Image forming device

Abstract

To reduce temperature unevenness in the longitudinal direction of a fixing device.SOLUTION: An image forming apparatus comprises: a pair of sub thermistors 60, 61 that are provided at positions corresponding to both ends of a heating element 54b3; a pair of paper width sensors 31a that are provided on the upstream side of a fixing device 50 in a conveyance direction and provided at positions corresponding to both ends of a heating element 54b1; a pair of paper width sensors 31b that are provided on the upstream side of the fixing device 50 in the conveyance direction and provided at positions corresponding to both ends of a heating element 54b2; and a CPU 94 that determines the width of a recording material based on results of detection performed by the paper width sensors 31a, 31b and the sub thermistors 60, 61.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, such as a copying machine or a printer, that employs an electrophotographic method. [Background technology]

[0002] In a fixing device, when a sheet narrower than the width of the heater is continuously printed, a phenomenon called non-paper passing section temperature rise occurs in which the temperature of the fixing device gradually rises in the longitudinal area where the sheet does not pass. When the non-paper passing section temperature rise becomes significant, parts of the fixing device such as the film and pressure roller may be damaged. In order to prevent the non-paper passing section temperature rise, a configuration has been proposed in which multiple heating elements with different lengths are provided and the heating elements are selected according to the width of the sheet to reduce the non-paper passing section temperature rise. For example, Patent Document 1 discloses a configuration that includes multiple heating elements with different lengths, a first paper width sensor, and a second paper width sensor. Then, a configuration is disclosed in which a more appropriate heating element and throughput (number of printed sheets per unit time) are selected based on the detection results of the first paper width sensor and the second paper width sensor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2000-162919 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the conventional method, temperature unevenness may occur in the longitudinal direction of the fixing device. For example, if there is a "paper size mismatch" between the paper width specified by the user and the actual detection result by the paper width sensor, the appropriate heating element may not be selected, resulting in insufficient heat (power) supply at the edge of the paper.

[0005] The present invention has been made under such circumstances, and has an object to reduce temperature unevenness in the longitudinal direction of a fixing device. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0007] (1) An image forming apparatus having an image forming section which forms a toner image on a sheet of paper, and a fixing device which heats the sheet of paper on which the toner image has been formed by the image forming section in a fixing nip section and fixes the toner image to the sheet of paper, the fixing device having a heater including a first heating element, a second heating element having a length in the longitudinal direction shorter than that of the first heating element, and a third heating element having a length in the longitudinal direction shorter than that of the second heating element, a first rotating element heated by the heater, and a second rotating element which sandwiches the first rotating element between itself and the heater to form the fixing nip section, the image forming apparatus being configured to a first paper width detection means for detecting the width of the paper, the first paper width detection means being arranged upstream of the nip portion in the paper transport direction and on a position inside the first heating element and outside the second heating element in the longitudinal direction; a second paper width detection means for detecting the width of the paper, the second paper width detection means being arranged upstream of the fixing nip portion in the paper transport direction and on a position inside the second heating element and outside the third heating element in the longitudinal direction; and a temperature detection means for detecting the temperature of the fixing nip portion, the second paper width detection means being arranged in a position overlapping with the fixing nip portion in the paper transport direction and on a position inside the third heating element in the longitudinal direction. a determination means for determining a width of a sheet of paper based on detection results of the first paper width detection means, the second paper width detection means, and the temperature detection means; An image forming apparatus comprising: Effect of the Invention

[0009] According to the present invention, it is possible to reduce temperature unevenness in the longitudinal direction of the fixing device. [Brief description of the drawings]

[0010] [Figure 1] Configuration diagram of image forming apparatus according to first and second embodiments [Diagram 2]Block diagram of an image forming apparatus according to first and second embodiments [Diagram 3] Schematic cross-sectional view of fixing devices according to first and second embodiments. [Figure 4] Schematic diagram and cross-sectional diagram of the heater of Example 1 [Diagram 5] Schematic diagram of a power control circuit of a fixing device according to a first embodiment of the present invention. [Figure 6] FIG. 1 is a schematic diagram showing the longitudinal positional relationship between a paper width sensor and a heating element in the first embodiment. [Figure 7] 1 is a flowchart showing a control process of a fixing device according to a first embodiment of the present invention; [Figure 8] FIG. 1 shows the printing speed (number of sheets) for each paper type in Example 1. [Figure 9] FIG. 11 is a schematic diagram showing the positional relationship in the longitudinal direction between the paper width sensor and the heating element in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the following examples, passing a sheet through a fixing nip is referred to as "passing the sheet." In addition, an area where the heating element is generating heat and where the sheet does not pass is referred to as a non-sheet passing area (or non-sheet passing section), and an area where the sheet passes is referred to as a sheet passing area (or sheet passing section). Furthermore, a phenomenon in which the temperature of the non-sheet passing area becomes higher than that of the sheet passing area is referred to as non-sheet passing section temperature rise. EXAMPLES

[0012] [Image forming equipment] Fig. 1 is a configuration diagram showing an in-line type color image forming apparatus, which is an example of an image forming apparatus equipped with the fixing device of the first embodiment. The operation of the electrophotographic type color image forming apparatus will be described with reference to Fig. 1. The first station is a station for forming a yellow (Y) toner image, and the second station is a station for forming a magenta (M) toner image. The third station is a station for forming a cyan (C) toner image, and the fourth station is a station for forming a black (K) toner image.

[0013] In the first station, the photosensitive drum 1a, which is an image carrier, is an OPC photosensitive drum. The photosensitive drum 1a is a metal cylinder on which multiple layers of functional organic materials, such as a carrier generation layer that is photosensitive to light and generates charges, and a charge transport layer that transports the generated charges, are laminated, and the outermost layer has low electrical conductivity and is almost insulated. A charging roller 2a, which is a charging means, is abutted against the photosensitive drum 1a, and as the photosensitive drum 1a rotates, it uniformly charges the surface of the photosensitive drum 1a while rotating in accordance with the rotation of the photosensitive drum 1a. A voltage in which a DC voltage or an AC voltage is superimposed is applied to the charging roller 2a, and the photosensitive drum 1a is charged by generating discharge in a small air gap on the upstream and downstream sides of the rotation direction from the nip portion between the charging roller 2a and the surface of the photosensitive drum 1a. The cleaning unit 3a is a unit that cleans toner remaining on the photosensitive drum 1a after transfer, which will be described later. The developing unit 8a, which is a developing means, is composed of a developing roller 4a, a non-magnetic one-component toner 5a, and a developer coating blade 7a. The photosensitive drum 1a, the charging roller 2a, the cleaning unit 3a, and the developing unit 8a are integrated into a process cartridge 9a that is detachably attached to the image forming apparatus.

[0014] The exposure device 11a, which is an exposure means, is composed of a scanner unit or an LED (light-emitting diode) array that scans laser light using a polygon mirror, and irradiates the photosensitive drum 1a with a scanning beam 12a modulated based on an image signal. The charging roller 2a is connected to a charging high voltage power supply 20a, which is a voltage supply means to the charging roller 2a. The developing roller 4a is connected to a developing high voltage power supply 21a, which is a voltage supply means to the developing roller 4a. The primary transfer roller 10a is connected to a primary transfer high voltage power supply 22a, which is a voltage supply means to the primary transfer roller 10a. The above is the configuration of the first station, and the second, third, and fourth stations have the same configuration. For the other stations, parts having the same functions as the first station are given the same reference numerals, and the suffixes b, c, and d are added to the reference numerals for each station. In the following description, the suffixes a, b, c, and d are omitted except when a specific station is described.

[0015] The intermediate transfer belt 13 is supported by three rollers, a secondary transfer counter roller 15, a tension roller 14, and an auxiliary roller 19, as tension members. A force in a direction to stretch the intermediate transfer belt 13 is applied only to the tension roller 14 by a spring, so that an appropriate tension force is maintained on the intermediate transfer belt 13. The secondary transfer counter roller 15 rotates by receiving a rotation drive from a main motor (not shown), and the intermediate transfer belt 13 wound around the outer periphery rotates. The intermediate transfer belt 13 moves at approximately the same speed in a forward direction (for example, clockwise direction in FIG. 1) as the photosensitive drums 1a to 1d (for example, rotating counterclockwise in FIG. 1). The intermediate transfer belt 13 also rotates in the direction of the arrow (clockwise direction), and the primary transfer roller 10 is disposed on the opposite side of the photosensitive drum 1 with the intermediate transfer belt 13 in between, and rotates following the movement of the intermediate transfer belt 13. The position where the photosensitive drum 1 and the primary transfer roller 10 are in contact with each other across the intermediate transfer belt 13 is called the primary transfer position. The auxiliary roller 19, the tension roller 14, and the secondary transfer opposing roller 15 are electrically grounded. Note that the primary transfer rollers 10b to 10d in the second to fourth stations have the same configuration as the primary transfer roller 10a in the first station, so a description thereof will be omitted.

[0016] Next, the image forming operation of the image forming apparatus of the first embodiment will be described. When the image forming apparatus receives a print command in a standby state, it starts the image forming operation. The photosensitive drum 1, the intermediate transfer belt 13, etc. start to rotate in the direction of the arrow at a predetermined process speed by a main motor (not shown). The photosensitive drum 1a is uniformly charged by the charging roller 2a to which a voltage is applied by the charging high-voltage power supply 20a, and then an electrostatic latent image according to image information is formed by the scanning beam 12a irradiated from the exposure device 11a. The toner 5a in the developing unit 8a is negatively charged by the developer application blade 7a and applied to the developing roller 4a. Then, a predetermined developing voltage is supplied to the developing roller 4a from the developing high-voltage power supply 21a. When the photosensitive drum 1a rotates and the electrostatic latent image formed on the photosensitive drum 1a reaches the developing roller 4a, the electrostatic latent image is visualized by the negative toner adhering thereto, and a toner image of a first color (for example, Y (yellow)) is formed on the photosensitive drum 1a. The stations (process cartridges 9b-9d) for the other colors M (magenta), C (cyan), and K (black) operate in the same way. Electrostatic latent images are formed on the photosensitive drums 1a-1d by exposure while delaying the write start signal from a controller (not shown) at a fixed timing according to the distance between the primary transfer positions of each color. A high DC voltage of the opposite polarity to the toner is applied to each of the primary transfer rollers 10a-10d. Through the above process, the toner images are transferred in order to the intermediate transfer belt 13 (hereinafter referred to as primary transfer), and a multiple toner image is formed on the intermediate transfer belt 13.

[0017] Thereafter, in accordance with the formation of the toner image, the paper P, which is a recording material loaded in the paper feed cassette 16, is conveyed along the conveying path Y. Specifically, the paper P is fed (picked up) by the paper feed roller 17 which is driven to rotate by a paper feed solenoid (not shown). At that time, an ON / OFF signal of the paper width sensor 31, which is a plurality of paper width detection means, is output to a CPU 94 described later depending on whether the paper P passes (cuts) or does not pass (cuts) through the paper width sensor 31. The CPU 94 determines the paper width of the paper P based on the detection result of the paper width sensor 31. Here, the paper width refers to the direction approximately perpendicular to the conveying direction of the paper P, in other words, the length in the longitudinal direction of the heating element 54b described later. The paper width sensor 31 will be described in detail later.

[0018] The fed paper P is conveyed to a registration roller (hereinafter referred to as a registration roller) 18 by a conveying roller. The paper P is conveyed to a transfer nip portion, which is a contact portion between the intermediate transfer belt 13 and a secondary transfer roller 25, by the registration roller 18 in synchronization with the toner image on the intermediate transfer belt 13. A voltage of a polarity opposite to that of the toner is applied to the secondary transfer roller 25 by a secondary transfer high voltage power source 26, and the four-color multi-layered toner image carried on the intermediate transfer belt 13 is transferred collectively onto the paper P (onto a recording material) (hereinafter referred to as a secondary transfer). The members (e.g., the photosensitive drum 1, etc.) that contributed to the formation of an unfixed toner image on the paper P function as an image forming means. On the other hand, after the secondary transfer is completed, the toner remaining on the intermediate transfer belt 13 is cleaned by a cleaning unit 27. After the secondary transfer is completed, the paper P is conveyed to a fixing device 50, which is a fixing means, and the toner image is fixed and discharged to a discharge tray 30 as an image formed product (print, copy). The film 51, the nip forming member 52, the pressure roller 53, and the heater 54 of the fixing device 50 will be described later.

[0019] [Block diagram of image forming device] FIG. 2 is a block diagram for explaining the operation of the image forming apparatus, and the printing operation of the image forming apparatus will be explained with reference to this diagram. PC 110, which is a host computer, outputs a print command to a video controller 91 inside the image forming apparatus, and plays a role in transferring image data of a print image to the video controller 91. At that time, the size of the image data (hereinafter referred to as image size) is determined according to the paper size (hereinafter referred to as designated paper size) specified by PC 110, which is a specification means. Note that the paper size input from an input unit (not shown) provided in the image forming apparatus may be set as the specified paper size, and in this case, the input unit corresponds to the specification means. In the first embodiment, the image size is the size obtained by subtracting 5 mm paper edge margins and 10 mm on both ends from the specified paper size.

[0020] The video controller 91 converts image data from the PC 110 into exposure data and transfers it to the exposure control device 93 in the engine controller 92. The exposure control device 93 is controlled by the CPU 94, and turns the exposure data on and off and controls the exposure device 11. The size of the exposure data is determined by the image size. When the CPU 94, which is the control means, receives a print command, it starts the image formation sequence.

[0021] The engine controller 92 is equipped with a CPU 94, a memory 95, etc., and performs pre-programmed operations. The high voltage power supply 96 is composed of the charging high voltage power supply 20, the developing high voltage power supply 21, the primary transfer high voltage power supply 22, and the secondary transfer high voltage power supply 26 described above. The power control unit 97 is composed of a bidirectional thyristor (hereinafter referred to as a triac) 56, a heating element switch 57 as a switching means for exclusively selecting a heating element to which power is supplied, etc. The power control unit 97 selects a heating element that generates heat in the fixing device 50 and determines the amount of power to be supplied. The drive unit 98 is composed of a main motor 99, a fixing motor 100, etc. The sensor 101 is composed of fixing temperature sensors 59, 60, 61 that detect the temperature of the fixing device 50, a paper width sensor 31 that detects the width of the paper P, etc., and the detection result of the sensor 101 is transmitted to the CPU 94. The CPU 94 obtains the detection result of the sensor 101 in the image forming apparatus, and controls the exposure device 11, the high voltage power supply 96, the power control unit 97, and the drive unit 98. As a result, the CPU 94 performs processes such as forming an electrostatic latent image, transferring the developed toner image, and fixing the toner image onto the paper P, thereby controlling the image forming process in which the exposure data is printed as a toner image on the paper P. Note that the image forming apparatus to which the present invention is applied is not limited to the image forming apparatus having the configuration described in Fig. 1, but may be any image forming apparatus capable of printing on paper P of different widths and equipped with a fixing device 50 having a heater 54 described later.

[0022] [Fixing device] Next, the configuration of the fixing device 50 in the first embodiment will be described with reference to FIG. 3. Here, the longitudinal direction refers to the direction of the rotation axis of the pressure roller 53, which is approximately perpendicular to the transport direction of the paper P described later. Also, the length of the paper P in the direction (longitudinal direction) approximately perpendicular to the transport direction is called the width. FIG. 3 is a cross-sectional schematic diagram of the fixing device 50. Also, FIG. 4(a) is a schematic diagram of the heater 54, FIG. 4(b) is a cross-sectional schematic diagram of the heater, and FIG. 5 is a circuit schematic diagram of the power control unit 97 of the heater 54 of the fixing device 50. Also, FIG. 4(b) is a diagram showing a cross-section of the heater 54 at the center line of the heating elements 54b1a, 54b1b, 54b2, and 54b3 in the longitudinal direction and the center line of the paper P transported to the fixing device 50 (the dashed line a in FIG. 4(a)). Hereinafter, the line a will be called the reference line a.

[0023] 3, paper P carrying an unfixed toner image Tn is heated while being transported from left to right in the figure in the fixing nip N, thereby fixing the toner image Tn to the paper P. Fixing device 50 in the first embodiment is composed of cylindrical film 51, nip forming member 52 that holds film 51, pressure roller 53 that forms fixing nip N together with film 51, and heater 54 for heating paper P.

[0024] The film 51, which is the first rotating body, is a fixing film serving as a heating rotating body. In the first embodiment, for example, polyimide is used as the base layer. An elastic layer made of silicone rubber and a release layer made of PFA are used on the base layer. In order to reduce the frictional force generated between the film 51 and the nip forming member 52 and heater 54 due to the rotation of the film 51, grease is applied to the inner surface of the film 51.

[0025] The nip forming member 52 guides the film 51 from the inside and plays a role of forming a fixing nip N between the film 51 and the pressure roller 53. The nip forming member 52 is a member having rigidity, heat resistance, and heat insulation properties, and is made of a liquid crystal polymer or the like. The film 51 is fitted onto the outside of the nip forming member 52. The pressure roller 53, which is a second rotating body, is a roller serving as a pressure rotating body. The pressure roller 53 is composed of a core metal 53a, an elastic layer 53b, and a release layer 53c. The pressure roller 53 is rotatably held at both ends and is driven to rotate by a fixing motor 100 (see FIG. 2). The film 51 is rotated by the rotation of the pressure roller 53. The heater 54, which is a heating member, is held by the nip forming member 52 and is in contact with the inner surface of the film 51. The substrate 54a, the heating elements 54b1a (54b1), 54b1b (54b1), 54b2, 54b3, the protective glass layer 54e, and the fixing temperature sensors 59, 60, 61 will be described later.

[0026] (heater) The heater 54 will be described in detail with reference to FIG. 4(a). The heater 54 includes a substrate 54a, a first heating element 54b1a, a fourth heating element 54b1b, a second heating element 54b2, a third heating element 54b3, a conductor 54c, contacts 54d1-54d4, and a protective glass layer 54e. Hereinafter, the heating elements 54b1a, 54b1b, 54b2, and 54b3 may be collectively referred to as heating element 54b. In addition, the heating elements 54b1a and 54b1b, which have approximately the same length in the longitudinal direction, may be collectively referred to as heating element 54b1. The substrate 54a uses alumina (Al2O3), which is a ceramic. Alumina (Al2O3), aluminum nitride (AlN), zirconia (ZrO2), silicon carbide (SiC), and the like are widely known as ceramic substrates. Among them, alumina (Al2O3) is inexpensive and easily available industrially. In addition, a metal having excellent strength may be used for the substrate 54a, and stainless steel (SUS) is preferably used as the metal substrate because it is inexpensive and has excellent strength. Whether a ceramic substrate or a metal substrate is used as the substrate 54a, an insulating layer may be provided if the substrate is conductive. Heating elements 54b1a, 54b1b, 54b2, and 54b3, a conductor 54c, and contacts 54d1 to 54d4 are formed on the substrate 54a. A protective glass layer 54e is formed thereon to ensure insulation between the heating elements 54b1a, 54b1b, 54b2, and 54b3 and the film 51.

[0027] Heating elements 54b have different longitudinal lengths (hereinafter also referred to as sizes). Heating elements 54b1a and 54b1b have a first longitudinal length L1=222 mm, heating element 54b2 has a second longitudinal length L2=188 mm, and heating element 54b3 has a third longitudinal length L3=154 mm. The lengths L1, L2, and L3 have a relationship of L1>L2>L3.

[0028] In addition, the largest paper width (hereinafter referred to as the maximum paper width) C of the paper P that can be used in the image forming apparatus of the first embodiment is 216 mm, and the smallest paper width (hereinafter referred to as the minimum paper width) is 76 mm. Therefore, L1 is a length that allows the heating element 54b1 to fix an image size (206 mm) of the maximum paper width C (216 mm). The heating element 54b1 is electrically connected to the contact 54d2, which is the second contact, and the contact 54d4, which is the fourth contact, via the conductor 54c, and the heating element 54b2 is electrically connected to the contacts 54d2 and 54d3 via the conductor 54c. The heating element 54b3 is electrically connected to the contact 54d1, which is the first contact, and the contact 54d3, which is the third contact, via the conductor 54c. Here, the heating elements 54b1a and 54b1b have the same length, and are always used approximately at the same time. The heating element 54b1a is provided at one end in the short side direction of the substrate 54a, and the heating element 54b1b is provided at the other end in the short side direction of the substrate 54a. The heating elements 54b2 and 54b3 are provided symmetrically with respect to the center in the short side direction between the heating elements 54b1a and 54b2b in the short side direction of the substrate 54a.

[0029] The fixing temperature sensors 59, 60, and 61 are thermistors. The fixing temperature sensor 59 will be described as a representative with reference to FIG. 4(b). The fixing temperature sensor 59, which is the first temperature detection means, is composed of a main thermistor element 59a, a holder 59b, ceramic paper 59c, and an insulating resin sheet 59d. The ceramic paper 59c serves to inhibit heat conduction between the holder 59b and the main thermistor element 59a. The insulating resin sheet 59d serves to physically and electrically protect the main thermistor element 59a. The main thermistor element 59a is a temperature detection means whose output value changes according to the temperature of the heater 54, and is connected to the CPU 94 by a dumet wire (not shown) and wiring. The main thermistor element 59a detects the temperature of the heater 54 and outputs the detection result to the CPU 94.

[0030] The fixing temperature sensor 59 is located on the opposite side of the protective glass layer 54e with respect to the substrate 54a, and is installed at the position of the reference line a (the position corresponding to the center) in the longitudinal direction of the heating element 54b, and is in contact with the substrate 54a. The CPU 94 controls the temperature during the fixing process based on the detection result of the fixing temperature sensor 59. The above is an explanation of the configuration of the fixing temperature sensor 59 which is the main thermistor. The fixing temperature sensors 60 and 61 which are a pair of second temperature detection means functioning as sub-thermistors have the same configuration as the fixing temperature sensor 59, but are arranged at different positions in the longitudinal direction. Hereinafter, the fixing temperature sensor 59 will be referred to as the main thermistor 59, and the fixing temperature sensors 60 and 61 will be referred to as the sub-thermistors 60 and 61. The sub-thermistors 60 and 61 will also be referred to as the sub-thermistor pair 60 and 61.

[0031] [Thermistor placement] In FIG. 4(a), the dashed lines indicate that the main thermistor 59 and the sub-thermistors 60 and 61 are disposed on the rear surface of the substrate 54a, and indicate the positions where the main thermistor 59 and the sub-thermistors 60 and 61 contact the substrate 54a. The CPU 94 controls the temperature of the heater 54 based on the detection result of the main thermistor 59. The main thermistor 59 is disposed on a reference line a, which is the center line of the heating elements 54b1, 54b2, and 54b3 in the longitudinal direction and the center line of the paper P transported to the fixing device 50. The sub-thermistor elements 60a and 61a of the sub-thermistors 60 and 61 serve both as temperature detection means and as paper width detection means corresponding to the heating element 54b3, which is the heating element with the smallest width. The sub-thermistor elements 60a and 61a are disposed symmetrically with respect to the reference line a. In other words, the sub-thermistor elements 60a and 61a are disposed at positions corresponding to both ends of the heating element 54b3. The distance S3 between the sub-thermistor element 60a and the sub-thermistor element 61a is S3=142 mm, and L3>S3.

[0032] The detection of the width of the paper P by the sub-thermistor elements 60a and 61a is performed based on a change in temperature detected when the paper P passes through the fixing nip N. Specifically, the CPU 94, which is a determination means, determines the width of the paper P based on the temperature difference between the temperature detected by the main thermistor element 59a and the temperature detected by the sub-thermistor elements 60a and 61a when the paper P passes through the fixing nip N. For example, if the temperature detected immediately after the paper P reaches the fixing nip N of the fixing device 50 is higher than the temperature detected by the main thermistor element 59a by, for example, 10° C. or more, the CPU 94 makes the following determination. At this time, the CPU 94 determines that the paper P has not passed over the sub-thermistor elements 60a and 61a, and determines that the paper width of the paper P is narrower than S3. Note that when the paper P has not passed through the sub-thermistor elements 60a and 61a, the sub-thermistor 60 is OFF and the sub-thermistor 61 is OFF, as if they are sensor outputs. When the paper P is passing through the sub-thermistor elements 60a and 61a, the sub-thermistor 60 is ON and the sub-thermistor 61 is ON. The CPU 94 determines the timing when the paper P reaches the fixing nip N based on the timing when the leading edge of the paper P passes the paper width sensor 31 and the process speed (conveyance speed).

[0033] [Power Control Section] FIG. 5 is a schematic diagram of the heater 54 of the fixing device 50 and the power control circuit of the power control unit 97. The power control circuit of the fixing device 50 includes the heating elements 54b1, 54b2, and 54b3, the AC power source 55, the triac 56, and the heating element switch 57. The contacts 54d1 to 54d4 are connected to the heating element switch 57 for switching the power supply path. Since the heating element 54b that generates heat is switched by switching the power supply path by the heating element switch 57, switching the power supply path is also expressed as switching the heating element 54b. In the first embodiment, the heating element switch 57 is specifically electromagnetic relays 57a and 57b having a C-contact configuration. The triac 56 is a triac that supplies or cuts off the power supply from the AC power source 55 to the heating elements 54b1, 54b2, and 54b3 by being conductive or non-conductive. The CPU 94 calculates the power required to make the heater 54 at a predetermined temperature (a target temperature required for fixing) based on the temperature information reported from the main thermistor 59, and issues an instruction to make the triac 56 conductive or non-conductive.

[0034] The electromagnetic relay 57a has a contact 57a1 connected to the first pole of the AC power supply 55 via the triac 56, a contact 57a2 connected to the contact 54d4, and a contact 57a3 connected to the contact 54d3. The electromagnetic relay 57a is controlled by the engine controller 92 to be in one of two states: a state in which the contact 57a1 and the contact 57a2 are connected, and a state in which the contact 57a1 and the contact 57a3 are connected. The electromagnetic relay 57b has a contact 57b1 connected to the second pole of the AC power supply 55, a contact 57b2 connected to the contact 54d2, and a contact 57b3 connected to the contact 54d1. The electromagnetic relay 57b is controlled by the engine controller 92 to be in one of two states: a state in which the contact 57b1 and the contact 57b2 are connected, and a state in which the contact 57b1 and the contact 57b3 are connected.

[0035] For example, when contacts 57a1 and 57a2 are connected and contacts 57b1 and 57b2 are connected, power is supplied to heating element 54b1. For example, when contacts 57a1 and 57a3 are connected and contacts 57b1 and 57b3 are connected (the state of FIG. 5), power is supplied to heating element 54b3. For example, when contacts 57a1 and 57a3 are connected and contacts 57b1 and 57b2 are connected, power is supplied to heating element 54b2.

[0036] (About switching of heating elements and current ratio) The electromagnetic relays 57a and 57b are a heating element control means for controlling the ratio of power supply to the plurality of heating elements 54b (hereinafter referred to as energization ratio). The heating element switchers 57a and 57b receive a signal from the CPU 94 and exclusively select which heating element 54b to supply power to among the heating elements 54b1, 54b2, and 54b3. In order to realize a predetermined energization ratio, the CPU 94 switches the electromagnetic relays 57a and 57b to allocate the time of power supply to the heating elements 54b1, 54b2, and 54b3 (hereinafter referred to as energization time). The CPU 94 switches the electromagnetic relays 57a and 57b every predetermined period of the AC power source 55, for example, every four periods. For example, when the energization ratio to the heating elements 54b1a and 54b1b is 2 and the energization ratio to the heating element 54b2 is 8, the CPU 94 controls as follows. First, the CPU 94 uses the electromagnetic relays 57a and 57b to maintain the state of connection to the heating element 54b1 for 4 cycles x 2 = 8 cycles. After that, the CPU 94 switches the electromagnetic relays 57a and 57b to maintain the state of connection to the heating element 54b2 for 4 cycles x 8 = 32 cycles, and then repeats the operation of connecting to the heating element 54b1 again. In this way, when the power supply ratio to the heating elements 54b1a and 54b1b is 2 and the power supply ratio to the heating element 54b2 is 8, hereinafter, the power supply ratio will be expressed as 2:8.

[0037] In the first embodiment, the predetermined current distribution ratio is realized by distributing the current distribution time to the heating elements 54b, but the method of distributing the power supply to the heating elements 54b is not limited to this. The amount of power supplied to each heating element 54b may be distributed by any one of time, voltage, and current, or a combination of these. For example, a triac may be provided for each heating element 54b as the heating element control means, and the predetermined current distribution ratio may be realized by controlling the amount of current supplied to each heating element 54b by switching the conduction / non-conduction of each triac by the CPU 94.

[0038] [Positional relationship between heating element and paper width sensor] The positional relationship between each heating element 54b and the paper width sensor 31 will be described with reference to FIG. 6. FIG. 6 is a schematic diagram showing the positional relationship of the image forming apparatus on the transport path Y of the paper P. On the transport path Y, the paper feed cassette 16, the paper width sensor 31, the registration roller 18, the secondary transfer roller 25, and the fixing device 50 are arranged in this order from the upstream in the transport direction of the paper P. FIG. 6 shows only the heater 54 as the fixing device 50. In addition, in the first embodiment, three pairs of paper width sensors 31 corresponding to each heating element 54b are provided. As the paper width sensor 31 corresponding to the heating element 54b1 having the maximum width among the heating elements 54b, a pair of paper width sensors 31a1 and 31a2, which are the first paper width detection means, are arranged at an interval S1. That is, the paper width sensors 31a1 and 31a2 are a pair of sensors provided upstream of the fixing device 50 in the transport direction of the paper P that is approximately perpendicular to the longitudinal direction, and provided at positions corresponding to both ends of the heating element 54b1a. As the paper width sensor 31 corresponding to the second longest heating element 54b2 among the heating elements 54b, a pair of paper width sensors 31b1 and 31b2, which are second paper width detection means, are arranged at a distance S2. Hereinafter, the paper width sensors 31a1 and 31a2 will be referred to as the paper width sensor pair 31a, and the paper width sensors 31b1 and 31b2 will also be referred to as the paper width sensor pair 31b. In addition, the sub-thermistor elements 60a and 61a, which function as the paper width sensor corresponding to the heating element 54b3 with the smallest width among the heating elements 54b and are used for temperature control, are arranged at a distance S3 in the fixing nip portion N of the fixing device 50 as described in FIG. Hereinafter, the sub-thermistor elements 60a and 61a will also be included in the paper width sensor 31.

[0039] The three pairs of paper width sensors 31 described above are arranged symmetrically with respect to a reference line a, which is the center line of the longitudinal direction of the paper P and also the center line of the heating elements 54b1, 54b2, 54b3. The relationship between the lengths L1, L2, L3 of the heating elements 54b, the intervals S1, S2, S3 between the paper width sensors 31, and the maximum paper width C that can be arranged in the paper feed cassette 16 is as follows: L1(222mm)>C(216mm)>S1(198mm)>L2(188mm)>S2(170mm)>L3(154mm)>S3(142mm) That is, the paper width sensor 31 corresponding to each heating element 54b is disposed inside the corresponding heating element 54b in the longitudinal direction.

[0040] Thus, in the first embodiment, the sub-thermistor elements 60a, 61a, which are the paper width sensor 31 located innermost in the longitudinal direction among the multiple paper width sensors 31, also serve as a temperature detection means for detecting the temperature of the heater 54. The sub-thermistor elements 60a, 61a, which are the paper width sensor 31, are disposed in the fixing nip N. Meanwhile, the paper width sensor pair 31a, 31b, which are the paper width sensors 31 other than the paper width sensor 31 (thermistors 60a, 61a) located innermost in the longitudinal direction, are disposed upstream of the fixing nip N in the transport direction of the paper P.

[0041] The sub-thermistor elements 60a, 61a detect the paper width of the paper P when the leading edge of the first sheet of continuous printing reaches the fixing nip N of the fixing device 50. The detection result of the paper width of the paper P at the fixing nip N is used to control the heating elements 54b for the second and subsequent sheets of continuous printing. Furthermore, the paper width is detected by the paper width sensor pair 31a, 31b when the leading edge of the paper P reaches the paper width sensor pair 31a, 31b, and the detection result is used to control the heating elements 54b for the first and subsequent sheets of printing.

[0042] [Control of heating elements] Next, the operation of the fixing device 50 in the first embodiment will be described with reference to FIG. 7 and Table 1. Here, assuming that the width of the paper P is p, the paper P with a paper width p of the maximum paper passing width C (216 mm) ≧ p ≧ S1 (198 mm) is called a large size paper, which is a first recording material having a width in the first range. The paper P with a paper width p of S1 (198 mm) > p ≧ S2 (170 mm) is called a medium size paper, which is a second recording material having a width in the second range. The paper P with a paper width p of S2 (170 mm) > p ≧ S3 (142 mm) is called a small size paper, which is a third recording material having a width in the third range. The paper P with a paper width p of S3 (142 mm) > p ≧ the minimum paper passing width (76 mm) is called a very small size paper, which is a fourth recording material having a width in the fourth range. Representative large size paper is LTR, A4, medium size paper is B5, small size paper is A5, and extra small size paper is a postcard.

[0043] In Table 1, "ON" indicates a state in which each paper width sensor 31 detects the "presence" of paper P, and "OFF" indicates a state in which each paper width sensor 31 detects the "absence" of paper P. Also, "TPD" in Table 1 stands for Through Put Down, which means reducing the throughput (number of printed sheets per unit time), in other words, reducing the transport speed of paper P. Hereinafter, control that reduces throughput may also be referred to as TPD control.

[0044] 7 shows the sequence from receiving a print command to the end of printing. In step (hereinafter, referred to as S) 101, the CPU 94 receives a print command from the PC 110. In S102, the CPU 94 acquires the detected temperature of the main thermistor 59 of the fixing device 50. When the CPU 94 determines that the detected temperature of the main thermistor 59 is, for example, 70° C. or higher, it determines that the state of the fixing device 50 is Hot, and when it determines that the detected temperature is less than 70° C. (less than a predetermined temperature), it determines that the state of the fixing device 50 is Cold.

[0045] (Hot and Cold states) Here, when the CPU 94 determines in S102 that the fixing device 50 is in a cold state, the fixing operation is performed on the first three sheets of paper P of the continuous printing using only the heating element 54b1, regardless of the detection result of the paper width or the specified paper size. Then, the CPU 94 controls the fourth and subsequent sheets of paper P at the energization ratio shown in Table 1. Also, when the CPU 94 determines in S102 that the fixing device 50 is in a hot state, the CPU 94 controls the first and subsequent sheets of paper P at the energization ratio shown in Table 1. The reason for performing the fixing process on the first three sheets of paper P in the cold state using only the heating element 54b1 is to melt the grease in the film 51 uniformly in the longitudinal direction of the fixing nip portion N. If temperature unevenness occurs in the longitudinal direction of the film 51 and low temperature portions are formed, the sliding resistance of the film 51 becomes non-uniform in the longitudinal direction, and the film 51 may be deformed. As will be described later with reference to Table 1, a certain percentage of current is passed to the widest heating element, 54b1, even in the Cold state after the fourth sheet and in the Hot state, preventing the film 51 from being deformed due to uneven sliding resistance caused by a temperature drop in the fixing nip N.

[0046] Returning to the explanation of the flowchart in FIG. 7, in S103, the CPU 94 acquires the paper size designated by the user. When the paper P is conveyed from the paper feed cassette 16, in S104, the CPU 94 acquires the detection result of the paper width of the paper P by the paper width sensor 31 (hereinafter referred to as the paper width detection result). Note that the paper width sensor 31 here refers to the pair of paper width sensors 31a, 31b provided upstream of the fixing nip N. In S105, the CPU 94 sets the power supply ratio based on the paper size designated by the user acquired in S103 and the paper width detection result acquired in S104. In S106, the CPU 94 determines whether or not it is the timing when the first sheet of paper P has reached the fixing nip N based on the detection result of the pair of paper width sensors 31a, 31b and the process speed. In S106, if the CPU 94 determines that it is not the time when the paper P has reached the fixing nip portion N, the process returns to S106, and if it determines that it is the time when the paper P has reached the fixing nip portion N, the process proceeds to S107.

[0047] In S107, the CPU 94 acquires (determines) the paper width of the paper P based on the detection result by the sub-thermistor elements 60a and 61a as the paper width sensor 31. In S108, the CPU 94 determines whether or not it is necessary to change the power supply ratio based on the specified paper size acquired in S103 and the paper width detection result acquired in S107. If the CPU 94 determines in S108 that it is necessary to change the power supply ratio, the process proceeds to S109, and if it determines that it is not necessary, the process proceeds to S110. As described above, if the CPU 94 determines that the state is Cold in S102, control is performed using the heating element 54b1 for up to the third sheet of paper P, regardless of the paper width detection result or the specified paper size. In S109, the CPU 94 changes the power supply ratio and proceeds to S110. In S110, the CPU 94 determines whether or not the continuous printing has ended, and if it determines that it has not ended, the process returns to S107, and if it determines that it has ended, the process ends.

[0048] [Control of current ratio] Next, the control of the power supply ratio by the CPU 94 shown in Table 1 will be described. In Table 1, the paper width detection result is expressed as ON when each paper width sensor 31 detects the arrival of the paper P, and OFF when it does not detect it. For example, when it is determined that the paper P has passed over all the paper width sensors 31 (all ON), the paper width detection result is determined to be a large size paper. Furthermore, when the specified paper size is a large size paper, the control of (a) in Table 1 is performed. Also, the numbers shown in Table 1 indicate the ratio of power supply to each heating element 54b1, 54b2, and 54b3 (hereinafter referred to as the power supply ratio). For example, (a) shows 10:0:0, which indicates that the power supply ratio to the heating element 54b1 is 10, the power supply ratio to the heating element 54b2 is 0, and the power supply ratio to the heating element 54b3 is 0. As described above, for example, a power supply ratio of 2:3:5 means that the control is performed as follows. In other words, this means that power is supplied to heating element 54b1 for two cycles of the AC voltage of AC power supply 55, then heating element 54b is switched to supply power to heating element 54b2 for three cycles, and then heating element 54b is switched to supply power to heating element 54b3 for five cycles.

[0049] [Table 1]

[0050] (Table 1: Selection of heating element 54b in hot state in Example 1) (When the paper width detection result matches the specified paper size) First, we will explain the cases where the paper width detection result and the specified paper size match. This corresponds to (a), (f), (k), and (p) in Table 1.

[0051] (a) When both the paper width detection result and the specified paper size are large size paper, the power ratio is 10:0:0 as shown in (a), and the fixing operation is performed using only the heating element 54b1. Note that the paper width detection result for large size paper is that the paper width sensor pair 31a is ON, the paper width sensor pair 31b is ON, and the sub-thermistor elements 60a and 61a are ON.

[0052] (f) When both the paper width detection result and the specified paper size are medium size paper, the power ratio is in the range of 4:6:0 to 2:8:0 as shown in (f), and the ratio is controlled according to the specified paper size. In either case, the heating elements 54b1 and 54b2 are heated at a constant power ratio, and the heating element 54b3 is not heated. Note that the paper width detection result for medium size paper is that the paper width sensor pair 31a is OFF, the paper width sensor pair 31b is ON, and the sub-thermistor elements 60a and 61a are ON.

[0053] Here, the heat generated by the heating element 54b1 is also supplied to the outer region of the medium-sized paper. In the outer region of the medium-sized paper, the heat is not absorbed by the paper P or toner, so the temperature is likely to rise excessively. Therefore, if the power supply ratio of the heating element 54b1 is increased, the temperature rise in the non-paper passing portion becomes more noticeable. If the temperature rise in the non-paper passing portion becomes noticeable, the film 51 or the pressure roller 53 may be damaged, so it is necessary to reduce the throughput by spacing the paper P so that the temperature rise in the non-paper passing portion is below a certain temperature. On the other hand, the heating element 54b2 supplies heat only to the inner region of the medium-sized paper, which is advantageous for the temperature rise in the non-paper passing portion. However, if toner is formed on the paper P in an area of ​​the heating element 54b2 that is equal to or larger than the width L2, there is a risk of insufficient heat supply in the area of ​​the width L2 or larger. Therefore, if the energization ratio of the heating element 54b2 is increased, a phenomenon called poor fixing occurs at the edge of the paper, where the toner is not fixed, and the toner adheres to the film 51, soiling the fixing device 50, and may be ejected onto the following paper, soiling the paper P. For these reasons, it is preferable to increase the energization ratio of the heating element 54b1 as the paper width of the paper P increases, and to increase the energization ratio of the heating element 54b2 as the paper width decreases, even for the same medium-sized paper. In the first embodiment, the energization ratio is set between 4:6:0 and 2:8:0 according to the designated paper size, even for medium-sized paper. By setting it in this way, regardless of the designated paper size, damage to the film 51 or the pressure roller 53 due to a rise in temperature in the non-paper passing area and poor fixing at the edge of the paper did not occur.

[0054] (k) When both the paper width detection result and the specified paper size are small size paper, the energization ratio is in the range of 2:3:5 to 2:0:8 as shown in (k), and the ratio control is performed according to the specified paper size. In this case, the heating elements 54b1, 54b2, and 54b3 are heated at a constant energization ratio. The purpose of energizing the heating element 54b1 is to prevent the deformation of the film 51 due to the uneven sliding resistance in the longitudinal direction described above, and the energization ratio is constant at 2 regardless of the paper width of the small size paper. The energization ratio of the heating elements 54b2 and 54b3 differs depending on the specified paper size. Specifically, in order to suppress the temperature rise of the non-paper passing part as much as possible and to increase the throughput, the energization ratio of the heating element 54b2 is increased and the energization ratio of the heating element 54b3 is decreased as the paper width of the paper P becomes larger. The energization ratio is set to be between 2:3:5 to 2:0:8 depending on the specified paper size. In addition, the paper width detection result for small size paper is that the paper width sensor pair 31a is OFF, the paper width sensor pair 31b is OFF, and the sub-thermistor elements 60a and 61a are ON.

[0055] (p) When both the paper width detection result and the specified paper size are very small size paper, as shown in (p), the power supply ratio is 2:0:8, and TPD control is performed according to the paper width of paper P. The power supply ratio to heating element 54b1 is 2, and the power supply ratio to heating element 54b3 is 8. Note that the paper width detection result for very small size paper is that the paper width sensor pair 31a is OFF, the paper width sensor pair 31b is OFF, and the sub-thermistor elements 60a and 61a are OFF.

[0056] Here, consider a case where the minimum paper width detection means is a paper width sensor rather than a thermistor like the other paper width detection means (paper width sensor pair 31a, 31b). In that case, it is possible to detect whether the paper width of paper P is a small size paper or larger or a very small size paper, but it is not possible to detect any size smaller than the very small size paper. Therefore, in order to prevent deformation of film 51 and pressure roller 53 due to the temperature rise in the non-paper passing portion, it is necessary to uniformly reduce the throughput to match the minimum paper width paper P, which is where the temperature rise in the non-paper passing portion is most noticeable, which reduces productivity.

[0057] On the other hand, in the first embodiment, the sub-thermistor elements 60a and 61a, which are the innermost paper width detection means and also serve as the temperature detection means, are arranged in the fixing nip portion N. Also, the pair of paper width sensors 31a and 31b, which are the other paper width detection means, are arranged upstream of the fixing nip portion N. With such a configuration, it is possible to always detect the temperature rise in the non-paper passing portion when printing on very small size paper. And, even with the same very small size, it is possible to select an appropriate throughput according to the paper size (paper width of the paper P) and maximize productivity. In the first embodiment, when one of the detected temperatures of the sub-thermistors 60 and 61 exceeds, for example, 230°C, the paper interval is widened to reduce the throughput and prevent the deformation of the film 51. Here, the paper interval refers to the interval between the rear end of a specific paper P and the leading end of the following paper P being transported following the specific paper P. Even with the same very small size paper, the smaller the width of the paper P, the greater the temperature rise in the non-paper passing portion, and the greater the throughput reduction.

[0058] [Number of Outputs] FIG. 8 shows the number of sheets output when printing different sizes of paper P for one minute from a state where the detected temperature of the main thermistor 59 of the fixing device 50 is 75° C. (i.e., Hot state). In FIG. 8, the horizontal axis shows time (minutes) and the vertical axis shows the number of sheets. The solid line shows postcards, the dashed line shows A6 paper P, the dashed line with a large pitch shows invoices, and the dashed line shows A5. For example, in the case of postcards, the detected temperature of the sub-thermistors 60 and 61 exceeds 230° C. about 10 seconds after the start of printing, and the throughput is reduced, so the number of sheets output after one minute is 15 sheets. On the other hand, in the case of A5, the throughput is not reduced and the number of sheets printed after one minute is 44 sheets. In addition, in the case of invoices, the number is 40 sheets, and in the case of A6, the number is 25 sheets. From the graph shown in FIG. 8, it can be seen that the number of sheets printed after one minute, in other words the output speed, decreases as the width of the paper becomes narrower, and an appropriate throughput can be selected according to the paper width. Moreover, for any type of paper, when the temperatures detected by the sub-thermistors 60, 61 were kept within 230° C., the members of the fixing device 50, such as the film 51 and the pressure roller 53, did not deform.

[0059] (If the detected paper width is smaller than the specified paper size) Next, we will explain the case where the paper width detection result is smaller than the specified paper size. This corresponds to (e), (i), (j), (m), (n), and (o) in Table 1. In the first embodiment, when the paper width detection result is smaller than the specified paper size, the power supply ratio to the heating element 54b, which has a width in the longitudinal direction larger than the width determined by the paper width detection result, is increased compared to when the specified paper size is the same as the paper width detection result.

[0060] (e) When the paper width detection result is a medium size paper and the specified paper size is a large size paper, the power distribution ratio is 10:0:0 as shown in (e), and uniform TPD control is performed. In this case, power is distributed only to the heating element 54b1. When the specified paper size is a large size paper, the image size is a maximum of 206 mm. When the paper width detection result is a medium size, the paper width is S2 (170 mm) or more and S1 (198 mm) or less, so the paper width < image size. In this case, depending on the image data, a toner image may be loaded on the paper P in the marginal part as well. In this case, if the fixing operation is performed with the same power distribution ratio as when the paper width detection result and the specified paper size match in (f) and are correct, excessive heat may be taken by the toner image in the marginal part, and fixing failure may occur at the edge of the paper. However, in the first embodiment, the power distribution ratio to the heating element 54b1 corresponding to the large size paper larger than the medium size paper determined by the paper width detection result is increased compared to the case (f) where the specified paper size is as the paper width detection result. Therefore, poor fixing did not occur even in the marginal area. In addition, the throughput reduction suppressed the temperature rise in the non-paper passing area, and the film 51 did not deform.

[0061] (i) When the paper width detection result is small size paper and the specified paper size is large size paper, the fixing operation is performed with a power supply ratio of 2:8:0 as shown in (i), and uniform TPD control is performed. Since the power supply ratio to the heating element 54b2 is higher than in the case of (k) where the specified paper size was the same as the paper width detection result, no fixing failure occurred in the margins. In this case, as in the case of (e), a uniform throughput reduction is performed. In the case of (i), no fixing failure or deformation of the film 51 due to temperature unevenness occurred. (j) When the paper width detection result is small size paper and the specified paper size is medium size paper, the same as in the case of (i).

[0062] (m) When the paper width detection result is extra small size paper and the specified paper size is large size paper, the fixing operation is performed according to the number of printed sheets, and the TPD control is performed according to the paper width. Specifically, the fixing operation is performed with a current distribution ratio of 2:8:0 for the first sheet of continuous printing, and the fixing operation is performed with a current distribution ratio of 2:0:8 for the second sheet and after. Since only the detection result by the paper width sensor pair 31a, 31b can be used for the first sheet of paper P, the CPU 94 can determine that the paper P being conveyed is small size or smaller, but cannot determine whether it is small size paper or extra small size paper. Therefore, even if the first sheet is small size paper, the current distribution ratio is set to the same as (i) to prevent poor fixing in the margins. From the second sheet onwards in the continuous printing, the sub-thermistor elements 60a and 61a detect that the paper is a very small size paper, so in order to suppress the temperature rise in the non-paper passing area, the power supply ratio of the heating element 54b2 is lowered and the power supply ratio of the heating element 54b3 is increased, changing it from 2:8:0 to 2:0:8. Furthermore, in order to prevent the temperature rise in the non-paper passing area, the throughput is reduced according to the detected temperature of the sub-thermistor elements 60a and 61a, in other words, according to the paper width of the paper P. Specifically, the throughput is reduced when the detected temperature of either one of the sub-thermistor elements 60a and 61a reaches 230°C. For the first sheet, the power supply ratio to the heating element 54b2 is increased compared to the case (p) where the specified paper size was the same as the paper width detection result, so no fixing failure occurred even in the marginal area. (n) When the paper width detection result is a very small size paper and the specified paper size is a medium size paper, the same as the case (m).

[0063] (o) If the paper width detection result is extra small size paper and the specified paper size is small size paper, the same control as (k) is performed on the first sheet of paper P (paper P for which it has not been determined whether it is small size paper or extra small size paper). After that, for the second and subsequent sheets of paper P (paper P determined to be extra small size paper), the power ratio is set to 2:0:8 and TPD control is performed according to the detection result of sub-thermistor elements 60a, 61a, in other words, according to the paper width of paper P.

[0064] (If the detected paper width is larger than the specified paper size) Next, we will explain the cases where the detected paper width is larger than the specified paper size. This corresponds to (b), (c), (d), (g), (h), and (l) in Table 1.

[0065] (b) When the paper width detection result is large size paper and the specified paper size is medium size paper, the power supply ratio is 10:0:0, and the fixing operation is performed using only the heating element 54b1. Since the paper width to be conveyed is wider than the specified paper size, the area outside the specified paper size loses extra heat and the temperature becomes lower than when the paper P is conveyed exactly according to the specified paper size. As a result, the heat within the image size of the specified paper size also moves to the relatively low temperature area on the outside. Therefore, if the power supply ratio is set to match the specified paper size (4:6:0 to 2:8:0 as in (f)), the temperature of the fixing nip portion N at the end of the image size will drop, and there is a risk of causing fixing failure. Furthermore, the drop in temperature of the fixing nip portion N may cause unevenness in the sliding resistance inside the film 51 in the longitudinal direction, which may cause the film 51 to deform. In order to prevent fixing failure at the end and deformation of the film 51, the following control is required. In other words, before the paper P reaches the fixing nip N, it is determined whether the specified paper size is large size paper or medium size or smaller, and furthermore, it is necessary to increase the power supply ratio to the outer heating element 54b compared to when the paper width detection result is the specified paper size.

[0066] In the first embodiment, the pair of paper width sensors 31a, 31b, which are paper width detection means other than the innermost paper width detection means, are arranged upstream of the fixing nip N. Therefore, it is possible to detect that the passed paper P is wider than the designated paper size before the paper P reaches the fixing nip N. In addition, in the first embodiment, the power supply ratio to the larger heating element 54b is increased to 10:0:0 compared to the power supply ratio when the paper width detection result is the designated paper size (in this case (f)). With such a configuration and control, poor fixing or deformation of the film 51 due to a drop in temperature in the fixing nip N did not occur.

[0067] In addition, in the first embodiment, the power ratio is set to 10:0:0, but it may be set to 10:0:0 or less or 4:6:0 or more within a range where poor fixing or deformation of the film 51 does not occur. A drop in temperature leads to poor fixing or deformation of the film 51. However, if the ratio of the heating element 54b1 is increased too much, the temperature of the fixing nip N near the edge of the paper without an image increases, resulting in energy loss. In other words, by lowering the ratio of the heating element 54b1 as much as possible, for example to 4:6:0, the effect of reducing power consumption can be obtained. (c) When the paper width detection result is large size paper and the specified paper size is small size paper, and (d) when the paper width detection result is large size paper and the specified paper size is extra small size paper, the same is true for (b).

[0068] (g) If the paper width detection result is medium size paper and the specified paper size is small size paper, the fixing operation is performed with a current ratio of 2:8:0. (h) If the paper width detection result is medium size paper and the specified paper size is extra small size paper, the same as (g) is performed.

[0069] (l) When the paper width detection result is small size paper and the specified paper size is extra small size paper, the fixing operation is performed with a power ratio of 2:0:8. As in this configuration, the innermost paper width detection means does not need to be located upstream of the fixing nip N. This is because, regardless of whether the innermost paper width detection means is ON or OFF, the fixing operation is performed using the heating element 54b3 with the smallest width, so that even if the specified paper size is incorrect between small size paper and extra small size paper, the temperature of the fixing nip N is unlikely to drop.

[0070] As described above, in the first embodiment, the paper width detection means that is the innermost in the longitudinal direction among the multiple paper width detection means is disposed in the fixing nip portion N, and the paper width detection means other than the paper width detection means that is the innermost in the longitudinal direction are disposed upstream of the fixing nip portion N. With such a configuration, if the paper width of the paper P is larger than the specified paper size, it is possible to detect an error in the specified paper size before the paper P reaches the fixing nip portion N, and it is also possible to detect a temperature rise in the non-paper passing portion when a narrow paper P is passed through. Therefore, even if the specified paper size is incorrect, it is possible to reduce temperature unevenness in the longitudinal direction, thereby preventing poor fixing and deformation of the film 51, while selecting an appropriate throughput according to the width of the small size paper and maximizing productivity.

[0071] In addition, in the first embodiment, when the paper width detection result is smaller than the specified paper size, the power supply ratio to the heating element 54b having a width in the longitudinal direction larger than the width determined by the paper width detection result is increased compared to when the specified paper size is the same as the paper width detection result. By doing so, even if the specified paper size is incorrect, it is possible to reduce temperature unevenness in the longitudinal direction and prevent poor fixing at the paper edge and film deformation.

[0072] As described above, in the first embodiment, the width of the paper P is determined based on the detection results of the paper width sensors 31a1, 31a2, the paper width sensors 31b1, 31b2, and the fixing temperature sensors 60, 61. In other words, the configuration controls the ratio of the power supplied to the heating elements 54b1a, 54b2, and 54b3 based on the detection results of the paper width sensors 31a1, 31a2, the paper width sensors 31b1, 31b2, and the fixing temperature sensors 60, 61.

[0073] As described above, according to the first embodiment, it is possible to reduce temperature unevenness in the longitudinal direction of the fixing device. EXAMPLES

[0074] Example 2 will be described with reference to Fig. 9. Example 2 is different from Example 1 in that a sub-thermistor pair serving as both paper width detection means and temperature detection means is added to each of heating elements 54b1 and 54b2, and a paper width sensor pair serving as paper width detection means is added to heating element 54b3. Note that only the differences from the above-mentioned examples will be described here.

[0075] [Positional relationship between heating element and paper width sensor] The relationship between each heating element 54b and each paper width sensor 31 will be described with reference to Fig. 9. The sub-thermistors 62-65 function as temperature detection means and have sub-thermistor elements 62a-65a. The sub-thermistor elements 62a and 63a serve both as a third temperature detection means and as a paper width detection means corresponding to the heating element 54b1, and are arranged symmetrically with respect to the reference line a at a distance S1 = 198 mm. It can also be said that the sub-thermistor elements 62a and 63a are provided at positions corresponding to both ends of the heating element 54b1a.

[0076] The sub-thermistor elements 64a, 65a serve both as a fourth temperature detection means and as a paper width detection means corresponding to the heating element 54b2, and are arranged symmetrically with respect to the reference line a at a distance S2=170 mm. It can be said that the sub-thermistor elements 64a, 65a are provided at positions corresponding to both ends of the heating element 54b2. The sub-thermistor elements 62a-65a are arranged in the center of the heater 54 in the short direction, similar to the sub-thermistor elements 60a, 61a. All the sub-thermistor elements 60a-65a output the detection results to the CPU 94, and the CPU 94 judges the paper width of the paper P based on the change in temperature detected by the sub-thermistor elements 60a-65a when the paper P passes through, similar to the first embodiment.

[0077] The paper width sensors 31c1 and 31c2, which are the third paper width detection means, function as paper width detection means corresponding to the heating element 54b3. The paper width sensors 31c1 and 31c2 are arranged symmetrically with respect to the reference line a at a distance S3, and are arranged at approximately the same position as the paper width sensors 31a1 to 31b2 on the transport path Y. The paper width sensors 31c1 and 31c2 are also referred to as a paper width sensor pair 31c.

[0078] As described above, in the second embodiment, a total of six pairs of paper width detection means corresponding to each heating element 54b are provided. Specifically, the paper width sensor pairs 31a, 31b, and 31c are disposed upstream of the fixing nip portion N, and the sub-thermistor pairs 60-65, which also serve as temperature detection means, are disposed in the fixing nip portion N. In this case, the paper width sensor 31a1 corresponds to the sub-thermistor element 62a, and the paper width sensor 31a2 corresponds to the sub-thermistor element 63a. The paper width sensor 31b1 corresponds to the sub-thermistor element 64a, and the paper width sensor 31b2 corresponds to the sub-thermistor element 65a. The paper width sensor 31c1 corresponds to the sub-thermistor element 60a, and the paper width sensor 31c2 corresponds to the sub-thermistor element 61a.

[0079] [Control of current ratio] Next, the operation of the fixing device 50 in the second embodiment will be described with reference to Table 2. Table 2 shows the control of the power supply ratio when the fixing device 50 is in a hot state. Here again, only the points different from the first embodiment will be described.

[0080] [Table 2]

[0081] (Table 2: Selection of heating element 54b in hot state in Example 2) (When the paper width detection result matches the specified paper size) (f) When both the paper width detection result and the specified paper size are medium size paper, the heating elements 54b1 and 54b2 are heated at a constant power ratio of 4:6:0 to 2:8:0, and the ratio is controlled according to the paper width. At this time, the wider the paper P is among medium size paper, the smaller the temperature rise in the non-paper passing portion, and the narrower the paper is, the larger the temperature rise is. Therefore, in the first embodiment, if the specified paper size is incorrect even among medium size paper, a temperature drop occurs at the end, which may cause poor fixing. However, in the second embodiment, the sub-thermistor elements 62a and 63a arranged at a position where the paper P does not pass can detect the temperature rise in the non-paper passing portion and the temperature drop at the end. Therefore, even if the specified paper size is incorrect among medium size paper, it is possible to select an appropriate power ratio.

[0082] For example, consider the case where B5 (ISO) size (paper width 176 mm) is specified and 16K size (paper width 195 mm) is passed. In this case, the paper width sensors 31b1 and 31b2 cannot detect the difference between the specified paper size and the paper P that is actually being conveyed. That is, the paper width sensors 31b1 and 31b2 are both ON for both B5 paper P and 16K paper P, and it is not possible to distinguish the difference in size of medium-sized paper. In the second embodiment, when B5 size is specified, paper passing is started with a power ratio of 2:8:0. Then, since 16K, which is actually wider than B5, is actually passed, the temperature of the end of the fixing nip N drops compared to when B5 paper P is normally passed.

[0083] In the second embodiment, when the temperature of either of the sub-thermistor elements 62a and 63a drops by 40°C or more compared to the main thermistor element 59a, the energization ratio of the heating element 54b1 is increased and the energization ratio of the heating element 54b2 is decreased. By performing this control, the control started with the energization ratio of 2:8:0 gradually becomes the energization ratio of 4:6:0 by, for example, the fourth sheet of paper, and the temperature drop in the fixing nip portion N is eliminated. On the other hand, when a 16K sheet of paper P is specified but a B5 sheet of paper P is passed, the temperature rise in the non-paper passing portion becomes large. In the second embodiment, when the temperature of either of the sub-thermistor elements 62a and 63a reaches 230°C, the energization ratio of the heating element 54b1 is decreased and the energization ratio of the heating element 54b2 is increased. Therefore, the control that was initially started with a current ratio of 4:6:0 to match the specified 16K gradually changed to a current ratio of 2:8:0 by the fourth sheet of paper passing, for example, and the temperature rise in the non-paper passing areas in the fixing nip N was eliminated.

[0084] (k) When both the paper width detection result and the specified paper size are small size paper, control is started with a power distribution ratio that matches the specified paper size, as in the first embodiment. In addition, in the second embodiment, as in (f), even if there is a difference in size of small size paper, the temperature change can be detected by the sub-thermistor elements 64a, 65a arranged in a position where the paper P does not pass. Therefore, from the second sheet onwards, it is possible to pass the same small size paper at an appropriate power distribution ratio according to the paper width. Note that (a) and (p) are the same as in Table 1, and explanations are omitted.

[0085] (If the detected paper width is smaller than the specified paper size) (e) A case where the paper width detection result is a medium size paper and the specified paper size is a large size paper will be described. In the first embodiment, when the paper width detection result is a medium size, paper P with a paper width of S2 (170 mm) or more and S1 (198 mm) or less cannot be identified. For this reason, the power distribution ratio is set to 10:0:0 so that the toner image in the margin can be fixed even if the paper P with the maximum paper width among medium size papers is passed through, and the throughput is reduced by leaving a gap between the papers to reduce the temperature rise in the non-paper passing portion (Table 1). In the second embodiment, the sub-thermistor elements 62a and 63a arranged in the range where the paper P does not pass can detect the temperature rise in the non-paper passing portion, so that the power distribution ratio can be changed according to the paper width, as in the case of (f). The power distribution ratio is initially set to 10:0:0, and the power distribution ratio of the heating element 54b1 is reduced and the power distribution ratio of the heating element 54b2 is increased according to the temperature rise in the non-paper passing portion.

[0086] For example, when B5 size paper is passed, in the second embodiment, (f) if the designated paper size is also B5 size, the power supply ratio is started at 2:8:0, but (e) if the designated paper size is LTR size, the power supply ratio is started at 10:0:0. After that, when the temperature rise of the non-paper passing portion becomes large and either of the sub-thermistor elements 62a and 63a reaches 230°C, the power supply ratio to the heating element 54b1 is lowered and the power supply ratio to the heating element 54b2 is raised. For example, when the fourth sheet of paper is passed, the power supply ratio becomes about 2:8:0, the non-paper passing portion temperature rise is eliminated, and thereafter, no damage to the film 51 or the pressure roller 53 due to the non-paper passing portion temperature rise occurs. Furthermore, since the thermistor elements 62a and 63a at the end are kept at about 230°C, no fixing failure occurs even when toner is piled up in the marginal portion. Thus, in the second embodiment, it is possible to eliminate temperature unevenness without reducing throughput, compared to the first embodiment.

[0087] (i) When the paper width detection result is small size paper and the specified paper size is large size paper, printing begins with a power ratio of 2:8:0. Thereafter, when the detection result of sub-thermistor elements 64a, 65a exceeds 230°C, the power ratio of heating element 54b2 is decreased and the power ratio of heating element 54b3 is increased. In this case, as in the case of (e), the temperature unevenness can be eliminated without reducing throughput, and there is no occurrence of poor fixing or damage to components due to temperature rise in non-paper passing areas.

[0088] (j) When the paper width detection result indicates a small size paper and the specified paper size is a medium size paper, the same control as in (i) is performed. In this case, as in (i), the temperature unevenness can be eliminated without reducing the throughput, and there is no occurrence of fixing failure or damage to components due to temperature rise in non-paper passing areas.

[0089] (m) A case will be described where the paper width detection result is a very small size paper and the specified paper size is a large size paper. In the first embodiment, the paper width detection means corresponding to the heating element 54b3 was only the sub-thermistors 60a and 61a. Therefore, it was not possible to distinguish whether the paper width was a small size paper or a very small size paper until the first sheet of paper P passed through the fixing nip N. Therefore, the control was started from the current distribution ratio of 2:8:0 for small size paper, and after the first sheet of paper P passed through the fixing nip N, it was switched to the current distribution ratio of 2:0:8 for very small size paper.

[0090] In the second embodiment, not only the sub-thermistors 60a and 61a but also the paper width sensors 31c1 and 31c2 are added upstream as a paper width detection means corresponding to the heating element 54b3. Therefore, the CPU 94 can determine that the paper width is a very small size paper before the first paper P reaches the fixing nip N. Therefore, it is possible to control according to the current ratio of 2:0:8 corresponding to very small size paper from the beginning, and then the throughput is reduced according to the temperature rise of the sub-thermistor elements 60a and 61a. In the second embodiment, the heat generated in the range where the first paper P does not pass is further reduced, the temperature rise of the non-paper passing part is reduced, and the timing of the throughput reduction can be delayed, and the initial productivity is improved. (n) and (o) are the same as (m). Note that when the paper width detection result is larger than the specified paper size, that is, (b), (c), (d), (g), (h), and (l), the control is the same as that described in Table 1 of the first embodiment, and the description will be omitted.

[0091] As described above, in addition to the effects of Example 1, Example 2 can further improve productivity when the paper sizes do not match, and can suppress temperature unevenness in the longitudinal direction of the fixing device 50. As described above, Example 2 can reduce temperature unevenness in the longitudinal direction of the fixing device. [Explanation of symbols]

[0092] 31a1, 31a2, 31b1, 31b2 Paper width sensor 50 Fixing device 51 Film 53 Pressure roller 54 Heater 54b1, 54b2, 54b3 Heating element 59 Main thermistor 60, 61 Sub-thermistor 94 CPU

Claims

1. An image forming apparatus having an image forming unit that forms a toner image on a sheet of paper, and a fixing device that heats the sheet of paper on which the toner image has been formed by the image forming unit in a fixing nip portion, thereby fixing the toner image to the sheet of paper, The fixing device is a heater including a first heating element, a second heating element having a length in a longitudinal direction shorter than that of the first heating element, and a third heating element having a length in the longitudinal direction shorter than that of the second heating element; a first rotating body heated by the heater; a second rotating body that sandwiches the first rotating body between itself and the heater to form the fixing nip portion; having The image forming apparatus includes: a first paper width detection unit for detecting a width of the paper, the first paper width detection unit being disposed upstream of the fixing nip portion in a paper transport direction, on an inner side of the first heating element and on an outer side of the second heating element in the longitudinal direction; a second paper width detection unit for detecting a width of the paper, the second paper width detection unit being disposed upstream of the fixing nip portion in a paper transport direction, on an inner side of the second heating element and on an outer side of the third heating element in the longitudinal direction; a temperature detector that detects a temperature of the fixing nip portion, the temperature detector being disposed at a position overlapping with the fixing nip portion in a paper conveying direction and on an inner side in the longitudinal direction of the third heating element; a determination means for determining a width of a sheet of paper based on detection results of the first paper width detection means, the second paper width detection means and the temperature detection means; An image forming apparatus comprising:

2. 2. The image forming apparatus according to claim 1, wherein the temperature detection means also serves as a paper width detection means for detecting a width of the paper.

3. the temperature detection means includes a first thermistor and a second thermistor; 2. The image forming apparatus according to claim 1, wherein the first thermistor and the second thermistor are disposed at positions corresponding to both ends of the third heating element in the longitudinal direction.

4. 4. The image forming apparatus according to claim 3, wherein the temperature detection means further comprises a third thermistor disposed between the first thermistor and the second thermistor in the longitudinal direction.

5. 2. The image forming apparatus according to claim 1, further comprising a control unit for determining a size of the paper based on the temperature detected by the temperature detection unit.

6. 2. The image forming apparatus according to claim 1, further comprising a control unit that controls a temperature of the fixing nip portion based on the temperature detected by the temperature detection unit.

7. 2. The image forming apparatus according to claim 1, further comprising a control unit that controls a ratio of power supply to a heating element in the heater based on the temperature detected by the temperature detection unit.

8. 2. The image forming apparatus according to claim 1, wherein the first paper width detection means and the second paper width detection means are disposed at positions overlapping each other in a paper transport direction.

9. 2. The image forming apparatus according to claim 1, wherein said first paper width detection means comprises a pair of paper width sensors provided at positions corresponding to both ends of said first heat generating element.

10. 2. The image forming apparatus according to claim 1, wherein said second paper width detection means comprises a pair of paper width sensors provided at positions corresponding to both ends of said second heat generating element.

11. 2 . The image forming apparatus according to claim 1 , wherein the heater includes a substrate on which the first heating element, the second heating element, and the third heating element are arranged in the transport direction.

12. 2. The image forming apparatus according to claim 1, wherein the first rotating body is a film.

13. the heater is provided so as to be in contact with an inner surface of the film, 13. The image forming apparatus according to claim 12, wherein the fixing nip portion is formed by the heater and the second rotating body via the film.

Images