The present invention relates to an image heating apparatus (device) which heats a toner image on a sheet of recording medium.
In recent years, a fixing apparatus (image heating apparatus) mounted in an electrophotographic image forming apparatus has been increased in process speed, and also, has come to be required to deal with a thick sheet of recording medium (card stock or the like). Thus, it has become rather difficult to keep the fixation roller (rotational component) stable in temperature at a proper level. Thus, in the case of the fixing apparatus disclosed in Japanese Laid-open Patent Application No. 2005-316421, it is provided with an external heating means (two external heat rollers) which are placed in contact with the peripheral surface of the fixation roller to heat the fixation roller.
Further, in the case of the fixing apparatus disclosed in Japanese Laid-open Patent Application No. 2010-134072, it employs an external heat belt. More concretely, an external heat belt is suspended and kept stretched by a pair of two rollers, and is placed in contact with the fixation roller to create an area of contact, between itself and fixation roller, in which the external heat belt can heat the fixation roller. Thus, this fixing apparatus which employs an external heat belt can heat the fixation roller more efficiently than the fixing device which employs the pair of external heat rollers.
The object of the present invention is to improve the fixing device disclosed in Japanese Laid-open Patent Application No. 2010-134072.
According to an aspect of the present invention, there is provided an image heating apparatus comprising first and second rotatable members configured and positioned to heat a toner image on a sheet therebetween; an endless belt configured and positioned to heat an outer surface of said first rotatable member while being rotated by rotation of said first rotatable member; first and second rollers provided inside said endless belt in this order along a rotational moving direction of said first rotatable member and configured and positioned to urge said endless belt toward said first rotatable member; a holding mechanism configured and positioned to rotatably hold said first roller and said second roller, said first roller being movable relative to said second roller; an urging mechanism configured and positioned to urge said holding mechanism toward said first rotatable member; and an urging member configured and positioned to urge said first roller in a direction away from said second roller.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Hereinafter, the embodiments of the present invention are described in detail with reference to the appended drawings.
<Image Forming Apparatus>
Referring to
In the image formation section Pa, a yellow toner image is formed on the photosensitive drum 3a, and is transferred (primary transfer) onto the intermediary transfer belt 130. In the image formation section Pb, a magenta toner image is formed on the photosensitive drum 3b, and is transferred (primary transfer) onto the intermediary transfer belt 130. In the image formation sections Pc, and Pd, cyan toner image and black toner image are formed on the photosensitive drums 3c and 3d, respectively, and are sequentially transferred (primary transfer) onto the intermediary transfer belt 130.
Each of the sheets P of recording medium in a recording medium cassette 10 is taken out of the cassette 10 one by one, and is kept on standby at a pair of registration rollers 12, which convey each sheet P of recording medium to the secondary transfer station T2, with such a timing that the sheet P arrives at the same time as the toner images on the intermediary transfer belt 130. Then, the sheet P is conveyed through the secondary transfer station T2. While the sheet P is conveyed through the secondary transfer station T2, the four toner images, different in color, on the intermediary transfer belt 130 are transferred (secondary transfer) onto the sheet P. Then, the sheet P is conveyed to the fixing device 9, in which the sheet P and the toner images thereon are subjected to heat and pressure. Thus, the toner images become fixed to the sheet P. Then, the sheet P is discharged into the tray 7 which is outside the main assembly of the image forming apparatus 100.
In a case where the image forming apparatus 100 is in the two-sided printing mode, after the fixation of the toner images onto the first surface of the sheet P of recording medium by the fixing device 9, the sheet P is guided by the flapper 16, into the reversal passage 18 in which the sheet P is guided by the reversal roller 17 into the two-sided print passage 19. Then, the sheet P is again kept on standby by the registration rollers 12. Then, it is sent by the registration rollers 12 into the secondary transfer station T2, in which toner images are transferred onto the second surface of the sheet P. Then, the transferred toner images on the second surface of the sheet P are fixed by the fixing device 9. Then, the sheet P, which has fixed toner images on both of its first and second surfaces, is discharged into the external tray 7.
The image formation sections Pa, Pb, Pc and Pd are virtually the same in structure, although their developing devices 1a, 1b, 1c and 1d, respectively, are different in the color of the toner they use. Thus, only the image formation section Pa is described in order to not repeat the same description.
The image formation station Pa is made up of the photosensitive drum 3a, and drum processing means, more specifically, charge roller 2a, exposing device 5a, developing device 1a, a primary transfer roller 6a, and a drum cleaning device 4a, which are disposed in the adjacencies of the peripheral surface of the photosensitive drum 3a. The photosensitive drum 3a is made up of a cylindrical substrate formed of aluminum, and a photosensitive layer formed on the peripheral surface of the cylindrical substrate.
The charge roller 2a uniformly charges the peripheral surface of the photosensitive drum 3a to a preset potential level. The exposing device 5a writes an electrostatic image of the image to be formed, on the peripheral surface of the photosensitive drum 3a, by scanning the peripheral surface of the photosensitive drum 3a with the beam of layer light it emits. The developing device 1a develops the electrostatic image on the peripheral surface of the photosensitive drum 3a into a toner image. The primary transfer roller 6a transfers (primary transfer) the toner image on the peripheral surface of the photosensitive drum 3a, by being supplied with electrical voltage.
The drum cleaning device 4a recovers the transfer residual toner, that is, the toner which has escaped from being transferred onto the intermediary transfer belt 130 and is remaining adhered to the peripheral surface of the photosensitive drum 3a, by rubbing the peripheral surface of the photosensitive drum 3a with its cleaning blade. The belt cleaning device 15 recovers the transfer residual toner, that is, the toner which has escaped from being transferred onto a sheet P of recording medium in the secondary transfer station T2, and is remaining adhered to the intermediary transfer belt 130.
<Fixing Device>
Referring to
The pressure roller 102 is made up of a metallic core 102a, an elastic layer 102b formed on the peripheral surface of the metallic core 102a, and a parting layer 102c formed on the peripheral surface of the elastic layer 102b. The pressure roller 102 is rotated in the direction indicated by an arrow mark B, at a preset process speed by the driving mechanism 141. The pressure roller 102 is placed in contact with, or separated from, the fixation roller 101 by an unshown pressure application mechanism which employs an eccentric cam. The unshown pressure application mechanism forms a nip N between the pressure roller 102 and fixation roller 101, by pressing the pressure roller 102 upon the fixation roller 101 with the application of a preset amount of pressure.
A halogen heater 111 is stationarily disposed in the hollow of the metallic core 101a of the fixation roller 101. A thermistor 121 is disposed in contact with the fixation roller 101 to detect the surface temperature of the fixation roller 101. A control section 140 keeps the surface temperature of the fixation roller 101 at a preset target level, which corresponds to the recording medium type, by turning on or off the halogen heater 111 in response to the temperature of the fixation roller 101 detected by the thermistor 121.
A halogen heater 112 is stationarily disposed in the hollow of the metallic core 102a of the pressure roller 102. A thermistor 122 is disposed in contact with the pressure roller 102 to detect the surface temperature of the pressure roller 102. A control section 140 keeps the surface temperature of the pressure roller 102 at a preset target level, which corresponds to the recording medium type, by turning on or off the halogen heater 112 in response to the temperature of the pressure roller 102 detected by the thermistor 122.
<External Heat Belt>
An image forming apparatus is required of a high level of productivity (print count per unit length of time) even if the recording medium used for image formation is a sheet of cardstock, which is relatively large in basis weight (weight per unit area). Thus, one of the desirable methods for increasing an image forming apparatus in productivity is to increase the fixing device of the image forming apparatus in the speed with which it processes each sheet P of recording medium. However, recording medium which is large in basis weight is substantial in the amount of heat it robs from the fixing device. Thus, the amount of heat required to fix an unfixed toner image on a sheet of recording medium which is substantial in basis weight is substantially larger than that required to fix an unfixed toner image on a sheet of recording medium which is less in basis weight. Therefore, the fixing device 9 is structured so that the external heat belt (endless belt) 105 can be placed in contact with, or separated from, the fixation roller 101. The fixation roller 101 is externally heated by the pressing of the external heat belt 105 upon the peripheral surface of the fixation roller 101.
Referring to
The external heat belt 105 is an endless belt for externally heating the fixation roller 101. It is placed in contact with the peripheral surface of the fixation roller 101, forming thereby a nip Ne. It is made up of a substrative layer made of metal such as stainless steel or nickel, or resin such as polyimide, and a heat resistant surface layer formed of fluorinated resin, on the outward surface of the substrative layer to prevent toner from adhering to the external heat belt 105. As the fixation roller 101 rotates, the external heat belt 105 is rotated in the direction indicated by an arrow mark C, by the friction between the fixation roller 101 and external heat belt 101.
A heater (halogen heater) 113, which is an example of a means for heating the upstream roller, heats the upstream roller 103 to a preset temperature level. The upstream roller 103 is made up of a cylindrical hollow component formed of a metallic substance such as aluminum, iron, stainless steel, etc., which is high in thermal conductivity, and a surface layer formed on the peripheral surface of the cylindrical component by coating the peripheral surface of the cylindrical component with a substance such rubber, resin, etc., which are excellent in terms of parting properties. The aforementioned halogen heater 113 is stationarily disposed at the center of the hollow of the upstream roller 103, in terms of the radius direction of the roller 103, in such a manner that it extends from one lengthwise end of the roller 103 to the other. A thermistor 123 is disposed so that it remains in contact with the portion of the external heat belt 105, which is in contact with the upstream roller 103. It detects the temperature of the upstream roller 103. The control section 140 keeps the temperature of the upstream roller 103 at the preset target level, by turning on or off the halogen heater 113 in response to the temperature of the upstream roller 103 detected by the thermistor 123.
A heater (halogen heater) 114, which is an example of a means for heating the downstream roller 104, heats the downstream roller 104 to a preset temperature level. The downstream roller 104 is made up of a cylindrical hollow component formed of a metallic substance such as aluminum, iron, stainless steel, etc., which is high in thermal conductivity, and a surface layer formed on the peripheral surface of the cylindrical component by coating the peripheral surface of the cylindrical component with a substance such rubber, resin, etc., which is excellent in terms of parting properties. The aforementioned halogen heater 114 is stationarily disposed at the center of the hollow of the downstream roller 104, in terms of the radius direction of the roller 104, in such a manner that it extends from one lengthwise end of the roller 104 to the other. A thermistor a thermistor 124 is disposed so that it remains in contact with the portion of the external heat belt 105, which is in contact with the downstream roller 104, and detects the temperature of the downstream roller 104. The control section 140 keeps the temperature of the downstream roller 104 at the preset target level, by turning on or off the halogen heater 114 in response to the temperature of the downstream roller 104 detected by the thermistor 124.
The target temperature levels set for controlling the upstream and downstream rollers 103 and 104 in temperature is set higher than the target temperature level for controlling the temperature of the fixation roller 101, for the reason that when the surface temperatures of the upstream roller 103 and downstream roller 104 are higher than the surface temperature of the fixation roller 101, the efficiency with which heat is transferred from the upstream and downstream rollers 103 and 104 to the fixation roller 101, as the fixation roller 101 reduces in surface temperature, is higher than when the upstream and downstream rollers 103 and 104 are the same in surface temperature as the fixation roller 101. For example, in a case where the target temperature level for the fixation roller 101 is set to 165° C. in an image forming operation in which cardstock is used as recording medium, the target temperature levels for the upstream and downstream roller 103 and 104 are set to 230° C. That is, the surface temperatures of the upstream and downstream rollers 103 and 104 are kept higher by 75° C. than the surface temperature of the fixation roller 101.
While the fixing device 9 is kept on standby for the next image formation job, the external heat belt 105 is kept separated from the fixation roller 101. Referring to
Next, referring to
The external heat belt 105 is enabled to take a position in which it is kept in contact with the fixation roller 101 by a belt positioning mechanism 200, and a position in which it is kept away from the fixation roller 101 by the belt positioning mechanism 200. The belt positioning mechanism 200 doubles as the mechanism for keeping the upstream and downstream rollers 103 and 104 pressed against the fixation roller 101 with the presence of the external heat belt 105 between the two rollers 103 and 104, and the fixation roller 101. A pressure application frame (pressing mechanism) 201 is pivotally movable relative to the frame 9f of the fixing device 9, about a shaft (pivot) 203 by which the pressure application frame 201 is supported.
There is disposed a compression spring (pressure application mechanism) 204 between the opposite end portion of the pressure application frame 201 from the shaft (pivot) 203, and the fixing device frame 9f. The compression spring 204 presses the opposite end portion of the pressure application frame 201 from the shaft 203, pressing thereby the upstream and downstream rollers 103 and 104 against the fixation roller 101. The total amount of pressure, which the compression spring 204 generates between the upstream and downstream rollers 103 and 104, and the fixation roller 101, with the presence of the external heat belt 105 between the two rollers 103 and 104, and the fixation roller 101, is 392N (roughly 40 kgf).
A pressure removal cam 205 is in contact with the bottom surface of the opposite end portion of the pressure application frame 201 from the shaft 203. The control section 140 causes the opposite end portion of the pressure application frame 201 from the shaft 201, to move upward or downward by rotating the pressure removal cam 205 about the shaft 205a. As the pressure application cam 205 is kept separated from the pressure application frame 201, the compression spring 204 presses downward the opposite end portion of the pressure application frame 201 from the shaft 203, causing thereby the external heat belt 105 to be pressed upon the fixation roller 101. As the pressure removal cam 205 moves upward the pressure application frame 210 while compressing the compression spring 204, the external heat belt 105 is separated from the fixation roller 101.
Referring to
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<Comparison of Fixing Device in First Embodiment to First Example of Comparative Fixing Device>
By the way, in a case of the fixing device 9 in this embodiment which employs the external heat belt 105, in order to ensure that the fixation roller 101 remains stable in its surface temperature, it is important that the external heat belt 105 is kept airtightly in contact with the peripheral surface of the fixation roller 101 (across entirety of area of contact between external heat belt 105 and fixation roller 101, and with no interruption). If the contact between the fixation roller 101 and external heat belt 105 fails to remain airtight, it becomes impossible for heat to be supplied from the external heat belt 105 to the fixation roller 101 by a sufficient amount. Consequently, the fixation roller 101 reduces in surface temperature, which causes the image forming apparatus 100 to reduce in image quality; the image forming apparatus 100 outputs images which are nonuniform in glossiness, and/or toner fails to be satisfactorily fixed to recording medium.
However, when the image forming apparatus 100 is kept on standby while it is waiting for an image formation command, it is desired that the external heat belt 105 remains completely separated from the fixation roller 101, and the fixation roller 101 is kept as high as possible in temperature by being heated by the heater (halogen heater) 111 placed in the fixation roller 101. If the metallic core 101a is low in temperature, except for its surface layer which is kept high in temperature by the external heat belt 105, the fixation roller 101 abruptly drops in temperature as it comes into contact with recording medium.
Referring to
In this embodiment, therefore, in order to make the fixing device 9 smaller, in the amount by which the external heat belt 105 slackens as it is separated from the fixation roller 101, than the fixing device 9H, or the first example of comparative fixing device, a pair of compression springs which function as a pressure generating components are disposed between the two rollers by which the external heat belt 105 is suspended. Further, the downstream roller, in terms of the rotational direction of the fixation roller 101, of the two rollers by which the external heat belt 105 is suspended, is made changeable in position. Therefore, the fixing device 9 in this embodiment is smaller in the distance by which the external heat belt 105 has to be moved to be kept completely separated from the fixation roller 101, and also, in the amount of pressure necessary to keep the external heat belt 105 airtightly in contact with the fixation roller 101, than the fixing device 9H, or the first example of comparative fixing device. Further, the fixing device 9 in this embodiment is smaller in the fluctuation of the tension of the external heat belt 105 than the fixing device 9H. Therefore, the former is more stable in the circular movement of the external heat belt 105 than the latter.
Referring to
Next, referring to
The compression spring (coil spring) 301 which is an example of a pressure generating component presses the upstream roller 103 in the direction to move the roller 103 away from the downstream roller 104. The compression spring 301 is placed between the bearing for the upstream roller 103 and the bearing for the downstream roller 104 so that it exerts its force upon both bearings. The compression spring 301 is supported by the opposite one of the aforementioned two sections of the bearing holder 220, from the fixation roller 101. The compression spring 301 is stationarily disposed between the bearing for the upstream roller 103 and the bearing for the downstream roller 104.
Next, referring to
In the case of the fixing device 9, in this embodiment, structured to heat its fixation roller 101 with the use of the external heat belt 105, the external heat belt 105 is kept tensioned by the placement of a means for tensioning the external heat belt 105, between the axle of the upstream roller 103 and the axle of the downstream roller 104. Further, the axle of the downstream roller 104 is fixed (held) so that it is virtually unchangeable in position, and the axle of the upstream roller 103 is held so that it can be moved toward, or away from, the downstream roller 104, to minimize the distance by which the external heat belt 105 has to be moved away from the fixation roller 101 to be kept completely separated from the peripheral surface of the fixation roller 101. Therefore, the amount of pressure to be applied to keep the external heat belt 105 airtightly in contact with the fixation roller 101, in this embodiment, does not need to be as high as that to be applied to keep the external heat belt 105 of the fixing device 9H, or the first example of comparative fixing device. Further, the external heat belt 105 in this embodiment is stabler in its circular movement than the external heat belt 105 of the first comparative fixing device 9H.
Referring to
Next, referring to
The bearing holder 220 holds the bearing 103e for the upstream roller 103, and the bearing 104e for the downstream roller 104, between the top and bottom sections 221 and 222. As a connective pin 224 is removed, the bottom section 222 of the bearing holder 220 can be pivotally moved about a shaft (pivot) 223 to allow the bearings 103e and 104e to be removed. Next, referring to
The bearing 103e for the upstream roller 103 is loosely confined by the top and bottom sections 221 and 222 of the bearing holder 220. There is provided gaps 303a and 303b between the bearing 103e and top section 221, allowing thereby the bearing 103e to horizontally move when the external heat belt 105 is placed in contact with, or separated from, the fixation roller 101.
The bearing 104e for the downstream roller 104 is firmly confined by the top and bottom sections 221 and 222 of the bearing holder 220. That is, no gap is provided between the bearing 104e and the top section 221. Thus, the axial line of the bearing 104e is not movable at all relative to the bearing holder 220.
As the external heating unit in the first embodiment is moved toward the fixation roller 101 to place the external heat belt 105 in contact with the fixation roller 101, the bearing holder 220 allows the upstream roller 103 to moves downstream in terms of the rotational direction of the fixation roller 101, while it does not allow the downstream roller 104 to change in position. Thus, it can be prevented that when the external heating unit is moved away from the fixation roller 101, the external heat belt 105 slackens and remains in contact with the fixation roller 101 as shown in
In comparison, referring to
Referring to
As the external heat belt 105 is moved away from the fixation roller 101, the bearing holder 220 in the first embodiment allows the upstream roller 103 to move upstream in terms of the rotational direction of the fixation roller 101 without allowing the downstream roller 104 to change in position. Therefore, the fixing device 9 in the first embodiment is superior to the fixing device 9H, or the first example of comparative fixing device, in terms of the airtightness between the external heat belt 105 and fixation roller 101, when the external heat belt 105 is kept in contact with the fixation roller 101. Further, as the external heat belt 105 is moved away from the fixation roller 101, the downstream roller 104 is moved away from the upstream roller 103, continuing thereby to provide the external heat belt 105 with a preset amount of tension, preventing thereby the external heat belt 105 from slackening.
<Comparison between Fixing Device 9 in First Embodiment, and Second Example of Comparative Fixing Device>
Referring to
In the case of the second example of comparative fixing device, the upstream roller is allowed to change in position as a tension roller. Therefore, the downstream roller is subjected to such force that acts in the direction to move the downstream roller away from the upstream roller. That is, such force that acts in the direction to increase the distance between the axle of the upstream roller 103 and the axle of the downstream roller 104, even though when the external heat belt 105 is in contact with the fixation roller 101, the distance between the axle of the upstream roller 103 and the axle of the downstream roller 104 has to be less than when the external heat belt 105 is kept separated from the fixation roller 101.
That is, as the external heat belt 105 is circularly moved by the rotation of the fixation roller 101, the downstream roller 104 is subjected to such force that works in the direction indicated by an arrow mark X. Consequently, the distance between the axle of the upstream roller 103 and the axle of the downstream roller 104 is slightly increased, increasing thereby the external heat belt 105 in tension. Thus, the area of contact between the external heat belt 105 and fixation roller 101 reduces in size. Therefore, in order to keep the external heat belt 105 airtightly in contact with the fixation roller 101 by the preset amount of nip length, against the above described additional amount of tension, the compression spring 204 has to be increased in the amount of pressure it can generate. However, increasing the compression spring 204 in the amount of pressure it can generate increases the amount of the stress to which the external heat belt 105 is subjected. This is not desirable.
Further, as the external heat belt 105 is circularly moved by the rotation of the fixation roller 101, the downstream roller 104 might oscillate in the direction parallel to the direction indicated by the arrow mark X, which in turn might change the external heat belt 105 in tension. The change in the tension of the external heat belt 105 changes the contact pressure between the external heat belt 105 and fixation roller 101, at the upstream roller 103 and downstream roller 104, causing thereby the entirety of the nip Ne to change in the state of contact between the external heat belt 105 and fixation roller 101. The change in the nip Ne in the state of contact between the external heat belt 105 and fixation roller 101 makes the external heat belt 105 nonuniform in temperature, which is undesirable.
In comparison, in the first embodiment, the fixing device 9 is structured so that the endless belt suspended and kept stretched by two rollers is placed in contact with, or separated from, the fixation roller 101, and also, so that one of the rollers by which the endless belt is suspended is utilized as a tension roller, and is kept pressed in the direction to increase the distance between itself and the other roller. Further, the axle of the downstream roller is fixed in position.
Referring to
Also in the case of the fixing device 9 in the first embodiment, the downstream roller 104 is fixed in position by the bearing holder 220. Therefore, it does not move when the external heat belt 105 is placed in contact with the fixation roller 101. However, gaps are provided between the bearing of the upstream roller 103 and bearing holder 220. Therefore, as the external heat belt 105 is placed in contact with the fixation roller 101, the upstream roller 103 is allowed to move to the position in which it causes the external heat belt 105 to airtightly contact the fixation roller 101, by the desired nip length, while reducing the distance 303b between the top section 221 of the bearing holder 220 and the bearing of the upstream roller 103 to virtual zero.
Further, as the external heat belt 105 is circularly moved by the rotation of the fixation roller 101, the upstream roller 103 is subjected to such force that acts in the direction indicated by an arrow mark Y, improving the area of contact between the external heat belt 105 and fixation roller 101 in airtightness. That is, even if the compression spring 204 is not increased in the amount of pressure it generates, it is ensured that the external heat belt 105 is kept airtightly in contact with the fixation roller 101. Further, as the upstream roller 103 is moved in the direction indicated by the arrow mark Y, the external heat belt 105 is reduced in the amount of tensional stress, which is desirable. Since it is unnecessary to increase the compression spring 204 in the amount of pressure it generates, the amount of the stress to which the external heat belt 105 in the first embodiment is subjected is smaller than that to which the external heat belt 105 of the second example of comparative fixing device 9I, Therefore, the external heat belt 105 in this embodiment is longer in service life than the external heat belt 105 of the second example of comparative fixing device.
The inward movement of the bearing 103e of the upstream roller 103 is regulated by the top section 221 of the bearing holder 220, so that it is not allowed to move inward of the external heating unit beyond position where it keeps the external heat belt 105 airtightly in contact with the fixation roller 101 by the present nip width (heat transfer area width). Therefore, it does not occur that because the upstream roller 103 is limitlessly pulled in the direction of the arrow mark Y, the external heat belt 105 fails to be kept airtightly in contact with the fixation roller 101, by the preset nip width.
In the case of the fixing device 9 in the first embodiment structured to supply the fixation roller 101 with supplemental heat, pressure is applied between the axle of the upstream roller 103 and downstream roller 104 in such a direction to increase the distance between the two rollers 103 and 104. Further, the upstream roller 103 is enabled to change in position, while the downstream roller 104 is fixed in position. Therefore, it does not require a large amount of pressure to keep the external heat belt 105 airtightly in contact with the fixation roller 101, and also, the external heat belt 105 remains stable in its circular movement. In addition, the external heat belt 105 suspended and kept tensioned by the upstream roller 103 and downstream roller 104 does not slacken when the external heat belt 105 is moved away from the fixation roller 101 as the image forming apparatus 100 is put on standby. Further, as the external heat belt 105 is separated from the fixation roller 101, it remains tensioned, being not allowed to slacken. Therefore, the fixing device 9 in this embodiment is smaller in the distance by which the external heating unit has to be moved away from the fixation roller 101 to keep the external heat belt 105 separated from the fixation roller 101, than the second example of comparative fixing device. Therefore, the former is smaller in the amount of space necessary for the positional change of the external heat belt 105, being therefore less in the vertical dimension, than the latter. That is, as is evident from the detailed description of the comparison between the fixing device 9 in the first embodiment and second example of comparative fixing device, the present invention can eliminate the problem which the second example of comparative fixing device suffers, that is, the problem that the slackening of the external heat belt 105, which occurs as the external heat belt 105 is separated from the fixation roller 101, requires a substantial amount of space for the positional movement of the external heat belt 105.
The fixing device 9 in the first embodiment is smaller in the distance by which the external heating unit has to be moved away from the fixation roller 101 to separate the external heat belt 105 from the fixation roller 101, and also, the amount of pressure to be applied to keep the external heat belt 105 airtightly in contact with the fixation roller 101, and is stabler in the circular movement of the external heat belt 105, than the second example of the comparative fixing device. Further, in the case of the fixing device 9 in the first embodiment, the force generated by the rotation of the fixation roller 101 does not act in the direction to widen the distance between the axle of the upstream roller 103 and the axle of the downstream roller 104. Therefore, the fixing device 9 in this embodiment is smaller in the amount of the pressure required to keep the external heat belt 105 airtightly in contact with the fixation roller 101 than the second example of comparative fixing device. The stability in the circular movement of the external heat belt 105 is essential to the stability in the steering of the external heat belt 105, which will be described next.
<Steering Mechanism>
Referring to
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Referring to
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Therefore, there is a relationship between the angle θ between the fixation roller 101 and external heat belt 105, and the speed of the lateral shift of the external heat belt 105. Thus, the speed at which the external heat belt 105 laterally shifts along the upstream roller 103 and downstream roller 104 can be controlled by externally controlling the angle θ between the fixation roller 101 and external heat belt 105.
In a case where the shaft 207a is moved in the direction indicated by the arrow mark H from a point at which the amount of the force which acts in the direction to laterally shift the external heat belt 105 is zero, the force which acts in the direction to shift the external heat belt 105 rearward of the fixation roller 101 (direction indicated by arrow mark M) increases. In a case where the shaft 207a is moved in the direction indicated by the arrow mark J from a point at which the amount of the force which acts in the direction to laterally shift the external heat belt 105 is zero, the force which acts in the direction to shift the external heat belt 105 frontward of the fixation roller 101 (direction indicated by arrow mark L) increases. That is, the direction in which the external heat belt 105 shifts can be controlled by moving the shaft 207a in the direction indicated by the arrow mark H or J.
<Sensor for Detecting Amount of Lateral Shift of External Heat Belt>
Referring to
The roller 128 is in contact with one of the edges of the external heat belt 105. A torsional spring 131 provides the arm 129 with a preset amount of torque, keeping thereby the roller 128 torqued in the direction indicated by an arrow mark Q. Thus, as the external heat belt 105 shifts in the direction indicated by the arrow mark Q, the linkage 138 is moved in the direction indicated by an arrow mark P. As the external heat belt 105 shifts in the direction indicated by the arrow mark R, the linkage 138 is moved in the direction indicated by an arrow mark O.
There are disposed a pair of photo-interrupters 133 and 134, in line with the sensor flag 132. As they detect one of the four edges of a pair of slits with which the sensor flag 132 is provided, it reverses its output. The fixing device 9 is structured so that there is positional correlation between the four edges of the sensor flag 132 and one of the lateral edges of the external heat belt 105. For example, the photo-interrupters 132 and 133 are positioned so that the external heat belt 105 oscillates in its widthwise direction by an amplitude of 5 mm.
Next, referring to
Referring to
<Control of Steering Mechanism>
Referring to
As the external heat belt 105 shifts frontward to a preset point, the control section 140 moves the shaft 207a in the direction indicated by the arrow mark H, by activating the motor 125. Consequently, such force that acts in the direction to shift the external heat belt 105 rearward is generated. On the other hand, as the external heat belt 105 shifts rearward to a preset point, the control section 140 moves the shaft 207a in the direction indicated by the arrow mark J, by activating the motor 125. Consequently, such force that acts in the direction to shift the external heat belt 105 frontward is generated.
Referring to
Referring to
Next, referring to
If the photo-interrupter 133 is turned off by the frontward shifting of the external heat belt 105 (Yes in S17), the control section 140 shifts the shaft 207a in the direction to cause the external heat belt 105 to shift rearward, by activating the motor 125 (S18). If the photo-interrupter 134 is turned off by the rearward shifting of the external heat belt 105 (Yes in S19), the control section 140 shifts the shaft 207a in the direction to cause the external heat belt 105 to shift frontward, by activating the motor 125 (S20).
The control section 140 continues to controls the lateral shifting of the external heat belt 105 (S17-S21) until the image formation job is ended (No in S21). As the image formation job is ended (Yes in S21), the control section 140 moves the external heat belt 105 away from the fixation roller 101 by rotating the pressure removal cam 205 (S22).
Then, the control section 140 places the worm wheel 118 in the home position (reducing thereby angle θ of fixation roller 101 relative to upstream roller 103 and downstream roller 104 to virtually zero (0°)), by activating the motor 125 while checking whether the worm wheel 118 is in the home position or not, with the use of the photo-interrupter 135. Then, it stops the motor 125 (S23).
<Cleaning Roller>
It is desired that the cleaning roller 108 is positioned so that as the cleaning roller 108 is placed in contact with the external heat belt 105, it is pressed against the upstream roller 103 or downstream roller 104 with the presence of the external heat belt 105 between the cleaning roller 108 and the upstream roller 103 or downstream roller 104. With the external heat belt 105 being supported by the upstream roller 103 and downstream roller 104, from the inward side of the belt loop, the fixing device 9 is increased in the airtightness of the area of contact between the cleaning roller 108 and external heat belt 105, which in turn increases the cleaning roller 108 in cleaning performance.
Next, referring to
The fluctuation in the distance between the axle of the upstream roller 103 and the axle of the downstream roller 104 changes the external heat belt 105 in tension, which in turn is likely to make the external heat belt 105 unstable in circular movement. With the external heat belt 105 being unstable in its circular movement, it is difficult to reliably control the lateral shift of the external heat belt 105, with the use of the above-described belt steering mechanism.
The change in position, and/or vibration, of the upstream roller 103, which is attributable to the rotation of the fixation roller 101, occurs at both lengthwise ends of the upstream roller 103. Further, the occurrence of the change in position, and/or vibration, of the upstream roller 103 at one lengthwise end of the upstream roller 103 is independent from that at the other end. Therefore, the distance between the axle of the upstream roller 103 and the axle of the downstream roller 104 is likely to be nonuniform in terms of the lengthwise direction. If the distance between the axle of the upstream roller 103 and the axle of the downstream roller 104 become nonuniform in terms of the lengthwise direction, the nonuniformity affects the lateral shift of the external heat belt 105, interfering thereby the control of the lateral shift of the external heat belt 105 by the above described belt steering mechanism. In the second embodiment of the present, therefore, the cleaning roller 108 is pressed against the downstream roller 104, the axle of which is fixed to the roller supporting frame 206.
The axle of the downstream roller 104 and the axle of the cleaning roller 108 are fixed to the roller supporting frame 206. Therefore, even if the external heat belt 105 drastically changes in tension, the distance between the axle of the cleaning roller 108 and the axle of the downstream roller 104 is kept stable. Therefore, the pressure which the cleaning roller 108 applies to the downstream roller 104 through the external heat belt 105 does not change. Therefore, the load to which the external heat belt 105 is subjected as it is circularly moved by the rotation of the downstream roller 104 remains stable. Therefore, the external heat belt 105 remains stable in its circular movement.
In the first embodiment, the fixing device 9 is structured so that the upstream roller 103 is changeable in position. Therefore, it is unlikely for the external heat belt 105 to change in tension. Therefore, the external heat belt 105 is stable in its circular movement in the first place. Thus, the structural arrangement for the fixing device 9 in the second embodiment is synergetic to that in the first embodiment in terms of the stabilization of the circular movement of the external heat belt 105. In the case of the second embodiment, therefore, the external heat belt 105 is kept even more airtightly in contact with the fixation roller 101 than in the case of the first embodiment. Therefore, the external heating unit in the second embodiment is more uniform in the heat transfer, and higher in heat transfer efficiency, across the area of contact between the external heat belt 105 and fixation roller 101, than the external heating unit in the first embodiment. Therefore, the former is less in the fluctuation of the surface temperature of the fixation roller 101, which occurs as recording medium is conveyed through the nip Ne, than the latter.
Referring to
A referential code L2 stands for the maximum range, in which an image can be formed, that is, the maximum range in which toner adheres to the external heat belt 105. A referential code L3 stands for the range in which the edges of the external heat belt 105 are kept by the mechanism for steering the external heat belt 105. A referential code L1 stands for the range in which toner adheres to the external heat belt 105 while the external heat belt 105 is steered by the steering mechanism. A referential code L4 stands for the range in which the external heat belt 105 is kept by the belt steering mechanism.
The outwardly facing surface of the external heat belt 105 has to be cleaned across the entirety of its image formation range by the cleaning roller 108. Therefore, the length L of the cleaning roller 108 is greater than the length L2 of the maximum image formation range. Further, in order to prevent the lateral edges of the external heat belt 105 from being moved inward of the lengthwise ends of the cleaning roller 108 by the steering of the external heat belt 105, the length L of the cleaning roller 108 is made less than the distance L between the point at which the front lateral edge of the external heat belt 105 is when the external heat belt 105 is in the most rearward position, and the point at which the rear lateral edge of the external heat belt 105 is when the external heat belt 105 is in the most frontward position, for the following reason.
That is, the moment one of the lengthwise ends of the cleaning roller 108 moves out of the range in which the external heat belt 105 is allowed to snake, the external heat belt 105 suddenly changes in terms of how it laterally shifts. Further, if the external heat belt 105 is changed in the direction of it lateral oscillation while one of the lengthwise ends of the cleaning roller 108 is outside the range in which the external heat belt 105 is allowed to snake, the edge portion of the external heat belt 105 bite into the cleaning roller 108, which is relatively soft. Thus, it is possible that the external heat belt 105 will change in the manner in which it laterally shifts, and/or that it will become impossible for the external heat belt 105 to be controlled in lateral movement as intended.
This is why the length L of the cleaning roller 108 is set to satisfy an inequality: L1>L>L2, and the cleaning roller 108 is disposed as shown in
In the second embodiment, the cleaning roller 108 is pressed against the downstream roller 104 with the presence of the external heat belt 105 between itself and the downstream roller 104. Therefore, the pressure applied to the external heat belt 105 by the downstream roller 104 and cleaning roller 108 is uniform across the lengthwise direction of the fixing device 9. Therefore, the cleaning roller 108 does not significantly affect the control of the external heat belt 105 by the above-described belt steering mechanism in terms of the lateral shift. Therefore, it is assured that the external heat belt 105 remains stable in its circular movement.
Also in the second embodiment, the distance between the axle of the cleaning roller 108 and the axle of the downstream roller 104 does not change. Therefore, the external heat belt 105 does not change in tension. Therefore, it does not occur that the external heat belt 105 is made unstable in its circular movement by the instability in the tension of the external heat belt 105, which is attributable to the change in the distance between the axle of the cleaning roller 108 and the axle of the downstream roller 104. Therefore, the external heat belt 105 remains stable in its circular movement. Therefore, the external heat belt 105 can be reliably controlled in its lateral movement by the above-described belt steering mechanism.
Also in the second embodiment, the length L of the cleaning roller 108 is set as shown in
The foregoing is the description of the embodiments of the present invention. However, the present invention can be embodied in the different forms from those in the preceding embodiments, by partially or entirely replacing the structural components of the fixing devices in the preceding embodiments, with some of the known structural components.
That is, the means for heating the fixation roller, external heat belt, etc., does not need to be limited to a halogen heater. For example, a heating means based on the electromagnetic induction may be employed in place of the halogen heater. Further, a fixation belt may be employed in place of the fixation roller.
In the foregoing description of the embodiments of the present invention, the image heating apparatus (device) was described as a fixing apparatus (device). However, the present invention is also applicable to an apparatus for heating an incompletely fixed image or a fixed image to adjust the image in surface properties such as glossiness. Further, it is also applicable to an apparatus for flattening a sheet of recording medium after the fixation of an image to the sheet of recording medium caused the sheet to curl. Not only can an image heating apparatus in accordance with the present invention be employed as a part of an image forming apparatus, but also, can be employed as a heating apparatus or unit which is independently operated from an image forming apparatus. Further, not only is the present invention compatible with a full-color image forming apparatus, but also, a black-and-white image forming apparatus. Further, not only is the present invention applicable to a printer such as those in the preceding embodiments, but also, various image forming apparatuses other than the printer. That is, the present invention is compatible with a copying machine, a facsimile machine, etc., and a multifunction image forming apparatus capable of performing two or more functions of the preceding image forming apparatuses.
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. 210919/2012 filed Sep. 25, 2012, which is hereby incorporated by reference.
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