Image formation device

Abstract

To provide an image formation device capable of inhibiting generation of image defect such as white point image even if a thermally expanded pressure roller reduces warpage amount of a recording material to be transported between a transfer device and a fixation device, irrespective of simple device configuration and control configuration.SOLUTION: An image formation device 100 comprises: a transfer power source 80 supplying transfer bias which is a predetermined voltage or current to a transfer device 11; and temperature detection means (121, 62) for detecting the temperature of a pressure roller 32. The image formation device 100 corrects a transfer bias value from the transfer power source 80 to the transfer device 11 when performing transfer processing on the basis of detection temperature detected by the temperature detection means (121, 62).SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present disclosure relates to image forming apparatuses such as copiers, multifunction machines, printers, and facsimile machines. [Background technology]

[0002] Conventional image forming apparatuses equipped with a transfer device that transfers a toner image onto a recording material such as recording paper, and a fixing device having a heat source that heats and fixes the toner image transferred onto the recording material onto the recording material using a fixing member and a pressure roller have the following disadvantages.

[0003] FIG. 11 is a side view that shows a schematic view of a transfer device 11X and a fixing device 12X in a conventional image forming apparatus 100X.

[0004] In the conventional image forming apparatus 100X, the conveying speed of the recording material P by the transfer device 11X hardly changes, whereas the conveying speed of the recording material P by the fixing device 12X changes depending on the temperature of the fixing member and the pressure roller. The range of fluctuation of the conveying speed of the recording material P in the fixing device 12X may reach 2 to 3%. When the conveying speed of the recording material P in the fixing device 12X becomes faster than the conveying speed of the transfer device 11X, the recording material P conveyed between the transfer device 11X and the fixing device 12X may be pulled by the fixing device 12X and become unbent. In such a state, since the conveying force of the recording material P is larger in the fixing device 12X, the recording material P passes through the transfer device 11X at a speed faster than the conveying speed of the recording material P in the transfer device 11X, and there is a risk of the toner image being rubbed and causing a transfer failure.

[0005] One approach to this problem is to increase the amount of bending of the recording material P transported between the transfer device 11X and the fixing device 12X. In this case, however, if the amount of bending of the recording material P becomes too great, the toner transferred to the recording material P will scatter due to the influence of frictional charging of the recording material P with the guide members 20X, 20X made of a non-conductive material such as resin that guide the recording material P, and this will easily result in image defects such as toner scattering.

[0006] Therefore, Patent Document 1 describes a configuration in which an air suction means with variable air volume is provided to move the position of the recording surface of the recording material transported between a transfer device (a transfer section where a toner image is transferred to the recording material P) and a fixing device (a fixing section where the toner image is fixed to the recording material P) in a direction perpendicular to the recording surface, and when the fixing temperature conditions are such that the transport speed in the fixing device is fast, the air suction means is controlled to increase the loop (deflection) of the recording material before it enters the fixing device. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2004-151477 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, the configuration described in Patent Document 1 requires the provision of a separate component such as an air suction means, and the amount of suction by the air suction means needs to be changed when transporting stiff recording material such as postcards and when transporting weak recording material such as thin paper, complicating the device configuration and control configuration.

[0009] In addition, when a recording material P that is difficult to bend (has a strong stiffness), such as a postcard or thick paper, is conveyed, even if the suction force is changed by the air suction means, a sufficient loop cannot be formed in the recording material P, and the recording material P conveyed between the transfer device 11X and the fixing device 12X may come too close to the image carrier (photoconductor drum 3X). If the recording material P comes too close to the image carrier (photoconductor drum 3X), even if the toner image is not rubbed off, localized discharge from the toner image transferred to the recording material P to the image carrier or re-transfer in which the toner on the recording material P is transferred back to the image carrier may occur when the recording material P passes through the transfer device 11X, and image defects may occur in which the toner image transferred to the recording material P has white spots here and there, so-called white spots (white spot image). This phenomenon is particularly likely to occur downstream in the conveying direction H of the recording material P.

[0010] Therefore, the present disclosure aims to provide an image forming device that has a simple device configuration and control configuration and can suppress the occurrence of image defects such as white spot images even if the amount of bending of the recording material transported between the transfer device and the fixing device is reduced due to thermal expansion of the pressure roller. [Means for solving the problem]

[0011] In order to solve the above problem, the image forming apparatus of the present disclosure comprises a transfer device that performs a transfer process to transfer a toner image formed on an image carrier to a sheet-shaped recording material, and a fixing device having a heat source that heats and fixes the toner image transferred to the recording material to the recording material using a fixing member and a pressure roller, and performs transport control so that there is a difference in the transport speed of the recording material in the transfer device and the fixing device in order to cause bending of the recording material transported between the transfer device and the fixing device.The image forming apparatus comprises a transfer power supply that supplies a transfer bias, which is a predetermined voltage or current, to the transfer device, and a temperature detection means that detects the temperature of the pressure roller, and corrects the value of the transfer bias supplied from the transfer power supply to the transfer device when performing the transfer process based on the detected temperature detected by the temperature detection means. Effect of the Invention

[0012] According to the present disclosure, even with a simple device configuration and control configuration, it is possible to suppress the occurrence of image defects such as white spot images even if the amount of deflection of the recording material transported between the transfer device and the fixing device is reduced due to thermal expansion of the pressure roller. [Brief description of the drawings]

[0013] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2A] 2 is a perspective view of a fixing device in the image forming apparatus shown in FIG. [Figure 2B] FIG. 2B is a cross-sectional view taken along line AA shown in FIG. 2A. [Figure 3A] 2 is a perspective view of one end portion in a rotation axis direction of a fixing belt of the fixing device according to the present embodiment, as viewed from the front side. FIG. [Figure 3B] 11 is a perspective view of the other end portion in the rotation axis direction of the fixing belt of the fixing device according to the embodiment, as viewed from the rear side. FIG. [Figure 4] 11 is a perspective view showing a fixing belt supported by one holding member and the other holding member as viewed from the front side. FIG. [Diagram 5] 1 is a schematic block diagram showing a control configuration for correcting a value of a transfer bias based on a detected temperature of a pressure roller in an image forming apparatus; [Figure 6] 6 is a flowchart showing a process flow of an example of a correction operation according to the first embodiment. [Figure 7] 10 is a flowchart showing a process flow of an example of a correction operation according to the second embodiment. [Figure 8A] 13 is a flowchart showing a process flow of an example of a correction operation according to the third embodiment. [Figure 8B] 13 is a flowchart showing a processing flow of another example of the correction operation according to the third embodiment. [Figure 9A] 13 is a flowchart showing a process flow of an example of a correction operation according to the fourth embodiment. [Figure 9B] 13 is a flowchart showing a process flow of another example of the correction operation according to the fourth embodiment. [Figure 10] 13 is a flowchart showing an example of a correction operation according to the fifth embodiment. [Figure 11] FIG. 1 is a side view showing a schematic diagram of a transfer device and a fixing device in a conventional image forming apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals. The names and functions of the components are also the same. Therefore, detailed description thereof will not be repeated.

[0015] [Image forming device] FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus 100 according to the present embodiment.

[0016] 1, the image forming apparatus 100 is a multifunction machine having a copy function, a scanner function, a facsimile function, and a printer function, and reads an original G with an image reading device 102 to convert it into electronic data, and transmits the electronic data of the image data to the outside. The image forming apparatus 100 forms a color or monochrome toner image on a recording material P such as recording paper based on the image data of the original G converted into electronic data by the image reading device 102 or image data received from the outside. The image forming apparatus 100 may be a monochrome image forming apparatus. The image forming apparatus 100 may also be a color image forming apparatus of another type.

[0017] An original feeding device 160 (automatic original conveying device) is provided above the image reading unit 130 and is supported so as to be openable and closable relative to the image reading unit 130. The image reading unit 102 reads the original G conveyed by the original feeding device 160. The original feeding device 160 includes an original placement tray 161 on which the original G is placed, and an original discharge tray 162 on which the read original G is stacked. The original feeding device 160 conveys one or more originals G placed on the original placement tray 161 one by one in order onto the original reading unit 130b in the image reading unit 130, and discharges them onto the original discharge tray 162. The image reading unit 130 is also provided with an original placement table 130a on which the original G is placed. The image reading unit 102 reads the original G placed on the original placement table 130a. When the document feeder 160 is opened, the document placement table 130a above the image reading unit 130 is opened, allowing the document G to be placed manually on the document feeder 160. The image reading unit 130 reads the document G transported by the document feeder 160 with the scanning optical system 130c positioned at a reading position below the document reading unit 130b, or scans the scanning optical system 130c to read the document G placed on the document placement table 130a, and generates image data.

[0018] The image forming apparatus main body 101 includes an optical scanning device 1, a developing device 2, a photosensitive drum 3, a drum cleaning device 4, a charger 5, an intermediate transfer belt device 70, a transfer device 11 (secondary transfer device), a fixing device 12, a sheet transport path S, a paper feed cassette 18, and a sheet discharge tray 141.

[0019] The image forming apparatus 100 handles image data corresponding to color images using the colors black (K), cyan (C), magenta (M), and yellow (Y), or monochrome images using a single color (e.g., black). The image forming section 50 of the image forming apparatus 100 includes a developing device 2, a photoconductor drum 3, a drum cleaning device 4, and a Four chargers 5 are provided, each corresponding to black, cyan, magenta, and yellow, forming four image stations Pa, Pb, Pc, and Pd.

[0020] The charger 5 uniformly charges the surface of the photosensitive drum 3 to a predetermined potential. The optical scanning device 1 exposes the surface of the photosensitive drum 3 to light to form an electrostatic latent image. The developing device 2 develops the electrostatic latent image on the surface of the photosensitive drum 3 to form a toner image on the surface of the photosensitive drum 3. The drum cleaning device 4 removes and collects residual toner on the surface of the photosensitive drum 3. Through the series of operations described above, a toner image of each color is formed on the surface of each photosensitive drum 3.

[0021] The intermediate transfer belt device 70 includes an intermediate transfer roller 6, an endless intermediate transfer belt 71, an intermediate transfer driving roller 72, an intermediate transfer driven roller 73, and a cleaning device 9. Four intermediate transfer rollers 6 are provided inside the intermediate transfer belt 71 so as to form four types of toner images corresponding to the respective colors. The intermediate transfer rollers 6 transfer the toner images of the respective colors formed on the surfaces of the photosensitive drums 3 onto the intermediate transfer belt 71 which moves in a circular movement direction C.

[0022] The intermediate transfer belt 71 is stretched around an intermediate transfer drive roller 72 and an intermediate transfer driven roller 73. In the image forming apparatus 100, the toner images of each color formed on the surface of each photoconductor drum 3 are sequentially transferred to the surface of the intermediate transfer belt 71 and superimposed on each other to form a color toner image on the surface of the intermediate transfer belt 71.

[0023] The transfer device 11 forms a transfer nip portion TN between the secondary transfer roller 11a and the intermediate transfer belt 71, and conveys the recording material P conveyed through the sheet conveying path S by pinching it in the transfer nip portion TN. When the recording material P passes through the transfer nip portion TN, the toner image on the surface of the intermediate transfer belt 71 is transferred to the recording material P by the transfer device 11, and the recording material P is conveyed to the fixing device 12. The cleaning device 9 removes and collects waste toner remaining on the surface of the intermediate transfer belt 71 without being transferred to the recording material P.

[0024] The fixing device 12 includes a fixing belt 31 (one example of a fixing member) that rotates while sandwiching the recording material P, and a pressure roller 32. The fixing device 12 sandwiches the recording material P, onto which the toner image has been transferred, in a fixing nip portion FN between the fixing belt 31 and the pressure roller 32, and applies heat and pressure to fix the toner image to the recording material P. Although not shown in FIG. 1, the fixing device 12 includes components other than the fixing belt 31 and the pressure roller 32. Details of the fixing device 12 will be described later.

[0025] The paper feed cassette 18 is a cassette for storing the recording material P used for image formation, and is provided below the optical scanning device 1. The recording material P is pulled out from the paper feed cassette 18 by the pickup roller 16 and conveyed to the sheet conveying path S. The recording material P conveyed to the sheet conveying path S passes through the transfer device 11 and the fixing device 12, is conveyed to the discharge roller 17, and is discharged to the sheet discharge tray 141 in the discharge section 140. The conveying roller 13, the registration roller 14, and the discharge roller 17 are arranged on the sheet conveying path S. The conveying roller 13 promotes the conveyance of the recording material P. The registration roller 14 temporarily stops the recording material P and aligns the leading edge of the recording material P. The registration roller 14 conveys the stopped recording material P in accordance with the timing of the toner image on the intermediate transfer belt 71.

[0026] In the example shown in FIG. 1, there are two paper feed cassettes 18, but the present invention is not limited to this, and one or three or more paper feed cassettes 18 may be provided.

[0027] Furthermore, when forming an image on the back side of the recording material P in addition to the front side, the image forming apparatus 100 conveys the recording material P in the reverse direction from the discharge rollers 17 to a sheet inversion path Sr. The image forming apparatus 100 inverts the recording material P conveyed in the reverse direction and guides it again to the registration rollers 14. The image forming apparatus 100 also forms an image on the back side of the recording material P guided to the registration rollers 14 in the same manner as on the front side, and conveys the recording material P to the sheet discharge tray 141.

[0028] [Fixing device] Fig. 2A is a perspective view of the fixing device 12 in the image forming apparatus 100 shown in Fig. 1. Fig. 2B is a cross-sectional view taken along the line AA shown in Fig. 2A.

[0029] The fixing device 12 further includes a support member 33, a fixing pad 34, a heat source 36, a reflecting member 37 (a reflecting plate in this example), a fixing member temperature detection unit 38 (a fixing belt temperature sensor, a thermopile in this example), a peeling plate 39, a thermostat 40, and a pressure roller temperature detection unit 121 (a pressure roller temperature sensor, a thermistor in this example). The support member 33, the fixing pad 34, the heat source 36, and the reflecting member 37 are provided inside the fixing belt 31. The fixing belt 31 is an endless (annular) flexible belt that is rotatable about a rotation axis α along a width direction E perpendicular to a conveying direction H of the recording material P.

[0030] The fixing pad 34 is made of, for example, resin, and is formed in a long plate shape extending in the rotation axis direction W of the fixing belt 31. The fixing pad 34 includes a fixing pad main body 340 and a sliding sheet 341 provided on the surface of the fixing pad main body 340 that comes into sliding contact with the fixing belt 31. The fixing pad 34 is preferably made of a heat-resistant resin material, and for example, a heat-resistant resin material such as liquid crystal polymer (LCP) or PEEK (polyether ether ketone) can be used.

[0031] The support member 33 is a member that supports the fixing pad 34. Both ends of the support member 33 in the rotation axis direction W are fixed to the fixing device main body 12a (main body frame). A reflecting member 37 is provided on at least the surface of the support member 33 facing the heat source 36.

[0032] The heat source 36 is a member for heating the fixing belt 31, and extends in the direction W of the axis of rotation of the fixing belt 31. The heat source 36 can be, for example, a lamp heater such as a halogen lamp. The fixing belt 31 is heated to a predetermined fixing temperature (for example, 150°C to 200°C, 150°C in this example) by the heat source 36. Therefore, not only the fixing belt 31 but also the sliding sheet 341 and the like have heat resistance to the above temperature.

[0033] The reflecting member 37 is disposed so as to cover at least the surface of the supporting member 33 facing the heat source 36, and in this example, is formed of a thin metal plate. This allows the fixing belt 31 to be efficiently heated. The reflecting member 37 is fixed to the supporting member 33.

[0034] The pressure roller 32 is disposed at a position facing the fixing pad 34 with the fixing belt 31 interposed therebetween. The pressure roller 32 rotates in a second rotation direction R2 about a rotation axis parallel to the rotation axis α of the fixing belt 31, and extends parallel to the fixing belt 31. The pressure roller 32 presses the fixing belt 31 toward the fixing pad 34 to form a fixing nip FN between the pressure roller 32 and the fixing belt 31. The pressure roller 32 may be configured, for example, as a roller member in which the surface of a cylindrical core material formed of a metal such as aluminum is covered with an elastic material such as rubber. The outer diameter of the pressure roller 32 is not limited thereto, but is set to 30 mm in this example.

[0035] A driving force from a rotation drive unit 90 such as a motor (see FIG. 5 described later) is transmitted to the pressure roller 32 via a drive transmission mechanism such as gears (not shown). The pressure roller 32 is driven to rotate by receiving the driving force from the rotation drive unit 90. As the pressure roller 32 is driven to rotate, the fixing belt 31 is driven to rotate in a first rotation direction R1 opposite to the second rotation direction R2 of the pressure roller 32. In other words, the pressure roller 32 forms a fixing nip portion FN by contacting the surface of the fixing belt 31, and transmits a driving force to the fixing belt 31 via the fixing nip portion FN, thereby rotating the fixing belt 31.

[0036] The fixing member temperature detection section 38 detects the surface temperature of the fixing belt 31. In the fixing device 12, the temperature of the heat source 36 is controlled so that the fixing belt 31 reaches the fixing temperature (150° C. in this example) based on the detection signal detected by the fixing member temperature detection section 38. In this example, the fixing member temperature detection section 38 is fixed to the image forming apparatus main body 101 (main body frame) (see FIG. 1). The fixing member temperature detection section 38 detects the surface temperature of the fixing belt 31 in a non-contact manner.

[0037] The peeling plate 39 is disposed in the vicinity of the fixing nip portion FN on the downstream side in the first rotation direction R1 of the fixing belt 31, and prevents the recording material P from being wrapped around the fixing belt 31.

[0038] The thermostat 40 cuts off the power supply to the heat source 36 when the heat source 36 becomes abnormally hot. More specifically, the thermostat 40 is electrically connected to a power line (not shown) that supplies power to the heat source 36, and is configured to directly cut off the power supply to the heat source 36. The thermostat 40 is fixed to the fixing device main body 12a (main body frame). The thermostat 40 operates to cut off the power supply to the heat source 36 when a predetermined reaction temperature (operating temperature, rated temperature) is reached.

[0039] The pressure roller temperature detection unit 121 detects the surface temperature of the pressure roller 32. In this example, the pressure roller temperature detection unit 121 is fixed to the fixing device main body 12a (main body frame) in a state of contact with the surface of the pressure roller 32. The pressure roller temperature detection unit 121 detects a passing area of ​​the pressure roller 32 through which the recording material P passes (in this example, the central area in the rotation axis direction W of the fixing belt 31).

[0040] 3A and 3B are perspective views of end portions of one side W1 and the other side W2 in the rotation axis direction W of the fixing belt 31 of the fixing device 12 according to the present embodiment as viewed from the front and rear, respectively. Also, Fig. 4 is a perspective view of the fixing belt 31 supported by one holding member 411 and the other holding member 412 as viewed from the front.

[0041] 3A to 4, the fixing device 12 includes a pair of holding members 411, 412 (41) (belt guide members) and a fixing belt 31 rotatably provided on the outer circumferential surfaces 401a of the pair of holding members 411, 412 (41) about a rotation axis α (see FIG. 2B). The pair of holding members 411, 412 (41) are fixed to the fixing device main body 12a (main body frame) (see FIG. 2B).

[0042] The pair of holding members 411, 412 (41) hold both end portions in the rotation axis direction W of the inner surface 31a of the fixing belt 31. Of the pair of holding members 411, 412 (41), one holding member 411 holds an end portion on one side W1 (front side, operation side) in the rotation axis direction W of the inner surface 31a of the fixing belt 31, and the other holding member 412 holds an end portion on the other side W2 (rear side) in the rotation axis direction W of the inner surface 31a of the fixing belt 31.

[0043] [Control configuration] FIG. 5 is a schematic block diagram showing a control configuration for correcting the value of the transfer bias based on the detected temperature of the pressure roller 32 in the image forming apparatus 100. As shown in FIG.

[0044] The image forming apparatus 100 includes a transfer device 11 that performs a transfer process to transfer a toner image formed on a photosensitive drum 3 onto a recording material P, a fixing device 12 that heats and fixes the toner image transferred onto the recording material P onto the recording material P using a fixing member (in this example, a fixing belt 31) and a pressure roller 32, and a transfer power supply 80 that supplies a transfer bias, which is a predetermined voltage or current, to the transfer device 11.

[0045] 5, the control unit 60 has a processing unit 60a consisting of a computer such as a CPU (Central Processing Unit), and a storage unit 60b including a non-volatile memory such as a ROM (Read Only Memory) and a volatile memory such as a RAM (Random Access Memory). The control unit 60 controls the operation of various components by the processing unit 60a loading a control program stored in advance in the ROM of the storage unit 60b onto the RAM of the storage unit 60b and executing it.

[0046] The pressure roller temperature detection unit 121 is electrically connected to an input system of the control unit 60. This allows the control unit 60 to detect the detected temperature (surface temperature) of the pressure roller 32 based on a detection signal sent from the pressure roller temperature detection unit 121.

[0047] The heat source 36 is electrically connected to an output system of the control unit 60. This allows the control unit 60 to control the operation of the heat source 36 by sending a heat source operation control signal to the heat source 36.

[0048] The transfer power supply 80 has an input terminal of a signal system electrically connected to an output system of the control unit 60. This allows the control unit 60 to control the operation of the transfer power supply 80 by sending a transfer power supply operation control signal to the transfer power supply 80. In addition, the transfer power supply 80 has an output terminal of a power system electrically connected to the transfer device 11. This allows the transfer power supply 80 to supply a transfer bias to the transfer device 11 under instructions from the control unit 60.

[0049] The rotation drive unit 90 is electrically connected to an output system of the control unit 60. This allows the control unit 60 to control the operation of the rotation drive unit 90 by transmitting a rotation drive unit operation control signal to the rotation drive unit 90.

[0050] (About this embodiment) The fixing device 12 has a heat source 36, a fixing belt 31 heated by the heat source 36, a fixing pad 34 (see FIG. 2B) provided inside the fixing belt 31, and a pressure roller 32 that presses the fixing belt 31 against the fixing pad 34.

[0051] The control unit 60 includes a transport control unit 61, a temperature detection control unit 62, a correction control unit 63, and a judgment control unit 64. The pressure roller temperature detection unit 121 (see FIG. 2B) and the temperature detection control unit 62 constitute a temperature detection means for detecting the temperature of the pressure roller 32.

[0052] The transport control unit 61 performs transport control so that there is a difference in the transport speed of the recording material P (relative to each other) in the transfer device 11 and the fixing device 12, in order to cause bending of the recording material P transported between the transfer device 11 and the fixing device 12.

[0053] The temperature detection control unit 62 detects a detection temperature (the surface temperature of the pressure roller 32) according to a detection signal detected by the pressure roller temperature detection unit 121. The surface temperature of the pressure roller 32 may be, for example, a temperature detected within a predetermined detection time (for example, the time during which one sheet of recording material P passes through the pressure roller 32), preferably an average value of a plurality of detection temperatures sampled within the predetermined detection time.

[0054] The judgment control section 64 judges whether the amount of bending of the recording material P during conveyance is below a reference amount of bending having a predetermined range based on the detected temperature of the pressure roller 32 detected by the temperature detection control section 62. When the pressure roller 32 of the fixing device 12 is heated to the inside, the outer diameter of the pressure roller 32 increases due to thermal expansion of the pressure roller 32, the conveyance speed of the recording material P increases, and the amount of bending of the recording material P conveyed between the transfer nip portion TN of the transfer device 11 and the fixing nip portion FN of the fixing device 12 decreases (the recording material P becomes taut). Then, as described above, image defects such as white spots may occur. The relationship between the change in the outer diameter of the pressure roller 32 due to the temperature change of the pressure roller 32 and the amount of bending of the recording material P during conveyance is obtained in advance by experiment. Similarly, the amount of bending of the recording material P that does not cause image defects such as white spots is also obtained by experiment, but there is a certain range for the amount of bending of the recording material P during conveyance that does not cause image defects. For this reason, in this embodiment, the amount of sagging of the recording material P during conveyance at which no image defects occur is stored in the storage unit 60b as a reference amount of sagging having a predetermined range together with the temperature condition of the pressure roller 32. Therefore, the determination control unit 64 can determine whether the amount of sagging of the recording material P during conveyance is below the reference amount of sagging having a predetermined range based on the detected temperature of the pressure roller 32.

[0055] When the determination control unit 64 determines that the amount of sag of the recording material P being conveyed falls below a reference amount of sag having a predetermined range, the correction control unit 63 corrects the value of the transfer bias, which is a predetermined voltage or current supplied from the transfer power source 80 to the transfer device 11 when performing the transfer process in the transfer device 11. In detail, when the determination control unit 64 determines that the amount of sag of the recording material P falls below the reference amount of sag range, the correction control unit 63 performs a correction to lower the value of the reference transfer bias (reference transfer voltage and / or reference transfer current, in this example, reference transfer current Is) supplied to the transfer device 11 when the toner image is transferred to the recording material P. The reference transfer current Is is stored in advance in the storage unit 60b (see FIG. 5), and is set to, for example, 95% within a range in which a predetermined transfer efficiency (the ratio of toner transferred from the image carrier to the recording material P) exceeds, for example, 85% as a reference. When the amount of sag of the recording material P conveyed by the pressure roller 32 is within the reference amount of sag, no image defects such as white spots are generated even if the reference transfer current Is is supplied to the transfer device 11. Here, if the judgment control unit 64 judges that the amount of sagging of the recording material P during conveyance is below the reference amount of sagging, the recording material P in the transfer device 11 approaches too close to the intermediate transfer belt 71, which is an image carrier, and even if the toner image is not rubbed, when the recording material P passes through the transfer device 11, localized discharge from the toner image transferred to the recording material P to the intermediate transfer belt 71, or retransfer in which the toner on the recording material P is transferred again to the intermediate transfer belt 71 side, may occur, and image defects such as white spots may occur in the toner image transferred to the recording material P. However, even if the amount of sagging of the recording material P during conveyance falls below the reference amount of sagging and the recording material P approaches the intermediate transfer belt 71, which is an image carrier in the transfer device 11, the correction control unit 63 reduces the value of the transfer bias from the value set during normal transfer operation (reference transfer bias value) to a value at which the above-mentioned retransfer or discharge does not occur, so that the transfer efficiency does not fall below the reference, thereby making it possible to suppress the occurrence of image defects such as white spots.

[0056] In this regard, while the configuration described in Patent Document 1 complicates the device configuration and control configuration, the present embodiment has a simple device configuration and control configuration because the value of the transfer bias supplied to the transfer device 11 is corrected based on the detected temperature detected by the temperature detection means (121, 62). Moreover, the amount of sagging of the recording material P that would cause image defects such as white spots can be detected from the detected temperature detected by the temperature detection means (121, 62). Therefore, even if the amount of sagging of the recording material P during conveyance falls below a reference amount of sagging, the value of the transfer bias supplied to the transfer device 11 can be corrected, for example, by lowering the voltage and / or current (current in this example), to effectively prevent image defects such as white spots.

[0057] Incidentally, in the fixing device 12 of the pad fixing type according to the present embodiment, the fixing members include a heat source 36, a fixing belt 31 heated by the heat source 36, and a fixing pad 34 provided inside the fixing belt 31, and a pressure roller 32 presses the fixing belt 31 against the fixing pad 34. In such a fixing device 12, a corner is formed at an end portion on the upstream side of the fixing pad 34 in the rotation direction of the fixing belt 31 on the fixing belt 31 (heating side) side of the fixing nip portion FN. As a result, the winding state of the recording material P around the fixing belt 31 becomes unstable at the corner at the end portion of the fixing nip portion FN, making it more likely that image defects such as white dot images will occur.

[0058] In this regard, in the fixing device 12 using the pad fixing method, the value of the transfer bias supplied to the transfer power source 80 is corrected based on the detected temperature detected by the temperature detection means (121, 62), which is particularly effective in effectively preventing image defects such as white dot images.

[0059] As described above, when the judgment control unit 64 determines that the amount of deflection of the recording material P is below the reference amount of deflection, the correction control unit 63 corrects the value of the transfer bias supplied to the transfer device 11, thereby reliably preventing the occurrence of image defects such as white dot images.

[0060] (First embodiment) -When the pressure roller has not been heated for some time- Incidentally, when the pressure roller 32 is heated in a state where the entire temperature including the surface and inside of the pressure roller 32 is at room temperature (for example, 15°C to 30°C) or a temperature close to room temperature [a predetermined temperature (for example, 50°C) lower than the fixing temperature (for example, 150°C)], the temperature near the surface of the pressure roller 32 rises at the beginning, and the pressure roller 32 starts to thermally expand, but the pressure roller 32 does not thermally expand to an extent that the circumferential speed exceeds the allowable range. At this time, the temperature change per unit time of the detected temperature of the pressure roller 32 is large. When the pressure roller 32 is continuously heated, the inside of the pressure roller 32 starts to be heated, and the temperature change per unit time of the detected temperature of the pressure roller 32 gradually decreases. Therefore, when the temperature change per unit time of the detected temperature of the pressure roller 32 becomes small to a certain extent, it can be considered that the pressure roller 32 has thermally expanded to an extent that the circumferential speed of the pressure roller 32 exceeds the allowable range.

[0061] In this regard, in the present embodiment, the determination of whether the amount of sagging of the recording material P is below the reference amount of sagging is a determination of whether the amount of temperature change per unit time of the detected temperature of the pressure roller 32 is equal to or less than a predetermined judgment temperature change amount during continuous image formation (hereinafter, image formation may be referred to as printing). In this example, this determination is a determination of whether the detected temperature difference ΔTa [Ta(N-2)-Ta(N-1)] between the detected temperature Ta(N-2) of the pressure roller 32 during the conveyance of the preceding (N-2)th sheet of recording material P by the pressure roller 32 and the detected temperature Ta(N-1) of the pressure roller 32 during the conveyance of the succeeding (N-1)th sheet of recording material P by the pressure roller 32 is within a predetermined judgment temperature difference ΔTas (for example, ±5° C.) during continuous image formation. The judgment temperature difference ΔTas is stored in advance in the storage unit 60b (see FIG. 5).

[0062] When the correction control unit 63 determines that the temperature change amount (in this example, the detected temperature difference ΔTa) is equal to or smaller than the judgment temperature change amount (within the judgment temperature difference ΔTas in this example), it considers that the pressure roller 32 has thermally expanded to such an extent that the circumferential speed of the pressure roller 32 exceeds the allowable range. That is, it determines that the outer diameter of the pressure roller 32 becomes larger than the allowable outer diameter, and therefore the circumferential speed of the pressure roller 32 becomes faster than the allowable circumferential speed, and the amount of bending of the recording material P transported between the transfer device 11 and the fixing device 12 is smaller than the reference amount of bending, that is, during a continuous image forming process in which images are formed by sequentially transferring a plurality of recording materials P, if the amount of temperature change per unit time of the detected temperature detected sequentially is smaller than a predetermined judgment amount of temperature change, it determines that the amount of bending occurring in the recording material P being transported is smaller than the reference amount of bending, and performs a correction to the transfer bias (a correction to reduce the transfer bias to be lower than the reference transfer bias).

[0063] In this configuration, when the pressure roller 32 is heated from a state in which the entire temperature of the pressure roller 32, including the surface and inside, is at room temperature or close to room temperature, it is possible to reliably detect the degree of thermal expansion of the pressure roller 32, which is the amount of bending of the recording material P to the extent that image defects such as white spots occur. This makes it possible to reliably prevent the occurrence of image defects such as white spots.

[0064] Second embodiment -When the pressure roller is already warmed up to a certain extent- Incidentally, when not much time has passed between the end of the previous job and the start of the next job and the overall temperature of the pressure roller 32, including its surface and interior, is at the fixing temperature (e.g., 150°C) or a temperature close to the fixing temperature [a specified temperature higher than room temperature (e.g., 15°C to 30°C)], it can be considered that the pressure roller 32 has thermally expanded to such an extent that the peripheral speed of the pressure roller 32 exceeds the allowable range.

[0065] In this regard, in the present embodiment, the judgment as to whether the amount of flexure of the recording material P is below the reference amount of flexure is whether the detected temperature Tb of the pressure roller 32 before the start of the image forming process is equal to or higher than a predetermined first judgment temperature Tbs (e.g., 130° C.) and the detected temperature Tb of the pressure roller 32 equal to or higher than the first judgment temperature Tbs is maintained within a predetermined judgment temperature fluctuation range Tbr (e.g., ±1° C.) for a predetermined first judgment time tb (e.g., 10 seconds). In other words, the judgment is whether the detected temperature of the pressure roller 32 detected by the pressure roller temperature detection unit 121, which is a temperature detection means, before the start of the image forming process in which the recording material P is transferred to form an image is equal to or higher than the predetermined first judgment temperature Tbs and the detected temperature detected by the pressure roller temperature detection unit 121 during the image forming process is maintained within a predetermined judgment temperature fluctuation range Tbr for the first judgment time. The first judgment temperature Tbs, the judgment temperature fluctuation range Tbr, and the first judgment time tb are stored in advance in the storage unit 60b (see FIG. 5).

[0066] When the judgment control unit 64 determines that the detected temperature of the pressure roller 32, which is equal to or higher than the first judgment temperature Tbs, is maintained within the judgment temperature fluctuation range Tbr for the first judgment time tb, the correction control unit 63 performs a correction to the transfer bias (a correction to lower the transfer bias below the reference transfer bias) because the pressure roller 32 has thermally expanded to such an extent that the circumferential speed of the pressure roller 32 exceeds the allowable range, the outer diameter of the pressure roller 32 has become larger than the allowable outer diameter, and ultimately the circumferential speed of the pressure roller 32 has become faster than the allowable circumferential speed, and the amount of deflection of the recording material P is below the reference deflection amount.

[0067] In this configuration, when not much time has passed between the end of the previous job and the start of the next job and the overall temperature, including the surface and interior of the pressure roller 32, is at or close to the fixing temperature, it is possible to reliably detect the degree of thermal expansion of the pressure roller 32, which results in the amount of bending of the recording material P that causes image defects such as white spots, and to reliably prevent the occurrence of image defects such as white spots.

[0068] Third embodiment - When printing on a larger size recording material after continuous printing on smaller size recording material - Incidentally, during continuous image formation processing in which a recording material P (e.g., A4 vertical size, B5 size, etc.) with a small width size equal to or smaller than a judgment width size Es, which is a predetermined length in the width direction E (rotation axis direction W) perpendicular to the conveying direction H of the recording material P, is image-formed for a predetermined judgment number Cs or more (e.g., 3 sheets), the temperature of the non-passing area (non-passing area at both ends in the axial direction in this example) of the small width recording material P in the width direction E becomes higher than the temperature of the passing area by the amount of the recording material P not passing through the non-passing area. In other words, when image formation processing including transfer processing is performed on a recording material whose width, which is the length in the direction perpendicular to the conveying direction of the recording material P, is smaller than the judgment width size Es (judgment width), which is a predetermined length, for a predetermined number of sheets or more of judgment number Cs, the temperature of the non-passing area (non-passing area at both ends in the axial direction in this example) of the recording material P becomes higher than the temperature of the passing area by the amount of the recording material P not passing through the non-passing area. In this case, it can be considered that the non-passing area of ​​the pressure roller 32 is thermally expanded to such an extent that the peripheral speed of the non-passing area exceeds the allowable range. In this case, the non-passing area is thermally expanded to such an extent that the peripheral speed of the non-passing area exceeds the allowable range, so that the outer diameter of the non-passing area increases, and the amount of bending of the recording material P transported between the transfer nip portion TN of the transfer device 11 and the fixing nip portion FN of the fixing device 12 decreases.

[0069] In such a state, image forming processing may be performed on a recording material P having a large size (for example, A3 or A4 landscape size) larger than the determination width size Es.

[0070] In this way, as an example of a mode in which image forming processing is performed on a large-size recording material P larger than the judgment width size Es after continuous image forming processing of the judgment number Cs or more of the small-size recording material P, [1a] In a case where the image forming apparatus 100 is equipped with a narrow-size (A4 portrait) paper feed cassette 18 and a wide-size (A4 landscape) paper feed cassette 18, when the recording material P in the narrow-size paper feed cassette 18 becomes empty during continuous image formation processing of the narrow-size recording material P in the narrow-size paper feed cassette 18, the narrow-size paper feed cassette 18 is switched to convert the narrow-size image (A4 portrait image) into a large-size image (A4 landscape image) through image processing, and image formation processing is performed on the large-size recording material P in the wide-size paper feed cassette 18. [1b] In badge processing of a plurality of jobs, an image forming process is performed on a plurality of sheets of recording material P having a narrow size (A4 portrait size) in a preceding job, and then an image forming process is performed on a plurality of sheets of recording material P having a wide size (A3 size) in a subsequent job. [1c] A mode in which, in a case of a plurality of original documents G to G (mixed original documents) of different sizes, a plurality of original documents G to G of a narrow size (A4 portrait size) are read in one job by an automatic image reading device 102, followed by a plurality of original documents G to G of a large size (A3 size), and image formation processing is performed on a plurality of recording materials P to P of a narrow size (A4 portrait size) followed by a plurality of recording materials P to P of a large size (A3 size), Examples include:

[0071] In this regard, in the present embodiment, the determination of whether the amount of sagging of the recording material P is below the reference amount of sagging is a determination of whether or not an image forming process is performed on a recording material P of a large size (e.g., A4 landscape size) larger than the small width size after continuous image forming process in which a recording material P of a small width size (e.g., A4 portrait size) equal to or smaller than a predetermined judgment width size Es, which is a length in the width direction E perpendicular to the conveying direction H of the recording material P, is subjected to image forming process for a predetermined judgment number Cs (e.g., 3 sheets) or more. In other words, the determination is a determination of whether or not an image forming process including a transfer process is performed on a recording material P of which the width, which is the length in the direction perpendicular to the conveying direction of the recording material P, is smaller than a predetermined judgment width size Es (judgment width), for a predetermined number of sheets or more of judgment number Cs. The judgment width size Es and the judgment number Cs are stored in advance in the storage unit 60b (see FIG. 5).

[0072] The determination control unit 64 determines that when continuous image formation processing is performed for the determination number Cs or more of the small-width recording material P (e.g., three sheets), the non-passing area of ​​the pressure roller 32 thermally expands to such an extent that the circumferential speed of the non-passing area exceeds the allowable range, and the outer diameter of the non-passing area of ​​the pressure roller 32 becomes larger than the allowable outer diameter. Then, when performing image formation processing for a large-size recording material P after continuous image formation processing for the determination number Cs or more of the small-width recording material P, the determination control unit 64 determines that the circumferential speed of the non-passing area of ​​the pressure roller 32 becomes faster than the allowable circumferential speed, and the amount of bending of the recording material P falls below the reference amount of bending. When the determination control unit 64 determines that the amount of bending of the recording material P being conveyed falls below the reference amount of bending, the correction control unit 63 performs correction to the transfer bias (correction to reduce the transfer bias to be lower than the reference transfer bias).

[0073] With this configuration, when performing image forming processing on a large-size recording material P immediately after continuous image forming processing of a small-size recording material P for a determination number Cs or more, the occurrence of image defects such as white dot images can be reliably prevented.

[0074] (Third embodiment-1) However, the thermal expansion state of the non-passage area of ​​the pressure roller 32 to such an extent that the peripheral speed of the non-passage area exceeds the allowable range may continue for a while. Therefore, if the correction of the transfer bias is immediately terminated, the correction of the transfer bias will not be performed even though the pressure roller 32 has expanded in the non-passage area to such an extent that the peripheral speed of the non-passage area of ​​the pressure roller 32 exceeds the allowable range and the bending amount of the recording material P continues to be below the reference bending amount.

[0075] In this regard, in the present embodiment, when image forming processing of a large-size recording material P (e.g., A4 landscape size) is performed after continuous image forming processing of recording materials P of a small width size (e.g., A4 portrait size) for a number of sheets Cs or more, the correction control unit 63 continues the correction for the transfer processing until image forming processing of a predetermined operating period is performed. Here, the predetermined operating period can be, for example, a passing time (about 30 seconds) for passing 10 consecutive A4 landscape recording materials P in the case of an A4 landscape process speed of 120 mm / sec (machine speed is about 20 sheets / min), or a period until the job is completed.

[0076] In this way, even if the pressure roller 32 expands in the non-passage area for a while to the extent that the peripheral speed of the non-passage area of ​​the pressure roller 32 exceeds the allowable range and the deflection amount of the recording material P continues to be below the standard deflection amount range, the occurrence of image defects such as white dot images can be reliably prevented even if the image formation process for the next recording material P is started in such a state.

[0077] (Fourth embodiment) - When the pressure roller is rotated idly - Incidentally, when the pressure roller 32 continues to rotate idly without the recording material P passing therethrough (the pressure roller 32 rotates in a heated state and without the recording material P passing therethrough) for a certain period of time during the image formation process on the recording material P, the temperature of the entire pressure roller 32, including the surface and the inside, increases. In this case, it can be considered that the pressure roller 32 has thermally expanded to such an extent that the peripheral speed of the pressure roller 32 exceeds the allowable range. When this happens, the pressure roller 32 thermally expands to such an extent that the peripheral speed of the pressure roller 32 exceeds the allowable range, so that the outer diameter of the pressure roller 32 increases, and the amount of bending of the recording material P transported between the transfer nip TN of the transfer device 11 and the fixing nip FN of the fixing device 12 decreases.

[0078] Examples of the manner in which the pressure roller 32 continues to rotate idly to a certain extent include the following: [2a] A state in which the pressure roller 32 rotates idly when adjusting the toner concentration of the developer in the developing tank of the developing device 2, [2b] A state in which the pressure roller 32 rotates idly when a process condition adjustment process (process control) is performed based on the image densities of a plurality of detection target image patterns formed on the image carrier (71), Examples include:

[0079] Here, the process condition adjustment process refers to a process (image quality adjustment process) that adjusts various image formation conditions (process conditions) such as charging potential, exposure amount, development bias potential, and transfer potential to respond to fluctuations in image quality due to changes over time (for example, the environment in which the image forming apparatus 100 is installed, changes in the materials of the developer, etc. over time, etc.), in order to effectively prevent fluctuations in image quality regardless of changes over time.

[0080] In this regard, in the present embodiment, the determination of whether the amount of sagging of the recording material P is below the reference amount of sagging is a determination of whether the pressure roller 32 has rotated idly for a predetermined second judgment time tc (e.g., 30 seconds) or more after the detection temperature Tc of the pressure roller 32 during the image formation process of the recording material P reaches a predetermined second judgment temperature Tcs (e.g., 120° C.) or more. In other words, it is a determination of whether the detection temperature Tc is equal to or higher than the predetermined second judgment temperature Tcs and the pressure roller 32 has rotated for the predetermined second judgment time tc or more in a state in which the recording material P is not conveyed. The second judgment temperature Tcs and the second judgment time tc are stored in advance in the storage unit 60b (see FIG. 5).

[0081] When the pressure roller 32 rotates idle for a second judgment time (e.g., 30 seconds) or more after the detected temperature of the pressure roller 32 reaches a second judgment temperature (e.g., 120° C.), the judgment control unit 64 judges that the pressure roller 32 has thermally expanded to such an extent that the circumferential speed of the pressure roller 32 exceeds the allowable range, and the outer diameter of the pressure roller 32 has become larger than the allowable outer diameter. When the pressure roller 32 rotates idle for a second judgment time or more after the detected temperature of the pressure roller 32 reaches a second judgment temperature or more, the judgment control unit 64 judges that the circumferential speed of the pressure roller 32 has become faster than the allowable circumferential speed, and the amount of bending of the recording material P is below the reference bending amount range. When the judgment control unit 64 judges that the amount of bending of the recording material P being conveyed is below the reference bending amount, the correction control unit 63 corrects the transfer bias (corrects the transfer bias to be lower than the reference transfer bias).

[0082] In this configuration, when the pressure roller 32 continues to rotate idly for a certain period of time, the occurrence of image defects such as white dot images can be reliably prevented.

[0083] (Fourth embodiment-1) However, the thermal expansion state of the pressure roller 32 to the extent that the peripheral speed of the pressure roller 32 exceeds the allowable range may continue for a while. Therefore, if the correction of the transfer bias is immediately terminated, the correction of the transfer bias will not be performed even though the pressure roller 32 has expanded to the extent that the peripheral speed of the pressure roller 32 exceeds the allowable range and the bending amount of the recording material P continues to be below the reference bending amount.

[0084] In this regard, in this embodiment, when the pressure roller 32 rotates idly for a second judgment time tc (e.g., 30 seconds) or more after the detection temperature Tc of the pressure roller 32 reaches a second judgment temperature Tcs (e.g., 120° C.) or more, if the judgment control unit 64 judges that the amount of sagging of the recording material P is below the reference sagging amount range, the correction control unit 63 continues to correct the transfer bias for a predetermined operating period after the image forming process. Here, the predetermined operating period is the same as that exemplified in the third embodiment-1.

[0085] In this way, even if the pressure roller 32 expands for a while to such an extent that the peripheral speed of the pressure roller 32 exceeds the allowable range and the deflection amount of the recording material P continues to be below the standard deflection amount range, the occurrence of image defects such as white dot images can be reliably prevented.

[0086] Fifth embodiment - When printing on large conveying size recording material - In the embodiment of the present disclosure, in order to cause bending of the recording material P transported between the transfer device 11 and the fixing device 12, transport control is performed to make the transport speed of the recording material P in the transfer device 11 and the fixing device 12 different. This transport control is performed by the transport control unit 61, and is control to make the fixing transport speed of the recording material P in the fixing device 12 slower than the transfer transport speed of the recording material P in the transfer device 11 [to make the speed a predetermined speed ratio (for example, more than 97% and less than 100%) of the transfer transport speed].

[0087] For example, in a transport control in which the fixing transport speed is made slower than the transfer transport speed, when the recording material P enters the fixing nip portion FN, it is in a bent state due to gravity, and initially, the amount of bending is within the range of the reference bending amount.

[0088] However, when a recording material P of a large conveying size (e.g., A3 size) whose length in the conveying direction H of the recording material P is longer (larger) than a predetermined judged conveying size Hs is conveyed by the thermally expanded pressure roller 32 and fixing member (31), the amount of bending of the recording material P of the large conveying size conveyed between the transfer device 11 and the fixing device 12 in the conveying direction H is maintained within the range of the reference bending amount, but the amount of bending of the recording material P of the large conveying size conveyed between the transfer device 11 and the fixing device 12 in the conveying direction H may fall below the reference bending amount in the downstream portion. This phenomenon occurs because the time during which a state in which there is a difference in the conveying speed of the recording material P in the transfer device 11 and the fixing device 12 is maintained, in other words, the time during which the recording material P is conveyed by both the transfer device 11 and the fixing device 12, becomes longer for the recording material P with a longer length in the conveying direction H. In other words, the effect of the speed difference of the recording material P changes depending on the length of time during which the speed difference is maintained. Therefore, if the transfer bias is corrected in the upstream portion in the conveying direction H of the recording material P of the large conveying size, the correction operation for the transfer bias becomes useless.

[0089] In this regard, in the present embodiment, when the length (conveying size) of the recording material P in the conveying direction H is a large conveying size (e.g., A3 size) that is larger than a predetermined length, a judged conveying size Hs (e.g., B4 size), the correction of the transfer bias is performed on the downstream side portion (e.g., a portion downstream of half, 1 / 3, 1 / 4, or 1 / 5) of the large conveying size recording material P in the conveying direction H. In other words, the transfer bias is corrected for the recording material P whose length in the conveying direction H of the recording material P is longer than a predetermined length, a judged length (judgment conveying size Hs). The judgement conveying size Hs is stored in the storage unit 60b (see FIG. 5) in advance. By controlling as described above, it is possible to avoid wasteful correction operation of the transfer bias on the upstream side portion in the conveying direction H of the large conveying size recording material P, and to reliably prevent the occurrence of image defects such as white spots.

[0090] [Control example] -An example of the correction operation according to the first embodiment (when a certain amount of time has passed since the pressure roller 32 was no longer heated)- FIG. 6 is a flowchart showing a process flow of an example of the correction operation according to the first embodiment.

[0091] 6, when the control unit 60 receives a job of X sheets (X is an integer equal to or greater than 3) (S11), it determines whether the number of printed sheets N is the third or greater (S12), and if the number of printed sheets is less than the third (S12: No), it prints the entire recording material P with the reference transfer current Is (S4) and proceeds to S6. On the other hand, if the number of printed sheets is the third or greater (S12: Yes), the control unit 60 determines the amount of sagging of the recording material P for the Nth printed sheet (S13).

[0092] That is, the control unit 60 judges whether the detected temperature difference ΔTa [Ta(N-2)-Ta(N-1)] between the detected temperature Ta(N-2) of the pressure roller 32 while the (N-2)th sheet of recording material P is being conveyed by the pressure roller 32 and the detected temperature Ta(N-1) of the pressure roller 32 while the (N-1)th sheet of recording material P is being conveyed by the pressure roller 32 is within the judgment temperature difference ΔTas (in this example, ±5° C.) (S13). If the control unit 60 judges that the detected temperature difference ΔTa exceeds the judgment temperature difference ΔTas (S13: No), it proceeds to S4. On the other hand, if the control unit 60 judges that the detected temperature difference ΔTa is within the judgment temperature difference ΔTas (S13: Yes), it prints the entire recording material P with a transfer current lowered than the reference transfer current Is (S5). Next, the control unit 60 determines whether the number of printed sheets N is the Xth sheet (S6), and if the number of printed sheets N is not the Xth sheet (S6: No), it proceeds to S12, whereas if the number of printed sheets N is the Xth sheet (S6: Yes), it ends the process.

[0093] -An example of the correction operation according to the second embodiment (when the pressure roller 32 is warmed to some extent)- FIG. 7 is a flowchart showing a process flow of an example of the correction operation according to the second embodiment.

[0094] In the example of the correction operation according to the second embodiment, the same processes as those in the example of the correction operation according to the first embodiment are denoted by the same reference numerals, and the description thereof will be omitted.

[0095] In one example of the correction operation according to the second embodiment, as shown in FIG. 7, when the control unit 60 receives a job for X sheets (X is an integer equal to or greater than 1) (S21), it determines the amount of sagging of the recording material P for the Nth printed sheet (S23).

[0096] That is, in S23, the control unit 60 judges whether the detected temperature Tb of the pressure roller 32 is equal to or higher than the first judgment temperature Tbs (130° C. in this example) (S231), and if it judges that the detected temperature Tb is lower than the first judgment temperature Tbs (S231: No), it proceeds to S4. On the other hand, if the control unit 60 judges that the detected temperature Tb is equal to or higher than the first judgment temperature Tbs (S231: Yes) and then judges that the detected temperature Tb has not been maintained within the judgment temperature fluctuation range Tbr (±1° C. in this example) for the first judgment time tb (for example, 10 seconds) (S232: No), it proceeds to S4. Also, if the control unit 60 judges that the detected temperature Tb is equal to or higher than the first judgment temperature Tbs (S231: Yes) and then judges that the detected temperature Tb has been maintained within the judgment temperature fluctuation range Tbr for the first judgment time tb (S232: Yes), it proceeds to S5.

[0097] -An example of a correction operation according to the third embodiment (when printing on a large-size recording material P after continuous printing on a small-size recording material P)- FIG. 8A is a flowchart showing a process flow of an example of a correction operation according to the third embodiment.

[0098] In the example of the correction operation according to the third embodiment, the same processes as those in the examples of the correction operation according to the first and second embodiments are denoted by the same reference numerals, and the description thereof will be omitted.

[0099] In one example of the correction operation according to the third embodiment, as shown in FIG. 8A, when the control unit 60 receives a job of X sheets (X is an integer equal to or greater than the determination number Cs) (S31), it determines the amount of sagging of the recording material P for the Nth printed sheet (S33).

[0100] That is, in S33, the control unit 60 judges whether or not continuous printing processing of the recording material P of a narrow size (for example, A4 portrait size) has been performed for a number of sheets Cs (three sheets in this example) or more (S331). If the control unit 60 judges that continuous printing processing has not been performed (S331: No), the process proceeds to S4. On the other hand, if the control unit 60 judges that continuous printing processing has been performed (S331: Yes) and then subsequently print processing of a recording material P of a narrow size (S332: No), the process proceeds to S4. Also, if the control unit 60 judges that continuous printing processing has been performed (S331: Yes) and then subsequently print processing of a recording material P of a wide size (for example, A4 landscape size), the process proceeds to S5.

[0101] -Another example of the correction operation according to the third embodiment- FIG. 8B is a flowchart showing the flow of processing of another example of the correction operation according to the third embodiment.

[0102] Another example of the correction operation according to the third embodiment shown in FIG. 8B is the example of the correction operation according to the third embodiment shown in FIG. 8A, in which processes S321 and S322 are added between processes S31 and S33, and further processes S34 to S37 are added between processes S5 and S6.

[0103] In this other example of the correction operation according to the third embodiment, the same processes as those in the example of the correction operation according to the third embodiment are denoted by the same reference numerals, and the description thereof will be omitted.

[0104] In another example of the correction operation according to the third embodiment, as shown in Fig. 8B, when the control unit 60 receives a job of X sheets (X is an integer equal to or greater than the determination sheet count Cs) (S31), the control unit 60 resets the thermal expansion flag FL to 0 (S321). Here, the thermal expansion flag FL is stored in the memory unit 60b (see Fig. 5).

[0105] Next, the control unit 60 determines whether the thermal expansion flag FL is set to 1 (S322), and if it determines that the thermal expansion flag FL is 0 (S322: No), it determines the amount of sagging of the recording material P of the Nth printed sheet (S33). On the other hand, if the control unit 60 determines that the thermal expansion flag FL is 1 (S322: Yes), it proceeds to S5.

[0106] After the process of S5, the control unit 60 counts up the corrected number count Y by 1 (S34) and judges whether the corrected number count Y has reached a predetermined judgment number count Ys (10 in this example) (S35). If the corrected number count Y has not reached the judgment number count Ys (S35: No), the control unit 60 sets the thermal expansion flag FL to 1 (S36) and proceeds to S6. On the other hand, if the corrected number count Y has reached the judgment number count Ys (S35: Yes), the control unit 60 resets the thermal expansion flag FL to 0 (S37) and proceeds to S6. The judgment number count Ys is stored in advance in the storage unit 60b (see FIG. 5).

[0107] -An example of a correction operation according to the fourth embodiment- FIG. 9A is a flowchart showing a process flow of an example of a correction operation according to the fourth embodiment.

[0108] In the example of the correction operation according to the fourth embodiment, the same processes as those in the examples of the correction operation according to the first to third embodiments are denoted by the same reference numerals, and the description thereof will be omitted.

[0109] In one example of the correction operation according to the fourth embodiment, as shown in FIG. 9A, when the control unit 60 receives a job (S41), it determines the amount of sagging of the recording material P of the Nth printed sheet (S43).

[0110] That is, in S43, the control unit 60 judges whether the detected temperature Tc of the pressure roller 32 during the printing process of the recording material P reaches the second judgment temperature Tcs (120° C. in this example) or more (S431). If the control unit 60 judges that the detected temperature Tc has not reached the second judgment temperature Tcs or more (S431: No), the control unit 60 proceeds to S4. On the other hand, if the control unit 60 judges that the detected temperature Tc has reached the second judgment temperature Tcs or more (S431: Yes) and judges that the pressure roller 32 has not been idling for the second judgment time tc (30 seconds in this example) or more (S432: No), the control unit 60 proceeds to S4. Also, if the control unit 60 judges that the detected temperature Tc has reached the second judgment temperature Tcs or more (S431: Yes) and judges that the pressure roller 32 has been idling for the second judgment time tc or more (S432: Yes), the control unit 60 proceeds to S5.

[0111] -Another example of the correction operation according to the fourth embodiment- FIG. 9B is a flowchart showing the flow of processing of another example of the correction operation according to the fourth embodiment.

[0112] Another example of the correction operation according to the fourth embodiment shown in FIG. 9B is the example of the correction operation according to the fourth embodiment shown in FIG. 9A, in which processes S421 and S422 are added between processes S41 and S43, and further processes S44 to S47 are added between processes S5 and S6.

[0113] In this other example of the correction operation according to the fourth embodiment, the same processes as those in the example of the correction operation according to the fourth embodiment are denoted by the same reference numerals, and the description thereof will be omitted.

[0114] In another example of the correction operation according to the fourth embodiment, as shown in FIG. 9B, when the control unit 60 accepts a job (S41), it resets the thermal expansion flag FL to 0 (S421).

[0115] Next, the control unit 60 judges whether the thermal expansion flag FL is 1 or not (S422), and if it is judged that the thermal expansion flag FL is 0 (S422: No), it judges the amount of sagging of the recording material P of the Nth printed sheet (S43). On the other hand, if the control unit 60 judges that the thermal expansion flag FL is 1 (S422: Yes), it proceeds to S5.

[0116] After the process of S5, the control unit 60 counts up the corrected number count Y by 1 (S44) and judges whether the corrected number count Y has reached the judgment number count Ys (10 in this example) (S45). If the corrected number count Y has not reached the judgment number count Ys (S45: No), the control unit 60 sets the thermal expansion flag FL to 1 (S46) and proceeds to S6. On the other hand, if the corrected number count Y has reached the judgment number count Ys (S45: Yes), the control unit 60 resets the thermal expansion flag FL to 0 (S47) and proceeds to S6.

[0117] -An example of a correction operation according to the fifth embodiment- FIG. 10 is a flowchart showing an example of the correction operation according to the fifth embodiment.

[0118] An example of the correction operation according to the fifth embodiment shows another example of the process of S5 in the correction operations according to the first to fourth embodiments.

[0119] In the processing example of Figure 10, after determining the amount of bending (S13, S23, S33, S43), the control unit 60 determines (S51) whether the transport direction size of the recording material P is a large transport size (A3 size in this example) larger than the determined transport size Hs (B4 size in this example).

[0120] When the size of the recording material P in the transport direction is the large transport size (S51: Yes), the control unit 60 prints the downstream portion of the large transport size recording material P in the transport direction H (in this example, the portion downstream of the half) with a transfer current lower than the reference transfer current Is (S52). On the other hand, when the size of the recording material P in the transport direction is a small transport size that is equal to or smaller than the judgment transport size Hs (S51: No), the control unit 60 prints the entire small transport size recording material P with the reference transfer current Is (S53).

[0121] Then, after processing S52 or S53 (see Figures 6 to 8A and 9A), the control unit 60 transitions to processing S6 in an example of correction operation according to the first embodiment to an example of correction operation according to the fourth embodiment, transitions to processing S34 in another example of correction operation according to the third embodiment (see Figure 8B), and transitions to processing S44 in another example of correction operation according to the fourth embodiment (see Figure 9B).

[0122] The present disclosure is not limited to the above-described embodiments, and can be implemented in various other forms. Therefore, the embodiments are merely illustrative in all respects and should not be interpreted as being limited. The scope of the present disclosure is indicated by the claims, and is not restricted in any way by the text of the specification. Furthermore, all modifications and changes within the scope of the claims are within the scope of the present disclosure. [Explanation of symbols]

[0123] 100 Image forming device 11 Transcription device 11a Secondary transfer roller 12 Fixing device 121 Pressure roller temperature detector 36 Heat source 60 Control section 60a Processing section 60b Storage section 61 Transport control section 62 Temperature detection control section 63 Correction control section 64 Judgment control unit 70 Intermediate transfer belt device 71 Intermediate transfer belt 72 Intermediate transfer drive roller 73 Intermediate transfer driven roller 80 Transfer power supply 90 Rotation drive unit Cs judgment number E Width direction Es judgment width size FL Thermal expansion flag FN Fixing nip H Transport direction Hs Judgment conveyance size N Number of prints P Recording material TN Transfer nip Ta Detection temperature Tb Detection temperature Tbr Judgment temperature fluctuation range Tbs 1st judgment temperature Tc Detection temperature Tcs 2nd judgment temperature W Rotation axis direction Y correction number count Ys Judgment Count tb First judgment time tc 2nd judgment time ΔTa Detected temperature difference ΔTas Judgment temperature difference α Rotation axis

Claims

1. a transfer device that performs a transfer process for transferring a toner image formed on an image carrier onto a sheet-shaped recording material; a fixing device having a heat source, which heats and fixes the toner image transferred onto the recording material to the recording material by a fixing member and a pressure roller, an image forming apparatus that performs conveyance control so that there is a difference in conveyance speed of the recording material between the transfer device and the fixing device in order to cause a flexure in the recording material conveyed between the transfer device and the fixing device, a transfer power supply that supplies a transfer bias, which is a predetermined voltage or current, to the transfer device; a temperature detection means for detecting a temperature of the pressure roller, an image forming apparatus comprising: an image forming unit that corrects a value of the transfer bias supplied from the transfer power source to the transfer device when performing the transfer process, based on the detected temperature detected by the temperature detection means;

2. 2. The image forming apparatus according to claim 1, a determination control unit that determines whether or not a deflection amount generated in the recording material during conveyance falls below a reference deflection amount having a predetermined range based on the detected temperature detected by the temperature detection unit, and a correction control unit that corrects a value of the transfer bias, An image forming apparatus characterized in that, when the judgment control unit determines that the amount of bending of the recording material during transport falls below the reference amount of bending, the correction control unit corrects the value of the transfer bias supplied to the transfer power source.

3. 3. The image forming apparatus according to claim 2, The image forming apparatus is characterized in that the judgment control unit judges that the amount of deflection occurring in the recording material is below a reference amount of deflection when the temperature change amount per unit time of the detected temperature detected sequentially during a continuous image formation process in which the transfer process is performed sequentially on multiple recording materials to form images is less than a predetermined judgment temperature change amount.

4. 3. The image forming apparatus according to claim 2, The image forming apparatus is characterized in that, during a continuous image forming process in which the transfer process is performed sequentially on a plurality of recording materials to form an image, when a detected temperature difference, which is the difference between the detected temperature detected when a preceding recording material is being subjected to the transfer process and the detected temperature detected when a succeeding recording material is being subjected to the transfer process, is within a predetermined value, the judgment control unit determines that the amount of deflection of the recording material is below the reference amount of deflection.

5. 3. The image forming apparatus according to claim 2, The image forming apparatus is characterized in that the judgment control unit judges that the amount of deflection of the recording material falls below the reference amount of deflection when the detected temperature detected by the temperature detection means before the image forming process is started, in which the transfer process is performed on the recording material to form an image, is equal to or higher than a predetermined first judgment temperature, and when the detected temperature detected by the temperature detection means during the image forming process is maintained within a predetermined judgment temperature fluctuation range for a first judgment time.

6. 3. The image forming apparatus according to claim 2, The image forming apparatus is characterized in that the judgment control unit judges that the amount of deflection of the recording material is below the reference amount of deflection when the transfer process is performed on a recording material whose width, which is the length in a direction perpendicular to the transport direction of the recording material, is smaller than a predetermined length, a judgment width, for a predetermined number of sheets or more, and then the transfer process is performed on a recording material whose width is larger than the judgment width.

7. 3. The image forming apparatus according to claim 2, The image forming apparatus is characterized in that the judgment control unit judges that the amount of deflection of the recording material falls below the reference deflection amount when the detected temperature is equal to or higher than a predetermined second judgment temperature and the pressure roller rotates for a predetermined second judgment time or longer without the recording material being transported.

8. 8. The image forming apparatus according to claim 6, An image forming apparatus characterized in that, when the judgment control unit determines that the amount of deflection of the recording material is below the reference amount of deflection and the image formation process including the transfer process is continued, the correction control unit continues to supply the corrected transfer bias value to the transfer device for a predetermined operating period.

9. 2. The image forming apparatus according to claim 1, When the transfer process is performed on the recording material, the length of the recording material in the conveying direction is longer than a predetermined determination length, The image forming apparatus is characterized in that the correction control unit changes the value of the transfer bias after the recording material begins to be transported by the fixing device and before the transport of the recording material by the transfer device is completed, maintains the changed value of the transfer bias unchanged until the transport of the recording material by the transfer device is completed, and returns it to the original value when the transport of the recording material by the transfer device is completed.

10. 2. The image forming apparatus according to claim 1, the fixing device includes a fixing belt that is heated by the heat source, and a fixing pad that is provided on an inner side of the fixing belt, The image forming apparatus according to claim 1, wherein the pressure roller presses the fixing belt against the fixing pad.

Images