JP7799658B2 - Fixing device and image forming apparatus - Google Patents

Description

The present invention relates to a fixing device and an image forming apparatus that fixes a toner image on a recording material by applying heat and pressure to the recording material.

In recent years, the market for on-demand printing has expanded, allowing for the printing of commercial printed materials such as catalogs, posters, and pamphlets in the required number of copies, as well as the continuous printing of various invoices, direct mail, and other documents while changing some of the printed content for each customer. Furthermore, there is a growing demand for faster printing times for electrophotographic image forming devices used in on-demand printing.

To achieve such faster printing times, it is necessary to maximize the heating efficiency of the recording material when fixing the toner image to the recording material. To maximize heating efficiency, a fixing device with a wide fixing nip (wide nip) has been proposed, using a belt member, backup member (pad), and pressure member.

In a fixing device with such a wide nip, preventing slippage between the belt member and the recording material, and between the pressure member and the recording material at the nip forming area, is essential to ensuring excellent image quality on the recording material.

In other words, the frictional force between the backup member and the inner surface of the belt member must be sufficiently small relative to the frictional force between the recording material and the belt member and the frictional force between the recording material and the pressure member.

In order to reduce the friction between the backup member and the belt member, Patent Document 1 proposes a system that uses a sliding member between the backup member and the belt member.

Surface materials and surface shapes for the sliding member have also been proposed to minimize the coefficient of friction on the surface of the sliding member. For example, in Patent Document 2, a low-friction material is used for the surface of the sliding member, and the frictional force of the sliding member is reduced by creating an uneven surface shape.

In addition, in Patent Document 3, multiple uneven shapes are provided on the sliding members inside and outside the nip to reduce the frictional force of the sliding members.

JP 2017-181948 A Patent No. 5119724 Japanese Patent Application Laid-Open No. 2020-52354

In the fixing configuration described above, if foreign matter enters the uneven protrusions on the sliding member while the fixing device is in operation, the inside surface of the belt can be damaged by the foreign matter, resulting in image defects.

The present invention has been developed in light of the above circumstances, and aims to provide a fixing device and image forming apparatus that prevents foreign matter from entering the sliding members of the fixing device and prevents the occurrence of image defects.

In order to achieve the above object, the present invention provides a fixing device for fixing a toner image on a recording material, the fixing device comprising: a rotatable fixing belt that sandwiches and conveys the recording material between itself and a pressure contact member, and fixes the toner image on the recording material; a sliding member that slides on the inner circumferential surface of the fixing belt at a position where the fixing belt sandwiches and conveys the recording material; and a lubricant that flows between the fixing belt and the sliding member, the sliding member having a protrusion with which the fixing belt slides, the protrusion being 10 mm in diameter and located upstream in the conveyance direction of the recording material from the position where the fixing belt slides. a fixing device and an image forming apparatus equipped with the fixing device, wherein the height h of the protrusion from the tip of the first protrusion at a portion 100 μm away from the fixing belt on the upstream side in the recording material transport direction is 35 μm≦h≦100 μm, the sliding member has a first protrusion located in a first region in the recording material transport direction and a second protrusion located in a second region downstream of the first region in the recording material transport direction, the first protrusion and the second protrusion are arranged in positions where they overlap when projected in the recording material transport direction, the height h1 ' of the _first protrusion from the tip of the first protrusion at a portion 100 μm away from the position where the fixing belt slides on the upstream side in the recording material transport direction is 35 _μm ≦ _h1 '≦100 μm, and the height h2 ' of the second protrusion from the tip of the second protrusion at a portion 100 μm away from the position where the fixing belt slides on the upstream side in the recording material transport direction is ₅ μm≦ h2 '≦30 μm.

This invention provides a fixing device and image forming apparatus that prevent foreign matter from entering the sliding members of the fixing device and prevent image defects from occurring.

1 is a schematic front cross-sectional view of an image forming apparatus according to a first embodiment of the present invention. 2 is a schematic front cross-sectional view of the fixing device of FIG. 1 according to the first embodiment of the present invention; 3 is a detailed view of a sliding member in the fixing device of FIG. 2 according to the first embodiment of the present invention. 3 is an enlarged front cross-sectional view of a fixing belt and a sliding member in the fixing device of FIG. 2 according to the first embodiment of the present invention. FIG. 5A and 5B are schematic diagrams illustrating the intrusion of foreign matter into a sliding member according to the first embodiment of the present invention and the resulting image defects. 3 is an enlarged front cross-sectional view of the vicinity of a tip of one protrusion of the sliding member according to the first embodiment of the present invention. FIG. 4 is a graph showing the height from the tip of a protrusion of a protrusion of a sliding member according to the first embodiment of the present invention. 10 is a table showing the results of an investigation into the state of foreign matter intrusion when the height from the tip of the protrusion of the sliding member according to the first embodiment of the present invention is changed. 10 is a table showing the results of an investigation into the state of foreign matter intrusion when the viscosity of the lubricant of the fixing belt and the paper conveyance speed are changed according to the second embodiment of the present invention; 10A and 10B are explanatory views illustrating a state in which the sliding layer of the protrusions in the sliding member according to the third embodiment of the present invention is worn away. 10A and 10B are detailed views of the configuration of a sliding member and the arrangement of protrusions according to a third embodiment of the present invention. 10 is a graph showing the height from the tip of a protrusion of a sliding member according to a third embodiment of the present invention. 10 is a table showing the results of examining the wear state of the sliding layer of the protrusions when the height from the tip of the protrusion of the slide member according to the third embodiment of the present invention is changed. 10 is a graph showing the results of a study on the recovery rate of the thickness of the lubricant at the tip of the projection when the interval between the projections of the sliding member according to the fourth embodiment of the present invention is changed. 13A and 13B are explanatory diagrams showing the relationship between the positional relationship of the sliding member with the pressure roller and the contact pressure with the fixing belt according to the fifth embodiment of the present invention.

Preferred embodiments of the present invention will be described in detail below with reference to the drawings. Note that in the following embodiments, an electrophotographic full-color image forming apparatus having multiple photosensitive drums will be described as an example, but 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 components described in the following embodiments are merely examples, and various conditions such as the configuration, function, dimensions, materials, shape, and relative arrangement of the apparatus to which the present invention is applied can be modified or changed as appropriate within the scope of the spirit of the present invention, and are not limited to the following embodiments.

First Embodiment
FIG. 1 is a schematic front cross-sectional view of a full-color image forming apparatus according to a first embodiment of the present invention.

In Figure 1, the image forming device 1 includes an image reading unit 2 and an image forming device main body 3.

The image reading unit 2 reads a document placed on the document glass 21. Light emitted from a light source 22 is reflected by the document and forms an image on a CCD sensor 24 via optical elements 23 such as lenses. The optical unit consisting of the light source 22, optical elements 23, and CCD sensor 24 scans in the direction of arrow E, causing the CCD sensor 24 to convert the document into an image signal consisting of a line-by-line electrical signal data sequence. The image signal obtained by the CCD sensor 24 is sent to the image forming device main body 3, where it is processed in the control unit 30 in accordance with the image processing requirements of each image forming unit, as described below. The control unit 30 also receives external inputs as image signals from external host devices such as print servers.

The image forming device main body 3 has multiple image forming units Pa, Pb, Pc, and Pd, and each image forming unit forms an image based on the image signal described above.

Specifically, the image signal is first converted into a PWM (pulse width modulation) laser beam by the control unit 30. 31, an exposure device, scans the laser beam in accordance with the image signal. The laser beam is then irradiated onto the photosensitive drums 200a-200d, which serve as image carriers in each image forming unit 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 images of the corresponding color. Note that image forming units Pa to Pd have the same configuration, so the following will explain the configuration of Y image forming unit Pa. The configurations of the other image forming units will be indicated by assigning the same numbers as the components of Y image forming unit Pa to the components corresponding to Y image forming unit Pa, and by replacing the suffix "a" in the numbers of the components of Y image forming unit Pa with b to d, and will not be explained further.

In the Y image forming unit Pa, reference numeral 200a denotes a photosensitive drum, on whose surface a toner image is formed based on an image signal, as described below.

Reference numeral 201a denotes a primary charger, which charges the surface of photosensitive drum 200a to a predetermined potential to prepare for the formation of an electrostatic latent image. A laser beam from polygon scanner 31 forms an electrostatic latent image on the surface of photosensitive drum 200a, which has been charged to a predetermined potential. Reference numeral 202a denotes a developer, which develops the electrostatic latent image on photosensitive drum 200a to form a toner image. Reference numeral 203a denotes a transfer roller, which discharges from the back of intermediate transfer belt 204 and applies a primary transfer bias of opposite polarity to the toner, transferring the toner image on photosensitive drum 200a onto intermediate transfer belt 204. After transfer, the surface of photosensitive drum 200a is cleaned by cleaner 207a.

The toner image on the intermediate transfer belt 204 is transported to the next image forming unit, where the toner images of each color formed in each image forming unit are transferred in the order of Y, M, C, and Bk, forming a four-color image superimposed on its surface. The toner image that has passed through the Bk image forming unit Pd is secondarily transferred to paper as a recording material in the secondary transfer unit consisting of a pair of secondary transfer rollers 205 and 206 by applying a secondary transfer electric field of opposite polarity to the toner image on the intermediate transfer belt 204. Paper fed from paper feed cassette 8 or 9 waits in the registration unit 208, and then the timing is controlled to align the paper with the toner image on the intermediate transfer belt, before being transported from the registration unit 208.

The toner image transferred onto the paper is fixed to the paper by the fixing device G, which also serves as an image heating device. After passing through the fixing device G, the paper with the fixed toner image is ejected from the machine and stacked on the paper output tray 7.

On the other hand, in the case of a double-sided job, once the transfer and fixing of the toner image on the first side (side 1) of the image formation is complete, the fixed paper is turned over through an inversion section located inside the image formation device, and the toner image on the second side (side 2) of the image formation is transferred and fixed, after which the paper is ejected from the machine and stacked on the paper output tray 7.

Next, we will explain the details of the configuration of the fixing device G in Figure 1.

Figure 2 is a schematic front cross-sectional view showing the overall configuration of the fixing device G in Figure 1. Note that in the following figures, the direction of arrow X indicates the conveyance direction of the recording material (not shown in the figure), the direction of arrow Y indicates the width direction of the recording material, and the direction of arrow Z indicates the pressure direction of the fixing belt. The dotted line area is an enlarged view of the nip portion N.

In Figure 2, fixing device G is a belt heating type fixing device, and includes a fixing belt 301 as an endless, rotatable heating rotor, a pressure pad 303 for supporting the fixing member, a stay 302 for supporting the pressure pad 303, a sliding member 304 arranged to cover the pressure pad 303, a heating roller 307, and a pressure roller 305 as a pressure rotor that faces the fixing belt 301 and forms a nip portion N together with the fixing belt 301.

The fixing belt 301 has thermal conductivity, heat resistance, and other properties, and is a thin, cylindrical shape. In this embodiment, as shown in the enlarged dotted line view, the fixing belt 301 has a three-layer structure consisting of a base layer 301a, an elastic layer 301b formed on the outer periphery of the base layer 301a, and a release layer 301c formed on the outer periphery of the elastic layer 301b. The base layer 301a is 80 μm thick and made of polyimide resin (PI), the elastic layer 301b is 300 μm thick and made of silicone rubber, and the release layer 301c is 30 μm thick and made of PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin) as a fluororesin. The fixing belt 301 is stretched between a pressure pad 303 and a heating roller 307. The outer diameter of the fixing belt 301 is 150 mm.

The pressure pad 303 is pressed against the fixing belt 301 by a pressure roller 305, which serves as a pressure contact member, with the fixing belt 301 sandwiched between them. The pressure pad 303 is made of LCP (liquid crystal polymer) resin. A sliding member 304 is interposed between the pressure pad 303 and the fixing belt 301.

The detailed configuration of the sliding member 304 is shown in Figure 3. Figure 3(A) is a cross-sectional view of the sliding member 304 in the recording material conveyance direction, and Figure 3(B) is a view of the sliding member 304 viewed from the side of the contact surface with the fixing belt 301.

In Figure 3, the sliding member 304 of this embodiment, as shown in Figure 3(A), is composed of a base layer 304a, protrusions 304b, and a sliding layer 304c. The base layer 304a only needs to have sufficient heat resistance and strength. Materials such as SUS, copper, aluminum, and engineering plastics (PI, PEEK, LCP, etc.) are desirable. In this embodiment, 1.3 mm thick SUS is used. The sliding member 304 has a flat top surface and is formed with cylindrical protrusions 304b. The sliding layer 304c is preferably provided with a material (fluorine coating, PTFE, PFA, etc.) to achieve low friction. In this embodiment, it is coated with a 20 μm thick PTFE (polytetrafluoroethylene) coating.

In Figure 2, a lubricant is further applied to the inner surface of the fixing belt 301, allowing the fixing belt 301 to slide smoothly against the sliding member 304. Silicone oil is used as the lubricant. A stay 302 is used to supplement the strength of the pressure pad 303. In this embodiment, the sliding layer 304c is provided on the base material layer 304a, but an adhesive layer may also be provided between the base material layer 304a and the sliding layer 304c. By using an adhesive layer in this way, it is possible to achieve good adhesive strength between the base material layer a and the sliding layer c when using a metal material such as SUS, copper, or aluminum as the material for the base material layer 304a.

In addition, this embodiment employs a configuration in which the sliding member 304 is fixed to the pressure pad 303. For example, both ends of the sliding member 304 in the direction of arrow Y (the recording material width direction) are fixed to the pressure pad 303 with screws or the like. Although not shown here, the sliding member 304 may be integral with the pressure pad 303.

The heating roller 307 is a 1 mm thick stainless steel pipe with a halogen heater 306 installed inside as a heat source, capable of generating heat up to a predetermined temperature. The fixing belt 301 is heated by the heating roller 307.

The heating roller 307 has a rotation center at one end or near the center, and rotates relative to the fixing belt 301 to generate a tension difference between the front and rear, controlling the position of the fixing belt 310 in the main scanning direction. The heating roller 307 is also biased by a spring supported on the frame of the heating unit (not shown), and serves as a tension roller that applies a predetermined tension to the fixing belt 301.

The pressure roller 305 is a roller formed with an elastic layer 305b on the outer periphery of a core metal shaft 305c, as shown in the enlarged dotted line, and a release layer 305a on the outer periphery of the elastic layer 305b. The core metal shaft 305c is made of a 72 mm diameter SUS member, the elastic layer 305b is made of conductive silicone rubber with a thickness of 8 mm, and the release layer 305a is made of a 100 μm thick fluororesin PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin). The pressure roller 305 is axially supported by the fixing frame (not shown) of the fixing device G, and a gear is fixed to one end. It is connected to a drive source (not shown) via this gear and is driven to rotate.

In the nip N formed between the fixing belt 301 and pressure roller 305, the toner image is heated while the recording material carrying the toner image is nipped and transported. In this way, the fixing device G fixes the toner image to the recording material while nipping and transporting the recording material. Therefore, the nip N must be able to both apply heat and pressure and transport the recording material. A drive source (not shown) presses the pressure roller 305 against the pressure pad 303 via the fixing belt 301. The pressure force (NF) at the nip N during printing is 1600 N, and the width of the nip N in the direction of arrow X (paper transport direction) is set to 24.5 mm, and the width in the direction of arrow Y (paper width direction) is set to 326 mm.

Next, we will explain the sliding layer 304c of the sliding member 304 when the fixing device G is driven.

Figure 4 is an enlarged front cross-sectional view of the fixing belt 301 and sliding member 304 of the fixing device G.

In Figure 4, when the fixing device G is driven, the fixing belt 301 moves relative to the sliding member 304 in the direction of arrow L in the figure, causing the sliding layer 304c of the sliding member 304 to rub against the base layer 301a of the fixing belt 301.

Figure 5 is a schematic diagram illustrating the intrusion of foreign matter into sliding member 304 and the resulting image defects. Figure 5(A) shows foreign matter D having infiltrated one of protrusions 304b of sliding member 304 and become entangled at the tip of protrusion 304b. When foreign matter D becomes entangled at the tip of protrusion 304b, the entangled foreign matter D damages the inner surface of fixing belt 301, reducing the pressure applied to toner and decreasing gloss. Such locally low gloss areas can affect the image in the form of gloss streaks as shown in Figure 5(B). Therefore, to prevent such image defects, it is important to prevent foreign matter from entering the protrusions of sliding member 304.

Figure 6 is an enlarged front cross-sectional view of the vicinity of the tip of one of the protrusions 304b on the sliding member 304. Figure 6(A) shows the shape of the tip of the protrusion 304b with a gentle slope, while Figure 6(B) shows the shape of the tip of the protrusion 304b with a steep slope.

When the tip of the protrusion 304b on the sliding member 304 is gently inclined, a buoyancy F is generated on the sliding member 304 when the lubricant O on the inner surface of the fixing belt 301 flows into the protrusion 304b, creating a gap between the protrusion 304b of the sliding member 304 and the base layer 301a of the fixing belt 301 (= wedge effect). If a foreign object D flows in under this condition, the foreign object D will penetrate the tip of the protrusion 304b.

On the other hand, when the tip of the protrusion 304b on the sliding member 304 is steeply inclined, less lubricant O on the inner surface of the fixing belt 301 flows into the protrusion 304b, and the buoyancy F generated on the sliding member 304 is small, so the protrusion 304b of the sliding member 304 and the base layer 301a of the fixing belt 301 come into contact, preventing foreign matter D from entering the protrusion tip of the protrusion 304b.

Therefore, in order to prevent the intrusion of foreign matter, it is effective to design the protrusion shape of the protrusion portion 304b on the sliding member 304 so that the tip of the protrusion is steeply inclined.

Here, we will explain the method used to examine the effectiveness of the configuration of the protrusion 304b of the sliding member 304, which is effective in preventing the intrusion of foreign matter in this embodiment, and the results of that investigation.

Prepare multiple levels of sliding members with different projection shapes for the protrusion portion 304b of the sliding member 304.

As shown in Figure 7, the protrusion shape of the protrusion 304b is such that the height h from the tip of the protrusion 304b at part B, which is 100 μm upstream in the paper transport direction from the position of contact part A where the base layer 301a of the fixing belt 301 slides against the protrusion 304b, is 10 μm to 120 μm.

These sliding members 304 were then sequentially replaced with the fixing device G, and a black solid image was printed on plain paper to check whether any image defects occurred. The lubricant on the inner surface of the fixing belt 301 had a viscosity η of 1000 ^mm /s, and the paper transport speed v was set to 435 mm/s.

Next, the results of the study will be explained using the table in Figure 8.

Under conditions 1-1 and 1-2, streaks appeared in the image, and it was found that foreign matter that had entered the tip of protrusion 304b had damaged the inner surface of fixing belt 301. Under condition 1-3, slight streaks appeared in the image, and under conditions 1-4 to 1-9, no streaks were observed in the image, and no damage was observed on the inner surface of fixing belt 301. This is thought to be because the wedge effect described above changes the gap between the tip of protrusion 304b and base layer 301a of fixing belt 301, and foreign matter enters when the gap becomes larger than the size of the foreign matter.

Furthermore, when the sliding member 304 was observed after the image was printed, no change was observed compared to before printing under conditions 1-1 to 1-8, but under condition 1-9, it was confirmed that the sliding layer 304c at the tip of the protrusion 304b of the sliding member 304 had peeled off. This is thought to be because the inclination of the protrusion tip of the protrusion 304b became too great, reducing the inflow of lubricant and causing the lubricant to run out at the contact surface between the tip of the protrusion 304b and the base layer 301a of the fixing belt 301.

Therefore, by setting the shape of the protrusion 304b to 35 μm≦h≦100 μm, durability can be maintained and image defects can be prevented.

Second Embodiment
Next, in order to prevent the intrusion of smaller foreign matter, the shape of the protrusion 304b was set to the above-mentioned 35 μm≦h≦100 μm, and the effects on the image of the viscosity η [mm ² /s] of the lubricant used on the inner surface of the fixing belt 301 and the paper conveying speed v [mm/s] were examined.

Generally, the greater the viscosity η of the lubricant used on the inner surface of the fixing belt 301 x the paper conveyance speed v, the more easily the lubricant can penetrate between the protrusions 304b of the sliding member 304 and the base layer 301a of the fixing belt 301, creating a gap that allows foreign matter to easily enter due to the wedge effect described above.

Here, we will explain the method used to examine the effect of the viscosity η of the lubricant used on the inner surface of the fixing belt 301 according to this embodiment, which is effective in preventing the intrusion of smaller foreign objects, multiplied by the paper conveyance speed v, and the results of that examination.

In this embodiment, several levels of lubricant with a viscosity η of 300 to 3000 mm ² /s were prepared for use on the inner surface of the fixing belt 301, and the state of the image was confirmed when the paper conveying speed v was changed in the range of 50 to 435 mm/s.

To make it easier to check the effects of foreign matter, a solid black image was printed on coated paper, on which image defects are more noticeable, and the image was checked. The shape of the protrusion 304b used was a sliding member with the above-mentioned h = 40 μm.

Next, we will explain the results of our study using the table in Figure 9.

Under conditions 2-1 to 2-3, slight streaks were observed in the image, but no damage was found on the inner surface of the fixing belt 301. This is because tiny foreign matter had entered the tip of the protrusion 304b, which is small enough not to affect plain paper. It is thought that the gloss increased in areas where pressure was locally increased due to the entry of foreign matter, resulting in gloss streaks appearing in the image. Under conditions 2-4 to 2-6, no streaks were observed in the image, and no damage was found on the inner surface of the fixing belt 301.

Therefore, by setting the relationship between the viscosity η [mm ² /s] of the lubricant used on the inner surface of the fixing belt 301 and the paper conveying speed v [mm/s] to η×v≦2×10 ⁵ mm ³ /s ² , it is possible to further improve the accuracy of preventing image defects.

Third Embodiment
In the first embodiment, the height h of the protrusion 304b of the sliding member 304 is set within a specified range, and in the second embodiment, the viscosity η of the lubricant and the paper conveying speed v are set within a specified range to prevent foreign matter from entering the protrusion 304b of the sliding member 304. However, when the contact pressure between the fixing belt 301 and the protrusion 304b is high, it is necessary to improve durability.

Figure 10 is an explanatory diagram showing the state in which the sliding layer 304c of the protrusion 304b of the sliding member 304 wears.

In Figure 10, sliding layer 304c of protrusion 304b gradually wears from the state shown in Figure 10(A) as the printing operation of the image forming device progresses, and the film thickness of sliding layer 304c at the tip of protrusion 304b decreases as shown in Figure 10(B). As the printing operation of the image forming device progresses further, protrusion 304b becomes exposed as shown in Figure 10(C), and base layer 301a of fixing belt 301, which has a higher coefficient of friction than sliding layer 304c, comes into direct contact with protrusion 304b, causing an increase in frictional force between fixing belt 301 and sliding member 304, increasing the driving torque and bringing the product to the end of its life.

This wear rate actually becomes faster if, as in the first and second embodiments, it is difficult for the lubricant to penetrate between the protrusions 304b of the sliding member 304 and the base layer 301a of the fixing belt 301.

Therefore, in the configuration of the first embodiment, if the thickness of the lubricant used on the inner surface of the fixing belt 301 at the tip of the protrusion 304b can be ensured to be sufficient, it is possible to reduce the frictional force between the fixing belt 301 and the sliding member 304, slow down the wear rate of the sliding layer 304c, and improve the durability and lifespan.

To achieve this, in this embodiment, as shown in Figure 11 (A), the sliding member is divided into two regions: region 701, where the protrusions are located most upstream in the paper transport direction, and region 702, which is located downstream of region 701. In region 701, the protrusions 304b are shaped to prevent the intrusion of foreign matter, and in region 702, the protrusions 304b are shaped to ensure the thickness of the lubricant, thereby achieving both prevention of image defects and improved durability.

As shown in Figure 11(A), all protrusions located in areas 701 and 702 are positioned so that they overlap when projected in the paper transport direction. In other words, as shown by the dashed line in Figure 11(A), protrusion 702a is located downstream of protrusion 701a in the paper transport direction at the same position. Similarly, protrusion 702b is located downstream of protrusion 701b in the paper transport direction at the same position.

This is to prevent foreign matter D from entering the protrusions located in area 702 if it flows downstream in the paper transport direction, passing by the protrusions located in area 701, without entering the protrusions located in area 701. If the protrusions are located in places where they do not overlap when projected in the paper transport direction, as shown in Figure 11(b), there is a risk that foreign matter D passing by the protrusions located in area 701 will enter the protrusions located in area 702.

In this embodiment, the configuration shown in Figure 11(A) is used to ensure that the thickness of the lubricant used on the inner surface of the fixing belt 301 is sufficient for the protrusions located in the area 702 where foreign matter is prevented from entering, thereby slowing the wear rate of the sliding layer 304c.

In other words, the greater the height h of the protrusion located in region 702, the more difficult it becomes to ensure a sufficient thickness of lubricant at the tip of the protrusion. Therefore, the following considerations were conducted to determine a protrusion shape that would ensure a sufficient thickness of lubricant.

Prepare multiple levels of sliding members with different projection shapes for the protrusions 304b of the sliding members 304 located in areas 701 and 702.

As shown in Figure 12, the protrusion shape of the protrusion portion 304b located in region 701 is such that the height h1' from the tip of the protrusion at part _B , which is 100 μm upstream in the paper transport direction from the contact point A with the base layer 301a of the fixing belt 301, is 60 μm, which is the range in which foreign matter does not enter as shown in the first embodiment, and in contrast, the protrusion portion 304b located in region 702 is such that the height _h2 ' from the tip of the protrusion at part B, which is 100 μm upstream in the paper transport direction from the contact point A with the base layer 301a of the fixing belt 301, is 2 μm to 35 μm.

These sliding members 304 were then sequentially swapped into the fixing device G mounted on the image forming apparatus shown in Figure 1, and a drive durability test was conducted. The drive durability test was conducted in a mode in which the pressure roller 305 alternately repeated states of contact and non-contact with the fixing belt 301, with the design target time in this mode being 240 hours. If the drive torque exceeded a preset upper limit within the design target time, the drive durability test was terminated; if the drive torque did not exceed the upper limit within the design target time, the drive durability test was terminated after the design target time had elapsed.

The upper limit of the drive torque was set to 300 mNm, which is the limit at which slippage can cause defective images and damage to the drive gear. The viscosity η of the lubricant was set to 1000 ^mm /s, and the paper transport speed v was set to 435 mm/s.

Next, the results of the study will be explained using the table in Figure 13.

Under condition 3-1, the drive torque exceeded the threshold value of 300 mNm within the durability design target time, and when the condition of the sliding member 304 was checked, it was observed that the sliding layer 304c at the tip of the protrusion 304b had disappeared. Furthermore, the inner surface of the base layer 301a of the fixing belt 301 was also severely damaged. This is thought to be because the disappearance of the sliding layer 304c exposed the protrusion 304b, which then excessively rubbed against the inner surface of the fixing belt.

Under condition 3-2, the drive torque did not exceed the threshold value within the durability design target time, but the final torque value was 280 mNm, rising close to the upper limit. At this time, the sliding layer 304c of the sliding member 304 had partially disappeared, and minor streak-like scratches were found on the inner surface of the fixing belt 301.

Under conditions 3-1 and 3-2, the height _h2 ' of the tip of the protrusion 304b became too small, narrowing the gap between the base layer 301a of the fixing belt 301, making it difficult for the lubricant to penetrate, causing wear powder to accumulate on the sliding layer 304c, and leading to an increase in torque.

For conditions 3-3 to 3-5, the final torque value was sufficiently lower than the threshold value, and the sliding layer 304c remained sufficiently intact. Furthermore, no damage was observed on the inner surface of the fixing belt 301. This is thought to be because the protrusions 304b located in region 702 have a shape with a height _h2 ' at the tip of the protrusion, which ensures a sufficient lubricant thickness through the wedge effect, thereby reducing the wear rate of the sliding layer 304c.

On the other hand, under condition 3-9, as with condition 3-1, the drive torque exceeded the threshold value of 300 mNm within the durability design target time, and the sliding layer 304c at the tip of the protrusion disappeared. This is thought to be because the height _h2 ', which is the inclination of the tip of the protrusion 304b, became large, preventing sufficient flow of lubricant, and the lubricant at the contact surface between the tip of the protrusion 304b and the fixing belt 301 was depleted, resulting in partial contact.

From the above, by setting the shape of the protrusion 304b located in region 701 to 35 μm≦h ₁ '≦100 μm as shown in the first embodiment, and by setting the shape of the protrusion located in region 702 to 5 μm≦h ₂ '≦30 μm, it is possible to prevent image defects and improve durability at the same time.

Fourth Embodiment
When the lubricant used on the inner surface of fixing belt 301 passes over protrusions 304b of sliding member 304, the lubricant is scraped off by protrusions 304b, temporarily thinning the thickness of the lubricant. A certain amount of time is required for the thickness of the lubricant scraped off by passing over protrusions 304b to recover to the same level as the initial thickness before it reaches the next protrusion 304b it passes over.

In the third embodiment, it was shown that it is possible to prevent image defects and improve durability at the same time. However, in cases where the contact pressure with the fixing belt is higher or in order to further extend the durability life in the future, by setting the protrusion distance d [mm] between the protrusion 701a in region 701 and the protrusion 701b in region 702 shown in FIG. 11(A) in the third embodiment and the time t _d [s] required when the paper travels at a conveying speed v [mm/s] within a specified range, it is possible to ensure sufficient thickness of the lubricant in the protrusions, and further improvement in the durability of the fixing device can be expected.

To investigate the effect on lubricant thickness when the protrusion spacing d is changed, we prepared several levels of sliding members with protrusion spacing d = 1 to 4 mm and conducted the following study.

11A, the thickness of the lubricant flowing into the protrusion 701a located at the most upstream of the nip was set to 100%, and the thickness of the lubricant flowing into the next protrusion 702a after passing through the protrusion 701a (=thickness recovery rate) was measured. The viscosity η of the lubricant was set to 1000 ^mm /s, and the paper transport speed v was set to 435 mm/s.

Next, the results of this study will be explained using the graph in Figure 14.

As the protrusion spacing d increases, the thickness recovery rate gradually increases, reaching 100% when d = 3.5 mm. Therefore, to fully recover the thickness of the lubricant scraped off by the protrusions, d ≥ 3.5 mm is sufficient.

Therefore, the time _td [s] required for the paper to travel at a transport speed v [mm/s] over a distance d [mm] between the protrusions can be expressed as follows: _td = d/v
v=435mm/s, d≧3.5mm
Therefore, t _d ≧ 3.5/435 = 0.008 s
By satisfying the above relationship, it is possible to prevent image defects and further improve durability.

Fifth Embodiment
In the configuration of the third embodiment, when the state of the sliding member 304 after the durability design target time was checked, it was found that the sliding layer 304c was more worn at the protrusions located in the region 701 than at the protrusions located in the region 702. This is thought to be because the lubricant of the fixing belt 301 is less likely to penetrate into the protrusions located in the region 701 than into the protrusions located in the region 702.

Therefore, if the pressure applied to the protrusions located in region 701 can be reduced, the wear rate of sliding layer 304c will slow down, and further improvements in durability can be expected.

Figure 15(A) is a schematic diagram showing the positional relationship between the sliding member 304, the fixing belt 301, and the pressure roller 305 facing the fixing belt 301 in this embodiment.

In Figure 15 (A), the protrusions located in area 701 contact only the fixing belt 301 and are located outside the area that is pressed against the pressure roller 305, while the protrusions located in area 702 contact the fixing belt 301 and the pressure roller 305 that faces it via the fixing belt 301, and are located within the area that is pressed against the pressure roller 305.

As a result, as shown in Figure 15 (B), the protrusions located in area 701 can reduce the contact pressure with the fixing belt 301 compared to the protrusions located in area 702, thereby improving durability.

In FIG. 15(A), among the protrusions located in region 702, those located in region 702' that are not included in the nip forming portion located downstream in the paper transport direction may be in contact with the pressure roller 305 that faces it across the fixing belt 301. In other words, the protrusions located in region 702' may be included in the nip forming portion.

Furthermore, since the protrusions located in area 702' do not affect the intrusion of foreign matter, if the protrusions located in area 702' are not in contact with the pressure roller 305, the protrusion height of the protrusions located in 702' can be selected from the protrusion height of the protrusions in area 701 or area 702.

REFERENCE SIGNS LIST 1... image forming apparatus 301... fixing belt 301a... base layer 301b... elastic layer 301c... release layer 302... stay 303... pressure pad 304... sliding member 304a... base layer 304b... protrusions 304c... sliding layer 305... pressure roller 306... halogen heater 307... heating roller D... foreign matter G... fixing device N... nip portion O... lubricant

Claims (6)

In a fixing device that fixes a toner image on a recording material,
a rotatable fixing belt that sandwiches and conveys a recording material between itself and a pressure contact member, and fixes a toner image onto the recording material;
a sliding member that slides on an inner circumferential surface of the fixing belt at a position where the fixing belt nip and conveys the recording material;
a lubricant that flows between the fixing belt and the sliding member;
and
the sliding member has a protrusion with which the fixing belt slides,
the protrusion has a height h of 35 μm≦h≦100 μm from the tip of the protrusion at a position 100 μm away from the position with which the fixing belt slides toward the upstream side in the conveyance direction of the recording material,
the sliding member includes a first protrusion located in a first region in the recording material conveyance direction, and a second protrusion located in a second region downstream of the first region in the recording material conveyance direction,
the first protrusion and the second protrusion are disposed at positions where they overlap when projected in a recording material conveyance direction,
a height h1 _' of the first protrusion from a tip of the first protrusion at a portion 100 μm away from a position with which the fixing belt is in sliding contact with the fixing belt on the upstream side in the conveyance direction of the recording material is ₃₅ μm≦ h1 '≦100 μm;
The fixing device is characterized in that the height h2 ' from the tip of the second protrusion at a portion 100 _μm upstream of the position where the fixing belt slides in contact with the fixing belt in the conveying direction of the recording material is 5 μm _≦ h2 '≦30 μm. 2. The fixing device according to claim 1, wherein the relationship between the viscosity η [ mm ² /s ] of the lubricant and the conveying speed v [mm/s] of the recording material is η×v≦2× 10 ⁵ mm ³ /s ² . _2. The fixing device according to claim 1, wherein the time td [s] during which the recording material is transported between the first protrusions and the second protrusions satisfies td _≧ 0.008 s. the pressing member presses the fixing belt against the sliding member,
the first protrusion of the sliding member is provided outside a region that is pressed against the pressing member,
2. The fixing device according to claim 1, wherein the second protrusion of the sliding member is provided within a region that is pressed against the pressing member. The fixing device described in claim 1, characterized in that the first protrusion of the sliding member has a lower contact pressure with the fixing belt than the second protrusion of the sliding member. The fixing device according to any one of claims 1 to 5,
an image forming unit that forms a toner image fixed by the fixing device on a recording material;
An image forming apparatus comprising: