Image forming device

Abstract

To improve print quality.SOLUTION: A color printer 1 comprises: a printing rate assessment unit 74 for assessing a printing rate TC of an image formed in a printing rate calculation region PCS which is the region of a recording medium P that corresponds to a cell eye region CS regarding a main scan direction Drm; and a fixation control unit 64 which, when the printing rate TC of the printing rate calculation region PCS is a high printing rate greater than or equal to a printing rate threshold, sets a set temperature TmpS of a heating cell 40 which is adjacent to the cell eye region CS corresponding to the printing rate calculation region PCS, on at least one side in the main scan direction Drm, to be higher than the original set temperature TmpS of the heating cell 40.SELECTED DRAWING: Figure 10

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, and is suitably applied to a fixing unit mounted in, for example, an electrophotographic image forming apparatus. [Background technology]

[0002] Conventionally, in electrophotographic image forming apparatuses, those which charge the surface of a photosensitive drum, expose the surface of the charged photosensitive drum to light to form an electrostatic latent image, attach toner to the electrostatic latent image to form a toner image, transfer the toner image to a medium, and fix it by a fixing device have been widely used. Such fixing devices include a heating rotator heated by a heating source such as a heater, and a pressure rotator which presses the heating rotator to form a nip portion between the heating rotator and the heating rotator. There are various sizes of recording media to be printed, and for example, when printing on a narrow recording medium, the temperature rise of the non-paper passing portion becomes a problem. For this reason, some image forming apparatuses divide the heat generating portion of the heating source into multiple portions along the main scanning direction, and independently control the ON / OFF of each heat generating portion (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-197653 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in a heat source composed of multiple heat generating parts, in order to ensure insulation between the heat generating parts, it is necessary to provide a space between the heat generating parts and an inter-heating part area between adjacent heat generating parts. This inter-heating part area generates less heat because there is no heat generating resistive heating wiring that forms the heat generating part. For this reason, the fixability of the inter-heating part area is worse than other areas, and there is a possibility that poor fixation (cold offset) may occur, especially when printing with a high print rate.

[0005] The present invention has been made in consideration of the above points, and aims to propose an image forming apparatus capable of improving print quality. [Means for solving the problem]

[0006] In order to solve the above problems, the image forming apparatus of the present invention has a heating source having a plurality of heating portions arranged along a main scanning direction and divided from one another, with a heating portion inter-area formed between adjacent heating portions in the main scanning direction, a heating rotor heated by the heating source, and a pressure rotor in contact with an outer circumferential surface of the heating rotor, and has a fixing device that holds a recording material carrying an image in a nip portion formed between the heating rotor and the pressure rotor, and heats and fixes the image by passing the recording material through the nip portion, The region is surrounded by a range in the main scanning direction that is the same as the region between the heat generating portions on the recording material, and a range from one end to the other end in the sub-scanning direction perpendicular to the main scanning direction on the recording material. The printer is provided with a printing rate determination unit that determines the printing rate of an image formed in a printing rate calculation area, which is a region, and a control unit that, when the printing rate of the printing rate calculation area is a high printing rate equal to or higher than a predetermined threshold value, sets the set temperature of a heating unit adjacent to at least one side of the inter-heating unit area corresponding to the printing rate calculation area in the main scanning direction to a temperature higher than the original set temperature of the heating unit.

[0007] According to the present invention, the amount of heat generated in the inter-heating portion regions of the heating rotator can be compensated for by the heat generating portions adjacent to the inter-heating portion regions, and a decrease in temperature in the inter-heating portion regions can be suppressed. Effect of the Invention

[0008] According to the present invention, it is possible to realize an image forming apparatus capable of improving print quality. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a right side view showing the configuration of the color printer. [Diagram 2] 1A and 1B show the configuration of the fixing unit, in which FIG. [Diagram 3]4A and 4B are bottom views showing the configuration of a heater, where (A) is the overall configuration of the heater, and (B) is a partially enlarged view of (A). [Figure 4] FIG. 2 is a block diagram showing a control configuration of the color printer. [Diagram 5] FIG. 11 is a diagram showing a printing rate and a fixing limit temperature. [Figure 6] FIG. 4 is a diagram showing the temperature distribution of a heater. [Figure 7] FIG. 13 is a diagram showing a range for determining a printing rate. [Figure 8] 11 is a graph showing the relationship between the ON duty of a heat generating cell and the temperature of a fixing belt. [Figure 9] 4 is a table showing the ON duty of heat generating cells according to the first embodiment. [Figure 10] FIG. 4 is a diagram showing a temperature distribution of a heater according to the first embodiment. [Figure 11] FIG. 13 is a diagram showing a temperature distribution of a heater according to a comparative example. [Figure 12] 5 is a timing chart showing the control timing of the ON duty of the heat generating cells according to the first embodiment. [Figure 13] 13 is a table showing the ON duty of heat generating cells according to the second embodiment. [Figure 14] FIG. 11 is a diagram showing a temperature distribution of a heater according to the second embodiment. [Figure 15] 10 is a timing chart showing the control timing of the ON duty of a heat generating cell according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, modes for carrying out the invention (hereinafter referred to as embodiments) will be described with reference to the drawings. 1. First embodiment [1-1. Configuration of image forming device] As shown in FIG. 1, the color printer 1 is a color electrophotographic printer that prints a desired color image on a recording medium P, for example, A3 size or A4 size. In the following description, the left end portion in FIG. 1 is the front of the color printer 1, and the up-down direction, left-right direction, and front-rear direction are defined when viewed from the front. In the following description, a position close to the recording medium storage unit 4 as viewed from an arbitrary position on the conveying path 5 along which the recording medium P is conveyed, or a direction toward the recording medium storage unit 4, is referred to as upstream. In the following description, a position close to the discharge stacker unit 22 on which the recording medium P is discharged and stacked, or a direction toward the discharge stacker unit 22, as viewed from an arbitrary position on the conveying path 5, is referred to as downstream. Furthermore, the direction from upstream to downstream is referred to as the conveying direction. In addition, the left-right direction perpendicular to the conveying direction and the thickness direction perpendicular to the paper surface of the conveyed recording medium P is also referred to as the conveying width direction.

[0011] This color printer 1 is composed of a main body 2 and an openable and closable top cover 3. A recording medium storage section 4 capable of storing a plurality of recording media P is provided at the bottom inside the main body 2. A paper feed roller 6 is provided at the upper front end of the recording medium storage section 4 for separating the recording media P stored in a stacked state in the recording medium storage section 4 one by one and feeding the recording media P to a conveying path 5. A first registration roller 7 and a second registration roller 8 are conveying rollers for conveying the recording media P fed by the paper feed roller 6 from the recording medium storage section 4 to an image forming section 9. The image forming section 9 is composed of image forming units 14Y, 14M, 14C, and 14K.

[0012] The IN1 sensor 10 is disposed upstream of the first registration roller 7 and is a transport system sensor that detects the arrival of the recording medium P. The IN2 sensor 11 is disposed upstream of the second registration roller 8 and is a transport system sensor that detects the arrival of the recording medium P. The WR sensor 12 is disposed downstream of the second registration roller 8 and is a transport system sensor that detects the recording medium P and detects the timing when the recording medium P reaches the image forming unit 9.

[0013] A conveyor belt 13 is provided above the recording medium storage unit 4 within the main body 2, so as to traverse the main body 2 from front to rear. The conveyor belt 13 is stretched so as to go around front and rear rollers each having an elongated cylindrical shape with its central axis oriented in the left-right direction, and is disposed opposite the lower side of the image forming unit 14. The recording medium P is placed on its upper surface and conveyed downstream in the rear direction.

[0014] On the other hand, above the conveyor belt 13, four image forming units 14Y, 14M, 14C, and 14K (hereinafter, these are also collectively referred to as image forming units 14) are arranged in order from the front side to the rear side. That is, the image forming units 14 of each color are arranged in a so-called tandem system. The image forming units 14Y, 14M, 14C, and 14K correspond to the colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. The image forming units 14Y, 14M, 14C, and 14K are configured similarly to each other, and only the colors of the corresponding toners are different. The image forming unit 14 is formed in a box shape that is relatively long in the left-right direction so as to correspond to the left-right width of the recording medium P.

[0015] Further, toner cartridges 15Y, 15M, 15C and 15K (hereinafter, collectively referred to as toner cartridges 15) are provided above the image forming units 14Y, 14M, 14C and 14K, respectively. The toner cartridges 15 are hollow containers that are long in the left-right direction, and contain powdered toner of each color, and also incorporate a predetermined stirring mechanism.

[0016] Further, LED heads 16Y, 16M, 16C, and 16K (hereinafter, collectively referred to as LED heads 16) are provided in the main body 2 so as to correspond to the image forming units 14Y, 14M, 14C, and 14K, respectively. The LED head 16 is configured as a rectangular parallelepiped elongated in the left-right direction, and a plurality of LEDs (Light Emitting Diodes) are arranged in the left-right direction inside the LED head 16, and each LED is caused to emit light in a light emission pattern according to image data supplied from an exposure control unit 62 (FIG. 4). The LED head 16 is supported by the top cover 3, and when the top cover 3 is closed, the photosensitive drum 17 is arranged very close to the LED head 16, and exposure processing is performed by the light from the LED head 16.

[0017] The image forming units 14Y, 14M, 14C, and 14K are provided with photosensitive drums 17Y, 17M, 17C, and 17K (hereinafter, these are also collectively referred to as photosensitive drums 17). The photosensitive drums 17 can create electrostatic latent images on their surfaces according to print data by electrostatic force, and a toner image is formed by toner supplied from a toner cartridge 15.

[0018] Transfer rollers 18Y, 18M, 18C and 18K (hereinafter collectively referred to as transfer rollers 18) are provided at four locations directly below the image forming units 14Y, 14M, 14C and 14K. That is, the transfer rollers 18 are disposed facing the photosensitive drum 17 with the conveyor belt 13 interposed therebetween. The transfer rollers 18 are configured to be capable of being charged, and transfer the toner image formed on the surface of the photosensitive drum 17 onto the recording medium P. The photosensitive drum 17 and transfer rollers 18 are subjected to a high voltage applied from a high voltage power supply 24, which will be described later, to perform electrophotographic processes such as electrostatic charging, development and transfer.

[0019] The motor control unit 63 (FIG. 4) supplies toner from the toner cartridge 15 to the image forming units 14. At the same time, the exposure control unit 62 (FIG. 4) causes the LED head 16 to emit light so as to form an emission pattern corresponding to the image data supplied from the PC 54 (FIG. 4). In response to this, each image forming unit 14 uses the toner supplied from the toner cartridge 15 to form a toner image corresponding to the emission pattern of the LED head 16, and transfers this toner image onto the recording medium P. As a result, four color toner images corresponding to the image data are sequentially transferred onto the recording medium P being transported by the transport belt 13.

[0020] The FUSER-IN sensor 19 is disposed in the vicinity of the upstream side of the fixing unit 20, and is a transport sensor for detecting the recording medium P.

[0021] The fixing device 20 is provided downstream of the image forming unit 14. The fixing device 20 heats the fixing belt 30 (FIG. 2) and rotates the fixing belt 30 and the pressure roller 32 (FIG. 2) in a predetermined direction under the control of the fixing control unit 64 (FIG. 4). As a result, the fixing device 20 applies heat and pressure to the delivered recording medium P, i.e., the recording medium P on which the four color toner images are superimposed, to fix the toner, and delivers the recording medium P further downstream.

[0022] The EXIT sensor 21 is installed downstream of the fixing unit 20 and is a travel system sensor that detects the recording medium P and detects that the recording medium P has been discharged from the fixing unit 20. The discharge stacker unit 22 is provided outside the main body unit 2 and accumulates the recording medium P on which an image has been formed. The density sensor 23 is an optical sensor that reads a special pattern created on the conveyor belt 13 and is used for print quality maintenance operations such as density correction.

[0023] The high-voltage power supply 24 is a power supply that generates a high voltage to be applied to the photosensitive drum 17 and the transfer roller 18. The low-voltage power supply 25 is an AC-DC power supply that converts a commercial AC power supply into a DC power supply. This low-voltage power supply 25 supplies DC power of 3.3 [V], 5 [V], 24 [V], etc. to each board (not shown). The low-voltage power supply 25 also supplies AC 100 [V] to the heater 31 (FIG. 3) of the fixing unit 20 via a triac 38 described later.

[0024] Here, the above-mentioned transport system sensors (also called paper sensors) (IN1 sensor 10, IN2 sensor 11, WR sensor 12, FUSER-IN sensor 19, and EXIT sensor 21) and density sensor 23 are connected to the process control unit 60 (FIG. 4) via cables. Also, each roller (each roller in the fixing unit 20, the paper feed roller 6, the first registration roller 7, the second registration roller 8, the photosensitive drum 17, and the transfer roller 18) is mechanically driven by an actuator (not shown), and is capable of transporting the recording medium P downstream of the color printer 1.

[0025] Meanwhile, a display unit 26 is provided on the front side of the exterior of the main body 2. The display unit 26 is composed of a liquid crystal display panel and switches, etc., and is capable of displaying the status of the color printer 1 and allowing the user to input operations. The display unit 26 is connected to the process control unit 60 (FIG. 4) via a cable. The liquid crystal display panel is capable of displaying, for example, 24 characters by 2 lines of characters (letters).

[0026] [1-2. Configuration of the fixing unit] 2, the fixing unit 20 includes a fixing belt 30, a heater 31, a pressure roller 32, temperature sensors 33, 34, and 35, a heat diffusion member 36, a heater holder (not shown), and a fixed support member (not shown). Note that FIG. 2(A) is a diagram including a power supply.

[0027] The fixing belt 30 is an endless belt-like body, and the inside is supported by a fixed support member. The fixing belt 30 has a base material, a rubber layer, and a release layer formed from the inner circumference side to the outer circumference side, and for example, the base material is polyimide, the rubber layer is silicone rubber, and the release layer is fluororesin PFA or other materials. The heater 31 is disposed on the inner circumference side of the fixing belt 30, and is a heat source composed of a rectangular base material and a heat generating cell 40 (FIG. 3) formed on the base material (details will be described later).

[0028] The pressure roller 32 is disposed facing the underside of the fixing belt 30, and forms a nip between the pressure roller 32 and the fixing belt 30. The pressure roller 32 is configured such that the peripheral surface of a cylindrical member made of a metal material is covered with a rubber elastic layer, and the rubber elastic layer is made of a material such as silicone rubber, for example.

[0029] The heat diffusion member 36 is disposed in the left-right direction (longitudinal direction) between the heater 31 and the fixing belt 30, and is, for example, a plate-shaped member made of a material with high thermal conductivity such as aluminum. The heat diffusion member 36 diffuses the temperature evenly in the longitudinal direction and the width direction (the front-rear direction in which the recording medium P is transported).

[0030] The temperature sensor 33 is a non-contact type temperature detection sensor, which is installed in a paper passing area at the center of the fixing belt 30 in the transport width direction, and detects the fixing belt temperature, which is the surface temperature of the fixing belt 30. The temperature sensors 34 and 35 are contact type thermistors, which are installed in non-paper passing areas at both ends of the pressure roller 32 in the transport width direction, which are areas where the recording medium P does not pass, and detect the surface temperature of the pressure roller 32.

[0031] The AC nominal voltage 37 is, for example, an AC input voltage of AC 100 V. Triacs 38a, 38b, 38c, 38d, and 38e (hereinafter collectively referred to as triacs 38) are semiconductor switching elements used for power control of the heater 31, and are turned on and off by a fixing control unit 64. The fixing control unit 64 will be described in detail later, but it controls the temperature of the fixing belt 30 to be optimal based on the detection results of the temperature sensors 33, 34, and 35.

[0032] [1-3. Heater configuration] As shown in Fig. 3(A), the heater 31 has a plurality of heat generating cells 40a, 40b, 40c, 40d, and 40e (hereinafter collectively referred to as heat generating cells 40) arranged in order from left to right along the longitudinal direction of the heater 31 (i.e., the transport width direction or the main scanning direction (hereinafter also referred to as the heater longitudinal direction)) on a rectangular base. For the sake of simplicity, Fig. 3(B) shows an enlarged view of heat generating cells 40a, 40b, and 40c out of heat generating cells 40a, 40b, 40c, 40d, and 40e. In this embodiment, the widths of all heat generating cells 40a, 40b, 40c, 40d, and 40e in the heater longitudinal direction are the same.

[0033] Each of the heat generating cells 40a, 40b, and 40c is composed of a resistance heating wiring 41a, 41b, and 41c that generates heat when power is supplied. Hereinafter, the resistance heating wirings 41a, 41b, 41c, 41d (not shown), and 41e (not shown) are also collectively referred to as resistance heating wiring 41. The resistance heating wirings 41d and 41e are resistance heating wirings 41 corresponding to the heat generating cells 40d and 41e, respectively. The resistance heating wiring 41 occupies a predetermined region in the conveying direction perpendicular to the conveying width direction by being folded back multiple times at both ends in the conveying width direction, which is a wiring extending along the conveying width direction. The common power supply wiring 42 is a common line commonly connected to the resistance heating wirings 41a, 41b, and 41c of the heat generating cells 40a, 40b, and 40c, respectively, and the opposite side of the connection point with the resistance heating wirings 41a, 41b, and 41c is connected to the AC nominal voltage 37 (FIG. 2). The power supply wirings 43a, 43b, and 43c are connected to the resistance heating wirings 41a, 41b, and 41c, respectively, on the side opposite to the common power supply wiring 42. In addition, the power supply wirings 43a, 43b, and 43c are connected to the triacs 38a, 38b, and 38c (FIG. 2) on the side opposite to the connection points with the resistance heating wirings 41a, 41b, and 41c, respectively. In this manner, in the heater 31, five sets, namely, a set of the resistive heating wiring 41a, the power supply wiring 43a, and the triac 38a, a set of the resistive heating wiring 41b, the power supply wiring 43b, and the triac 38b, a set of the resistive heating wiring 41c, the power supply wiring 43c, and the triac 38c, a set of the resistive heating wiring 41d, the power supply wiring 43d, and the triac 38d (none of which are shown), and a set of the resistive heating wiring 41e, the power supply wiring 43e, and the triac 38e (none of which are shown), form independent circuits, and these five sets of circuits are connected to the common power supply wiring 42. For this reason, in the heater 31, each of the heat generating cells 40a to 40e can be controlled independently.

[0034] [1-4. Color printer control configuration] As shown in FIG. 4, the color printer 1 is controlled by a controller control unit 50. The controller control unit 50 has a CPU 51, a ROM (Read Only Memory) 52, and a RAM (Random Access Memory) 53, and the CPU 51, ROM 52, and RAM 53 are connected to each other by an internal bus. The CPU 51 controls the RAM 53 and a process control unit 60 according to a print processing program stored in the ROM 52. The ROM 52 is an area for storing the print processing program, and is a non-volatile memory capable of retaining data even when the color printer 1 is turned off. The RAM 53 is a volatile memory that stores print data input from a PC 54, and the data is erased when the color printer 1 is turned off. The controller control unit 50 is connected to the PC 54 external to the color printer 1.

[0035] The PC 54 is, for example, a personal computer, and has a PC display unit 55 and a PC input unit 56. The PC 54 creates print data and transmits the print data to the color printer 1 via a communication interface consisting of a USB (Universal Serial Bus) or a LAN (Local Area Network), etc. The PC 54 also receives instructions issued by the color printer 1 via the communication interface. The PC display unit 55 is made up of a liquid crystal display or the like, and displays a print image created by an application (not shown) as well as instructions issued by the color printer 1. The PC input unit 56 is made up of a keyboard, mouse, etc., and creates an image of the print data via an application (not shown) as well as inputs responses to instructions issued by the color printer 1.

[0036] The process control unit 60 has a high voltage control unit 61, an exposure control unit 62, a motor control unit 63, and a fixing control unit 64, and controls the printing process such as conveyance of the recording medium P, charging, developing, transferring, and fixing.

[0037] The high voltage control unit 61 has a supply voltage control unit 66, a development voltage control unit 67, a charging voltage control unit 68, and a transfer control unit 69, and appropriately controls the voltages applied to the various rollers in order to transfer the toner to the recording medium P. The supply voltage control unit 66 controls the supply voltage applied to the supply roller 70. The development voltage control unit 67 controls the development voltage applied to the development roller 71. The charging voltage control unit 68 controls the charging voltage applied to the charging roller 72. The transfer control unit 69 controls the transfer voltage applied to the transfer roller 18. The exposure control unit 62 controls the exposure of the LED head 16.

[0038] The motor control unit 63 controls and rotates the main motor 76 in the color printer 1, thereby driving each roller in the image forming unit 14, each roller in the fixing unit 20, the paper feed roller 6, the first registration roller 7, and the second registration roller 8.

[0039] The fixing control unit 64 controls the ON / OFF of the triacs 38a-38e of the low voltage power supply 25 based on the detection results of the temperature sensors 33, 34, and 35, thereby supplying power to the heater 31 from the AC nominal voltage 37 and controlling the surface temperature of the fixing belt 30 to a predetermined appropriate temperature. The printing rate determination unit 74 holds ON duty parameters of the heat generating cells 40a-40e of the heater 31, determines the printing rate of each area from the information of the print data expanded in the RAM 53, and determines the ON duty of each heat generating cell based on the determination result.

[0040] [1-5. Print rate and fixing limit temperature] First, the current problems will be described with reference to Figures 5 and 6. Figure 5 shows the relationship between the printing rate (toner concentration) and the fixing limit temperature TmpL.

[0041] First, the characteristics of the good fixing property range Rgf of the fixing unit 20 will be described. The good fixing property range Rgf is a temperature range between the hot limit temperature TmpU and the fixing limit temperature TmpL. The hot limit temperature TmpU is a temperature above which a printing defect such as hot offset occurs when the fixing belt temperature is higher than this temperature, and is set to 190[°C] in this embodiment. The fixing limit temperature TmpL is a temperature below which a printing defect such as a fixing defect occurs when the fixing belt temperature is lower than this temperature. This good fixing property range Rgf differs for each recording medium P and also differs for each printing rate TC of the print data. Here, the printing rate TC refers to the toner concentration of the image formed on the recording medium P. The set temperature TmpS is a set temperature of the fixing belt 30 that is set based on an experiment so that it falls within a good fixing property range even if an overshoot or a temperature drop occurs.

[0042] In this explanation, yellow (Y), magenta (M), and cyan (C) will be described as examples. When the print rate TC is 50%, such as magenta (M) 50% (monochrome 50% or magenta (M) 25% + yellow (Y) 25% (mixed colors 50%]), the fixing limit temperature TmpL is 145°C. When the print rate TC is 100%, such as magenta (M) 100% (monochrome 100% or magenta (M) 50% + yellow (Y) 50% (mixed colors 100%]), the fixing limit temperature TmpL is 150°C. Furthermore, when the print ratio TC is 200[%], such as magenta (M) 100[%] + yellow (Y) 100[%] (mixture of 200[%]), the fixing limit temperature TmpL is 160[°C]. Furthermore, when the print ratio TC is 240[%], such as cyan (C) 80[%] + magenta (M) 80[%] + yellow (Y) 80[%] (mixture of 240[%]), the fixing limit temperature TmpL is 160[°C].

[0043] In this way, when the printing rate TC becomes higher, the fixing limit temperature TmpL becomes higher and the good fixing range Rgf becomes narrower. In FIG. 5, for the sake of simplicity, only examples of printing rates TC240[%], TC200[%], TC100[%] and TC50[%] are shown, but experiments have shown that if the printing rate TC is less than 200[%], the fixing limit temperature TmpL is 150[°C] or less. In this embodiment, the printing rate threshold for determining whether the printing rate TC is a high printing rate or a low printing rate is set to 200[%], and the printing rate TC<200[%] is a low printing rate, and the printing rate TC≧200[%] is a high printing rate.

[0044] [1-6. Heater temperature distribution] FIG. 6 shows the temperature distribution of the heater 31. In this temperature distribution of the heater 31, the horizontal axis is the position in the heater longitudinal direction, and the vertical axis is the fixing belt temperature TmpF. In the actual color printer 1, the temperature sensor 33 (FIG. 2(A)) is installed only at the center of the fixing belt 30 in the conveying width direction, but the fixing belt temperature TmpF in FIG. 6 is a detection value obtained by installing multiple temperature sensors 33 along the heater longitudinal direction for the experiment. The thick solid line in the graph is a temperature distribution line L1 showing the temperature distribution of the measured fixing belt temperature TmpF over the heater longitudinal direction. In addition, the set temperature TmpS is 165[°C], the hot limit temperature TmpU is 190[°C], the high print rate fixing limit temperature TmpLu which is the fixing limit temperature TmpL in the case of a high print rate is 160[°C], and the low print rate fixing limit temperature TmpLl which is the fixing limit temperature TmpL in the case of a low print rate is 150[°C]. Hereinafter, the high print rate fixing limit temperature TmpLu and the low print rate fixing limit temperature TmpLl will also be collectively referred to as fixing limit temperature TmpL.

[0045] In order to arrange the heat generating cells 40a to 40e, each of which is independently powered, in series, it is necessary to provide a gap between adjacent heat generating cells 40 to maintain insulation. For this reason, in the heater 31, a cell eye region CSa that is a gap is arranged between the heat generating cells 40a and 40b, a cell eye region CSb that is a gap is arranged between the heat generating cells 40b and 40c, a cell eye region CSc that is a gap is arranged between the heat generating cells 40c and 40d, and a cell eye region CSd that is a gap is arranged between the heat generating cells 40d and 40e. Hereinafter, the cell eye regions CSa, CSb, CSc, and CSd are collectively referred to as the cell eye regions CS.

[0046] Here, a rectangular region surrounded by a dashed line in the heat generating cell 40b that does not include the cell eye region CS is defined as region AR1, and a rectangular region surrounded by a dashed line in the heat generating cells 40b and 40c that includes the cell eye region CS and has the same area as the region AR1 is defined as region AR2. If the proportion of the resistance heating wiring 41 (FIG. 3) in the region AR1 is X and the proportion of the resistance heating wiring 41 (FIG. 3) in the region AR2 is Y, then the region AR2 includes the cell eye region CSb, so that the relationship is X>Y. That is, when the amount of heat generated Qx(J) in the region AR1 is compared with the amount of heat generated Qy(J) in the region AR2, Qx>Qy holds, and the amount of heat generated in the region AR2 is less than that in the region AR1.

[0047] Therefore, when the color printer 1 supplies power to each of the heat generating cells 40a-40e of the heater 31 and controls the set temperature TmpS to 165[°C], the temperature distribution of the heater 31 becomes the temperature distribution shown by the temperature distribution line L1 in Fig. 6. As shown by this temperature distribution line L1, the fixing belt temperature TmpF in the area of ​​each heat generating cell 40 not including the cell mesh area CS becomes 165[°C], but the fixing belt temperature TmpF in the cell mesh area CS between the heat generating cells 40a-40e becomes low due to insufficient heat generation, and although it exceeds the low print rate fixing limit temperature TmpLl (150[°C]), it falls below the high print rate fixing limit temperature TmpLu (160[°C]). For example, consider a case in which, on an A3 size recording medium P, an image IMG200 with a high printing rate TC of 200[%] (yellow (Y) 100[%] + magenta (M) 100[%] red) is formed in the range including the range of the heat generating cell 40c and the cell eye region CSb in the heater longitudinal direction, and an image IMG100 with a low printing rate TC of 100[%] (magenta (M) 100[%]) is formed in the other range in the heater longitudinal direction, and the two are transported to the fixing device 20.

[0048] In this case, in the region AR3, which is the region other than the cell-eye region CSb in the range where the image IMG200 is formed in the heater longitudinal direction, the temperature distribution line L1 is maintained at a temperature higher than the high print rate fixing limit temperature TmpLu, so cold offset does not occur. On the other hand, in the region AR4, which is the region of the cell-eye region CSb in the range where the image IMG200 is formed in the heater longitudinal direction, the temperature distribution line L1 falls below the high print rate fixing limit temperature TmpLu due to a temperature drop, so cold offset occurs. In addition, if the set temperature TmpS is set to a high temperature from the beginning, such as 180°C, in order to avoid cold offset in the cell-eye region CS, the temperature will be set high even in the region other than the region AR4 in the heater longitudinal direction where the fixing belt temperature TmpF is sufficiently secured, resulting in wasted power consumption and inconvenience such as a high fixing belt temperature TmpF in the non-paper-passing region, where the recording medium P does not pass through.

[0049] [1-7. Range for determining print rate] Next, the present embodiment for solving the above-mentioned problems will be described with reference to Figs. 7 to 12. First, the range for calculating and determining the printing rate TC will be described with reference to Fig. 7. In the following, the areas for calculating the printing rate TC corresponding to the cell regions CSa, CSb, CSc, and CSd in the heater 31 in the recording medium P (for example, A3 size recording medium Pa3) are the printing rate calculation areas PCSa, PCSb, PCSc, and PCSd (hereinafter, these are also collectively referred to as the printing rate calculation areas PCS), which are the colored areas in Fig. 7. The position of the distance L0 from the left end of the heat generating cell 40a toward the right side along the main scanning direction Drm (transport width direction) is the left uppermost point (origin OC) where an image is formed, the heater longitudinal direction is the main scanning direction Drm (transport width direction), and the direction perpendicular to the heater longitudinal direction is the sub-scanning direction Drs (transport direction). The width of the heat generating cells 40a to 40e of the heater 31 in the main scanning direction Drm is defined as a heat generating cell width La, and the width of each cell region CS in the main scanning direction Drm is defined as a cell region width Lb.

[0050] At this time, the range of the main scanning direction Drm with the origin OC as the reference in each printing rate calculation area PCS is as follows: Printing rate calculation area PCSa...(La-L0)~(La-L0+Lb) range Printing rate calculation area PCSb...(2La-L0+Lb)~(2La-L0+2Lb) range Printing rate calculation area PCSc…(3La-L0+2Lb)~(3La-L0+3Lb) range Print rate calculation area PCSd...(4La-L0+3Lb)~(4La-L0+4Lb) range

[0051] Furthermore, the range of the sub-scanning direction Drs based on the origin OC in each printing rate calculation area PCS is the length of the sub-scanning direction Drs of the recording medium P (for example, the A3 size recording medium Pa3). In this way, the color printer 1 calculates the printing rate TC by setting the range of the main scanning direction Drm to the cell eye area width region Lb of each cell eye area CS, and setting the range of the sub-scanning direction Drs to the length from one end to the other end of the sub-scanning direction Drs of the recording medium P.

[0052] [1-8. Relationship between the ON Duty of each heating cell and the fixing belt temperature] Next, the ON duty of the heat generating cell 40 (hereinafter also referred to as the heat generating cell ON duty) will be described. The ON duty of the heat generating cell 40 is the ratio of the time during which the heat generating cell 40 is ON to the time during which the fixing control unit 64 controls the triacs 38a to 38e to be ON / OFF. If the ON time is Ton and the OFF time is Toff, then the ON duty of the heat generating cell 40=Ton / (Ton+Toff). If the ON duty of the heat generating cell 40 is high, the time during which the heat generating cell 40 is ON will be longer. In other words, the color printer 1 controls the temperature of the heat generating cell 40 to be higher the longer the ON duty of the heat generating cell 40 is kept ON.

[0053] FIG. 8 shows the relationship between the ON duty of the heat generating cell 40 and the fixing belt temperature TmpF. In the graph of FIG. 8(A), the horizontal axis is the heat generating cell ON duty, and the vertical axis is the fixing belt temperature TmpF in the cell region CS in the main scanning direction Drm. As shown in the table of FIG. 8(B), α0 is the heat generating cell ON duty that can maintain the fixing belt temperature TmpF of the set temperature TmpS in the cell region CS in the main scanning direction Drm, and can ensure that the fixing belt temperature TmpF exceeds the low print rate fixing limit temperature TmpLl in the cell region CS in the main scanning direction Drm. α2 is the heat generating cell ON duty that can ensure that the fixing belt temperature TmpF does not exceed the hot limit temperature TmpU in the cell region CS in the main scanning direction Drm, and can ensure that the fixing belt temperature TmpF exceeds the high print rate fixing limit temperature TmpLu in the cell region CS in the main scanning direction Drm. α1 is a heating cell ON duty that maintains the fixing belt temperature TmpF intermediate between the set temperature TmpS and the hot limit temperature TmpU in the cell region CS in the main scanning direction Drm, and maintains the fixing belt temperature TmpF intermediate between the high print rate fixing limit temperature TmpLu and the low print rate fixing limit temperature TmpLl in the cell region CS in the main scanning direction Drm. The magnitude relationship of the heating cell ON duty is α2>α1>α0.

[0054] [1-9. Setting the ON Duty of each heat generating cell] Next, the ON duty setting of each heating cell 40 according to the first embodiment will be described using table TB1 of FIG. 9 stored in the ROM 52. Table TB1 is a matrix of ON duty settings of each heating cell 40a-40e corresponding to the printing rate TC of the cell region CSa, CSb, CSc, and CSd. In this table TB1, "low" in "printing rate of cell region" indicates a low printing rate, and "high" indicates a high printing rate. Here, the threshold value for determining the high printing rate and the low printing rate in the printing rate TC is 200[%]. That is, when the printing rate TC is 200[%] or more, the printing rate is high, and when the printing rate TC is less than 200[%], the printing rate is low. In the following, for the sake of simplicity, the case of case 4 surrounded by a thick line will be described. In case 4, the cell region CSb has a high printing rate, and the other cell regions CSa, CSc, and CSd have a low printing rate. The ON duty of the heat generating cells 40b and 40c adjacent to the cell eye region CSb is set to α2, and the ON duty of the heat generating cells 40a, 40d, and 40e other than the heat generating cell 40 whose ON duty is set to α2 is set to α0.

[0055] [1-10. Heater temperature distribution in case 4] Fig. 10, which corresponds to Fig. 6, shows the heater temperature distribution according to the first embodiment in case 4. A detailed description of the graph is omitted since it is the same as Fig. 6. As in Fig. 6, consider a case where, on an A3 size recording medium Pa3, an image IMG200 with a print rate of TC200[%] (red of yellow (Y) 100[%] + magenta (M) 100[%]) is formed in a range including the range of heat generating cell 40c and cell eye region CSb in the heater longitudinal direction, and an image IMG100 with a print rate of TC100[%] (magenta (M) 100[%]) is formed in the other range in the heater longitudinal direction.

[0056] In this case, the cell eye region CSb is determined to have a high printing rate because it has a printing rate of TC200[%], and the other cell eye regions CSa, CSc, and CSd are determined to have a low printing rate because they have a printing rate of TC100[%]. The ON duty of each heating cell 40 is set according to table TB1 (FIG. 9), with the ON duty of heating cells 40b and 40c adjacent to the cell eye region CSb in the main scanning direction Drm being set to α2, and the ON duty of the other heating cells 40a, 40d, and 40e being set to α0.

[0057] Since the region of the heating cells 40b and 40c is maintained at a fixing belt temperature TmpF that does not exceed the hot limit temperature TmpU, the fixing property of the image IMG200 with a printing rate TC200 [%] formed in the range of the heating cells 40c in the main scanning direction Drm is not a problem. In the cell eye region CSb, the ON duty of the heating cells 40b and 40c arranged on both sides of the cell eye region CSb in the main scanning direction Drm is set to α2, which is higher than α0, so the fixing belt temperature TmpF rises due to the temperature rise of the heating cells 40b and 40c compared to when the ON duty of the heating cells 40b and 40c is set to α0. Therefore, since the cell eye region CSb is maintained at a fixing belt temperature TmpF that does not fall below the high printing rate fixing limit temperature TmpLu, the fixing property of the image IMG200 with a printing rate TC200 [%] formed in the cell eye region CSb is also good, and cold offset does not occur.

[0058] The temperature in the region of heat generating cell 40a is higher than the ON Duty of adjacent heat generating cell 40b, which is set to α2, and the fixing belt temperature TmpF increases toward heat generating cell 40b. This is because the ON Duty of adjacent heat generating cell 40b is set to α2, which is higher than α0. As a result, the fixing belt temperature TmpF in the cell mesh region CSa also increases.

[0059] The fixing belt temperature TmpF of the cell region CSa is below the high print rate fixing limit temperature TmpLu but above the low print rate fixing limit temperature TmpLl, and the image IMG100 formed in the range between the heat generating cell 40a and the cell region CSa in the main scanning direction Drm has a printing rate of TC100[%], so cold offset does not occur and there is no problem. Similarly, the image IMG100 formed in the range between the heat generating cell 40d and the cell region CSc in the main scanning direction Drm also has a printing rate of TC100[%], so there is no problem.

[0060] Since the ON duty of the heat generating cell 40e is set to α0, the set temperature TmpS is maintained in the heat generating cell 40e region. Since the cell mesh region CSd is maintained at a temperature higher than the low print rate fixing limit temperature TmpLl, cold offset does not occur and there is no problem.

[0061] For reference, a case where the print rate TC is determined not in the cell region CS but in the heat cell 40 region in the main scanning direction Drm, and the ON duty of only the heat cells 40 determined to have a high print rate is set high will be described with reference to FIG. 11 in which an image similar to FIG. 10 is formed. Since the print rate TC of the heat cell 40c range in the main scanning direction Drm is 200[%], it is determined to be a high print rate, and the ON duty of only the heat cell 40c in the main scanning direction Drm is set to α2, and the ON duty of the other heat cells 40a, 40b, 40d, and 40e is set to α0. Since the heat cell 40c region is maintained at a temperature not exceeding the hot limit temperature TmpU, there is no problem with the fixability of the image IMG200 with a print rate TC200[%] in the heat cell 40c range in the main scanning direction Drm. However, although the fixing belt temperature TmpF in the cell region CSb rises due to the heat generation cells 40c, it does not exceed the high print rate fixing limit temperature TmpLu and falls below it. As a result, the image IMG200 with a print rate of TC200 [%] in the range of the cell region CSb in the main scanning direction Drm becomes cold offset.

[0062] [1-11. Operation] The operation of color printer 1 according to the first embodiment will be described again with reference to Fig. 12. As an operation example, a case where the following print job is received will be described. Print job 1 (A3 size, 6 consecutive pages) Page 1 and page 3: Image IMG with high print ratio (200[%]) in cell area CSb, and image with low print ratio in other cell area CS Page 2 and page 6: Images with low print ratio over the entire range in the main scanning direction Drm Page 4: Image IMG with high print ratio (200[%]) in cell area CSc, and image with low print ratio in other cell area CS Page 5: Image IMG with high print ratio (200[%]) over the entire range in the main scanning direction Drm Print job 2 (A3 size, 3 consecutive pages) Page 1 and page 3: Images with low print ratio over the entire range in the main scanning direction Drm Page 2: Image IMG with high print ratio (200[%]) in cell eye areas CSa and CSd, and image with low print ratio in other cell eye areas CS

[0063] 12 shows how an A3 size recording medium P is fed in the direction of the arrow through the fixing unit 20. The first six pages are print job 1 and the next three pages are print job 2.

[0064] The timing charts in the center of the top and bottom of FIG. 12 are as follows: ChartA:…Print rate determination Chart B: Paper sensor ON / OFF (ON: paper present, OFF: no paper present) The ON timing is determined by the FUSER-IN sensor 19, and the OFF timing is determined by the EXIT sensor 21. Chart C: ON Duty setting of heating cell 40e Chart D: ON Duty setting of heating cell 40d Chart E: ON Duty setting of heating cell 40c Chart F: ON Duty setting of heating cell 40b Chart G: ON Duty setting of heating cell 40a

[0065] The timing chart in the lower part of Fig. 12 shows the transition of the fixing belt temperature TmpF of each heat generating cell 40 along the vertical axis over the time course of timings T1 to T22 along the horizontal axis, and is drawn in synchronism with the timing chart in the center of the top and bottom of Fig. 12. Note that for convenience of explanation, this timing chart is drawn such that the positions of the heat generating cells 40 are slightly shifted in the vertical direction even though they have the same fixing belt temperature TmpF.

[0066] At timing T1, color printer 1 receives print job 1 sent from PC 54. CPU 51 converts the R, G, and B image data from received print job 1 into density data for the developing colors Y, M, C, and K, and stores the data in RAM 53. Printing rate determination unit 74 determines the page, position, and printing rate TC on which the image is to be formed from the stored image data, and determines the ON duty of each heating cell.

[0067] At timing T2, the fixing control unit 64 starts fixing control based on the ON duty determined by the printing rate determination unit 74. Since the first page of print job 1 has an image IMG with a high printing rate (200[%]) formed in the cell region CSb and an image with a low printing rate formed in the other range, the combination of ON duties shown in case 4 of table TB1 (FIG. 9) is set and controlled for each heat generating cell 40. That is, the ON duty of heat generating cells 40b and 40c is α2, and the ON duty of the other heat generating cells 40 is α0. The temperature of each heat generating cell 40 rises, and the heat generating cells 40b and 40c have a fixing belt temperature TmpF that exceeds the high printing rate fixing limit temperature TmpLu, and the other heat generating cells 40 have a fixing belt temperature TmpF that exceeds the low printing rate fixing limit temperature TmpLl.

[0068] At timing T3, the first page reaches the fixing unit 20 and fixing begins. A temperature drop occurs near the beginning of the recording medium P because heat is absorbed by the recording medium P, but the set ON Duty is designed to a value that takes the temperature drop into consideration, so the fixing belt temperature TmpF of each heat-generating cell 40 does not fall below the corresponding fixing limit temperature TmpL. The ON Duty of each heat-generating cell 40 is controlled to the set ON Duty until the recording medium P turns off the EXIT sensor 21, that is, until it leaves the fixing unit 20.

[0069] At timing T4, when fixing of the first page is completed, the ON duty of the second page is set. Since the second page is an image with a low printing rate over the entire range of the main scanning direction Drm, the combination of ON duties shown in case 0 of table TB1 (FIG. 9) is set and controlled for each heating cell 40. The ON duty of heating cells 40b and 40c is switched from α2 to α0. The ON duty of the other heating cells 40 remains at α0. The fixing belt temperature TmpF of heating cells 40b and 40c gradually decreases until it reaches a temperature that does not fall below the low printing rate fixing limit temperature TmpLl.

[0070] At timing T5, the second page reaches the fixing unit 20 and is fixed. At timing T6, when fixing of the second page is completed, the ON Duty for the third page is set. Since the third page again has an image IMG with a high print ratio (200[%]) in the cell region CSb and an image with a low print ratio in the other range, the combination of ON Duty shown in Case 4 of Table TB1 (FIG. 9) is set and controlled for each heating cell 40.

[0071] At timing T7, the third page reaches the fixing unit 20 and is fixed. At timing T8, when fixing of the third page is completed, the ON duty of the fourth page is set. Since the fourth page has an image IMG with a high print ratio (200[%]) in the cell region CSc and an image with a low print ratio in the other range, the combination of ON duties shown in case 2 of table TB1 (FIG. 9) is set and controlled for each heat generating cell 40. The ON duty of heat generating cells 40c and 40d is α2, and the ON duty of the other heat generating cells 40 is α0. The ON duty of heat generating cell 40c remains α2, and the ON duty of heat generating cell 40d is switched from α0 to α2. The ON duty of heat generating cell 40b is switched from α2 to α0.

[0072] At timing T9, the fourth page reaches the fixing unit 20 and is fixed. At timing T10, when fixing of the fourth page is completed, the ON duty for the fifth page is set. Since the fifth page is an image IMG with a high printing ratio (200[%]) over the entire range in the main scanning direction Drm, the combination of ON duties shown in case 15 of table TB1 (FIG. 9) is set and controlled for each heat cell 40. The ON duty of heat cells 40c and 40d remains at α2, and the ON duties of the other heat cells 40a, 40b, and 40e are switched from α0 to α2.

[0073] At timing T11, the fifth page reaches the fixing unit 20 and is fixed. At timing T12, when fixing of the fifth page is completed, the ON duty for the sixth page is set. Since the sixth page is an image with a low printing ratio over the entire range in the main scanning direction Drm, the combination of ON duties shown in case 0 of table TB1 (FIG. 9) is set and controlled for each heat cell 40. The ON duty of all heat cells 40 is switched from α2 to α0.

[0074] At timing T13, the sixth page reaches the fixing unit 20 and is fixed. At timing T14, printing of print job 1 is completed and the settings of each heat generating cell 40 are cleared. Timings T15 to T22 are operations from when print job 2 is received until printing is completed, and are the same as for print job 1, so a description thereof is omitted.

[0075] [1-12. Effects, etc.] In the color printer 1 configured as above, the print rate TC of the image formed in the print rate calculation area PCS, which is the same range in the main scanning direction Drm as each cell eye area CS on the recording medium P, is determined by the print rate determination unit 74, and when the print rate TC of the print rate calculation area PCS is a high print rate equal to or higher than a predetermined print rate threshold value, the fixing control unit 64 sets the ON Duty of the heat generating cells 40 adjacent to both sides of the main scanning direction Drm for the cell eye area CS, which has the same range in the main scanning direction Drm as the print rate calculation area PCS, higher than the original ON Duty of the heat generating cells 40, thereby making the set temperature TmpS of the heat generating cells 40 higher than the original set temperature TmpS.

[0076] Therefore, in the color printer 1, the heat generation amount of the cell region CS in the main scanning direction Drm of the fixing belt 30 can be compensated for by the heat generating cells 40 adjacent to the cell region CS, and the temperature drop of the cell region CS can be suppressed. This allows the color printer 1 to prevent poor fixing (cold offset) caused by insufficient heat generation in the cell region CS when printing at a high printing rate TC, and obtain good printing results.

[0077] Here, it is conceivable that by setting all the heat-generating cells 40 to a temperature (e.g., 180°C) higher than the original set temperature TmpS (e.g., 160°C) between the high-print-rate fixing limit temperature TmpLu and the hot limit temperature TmpU, it would be possible to prevent a drop in temperature in the cell-eye area CS and maintain fixability. However, in that case, the fixing unit 20 heats the heat-generating cells 40 to an unnecessarily high temperature even when printing an image with a low print rate TC, resulting in a waste of power.

[0078] In response to this, the color printer 1, only when the printing rate calculation area PCS has a high printing rate, makes the set temperatures TmpS of only the heating cells 40 adjacent to both sides of the cell region CS, whose range in the main scanning direction Drm is the same as that of the printing rate calculation area PCS, higher than the original set temperatures TmpS of the heating cells 40. As a result, the color printer 1 can prevent the heating cells 40 from being heated to an unnecessarily high temperature when printing an image with a low printing rate TC, thereby saving power.

[0079] According to the above configuration, the color printer 1 has a heater 31 having heat cells 40 as heat generating parts arranged in the main scanning direction Drm and divided from each other, with cell mesh areas CS formed as inter-heating part areas between adjacent heat cells 40 in the main scanning direction Drm, a fixing belt 30 heated by the heater 31, and a pressure roller 32 in contact with the outer peripheral surface of the fixing belt 30, and a fixation unit 20 is provided in a nip area formed between the fixing belt 30 and the pressure roller 32, which holds a recording medium P as a recording material carrying an image and passes the recording medium P through the nip area to heat and fix the image. The color printer 1 also has a printing rate determination unit 74 that determines the printing rate TC of an image formed in a printing rate calculation area PCS, which is an area on the recording medium P that corresponds to the cell eye area CS in the main scanning direction Drm, and a fixing control unit 64 that, when the printing rate TC of the printing rate calculation area PCS is a high printing rate that is equal to or higher than a predetermined printing rate threshold, sets the set temperature TmpS of a heat generating cell 40 adjacent to at least one side of the cell eye area CS corresponding to the printing rate calculation area PCS in the main scanning direction Drm to be higher than the original set temperature TmpS of the heat generating cell 40.

[0080] As a result, the color printer 1 can compensate for the amount of heat generated in the cell region CS of the fixing belt 30 by the heat generating cells 40 adjacent to the cell region CS, and can suppress a drop in temperature in the cell region CS.

[0081] 2. Second embodiment [2-1. Configuration of image forming device] A color printer 101 (FIGS. 1 and 4) according to the second embodiment differs from color printer 1 according to the first embodiment in that table TB101 shown in FIG. 13 is stored in ROM 52 instead of table TB1 shown in FIG. 9, but is otherwise configured similarly.

[0082] [2-2. Setting the ON Duty of each heat generating cell] Next, the ON duty setting of each heating cell 40 according to the second embodiment will be described using table TB101 in FIG. 13 corresponding to table TB1 (FIG. 9) stored in the ROM 52. Table TB101 is a matrix of ON duty settings of each heating cell 40a-40e corresponding to the printing rate TC of the cell eye regions CSa, CSb, CSc, and CSd. Here, the threshold value for determining the high printing rate and the low printing rate at the printing rate TC is 200[%]. That is, when the printing rate TC≧200[%], the printing rate is high, and when the printing rate TC<200[%], the printing rate is low. In the following, for the sake of simplicity, the case of case 4 surrounded by a thick line will be described. In case 4, the cell eye region CSb has a high printing rate, and the other cell eye regions CSa, CSc, and CSd have a low printing rate. First, the ON duty of the heating cells 40b and 40c adjacent to the cell eye region CSb is set to α2. Moreover, the ON duty of the heating cell 40 adjacent to the heating cell 40 with the ON duty set to α2 in the direction away from the high print rate cell eye region CSb is set to α1. Here, the ON duty of the heating cell 40a on the left of the heating cell 40b and the heating cell 40d on the right of the heating cell 40c are set to α1. Furthermore, the ON duty of the heating cell 40e, which is a heating cell 40 other than the heating cell 40 with the ON duty set to α2 or α1, is set to α0.

[0083] Thus, table TB101 differs from table TB1 (FIG. 9) according to the first embodiment in that instead of the two ON Duty patterns of α0 and α2, the heat generating cell ON Duty has three patterns of α0, α1, and α2, and also in that the ON Duty of the heat generating cell 40 next to the heat generating cell 40 whose heat generating cell ON Duty is set to α2 is set to α1 instead of α0.

[0084] [2-3. Heater temperature distribution in case 4] Fig. 14, which corresponds to Fig. 10, shows the heater temperature distribution according to the second embodiment for case 4. A detailed description of the graph is omitted as it is the same as Fig. 10. As in Fig. 10, consider a case where, on an A3 size recording medium Pa3, an image IMG200 with a print rate of TC200[%] (red of yellow (Y) 100[%] + magenta (M) 100[%]) is formed in a range including the range of heat generating cell 40c and cell eye region CSb in the heater longitudinal direction, and an image IMG100 with a print rate of TC100[%] (magenta (M) 100[%]) is formed in the other range in the heater longitudinal direction.

[0085] In this case, the cell eye region CSb is determined to have a high printing rate because it has a printing rate of TC200[%], and the other cell eye regions CSa, CSc, and CSd are determined to have a low printing rate because they have a printing rate of TC100[%]. The ON duty of each heating cell 40 is set according to table TB101 (FIG. 13), and the ON duty of heating cells 40b and 40c adjacent to the cell eye region CSb in the main scanning direction Drm is set to α2, the ON duty of heating cells 40a and 40d adjacent to the heating cell 40 with the ON duty set to α2 in the direction away from the high printing rate cell eye region CSb is set to α1, and the ON duty of the other heating cell 40e is set to α0.

[0086] Since the region of the heating cells 40b and 40c is maintained at a fixing belt temperature TmpF that does not exceed the hot limit temperature TmpU, the fixing property of the image IMG200 with a printing rate TC200 [%] formed in the range of the heating cells 40c in the main scanning direction Drm is not a problem. In the cell eye region CSb, the ON duty of the heating cells 40b and 40c arranged on both sides of the cell eye region CSb in the main scanning direction Drm is set to α2, which is higher than α0, so the fixing belt temperature TmpF rises due to the temperature rise of the heating cells 40b and 40c compared to when the ON duty of the heating cells 40b and 40c is set to α0. Therefore, since the cell eye region CSb is maintained at a fixing belt temperature TmpF that does not fall below the high printing rate fixing limit temperature TmpLu, the fixing property of the image IMG200 with a printing rate TC200 [%] formed in the cell eye region CSb is also good, and cold offset does not occur.

[0087] The temperature in the region of the heat generating cells 40a and 40d is distributed such that the fixing belt temperature TmpF becomes higher as it approaches the heat generating cells 40b and 40c because the ON duty of the adjacent heat generating cells 40b and 40c is set to α2, which is higher than α1. Here, the ON duty of the heat generating cells 40a and 40d themselves is also set to α1, which is higher than α0, so the fixing belt temperature TmpF is maintained at a higher temperature than in the first embodiment. As a result, the fixing belt temperature TmpF in the cell mesh regions CSa and CSc also becomes higher in conjunction with this, compared to the first embodiment.

[0088] Therefore, the fixing belt temperature TmpF in the cell region CSa and CSc is approximately equal to the high print rate fixing limit temperature TmpLu and exceeds the low print rate fixing limit temperature TmpLl, and the image IMG100 formed in the cell region CSa in the main scanning direction Drm has a printing rate of TC100[%], so cold offset does not occur and there is no problem. Also, the image IMG100 formed in the cell region CSc in the main scanning direction Drm has a printing rate of TC100[%], so there is no problem.

[0089] Since the ON duty of the heat generating cell 40e is set to α0, the set temperature TmpS is maintained in the heat generating cell 40e region. The cell eye region CSd is maintained at a temperature higher than the low print rate fixing limit temperature TmpLl, so cold offset does not occur and there is no problem. Furthermore, in the cell eye region CSd, the ON duty is set to α1, and the fixing belt temperature TmpF is distributed higher as it approaches the adjacent heat generating cell 40d.

[0090] [2-4. Operation] Figure 15, which corresponds to Figure 12, shows the operation of color printer 101 of the second embodiment. This operation is basically the same as that of color printer 1 of the first embodiment, so a description thereof will be omitted.

[0091] [2-5. Effects, etc.] In the above configuration, color printer 101 differs from color printer 1 according to the first embodiment in that, in addition to α0 and α2, the ON duty of heat-generating cells 40 is set to α1 such that the relationship of α0<α1<α2 is satisfied, and the heat-generating cell 40 next to the heat-generating cell 40 whose ON duty is set to α2 is set to α1, which is the ON duty intermediate between α2 and α0. That is, in the color printer 101, when the printing rate calculation area PCS has a high printing rate, the set temperature TmpS of the nearest heating cell, which is the heating cell 40 adjacent to both sides in the main scanning direction Drm of the cell region CS having the same range in the main scanning direction Drm as the printing rate calculation area PCS, is set higher than the original set temperature TmpS of the nearest heating cell, and the set temperature TmpS of the heating cell 40 adjacent to the nearest heating cell on the side away from the cell region CS in the main scanning direction Drm is set higher by an amount of increase that is smaller than the original set temperature of the heating cell 40 than the amount of increase from the original set temperature TmpS of the nearest heating cell. Therefore, the color printer 101 can reduce the temperature difference in the fixing belt temperatures TmpF between adjacent heating cells 40, and can further reduce temperature unevenness, compared to the color printer 1 according to the first embodiment.

[0092] In addition, color printer 101 according to the second embodiment can achieve the same effects as color printer 1 according to the first embodiment.

[0093] 3. Other embodiments In the above-described first embodiment, a description has been given of a case where, when the printing rate TC of the cell region CS is equal to or higher than the printing rate threshold, the set temperature TmpS of the heating cells 40 adjacent to the cell region CS on both sides in the main scanning direction Drm is set higher than the original set temperature TmpS of the heating cells 40. The present invention is not limited to this, and, when the printing rate TC of the cell region CS is equal to or higher than the printing rate threshold, the set temperature TmpS of the heating cells 40 adjacent to the cell region CS on at least one side in the main scanning direction Drm may be set higher than the original set temperature TmpS of the heating cells 40.

[0094] In the above embodiment, the ON duty of the triacs 38a to 38e is controlled to adjust the amount of heat generated by the heat generating cell 40. However, the present invention is not limited to this, and the amount of heat generated by the heat generating cell 40 may be changed by various other methods.

[0095] Furthermore, in the above-described first embodiment, the ON duty of the heat generating cells 40 is classified into two types, α0 and α2, and in the second embodiment, the ON duty of the heat generating cells 40 is classified into three types, α0, α1, and α2. However, the present invention is not limited to this, and the ON duty of the heat generating cells 40 may be classified into various numbers of four or more types.

[0096] Furthermore, in the above-described embodiment, the range in which the printing rate TC is calculated is the range in the main scanning direction Drm of the cell eye region width Lb of each cell eye region CS, and the range in the sub-scanning direction Drs is the length from one end to the other end in the sub-scanning direction Drs of the recording medium P. The present invention is not limited to this, and the range in the sub-scanning direction Drs for calculating the printing rate TC may be any range in the sub-scanning direction Drs, from one print line to multiple pages in one print job.

[0097] Furthermore, in the above-mentioned embodiment, the present invention has been described as being applied to the color printer 1 or 101 equipped with a heater 31 having heat generating cells 40 formed by folding back the resistive heating wiring 41 extending along the main scanning direction Drm multiple times at both ends in the main scanning direction Drm. The present invention is not limited to this, and the present invention may be applied to color printers equipped with heaters 31 having heat generating cells 40 having various other structures as long as there is a cell stitch region CS between the heat generating cells 40 adjacent to each other in the main scanning direction Drm.

[0098] Furthermore, in the above-described embodiment, the present invention has been described as being applied to the color printer 1 or 101 equipped with a heater 31 having heat cells 40 divided into five along the main scanning direction Drm. The present invention is not limited to this, and the present invention may be applied to a color printer equipped with a heater 31 having heat cells 40 divided into various numbers, such as four or less or six or more, along the main scanning direction Drm.

[0099] Furthermore, in the above-described embodiment, the present invention has been described as being applied to the color printer 1 or 101 of a so-called direct transfer type, in which the toner image is directly transferred from the photosensitive drum 17 to the recording medium P. However, the present invention is not limited to this, and the present invention may also be applied to an image forming apparatus of a so-called intermediate transfer type (or secondary transfer type), in which the toner images of each color are transferred from the photosensitive drum 17 to an intermediate transfer belt as a medium so as to be superimposed one on top of the other, and the toner images are then transferred from this intermediate transfer belt to the recording medium P.

[0100] Furthermore, in the above-described embodiment, the present invention has been described as being applied to the color printer 1 or 101 that uses a developer for one-component development. However, the present invention is not limited to this, and may be applied to an image forming apparatus that uses a developer for two-component development, which is a method in which a carrier and a toner are mixed together and friction between the carrier and the toner is used to impart an appropriate amount of charge to the toner.

[0101] Furthermore, in the above-described embodiment, the present invention has been described as being applied to a tandem color printer 1 or 101 having image forming units 14 of each color arranged in series along the front-to-rear direction. However, the present invention is not limited to this, and may be applied to various other types of image forming apparatuses, such as a four-cycle type.

[0102] Furthermore, in the above-described embodiment, the present invention has been described as being applied to a color printer 1 or 101 that has four image forming units 14 corresponding to the colors of yellow, magenta, cyan, and black and forms color images using toner of four colors. However, the present invention is not limited to this, and the present invention may be applied to an image forming device that has three or less image forming units 14 or five or more image forming units 14 depending on the number of colors of toner used in the image forming device, or an image forming device that has one image forming unit and performs monochrome printing.

[0103] Furthermore, in the above-described embodiment, the present invention has been described as being applied to the color printer 1 or 101, which is a single-function printer. The present invention is not limited to this, and may be applied to image forming devices having various other functions, such as an MFP (Multi Function Peripheral) having the functions of a copier or facsimile machine. The present invention may also be applied to various electronic devices that have a fixing device that uses a heater divided into multiple units to perform heating and form images on recording media such as paper.

[0104] Furthermore, the present invention is not limited to the above-mentioned embodiments and other embodiments. That is, the scope of application of the present invention also extends to embodiments in which the above-mentioned embodiments and the other embodiments are combined in part or in whole as desired. The scope of application of the present invention also extends to embodiments in which a part of the configuration described in any of the above-mentioned embodiments and other embodiments is extracted and replaced or diverted with a part of the configuration of any of the above-mentioned embodiments and other embodiments, or an embodiment in which a part of the extracted configuration is added to any of the embodiments.

[0105] Furthermore, in the above-described embodiment, the fixing unit 20 is constituted by the heater 31 as a heat source, the fixing belt 30 as a heating rotator, and the pressure roller 32 as a pressure rotator, and the color printer 1 is constituted by the fixing unit, the print rate determination unit 74 as a print rate determination unit, and the fixing control unit 64 as a control unit. The present invention is not limited to this, and the fixing unit may be constituted by a heat source, a heating rotator, and a pressure rotator having various other configurations, and the image forming apparatus may be constituted by the fixing unit, the print rate determination unit having various other configurations, and the control unit. [Industrial Applicability]

[0106] The present invention can be used, for example, in electrophotographic printers. [Explanation of symbols]

[0107] 1: color printer, 2: main body, 3: top cover, 4: recording medium storage section, 5: transport path, 6: paper feed roller, 7: first registration roller, 8: second registration roller, 9: image forming section, 10: IN1 sensor, 11: IN2 sensor, 12: WR sensor, 13: transport belt, 14: image forming unit, 15: toner cartridge, 16: LED head, 17: photosensitive drum, 18: transfer roller, 19: F USER-IN sensor, 20... fuser, 21... EXIT sensor, 22... discharge stacker, 23... density sensor, 24... high voltage power supply, 25... low voltage power supply, 26... display, 30... fuser belt, 31... heater, 32... pressure roller, 33, 34, 35... temperature sensor, 36... heat diffusion member, 37... AC nominal voltage, 38... triac, 40... heat generating cell, 41... resistance heat generating wiring, 42... common power supply wiring, 43... power supply wiring, 50 ......Controller control unit, 51...CPU, 52...ROM, 53...RAM, 54...PC, 55...PC display unit, 56...PC input unit, 60...Process control unit, 61...High voltage control unit, 62...Exposure control unit, 63...Motor control unit, 64...Fuser control unit, 66...Supply voltage control unit, 67...Development voltage control unit, 68...Charge voltage control unit, 69...Transfer control unit, 70...Supply roller, 71...Development roller, 72...Charge roller, 74...Printing rate determination unit, 76...Main motor, CS...Cell eye area, PCS...Printing rate calculation area, TmpU...Hot limit temperature, TmpL...Fuser limit temperature, TmpLu...High printing rate fixing limit temperature, TmpLl...Low printing rate fixing limit temperature, TmpS...Set temperature, Rgf...Good fixing range, TmpF...Fuser belt temperature, L1...Temperature distribution line, Drm...Main scanning direction, Drs...Sub-scanning direction, Pa3...A3 size recording medium, P...Recording medium.

Claims

1. a heat source having a plurality of heat generating portions arranged along a main scanning direction and divided from one another, with an inter-heat generating portion region being formed between the heat generating portions adjacent to each other in the main scanning direction; A heating rotor heated by the heat source; A pressure rotating body that contacts an outer circumferential surface of the heating rotating body, a fixing device which holds a recording material carrying an image in a nip portion formed between the heating rotary body and the pressure rotary body, passes the recording material through the nip portion, and heat-fixes the image; An image forming apparatus having a printing rate determination unit that determines a printing rate of an image formed in a printing rate calculation area that is an area surrounded by a range in the main scanning direction that is the same as the area between the heat generating parts on the recording material and a range from one end to the other end on the recording material in a sub-scanning direction perpendicular to the main scanning direction; a control unit that, when the printing rate of the printing rate calculation area is a high printing rate equal to or higher than a predetermined threshold value, sets a set temperature of the heating unit adjacent to at least one side of the inter-heating unit area corresponding to the printing rate calculation area in the main scanning direction higher than an original set temperature of the heating unit; An image forming apparatus comprising:

2. The control unit is When the printing rate calculation area is the high printing rate, the set temperatures of the heating parts adjacent to the inter-heating part area corresponding to the printing rate calculation area on both sides in the main scanning direction are set higher than the original set temperatures of the heating parts. The image forming apparatus according to claim 1 .

3. The control unit is When the printing rate calculation region is the high printing rate, the set temperature of the nearest heat generating portion, which is the heat generating portion adjacent on both sides in the main scanning direction to the inter-heating portion region corresponding to the printing rate calculation region, is set to be higher than the original set temperature of the nearest heat generating portion, thereby suppressing a decrease in temperature of the region of the heating rotator corresponding to the inter-heating portion region, compared to a case in which the nearest heat generating portion is at the original set temperature. The image forming apparatus according to claim 2 .

4. The control unit is When the printing rate calculation region is the high printing rate, the set temperature of the nearest heat generating portion is set higher than the original set temperature of the nearest heat generating portion, thereby keeping the temperature of the region of the heating rotator corresponding to the region between the heat generating portions higher than a high printing rate fixing limit temperature, which is the lower limit temperature of the heating rotator in the case of the high printing rate. The image forming apparatus according to claim 3 .

5. The control unit is When the printing rate calculation region is the high printing rate, the set temperature of a nearest adjacent heating portion, which is the heating portion adjacent on both sides in the main scanning direction to the inter-heating portion region corresponding to the printing rate calculation region, is set higher than the original set temperature of the nearest adjacent heating portion, and the set temperature of the heating portion adjacent to the nearest adjacent heating portion on the side away from the inter-heating portion region is set higher than the original set temperature of the heating portion by an increase amount that is smaller than the increase amount from the original set temperature of the nearest adjacent heating portion. The image forming apparatus according to claim 2 .

6. The control unit is The power supplied to each of the heat generating parts can be individually controlled to be turned on and off, and the set temperature of the heat generating part can be increased by lengthening the time for which the power is turned on relative to the time for which the power is turned off. The image forming apparatus according to claim 1 .

7. The original set temperature is a temperature higher than the lower limit temperature of the heating rotatable body in the case of the high printing rate. The image forming apparatus according to claim 1 .

Images