Image forming apparatus

Abstract

To solve the problem that fixation of wide recording paper in succession to narrow recording paper often causes hot off-set or cold off-set at an end part, in which case a time for resolving off-set is required and printing efficiency is lowered.SOLUTION: An image forming apparatus has: a fixation belt 51; a fixation part 16 which has a central resistor 52a placed in a passage area of B5 and A4 recording paper 3 and an end part resistor 52b arranged in an area where an area A4 recording paper 3a passes and an area it does not pass are mixed; a temperature sensor 61 detecting temperature of an area of the central resistor 52a; and temperature control parts (160, 67, 23) which are provided with a printing rate determination part 162 and a narrow / wide width switch determination part 161 and control temperature of the heater 52. When printing the A4 recording paper after a B5 recording paper, a ratio of power to be supplied for the end part resistor 52b is changed during fixation on the B5 recording paper 3b.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a configuration of a fixing device in an image forming apparatus and a temperature control method. [Background Art]

[0002] Conventionally, as this type of apparatus, there has been an apparatus provided with a plurality of heaters divided in a longitudinal direction inside a heating roller, and performing temperature control on corresponding heaters in accordance with a printed image area (for example, refer to Patent Document 1). [Prior Art Documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2020-13069 (Page 7, Figure 2) [Summary of the Invention] [Problem to be Solved by the Invention]

[0004] When fixing on a wider recording sheet after continuing temperature control suitable for fixing on a narrow recording sheet, hot offset or cold offset may occur at the edge due to temperature rise or temperature drop in the non-sheet passing area of the narrow recording sheet. In addition, in order to prevent these problems, it is necessary to provide a time period for eliminating these offsets at the timing of switching between narrow / wide paper widths, which leads to a decrease in printing efficiency. [Means for Solving the Problem]

[0005] An image forming apparatus according to the present invention is an image forming apparatus capable of printing on recording media of different sizes, A fixing unit comprising a rotating member, a main heating element positioned in the longitudinal direction of the rotating member within the passage area of ​​a first recording medium and a second recording medium wider than the first recording medium, a sub-heating element positioned in a region where the second recording medium has both passage areas and non-passage areas, and a pressing member in contact with the outer circumferential surface of the rotating member, wherein a nip portion formed between the rotating member and the pressing member holds the first recording medium or the second recording medium on which the toner image has been transferred, passes through it, and heats and fixes the toner image; A temperature detection means for detecting the temperature of the region of the rotating member facing the main heating element, The system includes a print density determination unit that determines the print density of the toner image, a narrow / wide determination unit that determines the width of the recording medium to be printed in the longitudinal direction, and a temperature control unit that controls the on / off switching of power supplied to the main heating element and the sub-heating element based on the temperature detected by the temperature detection means. When the temperature control unit prints the second recording medium after the first recording medium, the temperature control unit controls the second recording medium The temperature of the rotating member corresponding to the passage region falls within a predetermined temperature range. As described above, during the fixing of the recording medium 1, Depending on the print density and number of printed pages of the first recording medium, The ratio of the power supplied to the sub-heater to the power supplied to the main heating element is Control, When the number of printed pages on the first recording medium is two or more, the ratio becomes smaller as the print density increases, and the ratio becomes larger as the print density decreases. It is characterized by the following: [Effects of the Invention]

[0006] According to the present invention, even when printing on a wider recording medium is performed after continuing temperature control suitable for fixing a narrow recording medium, the fixing temperature in the paper feeding area can be brought within a range suitable for the wider recording medium once the fixing of the narrow recording paper is completed, thereby resolving the problem. [Brief explanation of the drawing]

[0007] [Figure 1] This is a diagram showing the main components of a color printer as an image forming apparatus according to Embodiment 1 of the present invention. [Figure 2]This is a schematic diagram illustrating the internal structure of the fixing unit. (a) is a diagram showing the relationship between the fixing belt and pressure rollers, the control circuit that controls their temperature, and the passage areas for different sized recording papers, as viewed from the paper feeding side of the fixing unit. (b) is a plan view of the heater. [Figure 3] This is a diagram showing the main components of the fixing section as viewed from the axis of rotation of the pressure roller. [Figure 4] This is a block diagram showing the configuration of the control system for a color printer. [Figure 5] This is an image illustrating the temperature distribution along the longitudinal direction (Y direction) of the fixing belt surface when fixing recording paper of various sizes. [Figure 6] This diagram illustrates the temperature changes of a temperature sensor as the recording paper passes through the fuser section. (a) shows the positional relationship between the arrangement area of ​​the heater's central resistor and pair of end resistors in the longitudinal direction (Y direction) of the fuser section and the respective passage areas for B5 and A4 recording paper. (b) is a graph showing the temperature changes as each recording paper passes through the nip section of the fuser section. [Figure 7] This diagram illustrates the temperature changes and duty cycle of a temperature sensor when low-toner-density recording paper passes through the fuser section. (a) shows the positional relationship between the arrangement area of ​​the heater's central resistor and pair of end resistors in the longitudinal direction (Y direction) of the fuser section and the respective passage areas for B5 and A4 recording paper. (b) shows the temperature change graph for each area as each sheet of recording paper passes through the nip section of the fuser section. (c) shows the duty cycle of the heater as each sheet of paper passes through the nip section of the fuser section. [Figure 8] This diagram illustrates the temperature changes and duty cycle of a temperature sensor when recording paper with a high toner density passes through the fuser section. (a) shows the positional relationship between the arrangement area of ​​the heater's central resistor and pair of end resistors in the longitudinal direction (Y direction) of the fuser section and the respective passage areas for B5 and A4 recording paper. (b) shows the temperature change graph for each area as each sheet of recording paper passes through the nip section of the fuser section. (c) shows the same duty cycle of the heater as each sheet of paper passes through the nip section of the fuser section. [Figure 9]This diagram illustrates the temperature changes and duty cycle of a temperature sensor when recording paper with a low toner density passes through the fuser section in the present invention. (a) shows the positional relationship between the arrangement region of the heater's central resistor and a pair of end resistors in the longitudinal direction (Y direction) of the fuser section and the respective passage regions for B5 and A4 recording paper. (b) shows a temperature change graph for each region as each sheet of recording paper passes through the nip section of the fuser section. (c) shows the duty cycle of the heater as each sheet of paper passes through the nip section of the fuser section. [Figure 10] This diagram illustrates the temperature changes and on-duty cycle of a temperature sensor when a recording paper with a high toner density passes through the fuser section in the present invention. (a) shows the positional relationship between the arrangement region of the heater's central resistor and a pair of end resistors in the longitudinal direction (Y direction) of the fuser section and the respective passage regions for B5 and A4 recording paper. (b) shows a temperature change graph for each region as each recording paper passes through the nip section of the fuser section. (c) shows the on-duty cycle of the heater as each sheet of paper passes through the nip section of the fuser section. [Modes for carrying out the invention]

[0008] Embodiment 1. Figure 1 is a diagram showing the main components of a color printer 1 as an image forming apparatus according to Embodiment 1 of the present invention. As shown in the figure, the color printer 1 consists of a main body 1a and a top cover 1b.

[0009] Inside the main unit 1a, the recording paper storage unit 4, paper feed roller 5, travel system sensor 6, first registration roller 7, travel system sensor 8, second registration roller 9, travel system sensor 10, image forming unit 2Y for yellow (Y), image forming unit 2M for magenta (M), image forming unit 2C for cyan (C), image forming unit 2K for black (K), fuser unit 16, travel system sensor 17, and discharge stacker unit 18 are arranged in order from the upstream side of the transport path for the recording paper 3, which serves as the recording medium.

[0010] Furthermore, as the recording media herein, there are a wide-size (A4 in this case) recording sheet 3a accommodated in a recording sheet storage section 4a as described later, and a narrow-size (B5 in this case) recording sheet 3b accommodated in an additional recording sheet storage section 4b. When there is no particular need to distinguish between them, they may be referred to as the recording sheet 3. Also, with respect to the recording sheet storage section 4a, the additional recording sheet storage section 4b, the sheet feed roller 5a and the sheet feed roller 5b, when there is no particular need to distinguish between them, they may be respectively referred to as the recording sheet storage section 4 and the sheet feed roller 5.

[0011] Transfer rollers 15Y, 15M, 15C, 15K (which may be referred to as the transfer roller 15 when there is no particular need to distinguish between them) are disposed at positions facing the respective image forming units 2Y, 2M, 2C, 2K (which may be referred to as the image forming unit 2 when there is no particular need to distinguish between them). A conveyance belt unit 28 including a conveyance belt 11 that conveys the recording sheet 3 therethrough is provided in the conveyance path where the four transfer rollers 15 and the image forming units 2 are disposed to face each other respectively.

[0012] The recording sheets 3 to be printed by the color printer 1 are stored in a stacked state of a plurality of sheets by the recording sheet storage section 4. The sheet feed roller 5 feeds the recording sheets 3 one by one from the recording sheet storage section 4, and the first registration roller 7 and the second registration roller 9 convey the recording sheet 3 fed from the recording sheet storage section 4 by the sheet feed roller 5 to the conveyance belt unit 28.

[0013] In the conveyance path of the recording sheet 3, the traveling system sensor 6 and the traveling system sensor 8 installed immediately before the first registration roller 7 and the second registration roller 9 detect the arrival of the recording sheet 3 to obtain the operation timing for each registration roller, and the traveling system sensor 10 disposed downstream of the second registration roller 9 detects the arrived recording sheet 3, thereby detecting the timing of image formation on the recording sheet 3.

[0014] The image forming units 2Y, 2M, 2C, and 2K respectively include corresponding photosensitive drums 14Y, 14M, 14C, and 14K (may be referred to as photosensitive drum 14 when there is no particular need for distinction), and toner cartridges 13Y, 13M, 13C, and 13K serving as developer accommodating portions (may be referred to as toner cartridge 13 when there is no particular need for distinction).

[0015] Each photosensitive drum 14 is a photosensitive drum capable of forming an electrostatic latent image corresponding to print data on the drum surface by electrostatic force for each color. The electrostatic latent image is developed with a developer such as toner of a corresponding color, and holds a toner image to be transferred onto the recording sheet 3.

[0016] Since the image forming unit 2 and the toner cartridge 13 each require replacement as replaceable parts, the image forming unit 2 is configured to be detachably attachable to the main body of the color printer 1, and the toner cartridge 13 is configured to be detachably attachable to the main body of the image forming unit 2, respectively.

[0017] The respective transfer rollers 15Y, 15M, 15C, 15K are disposed to face the respective photosensitive drums 14Y, 14M, 14C, 14K via the conveyor belt 11, and sequentially superimpose and transfer the toner images of respective colors formed on the outer peripheral surfaces of the respective photosensitive drums 14Y, 14M, 14C, 14K onto the image forming surface of the recording sheet 3 conveyed by the conveyor belt 11. A high voltage is applied to the photosensitive drum 14 and the transfer roller 15 by a high-pressure control unit 180 (see FIG. 4) disposed in the main body 1a, thereby enabling electrostatically performed electrophotographic processes such as charging, developing, and transferring.

[0018] The fixing unit 16 fixes the toner image formed on the recording sheet 3 by heat and pressure. In the conveyance path of the recording sheet 3, a traveling system sensor 17 disposed downstream of the fixing unit 16 detects the recording sheet 3 with the fixed toner image, thereby enabling detection that the recording sheet 3 has finally been discharged to the discharge stacker unit 18.

[0019] Each of the aforementioned travel system sensors 6, 8, 10, and 17 is connected via a cable to the process control unit 150 (see Figure 4), which will be described later. In addition, each roller (paper feed roller 5, first registration roller 7, second registration roller 9, photoreceptor drum 14, transfer roller 15, and fixing unit 16) is mechanically driven by actuators (not shown), enabling the recording paper 3 to be transported downstream along the transport path.

[0020] On the other hand, the top cover section 1b is equipped with LED heads 24Y, 24M, 24C, and 24K (referred to as LED head 24 unless otherwise specified) and a display section 20.

[0021] The LED heads 24Y, 24M, 24C, and 24K are positioned to face the photoreceptor drums 14K, 14Y, 14M, and 14C of each image forming unit 2, respectively. They are exposure units that irradiate light according to the received print data and are supported so as to be displaceable without obstructing the opening and closing of the top cover 1b. When the top cover 1b is closed, the LED heads 24 can expose the surface of the photoreceptor drum 14 in close proximity and are connected to the main body 1a of the color printer 1 via a cable.

[0022] The display unit 20 is a printed circuit board consisting of a liquid crystal display panel and switches, etc., and enables the status display of the color printer 1 and user input operations. The liquid crystal display panel shall, for example, display 24 characters x 2 lines of characters.

[0023] The density sensor 22 is an optical sensor that reads a special pattern created on the conveyor belt 11 in order to perform print quality maintenance operations such as density correction. The low-voltage control unit 23 (see Figure 4) is an AC-DC power supply that converts commercial AC power to DC power. It supplies DC power such as 3.3V, 5V, and 24V to each circuit board (not shown). In addition, as will be described later, it supplies a nominal AC voltage 67 (AC100V) to the heater 52 (see Figure 2) of the fixing unit 16 via triacs 65 and 66.

[0024] The color printer 1 is also equipped with a removable paper storage unit 4a in the upper section and an additional paper storage unit 4b in the lower section. Here, the upper paper storage unit 4a stores A4 paper 3a as a second recording medium, and the additional paper storage unit 4b in the lower section stores B5 paper 3b as a first recording medium. In response to a print request, either A4 paper 3a or B5 paper 3b is selectively fed into the paper transport path. The paper storage units 4 installed in each section are detected by a cassette detection unit (not shown), and the paper size information of each paper storage unit 4 is transmitted to a process control unit 150 (see Figure 4), which will be described later.

[0025] In Figure 1, the X, Y, and Z directions represent the transport direction of the recording paper 3 as it passes through the image forming units 2Y, 2M, 2C, and 2K. The X direction is the transport direction of the photoreceptor drums 14Y, 14M, 14C, and 14K, the Y direction is the rotation axis direction of these drums, and the Z direction is the direction perpendicular to both of these axes. Furthermore, when the X, Y, and Z directions are shown in other figures described later, these directions represent the common direction. That is, the XYZ directions in each figure indicate the arrangement direction of the drawing portion of each figure when it constitutes the color printer 1 shown in Figure 1. Here, it is assumed that the Z direction is approximately vertical.

[0026] Figure 2 is a schematic diagram illustrating the internal structure of the fixing unit 16. Figure 2(a) is an explanatory diagram showing the relationship between the fixing belt 51 and pressure roller 53, the control circuit for temperature control, and the passage areas for recording paper 3 of different sizes, as viewed from the side where the recording paper 3 is fed into the fixing unit 16. Figure 2(b) is a plan view of the heater 52. Figure 3 is a diagram showing the main components of the fixing unit 16 as viewed from the direction of the rotation axis of the pressure roller 53.

[0027] As shown in these figures, the fixing unit 16 consists of a fixing belt 51, a heater 52, a pressure roller 53, temperature sensors 61, 62, 63, a heat diffusion member 54, and a heater holder and fixing support member (not shown).

[0028] The fixing belt 51, which functions as a rotating member, belt member, or fixed rotating member, is an endless strip-shaped member and is supported from the inside by a fixed support member (not shown). The fixing belt 51 has a structure in which a base material, a rubber layer, and a release layer are provided from the inside. For example, the base material is polyimide, the rubber layer is silicone rubber, and the release layer is made of a fluororesin such as PFA.

[0029] The heater 52, which serves as a heat source positioned inside the fixing belt 51, is composed of a rectangular base 52f extending in the longitudinal direction (Y direction) of the fixing belt 51 and a resistor that serves as a heating element formed on the base 52f. The resistor is divided into a central resistor 52a, which serves as the main heating element in the center, and two end resistors 52b, which serve as sub-heating elements, positioned adjacent to both ends of the resistor.

[0030] The central resistor 52a has a width that can support a narrow recording medium (in this case, the width of A5 recording paper 3c placed horizontally), while the resistor 25a and the two end resistors 52b together have a width that can support a wide recording medium (in this case, the width of A4 recording paper 3a placed horizontally). The central resistor 52a and the end resistors 52b are independently supplied with power from the AC nominal voltage 67 by the on / off controlled triacs 65 and 66, as described later, and generate heat. The AC nominal voltage 67 is the AC input voltage, which is AC 100V in this case.

[0031] The pressure roller 53, which serves as a pressurizing member or pressurizing rotating member, has a cylindrical member made of metal material with a rubber elastic layer covering its circumferential surface. It is positioned opposite the fixing belt 51 and forms a nip portion 16a (see Figure 6) with it. The material of the rubber elastic layer is, for example, silicone rubber. The pressure roller 53 is rotated in the direction of the arrow shown in Figure 3, thereby rotating the fixing belt 51 that it presses against in the direction of the arrow and transporting the recording paper 3 held by the nip portion in the X direction.

[0032] The heat diffusion member 54 is positioned between the heater 52 and the fixing belt 51 for the purpose of uniformly diffusing the temperature in the longitudinal direction (Y direction) and the width direction (X direction) of the fixing belt 51. The material of the heat diffusion member 54 is, for example, aluminum, which has high thermal conductivity.

[0033] The temperature sensor 61, used as a means of temperature detection, is a non-contact type temperature sensor. It is installed in the central paper feeding area and detects the surface temperature of the fixing belt 51. The temperature sensors 62 and 63 are contact type thermistors and are installed at both ends of the non-paper feeding area (the area through which the recording paper 3 does not pass) and detect the surface temperature of the pressure roller 53.

[0034] One end terminal of the AC nominal voltage 67 is connected to terminal 52c, which is connected to one end of the central resistor 52a and the two end resistors 52b. The other end terminal of the AC nominal voltage 67 is connected to the movable terminals of the triacs 65 and 66. The fixed terminal of the triac 65 is connected to terminal 52e, which is connected to the other end of the two end resistors 52b. The fixed terminal of the triac 66 is connected to terminal 52d, which is connected to the other end of the central resistor 52a.

[0035] The triacs 65 and 66 are semiconductor switching elements used for power control of the heater 52. As will be described later, they are switched on / off by the fixing control unit 160 (Figure 4) based on the detection result of the temperature sensor 61 so that the temperature of the fixing belt 51 is optimized.

[0036] Figure 4 is a block diagram showing the configuration of the control system for the color printer 1. In this figure, the PC 100, acting as a higher-level device, has a PC display unit 101 and a PC input unit 102, creates print data, and transmits the created print data to the color printer 1 via an interface such as USB or LAN. The PC 100 also receives instructions issued from the color printer 1 via the aforementioned interface.

[0037] The PC display unit 101 is a liquid crystal display or the like for displaying print images created by an application (not shown) and for displaying instructions issued from the color printer 1. The PC input unit 102 is a keyboard, mouse, or the like for creating print data images via an application (not shown) and for inputting responses to instructions issued from the color printer 1.

[0038] The central control unit 110 comprises a CPU 111, ROM 112, and RAM 113, and these components are connected by an internal bus. The CPU 111 controls the RAM 113 and the process control unit 150 according to the printing program stored in ROM 112. ROM 112 is a memory area for storing the printing program and is a non-volatile memory that can retain data even when the power to the color printer 1 is turned off. RAM 113 is a volatile memory that stores print data input from the PC 100, and the data is erased when the power to the color printer 1 is turned off.

[0039] The process control unit 150 receives position information of the recording paper 3 from a group of travel sensors 192 consisting of travel sensors 6, 8, 10, and 17, and has a high-voltage control unit 180, an exposure control unit 185, a motor control unit 170, and a fixing control unit 160, and properly controls the transport of the recording paper 3 and the printing process such as charging, developing, transferring, and fixing. The high-voltage control unit 180 and the exposure control unit 185 properly control the voltage applied to various rollers and the exposure by the exposure means in order to transfer the toner image onto the recording paper 3.

[0040] Specifically, the charging voltage control unit 184 applies a charging voltage to the charging roller 33 to charge the surface of the photoreceptor drum 14 to an appropriate voltage; the exposure control unit 185 uses the LED head 24 as an exposure means to irradiate the surface of the charged photoreceptor drum 14 with light according to the received print data to form an electrostatic latent image; the developing voltage control unit 183 applies a developing voltage to the developing roller 32 to develop the electrostatic latent image (toner image); the supply voltage control unit 182 applies a supply voltage to the supply roller 31 to supply toner to the developing roller 32; and the transfer control unit 181 applies a transfer voltage to the transfer roller 15 to transfer the toner development formed on the surface of the photoreceptor drum 14 to the recording paper 3.

[0041] The fixing control unit 160 is a control unit for adjusting the surface temperature of the fixing belt 51. Based on the detection result of the temperature sensor 61, it supplies power to the heater 52 from the AC nominal voltage 67 by turning on / off the triacs 65, 66, etc. of the low-voltage control unit 23, thereby controlling the surface temperature of the fixing belt 51 to a predetermined temperature. As will be described later, the triacs 65, 66, which maintain a table of set temperature, on-duty correction parameters, etc., and perform fixing control according to these parameters, have their fixed terminals and movable terminals conductive when they are ON. Note that the fixing control unit 160, the AC nominal voltage 67, and the low-voltage control unit 23 correspond to the temperature control unit.

[0042] The narrow / wide switching determination unit 161, which acts as a narrow / wide determination unit, holds the on-duty correction parameter and, when print jobs of different paper widths are printed consecutively, performs fixing control when switching from narrow to wide, measures the number of narrow prints, and further transmits the paper width information at the time of switching to the motor control unit 170.

[0043] The print rate determination unit 162 holds the on-duty correction parameters of the heating cells of the heater 52, determines the print rate of each data area from the print data information expanded in the RAM 113, and determines the on-duty cycle of each heating cell based on the determination result.

[0044] The motor control unit 170 controls and rotates the main motor 191 located inside the color printer 1. Specifically, it drives the image forming unit 2, the fuser unit 16, the first register roller 7, and the second register roller 9. Furthermore, based on the paper width information from the narrow / wide switching determination unit 161, it selects either the paper feed roller 5a of the paper storage unit 4a that accommodates A4 paper 3a or the paper feed roller 5b of the additional paper storage unit 4b that accommodates B5 paper 3b, and feeds out the selected paper 3.

[0045] In the above configuration, the basic printing operations performed by color printer 1 will first be explained with reference to Figures 1 to 4.

[0046] When the color printer 1 receives a print command from the PC 100, which acts as a higher-level device, the central control unit 110 controls each operation based on the control program and executes the print operation. First, the actuators such as the main motor 191 are controlled, and the paper feed roller 5, the first registration roller 7, and the second registration roller 9 are mechanically driven, transporting the recording paper 3 from the recording paper storage unit 4 into the interior of the color printer. The driving timing of the first registration roller 7 and the second registration roller 9 is determined by the position of the transported recording paper 3, which is detected by the travel system sensors 6 and 8.

[0047] When the leading edge of the recording paper 3 is detected by the transport sensor 10, the printing process starts. This controls the high-voltage control unit 180, which applies the voltage necessary for charging and developing the photoreceptor drum 14 to the charging roller 33, developing roller 32, etc., of the image forming unit 2. The drum surface of the photoreceptor drum 14 is uniformly charged by the charging roller 33, and then exposed by recording light from the LED head 24 corresponding to the print data signal from the PC 100. On the exposed drum surface of the photoreceptor drum 14, the print data is formed as an electrostatic latent image. At this time, the photoreceptor drum 14 and the transfer roller 15 are driven by their respective actuators to transport the recording paper 3 downstream (in the direction of arrow X).

[0048] The electrostatic latent image formed on the drum surface of the photoreceptor drum 14 is developed by the developing roller 32 using toner supplied from the toner cartridge 13, forming a toner image on the drum surface of the photoreceptor drum 14. A transfer voltage according to desired conditions is applied to the transfer roller 15 from the transfer control unit 181, and the toner image formed on the photoreceptor drum 14 is transferred by this transfer roller 15 to the surface of the transported recording paper 3. If the print data is color image data, the image forming units 2 and transfer rollers 15 corresponding to each color (Y), (M), (C), and (K) perform process operations for each color, and toner images of each color are sequentially superimposed and transferred onto the surface of the recording paper 3 to form a color image.

[0049] The recording paper 3 on which the image has been formed is transported to the fixing unit 16, where it is fixed by heat and pressure. The fixing unit 16 has a fixing belt 51 equipped with a heater 52 and a pressure roller 53. The fixing temperature is detected by a temperature sensor 61, and the temperature is controlled so that fixing is performed at an appropriate temperature, as will be described later. The recording paper 3 that has gone through the above printing process passes through the travel system sensor 17 and is discharged to the discharge stacker unit 18.

[0050] Next, we will explain the fixing operation by the fixing unit 16, but first we will explain the problems that existed before implementing the present invention.

[0051] <Regarding the rise in temperature of paper that is not being fed> Figure 5 is an image of the temperature distribution in the longitudinal direction (Y direction) on the surface of the fixing belt 51 when fixing each size of recording paper 3. In the figure, temperature distribution 81 is the temperature distribution when printing A5 recording paper 3c (landscape orientation), temperature distribution 82 is the temperature distribution when printing A4 recording paper 3a (landscape orientation), and temperature distribution 83 is the temperature distribution when printing B5 recording paper 3b (landscape orientation).

[0052] The fixing unit 16 fixes the toner image formed on the recording paper 3 using heat and pressure. The surface temperature of the fixing belt 51 is detected by a non-contact temperature sensor 61 located in the center of the longitudinal direction, and the fixing control unit 160 controls the power supplied from the low-pressure control unit 23 to the heater 52 by turning it on and off via triacs 65 and 66 to maintain a predetermined set temperature. The predetermined temperature is the set temperature determined for each medium.

[0053] The heater 52 is divided into a central resistor 52a and end resistors 52b at both ends, and is connected to a nominal AC voltage (100V) 67 and triacs 65 and 66. As shown in Figure 2, the triac 66 is connected to the central resistor 52a, and the triac 65 is connected to the two end resistors 52b, allowing the central and end sections to be controlled independently.

[0054] The division position between the central resistor 52a and each end resistor 52b is determined by the size of the target recording paper 3. Here, as shown in Figures 2 and 5, the length of the central resistor 52a is set to match the width of an A5 recording paper 3c (landscape orientation), and the maximum length of the two end resistors 52b combined supports the width of an A4 recording paper 3a (landscape orientation). The central resistor 52a is driven on duty cycle so that the fixing belt 51 reaches the set temperature, while the two end resistors 52b are driven on duty cycles that are calculated by multiplying the on-duty cycle of the central resistor 52a by a different coefficient depending on the size of the recording paper 3.

[0055] In other words, the on-duty cycle Ds[%] of the two end resistors 52b is determined by the following formula, where Dm[%] is the actual on-duty cycle of the central resistor 52a and α is the paper size coefficient due to the paper size. Ds = Dm × α ... (1) Note that the on-duty cycles Dm[%] and Ds[%] here represent the ratio of the voltage applied to each resistor per given time.

[0056] As shown in Table 1, for example, the paper size coefficient for A4 size is α1, the paper size coefficient for B5 size is α2, and the paper size coefficient for A5 size is α3, and the relationship is 1 ≥ α1 > α2 > α3. [Table 1]

[0057] As shown in the temperature distributions 81, 82, and 83 in Figure 5, by separating and independently controlling the central resistor 52a and each end resistor 52b of the heater 52, the temperature rise in the non-paper-feeding area can be suppressed when printing recording paper 3 smaller than the size of A5 recording paper 3c (landscape orientation). However, in the case of recording paper 3 smaller than the size of A4 recording paper 3a (landscape orientation) and with a width greater than the length of the central resistor 52a, for example, B5 recording paper 3b (landscape orientation), a larger portion of the end resistors 52b is exposed relative to the paper width, resulting in a greater temperature rise in the non-paper-feeding area. In the following explanation, it will be assumed that the recording paper 3 is printed in a landscape orientation.

[0058] <Regarding the set temperature> Figure 6 is an explanatory diagram showing the temperature change of the temperature sensor 61 as the recording paper 3 passes through the fixing section 16. Figure 6(a) shows the positional relationship between the arrangement area of ​​the central resistor 52a and the pair of end resistors 52b of the heater 52 and the passing areas of the B5 recording paper 3b and A4 recording paper 3a in the longitudinal direction (Y direction) of the fixing section 16. Figure 6(b) is a temperature change grab as each recording paper passes through the nip section 16a of the fixing section 16.

[0059] The time on the time axis in Figure 6(b) indicates the time when the B5 recording paper 3b or A4 recording paper 3a passes the nip portion 16a of the fixing unit 16. Therefore, the nip portion 16a shown in Figure 6(a) moves relatively to the right as time progresses. For example, at time t01, it indicates that the leading edge of the B5 recording paper 3b has reached the nip portion 16a, and at time t03, it indicates that the leading edge of the A4 recording paper 3a has reached the nip portion 16a.

[0060] As shown in Figure 6(b), the set temperature Tf is determined to fall within the optimal temperature range required for fixing. Table 2 shows numerical examples of the upper and lower limits of the optimal temperature range, the set temperature, and the corresponding printing speed for each type of paper. [Table 2]

[0061] The optimal temperature range is defined by an upper temperature limit Tu (180°C in this case) above which printing defects such as hot offset occur, and a lower temperature limit Tl (150°C in this case) below which printing defects such as fixing defects occur. This optimal temperature range differs for each type of paper when the printing speed is different, and the set temperature Tf is experimentally determined so that it remains within the optimal temperature range even if overshoot, temperature drop, etc. occur.

[0062] Here, the recording paper 3 to be printed is A4 recording paper 3a and B5 recording paper 3b of similar thickness, and each recording paper is set to the same printing speed (35 ppm), upper limit 180°C / lower limit 150°C, and set temperature 165°C. In this embodiment, the color printer 1 supports a maximum print size of A4 landscape and a maximum printing speed of 35 ppm. Furthermore, it is possible to print while switching paper widths, and it is designed to be able to print about a dozen or so sheets of narrow media such as B5 size at a printing speed of 35 ppm.

[0063] Furthermore, in the temperature transition graph in Figure 6(b), the rear end of the B5 recording paper 3b passes through the nip section 16a at time t03, and the rear end of the A4 recording paper 3a passes through the nip section 16a at time t04. As a control to end printing, the heater 52 is turned off, and the temperature gradually drops below the optimal temperature range.

[0064] In this case, we will consider the problems that arise when switching between printing on narrow paper, such as B5 size, and wide paper, such as A4 size.

[0065] <Switching from narrow to wide width when toner density is low> Figure 7 is an explanatory diagram showing the temperature changes and on-duty cycle of the temperature sensor 61 as the recording paper 3 passes through the fixing unit 16. Figure 7(a) shows the positional relationship between the arrangement area of ​​the central resistor 52a and the pair of end resistors 52b of the heater 52 and the passing areas of the B5 recording paper 3b and A4 recording paper 3a in the longitudinal direction (Y direction) of the fixing unit 16. Figure 7(b) shows the temperature change grab for each area as each recording paper passes through the nip portion 16a of the fixing unit 16. Figure 7(c) shows the on-duty cycle of the heater 52 as each paper passes through the nip portion 16a of the fixing unit 16.

[0066] The times on the time axis in Figures 7(b) and (c) indicate the time when the B5 recording paper 3b or A4 recording paper 3a passes the nip portion 16a of the fixing unit 16. Therefore, the nip portion 16a shown in Figure 7(a) moves relatively to the right as time progresses. For example, at time t1, the leading edge of the B5 recording paper 3b has reached the nip portion 16a; at time t2, the trailing edge of a few sheets of B5 recording paper 3b has reached the nip portion 16a; and at time t3, the leading edge of the A4 recording paper 3a has reached the nip portion 16a.

[0067] Referring to Figure 7, we will explain the problems that arise when switching from narrow paper to wide paper, specifically the problems that occur when printing on multiple sheets of B5 recording paper 3b with low toner density, followed by printing on multiple sheets of A4 recording paper 3a. In Figure (b), the solid black line represents the temperature change in region A (the region of the central resistor 52a that passes through both B5 and A4), i.e., the temperature detected by the temperature sensor 61. The dotted line represents the temperature change in region B (the region of the end resistor 52b that passes through both B5 and A4), and the dashed line represents the temperature change in region C (the region of the end resistor 52b that does not pass through B5 but does pass through A4). Figure (c) shows the on-duty cycles of the central resistor 52a and the end resistors 52b, and further, the on-duty cycle Ds[%] of the end resistor 52b is shown with the paper size coefficients α1, α2, and α3 used at that timing.

[0068] Furthermore, the temperatures in regions B and C were detected for analysis purposes using test temperature sensors placed in each region under the same conditions as temperature sensor 61. These temperature sensors are not originally equipped in color printer 1.

[0069] At time t0, a print job for B5 recording paper 3b is received, and the temperature is raised from room temperature towards the set temperature of 165°C. To print B5 recording paper 3b, the paper size coefficient of the edge resistor 52b becomes α2, which is lower than the on-duty cycle Dm[%] of the central resistor 52a in region A, so region A becomes the hottest. At this time, region B is hotter than region C because the heat transfer from region A makes the temperature of region B higher than that of region C.

[0070] When the temperature detected by the temperature sensor 61 reaches the printable temperature (approximately -5°C from the set temperature), the feeding of B5 recording paper 3b begins, and at time t1, the B5 recording paper 3b reaches the nip portion 16a of the fuser unit 16. In areas A and B, through which the B5 recording paper 3b passes, a temperature drop occurs due to the recording paper 3, but the control is maintained to keep the set temperature. At this time, it is assumed that low-toner density text or similar material is printed on the B5 recording paper 3b. The B5 recording paper 3b does not pass through area C, but the edge resistor 52b is controlled by a paper size coefficient α2 corresponding to the B5 size, so the temperature rises.

[0071] At time t2, printing of multiple B5 recording sheets 3b is completed, but a print job for the next A4 recording sheet 3a has been received. At the same set temperature of 165°C, at time t3, after the shortest possible waiting time T1, the A4 recording sheet 3a, which has just started to feed, reaches the nip portion 16a of the fuser unit 16. Regions A and B are maintained at the set temperature, and region C, which is the region through which A4 size paper passes, also gradually settles to near the set temperature.

[0072] However, at time t2, when printing multiple sheets of B5 recording paper 3b is completed, the temperature of region C, which is not passed through by the B5 recording paper 3b, exceeds the upper limit temperature Tu of the good temperature range, and remains above the upper limit temperature Tu even at time t3, when fixing of A4 recording paper 3a begins.

[0073] Therefore, in order to prevent the temperature of area C, which is the A4 size paper feeding area, from exceeding the upper limit temperature Tu of the good temperature range when switching from B5 recording paper 3b to A4 recording paper 3a, it is necessary to ensure a sufficient waiting time T1 during the switch. If the waiting time T1 is not sufficiently ensured for reasons such as shortening the printing time, the temperature of area C will remain above the upper limit temperature Tu (180°C in this case), making it more likely for printing defects such as hot offset to occur in some areas of the A4 recording paper 3a.

[0074] Furthermore, if the printing after time t1 is only one sheet of B5 recording paper 3b, and then a request to print A4 recording paper 3a comes immediately afterward, the temperature rise in region C will be slow. Conversely, the temperature in region C will not reach the lower limit temperature Tl (150°C in this case), making it more likely for printing defects such as cold offset due to insufficient temperature to occur.

[0075] <Switching from narrow to wide when toner density is high> Figure 8 is an explanatory diagram showing the temperature changes and on-duty cycle of the temperature sensor 61 as the recording paper 3 passes through the fixing unit 16. Figure 8(a) shows the positional relationship between the arrangement area of ​​the central resistor 52a and the pair of end resistors 52b of the heater 52 and the respective passage areas of the B5 recording paper 3b and A4 recording paper 3a in the longitudinal direction (Y direction) of the fixing unit 16. Figure 8(b) shows the temperature change grab for each area as each recording paper passes through the nip portion 16a of the fixing unit 16. Figure 8(c) shows the on-duty cycle of the heater 52 as each paper passes through the nip portion 16a of the fixing unit 16.

[0076] Referring to Figure 8, we will explain the problems that arise when switching from narrow paper to wide paper when printing with toner density (200% print density) on narrow paper. Note that the time axis in Figures 8(b) and (c) and the positional relationship of the nip portion 16a in Figure 8(a), the relationship between each line in Figure 8(b) and the printing area in Figure 8(a), and the method of displaying the on-duty cycle and paper size coefficient in Figure 8(c) are the same as in Figure 7, so we will omit the explanation here.

[0077] At time t0, a print job for B5 recording paper 3b is received, and the temperature is raised from room temperature towards the set temperature of 165°C. To print B5 recording paper 3b, the paper size coefficient of the on-duty cycle Ds[%] of the edge resistor 52b becomes α2, which is lower than the on-duty cycle Dm[%] of the central resistor 52a in region A, so region A becomes the hottest. At this time, region B is hotter than region C because the heat transfer from region A makes the temperature of region B higher than that of region C.

[0078] When the temperature detected by the temperature sensor 61 reaches the printable temperature (approximately -5°C from the set temperature), the feeding of B5 recording paper 3b begins, and at time t1, the B5 recording paper 3b reaches the nip portion 16a of the fuser unit 16. Although a temperature drop occurs in areas A and B through which the B5 recording paper 3b passes, the control is maintained to preserve the set temperature.

[0079] At this time, if the B5 recording paper 3b is printed with a toner density of 200%, for example, blue, across the entire width of the recording medium, the on-duty cycle Dm[%] of the central resistor 52a in area A increases compared to that of a low toner density, and consequently, the on-duty cycle Ds[%] of the end resistors 52b also increases compared to that of a low toner density. This is because the increased print density requires more energy to maintain the set temperature. In area C, which the B5 recording paper 3b does not pass through, the temperature rises even further than at the low toner density due to the increased on-duty cycle Ds[%] of the end resistors 52b.

[0080] At time t2, printing of multiple B5 recording sheets 3b is completed, but the print signal for the next A4 recording sheet 3a has been received. At the same set temperature of 165°C, at time t3, after the shortest possible waiting time T1, the A4 recording sheet 3a, which has just begun to feed, reaches the nip portion 16a of the fuser unit 16. Regions A and B are maintained at the set temperature, and region C, which is the region through which A4 size paper passes, also gradually settles to near the set temperature.

[0081] However, at time t2, when printing multiple sheets of B5 recording paper 3b is completed, the temperature of area C, which is not passed through by the B5 recording paper 3b, exceeds the upper limit temperature Tu of the good temperature range, even more so than at low toner density, and remains above the upper limit temperature Tu at time t3, when fixing of A4 recording paper 3a begins.

[0082] Therefore, in order to prevent the temperature of area C, which is the A4 size paper feeding area, from exceeding the upper limit temperature Tu of the good temperature range when switching from B5 size recording paper 3b to A4 size recording paper 3a, it is necessary to ensure a sufficiently long waiting time T1 during the switch compared to when using low toner density. If the waiting time T1 is not sufficiently ensured for reasons such as shortening the printing time, the temperature of area C will remain above the upper limit temperature Tu (180°C in this case), making it more likely for printing defects such as stronger hot offset to occur in some areas of the A4 recording paper 3a than when printing with B5 size and low toner density.

[0083] Next, the printing operation of the color printer 1 according to the present invention, which solves the above-mentioned problems, will be described below with reference to Figures 9 and 10.

[0084] <Switching from narrow to wide width when toner density is low> Figure 9 is an explanatory diagram showing the temperature changes and on-duty cycle of the temperature sensor 61 as the recording paper 3 passes through the fixing unit 16. Figure 9(a) shows the positional relationship between the arrangement area of ​​the central resistor 52a and the pair of end resistors 52b of the heater 52 and the passing areas of the B5 recording paper 3b and A4 recording paper 3a in the longitudinal direction (Y direction) of the fixing unit 16. Figure 9(b) shows the temperature change grab for each area as each recording paper passes through the nip portion 16a of the fixing unit 16. Figure 9(c) shows the on-duty cycle of the heater 52 as each paper passes through the nip portion 16a of the fixing unit 16.

[0085] Referring to Figure 9, the printing operation when switching from narrow paper to wide paper when printing with low toner density on narrow paper using the color printer 1 according to the present invention will be explained. Note that the time axis in Figures 9(b) and (c) and the positional relationship of the nip portion 16a in Figure 9(a), the relationship between each line in Figure 9(b) and the printing area in Figure 9(a), and the method of displaying the on-duty cycle in Figure 9(c) are the same as in Figure 7, so the explanation here will be omitted. However, here the on-duty cycle Ds[%] of the end resistor 52b also shows the correction coefficient β[%] which will be described later, in addition to the paper size coefficients α1, α2, and α3 used at that timing.

[0086] The on-duty cycle Ds[%] of the end resistor 52b is calculated by the following formula, where Dm[%] is the actual on-duty cycle of the central resistor 52a, α is the paper size coefficient due to the paper size, and β[%] is the correction coefficient. Ds = Dm × α + β ... (2) Here, if we denote the ratio of the power supplied to the end resistors 52b to the power supplied to the central resistor 52a as ra, ra = (Dm × α + β) / Dm ... (3) This is the result.

[0087] Table 3 shows numerical examples of the correction coefficient β[%] for the on-duty cycle Ds[%] of the end resistor 52b, which is set according to various conditions. [Table 3] As shown in the table, the correction coefficient β[%] is set to a suitable value in advance according to the number of prints and toner density (hereinafter referred to as print density) when printing on narrow paper (in this case, B5 recording paper 3b). By substituting this value into the above formula (2), the on-duty cycle of the edge resistor 52b is adjusted so that the temperature in each region A, B, and C of the fuser unit 16 remains within a good temperature range, even when switching from narrow to wide paper size.

[0088] Referring to Figure 9, the operation when printing multiple sheets of A4 recording paper 3a after printing multiple sheets of B5 recording paper 3b at a low print density will be explained. In Figure 9(b), the solid black line represents the temperature change in region A (the region of the central resistor 52a that passes through both B5 and A4), i.e., the temperature detected by the temperature sensor 61. The dotted line represents the temperature change in region B (the region of the end resistor 52b that passes through both B5 and A4), and the dashed line represents the temperature change in region C (the region of the end resistor 52b that does not pass through B5 but does pass through A4). Figure 9(c) shows the on-duty cycle Ds[%] of the central resistor 52a and the end resistors 52b, and further, the on-duty cycle Ds[%] of the end resistor 52b shows the paper size coefficients α1, α2, α3 and the correction coefficient β used at that timing.

[0089] At time t0, a print job for B5 size recording paper 3b is received, and the temperature is raised from room temperature towards the set temperature of 165°C. In this case, in equation (2) above, the paper size coefficient α becomes α2 to print B5 size, and the correction coefficient β is set to +10% as indicated in the warm-up section (indicated as Wup in Table 3), as shown in Table 3. As a result, the temperatures of regions B and C also rise by that amount (see comparison in Figure 7(b)). Note that here the temperature is raised during the warm-up for B5 size printing, but it could also be done just before A4 size printing. Furthermore, if the temperature of region C is considered to be within a good temperature range before A4 size printing as a result of repeated B5 printing, this temperature increase is unnecessary.

[0090] When the temperature detected by the temperature sensor 61 reaches the printable temperature (approximately -5°C from the set temperature), the feeding of B5 recording paper 3b begins, and at time t1, the B5 recording paper 3b reaches the nip portion 16a of the fuser unit 16. In areas A and B through which the B5 recording paper 3b passes, a temperature drop occurs due to the recording paper 3, but the control is maintained to keep the set temperature. At this time, printing of characters or other text with a print density of 20% or less is performed on the B5 recording paper 3b.

[0091] Although the B5 recording paper 3b does not pass through region C, the edge resistor 52b is controlled by the on-duty cycle Ds[%] calculated using the paper size coefficient α2 and correction coefficient β(+10%) corresponding to the B5 size in equation (2) above, so the temperature rises (see comparison in Figure 7(b)).

[0092] At time t11, if the printing of the first B5 recording sheet 3b is completed and the next print is also a B5 recording sheet 3b (second sheet) with a print density of 20% or less, the value of the correction coefficient β is changed from +10% to +5% as shown in Table 3. In this way, from time t11 until the printing of the B5 recording sheets 3b is completed, the value of the correction coefficient β shown in Table 3 is selected according to the number of B5 recording sheets 3b printed and the print density at that time, and the on-duty cycle Ds[%] of the end resistor 52b is continuously calculated using the above equation (2), so the on-duty cycle Ds[%] decreases in a stepwise manner as shown in Figure (c).

[0093] At time t2, printing of multiple B5 recording sheets 3b is completed. However, during the printing of the B5 recording sheets 3b, the on-duty cycle Ds[%] of the edge resistor 52b was adjusted to control region C so that it remained within the good fixing range (see comparison in Figure 7(b)). Therefore, even when the A4 recording sheet 3a, which has just started to be fed, reaches the nip portion 16a of the fixing unit 16 at time t3, after the shortest possible waiting time T1, regions A, B, and C continue to be maintained within the good fixing temperature range.

[0094] As described above, when switching from B5-sized recording paper 3b to A4-sized recording paper 3a, the temperature of area C, which is the A4-sized paper feeding area, can be controlled so that it does not exceed the upper limit of the good temperature range. Therefore, even with the minimum waiting time T1, printing defects such as hot offset can be prevented on the A4 recording paper 3a. Furthermore, even if only one sheet of B5 recording paper 3b is printed after time t1, and a print request for A4 recording paper 3a comes immediately afterward, the temperature of area C quickly reaches above the lower limit temperature Tl, thus preventing cold offset due to insufficient temperature.

[0095] <Switching from narrow to wide when toner density is high> Figure 10 is an explanatory diagram showing the temperature changes and on-duty cycle of the temperature sensor 61 as the recording paper 3 passes through the fixing unit 16. Figure 10(a) shows the positional relationship between the arrangement area of ​​the central resistor 52a and the pair of end resistors 52b of the heater 52 and the passing areas of the B5 recording paper 3b and A4 recording paper 3a in the longitudinal direction (Y direction) of the fixing unit 16. Figure 10(b) shows the temperature change grab for each area as each recording paper passes through the nip portion 16a of the fixing unit 16. Figure 10(c) shows the on-duty cycle of the heater 52 as each paper passes through the nip portion 16a of the fixing unit 16.

[0096] Referring to Figure 10, the printing operation when switching from narrow paper to wide paper when performing high-density printing with a high toner density using the color printer 1 according to the present invention will be explained. Note that the time axis in Figures 9(b) and (c) and the positional relationship of the nip portion 16a in Figure 9(a), the relationship between each line in Figure 9(b) and the printing area in Figure 9(a), and the method of displaying the on-duty cycle in Figure 9(c) are the same as in Figure 7, so their explanation will be omitted here. However, here, the on-duty cycle Ds[%] of the end resistor 52b also shows the correction coefficient β[%] in addition to the paper size coefficients α1, α2, and α3 used at that timing.

[0097] Here too, the on-duty cycle Ds[%] of the end resistor 52b is calculated by equation (2) above, and even when switching from a narrow width to a wide width, the temperature in each region A, B, and C of the fixing section 16 is adjusted to stay within a good temperature range.

[0098] Referring to Figure 10, the operation when printing multiple sheets of A4 recording paper 3a after high-print-rate printing on multiple sheets of B5 recording paper 3b will be explained. In Figure 10(b), the solid black line represents the temperature change in region A (the region of the central resistor 52a that passes through both B5 and A4), i.e., the temperature detected by the temperature sensor 61. The dotted line represents the temperature change in region B (the region of the end resistor 52b that passes through both B5 and A4), and the dashed line represents the temperature change in region C (the region of the end resistor 52b that does not pass through B5 but does pass through A4). Figure 10(c) shows the on-duty cycles of the central resistor 52a and the end resistors 52b, and further, the on-duty cycle Ds[%] of the end resistor 52b shows the paper size coefficients α1, α2, α3 and the correction coefficient β used at that timing.

[0099] At time t0, a print job for B5 size paper 3b is received, and the temperature is raised from room temperature towards the set temperature of 165°C. In this case, in equation (2) above, the paper size coefficient α becomes α2 to print B5 size, and the correction coefficient β is set to +10% as indicated in the warm-up section (indicated as Wup in Table 3), as shown in Table 3. As a result, the temperatures of regions B and C also rise by that amount (see comparison in Figure 8(b)). Note that here the temperature is raised during the warm-up for B5 size printing, but it could also be done just before A4 size is printed.

[0100] When the temperature detected by the temperature sensor 61 reaches the printable temperature (approximately -5°C from the set temperature), the feeding of B5 recording paper 3b begins, and at time t1, the B5 recording paper 3b reaches the nip portion 16a of the fuser unit 16. In areas A and B through which the B5 recording paper 3b passes, a temperature drop occurs due to the recording paper 3, but the control is maintained to keep the set temperature. At this time, printing of 100% or more of a color such as blue is performed on the B5 recording paper 3b.

[0101] At this time, if printing with a print density of 100% or more is performed on the B5 recording paper 3b, the on-duty cycle Dm[%] of the central resistor 52a in area A increases compared to when printing with a low print density, and consequently, the on-duty cycle Ds[%] of the end resistors 52b also increases compared to when printing with a low toner density. This is because the energy required to maintain the set temperature increases due to the higher print density.

[0102] Although the B5 recording paper 3b does not pass through region C, the edge resistor 52b is controlled by the on-duty cycle Ds[%] calculated using the paper size coefficient α2 and correction coefficient β(+10%) corresponding to the B5 size in equation (2) above, so the temperature rises (see comparison in Figure 8(b)).

[0103] At time t11, if the printing of the first B5 recording sheet 3b is completed and the next print is also a B5 recording sheet 3b (second sheet) with a print density of 100% or more, the value of the correction coefficient β is changed from +10% to 0.0% as shown in Table 3. In this way, from time t11 until the printing of the B5 recording sheets 3b is completed, the value of the correction coefficient β shown in Table 3 is selected according to the number of B5 recording sheets 3b printed and the print density at that time, and the on-duty cycle Ds[%] of the end resistor 52b is continuously calculated using the above equation (2), so the on-duty cycle Ds[%] decreases in a stepwise manner as shown in Figure (c).

[0104] At time t2, printing of multiple B5 recording sheets 3b is completed. However, during the printing of the B5 recording sheets 3b, the on-duty cycle Ds[%] of the edge resistor 52b was adjusted to control region C so that it also remained within the good fixing range (see comparison in Figure 8(b)). Therefore, even when the A4 recording sheet 3a, which has just started to be fed, reaches the nip portion 16a of the fixing unit 16 at time t3, after the shortest possible waiting time T1, regions A, B, and C are continuously maintained within the good fixing range.

[0105] As described above, when switching from B5 recording paper 3b to A4 recording paper 3a, the temperature of area C, which is the A4 size paper feeding area, can be controlled so as not to exceed the upper limit of the good temperature range. Therefore, even with the shortest waiting time, it is possible to prevent printing defects such as hot offset from occurring on the A4 recording paper 3a.

[0106] As described above, when switching from B5 size to A4 size, the temperature of area C, which is the A4 paper feeding area, can be controlled so that it does not exceed the upper limit of the good range for A4 size. Therefore, even with the minimum waiting time T1, printing is possible without hot offset at the edges of the A4 size. Furthermore, even if the printing after time t1 is only one sheet of B5 record paper 3b, and a request to print A4 record paper 3a comes immediately afterward, the temperature of area C quickly reaches above the lower limit temperature Tl, thus preventing cold offset due to insufficient temperature.

[0107] In this embodiment, the heater 52 is configured with a central resistor 52a and a pair of end resistors 52b arranged adjacent to both ends thereof. However, it is not limited to this configuration. In cases where recording media of various sizes are fed using the edges of the paper feeding area as a reference, it may be configured with a main resistor corresponding to the central resistor 52a and sub-resistors corresponding to the end resistors 52b. Various configurations are possible.

[0108] As described above, with the color printer 1 of this embodiment, even when printing (fixing) multiple sheets of narrow recording paper and then printing (fixing) a wide recording paper, the fixing temperature can be set to a temperature suitable for fixing the next wide recording paper at the stage when the fixing of the narrow recording paper is completed. Therefore, there is no need to allow an extra time interval when switching between narrow and wide, and a decrease in printing efficiency can be prevented.

[0109] Furthermore, although terms such as "up," "down," "left," "right," "front," and "back" were used in the above-mentioned claims and descriptions of embodiments, these are for convenience only and do not limit the absolute positional relationships in the state in which the image forming apparatus is arranged. [Industrial applicability]

[0110] In the embodiments described above, an example of the present invention being used in an image forming apparatus as a color printer was shown. However, the present invention is not limited to this, and can also be used in image processing devices such as copiers, facsimile machines, and MFPs that are equipped with a fuser. Furthermore, although a color printer has been described, a monochrome printer may also be used. [Explanation of Symbols]

[0111] 1 Color printer, 1a Main unit, 1b Top cover, 2 Image forming unit, 3 Recording paper, 3a A4 recording paper, 3b B5 recording paper, 3c A5 recording paper, 4 Recording paper storage unit, 4b Additional recording paper storage unit, 5 Paper feed roller, 6 Travel system sensor, 7 First registration roller, 8 Travel system sensor, 9 Second registration roller, 10 Travel system sensor, 11 Transfer belt, 13 Toner cartridge, 14 Photoconductor drum, 15 Transfer roller, 16 Fixing unit, 16a Nip unit, 17 Travel system sensor, 18 Ejection stacker unit, 19 High-voltage power supply, 20 Display unit, 22 Density sensor, 23 Low-voltage control unit, 24 LED head, 28 Conveyor belt unit, 31 Supply roller, 32 Developing roller, 33 Charging roller, 51 Fixing belt, 52 Heater, 52a Central resistor, 52b End resistors, 52c Terminal, 52d Terminal, 52e Terminal, 52f Base, 53 Pressure roller, 54 Heat diffusion member, 61 Temperature sensor, 62 Temperature sensor, 63 Temperature sensor, 65 Triac, 66 Triac, 67 AC nominal voltage, 100 PC, 101 PC display unit, 102 PC input unit, 110 Central control unit, 111 CPU, 112 ROM, 113 RAM, 150 Process control unit, 160 Fixing control unit, 161 Narrow / wide switching determination unit, 162 Printing rate determination unit, 170 Motor control unit, 180 High voltage control unit, 181 Transfer control unit, 182 Supply voltage control unit, 183 Development voltage control unit, 184 Charging voltage control unit, 185 Exposure control unit, 191 Main motor, 192 Travel system sensor group.

Claims

1. In an image forming apparatus capable of printing on recording media of different sizes, A fixing unit comprising a rotating member, a main heating element positioned in the longitudinal direction of the rotating member within the passage area of ​​a first recording medium and a second recording medium wider than the first recording medium, a sub-heating element positioned in a region where the second recording medium has both passage areas and non-pass areas, and a pressing member in contact with the outer circumferential surface of the rotating member, wherein a nip portion formed between the rotating member and the pressing member holds the first recording medium or the second recording medium on which the toner image has been transferred, passes through it, and heats and fixes the toner image; A temperature detection means for detecting the temperature of the region of the rotating member facing the main heating element, The system includes a print ratio determination unit that determines the print ratio of the toner image, and a narrow / wide determination unit that determines the width of the recording medium to be printed in the longitudinal direction, and a temperature control unit that controls the on / off of the power supplied to the main heating element and the sub-heating element based on the temperature detected by the temperature detection means. It has, When printing the second recording medium after the first recording medium, the temperature control unit controls the ratio of power supplied to the sub-heating element to the power supplied to the main heating element during the fixing of the first recording medium, according to the print density and number of printed sheets of the first recording medium, so that the temperature of the rotating element corresponding to the passage area of ​​the second recording medium stays within a predetermined temperature range. When the number of printed pages on the first recording medium is two or more, the ratio becomes smaller as the print density increases, and the ratio becomes larger as the print density decreases. An image forming apparatus characterized by the following features.

2. The image forming apparatus according to claim 1, characterized in that the main heating element is a central resistor positioned in the center in the longitudinal direction, and the sub-heating elements are a pair of end resistors positioned adjacent to both ends of the central resistor.

3. When the ratio is denoted as ra, the on-duty cycle for the central resistor as Dm [%], the paper size coefficient set based on the size of the recording medium to be printed as α (1 ≥ α), and the correction coefficient as β [%], the ratio ra is given by the following formula ra=(Dm×α+β) / Dm The image forming apparatus according to claim 2, characterized in that the correction coefficient β is obtained by and changes during the fixing of the first recording medium.

4. The image forming apparatus according to claim 3, characterized in that the correction coefficient β decreases in stages as the number of first recording media to be fixed increases.

5. The image forming apparatus according to claim 3, characterized in that the correction coefficient β changes depending on the printing rate of each data area of ​​the print data on the first recording medium and decreases in steps as the printing rate increases.

6. The image forming apparatus according to claim 1, characterized in that the ratio decreases in stages as the number of first recording media to be fixed increases.

7. An image forming apparatus capable of printing on recording media of different sizes, A fixing unit comprising a rotating member, a main heating element positioned in the longitudinal direction of the rotating member within the passage area of ​​a first recording medium and a second recording medium wider than the first recording medium, a sub-heating element positioned in a region where the second recording medium has both passage areas and non-pass areas, and a pressing member in contact with the outer circumferential surface of the rotating member, wherein a nip portion formed between the rotating member and the pressing member holds the first recording medium or the second recording medium on which the toner image has been transferred, passes through it, and heats and fixes the toner image; A temperature detection means for detecting the temperature of the region of the rotating member facing the main heating element, The system includes a print ratio determination unit that determines the print ratio of the toner image, and a narrow / wide determination unit that determines the width of the recording medium to be printed in the longitudinal direction, and a temperature control unit that controls the on / off of the power supplied to the main heating element and the sub-heating element based on the temperature detected by the temperature detection means. It has, When printing the second recording medium after the first recording medium, the temperature control unit controls the ratio of power supplied to the sub-heating element to the power supplied to the main heating element during the fixing of the first recording medium, according to the print density and number of printed sheets of the first recording medium, so that the temperature of the rotating element corresponding to the passage area of ​​the second recording medium stays within a predetermined temperature range. The main heating element is a central resistor positioned in the center in the longitudinal direction, and the sub-heating elements are a pair of end resistors positioned adjacent to both ends of the central resistor. When the ratio is denoted as ra, the on-duty cycle for the central resistor as Dm [%], the paper size coefficient set based on the size of the recording medium to be printed as α (1 ≥ α), and the correction coefficient as β [%], the ratio ra is given by the following formula ra=(Dm×α+β) / Dm Calculated by, The correction coefficient β corresponding to the print density of the first recording medium at a predetermined number of printed sheets of the first recording medium is predetermined. When the number of printed sheets of the first recording medium reaches a predetermined number, the power supplied to the main heating element and the sub-heating element is controlled to turn on and off so that the ratio decreases in stages according to the gradual increase in the print density of the first recording medium at the predetermined number of sheets, and the power supplied to the main heating element and the sub-heating element is controlled to turn on and off so that the ratio increases in stages according to the gradual decrease in the print density of the first recording medium at the predetermined number of sheets. An image forming apparatus characterized by the following features.

8. An image forming apparatus capable of printing on recording media of different sizes, A fixing unit comprising a rotating member, a central resistor positioned in the longitudinal direction of the rotating member within a passage area for a first recording medium and a second recording medium wider than the first recording medium, a pair of end resistors positioned in a region adjacent to both ends of the central resistor in a region where the second recording medium has both passage areas and non-passage areas, and a pressing member in contact with the outer circumferential surface of the rotating member, wherein a nip portion formed between the rotating member and the pressing member holds and passes through the first recording medium or the second recording medium on which a toner image has been transferred, and heats and fixes the toner image, A temperature sensing means for detecting the temperature of the region of the rotating member facing the central resistor, The system includes a print density determination unit that determines the print density of the toner image, and a narrow / wide determination unit that determines the width of the recording medium to be printed in the longitudinal direction, and a temperature control unit that controls the on / off switching of power supplied to the central resistor and the pair of end resistors based on the temperature detected by the temperature detection means. It has, When printing the second recording medium after the first recording medium, the temperature control unit controls the ratio of the power supplied to the pair of end resistors to the power supplied to the central resistor according to the print density and number of prints of the first recording medium, so that the temperature of the rotating member corresponding to the passage area of ​​the second recording medium stays within a predetermined temperature range during the fixing of the first recording medium. When the ratio is denoted as ra, the on-duty cycle for the central resistor as Dm [%], the paper size coefficient set based on the size of the recording medium to be printed as α (1 ≥ α), and the correction coefficient as β [%], the ratio ra is given by the following formula ra=(Dm×α+β) / Dm Calculated by, The correction coefficient β is smaller as the print density of the first recording medium increases, and larger as the print density of the first recording medium decreases. An image forming apparatus characterized by the following features.

Images