JP2007240622A - Image heating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To rationalize pressure distribution in an image heating nip, even when the image heating device is constituted for lengthening an image heating nip. <P>SOLUTION: In the image heating device, a pressurization pad 27 is contacted and arranged to a pressurization roller 26 with a small sliding load and a fixing pad 24 is not contacted to the fixing roller 23 with a large sliding load by separating by predetermined gap. As a result, the fixing nip is continuously formed along the sheet conveyance direction, without causing a large pressure omission part to be generated. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image heating device that heats an image formed on a recording material, and a fixing device that fixes an unfixed image formed on a recording material.

  In an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, a toner image is formed on a sheet, and the image is formed by fixing the image by heating and pressing it. As such a fixing device, a roller fixing method in which a fixing nip is formed by pressing a pressure roller against a fixing roller having a heater therein to perform fixing has been conventionally employed.

  By the way, in order to increase the glossiness of the image and increase the speed of image formation, it is preferable to lengthen the sheet nip passage time and sufficiently melt the toner. In the case of the roller fixing method, in order to achieve this, the roller diameter must be increased, and the fixing device is increased in size.

Therefore, a belt fixing method has been proposed that can achieve a sufficient nip width (length in the sheet conveying direction) while achieving a reduction in size and speed of the apparatus as compared with the roller fixing method (patent). Reference 1). In this proposal, a fixing belt and a pressure belt facing each other are provided, and the fixing is performed while the sheet is nipped and conveyed between the two belts. Therefore, a sufficient nip width is obtained as compared with the prior art.
JP 2004-341346 A

  However, the configuration of Patent Document 1 has the following problems. That is, in the fixing nip formed between both belts, a high pressure portion pressed by the suspension rollers of both belts, a low pressure portion pressed only by the fixing pad, There is a pressure-relieved part where nothing is supported from both sides of the belt.

  For this reason, when an unfixed toner image is to be fixed, a difference occurs in the sheet conveyance speed between the high-pressure portion and the portion where the pressure is lost. Since the belt has a certain degree of flexibility, there arises a problem that the belt expands and contracts due to a difference in conveyance speed, and an image on the sheet shifts.

  In addition, the water pressure generated at the time of fixing cannot be suppressed at the pressure relief portion. Therefore, uneven gloss due to the contact state between the toner surface and the belt surface in the portion where the air or water vapor is accumulated and the portion where it is not in the width direction (direction perpendicular to the sheet conveying direction) of the pressure relief portion is different. appear. In particular, it tends to occur in a sheet having low air permeability such as coated paper.

  Therefore, an object of the present invention is to provide an image heating apparatus and a fixing apparatus that can optimize the pressure distribution in the image heating nip even when the image heating nip is configured to be long.

  Another object of the present invention is to provide an image heating device and a fixing device capable of preventing the occurrence of image defects such as uneven gloss.

  According to the present invention, the above object can be achieved.

  According to a first aspect of the present invention, a first belt and a second belt that sandwich and convey a recording material carrying an image, and a nip formation for forming an image heating nip between the first belt and the second belt. The image forming apparatus includes a high-friction roller that drives the first belt, a low-friction roller that is disposed opposite to the high-friction roller and pressurizes the second belt, and a high-friction roller. A first fixing member arranged in non-contact with the friction roller and pressurizing the first belt, and a second fixing member facing the first fixing member and arranged in contact with the low friction roller and pressurizing the second belt The high friction roller and the first fixing member are in a non-contact area with the first belt inner surface, and the most downstream portion in the recording material conveyance direction in the second belt contact portion of the second fixing member is the first belt. And pressurizing via the second belt .

  According to the present invention, even when the image heating nip is lengthened, the pressure distribution in the image heating nip can be optimized. Furthermore, the pressure distribution in the image heating nip can be optimized. Therefore, occurrence of image defects such as uneven gloss can be prevented.

  Next, details of the present invention will be described in accordance with the description of the embodiments.

  Hereinafter, the present invention will be described more specifically with reference to examples. Although these examples are examples of embodiments to which the present invention can be applied, the present invention is not limited to these examples, and various modifications are possible within the scope of the idea of the present invention.

(First embodiment)
First, the overall configuration of the image forming apparatus will be described with reference to FIG.

  The image forming apparatus shown in FIG. 6 is an image forming apparatus (so-called printer) that employs an electrophotographic system.

  The image forming apparatus 1 is roughly divided into an image forming unit for forming a toner image on a sheet as a recording material, and a fixing device as an image heating apparatus for fixing the toner image formed on the sheet by heating and pressing. It has been.

  First, the image forming means is provided with a device described below. A charger 3 as a charging unit is provided around the photosensitive drum 2 as an image carrier, and the surface of the photosensitive drum 2 is uniformly charged by the charger 3. Then, an electrostatic latent image is formed on the photosensitive drum 2 by irradiating light 5 corresponding to the image from an exposure device 4 as an exposure unit. This electrostatic latent image is developed by a developing device 6 as developing means to form a toner image. On the other hand, the sheet S is stored in a feeding cassette 9 at the lower part of the apparatus and fed by a feeding roller 10. The sheet is conveyed in synchronism with the toner image on the photosensitive drum 2 by a pair of registration rollers 11 as conveying means. The toner image on the photosensitive drum is electrostatically transferred onto a sheet by a transfer roller 7 as a transfer unit, and is conveyed to the fixing device A. Thereafter, the toner remaining on the photosensitive drum 2 is removed by a cleaning device 8 as a cleaning unit.

  Then, the toner image formed on the sheet S by the image forming means is fixed on the sheet by being heated and pressed in the fixing device A as an image heating device. Thereafter, the sheet S on which the toner image is fixed is conveyed and discharged by the discharge roller pair 12 to the discharge tray 13 at the upper part of the apparatus.

  FIG. 1 is a cross-sectional view of a fixing device A according to this embodiment, and FIG. 2 is a cross-sectional view illustrating a comparative example. As shown in the figure, the fixing device A includes a fixing belt (fixing means) 20 as an example of a first belt and a pressure belt (pressure means) 21 as an example of a second belt. .

  The fixing belt 20 has an inner diameter of 40 mm and a thickness of 75 μm as a base layer, and an elastic layer is provided on the outer periphery of the base layer with a thickness of 300 μm. As a material of the elastic layer, a known elastic material can be used, and for example, silicone rubber, fluorine rubber, or the like can be used. In this embodiment, silicone rubber is used, the hardness is JIS-A 20 degrees, and the thermal conductivity is 0.8 W / mK. Due to the deformation of the elastic layer, it is possible to prevent the sheet from being wound around the fixing belt 20 and to obtain good separation performance from the belt. Further, on the outer periphery of the elastic layer, a fluororesin layer (for example, PFA or PTFE) is provided with a thickness of 30 μm as a surface release layer.

  The pressure belt 21 has an inner diameter of 40 mm and a thickness of 75 μm as a base layer, and the surface is provided with a PFA tube made of a fluororesin with a thickness of 30 μm as a release layer.

  The fixing belt 20 is stretched by a heating roller 22 as a suspension roller and a fixing roller 23. The heating roller 22 is a 1 mm-thick iron hollow roller having an outer diameter of 20 mm and an inner diameter of 18 mm, and a halogen heater 22a as a heating means is disposed inside. The heating roller also has a function as a tension roller.

  The fixing roller 23 is a high-friction elastic roller in which a silicone rubber layer as an elastic layer is provided on an iron alloy core metal having an outer diameter of 20 mm and a diameter of 18 mm. By providing the elastic layer in this way, it is possible to satisfactorily transmit the driving force input from the driving source (motor) via the driving gear train to the fixing belt and to ensure separation of the sheet from the fixing belt. A fixing nip can be formed. As will be described later, the pressure of the fixing nip is set to a maximum value in the region suspended by the fixing roller 23.

  The hardness of the silicone rubber is JIS-A 15 degrees, and the thermal conductivity is 0.8 W / mK. The silicone rubber layer is also effective in shortening the warm-up time because heat conduction to the inside is reduced.

  The fixing belt 20 is rotated by driving the fixing roller 23 by a motor, and the fixing belt 20 is rotated by friction between the silicone rubber surface of the fixing roller 23 and the inner surface polyimide layer of the fixing belt 20.

  A fixing pad 24 as a first fixing member that presses the fixing belt 20 toward the pressure belt 21 is disposed in close contact with the fixing roller 23 in a non-contact manner. In this example, the separation distance (gap) at the closest portion between the fixing roller 23 and the fixing pad 24 is set to 1 mm.

  The fixing pad 24 is made of resin having a thickness of 3 mm and a width of 12 mm, specifically, PPS (polyphenylene sulfide resin). However, the fixing pad 24 is not limited to a resin and may be a rigid body such as a metal as long as it does not have a rubber elastic layer and has high rigidity.

  On the surface of the fixing pad 24, a cover in which a glass fiber cloth as a slidable sheet is coated with a fluororesin is provided in order to reduce the frictional resistance with the inner surface of the sliding fixing belt 20. Since the pad cover suppresses the driving torque of the fixing roller 23, the belt can be stably rotated without increasing the size of the motor.

  The pressure belt 21 is stretched between a tension roller 25 and a pressure roller 26. The tension roller 25 has an iron alloy cored bar having an outer diameter of 20 mm and a diameter of 16 mm, and a silicone sponge layer is provided to reduce the thermal conductivity and reduce the thermal conduction from the pressure belt 21. is there. The pressure roller 26 is a low-sliding rigid roller made of an iron alloy having an outer diameter of 20 mm and an inner diameter of 16 mm and a thickness of 2 mm.

  A pressure pad 27 as a second fixing member for pressing the pressure belt 21 toward the fixing belt 20 (fixing pad 24) is disposed in contact with the pressure roller 26 which is a rigid roller.

  In order to form a fixing nip as an image heating nip, the pressure roller 26 of the pressure belt 21 has both ends of the rotating shaft directed in the direction of the arrow in FIG. Pressurized toward

  Here, a characteristic configuration of the invention will be described with reference to FIG. The fixing roller 23 and the fixing pad 24 form the tip of the pressure pad in the most downstream direction in the sheet S conveyance direction in order to obtain a continuous pressure distribution without a pressure relief portion in one fixing nip along the sheet conveyance direction. The feature of the present invention is that the pressure pad is set so as to pressurize in the direction of arrow (1) in the figure while being arranged at a position facing the gap part (G in the figure).

  The pressure pad 27 has a thickness of 3 mm and a width of 15 mm, and a distal end portion 27a at the most downstream portion in the transport direction is formed in an R shape made of a rigid body. In this embodiment, the tip 27a made of an R shape is configured such that a wire made of SUS is provided over substantially the entire area along the longitudinal direction of the pressure pad (the sheet width direction orthogonal to the sheet conveying direction). The other part 27b is made of heat-resistant silicone rubber (JIS-A 10 °) as an elastic body. The wire is fixed to the heat resistant silicone rubber.

  Further, the surface of the pressure pad 27 is made of a glass fiber as a low-sliding sheet in order to reduce the frictional resistance between the inner surface of the sliding pressure belt 21 and the pressure roller 26, similarly to the fixing pad 24. A cover (not shown) in which the cloth is coated with a fluororesin is provided.

  As shown in FIG. 3, it is necessary to set the position of the entire pressure pad so that the nip forming surface (upper side in the figure) of the tip 27a enters the fixing belt side from the nip forming surface of the pressure roller. There is. Therefore, the lower surface (pressure roller side) of the distal end portion 27a is set so as to come into contact with the pressure roller, and at that time, the following is satisfied.

R-((R + r) ^ 2-g ^ 2) ^ 0.5 <r ⇔ g ^ 2 <4 · R · r (Formula 1)
R: Pressure roller radius
R: radius of the r shape at the tip of the pressure pad g: distance from the nip between rollers to the center of the pressure pad tip R
As the conditions for g, if 1 and g are too large, the tip 27a contacts the fixing pad and the G region cannot be pushed up. That is, g <G
If 2 and g are small, the leading end 27a may be caught in the roller nip when the fixing device is driven, and the pressure pad may be damaged. That is, g needs to be equal to or greater than the pressure generation region between the rollers.

From the nip measurement at the roller, g> 1.5
Further, as the condition of r, when g is in the range of 1 or 2 above, when the leading end 27a comes into contact with the fixing roller via the belt, the pressure between the rollers falls below the setting, and there is a problem of sheet S discharge (separation). Will occur.

  Considering the above, g is preferably set near the center of the G region between the fixing roller and the fixing pad. Needless to say, it is effective to dispose in the vicinity of the center of the G region in order to effectively improve the pressure loss.

  Since the minimum gap between the fixing roller and the fixing pad is set to 1 mm, G is about 5.5 mm in the fixing belt conveyance direction.

  The pressure roller diameter is 20 mm in this embodiment.

  g may be in the range of 1.5 <g <5.5.

  In this embodiment, the center value is set to 3.5 mm.

  At this time, the radius of the pressure pad tip 27a may be about 0.31 mm or more.

  Further, the radius of the pressure pad tip 27a may be about 0.6 mm or less so that the tip 27a does not contact the fixing roller 23 via the fixing and pressure belt.

  FIG. 4 shows the nip internal pressure distribution in the belt conveyance direction when the radius of the tip portion 27a is set to 0.31, which is the lower limit. As can be seen from the figure, a large pressure drop occurs before the downstream peak.

  Therefore, in this embodiment, the R-shaped radius of the tip 27a is set to 0.4 (SUS wire diameter 0.6 mm sliding sheet 0.1 mm) so that pressure is generated in the G region without any problem in the operation of the fixing device A. FIG. 5 shows the pressure distribution without nip in the belt conveyance direction at that time. Compared with the pressure distribution in FIG. 4, it can be seen that the pressure drop is greatly improved before the most downstream peak.

  With the above configuration, as shown in FIG. 3, a force is generated in the direction of the elastic portion 27b arrow (3) of the pressure pad 27 when the fixing belt and the pressure belt are pressed, and the pressure roller and the tip The force is distributed in the direction of the arrow (2) at the contact portion with the portion 27a. As a result, it is possible to generate a pressing force in the fixing belt direction (arrow (1)) at the tip portion 27a.

  As a result, not only between the fixing roller 23 and the pressure roller 26 but also between the fixing pad 24 and the pressure pad 27, the pressure is fixed along the sheet conveyance direction without causing a large pressure loss portion. The nip can be formed continuously.

  The friction coefficient of the pressure roller 26 against the pressure pad 27 (with the cover described above) is 200 ° C. at an atmospheric temperature and 50 N at a load in a thrust type frictional wear test using a test piece having a predetermined shape. It was 0.15 when the speed was measured under the condition of 30 mm / s.

  On the other hand, as described above, the fixing roller 23 is an elastic roller having a large friction coefficient in order to satisfy the function as a driving roller. Therefore, by disposing the fixing pad 24 close to the fixing roller 23 in a non-contact manner, the conveyance torque required to drive the fixing roller 23 is prevented from becoming too large, and the conveyance performance of the fixing belt is stabilized. It contributes to making it.

  As described above, the fixing pad 24, the fixing roller 23, the pressure pad 27, and the pressure roller 26 as the nip forming means for forming the fixing nip are arranged along the sheet conveying direction between the fixing belt 20 and the pressure belt 21. Thus, one fixing nip can be formed satisfactorily.

  The fixing belt 20 is rotationally driven in the direction of arrow X in FIG. 1 by rotating the fixing roller 23 by a motor at least during execution of image formation. The peripheral speed of the fixing belt 20 is slightly lower than the transport speed of the sheet S transported from the image forming unit side to form a loop on the sheet.

  The pressure belt 21 rotates in the arrow Y direction following the fixing belt 20. Here, the configuration is such that the most downstream part of the fixing nip (the part where the pressure distribution (sheet conveying direction) in the fixing nip is maximum) is conveyed with the fixing belt 20 and the pressure belt 21 sandwiched by a pair of rollers. Belt slip can be prevented. In this embodiment, the peripheral speed of the fixing belt 20 is 300 mm / sec, and it is possible to fix 70 A4 size full color images per minute.

  In a state where the fixing belt 20 rises to a predetermined fixing temperature and is temperature-controlled, the sheet S having the unfixed toner image T is conveyed to the fixing nip between the fixing belt 20 and the pressure belt 21. The sheet S is introduced with the surface carrying the unfixed toner image facing the fixing belt 20 side. The unfixed toner image T on the sheet S is nipped and conveyed while being in close contact with the outer peripheral surface of the fixing belt 20, so that heat is mainly applied from the fixing belt 20 and the surface of the sheet S is subjected to pressure. To be established.

  Further, since the fixing roller 23 in the fixing belt 20 is an elastic roller having a rubber layer, and the pressure roller 26 in the pressure belt 21 is a rigid roller made of iron alloy, the fixing belt 20, the pressure belt 21, The deformation of the fixing roller 23 is large at the fixing nip exit. As a result, the fixing belt is also greatly deformed, and the sheet S carrying the toner image is separated from the fixing belt 20 by the curvature thereof by its own strain.

  By adopting such a configuration, a pressure relief portion does not occur in the fixing nip while stably securing the driving of the fixing belt and the separation of the sheet.

  As described above, in this embodiment, the fixing pad is disposed in a non-contact manner close to the elastic fixing roller, and the pressure pad is disposed in contact with the rigid pressure roller, and the fixing roller. By disposing a gap generated by non-contact between the fixing pad and the fixing pad, a large fixing nip is formed, but there is no large pressure-out portion, thereby preventing image defects such as image misalignment and uneven gloss. It becomes possible.

  Further, although an example in which a halogen heater is employed as a heating source for the fixing belt has been described, a heat source (excitation coil) of an electromagnetic induction heating method with high energy efficiency may be employed. In this case, the fixing belt has a configuration including a conductive layer that generates electromagnetic induction heat by the magnetic flux generated from the exciting coil.

1 is a cross-sectional view of a fixing device according to a first embodiment. It is sectional drawing explaining the characteristic structure of 1st Example. 1 is a diagram illustrating a fixing device according to a first embodiment. It is a figure explaining the structure of 1st Example. It is a figure explaining the structure of 1st Example. 1 is a diagram illustrating an overall configuration of an image forming apparatus.

Explanation of symbols

A Fixing device S Sheet 1 Image forming device 2 Photosensitive drum 3 Charger 4 Optical device 5 Light 6 Developer 7 Transfer roller 8 Cleaning device 9 Feed cassette 10 Feed roller 11 Registration roller pair 12 Discharge roller pair 13 Discharge tray 20 Fixing belt 21 Pressure belt 22 Heating roller 22a Halogen heater 23 Fixing roller 24 Fixing pad 25 Tension roller 26 Pressure roller 27 Pressure pad

Claims (7)

An image heating apparatus having first and second belts for nipping and conveying a recording material carrying an image, and nip forming means for forming an image heating nip between the first and second belts In
The nip forming means includes a high friction roller that drives the first belt, and a low friction roller and a high friction roller that are disposed opposite to the high friction roller and pressurize the second belt.
A first fixing member that is arranged in a non-contact manner and pressurizes the first belt; and a second fixing member that faces the first fixing member and that is arranged in contact with the low friction roller and pressurizes the second belt. ,
In the non-contact area between the high friction roller and the first belt inner surface of the first fixing member, the most downstream portion in the recording material conveyance direction in the second belt contact portion of the second fixing member is the first belt and the second belt An image heating apparatus, wherein pressure is applied via a belt.   2. The image heating according to claim 1, wherein the most downstream portion in the recording material conveyance direction in the second belt contact portion of the second fixing member is disposed in contact with the low friction roller and the inner surface of the second belt. apparatus.   3. The image heating apparatus according to claim 1, wherein the second fixing member has an R shape at the most downstream side in the recording material conveyance direction at the second belt contact portion of the second fixing member.   2. The second fixing member is formed of a rigid body in the recording material conveyance direction most downstream portion R shape in the second belt contact portion of the second fixing member, and an elastic body other than the R portion. The image heating apparatus according to claim 3.   The second fixing member is formed of a rigid body in the recording material conveyance direction most downstream portion R shape in the second belt contact portion of the second fixing member, an elastic body other than the R portion, and at least a second belt. The image heating apparatus according to claim 1, wherein the contact portion is covered with a low sliding material. The field distance g from the center in the conveyance direction of the nip formed by the high friction roller and the low friction roller to the second belt inner surface contact position at the most downstream portion in the recording material conveyance direction of the second fixing member, and the second fixing member When the radius of the R shape at the most downstream part in the recording material conveyance direction is r and the radius of the low sliding roller is R1,
g ^ 2 <4 · R1 · r
The image heating apparatus according to claim 1, wherein:   The most downstream portion in the recording material conveyance direction in the second belt contact portion of the second fixing member is pressurized with the high friction roller and the first fixing member via the first belt and the second bell. The image heating apparatus according to claim 1, wherein the image heating apparatus is not in contact with the image heating apparatus.

Images