Image forming apparatus

Abstract

To control a rotation speed of a fixing unit according to a change in an interval of passage of a recording material.SOLUTION: An image forming apparatus 100 comprises: a transfer unit 4 that transfers a toner image to a recording material P conveyed in a conveyance direction CD; a fixing unit 5 that is arranged downstream of the transfer unit in the conveyance direction, has a cylindrical rotating member 132 conveying the recording material and heating means 133, and fixes the toner image to the recording material conveyed from the transfer unit; a fixing driving unit 403 that drives the cylindrical rotating member; and a control unit 301 that controls the fixing driving unit to control a rotation speed of the cylindrical rotating member. In forming images on a plurality of recording material, when the interval between passage in the fixing unit of a rear end part of a preceding recording material and passage of a leading end of a subsequent recording material is increased, the control unit controls the fixing driving unit so that a second rotation speed of the cylindrical rotating member in conveying the subsequent recording material becomes smaller than a first rotation speed of the cylindrical rotating member in conveying the preceding recording material.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to an image forming apparatus having a fixing unit that fixes a toner image onto a recording material.

Background Art

[0002] Here, an electrophotographic image forming apparatus forms an image on a recording material using an electrophotographic image forming process. Examples of electrophotographic image forming apparatuses include electrophotographic copiers (e.g., digital copiers), electrophotographic printers (e.g., color laser beam printers, color LED printers, etc.), MFPs (multifunction peripherals), facsimile apparatuses, and printing machines. The electrophotographic image forming apparatus (hereinafter referred to as the image forming apparatus) includes not only an image forming apparatus that forms a color image but also an image forming apparatus that forms a monochrome image.

[0003] In the image forming process of the image forming apparatus, the surface of the image carrier is uniformly charged by a charger. The exposure device exposes the uniformly charged surface of the image carrier according to image data to form an electrostatic latent image on the surface of the image carrier. The developing device develops the electrostatic latent image using toner to form a toner image (visible image). The transfer unit transfers the toner image onto the recording material. The fixing unit heats and presses the toner image on the recording material to fix the toner image onto the recording material, and an image is formed on the recording material.

[0004] Here, the fixing unit is constituted by, for example, a pressure roller and a fixing roller as a pair of rotators and a heater. Since the pressure roller is an elastic roller provided with an elastic member such as silicone rubber on the outer peripheral portion, when heated by the heater, the pressure roller thermally expands. As a result, it is known that the conveyance speed of the recording material by the fixing unit changes (Patent Document 1). When the speed difference between the conveyance speed of the recording material by the transfer unit and the conveyance speed of the recording material by the fixing unit changes, the recording material is pulled by the fixing unit, or the bending shape of the recording material changes between the fixing unit and the transfer unit. As a result, various image defects occur, the recording material rubs against the conveyance guide and is damaged, or conveyance failure of the recording material occurs.

[0005] Therefore, an image forming apparatus has been proposed that detects the transport speed of the recording material by the fixing unit and controls the drive speed of the pressure roller to compensate for changes in transport speed due to thermal expansion of the pressure roller. Patent Document 2 discloses detecting the amount of deflection of the recording material between the transfer unit and the fixing unit using a sensor and controlling the acceleration / deceleration of the drive speed of the fixing unit based on the deflection detection result. Patent Document 3 also discloses detecting the passage time from the leading end to the trailing end of the recording material using a sensor installed downstream of the fixing unit in the transport direction of the recording material and correcting the transport speed of the fixing unit for subsequent recording material. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2015-114551 [Patent Document 2] Japanese Patent Publication No. 2017-207648 [Patent Document 3] Japanese Patent Publication No. 2015-108799 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in control systems that correct the transport speed of the fixing unit based on the detection result of the amount of deflection of the recording material between the transfer unit and the fixing unit, as described in Patent Document 2, a transport section is required in which the amount of deflection of the recording material can be stably detected. If the length of the recording material in the transport direction is not very long compared to the distance between the transfer unit and the fixing unit, the transport distance of the recording material from when the leading edge of the recording material reaches the fixing unit until the trailing edge passes through the transfer unit is short, which may result in unstable deflection and a decrease in the accuracy of deflection detection. Furthermore, after the trailing edge of the recording material passes through the transfer unit, the deflection shape of the recording material is not maintained, which may prevent the appropriate correction of the transport speed of the fixing unit. Therefore, when the length of the recording material in the transport direction is this short, the section in which the transport speed of the fixing unit can be effectively controlled based on the detection result of the amount of deflection of the recording material is substantially limited to a portion of the length of the recording material.

[0008] On the other hand, according to Patent Document 3, the passage time from the leading end to the trailing end of the preceding recording material is detected, and the transport speed of the subsequent recording material by the fixing unit is corrected based on the detected passage time. Therefore, it is possible to control the transport speed of the fixing unit regardless of the length of the recording material. However, if the distance between multiple recording materials passing through the fixing unit changes, correcting the transport speed of the subsequent recording material based on the detected passage time of the preceding recording material may lead to an error in the amount of correction for the following reasons.

[0009] In electrophotographic image forming apparatuses, continuous operation causes changes in the internal environment and the state of the developer. Therefore, interrupt control is performed during continuous printing to make various corrections. During such interrupt control, the transport of the recording material is temporarily suspended, preventing the recording material, which absorbs heat from the fuser roller, from passing through the fuser section, causing the pressure roller to expand rapidly. As a result, when the gap between the preceding and succeeding recording material is increased by the interrupt control, the transport speed of the succeeding recording material by the fuser section increases sharply compared to the transport speed of the preceding recording material. In such cases, it is not possible to appropriately correct the transport speed of the succeeding recording material based on the detection result of the passage time of the preceding recording material. Means for solving this problem

[0010] An image forming apparatus according to one embodiment of the present invention is: A transfer unit that transfers a toner image to a recording material being transported in the transport direction, A fixing unit is located downstream of the transfer unit in the aforementioned transport direction and has a cylindrical rotating member for transporting the recording material and a heating means, and fixes the toner image onto the recording material transported from the transfer unit, A fixing drive unit that drives the cylindrical rotating member, The system includes a control unit that controls the fixing drive unit in order to control the rotational speed of the cylindrical rotating member, The control unit is characterized in that, when forming an image on multiple recording materials, if the distance between the rear end of a preceding recording material and the front end of a succeeding recording material passing through the fixing unit becomes long, the second rotational speed of the cylindrical rotating member when transporting the succeeding recording material becomes smaller than the first rotational speed of the cylindrical rotating member when transporting the preceding recording material. [Effects of the Invention]

[0011] According to the present invention, the rotation speed of the fixing unit can be controlled in accordance with changes in the recording material passage interval. [Brief explanation of the drawing]

[0012] [Figure 1] Cross-sectional view of an image forming apparatus. [Figure 2] Block diagram of the control system of an image forming apparatus. [Figure 3] Enlarged view of the transport path from the registration section to the fixing section. [Figure 4] A flowchart illustrating the drive control of the fixing unit, performed by the control unit. [Figure 5] This diagram shows the change in the transport speed of the recording material by the fixing unit during continuous paper feeding. [Figure 6] A diagram showing the interval between recording materials passing through during continuous paper feeding. [Figure 7] This figure shows the relationship between the interval between recording material passages during continuous paper feeding and the rate of increase in the transport speed of the fixing unit. [Figure 8A] A flowchart illustrating the control operation for correcting the rotational speed of the fixing unit based on the recording material passage interval. [Figure 8B] A flowchart illustrating the control operation for correcting the rotational speed of the fixing unit based on the recording material passage interval. [Figure 9A] A flowchart illustrating the control operation for correcting the rotational speed of the fixing unit based on the temperature estimation of the pressure roller. [Figure 9B] A flowchart illustrating the control operation for correcting the rotational speed of the fixing unit based on the temperature estimation of the pressure roller. [Figure 10A] A flowchart illustrating the control operation for correcting the rotational speed of the fuser unit based on the temperature estimation of the pressure roller while the print job is stopped. [Figure 10B] Flowchart showing the control operation of correcting the rotation speed of the fixing unit based on the estimated temperature of the pressure roller during a print job stop. [Figure 10C] Flowchart showing the control operation of correcting the rotation speed of the fixing unit based on the estimated temperature of the pressure roller during a print job stop. [Figure 10D] Flowchart showing the control operation of correcting the rotation speed of the fixing unit based on the estimated temperature of the pressure roller during a print job stop.

Embodiment for Carrying Out the Invention

[0013] Hereinafter, embodiments will be described with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described in the following embodiments are not intended to limit the embodiments to only those, unless there are particularly limiting descriptions.

Embodiment

[0014] (Configuration and Operation of Image Forming Apparatus) FIG. 1 is a cross-sectional view of an image forming apparatus 100. The image forming apparatus 100 is an intermediate transfer type tandem color digital printer having four image forming units 1Y, 1M, 1C, and 1K. FIG. 2 is a block diagram of a control system 2 of the image forming apparatus 100. The image forming units 1Y, 1M, 1C, and 1K form toner images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. The configurations and operations of the image forming units 1Y, 1M, 1C, and 1K are substantially the same except that the colors of the toner are different. Therefore, when no particular distinction is required in the following description, these elements will be described generically.

[0015] The image forming apparatus 100 includes a housing 101. The housing 101 incorporates each mechanism constituting the image forming units 1Y, 1M, 1C, and 1K, and a control board housing section (not shown) that houses a CPU as a control unit 301 (control means) for controlling each image forming process by each mechanism. Further, an environmental sensor 120 for acquiring the temperature and humidity as an external environment is disposed on the housing 101.

[0016] The four photoreceptor drums 105, acting as image carriers, are each rotated clockwise in Figure 1. The charging roller 111 charges the surface of the photoreceptor drums 105 with a uniform charge. The control unit 301 inputs yellow, magenta, cyan, and black image data to the laser scanner 107 (exposure device), which acts as an exposure means. The laser scanner 107 irradiates the surface of each photoreceptor drum 105 with laser light modulated according to the image data of each color to form an electrostatic latent image.

[0017] The developing device 112 develops the electrostatic latent image formed on the surface of the photoreceptor drum 105 with toner (developer) of each color. That is, the developing device 112 attaches toner charged with the same polarity as the charge polarity of the photoreceptor drum 105 to the electrostatic latent image, making the electrostatic latent image visible as yellow, magenta, cyan, and black toner images (reverse development). The toner images formed on the photoreceptor drum 105 are sequentially transferred by the primary transfer roller 108 to the intermediate transfer belt 106, which rotates counterclockwise in Figure 1 (primary transfer), and are superimposed on the intermediate transfer belt 106 to form a full-color toner image. At this time, a primary transfer voltage (primary transfer bias), which is a DC voltage with the opposite polarity to the charge polarity of the toner during development, is applied to the primary transfer roller 108. Toner remaining on the photoreceptor drum 105 after primary transfer is removed and recovered by the drum cleaner 109.

[0018] The lower part of the image forming apparatus 100 is provided with a plurality of storage compartments 113 for accommodating recording material P. The recording material is a sheet on which an image is formed by the image forming apparatus 100, and can be, for example, plain paper, recycled paper, cardboard, second original drawing paper, OHP paper, colored paper, pre-punched paper, pre-printed paper, plastic sheet, letterhead, or label paper. The recording material P fed from the storage compartments 113 is transported to the registration section 3. The registration section 3 is composed of a pair of registration rollers 115. The leading edge of the recording material P abuts against the nip portion of the stationary pair of registration rollers 115, forming a flex (loop) in the recording material P and correcting the skewness of the recording material P.

[0019] A secondary transfer section 4 (transfer section) is provided downstream of the registration section 3 in the transport direction CD of the recording material P. The registration section 3 is located upstream of the secondary transfer section 4 in the transport direction CD of the recording material P and transports the recording material P to the secondary transfer section 4. The secondary transfer section 4 has a secondary transfer inner roller 1061 and a secondary transfer outer roller 114. An intermediate transfer belt 106 is sandwiched between the secondary transfer inner roller 1061 and the secondary transfer outer roller 114, and a secondary transfer nip section ST is formed between the intermediate transfer belt 106 and the secondary transfer outer roller 114. The registration roller 115 is driven by a registration drive motor 401 (registration drive unit). The control unit 301 inputs a drive signal to the registration drive motor 401 to start the rotation of the registration roller 115 so that the leading edge of the recording material P at the secondary transfer nip section ST coincides with the leading edge of the toner image on the intermediate transfer belt 106. The recording material P is transported to the secondary transfer section 4 by the registration roller 115.

[0020] When forming images on multiple recording materials, the control unit 301 stops the registration drive motor 401 at least once after the trailing end of the preceding recording material has passed through the registration unit 3, and then restarts the drive to transport the subsequent recording material to the secondary transfer unit 4.

[0021] The toner image on the intermediate transfer belt 106 is transferred to the recording material P by the secondary transfer outer roller 114 in the secondary transfer section 4 (secondary transfer). At this time, a secondary transfer voltage (secondary transfer bias), which is a DC voltage with the opposite polarity to the charge polarity of the toner during development, is applied to the secondary transfer outer roller 114. After secondary transfer, the toner remaining on the intermediate transfer belt 106 is removed and recovered by the intermediate transfer belt cleaner 117.

[0022] In the transport direction CD of the recording material P, the fixing unit 5 is located downstream of the secondary transfer unit 4. The recording material P onto which the toner image has been transferred is transported to the fixing unit 5. The fixing unit 5 has a pair of rotating bodies, a fixing roller 131 and a pressure roller 132, and a heater 133 as a heating element (heating means) located inside the fixing roller 131. The fixing roller 131 and the pressure roller 132 press against each other to form a fixing nip FN. The toner image transferred onto the recording material P is held between the fixing roller 131 and the pressure roller 132 in the fixing nip FN, and while being transported, it is heated and pressurized and fixed to the recording material P.

[0023] The fixing roller 131 has a rubber layer with high thermal conductivity formed on a thin tubular metal base layer. A fluororesin release layer is formed on the surface of the rubber layer. The pressure roller 132 is a cylindrical rotating member with a flexible elastic layer, such as silicone rubber, on the outside of a solid or hollow cylindrical metal shaft made of iron or aluminum. A fluororesin release layer is formed on the surface of the elastic layer. The pressure roller 132 is strongly pressed against the fixing roller 131 to improve the fixing performance of the toner image, forming a planar fixing nip FN.

[0024] The fixing nip section FN is heated by a heater 133 located inside the fixing roller 131. The surface temperature of the fixing roller 131 is detected by a fixing roller temperature sensor 134 (temperature detector). The surface temperature information of the fixing roller 131 detected by the fixing roller temperature sensor 134 is input to the control unit 301 as a detection signal. Based on the input temperature information, the control unit 301 outputs a power control signal to the heater 133 and controls the power supplied to the heater 133 to maintain the surface temperature of the fixing roller 131 at a predetermined temperature. The control unit 301 can control the heater 133 to maintain at least a first temperature for fixing the toner image to the recording material and a second temperature lower than the first temperature. The second temperature is, for example, the ambient temperature. The fixing roller 131 may be a tubular film or a cylindrical rotating member.

[0025] The pressure roller 132 of the fixing unit 5 is driven by the fixing drive motor 403 (fixing drive unit). The control unit 301 outputs a drive signal to the fixing drive motor 403, which rotates the pressure roller 132 and transports the recording material P that is held between the fixing nip FN between the fixing roller 131 and the pressure roller 132. The recording material P on which the toner image has been fixed by the fixing unit 5 is discharged onto the discharge tray 143 by the discharge unit 141 or discharge unit 142.

[0026] (Fusing drive control according to the timing of the recording material transport) Next, the fixing drive control according to the transport timing of the recording material P will be described. Figure 3 is an enlarged view of the transport path 6 from the registration section 3 to the fixing section 5. The deflection amount detection unit 200 (transport speed detection means) that detects the amount of deflection of the recording material P between the secondary transfer section 4 and the fixing section 5 is provided in the transport path 6 between the secondary transfer section 4 and the fixing section 5. The deflection amount detection unit 200 also functions as a transport speed detection means that detects the transport speed when the recording material is transported between the secondary transfer section 4 and the fixing section 5 based on the deflection amount detection result. The deflection amount detection unit 200 has a deflection amount detection flag 201 and a deflection amount sensor 202. The deflection amount detection flag 201 is biased to rotate toward the transport path 6 by the weak force of a flag spring (not shown) as a biasing member. The deflection amount sensor 202 is a transmissive photosensor (photointerrupter sensor) consisting of a light-emitting element (not shown) and a light-receiving element (not shown) arranged facing each other. When light reaches the light-receiving element (not shown) from the light-emitting element (not shown), the deflection sensor 202 outputs an OFF signal. When the light from the light-emitting element (not shown) is blocked by the deflection detection flag 201, the deflection sensor 202 outputs an ON signal.

[0027] When the recording material P is held and transported by both the secondary transfer nip ST of the secondary transfer unit 4 and the fixing nip FN of the fixing unit 5, the recording material P flexes due to the speed difference between the transport speed of the secondary transfer unit 4 and the transport speed of the fixing unit 5. When the recording material P flexes in the transport path 6 between the secondary transfer unit 4 and the fixing unit 5, one end 201a of the flexure detection flag 201 is pushed by the recording material P. The one end 201a of the flexure detection flag 201 rotates away from the transport path 6 against the biasing force of the flag spring (not shown) according to the amount of flexure of the recording material P. Consequently, the other end 201b of the flexure detection flag 201 enters between the light-emitting element (not shown) and the light-receiving element (not shown) of the flexure sensor 202. As shown in Figure 3(a), when the amount of deflection of the recording material P exceeds a predetermined amount (predetermined range), the other end 201b of the deflection amount detection flag 201 blocks the optical axis of the deflection amount sensor 202, and the output signal of the deflection amount sensor 202 changes from an OFF signal to an ON signal. That is, when the output signal of the deflection amount sensor 202 is an ON signal, the amount of deflection of the recording material P is greater than the predetermined amount, as shown in Figure 3(a). When the output signal of the deflection amount sensor 202 is an OFF signal, the amount of deflection of the recording material P is less than the predetermined amount, as shown in Figure 3(b), or the recording material P is not in the detection position of the deflection amount detection unit 200.

[0028] The deflection detection unit 200 should detect at least two states: when the recording material has a first deflection, and when the recording material has a second deflection greater than the first deflection. The control unit 301 should control the rotation speed of the fixing drive motor 403 so that the deflection of the recording material falls within a predetermined range based on the deflection detection result (detection information) of the deflection detection unit 200. The control unit 301 should calculate the average rotation speed from the detection time of the first deflection and the rotation speed during that time, and from the detection time of the second deflection and the rotation speed during that time.

[0029] Next, the drive control of the fuser unit 5 by the control unit 301 will be explained with reference to Figure 4. Figure 4 is a flowchart showing the drive control of the fuser unit 5 performed by the control unit 301. When the control unit 301 receives a print job (S1), it outputs a power control signal to the heater 133 to start controlling the fixing temperature of the fuser roller 131 (S2). Subsequently, the control unit 301 starts driving the pressure roller 132 of the fuser unit 5 with the fuser drive motor 403 (S3). At this time, the pressure roller 132 is rotated at a rotational speed of Nm. The control unit 301 determines whether or not the fuser unit 5 has entered a standby state based on the detection signal from the fuser roller temperature sensor 134 (S4). Specifically, when it is detected that the surface temperature of the fuser roller 131 has reached a predetermined temperature based on the detection signal from the fuser roller temperature sensor 134, it is determined that the fuser unit 5 has entered a standby state.

[0030] When the fixing unit 5 enters standby mode (YES in S4), the control unit 301 drives the image forming drive motor 402 (transfer drive unit) that drives the image forming units 1Y, 1M, 1C, and 1K and the secondary transfer unit 4, and starts the image forming operation (S5). As described above, the recording material P is transported to the fixing unit 5 after the toner image on the intermediate transfer belt 106 is transferred in the secondary transfer unit 4. At this time, the control unit 301 determines whether a predetermined time Ti has elapsed since the exposure start signal (S6). If a predetermined time Ti has elapsed since the exposure start signal (YES in S6), the control unit 301 switches the rotation speed of the pressure roller 132 of the fixing unit 5 from rotation speed Nm to rotation speed Ni (S7). The predetermined time Ti corresponds to the time from when the exposure start signal is output until the leading edge of the recording material P reaches the fixing unit 5. When the pressure roller 132 of the fixing unit 5 rotates at rotational speed Ni, the transport speed Vi of the recording material P by the fixing unit 5 is slightly slower than the transport speed Vs of the recording material P by the secondary transfer unit 4 (hereinafter referred to as the intermediate transfer belt speed). As a result, the recording material P is transported between the secondary transfer unit 4 and the fixing unit 5 in a slightly slack state, thereby suppressing disturbance of the toner image transferred from the intermediate transfer belt 106 to the recording material P by the secondary transfer unit 4. In this embodiment, the rotational speed Ni of the pressure roller 132 is set to 97% of the reference rotational speed N0 of the pressure roller 132, which corresponds to the intermediate transfer belt speed Vs. Alternatively, the rotational speed of the pressure roller 132 may be set to Ni by setting the rotational speed Nm at the start of driving the pressure roller 132, so that the rotational speed of the pressure roller 132 is not switched.

[0031] When the leading edge of the recording material P reaches the fixing nip section FN, the recording material P begins to flex due to the speed difference between the transport speed of the secondary transfer section 4 and the transport speed of the fixing section 5. The control unit 301 determines whether a predetermined time Ts has elapsed since the exposure start signal was output (S8). When the predetermined time Ts has elapsed since the exposure start signal was output (YES in S8), the control unit 301 starts flex control based on the amount of flex of the recording material P detected by the flex amount detection unit 200 (S9). In flex control, the control unit 301 selectively switches the rotation speed of the pressure roller 132 between rotation speed Nh and rotation speed Nl according to the detection result of the flex amount detection unit 200. When the pressure roller 132 rotates at rotation speed Nh, the transport speed of the fixing section 5 becomes faster than the intermediate transfer belt speed Vs. When the pressure roller 132 rotates at rotation speed Nl, the transport speed of the fixing section 5 becomes slower than the intermediate transfer belt speed Vs.

[0032] When the output signal of the deflection detection unit 200 is an ON signal, the amount of deflection of the recording material P is greater than a predetermined amount, so the rotation speed of the pressure roller 132 is switched to rotation speed Nh so that the transport speed of the fixing unit 5 becomes faster than the intermediate transfer belt speed Vs in order to reduce the amount of deflection. When the output signal of the deflection detection unit 200 is an OFF signal, the amount of deflection of the recording material P is less than a predetermined amount, so the rotation speed of the pressure roller 132 is switched to rotation speed Nl so that the transport speed of the fixing unit 5 becomes slower than the intermediate transfer belt speed Vs in order to increase the amount of deflection. In this embodiment, the rotation speeds Nh and Nl of the pressure roller 132 are set to 101% and 95% of the reference rotation speed N0, respectively. In this way, in deflection control, the amount of deflection of the recording material P is controlled to be approximately constant at a predetermined amount by switching the rotation speed of the pressure roller 132 of the fixing unit 5. In this embodiment, the rotation speed of the pressure roller 132 is first switched to rotation speed Nl at a predetermined time Ts elapsed from the time the exposure start signal is output.

[0033] When deflection control is initiated, the recording material P is being transported by the registration section 3, the secondary transfer section 4, and the fixing section 5. As the recording material P is transported further and its rear end passes the registration section 3, the recording material P is transported by the secondary transfer section 4 and the fixing section 5. In the transport direction CD, the portion of the recording material P upstream of the fixing section 5 is held only by the secondary transfer section 4. As described above, the recording material P is transported with a slight slack between the secondary transfer section 4 and the fixing section 5, so a restoring force is generated due to the bending rigidity of the recording material P. This restoring force acts on the recording material P as a force that pushes the portion of the recording material P in the secondary transfer section 4 in the opposite direction to the transport direction CD. When the recording material P is being transported by the registration section 3, the secondary transfer section 4, and the fixing section 5, the force due to the deflection of the portion of the recording material P between the secondary transfer section 4 and the fixing section 5 cancels out the force due to the deflection of the portion of the recording material P between the registration section 3 and the secondary transfer section 4. However, when the rear end of the recording material P passes the registration section 3, the balance of forces is disrupted, and the recording material P may slip in the opposite direction to the transport direction CD relative to the intermediate transfer belt 106 due to the force caused by the deflection of the portion of the recording material P between the secondary transfer section 4 and the fixing section 5. As a result, streak-like image defects may occur at a position on the front end side of the recording material P, corresponding to the distance between the registration section 3 and the secondary transfer section 4 from the rear end of the recording material P.

[0034] Therefore, in this embodiment, the deflection control is terminated before the trailing end of the recording material P passes the registration section 3, and the transport speed of the fixing section 5 is switched to minimize the deflection between the secondary transfer section 4 and the fixing section 5. Specifically, the control unit 301 determines whether a predetermined time Tr has elapsed since the exposure start signal was output (S10). The predetermined time Tr corresponds to the time from when the exposure start signal is output until just before the trailing end of the recording material P passes the registration section 3. When the predetermined time Tr has elapsed since the exposure start signal was output (YES in S10), the control unit 301 terminates the deflection control (S11). At this timing, the control unit 301 switches the rotation speed of the fixing section 5 from rotation speed Nh or rotation speed Nl to rotation speed Nr so that the transport speed of the fixing section 5 becomes faster than the intermediate transfer belt speed Vs (S11). In this embodiment, the rotation speed Nr is set to 102% of the reference rotation speed N0. Furthermore, the control unit 301 switches the rotational speed Vregi of the registration roller 115 to a lower rotational speed (S12) so that the transport speed of the recording material P by the registration unit 3 is slower than the intermediate transfer belt speed Vs. This reduces the amount of deflection of the portion of the recording material P upstream of the secondary transfer unit 4 and the amount of deflection of the portion of the recording material P downstream of the secondary transfer unit 4 just before the rear end of the recording material P passes through the registration unit 3. As a result, fluctuations in the force acting on the secondary transfer unit 4 before and after the rear end of the recording material P passes through the registration unit 3 are suppressed.

[0035] The control unit 301 determines whether the rear end of the recording material P has passed through the registration section 3 (S13). If the rear end of the recording material P has passed through the registration section 3 (YES in S13), the rotation speed of the fixing section 5 is switched from rotation speed Nr to a rotation speed Nt that is lower than the reference rotation speed N0 (S14). This eliminates the deflection of the recording material P between the secondary transfer section 4 and the fixing section 5, preventing the fixing section 5 from pulling on the recording material P, and controlling the amount of deflection of the recording material P so that it is an appropriate amount when the rear end of the recording material P passes through the secondary transfer section 4. The rotation speeds Ni, Nh, Nl, Nr, and Nt (first rotation speeds) of the pressure roller 132 of the fixing section 5 are set based on the reference rotation speed N0 stored as an initial value in the memory unit 302.

[0036] When the rear end of the recording material P passes the secondary transfer section 4, the fixing section 5 continues to transport the recording material P at the same rotational speed Nt. The control unit 301 determines whether the rear end of the recording material P has passed the fixing nip section FN (S15). If the rear end of the recording material P has passed the fixing nip section FN (YES in S15), the control unit 301 determines whether there is a subsequent recording material P2 (S16). If there is a subsequent recording material P2 (YES in S16), the control unit 301 proceeds to S6. At a predetermined time Ti elapsed since the exposure start signal for the subsequent recording material P2 was output (S6), the control unit 301 switches the rotational speed of the fixing section 5 from rotational speed Nt to rotational speed Ni (S7). On the other hand, if there is no subsequent recording material P2 (NO in S16), the control unit 301 determines whether the recording material P has been discharged from the discharge section 141 or discharge section 142 (S17). When it is determined that the recording material P has been discharged from the discharge unit 141 or 142 (YES in S17), the control unit 301 stops driving each part, including the fixing unit 5, and stops supplying power to the heater 133 (S18). The control unit 301 terminates the print operation (S19).

[0037] (Changes in the transport speed of the fixing section due to thermal expansion of the pressure roller and their impact on the image) As mentioned above, the rotational speed of the fixing unit 5 is precisely controlled to accelerate and decelerate sequentially to predetermined values ​​(rotational speeds Nm, Ni, Nh, Nl, Nr, Nt) for each of the multiple transport sections corresponding to the transport timing of the recording material (predetermined timing). In the deflection amount control (S9~S11), the control unit 301 controls the rotational speed of the fixing unit 5 based on the amount of deflection of the recording material between the secondary transfer unit 4 and the fixing unit 5, that is, the speed difference between the transport speed of the recording material by the fixing unit 5 and the transport speed of the recording material by the secondary transfer unit 4. However, on the other hand, the transport speed of the recording material P by the fixing unit 5 fluctuates greatly due to the thermal expansion of the pressure roller 132. Specifically, the transport speed of the recording material by the fixing unit 5 becomes faster than the predetermined rotational speed by the amount of thermal expansion of the pressure roller 132.

[0038] Figure 5 shows the change in the transport speed of the recording material by the fuser unit 5 during continuous paper feeding. Figure 5 shows the change in the transport speed of the recording material by the fuser unit 5 when high-quality paper (A3 size) with a basis weight of 80 gsm (grams per square meter) is continuously fed as the recording material. The vertical axis represents the percentage increase in the transport speed of the recording material by the fuser unit 5 during continuous paper feeding. The horizontal axis represents the number of printed pages (sheets) of the recording material. The image forming apparatus 100 was set up in a typical office environment (room temperature 23°C, humidity approximately 50%), and temperature control of the fuser unit 5 was started. When the temperature of the fuser unit 5 reached the temperature at which the toner image could be fixed, the transport of the recording material began. The rotation speed of the pressure roller 132 was controlled to be constant during continuous paper feeding. However, as shown in Figure 5, it can be seen that shortly after the continuous transport of the recording material began, the transport speed of the recording material by the fuser unit 5 gradually increased. This change in the transport speed of the recording material by the fixing unit 5 is caused by the gradual thermal expansion of the pressure roller 132, which is heated by the heater 133. Furthermore, as can be seen from Figure 5, the transport speed of the recording material by the fixing unit 5 shows a change in which the transport speed increases sharply, for example, at intervals of approximately every 50 pages, and then converges back to the original gradual increase trend shown by the dashed line. This means that the pressure roller 132 expanded sharply when the transport speed increased sharply.

[0039] Generally, the image forming apparatus 100 performs various correction operations to ensure that the image forming units 1Y, 1M, 1C, and 1K and the recording material transport unit perform the desired operations appropriately in response to environmental changes and changes over time. For example, the image forming apparatus 100 forms a predetermined toner image (patch) on the intermediate transfer belt 106 according to the purpose of correction, reads the toner image on the intermediate transfer belt 106 with a reflective optical sensor, calculates the correction amount from the error relative to the target value, and adjusts the control input value. In addition, the image forming apparatus 100 performs adjustment operations that do not involve the formation of a toner image, such as adjusting the amount of toner supplied to the developing unit 112. For example, if the amount of toner in the developing unit 112 falls below a predetermined amount due to an excessive amount of toner being consumed when forming an image on the recording material, the print operation is interrupted, toner is supplied intensively from the toner container 116 to the developing unit 112, and the amount of toner in the developing unit 112 is adjusted. Such adjustment operations are performed when the image forming apparatus 100 is powered on, immediately before the start of image formation, or before the print operation is completed and the main unit stops. To respond to changes in the environment inside the image forming apparatus 100 and changes in the state of the developer, etc., due to continuous operation, it is also implemented as interrupt control during continuous printing (hereinafter also referred to as inter-recording material control).

[0040] Figure 6 shows the interval between recording materials passing during continuous paper feeding. Figure 6 shows the interval between recording materials passing through the fixing unit 5 (the interval between preceding and succeeding recording materials). The vertical axis represents the recording material passing interval (seconds) during continuous paper feeding. The horizontal axis represents the number of printed pages (sheets) of the recording material. As can be seen from Figure 6, the interval between preceding and succeeding recording materials is sporadically wider than the normal recording material passing interval of approximately 2 seconds. In Figure 6, the time in the portion where the recording material passing interval is longer than the normal recording material passing interval (normal interval) corresponds to the recording material control time. Referring to Figures 5 and 6, it can be seen that the transport speed of the fixing unit 5 increases at the timing when the interval between preceding and succeeding recording materials widens. This is for the following reason: In the normal recording material passing interval, most of the heat inflow from the heater 133 is successively absorbed by the continuously transported recording materials. In contrast, during inter-recording material control, the recording material P does not pass through the fixing unit 5, so the pressure roller 132 continues to be heated by the heating from the heater 133 and the heat contained in the fixing roller 131. As a result, the pressure roller 132 expands, and the transport speed of the fixing unit 5 increases. Taking these effects into consideration, if the passage of the subsequent recording material is delayed by a predetermined time or more, control may be implemented to reduce or stop the power supply to the heater 133. However, since the heater 133 and fixing roller 131, once heated, do not cool down immediately, the surface temperature of the pressure roller 132 still rises. As a result, the pressure roller 132 expands rapidly during inter-recording material control, and the transport speed of the recording material by the fixing unit 5 after inter-recording material control temporarily increases. Subsequently, by transporting the recording material continuously at normal intervals, the temperature of the pressure roller 132 gradually decreases to its original temperature, and the transport speed of the recording material by the fixing unit 5 changes back to its original gradual increase.

[0041] In this embodiment, the diameter of the pressure roller 132 is 30 mm, and the thickness of the rubber layer is approximately 3 mm. The change in the transport speed of the fixing unit 5 is approximately 0.75% with a gradual increase in speed, approximately 0.3% temporarily during recording material control for about 10 seconds, and approximately 0.6% during recording material control for about 20 seconds. The distance from the secondary transfer unit 4 to the fixing unit 5 is 100 mm. The distance that the fixing unit 5 transports the recording material from the leading edge of an A3 size (transport length 420 mm) recording material to the trailing edge passing through the secondary transfer unit 4 increases by 3.2 mm for every 1% increase in the transport speed of the fixing unit 5.

[0042] As mentioned above, in order to avoid disrupting the transfer of the toner image from the intermediate transfer belt 106 to the recording material in the secondary transfer section 4, the recording material is transported between the secondary transfer section 4 and the fixing section 5 in a slightly slack state. Therefore, even if the transport speed in the fixing section 5 changes due to the thermal expansion of the pressure roller 132, the fluctuation in the transport distance of the recording material by the fixing section 5 is absorbed by the deflection of the recording material, and the fixing section 5 is designed not to pull on the recording material during secondary transfer.

[0043] However, in order to allow large fluctuations in the transport speed of the fixing unit 5, it is necessary to set a large deflection of the recording material between the secondary transfer unit 4 and the fixing unit 5. As mentioned above, when the recording material is transported in a slack state, the restoring force due to the bending rigidity of the recording material acts on the secondary transfer unit 4 in the opposite direction to the transport direction CD. Similarly, the restoring force of the recording material acts on the fixing unit 5 as a force in the transport direction CD. However, in the fixing unit 5, the pressure roller 132 is strongly pressed against the fixing roller 131, and the clamping force on the recording material is greater than that of the secondary transfer unit 4. Therefore, especially in the case of recording material with high bending rigidity (for example, cardboard), the restoring force of the recording material is large relative to the amount of deflection, and slippage is likely to occur between the intermediate transfer belt 106 and the recording material P in the secondary transfer unit 4.

[0044] When slippage occurs between the intermediate transfer belt 106 and the recording material, and a relative speed difference is created between the intermediate transfer belt 106 and the recording material, the length of the toner image transferred onto the recording material P changes relative to the length of the toner image on the intermediate transfer belt 106 in the transport direction CD. That is, the magnification of the image on the recording material in the transport direction CD changes. When the recording material is pushed back against the intermediate transfer belt 106 by the restoring force of the recording material, and the transport speed of the recording material by the secondary transfer unit 4 becomes slower than the transport speed of the intermediate transfer belt 106, the length of the image on the recording material in the transport direction CD becomes shorter. In addition, if the slippage between the intermediate transfer belt 106 and the recording material P changes abruptly, the image may expand or contract locally, causing streaky image defects. Furthermore, if the rear end of the recording material passes the secondary transfer unit 4 while there is excessive deflection of the recording material between the secondary transfer unit 4 and the fixing unit 5, the holding of the recording material is released and the posture of the recording material changes abruptly. At this time, there is a risk of problems such as toner adhering to and staining the edge (cut surface) of the trailing end of the recording material (edge ​​staining), or the toner image becoming distorted before fixing.

[0045] Thus, it is not possible to set the amount of deflection of the recording material between the secondary transfer unit 4 and the fixing unit 5 to be excessively large, and it is desirable that the amount of deflection of the recording material be precisely managed and controlled. To achieve this, it is necessary to suppress fluctuations in the transport speed of the fixing unit 5 relative to the transport speed of the secondary transfer unit 4.

[0046] (Rotational speed correction of the anchoring unit based on the deflection amount control result) According to this embodiment, the average rotational speed Nave, which is the average value of the rotational speed of the pressure roller 132 of the fixing unit 5, can be calculated from the transport speed switching control operation of the fixing unit 5 in deflection amount control based on the deflection amount detection result of the recording material P. As described above, in the control operation from step S9 to step S11 shown in Figure 4, the control unit 301 switches the rotational speed of the pressure roller 132 between rotational speed Nh and rotational speed Nl according to the detection result of the deflection amount detection unit 200. At the same time, the control unit 301 calculates the integrated drive time Th by integrating the drive time (rotation time) at rotational speed Nh and the integrated drive time Tl by integrating the drive time (rotation time) at rotational speed Nl. The integrated drive time Tl is the integrated drive time (detection time) of driving the fixing unit 5 at rotational speed Nl when the recording material has a first deflection amount. The cumulative drive time Th is the cumulative drive time (detection time) during which the fixing unit 5 is driven at a rotational speed Nh when the recording material has a second deflection greater than a first deflection. The control unit 301 calculates the average rotational speed Nave of the pressure roller 132 during deflection control from the rotational speed Nh, rotational speed Nl, cumulative drive time Th, and cumulative drive time Tl using the following formula (1).

number

[0047] The integrated drive time Th and integrated drive time Tl are the sum of the drive times for each control operation from step S9 to step S11. In this embodiment, deflection amount control is started when a predetermined time Ts has elapsed from the time the exposure start signal is output, and ends when a predetermined time Tr has elapsed from the time the exposure start signal is output. Therefore, the integrated drive time Th, integrated drive time Tl, predetermined time Ts, and predetermined time Tr satisfy the relationship shown by the following equation (2).

number

[0048] The average rotational speed Nave is the result of a control operation that keeps the deflection amount of the recording material P between the secondary transfer unit 4 and the fixing unit 5 approximately constant. Therefore, it can be said that the rotational speed corresponds to the intermediate transfer belt speed Vs at that point in time, i.e., the state of the fixing unit 5, i.e., the pressure roller 132. Accordingly, the preset reference rotational speed N0 of the pressure roller 132 is corrected by a correction value ΔN0, which is the difference between the average rotational speed Nave and the reference rotational speed N0. At the same time, the correction value ΔN0 for the reference rotational speed N0, which has been pre-stored in the storage unit 302, is stored in the storage unit 302.

[0049] However, the calculation of the correction value ΔN0 for the reference rotation speed N0 based on the results of the control operations from step S9 to step S11 for the first recording material P1 is not timely enough for the control operations from step S11 onward (rotation speeds Nr and Nt) for the same recording material P1. Therefore, the calculated correction value ΔN0 is used as the correction value for the rotation speeds Ni, Nr, and Nt (second rotation speed) for the second recording material P2. Similarly, the new correction value ΔN0 calculated from the results of the control operations for the second recording material P2 is used as the correction value for the rotation speeds Ni, Nr, and Nt for the third recording material P3.

[0050] In other words, the control unit 301 controls the system so that the longer the distance between the rear end of the preceding recording material and the front end of the succeeding recording material passing through the fixing unit 5, the greater the difference between the second rotational speed for transporting the succeeding recording material and the first rotational speed for transporting the preceding recording material.

[0051] In this embodiment, the rotational speed of the subsequent recording material is corrected using the result of the control operation on the preceding recording material. However, the rotational speed of the subsequent recording material may also be corrected based on the result of the control operation on multiple preceding recording materials. In this embodiment, the rotational speeds Ni, Nr, and Nt of the subsequent recording material are corrected based on the result of the control operation on the preceding recording material. However, the rotational speeds Nh and Nl used for deflection control of the subsequent recording material do not necessarily need to be corrected based on the result of the control operation on the preceding recording material. This is because the ratio of the integrated drive time Tl of rotational speed Nl to the integrated drive time Th of rotational speed Nh fluctuates, and therefore does not significantly affect the average rotational speed Nave. However, if the change in the transport speed of the recording material by the fixing unit 5 becomes excessive, it becomes impossible to control the deflection amount to be approximately constant, in which case correction of the rotational speeds Nh and Nl in deflection control becomes necessary.

[0052] The correction value ΔN0 for the reference rotation speed N0 of the fixing unit 5 can be calculated each time a sheet of recording material is passed through the fixing unit 5. However, if the correction value ΔN0 is excessively small, the correction effect is practically negligible. Therefore, a threshold value for the correction value ΔN0 may be set, and the correction may be suspended if the correction value ΔN0 is below the threshold value. However, as mentioned above, in the case of thick paper or wide recording material where the restoring force of the recording material against the amount of deflection is large (the bending rigidity of the CD in the transport direction of the recording material is large), the correction value ΔN0 is more likely to affect the image compared to softer recording material. Therefore, the rotation speed of the fixing unit 5 may be corrected at an appropriate frequency by switching the threshold value according to the type of recording material and paper width to be printed.

[0053] As described above, the average rotational speed Nave of the pressure roller 132 can be calculated based on the deflection detection result of the recording material between the secondary transfer unit 4 and the fixing unit 5. From the average rotational speed Nave, a correction value ΔN0 for the rotational speed of the pressure roller 132 can be calculated according to the state of the pressure roller 132 at that time. The rotational speed of the pressure roller 132 of the fixing unit 5 can be corrected based on the correction value ΔN0.

[0054] (Rotation speed correction of the fixing unit based on the interval between recording materials passing) As mentioned above, the correction of the rotational speed of the fixing unit 5 on the subsequent recording material based on the deflection detection result for the preceding recording material is effective in counteracting the gradual speed increase trend shown by the dashed line in Figure 5. However, when the distance between the preceding and succeeding recording materials widens due to the execution of inter-recording material control, causing the pressure roller 132 to rapidly expand due to heat and resulting in a discontinuous increase in the transport speed of the fixing unit 5, the aforementioned rotational speed correction cannot cope. Therefore, in this embodiment, the rotational speed of the fixing unit 5 is corrected based on the recording material passing interval.

[0055] Figure 7 shows the relationship between the recording material passage interval during continuous paper feeding and the rate of increase of the transport speed of the fixing unit 5. The vertical axis represents the rate of increase (%) of the transport speed of the recording material by the fixing unit 5 during continuous paper feeding. The horizontal axis represents the recording material passage interval (seconds). The recording material passage interval is the interval between recording materials passing through the fixing unit 5 during continuous paper feeding as shown in Figure 6 (the interval between the preceding recording material and the succeeding recording material). As can be seen from Figure 7, the rate of increase of the transport speed of the succeeding recording material relative to the transport speed of the preceding recording material by the fixing unit 5 increases with increasing recording material passage interval, and the rate of increase of the transport speed with respect to increasing recording material passage interval gradually slows down. Therefore, using the relationship shown in Figure 7, the rate of increase of the transport speed of the succeeding recording material is estimated from the recording material passage interval, and the rotation speed of the fixing unit 5 is reduced to offset the increase in transport speed. For example, suppose the recording material passage interval is 10 seconds due to the execution of inter-recording material control. At this time, if the subsequent recording material is transported at the same rotational speed as the preceding recording material using the pressure roller 132, it can be estimated from the relationship in Figure 7 that the transport speed of the subsequent recording material will be approximately 0.45% faster than the transport speed of the preceding recording material. Therefore, by reducing the fixing rotational speed by 0.45% before the subsequent recording material reaches the fixing nip section FN, the speed increase due to thermal expansion of the pressure roller 132 can be offset.

[0056] Figures 8A and 8B are flowcharts illustrating the control operation of the fixing unit 5's rotational speed correction based on the recording material passage interval. Note that while Figure 4 shows the fixing unit's drive control using the transport of the first recording material from the start of the print operation as an example, the drive control for subsequent recording materials is the same as that shown in Figure 4 if the recording material passage interval is the normal interval during continuous operation. In Figures 8A and 8B, the same symbols are used for the same steps as in Figure 4, and detailed explanations are omitted.

[0057] Steps S1 to S19 in Figures 8A and 8B are the same as steps S1 to S19 in Figure 4, so their explanation is omitted. Referring to Figure 8B, in S16, the control unit 301 determines whether or not there is a subsequent recording material. If there is a subsequent recording material (YES in S16), the control unit 301 proceeds to S101. The control unit 301 determines whether or not to perform inter-recording material control (S101). If the control unit 301 determines that inter-recording material control should be performed between the i-th recording material and the j-th (j=i+1) recording material in a continuous printing operation on multiple recording materials (YES in S101), the process proceeds to S102 and S105. After the i-th recording material Pi passes through the registration unit 3, the control unit 301 stops the registration roller 115 (S102) and temporarily interrupts the feeding of the j-th recording material Pj to the secondary transfer unit 4. At the same time, the control unit 301 starts measuring the stopping time of the registration roller 115 (S102).

[0058] When the trailing edge of the i-th recording material Pi passes the fixing nip section FN, the control unit 301 switches the rotation speed of the pressure roller 132 of the fixing section 5 to rotation speed Nm at a predetermined timing (S103). The control unit 301 reduces the power supply to the heater 133 as temperature control during inter-recording material control (S104). Meanwhile, after the image forming sections 1Y, 1M, 1C, and 1K form a toner image on the recording material Pi, the control unit 301 continues to start the inter-recording material control operation by the image forming sections 1Y, 1M, 1C, and 1K (S105). In inter-recording material control, for example, a predetermined toner image (patch) is formed on the intermediate transfer belt 106 to match the purpose of correction. The toner image on the intermediate transfer belt 106 is read by a reflective optical sensor, and a correction amount is calculated from the error of the reading value relative to the target value. The control unit 301 adjusts the control input based on the correction amount. The control unit 301 determines whether or not inter-recording material control has been completed (S106). If inter-recording material control has been completed (YES in S106), the control unit 301 restores power supply to the heater 133 to heat the fixing roller 131 and restarts fixing temperature control (S107). The control unit 301 returns the process to S4.

[0059] Referring to Figure 8A, when the standby state of the fixing unit 5 is detected (YES in S4), the image formation operation on the j-th recording material Pj is started (S5). Once the image formation operation on the j-th recording material Pj has started (S5), the control unit 301 proceeds to S108. The control unit 301 starts the rotation of the registration roller 115 and ends the measurement of the stop time of the registration roller 115 (S108). The timing of the start of rotation of the registration roller 115 is adjusted by calculating backward from the time from when the exposure start signal is output until the toner image reaches the secondary transfer unit 4, and the transport time of the recording material from the registration unit 3 to the secondary transfer unit 4. Once the rotation of the registration roller 115 has started, the transport of the recording material Pj has started. At this time, the control unit 301 ends the measurement of the stop time of the registration roller 115 that was started in step S102 and calculates the stop time as the recording material passing interval. The difference between the stopping time of the registration roller 115 as the recording material passage interval calculated here and the stopping time as the normal interval during continuous paper feeding is used to calculate the recording material control time required for controlling the intervals between recording materials.

[0060] The control unit 301 calculates the rate of increase of the transport speed by the fixing unit 5, Vf_rate, based on the interval between recording materials, from the relationship shown in Figure 7 stored in the memory unit 302 (S109). The reference rotation speed N0(j) of the fixing unit 5 for the recording material Pj immediately after the control between recording materials is set by the following equation (3), and the reference rotation speed N0 is corrected (S110).

number

[0061] Here, N0(i) is the reference rotational speed of the fixing unit 5 with respect to the recording material Pi immediately before inter-recording material control. By setting the reference rotational speed N0(j) of the fixing unit 5 with respect to the recording material Pj, the reference rotational speed of the fixing unit 5 is corrected from the reference rotational speed N0(i) to the reference rotational speed N0(j). Since the reference rotational speed of the fixing unit 5 with respect to the recording material Pj has been corrected to the reference rotational speed N0(j), the rotational speed of the fixing unit 5 that can be switched in subsequent steps is set low to offset the speed increase due to thermal expansion of the pressure roller 132. In this embodiment, the rotational speeds Ni, Nr, and Nt of the fixing unit 5 are set as ratios to the reference rotational speed N0, so the rotational speeds Ni, Nr, and Nt are corrected by correcting the reference rotational speed N0. The value (initial value) stored in the storage unit 302 as the reference rotation speed N0 of the fixing unit 5 is not overwritten, and the difference between the reference rotation speed N0(j) obtained from equation (3) and the initial reference rotation speed N0 is stored in the storage unit 302 as a correction value ΔN0.

[0062] Figure 7 shows the relationship between the recording material passing interval and the rate of increase in the recording material transport speed by the fixing unit 5. However, this relationship differs depending on the configuration and material of the fixing unit 5, so this embodiment is not limited to the values ​​and curves shown in Figure 7. Furthermore, even with the same fixing unit 5, the relationship in Figure 7 changes according to the amount of power supplied to the heater 133 (amount of heat supplied) during recording material control and the temperature of the fixing roller 131 during recording material passage (amount of power supplied to the heater 133 during recording material passage). Therefore, when changing the amount of power supplied depending on the type of recording material, it is advisable to store the relationship between the recording material passing interval and the rate of increase in the recording material transport speed by the fixing unit 5 in the storage unit 302 according to the amount of power supplied. When correcting the reference rotational speed N0 of the fixing unit 5, it is advisable to correct the reference rotational speed N0 by switching the relationship between the recording material passing interval and the rate of increase in the recording material transport speed by the fixing unit 5 according to the amount of power supplied.

[0063] When inter-recording material control is performed, the interval between recording materials passing is obtained from the stopping time of the registration roller 115. However, the method of obtaining the interval between recording materials passing is not limited to this. The interval between recording materials passing may also be obtained from the rotation start timing when the registration roller 115 starts rotating to transport the recording material to the secondary transfer unit 4, and the stopping timing when the registration roller 115 stops after the recording material has passed. Alternatively, the interval between recording materials passing may be obtained using the time from when the exposure start signal is output for the recording material Pi immediately before inter-recording material control until when the exposure start signal is output for the recording material Pj immediately after inter-recording material control.

[0064] Alternatively, the recording material passage interval may be obtained by measuring the distance between the passing distances of recording material Pi and recording material Pj at predetermined positions. For example, a recording material sensor 151 (recording material tip detection means) may be provided in the transport path 6 from the registration unit 3 to the fixing unit 5. The recording material sensor 151 may detect the passing timing of the tip of recording material Pi and the tip of recording material Pj, and the recording material passage interval may be obtained based on the detection result. Alternatively, a recording material sensor 152 (recording material rear end detection means) may be provided in the transport path 7 from the position of the rear end of the recording material waiting in the registration unit 3 to the secondary transfer unit 4. The recording material sensor 152 may be positioned in the transport path 7 from a position upstream of the length of the recording material from the registration unit 3 in the transport direction CD to the secondary transfer unit 4. The recording material sensor 152 may detect the passing timing of the rear end of recording material Pi and the rear end of recording material Pj, and the recording material passage interval may be obtained based on the detection result. However, when using the timing of the leading or trailing end of the recording material, it is best to complete the detection of the passing timing and the correction of the reference rotation speed at least before the leading end of the recording material reaches the fixing nip section FN.

[0065] For the kth (k=j+1) recording material Pk following the jth recording material Pj, the recording material passing interval returns to the normal interval. Therefore, when calculating the rate of increase in the transport speed of the kth recording material Pk relative to the transport speed of the jth recording material Pj based on the recording material passing interval, it becomes 0%. However, the pressure roller 132, which has undergone thermal expansion during inter-recording material control, may not return to its state before inter-recording material control after passing just one recording material Pj. Therefore, in order to appropriately offset the increase in the transport speed of the fixing unit 5 for the second and subsequent recording materials after inter-recording material control, a correction is made according to the number of recording materials that have passed since inter-recording material control.

[0066] The pressure roller 132, which is temporarily heated by the inter-recording material control, gradually returns to its original temperature. In this case, the temperature change of the pressure roller 132 can be considered as the impulse response of a first-order lag system. Here, Vf_rate(1) is the rate of increase in the transport speed of the subsequent recording material Pk relative to the transport speed of the first recording material Pj after the inter-recording material control. When recording materials are transported at normal intervals (approximately the same intervals) after the inter-recording material control, the rate of increase in the transport speed of the n+1 recording material Pj+n+1 relative to the nth recording material Pj+n after the inter-recording material control, Vf_rate(n), is expressed by the following equation (4).

number

[0067] Here, the increase rate Vf_rate(1) is the result calculated in step S109. The constant T is a time constant and is a parameter that determines the rate of speed reduction exp(-1 / T) for each recording material conveyed after inter-recording material control. From equation (4), approximately after 5T+1 materials, the conveying speed by the pressure roller 132 returns to the original conveying speed. The constant T is determined experimentally in advance and stored in the memory unit 302.

[0068] The control unit 301 may also gradually increase or decrease the rotation speed of the fixing unit 5 in accordance with the number of recording materials passing through, even when transporting multiple recording materials continuously at approximately the same intervals and forming an image. In addition, the control unit 301 may control the total amount of change in the rotation speed of the fixing unit 5 while passing a predetermined number of recording materials through the fixing unit 5 at approximately the same intervals, so that it decreases as the number of recording materials passing through increases.

[0069] In Figure 8B, when the j-th recording material Pj passes the fixing nip section FN, the control unit 301 counts up the number of recording materials to be transported after inter-recording material control (S111). The control unit 301 determines whether the count value of the number of recording materials to be transported after inter-recording material control is less than 5T+1 (S112). If the count value of the number of recording materials to be transported after inter-recording material control is less than 5T+1 (YES in S112), the control unit 301 proceeds to S113. For each recording material transported, the control unit 301 calculates the increase rate Vf_rate(n) of the transport speed of the fixing unit 5 and the reference rotation speed N0(n) of the fixing unit 5 using the following equations (5) and (6), and corrects the reference rotation speed N0 of the fixing unit 5 (S113).

number

number

[0070] This allows the increase in the transport speed of the fixing unit 5 to be appropriately offset for the second and subsequent recording materials after inter-recording material control. After that, the control unit 301 returns to processing S6. On the other hand, if the count value of the number of recording materials transported after inter-recording material control is 5T+1 or more (NO in S112), the control unit 301 proceeds to processing S114. For recording materials from 5T+1 onwards after inter-recording material control, even if the calculation in step S113 is repeated, Vf_rate(n) ≈ 0, and the reference rotation speed N0 of the fixing unit 5 is hardly updated. Therefore, the control unit 301 sets the correction value ΔN0 to 0 (ΔN0=0) (S114). After that, the control unit 301 returns to processing S6. After deflection amount control is performed from S9 to S11, the control unit 301 calculates the average rotation speed Nave of the pressure roller 132 from equation (1) based on the deflection amount detection result (S115). The control unit 301 calculates a correction value ΔN0, which is the difference between the average rotational speed Nave and the reference rotational speed N0, and corrects the reference rotational speed N0 (S116). Therefore, the reference rotational speed N0 converges to the average rotational speed Nave.

[0071] In other words, if the interval between the trailing end of a preceding recording material and the leading end of a subsequent recording material passing through the fixing unit 5 becomes longer due to the control between recording materials, and image formation continues on multiple recording materials, the control unit 301 performs the aforementioned control. As a result, the control unit 301 changes the rotation speed of the fixing unit 5 for the multiple recording materials transported by the fixing unit 5 after the interval between recording materials passing through the fixing unit 5 has become longer, so that it increases from the second rotation speed each time a recording material is transported. As a result, the rotation speed of the fixing unit 5 converges toward the first rotation speed. The control unit 301 controls the fixing drive motor 403 so that the amount by which the rotation speed of the fixing unit 5 is increased (speed increase) each time a recording material is transported, starting from the recording material being transported by the fixing unit 5 at the second rotation speed, decreases each time a recording material is transported.

[0072] The control unit 301 sequentially switches the rotation speed of the fixing unit 5 for each of several transport sections according to predetermined timings while the fixing unit 5 transports the recording material from the leading edge to the trailing edge. When forming images on multiple recording materials, if the interval between the trailing edge of the preceding recording material and the leading edge of the succeeding recording material passing the fixing unit 5 becomes long, the control unit 301 performs the following control: The control unit 301 controls the rotation speed of at least one of the transport sections (Ni, Nh, Nl, Nr, Nt) by the fixing unit 5 for the succeeding recording material to be smaller than the rotation speed of the same transport section for the preceding recording material.

[0073] When the control unit 301 transports multiple recording materials continuously at approximately the same interval to form an image, it is preferable to control the rotation speed of the fixing unit 5 when transporting the subsequent recording material based on the transport speed detection result of the preceding recording material obtained from the deflection amount detection result of the deflection amount detection unit 200. When the interval between the rear end of the preceding recording material and the front end of the subsequent recording material passing the fixing unit 5 becomes longer, the control unit 301 is preferable to control the rotation speed of the fixing unit 5 when transporting the subsequent recording material so that it becomes smaller than the rotation speed of the fixing unit 5 when transporting the preceding recording material.

[0074] The rotational speed of the fixing unit 5, which the control unit 301 changes according to the interval between the rear end of the preceding recording material and the front end of the following recording material passing through the fixing unit, may include at least one of the rotational speeds Ni, Nr, and Nt. The rotational speeds Ni, Nr, and Nt are the rotational speeds in the transport section other than the transport section in which the rotational speed of the fixing unit 5 is varied to rotational speeds Nh and Nl based on the deflection detection result (detection information) of the deflection detection unit 200 so that the deflection of the recording material falls within a predetermined range.

[0075] According to Example 1, even if the pressure roller 132 undergoes rapid thermal expansion due to the recording material passing interval becoming longer than the normal interval through recording material control, the rotational speed of the pressure roller 132 can be corrected to stabilize the conveying speed of the recording material by the fixing unit 5. According to Example 1, the rotational speed of the fixing unit 5 can be controlled in accordance with the change in the recording material passing interval. [Examples]

[0076] The following describes Example 2. In Example 2, structures similar to those in Example 1 are given the same reference numerals and their descriptions are omitted. The configuration and operation of the image forming apparatus 100 in Example 2, as well as the fixing drive control according to the timing of the transport of the recording material, are the same as in Example 1, so their descriptions are omitted. The rotational speed correction of the fixing unit 5 in Example 2 differs from that in Example 1. The differences will be mainly described below.

[0077] (Rotation speed correction of the fixing part based on temperature estimation of the pressure roller) As mentioned above, correcting the transport speed of the subsequent recording material based on the deflection detection result of the preceding recording material is effective against the gradual speed increase trend shown by the dashed line in Figure 5, and can counteract the speed increase due to the expansion of the pressure roller 132. However, for recording material where the length in the transport direction CD is not very long relative to the distance from the secondary transfer section 4 to the fixing section 5, the trailing end of the recording material passes the secondary transfer section 4 shortly after the leading end reaches the fixing section 5. Therefore, a sufficient transport section (control time from S9 to S11 in Figure 4 or Figure 8A) can not be secured in which the two rotation speeds Nh and Nl are switched and controlled based on the deflection detection result of the recording material between the secondary transfer section 4 and the fixing section 5. As a result, the deflection amount of the recording material is unstable, the detection accuracy decreases, and the calculation result of the average rotation speed Nave by equation (1) becomes inappropriate. Therefore, in Example 2, the temperature of the pressure roller 132 is estimated and the rotational speed of the pressure roller 132 is corrected based on the heat balance of the amount of heat supplied by the heater 133 (amount of power supplied), the amount of heat absorbed by the recording material passing through the fixing section 5, and the amount of heat dissipated from the pressure roller 132 to the atmosphere.

[0078] Let ΔQin be the amount of heat supplied from the heater 133 to the pressure roller 132 while one recording material passes through the fixing nip section FN. Let ΔQout be the amount of heat removed from the pressure roller 132 while one recording material passes through. Let Tf be the estimated temperature of the pressure roller 132. Let T0 be the ambient temperature around the pressure roller 132. Let Δtp be the recording material passing interval between the nth recording material and the (n+1)th recording material. The temperature change ΔTf of the pressure roller 132 when conveying the (n+1)th recording material, relative to the temperature of the pressure roller 132 when conveying the nth recording material, is expressed by the following equation (7).

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[0079] Here, the heat capacity C, heat transfer coefficient h to the atmosphere, and surface area S of the pressure roller 132 are parameters that are stored in the memory unit 302 in advance according to the configuration of the fixing unit 5. The amount of heat ΔQout removed from the pressure roller 132 is appropriately determined by multiplying the amount of heat per unit length in the transport direction CD for each type of recording material stored in the memory unit 302 by the length of the transport direction CD of the recording material. The recording material passage interval Δtp is obtained from the stop time of the registration roller 115, the interval of the exposure start signals, or the time interval of the recording material passing at a predetermined position, as described in Example 1. When forming images on multiple recording materials, the recording material passage interval Δtp may be obtained from the interval between the exposure start signal for the preceding recording material and the exposure start signal for the subsequent recording material. When forming images on multiple recording materials, the recording material passage interval Δtp may be obtained from the stop time, drive start interval, or drive stop interval of the registration drive motor 401. The ambient temperature T0 around the pressure roller 132 is obtained from an environmental sensor (temperature and humidity sensor) 120 provided in the image forming apparatus 100. Alternatively, the ambient temperature T0 around the pressure roller 132 may be obtained from a temperature sensor located near the pressure roller 132, separate from the environmental sensor 120.

[0080] If α is the coefficient of linear expansion of the pressure roller 132 with respect to the temperature change ΔTf, then the diameter d of the pressure roller 132 increases by αΔTf × d, and the transport speed of the fixing unit 5 is approximately proportional to the temperature of the pressure roller 132. Therefore, the rate of increase Vf_rate of the transport speed Vf(n+1) of the (n+1) recording material Pn+1 with respect to the transport speed Vf(n) of the nth recording material Pn is expressed by the following equation (8).

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[0081] Here, Vf0 is the transport speed of the fixing unit 5, which is in standby mode at a reference rotation speed N0 pre-stored in the memory unit 302. k is an increase coefficient that represents the amount of increase in transport speed with respect to the change in the diameter d of the pressure roller 132. The transport speed Vf0 and the increase coefficient k are parameters that are pre-stored in the memory unit 302.

[0082] Next, the control operation of the fixing unit 5 using equations (7) and (8) will be explained with reference to Figures 9A and 9B. Figures 9A and 9B are flowcharts showing the control operation of the rotational speed correction of the fixing unit 5 based on the temperature estimation of the pressure roller 132. In Figures 9A and 9B, the same reference numerals are used for steps that are the same as those in Figures 4, 8A, and 8B, and their explanations are omitted.

[0083] The control unit 301 starts controlling the fixing temperature of the fixing roller 131 (S2), and when the surface temperature of the fixing roller 131 reaches a predetermined temperature and the fixing unit 5 enters a standby state (YES in S4), the image forming unit starts its imaging operation (S5). The reference rotation speed N0(1) of the fixing unit 5 for the first recording material P1 is an initial value stored in the storage unit 302. Similar to Embodiment 1, the rotation speeds Ni, Nr, and Nt of the pressure roller 132 while the recording material is passing are set by their respective ratios to the reference rotation speed N0. Therefore, the rotation speed of the pressure roller 132 from before the leading edge of the recording material P1 reaches the fixing nip FN until the trailing edge of the recording material P1 passes through the fixing nip FN can be uniquely determined.

[0084] In Example 1, after the deflection control of the first recording material P1 is completed, the rotation speed of the pressure roller 132 is switched to rotation speed Nr (S11), and the control unit 301 calculates the average rotation speed Nave of the pressure roller 132 from equation (1) based on the deflection detection result (S115). Then, the control unit 301 calculates a correction value ΔN0, which is the difference between the average rotation speed Nave and the reference rotation speed N0(1) of the first recording material P1, and sets the reference rotation speed N0(2) for the second recording material P2 based on the correction value ΔN0 (S116). In contrast, in Example 2, equations (7) and (8) are used instead of equation (1). The control unit 301 obtains the amount of heat ΔQin supplied from the heater 133 to the pressure roller 132 while the first recording material P1 passes through the fixing nip section FN (S201). The control unit 301 calculates the temperature change ΔTf of the pressure roller 132 from equation (7) using the heat quantity ΔQin (S202). The control unit 301 calculates the rate of increase Vf_rate of the transport speed of the second recording material P2 from equation (8) using the temperature change ΔTf (S203). The control unit 301 calculates the reference rotation speed N0(2) of the fixing unit 5 for the second recording material P2 from equation (3) using the rate of increase Vf_rate, and corrects the reference rotation speed N0 to offset the rate of increase Vf_rate (S204). The subsequent control operation for correcting the rotation speed of the fixing unit 5 is the same as in Embodiment 1 shown in Figures 8A and 8B. In Embodiment 2, the value (initial value) stored in the storage unit 302 as the reference rotation speed N0 of the fixing unit 5 is not overwritten, and the difference between the reference rotation speed N0(j) obtained from equation (3) and the initial reference rotation speed N0 is stored in the storage unit 302 as the correction value ΔN0.

[0085] According to Example 2, even for recording materials with a short transport direction CD length, where it is difficult to correct the rotational speed of the fixing unit 5 using the deflection detection result, the transport speed of the fixing unit 5 can be appropriately corrected by estimating the temperature change of the pressure roller 132. Depending on the length of the recording material, the rotational speed correction using the deflection detection result in Example 1 and the rotational speed correction using the temperature change ΔTf of the pressure roller 132 in Example 2 may be switched to control the rotational speed of the fixing unit 5. According to Example 2, the rotational speed of the fixing unit 5 can be controlled in accordance with changes in the distance between the passing recording materials. [Examples]

[0086] The following describes Example 3. In Example 3, the same reference numerals are used for structures similar to those in Example 1, and their descriptions are omitted. The configuration and operation of the image forming apparatus 100 in Example 3 are the same as in Example 1, so their descriptions are omitted. The rotation speed compensation of the fixing unit 5 in Example 3 differs from that in Example 1. The differences will be mainly described below.

[0087] (Fusing unit rotation speed correction based on pressure roller temperature estimation at the start of the print job) Examples 1 and 2 describe the fuser drive control from the acceptance of a print job to the completion of the print operation. When the image forming operation is started when the temperature of the pressure roller 132 is approximately the same as the ambient temperature, the rotation speed of the fuser unit 5 (third rotation speed) is set based on the reference rotation speed N0 stored as an initial value in the memory unit 302 when passing the first recording material P1 through the fuser unit 5. This is effective in cases where an image forming apparatus 100 installed in a typical office environment occasionally prints on a small number of recording materials.

[0088] However, when performing a print operation that continuously feeds a large amount of recording material, as can be seen from the rate of increase in the transport speed of the fixing unit 5 shown in Figure 5, the pressure roller 132 expands significantly immediately after the print operation, resulting in a large temperature rise compared to the initial state. As mentioned above, the pressure roller 132 has an elastic layer such as silicone rubber on the outside of a solid or hollow cylindrical metal shaft, and a release layer of fluororesin is formed on the surface of the elastic layer. Since the heat capacity of the pressure roller 132 is large, even if the power supply to the heater 133 is stopped, the temperature of the pressure roller 132 does not immediately return to the original ambient temperature. Therefore, for example, if the next print operation is performed without much time having passed after continuously feeding a large amount of recording material, the pressure roller 132 will be reheated during natural cooling. In this case, if fixing drive control is performed for the first sheet of recording material using the reference rotation speed N0 that is pre-stored in the memory unit 302, the temperature of the pressure roller 132 will be higher than expected, and because the diameter of the pressure roller 132 is large, the transport speed of the recording material will become faster.

[0089] In Figure 5, an example is shown where the transport speed of the fixing unit 5 gradually increases with the continuous feeding of the recording material, but it may also gradually decrease. For example, if in the previous print job an image is formed on recording material that requires a large amount of heat to fix the toner image, causing the temperature of the pressure roller 132 to rise, and then in the next print job an image is formed on recording material that requires less heat, the temperature of the pressure roller 132 will gradually decrease.

[0090] Therefore, in Example 3, the temperature drop of the pressure roller 132 is estimated based on the elapsed time after the print operation is completed and heating by the heater 133 is stopped, and the rotation speed of the fixing unit 5 to the first recording material during the next print operation (fourth rotation speed) is corrected.

[0091] When the printing operation is completed and the power supply to the heater 133 is stopped, the temperature change of the pressure roller 132 is expressed by the following equation (9).

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[0092] Considering that the ambient temperature T0 around the pressure roller 132 also changes over time, equation (9) can be expressed as a difference equation by the following equation (10).

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[0093] Next, the control operation of the fuser unit 5 using equation (10) will be explained with reference to Figures 10A, 10B, 10C, and 10D. Figures 10A, 10B, 10C, and 10D are flowcharts showing the control operation of the rotational speed correction of the fuser unit 5 based on the temperature estimation of the pressure roller 132 while the print job is stopped. Note that in Figures 10A, 10B, 10C, and 10D, the same reference numerals are used for operations that are the same as those in Figures 4, 8A, 8B, 9A, and 9B, and their explanation is omitted.

[0094] The description of Embodiment 3 begins with the ejection of the last recording material from the previous print job. Referring to Figure 10D, in S16, the control unit 301 determines whether or not there is any subsequent recording material. If it is determined that there is no subsequent recording material (NO in S16), the control unit 301 determines whether or not the last recording material has been ejected (S17). If the last recording material has been ejected (YES in S17), the control unit 301 stops the driving of each part, including the fixing unit 5, and the power supply to the heater 133 (S18), and ends the print operation (S19). At the same time, the control unit 301 starts measuring the power supply stop time ts to the heater 133 (S301). Subsequently, at each preset update interval Δts, the control unit 301 obtains the ambient temperature T0 around the pressure roller 132 using the environmental sensor 120 and calculates the estimated temperature Tf of the pressure roller 132 from equation (10). Specifically, the control unit 301 determines whether an update interval Δts has elapsed since the last update of the estimated temperature Tf of the pressure roller 132 (S302). If it is determined that the update interval Δts has elapsed (YES in S302), the control unit 301 obtains the ambient temperature T0 around the pressure roller 132 based on the detection result of the environmental sensor 120 (S303). The control unit 301 calculates the estimated temperature Tf of the pressure roller 132 from equation (10) using the power supply downtime ts, the update interval Δts, and the ambient temperature T0 (S304).

[0095] The control unit 301 sequentially saves the updated latest estimated temperature Tf of the pressure roller 132 in the storage unit 302 (S305). The control unit 301 determines whether the ambient temperature T0 around the pressure roller 132 is within a predetermined temperature range and whether the estimated temperature Tf of the pressure roller 132 is below the predetermined temperature (S306). If the ambient temperature T0 is not within the predetermined temperature range or the estimated temperature Tf is greater than the predetermined temperature (NO in S306), the control unit 301 returns the process to S302 and updates the ambient temperature T0 and estimated temperature Tf after the update interval Δts has elapsed. If the ambient temperature T0 is within the predetermined temperature range and the estimated temperature Tf is below the predetermined temperature (YES in S306), the update of the estimated temperature Tf of the pressure roller 132 according to equation (10) is stopped (S307). In Example 3, when the ambient temperature T0 around the pressure roller 132 is within a predetermined temperature range of 20 to 27°C and the estimated temperature Tf of the pressure roller 132 falls below the predetermined temperature of 27°C, the control unit 301 stops updating the estimated temperature Tf.

[0096] The previous print job has finished and a new print job has started. Referring to Figure 10A, when the control unit 301 receives a new print job (S1), it obtains the latest estimated temperature Tf of the pressure roller 132 from the storage unit 302 (S308). The control unit 301 proceeds to processing S311 and S309. The control unit 301 outputs a power control signal to the heater 133 to start fixing temperature control of the fixing roller 131 and starts measuring the fixing temperature control time tm (S311). The control unit 301 starts driving the pressure roller 132 of the fixing unit 5 with the fixing drive motor 403 (S3). Meanwhile, the control unit 301 determines whether the estimated temperature Tf of the pressure roller 132 is below a predetermined temperature (S309). If the estimated temperature Tf is below a predetermined temperature (YES in S309), the control unit 301 reads the reference rotation speed N0 that has been previously stored as an initial value in the storage unit 302 to set the rotation speed of the pressure roller 132 thereafter (S310). The control unit 301 proceeds to S4.

[0097] If the estimated temperature Tf of the pressure roller 132 has not returned to the predetermined temperature and the estimated temperature Tf of the pressure roller 132 is greater than the predetermined temperature (NO in S309), the control unit 301 corrects the reference rotation speed N0 taking into account the estimated temperature Tf of the pressure roller 132. The control unit 301 determines whether the fixing unit 5 has entered a standby state based on the detection signal from the fixing roller temperature sensor 134 (S312). When the fixing unit 5 enters a standby state (YES in S312), the control unit 301 ends the measurement of the fixing temperature control time tm until the standby state and obtains the required time Δtm (S313). The control unit 301 obtains the amount of heat ΔQin_m supplied from the heater 133 to the pressure roller 132 until the standby state (S314). The control unit 301 calculates the estimated temperature Tf(tm+Δtm) of the pressure roller 132 when the first recording material P1 is passed through the fixing unit 5 from the following equation (11) (S315).

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[0098] The control unit 301 uses the estimated temperature Tf(tm+Δtm) of the pressure roller 132 to calculate the rate of increase in transport speed Vf_rate while passing the first recording material P1 through the fixing unit 5 from the following equation (12) (S316).

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[0099] Here, Tf0 is a parameter that is pre-stored in the memory unit 302 as the standard temperature of the pressure roller 132 when the temperature control of the fixing unit 5 is started and the unit enters a standby state after being left in an office environment. The control unit 301 calculates the reference rotation speed N0(1) for the first recording material P1 from the following equation (13) and corrects the reference rotation speed N0 (S317).

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[0100] In Embodiment 3, the value of the reference rotation speed N0 of the fixing unit 5, which has been stored in the storage unit 302 in advance (initial value), is not rewritten. Instead, the difference between this value and N0(1) calculated by equation (13) is stored in the storage unit 302 as a correction value ΔN0 relative to the initial value.

[0101] In Example 3, an example of correcting the reference rotational speed N0 using equations (11), (12), and (13) was described, but this is not the only method. For example, for simplicity, the relationship between the estimated temperature Tf(tm+Δtm) of the pressure roller 132 when temperature control by the heater 133 is restarted and the rate of increase of the transport speed of the first recording material Vf_rate may be stored in the storage unit 302 in advance as a table. Alternatively, the relationship between the rate of increase of the transport speed Vf_rate and the correction value ΔN0 of the reference rotational speed N0 may be stored in the storage unit 302 in advance as a table. Or, the relationship between the estimated temperature Tf(tm+Δtm) of the pressure roller 132 when temperature control by the heater 133 is restarted and the correction value ΔN0 of the reference rotational speed N0 may be stored in the storage unit 302 in advance as a table.

[0102] In Example 3, Figure 10B shows a control operation in which the reference rotation speed N0 is corrected (S115, S116) using the deflection detection result described in Example 1 in response to the increase in transport speed due to thermal expansion of the pressure roller 132 during continuous paper feeding. However, this is not the only option, and the correction of the reference rotation speed N0 of the fixing unit 5 based on the temperature estimation of the pressure roller 132 described in Example 2 (S201~S204 in Figure 9B) may also be performed. Alternatively, the rotation speed of the fixing unit 5 may be controlled by switching between the two methods depending on the length of the recording material.

[0103] According to Example 3, even if a print operation is performed again before the temperature of the pressure roller 132 returns to normal after the print operation is completed and heating by the heater 133 is stopped, the temperature drop of the pressure roller 132 between print jobs is estimated. Based on the estimated temperature of the pressure roller 132, the rotation speed of the fuser unit 5 can be appropriately corrected from the first recording material. According to Example 3, even if the next print job is started before the temperature of the pressure roller 132 returns to ambient temperature after the completion of the previous print job, the rotation speed of the fuser unit 5 for the first recording material can be appropriately corrected.

[0104] According to Examples 1 to 3, the transport speed of the fixing unit 5 can be controlled even for recording materials with a short transport direction CD length, which is difficult to stably detect the amount of deflection, and also to cope with the rapid change in the diameter of the pressure roller 132 when the gap between the passing recording materials widens. This suppresses image defects, damage to the recording material, and transport defects. According to Example 3, the rotation speed of the fixing unit 5 can be controlled in accordance with changes in the gap between the passing recording materials.

[0105] <Other examples> The present invention can also be realized by supplying a program that implements one or more of the functions of each of the above embodiments to a system or device via a network or storage medium, and by having one or more processors in the computer of that system or device read and execute the program. It can also be realized by a circuit that implements one or more functions (for example, an application-specific integrated circuit (ASIC) or a programmable gate array (FPGA)). [Explanation of Symbols]

[0106] 4. Secondary transfer section 5. Fixing section 100...Image forming apparatus 132... Pressure roller 133... Heater 301... Control Unit 403... Fuser drive motor CD transport direction P... Recording materials

Claims

1. a transfer section that transfers a toner image onto a recording material that is conveyed in a conveyance direction; a fixing section that is disposed downstream of the transfer section in the transport direction, the fixing section having a cylindrical rotating member that transports the recording material and a heating means, and that fixes the toner image onto the recording material transported from the transfer section; a fixing drive unit that drives the cylindrical rotating member; a control unit that controls the fixing drive unit to control the rotation speed of the cylindrical rotation member; Equipped with The control unit controls the fixing drive unit when forming images on multiple recording materials, based on the time from when the trailing end of a preceding recording material passes through the fixing unit to when the leading end of a subsequent recording material following the preceding recording material passes through the fixing unit, so that the second rotational speed of the cylindrical rotating member when transporting the subsequent recording material by the cylindrical rotating member is smaller than the first rotational speed of the cylindrical rotating member when transporting the preceding recording material by the cylindrical rotating member.

2. 2. The image forming apparatus according to claim 1, wherein the control unit controls the fixing drive unit so that the difference between the first rotation speed and the second rotation speed becomes larger as the time from when the trailing end of the preceding recording material passes through the fixing unit to when the leading end of the succeeding recording material passes through the fixing unit becomes longer.

3. The image forming apparatus according to claim 1 or 2, characterized in that after the time from when the trailing end of the preceding recording material passes through the fixing unit to when the leading end of the succeeding recording material passes through the fixing unit becomes long, the control unit changes the rotational speed of the cylindrical rotating member from the second rotational speed to increase each time a recording material is transported, and controls the fixing drive unit so that the rotational speed converges toward the first rotational speed.

4. The image forming apparatus according to claim 3, characterized in that the control unit controls the fixing drive unit so that the change width by which the rotational speed of the cylindrical rotating member is changed from the second rotational speed to an increased value each time a sheet of recording material is transported decreases each time a sheet of recording material is transported.

5. the control unit controls the fixing drive unit so as to sequentially switch the rotation speed of the cylindrical rotating member for each of a plurality of conveying sections according to a predetermined timing while the leading end to the trailing end of the recording material passes through the fixing unit; The image forming apparatus according to any one of claims 1 to 4, characterized in that when the time from when the trailing end of the preceding recording material passes through the fixing unit to when the leading end of the following recording material passes through the fixing unit becomes long, the control unit controls the fixing drive unit so that the rotational speed of at least one of the plurality of conveying sections for the following recording material becomes slower than the rotational speed of the same conveying section for the following recording material.

6. The image forming apparatus according to any one of claims 1 to 5, characterized in that, when the multiple recording materials are continuously transported at approximately the same time intervals to form the images on the multiple recording materials, the control unit gradually changes the rotational speed of the cylindrical rotating member to either increase or decrease depending on the number of recording materials passing through the fixing unit, and controls the fixing drive unit so that the total amount of change in the rotational speed while a predetermined number of recording materials are transported at approximately the same time intervals decreases as the number of recording materials passing through increases.

7. a conveying speed detection unit disposed between the transfer unit and the fixing unit, the conveying speed detection unit detecting a conveying speed of the recording material when the recording material is conveyed between the transfer unit and the fixing unit; The image forming apparatus according to any one of claims 1 to 6, characterized in that, when the multiple recording materials are transported continuously at approximately the same time intervals to form the images on the multiple recording materials, the control unit controls the fixing drive unit so as to control the rotational speed of the cylindrical rotating member when transporting the subsequent recording material based on the transport speed detection result of the preceding recording material.

8. the conveying speed detection unit includes a slack amount detection unit that detects a slack amount of the recording material between the transfer unit and the fixing unit, the sag amount detection unit detects at least a state in which the recording material has a first sag amount and a state in which the recording material has a second sag amount that is larger than the first sag amount, the control unit controls the fixing drive unit to change the rotation speed of the cylindrical rotation member based on a deflection amount detection result of the deflection amount detection unit so that the deflection amount falls within a predetermined range; 8. The image forming apparatus according to claim 7, wherein the control unit calculates an average rotational speed of the cylindrical rotating member from a detection time and the rotational speed while the recording material is in a state having the first amount of sagging, and a detection time and the rotational speed while the recording material is in a state having the second amount of sagging.

9. The image forming apparatus according to claim 8, characterized in that the rotational speed of the cylindrical rotating member, which is changed according to the time from when the trailing end of the preceding recording material passes through the fixing section to when the leading end of the succeeding recording material passes through the fixing section, includes at least the rotational speed of the cylindrical rotating member in a conveying section other than a conveying section in which the rotational speed of the cylindrical rotating member is changed so that the amount of deflection falls within the specified range based on the deflection amount detection result of the deflection amount detection section.

10. The image forming apparatus of any one of claims 1 to 9, characterized in that, when forming the image on the multiple recording materials, the control unit obtains the time from when the trailing end of the preceding recording material passes through the fixing unit to when the leading end of the subsequent recording material passes through the fixing unit from the time from an exposure start signal for the preceding recording material to an exposure start signal for the subsequent recording material.

11. a registration unit disposed upstream of the transfer unit in the transport direction and configured to transport the recording material to the transfer unit; a registration driving unit that drives the registration unit, the control unit, when forming the images on the plurality of recording materials, controls the registration drive unit to stop the registration drive unit at least once after the trailing end of the preceding recording material has passed through the registration unit, and then starts driving the registration drive unit again to transport the succeeding recording material to the transfer unit; The image forming apparatus according to any one of claims 1 to 9, characterized in that the control unit obtains the time from when the trailing end of the preceding recording material passes through the fixing unit to when the leading end of the succeeding recording material passes through the fixing unit from the stop time, drive start interval or drive stop interval of the registration drive unit.

12. a registration unit disposed upstream of the transfer unit in the transport direction and configured to transport the recording material to the transfer unit; a recording material leading edge detection unit that is disposed on a conveyance path from the registration unit to the fixing unit and detects a passing timing of the leading edge of the recording material; The image forming apparatus according to any one of claims 1 to 9, characterized in that, when forming the image on the multiple recording materials, the control unit obtains the time from when the trailing end of the preceding recording material passes through the fixing unit to when the leading end of the succeeding recording material passes through the fixing unit from the detection result of the recording material leading end detection means.

13. a registration unit disposed upstream of the transfer unit in the transport direction and configured to transport the recording material to the transfer unit; a recording material trailing edge detection unit that is disposed on a conveying path from a position upstream of the registration unit by a length of the recording material in the conveying direction to the transfer unit and detects a passing timing of the trailing edge of the recording material, The image forming apparatus according to any one of claims 1 to 9, characterized in that, when forming the image on the multiple recording materials, the control unit obtains the time from when the trailing end of the preceding recording material passes through the fixing unit to when the leading end of the succeeding recording material passes through the fixing unit from the detection result of the recording material trailing end detection means.

14. a transfer section that transfers a toner image onto a recording material that is conveyed in a conveyance direction; A transfer driving unit that drives the transfer unit; a fixing section that is disposed downstream of the transfer section in the transport direction, the fixing section having a cylindrical rotating member that transports the recording material and a heating means, and that fixes the toner image onto the recording material transported from the transfer section; a fixing drive unit that drives the cylindrical rotating member; a control unit that controls the fixing drive unit to control the rotation speed of the cylindrical rotation member; Equipped with the control unit controls the heating unit so that the temperature of the heating unit becomes at least a first temperature at which the toner image is fixed onto the recording material, and a second temperature lower than the first temperature; when an image forming operation for forming images on a plurality of recording materials is started from a state in which the temperature of the cylindrical rotating member is substantially equal to an environmental temperature, the control unit controls the fixing drive unit so that the rotation speed of the cylindrical rotating member for a first sheet of recording material becomes a third rotation speed; an image forming apparatus, characterized in that the control unit stops the transfer drive unit after the image forming operation is completed, controls the heating means to change the temperature of the heating means from the first temperature to the second temperature, and then resumes driving of the transfer drive unit to form an image on at least one sheet of recording material, controls the heating means to return the temperature of the heating means to the first temperature, and controls the fixing drive unit so that the rotational speed of the cylindrical rotating member becomes a fourth rotational speed lower than the third rotational speed.

15. the second temperature is the environmental temperature; 15. The image forming apparatus according to claim 14, wherein the control unit stops the supply of power to the heating unit in order to change the temperature of the heating unit to the second temperature.

16. 16. The image forming apparatus according to claim 14, wherein the control unit controls the fixing drive unit so that the fourth rotation speed is greater than the rotation speed of the cylindrical rotating member for the last recording material in the image forming operation.

Images