Patent Grant
8326201
The present invention relates to a fixing apparatus (fixing device) mounted in an image forming apparatus such as an electrophotographic copying machine, an electrophotographic printer, and the like.
As a fixing apparatus mounted in an image forming apparatus such as an electrophotographic copying machine, printer, and the like, there is a fixing apparatus of the belt type. A fixing apparatus of the belt type uses a flexible endless belt to form a nip through which recording media, such as a sheet of recording paper, an OHP sheet, and the like, which is bearing a toner image, is conveyed while remaining pinched in the nip. Therefore, a fixing apparatus of the belt type can make its nip wider in the direction parallel to the recording medium conveyance direction than a fixing apparatus which uses only rollers. Thus, even if it is increased in recording medium conveyance speed, it can still heat toner (toner image) for a sufficient length of time, making it possible to increase an image forming apparatus in printing speed.
Japanese Laid-open Patent Applications 2001-356625, and 2006-091501 disclose fixing apparatuses made up of a combination of an endless belt and a roller.
This fixing apparatus has a fixation roller 1, a pressure pad 4, a pressure belt 2, and a halogen heater 5. The fixation roller 1 has an elastic layer 16, which is its outermost layer. The elastic layer 16 is formed of rubber. The pressure belt 2 is an endless belt, and is suspended in such a manner that it is held against the peripheral surface of the fixation roller 1 by the pressure pad 4, creating a fixation nip N (fixation nip), while it is moved between the fixation roller 1 and the pressure pad 4. The halogen heater 5 is in the fixation roller 1. The pressure pad 4 is formed of such a substance that it is low in thermal conductivity and is harder than the elastic layer 1b of the fixation roller 1. For example, it is formed of a heat resistant resin or ceramic. After the formation of an unfixed toner image T on a sheet of recording medium P, the sheet of recording medium P (which hereafter will be referred to simply as recording medium P) is conveyed through the fixation nip N of the fixing apparatus in the direction indicated by an arrow mark C. As the recording medium P is conveyed through the fixation nip N, the unfixed toner image T is thermally fixed to the surface of the recording medium P.
In a case where a sheet of thick and coated paper or the like is used as a recording medium for the fixing apparatus in
Referring to
While a sheet of coated thick paper is conveyed through the fixation nip N, the portion of the pressure pad 4, which comes into contact with the sheet, remains virtually unchanged in shape; its curvature remains virtually matched with the curvature of the peripheral surface of the fixation roller 1 as is shown in
The pressure distribution in the fixation nip N was measured with the use of a pressure distribution measuring system PINCH (Nitta Co., Ltd).
Because of the presence of the sheet, the internal pressure of the fixation nip N when the combination of the pressure sensor and a sheet of coated thin paper (80 g/m2 in basic weight) was conveyed through the fixation nip N is greater than the internal pressure of the fixation nip N when the pressure sensor alone was conveyed through the fixation nip N. However, it did not occur that the pressure becomes lower across a certain portion of the fixation nip N in the recording medium conveyance direction.
On the other hand, the pressure distribution in the fixation nip N when a sheet of coated thick paper is conveyed through the fixation nip N is substantially affected by the presence of the recording medium. That is, the pressure distribution becomes such that the internal pressure of the fixation nip N is higher across the end portions of the fixation nip N in terms of the direction C in which the sheet is conveyed through the fixation nip N, and lower across the center portion of the fixation nip N. In other words, while a sheet of coated thick paper is conveyed through the fixation nip N, the center portion of the fixation nip N, in terms of the recording medium conveyance direction C, reduces in pressure. Such a phenomenon, as this one, that as a sheet of recording medium is conveyed through the fixation nip N, the entrance portion of the fixation nip N becomes higher in pressure than the following portion (center portion) of the fixation nip N, is generally called “pressure dip”.
“Pressure dip”, such as the above-described one, occurs when the toner image on a sheet of coated paper, which is low in air permeability, is fixed. Thus, as a sheet of coated recording paper, which is low in air permeability, is conveyed through the fixation nip N, not only the air and/or water vapor in the fixation nip N, remains in the “pressure dipped” portion of the fixation nip N, and creates gaps between the sheet of coated recording paper and fixation roller 1, but also, disturbs the toner image before the toner image becomes fixed. Therefore, “image deviation”, which is such a phenomenon that a part or parts of the toner image are displaced as they are fixed, and/or “glossiness nonuniformity”, which is attributable to the instability in the contact between the fixation roller 1 and the sheet of coated paper, is likely to occur. That is, “pressure dip” is likely to cause the formation of abnormal images.
A sheet of ordinary recording paper is relatively large in air permeability. Therefore, when a sheet of ordinary recording paper is used as the recording medium, the air and water vapor in the fixation nip N are allowed to escape. Therefore, it is thought that when a sheet of ordinary recording paper is used as the recording medium, “image deviation” and “glossiness nonuniformity” do not occur often.
This fixing apparatus is made up of a fixation roller 1, a tension roller 22, and a pressure belt 2. The fixation roller 1 has an elastic layer 1b, as the peripheral layer, which is formed of rubber. The pressure belt 2 is disposed so that the peripheral surface of the fixation roller 1 is partially wrapped by the pressure belt 2, and also, that the pressure belt 2 is provided a preset amount of tension by the tension roller 22. The fixation nip N of this fixing apparatus is between the pressure belt 2 and fixation roller 1.
The pressure belt 2 is suspended by three rollers, that is, a separation roller 21, an entrance roller 23, and the tension roller 22, in a manner to partially wrap around them. The separation roller 21 and pressure pad 4 are kept pressed against the fixation roller 1, creating the fixation nip N, which is substantially wider than that of a fixing apparatus of the roller type.
The fixing apparatus in
As the method for dealing with a wide range of recording mediums in terms of thickness, from a sheet of thin recording medium (paper) to a sheet of thick recording medium, this fixing apparatus is structured so that its pressure pad 4 is changeable in the angle of contact relative to its fixation roller 1 to change its pressure distribution, as shown in
As will be evident from the pressure distributions in
The reason why these unwanted phenomena do not occur is as follows. The elastic portion of the pressure pad 4 is deformed by the sheet of thick recording paper. Therefore, the portion of the elastic layer 1b of the fixation roller 1, which is in the fixation nip N, is evenly deformed across the entire range of the fixation nip N.
As described above, what is necessary to prevent “image deviation” and “glossiness nonuniformity” is to structure a fixing apparatus so that the “pressure dip” does not occur in the fixation nip N, that is, to structure a fixing apparatus so that the internal pressure distribution of the fixation nip N becomes such that the internal pressure is lowest at the entrance of the fixation nip N, and gradually increases toward the exit.
However, both the fixing apparatus in
For example, in the case of the fixing apparatus in
In other words, the fixing apparatus in
The present invention was made in consideration of the above-described problems, and its primary object is to provide a fixing apparatus which does not reduce a toner image in density while it is fixing the toner image.
According to an aspect of the present invention, there is provided a fixing apparatus for heat-fixing an unfixed toner image formed on a recording material, the fixing apparatus comprising a rotatable roller; an endless belt contacted to the roller; a pressing member contacted to an inner surface of the endless belt and cooperating with the roller to nip the endless belt, wherein the recording material on which the unfixed toner image is formed is heated while being nipped and fed by a nip between the roller and the endless belt, wherein such a surface of the pressing member as is opposed to the inner surface of the endless belt is curved substantially in the same direction as a curved surface of the roller, wherein the nip includes a first nip region in which the belt is not pressed by the pressing member and the endless belt is contacted to the roller by an elastic force of the endless belt which is in a slack state, and a second nip region in which the endless belt is contacted to the roller by pressing of the pressing member, and wherein the nip starts with the first nip region with respect to a feeding direction of the recording material, wherein the second nip region is continuous with the first nip region.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.
Hereinafter, the preferred embodiments of the present invention will be described with reference to the appended drawings.
(1-1) Example of Image Forming Apparatus
The image forming apparatus 100 in this preferred embodiment of the present invention has an electrophotographic photosensitive member 101 as an image bearing member. The electrophotographic photosensitive member 101 is in the form of a drum (and therefore, it will be referred to as photosensitive drum 101 hereafter). The photosensitive drum 101 is rotatably supported by the primary frame of main assembly 100A of the image forming apparatus 100, and is rotationally driven by a driving means (unshown) in the direction indicated by an arrow mark, at a preset process speed. The image forming apparatus 100 has also a charge roller 102 (charging means), a laser exposing apparatus 103 (exposing means), a developing apparatus 105 (developing means), a transfer roller 106 (transferring means), and a cleaning apparatus 107 (cleaning means), which are disposed in the listed order in the adjacencies of the peripheral surface of the photosensitive drum 101.
While the photosensitive drum 101 is rotated, the peripheral surface of the photosensitive drum 101 is uniformly charged to a preset potential level and polarity by the charge roller 102. Then, the peripheral surface of the photosensitive drum 101 is exposed by the laser exposing apparatus 103; the peripheral surface of the photosensitive drum 101 is scanned by a beam of laser light L projected upon the peripheral surface from the laser exposing apparatus 103 by way of a mirror 104, and the like, while being modulated with the information of the image to be formed. Thus, electrical charge is removed from the exposed (scanned) points of the charged peripheral surface of the photosensitive drum 101. Consequently, an electrostatic latent image (electrostatic image), which reflects the image information, is formed on the peripheral surface of the photosensitive drum 101. The electrostatic latent image is developed with toner (developer) by the developing apparatus 105 which has a development roller 105a. That is, the developing apparatus 105 causes toner to adhere to the electrostatic latent image on the peripheral surface of the photosensitive drum 101 by applying a development bias to the development roller 105a. As a result, the electrostatic latent image is turned into a visible image, or an image formed of toner (developer image).
Meanwhile, a sheet of recording medium (paper) P is fed into the main assembly 100A of the image forming apparatus 100, with a preset timing, from a recording medium feeder cassette 108 by a feeder-and-conveyer roller 109. Then, the sheet of recording medium P is conveyed by a pair of conveyance rollers 110 to a transfer nip Tn, which is the nip between the photosensitive drum 101 and transfer roller 106. Then, the sheet of recording medium P is conveyed through the transfer nip Tn while remaining pinched between the photosensitive drum 101 and transfer roller 106. While the sheet of recording medium P is conveyed through the transfer nip Tn, a transfer bias is applied to the transfer roller 106. Thus, the toner image on the peripheral surface of the photosensitive drum 101 is transferred onto the sheet of recording medium P as if it is peeled away from the peripheral surface of the photosensitive drum 101.
After bearing the toner image in the transfer nip Tn, the sheet of recording medium P (which hereafter may be referred to simply as recording medium P) is separated from the peripheral surface of the photosensitive drum 101, and is conveyed along a recording medium conveyance guide 111 to the fixing apparatus 112, which thermally fixes the toner image to the surface of the recording medium P by applying heat and pressure to the toner image on the recording medium P. After coming out of the fixing apparatus, the recording medium P is conveyed by roller pairs 113, 113 to a pair of discharge rollers 114. Then, it is discharged by the discharge rollers 114 onto a delivery tray 115, which is a part of the top surface of the main assembly 100A of the image forming apparatus 100.
After the transfer of the toner image, the peripheral surface of the photosensitive drum 101 is cleaned by the cleaning apparatus 107; adherents, such as the transfer residual toner, on the peripheral surface of the photosensitive drum 101 are removed by a cleaning blade 107a of the cleaning apparatus 107 to prepare the peripheral surface of the photosensitive drum 101 for the following image formation cycle.
(1-2) Fixing Apparatus
In the following description of the fixing apparatus 112, the lengthwise direction of the fixing apparatus 112, and the lengthwise direction of the structural components the fixing apparatus 112, are the direction which is parallel to the surface of the recording medium P and is perpendicular to the recording medium conveyance direction C. The widthwise direction therefor is the direction which is parallel to the surface of the recording medium P and is parallel to the recording medium conveyance direction C. The “width” of the recording medium P means the measurement of the shorter edge of the recording medium P.
The fixing apparatus 112 in this embodiment has: the fixation roller 1 as a rotatable roller; the pressure belt 2 which is an endless belt; a pressure pad 4 as a pressure applying member; a halogen heater 5 as a heating means (heating member); and a belt guide 7; etc. The fixation roller 1, pressure belt 2, pressure pad 4, heater 5, and belt guide 7 are all long and narrow members, the lengthwise direction of which is parallel to the abovementioned lengthwise direction of the fixing apparatus 112.
(1-2-1) Description of Fixation Roller
The fixation roller 1 is made up of a metallic core 1a, an elastic layer 1b, and a separation layer 1c. The metallic core 1a is made of iron, and is a hollow and cylindrical member which is 37.5 mm in internal diameter, 38.5 mm in external diameter, and 0.5 mm in wall thickness. The elastic layer 1b is formed of silicone rubber, and is 0.5 mm in thickness. It covers the entirety of the peripheral surface of the metallic core 1a except for the lengthwise end portions of the metallic core 1a. The separation layer 1c covers the peripheral surface of the elastic layer 1b to a thickness in a range of 10-50 μm. It is formed of a fluorinated resin, such as PFA, PTFE, and the like. This fixation roller 1 is 40 mm in external diameter. The inward surface of the metallic core 1a of the fixation roller 1 is coated black to make it easier for the metallic core 1a to absorb the radiant heat from the heater 5 disposed in the hollow of the metallic core 1a. The fixation roller 1 is rotatably supported at its lengthwise end portions by a frame (unshown) of the fixing apparatus 112, with the presence of bearings between the metallic core 1a and the frame.
(1-2-2) Description of Pressure Belt (Endless Belt)
The endless belt 2 has: a substrate layer 2a which is endless; and a separation layer 2b which covers the entirety of the outward surface of the substrate layer 2a. That is, the surface of the substrate layer 2a, which faces the fixation roller 1, is covered with the separation layer 2b. The substrate layer 2a is electrically furnaced of such a metal as nickel, SUS (stainless steel), or formed of such a heat resistance resin as polyimide. If the substrate layer 2a is an electrically furnaced metallic belt, its thickness is in a range of 50-150 μm. If it is formed of a heat resistant resin, its thickness is in a range of 50-300 μm. Thus, the endless belt 2 is provided with a proper amount of rigidity (stiffness). The separation layer 2b is roughly 10-50 μm in thickness, and is formed of a fluorinated resin, such as PFA, PTFE (polytetrafluoroethylene). It is in the form of a piece of tube which wraps around the pressure belt 2, or coated on the outward surface of the substrate layer 2a. The pressure belt 2 is suspended in such a manner that it loosely contacts the pressure pad 4 and belt guide 7. That is, the pressure belt 2 is not tensioned by a tension roller or the like, being therefore loosely fitted. The external diameter of the pressure belt 2 is 40 mm. Its length is roughly the same as that of the fixation roller 1.
(1-2-3) Description of Pressure Pad (Pressure Applying Member) and Belt Guide)
The pressure pad 4 is formed so that its length is the same as that of the pressure belt 2. It is disposed so that it applies pressure on the fixation roller 1, with the presence of pressure belt 2 between the pressure pad 4 and fixation roller 1. It has a pressure pad shaft 3 (rotational axis), which is on the exit side in terms of recording medium conveyance direction C (downstream side in terms of recording medium conveyance direction). The pressure pad 4 is enabled to freely rotate about this pressure pad shaft 3. The pressure pad shaft 3 is rotatably supported at its lengthwise ends, with the presence of bearings between the pressure pad shaft 3 and the apparatus frame. As the material for the pressure pad 4, a substance which is low in thermal conductivity, and is harder than the material for the elastic layer 1b of the fixation roller 1, is used. In this embodiment, a heat resistance resin or ceramic is used as the material for the pressure pad 4.
The surface layer of the pressure pad 4, which remains in contact with the inward surface of the pressure belt 2, is formed of fluorinated resin, such as PFA, PTFE (polytetrafluoroethylene), and is roughly 10-50 μm in thickness. This surface layer is provided to reduce the frictional resistance between the surface of the pressure pad 4 and the inward surface of the pressure belt 2. Incidentally, in order to reduce the frictional resistance between the surface of the pressure pad 4 and the inward surface of the pressure belt 2, a heat resistant lubricant such as fluorinated grease or the like may be coated on the inward surface of the pressure belt 2.
The surface of the pressure pad 4, which faces the fixation roller 1 (surface of pressure pad 4, which faces inward surface of pressure belt 2), is provided with such a curvature that matches the curvature of the peripheral surface of the fixation roller 1 (its curvature is the same in direction as that of fixation roller 1). More concretely, the radius of curvature of the abovementioned surface of the pressure pad 4 is 35 mm, which is larger than the radius (20 mm) of the peripheral surface of the fixation roller 1.
The belt guide 7 is below the pressure pad 4, and is disposed so that it opposes the pressure pad 4. It is held by the apparatus frame, at its length lengthwise ends. The surface of the belt guide 7, which is in contact with the inward surface of the pressure belt 2, is provided with such a curvature that matches that of the inward surface of the pressure belt 2.
The entrance portion (upstream portion) of the pressure pad 4, in terms of recording medium conveyance direction C, is kept slightly pressed in the direction (which is perpendicular to recording medium conveyance direction) indicated by an arrow mark P2 in
(1-2-4) Pressure Nip (Second Nip) and Pre-nip (First Nip)
Referring to
The pre-nip N1 described above is formed by the contact between the flexible and deformable pressure belt 2, and the cylindrical fixation roller 1. Therefore, the internal pressure of the pre-nip N1 is very low, and the pressure of the pre-nip N1 is roughly uniform. Thus, the pre-nip N1 remains stable in terms of the state of contact between the pressure belt 2 and the peripheral surface of the fixation roller 1. Further, the pre-nip N1 is formed so that it is continuous with “pressure nip N2 which is formed by placing the portion of the pressure belt 2, which is backup by the pressure pad 4, in contact with the peripheral surface of the fixation roller 1”. Therefore, while the recording medium P is conveyed, remaining pinched by the pressure belt 2 and fixation roller 1, through the fixation nip N, the airtight contact between the recording medium P and fixation roller 1, and the airtight contact between the recording medium P and pressure belt 2, are maintained throughout the entirety of the fixation nip N, that is, the combination of the pre-nip N1 and pressure nip N2. Therefore, the internal pressure of the fixation nip N gradually increases from the nip entrance toward the point at which the internal pressure becomes highest, without decreasing between the nip entrance and the point with the highest pressure. That is, the fixation nip N begins at the pre-nip N1, and continues to the exit end of the pressure nip N2, in terms of the recording medium conveyance direction C. The fixation nip N is the nip which was formed by placing the peripheral surface of the fixation roller 1 and the outward surface of the pressure belt 2 in contact with each other by the pressure pad 4.
As described above, the fixation nip N where the fixation roller 1 and pressure belt 2 remain in contact with each other has: the pre-nip (first nip) N1 which does not have the backup from the pressure pad 4, and in which the pressure belt 2 is kept in contact with the fixation roller 1 by the resiliency of the pressure belt 2; and the pressure nip (second nip) N2 in which the pressure belt 2 remains in contact with the fixation roller 1 by being back up by the pressure pad 4. Further, in terms of the recording medium conveyance direction C, the fixation nip N begins from the first nip N1. Further, the first nip N1 and second nip N2 are continuous.
(1-2-5) Description of Thermal Fixing Operation of Fixing Apparatus
As the driving gear (unshown) attached to one of the lengthwise ends of the metallic core 1a of the fixation roller 1 is rotationally driven by a motor (unshown), the fixation roller 1 is rotated in the direction indicated by an arrow mark A at a preset peripheral velocity (
To the heater 5, electric power begins to be supplied by an electric power controlling portion 120 (as an electric power controlling means) (
While the fixation roller 1 and pressure belt 2 are rotated, and electric power is supplied to the heater 5 as described above, a recording medium P, which has a toner image T on its surface, is conveyed through the pre-nip N1, being positioned so that its surface having the toner image T faces upward.
The recording medium P is conveyed through the pre-nip N1, while remaining weakly and evenly pinched between the fixation roller 1 and pressure belt 2 throughout the pre-nip N1, by the fixation roller 1 and the resiliency of pressure belt 2.
While the recording medium P is conveyed through the pre-nip N1, it is preheated from both the toner image bearing surface side, that is, the fixation roller side, and the side having no toner image, that is, the pressure belt side, by the pre-heated fixation roller 1 and pressure belt 2, respectively. Referring to
In the pre-nip N1, the recording medium P remains pinched between the peripheral surface of the fixation roller 1 and outward surface of the pressure belt 2 by the fixation roller 1 and the “resiliency of the pressure belt 2” as described above. Therefore, the top and bottom surfaces of the recording medium P are uniformly pressed by a very weak pressure, and preheated across their entireties.
The toner image T on the recording medium P is sufficiently heated for the toner image T to reach the preset fixation temperature level, in the pre-nip N1 which is continuous to the pressure nip N2. In the pressure nip N1, the toner image T on the recording medium P is conveyed while remaining under the pressure from the peripheral surface of the fixation roller 1 and the outward surface of the fixation roller 1. Thus, the toner image T on the recording medium P is thermally fixed to the surface of the recording medium P so that the toner image T remains satisfactorily fixed to the recording medium P, and also, that the toner image T has a proper amount of glossiness. That is, the fixing apparatus 112 is set up so that the time for sufficiently melting the toner image T while hardly applying pressure to the toner image T in the pre-nip N1 can be secured, and then, the toner image T can be heated and pressed in the pressure nip N2 to fix the toner image T to the recording medium P. The temperature distribution and pressure distribution, which can achieve the above-described object, can be obtained by the combination of the pre-nip N1 and pressure nip N2. Therefore, the occurrence of the failure of the fixation of the toner image T, blisters, offsets, and the like, can be greatly reduced.
Then, the recording medium P is discharged from the pressure nip N2.
(1-2-6) Description of Recording Medium Passage Through Pre-Nip and Pressure Nip
As a thin recording medium P is introduced into the fixation nip N, the pressure pad 4 rotationally moves about the pressure pad shaft 3, which is on the exit side of the pressure nip N2, in terms of the recording medium conveyance direction C, in such a manner that the entrance side 4a of the pressure pad 4 rotationally moves downward by a preset amount (
On the other hand, as a thick recording medium P is conveyed through the fixation nip N as shown in
(1-2-7) Description of Pressure Distribution of Fixation Nip
The pressure distribution of the fixation nip N was measured with the use of a pressure distribution measuring system PINCH (product of Nitta Co., Ltd.).
One of the characteristic features of the fixing apparatus 112 in the first preferred embodiment is that the internal pressure of the pre-nip N1 is very low compared to the pressure peak in the pressure nip N2, which is generated by pressing the fixation roller 1 and pressure pad 4 relative to each other. The reason why the internal pressure in the pre-nip N1 is lower than the internal pressure in the pressure nip N2 is that in the pre-nip N1, the pressure belt 2 is kept in contact with the fixation roller 1 only by the resiliency of the pressure belt 2. That is, the fixing apparatus 112 is structured so that while a sheet of recording medium P is conveyed through the fixation nip N, the pressure to which the sheet of recording medium P is subjected does not reduce.
Compared to the internal pressure of the fixation nip N when only the pressure sensor is in the fixation nip N, the internal pressure of the fixation nip N when a sheet of thin recording medium P is in the fixation nip N is higher because of the presence of the thin recording medium P. As a thin recording medium P is conveyed through the fixation nip N, the pressure pad 4 is adjusted in position in such a manner that its entrance side 4a in terms of the recording medium conveyance direction C rotationally is moved downward by the preset amount. Therefore, the width of the pre-nip N1 of the fixation nip N, and the width of the pressure nip N2 of the fixation nip N become approximately the same. Thus, the width of the pre-nip N1 of the fixation nip N, and the pressure nip N2 of the fixation nip N, are kept approximately constant whether the recording medium is conveyed through the fixation nip N or not.
Further, compared to the pressure distribution of the fixation nip N when a thin recording medium P is in the fixation nip N, the pressure of the fixation nip N when a thick recording medium P is in the fixation nip N is higher because of the difference in thickness between the thick and thin recording mediums P. Further, even when a thick recording medium P is conveyed through the fixation nip N, the pressure pad 4 is adjusted in position in such a manner that its entrance side 4a in terms of the recording medium conveyance direction C rotationally moves downward by the preset amount. Therefore, the width of the pre-nip N1 of the fixation nip N, and the width of the pressure nip N2 of the fixation nip N, become roughly the same as those when the thin recording medium P is conveyed.
Next, another example of a fixing apparatus 112 in accordance with the present invention will be described.
In the description of the second preferred embodiment of the present invention, the members, portions, and the like, of the fixing apparatus 112, which are the same as those in the first preferred embodiment, are given the same referential codes as those given in the description of the first preferred embodiment, and they will not be described here, and so will be the comparable members, portions, and the like, in the third preferred embodiment of the present invention.
The fixing apparatus 112 in this embodiment is basically the same in structure, except that instead of having the above described springs 8, the fixing apparatus 112 in this embodiment has a driving mechanism M (driving means) for rotationally moving the pressure pad 4 according to the type and basic weight of the recording medium P conveyed through the fixation nip N while remaining pinched between the fixation roller 1 and pressure belt 2.
In the case of the fixing apparatus 112 in this embodiment, therefore, the width of the pre-nip N1 of the fixation nip N, and the width of the pressure nip N2 of the fixation nip N, are kept approximately constant regardless of the type and basic weight of the recording medium P.
(2-1) Description of Driving Mechanism
The driving mechanism M has a gear 10 (gear to be driven), a driving gear 11, and a pressure pad motor (driving force source). The gear 10 is attached to one of the lengthwise ends of the pressure pad shaft 3. This gear 10 meshes with the driving gear 11 attached to the output shaft of the pressure pad motor 12. The pressure pad motor 12 is solidly attached to the apparatus frame. The pressure pad motor 12 is controlled by a MPU (microprocessor unit 130) as a controlling means for controlling the entirety of the image forming apparatus 100.
The MPU 130 obtains the type of recording medium P conveyed through the fixation nip N, from the information regarding the recording medium selection inputted by a user through a control panel of the image forming apparatus 100, information regarding the recording medium selection by a media center, external input from a personal computer or the like. The MPU 130 determines, based on the obtained information regarding the recording medium P, the basic weight of the recording medium P, with reference to a preset table or the like. Then, the MPU 130 makes the image forming apparatus 100 operate in the proper mode for the recording medium P. For example, in a case where the MPU 130 determines that the recording medium P to be used is ordinary paper which is 75 g/cm2 in basic weight, it places the pressure pad 4 in the normal mode position, based on the basic weight table for ordinary paper. In a case where the MPU 130 determines that ordinary paper which is 220 g/m2 in basic weight is going to be used, it places the pressure pad 4 in the thick paper mode position. In this embodiment (second preferred embodiment), in a case where ordinary paper or coated paper is used as the recording medium P, the normal mode is selected when the recording medium P is no more than 150 g/m2 in basic weight, whereas in a case where the recording medium P is no less than 150 g/m2 in basic weight, the thick medium mode is selected.
In the case of the fixing apparatus 112 in this embodiment (second preferred embodiment), its home position is the position in which the pressure pad 4 is while the MPU 130 is carrying out the normal mode (
Also in the case of the fixing apparatus 112 in the second preferred embodiment, the pressure distribution of the fixation nip N was measured with the use of the pressure distribution measuring system PINCH (product of Nitta Co., Ltd.).
The pressure distribution of the fixation nip N in this embodiment which was measured in the normal mode when the pressure sensor and a sheet of thin recording medium P (ordinary paper which is 75 g/cm2, for example, in basic weight) were remaining pinched in the fixation nip N is the same as that when the pressure sensor and a sheet of thin recording medium P (ordinary paper which is 75 g/cm2, for example, in basic weight) were remaining pinched in the fixation nip N of the fixing apparatus 112 in the first preferred embodiment (
Next, another (third) preferred embodiment of the present invention will be described.
The fixing apparatus 112 in this embodiment is a fixing apparatus 112 which uses an electromagnetic heating method.
The fixing apparatus 112 in this embodiment has: the fixation belt 13 which is an endless belt; a pressure roller 14; a pressure pad 4 as a pressure applying member; a magnetic flux generating means (coil unit) 15 as a heating means (heating member); and a belt guide 7. The fixation belt 13, pressure roller 14, coil unit 15, pressure pad 4, and belt guide 7 are all long and narrow members, the lengthwise direction of which is parallel to the abovementioned lengthwise direction of the fixing apparatus 112.
(3-1) Description of Fixation Belt
The fixation belt 13 has: a metallic layer 13a (substrate layer), as a heat generating layer based on electromagnetic induction, which is in the form of an endless belt and is formed of a magnetic metallic substance; an elastic layer 13b layered on the outward surface of the metallic layer 13, and is formed of such a substance as silicone rubber; and a separation layer 13c layered on the outward surface of the elastic layer 13b. The external diameter of the fixation belt 13 is 45 mm. It is loosely fitted around the combination of the pressure pad 4 and belt guide 7. The metallic layer 13a itself has a proper amount of rigidity (stiffness).
The metallic layer 13a is in the form of an endless belt, and is 50 μm in thickness. It is formed of a substance, such as nickel, which is excellent in electrical conductivity. It generates heat, based on eddy current loss attributable to the magnetic flux generated by an induction coil 15c of the magnetic flux generating means 15, which will be described later. In order to obtain an excellent fixed image when fixing an unfixed color image, the elastic layer 13b is formed of silicone rubber, and is made to be 300 μm in thickness. The separation layer 13c is roughly 10-50 μm in thickness, and is formed of a fluorinated resin, such as PFA, PTFE (polytetrafluoroethylene). It is made in the form of a piece of tube which wraps around the fixation belt 13, or coated on the outward surface of the elastic layer 13b.
In the third preferred embodiment, the fixation belt 13 is provided with a layer 13d, which is on the inward surface of the metallic layer 13a, which is in contact with the surface of the pressure pad 4 and the surface of the belt guide 7. The layer 13d is for making it easier for the fixation belt 13 to slide on the surface of the pressure pad 4 and the surface of the belt guide 7. The layer 13 is formed by coating the inward surface of the metallic layer 13a with a highly heat resistant resin, such as polyimide, polyamide, polyimide-amide, and the like. The external diameter of the fixation belt 13 is 45 mm.
The length of the fixation belt 13 is approximately the same as that of the pressure roller 14.
(3-2) Description of Magnetic Flux Generating Means
The magnetic flux generating means 15 has a casing 15a for its inductive heating means, a magnetic core 15b (which hereafter will be referred to simply as core 15b), and the induction coil 15c for heating (which hereafter will be referred to simply as coil). The core 15b is formed of a single or multiple layers of ferrite, for example. The length of the core 15b is roughly the same as that of the fixation belt 13. The coil 15c is formed of a piece of copper wire, the surface of which is covered with a self-welding insulative substance. It is convolutely wound multiple times in the form of a narrow and flat sheet of thick paper. It is disposed in parallel to the core 15b, covering the core 15b.
The casing 15a for the inductive heating means 15 is formed of a dielectric resin. It is a long, narrow, and thin box, in which the coil 15c and core 15b are stored. The core 15b is disposed in the casing 15a in such a manner that the magnetic flux does not leak through the portions of the casing 15a, which do not face the outward surface of the fixation belt 13. The casing 15a is in such a shape that its cross section at a plane parallel to the moving direction of the fixation belt 13 is arcuate. The casing 15a is disposed close to the outward surface of the fixation belt 13, with the presence of a preset amount of distance between the casing 15a and outward surface of the fixation belt 13. The casing 15a is held to the apparatus frame, by its lengthwise ends.
(3-3) Description of Pressure Roller
The pressure roller 14 is an elastic roller. It has: a metallic core 14a which is formed of SUS (stainless steel) and is 30 mm in diameter; a 5 mm thick elastic layer 14b formed of silicone rubber, on the peripheral surface of the metallic core 14a; and a 10-50 μm thick separation layer 14c formed of fluorinated resin, on the peripheral surface of the elastic layer 14b. The fluorinated material for the separation layer 14c is PFA, PTFE, or the like. The external diameter of the pressure roller 14 is 40 mm.
(3-4) Description of Pressure Pad and Belt Guide
The pressure pad 4 is approximately the same in length as the fixation belt 13, and is disposed so that it applies pressure to the pressure roller 14, with the presence of fixation belt 13 between the pressure pad 4 and pressure roller 14. The pressure pad 4 has a pressure pad shaft 3 (rotational axis), which is on the exit side in terms of recording medium conveyance direction C (downstream side in terms of recording medium conveyance direction C). The fixing apparatus 112 is structured so that the pressure pad 4 is allowed to rotate about the pressure pad shaft 3. The pressure pad 4 is made roughly the same in length as the fixation belt 13, and is disposed so that it applies pressure to the pressure roller 14, with presence of the fixation belt 13 between the pressure pad 4 and pressure roller 14. The pressure pad 4 has the pressure pad shaft 3 (rotational axis), which is in the exit side of the pressure pad 4 in terms of the recording medium conveyance direction C (downstream side in terms of recording medium conveyance direction C). The pressure pad 4 is rotationally movable about the pressure pad shaft 3.
The surface layer of the pressure pad 4, which is in contact with the inward surface of the fixation belt 13, is formed of a fluorinated resin, such as PFA, PTFE (polytetrafluoroethylene), and the like, and is roughly 10-50 μm in thickness. The presence of this fluorinated layer reduces the frictional resistance between the surface of the pressure pad 4 and the inward surface of the fixation belt 13. Further, in order to reduce the frictional resistance between the surface of the pressure pad 4 and the inward surface of the fixation belt 13, the inward surface of the fixation belt 13 may be coated with a heat resistance lubricant such as fluorinated grease.
The pressure pad 4 is structured so that its surface which faces the fixation belt 13 is provided with such an inward curvature that matches the curvature of the peripheral surface of the fixation belt 13. More concretely, the radius of curvature of the curved surface of the pressure pad 4 is 35 mm, being larger than the radius (20 mm) of the peripheral surface of the fixation roller 1.
The belt guide 7 is disposed above the pressure pad 4, and opposes the pressure pad 4. The surface of the belt guide 7, which is in contact with the fixation belt 13, is provided with such a curvature that matches the curvature of the inward surface of the fixation belt 13.
The entrance end of the pressure pad 4 in terms of the recording medium conveyance direction C (upstream end in terms of recording medium conveyance direction) is kept weakly pressed in the direction indicated by an arrow mark P2 (direction perpendicular to recording medium conveyance direction C) in
(3-5) Description of Pressure Nip and Pre-Nip
Referring to
The pre-nip N1 described above is formed by the contact between the flexible and deformable fixation belt 13, and the cylindrical pressure roller 14. Therefore, pressure in the pre-nip N is roughly uniform. Thus, the pre-nip N1 remains stable in terms of the state of contact between the fixation belt 13 and the peripheral surface of the pressure roller 14. Further, the pre-nip N1 is formed so that it is continuous with the pressure nip N2 which is formed by placing the portion of the fixation belt 13, which is backup by the pressure pad 4, in contact with the peripheral surface of the pressure roller 14. Therefore, while the recording medium P is conveyed, remaining pinched by the fixation belt 13 and pressure roller 14, through the fixation nip N, the airtight contact between the recording medium P and pressure roller 14, and the airtight contact between the recording medium P and fixation belt 13, are maintained through the entirety of the fixation nip N. That is, the fixation nip N begins at the pre-nip N1, and continues to the exit end of the pressure nip N2, in terms of the recording medium conveyance direction C.
(3-6) Description of Thermal Fixing Operation of Fixing Apparatus
As the driving gear (unshown) attached to one of the lengthwise ends of the metallic core 14a of the pressure roller 14 is rotationally driven by a fixation motor (unshown), the pressure roller 14 is rotated in the direction indicated by an arrow mark B at a preset peripheral velocity (
To the coil 15c, an alternating electric current, which is 10 K-1 MHz in frequency, begins to flow from an unshown exciter circuit which is under the control of an electric power controlling portion 120, slightly before or after, or at the same time as, the starting of the rotation of the pressure roller 14. Thus, the metallic layer 13a of the metallic fixation belt 13 is heated by magnetic induction. As electric current flows through the coil 15c, the magnetic flux, which is to be supplied to the fixation belt 13, is generated. This magnetic flux is absorbed by the metallic layer 13a of the fixation belt 13, in the area where the casing 15a for the inductive heating means opposes the fixation belt 13. Thus, the metallic layer 13a generates heat because its specific resistance. The heat generated in the metallic layer 13a transmits to the surface of the fixation belt 13, and also, transmits inward of the fixation belt 13. Then, the heat transmits from the fixation belt 13 through the pressure nip N2 and pre-nip N1 to the pressure roller 14, which is rotating. Thus, the pressure roller 14 is heated. The temperature of the fixation belt 13 is detected by a temperature detection element 6 (
While the fixation belt 13 and pressure roller 14 are rotated, and electric current flows through the coil 15c as described above, the recording medium P, on which the toner image T is present, is conveyed through the pre-nip N1, being positioned so that its surface having the toner image T faces upward.
The recording medium P is conveyed through the pre-nip N1 while remaining gently pinched between the fixation belt 13 and pressure roller 14 by the resiliency of the fixation belt 13.
While the recording medium P is conveyed through the pre-nip N1, it is preheated from both the toner image bearing surface side, that is, the fixation roller side, and the side having no toner image, that is, the pressure belt side, by the pre-heated pressure roller 14 and fixation belt 13, respectively. Referring to
In the pre-nip N1, the recording medium P remains pinched between the peripheral surface of the pressure roller 14 and the outward surface of the fixation belt 13 by the “resiliency of the fixation belt 13” as described above. Therefore, the top and bottom surfaces of the recording medium P are uniformly pressured and preheated across their entirety.
The toner image T on the recording medium P is sufficiently heated for the toner image T to reach the fixation temperature level, in the pre-nip N1 which is continuous to the pressure nip N2. In the pressure nip N2, the toner image T on the recording medium P is conveyed through the pressure nip N2 while remaining under the pressure from the outward surface of the fixation belt 13 and the peripheral surface of the pressure roller 14. Thus, the toner image T on the recording medium P is thermally fixed to the surface of the recording medium P so that the toner image T remains satisfactorily fixed, and also, that the toner image T has a proper level of glossiness. That is, such temperature distribution and pressure distribution that fix the toner image T with the application of pressure after securing a sufficient length of time for satisfactorily melting the toner image T in the pre-nip N1 are obtained by the combination of the pre-nip N1 and pressure nip N2. Therefore, the fixing apparatus 112 in this embodiment is significantly less in the occurrence of the failure of the fixation of the toner image T, blisters, offsets, and the like, than a conventional fixing apparatus.
Then, the recording medium P is discharged from the pressure nip N2.
In the case of a fixing apparatus, such as the fixing apparatus 112 in this embodiment, which directly heats the fixation belt 13 with the use of an inductive heating method, the component to be heated is only the fixation belt 13. Therefore, it is substantially shorter in warm-up time than a conventional fixing apparatus.
Also in the case of the fixing apparatus 112 in this embodiment (third preferred embodiment), the pressure distribution in its fixation nip N was measured with the use of the pressure distribution measuring system PINCH (product of Nitta Co., Ltd.).
The pressure distribution in its fixation nip N measured when the combination of the pressure sensor and a sheet of thin recording medium (ordinary paper which is 75 g/m2 in basic weight) was conveyed through the fixation nip N while remaining pinched in the fixation nip was the same as the pressure distribution (
[Tests]
The fixing apparatuses in the first to third preferred embodiment, and the first and second comparative fixing apparatuses were evaluated in nonuniformity in glossiness, and density, by outputting the following toner image samples.
Unfixed toner image samples were formed on recording medium P so that the amount of toner on the recording medium P became 0.45±0.01 mg/m2. The fixing apparatuses were adjusted so that the surface temperature level of their fixation roller 1 or fixation belt 13 became 180° C. Then, the unfixed toner image samples were thermally fixed to the recording mediums P by conveying them through the fixation nip N of each of the above described fixing apparatuses at a process speed of 300 mm/sec.
The first comparative fixing apparatus shown in
The second comparative fixing apparatus shown in
The operation of the pressure pad 4 of the second comparative fixing apparatus when the fixing apparatus was in the thick recording medium mode was opposite to that of the pressure pad 4 of the fixing apparatus 112 in the second preferred embodiment. Also in the case of the second comparative fixing apparatus, the width of its fixation nip N when it was in the medium thickness recording medium mode and thick recording medium mode were the same as the width of the fixation nip N of the fixing apparatus 112 in each of the first to third preferred embodiments.
[Toner Image Evaluation in Glossiness Nonuniformity]
The toner images fixed under the above described conditions were evaluated in terms of glossiness nonuniformity. More concretely, sheets of coated paper, which were A4 in size and 80 g/cm2 in basic weight, sheets of coated paper, which were A4 in size and 148 g/m in basic weight, and sheets of coated paper, which were A4 in size and 300 g/m2 in basic weight, were used as the recording mediums P, and the toner images formed on the recording mediums P were visually evaluated in their nonuniformity in glossiness after their thermal fixation. The results are given in Table 1.
It is evident from the results given in Table 1 that in the case of the fixing apparatuses in the first to third preferred embodiments and the second comparative fixing apparatus, no glossiness nonuniformity occurred when the above described three different coated papers were used, but, in the case of the first comparative fixing apparatus, the glossiness nonuniformity occurred when the coated thick papers, which were 300 g/m2 in basic weight, were used.
As for the cause of the above described results, the “pressure dip” occurred when a sheet of coated thick paper, which was 300 g/m2 in basic weight, was conveyed through the first comparative fixing apparatus. In the case of the fixing apparatuses different in structure from the first comparative fixing apparatus, no glossiness nonuniformity occurred when the above-described three coated papers, which were different in basic weight, were used.
[Toner Image Evaluation in Density]
The toner images fixed under the above described conditions were evaluated in terms of density. More concretely, sheets of ordinary paper, which were A4 in size and 75 g/cm2 in basic weight, sheets of ordinary paper, which were A4 in size and 150 g/m2 in basic weight, sheets of ordinary paper, which were A4 in size and 220 g/m2 in basic weight, and sheets of ordinary paper, which were A4 in size and 300 g/m2 in basic weight, were used as the recording mediums P, and 10 of each of the above described ordinary papers different in basic weight, on which the unfixed toner image was present, were conveyed in succession through the fixation nip N of each fixing apparatus. Then, the reflective density of the thermally fixed toner image on each of 10 sheets of ordinary paper was measured. Then, the average reflective density of the 10 fixed toner images was obtained and evaluated. As the evaluation of the image density, X-Rite (product of X-Rite Co., Ltd.) was used.
The results of the image density evaluation made with the use of the following evaluation symbols are given in Table 2. The fixing apparatus which was higher than G in evaluation had no problem.
It is evident from the results given in Table 2 that in the case of the first comparative fixing apparatus, its reflection density was slightly low when ordinary paper which is 75 g/m2 in basic weight, and ordinary paper which is 150 g/m2 in basic weight, were used, and the reflection density was very low when ordinary paper which was 220 g/m2 in basic weight, and ordinary paper which was 300 g/m2 in basic weight, were used. This reduction in reflection density occurred because in the case of the first comparative fixing apparatus, the length of time a toner image remained under pressure was long. In other words, the toner image was excessively pressed. Therefore, the toner on the surface of the ordinary paper infiltrated into the fibrous surface layers. Thus, the fibers appeared at the surface of the toner image, and caused the toner image to reduce in reflection density.
In the case of the second comparative fixing apparatus, when ordinary paper which was 220 g/m2 in basic weight, and ordinary paper which was 300 g/m2 in basic weight, were used, the reflection density was slightly lower compared to the fixing apparatuses 112 in the first to third preferred embodiments, although the reflection density was not low enough to create problems in practical terms. This reduction in reflection density occurred for the following reason. That is, in a case where the second comparative fixing apparatus was used with thick ordinary paper, the internal pressure of its fixation nip N was increased by the thick ordinary paper, and therefore, the amount of the pressure applied to the toner image in the fixation nip N of this fixing apparatus was slightly higher than those applied in the fixation nips N of the fixing apparatuses 112 in the first to third preferred embodiments. Therefore, the fibrous surface layer of the thick ordinary paper was partially infiltrated by the toner. Thus, the fibers appeared at the top surface of the toner image, and therefore, slightly reduced the toner image in image density.
In the case of the fixing apparatuses 112 in the first to third preferred embodiments, the infiltration of toner into the fibrous surface layer of recording medium was not as bad as that in the case of the first and second comparative fixing apparatus, and therefore, the images outputted by the former were higher in reflective density than those outputted by the latter. This occurred for the following reason. In the case of the fixing apparatuses 112 in the first to the third preferred embodiments, the width of the pre-nip N1 of the fixation nip N, and the width of the pressure nip N2 of the fixation nip N, were kept constant whether the recording medium P conveyed through the fixation nip N was thin or thick (
As will be evident from the preceding description of the first to third preferred embodiments of the present invention, the present invention can provide a wider fixation nip which can deal with the increase in the image formation speed, can deal with various recording mediums different in thickness, for example, thin recording medium as well as thick recording medium, can maintain a high level of fixation performance, can achieve a high level of image density, and can prevent the formation of abnormal images, that is, the images suffering from “image deviation” and “glossiness nonuniformity”.
While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.
This application claims priority from Japanese Patent Application No. 002379/2009 filed Jan. 8, 2009 which is hereby incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-002379 | Jan 2009 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6459878 | Tomoyuki et al. | Oct 2002 | B1 |
| 7962084 | Tanaka et al. | Jun 2011 | B2 |
| 8080359 | Matsubara et al. | Dec 2011 | B2 |
| 8126384 | Baba | Feb 2012 | B2 |
| 20080166653 | Matsubara et al. | Jul 2008 | A1 |
| Number | Date | Country |
|---|---|---|
| 8-123241 | May 1996 | JP |
| 10-198204 | Jul 1998 | JP |
| 10-228195 | Aug 1998 | JP |
| 10-228200 | Aug 1998 | JP |
| 2001-356625 | Dec 2001 | JP |
| 2006-78578 | Mar 2006 | JP |
| 2006-91214 | Apr 2006 | JP |
| 2006-91501 | Apr 2006 | JP |
| 2007-72364 | Mar 2007 | JP |
| 2008-191652 | Aug 2008 | JP |
| 2008-241843 | Oct 2008 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20100172677 A1 | Jul 2010 | US |
