Fixing device

Abstract

To prevent the temperature from rising to an abnormal level while a rotary heater is not rotating.SOLUTION: A control unit detects a rate of change in electrical resistance of a heat generating layer and, when the rate of change in electrical resistance for power being supplied is greater than a threshold, limits or stops the supply of power to a rotary heater.SELECTED DRAWING: Figure 8

Description

Technical Field

[0001] The present invention relates to a fixing device mounted on an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer.

Background Art

[0002] As one form of a fixing device mounted on an electrophotographic printer or the like, a fixing device has been proposed that uses a cylindrical film (also referred to as a belt) having a resistive heating layer and heats the film by passing an electric current through the resistive heating layer. Patent Document 1 discloses a fixing device in which electrical contacts are provided at the ends of the film and the film is heated by passing an electric current through the film in the direction of the rotation axis of the film. Patent Document 2 discloses an induction heating type fixing device in which an exciting coil and a magnetic core are arranged in the internal space of the film, and an electric current flowing in the circumferential direction of the film is generated in the film by electromagnetic induction.

[0003] Since the means for detecting the temperature of the film may not be able to correctly detect the temperature when the recording material is wound around the film, it is desirable to provide it in the internal space of the film. Patent Document 3 discloses a temperature sensor of an in-film type using a thermistor element.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] The film that contacts the toner image on the recording material is thin and thus has a small heat capacity. When the fixing device is operating normally, the film rotates while in contact with the pressure roller and generates heat, so heat is sequentially taken away by the internal members of the film, the pressure roller, etc. In this way, the rate of temperature rise of the film is suppressed by the amount of heat taken away, so the temperature sensor that detects the temperature of the film can easily follow the rate of temperature rise of the film.

[0006] However, when an abnormal state occurs where the film slips and does not rotate, less heat is taken away by the pressure roller, etc., and the rate of temperature rise of the film becomes extremely fast. In such a case, the response of the temperature sensor may lag behind the temperature rise of the film, and there is also a possibility that the operation of the safety mechanism of the device may be delayed.

Means for Solving the Problem

[0007] The present invention for solving the above problems is When the temperature rises, the resistance value increases. a rotary heating element having a heat generating layer, a power supply circuit for supplying power to the rotary heating element, and a control unit for controlling the power supply to the rotary heating element. The heat generating layer is heated by the power supplied to the rotary heating element, and in a fixing device that uses this heat to fix the toner image on the recording material to the recording material, the control unit It is the amount of change in electrical resistance per unit time. detects the rate of change of the electrical resistance of the heat generating layer, and when The above-mentioned the rate of change of the electrical resistance is greater than a threshold value with respect to the supplied power, Judging that the rotation of the rotating heating element has stopped, the power supply to the rotary heating element is restricted or stopped, which is characterized.

Effects of the Invention

[0008] According to the present invention, it is possible to suppress the temperature of the rotary heating element from rising to an abnormal temperature in a state where the rotation has stopped.

Brief Description of the Drawings

[0009]

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Mode for Carrying Out the Invention

[0010] [Example] 1. Description of the printer First, the printer, which is an image forming apparatus, will be described with reference to FIG. 1. A cassette 2 is retractably housed in the lower part of the printer 1. The cassette 2 accommodates a recording material P. The recording material P in the cassette 2 is separated one by one by a separation roller 3 and fed to a registration roller 4. The printer 1 includes image forming units 5Y, 5M, 5C, and 5K corresponding to yellow, magenta, cyan, and black colors respectively. The image forming unit 5Y is provided with a photosensitive member 6Y and a charging unit 7Y that uniformly charges the surface of the photosensitive member 6Y. The photosensitive member 6Y charged by the charging unit 7Y is scanned by a laser beam corresponding to image information emitted from a scanner unit 8. Thereby, an electrostatic latent image corresponding to the image information is formed on the photosensitive member 6Y. The electrostatic latent image is developed by toner supplied from a developing unit 9Y. The toner image on the photosensitive member 6Y is transferred to an electrostatic transfer belt 10 by a primary transfer unit 11Y. Similarly, toner images are formed in the other image forming units 5M, 5C, and 5K, and the four-color toner images superimposed on the electrostatic transfer belt 10 are transferred to the recording material P by a secondary transfer unit 12. The toner image transferred onto the recording material is fixed to the recording material P by a fixing unit A. Thereafter, the recording material P passes through a conveying unit 13 and is discharged to a stacking unit 14.

[0011] 2. Description of Fixing Measures (Fixing Unit) The fixing device A is a fixing device using an electromagnetic induction heating method. FIG. 2 is a cross-sectional view of the fixing device A, and FIG. 3 is a perspective view of the fixing device A.

[0012] The cylindrical fixing film (rotating heating element) 20 has a base layer 20a, a heating layer 20b, an elastic layer 20c, and a release layer 20d. The material of the base layer 20a is an insulating heat-resistant resin such as polyimide, polyamideimide, PEEK, or PES. Its size is an inner diameter of 30 mm, a length of 240 mm, and a thickness of about 50 μm. The material of the heating layer 20b is an alloy such as iron, copper, silver, aluminum, nickel, chromium, tungsten, SUS304 (stainless steel) containing these, or nichrome. Or it is a conductor such as CFRP (carbon fiber reinforced plastic) or carbon nanotube resin, and a material with a large absolute value of the temperature coefficient of resistance is preferred. As a method for forming the heating layer 20b, there are means such as coating, plating, sputtering, and vapor deposition. The heating layer 20b of this example is formed by electrolytic plating with copper to a thickness of about 2 μm. The material of the elastic layer 20c is silicone rubber, fluororubber, fluorosilicone rubber, etc., and a material with excellent heat resistance and thermal conductivity is preferred. The elastic layer 20c of this example is silicone rubber with a thickness of about 200 μm. The material of the release layer 20d is preferably a material with good releasability and heat resistance such as PFA, PTFE, or FEP. In this example, the release layer 20d is formed with a PFA resin tube having a thickness of about 15 μm. A film guide member 25 formed of a heat-resistant resin such as PPS is in contact with the inner surface of the fixing film 20.

[0013] The pressure roller 21 is composed of a core metal 21a and an elastic layer 21b formed and coated in a roller shape concentrically and integrally around the core metal, and a release layer 21c is provided on the surface layer. The elastic layer 21b is preferably made of a material with good heat resistance such as silicone rubber, fluororubber, fluorosilicone rubber, etc. Both ends of the core metal 21a are held via conductive bearings on side plates (not shown) which are part of the chassis of the device. Also, by providing pressure springs 24a and 24b respectively between both ends of a metal stay 22 for ensuring the rigidity of the fixing device A and the spring receiving members 23a and 23b of the aforementioned chassis, the stay 22 is pressed in the direction towards the pressure roller 21. In the fixing device A of this embodiment, a pressing force of approximately 100 N to 300 N (approximately 10 kgf to approximately 30 kgf) is applied to the stay 22. Thereby, a fixing nip portion N is formed by the film guide member 25 and the pressure roller 21 via the fixing film 20. The pressure roller 21 is driven by a motor (not shown), and the fixing film 20 rotates following the rotation of the pressure roller 21.

[0014] Inside the internal space of the stay 22 with a U-shaped cross-section, a magnetic core 26 is inserted. FIG. 4 is a perspective view of the magnetic core 26, and an exciting coil 27 is wound around the outer periphery of the magnetic core 26. The magnetic core 26 has a cylindrical shape and an end shape, and is disposed substantially at the center of the fixing film 20 in the radial direction of the fixing film 20. Thus, inside the fixing film 20, an exciting coil 27 wound so as to form a spiral portion where the spiral axis is substantially parallel to the axial direction of the fixing film 20, and an end-shaped magnetic core 26 disposed inside the spiral portion are arranged. The magnetic core 26 serves to induce the magnetic force lines (magnetic flux) of the alternating magnetic field generated by the exciting coil 27 and form a path (magnetic path) for the magnetic force lines. The material of the magnetic core 26 is preferably a material with low hysteresis loss and high relative permeability, for example, a high-permeability ferromagnetic material such as fired ferrite or ferrite resin. The cross-sectional shape of the magnetic core 26 may be any shape that can be accommodated in the hollow portion of the fixing film 20. Although the cross-section does not necessarily have to be circular, a shape that can have as large a cross-sectional area as possible is preferred. In this embodiment, the diameter of the magnetic core 26 is 10 mm and the length is 280 mm. The exciting coil 27 is formed by spirally winding a copper wire (single conductor) with a diameter of 1 to 2 mm coated with heat-resistant polyamideimide around the magnetic core 26. The number of turns is 20. The spiral axis of the exciting coil 27 is parallel to the axial direction of the magnetic core 26. When a high-frequency current is passed through the exciting coil 27, an induced current flows through the heating layer 20b and the heating layer 20b generates heat by the principle described later.

[0015] The temperature of the fixing film 20 is detected by a temperature sensor 30. The temperature sensor 30 includes a leaf spring 30a with one end fixed to the stay 22, a thermistor (temperature detection element) 30b installed at the other end of the leaf spring 30a, and a sponge 30c interposed between the leaf spring 30a and the thermistor 30b. The surface of the thermistor 30b is covered with a polyimide tape having a thickness of 50 μm to ensure electrical insulation. The sponge 30c functions as a heat insulating material for the thermistor 30b and also has a function of fitting the thermistor 30b flexibly to the fixing film 20 which is the measurement object.

[0016] The thermistor 30b covered with the polyimide tape is in contact with the inner surface of the fixing film 20 due to the elasticity of the leaf spring 30a. The output (voltage value) of the thermistor 30b is A / D converted and input to the control circuit (control unit) 100 (see FIG. 1), and the control circuit 100 detects the temperature based on the input voltage value. When fixing the toner image at the fixing nip portion N, the control unit 100 controls the power supplied to the excitation coil 27 so that the temperature of the fixing film 20 reaches the target temperature suitable for the fixing process.

[0017] 3. Explanation of the heating principle FIG. 5 is a conceptual diagram showing an instant when the current flowing in the direction of arrow I1 increases with respect to the excitation coil 27. In the fixing device A of this example, when a high-frequency current flows through the excitation coil 27, most (90% or more) of the magnetic flux emitted from one end of the magnetic core 26 passes outside the fixing film 20 and returns to the other end of the magnetic core 26, forming a magnetic field. When such a magnetic field is formed, an induced current is generated in the circumferential direction of the fixing film 20 (the heating layer 20b). In the figure, S indicates a part of the induced current (circulating current) flowing through the heating layer 20b.

[0018] As described above, the fixing device A includes a fixing film 20 having a heating layer 20b, a power supply circuit (FIG. 8) for supplying power to the fixing film 20, and a control unit 100 for controlling the power supply to the fixing film. Then, the heating layer 20b is heated by the power supplied to the fixing film 20, and the heat is used to fix the toner image on the recording material P to the recording material P.

[0019] 4. Explanation of the method for detecting the stopped heating state Next, a method for detecting the stopped heating state will be described. The electrical energy (power) input from the power supply is finally converted into heat energy by Joule heating in the heating layer 20b of the fixing film 20. When power is applied to the excitation coil 27 while the fixing film 20 is rotating normally, the Joule heat generated by the circulating current flowing through the heating layer 20b raises the temperature of the fixing film 20 itself, the pressure roller 21, and the film guide member 25.

[0020] When power is supplied with the fixing film 20 not rotating, most of the thermal energy generated in the heating layer 20b becomes energy for raising the temperature of only the fixing film 20. In this case, since the fixing film 20 has a small heat capacity, the temperature rising rate of the fixing film 20 becomes extremely fast.

[0021] FIG. 6 shows the temperature change of the surface of the fixing film 20 when a constant voltage is applied. The solid line represents the temperature change in the normal state of rotational heating, and the dotted line represents the temperature change in the abnormal state of stopped heating. As can also be seen from FIG. 6, the temperature rising rate is fast in the case of stopped heating. That is, it is possible to determine whether it is in the normal state of rotational heating or the abnormal state of stopped heating based on the temperature rising rate of the fixing film 20. Since the heating layer 20b is formed of a conductor, its electrical resistance value has temperature dependence. Therefore, if the change rate of the electrical resistance with respect to the applied power can be captured, it is possible to determine whether it is in the normal state of rotational heating or the abnormal state of stopped heating.

[0022] Note that if a material whose electrical resistance changes according to temperature is adopted as the material of the heating layer 20b, it is possible in principle to determine the abnormal state. However, since the change rate of the electrical resistance becomes small when the absolute value of the temperature coefficient of resistance is small, high detection accuracy is required for the means for detecting the change rate of the electrical resistance. Therefore, it is desirable that the change rate of the electrical resistance is about 10%. When the temperature coefficient of resistance is 550×10 -6 / °C, the change rate of the electrical resistance when the temperature rises from 20°C to 200°C is about 10%. If the temperature coefficient of resistance is 1100×10 -6 / °C or more, the change rate is about 20%, which is more desirable. In this example, copper plating is adopted as the heating layer 20b, and when its temperature coefficient of resistance was measured, it was about 1500×10 -6 / °C.

[0023] Next, a method for capturing the change rate of the electrical resistance with respect to the applied power will be described.

[0024] FIG. 7 shows the time change of the surface temperature (solid line in the figure) and the power consumption (dotted line in the figure) of the fixing film 20 when a constant power supply voltage is applied from the power supply while the fixing film 20 is rotating normally. It can be seen that the temperature rises with time and the power consumption decreases. The fact that the power consumption decreases despite the applied power supply voltage being a constant value means that the electrical resistance of the fixing film 20 increases with temperature and the power supply current flowing through the power supply decreases.

[0025] To obtain the electrical resistance of the fixing film 20, although it is necessary to know the film voltage and film current applied to the fixing film 20, it is not possible to connect a voltage detection circuit or a current detection circuit to the fixing film 20. However, even without directly measuring the film voltage and film current, if the power supply voltage and power supply current can be measured, it is possible to capture the change rate of the electrical resistance of the fixing film 20.

[0026] FIG. 8 is a circuit diagram of an inverter power supply with a full-bridge configuration in the fixing device A of this embodiment. An alternating voltage is applied to the excitation coil 27 by the inverter power supply. The input voltage (commercial voltage) is full-wave rectified by a diode bridge circuit and then smoothed by a smoothing capacitor 81 to be converted into a DC voltage. After passing through an LC noise filter 82, the DC voltage is converted into a high-frequency square wave voltage by switching transistors TR1 to TR4 as four driving elements at a high frequency of about several tens of kHz. The temperature control of the fixing film 20 is executed by varying the driving frequency of transistors TR1 to TR4. When raising the temperature of the fixing film 20, the driving frequency is lowered, and when lowering the temperature, the driving frequency is raised for control.

[0027] As shown in FIG. 8, by providing a current detection circuit at the GND terminal of the power supply circuit, the power supply current (output current) can be detected. Also, as shown in FIG. 8, by providing a voltage detection circuit at the output terminal of the power supply circuit, the power supply voltage output to the fixing device A can be detected. Since the power supply voltage and power supply current can be measured in this way, it is possible to capture the change rate of the electrical resistance of the fixing film 20.

[0028] As a method for calculating the power supply voltage, there is another method in which the voltage detection circuit is installed at a position other than the output end shown in FIG. 8 (the position in FIG. 11) for calculation. Before that, first, the operations of the power supply circuits shown in FIGS. 8 and 11 will be further described. The difference between the circuit in FIG. 8 and the circuit in FIG. 11 is only the connection position of the voltage detection circuit.

[0029] FIG. 9 is a diagram for explaining the case where the driving frequencies of transistors TR1 to TR4 in the power supply circuit of FIG. 8 are variable. The dotted line is the voltage waveform, and the solid line is the current waveform. FIG. 9(b) is an example in which the driving frequency is set to twice that of FIG. 9(a). In a case where the temperature of the fixing film 20 is lowered, by increasing the driving frequency, the peak value of the current waveform becomes smaller, and the input power also becomes smaller. Changing the driving frequency is equivalent to changing the magnitude of the output voltage.

[0030] As a method for varying the magnitude of the output voltage, there is also a method of varying the duty ratio of the square wave. FIG. 10 is a diagram for explaining the case where the driving duty ratio is variable. The dotted line is the voltage waveform, and the solid line is the current waveform. FIG. 10(b) is an example in which the duty ratio is set to half that of FIG. 10(a). By reducing the duty ratio, the peak value of the current waveform becomes smaller, and the input power also becomes smaller. Changing the duty ratio is equivalent to changing the magnitude of the output voltage.

[0031] Next, another method for calculating the power supply voltage will be described. FIG. 11 shows an example in which the voltage detection circuit is provided at the subsequent stage of the smoothing capacitor. When the voltage detection circuit is placed at this position, what can be detected is the rectified and smoothed DC voltage, so voltage detection can be performed more simply than in the example of FIG. 8. Although the voltage value detected at this position is different from the high-frequency square wave voltage value finally output, it corresponds to the peak value of the square wave. Therefore, when the magnitude of the output voltage is variable by the driving frequency, the output voltage waveform can be estimated from the frequency and the peak value. That is, by combining the driving frequency information input from the driving circuit and the output result of the voltage detection circuit, the finally output voltage value can be calculated. Also, when the duty ratio is variable, the output voltage value can be calculated from the duty ratio information and the voltage detection result.

[0032] FIG. 12 shows the relationship between the power calculated from the detected voltage and current and the resistance increase rate per second of energization. The solid line represents the normal state (rotating heating state) where the fixing film 20 is rotating and generating heat, and the dotted line represents the abnormal state where the fixing film is not rotating but generating heat. It can be seen that the change rate of the electrical resistance is larger in the abnormal state (stopped heating state) than in the normal state. Therefore, the control unit 100 detects the change rate of the electrical resistance of the heating layer 20b, and when the change rate of the electrical resistance is larger than the threshold value with respect to the supplied power, it determines that it is in the stopped heating state and restricts or stops the power supply to the fixing film 20.

[0033] If the power input to the fixing film 20 is always constant, it is possible to determine whether it is in the rotating heating state or the stopped heating state by determining whether the change rate of the electrical resistance with respect to that power value is less than a predetermined threshold value or greater than or equal to the threshold value. However, it is rare for a constant power to be always input. For example, even when the voltage is constant, the electrical resistance value of the heating layer 20b increases with the temperature change, so the consumed power decreases. Therefore, the power integration value for a predetermined time is obtained, the average power is calculated, and then it may be determined whether it is in the rotating heating state or the stopped heating state from the change rate of the electrical resistance with respect to the average power.

[0034] In addition, a sequence for applying a constant power may be provided. For example, when adjusting the fixing device A to a predetermined temperature at the start of printing, by setting the amount of power applied for only a predetermined time to be constant and obtaining the rate of change of electrical resistance during that period, it becomes possible to determine whether it is in a rotational heating state or a stop heating state. FIG. 13 is an example of the power transition when the period PA is controlled with a constant power. When a print signal is input and power supply starts, the output is fixed at 600 W for the first 2 seconds (period PA). The rate of change of resistance is measured during this period PA. After the period PA, power control aiming at the target temperature during the fixing process is executed. Since the fixing film 20 has not reached the target temperature, initially the supplied power is almost the maximum power (period PB). It can be seen that the supplied power is throttled when the target temperature is reached (period PC). Then, when the recording material P enters the fixing nip portion N, heat is taken away from the recording material P, so the supplied power increases (period PD).

[0035] When providing a sequence for applying a constant power, attention is required for the amount of power applied. For example, when setting the power applied during the period PA to the maximum power, there is no problem if the fixing device A is in a cooled state at the start of power application, but if it is warm, there is a risk that the temperature of the fixing film 20 will become higher than the target temperature during the fixing process during the constant power application. On the other hand, if the power applied during the period PA is reduced too much, the rate of change of resistance also becomes small, resulting in a problem with detection accuracy. As mentioned above, the larger the temperature coefficient of resistance, the easier the detection. Therefore, when the absolute value of the temperature coefficient of resistance is 550×10 -6 / °C, it is not preferable to greatly reduce the constant power value, but if the absolute value of the temperature coefficient of resistance is 1100×10 -6 / °C, sufficient detection accuracy can be ensured even if the constant power value is set to about half of the maximum power. In this way, while avoiding the temperature of the fixing film 20 becoming too high with respect to the target temperature during the fixing process, it is also possible to detect abnormal temperature rise during stop heating.

[0036] Note that the above-described method for determining the rotation stop state and the stop heating state is not limited to a fixing device that generates heat by non-contact power feeding by electromagnetic induction, but is also an effective technique for a fixing device that generates heat by contact power feeding.

Explanation of Symbols

[0037] A fixing device 20 Fixing film 21 Pressing roller 26 Magnetic core 27 Excitation coil

Claims

1. A fixing device having a rotary heating element with a heating layer whose resistance value increases as the temperature rises, a power supply circuit for supplying power to the rotary heating element, and a control unit for controlling the power supply to the rotary heating element, wherein the heating layer is heated by the power supplied to the rotary heating element, and the toner image on the recording material is fixed to the recording material using this heat. The control unit detects a rate of change of electrical resistance, which is a change amount of the electrical resistance of the heating layer per unit time. When the rate of change of the electrical resistance is greater than a threshold value with respect to the supplied power, it is determined that the rotation of the rotary heating element has stopped, and the power supply to the rotary heating element is restricted or stopped. A fixing device characterized by this.

2. The absolute value of the temperature coefficient of resistance of the heating layer is 550×10 －６ / °C or more, and the fixing device according to claim 1, characterized in that.

3. A period for continuously supplying a constant power to the rotary heating element is provided, and when the rate of change of the electrical resistance during this period is greater than a threshold value, the power supply is restricted or stopped. The fixing device according to claim 1 or 2, characterized by this.

4. The value of the constant power is smaller than the maximum power supplied from the power supply circuit. The fixing device according to claim 3, characterized by this.

5. The fixing device has an exciting coil wound so as to form a spiral portion inside the rotary heating element and having a spiral axis substantially parallel to the axial direction of the rotary heating element, and an end-shaped magnetic core disposed inside the spiral portion. The power supply circuit generates an induced current in the circumferential direction in the heating layer by applying an alternating voltage to the exciting coil. The fixing device according to any one of claims 1 to 4, characterized by this.

6. The fixing device has a diode bridge circuit and a smoothing capacitor for converting an input commercial voltage into a DC voltage, and an inverter power supply having a full bridge configuration having four driving elements for converting the DC voltage into a square wave voltage. The control unit calculates the rate of change of the electrical resistance from the output voltage and output current of the inverter power supply. The fixing device according to claim 5, characterized by this.

7. The fixing device has a voltage detection circuit for detecting the DC voltage, and the control unit calculates the output voltage of the inverter power supply from the DC voltage detected by the voltage detection circuit and the driving frequency of the driving element. The fixing device according to claim 6, characterized by this.

8. The fixing device has a voltage detection circuit that detects the DC voltage, and the control unit calculates the output voltage of the inverter power supply from the DC voltage detected by the voltage detection circuit and the drive duty ratio of the drive element. The fixing device according to claim 6, characterized in that.

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