Image forming apparatus

Abstract

To provide an image forming apparatus that can control a bending amount according the various types of sheets with a simple configuration.SOLUTION: An image forming apparatus comprises: a transfer unit that conveys, while sandwiching, a sheet and transfers a toner image to the sheet; a fixing unit that conveys, while sandwiching, the sheet and fixes the toner image transferred by the transfer unit to the sheet; a detection unit that is arranged between the transfer unit and the fixing unit in a sheet conveyance direction, and detects the amount of bending of the sheet sandwiched by the transfer unit and the fixing unit; and a control unit that controls the convey speed of the sheet conveyed by the fixing unit on the basis of the result of detection made by the detection unit. The detection unit has a rotating part that is pressed by the sheet sandwiched by the transfer unit and the fixing unit to rotate, and a rotary encoder that outputs a pulse signal according to the amount of rotation of the rotating part.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus for forming an image on a sheet. [Background technology]

[0002] Conventionally, a loop detection sensor has been proposed for detecting whether or not a loop formed on a sheet by a transfer unit and a fixing roller has reached a certain amount (see Patent Document 1). The loop detection sensor has a mechanical flag that rotates when it comes into contact with a sheet, and a photointerrupter that can transition between a light-shielding state and a light-transmitting state as the mechanical flag rotates.

[0003] Also, an image forming apparatus has been proposed in which two light-transmitting loop detection sensors are provided between a secondary transfer unit and a fixing device (see Patent Document 2). The image forming apparatus includes an actuator that rotates by contacting a sheet, and the actuator has two protrusions that can block the optical axes of the two loop detection sensors. These two loop detection sensors output an OFF signal when the optical axes are blocked by the protrusions, and output an ON signal when the optical axes are open. This image forming apparatus can detect four types of loop amounts by combining the signals of these two loop detection sensors. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2005-181507 A [Patent Document 2] JP 2007-041188 A Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, there are various types of sheets used in image forming apparatuses, such as thin paper, regular paper, and thick paper. It is known that the appropriate loop amount varies depending on the type of sheet. However, the loop detection sensor in Patent Document 1 can only detect one type of loop amount, and it is difficult to control the loop amount (deflection amount) according to various types of sheets.

[0006] Furthermore, the image forming apparatus described in Patent Document 2 requires that the two loop detection sensors and the actuator having two protruding pieces be positioned so as not to cause erroneous detection, resulting in a complex configuration.

[0007] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS An object of the present invention is to provide an image forming apparatus that is capable of controlling the amount of sagging in accordance with various types of sheets with a simple configuration. [Means for solving the problem]

[0008] The present invention is characterized in that an image forming apparatus includes a transfer unit that clamps a sheet while transporting it and transfers a toner image onto the sheet, a fixing unit that clamps a sheet while transporting it and fixes the toner image transferred by the transfer unit onto the sheet, a detection unit that is arranged between the transfer unit and the fixing unit in the sheet transport direction and detects the amount of deflection of the sheet clamped by the transfer unit and the fixing unit, and a control unit that controls the sheet transport speed by the fixing unit based on the detection result of the detection unit, wherein the detection unit has a rotating unit that rotates when pressed against the sheet clamped by the transfer unit and the fixing unit, and a rotary encoder that outputs a pulse signal according to the amount of rotation of the rotating unit. Effect of the Invention

[0009] According to the present invention, the amount of bending can be controlled in accordance with various types of sheets with a simple configuration. [Brief description of the drawings]

[0010] [Figure 1] 1 is an overall schematic view showing a cross-sectional configuration of an image forming apparatus. [Diagram 2] FIG. [Diagram 3] FIG. [Figure 4] FIG. [Diagram 5] FIG. [Figure 6] 11 is a graph showing a change in the amount of deflection of a sheet when deflection amount control is performed. [Figure 7] FIG. 4 is a schematic diagram showing a detection unit according to a comparative example. [Figure 8] 5A to 5C are schematic diagrams showing the operation of a detection unit according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [Overall configuration] 1 is an overall schematic diagram showing a cross-sectional configuration of an image forming apparatus 100 according to the present invention. The image forming apparatus 100 has an image forming section 140 that forms an image on a sheet S, which is a recording material, a feeding unit 110, a fixing device 150, and an image reading device 102. The image forming apparatus 100 also has an apparatus main body 101, which is a housing that houses the image forming section 140.

[0012] The image forming section 140 is an electrophotographic unit of an intermediate transfer tandem type in which image forming stations Y, M, C, and Bk for forming four-color toner images are arranged along an intermediate transfer belt 145 .

[0013] The sheets S are stored in a cassette 111 provided at the bottom of the apparatus main body 101, and are fed one by one by a feeding unit 110. The feeding unit 110 used may have, for example, a feeding roller that feeds the sheets S, and a separation roller that is disposed in contact with the feeding roller and separates the sheets S fed by the feeding roller from the other sheets S by applying a frictional force to the sheets S. Note that, as the sheet S which is the recording material, various sheets of different sizes and materials can be used, such as paper such as plain paper and thick paper, sheet materials with surface treatments such as plastic film, cloth, and coated paper, and sheet materials with special shapes such as envelopes and index paper.

[0014] The sheet S fed from the feeding unit 110 has its skew corrected by the skew correction device 120, and is transported toward the transfer nip 130 at a timing synchronized with the toner image formation process by the image forming section 140. The transfer nip 130 as a transfer section is a nip portion formed between a secondary transfer inner roller 131 and a secondary transfer outer roller 132 that are substantially opposed to each other with an intermediate transfer belt 145 sandwiched therebetween, and transports the sheet while sandwiching it.

[0015] In parallel with the above-described conveyance process of the sheet S to the transfer nip 130, the image forming unit 140 executes a toner image forming process. Each of the image forming stations Y, M, C, and Bk of the image forming unit 140 includes a photosensitive drum 141, which is a drum-shaped image carrier (electrophotographic photosensitive member), a charging unit such as a charging roller, and a developing unit 143 as a developing means. The image forming unit 140 also includes an exposure device 142 disposed below the four photosensitive drums 141. In the toner image forming process, the charging unit uniformly charges the surface of the photosensitive drum 141, and the exposure device 142 exposes the photosensitive drum 141 based on a signal of image information to be formed, thereby writing an electrostatic latent image on the surface of the photosensitive drum 141. This electrostatic latent image is developed by toner supplied from the developing unit 143 to become a monochromatic toner image. As a result, four toner images of yellow, magenta, cyan, and black are formed on the surfaces of the four photosensitive drums 141.

[0016] The intermediate transfer belt 145 is driven to rotate in a counterclockwise direction in FIG. 1. The toner images carried on the four photosensitive drums 141 are primarily transferred by the primary transfer roller 144 to the intermediate transfer belt 145 in sequence so as to be superimposed on one another. As a result, a full-color toner image is finally formed on the intermediate transfer belt 145, and is carried by the intermediate transfer belt 145 and conveyed to the transfer nip 130. Then, the toner image is secondarily transferred from the intermediate transfer belt 145 to the sheet S by the pressure and electrostatic bias in the transfer nip 130.

[0017] The sheet S that has passed through the transfer nip 130 is conveyed to the fixing device 150. The fixing device 150 has a fixing roller 155 with a built-in heater, and a pressure roller 156 that contacts the fixing roller 155 with a predetermined pressure. The fixing roller 155 is driven by a driving source such as a motor (not shown), and the pressure roller 156 rotates following the fixing roller 155. The fixing device 150 applies pressure and heat to the toner image on the sheet S while nipping and conveying the sheet S with a fixing nip 157 as a fixing section formed by the fixing roller 155 and the pressure roller 156. This melts the toner, and the toner is fixed after passing through the fixing nip, thereby obtaining an image fixed on the sheet S.

[0018] The sheet S that has passed through the fixing device 150 is guided by the first guide member 151 to either a path toward the first discharge roller pair 160 or a path toward the second discharge roller pair 161. When images are formed on both sides of the sheet S, the sheet S with an image formed on the first side is guided by the first guide member 151 toward the second discharge roller pair 161 and is conveyed toward the outside of the machine by the second discharge roller pair 161. When the rear end of the sheet S in the conveying direction passes through the second guide member 152, the second discharge roller pair 161 reverses the conveying direction of the sheet S and sends the sheet S to the double-sided conveying path 180. The part of the sheet S that protrudes outside the device main body 101 during the reversing operation by the second discharge roller pair 161 is supported by the second discharge tray 171. The sheet S that reaches the skew correction device 120 again via the double-sided conveying path 180 passes through the transfer nip 130 and the fixing device 150, after which an image is formed on the second side.

[0019] When discharging the sheet S, the sheet S sent out from the fixing unit 150 is guided by the first guide member 151 to the first discharge roller pair 160, and is discharged to the outside of the apparatus main body 101 by the first discharge roller pair 160. A first discharge tray 170 is provided on the upper part of the apparatus main body 101, and the sheet S discharged by the first discharge roller pair 160 is stacked on the first discharge tray 170. The upper surfaces of the first discharge tray 170 and the second discharge tray are inclined upward toward the downstream in the sheet discharge direction. The sheet S stacked on the first discharge tray 170 or the second discharge tray 171 slides upstream in the sheet discharge direction along the inclination of the first discharge tray 170 or the second discharge tray 171 due to its own weight. An alignment surface extending in the vertical direction is provided on the upstream side of the first discharge tray 170 and the second discharge tray 171 in the sheet discharge direction. The rear end of the sheet S sliding along the inclination of the first discharge tray 170 or the second discharge tray 171 hits the alignment surface, thereby aligning the position of the sheet stack loaded on the first discharge tray 170 or the second discharge tray 171.

[0020] The image forming apparatus 100 includes an image reading device 102 mounted on the upper part of the apparatus main body 101. The image reading device 102 includes a platen glass on which an original is placed, and an image sensor that reads the image of the original through the platen glass. The image reading device 102 also includes an automatic document feeder that feeds the originals set in the document tray one by one and allows the image to be read by the image sensor. The image forming apparatus 100 of this embodiment has a so-called internal discharge type configuration in which an internal discharge space 190 for the sheet S is provided between the image forming unit 140 and the image reading device 102 in the vertical direction. The internal discharge type configuration has an advantage that the area occupied by the image forming apparatus 100 as viewed from above can be made smaller than a configuration in which, for example, the first discharge tray 170 is provided on the side of the apparatus main body 101 and the discharge space for the sheet is disposed on the side of the apparatus main body 101.

[0021] Further, the image forming unit 140 described above is one example of an image forming unit, and for example, a direct transfer type electrophotographic unit in which a toner image formed on a photoreceptor is transferred to a sheet without an intermediate transfer body may be used.

[0022] [Detection unit] Next, the detection unit 200 disposed between the transfer nip 130 and the fixing nip 157 in the sheet conveying direction D1 and its surrounding configuration will be described. Fig. 2 is a cross-sectional view showing the detection unit 200. Fig. 3 is a perspective view showing the detection unit 200 and a conveying guide 210. Fig. 4 is a side view showing the detection unit 200 in a standby state. Fig. 5 is a side view showing the detection unit 200 in a detection state.

[0023] 2, the sheet S is conveyed in the sheet conveying direction D1 by the transfer nip 130. A conveying guide 210 and a detection unit 200 are disposed between the transfer nip 130 and the fixing nip 157 in the sheet conveying direction D1. The conveying guide 210 is disposed only on the non-image side of the sheet S so as not to disturb the unfixed image on the sheet S. In other words, no conveying guide is provided on the image side where the sheet S abuts against the intermediate transfer belt 145, and the image side of the sheet S does not rub against any member of the apparatus main body 101 between the transfer nip 130 and the fixing nip 157.

[0024] The conveying guide 210 as a guide member is composed of a first guide portion 210a, a second guide portion 210b, and a third guide portion 210c. The first guide portion 210a, the second guide portion 210b, and the third guide portion 210c may be integrally formed with each other or may be formed separately. The first guide portion 210a supports a plurality of driven rollers 250 that are driven to rotate while rubbing against the non-image surface of the sheet S. The plurality of driven rollers 250 are arranged side by side in the width direction W as shown in FIG.

[0025] The conveying guide 210 is configured so that the sheet S is not guided in a straight line between the transfer nip 130 and the fixing nip 157, but can bend toward the conveying guide 210 as shown by the dashed line in FIG. 2. In this manner, the conveying guide 210 forms a loop forming space SP in which the sheet S can bend. Here, in this embodiment, the term "bending" or "forming a loop" refers to the sheet S being curved rather than straight when viewed in the direction of the rotation axis of the outer secondary transfer roller 132. That is, in this embodiment, "loop" refers to the bending of the sheet S.

[0026] As shown in FIG. 3, the detection unit 200 is disposed at the center of the loop formation space SP in the width direction W. In other words, the detection unit 200 is disposed at the center of the conveying path between the transfer nip 130 and the fixing nip 157 in the width direction W. The width direction W is a direction perpendicular to the sheet conveying direction D1 and parallel to the rotation axis direction of the secondary transfer outer roller 132 and the pressure roller 156. The detection unit 200 can detect the amount of sagging of the sheet S in the loop formation space SP. The amount of sagging indicates the amount (distance) of sagging of the sheet S from a linear state toward the loop formation space SP as viewed in the width direction W. For example, the amount of sagging can be expressed as the distance from a straight line connecting the contact points between the multiple driven rollers 250 and the sheet S and the fixing nip 157 to the apex of the sagging (loop) formed in the sheet S.

[0027] The detection unit 200 is disposed at a position approximately midway between the transfer nip 130 and the fixing nip 157 in the sheet conveying direction D1, and can accurately detect the amount of sagging of the sheet S. In addition, the detection unit 200 is disposed at the center in the width direction W of the loop formation space SP, and can therefore detect the amount of sagging of sheets of sizes ranging from the minimum size to the maximum size usable by the image forming apparatus 100.

[0028] 4, the detection unit 200 has a first flag 201 as a first rotating member, a second flag 205 as a second rotating member, and a sensor 207. The first flag 201 is supported rotatably about a first rotating shaft 203, and has an abutment portion 201a, a butting portion 201b, and a spring support portion 201c. The abutment portion 201a is configured to be able to abut or rub against the second flag 205, and one end of a first flag spring 202 is attached to the spring support portion 201c.

[0029] The first flag spring 202 as the first biasing member is composed of, for example, a torsion coil spring, and the coil portion of the first flag spring 202 is supported by the first rotating shaft 203. One end portion extending in one direction from the coil portion of the first flag spring 202 is engaged with the spring support portion 201c, and the other end portion extending in the other direction from the coil portion is engaged with a fixed member of the device body 101. By this first flag spring 202, the first flag 201 is biased in the F1 direction as the first rotation direction around the first rotating shaft 203.

[0030] The second flag 205 is supported rotatably around a second rotating shaft 206 different from the first rotating shaft 203, and has a detected portion 205b. The second flag spring 251 as a second biasing member is, for example, a torsion coil spring, and the coil portion of the second flag spring 251 is supported by the second rotating shaft 206. One end portion extending from the coil portion of the second flag spring 251 is engaged with a spring support portion (not shown) of the second flag 205, and the other end portion extending from the coil portion is engaged with a fixed member of the device body 101. The second flag spring 251 biases the second flag 205 in the F2 direction as a third rotation direction around the second rotating shaft 206.

[0031] As shown in FIG. 4, when the detection unit 200 is in the standby state, at the contact point T between the abutting portion 201a and the second flag 205, the biasing force F20 by the second flag spring 251 is set to be larger than the biasing force F10 by the first flag spring 202. That is, it satisfies F20 > F10. For this reason, the first flag 201 is biased in the CR1 direction opposite to the F1 direction about the first rotation axis 203, and is held in a state where the abutting portion 201b of the first flag 201 abuts against the stopper 204. The stopper 204 is supported by a fixing member of the apparatus main body 101, and regulates the rotation of the first flag 201 in the CR1 direction as the fourth rotation direction. At this time, the first flag 201 and the second flag 205 are respectively located at the standby positions.

[0032] As shown in FIG. 3, an opening 252 is provided in the first guide portion 210a, and the first flag 201 located at the standby position protrudes from the opening 252 into the loop formation space SP. Since the first flag 201 protrudes into the loop formation space SP in this way, it can contact the sheet S conveyed by the transfer nip 130 and the fixing nip 157.

[0033] As shown in FIG. 5, the first flag 201 is pressed by the sheet S sandwiched between the transfer nip 130 and the fixing nip 157, and rotates in the direction of arrow B (F1 direction) about the first rotation axis 203. In this way, when the first flag 201 rotates from the standby position in the direction of arrow B (F1 direction), it is assumed that the detection unit 200 is in the detection state.

[0034] Let the force in the F1 direction received by the first flag 201 from the sheet S be F31. When the detection unit enters the detection state, it satisfies F20 < F10 + F31. Then, as shown in FIG. 5, the abutting portion 201a of the first flag 201 presses the second flag 205, so that the second flag 205 rotates about the second rotation axis 206 in the direction of arrow C as the second rotation direction opposite to the F2 direction. The direction of arrow C is opposite to the direction of arrow B (F1 direction).

[0035] The first flag 201 and the second flag 205 stop at the contact point T when the biasing force F11 by the first flag spring 202, the biasing force F21 by the second flag spring 251, and the force F31 by the sheet S satisfy F21=F11+F31. The positions where the first flag 201 and the second flag 205 stop after rotating from their respective standby positions are set as detection positions.

[0036] In this way, the second flag 205 rotates from the standby position to the detection position in conjunction with the first flag 201 being pressed and rotated by the sheet S. The first flag 201 and the second flag 205 constitute a rotation unit 240 that rotates when pressed by the sheet S. The sensor 207 can detect the amount of rotation of the second flag 205 from the standby position to the detection position. For example, the sensor 207 is composed of a photointerrupter having a light-emitting element and a light-receiving element, and the sensor 207 and the detected portion 205b formed on the second flag 205 constitute a photoelectric rotary encoder 260. The light-emitting element of the photointerrupter is, for example, a light-emitting diode, and the light-receiving element is, for example, a phototransistor.

[0037] When the rotary encoder 260 is a transmission type encoder, the detected portion 205b has a plurality of slits that can transmit light emitted from the light-emitting element. When the rotary encoder 260 is a reflection type encoder, the detected portion 205b has a plurality of irregularities or slits that can or cannot reflect light emitted from the light-emitting element. The light-receiving element of the sensor 207 outputs a current according to the amount of light received, and the waveform shaping circuit in the rotary encoder 260 converts the waveform of the current into a pulse signal and outputs it as a voltage signal. In other words, the rotary encoder 260 outputs a pulse signal according to the amount of rotation of the rotating portion 240.

[0038] In this way, the rotary encoder 260 outputs a pulse signal according to the amount of rotation of the second flag 205, which rotates in conjunction with the first flag 201, so that the detection unit 200 can detect the amount of bending of the sheet. Note that the resolution of the rotary encoder 260 may be set arbitrarily, and the rotary encoder 260 of the detection unit 200 can detect the amount of loop of at least three or more sheets S.

[0039] [Deflection control] Next, the deflection amount control (loop amount control) of the sheet S by the control unit 300 will be described. FIG. 6 is a graph showing the change in the deflection amount of the sheet S when the deflection amount control is performed. The control unit 300 (see FIG. 1) has a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Registered Domain Access Memory). The CPU reads out and executes various programs stored in the ROM. The RAM is used as a working area for the CPU. The control unit 300 can perform deflection amount control that controls the sheet conveying speed by the fixing nip 157 based on the detection result of the detection unit 200.

[0040] In this embodiment, the conveying speed of the sheet S through the fixing nip 157 is set to be slightly slower than the conveying speed of the sheet S through the transfer nip 130. Therefore, the sheet S conveyed through the transfer nip 130 is conveyed so as to gradually bend after reaching the fixing nip 157. This is shown in the first region X1 of FIG. 6. The solid line in FIG. 6 shows the control of the amount of bending when thin paper with a relatively low stiffness is conveyed, and it is considered appropriate for thin paper to be conveyed with an amount of bending of value L1 between the transfer nip 130 and the fixing nip 157. If the conveying speed of the sheet S through the transfer nip 130 is speed V1 and the conveying speed of the sheet S through the fixing nip 157 is speed V2, in the first region X1, <V1となっている。

[0041] Then, as shown in the second region Y1 of FIG. 6, when the control unit 300 determines that the amount of deflection of the sheet S has reached the amount of deflection (L1) suitable for thin paper based on the detection result of the detection unit 200, the control unit 300 controls the fixing device 150 so that the speed V2 becomes the same value as the speed V1. Further, as shown in the third region Z1 of FIG. 6, the control unit 300 reduces the amount of deflection based on the detection result of the detection unit 200 until there is still a certain amount of deflection slightly before the rear end of the sheet S passes through the transfer nip 130. That is, in the third region Z1, the control unit 300 controls the fixing device 150 so that V1 < V2, and after the predetermined amount of deflection is reached, the control unit 300 controls the fixing device 150 so that V1 = V2.

[0042] In this way, the control unit 300 controls the fixing device 150 based on the detection result of the detection unit 200 so that the amount of deflection set according to the type of the sheet S is formed on the sheet S for at least a predetermined time. In FIG. 6, the value L3 is, for example, the amount of deflection set when transporting thick paper with relatively high stiffness, and the value L2 is the amount of deflection set when transporting plain paper with a stiffness intermediate between that of thin paper and thick paper.

[0043] Note that the thin paper is, for example, a sheet with a basis weight of 52 to 59 [g / m 2 , the plain paper is, for example, a sheet with a basis weight of 64 to 105 [g / m 2 , and the thick paper is, for example, a sheet with a basis weight of 106 to 300 [g / m 2 .

[0044] By the way, when performing primary transfer from the photosensitive drum 141 to the intermediate transfer belt 145, if the intermediate transfer belt 145 receives an external force from the sheet S at the transfer nip 130, there is a possibility that the position of the toner image transferred from the photosensitive drum 141 to the intermediate transfer belt 145 will shift. As a result, color misregistration may occur in the full-color toner image completed by overlapping the four-color toner images, resulting in image defects. Therefore, it is preferable that the intermediate transfer belt 145 is not subjected to an external force from the sheet S clamped by the transfer nip 130 and the fixing nip 157.

[0045] Furthermore, when the sheet S passes through the fixing nip 157, if the posture of the sheet S is unstable upstream of the fixing nip 157 in the sheet conveying direction D1, wrinkles may occur on the sheet S. Furthermore, if the sheet S sandwiched between the transfer nip 130 and the fixing nip 157 is in a pulled state, an unfixed image on the sheet S may become distorted when the trailing end of the sheet S passes through a pair of rollers located upstream of the transfer nip 130 in the sheet conveying direction D1. To prevent such image defects and wrinkles on the sheet, there is an appropriate amount of bending depending on the type, size, stiffness, etc. of the sheet S.

[0046] For example, generally, a sheet with higher stiffness has a larger reaction force in a bent state, and the intermediate transfer belt 145 receives a larger external force from the sheet S. For this reason, in the present embodiment, the amount of bending is set to target values ​​L1, L2, and L3 for each of thin paper, plain paper, and thick paper, and the amount of bending can be controlled to be an appropriate amount of bending according to the type of sheet. That is, when conveying a sheet S with a first stiffness, the control unit 300 controls the conveying speed of the sheet S through the fixing nip 157 so that the amount of bending of the sheet S becomes the first amount of bending. Also, when conveying a sheet S with a second stiffness smaller than the first stiffness, the control unit 300 controls the conveying speed of the sheet S through the fixing nip 157 so that the amount of bending of the sheet S becomes the second amount of bending larger than the first amount of bending.

[0047] This makes it possible to suppress image defects and wrinkles in the sheet even when various types of sheets are conveyed. The reason why the amount of sagging of the sheet S can be controlled to target different values ​​depending on the type of the sheet S is that the rotary encoder 260 outputs a pulse signal according to the amount of rotation of the second flag 205 and can detect the amount of sagging of the sheet S in real time with high accuracy.

[0048] In this embodiment, the target deflection amount is set in three stages, L1, L2, and L3, but is not limited to this. For example, four or more stages of deflection amount may be set depending on the type, size, and stiffness of the sheet.

[0049] [Comparative Example] Here, a detection unit 400 according to a comparative example will be described with reference to Fig. 7. Fig. 7 is a schematic diagram showing a detection unit 400 according to a comparative example. Note that the configuration other than the detection unit 400 is similar to that of the above-mentioned embodiment, so that the same configuration as that of the present embodiment is denoted by the same reference numerals in Fig. 7 and description thereof will be omitted.

[0050] 7, the detection unit 400 has a flag 305 that can rotate about a rotation axis 305a by being pressed by the sheet S sandwiched between the transfer nip 130 and the fixing nip 157, and a sensor 207. That is, the detection unit 400 according to the comparative example has only one flag, unlike the present embodiment in which two flags are provided.

[0051] The flag 305 is biased in the direction of the arrow C by a flag spring (not shown) and is positioned at the standby position by hitting a stopper (not shown). That is, the flag 305 always tries to return to the standby position with a predetermined biasing force of the flag spring. This predetermined biasing force of the flag spring acts as resistance to the sheet S pressing the flag 305.

[0052] In particular, in the section between the transfer nip 130 and the fixing nip 157, since the conveying guide 210 is provided only on the non-image side of the sheet S, depending on the stiffness of the sheet S, the force to push in the flag 305 may be insufficient, and the amount of bending of the sheet S may not be detected. In particular, with a sheet with a relatively low stiffness such as thin paper, the flag 305 may not be pushed in from the standby position against the biasing force of the flag spring, and there is a risk of detection failure by the detection unit 400.

[0053] [Functions of the 1st and 2nd flags] FIG. 8 is a schematic diagram showing the operation of the detection unit 200 according to this embodiment. In this embodiment, as shown in FIGS. 4, 5, and 8, the first flag 201 is biased in the F1 direction by the first flag spring 202, and the second flag 205 is biased in the F2 direction opposite to the F1 direction by the second flag spring 251. The first flag 201 and the second flag 205 are biased by the first flag spring 202 and the second flag spring 251 so as to press against each other. The first flag spring 202 biases the first flag 201 in the same direction as the direction in which the bent sheet S presses the first flag 201. In this embodiment, when the detection unit 200 is in a standby state, the biasing force F10 by the first flag spring 202 is set to be slightly smaller than the biasing force F20 by the second flag spring 251.

[0054] With this configuration, the sheet S pressing the first flag 201 can press the first flag 201 with a small force. That is, the force F31 that the first flag 201 receives from the sheet S, which is necessary for the first flag 201 to rotate in the F1 direction from the standby position, is sufficiently small. In other words, the sheet S contacting the first flag 201 receives a small resistance force from the first flag 201 when pressing the first flag 201, and even a sheet such as thin paper with low stiffness can reliably press the first flag 201. This reduces erroneous detection by the detection unit 200.

[0055] As described above, in the present embodiment, the amount of flexure of the sheet S is detected in multiple stages by the detection unit 200 including the rotary encoder 260, so that the amount of flexure can be controlled according to various types of sheets with a simple configuration. In addition, the rotary encoder 260 is configured to be relatively small, so that the degree of freedom in arrangement can be improved.

[0056] In addition, since the first flag 201 is biased by the first flag spring 202 in the same direction as the direction in which the first flag 201 rotates when pressed by the sheet S, the sheet S can press the first flag 201 from the standby position with a slight force. Therefore, even if the sheet S is, for example, thin paper with low stiffness, it is possible to reduce erroneous detection by the detection unit 200 and perform favorable control of the amount of flexure.

[0057] Furthermore, the first flag 201 and the second flag 205 are biased to contact each other by the biasing forces of the first flag spring 202 and the second flag spring 251, so that the first flag 201 and the second flag 205 are always in contact with each other. Therefore, the rotation of the first flag 201 is immediately transmitted to the second flag 205, and the sensor 207 can detect the amount of bending of the sheet S with high accuracy by detecting the amount of rotation of the second flag 205.

[0058] <Other embodiments> In the image forming apparatus of the present embodiment, the conveying path from the feeding unit 110 to the first discharge roller pair 160 and the second discharge roller pair 161 extends along the vertical direction, but is not limited thereto. For example, the present invention may be applied to an image forming apparatus in which at least a part of the conveying path conveys the sheet S in the horizontal direction.

[0059] In the present embodiment, the fixing unit 150 is configured by the fixing roller 155 and the pressure roller 156, but is not limited thereto. For example, instead of the fixing roller 155, an endless belt or film having a built-in heater may be used, or a belt having a heat generating layer that heats by electromagnetic induction may be used.

[0060] Further, the detection unit 200 has two flags, the first flag 201 and the second flag 205, but may have three or more flags. Furthermore, the detection unit 200 may have only one flag 305, as shown in the comparative example of FIG. 7. Further, only one detection unit 200 is disposed in the approximate center of the loop formation space SP in the width direction W, but multiple detection units 200 may be disposed in the loop formation space SP. More preferably, the two detection units 200 may be disposed in positions symmetrical with respect to the center of the loop formation space SP in the width direction W. This makes it possible to detect the bending of the sheet S in a twisted state.

[0061] In addition, in the present embodiment, the stopper 204 is disposed so as to abut against the first flag 201, but this is not limiting. For example, the stopper 204 may be disposed so as to abut against the second flag 205. That is, the stopper 204 may abut against either the first flag 201 or the second flag 205, thereby positioning the first flag 201 and the second flag 205 at their respective standby positions.

[0062] Furthermore, if the biasing forces of the first flag spring 202 and the second flag spring 251 are balanced in the natural state and the first flag 201 and the second flag 205 are held in the standby position, the stopper 204 may be omitted.

[0063] In the present embodiment, the second flag 205 is pushed by the first flag 201 rotating in the direction of the arrow B (F1 direction) to rotate in the direction of the arrow C opposite to the direction of the arrow B (F1 direction), but this is not limited thereto. For example, the second flag 205 may be pushed by the first flag 201 rotating in the direction of the arrow B (F1 direction) to rotate in the same direction (F2 direction) as the direction of the arrow B (F1 direction). In this case, the second flag spring 251 biases the second flag 205 in the direction of the arrow C. [Explanation of symbols]

[0064] 130: transfer unit (transfer nip) / 157: fixing unit (fixing nip) / 200: detection unit / 210: guide member (conveyance guide) / 240: rotation unit / 252: opening / 260: rotary encoder / 300: control unit / D1: sheet conveyance direction / S: sheet

Claims

1. a transfer section that conveys the sheet while sandwiching it and transfers the toner image onto the sheet; a fixing section that conveys the sheet while sandwiching it and fixes the toner image transferred by the transfer section onto the sheet; a detection unit disposed between the transfer unit and the fixing unit in a sheet conveying direction, the detection unit detecting an amount of slack in the sheet being sandwiched between the transfer unit and the fixing unit; a control unit that controls a sheet conveying speed by the fixing unit based on a detection result of the detection unit, The detection unit includes a rotation unit that rotates by being pressed against the sheet sandwiched between the transfer unit and the fixing unit, and a rotary encoder that outputs a pulse signal according to an amount of rotation of the rotation unit.

1. An image forming apparatus comprising:

2. the rotating unit includes a first rotating member that rotates when pressed by a sheet sandwiched between the transfer unit and the fixing unit, and a second rotating member that rotates when pressed by the first rotating member, The rotary encoder outputs a pulse signal corresponding to an amount of rotation of the second rotating member.

2. The image forming apparatus according to claim 1,

3. the detection unit is disposed at a center portion in a width direction perpendicular to the sheet conveying direction in a conveying path between the transfer unit and the fixing unit; 3. The image forming apparatus according to claim 2,

4. a guide member that forms the conveying path and guides the sheet; The first rotating member is disposed so as to protrude into the transport path from an opening provided in the guide member.

4. The image forming apparatus according to claim 3.

5. the control unit, when conveying a sheet having a first stiffness, controls a sheet conveying speed by the fixing unit so that the amount of bending of the sheet becomes a first amount of bending, and, when conveying a sheet having a second stiffness smaller than the first stiffness, controls a sheet conveying speed by the fixing unit so that the amount of bending of the sheet becomes a second amount of bending larger than the first amount of bending.

5. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

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