JP7580978B2 - Image forming device - Google Patents

Description

The present invention relates to image forming devices that use electrophotographic technology, such as printers, copiers, facsimiles, and multifunction devices.

The image forming apparatus is equipped with a fixing device that applies heat and pressure to a recording material on which an unfixed toner image is formed, thereby fixing the toner image to the recording material. As a fixing device, for example, a film heating type fixing device has been proposed that includes a rotating endless fixing film, a heater that contacts the inner peripheral surface of the fixing film, and a pressure roller that contacts the fixing film from the outside to form a fixing nip portion (Patent Document 1). In this fixing device, the heater generates heat in response to the supply of power, and the heat of the heater is transferred to the fixing film, heating the fixing film. As the heater, for example, a ceramic heater with a low thermal capacity is used. Then, when the recording material on which an unfixed toner image is formed is sandwiched and conveyed to the fixing nip portion, heat and pressure are applied, and the toner image is fixed to the recording material.

When the recording material is transported toward the fixing nip, multiple sheets of recording material may be transported in an overlapping state, stuck together due to moisture or static electricity (this is called double feeding). When multiple sheets of recording material are transported toward the fixing nip, a gap is created between the fixing film and the pressure roller near the edge of the recording material in the width direction that intersects with the transport direction of the recording material, making it difficult for the pressure roller to absorb heat from the heater. When this happens, the heater becomes locally hot, and in some cases there is a risk that the heater itself may be damaged or part of the fixing film may melt.

Therefore, in the device described in Patent Document 1, a thermistor is provided on the non-sliding surface side of the heater that does not come into contact with the fixing film in order to detect local temperature changes in the heater caused by double feeding of recording materials. The heater temperature change according to the detection result of this thermistor is compared with a reference value to detect double feeding of recording materials. Then, when double feeding of recording materials is detected, the power supply to the heater is adjusted to suppress the temperature rise in the heater so that the heater does not become locally too hot.

JP 2002-296962 A

However, when adjusting the heater temperature based on the heater temperature change according to the detection result of the thermistor, the heater temperature change may differ for each fixing device. For example, the heater temperature change according to the detection result of the thermistor may differ depending on the assembly accuracy of the fixing film, pressure roller, heater, thermistor, etc., and the variation in accuracy of each of these parts. As such, in the past, even if multiple recording materials were actually being fed, the multiple feeding of recording materials was not detected and the heater temperature rise was not suppressed, resulting in the heater becoming locally hot. Conversely, even if multiple recording materials were not actually being fed, the multiple feeding of recording materials was detected and the heater temperature rise was suppressed, resulting in the fixing film not being heated sufficiently.

The present invention was made in consideration of the above problems, and aims to provide an image forming device that uses a thermistor to detect changes in heater temperature caused by overlapping feeding of recording materials, and can properly suppress the rise in heater temperature based on this.

An image forming apparatus according to one embodiment of the present invention has an apparatus main body, an image forming means for forming a toner image on a recording material, a first rotating body, a second rotating body that contacts the first rotating body to form a nip portion for nipping and transporting the recording material, and a heating means for heating the first rotating body, and further includes a fixing means for fixing the toner image formed on the recording material by the image forming means onto the recording material, and a temperature detection means provided in the fixing means for detecting the temperature of the heating means outside a passing region through which a recording material of a minimum size that can be transported passes and within a heating region for heating the first rotating body in a width direction intersecting the transport direction of the recording material at the nip portion, and a temperature detection means for detecting a temperature of the heating means when a temperature detected by the temperature detection means while the recording material is passing through the nip portion becomes higher than a reference temperature. and a memory means provided in the fixing means and storing a temperature rise rate per unit time of the temperature detection means specific to the fixing means, wherein the control means sets the reference temperature to a first temperature when the temperature rise rate stored in the memory means is a first rise rate and the temperature rise rate per unit time based on the temperature detected by the temperature detection means is a first detected temperature rise rate , and sets the reference temperature to a second temperature lower than the first temperature when the temperature rise rate stored in the memory means is a second rise rate smaller than the first rise rate and the temperature rise rate per unit time based on the temperature detected by the temperature detection means is the first detected temperature rise rate .

According to the present invention, the temperature change of the heating means caused by the overlapping of recording materials is detected by the temperature detection means, and the temperature rise of the heating means can be easily suppressed based on the detection result.

1 is a schematic diagram showing an image forming apparatus according to an embodiment of the present invention. FIG. FIG. 4A is a schematic diagram showing the sliding surface side of a heater, FIG. 4B is a schematic diagram showing the non-sliding surface side of a heater, and FIG. 4C is an enlarged cross-sectional view showing a transverse section of a heater. FIG. 4 is a control block diagram showing a heater temperature control system of the control unit. 4 is a flowchart showing a heater-off temperature changing process according to the embodiment. 5 is a timing chart for explaining a heater-off temperature changing process. 6 is a graph showing changes in heater temperature over time.

[Image forming apparatus]
An image forming apparatus suitable for using the fixing device of this embodiment will be described with reference to Fig. 1. The image forming apparatus (hereinafter, referred to as printer 100) shown in Fig. 1 forms an image on a recording material P according to image information sent from an output device (not shown), such as a host computer or a document reader, provided outside the main body 100A of the printer 100. The surface of the photosensitive drum 101 is a photoconductive layer mainly composed of an organic photoreceptor, and is rotated at a predetermined process speed in the direction of arrow A (clockwise here) by a drive mechanism (not shown) provided in the main body 100A.

The outer peripheral surface of the photosensitive drum 101 is uniformly charged to a predetermined potential by the charging roller 102, and then an electrostatic latent image corresponding to the image information from the output device is formed on the outer peripheral surface by the laser light 103 output from the exposure device 104. This electrostatic latent image is developed into a toner image by the developing device 105 using a developer (specifically, toner T).

The recording material P is taken out by the paper feed roller 112 from the paper feed cassette 107, which has multiple sheets stacked on it, and is transported through the transport path B by the pair of registration rollers 113 toward the transfer nip N1 between the transfer roller 108 and the photosensitive drum 101 at a predetermined timing. However, at this time, multiple sheets of recording material P may be transported in an overlapping state, stuck together due to moisture or static electricity (so-called double feeding).

When the recording material P being transported in this manner passes through the transfer nip portion N1, a transfer voltage is applied to the transfer roller 108, which rotates at a predetermined process speed, and the toner image on the photosensitive drum 101 is transferred onto the recording material P. Residual toner that remains on the photosensitive drum 101 without being transferred to the recording material P during this transfer process is removed from the photosensitive drum 101 by the cleaning blade 109 and collected in the cleaning device 110. Examples of the recording material P include sheet materials such as paper, plastic film, and cloth.

The recording material P that has passed through the transfer nip portion N1 is transported to the fixing device 111, where heat and pressure are applied to fix the toner image onto the recording material P (specifically, at the fixing nip portion N2). The recording material P that has passed through the fixing device 111 then passes through the transport path C and is discharged with the image forming surface (printing surface) facing up onto the discharge tray 114 provided outside the device main body 100A. Alternatively, the recording material P that has passed through the fixing device 111 passes through the transport path D and is discharged with the image forming surface facing down onto the loading section 115 provided on the top surface of the device main body 100A.

In this embodiment, the photosensitive drum 101, charging roller 102, exposure device 104, developing device 105, and transfer roller 108 form an image forming unit 300 as an image forming means for forming a toner image on a recording material P. In this embodiment, the operation of each part, such as the image forming unit 300 and the fixing device 111, is controlled by a control unit 120 provided in the apparatus main body 100A.

[Fixing device]
Next, the fixing device 111 as a fixing means will be described with reference to FIG. 2. The fixing device 111 of this embodiment is a film heating type heating device, and is provided in the apparatus main body 100A in a replaceable manner. As shown in FIG. 2, the fixing device 111 has a heater 305 as a heating means, an endless fixing film 303 as a first rotating body, and a cylindrical or approximately cylindrical rotatable pressure roller 302 as a second rotating body. The fixing device 111 also has thermistors 301a and 301b as temperature detection means for detecting the temperature of the heater 305, and a heater holder 304 that supports the heater 305 and also serves as a guide for the movement of the fixing film 303 in a predetermined direction. Furthermore, in this embodiment, the fixing device 111 is equipped with an IC memory 205 that can store various information related to the fixing device 111 in advance, for example, before shipping from the factory.

The heater 305 will be described with reference to Figs. 3(a) to 3(c). As shown in Fig. 3(a), the heater 305 is a ceramic heater having a horizontally elongated planar heating element with low heat capacity, with the longitudinal direction being the width direction intersecting with the conveying direction of the recording material P. The heater 305 has a heating element 305c provided on one surface of a heater substrate 305a whose main component is a highly thermally conductive ceramic such as alumina (Al2O3) or aluminum nitride (AlN).

In this embodiment, for example, two heating elements 305c each having a thickness of 20 μm, a width of 2 mm, and a length of 230 mm are provided on one surface of a heater substrate 305a having a thermal conductivity of 100 W/(m·K) and a thickness of 1 mm, a width of 9 mm, and a length of 280 mm. As shown in FIG. 3(a) and FIG. 3(c), the two heating elements 305c are coated and baked by screen printing so as to be parallel to each other with a gap of 0.5 mm, using an electrical resistance material such as TaSiO2, AgPd, Ta2N, RuO2, or nichrome. The one surface side (sliding surface side) of the heater substrate 305a on which the heating elements 305c are formed is coated with a protective layer 305b mainly composed of glass or fluororesin to protect the heating elements 305c from sliding with the fixing film 303. As shown in FIG. 3(a), these two heating elements 305c are electrically connected to the conductive electrode 306 at one end, and are electrically connected to the triac 200 via the conductive electrode 306 at the other end.

In this embodiment, thermistors 301a and 301b are arranged on the non-sliding surface side of heater 305 with fixing film 303, as shown in FIG. 3B. Thermistor 301a is arranged in the center of heater 305 in the longitudinal direction, and thermistor 301b is arranged at a position "115 mm" away from the center of heater 305 in the longitudinal direction. In detail, thermistor 301a is arranged in the minimum size passing area through which all sizes of recording material P passing through fixing nip portion N2 pass in the longitudinal direction of heater 305, and detects the temperature of heater 305 in the passing area. On the other hand, thermistor 301b is arranged outside (outside the passing area) the passing area through which the minimum size of conveyable recording material P passes and inside (inside the heating area) the heating area of heating element 305C that heats fixing film 303, and detects the temperature of heater 305 at that position.

Returning to FIG. 2, the inner circumference of the fixing film 303 is slightly longer than the approximate outer circumference of the heater holder 304, so that the fixing film 303 can be fitted around the heater holder 304 without tension. The fixing film 303 has a low thermal capacity, which improves the start-up (quick start) of the temperature rise caused by heating by the heater 305. For example, the fixing film 303 may have a single-layer structure with a film thickness of "400 μm or less", preferably "30 to 80 μm", and made primarily of heat-resistant materials such as PTFE, PFA or FEP. Alternatively, there is a multi-layer structure in which a base layer with a thickness of "approximately 30 μm" mainly composed of polyimide, polyamideimide, PEEK, PES, PPS, etc., SUS, metal material such as nickel, etc., is formed, a silicone rubber elastic layer with a thickness of "300 μm" on the outer peripheral surface of the base layer, and a release layer with a thickness of "approximately 20 μm" mainly composed of PTFE, PFA, FEP, etc. is formed on the elastic layer. In this embodiment, a multi-layer film member with a diameter of "25 mm" and a total thickness of "350 μm" is used as the fixing film 303.

The pressure roller 302 is an elastic roller with a core metal 302a provided with an elastic layer 302b of silicone rubber, fluororubber, or the like to reduce the hardness. In addition, in order to improve the surface properties and the releasability to the toner T, a fluororesin layer such as PTFE, PFA, or FEP may be provided on the outer peripheral surface of the elastic layer 302b. The pressure roller 302 is pressed toward the heater 305 via the fixing film 303 by a pressure mechanism (not shown) provided outside the fixing device 111. As a result, a fixing nip portion N2 is formed between the fixing film 303 and the pressure roller 302, which sandwiches and conveys the recording material P while applying heat and pressure, thereby fixing the toner image to the recording material P. That is, when the recording material P passes through the fixing nip portion N2, it is sandwiched and conveyed between the fixing film 303 and the pressure roller 302, and the unfixed toner image formed on the recording material P at that time is melted by heat and fixed on the recording material P.

In this embodiment, the pressure roller 302 is driven to rotate in the direction of arrow Y (counterclockwise in this case) by receiving a driving force from a driving mechanism (not shown) provided outside the fixing device 111. The fixing film 303 rotates in the direction of arrow X (clockwise) at approximately the same peripheral speed as the pressure roller 302 by the rotational driving force transmitted from the pressure roller 302 by the frictional force generated in the fixing nip portion N2 (so-called pressure roller driving method). At that time, the inner peripheral surface of the fixing film 303 slides against the heater 305.

The heater holder 304 holds the heater 305 in a pressed state toward the fixing film 303. The heater holder 304 is formed so that its cross-sectional shape is a semicircular arc shape, and regulates the rotational trajectory of the fixing film 303 from the inside. The heater holder 304 is formed from a highly heat-resistant resin such as "Zenite 7755 (DuPont)".

Next, the power supply control to heater 305 (more specifically, heating element 305C) will be described using FIG. 4 with reference to FIGS. 2 to 3(b). FIG. 4 is a control block diagram showing a heater temperature control system of control unit 120. In addition to those shown in the figure, control unit 120 is also connected to various motors that drive each component such as image forming unit 300, and various power sources that apply voltage. However, as this is not the main focus of the invention, illustration and description of these will be omitted here.

The control unit 120 as a control means performs various controls of the printer 100 (see FIG. 1), such as image formation operations, and has, for example, a CPU (Central Processing Unit) 203 and a memory 204. The memory 204 is composed of a ROM (Read Only Memory) and a RAM (Random Access Memory), and stores various programs and data for controlling the printer 100. The CPU 203 can execute various programs, such as an image formation job (not shown) stored in the memory 204, and can operate the printer 100 to form an image on the recording material S. In this embodiment, at that time, the power supply to the heater 305 is controlled so that the temperature of the heater 305 can be adjusted. The memory 203 can temporarily store the results of calculations associated with the execution of various programs.

As shown in FIG. 4, the control unit 120 is connected to a triac 200, an A/D conversion circuit 202, an IC memory 205, and the like via an input/output interface. The heating element 305C is adapted to receive power from a commercial power source 201 via the triac 200, and the power supply from the commercial power source 201 to the heating element 305C is controlled by the CPU 203.

The temperature detected by thermistors 301a and 301b is converted to digital information by A/D conversion circuit 202 and input to CPU 203 as temperature information of heating element 305C. CPU 203 compares the input temperature information of heating element 305C with the target temperature stored in memory 204, and performs PID control on the power supply from commercial power source 201 to heating element 305C via triac 200 to adjust the temperature of heating element 305C to the target temperature.

The CPU 203 monitors the temperature information of the heating element 305C at predetermined intervals, and can correct the power supplied to the heating element 305C at predetermined intervals. In this embodiment, wave number control is used to select whether or not the commercial power source 201 supplies power to the heating element 305C for each half-wave of the AC power output from the commercial power source 201 during a predetermined period. In addition to wave number control, the amount of power supplied from the commercial power source 201 to the heating element 305C over a predetermined period can also be adjusted using phase control, which determines the phase range for each half-wave of the AC power output from the commercial power source 201.

The thermistor 301b is provided to detect double feeding of the recording material P, and the temperature (analog information) detected by the thermistor 301b is converted to digital information by the A/D conversion circuit 202 and input to the CPU 203. The CPU 203 performs double feeding detection control based on the input temperature information of the heating element 305C.

That is, in this embodiment, double feed detection is performed by detecting the temperature change of the heater 305 within a predetermined time while the recording material P is passing through the fixing nip N2 according to the detection result of thermistor 301b. Specifically, when the difference (= T2 - T1) between the detected temperature T1 of thermistor 301b at the time when the leading edge of the recording material P is detected by sensor 307 arranged near the paper discharge side of the fixing nip N2 shown in FIG. 2 and the detected temperature T2 of thermistor 301b while the recording material P is passing through the fixing nip N2 reaches a predetermined temperature or more, it is determined that the recording material P has been double fed and double feed detection is performed. This double feed detection is performed by the CPU 203.

The temperature (analog information) detected by the above thermistors 301a and 301b is converted to digital information by the A/D conversion circuit 202 and then input to the CPU 203. In this embodiment, the CPU 203 converts the input digital information back to analog information, and the analog information can be used to determine the temperature rise rate per unit time [°C/0.1s] of thermistor 301b (referred to as the detected temperature slope ΔT for distinction). Since analog information and digital information are not proportional, calculating the detected temperature slope ΔT using analog information results in smaller errors than calculating the detected temperature slope ΔT using digital information.

In this embodiment, the control unit 120 is connected to the IC memory 205 provided in the fixing device 111 via an input/output interface. The "stored temperature gradient ΔT0" of thermistor 301b is stored in advance in the IC memory 205. The "stored temperature gradient ΔT0" is the temperature rise rate per unit time [°C/0.1s] of thermistor 301b, which is measured in advance when the fixing film 303, pressure roller 302, heater 305, thermistor 301b, etc. are installed in the fixing device 111. Therefore, the "stored temperature gradient ΔT0" stored in the IC memory 205 may differ for each attached fixing device 111.

[Heater off temperature change process]
Next, the heater-off temperature change process of this embodiment will be described with reference to Fig. 5 along with Fig. 1, Fig. 2, and Fig. 4. This heater-off temperature change process is started by the control unit 120 when an image forming job is started, and is ended when the image forming job is ended.

5, the control unit 120 acquires an image forming job start command (print command) sent from an output device or an operation unit (not shown) that can be input by a user (S1). When the control unit 120 acquires a print command, the control unit 120 acquires the above-mentioned "stored temperature gradient ΔT0" from the IC memory 205 (step S2). The control unit 120 acquires the "detected temperature T0" of the thermistor 301b at the timing when the leading edge (downstream end in the transport direction) of the recording material P (S3, S4) that has been taken out of the paper feed cassette 107 and has a toner image transferred thereon enters the fixing nip portion N2 (S5, S6). The control unit 120 can detect that the recording material P has entered the fixing nip portion N2 by a signal from an entrance sensor (not shown) provided upstream of the fixing device 111 in the transport direction. If an entrance sensor is not provided, the control unit 120 can predict the timing at which the recording material P enters the fixing nip N2, for example, from the transport distance of the recording material P from the registration roller pair 113 to the fixing nip N2 and the above-mentioned process speed.

In addition, while the recording material P is passing through the fixing nip N2, the control unit 120 acquires the "detected temperature T0" of thermistor 301b (see S6), and then acquires the "detected temperature Tn" of thermistor 301b n seconds later (S7). Furthermore, the control unit 120 acquires the "detected temperature Tn+1" of thermistor 301b n+1 seconds later (S8). That is, in this embodiment, the control unit 120 acquires the temperature of thermistor 301b every n seconds (e.g., 0.1 seconds) from when the recording material P enters the fixing nip N2 until it passes through the fixing nip N2. Then, the control unit 120 calculates the "detected temperature slope ΔTn+1 (=detected temperature Tn+1-detected temperature Tn)" according to the acquired "detected temperature Tn" and "detected temperature Tn+1" (S9).

The control unit 120 determines whether the acquired "detection temperature Tn+1" (see S8) of thermistor 301b, i.e., the heater temperature, is lower than the "predetermined temperature Ta" (e.g., 260°C) (S10). If the "detection temperature Tn+1" (heater temperature) of thermistor 301b is lower than the "predetermined temperature Ta" (Yes in S10), the control unit 120 jumps to the process of step S15. If the "detection temperature Tn+1" (heater temperature) of thermistor 301b is equal to or higher than the "predetermined temperature Ta" (No in S10), the control unit 120 determines that this is a local temperature change of the heater 305 caused by the double feeding of the recording material P, detects the double feeding of the recording material P, and proceeds to the process of step S11.

When the control unit 120 detects the double feeding of the recording material P, it is necessary to stop the power supply to the heater 305 to suppress the temperature rise of the heater 305. For this purpose, the heater-off temperature (Toff1 (°C)) stored in advance in the memory 204 is set as the reference temperature for stopping the power supply to the heater 305. However, depending on the temperature rise rate per unit time of the thermistor 301b, even if the heater 305 is turned off based on the heater-off temperature (Toff1 (°C)), the temperature rise cannot be suppressed and the heater 305 may become hotter. Therefore, in this embodiment, the heater-off temperature set as the reference temperature can be changed according to the "stored temperature gradient ΔT0" stored in the IC memory 205. By doing so, even if the temperature rise rate per unit time of the thermistor 301b is different, the power supply to the heater 305 can be appropriately stopped to suppress the temperature rise of the heater 305.

Returning to the explanation of FIG. 5, when the control unit 120 detects a double feed of the recording material P, it obtains the "storage temperature gradient ΔT0" from the IC memory 205 and judges whether any of the following conditions are satisfied. As a first condition, the control unit 120 judges whether "storage temperature gradient ΔT0≦T1 (°C/0.1s)" and "detection temperature gradient ΔTn+1 (see S9)>α (°C/0.1s)" is satisfied (S11). As a second condition, the control unit 120 judges whether "T1<storage temperature gradient ΔT0≦T2 (°C/0.1s)" and "detection temperature gradient ΔTn+1>β (°C/0.1s)" is satisfied (S12). As a third condition, the control unit 120 judges whether "storage temperature gradient ΔT0>T2 (°C/0.1s)" and "detection temperature gradient ΔTn+1>γ (°C/0.1s)" is satisfied (S13). For example, α=4, β=6, γ=8, T1=4, and T2=6 (see Table 1 below).

If none of the above three conditions are met (S11, S12, and S13 are all No), the control unit 120 proceeds to step S15. In this case, the power supply to the heater 305 is stopped according to the heater-off temperature (Toff1) that is set as the reference temperature.

On the other hand, if any of the above three conditions is met (Yes in any of S11, S12, and S13), the control unit 120 changes the heater-off temperature set as the reference temperature (S14). Here, the reference temperature is set from the heater-off temperature (Toff1) stored in the memory 204 to a heater-off temperature (Toff2 (°C)) lower than the heater-off temperature (Toff1). For example, "Toff1 = 285 (°C)" and "Toff2 = 260 (°C)" (see Table 1 described later). Then, the process proceeds to step S15.

In the process of step S15, the control unit 120 judges whether the trailing end (upstream end in the conveying direction) of the recording material P has passed through the fixing nip portion N2 (S15). If the trailing end of the recording material P has not passed through the fixing nip portion N2 (No in S15), the control unit 120 returns to the process of step S7 and repeats the processes of steps S7 to S14. If the trailing end of the recording material P has passed through the fixing nip portion N2 (Yes in S15), the control unit 120 sets the reference temperature to the heater-off temperature (Toff1 (°C)) previously stored in the memory 204. In other words, if the reference temperature has been changed to the heater-off temperature (Toff2 (°C)) (see S14), after the trailing end of the recording material P has passed through the fixing nip portion N2, the control unit 120 sets the reference temperature back to the heater-off temperature (Toff1) before the change. This is because, when a double feed of the recording material P is detected, the recording material P next conveyed toward the fixing nip portion N2 may not be a double feed.

Then, the control unit 120 determines whether or not to end the image formation job being executed (S17). If the image formation job is not to be ended (No in S17), the control unit 120 returns to the process of step S5. On the other hand, if the image formation job is to be ended (Yes in S17), the control unit 120 ends the heater-off temperature change process of this embodiment.

An example of the reference temperatures set by the above-mentioned heater-off temperature changing process is shown in Table 1. In Table 1, "Toff1=285" and "Toff2=260° C."

As shown in Table 1, when the "memory temperature gradient ΔT0" (temperature rise rate) stored in the IC memory 205 is a first rise rate (e.g., 5 (°C/0.1 sec)), the reference temperature is set to the first temperature (285°C). When the "memory temperature gradient ΔT0" is a second rise rate (e.g., 3 (°C/0.1 sec)) that is smaller than the first rise rate, the reference temperature is set to the second temperature (260°C) that is lower than the first temperature. When the temperature rise rate per unit time of the detected temperature detected by thermistor 301b while the recording material P is passing through the fixing nip portion N2 (detected temperature gradient ΔTn+1) is larger than the "memory temperature gradient ΔT0" stored in the IC memory 205, the reference temperature is lowered.

In this embodiment, as long as the relationships of "α<β<γ", "T1<T2", and "Toff1>Toff2" are satisfied, these are not limited to the values listed in Table 1. For example, the above threshold values (α, β, γ) to be compared with the "detected temperature gradient ΔTn+1" may be changed between the case where the basis weight of the recording material P is "105 (g/m ² )" and the case where the basis weight is "300 (g/m ² )". That is, when the recording material P is double-fed, the larger the basis weight of the recording material P, the more likely a gap is to be generated between the fixing film 301 and the pressure roller 302, and therefore the "detected temperature gradient ΔTn+1" (see S9) becomes large. Therefore, the larger the basis weight of the recording material P, the lower the reference temperature may be set by comparing with the larger threshold values (α, β, γ). Furthermore, the value of the "predetermined temperature Ta" (see S10) may be changed depending on the basis weight of the recording material P and the width direction length of the recording material P.

The threshold values (α, β, γ) compared to the "detection temperature slope ΔTn+1" may be changed according to the "detection temperature Tn+1" when the leading edge of the recording material P passes through the fixing nip N2. In other words, if the heater temperature is high when the leading edge of the recording material P passes through the fixing nip N2, it is better to stop the power supply to the heater 305 as soon as possible, so the reference temperature may be set lower by comparing with a smaller threshold value (α, β, γ).

Next, the heater-off temperature change process of this embodiment will be described using the timing chart shown in FIG. 6. FIG. 6 will be described taking as an example the case where the "stored temperature gradient ΔT0≦T1" stored in IC memory 205. From the top, FIG. 6 shows the presence or absence of recording material P in the fixing nip N2, i.e., whether it is passing through or not, the "detected temperature Tn+1" of thermistor 301b, the "detected temperature gradient ΔTn+1" of thermistor 301b, and the reference temperature. The heater-off temperature (Toff1) previously stored in memory 204 is "285°C", which is initially set as the reference temperature.

When the "detection temperature gradient ΔTn+1" is greater than "4 (°C/0.1s)" and the "detection temperature Tn+1" of thermistor 301b is greater than "260°C", the reference temperature is changed from "285°C" to "260°C". That is, in the case of double feeding of recording material P, the heater temperature continues to rise locally while the recording material P is passing through the fixing nip N2. In order to prevent the heater temperature from reaching an unacceptably high temperature, the reference temperature that is the basis for forcibly stopping the heating of the fixing film 303 by the heater 305 is lowered until the recording material P passes through the fixing nip N2. Then, when the recording material P passes through the fixing nip N2, the reference temperature is returned from "260°C" to "285°C". In this way, in this embodiment, when the setting of the reference temperature is changed, the changed reference temperature is used until the recording material P passes through the fixing nip N2.

In this embodiment, the reference temperature is changed stepwise (from 285°C to 260°C), but this is not limiting. For example, the reference temperature may be changed continuously according to the magnitude of the "detected temperature gradient ΔTn+1" of thermistor 301b. As a specific example, the reference temperature may be lowered by a predetermined amount of change (for example, 1°C) each time the "detected temperature gradient ΔTn+1" increases by "1°C/0.1s." In this case, the predetermined amount of change is stored in IC memory 205.

The effect of this embodiment will be described using Fig. 7 with reference to Fig. 2. A1 and A2 in Fig. 7 show the temperature transition of the heater 305 detected by the thermistor 301b during overlap feeding when four sheets of recording material P having LGL size (216 mm x 356 mm) and basis weight "105 g/ ^m2 " are overlapped and pass through the fixing nip portion N2. A1 shows a conventional example, and A2 shows this embodiment. A3 in Fig. 7 shows the temperature transition of the heater 305 detected by thermistor 301b during normal times when only one sheet of recording material P having LGL size (216 mm x 356 mm) and basis weight "105 g/ ^m2 " passes through the fixing nip portion N2.

The error temperature (e.g., 297°C) shown in FIG. 7 is a temperature at which the heater 305 itself may be damaged or part of the fixing film 303 may melt. Therefore, it is necessary to manage the temperature of the heater 305 so that it does not reach the error temperature, and in this embodiment, the heater 305 is turned off before it reaches the error temperature. However, in the case of the conventional example, as shown in A1, the reference temperature is set to "285°C", so the heater 305 is not turned off until the "detection temperature Tn+1" of thermistor 301b detects "285°C". Even if the "detection temperature Tn+1" detects "285°C" and the heater 305 is immediately turned off, the temperature of the heater 305 will rise to the error temperature, causing the heater 305 itself to be damaged or part of the fixing film 303 to melt. This is because it is affected by the heat stored in the heater 305 and thermistor 301b, and because a larger gap than normal occurs between the fixing film 301 and the pressure roller 302 when the recording material P is being fed in multiple layers, preventing the heat from escaping to the pressure roller 302.

On the other hand, as shown in A2, in the case of this embodiment, if the "detection temperature gradient ΔTn+1" of thermistor 301b is "4°C/0.1s", the reference temperature is set to "260°C". Therefore, if the "detection temperature Tn+1" detects "260°C" and heater 305 is turned off, the temperature of heater 305 will not rise to the error temperature even if it is affected by heat stored in heater 305 or thermistor 301b. Note that, in the normal state shown in A3, since the "detection temperature gradient ΔTn+1" of thermistor 301b is smaller than "4°C/0.1s", even if heater 305 is turned off at "285°C", the temperature of heater 305 will not rise to the error temperature.

As described above, in this embodiment, the temperature rise rate per unit time of the thermistor 301b (stored temperature gradient ΔT0) measured in advance while the thermistor 301b is attached to the fixing device 111 is stored in the IC memory 205 provided in the fixing device 111. Then, in order to suppress the temperature rise of the heater 305 during double feeding of the recording material P, the reference temperature at which the heater 305 is turned off is changed according to the "stored temperature gradient ΔT0" stored in the IC memory 205. In other words, by changing the reference temperature according to the "stored temperature gradient ΔT0" already stored in the IC memory 205, it is possible to appropriately stop the power supply to the heater 305 and suppress the temperature rise of the heater 305. As a result, even if double feeding of the recording material P occurs, the heater 305 does not become so hot that the heater 305 itself is damaged or a part of the fixing film 303 melts.

Although the present embodiment has been described above, the numerical values shown in the above embodiment are merely examples and are not limited to these numerical values. The numerical values described above can be appropriately selected within the scope of the application of the present invention. Furthermore, the configuration of the fixing device can be appropriately changed within the scope of the application of the present invention.

For example, the fixing device may be applied to a roller fixing method in which a heater 305 is provided on the pressure roller 302 side to heat the fixing film 303, or an IH fixing method in which the fixing film 303 is heated by electromagnetic induction of a coil instead of a ceramic heater. Furthermore, the fixing film 303 is not limited to a configuration in which its inner surface is supported by the heater 305 and driven by the pressure roller 302. For example, the fixing film 303 may be stretched over multiple rollers and driven by one of these multiple rollers in a unit drive method. Furthermore, to form the fixing nip portion N2, instead of using a roller member such as the pressure roller 302 described above, a pressure belt in which an endless belt is stretched over multiple rollers may be used.

In the above embodiment, a direct transfer method is described in which a toner image formed on a photosensitive drum is directly transferred to a recording material, but the present invention can also be applied to an intermediate transfer method in which a toner image formed on a photosensitive drum is transferred to an intermediate transfer belt.

100...image forming apparatus (printer), 100A...apparatus main body, 111...fixing means (fixing device), 120...control means (control unit), 205...storage means (IC memory), 300...image forming means (image forming unit), 301b...temperature detection means (thermistor), 302...second rotating body (pressure roller), 303...first rotating body (fixing film), 305...heating means (heater), N2...nip portion (fixing nip portion), P...recording material

Claims (5)

A device body,
an image forming means for forming a toner image on a recording material;
a fixing unit that includes a first rotating body, a second rotating body that contacts the first rotating body to form a nip portion that sandwiches and conveys a recording material, and a heating unit that heats the first rotating body and fixes a toner image formed on the recording material by the image forming unit onto the recording material;
a temperature detection means provided in the fixing means, the temperature detection means detecting a temperature of the heating means in a heating area for heating the first rotating body, outside a passing area through which a conveyable recording material of a minimum size passes, in a width direction intersecting a conveying direction of the recording material in the nip portion;
a control means for stopping heating by the heating means when a temperature detected by the temperature detection means becomes higher than a reference temperature while the recording material is passing through the nip portion;
a storage means provided in the fixing means and storing a temperature rise rate per unit time of the temperature detection means specific to the fixing means,
The control means
When the temperature rise rate stored in the memory means is a first rise rate and the temperature rise rate per unit time based on the temperature detected by the temperature detection means is a first detected temperature rise rate , the reference temperature is set to a first temperature;
When the temperature rise rate stored in the memory means is a second rise rate which is smaller than the first rise rate, and the temperature rise rate per unit time based on the temperature detected by the temperature detection means is the first detected temperature rise rate , the reference temperature is set to a second temperature which is lower than the first temperature.
1. An image forming apparatus comprising: The control means
When the temperature rise rate stored in the memory means is the second rise rate, and the temperature rise rate per unit time based on the temperature detected by the temperature detection means is a second detected temperature rise rate that is smaller than the first detected temperature rise rate, the reference temperature is set to the first temperature.
2. The image forming apparatus according to claim 1, the control unit returns the reference temperature to the temperature before the change after the upstream end of the recording material in the conveying direction passes through the nip portion.
3. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction. The first rotating body is an endless film member,
the heating means contacts an inner circumferential surface of the film member to heat the film member;
4. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction. The fixing means is provided in the main body of the apparatus so as to be freely replaceable.
5. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

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