JP5893968B2 - Fixing apparatus and image forming apparatus having the same - Google Patents

Description

  The present invention relates to a fixing device used in an electrophotographic image forming apparatus and an image forming apparatus having the same.

  As a fixing device used in an electrophotographic image forming apparatus such as a copying machine or a printer, a heat roller fixing type fixing device is frequently used. The fixing device of the heat roller fixing system includes a roller pair (fixing roller and pressure roller) that are in pressure contact with each other, and the roller is heated by a heating unit including a halogen heater or the like disposed in both or one of the roller pairs. After the pair is heated to a predetermined temperature (fixing temperature), the recording paper on which the unfixed toner image is formed is fed to the pressure contact part (fixing nip part) of the roller pair, and the heat and pressure are passed through the pressure contact part. Thus, the toner image is fixed.

  By the way, in a fixing device provided in a color image forming apparatus, it is common to use an elastic roller provided with an elastic layer made of silicon rubber or the like on the surface of the fixing roller. By using an elastic roller as the fixing roller, the surface of the fixing roller is elastically deformed according to the unevenness of the unfixed toner image and comes into contact so as to cover the toner image surface. It is possible to perform heat fixing on an unfixed toner image satisfactorily.

  Further, due to the effect of releasing the distortion of the elastic layer at the fixing nip, it is possible to improve the releasability for color toners that are more likely to be offset than in monochrome.

  Further, since the nip shape of the fixing nip portion is convex upward (on the fixing roller side) (so-called reverse nip shape), it is possible to improve the paper peeling performance without using a peeling means such as a peeling claw. Paper can be peeled off (self-stripping), and image defects caused by the peeling means can be eliminated.

  By the way, in such a color fixing device, it is necessary to widen the nip width of the fixing nip portion in order to cope with a high speed. There are two methods for increasing the nip width: a method of increasing the elastic layer of the fixing roller and a method of increasing the diameter of the fixing roller.

  However, since the thermal conductivity of the elastic layer is very low, when there is a heating means inside the fixing roller as in the past, the fixing roller temperature follows the fixing roller with a thick elastic layer when the process speed is increased. There is a problem that it will not.

  On the other hand, when the diameter of the fixing roller is increased, there are problems such as a longer warm-up time and increased power consumption.

  In order to solve such a problem, in recent years, in a color fixing device, as disclosed in Patent Document 1, a fixing belt is stretched between a fixing roller and a heating roller, and fixing is performed via the fixing belt. A belt fixing method in which a roller and a pressure roller are pressed against each other has been used.

  In this belt fixing device, since the fixing belt having a small heat capacity is heated, the warm-up time is short, and it is not necessary to incorporate a heat source such as a halogen lamp in the fixing roller, so that a low hardness elastic layer made of sponge rubber or the like is thickened. It is possible to provide a wide nip width.

JP 2004-286922 A

  However, such a conventional belt fixing device has the following problems.

  That is, in the configuration of Patent Document 1, a fixing member is provided separately from the driving roller in order to enlarge the fixing nip. As a result, a wide nip can be achieved, but the individually provided fixing member becomes a heat load and the warm-up time is extended, leading to an increase in power consumption.

  The present invention has been made in view of the above-described conventional problems. The object of the present invention is to provide a fixing device that has excellent thermal efficiency, low power consumption, and does not cause fixing failure, and an image forming apparatus including the same. Is to provide.

To achieve the above object, in the present invention, a fixing roller rotatably supported, an endless fixing belt wound around the fixing roller, a heating unit for heating the fixing belt to a predetermined temperature, A pressure roller which is pressed against the fixing roller via the fixing belt and is rotatably supported, and the recording material is passed through a nip formed by the fixing roller and the pressure roller via the fixing belt. A fixing device for fixing an upper unfixed toner image, wherein a gap is provided between the fixing roller and the fixing belt on the exit side in the recording material conveyance direction of the nip so that the nip exit side of the fixing belt is separated from the fixing roller. around the pressure roller side Te according virtual nip portion is formed, the heating unit is rotated friendly disposed on the inner surface of the fixing belt to suspending the fixing belt together with the fixing roller The fixing belt is stretched between the heating roller and the fixing roller, and the distance between the axes of the heating roller and the fixing roller can be adjusted. The tension of the fixing belt can be adjusted, and the gap is formed by adjusting the tension of the fixing belt.

  According to the above configuration, the fixing belt is arranged to be wound around the pressure roller side by the gap provided on the nip outlet side. Thereby, in addition to the actual nip formed by the fixing roller and the pressure roller, a virtual nip is formed by winding the fixing belt around the pressure roller, and a wide nip can be achieved. As a result, the heat supply amount increases and the fixable temperature range can be shifted to the low temperature side, so that the set temperature can be lowered and a fixing device with low power consumption can be provided.

  Here, the virtual nip refers to a portion where the fixing belt is separated from the fixing roller and is wound around the pressure roller without receiving interference from the fixing roller. A substantial nip width is formed by adding the virtual nip and the actual nip formed by the fixing roller and the pressure roller.

  As a method for forming a gap between the fixing roller and the fixing belt on the nip exit side, it can be formed by adjusting the tension of the fixing belt. The adjustment of the tension of the fixing belt can most easily form the gap. When the tension of the fixing belt is increased, the fixing belt is wound around the fixing roller. Conversely, when the tension of the fixing belt is reduced, a gap can be formed on the nip exit side of the fixing belt. However, if the tension of the fixing belt is excessively loosened, the driving force for driving the fixing belt in a circular manner is not transmitted to the fixing belt, and the fixing belt does not move around.

  In this example, the tension of the fixing belt suspended on the fixing roller is set to 2.5 kgf or less, so that the tension is set lower than a normal tension (for example, 3.6 kgf).

  As a result, by setting the belt tension of the fixing belt to a predetermined value or less, it is possible to widen the gap on the nip outlet side and enlarge the virtual nip, thereby achieving a wider nip. As a result, the amount of heat supply increases and the fixable temperature range can be shifted to the low temperature side, so that the set temperature can be lowered and a fixing device with low power consumption can be provided.

  Further, the lower limit value of the tension of the fixing belt may be 0.7 kgf or more. If it is less than 0.7 kgf, the rotational driving force of the fixing belt may not be transmitted to the fixing belt.

  In addition to the above, as a method for forming a gap between the fixing roller and the fixing belt on the nip exit side, a relative speed difference (rotational speed difference) is provided between the fixing roller and the pressure roller. A gap can be formed between the fixing belt and the fixing roller on the nip exit side. In addition to adjusting the tension of the fixing belt, a method of providing a speed difference between the fixing roller and the pressure roller may be combined to form the fixing belt gap on the nip exit side.

  The size of the gap on the nip exit side in the recording material conveyance direction of these fixing belts, in other words, how much the virtual nip width is set depends on the degree of tension adjustment of the fixing belt, but as another influencing factor, pressurization Degree of pressing of the fixing roller by the roller, a heating part opposite to the fixing roller diameter, particularly the heating roller diameter, the distance between the fixing roller and the heating roller, and the physical properties of the fixing belt (particularly, tensile strength, elastic force) , Restoring force, etc.) and physical properties of the fixing roller and pressure roller (especially physical properties of the elastic layer on the surface). In this example, the tension of the fixing belt is adjusted under the same conditions as those in the prior art.

  In addition, although the structure provided with the heater can be illustrated for a heating part, the arrangement | positioning should just be a structure arrange | positioned inside and / or the outside of a fixing belt. Further, the heating unit may be either a fixed type or a rotary type such as a heating roller. These heating units can be arranged on the inner surface side of the fixing belt so as to suspend the fixing belt together with the fixing roller. Thereby, the heating part can function as a suspension part of the fixing belt.

In particular, the heating unit is a heating roller that is rotatably supported, and the fixing belt can be configured to be stretched between the heating roller and the fixing roller. Thus, the fixing belt can be stretched between the heating roller and the fixing roller and smoothly moved between both rollers without preparing a separate member for suspending the fixing belt.

  In this case, the distance between the axes of the heating roller and the fixing roller can be adjusted, and the tension of the fixing belt can be adjusted by adjusting the distance between the axes. As a result, the tension of the fixing belt can be adjusted without preparing a member for adjusting the tension of the fixing belt.

  The distance between the axes of the heating roller can be adjusted by adjusting the urging force of the urging spring added to the heating roller.

  The fixing belt has a multilayer structure, and a configuration in which the surface layer of the fixing belt is formed of a fluororesin tube can be employed.

  By making the fixing belt have a multilayer structure, a fixing belt having functions of strength, elasticity and releasability can be achieved. Then, by forming the surface layer of the fixing belt with a fluororesin tube, stable releasability can be secured, durability and scratch resistance are improved, and a long life can be achieved.

  As a multilayer of the fixing belt, a synthetic resin or a metal material can be used for the base layer. For example, the fixing belt can be a metal belt having a base layer made of a metal material.

  By using a fixing belt having a base layer of a metal material, it is possible to achieve higher thermal conductivity than a conventional belt such as a polyimide base material. As an example, when the polyimide is changed to nickel steel, the heat conductivity can be greatly increased (polyimide: 0.37 W / mK⇒nickel steel: 59 to 90 W / mK), and the fixing belt can be heated uniformly without unevenness. Therefore, good fixability can be ensured. Further, since the mechanical strength is increased by changing from a synthetic resin material to a metal material, durability is improved and a long life can be achieved.

  The above fixing device can be employed in an image forming apparatus. Accordingly, by using a fixing belt having a small heat capacity as a fixing member, it is possible to provide a fixing device with low power consumption and an image forming apparatus using the same.

  As described above, according to the present invention, the fixing belt is arranged to be wound around the pressure roller side by the gap provided on the nip outlet side, so that in addition to the actual nip formed by the fixing roller and the pressure roller, Thus, a virtual nip formed by winding the fixing belt around the pressure roller is added, and a wide nip can be achieved. As a result, the amount of heat supplied by the fixing belt increases, and the fixable temperature range can be shifted to the low temperature side, so that the set temperature can be lowered, and the fixing device with low power consumption and an image provided with the fixing device A forming apparatus can be provided.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. 1 is a schematic configuration diagram of a fixing device according to an embodiment of the present invention. FIG. 2 is an enlarged view of a main part of the fixing device.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. As shown in the figure, the image forming apparatus 100 forms a multicolor or single color image on a predetermined recording material (sheet-like paper or OHP sheet) according to image data transmitted from the outside.

  The image forming apparatus 100 of this example includes four image forming units pa, pb, pc, pd, an intermediate transfer belt 11, a transfer unit 14, a fixing device 15, a paper feed tray 16, a manual paper feed unit 17, and a paper discharge tray. 18 is provided.

  The four image forming units pa to pd include developing units 102a to 102d, photosensitive drums 101a to 101d, chargers 103a to 103d, and cleaner units 104a to 104d, respectively. Image data handled in this example corresponds to a color image using each color of black (K), cyan (C), magenta (M), and yellow (Y). Accordingly, each of the image forming units 101a to 101d executes an image forming process for any color component of black (K), cyan (C), magenta (M), and yellow (Y).

  Specifically, black is used for the photosensitive drum 101a of the image forming unit pa, cyan is used for the photosensitive drum 101b of the image forming unit pb, magenta is used for the photosensitive drum 101c of the image forming unit pc, and the photosensitive drum 101d of the image forming unit pd. Then, an electrostatic latent image and a developer image relating to the yellow color component are formed.

  Of the four image forming units pa, pb, pc, pd, the image forming unit pa will be described. The image forming unit pa includes a developing device 102a, a photosensitive drum 101a, a charger 103a, and a cleaner unit 104a. The photosensitive drum 101 a is disposed on the upper part of the image forming apparatus 100. The charger 103a is for uniformly charging the surface of the photosensitive drum 101a to a predetermined potential. In this example, a roller-type charger 103a is used. However, a brush-type charger or a charger-type charger may be used as the charger 103a.

  The developing device 102a visualizes the electrostatic latent image formed on the photosensitive drum 101a with black (K) toner. The cleaner unit 104a removes and collects toner remaining on the surface of the photosensitive drum 101a after development and image transfer.

  Since the other image forming units pb, pc, pd have the same configuration as the image forming unit pa, the description thereof is omitted. The developing devices 102b, 102c, and 102d of the image forming units pb, pc, and pd contain cyan (C), magenta (M), and yellow (Y) toners, respectively.

  In this example, the optical system unit E for exposing the four photosensitive drums 101a to 101d is combined into one unit. This optical system unit E includes a laser beam light source (not shown), a deflector 4 such as a polygon mirror, a folding mirror 8 that guides the deflected beams to four sets of photosensitive drums 101a to 101d, and the like. The deflected beam scans the surfaces of the photosensitive drums 101a, 101b, 101c, and 101d through the folding mirror 8 and the like.

  In this manner, the optical system unit E exposes the photosensitive drums 101a to 101d charged according to the input image data, thereby forming an electrostatic latent image according to the image data on the surface thereof. Have Here, a laser scanning unit (LSU) is used as the optical system unit E. However, for example, an EL or LED writing head in which light emitting elements are arranged in an array may be used.

  The intermediate transfer belt 11 disposed above the photosensitive drums 101a, 101b, 101c, and 101d is stretched between the tension rollers 11a and 11b in a tensioned state. The intermediate transfer rollers 13a, 13b, 13c, and 13d are pressed against the photosensitive drums 101a, 101b, 101c, and 101d through the intermediate transfer belt 11, respectively.

  The transfer roller 14 constituting the transfer unit is in pressure contact with the tension roller 11 a via the intermediate transfer belt 11. The toner remaining on the intermediate transfer belt 11 is collected by a waste toner box 12.

  The intermediate transfer belt 11 is formed endlessly using a film having a thickness of about 100 μm to 150 μm, and is provided so as to be in contact with each of the photosensitive drums 101 a to 101 d. Further, the toner images of the respective colors formed on the photosensitive drums 101a to 101d are sequentially transferred to the intermediate transfer belt 11 in a superimposed manner. The intermediate transfer belt 11 has a function of supporting a multicolor toner image on the surface thereof.

  Transfer of the toner image from the photosensitive drums 101a to 101d to the intermediate transfer belt 11 is performed by intermediate transfer rollers 13a to 13d that are in contact with the back side of the intermediate transfer belt 11. A high voltage transfer bias is applied to the intermediate transfer rollers 13a to 13d in order to transfer the toner image. In this example, since the charging polarity of the toner is negative, a positive high voltage is used as the transfer bias.

  The intermediate transfer rollers 13a to 13d are rollers whose base is a metal (for example, stainless steel) shaft having a diameter of 8 to 10 mm and whose surface is covered with a conductive elastic material (for example, EPDM, urethane foam, or the like). This conductive elastic material is used to uniformly apply a high voltage to the intermediate transfer belt 11. In this example, a roller shape is used as the transfer electrode, but a brush or the like may be used instead.

  The transfer roller 14 is disposed so as to press the tension roller 11 a through the intermediate transfer belt 11. Toner images (developer images) formed on the respective photosensitive drums 101 a to 101 d in accordance with the respective hues are stacked on the intermediate transfer belt 11. Subsequently, the stacked toner images are guided to a contact position between the sheet-like recording material and the intermediate transfer belt 11 by the rotation of the intermediate transfer belt 11, and transferred onto the recording material by a transfer bias applied to the transfer roller 14. The

  The intermediate transfer belt 11 and the transfer roller 14 are pressed against each other so that a predetermined amount of nip is formed. In order to obtain the nip constantly, the transfer roller 11 uses either one of the transfer roller 14 or the intermediate tension roller 11a as a hard material (metal or the like), and the other is a soft material such as an elastic roller (an elastic rubber roller, Alternatively, a foaming resin roller or the like is used.

  A paper feed tray 16 disposed in the lower part of the image forming apparatus 100 is a tray for storing sheet-like recording materials used for image formation. The image forming units pa to pd and the optical system unit E It is provided on the lower side. Further, the image forming apparatus 100 is provided with a recording material conveyance path S for sending the recording material on the paper feed tray 16 to the paper discharge tray 18 via the transfer roller 14 and the fixing device 15. Along the recording material conveyance path S, a pickup roller 16a, a registration roller 19, a transfer roller 14, a fixing device 15, a conveyance roller r for conveying the recording material, and the like are disposed.

  The conveyance roller r is a small roller for promoting and assisting conveyance of the paper, and a plurality of conveyance rollers r are provided along the recording material conveyance path S. The pickup roller 16 a is a pull-in roller that is provided at the end of the paper feed tray 16 and supplies recording materials from the paper feed tray 16 one by one to the recording material conveyance path S. The registration roller 19 temporarily holds the recording material conveyed in the recording material conveyance path S, and transfers the recording material at a timing when the leading edge of the toner image on the photosensitive drums 101a to 101d is aligned with the leading edge of the recording material. 14 and the tension roller 11a.

  In addition to the paper feed cassette 16 that stores the recording material, the image forming apparatus 100 is provided with a manual feed tray 17 that is used when a user prints a small number of sheets. Pickup rollers 17 a and 16 a are arranged in the vicinity of the paper feed cassette 16 and the manual feed tray 17. The pickup rollers 17a and 16a feed the recording materials one by one to the recording material conveyance path S.

  The recording material conveyed from the paper feed cassette 16 is conveyed to the registration roller 14 by the conveyance roller r in the conveyance path. The recording material is conveyed between the transfer roller 14 and the tension roller 11a at the timing when the leading edge of the recording material and the leading edge of the image information on the intermediate transfer belt 11 are aligned, and the toner image carried on the intermediate transfer belt 11 is recorded. It is transferred to the material. Subsequently, the recording material passes through the fixing device 15 so that the unfixed toner on the recording material is melted and fixed by heat. Thereafter, the recording material is discharged from the paper discharge roller 18a onto the paper discharge tray 18 through the conveyance roller r.

  On the other hand, the recording material loaded on the manual paper feed tray 17 is fed by a pickup roller 17a, reaches a registration roller 19 through a plurality of transport rollers r, and thereafter is fed from a paper feed cassette 16. The paper is discharged to the paper discharge tray 18 through the same path as the material. At this time, when the print request content is a double-sided print request, the rear end of the paper that has finished single-sided printing and passed through the fixing device 15 as described above is sandwiched by the paper discharge roller 18a, and the paper discharge roller 18a rotates in the reverse direction. Thus, after being guided to the conveyance roller r, the back side printing is performed through the registration roller 19 and then discharged to the discharge tray 18.

  The image forming apparatus 100 includes the fixing device 15 according to the present invention. FIG. 2 is a schematic configuration diagram of the fixing device. As shown in FIG. 2, the fixing device 15 includes an endless fixing belt 15a, a fixing roller 15b and a heating unit 15c that suspends the fixing belt 15a, and pressure applied to the fixing roller 15b via the fixing belt 15a. And a roller 15d. Then, the recording material is passed through the nip N formed by the fixing roller 15b and the pressure roller 15d via the fixing belt 15a to fix the unfixed toner image on the recording material. At this time, by providing a gap 25 between the fixing roller 15b and the fixing belt 15a at the exit side of the nip N in the recording material conveyance direction, the nip exit side is added away from the fixing roller 15b in the recording material conveyance direction of the fixing belt 15a. A virtual nip portion N1 is formed to be wound around the pressure roller 15d.

The specific configuration of the fixing device 15 is as follows. The fixing device 15 includes a fixing belt 15a having a diameter of 75 mm, a fixing roller 15b having a diameter of 50 mm, a pressure roller 15d having a diameter of 50 mm, and a heating roller having a diameter of 35 mm as a heating unit.
The heating member 15c is not limited to the heating roller, and may be formed, for example, in a circular arc shape so that the fixing belt 15a can be smoothly slid on the circular arc surface.

  The fixing roller 15b has a substantially cylindrical shape, and is made of a hollow or solid metal core 15b1 made of SUS or the like as a rigid body, and an elastic made of a foam rubber material formed on the outer periphery of the core 15b1. The layer 15b2 has a two-layer structure. The fixing roller 15b is rotatably supported by the fixing device main body and is rotationally driven by a motor (not shown). For the elastic layer 15b2 of the fixing roller 15b, a heat-resistant rubber material such as silicone rubber or fluorine rubber is suitable. In this example, a 15 mm thick silicone sponge rubber layer is formed as an elastic layer 15b2 on a solid metal core 15b1 having a diameter of 20 mm.

  The pressure roller 15d has a substantially cylindrical shape, and includes a hollow metal core 15d1, an elastic layer 15d2 such as silicone rubber, and a release layer (not shown) such as fluororesin in order from the inside. . For the elastic layer 15d2, a rubber material having heat resistance such as silicone rubber and fluorine rubber is suitable. A fluororesin such as PFT (copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether) or PTFE (polytetrafluoroethylene) is suitable for the release layer. In this example, a 2 mm-thick silicone rubber layer is formed as an elastic layer 15d2 on the surface of a hollow cored bar 15d1 having a diameter of 46 mm, and a 30 μm-thick PFA tube is further coated thereon as a release layer. . A heater lamp 21 such as a halogen lamp is disposed inside the pressure roll 15d.

  The heating roller 15c includes a substantially cylindrical cored bar made of a metal such as aluminum, an infrared absorption layer (not shown) is formed on the inner peripheral surface, and a release layer (such as PFA) is formed on the outer peripheral surface. (Not shown) is formed. The infrared absorption layer is formed as a black paint layer in which a carbon black-containing paint is applied to the inner surface of the core metal, thereby improving heating efficiency. In this example, a release layer made of PFA is formed on the surface of a hollow metal core having a thickness of 0.7 mm. A heater lamp 21 such as a halogen lamp is disposed inside the heating roll 15c.

  The fixing belt 15a is formed on an outer surface of a flexible hollow cylindrical base layer 15a1 made of a heat-resistant resin such as polyimide or a metal material such as stainless steel or nickel, and an elastomer material (for example, silicone rubber) having excellent heat resistance and elasticity. ) And a surface layer 15a3 (release layer) made of a fluororesin having excellent heat resistance and releasability is formed on the outer surface of the belt elastic layer 15a2. Yes. Examples of the fluororesin include PFA and PTFE.

  Further, the fixing belt 15a may be made to internally add carbon black or the like to the polyimide of the base layer 15a1. Thereby, charging of the fixing belt 15a can be prevented, and electrostatic offset can be reduced.

  The surface layer 15a3 of the fixing belt is preferably formed of a tube made of a fluororesin. By covering the surface of the belt elastic layer 15a2 with a fluororesin tube to form a surface layer, stable release properties can be secured, durability and scratch resistance are good, and a long life can be achieved.

  In this example, a 200 μm thick silicone rubber layer is formed as a belt elastic layer 15a2 on a 50 μm cylindrical nickel base material as a base layer 15a1, and further coated with a 30 μm thick PFA tube to form a surface layer 15a3. .

  As shown in FIG. 2, the fixing belt 15a is stretched in a state where a predetermined tension is applied between the fixing roller 15b and the heating roller 15c. Each of the heating roller 15c and the pressure roller 15d is provided with a heater lamp 21, and the fixing belt 15a has a predetermined fixing temperature by a control unit (not shown) based on a signal from a temperature detector (not shown). Thus, the output to the heater lamp 21 is controlled.

  As shown in FIG. 3, the pressure roller 15d is disposed opposite to the fixing roller 15b with the fixing belt 15a interposed therebetween, and presses the fixing roller 15b. Thereby, between the fixing belt 15a and the pressure roller 15d, the fixing roller 15b and the pressure roller 15d come into contact with each other via the fixing belt 15a. This contact portion is a fixing nip portion N. In the fixing nip portion N, the recording material enters from the X direction, and the recording material is thermocompression-bonded by the fixing roller 15b and the pressure roller 15d, so that the multicolor toner image transferred onto the paper is melted, mixed, and pressed. And heat-fixing to the paper.

  At this time, as shown in FIG. 3, between the fixing roller 15b and the fixing belt 15a on the exit side in the recording material conveying direction of the nip portion N formed by the fixing roller 15b and the pressure roller 15d via the fixing belt 15a. An air gap 25 is provided in Accordingly, a virtual nip portion N1 is formed in which the nip exit side of the fixing belt 15a is separated from the fixing roller 15b and is wound around the pressure roller 15d.

  In this example, the gap 25 is formed by adjusting the tension of the fixing belt 15a. By setting the belt tension of the fixing belt 15b to a predetermined value or less, the gap 25 on the nip outlet side can be widened to enlarge the virtual nip N1, and a wider nip can be achieved. As a result, the heat supply amount increases and the fixable temperature range can be shifted to the low temperature side, so that the set temperature can be lowered and a fixing device with low power consumption can be provided.

  The tension of the fixing belt 15a can be achieved by adjusting the distance L between the axes of the fixing roller 15b and the heating roller 15c. For example, as shown in FIG. 2, the adjusting device 26 that adjusts the inter-axis distance L between the fixing roller 15b and the heating roller 15c can move the heating roller 15c in a direction in which the axes are close to and away from the fixing roller 15b. And a biasing spring 27 that biases the heating roller 15c in a direction away from the fixing roller 15b, and a regulating screw 28 that regulates the heating roller 15c from moving away from the fixing roller 15b is applied to the shaft 29 of the heating roller. Keep in touch. By moving the regulating screw 28 in the axial direction, the axial position of the heating roller 15c is determined by the balance with the urging force of the urging spring 27. In FIG. 2, the shaft 29 of the heating roller 15 c is extracted and drawn on the upper side of the drawing for easy understanding of the configuration of the adjusting device 26.

  When the axial position of the heating roller 15c is determined, the distance between the fixing roller 15b and the heating roller 15c is determined, and the tension of the fixing belt 15a wound around the fixing roller 15b and the heating roller 15c is also determined. .

  Here, the rotational movement of the regulating screw 28 in the axial direction may be performed manually or by a driving mechanism such as a motor through a control unit (not shown). Thereby, the center distance between the heating roller 15c and the fixing roller 15b can be freely changed.

  Table 1 shows the relationship between the nip width (N + N1) and the fixing temperature (set temperature) by changing the belt tension of the fixing belt 15a in three stages. The fixing temperature was measured with a temperature detector such as a thermistor facing the surface temperature of the fixing belt 15a. The belt tension was obtained by measuring the load applied to the shaft portion of the heating roller 15c. The belt tension was changed in three stages of 0.7 kgf, 2.5 kgf, and 3.6 kgf to obtain measurement data of the nip width and the fixing temperature. The fixing temperature was changed from 160 degrees to 200 degrees at intervals of 5 degrees, and the state of the toner fixed on the recording material at that time was subjected to sensory evaluation. “◯” indicates a case where there is no problem in image density and image quality, and “X” indicates a case where the image density is lowered or the image quality is inferior.

  As shown in Table 1, when the fixing belt 15a has a normal belt tension of 3.6 kgf, there is no gap between the fixing belt 15a and the fixing roller 15b on the exit side of the fixing nip portion N. The nip width was 10.5 mm. When the normal nip width is 10.5 mm, good results are obtained when the fixing temperature is in the range of 175 ° to 200 °. Below 170 °, the toner is not sufficiently melted and the toner image adheres to the fixing belt 15a. A phenomenon (so-called low temperature offset phenomenon) occurs, and image density unevenness occurs.

  When the belt tension of the fixing belt 15a is 2.5 kgf, a gap 25 is formed between the fixing belt 15a and the fixing roller 15b on the exit side of the fixing nip portion N, and a virtual nip N1 is generated. The nip width was 11.0 mm. When the substantial nip width is 11.0 mm, good results can be obtained when the fixing temperature is in the range of 165 ° to 190 °, low temperature offset occurs at 160 ° or less, and conversely high temperature offset phenomenon at 195 ° or more. Occurs, and an image defect occurs.

  When the belt tension of the fixing belt 15a is 0.7 kgf, a gap 25 is formed between the fixing belt 15a and the fixing roller 15b on the exit side of the fixing nip portion N, and a virtual nip N1 is generated. The nip width was 13.2 mm. When the substantial nip width is 13.2 mm, good results are obtained when the fixing temperature is in the range of 165 ° to 190 °. A low temperature offset occurs when the temperature is 160 ° or less, and a high temperature offset phenomenon occurs when the temperature is 195 ° or more. Then, an image defect occurs.

  In this way, by adjusting the belt tension of the fixing belt 15a and providing the gap 25 between the fixing belt 15a and the fixing roller 15b on the nip exit side between the fixing roller and the pressure roller, the fixing belt 15a is wound around the pressure roller side, and a virtual nip N1 is added to form a wide nip. Therefore, the amount of heat supplied by the fixing belt 15a is increased, and as shown in Table 1, the fixable temperature range can be shifted to the low temperature side, so that the set temperature can be lowered and the fixing with low power consumption. An apparatus and an image forming apparatus including the apparatus can be provided.

  Note that the present invention is not limited to the above-described embodiment, and it is needless to say that many modifications can be made within the scope of the present invention. For example, in the above embodiment, the gap between the fixing belt and the fixing roller is formed on the nip exit side by adjusting the belt tension of the fixing belt. By providing a relative speed difference, the bending of the fixing belt is formed in the downstream region (nip exit side) in the circumferential direction of the fixing belt, and the virtual nip around which the fixing belt is wound around the pressure roller is increased. May be.

E Optical system unit N Nip part N1 Virtual nip S Recording material conveying path pa, pb, pc, pd Image forming unit r Conveying roller 8 Folding mirror 11 Intermediate transfer belt 11a, 11b Tension roller 12 Waste toner boxes 13a, 13b, 13c, 13d Intermediate transfer roller 14 Transfer unit (transfer roller)
15 fixing device 15a fixing belt 15a1 base layer 15a2 elastic layer 15a3 surface layer (release layer)
15b Fixing roller (fixing member)
15b1 Core 15b2 Elastic layer 15c Heating roller (heating member)
15d Pressure roller (Pressure member)
15d1 Core 15d2 Elastic layer 16 Paper feed tray 17 Manual paper feed units 16a and 17a Pickup roller 18 Paper discharge tray 19 Registration roller 21 Heater lamp 25 Air gap 26 Adjusting device 27 Biasing spring 28 Regulating screw 100 Image forming devices 102a to 102d Development Devices 101a to 101d Photosensitive drums 103a to 103d Chargers 104a to 104d Cleaner units

Claims (5)

A fixing roller that is rotatably supported, an endless fixing belt wound around the fixing roller, a heating unit that heats the fixing belt to a predetermined temperature, and can be rotated by being pressed against the fixing roller via the fixing belt. A fixing device that includes a pressure roller supported and fixes a non-fixed toner image on the recording material by passing the recording material through a nip formed by the fixing roller and the pressure roller via the fixing belt. By providing a gap between the fixing roller and the fixing belt at the exit side of the nip in the recording material conveyance direction, a virtual nip portion is formed in which the nip exit side of the fixing belt is separated from the fixing roller and is wound around the pressure roller side And
The heating unit is a heating roller that is disposed on the inner surface side of the fixing belt so as to suspend the fixing belt together with the fixing roller and is rotatably supported
The fixing belt is stretched between the heating roller and the fixing roller,
The heating roller and the fixing roller can be adjusted in the distance between the axes, the tension in the fixing belt can be adjusted by adjusting the distance between the axes, and the gap is formed by adjusting the tension in the fixing belt. a fixing device, characterized in that that. The fixing device according to claim 1, wherein the fixing belt has a multilayer structure, and a surface layer of the fixing belt is formed of a fluororesin tube. The fixing belt fixing device according to claim 1 or 2, characterized in that a metal belt having a base layer made of a metal material. The tension of the fixing belt suspending the fixing roller fixing device according to any of claims 1-3, characterized in that the following 2.5 kgf. An image forming apparatus characterized by using the fixing device according to any one of claims 1-4.

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