JP2019197239A - Fixing device, fixing method, and image forming apparatus - Google Patents

Abstract

To eliminate temporal loss for cleaning, and remove a toner fastened to rollers of a fixing device through a regular printing operation without arranging a large-scale cleaning member.SOLUTION: There is provided a fixing device that passes a transfer medium P carrying a toner image through a nip part N that is formed between two rollers 21 and 31 on a driving side and a driven side having their outer peripheral surfaces in pressure contact with each other to fix the toner image to the transfer medium P. The shearing force acting on the fixing nip during rotation of the two rollers is 15 N or more and 25 N or less.SELECTED DRAWING: Figure 6A

Description

The present invention relates to a fixing device and a fixing method for fixing an image on a transfer medium, and an image forming apparatus including the fixing device.

In an electrophotographic image forming apparatus such as a printer or a copying machine, a transfer medium such as a sheet on which a toner image is formed is passed through a fixing nip formed between a pressure roller and a fixing roller. A fixing device that fixes a toner image on a transfer medium is used.

The toner (toner image) on the transfer medium is fixed on the transfer medium by being melted by heat applied from one or both rollers. However, a part of the toner may adhere to one or both of the rollers due to excessive or insufficient heat amount or electrical action.

When such a fixed toner is present, as shown in FIG. 13, the toner releasability (fixability of the toner to the transfer medium) is reduced at the fixed toner 200 portion, and the toner on the fixing roller 21 in the subsequent fixing process. An offset image (also referred to as “black spot image”) 201 is generated in which the image is transferred onto the transfer medium P at a circumferential pitch. Further, when a transfer medium containing a large amount of filler such as calcium carbonate is used, the filler is likely to adhere to the fixing roller 21, so that the offset image is likely to occur.

In order to suppress the occurrence of such an offset image, for example, in Patent Document 1 (Japanese Patent Laid-Open No. 2002-40860), the transfer material moves between the surfaces of the fixing member and the pressure member before reaching the fixing nip. A method for creating a speed difference has been proposed. Japanese Patent Application Laid-Open No. 2009-37078 proposes a method of installing a cleaning web and a cleaning roller for removing fixed toner on the surface of a pressure member.

The method of Patent Document 1 generates fixed toner removal force (peeling force) due to the difference in moving speed between the fixing member and the pressure member. The magnitude of this force is caused by paper dust (toner filler, etc.). It is not sufficient for removing a large amount of fixed toner. In some cases, the fixed toner is transferred from one of the rollers to the other and is not finally removed. Further, the fixed toner cannot be removed while the transfer material passes through the fixing nip.

The method of Patent Document 2 increases the size of the apparatus due to the cleaning web and the cleaning roller, resulting in an increase in cost. Furthermore, the toner collected by the cleaning roller accumulates and solidifies, generating abnormal noise, or the transfer medium is soiled by “toner dropping” in which the toner accumulated in a predetermined amount or more on the cleaning roller melts. Further, in both Patent Documents 1 and 2, it is necessary to execute a predetermined cleaning sequence different from the normal printing operation, so that time loss occurs.

In view of the above, an object of the present invention is to remove toner adhered to a roller of a fixing device by a normal printing operation without a time loss for cleaning and without disposing a large cleaning member.

In order to solve the above-mentioned problems, the present invention provides the toner by passing a transfer medium holding a toner image through a fixing nip formed between two driving and driven rollers whose outer peripheral surfaces are in pressure contact with each other. In the fixing device for fixing an image on the transfer medium, a shearing force acting on the fixing nip when the two rollers are rotated is 15 N or more and 25 N or less.

In the present invention, as described above, since the shearing force acting on the fixing nip when the two rollers rotate is 15 N or more and 25 N or less, the toner adhering to the roller is transferred to the transfer medium side by the shearing force during the normal printing operation. Can be removed.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. 1 is a schematic cross-sectional view showing a type of a fixing device to which the present invention can be applied. 1 is a schematic configuration diagram of a fixing device according to a first embodiment of the present invention. FIG. 5 is a shaft end view showing two examples of a pressure roller sliding bearing of the fixing device according to the first embodiment of the present invention. (A) is a sectional view of the shaft end of the sliding bearing used in the fixing device of the first embodiment, (b) is an enlarged sectional view of the initial shaft end of the sliding bearing, and (c) is an enlarged sectional view of the sliding shaft end of the sliding shaft over time. FIG. (A) is a sectional view of an initial shaft end of a sliding bearing used in the fixing device of the first embodiment, and (b) is a sectional view of a sliding shaft end of the sliding shaft. (A) is a sectional view of an initial shaft end of a sliding bearing used in the fixing device of the first embodiment, and (b) is a sectional view of a sliding shaft end of the sliding shaft. It is a schematic sectional drawing which shows the shearing force which arises between a fixing roller and a pressure roller. FIG. 6 is a schematic cross-sectional view illustrating a state where the fixing toner on the fixing roller is removed by a shearing force. It is a schematic sectional drawing which shows a mode that the fixed toner of a pressure roller is removed by a shearing force. It is a figure which shows the change of the shear force and the offset image generation rate accompanying the increase in the number of paper passing. It is a figure which shows the change of the torque accompanying the increase in the number of paper passing. It is a figure which shows schematic structure of a torque measuring device. It is a schematic block diagram of the fixing device which concerns on 2nd Embodiment of this invention. It is a schematic block diagram of the fixing device which concerns on 3rd Embodiment of this invention. It is a schematic block diagram of the fixing device which concerns on 4th Embodiment of this invention. It is a schematic sectional drawing of the fixing device which concerns on 5th Embodiment of this invention. It is a schematic block diagram which shows embodiment provided with the cleaning means. It is a figure which shows the generation | occurrence | production state of the offset image by fixed toner.

Hereinafter, embodiments including the first to fifth embodiments of the present invention will be described with reference to the accompanying drawings. In addition, about components, such as a member and a component which have the same function or shape in each drawing, after having demonstrated once by attaching | subjecting the same code | symbol as much as possible, the description is abbreviate | omitted.

(Schematic configuration of a color printer as an image forming apparatus)
As shown in FIG. 1, the image forming apparatus 1 of the present embodiment is a tandem color printer. Four bottles 102Y, 102M, 102C, and 102K corresponding to the respective colors (yellow, magenta, cyan, and black) are detachably (replaceable) installed in the bottle housing unit 101 above the image forming apparatus main body. Yes.

An intermediate transfer unit 85 is disposed below the bottle housing portion 101. Image forming units 4Y, 4M, 4C, and 4K corresponding to the respective colors (yellow, magenta, cyan, and black) are arranged in parallel so as to face the intermediate transfer belt 78 of the intermediate transfer unit 85. Photosensitive drums 5Y, 5M, 5C, and 5K are disposed in the image forming units 4Y, 4M, 4C, and 4K, respectively.

Further, around each of the photosensitive drums 5Y, 5M, 5C, and 5K, a charging unit 75, a developing unit 76, a cleaning unit 77, a charge eliminating unit, and the like are disposed. Then, an image forming process (charging process, exposure process, developing process, transfer process, cleaning process) is performed on each of the photoconductive drums 5Y, 5M, 5C, and 5K. An image of each color is formed on 5K. The photosensitive drums 5Y, 5M, 5C, and 5K are rotationally driven clockwise in FIG. 1 by a drive motor.

Then, the surfaces of the photosensitive drums 5Y, 5M, 5C, and 5K are uniformly charged at the position of the charging unit 75 (charging process). Thereafter, the surfaces of the photosensitive drums 5Y, 5M, 5C, and 5K reach the irradiation position of the laser light L emitted from the exposure unit 3, and electrostatic latent images corresponding to the respective colors are formed by exposure scanning at this position. (Exposure process).

Thereafter, the surfaces of the photosensitive drums 5Y, 5M, 5C, and 5K reach a position facing the developing unit 76, and the electrostatic latent image is developed at this position to form toner images of each color (development process). ). Thereafter, the surfaces of the photosensitive drums 5Y, 5M, 5C, and 5K reach positions facing the intermediate transfer belt 78 and the primary transfer bias rollers 79Y, 79M, 79C, and 79K, and the photosensitive drums 5Y, 5M are located at this position. The toner images on 5C and 5K are transferred onto the intermediate transfer belt 78 (primary transfer step).

At this time, a small amount of untransferred toner remains on the photosensitive drums 5Y, 5M, 5C, and 5K. Thereafter, the surfaces of the photoconductive drums 5Y, 5M, 5C, and 5K reach a position facing the cleaning unit 77. At this position, the untransferred toner remaining on the photosensitive drums 5Y, 5M, 5C, and 5K is mechanically collected by the cleaning blade of the cleaning unit 77 (cleaning process).

Finally, the surfaces of the photoconductive drums 5Y, 5M, 5C, and 5K reach a position facing the charge eliminating unit, and the residual potential on the photoconductive drums 5Y, 5M, 5C, and 5K is removed at this position. Thus, a series of image forming processes performed on the photosensitive drums 5Y, 5M, 5C, and 5K is completed.

Thereafter, the toner images of the respective colors formed on the respective photosensitive drums through the developing process are transferred onto the intermediate transfer belt 78 in an overlapping manner. In this way, a color image is formed on the intermediate transfer belt 78.

Here, the intermediate transfer unit 85 includes an intermediate transfer belt 78, four primary transfer bias rollers 79Y, 79M, 79C, and 79K, a secondary transfer backup roller 82, a cleaning backup roller 83, a tension roller 84, and an intermediate transfer cleaning unit 80. , Etc.

The intermediate transfer belt 78 is stretched and supported by the three rollers 82 to 84 and is endlessly moved in the counterclockwise direction in FIG. The four primary transfer bias rollers 79Y, 79M, 79C, and 79K sandwich the intermediate transfer belt 78 with the photosensitive drums 5Y, 5M, 5C, and 5K, respectively, thereby forming primary transfer nips.

Then, a transfer bias reverse to the polarity of the toner is applied to the primary transfer bias rollers 79Y, 79M, 79C, and 79K. The intermediate transfer belt 78 travels in the direction of the arrow and sequentially passes through the primary transfer nips of the primary transfer bias rollers 79Y, 79M, 79C, and 79K. In this way, the toner images of the respective colors on the photosensitive drums 5Y, 5M, 5C, and 5K are primarily transferred while being superimposed on the intermediate transfer belt 78.

Thereafter, the intermediate transfer belt 78 onto which the toner images of the respective colors are transferred in an overlapping manner reaches a position facing the secondary transfer roller 89. At this position, the secondary transfer backup roller 82 sandwiches the intermediate transfer belt 78 between the secondary transfer roller 89 and forms a secondary transfer nip. The four color toner images formed on the intermediate transfer belt 78 are transferred onto the transfer medium P conveyed to the position of the secondary transfer nip.

At this time, untransferred toner that has not been transferred to the transfer medium P remains on the intermediate transfer belt 78. Thereafter, the intermediate transfer belt 78 reaches the position of the intermediate transfer cleaning unit 80. At this position, the untransferred toner on the intermediate transfer belt 78 is collected.

Thus, a series of transfer processes performed on the intermediate transfer belt 78 is completed. The above is the description of the configuration and operation of the image forming unit. Here, the transfer medium P transported to the position of the secondary transfer nip is transported from the paper feed unit 12 disposed below the apparatus main body via the paper feed roller 97, the timing roller pair 98, and the like. Is.

Specifically, a plurality of transfer media P such as transfer paper are stored in the paper supply unit 12 in a stacked manner. When the paper feed roller 97 is driven to rotate counterclockwise in FIG. 1, the uppermost transfer medium P is fed toward the timing roller pair 98. The transfer medium P conveyed to the timing roller pair 98 temporarily stops at the position of the roller nip of the timing roller pair 98 that has stopped rotating.

Then, the timing roller pair 98 is driven to rotate in synchronization with the color image on the intermediate transfer belt 78, and the transfer medium P is conveyed toward the secondary transfer nip. Thus, a desired color image is transferred onto the transfer medium P.

Thereafter, the transfer medium P on which the color image is transferred at the position of the secondary transfer nip is conveyed to the position of the fixing device 20. At this position, the color image transferred onto the surface of the intermediate transfer belt 78 is fixed on the transfer medium P by heat and pressure generated by the fixing roller 21 and the pressure roller 31.

Thereafter, the transfer medium P is discharged out of the apparatus through a pair of paper discharge rollers 99. The transfer medium P discharged out of the apparatus by the discharge roller pair 99 is sequentially stacked on the stack unit 100 as an output image. Thus, a series of image forming processes in the image forming apparatus 1 is completed.

(Basic configuration of fixing device)
Next, a basic configuration of a fixing device which is a type of the fixing device 20 described above and to which the present invention is applicable will be described with reference to FIG. The fixing device 20 of FIG. 2 includes two rollers, that is, a fixing roller 21 and a pressure roller 31 that contact each other to form a nip portion N as a fixing nip. Inside the fixing roller 21, a halogen heater 24 is disposed as a heating unit that heats the fixing roller 21. The heating means may be an external heating method in which the fixing roller 21 is heated from the outer peripheral surface side instead of the halogen heater 24. Further, the fixing roller 21 and the pressure roller 31 are configured to be rotatable in a direction of an arrow in the figure by a rotation driving unit such as a motor.

The fixing roller 21 is a cylindrical member in which the periphery of the heat conductive substrate is covered with a covering layer having releasability. As the heat conductive substrate, a carbon steel material or aluminum material having a required mechanical strength and good heat conductivity is mainly used. The outer (outer peripheral surface) coating layer is formed of a material having good releasability from the toner, high thermal conductivity, and high durability. For example, those coated with a fluororesin (PFA) tube, those coated with a fluororesin (PFA or PTFE) paint, or those formed with a silicone rubber layer or a fluororubber layer are used as the coating layer.

The pressure roller 31 is a cylindrical member that includes a cored bar, an elastic layer formed on the outer side (outer periphery) of the cored bar, and a covering layer that covers the elastic layer. For example, STKM or the like is used as the core metal, and silicone rubber, fluorine rubber, or a foam thereof is used as the elastic layer. The covering layer is formed of, for example, a tube of heat-resistant fluororesin such as PFA or PTFA rich in releasability.

The pressure roller 31 is urged toward the fixing roller 21 by an urging mechanism B using a spring or the like. The urging mechanism B includes a compression spring 28 and an urging lever 29 that can swing in the left-right direction about the fulcrum portion 29a. By pressing the front end portion of the urging lever 29 with the compression spring 28, the intermediate portion 29 b of the urging lever 29 is pressed toward the rotating shaft portion 31 a of the pressure roller 31.

A claw-like separation member 23 having a sharp tip is disposed opposite to the fixing roller 21 on the downstream side in the transfer medium conveyance direction from the nip portion N (upper side in FIG. 2). In the present embodiment, four separation members 23 are arranged in the axial direction of the fixing roller 21. However, the number of separating members 23 is not limited to four as long as it is plural.

As a material of the separation member 23, a material having good release properties and slidability such as PFA, PEK, and PEEK is mainly used. Further, the surface of the separating member 23 may be coated with a material having good mold release and sliding properties such as PFA and Teflon (registered trademark).

Each separation member 23 is provided with a contact direction biasing means. As the contact direction biasing means, for example, a compression coil spring or a tension coil spring can be used, but other biasing means are used as the contact direction biasing means depending on various conditions such as installation space and manufacturing cost. It is also possible to do. By this abutting direction urging means, each separating member 23 is urged in a direction to abut against the fixing roller 21.

Around the fixing roller 21, a thermistor 25 as temperature detecting means, a thermostat for preventing abnormal temperature, and the like are disposed. The surface temperature of the fixing roller 21 is controlled by the detection signal from the thermistor 25 so as to be maintained within a predetermined temperature range.

(First embodiment)
FIG. 3 is a schematic configuration diagram of the fixing device according to the first embodiment of the present invention. As shown in FIG. 3, a gear portion 21a is provided continuously at one end portion of the fixing roller 21 in the circumferential direction, and a drive gear 41 provided on a rotational drive means 40 such as a motor is provided on the gear portion 21a. They are engaged and connected. When the rotation driving means 40 is driven, a driving force is applied to the fixing roller 21 through the gear portion 21a, and the fixing roller 21 is rotationally driven.

On the other hand, the pressure roller 31 is rotatably supported by a sliding bearing 42, and the fixing roller 21 is driven to rotate together with the rotation of the fixing roller 21. A sheet passing area is formed with a predetermined width at the center in the width direction of the outer peripheral surface of the pressure roller 31, and a narrow non-sheet passing area exists on both the left and right sides of the sheet passing area.

In the first embodiment, as will be described in detail below, a braking force is applied to the pressure roller 31 using friction of the rotating shaft portion 31a of the pressure roller 31 by the sliding bearing 42. That is, in order to form the nip portion N with a predetermined width, a load is applied between the fixing roller 21 and the pressure roller 31 by the urging mechanism B as shown in FIG. For this reason, a force to push back from the fixing roller 21 as a reaction force of the load is received by the rotating shaft portion 31a of the pressure roller 31, and a bearing is provided between the rotating shaft portion 31a and the sliding bearing 42. Friction occurs.

In general, a rolling bearing or a sliding bearing is used as the bearing of the fixing roller 21 or the pressure roller 31, but the sliding bearing 42 is used in the first embodiment of the present invention. The sliding bearing 42 has a larger bearing friction than the rolling bearing and tends to generate a rotational load. This bearing friction or rotational load generates a circumferential component (15 to 25 N) of a shearing force which will be described later.

That is, a shearing force having a size of 15 N or more and 25 N or less is applied to the nip portion N by bearing friction or rotational load acting on the pressure roller 31 on the driven side. Factors (parameters) that affect the shear force include nip width, load applied between rollers, roller shaft length, frictional force between rollers, and roller rotation load (bearing friction, brake). Further, examples of the rotational load (bearing friction) of the roller include scraping of convex portions 42a to 42c and a skin layer of the sliding bearing 42 described later.

As the sliding bearing 42, a U-shaped sliding bearing 42 as shown in FIG. 4A or a cylindrical sliding bearing 42 as shown in FIG. 4B can be used. As the material of the sliding bearing 42, for example, TFE {fluororesin (tetrafluoroethylene)}, PI (polyimide), PAI (polyamideimide), PPS (polyphenylene sulfide), or the like can be used.

As shown in FIGS. 4B to 4C, convex portions 42 a, 42 b and 42 c are formed on the sliding surface of the shaft bearing 42. These convex portions 42a to 42c all have a V-shaped tip and form a triangular prism shape. The V-shaped tip is gradually worn by friction with the rotating shaft portion 31a (made of iron). The regions 42a1, 42b1, and 42c1 where the rotating shaft portion 31a comes into surface contact with each other over time increase. The friction coefficient increases over time due to the increase in the contact area and the generation of wear powder due to wear.

In order to avoid damage to the rotating shaft portion 31a due to stress concentration of the convex portions 42a to 42c due to a high load setting of the urging mechanism B or the like, a region 42a1 in surface contact with a predetermined area in the initial state of the sliding bearing 42. 42b1 and 42c1 may be formed from the beginning (the convex portions 42a to 42c are changed from a triangular prism shape to a trapezoidal shape). In this case as well, the areas of the regions 42a1, 42b1, and 42c1 remain unchanged with the operating time.

Specifically, in the sliding bearing 42 of FIG. 4B, a plurality of convex portions 42a are formed continuously (zigzag or jagged) in the circumferential direction of the shaft hole sliding surface. The tip portions of the convex portions 42a are in sliding contact with the outer peripheral surface of the rotating shaft portion 31a. Each convex portion 42 a extends by a predetermined length in the axial direction of the sliding bearing 42.

When the sharp tip of the convex portion 42a is worn by friction with the rotating shaft portion 31a, that is, as the number of sheets passing increases, the surface contact area 42a1 gradually expands. At the same time, the initial friction of the bearing due to the small initial contact area of the slide bearing 42 is stably maintained as the initial friction without decreasing or maintains a slight increase tendency, and as a result, the initial driving torque of the fixing roller 21 does not decrease. Maintain stable or maintain a slight increase tendency.

In the plain bearing 42 of FIG. 4C, a convex portion 42b is formed so as to approach an edge portion on one side in the axial direction of the shaft hole. The convex portion 42b can be formed close to either the left or right side, but it is desirable that the convex portion 42b be formed close to the pressure roller 31 for stable support as shown. The convex part 42b is continuously formed in the circumferential direction of the shaft hole as shown in FIG. 4C (a), and the opposite side in the axial direction is inclined in a tapered shape.

The tip of the convex portion 42b is in sliding contact with the outer peripheral surface of the rotary shaft portion 31a at a contact angle of about 180 °, and the tip portion of the convex portion 42b is worn by friction with the rotary shaft portion 31a (increase in the number of sheets passed). Then, as shown in FIG. 4C (b), the surface contact area 42b1 gradually expands. At the same time, the initial friction of the bearing due to the small initial contact area of the slide bearing 42 is stably maintained as the initial friction without decreasing or maintains a slight increase tendency, and as a result, the initial driving torque of the fixing roller 21 does not decrease. Maintain stable or maintain a slight increase tendency.

In the sliding bearing 42 of FIG. 4D, a convex portion 42c is formed at the axial center portion of the shaft hole. The convex portion 42c is formed continuously in the circumferential direction of the shaft hole as shown in FIG. 4D (a), and both sides in the axial direction are symmetrical and inclined in a tapered shape. By forming the convex portion 42 c at the central portion in the axial direction, it is possible to suppress the bearing collapse in which the axis of the sliding bearing 42 is inclined with respect to the axis of the pressure roller 31. In addition, the sliding bearing 42 can be made to have left and right common parts, so that the number of parts and parts cost can be reduced and the number of assembly confirmation steps can be reduced, and the quality can be stabilized by preventing assembly errors.

The tip of the convex portion 42c is in sliding contact with the outer peripheral surface of the rotating shaft portion 31a at a contact angle of about 180 °. Then, when the tip of the convex portion 42a is worn by friction with the rotating shaft portion 31a (increase in the number of sheets passed), the surface contact area 42c1 gradually expands as shown in FIG. 4D (b). At the same time, the initial friction of the bearing due to the small initial contact area of the slide bearing 42 is stably maintained as the initial friction without decreasing or maintains a slight increase tendency, and as a result, the initial driving torque of the fixing roller 21 does not decrease. Maintain stable or maintain a slight increase tendency. Thus, the convex portions 42a to 42c formed on the shaft hole sliding surface of the sliding bearing 42 are worn by the rotation of the rotary shaft portion 31a to maintain or increase the rotational torque.

Since the pressure roller 31 is driven and rotated as a result of the rotation drive of the fixing roller 21, when a braking force is applied to the pressure roller 31 by the slide bearing 42, the pressure roller 31 is driven and rotated by the fixing roller 21 rotated as shown in FIG. Shear forces F1 and F2 as indicated by arrows are generated in the nip portion N between the pressure roller 31 and the pressure roller 31. F2 is a conjugate shear force and is equal to and opposite to the shear force F1.

(Removal of fixed toner by shearing force)
When the transfer medium P passes through the nip portion N where the shearing forces F1 and F2 exist in this way, as shown in FIGS. 6A and 6B, between the transfer medium P and the fixing roller 21, and between the transfer medium P and the pressure. Shear forces F1 and F2 act between the rollers 31. That is, as shown in FIG. 6A (a), when the fixing toner 203 is present on the surface of the fixing roller 21, the transfer medium P is passed through the nip portion N, and as shown in FIG. 203 is scraped to the transfer medium P by the downward shearing force F2.

Then, as shown in FIG. 6A (c), the fixed toner 203 scraped to the transfer medium P is discharged together with the transfer medium P to the outside of the apparatus. Note that since the amount of the fixed toner transferred onto the transfer medium P is very small, the image quality is not deteriorated.

Further, as shown in FIG. 6B (a), when the fixing toner 203 is present on the surface of the pressure roller 31, the transfer medium P is passed through the nip portion N, and as shown in FIG. The fixed toner 203 is scraped off the transfer medium P by the upward shearing force F1. Then, as shown in FIG. 6B (c), the fixed toner 203 scraped off by the transfer medium P is discharged out of the apparatus together with the transfer medium P.

(Shear force and torque magnitude)
In the embodiment of the present invention, the magnitude of the circumferential direction (roller rotation direction) component of the shearing force is set to be 15N or more and 25N or less. In order to generate a shearing force having such a magnitude, the torque of the fixing roller 21 is set to 0.2 N · m or more and 0.3 N · m or less.

Here, the values of the shearing force 15N to 25N and the torque 0.2N · m to 0.3N · m are values at the time of non-sheet passing before the transfer medium P passes through the nip portion N. This is because it is extremely difficult to measure the shearing force acting on the transfer medium P and the torque of the fixing roller 21 when the transfer medium P passes through the nip portion N.

In the embodiment of the present invention, the shearing force that acts on the transfer medium P when the transfer medium P passes through the nip portion N is the shearing force that acts on the nip portion N when the transfer medium P does not pass before passing the transfer medium P. It is set to be always larger than. Therefore, at least the shearing force 15N to 25N acts on the transfer medium P at the time of paper passing.

The shear force and torque have a certain correlation. That is, since the value obtained by dividing the torque by the roller radius is the shearing force, for example, if the roller diameter is 26 mm (radius 13 mm), the torque can be obtained by multiplying the shearing force by the radius 13 mm.

Therefore, when the shearing force is 15 N, the torque is 15 N × 0.013 m = 0.195 N · m, and when the shearing force is 25 N, the torque is 25 N × 0.013 m = 0.325 N · m. Since the roller diameter is considered to be constant without changing over time, the shearing force increases as the torque increases, and the shearing force decreases as the torque decreases. The reason why the upper and lower limits of the shearing force and torque are set in this way will be described next with reference to FIGS. 7A and 7B.

(Upper / lower limit of shear force)
FIG. 7A is a test result in which the relationship between the magnitude of the circumferential component of the shear force and the occurrence rate of the offset image caused by the fixed toner is examined. This test was performed using two fixing devices for A4 size. The first fixing device uses the sliding bearing (U-shaped / round shape in FIG. 10A) according to the embodiment of the present invention, and the second fixing device uses a conventional sliding bearing (U-shaped / round shape in FIG. 10A). It was used. There was no significant difference in test results between the U-shape and the round shape. In FIG. 7A, solid lines A1 and A2 indicate the magnitudes of the circumferential components of the shearing force generated between the fixing roller and the pressure roller, respectively. The solid line A1 is from the first printer, and the solid line A2 is the second. This is due to the printer. Broken lines B1 and B2 indicate the occurrence rates of offset images caused by the toner adhering to the fixing roller.

When the shearing force is A1, the offset image generation rate is B1, and when the shearing force is A2, the offset image generation rate is B2. Further, the horizontal axis of FIG. 7A indicates the cumulative number of sheets (the cumulative number of sheets that have been passed) through which the transfer medium has passed through the nip portion.

From the result of FIG. 7A, it can be seen that in the case of A1 in which the shearing force is in the range of 15N to 25N, the occurrence rate of the offset image is always 0% as indicated by the broken line B1. In the case of A1 having a shearing force of 15 N or more, it is considered that as a result of obtaining a sufficient fixed toner removal effect, the accumulation of fixed toner on the fixing roller was suppressed, and no offset image was generated. According to another test, it was found that when the shearing force exceeds 25 N, the transfer medium is likely to be wrinkled.

On the other hand, in the case of A2 in which the shearing force is less than 15 N, as shown by the broken line B2, it can be seen that the occurrence rate of the offset image increases as the number of sheets to be passed increases. In the case of A2, since the shearing force is small, a sufficient fixing toner removal effect cannot be obtained, and the fixing toner accumulates on the fixing roller as the number of sheets passing increases, which is considered to cause an offset image. Therefore, by maintaining the size of the circumferential component of the shearing force in the range of 15N to 25N, the accumulation of the fixed toner on the fixing roller is highly suppressed to prevent the offset image, and the wrinkle of the transfer medium is reduced. It was also found that it can be prevented.

In FIG. 7A, in the case of A2 in which an offset image is generated, in the initial state where the cumulative number of sheets passed is small (less than about 500 sheets), the shearing force is 15 N or more and A1 in which no offset image is generated. Although it was almost the same, after that, the shear force decreased rapidly. In this case, different materials were used as the slide bearings for supporting the pressure roller in the case of A1 and in the case of A2, but since both were new in the initial state, the characteristics of the rotational load due to the difference in material This is probably because there was not much difference in (effect on shearing force).

That is, since the surface of the plain bearing was covered with the skin layer in the new state, the difference due to the difference in the material of each plain bearing did not appear clearly, and then the skin layer was shaved and the characteristics of the material itself appeared. It is considered that a difference occurred in the shearing force. From this, in a fixing device using a new or equivalent sliding bearing, it is difficult to determine whether the shearing force is 15 N or more in the initial state, and at least the cumulative number of sheets to be passed is 1000 sheets. It is better to judge in the above state. Whether or not the shearing force is 25 N or less is preferably determined in a state where the number of sheets to be passed is 10,000 or less.

(Torque upper / lower limit)
FIG. 7B is a test result in which the relationship between the change in the torque of the fixing roller 21 accompanying the increase in the cumulative number of sheets to be passed and the occurrence of fixed toner is examined. This test was also performed using the same two A4 size fixing devices used in the test of FIG. 7A (the diameter of the fixing roller 21 is 26 mm). A dotted line indicates the case of the second fixing device using a conventional sliding bearing (U-shaped / round shape in FIG. 4A) as a pressure roller bearing, and a solid line indicates the sliding bearing (in FIG. 4A in FIG. 4A). A case of a first fixing device using a U-shape / round shape is shown. There was no significant difference in test results between the U-shape and the round shape.

The initial torque of the plain bearing is relatively high at 0.25 N · m for both the dotted line and the solid line, but with conventional slide bearings, the torque decreases to about 0.15 N · m at the beginning of the printing life (100,000 sheets passed). stable. The reason for the torque reduction to about 0.15 N · m is that the surface layer of the shaft hole sliding surface is initially scraped, and the scraped powder adheres to the periphery of the rotating shaft portion 31 a, serving as a cushioning material / lubricant. This is probably due to this. However, when the torque fell below 0.2 N · m, an offset image with black spots was generated.

On the other hand, according to the shape of the sliding roller sliding bearing used in the embodiment of the present invention, the torque slightly increased from 0.25 N · m at the beginning of the printing life (100,000 sheets passed). After that, the overall increase trend was maintained moderately, but it was in the range of 0.3 N · m or less even at the end of durability (500,000 sheets passed). According to another test, it was found that when the torque exceeds 0.3 N · m, the load on the drive motor increases, causing abnormal noise and damage to parts.

From the above results, according to the embodiment of the present invention, by using the sliding bearing while scraping it and keeping the torque in the range of 0.2 to 0.3 N · m, an offset image of black spot adhesion does not occur. As a result, it was found that an image forming apparatus having a stable operation can be obtained without abnormal noise or damage to parts.

(Torque measuring device)
The torque Tr generated in the fixing roller 21 is a total torque generated in the fixing roller 21 when the sheet does not pass before the transfer medium P passes through the nip portion N. The total torque of the fixing roller 21 can be measured by, for example, a torque measuring device 50 as shown in FIG.

A torque measuring device 50 shown in FIG. 8 includes a torque converter 51, a motor 52, a signal conditioner 53, a computer 54, and a fixed base 55. A torque converter 51 and a motor 52 are installed on the fixed base 55, and a computer 54 is connected to the torque converter 51 via a signal conditioner 53. A drive gear 56 is provided at the tip of the rotating shaft of the motor 52 that passes through the torque converter 51.

In order to measure the total torque of the fixing roller 21, first, the fixing roller 21 is fixed to the fixing base 55 together with the fixing device 20, and a gear portion 21 a provided at one end portion in the axial direction of the fixing roller 21 is connected to the drive gear 56. To do. In this state, the motor 52 is driven, and the total torque generated at the fixing roller 21 at that time is measured by the torque converter 51. Then, the measurement data is converted into a predetermined signal by the signal conditioner 53 and input to the computer 54 to calculate the total torque.

The fixing roller 21 and the pressure roller 31 are assigned by substituting the value Tr of the total torque of the fixing roller 21 and the average radius R of the fixing roller 21 into the calculation formula of Fr = Tr / R. It is possible to calculate the value Fr of the circumferential component of the shear force generated between the two. And the magnitude | size of a torque, a roller radius, etc. should just be adjusted so that the value Fr of the circumferential direction component of the obtained shear force may become the range of 15N or more and 25N or less.

The above is a case where the fixing roller 21 is a driving roller and the pressure roller 31 is a driven roller. On the contrary, when the pressure roller 31 is a driving roller and the fixing roller 21 is a driven roller, the total torque of the pressure roller 31 is calculated in the same manner. Then, using the torque value and the average radius of the pressure roller 31 (at the nip portion N), the value Fr of the circumferential component of the shearing force is calculated.

(Second Embodiment)
FIG. 9 is a schematic configuration diagram of a fixing device according to a second embodiment of the present invention. In the second embodiment, instead of the bearing 42 shown in FIGS. 4B to 4D described above as a bearing for the pressure roller 31, a rolling bearing or a sliding bearing having a small general bearing friction is used. Instead, in the second embodiment, the brake pad 32 is brought into sliding contact with the non-sheet passing area of the pressure roller 31.

The brake pad 32 is pressed against the non-sheet passing region by a brake spring 33 with a predetermined pressure. By pressing the brake pad 32 against the non-sheet passing region, it is possible to prevent the brake pad 32 from being stained with toner and to prevent the stain from flowing back to the transfer medium P.

(Third embodiment)
Next, a third embodiment of the present invention will be described with reference to FIG. In the third embodiment, the compression spring 28 that urges the pressure roller 31 against the fixing roller 21 is also used as a brake spring (brake spring 33 in FIG. 9).

That is, the brake pad 61 is attached to the front end portion of the urging lever 29, and the brake pad 61 is brought into sliding contact with the non-sheet passing regions at both ends of the outer peripheral surface of the pressure roller 31. By omitting the brake spring 33 of FIG. 9 and integrating the brake pad 61 with the biasing lever 29, the cost can be reduced by reducing the number of parts. Since the pressing force by the brake pad 61 also acts on the fixing roller 21 via the pressure roller 31, it is not always necessary to bring the intermediate portion 29b of the urging lever 29 into contact with the rotating shaft portion 31a. Even when the pressure of the compression spring 28 is changed to change the pressure of the nip portion N, the pressure of the brake pad 61 can be maintained within a predetermined range.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described with reference to FIG. 11A. In the fourth embodiment, the pressing force by the brake pad 32 does not affect the pressure of the nip portion N. That is, in FIG. 11A, the brake pad 32 urged by the brake spring 33 from the axial direction is brought into sliding contact with both axial end surfaces of the pressure roller 31.

In this case, it is not necessary to secure a certain width of the non-sheet passing region in order to apply the brake pad 32, and the pressure roller 31 can be downsized. The brake pad 32 may be brought into sliding contact with only one axial end surface of the pressure roller 31. According to the fourth embodiment of FIG. 11A, it is possible to avoid the pressing force by the brake pad 32 from affecting the pressure of the nip portion N, and the pressure gradient in the axial direction of the nip portion N (right and left) by the pressing force by the brake pad 32. (Deviation) can be prevented from occurring.

(Fifth embodiment)
Next, a fifth embodiment of the present invention will be described with reference to FIG. 11B. In the fifth embodiment, the pressing force by the brake pad 32 does not affect the pressure of the nip portion N. That is, in a non-sheet passing region at both ends of the outer peripheral surface of the pressure roller 31, a direction perpendicular to a straight line connecting the centers of the fixing roller 21 and the pressure roller 31 (a direction parallel to a tangential direction of the fixing nip portion N). The brake pads 32 urged by the brake springs 33 are brought into sliding contact with each other. According to the fifth embodiment of FIG. 11B, it is possible to avoid the pressing force by the brake pad 32 from affecting the pressure of the nip portion N, and the pressure gradient in the axial direction of the nip portion N (right and left) by the pressing force by the brake pad 32. (Deviation) can be prevented from occurring.

In the embodiment of the present invention, since the shearing force is applied during the sheet passing, the toner removing action is greatly enhanced by the so-called file effect on the surface of the transfer medium P, compared to the case where the shearing force is applied during the non-sheet passing. Each time the transfer medium P passes through the nip portion N, the toner removing action is exhibited. For this reason, there is no time loss compared to a configuration in which toner removal is performed in a predetermined cleaning sequence and toner removal is not performed during paper passing as in the above-mentioned Patent Document 1, and moreover, deposits are removed more frequently. It is possible to effectively suppress the deposits of adhered toner and the like on the roller.

In particular, the effect of the present invention as described above can be expected to be great when a transfer medium containing a large amount of filler such as calcium carbonate is used. In addition, a great effect can be expected when a toner to which silica particles containing silicone oil are externally added is used. In this type of toner, for example, 2 parts of hydrophobic silica RY50 (manufactured by Aerosil) containing or coated with silicone oil on the surface is added to 100 parts of pulverized toner or polymerized toner, and the peripheral speed is 40 m / min with a 20 L Henschel mixer. for 5 minutes, and then sieved with a sieve having an opening of 75 microns.

Although the first to fifth embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications can be made. For example, in the above-described embodiment, the fixing roller 21 is the driving side and the pressure roller 31 is the driven side. However, the pressure roller 31 may be the driving side and the fixing roller 21 may be the driven side. In this case, the shearing force described above is applied to the nip portion N by applying a rotational load to the fixing roller 21 on the driven side.

Further, as shown in FIGS. 12 (a) and 12 (b), if necessary, a cleaning unit for removing the fixed toner on the fixing roller or the pressure roller may be provided. In this case, even if the cleaning unit is provided, the amount of the fixed toner removed and collected by the cleaning unit can be reduced by the amount of the fixed toner removed by the transfer medium. For this reason, it is possible to prevent problems such as generation of abnormal noise due to solidification of the fixed toner collected by the cleaning unit and contamination of the transfer medium due to dissolution of the fixed toner from the cleaning unit.

In addition, the brake pad may not always be in contact, but may be disposed so as to be able to contact and separate from the roller to be braked (ON-OFF operation is possible), and may be ON only when the fixed toner is removed. In that case, instead of plain paper, a special cleaning paper having a so-called file effect may be used as the transfer medium P for removing fixed toner.

Further, the image forming apparatus is not limited to the color image forming apparatus as shown in FIG. 1 and may be a monochrome image forming apparatus. Further, the image forming apparatus to which the present invention can be applied includes a printer, a copying machine, a facsimile, or a complex machine thereof.

1: Image forming device 3: Exposure unit 4Y-4K: Image forming unit 5Y-5K: Photosensitive drum 12: Paper feeding unit 20: Fixing device 21: Fixing roller 21a: Gear unit 22: Heating roller 23: Separating member 24: Halogen heater 25: thermistor 26: nip forming member 27: fixing belt 28: compression spring 29: urging lever 29a: fulcrum portion 29b: intermediate portion 31: pressure roller 31a: rotating shaft portion 32: brake pad 33: brake spring 40 : Rotation drive means 41: Drive gear 42: Sliding bearings 42a, 42b, 42c: Convex part
42a1, 42b1, 42c1: Area of surface contact 50: Torque measuring device 51: Torque converter 52: Motor 53: Signal conditioner 54: Computer 55: Fixing base 56: Drive gear 61: Brake pad 75: Charging part 76: Development Section 77: Cleaning section 78: Intermediate transfer belt 79Y-79K: Primary transfer bias roller 80: Intermediate transfer cleaning section 82: Secondary transfer backup roller 83: Cleaning backup roller 84: Tension roller 85: Intermediate transfer unit 89: Secondary Transfer roller 97: Paper feed roller 98: Timing roller pair 99: Paper discharge roller pair 100: Stack part 101: Bottle storage part 102Y-102K: Toner bottle 200: Adhering toner 203: Adhering toner L: Laser light N: Nip part P : Transfer media

Japanese Patent Laid-Open No. 2002-40860 JP 2009-37078 A

Claims (14)

In a fixing device for fixing a toner image to the transfer medium by passing the transfer medium holding the toner image through a fixing nip formed between two rollers on the driving side and the driven side whose outer peripheral surfaces are in pressure contact with each other ,
A fixing device, wherein a shearing force acting on the fixing nip when the two rollers rotate is 15 N or more and 25 N or less.   The fixing device according to claim 1, further comprising a sliding bearing that supports a rotating shaft portion of the driven roller.   The fixing device according to claim 2, wherein a convex portion is formed on a sliding surface of the shaft hole of the sliding bearing.   The shearing force generated in the fixing nip when the number of sheets of the transfer medium passing through the fixing nip is 1000 or more and 10,000 or less is 15N or more and 25N or less. The fixing device according to claim 1.   The shearing force acting on the transfer medium when the transfer medium passes through the fixing nip is greater than the shearing force acting on the fixing nip when the transfer medium does not pass before passing the transfer medium. The fixing device according to any one of claims 1 to 4.   2. The fixing device according to claim 1, wherein a brake pad is provided in sliding contact with the driven roller, and the rotational load is applied by the brake pad.   The fixing device according to claim 6, wherein the brake pad is slidably brought into contact with a non-sheet passing region of an outer peripheral surface of the driven roller.   8. The fixing device according to claim 7, wherein the brake pad is brought into sliding contact with the non-sheet passing region from a direction parallel to a tangential direction of the fixing nip.   The fixing device according to claim 6, wherein the brake pad is slidably brought into contact with one end surface or both end surfaces of the driven roller in the axial direction.   An image forming apparatus comprising the fixing device according to claim 1. In a fixing method for fixing a toner image to the transfer medium by passing the transfer medium holding the toner image through a fixing nip formed between two rollers, a driving side and a driven side, whose outer peripheral surfaces are pressed against each other. ,
A fixing method, wherein a shearing force acting on the fixing nip when the two rollers are rotated is 15 N or more and 25 N or less.   12. The fixing method according to claim 11, wherein when the number of sheets of the transfer medium passing through the fixing nip is 1000 or more and 10,000 or less, a shearing force generated in the fixing nip is 15 N or more and 25 N or less.   The shearing force acting on the transfer medium when the transfer medium passes through the fixing nip is greater than the shearing force acting on the fixing nip when the transfer medium does not pass before passing the transfer medium. The fixing method according to claim 11.   The fixing method according to claim 11, wherein the shearing force is applied by applying a braking force to the driven roller.