JP2012133034A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing generation of damages on a surface of a fixing member caused by paper powder or the like with neither increasing the size of a device body nor using other members.SOLUTION: An image forming apparatus comprises: sheet conveyance means for conveying a sheet on which an image is to be formed; an image forming part for forming an image by developing an electrostatic image formed on an image carrier with toner and transferring the formed toner image on the conveyed sheet; and a fixing device for fixing the toner image on the sheet with a heating member for heating the sheet having the toner image transferred thereon and a pressing member for pressing the sheet toward the heating member. The image forming apparatus forms on a top end margin or a rear end margin in the sheet conveyance direction a cleaning image pattern for removing foreign matter attached to the heating member and heats the sheet with the fixing device.

Description

  The present invention relates to an image forming apparatus having a fixing device, such as an electrophotographic copying machine or a printer.

  In image forming apparatuses such as electrophotographic copying machines and printers, in recent years, film heating type fixing devices are widely used as fixing means for fixing a toner image transferred onto a sheet. In this film heating and fixing method, an endless fixing film can be rotated along a guide and a nip is formed by pressing a rotatable pressure roller, and a sheet is heated when the sheet passes through the nip portion. The toner image is fixed on the sheet by applying pressure. In this film heat fixing method, the temperature rise time of the surface temperature of the film member can be shortened by sliding the inside of the fixing film and the heating resistor directly, and the power consumption during standby is kept as low as possible. It becomes possible.

  In recent years, the spread of printers, copiers and the like has become widespread worldwide. Along with this, the types of sheets used are also diversified, and their characteristics often vary depending on the region. Among them, depending on the region, there is a region that uses a sheet containing a large amount of heavy calcium carbonate as a filler. Also, in recent years, in order to improve the texture of the sheet, the filling amount of calcium carbonate tends to increase due to reasons such as high whiteness, excellent opacity, and low cost.

  However, paper powder containing calcium carbonate as a main component has a frictional charging characteristic such that it is strongly positively charged by friction with a fluororesin as compared with paper powder containing other fillers such as kaolin and talc. For this reason, the paper dust is easily electrostatically adsorbed on the surface of the fixing film whose surface is covered with a fluorine-based resin such as PFA or PTFE.

  If a large amount of the paper powder containing calcium carbonate as a main component adheres to the fixing film and is pressed by the pressure roller, scratches may occur on the surface of the fixing film.

  In order to solve this problem, conventionally, a method has been used in which a cleaning member such as a web, a roller, or a brush is brought into contact with the surface of the fixing film to directly remove the adhered matter (Patent Documents 1 and 2). .

  Further, by using a sheet on which a solid black image is recorded on one side and passing the fixing nip portion with the recording surface of the sheet directed toward the pressure roller, the pressure roller is made to adsorb paper powder or the like to the molten toner. A method of cleaning the dirt is also proposed (Patent Document 3).

JP 10-161450 A JP 2001-117407 A JP-A-2-160276

  However, in the method using the cleaning member, the cleaning member is brought into direct contact with the fixing film to peel off the adhered matter, so that the fixing film may be scratched. The type may be deteriorated. For this reason, toner adheres to the surface of the fixing film, and the adhered toner deteriorates the thermal conductivity and lowers the fixing property. Further, in the system using the cleaning member, since the cleaning mechanism for the fixing film and the pressure roller is added, the apparatus configuration is complicated and the apparatus is prevented from being downsized.

  On the other hand, in the method using a sheet on which a solid black image is recorded, a cleaning sheet is separately required, and this sheet has to be disposed after use.

  The present invention has been made in view of the above problems, and its purpose is to prevent the surface of the fixing member from being scratched due to paper dust or the like without increasing the size of the apparatus body and using other members. An image forming apparatus is obtained.

  In order to achieve the above object, a typical configuration according to the present invention includes: a sheet conveying unit that conveys a sheet on which an image is formed; and an electrostatic image formed on the image carrier is developed with toner. The toner image is fixed to the sheet by an image forming unit that forms an image by transferring the image onto a conveyed sheet, a heating member that heats the sheet on which the toner image is transferred, and a pressing member that presses the sheet against the heating member. And a fixing device that forms a cleaning image pattern on a leading edge portion or a trailing edge margin portion in the conveyance direction of the sheet, and heats the sheet with the fixing device.

  In another configuration of the present invention, a sheet conveying unit that conveys a sheet on which image formation is performed, and an electrostatic image formed on the image carrier are developed with toner, and the formed toner image is transferred to the conveyed sheet. An image forming unit for forming an image, a heating member for heating the sheet on which the toner image is transferred, a fixing device for fixing the toner image to the sheet by a pressing member for pressing the sheet against the heating member, and one side An image forming apparatus comprising: a sheet reversing and conveying unit that conveys the sheet so that the sheet recorded on the surface of the sheet can be reversed and recorded on the other side of the sheet. In the case of recording only on the surface, the cleaning image pattern is formed on the other surface of the sheet.

  In the present invention, a cleaning image pattern is formed on the leading edge portion or the trailing edge margin portion of the sheet, and this is heated by a fixing device, whereby the toner on which the cleaning image pattern is formed is melted, and this molten toner Thus, paper dust and the like attached to the fixing member can be collected.

  Further, in the case of single-sided recording, paper dust or the like can be collected without making the cleaning image pattern conspicuous by forming the cleaning image pattern on the surface where no image is formed.

  For this reason, in the present invention, paper dust and the like adhering to the fixing member can be collected and the occurrence of scratches on the fixing member can be prevented without increasing the size and complexity of the apparatus main body.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. 1 is a schematic configuration diagram illustrating a fixing device according to an embodiment of the present invention. It is a graph showing RMS transition in a solid white and a printing rate 0.5% chart. It is explanatory drawing of embodiment which divides and forms the cleaning image pattern in the sheet | seat front end concerning 1st Embodiment. It is a flowchart which shows the procedure which forms the cleaning image pattern which concerns on 1st Embodiment. It is a flowchart which shows the procedure which forms the cleaning image pattern which concerns on 2nd Embodiment. It is explanatory drawing of embodiment which forms the cleaning image pattern which concerns on 2nd Embodiment. It is explanatory drawing of embodiment which forms the cleaning image pattern which concerns on 3rd Embodiment.

  Next, an image forming apparatus according to an embodiment for carrying out the present invention will be described with reference to the drawings.

[First Embodiment]
<Overall configuration of image forming apparatus>
FIG. 1 is a schematic configuration diagram showing a configuration of an image forming apparatus according to the present embodiment. First, the overall configuration of the image forming apparatus will be described together with the image forming operation with reference to FIG.

  An image forming apparatus A according to the present embodiment has an image forming unit that develops an electrostatic image formed on an image carrier with toner and transfers the formed toner image to a conveyed sheet to form an image. Is a copier using

  The image forming unit includes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 1 that is an image carrier, and the photosensitive drum 1 is charged by a charging roller 2 that is a charging unit. . Next, an electrostatic image corresponding to the image information is formed by irradiating the photosensitive drum 1 with laser light corresponding to the image information from the optical means 3, and this electrostatic image is developed by the developing device 4 using toner. The visible image is formed as a toner image.

  On the other hand, in synchronization with the formation of the toner image, the sheet is conveyed to the image forming unit by a sheet conveying unit. That is, the sheet 6 in the feeding cassette 5 set at the lower part of the apparatus main body is conveyed to the toner image transfer position of the image forming unit by the pickup roller 7 and the conveying roller 8. A transfer roller 9 as a transfer unit is disposed at the transfer position, and the toner image on the photosensitive drum 1 is transferred to the sheet 6 by applying a transfer bias.

  The sheet 6 having received the transfer of the toner image is conveyed to the fixing device 10, and when passing through the fixing device 10, heat and pressure are applied and the toner image is melted and fixed on the sheet. In the case of single-sided recording, the sheet 6 on which the toner image has been fixed is conveyed to a discharge tray 12 by a discharge roller 11 and discharged. On the other hand, in the case of double-sided recording, the sheet is conveyed by the sheet reversing and conveying means 13 so that the sheet recorded on one side is reversed and conveyed again to the image transfer position, and recording is performed on the other side. Thereafter, the paper is discharged to the discharge tray 12 by the discharge roller 11.

  The photosensitive drum 1 after transferring the toner image to the sheet 6 is subjected to the next image forming process after the toner remaining on the photosensitive drum 1 is removed by the cleaning unit 14.

<Fixing device>
Next, the configuration of the fixing device 10 of this embodiment will be described. FIG. 2 shows a schematic configuration of the fixing device 10 in the present embodiment. The fixing device 10 in the present embodiment is a film heating type fixing device.

  The fixing device 10 of the present embodiment includes a fixing assembly 90 as a heating member that heats a sheet onto which a toner image has been transferred, and a pressure roller 80 as a pressing member that presses the sheet against the fixing unassembly 90. Yes. The pressure roller 80 is in pressure contact with the fixing assembly 90 to form a heating nip portion N, and the pressure roller 80 rotates in the direction of the arrow in FIG. 2 so that the fixing assembly 90 is driven in the direction of the arrow in FIG. It is configured to rotate.

  The fixing assembly 90 includes a heater 91, a heat insulating holder 92, and a fixing film 93 that is a heating rotator. The heater 91 is held by the heat insulating holder 92 and is provided at a position that slides with the inner peripheral surface of the fixing film 93.

  As the heater 91, a plate having a low heat capacity is used. In this embodiment, the surface of an insulating ceramic substrate such as alumina or aluminum nitride is screen-printed with an energization heating resistance layer such as Ag / Pd (silver palladium), RuO2, Ta2N along the longitudinal direction of the substrate. Is used. In addition, a configuration in which a protective layer such as a glass layer that protects the energization heating resistance layer may be provided on the sliding surface with the inner peripheral surface of the fixing film 93 within a range that does not impair the thermal efficiency.

  The heat insulating holder 92 holds the heater 91 and is formed of a heat resistant resin such as a liquid crystal polymer, a phenol resin, PPS, or PEEK. In addition, it holds the heater 91 and also has a function of guiding the rotation of the fixing film 93.

  In addition, the fixing film 93 in the present embodiment has a multilayer structure formed of a base layer and a surface layer. As the base layer, a heat-resistant resin such as polyimide, polyamideimide, or PEEK, or a pure metal such as SUS, Al, Ni, Cu, or Zn having heat resistance and high thermal conductivity, an alloy, or the like is used.

  As the surface layer, resins such as PTFE (polytetrafluoroethylene), PFA (tetrafluoroethylene perfluoroalkyl vinyl ether copolymer), and FEP (tetrafluoroethylene hexafluoropropylene copolymer) are used. Further, a resin such as ETFE (ethylene tetrafluoroethylene copolymer), CTFE (polychlorotrifluoroethylene), PVDF (polyvinylidene fluoride), or the like may be used. By configuring the surface layer of the fixing film 93 in this way, the releasability of the fixing film 93 is improved.

  The pressure roller 80 includes a metal core 81 made of SUS, SUM, Al or the like, and an elastic layer 82 formed on the outer side of the core 81.

  In the present embodiment, as the elastic layer 82, there is an elastic solid rubber layer formed of heat-resistant rubber such as silicone rubber or fluorine rubber, or an elastic sponge rubber layer formed by foaming silicone rubber in order to have a more heat insulating effect. Used. Moreover, the structure using the elastic foam rubber layer which disperse | distributed the hollow filler (microballoon etc.) in the silicone rubber layer, gave the gas part in hardened | cured material, and improved the heat insulation effect may be sufficient.

  Furthermore, a configuration in which a release layer such as perfluoroalkoxy resin (PFA) or polytetrafluoroethylene resin (PTFE) is formed on the surface of the elastic layer 82 may be employed.

  The pressure roller 80 is pressed against the fixing film 93 by a spring (not shown) to obtain a heating nip portion N. Further, a driving force rotating in the direction of the arrow in FIG. 2 is obtained by a driving gear (not shown) provided at the end of the cored bar 81. The driving force is transmitted from a motor (not shown) in accordance with a command from a CPU (not shown) that controls the control means. As the pressure roller 80 is driven to rotate, the fixing film 93 is driven to rotate by a frictional force with the pressure roller 80.

  In the present embodiment, a lubricant such as a fluorine-based or silicone-based heat-resistant grease is interposed between the fixing film 93 and the heater 91, so that the frictional resistance can be suppressed to a low level. The configuration allows smooth rotation.

  Further, the temperature control of the heater 91 is performed by controlling the duty ratio, wave number, etc. of the voltage applied by the CPU (not shown) to the energized heating resistor layer in accordance with a signal from a temperature detection element such as a thermistor (not shown) provided on the back surface of the ceramic substrate. Is determined and appropriately controlled. According to this configuration, the temperature of the heating nip portion can be maintained at a desired fixing set temperature.

  A sheet 6 holding an unfixed toner image is appropriately supplied to the fixing device 10 configured as described above at a predetermined timing, and is conveyed to the heating nip portion N for heat fixing.

<Cleaning configuration of fixing device>
Next, the cleaning configuration of the fixing device of the present embodiment will be described.

  As described above, when a large amount of paper dust adheres to the fixing film 93, there is a possibility that the portion may be damaged. For example, FIG. 3 shows the result of examining the state of scratches generated on the fixing film 93 when the inventor continuously passes sheets with a printing rate of 0% (solid white image) to the fixing device 10.

  Graph A in FIG. 3 shows the results when A4 size plain white plain paper is passed with the longitudinal direction being the transport direction. The horizontal axis represents the number of sheets passed (x1000), and the vertical axis represents the number of scratches generated on the fixing film 93. Here, the numerical value representing the occurrence of flaws in the fixing film is expressed by the RMS value (mean square root) of the surface of the fixing film, and the unit is μm. In this test, as an example, when the fixing film 93 using a resin such as PTFE or PFA is used for the surface layer, if the RMS value exceeds 1 μm, the fixing film 93 is scratched, and an abnormal image It is set as the threshold that occurs.

  As can be seen from the results in FIG. 3, the solid white image without any toner transferred has an RMS value exceeding the threshold value of 1 μm from the vicinity of about 40,000 sheets, and film scratches have occurred. I understand.

  On the other hand, as shown in graph B of FIG. 3, when a sheet having a printing rate of 0.5% transferred with respect to the sheet size in the sheet conveying direction is passed, 150,000 sheets are passed. Also, the RMS value was a threshold value of 1 μm or less, and no film scratch was generated. This is because the transferred toner is heated and melted by the fixing device, and the molten toner adsorbs and removes the paper dust adhering to the fixing film 93. Even if the amount of applied toner is small, the cleaning effect is obtained. This is because Note that the number of print dots in the conveyance direction of a sheet with a print rate of 0.5% corresponds to 30 to 40 dots if the resolution setting during printing is 600 dpi × 600 dpi.

  Therefore, in the present embodiment, toner serving as a cleaning image pattern (hereinafter referred to as “image pattern”) is transferred to at least one blank portion (non-image forming region) at the leading or trailing end of the sheet in the conveyance direction. Then, the toner is heated at the nip portion of the fixing device to collect foreign matters due to paper dust on the fixing film through the toner, prevent damage to the fixing film 93, and suppress generation of scratches. .

  Therefore, the image pattern is formed to adsorb and remove foreign matters such as paper dust attached to the fixing film 93 on the sheet by the molten toner when the toner forming the pattern is heated and melted by the fixing device 10. Transfer toner pattern. The image pattern of the present embodiment is a thin line-shaped toner pattern formed in the sheet width direction on the leading edge margin or trailing edge margin of the sheet.

  Further, the image pattern needs to prevent the toner from being visually noticeable. Therefore, in this embodiment, for example, as shown in FIG. 4, when the image pattern 15 is formed in the margin at the leading end of the sheet, the printing rate per unit area is reduced and the sheet width direction (crosses the sheet conveyance direction). The pattern divided into a plurality of (direction) is printed at a position shifted in the sheet width direction. Note that a pattern such as a printing rate, a shape, and a transfer position is stored in advance in the storage unit (memory) (not shown) in the image forming apparatus as the image pattern.

  FIG. 5 is a flowchart showing a procedure for executing the cleaning method. The cleaning procedure of this embodiment will be described based on this flowchart.

  First, from the start of recording, the image area in the sheet conveyance direction in the print area in the sheet width direction is stored and integrated (S1).

  Next, for each predetermined area in the sheet width direction, it is determined whether or not the area of the solid white image in the sheet conveyance direction is equal to or larger than a preset setting value (threshold value) area (S2). In this embodiment, the number of image dots is counted as toner measuring means for measuring the amount of toner transferred to the sheet by image formation. Specifically, the number of image dots in the sheet conveyance direction in the region is counted and integrated, and the length in the conveyance direction of the sheet passing through the fixing device is counted and integrated. Then, when the number of image dots with respect to the length in the transport direction is equal to or smaller than a predetermined set value, that is, when the solid white portion exceeds the set value region, an image pattern for applying toner to the margin portion of the sheet Is formed (S3).

  In the present embodiment, it is assumed that if 40,000 sheets of plain white images are passed through, the fixing film 93 is damaged, and when the solid white image is conveyed by an amount corresponding to 400 sheets, the leading edge margin of the sheet is printed. An image pattern is formed. As shown in FIG. 4, this image pattern is divided into four parts in the sheet width direction, and is line-shaped with a printing rate of 0.5% with respect to the sheet conveying direction dimension at positions shifted sequentially in the sheet width direction. The image pattern 15 is formed. Then, a total of 400 sheets on which the image pattern 15 is formed are heated while being conveyed by the fixing device (S4).

  As described above, in the present embodiment, when the amount of toner transferred to the sheet by image formation is smaller than the set value with respect to a predetermined number of sheets, the image pattern is formed on the leading edge portion in the sheet conveyance direction. Form. As a result, paper dust adhering to the fixing film 93 caused by passing the sheet of the solid white image is removed, and the occurrence of scratches due to the paper dust on the fixing film 93 is prevented.

  Note that the image pattern 15 formed on the leading edge of the sheet does not necessarily have to be divided as shown in FIG. 4, but it is more per sheet to form the image pattern 15 so as to be shifted in the width direction of a plurality of sheets. This is preferable because the image pattern 15 becomes inconspicuous. Further, the number of divisions may be more than four.

  Further, when the image pattern is divided and formed so as to be sequentially shifted in the sheet width direction with respect to a plurality of sheets, a part of the image pattern overlaps in the sheet width direction and the length of the divided image pattern is The total may be formed to be equal to or greater than the width of the maximum width sheet to be conveyed. That is, the image pattern is composed of a combination of patterns divided into a plurality of sheets in the sheet width direction intersecting the sheet conveyance direction, and one of the divided patterns is recorded on one sheet to be image-formed, The pattern is formed by the sheet. The total length of the divided patterns in the sheet width direction is formed to be equal to or greater than the length in the width direction of the maximum width sheet to be conveyed. Thereby, by overlapping, a portion where an image pattern is not formed in the sheet width direction can be surely eliminated, and paper dust can be more reliably removed corresponding to the maximum width sheet. .

[Second Embodiment]
Next, an image forming apparatus according to a second embodiment will be described with reference to FIGS. Note that the basic configuration of the apparatus of the present embodiment is the same as that of the above-described embodiment, and thus redundant description is omitted. Only the configuration that is a feature of the present embodiment will be described here.

  In the above-described embodiment, an example in which an image pattern is formed so as to cover at least one of the leading edge and the trailing edge of the sheet and the entire sheet width direction when the solid white image is equal to or larger than the set value region. Here, when the solid white image is equal to or larger than the set value region, an image pattern is formed only in the region that is the solid white image in the sheet width direction.

  A specific description will be given based on the flowchart of FIG. In this embodiment, step 23 is different from the flowchart (FIG. 5) shown in the first embodiment.

  First, from the start of recording, the image area in the sheet conveyance direction in the print area in the sheet width direction is stored and integrated (S21).

  Next, it is determined whether or not the solid white image in the sheet conveying direction is greater than or equal to a preset set value (threshold) region for each predetermined region in the sheet width direction (S22). Specifically, the number of image dots in the sheet conveyance direction in the region is counted, and the length in the conveyance direction of the sheet passing through the fixing device is counted. Then, when the count value of the number of image dots with respect to the count value of the conveyance direction length is equal to or smaller than a predetermined set value, that is, when the solid white portion exceeds a preset set value region, An image pattern 15 is formed at the leading edge of the sheet (S23).

  In this embodiment, assuming that 40,000 sheets of plain white images are passed through, the fixing film 93 is damaged, and when the solid white images are conveyed by an amount corresponding to 400 sheets, as shown in FIG. Then, an image pattern is formed in a margin portion at the front end of the sheet in a predetermined region in the sheet width direction where the solid white portion is located (the same position in the sheet width direction). This image pattern is a line-shaped image pattern 15 having a printing rate of 0.5% with respect to the dimension in the sheet conveyance direction, and the sheet on which the image pattern is formed is heated while being conveyed by a total of 100 fixing devices (S24). As a result, paper dust adhering to the fixing film 93 due to the passing of the solid white image is removed, and the occurrence of scratches due to the paper dust on the fixing film 93 is prevented.

[Third Embodiment]
Next, an image forming apparatus according to a third embodiment will be described with reference to FIG. In this embodiment, the basic configuration of the apparatus is the same as that of the above-described embodiment, and therefore, a duplicate description is omitted. Here, only a configuration that is a feature of this embodiment will be described.

  In the above-described embodiment, the image pattern is formed on the leading edge or the trailing edge of the sheet. However, in the present embodiment, when image formation on the sheet is recorded only on one surface (front surface), the image pattern is formed on the sheet. It is formed on the other side surface (back surface).

  In the case of single-sided recording, the sheet on which the image is formed on the front surface is reversed and conveyed by the sheet reversing and conveying means 13, and as shown in FIG. To form.

  Note that when the image pattern 15 is formed on the back surface as in the present embodiment, the image pattern 15 may not be the leading edge or the trailing edge of the sheet, and the degree of freedom of the position where the image pattern is formed increases. Further, when an image pattern is formed on the back surface, characters or the like may be printed as the image pattern. For example, an IP address of a personal computer that outputs an image as an image pattern may be printed. This makes it possible to specify the printed out personal computer.

DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum 2 ... Charging roller 3 ... Optical means 4 ... Developing device 5 ... Feed cassette 6 ... Sheet 7 ... Pick-up roller 8 ... Conveying roller 9 ... Transfer roller 10 ... Fixing device 11 ... Discharge roller 12 ... Discharge tray 13 ... Sheet reversal conveying means 14 ... Cleaning means 15 ... Cleaning image pattern 80 ... Pressure roller 81 ... Core metal 82 ... Elastic layer 90 ... Fixing assembly 91 ... Heater 92 ... Heat insulation holder 93 ... Fixing film

Claims (7)

Sheet conveying means for conveying a sheet on which image formation is performed;
An image forming section that develops an electrostatic image formed on the image carrier with toner and transfers the formed toner image to a conveyed sheet to form an image; and
A fixing device that fixes the toner image on the sheet by a heating member that heats the sheet to which the toner image is transferred and a pressing member that presses the sheet against the heating member;
An image forming apparatus having
An image forming apparatus comprising: forming a cleaning image pattern for removing foreign matter adhering to the heating member on a leading edge margin or a trailing edge margin of the sheet in the conveyance direction; and heating the sheet by the fixing device. . Sheet conveying means for conveying a sheet on which image formation is performed;
An image forming section that develops an electrostatic image formed on the image carrier with toner and transfers the formed toner image to a conveyed sheet to form an image; and
A fixing device that fixes the toner image on the sheet by a heating member that heats the sheet to which the toner image is transferred and a pressing member that presses the sheet against the heating member;
A sheet reversing and conveying means for conveying the sheet so that the sheet recorded on the one side surface can be reversed and recorded on the other side surface;
An image forming apparatus having
An image forming apparatus, wherein when the image formation on the sheet is recorded only on one side surface, the cleaning image pattern is formed on the other side surface of the sheet.   3. The cleaning image pattern according to claim 1, wherein the cleaning image pattern is formed when an amount of toner transferred to the sheet by image formation is smaller than a preset value for a predetermined number of sheets. Image forming apparatus.   The cleaning image pattern is composed of a combination of patterns divided into a plurality of sheets in the sheet width direction intersecting the sheet conveyance direction, and one of the divided patterns is recorded on one sheet to be image-formed. 4. The image forming apparatus according to claim 1, wherein the pattern is formed by the sheet.   The plurality of cleaning image patterns partially overlap in the sheet width direction, and the total length of the cleaning image patterns is equal to or greater than the length in the width direction of the maximum sheet to be conveyed. The image forming apparatus according to claim 4. When forming an image on the sheet,
Count and accumulate the number of image dots in the sheet conveyance direction, count and accumulate the conveyance direction length of the sheet passing through the fixing device,
When the count value of the number of dots with respect to the count value of the length in the conveyance direction of the sheet is equal to or less than a preset value, the sheet conveyance direction at a position corresponding to the number of dots that is equal to or less than the set value The image forming apparatus according to claim 1, wherein the cleaning image pattern is formed in a blank portion.   The image forming apparatus according to claim 6, wherein the cleaning image pattern is continuously formed on a predetermined number of sheets.

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