Image forming apparatus

An image forming apparatus includes: a transfer portion for transferring an image onto a sheet at a transfer nip; a fixing portion for fixing the image on the sheet at a fixing nip; a sheet feeding guide, provided between the transfer portion and the fixing portion, having a sheet guide surface; a sheet detecting portion provided between the transfer portion and the fixing portion; 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 sandwiched at both of the transfer nip and the fixing nip is maintained in a predetermined feeding attitude. The sheet guide surface of the sheet feeding guide has a most recessed region in a region between the transfer portion and the sheet detecting portion.

FIELD OF THE INVENTION AND RELATED ART

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.

SUMMARY OF THE INVENTION

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.

DESCRIPTION OF THE EMBODIMENTS

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.

(1) Image Forming Apparatus

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.

FIG. 7is a schematic front view of an example of the image forming apparatus according to this embodiment.

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 portion14and the like.

In the image forming portion B, a process cartridge8is detachably mounted in an image forming apparatus main assembly C constituting a casing of the image forming apparatus. The process cartridge8is constituted by integrally assembling a drum-shaped electrophotographic photosensitive member as an image bearing member (hereinafter referred to as a photosensitive drum)9, a charger10, a developing device11, a cleaner12and the like into a unit. The charger10electrically charges the photosensitive drum9. The developing device11develops an electrostatic latent image on the photosensitive drum9with a toner. The cleaner12removes a residual toner remaining on the photosensitive drum9, and the residual toner is accommodated in a residual toner chamber (not shown).

The photosensitive drum9is rotationally driven in an arrow direction at a predetermined peripheral speed. The charger10uniformly charges an outer peripheral surface of the rotating photosensitive drum9to 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 unit13as an exposure device, so that the charged surface of the photosensitive drum9is 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 drum9. This electrostatic latent image is developed, as a toner image, with the toner by the developing device11.

On the other hand, sheets P stacked on a sheet mounting table in a sheet feeding tray1are picked up from an uppermost sheet one by one by rotation of a sheet feeding roller3, and then the picked-up sheet P is fed to a registration portion by feeding rollers4and5. The sheet P is subjected to uniformization of a feeding direction thereof at the registration portion consisting of registration rollers6and7, and thereafter is gradually fed to a transfer portion22constituted by the photosensitive drum9and a transfer roller2.

At the transfer portion22, a transfer nip N1is formed by the surface of the photosensitive drum9and the surface of the transfer roller2, 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 drum9is transferred onto the sheet P by a transfer bias applied to the transfer roller2. The sheet P after the toner image transfer thereon is completed is gradually fed to a fixing nip N2roughly along a sheet feeding guide18provided between the transfer portion22and the fixing portion14.

At the fixing portion14, the fixing nip N2is formed by a cylindrical fixing roller (rotatable heating member)30and a pressing roller (rotatable pressing member)19, and the sheet P fed from the transfer portion22is nipped and fed through the fixing nip N2. Further, in a feeding process of the sheet P, heat of the fixing roller30heated by a halogen lamp31provided inside the fixing roller30is 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 portion22to the fixing portion14, a curve sensor (sheet detecting portion)20provided inside the sheet feeding guide18is constituted to fall white following sliding with the sheet P. The curve sensor20is 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 N1and the fixing nip N2. The curve sensor20has a structure as shown inFIG. 18and includes a photo-sensor PS and a flag portion20F rotatable in contact with the sheet P. The photo-sensor PS includes an emitting portion LE for emitting the light and a light receiving portion LD for receiving the light, and a state in which the light from the emitting portion LE reaches the light receiving portion LD is a sensor OFF state (i.e., a state in which a controller100recognizes no sheet as described later). The flag portion20F is provided with a contact portion20bto which the sheet P is contacted, a rotation shaft20aand a light blocking portion LB for blocking the light from entering the light receiving portion LD of the photo-sensor PS. The flag portion20F is urged toward a home position by a spring24as shown inFIG. 16. Accordingly, before the sheet P abuts against the flag portion20F, the flag portion20F is kept in an attitude (home position) shown in (a) ofFIG. 18, and when the sheet P abuts against the flat portion20F, the flag portion20F is gradually rotated about the shaft20ainto attitudes shown in (b) and (c) ofFIG. 18. When the contact with the sheet P is eliminated, the attitude of the flag portion20F is returned from the attitude of (c) ofFIG. 18to the attitude of (a) ofFIG. 18by the urging force of the spring24. Each of (b) and (c) ofFIG. 8shows a state in which the light blocking portion LB blocks the incidence of the light into the light receiving portion LD, and this state is a sensor ON state (i.e., a state in which the controller100recognizes the presence of the sheet P). The attitude of the flag portion20F shown in (b) ofFIG. 18is an attitude immediately after the light blocking portion LB blocks the light and is an attitude of a boundary of turning-on of the sensor. The sensor is in an ON state from this attitude to the attitude of (c) ofFIG. 18.

Here, with reference toFIG. 1, a control system for controlling the sheet feeding speed at each of the transfer portion22and the fixing portion14will be described. An output signal from the photo-sensor PS is fetched into a feeding speed controller (control means)100constituted by a CPU and memories such as ROM and RAM. The feeding speed controller100effects, on the basis of the output signal, control of either one or both of a transfer portion driving motor M1and a fixing portion driving motor M2, so that the sheet feeding speed in at least one of the transfer portion22and the fixing portion14.

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 sensor20.

Specifically, when the sheet P in a sandwiched state between the fixing portion14and the transfer portion22turns on the sensor20, the sensor20is in an excessively loosen state, and therefore the sheet feeding speed in at least one of the transfer portion22and the fixing portion14is changed so as to eliminate the loosening of the sheet P. Thereafter, when the sensor20is 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 portion22and the fixing portion14is changed so as to loosen the sheet P. Such control in which the turning-on and the turning-off of the sensor20are repeated is effected, and therefore the attitude of the flag portion20F shown in (b) ofFIG. 18is the control point (boundary of speed adjustment). Further, a sheet feeding attitude as a control target is Ptarget shown in ofFIG. 1, and the attitude of the flag portion20F in this feeding attitude corresponds to the attitude shown in (b) ofFIG. 18.

The sheet P passing through the fixing portion14is discharged onto a sheet discharge tray17provided at an upper portion of an apparatus main assembly C by an intermediary sheet discharging roller pair15, a sheet discharging roller pair16, 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 path27for double-side printing by a feeding path switching mechanism (not shown) provided downstream of the fixing portion14with respect to a recording material (sheet) feeding direction. Then, in the feeding path27, 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 rollers6and7, fed to the transfer portion22. Then, the toner image is transferred from the surface of the photosensitive drum9onto the sheet P at the transfer nip N1of the transfer portion22, and thereafter the sheet P is fed to the fixing portion14. Then, the toner image transferred from the surface of the photosensitive drum9is heat-fixed on the sheet P at the fixing nip N2of the fixing portion14.

The sheet P passing through the fixing portion14is discharged onto the sheet discharge tray17provided at the upper portion of the apparatus main assembly C by the intermediary sheet discharging roller pair15, the sheet discharging roller pair16, and the like.

(2) Structure of Sheet Feeding Guide17in Comparison Example

A constitution of a sheet feeding guide18, in Comparison Example, for guiding the sheet P from the time when the sheet P passes through the transfer portion22to the time when the sheet P enters the fixing portion14will be described.

FIG. 8is a perspective view of the sheet feeding guide18in Comparison Example as seen from obliquely above the sheet feeding guide18in the upstream side of the sheet feeding direction.

The sheet feeding guide18has a guide surface18afor guiding the feeding of the sheet P, and the guide surface18ais constituted by a curved surface smoothly connecting the transfer portion22and the fixing portion14. The guide surface18ais provided with a plurality of ribs28provided in parallel along the sheet feeding direction thereof, and by a shape of the ribs, the guide surface18ais configured to be decreased in contact surface with the back surface of the sheet P.

At a central portion of the sheet feeding guide18with respect to a direction perpendicular to the sheet feeding direction, a curve sensor (sheet detecting portion)20is provided. The curve sensor20is set so as to be in stand-by in a predetermined position (home position) where the curve sensor20is projected from the guide surface18ain a certain amount by a spring24. Further, the curve sensor20is rotatable with the slide of the sheet P.

In a downstream side of the sheet feeding guide18with respect to the sheet feeding direction, a plurality of rollers29are provided smoothly rotatably over a widthwise direction of the sheet P. These rollers29not only have the function of smoothly delivering the sheet P to an entrance guide21toward the inside of the fixing portion14but also reduce a degree of abrasion of the ribs28due to sliding between the sheet feeding guide18and the back surface of the sheet P.

Next, with reference toFIGS. 9 and 10, a behavior of the sheet P passing through the sheet feeding guide18in Comparison Example will be described in detail.

FIG. 9is a schematic view showing the behavior of the sheet P passing through the sheet feeding guide18in Comparison Example. InFIG. 9, between the transfer portion22and the fixing portion14, the sheet feeding guide18having the guide surface18aas a smoothly curved surface and the entrance guide21for introducing the sheet P into the fixing nip N2are provided. Further, above the guide surface18aof the sheet feeding guide18, an upper feeding guide23for regulating the feeding of the sheet P into the fixing nip N2is provided.

As shown inFIG. 9, the sheet P on which an unfixed toner image (not shown) transferred at the transfer portion22is carried is fed roughly along the guide surface18aof the sheet feeding guide18. When the sheet P is further fed, the sheet P passes through the curve sensor20and the entrance guide21and then is nipped at the fixing nip N2to be placed in a state as shown inFIG. 10. At this time, the curve sensor20contacts the back surface (opposite from the unfixed toner image-carrying surface) of the sheet P. Then, the feeding speed controller100adjusts the sheet feeding speed at the fixing portion14so as to maintain the control point of the curve sensor20and thus intends to feed the sheet P while maintaining the sheet attitude in the target attitude by the transfer nip N1and the fixing nip N2.

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.

FIG. 11shows an example of a state in which the sheet P causes the non-uniform curve.FIG. 11is a top plan view of the sheet P as seen from above the sheet feeding guide18shown inFIG. 10. InFIG. 11, a state of the case where the right-side corner portion of the leading end of the sheet P enters the fixing nip N2earlier is shown, thus assuming a state such that the sheet P is obliquely distorted (non-uniformly curved state).

FIG. 12shows an example of contact of the sheet P, causing the non-uniform curve with the sheet feeding guide18. When the non-uniform curve is generated on the sheet P, a part of the sheet P contacts the guide surface18aof the sheet feeding guide18in some cases although the part should be originally floated from the guide surface18a(“a” inFIG. 18). At this time, the sheet P and the curve sensor20are spaced from each other. In such a state, with respect to the control of the curve sensor20, the feeding speed controller100discriminates that the fixing portion14pulls the sheet P more than necessary, and increases the sheet feeding speed at the fixing portion. As a result, the state of the non-uniform curve becomes worse, and the sheet P is further flexed over the guide surface18aas shown inFIG. 13, so that the unfixed toner image-carrying surface of the sheet P slides with the upper feeding guide23(“b” inFIG. 13) and thus image defect occurs.FIG. 13is an illustration of the case where the non-uniform curve of the sheet P is accelerated.

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 surface18aof the sheet feeding guide18,

2) The attitude of the sheet P is disordered by the contact with the guide surface18ato space the sheet P from the curve sensor20, and

3) The curve sensor20erroneously detects the attitude of the sheet P, so that the non-uniform curve is further accelerated.

The sheet P shown inFIG. 13shows a profile of the sheet P at the right-side end portion with respect to the widthwise direction of the sheet P, and the sheet P is distorted in actuality, and therefore is not uniform with respect to the widthwise direction of the sheet P. In either case, it is characterized that a portion where a distance between the sheet P and the sheet feeding guide18when the non-uniform curve is generated is closest is either one of the left and right end portions with respect to the widthwise direction of the sheet P.

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.

(3) Structure of Sheet Feeding Guide18in Embodiment 1

Therefore, in Embodiment 1, a constitution in which the sheet P does not contact the sheet feeding guide18even when the sheet P causes the non-uniform curve was employed. Specifically, the guide surface18aof the sheet feeding guide18was set at a low level so as not to cause separation of the sheet P from the curve sensor20due to the contact of the sheet P with the sheet feeding guide18.

FIG. 14is a schematic view showing the sheet feeding guide18configured to prevent the speed P causing the non-uniform curve from contacting the sheet feeding guide18. InFIG. 14, the guide surface18aof the sheet feeding guide18in the constitution of Comparison Example is also indicated by a chain double-dashed line. As shown inFIG. 14, the flexed portion (“A” inFIG. 14) of the non-uniform curved sheet P is positioned below the guide surface indicated by the chain double-dashed line as the guide surface18ain Comparison Example, but does not reach a guide surface18bin Embodiment 1 indicated by a solid line. In this state, there is no extreme disorder of the attitude of the sheet P, and therefore the curve sensor20follows the sliding with the back surface of the sheet P. As a result, the sheet feeding speed control at the fixing portion14is properly effected, and thus there is no generation of the acceleration of the non-uniform curve.

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 surface18bwas set so as to be lower than that of the guide surface18ain Comparison Example by about 5 mm at the maximum.

On the other hand, by lowering the guide surface of the sheet feeding guide18, also a feeding path of the leading end of the sheet P is changed.FIG. 15shows a behavior of the leading end of the sheet P (sheet end) before and after the change in level of the guide surface of the sheet feeding guide18. The sheet end feeding path before the change is represented by a broken line P1, and the sheet end feeding path after the change is represented by a solid line P2.

As shown inFIG. 15, the feeding path of the sheet end passing through the transfer portion22in this embodiment assumes a behavior such that the sheet end moves along the feeding path lower than the guide surface in Comparison Example. At this time, the sheet end contacts the upstream surface20bof the curve sensor20with respect to the sheet feeding direction. The shaft20ais a rotation supporting point of the curve sensor20. The curve sensor20(flag portion20F) is urged by the spring (elastic member)24in a direction opposite to the sheet feeding direction, so that the sheet P moves against a spring force of the spring24while rotating the curve sensor20in the sheet feeding direction. That is, compared with the guide surface18abefore the change in level, a force necessary to pass the sheet P through the curve sensor20is increased.

FIG. 16shows a behavior of the curve sensor20before and after the change in level of the guide surface of the sheet feeding guide18.FIG. 17is an enlarged perspective view of the curve sensor20positioned in a stand-by position.

As shown inFIG. 16, after a trailing end of a (preceding) sheet P3passes through the curve sensor20, the curve sensor20is returned to the stand-by position (indicated by a solid line inFIG. 16) by the spring force of the spring24(FIG. 17). In some cases, before this returning operation of the curve sensor20is completed, a leading end of a subsequent sheet P4abuts against the surface20bof the curve sensor20. In such cases, an impact applied to the leading end of the subsequent sheet P4is further increased.

This is because a feeding position of the trailing end of the sheet P3is also lowered by lowering the guide surface to result in an increase in rotation amount of the curve sensor20and 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 sensor20. That is, a rotation locus of the curve sensor20inFIG. 16is increased from C1 to C2.

In this way, when the impact between the leading end of the subsequent sheet P4and the curve sensor20during the contact therebetween is increased, there is also a possibility that the unfixed toner image in the leading end side of the subsequent sheet P4is scattered.

Therefore, in this embodiment, a constitution in which also the impact when the leading end contacts the flag portion20F is suppressed was employed. InFIG. 1, (a) and (b) show the sheet feeding guide18in this embodiment.FIG. 2is an enlarged perspective view of the sheet feeding guide18shown inFIG. 1.

Specifically, as shown inFIG. 1, the guide surface18cof the sheet feeding guide18in the neighborhood of the curve sensor20is projected relative to the guide surface18bat portions other than the neighborhood of the curve sensor20. That is, with respect to the sheet feeding direction, the guide surface in the position where the sheet detecting portion (curve sensor)20is provided is shaped, with respect to a direction perpendicular to the sheet feeding direction, such that a first region where the curve sensor20is provided is higher (in level) than second regions18beach remoter from the curve sensor20than the first region18c. A height position of the guide surface18cin the neighborhood of the curve sensor20is kept at the same level as the guide surface18ain Comparison Example. The projected-shaped guide surface18cis provided with a stepped portion18c1at each of ends thereof with respect to the widthwise direction of the sheet P (FIGS. 2 and 4).

By forming the guide surface18cin the projected shape, a tilt angle of the curve sensor20during 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 sensor20before the returning operation of the curve sensor20is completed. As a result, scattering of the unfixed toner image in the leading end side of the subsequent sheet was suppressed.

Further, as shown inFIG. 2, the rotatable roller20dwas disposed at the end portion, of the curve sensor20, as a sliding portion (sheet sliding position) of the curve sensor20with the sheet P. As a result, a feeding property of the sheet P is stabilized. In addition, a degree of the sliding between the curve sensor20and the sheet P is minimized so that also an increase in amount of triboelectric charges generated by the sliding of the sheet P on the curve sensor20is suppressed and thus the unfixed toner image on the sheet P is prevented from being electrically discharged and disordered.

FIG. 3is a perspective view of the sheet feeding guide18in this embodiment. As shown inFIG. 3, end portions of the guide surfaces18bwhere a distance therefrom to the sheet P during generation of the non-uniform curve are lower (in level) than the guide surface18cin the neighborhood of the curve sensor20in order to prevent the end portions from sliding with the widthwise end portions of the sheet P when the sheet P causes the non-uniform curve. In this embodiment, the projected-shaped guide surface18cin the neighborhood of the curve sensor20was about 80 mm in dimension (width) with the flag of the curve sensor20as the center (FIGS. 3 and 5). InFIG. 5, the center of the flag is represented by20c. For reference, a B-B cross section inFIG. 3is shown inFIG. 4.

FIG. 5is an illustration of a rib shape of the sheet feeding guide18, and is a top plan view of the sheet feeding guide18having the guide surfaces18band18c.

The shape of the plurality of ribs28aprovided on the guide surfaces18bare, as shown inFIG. 5, disposed in a line-symmetrical manner with respect to the sheet widthwise center line20cand extend radially. That is, the ribs28aextend outward in an open direction toward the downstream side of the sheet feeding direction. This is because the influence of the sliding of the sheet P, at the widthwise end portions thereof, with the ribs28on the sheet P feeding performance is eliminated. For example, the sheet P after the fixing causes curl in some cases. When the sheet P having the curl is turned upside down for performing the double-side printing and then is fed again from the transfer portion22to the fixing portion14, also positions of the widthwise end portions of the sheet P are liable to vary not a little.

As in this embodiment, the ribs28aare 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 surfaces28a1of the ribs28ais eliminated. For that reason, it is possible to prevent the sheet P from being caught by the ribs28ato skew and from causing corner creases.

However, within the projected-shaped guide surface18cin the neighborhood of the curve sensor20, the ribs28care shaped in a straight shape in parallel to the feeding direction of the sheet P. A dimension of the projected-shaped guide surface18cis made smaller than a width of a minimum regular-sized sheet P usable in the image forming apparatus. In a region of the guide surface18cin which there is no fear of the sliding with the widthwise end portions of the sheet P, by shaping the ribs28bso 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 ribs28bwhen the sheet P passes through the sheet feeding guide18to 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 sensor20.

As described above, by constituting the guide surfaces18band18cof the sheet feeding guide18as in the above-described manner, the impact applied from the curve sensor20onto the sheet P was capable of being ensured so as to be comparable to or more than that in Comparison Example. Further, the guide surfaces18bcorresponding 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 surface18cin the neighborhood of the curve sensor20, and therefore also a degree of tolerance with respect to the non-uniform curve of the sheet P can be maintained.

Here, as shown inFIG. 1, the position of the curve sensor20is downstream of intersection point X of a nip line25of the transfer nip N1and a nip line26of the fixing nip N2with respect to the sheet feeding direction. In other words, the intersection point X is positioned between the transfer nip N1and the curve sensor20. Here, the nip line refers to a line segment which is perpendicular to a line segment connecting roller centers for an associated one of the transfer nip N1and the fixing nip N2and which is a tangential line of the associated rollers.

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 guide18caused due to the non-uniform curve is highest by disposing the guide surface18bsuch that a point substantially below the intersection line X is a lowest point of the guide surface18b. Further, the curve sensor20is 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) ofFIG. 1. Further, the region H may more preferably be provided substantially just below the intersection line X of the nip lines of the transfer and fixing nips. Incidentally, the region H in Comparison Example shown inFIG. 8is positioned in the downstream side of the sheet detecting portion with respect to the sheet feeding direction.

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) ofFIG. 1is a largest (deepest) distance Dmax. In the image forming apparatus in this embodiment, a length of the guide surface (i.e., a length of a curved surface along the guide surface) from the transfer nip to the fixing nip is 170 mm, but the region H where the distance is Dmax may preferably be 20-50 mm. Further, the distance Dmax may preferably be 10-20 mm. In the image forming apparatus in this embodiment, the length of the region H (with respect to the sheet feeding direction) is set at 30 mm, and Dmax is set at 12 mm.

Further, with respect to the sheet P passing through the transfer portion22, by passing the leading end of the sheet P through the guide surface18band then the curve sensor20without directly contacting the curve sensor20, 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 sensor20.

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 guide18. 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 sensor20, 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 sensors20with 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 surface18cof the sheet feeding guide18is 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 surface18cof the sheet feeding guide18is the projected shape having the stepped portions18c1in both sides thereof with respect to the sheet feeding direction as shown inFIG. 4, but in this embodiment, the shape of the guide surface18cis a projected shape providing a smoothly curved surface as a whole as shown inFIG. 6.

FIG. 6is a schematic view showing a longitudinal cross-section of the guide surface18cof the sheet feeding guide18in the image forming apparatus in this embodiment. A cross-sectional direction of the guide surface18cshown inFIG. 6corresponds to the B-B cross section inFIG. 3.

As in this embodiment, even when the guide surface18cis 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 guide18.

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 surface18cin cross-sectional shape, and therefore the shape of the guide surface18calso 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 surface18cof the sheet feeding guide18may 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 sensor20without causing a heavy impact which otherwise might be caused by a conventional contact between the sheet P and the sheet feeding guide18even when the sheet P causes the non-uniform curve. Further, the feeding attitude of the sheet P from the transfer portion22to the fixing portion14can 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.

Another Embodiment

In place of the fixing roller30of the fixing portion14, 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.

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.