by the separation property detection means;
It is characterized by.

  In the image forming apparatus, since the tension of the fixing belt and the protrusion amount of the pad member on the downstream side in the sheet passing direction of the nip portion are switched based on the separation property of the recording material, the separation property is lowered. The separation performance with respect to the recording material is improved, and the deterioration of the durability of the fixing belt and the like is suppressed.

  According to the present invention, it is possible to improve the separation performance of the recording material without reducing the durability of the fixing belt or the like as much as possible.

1 is a schematic configuration diagram illustrating a color image forming apparatus according to an embodiment. 2A and 2B are cross-sectional views illustrating a fixing device, in which FIG. 1A illustrates a case where separability is normal, and FIG. 2B illustrates a case where separability decreases. FIG. 2 is a plan view of the fixing device. FIG. 2 is a block diagram illustrating a control unit of the image forming apparatus. 2A and 2B are cross-sectional views illustrating a state of a lubricant in the fixing device, in which FIG. 1A illustrates a case where separability is normal, and FIG. 2B illustrates a case where separability decreases.

  Embodiments of an image forming apparatus according to the present invention will be described below with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the same member and part in each figure, and the overlapping description is abbreviate | omitted.

(Outline of color image forming apparatus, see FIG. 1)
First, a schematic configuration of an electrophotographic color image forming apparatus 1 according to an embodiment will be described with reference to FIG. This image forming apparatus 1 is provided with an image reading unit 80 in the upper part, and forms a color image by a tandem method. That is, four image forming units 10 (an image forming unit 10y for forming a yellow image, an image forming unit 10m for forming a magenta image, an image forming unit 10c for forming a cyan image, and a black image are formed. Image forming units 10k) are juxtaposed, and the toner images of the respective colors formed on the respective photoconductors 11 are transferred / synthesized by the electric field applied from the transfer roller 32 onto the intermediate transfer belt 31 (primary transfer). To do. Thereafter, the composite toner image is secondarily transferred onto the recording material by the electric field applied from the transfer roller 35.

  Around each photoconductor 11, a charging roller 12, a developing device 13 including a developing roller 13a, a cleaning blade 14 for residual toner, and the like, and a laser scanning optical unit 20 are further arranged. The process of forming a toner image on the photoconductor 11 by electrophotographic method with this type of image forming unit 10, performing primary transfer onto the intermediate transfer belt 31, and then performing secondary transfer onto the recording material is well known and explained. Is omitted.

  The recording materials are stacked and accommodated in a paper feed cassette 50 and are fed one by one by a paper feed roller 51. The fed recording material is conveyed to the secondary transfer section via the registration roller pair 52, and the toner image is secondarily transferred. Thereafter, the recording material is heated and fixed with toner by the fixing device 60, and is discharged from the discharge roller pair 56 to the discharge portion 57.

(Fixing device, see FIGS. 2 and 3)
2A, the fixing device 60 includes at least one holding roller 61, a pad member 62, a fixing belt 63 stretched endlessly on the holding roller 61 and the pad member 62, and a fixing belt. And a pressure roller 64 that presses the pad member 62 through 63, and the holding roller 61 has a heat source 65 incorporated therein. In this fixing device 60, a nip portion formed between the fixing belt 63 held by the pad member 62 and the pressure roller 64 is heated to a predetermined fixing temperature, and the recording material S is placed in the nip portion. By passing in the direction of arrow B, the unfixed toner on the recording material S is heated and fixed.

  The holding roller 61 is heated by the heat source 65, and the fixing belt 63 is heated at the contact portion with the holding roller 61 while rotating in the arrow D direction. When the heated portion reaches the nip portion, the pad member 62 and the pressure roller 64 are heated, and the nip portion is held at a necessary fixing temperature.

  Further, the fixing device 60 is provided on the pad member 62, and can protrude toward the contact surface side of the fixing belt 63 with respect to the pressure roller 64 on the downstream side in the recording material passage direction B of the nip portion. The projection amount switching means 67 for switching the projection amount 66 and the tension switching means 68 for switching the tension of the fixing belt 63 are provided.

  Hereinafter, the configuration, material, and size of the fixing belt 63 and the rollers 61 and 64 will be described in detail, but the present invention is not limited thereto. The fixing belt 63 is formed by coating the outer peripheral surface of a heat-resistant substrate such as a polyimide resin having a thickness of 70 μm with a heat-resistant silicon rubber having a thickness of 200 μm, and a perfluoroalkoxy tube having a thickness of 30 μm as a release layer on the outer peripheral surface. The one coated with is used. The outer diameter of the fixing belt 63 is 80 mm, and the width is 340 mm (see FIG. 3).

  The holding roller 61 is a cylindrical hollow rotating body made of aluminum and having a thickness of 2 mm, and is configured as a hard roller whose outer peripheral surface is covered with a heat-resistant resin tube, and has an outer diameter of 47 mm. A halogen lamp is used as the heat source 65. Note that the heat source 65 does not necessarily have to be built in the holding roller 61, and an induction heating heating element using a heater or an excitation coil that directly heats the fixing belt 63 from the outside may be used. Further, a temperature sensor (not shown) for detecting the surface temperature of the fixing belt 63 is provided.

  The pad member 62 has a contact surface that forms a nip portion facing the pressure roller 64, is made of a heat-resistant resin substrate such as a liquid crystal polymer or polyphenylene sulfide, and the nip portion (contact surface) has a thickness of 100 μm. It is coated with fluoroethylene.

  The protrusion 66 is provided so as to be movable in a direction perpendicular to the sheet passing direction B so as to be able to protrude toward the contact surface side of the fixing belt 63 with respect to the pressure roller 64 on the downstream side in the sheet passing direction B of the nip portion. ing. The protrusion amount switching means 67 is composed of an actuator 67a that presses behind the protrusion 66, a cam 67b, and a drive source (not shown). The drive source is a combination of a solenoid, a motor and a clutch, and the protrusion 66 is moved forward and backward with respect to the nip portion via the actuator 67a by changing the rotation angle of the cam 67b. Thereby, the protrusion amount of the protrusion part 66 is switched. FIG. 2A shows the position of the protrusion 66 where the separation of the recording paper S is normal, and FIG. 2B shows the position when the separation is lowered (detailed later). Yes.

  The pad member 62 is 30 mm in the height direction (paper passing direction B), and the width dimension is 370 mm (see FIG. 3). As shown in FIG. 3, the protrusion 66 is set to have a width dimension larger than that of the pad member 62. By setting the width dimension of the protruding portion 66 to be larger than that of the fixing belt 63 and the pad member 62, a portion protruding outside the fixing belt 63 and the pad member 62 is pressed by the actuator 67 a to advance and retract the protruding portion 66. .

  A lubricant 70 (see FIG. 5) is applied between the pad member 62 and the fixing belt 63. As the lubricant 70, dimethyl silicone oil or fluorine grease can be preferably used, and a small amount of oil film is formed between the pad member 62 and the fixing belt 63, or the inner peripheral surface of the fixing belt 63. Is held in. The lubricant 70 enters the contact surface of the pad member 62 little by little as the fixing belt 63 rotates, so that the lubricant 70 is held as an oil film between the fixing belt 63 and the pad member 62.

  The pressure roller 64 is a soft roller in which a metal cored bar 64a is covered with a heat-resistant silicone rubber 64b having a thickness of 3 mm, and an outer peripheral surface of the silicone rubber 64b is covered with a perfluoroalkoxy tube 64c having a thickness of 30 μm. The outer diameter is 50 mm.

  The tension switching means 68 of the fixing belt 63 includes a tension spring 68a. That is, the support shaft (not shown) of the holding roller 61 is arranged to be movable in the direction of arrow E in FIG. The front end of the tension spring 68a is locked to the support shaft, and the rear end is connected to a drive source. The drive source is a solenoid, a motor, or the like, and the tension of the fixing belt 63 is switched by changing the urging force applied to the support shaft by the tension spring 68a.

  If the holding roller 61 is disposed slightly below the pad member 62, the fixing belt 63 can be pulled more on the exit side in the sheet passing direction of the nip portion, and the radius of curvature of the fixing belt 63 on the exit side can be reduced. It is preferable in terms of performance.

  In the fixing device 60 having the above configuration, when the pressure roller 64 is rotationally driven in the direction of arrow C by a driving means (not shown), the fixing belt 63 and the holding roller 61 are rotated in the direction of arrow D by the respective frictional forces. The fixing belt 63 is indirectly heated by the heat source 65. The pressure roller 64 is elastically pressed against the pad member 62 via the fixing belt 63 by an urging means (not shown). As a result, the silicon rubber 64b and the tube 64c of the pressure roller 64 are partially bent inward to form a fixing nip portion. The recording material S passes through the nip portion in the direction of arrow B with the toner image forming surface facing the pad member 62, whereby the toner is heated and fixed.

(Control unit, see FIG. 4)
With respect to the fixing device 60, the image forming apparatus 1 includes a separation property detecting unit that detects separation property of the recording material from the fixing nip portion during image printing, and a control unit. The control means controls the protrusion amount switching means 67 and the tension switching means 68 based on the information on the separation property by the separation property detection means to switch the protrusion amount of the protrusion 66 and the tension of the tension spring 68a.

  Specifically, as shown in FIG. 4, various types of information related to separability from the separability detection unit are input to the CPU 101 constituting the control unit 100. The information relating to the separability includes the type of recording material (whether plain paper or thin paper) from the recording material information unit 111, the durability state of the fixing device 60 from the durability state information unit 112, and the print from the image pattern information unit 113. Image pattern coverage, and environmental humidity from the use environment information unit 114. There are various types of information related to separability other than these. Further, the CPU 101 controls the protrusion amount switching means 67 and the tension switching means 68.

From the recording material information section 111, the amount and type of recording material are input. Since the recording material has a smaller amount of wrinkle and is thinner, the separation property is worse. Therefore, for example, the CPU 101 determines that the separation amount is poor when the amount of wrinkle is 60 g / m ² or less. Also, it is determined that a paper having a smooth surface such as coated paper has poor separability. From the durability state information section 112, the usage period of the fixing belt 63 (value converted to the number of prints) is input. Since the surface of the fixing belt 63 is smoother as the usage period is shorter, the CPU 101 determines that the separation property is poor when the number of printed sheets is within 1000 from a new one.

  The print pattern coverage is input from the image pattern information unit 113. As the image pattern having a higher density is arranged closer to the front end in the sheet passing direction B, the separability becomes worse. Therefore, the CPU 101 determines that the separability is poor when the image coverage of 80% or more exists in an area within 3 mm from the front end in the sheet passing direction B. From the use environment information unit 114, the humidity around the fixing device 60 is input. When the humidity increases, the recording material becomes weaker and the separability deteriorates. Therefore, the CPU 101 determines that the separability is poor when the humidity is 70% or more.

(Separability, protrusion amount and tension setting)
The protruding amount of the protruding portion 66 is set in a range that does not cause great damage to the fixing belt 63 due to bending in a normal case where separation is not disadvantageous. In this embodiment, when the separability is normal, the protrusion 66 is set at a position where the tip protrudes 0.2 mm from the contact surface of the pad member 62. The protruding amount may be zero or negative. When the separability is lowered, the protrusion 66 is set at a position where the tip protrudes 0.5 mm from the contact surface of the pad member.

  The tension of the tension spring 68a is set to an upper limit within a range in which the pressure between the pad member 62 and the fixing belt 63 does not adversely affect the durability due to friction between the two, and the adhesion between the fixing belt 63 and the holding roller 61 is set. The lower limit is set within a range where the state does not deteriorate and the heating of the fixing belt 63 is not adversely affected. In this embodiment, the tension is set to 40N when the separability is normal, and is set to 60N when the separability is lowered.

  When the CPU 101 determines that the separability is disadvantageous by combining one or more pieces of information from the information units 111 to 114, the CPU 101 controls the protruding amount switching unit 67 and the tension switching unit 68 to project the protruding unit 66. The amount is increased and the tension of the tension spring 68a is increased. FIG. 2B shows this state. That is, when the protrusion amount of the protrusion 66 is set to be large, the protrusion amount of the fixing belt 63 on the downstream side in the sheet passing direction is increased, and when the tension of the fixing belt 63 is set to be large, the fixing bell 63 and 63 at the outlet side of the nip portion. Is bent more greatly (the radius of curvature becomes smaller), so that the separability is improved by the stiffness of the recording material.

  Incidentally, when the separability is lowered, the protrusion amount of the protrusion 66 is set in a range that does not cause damage such as bending of the fixing belt 63 due to bending in a short-term use, and the tension of the tension spring 68a is set to the nip. The bulge due to the elasticity of the fixing belt 63 on the exit side of the unit is set to be reduced or eliminated. As a result, when the separability decreases, the radius of curvature on the exit side of the nip portion of the fixing belt 63 becomes smaller than when the separability is normal, and the separability is improved.

  On the other hand, as the radius of curvature of the fixing belt 63 decreases, the damage to the fixing belt 63 increases. Further, as the tension increases, the contact pressure between the pad member 62 and the fixing belt 63 increases, and the oil film of the lubricant interposed between the two becomes thinner and the wear is promoted. Therefore, it is not desirable from the viewpoint of durability to operate the fixing device 60 for a long time in a state where the separability is improved. In the present embodiment, only when the separability is unfavorable, the protrusion amount and the tension are switched to operate, thereby making it possible to achieve both improvement in separability and maintenance of durability.

(Supply of lubricant, see Fig. 5)
As described above, the lubricant 70 is interposed between the pad member 62 and the fixing belt 63, and both wear due to friction is suppressed. However, the wear of the pad member 62 and the fixing belt 63 does not proceed uniformly as a whole, but a portion where the contact pressure is large is locally formed, and the oil film of the lubricant 70 becomes thin at that portion, thereby promoting wear. .

  When the projection amount and tension are set normally, as shown in FIG. 5A, the projection amount of the projection portion 66 is zero or small, so the contact pressure P is the end of the pad member 62, that is, the inlet side of the nip portion. The distribution becomes higher on the exit side. In a portion where the contact pressure P on the inlet side or outlet side of the nip portion is high, the amount of the lubricant 70 held is small and wear is promoted, which is disadvantageous in terms of durability. Most of the lubricant 70 is held immediately before the fixing belt 63 is wound around the pad member 62, or is held on the inner surface of the fixing belt 63 that does not face the pad member 62. By entering the surface little by little, an oil film is formed between the pad member 62 and the fixing belt 63. However, in the normal setting, since the tension of the fixing belt 63 is small and the radius of curvature is large, the angle α between the pad member 62 and the fixing belt 63 at the intrusion portion is large, and the lubricant 70 is applied to the pad member 62. The amount of intrusion is relatively small. Further, since the contact pressure P on the inlet side of the nip portion is high, the amount of oil film retained is also reduced, and the lubricant 70 tends to be insufficient.

  On the other hand, when the separability decreases and the protruding amount and tension are set large, the contact pressure P increases at the protruding portion 66 as shown in FIG. 5B, and the inlet side or outlet side of the nip portion. The distribution decreased by. As the contact pressure P on the inlet side or outlet side of the nip portion decreases, the amount of the lubricant 70 held in this portion increases. Further, as the tension of the fixing belt 63 increases, the angle α between the pad member 62 and the fixing belt 63 at the intrusion portion decreases, and the amount of the lubricant 70 entering the pad member 62 due to a so-called wedge effect. Will increase. As a result, the penetration amount and the retention amount of the lubricant 70 are increased at the nip portion, particularly at the inlet side of the nip portion, and the formation state of the oil film is improved. After the printing process is performed under conditions where the separability is unfavorable, when the unfavorable conditions are resolved and the normal setting is restored, the contact pressure P becomes high, so the oil film of the lubricant 70 gradually becomes thinner. However, the lubricant 70 is temporarily increased during operation in a state where the separability is disadvantageous, so that the lubricant 70 is replaced, and waste due to scraping of the belt 63 and the pad member 62 is reduced. That is, since the separability is disadvantageous, the effect of improving the oil film state of the lubricant 70 is continued by appropriately performing the operation with the protruding amount and the tension increased.

  Therefore, driving the fixing device 60 under the condition that the separation property is disadvantageous causes the durability of the pad member 62 and the fixing belt 63 to deteriorate when the driving is performed for a long period of time. However, the short-term driving by switching to increase the protrusion amount and the tension supplies the lubricant 70 to the disadvantageous part of the pad member 62 compared to the case of driving only in the normal state. As a result of the increase, the effect of suppressing the wear of the pad member 62 and the fixing belt 63 is produced.

  Therefore, even if the protrusion amount and tension are not switched within a predetermined driving period, the oil film improvement effect (wear suppression effect) by the supply of the lubricant 70 as described above is anticipated and constant. It is preferable that the projection amount and the tension be switched while the fixing device 60 is operating at the interval of. For example, every time the number of printed sheets reaches 10,000, the state is switched to a state where the separation property is disadvantageous for about one minute. In this case, it is preferable to drive the fixing device 60 in a heated state during the switching operation.

(Other examples)
The image forming apparatus according to the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the gist thereof.

  In particular, the image forming apparatus may be a multifunction machine having a communication function, a facsimile function, and the like in addition to a printer having only a print function. Further, the detailed configuration of the fixing device or the like is arbitrary.

  As described above, the present invention is useful for an image forming apparatus, and is particularly excellent in that the separation performance of a recording material is improved without reducing the durability of a fixing belt or the like as much as possible.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 60 ... Fixing apparatus 61 ... Holding roller 62 ... Pad member 63 ... Fixing belt 64 ... Pressure roller 65 ... Heat source 66 ... Protruding part 67 ... Projection amount switching means 68 ... Tension switching means 101 ... CPU (control means) )
111 ... Recording material information part 112 ... Durability state information part 113 ... Image pattern information part 114 ... Usage environment information part

Claims (3)

And at least one holding roller, a pad member, a fixing belt stretched endlessly on the holding roller and the pad member, and a pressure roller that presses the pad member via the fixing belt. The nip portion formed between the fixing belt held by the pad member and the pressure roller is heated to a predetermined fixing temperature, and the recording material is passed through the nip portion so that the recording material is placed on the recording material. It has a fixing device that heats and fixes unfixed toner,
The fixing device is provided on the pad member, and is capable of projecting to a contact surface side of the fixing belt with respect to the pressure roller on the downstream side of the nip portion in the recording material passing direction, and a projecting amount of the projecting portion A protrusion amount switching means for switching the tension, and a tension switching means for switching the tension of the fixing belt,
Furthermore, a separation property detecting means for detecting information relating to the separation property from the nip portion of the recording material at the time of image printing, and a control means,
The control means switches the protrusion amount and the tension by controlling the protrusion amount switching means and the tension switching means based on the information on the separation property by the separation property detection means ,
When the control means determines a decrease in separability based on the information of the separability detection means, the control means increases the protrusion amount and the tension .
An image forming apparatus. And at least one holding roller, a pad member, a fixing belt stretched endlessly on the holding roller and the pad member, and a pressure roller that presses the pad member via the fixing belt. The nip portion formed between the fixing belt held by the pad member and the pressure roller is heated to a predetermined fixing temperature, and the recording material is passed through the nip portion so that the recording material is placed on the recording material. It has a fixing device that heats and fixes unfixed toner,
The fixing device is provided on the pad member, and is capable of projecting to a contact surface side of the fixing belt with respect to the pressure roller on the downstream side of the nip portion in the recording material passing direction, and a projecting amount of the projecting portion A protrusion amount switching means for switching the tension, and a tension switching means for switching the tension of the fixing belt,
Furthermore, a separation property detecting means for detecting information relating to the separation property from the nip portion of the recording material at the time of image printing, and a control means,
The control means switches the protrusion amount and the tension by controlling the protrusion amount switching means and the tension switching means based on the information on the separation property by the separation property detection means ,
The control unit is configured so that the fixing device is operating when the projection amount and the tension are not switched based on information from the separability detection unit within a predetermined driving period. Switching the amount of protrusion and the tension ,
An image forming apparatus. 2. The control unit according to claim 1, wherein the control unit determines the separability based on at least one piece of information among a recording material type, a use environment, an image pattern to be printed, and a durability state of the fixing device. Item 3. The image forming apparatus according to Item 2.

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