JP2018060167A - Fixing device, method for controlling fixing device, and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce deformation of a rotation unit that rotates to heat a developer image on a recording material in a fixing device.SOLUTION: A fixing device 11 comprises: a film 22; a heater 23 that heats the film 22; a pressure roller 24 that sandwiches a sheet P with the film 22; and a CPU 80 that controls heating performed by the heater 23 after the film 22 stops according a heating temperature of the film 22 in a rotation state heated by the heater 23.SELECTED DRAWING: Figure 12

Description

  The present invention relates to a fixing device suitable for an image forming apparatus that forms an image on a recording medium using, for example, an electrophotographic method, and a control method for the fixing device. The present invention also relates to an image forming apparatus such as an electrophotographic copying machine, a laser beam printer, and a facsimile provided with the fixing device.

  As a fixing device mounted on an electrophotographic image forming apparatus, there are a heater, a film (rotation unit) that rotates while being in contact with the heater, and a pressure roller (pressure member) that rotates by pressing the film. A configuration with which it is provided is known. In this configuration, a recording material carrying an unfixed toner image (developer image) is heated while being nipped and conveyed in a fixing nip formed by a film and a pressure roller, whereby an image on the recording material is recorded. Fixed to the material.

  Here, it is ideal that all the unfixed toner images on the recording material are appropriately heated and melted and fixed. However, when there is toner that cannot be melted by heat, toner that has melted too much, toner that electrostatically adheres to the pressure roller or film, etc., the toner has transferred to the pressure roller or film, and the toner has transferred to the film. Is transferred to the pressure roller between sheets.

  When the fixing operation is repeated in this state, toner transfer to the pressure roller is accumulated. When the predetermined accumulation amount is exceeded, the toner on the pressure roller adheres to the back surface of the next recording material, and noticeable toner contamination occurs on the back surface of the recording material.

  Therefore, in Patent Document 1, after the fixing operation is completed, the film is stopped and heated until the film reaches a temperature equal to or higher than the softening point of the toner, thereby discharging the toner on the pressure roller to the film. A configuration for performing control has been proposed. By performing such discharge control, the pressure roller can be cleaned and toner contamination on the back surface of the recording material can be suppressed.

Japanese Patent Laid-Open No. 11-344894

  However, when the film is continuously heated in a state where the film is stopped as in the configuration described in Patent Document 1, the temperature of the film is greatly increased only in the fixing nip portion in contact with the heater. The temperature of the part does not change greatly with the ambient temperature. In this way, when the pressure roller is suddenly driven with a temperature difference between the fixing nip portion and other portions in the rotation direction of the film, the film is deformed and recessed as described below. Traces may occur.

  FIG. 27 is a schematic diagram of a film for explaining the deformation mechanism of the film. FIG. 27A is a diagram showing a state in which the heater is heated while the film is stopped (non-rotating). FIG. 27B is a diagram when the film is driven to rotate by rotating the pressure roller from the state of FIG.

  As shown in FIG. 27A, when the heater is heated while the film is stopped, the film locally expands in the vicinity of the fixing nip (broken line), and the other part (solid line) is heated. Does not swell. For this reason, in the rotation direction (circumferential direction) of the film, thermal stress is applied in the vicinity of the boundary between the portion that thermally expands and the portion that does not thermally expand, causing distortion in the film. As the temperature difference between the inside and outside of the nip portion of the film increases, the amount of strain increases with a difference in the amount of expansion.

  Next, as shown in FIG. 27 (b), when the film rotates in a state where thermal stress is applied, the film is pulled by the pressure roller, and stress is applied near the boundary between the portion that does not thermally expand and the portion that does not thermally expand. Furthermore, the film is concentrated, and the film is permanently deformed to generate dent marks.

  When the fixing process is performed with the dent marks in this way, the film surface and the recording material do not come into contact with each other at the dent marks, so that the heat is not transferred to the toner and the fixing becomes insufficient, and the image is whitened. The image becomes defective. In particular, such an image defect occurs remarkably in a low temperature environment where it is relatively difficult to ensure the fixing property. Further, if the film is continuously used in the state where the dent marks are generated, the dent marks may be bent repeatedly and the film may be broken.

  Accordingly, the present invention has been made in view of such a situation, and an object thereof is to provide a fixing device capable of suppressing deformation of a rotating unit that rotates and heats a developer image on a recording material. .

  In order to achieve this object, a fixing device according to the present invention includes a rotation unit, a heating unit that heats the rotation unit, a pressure unit that sandwiches and conveys a recording material between the rotation unit, and the heating unit. Control means for controlling heating by the heating unit after the rotation of the rotary unit is stopped according to a heating temperature in a rotating state of the rotary unit heated by the unit.

  According to the present invention, it is possible to suppress deformation of the rotating unit that rotates and heats the developer image on the recording material.

1 is a schematic cross-sectional view of an image forming apparatus. 1 is a schematic cross-sectional view of a fixing device. It is a top view of a heater substrate. 2 is a block diagram illustrating a configuration of a control unit of the image forming apparatus. FIG. It is a circuit diagram which shows the electricity supply control path | route of a heater. It is a table | surface which shows the experimental result of a film dent trace generation | occurrence | production experiment. It is a flowchart of start-up control. 5 is a graph showing the transition of the temperature inside the nip and the temperature outside the nip when the start-up control is executed. It is a flowchart of post-rotation control. It is a graph which shows transition of the nip inside temperature and the nip outside temperature of a film when post-rotation control is performed. It is a flowchart of discharge control. It is a graph which shows transition of the nip inside temperature and the nip outside temperature of a film when discharging control is performed. It is a flowchart of start-up control. It is a graph which shows transition of the nip inside temperature and the nip outside temperature of a film when starting control is performed. 5 is a flowchart of fixing operation, post-rotation control, and discharge control. 6 is a graph showing a transition of a nip inner temperature and a nip outer temperature of a film from a fixing operation to a fixing standby state. 7 is a flowchart illustrating control when an image forming job signal is received during discharge control. It is a flowchart of control which calculates the nip outside temperature of a film. 6 is a graph showing a transition of a nip inner temperature and an outer nip temperature of a film from a fixing operation until a next image forming job signal is received. It is a flowchart of control which calculates the nip outside temperature of a film. 6 is a graph showing a transition of a nip inner temperature and an outer nip temperature of a film from a fixing operation until a next image forming job signal is received. It is the schematic diagram which showed typically the deformation | transformation by the thermal expansion of the film when the width | variety of a fixing nip part is narrow and wide. 7 is a flowchart illustrating control when an image forming job signal is received during discharge control. 6 is a table showing a table in which a width of a fixing nip portion in a sheet conveyance direction and a threshold value related to a temperature difference between the inside and outside of a nip when a pressure roller is driven are associated with each other. 6 is a graph showing the relationship between the number of fixings of the fixing device and the width of the fixing nip portion. 7 is a flowchart illustrating control when an image forming job signal is received during discharge control. It is a schematic diagram of the film and pressure roller for demonstrating the conventional subject.

(First embodiment)
<Image forming apparatus>
Hereinafter, the overall configuration of the image forming apparatus A including the fixing device according to the first embodiment of the present invention will be described with reference to the drawings together with the operation during image formation. The type, shape, arrangement, number, etc. of the members are not limited to those in the following embodiments, and are appropriately replaced within the scope not departing from the gist of the invention, such as appropriately replacing the constituent elements with those having the same effects. It can be changed.

  As shown in FIG. 1, the image forming apparatus A includes an image forming unit that transfers a toner image onto a sheet P as a recording material, a sheet feeding unit that supplies the sheet P to the image forming unit, and a toner image on the sheet P. A fixing unit for fixing.

  The image forming unit includes a photosensitive drum 1, a charging roller 2, a laser scanner unit 3, a developing device 4, a transfer roller 5, and the like.

  In the image formation, when the CPU 80 shown in FIG. 4 receives the image forming job signal, the sheet P stacked and stored in the sheet stacking unit 9 is sent to the registration roller 7 by the feeding roller 6. Thereafter, after performing timing correction with the image forming unit, the sheet P is sent to the image forming unit by the registration roller 7.

  On the other hand, in the image forming unit, a charging bias is applied to the charging roller 2 to charge the surface of the photosensitive drum 1 in contact with the charging roller 2. Then, the laser scanner unit 3 emits laser light L from a light source (not shown) provided therein, and irradiates the photosensitive drum 1 with the laser light. As a result, the potential of the photosensitive drum 1 is partially reduced, and an electrostatic latent image corresponding to image information is formed on the surface of the photosensitive drum 1.

  Thereafter, a developing bias is applied to the developing sleeve 4a included in the developing device 4, thereby causing the toner to adhere to the electrostatic latent image formed on the surface of the photosensitive drum 1 from the developing sleeve 4a, and a toner image (developer image). Is formed. The toner image formed on the surface of the photosensitive drum 1 is sent to a transfer nip portion formed between the photosensitive drum 1 and the transfer roller 5. When the toner image arrives at the transfer nip portion, a transfer bias having a polarity opposite to that of the toner is applied to the transfer roller 5 and the toner image is transferred to the sheet P.

  Thereafter, the sheet P to which the toner image has been transferred is sent to the fixing device 11 and is heated and pressurized by the fixing device 11 to perform a fixing operation. The toner image on the sheet P (on the recording material) is permanently applied to the sheet P. It is fixed. Thereafter, the sheet P is conveyed by the discharge roller 13 and discharged to the discharge tray 15.

<Fixing device>
Next, the configuration of the fixing device 11 will be described.

  FIG. 2 is a schematic cross-sectional view of the fixing device 11. As shown in FIG. 2, the fixing device 11 includes a heating unit 14 that heats the toner image carried on the sheet P and melts the toner to fix the toner image on the sheet P. The heating unit 14 includes a pressure roller 24 (pressure means) that pressurizes the film 22 and sandwiches and conveys the sheet P together with the film 22.

  The pressure roller 24 includes a cored bar 24a that is a rotating shaft, an elastic layer 24b provided around the cored bar 24a, and an outermost release layer 24c provided around the elastic layer 24b. The Both ends of the cored bar 24a are rotatably supported, and a gear (not shown) disposed on the end side is rotated by receiving a driving force from the fixing motor 86 (see FIG. 4). Rotates. In the pressure roller 24, both ends of the cored bar 24a are pressed against the film 22 with a force of 120 N by a pressure spring (not shown). As a result, the pressure roller 24 presses the film 22.

  In the present embodiment, the core metal 24a is made of aluminum, the elastic layer 24b is made of silicon rubber, and the release layer 24c is made of a PFA tube. The outer diameter of the pressure roller was 30 mm, the thickness of the release layer was 50 μm, and the total length in the longitudinal direction of the rubber was 330 mm.

  The heating unit 14 includes a film 22, a guide member 21 that holds the film 22, a U-shaped stay 31, a heater 23 that heats the film 22, a thermistor 25 (temperature detection means), a non-contact thermometer 89 (see FIG. 4), and the like. Prepare.

  The film 22 (rotating unit) is an endless cylindrical heat-resistant film-like member, and is fitted around a guide member 21 having a vertical cross-sectional shape formed of a liquid crystal polymer, and rotates the pressure roller 20. Driven by frictional force. In other words, in the present embodiment, the fixing motor 86 that rotates by transmitting the driving force to the pressure roller 24 is a driving unit that rotates the film 22.

  The inner peripheral length of the film 22 is about 3 mm larger than the outer peripheral length of the guide member 21, and the film 22 is fitted on the guide member 21 with a margin in the peripheral length. Further, a lubricant (not shown) is applied between the inner peripheral surface of the film 22 and the outer peripheral surface of the guide member 21, so that sliding resistance when the guide member 21 and the inner peripheral surface of the film 22 rotate in contact with each other is applied. Is reduced.

  Moreover, the film 22 is comprised from three layers, the base layer used as a base material, the surface layer which covers the surface of a base layer, and the contact bonding layer which adhere | attaches a surface layer and a base layer. The base layer is a 40 μm-thick stainless steel film, and its outer peripheral surface is coated with PFA. The outer diameter of the film 22 is 30 mm, and the total length in the longitudinal direction, which is the rotational axis direction of the pressure roller 24, is set to 340 mm.

  The film 22 preferably has a film thickness of 100 μm or less in order to reduce the heat capacity and increase the startup time. In addition to stainless steel, the base layer may be made of metal such as nickel or resin such as polyimide. In addition, in order to ensure releasability from the toner, other fluororesin such as PTFE may be used for the surface layer instead of PFA. Moreover, although the dent mark of the film 22 mentioned above generate | occur | produces also with a resinous film, it is easy to generate | occur | produce notably with the case of a metal film. This is because a metal having a relatively low flexibility, such as a metal, will be permanently left with a dent mark once locally deformed.

  The U-shaped stay 31 is an elongated U-shaped metal extending in the longitudinal direction, and is disposed on the upper side of the guide member 21. The U-shaped stay 31 applies pressure uniformly to the guide member 21, and has strength against the pressure of the guide member 21 by the pressure roller 24. Further, the thermal conductivity in the longitudinal direction is improved to improve the temperature unevenness in the longitudinal direction. Because of these functions, a metal having high strength and excellent thermal conductivity is generally used as a material. In this embodiment, a galvanized steel sheet is used as the material.

  The heater 23 is disposed inside the film 22 in the fixing nip portion so as to contact (oppose) the inner peripheral surface of the film 22 and heat the film 22 from the inner peripheral surface side. The heater 23 includes a heating resistor 26 (heating source) made of ceramic that is fitted in a groove portion of a heater substrate 27 made of aluminum nitride and is insulated and held to generate heat when energized. Further, the heating resistor 26 is covered with a glass coat 28 in order to ensure insulation. Further, on the contact surface side of the heater substrate 27 with the film 22, a polyimide coating 30 is printed by 10 μm in order to ensure slidability with the film 22. Further, a lubricant is applied between the film 22 and the polyimide coating 30 to further improve the slidability when the film 22 rotates. The heater substrate 27 is fitted and held in a concave groove formed in the longitudinal direction on the surface of the guide member 21 on the side of the pressure roller 24, thereby holding the heater 23 via the heater substrate 27. 21 is fixed.

  A thermistor 25 (first temperature detecting means) for measuring the temperature of the heater 23 is disposed on the side of the heater substrate 27 facing the guide member 21. The thermistor 25 is provided with a heat insulating layer on a support (not shown), an element of the chip thermistor is fixed on the heat insulating layer, and the element is pressed against the heater substrate 27 with a predetermined pressurizing force. It is in contact with the heater substrate 27.

  Here, as described above, the heater 23 is in contact with the film 22, and the temperature of the contact area of the film 22 with the heater 23 can be almost the same as the temperature of the heater 23. That is, the thermistor 25 is a first detection means and heater temperature sensor that measures and detects the temperature of the contact area of the film 22 with the heater 23. In this embodiment, the contact region of the film 22 with the heater 23 is inside the fixing nip portion, and the temperature of the contact region and the temperature of the fixing nip portion substantially coincide with each other. This is called the internal temperature.

  Further, the non-contact thermometer 89 measures the temperature of the area where the film 22 is not in contact with the heater 23. That is, the non-contact thermometer 89 is a non-contact area temperature sensor that measures the temperature of the non-contact area of the film 22 with the heater 23. Specifically, the temperature on the sheet contact surface side of the film 22 at a position (point S in FIG. 2) inclined by τ ° (30 ° in this embodiment) along the curvature of the film 22 from the fixing nip portion is measured. In this embodiment, since the non-contact area of the film 22 with the heater 23 is outside the fixing nip portion, the temperature of this non-contact area is hereinafter referred to as a nip outside temperature. The temperature difference between the nip internal temperature and the nip external temperature is referred to as the nip internal / external temperature difference.

  FIG. 3 is a diagram illustrating the configuration of the heater substrate 27 facing the guide member 21 (FIG. 3A) and the film 22 contact surface side (FIG. 3B). As shown in FIG. 3, two heating resistors 26 are arranged in parallel on the side of the heater substrate 27 facing the guide member 21. In addition, power supply portions 33 (33a, 33b) for supplying power to the heating resistor 26 are arranged.

  Three thermistors 25 are installed in the longitudinal direction on the side of the heater substrate 27 facing the guide member 21. Among these, the main thermistor 25a near the center in the longitudinal direction is arranged in the passage region of the minimum width size sheet P through which all the sheets P always pass in the sheet width direction orthogonal to the conveyance direction of the sheet P. . The first sub-thermistor 25b is disposed in a region corresponding to a non-passing region when an A4 size sheet P is passed in the R direction in the sheet width direction, and the second sub thermistor 25c is configured to apply the B5 size sheet P in the R direction. It is arranged in a region corresponding to a non-passing region when it is passed.

  Then, the temperature of the passing region of the sheet P is detected by the main thermistor 25a, and the temperature of the non-passing region when the small size sheet such as A4R or B5R is passed is detected by the sub-thermistors 25b and 25c. This prevents an abnormal temperature rise in the non-passing area during small size continuous paper feeding.

  On the heater substrate 27, a thermo switch 32 (see FIG. 5) is disposed at a position symmetrical to the main thermistor 25a with respect to the central portion in the longitudinal direction. The thermo switch 32 is a switch that functions as a safety device when the heater 23 is excessively heated due to a failure of the thermistor 25 or a control unit, and a bimetal is incorporated therein. When the bimetal reaches a predetermined temperature, the bimetal is deformed and the power supply to the heating resistor 26 is interrupted.

<Control unit>
Next, the configuration of the control unit of the image forming apparatus A will be described, particularly focusing on the part related to the control of the fixing device 11.

  FIG. 4 is a block diagram illustrating a partial configuration of the control unit of the image forming apparatus A. As shown in FIG. 4, the control unit includes a CPU 80 (control means, setting means), a RAM 81, and a ROM 82. The CPU 80 is connected to the heater 23, the operation unit 83, the environment sensor 88 (environment detection means), the non-contact thermometer 89, the fixing motor 86, and the like.

  The ROM 82 stores various programs such as a temperature control program and a power supply control program, and fixing temperature information. The CPU 80 performs various arithmetic processes based on programs stored in the ROM 82. The RAM 81 is used as a work area in the arithmetic processing of the CPU 80.

  The operation unit 83 outputs an external operation instruction input by a user or the like to the CPU 80. The fixing motor 86 rotates the pressure roller 24 according to the control of the CPU 80.

  The environment sensor 88 is disposed in the image forming apparatus main body, detects the ambient temperature (in-machine temperature) of the image forming apparatus A, and outputs it to the CPU 80. The non-contact thermometer 89 detects the temperature outside the nip of the film 22 and outputs it to the CPU 80. The thermistor 25 detects the temperature in the nip of the film 22 through the temperature of the heater 23 and the temperature of the heater 23 and outputs the detected temperature to the CPU 80. Based on the temperature information and the like, the CPU 80 controls the temperature of the heater 23 and the driving of the fixing motor 86 as described later.

  Next, energization control of the heater 23 during image formation will be described.

  FIG. 5 is a diagram illustrating an energization control path of the heater. As shown in FIG. 5, when the CPU 80 receives the image forming job signal, the CPU 80 turns on the triac 42, whereby the heating resistor 26 and the power feeding unit 33b are fed from the power supply unit 33a through the thermo switch 32 from the AC power source 43. Is energized.

  By this energization, the entire heating resistor 26 generates heat and the temperature rises. The temperature of the heater substrate 27 heated in accordance with the temperature rise is detected by A / D converting the output of the thermistor 25 and energized until the temperature of the heater substrate 27, that is, the temperature of the heater 23 reaches the target temperature. Continue.

  That is, when the heater 23 reaches the target temperature, the temperature of the heater 23 is controlled by controlling the power supplied to the heater 23 by the triac 42 based on the output signal from the thermistor 25 by phase control or wave number control. Specifically, the CPU 80 controls the triac 42 so as to raise the temperature of the heating resistor 26 when the temperature detected by the thermistor 25 is lower than the set temperature, and to lower the temperature of the heating resistor 26 when higher than the set temperature. To keep the heater 23 at the set temperature. When the image forming operation is finished, the triac 42 is turned off and the energization of the heater 23 is finished.

<Film dent trace generation experiment>
Next, the result of the experiment for generating dent marks on the film 22 will be described.

  As described above, the dent mark of the film 22 is generated by a distortion caused by a thermal stress generated in the film 22 due to a temperature difference in the rotation direction (circumferential direction) of the film 22, and a driving force is applied to the film 22 there. In this experiment, the strain amount of the film 22 at the fixing nip portion is measured when the temperature difference between the inside and outside of the nip of the film 22 is changed between 80 ° C. and 100 ° C. with the film 22 and the pressure roller 24 stopped. . Thereafter, the pressure roller 24 was driven to rotate the film 22 to confirm whether or not a dent mark was generated on the film 22.

  As for the temperature in the nip, the temperature on the contact surface side with the sheet P at the substantially central portion in the sheet conveyance direction of the fixing nip is measured, and the position where the non-contact thermometer described above is arranged for the temperature outside the nip (FIG. 2). The temperature of the contact surface side with the sheet P of the film 22 at the point S) was measured. Regarding the amount of strain, the amount of change in the shape of the film 22 before and after heating (the length of the arrow h shown in FIG. 22) was measured.

  FIG. 6 shows the experimental results. As shown in FIG. 6, in this experiment, when the temperature difference between the inside and outside of the nip of the film 22 becomes 95 ° C. or more, the strain amount becomes 50 μm or more. It was verified that it occurred. Therefore, in the following, in order to suppress deformation of the film 22 (occurrence of dent marks), control described later is performed so that the temperature difference between the inside and outside of the nip of the film 22 is less than 95 ° C.

<Start-up control>
First, start-up control when the image forming job signal is received and the heater 23 is raised to a set temperature will be described with reference to the flowchart shown in FIG. In the present embodiment, the temperature at which the lubricant applied between the polyimide coating 30 and the film 22 of the heater 23 starts to melt and the lubricity can be ensured is 80 ° C.

  As shown in FIG. 7, when an image forming job signal is first received (S1), energization of the heater 23 is started with the film 22 stopped (S2). Next, when the temperature of the heater 23 detected by the main thermistor 25a reaches 85 ° C. (S3), the driving of the fixing motor 86 is started (S4), and the pressure roller 24 is rotated to rotate the film 22. That is, the CPU 80 acquires the detection result of the temperature of the heater 23 by the main thermistor 25a, and starts driving the fixing motor 86 when the temperature of the heater 23 reaches 85 ° C. Thereafter, when the heater 23 reaches the set temperature, the sheet P is passed through the fixing nip portion to perform a fixing operation (S5).

  FIG. 8 is a graph showing the transition of the temperature in the nip and the temperature outside the nip of the film when the start-up control is executed in an environment of 25 ° C. As shown in FIG. 8, when an image forming job signal is received, the film 22 is stopped and heated, and the temperature in the nip of the film 22 rises. At this time, since the film 22 is in a non-rotating state, the temperature outside the nip does not rise to the ambient temperature.

  Next, when the heater is heated to 85 ° C., driving of the fixing motor 56 is started and the film 22 rotates. As a result, the temperature outside the nip of the film 22 increases. Here, when the temperature detected by the thermistor reaches 210 ° C., the fixing operation is performed. At this time, the temperature in the nip is about 200 ° C.

  By such control, even in a low temperature environment such as a 0 ° C. environment, the temperature difference between the inside and outside of the nip of the film 22 becomes 85−0 = 85 ° C., and the temperature difference between the inside and outside of the nip can be suppressed to 95 ° C. or less. That is, in the start-up control, the rotation of the film 22 is started when the temperature difference between the nip inside and outside the film 22 is equal to or less than a predetermined value, so that the temperature difference in the rotation direction of the film 22 is equal to or less than the predetermined value when the film 22 rotates. Can be suppressed. For this reason, the friction between the film 22 and the heater 23 at the start of driving can be reduced by dissolving the lubricant while suppressing the occurrence of the dent marks on the film 22.

  In the present embodiment, the control for starting the fixing motor 86 when the detected temperature of the main thermistor 25a is 85 ° C. is described, but the present invention is not limited to this. In other words, the control is performed to rotate the film 22 in a temperature range in which the film 22 and the heater 23 are lubricated while ensuring the lubricity of the lubricant applied and the film 22 is prevented from being generated when the film 22 is rotated. Thus, the same effect as described above can be obtained.

<Post-rotation control>
Next, post-rotation control performed after the fixing operation will be described.

  If the rotation of the pressure roller 24 and the film 22 is stopped immediately after the completion of the fixing operation, both of them are at a high temperature, and there is a possibility that both stick to the fixing nip portion. If rotation is started again with both attached, the fluorine coat, fluorine tube, etc. on the surface layer of the film 22 will be peeled off, and toner will adhere to the pressure roller 24 and the film 22, causing image smearing. .

  Further, a charge-up in which the pressure roller is charged may occur due to friction with the sheet P during the fixing operation. When the pressure roller 24 is charged up to the same polarity as the toner, the toner adheres to the film 22 side, and the sheet P on which the toner image is next fixed is soiled.

  Therefore, after the fixing operation, the pressure roller 24 and the film 22 are rotated to cool the both, and the post-rotation control is performed to neutralize the pressure roller 24.

  First, conventional post-rotation control will be described. Conventionally, after the fixing operation is completed, the heater 23 is turned off and only the rotation control is performed to cool the film 22 and the pressure roller 24. The rotation control time is 20 seconds when the basis weight of the sheet P for fixing is large, and is 2.5 seconds when the basis weight is small. This is because the electrical resistance of the sheet P increases as the basis weight increases, and the pressure roller 24 is easily charged up by friction with the sheet P. Therefore, when the basis weight of the sheet P is large, the post-rotation time is lengthened so that the film 22 that is more conductive than the sheet P is brought into contact with the pressure roller 24 for a long time to perform sufficient static elimination.

  Next, post-rotation control of this embodiment will be described with reference to the flowchart shown in FIG.

As shown in FIG. 9, first, after completion of the fixing operation (S21), it is determined whether or not the basis weight of the sheet P for fixing, that is, the basis weight of the sheet P on which the toner image is fixed is greater than or equal to a predetermined amount (S22). . In this embodiment, it is determined whether the basis weight of the sheet P is 90 g / m ² or more. The basis weight of the sheet P is read based on the type of the sheet P set by the user on the operation unit 83 (see FIG. 4).

If the basis weight of the sheet P is less than 90 g / m ² , the heater is turned off (S23), the pressure roller 24 and the film 22 are rotated for 2.5 seconds (S24), and then the fixing motor 86 is turned on. The drive is turned off (S28), and the post-rotation control is terminated.

On the other hand, when the basis weight of the sheet P is 90 g / m ² or more, the pressure roller 24 and the film 22 are rotated for 20 seconds in the same manner as in the past in order to neutralize the pressure roller 24. At this time, the pressure roller 24 and the film 22 are rotated while the energization of the heater 23 is continued for the first 10 seconds (S25). Note that the temperature of the heater 23 at this time is controlled to a temperature control temperature during the fixing operation.

  Thereafter, the heater 23 is turned off (S26), and the pressure roller 24 and the film 22 are rotated for 10 seconds (S27). Thereafter, the driving of the fixing motor 86 is turned off (S28), and the post-rotation control is ended.

FIG. 10 is a graph showing the transition of the temperature inside and outside the nip of the film 22 when the discharge control described above is performed after the post-rotation control. Here, FIG. 10A shows a temperature transition when the conventional post-rotation control is performed, and FIG. 10B shows a temperature transition when the post-rotation control of the present embodiment is performed. This temperature transition is a temperature transition after the fixing operation of five sheets P having a basis weight of 100 g / m ² and a heater temperature adjustment of 210 ° C. in a low temperature environment of 0 ° C. in the fixing operation. In this graph, the 0 second time point is the post-rotation control start time point after the end of the fixing operation.

  As shown in FIG. 10, in the conventional control, the energization to the heater 23 is cut off at the time of starting the post-rotation control, so that both the nip internal temperature and the nip external temperature of the film 22 are reduced, and the nip internal / external temperature difference. Is getting smaller. Thereafter, when stop heating is performed during discharge control, the temperature inside the nip of the film 22 increases rapidly, but the temperature outside the nip continues to decrease. For this reason, the temperature difference between the inside and outside of the nip increases during the discharge control, and when the image forming job is received and the fixing motor 86 is driven during the subsequent discharge control or immediately after the discharge control, a dent mark is generated on the film 22.

  On the other hand, in the control of this embodiment, since the rotation is performed while energizing the heater for the first 10 seconds after the start of the post-rotation control, the temperature in the nip of the film 22 at the end of the post-rotation control is compared with the conventional control. High temperature. For this reason, even if stop heating is subsequently performed under the discharge control, the temperature difference between the inside and outside of the nip of the film 22 becomes less than 95 ° C., and even when the fixing motor 86 is driven at this time, the trace of the recess of the film 22 is suppressed. .

  Thus, by continuing energization without immediately turning off the heater 23 in the post-rotation control, the temperature in the nip of the film 22 at the end of the post-rotation control can be increased, and then stop heating is performed. Also, the temperature difference between the inside and outside of the nip can be reduced. That is, by controlling the temperature of the heater 23 so that the temperature difference between the inside and outside of the nip of the film 22 becomes small when the film 22 is not rotated, even if the fixing motor 86 is turned on after that, the dent mark of the film 22 is generated. Can be suppressed.

  In addition, since the film 22 and the pressure roller 24 are cooled without energizing the heater 23 in the latter half 10 seconds, it is possible to prevent the film 22 and the pressure roller 24 from sticking to each other. Further, even if the heater 23 is turned on while being energized, the electrical resistance on the surface of the film 22 and the pressure roller 24 does not change greatly. Therefore, the neutralization effect of the pressure roller 24 does not change and the pressure roller 24 is charged up. It is possible to prevent the toner from being smeared.

<Discharge control>
Next, the discharge control for cleaning the pressure roller 24 after the completion of the post-rotation control will be described.

  In the discharge control, when the fixing motor 86 is stopped, the film 22 is heated by raising the temperature of the heater 23 until the film 22 reaches a temperature equal to or higher than the softening point of the toner, and the toner on the pressure roller 24 is transferred to the film 22. In this control, the pressure roller 24 is cleaned by the transfer. As a result, during the next fixing operation, the toner is gradually transferred from the film 22 to the surface of the sheet P, and this is repeated to prevent toner accumulation on the pressure roller 24 and to suppress conspicuous toner contamination on the back surface of the sheet P. To do.

  First, conventional discharge control will be described. In the conventional control, when the driving of the fixing motor 86 is turned off after completion of the post-rotation control, first, the energization of the heater 23 is started. Thereafter, energization is continued until the main thermistor 25a detects 190 ° C. After reaching 190 ° C., PI control is performed by controlling the temperature at 190 ° C. by the main thermistor 25a. Then, the energization of the heater 23 is turned off after the elapse of 5 seconds after the heater 23 detects 190 ° C. As a result, the toner on the pressure roller 24 is transferred to the film 22.

  Next, the discharge control of this embodiment will be described with reference to the flowchart shown in FIG. In this embodiment, the softening point of the toner is assumed to be 160 ° C.

  As shown in FIG. 11, first, when the fixing motor 86 is turned off and the post-rotation control is completed, the energization to the heater 23 is turned on and the discharge control is started (S31).

  Next, when the temperature adjustment temperature of the heater 23 during the fixing operation is 210 ° C. or higher (first temperature), the temperature adjustment temperature of the heater 23 during discharge control is set to 190 ° C. (second temperature) (S32, S33). ). On the other hand, when the temperature adjustment temperature of the heater 23 during the fixing operation is 190 ° C. or higher and lower than 210 ° C. (third temperature), the temperature adjustment temperature during discharge control is set to 180 ° C. (fourth temperature) (S34, S35). ). If the temperature adjustment temperature of the heater 23 is less than 190 ° C., the temperature adjustment temperature during discharge control is set to 170 ° C. (S34, S36). In general, the temperature control temperature of the heater 23 is set to be high for securing a fixing property in the sheet P having a large basis weight, and is set to be low in order to prevent hot offset of toner in the small sheet P. .

  Next, 5 seconds after the set temperature is reached (S37), the heater 23 is turned off (S38), the discharge control is terminated, and a fixing standby state is entered.

FIG. 12 shows the film 22 from the fixing operation to the post-rotation control and the discharge control when the basis weight of the sheet P for fixing and the set temperature control temperature of the heater 23 during the fixing operation are changed in an environment of 0 ° C. It is a graph which shows transition of the nip inside temperature and nip outside temperature. Here, FIG. 12A shows the conventional discharge control and the discharge control according to the present embodiment at a basis weight of the sheet P for fixing of 80 g / m ² and the set temperature control temperature 210 ° C. of the heater 23 during the fixing operation. It is temperature transition at the time. FIG. 12B shows a case where the conventional discharge control and the discharge control of the present embodiment are performed at a sheet basis weight of 60 g / m ² for fixing and a temperature control temperature 190 ° C. of the heater 23 during the fixing operation. It is temperature transition.

As shown in FIG. 12A, when the basis weight of the sheet P for fixing is 80 g / m ² , the temperature control temperature during the discharge control is 190 ° C. in both the present embodiment and the conventional control. The temperature transition is the same between the control and the control of the present embodiment. Specifically, the temperature inside and outside the nip of the film 22 decreases during the post-rotation operation after the fixing operation is completed. Thereafter, the discharge control starts and the temperature in the nip of the film 22 rises and the temperature is controlled to 190 ° C. On the other hand, since the temperature outside the nip continues to decrease during the discharge control, the temperature difference inside and outside the film nip at the end of the discharge control is 80 ° C. At this time, since the temperature difference between the inside and outside of the nip is within 95 ° C., even if the film is rotated by driving the pressure roller in this state, no film dent marks are generated.

On the other hand, as shown in FIG. 12 (b), when the basis weight of the sheet for fixing is 60 g / m ² and the set temperature adjustment temperature of the heater during fixing operation is 190 ° C., the basis weight is 80 g / m ^2. Since the temperature control temperature of the heater is low, the amount of heat stored in the film 22 in the fixing operation is small. For this reason, the temperature of the film at the end of the post-rotation control is lowered overall. In this case, in the conventional control, when the temperature in the nip of the film rises and the temperature is controlled to 190 ° C. after the discharge control is started, the temperature difference between the inside and outside of the film nip at the end of the discharge control becomes 100 ° C. . For this reason, when the driving of the motor is started at the end of the discharge control, the temperature difference is larger than 95 ° C., so that film dent marks are generated.

  On the other hand, in the control of the present embodiment, the temperature outside the nip of the film shows a transition similar to that of the conventional control, but the temperature control temperature of the heater during the discharge control changes according to the temperature control temperature during the fixing operation, and 180 It becomes ℃. For this reason, the temperature difference inside and outside the film nip at the end of the discharge control is 90 ° C., and no film dent marks are generated even if the drive to the motor is started at the end of the discharge control.

  In this way, the temperature control temperature of the heater during discharge control is changed based on the temperature control temperature of the heater during the fixing operation, and the temperature difference between the temperature inside and outside the nip of the film during discharge control is reduced. That is, the temperature of the heater is controlled so that the temperature difference between the inside and outside of the nip is not more than a predetermined value when the film is not rotated. Thereby, even when an image forming job is received after that and the motor is driven, it is possible to suppress the occurrence of dent marks on the film.

  In the present embodiment, the configuration using the heater 23 as the heating unit has been described. However, the present invention is not limited to this. For example, without using a heater as a heating unit, an IH coil facing the film 22 may be provided and the film itself may generate heat.

(Second Embodiment)
Next, a second embodiment of the image forming apparatus including the fixing device according to the present invention will be described with reference to the drawings. About the part which overlaps with the said 1st Embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

  In the first embodiment, in the start-up control, the fixing motor 86 is driven when the main thermistor detects 85 ° C., and the rotation of the pressure roller 24 and the film 22 is started. However, if the fixing operation is not performed for a long time in an ultra-low temperature environment such as an environment of −15 ° C., the temperature of the film 22 decreases to near the −15 ° C. environment. In this case, in the control in which the fixing motor 86 is driven at 85 ° C. during the start-up control, the temperature difference between the inside and outside of the nip of the film 22 becomes 95 ° C. or more, and a dent mark may be generated.

  Therefore, in the present embodiment, the driving start temperature of the fixing motor 86 is changed based on the detected temperature of the main thermistor 25a, the elapsed time since the previous image forming job was received, and the detected temperature of an environmental sensor (not shown). Hereinafter, the start-up control of the present embodiment will be described with reference to the flowchart shown in FIG.

  As shown in FIG. 13, first, when an image forming job signal is received (S41), energization of the heater 23 is turned on (S42). Next, the ambient temperature is detected by the environmental sensor 88 (S43). Next, it is determined whether or not the ambient temperature is lower than a predetermined temperature (0 ° C. in the present embodiment) (S44).

  Here, when the ambient temperature is higher than 0 ° C., it is not an ultra-low temperature environment, and therefore the driving of the fixing motor 86 is started when 85 ° C. is detected as in the control of the first embodiment (S45, S50).

  On the other hand, if the ambient temperature is less than 0 ° C., it is determined whether or not 45 minutes or more have elapsed since the previous reception of the image forming job signal (S46). Here, when 45 minutes or more have elapsed, it is considered that the temperature of the film 22 is also equal to the ambient temperature. Therefore, when the main thermistor 25a detects the detected temperature + 85 ° C. of the environmental sensor 88, the fixing motor 86 Driving is started (S47, S50).

  On the other hand, if 45 minutes or more have not elapsed, it is determined whether or not the detected temperature of the main thermistor 25a is less than 0 ° C. (S48). Here, when the detected temperature is less than 0 ° C., it is considered that the temperature of the film 22 is almost equal to the detected temperature. Therefore, when the main thermistor 25a detects the detected temperature + 85 ° C., the fixing motor 86 is driven. Start (S49, S50).

  On the other hand, when the detected temperature of the main thermistor 25a is 0 ° C. or higher, the driving of the fixing motor 86 is started when the main thermistor 25a detects 85 ° C. (S45, S50).

  FIG. 14 shows the nip of the film 22 at the start of driving of the fixing motor 86 when the start-up control of the first embodiment and the start-up control of the present embodiment are performed under various environments of −15 ° C. to 35 ° C. It is a graph which shows the result of having measured the internal and external temperature difference. Note that the fixing device 11 is left until it is adjusted to room temperature.

  As shown in FIG. 14, in the control of the first embodiment, since the fixing motor 86 is driven at 85 ° C. even in the environment of −15 ° C., the temperature difference between the inside and outside of the nip portion of the film 22 is 85 − (− 15) = 100. May cause dent marks. On the other hand, in the control of the present embodiment, even when the fixing device 11 is placed in an ultra-low temperature environment such as a −15 ° C. environment, the fixing motor 86 sets 85 + (− 15) = 70 ° C. to the main thermistor 25a. Drive is started at the time when is detected. For this reason, the temperature difference between the inside and outside of the nip of the film 22 is 85 ° C. and is within 95 ° C. As described above, the control of changing the driving start temperature of the fixing motor 86 at the start-up control according to the environmental temperature can suppress the generation of the dent marks on the film 22.

  In this embodiment, the driving of the fixing motor 86 is started when the temperature difference between the inside and outside of the nip of the film 22 falls within a predetermined range. However, the fixing motor 86 is in a state where the temperature difference between the inside and outside of the nip exceeds the predetermined range. When starting this driving, the driving may be performed gradually (intermittently), or may be driven at a slower acceleration and slower speed than during image formation.

(Third embodiment)
Next, a third embodiment of an image forming apparatus including the fixing device according to the present invention will be described with reference to the drawings. About the part which overlaps with the said 1st Embodiment and 2nd Embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

In the fixing device 11, when the sheet P relating to fixing is thin, for example, with a basis weight of 50 g / m ² , the temperature adjustment temperature of the heater 23 is lowered by half-speed driving rotation in order to prevent sheet clogging or sheet winding. In general, the fixing operation is performed. In this case, since the temperature control temperature of the heater 23 is set to be lower, the temperature of the film 22 becomes lower between the post-rotation control and the discharge control.

  On the other hand, when the basis weight of the sheet P for fixing is small, the amount of heat taken away by the sheet P is relatively small, and the fixability of the toner to the sheet P tends to be good. For this reason, the amount of toner accumulated on the surface of the pressure roller 24 tends to be relatively small, and there is little need to execute discharge control.

  Therefore, in the present embodiment, whether or not to perform discharge control is determined according to the temperature control temperature of the heater 23 during the fixing operation. Hereinafter, the control of this embodiment will be described with reference to the flowchart shown in FIG.

  As shown in FIG. 15, first, when the driving of the fixing motor 86 is turned off and the post-rotation control is finished (S51), it is determined whether or not the temperature control temperature of the heater 23 during the fixing operation is not less than a predetermined value (S52). In this embodiment, it is determined whether it is 170 degreeC. The numerical value of 170 ° C. is a numerical value that can be appropriately changed according to the environment or the like.

  Here, when the temperature adjustment temperature of the heater 23 is less than 170 ° C., there is little necessity for the discharge control for the above-described reason, so the fixing standby state is entered without performing the discharge control (S61). On the other hand, when the temperature adjustment temperature of the heater 23 is 170 ° C. or higher, the discharge standby control similar to that in the first embodiment is performed, and then the fixing standby state is entered (S53 to S61). That is, the CPU 80 controls the heating by the heater 23 after the film 22 is stopped according to the heating temperature in the rotating state of the film 22. Specifically, when the temperature control temperature of the heater 23 in the rotating state of the film 22 is 170 ° C. or more (predetermined value or more), the heater 22 is heated after the film 22 is stopped, and is less than 170 ° C. (predetermined) If it is less than the value, heating by the heater 23 is not performed.

  FIG. 16 shows the temperature in the nip of the film 22 from the fixing operation to the fixing standby state and the outside of the nip at a temperature adjustment temperature of 160 ° C. of the heater 23 during the fixing operation, with the sheet P relating to fixing being thin paper. It is a graph which shows transition of temperature. Here, FIG. 16A shows a temperature transition when the control of the first embodiment is performed, and FIG. 16B shows a temperature transition when the control of the present embodiment is performed.

  First, as shown in FIG. 16A, in the control according to the first embodiment, the temperature of the heater 23 during the fixing operation is as low as 160 ° C., so the temperature of the film 22 after the post-rotation control is also low. For this reason, even when the discharge control is executed at the lowest temperature control temperature of 170 ° C., the temperature difference between the inside and outside of the nip of the film 22 at the end of the discharge control becomes as large as 120 ° C.

  On the other hand, in the control of this embodiment, when the temperature is 170 ° C. or less, the discharge control is not executed and the fixing standby state is entered. Therefore, the temperature difference between the inside and outside of the nip of the film 22 is increased by stop heating during the discharge control. Absent. Therefore, even after entering the fixing standby state, the temperature difference between the inside and outside of the nip of the film 22 remains small. Therefore, even when the image forming job signal is subsequently received and the fixing motor 86 is driven, the temperature difference between the inside and outside of the nip of the film 22 when the fixing motor 86 is driven is less than 95 ° C. Can be suppressed.

(Fourth embodiment)
Next, a fourth embodiment of an image forming apparatus including the fixing device according to the present invention will be described with reference to the drawings. About the part which overlaps with said 1st-3rd embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

  Conventionally, when an image forming job signal is received during discharge control, the discharge control is stopped and the operation proceeds to an image forming operation. In the fixing device 11, the fixing motor 86 is driven and the pressure roller 24 and the film 22 are rotated. . However, since the stop heating is performed in the discharge control, the temperature difference between the inside and outside of the nip of the film 22 is large. If the film 22 rotates in this state, a dent mark may be generated.

  Therefore, in this embodiment, when an image forming job signal is received during the discharge control, the start of the image forming operation is waited until the temperature difference between the inside and outside of the nip of the film 22 becomes a predetermined value or less. Hereinafter, the control of this embodiment will be described with reference to the flowchart shown in FIG.

  As shown in FIG. 17, first, after the post-rotation control is completed after the fixing operation, the energization of the heater 23 is turned on while the film 22 is not rotating, and the discharge control is started (S71). Next, when the image forming job signal is not received during the discharge control, the heater 23 is de-energized after 5 seconds after the heater 23 reaches the predetermined set temperature as usual (S72 to S74), and the discharge control is performed. finish.

  On the other hand, when an image forming job signal is received during discharge control, first, the main thermistor 25a and the non-contact thermometer 89 detect the temperature inside the nip and the temperature outside the nip of the film 22 and calculate the temperature difference between the inside and outside the nip (S72, S75). , S76, S77). Next, it is determined whether or not the temperature difference between the inside and outside the nip of the film 22 is greater than or equal to a predetermined value (S78). In the present embodiment, it is determined whether or not the temperature difference between the inside and outside of the nip of the film 22 is 90 ° C. or more.

  Here, when the temperature difference between the inside and outside of the nip of the film 22 is less than 90 ° C., the driving of the fixing motor 86 is turned on (S79), and the image forming operation is executed (S87).

  On the other hand, when the temperature difference between the inside and outside of the nip of the film 22 is 90 ° C. or more, the heater 23 is turned off and cooled without performing the image forming operation immediately (S80). Thereafter, the temperature difference inside and outside the nip of the film 22 is detected again in the same manner as above (S82 to S84). When the temperature is within 90 ° C., the heater 23 is turned on (S85) and the fixing motor is turned on ( S86), an image forming operation is executed (S87).

  As described above, when the CPU 80 receives a signal for driving the fixing motor 86 in a state where the temperature difference between the inside and outside the nip of the film 22 being discharged is greater than or equal to a predetermined value, the CPU 80 waits and cools until the temperature difference between the outside and inside the nip becomes less than a predetermined value. Then, the fixing motor 86 is driven. That is, when the CPU 80 receives a signal for rotating the film 22 while the stopped film 22 is heated by the heater 23, the CPU 80 starts the rotating operation when determining that the temperature difference between the inside and outside of the nip is equal to or less than a predetermined value. When it is determined that the value is larger than the predetermined value, the rotation operation is restricted. As a result, the temperature difference between the inside and outside of the nip during the rotation of the film 22 can be reduced, and the occurrence of concave marks on the film 22 can be suppressed.

(Fifth embodiment)
Next, a fifth embodiment of the image forming apparatus A according to the present invention will be described with reference to the drawings. About the part which overlaps with the said 1st-4th embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

  In the present embodiment, in the discharge control of the fourth embodiment, the temperature outside the nip of the film 22 is not measured by a non-contact thermometer (not shown), but calculated based on the amount of change in the nip temperature per unit time. To do. That is, the detection of the temperature outside the nip of the film 22 in step S76 and step S83 described in the fourth embodiment is performed by the control described later, and the other control is the same as the control of the fourth embodiment. Hereinafter, the calculation procedure of the temperature outside the nip of the film 22 of the present embodiment will be described with reference to the flowchart shown in FIG. 18 and the graph showing the transition of the temperature inside the nip and the temperature outside the nip shown in FIG.

  As shown in FIG. 18, when the post-rotation control is started by turning off the heater 23 after the image forming operation is completed, the start time of the post-rotation control is recorded in the ROM 82, and the temperature in the nip of the film 22 is set. It is detected by the main thermistor 25a and stored in the ROM 82 (S91). Next, when the driving of the fixing motor 86 is turned off to end the post-rotation control, the post-rotation control end time is recorded in the ROM 82, and the nip temperature of the film 22 is detected by the main thermistor 25a and stored in the ROM 82. (S92).

  Next, based on the amount of change in the nip temperature of the film 22 during the post-rotation control and the time for the post-rotation control, the amount of change in the nip temperature per unit time in the post-rotation control is expressed as a temperature decrease rate η (FIG. 19). Reference) is calculated (S93). In this embodiment, the post-rotation control is executed for 2 seconds, and the temperature in the nip of the film 22 changes from 190 ° C. to 120 ° C., so that the temperature change rate η = 35.

  Here, the temperature decrease rate η and the temperature decrease rate α (see FIG. 19), which is the amount of change in the temperature outside the nip of the film 22 per unit time in the discharge control, are preliminarily experimentally set to a temperature decrease rate α = 0.286η. I know that there is a relationship. Therefore, the temperature decrease rate η (= 35) is substituted into this equation, and the temperature decrease rate α = 0.286 × 35 = 10 of the temperature outside the nip in the discharge control is calculated (S94).

  Further, as described above, in the post-rotation control, the temperature inside the nip of the film 22 and the temperature outside the nip become substantially equal when a certain time has elapsed. In this embodiment, as shown in FIG. 19, the temperature inside the nip of the film 22 and the temperature outside the nip are substantially equal after 2 seconds from the start of the post-rotation control (at the end of the post-rotation control). That is, the temperature in the nip of the film 22 at the end of the rotation control detected in step S2 is substantially the same as the temperature outside the nip of the film 22 at the start of the discharge control.

  For this reason, the temperature outside the nip of the film 22 can be determined based on the elapsed time from the start of discharge control (= end of post-rotation control). That is, assuming that the elapsed time from the start of the discharge control is T, and the in-nip temperature β of the film 22 at the start of the discharge control, the outside nip temperature θ of the film 22 is calculated by the following equation 1 (S95).

  θ = β− (αT) (Formula 1)

  For example, as shown in FIG. 19, when the temperature 22 in the nip of the film 22 at the start of the discharge control is 120 ° C. and an image forming job signal is received 4 seconds after the start of the discharge control, the temperature decrease rate α = 10. The temperature in the nip θ = 120− (4 × 10) = 80 ° C.

  In this way, the temperature outside the nip of the film 22 is not measured by a temperature sensor such as a non-contact thermometer, but is calculated based on the temperature detected by the temperature sensor that detects the temperature inside the nip of the film 22. The cost can be reduced by reducing the number of points.

(Sixth embodiment)
Next, a sixth embodiment of an image forming apparatus including the fixing device according to the present invention will be described with reference to the drawings. About the part which overlaps with the said 1st-5th embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

  In the present embodiment, in the discharge control of the fourth embodiment, the temperature outside the nip of the film 22 is not measured by a non-contact thermometer (not shown), but calculated based on the amount of change in the nip temperature per unit time. To do. That is, the detection of the temperature outside the nip of the film 22 in step S76 and step S83 described in the fourth embodiment is performed by the control described later, and the other control is the same as the control of the fourth embodiment. Hereinafter, the calculation procedure of the temperature outside the nip of the film 22 of the present embodiment will be described with reference to the flowchart shown in FIG. 20 and the graph showing the transition of the temperature inside the nip and the temperature outside the nip shown in FIG.

  As shown in FIG. 20, first, after completion of the post-rotation control, a cooling period is provided in which the energization of the heater 23 and the driving of the fixing motor 86 are turned off without starting the start-up control immediately. At this time, the ROM 82 stores the time at the start of the cooling period (when the heater energization and the motor drive are both turned off) and the temperature in the nip of the film 22 at the start of the cooling period (S101). The nip temperature is detected by the main thermistor 25a.

  Next, after elapse of a predetermined time, the heater 23 is turned on, and discharge control is started. That is, the start of the discharge control is the same as the end of the cooling period. At this time, the time at the start of the discharge control (at the end of the cooling period) and the in-nip temperature of the film 22 detected by the main thermistor 25a are stored in the ROM 82 (S102).

  Next, the amount of change per unit time of the nip temperature of the film 22 during the cooling period is calculated as the temperature change rate ε (S103). As shown in FIG. 21, in this embodiment, the temperature in the nip of the film 22 at the start of the cooling period was 120 ° C., and the temperature in the nip at the end of the cooling period was 110 ° C. The cooling period is 1 second. Therefore, the temperature change rate ε is (120−110) / 1 = 10.

  As described above, in the post-rotation control, the temperature inside the nip of the film 22 and the temperature outside the nip become substantially equal after a certain period of time. In this embodiment, at the end of the post-rotation control, the temperature inside the nip and the temperature outside the nip of the film 22 are substantially equal (FIG. 21). Further, during the cooling period, the energization of the heater 23 and the driving of the fixing motor 86 are turned off, so that the temperature inside the nip and the temperature outside the nip remain substantially the same. That is, the temperature in the nip of the film 22 at the start of discharge control (at the end of the cooling period) detected in step S102 is substantially equal to the temperature outside the nip.

  When the heater 23 is turned on at the start of discharge control and stopped heating is performed, the temperature inside the nip of the film 22 increases, but the temperature outside the nip decreases at the same rate of temperature change as in the cooling period. That is, the temperature decrease rate Ψ, which is the amount of change in the temperature outside the nip of the film 22 per unit time in the discharge control, and the temperature change rate ε of the temperature in the nip of the film 22 during the cooling period are the same (FIG. 21). That is, since the temperature decrease rate ψ = temperature decrease rate ε is established, the CPU 80 sets the value of the temperature decrease rate ψ as the value of the temperature decrease rate ε (S104). This result is also found from the experimental results.

  For this reason, if the elapsed time from the start of the discharge control (= the end of the cooling period) is determined, the temperature outside the nip of the film 22 is determined. In other words, the elapsed time from the start of the discharge control is T, and the in-nip temperature β of the film 22 at the start of the discharge control, and the outside nip temperature γ of the film 22 is calculated by the following equation (S105).

  γ = β− (ΨT) (Expression 2)

  For example, as shown in FIG. 21, when the in-nip temperature β of the film 22 at the start of the discharge control is 110 ° C. and an image forming job signal is received 3 seconds after the start of the discharge control, the temperature decrease rate Ψ = 10, so the film The temperature in the nip θ = 110− (3 × 10) = 80 ° C.

  In this way, the temperature outside the nip of the film 22 is not measured by a temperature sensor such as a non-contact thermometer, but is calculated based on the temperature detected by the temperature sensor that detects the temperature inside the nip of the film 22. The cost can be reduced by reducing the number of points.

(Seventh embodiment)
Next, a seventh embodiment of an image forming apparatus including the fixing device according to the present invention will be described with reference to the drawings. About the part which overlaps with the said 1st-6th embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

  FIG. 22 is a schematic diagram schematically showing deformation due to thermal expansion of the film 22 when the width of the fixing nip portion is narrow (FIG. 22A) and wide (FIG. 22B). As shown in FIG. 22, when the width of the fixing nip portion is wide, the amount of elongation of the film 22 due to thermal expansion is greater than when the width is narrow, and the amount of strain at the temperature boundary surface of the film 22 is also large. Since the fixing nip portion has a width not only in the sheet conveying direction in the fixing device 11 but also in the rotation axis direction of the pressure roller 24, the deformation of the film 22 occurs in both directions. Thus, when the amount of strain increases, the film 22 is easily permanently deformed, and a dent mark is easily generated. For this reason, in order to suppress the occurrence of a dent mark on the film 22, it is necessary to reduce the temperature difference between the inside and outside of the nip when the pressure roller 24 is driven when the width of the fixing nip portion is wide than when it is narrow. .

  Therefore, in this embodiment, the temperature difference between the inside and outside of the nip of the film 22 when the pressure roller 24 is driven is set according to the width of the fixing nip portion. Thereby, generation | occurrence | production of the dent mark of the film 22 can be suppressed. Hereinafter, the control of this embodiment will be described with reference to the flowchart shown in FIG.

  As shown in FIG. 23, when the post-rotation control is finished after the fixing operation, the heater 23 is turned on while the film 22 is not rotated, and the discharge control is started (S111). Next, when an image forming job signal is not received during the discharge control, the heater 23 is de-energized after 5 seconds after the heater 23 reaches a predetermined set temperature as usual (S112 to S114), and the discharge control is performed. finish.

  On the other hand, when an image forming job signal is received during discharge control, first, the main thermistor 25a and the non-contact thermometer 89 detect the nip internal temperature and the nip external temperature of the film 22, and calculate the nip internal / external temperature difference (S112, S115). ~ S117).

  Next, the CPU 80 acquires the width information of the fixing nip portion from the ROM 82 (S118). Since the width of the fixing nip varies from one unit to another due to variations in the members, the width information is stored in the ROM 82 in advance at the time of shipment. In the present embodiment, the width of the fixing nip portion in the sheet conveyance direction (the rotation direction of the film 22) at the time of shipment is 9.0 mm.

  Next, the CPU 80 obtains a table μ (FIG. 24) in which the width N of the fixing nip portion in the sheet conveying direction and the threshold value ν (predetermined temperature) relating to the temperature difference between the inside and outside of the film 22 when the pressure roller 24 is driven. The threshold value ν is set with reference to (S119). The table μ is stored in the ROM 82 in advance. Further, as shown in FIG. 24, in the table μ, the threshold value ν is set to be smaller when the width of the fixing nip portion is large than when it is small. In this embodiment, since the width N of the fixing nip portion in the sheet conveyance direction is 9.0 mm, the threshold value ν is set to 80 ° C.

  Next, the CPU 80 determines whether or not the nip inside / outside temperature difference of the film 22 is greater than or equal to the threshold value ν (S120). That is, in this embodiment, it is determined whether or not the in-nip μ outside temperature difference of the film 22 is 80 ° C. or more.

  Here, when the temperature difference between the inside and outside of the nip of the film 22 is less than 80 ° C., the driving of the fixing motor 86 is turned on (S127), and the image forming operation is executed (S129).

  On the other hand, when the temperature difference between the inside and outside of the nip of the film 22 is 80 ° C. or more, the heater 23 is turned off and cooled without performing the image forming operation immediately (S123). Thereafter, similarly to the above, the temperature difference between the inside and outside of the nip of the film 22 is detected again (S124 to S126). When the temperature is within 80 ° C., the heater 23 is turned on (S127), and the fixing motor is turned on ( S128), an image forming operation is executed (S129).

  In this way, by setting the temperature difference between the inside and outside of the nip of the film 22 when the pressure roller 24 is driven according to the width of the fixing nip, even in a fixing device having a wide fixing nip, Occurrence can be suppressed.

  In this embodiment, the threshold value ν is set based on the width of the fixing nip in the sheet conveyance direction. However, the present invention is not limited to this, and the threshold value ν is set based on the width of the pressure roller 24 in the rotation axis direction. May be.

(Eighth embodiment)
Next, an eighth embodiment of an image forming apparatus including the fixing device according to the present invention will be described with reference to the drawings. About the part which overlaps with the said 1st-7th embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

  FIG. 25 is a graph showing the relationship between the number of fixed sheets of the fixing device 11 and the width of the fixing nip portion. As shown in FIG. 25, the width of the fixing nip portion gradually increases as the number of fixing sheets increases due to softening deterioration of the rubber of the pressure roller 24. Line A, line B, and line C indicate changes in the width of the fixing nip portion of different fixing devices 11, and as described above, the width of the fixing nip portion differs depending on the member.

  Therefore, in this embodiment, the width of the fixing nip portion is determined, and the temperature difference between the inside and outside of the film 22 when the pressure roller 24 is driven is set according to the determined width of the fixing nip portion. Hereinafter, the control of this embodiment will be described with reference to the flowchart shown in FIG.

  As shown in FIG. 26, after the post-rotation control is ended after the fixing operation, the heater 23 is energized with the film 22 in the non-rotating state, and the discharge control is started (S131). Next, when the image forming job signal is not received during the discharge control, the heater 23 is turned off after 5 seconds after the heater 23 reaches the predetermined set temperature as usual (S132 to S134), and the discharge control is performed. finish.

  On the other hand, when an image forming job signal is received during the discharge control, first, the main thermistor 25a and the non-contact thermometer 89 detect the temperature inside the nip and the temperature outside the nip of the film 22, and calculate the temperature difference between the inside and outside the nip (S132, S135). ~ S137).

  Next, the CPU 80 obtains the width information of the fixing nip portion at the time of shipment and the current number of formed images (S138). Note that the width information of the fixing nip portion is stored in the ROM 82 in advance at the time of shipment. In this embodiment, the width N of the fixing nip portion in the sheet conveyance direction at the time of shipment is 9.5 mm. Based on these pieces of information, the current width of the fixing nip is determined as described below (S139).

In this embodiment, when the number of formed images is n, the increase amount Δ of the fixing nip width is experimentally confirmed to be Δ = 2 × 10 ⁻⁵ × n (mm). For this reason, for example, when it is assumed that the current number of formed images is 50,000, the width N of the fixing nip portion in the current sheet conveyance direction is determined to be 10.5 mm. That is, the CPU 80 determines that the width of the fixing nip portion is larger as the cumulative number of sheets fixed by the fixing device 11 is larger.

  Here, in the present embodiment, as in the seventh embodiment, the width N of the fixing nip portion in the sheet conveyance direction and the threshold value ν (predetermined temperature) relating to the temperature difference between the inside and outside of the nip of the film 22 when the pressure roller 24 is driven. The associated table μ (FIG. 24) is stored in the ROM 82 in advance. Accordingly, the CPU 80 next sets the threshold value ν with reference to the table μ based on the determined width of the fixing nip portion (S140). In the present embodiment, the threshold value ν is set to 70 ° C.

  Next, it is determined whether or not the temperature difference inside and outside the nip of the film 22 is greater than or equal to a threshold value ν (S141). That is, in this embodiment, it is determined whether or not the nip inner / outer temperature difference of the film 22 is 70 ° C. or more.

  Here, when the temperature difference between the inside and outside of the nip of the film 22 is less than 70 ° C., the fixing motor 86 is turned on (S142), and the image forming operation is executed (S150).

  On the other hand, when the temperature difference between the inside and outside of the nip of the film 22 is 70 ° C. or more, the heater 23 is turned off and cooled without performing the image forming operation immediately (S143). Thereafter, the temperature difference inside and outside the nip of the film 22 is detected again in the same manner as above (S145 to S147). When the temperature is within 70 ° C., the heater 23 is turned on (S148), and the fixing motor is turned on ( S149), an image forming operation is executed (S150).

  In this way, the temperature difference between the inside and outside of the nip of the film 22 when the pressure roller 24 is driven is set according to the determined width of the fixing nip portion, so that the width of the fixing nip portion changes depending on the use situation. In addition, the occurrence of dent marks on the film 22 can be suppressed.

  In this embodiment, the threshold value ν is set based on the width of the fixing nip in the sheet conveyance direction. However, the present invention is not limited to this, and the threshold value ν is set based on the width of the pressure roller 24 in the rotation axis direction. May be.

  In addition to the method for detecting the temperature outside the nip of the film 22 described in the first to eighth embodiments, for example, a temperature transition table of the temperature outside the nip of the film 22 may be stored in the ROM 82 in advance. The same effect as described above can be obtained.

11: Fixing device 22 ... Film (rotary unit)
23 ... Heater 24 ... Pressure roller (Pressure member)
25 ... Thermistor (first detection means, heater temperature sensor)
80 ... CPU (control means, setting means)
86. Fixing motor (driving means)
88 ... Environmental sensor (environment detection means)
89 ... Non-contact thermometer (second detection means, non-contact region temperature sensor)
A: Image forming apparatus P: Sheet (recording material)

Claims (18)

A rotating unit;
A heating unit for heating the rotating unit;
Pressurizing means for nipping and conveying the recording material to and from the rotating unit;
Control means for controlling the heating by the heating unit after the rotation unit is stopped according to the heating temperature in the rotation state of the rotation unit heated by the heating unit;
A fixing device.   When the heating temperature in the rotating state of the rotating unit is equal to or higher than a predetermined value, the control means performs predetermined heating by the heating unit after stopping the rotating unit, and the heating temperature in the rotating state of the rotating unit. 2. The fixing device according to claim 1, wherein when the temperature is less than the predetermined value, the predetermined heating by the heating unit after the rotation unit is stopped is not performed. Environment detection means for detecting the ambient temperature of the fixing device;
Temperature detecting means for detecting the temperature of the heating unit;
Further comprising
The control means starts rotation of the rotating unit in a stopped state based on the ambient temperature of the fixing device, the elapsed time from the previous image forming operation, and the temperature of the heating unit. The fixing device according to claim 1. A rotating unit;
A heating unit for heating the rotating unit;
Pressurizing means for nipping and conveying the recording material to and from the rotating unit;
When it is determined that a temperature difference in the rotation direction of the rotating unit is equal to or less than a predetermined value when starting the rotating operation of the rotating unit while the rotating unit is being heated by the heating unit, Performing a rotation operation, and when it is determined that the temperature difference is larger than a predetermined value, a control means for regulating the predetermined rotation operation;
A fixing device.   The control means determines the temperature difference based on a temperature of a contact area of the rotating unit with the pressurizing means and a temperature of a non-contact area that is away from the contact area in the rotation direction and does not contact the pressurizing means. The fixing device according to claim 4, wherein the fixing device is determined.   The said control means sets the said predetermined value regarding the said temperature difference of the said rotation unit small when the width | variety of the said rotation direction of the said contact area is large rather than the case where it is small. Fixing device.   The fixing device according to claim 6, wherein the control unit determines that the width of the contact area in the rotation direction is larger as the cumulative number of the recording materials on which the fixing has been performed is larger.   And a first detection means for detecting the temperature of the contact area of the rotating unit; and a second detection means for detecting the temperature of the non-contact area, wherein the control means is the first detection means and the second detection means. The fixing device according to claim 5, wherein the temperature difference is determined based on a detection result of a detection unit.   The fixing device according to claim 8, wherein the first detection unit is a temperature sensor that measures a temperature of the heating unit.   The fixing device according to claim 8, wherein the second detection unit is a non-contact region temperature sensor that detects a temperature of the non-contact region of the rotating unit.   The second detection means determines the temperature of the non-contact region of the rotating unit based on the temperature detected by the first detection means and the amount of change per unit time of the temperature detected by the first detection means. The fixing device according to claim 8, wherein the fixing device is a fixing device.   The fixing device according to claim 4, wherein the control unit starts rotation of the rotating unit when the temperature difference of the rotating unit is equal to or less than a predetermined value. .   When the control means receives a signal for rotating the rotating unit in a state where the temperature difference of the rotating unit is larger than a predetermined value, the control unit starts rotating the rotating unit until the temperature difference becomes a predetermined value or less. The fixing device according to claim 12, wherein the fixing device is not. A rotating unit;
A heating unit for heating the rotating unit;
Pressurizing means for nipping and conveying the recording material to and from the rotating unit;
When changing the rotation unit from the rotation state to the stop state, if the heating temperature by the heating unit in the rotation state is the first temperature, the heating temperature by the heating unit in the stop state is set to the second temperature, and heating in the rotation state When the heating temperature by the unit is the third temperature lower than the first temperature, setting means for setting the heating temperature by the heating unit in the stopped state to a fourth temperature lower than the second temperature;
A fixing device.   The fixing device according to claim 1, wherein the rotating unit is a cylindrical film. An image forming unit for forming an image;
The fixing device according to claim 1, wherein an image formed by the image forming unit is fixed on a recording material.
An image forming apparatus comprising: A rotating unit;
A heating unit for heating the rotating unit;
Pressurizing means for nipping and conveying the recording material to and from the rotating unit;
And a fixing device control method for heating and fixing the developer image on the recording material to the recording material while sandwiching and conveying the recording material by the rotating unit and the pressurizing unit,
Heating the rotating unit in a rotating state by the heating unit;
Heating the rotating unit in a stopped state at a temperature corresponding to the heating temperature in the rotating state;
A control method for a fixing device. A rotating unit;
A heating unit for heating the rotating unit;
Pressurizing means for nipping and conveying the recording material to and from the rotating unit;
And a fixing device control method for heating and fixing the developer image on the recording material to the recording material while sandwiching and conveying the recording material by the rotating unit and the pressurizing unit,
A determination step of determining a temperature difference in the rotation direction of the rotating unit when starting the rotating operation of the rotating unit while the rotating unit is being heated by the heating unit.
When the temperature difference is determined to be less than or equal to a predetermined value, a predetermined rotation operation is performed. When the temperature difference is determined to be greater than the predetermined value, the predetermined rotation operation is performed. A control process for controlling to regulate,
A control method for a fixing device.

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