JP5489939B2 - Fixing heater, fixing device, and image forming apparatus - Google Patents

Description

  The present invention relates to a fixing heater, a fixing device, and an image forming apparatus for heating and fixing a toner image formed on a sheet by an electrophotographic system such as a copying machine, a printer, and a facsimile machine.

  An electrophotographic image forming apparatus that forms an image based on an electrophotographic method can easily form an image having good image quality, and is widely used in a copying machine, a printer, a facsimile machine, a multifunction machine, and the like.

  As a means for performing the fixing process of the electrophotographic image forming apparatus, for example, a heat roller fixing type fixing device is used. The heat roller fixing method includes a fixing roller and a pressure roller. The fixing roller and the pressure roller are a pair of rollers in pressure contact with each other. In a fixing device provided in an image forming apparatus capable of full color printing, a fixing roller having an elastic layer made of, for example, silicone rubber or the like is used. With the roller provided with this elastic layer, the recording medium and the fixing roller can be peeled off without using a peeling claw in the fixing step. However, when the elastic layer is enlarged in order to take a wide nip portion in order to cope with a higher speed, the heat capacity of the elastic layer becomes larger, causing problems such as a delay in warm-up time and an increase in power consumption.

  Therefore, a belt-type fixing method has been devised. In the belt-type fixing method, a heater is directly in contact with a fixing belt having a small heat capacity and an elastic layer, and the back surface of the fixing belt (the surface on the back side with respect to the surface on which the paper abuts). This configuration eliminates the need for a heater to be built in the fixing roller, so the fixing roller can be provided with a thick, low-hardness elastic layer made of sponge rubber or the like, so that a wide fixing nip can be secured and warm-up can be ensured. Shortening and energy saving.

  In the fixing process, when a recording medium having a size smaller than the maximum sheet passing width of the fixing device (hereinafter referred to as a small size paper) is continuously passed, a paper passing portion through which the small size paper passes in the fixing nip portion is Because the temperature is recovered by being heated by the heating means as much as the heat is taken away, while the non-sheet passing portion outside the small size paper is heated by the heater even though the heat is not taken away, A phenomenon occurs in which the temperature of the non-sheet passing portion abnormally increases. When this phenomenon occurs, if normal size paper is passed immediately after that, it may cause high temperature offset due to an abnormal temperature rise portion or paper wrinkles. Therefore, there has been proposed a heater configured to heat only a portion corresponding to the paper size.

  In Patent Document 1, through holes are respectively formed on resistance heating elements at locations corresponding to a plurality of types of paper widths of copy paper, and electrodes are respectively provided at the substrate back side end portions of these through holes. A fixing heater is disclosed in which the length of the heat generating portion of the resistance heating element can be appropriately adjusted by selectively energizing.

  Further, Patent Document 2 includes a plurality of branched resistance heating elements and energization switching means for switching energization at the branch end portions of the resistance heating elements, and the energization of the branch end portions according to the detected paper size. It is disclosed that switching is performed when the resistance heating element is not energized.

Japanese Patent Application Laid-Open No. 8-152795 JP-A-6-348172

  However, in the case of the configuration as in Patent Document 1, the heat generation part corresponding to the small size paper and the heat generation part corresponding to the normal size paper have different inter-electrode resistance values. Different. In general, the power required for the fixing process of normal size paper and small size paper is smaller for small size paper. Therefore, the resistance of the location corresponding to the small size paper must be formed to be larger or the same as that of the normal size paper, leading to a complicated structure of the heater itself and an increase in cost. End up.

  In addition, when the configuration as in Patent Document 2 is adopted, the resistance value of the energized portion greatly changes in conjunction with the paper size, and if the resistance value is set so that a predetermined power can be ensured for the entire heater, it is compatible with small size paper. Since the resistance value of the portion becomes too small, harmful effects such as excess current occur.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a fixing heater, a fixing device, and an image forming apparatus that can adjust the power consumption corresponding to the paper size with a simple configuration without requiring a complicated structure or control. .

A first invention is a fixing heater for fixing a toner by heating a sheet to be conveyed,
A long plate-like substrate whose longitudinal direction is the direction perpendicular to the sheet conveyance, a resistance heating element formed on the substrate, and a sheet width in the direction perpendicular to the sheet conveyance for each sheet size that can be conveyed on the resistance heating element An electrode for energization provided in pairs in the longitudinal direction of the substrate at a distance corresponding to
The electrode pair having the inter-electrode distance corresponding to the paper width is disposed between the electrode pair having the inter-electrode distance corresponding to the paper width larger than the inter-electrode distance. The electrode pair having a distance between electrodes corresponding to the paper width of the paper is energized , and the electrode width, which is the length in the paper transport direction, is changed according to the paper width of each paper size .

According to a second aspect of the present invention, in the fixing heater of the first aspect , when the fixing belt is heated by the resistance heating element and moved to the paper in the paper conveyance direction and fixed, each center of the electrode in the paper conveyance direction is The fixing belt is shifted from the center of the substrate in the sheet conveyance direction to the traveling direction side of the fixing belt.

According to a third aspect of the present invention, in the fixing heater of the first aspect , when the fixing belt is heated by the resistance heating element and moved to the paper in the paper conveyance direction for fixing, the driving member for rotating the fixing belt The electrode width of one electrode of the electrode pair on the side where is attached is larger than the electrode width of the other electrode forming the pair.

According to a fourth invention, in the fixing heater of the first, second or third invention, the electrode width (Wi) of the electrodes, the interelectrode distance (Li) of the electrode pair including the electrodes, and the distance between the electrodes Although it is larger than the distance (Li), in the relationship between the electrode width (Wo) of the electrodes of the electrode pair and the inter-electrode distance (Lo),
Li / Wi ≧ Lo / Wo
The above relational expression is satisfied.

According to a fifth aspect of the present invention, there is provided a fixing heater for fixing toner by heating a sheet to be conveyed, and is formed on a long plate-like substrate having a longitudinal direction in a direction perpendicular to the sheet conveying direction. A resistance heating element, and an electrode for energization provided in a pair in the longitudinal direction of the substrate at a distance corresponding to the sheet width in the direction perpendicular to the sheet conveyance in each sheet size that can be conveyed on the resistance heating element; The electrode pair having an inter-electrode distance corresponding to the paper width is disposed between the electrode pair having an inter-electrode distance corresponding to the paper width larger than the inter-electrode distance and fixing the conveyed paper. In this case, the electrode pair having a distance between the electrodes corresponding to the sheet width of the transport sheet is energized, the electrode width of each electrode is the same, and the electrode and the electrode adjacent thereto are included in different electrode pairs. The electrode and its adjacent The resistance per unit length of the resistance heating element between the electrodes is smaller than the resistance per unit length of the resistance heating element located on the inner side of the electrode pair to which the electrodes belong. It is characterized by.

According to a sixth aspect of the present invention, there is provided a fixing device for fixing the toner by heating the conveyed paper.
The fixing heater according to any one of the first to fourth aspects of the present invention, switching means for switching the connection between the electrode pairs of the fixing heater, and control means for controlling the switching means to connect the electrode pairs and energize them. Prepared,
The control unit connects the electrode pairs having an inter-electrode distance corresponding to a conveyance sheet width by the switching unit.

The invention of seventh switching means for switching a fixing device for heating the sheet to be conveyed to fix the toner, and the fifth inventions of the fixing heater, a connection between the electrode pair of the fixing heater, Control means for controlling the switching means to connect and energize the electrode pairs, and the control means uses the switching means to connect all the electrode station pairs having an inter-electrode distance corresponding to the conveyance paper width or less. It is characterized by connecting.

According to an eighth aspect of the present invention, in the fixing device according to the sixth aspect , the control means applies duty current to the resistance heating element separated by the electrodes so as to uniformly heat the resistance heating element corresponding to the conveyance sheet width. It is characterized by controlling.

According to a ninth aspect , in the fixing device according to the eighth aspect , the control means performs duty energization control of the resistance heating elements corresponding to both ends of the resistance heating elements corresponding to the conveyance sheet width. Features.

Further, the tenth aspect of the present invention includes a fixing belt for fixing an unfixed toner image on the recording medium, a heating member for heating the fixing belt, and a pressure member to assist the fixing pressurizing the fixing belt The heating member includes the fixing heater according to any one of the first to fifth aspects , wherein the fixing belt is formed in an endless shape and is heated by being suspended at least by the fixing heater, The fixing belt is in contact with the entire width direction of the fixing belt.

The invention of eleventh and a one Kano fixing device of the sixth aspect of the present invention from 10 to heat fixing and image forming means for forming a toner image on the conveyed sheet, the formed toner image to the sheet An image forming apparatus.

  The present invention realizes measures for increasing the edge temperature and saving energy by switching the heating region according to the paper size.

  Further, the resistance value (power) of the heat generating region can be adjusted with a simple configuration in which the width of the electrode is changed. Furthermore, since the heat radiation on the driving side increases, the end portion temperature drop is prevented by increasing the heat generation area in the shorter direction than the other side.

  The equal sign of Li / Wi ≧ Lo / Wo means that the resistance values are equal, that is, the supplied power is equal in any region. The inequality sign is the size of the recording medium itself if it has the same thickness and material configuration. Smaller means less heat capacity. That is, since the electric power for heating on the inner side may be smaller than that on the outer side, the inner resistance may be increased. As described above, it is possible to realize a heater that can obtain an amount of heat corresponding to the size by a simple method.

Further, when the width L of the resistance heating element in a certain region and the resistance ρ per unit length of the resistance heating element in that region are given,
(Formula) ρ = α × 1 / L ^ 2
If the heater is configured so as to satisfy (α is a constant), the power consumption per unit length is equal in all regions, and heating can be performed uniformly in the longitudinal direction.

  In addition, by connecting all the electrode station pairs having an inter-electrode distance corresponding to the conveyance paper width or less by the switching means, the resistance between the electrodes is connected in parallel, thereby supporting the center reference with a simple control system. it can.

  Also, by performing duty control, the power consumption per unit length is equal in all regions, and heating can be performed uniformly in the longitudinal direction.

1 is a schematic diagram showing an internal structure of a dry electrophotographic color image forming apparatus according to the present invention. 1 is a cross-sectional view illustrating a fixing device according to a first embodiment. It is a figure which shows the fixing heater which concerns on 1st Embodiment. It is a figure which shows the other fixing heater which concerns on 1st Embodiment. It is a figure which shows the further another fixing heater which concerns on 1st Embodiment. It is a control block diagram showing an image forming apparatus. FIG. 6 is a cross-sectional view illustrating a fixing device according to a second embodiment. It is a figure which shows the fixing heater which concerns on 2nd Embodiment. It is a figure which shows the equalization circuit of the fixing heater which concerns on 2nd Embodiment. It is a figure which shows the fixing heater which concerns on 3rd Embodiment. It is a figure which shows the fixing heater provided with many electrodes. It is a figure which shows the equalization circuit of the fixing heater of FIG.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 shows an internal structure of a dry electrophotographic color image forming apparatus according to the present invention. An optical system unit E, four sets of visible image forming units pa, pb, pc, pd, an intermediate transfer belt 11 are shown. A secondary transfer unit 14, a fixing device 15, an internal paper feed unit 16, and a manual paper feed unit 17.

  In the visible image forming unit pa, a charging unit 103a, a developing unit 102a, and a cleaning unit 104a are arranged around a photosensitive member 101a serving as a toner image carrier. The primary transfer unit 13 a is disposed via the intermediate transfer belt 11. The other three sets of visible image forming units pb, pc, and pd have the same configuration as that of the visible image forming unit pa. The developing units of each unit include yellow (Y), magenta (M), cyan (C), Each color toner of black (B) is accommodated.

  The optical system unit E is arranged so that the data from the light source 4 reaches the four sets of photoconductors 101a, 101b, 101c, and 101d. The intermediate transfer belt 11 is disposed without being bent by the tension rollers 11a and 11b. The waste toner box 12 is disposed on the tension roller 11b side, and the secondary transfer unit 14 is disposed on the tension roller 11a side. The fixing unit 15 is disposed downstream of the secondary transfer unit 14.

The image forming process is as follows.
After the surface of the photosensitive drum 101a is uniformly charged by the charging unit 103a, the surface of the photosensitive drum 101a is laser-exposed according to image information by the optical system unit E to form an electrostatic latent image. The charging unit 103a employs a charging roller system in order to charge the surface of the photosensitive drum 101a uniformly and without generating ozone as much as possible. Thereafter, a toner image is developed on the electrostatic latent image on the photosensitive drum 101a by the developing unit 102a, and this visualized toner image is intermediated by the temporary transfer unit 13a to which a bias voltage having a polarity opposite to that of the toner is applied. Transfer onto the transfer belt 11. The other three sets of visible image forming units pb, pc, and pd operate in the same manner and are sequentially transferred onto the intermediate transfer belt 11. The toner image on the intermediate transfer belt 11 is conveyed to the secondary transfer unit 14, and is separately supplied to the recording paper fed from the paper feed roller 16 a of the internal paper feed unit 16 or the paper feed roller 17 a of the manual paper feed unit 17. Transfer is performed by applying a bias voltage having a polarity opposite to that of the toner. The toner image on the recording paper is conveyed to the fixing device 15, heated by the heat generating fixing belt, fused onto the recording paper, and discharged to the outside.

<First Embodiment>
Next, the fixing device according to the first embodiment will be described with reference to FIG. FIG. 2 is a cross-sectional view seen from the side of the paper.

  The fixing device 15 in this embodiment includes an endless fixing belt 21, a fixing roller 22, a heater 23, a thermistor 24 for detecting the temperature of the fixing belt 21, and a pressure roller 25. The fixing belt 21 is wound around a fixing roller 22 and a heater 23. Although the heater 23 will be described in detail later, it is held by a holding member (not shown).

  The fixing belt 21 includes a base material layer, an elastic layer, and a release layer. In the present embodiment, the belt is an endless belt having an outer shape of 50φ and a width of 320 mm without being mounted.

  The base material layer is formed in a hollow cylindrical shape using an insulating heat-resistant resin such as polyimide, and the thickness is desirably about 50 μm.

The elastic layer is formed of heat-resistant fluoro rubber or silicone rubber on the insulating base layer, and has a thickness of 150 μm in this embodiment.
Further, a release layer made of a fluorine-based synthetic resin is formed on the elastic layer. In this embodiment, a PFA tube having a thickness of 30 μm is used.

  The heater 23 is disposed on the back side of the fixing belt 21 (the side not in contact with the paper), and generates heat when energized to heat the fixing belt 21. The heated portion of the fixing belt 21 is moved by the fixing roller 22 to heat and fix the paper sandwiched between the fixing roller 22 and the pressure roller 25. The detailed structure of the heater 23 will be described later.

  The thermistor 24 detects the temperature of the surface of the fixing belt 21 (the surface in contact with the paper) using a contact type or a non-contact type, and performs temperature control based on the detected temperature. In the case of a contact thermistor, the surface of the fixing belt is scraped due to friction with the fixing belt 21, which affects the fixed image and lowers the function of the fixing belt 21 itself. Therefore, a non-contact thermistor such as an infrared absorption type is mounted. desirable. Further, by mounting a plurality of thermistors 24 in the width direction of the fixing belt 21, temperature control is performed with higher accuracy. Although the temperature of the surface of the fixing belt 21 is detected in FIG. 2, the temperature of the fixing belt 21 is detected from the inside of the belt by mounting the thermistor 24 inside the fixing belt 21, or the heater 23 is directly heated. It may be detected.

  The fixing roller 22 has a substantially cylindrical shape, and has a two-layer structure in which a core metal and an elastic layer are formed from the center of the substantially cylindrical shape toward the outer periphery. A metal such as iron, stainless steel, aluminum or copper, or an alloy thereof is used for the core metal. A rubber material having heat resistance such as silicon rubber or fluorine rubber is suitable for the elastic layer. In this embodiment, the fixing roller 22 has a diameter of 30 mm. Stainless steel with a diameter of 20 mm is used for the core metal, and silicon sponge rubber with a thickness of 5 mm is used for the elastic layer. Support shafts on both sides of the fixing roller 22 are rotatably supported by a main frame (not shown) via bearings.

  The pressure roller 25 has a substantially cylindrical shape and has a three-layer structure in which a metal core, an elastic layer, and a release layer are formed from the center of the substantially cylindrical shape toward the outer periphery. A metal such as iron, stainless steel, aluminum copper, or an alloy thereof is used for the core metal. A rubber material having heat resistance such as silicon rubber or fluorine rubber is suitable for the elastic layer. A fluororesin such as PFA (copolymer with tetrafluoroethylene alkyl vinyl ether) or PTFE (polytetrafluoroethylene) is suitable for the release layer. In this embodiment, the outer diameter of the pressure roller 25 is 30.1 mm, and iron (STKM) having an inner diameter of 26 mm and a wall thickness of 1 mm is used for the core metal. Silicon elastic rubber having a thickness of 1 mm is used for the elastic layer. A PFA tube having a thickness of 50 μm is used for the release layer. The pressure roller 25 is disposed on the fixing roller 22 so as to face the fixing roller 22 via the fixing belt 21, and is rotated by a motor (not shown). Support shafts on both sides of the pressure roller 25 (not shown) are rotatably supported by a support frame (not shown) via a bearing. One support shaft is connected to the motor shaft of the motor via a gear. When the rotation of the motor is controlled by a control device (not shown), the pressure roller 25 rotates. The pressure roller 25 and the fixing belt 21 form a nip portion 26 by applying a predetermined pressure (for example, 400 N) to the support frame. In the present embodiment, the width of the nip portion 26 is 7 mm.

  With the above configuration, when the pressure roller 25 is driven to rotate in the R2 direction in FIG. 2, the fixing belt 21 that is in pressure contact is driven to rotate in the R1 direction, which is opposite to the R2 direction. The fixing process is performed by feeding the paper 28 on which the unfixed toner 27 is transferred in the direction P. Accordingly, the moving direction (R1 direction) of the fixing belt 21 and the paper transport direction (P direction) are the same in the nip portion 26 for fixing.

Next, details of the configuration of the heater of this embodiment will be described with reference to FIGS. 3 to 5. As shown in FIG. 3A, the heater substrate 31 is made of a material having heat resistance and electrical insulation, such as alumina (Al 2 O 3) ceramics, quartz glass, etc., and is formed in an elongated plate shape (long plate shape). ). In order to improve the slidability with the fixing belt 21, as shown in FIG. 3B, the substrate 31 is formed in an arc shape in the short direction (direction perpendicular to the longitudinal direction). Note that a straight plate shape may be used instead of the arc shape.
Here, the longitudinal direction of the substrate 31 is arranged in the same direction as the conveyance perpendicular direction of the paper to be fixed, and the short direction of the substrate is arranged in the same direction as the conveyance direction of the paper. Accordingly, the sheet width of the sheet to be conveyed is the length in the direction perpendicular to the sheet conveyance.

  A resistance heating element layer 32 that generates heat when energized is uniformly formed on the substrate 31. The resistance heating element layer 32 is made of a fired silver / palladium alloy powder or a coating of a resistance heating element coating made of carbon.

  Furthermore, the electrode pairs 33a, 33b, 33c, and 33d for energizing the resistance heating element layer 32 are arranged as electrode pairs having a distance between electrodes corresponding to the paper width in accordance with the paper size. The electrode pair having the inter-electrode distance corresponding to the paper width is disposed between the electrode pair having the inter-electrode distance corresponding to the paper width larger than the inter-electrode distance. Accordingly, there is an electrode pair 33b between the electrode pair 33a and an electrode pair 33c between the electrode pair 33b. Thus, by energizing the electrode pair corresponding to the sheet width of the transport sheet, only the resistance heating element layer 32 corresponding to the sheet width generates heat. In this case, although not shown, a switch (switching means) for energizing connection is provided for each electrode pair, and switching according to the sheet width is performed by turning this on / off. That is, the distance between the electrodes is 297 mm for A4 size paper width, 182 mm for B5 size paper width, and so on. With this configuration, only necessary portions can be heated, and abnormal temperature rise at the end portion can be prevented and power consumption can be reduced.

However, when the electrode width is the same with respect to the short direction of the substrate 31 (paper conveyance direction or fixing belt conveyance direction),
R = ρs × L ÷ W [Ω] (1)
As the distance between the electrodes of the electrode pair decreases, the resistance value of the resistance heating element 32 decreases. When the same voltage is applied, the power consumption varies depending on the heating location (where R is the electrode). The resistance value between the electrodes, ρs is the sheet resistance of the resistance heating layer, L is the distance between the electrodes, and W is the width of the electrodes in the short-side direction of the substrate).

  Therefore, in the present embodiment, the above-mentioned problem is solved by changing the width of the electrode in the short side direction of the substrate (paper conveyance direction or fixing belt conveyance direction) depending on the position where the electrode is arranged.

In the present embodiment, the resistance heating element layer 32 is formed using a material mainly composed of carbon and having a sheet resistance of 0.68Ω / □. Now, in FIG. 3A, when LA [mm] is A4 size paper width, that is, 297 mm, and the heater substrate short direction length WA of the electrode 33a is 20 mm, the resistance value between the electrodes 33a is 10Ω, and 100V A power of 1000 W can be obtained by applying a voltage. Subsequently, when the distance LB between the electrodes 33b provided between the electrodes 33a is 182 mm, which is the vertical width of the B5 size paper, the width WB of the electrode 33b is 12.2 mm, as can be seen from the relationship of the expression (1). Then, the resistance value between the electrodes 33b becomes 10Ω. Therefore, if the same voltage of 100 V is applied, power consumption of 1000 W can be obtained. Similarly, if LC is 148 mm in length of A5 size paper, the same result as above can be obtained by setting WC to 10 mm.
In this way, by setting the electrode width so that the resistance value is common even if the length of the heater substrate in the short direction of each heater pair is different, heating between the electrode pairs becomes a constant power consumption, and energization heating control is performed. It becomes easy.

  Further, as shown in FIG. 4, the center of each electrode in the short direction of the substrate (paper conveyance direction or fixing belt conveyance direction) may be shifted from the short direction center of the substrate 31. That is, the centers of the electrodes 34a, 34b, and 34c are shifted in a direction approaching the nip portion 26 in FIG. 2 (fixing belt conveyance destination direction). As a result, the heating region gets closer to the nip portion 26, the loss of heat dissipation during rotation can be reduced, and further improvement in fixability and energy saving can be realized.

  In general, the temperature balance in the width direction of the fixing belt 21 may be lost due to convection in the machine or heat radiation from the surface of the fixing belt 21. In particular, the drive side to which the drive gear is attached has a greater heat dissipation than the other, and a temperature difference is noticeable immediately after warm-up and the like, causing a cold offset. As a countermeasure, as shown in FIG. 5, the width WA of the drive-side electrodes 35a, 35b, 35c is made larger than the width WA 'of the other electrodes 36a, 36b, 36c, that is, WA> WA'. The wider the electrode width, the larger the heat generation area, and even if the amount of heat radiation increases, a balance can be achieved and the temperature of the entire belt can be balanced.

  Although not shown, in the heater 23 of the present embodiment, the surface contacting the fixing belt 21 may be coated with fluorine (such as PTFE coating) to improve the slidability with the fixing belt 21. Further, in this embodiment, the resistance heating element 32 is formed on the surface of the substrate 31 opposite to the side in contact with the fixing belt 21, but is formed on the surface of the substrate 31 in contact with the fixing belt 21. You may do it.

  Next, the operation of this embodiment will be described with reference to the block diagram of FIG.

  The image forming apparatus 1 is, for example, a multifunction machine including a scanner 41, a printer 42, and a peripheral device 43. The image forming apparatus 1 reads a document image 51, converts the read document image into an appropriate electrical signal, and generates image data. An image processing unit 52, an image forming unit 53 that prints out the generated image data, an electrode pair connection switching (that is, switching of a heating portion of the heater) and a fixing control unit 54 that controls driving of the fixing unit 15, and post-processing A peripheral device control unit 55 that controls a peripheral device 43 such as a finisher or a sorter that is a device, an input unit 56 that is an operation unit of the image forming apparatus 1, a display unit 57, a storage unit 58 that stores data, and the entire device are controlled. A control unit 59 is provided.

  Immediately after turning on the power or returning from the sleep mode, the fixing control unit 54 starts driving rotation of the pressure roller 25 and simultaneously selects an electrode that generates heat from the entire heater 23, and heats the entire heater 23 by energization. The fixing belt 21 is heated.

  The fixing control unit 54 continuously energizes the entire resistance heating element 32 while detecting the temperature by the mounted thermistor 24, and when the set temperature reaches a preset temperature recorded in the storage unit 58, the fixing control unit 54 adjusts the temperature to a predetermined temperature. Enter standby mode. That is, the resistance heating element 32 is heated to a predetermined temperature by energizing an electron pair corresponding to the maximum sheet width.

  When there is a print signal, the fixing control unit 54 connects the electrode pairs corresponding to the sheet width by switches according to the sheet size, and sets the resistance heating element 32 between the electrode pairs to the fixing set temperature. As described above, the fixing process is started by energizing and heating in accordance with the detected temperature of the thermistor 24. This fixing set temperature is also stored in the storage unit 58, and the energization value that is controlled to be the fixing set temperature corresponding to the temperature detected by the thermistor 24 is also stored in the storage unit 58. Thus, only a portion necessary for fixing can be heated with a simple configuration, and uniform heating can be performed over the entire heating area.

  When continuously passing small-size paper, if the temperature of the portion through which small-size paper does not pass is lower than a predetermined fixing temperature (10 ° C. in this embodiment), the heater 23 is switched to overall heating, and the fixing belt 21 Correct the temperature drop at the end of the. As a result, even when there is a normal size print signal immediately after the small size paper, the fixing process can be performed immediately without waiting for the temperature of the fixing belt 21 to rise.

<Second Embodiment>
FIG. 7 is a cross-sectional view showing the fixing device of this embodiment.
The basic structure is the same as that of the fixing device of FIG. 2, and the same parts are denoted by the same reference numerals and the description thereof is saved. The fixing device shown in FIG. 7 is different from the fixing device shown in FIG.

  Next, details of the configuration of the heater of the present embodiment will be described with reference to FIG. The substrate 61 of the heater 60 is made of a material having heat resistance and electrical insulation made of, for example, alumina (Al 2 O 3) ceramics or quartz glass, and has an elongated plate shape (long plate shape). Unlike the first embodiment, this embodiment has a straight plate shape, but may be formed in an arc shape in the same manner as the first embodiment in order to improve the slidability with the belt.

  A resistance heating element layer 62 that generates heat when energized on the substrate 61 is uniformly formed. The resistance heating element layer 62 is made of a fired silver / palladium alloy powder or a coating of a resistance heating element coating made of carbon.

  Furthermore, the electrode pairs 63a and 63b for energizing the resistance heating element layer 62 are configured such that only the portion corresponding to the paper width generates heat according to the paper size. Similar to the first embodiment, the electrode pair having an inter-electrode distance corresponding to the paper width is disposed between the electrode pair having an inter-electrode distance corresponding to the paper width larger than the inter-electrode distance. That is, in addition to the electrode pair 63b at both ends, the electrode pair 63a is provided in the middle portion of the resistance heating element 62, so that the heating region 1 and the heating region 2 can be divided. Further, by providing the electrodes in the intermediate portion and connecting the electrode pairs in this way, the resistance heating elements 62 in each region are in an electrically parallel relationship as in the equivalent circuit of FIG.

  In the present embodiment, the heater substrate 61 has a total length of 330 mm, and the length in the paper conveyance direction (short direction length) is 20 mm. The resistance heating element 62 is formed on the substrate 61 with a length of 320 mm, which is the same length as the fixing belt 21, and the length in the short direction is 10 mm. The electrodes 63a and 63b are 5 mm × 10 mm, and are arranged on the resistance heating element 62 so that L2 is 320 mm and L1 is 200 mm. The sheet resistance of the resistance heating element 62 is 1Ω / □, and the resistance value R1 of the heating region 64 is 20Ω. Heated with a power consumption of 500 W by an AC power supply 76 of 100 V, the power consumption per unit length in the longitudinal direction is 2.5 W / mm.

  On the other hand, the resistance value R2 of the heating region 65 is 12Ω, and when the same AC voltage of 100 V is used, the power consumption is 833.3 W, and the power consumption per unit length in the longitudinal direction is 6.9 W / mm, which is larger than the heating region. This makes it difficult for the fixing belt 21 to be heated uniformly.

  Therefore, the energization amount to the heating region 65 is adjusted by duty control. The duty control is power control based on the ratio of the energization time during which energization is turned on and the non-energization time during which it is turned off. In the present embodiment, the switch 68 is turned on for a time corresponding to 36% of the interval period (lighting period), and the remaining time corresponding to 64% is turned off. As a result, the average power consumption of the heating region is 300 W, the average power consumption per unit length in the longitudinal direction is 2.5 W / mm, and the average power consumption per unit length of the entire heater is equal. Therefore, the fixing belt 21 can be heated uniformly, and an unfixed image can also be heated uniformly.

  In addition, when fixing an unfixed image on a small size paper such as a B5 size vertical thread (width is 182 mm), only the switch 67 is turned on and the switch 68 is always turned off during the paper passing.

  These connection and energization controls are performed by the fixing controller 54 in the block diagram of FIG. Control from when the power is turned on until the set temperature is reached is the same as in the first embodiment. When there is a print signal, the fixing control unit 54 connects all the electrode station pairs having an inter-electrode distance corresponding to the conveyance paper width or less by the switching means, and the resistance value of the resistance heating element between the adjacent electrodes. Are connected in parallel. Thus, duty energization control is performed so that the power consumption per unit length in the longitudinal direction of the resistance values connected in parallel in the conveyance sheet width is constant. Since the fixing set temperature and the energization value to be set to the fixing set temperature according to the detected temperature of the thermistor 24 are stored in the storage unit 58, the fixing control unit 54 performs temperature control according to these values in the storage unit 58. I do. Thus, only a portion necessary for fixing can be heated with a simple configuration, and uniform heating can be performed over the entire heating area.

<Third Embodiment>
Next, a third embodiment will be described. Since the configurations of the image forming apparatus and the fixing device are the same as those in the second embodiment, a detailed description thereof will be omitted with reference to FIG. 10. As in the first embodiment, the heater substrate 81 is made of a material having heat resistance and electrical insulation, such as alumina (Al 2 O 3) ceramics or quartz glass, and has an elongated plate shape (long plate shape).

  Resistance heating element layers 72 and 73 that generate heat when energized on the substrate 71 are uniformly formed. The resistance heating element layers 72 and 73 are made of a fired silver / palladium alloy powder or a resistance heating element coating made of carbon.

  Furthermore, the electrodes 74a and 74b for energizing the resistance heating element layers 72 and 73 are configured to generate heat only in a portion corresponding to the paper width in accordance with the paper size. Similar to the first embodiment, the electrode pair having an inter-electrode distance corresponding to the paper width is disposed between the electrode pair having an inter-electrode distance corresponding to the paper width larger than the inter-electrode distance. In other words, in addition to the electrodes 74b at both ends, the electrode 74a is provided in the middle portion of the resistance heating element, whereby the heating region 75 and the heating region 76 can be divided. Further, by providing the electrodes in the intermediate portion and connecting the electrode pairs in this way, the resistance heating elements 72 and 73 in each region are electrically in parallel as in the equivalent circuit of FIG.

  In the present embodiment, the heater substrate 71 has a total length of 330 mm, and the length in the paper conveyance direction (short direction length) is 20 mm. The resistance heating elements 72 and 73 are formed on the substrate 71 with a length of 320 mm, which is the same length as the fixing belt 21, and the length in the short direction is 10 mm. The electrodes 74a and 74b are 5 mm × 10 mm, and are arranged on the resistance heating elements 72 and 73 so that L2 is 320 mm and L1 is 200 mm. The sheet resistance of the resistance heating element 72 in the heating region 75 is 1Ω / □, and the resistance value R1 of the heating region 75 is 20Ω. Heated with a power consumption of 500 W by an AC power supply of 100 V, the power consumption per unit length in the longitudinal direction is 2.5 W / mm.

  The sheet resistance of the resistance heating element 73 in the heating region 76 is adjusted to be equal to the power consumption per unit length of the heating region 75. If the sheet resistance of the heating region 76 is 2.8Ω / □, the resistance value of the resistance heating element 73 in the heating region 76 is 33.3Ω. Therefore, the power consumption per unit length is 2.5 W / mm, and the entire heater can be uniformly heated without requiring complicated control.

  These connection and energization controls are performed by the fixing controller 54 in the block diagram of FIG. Control from when the power is turned on until the set temperature is reached is the same as in the first embodiment. When there is a print signal, the fixing control unit 54 connects all the electrode station pairs having an inter-electrode distance corresponding to the conveyance paper width or less by the switching means, and the resistance value of the resistance heating element between the adjacent electrodes. Are connected in parallel. The fixing control unit 54 energizes the electrodes. The sheet resistance is adjusted so that the power consumption per unit length in the longitudinal direction of the resistance value connected in parallel in the conveyance paper width is constant. It becomes. Since the fixing set temperature and the energization value to be set to the fixing set temperature according to the detected temperature of the thermistor 24 are stored in the storage unit 58, the fixing control unit 54 performs temperature control according to these values in the storage unit 58. I do. Thus, only a portion necessary for fixing can be heated with a simple configuration, and uniform heating can be performed over the entire heating area.

  Further, as shown in FIGS. 11A and 11B, the resistance heating element 82 may be divided into heating regions by three or more electrodes 83a, 83b, 83c, and 83d. Since the resistance heating element 82 in FIG. 11A is formed with a constant sheet resistance, duty control is performed as in the second embodiment to keep power consumption constant. In FIG. 11B, the sheet resistance of the resistance heating element 82 between adjacent electrodes may be adjusted to make the power consumption constant. FIG. 12 shows an equivalent circuit when the resistance heating element is divided into i heating regions by electrodes. A power source 91, switches 92a to 92i, and resistors R1 to Ri are included.

  As shown in FIGS. 11 and 12, the connection of the fixing control unit and the control of energization shown in FIG. 6 are controlled in the second and third embodiments even when the resistance heating element is divided by a large number of electrodes. Can be done as described in.

DESCRIPTION OF SYMBOLS 15 Fixing device 21 Fixing belt 22 Fixing roller 23 Heater 24 Thermistor 25 Pressure roller 26 Nip part 27 Unfixed toner 28 Paper 31 Substrate 32 Resistance heating element layer 33a, 33b, 33c, 33d Electrode pair 54 Fixing control part 58 Storage part 60 Heater 61 Substrate 62 Resistance heating element layer 63a Electrode pair 63a, 63b Electrode pair 64, 65 Heating area 67 Switch 68 Switch 71 Substrate 72, 73 Resistance heating element layer 74a, 74b Electrode 75, 76 Heating area 78 Switch 81 Substrate 82 Resistance heating Body 83a, 83b, 83c, 83d Electrode 91 Power supply

Claims (11)

In the fixing heater for fixing the toner by heating the conveyed paper,
A long plate-like substrate whose longitudinal direction is the direction perpendicular to the sheet conveyance;
A resistance heating element formed on the substrate;
On the resistance heating element, electrodes for energization provided in pairs in the longitudinal direction of the substrate at a distance corresponding to the sheet width in the direction perpendicular to the sheet conveyance in each sheet size that can be conveyed,
With
The electrode pair having an interelectrode distance corresponding to the paper width is disposed between the electrode pair having an interelectrode distance corresponding to a paper width larger than the interelectrode distance,
When fixing the transported paper, energize the electrode pair having a distance between electrodes corresponding to the paper width of the transport paper ,
A fixing heater characterized in that an electrode width, which is a length in a paper transport direction, is changed according to a paper width of each paper size . When the fixing belt is heated by the resistance heating element and moved to the sheet in the sheet conveyance direction for fixing, the center of the electrode in the sheet conveyance direction is on the side of the substrate in the sheet conveyance direction from the center of the substrate in the sheet conveyance direction. The fixing heater according to claim 1 , wherein the fixing heater deviates. When the fixing belt is heated by the resistance heating element to move to the paper in the paper conveyance direction and is fixed, the electrode width of one electrode of the electrode pair on the side where the driving member for rotating the fixing belt is attached The fixing heater according to claim 1 , wherein is larger than an electrode width of the other electrode of the pair. The electrode width (Wi) of the electrode, the interelectrode distance (Li) of the electrode pair including the electrode, and the electrode width (Wo) of the electrode of the electrode pair that is larger than the interelectrode distance (Li) In relation to the distance (Lo),
Li / Wi ≧ Lo / Wo
Fixing heater according to claim 1, 2 or 3 characterized by satisfying the relational expression. In the fixing heater for fixing the toner by heating the conveyed paper,
A long plate-like substrate whose longitudinal direction is the direction perpendicular to the sheet conveyance;
A resistance heating element formed on the substrate;
On the resistance heating element, electrodes for energization provided in pairs in the longitudinal direction of the substrate at a distance corresponding to the sheet width in the direction perpendicular to the sheet conveyance in each sheet size that can be conveyed,
With
The electrode pair having an interelectrode distance corresponding to the paper width is disposed between the electrode pair having an interelectrode distance corresponding to a paper width larger than the interelectrode distance,
When fixing the transported paper, energize the electrode pair having a distance between electrodes corresponding to the paper width of the transport paper,
The electrode width of each electrode is the same,
When the electrode and the electrode adjacent to the electrode are included in different electrode pairs, the resistance per unit length of the resistance heating element between the electrode and the adjacent electrode is the electrode pair to which the electrodes belong. Fixing heater you being smaller than the unit per length resistance of the resistance heating element on the inside towards the inter-electrode distance is smaller among. In a fixing device for fixing toner by heating conveyed paper,
A fixing heater according to any one of claims 1 to 4 ,
Switching means for switching the connection between the electrode pairs of the fixing heater;
Control means for controlling the switching means to connect the electrode pair and energize;
With
The fixing device is characterized in that the control unit connects the electrode pairs having an inter-electrode distance corresponding to a conveyance sheet width by the switching unit. In a fixing device for fixing toner by heating conveyed paper,
A fixing heater according to claim 5 ;
Switching means for switching the connection between the electrode pairs of the fixing heater;
Control means for controlling the switching means to connect the electrode pair and energize;
With
The fixing unit is characterized in that the control unit connects all the electrode station pairs having an inter-electrode distance corresponding to a conveyance sheet width or less by the switching unit. The fixing device according to claim 6 , wherein the control unit performs duty energization control on the resistance heating element divided by the electrodes so as to uniformly heat the resistance heating element corresponding to the conveyance sheet width. The fixing device according to claim 8 , wherein the control unit performs duty energization control on the resistance heating elements corresponding to both ends of the resistance heating elements corresponding to the conveyance sheet width. A fixing belt for fixing an unfixed toner image on a recording medium;
A heating member for heating the fixing belt;
A pressure member that presses the fixing belt to assist fixing;
With
The heating member includes the fixing heater according to any one of claims 1 to 5 ,
The fixing belt is formed in an endless shape and heated by being suspended by at least the fixing heater,
The fixing device, wherein the fixing heater is in contact with the entire width direction of the fixing belt. Image forming means for forming a toner image on the conveyed paper;
A fixing device according to any one of The formed toner image from the claims 6 to heat fixed to the sheet 10,
An image forming apparatus comprising: