JP7397405B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus.

2. Description of the Related Art Electrophotographic image forming apparatuses that form images using toner are known as image forming apparatuses such as copying machines and printers.

Generally, an electrophotographic image forming apparatus is equipped with a fixing device that fixes a toner image transferred to paper onto the paper {see, for example, Patent Document 1 (Japanese Patent Laid-Open No. 6-75453)}. . The fixing device includes a heating member such as a heater that heats the paper, and the image is fixed by melting the toner on the paper by the heat of the heating member.

Incidentally, the fixability of toner to paper depends on the temperature at which the toner is heated. Therefore, in order to obtain uniform fixing performance, it is desirable that the temperature of the heating member that heats the paper is uniform throughout the paper.

However, depending on the characteristics of the heating member and how it is used, it may be difficult to maintain the temperature of the heating member uniformly across the width of the paper. In such a case, there will be variations in the fixability of the image, and there is a concern that there will be variations in the image density after the fixing process.

In order to solve the above problems, the present invention includes an image carrier, a charging means for charging the surface of the image carrier, and an electrostatic latent image by exposing the charged surface of the image carrier to light based on image information. an exposure means for forming a toner image on the image bearing member; a developing means for forming a toner image by attaching toner to the electrostatic latent image on the image bearing member; a transfer means for transferring the toner image on the image bearing member to a recording medium; a fixing unit that fixes the toner image on the recording medium by heating the recording medium; and calculating the gradation rate of the image for each of a plurality of blocks obtained by dividing an image forming area on the recording medium across the width direction of the recording medium. gradation rate calculation means for comparing the gradation rates calculated by the gradation rate calculation means between the blocks; and exposure controlling the exposure amount of the exposure means for each block. and a control unit, comparing the gradation rate of each block, and if there are blocks with the same gradation rate, the temperature of the fixing unit is relatively high among the blocks with the same gradation rate. The image forming apparatus is characterized in that the exposure amount of a block is lowered than a preset exposure amount.

According to the present invention, it is possible to suppress variations in image density caused by variations in heating temperature.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. FIG. 1 is a schematic configuration diagram of a fixing device according to an embodiment. FIG. 3 is a perspective view of the fixing device. FIG. 3 is an exploded perspective view of the fixing device. FIG. 3 is a perspective view of a heating unit included in the fixing device. FIG. 3 is an exploded perspective view of the heating unit. FIG. 2 is a plan view of a heater according to the present embodiment. FIG. 3 is an exploded perspective view of the heater. FIG. 3 is a perspective view showing a state in which a connector is connected to the heater. FIG. 3 is a plan view of the heater. FIG. 7 is a diagram showing the amount of heat generated by the feeder line for each block when all the resistance heating elements generate heat. FIG. 7 is a diagram showing the amount of heat generated by the power supply line for each block when only some of the heat generating parts generate heat. FIG. 3 is an image diagram showing a comparison of the amount of toner adhesion after fixing in a high temperature area and a low temperature area. 7 is a graph showing the relationship between toner adhesion amounts before and after fixing. FIG. 7 is an image diagram showing a comparison of the amount of toner after fixing when the amount of toner is increased in a low temperature area. FIG. 7 is an image diagram showing a comparison of the amount of toner after fixing when the amount of toner is reduced in a high temperature area. FIG. 2 is a block diagram showing a control system for adjusting image density. FIG. 3 is a diagram showing an example of a plurality of blocks obtained by dividing an image forming area. FIG. 3 is a diagram illustrating an example of divided areas in which a plurality of blocks are further divided in the paper conveyance direction. 7 is a graph showing the relationship between fixing temperature and toner adhesion amount after fixing for each gradation rate. It is a figure which shows the example which provided the temperature sensor in all the resistance heating elements. It is a figure which shows the example which provided the temperature sensor in some resistance heating elements. 7 is a graph showing the relationship between exposure amount and toner adhesion amount before fixing. FIG. 3 is a diagram showing a control flow of exposure amount. FIG. 3 is a diagram for explaining a miniaturized heater. It is a figure which shows the structure of another heater. FIG. 3 is a diagram showing the configuration of another fixing device. FIG. 7 is a diagram showing the configuration of another fixing device. FIG. 7 is a diagram showing the configuration of yet another fixing device.

Hereinafter, the present invention will be explained based on the accompanying drawings. In each drawing for explaining the present invention, components such as members and components having the same function or shape are given the same reference numerals as much as possible so that they can be easily distinguished. Omitted.

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention.

The image forming apparatus 100 shown in FIG. 1 includes an image forming section 200, a transfer section 300, a fixing section 400, a recording medium supply section 500, and a recording medium ejection section 600.

The image forming section 200 is provided with four image forming units 1Y, 1M, 1C, and 1Bk and an exposure device 6. Each of the image forming units 1Y, 1M, 1C, and 1Bk is configured to be detachable from the main body of the image forming apparatus. Furthermore, each of the image forming units 1Y, 1M, 1C, and 1Bk has basically the same configuration except that it accommodates developers of different colors of yellow, magenta, cyan, and black corresponding to the color separation components of a color image. It is. Specifically, each of the image forming units 1Y, 1M, 1C, and 1Bk includes a photoreceptor 2, which is an image carrier that carries an image on its surface, and a charging roller 3, which is a charging means that charges the surface of the photoreceptor 2. The photoreceptor 2 includes a developing device 4 that is a developing device that forms a toner image on the photoreceptor 2, and a cleaning blade 5 that is a cleaning device that cleans the surface of the photoreceptor 2. Further, the exposure device 6 is an exposure means (writing means) that exposes the charged surface of the photoreceptor 2 based on image information.

The transfer unit 300 is provided with a transfer device 8 as a transfer unit that transfers an image onto paper, which is a recording medium. Note that the recording medium on which the image is formed (transferred) may be paper (including plain paper, cardboard, thin paper, coated paper, label paper, envelope, etc.), or a resin sheet such as an OHP sheet. . The transfer device 8 includes an intermediate transfer belt 11 , four primary transfer rollers 12 , and a secondary transfer roller 13 . Each primary transfer roller 12 is in contact with a different photoreceptor 2 via the intermediate transfer belt 11. As a result, a primary transfer nip is formed between the intermediate transfer belt 11 and each photoreceptor 2, where the intermediate transfer belt 11 and each photoreceptor 2 are in contact with each other. On the other hand, the secondary transfer roller 13 contacts one of the plurality of rollers stretching the intermediate transfer belt 11 via the intermediate transfer belt 11, and forms a secondary transfer nip between the secondary transfer roller 13 and the intermediate transfer belt 11. There is.

The fixing unit 400 is provided with a fixing device 9 as a fixing means for fixing an image on paper. The configuration of the fixing device 9 will be explained in detail later.

The recording medium supply unit 500 is provided with a paper feed cassette 14 that accommodates paper P, and a paper feed roller 15 that feeds the paper P from the paper feed cassette 14.

The recording medium ejection section 600 is provided with a pair of paper ejection rollers 17 that eject the paper to the outside of the image forming apparatus, and a paper ejection tray 18 on which the paper ejected by the paper ejection rollers 17 is placed.

Next, the printing operation of the image forming apparatus 100 according to this embodiment will be described with reference to FIG.

When an instruction to start a printing operation is given, the photoreceptors 2 of each image forming unit 1Y, 1M, 1C, and 1Bk and the intermediate transfer belt 11 start rotating. Further, as the paper feed roller 15 rotates, the paper P is sent out from the paper feed cassette 14. The fed paper P comes into contact with a pair of timing rollers 16 and is temporarily stopped.

In each of the image forming units 1Y, 1M, 1C, and 1Bk, first, the surface of the photoreceptor 2 is charged to a uniform high potential by the charging roller 3. Next, the exposure device 6 exposes the surface (charged surface) of each photoconductor 2 based on the image information of the document read by the document reading device or the print image information instructed to print from the terminal. As a result, the potential of the exposed portion decreases, and an electrostatic latent image is formed on the surface of each photoreceptor 2. Then, toner is supplied from the developing device 4 to this electrostatic latent image, and a toner image is formed on each photoreceptor 2. When the toner images formed on each photoreceptor 2 reach the primary transfer nip (the position of the primary transfer roller 12) as each photoreceptor 2 rotates, they are transferred onto the rotating intermediate transfer belt 11 so as to overlap one another. be done. In this way, a full-color toner image is formed on the intermediate transfer belt 11. Further, after the toner image is transferred from the photoreceptor 2 to the intermediate transfer belt 11, the toner remaining on each photoreceptor 2 is removed by the cleaning blade 5.

The toner image transferred onto the intermediate transfer belt 11 is conveyed to the secondary transfer nip (the position of the secondary transfer roller 13) as the intermediate transfer belt 11 rotates, and onto the paper P conveyed by the timing roller 16. transcribed into. Then, the paper P onto which the toner image has been transferred is conveyed to the fixing device 9, and the toner image is fixed onto the paper P by the fixing device 9. Thereafter, the paper P is discharged to the paper discharge tray 18 by the paper discharge roller 17, and a series of printing operations is completed.

The above explanation of the printing operation is about the operation when forming a full-color image, but it is also possible to use one of the four image forming units to form a monochrome image, or use two or three image forming units. It is also possible to use the imaging unit to form two or three color images.

Next, the configuration of the fixing device 9 according to this embodiment will be described.

As shown in FIG. 2, the fixing device 9 according to the present embodiment includes a fixing belt 20, a pressure roller 21, a heater 22, a heater holder 23, a stay 24, and a temperature sensor 19.

The fixing belt 20 is a rotating member disposed on the unfixed image bearing surface side of the paper P, and is a fixing member that fixes the unfixed image to the paper P. The fixing belt 20 is, for example, an endless belt member having a cylindrical base having an outer diameter of 25 mm and a thickness of 40 to 120 μm. In addition to polyimide, the material of the base body may be a heat-resistant resin such as PEEK, or a metal material such as nickel or SUS. Further, in order to enhance durability and ensure mold releasability, a mold release layer made of a fluororesin such as PFA or PTFE may be provided on the outer peripheral surface of the base. Furthermore, an elastic layer made of rubber or the like may be provided between the base and the release layer. Furthermore, a sliding layer made of polyimide, PTFE, or the like may be provided on the inner peripheral surface of the base.

Pressure roller 21 is a facing member arranged to face the outer peripheral surface of fixing belt 20 . Further, the pressure roller 21 is also a pressure member that is pressed against the outer peripheral surface of the fixing belt 20 to form a nip portion N between the pressure roller 21 and the fixing belt 20 . The pressure roller 21 has an outer diameter of 25 mm, for example, and includes a core made of iron, an elastic layer made of silicone rubber provided on the outer circumference of the core, and a fluorine rubber layer provided on the outer circumference of the elastic layer. It is composed of a roller or the like having a mold release layer made of resin.

The heater 22 is a heating member that is disposed inside the fixing belt 20 and heats the fixing belt 20 and the paper via the fixing belt 20. In this embodiment, the heater 22 includes a plate-shaped base material 50, a first insulating layer 51 provided on the base material 50, a conductive layer 52 provided on the first insulating layer 51, and a conductive layer 52 provided on the first insulating layer 51. and a second insulating layer 53 covering the. The conductor layer 52 has a heat generating section 60.

The base material 50 is made of, for example, a metal material such as stainless steel (SUS), iron, or aluminum. Moreover, as the material of the base material 50, it is also possible to use ceramic, glass, etc. in addition to metal materials. When the base material 50 is made of an insulating material such as ceramic, the first insulating layer 51 between the base material 50 and the conductor layer 52 can be omitted. On the other hand, metal materials have excellent durability against rapid heating and are easy to process, so they are suitable for reducing costs. Among metal materials, aluminum and copper are particularly preferable because they have high thermal conductivity and are less likely to cause temperature unevenness. Additionally, stainless steel has the advantage that it can be manufactured at a lower cost than these.

Each of the insulating layers 51 and 53 is made of an insulating material such as heat-resistant glass, for example. Additionally, ceramic, polyimide, or the like may be used as these materials. Further, an additional insulating layer may be provided on the surface of the base material 50 opposite to the surface on which the first insulating layer 51 and the second insulating layer 53 are provided.

In this embodiment, the heat generating part 60 is arranged closer to the nip part N than the base material 50, but on the contrary, the base material 50 may be arranged closer to the nip part N than the heat generating part 60. . However, in that case, the heat of the heat generating section 60 will be transferred to the fixing belt 20 via the base material 50, so it is desirable that the base material 50 be made of a material with high thermal conductivity such as aluminum nitride. .

Further, in this embodiment, in order to increase the efficiency of heat transfer from the heater 22 to the fixing belt 20, the heater 22 is arranged so as to be in direct contact with the inner circumferential surface of the fixing belt 20. Further, the present invention is not limited to this, and the heater 22 may be arranged so as to be in non-contact with the fixing belt 20 or in indirect contact with the fixing belt 20 via a low-friction sheet or the like. Further, the contact point of the heater 22 with the fixing belt 20 may be the outer circumferential surface of the fixing belt 20. However, in order to avoid deterioration in fixing quality due to scratches on the outer circumferential surface of the fixing belt 20, it is desirable that the surface that the heater 22 contacts is the inner circumferential surface of the fixing belt 20.

The heater holder 23 is a holding member that holds the heater 22 inside the fixing belt 20. Since the heater holder 23 tends to reach a high temperature due to the heat of the heater 22, it is desirable that the heater holder 23 is made of a heat-resistant material. In particular, when the heater holder 23 is made of a heat-resistant resin with low thermal conductivity such as LCP or PEEK, heat transfer from the heater 22 to the heater holder 23 is suppressed while ensuring the heat resistance of the heater holder 23. It is possible to heat the fixing belt 20 efficiently.

The stay 24 is a reinforcing member arranged inside the fixing belt 20. By supporting the surface of the heater holder 23 opposite to the surface on the nip portion N side by the stay 24, bending of the heater holder 23 due to the pressing force of the pressure roller 21 is suppressed. As a result, a nip portion N having a uniform width is formed between the fixing belt 20 and the pressure roller 21. In order to ensure its rigidity, the stay 24 is preferably formed of a ferrous metal material such as SUS or SECC.

The temperature sensor 19 is a temperature detection means for detecting the temperature of the heater 22. By controlling the output of the heater 22 based on the detection result of the temperature sensor 19, the temperature of the fixing belt 20 is maintained at a desired temperature (fixing temperature). The temperature sensor 19 may be either a contact type or a non-contact type. For example, as the temperature sensor 19, a known temperature sensor such as a thermopile, thermostat, thermistor, or NC sensor can be used.

In the fixing device 9 according to the present embodiment, when the printing operation is started, power is supplied to the heater 22, so that the heat generating section 60 generates heat, and the fixing belt 20 is heated. Further, the pressure roller 21 is rotationally driven, and the fixing belt 20 starts to rotate in a driven manner. Then, when the temperature of the fixing belt 20 reaches a predetermined target temperature (fixing temperature), as shown in FIG. By conveying the unfixed toner to the nip portion N, the unfixed toner is heated and pressurized, and the toner image is fixed to the paper P.

FIG. 3 is a perspective view of the fixing device 9 according to this embodiment, and FIG. 4 is an exploded perspective view thereof.

As shown in FIGS. 3 and 4, the fixing device 9 according to this embodiment includes a device frame 40 formed in a rectangular frame shape. The device frame 40 includes a first device frame 25 that integrally has a pair of side walls 28 and a front wall 27, and a second device frame 26 that has a rear wall 29. The first device frame 25 and the second device frame 26 are assembled by engaging a plurality of engagement protrusions 28a provided on a pair of side walls 28 with a plurality of engagement holes 29a provided in a rear wall 29. It will be done.

The fixing belt 20 and the pressure roller 21 are supported by a pair of side walls 28 . For this reason, each side wall portion 28 is provided with an insertion groove 28b through which the rotating shaft of the pressure roller 21 and the like are inserted. The insertion groove 28b is open at one end (the rear wall 29 side), and has an abutting portion that is not open at the opposite end. A bearing 30 that rotatably supports the rotating shaft of the pressure roller 21 is provided at this abutting portion. When the pressure roller 21 is supported by each side wall 28 , the drive transmission gear 31 as a drive transmission member provided at one end of the pressure roller 21 in the axial direction is exposed to the outside of the side wall 28 . It will be placed in As a result, when the fixing device 9 is mounted on the image forming apparatus main body, the drive transmission gear 31 is connected to a gear provided on the image forming apparatus main body, and becomes in a state in which the driving force from the drive source can be transmitted. Further, instead of the drive transmission gear 31, a drive transmission member such as a pulley or a coupling mechanism that stretches the drive transmission belt may be used.

A pair of support members 32 that support the fixing belt 20, the stay 24, and the like are provided at both ends of the fixing belt 20 in the longitudinal direction. Each support member 32 is formed with a guide groove 32a. As shown in FIG. 4, when the pair of support members 32, the fixing belt 20, the stay 24, the heater holder 23, and the heater 22 are assembled, the guide groove 32a of each support member 32 is inserted into the insertion groove 28b of each side wall portion 28. The fixing belt 20 , the stay 24 , the heater holder 23 , and the heater 22 are supported by the side walls 28 by assembling each supporting member 32 to each side wall 28 along the edge of the fixing belt 20 , stay 24 , heater holder 23 , and heater 22 . Further, each support member 32 is biased by a pair of springs 33 as biasing members provided between the support member 32 and the rear wall portion 29, so that the fixing belt 20 is pressed against the pressure roller 21, and the nip part is formed.

Further, the rear wall portion 29 is provided with a hole portion 29b serving as a positioning portion for positioning the fixing device main body with respect to the image forming apparatus main body. On the other hand, the main body of the image forming apparatus is provided with a protrusion 101 (see FIG. 4) as a positioning section. When the protrusion 101 is inserted into the hole 29b of the fixing device 9, the protrusion 101 and the hole 29b are fitted, and the fixing device main body is positioned with respect to the image forming apparatus main body. Note that the position where the hole 29b is provided is preferably a position closer to one end of the rear wall portion 29 than the center in the longitudinal direction. By providing the holes 29b in such positions, expansion and contraction in the longitudinal direction due to temperature changes is allowed on the end side where the holes 29b are not provided, and it is possible to suppress distortion of the device frame 40. .

FIG. 5 is a perspective view of a heating unit in which the heater 22 and the like are supported by a pair of support members 32, and FIG. 6 is an exploded perspective view of the heating unit.

As shown in FIG. 5, the heater 22 and the heater holder 23 are elongated members that extend in the left-right direction in FIG. The heater 22 and the heater holder 23 are incorporated into the fixing device, and when the fixing device is installed in the image forming apparatus, the longitudinal direction of the heater 22 and the heater holder 23 is in the width direction U (hereinafter referred to as " (Sometimes referred to as "the width direction of the recording medium.") Further, as shown in FIG. 6, like the heater 22 and the heater holder 23, the stay 24 is also arranged longitudinally in the recording medium width direction U. In this specification, "recording medium width direction," "heater longitudinal direction," "substrate longitudinal direction," and "fixing belt longitudinal direction" all mean the same direction.

As shown in FIGS. 5 and 6, the heater holder 23 is provided with a rectangular housing recess 23a for housing the heater 22. As shown in FIGS. The accommodation recess 23a is formed to have substantially the same shape and size as the heater 22. However, the longitudinal dimension L2 of the accommodation recess 23a is set to be slightly longer than the longitudinal dimension L1 of the heater 22. Therefore, even if the heater 22 extends in its longitudinal direction due to thermal expansion, interference between the heater 22 and the housing recess 23a can be avoided.

The pair of support members 32 includes a C-shaped belt support portion 32b that is inserted inside the fixing belt 20 and supports the fixing belt 20, and a C-shaped belt support portion 32b that is inserted into the inside of the fixing belt 20 and supports the fixing belt 20, and a C-shaped belt support portion 32b that is in contact with an end surface of the fixing belt 20 to move the fixing belt 20 in the longitudinal direction ( It has a flange-like belt regulating part 32c that regulates the shift of the heater holder 23 and the stay 24, and a support recess 32d into which the heater holder 23 and the stay 24 are inserted and supported near both ends in the longitudinal direction. The fixing belt 20 is supported by a so-called free belt method in which no tension is applied in the circumferential direction (belt rotation direction) when the belt is not rotating by inserting the belt support portions 32b at both ends in the longitudinal direction. Ru.

Further, as shown in FIGS. 5 and 6, a positioning recess 23e serving as a positioning portion is provided on one end side of the heater holder 23 from the center in the longitudinal direction. The heater holder 23 and the support member 32 are positioned by fitting the fitting portion 32e of the left support member 32 in FIGS. 5 and 6 into the positioning recess 23e. On the other hand, the right support member 32 in FIGS. 5 and 6 is not provided with a fitting portion 32e, and is not positioned with respect to the heater holder 23 in the longitudinal direction. In this way, by positioning the heater holder 23 with respect to the support member 32 only on one side in the longitudinal direction of the heater holder 23, expansion and contraction of the heater holder 23 in the longitudinal direction due to temperature changes is allowed.

Further, as shown in FIG. 6, step portions 24a are provided near both ends of the stay 24 in the longitudinal direction to restrict movement of the stay 24 with respect to each support member 32. Each stepped portion 24a abuts against the support member 32, thereby restricting movement of the stay 24 in the longitudinal direction with respect to the support member 32. However, at least one of these step portions 24a is arranged with a gap (backlash) in between with respect to the support member 32. In this manner, at least one stepped portion 24a is disposed with a gap between it and the support member 32, thereby allowing the stay 24 to expand and contract as the temperature changes.

FIG. 7 is a plan view of the heater 22 according to this embodiment, and FIG. 8 is an exploded perspective view thereof.

As shown in FIG. 8, on the base material 50 of the heater 22, a plurality of resistance heating elements 59 constituting the heating section 60 are arranged with the first insulating layer 51 interposed therebetween. The resistance heating elements 59 are arranged in a line across the longitudinal direction Z of the base material 50, which is also the width direction U of the recording medium. The conductor layer 52 is provided with a plurality of resistance heating elements 59, a plurality of electrode parts 61, and a plurality of power supply lines (conductive parts) 62. Each resistance heating element 59 is electrically connected to any two of the plurality of electrode sections 61 via a plurality of power supply lines 62 . As shown in FIG. 7, the entirety of each resistance heating element 59 and most of each power supply line 62 are covered with the second insulating layer 53 to ensure insulation. Furthermore, since the resistance heating elements 59 are arranged at intervals, an insulating region (second insulating layer 53) is interposed between adjacent resistance heating elements 59. On the other hand, each electrode part 61 is hardly covered with the second insulating layer 53 and is exposed so that a connector described later can be connected thereto.

The resistance heating element 59 can be formed, for example, by applying a paste prepared by mixing silver palladium (AgPd), glass powder, etc. onto the base material 50 by screen printing or the like, and then firing the base material 50. . Further, as the material of the resistance heating element 59, a resistance material such as a silver alloy (AgPt) or ruthenium oxide (RuO ₂ ) may be used.

The electrode portion 61 and the power supply line 62 are made of a conductor having a resistance value smaller than that of the resistance heating element 59. For example, the electrode portion 61 and the power supply line 62 are formed by screen printing a material such as silver (Ag) or silver palladium (AgPd) on the base material 50.

FIG. 9 is a perspective view showing a state in which a connector 70 as a power supply member is connected to the heater 22.

As shown in FIG. 9, the connector 70 includes a housing 71 made of resin and a plurality of contact terminals 72 provided on the housing 71. Each contact terminal 72 is composed of a leaf spring. Further, a power supply harness 73 is connected to each contact terminal 72.

As shown in FIG. 9, the connector 70 is attached so as to sandwich the heater 22 and heater holder 23 together. The heater 22 and heater holder 23 are thereby held together by the connector 70. In addition, in this state, the tips (contact portions 72a) of each contact terminal 72 of the connector 70 are elastically contacted (press-contacted) with the corresponding electrode portions 61, so that each contact terminal 72 and each electrode portion 61 are connected to each other. are electrically connected. Further, the connector 70 is similarly connected to the electrode portion 61 at the end opposite to the longitudinal end of the heater 22 shown in FIG. This allows power to be supplied to the heat generating section 60 from the power source provided in the image forming apparatus via the connector 70.

Hereinafter, the configuration of the heater 22 according to this embodiment will be explained in more detail based on FIG. 10.

As shown in FIG. 10, the heater 22 according to this embodiment includes seven resistance heating elements 59A to 59G, three electrode parts 61A to 61C, and four power supply lines 62A to 62D connecting these. It is provided. Of the three electrode parts 61A to 61C, two of the electrode parts 61A and 61C are arranged closer to one end (on the left side in FIG. 10) of the base material 50 in the longitudinal direction Z than each of the resistance heating elements 59A to 59G. One electrode portion 61B is arranged closer to the other end of the base material 50 in the longitudinal direction Z (on the right side in FIG. 10) than each of the resistance heating elements 59A to 59G. Each of the resistance heating elements 59A to 59G is electrically connected to one of two electrode parts 61A and 61C arranged on one end side and one electrode part 61B arranged on the other end side. There is.

Specifically, among the seven resistance heating elements 59A to 59G, each of the resistance heating elements 59B to 59F other than both ends is connected in parallel to the first electrode portion 61A via the first feed line 62A, and is connected to the second feed line 61A in parallel. It is connected in parallel to the second electrode section 61B via an electric wire 62B. On the other hand, the resistance heating elements 59A and 59G at both ends are connected in parallel to the third electrode portion 61C via the third feed line 62C or the fourth feed line 62D, and are connected to the second electrode portion 61C via the second feed line 62B. It is connected in parallel to the electrode section 61B.

With such a connection structure, in this embodiment, the first heat generating section 60A is made up of each of the resistance heating elements 59B to 59F other than both ends, and the first heat generation section is made up of each of the resistance heating elements 59A and 59G at both ends. It is possible to control the heat generation of the two heat generating parts 60B independently of each other. Specifically, when a voltage is applied to the first electrode section 61A and the second electrode section 61B to generate a potential difference between the two electrode sections 61A and 61B, each of the resistance heating elements 59B to 59F other than both ends is energized. , only the first heat generating section 60A generates heat. On the other hand, when a voltage is applied to the third electrode section 61C and the second electrode section 61B to generate a potential difference between the two electrode sections 61C and 61B, each of the resistance heating elements 59A and 59G at both ends is energized. Only the second heat generating section 60B generates heat. Further, when a voltage is applied to all the electrode parts 61A to 61C to generate a potential difference between the first electrode part 61A and the second electrode part 61 and between the third electrode part 61C and the second electrode part 61B, respectively. Since all the resistance heating elements 59A to 59G are energized, both the first heating section 60A and the second heating section 60B generate heat. For example, when passing a relatively small width paper of A4 size (paper passing width: 210 mm) or less, only the first heating section 60A generates heat, and when passing a relatively large paper of A3 size (paper passing width: 297 mm) or more. When passing paper of a width size, by generating heat in the second heat generating part 60B in addition to the first heat generating part 60A, it is possible to create a heat generating area according to the width of the paper.

Here, temperature variations (temperature distribution deviations) occurring in the heater 22 according to the present embodiment will be explained.

Generally, in a heater in which a resistive heating element as described above is connected to an electrode part via a power supply line, when the resistance heating element is made to generate heat, the power supply line also generates a small amount of heat due to energization of the power supply line. Therefore, depending on the heat generation distribution of the power supply line, there is a possibility that the temperature distribution of the heater may vary. In particular, as the speed of image forming apparatuses increases, if the current flowing through the heating element is increased in order to increase the amount of heat generated, the amount of heat generated in the power supply line also increases, and the effect thereof cannot be ignored.

In FIG. 11, when a 20% current flows through all the resistance heating elements 59A to 59G, each power supply line 62A, 62B, 62D generated in each block partitioned for each resistance heating element 59A to 59G. Shows the calorific value and its total value. Here, if the direction Y (see FIG. 10) that intersects the longitudinal direction Z along the surface of the base material 50 on which the resistance heating element 59 is provided is referred to as the "lateral direction" of the base material 50, this embodiment In the configuration, the portions of each power supply line 62A, 62B, and 62D that extend in the transverse direction Y are short, and the amount of heat generated in that portion is negligible, so it is ignored, and only the amount of heat generated in the portions that extend in the longitudinal direction Z is calculated. are doing. Further, since the amount of heat generation (W) is expressed by the following formula (1), the amount of heat generation shown in the table of FIG. 11 is conveniently calculated as the square of the current (I) flowing through each power supply line. Therefore, the calculated calorific value is just a simply calculated value and is different from the actual calorific value.

The method for calculating the amount of heat generated will be explained using the first and second blocks in FIG. 11 as an example. In the first block, the current flowing through the first power supply line 62A is 100%, and the current flowing through the fourth power supply line 62D is 100%. Since it is 20%, the total value of each square, 10400 (10000+400), is the total amount of heat generated by the power supply lines in the first block. In addition, in the second block, the current flowing through the first power supply line 62A is 80%, the current flowing through the second power supply line 62B is 20%, and the current flowing through the fourth power supply line 62D is 20%, so the square of these The total value of 7200 (6400+400+400) is the total amount of heat generated by the power supply lines in the second block. In addition, the amount of heat generated in other blocks is calculated in the same manner.

The graph in FIG. 11 shows the total calorific value of each block on the vertical axis. As can be seen from this graph, the total amount of heat generated by each feeder line is large in the blocks on both ends, and on the contrary, it is low in the blocks in the center. In addition, the total amount of heat generated by each feeder line in blocks that are symmetrical about the center (for example, the first block and the seventh block) is also different. As described above, since there is variation in the heat generation distribution of the power supply line over the longitudinal direction Z of the base material, this variation causes variation in the heat generation distribution of the heater. As a result, the fixability of the toner is affected, resulting in variations in image density after fixing, and there is a possibility that image quality may deteriorate.

Furthermore, such temperature variations due to heat generation in the power supply line occur not only when all the resistance heating elements are made to generate heat (as shown in Figure 11), but also when some of the resistance heating elements are made to generate heat. It is possible. Particularly, as heaters become smaller and image forming apparatuses become faster, if unintended shunts occur in the power supply line, temperature variations may become noticeable. In addition, unintended shunts can occur when the resistance value of the power supply line increases as a result of reducing the width of the power supply line in the width direction of the heater in order to make the heater smaller, or when the image forming device is made faster. Therefore, when the resistance value of the resistive heating element is decreased in order to increase the amount of heat generated by the resistive heating element, it is more likely to occur. In other words, if the resistance value of the power supply line and the resistance value of the resistive heating element become relatively close due to miniaturization and speed increase, current may be applied to paths that were not previously energized (unintentional shunt may occur). (possible) state.

For example, as shown in FIG. 12, when only each of the resistance heating elements 59B to 59F (first heating section 60A) other than both ends is energized, the current passing through the second resistance heating element 59B from the left in the figure is An unintended shunt may occur in which the current also flows to the side opposite to the second electrode portion 61B (to the left in the figure) at the branch portion X of the second feeder line 62B. The shunted current passes through the resistance heating element 59A at the left end in FIG. 12, and further passes through the resistance heating element 59G at the right end via the third power supply line 62C, the third electrode section 61C, and the fourth power supply line 62D. , joins the second power supply line 62B.

In this way, the unintended branch flow reaches the second power supply line 62B from the branch portion X through the path shown by the dashed line K3 in FIG. Further, such unintended shunting may occur if the conductive path of the heater 22, such as the heater 22 according to the present embodiment, connects each of the resistance heating elements 59B to 59F (first heating section 60A) other than both ends and the first electrode section. 61A, and a second electrode portion 61B extending from each resistance heating element 59B to 59F other than both ends in the first direction S1 (right direction in FIG. 12) of the longitudinal direction of the heater 22. A second conductive path K2 is connected to the second conductive path K2, and a second conductive path K2 is branched from the second conductive path K2 to a second direction S2 (left direction in FIG. 12) opposite to the first direction S1, and a second conductive path K2 is connected to the This can occur if the configuration has at least two conductive paths K2 or a third conductive path K3 connected to the second electrode portion 61B. In the present embodiment, the third conductive path K3 includes a part of the second power supply line 62B (the part on the left side in FIG. 12 from the branch part X), the third power supply line 62C, and the fourth power supply line 62C. In addition to the electric wire 62D, each resistance heating element 59A, 59G (second heating part 60B) at both ends and a third electrode part 61C are included, but the third conductive path K3 does not connect the resistance heating element or the electrode part. Even if there is only a feeder line that does not contain a power supply, unintended shunts may occur.

The table and graph in FIG. 12 show the amount of heat generated in each of the power supply lines 62A, 62B, and 62D for each block and the total value thereof when an unintended shunt occurs. In this example, when 20% of the current flows evenly to each of the resistance heating elements 59B to 59F other than both ends, some of the current is diverted by 5% at the branch part X, and the block that generates heat (the second The amount of heat generated by each power supply line 62A, 62B, and 62D for each block (block to sixth block) is calculated. Note that the method for calculating the calorific value is the same as the method explained in the example shown in FIG.

As shown in the table and graph in FIG. 12, in this case as well, the total calorific value of the feeder line is large in the blocks on both ends, and on the contrary, it is low in the block on the center side, causing variations. However, in the case of FIG. 12, contrary to FIG. 11, the temperature of the block on the left side of the graph is higher than that of the block on the right side of the graph. Although FIGS. 11 and 12 show that the current flows in one direction, the current that flows through the heater 22 may be direct current or alternating current. In any case, if such variations in the temperature distribution of the power supply line occur, variations will occur in the image density after fixing, and there is a possibility that the image quality will deteriorate.

Therefore, in the image forming apparatus according to the present embodiment, the following measures are taken in order to suppress variations in image density caused by variations in the temperature distribution of the heater as described above.

As described above, if there are variations in the temperature distribution of the heater, the toner fixing performance is affected. Specifically, since the toner is heated at a high temperature in the portion of the heater where the temperature is high, high fixability is obtained and the toner is difficult to peel off from the paper. On the other hand, in areas where the toner temperature is low, the toner is heated at a low temperature, resulting in poor fixing properties and the toner becoming easily peeled off from the paper. Therefore, as shown in FIG. 13, which visualizes the toner adhesion amount, even if the same amount (same density) of toner T is attached to the paper P and the fixing process is performed, if there is variation in the temperature distribution of the heater, Since the toner T is more easily peeled off from the paper P in the lower temperature part L than in the higher temperature part H, less toner remains on the paper P. As a result, the image density decreases in the low-temperature portion L, causing variations in image density.

Here, as a method of increasing the image density, there is a method of increasing the amount of toner adhering to the paper before fixing, that is, the amount of toner transferred from the photoreceptor, intermediate transfer belt, etc. to the paper in advance. As shown in FIG. 14, the amount of toner adhering to the paper before fixing and the amount of toner adhering to the paper after fixing have a substantially proportional relationship. Therefore, if the amount of toner adhering before fixing is increased, the amount of toner adhering after fixing also tends to increase.

However, since the amount of toner attached after fixing depends on the heating temperature of the toner, as shown in FIG. The maximum amount remains unchanged. Therefore, even if the amount of toner adhered to the low-temperature portion L is increased, more toner will only be peeled off from the paper, and the image density will not increase.

On the other hand, as shown in FIG. 16, in the high-temperature portion H, by reducing the toner adhesion amount before fixing from t0 to t1, it is possible to reduce the toner adhesion amount after fixing (lower the image density). It is. Therefore, in the present invention, paying attention to this point, by reducing the amount of toner adhesion in the high-temperature area H in advance, the density difference with the low-temperature area L can be reduced, and variations in image density can be suppressed. ing.

FIG. 17 is a block diagram showing a control system for adjusting image density.

As shown in FIG. 17, the image forming apparatus according to the present embodiment includes a gradation processing unit 41, a gradation rate calculation unit 42, a gradation rate comparison unit 43, a toner adhesion amount calculation unit 44, and a toner adhesion amount calculation unit 44. It includes an amount comparison means 45 and an exposure control means 46.

The gradation processing means 41 is a means for performing gradation processing on image information input from a document reading device or a terminal. For the input image information, a predetermined gradation is appropriately set based on information set in advance for each type of image such as text or a photograph and for each model of the image forming apparatus.

The gradation rate calculating means 42 is a means for calculating the gradation rate of the image for each of a plurality of blocks on the paper based on the gradation information from the gradation processing means 41. In this embodiment, as shown in FIG. 18, the largest image forming area J on the paper is divided into seven blocks J1 to 7 in the width direction of the paper for each resistance heating element 59A to 59G of the heater 22. The block is divided into blocks J7, and the gradation rate is calculated for each of these blocks J1 to J7. Note that the number of divided blocks is not limited to seven, but can be arbitrarily set to at least two or more. The gradation rate is determined as follows.

First, as shown in FIG. 19, the seven blocks J1 to J7, which are divided for each of the resistance heating elements 59A to 59G, are further divided into a plurality of (for example, seven) areas across the paper conveyance direction V. The gradation rate is calculated for each divided area g. The gradation rate for each divided area g can be calculated using the following formula (2).

"X gradation per unit pixel" in the above formula (2) means the gradation per unit pixel in the printed portion within one divided area. The term "X gradation" is used here because gradation generally varies depending on the type of input image such as text, photographs, and graphs, and gradation varies depending on the image design ( This is because it cannot be determined uniformly because it varies depending on the toner color, image processing method, etc.). Therefore, for "X gradation", the optimum value set for each model is used depending on the type of image.

Further, the "number of printed pixels" in the above formula (2) is the number of pixels of the portion printed within the same one divided area. Furthermore, "maximum gradation per unit pixel" and "number of pixels in the entire divided area" mean the maximum gradation in the same one divided area and the total number of pixels.

In order to calculate the gradation rate for each of the blocks J1 to J7, first, the gradation rate for each divided area is calculated using the above equation (2). Then, among the gradation rates of each divided area calculated within the same block, the maximum gradation rate is specified as the gradation rate of that block.

In addition, if the image formed in one divided area includes text, photographs, or graphs, the "X gradation per unit pixel" in the above formula (2) should be changed to the type of text, photograph, or graph. The gradation rate for each divided area may be calculated using the following formula (3). Note that in this case, the "maximum gradation per unit pixel" in equation (3) is constant regardless of the type of input image.

Furthermore, if the "maximum gradation per unit pixel" is set to a different value for each character, photo, or graph, the gradation rate for each divided area can be calculated using formula (4) below. .

The gradation rate comparing means 43 is a means for comparing the gradation rates calculated by the gradation rate calculating means 42 among the blocks J1 to J7. Comparison of gradation rates can be performed for all blocks J1 to J7, or for some blocks.

The toner adhesion amount calculation means 44 calculates the temperature of each block J1 to J7 based on the gradation rate calculated by the gradation rate calculation means 42 and the temperature of the heater 22 detected by the temperature sensor 19 shown in FIG. This is means for calculating the amount of toner adhering to the corresponding paper after fixing. As shown in FIG. 20, the amount of toner adhering to the paper after fixing varies depending on the temperature of the paper heated by the heater (fixing temperature) and the gradation rate of the image. In FIG. 20, the amount of toner adhesion after fixing is displayed separately for each gradation rate of the image, and for any gradation rate, as the temperature (fixing temperature) increases, the amount of toner adhesion after fixing increases. is increasing. The toner adhesion amount calculation means 44 determines which gradation rate (graph) in FIG. 20 the image of each block J to J7 has from the calculated gradation rate, and further determines which gradation rate (graph) in FIG. 20 the image of each block J to J7 has. From the temperature of J7, the toner adhesion amount after fixing corresponding to that temperature is calculated.

Furthermore, in this embodiment, as shown in FIG. 21, the temperature sensors 19 are connected to all One each is provided corresponding to the resistance heating elements 59A to 59G. Furthermore, by detecting the temperature of some blocks, if it is possible to calculate the temperature (estimated value) of other blocks based on the detected temperature information, as in the example shown in FIG. The temperature sensor 19 may be provided only on the resistance heating elements 59A, 59D, and 59G. In this case, the number of temperature sensors 19 installed can be reduced, making it possible to achieve downsizing and cost reduction. Alternatively, the toner adhesion amount may be calculated by predicting all the temperatures of each block without providing the temperature sensor 19 at all.

The toner adhesion amount comparison means 45 is a means for comparing the toner adhesion amounts after fixing calculated by the toner adhesion amount calculation means 44 between the blocks J1 to J7. Comparison of toner adhesion amounts after fixing can be performed for all blocks J1 to J7, or for some blocks.

The exposure control means 46 is a means for controlling the exposure amount of the exposure device 6 shown in FIG. The exposure control means 46 controls the exposure amount based on the image information subjected to gradation processing by the gradation processing means 41, the comparison result of the gradation rate, and the comparison result of the toner adhesion amount after fixing.

FIG. 23 shows the amount of toner adhered to paper (the amount of toner adhered before fixing) of each color of yellow, magenta, cyan, and black when the exposure amount is changed. As shown in FIG. 23, for any color toner, when the exposure amount is increased, the amount of toner adhesion to the paper increases, and conversely, when the exposure amount is decreased, the amount of toner adhesion to the paper is decreased. In this way, by adjusting the exposure amount, the amount of toner adhering to the photoreceptor changes, so it is also possible to adjust the amount of toner adhesion (image density) after fixing.

The exposure control flow will be described below with reference to FIG. 24.

In the exposure amount control flow shown in FIG. 24, first, the gradation rate, heater temperature, and toner adhesion amount after fixing are determined for each block to extract the block to be compared among the blocks J1 to J7. Steps (Step 1 to Step 5) of acquiring each piece of information are performed.

Specifically, as shown in FIG. 24, when a printing request is made to the image forming apparatus, first, the gradation processing means 41 performs gradation processing on the input image (Step 1). The gradation rate calculation means 42 calculates the gradation rate for each block J1 to J7 based on the image information obtained (Step 2).

Furthermore, when the above-mentioned printing request is received, the heater of the fixing device starts generating heat (Step 3), and when the temperature of the heater reaches a predetermined fixing temperature (temperature at which the image can be fixed), the temperature of each block J1 to J7 is is detected by the temperature sensor 19 (Step 4). Next, the toner adhesion amount calculation means 44 calculates the toner adhesion amount after fixing for each block J1 to J7 based on the calculated gradation rate and the detected temperature (Step 5). In this manner, in this embodiment, first, each piece of information about the gradation rate, heater temperature, and toner adhesion amount after fixing is acquired for each block J1 to J7.

Next, a process of extracting blocks to be compared (Steps 6 to 9) is performed.

Specifically, out of each block J1 to J7, the block whose heater temperature is the lowest is extracted, and the gradation rate of the block with the lowest temperature is compared with the gradation rate of other blocks to determine whether the block is the same. It is determined whether there is a block with a gradation rate (Step 6). Here, the reason for determining whether there are blocks with the same gradation rate is that variations in image density cannot be compared or are difficult to compare unless the blocks have the same gradation rate. Furthermore, the fact that it is difficult to compare image densities does not result in a significant deterioration in image quality that can be visually discerned, so there is little need to perform density adjustment. Therefore, blocks having the same gradation rate are extracted as comparison targets. Also, the reason why the block with the lowest temperature is extracted as a comparison target is that the temperature difference with respect to other blocks is large, and the variation in image density associated with this is also large, so priority is given to improving the density difference with such a large variation. Because it should.

For example, to explain the temperature distribution of the heater shown in FIG. 11 as an example, in this case, the block heated by the heater at the lowest temperature is the third block or the fourth block. In this way, if there are a plurality of blocks having the same minimum temperature, the fourth block located at the center of the image forming area in the width direction of the recording medium is selected as the comparison target. Here, the reason why the fourth block is selected is that variations in image density are more noticeable in the center of the image forming area than in parts other than the center. Note that the third block is not limited to this, and the third block may be selected depending on the type of image or other image forming conditions.

The gradation rate comparison means 43 determines whether there is a block with the same gradation rate as the block with the lowest temperature. As a result, if there is a block with the same gradation rate as the block with the lowest temperature, the toner adhesion amount comparing means 45 further compares the toner adhesion amount after fixing between blocks with the same gradation rate ( Step 7).

On the other hand, if there is no block with the same gradation rate as the block with the lowest temperature, in order to find the next block to be compared, the blocks arranged symmetrically with respect to the center of the image forming area in the width direction of the recording medium are compared. The gradation rates are compared (Step 8). The reason why symmetrically arranged blocks are compared is that if there is variation in image density between symmetrical parts, the variation tends to be noticeable. In this case as well, the comparison of the gradation rates is performed by the gradation rate comparison means 43.

Then, as a result of comparing the gradation rates of symmetrically arranged blocks, if there are no blocks with the same gradation rate, it is assumed that the block to be compared cannot be extracted (the variation in image density is (Step 14), image formation is started without adjusting the exposure amount (Step 14). On the other hand, if there are blocks with the same gradation rate, the toner adhesion amount comparing means 45 further compares the amount of toner adhesion after fixing between the blocks with the same gradation rate (Step 8).

As mentioned above, in each case where blocks to be compared are extracted, the toner adhesion amount after fixing of blocks with the same gradation rate is compared (Step 7, Step 9). If they are the same, the image densities are also the same (there is no variation in image density), so the exposure amount is not adjusted (Steps 11 and 13). Note that determining whether the toner adhesion amounts after fixing are the same is not limited to determining whether they exactly match or not, but whether or not they match within a certain range including errors. You can judge.

On the other hand, if the toner adhesion amounts after fixing are different from each other, it is determined that there is a risk of variation in image density, and the exposure amount is adjusted (Step 10, Step 12).

Adjustment of the exposure amount is performed by the exposure control means 46. Specifically, the exposure amount of a block where a large amount of toner is expected to adhere after fixing, that is, a block whose temperature is relatively high, is controlled to be lower than a preset exposure amount. That is, when a block with the lowest temperature is selected as a comparison target, control is performed to lower the exposure amount of the extracted block (a block that is not the lowest temperature) than a preset exposure amount. Further, when blocks arranged symmetrically are extracted for comparison, the exposure amount of the block with a higher temperature is controlled to be lower than the preset exposure amount.

In this way, by controlling the exposure amount to be lower than the preset exposure amount in blocks with relatively high temperatures among blocks with the same gradation rate, as shown in FIG. The amount t1 of toner deposited in the portion H can be made smaller than the amount t0 of toner that is set in advance based on image information (gradation information). As a result, the amount of toner t2 attached to the high-temperature area H after fixing can be made closer to the toner amount t3 attached to the low-temperature area L after fixing, so the temperature difference between the high-temperature area H and Variations in image density between the low portion L can be suppressed.

In the above control flow, when extracting blocks to be compared, it is first determined whether there is a block with the same gradation rate as the block with the lowest temperature, and if there is no block with the same gradation rate, the next block is extracted. , it is determined whether there are blocks with the same gradation rate among the blocks that are arranged symmetrically, but even if there is a block with the same gradation rate as the lowest temperature, the block that is arranged symmetrically still has the same gradation rate. It may also be determined whether there are blocks with the same gradation rate. In that case, the exposure amount may be adjusted for both the block having the same gradation rate as the lowest temperature and the blocks arranged symmetrically. Alternatively, the control flow may perform only one of the steps of searching for a block with the same gradation rate as the block with the lowest temperature, or searching for a block with the same gradation rate among blocks arranged symmetrically. .

Further, the exposure amount adjustment described above may be performed for each color toner (all colors) forming a color image, or may be performed for only some color toners. Furthermore, when the same image is to be continuously formed on a plurality of sheets, the image formation on subsequent sheets may be performed using the exposure amount that was set when the image was formed on the first sheet.

As described above, according to the present invention, even if there is variation in temperature in the longitudinal direction of the heater, by adjusting the exposure amount based on the temperature distribution of the heater, the image due to the temperature distribution of the heater can be improved. This makes it possible to suppress variations in concentration. This makes it possible to improve the problem of image quality deterioration caused by variations in temperature distribution of the heater. In addition, by being able to improve the problem of image quality deterioration caused by variations in the temperature distribution of the heater, it is possible to improve It can also be configured.

Therefore, the present invention can be expected to be particularly effective when applied to an image forming apparatus equipped with the following small-sized heater. Specifically, the ratio (R/Q) of the width direction dimension R of the resistance heating element 59 to the width direction dimension Q of the base material 50 as shown in FIG. 25 is 25% or more. Great effects can be expected when the invention is applied. Furthermore, if the heater 22 has such a dimension ratio (R/Q) in the transverse direction of 40% or more, the effect of applying the present invention will be even greater. In the example shown in FIG. 25, since the base material 50 of the heater 22 is formed in a rectangular shape, the width direction dimension Q of the base material 50 is the same at any longitudinal position; If there are irregularities and the transverse direction dimension Q changes depending on the position in the longitudinal direction Z, the portion where the transverse direction dimension Q of the base material 50 is the minimum within the longitudinal direction range where the resistance heating element 59 is arranged. It is only necessary that the dimension ratio (R/Q) in the transverse direction holds true for .

Further, in order to suppress temperature variations in the heater, a resistance heating element having PTC characteristics may be used. The PTC characteristic is a characteristic in which the resistance value increases as the temperature increases (when a constant voltage is applied, the heater output decreases). By using a heat generating part having PTC characteristics, it is possible to start up quickly at low temperatures due to high output, and to suppress excessive temperature rise at high temperatures due to low output. For example, by setting the TCR coefficient of the PTC characteristic to about 300 to 4000 ppm/degree, it is possible to reduce the cost while ensuring the resistance value necessary for the heater. More preferably, the TCR coefficient is 500 to 2000 ppm/degree.

The temperature coefficient of resistance (TCR) can be calculated using the following formula (5). In equation (5), T0 is a reference temperature, T1 is an arbitrary temperature, R0 is a resistance value at the reference temperature T0, and R1 is a resistance value at the arbitrary temperature T1. For example, in the above-described heater 22 shown in FIG. 10, the resistance value between the first electrode part 61A and the second electrode part 61B is 10Ω (resistance value R0) at 25°C (reference temperature T0), and at 125°C If it is 12Ω (resistance value R1) at (arbitrary temperature T1), the resistance temperature coefficient is 2000 ppm/° C. from equation (5).

Further, the heater included in the fixing device is not limited to the heater 22 having a block-shaped (square-shaped) resistance heating element 59 as shown in FIG. The heater 22 having a heating element 59 or a heater having a resistance heating element of other shapes may be used.

Further, the fixing device included in the image forming apparatus is not limited to the above-described fixing device, but may be a fixing device as shown in FIGS. 27 to 29. The configuration of each fixing device shown in FIGS. 27 to 29 will be briefly described below.

The fixing device 9 shown in FIG. 27 differs from the above-described fixing device in that a pressure roller 90 is arranged on the opposite side of the fixing belt 20 from the pressure roller 21 side. In this case, the fixing belt 20 is sandwiched between the pressing roller 90 and the heater 22 and is heated. On the other hand, on the pressure roller 21 side, a nip forming member 91 is arranged on the inner periphery of the fixing belt 20 . The nip forming member 91 is supported by the stay 24, and forms a nip portion N with the fixing belt 20 sandwiched between the nip forming member 91 and the pressure roller 21.

Next, in the fixing device 9 shown in FIG. 28, the above-described pressure roller 90 is omitted, and in order to ensure the circumferential contact length between the fixing belt 20 and the heater 22, the heater 22 It is formed into an arc shape. The rest of the configuration is the same as the fixing device 9 shown in FIG. 27.

Next, in the fixing device 9 shown in FIG. 29, a pressure belt 92 is provided in addition to the fixing belt 20, and a heating nip (first nip portion) N1 and a fixing nip (second nip portion) N2 are configured separately. has been done. That is, the nip forming member 91 and the stay 93 are also arranged on the side opposite to the fixing belt 20 side with respect to the pressure roller 21, and the pressure belt 92 is arranged so as to include the nip forming member 91 and the stay 93. ing. The rest of the structure is the same as that of the fixing device 9 shown in FIG. 2.

By applying the present invention to such an image forming apparatus equipped with a fixing device as shown in FIGS. 27 to 29, variations in image density caused by variations in the temperature distribution of the heater can be suppressed, and miniaturization can be achieved. It will be able to handle higher speeds.

Further, the present invention is applicable not only to a color image forming apparatus as shown in FIG. 1 but also to a monochrome image forming apparatus.

2 Photoreceptor (image carrier)
3 Charging roller (charging means)
4 Developing device (developing means)
6 Exposure device (exposure means)
8 Transfer device (transfer means)
9 Fixing device (fixing means)
19 Temperature sensor (temperature detection means)
22 Heater (heating member)
41 Gradation processing means 42 Gradation rate calculation means 43 Gradation rate comparison means 44 Toner adhesion amount calculation means 45 Toner adhesion amount comparison means 46 Exposure control means 50 Base material 59 Resistance heating element (heating element)
60 Heat generating part 61 Electrode part 62 Power supply line (conductive part)
100 Image forming apparatus J Image forming area J1 to J7 Block K1 First conductive path K2 Second conductive path K3 Third conductive path S1 First direction S2 Second direction

Japanese Patent Application Publication No. 6-75453

Claims (10)

an image carrier;
Charging means for charging the surface of the image carrier;
exposure means for exposing the charged surface of the image carrier to form an electrostatic latent image based on image information;
a developing means for depositing toner on the electrostatic latent image on the image carrier to form a toner image;
a transfer means for transferring the toner image on the image carrier to a recording medium;
a fixing unit that heats the recording medium to fix the toner image on the recording medium;
gradation rate calculation means for calculating the gradation rate of an image for each of a plurality of blocks obtained by dividing the image forming area on the recording medium in the width direction of the recording medium;
gradation rate comparison means for comparing the gradation rates calculated by the gradation rate calculation means between the blocks;
exposure control means for controlling the exposure amount of the exposure means for each block;
Equipped with
Comparing the gradation rates for each block, and if there are blocks with the same gradation rate, setting in advance the exposure amount of the block whose temperature of the fixing means is relatively high among the blocks with the same gradation rate. An image forming device that lowers the exposure amount below the specified exposure amount. Compare the gradation rate of the block with the lowest temperature among the plurality of blocks with the gradation rate of other blocks, and if there are blocks with the same gradation rate, calculate the exposure amount of the block that is not the lowest temperature. The image forming apparatus according to claim 1, wherein the exposure amount is lower than a preset amount. The gradation rates of the blocks arranged symmetrically with respect to the center of the recording medium width direction of the image forming area among the plurality of blocks are compared, and if the gradation rates are the same, the relative temperature 3. The image forming apparatus according to claim 1, wherein the exposure amount of a block with a high value is lower than a preset exposure amount. toner adhesion amount calculation means for calculating the amount of toner adhesion after fixing to the recording medium according to the temperature of the fixing means;
toner adhesion amount comparison means for comparing the post-fixing toner adhesion amounts calculated by the toner adhesion amount calculation means between the blocks;
Equipped with
The gradation rate for each block is compared, and if there are blocks with the same gradation rate, the toner adhesion amount after fixing is further compared between blocks with the same gradation rate, and blocks with a relatively high temperature are compared. 2. The exposure amount of the block having a relatively high temperature is lowered from a preset exposure amount so that the amount of toner adhesion after fixing approaches the amount of toner adhesion after fixing of the block having a relatively low temperature. 3. The image forming apparatus according to any one of 3. The fixing means includes a heating member that heats the recording medium,
The heating member is
a base material arranged longitudinally in the width direction of the recording medium;
a plurality of heating elements arranged in line in the longitudinal direction of the base material;
a plurality of electrode parts;
a conductive part that connects the plurality of heating elements and the plurality of electrode parts;
The image forming apparatus according to any one of claims 1 to 4. The electrode part has a first electrode part and a second electrode part,
The conductive part includes a first conductive path connecting the heating element and the first electrode part, and a first conductive path extending from the heating element in a first longitudinal direction of the base material and connected to the second electrode part. a second conductive path branched from the second conductive path in a second direction opposite to the first direction and connected to the second conductive path or the second electrode portion without going through the first conductive path; The image forming apparatus according to claim 5, comprising at least a part of the third conductive path to be connected. The image forming apparatus according to claim 5 or 6, wherein the block is an area divided for each of the plurality of heating elements. The image forming apparatus according to any one of claims 1 to 7, further comprising a plurality of temperature detection means for detecting temperatures of the plurality of blocks. comprising temperature detection means for detecting the temperature of only some of the blocks,
The image forming apparatus according to claim 8, wherein the temperatures of the other blocks are calculated based on the detected temperature information of the some of the blocks. When the same image is successively formed on a plurality of recording media, the exposure amount set at the time of image formation on the first recording medium is used to form images on subsequent recording media. The image forming apparatus according to any one of the above.

Images