Patent Grant
9146520
The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer or a facsimile machine.
The image forming apparatus such as the copying machine, the laser beam printer or the facsimile machine is constituted so as to form an image on a sheet such as plain paper or resin-coated paper by using electrophotography in which a developer consisting of fine powder is controlled so as to be electrostatically attracted to the sheet.
Specifically, an electrostatic latent image is formed on another peripheral surface of a photosensitive drum or a photosensitive belt as an image bearing member and then is developed with a toner or the like as the developer to be visualized. The sheet is nipped and fed at a transfer nip of a transfer portion formed by the image bearing member and a transfer member. In a feeding process, a toner image on the image bearing member surface is transferred onto the sheet by the transfer member and then is carried on the sheet. The toner image is fixed on the sheet by applying heat and pressure to the sheet while nipping and feeding a sheet end, fed from the transfer portion, through a fixing nip of a fixing device (apparatus).
By successively performing these steps, the image is formed on the sheet.
As a fixing device for heat-fixing, on the sheet surface as a fixed image, an unfixed image (toner image), of intended image information, formed and carried on the sheet by a transfer type or a direct type, a fixing device of a heating roller type (heater roller type) or a film type has been put into practical use.
In the fixing device of the heating roller type, the fixing nip is formed by a fixing roller and a pressing roller, and the toner image is heated and fixed on the sheet while nipping and feeding the sheet, on which the unfixed toner image is carried, through the fixing nip. In the fixing device of the film type, the fixing nip is formed by a fixing film and the pressing roller, and the toner image is heated and fixed on the sheet while nipping and feeding the sheet, on which the unfixed toner image is carried, through the fixing nip.
In the above-described fixing devices, a temperature of the fixing device is kept at a predetermined temperature so as to heat-fix the toner image. However, the fixing device temperature varies depending on a thickness of the sheet introduced into (passed through) the fixing nip, a sheet feeding speed, a sheet interval during sheet passing of a plurality of sheets, and an operation state of the image forming apparatus. By the temperature change, an outer diameter of the fixing roller is changed, and therefore with this change, the feeding speed of the sheet passing through the fixing nip is also changed.
Here, when the sheet feeding speed at the fixing nip is slower than the sheet feeding speed at the transfer nip, excessive curve (also called a loop) is formed on the sheet between the fixing device and the transfer portion. Further, when such an excessive curve is formed, the unfixed toner image on the sheet contacts and rubs a sheet feeding guide provided between the fixing device and the transfer portion, so that image defect and image disorder during the transfer are caused.
On the other hand, when the sheet feeding speed at the fixing nip is faster than the sheet feeding speed at the transfer nip, the sheet is in a tension state between the fixing device and the transfer portion. For that reason, in some cases, the image on the sheet is elongated and disorder of the unfixed toner image is caused by an impact when a trailing end of the sheet comes out of the transfer portion.
Therefore, as one of methods for solving the above problems, as disclosed in Japanese Patent No. 4795110, a detecting means for detecting a curve amount of the sheet (hereinafter referred to as a curve sensor) is provided at a central portion, with respect to a sheet width direction perpendicular to the sheet feeding direction, between the transfer portion and the fixing device. Then, on the basis of an output signal of the curve sensor, sheet feeding speed control at the fixing portion and the transfer portion is effected, so that the sheet is fed while maintaining the curve amount in a proper state.
Further, as one of problems which cannot be solved by the above-described image forming apparatus, there is a phenomenon of non-uniform curve feeding such that the curve amount is different with respect to the sheet width direction.
By the influence of the type of the sheet and a specific fixing condition or with a difference in amount per unit area of the toner image or a difference in pressure balance at the transfer portion with respect to the sheet width direction, when a difference in timing when a leading end of the sheet enters the fixing nip is generated between left and right portions, the sheet causes the non-uniform curve. When the non-uniform curve is generated, an attitude of the sheet is disordered, so that the non-uniform curve state cannot be accurately detected by the curve sensor disposed at only the widthwise central portion of the sheet. For that reason, proper control of the curve amount cannot be effected, so that the sheet is slewing-fed by landing thereof on a feeding guide in one side, and generation of creases and generation of scattering of the toner image by an impact during elimination of the curve were caused.
Therefore, as a method for solving the problems, as disclosed in Japanese Laid-Open Patent Application (JP-A) 2007-52112, a plurality of curve sensors are provided, with respect to the sheet width direction, inside the feeding guide between the transfer portion and the fixing device. In this method, even when the non-uniform curve is generated on the sheet, the attitude of the sheet is detected by any of the plurality of curve sensors, and on the basis of a result of the detection, the sheet feeding speed in the fixing device is switched to control the curve amount.
However, in this method, the plurality of the sensors are required, thus leading to an increase in cost.
The present invention has been accomplished in view of the above-described problems. A principal object of the present invention is to provide an image forming apparatus capable of stabilizing a sheet feeding attitude with a simple constitution.
According to an aspect of the present invention, there is provided an image forming apparatus comprising: a transfer portion for transferring an image onto a sheet while feeding the sheet through a transfer nip; a fixing portion for fixing the image on the sheet while feeding the sheet, fed from the transfer portion, through a fixing nip; a sheet feeding guide, provided between the transfer portion and the fixing portion, having a guide surface for guiding feeding of the sheet; a sheet detecting portion, provided at a position between the transfer portion and the fixing portion, for detecting the sheet; and a controller for controlling a sheet feeding speed of at least one of the transfer portion and the fixing portion depending on an output of the sheet detecting portion so that a feeding attitude of the sheet fed while being sandwiched at both of the transfer nip and the fixing nip is maintained in a predetermined feeding attitude, wherein the guide surface of the sheet feeding guide has a most recessed region in a region between the transfer portion and the sheet detecting portion.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.
In
In
Embodiments of the present invention will be described specifically with reference to the drawings. Although the following embodiments are preferred embodiments of the present invention, the present invention is not limited to the following embodiments. Within the scope of the present invention, various constituent elements can be replaced with other known constituent elements.
An image forming apparatus forms an image on a recording material (hereinafter referred to as a sheet), such as plain paper or an OHP sheet of various types having regular and irregular sizes, by using an appropriate image forming process, and then outputs an image-formed product.
The image forming apparatus in this embodiment is an ordinary image forming apparatus including a sheet feeding guide, and is a laser beam printer using an electrophotographic process.
The image forming apparatus in this embodiment includes a sheet feeding portion A, an image forming portion B, a fixing portion 14 and the like.
In the image forming portion B, a process cartridge 8 is detachably mounted in an image forming apparatus main assembly C constituting a casing of the image forming apparatus. The process cartridge 8 is constituted by integrally assembling a drum-shaped electrophotographic photosensitive member as an image bearing member (hereinafter referred to as a photosensitive drum) 9, a charger 10, a developing device 11, a cleaner 12 and the like into a unit. The charger 10 electrically charges the photosensitive drum 9. The developing device 11 develops an electrostatic latent image on the photosensitive drum 9 with a toner. The cleaner 12 removes a residual toner remaining on the photosensitive drum 9, and the residual toner is accommodated in a residual toner chamber (not shown).
The photosensitive drum 9 is rotationally driven in an arrow direction at a predetermined peripheral speed. The charger 10 uniformly charges an outer peripheral surface of the rotating photosensitive drum 9 to a predetermined polarity and a predetermined potential. Laser light L subjected to ON/OFF modulation corresponding to image information to be printed is outputted from a laser scanner unit 13 as an exposure device, so that the charged surface of the photosensitive drum 9 is subjected to main scanning exposure. As a result, the electrostatic latent image corresponding to the image information to be printed is written (formed) on the surface of the rotating photosensitive drum 9. This electrostatic latent image is developed, as a toner image, with the toner by the developing device 11.
On the other hand, sheets P stacked on a sheet mounting table in a sheet feeding tray 1 are picked up from an uppermost sheet one by one by rotation of a sheet feeding roller 3, and then the picked-up sheet P is fed to a registration portion by feeding rollers 4 and 5. The sheet P is subjected to uniformization of a feeding direction thereof at the registration portion consisting of registration rollers 6 and 7, and thereafter is gradually fed to a transfer portion 22 constituted by the photosensitive drum 9 and a transfer roller 2.
At the transfer portion 22, a transfer nip N1 is formed by the surface of the photosensitive drum 9 and the surface of the transfer roller 2, and the sheet P is nipped and fed through the transfer nip N1. Further, in a feeding process of the sheet P, the toner image on the surface of the photosensitive drum 9 is transferred onto the sheet P by a transfer bias applied to the transfer roller 2. The sheet P after the toner image transfer thereon is completed is gradually fed to a fixing nip N2 roughly along a sheet feeding guide 18 provided between the transfer portion 22 and the fixing portion 14.
At the fixing portion 14, the fixing nip N2 is formed by a cylindrical fixing roller (rotatable heating member) 30 and a pressing roller (rotatable pressing member) 19, and the sheet P fed from the transfer portion 22 is nipped and fed through the fixing nip N2. Further, in a feeding process of the sheet P, heat of the fixing roller 30 heated by a halogen lamp 31 provided inside the fixing roller 30 is applied to the sheet P to melt the toner image, so that the toner image is fixed on the sheet P by pressure at the fixing nip N2.
When the sheet P is fed from the transfer portion 22 to the fixing portion 14, a curve sensor (sheet detecting portion) 20 provided inside the sheet feeding guide 18 is constituted to fall white following sliding with the sheet P. The curve sensor 20 is used for detecting a curved state (feeding attitude) of the sheet P generated by a difference in feeding speed of the sheet P between the transfer nip N1 and the fixing nip N2. The curve sensor 20 has a structure as shown in
Here, with reference to
In this way, by changing the sheet feeding speed, the sheet P is fed so as to be maintained at a control point of the curve sensor 20.
Specifically, when the sheet P in a sandwiched state between the fixing portion 14 and the transfer portion 22 turns on the sensor 20, the sensor 20 is in an excessively loosen state, and therefore the sheet feeding speed in at least one of the transfer portion 22 and the fixing portion 14 is changed so as to eliminate the loosening of the sheet P. Thereafter, when the sensor 20 is turned off, the sheet P is in an excessively less state of the loosening of the sheet P, and therefore the sheet feeding speed in at least one of the transfer portion 22 and the fixing portion 14 is changed so as to loosen the sheet P. Such control in which the turning-on and the turning-off of the sensor 20 are repeated is effected, and therefore the attitude of the flag portion 20F shown in (b) of
The sheet P passing through the fixing portion 14 is discharged onto a sheet discharge tray 17 provided at an upper portion of an apparatus main assembly C by an intermediary sheet discharging roller pair 15, a sheet discharging roller pair 16, and the like.
An operation when one-side printing on the sheet P is performed is as described above.
When double-side printing on the sheet P is performed, the sheet P is introduced into a feeding path 27 for double-side printing by a feeding path switching mechanism (not shown) provided downstream of the fixing portion 14 with respect to a recording material (sheet) feeding direction. Then, in the feeding path 27, a switch-back operation of the sheet P is performed, so that the sheet P is turned upside down and then is fed again to the registration portion.
The sheet P fed to the registration portion is, after the feeding direction thereof is uniformized by the registration rollers 6 and 7, fed to the transfer portion 22. Then, the toner image is transferred from the surface of the photosensitive drum 9 onto the sheet P at the transfer nip N1 of the transfer portion 22, and thereafter the sheet P is fed to the fixing portion 14. Then, the toner image transferred from the surface of the photosensitive drum 9 is heat-fixed on the sheet P at the fixing nip N2 of the fixing portion 14.
The sheet P passing through the fixing portion 14 is discharged onto the sheet discharge tray 17 provided at the upper portion of the apparatus main assembly C by the intermediary sheet discharging roller pair 15, the sheet discharging roller pair 16, and the like.
A constitution of a sheet feeding guide 18, in Comparison Example, for guiding the sheet P from the time when the sheet P passes through the transfer portion 22 to the time when the sheet P enters the fixing portion 14 will be described.
The sheet feeding guide 18 has a guide surface 18a for guiding the feeding of the sheet P, and the guide surface 18a is constituted by a curved surface smoothly connecting the transfer portion 22 and the fixing portion 14. The guide surface 18a is provided with a plurality of ribs 28 provided in parallel along the sheet feeding direction thereof, and by a shape of the ribs, the guide surface 18a is configured to be decreased in contact surface with the back surface of the sheet P.
At a central portion of the sheet feeding guide 18 with respect to a direction perpendicular to the sheet feeding direction, a curve sensor (sheet detecting portion) 20 is provided. The curve sensor 20 is set so as to be in stand-by in a predetermined position (home position) where the curve sensor 20 is projected from the guide surface 18a in a certain amount by a spring 24. Further, the curve sensor 20 is rotatable with the slide of the sheet P.
In a downstream side of the sheet feeding guide 18 with respect to the sheet feeding direction, a plurality of rollers 29 are provided smoothly rotatably over a widthwise direction of the sheet P. These rollers 29 not only have the function of smoothly delivering the sheet P to an entrance guide 21 toward the inside of the fixing portion 14 but also reduce a degree of abrasion of the ribs 28 due to sliding between the sheet feeding guide 18 and the back surface of the sheet P.
Next, with reference to
As shown in
However, at this time, in some cases, the above-described phenomenon of the non-uniform curve of the sheet P occurs. This phenomenon occurs in the case where a difference in entrance timing of the sheet P between left and right leading end corner portions of the sheet P is generated by the influence of the type of the sheet P and a fixing condition or with a difference in amount per unit area of the toner image between left and right end portions of the sheet P with respect to the sheet widthwise direction or with a difference in pressure balance at the transfer portion.
As described above, a process until the non-uniform curve of the sheet P causes the image defect problem includes:
1) The non-uniformly curved portion of the sheet P contacts the guide surface 18a of the sheet feeding guide 18,
2) The attitude of the sheet P is disordered by the contact with the guide surface 18a to space the sheet P from the curve sensor 20, and
3) The curve sensor 20 erroneously detects the attitude of the sheet P, so that the non-uniform curve is further accelerated.
The sheet P shown in
The generation of such non-uniform curve is conspicuous with an increasing size of the sheet P, for the reason such as constraint of the structure of the image forming apparatus or the like, it is difficult to prevent the generation of the non-uniform curve in many cases.
Therefore, in Embodiment 1, a constitution in which the sheet P does not contact the sheet feeding guide 18 even when the sheet P causes the non-uniform curve was employed. Specifically, the guide surface 18a of the sheet feeding guide 18 was set at a low level so as not to cause separation of the sheet P from the curve sensor 20 due to the contact of the sheet P with the sheet feeding guide 18.
As described above, with a lower guide surface, a degree of tolerance with respect to the non-uniform curve becomes larger, but it is preferable that the lowering level of the guide surface is limited to a necessary minimum level since a large lowering level leads to upsizing of the image forming apparatus. In this embodiment, the level of the guide surface 18b was set so as to be lower than that of the guide surface 18a in Comparison Example by about 5 mm at the maximum.
On the other hand, by lowering the guide surface of the sheet feeding guide 18, also a feeding path of the leading end of the sheet P is changed.
As shown in
As shown in
This is because a feeding position of the trailing end of the sheet P3 is also lowered by lowering the guide surface to result in an increase in rotation amount of the curve sensor 20 and therefore a time required for returning the position of the curve sensor to the stand-by position is also increased with the increased rotation amount of the curve sensor 20. That is, a rotation locus of the curve sensor 20 in
In this way, when the impact between the leading end of the subsequent sheet P4 and the curve sensor 20 during the contact therebetween is increased, there is also a possibility that the unfixed toner image in the leading end side of the subsequent sheet P4 is scattered.
Therefore, in this embodiment, a constitution in which also the impact when the leading end contacts the flag portion 20F is suppressed was employed. In
Specifically, as shown in
By forming the guide surface 18c in the projected shape, a tilt angle of the curve sensor 20 during the sheet passing is maintained at a necessary minimum level. Further, even when a continuous feeding interval of the sheets P is minimized in order to increase a print possessing speed of the image forming apparatus, the subsequent sheet is prevented from abutting against the curve sensor 20 before the returning operation of the curve sensor 20 is completed. As a result, scattering of the unfixed toner image in the leading end side of the subsequent sheet was suppressed.
Further, as shown in
The shape of the plurality of ribs 28a provided on the guide surfaces 18b are, as shown in
As in this embodiment, the ribs 28a are disposed radially with respect to the sheet feeding direction, so that even when the widthwise end portions of the sheet P are positioned in any positions, a state in which the widthwise end portions of the sheet continuously slide with end surfaces 28a1 of the ribs 28a is eliminated. For that reason, it is possible to prevent the sheet P from being caught by the ribs 28a to skew and from causing corner creases.
However, within the projected-shaped guide surface 18c in the neighborhood of the curve sensor 20, the ribs 28c are shaped in a straight shape in parallel to the feeding direction of the sheet P. A dimension of the projected-shaped guide surface 18c is made smaller than a width of a minimum regular-sized sheet P usable in the image forming apparatus. In a region of the guide surface 18c in which there is no fear of the sliding with the widthwise end portions of the sheet P, by shaping the ribs 28b so as to extend in parallel to the feeding direction of the sheet P, the following effect is obtained. That is, an effect of minimizing a slidable section between the sheet P and the ribs 28b when the sheet P passes through the sheet feeding guide 18 to stabilize also the attitude of the sheet P with respect to the widthwise direction of the sheet P when the leading end of the sheet P enters the curve sensor 20.
As described above, by constituting the guide surfaces 18b and 18c of the sheet feeding guide 18 as in the above-described manner, the impact applied from the curve sensor 20 onto the sheet P was capable of being ensured so as to be comparable to or more than that in Comparison Example. Further, the guide surfaces 18b corresponding to the end portions of the sheet P, with respect to the sheet feeding direction, where the non-uniform curve amount of the sheet P is largest are made lower in level than the guide surface 18c in the neighborhood of the curve sensor 20, and therefore also a degree of tolerance with respect to the non-uniform curve of the sheet P can be maintained.
Here, as shown in
This is based on the following reason. The non-uniform curve of the sheet P is generated roughly from the intersection point X of the nip lines as a starting point, and therefore an effect of preventing the contact between the sheet P and the sheet feeding guide 18 caused due to the non-uniform curve is highest by disposing the guide surface 18b such that a point substantially below the intersection line X is a lowest point of the guide surface 18b. Further, the curve sensor 20 is disposed in the downstream side, with respect to the sheet feeding direction, where the behavior of the sheet P is stable relative to that at the intersection point X of the nip lines as the starting point of the non-uniform curve, whereby it is possible to detect the sheet attitude and the curve amount of the sheet P with high accuracy. Therefore, the guide surface of the sheet feeding guide may preferably have a most deeply recessed region in a region between the transfer portion and the sheet detecting portion. The most deeply recessed region is indicated as H in (a) and (b) of
Incidentally, the most deeply recessed region H is a region where a distance (depth) from the target attitude (predetermined feeding attitude) Ptarget of the sheet P shown in (b) of
Further, with respect to the sheet P passing through the transfer portion 22, by passing the leading end of the sheet P through the guide surface 18b and then the curve sensor 20 without directly contacting the curve sensor 20, it is also possible to obtain an effect of alleviating the impact at the time of contact the back surface of the leading end of the sheet P and the curve sensor 20.
By the above constitution, even when the sheet P causes the non-uniform curve, the sheet P can be fed while stably maintaining the attitude thereof without contacting the sheet feeding guide 18. By suppressing the impact at the time of the contact between the back surface of the leading end of the sheet P and the curve sensor 20, the unfixed toner image on the sheet P is prevented from being disordered. It became possible to solve both of problems of the abrasion of the sheet P with the upper feeding guide caused due to the non-uniform curve of the sheet P and the scattering of the image generated in the leading end side of the subsequent sheet without providing a plurality of curve sensors 20 with respect to the widthwise direction of the sheet P.
Another embodiment of the image forming apparatus will be described. The image forming apparatus in this embodiment has the same constitution as that of the image forming apparatus in Embodiment 1 except that a shape of the guide surface 18c of the sheet feeding guide 18 is different from that in Embodiment 1. The same constitution as that of the image forming apparatus in Embodiment 1 will be described while quoting the explanation of the constitution of the image forming apparatus in Embodiment 1.
In the image forming apparatus in Embodiment 1, the shape of the guide surface 18c of the sheet feeding guide 18 is the projected shape having the stepped portions 18c1 in both sides thereof with respect to the sheet feeding direction as shown in
As in this embodiment, even when the guide surface 18c is shaped in the projected shape providing the smoothly curved surface in the entire region with respect to the sheet feeding direction, a sufficient distance of the widthwise end portions of the sheet P is ensured from an ordinary feeding position. For that reason, even in the case where the sheet P causes the non-uniform curve, it is possible to prevent the contact between the sheet P and the sheet feeding guide 18.
Further, when the small-sized sheet P which does not readily cause the non-uniform curve relative to other sheets P is fed, the leading end of the sheet P is fed along the smooth guide surface 18c in cross-sectional shape, and therefore the shape of the guide surface 18c also contributes to sheet feeding stability.
In this way, depending on the sheet size used in the image forming apparatus, a profile of the cross-sectional shape of the guide surface 18c of the sheet feeding guide 18 may preferably be selected appropriately.
As described above, the image forming apparatuses in Embodiments 1 and 2 are capable of feeding the sheet P while stably maintaining the attitude of the sheet P by the curve sensor 20 without causing a heavy impact which otherwise might be caused by a conventional contact between the sheet P and the sheet feeding guide 18 even when the sheet P causes the non-uniform curve. Further, the feeding attitude of the sheet P from the transfer portion 22 to the fixing portion 14 can be stabilized while maintaining a simple constitution without inviting an increase in cost, so that it is possible to compatibly realize a stable feeding performance of the sheet P and a high image quality.
In place of the fixing roller 30 of the fixing portion 14, a rotatable heating member such as a fixing film or a fixing belt may also be used. In this case, as a heating member for heating the rotatable heating member, a ceramic heater or a coil for generating magnetic flux can be appropriately used.
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 purpose of the improvements or the scope of the following claims.
This application claims priority from Japanese Patent Applications Nos. 029062/2013 filed Feb. 18, 2013 and 012361/2014 filed Jan. 27, 2014, which are hereby incorporated by reference.
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