Image forming device

Abstract

To control the timing to change a target temperature according to the target temperatures of a preceding page and a subsequent page.SOLUTION: An image forming apparatus is used which comprises a fixing unit that heats a recording material at a fixing nip part to fix a toner image, and a control unit that changes a target temperature determined for every recording material. When the target temperature of a first recording material is a first temperature, the target temperature of a second recording material subsequent to the first recording material is a second temperature higher than the first temperature, and the difference between the second temperature and the first temperature is a first value, the control unit starts changing the target temperature from the time earlier by a first period than the arrival of the second recording material at the fixing nip part, and when the target temperature of a third recording material is a third temperature higher than the first temperature, the target temperature of a fourth recording material subsequent to the third recording material is a fourth temperature higher than the third temperature, and the difference between the fourth temperature and the third temperature is the first value, the control unit starts changing the target temperature from the time earlier by a second period than the arrival of the fourth recording material at the fixing nip part. The second period is shorter than the first period.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] Electrophotographic image forming devices, such as laser printers and digital copiers, are widely used. In such image forming devices, when a toner image is heated and fixed to a recording material, power consumption can be reduced by setting an appropriate target temperature for each page in accordance with the amount of toner on the image calculated from image data.

[0003] Patent Document 1 discloses a technology that achieves both energy conservation and maintaining print productivity when the target temperature of a subsequent page is higher than that of the preceding page during continuous printing. Patent Document 1 uses information on whether uneven glossiness may occur when the target temperature is increased within the preceding page, and if uneven glossiness does not occur, the target temperature is increased within the preceding page and also between sheets, so that the desired target temperature is reached at the start of the subsequent page. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5900474 Summary of the Invention [Problem to be solved by the invention]

[0005] In the image forming apparatus described in Patent Document 1, the target temperature is increased within the preceding page if no gloss unevenness occurs, but depending on the timing of increasing the target temperature, there is a possibility that power may be supplied.

[0006] The present invention has been made in view of the above-mentioned problems, and has as its object to control the timing for switching the target temperature in accordance with the target temperatures of the preceding page and the succeeding page. [Means for solving the problem]

[0007] The present invention employs the following configuration: a fixing unit that heats a recording material on which a toner image based on image data is formed and conveyed in a fixing nip portion, thereby fixing the toner image to the recording material; a control unit that determines a target temperature when the fixing unit heats the toner image for each recording material based on the image data and controls a timing for switching the target temperature; An image forming apparatus comprising: the control unit starts switching the temperature to the second temperature from a timing a first period before the second recording material reaches the fixing nip portion when a target temperature of the first recording material is a first temperature, a target temperature of the second recording material to be fixed after the first recording material is a second temperature higher than the first temperature, and a difference between the second temperature and the first temperature is a first value; when a target temperature of a third recording material is a third temperature higher than the first temperature, a target temperature of a fourth recording material to be fixed after the third recording material is a fourth temperature higher than the third temperature, and a difference between the fourth temperature and the third temperature is the first value, starting switching to the fourth temperature from a timing a second period before the fourth recording material reaches the fixing nip portion; The image forming apparatus is characterized in that the second period is shorter than the first period. [Effects of the Invention]

[0008] According to the present invention, the timing for switching the target temperature can be controlled according to the target temperatures of the preceding page and the succeeding page. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view showing a configuration of an image forming apparatus according to an embodiment of the present invention; [Figure 2] 1 is a functional block diagram relating to control of an image forming apparatus according to an embodiment of the present invention; [Figure 3]1 is a cross-sectional view showing a configuration of a heat fixing device according to an embodiment of the present invention; [Figure 4] A diagram illustrating a conventional target temperature control sequence. [Figure 5] FIG. 10 is a diagram illustrating division of image data according to an embodiment. [Figure 6] FIG. 10 is a diagram showing the relationship between the width of vertical band-shaped print and the correction amount of the target temperature in the embodiment. [Figure 7] FIG. 10 is a diagram showing the relationship between the length of vertical band-shaped printing and the correction amount of the target temperature in the embodiment. [Figure 8] FIG. 10 is a diagram showing an image for evaluation according to an embodiment. [Figure 9] Conventional control diagram for printing low to high print images [Figure 10] FIG. 10 is an explanatory diagram of control for printing low- to high-print images according to an embodiment. [Figure 11] Conventional control diagram for printing medium to high print images [Figure 12] FIG. 10 is an explanatory diagram of control for printing medium to high print images according to an embodiment. [Figure 13] FIG. 1 is a flow chart for explaining a process according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the embodiments may be changed as appropriate depending on the configuration of the device to which the invention is applied and various conditions, and the scope of the present invention is not limited to the following embodiments.

[0011] [Example 1] <Image forming device> 1 shows a schematic cross-sectional view of an image forming apparatus 100 of this embodiment. Here, a laser printer is used as an example of the image forming apparatus 100. The present invention can be applied to printers other than laser printers, such as LED printers, and image forming apparatuses that use electrophotography or electrostatic recording, such as digital copiers.

[0012] The image forming apparatus 100 generally includes an image forming unit 50 and a printer control device 304. The image forming unit 50 includes a photosensitive drum 1, a charging roller 2, a laser scanner 3, a developing device 4, a transfer roller 5, a heat fixing device 6 as a fixing unit, and a cleaning device 7. The image forming unit 50 forms a toner image on a recording material P according to image data under the control of the printer control device 304 as a control unit. The image forming apparatus also includes a paper feed tray 101, paper feed rollers 102, transport rollers 103, a top sensor 104, a paper discharge sensor 105, paper discharge rollers 106, a paper discharge tray 107, etc.

[0013] The photosensitive drum 1 is a drum-shaped electrophotographic photosensitive member, and is configured by providing a photosensitive material such as OPC (organic photoconductor), amorphous silicon, etc. on a cylindrical drum base made of aluminum alloy, nickel, etc. The photosensitive drum 1 is driven to rotate at a predetermined process speed (circumferential speed) in the direction of arrow R1 by a driving means (not shown).

[0014] The charging roller 2 uniformly charges the surface of the photosensitive drum 1 to a predetermined polarity and potential. Then, the laser scanner 3 irradiates the charged photosensitive drum 1 with a laser beam E, forming an electrostatic latent image on the surface of the photosensitive drum. At this time, the laser scanner 3 performs scanning exposure in the longitudinal direction of the photosensitive drum 1, with ON / OFF control according to image data, and removes charge from the exposed area.

[0015] The developing device 4 develops the formed electrostatic latent image to make it visible. As a developing method, in addition to the jumping development method of this embodiment, a two-component development method, a contact development method, etc. may be used. Alternatively, a combination of image exposure and reversal development may be used. The developing roller 41 of the developing device 4 deposits toner onto the electrostatic latent image on the photosensitive drum 1 to form a toner image.

[0016] The toner image on the photosensitive drum 1 is transferred onto the surface of a recording material P. The recording material P is fed one sheet at a time from a state in which it is stored in a paper feed tray 101 by a paper feed roller 102, and is supplied to a transfer nip portion Nt between the photosensitive drum 1 and a transfer roller 5 via a conveying roller 103 and the like.

[0017] The leading edge of the recording material P is detected by a top sensor 104. The printer control device 304 acquires the timing at which the leading edge of the recording material P reaches the transfer nip Nt from the positions of the top sensor 104 and the transfer nip Nt, and the conveying speed of the recording material P. Then, the transfer roller 5 applies a transfer bias onto the recording material P that has been fed and conveyed at a predetermined timing, thereby transferring the toner image on the photosensitive drum 1.

[0018] The recording material P onto which the toner image has been transferred is transported to a heat fixing device 6. The heat fixing device 6 applies heat and pressure to the recording material P while nipping and transporting it at a fixing nip Nf between the film unit 10 and the pressure roller 20. This fixes the toner image onto the surface of the recording material P. Thereafter, the recording material P is discharged onto a paper discharge tray 107 formed on the top surface of the image forming apparatus 100 by a paper discharge roller 106. A paper discharge sensor 105 detects the timing at which the leading and trailing ends of the recording material P pass by, thereby monitoring the occurrence of a jam or the like.

[0019] On the other hand, the cleaning device 7 removes residual toner (toner that remains without being transferred to the recording material P) on the surface of the photosensitive drum 1 after the toner image has been transferred, using a cleaning blade 71. The removed residual toner is used for the next image formation.

[0020] The image forming apparatus 100 repeats the above operations to continuously form images. The image forming apparatus 100 of this embodiment can form images with a resolution of 600 dpi at 35 sheets per minute (LTR vertical feed: process speed approximately 200 mm / s, paper interval 66 mm), and has a lifespan of 100,000 sheets.

[0021] <Printer control device> 2(a) will be used to explain the printer control device 304 provided in the image forming apparatus 100. As shown in FIG. 2(a), the printer control device 304 and the host computer 300 configure a printer system (image forming system).

[0022] The host computer 300 is an information processing device that has instructions from a user and image data that is the source of an image to be formed. The printer control device 304 controls the image forming apparatus 100 using information received through communication with the host computer 300. The host computer 300 may be, for example, a server or personal computer on a network such as the Internet or a local area network (LAN), or may be a mobile information terminal such as a smartphone or tablet terminal. The printer control device 304 is broadly divided into a controller 301 and an engine control unit 302.

[0023] The controller 301 has an image processing unit 303 and a controller interface 305. The controller interface 305 performs internal and external communication with the printer control device 304. The image processing unit 303 processes image data received from the host computer 300 via the controller interface 305. The image data processing includes character code Examples include bitmapping of images and halftoning of grayscale images.

[0024] The controller 301 also transmits image data to a video interface 310 of the engine control unit 302 via a controller interface 305. The image data in this embodiment also includes information about a target temperature calculated by the image processing unit 303 to maintain the temperature of the heater 11. The method for calculating the target temperature will be described in detail later.

[0025] The engine control unit 302 includes a video interface 310, a CPU (Central Processing Unit) 311, a ROM (Read Only Memory) 312, and a RAM (Random Access Memory) 313. )313, ASIC (Application Specific Integrated Circuit, The controller 301 transmits information about the timing at which the laser scanner 3 is turned on to the ASIC 314, and transmits print mode and image size information to the CPU 311. The controller 301 transmits information about the timing at which the laser scanner 3 is turned on to the ASIC 314, and transmits print mode and image size information to the CPU 311.

[0026] The CPU 311 performs various controls of the engine control unit 302 using the ROM 312 and RAM 313 in accordance with programs, user instructions, etc. The CPU 311 may be a single processor or a multi-processor configuration. The controller 301 transmits print commands, cancel commands, etc. to the engine control unit 302 in response to instructions from the user using the host computer 300, and controls operations such as starting and stopping printing operations.

[0027] FIG. 2B illustrates the engine control unit 302 of this embodiment in terms of functional blocks. The engine control unit 302 has, as functional blocks, a fixing control unit 320, a paper feed / conveyance control unit 330, and an image formation control unit 340. The CPU 311 performs processes such as storing information in the RAM 313, using programs stored in the ROM 312 or the RAM 313, and referencing information stored in the ROM 312 or the RAM 313, as necessary. Such processes by the CPU 311 cause the engine control unit 302 to function as each unit shown in FIG. 2B. The functional blocks may be considered as program modules executed by the engine control unit 302.

[0028] The fixing control unit 320 controls the temperature of the heat fixing device 6. The paper feed conveyance control unit 330 controls the operation interval of the paper feed roller 102. The image formation control unit 340 performs process speed control, development control, charging control, transfer control, etc. Some or all of the processes performed by the image forming apparatus 100 (for example, processes performed by the engine control unit 302 and the image processing unit 303) may be performed by a processing device such as the host computer 300 or a server (not shown) on a network. Furthermore, some or all of the processes performed by the engine control unit 302 may be performed by the image processing unit 303, or some or all of the processes performed by the image processing unit 303 may be performed by the engine control unit 302.

[0029] <Heat fixing device> The heat fixing device 6 will be described using Figure 3. The heat fixing device 6 of this embodiment is of a film heating type and is made up of a film unit 10 as a heating device and a pressure roller 20. The film unit 10 is made up of a heat-resistant fixing film 13 which is a heating rotatable body as a heat transfer member, a heater 11 which is a heating member, and a holder 12 which is a heater holding member. The heater 11 is provided inside the fixing film 13. The pressure roller 20 is provided opposite the film unit 10.

[0030] The heat fixing device 6 nip-conveys the recording material P on which the toner image t is formed in the fixing nip Nf formed between the fixing film 13 and the pressure roller 20, whereby the toner image t, which is conveyed together with the fixing film 13, is fixed onto the recording material P. As long as the heat fixing device 6 can fix the toner image on the recording material, the configuration of the heat fixing device 6 is not limited to that of this embodiment.

[0031] A thermistor 14 as a temperature detection member is disposed in contact with the surface of the heater 11 opposite to the sliding surface with the fixing film 13. Based on the temperature detected by the thermistor 14, the engine control unit 302 controls the current that the fixing control unit 320 passes through the heater 11 so that the temperature of the heater 11 becomes a desired temperature.

[0032] (fixing film) The fixing film 13 is a composite film formed by coating or tube-coating a release layer such as PFA, PTFE, or FEP directly or via a primer layer on the surface of a thin metal tube such as SUS. Instead of a metal tube, a base layer formed by kneading a heat-resistant resin such as polyimide with a thermally conductive filler such as graphite into a cylindrical shape may be used. In this embodiment, a fixing film 13 having a polyimide base layer coated with PFA is used. The fixing film 13 in this embodiment has a total thickness of 80 μm and a peripheral length of 56 mm. Because the fixing film 13 rotates while rubbing against the internal heater 11 and holder 12, it is necessary to minimize the frictional resistance between the fixing film 13 and the heater 11 / holder 12. In this embodiment, a small amount of lubricant such as heat-resistant grease is applied to the surfaces of the heater 11 and holder 12 to enable smooth rotation of the fixing film 13.

[0033] (Pressure roller) The pressure roller 20 has a core 21, an elastic layer 22, and a release layer 23. The elastic layer 22 is formed by foaming heat-resistant rubber such as insulating silicone rubber or fluororubber on the core 21 made of iron or the like. An adhesive layer of RTV silicone rubber (not shown) that has been primer-treated and has adhesive properties is applied to the elastic layer 22. The release layer 23 is then formed on the elastic layer 22 via the adhesive layer. The release layer 23 may be, for example, PFA, PTFE, FEP, or the like, covered or coated with a tube in which a conductive agent such as carbon is dispersed.

[0034] In this embodiment, the pressure roller 20 has an outer diameter of 20 mm and a hardness of 48° (Asker-C 600 g load). The pressure roller 20 is pressed at 147 N (15 kgf) from both longitudinal ends by a pressure means (not shown). This forms a fixing nip Nf necessary for heat fixing. The pressure roller 20 is also rotated in the direction of arrow R2 in FIG. 3 (counterclockwise on the paper) by a rotation drive means (not shown) via a core metal 21 from both longitudinal ends. This causes the fixing film 13 to rotate outside the holder 12 in the direction of arrow R3 in FIG. 3 (clockwise on the paper).

[0035] (heater) The heater 11 is provided inside the fixing film 13. The heater 11 has a substrate (insulating substrate) 113 made of alumina or aluminum nitride, which is a ceramic, and a resistance heating layer (heating element) 112 formed on the substrate 113. The resistance heating layer 112 is covered with a thin overcoat glass 111 to improve insulation and wear resistance, and the overcoat glass 111 is in contact with the inner circumferential surface of the fixing film 13. The overcoat glass 111 has excellent voltage resistance and wear resistance, and is configured and arranged to slide on the fixing film 13.

[0036] In this embodiment, the overcoat glass 111 has a thermal conductivity of 1.0 W / m·K, a withstand voltage characteristic of 2.5 kV or more, and a film thickness of 70 μm. In this embodiment, the material of the substrate 113 is alumina, and its dimensions are a width of 6.0 mm, a length of 260.0 mm, and a thickness of 1.00 mm. The thermal expansion coefficient of the substrate 113 is 7.6×10 -6 / °C. In this embodiment, the resistive heating layer 112 is made of a silver-palladium alloy. The total resistance of the resistive heating layer 112 is 20 Ω, and the temperature dependence of resistivity is 700 ppm / ℃.

[0037] (holder) The holder 12 is a member that holds the heater 11 and is also a heat insulating stay holder that prevents heat from radiating to the rear side of the fixing nip portion Nf. The holder 12 is made of a liquid crystal polymer, phenolic resin, PPS (polyphenylene sulfide), PEEK (polyether ether ketone), or the like. The fixing film 13 is fitted onto the holder 12 with a certain amount of slack and is arranged to be rotatable. The holder 12 in this embodiment is made of a liquid crystal polymer material that has a heat resistance of 260°C and a thermal expansion coefficient of 6.4×10 -5 / ℃.

[0038] <Engine control unit> The engine control unit 302 controls the heater 11 to a predetermined target temperature based on the temperature detected by the thermistor 14 in accordance with a control program. To achieve this, the engine control unit 302 controls the power supplied to the heater 11 so that the heater 11 maintains the target temperature. The engine control unit 302 is an example of a control unit. A preferred control method is PID control, which consists of a proportional term, an integral term, and a differential term. The following equation (1) shows this control formula. f(t)=α1×e(t)+α2×Σe(t)+α3×(e(t)-e(t-1)) …(1) Here, each item is as follows: t: control timing f(t): The ratio of heater power on time within the control period at control timing (t) (1 or more means full lighting) e(t): The temperature difference between the target temperature and the actual temperature at the current control timing (t) e(t-1): The temperature difference between the target temperature and the actual temperature at the previous control timing (t-1) α1 to α3: Gain constants α1: P (proportional) term gain α2: I (integral) term gain α3: D (differential) term gain

[0039] The first to third terms on the right side of equation (1) correspond to proportional control, integral control, and differential control, respectively. α1 to α3 are proportional coefficients used to weight the increase or decrease in the power-on time ratio of heater 11 within a control cycle. Setting α1 to α3 according to the characteristics of the heat-fixing device 6 enables appropriate temperature control. Engine control unit 302 determines the power-on time of heater 11 within a control cycle according to the value of f(t) and drives a heater power-on time control circuit (not shown) to determine the output power of heater 11. If the D term is not required, the D term gain may be set to 0, allowing for PI control in which only the P and I terms function. In this embodiment, the control timing is updated every 100 msec, with the P term gain (α1) set to 0.05°C-1, the I term gain set to 0.01°C-1 (α2), and the D term gain set to 0.001°C-1 (α3). In this embodiment, when the f(t) value is 1, the energization time within the control period is the maximum, and when the calculation result is greater than 1, the maximum energization time within the control period is set.

[0040] 4 shows a control sequence of the target temperature of the heater 11 by the conventional engine control unit 302. During pre-rotation (the period from the start of the printing operation until the leading edge of the first sheet of recording material enters the fixing nip Nf), the engine control unit 302 controls the power supply to the heater 11 to maintain the target temperature T0. The target temperature T0 here is set to 170°C. Then, before the first sheet of recording material (first recording material, also referred to as "preceding sheet") is passed through, the target temperature is switched to T1 (first temperature) for the first sheet of recording material.

[0041] During the preceding sheet passing (the period from when the leading edge of the first sheet of recording material enters the fixing nip portion Nf until the trailing edge of the first sheet of recording material leaves the fixing nip portion Nf), the engine control unit 302 sets the target temperature T1. The power supply to the heater 11 is controlled to maintain the target temperature T1 during paper passage within a range of 170° C. or higher and 204° C. or lower, and is determined by a calculation method to be described later.

[0042] During the paper interval (the period from when the trailing edge of the preceding paper leaves the fixing nip portion Nf until the succeeding paper enters the fixing nip portion Nf), the engine control unit 302 controls the power supply to the heating heater 11 to maintain the target temperature T1, and then switches to the target temperature T2 (second temperature) for the second sheet of recording material midway through the paper interval before the second sheet of recording material (second recording material, also written as "successing paper") is passed through.

[0043] Next, while the succeeding sheets are passing (from the time when the leading edge of the second sheet of recording material enters the fixing nip Nf until the trailing edge of the second sheet of recording material leaves the fixing nip Nf), the engine control unit 302 controls the power supply to the heater 11 to maintain the target temperature T2. The target temperature T2 during paper passing is in the range of 170°C to 204°C, similar to T1, and is determined by a calculation method described later.

[0044] <Image processing unit> (Target temperature is calculated from image data) The image processing unit 303 has a processor such as a CPU and memories such as ROM and RAM. Note that an information processing device that functions as the engine control unit 302 may also function as the image processing unit 303. In addition to halftoning the grayscale image, the image processing unit 303 also performs processing to calculate a target temperature from image data. In the following example, the processing of the image processing unit 303 when a toner image according to image data is formed on the surface of one sheet of recording material P will be described.

[0045] In determining the target temperature based on the image density information of the divided regions, the image processing unit 303 divides the image data into areas and regions, and then classifies each region into seven representative values. Next, the classified representative values are converted into temperature addition amounts for each region, and then added in the sub-scanning direction. The maximum value is then selected from the addition values for multiple main scanning areas, and this value is added to the base temperature control to calculate the target temperature T. Each step will be explained below in order. Note that the target temperature calculation method is not limited to this, and it is sufficient if a temperature corresponding to the print volume can be determined.

[0046] <Image data division> The division of image data by the image processing unit 303 will be described with reference to Figure 5. In the following description, the "sub-scanning direction" is the direction in which the recording material P is conveyed, and the "main scanning direction" is the direction perpendicular to the sub-scanning direction. As shown in the figure, a "sub-scanning area" is each area obtained by dividing the image data so that it is continuous in the sub-scanning direction, and a "main scanning area" is each area obtained by dividing the image data so that it is continuous in the main scanning direction.

[0047] (Main scanning area division process) The image processing unit 303 divides the entire image data in the main scanning direction to set main scanning areas. In this embodiment, the number of divisions is four. Here, when LTR-sized paper (short side 216 mm) is fed to the heat fixing device, the center of the paper is set as the origin on the heat fixing device, and the coordinate is 0 mm. The left side of the transport direction is defined as negative, and the right side is defined as positive. In this embodiment, each main scanning area is set as shown in Table 1 and Figure 5. That is, main scanning area MS1 is in the range of -108 mm to -54 mm, main scanning area MS2 is -54 mm to 0 mm, main scanning area MS3 is 0 mm to +54 mm, and main scanning area MS4 is +54 mm to +108 mm. [Table 1]

[0048] (Sub-scanning area division process) The image processing unit 303 divides the entire image data in the sub-scanning direction to define sub-main scanning areas. In this embodiment, the number of divisions is five. The image start position is set as the origin on the heat fixing device, with its coordinate set at 0 mm. In this embodiment, each sub-scanning area is set as shown in Table 2 and FIG. 5. Specifically, the sub-scanning area SS1 ranges from 0 mm to 56 mm, the sub-scanning area SS2 ranges from 56 mm to 112 mm, the sub-scanning area SS3 ranges from 112 mm to 168 mm, the sub-scanning area SS4 ranges from 168 mm to 224 mm, and the sub-scanning area SS5 ranges from 224 mm to 280 mm. The sub-scanning area range is set to 56 mm so that the length of the sub-scanning area in the sub-scanning direction approximately matches the circumferential length of the fixing film 13 in this embodiment. The reason for this length will be described later in the process of determining the target temperature T. Here, "approximately the same" does not necessarily mean that the lengths are exactly the same, but it is preferable that they match to a degree that is effective in suppressing temperature drops. [Table 2]

[0049] (Region setting process) The image processing unit 303 sets one area defined by the main scanning area and the sub-scanning area as a region. Hereinafter, the range defined by the main scanning area MSn and the sub-scanning area SSk will be referred to as "region R(k,n)."

[0050] <Regional ranking> The image processing unit 303 calculates the printing amount within the region R(k, n).

[0051] (High density pixel count) First, the image processing unit 303 extracts high-density pixels having a gray density of 4% or more in each region, and then counts the total number of high-density pixels in the region R(k,n) as N(k,n).

[0052] Then, the image processing unit 303 classifies the total number N(k,n) of high-density pixels in the region R(k,n) into seven ranks from rank 0 to rank 6 based on Table 3. [Table 3]

[0053] The print amount rank within region R(k,n) calculated in this way is called Rank(k,n). By using the above processing procedure, the print amount information for the entire image data can be aggregated into seven-level rank information for each of the 20 regions.

[0054] <Determining the target temperature T> Next, the image processing unit 303 determines the target temperature T based on the rank of the print amount of each region. The assumed print shape and related phenomena will be explained below.

[0055] (Estimated image and effect of temperature drop) Before describing the specific processing details, let's first consider image data in which vertical bands are printed for each rank, as an assumed image that is significantly affected by temperature drops. That is, once the image processing unit 303 determines the rank of the print volume for each region, it assumes that it will print a rectangular print (hereafter referred to as "vertical band print") that fills the sub-scanning direction within that region, with a width of the number of pixels based on that rank. Then, it assumes a target temperature at which the vertical band print can be sufficiently fixed.

[0056] For example, if the length in the sub-scanning direction is 56.5 mm, the width of the vertical band-shaped print in the main scanning direction is assumed as follows: 0.042 mm for rank 0, 1 mm for rank 1, 2 mm for rank 2, 4 mm for rank 3, 8 mm for rank 4, 16 mm for rank 5, and the entire width of the region in the main scanning direction for rank 6. The reason for this assumption is that such vertical band-shaped print requires the highest target temperature T for a given print volume rank. In other words, when toner is arranged in a vertical band, heat is continuously removed from specific positions in the main scanning direction of the components (such as the fixing film 13 and heater 11) responsible for heating in the thermal fixing device 6. This causes the temperature in those areas to drop, resulting in reduced fixing performance. Therefore, the target temperature T must be increased to compensate for the reduced heat.

[0057] If the width of the vertical band in the main scanning direction is thin, this temperature drop phenomenon can be almost ignored because it is compensated for by the heat flowing in from the surrounding components. However, the thicker the vertical band, the more difficult it is for heat to flow to the center of the vertical band, so the degree of temperature drop becomes larger and cannot be ignored, and a higher target temperature T is required.

[0058] Figure 6 shows the relationship between the width of a vertical band in the main scanning direction and the correction amount for the target temperature T. Here, the target temperature T required to fix a vertical line 0.042 mm wide and 56.5 mm long in the transport direction is taken as the reference. In this case, the target temperature T required to fix a vertical line 1 mm wide is 2°C higher. Also, the target temperature T required to fix a vertical band 16 mm wide is 4°C higher. Note that the wider the width in the main scanning direction, the slower the rate of increase in the target temperature T becomes, and when the width exceeds 58 mm, the temperature rises from outside the vertical band. Since the influence of heat inflow from the sensor is almost eliminated, further temperature correction is not required.

[0059] In the description of this embodiment, a basic value of the target temperature is set, and a correction amount (addition amount) to the basic value is calculated based on image data. However, this method is not limited to this as long as the target temperature can ultimately be calculated based on image data. For example, a method may be used in which the target temperature is calculated directly based on image data without setting a basic value or correction amount.

[0060] This temperature drop phenomenon becomes more pronounced as the length of the vertical band in the sub-scanning direction increases, and is particularly noticeable when the length in the sub-scanning direction exceeds a constant multiple of the circumferential length of the fixing film 13. Figure 7 is a graph showing the relationship between the length of the vertical band in the transport direction (sub-scanning direction) and the correction amount of the target temperature T required to compensate for the temperature drop.

[0061] For a vertical band 0.042 mm wide in the main scanning direction, the required correction amount for the target temperature T remains the same whether the length in the sub-scanning direction is 56.5 mm or 287 mm, which corresponds to the image length within an A4 sheet. This is because with a width of around 0.042 mm, there is sufficient heat inflow from the surroundings, so local temperature drops in components can be ignored.

[0062] On the other hand, in the case of a vertical band 1 mm wide in the main scanning direction, the degree of temperature drop of the components becomes greater, so the required correction amount for the target temperature T increases in proportion to the length in the transport direction. In this case, as shown in Figure 7, when the length exceeds a constant multiple of the circumferential length of the fixing film 13, the required correction amount for the target temperature T increases significantly. This is because the rotating fixing film 13 comes into contact with the toner and performs fixing in a state where heat has been absorbed by the vertical band one revolution before.

[0063] Therefore, as described above, if the length in the sub-scanning direction in the sub-scanning area division is made to approximately match the circumferential length of the fixing film 13, calculations can be performed that reflect this phenomenon, thereby achieving a greater power consumption reduction effect. Here, "approximately matching the length in the sub-scanning direction and the circumferential length of the fixing film 13" means that the two do not need to be exactly the same length, as long as they match to an extent that the effect of temperature drop can be ignored.

[0064] (Calculation of target temperature T) Based on the above assumptions, we will now describe a specific method for calculating the target temperature T. The target temperature T is calculated by using the temperature when the region's print volume rank is 0 as the base and determining the correction amount required when the region's print volume is other than 0 as an additional amount ΔT.

[0065] First, in this embodiment, the temperature required to fix a vertical band with a width of 0.042 mm, which corresponds to rank 0, is 170°C. The additional amount required when each region is ranked other than 0 is defined based on FIG. 6, as shown in Table 4. Based on this, the print amount rank of region R(k,n) is converted to the additional amount ΔT(k,n). [Table 4]

[0066] Next, the addition amount ΔT(k, n) is added for five consecutive regions (region rows) in the sub-scanning direction, and ΔT is used as a candidate value for the correction amount of the target temperature. MSn That is, for the five main scanning areas where n=1 to 4, ΔT(1,n), ΔT(2,n), ΔT(3,n), ΔT(4,n), and ΔT(5,n) are added together to obtain ΔT MSn This corresponds to the proportional increase in the required target temperature T when vertical bands corresponding to the print amount ranks are arranged in each of the five regions connected in the sub-scanning direction. In other words, the additional amount ΔT(k,n) is calculated as the candidate value ΔT MSn is a conversion value from the image density within the region to calculate

[0067] Therefore, the four calculated candidate values ΔT MS1 , ΔT MS2 , ΔT MS3 , ΔT MS4 The target temperature T is calculated by adding the base temperature (170°C in this case) to the maximum of these.

[0068] (Evaluation example) An evaluation example will be described below to confirm that the determination method of this embodiment can achieve the desired power consumption reduction effect. Four types of images are shown in Figures 8(a) to 8(d). Figure 8(a) shows a text image with a small amount of printing, Figure 8(b) shows an image with a large amount of printing at the leading edge, Figure 8(c) shows a 10% halftone image, and Figure 8(d) shows an image with a full solid black image. Based on the determination method of this embodiment, the target fixing temperatures for these images are determined, and the presence or absence of fixing defects and power consumption are evaluated.

[0069] First, the target temperature is determined for the image in Fig. 8(a). The print amount rank information calculated from the image is shown in Table 5. [Table 5]

[0070] Next, when this print volume rank is converted into the temperature addition amount ΔT for each region and each region column, the result is as shown in Table 6. As a result, the target temperature for this evaluation image is 170°C plus a correction value of 2.5°C, rounded up to the nearest integer, resulting in 173°C. [Table 6]

[0071] Similarly, the target temperature is determined for the image in Fig. 8(b). The print amount rank information calculated from the image is shown in Table 7. [Table 7]

[0072] Next, when this print volume rank is converted into the temperature addition amount ΔT for each region and each region column, the result is as shown in Table 8. As a result, the target temperature for this evaluation image is 178°C, which is 170°C plus a correction value of 7.5°C and rounded up to the nearest whole number. [Table 8]

[0073] Similarly, the target temperature is determined for the image in Fig. 8(c). The print amount rank information calculated from the image is shown in Table 9. [Table 9]

[0074] Next, when this print volume rank is converted into the temperature addition amount ΔT for each region and each region column, the result is as shown in Table 10. As a result, the target temperature for this evaluation image is 188°C, which is 170°C plus the correction value of 17.7°C and rounded up to the nearest whole number. [Table 10]

[0075] Similarly, the target temperature is determined for the image in Fig. 8(d). The print amount rank information calculated from the image is shown in Table 11. [Table 11]

[0076] Next, when this print amount rank is converted into the temperature addition amount ΔT for each region and each region row, it becomes as shown in Table 12. As a result, the target temperature of this evaluation image is 170°C with a correction value of 22 Add .5°C and round up to the nearest decimal point to get 193°C. [Table 12]

[0077] The target temperatures shown in Figures 8(a) to 8(d) are summarized in Table 13. [Table 13]

[0078] (Comparative Example 1-1) Comparative Example 1-1 shows a conventional example in which images (a) and (b) are printed consecutively. The target temperature (first temperature) for image (a) on the preceding sheet is 173°C, and the target temperature (second temperature) for image (b) on the succeeding sheet is 178°C, with a target temperature difference between the preceding and succeeding sheets of Δ5°C.

[0079] Figure 9 shows the changes in target temperature and film temperature. The film temperature was measured using a radiation thermometer to measure the film surface temperature downstream of the fixing nip Nf. If the film temperature is above 168°C, fixability can be ensured. In Comparative Example 1-1, as shown by the arrow in Figure 9, the target temperature for the preceding sheet is switched to the target temperature for the succeeding sheet 33 mm before the leading edge of the succeeding sheet in the middle of the sheet gap (165 msec before).

[0080] In Comparative Example 1-1, the print rate of image (a) on the preceding paper is low, and the target temperature is also low. Therefore, the power input to the fixing heater is small and the amount of heat absorbed by the fixing members is also small, so immediately after the preceding paper passes, the amount of heat stored in the film unit 10 and pressure roller 20 is small. In this state, when a high-print image like image (b) is printed on the leading edge of the succeeding paper, the film temperature drops rapidly due to heat absorption by the toner and paper.

[0081] As shown in Figure 9, even though the target temperature was switched from that of the preceding paper to that of the succeeding paper 33 mm before the leading edge of the succeeding paper and the target temperature was set high, the film temperature at the leading edge of the succeeding paper was unable to keep up, causing a sudden drop in temperature. As a result, the film temperature temporarily fell below 168°C, causing poor fixing in the high-print area of the succeeding paper.

[0082] (Example 1-1) Example 1-1 shows the case where image (a) and image (b) are printed consecutively. As in Comparative Example 1-1, the target temperature (first temperature) for image (a) on the preceding sheet is 173°C, the target temperature (second temperature) for image (b) on the succeeding sheet is 178°C, and the target temperature difference between the preceding and succeeding sheets is Δ5°C.

[0083] In Example 1-1, the print rate of image (a) on the preceding paper is low, and the target temperature is also low. Therefore, the power input to the fixing heater is small and the amount of heat absorbed by the fixing members is also small, so immediately after the preceding paper passes through, the amount of heat stored in the film unit 10 and pressure roller 20 is small. In this state, if a high-print image like image (b) is printed at the leading edge of the succeeding paper, it is expected that the heat absorption by the toner and paper will cause a rapid drop in the film temperature, resulting in poor fixing.

[0084] Therefore, in this embodiment, the target temperature of the preceding sheet is switched to the target temperature of the succeeding sheet at the timing when the position 5 mm before the trailing edge of the preceding sheet, as indicated by the arrow in FIG. 10, approaches the fixing nip Nf. By switching to the target temperature of the succeeding sheet earlier than in Comparative Example 1-1, the amount of heat stored is ensured by the time the leading edge of the succeeding sheet enters the fixing nip Nf. The target temperature is switched at the margin of the trailing edge of the preceding sheet. In other words, even if the target temperature is switched early, it is switched after the portion of the preceding sheet on which the toner image is formed has passed the fixing nip Nf. Therefore, the image on the preceding sheet is not affected.

[0085] In the control of FIG. 10, the target temperature for the preceding sheet (first recording material) is set to a first temperature, and the target temperature for the succeeding sheet (second recording material) is set to a second temperature. In this embodiment, the second temperature is higher than the first temperature. The difference between the second temperature and the first temperature, Δ5°C, is set to a first value. Furthermore, if the first period is set to F1, the target temperature is switched from timing (r11) in the first period before timing (r12) when the succeeding sheet (second recording material) reaches the fixing nip portion Nf.

[0086] As can be seen from the transition of film temperature shown in Figure 10, the film temperature rises between the preceding and succeeding sheets, and heat accumulates during this period. Even when the leading edge of the succeeding sheet enters the fixing nip Nf, the film temperature does not drop, and a temperature of 168°C or higher is maintained. As a result, poor fixing does not occur in the high-print areas of the succeeding sheet.

[0087] (Comparative Example 1-2) Comparative Example 1-2 shows a conventional example in which images (c) and (d) are printed consecutively. The target temperature (third temperature) for image (c) on the preceding sheet (third recording material) is 188°C, and the target temperature (fourth temperature) for image (d) on the succeeding sheet (fourth recording material) is 193°C. The target temperature difference between the preceding and succeeding sheets is Δ5°C, the same as in Comparative Example 1-1 and Example 1-1. In Comparative Example 1-2, the print rate of image (c) on the preceding sheet is high, and the determined target temperature is also high. Therefore, because the power input to the fixing heater is large and the heat absorption of the fixing member is also large, the heat accumulation amount in the film unit 10 and pressure roller 20 is large immediately after the preceding sheet passes. In this state, when a high-print image such as image (d) is printed on the succeeding sheet, an even higher target temperature is set, resulting in a large rise in film temperature at the leading edge of the succeeding sheet.

[0088] As shown in Figure 11, when the target temperature of the preceding paper is switched to the target temperature of the succeeding paper 33 mm before the leading edge of the succeeding paper (165 msec before in time), the film temperature at the leading edge of the paper is much higher than 168°C, and since there is a large amount of accumulated heat, the film temperature continues to decrease slowly from the leading edge to the middle of the succeeding paper. A large amount of power is being input, resulting in a state of high power consumption.

[0089] (Example 1-2) Example 1-2 shows the case where images (c) and (d) are printed consecutively. The target temperature (third temperature) for image (c) on the preceding sheet (third recording material) is 188°C, and the target temperature (fourth temperature) for image (d) on the succeeding sheet (fourth recording material) is 193°C. The target temperature difference between the preceding and succeeding sheets is Δ5°C, the same as in Comparative Example 1-1 and Example 1-1. In Example 1-2, the print rate of image (c) on the preceding sheet is high, and the determined target temperature is also high. Therefore, because the power input to the fixing heater is large and the heat absorption of the fixing members is also large, the heat accumulation in the film unit 10 and pressure roller 20 is high immediately after the preceding sheet passes. In this state, even if a high-print image such as image (d) is printed on the succeeding sheet, the film temperature does not drop rapidly due to heat absorption by the toner and paper, and fixing failure does not occur.

[0090] Therefore, in this embodiment, the target temperature is switched from the target temperature of the preceding sheet to the target temperature of the succeeding sheet at the timing when the leading edge of the succeeding sheet enters the fixing nip Nf, as indicated by the arrow in Figure 12. Because there is sufficient heat stored after the preceding sheet has passed through, the timing of switching to the target temperature of the succeeding sheet is delayed as much as possible to reduce power consumption. In this way, the target temperature is switched at the latest, simultaneously with the leading edge of the succeeding sheet entering the fixing nip.

[0091] The power input to the heater 11 when 50 sheets of images (c) and (d) were fed continuously was measured with a wattmeter, and the result was 15.5 Wh in Comparative Example 1-2 and 15.2 Wh in Example 1-2, as shown in Table 14. In other words, this example enabled a reduction of 0.3 Wh in power consumption when 50 sheets were fed continuously. [Table 14]

[0092] In the control of Figure 12, the target temperature of the preceding sheet (third recording material) is set to the third temperature. At this time, the third temperature is 188°C, which is higher than the target temperature (first temperature) of 173°C of the preceding sheet (first recording material) in Example 1-1. Furthermore, in Figure 12, the target temperature of the succeeding sheet (fourth recording material) is set to the fourth temperature. At this time, the fourth temperature is 193°C, so the fourth temperature is higher than the third temperature, and the difference between the third temperature and the fourth temperature, Δ5°C, is the first value, as in Example 1-1.

[0093] In this embodiment 1-2 under these conditions, the target temperature is switched at the same timing (r22) as when the subsequent sheet (fourth recording material) reaches the fixing nip Nf. If the length of the second period is set to 0 seconds, the target temperature is switched from r22 to the timing before the second period. From the above, it can be said that the control in embodiment 1-2 is control in which the second period is shorter than the first period when the third temperature is higher than the first temperature, the fourth temperature is higher than the third temperature, and the difference between the fourth temperature and the third temperature is the same first value as in embodiment 1-1.

[0094] (Processing flow) An example of processing control including switching between Examples 1-1 and 1-2 will be described below with reference to the flow chart in Fig. 13. The engine control unit 202 calculates the target temperature T1 of the preceding sheet in step S101, and calculates the target temperature T2 of the succeeding sheet in step S102. In S103, it is determined whether T2>T1. If the determination result is NO, the target temperature is switched at the normal timing. For example, if the initial value of the switching timing is midway between the timing of the preceding sheet passing and the timing of the succeeding sheet entering, then the timing is set to that point.

[0095] On the other hand, if the determination result in S103 is YES, the process proceeds to step S104, where it is determined whether the printing rate of the preceding sheet is equal to or less than a predetermined threshold Th. If the printing rate is equal to or less than the threshold Th, the process proceeds to S105, where the switching timing is made earlier than normal. This performs processing that emphasizes suppressing temperature drops, as shown in FIG. 10. On the other hand, if the printing rate is greater than the threshold Th in S104, the process proceeds to S106, where the switching timing is made later than normal. This performs processing to reduce power consumption, as shown in FIG. 12. The determination in S104 may be replaced by a determination of whether the target temperature T1 of the preceding sheet is equal to or less than a predetermined threshold.

[0096] Then, the preceding sheet is fixed in step S107, inter-sheet processing is performed in step S108, and the succeeding sheet is fixed in step S109. The target temperature is switched at the timing determined in step S105 or S106 among steps S107 to S109.

[0097] (effect) As explained in Examples 1-1 and 1-2, when the target temperature of the succeeding sheet is higher than the target temperature of the preceding sheet, the film temperature of the succeeding sheet can be appropriately controlled by setting the timing of switching the target temperature based on the target temperature of the preceding sheet. As a result, poor fixing due to a drop in temperature can be prevented even in the case of Example 1-1, and power consumption can be reduced even in the case of Example 1-2. Note that the values of the target temperature T of the preceding and succeeding sheets are not limited to the above example and can be set appropriately depending on the device configuration and performance. Furthermore, the target temperature switching control of the present invention may be performed when the difference D between the target temperatures of the preceding and succeeding sheets is a predetermined temperature difference or more (for example, 5°C or more). The threshold value in this case can also be set appropriately depending on the device configuration and performance.

[0098] (Variation) In the above examples, in Example 1-1, where the target temperature of the preceding sheet is low, the timing for switching the target temperature is advanced until the preceding sheet is passing, as shown in Figure 10. In Example 1-2, where the target temperature of the preceding sheet is high, the timing for switching the target temperature is delayed until the subsequent sheet enters, as shown in Figure 12. However, the timing for switching the target temperature is not limited to these examples, and can be changed depending on the target temperature of the preceding sheet.

[0099] For example, in addition to the timings shown in Figures 10 and 12, switching may be performed at various timings between sheets depending on the target temperature of the preceding sheet. For example, as shown in Table 15, when the target temperature is "slightly low," "normal," or "slightly high," the fixing temperature can be precisely controlled by switching at "early timing," "midway between the preceding and succeeding sheets," or "slightly late timing." Note that the terms "high," "low," "early," and "late" used in this modification are relative and do not limit the target temperature or switching timing. The relationship between the target temperature and switching timing may be determined by setting a threshold value for the target temperature and gradually switching the timing, or by using a mathematical formula that represents a function of the target temperature and switching timing. [Table 15]

[0100] As described above, according to the present invention, when the difference in target temperature between the preceding and succeeding sheets is equal to or greater than a certain value, the timing of switching the target temperature from the preceding sheet to the succeeding sheet is changed according to the target temperature of the preceding sheet. As a result, the timing of switching the target temperature can be suitably controlled according to the target temperatures of the preceding and succeeding pages. For example, the present invention can provide an image forming apparatus that consumes less power while preventing poor fixing without increasing the inter-sheet period. [Explanation of symbols]

[0101] 6: heat fixing device, Nf: fixing nip portion, 50: image forming portion, 100: image forming apparatus, 302: engine control portion, 303: image processing portion

Claims

1. a fixing unit that heats a recording material on which a toner image based on image data is formed and conveyed in a fixing nip portion, thereby fixing the toner image to the recording material; a control unit that determines a target temperature when the fixing unit heats the toner image for each recording material based on the image data and controls a timing for switching the target temperature; An image forming apparatus comprising: the control unit starts switching to the second temperature from a timing a first period before the second recording material reaches the fixing nip portion when a target temperature of the first recording material is a first temperature, a target temperature of the second recording material to be fixed after the first recording material is a second temperature higher than the first temperature, and a difference between the second temperature and the first temperature is a first value; when a target temperature of a third recording material is a third temperature higher than the first temperature, a target temperature of a fourth recording material to be fixed after the third recording material is a fourth temperature higher than the third temperature, and a difference between the fourth temperature and the third temperature is the first value, starting switching to the fourth temperature from a timing a second period before the fourth recording material reaches the fixing nip portion; The image forming apparatus is characterized in that the second period is shorter than the first period.

2. The control unit controls the timing of switching the target temperature so that the timing is after the portion of the first recording material on which the toner image is formed has passed through the fixing nip portion.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. The control section controls the timing of switching the target temperature so that the timing is before or at the same time as the leading edge of the second recording material enters the fixing nip section.

3. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. The control unit controls the timing of switching the target temperature when a difference between the second temperature and the first temperature is equal to or greater than a predetermined temperature difference.

4. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

5. The control unit controls the timing of switching the target temperature in stages according to the first temperature.

5. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

6. When the second temperature is lower than the first temperature, the control unit does not change the timing of switching the target temperature from an initial value.

6. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

7. The initial value of the timing of switching the target temperature by the control unit is set during sheet-to-sheet processing, which is a period from when the trailing edge of the first recording material leaves the fixing nip portion until when the second recording material enters the fixing nip portion.

7. The image forming apparatus according to claim 6, wherein the image forming apparatus is a recording medium.

Images