JP5940888B2 - Fixing apparatus and image forming apparatus having the same - Google Patents

Description

  The present invention relates to a fixing device that heats and fixes an unfixed toner image on a recording medium in an image forming apparatus such as a copying machine, a printer, and a facsimile machine.

  As a fixing device mounted on this type of image forming apparatus, there is a roller pair type fixing device that performs fixing by heat melting by passing an unfixed toner image on a recording medium between a pair of heated rollers. Are known. In this roller-pair type fixing roller, an elastic layer is formed on the surface of a hollow metal core made of metal such as aluminum, a halogen lamp is disposed as a heating source inside the metal core, and a temperature provided on the surface of the fixing roller. The temperature control circuit controls the halogen lamp on and off by a signal from the sensor so as to keep the temperature of the surface of the fixing roller constant.

  As the pressure roller facing the fixing roller, a pressure roller provided with a heat-resistant elastic layer such as silicon rubber as a coating layer is used. When the pressure roller is brought into pressure contact with the fixing roller, a predetermined nip region is formed by elastic deformation of the elastic layer of the pressure roller. The unfixed toner image on the recording medium is passed through this nip area, and the toner image is fused by heat melting.

  Here, when fixing is performed at a higher speed by using a roller pair type fixing device, the toner image cannot be fixed unless heat is applied to the toner image as much as the fixing at a low speed. For this purpose, it is necessary to increase the nip width as the fixing speed increases. To widen the nip width, there are methods such as 1) increasing the load between both rollers, 2) increasing the thickness of the elastic layer of the fixing roller, and 3) increasing the roller diameter.

  However, as described in 1) above, if the load between both rollers is increased to widen the nip width, the rollers will bend and the nip width will not be formed uniformly in the axial direction, affecting the transportability and image quality. give. If the elastic layer of the fixing roller is thickened to widen the nip width as in 2) above, or the roller diameter is increased to widen the nip width as in 3), there is no problem in image quality. As a result, there arises a problem that the warm-up time becomes longer as the fixing device becomes larger and the heat capacity increases.

  In order to solve these problems and cope with higher speed, a belt nip type fixing device using an endless belt has appeared. Patent Document 1 includes a fixing roller having an elastic layer, a pressure belt, and a pressing member that is disposed on the inner surface of the pressure belt and presses the pressure belt against the fixing roller, and is provided between the pressure belt and the fixing roller. In the nip area formed in the area, the fixing roller surface is locally distorted by providing a mechanism to move and support the pressing member so that the pressure downstream in the conveyance direction of the recording medium has a higher pressure distribution than other parts. However, the peelability of the recording medium is improved. Accordingly, the recording medium can be stably peeled from the pressure belt while maintaining high fixing performance even when the speed is increased.

Japanese Patent No. 4779622

  By the way, the fixing device of Patent Document 1 has a fixing roller and a fixing belt, has a pressing member on the inner surface of the fixing belt, and presses so that the downstream pressure in the nip region has a higher pressure distribution than other portions. By providing a mechanism that supports the member in a movable manner, the surface of the fixing roller is locally distorted, and the peelability of the recording medium is improved. This is necessary. When a large load is applied, the pressing member (fixing member) and the inner surface of the fixing belt rub against each other strongly, and there is a problem in durability of the fixing belt.

  Further, in order to distort the surface of the fixing roller, it is necessary to increase the surface rubber layer of the fixing roller, so that the warm-up time becomes long, resulting in poor energy saving performance.

  SUMMARY OF THE INVENTION In view of the above, an object of the present invention is to provide a fixing device with a belt nip type fixing device that has a short warm-up time, maintains fixing performance, and has good peelability.

In order to achieve the above object, the present invention provides a rotatable fixing belt, a heat source for heating the fixing belt, a pressure roller provided opposite to the fixing belt, and an inside of the fixing belt. A fixing nip portion for fixing an unfixed toner image on the recording medium by pressing the pressing portion against the pressure roller via the fixing belt. In the formed fixing device, the pressing portion includes a pressure pad disposed so as to rub against an inner peripheral surface of the fixing belt, and a pad that presses and supports a surface opposite to the nip forming surface of the pressure pad. A support member, and a plurality of urging members that are interposed between the opposite surface of the pressure pad and the pad support material and arranged in the conveyance direction of the recording medium. The nip forming surface of the pressure roller It is formed on the concave curved surface corresponding to the rate, and the support surface opposite to the surface in which the biasing member of the pressure pad is a flat surface, further, on the downstream side in the transport direction of the recording medium in the fixing nip portion as the pressure is higher than the upstream side, the thickness of the pressing direction in the press pad is, as toward the downstream side than the upstream side in the conveying direction of the recording medium becomes thicker, the opposite side of the press pad The supporting surface of the urging member is formed in a stepped shape with a flat surface of the supporting surface on the upstream side and a flat surface of the supporting surface on the downstream side .

  According to the above configuration, the pressure pad arranged so as to rub against the inner peripheral surface of the fixing belt has a nip forming surface formed in a concave curved surface corresponding to the curvature of the pressure roller. The curvature of the toner greatly changes at the nip exit, and the recording medium can be prevented from being wound around the fixing belt.

  In addition, with respect to the thickness in the pressing direction of the pressure pad in the fixing nip portion, the downstream side in the conveyance direction of the recording medium is made thicker on the downstream side than the upstream side in the conveyance direction. Therefore, it is possible to increase the pressure on the downstream side of the nip portion with a simple configuration, to deform the fixing belt at the nip outlet, and to greatly change the direction of the belt conveyance direction at the nip outlet, and to prevent the curvature peeling. It is easy to do.

Here, as a mode in which the thickness in the pressing direction of the pressure pad is increased on the downstream side in the transport direction, the surface opposite to the nip forming surface is an inclined surface so that the thickness gradually increases from the upstream side to the downstream side in the transport direction. Although a configuration or a configuration in which the thickness is increased in a staircase shape is conceivable, a configuration in which the thickness is increased in a staircase shape as described below is preferable.

  In addition, it is preferable to arrange a plurality of urging members on the surface opposite to the pressure pad in the transport direction so that the pressure pad can be pressed on the average in the transport direction. In this case, in the pressure pad, since the support surface of the urging member is preferably a flat surface, a stepped configuration can be suitably employed. Further, as a measure for increasing the pressure on the downstream side in the transport direction, a configuration in which the spring constants of a plurality of urging members (springs) arranged in the transport direction are set higher toward the downstream can be employed. The configuration in which the spring constant is increased toward the downstream side can be employed in combination with the configuration in which the thickness in the pressing direction of the pressure pad is increased on the downstream side in the conveyance direction. As another measure, it is also possible to adopt a configuration in which a member having a specific gravity in the transport direction is combined with a surface opposite to the nip forming surface of the pressure pad.

  Further, the curvature (1 / r) of the nip forming surface of the pressure pad is 1 / r where the curvature (1 / R) of the pressure roller (R is the radius of the pressure roller) and the thickness t of the fixing belt are 1 / r. It is preferable to adopt a configuration set to <1 / (R + t).

  According to the above relational expression, the curvature radius r of the concave curved surface of the pressure pad is larger than the curvature radius (R + t) obtained by adding the curvature radius R of the pressure roller and the thickness t of the fixing belt. Since the pressure roller and the pressure pad are securely in contact with each other in the center area in the conveyance direction of the nip forming surface of the pressure pad, the unfixed toner image can be reliably melted by contact, and the fixing performance is improved. can do. On the other hand, when 1 / r ≧ 1 / (R + t), both end portions are locally in contact with the pressure roller in the conveyance direction of the pressure pad, and the central portion of the pressure pad (recording) No pressure is applied to the central portion in the nip region in the medium conveyance direction, which may cause a fixing failure.

  In addition, a belt guide that holds the rotation of the fixing belt in the vicinity of the inner peripheral surface of the fixing belt other than the fixing nip portion is provided, and the curvature of the surface of the belt guide and the curvature of the inner diameter of the fixing belt are substantially the same, A configuration in which the shape of the belt guide is a perfect circle can be employed.

  By providing the perfect belt guide, the rotating fixing belt rubs against the belt guide, and the looseness of the fixing belt itself can be reduced as much as possible. As a result, the fixing belt can rotate without slack at the nip outlet, so that the curvature of the fixing belt at the nip outlet can be increased and the peelability of the recording medium can be improved. Furthermore, by making the shape of the belt guide a perfect circle, it is possible to reduce the load on rotating the fixing belt.

  In this case, the rotation of the fixing belt can be driven by rotating the pressure roller side to drive the fixing belt side by rubbing against the fixing belt at the fixing nip portion, or a driving roller on the outer surface of the fixing belt. The power may be transmitted by sliding with the drive roller.

  Further, in the image forming apparatus provided with the above-described fixing device, the warm-up time of the image forming apparatus can be shortened, and power consumption during standby is not necessary, so that an image forming apparatus with low power consumption can be obtained. Can do.

  As described above, according to the present invention, since the belt nip type fixing belt is used, the warm-up time can be shortened, and the pressure on the downstream side in the conveyance direction of the recording medium in the fixing nip portion is increased from the upstream side. Since the thickness of the pressure pad in the pressing direction is set to be thicker on the downstream side so that the pressure becomes higher, the fixing belt can be greatly deformed at the nip outlet, and the curvature of the recording medium is peeled off. Make it easier to do. Further, since the curvature of the concave curved surface of the pressure pad is smaller than the curvature of the pressure roller [1 / (radius R of pressure roller + thickness t of fixing belt)], at least the nip of the pressure pad is formed. Since the pressure roller and the pressure pad are reliably brought into pressure contact with each other at the center in the transport direction of the surface, the unfixed toner image can be reliably contacted and melted, so that the fixing performance can be improved.

1 is a cross-sectional view illustrating an internal structure of an image forming apparatus according to an embodiment of the present invention. FIG. 3 is a cross-sectional view of a fixing device. It is principal part sectional drawing of a pressurization pad part.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention and a fixing device mounted thereon will be described with reference to the drawings. FIG. 1 shows the internal structure of a dry electrophotographic color image forming apparatus. This image forming apparatus forms, for example, a multicolor or single color image on a predetermined recording medium based on image data transmitted from each terminal device on the network. The visible image forming unit 50 (50Y / 50M / 50C / 50B), a recording medium transport unit 30, a fixing device (heating device) 40, and a supply tray 57.

  In the image forming apparatus shown in FIG. 1, four visible image forming units 50Y, 50M, 50C, and 50B are provided for each color of yellow (Y), magenta (M), cyan (C), and black (B). It is installed side by side. The visible image forming unit 50Y forms an image using yellow (Y) toner, the visible image forming unit 50M forms an image using magenta (M) toner, and the visible image forming unit 50C. Performs image formation using cyan (C) toner, and the visible image forming unit 50B performs image formation using black (B) toner. The four sets of visible image forming units are arranged in a so-called tandem manner along a recording medium conveyance path that connects the supply tray 57 of the recording medium P and the fixing device 40.

  The four sets of visible image forming units 50 have substantially the same configuration. That is, each visible image forming unit 50 includes a photosensitive drum 51, a charger 52, a laser beam irradiation unit 53, a developing unit 54, a transfer roller 55, and a drum cleaner unit 56, and has been conveyed. Each color toner is multiplex-transferred to the sheet-like recording medium P.

  Here, the photosensitive drum 51 carries an image to be formed. The charger 52 uniformly charges the surface of the photosensitive drum 51 to a predetermined potential. The laser beam irradiation unit 53 exposes the surface of the photosensitive drum 51 charged by the charger 52 in accordance with image data input to the image forming apparatus, and forms an electrostatic latent image on the surface of the photosensitive drum 51. .

    The developing device 54 visualizes the electrostatic latent image formed on the surface of the photosensitive drum 51 with toner of each color. The transfer roller 55 is applied with a bias voltage having a polarity opposite to that of the toner, and transfers the formed toner image to the recording medium P conveyed by the recording medium conveyance unit 30 described later. The drum cleaner unit 56 removes and collects toner remaining on the surface of the photosensitive drum 51 after the development processing in the developing device 54 and the transfer of the image formed on the photosensitive drum 51. The transfer of the toner image to the recording medium P as described above is repeated four times for four colors.

  The recording medium conveyance unit 30 includes a driving roller 31, an idling roller 32, and a conveyance belt 33, and the recording medium P is formed on the recording medium P so that a toner image is formed by the visible image forming unit 50. It is to be transported.

  The drive roller 31 and the idling roller 32 stretch the endless transport belt 33. The drive roller 31 is rotated by being controlled at a predetermined peripheral speed, thereby rotating the endless transport belt 33. Yes. The conveyance belt 33 generates static electricity on the outer surface, and conveys the recording medium P while electrostatically adsorbing it.

  In this way, after the toner image is transferred to the recording medium P while being transported to the transport belt 33, the recording medium P is peeled off from the transport belt 33 by the curvature of the driving roller 31 and transported to the fixing device 40.

  The fixing device 40 applies a suitable heat and pressure to the recording medium P, dissolves the toner, and fixes it to the recording medium, thereby forming a robust image.

  Here, the fixing device 40 will be described in detail with reference to FIG. FIG. 2 is a schematic view of the fixing device 40. The fixing device 40 includes a fixing belt 1, a belt guide 2, a halogen lamp 5 as a heating source, a pressure roller 6, and a pressing unit 7.

  The fixing belt 1 is disposed so that its inner peripheral surface is close to the surface of the cylindrical belt guide 2. Further, in the inner peripheral area of the fixing belt 1, a pressing portion 7 that presses the fixing belt 1 toward the pressure roller is provided in the fixing nip portion A other than the installation area of the belt guide 2.

  In the pressing portion 7, the pad support member 4, which is a constituent member, applies a load to the pressure pad 3 through the biasing members 13 and 14, so that the surface of the pressure pad 3 is applied to the inner peripheral surface of the fixing belt 1. The (nip forming surface) is rubbed and the pressure pad 3 presses the pressure roller 6 via the fixing belt 1 to form the fixing nip portion A. By passing the recording medium P on which the unfixed toner image is formed through the fixing nip A, the toner S is melted and pressed against the recording medium P to fix the image on the recording medium P. After fixing, the toner is welded to the recording medium P, and gloss is obtained.

  In the fixing belt 1, an elastic layer is formed on a base film made of a polyimide resin having excellent heat resistance to improve the contact property with the toner image, and the releasability from the toner image is formed on the elastic layer. A release layer is formed to improve the resistance. In the fixing belt 1 of this example, an elastic layer having a thickness of 150 μm is formed on a polyimide resin film having an outer diameter of 50 mm and a thickness of 90 μm, and a release layer (PTFE) having a thickness of 10 μm is further formed thereon. The thin belt has a thickness of about 250 μm.

  The thickness and material of the fixing belt 1 are not limited to this, and a belt made of metal such as Ni (nickel), stainless steel, or iron may be used. If a metal belt is used, the belt itself becomes hard and the belt curvature is difficult to be applied. However, if there is no problem in peelability, the use of the metal belt improves the durability of the belt itself.

  Any material may be used for the elastic layer of the fixing belt 1 as long as it has heat resistance and elasticity. In this example, silicon rubber is used. The material of the release layer is not particularly limited as long as it has excellent heat resistance and durability, and excellent release properties from the toner, such as PFA (a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether), PTFE, and the like. A fluorine-based material such as (polytetrafluoroethylene) may be used.

  A temperature detection element (thermistor) 8 is in contact with the fixing belt 1 immediately before the fixing nip portion A. In response to the temperature signal from the temperature detecting element 8, a temperature control circuit (not shown) controls the on / off of the halogen lamp 5, which is a heating source installed in the fixing belt 1, so that the fixing belt 1 is brought to a predetermined fixing temperature. It is controlled to become.

  Next, the pressing unit 7 presses and supports the pressure pad 3 disposed so as to rub against the inner peripheral surface of the fixing belt 1 and the surface 12 opposite to the nip forming surface 11 of the pressure pad 3. A support member 4 and a plurality of urging members 13 and 14 interposed between the surface 12 on the opposite side of the pressure pad 3 and the pad support material 4 are provided.

  The pressure pad 3 is installed on the inner peripheral surface of the fixing belt 1, and the fixing belt 1 rotates while rubbing the surface of the pressure pad 3 and the inner peripheral surface of the fixing belt 1. The nip forming surface 11 of the pressure pad 3 is formed in a concave curved surface corresponding to the curvature of the pressure roller 6. The thickness in the pressing direction of the pressure pad 3 in the fixing nip portion A is the transport direction so that the pressure on the downstream side in the transport direction of the recording medium P in the fixing nip portion A is higher than that on the upstream side of the nip portion. It is formed so that the downstream side is thicker than the upstream side.

  Specific examples of the pressure pad 3 are as follows. That is, the pressure pad 3 is formed by laminating the elastic layer 10 and the release layer 11 on the nip forming surface side of the base material 9 as shown in FIG. The base material 9 is formed in a substantially rectangular parallelepiped shape, and its nip forming surface side is formed in a concave curved surface symmetrical in FIG. 3 centering on the central portion in the conveyance direction of the recording medium P. On the surface 12 opposite to the nip forming surface of the base material 9, a stepped step portion is formed so that the downstream side in the conveyance direction of the recording medium P is thicker than the upstream side by a thickness T. The opposite surface 12 of the substrate 9 is a flat surface and functions as a spring seat for the biasing members 13 and 14 such as a compression coil spring.

  In the pressure pad 3 of this example, the elastic layer 10 having a heat resistance of 2 mm in thickness is formed on the concave curved surface of the base material 9 such as aluminum having a thickness on the upstream side in the sheet conveyance direction (the thickness on the nip portion entrance side). Laminated along a curved surface. The base material 9 is made of a metal material such as aluminum having good thermal conductivity so that the radiant heat from the halogen lamp 5 can be easily absorbed. A surface 12 opposite to the nip forming surface of the base material 9 is formed in a step shape in the vicinity of the substantially central portion in the transport direction, and the downstream side in the transport direction is thicker by 2 mm than the upstream side. Thereby, the downstream load is increased in the transport direction.

  The elastic layer 10 only needs to have heat resistance and elasticity. In this example, silicon rubber (thickness 2 mm) is used. A release layer 11 is provided on the surface of the elastic layer 10 in order to reduce the friction coefficient. The release layer 11 is coated with a fluorine material (thickness 10 μm) such as PFA (copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether) or PTFE (polytetrafluoroethylene). With this coating layer, the wear of the surface of the pressure pad 3 and the inner peripheral surface of the fixing belt 1 is improved, and durability can be provided.

  The surface of the pressure pad 3 on which the release layer 11 is coated is a nip forming surface. This surface is a concave curved surface, and is formed by pressure contact with the pressure roller 6 via the fixing belt 1. The part is formed along the curved surface of the pressure roller 6.

  Here, when the radius of curvature r of the nip forming surface of the pressure pad 3, the thickness t of the fixing belt, and the radius of the pressure roller 6 are R, the following relational expression [1 / r <1 // (R + t) ] Is established. That is, the curvature of the concave curved surface at the fixing nip portion of the pressure pad 3 is made smaller than the curvature of the pressure roller 6 [1 / (radius R of pressure roller + thickness t of fixing belt)]. As a result, the pressure roller 6 and the pressure pad 3 can be surely brought into pressure contact with each other at least at the center in the conveyance direction of the pressure pad 3 at the fixing nip, and the unfixed toner image can be reliably melted by contact. The performance can be improved. Conversely, when the relational expression 1 / r ≧ 1 / (R + t) holds, both ends of the pressure pad 3 are in local contact with the pressure roller 6, and the central portion of the pressure pad 3 (sheet conveyance direction) No pressure is applied to the central part in the nip area at the bottom of the nip, resulting in poor fixing.

  The press pad 3 is pressed by the biasing members 13 and 14 from the pad support member 4 on the surface 12 opposite to the rubbing surface with the fixing belt 1. The urging members 13 and 14 are compression coil springs, and are arranged in a plurality (two in this example) in the conveyance direction of the recording medium P in the fixing nip portion. The fixing nip can be reliably pressed.

  Here, both springs 13 and 14 arranged in two rows in the conveyance direction of the recording medium P are set to the same spring constant. Since the thickness in the pressing direction downstream of the pressure pad 3 in the conveying direction is large, the load on the downstream side is increased, the nip forming surface is pressed with a higher pressure than the upstream side, and fixing is performed on the exit side of the fixing nip portion. The elastic layer (silicone rubber) of the belt 1 is deformed, and the direction of the conveyance direction of the fixing belt 1 at the nip exit is greatly changed to facilitate the peeling of the curvature. The springs 13 and 14 use a plurality of similar springs in the direction perpendicular to the conveyance direction of the recording medium (the pressure roller axial direction), and are installed so as to apply a load uniformly in the axial direction of the fixing nip portion. ing.

  The compression coil springs 13 and 14 have the same spring constant in the recording medium conveyance direction, but the spring constant of the compression coil spring 13 on the downstream side is made larger than the compression coil spring 14 on the upstream side. The load applied to the fixing nip portion 11 may be increased.

  The pad support member 4 is provided with compression coil springs 13 and 14 on a plane parallel to the recording medium conveyance direction. Accordingly, the base 9 of the pressure pad 3 has a larger thickness on the downstream side of the substrate 9 in the conveyance direction of the recording medium, so that the load on the downstream side is larger. The pad support member 4 has a thickness of 10 mm, and a metal material having a high elastic modulus such as SUS or iron is used as the material, and a material that does not bend even when a load is applied is selected.

A total load of 20 N is applied to the pressure roller 6 from the pad support member 4 through the compression coil springs 13 and 14 and from the pressure pad 3 through the fixing belt 1. Since the pressing portion 7 applies a load to the pressure roller 6, in the free state, the pressing portion 7 is outside the belt movement locus (perfect circular virtual circle) formed by the cylindrical belt guide 2 (pressure roller). However, in the installed state, the inlet and outlet of the fixing nip portion A are positioned in the vicinity of a substantially circular virtual circle due to the pressure contact between the pressure pad 3 and the pressure roller 6. become. Therefore, the fixing belt 1 has a convex curvature along the belt guide 2 at the perfect belt guide 2 portion, but moves along the concave curved surface at the fixing nip portion A, and thus has a concave shape. The curvature is. For this reason, the curvature of the fixing belt 1 greatly changes at the entrance and exit of the fixing nip portion A, and the peelability of the recording medium is improved.

  The belt guide 2 is a true cylindrical guide made of metal such as iron or SUS having a thickness of 0.1 to 0.5 mm, and its outer diameter is substantially the same as the inner diameter of the fixing belt 1 (0.1 from the belt inner diameter). ˜0.5 mm smaller). By making the belt guide 2 into a true cylindrical shape, the inner surface of the fixing belt 1 is always rubbed in close proximity to the surface of the belt guide 2 so that no load is applied locally and the fixing belt 1 is stable. Rotation can be secured.

If the outer diameter of the belt guide 2 is larger than the above value (the difference from the inner diameter of the belt is smaller than 0.1 mm), the fixing belt 1 does not rotate smoothly and rotation failure occurs. On the other hand, if the value is smaller than the above value (less than 0.5 mm from the inner diameter of the fixing belt), the fixing belt 1 is loosened and rotates, so that the fixing belt 1 is loosened at the fixing nip outlet, and the curvature is large at the nip outlet of the fixing belt 1. Since there is no change, there is a risk that defective peeling may occur when an unfixed image is fixed.

  Next, the pressure roller 6 will be described in detail. The pressure roller 6 is formed on the metal core 15 in the order of the elastic layer 16 and the release layer 17. The core metal 15 is made of aluminum having good thermal conductivity. The material is not limited to this, and may be made of iron metal. A halogen lamp 5 is installed as a heating source inside the cored bar 15. The core metal 15 of this example is a hollow one having an outer diameter of 26 mm.

  The elastic layer 16 uses heat-resistant silicon rubber. The elastic layer 16 of this example is set to a thickness of 2 mm. The material of the release layer 17 may be any material as long as it is excellent in heat resistance and durability and has excellent release properties from the toner, and a fluorine-based material such as PFA or PTFE may be used. In this embodiment, a PFA tube having a thickness of about 30 μm is used. The pressure roller 6 thus configured has an outer diameter of 30 mm and a surface hardness (Asker C hardness) of 75 degrees.

  Here, the nip shape formed by the pressure contact with the pressure pad 3 is a downward nip shape (concave curved surface). This is because, when a color image is fixed, self-peeling (forced peeling assisting means such as a peeling nail is not required and peeling is performed with a squeeze of a recording medium) is facilitated, and peeling can be done without depending on the peeling nail as much as possible. On the other hand, if the nip shape is directed upward, the recording medium discharged from the nip portion is discharged along the surface of the fixing belt 1, so that sufficient self-peeling property cannot be obtained.

  The pressure roller 6 has a temperature detection element (thermistor) 8 in contact with the surface. The pressure roller 6 and the fixing belt 1 are subjected to on / off control of the halogen lamp 5 based on the detected temperature of the temperature detecting element (thermistor) 8 in contact therewith, and are controlled to a predetermined temperature. Yes.

  Further, a gear (not shown) is connected to the axial end of the pressure roller 6, and a driving force from a separately installed motor is transmitted to the gear to rotate the pressure roller 6.

  In the above configuration, when the pressure roller 6 is rotationally driven, the fixing belt 1 that slides on the pressure roller 6 is also driven and slides on the outer circumferential surface of the perfect belt guide 2. At this time, the belt guide 2 is formed in a perfect circle shape, and the curvature of the inner diameters of the belt guide 2 and the fixing belt is substantially the same, so the load on rotating the fixing belt 1 is reduced. In addition, since the rotating fixing belt 1 slides on the belt guide, the fixing belt 1 can be rotated without slacking at the nip exit, with the slackness of the fixing belt itself being reduced as much as possible. Therefore, the curvature of the fixing belt 1 at the nip exit can be increased, and the peelability of the recording medium P can be improved.

  In addition, since the thickness of the pressure pad 3 in the pressing direction is thicker on the downstream side than on the upstream side in the transport direction, the pressure can be increased on the downstream side of the nip portion with a simple configuration. By deforming the fixing belt, the direction of the conveying direction of the belt at the nip exit can be greatly changed, so that the curvature peeling can be easily performed.

  Furthermore, the curvature (1 / r) of the nip forming surface of the pressure pad 3 is 1 / r where the curvature (1 / R) of the pressure roller (R is the radius of the pressure roller) and the thickness t of the fixing belt are 1 / r. Since r <1 / (R + t) is set, the pressure roller 6 and the pressure pad 3 are surely brought into pressure contact at least in the central region in the conveyance direction of the nip forming surface of the pressure pad 3. Thus, the unfixed toner image S can be reliably contacted and melted, and the fixing performance can be improved.

  In the image forming apparatus having the fixing device 40, the belt nip method is adopted, so that the warm-up time can be shortened and the power consumption during standby is not required. A device can be obtained.

Next, Table 1 shows the results of evaluating the peelability of the recording medium by passing the A4 size sheet-like recording medium P on which the unfixed toner image is placed using the fixing device 40 of FIG. As the evaluation method of releasability, “x” indicates that the film is wound around the fixing device 40 or peels off slightly, and “◯” indicates that the film is peeled without problems.

A method for preparing a sample of an unfixed toner image will be described. As the toner, an oilless toner for color (polyester resin) containing wax was used. The particle diameter (volume average particle diameter) of the toner is 6.5 μm, and the adhesion amount of this toner per unit area of the recording medium is 1.0 mg / cm ² . The recording medium used was A4 size plain paper with a basis weight of 65 g / m ² . The experiment was conducted with the process speed being 120 mm / s and the surface temperature (fixing temperature) of the fixing belt 1 being arbitrarily changed between 130 degrees and 180 degrees.

As a comparative example, an example in which the base material 9 of the pressure pad 3 has the same thickness (2 mm) on the upstream side and the downstream side in the recording medium conveyance direction in the fixing device 40 of FIG.

  As shown in Table 1, in the configuration of this example, even when the surface temperature (fixing temperature) of the fixing belt 1 is arbitrarily changed from 130 degrees to 180 degrees, good results are obtained for the peelability of the recording medium. Obtained. On the other hand, in the configuration of the comparative example, there is no problem with releasability from 130 degrees to 150 degrees, but at a fixing temperature of 160 degrees to 180 degrees, the molten toner remains on the fixing belt 1 and there is a problem with releasability. was there.

  Note that the present invention is not limited to the above-described embodiment, and it is needless to say that many modifications and changes can be made within the scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Fixing belt 2 Belt guide 3 Pressure pad 4 Pad support member 5 Halogen lamp 6 Pressure roller 7 Press part 8 Temperature detection element (thermistor)
9 Metal base material 10 Elastic layer 11 Release layer 12 Opposite surfaces 13 and 14 Elastic member (spring)
DESCRIPTION OF SYMBOLS 15 Core metal 16 Elastic layer 17 Release layer 30 Recording medium conveyance part 31 Drive roller 32 Idling roller 33 Conveyor belt 40 Fixing device 50 Visible image formation unit 51 Photosensitive drum 52 Charger 53 Laser beam irradiation part 54 Developer 55 Transfer Roller 56 Cleaner unit 57 Supply tray

Claims (4)

A rotatable fixing belt, a heating source for heating the fixing belt, a pressure roller provided to face the fixing belt, and a fixing belt provided inside the fixing belt to press the fixing belt toward the pressure roller. A fixing device having a fixing nip portion that fixes an unfixed toner image on a recording medium by pressing the pressing portion against a pressure roller via a fixing belt.
The pressing portion includes a pressure pad arranged so as to rub against the inner peripheral surface of the fixing belt, a pad support member for pressing and supporting a surface opposite to the nip forming surface of the pressure pad, and the pressure A plurality of urging members interposed between the opposite surface of the pad and the pad support material and arranged in the conveyance direction of the recording medium ,
The pressure pad has a nip forming surface formed in a concave curved surface corresponding to the curvature of the pressure roller, and a support surface of the urging member, which is the opposite surface of the pressure pad, is a flat surface. further, the pressure on the downstream side in the transport direction of the recording medium in the fixing nip portion to be higher than the upstream side, the thickness of the pressing direction in the press pad is on the downstream side than the upstream side in the conveying direction of the recording medium The supporting surface of the urging member on the opposite surface of the pressure pad is formed in a stepped manner with the flat surface of the supporting surface on the upstream side and the flat surface of the supporting surface on the downstream side so that the thickness is thicker A fixing device. A belt guide for holding the rotation of the fixing belt in the vicinity of the inner peripheral surface of the fixing belt other than the fixing nip portion is provided;
The fixing device according to claim 1, wherein a curvature of the belt guide surface and a curvature of the inner diameter of the fixing belt are substantially the same, and the shape of the belt guide is a perfect circle.   When the curvature (1 / r) of the nip forming surface of the pressure pad, the curvature (1 / R) of the pressure roller (R is the radius of the pressure roller), and the thickness t of the fixing belt, 1 / r <1 The fixing device according to claim 1, wherein the fixing device is set to / (R + t). An image forming apparatus comprising the fixing device according to claim 1.