Image forming apparatus

Abstract

To prevent an excessive increase in the temperature of part of a plurality of heater blocks in a fixing device due to an error in size information of a sheet.SOLUTION: A heater 53 has a main heater block 61 arranged in a reference area corresponding to a sheet with a reference size, and sub heater blocks 62 arranged in outside areas. An outside sheet sensor 72a detects the sheet at a position in the outside area between a transfer device 44 and a fixing device 5. A control unit 8 controls a power supply state to be one of a first power supply state and a second power supply state according to target size information representing the size of a target sheet. When the target size information represents a size exceeding the reference size, the control unit 8 controls the conveyance interval between the target sheet and a subsequent sheet subsequent to the target sheet to be one of a first interval and a second interval longer than the first interval, according to whether the outside sheet sensor 72a detects the target sheet or not.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus equipped with a fixing device including a plurality of heater blocks whose power supply state can be switched according to the size of a sheet. [Background technology]

[0002] An electrophotographic image forming apparatus includes a transfer device and a fixing device. The transfer device transfers a toner image onto a sheet being conveyed. The fixing device fixes the toner image onto the sheet by applying heat and pressure to the toner image on the sheet.

[0003] It is also known that the heater of the fixing device includes a plurality of electrodes that are divided in the width direction perpendicular to the sheet conveying direction (see, for example, Patent Document 1). As a result, the heater is divided into a plurality of heater blocks whose power supply state is switched according to the size of the sheet. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-299205 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the image forming apparatus, the control device acquires size information that indicates the size of the sheet being conveyed, and switches the power supply state to the plurality of heater blocks according to the size indicated by the size information.

[0006] If the size information is erroneous information indicating a size larger than the actual size of the sheet, a situation will arise in which the amount of heat dissipated to the sheet in some of the multiple heater blocks is smaller than the expected amount of heat.

[0007] In addition, the sheet may be conveyed in a state shifted in the width direction from the expected position, and in this case, a situation occurs in which the amount of heat radiated to the sheet is smaller than the expected amount of heat in some of the heater blocks.

[0008] When a continuous printing process is being performed to form images sequentially on multiple sheets, if the size information is incorrect or if the multiple sheets are transported with a widthwise shift, there is a risk that some of the multiple heater blocks will rise to a temperature higher than the expected temperature.

[0009] The object of the present invention is to provide an image forming apparatus that can prevent the temperature of some of the multiple heater blocks in a fixing device from rising excessively due to incorrect sheet size information or a shift in the sheet transport position when continuous printing processing is being performed. [Means for solving the problem]

[0010] According to one aspect of the present invention, an image forming apparatus includes a sheet conveying device, a transfer device, a fixing device, a sheet detection unit, and a control device. The sheet conveying device is capable of conveying a plurality of sheets in sequence. The transfer device transfers a toner image onto each of the plurality of sheets being conveyed. The fixing device has a heater for heating the toner image on each of the plurality of sheets being conveyed in sequence from the transfer device, and a pressure unit for pressing each of the plurality of sheets toward the heater. The sheet detection unit is disposed upstream of the fixing device in a sheet conveying direction, and detects each of the plurality of sheets being conveyed. The control device controls power supply to the heater and the sheet conveying device. The heater has a main heater block disposed in a reference area corresponding to the sheet of a reference size in a main direction intersecting the sheet conveying direction, and a sub-heater block disposed in an outer area adjacent to the reference area in the main direction and corresponding to the sheet of a size larger than the reference size. The sheet detection unit includes an outer sheet sensor for detecting the sheet at a position in the outer area in the main direction between the transfer device and the fixing device. The control device acquires target size information representing the size of a target sheet that will arrive at the fixing device. The control device further controls a power supply state for the heater to one of a first power supply state in which power is supplied only to the main heater block and a second power supply state in which power is supplied to the main heater block and the sub-heater block, depending on whether the target size information represents a size within the reference size or a size exceeding the reference size. The control device further controls a conveying interval between the target sheet and a next sheet following the target sheet to one of a first interval and a second interval longer than the first interval, depending on whether the outer sheet sensor detects the target sheet when the target size information represents a size exceeding the reference size. Effect of the Invention

[0011] According to the present invention, it is possible to provide an image forming apparatus that can prevent the temperature of some of the multiple heater blocks in a fixing device from rising excessively due to incorrect sheet size information or a shift in the sheet transport position when continuous printing processing is being performed. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing the configuration of an image forming apparatus according to an embodiment. [Diagram 2] FIG. 2 is a configuration diagram of the fixing device in the image forming apparatus according to the embodiment. [Diagram 3] FIG. 3 is a block diagram showing the configuration of a control device in the image forming apparatus according to the embodiment. [Figure 4] FIG. 4 is a configuration diagram of a heater and a sheet detection unit in the image forming apparatus according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the following embodiment is an example of the present invention, and is not intended to limit the technical scope of the present invention.

[0014] [Configuration of image forming apparatus 10] As shown in FIG. 1, an image forming apparatus 10 includes a main body 1, a sheet conveying device 3, a printing device 4, and a control device 8. The main body 1 includes a sheet conveying device 3, a printing device 4, and a control device 8.

[0015] The printing device 4 executes a printing process for forming an image on a sheet 9. The printing device 4 executes the printing process by an electrophotographic method. The sheet 9 is an image forming medium such as paper or a sheet-like resin member.

[0016] The sheet transport device 3 includes a sheet sending device 30 and a plurality of transport roller pairs 31. The sheet sending device 30 sends out the sheet 9 stored in the sheet storage unit 2 to a transport path 300 in the main body 1. The transport path 300 forms a passage along which the sheet 9 is transported.

[0017] The plurality of transport roller pairs 31 are rotationally driven by a motor (not shown). The plurality of transport roller pairs 31 rotate to transport the sheet 9 along the transport path 300, and further discharge the sheet 9 onto the discharge tray 101.

[0018] The sheet 9 passes through a transfer position P1 and a fixing position P2 on the conveying path 300 and is discharged onto a discharge tray 101.

[0019] In the following description, the sheet conveying direction along the conveying path 300 is referred to as a conveying direction D01. The fixing position P2 is a position downstream of the transfer position P1 in the conveying direction D01.

[0020] Further, a direction intersecting the transport direction D01 in the transport path 300 is referred to as a main direction D1. In this embodiment, the main direction D1 is a direction perpendicular to the transport direction D01. The main direction D1 is the main scanning direction in the printing process.

[0021] The printing device 4 forms a toner image on the sheet 9 transported along the transport path 300. The toner image is a developer image using toner as a developer. The toner is an example of the granular developer.

[0022] The printing device 4 includes one or more image forming units 4x, an optical scanning device 40, a transfer device 44, and a fixing device 5. The image forming unit 4x includes a drum-shaped photoconductor 41, a charging device 42, a developing device 43, a drum cleaning device 45, and the like.

[0023] 1 is a tandem color image forming apparatus, and the printing device 4 includes four image forming units 4x corresponding to the toners of the four colors, yellow, cyan, magenta, and black.

[0024] In the image forming unit 4x, the photoconductor 41 rotates, and the charging device 42 charges the surface of the photoconductor 41. Furthermore, the optical scanning device 40 writes an electrostatic latent image on the surface of the photoconductor 41 by scanning with a laser beam.

[0025] Furthermore, the developing device 43 develops the electrostatic latent image into the toner image by supplying the toner to the surface of the photoconductor 41. The photoconductor 41 is an example of an image carrier that rotates while carrying the toner image.

[0026] The transfer device 44 transfers the toner image onto the sheet 9 at a transfer position P1 of the transport path 300. The transfer device 44 includes an intermediate transfer belt 441, four primary transfer devices 442 corresponding to the four image forming units 4x, a secondary transfer device 443, and a belt cleaning device 444.

[0027] In the transfer device 44, the primary transfer device 442 transfers the toner image on the surface of the photoconductor 41 onto the surface of the intermediate transfer belt 441. As a result, a color toner image is formed on the surface of the intermediate transfer belt 441.

[0028] The secondary transfer device 443 transfers the toner image formed on the intermediate transfer belt 441 onto the sheet 9 in the transport path 300 .

[0029] When the image forming apparatus 10 is a monochrome image forming apparatus, the secondary transfer device 443 transfers the toner image on the photoconductor 41 onto the sheet 9 in the transport path 300 .

[0030] The drum cleaning device 45 removes the waste toner remaining on the surface of the photoconductor 41. The belt cleaning device 444 removes the waste toner remaining on the intermediate transfer belt 441.

[0031] [Fixing device 5] The fixing device 5 applies heat and pressure to the toner image on the sheet 9 while transporting the sheet 9 at a fixing position P2 on the transport path 300. In this way, the fixing device 5 fixes the toner image on the sheet 9 to the sheet 9.

[0032] 2, the fixing device 5 includes a pressure roller 50, a fixing member 51, a support member 52, a heater 53, a biasing mechanism 54, and a temperature detection unit 55. The pressure roller 50, the fixing member 51, the support member 52, and the heater 53 are each disposed along the main direction D1 at a fixing position P2.

[0033] The fixing member 51 is a flexible cylindrical member. In other words, the fixing member 51 is an endless belt-like flexible cylindrical body. For example, the fixing member 51 is a cylindrical film member. The fixing member 51 is rotatably supported by the support member 52.

[0034] The pressure roller 50 is pressed against the fixing member 51 to form a nip Np1 between the pressure roller 50 and the fixing member 51 (see FIG. 2).

[0035] The pressure roller 50 presses the sheet 9 passing through the fixing position P2 against the fixing member 51. As a result, the sheet 9 is pressed toward the heater 53. The pressure roller 50 is an example of a pressure unit that presses the sheet 9 toward the heater 53.

[0036] The supporting member 52 rotatably supports the fixing member 51. The supporting member 52 further supports a heater 53. The supporting member 52 has a facing portion 52a that faces the pressure roller 50 with the fixing member 51 interposed therebetween. The facing portion 52a contacts the inner surface of the fixing member 51.

[0037] The heater 53 is incorporated in the facing portion 52a. As a result, the heater 53 is supported by the support member 52 in a state along the main direction D1. The pressure roller 50, the fixing member 51, and the support member 52 are formed to extend in the main direction D1. The heater 53 is in contact with the inner surface of the fixing member 51.

[0038] The urging mechanism 54 includes a pressing member 541 and a spring 542. The spring 542 elastically urges the facing portion 52a toward the pressure roller 50 via the pressing member 541. That is, the urging mechanism 54 elastically urges the fixing member 51 toward the pressure roller 50 via the support member 52.

[0039] The pressure roller 50 is rotated by being driven by a motor (not shown). The pressure roller 50 rotates the fixing member 51. As the fixing member 51 rotates, the inner surface of the fixing member 51 slides against the heater 53 and the opposing portion 52a. A lubricant is applied to the inner surface of the fixing member 51.

[0040] The heater 53 heats the toner image on the sheet 9 conveyed from the transfer device 44. Specifically, the heater 53 heats the sheet 9 through a portion of the fixing member 51 that forms the nip Np1.

[0041] The temperature detection unit 55 detects the temperature of the heater 53. The temperature detected by the temperature detection unit 55 is used for fixing temperature control. The fixing temperature control is a feedback control that controls the power supplied to the heater 53 by comparing the temperature detected by the temperature detection unit 55 with a preset target temperature.

[0042] [Control device 8] The control device 8 executes various data processing and controls devices such as the sheet conveying device 3 and the printing device 4. The objects of control of the control device 8 include the conveyance of the sheet 9 by the sheet conveying device 3 and the power supply to the heater 53 of the fixing device 5.

[0043] 3, the control device 8 includes a central processing unit (CPU) 81 and peripheral devices. The peripheral devices include a random access memory (RAM) 82, a secondary storage device 83, a signal interface 84, and the like.

[0044] Furthermore, the control device 8 includes a communication device 85 and a power supply circuit 86 .

[0045] The CPU 81 is a processor that executes various types of data processing and control by executing computer programs.

[0046] The RAM 82 is a computer-readable volatile storage device that temporarily stores the computer programs executed by the CPU 81 and data output and referenced in the course of the CPU 81 executing various processes.

[0047] The CPU 81 includes a plurality of processing modules that are realized by executing the computer programs, including a main control unit 8a, a heater control unit 8b, and a print control unit 8c.

[0048] The main control unit 8a executes control such as start control for starting various processes in response to an operation on an operating device (not shown).

[0049] The heater control section 8b controls the amount of power supplied to the heater 53 by controlling the fixing temperature. The heater control section 8b adjusts the amount of power supplied to the heater 53 by controlling the power supply circuit 86.

[0050] The power supply circuit 86 supplies power to the heater 53 in accordance with a power supply command from the heater control unit 8b.

[0051] The print control unit 8c controls the sheet transporting device 3. Furthermore, in synchronization with the transport of the sheet 9 by the sheet transporting device 3, the print control unit 8c causes the printing device 4 to execute the print process.

[0052] The control device 8 controls the power supply to the heater 53 and the sheet conveying device 3 through the action of the heater control section 8b and the print control section 8c, and further controls the image forming section 4x and the transfer device .

[0053] The secondary storage device 83 is a computer-readable non-volatile storage device. For example, a flash memory or a hard disk drive or both may be used as the secondary storage device 83.

[0054] The signal interface 84 converts detection signals output by various sensors into digital data, and transmits the digital data to the CPU 81. Furthermore, the signal interface 84 converts control commands output by the CPU 81 into control signals, and transmits the control signals to the devices to be controlled.

[0055] The communication device 85 executes communication with other devices, such as a host device that transmits a print job to the image forming apparatus 10. The CPU 81 communicates with the other devices through the communication device 85.

[0056] In this embodiment, the heater 53 is a planar heater having a plurality of resistors 6 (see FIG. 4).

[0057] The heater 53 includes a substrate 6x, a plurality of heater blocks 60, a plurality of power supply electrodes 600, and a ground electrode 610 (see FIG. 4).

[0058] The substrate 6x is a non-conductive film. A plurality of heater blocks 60, a plurality of power supply electrodes 600 and a ground electrode 610 are formed on the substrate 6x.

[0059] Each of the heater blocks 60 has a plurality of resistors 6 arranged in the main direction D1. Each of the resistors 6 is a heating element that generates heat when power is supplied thereto.

[0060] In each drawing, the sub-direction D2 is a direction along the conveying direction D01. The sub-direction D2 is a direction intersecting the main direction D1. The main direction D1 is the longitudinal direction of the heater 53. The sub-direction D2 is the lateral direction of the heater 53.

[0061] The multiple power supply electrodes 600 are connected to first ends of the multiple heater blocks 60 in the secondary direction D2. The ground electrode 610 is connected to second ends of the multiple heater blocks 60 in the secondary direction D2.

[0062] In this embodiment, the sheet conveying device 3 conveys the sheet 9 with the center of the sheet 9 in the main direction D1 as a reference.

[0063] The heater 53 is divided into a main heater block 61 and a pair of sub-heater blocks 62 (see FIG. 4). The main heater block 61 is disposed in a reference region in the main direction D1. The pair of sub-heater blocks 62 are disposed in a pair of outer regions adjacent to the reference region in the main direction D1, respectively.

[0064] The reference area is an area corresponding to a first size sheet 9a, which is a sheet 9 of a predetermined reference size. The pair of outer areas are areas corresponding to a second size sheet 9b, which is a sheet 9 of a size larger than the reference size.

[0065] In Fig. 4, various sheets 9 including a first size sheet 9a and a second size sheet 9b are shown by imaginary lines (two-dot chain lines). In Fig. 4, a center line L1 is a straight line that passes through the center of the reference area in the main direction D1 and is aligned along the conveying direction D01.

[0066] The first size sheet 9a passes through the reference area at the fixing position P2, and the second size sheet 9b passes through the reference area and the pair of outer areas at the fixing position P2.

[0067] The multiple power supply electrodes 600 include a main electrode 601 connected to the first end of the main heater block 61 and a pair of sub-electrodes 602 connected to the first ends of a pair of sub-heater blocks 62 (see FIG. 4).

[0068] The power supply circuit 86 can supply power individually to the main heater block 61 and the sub-heater block 62 through one or both of the main electrode 601 and the pair of sub-electrodes 602 .

[0069] Incidentally, in the image forming apparatus 10, the control device acquires information indicating the size of the sheet 9 being conveyed, and switches the power supply state to the multiple heater blocks 60 according to the size indicated by the acquired information.

[0070] In this embodiment, sheet size information indicating the size of the sheet storage unit 2 and the sheet 9 stored in the sheet storage unit 2 is stored in advance in the secondary storage device 83. In addition, the print job includes supply source information specifying the sheet storage unit 2 that is the supply source of the target sheet 9x.

[0071] The sheet size information corresponding to the supplier information included in the print job is target size information that indicates the size of the target sheet 9x.

[0072] When the print process is performed, the heater control unit 8b acquires from the secondary storage device 83 the target size information corresponding to the target sheet 9x.

[0073] Furthermore, the heater control unit 8b controls the power supply state to the heater 53 to one of a first power supply state and a second power supply state depending on whether the target size information represents a size within the reference size or a size exceeding the reference size.

[0074] The first power supply state is a state in which power is supplied only to the main heater block 61. The second power supply state is a state in which power is supplied to the main heater block 61 and the pair of sub-heater blocks 62.

[0075] If the target size information is erroneous information indicating a size larger than the actual size of the target sheet 9x, a situation will arise in which the amount of heat dissipated to the target sheet 9x in the pair of sub-heater blocks 62 is smaller than the expected amount of heat.

[0076] In addition, there may be cases where the target sheet 9x is conveyed in a state shifted from the expected position in the main direction D1. In this case, a situation occurs in which the amount of heat dissipated to the target sheet 9x by one of the pair of sub-heater blocks 62 is smaller than the expected amount of heat.

[0077] There are cases where the image forming apparatus 10 performs a continuous printing process. When the continuous printing process is performed, the sheet conveying device 3 sequentially conveys a plurality of sheets 9. The transfer device 44 transfers the toner image onto each of the plurality of sheets 9 being conveyed.

[0078] Furthermore, in the fixing device 5 , the heater 53 heats the toner image on each of the plurality of sheets 9 conveyed in sequence from the transfer device 44 , and the pressure roller 50 presses each of the plurality of sheets 9 toward the heater 53 .

[0079] When the continuous printing process is being performed, if the target size information is erroneous, or if the multiple sheets 9 are transported in a misaligned state in the main direction D1, there is a risk that one or both of the pair of sub-heater blocks 62 will rise above the expected temperature.

[0080] That is, when the continuous printing process is being performed, there is a risk that the temperature of some of the heater blocks 60 will rise too high due to an error in the target size information or a deviation in the transport position of the target sheet 9x.

[0081] The image forming apparatus 10 has a configuration for preventing the temperature of some of the heater blocks 60 from rising too high when the continuous printing process is being performed. The configuration will be described below.

[0082] In the fixing device 5, the temperature detection unit 55 includes a main temperature sensor 55a and a sub-temperature sensor 55b (see FIG. 4). For example, each of the main temperature sensor 55a and the sub-temperature sensor 55b is a thermistor.

[0083] The main temperature sensor 55a detects the temperature of the main heater block 61. The sub-temperature sensor 55b detects the temperature of one of the pair of sub-heater blocks 62.

[0084] The pair of sub-heater blocks 62 includes a first sub-heater block 62a and a second sub-heater block 62b (see FIG. 4). The first sub-heater block 62a is disposed adjacent to the main heater block 61 on a first side SD1 in the main direction D1. The second sub-heater block 62b is disposed adjacent to the main heater block 61 on a second side SD2 in the main direction D1.

[0085] The pair of outer regions includes a first outer region adjacent to a first side SD1 in the main direction D1 with respect to the reference region, and a second outer region adjacent to a second side SD2 in the main direction D1 with respect to the reference region.

[0086] The first sub-heater block 62a is disposed in the first outer region in the main direction D1, and the second sub-heater block 62b is disposed in the second outer region in the main direction D1. The sub-temperature sensor 55b detects the temperature of the first sub-heater block 62a.

[0087] In this embodiment, in the first power supply state, the heater control unit 8b controls the amount of power supplied to the main heater block 61 by the feedback control of the temperature detected by the main temperature sensor 55a.

[0088] Similarly, in the second power supply state, the heater control section 8b controls the amount of power supplied to the main heater block 61 and the pair of sub-heater blocks 62 by the feedback control of the temperature detected by the main temperature sensor 55a.

[0089] Furthermore, the heater control unit 8b stops the power supply to the main heater block 61 when the temperature detected by the main temperature sensor 55a exceeds a preset upper limit temperature. Similarly, the heater control unit 8b stops the power supply to the sub-heater block 62 when the temperature detected by the sub-temperature sensor 55b exceeds the upper limit temperature.

[0090] The heater control unit 8b may stop supplying power to the main heater block 61 and the sub-heater block 62 when the temperature detected by at least one of the main temperature sensor 55a and the sub-temperature sensor 55b exceeds the upper limit temperature.

[0091] The fixing device 5 further includes two thermal protectors 56 (see FIG. 4). Each of the thermal protectors 56 is a bimetal thermostat.

[0092] The two thermal protectors 56 include a first thermal protector 56a and a second thermal protector 56b (see FIG. 4). The first thermal protector 56a is disposed in a main heater block 61 of the heater 53. The second thermal protector 56b is disposed in a second sub-heater block 62b of the heater 53.

[0093] The first thermal protector 56a electrically cuts off the power supply line to the main heater block 61 when the main heater block 61 exceeds an allowable temperature. The second thermal protector 56b electrically cuts off the power supply line to the pair of sub-heater blocks 62 when the second sub-heater block 62b exceeds the allowable temperature.

[0094] In the example shown in FIG. 3, the control device 8 further includes a first interruption circuit 87a electrically connected to the first thermal protector 56a, and a second interruption circuit 87b electrically connected to the second thermal protector 56b.

[0095] The first thermal protector 56a is activated when the temperature exceeds the allowable temperature, and the first cutoff circuit 87a electrically cuts off the power supply line to the main heater block 61 as a result of the first thermal protector 56a being activated.

[0096] The second thermal protector 56b is activated when the allowable temperature is exceeded, and the second cutoff circuit 87b electrically cuts off the power supply line to the pair of sub-heater blocks 62 as a result of the second thermal protector 56b being activated.

[0097] The second thermal protector 56b is disposed at a position that is line-symmetrical with respect to the position of the sub-temperature sensor 55b with respect to the center line L1 of the second sub-heater block 62b.

[0098] In the above case, when the target sheet 9x is conveyed in a state shifted to the first side SD1 in the main direction D1, the target sheet 9x is likely to deviate from the position of the second thermal protector 56b. In this case, the second sub-heater block 62b is likely to reliably detect the state in which the temperature of the second sub-heater block 62b exceeds the allowable temperature.

[0099] On the other hand, when the target sheet 9x is conveyed in a state shifted to the second side SD2 in the main direction D1, the target sheet 9x is likely to deviate from the position of the sub-temperature sensor 55b. In this case, the state in which the first sub-heater block 62a exceeds the upper limit temperature is likely to be reliably detected by the sub-temperature sensor 55b.

[0100] Therefore, when the position of the target sheet 9x in the main direction D1 is shifted, the power supply to the pair of sub-heater blocks 62 is reliably cut off before the temperature of the first sub-heater block 62a or the second sub-heater block 62b increases excessively.

[0101] In this embodiment, the first thermal protector 56a is disposed at a position that is line-symmetrical with respect to the position of the main temperature sensor 55a with respect to the center line L1 of the main heater block 61 as a reference.

[0102] For example, the main temperature sensor 55a and the first thermal protector 56a are disposed at positions corresponding to both ends in the main direction D1 of a seat 9 of a minimum assumed size.

[0103] The image forming apparatus 10 further includes a sheet detection unit 7 (see FIGS. 1 and 4) disposed upstream of the fixing device 5 in the conveying direction D01. When the continuous printing process is performed, the sheet detection unit 7 detects each of the multiple sheets 9 being conveyed.

[0104] In this embodiment, the sheet detection unit 7 includes a main sheet sensor 71 and a pair of outer sheet sensors 72 (see FIG. 4).

[0105] For example, each of the main sheet sensor 71 and the pair of outer sheet sensors 72 includes a swinging member and a photosensor. The swinging member is supported so as to be swingable, and swings when it comes into contact with each of the conveyed sheets 9. The photosensor detects that the swinging member has swung.

[0106] The main sheet sensor 71 detects each sheet 9 at a position in the reference region in the main direction D1 between the transfer device 44 and the fixing device 5. Each of the pair of outer sheet sensors 72 detects each sheet 9 at a position in each of the pair of outer regions in the main direction D1 between the transfer device 44 and the fixing device 5.

[0107] The pair of outer sheet sensors 72 includes a first outer sheet sensor 72a and a second outer sheet sensor 72b (see FIG. 4). The first outer sheet sensor 72a detects the sheet 9 in the first outer region of the main direction D1. The second outer sheet sensor 72b detects the sheet 9 in the second outer region of the main direction D1.

[0108] When the continuous printing process is performed, the print control unit 8c executes the transport interval control if the target size information indicates a size exceeding the reference size.

[0109] When the target sheet 9x of the specific size is transported in its original position in the main direction D1, both ends of the target sheet 9x in the main direction D1 pass through the positions of the pair of outer sheet sensors 72.

[0110] The conveying interval control is a control for setting the conveying interval between the target sheet 9x and the next sheet 9y to one of a first interval and a second interval depending on whether or not the target sheet 9x is detected by the pair of outer sheet sensors 72. The second interval is longer than the first interval.

[0111] The next sheet 9y is the sheet 9 that is conveyed following the target sheet 9x in the continuous printing process (see FIG. 4).

[0112] In the conveying interval control, when both the first outer sheet sensor 72a and the second outer sheet sensor 72b detect the target sheet 9x, the print control unit 8c controls the conveying interval between the target sheet 9x and the next sheet 9y to the first interval.

[0113] In the conveying interval control, when at least one of the first outer sheet sensor 72a and the second outer sheet sensor 72b does not detect the target sheet 9x, the print control unit 8c controls the conveying interval between the target sheet 9x and the next sheet 9y to the second interval.

[0114] When the target sheet 9x is conveyed at a position shifted in the main direction D1, it is likely that one of the pair of outer sheet sensors 72 will not detect the target sheet 9x. Also, when the target size information is erroneous information indicating a size larger than the actual size of the target sheet 9x, it is likely that one or both of the pair of outer sheet sensors 72 will not detect the target sheet 9x.

[0115] On the other hand, by widening the transport interval between the target sheet 9x and the next sheet 9y, the amount of power supplied to the heater 53 from when the target sheet 9x passes through the fixing position P2 until when the next sheet 9y reaches the fixing position P2 is reduced.

[0116] By carrying out the transport interval control, it is possible to prevent the temperature of one or both of the pair of sub-heater blocks 62 from increasing too much due to a positional deviation of the target sheet 9x or an error in the target size information.

[0117] In this embodiment, the sub-temperature sensor 55b is disposed at a position overlapping with the position of the first outer sheet sensor 72a in the main direction D1. In the example shown in Fig. 4, the sub-temperature sensor 55b is disposed at the same position as the first outer sheet sensor 72a in the main direction D1. As a result, the detected temperature of the sub-temperature sensor 55b indicates a temperature that reflects the state of whether the target sheet 9x passes through the position of the first outer sheet sensor 72a or not.

[0118] In this embodiment, the second thermal protector 56b is disposed at a position overlapping with the position of the second outer sheet sensor 72b in the main direction D1. In the example shown in Fig. 4, the second thermal protector 56b is disposed at the same position as the position of the second outer sheet sensor 72b in the main direction D1. As a result, the second thermal protector 56b operates according to a temperature that reflects a state of whether the target sheet 9x passes through the position of the second outer sheet sensor 72b or not.

[0119] [First application example] Next, a description will be given of a first application example of the image forming apparatus 10. Below, the differences between this application example and the image forming apparatus 10 will be described.

[0120] In this application example, in the first power supply state, the heater control section 8b controls the amount of power supplied to the main heater block 61 by the feedback control of the temperature detected by the main temperature sensor 55a. This control is the same as the control in the image forming apparatus 10.

[0121] On the other hand, in the second power supply state, the heater control unit 8b controls the amount of power supplied to the main heater block 61 by the feedback control of the temperature detected by the main temperature sensor 55a, but performs different control on the amount of power supplied to the pair of sub-heater blocks 62.

[0122] In the second power supply state, the heater control section 8b in this application example controls the amount of power supplied to the pair of sub-heater blocks 62 by the feedback control of the temperature detected by the sub-temperature sensor 55b.

[0123] When this application example is adopted, the same effects as when the image forming apparatus 10 is adopted can be obtained.

[0124] [Second application example] Next, a description will be given of a second application example of the image forming apparatus 10. Below, the differences between this application example and the image forming apparatus 10 will be described.

[0125] In this application example, the sheet conveying device 3 conveys the sheet 9 based on one end of the main direction D1 of the sheet 9. Therefore, in this application example, the heater 53 is divided into one main heater block 61 and one sub-heater block 62.

[0126] In this application example, the sub-temperature sensor 55b and the second thermal protector 56b are disposed at different positions in one sub-heater block 62.

[0127] In this application example, the second outer sheet sensor 72b of the pair of outer sheet sensors 72 in the image forming apparatus 10 is not provided.

[0128] When this application example is adopted, the same effects as when the image forming apparatus 10 is adopted can be obtained. [Explanation of symbols]

[0129] 3: Sheet transport device 4: Printing device 4x: Image forming section 5: Fixing device 8: Control device 10: Image forming device 44: Transcription device 50: Pressure roller 53: Heater 55a: Primary temperature sensor 55b: Sub temperature sensor 56a: First thermal protector 56b: Secondary thermal protector 60: Heater block 61: Main heater block 62: Secondary heater block 62a: First auxiliary heater block 62b: Second sub-heater block 71: Main seat sensor 72: Outer seat sensor 72a: First outer seat sensor 72b: Second outer seat sensor

Claims

1. a sheet conveying device capable of sequentially conveying a plurality of sheets; a transfer device that transfers a toner image onto each of the plurality of sheets being conveyed; a fixing device having a heater for heating the toner image on each of the plurality of sheets conveyed from the transfer device and a pressure unit for pressing each of the plurality of sheets toward the heater; a sheet detection unit disposed upstream of the fixing device in a sheet conveying direction, the sheet detection unit detecting each of the plurality of sheets being conveyed; a control device for controlling power supply to the heater and the sheet conveying device, the heater includes a main heater block disposed in a reference area corresponding to the sheet of a reference size in a main direction intersecting the sheet conveying direction, and a sub-heater block disposed in an outer area adjacent to the reference area in the main direction and corresponding to the sheet of a size larger than the reference size, the sheet detection unit includes an outer sheet sensor that detects the sheet at a position in the outer region in the main direction between the transfer device and the fixing device, The control device acquires target size information representing a size of a target sheet that will arrive at the fixing device; Furthermore, the control device controls a power supply state for the heater to one of a first power supply state in which power is supplied only to the main heater block and a second power supply state in which power is supplied to the main heater block and the sub-heater block, depending on whether the target size information indicates a size within the reference size or a size exceeding the reference size, Furthermore, in an image forming apparatus, when the target size information represents a size larger than the reference size, the control device controls the conveying interval between the target sheet and the next sheet following the target sheet to one of a first interval and a second interval longer than the first interval, depending on whether the outer sheet sensor detects the target sheet or not.

2. a main temperature sensor for detecting a temperature of the main heater block; 2. The image forming apparatus according to claim 1, wherein the control device controls the amounts of power supplied to the main heater block and the sub-heater block by feedback control of the temperature detected by the main temperature sensor.

3. a main temperature sensor for detecting the temperature of the main heater block; a sub-temperature sensor for detecting the temperature of the sub-heater block; 2. The image forming apparatus according to claim 1, wherein the control device controls the amount of power supplied to the main heater block by feedback control of the temperature detected by the main temperature sensor, and controls the amount of power supplied to the sub-heater block by feedback control of the temperature detected by the sub-temperature sensor.

4. a main temperature sensor for detecting the temperature of the main heater block; a sub-temperature sensor for detecting the temperature of the sub-heater block; a first thermal protector disposed in the main heater block and electrically cutting off a power supply line to the main heater block when the main heater block exceeds an allowable temperature; a second thermal protector that is disposed in the sub-heater block and electrically cuts off a power supply line to the sub-heater block when the sub-heater block exceeds a tolerable temperature; 4. The image forming apparatus according to claim 1, wherein the control device stops power supply to the main heater block and / or the sub-heater block when the detected temperatures of the main temperature sensor and / or the sub-temperature sensor exceed a preset upper limit temperature.

5. the outer region includes a first outer region and a second outer region adjacent to either side of the reference region; the sub-heater blocks include a first sub-heater block disposed in the first outer region and a second sub-heater block disposed in the second outer region, The sub-temperature sensor detects the temperature of the first sub-heater block, The image forming apparatus according to claim 4 , wherein the second thermal protector is disposed in the second sub-heater block.

6. 6. The image forming apparatus according to claim 5, wherein the second thermal protector is disposed at a position that is line-symmetrical with respect to a position of the sub-temperature sensor with respect to a center line that passes through a center of the reference area in the main direction and is aligned along the sheet transport direction.

Images