Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent an image defect.SOLUTION: An image forming apparatus comprises: a conveying unit that conveys, while sandwiching, a sheet in a sheet conveyance direction; a transfer unit that transfers a toner image to the sheet; a fixing unit that 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 the sheet conveyance direction, and detects, in multiple stages, the amount of bending of the sheet sandwiched by the transfer unit and the fixing unit; and a control unit that controls the conveyance speed of the sheet at the fixing unit on the basis of a detection result from the detection unit. The control unit can execute, while one sheet is sandwiched by the conveying unit, the transfer unit and the fixing unit, a mode for performing first processing of controlling the conveyance speed of the sheet at the fixing unit so as to adjust the bending amount to a first bending amount, and second processing executed after the first processing and of controlling the conveyance speed of the sheet at the fixing unit so as to adjust the bending amount to a second bending amount smaller than the first bending amount.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to an image forming apparatus that forms an image on a sheet.

Background Art

[0002] Conventionally, an image forming apparatus has been proposed that is provided with a loop detection sensor disposed between a transfer unit and a fixing device and that detects a loop formed on a sheet (see Patent Document 1). The loop detection sensor includes a photo interrupter and an actuator that rotates when pressed by the conveyed sheet. The photo interrupter is turned ON or OFF according to the displacement of the actuator. The conveyance speed of the sheet by the fixing device is controlled to a speed slower than the process speed or a speed faster than the process speed based on the detection result of the photo interrupter. Thereby, the amount of loop formed on the sheet is controlled so as to maintain a certain amount.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the image forming apparatus described in Patent Document 1, a sheet may form a loop also between the transfer unit and a conveyance unit provided upstream of the transfer unit in the sheet conveyance direction. In this case, the reaction force of the loop formed between the conveyance unit and the transfer unit and the reaction force of the loop formed between the transfer unit and the fixing device act so as to cancel each other out at the transfer unit. However, when the trailing edge of the sheet passes through the conveyance unit, the force acting on the sheet at the transfer unit changes abruptly, and the sheet may slip with respect to the transfer unit. When the sheet slips, the image on the sheet is disturbed, causing image defects.

[0005] An object of the present invention is to provide an image forming apparatus capable of suppressing image defects.

Means for Solving the Problems

[0006] In the image forming apparatus of the present invention, a conveyance unit that conveys a sheet in the sheet conveyance direction while sandwiching the sheet, a transfer unit that is disposed downstream of the conveyance unit in the sheet conveyance direction, conveys the sheet while sandwiching the sheet, and transfers a toner image to the sheet, and a fixing unit that is disposed downstream of the transfer unit in the sheet conveyance direction, conveys the sheet while sandwiching the sheet, and fixes the toner image transferred by the transfer unit to the sheet, a detection unit that is disposed between the transfer unit and the fixing unit in the sheet conveyance direction and detects the amount of deflection of the sheet sandwiched by the transfer unit and the fixing unit in multiple stages, and a control unit that controls the conveyance speed of the sheet by the fixing unit based on the detection result of the detection unit. The control unit executes a first process of controlling the conveyance speed of the sheet by the fixing unit so that the amount of deflection becomes a first amount of deflection while one sheet is sandwiched by the conveyance unit, the transfer unit, and the fixing unit, and a second process that is executed after the first process and controls the conveyance speed of the sheet by the fixing unit so that the amount of deflection becomes a second amount of deflection smaller than the first amount of deflection. It is characterized in that it is possible to execute a mode that performs the above.

Advantages of the Invention

[0007] According to the present invention, image defects can be suppressed.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Mode for Carrying Out the Invention

[0009] [Overall Configuration] FIG. 1 is an overall schematic view showing a cross-sectional configuration of an image forming apparatus 100 according to the present invention. The image forming apparatus 100 includes an image forming unit 150 that forms an image on a sheet S as a recording material, a feeding unit 110, a fixing unit 160, and an image reading device 102. Further, the image forming apparatus 100 includes a device main body 101 that is a housing for accommodating the image forming unit 150.

[0010] Note that the image forming apparatus includes a printer, a copier, a facsimile machine, and a multifunction machine, and refers to a device that forms an image on a sheet used as a recording medium based on image information input from an external PC or image information read from a document. In addition, in the case where an image forming apparatus has an option feeder, an image reading device, a sheet processing device, etc. connected to a main body having an image forming function, the entire system to which such attached devices are connected is also a type of image forming apparatus.

[0011] The image forming unit 150 is an intermediate transfer tandem type electrophotographic unit in which image forming stations Y, M, C, and Bk that form four-color toner images are arranged along an intermediate transfer belt 155. Note that in the present embodiment, the image forming unit 150 forms a full-color toner image, but is not limited thereto. For example, the image forming unit 150 may be configured to form a monochrome image on the sheet S.

[0012] The sheet S is housed in a cassette 111 provided at the lower part of the apparatus main body 101, and is fed one by one by a feeding unit 110. In the present embodiment, two feeding units 110 are provided side by side in the vertical direction. The feeding unit 110 includes, for example, a feeding roller for feeding the sheet S, and a separating roller that is disposed in contact with the feeding roller and separates other sheets S from the sheet S fed by the feeding roller by applying a frictional force to the sheet S. As the sheet S, which is a recording material, various sheets with different sizes and materials can be used, such as paper such as plain paper and cardboard, plastic film, cloth, a sheet material with a surface treatment such as coated paper, and a sheet material with a special shape such as an envelope and index paper.

[0013] The sheet S fed from the feeding unit 110 is conveyed to a pre-registration roller pair 121 by a pulling roller pair 114. The pre-registration roller pair 121 conveys the sheet S to a registration roller pair 131. The registration roller pair 131 is composed of a roller pair that forms a nip N3. The sheet S is corrected for skew by the nip N3 of the registration roller pair 131 as a conveying unit, and is conveyed toward the secondary transfer unit 156 at a timing synchronized with the toner image forming process by the image forming unit 150.

[0014] The secondary transfer unit 156 includes an intermediate transfer belt 155, and a secondary transfer inner roller 141 and a secondary transfer outer roller 142 that are substantially opposed to each other with the intermediate transfer belt 155 interposed therebetween. A transfer nip N1 is formed by the intermediate transfer belt 155 and the secondary transfer outer roller 142. The secondary transfer unit 156 as a transfer unit conveys the sheet S while sandwiching it at the transfer nip N1.

[0015] In parallel with the conveyance process of the sheet S up to the transfer nip N1 described above, the image forming unit 150 executes a toner image forming process. Each image forming station Y, M, C, Bk of the image forming unit 150 includes a photosensitive drum 151 which is a drum-shaped image carrier (electrophotographic photoreceptor), a charging unit such as a charging roller, and a developing device 153. Further, the image forming unit 150 has a laser scanner 152 disposed below the four photosensitive drums 151. In the toner image forming process, the charging unit uniformly charges the surface of the photosensitive drum 151, and the laser scanner 152 exposes the photosensitive drum 151 based on the signal of the image information to be formed, writing an electrostatic latent image on the surface of the photosensitive drum 151. This electrostatic latent image is developed by the toner supplied from the developing device 153 to become a monochromatic toner image. As a result, toner images of four colors, yellow, magenta, cyan, and black, are formed on the surfaces of the four photosensitive drums 151.

[0016] The intermediate transfer belt 155 as an image carrier is rotationally driven in the direction of arrow A in FIG. 1. The toner images carried on the four photosensitive drums 151 are sequentially primary transferred onto the intermediate transfer belt 155 so as to overlap each other by the primary transfer roller 154. As a result, finally, a full-color toner image is formed on the intermediate transfer belt 155, carried on the intermediate transfer belt 155, and conveyed to the transfer nip N1. Then, due to the pressure and electrostatic bias at the transfer nip N1, the toner image is secondary transferred from the intermediate transfer belt 155 to the sheet S.

[0017] The sheet S that has passed through the transfer nip N1 is conveyed to the fixing device 160. The fixing device 160 includes a fixing roller 158 with a built-in heater and a pressure roller 159 that contacts the fixing roller 158 with a predetermined pressing force. The fixing roller 158 is driven by a fixing drive motor 163 (see FIG. 4), and the pressure roller 159 rotates in a driven manner following the fixing roller 158. The fixing device 160 sandwiches and conveys the sheet S at the fixing nip N2 as a fixing portion formed by the fixing roller 158 and the pressure roller 159, and applies pressure and heat to the toner image on the sheet S. As a result, the toner melts, and after passing through the fixing nip N2 as the fixing portion, the toner adheres, thereby obtaining an image fixed on the sheet S.

[0018] The sheet S that has passed through the fixing device 160 is conveyed by a discharge conveyance unit 170 including a pair of discharge rollers 171. Then, the sheet S is discharged to the outside of the apparatus main body 101 by the pair of discharge rollers 171. A discharge tray 180 is provided above the apparatus main body 101, and the sheet S discharged by the pair of discharge rollers 171 is stacked on the discharge tray 180. The upper surface of the discharge tray 180 is inclined upward toward the downstream in the sheet discharge direction. The sheet S stacked on the discharge tray 180 slides upstream in the sheet discharge direction along the inclination of the discharge tray 180 due to its own weight. An alignment surface extending in the vertical direction is provided on the upstream side of the discharge tray 180 in the sheet discharge direction. When the rear end of the sheet S sliding along the inclination of the discharge tray 180 abuts against the alignment surface, the position of the stack of sheets S stacked on the discharge tray 180 is aligned.

[0019] When forming images on both sides of the sheet S, when the rear end of the sheet S with an image formed on the first side passes through the branch portion 172, the pair of discharge rollers 171 reverses the conveyance direction of the sheet S and feeds the sheet S into the duplex conveyance unit 190. The sheet S conveyed by the duplex conveyance unit 190 is conveyed again to the transfer nip N1 by the pair of extraction rollers 115, the pair of pre-registration rollers 121, and the pair of registration rollers 131. Then, the sheet S forms an image on the second side by passing through the transfer nip N1 and the fixing device 160, and is discharged to the discharge tray 180.

[0020] Note that the sheet S may be fed not only from the cassette 111 but also from the multi-tray 113. The sheets stacked on the multi-tray 113 are conveyed by the pair of conveyance rollers 112 through the pair of extraction rollers 115 to the pair of pre-registration rollers 121. The subsequent conveyance path of the sheet is the same as described above.

[0021] Incidentally, 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 document table glass on which a document is placed, and an image sensor that reads an image of the document through the document table glass. Further, the image reading device 102 includes an automatic document feeder for feeding the documents set on the document tray one by one and causing the image sensor to read the images. The image forming apparatus 100 of the present embodiment has a so-called in-body discharge type configuration in which an in-body discharge space 200 for the sheet S is provided between the image forming unit 150 and the image reading device 102 in the vertical direction. The in-body discharge type configuration has an advantage that the occupied area of the image forming apparatus 100 as viewed from above can be reduced as compared with a configuration in which, for example, a discharge tray 180 is provided on the side portion of the apparatus main body 101 and the discharge space for the sheet is arranged on the side of the apparatus main body 101.

[0022] Further, the above-described image forming unit 150 is an example of an image forming unit, and for example, a direct transfer type electrophotographic unit that transfers a toner image formed on a photoreceptor to a sheet without using an intermediate transfer member may be used.

[0023] [Bending Sensor] Next, with reference to FIGS. 2 and 3, the bending sensor 162 and its peripheral configuration will be described. FIG. 2 is a schematic diagram showing the bending sensor 162 and its peripheral configuration. FIG. 3 is an enlarged view showing the bending sensor 162.

[0024] As shown in FIG. 2, a registration roller pair 131 is disposed upstream of the transfer nip N1 in the sheet conveyance direction CD, and a leading edge detection sensor 132 is disposed upstream of the registration roller pair 131 in the sheet conveyance direction CD. The leading edge detection sensor 132 as a leading edge detection unit detects the position of the leading edge of the sheet S. A deflection sensor 162 as a detection unit is disposed between the transfer nip N1 and the fixing nip N2 in the sheet conveyance direction CD, and the deflection sensor 162 detects the amount of deflection of the sheet sandwiched between the transfer nip N1 and the fixing nip N2 in multiple stages.

[0025] In addition, in the sheet conveyance direction CD, a deflection forming space SP1 is provided between the registration roller pair 131 and the transfer nip N1, and a deflection forming space SP2 is provided between the transfer nip N1 and the fixing nip N2. The sheet S can be deflected in the deflection forming spaces SP1 and SP2, and the region of the sheet S located in the deflection forming space SP1 is defined as the first deflection region 211, and the region of the sheet S located in the deflection forming space SP2 is defined as the second deflection region 212.

[0026] Here, in the present embodiment, when viewed in the rotational axis direction of the secondary transfer outer roller 142 as an opposing member, the state in which the sheet S is not linear but curved is referred to as "deflected" or "forming a loop". The common tangent P1 at the nip N3 of the registration roller pair 131 intersects the common tangent P2 of the intermediate transfer belt 155 and the secondary transfer outer roller 142 at the transfer nip N1. Therefore, the sheet S can be deflected in the deflection forming space SP1 and is guided to the transfer nip N1 by the intermediate transfer belt 155.

[0027] The deflection sensor 162 is disposed at the center of the deflection forming space SP2 in the width direction orthogonal to the sheet conveyance direction CD. In other words, the deflection sensor 162 is disposed at the center of the conveyance path between the transfer nip N1 and the fixing nip N2 in the width direction. The width direction is a direction orthogonal to the sheet conveyance direction D1 and parallel to the rotation axis direction of the secondary transfer outer roller 142 and the pressure roller 159. The deflection sensor 162 can detect the amount of deflection of the sheet S in the deflection forming space SP2. The amount of deflection indicates the amount (distance) by which the sheet S is deflected toward the deflection forming space SP2 from the straight state when viewed in the width direction. For example, the amount of deflection can be expressed as the distance from the straight line connecting the transfer nip N1 and the fixing nip N2 to the apex of the deflection (loop) formed in the sheet S.

[0028] Further, since the deflection sensor 162 is disposed at the center in the width direction of the deflection forming space SP2, it can detect the amount of deflection of sheets from the minimum size to the maximum size that can be used in the image forming apparatus 100.

[0029] As shown in FIG. 3, the deflection sensor 162 has a flag 231 that can rotate about a rotation axis 233 and a sensor 232, which are pressed by the sheet S sandwiched between the transfer nip N1 and the fixing nip N2. The flag 231 as a rotating part is biased in the direction of arrow D by a spring (not shown), and is positioned at the standby position when the abutting part 234 provided on the flag 231 abuts against the sensor 232 or another stopper (not shown) different from the sensor 232. The flag 231 positioned at the standby position is located at a position where it is pressed by the sheet S conveyed by the transfer nip N1.

[0030] The flag 231 rotates about the rotation axis 233 in the direction of arrow B opposite to the direction of arrow D by being pressed by the sheet S sandwiched between the transfer nip N1 and the fixing nip N2. Thus, when the flag 231 rotates from the standby position in the direction of arrow B and moves to the detection position, the deflection sensor 162 is assumed to be in the detection state.

[0031] Then, the sensor 232 can detect the amount of rotation in the direction of arrow B from the standby position to the detection position of the flag 231. For example, the sensor 232 has a photo interrupter having a light emitting element and a light receiving element, and is composed of an optical rotary encoder. The light emitting element of the photo interrupter is, for example, a light emitting diode, and the light receiving element is, for example, a photo transistor.

[0032] When the sensor 232 is a transmissive encoder, the detected portion 231a of the flag 231 has a plurality of slits that can transmit the light emitted from the light emitting element. When the sensor 232 is a reflective encoder, the detected portion 231a has a plurality of unevenness or slits that can reflect or cannot reflect the light emitted from the light emitting element. The light receiving element of the sensor 232 outputs a current according to the amount of received light, and the waveform shaping circuit in the sensor 232 converts the waveform of the current into a pulse signal and outputs it as a voltage signal. In other words, the sensor 232 outputs a pulse signal according to the amount of rotation of the flag 231.

[0033] Thus, since the sensor 232 outputs a pulse signal according to the amount of rotation of the flag 231, the deflection sensor 162 can detect the amount of deflection of the sheet S. Note that the resolution of the deflection sensor 162 may be arbitrarily set, and the deflection sensor 162 can detect the amount of deflection of at least two or more sheets S.

[0034] [Control Block] FIG. 4 is a block diagram showing the control block of the image forming apparatus 100. The image forming apparatus 100 includes a control unit 201. The control unit 201 has a CPU (Central Processing Unit) 201a, a ROM (Read Only Memory) 201b, and a RAM (Random Access Memory) 201c. The CPU 201a reads and executes various programs stored in the ROM 201b. The RAM 201c is used as a work area for the CPU 201a.

[0035] The control unit 201 is connected to the tip detection sensor 132, the registration drive motor 133, the transfer drive motor 157, the fixing drive motor 163, the deflection sensor 162, and the operation unit 202. The registration drive motor 133 drives the registration roller pair 131. The transfer drive motor 157 drives the secondary transfer inner roller 141. The fixing drive motor 163 drives the fixing roller 158. By controlling these registration drive motor 133, transfer drive motor 157, and fixing drive motor 163, the control unit 201 can control the conveyance speed of the sheet S at the registration roller pair 131, the transfer nip N1, and the fixing nip N2. In particular, the control unit 201 controls the conveyance speed of the sheet (fixing conveyance speed) by the fixing nip N2 based on the detection result of the deflection sensor 162.

[0036] Note that the transfer drive motor 157 and the fixing drive motor 163 may drive the secondary transfer outer roller 142 and the pressure roller 159, respectively. That is, the transfer drive motor 157 and the fixing drive motor 163 may be configured to be able to control the conveyance speed of the sheet S at the transfer nip N1 and the fixing nip N2, respectively.

[0037] The operation unit 202 is composed of a touch panel, physical buttons, etc. attached to the exterior of the image forming apparatus 100. The user can input information such as the size and basis weight of the sheet S used in the image forming apparatus 100 by operating the operation unit 202.

[0038] [Deflection Amount Control] Next, with reference to FIGS. 5 to 8, the deflection amount control (loop amount control) of the sheet S by the control unit 201 will be described. FIGS. 5 and 6 are flowcharts showing the deflection amount control. FIG. 7 is a graph showing the change in the deflection amount of the sheet S when the deflection amount control is performed. FIG. 8 is a diagram showing the state when the rear end Sb of the sheet S passes through the registration roller pair 131.

[0039] As shown in FIG. 5, before starting a print job, the user inputs information such as the size and basis weight of the sheet S to be used from the operation unit 202 (step S11). Then, after the information is input, the print job is started (step S12). Then, the control unit 201 controls the fixing drive motor 163 so that the conveyance speed of the sheet S at the fixing nip N2 becomes the speed VLo0 (step S13). Hereinafter, the conveyance speed of the sheet S at the fixing nip N2 is referred to as the fixing conveyance speed. The fixing conveyance speed is the same as the peripheral speed of the fixing roller 158 and the pressure roller 159, and is controlled by adjusting the rotational speed of the fixing drive motor 163.

[0040] In the present embodiment, the speed VLo0 is lower than the speed VM which is the conveyance speed of the sheet S at the transfer nip N1, and is configured such that the sheet S is not pulled by the fixing nip N2 in the second bending region 212 (see FIG. 2). Thereby, scattering of the unfixed toner image transferred onto the sheet S can be suppressed.

[0041] Next, the control unit 201 abuts the leading edge of the sheet S against the nip N3 of the registration roller pair 131 in the stopped state to correct the skew of the sheet S, and then drives the registration drive motor 133 to rotate the registration roller pair 131 (step S14). Thereby, the sheet S is conveyed toward the transfer nip N1 and the fixing nip N2. Let the time when the registration roller pair 131 starts to rotate be the time t0.

[0042] Thereafter, the control unit 201 determines whether the time t1 has been reached (step S15). As shown in FIG. 2, the time t1 is the time when the leading edge Sa of the sheet S has passed through the fixing nip N2 and the trailing edge Sb of the sheet S has not passed through the nip N3 of the registration roller pair 131. The times t1 to t4 appearing below are calculated based on the time t0, the distances between the rollers, the length of the sheet in the sheet conveyance direction CD, and the rotational speeds of the rollers. And when the control unit determines that the time t1 has been reached (step S15: Yes),

[0043] Proceed to step S16. That is, the control unit 201 rotates the fixing roller 158 and the pressure roller 159 so as to maintain the fixing conveyance speed at the speed VLo0 until at least time t1. In the present embodiment, the speed VLo0 is set such that the amount of deflection in the second deflection region 212 at time t1 is slightly smaller than the first amount of deflection L1.

[0044] Next, the control unit 201 determines whether or not the amount of deflection in the second deflection region 212 is equal to or greater than the first amount of deflection L1 (step S16). The amount of deflection in the second deflection region 212 is constantly detected by the deflection sensor 162. If it is determined that the amount of deflection in the second deflection region 212 is equal to or greater than the first amount of deflection L1 (step S16: Yes), the control unit 201 sets the fixing conveyance speed to the speed VHi1 as the first speed (step S17) and proceeds to step S19. If it is determined that the amount of deflection in the second deflection region 212 is less than the first amount of deflection L1 (step S16: No), the control unit 201 sets the fixing conveyance speed to the speed VLo1 as the second speed (step S18) and proceeds to step S19. The first amount of deflection L1 is an appropriate amount of deflection for stably conveying the unfixed toner on the sheet.

[0045] The speed VHi1 is faster than the speed VM which is the conveyance speed of the sheet S at the transfer nip N1, and the speed VLo1 is slower than the speed VM. That is, when the fixing conveyance speed is set to the speed VHi1, the amount of deflection in the second deflection region 212 decreases, and when the fixing conveyance speed is set to the speed VLo1, the amount of deflection in the second deflection region 212 increases. In other words, the control unit 201 adjusts the amount of deflection in the second deflection region 212 by switching the fixing conveyance speed between the speed VHi1 and the speed VLo1.

[0046] Next, the control unit 201 determines whether the time has reached t2 (step S19). The time t2 is after the time t1, and the sheet S is sandwiched by the transfer nip N1 and the fixing nip N2, and the rear end Sb of the sheet S has not passed through the nip N3 of the registration roller pair 131. If the time has not reached t2 (step S19: No), the process returns to step S16. That is, from time t1 to time t2, the control unit 201 repeats the processes of steps S16 to S19, and controls the fixing conveyance speed so that the amount of deflection in the second deflection region 212 maintains the first amount of deflection L1. At this time, as shown in FIG. 2, at the transfer nip N1, the reaction force 221 of the deflection of the sheet S in the first deflection region 211 and the reaction force 222 of the deflection of the sheet S in the second deflection region 212 act so as to cancel each other out.

[0047] If it is determined that the time has reached t2 (step S19: Yes), the control unit 201 determines whether the basis weight of the sheet S input by the operation unit 202 is equal to or greater than the threshold basis weight w1, as shown in FIG. 6 (step S20). If it is determined that the basis weight of the sheet S is equal to or greater than the threshold basis weight w1 (step S20: Yes), the control unit 201 determines whether the amount of deflection in the second deflection region 212 is equal to or greater than a second amount of deflection L2 that is smaller than the first amount of deflection L1 (step S21).

[0048] If it is determined that the amount of deflection in the second deflection region 212 is equal to or greater than the second amount of deflection L2 (step S21: Yes), the control unit 201 sets the fixing conveyance speed to the speed VHi1 (step S22) and proceeds to step S27. If it is determined that the amount of deflection in the second deflection region 212 is less than the second amount of deflection L2 (step S21: No), the control unit 201 sets the fixing conveyance speed to the speed VLo1 (step S23) and proceeds to step S27.

[0049] On the other hand, if it is determined that the basis weight of the sheet S is less than the threshold basis weight w1 (step S20: No), the control unit 201 determines whether the amount of deflection in the second deflection region 212 is equal to or greater than the first amount of deflection L1 (step S24).

[0050] When it is determined that the amount of deflection in the second deflection region 212 is equal to or greater than the first deflection amount L1 (step S24: Yes), the control unit 201 sets the fixing conveyance speed to the speed VHi1 (step S25), and proceeds to step S27. When it is determined that the amount of deflection in the second deflection region 212 is less than the first deflection amount L1 (step S24: No), the control unit 201 sets the fixing conveyance speed to the speed VLo1 (step S26), and proceeds to step S27.

[0051] Next, in step S27, the control unit 201 determines whether the time has reached t3 (step S27). The time t3 is after the time t2 and is the time when the trailing edge Sb of the sheet S passes through the nip N3 of the registration roller pair 131. If the time has not reached t3 (step S27: No), the process returns to step S20. That is, from time t2 to time t3, the control unit 201 repeats the processes of steps S21 to S23 for the sheet S having a basis weight equal to or greater than the threshold basis weight w1, and controls the fixing conveyance speed so that the amount of deflection in the second deflection region 212 maintains the second deflection amount L2. Further, the control unit 201 repeats the processes of steps S24 to S26 for the sheet S having a basis weight less than the threshold basis weight w1, and controls the fixing conveyance speed so that the amount of deflection in the second deflection region 212 maintains the first deflection amount L1.

[0052] For example, FIG. 7 shows the amount of deflection in the second deflection region 212 when the amount of deflection control is executed for the sheet S having a basis weight equal to or greater than the threshold basis weight w1. The first deflection amount L1 is maintained from time t1 to time t2, and the second deflection amount L2 is maintained from time t2 to time t3.

[0053] When it is determined in step S27 that the time has reached t3 (step S27: Yes), the control unit 201 sets the fixing conveyance speed to the speed VM (step S28). In the present embodiment, in step S28, the fixing conveyance speed is set to the same speed VM as the conveyance speed of the sheet S at the transfer nip N1, but the present invention is not limited to this. For example, in step S28, the fixing conveyance speed may be set to any of the speeds VLo0, VHi1, and VLo1.

[0054] Next, the control unit 201 determines whether the time has reached t4 (step S29). The time t4 is after the time t3 and is the time when the trailing edge Sb of the sheet S passes through the fixing nip N2. If the time has not reached t4 (step S29: No), the process returns to step S28. When it is determined that the time has reached t4 (step S29: Yes), the control unit 201 stops the fixing drive motor 163 and stops the rotation of the fixing roller 158 and the pressure roller 159 (step S30). Thereafter, the sheet S is discharged outside the machine by the discharge roller pair 171, and the print job ends (step S31), and the deflection amount control also ends.

[0055] Incidentally, when the trailing edge Sb of the sheet S passes through the nip N3 of the registration roller pair 131 at time t3, as shown in FIG. 8, the deflection of the sheet S in the first deflection region 211 is released. Then, the reaction force 221 of the deflection of the sheet S in the first deflection region 211 (see FIG. 2) that acts in the direction of canceling the reaction force 222 of the deflection of the sheet S in the second deflection region 212 disappears. As a result, the force acting on the sheet S at the transfer nip N1 changes abruptly. The reaction force 221 tends to be larger as the stiffness of the sheet S is higher when the amount of deflection is the same. Therefore, when the trailing edge of a sheet such as thick paper with a large basis weight passes through the nip N3 of the registration roller pair 131, the force acting on the sheet at the transfer nip N1 changes more abruptly, and there is a risk that the sheet slips with respect to the transfer nip N1.

[0056] Therefore, in the present embodiment, for the sheet S with a basis weight input at the operation unit 202 that is equal to or greater than the threshold basis weight w1, before the trailing edge Sb exits the nip N3 of the registration roller pair 131, the fixing conveyance speed is controlled so that the amount of deflection becomes the second amount of deflection L2. More specifically, while one sheet S with a basis weight equal to or greater than the threshold basis weight w1 is being sandwiched between the registration roller pair 131, the transfer nip N1, and the fixing nip N2, the control unit 201 can execute a first mode (mode) that performs a first process and a second process. Further, when a sheet S with a basis weight less than the threshold basis weight w1 is being conveyed, the control unit 201 can execute a second mode that continues the first process until at least the trailing edge Sb of the sheet S exits the registration roller pair 131. In the second mode, since the stiffness of the sheet S is lower than that in the first mode, the reaction force 222 is also relatively low and suppressed at the first amount of deflection L1. Therefore, the amount of deflection can be maintained at the first amount of deflection L1, and the unfixed toner on the sheet can be stably conveyed.

[0057] The first process is a process of controlling the conveyance speed (fixing conveyance speed) of the sheet by the fixing nip N2 so that the amount of deflection in the second deflection region 212 becomes the first amount of deflection L1. The second process is a process of controlling the conveyance speed (fixing conveyance speed) of the sheet by the fixing nip N2 so that the amount of deflection in the second deflection region 212 becomes the second amount of deflection L2. That is, the above first mode is executed when conveying a sheet with a first stiffness, and includes steps S16 to S19 corresponding to the first process and steps S20 to S23, S27 corresponding to the second process. Further, the above second mode is executed when conveying a sheet with a second stiffness that is smaller than the first stiffness, and includes steps S16 to S19, S20, S24 to S27 corresponding to the first process.

[0058] By the second process (steps S20 to S23, S27) in the first mode, before the rear end Sb of the sheet S passes through the nip N3 of the registration roller pair 131, the amount of deflection in the second deflection region 212 can be set to a relatively small second deflection amount L2. Thereby, the reaction force 222 of the deflection of the sheet S in the second deflection region 212 is reduced, and the change in the force acting on the sheet S at the transfer nip N1 when the rear end Sb of the sheet S passes through the nip N3 of the registration roller pair 131 can be reduced. Therefore, the slip of the sheet S at the transfer nip N1 can be reduced, and image defects such as image distortion caused by the slip can be suppressed. In addition, images can be formed well on various media such as thick paper with high stiffness, thin paper with low stiffness, and plain paper.

[0059] <Other Embodiments> In the present embodiment, when the basis weight of the sheet S input by the operation unit 202 is equal to or greater than the threshold basis weight w1, the second process of controlling the fixing conveyance speed so that the amount of deflection in the second deflection region 212 becomes the second deflection amount L2 is executed, but the present invention is not limited to this. For example, regardless of the basis weight of the conveyed sheet S, the second process may be executed.

[0060] Also, in the present embodiment, in the first mode, the amount of deflection in the second deflection region 212 is changed in two steps from the first deflection amount L1 to the second deflection amount L2, but the present invention is not limited to this. For example, the amount of deflection in the second deflection region 212 may be changed in three or more steps.

[0061] Also, in the present embodiment, it has been determined whether or not to execute the second process based on the basis weight of the sheet S input by the operation unit 202, but the present invention is not limited to this. For example, a media sensor capable of detecting at least one of the basis weight, thickness, and surface property of the sheet S may be arranged in the cassette 111 or the sheet conveyance path, and it may be determined whether or not to execute the second process based on the detection result of the media sensor. In any case, when conveying a sheet with the first stiffness, the first mode may be executed, and when conveying a sheet with a second stiffness smaller than the first stiffness, the second mode may be configured to be executed.

[0062] Also, in this embodiment, an encoder-type sensor is applied to the deflection sensor 162, but the present invention is not limited thereto. The deflection sensor 162 only needs to be able to detect the amount of deflection of at least two or more sheets S. For example, it may include two or more photo-interrupters that detect the rotation position of the flag 231.

[0063] Also, in this embodiment, the fixing device 160 is composed of the fixing roller 158 and the pressure roller 159, but the present invention is not limited thereto. For example, instead of the fixing roller 158, a fixing film with a built-in heater may be applied. At this time, the heater may be in direct contact with the fixing film or may be in contact with the film through a sheet material with high thermal conductivity such as ferroalloy or aluminum. Also, instead of the fixing roller 158, a fixing belt having a heat-generating layer may be applied, and the heat-generating layer may be heated by electromagnetic induction. Also, the type of the heater is not particularly limited, and a ceramic heater, a halogen heater, etc. can be applied.

[0064] Also, in this embodiment, the flag 231 is configured to be rotatable about the rotation axis 233, but the present invention is not limited thereto. That is, the flag 231 only needs to move by being pressed by the sheet S sandwiched between the transfer nip N1 and the fixing nip N2, and may slide, for example. Then, the sensor 232 only needs to change the output value according to the amount of movement of the flag 231. Also, the deflection sensor 162 may include a second flag interlocked with the flag 231, and the sensor 232 may detect the amount of rotation of the second flag.

[0065] The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiment to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program. Also, it can be realized by a circuit (for example, ASIC) that realizes one or more functions.

Explanation of Reference Numerals

[0066] 131: Conveyor section (Registration roller pair) / 132: Tip detection section (Tip detection sensor) / 142: Opposing member (Secondary transfer outer roller) / 155: Image carrier (Intermediate transfer belt) / 156: Transfer section (Secondary transfer section) / 162: Detection section (Deflection sensor) / 231: Rotating section (Flag) / 232: Rotary encoder (Sensor) / CD: Sheet conveyance direction / L1: First deflection amount / L2: Second deflection amount / N1: Transfer nip / N2: Fixing section (Fixing nip) / N3: Nip / P1, P2: Common tangent / S: Sheet / Sb: Rear end / VHi1: First speed (Speed) / VLo1: Second speed (Speed)

Claims

1. A conveying unit that conveys a sheet while sandwiching the sheet in the sheet conveying direction; A transfer unit that is disposed downstream of the conveying unit in the sheet conveying direction, conveys the sheet while sandwiching the sheet, and transfers a toner image onto the sheet; A fixing unit that is disposed downstream of the transfer unit in the sheet conveying direction, conveys the sheet while sandwiching the sheet, and fixes the toner image transferred by the transfer unit onto the sheet; A detection unit that is disposed between the transfer unit and the fixing unit in the sheet conveying direction, and detects the amount of deflection of the sheet sandwiched by the transfer unit and the fixing unit in multiple stages; A control unit that controls the conveying speed of the sheet by the fixing unit based on the detection result of the detection unit, and includes: The control unit executes a first process of controlling the conveying speed of the sheet by the fixing unit so that the amount of deflection becomes a first amount of deflection while one sheet is sandwiched by the conveying unit, the transfer unit, and the fixing unit, and a second process that is executed after the first process and controls the conveying speed of the sheet by the fixing unit so that the amount of deflection becomes a second amount of deflection smaller than the first amount of deflection, and can execute a mode of performing the second process; An image forming apparatus characterized by the above.

2. In the mode, the control unit continues the second process until at least the rear end of the sheet exits the conveying unit after the first process. The image forming apparatus according to claim 1, characterized by the above.

3. The mode is a first mode. When conveying a sheet with a first stiffness, the control unit executes the first mode, and when conveying a sheet with a second stiffness smaller than the first stiffness, the control unit executes a second mode in which the first process is continued until at least the rear end of the sheet exits the conveying unit. The image forming apparatus according to claim 1, characterized by the above.

4. The control unit adjusts the amount of deflection by switching the conveying speed of the sheet by the fixing unit between a first speed faster than the conveying speed of the sheet by the transfer unit and a second speed slower than the conveying speed of the sheet by the transfer unit. The image forming apparatus according to claim 1, characterized by the above.

5. The detection unit includes a rotating unit that rotates by being pressed against the sheet sandwiched by the transfer unit and the fixing unit, and a rotary encoder that outputs a pulse signal corresponding to the amount of rotation of the rotating unit. The image forming apparatus according to claim 1, characterized by the above.

6. The conveying unit includes a pair of rollers, and corrects skewing of the sheet by abutting the sheet against the nip of the pair of rollers in a stopped state. The image forming apparatus according to claim 1, characterized in that.

7. The transfer unit includes an image carrier that carries a toner image, and an opposing member that faces the image carrier and forms a transfer nip together with the image carrier. A common tangent of the pair of rollers at the nip intersects a common tangent of the image carrier and the opposing member at the transfer nip. The image forming apparatus according to claim 6, characterized in that.

8. The sheet sandwiched between the conveying unit, the transfer unit, and the fixing unit is bent between the conveying unit and the transfer unit and also between the transfer unit and the fixing unit. The image forming apparatus according to any one of claims 1 to 7, characterized in that.

9. The image forming apparatus further includes a leading edge detection unit that is disposed upstream of the conveying unit in the sheet conveying direction and detects the position of the leading edge of the sheet. The image forming apparatus according to claim 1, characterized in that.

Images