Fixing device and image forming apparatus

Abstract

To maintain good fixability to less likely to cause flicker.SOLUTION: A fixing device is provided with a fixing roller 21 (fixing member) that heats a toner image T to fix it to a surface of a sheet P, and a pressure roller 31 (pressure member) that is in pressure contact with the fixing roller 21 to form a nip part through which the sheet P is conveyed. The fixing device is further provided with a plurality of center part heaters 26A-26C in the same shape that are formed to be able to heat a center part in a width direction of the fixing roller 21 and are different in wattage from each other, and a plurality of end heaters 27A-27C in the same shape that are formed to be able to heat both ends in the width direction of the fixing roller 21 and are different in wattage from each other.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a fixing device that heats a toner image to fix it on the surface of a sheet, and to an image forming apparatus including the fixing device, such as a copying machine, a printer, a facsimile, or a combination machine thereof. [Background technology]

[0002] 2. Description of the Related Art Conventionally, in a fixing device of an image forming apparatus such as a copier or a printer, a technique of installing a rod-shaped heater in a hollow portion of a fixing member (fixing roller) has been known (see, for example, Patent Document 1).

[0003] More specifically, in the fixing device, a pressure roller (pressure member) is pressed against a fixing roller (fixing member) to form a nip portion (fixing nip) through which a sheet (paper) is transported. The fixing roller is heated by a heater installed inside. The toner image on the sheet transported to the nip portion is fixed onto the sheet by the heat received from the fixing roller and the pressure of the nip portion.

[0004] Meanwhile, Patent Document 1 discloses a technique in which two heaters with different power capacities are installed inside a heating roller (fixing member). When forming a monochrome image, the heater with the larger power capacity is turned on, and when forming a color image, the heater with the smaller power capacity is turned on. Summary of the Invention [Problem to be solved by the invention]

[0005] Conventional fixing devices have been unable to sufficiently reduce the occurrence of flicker (a phenomenon in which fluorescent lights flicker in a room in which a device connected to a commercial power source is installed) while maintaining good fixing performance.

[0006] The present invention has been made in order to solve the above-mentioned problems, and has an object to provide a fixing device and an image forming apparatus in which good fixing performance is maintained and flicker is unlikely to occur. [Means for solving the problem]

[0007] The fixing device of the present invention includes a fixing member that heats a toner image and fixes it on the surface of a sheet, a pressure member that presses against the fixing member to form a nip portion through which the sheet is conveyed, a plurality of center heaters that are formed to be able to heat the center portion in the width direction of the fixing member and have different wattages and the same shape, and a plurality of end heaters that are formed to be able to heat both ends in the width direction of the fixing member and have different wattages and the same shape. one center heater among the plurality of center heaters and one end heater among the plurality of end heaters are combined as a heater set in order of wattage, and the fixing member is heated by one heater set selected from the plurality of heater sets, and at least one of the following three heating controls is executed: when the ambient temperature is low, the fixing member is heated by the heater set with a higher wattage than when the ambient temperature is high; when the voltage applied to the heater set is low, the fixing member is heated by the heater set with a higher wattage than when the voltage applied to the heater set is high; and when the thickness of the sheet transported to the nip portion is thick, the fixing member is heated by the heater set with a higher wattage than when the thickness of the sheet transported to the nip portion is thin. It is something. Effect of the Invention

[0008] According to the present invention, it is possible to provide a fixing device and an image forming apparatus that maintain good fixing performance and are less likely to cause flicker. [Brief description of the drawings]

[0009] [Figure 1] 1 is an overall configuration diagram showing an image forming apparatus according to an embodiment of the present invention; [Diagram 2] FIG. 2 is a diagram illustrating a configuration of a fixing device. [Diagram 3] FIG. 2 is a schematic diagram showing the fixing roller in the width direction. [Figure 4] FIG. 2 is an enlarged view showing the inside of the fixing roller. [Diagram 5] FIG. 11 is a table showing the relationship between the ambient temperature and the heater used. [Figure 6] FIG. 11 is a table showing the relationship between applied voltage and heater used. [Figure 7] FIG. 11 is a table showing the relationship between the sheet type and the heater used. [Figure 8] 13 is a diagram showing a configuration of a fixing device as a modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and the duplicated description will be appropriately simplified or omitted.

[0011] First, the overall configuration and operation of an image forming apparatus 1 will be described with reference to FIG. In FIG. 1, reference numeral 1 denotes a tandem type color copier as an image forming apparatus, reference numeral 2 denotes a writing unit that emits laser light based on input image information, reference numeral 3 denotes a document transport unit that transports a document D to a document reading unit 4, and reference numeral 4 denotes a document reading unit that reads image information of the document D. Additionally, 7 indicates a paper feed section in which sheets P such as paper are stored, 9 indicates a registration roller (timing roller) that adjusts the timing of transporting sheets P, and 11Y, 11M, 11C, and 11BK indicate photosensitive drums on which toner images of each color (yellow, magenta, cyan, and black) are formed.

[0012] Further, reference numeral 12 denotes a charging section which charges the surface of each photoconductor drum 11Y, 11M, 11C, and 11BK, reference numeral 13 denotes a developing section which develops the electrostatic latent image formed on the surface of each photoconductor drum 11Y, 11M, 11C, and 11BK, reference numeral 14 denotes a primary transfer roller which transfers the toner image formed on the surface of each photoconductor drum 11Y, 11M, 11C, and 11BK onto the surface of sheet P, and reference numeral 15 denotes a cleaning section which collects untransferred toner remaining on the surface of each photoconductor drum 11Y, 11M, 11C, and 11BK. In addition, reference numeral 16 denotes an intermediate transfer belt cleaning section that cleans the intermediate transfer belt 17, reference numeral 17 denotes the intermediate transfer belt onto which multiple color toner images are transferred in superimposed order, reference numeral 18 denotes a secondary transfer roller for transferring the color toner image on the intermediate transfer belt 17 onto the sheet P, and reference numeral 20 denotes a fixing device that fixes the toner image (unfixed image) on the sheet P.

[0013] Hereinafter, the operation (printing operation) during normal color image formation in the image forming apparatus will be described. First, the document D is transported from the document table by the transport rollers of the document transport unit 3 and placed on the contact glass 5 of the document reading unit 4. Then, the image information of the document D placed on the contact glass 5 is optically read by the document reading unit 4.

[0014] More specifically, the document reading unit 4 scans the image of the document D on the contact glass 5 while irradiating it with light emitted from an illumination lamp. The light reflected by the document D is then imaged on a color sensor via a group of mirrors and a lens. The color image information of the document D is read by the color sensor for each color separation light of RGB (red, green, blue), and then converted into an electrical image signal. Furthermore, based on the RGB color separation image signal, the image processing unit performs processing such as color conversion processing, color correction processing, and spatial frequency correction processing to obtain color image information of yellow, magenta, cyan, and black.

[0015] Then, the image information for each color of yellow, magenta, cyan, and black is transmitted to the writing unit 2. Then, from the writing unit 2, laser light (exposure light) based on the image information for each color is emitted toward the surfaces of the corresponding photoconductor drums 11Y, 11M, 11C, and 11BK.

[0016] Meanwhile, the four photoconductor drums 11Y, 11M, 11C, and 11BK each rotate in the counterclockwise direction in Fig. 1. First, the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK are uniformly charged at the portions facing the charging unit 12 (charging process). Thus, a charging potential is formed on the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK. Thereafter, the surfaces of the charged photoconductor drums 11Y, 11M, 11C, and 11BK reach the irradiation positions of the respective laser beams (exposure process). More specifically, in the writing unit 2, laser beams corresponding to the image signals are emitted from four light sources, one for each color. Each laser beam passes through a separate optical path for each color component of yellow, magenta, cyan, and black.

[0017] The laser light corresponding to the yellow component is irradiated onto the surface of the first photoconductor drum 11Y from the left side of the drawing. At this time, the laser light of the yellow component is scanned in the direction of the rotation axis (main scanning direction, width direction) of the photoconductor drum 11Y by a polygon mirror rotating at high speed. In this way, an electrostatic latent image corresponding to the yellow component is formed on the photoconductor drum 11Y after it has been charged by the charging unit 12.

[0018] Similarly, the laser light corresponding to the magenta component is irradiated onto the surface of the second photoconductor drum 11M from the left on the paper, forming an electrostatic latent image corresponding to the magenta component. The laser light corresponding to the cyan component is irradiated onto the surface of the third photoconductor drum 11C from the left on the paper, forming an electrostatic latent image of the cyan component. The laser light corresponding to the black component is irradiated onto the surface of the fourth photoconductor drum 11BK from the left on the paper, forming an electrostatic latent image of the black component.

[0019] Thereafter, the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK on which the electrostatic latent images of each color are formed reach positions facing the developing unit 13. Then, toner of each color is supplied from each developing unit 13 to the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK, and the latent images formed on the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK are developed (this is the developing process). Thereafter, the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK after the developing process reach the portions facing the intermediate transfer belt 17. Here, primary transfer rollers 14 are provided at each of the facing portions so as to abut against the inner circumferential surface of the intermediate transfer belt 17. Then, at the position of the primary transfer rollers 14, the toner images of each color formed on the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK are transferred onto the intermediate transfer belt 17 in order, superimposed on top of each other (this is the primary transfer process).

[0020] After the transfer process, the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK each reach a position facing the cleaning unit 15. Then, the cleaning unit 15 collects untransferred toner remaining on the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK (this is the cleaning process). Thereafter, the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK pass through a charge removing section, and a series of image forming processes on the photoconductor drums 11Y, 11M, 11C, and 11BK are completed.

[0021] Meanwhile, the intermediate transfer belt 17, onto which the toner of each color is transferred (carried) in an overlapping manner on the surfaces of the photoconductor drums 11Y, 11M, 11C, and 11BK, travels in the clockwise direction in FIG. 1 and reaches a position facing the secondary transfer roller 18. Then, at the position facing the secondary transfer roller 18, the color toner images carried on the intermediate transfer belt 17 are transferred onto the sheet P (secondary transfer process). Thereafter, the surface of the intermediate transfer belt 17 reaches the position of the intermediate transfer belt cleaning unit 16. Then, the untransferred toner adhering to the intermediate transfer belt 17 is collected by the intermediate transfer belt cleaning unit 16, and the series of transfer processes on the intermediate transfer belt 17 is completed.

[0022] Here, the sheet P conveyed between the intermediate transfer belt 17 and the secondary transfer roller 18 (secondary transfer nip) is conveyed from the paper feed section 7 via the registration rollers 9 and the like. Specifically, the sheet P fed by a feed roller 8 from a feed unit 7 storing the sheet P passes through a transport path and is then guided to a registration roller 9. The sheet P that has reached the registration roller 9 is transported in a timely manner toward the secondary transfer nip.

[0023] Then, the sheet P onto which the full-color image has been transferred is guided by a conveyor belt to the fixing device 20. In the fixing device 20, the color image (toner image) is fixed onto the surface of the sheet P at a nip portion (fixing nip) between a fixing roller and a pressure roller (fixing process). Then, the sheet P after the fixing process is discharged as an output image to the outside of the apparatus main body 1 by a paper discharge roller, and a series of image forming processes (printing operations) is completed.

[0024] Next, the configuration and operation of the fixing device 20 installed in the image forming apparatus main body 1 will be described in detail with reference to FIG. 2 etc. As shown in FIG. 2, the fixing device 20 is composed of a fixing roller 21 as a fixing member, a heater group 25 as a heating means, a pressure roller 31 as a pressure member, a central temperature sensor 40 as a central temperature detection means for detecting the temperature (surface temperature) of the central portion of the width of the fixing roller 21, and an end temperature sensor 41 as an end temperature detection means for detecting the temperature (surface temperature) of the end portion of the width of the fixing roller 21.

[0025] Here, the fixing roller 21 (fixing rotor) as the fixing member is a multi-layer roller member in which a coating layer 21b (a laminate of an elastic layer and a release layer) is formed on a hollow core metal 21a made of a metal material such as stainless steel, and is pressed against a pressure roller 31 as a pressure member to form a nip portion (fixing nip). The elastic layer in the covering layer 21b of the fixing roller 21 is made of an elastic material such as fluororubber, silicone rubber, or foamed silicone rubber. The release layer in the covering layer 21b of the fixing roller 21 is made of PFA (tetrafluoroethylene perfluoroalkylvinylether copolymer resin) or the like. By providing a release layer on the surface layer of the fixing roller 21, releasability (peelability) for the toner T (toner image) is ensured. The fixing roller 21 is rotated clockwise in FIG. 2 by a drive motor 51.

[0026] A heater group 25 serving as a heating means is fixedly provided inside (hollow portion) of the fixing roller 21 (core metal 21a) having a hollow structure. The heater group 25 is made up of a plurality of heaters 26A-26C, 27A-27C (halogen heaters) formed in a rod shape, and both ends of the heaters are fixed to the side plates of the fixing device 20. When the main switch of the image forming apparatus main body 1 is turned on (power on), power is supplied from a power supply unit 91 to one heater set 25A-25C selected from the heater group 25. The fixing roller 21 is heated by radiant heat from the heater sets 25A-25C whose output is controlled (PID controlled) by a control unit 90, and heat is applied to the toner image T on the sheet P from the surface of the heated fixing roller 21. The heater group 25 (heater sets 25A to 25C) will be described in detail later.

[0027] The pressure roller 31 (pressure rotating body) serving as a pressure member is a roller member mainly composed of a core metal 32 and an elastic layer 33 serving as a coating layer that covers the outer circumferential surface of the core metal 32. The elastic layer 33 (covering layer) of the pressure roller 31 is made of a material such as silicone rubber, foamed silicone rubber, etc. A thin release layer made of PFA or the like may be provided on the surface of the elastic layer 33, and the cover layer may be formed by the elastic layer 33 and the release layer. Then, the pressure roller 31 is pressed against the fixing roller 21 by a pressure mechanism (not shown). In this way, a desired nip portion is formed between the pressure roller 31 and the fixing roller 21. The pressure roller 31 is rotated counterclockwise in FIG. 2 in accordance with the rotation of the fixing roller 21.

[0028] The fixing device 20 configured as above operates as follows. When the main switch of the apparatus main body 1 is turned on, an AC voltage is applied (power is supplied) from the power supply unit 91 to the heater set (which is one of the three heater sets 25A to 25C). Then, when a print command (print request) is input, the fixing roller 21 starts to rotate clockwise by a drive motor (drive mechanism) (not shown), and the pressure roller 31 starts to rotate counterclockwise. After that, the sheet P is fed from the paper feed unit 7, and the toner image on the intermediate transfer belt 17 is carried on the sheet P as an unfixed image at the position of the secondary transfer roller 18. The sheet P carrying the unfixed image T (toner image) is transported in the direction of the arrow in FIG. 2 and is fed into the nip portion between the fixing roller 21 and the pressure roller 31, which are in a pressure-contact state. Then, the toner image T is fixed on the surface of the sheet P by the heating by the fixing roller 21 and the pressing force of the fixing roller 21 and the pressure roller 31. After the fixing process, the sheet P is sent out from the nip portion in the direction of the arrow by the rotating fixing roller 21 and the pressure roller 31.

[0029] The characteristic configuration and operation of the fixing device 20 (image forming apparatus 1) in this embodiment will be described in detail below. As described above with reference to FIG. 2 and the like, the fixing device 20 in this embodiment is provided with a heater group 25 as heating means for heating the fixing roller 21 serving as a fixing member. 3, 4, etc., the heater group 25 is made up of a plurality of central heaters 26A to 26C and a plurality of end heaters 27A to 27C.

[0030] The central heaters 26A to 26C are all halogen heaters and are formed to be capable of heating the central portion (range N in FIG. 3) in the width direction of the fixing roller 21 (fixing member). The central heaters 26A to 26C have different wattages and are the same shape. Specifically, in this embodiment, three center heaters 26A to 26C having the same outer shape such as widthwise length, outer diameter, and heater range are installed in the hollow part of the fixing roller 21. Although the center heaters 26A to 26C are formed so that the widthwise length is almost equal to the widthwise length of the fixing roller 21, the part that functions as a heater (main heater part) is formed only in the center. In addition, the wattage of the first center heater 26A is set to 850W (watts), the wattage of the second center heater 26B is set to 800W, and the wattage of the third center heater 26C is set to 750W.

[0031] The end heaters 27A to 27C are all halogen heaters and are formed to be capable of heating both ends in the width direction of the fixing roller 21 (fixing member) (a range slightly wider than the range M in FIG. 3 excluding the range N). The end heaters 27A to 27C have different wattages and are the same shape. Specifically, in this embodiment, three end heaters 27A to 27C having the same outer shape such as widthwise length, outer diameter, and heater range are installed in the hollow part of the fixing roller 21. Although the end heaters 27A to 27C are formed so that the widthwise length is almost equal to the widthwise length of the fixing roller 21, the parts that function as heaters (main heater parts) are formed only at both ends. In addition, the wattage of the first end heater 27A is set to 650W (watts), the wattage of the second end heater 27B is set to 600W, and the wattage of the third end heater 27C is set to 550W.

[0032] In this embodiment, the range N of the "widthwise center" of the fixing roller 21 is the widthwise center position X (see FIG. 3) or its vicinity, and is the paper passing area for the smallest size of sheet P that can be passed, that is, the portion that can be the paper passing area for all sizes of sheet P that can be passed. The center temperature sensor 40 is disposed in this range N (the center position X in this embodiment). Moreover, in the range (MN) of the "both ends in the width direction" of the fixing roller 21 described above, the range M includes the paper passing area of ​​the maximum size of the sheet P that can be passed, and is set to a range slightly wider than that. And, the end temperature sensor 41 is disposed in this range M (only one end side in this embodiment).

[0033] In this embodiment, one central heater among the plurality of central heaters 26A to 26C and one end heater among the plurality of end heaters 27A to 27C are combined as heater sets 25A to 25C in order of wattage. Specifically, in this embodiment, three heater sets 25A to 25C are provided. The first heater set 25A is made up of a first center heater 26A (850 W) and a first end heater 27A (650 W), which are a combination of heaters with the largest wattages. The second heater set 25B is made up of a second center heater 26B (800 W) and a second end heater 27B (600 W), which are a combination of medium wattage heaters. The third heater set 25C is made up of a third center heater 26C (750 W) and a third end heater 27C (550 W), which are a combination of heaters with the smallest wattage.

[0034] In this embodiment, the fixing roller 21 (fixing member) is heated by one heater set selected from the plurality of heater sets 25A to 25C. In detail, based on predetermined conditions, the control unit 90 applies voltage to the power supply unit 91 and controls the switching of the switching circuit 93 to perform the fixing process by applying voltage only to the first heater set 25A, the fixing process by applying voltage only to the second heater set 25B, or the fixing process by applying voltage only to the third heater set 25C.

[0035] Furthermore, when any of the heater sets 25A to 25C is used for the fixing process, the central heaters 26A to 26C are controlled to be turned on and off based on the detection result of the roller surface temperature by the central temperature sensor 40 (central temperature detection means) that faces the surface of the fixing roller 21 without contacting it. In detail, an AC voltage is applied from the power supply unit 91 to one central heater (for example, the first central heater 26A) selected for a current-carrying time determined based on the detection result of the central temperature sensor 40 (thermopile). By controlling the output of the central heater in this way, the temperature (fixing temperature) at the central part of the fixing roller 21 can be adjusted and controlled to a desired temperature (target control temperature). Similarly, when any of the heater sets 25A to 25C is used for the fixing process, the end heaters 27A to 27C are controlled to be turned on and off based on the detection result of the roller surface temperature by the end temperature sensor 41 (end temperature detection means) that faces the surface of the fixing roller 21 without contacting it. In detail, an AC voltage is applied from the power supply unit 91 to one selected end heater (for example, the first end heater 27A) for a current-carrying time determined based on the detection result of the end temperature sensor 41 (thermopile). By controlling the output of the end heater in this way, the temperature (fixing temperature) at both ends of the fixing roller 21 can be adjusted and controlled to a desired temperature (target control temperature). By performing temperature control based on the detection results of the central temperature sensor 40 and temperature control based on the detection results of the end temperature sensor 41, the fixing roller 21 can be adjusted and controlled to a desired temperature across the entire width direction.

[0036] In this embodiment, when the ambient temperature is low, the fixing roller 21 (fixing member) is controlled to be heated by the heater sets 25A to 25C having a higher wattage than when the ambient temperature is high. More specifically, with reference to FIGS. 1 and 2, image forming apparatus 1 is provided on its exterior with a thermometer 70 (ambient temperature detection means) for detecting the ambient temperature. As shown in FIG. 5, when the temperature T detected by the thermometer 70 is equal to or lower than the first temperature T1 (low temperature), the fixing process is performed using the first heater set 25A (first central heater 26A, first end heater 27A) which is the combination with the highest wattage and the highest heat generation. On the other hand, when the temperature T detected by the thermometer 70 is higher than the first temperature T1 and lower than the second temperature T2 (>T1) (normal temperature), the fixing process is performed using the second heater set 25B (second central heater 26B, second end heater 27B) which is the combination with a medium wattage and a medium heat generation. Furthermore, when the temperature T detected by the thermometer 70 is equal to or higher than the second temperature T2 (high temperature), the fixing process is performed using the third heater set 25C (third central heater 26C, third end heater 27C) which is the combination with the lowest wattage and the lowest heat generation.

[0037] The reason for this control is that at low temperatures the heater capacity tends to be insufficient, making it difficult to obtain the amount of heat required to maintain the fixing roller 21 at the desired temperature, and at high temperatures the heater capacity tends to be excessive, causing frequent on-off switching in order to maintain the fixing roller 21 at the desired temperature, making flicker more likely to occur. In this embodiment, the occurrence of these problems is reduced because an optimal heater is selected from among the three heater sets 25A to 25C having different wattages (heat generation amounts) according to the ambient temperature. Specifically, since the fixing process is performed using the first heater set 25A with high heating capacity at low temperatures, it is easy to obtain the amount of heat required to maintain the fixing roller 21 at a desired temperature. Also, since the fixing process is performed using the third heater set 25C with low heating capacity at high temperatures, the heater is easily turned on continuously without being frequently turned on and off to maintain the fixing roller 21 at a desired temperature, so flicker is less likely to occur.

[0038] Furthermore, in this embodiment, when the voltage applied from power supply unit 91 to heater sets 25A-25C is low, the fixing roller 21 (fixing member) is controlled to be heated by heater sets 25A-25C having a higher wattage than when the voltage applied from power supply unit 91 to heater sets 25A-25C is high. More specifically, referring to FIG. 2, image forming apparatus 1 is provided with a voltage detection section 92 that detects the magnitude of the voltage applied from power supply section 91 to heater sets 25A to 25C. 6, when the voltage V detected by the voltage detection unit 92 is equal to or lower than the first voltage V1 (at low voltage such as during a voltage drop), the fixing process is performed using the first heater set 25A (first central heater 26A, first end heater 27A) which is the combination with the highest wattage and the highest heat generation. On the other hand, when the voltage V detected by the voltage detection unit 92 is higher than the first voltage V1 and lower than the second voltage V2 (>V1) (at normal voltage), the fixing process is performed using the second heater set 25B (second central heater 26B, second end heater 27B) which is the combination with medium wattage and medium heat generation. Furthermore, when the voltage V detected by the voltage detection unit 92 is equal to or higher than the second voltage V2 (at high voltage such as when the voltage is rising), the fixing process is performed using the third heater set 25C (third central heater 26C, third end heater 27C), which is the combination with the lowest wattage and heat generation.

[0039] The reason for this control is that when the voltage is low, the heater capacity tends to be insufficient, making it difficult to obtain the amount of heat required to maintain the fixing roller 21 at the desired temperature, and when the voltage is high, the heater capacity tends to be excessive, causing the heater to frequently cycle on and off in order to maintain the fixing roller 21 at the desired temperature, making flicker more likely to occur. In the present embodiment, the optimum heater is selected from among the three heater sets 25A to 25C having different wattages (calorific value) according to the applied voltage, and thus the occurrence of these problems is reduced. Specifically, since the fixing process is performed using the first heater set 25A with high heating capacity at low voltages, it is easy to obtain the amount of heat required to maintain the fixing roller 21 at a desired temperature. Also, since the fixing process is performed using the third heater set 25C with low heating capacity at high voltages, the heaters are not frequently turned on and off to maintain the fixing roller 21 at a desired temperature, but are easily turned on continuously, which makes it difficult for flicker to occur.

[0040] In addition, in this embodiment, when the thickness of the sheet P transported to the nip portion (fixing nip) is thick, the fixing roller 21 (fixing member) is controlled to be heated by the heater sets 25A to 25C having a higher wattage than when the thickness of the sheet P transported to the nip portion (fixing nip) is thin. In detail, referring to Figures 1 and 2, when the user operates the operation panel 100, which functions as a thickness detection means, to input information regarding the type (thickness) of the sheet P to be passed through, the control unit 90 grasps the thickness of the sheet P based on that information. 7, when the type of sheet P being passed is thick paper (when thick paper is passed), the fixing process is performed using the first heater set 25A (first center heater 26A, first end heater 27A) which is the combination with the highest wattage and the highest heat generation. On the other hand, when the type of sheet P being passed is plain paper (when plain paper is passed), the fixing process is performed using the second heater set 25B (second center heater 26B, second end heater 27B) which is the combination with a medium wattage and a medium heat generation. Furthermore, when the type of sheet P being passed is thin paper (when thin paper is passed), the fixing process is performed using the third heater set 25C (third center heater 26C, third end heater 27C) which is the combination with the lowest wattage and the lowest heat generation.

[0041] The reason for this type of control is that when thick paper is passed through, the heater capacity tends to be insufficient, making it difficult to obtain the amount of heat required to maintain the fixing roller 21 at the desired temperature, and when thin paper is passed through, the heater capacity tends to be excessive, causing the heater to cycle on and off frequently in order to maintain the fixing roller 21 at the desired temperature, making it more likely to cause flicker. In the present embodiment, the optimum heater set is selected from the three heater sets 25A to 25C having different wattages (calorific value) according to the thickness of the sheet P, and therefore the occurrence of these problems is reduced. Specifically, when thick paper is passed, the fixing process is performed using the first heater set 25A with high heating capacity, making it easier to obtain the amount of heat required to maintain the fixing roller 21 at the desired temperature. Also, when thin paper is passed, the fixing process is performed using the third heater set 25C with low heating capacity, so the heater is more likely to be continuously lit without frequent on / off switching to maintain the fixing roller 21 at the desired temperature, making it less likely to cause flicker.

[0042] When the control conditions described with reference to FIGS. 5 to 7 overlap, it may not be possible to select the optimal one from among the three heater sets 25A to 25C if things remain as they are. For example, when the ambient temperature T is equal to or lower than the first temperature T1 and the applied voltage V is equal to or higher than the second voltage V2, it is not possible to determine whether to select the first heater set 25A or the third heater set 25C. In this embodiment, in order to prevent such a problem, priorities are set for three conditions (ambient temperature T, applied voltage V, and thickness of sheet P) for selecting the optimal one of the three heater sets 25A to 25C. For example, among the three conditions, the conditions with the highest priority are the ambient temperature T, the applied voltage V, and the thickness of sheet P. Therefore, in the above example (where the ambient temperature T is equal to or lower than the first temperature T1 and the applied voltage V is equal to or higher than the second voltage V2), the first heater set 25A is selected. Also, instead of such control based on priority, when the three control conditions overlap, control may be performed to select the most suitable one of the three heater sets 25A to 25C based on a predetermined program.

[0043] Thus, the fixing device 20 in this embodiment is provided with a plurality of center heaters 26A-26C having different wattages but the same shape, and a plurality of end heaters 27A-27C having different wattages but the same shape. This makes it possible to reduce the occurrence of flicker (a phenomenon in which fluorescent lights flicker in a room where a device connected to a commercial power source is installed) while maintaining good fixability. By providing multiple heater sets 25A-25C (center heaters 26A-26C, end heaters 27A-27C) with the same shape but different wattages, it is possible to select the most suitable heater set each time depending on the difference in conditions such as the ambient temperature T, applied voltage V, and sheet type while keeping the same control conditions such as the target fixing temperature. Therefore, it is possible to create a situation in which the fixing process is performed by switching between three fixing devices with different wattage (heat amount) settings with one fixing device. Therefore, the fixing process can be performed with the optimal wattage (heat amount) to maintain good and stable fixability of the fixed image, and the occurrence of flicker can be reduced by controlling the number of on / off switching times to be reduced at the optimal wattage. Furthermore, by providing multiple central heaters 26A-26C and multiple end heaters 27A-27C rather than providing multiple continuous heaters that are not divided into a central portion and both ends across the width, the fixing roller 21 can be heated efficiently and in a balanced manner across the width even if there is a difference in the temperature rise characteristics between the both ends and the central portion of the fixing roller 21.

[0044] In particular, in this embodiment, heater sets 25A-25C are configured so that the wattage of center heaters 26A-26C is higher than the wattage of end heaters 27A-27C. Specifically, in the first heater set 25A, the wattage (850 W) of the first central heater 26A is set higher than the wattage (650 W) of the first end heater 27A, in the second heater set 25B, the wattage (800 W) of the second central heater 26B is set higher than the wattage (600 W) of the second end heater 27B, and in the third heater set 25C, the wattage (750 W) of the third central heater 26C is set higher than the wattage (550 W) of the third end heater 27C. This is because the temperature of both ends of the fixing roller 21 increases more easily than that of the center due to heat transfer from the center. With this configuration, the fixing roller 21 can be heated efficiently and in a balanced manner across its width.

[0045] <Modification> As shown in FIG. 8, the fixing device 20 in the modified example is a belt-type fixing device (a fixing device using a fixing belt 22 as a fixing member), and includes a heater group 25 (a plurality of heater sets 25A-25C) as a heating means disposed inside a heating roller 24. In detail, the fixing belt 22 as a fixing section member is stretched and supported by a plurality of roller members such as a fixing auxiliary roller 23, a heating roller 24, and a tension roller. The fixing auxiliary roller 23 is pressed against a pressure roller 31 via the fixing belt 22 to form a nip portion. Furthermore, a heater group 25 for indirectly heating the fixing belt 22 as a fixing member is fixedly disposed inside the heating roller 24 having a hollow structure. In the modified example, the heater group 25 is also composed of a plurality of central heaters 26A to 26C and a plurality of end heaters 27A to 27C. This maintains good fixation and reduces the occurrence of flicker.

[0046] As described above, the fixing device 20 in this embodiment is provided with the fixing roller 21 (fixing member) that heats the toner image T and fixes it on the surface of the sheet P, and the pressure roller 31 (pressure member) that forms a nip portion through which the sheet P is conveyed by pressing against the fixing roller 21. Also provided are a plurality of center heaters 26A-26C of the same shape and different wattages that are formed to be able to heat the center portion in the width direction of the fixing roller 21, and a plurality of end heaters 27A-27C of the same shape and different wattages that are formed to be able to heat both ends in the width direction of the fixing roller 21, respectively. This maintains good fixation and reduces the occurrence of flicker.

[0047] In this embodiment, the pressure roller 31 is used as the pressure member, but the pressure member is not limited to this, and for example, a pressure belt can also be used as the pressure member. In this embodiment, the end temperature sensor 41 is disposed at the left end with respect to the central position X in FIG. In these cases, the same effects as those of this embodiment can be obtained.

[0048] It is clear that the present invention is not limited to the present embodiment, and that the present embodiment may be modified as appropriate within the scope of the technical concept of the present invention in addition to the modifications suggested in the present embodiment. Furthermore, the number, position, shape, etc. of the components are not limited to the present embodiment, and may be any number, position, shape, etc. suitable for implementing the present invention.

[0049] In this specification, the term "width direction" is defined as a direction perpendicular to the sheet conveying direction. In the present specification, the term "sheet" is defined to include not only paper (paper), but also all sheet-like recording media such as coated paper, label paper, OHP sheets, and film sheets. [Explanation of symbols]

[0050] 1 Image forming apparatus (image forming apparatus main body), 20 Fixing device, 21 Fixing roller (fixing member), 25 heater group, 25A~25C heater set, 26A~26C Center heater, 27A~27C End heaters, 31 pressure roller (pressure member), 40 central temperature sensor (central temperature detection means), 41 end temperature sensor (end temperature detection means), 70 Thermometer (ambient temperature detection means), P sheet (recording medium).

Prior Art Documents

Patent Documents

[0051]

Patent Document 1

Claims

1. a fixing member for heating the toner image to fix it on the surface of the sheet; a pressing member that forms a nip portion through which a sheet is conveyed by being pressed against the fixing member; a plurality of center heaters each having a different wattage and the same shape, each of which is formed to be able to heat a center portion in a width direction of the fixing member; a plurality of end heaters each having a different wattage and the same shape, each end heater being formed to be able to heat both ends in the width direction of the fixing member; Equipped with one center heater of the plurality of center heaters and one end heater of the plurality of end heaters are combined into a heater set in order of wattage size; heating the fixing member with one heater set selected from the plurality of heater sets; a heating control for heating the fixing member with a heater set having a higher wattage when the ambient temperature is low, compared to when the ambient temperature is high; a heating control in which, when a voltage applied to the heater set is low, the fixing member is heated by a heater set having a higher wattage than when a voltage applied to the heater set is high; a heating control for heating the fixing member by a heater set having a higher wattage when the thickness of the sheet conveyed to the nip portion is thicker than when the thickness of the sheet conveyed to the nip portion is thin; The fixing device performs at least one of the above three heating controls.

2. The center heater and the end heater are both halogen heaters, 2. The fixing device according to claim 1, wherein the fixing member is a fixing roller having a hollow portion, and the plurality of center heaters and the plurality of end heaters are disposed in the hollow portion.

3. A fixing device as described in claim 1 or claim 2, characterized in that the heater set is configured so that the wattage of the central heater is higher than the wattage of the end heaters.

4. An image forming apparatus comprising a fixing device according to any one of claims 1 to 3.

Images