Image forming apparatus

Abstract

To appropriately prevent an increase in the temperature of a non-passage part, while maintaining the number of sheets to be processed per unit time.SOLUTION: An image forming apparatus has a first rotating body, a second rotating body that is in contact with the first rotating body to form a nip part and conveys a sheet at a predetermined conveying speed; heating means that heats the sheet on which a toner image is formed, with the first rotating body therebetween; and control means. The control means determines a history value of the amount of toner transferred to end areas of the plurality of sheets that are continuously conveyed, the end areas extending in parallel with a conveyance direction of the plurality of sheets, and adjusts the number of sheets to be heated per unit time according to the history value.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

[0002] An electrophotographic fixing device heats a sheet and a toner image to fix the toner image onto the sheet. Since sheets come in a variety of sizes, the width of the fixing device is designed to allow sheets of the largest possible width to pass through. Therefore, when a sheet narrower than the maximum width passes through the fixing device, a local temperature rise occurs at the edges of the fixing device in the width direction. This is called a temperature rise in non-passage areas.

[0003] If a temperature rise occurs in the non-passage area and the device is used in a state where the temperature at the edge is high, the lifespan of the components that make up the fixing device may be shortened and problems may occur with the conveyance of the recording material. Patent Document 1 proposes suppressing the temperature rise in the non-passage area by increasing the feeding interval between the preceding sheet and the succeeding sheet. Patent Document 2 proposes increasing the throughput if the print rate in the edge area is 0%, and decreasing the throughput if the print rate is not 0%. This is said to suppress hot offset in the edge area of ​​the image. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-305188 [Patent Document 2] Japanese Patent Application Publication No. 2022-113367 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, the speed of image forming devices has increased, leading to higher temperatures in fixing devices. Accordingly, there is a need to extend the feeding interval not only for narrow sheets but also for wide sheets. When the feeding interval is extended, the number of sheets heated (number of sheets processed) per unit time decreases, resulting in a decrease in the productivity of the image forming device. In the method disclosed in Patent Document 2, the number of sheets processed per unit time for each sheet is set based on the printing rate of the edge region of that sheet. However, because the printing rate of the preceding sheet is not taken into consideration, the number of sheets processed per unit time is often reduced more than necessary. Therefore, an object of the present invention is to appropriately suppress the temperature rise in the non-passage area while maintaining the number of sheets processed per unit time. [Means for solving the problem]

[0006] The present invention is, for example, an image forming means for forming a toner image on a sheet; A first rotating body; a second rotating body that contacts the first rotating body to form a nip portion and conveys the sheet at a predetermined conveying speed; a heating unit that heats the sheet on which the toner image is formed via the first rotating body; a control means for controlling the number of sheets heated per unit time by the heating means, The control means is an end region extending parallel to the transport direction of a plurality of sheets transported continuously, and calculates a history value of the amount of toner transferred to each end region of the plurality of sheets, and adjusts the number of sheets heated per unit time in accordance with the history value. [Effects of the Invention]

[0007] According to the present invention, it is possible to appropriately suppress the temperature rise in the non-passage portion while maintaining the number of sheets processed per unit time. [Brief explanation of the drawings]

[0008] [Figure 1] Schematic cross-sectional view of an image forming apparatus [Figure 2] Schematic cross-sectional view of a fixing device [Figure 3] FIG. 1 is a diagram illustrating a sheet transport position and a heater position; [Figure 4] Diagram explaining the controller [Figure 5] Flowchart showing a control method according to the first embodiment [Figure 6] FIG. 10 is a diagram illustrating an image formed in an edge region. [Figure 7] 1 is a diagram illustrating the print rate, toner amount, and saturation amount, and a diagram illustrating the relationship between the feeding interval and the correction coefficient; [Figure 8] Diagram explaining the timing of history acquisition [Figure 9] A diagram explaining the relationship between continuous printing time and the standard value of the non-passage area temperature [Figure 10] FIG. 10 is a diagram illustrating the degree of contribution of the temperature rise in the non-passing area and the temperature drop due to the toner formed in the edge area. [Figure 11] Diagram explaining print conditions [Figure 12] 1 is a diagram illustrating the relationship between the number of prints and the history value, and a diagram illustrating the relationship between the number of prints and the temperature of the non-passage portion. [Figure 13] Diagram explaining the heat propagation model [Figure 14] FIG. 10 is a diagram illustrating the effect of a toner image formed in an edge area on the temperature of the non-passing portion for each sheet size. [Figure 15] Diagram explaining the controller [Figure 16] Flowchart showing a control method according to the second embodiment [Figure 17] A diagram explaining division of edge regions [Figure 18] A diagram for explaining an index determination unit [Figure 19] Flowchart showing a control method according to the third embodiment [Figure 20] Diagram explaining test images [Figure 21] FIG. 10 is a diagram illustrating the relationship between the test image and the temperature of the non-passing area. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0010] Example 1 (1) Image forming device As shown in FIG. 1, the image forming apparatus 1 is a laser printer that forms an image on a sheet P using an electrophotographic system. The image forming apparatus 1 may be implemented in a multifunction peripheral, a copier, or a facsimile machine. The sheet P may also be called a recording material, recording paper, or transfer material. The image forming apparatus 1 operates by receiving power from an AC power source 30. The image forming unit 20 is formed of the following components.

[0011] The photosensitive drum 19 is an image carrier (electrophotographic photosensitive member) that rotates while carrying an electrostatic latent image and a toner image. The cleaning blade 18 cleans the surface of the photosensitive drum 19. The charging roller 16 is a charging member that charges the surface of the photosensitive drum 19. The charging roller 16 may be replaced with a charging wire. The laser scanner 11 irradiates the surface of the photosensitive drum 19 with laser light corresponding to an image signal to form an electrostatic latent image. The developing roller 17 develops the electrostatic latent image with toner held in a toner container inside the process cartridge 10 to form a toner image. The process cartridge 10 includes the photosensitive drum 19, the charging roller 16, the developing roller 17, and the cleaning blade 18. The photosensitive drum 19 is driven to rotate by a motor M1. This transports the toner image to the transfer nip N1. The transfer nip N1 is a nip formed by the contact between the photosensitive drum 19 and the transfer roller 12.

[0012] The feeding cassette 21 is a storage container that stores a plurality of sheets P. The feeding roller 22 is driven to rotate by a motor M1 and feeds the sheet P to a conveying path. The conveying roller pair 23 is driven to rotate by the motor M1 and conveys the sheet P to the transfer nip N1.

[0013] The top sensor 24 is installed on the transport path between the transport roller pair 23 and the transfer nip N1 and detects the timing of the passage of the leading edge of the sheet P transported from the transport roller pair 23. The controller 40 adjusts the timing of writing the electrostatic latent image by the laser scanner 11 according to the timing of the leading edge of the sheet P detected by the top sensor 24. That is, the write timing is controlled so that the leading edge of the toner image on the photosensitive drum 19 also reaches the transfer nip N1 at the same time that the leading edge of the sheet P reaches the transfer nip N1. The controller 40 has a CPU 41, a ROM 42, and a RAM 43. The CPU 41 executes various programs stored in the ROM 42 to control various operations related to image formation while using the RAM 43 as a working area. The ROM 42 is a non-transitory storage medium that stores the control programs for the image forming apparatus 1.

[0014] At the transfer nip N1, the toner image is transferred from the photosensitive drum 19 to the sheet P. The sheet P is transported to the fixing device 13 as the photosensitive drum 19 and the transfer roller 12 rotate.

[0015] The fixing device 13 has a fixing film 14 as a fixing member and a pressure roller 15 as a pressure member. The pressure roller 15 is driven to rotate by a motor M1. The fixing film 14 rotates following the pressure roller 15. A fixing nip N2 formed by the contact between the pressure roller 15 and the fixing film 14 sandwiches and conveys the sheet P. During this time, the temperature (fixing temperature) of the fixing film 14 is controlled by a CPU 41 so that it becomes a target temperature. The toner image on the sheet P is heated by the fixing film 14, thereby fixing the toner image to the sheet P. After passing through the fixing device 13, the sheet P is conveyed to a pair of discharge rollers 26 that discharge the sheet P. The pair of discharge rollers 26 is driven to rotate by a motor M1 and discharges the sheet P to a discharge tray provided at the top of the image forming apparatus 1.

[0016] In double-sided printing, the discharge roller pair 26 switches from forward rotation to reverse rotation, and the sheet P with an image formed on its first side is conveyed to the conveyance path 31 for double-sided printing. This is called switchback reversal, and is a method of switching the image forming side of the sheet P from the first side to the second side. The conveyance roller pairs 32 and 33 provided in the conveyance path 31 convey the sheet P and hand it over to the conveyance roller pair 23. An image is then formed on the second side of the sheet P as well.

[0017] Image forming apparatus 1 can print black and white images on, for example, A4 size (210mm x 297mm) plain paper at a feed speed of 240mm / sec. This corresponds to a throughput of approximately 43 sheets / min. Image forming apparatus 1 may also be capable of color printing or multi-color printing.

[0018] (2) Fixing device 2, the fixing device 13 has a fixing film 14, a pressure roller 15, a nip forming member 64, and a pressure stay 63. The nip forming member 64 has a heater 60 and a heater holder 61. An arrow D1 indicates the conveyance direction of the sheet P. An arrow R1 indicates the rotation direction of the pressure roller 15. An arrow R2 indicates the rotation direction of the fixing film 14.

[0019] The fixing film 14 is a flexible, cylindrical (endless) film-like member. The thickness of the fixing film 14 may be, for example, 450 micrometers (um) or less and 20 um or more. A smaller heat capacity of the fixing film 14 shortens the wait time (first print-out time). A heat-resistant single-layer film may be used as the fixing film 14. Alternatively, a multi-layer film may be used as the fixing film 14. The multi-layer film has, for example, a film base layer and a coating layer. In Example 1, a film base layer made of a polyimide film and a coating layer made of perfluoroalkoxyalkane (PFA) are used. The thickness of the film base layer is, for example, approximately 60 um. The thickness of the coating layer is, for example, approximately 14 um. The outer diameter of the fixing film 14 is, for example, 24 mm. Instead of a resin material, a metal material such as stainless steel (SUS) may be used as the film base layer. Furthermore, to improve image quality, a heat-resistant rubber such as silicone rubber may be formed between the film base layer and the coating layer.

[0020] The pressure roller 15 has a core 151, an elastic layer 152, and a surface layer 153. The core 151 may be, for example, an aluminum core. The elastic layer 152 may be, for example, silicone rubber. The surface layer 153 may have a thickness of, for example, about 50 μm, and be made of PFA. The outer diameter of the pressure roller 15 may be, for example, 25 mm. The thickness of the elastic layer 152 may be, for example, about 3 mm.

[0021] The heater 60 is a plate-shaped heat-generating member that contacts the inner circumferential surface of the fixing film 14 and rapidly heats the fixing film 14. The heater 60 has a plate shape with low heat capacity. The heater 60 may have a heat-generating resistance layer and an insulating ceramic substrate. The ceramic substrate is made of alumina or aluminum nitride. The heat-generating resistance layer is made of silver palladium (Ag / Pd), ruthenium dioxide (RuO2), tantalum nitride (Ta2N), or the like. A glass layer may be provided on the heat-generating resistance layer as an insulating protective layer. The temperature of the heater 60 is detected by a temperature sensor (thermistor 62) that is in contact with the back surface of the ceramic substrate.

[0022] The heater holder 61 is disposed inside the fixing film 14. The heater holder 61 holds the heater 60. The pressure stay 63 is made of a rigid member such as metal, and applies pressure received from a spring or the like (not shown) to the pressure roller 15 via the heater holder 61. This pressure forms a fixing nip N2 of a predetermined area between the nip forming member 64 and the pressure roller 15.

[0023] 2, the heater 60 is in direct contact with the inner circumferential surface of the fixing film 14, but this is merely an example. A plate-like or sheet-like member with high thermal conductivity (e.g., a sheet-like member made of an iron alloy or aluminum) may be disposed between the heater 60 and the fixing film 14. The heater 60 may heat the fixing film 14 via a sliding member that slides against the inner circumferential surface of the fixing film 14.

[0024] When a print signal is input from an external input device such as an image scanner or a host computer, the controller 40 controls the motor M1 to rotate the pressure roller 15. The pressure roller 15 transmits a rotational force to the fixing film 14, causing it to rotate.

[0025] The controller 40 controls the power supplied from the AC power supply 30 to the heater 60 to maintain the temperature detected by the thermistor 62 at the target temperature. A triac may be used to control the AC.

[0026] When the fixing film 14 is rotated by the pressure roller 15 and the temperature of the heater 60 reaches a predetermined target temperature, the sheet P onto which the toner image has been transferred is transported to the fixing nip N2. As the sheet P is transported through the fixing nip N2, heat from the heater 60 is applied to the sheet P via the fixing film 14. In other words, the unfixed toner image on the sheet P is heated and pressurized, and is fixed to the sheet P. After passing through the fixing nip N2, the sheet P is separated from the fixing film 14 and transported further.

[0027] (3) Temperature rise in non-passing areas 3 shows the relationship between the heater 60 and the transport position of an A4-sized sheet P. A heating element 301 is formed on a substrate 300. The width L1 of the heating element 301 is 220 mm, which corresponds to the maximum length of the sheet P (LTR size) assumed in the design.

[0028] An A4-sized sheet P (width L2 = 210 mm) may pass through the fixing device 13. In this case, an area NPL (5 mm) from the left edge of the heating element 301 to the left edge of the sheet P and an area NPR (5 mm) from the right edge of the heating element 301 to the right edge of the sheet P do not come into contact with the sheet P via the fixing film 14. Hereinafter, the areas NPL and NPR are referred to as non-passing areas. An area of ​​the heating element 301 that indirectly comes into contact with the sheet P via the fixing film 14 is referred to as a passing area. The concepts of passing area and non-passing area apply to the heating element 301, the fixing film 14, and the pressure roller 15, respectively.

[0029] When toner images are formed continuously on multiple A4-sized sheets P, the temperature of the non-passage areas NPL and NPR becomes higher than the temperature of the passage areas. To maintain the temperature of the heater 60 at the target temperature, the same power is supplied to the entire heating element 301. The heat generated in the passage areas of the heating element 301 is consumed to melt the toner. On the other hand, the heat generated in the non-passage areas of the heating element 301 is not consumed to melt the toner. As a result, the temperature of the passage areas is maintained at the target temperature, but the temperature of the non-passage areas becomes higher than the target temperature. This is a phenomenon called non-passage area temperature rise.

[0030] (4) Controller block diagram 3, X indicates the center of the sheet P in the width direction. In this example, the center of the fixing device 13 in the width direction also coincides with X. Regardless of the width of the sheet P, the sheet P is transported so that the center of the sheet P coincides with the center of the fixing device 13, regardless of the width of the sheet P. Therefore, the temperature of the non-passing portion NP of the fixing film 14 becomes higher than the temperature of the passing portion.

[0031] The temperature of the non-passage portion NP is affected by the temperature of the passing portion located inside the non-passage portion NP. The temperature of the passing portion is affected by the amount of toner adhering to the edge region E of the sheet P. When toner images are formed continuously on multiple sheets P, the temperature of the non-passage portion NP when the i-th sheet P passes is cumulatively affected by the amount of toner adhering to each edge region E of the first to i-1th sheets P. Therefore, the CPU 41 accumulates the amount of toner in each edge region E of the multiple sheets P and predicts the temperature of the non-passage portion based on the accumulated value (history value), thereby adjusting the throughput. Note that it is not necessary to estimate a specific temperature value; it is sufficient to estimate a value correlated with the temperature. Throughput is the number of sheets P fixed per unit time. However, the conveying speed of the sheet P or the feeding interval between the subsequent sheet P and the preceding sheet P may also be understood as throughput.

[0032] FIG. 4 shows functions realized by the CPU 41. The index determination unit 400 determines an index for reducing the temperature of the non-passage portion NP in the fixing device 13. For example, the index increases as the amount of toner transferred to the edge region E of the sheet P increases. Furthermore, the index determined for each of the multiple sheets P that pass through the fixing device 13 in succession is accumulated. Therefore, the index determination unit 400 may be referred to as an accumulation unit or a history unit. The temperature estimation unit 420 estimates the temperature of the non-passage portion NP. The correction unit 410 corrects the temperature of the non-passage portion NP estimated by the temperature estimation unit 420 with the index determined by the index determination unit 400. This enables the temperature of the non-passage portion NP to be estimated with high accuracy. The SP determination unit 407 determines the throughput (e.g., conveying speed V, feeding interval G) based on the temperature of the non-passage portion NP. Furthermore, in the first embodiment, the following physical quantities are defined. The feeding interval G may be referred to as the conveying interval. The conveying interval is the distance or time from the trailing edge of the preceding sheet P to the leading edge of the succeeding sheet P.

[0033] H is the cumulative value (history value) of the amount of toner transferred to the edge region E. It is assumed that the first to Nth sheets P pass through the fixing device 13 consecutively. In this case, the history value H at the time when the leading edge of the i-th sheet P arrives at the fixing device 13 is expressed as H_top. The history value H of the right edge region ER is expressed as HR. The history value H of the left edge region EL is expressed as HL. In this way, R indicates the right region. L indicates the left region.

[0034] Q is the amount of toner adhering to the edge region E of the i-th sheet P. QR is the amount of toner adhering to the edge region ER of the i-th sheet P. QL is the amount of toner adhering to the edge region EL of the i-th sheet P.

[0035] U is the saturation value of the amount of temperature drop (dropping capacity) of the fixing film 14 caused by the heat being taken away from the fixing film 14 by the toner on the sheet P. The higher the printing rate in the edge region E, the larger the saturation value U. When the printing rate in the edge region E is 0%, the saturation value U is 0. When the printing rate in the edge region E is 100%, the saturation value U is at its maximum value. UR is the saturation value for the right edge region ER. UL is the saturation value for the left edge region EL.

[0036] H_bottom is the history value H at the time when the trailing edge of the i-th sheet P exits the fixing device 13. HR_bottom is the toner history for the right edge region ER. HL_bottom is the toner history for the left edge region EL.

[0037] Tmax_s is an index related to the temperature at the non-passage portion NP when there is no toner at all in the edge region E. Tmax_s may also be called a standard value or a reference value. This index may also be called a non-passage portion temperature rise count. Tmax_c is an index related to the temperature at the non-passage portion NP when the i-th sheet P passes through the fixing device 13.

[0038] The HR_top acquisition unit 401 acquires the HR_top for the i-th sheet P. When i is 1, there is no preceding sheet P, so HR_top is 0 (zero). When i is 2 or greater, HR_top is determined based on the H_bottom for the preceding sheet P and the throughput (e.g., feeding interval G). The QR acquisition unit 402 acquires the amount of toner QR used in the edge region ER of the i-th sheet P by analyzing image data sent from a host computer or the like. The UR acquisition unit 403 acquires a saturation value UR based on the amount of toner QR. The HR_bottom acquisition unit 404 acquires HR_bottom based on the HR_top and the saturation value UR.

[0039] The HL_top acquisition unit 411 acquires HL_top for the i-th sheet P. When i is 1, there is no preceding sheet P, so HL_top is 0 (zero). When i is 2 or greater, HL_top is determined based on H_bottom for the preceding sheet P and the throughput (e.g., feeding interval G). The QL acquisition unit 412 acquires the amount of toner QL used in the edge region EL of the i-th sheet P by analyzing image data sent from a host computer or the like. The UL acquisition unit 413 acquires a saturation value UL based on the amount of toner QL. The HL_bottom acquisition unit 414 acquires HL_bottom based on HL_top and the saturation value UL.

[0040] The H determination unit 405 determines the smaller history value H between HR_bottom and HL_bottom. In other words, the one with the lower temperature reduction capability is selected between HR_bottom and HL_bottom. This will make it less likely that the temperature of the non-passage portion NP will be underestimated.

[0041] The Tmax_s acquisition unit 421 acquires the maximum temperature of the fixing film 14 when M sheets P having no toner images in the edge regions ER and EL are continuously fed into the fixing device 13.

[0042] The Tmax_c acquisition unit 406 corrects the maximum temperature Tmax_s with the history value H to acquire the non-passage portion temperature Tmax_c. The Sp determination unit 407 determines the throughput (e.g., feeding interval G) based on the non-passage portion temperature Tmax_c. The throughput (e.g., feeding interval G) affects the history value H. Therefore, the throughput (e.g., feeding interval G) is supplied to the HR_top acquisition unit 401 and the HL_top acquisition unit 411.

[0043] (5) Flowchart 5 shows a control method executed by the CPU 41 in accordance with a control program. Here, when a plurality of sheets P are continuously fed, the amount by which the temperature of the fixing film 14 is reduced due to toner adhering to the edge region E is taken into consideration as the toner history (history value H) and the throughput is adjusted. When a print signal is input to the image forming apparatus 1, the CPU 41 executes the following process.

[0044] ●S501....Getting tip history value The CPU 41 (HR_top acquisition unit 401, HL_top acquisition unit 411) acquires the history values ​​HR_top and HL_top of the toner amount immediately before the leading edge of the i-th sheet P enters the fixing device 13. The temperature of the fixing film 14 when the sheet P without a toner image is heated by the fixing device 13 is adopted as a reference value. The history value H is the difference between the surface temperature of the fixing film 14 when the sheet P with a toner image formed thereon passes through the fixing device 13 and the reference value. This difference indicates the amount of temperature drop caused by the toner image.

[0045] The i-th sheet P is the sheet P about to enter the fixing device 13. The preceding sheet is the (i-1)th sheet P.

[0046] As described above, in the first embodiment, the history value HR_top for the edge region ER and the history value HL_top for the edge region EL are acquired. As shown in FIG. 3, the edge region AER is an area of ​​a predetermined width (e.g., 10 mm) located at the right edge of an A4-sized sheet P. The edge region EL is an area of ​​a predetermined width (e.g., 10 mm) located at the left edge of an A4-sized sheet P. When i=1, the history values ​​HR_top and HL_top are each set to an initial value (e.g., 0). When i is 2 or greater, the history values ​​HR_top and HL_top are determined according to the history value H obtained for the (i-1)th sheet P.

[0047] S502: Obtaining the amount of toner adhering to the edge area The CPU 41 (QR acquisition unit 402, QL acquisition unit 412) acquires the toner amounts in the edge region E of the i-th sheet P. For example, the CPU 41 calculates the toner amounts QR and QL in the edge regions ER and EL, respectively, from image information (image position and density) received from a host computer or an image scanner. Here, the toner amounts QR and QL are calculated for each sheet P. The toner amounts QR and QL may be the mass of the toner or may be a ratio to the mass of the toner under standard conditions. For example, the image under standard conditions is a toner image of maximum density formed in the edge region E (a region 10 mm wide x 297 mm long) of an A4-size sheet P. As an example, the toner amount under standard conditions is expressed as 250.

[0048] Figure 6 shows an example of toner amounts. A toner image is formed in the edge region ER of image Im1 with a printing rate of 100%. The toner amount QR in this case is 250. A toner image is formed in the edge region EL of image Im1 with a printing rate of 75%. The toner amount QL in this case is 188.

[0049] A toner image is formed in the edge region ER of image Im2 at a printing rate of 50%. The toner amount QR in this case is 125. A toner image is formed in the edge region EL of image Im2 at a printing rate of 0%. The toner amount QL in this case is 0.

[0050] A toner image is formed in the edge region ER of image Im3 at a printing rate of 66%. The toner amount QR in this case is 165. A toner image is formed in the edge region EL of image Im3 at a printing rate of 40%. The toner amount QL in this case is 100.

[0051] ●S503....Get rear end history value The CPU 41 acquires the history values ​​HR_bottom and HL_bottom of the toner amount immediately after the trailing edge of the i-th sheet P passes through the fixing device 13. For example, the history values ​​HR_bottom and HL_bottom are calculated based on the toner amounts QR and QL and the history values ​​HR_top and HL_top, respectively. As described above, the UR acquisition unit 403 acquires the saturation value UR from the toner amount QR. The UL acquisition unit 413 acquires the saturation value UL from the toner amount QL. The saturation value U may be calculated using the following equation.

[0052] UR=c1×QR×QR + c2×QR (1) UL=c1×QL×QL + c2×QL ···(2) Here, the coefficients c1 and c2 are determined through experiments or simulations. c1 is, for example, −0.000103. c2 is, for example, 0.0758.

[0053] 7A shows an example of the saturation value U determined based on the toner amount Q. According to this example, the greater the toner amount Q, the greater the saturation value U. The more toner carried on the sheet P, the greater the amount of heat absorbed by the toner from the fixing film 14. In other words, the greater the amount of toner, the greater the decrease in surface temperature (saturation value).

[0054] Equations (1) and (2) are equations that approximate the amount of decrease in temperature of the fixing film 14 in the first embodiment. However, equations (1) and (2) and coefficients c1 and c2 are merely examples. The coefficients c1 and c2 may have different values. Furthermore, the equations themselves may be different.

[0055] The HR_bottom acquisition unit 404 and the HL_bottom acquisition unit 414 acquire the history values ​​HR_bottom and HL_bottom based on the history values ​​HR_top and HL_top and the saturation values ​​UR and UL.

[0056] 8 shows the timing at which the history values ​​HR_top, HL_top, HR_bottom, and HL_bottom are acquired. The history values ​​HR_bottom and HL_bottom may be calculated using the following equations.

[0057] HR_bottom = HR_top + (UR-HR_top)×c3 ···(3) HL_bottom = HL_top + (UL-HL_top)×c3 ···(4) Here, the coefficient c3 is determined through an experiment or a simulation, and is, for example, 0.466.

[0058] The history value H_bottom when the trailing edge of the i-th sheet P passes through the fixing device 13 is calculated by adding the temperature drop amount of the i-th sheet P to the history value H_top before the i-th sheet P is heated in the fixing device 13. Here, the temperature drop amount is calculated by multiplying the difference between the saturation value U and the history value H_top before the i-th sheet P is heated in the fixing device 13 by a coefficient c3.

[0059] The formulas (3) and (4) and the value of the coefficient c3 are merely examples. The coefficient c3 may have a different value. Furthermore, the formula itself may be different.

[0060] The H determination unit 405 determines the smaller of the history value HR_bottom and the history value HL_bottom as the history value H of the i-th sheet P. By using the smaller history value H, the higher temperature is identified between the temperature of the right non-passage portion NPR and the temperature of the left non-passage portion NPL. ●S504....Acquisition of standard value of passing part temperature The CPU 41 (temperature estimation unit 420) acquires a standard value Tmax_s of the temperature of the non-passage portion up to the i-th sheet P. The standard value Tmax_s is the maximum temperature of the non-passage portion NP.

[0061] FIG. 9 shows the relationship between the standard value Tmax_s and the continuous printing time. It is assumed here that A4-sized sheets P, completely devoid of toner images in both the edge region ER and the edge region EL, are continuously transported at 43 sheets per minute. A table showing the relationship shown in FIG. 9 is stored in the ROM 42. Therefore, the CPU 41 can obtain the standard value Tmax_s corresponding to the continuous printing time or the number of sheets to be continuously printed by referring to this table. For example, if the continuous printing time is 30 seconds, Tmax_s is estimated to be 270°C. If the continuous printing time is 60 seconds, Tmax_s is estimated to be 277°C. Instead of the table, a formula obtained by approximating the relationship shown in FIG. 9 may be used.

[0062] ●S505···Temperature correction for non-passing areas The CPU 41 (correction unit 410) corrects the standard value Tmax_s of the non-passage portion temperature with the toner amount history value H. As a result, the corrected non-passage portion temperature Tmax_c is obtained.

[0063] Figure 10(A) shows the temperature distribution in the width direction of the fixing film 14. The vertical axis represents temperature. The horizontal axis represents position in the width direction of the fixing film 14. The dashed line represents the temperature distribution under standard conditions. As shown in Figure 10(B), under standard conditions, image ImX is formed on sheet P. Here, image ImX is an image in which no toner is transferred to the edge regions ER and EL of sheet P (printing rate 0%). The solid line represents the temperature distribution in the case where image ImY is formed on sheet P. Image ImY is an image in which toner adheres to the edge regions ER and EL of sheet P (printing rate 100%).

[0064] 10A, the temperature of the passage portion is maintained at the target temperature (205°C). However, when the image ImY is formed on the sheet P, the temperatures of the end regions ER and EL of the fixing film 14 are lower by the history values ​​HR and HL compared to the temperatures corresponding to the image ImX.

[0065] On the other hand, the temperature of the non-passage portions NPR and NPL of the fixing film 14 corresponding to the image ImX is the standard value Tmax_s. The temperature of the non-passage portions NPR and NPL of the fixing film 14 corresponding to the image ImY is lower than the standard value Tmax_s by the history values ​​HR and HL. This is because the non-passage portions NPR and NPL of the fixing film 14 are close to the edge regions ER and EL of the sheet P. Therefore, Tmax_s is corrected by the following equation.

[0066] Tmax_c = Tmax_s - c4×H ···(5) Here, the coefficient c4 is, for example, 1.0. In this way, the CPU 41 may obtain the corrected non-passage portion temperature Tmax_c by subtracting the correction value (c4×H) from the standard value Tmax_s. The coefficient c4 is merely an example, and may be another value obtained by experiment or simulation. For example, the coefficient c4 may be set appropriately depending on the structure of the fixing device 13.

[0067] ●S506...Throughput adjustment The CPU 41 (Sp determination unit 407) determines the throughput according to the corrected non-passage portion temperature Tmax_c. For example, the Sp determination unit 407 may determine whether the corrected non-passage portion temperature Tmax_c exceeds the heat resistance threshold Tth (e.g., 265°C) of the fixing film 14. If the non-passage portion temperature Tmax_c exceeds the heat resistance threshold Tth, the Sp determination unit 407 reduces the throughput. For example, if the normal throughput is 43 sheets per minute, the throughput may be reduced to 20 sheets per minute. The throughput may be reduced by reducing the conveying speed V or widening the feeding interval G. If the non-passage portion temperature Tmax_c does not exceed the heat resistance threshold Tth, the Sp determination unit 407 maintains or increases the throughput. For example, if the current throughput is the normal throughput, the current throughput is maintained. If the current throughput is the reduced throughput, the current throughput is increased. This makes it possible to increase the number of sheets processed per unit time (the number of sheets on which images are formed) while protecting the fixing film 14.

[0068] In this example, two levels are provided for throughput, but three or more levels may be provided. For example, if there are n levels, the throughput can be adjusted by n-1 thresholds.

[0069] ●S507...Print completion determination The CPU 41 compares the number of sheets to be printed specified by the print job with the number of printed sheets P to determine whether there are any subsequent sheets P. If there are no subsequent sheets P, the CPU 41 ends the print job.

[0070] If a subsequent sheet P exists, the CPU 41 returns from S507 to S501. Next, in S501, the CPU 41 calculates the history values ​​HR_top and HL_top for the (i+1)th sheet P in consideration of the history values ​​HR_bottom and HL_bottom obtained for the i-th sheet P and the throughput (e.g., feeding interval G).

[0071] In Example 1, a black-and-white image was printed on one side of an A4-sized sheet of plain paper (210 mm x 297 mm) at a conveying speed of 240 mm / sec (43 sheets / min). The feeding interval G is the normal feeding interval G0 (e.g., 39 mm). However, Example 1 may also be applied to double-sided printing. In double-sided printing, the sheet P needs to be turned over, so the feeding interval G may be longer than G0. Furthermore, the feeding interval G may be longer than G0 due to feeding control, discharge control, or stacking control. In this case, the history value H_top of the (i+1)th sheet P to be printed next is affected by the feeding interval G. Therefore, the feeding interval G needs to be corrected according to equations (6) and (7).

[0072] HR_top = HR_bottom × c5 ···(6) HL_top = HL_bottom × c5 ···(7) FIG. 7(B) shows the correction coefficient c5 with respect to the feeding interval G. In this way, the correction coefficient c5 is determined based on the feeding interval G between the preceding sheet P and the succeeding sheet P. The history value H_bottom represents the amount of temperature decrease in the fixing film 14 caused by toner. Because no sheet P passes through the fixing film 14 during the period corresponding to the feeding interval G, no toner acts on the fixing film 14. Therefore, the history value H_top decreases toward the reference value (e.g., 0) when a sheet P on which no toner image is formed is heated. As shown in FIG. 7(B), the correction coefficient c5 decreases as the feeding interval G increases. In other words, the history value H_top decreases as the feeding interval G increases.

[0073] The normal feeding interval G0 is 39 mm. In this case, the correction coefficient c5 is 1.0. In this case, the history value H_top of the (i+1)th sheet P is equal to the history value H_bottom of the i-th sheet P. Note that the correction coefficient c5 shown in FIG. 7B is merely an example. The correction coefficient c5 may be appropriately determined by experiment or simulation.

[0074] (6) Effects of Example 1 Figure 11 shows print conditions I, II, and III that were adopted to confirm the effects of Example 1. Print condition I involves repeatedly printing image Im4 on multiple sheets P. Print condition II involves repeatedly printing image Im5 on multiple sheets P. Print condition III involves alternately printing images Im4 and Im5 on multiple sheets P. For all print conditions, the feed interval G is 30 mm.

[0075] Figure 12(A) shows the progress of the history value H for each print condition. The vertical axis represents the history value H. The horizontal axis represents the number of prints. Under print condition I, there is no toner image in the edge regions ER and EL. Therefore, the history value H is 0. Under print condition II, there is a toner image with a 100% coverage in the edge regions ER and EL. Therefore, the history value H increases as the number of prints of sheet P increases. As a result, the history value H for the tenth sheet P reached 12.3. Under print condition III, the history value H varied, but reached a maximum of 8.0.

[0076] FIG. 12(B) shows the non-passage portion temperature Tmax_c for each print condition. The vertical axis shows the non-passage portion temperature, and the horizontal axis shows the number of prints. Under print condition I, the non-passage portion temperature Tmax_c increases as the number of prints increases. When the number of prints reaches 12, the non-passage portion temperature Tmax_c exceeds the heat resistance threshold Tth (e.g., 265°C). Therefore, the feeding interval G is increased for the 13th and subsequent sheets P.

[0077] Under print condition II, the non-passage portion temperature Tmax_c exceeds the heat resistance threshold Tth when the number of printed sheets reaches 18. Therefore, the feeding interval G is extended for the 19th and subsequent sheets P.

[0078] Under print condition III, the non-passage portion temperature Tmax_c does not exceed the heat resistance threshold Tth even when the number of printed sheets reaches 30. Therefore, the feeding interval G is not changed, and high throughput is maintained.

[0079] As described above, according to the first embodiment, the amount of toner (history value H) transferred to the edge regions ER and EL of the preceding sheet P is reflected in the history value H of the toner amount of the succeeding sheet P. In other words, the amount of decrease in the temperature of the non-passage portion due to the toner in the preceding sheet P is taken into consideration, and the temperature of the non-passage portion of the succeeding sheet P is estimated or predicted. As a result, the number of sheets processed per unit time is maintained while the temperature rise of the non-passage portion is appropriately suppressed.

[0080] In the first embodiment, the temperature of the non-passage portion is acquired according to the printing time, but this is merely an example. The temperature of the non-passage portion of the fixing film 14 may be estimated using a heat propagation model 1300 as shown in FIG. 13. The heat propagation model 1300 is a simplified representation of heat conduction between the components constituting the fixing device 13. The arrows in FIG. 13 indicate the heat propagation paths between the components that are in contact with each other. The CPU 41 may estimate the temperature of the non-passage portion in real time by using the heat propagation model 1300. Note that a program corresponding to the heat propagation model 1300 is stored in the ROM 42.

[0081] <Example 2> The second embodiment is a method for controlling the throughput by further taking into consideration the size of the sheet P. For example, if the length (width) of the sheet P in a direction perpendicular to the conveying direction D1 of the sheet P is within a predetermined range, the method for determining the throughput described in the first embodiment is used. On the other hand, if the width of the sheet P is small, the throughput is determined by a different method. Note that matters already described in the first embodiment are omitted in the second embodiment.

[0082] (1) Basic Concept Figure 14(A) shows the temperature change in the non-passing portion NPL when continuous printing is performed on an A4-sized (width L2 = 210 mm) sheet P. The vertical axis represents temperature. The horizontal axis represents the distance (position) from the center of the fixing film 14. The center of the fixing film 14 is represented as 0 mm. The solid line (image Im7) shows the temperature change when a toner image is formed in the edge region EL of the sheet P. The printing rate of the toner image in the edge region EL is 100%. The dashed line (image Im6) shows the temperature change when a toner image is formed in the edge region EL of the sheet P. The printing rate of the toner image in the edge region EL is 0%.

[0083] According to FIG. 14(A), a temperature rise occurs at a position approximately 105 mm to 110 mm away from the center of the fixing film 14. The position where the temperature is highest is approximately 107 mm to 108 mm away from the center of the fixing film 14. For image Im7, a toner image is formed in the edge region EL of the sheet P. Furthermore, the position where the toner image is formed is close to the position where the temperature is highest. Therefore, the toner image lowers the maximum temperature. In particular, the maximum temperature for image Im7 is lower than the maximum temperature for image Im6, where no toner image is formed in the edge region EL.

[0084] Figure 14(B) shows the temperature change in the non-passing section NPL when continuous printing is performed on an A5-sized (width L2 = 148 mm) sheet P. The vertical axis represents temperature. The horizontal axis represents the distance (position) from the center of the fixing film 14. The center of the fixing film 14 is represented as 0 mm. The solid line (image Im9) shows the temperature change when a toner image is formed in the edge region EL of the sheet P. The printing rate of the toner image in the edge region EL is 100%. The dashed line (image Im8) shows the temperature change when a toner image is formed in the edge region EL of the sheet P. The printing rate of the toner image in the edge region EL is 0%.

[0085] For an A5-sized sheet P, a temperature rise occurs in the non-passing area at a position approximately 74 mm to 110 mm away from the center of the fixing film 14. The position of the highest temperature is approximately 90 mm away from the center of the fixing film 14. Comparing the solid line and the dashed line, it can be seen that for an A5-sized sheet P, the highest temperature does not change regardless of whether or not there is a toner image in the edge region EL.

[0086] Therefore, it can be said that the width of the sheet P for which the first embodiment works effectively is 207 mm or more and 213 mm or less. Therefore, in the second embodiment, the control mode is switched depending on the width of the sheet P.

[0087] (2) Controller FIG. 15 shows functions realized by the CPU 41 of the second embodiment. Compared to the first embodiment, in the second embodiment, a width acquisition unit 1501 acquires the size (width) of the sheet P from a size sensor 1502, an input device 1503, or a host computer 1504 provided in the image forming apparatus 1. The size sensor 1502 is a sensor provided in the feeding cassette 21 or the conveying path that detects the width of the sheet P. The input device 1503 is a touch sensor or input keys provided on the operation unit of the image forming apparatus 1. A user can specify the size of the sheet P through the input device 1503. The host computer 1504 is a computer that transmits a print job to the image forming apparatus 1. The host computer 1504 can specify the size of the sheet P through a printer driver. A determination unit 1505 determines whether the width of the sheet P is within a predetermined range (e.g., 207 to 213 mm) and outputs the determination result to the Sp determination unit 407. If the width of the sheet P is within a predetermined range, the Sp determination unit 407 determines the throughput according to the method described in the first embodiment. If the width of the sheet P is outside the predetermined range, the Sp determination unit 407 determines the throughput according to a different method. The different method includes, for example, determining the throughput according to the width of the sheet P. The ROM 42 may store a table linking the width with the throughput. The Sp determination unit 407 may obtain the throughput (feeding interval G or conveying speed V) corresponding to the width by referring to the table.

[0088] (3) Flowchart 16 shows a control method of the embodiment 2. The CPU 41 executes the following processes in accordance with a control program stored in the ROM 42.

[0089] In S1601, the CPU 41 (width acquisition unit 1501) acquires the size (width) of the sheet P. Here, the width of the sheet P is the length of the sheet P in the longitudinal direction of the fixing film 14 (direction perpendicular to the conveying direction D1).

[0090] In S1602, the CPU 41 (determination unit 1505) determines whether the size (width) of the sheet P is within a predetermined range. If the size (width) of the sheet P is within the predetermined range, the CPU 41 advances the process from S1602 to S501, and executes the control method (S501 to S507) described in the first embodiment. If the size (width) of the sheet P is not within the predetermined range, the CPU 41 advances the process from S1602 to S1603.

[0091] In S1603, the CPU 41 (Sp determination unit 407) determines the throughput according to the width of the sheet P, and executes printing at the determined throughput.

[0092] In the second embodiment, the control mode can be switched depending on the width of the sheet P. For example, if the width of the sheet P is within a predetermined range, the control method described in the first embodiment is applied. As a result, the effects described in the first embodiment are achieved. On the other hand, if the width of the sheet P is outside the predetermined range, a uniform throughput according to the width of the sheet P is applied. This will protect the fixing film 14 from heat and extend the life of the fixing device 13.

[0093] Example 3 In Example 1, the temperature of the non-passage portion NP is estimated based on the amount of toner used in the edge regions ER and EL in the width direction of the sheet P. Example 3 is an improved version of Example 1, in which the edge regions ER and EL are further divided into multiple sub-regions, and the amount of toner used (history value H) is calculated for each sub-region. As a result, Example 3 will have improved accuracy in estimating the temperature of the non-passage portion NP compared to Example 1, and will also have improved accuracy in determining the throughput. Matters already explained in Example 1 will be omitted in Example 3. In other words, the explanation of Example 1 is applied to Example 3.

[0094] (1) Sub-areas 17 shows sub-regions ER1, ER2, EL1, and EL2 of an A4-sized sheet P. Here, for convenience of explanation, the edge region ER is divided into sub-regions ER1 and ER2. The edge region EL is divided into sub-regions EL1 and EL2. Furthermore, each of the edge regions ER and EL may be divided into three or more sub-regions.

[0095] Sub-region ER1 is, for example, a region extending 5 mm from the right edge of sheet P. Sub-region ER2 is, for example, a region starting 5 mm from the right edge of sheet P and ending 100 mm from the right edge of sheet P. Similarly, sub-region EL1 is, for example, a region extending 5 mm from the left edge of sheet P. Sub-region EL2 is, for example, a region starting 5 mm from the left edge of sheet P and ending 100 mm from the left edge of sheet P.

[0096] For the sub-regions ER1, ER2, EL1, and EL2, history values ​​HR1_top, HR2_top, HL1_top, and HL2_top are calculated, respectively. For the sub-regions ER1, ER2, EL1, and EL2, toner amounts QR1, QR2, QL1, and QL2 are calculated, respectively. Furthermore, saturation values ​​UR1, UR2, UL1, and UL2 are calculated for the sub-regions ER1, ER2, EL1, and EL2. Furthermore, history values ​​HR1_bottom, HR2_bottom, HL1_bottom, and HL2_bottom are calculated for the sub-regions ER1, ER2, EL1, and EL2, respectively.

[0097] The CPU 41 calculates the history value HR_bottom from the history values ​​HR1_bottom and HR2_bottom, and further calculates the history value HL_bottom from the history values ​​HL1_bottom and HL2_bottom.

[0098] HR_bottom = c6×HR1_bottom + c7×HR2_bottom ···(8) HL_bottom = c6×HL1_bottom + c7×HL2_bottom ···(9) For example, coefficient c6 is 0.75. Coefficient c7 is 0.25. Coefficients c6 and c7 are determined by experiment or simulation. Coefficients c6 and c7 indicate the contribution (temperature reduction capability) of the toner image formed in the sub-region to reducing the temperature of the non-passing portion NP. The distance between sub-region ER1 and non-passing portion NPR is shorter than the distance between sub-region ER2 and non-passing portion NPR. Therefore, the temperature reduction capability of the toner image formed in sub-region ER1 is higher than the temperature reduction capability of the toner image formed in sub-region ER2. Therefore, coefficient c6 is greater than coefficient c7.

[0099] (2) Controller 18 shows an index determination unit 1800 according to a third embodiment. The index determination unit 1800 is used in place of the index determination unit 400. An HR1_bottom acquisition unit 1810 acquires HR1_bottom for the sub-region ER1. An HR2_bottom acquisition unit 1820 acquires HR2_bottom for the sub-region ER2. An HL1_bottom acquisition unit 1830 acquires HL1_bottom for the sub-region EL1. An HL2_bottom acquisition unit 1840 acquires HL2_bottom for the sub-region EL2.

[0100] The HR1_bottom acquisition unit 1810, the HR2_bottom acquisition unit 1820, the HL1_bottom acquisition unit 1830, and the HL2_bottom acquisition unit 1840 have the same internal configuration. Therefore, only the internal configuration of the HR1_bottom acquisition unit 1810 is illustrated in FIG.

[0101] The H_top acquisition unit 1801 acquires the history value H_top in the edge region E. The specific operation of the H_top acquisition unit 1801 is similar to the operation of the HR_top acquisition unit 401 and the HL_top acquisition unit 411.

[0102] The Q acquisition unit 1802 acquires the toner amount Q in the edge region E. The specific operation of the Q acquisition unit 1802 is similar to the operation of the QR acquisition unit 402 and the QL acquisition unit 412.

[0103] The U acquisition unit 1803 acquires the saturation value U in the edge region E. The specific operation of the U acquisition unit 1803 is similar to the operation of the UR acquisition unit 403 and the UL acquisition unit 413.

[0104] The H_bottom acquisition unit 1804 acquires the history value H_bottom in the edge region E.

[0105] The HR_bottom acquisition unit 1805 applies equation (8) to HR1_bottom output from the HR1_bottom acquisition unit 1810 and HR2_bottom output from the HR2_bottom acquisition unit 1820 to acquire HR_bottom.

[0106] The HL_bottom acquisition unit 1806 applies equation (9) to HL1_bottom output from the HL1_bottom acquisition unit 1830 and HL2_bottom output from the HL2_bottom acquisition unit 1840 to acquire HR_bottom.

[0107] The H determination unit 405 determines the history value H based on the HR_bottom output from the HR_bottom acquisition unit 1805 and the HL_bottom output from the HL_bottom acquisition unit 1806 .

[0108] (3) Flowchart 19 shows a control method of the third embodiment. The same reference numerals are given to the processes common to the control method of the first embodiment, and the description thereof is incorporated in the third embodiment.

[0109] In S1901, the CPU 41 (H_top acquisition unit 1801) acquires the history values ​​HR1_top, HR2_top, HL1_top, and HL2_top of the toner amounts of the sub-regions immediately before the leading edge of the i-th sheet P enters the fixing device 13.

[0110] In S1902, the CPU 41 (Q acquisition unit 1802) acquires the toner amounts QR1, QR2, QL1, and QL2 in the sub-areas ER1, ER2, EL1, and EL2 of the i-th sheet P.

[0111] In S1903, the CPU 41 (H_bottom acquisition unit 1804) acquires the history values ​​HR_bottom and HL_bottom of the toner amounts immediately after the rear edge of the i-th sheet P passed through the fixing device 13. First, the U acquisition unit 1803 acquires the saturation value U from the toner amount Q. The saturation value UR1 is calculated by substituting the toner amount QR1 into equation (1). The saturation value UR2 is calculated by substituting the toner amount QR2 into equation (1). The saturation value UL1 is calculated by substituting the toner amount QL1 into equation (2). The saturation value UL2 is calculated by substituting the toner amount QL2 into equation (2).

[0112] HR1_bottom is calculated by substituting HR1_top and UR1 into formula (3). HR2_bottom is calculated by substituting HR2_top and UR2 into formula (3). HL1_bottom is calculated by substituting HL1_top and UL1 into formula (4). HL2_bottom is calculated by substituting HL2_top and UL2 into formula (4).

[0113] Furthermore, HR_bottom is calculated by substituting HR1_bottom and HR2_bottom into equation (8). HL_bottom is calculated by substituting HL1_bottom and HL2_bottom into equation (9). The subsequent processing is as described in the first embodiment.

[0114] The third embodiment can achieve the same effects as the first embodiment. Furthermore, the third embodiment can control the throughput more accurately than the first embodiment, and can also protect the fixing device 13.

[0115] FIG. 20 shows test images Im10 to Im12 used to confirm the effects of Example 3. Test images Im10 to Im12 were all printed on an A4-sized sheet P. In test image Im10, no toner image was formed in sub-regions ER1, ER2, EL1, and EL2. In test image Im11, a maximum density black image (solid black) was formed in sub-regions ER1 and EL1. Note that in sub-regions ER1 and EL1, the areas extending 5 mm from the leading edge and 5 mm from the trailing edge are blank spaces, and no toner image was formed. The print rate for sub-regions ER1 and EL1 was 97%. The print rate for sub-regions ER2 and EL2 was 0%. In test image Im12, a maximum density black image (solid black) was formed in sub-regions ER2 and EL2. Note that in sub-regions ER2 and EL2, the areas extending 5 mm from the leading edge and 5 mm from the trailing edge are blank spaces, and no toner image was formed. The print rate of the sub-areas ER2 and EL2 is 97%. The print rate of the sub-areas ER1 and EL1 is 0%.

[0116] 21 shows the temperature transition of the non-passage portion NPL when test images Im10 to Im12 are each continuously printed on K sheets P. Test image Im10 has no toner image in sub-regions EL1 and EL2, so the temperature of the non-passage portion NPL is the highest.

[0117] In the test image Im11, a toner image exists in the sub-region EL1, and no toner image exists in the sub-region EL2. Since the sub-region EL1 is adjacent to the non-passage portion NPL, it has the greatest effect of lowering the temperature of the non-passage portion NPL.

[0118] In the test image Im12, a toner image exists in the sub-region EL2, but does not exist in the sub-region EL1. Since the sub-region EL2 is far from the non-passage portion NPL, it is less effective in reducing the temperature of the non-passage portion NPL.

[0119] In this way, the edge region E is divided into multiple sub-regions, and the history value H is calculated based on the distance from the edge of the sheet P to each sub-region and the amount of toner in each sub-region. As a result, it is possible to accurately calculate the temperature of the non-passage portion NP. As a result, it is possible to achieve a high level of balance between improving throughput and protecting the fixing device 13.

[0120] The formulas and coefficients used in the third embodiment are merely examples. Other formulas and coefficients may be used. The modifications described in the first embodiment are also applicable to the third embodiment. The third embodiment may be combined with the second embodiment. In this case, steps S501 to S503 described in FIG. 16 are replaced with steps S1901 to S1903.

[0121] <Technical ideas derived from examples> (Item 1) an image forming means for forming a toner image on a sheet; A first rotating body; a second rotating body that contacts the first rotating body to form a nip portion and conveys the sheet at a predetermined conveying speed; a heating unit that heats the sheet on which the toner image is formed via the first rotating body; a control means for controlling the number of sheets heated per unit time by the heating means, The control means is an end region extending parallel to the transport direction of multiple sheets transported continuously, and calculates a history value of the amount of toner transferred to each end region of the multiple sheets, and adjusts the number of sheets heated per unit time in accordance with the history value.

[0122] The pressure roller 15 is an example of a first rotating body. The fixing film 14 is an example of a second rotating body. The heater 60 is an example of a heating means. The controller 40 and the CPU 41 are examples of a control means. In the sheet P, the edge regions ER and EL each extend parallel to the conveying direction D1. According to Examples 1 to 3, the number of sheets heated per unit time (number of sheets on which images are formed) is adjusted according to the history value H of the amount of toner transferred to the edge regions ER and EL. This appropriately suppresses the temperature rise in the non-passage areas while maintaining the number of sheets processed per unit time. (Item 2) The image forming apparatus described in item 1 is characterized in that the control means estimates the temperature of a non-passage portion of the nip portion through which the sheet does not pass based on the history value, and adjusts the number of sheets heated per unit time so that the estimated temperature does not exceed the heat resistance temperature of the first rotating body.

[0123] The heat resistance threshold Tth of the fixing film 14 is an example of the heat resistance temperature of the first rotating body. (Item 3) The image forming apparatus according to item 2, characterized in that the control means estimates the temperature of the non-passing section lower as the amount of toner indicated by the history value increases, and estimates the temperature of the non-passing section higher as the amount of toner indicated by the history value decreases.

[0124] The more toner there is, the more heat is absorbed by the heater 60 by the toner. When the heat of the passing portion adjacent to the non-passing portion is absorbed by the toner, the temperature of the passing portion drops. Here, heat is transferred from the non-passing portion to the passing portion as the fixing film 14 tries to maintain thermal equilibrium, and the temperature of the non-passing portion also drops. In other words, the temperature of the non-passing portion also drops depending on the amount of toner in the edge region. (Item 4) the history value indicates a degree of contribution of the toner image formed in the end region to a decrease in the temperature of the non-passage portion, The image forming apparatus described in item 3 is characterized in that the end region is divided into multiple sub-regions, and the contribution of the toner images formed in each of the multiple sub-regions to lowering the temperature of the non-passing section decreases as the distance from the end of the sheet to each sub-region increases, and increases as the distance decreases.

[0125] In this way, the edge region may be divided into a number of sub-regions, and the history value may be determined depending on the distance from the edge to the sub-region, which will result in a more accurate estimation of the temperature of the non-passed portion. (Item 5) The history value when the trailing edge of the i-th sheet among the plurality of sheets exits the nip portion is the history values ​​accumulated from the first sheet to the (i-1)th sheet among the plurality of sheets; a contribution to the temperature decrease of the edge region by the i-th sheet passing through the nip; and 3. The image forming apparatus according to item 2, wherein the information is acquired based on the above.

[0126] H_bottom is an example of a history value when the trailing edge of the i-th sheet exits the nip. H_top is an example of a history value accumulated from the first sheet to the (i-1)-th sheet. As suggested by equations (3) and (4), (U-H_top) x c3 is an example of the contribution of the i-th sheet to the decrease in temperature of the edge region E. (Item 6) The image forming apparatus described in item 5 is characterized in that the contribution is calculated by multiplying the difference between the saturation value of the temperature drop amount for the non-passage portion and the history value accumulated from the first sheet to the i-1th sheet by a predetermined first coefficient.

[0127] As indicated by equations (3) and (4), coefficient c3 is an example of the first coefficient. (Item 7) 7. The image forming apparatus according to item 6, wherein the saturation value is the sum of a first product obtained by multiplying the square of the amount of toner in the end region by a second coefficient and a second product obtained by multiplying the amount of toner in the end region by a third coefficient.

[0128] As indicated by equations (1) and (2), coefficient c1 is an example of a second coefficient, and coefficient c2 is an example of a third coefficient. (Item 8) The history values ​​accumulated from the first sheet to the (i-1)th sheet among the plurality of sheets are: the history value at the timing when the trailing edge of the (i-1)th sheet passes through the nip portion; A fourth coefficient according to the distance between the rear end of the (i-1)th sheet and the front end of the i-th sheet; 8. The image forming apparatus according to any one of items 5 to 7, characterized in that the image forming apparatus is obtained based on the above.

[0129] As indicated by equations (6) and (7), H_bottom is an example of a history value at the timing when the trailing edge of the (i-1)th sheet passes through the nip portion. The coefficient c5 is an example of a fourth coefficient. (Item 9) 9. The image forming apparatus according to any one of items 2 to 8, wherein the temperature of the non-passage portion is estimated by subtracting the history value from a standard value of the temperature of the non-passage portion.

[0130] As explained in relation to equation (5), the maximum temperature Tmax_s of the non-passing portion NP when the printing rate of the edge region E is 0% is an example of a standard value. (Item 10) The image forming apparatus according to any one of items 1 to 9, characterized in that the control means estimates the temperature of a non-passage portion of the nip portion through which the sheet does not pass based on the width of the sheet, which is the length of the sheet in a direction perpendicular to the conveying direction of the sheet, and the history value, and adjusts the number of sheets heated per unit time so that the estimated temperature does not exceed the heat resistance temperature of the first rotating body.

[0131] As explained in the second embodiment, by taking the width of the sheet P into consideration, it becomes possible to estimate the temperature of the non-passage portion NP with higher accuracy. (Item 11) The control means If the width is within a predetermined range, the temperature of the non-passage portion is estimated based on the history value, and the number of sheets to be heated per unit time is adjusted according to the temperature of the non-passage portion; Item 11. The image forming apparatus according to item 10, wherein if the width is not within the predetermined range, the number of sheets heated per unit time is adjusted in accordance with the width.

[0132] As explained in the second embodiment, 207 mm or more and 213 mm or less is an example of the predetermined range. (Item 12) 12. The image forming apparatus according to any one of items 1 to 11, wherein the number of sheets heated per unit time is increased as the history value is larger, and is reduced as the history value is smaller. (Item 13) 13. The image forming apparatus according to any one of items 1 to 12, wherein the control unit acquires the history values ​​for a first end region along a first side parallel to the conveying direction of the sheet, and a second end region along a second side parallel to the conveying direction of the sheet, among the four sides of the sheet, and adjusts the number of sheets heated per unit time based on the smaller history value of the history value for the first end region and the history value for the second end region. (Item 14) 14. The image forming apparatus according to any one of items 1 to 13, wherein the number of sheets heated per unit time is adjusted by changing the conveying interval between the preceding sheet and the succeeding sheet. (Item 15) 15. The image forming apparatus according to any one of items 1 to 14, wherein the number of sheets heated per unit time is adjusted by changing the conveying speed of the sheet.

[0133] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0134] 20: Image forming unit, 13: Fixing device, 14: Fixing film, 15: Pressure roller, 60: Heater, 40: Controller

Claims

1. an image forming means for forming a toner image on a sheet; A first rotating body; a second rotating body that contacts the first rotating body to form a nip portion and conveys the sheet at a predetermined conveying speed; a heating unit that heats the sheet on which the toner image is formed via the first rotating body; a control means for controlling the number of sheets heated per unit time by the heating means, The control means is an end region extending parallel to the transport direction of multiple sheets transported continuously, and calculates a history value of the amount of toner transferred to each end region of the multiple sheets, and adjusts the number of sheets heated per unit time in accordance with the history value.

2. The image forming apparatus according to claim 1, characterized in that the control means estimates the temperature of a non-passage portion of the nip portion through which the sheet does not pass based on the history value, and adjusts the number of sheets heated per unit time so that the estimated temperature does not exceed the heat resistance temperature of the first rotating body.

3. The image forming apparatus according to claim 2, wherein the control means estimates the temperature of the non-passing portion lower as the amount of toner indicated by the history value increases, and estimates the temperature of the non-passing portion higher as the amount of toner indicated by the history value decreases.

4. the history value indicates a degree of contribution of the toner image formed in the end region to a decrease in the temperature of the non-passage portion, The image forming apparatus of claim 3, characterized in that the end region is divided into multiple sub-regions, and the contribution of the toner images formed in each of the multiple sub-regions to lowering the temperature of the non-passing section decreases as the distance from the end of the sheet to each sub-region increases, and increases as the distance decreases.

5. The history value when the trailing edge of the i-th sheet of the plurality of sheets exits the nip portion is the history values ​​accumulated from the first sheet to the (i-1)th sheet among the plurality of sheets; a contribution to a temperature decrease in the edge region by the i-th sheet passing through the nip; and 3. The image forming apparatus according to claim 2, wherein the image forming apparatus is configured to acquire the image data based on the acquired image data.

6. The image forming apparatus described in claim 5, characterized in that the contribution is calculated by multiplying the difference between the saturation value of the temperature drop for the non-passage portion and the history value accumulated from the first sheet to the i-1th sheet by a predetermined first coefficient.

7. 7. The image forming apparatus according to claim 6, wherein the saturation value is the sum of a first product obtained by multiplying the square of the amount of toner in the edge region by a second coefficient and a second product obtained by multiplying the amount of toner in the edge region by a third coefficient.

8. The history values ​​accumulated from the first sheet to the (i-1)th sheet are the history value at the timing when the trailing edge of the (i-1)th sheet passes through the nip portion; A fourth coefficient according to the distance between the rear end of the (i-1)th sheet and the front end of the i-th sheet; 6. The image forming apparatus according to claim 5, wherein the image forming speed is determined based on the following equation.

9. 3. The image forming apparatus according to claim 2, wherein the temperature of the non-passage portion is estimated by subtracting the history value from a standard value of the temperature of the non-passage portion.

10. The image forming apparatus described in claim 1, characterized in that the control means estimates the temperature of the non-passage portion of the nip portion through which the sheet does not pass based on the width, which is the length of the sheet in a direction perpendicular to the conveying direction of the sheet, and the history value, and adjusts the number of sheets heated per unit time so that the estimated temperature does not exceed the heat resistance temperature of the first rotating body.

11. The control means If the width is within a predetermined range, the temperature of the non-passage portion is estimated based on the history value, and the number of sheets to be heated per unit time is adjusted according to the temperature of the non-passage portion; 11. The image forming apparatus according to claim 10, wherein if the width is not within the predetermined range, the number of sheets heated per unit time is adjusted in accordance with the width.

12. 2. The image forming apparatus according to claim 1, wherein the number of sheets heated per unit time is increased as the history value increases, and is reduced as the history value decreases.

13. The image forming apparatus of claim 1, characterized in that the control means acquires the history values ​​for each of the four sides of the sheet, namely, a first end region along a first side parallel to the conveying direction of the sheet, and a second end region along a second side parallel to the conveying direction of the sheet, and adjusts the number of sheets heated per unit time based on the smaller history value of the history value for the first end region and the history value for the second end region.

14. 2. The image forming apparatus according to claim 1, wherein the number of sheets heated per unit time is adjusted by changing a conveyance interval between a preceding sheet and a succeeding sheet.

15. 2. The image forming apparatus according to claim 1, wherein the number of sheets heated per unit time is adjusted by changing a conveying speed of the sheet.

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