Image forming apparatus

Abstract

Even if the recording medium is transported unevenly, a small number of temperature sensors can prevent excessive temperature rise in the heating element. [Solution] When the temperature difference between the temperature detected by the first temperature sensor 53 in the region R through which the smallest size paper Ps in the heating device 52 passes and the temperature detected by the second temperature sensor 54 between one end of the heating device 52 and the end of region R on the side of that end is smaller than the first threshold temperature difference (YES in S9), and the paper P passing between the other end of the heating device 52 and the end of region R on the side of that end is not detected by the paper detection sensor 55 (NO in S10), the control unit 100 reduces the number of sheets of paper P per unit time that are continuously transported to the nip section N (S12).

Description

Technical Field

[0001] The present invention relates to an image forming apparatus including a fixing unit that thermally presses and fixes a toner image formed on a recording medium.

Background Art

[0002] An electrophotographic image forming apparatus includes a fixing device that fixes a toner image transferred onto a recording medium. The fixing device generally includes a heating roller extending in a direction orthogonal to the conveyance direction of the recording medium by a conveyance unit, a pressure roller extending in the orthogonal direction, disposed opposite to the heating roller, and forming a nip portion through which the recording medium to be fixed is inserted between the heating roller, and a heating device extending from one end portion to the other end portion of the heating roller in the orthogonal direction and disposed inside the heating roller to heat the heating roller. In the nip portion, there is one that fixes the toner image on the recording medium by heating and pressing (thermal pressure bonding).

[0003] In an image forming apparatus including a fixing device, for example, when continuously printing on a small-sized recording medium at the same printing interval as a large-sized recording medium, the temperature of a region (non-paper-passing portion) of the heating roller through which the recording medium does not pass rises excessively. When printing on a large-sized recording medium in a state where the temperature of the non-paper-passing portion is excessively high, there occurs a problem that the toner melts too much in a portion of the large-sized recording medium corresponding to the non-paper-passing portion of the small-sized recording medium.

[0004] As a countermeasure against the above problem, throughput down (TPD) control is performed to reduce the productivity of the image forming process (the specified number of printed sheets per unit time for each paper type used), such as by extending the paper interval. For example, when the end temperature of the heating roller reaches a certain temperature or higher, throughput is decreased, so the number of sheets passing through per unit time decreases, and thus the amount of power input to the heating device per unit time decreases. Thereby, excessive temperature rise of the non-paper-passing portion of the heating roller can be prevented. As such a fixing device, for example, there is a fixing device disclosed in Patent Document 1.

[0005] For example, the fuser unit is equipped with three temperature sensors that detect the temperature of the heating element in a direction perpendicular to the direction in which the recording medium is transported to the nip. The central temperature sensor detects the temperature at the part of the heating device corresponding to the area through which all sizes of recording media pass on the heating roller, while the temperature sensors at both ends detect the temperature at the parts of the heating device corresponding to the area through which large-sized recording media pass but small-sized recording media do not, thereby controlling the temperature at three points on the heating device. Power is supplied to the heating device when the temperature detected by the central temperature sensor is maintained at a predetermined fixing temperature. When the temperature detected by the temperature sensors at both ends reaches the throughput reduction temperature, the PPM (Pages per Minute) is reduced, and if it subsequently falls below the throughput reduction temperature, the PPM is increased according to the amount of temperature decrease per predetermined time. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-106558 [Overview of the project] [Problems that the invention aims to solve]

[0007] In the fixing device described above, temperature sensors are provided at both ends, and control is performed to lower the PPM based on the temperature detected by the temperature sensors at both ends. However, it is desirable to be able to prevent excessive temperature rise in the heating device portion corresponding to the non-paper-feeding section of the heating roller with a small number of temperature sensors. In particular, when recording media such as small-sized recording media are transported to the nip section unevenly in a direction perpendicular to the direction in which the recording media is transported to the nip section, it is desirable to be able to prevent excessive temperature rise in the heating device portion corresponding to the non-paper-feeding section of the heating roller with a small number of temperature sensors.

[0008] The present invention has been made in view of the above circumstances, and aims to prevent excessive temperature rise in the heating device portion corresponding to the non-paper-feeding portion of the heating roller, even if the recording medium is transported to the nip portion unevenly in a direction perpendicular to the transport direction of the recording medium to the nip portion, using a small number of temperature sensors. [Means for solving the problem]

[0009] An image forming apparatus according to one aspect of the present invention is an image forming apparatus comprising a fixing unit, a control unit, and a transport unit for transporting a recording medium to the fixing unit, wherein the fixing unit comprises a heating roller extending in an orthogonal direction perpendicular to the transport direction of the recording medium by the transport unit, a pressure roller extending in the orthogonal direction and positioned opposite the heating roller, forming a nip portion through which the recording medium to be fixed is inserted between the heating roller and the heating roller, and a heating device extending from one end to the other end of the heating roller in the orthogonal direction and positioned inside the heating roller, and heating the heating roller, wherein the transport unit transports the recording medium of a predetermined minimum size to a predetermined region of the heating roller and the pressure roller, including the center of the heating roller and the pressure roller in the orthogonal direction, and the fixing unit further comprises a first temperature sensor that detects the temperature in the predetermined region of the heating device as a first temperature, and one end of the heating device in the orthogonal direction and the heating device The system includes a second temperature sensor that detects the temperature at a position from one end to the end of the predetermined region on the side of that one end as a second temperature, and a paper detection sensor positioned in the nip section from the other end of the heating device in the orthogonal direction to the end of the predetermined region on the side of that other end, which detects the presence or absence of the recording medium passing through the nip section. The control unit energizes the heating device to control the first temperature to a preset fixing temperature, starts the continuous transport of the recording medium to the nip section by the transport unit at a predetermined number of sheets per hour, and when the temperature difference between the first temperature and the second temperature is smaller than a predetermined first threshold temperature difference and the paper detection sensor determines that the recording medium has not been detected, it changes the continuous transport of the recording medium to the nip section by the transport unit at a predetermined number of sheets per hour, which is less than the first number, to a predetermined second number. [Effects of the Invention]

[0010] According to the present invention, a paper detection sensor is provided at a position in the nip section that extends from the other end of the heating device in the orthogonal direction to the other end of a predetermined region, and detects the presence or absence of the recording medium passing through the nip section. The control unit, when the temperature difference between the first temperature and the second temperature is smaller than a predetermined first threshold temperature difference and no recording medium is detected by the paper detection sensor, changes the predetermined number of sheets per unit of time for continuous transport of recording media to the nip section by the transport unit to a second number, which is less than the initial first number. As a result, even if recording media such as small-sized recording media are transported to the nip section with a bias toward the side with the second temperature sensor in the orthogonal direction, an excessive temperature rise in the part of the heating device corresponding to the non-paper-passing part of the heating roller can be prevented with a small number of temperature sensors. [Brief explanation of the drawing]

[0011] [Figure 1] This is a cross-sectional view showing the configuration of an image forming apparatus according to one embodiment of the present invention. [Figure 2] Figure 1 is a functional block diagram showing the electrical configuration of the image forming apparatus. [Figure 3] Figure 1 is a cross-sectional view showing the configuration of the fixing section. [Figure 4] Figure 3 is a schematic diagram showing the arrangement of the first temperature sensor, second temperature sensor, and paper detection sensor in the fixing unit. [Figure 5] Figure 3 is a schematic diagram showing the arrangement of the first temperature sensor, second temperature sensor, and paper detection sensor in the fixing unit. [Figure 6] Figure 2 is a flowchart showing the processing procedure of the productivity control process by the control unit. [Figure 7] Figure 2 is a flowchart showing the processing procedure of the productivity control process by the control unit. [Modes for carrying out the invention]

[0012] Hereinafter, an image forming apparatus according to one embodiment of the present invention will be described with reference to the drawings.

[0013] Figure 1 is a cross-sectional view showing the configuration of an image forming apparatus 1 according to one embodiment of the present invention. Figure 2 is a functional block diagram showing the electrical configuration of the image forming apparatus 1 of Figure 1. The image forming apparatus 1 is a multifunction device equipped with multiple functions such as a copy function, a transmission function, a printer function, and a facsimile function.

[0014] As shown in Figure 1, the image forming apparatus 1 comprises a document transport unit 6, a document reading unit 11, an image forming unit 12, a fixing unit 13, a paper feeding unit 14, a display unit 15, an operation unit 16, and a transport unit 17.

[0015] The document transport unit 6 transports each document placed on the document glass to the reading position of the image reading unit 11. The document transport unit 6 is configured to open and close the platen glass 7 by rotating vertically around the support axis at the back of the paper surface as shown in Figure 1. The document transport unit 6 also functions as a cover that holds down the document on the platen glass 7 from above.

[0016] The image reading unit 11 is a scanner that optically reads a document and generates image data showing the document image. The image reading unit 11 reads a document that is transported by the document transport unit 6 or a document that is placed on the platen glass 7.

[0017] The image forming unit 12 includes a photosensitive drum, a charging device, an exposure device, a developing device, and a transfer device. The exposure device includes an LSU (Laser Scanning Unit). The image forming unit 12 forms a toner image on the paper P conveyed along the conveyance path T by the conveyance unit 17 based on the image data. When performing color printing, the magenta image forming unit 12M, the cyan image forming unit 12C, the yellow image forming unit 12Y, and the black image forming unit 12Bk of the image forming unit 12 form magenta, cyan, yellow, and black toner images on the photosensitive drum 121 through the processes of charging, exposure, and development. The formed toner images of each color are transferred onto the intermediate transfer belt 125 by the primary transfer roller 126 and overlapped to form a color toner image. The overlapped color toner image is transferred onto the paper P conveyed in the conveyance path T by the secondary transfer roller 127. When performing monochrome printing, the black image forming unit 12Bk of the image forming unit 12 forms a black toner image on the photosensitive drum 121 through the processes of charging, exposure, and development. The formed black toner image is transferred onto the intermediate transfer belt 125 by the primary transfer roller 126. The monochrome toner image transferred onto the intermediate transfer belt 125 is transferred onto the paper P conveyed in the conveyance path T by the secondary transfer roller 127.

[0018] The fixing unit 13 heats and presses the paper P with the toner image formed thereon to fix the toner image on the paper P. The paper P with the toner image fixed thereon is discharged to the discharge tray 8. Note that the fixing unit 13 is an example of the fixing device in the claims.

[0019] The paper feeding unit 14 includes a manual feed tray, a plurality of paper feeding cassettes, a pickup roller, and a paper feeding motor. The paper feeding unit 14 feeds the paper P accommodated in any of the plurality of paper feeding cassettes or the paper placed on the manual feed tray one by one by the pickup roller rotated by the drive of the paper feeding motor, and feeds it to the conveyance path T. Note that the paper P is not limited to a paper medium, and may be, for example, an OHP (Overhead Projector) sheet. The paper P and the OHP sheet are an example and another example of the recording medium in the claims.

[0020] The display unit 15 includes a liquid crystal display or an organic EL (Organic Light-Emitting Diode) display. The display unit 15 displays various screens. The operation unit 16 includes a plurality of hard keys such as a start key for inputting an execution start instruction for copy processing or scan processing, etc., and a touch panel disposed over the display unit 15. An instruction from the user is input to the operation unit 16.

[0021] The conveyance unit 17 conveys the paper P to the image forming unit 12 and the fixing unit 13, and includes a pair of conveyance rollers 17A, a pair of discharge rollers 17B, a registration roller 17C, etc., and a conveyance motor. When the pair of conveyance rollers 17A, the pair of discharge rollers 17B, the registration roller 17C, etc. rotate by the drive of the conveyance motor, the paper P fed by the paper feeding unit 14 is conveyed along the conveyance path T to the image forming unit 12, the fixing unit 13, etc.

[0022] As shown in FIG. 2, the image forming apparatus 1 further includes a control unit 10, a storage unit 18, an image processing unit 19, an image memory 20, a facsimile communication unit 21, and a communication unit 22. <000010​​​​ The control unit 10 controls the operation of each part of the image forming apparatus 1.

[0025] The storage unit 18 is a large-capacity storage device for storing various types of data, such as an HDD (Hard Disk Drive) or SSD (Solid State Drive), and stores various computer programs, such as various control programs, for realizing the operation of the image forming apparatus 1. In this embodiment, the storage unit 18 has a productivity control program installed on it for the control unit 10 to execute the productivity control processing shown in Figures 6 and 7.

[0026] The image processing unit 19 performs image processing (rasterization, color conversion, rotation / aggregation, resolution conversion, screen processing, etc.) on the image data. The image memory 20 temporarily stores the image data. The facsimile communication unit 21 transmits and receives image data via a public network. The communication unit 22 includes a communication module such as a LAN (Local Area Network) board. The control unit 10 communicates data with external devices such as a host PC (Personal Computer) 23 connected to the network via the communication unit 22.

[0027] The control unit 10 includes a processor, RAM (Random Access Memory), and ROM (Read Only Memory), etc. The processor is, for example, a CPU (Central Processing Unit), an MPU (Micro-Processing Unit), or an ASIC (Application Specific Integrated Circuit).

[0028] The control unit 10 functions as a control unit 100 that controls the operation of the document transport unit 6, document reading unit 11, image forming unit 12, fixing unit 13, paper feeding unit 14, display unit 15, operation unit 16, and transport unit 17, etc., by having the processor execute a control program, including a productivity control program, etc., stored in the ROM or storage unit 18. However, the control unit 100 can also be configured by hardware circuits, rather than by operation according to a control program such as a productivity control program by the control unit 10. The same applies to each embodiment below unless otherwise specified.

[0029] When the image forming apparatus 1 performs a document reading operation, under the control of the control unit 100, the document reading unit 11 optically reads the image of the document being transported by the document transport unit 6 or the document placed on the platen glass 7, and generates image data. The image data generated by the document reading unit 11 is stored in the image memory 20.

[0030] When the image forming apparatus 1 performs an image forming operation, under the control of the control unit 100, the paper feed motor of the paper feed unit 14 drives the pickup roller, which pulls out one sheet of paper P at a time. The transport motor of the transport unit 17, also controlled by the control unit 100, drives the transport roller pair 17A, the discharge roller pair 17B, and the registration roller 17C, etc., which rotate the transport roller pair 17A, the discharge roller pair 17B, and the registration roller 17C, etc., causing the paper P pulled out by the paper feed unit 14 to be transported along the transport path T to the image forming unit 12, then further transported along the transport path T to the fuser unit 13, and finally transported along the transport path T to be discharged into the discharge tray 8. Under the control of the control unit 100, the image processing unit 19 performs image processing on the image data generated by the above document reading operation and stored in the image memory 20, the image data stored in the storage unit 18, or the image data received from a network-connected computer, etc. Based on the image data processed by the image processing unit 19, the image forming unit 12 forms a toner image on the paper P that is transported to the image forming unit 12 along the transport path T. Under the control of the control unit 100, the fixing unit 13 heats and pressurizes the paper P on which the toner image has been formed, which is transported to the fixing unit 13 along the transport path T, to fix the toner image to the paper P. The paper P, on which the fixing process is complete, is discharged to the discharge tray 8 along the transport path T.

[0031] Figure 3 is a cross-sectional view showing the configuration of the fuser unit 13 in Figures 1 and 2. Figure 4 is a schematic diagram showing the arrangement of the first temperature sensor 53, the second temperature sensor 54, and the paper detection sensor 55 in the fuser unit 13 of Figure 3. Figure 5 is a schematic diagram showing the arrangement of the first temperature sensor 53, the second temperature sensor 54, and the paper detection sensor 55 in the fuser unit 13 of Figure 3.

[0032] In Figures 4 and 5, the paper Ps is the smallest size of paper P in the orthogonal direction perpendicular to the direction in which paper P is transported to the nip section N (the direction indicated by arrow L in Figures 4 and 5) (this orthogonal direction is the direction parallel to the surface of paper P among the orthogonal directions perpendicular to the transport direction; the direction indicated by arrow M in Figures 4 and 5). Hereafter, the size of paper P in the above orthogonal direction will be appropriately referred to as "S", and the size of the smallest size paper Ps in the above orthogonal direction will be appropriately referred to as "Ss".

[0033] The fixing unit 13 comprises a heating roller 31, a pressure roller 32, a housing 34, and a rotary drive unit 41 (see Figure 2). The heating roller 31 and the pressure roller 32 are housed within the resin housing 34.

[0034] The pressure roller 32 is constructed, for example, by providing an elastic layer of silicone rubber to an iron core and coating its outer surface with a 50 μm thick PFA resin film. The pressure roller 32 is pivotally supported within the housing 34 so as to be rotatable counterclockwise in the plane of the paper shown in Figure 3. During the fixing process, the rotation axis 33 of the pressure roller 32 is rotated by a rotation drive unit 41, which consists of a motor or the like, under the control of the control unit 100. The pressure roller 32 extends in a direction perpendicular to the direction of transport of the paper P by the transport unit 17 (the direction indicated by the arrow L in Figures 4 and 5) (the direction indicated by the arrow M in Figures 4 and 5), and is positioned opposite the heating roller 31 (heating member 51 provided on the heating roller 31). A nip section N is formed between the heating roller 31 (heating member 51 provided on the heating roller 31) and the pressure roller 32 through which the paper P to be fixed is inserted. The heating roller 31 and the pressure roller 32 fix the toner image to the paper P inserted in the nip section N.

[0035] The heating roller 31 extends in a direction perpendicular to the direction of transport of the paper P by the transport unit 17 (the direction indicated by arrow L in Figures 4 and 5) (the direction indicated by arrow M in Figures 4 and 5), and comprises a cylindrical belt-shaped heating member 51, a heating device 52, a first temperature sensor 53 (see Figures 4 and 5), a second temperature sensor 54 (see Figures 4 and 5), a paper detection sensor 55 (see Figures 4 and 5), a holding member 56, and a pressing member 57. The heating device 52, the first temperature sensor 53, the second temperature sensor 54, the holding member 56, and the pressing member 57 are arranged inside the heating member 51, while the paper detection sensor 55 is arranged outside the heating member 51.

[0036] The heating element 51 is constructed, for example, by coating the inner and outer surfaces of a stainless steel belt with an outer diameter of 30 mm and a thickness of 0.03 mm with fluororesin. The heating element 51 is pivotally supported within the housing 34 so as to be rotatable clockwise in the plane of Figure 3. The heating element 51 is pressed against the pressure roller 32 by a pressing member 57 made of a heat-resistant resin pad, and rotates in conjunction with the rotation of the pressure roller 32.

[0037] The heating device 52 is positioned inside the heating roller 31, extending from one end to the other in a direction perpendicular to the direction of transport of the paper P by the transport unit 17 (indicated by arrow L in Figures 4 and 5) (indicated by arrow M in Figures 4 and 5), and heats the heating roller 31. For example, it is a ceramic heater. More specifically, the heating device 52 is positioned inside the heating member 51, facing the inner circumferential surface of the heating member 51, and heats the inner circumferential surface of the heating member 51. The heating device 52 performs heating operations during the fixing process under the control of the control unit 100, and stops heating operations after the fixing process is completed.

[0038] The holding member 56 holds the heating device 52. For example, the holding member 56 holds the heating device 52 in a state where the heating device 52 is housed in a housing recess of a size that can accommodate the heating device 52.

[0039] As shown in Figure 4, the transport unit 17 transports a predetermined minimum size of paper P to a predetermined region R of the heating roller 31 and pressure roller 32, which includes the center of the heating roller 31 and pressure roller 32 in an orthogonal direction (indicated by arrow M in Figure 4) perpendicular to the transport direction of the paper P by the transport unit 17 (indicated by arrow L in Figure 4). Figure 4 shows how the minimum size of paper Ps is transported to the nip section N such that the center of the minimum size of paper Ps in the orthogonal direction passes through the center of the heating roller 31 and pressure roller 32 in the orthogonal direction (it is transported to the nip section N without bias to either the orthogonal direction), and in this embodiment, the region R is the area through which the minimum size of paper Ps passes in this case.

[0040] The first temperature sensor 53 is, for example, a thermistor, which detects the temperature within a predetermined region R in the heating device 52 as the first temperature and outputs a detection signal indicating the first temperature to the control unit 100. The first temperature sensor 53 is inserted into a through hole formed in the holding member 56 and contacts the heating device 52. In this embodiment, the first temperature sensor 53 is positioned at the center of the heating roller 31 and the pressure roller 32 in an orthogonal direction (direction indicated by arrow M in Figure 4) that is perpendicular to the direction in which the paper P is transported by the transport unit 17 (direction indicated by arrow L in Figure 4).

[0041] The second temperature sensor 54 is, for example, a thermistor and detects the temperature of one end 52a of the heating device 52 in an orthogonal direction (indicated by arrow M in Figure 4) perpendicular to the direction in which the paper P is transported by the transport unit 17 (indicated by arrow L in Figure 4), and the temperature of the heating device 52 at a position from one end 52a to the end Ra on the side of the predetermined region R, and outputs a detection signal indicating the second temperature to the control unit 100. The second temperature sensor 54 is inserted into a through hole formed in the holding member 56 and contacts the heating device 52. In this embodiment, the second temperature sensor 54 is positioned at a distance A from the center of the paper P, which is transported without bias to the nip section N in an orthogonal direction (indicated by arrow M in Figure 4) perpendicular to the direction in which the paper P is transported by the transport unit 17 (indicated by arrow L in Figure 4), toward one end 52a of the heating device 52 in an orthogonal direction. The second temperature sensor 54 detects the second temperature of the heating device 52 corresponding to the area of ​​the heating roller 31 that the smallest size paper Ps does not pass through (non-paper-feeding area), as shown in Figure 4, or it detects the second temperature of the heating device 52 corresponding to the area of ​​the heating roller 31 that the smallest size paper Ps passes through (paper-feeding area), as shown in Figure 5. Figure 5 shows how the smallest size paper Ps is transported to the nip section N with its center in the orthogonal direction offset by A-Ss / 2 or more toward one end 52a of the heating device 52 in the orthogonal direction.

[0042] The paper detection sensor 55 is a reflective light sensor, for example, a combination of an infrared photo IC (Integrated Circuit) and an infrared LED (Light Emitting Diode). It detects the presence or absence of paper P passing through a position in the nip section N, which is from the other end 52b of the heating device 52 to the end Rb on the other end 52b side of a predetermined region R, in an orthogonal direction (indicated by arrow M in Figure 4) perpendicular to the direction in which paper P is transported by the transport unit 17 (indicated by arrow L in Figure 4), and outputs a detection signal indicating the detection result to the control unit 100. In this embodiment, the paper detection sensor 55 is positioned at a distance B away from the center of the paper P that is transported without bias to the nip section N in an orthogonal direction (indicated by arrow M in Figure 4) perpendicular to the direction in which paper P is transported by the transport unit 17 (indicated by arrow L in Figure 4), toward the other end 52b of the heating device 52.

[0043] Furthermore, the fixing unit 13 is equipped with a transport guide member 61. The transport guide member 61 is, for example, flat and is a guide member that guides the paper P moving from the transport path T toward the fixing unit 13 to the nip section N, and is provided with a gap between it and the pressure roller 32 in the transport direction of the paper P. The paper P is guided by the transport guide member 61 and transported to the nip section N of the fixing unit 13.

[0044] In the fixing device 13 described above, for example, if a sheet of paper P (hereinafter referred to as "paper Pab" as appropriate) whose size C in the orthogonal direction perpendicular to the transport direction of the paper P by the transport unit 17 is smaller than A + B is transported to the nip section N with an offset of AC / 2 or more toward one end 52a of the heating device 52 in the orthogonal direction, the second temperature sensor 54 detects the second temperature of the part of the heating device 52 corresponding to the area of ​​the heating roller 31 through which the paper Pab passes (paper feeding section), and the paper detection sensor 55 does not detect the paper Pab. Also, if the paper Pab is transported to the nip section N with an offset of less than AC / 2 toward one end 52a of the heating device 52 in the orthogonal direction, the second temperature sensor 54 detects the second temperature of the part of the heating device 52 corresponding to the area of ​​the heating roller 31 through which the paper Pab does not pass (non-paper feeding section).

[0045] The control unit 100 supplies power to the heating device 52 to control the first temperature of the heating device 52, which is detected by the first temperature sensor 53, to a preset fixing temperature.

[0046] Furthermore, the control unit 100 starts the continuous transport of paper P to the nip section N by the transport section 17 at a predetermined number of sheets per hour, which is a predetermined first number.

[0047] Furthermore, when the control unit 100 determines that the temperature difference between the first temperature of the heating device 52 detected by the first temperature sensor 53 and the second temperature of the heating device 52 detected by the second temperature sensor 54 is smaller than a predetermined first threshold temperature difference, and that the paper detection sensor 55 has not detected any paper P, it changes the predetermined number of sheets per unit time for continuous transport of paper P to the nip section N by the transport unit 17 to a predetermined second number which is less than the first number. When the predetermined number of sheets per unit time for continuous transport of paper P to the nip section N is set to a predetermined second number which is less than the first number, the amount of heat absorbed by the paper P on which the toner image is formed decreases. As a result, the power supplied to the heating device 52 to maintain the first temperature of the heating device 52 at the fixing temperature decreases, and consequently, the temperature rise of the part of the heating device 52 corresponding to the area of ​​the heating roller 31 that paper P does not pass through (non-paper-passing area) is suppressed.

[0048] Furthermore, when the control unit 100 determines that the temperature difference is smaller than the first threshold temperature difference and that the paper detection sensor 55 has detected the paper P, it maintains the predetermined number of sheets per unit of time for continuous transport of the paper P to the nip section N by the transport section 17 at the first number.

[0049] Furthermore, when the control unit 100 determines that the temperature difference has become greater than a predetermined second threshold temperature difference, which is greater than the first threshold temperature difference, it changes the predetermined number of sheets of paper P per unit time that the paper transport unit 17 continuously transports to the nip section N to a predetermined third number that is less than the first number, regardless of the detection result of the paper detection sensor 55 regarding the presence or absence of paper P.

[0050] Furthermore, the control unit 100 determines whether the temperature difference is smaller than the first threshold temperature difference and whether paper has been detected by the paper detection sensor 55 when the number of sheets of paper P continuously transported to the nip section N by the transport unit 17 exceeds a predetermined threshold number. The threshold number is the number of sheets at which, when the continuous transport of paper P to the nip section N by the transport unit 17 is performed at the predetermined first number of sheets per predetermined time, the temperature of the heating device 52 corresponding to the above position in the nip section N where the presence or absence of passing paper P is detected by the paper detection sensor 55 falls within a predetermined allowable temperature (for example, the allowable temperature is set within a temperature range of 320°C to 350°C). This threshold number is determined in advance through experiments or simulations.

[0051] When the transport unit 17 transports the paper P in the second number of sheets, the control unit 100 transports the paper P by setting the distance between the continuously transported sheets of paper P to a predetermined second distance which is longer than the predetermined first distance when transporting in the first number of sheets.

[0052] The control of the control unit 100 with respect to the fixing unit 13 described above will be explained below. The control unit 100 drives the heating device 52 using PID (Proportional-Integral-Differential Controller) control so that the first temperature of the heating device 52 detected by the first temperature sensor 53 becomes the fixing temperature. The fixing temperature is set within a temperature range of, for example, 230°C to 270°C. The control unit 100 supplies power to the heating device 52, and when the first temperature of the heating device 52 detected by the first temperature sensor 53 rises above the fixing temperature, it reduces the power supplied to the heating device 52. Then, when the first temperature falls below the fixing temperature, it increases the power supplied to the heating device 52. In this way, the control unit 100 energizes the heating device 52 to control the first temperature to the fixing temperature.

[0053] Figure 6 is a flowchart showing the processing procedure of the productivity control process performed by the control unit 100 in Figure 2. Figure 7 is also a flowchart showing the processing procedure of the productivity control process performed by the control unit 100 in Figure 2. The control unit 10 operates as the control unit 100 that performs the productivity control process shown in Figures 6 and 7 by executing the productivity control program installed in the storage unit 18.

[0054] The control unit 100 starts the continuous transport of paper P to the nip section N by the transport unit 17 at a predetermined number of sheets per hour, which is a predetermined first number (step S1). Under the control of the control unit 100, the paper feeding unit 14 and the transport unit 17 transport the predetermined first number of sheets of paper P per hour to the nip section N by driving the paper feeding motor and the transport motor in accordance with the predetermined first number of sheets per hour.

[0055] Following the processing in step S1, the control unit 100 determines whether or not printing has finished (step S2). If the control unit 100 determines in the determination process of step S2 that printing has finished (S2: YES), the productivity control processing by the control unit 100 ends.

[0056] If the control unit 100 determines in the determination process of step S2 that printing has not finished (S2: NO), the control unit 100 determines whether the temperature difference between the first temperature of the heating device 52 detected by the first temperature sensor 53 and the second temperature of the heating device 52 detected by the second temperature sensor 54 is greater than or equal to the second threshold temperature difference (step S3). The second threshold temperature difference is set, for example, within a temperature range of 40°C to 50°C.

[0057] In the determination process of step S3, if the control unit 100 determines that the temperature difference is equal to or greater than the second threshold temperature difference (S3: YES), the control unit 100 changes the predetermined number of sheets of paper P continuously transported by the transport unit 17 to the nip section N to a predetermined third number that is less than the first number (step S4). Under the control of the control unit 100, the paper feeding unit 14 and the transport unit 17 transport the predetermined third number of sheets of paper P per unit of time to the nip section N by driving the paper feeding motor and the transport motor in accordance with the predetermined third number of sheets per unit of time. For example, the distance between continuously transported sheets of paper P corresponding to the predetermined third number of sheets per unit of time is made greater than the distance between continuously transported sheets of paper P corresponding to the predetermined first number of sheets per unit of time.

[0058] Following the processing in step S4, the control unit 100 determines whether or not printing has finished (step S5). If the control unit 100 determines in the determination process of step S5 that printing has finished (S5: YES), the productivity control processing by the control unit 100 ends.

[0059] If the control unit 100 determines in the determination process of step S5 that printing is not finished (S5:NO), the control unit 100 determines whether the temperature difference between the first temperature of the heating device 52 detected by the first temperature sensor 53 and the second temperature of the heating device 52 detected by the second temperature sensor 54 is less than a predetermined third threshold temperature difference which is less than the second threshold temperature difference (step S6). The third threshold temperature difference is set, for example, within a temperature range of 10°C to 15°C. If the control unit 100 determines in the determination process of step S6 that the temperature difference is not less than the third threshold temperature difference (i.e., the temperature difference is greater than or equal to the third threshold temperature difference) (S6:NO), the control unit 100 returns to the process of step S5.

[0060] In the determination process of step S6, if the control unit 100 determines that the temperature difference is smaller than the third threshold temperature difference (S6: YES), the control unit 100 changes the predetermined number of sheets per unit time for continuous transport of paper P to the nip section N by the transport unit 17 to the first number of sheets (step S7). Under the control of the control unit 100, the paper feeding unit 14 and the transport unit 17 transport the predetermined first number of sheets per unit time to the nip section N by driving the paper feeding motor and the transport motor in accordance with the predetermined first number of sheets per unit time. After the processing of step S7, the control unit 100 returns to the processing of step S2.

[0061] If, in the determination process of step S3, the control unit 100 determines that the temperature difference is not greater than or equal to the second threshold temperature difference (i.e., the temperature difference is less than the second threshold temperature difference) (S3:NO), the control unit 100 determines whether the number of sheets of paper P continuously transported to the nip section N from the start of printing (hereinafter, as appropriate, referred to as "number of continuously transported sheets") is greater than or equal to the threshold number (step S8). If, in the determination process of step S8, the control unit 100 determines that the number of continuously transported sheets is not greater than or equal to the threshold number (i.e., the number of continuously transported sheets is less than the threshold number) (S8:NO), the control unit 100 returns to the process of step S2.

[0062] If the control unit 100 determines in the determination process of step S8 that the number of continuously transported sheets is equal to or greater than the threshold number (S8:YES), the control unit 100 determines whether the temperature difference between the first temperature of the heating device 52 detected by the first temperature sensor 53 and the second temperature of the heating device 52 detected by the second temperature sensor 54 is smaller than the first threshold temperature difference (step S9). The first threshold temperature difference is set, for example, within a temperature range of 10°C to 15°C. If the control unit 100 determines in the determination process of step S9 that the temperature difference is not smaller than the first threshold temperature difference (i.e., the temperature difference is equal to or greater than the first threshold temperature difference) (S9:NO), the control unit 100 returns to the process of step S2.

[0063] In the determination process of step S9, if the control unit 100 determines that the temperature difference is smaller than the first threshold temperature difference (S9: YES), the control unit 100 determines whether or not the paper P passing through the nip section N has been detected by the paper detection sensor 55 (step S10).

[0064] In the determination process of step S10, if the control unit 100 determines that a sheet of paper P has been detected by the paper detection sensor 55 (S10: YES), the control unit 100 maintains the predetermined number of sheets per unit of time for continuous transport of the sheet of paper P to the nip section N by the transport section 17 to the first sheet (step S11), and returns to the process of step S2.

[0065] In the determination process of step S10, if the control unit 100 determines that no paper P has been detected by the paper detection sensor 55 (S10: NO), the control unit 100 changes the predetermined number of sheets per unit time for continuous transport of paper P to the nip section N by the transport unit 17 to a predetermined second number that is less than the first number (step S12). Under the control of the control unit 100, the paper feeding unit 14 and the transport unit 17 transport the predetermined second number of sheets of paper P per unit time to the nip section N by driving the paper feeding motor and the transport motor in accordance with the predetermined second number of sheets per unit time. For example, the distance between continuously transported sheets of paper P corresponding to the predetermined second number of sheets per unit time is made greater than the distance between continuously transported sheets of paper P corresponding to the predetermined first number of sheets per unit time. Note that the third number in step S4 and the second number in step S12 may be the same or different.

[0066] Following the processing in step S12, the control unit 100 determines whether or not printing has finished (step S13). If the control unit 100 determines in the determination process of step S13 that printing has not finished (S13: NO), the control unit 100 returns to the processing in step S13. On the other hand, if the control unit 100 determines in the determination process of step S13 that printing has finished (S13: YES), the productivity control processing by the control unit 100 ends.

[0067] If the paper P is transported to the nip section N in a direction perpendicular to the direction in which the paper P is transported to the nip section N, and is biased toward one end 52a of the heating device 52, the paper P will pass through the part of the nip section N corresponding to the temperature detection point of the heating device 52 by the second temperature sensor 54, but will not pass through the point where the presence or absence of the paper P is detected by the paper detection sensor 55. Under these circumstances, if the continuous transport of the paper P to the nip section N by the transport unit 17 continues at the predetermined first number of sheets per predetermined time, there is a risk that the temperature of the part of the heating device 52 corresponding to the area of ​​the heating roller 31 that the paper P does not pass through (non-paper-feeding area) will rise excessively.

[0068] Under the above circumstances, the temperature difference between the first temperature of the heating device 52 detected by the first temperature sensor 53 and the second temperature of the heating device 52 detected by the second temperature sensor 53 is smaller than the first threshold temperature difference, and the paper P is not detected by the paper detection sensor 55.

[0069] Furthermore, even under the above circumstances, the temperature of the heating device 52 corresponding to the area of ​​the heating roller 31 that the paper P does not pass through (non-paper-passing area) does not rise excessively until the number of sheets of paper P continuously transported to the nip section N from the start of printing exceeds a threshold number.

[0070] According to the above embodiment, when the number of sheets of paper P continuously transported to the nip section N exceeds a threshold number, and the temperature difference between the first temperature of the heating device 52 detected by the first temperature sensor 53 and the second temperature of the heating device 52 detected by the second temperature sensor 53 is smaller than the first threshold temperature difference, and the paper detection sensor 55 does not detect any paper P, the predetermined number of sheets per unit time for continuous transport of paper P to the nip section N by the transport section 17 is changed to a predetermined second number, which is less than the predetermined first number. This prevents a decrease in the productivity of printing by the image forming apparatus 1 until the number of sheets of paper P continuously transported to the nip section N exceeds a threshold number, while preventing the temperature of the part of the heating device 52 corresponding to the area of ​​the heating roller 31 that does not have paper P passing through (non-paper-passing area) from rising excessively after the number of sheets of paper P continuously transported to the nip section N exceeds a threshold number.

[0071] Furthermore, when the number of sheets of paper P continuously transported to the nip section N exceeds a threshold number, and the temperature difference is smaller than the first threshold temperature difference, and the paper P is detected by the paper detection sensor 55, the predetermined number of sheets per unit time for continuous transport of paper P to the nip section N by the transport section 17 can be maintained at the first number, thereby preventing a decrease in the productivity of printing by the image forming apparatus 1.

[0072] Furthermore, when the temperature difference exceeds a predetermined second threshold temperature difference, the predetermined number of sheets per unit time for continuous transport of paper P to the nip section N by the transport section 17 is changed to a predetermined third number that is less than the first number, thereby preventing the temperature of the heating device 52 in the area of ​​the heating roller 31 that does not pass through (non-paper-passing section) from rising excessively.

[0073] It should be noted that the present invention is not limited to the configuration of the above embodiments and various modifications are possible. Furthermore, the configuration and process shown in the above embodiments using Figures 1 to 7 are merely one embodiment of the present invention and are not intended to limit the present invention to such configuration and process. [Explanation of symbols]

[0074] 1. Image forming apparatus 12 Image forming unit 13 Fixing section 31 Heating rollers 32 Pressure Rollers 51 Heating element 52 Heating device 53. First temperature sensor 54 Second temperature sensor 55 Paper detection sensor 100 Control Unit

Claims

1. An image forming apparatus comprising a fixing unit, a control unit, and a transport unit for transporting a recording medium to the fixing unit, The fixing part is, A heating roller extending in a direction perpendicular to the direction in which the recording medium is transported by the transport unit, A pressure roller extending in the orthogonal direction, positioned opposite the heating roller, and forming a nip portion through which the recording medium to be fixed is inserted between it and the heating roller, The heating device provided on the heating roller extends from one end to the other end of the heating roller in the orthogonal direction and is positioned inside the heating roller, and heats the heating roller. The transport unit transports the recording medium of a predetermined minimum size to a predetermined area of ​​the heating roller and the pressure roller, including the center of the heating roller and the pressure roller in the orthogonal direction. The aforementioned fixing part further, A first temperature sensor that detects the temperature within the predetermined region in the heating device as the first temperature, A second temperature sensor detects the temperature at one end of the heating device in the orthogonal direction, and the temperature at a position from that end of the heating device to the end of the predetermined region on the side of that end, as a second temperature. The system includes a paper detection sensor that detects the presence or absence of the recording medium passing through a position in the nip portion that extends from the other end of the heating device in the orthogonal direction to the other end of the predetermined region on the other end side, The control unit, The heating device is energized to control the first temperature to a preset fixing temperature. The continuous transport of the recording medium to the nip section by the transport section is started at a predetermined number of sheets per unit of time, starting with a predetermined first number of sheets. An image forming apparatus that, when the temperature difference between the first temperature and the second temperature is smaller than a predetermined first threshold temperature difference, and the paper detection sensor determines that the recording medium has not been detected, changes the predetermined number of sheets per unit of time for continuous transport of the recording medium to the nip section by the transport section to a predetermined second number which is less than the first number.

2. The image forming apparatus according to claim 1, wherein when the control unit determines that the temperature difference is smaller than the first threshold temperature difference and that the recording medium has been detected by the paper detection sensor, it maintains the predetermined number of sheets per unit of time for continuous transport of the recording medium to the nip section by the transport section to be the first number.

3. When the number of recording media continuously transported to the nip section by the transport section exceeds a predetermined threshold number, the control unit determines whether the temperature difference is smaller than the first threshold temperature difference and whether the recording media is detected by the paper detection sensor. The image forming apparatus according to claim 1 or claim 2, wherein the threshold number of sheets is the number of sheets at which the temperature of the heating device corresponding to the position in the nip section where the presence or absence of the passing recording medium is detected by the paper detection sensor falls within a predetermined allowable temperature when the continuous transport of the recording medium to the nip section by the transport section is performed at the predetermined first number of sheets per predetermined time.

4. The image forming apparatus according to claim 1, wherein when the control unit determines that the temperature difference has become greater than or equal to a predetermined second threshold temperature difference which is greater than the first threshold temperature difference, it changes the predetermined number of sheets per unit of time for continuous transport of the recording medium to the nip section by the transport section to a predetermined third number which is less than the first number, regardless of the detection result of the paper detection sensor regarding the presence or absence of the recording medium.

5. The image forming apparatus according to claim 1, wherein when the transport unit transports the recording medium in the second number of sheets, the transport unit transports the recording medium with the distance between sheets of the recording medium being transported in a continuous manner set to a predetermined second distance that is longer than a predetermined first distance when transporting in the first number of sheets.

6. The heating roller is equipped with a heating element that is shaped like a cylindrical belt, The fixing device according to claim 1, wherein the heating device is disposed inside the heating member and heats the inner circumferential surface of the heating member.

Images