Image forming apparatus

Abstract

To accurately heat a fixing belt to a proper fixing temperature by a sub-heater in a fixing device having a main heater and the sub-heater juxtaposed in a width direction orthogonal to the traveling direction of the fixing belt.SOLUTION: A fixing device 13 includes a main heater 211 disposed at a center part in a rotation axis RG1 direction of a fixing belt, two sub-heaters 212213 disposed at both ends, and cooling fans 151 for cooling the ends of the heaters. When a recording paper sheet P to be fixed does not reach a temperature detection position TP of a thermosensor 232 for detecting the temperature of a sub-heater 212, the target temperature of the sub-heater 212213 is set high to keep the temperature of a paper passing area A21 within a fixable temperature range.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus equipped with a fixing device that fixes an image formed on a recording medium such as recording paper by thermocompression bonding. [Background technology]

[0002] An electrophotographic image forming apparatus includes a fixing device that fixes an image formed on a recording medium. The fixing device includes a cylindrical, rotatable fixing belt, a pressure member disposed opposite the fixing belt and forming a nip between the fixing belt and the recording medium to be fixed, and a heater that heats the fixing belt. The image formed on the recording medium is fixed to the recording medium in the nip by applying heat and pressure (thermocompression bonding). The fixing device also generally includes a temperature sensor that detects the heater temperature to maintain the temperature of the fixing belt, which holds and heats the recording medium, at an appropriate temperature. The heater is divided in the width direction and includes a main heater located in the center of the fixing belt and two sub-heaters located at both ends of the fixing belt. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6929033 Summary of the Invention [Problem to be solved by the invention]

[0004] When heating is performed by heaters in which the main heater and sub-heaters are arranged in the width direction as described above, the edges of the recording medium in the width direction can be heated efficiently.

[0005] When the size of the recording medium passing through the fixing device is relatively small, the center of the nip portion of the fixing device in the direction of the rotation axis of the fixing belt (the width direction perpendicular to the running direction of the fixing belt and the transport direction of the recording medium) becomes the paper-passing area through which the recording medium passes, but the area heated by the sub-heater outside of this may become a non-paper-passing area through which the recording medium does not pass.

[0006] When a recording medium passes through the fixing device, heat from the fixing device is absorbed by the recording medium. Therefore, when a small-sized recording medium passes through the fixing device, the temperature difference between the center (paper passing area) where heat is absorbed by the recording medium and both ends (non-paper passing areas) where almost no heat is absorbed by the recording medium becomes large, causing excessive temperature rise at both ends of the fixing device.

[0007] To prevent excessive heating at both ends of the fixing device, a cooling fan can be installed to lower the temperature of the heater in the area where excessive heating occurs. However, depending on the size of the recording medium, if the recording medium has a specific size in which the width dimension thereof reaches the position of the sub-heater in the width direction but does not reach the temperature detection position of the second temperature sensor that detects the temperature of the sub-heater in the width direction, controlling the heater based on the temperature detected by the second temperature sensor may result in the temperature of the paper passing area of ​​the recording medium that reaches the position of the sub-heater in the width direction dropping too much, resulting in poor fixing.

[0008] The present invention has been made in consideration of the above circumstances, and aims to enable the sub-heater to accurately heat to an appropriate fixing temperature in a fixing device having a main heater and a sub-heater arranged side by side in the width direction perpendicular to the running direction of the fixing belt. [Means for solving the problem]

[0009] According to one aspect of the present invention, there is provided an image forming apparatus comprising: a fixing device including a cylindrical rotatable fixing belt; a pressure member disposed opposite the fixing belt and forming a nip portion between the fixing belt and the pressure member, through which a recording medium to be fixed is inserted; and a heater extending in a width direction perpendicular to a direction of rotation of the fixing belt and heating the fixing belt, wherein the heater is divided in the width direction into a main heater disposed in a central portion of the fixing belt in the width direction and two sub-heaters disposed at both ends of the fixing belt in the width direction, and the image forming apparatus further comprises a first temperature sensor that detects the temperature of the main heater, a second temperature sensor that detects the temperature of the sub-heater, and a cooling mechanism that has a cooling fan and cools both ends of the heater in the width direction, and controls driving of the cooling fan and calculates a first detected temperature detected by the first temperature sensor when the cooling fan is driven as a predetermined first target temperature that is an appropriate fixing temperature. and a control unit that controls driving of the main heater to set a second detected temperature by the second temperature sensor when the cooling fan is driven to a predetermined second target temperature; and a setting unit that sets the second target temperature to a predetermined first set temperature when the recording medium transported by the fixing belt is a large size that approaches the temperature detection position of the second temperature sensor in the width direction perpendicular to the transport direction, and sets the second target temperature to a predetermined second set temperature that is higher than the first set temperature when the size of the recording medium in the width direction is a specific size that approaches the position of the sub-heater in the width direction but does not approach the temperature detection position of the second temperature sensor in the width direction, wherein the second set temperature is a predetermined temperature at which the temperature of the paper passing area of ​​the recording medium that approaches the position of the sub-heater in the width direction is maintained at an appropriate fixing temperature. [Effects of the Invention]

[0010] According to the present invention, in a fixing device having a main heater and a sub-heater arranged side by side in the width direction perpendicular to the running direction of the fixing belt, heating to an appropriate fixing temperature by the sub-heater can be performed accurately. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a functional block diagram illustrating a schematic main internal configuration of an image forming apparatus according to an embodiment of the present invention; [Figure 2] FIG. 2 is a cross-sectional view schematically illustrating an example of a fixing device provided in an image forming apparatus. [Figure 3] FIG. 2 is a plan view schematically showing a part of the fixing device and its periphery. [Figure 4] 10A and 10B are diagrams illustrating a case where a recording medium does not pass a temperature detection position of a temperature sensor that detects the temperature of a sub-heater. [Figure 5] 10A and 10B are diagrams illustrating a case where a recording medium passes a temperature detection position of a temperature sensor that detects the temperature of a sub-heater. [Figure 6] 6 is a flowchart illustrating an example of a process related to temperature control of a fixing device performed by a control unit of an image forming apparatus. [Figure 7] Graph (A) shows an example of the temperature change of the heater when the recording medium does not pass the temperature detection position of the temperature sensor that detects the temperature of the sub-heater, and graph (B) shows a comparative example. [Figure 8] 10 is a graph showing an example of a change in the temperature of the heater when a recording medium passes a temperature detection position of a temperature sensor that detects the temperature of the sub-heater. DETAILED DESCRIPTION OF THE INVENTION

[0012] An image forming apparatus according to one embodiment of the present invention will be described below with reference to the drawings. Fig. 1 is a functional block diagram showing the main internal configuration of an image forming apparatus according to one embodiment of the present invention. Image forming apparatus 1 is a multifunction peripheral that combines multiple functions, such as a copy function, a printer function, a scanner function, and a facsimile function.

[0013] The image forming apparatus 1 includes a control unit 10, a document feeding section 6, a document reading section 5, an image forming section 12, a fixing device 13, a cooling mechanism 15, a paper feeding section 14, an operation section 47, and a memory section 8.

[0014] The document feeder 6 is configured to be openable and closable by a hinge or the like (not shown) on the top surface of the document reading unit 5, and functions as a document pressing cover when reading a document placed on a platen glass (not shown). The document feeder 6 is also an ADF (Auto Document Feeder), and is equipped with a document placement tray (not shown), and supplies documents placed on the document placement tray to the document reading unit 5.

[0015] The following describes the case where a document is read by the image forming apparatus 1. The document reading unit 5 optically reads an image of a document supplied to the document reading unit 5 by the document feeding unit 6 or a document placed on the platen glass, and generates image data. The image data generated by the document reading unit 5 is stored in an image memory (not shown) or the like.

[0016] The following describes an image forming operation performed by image forming apparatus 1. Image forming unit 12 forms a toner image on recording paper as a recording medium fed from paper feed unit 14 based on image data generated by a document reading operation, image data stored in an image memory or the like, image data received from a computer connected via a network, and the like.

[0017] The fixing device 13 applies heat and pressure to the recording paper on which the toner image has been formed by the image forming unit 12, fixing the toner image to the recording paper, and the recording paper after the fixing process is discharged to an output tray (not shown). The paper feed unit 14 is equipped with a paper feed cassette. The cooling mechanism 15 has a cooling fan 151 and cools a heater 21 (FIG. 2) provided in the fixing device 13, which will be described later.

[0018] 2 is a cross-sectional view schematically illustrating an example of the fixing device 13. The fixing device 13 includes a fixing belt 20, a pressure member 30, a heater 21, a holding member 22, a temperature sensor 23, a reinforcing member 24, a biasing member 25, and a conveying guide 40.

[0019] The fixing belt 20 is a cylindrical, rotatable, endless belt that heats a recording medium (recording paper P) on which a toner image is formed. The fixing belt 20 is rotatable about a first rotation shaft RG1 that extends in the depth direction of the paper in FIG. 2, and extends in the direction of the first rotation shaft RG1.

[0020] The pressure member 30 is a roller that can rotate around a second rotation axis RG2 that is parallel to the first rotation axis RG1, and extends in the direction of the second rotation axis RG2. The pressure member 30 forms a nip N between the pressure member 30 and the fixing belt 20, where a recording paper P as a recording medium is sandwiched and transported, and rotates the fixing belt 20. The recording paper P on which the toner image has been formed is guided by a transport guide 40, and is transported and sandwiched at the nip N. The arrow CD indicates the transport direction of the recording paper P.

[0021] The fixing belt 20 rotates in a first rotation direction R1 (clockwise in Figure 2, the running direction of the fixing belt 20) around the first rotation axis RG1, following the rotation of the pressure member 30 in a second rotation direction R2 (counterclockwise in Figure 2) around the second rotation axis RG2.

[0022] The heater 21 heats the fixing belt 20. The heater 21 is a planar heater extending along the direction of the first rotation axis RG1, and is disposed inside the fixing belt 20, facing the inner circumferential surface 201 of the fixing belt 20. The heater 21 is, for example, a ceramic heater with a relatively low heat capacity, and includes a ceramic substrate and a resistance heating element.

[0023] The holding member 22 holds the heater 21. The holding member 22 is a heat-resistant resin member that extends along the direction of the first rotation axis RG1 and has a U-shaped cross section.

[0024] The temperature sensor 23 faces the heater 21 and detects the temperature of the heater 21. The temperature sensor 23 is inserted into a through-hole formed in the holding member 22 and abuts against the heater 21. The temperature sensor 23 is, for example, a thermistor. Note that the temperature sensor 23 does not have to be of a type that contacts the heater 21, but may be of a non-contact type.

[0025] The reinforcing member 24 is a metal stay that extends along the first rotation axis RG1 and has an inverted U-shaped cross section. The reinforcing member 24 is fixed to the holding member 22 so as to reinforce the holding member 22.

[0026] The biasing member 25 is disposed between the temperature sensor 23 and the reinforcing member 24, and biases the temperature sensor 23 in a direction from the temperature sensor 23 toward the heater 21. The biasing member 25 is, for example, a coil spring.

[0027] 3 is a plan view schematically showing a part of the fixing device 13 and its surroundings. The substrate 50 is, for example, a ceramic substrate, and has two surfaces parallel to each other. The rear surface is a heat generating surface 502.

[0028] A main heater 211, a first sub-heater 212, and a second sub-heater 213 are provided on the heat generating surface 502. The heater 21 (FIG. 2) is made up of the main heater 211, the first sub-heater 212, and the second sub-heater 213. The main heater 211 is disposed at the center of the fixing belt 20 in the width direction, which is perpendicular to the running direction of the fixing belt 20 and the transport direction of the recording paper P. The first sub-heater 212 and the second sub-heater 213 are disposed at each end of the fixing belt 20 in the width direction.

[0029] The main heater 211 is arranged in the center of the substrate 50 in the direction of the first rotation axis RG1, the first sub-heater 212 is arranged at one end in the direction of the first rotation axis RG1, and the second sub-heater 213 is arranged at the other end in the direction of the first rotation axis RG1.

[0030] The length L1 in the longitudinal direction (width direction) of the main heater 211 is, for example, the same as the short side (210 mm) of an A4 size paper, which is frequently used in business situations. The length L2 from the outer end of the first sub-heater 212 in the longitudinal direction (width direction) to the outer end of the second sub-heater 213 in the longitudinal direction is, for example, about 20 mm longer than the short side (297 mm) of an A3 size paper.

[0031] As shown in FIG. 2, the fixing device 13 includes a temperature sensor 23 that detects the temperature of the heater 21. As shown in FIG. 3, the temperature sensor 23 includes a first temperature sensor 231 that detects the temperature of the main heater 211 and a second temperature sensor 232 that detects the temperature of the first sub-heater 212. The second temperature sensor 232 may also be provided in the second sub-heater 213, but this embodiment describes a configuration in which the second temperature sensor 232 is provided only in the first sub-heater 212. That is, the first temperature sensor 231 is disposed at a position in the width direction where the main heater 211 is disposed. The second temperature sensor 232 is disposed at a position in the width direction where the first sub-heater 212 is disposed.

[0032] The first temperature sensor 231 is disposed on the non-heating surface 501 side of the substrate 50 (the surface opposite the heating surface 502) at a position facing the main heater 211. The second temperature sensor 232 is disposed on the non-heating surface 501 side of the substrate 50 at a position facing the first sub-heater 212.

[0033] The paper passing area A2 indicates the area of ​​the heater 21 in the width direction through which the recording paper P passes. The non-paper passing area A1 indicates the area of ​​the heater 21 (FIG. 2) in the width direction through which the recording paper P does not come. Furthermore, the paper passing area A21 in the width direction is the area through which the recording paper P comes within the area where the first sub-heater 212 and the second sub-heater 213 are arranged.

[0034] The cooling fan 151 cools both ends of the heater 21 (Figure 2) in the direction of the first rotation axis RG1 (the ends of the area where the main heater 211 is located and the entire area where the first sub-heater 212 and the second sub-heater 213 are located).

[0035] Fig. 4 is a diagram illustrating a case where the recording medium does not come close to the temperature detection position of the second temperature sensor 232 in the width direction. Fig. 5 is a diagram illustrating a case where the recording medium passes the temperature detection position of the second temperature sensor 232.

[0036] Length L3 indicates the distance between the center position CP in the direction of the first rotation axis RG1 (width direction) of the main heater 211 and the temperature detection position TP of the second temperature sensor 232. The large size SZ is used to determine whether the recording paper P will pass through the temperature detection position TP, and is the smallest size in the width direction of the recording paper P that will pass through the temperature detection position TP of the second temperature sensor 232. For example, the large size SZ is set to be twice the length L3.

[0037] As shown in Fig. 4, if the width W of the recording paper P is less than the large size SZ, the recording paper P does not reach the temperature detection position TP in the width direction. On the other hand, as shown in Fig. 5, if the width W of the recording paper P is equal to or greater than the large size SZ, the recording paper P reaches the temperature detection position TP in the width direction.

[0038] Returning now to the description of each part of image forming apparatus 1 with reference to Fig. 1, operation unit 47 accepts instructions, such as an instruction to perform an image forming operation, from the operator regarding various operations and processes that image forming apparatus 1 can perform. Operation unit 47 is equipped with display unit 473 that displays operation guides and the like for the operator. Operation unit 47 also accepts input of instructions from the user via a touch panel provided in display unit 473, based on the user's operation (touch operation) on the operation screen displayed on display unit 473.

[0039] The display unit 473 is composed of an LCD (Liquid Crystal Display) or the like. The display unit 473 is equipped with a touch panel. When the operator touches a button or key displayed on the screen, the touch panel receives an instruction associated with the touched position.

[0040] The storage unit 8 is a large-capacity storage device such as a hard disk drive (HDD) or a solid state drive (SSD), and stores various control programs and the like.

[0041] The control unit 10 includes a processor, a RAM (Random Access Memory), a ROM (Read Only Memory), and a dedicated hardware circuit. The processor is, for example, a CPU (Central Processing Unit), an ASIC (Application Specific Integrated Circuit), or an MPU (Micro Processing Unit). The control unit 10 includes a control unit 100 and a setting unit 101.

[0042] The control unit 10 functions as a control unit 100 and a setting unit 101 by the operation of the processor in accordance with a control program stored in the storage unit 8. However, the control unit 100 and the like can also be configured by hardware circuits, without relying on the operation of the control unit 10 in accordance with a control program. Unless otherwise stated below, the same applies to each embodiment.

[0043] The control unit 100 is responsible for overall operational control of the image forming apparatus 1. The control unit 100 is connected to the document feed unit 6, the document reading unit 5, the image forming unit 12, the fixing device 13, the cooling mechanism 15, the paper feed unit 14, the operation unit 47, and the storage unit 8, and controls the driving of each of these units. For example, the control unit 100 executes various processes required for image formation by the image forming apparatus 1.

[0044] Furthermore, the control unit 100 controls the driving of the main heater 211 so that the first detected temperature D1 by the first temperature sensor 231 becomes a predetermined first target temperature T1, which is an appropriate fixing temperature. Furthermore, the control unit 100 controls the driving of both the first sub-heater 212 and the second sub-heater 213 so that the second detected temperature D2 by the second temperature sensor 232 becomes a predetermined second target temperature T2 (different from the first target temperature T1). Furthermore, the control unit 100 controls the driving of the cooling fan 151.

[0045] For example, the control unit 100 drives the main heater 211 using PID (Proportional Integral Differential) control so that the first detected temperature D1 becomes the first target temperature T1, and drives both the first sub-heater 212 and the second sub-heater 213 using PID control so that the second detected temperature D2 becomes the second target temperature T2.

[0046] When the size W of the recording sheet P in the width direction perpendicular to the conveyance direction CD is equal to or larger than the large size SZ, the setting unit 101 sets the second target temperature T2 to a predetermined first set temperature PT1. Furthermore, when the size W of the recording sheet P in the width direction is smaller than the large size SZ and the recording sheet P reaches the position of the first sub-heater 212 in the width direction and is a specific size, the setting unit 101 sets the second target temperature T2 to a predetermined second set temperature PT2 higher than the first set temperature PT1. The first set temperature PT1 is a predetermined appropriate fixing temperature (e.g., 180°C). The second set temperature PT2 is a predetermined temperature higher than the appropriate fixing temperature at which the temperature of the sheet passing area (sheet passing area A21 in FIG. 4 ) of the recording sheet P that reaches the positions of the first sub-heater 212 and the second sub-heater 213 in the width direction is maintained at the appropriate fixing temperature (e.g., 180°C). The predetermined temperature is assumed to be an appropriate value that is detected in advance through experiments or the like and set in the setting unit 101 in advance.

[0047] The setting unit 101 sets whether to set the second target temperature T2 to the first set temperature PT1 or the second set temperature PT2 in accordance with an instruction input to the operation unit 47. Alternatively, if the control unit 100 stores the size of the recording paper stored in the paper feed unit 14, the setting unit 101 sets the second target temperature T2 to the second set temperature PT2 if the stored recording paper size is a predetermined size that reaches the positions of the first sub-heater 212 and the second sub-heater 213 in the width direction.

[0048] Next, an example of a process related to temperature control of the fixing device 13 by the image forming apparatus 1 will be described with reference to the flowchart shown in Fig. 6. This process is performed when the operation unit 47 receives a copy instruction or the like and executes an image forming process on the recording paper P.

[0049] First, the setting unit 101 sets the first target temperature T1 to the first set temperature PT1 (S1). Next, the setting unit 101 determines whether the width W of the recording paper P to be fixed is equal to or larger than the large size SZ (S2).

[0050] If the setting unit 101 determines that the widthwise dimension W of the recording sheet P is less than the large size SZ and that the widthwise dimension of the recording sheet P is approaching the position of the first sub-heater 212 in the width direction (as shown in the example of FIG. 4, the recording sheet P is approaching the arrangement position of the first sub-heater 212 but has not approached the temperature detection position TP) (NO in S2), the setting unit 101 sets the second target temperature T2 to the second set temperature PT2 (S3). Furthermore, the setting unit 101 sets the operation start temperature TS of the cooling fan 151 to a predetermined third set temperature PT3 that is equal to or higher than the second set temperature PT2 or that is higher than the second set temperature PT2 (S4).

[0051] Next, the control unit 100 starts driving control of the main heater 211 so that the first detected temperature D1 by the first temperature sensor 231 becomes the first target temperature T1 (=first set temperature PT1) (S5), and starts driving control of the first sub-heater 212 and the second sub-heater 213 so that the second detected temperature D2 by the second temperature sensor 232 becomes the second target temperature T2 (=second set temperature PT2) (S6). Note that in S6, if the second temperature sensor 232 is also provided in the second sub-heater 213, the control unit 100 drives and controls the first sub-heater 212 and the second sub-heater 213, for example, using the average value of the second temperature sensors 232 of both the first sub-heater 212 and the second sub-heater 213 as the second detected temperature D2 by the second temperature sensor 232.

[0052] Then, the control unit 100 determines whether the second detected temperature D2 by the second temperature sensor 232 has become equal to or higher than the operation start temperature TS (= second set temperature PT2 or third set temperature PT3) (S7), and if it determines that the second detected temperature D2 has become equal to or higher than the operation start temperature TS (YES in S7), it controls the driving of the cooling fan 151 to operate the cooling fan 151 (S8).

[0053] Then, the control unit 100 determines whether the image formation process is completed (S9), and if it determines that the image formation process is not completed (NO in S9), the process returns to S5. The control unit 100 continues to control the drive of the main heater 211 so that the first detected temperature D1 by the first temperature sensor 231 becomes the first target temperature T1 (=first set temperature PT1) (S5), and to control the drive of the first sub-heater 212 and the second sub-heater 213 so that the second detected temperature D2 by the second temperature sensor 232 becomes the second target temperature T2 (=second set temperature PT2) (S6).

[0054] Figure 7(A) is a graph showing an example of the temperature change of the heater 21 when the widthwise size W of the recording paper P is less than the large size SZ and the widthwise size of the recording paper P approaches the position of the first sub-heater 212 in the widthwise direction, that is, when the recording paper P does not pass through the temperature detection position TP, as shown in the example of Figure 4.

[0055] The solid line (thick line) in the graph indicates the change in the first detected temperature D1 (temperature of the main heater 211) detected by the first temperature sensor 231. Due to the above-described control of the main heater 211 by the control unit 100, the temperature of the main heater 211 is maintained at the first set temperature PT1 (first target temperature T1) during the image formation processing period, and falls within the temperature range in which fixing is possible.

[0056] The solid line (thin line) shows the change in the second detected temperature D2 (temperature of the non-paper passing area A1 of the first sub-heater 212) by the second temperature sensor 232, and due to the above-mentioned control of the first sub-heater 212 and the second sub-heater 213 by the control unit 100, the temperature is maintained at the second set temperature PT2 (second target temperature T2), and no overheating occurs.

[0057] The dashed line indicates the change in temperature of the first sub-heater 212 in the paper passage area A21, which cannot be detected by the second temperature sensor 232. The control unit 100 controls the drive of both the first sub-heater 212 and the second sub-heater 213 so that the second detected temperature D2 by the second temperature sensor 232 becomes the second target temperature T2. However, because no recording paper P is present at the temperature detection position of the second temperature sensor 232, the temperature detected by the second temperature sensor 232 becomes higher than the temperature of the paper passage area A21. Therefore, when the control unit 100 controls the drive of both the first sub-heater 212 and the second sub-heater 213 so that the second detected temperature D2 by the second temperature sensor 232 becomes the second target temperature T2, the temperature detected by the second temperature sensor 232 remains at the second detected temperature D2. However, because heat is lost from the paper passage area A21 as the recording paper P passes through, the temperature of the paper passage area A21 drops after the cooling fan 151 starts operating. Here, the second set temperature PT2 is a predetermined temperature at which the temperature of the paper passage area where the recording paper P reaches the positions of the first sub-heater 212 and the second sub-heater 213 in the width direction is maintained at the first target temperature T1, which is the appropriate fixing temperature, and therefore the temperature of the paper passage area A21 drops as described above, but maintains the first target temperature T1, which is the appropriate fixing temperature (FIG. 7B). In other words, the temperature of the paper passage area A21 is maintained at a temperature close to the first set temperature PT1 during the image formation processing period, and is maintained within a temperature range where fixing is possible. In the non-paper passage area A1 of the first sub-heater 212 and the second sub-heater 213, excessive temperature rise is suppressed by the air from the cooling fan 151.

[0058] 7B is a graph showing, as a comparative example, an example of the temperature change of the heater 21 when the recording paper P does not pass through the temperature detection position TP. Here, the second target temperature T2' is set by considering only that the second detected temperature D2 by the second temperature sensor 232 is set as the second target temperature T2, without considering the temperature drop in the paper passage area A21, and is therefore set at a temperature lower than the second set temperature PT2. As a result, the temperature in the paper passage area A21 drops too much, and during the image formation process, it drops significantly below the first set temperature PT1, which is the appropriate fixing temperature, and falls outside the temperature range in which fixing is possible.

[0059] In other words, in this embodiment, in order to maintain the temperature of the paper passage area A21 at a temperature close to the first set temperature PT1 during the image formation processing period, i.e., within the temperature range where fixing is possible, the second set temperature PT2 is set to the above-mentioned predetermined temperature which is higher than the second target temperature T2' of the above-mentioned comparative example (a temperature which is set without taking into account the above-mentioned temperature decrease in the paper passage area A21, and only taking into account that the second detected temperature D2 by the second temperature sensor 232 is the second target temperature T2).

[0060] Returning to the explanation with reference to the flowchart in Fig. 6, if the setting unit 101 determines in S2 that the width W of the recording paper P is equal to or larger than the large size SZ (YES in S2), it sets the second target temperature T2 to the first set temperature PT1 (S13) and sets the operation start temperature TS of the cooling fan 151 to the first set temperature PT1 (S14).

[0061] Next, the control unit 100 controls the drive of the main heater 211 so that the first detected temperature D1 by the first temperature sensor 231 becomes the first target temperature T1 (= first set temperature PT1) (S5), and controls the drive of the first sub-heater 212 and the second sub-heater 213 so that the second detected temperature D2 by the second temperature sensor 232 becomes the second target temperature T2 (= first set temperature PT1) (S6).

[0062] The control unit 100 determines whether the second detected temperature D2 by the second temperature sensor 232 is equal to or higher than the operation start temperature TS (here, the first set temperature PT1) (S7), and if the second detected temperature D2 is equal to or higher than the operation start temperature TS (YES in S7), it keeps the cooling fan 151 running (S8), and if the second detected temperature D2 is lower than the operation start temperature TS (NO in S7), it stops the cooling fan 151 (S10). After this, the process proceeds to S9.

[0063] Figure 8 is a graph showing an example of the temperature change of the heater 21 when the widthwise size W of the recording paper P is equal to or larger than the large size SZ (as shown in the example of Figure 5, the recording paper P approaches the temperature detection position TP in the widthwise direction).

[0064] The solid line (thick line) in the graph indicates the change in the first detected temperature D1 (temperature of the main heater 211) detected by the first temperature sensor 231, and the temperature of the main heater 211 is maintained at the first set temperature PT1 (first target temperature T1) during the image formation processing period, and is maintained within a temperature range where fixing is possible.

[0065] The solid line (thin line) shows the change in the second detected temperature D2 (temperature of the paper passage area A21 of the first sub-heater 212) by the second temperature sensor 232, and the temperature of the paper passage area A21 of the first sub-heater 212 is maintained at the first set temperature PT1 (second target temperature T2) during the image formation processing period, and is maintained within a temperature range where fixing is possible.

[0066] Since the second sub-heater 213 has the same environmental conditions as the first sub-heater 212, the temperature of the paper passing area A21 of the second sub-heater 213 is also maintained at the first set temperature PT1, and is maintained within a temperature range where fixing is possible. In the non-paper passing area A1 of the first sub-heater 212 and the second sub-heater 213, excessive temperature rise is suppressed by the wind from the cooling fan 151.

[0067] 6, when the control unit 100 determines that the image forming process is complete (YES in S9), the process ends. Also, if the cooling fan 151 is being driven at this point, the cooling fan 151 is stopped (S15). After this, the process ends.

[0068] According to the above embodiment, the cooling fan 151 prevents excessive temperature rise in the fixing device 13, and when the width W of the recording sheet P is less than the large size SZ and the width size of the recording sheet P approaches the position of the first sub-heater 212 in the width direction, the temperatures of the first sub-heater 212 and the second sub-heater 213 in the paper passing area A21 can be maintained at an appropriate fixing temperature. As a result, according to this embodiment, in the fixing device 13 having the main heater 211, first sub-heater 212, and second sub-heater 213 arranged side by side in the width direction, heating to an appropriate fixing temperature by the first sub-heater 212 and the second sub-heater 213 can be performed accurately.

[0069] In another embodiment, the cooling mechanism 15 may be configured such that the airflow from the cooling fan 151 increases in strength toward the outside in the width direction, i.e., the direction of the first rotation axis RG1 of the heater 21. For example, the airflow can be changed by devising a shape of a duct that directs the airflow from the cooling fan 151 toward the heater 21 and adjusting the distance from the duct outlet to the heater 21. This increases the airflow from the cooling fan 151 toward the non-paper passing area A1, where a temperature reduction is required, and decreases the airflow from the cooling fan 151 toward the paper passing area A21, where a temperature reduction is not required.

[0070] The present invention is not limited to the configuration of the above embodiment, and various modifications are possible. In addition, in the above embodiment, the configuration and processing shown in the above embodiment using Figures 1 to 8 are merely one embodiment of the present invention, and it is not intended that the present invention be limited to these configurations and processing. [Explanation of symbols]

[0071] 1. Image forming device 13 Fixing device 15 Cooling mechanism 20 Fixing belt 21 Heater 23 Temperature Sensor 30 Pressure member 100 control section 101 Setting section 151 Cooling fan 211 Main heater 212 1st sub-heater 213 Second sub-heater 231 First temperature sensor 232 Second temperature sensor

Claims

1. a cylindrical rotatable fixing belt; a pressure member disposed opposite the fixing belt and forming a nip portion between the fixing belt and the pressure member, through which a recording medium to be fixed is inserted; a fixing device having a heater extending in a width direction perpendicular to a rotation direction of the fixing belt and for heating the fixing belt, the heater is divided in the width direction and includes a main heater disposed in the center of the fixing belt in the width direction, and two sub-heaters disposed at both ends of the fixing belt in the width direction, a first temperature sensor that detects the temperature of the main heater; a second temperature sensor that detects the temperature of the sub-heater; a cooling mechanism having a cooling fan and configured to cool both ends of the heater in the width direction; a control unit that controls driving of the cooling fan, controls driving of the main heater so that a first temperature detected by the first temperature sensor when the cooling fan is driven becomes a predetermined first target temperature that is an appropriate fixing temperature, and controls driving of the sub-heater so that a second temperature detected by the second temperature sensor when the cooling fan is driven becomes a predetermined second target temperature; a setting unit that sets the second target temperature to a predetermined first set temperature when the recording medium transported by the fixing belt is large enough to approach the temperature detection position of the second temperature sensor in the width direction perpendicular to the transport direction, and sets the second target temperature to a predetermined second set temperature higher than the first set temperature when the size of the recording medium in the width direction is a specific size that approaches the position of the sub-heater in the width direction but does not approach the temperature detection position of the second temperature sensor in the width direction, The second set temperature is a predetermined temperature at which the temperature of a paper passage area of ​​the recording medium approaching the position of the sub-heater in the width direction is maintained at an appropriate fixing temperature.

2. The image forming apparatus of claim 1, wherein the control unit controls the driving of the cooling fan to operate the cooling fan when the second detected temperature reaches a predetermined third set temperature that is equal to or higher than the second set temperature, when the widthwise size of the recording medium is the specific size.

3. 3. The image forming apparatus according to claim 1, wherein the cooling mechanism is configured such that the airflow from the cooling fan becomes stronger toward the outer side in the width direction of the heater.

Images