JP2009025588A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device designed so that while the ability to fix toner is maintained, the Curie temperature of a magnetized metal which is the base material of a heat roller can be set far lower than that of a conventional one, and to provide an image forming apparatus incorporating the fixing device. <P>SOLUTION: The fixing device 14 of the image forming apparatus 100 includes: a fixing roller 21; an induction heat coil 23; a pressure roller 52; a thermistor 26a for detecting the temperature of a paper passage area; and a thermistor 26b for detecting the temperature of a paper non-passage area. When the passage of recording paper is started having the control temperature of the paper passage area as a predetermined temperature T1 and then the temperature of the paper non-passage area reaches a predetermined temperature T2 or above, the control temperature is set to a predetermined temperature T3 that is lower than T1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention inserts a sheet carrying an unfixed toner image into the nip of a heated roller pair, such as an electrophotographic copying machine, a facsimile machine or a printer, and heats and melts the unfixed toner to fix it on a recording sheet. The present invention relates to an image forming apparatus equipped with a fixing device.

  In an electrophotographic image forming apparatus, a heat source is built in at least one roller (fixing roller) of a roller pair forming a nip, and a sheet carrying an unfixed toner image in the nip of the roller pair heated by the heat source A heat roller fixing method is used in which toner is fixed to a recording sheet by inserting the toner.

  Also, as a heat roller fixing method, an endless fixing belt is stretched between the fixing roller and the heating roller, the heat source of the heating roller heats the fixing belt, and the fixing roller and the pressure roller nip through the fixing belt. A method of fixing an unfixed toner image on a recording sheet is also used. A surface layer made of silicon rubber or the like is formed on the surface layer of the pressure roller.

  As a heating method for a fixing roller or a heating roller (heat roller) incorporating such a heat source, a lamp method for heating with a lamp such as a halogen lamp is known. However, a shortening of the warm-up time and a request for energy saving are known. Therefore, an induction heating (IH: Induction Heating) method is proposed in which an alternating magnetic field is linked to a magnetic conductor to generate an eddy current.

  In the IH method, it has been proposed to use, for example, a magnetic shunt metal having a characteristic that the temperature does not rise above a predetermined temperature (Curie temperature) as a heat roller that generates heat by an induction heating coil. By using such a magnetic shunt metal, the temperature of the heat roller can be prevented from rising above a predetermined temperature, so that the fixing device is safe.

  Further, when small-size recording paper is continuously printed, heat is taken away from the paper passing area by the recording paper, whereas heat is not taken away from the non-paper passing area, so that the temperature of the non-paper passing area rises. If the temperature of the non-sheet passing region is excessively increased, hot offset may occur when a large size recording paper is continuously printed.

Therefore, a technique for suppressing hot offset has been proposed. For example, Patent Document 1 discloses a technique for setting the Curie temperature of a magnetic shunt metal to be equal to or higher than the fixing temperature and lower than the hot offset start temperature.
JP 2000-039797 A

  However, even if the curie temperature of the magnetic shunt metal is set below the hot offset start temperature and the hot offset of the recording paper can be suppressed, for example, when the temperature difference between the recording paper passing area and the non-passing area is large. If the surface temperature difference between the axial center and both ends of the pressure roller in contact with the heat roller becomes large, and there is a difference in the expansion coefficient of the silicon rubber or fluororesin layer on the surface of the pressure roller, On the other hand, a difference occurs in the outer diameter of the pressure roller, which may affect the formed image. For this reason, it is desired to minimize the temperature difference between the paper passing area and the non-paper passing area.

  In order to reduce such a temperature difference, it is necessary to bring the fixing temperature in the sheet passing area close to the Curie temperature. In order to suppress hot offset and to ensure the heat resistance of the member, it is desirable to set the Curie temperature low. On the other hand, since the toner needs to be sufficiently fixed, the fixing temperature cannot be lower than a predetermined temperature. Therefore, in order to satisfy these, it is necessary to set the Curie temperature to be low and to set the control temperature of the passing region for controlling the fixing temperature to be substantially constant near the Curie temperature.

  Here, since the magnetic shunt metal has non-magnetic characteristics when the temperature reaches the Curie temperature, the temperature rise in the non-sheet passing region is suppressed as described above. On the other hand, since the Curie temperature is not reached in the paper passing region, the magnetic shunt metal has magnetic properties. For this reason, when there is a sufficient temperature difference between the fixing temperature and the Curie temperature, the magnetic shunt metal contains a region having magnetic characteristics and a region having non-magnetic characteristics, and the induction heating coil The generated magnetic flux does not flow in the nonmagnetic region but flows in the magnetic region, so that the magnetic shunt metal as a whole is controlled to the Curie temperature or lower.

  However, when the control temperature is set close to the Curie temperature, the temperature of the actual passing region (fixing temperature) may become higher than the control temperature due to variations in the detection sensor of the fixing temperature. The actual Curie temperature may be lower than the set value due to variations in the ratio and the measured value of the Curie temperature. In such a case, the fixing temperature becomes equal to or higher than the Curie temperature, and not only the non-sheet passing area but also the sheet passing area has nonmagnetic characteristics. Then, since the entire magnetic shunt metal has non-magnetic characteristics, the magnetic flux flows through the whole, and the internal resistance of the magnetic shunt metal rises above the Curie temperature and may overshoot.

  For this reason, it is difficult to set the control temperature close to the Curie temperature, and it is also difficult to set the Curie temperature low.

  The present invention has been made in view of the above circumstances, and even if the control temperature of the passage region of the recording medium is set closer to the Curie temperature of the magnetic shunt metal that is the base of the heat roller, the fixing temperature is the Curie temperature. It is an object of the present invention to provide a fixing device capable of setting the Curie temperature lower while maintaining the toner fixability, and an image forming apparatus equipped with the fixing device.

  In order to achieve the above object, the present invention provides a heat roller based on a magnetic shunt metal, an induction heating coil that generates a magnetic flux for induction heating of the heat roller, and direct or heat transfer to the heat roller. And a pressurizing unit that forms a nip portion through which the recording medium is inserted, and a fixing device that fixes a toner image carried on the recording medium at the nip portion. Temperature detecting means for detecting the temperature of the non-passing area of the recording medium and the temperature of the passing area of the recording medium are controlled to a predetermined temperature T1 which is lower than the Curie temperature Tc of the magnetic shunt metal and equal to or higher than the fixing allowable temperature Tm. Then, the insertion of the recording medium is started, and when the detection result of the temperature detection means is equal to or higher than a predetermined temperature T2 higher than the Curie temperature Tc, the temperature of the passage region is lower than the T1, and Serial fixing allowable temperature Tm above, is characterized by comprising a temperature control means for controlling the predetermined temperature T3.

  Further, the present invention is characterized in that the temperature difference between the T1 and the Tc is less than 30 ° C.

  According to another aspect of the present invention, there is provided an image forming apparatus including the fixing device.

  According to the first configuration of the present invention, the temperature detection means is provided in the non-passing area of the recording medium, the control temperature of the passing area is lower than the predetermined Curie temperature Tc of the magnetic shunt metal that is the heat roller base material, and When the insertion of the recording medium is started by controlling at a predetermined temperature T1 which is equal to or higher than the fixing allowable temperature Tm and the temperature of the non-passing region becomes a predetermined temperature T2 higher than the Curie temperature Tc, the control temperature of the passing region is set from T1. By controlling the temperature to a predetermined temperature T3 that is low and equal to or higher than the fixable temperature Tm, the possibility that the fixing temperature exceeds the Curie temperature Tc can be further suppressed.

  Therefore, the Curie temperature Tc can be set lower, and the fixing capable of suppressing the influence on the image formation such as ensuring the heat resistance of the member and suppressing the hot offset while sufficiently satisfying the toner fixing property. An apparatus can be provided.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the temperature difference between the Curie temperature Tc and the control temperature T1 is less than 30 ° C., so that the control temperature T1 is further reduced to the Curie. The temperature can be brought close to the temperature Tc, and the toner fixability can be more sufficiently satisfied.

  According to the third configuration of the present invention, by mounting the fixing device having the first or second configuration in the image forming apparatus, the heat resistance of the member is ensured while satisfying the toner fixing property, and the hot Image formation capable of suppressing offset can be performed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing a fixing device according to a first embodiment of the present invention. 2 and 3 are plan views of the fixing device according to the present embodiment as viewed from the sheet feeding direction. FIG. 2 illustrates a case where a recording sheet having a maximum sheet passing size is inserted, and FIG. The case where a small-sized recording sheet is inserted is shown. 2 and 3 also show the temperature distribution in the axial direction of the fixing roller 21 (only one side: right side from the axis center).

  Here, for example, the fixing device 14 used for monochrome image formation is used. The fixing device 14 mainly includes a fixing roller 21 and a pressure roller 52.

  An induction heating coil 23 is disposed inside the fixing roller 21. The induction heating coil 23 includes a cylindrical bobbin 24, and a coil wire 25 wound around the outer peripheral surface of the bobbin 24 (radially wound). including. The coil wire 25 is made of, for example, a copper wire, and a high frequency magnetic flux is generated by applying a high frequency current. Due to this high-frequency magnetic flux, an induced eddy current is generated in the fixing roller 21, and the fixing roller 21 is heated by Joule heat generation due to eddy current loss due to the material resistivity of the fixing roller 21 itself. Details of the fixing roller 21 will be described later.

  The induction heating coil 23 is held inside the fixing roller 21 so as not to contact the roller, and the induction heating coil 23 does not rotate even when the fixing roller 21 is driven to rotate. In the bobbin 24, for example, a plurality of cores 27 made of ferrite that hardly self-heats and easily passes magnetic flux are disposed. The core 27 has substantially the same flat rectangular parallelepiped shape. The core 27 is arranged on the inner peripheral surface of the bobbin 24 at a substantially equal interval in the circumferential direction, with the maximum surface facing the center of the axis and one separated from the core 27. , Extending along the axial direction (paper surface direction in the drawing) so that the maximum surfaces face each other, and is fitted into the bobbin 24. The core 27 in each row may be divided into a plurality of parts in the axial direction.

  The pressure roller 52 is provided with an elastic member layer 52b made of sponge silicon rubber and an outer fluororesin (outermost surface) outside a cored bar 52a made of stainless steel. The thickness of the sponge silicon rubber can be 3 to 7 mm, for example. The pressure roller 52 is brought into contact with the fixing roller 21 to form a nip portion through which the recording paper (recording medium) is inserted. The pressure roller 52 contacts the fixing roller 21 and rotates according to the rotation of the fixing roller 21.

  Note that the induction heating coil 23 may also be disposed inside the pressure roller 52 in the same manner as the fixing roller 21. Further, although the pressure roller 52 is used in the present embodiment, the pressure unit may be a fixing member, a belt-like member, or the like, and is not particularly limited to the present embodiment.

  As shown in FIG. 1, thermistors 26 a and 26 b (temperature detection means) for detecting the surface temperature of the fixing roller 21 are in contact with the outer peripheral surface of the fixing roller 21. As shown in FIGS. 2 and 3, the thermistor 26a is disposed in the center in the axial direction of the fixing roller 21, and detects the temperature of the surface of the fixing roller 21 in the paper passing area (passing area) of the recording paper. .

  The thermistor 26b is disposed at one end of the fixing roller 21 (the right end in FIGS. 2 and 3) and detects the surface temperature of the fixing roller 21 in the non-sheet passing region. Here, for example, as shown in FIG. 2, the maximum sheet passing size W1 is set to A3 width (297 mm), and a position that is 297/2 mm away from the center position of the A3 width is further increased by 5 mm to the outside. The thermistor 26b is installed at the position where the width M is provided. The extra width M takes into account variations in the axial direction of the sheet passing. Further, the thermistor 26 b is installed inside the side plate 27.

  Thus, not only the maximum sheet passing width but also the recording sheet width W2 when the A4 width recording sheet shown in FIG. The detected temperature of the thermistor 26b can be set as the detected temperature of the common non-sheet passing area.

  Here, one thermistor 26a is disposed in the paper passing area of the recording paper and one thermistor 26b is disposed in the non-passing area. However, the number is not limited, and two or more thermistors 26a are disposed in the passing area. Two or more thermistors 26b can be provided in the non-passing region, and two or more thermistors 26a and 26b can be provided at the same time. The margin M can also be set as appropriate according to the apparatus configuration and the like.

  The thermistor 26a and the thermistor 26b transmit an output signal corresponding to the detection result to the control unit 35 described later. The temperature detection result of the sheet passing area by the thermistor 26a is used for temperature control to maintain the fixing property and keep the temperature of the passing area of the fixing nip portion substantially constant, which will be described later. On the other hand, the temperature detection result of the non-sheet passing area by the temperature detection sensor 26b is used for temperature control of the sheet passing area described later. In addition to the above, the thermistors 26a and 26b may be, for example, thermocouples, and are not particularly limited. The detection position can be appropriately set according to the apparatus configuration, and is not particularly limited.

  The fixing roller 21 has a cylindrical shape, and a highly conductive nonmagnetic metal layer 21b and a fluororesin layer 21c are formed on the surface of a magnetic shunt metal layer 21a using a magnetic shunt metal as a base material. The fixing roller 21 extends in a horizontal direction (paper surface direction in FIG. 1) orthogonal to the recording paper insertion direction, and an induction heating coil 23 extends in the axial direction (paper surface direction in FIG. 1) of the fixing roller 21 therein. ing. The fixing roller 21 is driven by a driving device (not shown) and rotates so that its peripheral speed is the same as the conveyance speed of the recording paper (recording medium). The details of the induction heating coil 23 will be described later.

  The magnetic shunt metal is a metal having magnetic properties, and loses magnetism and becomes non-magnetic when it reaches a predetermined temperature (Curie temperature) or higher. For this reason, when a region having magnetic characteristics and a region having non-magnetic characteristics are mixed in the magnetic shunt metal layer 21a, even if magnetic flux is generated by the induction heating coil 23, the magnetic flux mainly flows in the magnetic region. The magnetic flux flowing in the nonmagnetic region is small. Therefore, heat generation due to dielectric heating in the non-magnetic characteristic region is suppressed, and in such a region, the temperature hardly rises above the Curie temperature.

  Due to such characteristics, an unexpected temperature increase of the fixing roller 21 can be avoided, which is effective in terms of safety. In addition, when a small size recording paper is continuously printed, an excessive temperature rise in the non-sheet passing area is suppressed, and when a large size recording paper is continuously printed, hot offset can be suppressed. It is effective.

  The temperature distribution in the axial direction of the surface of the fixing roller 21 is shown in FIGS. 2 and 3. On the surface of the fixing roller 21, since the sheet passing area is deprived of heat by the recording paper, the non-sheet passing area is more temperature than the sheet passing area. Rises. For example, as shown in FIG. 2, when a recording sheet having the maximum recording sheet width is passed, since the non-sheet passing area is narrow, a lot of heat is taken away by the recording sheet, and the temperature rise in the non-sheet passing area is too large. Absent.

  On the other hand, as shown in FIG. 3, when the recording paper width is small, the non-sheet passing area is wide, and therefore the temperature rise in the non-sheet passing area is larger than that in the sheet passing area. However, since the paper passing area has magnetic characteristics and the non-paper passing area has non-magnetic characteristics, the non-paper passing area is kept below the Curie temperature due to the characteristics of the magnetic shunt metal 21a.

  As such a magnetic shunt metal layer 21a, for example, an alloy of iron (Fe) and nickel (Ni) can be used, and for example, iron: nickel is set at 70:30 (weight ratio) so as to have a predetermined Curie temperature. ). In addition, selection of the metal which comprises an alloy, and the structure ratio of the selected metal can be suitably set so that desired Curie temperature may be obtained.

  Further, the Curie temperature of the magnetic shunt metal layer 21a is equal to or higher than the toner melting temperature, and is equal to or higher than a predetermined allowable fixing temperature Tm (for example, about 170 ° C. to 200 ° C.) necessary for sufficiently fixing the toner. C. to about 220.degree. C.).

  The heat capacity of the magnetic shunt metal layer 21a is proportional to the thickness of the base material. Therefore, if the thickness of the magnetic shunt metal layer 21a is reduced, the heat capacity can be reduced and the heating efficiency can be improved. However, when the thickness is reduced to about 0.5 mm, the electrical resistance of the magnetic shunt metal layer 21a increases, and even if the temperature of the magnetic shunt metal layer 21a reaches the Curie temperature and becomes nonmagnetic, heat generation continues further. The temperature of the magnetic shunt metal layer 21a becomes equal to or higher than the Curie temperature and tends to overshoot.

  In order to avoid such overshoot and maintain Curie temperature performance, a highly conductive nonmagnetic metal layer 21b such as copper is laminated on the surface of the magnetic shunt metal layer 21a in the fixing roller 21. Further, a fluororesin layer 21c such as PFA is laminated on the surface of the highly conductive nonmagnetic metal layer 21b.

  Next, fixing using the fixing device 14 of the present embodiment will be described.

  FIG. 4 is a diagram conceptually showing the relationship between the control temperature of the sheet passing area and other temperatures. The Curie temperature Tc of the fixing roller 21 (the magnetic shunt metal layer 21a) needs to be set to be smaller than the hot offset temperature, the fluororesin used for the surface layer of the fixing roller 21, the heat resistance of the silicon rubber used for the surface layer of the pressure roller 52, and the like. If a temperature that satisfies these conditions simultaneously is a predetermined temperature Tu, then Tu> Tc. Further, the control temperature T1 of the sheet passing area needs to be lower than the Curie temperature Tc, that is, Tc> T1. Therefore, it is necessary to satisfy Tu> Tc> T1.

  By setting in this way, hot offset can be suppressed and the heat resistance of the member can be secured. However, if the temperature difference between the paper passing area and the non-paper passing area of the recording paper is large, the surface temperature difference between the central portion and both end portions of the pressure roller 52 becomes large. If a difference occurs in the expansion coefficient of silicon rubber or the like on the surface layer of the pressure roller 52 and a difference in the outer diameter of the pressure roller 52 in the axial direction, the formed image may be affected. For this reason, it is desirable to reduce the temperature difference between the sheet passing area and the non-sheet passing area (FIG. 4: temperature difference between the sheet passing area and the non-sheet passing area) as much as possible.

  On the other hand, the control temperature T1 needs to be equal to or higher than the allowable fixing temperature Tm, that is, T1 ≧ Tm. Therefore, the control temperature T1 needs to satisfy Tu> Tc> T1 ≧ Tm.

  For this reason, in order to ensure the heat resistance of the member while having sufficient fixability, it is necessary to set the Curie temperature Tc lower, that is, the control temperature T1 is made closer to the Curie temperature Tc. It is preferable.

  Here, as described above, when the temperature of the paper passing area is lower than the Curie temperature Tc, the paper passing area has magnetic characteristics, and the non-paper passing area has non-magnetic characteristics.

  However, when the control temperature T1 is brought close to the Curie temperature Tc, the Curie temperature Tc itself varies. In addition to this, the thermistor 26a that detects the temperature of the sheet passing area also varies. For example, the Curie temperature Tc of the magnetic shunt metal layer 21a is set to 210 ° C., the control temperature T1 of the sheet passing area is set to 200 ° C., the Curie temperature Tc has a variation of ± 5 ° C., and the thermistor 26a is also ± 5. If there is a variation of ° C., the Curie temperature Tc may actually be about 205 ° C., and the fixing temperature may actually be about 205 ° C.

  In such a case, the paper passing area has non-magnetic characteristics. At this time, since the non-sheet passing region also has nonmagnetic characteristics, the entire magnetic shunt metal layer 21a has nonmagnetic characteristics. Then, the magnetic flux selectively flowing in the paper passing area having magnetic characteristics flows in the entire magnetic shunt metal layer 21a. As a result, the magnetic shunt metal layer 21a continues to generate heat, and the temperature of the paper passing area is increased. There is a risk of the temperature exceeding the Curie temperature Tc. When the paper passing area rises to the Curie temperature Tc or higher, hot offset may occur or the heat resistance of the elastic member layer 52b of the pressure roller 52 may be affected.

  Since such a variation does not appear as a detection result of the thermistor 26a, even if the temperature of the paper passing area is appropriately controlled to the control temperature T1 based on the thermistor 26a, the temperature is actually the Curie temperature Tc. May be exceeded. Then, there is a possibility that an image defect with unknown cause may occur.

  On the other hand, when the temperature of the non-sheet passing area reaches the Curie temperature Tc before the temperature of the sheet passing area, the entire magnetic shunt metal 21a becomes non-magnetic, and when the sheet passing area reaches the Curie temperature, the Curie temperature Tc is reached. A higher predetermined temperature T2 has been reached. This is because the non-sheet passing area where heat is not taken away by the recording paper tends to have a relatively higher temperature than the sheet passing area.

  Therefore, printing is started by controlling the temperature of the paper passing area at the predetermined temperature T1, and the temperature of the non-paper passing area is detected by the thermistor 26b. If the detected temperature of the thermistor 26b is lower than the predetermined temperature T2, the paper passing area is It is judged to have a magnetic property lower than the Curie temperature. If the temperature detected by the thermistor 26b is equal to or higher than the predetermined temperature T2, the entire magnetic shunt metal layer 21a becomes non-magnetic regardless of the temperature detected by the thermistor 26a and the paper passing area exceeds the Curie temperature Tc. Therefore, the control temperature T1 of the sheet passing area is set to a predetermined temperature T3 lower than T1, and the fixing temperature is controlled.

  By lowering the control temperature of the sheet passing area from T1 to T3, which is lower than T1, the sheet passing area can be maintained lower than the Curie temperature Tc and the magnetic characteristics can be maintained. Further, the magnetic shunt metal layer 21a has magnetic characteristics and non-magnetic characteristics, and the temperature of the non-sheet passing region can be returned to the Curie temperature Tc or lower.

  Here, T2 can be set smaller than the predetermined temperature Tu. In this respect, since the Curie temperature Tc can be set low, it is not necessary to increase T2 excessively, and it is possible to sufficiently suppress the hot offset and maintain the heat resistance of the member. On the other hand, T3 needs to be equal to or higher than the fixing allowable temperature Tm. Further, since the control temperature T1 itself can be set high, it is not necessary to make T3 too low, and the toner fixing property can be sufficiently maintained.

  FIG. 5 is a block diagram illustrating an example of a control mechanism of the image forming apparatus according to the present embodiment. Portions common to FIGS. 1 to 3 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes an image forming unit 3, a fixing device 14, an image input unit 32, an AD conversion unit 33, a storage unit 34, a control unit 35, and an operation panel 36.

  When the image forming apparatus 100 is a copying machine, the image input unit 32 includes a scanning optical system equipped with a scanner lamp that illuminates the document during copying and a mirror that changes the optical path of reflected light from the document, and reflected light from the document. The image forming apparatus 100 is a printer. The image forming apparatus 100 includes a condenser lens that collects and forms an image and a CCD that converts the imaged image light into an electric signal. In this case, the reception unit receives image data transmitted from a personal computer or the like. The image signal input from the image input unit 32 is sent to the control unit 35, appropriately performs image processing such as gradation processing, and after being converted into a digital signal by the AD conversion unit 33, an image in the storage unit 34 to be described later. It is sent to the memory 40.

  The image forming unit 3 includes a photosensitive drum 5, a charging unit 4, an exposure unit 7, a developing unit 8, a transfer roller 13, and the like. Based on the image data converted by the AD conversion unit 33 and stored in the image memory 40. Then, an electrostatic latent image is formed on the photosensitive drum 5. The photosensitive drum 5, the transfer roller 13, and the fixing roller 21 (see FIG. 1) in the fixing device 14 are rotationally driven by a main motor (not shown), and the control unit 35 transmits a control signal to the main motor. The rotation and stop of the photosensitive drum 5, the transfer roller 13, and the fixing roller 21 are controlled.

  The storage unit 34 includes an image memory 40, a RAM 41, and a ROM 42, and the image memory 40 stores an image signal input by the image input unit 32 and digitally converted by the AD conversion unit 33, and is stored in the control unit 35. Send it out. The RAM 41 and ROM 42 store an image processing program, processing contents, and the like of the control unit 35. Further, a program (described later) for controlling a high-frequency current applied to the induction heating coil 23 in order to maintain the temperature of the sheet passing area at a predetermined temperature, control temperatures T1 and T3 serving as a reference for the control, and the fixing roller 21 The Curie temperature Tc and the predetermined temperature T2 are also stored.

  The control unit 35 is, for example, a central processing unit (CPU), and generally controls the image input unit 32, the image forming unit 3, the fixing device 14 and the like according to a set program, and is input from the image input unit 32. The image signal is converted into image data by scaling processing or gradation processing as necessary. The exposure unit 7 irradiates a laser beam based on the processed image data, and forms a latent image on the photosensitive drum 5. Further, the control unit 35 also controls each part of the image forming apparatus such as the image input unit 32, the charging unit 4, the exposure unit 7, and the developing unit 8 in accordance with the set program.

  Furthermore, the control unit 35 has a function of receiving an output signal from the thermistor 26b and increasing or decreasing the amount of high-frequency current applied to the induction heating coil 23 based on the predetermined temperature T2 stored in the storage unit 34. ing.

  The operation panel 36 includes an operation unit composed of a plurality of operation keys and a display unit (none of which is shown) for displaying setting conditions, apparatus statuses, and the like. For example, when the image forming apparatus 100 has a facsimile function, it is also used for various settings such as registering a facsimile transmission destination in the storage unit 34 and further reading and rewriting the registered transmission destination. Is done.

  Next, a control example of the image forming apparatus according to the present embodiment will be described with reference to FIGS. 1 to 3 and FIG.

  Here, A4 width recording paper is printed. Printing is started by setting the Curie temperature Tc of the magnetic shunt metal 21a to 210 ° C., for example, and the control temperature T1 of the paper passing area to 200 ° C., for example. As shown in FIG. 3, when A4 recording paper is continuously printed, the temperature detected by the thermistor 26b in the non-sheet passing area becomes high, and the non-sheet passing area of the fixing roller 21 is overheated with respect to the sheet passing area. . In this case, the magnetic shunt metal layer 21a has magnetic characteristics in the paper passing area of the recording paper from which heat is removed, and nonmagnetic characteristics in the non-paper passing area in which heat is not removed.

  Then, when printing is continued and the detected temperature T of the non-sheet passing area by the thermistor 26b becomes a predetermined temperature T2, for example, 230 ° C. or more, the control unit 35 sets the control temperature T1 to T3, which is lower than T1. For example, it is set to 190 ° C. Thereby, the temperature of the paper passing area can be made lower than the Curie temperature Tc. In addition, since the magnetic property region and the nonmagnetic property region are generated in the magnetic shunt metal layer 21a, the non-paper passing region can be returned to the vicinity of the Curie temperature Tc.

  According to this control example, when the temperature of the non-sheet passing area reaches T2, the temperature of the magnetic shunt metal layer 21a in the sheet passing area is equal to or higher than the Curie temperature Tc by changing the control temperature of the sheet passing area from T1 to T3. Can be suppressed. Further, by setting the difference between the control temperature T1 and the Curie temperature Tc to be less than 30 ° C., that is, Tc−T1 <30 ° C., the control temperature T1 can be brought closer to the Curie temperature Tc, and fixing failure occurs. Fear can be avoided.

  When printing recording paper having an A3 width, which is the maximum width of the recording paper, as shown in FIG. 2, since the non-sheet passing area is small, it is difficult to think that the entire magnetic shunt metal layer 21a has nonmagnetic characteristics. . However, even in this case, by performing the same control as described above, the non-magnetic characteristics of the entire magnetic shunt metal layer 21a can be suppressed, and the paper passing area and the non-paper passing area can be maintained below the Curie temperature.

  FIG. 6 is a flowchart showing a control procedure of this control example. The control will be described in accordance with the steps in FIG. 6 with reference to FIGS.

  First, the image forming apparatus 100 is turned on (step S1). After the printing conditions such as the number of prints and the printing magnification are manually set by the user through the operation panel 36 or an input operation from the personal computer, and the paper size is manually set by the input operation or automatically set by the document reading operation, the start button is turned on. Thus, a print start instruction (print signal) is input (step S2).

  When the print signal is input, a high-frequency current is applied to the induction heating coil 23 by the control unit 35 (step S3), and the temperatures of the recording paper passing area and the non-paper passing area in the fixing roller 21 by the thermistors 26a and 26b. Detection is started (step S4), the control unit 35 reads the control temperature T1 from the storage unit 34, sets the control temperature T1 as a reference value, and print processing so that the fixing temperature becomes T1 based on the detection result of the thermistor 26a. Is started (step S5).

  Next, the control unit 35 reads the predetermined temperature T2 stored in the storage unit 34, and first determines whether or not the non-sheet passing region detection temperature T by the thermistor 26b is equal to or higher than the predetermined temperature T2 (step S6). ). When T2 ≦ T, the control temperature T3 having a temperature lower than T1 is read from the storage unit 34, the control temperature T3 is set as a reference value, and the temperature of the paper passing area is T3 based on the detection result of the thermistor 26a. The amount of high-frequency current applied is adjusted so as to satisfy (Step S7), and the printing process is performed.

  If T2> T in step S6, the control unit 35 performs the printing process with the control temperature T1 set as a reference value as it is (step S8). Then, it is determined whether or not a predetermined number of printing processes have been completed (step S9). If printing is continuing, the process returns to step S6 and the same control is performed (steps S6 to S9). In addition, when the number of printed sheets set in step S2 has ended, the control unit 35 ends all the processes.

  In this manner, the thermistor 26b is provided in the non-sheet passing area, the temperature of the sheet passing area is controlled to T1, the insertion of the recording sheet is started, and the control temperature T1 when the detection result of the thermistor 26b becomes T2 or more. By setting T3 to T3, it is possible to set the control temperature T1 at the start of printing to a higher value and bring it closer to the Curie temperature Tc while keeping the temperatures of the paper passing area and the non-paper passing area at or below the Curie temperature. Further, if the control temperature T1 can be set higher, the control temperature T3 can be set higher accordingly, and the influence on the toner fixing property can be further avoided.

  FIG. 7 is a schematic side sectional view of a fixing device according to a second embodiment of the present invention. FIG. 8 is a block diagram illustrating an example of a control mechanism of the image forming apparatus according to the present embodiment. In the present embodiment, an induction heating coil 23 and a core 27 are disposed inside the fixing roller 21, and instead of directly contacting the fixing roller 21 to fix the recording paper, induction is performed at a position facing the surface of the heating roller 22. The heating coil 23 and the core 27 are provided, and the recording paper is fixed by the fixing roller 21 and the pressure roller 52 via the fixing belt 53. Here, for example, the fixing device 14 used for color image formation is used. Other configurations are the same as those in the first embodiment, and thus the description thereof is omitted.

  In the present embodiment, a fixing belt is stretched between the fixing roller 21 and the heating roller 22. An induction heating coil 23 is disposed on the opposite side of the fixing roller 21 across the heating roller 22. For example, the fixing roller 21 is formed by providing sponge silicon rubber constituting the heat-insulating elastic member layer 21b on the outside of a cored bar 21a made of stainless steel. The fixing roller 21 is driven by a driving device (not shown) and rotates so that its peripheral speed is the same as the conveyance speed of the recording paper (recording medium).

  The fixing belt 53 is an endless belt in which an elastic layer made of silicon rubber, fluorine resin, or the like is formed on the surface of a base made of heat-resistant resin such as polyimide. In addition, a rolled metal such as Ni (nickel) or SUS (stainless steel) can be used as the substrate. The fixing belt 53 may be, for example, a single layer or the like, and is not particularly limited. The fixing belt 53 is wound between the fixing roller 21 and the heating roller 22 and given a predetermined tension.

  The induction heating coil 23 has a semicircular shape in which the cross section is open toward the heating roller 22, the radius is larger than that of the heating roller 22, and surrounds the heating roller 22 with a predetermined interval. A bobbin 24 extending in parallel is disposed.

  On the outer surface of the bobbin 24, there is disposed a coil wire 25 that surrounds the periphery in the axial direction (paper surface direction in the drawing) and is spirally wound along the circumferential direction. Further, on the opposite side of the heating roller 22 across the bobbin 24 and the coil wire 25, the cross-section thereof is the same as that of the bobbin 24, the center of the circle is a semicircular core 27 having a radius larger than the bobbin 24 by a predetermined length. However, it is arranged in parallel with the bobbin 24 at a predetermined interval.

  The end of the nearest bobbin 24 and the end of the core 27 are connected to each other by a flat rectangular parallelepiped core 27, and the bobbin 24 and the core 27 are integrated to form a hollow shape. In addition, a flat core 27 is disposed in the direction of the center of the circle from the central portion in the circumferential direction on the outer peripheral surface of the bobbin 24 to the central portion in the circumferential direction on the inner peripheral surface of the core 27, and divides the hollow portion into two. .

  By applying a high frequency current to the coil wire 25, the heating roller 22 can generate heat, and the recording paper can be fixed at the nip portion between the fixing roller 21 and the pressure roller 52 via the fixing belt 53. Since other operations and control methods are exactly the same as those in the first embodiment, description thereof will be omitted.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, the A3 width and A4 width recording sheets have been described in the above embodiment, but the present invention can be applied to other sheets and combinations thereof in exactly the same manner. In the present embodiment, the recording paper is passed through the central portion in the axial direction of the fixing roller 21. However, the recording paper is passed through each size so as to be aligned with the left end or the right end of the fixing roller 21, for example. Also good.

  The image forming apparatus to which the present invention can be applied can be applied to various image forming apparatuses such as a digital copying machine, an analog copying machine, and a laser beam printer, regardless of whether it is for color or monochrome.

  The present invention includes a heat roller based on a magnetic shunt metal, an induction heating coil that generates a working magnetic flux for induction heating the heat roller, and abutting the heat roller directly or via a heat transfer means. A pressure unit that forms a nip portion through which the recording medium is inserted, and a fixing device that fixes a toner image carried on the recording medium at the nip portion. The temperature detecting means for detecting and the temperature of the passing area of the recording medium are controlled to a predetermined temperature T1 lower than the Curie temperature Tc of the magnetic shunt metal and not less than the allowable fixing temperature Tm, and the recording medium is inserted. And when the detection result of the temperature detection means becomes equal to or higher than a predetermined temperature T2 higher than the Curie temperature Tc, the temperature of the passage region is lower than T1 and equal to or higher than the allowable fixing temperature Tm. , In which and a temperature control means for controlling the predetermined temperature T3.

  Thereby, since the possibility that the temperature of the paper passing area exceeds the Curie temperature Tc is suppressed, the control temperature T1 of the paper passing area can be made closer to the Curie temperature Tc, and the Curie temperature Tc itself should be set lower. Can do. Therefore, it is possible to suppress the influence on the image formation such as maintaining the heat resistance of the heat generating member and suppressing the hot offset while sufficiently satisfying the toner fixing property.

  Further, by setting the difference between the control temperature T1 and the Curie temperature to be less than 30 ° C., the control temperature T1 can be made closer to the Curie temperature Tc, and the toner fixing property can be further improved. Can do. In addition, by mounting the above fixing device on the image forming apparatus, it is possible to form an image that maintains the heat resistance of the heat generating member, suppresses hot offset, and the like while sufficiently satisfying the toner fixing property. It becomes.

FIG. 3 is a schematic cross-sectional view showing the fixing device according to the first embodiment of the present invention. FIG. 4 is a plan view of the fixing device according to the present embodiment as viewed from the sheet passing direction, and illustrates a case where a recording sheet having a maximum sheet passing width is passed. FIG. 4 is a plan view of the fixing device according to the present embodiment as viewed from the sheet-feeding direction, and illustrates a case where a small-sized recording sheet is passed. These are figures which show notionally the relation between control temperature of a passage field, and other temperature. These are block diagrams showing an example of a control mechanism of the image forming apparatus according to the present embodiment. These are flowcharts showing an example (control example 1) of the control procedure of the image forming apparatus according to the present embodiment. These are schematic cross-sectional views showing a fixing device according to a second embodiment of the present invention. These are block diagrams showing an example of a control mechanism of the image forming apparatus according to the present embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 14 Fixing device 20 Fixing part 21 Fixing roller 21a Magnetic shunt metal layer 21b High electroconductive nonmagnetic metal layer 21c Fluorine resin layer 22 Heating roller 23 Dielectric heating coil 24 Bobbin 25 Coil wire 26a Thermistor 26b Thermistor (temperature detection means)
35 Control unit (control means)
52 Pressure roller (Pressure means)
53 Fixing belt 100 Image forming apparatus M Extra width W1 Maximum sheet passing width W2 Small size recording sheet width

Claims (3)

A heat roller using a magnetic shunt metal as a base material, an induction heating coil that generates a working magnetic flux for induction heating of the heat roller, and a contact with the heat roller directly or through heat transfer means to insert a recording medium A fixing device for fixing a toner image carried on a recording medium by the nip portion,
Temperature detecting means for detecting the temperature of the non-passing area of the recording medium in the heat roller;
The temperature of the recording medium passing area is controlled to a predetermined temperature T1 lower than the Curie temperature Tc of the magnetic shunt metal and equal to or higher than the allowable fixing temperature Tm, and insertion of the recording medium is started.
When the detection result of the temperature detection unit becomes equal to or higher than a predetermined temperature T2 higher than the Curie temperature Tc, the temperature of the passage region is controlled to a predetermined temperature T3 lower than the T1 and higher than the allowable fixing temperature Tm. Temperature control means to
A fixing device comprising:   The fixing device according to claim 1, wherein a temperature difference between the T1 and the Tc is less than 30 ° C.   An image forming apparatus comprising the fixing device according to claim 1.

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