Image heating device and image forming apparatus

Abstract

To improve efficiency in heating a recording material when heating an image formed on the recording material.SOLUTION: An image heating device has: an endless belt member that is stretched over a plurality of stretching members and is rotatably provided; a pressure member that is provided facing the belt member, is in contact with the belt member to form a nip, and sandwiches and conveys a recording material at the nip together with the belt member; and a heating member that is provided in contact or not in contact with the belt member and heats the belt member, where the image heating device heats an image formed on the recording material at the nip. When the length in the direction of rotation of the belt member that is in contact or not in contact with the heating member facing the belt member is defined as a heating length Lh, and the length in the direction of rotation of the belt member in contact with the pressure member as a nip length Ln, the heating length Lh is set to be a length satisfying the relationships of -2.5×Ln+75≤Lh≤95 and 16≤Ln≤27, relative to the nip length Ln.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image heating device that heats an image formed on a recording material, and an image forming apparatus. [Background technology]

[0002] In an image forming apparatus, a toner image is formed on a recording material, and the toner image is fixed onto the recording material by applying heat and pressure with a fixing device.

[0003] As one of conventional fixing devices, a belt-type fixing device is described in Patent Document 1. The fixing device described in Patent Document 1 has a heating rotor that heats an image on a recording material at a heating nip, an endless belt that forms a heating nip between itself and the heating rotor, and a pressure member that presses the belt against the heating rotor at the heating nip. This fixing device fixes the toner image to the recording material by heat and pressure while nipping and conveying the recording material carrying an unfixed toner image at the heating nip, which is the contact point between the belt and the rotor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-79036 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, the market for on-demand printing, in which printed materials such as posters and direct mail are printed when needed in the required number of copies, has expanded, and image forming apparatuses that handle this on-demand printing are required to print faster.To achieve this speedup, it is necessary to improve the efficiency of heating the recording material when fixing a toner image to the recording material.

[0006] One solution is to widen the heating nip of the fixing device. However, a wider heating nip requires a larger heat source to reheat the belt, which has dropped in temperature at the heating nip, and the fixing device tends to become larger. Furthermore, a larger fixing device requires a longer start-up time to heat the fixing device from a low temperature state (powered off or in sleep mode) to a predetermined temperature, leading to a deterioration in energy efficiency.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to improve the heating efficiency of a recording material when an image formed on the recording material is heated. [Means for solving the problem]

[0008] A typical configuration of the present invention is an image heating device that includes: an endless belt member that is rotatably mounted and suspended by a plurality of tension members; a pressure member that is disposed opposite the belt member and contacts the belt member to form a nip, and that sandwiches and transports a recording material together with the belt member at the nip; and a heating member that is disposed in contact or non-contact with the belt member and heats the belt member, and that heats an image formed on the recording material at the nip.The image heating device is characterized in that, when the length in the rotational direction of the belt member that the heating member faces in contact or non-contact with is defined as a heating length Lh, and the length in the rotational direction of the belt member that contacts the pressure member is defined as a nip length Ln, the heating length Lh is set to a length that satisfies the relationships -2.5 x Ln + 75≦Lh≦95 and 16≦Ln≦27 relative to the nip length Ln. [Effects of the Invention]

[0009] According to the present invention, it is possible to improve the heating efficiency of the recording material when heating an image formed on the recording material. [Brief explanation of the drawings]

[0010] [Figure 1] Schematic diagram of an image forming apparatus [Figure 2] Schematic diagram of a fixing device [Figure 3]Schematic diagram showing the belt heating length and fixing nip length of the fixing device [Figure 4] Schematic diagram showing the belt heating length and fixing nip length of the fixing device [Figure 5] Schematic diagram showing the belt heating length and fixing nip length of the fixing device [Figure 6] Schematic diagram showing the belt heating length and fixing nip length of the fixing device [Figure 7] Diagram showing the relationship between fixing nip length and belt surface temperature [Figure 8] Diagram showing the relationship between belt heating length and belt surface temperature [Figure 9] A diagram showing the relationship between the fixing nip length and the belt heating length [Figure 10] Diagram showing the relationship between belt heating length and rise time [Figure 11] A diagram showing the relationship between fixing nip length and drive torque [Figure 12] Diagram showing the range of fixing nip length and belt heating length DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described in detail with reference to the drawings. While the following describes an example in which the present invention is applied to an electrophotographic full-color image forming apparatus having multiple photosensitive drums, the present invention is not limited to this and can also be applied to various types of image forming apparatuses, monochrome image forming apparatuses, etc. Furthermore, the dimensions, materials, shapes, and relative positions of the components described in the following examples may be changed as appropriate depending on the configuration and various conditions of the apparatus to which the present invention is applied, and are not intended to limit the scope of the present invention to these alone.

[0012] Example 1 <Image forming device> The general configuration of the image forming apparatus according to this embodiment will be described with reference to Fig. 1. Fig. 1 is a schematic cross-sectional view showing the general configuration of the full-color image forming apparatus according to this embodiment.

[0013] The image forming apparatus 1 includes an image reading unit 2 and an image forming apparatus main body 3. The image reading unit 2 reads a document placed on a document glass 11. Light emitted from a light source 12 is reflected by the document and forms an image on a CCD sensor 14 via optical elements 13, such as a lens. The light source 12, optical elements 13, and CCD sensor 14 form an optical system unit capable of scanning in the direction indicated by the arrow. By scanning in the direction indicated by the arrow, this optical system unit converts the document into a line-by-line electrical signal data stream. The image signal obtained by the CCD sensor 14 is sent from a reader control unit 15 of the image reading unit 2 to the image forming apparatus main body 3, where it is subjected to image processing tailored to each image forming unit (described later) by a control unit 30. The control unit 30 also receives external inputs as image signals from an external host device, such as a print server. The control unit 30 also receives inputs from an operation unit 4 connected to the image forming apparatus main body 3 via a cable. The operation unit 4 includes a display unit for displaying information and is connected to the image forming apparatus main body 3 via a wired cable. Although the operation unit 4 is shown as being connected to the image forming apparatus main body 3 by wire, the present invention is not limited to this configuration, and the operation unit 4 may be connected wirelessly.

[0014] The image forming apparatus main body 3 includes multiple image forming units Pa, Pb, Pc, and Pd, each of which forms an image based on the image signal. Specifically, the image signal is converted into a laser beam that is PWM (pulse width modulation) controlled by a control unit 30. In FIG. 1, a polygon scanner 29 serving as an exposure device scans the laser beam in response to the image signal. The laser beam then irradiates the photosensitive drums 20a-20d serving as image carriers of the image forming units Pa-Pd. Note that Pa is a yellow (Y) image forming unit, Pb is a magenta (M) image forming unit, Pc is a cyan (C) image forming unit, and Pd is a black (Bk) image forming unit, each of which forms an image of the corresponding color. Since the image forming units Pa-Pd are substantially identical, the following describes the image forming unit Pa in detail, and the other image forming units Pb, Pc, and Pd are omitted. In the image forming unit Pa, a toner image is formed on the surface of the photosensitive drum 20a based on the image signal, as described below.

[0015] The primary charger 21a charges the surface of the photosensitive drum 20a to a predetermined potential to prepare for the formation of an electrostatic latent image. A laser beam from a polygon scanner 29 forms an electrostatic latent image on the surface of the photosensitive drum 20a, which has been charged to a predetermined potential. The developer 22a develops the electrostatic latent image on the photosensitive drum 20a to form a toner image. The transfer roller 23a discharges from the back of the intermediate transfer belt 24 and applies a primary transfer bias of opposite polarity to the toner, thereby performing primary transfer of the toner image on the photosensitive drum 20a onto the intermediate transfer belt 24. After transfer, the surface of the photosensitive drum 20a is cleaned by a cleaner 27a.

[0016] The toner image on the intermediate transfer belt 24 is then transported to the next image forming station, where the toner images of each color formed at each image forming station are sequentially transferred (primary transfer) in the order of Y, M, C, and Bk, forming a four-color image on the surface of the intermediate transfer belt 24. The toner image that has passed through the image forming station Pd is secondarily transferred to a recording material in a secondary transfer station comprised of a pair of secondary transfer rollers 25 and 26 by applying a secondary transfer electric field of opposite polarity to that of the toner image on the intermediate transfer belt 24. The recording material fed from cassettes 8 and 9 waits at a registration station 28, and then its timing is controlled to align the toner image on the intermediate transfer belt 24 with the recording material. The toner image on the recording material is then fixed to the recording material by a fixing device F, which also serves as an image heating device. The fixed recording material passes through the fixing device F and is then ejected from the printer. In the case of double-sided printing, the toner image is transferred and fixed to the first side (first surface) of the recording material. After fixing, the recording material passes through a reversing section provided inside the image forming apparatus, where the recording material is turned over and sent back to the image forming section. The toner image is then transferred and fixed onto the second side of the recording material. The recording material is then ejected outside the apparatus and stacked on the paper output tray 7.

[0017] <Fixing device> Next, the configuration of the belt heating type fixing device F according to this embodiment will be described. Fig. 2 is a schematic diagram of the overall configuration of the fixing device. In Fig. 2, the X direction indicates the conveyance direction of the recording material P, the Y direction indicates the width direction perpendicular to the conveyance direction of the recording material P, and the Z direction indicates the pressure direction of the pressure roller 305. In Fig. 2, the Y direction is perpendicular to the X direction, and the Z direction is perpendicular to the X and Y directions. The area surrounded by the dotted line in Fig. 2 represents an enlarged cross section of the fixing nip N in the fixing device F.

[0018] The fixing device F is an image heating device that heats an image formed on a recording material P. The fixing device F has a fixing belt 301 as a belt member, a pressure roller 305 as a pressure member, a pressure pad 303 as a support member, and a heating roller 307 as a heating member. The fixing device F further has a stay 302 and a sliding member 304.

[0019] (fixing belt) The fixing belt 301 has thermal conductivity, heat resistance, and the like, and is a thin cylindrical shape. In this embodiment, the fixing belt 301 has a three-layer structure consisting of a base layer 301a, an elastic layer 301b on the outer periphery of the base layer 301a, and a release layer 301c on the outer periphery of the elastic layer 301b. The base layer is 80 μm thick and is made of polyimide resin (PI). The elastic layer is 300 μm thick and is made of silicone rubber, and the release layer is 30 μm thick and is made of PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin) as a fluororesin.

[0020] The fixing belt 301 is an endless belt member that is rotatably provided and stretched around a plurality of tension members, namely, pressure pads 303 and heating rollers 307. The outer diameter of the fixing belt 301 is 100 mm.

[0021] (Pressure roller) The pressure roller 305 is provided opposite to the fixing belt 301 and contacts the fixing belt 301 to form a fixing nip N. The pressure roller 305 and the fixing belt 301 sandwich and convey the recording material P at the fixing nip N.

[0022] Pressure roller 305 is a rotatable pressure member having a shaft (metallic core layer) 305c, an elastic layer 305b on the outer periphery of shaft 305c, and a release layer 305a on the outer periphery of the elastic layer 305b. Shaft (metallic core layer) 305c is made of SUS material with a diameter of 72 mm. Elastic layer 305b is made of conductive silicone rubber with a thickness of 8 mm. Release layer 305a is made of PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin) as a fluororesin with a thickness of 100 μm. Pressure roller 305 is supported by the frame (not shown) of fixing device F, and a gear is fixed to one end of it. It is connected to a drive source (not shown) via the gear and is driven to rotate.

[0023] In the fixing nip N formed between the fixing belt 301 and the pressure roller 305, the recording material P carrying a toner image is sandwiched and conveyed while the toner image is heated. In this way, the fixing device F fixes the toner image to the recording material P while sandwiching and conveying the recording material P. Therefore, it is necessary to have both the function of applying heat and pressure and the function of conveying the recording material P. The pressure roller 305 is pressed against the pressure pad 303 via the fixing belt 301 by a driving source (not shown).

[0024] (Pressure pad) The pressure pad 303 is a support member that contacts the fixing belt 301 at the fixing nip N and supports the fixing belt 301. The pressure pad 303 is pressed against the pressure roller 305 with the fixing belt 301 sandwiched therebetween. The pressure pad 303 is made of LCP (liquid crystal polymer) resin.

[0025] The pressure pad 303 contacts the fixing belt 301 at a position facing the pressure roller 305 across the fixing belt 301, and also serves as a tension member that stretches the fixing belt 301. In other words, the pressure pad 303 is included in the multiple tension members that stretch the fixing belt 301.

[0026] The pressure pad 303 is covered with a sliding member 304. In other words, the sliding member 304 is interposed between the pressure pad 303 and the fixing belt 301.

[0027] In this embodiment, the sliding member 304 is composed of a base layer 304a and a sliding layer 304b on its surface. The base layer 304a only needs to have sufficient heat resistance and strength. The base layer 304a is preferably made of a metal material such as stainless steel, copper, or aluminum. In this embodiment, a 1.3 mm thick stainless steel plate was used. The sliding member 304 has an embossed portion formed to set the distance between the pressure pad 303 and the fixing belt 301 at 1.4 mm, thereby reducing the contact area with the fixing belt 301 and thereby reducing sliding resistance. The sliding layer 304b is preferably made of a material (such as a fluorine coating, PTFE, or PFA) to achieve low friction. In this embodiment, the base layer 304a is coated with a 20 μm thick PTFE (polytetrafluoroethylene) coating to form the sliding layer 304b. In this case, a lubricant is further applied to the inner surface of fixing belt 301 (the surface on the sliding member 304 side), so that fixing belt 301 slides smoothly against sliding member 304. Silicone oil is used as the lubricant.

[0028] In order to supplement the strength of the pressure pad 303, a stay 302 is used, and the pressure pad 303 is supported by the stay 302.

[0029] In this way, the pressure pad 303 is in contact with the fixing belt 301 via the sliding member 304. As a result, the sliding resistance of the fixing belt 301 is reduced by the sliding member 304, allowing the fixing belt 301 to rotate smoothly.

[0030] In this embodiment, sliding member 304 has a configuration in which sliding layer 304b is provided on base material layer 304a, but the present invention is not limited to this, and a configuration in which an adhesive layer is provided between base material layer 304a and sliding layer 304b is also acceptable. By using an adhesive layer in sliding member 304, it is possible to achieve good adhesive strength between base material layer 304a and sliding layer 304b when base material layer 304a is made of a metal material such as SUS, copper, or aluminum.

[0031] The sliding member 304 in this embodiment is configured to cover the pressure pad 303 regardless of whether it is inside or outside the fixing nip N. Although not shown here, it is acceptable as long as part of the fixing nip N is covered by the sliding member 304. In other words, it is acceptable to have a configuration in which the sliding member 304 is disposed only in the fixing nip N.

[0032] In this embodiment, the embossed portion of the sliding member 304 is molded over the entire area of ​​the sliding member 304. Although not shown in the following figures, it is acceptable as long as a part of the fixing nip N is covered with the embossed portion. In other words, it is acceptable to have a configuration in which the embossed portion of the sliding member 304 is disposed only in the fixing nip N.

[0033] Furthermore, in this embodiment, a configuration is adopted in which the sliding member 304 is fixed to the stay 302 (not shown). Although not shown here, the sliding member 304 may be integrated with the pressure pad 303. Also, a portion of the sliding member 304 may be fixed to the stay 302 or the pressure pad 303. For example, both ends of the sliding member 304 in the Y direction (width direction of the recording material) may be fixed to the pressure pad 303 with screws or the like.

[0034] (heating roller) The heating roller 307 is a heating member that comes into contact with the fixing belt 301 at a position different from the fixing nip N and heats the fixing belt 301 .

[0035] The heating roller 307 contacts the fixing belt 301 at a position different from the pressure pad 303 and also serves as a tension member for tensioning the fixing belt 301. In other words, the heating roller 307, together with the pressure pad 303, is included in the multiple tension members for tensioning the fixing belt 301.

[0036] Heating roller 307 is a stainless steel pipe with a thickness of 2.1 mm. Heating roller 307 is a rotating member that rotates in contact with fixing belt 301. Heating roller 307 has halogen heater 306 disposed therein as a heat source, and is capable of generating heat up to a predetermined temperature. Halogen heater 306 is controlled to a predetermined temperature in accordance with the temperature detected by temperature detection member 308 that is in contact with the outer surface of heating roller 307. Fixing belt 301 is heated by heating roller 307.

[0037] Although the above embodiment illustrates a configuration in which temperature detection member 308 is brought into contact with heating roller 307 to detect the temperature of heating roller 307, the present invention is not limited to this. For example, temperature detection member 308 may be brought into contact with fixing belt 301 to detect the temperature of fixing belt 301.

[0038] The heating roller 307 is a steering roller that has a rotation center near one end or the center in the longitudinal direction (width direction of the recording material) and rotates relative to the fixing belt 301. By rotating relative to the fixing belt 301, the heating roller 307 generates a tension difference between the front and rear sides, that is, one side and the other side in the width direction perpendicular to the rotation direction of the fixing belt 301, and controls the position of the fixing belt 301 in the main scanning direction. The heating roller 307 is also biased by a spring supported by a frame (not shown) of the fixing device F, and is a tension roller that applies a predetermined tension to the fixing belt 301.

[0039] <Definition of belt heating length and fixing nip length> Here, the belt heating length Lh and the fixing nip length Ln will be described using the fixing device F shown in Figures 3 to 6. Figures 3 to 6 are schematic diagrams showing the belt heating length Lh and the fixing nip length Ln of the fixing device F.

[0040] As described above, the fixing nip N is formed when the pressure roller 305 contacts the fixing belt 301. The length of the contact between the pressure roller 305 and the fixing belt 301 in the rotational direction of the fixing belt 301 is defined as the fixing nip length Ln. In other words, the fixing nip length Ln is the length of the fixing nip N in the rotational direction of the fixing belt 301.

[0041] Furthermore, a heating roller 307 that heats the fixing belt 301 is provided so as to be in contact or not in contact with the fixing belt 301. The length of the fixing belt 301 in the rotational direction that the heating roller 307 faces in contact or not in contact with is defined as the belt heating length Lh.

[0042] 3, the heating roller 307 is provided in contact with the fixing belt 301 at a position different from the pressure pad 303, stretches the fixing belt 301, and also serves as a stretching member for stretching the fixing belt 301. In this case, the length in the rotational direction of the fixing belt 301 where the heating roller 307 and the fixing belt 301 are in contact is defined as the belt heating length Lh.

[0043] 4 has a configuration including a plurality of heating rollers 307a and 307b. That is, the heating member that heats the fixing belt 301 includes a plurality of heating rollers 307a and 307b.

[0044] Each of the heating rollers 307a and 307b has the same configuration as the heating roller 307 described above, and halogen heaters 306a and 306c, which are heat sources, are provided inside the first heating roller 307a and the second heating roller 307b, which are rotating members, respectively.

[0045] The first heating roller 307a is provided in contact with the fixing belt 301 at a position different from the pressure pad 303, and also serves as a first tension member that stretches the fixing belt 301. The second heating roller 307b is provided in contact with the fixing belt 301 at a position between the first heating roller 307 and the pressure pad 303, and also serves as a second tension member that stretches the fixing belt 301.

[0046] In the fixing device F shown in FIG. 4, the first heating roller 307a and the second heating roller 307b are configured to contact the inner circumferential surface of the fixing belt 301.

[0047] In the case of the fixing device F shown in FIG. 4, the length of the fixing belt 301 in the rotation direction where the first heating roller 307a and the second heating roller 307b are in contact is defined as the belt heating length Lh.

[0048] 4, the fixing device F shown in Fig. 5 is configured to have a plurality of heating rollers 307a and 307b. In addition to the plurality of heating rollers 307a and 307b, the fixing device F further includes a tension roller 312, which is provided in contact with the fixing belt 301 at a position between the second heating roller 307b and the pressure pad 303 and tensions the fixing belt 301. However, in the fixing device F shown in Fig. 5, one of the first heating roller 307a and the second heating roller 307b contacts the inner circumferential surface of the fixing belt 301, and the other contacts the outer circumferential surface of the fixing belt 301.

[0049] Therefore, in the case of the fixing device F shown in Figure 5, as in the case of the fixing device F shown in Figure 4, the length in the rotational direction of the fixing belt 301 where the first heating roller 307a and the second heating roller 307b are in contact is defined as the belt heating length Lh.

[0050] In the fixing device F shown in Fig. 6, the heating roller 307 is provided in contact with the fixing belt 301 at a position different from the pressure pad 303, and serves as a tension member that stretches the fixing belt 301. In the fixing device F shown in Fig. 6, a heating member of an induction heating type is provided in addition to the tension member.

[0051] Specifically, the heating member includes an exciting coil 309 and a magnetic core 310. The exciting coil 309 and the magnetic core 310 are provided in a non-contact manner with respect to the fixing belt 301, and are provided in a position facing the heating roller 307 with the fixing belt 301 interposed therebetween. In this case, the length in the rotational direction of the fixing belt 301 facing the longer of the exciting coil 309 and the magnetic core 310 is defined as the belt heating length Lh.

[0052] 6 has a fixing roller 311 instead of the pressure pad 303. The fixing roller 311 is a support member that contacts the fixing belt 301 at the fixing nip N to support the fixing belt 301, and is a rotatable member.

[0053] Although the fixing nip length Ln is defined as the length of contact between the fixing belt 301 and the pressure roller 305 immediately before the recording material P is passed through, it may also be defined as the length of contact between the fixing belt 301 and the recording material P during image formation. In this case, the contact pressure between the fixing belt 301 and the pressure roller 305 is preferably 0.05 MPa or more at room temperature. As an example of measuring the pressure distribution in the fixing nip N, it can be measured using a tactile sensor (surface pressure distribution measurement system I-SCAN) manufactured by Nitta Corporation.

[0054] Furthermore, the support member that contacts the inner surface of the fixing belt 301 at a position facing the pressure roller 305 is not limited to the pressure pad 303, but may be a fixing roller 311 as a rotating member as shown in FIG. 6. A heat source may be installed inside the fixing roller 311 shown in FIG. 6 or inside the stay 302 shown in FIG. 2. In other words, the support member may also function as a heating member that heats the fixing belt. When a heat source is installed in this manner and the fixing roller 311 or the pressure pad 303 is heated to a temperature higher than that of the fixing belt 301, the fixing nip N can be considered both as the fixing nip length Ln and the belt heating length Lh. In this case, the length in the rotational direction of the fixing belt 301 where the heating roller 307 and the pressure pad 303 are in contact is defined as the belt heating length Lh.

[0055] Furthermore, both the belt heating length Lh and the fixing nip length Ln may change over time, for example, because the heating roller 307 oscillates about the center of rotation. In this case, the belt heating length Lh and the fixing nip length Ln in this embodiment are defined as the length when the heating roller 307 is positioned near the center of the rotation.

[0056] 4 and 5, a heat source is provided inside the heating roller, the heating roller serves as a heating member, and the heating member is in contact with the fixing belt. However, the present invention is not limited to this. For example, in the fixing device F shown in FIGS. 4 and 5, the heating member may be provided in a position facing heating rollers 307a and 307b across fixing belt 301, as in the configuration shown in FIG. 6. In other words, the heating member may be provided in a non-contact state with the fixing belt.

[0057] <Conditions for fixation> In order to fix the toner image formed on the recording material P, it is necessary to raise the temperature of the interface between the recording material and the toner immediately after it leaves the fixing nip N to a temperature at which the toner can be sufficiently melted or higher. To achieve this, it is necessary to maintain the temperature of the fixing belt 301 just before the fixing nip N at a certain level or higher.

[0058] In this embodiment, the fixing belt 301 is driven at a constant speed of 350 mm / sec so that A4-size recording materials P can be fed at a rate of 90 sheets per minute in a transverse feed. Figure 7 shows the relationship between the fixing nip length Ln (mm) and the minimum surface temperature (°C) of the fixing belt 301 required to fix a toner image on coated paper with a basis weight of 250 gsm. As shown in Figure 7, for example, if the fixing nip length Ln is 20 mm, the surface temperature of the fixing belt 301 immediately before the fixing nip N is 168°C or higher, and the toner image can be fixed. Furthermore, increasing the fixing nip length Ln increases the time required to heat the toner, so even if the surface temperature of the fixing belt 301 immediately before the fixing nip N is low, the toner can be heated to a temperature sufficient to melt the toner.

[0059] Furthermore, in order to continuously fix the recording material P, it is necessary to restore the surface temperature of the fixing belt 301, which has dropped as the recording material P passes through the fixing nip N. In this case, if the temperature of the heating roller 307 can be set high enough, the surface temperature of the fixing belt 301 can be restored to the required temperature even if the belt heating length Lh is short. However, there are limitations on the temperature of the heating roller 307, and it cannot be raised above a certain temperature. For example, when the temperature of the heating roller 307 is controlled by a contact-type temperature detection member 308, the heating roller 307 can only be heated up to the temperature at which the temperature detection member 308 is guaranteed to operate. Therefore, in order to raise the surface temperature of the fixing belt 301, in addition to the heating temperature of the heating roller 307, the belt heating length Lh is also important.

[0060] In this embodiment, a temperature detection member 308 with a guaranteed operating temperature of 224°C is used. Fig. 8 shows the results of measuring the surface temperature [°C] of the fixing belt 301 just before the fixing nip N while heating the heating roller 307 at the maximum temperature of 224°C and feeding 90 sheets of coated paper with a basis weight of 250 gsm per minute. The results in Fig. 8 show that if the fixing nip length Ln is 20 mm and the belt heating length Lh is 60 mm, the surface temperature of the fixing belt 301 can be maintained at 184°C, and that the surface temperature [°C] of the fixing belt 301 increases as the belt heating length Lh increases.

[0061] FIG. 9 shows the conditions for simultaneously satisfying the condition for the surface temperature [°C] of fixing belt 301 at which fixing is possible according to fixing nip length Ln (FIG. 7) and the condition for the belt heating length Lh at which the surface temperature [°C] of fixing belt 301 is maintained at that temperature (FIG. 8). FIG. 9 is a diagram showing the relationship between fixing nip length Ln and belt heating length Lh. The solid line in FIG. 9 indicates the boundary of belt heating length Lh at which the surface temperature of fixing belt 301 reaches the fixing possible temperature (the minimum temperature at which toner can be melted) when fixing nip length Ln is changed while heating roller 307 is maintained at the maximum temperature of 224 [°C].

[0062] However, even when the conditions shown in Figure 9 are all satisfied, i.e., fixation is possible, the glossiness of the image surface may be significantly reduced. For example, if the fixing belt length Ln is 14 mm, the belt heating length Lh is 61 mm, and the surface temperature of the fixing belt 301 is 185°C, the toner itself melts and is fixed to the recording material P. However, crater-like irregularities resembling air bubbles escaping from the surface of the toner layer result in a significant reduction in the glossiness of the image. This is presumably due to the process in which the spherical toner particles melt and collapse smoothly in the fixing nip N. The air spaces that were originally present in the voids between the spherical toner particles escape from the toner surface after passing through the fixing nip N, causing the image surface to become rough. Therefore, to avoid this problem, it is desirable to set the fixing nip length Ln to 16 mm or more. Note that when the fixing belt length Ln is 14 mm, the image glossiness did not decrease significantly.

[0063] From the above, it was found that in order for the fixing device F to fix the toner, the belt heating length Lh needs to be in the range above the solid line in Figure 9, and the fixing nip length Ln needs to be 16 mm or more.

[0064] That is, the fixing nip length Ln is set to Ln≧16 (a range in which the gloss of the image surface does not decrease).

[0065] <Start-up time> Next, let us consider the start-up time of the fixing device F. The start-up time is the time it takes for the fixing belt 301 to heat from a low temperature state (for example, when the power is off or in sleep mode) to a temperature at which image formation is possible. The start-up time is determined essentially by the amount of power available for the heating source of the fixing device F and the sum of the heat capacities of the fixing belt 301 and all the members in contact with the fixing belt 301 at start-up.

[0066] In this embodiment, the low temperature state is set to room temperature of 23°C, which is the installation environment of a typical image forming apparatus, the temperature at which image formation is possible is set to 155°C, at which a typical recording material with a basis weight of about 100 gsm can be printed, and the time required to heat from 23°C to 25°C is set as the rise time. The maximum power to the fixing device F is set to 2500W, which is the maximum power that can be supplied from a typical 200V, 15A outlet minus the power consumed by devices other than the fixing device F (approximately 500W).

[0067] The relationship between the rise time [sec] and the belt heating length Lh under these conditions is shown in Figure 10. The reason why the horizontal axis in Figure 10 is the belt heating length Lh is because the diameter of the heating roller 307 that contacts the fixing belt 301 and the circumferential length of the fixing belt 301 must be increased in proportion to the belt heating length Lh. On the other hand, by separating the pressure roller 305 from the fixing belt 301 during rise, the increase in the fixing nip length Ln and the accompanying increase in the heat capacity of the pressure roller 305 have almost no effect on the rise time.

[0068] In this embodiment, the target start-up time is 60 seconds. 60 seconds is the maximum recovery time compliance standard for a multifunction printer that can print 89 or more pages per minute, which is the International Energy Star Program standard. The results shown in Figure 10 show the start-up time (seconds) when the diameters of the heating roller 307 and fixing belt 301 are increased at a constant rate while the thicknesses are maintained, thereby increasing the belt heating length Lh.

[0069] For example, when the diameter of the heating roller 307 is 45 mm, the belt heating length Lh is 51 mm and the rise time is 41 seconds. When the diameter of the heating roller 307 is 75 mm, the belt heating length Lh is 83 mm and the rise time is 54 seconds.

[0070] From the above, it was found that increasing the heating belt length Lh increases the heat capacity and increases the rise time. Therefore, in order to keep the rise time to less than 60 seconds, the heating belt length Lh must be less than 96 mm.

[0071] That is, the belt heating length Lh is set to Lh≦95 (a range in which the rise time is 60 seconds or less in FIG. 10).

[0072] <Stable operation> On the other hand, in this embodiment, it was found that if the fixing nip length Ln is made longer than necessary, the fixing belt 301 cannot be driven stably. This is because the sliding member 304 that forms the fixing nip N slides against the inner surface of the fixing belt 301, and the driving torque of the fixing belt 301 increases in proportion to the increase in the fixing nip length Ln. FIG. 11 is a graph showing the relationship between the driving torque [mNm] and the fixing nip length Ln [mm]. In this embodiment, it was found that if the driving torque exceeds 310 [mNm], the fixing belt 301 cannot operate stably at 350 [mm / sec], and therefore the fixing nip length Ln cannot be made longer than 27.3 [mm].

[0073] That is, the fixing nip length Ln is set to Ln≦27 (a range of a driving torque of 310 [mNm] or less in FIG. 11).

[0074] As mentioned above, it was found that the fixing nip length Ln must be 16 mm or more and cannot be longer than 27.3 mm. Furthermore, it was found that in order for the fixing device F to fix the toner, the belt heating length Lh must be in the range above the solid line in Figure 9.

[0075] 9, when the fixing nip length Ln is 16 mm, the belt heating length Lh is 35 mm, and when the fixing nip length Ln is 27 mm, the belt heating length Lh is 7.5 mm.

[0076] That is, the belt heating length Lh [mm] is set to the range above the line connecting points (16, 35) and (27, 7.5) shown in Figure 12, and Lh ≧ -2.5 × Ln + 75 (the range of the line above the curve in Figure 9).

[0077] Summarizing the above, the relationship between the fixing nip length Ln and the belt heating length Lh that the fixing device F must satisfy is as follows:

[0078] Lh ≥ -2.5 × Ln + 75 (the range of the straight line above the curve in Figure 9) Ln≧16 (within the range where the gloss of the image surface does not decrease) Lh≦95 (Rise time is within 60 seconds in Figure 10) Ln≦27 (drive torque range of 310 [mNm] or less in Figure 11)

[0079] FIG. 12 shows the range that satisfies the above conditions, and the area enclosed by the straight lines is the fixing device F proposed in this embodiment.

[0080] As described above, in this embodiment, the belt heating length Lh is set to satisfy the relationships -2.5 x Ln + 75 ≦ Lh ≦ 95 and 16 ≦ Ln ≦ 27 relative to the fixing nip length Ln. This improves the heating efficiency of the recording material when heating the image formed on the recording material. This makes it possible to achieve both faster printing times and faster start-up times while preventing the device from becoming larger.

[0081] Furthermore, the fixing nip length Ln and the belt heating length Lh are set to lengths that satisfy the above relationship within the range up to the maximum temperature guaranteed for operation of the temperature detection member 308. This makes it possible to improve the heating efficiency of the recording material when heating the image formed on the recording material.

[0082] Note that, here, as an example of an image heating device that heats a toner image on a recording material, a fixing device that is mounted in an image forming apparatus such as a printer or copier and fixes a toner image on the recording material to the recording material is shown. However, the present invention is not limited to such fixing devices, and can also be applied to image heating devices such as gloss imparting devices that improve the glossiness of an image formed on a recording material. The present invention can also be applied to image heating devices that are not mounted in an image forming apparatus.

[0083] Furthermore, although an image forming apparatus including one fixing device has been exemplified here, the present invention is not limited to this, and the present invention is also effective in an image forming apparatus including multiple fixing devices.

[0084] Furthermore, while a copier has been exemplified as an image forming apparatus, the present invention is not limited thereto and may be applied to other image forming apparatuses, such as a printer or a facsimile machine, or to other image forming apparatuses, such as a multifunction peripheral that combines the functions of these. Furthermore, while the present invention has been described with reference to an image forming apparatus that uses an intermediate transfer member, sequentially transfers toner images of each color onto the intermediate transfer member in a superimposed manner, and then transfers the toner images carried on the intermediate transfer member to a recording material all at once, the present invention is not limited thereto. It may also be an image forming apparatus that uses a recording material carrier, sequentially transfers toner images of each color onto a recording material carried on the recording material carrier in a superimposed manner. Similar effects can be achieved by applying the present invention to these image forming apparatuses. [Explanation of symbols]

[0085] F...fixing device Lh: Belt heating length Ln: Fixing nip length N: Fixing nip P... Recording material Pa to Pd: Image forming section 1...Image forming device 301...Fuser belt 302 …Stay 303...Pressure pad 304 ...Sliding member 305...Pressure roller 306...Halogen heater 307...heating roller 308...Temperature sensing member 309...Excitation coil 310...Magnetic core 311 ... fuser roller 312 ... tension roller

Claims

1. an endless belt member that is stretched by a plurality of stretching members and is rotatably provided; a pressure member that is provided opposite the belt member, contacts the belt member to form a nip, and conveys a recording material while sandwiching it together with the belt member at the nip; a heating member that is provided in contact with or not in contact with the belt member and heats the belt member; an image heating device that heats an image formed on a recording material at the nip, When the length of the belt member in the rotational direction that the heating member faces in contact or non-contact is defined as a heating length Lh, and the length of the belt member in the rotational direction that the pressing member abuts against is defined as a nip length Ln, The heating length Lh is set relative to the nip length Ln, −2.5×Ln+75≦Lh≦95, and 16≦Ln≦27, The length should satisfy the relationship An image heating apparatus characterized by:

2. The contact pressure between the belt member and the pressure member is 0.05 MPa or more.

2. An image heating apparatus according to claim 1, wherein the image heating apparatus is a heater.

3. the plurality of tension members include a support member that contacts the belt member and supports the belt member at a position facing the pressure member with the belt member interposed therebetween; 2. An image heating apparatus according to claim 1, wherein the image heating apparatus is a heater.

4. the plurality of tension members include a tension member that is provided in contact with the belt member at a position different from the support member and tensions the belt member, The heating member also serves as the tension member, The length of the belt member in the rotational direction where the heating member is in contact with the belt member is defined as the heating length Lh.

4. An image heating apparatus according to claim 3.

5. the plurality of tension members include a tension member that is provided in contact with the belt member at a position different from the support member and tensions the belt member, the heating member is provided in a non-contact manner with respect to the belt member and is provided at a position facing the tension member with the belt member interposed therebetween, The length of the belt member in the rotation direction, which the heating member faces without contacting, is defined as the heating length Lh.

4. An image heating apparatus according to claim 3.

6. the plurality of tension members include a first tension member that is provided in contact with the belt member at a position different from the support member and that stretches the belt member, and a second tension member that is provided in contact with the belt member at a position between the first tension member and the support member and that stretches the belt member, the heating member includes a first heating member that also serves as the first tension member and a second heating member that also serves as the second tension member; The length of the belt member in the rotation direction where the first heating member and the second heating member are in contact is defined as the heating length Lh.

4. An image heating apparatus according to claim 3.

7. the first heating member and the second heating member are in contact with the inner circumferential surface of the belt member; 7. An image heating apparatus according to claim 6.

8. one of the first heating member and the second heating member contacts the inner circumferential surface of the belt member, and the other of the first heating member contacts the outer circumferential surface of the belt member; 7. An image heating apparatus according to claim 6.

9. The support member also serves as the heating member.

4. An image heating apparatus according to claim 3.

10. a temperature detection member for detecting the temperature of the heating member or the belt member; The nip length Ln and the heating length Lh are set to lengths that satisfy the relationship within a range up to a maximum temperature that ensures the operation of the temperature detection member.

2. An image heating apparatus according to claim 1, wherein the image heating apparatus is a heater.

11. An image forming apparatus comprising: an image forming section that forms an image on a recording material; and the image heating device according to claim 1 .

Images