JP5137621B2 - Image heating device - Google Patents

Description

  The present invention relates to an image heating apparatus suitable for use as an image heating and fixing apparatus mounted on an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer.

  As an image heating fixing device (fixing device) mounted on an image forming apparatus such as an electrophotographic copying machine or a printer, there is a belt type.

  The belt-type fixing device uses an endless belt to form a nip portion for heating while nipping and transporting a recording material such as an unfixed toner image or a recording material such as an OHP sheet. The width of the nip portion can be increased in the recording material conveyance direction. Therefore, even if the recording material conveyance speed is increased, the unfixed toner image can be heated for a sufficient time, and the printing speed can be increased.

  As a belt-type fixing device, a configuration in which a belt and a roller are combined and a configuration in which two belts are combined are proposed.

  As an example of a configuration in which a belt and a roller are combined, there is a fixing device disclosed in Patent Document 1. This is because a belt is wound around a fixing roller that has a heat source on the inside and is rotatable, and a nip portion is formed by contacting the belt with the fixing roller by a pressure member provided inside the belt. It is formed. Then, the recording material carrying the unfixed toner image is nipped and conveyed at the nip portion, and the unfixed toner image is fixed on the recording material.

  On the other hand, as an example of a configuration in which a belt and a belt are combined, there is a fixing device disclosed in Patent Document 2. The nip portion is formed by a fixing belt that is stretched between a driving roller and a driven roller, and a pressure belt that is stretched between a pair of rollers that press the respective rollers via the belt. The recording material carrying the unfixed toner image is nipped and conveyed at the nip portion, and the unfixed toner image is fixed on the recording material.

  Patent Document 3 discloses a fixing device in which a member having a function of pressing the belt from the inside of the belt is provided in the belt in order to reduce a pressure drop in the middle of the nip portion formed by contacting the belts. ing. This has two belts, a belt guide for guiding the inner circumference of each belt, and a roller disposed in each belt. Then, the rollers and the belt guides are pressed through the belt and the two belts are brought into contact with each other to form a nip portion. In this fixing device, it is possible to prevent the pressure at the center portion of the nip that pressurizes the belt guides from being relatively lowered as compared to the nip end portion that pressurizes the rollers.

As an invention for preventing the disturbance of the toner image caused by the water vapor generated from the recording material staying in the middle of the nip, a fixing device disclosed in Patent Document 4 can be cited. This is a heating plate that is provided inside the fixing belt and that contacts the inner peripheral surface of the fixing belt to heat the fixing belt, and contacts the outer peripheral surface of the fixing belt at a position facing the heating plate to form a nip portion. A pressing belt. The base material of the pressing belt is formed so that Young's modulus E (kgf / mm ² ) and thickness t (mm) satisfy E × t ³ ≧ 0.6. In this fixing device, in addition to a plurality of pressing members provided inside the pressing belt, the pressing belt is formed so as to satisfy a predetermined rigidity, so that the recording material is prevented from floating from the fixing belt. It is said that the occurrence of disturbance can be prevented.
JP 2001-356625 A JP-A-5-127551 JP 2005-173058 A JP-A-2005-300590

  The belt type fixing device is desired to suppress the floating of the recording material from the belt in order to prevent the unfixed toner image carried on the recording material from being disturbed.

  An object of the present invention is to make it possible to suppress the floating of a recording material from an endless belt in an image heating apparatus that forms a nip portion using two endless belts.

The configuration for achieving the above-described object has two endless belts and means for heating at least one endless belt of the two endless belts, and the outer peripheral surfaces of the two endless belts abut against each other. In the image heating apparatus that forms a nip portion and heats the recording material carrying a toner image at the nip portion while being nipped and conveyed,
The nip portion includes a pre-nip portion formed by regions of an endless belt that is not backed up by a pressure member, and a region of one endless belt that is backed up by the pressure member, in contact with the other endless belt. The two endless belts so as to form the pressure nip portion starting from the pre-nip portion and continuously downstream in the recording material conveyance direction. The Young's modulus E (kgf / mm ² ) and thickness t (mm) of at least one endless belt base layer, and the radius of curvature R (mm) before contact of the endless belt region corresponding to the pre-nip portion It is characterized by the relationship Et ³ /R≧0.05.

Further, the Young's modulus E (kgf / mm ² ) and thickness t (mm) of the at least one endless belt base layer, and the curvature radius R (mm) before contact of the endless belt region corresponding to the prenip portion are Et ^3. More preferably, the relationship is /R≧0.17.

  According to the present invention, in an image heating apparatus that forms a nip portion using two endless belts, it is possible to suppress the recording material from floating from the endless belt.

  The present invention will be described with reference to the drawings.

[Example 1]
(1) Example of Image Forming Apparatus FIG. 17 is a structural model diagram of an example of an image forming apparatus in which the image heating apparatus according to the present invention can be mounted as an image heating fixing apparatus. This image forming apparatus is a laser beam printer that forms an image on a recording material (for example, a recording material, an OHP sheet, etc.) using an electrophotographic image forming system.

  An image forming apparatus A shown in this embodiment includes a drum-type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 101 as an image carrier. The photosensitive drum 101 is rotatably supported by an image forming apparatus main body B that constitutes a casing of the image forming apparatus A, and is rotationally driven in a direction of an arrow at a predetermined process speed by a driving unit (not shown). Around the photosensitive drum 101, along the rotation direction, a charging roller (charging means) 102, a laser exposure device (exposure means) 103, a developing device (developing means) 105, a transfer roller (transfer means) 106, and a cleaning device. (Cleaning means) 107 are arranged in that order.

  During the rotation operation, the outer peripheral surface (surface) of the photosensitive drum 101 is uniformly charged to a predetermined potential and polarity by the charging roller 102. Then, the surface of the photosensitive drum 101 is scanned and exposed from the laser exposure device 103 to the laser L based on target image information through the mirror 104 and the like. As a result, the charge at the exposed portion is removed, and an electrostatic latent image (electrostatic image) corresponding to the image information is formed on the surface of the photosensitive drum 101. The electrostatic latent image is developed using toner (developer) by a developing device 105 having a developing roller 105a. In other words, the developing device 105 applies a developing bias to the developing roller 105 a and attaches toner to the electrostatic latent image on the surface of the photosensitive drum 101. Thereby, the electrostatic latent image is visualized (visualized) as a toner image (developed image).

  On the other hand, the recording material P is fed from the feeding cassette 108 by the feeding roller 109 at a predetermined timing, and the recording material P is transferred to the transfer nip Tn between the photosensitive drum 101 and the transfer roller 106 by the transport roller 110. Be transported. The recording material P is nipped and conveyed by the transfer nip Tn, and a transfer bias is applied to the transfer roller 106 in the conveyance process. As a result, the toner images on the surface of the photosensitive drum 101 are sequentially transferred onto the recording material P.

  The recording material P carrying the unfixed toner image at the transfer nip Tn is separated from the surface of the photosensitive drum 101 and conveyed to the image heating and fixing device 112 along the conveyance guide 111. The fixing device 112 applies heat and pressure to the unfixed toner image on the recording material P to heat and fix the unfixed toner image on the recording material P. The recording material P that has exited the fixing device 112 is transported to the discharge roller 114 by the transport roller 113, and is discharged to the discharge tray 115 on the apparatus main body B by the discharge roller 114.

  The surface of the photosensitive drum 101 after the transfer of the toner image is subjected to removal of adhering matter such as transfer residual toner by a cleaning blade 107a of the cleaning device 107, and used for the next image formation.

(2) Fixing Device In the following description, with respect to the fixing device or the members constituting the fixing device, the longitudinal direction is a direction orthogonal to the recording material conveyance direction on the surface of the recording material. The short side direction is a direction parallel to the recording material conveyance direction on the surface of the recording material. The width is a dimension in the short direction.

  FIG. 1 is a schematic cross-sectional view of an example of the fixing device 112. 2 is a cross-sectional view of the fixing device 112 taken along the line II-II shown in FIG. 3 is a cross-sectional view of the fixing device 112 taken along line III-III shown in FIG. 4 is a cross-sectional view of the fixing device 112 taken along line IV-IV shown in FIG.

  The fixing device 112 shown in this embodiment includes a fixing belt 11 and a pressure belt 12 as endless belts, a fixing roller 13 and a pressure roller 14 as pressure members, and a heating roller 16 and a tension roller 17 as rotating bodies. And having.

  The fixing device 112 includes a halogen heater 15 as a heating means (heating means) and a temperature detection element 18 such as a thermistor as a temperature detection means. Further, the fixing device 112 includes first frames 31L and 31R as support members for supporting the fixing roller 13, and second frames 33L and 33R as support members for supporting the pressure roller 14 (FIG. 2). . The fixing device 112 includes third frames 35L and 35R as support members for supporting the heating roller 16, and fourth frames 37L and 37R as support members for supporting the tension roller 17 (FIG. 4).

  The fixing belt 11 includes the fixing belt 11, the fixing roller 13, the heating roller 16, the heater 15, the temperature detection element 18, the first frames 31L and 31R that support the fixing roller, and the third frames 35L and 35R that support the heating roller. Unit U1 is configured.

  The pressure belt unit U2 is constituted by the pressure belt 12, the pressure roller 14, the tension roller 17, the second frames 33L and 33R for supporting the pressure roller 14 and the fourth frames 37L and 37R for supporting the tension roller. It is composed.

  In the fixing device 112 of this embodiment, in the fixing belt unit U1, the fixing roller 13 and the heating roller 16 are provided inside the fixing belt 11 arranged along the longitudinal direction of the fixing device 112, and the fixing roller 13 and the heating roller 16 are provided. Thus, the fixing belt 11 is supported.

  Further, in the pressure belt unit U 2, a pressure roller 14 and a tension roller 17 are provided inside the pressure belt 12 disposed along the longitudinal direction of the fixing device 112, and the pressure roller 14 and the tension roller 17 apply pressure. The pressure belt 12 is supported.

  The layer configuration of the fixing belt 11 and the pressure belt 12 will be described with reference to FIG.

  FIG. 5A is a cross-sectional view illustrating an example of a layer configuration of the fixing belt 11, and FIG. 5B is a cross-sectional view illustrating an example of a layer configuration of the pressure belt 12.

  Each of the fixing belt 11 and the pressure belt 12 has endless base layers 11a and 12a on the inner side, elastic layers 11b and 12b on outer peripheries of the base layers 11a and 12a, and outer peripheries of the elastic layers 11b and 12b. The release layers 11c and 12c are provided (FIGS. 5A and 5B). The base layers 11a and 12a as endless belt base layers are electroless belts made of metal such as nickel and SUS, or endless belts made of a heat resistant resin such as polyimide. The thickness of the base layers 11a and 12a is about 50 to 150 μm in the case of a metal electroformed belt, and about 100 to 300 μm in the case of a heat resistant resin. The belt itself has appropriate rigidity and flexibility. It is preferable to have. The range of specific appropriate values is determined by the conditions of the present invention and will be described in detail later. The elastic layers 11b and 12b are silicon rubber layers having a thickness of about 50 to 300 μm formed on the base layers 11a and 12a. The release layers 11c and 12c are fluororesin layers such as PFA and PTFE having a thickness of about 10 to 50 μm formed on the elastic layers 11b and 12b. The elastic layers 11b and 12c are formed by covering or coating the tube. 12b is formed.

  In this embodiment, the fixing belt 11 and the pressure belt 12 are configured as follows. That is, an endless belt made of a nickel layer having a thickness of 75 μm is used as the base layers 11a and 12a, and a silicon rubber layer having a thickness of 300 μm is formed as elastic layers 11b and 12b on the outer periphery of the base layers 11a and 12a. Further, the elastic layers 11b and 12b are covered with 50 μm PFA tubes as the release layers 11c and 12c. The outer diameter of both the fixing belt 11 and the pressure belt 12 is 55 mm.

  The fixing roller 13 and the pressure roller 14 are elastic rollers having an outer diameter of φ28 mm, in which elastic layers 13 b and 14 b made of a silicon sponge rubber layer having a thickness of 5 mm are provided on the outer periphery of a SUS core metal 13 a and 14 a having a diameter of 18 mm, respectively. It is. The Asker C hardness at this time is about 40 ° when 9.8 N (1 kgf) is applied.

  In this embodiment, the longitudinal dimensions of the elastic layers 13b and 14b of the fixing roller 13 and the pressure roller 14 are set to be slightly larger than the longitudinal dimensions of the fixing belt 11 and the pressure belt 12 ( Figure 2). The longitudinal dimensions of the elastic layers 13b and 14b of the fixing roller 13 and the pressure roller 14 are substantially the same as the longitudinal dimensions of the fixing belt 11 and the pressure belt 12, or the longitudinal dimensions of the fixing belt 11 and the pressure belt 12. There is no problem even if the dimension is set shorter than the dimension in the direction.

  The fixing roller 13 is rotatably supported by the first frames 31L and 31R through the bearings 32L and 32R at both ends of the cored bar 13a (FIG. 2).

  The pressure roller 14 is disposed below the fixing roller 13 in parallel with the fixing roller 13, and both ends of the cored bar 14a are rotatably supported by the second frames 33L and 33R via bearings 34L and 34R (FIG. 2).

  FIG. 6 is an explanatory diagram showing the relationship among the fixing roller 13, the heating roller 16, and the fixing belt 11. 6A shows a state in which the fixing belt 11 is wound around the fixing roller 13 and the heating roller 16 by the shortest path length of the fixing belt 11. FIG. 2B is a diagram illustrating a state in which the fixing belt 11 is wound around the fixing roller 13 and the heating roller 16 with a margin longer than the shortest path length of the fixing belt 11.

  The heating roller 16 is an aluminum hollow cylindrical body having a thickness of 1 mm and an outer diameter of 18 mm. The heating roller 16 is provided at a position where the fixing belt 11 wound around the fixing roller 13 protrudes obliquely upward on the upstream side in the recording material conveyance direction from the fixing roller 13.

  That is, the circumferential length of the fixing belt 11 is longer with a margin than the shortest path length of the fixing belt 11 when the fixing belt 11 is wound around the fixing roller 13 and the heating roller 16. Such a position is intentionally arranged. At that position, both ends of the heating roller 16 are rotatably supported by the third frames 35L and 35R via bearings 36L and 36R (FIG. 4). Alternatively, the bearings 36L and 36R at both ends of the heating roller 16 are attached with a spring or the like in a direction P1 (FIG. 1) away from the fixing roller 13 on an imaginary straight line L1 connecting the rotation center of the heating roller 16 and the rotation center of the fixing roller 13. It is supported by the third frames 35L and 35R in a biased state.

  That is, the fixing belt 11 is not stretched around the fixing roller 13 and the heating roller 16 as shown in FIG. 6A, but is fixed around the fixing roller 13 as shown in FIG. Loosely hung around the heating roller 16. Accordingly, the fixing belt 11 is formed with a slack portion 11 a between the fixing roller 13 and the heating roller 16 in the circumferential direction of the fixing belt 11.

  In this embodiment, the distance between the axial center position of the fixing roller 13 and the central position of the heating roller 16 is set to 46 [mm] to form the slack portion 11a. At this time, the radius of curvature R of the slack portion 11a, that is, the radius of curvature before contact of the fixing belt 11 region (endless belt region) corresponding to the pre-nip portion N1 described later is about 52 mm.

  Both ends of the halogen heater 15 provided inside the heating roller 16 are supported by heater support portions 35L1 and 35R1 provided on the third frames 35L and 35R. The inner surface of the heating roller 16 is painted black so that the radiant heat of the halogen heater 15 is easily absorbed.

  The heating roller 16 contacts a part of the outer peripheral surface (front surface) of the heating roller 16 with the inner peripheral surface (inner surface) of the fixing belt 11, and transmits heat from the halogen heater 15 to the fixing belt 11 from the contact region. Thus, the fixing belt 11 is heated. That is, the fixing belt 11 is heated by the halogen heater 15 through the heating roller 16.

  FIG. 7 is an explanatory diagram showing the relationship among the pressure roller 14, the tension roller 17, and the pressure belt 12. 7A shows a state in which the pressure belt 12 is wound around the pressure roller 14 and the tension roller 17 by the shortest path length of the pressure belt 12. (B) is a diagram showing a state in which the pressure belt 12 and the tension roller 17 are wound around the pressure belt 12 with a margin longer than the shortest path length of the pressure belt 12.

  The tension roller 17 is a roller having an outer diameter of φ18 mm, and has a configuration in which an elastic layer 17b made of a silicon sponge rubber layer having a thickness of 4 mm is provided on the outer periphery of a SUS φ10 mm cored bar 17a. The longitudinal dimension of the elastic layer 17 b is equal to the longitudinal dimension of the elastic layers 13 b and 14 b of the fixing roller 13 and the pressure roller 14. The tension roller 17 is provided at a position where the pressure belt 12 wound around the pressure roller 14 protrudes obliquely downward from the pressure roller 14 on the upstream side in the recording material conveyance direction.

  In other words, the tension roller 17 has a margin in the circumferential length of the pressure belt 12 than the shortest path length of the pressure belt 12 when the pressure belt 12 is wound around the pressure roller 14 and the tension roller 17. It is intentionally placed at a position where it can be long. At that position, both ends of the cored bar 17a of the tension roller 17 are rotatably supported by the fourth frames 37L and 37R via bearings 38L and 38R (FIG. 4). Alternatively, the bearings 38L and 38R at both ends of the cored bar 17a are on a virtual straight line L2 connecting the rotation center of the pressure roller 14 and the rotation center of the tension roller 17 in the direction P2 (FIG. 1) away from the pressure roller 14 Is supported by the fourth frames 37L and 37R.

  That is, the pressure belt 12 is not wound around a state where the pressure roller 14 and the tension roller 17 are tensioned as shown in FIG. 7A, but is pressed as shown in FIG. 7B. The roller 14 and the tension roller 17 are loosely wound around. Therefore, a slack portion 12 a is formed on the pressure belt 12 between the pressure roller 14 and the tension roller 17 in the circumferential direction of the pressure belt 12.

  In this embodiment, the distance between the axial center position of the pressure roller 14 and the central position of the tension roller 17 is set to 46 [mm] to form the slack portion 12a. At this time, the radius of curvature R of the slack portion 12a, that is, the radius of curvature before contact of the fixing belt 11 region (endless belt region) corresponding to a pre-nip portion N1 described later is about 52 mm.

  Next, details of a nip portion formed by the fixing belt 11 of the fixing belt unit U1 and the pressure belt 12 of the pressure belt unit U2 will be described.

  Hereinafter, in the configuration of the fixing device of the present embodiment, for convenience of explanation, the nip portion is described with the names “pre-nip portion” and “pressure nip portion” according to the functional role.

  The “pre-nip portion” is a nip portion formed by a belt portion where the fixing belt 11 and the pressure belt 12 are not in contact with the fixing roller 13 and the pressure roller 14, respectively (FIG. 1). That is, the pre-nip portion is a nip portion formed by regions of the fixing belt 11 and the pressure belt 12 (endless belt) that are not backed up by the fixing roller 13 and the pressure roller 14 (pressure member). The “pressure nip portion” is a nip portion formed in the pressure contact portion of the fixing roller 13 and the pressure roller 14 respectively disposed on the inner surfaces of the fixing belt 11 and the pressure belt 12 (FIG. 1). In other words, the pressure nip portion contacts the pressure belt 12 (the other endless belt) with the region of the fixing belt 11 (one endless belt) where there is a backup by the fixing roller 13 and the pressure roller 14 (pressure member). It is the nip part formed. A nip region including the “pre-nip portion” and the “pressure nip portion” is defined as a “total nip”.

  In the fixing belt unit U1 and the pressure belt unit U2, a first frame that supports the fixing roller 13 and a second frame that supports the pressure roller 14 are respectively provided with pressure springs 41L and 41R as pressure means. 42L and 42R are disposed (FIG. 2). The fixing roller 13 and the pressure roller 14 are urged toward each other by the pressure springs 41L / 41R, 42L / 42R. The fixing roller 13 and the pressure roller 14 are pressed against the outer peripheral surface (surface) of the fixing belt 11 by pressing the fixing belt 11 and the pressure belt 12 between the elastic layers 13b and 14b. The outer peripheral surface (surface) of the belt 14 comes into contact. That is, the fixing roller 13 and the pressure roller 14 have the outer peripheral surfaces of the fixing belt 11 and the pressure belt 14 in contact with each other. As a result, a pressure nip portion N2 is formed by contact between the surface of the fixing belt 11 and the surface of the pressure belt 12 (FIG. 1). In this embodiment, the total pressure applied to the fixing roller 13 and the pressure roller 14 by the pressure springs 41L, 41R, 42L, and 42R is 196 N (20 kgf), whereby the width of the pressure nip portion N2 is 5 mm. It is said.

  FIG. 8 is an explanatory diagram of the pre-nip portion N1 formed by the slack portions 11a and 12a of the fixing belt 11 and the pressure belt 12 when the pressure nip portion N2 is formed.

  As described above, the fixing belt unit U1 in the non-pressurized state has the slack portion 11a as shown in FIG. Further, the pressure belt unit U2 in the non-pressurized state has a slack portion 12a as shown in FIG. 7B.

  The fixing roller 13 and the pressure roller 14 are urged toward each other, and a pressure nip portion N <b> 2 is formed by contact between the fixing belt 11 and the pressure belt 12. Then, from the upper end of the pressure nip portion N2 in the recording material conveyance direction, a region that overlaps the slack portions 11a and 12a of the fixing belt 11 and the pressure belt 12 within a predetermined range, and an overlapping region (dotted line portion in FIG. 8). ) Occurs. In each of the slack portions 11a and 12a, the surface of the fixing belt 11 and the surface of the pressure belt 12 are in contact with each other in the overlapping region. As a result, the fixing belt 11 and the pressure belt 12 are appropriately deformed so as to be balanced in the circumferential direction. As a result, a pre-nip portion N1 is formed in the overlapping region (FIG. 1).

  Therefore, the nip pressure in the pre-nip portion N1 tries to return the contact state of the fixing belt 11 and the pressure belt 12 from the contact state shown in FIG. 1 to the non-contact state shown in FIGS. 6B and 7B, respectively. This is due to the elastic force of the fixing belt 11 and the pressure belt 12.

  That is, the nip pressure in the pre-nip portion N1 mainly returns to the shape in the non-contact state depending on the rigidity and flexibility of the respective base layers 11a and 12a of the fixing belt 11 and the pressure belt 12. This is due to the restoring force of 11 and 12 themselves. The width of the pre-nip portion N1 formed in this way is approximately 15 mm.

  The pre-nip portion N1 formed in this way is formed by contact between the fixing belt 11 and the pressure belt 12 which are flexible and deformable. Therefore, the pressure distribution is substantially uniform within the range of the pre-nip portion N1, and it is possible to maintain a stable contact state.

  Further, the pre-nip portion N1 is formed continuously with the pressure nip portion N2 formed by urging the fixing roller 13 included in the fixing belt 11 and the pressure roller 14 included in the pressure belt 12. Is done. Therefore, when the recording material is sandwiched and conveyed, the adhesion between the recording material P and the fixing belt 11 and the pressure belt 12 is maintained in the total nip including the pre-nip portion N1 and the pressure nip portion N2. . That is, the total nip starts from the pre-nip portion N1 and supports the fixing belt 11 and the pressure belt 12 which are two endless belts so that the pressure nip portion N2 is continuously formed downstream in the recording material conveyance direction. .

  The pressure distribution of the total nip in this example formed by the pre-nip portion N1 and the pressure nip portion N2 was measured using a pressure distribution measurement system PINCH manufactured by NITTA Corporation. FIG. 9 shows the measured pressure distribution.

  As shown in FIG. 9, since the fixing roller 13 and the pressure roller 14 are urged toward each other, the fixing belt 11 and the pressure belt 12 are formed in contact with each other at a position corresponding to this portion. The applied pressure is highest at the pressure nip portion N2.

  In contrast, in the pre-nip portion N1, the surface of the fixing belt 11 and the surface of the pressure belt 12 are in contact with each other only by the elastic force (restoring force) between the fixing belt 11 and the pressure belt 12. It is very low compared to the applied pressure at the nip portion N2. Further, in the pre-nip portion N1, since the fixing belt 11 provided with a rigid endless belt as the base layers 11a and 12a is in contact with the pressure belt 12, the pressure distribution is uniform.

(3) Heat Fixing Operation of Fixing Device When the driving gear G (FIG. 2) provided at the end of the core bar 14a of the pressure roller 14 is rotationally driven by the fixing motor M, the pressure roller 14 is set to a predetermined level. It is rotated in the direction of the arrow at the peripheral speed (FIG. 1). When the pressure roller 14 rotates, the rotation of the pressure roller 14 is transmitted to the pressure belt 12 at the pressure nip portion N2, and the pressure belt 12 and the tension roller 14 are rotated along with the rotation of the pressure roller 14. Move around 17 in the direction of the arrow. The rotation of the pressure belt 12 is transmitted to the tension roller 17, and the tension roller 17 is driven to rotate in the direction of the arrow as the pressure belt 12 rotates. In addition, the rotation of the pressure belt 12 is transmitted to the surface of the fixing belt 11 at the pressure nip portion N2, and the fixing belt 11 rotates around the fixing roller 13 and the heating roller 16 along with the rotation of the pressure belt 12. 12 and move in the direction of the arrow at the same speed. The rotation of the fixing belt 11 is transmitted to the heating roller 16, and the heating roller 16 is driven to rotate in the direction of the arrow as the fixing belt 11 rotates. In this embodiment, the circumferential movement speed (traveling speed) of the pressure belt 12 and the fixing belt 11 is 200 mm / s.

  Even in the state in which the fixing belt 11 and the pressure belt 12 are intentionally loosened as in this embodiment (FIGS. 6B and 7B), the fixing belt 11 and the pressure belt 12 are respectively The base layers 11a and 12a have rigidity and flexibility. From this, the fixing belt 11 and the pressure belt 12 rotate while maintaining the slackened state.

  Even during the heat fixing operation, the fixing belt 11 and the pressure belt 12 rotate while maintaining a slack shape. Therefore, undulation (swelling in the longitudinal direction of the belt) is less likely to occur than when the fixing belt 11 and the pressure belt 12 are rotated in a tensioned state (FIGS. 6A and 7A). Become. Therefore, there is an advantage that the surface of the fixing belt 11 can be brought into uniform contact with the recording material P.

  The heater 15 is energized by an energization control unit 41 (FIG. 4) serving as an energization control means before or after the rotational driving of the pressure roller 14 or simultaneously. As a result, the heater 15 generates heat, the heating roller 16 heats the heating roller 16 that is rotating, and the heating roller 16 heats the fixing belt 11 that is rotating. The heat of the fixing belt 11 is transmitted to the pressure belt 12 that is rotating through the pressure nip portion N2 and the pre-nip portion N1, and the pressure belt 12 is heated. The temperature of the heating roller 16 is detected by the temperature detection element 18 (FIG. 1), and the power supply control unit 41 controls the electric power supplied to the heater 15 based on the output signal S1 from the temperature detection element 18 so that the heater 15 Perform temperature control. In other words, the energization control unit 41 outputs from the temperature detection element 18 so as to maintain a predetermined set temperature (target temperature) at which the toner image T is heated to a temperature approximately equal to or higher than the outflow start temperature Tfb in the flow tester in the prenip portion N1. The energization to the heater 15 is controlled based on the signal S1.

  The outflow start temperature Tfb in the flow tester used in this example is determined under the following conditions.

  Using a flow tester CFT-500D (manufactured by Shimadzu Corporation), die hole diameter: 1 [mm], load value: 405 [kgf], temperature increase rate 4 [° C./min. The toner pellets are heated and melted out under the conditions of At this time, the temperature at the time when the toner started to flow out of the hole of the die was defined as “flowing start temperature Tfb”.

  In the fixing roller 13, the pressure roller 14, and the tension roller 17, the elastic layers 13b, 14b, and 17b are formed of a silicon sponge rubber layer having heat insulation. Therefore, the heat capacities of the members 13, 14, and 17 necessary for heating the fixing belt 11 and the pressure belt 12 for heat-fixing the unfixed toner image T on the recording material P can be reduced. Therefore, the image forming apparatus A equipped with the fixing device 112 of the present embodiment can shorten the time (FPOT: First Printout time) until the first image is output after the printer command is input. That is, the warm-up time can be shortened. In addition, the fixing device 112 according to the present exemplary embodiment can reduce power consumption during standby waiting for a printer command.

  In the state where the pressure belt 12 and the fixing belt 11 are rotated and the heater 15 is energized as described above, the recording material P carrying the toner image T is introduced into the prenip portion N1 with the toner image carrying surface facing upward. Is done.

  The recording material P is weakly and uniformly held by the elasticity (restoring force) of the fixing belt 11 and the pressure belt 12 by the fixing belt 11 and the pressure belt 12 in the pre-nip portion N1, and is conveyed in that state.

  At the same time, the recording material P is preheated from both the toner image carrying surface on the fixing belt 11 side and the toner image non-carrying surface on the pressure belt 12 side by the preheated fixing belt 11 and pressure belt 12. Since the pre-nip portion N1 is formed only by the contact between the fixing belt 11 and the pressure belt 12 as shown in FIG. 3, when the recording material P is sandwiched, the fixing belt 11 and the pressure belt are respectively the recording material. This is a region in contact with only P.

  That is, the fixing belt 11 in the pre-nip portion N1 comes into contact only with the toner image carrying surface of the recording material P and does not come into contact with the fixing roller 13 or other components.

Further, the pressure belt 12 in the pre-nip portion N1 is in contact only with the toner image non-carrying surface of the recording material P, and is not in contact with the pressure roller 14 or other components.
Accordingly, the heat held by the fixing belt 11 and the pressure belt 12 can be efficiently transmitted to the recording material P.

  As described above, the recording material P is sandwiched between the surface of the fixing belt 11 and the pressure belt 12 by the elasticity of the fixing belt 11 and the pressure belt 12, so that the surface of the recording material P is uniformly weakly pressed over the entire area. And preheated uniformly.

  The unfixed toner image T carried on the recording material P is sufficiently heated above the substantially outflow start temperature Tfb at the pre-nip portion N1, and is subsequently sandwiched between the surface of the fixing belt 11 and the surface of the pressure belt 12 at the pressure nip portion N2. Pressurized while being conveyed.

  As a result, the unfixed toner image T carried on the recording material P is heat-fixed on the surface of the recording material P as a fixed image having sufficient fixability and gloss (gloss).

  In other words, the temperature distribution and the pressure distribution at which the unfixed toner image T is pressure-fixed on the recording material P at the pressure nip portion N2 after securing the time for the unfixed toner image T to sufficiently melt at the prenip portion N1 are pre-nip portion. N1 and the pressure nip N2 can be obtained.

  Thereby, the occurrence of fixing failure, blistering, offset and the like of the unfixed toner image T can be greatly reduced. Then, the recording material P is discharged from the pressure nip portion N2.

  Through the heating and fixing process passing through the pre-nip portion N1 and the pressure nip portion N2, the temperature change of the toner image T carried on the recording material P at each point in the total nip was measured.

  The temperature profile was measured as follows. A thermocouple (for example, K-type thermocouple wire diameter 50 μm manufactured by Anritsu Keiki Co., Ltd.) having a small heat capacity of the temperature detection unit is attached on the recording material P, and the prenip portion of the fixing device 112 in which the recording material P is temperature-controlled. N1 and the pressure nip N2 were nipped and conveyed. And the electric potential difference signal was measured with Hioki Electric Co., Ltd. memory high coder (8842) from the thermocouple at that time.

  In this way, it is possible to measure the state of temperature change with respect to time when passing through the nip portion. By multiplying this by the conveyance speed of the recording material P, a temperature profile at each position in the fixing nip is obtained. Can do.

  FIG. 10 shows temperature profiles in the pre-nip portion N1 and the second pressure nip portion N2 of the present embodiment measured in this way. In this figure, the distribution of the applied pressure shown in FIG. 9 is overlapped with the temperature profile after the position at each point of the nip is matched in the horizontal axis direction.

  Further, in order to explain the melting and fixing process, a model diagram showing the state of the recording material P and the toner image T in the fixing process is shown in FIG.

  FIG. 11A is a diagram showing the state of the recording material P and the toner image T before entering the nip. Similarly, FIG. 11B shows the state immediately after passing through the prenip portion N1, and FIG. 11C shows the passage through the pressure nip portion N2. It is a model figure which shows the state immediately after.

  Hereinafter, the process of fixing the toner image T with the fixing device according to the embodiment of the present fixing device, the temperature profile in FIG. 10, the distribution of the applied pressure, and the state of the recording material P and the toner image T in FIG. It explains from.

  First, the recording material P carrying the toner image T is introduced into the pre-nip portion N1. In the pre-nip portion N1, as shown in FIG. 10, the toner image T is gradually preheated and its temperature rises. At this time, the temperature profile at the pre-nip portion N1 increases, and the inclination gradually becomes gentler toward the second half of the pre-nip portion N1, and shows a tendency to be saturated. At this time, as shown in FIG. 10, the temperature reaches substantially equal to or higher than the outflow start temperature Tfb in the flow tester within the range of the pre-nip portion N1.

  First, the reason why it is preferable that the temperature be approximately equal to or higher than the outflow start temperature Tfb in the flow tester is as follows.

  In order for the toner image T to be actually fixed to the recording material P with sufficient strength, the toner image T must be fixed with an anchor effect that penetrates at least the fibers of the recording material and is embedded and fixed between the fibers. In order to penetrate into the fibers of the recording material in this way, the toner must first be melted and deformed.

  From the measurement principle, the outflow start temperature Tfb in the flow tester indicates the temperature at which the toner starts to melt and deform. That is, in the flow tester, the temperature at which the toner begins to flow out of the die hole when the temperature is gradually increased with respect to the toner pellet in a state where a constant load is applied indicates the temperature at which the toner starts to melt and deform. Yes.

  This was confirmed experimentally as follows. When the recording material P carrying the toner image T was left unpressed and left in an environment below the outflow start temperature Tfb, the toner image T carried on the recording material did not change at all.

  On the other hand, when the toner image T is left in an environment of the outflow start temperature Tfb or higher, the toner image T carried on the recording material starts to melt, and it is apparent that the adhesion force to the recording material is improved.

  Next, the reason why it is desirable to set the temperature of the toner image T to Tfb or more, particularly in the pre-nip portion N1, is to make a sufficiently melted state before reaching the pressure nip portion N2 where pressure is applied. It is.

  The recording material P carrying the toner image T continues to receive pressure at the pressure nip portion N2 to which pressure is applied, so that the toner image T permeates the fibers of the recording material to obtain a fixed image. At this time, if the temperature in the pre-nip portion N1 has already reached a temperature equal to or higher than Tfb, the toner image T has a temperature that is substantially equal to or higher than the outflow start temperature Tfb over the entire pressure nip portion N2 to which the pressure is applied. Can be maintained. That is, since the toner image T is in a state of being melted and deformed over the entire area in the pressure nip portion N2, the pressure applied at the pressure nip portion N2 penetrates the toner image T into the recording material P without waste. Used to make

  On the other hand, if there is a portion in the pressure nip portion N2 where the toner does not reach the temperature equal to or higher than the outflow start temperature Tfb, the toner image T is in a state of not deforming. Is used as a force for pressing the toner to the recording material. In that case, the force for effectively penetrating the toner into the recording material P is reduced.

  As described above, in order to effectively use the applied pressure and fix the toner image T with the minimum necessary pressure, the toner image is spread over the entire pressure nip portion N2 where the pressure is applied. T is set to a temperature equal to or higher than Tfb. That is, it is preferable that the temperature of the toner image T is equal to or higher than the outflow start temperature Tfb in the flow tester in the pre-nip portion N1.

  Needless to say, the effect of the temperature of the toner image T not reaching Tfb in the pre-nip portion N1 does not mean that the effect is not lost. Even in this case, the temperature can be brought close to the most effective state by raising the temperature of the toner image T as much as possible in the pre-nip portion N1.

  By being preheated over time in this way, the toner image T is melted almost uniformly even in the thickness direction, and is in a good melted state from the upper layer to the lower layer of the toner layer. . At the same time, since almost no pressure is applied in the pre-nip portion N1, the toner image T on the recording material P exists in a molten state without much penetration into the recording material P as shown in FIG.

  As described above, after the pre-nip portion N1 is sufficiently melted, the recording material P reaches the pressure nip portion N2, and receives a pressure as shown in FIG.

  At this time, the toner image T reaches the outflow start temperature Tfb or more at the pre-nip portion N1. Therefore, since the toner image T is sufficiently melted in the thickness direction, the applied pressure can be effectively applied to allow the toner image T to permeate the recording material P appropriately. That is, since the toner image T is sufficiently melted, the fixing property can be secured by appropriately penetrating the fibers of the recording material P without applying a high pressing force. At this time, the toner image T is excessively recorded. Does not penetrate the fiber.

  The recording material P pressurized in the pressure nip portion N2 is discharged from the pressure nip portion N2 after the toner image T has permeated appropriately, thereby obtaining a fixed image having sufficient fixability (FIG. 11). (C)).

  The fixing process realized by the fixing device of the present embodiment that is fixed through the above-described process has the following three features.

  The first feature in the fixing process using the fixing device of this embodiment is that almost no pressure is applied until the temperature of the toner image T is sufficiently increased. When pressure is applied in a state where the toner is not melted, the applied pressure only presses the powdery toner against the recording material, and thus does not contribute to fixing at all. The pressure applied at this time is wasted. That is, in order to perform the fixing efficiently, it is necessary to apply a pressing force with the toner image T sufficiently melted.

  As shown in FIG. 9, no pressure is applied in the range of the pre-nip portion N1, which is the process of heating and heating the toner image T.

  In the present embodiment, a configuration in which preheating is performed without positively applying pressure to the recording material P carrying the toner image T in the region of the pre-nip portion N1 is sandwiched between the fixing belt 11 and the pressure belt 12. This is realized by forming the pre-nip portion N1.

  The second feature of the fixing device of the present embodiment is that the temperature of the toner image T is maintained at or above the outflow start temperature Tfb in the flow tester over the pressure nip portion N2 region where the applied pressure is applied. There is.

  As shown in FIG. 10, the temperature of the toner image T is maintained at a temperature equal to or higher than the outflow start temperature Tfb in the flow tester in the region of the pressure nip portion N <b> 2 where pressure is applied. As a result, the temperature of the toner image T reaches Tfb or higher, and the toner image T is pressed in a sufficiently melted state, whereby the fixing to the recording material P can be performed with the minimum necessary pressure.

  The reason why it is particularly preferable to set the pressure nip portion N2 to be equal to or higher than the “flow start temperature Tfb in the flow tester” is as described above.

  In this way, when the toner is sufficiently melted and pressed, the melted toner can be appropriately permeated into the fibers of the recording material and efficiently fixed without wasting the applied pressure.

  The third feature of the fixing device of the present embodiment is that the difference between the temperature of the upper layer of the toner image T and the temperature of the lower layer of the toner image T is small, that is, the toner melted almost uniformly in the thickness direction of the toner image T. It is a point to pressurize when it becomes.

  In the temperature profile of the fixing device of the present embodiment, the temperature of the toner image T rises at the pre-nip portion N1 as shown in FIG. 10 and reaches Tfb or more. The inclination of the temperature change of the toner image T gradually becomes gentle toward the second half of the pre-nip portion N1, and shows a tendency to be saturated. The fact that the temperature change tends to saturate in this way indicates that the temperature gradient in the vicinity thereof has decreased. That is, it is indicated that the heat of the fixing belt 11 is sufficiently transmitted to the recording material P side, and the temperature difference between the fixing belt 11 and the recording material P is getting smaller.

  This also applies to the thickness layer direction of the recording material P. That is, at this time, the temperature distribution in the thickness layer direction of the recording material P, and if the toner image T is actually carried, the temperature distribution in the thickness direction of the toner image T is reduced. ing. The fact that the temperature distribution in the thickness direction of the toner image T is small, that is, the melting state of the toner in the upper and lower layers of the toner image T is close, and the toner image T can be melted almost uniformly in the thickness direction. (FIG. 11 (b)).

  When the toner image T is melted in the same way in the upper layer and the lower layer as described above, it is pressurized in the pressure nip portion N2, so that the melted toner is appropriately permeated into the paper and at the same time sufficient gloss is realized. ing.

  As described above, the fixing device of the present embodiment sufficiently preheats the recording material P carrying the toner image T in the pre-nip portion N1, and there is little difference between the temperature of the toner image upper layer and the temperature of the toner image lower layer. After reaching the state, the toner image is pressurized in the pressure nip portion N2. That is, after the temperature difference between the temperature of the upper layer of the toner image T and the temperature of the lower layer of the toner image T is small, that is, the toner is almost uniformly melted in the thickness direction of the toner image T, the pressure nip N2 The toner image T is pressurized. As a result, the melted toner is appropriately permeated into the fibers of the recording material P to be fixed, and a sufficient gloss is obtained.

  The above effects could actually be obtained even within a range of about ± 5 ° C. with respect to the outflow start temperature Tfb in the flow tester. Therefore, if the “substantially outflow start temperature” is about the outflow start temperature Tfb ± 5 ° C., the same effect can be obtained.

  In the present embodiment, the halogen heater 15 is used as a heating source and is arranged in the heating roller 16 of the fixing belt unit U1 so as to heat the fixing belt 11. The arrangement of the halogen heater 15 as a heat source is not limited to this, and it may be arranged at the position of the fixing roller. Further, there is no problem even if it is provided in a plurality of rollers. Further, the heat supply to the surface of the fixing belt 11 may be heating by radiant heat, heating by hot air, or heating by induction heating when the belt base layer is a metal.

(4) Relationship between Young's Modulus, Thickness, and Curvature Radius of Belt Base Layer In the fixing device 112 of this embodiment, the Young's modulus E (kgf / mm ² ) of the belt base layers 11a and 12a is used in order to achieve higher image quality. ), Thickness t (mm), and radius of curvature R (mm) of the slack portion are set so as to satisfy the relationship Et ³ /R≧0.17. The reason why this relationship is necessary will be described below.

  In the pre-nip portion N1, when the recording material P carrying the toner image T is nipped and conveyed, the fixing belt 11, the pressure belt 12, and the recording material P must be in stable and uniform contact. At this time, if the contact becomes unstable and the recording material P is lifted from the belt, the toner image T is disturbed and an image defect occurs.

In the pre-nip portion N1, it is assumed that the endless belt is a rigid body having a certain width, length, thickness, and curvature. Considering the “thickness” having a large contribution rate to rigidity (hardness to deform) and the “curvature radius” for forming the slack portion, the stress σ of the rigid body is Young's modulus E, thickness t, radius of curvature. There is a relationship of σ∝E · t ³ / R with R. When a belt slack portion formed with a certain radius of curvature is brought into contact with the other belt, the slack portion pushes back the other belt with a stress determined by its shape, and the slack contact portion is formed in a state where the force is balanced. . At this time, the greater the stress of the slack portion, the stronger and evenly abuts against the other belt, and the floating of the recording material P can be suppressed.

FIG. 12 is a schematic cross-sectional view showing the relationship among the fixing belt 11, the pressure belt 12, and the recording material P in the vicinity of the pre-nip portion N1 when the relationship Et ³ /R≧0.05 is established. Here, a case where both the fixing belt 11 and the pressure belt 12 have slack portions and the values of Et ³ / R are equal is shown. The values of E, t, R, and Et ³ / R are, for example, E = 2 × 10 ⁴ kgf / mm ² , t = 0.075 mm, R = 52 mm, and Et ³ /R=0.17.

  When the recording material P passes through the pre-nip portion N1, the recording material P tries to push back the fixing belt 11 and the pressure belt 12 due to the stress generated in the tip entry portion or the curved portion. The force varies depending on the type, basis weight, thickness, etc. of the recording material P.

By setting Et ³ /R≧0.05, the fixing belt 11 and the pressure belt 12 can press the recording material P without losing the stress of the recording material P usable in this embodiment. Thereby, the floating of the recording material P from the surface of the fixing belt 11 and the pressure belt 12 can be suppressed, and the contact property between the fixing belt 11, the pressure belt 12 and the recording material P is stabilized. Therefore, the unfixed toner image T carried on the recording material P is not disturbed, and the unfixed toner image T can be fixed with good heat.

Further, when Et ³ /R≧0.17, it is possible to obtain the effect even with a recording material having a larger basis weight and thickness.

  FIG. 16A is a table showing the fixing state of the toner images T carried on various recording materials P when the Young's modulus E, thickness t, and curvature radius R of the belt base layer forming the slack portion are changed. is there.

FIG. 16B is an example of the recording material P used in this embodiment. When the image is good, it is indicated by ◯, and when the image is disturbed or gross unevenness is indicated by ×. The value of Ni is shown as an example of the metal material of the belt base layer, and the value of polyimide (PI) is shown as an example of the heat resistant resin material. Looking at the relationship between the value of Et ³ / R and the image, if Et ³ /R≧0.05, a good image is obtained with respect to the recording materials (1) and (2), and Et ³ / R. It was found that when ≧ 0.17, a good image was obtained even for the recording material (3).

FIG. 16C shows values of Young's modulus E, thickness t, radius of curvature R, and Et ³ / R of other materials constituting the belt. Compared with the curvature radius R = 50 mm, the value of Et ³ / R of the elastic layer and the release layer is sufficiently smaller than the value of the base layer (Ni, PI, etc.), so there is no problem even if ignored.

For the above reasons, the Young's modulus E, thickness t, and radius of curvature R of the belt base layers 11a and 12a in this embodiment are in the relationship of Et ³ /R≧0.05, preferably Et ³ /R≧0.17. It is necessary to become. Accordingly, it is possible to form the wide pre-nip portion N1 while reducing the heat capacity of the entire fixing device 112 with a minimum number of members without adding a member or the like that presses the fixing belt 11 and the pressure belt 12 from the inside of the belt. In addition, image defects such as image disturbance and gloss unevenness can be reduced for various recording materials P. Accordingly, it is possible to provide the fixing device 112 that is high speed, high image quality, and excellent in energy saving.

^Et 3 /R>0.05 in the fixing device 112 of this embodiment, preferably the relationship of ^Et 3 /R>0.17 the two belt in the fixing device of the belt type is in contact only with each other in the belt nip You may apply to the fixing device of the structure which forms a part. Even in this case, the same effect as that of the fixing device 112 of this embodiment can be obtained.

[Comparative Example 1]
FIG. 13 is a diagram illustrating a comparative example 1 of the fixing device 112 according to the present exemplary embodiment. Here, both the fixing belt 11 and the pressure belt 12 are schematic cross-sectional views in the vicinity of the pre-nip portion N1 when the relationship Et ³ /R≧0.05 does not hold.

In this comparative example, as in FIG. 12, both the fixing belt 11 and the pressure belt 12 have slack portions and the same value of Et ³ / R is shown.

  When the recording material P passes through the pre-nip portion N1, the recording material P is subjected to stress that pushes back the fixing belt 11 and the pressure belt 12 due to various factors. For example, when the leading end portion of the thick recording material P enters the prenip portion N1, the fixing belt 11 and the pressure belt 12 are slightly bent by receiving stress from the leading end portion of the recording material P. Further, the fixing belt 11 and the pressure belt 12 are subjected to stress due to a wrinkle generated in the recording material P due to a sudden change in moisture of the recording material P or unevenness of the conveying force. Further, the fixing belt 11 and the pressure belt 12 are also subjected to stress depending on the posture (how to bend) of the recording material P before entering the pre-nip portion N1.

When Et ³ /R≧0.05 does not hold, the fixing belt 11 and the pressure belt 12 are damaged by the stress received from the recording material P and are deformed. As a result, floating occurs between the recording material P, the fixing belt 11 and the pressure belt 12, and the unfixed toner image T is disturbed, or image defects such as gloss unevenness due to poor heat transfer are caused.

  This is a phenomenon that is particularly likely to occur because there is no member that presses the fixing belt 11 and the pressure belt 12 from the inside in the pre-nip portion N1, and the contact is made using the rigidity of the belt itself.

[Example 2]
Another example of the fixing device will be described. The same members / portions as those of the fixing device 112 of the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. The same applies to Example 3.

FIG. 14 is a schematic cross-sectional view showing the relationship between the fixing belt 11, the pressure belt 12, and the recording material P in the vicinity of the pre-nip portion N1 when the relationship Et ³ /R≧0.05 is established.

In the fixing device 112 shown in this embodiment, both the fixing belt 11 and the pressure belt 12 have slack portions, and the values of E · t ³ / R are intentionally changed. Here, the rigidity of the pressure belt 12 is relatively smaller than that of the fixing belt 11 by changing one or a plurality of values of E, t, and R. As a result, as shown in FIG. 14, the belt trajectory is curved to form a long pre-nip portion N1, and a sufficient preheating time can be taken even under high speed conditions. In the pre-nip portion N1, the pressure belt 12 having relatively low rigidity tends to follow the shape change of the fixing belt 11 having high rigidity. Therefore, with respect to disturbances such as vibration, the pre-nip portion N1 can be maintained more stably than in the case of belts having the same rigidity.

Note that the effect of the present invention can be obtained if the Et ³ / R values of the fixing belt 11 and the pressure belt 12 are 0.05 or more, preferably 0.17 or more, regardless of the magnitude relationship. .

[Example 3]
Another example of the fixing device will be described.

FIG. 15 is a schematic cross-sectional view showing the relationship between the fixing belt 11, the pressure belt 12, and the recording material P in the vicinity of the prenip portion N1 when the relationship Et ³ /R≧0.05 is established only in the fixing belt 11. .

  In the fixing device 112 shown in this embodiment, the fixing belt 11 is provided with a slack portion, and the pressure belt 12 is wound around the pressure roller 14 and the tension roller 17 in a tensioned state as shown in FIG. Has been. As shown in FIG. 15, the slack portion of the fixing belt 11 abuts the tensioned pressure belt 12 to form a pre-nip portion N1. Except for the above points, the configuration is the same as the fixing device 112 of the first embodiment.

In this way, even if either one of the fixing belt 11 and the pressure belt 12 is provided with a slack portion and the other is in a tensioned state, the belt provided with the slack portion has Et ³ /R≧0.05, The same effect can be obtained if the relationship Et ³ /R≧0.17 is satisfied.

1 is a cross-sectional model diagram of an example of a fixing device according to Embodiment 1. FIG. FIG. 2 is a cross-sectional view taken along the line II-II of the fixing device illustrated in FIG. 1. FIG. 3 is a cross-sectional view taken along line III-III of the fixing device illustrated in FIG. 1. FIG. 4 is a cross-sectional view taken along line IV-IV of the fixing device illustrated in FIG. 1. (A) is a sectional view showing an example of a layer configuration of a fixing belt, and (b) is a sectional view showing an example of a layer configuration of a pressure belt. (A) is a diagram showing a state in which the fixing belt is wound around the fixing roller and the heating roller with the shortest path length. FIG. It is a figure showing the state hung around. (A) is a diagram showing a state in which the pressure belt is wound around the pressure roller and the tension roller by the shortest path length, and (b) is a state in which the pressure belt is stretched between the pressure roller and the tension roller and the shortest path length It is a figure showing the state hung around with allowances. It is explanatory drawing of the pre-nip part formed of each slack part of a fixing belt and a pressure belt with formation of a pressure nip part. It is explanatory drawing showing the pressure distribution in a pre-nip part and a press nip part. FIG. 6 is an explanatory diagram illustrating a change in toner temperature in a pre-nip portion and a pressure nip portion in a heat fixing process. FIG. 6 is an explanatory diagram illustrating a toner melting state on a recording material. 5 is a schematic cross-sectional view illustrating a relationship between a fixing belt, a pressure belt, and a recording material in the vicinity of a nip portion when a relationship of E · t ³ /R≧0.05 is established in the fixing device according to Embodiment 1. FIG. FIG. 6 is a diagram illustrating a comparative example of the fixing device according to the first exemplary embodiment, and shows the relationship between the fixing belt, the pressure belt, and the recording material in the vicinity of the nip portion when the relationship of E · t ³ /R≧0.05 does not hold. It is the represented cross-sectional schematic diagram. In the fixing device according to the second exemplary embodiment, the fixing belt in the vicinity of the nip portion when E · t ³ /R≧0.05 is satisfied and the value of E · t ³ / R between the fixing belt and the pressure belt is different. 2 is a schematic cross-sectional view showing the relationship between the pressure belt and the recording material. In the fixing device according to the second exemplary embodiment, a schematic cross-sectional view showing the relationship between the fixing belt, the pressure belt, and the recording material in the vicinity of the nip portion when the relationship of E · t ³ /R≧0.05 holds only for the fixing belt. FIG. It is explanatory drawing showing an example of the Young's modulus of a belt base layer, thickness, a curvature radius, and the image quality for every recording material type. 1 is a configuration model diagram of an example of an image forming apparatus.

Explanation of symbols

11: fixing belt, 11a: belt base layer, 12: pressure belt, 12a: belt base layer, 15: halogen heater, 16: heating roller, 17: tension roller, N1: pre-nip portion, N2: pressure nip portion

Claims (2)

Two endless belts, and means for heating at least one endless belt of the two endless belts, the outer peripheral surfaces of the two endless belts are brought into contact with each other to form a nip portion, and In an image heating apparatus that heats a recording material carrying a toner image at the nip portion while nipping and conveying the recording material,
The nip portion includes a pre-nip portion formed by regions of an endless belt that is not backed up by a pressure member, and a region of one endless belt that is backed up by the pressure member, in contact with the other endless belt. The two endless belts so as to form the pressure nip portion starting from the pre-nip portion and continuously downstream in the recording material conveyance direction. The Young's modulus E (kgf / mm ² ) and thickness t (mm) of at least one endless belt base layer, and the radius of curvature R (mm) before contact of the endless belt region corresponding to the pre-nip portion An image heating apparatus having a relationship of Et ³ /R≧0.05. The Young's modulus E (kgf / mm ² ) and thickness t (mm) of the at least one endless belt base layer, and the curvature radius R (mm) before contact of the endless belt region corresponding to the pre-nip portion are Et ³ / R. The image heating apparatus according to claim 1, wherein a relationship of ≧ 0.17 is established.