Nip forming unit, fixing device, and image forming apparatus

Abstract

To restore a deformed part in a short time without damaging an endless belt.SOLUTION: A nip forming unit comprises: a rotatable, flexible endless belt 20; a nip forming member (heater 22) that is provided to be contactable with an inner peripheral surface of the endless belt; a heating member (heater 22) that heats the nip forming member; a pressure member that is in pressure contact with the nip forming member with the endless belt therebetween to form a nip for sandwiching and conveying a body to be conveyed; a separation member that separates the body to be conveyed passing through the nip from the endless belt; and driving means that rotates to drive the pressure member and the endless belt, wherein the driving means drives the pressure member and the endless belt in a normal direction, and thereby the body to be conveyed is conveyed through the nip. Before conveying the body to be conveyed, the driving means drives the pressure member and the endless belt in a reverse direction to sequentially move, to the nip, an exit adjacent part of the endless belt adjacent to an exit side of the nip and an entrance adjacent part adjacent to an entrance side, and heat the adjacent parts by heating means.SELECTED DRAWING: Figure 8

Description

Technical Field

[0006] , , , , , ,

[0005]

[0001] The present invention relates to a nip forming unit including a separating member for separating a conveyed body that has passed through a nip from an endless belt, a fixing device including the nip forming unit, and an image forming apparatus.

Background Art

[0002] In image forming apparatuses such as copiers and printers, as tensionless fixing devices using a cylindrical thin-walled low heat capacity heat-resistant resin film as disclosed in Patent Documents 1 to 3, for example, are known. This tensionless fixing device has high heat transfer efficiency and a fast startup of the device, so it is suitable for an on-demand system.

[0003] The fixing devices disclosed in Patent Documents 1 and 2 separate the paper from the fixing belt with a separating member fixedly arranged close to the fixing belt. When using such a separating member, if the distance between the fixing belt and the separating member is too small, the separating member is likely to contact the fixing belt and damage the belt, which causes abnormal images.

[0004] On the other hand, if the distance between the fixing belt and the separating member is too large, the paper passes through this large distance and wraps around the fixing belt, easily causing a paper jam. Therefore, it is necessary to bring the separating member as close as possible to the fixing belt within a range where it does not contact the fixing belt.

Summary of the Invention

Problems to be Solved by the Invention

[0007] When the fixing belt starts rotating (forward) in this kinked state, the rotational trajectory of the fixing belt fluctuates irregularly, sometimes becoming vertically elongated and sometimes horizontally elongated (rotational unevenness). This rotational unevenness is particularly noticeable when the fixing device is not sufficiently warmed up, such as immediately after the machine is powered on, because the bending rigidity of the fixing film is high.

[0008] Therefore, the fixing belt could come into contact with the downstream separation member, causing damage to the belt, which could lead to the generation of abnormal images. To address this, the fixing devices described in Patent Documents 1 and 2 reverse the fixing belt until the temperature of the pressurizing member reaches a predetermined temperature before the recording medium is fed through the nip, thereby preventing the fixing belt from coming into contact with the downstream separation member and removing any deformation of the fixing belt.

[0009] However, if the fixing belt is rotated in reverse until the temperature of the pressurizing member reaches a predetermined temperature, the fixing belt will be in contact with the nip forming member for a longer period, causing the sliding surface of the part to deteriorate prematurely and shortening the lifespan of the fixing device. In addition, since it takes a considerable amount of time for the temperature of the pressurizing member to reach a predetermined temperature, the job cannot be started during that time, which also causes a delay in the start of the job.

[0010] On the other hand, the fixing device described in Patent Document 3 prevents uneven gloss and streaks in the image caused by deformation (bending marks) of the belt by heating the portions adjacent to the nip entrance and exit of the fixing belt by inching them against the nip through forward and reverse rotation of the belt. This quickly removes the deformation (iron effect of the nip) of the portions adjacent to the entrance and exit of the fixing belt without requiring the fixing belt to rotate idle for a long time.

[0011] However, when the fixing belt is moved in an increment in the forward direction, there is a problem in that the deformation may come into contact with the downstream separation member, potentially damaging the belt and causing abnormal images.

[0012] Therefore, the objective of the present invention is to remove deformation kinks from an endless belt in a short time without damaging it. [Means for solving the problem]

[0013] To solve the above problems, the present invention provides a nip-forming unit comprising: a rotatable flexible endless belt; a nip-forming member provided so as to be in contact with the inner circumferential surface of the endless belt; a heating member for heating the nip-forming member; a pressurizing member for pressing against the nip-forming member via the endless belt to form a nip for gripping and conveying a conveyed object; a separation member for separating the conveyed object that has passed through the nip from the endless belt; and a driving means for rotationally driving the pressurizing member and the endless belt, wherein the conveyed object is conveyed by forward rotation of the pressurizing member and the endless belt by the driving means, and before conveying the conveyed object, the driving means by The pressurizing member and the endless belt It repeatedly reverses and pauses. As a result, the outlet adjacent portion of the endless belt adjacent to the exit side of the nip and the inlet adjacent portion adjacent to the inlet side are sequentially moved to the nip, and the heating component It is characterized by being heated. [Effects of the Invention]

[0014] According to the present invention, deformation kinks in the endless belt can be removed in a short time without damaging the belt. [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic diagram of an image forming apparatus according to an embodiment of the present invention. [Figure 2] This is a schematic diagram of the fixing device. [Figure 3] This is a perspective view of the heater, heater holder, and guide section. [Figure 4A] It is a plan view of a heater. [Figure 4B] It is a figure showing the atomic crystal structure of graphene. [Figure 4C] It is a figure showing the atomic crystal structure of graphite. [Figure 5] It is a figure showing a power supply circuit to a heater. [Figure 6A] It is a flowchart showing the control operation of a heater. [Figure 6B] It is a flowchart showing the pre-sheet-passing control operation of a fixing belt and a heater. [Figure 7] It is a figure showing a deformation tendency of a fixing belt. [Figure 8] It is a figure explaining the pre-sheet-passing control operation of a fixing belt. [Figure 9] It is a figure explaining the difference in the tendency-removing effect according to the belt temperature. [Figure 10] It is a figure showing another example of a conductive member. [Figure 11] It is a figure showing another example of a conductive member. [Figure 12] It is a perspective view of a conductive member having a bent portion and its periphery. [Figure 13] It is a figure showing an example of the longitudinal arrangement of a conductive member. [Figure 14] It is a figure showing an example where the longitudinal arrangement of the conductive member is different from that in FIG. 13. [Figure 15] It is a side cross-sectional view of a fixing device according to an embodiment in which a conductive member is provided in an insertion hole of a guide rib. [Figure 16] It is a side cross-sectional view of a fixing device according to an embodiment in which the extending direction of the conductive member is different. [Figure 17] It is a plan view of a heater. [Figure 18] It is a figure showing the power supply to a heater. [Figure 19] It is a plan view of a heater in which the shape of the resistance heating element is different from that in FIG. 17. [Figure 20] It is a plan view of a heater in which the shape of the resistance heating element is different from that in FIGS. 17 and 19. [Modes for carrying out the invention]

[0016] The present invention will be described below with reference to the attached drawings. In each drawing used to explain the present invention, components such as members and parts having the same function or shape will be given the same reference numerals to the extent possible so that they can be distinguished, and their description will be omitted after they have been described once.

[0017] (●Image forming apparatus) Figure 1 is a schematic diagram of an image forming apparatus according to one embodiment of the present invention. The image forming apparatus 100 shown in Figure 1 comprises four image forming units 1Y, 1M, 1C, and 1Bk that are detachable from the main body of the image forming apparatus.

[0018] Each image-forming unit 1Y, 1M, 1C, and 1Bk has the same configuration except that it contains different colored developers corresponding to the color separation components of a color image: yellow, magenta, cyan, and black. Specifically, each image-forming unit 1Y, 1M, 1C, and 1Bk comprises a drum-shaped photoreceptor 2 as an image carrier, a charging device 3 for charging the surface of the photoreceptor 2, a developing device 4 for supplying toner as a developer to the surface of the photoreceptor 2 to form a toner image, and a cleaning device 5 for cleaning the surface of the photoreceptor 2.

[0019] The image forming apparatus 100 also includes an exposure apparatus 6 that exposes the surface of each photoreceptor 2 to form an electrostatic latent image, a paper feed apparatus 7 that supplies paper P as a transported object or recording medium, a transfer apparatus 8 that transfers the toner image formed on each photoreceptor 2 to the paper P, a fixing apparatus 9 that acts as a nip forming unit to fix the toner image transferred to the paper P, and a paper discharge apparatus 10 that discharges the paper P outside the apparatus. Recording media include paper P (plain paper), as well as cardboard, postcards, envelopes, thin paper, coated paper (such as coated paper and art paper), tracing paper, OHP sheets, plastic films, prepregs, copper foil, etc.

[0020] The transfer device 8 includes an endless intermediate transfer belt 11 as an intermediate transfer body stretched by a plurality of rollers, four primary transfer rollers 12 as primary transfer members that transfer toner images on each photoreceptor 2 to the intermediate transfer belt 11, and a secondary transfer roller 13 as a secondary transfer member that transfers the toner images transferred on the intermediate transfer belt 11 to the paper P. Each of the plurality of primary transfer rollers 12 is in contact with the photoreceptor 2 via the intermediate transfer belt 11.

[0021] As a result, the intermediate transfer belt 11 and each photoreceptor 2 come into contact with each other, forming a primary transfer nip between them. Meanwhile, the secondary transfer roller 13 comes into contact with one of the rollers that tension the intermediate transfer belt 11 via the intermediate transfer belt 11. As a result, a secondary transfer nip is formed between the secondary transfer roller 13 and the intermediate transfer belt 11.

[0022] Furthermore, a paper transport path 14 is formed inside the image forming apparatus 100 through which the paper P fed from the paper feed device 7 is transported. A pair of timing rollers 15 are provided along this paper transport path 14, from the paper feed device 7 to the secondary transfer nip (secondary transfer roller 13).

[0023] Next, the printing operation of the image forming apparatus will be described with reference to Figure 1. When an instruction to start the printing operation is given, in each image forming unit 1Y, 1M, 1C, 1Bk, the photoreceptor 2 is driven to rotate clockwise as shown in Figure 1, and the surface of the photoreceptor 2 is charged to a uniform high potential by the charging device 3.

[0024] Next, based on the image information of the document read by the document scanner or the print information instructed from the terminal, the exposure device 6 exposes the surface of each photoreceptor 2, causing the potential of the exposed area to decrease and forming an electrostatic latent image. Then, toner is supplied from the developing device 4 to this electrostatic latent image, and a toner image is formed on each photoreceptor 2.

[0025] The toner images formed on each photoreceptor 2 reach the primary transfer nip (the position of the primary transfer roller 12) as each photoreceptor 2 rotates, and are transferred sequentially onto the intermediate transfer belt 11, which rotates counterclockwise as shown in Figure 1. The toner images transferred onto the intermediate transfer belt 11 are then transported to the secondary transfer nip (the position of the secondary transfer roller 13) as the intermediate transfer belt 11 rotates, and are transferred to the paper P that has been transported at the secondary transfer nip.

[0026] The paper P is supplied from the paper feeder 7. The paper P supplied from the paper feeder 7 is stopped by the timing roller 15, and then transported to the secondary transfer nip in time with the toner image on the intermediate transfer belt 11 reaching the secondary transfer nip. Thus, a full-color toner image is placed on the paper P. After the toner image is transferred, any toner remaining on each photoreceptor 2 is removed by each cleaning device 5.

[0027] The paper P onto which the toner image has been transferred is transported to the fuser unit 9, where the fuser unit 9 fixes the toner image onto the paper P. After that, the paper P is ejected from the device by the paper output unit 10, completing the series of printing operations.

[0028] (● Fixing device) Next, an embodiment of the fixing device 9 as a nip forming unit will be described. As shown in Figure 2, the fixing device 9 according to this embodiment includes a fixing belt 20 made of an endless belt, a pressure roller 21 as an opposing member that contacts the outer circumferential surface of the fixing belt 20 to form a fixing nip N, a heater 22 as a heating member that heats the fixing belt 20, a heater holder 23 as a holding member that holds the heater 22, a stay 24 as a support member that supports the heater holder 23, and a thermistor 25 as a temperature sensing means that detects the temperature of the fixing belt 20.

[0029] The fixing belt 20 has, for example, a tubular substrate made of polyimide (PI) with an outer diameter of 25 mm and a thickness of 40 to 120 μm. A release layer with a thickness of 5 to 50 μm is formed on the outermost layer of the fixing belt 20 using a fluororesin such as PFA or PTFE to enhance durability and ensure release properties.

[0030] An elastic layer made of rubber or the like with a thickness of 50 to 500 μm may be provided between the substrate and the release layer. Furthermore, the substrate of the fixing belt 20 is not limited to polyimide, but may also be a heat-resistant resin such as PEEK, or a metal substrate such as nickel (Ni) or SUS. The inner circumferential surface of the fixing belt 20 may be coated with polyimide, PTFE or the like as a sliding layer.

[0031] Furthermore, the fixing belt 20 can also be constructed with a base material, a surface layer, and an adhesive layer, without an elastic layer. Without an elastic layer, the overall rigidity of the belt is reduced, and it is prone to deformation when stopped, as will be described later.

[0032] The pressure roller 21 has, for example, an outer diameter of 25 mm and is composed of a solid iron core 21a, an elastic layer 21b formed on the surface of the core 21a, and a release layer 21c formed on the outside of the elastic layer 21b. The elastic layer 21b is made of silicone rubber and has a thickness of, for example, 3.5 mm. To improve release properties, it is desirable to form a release layer 21c on the surface of the elastic layer 21b, which is a fluororesin layer with a thickness of, for example, about 40 μm.

[0033] Furthermore, by making the belt diameter of the fixing belt 20 larger than the diameter of the pressure roller 21, the heater width 22 can be increased, thus enabling high-production machines. In addition, a larger belt diameter reduces the overall deformation of the fixing belt 20 relative to the nip width, which can suppress deformation tendencies and stabilize paper separation. However, if the heater width 22 is made too wide, the deformation tendencies will also increase. Therefore, it is best to set the heater width 22 to an appropriate size.

[0034] The pressure roller 21 is biased toward the fixing belt 20 by the biasing means, causing the pressure roller 21 to be pressed against the heater 22 via the fixing belt 20. As a result, a fixing nip N is formed between the fixing belt 20 and the pressure roller 21.

[0035] Therefore, the heater 22 also functions as a nip-forming member. The pressure roller 21 is configured to be rotationally driven by a drive mechanism, and when the pressure roller 21 rotates in the direction of the arrow in Figure 2, the fixing belt 20 rotates in conjunction with it. Because the fixing belt 20 rotates in conjunction with it, the diameter of the fixing belt 20 (belt diameter) must be configured to have a margin of error compared to the size of the inner members such as the heater 22 and heater holder 23. The drive mechanism for the pressure roller 21 and the heater 22 can be controlled by the control unit 220 shown in Figure 5, or the controller of the machine body, which will be described later.

[0036] The heater 22 is a planar heating element provided longitudinally across the width direction of the fixing belt 20, and is composed of a plate-shaped base material 30, a resistance heating element 31 provided on the base material 30, and an insulating layer 32 covering the resistance heating element 31. The heater 22 is in contact with the inner circumferential surface of the fixing belt 20 on the insulating layer 32 side, and the heat generated from the resistance heating element 31 is transferred to the fixing belt 20 via the insulating layer 32.

[0037] The insulating layer 32 is made of, for example, heat-resistant glass with a thickness of 75 μm. The insulating layer 32 covers the resistance heating element 31 and the power supply line 33, insulating and protecting them while maintaining sliding properties with the fixing belt 20.

[0038] In this embodiment, the resistive heating element 31 and the insulating layer 32 are provided on the fixing belt 20 side (fixing nip N side) of the base material 30. However, the resistive heating element 31 and the insulating layer 32 may be provided on the heater holder 23 side of the base material 30. In that case, since the heat from the resistive heating element 31 will be transferred to the fixing belt 20 via the base material 30, it is desirable that the base material 30 be made of a material with high thermal conductivity, such as aluminum nitride. Furthermore, by making the base material 30 of a material with good thermal conductivity, it is possible to sufficiently heat the fixing belt 20 even if the resistive heating element 31 is placed on the opposite side of the base material 30 from the fixing belt 20 side.

[0039] The heater holder 23 and the stay 24 are positioned on the inner circumference side of the fixing belt 20. The stay 24 is made of a metal channel material, and both ends of it are supported by the side plates of the fixing device 9. The heater holder 23 and the heater 22 held therein are supported by the stay 24, so that when the pressure roller 21 is pressed against the fixing belt 20, the heater 22 reliably receives the pressing force of the pressure roller 21 and stably forms the fixing nip N.

[0040] Since the heater holder 23 is prone to becoming hot due to the heat from the heater 22, it is desirable that it be made of a heat-resistant material. For example, if the heater holder 23 is made of a heat-resistant resin with low thermal conductivity such as LCP, heat transfer from the heater 22 to the heater holder 23 is suppressed, and the fixing belt 20 can be heated efficiently.

[0041] (●Paper separation mechanism) A paper separation mechanism 300 is located downstream (on the right side) of the fixing nip N. The paper separation mechanism 300 has a separation plate 310 as a separation member, and separates the paper from the fixing belt 20 using the separation plate 310.

[0042] The separation plate 310 can be made of a heat-resistant metal or resin. For example, stainless steel can be used as a heat-resistant metal. For example, polyimide or PEEK can be used as a heat-resistant resin.

[0043] The separation plate 310 may be made of a material other than metal or resin, as long as it has heat resistance. The separation plate 310 extends parallel to the axial direction of the fixing belt 20 with a width larger than the paper size, and both ends in the longitudinal direction are supported by a pair of left and right side plates.

[0044] (● Heater configuration) Figure 4 is a plan view of the heater 22 according to this embodiment. As shown in Figure 4A, the heater 22 according to this embodiment has a plurality of resistance heating elements 31 arranged at intervals in its longitudinal direction (belt width direction).

[0045] In other words, the heating section 35 is composed of multiple resistance heating elements 31, which divide it into multiple sections in the belt width direction. The heating section 35 can be divided into at least three or four or more sections, consisting of end heaters (end heating members) that heat both ends and a central heater (central heating member) that heats the central section. This allows for the selection of heaters to be energized according to the paper width, thereby suppressing overheating of the non-paper-feeding section of the fixing belt 20.

[0046] Each resistive heating element 31 is electrically connected in parallel to a pair of electrode portions 34 provided at both longitudinal ends of the base material 30 via a power supply line 33. The power supply line 33 is made of a conductor with a lower resistance value than the resistive heating element 31.

[0047] The gap between adjacent resistance heating elements 31 is preferably 0.2 mm or more, and more preferably 0.4 mm or more, from the viewpoint of ensuring insulation between the resistance heating elements 31. Furthermore, if the gap between adjacent resistance heating elements 31 is too large, a temperature drop is likely to occur in the gap, so from the viewpoint of suppressing temperature unevenness along the longitudinal direction, it is preferably 5 mm or less, and more preferably 1 mm or less.

[0048] The resistive heating element 31 is made of a material with PTC (positive temperature resistance coefficient) characteristics, and has the characteristic that its resistance increases (heater output decreases) as the temperature rises. Due to this characteristic, for example, when paper with a width smaller than the overall width of the heating section 35 is fed through, the heat from the fixing belt 20 is not absorbed by the paper in the area outside the paper width, so the temperature of the resistive heating element 31 in that area rises.

[0049] Since the voltage applied to the resistive heating element 31 is constant, as the temperature of the resistive heating element 31 outside the paper width rises and its resistance increases, the output (amount of heat generated) decreases relatively, suppressing the temperature rise at the edges. Furthermore, because multiple resistive heating elements 31 are electrically connected in parallel, the temperature rise in the non-paper-feeding area can be suppressed while maintaining the printing speed.

[0050] Furthermore, the heating element constituting the heating section 35 may be a type other than a resistance heating element having PTC characteristics. Also, the heating elements may be arranged in multiple rows in the short-side direction of the heater 22.

[0051] The resistive heating element 31 can be formed, for example, by coating a paste made of silver palladium (AgPd) or glass powder onto a substrate 30 using screen printing or the like, and then firing the substrate 30. In this embodiment, the resistance value of the resistive heating element 31 is set to 80Ω at room temperature.

[0052] In addition to the materials mentioned above, the resistive heating element 31 may also be made of a silver alloy (AgPt) or ruthenium oxide (RuO2) resistive material. The power supply line 33 and electrode section 34 can be made of silver (Ag) or silver-palladium (AgPd) formed by screen printing or the like.

[0053] (●Materials of the base material) As the material for the base material 30, ceramics such as alumina and aluminum nitride, which have excellent heat resistance and insulation properties, or non-metallic materials such as glass and mica are preferred. In this embodiment, an alumina base material with a width of 8 mm in the direction of the array intersection, a width of 270 mm in the direction of the array, and a thickness of 1.0 mm is used.

[0054] Alternatively, the base material 30 may be constructed by laminating an insulating material onto a conductive material such as a metal. Aluminum and stainless steel are preferred as the metal material for the base material 30 due to their low cost. By constructing the base material 30 from a stainless steel plate, cracking due to thermal stress can be suppressed. Furthermore, to improve the uniformity of the heater 22 and enhance image quality, the base material 30 may be constructed from a material with high thermal conductivity, such as copper, graphite, or graphene.

[0055] The insulating layer 32 is made of, for example, heat-resistant glass with a thickness of 75 μm. The insulating layer 32 covers the resistance heating element 31 and the power supply line 33, insulating and protecting them while maintaining sliding properties with the fixing belt 20.

[0056] Graphene is a flaky powder. As shown in Figure 4B, graphene consists of a planar hexagonal lattice structure of carbon atoms. A graphene sheet is a sheet of graphene, usually a single layer. The single layer of carbon may contain impurities.

[0057] Graphene may also have a fullerene structure. A fullerene structure is generally recognized as a compound in which an equal number of carbon atoms are fused in a cage-like manner in five-membered and six-membered rings to form a polycyclic structure, such as C60, C70, and C80 fullerenes or other closed cage-like structures having three-coordinate carbon atoms.

[0058] Graphene sheets are artificial materials and can be fabricated, for example, by chemical vapor deposition (CVD). Commercially available graphene sheets can also be used. The size, thickness, and number of layers of the graphite sheet (described later) of the graphene sheet are measured, for example, by a transmission electron microscope (TEM).

[0059] Furthermore, graphite, which has multiple layers of graphene, exhibits high thermal conductivity anisotropy. As shown in Figure 4C, graphite has a crystalline structure in which layers of condensed six-membered rings of carbon atoms are spread out in a planar manner, and these layers are stacked in multiple layers.

[0060] In this crystal structure, carbon atoms are bonded covalently to adjacent carbon atoms within a layer, and to van der Waals bonds between carbon atoms in different layers. The covalent bonds are stronger than van der Waals bonds, and there is a large anisotropy between the bonds within a layer and the bonds between layers.

[0061] By constructing the base material 30 from graphite, the heat transfer efficiency in the arrangement direction is greater than in the thickness direction (i.e., the stacking direction of the components), and heat transfer to the heater holder 23 can be suppressed. Therefore, temperature unevenness in the arrangement direction of the heater 22 can be efficiently suppressed, and the heat flowing out to the heater holder 23 can be minimized. In addition, by constructing the base material 30 from graphite, the base material 30 can be given excellent heat resistance, not oxidizing up to about 700 degrees Celsius.

[0062] The physical properties and dimensions of the graphite sheet can be appropriately changed according to the required functions of the base material 30. For example, the anisotropy of its thermal conductivity can be increased by using high-purity graphite or single-crystal graphite, or by increasing the thickness of the graphite sheet.

[0063] Furthermore, in order to increase the speed of the fixing device 9, a thin graphite sheet may be used to reduce the heat capacity of the fixing device 9. Also, if the width of the fixing nip N or heater 22 is large, the width of the substrate 30 in the arrangement direction may be increased accordingly.

[0064] From the viewpoint of increasing mechanical strength, it is preferable that the graphite sheet has 11 or more layers. Furthermore, the graphite sheet may partially consist of single-layer and multi-layer sections.

[0065] (●Power supply circuit) Figure 5 shows the power supply circuit to the heater according to this embodiment. As shown in Figure 5, in this embodiment, the power supply circuit to supply power to each resistive heating element 31 is configured by electrically connecting the AC power supply 200 and the electrode portion 34 of the heater 22. The power supply circuit is also provided with a triac 210 for controlling the amount of power supplied.

[0066] The amount of power supplied to each resistive heating element 31 is controlled by the control unit 220 via the triac 210 based on the temperature detected by the thermistor 25, which serves as a temperature sensing means. The control unit 220 is composed of a microcomputer that includes a CPU, ROM, RAM, I / O interface, etc.

[0067] In this embodiment, a thermistor 25, which serves as a temperature sensing means, is positioned in the central region of the heater 22 in the longitudinal direction, which is within the minimum paper feed width, and at one end of the heater 22 in the longitudinal direction. Furthermore, a thermostat 27, which serves as a power interruption means for interrupting the power supply to the resistance heating element 31 when the temperature of the resistance heating element 31 exceeds a predetermined temperature, is positioned at one end of the heater 22 in the longitudinal direction. The thermistor 25 and thermostat 27 contact the back surface of the base material 30 (the side opposite to the side on which the resistance heating element 31 is positioned) to detect the temperature of the resistance heating element 31.

[0068] (●Heater control operation) Next, the control operation of the heater according to this embodiment will be explained with reference to the flowchart in Figure 6A. First, when the printing operation is started in the image forming apparatus (S1 in Figure 6A), the control unit 220 starts supplying power from the AC power supply 200 to each resistive heating element 31 of the heater 22 (S2 in Figure 6A).

[0069] As a result, each resistance heating element 31 starts to generate heat, and the fixing belt 20 is heated. At this time, the temperature T4 of the resistance heating element 31 located in the central region of the heater 22 is detected by a thermistor (central thermistor) 25 located in the central region of the heater 22 (S3 in Figure 6A). Then, the control unit 220 controls the amount of power supplied to each resistance heating element 31 by the triac 210 based on the temperature T4 obtained from the central thermistor 25, so that each resistance heating element 31 reaches a predetermined temperature (S4 in Figure 6A).

[0070] At the same time, the temperature T8 of the resistance heating element 31 is also detected by a thermistor (end thermistor) 25 located on the longitudinal end of the heater 22 (S5 in Figure 6A). Then, it is determined whether the temperature T8 detected by the end thermistor 25 is above a predetermined temperature TN (T8≧TN) (S6 in Figure 6A). If it is below the predetermined temperature TN, the power supply to the heater 22 is cut off as an abnormal low temperature has occurred (wire breakage has occurred) (S7 in Figure 6A), and an error is displayed on the operation panel of the image forming apparatus (S8 in Figure 6A). On the other hand, if the detected temperature T8 is above the predetermined temperature TN, the printing operation is started as no abnormal low temperature has occurred (S9 in Figure 6A).

[0071] Furthermore, if the resistive heating element 31 is damaged or disconnected, making temperature control based on detection by the central thermistor 25 impossible, there is a risk that other resistive heating elements 31, including the resistive heating element 31 at the longitudinal end, may become abnormally hot. In that case, the thermostat 27 will activate when the resistive heating element 31 exceeds a predetermined temperature, cutting off the power supply to the resistive heating element 31 to prevent it from becoming abnormally hot. Note that before the start of energization (S2) in Figure 6A, the control shown in Figure 6B can be added, as will be described later. The control shown in Figure 6B can eliminate the deformation kinks A, B, and C of the fixing belt 20 shown in Figure 7.

[0072] In the fixing device 9 according to this embodiment, when the printing operation is started, the pressure roller 21 is driven to rotate, and the fixing belt 20 starts to rotate in response. Power is also supplied to the resistance heating element 31 of the heater 22, which heats the fixing belt 20. When the temperature of the fixing belt 20 reaches a predetermined target temperature (fixing temperature), as shown in Figure 2, the paper P on which the unfixed toner image is carried is transported between the fixing belt 20 and the pressure roller 21 (fixing nip N), so that the unfixed toner image is heated and pressurized and fixed to the paper P.

[0073] (●Deformation and removal of the fixing belt) As mentioned above, the fixing belt 20 can be made of a heat-resistant resin such as polyimide, but because it is thin, it is prone to deformation creases A, B, and C as shown in Figure 7 when it is stopped from rotating. When the fixing belt 20 is rotated with these deformation creases A, B, and C present, the trajectory of the fixing belt 20 will fluctuate.

[0074] In other words, at the fixing nip N while rotation is stopped, the fixing belt 20 is sandwiched between the planar heater 22 and the pressure roller 21, causing a flat plate-shaped deformation curve A. In addition, arc-shaped deformation curves B and C with large radii of curvature occur at the inlet and outlet adjacent parts of the fixing nip N. These deformation curves B and C become stronger (larger radius of curvature) when the sliding surface of the heater 22 is formed in a concave shape to improve paper separation.

[0075] Because there is a significant difference in cooling time between the fixing belt 20 and the heater 22, deformation quirks A, B, and C are more likely to occur. When the fixing belt starts to rotate (forward) with these deformation quirks A, B, and C present, there is no member inside the fixing belt 20 to restrict belt deformation, so the rotational trajectory of the fixing belt 20 fluctuates irregularly, such as from vertical to horizontal (causing fluttering), and the belt 20 comes into contact with the downstream separation plate 310.

[0076] Conventionally (Patent Documents 1 and 2), the fixing belt 20 was reversed until the pressure roller 21 reached a predetermined temperature to remove deformation kinks A, B, and C. However, this had problems such as shortening the lifespan of the fixing device 9 and delaying the start of the job. Therefore, in this embodiment, the rotation of the fixing belt 20 is controlled as shown in Figure 8. The rotation control in Figure 8 is performed based on the flowchart in Figure 6B, and the reference numerals (S1a to S1f) in the flowchart are used as needed. (● Rotation control of the fixing belt)

[0077] Figure 8 shows how to remove deformations C and B sequentially by reversing the fixing belt 20 which has deformations A, B, and C attached. First, as shown from the left end to the center of Figure 8, the fixing belt 20 is reversed (rotated clockwise) by a predetermined angle (S1a in Figure 6B). As a result, deformation C on the exit side arrives at the fixing nip N in Figure 2.

[0078] Here, the fixing belt 20 is temporarily stopped (S1b in Figure 6B), and the heater 22 is temporarily energized (S1c in Figure 6B). Note that when the fixing belt 20 is reversed, the deformation kinks C and B do not move downstream, so even when the deformation kinks C and B are large, the possibility of contact with the separation plate 310 can be reduced.

[0079] Subsequently, the fixing belt 20 is reversed (rotated clockwise) by a predetermined angle (S1d in Figure 6B). As a result, the deformation curve B on the inlet side now arrives at the fixing nip N in Figure 2. At this point, the fixing belt 20 is temporarily stopped (S1e in Figure 6B), and the heater 22 is temporarily energized (S1f in Figure 6B).

[0080] In this way, by reversing the fixing belt 20, stopping (heating), reversing, and stopping (heating), the deformation creases C and B can be removed in sequence. Furthermore, by only reversing the fixing belt 20 without rotating it forward at all, the possibility of the belt contacting and damaging the separation plate 310 can be reduced. In addition, since the rotation angle required to reverse the fixing belt 20 to remove the deformation creases C and B in sequence is only about 300°, the reduction in the lifespan of the fixing device 9 is minimal.

[0081] Furthermore, since the heat from the heater 22 can be concentrated on deformation creases B and C, deformation creases B and C can be removed in a short time. Therefore, the delay in starting the job can be minimized.

[0082] The rotational speed when rotating the fuser belt 20 forward by a predetermined angle should be slower than the rotational speed during printing. The rotational control shown in Figure 8 is performed before the heater 22 is fully energized for printing, and at this time the viscosity of the grease on the sliding parts such as the fuser nip N is high. Therefore, the sliding load on the fuser belt 20 is large, so the load on the drive system can be reduced by driving the fuser belt 20 at a low speed.

[0083] Next, we will consider the optimal temperature for heating the deformation creases B and C of the fixing belt 20 with the heater 22. In Figure 8, the heat from the heater 22 is concentrated on the deformation creases B and C, but in order to minimize the delay in starting the job, it is desirable to keep the heating by the heater 22 to the absolute minimum necessary.

[0084] (●Belt temperature and shape-removing effect) Figure 9 shows the correlation between the temperature of the fixing belt 20 and the deformation removal effect. As shown in the figure, it can be seen that no deformation removal effect is obtained when the belt temperature is below 50°C (X mark: 0% reduction in deformation).

[0085] In contrast, the deformation-removing effect begins to appear when the belt temperature reaches 75°C (△ mark: reduction of deformation greater than 0%). Furthermore, it can be seen that a deformation-removing effect is reliably obtained when the belt temperature reaches 100°C or higher (〇 mark: 100% reduction of deformation). From this, it can be seen that when the fixed belt 20 is temporarily suspended and energized, the energizing time should be such that the belt temperature reaches 75°C or higher, preferably 100°C or higher.

[0086] Once the temperature of the fixing belt 20 reaches 100°C, proceed to the next step immediately. However, the de-de-de-de-de-stamping effect varies depending on the type of base material and elastic layer of the fixing belt 20 used, as well as the type and thickness of the surface release layer. Therefore, the optimal temperature should be adjusted according to the type of fixing belt.

[0087] Furthermore, if the heater 22 has end heaters and a central heater, the temperature of the end heaters can be set higher than the temperature of the central heater when heating the deformation creases B and C of the fixing belt 20. Since the end heaters are prone to temperature degradation due to heat dissipation, they are set to a slightly higher temperature than the central heaters.

[0088] Furthermore, even when the fixing belt 20 is not rotating, the temperature of the fixing belt 20 and the pressure roller 21 may be high, and the fixing device 9 may not be in a cool state (below 60°C). In such cases, the possibility of deformation creases A, B, and C on the fixing belt 20 is low.

[0089] Therefore, the rotation control shown in Figure 8 may be performed only when the fixing device 9 is in a cold state (60°C or below). This avoids unnecessary rotation control of the fixing belt 20, suppresses shortening of the lifespan of the fixing device 9, and minimizes delays in starting the job.

[0090] (● Variations of thermistor placement) The thermistor 25 can also be installed, for example, on the upstream side in the rotational direction of the fixing belt 20 from the central position NA of the fixing nip N, in other words, on the inlet side of the fixing nip N, as shown in Figure 10. Since the inlet side of the fixing nip N is a region in which heat is particularly easily lost by the paper P, the thermistor 25 can detect the temperature of this part to ensure the fixing performance of the fixing device 9 and effectively suppress the fixing offset.

[0091] (● Modified examples of fixing devices and image forming apparatus) The fixing device of the present invention can also be configured as the modified examples shown in Figures 11 to 13. The configurations of each fixing device shown in Figures 11 to 13 will be briefly described below.

[0092] First, in the fixing device 9 shown in Figure 11, the pressure roller 44 is positioned on the side opposite to the pressure roller 21 relative to the fixing belt 20. The pressure roller 44 is an opposing rotating member that rotates opposite to the fixing belt 20, which is a rotating member. This pressure roller 44 and the heater 22 are configured to heat the fixing belt 20 by sandwiching it between them.

[0093] On the other hand, on the pressure roller 21 side, a nip-forming member 45 is positioned on the inner circumference of the fixing belt 20. The nip-forming member 45 is supported by a stay 24. The nip-forming member 45 and the pressure roller 21 form a fixing nip N by sandwiching the fixing belt 20.

[0094] Next, in the fixing device 9 shown in Figure 12, the aforementioned pressing roller 44 is omitted, and in order to ensure the circumferential contact length between the fixing belt 20 and the heater 22, the heater 22 is formed in an arc shape to match the curvature of the fixing belt 20. Otherwise, the configuration is the same as the fixing device 9 shown in Figure 11.

[0095] Finally, the fixing device 9 shown in Figure 13 will be described. The fixing device 9 consists of a heating assembly 92, a fixing roller 93 which is a fixing member, and a pressure assembly 94 which is an opposing member.

[0096] The heating assembly 92 includes the heater 22, heater holder 23, stay 24, and heating belt 120 as a rotating member, as described in the previous embodiment. The fixing roller 93 is an opposing rotating member that rotates opposite to the heating belt 120 as a rotating member.

[0097] Furthermore, the fixing roller 93 is composed of a solid iron core 93a, an elastic layer 93b formed on the surface of the core 93a, and a release layer 93c formed on the outside of the elastic layer 93b. In addition, a pressure assembly 94 is provided on the side of the fixing roller 93 that is opposite to the heating assembly 92.

[0098] The pressure assembly 94 includes a nip forming member 95 and a stay 96, and a pressure belt 97 is rotatably positioned to enclose these nip forming member 95 and stay 96. Then, paper P is passed through the fixing nip N2 between the pressure belt 97 and the fixing roller 93, and the image is fixed by heating and pressurizing it.

[0099] In the fixing apparatus shown in Figures 11 to 13 above, the amount of heat generated by the heater 22 is reduced in the divided region B between the resistance heating elements 31 of the heater 22. Therefore, as in the embodiment described above, by providing the temperature sensing element of the temperature sensing member at a position corresponding to the divided region B of the heater 22, the portion of the rotating member corresponding to the divided region can be sufficiently heated. This ensures sufficient image fixing performance and prevents the occurrence of problems such as fixing offset.

[0100] Furthermore, the present invention is not limited to fixing devices as described in the above embodiments, but can also be applied to drying devices for drying ink applied to paper, and even to heating devices such as laminators for heat-pressing a film as a covering member onto the surface of a sheet such as paper, and heat sealers for heat-pressing the sealing portion of packaging materials. By applying the present invention to such devices, the portion corresponding to the divided region of the rotating member can be sufficiently heated.

[0101] The image forming apparatus according to the present invention is not limited to the color image forming apparatus shown in Figure 1, but may also be a monochrome image forming apparatus, a copier, a printer, a facsimile, or a combination device thereof. For example, as shown in Figure 14, the image forming apparatus 100 of this embodiment includes an image forming means 50 consisting of a photosensitive drum, a paper transport unit consisting of a pair of timing rollers 15, a paper feed device 7, a fuser 9, a paper discharge device 10, and a reading unit 51. The paper feed device 7 has multiple paper trays, each of which accommodates paper of a different size.

[0102] The reading unit 51 reads the image of the original document Q. The reading unit 51 generates image data from the read image. The paper feed device 7 receives multiple sheets of paper P and feeds the paper P to the transport path. The timing roller 15 transports the paper P on the transport path to the image forming means 50.

[0103] The image forming means 50 forms a toner image on the paper P. Specifically, the image forming means 50 includes a photoreceptor drum, a charging roller, an exposure device, a developing device, a replenishment device, a transfer roller, a cleaning device, and a static elimination device.

[0104] The toner image, for example, shows the image of the original document Q. The fuser unit 9 heats and pressurizes the toner image to fix it to the paper P. The paper P with the fixed toner image is transported to the paper discharge unit 10 by transport rollers or the like. The paper discharge unit 10 discharges the paper P to the outside of the image forming apparatus 100.

[0105] Next, the fixing device 9 used in the image forming apparatus 100 shown in Figure 14 will be described. Configurations common to the fixing device of the previously described embodiment will be omitted from the description as appropriate.

[0106] As shown in Figure 15, the fixing device 9 includes a fixing belt 20, a pressure roller 21, a heater 22, a heater holder 23, a stay 24, a thermistor 25, etc. A fixing nip N is formed between the fixing belt 20 and the pressure roller 21. The nip width of the fixing nip N is 10 mm, and the linear speed of the fixing device 9 is 240 mm / s.

[0107] The fixing belt 20 comprises a polyimide substrate and a release layer, and does not have an elastic layer. The release layer is made of a heat-resistant film material, for example, made of fluororesin. The outer diameter of the fixing belt 20 is approximately 24 mm.

[0108] The pressure roller 21 includes a core metal 21a, an elastic layer 21b, and a release layer 21c. The outer diameter of the pressure roller 21 is formed to be 24-30 mm, and the thickness of the elastic layer 21b is formed to be 3-4 mm.

[0109] The heater 22 includes a base material, a heat insulating layer, a conductor layer containing a resistive heating element, and an insulating layer, and is formed with an overall thickness of 1 mm. The width Y in the direction of the arrangement intersection of the heaters 22 is 13 mm.

[0110] As shown in Figure 16, the conductor layer of the heater 22 comprises a plurality of resistive heating elements 31, a power supply line 33, and electrode portions 34A to 34C. In this embodiment as well, as shown in the enlarged view of Figure 16, a divided region B is formed in which the plurality of resistive heating elements 31 are divided in the direction of arrangement. However, although Figure 16 only shows the divided region B within the scope of the enlarged view, in reality, divided regions are provided between all of the resistive heating elements 31.

[0111] The resistive heating element 31 constitutes three heating sections 35A to 35C. By applying current to the electrode sections 34A and 34B, the heating sections 35A and 35C generate heat.

[0112] By applying current to the electrode sections 34A and 34C, the heating section 35B generates heat. For example, when performing a fixing operation on small-sized paper, the heating section 35B is heated, while when performing a fixing operation on large-sized paper, all heating sections can be heated.

[0113] As shown in Figure 17, the heater holder 23 holds the heater 22 in its recess 23b. The recess 23b is provided on the heater 22 side of the heater holder 23. The recess 23b consists of a surface 23b1 that is substantially parallel to the base material 30 and recessed on the stay 24 side compared to the other surfaces of the heater 22, a wall portion 23b2 provided on the inside of the heater holder 23 on both sides (or one side) in the arrangement direction of the heater holder 23, and a wall portion 23b3 provided on the inside of the heater holder 23 on both sides in the direction of the arrangement intersection.

[0114] The heater holder 23 has a guide portion 26. The heater holder 23 is made of LCP (liquid crystal polymer).

[0115] As shown in Figure 18, the connector 60 comprises a housing made of resin (e.g., LCP) and a plurality of contact terminals provided inside the housing. The connector 60 is attached by sandwiching the heater 22 and the heater holder 23 together from the front and back sides. In this state, each contact terminal makes contact (pressure contact) with each electrode portion of the heater 22, thereby electrically connecting the heating element 35 and the power supply provided in the image forming apparatus via the connector 60.

[0116] This allows power to be supplied from the power source to the heat-generating section 35. Note that, in order to ensure connection with the connector 60, at least a portion of each electrode section 34 is not covered by the insulating layer and is exposed.

[0117] The flanges 53 are provided on both sides of the anchoring belt 20 in the direction of arrangement and hold both ends of the anchoring belt 20 from the inside of the belt. The flanges 53 are fixed to the housing of the anchoring device 9. The flanges 53 are inserted into both ends of the stay 24 (see the direction of the arrows from the flanges 53 in Figure 18).

[0118] The mounting direction of the connector 60 to the heater 22 and heater holder 23 is the direction in which the heaters intersect (see the direction of the arrow from the connector 60 in Figure 18). When the connector 60 is mounted to the heater holder 23, a protrusion on one side of the connector 60 and the heater holder 23 may engage with a recess on the other side, and the protrusion may move relative to the other within the recess. The connector 60 is mounted to the heater 22 and heater holder 23 on one side in the arrangement direction, on the side opposite to the side where the drive motor for the pressure roller 21 is provided.

[0119] As shown in Figure 19, thermistors 25 are provided opposite the inner circumferential surface of the fixing belt 20, on the central side and the end side in the direction of arrangement of the fixing belt 20. The heater 22 is controlled based on the temperatures of the central side and the end side of the fixing belt 20 detected by the thermistors 25. One of these thermistors 25 is provided at a position corresponding to the dividing region between the resistance heating elements of the heater 22, as in the embodiment described above.

[0120] Thermostats 27 are provided on the center side and end side of the fixing belt 20, facing the inner circumferential surface of the fixing belt 20. If the temperature of the fixing belt 20 detected by the thermostats 27 exceeds a predetermined threshold, the power supply to the heater 22 is stopped.

[0121] Flanges 53 are provided at both ends of the fixing belt 20 in the direction of arrangement to hold each end of the fixing belt 20. The flanges 53 are formed of LCP (liquid crystal polymer).

[0122] As shown in Figure 20, the flange 53 is provided with a slide groove 53a. The slide groove 53a extends in the direction of contact with and separation of the fixing belt 20 from the pressure roller 21.

[0123] The engaging portion of the housing of the fixing device 9 engages with the slide groove 53a. As this engaging portion moves relative to the sliding groove 53a, the fixing belt 20 can move in the direction of approaching and moving away from the pressure roller 21.

[0124] In the fixing device 9 described above, by providing the temperature sensing element of the thermistor 25 at a position corresponding to the divided region B of the heater 22, the portion of the fixing belt 20 corresponding to the divided region can be sufficiently heated. This ensures sufficient image fixing performance and prevents the occurrence of defects such as fixing offset.

[0125] In particular, in image forming apparatuses that perform image forming operations using a single-color toner, hot offset is relatively less likely to occur compared to image forming apparatuses that perform image forming operations using multiple-color toners. Therefore, as in the present invention, even if the heating element is controlled based on the detection result of a temperature sensing element placed at a position corresponding to the divided region, image forming apparatuses that use a single-color toner have the advantage of being relatively less prone to hot offset.

[0126] (●Summary) Although embodiments of the present invention have been described above, it goes without saying that the present invention is not limited to the embodiments described above and can be modified in various ways. For example, only one of the portions adjacent to the inlet or outlet of the fixing belt 20 may be moved to the fixing nip N and heated by the heater 22.

[0127] Furthermore, although the fixing belt 20 of the fixing device 9 was described as an endless belt in the above embodiment, the endless belt may be a photoreceptor belt. That is, in an image forming apparatus that transfers a toner image carried on a photoreceptor belt as an image carrier onto a recording medium as a transported object, the recording medium is separated from the photoreceptor belt by the separation plate described above.

[0128] Alternatively, the endless belt may be the intermediate transfer belt 11 shown in Figure 1, which acts as an image carrier. That is, the recording medium, which is transported by passing through the nip between the intermediate transfer belt 11 and the secondary transfer roller 13, is separated from the intermediate transfer belt 11 by the aforementioned separation plate.

[0129] Similarly, the endless belt may be an intermediate transfer belt used in an inkjet-type image forming apparatus. Furthermore, in other inkjet-type image forming apparatuses, if a pressurizing member presses against a nip-forming member via the endless belt to form a nip, and the conveyed object passes through the nip for transport, the conveyed object after passing through the nip can be separated from the endless belt by the aforementioned separation plate.

[0130] Furthermore, the separation plate 310 can be arranged to move toward and away from the fixing belt 20. In addition to making the separation plate 310 rotatable, it may also be configured to move in parallel with the heater holder 23 in the direction of approaching and moving away from the fixing belt 20. [Explanation of Symbols]

[0131] 1Y, 1M, 1C, 1Bk: Image-forming unit; 2: Photoreceptor 3: Charging device 4: Developing device 5: Cleaning equipment 6: Exposure equipment 7: Paper feeder 8: Transfer device 9: Fixing unit 10: Paper output unit 11: Intermediate transfer belt 12: Primary transfer roller 13: Secondary transfer roller 14: Paper transport path 15: Timing roller 20: Fixing belt 21: Pressure roller 21a: Core metal 21b: Elastic layer 21c: Release layer 22: Heater 23: Heater holder 24: Stay 25: Thermistor 27: Thermostat 30: Base material 31: Resistive heating element 32: Insulating layer 33: Power supply line 34: Electrode part 34A~34C: Electrode section 35: Heating section 200: AC power supply 210: Triac 220: Control unit 300: Paper separation mechanism 310: Separation plate (separation member) P: Paper (transferred object) [Prior art documents] [Patent Documents]

[0132] [Patent Document 1] Patent No. 5305742 [Patent Document 2] Japanese Patent Publication No. 2009-288587 [Patent Document 3] Japanese Patent Publication No. 2006-163295

Claims

1. A rotatable, flexible, endless belt, A nip-forming member is provided so as to be able to contact the inner circumferential surface of the endless belt, A heating member for heating the nip-forming member, A pressurizing member that forms a nip for gripping and conveying an object to be conveyed by pressing it against the nip-forming member via the endless belt, A separating member for separating the conveyed object that has passed through the nip from the endless belt, The system comprises the aforementioned pressurizing member and a driving means for rotating the endless belt, In a nip-forming unit in which the object to be conveyed passes through the nip and is conveyed by forward-rotating the pressurizing member and the endless belt with the driving means, A nip forming unit characterized in that, before conveying the object to be conveyed, the driving means repeatedly drives the pressurizing member and the endless belt in reverse and pauses without rotating them in the forward direction, thereby sequentially moving the outlet adjacent portion of the endless belt adjacent to the exit side of the nip and the inlet adjacent portion adjacent to the inlet side of the nip to the nip, and heating them with the heating member.

2. The nip forming unit according to claim 1, characterized in that the pressurizing member and the endless belt are driven at a speed lower than the rotational speed when the pressurizing member and the endless belt are driven forward by the driving means.

3. The nip-forming unit according to claim 1 or 2, wherein the heating member comprises a central heating member that heats the central part of the nip-forming member in the longitudinal direction and end heating members that heat both ends in the longitudinal direction, and the temperature of the end heating members is made higher than the temperature of the central heating member when heating the inlet adjacent portion and the outlet adjacent portion.

4. A fixing device characterized in that the transported object is a recording medium carrying a developer, and the developer is fixed to the recording medium by passing the recording medium through the nip of the nip forming unit of claim 3.

5. An image forming apparatus characterized by having a fixing device according to claim 4.

Images