Image forming device

Abstract

To provide an image forming device capable of suppressing an image defect caused by rubbing of an image due to curvature of a recording material.SOLUTION: In an image forming device, a control unit sets the conveyance speed of a recording material in a fixing unit in accordance with information related to a difference between the right and left in image density in a width direction of the recording material orthogonal to a conveyance direction, in addition to the output of a curvature detection unit.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus that forms an image by electrophotography and fixes the formed image on a recording material. [Background technology]

[0002] An image forming apparatus such as an electrophotographic copying machine or a printer includes an image forming section that transfers a toner image carried by an image carrier to a recording material at a transfer nip section to form a toner image on the recording material. The image forming apparatus further includes a fixing section that heats the recording material on which an unfixed toner image has been formed in the image forming section while conveying the recording material at the nip section to fix the toner image onto the recording material.

[0003] In the above image forming apparatus, when the recording material is transported from the image forming section to the fixing section, if the fixing section transports the recording material faster than the image forming section, the recording material is pulled between the image forming section and the fixing section, which may result in distorted images during transfer, elongation of the image (sub-scanning magnification), or color shift in the case of a color image.

[0004] On the other hand, if the recording material is transported at a slower speed in the fixing section than in the image forming section, the recording material will curve (sag) between the image forming section and the fixing section, and if this curvature is too large, the image surface of the recording material will rub against the transport path, causing the image to be distorted.

[0005] Therefore, in order to suppress the pulling of the recording material and to optimize the degree of curvature (hereinafter referred to as the "curve amount"), it is necessary to control the difference in conveying speed between the image forming unit and the fixing unit.

[0006] Therefore, there is a device that provides a curvature sensor between the image forming unit and the fixing unit to detect the amount of curvature of the recording material, and controls the speed of the recording material conveyed by the fixing unit to accelerate or decelerate based on the results of the detection of the amount of curvature. This optimizes the amount of curvature of the recording material, prevents the recording material from being pulled by the image forming unit and the fixing unit, and suppresses excessive sagging.

[0007] However, in a configuration in which the curvature sensor is disposed at only one location in the center of the width direction of the recording material perpendicular to the conveying direction of the recording material, even if the amount of curvature in the center is appropriately controlled by controlling the acceleration / deceleration of the conveying speed of the recording material, the amount of curvature on the left and right sides of the center may not be appropriate. In particular, when a difference occurs between the left and right sides of an image in the width direction of the recording material, one of the right and left regions in the width direction of the recording material may be curvatured more, while the other region may be hardly curvatured at all. In other words, a state in which the difference in the amount of curvature between the right and left regions of the recording material is large (hereinafter referred to as "one-sided curvature") may occur.

[0008] In this way, when one side of the recording material is curved, there is a curve in the curvature detection area, but if the recording material is pulled in other areas, this cannot be detected, resulting in a problem of poor image quality due to the pulling of the recording material.

[0009] Patent Document 1 proposes a method of arranging multiple curvature sensors in the width direction of the recording material and detecting the amount of curvature at each position in the width direction. This method controls the conveying speed of the recording material in the fixing unit so that even if one side is curving, a proper amount of curvature is set based on the detection results of the left and right curvature sensors in the width direction of the recording material, preventing image defects caused by pulling the recording material. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2007-52112 A Summary of the Invention [Problem to be solved by the invention]

[0011] However, the configuration of Patent Document 1 requires multiple curvature sensors, which increases the cost of the device.

[0012] An object of the present invention is to provide an image forming apparatus that suppresses image defects caused by rubbing of an image due to a curved recording material. [Means for solving the problem]

[0013] In order to solve the above-mentioned problems, the present invention provides an image forming apparatus having an image forming unit that forms an image on a recording material, a fixing unit that has a pair of rotating bodies that form a fixing nip portion and heats the recording material while nip-conveying and transporting it in the fixing nip portion to fix the image formed on the recording material to the recording material, a curvature detection unit that is provided at a position between the image forming unit and the fixing unit in the transport direction of the recording material and detects the amount of curvature of the recording material between the image forming unit and the fixing unit, and a control unit that controls the transport speed of the recording material in the fixing unit in accordance with the output of the curvature detection unit to adjust the amount of curvature of the recording material, wherein the control unit sets the transport speed of the recording material in the fixing unit in accordance with information relating to the left-right difference in image density in the width direction of the recording material perpendicular to the transport direction, in addition to the output of the curvature detection unit.

[0014] The present invention also provides an image forming apparatus having an image forming unit that forms an image on a recording material, a fixing unit that has a pair of rotating bodies that form a fixing nip portion and that heats the recording material while nipping and transporting it in the fixing nip portion to fix the image formed on the recording material to the recording material, a curvature detection unit that is provided at a position between the image forming unit and the fixing unit in the transport direction of the recording material and detects the amount of curvature of the recording material between the image forming unit and the fixing unit, and a control unit that controls the transport speed of the recording material in the fixing unit in accordance with an output of the curvature detection unit to adjust the amount of curvature of the recording material, wherein the control unit sets the transport speed of the recording material in the fixing unit in accordance with information related to the surface properties of the recording material and information related to the left-right difference in image density in the width direction of the recording material perpendicular to the transport direction. Effect of the Invention

[0015] According to the present invention, it is possible to provide an image forming apparatus that suppresses image defects caused by rubbing of an image due to a curved recording material. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing an overall configuration of an image forming apparatus. [Diagram 2] FIG. 4 is a diagram showing the attitude of a recording material between a transfer unit and a fixing unit. [Diagram 3] 5 is a schematic diagram of a curvature detection flag and a curvature detection sensor. [Figure 4] FIG. 2 is a diagram showing left and right areas AL and AR in an image. [Diagram 5] FIG. 2 is a block diagram of a control system of the image forming apparatus. [Figure 6] 5 is a graph showing the rotation speed of the motor M2 and the amount of curvature of the recording material in the first embodiment. FIG. [Figure 7] 11 is a graph showing the rotation speed of a motor M2 and the amount of curvature of a recording material in a comparative example. [Figure 8] 6 is a diagram showing the amount of curvature in the paper width direction perpendicular to the conveying direction of the recording material. [Figure 9] 11 is a graph showing the rotation speed of the motor M2 and the amount of curvature of the recording material in the second embodiment. [Figure 10] 6 is a diagram showing the amount of curvature in the paper width direction perpendicular to the conveying direction of the recording material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] [Example 1] (Overall Description of Image Forming Apparatus 60) 1 is a diagram showing the configuration of an electrophotographic printer (image forming apparatus 60). A video controller 120 receives and processes image information and print instructions sent from an external device such as a personal computer. A control unit 40 is connected to the video controller 120, and controls each unit constituting the image forming apparatus in response to instructions from the video controller 120. When the video controller 120 receives a print instruction from an external device, image formation is performed through the following operations.

[0018] When a print command is received from an external device, the photosensitive drum 104 is rotated at a predetermined speed by the power of a drive motor (not shown), and is uniformly charged by a primary charger 105 during the rotation process. After that, a laser beam modulated according to an image signal is output from a laser beam scanner 106, and an electrostatic latent image is formed on the photosensitive drum 104. Reference numeral 107 denotes a developer, which attaches toner to the electrostatic latent image to visualize it as a toner image. The toner image formed on the photosensitive drum 104 is primarily transferred to an intermediate transfer body 103, which rotates in contact with the photosensitive drum 104, by a transfer bias applied to a primary transfer roller 102.

[0019] Here, the photosensitive drum 104, primary charger 105, laser beam scanner 106, and developer 107 are arranged for four colors, yellow (Y), magenta (M), cyan (C), and black (K), respectively. The toner images for the four colors are then transferred in a superimposed manner to the rotating intermediate transfer body 103 by the opposing roller 61 in the same procedure as described above. The toner images superimposed on the intermediate transfer body 103 are sent to the transfer nip portion N1 (see FIG. 2) between the intermediate transfer body 103 and a secondary transfer roller (transfer portion) 67 by the rotation of the intermediate transfer body 103. The above is the configuration of the image forming unit IFS.

[0020] Meanwhile, the recording material P loaded on the sheet stacking tray 68a of the feeding cassette 68 is picked up one by one by a feeding roller 69 driven at a predetermined control timing, and is sent to the registration section by a pair of conveying rollers 70. In the registration section, the leading edge of the recording material P is once received at a nip between a registration roller 71 and a roller 71a to correct the skew of the recording material P. A registration sensor 72 is also disposed in the registration section, and detects the arrival timing of the leading edge and the trailing edge of the recording material P, and the length of the recording material. After that, the recording material P is fed from the registration section to the transfer nip N1. The control section 40 controls the conveyance of the intermediate transfer body 103 so that the timing when the leading edge of the recording material P reaches the transfer nip N1 and the timing when the leading edge of the toner image on the intermediate transfer body 103 reaches the transfer nip N1 are synchronized.

[0021] The recording material P fed to the transfer nip portion N1 is nipped and transported between the intermediate transfer body 103 and the secondary transfer roller 67. Then, during the process of transporting the recording material P, the toner image on the intermediate transfer body 103 is transferred onto the recording material P by a transfer bias applied to the secondary transfer roller 67. The recording material P separated from the surface of the intermediate transfer body 103 is transported to a fixing unit 74 along a transport guide 73.

[0022] FIG. 2 shows a schematic diagram of the transfer nip portion N1 to the fixing portion 74. The fixing portion 74 has a cylindrical fixing film 83, a heater 84 arranged in the internal space of the fixing film, and a pressure roller 82 that forms the fixing nip portion N2 with the heater 84 through the fixing film 83. The fixing film 83 and the pressure roller 82 correspond to a pair of rotating bodies that form the fixing nip portion N2 that sandwiches and conveys the recording material P. The fixing film 83 has a base layer made of metal such as stainless steel, a rubber layer, and a release layer made of fluororesin. The heater 84 is a plate-shaped substrate (made of ceramic or metal with an insulating coating on the surface) on which a heating resistor is printed. The unfixed toner image is heated and fixed to the recording material P by applying heat and pressure to the recording material P carrying the unfixed toner image at the fixing nip portion N2. The recording material P discharged from the fixing portion 74 is conveyed to the discharge roller 75 (see FIG. 1). The discharge roller 75 then discharges the recording material P to the discharge tray 76 (see FIG. 1).

[0023] A movable regulating guide (not shown) for stacking various recording materials of different sizes is provided on the sheet stacking table 68a of the feeding cassette 68. By displacing the regulating guide in accordance with the size of the recording material P and stacking the recording material P on the sheet stacking table 68a, various recording materials of different sizes can be picked up one by one from the feeding cassette 68 by the paper feeding roller 69.

[0024] The image forming apparatus 60 of the first embodiment is an image forming apparatus 60 compatible with A4 size paper, and has a print speed of 63 sheets / minute (when printing on A4 size paper).

[0025] (Description of the state of the recording material P between the transfer nip portion N1 and the fixing nip portion N2) A conveying guide 73 and a fixing section entrance guide 79 are provided between the transfer nip N1 and the fixing nip N2, and smoothly guide the recording material P separated from the transfer nip N1 to the fixing nip N2. Furthermore, the conveying guide 73 and the fixing section entrance guide 79 are formed in a gently curved shape so that they can accommodate the recording material P if it curves between the transfer nip N1 and the fixing nip N2.

[0026] Next, the operation of the curvature detection flag 80 will be described in detail with reference to FIG. 3. The curvature detection unit is provided between the image forming unit IFS and the fixing unit 74 in the conveying direction of the recording material P, and detects the amount of curvature of the recording material between the image forming unit IFS and the fixing unit 74. The curvature detection flag 80 for detecting the amount of curvature of the recording material P is provided on the conveying surface of the conveying guide 73. The curvature detection flag 80 is made of a rod-shaped member that can swing around one end, and is disposed so that the other end protrudes from the conveying surface of the conveying guide 73. The curvature detection flag 80 is biased by a spring member (not shown), and when the recording material P is conveyed, it comes into contact with the recording material P at the center position in the width direction (hereinafter, paper width direction) perpendicular to the conveying direction, and swings according to the amount of curvature L formed on the recording material P. One end of the curvature detection flag 80, which swings according to the curvature amount L, blocks or opens a detection position S (optical path) of a curvature detection sensor 81 made of a photointerrupter for detecting whether the curvature amount L of the recording material P has exceeded a predetermined value Lsns. FIG. 3(a) shows the blocking position, and FIG. 3(b) shows the opening position. That is, the curvature detection sensor 81 turns on / off according to the swinging motion of the curvature detection flag 81. The curvature amount L is the distance between a straight line connecting the transfer nip portion N1 and the fixing nip portion N2 and the position of the recording material P at the position where the curvature detection flag 80 is provided in the recording material transport direction and the recording material width direction.

[0027] Making the conveying speed of the fixing nip N2 slower than that of the transfer nip N1 increases the amount of curvature L, and making the conveying speed of the fixing nip N2 faster than that of the transfer nip N1 decreases the amount of curvature L. In this way, the control unit 40 adjusts the amount of curvature of the recording material P by controlling the conveying speed of the recording material P in the fixing unit 74 in accordance with the output of the curvature detection unit.

[0028] (Acquisition of left-right difference in image density) The control unit 40 in the first embodiment sets the conveying speed of the recording material P at the fixing nip N2 in accordance with information from the curvature detection sensor 81 and information on the left-right difference in image density on the recording material P. Here, a method of acquiring information on the left-right difference in image density on the recording material P (information related to the left-right difference in image density in the width direction of the recording material P perpendicular to the conveying direction) will be described.

[0029] The video controller 120 converts the image density of each color of CMYK at each dot from the bitmap data into a toner amount equivalent value D (%). Specifically, the video controller 120 converts the image density into the toner amount equivalent value D using the following method.

[0030] Image data from an external device such as a host computer is received by a video controller 120 of the image forming apparatus 60, and is converted into bitmap data by image processing in the video controller 120. Note that the number of pixels in the image forming apparatus 60 in the first embodiment is 600 dpi, and the video controller 120 creates bitmap data (image density data for each color of CMYK) accordingly.

[0031] Furthermore, the video controller 120 converts the image density of each color of CMYK for each dot from the bitmap data into a toner amount equivalent value D(%). Specifically, from the image data converted to CMYK image data, the image densities d(C), d(M), d(Y), and d(K) of the colors C, M, Y, and K for each dot are obtained. Then, d(CMYK), which is the sum of the image densities d(C), d(M), d(Y), and d(K) of the colors, is calculated for each dot.

[0032] Here, the image information in the video controller 120 is an 8-bit signal, and the image densities d(C), d(M), d(Y), and d(K) per toner color are expressed in the range of minimum density 00h to maximum density FFh. The sum of these values, d(CMYK), is also an 8-bit signal.

[0033] This d(CMYK) value is then converted into a toner amount equivalent value D(%). Specifically, the minimum image density 00h per toner color is converted into 0%, and the maximum image density FFh into 100%. This toner amount equivalent value D(%) corresponds to the actual toner amount per unit area on the recording material P, and in the first embodiment, the toner amount on the recording material when the image density is FFh is 0.50 mg / cm. 2 =100%.

[0034] d (CMYK) is the total value of a plurality of toner colors, and the value of the toner amount conversion value D (%) may exceed 100%. In the image forming apparatus of the embodiment 1, the toner amount on the recording material P is 1.15 mg / cm for the entire solid image. 2 (equivalent to 230% of the toner amount conversion value D) is adjusted to be the upper limit.

[0035] FIG. 4 shows the left and right areas A and B of the image on the recording material P. L , A R 4 is a diagram showing an example of the conveyance direction of the recording material P in the image forming apparatus 60. In FIG. 4, an example of the recording material P being A4 size is shown. L , A R is an image area divided at the center of the paper width in the conveying direction of the recording material P.

[0036] The video controller 120 is L , A R The average value of the toner amount conversion value D (%) for each Ave (L), D Ave (R) (%) is calculated as the density of each region. In addition, the difference in image density between the left and right, ΔD, is calculated as |D Ave (L)-D Ave Calculate (R)|

[0037] (Bending control according to information from bending detection sensor and left / right image density difference) Next, the curvature control in the first embodiment will be described with reference to the block diagram of FIG. 5. The control unit 40 provided in the image forming apparatus 60 has a timer 41, a CPU 42, and a memory 43. The control unit 40 is connected to a registration sensor 72, a curvature detection sensor 81, and a motor M2. The timer 41 has timers 41a, 41b, and 41c, each of which counts the timing required for the control of the control unit 40 and the time when various controls are executed. The CPU 42 executes various calculations and the like required for the control of the control unit 40. The memory 43 records information required for control, such as the difference between the left and right image densities obtained from the video controller 120 and the passing timing of the leading and trailing ends of the recording material P detected by the registration sensor 72.

[0038] The curvature control will be described with reference to Fig. 2 and Fig. 5. The opposing roller 61 and the secondary transfer roller 67 are rotated by a motor M1. The secondary transfer roller 67 may be rotated by the frictional force with the intermediate transfer body 103. The pressure roller 82 of the fixing unit 74 is driven by a motor M2, and the fixing film 83 is rotated by the frictional force with the pressure roller 82. The motors M1 and M2 are each controlled by the CPU 42.

[0039] Here, the CPU 42 drives the opposing roller 61 and the secondary transfer roller 67 so that the recording material P is transported at a predetermined speed v0 (mm / s) through the transfer nip N1. The CPU 42 also controls the rotation speed of the motor M2 to control the recording material transport speed v (mm / s) through the fixing nip N2 in order to keep the amount of curvature of the recording material P between the transfer nip N1 and the fixing nip N2 within a predetermined range.

[0040] The curvature control switches between three sections when a sheet of recording material P is passed. Figure 6 shows the rotation speed (a) and curvature amount (b) of the motor M2 in three sections (sections 1 to 3) when one sheet of recording material P is passed.

[0041] Section 1 is a section from when the trailing edge in the transport direction of the recording material P of the previous page leaves the fixing nip portion N2 until the curvature detection sensor 81 is first turned ON. The time required for section 1 is defined as time t1. In section 1, the motor M2 rotates at a fixed rotation speed R2, which will be described later. At the end of section 1, the count of the timer 41a is reset to 0.

[0042] Section 2 (first period) is a section from when the leading edge of the recording material P in the conveying direction reaches the fixing nip N2 until a predetermined time t2 has elapsed. The period until the timer 41a counts the predetermined time t2 is determined as section 2.

[0043] In section 2, the rotation speed R of the motor M2 is switched to either the rotation speed R1 or R2 according to the detection signal of the curvature detection sensor 81, and the conveying speed v of the recording material P in the fixing nip portion N2 is controlled. That is, section 2 is a period in which the amount of curvature of the recording material is adjusted while switching between a rotation speed at which the conveying speed of the recording material P in the fixing nip portion N2 is faster than the conveying speed of the recording material P in the transfer nip portion N1 and a rotation speed at which the conveying speed is slower than the conveying speed of the recording material P in the fixing nip portion N2. When the detection signal of the curvature detection sensor 81 is on, R1 is selected as the rotation speed of the motor M2. The rotation speed R1 is a rotation speed at which the conveying speed v of the recording material P in the fixing nip portion N2 is faster than the conveying speed v0 in the transfer nip portion N1. The conveying speed of the recording material P in the fixing nip portion N2 at this time is v1 (mm / s). When the detection signal of the curvature detection sensor 81 is off, the rotation speed R2 of the motor M2 is selected. Rotational speed R2 is a rotational speed at which the conveying speed v of the recording material P at the fixing nip N2 is slower than the conveying speed v0 at the transfer nip N1. The conveying speed of the recording material P at the fixing nip N2 at this time is v2 (mm / s). In this way, by controlling the rotational speed of motor M2 with R1 and R2 in response to the detection signal of curvature detection sensor 81, the amount of curvature of the recording material P is controlled within a predetermined range.

[0044] In section 2, timer 41b counts the total time ΔtR1 during which the rotation speed of motor M2 is controlled by R1. Timer 41c counts the total time ΔtR2 during which the rotation speed of motor M2 is controlled by R2. At the end of section 2, the count of timer 41a is reset to 0, and an average speed Rsns is calculated from the time ratio of the rotation speeds R1 and R2 of motor M2 in section 2.

[0045] Section 3 (second period) is a period from the end of section 2 until the trailing end of the recording material P in the transport direction leaves the fixing nip N2. The timing Tc is the timing immediately before the trailing end of the recording material P in the transport direction leaves the transfer nip N1. Therefore, in section 3, there is a period in which the recording material P is sandwiched between both the transfer nip N1 and the fixing nip N2. The section in which the timer 41a counts the time t3 from the end of section 2 until the trailing end of the recording material P in the transport direction leaves the fixing nip N2 is determined to be section 3. In section 3, the rotation speed of the motor M2 is controlled at a constant speed by a speed Ropt obtained by multiplying the average speed Rsns of section 2 by a coefficient according to the difference in image density between the left and right sides. Here, the average speed Rsns corresponds to the curvature amount holding speed. ΔtR and ΔtR2 are read from the timers 41b and 41c, and the average speed Rsns is calculated based on the following formula (1) using the rotation speeds R1 and R2 of the motor M2. Rsns=(R1*ΔtR1+R2*ΔtR2) / (ΔtR1+ΔtR2) (Formula 1)

[0046] The control unit 40 calculates the left-right image difference coefficient K from a table based on Table 1, based on the left-right image density difference ΔD acquired from the video controller 120. The speed Ropt in section 3 is calculated by the following formula (2). Ropt = Rsns * K (Eq. 2)

[0047] [Table 1]

[0048] As described above, the larger the left-right difference in image density ΔD, the smaller the control speed Ropt in section 3 becomes, so that the amount of curvature L becomes larger than Lsns, and the recording material P is slackened. In this example, the reference values ​​for the left-right difference in image density ΔD are 50% and 100%, as shown in Table 1.

[0049] As described above, in the first embodiment, the conveying speed in the fixing nip N2 during section 3 is controlled based on the amount of curvature detected by the curvature detection sensor 81 and information on the left-right difference in image density, which is image information. That is, the control unit 40 sets the conveying speed of the recording material in the fixing unit 74 according to information on the left-right difference in image density in the width direction of the recording material P perpendicular to the conveying direction, in addition to the output of the curvature detection unit. More specifically, the control unit 40 sets the conveying speed in section 3 (second period) slower when the left-right difference in image density is large than when it is small.

[0050] (Effects of Example 1) The configuration of a comparative example for verifying the effect of the image forming apparatus 60 of the first embodiment will be described. The configuration of the comparative example controls the conveying speed in the fixing nip N2 only based on the information on the amount of curvature detected by the curvature detection sensor 81. In other words, the comparative example has a similar configuration structurally to the image forming apparatus 60 of the first embodiment, and differs only in the control in section 3 of the curvature control. The image on the recording material P used for the verification is in area A. L and area A R The image density is different.

[0051] 7 shows the rotation speed (a) of the motor M2 in sections 1 to 3 in the comparative example, and the amount of curvature (b) at the position of the curvature detection sensor 81. In section 3, the motor M2 is controlled at a speed Rsns at which the recording material P maintains the amount of curvature Lsns, regardless of the left-right image density difference ΔD.

[0052] Area A of recording material P L , area A R Although the image density is different, both areas are uniform images. Area A L Average toner amount conversion value D Ave (L) is 0(%), Area AR Average toner amount conversion value D Ave (R) was set to 150(%). The left-right image density difference ΔD(%) is calculated to be 150(%). In the first embodiment, the rotation speed of the motor M2 in the section 3 is controlled at Rsns*0.995, which is slower than the rotation speed controlled in the comparative example.

[0053] In the transfer nip portion N1, the recording material P is nipped and conveyed between the intermediate transfer body 103 and the secondary transfer roller 67, with the unfixed toner image therebetween. Therefore, when the amount of toner is large, minute slippage is likely to occur in the transfer nip portion N1, and the actual conveying speed in the transfer nip portion N1 decreases. L , A R Area A with a large amount of toner R On the side, the toner amount is small in area A. L When the conveying speed of the recording material P in the transfer nip portion N1 decreases, the conveying speed in the fixing nip portion N2 becomes larger than the conveying speed in the transfer nip portion N1, so that the amount of curvature tends to become smaller. R On the other hand, the curvature is smaller in the region A. L On the side where the recording material P is fixed to the fixing film 83, the amount of curvature is likely to be greater. If the amount of curvature falls below the film rubbing curvature amount Lng, the fixing film 83 and the recording material P come into contact with each other before the recording material P enters the fixing nip portion N2. In that case, the fixing film 83 rubs against the unfixed toner image on the recording material P, which is likely to cause image defects.

[0054] At timing Tc immediately before the trailing edge of the recording material P in the transport direction leaves the transfer nip portion N1, the curvature amounts of the recording material P in the first embodiment and the comparative example are compared. FIG. 8 shows a comparison diagram of the curvature amounts of the recording material P in the paper width direction in the first embodiment and the comparative example at timing Tc in section 3. Timing Tc is within section 3. In the first embodiment, as shown in FIG. 6B, the curvature amount of the recording material P at the position of the curvature detection sensor 81 is Lex, which is larger than Lsns. Also, in the comparative example, as shown in FIG. 7B, the curvature amount of the recording material P at the position of the curvature detection sensor 81 remains at Lsns.

[0055] In the comparative example, the amount of curvature of the recording material P is maintained at the central position where the curvature detection sensor 81 of the recording material P is located, but in the region A where the amount of toner is large R On the other hand, the curvature is smaller and falls below the film curvature Lng. R The toner image on the side of the recording material P is likely to rub against the fixing film 83, which is likely to cause image defects. On the other hand, the amount of curvature of the recording material P in the first embodiment is larger than the amount of curvature of the recording material P in the comparative example. Therefore, the amount of curvature of the recording material P in the first embodiment is larger in the area A where the amount of toner is large. R On the side of the film, a larger curvature amount than the film rubbing curvature amount Lng can be maintained, and the film is more stable in the region A than in the comparative example. R The toner image on the side is less likely to rub against the fixing film 83. In this way, the embodiment 1 has a configuration in which image defects are less likely to occur than the comparative example.

[0056] In Example 1, region A L , A R is an image area divided at the center position in the paper width direction of the recording material P, and area A L , A R For example, when the position of the curvature detection flag 80 of the curvature detection sensor 81 is different from the center position in the width direction of the recording material P, the area A L , A R It is preferable that the area A is divided by the position of the curvature detection flag 80 in the width direction. L , A R There may be gap areas between.

[0057] In the first embodiment, the toner amount per unit area is used as the information related to the left-right difference in image density, but the present invention is not limited to this, and image information correlated with the toner amount per unit area may be used. For example, the printing rate of the image (area on which the image is formed) may be used as the image density.

[0058] In addition, in the first embodiment, the speed in section 3 is adjusted using image information, but the present invention is not limited to this, and the speed in section 2 may be adjusted. For example, the rotation speeds R1 and R2 may be set so that the amount of curvature in section 2 (first period) is larger when the difference in image density between the left and right is large than when the difference is small.

[0059] [Example 2] Next, a description will be given of Example 2. The basic configuration and operation of the image forming apparatus of Example 2 are the same as those of Example 1, and elements having the same or equivalent functions and configurations as those of Example 1 are given the same reference numerals and detailed descriptions thereof will be omitted.

[0060] In the second embodiment, unlike the first embodiment, surface property information of the recording material P (information related to the surface property of the recording material) is obtained, and the left-right image difference coefficient K is determined using the surface property information and the left-right difference ΔD in image density.

[0061] The video controller 120 acquires paper type information of the recording material P from an external device. The control unit 40 acquires the paper type information from the video controller 120, and acquires surface property information of the recording material P from the paper type information. The control unit 40 determines the surface roughness level of the recording material P in two stages from the surface property information. If the recording material P is smooth paper and the surface roughness level is low, it is determined to be Lv1. If the recording material P is plain paper or rough paper, it is determined to be Lv2. In this example, the standard for surface roughness is whether the paper is smooth or not.

[0062] Further, the control unit 40 calculates the left-right image difference coefficient K from a table based on Table 2, based on the left-right image density difference ΔD and the surface roughness level of the recording material P.

[0063] [Table 2]

[0064] In this example, the reference values ​​for the left-right difference ΔD in image density are 50% and 100% as shown in Table 1. In case 1, where there is no toner on the recording material P and the surface property of the recording material P is rough, minute slippage is likely to occur in the fixing nip N2. Therefore, the actual conveying speed in the fixing nip N2 decreases, and the amount of curvature is likely to increase. On the other hand, in case 2, where there is toner on the recording material P, regardless of the surface property of the recording material P, the toner is crushed in the fixing nip N2 and is leveled on the surface of the recording material P. This makes it easier to transmit the conveying force to the recording material P via the toner, so minute slippage is unlikely to occur. Therefore, the amount of curvature is likely to be relatively small in case 2 compared to case 1.

[0065] In other words, when the difference ΔD between the left and right image densities is large and the surface of the recording material P is rough, the area A of the recording material P L , A R The difference in the amount of curvature becomes larger between the

[0066] The effect of the second embodiment will be described using the comparative example used in the first embodiment, based on an example in which the left-right difference in image density ΔD is 150(%).

[0067] 9 shows (a) the rotation speed of the motor M2 and (b) the amount of curvature of the recording material P when a recording material P with a roughness level Lv1 is used and when a recording material P with a roughness level Lv2 is used in Example 2. At timing Tc, the amount of curvature of the recording material P with a roughness level Lv1 is Lex1, and the amount of curvature of the recording material P with a roughness level Lv2 is Lex2.

[0068] FIG. 10 is a comparison diagram of the amount of curvature in the paper width direction of recording material P between Example 2 and the comparative example. FIG. 10 also shows the amount of curvature for recording material P with surface roughness levels Lv1 and Lv2. The difference in the amount of curvature in the paper width direction of recording material P is greater for Lv2 with rough surface compared to Lv1 with smooth surface. In the comparative example, the amount of curvature falls below the amount of curvature Lng for recording material P with both surface roughness levels Lv1 and Lv2. Therefore, in region A RThe toner image on the side is likely to rub against the fixing film 83, which may cause image defects. On the other hand, in the second embodiment, the amount of curvature of the recording material P does not fall below the amount of curvature Lng for both the surface roughness levels Lv1 and Lv2.

[0069] In this way, in the second embodiment, it is possible to control the amount of curvature appropriately by accurately predicting the one-sided curvature for the image and the type of paper of the recording material P. This results in a configuration in which image defects are less likely to occur than in the comparative example.

[0070] In the second embodiment, the surface property information is obtained from the paper type information input when the user issues an image formation instruction, but the present invention is not limited to this. The surface property information obtained by the control unit 40 may be obtained by providing a surface property detection sensor in the image forming apparatus 60 for detecting the surface property of the recording material P, and obtaining information from this sensor as the surface property information.

[0071] In the second embodiment, the left-right image difference coefficient K is determined only from the surface property information of the recording material P and the left-right difference ΔD in image density, but the present invention is not limited to this. For example, the stiffness of the recording material P varies depending on the basis weight, fiber direction, moisture absorption amount, etc. of the recording material. These factors also vary the ease with which the image rubs against the fixing film 83. The left-right image difference coefficient K may be determined by combining the surface property information of the recording material P and the left-right difference ΔD in image density, as well as recording material information such as the basis weight, fiber direction, moisture absorption amount, etc. of the recording material. [Explanation of symbols]

[0072] 40 Control section 61 Counter roller 67 Secondary transfer roller 74 Fixing section 81 Curvature detection sensor 103 Intermediate transfer body 104 Photosensitive drum

Claims

1. an image forming section for forming an image on a recording material; a fixing section including a pair of rotors forming a fixing nip section, the fixing nip section heats the recording material while nipping and conveying the recording material, and fixes an image formed on the recording material to the recording material; a curvature detection unit that is provided at a position between the image forming unit and the fixing unit in a conveying direction of the recording material and detects an amount of curvature of the recording material between the image forming unit and the fixing unit; a control unit that controls a conveying speed of the recording material in the fixing unit in response to an output of the curvature detection unit to adjust an amount of curvature of the recording material; In an image forming apparatus having The control unit sets the conveying speed of the recording material in the fixing unit in accordance with information relating to the left-right difference in image density in the width direction of the recording material perpendicular to the conveying direction, in addition to the output of the curvature detection unit.

2. the image forming unit has a transfer unit that forms a transfer nip portion for transferring an image onto a recording material; The image forming apparatus according to claim 1, characterized in that the control unit adjusts the amount of curvature of the recording material by switching between a speed at which the conveying speed of the recording material in the fixing nip portion is faster and a speed at which the conveying speed of the recording material in the transfer nip portion is slower than the conveying speed of the recording material in the transfer nip portion, and during a second period in which the recording material is clamped between both the transfer nip portion and the fixing nip portion, sets the conveying speed according to the output of the curvature detection unit and information related to the difference in image density between the left and right sides.

3. 3. The image forming apparatus according to claim 2, wherein the control unit sets the conveying speed in the second period to be slower when the difference between the left and right image densities is large than when the difference is small.

4. the image forming unit has a transfer unit that forms a transfer nip portion for transferring an image onto a recording material; The image forming apparatus according to claim 1, characterized in that during a first period in which the control unit adjusts the amount of curvature of the recording material by switching between a speed at which the conveying speed of the recording material in the fixing nip portion is faster and a speed at which the conveying speed of the recording material in the transfer nip portion is slower than the conveying speed of the recording material in the fixing nip portion, the control unit sets the conveying speed according to information related to the output of the curvature detection unit and the difference in image density between the left and right sides.

5. 5. The image forming apparatus according to claim 4, wherein the control unit sets the speed at which the image is increased and the speed at which the image is decreased so that the amount of bending in the first period is greater when the difference between the left and right image densities is large than when the difference is small.

6. 6. The image forming apparatus according to claim 1, wherein the pair of rotating members includes a fixing film and a pressure roller that forms the fixing nip between the fixing film and a pressure roller.

7. 7. The image forming apparatus according to claim 6, wherein the fixing section further includes a heater disposed in an internal space of the fixing film, and the fixing nip portion is formed by the heater and the pressure roller via the fixing film.

8. an image forming section for forming an image on a recording material; a fixing section including a pair of rotors forming a fixing nip section, the fixing nip section heats the recording material while nipping and conveying the recording material, and fixes an image formed on the recording material to the recording material; a curvature detection unit that is provided at a position between the image forming unit and the fixing unit in a conveying direction of the recording material and detects an amount of curvature of the recording material between the image forming unit and the fixing unit; a control unit that controls a conveying speed of the recording material in the fixing unit in response to an output of the curvature detection unit to adjust an amount of curvature of the recording material; In an image forming apparatus having The control unit sets the conveying speed of the recording material in the fixing unit in accordance with information relating to the surface properties of the recording material and information relating to the left-right difference in image density in the width direction of the recording material perpendicular to the conveying direction.

9. the image forming unit has a transfer unit that forms a transfer nip portion for transferring an image onto a recording material; The image forming apparatus according to claim 8, characterized in that the control unit adjusts the amount of curvature of the recording material by switching between a speed at which the conveying speed of the recording material in the fixing nip portion is faster and a speed at which the conveying speed of the recording material in the transfer nip portion is slower than the conveying speed of the recording material in the transfer nip portion, after a first period in which the control unit adjusts the amount of curvature of the recording material by switching between a speed at which the conveying speed of the recording material in the fixing nip portion is faster and a speed at which the conveying speed of the recording material in the transfer nip portion is slower than the conveying speed of the recording material in the transfer nip portion, and during a second period in which the recording material is clamped between both the transfer nip portion and the fixing nip portion, sets the conveying speed according to information related to the surface properties and information related to the difference in image density between the left and right sides.

10. The image forming apparatus according to claim 9, characterized in that the control unit sets the conveying speed in the second period slower when the left-right difference in image density is large and the surface roughness level is high than when the left-right difference in image density is large and the surface roughness level is low.

11. the image forming unit has a transfer unit that forms a transfer nip portion for transferring an image onto a recording material; The image forming apparatus according to claim 8, characterized in that the control unit sets the conveying speed according to information relating to the surface properties of the recording material and information relating to the difference in image density between the left and right sides during a first period in which the amount of curvature of the recording material is adjusted by switching between a speed at which the conveying speed of the recording material in the fixing nip portion is faster and a speed at which the conveying speed of the recording material in the transfer nip portion is faster and slower than the conveying speed of the recording material in the transfer nip portion.

12. The image forming apparatus according to claim 11, characterized in that the control unit sets the conveying speed in the first period slower when the left-right difference in image density is large and the surface roughness level is high than when the left-right difference in image density is large and the surface roughness level is low.

13. 13. The image forming apparatus according to claim 8, wherein the pair of rotating members includes a fixing film and a pressure roller that forms the fixing nip between the fixing film and a pressure roller.

14. 14. The image forming apparatus according to claim 13, wherein the fixing section further includes a heater disposed in an internal space of the fixing film, and the fixing nip portion is formed by the heater and the pressure roller via the fixing film.

Images