JP2018025645A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem in which: in an image forming apparatus including a fixing unit that detects the temperature of a heater with a thermistor, when there is a density difference in a density distribution in a direction orthogonal to a sheet conveyance, deviation occurs between a temperature detected by the thermistor and an actual temperature on a sheet, and thus an image may not be satisfactorily fixed to the sheet.SOLUTION: An image forming apparatus determines a density distribution in a direction orthogonal to a sheet conveyance direction, and rotates an image to be formed by 90 degrees to be thermally fixed to a sheet according to a result of the determination.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an image forming apparatus that develops a latent image formed on an electrophotographic photosensitive member with a developer, transfers the developed image onto a recording material, and fixes the image to form an image.

  In an electrophotographic image forming apparatus such as a copying machine, a printer, or a FAX machine, a transfer in which an unfixed toner image is transferred between a heating roller that is a heating member and a pressure roller that is a pressure member in pressure contact with the heating roller. It is equipped with a fixing unit that heats and fixes the material. This fixing device incorporates a heater, that is, a heating element, in a heating roller, and controls the lighting of the heater so as to reach a target temperature while detecting the roller surface temperature with a thermistor provided on the heater. There are roughly two types of heaters used in the fixing device: a halogen lamp (halogen heater) and a ceramic heater. The halogen heater is disposed at the center of the heating roller, and indirectly warms the heating roller. As a result, those using halogen heaters are characterized by a large heat capacity and a slow rise in temperature. For this reason, a fixing device using a halogen heater is lit intermittently to control the temperature at a constant temperature not only during copying (or recording) operation but also during standby (standby). On the other hand, an on-demand type fixing device using a ceramic heater has a feature that the temperature rises quickly because of its small heat capacity. For this reason, the heater is not turned on during standby, and the temperature is adjusted only during the copying operation. In either method, temperature control is performed by using a temperature detection element such as a thermistor so that the temperature of the fixing heater is detected and the target temperature is reached. This is because when the recording material onto which the developer has been transferred is fixed by the fixing unit with pressure and heat, the recording material is deprived of heat from the fixing heater, so that the target temperature is kept constant. is there. In a fixing device using a ceramic heater having a small heat capacity, this temperature change is particularly prominent. Therefore, fine control is required to maintain a constant temperature. Normally, one or more temperature sensing elements such as thermistors are arranged, and the (main) thermistor used for temperature control is located in the center of the heater so that the temperature on the recording material can be detected no matter what size recording material is conveyed. In order to detect the temperature at the heater end where the recording material may not pass depending on the size of the recording material, a (sub) thermistor is generally arranged near the heater end.

  When the recording medium passes through the fixing device, the heating member and the pressure member are deprived of heat by the recording medium, so that the temperature at the position where the recording medium has passed decreases, and there is a temperature difference from the position where the recording medium does not pass. Arise. In order to solve this problem, Patent Document 1 discloses a technique in which a plurality of heat generation patterns are provided on a heat-resistant substrate, and control is performed so that the temperature at a position where the recording medium does not pass is not excessively increased.

JP 2000-162906 A

  In Patent Document 1, attention is paid to the amount of heat taken away by the passage of the recording medium, and control is performed so that the temperature at the position where the recording medium does not pass is not excessively increased. However, the amount of heat deprived differs depending on the print pattern (density distribution of the formed image) printed on the recording medium. If the print pattern is uniform with respect to the entire paper, the amount of heat taken is also uniform, and the temperature as a result of temperature control by detecting the temperature with the thermistor is also uniform. However, for example, when a print pattern having a print rate that differs significantly in the direction (width direction) orthogonal to the conveyance direction of the recording medium is continuously printed, the temperature is detected even in the area through which the recording medium passes. There is a possibility that the temperature at the position of the thermistor is uneven and the temperature in other regions may be nonuniform. As a result, even if the temperature control is performed based on the temperature detected by the thermistor, the temperature at the position of the thermistor is appropriately controlled, but there is a problem that the temperature non-uniformity appears without being eliminated.

  If temperature non-uniformity occurs and the temperature of the region other than the thermistor becomes higher than the desired temperature, the curl amount of the recording medium increases, and the desired stackability can be obtained. There is a concern that the product cannot be obtained, and the parts are stressed at a temperature higher than expected, so that the intended product life cannot be obtained. When the temperature of the region other than the thermistor is lower than the desired temperature, there is a concern that an abnormal image may be output without obtaining the intended fixing property.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of obtaining good image fixing and a method for controlling the image forming apparatus.

  In the present invention, the object is achieved by providing an image forming apparatus having the following configuration.

Image forming means for forming an image on a sheet;
Fixing means for thermally fixing the image formed by the image forming means to the sheet while conveying the sheet;
Determination means for determining a density distribution in a direction orthogonal to the sheet conveyance direction of the image to be formed;
An image forming apparatus comprising: a control unit that rotates an image to be formed by 90 degrees according to a determination result of the determination unit and thermally fixes the image to the fixing unit.

  According to the configuration of the present invention, it is possible to perform good image fixing.

Schematic configuration diagram of an image forming apparatus Block diagram including engine control unit of image forming apparatus Thermistor distribution map and heater area inside the fuser Flow chart showing image forming process of image forming apparatus Flow chart showing control method of image rotation processing A diagram showing the number of jobs, the printing rate, and the presence or absence of rotation Figure showing image rotation image

<Outline of Embodiment>
The image forming apparatus described in this embodiment can be used as, for example, a copying machine, a printer, a facsimile machine, or a complex machine thereof. The image forming method of this embodiment can be used in such an image forming apparatus. In the image forming method and the image forming apparatus of the present embodiment, a toner image formed with a colorant such as toner based on a document image is formed on a recording sheet that is a sheet-like recording medium, and then the recording sheet The upper unfixed toner image is heat-fixed on the recording sheet. The image forming apparatus includes an image forming device and a fixing device in order to form an image in this way. With regard to this image forming method, for example, a toner image may be formed and fixed on a recording sheet using an image forming apparatus and a fixing apparatus as described below. The image forming apparatus forms a toner image based on a document image on a recording sheet. For example, a toner image based on an original image may be formed on a recording sheet by electrophotography. The image forming apparatus may form a monochrome image on a recording sheet or may form a color image on a recording sheet.

  The recording sheet on which the toner image based on the original image is formed by the image forming device is conveyed to a fixing device for fixing the toner image. The fixing device heat-fixes the toner image formed on the recording sheet by the image forming device on the recording sheet. The fixing device uses a temperature detection element such as a thermistor in the device to control the temperature so that the temperature is constant, and heat is applied to the unfixed toner image on the recording sheet and fixing is performed by applying pressure. You can do it. A conventionally known fixing device can be used. The fixing device has a fixing member such as a fixing roller for performing heat fixing. The fixing device may include, for example, two fixing rollers that press each other as a fixing member and a heater that heats one or both of the fixing rollers as a heat source. In this case, heat and pressure fixing can be performed by passing a recording sheet between two fixing rollers.

  As for the temperature of the fixing member for applying heat to the toner on the recording sheet, for example, as described above, the heater maintains a predetermined fixing temperature using a temperature detection element such as a thermistor for detecting the temperature of the heater in the fixing device. The temperature may be controlled as described above. Here, in this embodiment, one line of raster scanning is called main scanning, paper conveyance is called sub scanning, and the respective directions are called main scanning direction and sub scanning direction.

  In the image forming method and the image forming apparatus according to the present embodiment, the temperature distribution variation of the heater is reduced by rotating the image forming direction by 90 degrees so that the print density variation in the main scanning direction is reduced, and the recording sheet A toner image based on the original image is formed thereon.

  Here, to describe a general story, in an image forming apparatus, the orientation of a toner image formed on a recording sheet with respect to the recording sheet conveyance direction is usually determined in advance depending on the orientation of the original image. When the image forming apparatus is a copying machine, for example, the orientation of the toner image formed on the recording sheet is usually determined in advance according to the orientation of the original image when the original is placed on the original table. When the image forming apparatus is a printer connected to a host device such as a computer, the orientation of the toner image formed on the recording sheet according to the orientation of the original image when the original image is created by the host device. Is predetermined.

  In contrast, in the image forming method of the present embodiment, the toner is based on the original image on the recording sheet by rotating the image direction according to the print density distribution on the recording sheet, regardless of the original image direction. Form an image. Here, in order to detect the above-described print density, for example, dot count information represented by the number of dots for each density level of each region of the original image may be detected. When detecting the dot count information, for example, a device including a dot counter capable of counting the number of dots for each density level of the document image may be employed as the density detection device. The density detection device may detect the average image density of each region of the document image. The average image density of each area can be detected from the number of dots of each density of each area of the original image, that is, dot count information. The average image density of each area of the document image can be detected using, for example, one or more density sensors. It is determined whether or not the image is to be rotated by 90 degrees based on the print density detected by the print density sensor, which is a density detection means, and a preset print density and a threshold value for the number of printed sheets. The direction of rotation may be either. Although details of the image rotation determination criterion will be described later, when the toner image based on the original image is formed by rotation, variations in the temperature distribution of the heater can be further reduced. Details will be described with reference to the following embodiments.

<Detailed explanation>
DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings by way of preferred examples. The following embodiments do not limit the invention according to the scope of claims, and all combinations of features described in the embodiments are not necessarily essential to the solution means of the invention.

(First embodiment)
FIG. 1 shows a schematic configuration diagram of an image forming apparatus. The user can use the operation panel (not shown) at the top of the device to start copying, select the size of the recording sheet for copying the original image, select the number of copies, copy mode such as color copy or monochrome copy Can be selected. Recording sheets for copying original images are stored in sheet cassettes 6, 18, and 19. The cassette 18 stores A4 size paper in the horizontal direction in this example. In the cassette 19, A4 size paper is stored vertically. Note that the longitudinal recording sheet is a recording sheet in which the conveyance direction of the recording sheet is parallel to the long side direction of the recording sheet. The horizontal recording sheet is a recording sheet in which the recording sheet conveyance direction is parallel to the short side direction of the recording sheet. Similarly, a document in which the recording sheet conveyance direction and the long side direction are parallel is referred to as a vertically oriented document. A document in which the recording sheet conveyance direction and the short side direction are parallel is referred to as a landscape document.

  Of the recording sheets stored in the cassettes 6, 18, and 19, the original image is copied to the next recording sheet. When the user selects which recording sheet to copy from the operation panel, the document image is copied to the selected recording sheet. If such a selection is not made, a recording sheet having an appropriate size and orientation is selected according to the document size and orientation on the platen glass, and the document image is copied to the selected recording sheet. A sensor (not shown) for detecting the current size and orientation is arranged at the bottom of the platen glass, and a recording for copying a document image based on the size and orientation of the document detected by this sensor. A sheet is selected. For example, when copying an A4 portrait document, an A4 portrait recording sheet is selected. When there is no recording sheet suitable for the size and orientation of the document, this is displayed on a liquid crystal display panel (not shown) of the operation panel.

  The recording material M loaded in the selected paper feed cassette is fed by the paper feed roller 7. For example, the recording material M fed from the paper feed cassette 6 is transported by the transport roller 8 and is transported to the registration roller 11 while the transport state is detected by the transport sensor 9. When the leading edge of the recording material M reaches the pre-registration sensor 10, an electromagnetic clutch (not shown) that controls the rotational drive of the registration roller 11 is turned off, and the rotation of the registration roller 11 is stopped. When the recording material M is conveyed to the registration roller 11, the media sensor 17 detects the type of the recording material M.

  When the recording material M is conveyed to the registration roller 11, image formation is performed by the image forming unit. The image forming unit includes a photosensitive drum 2, a charging roller 3, a developing roller 4, and a cleaning blade 5. First, based on image information input to the controller of the image forming apparatus 100, a laser is emitted from the laser optical system 1 to the photosensitive drum 2 charged by the charging roller 3. The photosensitive drum 2 rotated by the laser is optically scanned in the main scanning direction to form an electrostatic latent image. When the latent image is formed, the developing roller 4 develops the latent image as a toner image. The recording material M is transported again, and the toner image formed on the photosensitive drum 2 is transferred onto the recording material M by the transfer roller 12. The photosensitive drum 2 to which the toner image is transferred is cleaned by a cleaning blade 5.

  The recording material M on which the toner image is transferred is conveyed to a fixing unit. The fixing unit is an on-demand fixing device including a fixing film 13, a heater 14, and a pressure roller 15. The fixing film 13 constitutes a fixing roller that is pressed against the pressure roller 15 that is rotationally driven, and forms a nip portion that sandwiches the sheet conveyed in the direction perpendicular to the axial direction at the pressing portion. When the fixing unit comes into contact with the sheet on which the colorant image is formed, the toner image is fixed on the recording material M, and the recording material M is discharged onto the paper discharge tray 16.

  The present embodiment relates to temperature control of the fixing unit portion in such an image forming apparatus.

<Configuration for control>
FIG. 2 is a block diagram of the image forming apparatus 100. A host computer 200 transmits image information and a print command to the controller. A controller 201 analyzes image information received from the host computer 200 and transmits the image information to the video interface unit 203. The controller 201 can communicate with the host computer 200 and the engine control unit 202. The controller 201 transmits a print reservation command, a print start command, and a video signal to the engine control unit 202 for each recording material via the video interface unit 203. Similarly, image information read by the image reading unit 205 such as a scanner is processed in the controller 201 and transmitted to the engine control unit via the video interface unit 203. Note that images that are formed are not limited to characters, but data other than characters such as vector graphic data described in vectors and image data composed of dots are also subject to printing. Includes non-character print data data such as vector graphics and images.

  The controller 201 performs print rate density detection by an internal dot counter processing unit based on print data sent from the host computer 200 or the image reading unit 205. The calculation of the printing rate density is performed for each area obtained by dividing one page of the document image into a predetermined number along the main scanning direction. The printing rate density may be, for example, the printing rate itself. Note that the printing rate is defined as “area of the area where the toner is transferred / area of the entire image forming area” indicating the toner application rate. Alternatively, instead of the printing rate, a value related to the number of dots of toner transferred to the recording material M or a value related to the number of dots of toner per unit area may be used. The reason why the number of dots can be used is that the number of dots serving as the denominator of the printing rate can be determined according to the size or unit area of the recording sheet. For example, when a value indicating the number of toner dots for each area transferred to the recording material M is received, a table showing the relationship between the size of the recording material M and the number of dots is used instead of the printing rate. You can also. When a value related to the number of toner dots per unit area is received, a value related to the number of toner dots per unit area may be averaged to replace the printing rate. In the present embodiment, the printing rate is acquired for each area (hereinafter simply referred to as an area) divided in the main scanning direction (that is, along the sub-scanning direction). If the total number of dots when printing in solid black for each area is specified in advance, the table shows the relationship between the number of dots received by that value or the printing ratio of the number of dots for each area Can be used to determine the printing rate of the area. As described above, the value relating to the printing rate may be a value that corresponds to or correlates with the printing rate. In the following description, for convenience, the value relating to the printing rate is described as being the printing rate itself. Therefore, the printing rate in the following description can be replaced with a value related to the printing rate.

  The number of dots may be the number of dots in the image after binarization processing. If the dot is not a binary value but a multi-value such as a quaternary value, for example, each dot may be converted into a value indicating the toner amount corresponding to the density value, and the value may be accumulated in the area to obtain a printing rate. . When the image to be formed is monochrome, the binarized dots may be counted as they are. However, in the case of a color image, for example, the total number of dots for all color components is used instead of the printing rate. Can do.

  In the present embodiment, as will be described later, image rotation processing is performed by the engine control unit 202. Since the reference for the presence or absence of rotation is the printing rate or the number of dots for each region, the number of dots or the printing rate calculated by the controller 201 is notified to the engine control unit 202 by a print reservation command or other commands, for example. Since the rotation process is performed in units of jobs as shown in FIG. 4, the controller 201 further notifies the engine control unit 202 of the start of the job and the number of sheets printed in the job. Further, information regarding whether or not post-processing is designated for the job is also notified from the controller 201 to the engine control unit 202 prior to the start of the job.

<Example of area division>
FIG. 3 is a view showing the arrangement of the heater 300 in the fixing unit, a thermo switch (SW) 301 for overtemperature protection, a temperature control main thermistor 302, and an end overtemperature detection sub-thermistor 303. It is also a diagram showing an example of a region obtained by dividing an image to be recorded in the main scanning direction, that is, along the sub-scanning direction. The CPU 204 controls the temperature so that the heater 300 reaches a predetermined temperature while reading the temperature information of the main thermistor 302. When image information is transmitted from the controller 201 to the video interface unit 203 of the engine control unit 202, the CPU 204 compares the maximum print rate yy% (305) and the minimum print rate xx% (306) in the area of FIG. Process. The maximum printing rate is the maximum value of the printing rate of each divided area. The minimum printing rate is the minimum value among the printing rates of each area divided in the page. In the example of FIG. 3, the areas are not all the same size, the area at one end (left end in the figure) is large, and the other areas are smaller (narrower) than the leftmost area and have the same size. It has become. One of the reasons for dividing in this way is that documents including characters and the like are often left-justified and many documents have a blank area on the right side. Accordingly, when the document is printed vertically as in the example of FIG. 3, it is considered that the leftmost area often has the maximum printing rate yy and the rightmost area has the minimum printing rate xx.

<Image formation processing procedure>
An actual processing example will be described with reference to the flowchart of FIG. The printing rate here can be controlled in the same manner even if it is a value related to the printing rate as described above. The procedure shown in FIG. 4 is executed by the engine control unit 202, particularly the CPU 204 in this embodiment.

In S101, the CPU 204 performs preliminary temperature control while reading the temperature information of the thermistor 207.
In step S102, the CPU 204 receives job data. In addition to the print data, the job data includes the number of prints of the job, the presence / absence of post-processing, and the print rate for each area.
In S103, it is determined whether or not post-processing is designated for the job to be processed. If there is post-processing such as stapling, punching or folding, the direction of the image formed in relation to the post-processing position such as punching or stapling cannot be changed carelessly, and the process branches to S106. If it is determined in S103 that no post-processing is designated, the process branches to S104.
In S104, it is determined whether the number of received jobs, that is, the number of sheets to be printed by one job is three or less. If it is determined in S104 that the number of prints of the received job is three or less, the process skips S105 and proceeds to the printing operation S106 without performing the printing rate calculation process. This is because if the number of prints is less than a predetermined number such as 3 sheets, the accumulated temperature difference is small, and it is considered difficult to obtain the advantage by rotating the image. If it is determined in S104 that the number of received jobs is greater than 3, the process branches to S105 to execute a subroutine “rotation determination process”. Although the threshold value is 3 here, other values may be used.

In S106, the received job is executed, an image is formed on the recording medium, and it is discharged. At this time, if the image is rotated in S105, the rotated image is printed.
In S107, it is determined whether or not all received jobs have been completed. If all jobs have not been completed, the process returns to S104 and the same flow is repeated again. If all jobs have been completed, all image forming processes are completed.

<Rotation determination process (S105)>
FIG. 5 is a flowchart (subroutine “rotation determination process” in S105) showing a method for determining the rotation of the recording sheet material when the received job has no post-processing and the number of printed sheets is three or more. .
In step S <b> 201, the CPU 204 performs a comparison calculation process of a difference between the printing rate of the maximum printing area 305 and the printing rate of the minimum printing area 306 and a predetermined threshold value. The predetermined threshold is less than 10% because the threshold in S207 described later is 10%, and is a value such as 3 to 5%, for example. If the difference is smaller than this threshold, the process is terminated without performing rotation. If the difference is larger than this threshold value, a threshold value corresponding to the number of jobs is further set, and if the difference in printing rate is equal to or greater than the threshold value, rotation is performed.
In S202, it is determined whether the number of jobs is 4-10. If the number of jobs is 4 to 10, the process proceeds to S203. If the difference between the printing ratio of the maximum printing area 304 and the printing ratio of the minimum printing area 305 exceeds 50% in S203, the process branches to S208 to rotate the image 90 degrees and perform image forming processing. If the number of jobs is not 4 to 10 in S202, the process proceeds to S204.
In S204, if the number of jobs is 10 to 30, the process proceeds to S205. If the difference between the printing ratio of the maximum printing area 304 and the printing ratio of the minimum printing area 305 exceeds 30% in S205, the process branches to S208 to rotate the image 90 degrees and perform image forming processing. In S204, if the number of jobs is not 10 to 30, the process proceeds to S206.
In S206, if the number of jobs is 30 or more, the process proceeds to S207. If the difference between the printing ratio of the maximum printing area 304 and the printing ratio of the minimum printing area 305 exceeds 10% in S207, the process branches to S208 to rotate the image 90 degrees and perform image forming processing. If the number of jobs is not 30 or more in S206, the calculation process is terminated as it is. In this way, the threshold of the printing rate is determined for each number of printed sheets, and the presence or absence of rotation is determined by comparison with the threshold. If the difference between the maximum and minimum printing ratios is less than or equal to the threshold value, the image does not rotate. Here, as the number of printed sheets increases, the temperature difference increases cumulatively even if the density difference between regions in one page is small. Therefore, the threshold value is reduced as the number of printed sheets increases. For convenience of explanation, the difference is the maximum and minimum difference. However, the difference is not necessarily the maximum and minimum. For example, if there is an area in the image of one page where the difference in the printing rate exceeds the threshold, the image Rotate.
In S208, it is assumed that all pages printed by the job to be processed are similarly rotated or not rotated. In S208, the received video data is rotated by 90 degrees, the sheet to be used is changed to a rotated sheet, and the tray holding the sheet is designated again.

  When the engine reservation unit 202 receives a print reservation command from the controller 201 via the video interface unit 203, the engine control unit 202 controls the CPU 204 to prepare for execution of printing in the order of the print reservation command, and receives the print start command. wait. When the print start command is received, the CPU 204 reads the temperature information of the thermistor 207 and controls the temperature of the heater so as to reach a predetermined temperature. When the heater reaches a predetermined temperature, the controller 201 outputs a / TOP signal as a reference timing for outputting a video signal to the controller 201, and starts a printing operation in accordance with a printing reservation command. That is, the procedure shown in FIG. 4 is started when a print start command is received.

  In the procedure of FIG. 5, the printing rate may be acquired for the first page of the job, or may be acquired as an accumulated value for all pages or representative pages. The procedure shown in FIG. 4 may be a procedure executed when one or a plurality of copies of one page of document per job is printed. In this case, even if a plurality of sheets are printed in one job, the contents are all the same. Therefore, if the printing rate is acquired only for one page to be printed and the rotation process of S208 is performed according to the conditions, the fixing device For example, the deviation of heat in the axial direction of the fixing roller and the pressure roller can be effectively reduced. In that case, the determination in S104 may be branched to S105 when the number of pages is 1 and the number of copies is greater than a predetermined number, for example, 3 copies.

  In addition, the rotation processing in S208 may be a rotation flag indicating that rotation is performed, for example, instead of the rotation processing itself. The sheet tray to be used at this time is also specified in accordance with the rotated page. In this case, the rotation flag is reset before S201. Then, the page rotation may be configured to refer to the rotation flag in S106 and form an image by rotating the page 90 degrees if it is on. In this way, the engine control unit 202 can perform rotation processing on the received data without holding all pages to be printed.

<Example of image rotation processing>
FIG. 6 illustrates the presence or absence of a rotation process according to the printing rate difference and the number of printed pages (that is, the number of jobs). The example of FIG. 6 follows the determination criteria in the procedure shown in FIG. For example, if the number of sheets is 4 or more and less than 10, if the printing rate exceeds 50%, the image is rotated and printed. Further, the threshold value of the number of jobs to be transferred to the rotation determination process described here and the threshold value for determining the printing rate are also examples in the configuration of the present embodiment, and are used for heating the melting point of the toner and the fixing device. Changes can be made as appropriate by changing the configuration of the power supply capacity, the pressure roller, and the like.

  FIG. 7 is a diagram for explaining the effect of rotation. If the image before image rotation is printed as it is, the printing rate gradually decreases from the left side to the right side of the sheet. Therefore, when fixing of this page is repeated, the axial direction of the heater or fixing roller (the horizontal direction in the figure) There is a temperature gradient where the temperature rises from the left side to the right side. If this is repeated over a plurality of pages, the temperature gradient becomes steep. Particularly, when the heater is controlled by the central temperature sensor, the higher temperature side of the heater becomes higher and it is difficult to eliminate the temperature gradient. Therefore, as shown in the right side of FIG. 7, the gradient of the printing rate with respect to the main scanning direction is eliminated by rotating the image by 90 degrees.

Although the control method using the printing rate has been described here as an example, the determination criterion is not limited to the printing rate. For example, the toner amount itself can be obtained by pixel count and compared with the threshold value.
In this way, by providing a plurality of threshold values for the result of calculating and comparing the maximum printing rate and the minimum printing rate of each area divided into an arbitrary number with respect to the main scanning direction of the document image, a recording sheet By rotating the, the temperature nonuniformity on the heater can be eliminated and a stable fixing property can be provided.

  In this embodiment, an example in which A4 portrait and A4 landscape sheets are installed in the cassette is shown. However, when a sheet rotated 90 degrees is not installed, a post-processing device such as a staple is connected. In the case where the stapling process cannot be performed at a desired position after the rotation, the image may not be rotated and the sheet conveyance interval may be increased to cool the heater. In this case, whether or not a sheet corresponding to the rotated direction can be supplied may be added as a condition for executing S105. For example, immediately before S103, it is determined whether or not it is possible to supply a sheet obtained by rotating the sheet size designated by the current job by 90 degrees. If it cannot be supplied, the process branches to S106, and if possible, the process branches to S103. Whether or not the sheet can be supplied may be determined based on the function of the image forming apparatus, or may be determined based on whether or not the corresponding sheet can actually be supplied. In the latter case, for example, it is determined that supply is not possible if there is a function but the corresponding cassette is not set.

  In the description of the embodiment, it has been described that printing of one job is collectively performed in S106, but it may be configured to print one sheet at a time and branch to S104 if there are remaining pages. In that case, the data of the printing rate determined by the procedure of FIG. 5 can be data relating to the print data to be printed.

  4 and 5 may be executed by the controller 201. In this case, since all processing relating to page rotation is performed by the controller 201, the engine control unit 202 may control image formation according to data received from the controller 201 as in the conventional case.

  The print rate determined in the procedure of FIG. 5 may be acquired for each area by dividing the first page of the job data, for example, but may be acquired for a plurality of pages if the job is a multi-page job. . In that case, all pages in the job may be targeted, or a part may be targeted. However, when printing multiple copies, it is of course sufficient to target only one copy.

  With the configuration and procedure described above, according to the image forming apparatus of the present embodiment, it is possible to prevent temperature deviation on the fixing member caused by uneven distribution of toner on the sheet. For example, when printing a plurality of copies of an image of one page, the same image is repeatedly printed, and a temperature drop at the toner adhesion portion and heating to compensate for the temperature drop are repeatedly performed on the fixing member. The expansion of the temperature difference in the axial direction (especially in the case of rollers) becomes significant. In such a case, the present embodiment is particularly effective in making the temperature of the fixing member uniform.

201: Controller (including dot counter processing unit), 202: Engine control unit, 203: Video I / F unit, 204: CPU, 305: Maximum printing rate area yy%, 306: Minimum printing rate area xx%

Claims (7)

Image forming means for forming an image on a sheet;
Fixing means for thermally fixing the image formed by the image forming means to the sheet while conveying the sheet;
Determination means for determining a density distribution in a direction orthogonal to the sheet conveyance direction of the image to be formed;
An image forming apparatus comprising: a control unit that rotates the formed image by 90 degrees according to a determination result of the determination unit and thermally fixes the image to the fixing unit. The determination unit determines a density difference in a direction orthogonal to a sheet conveyance direction of the formed image;
The image forming apparatus according to claim 1, wherein the control unit rotates the formed image by 90 degrees when the density difference is equal to or greater than a threshold value.   The image forming apparatus according to claim 1, wherein the threshold value is smaller as the number of pages on which an image is formed is larger.   The image forming apparatus according to claim 1, wherein the control unit does not rotate the formed image when post-processing is performed on the image-formed sheet.   5. The image forming apparatus according to claim 1, wherein the control unit does not rotate the formed image if the number of pages on which an image is formed is equal to or less than a reference number.   The said control means does not rotate the said formed image, when there is no sheet | seat used when rotating and outputting the said formed image. The image forming apparatus described in 1. In a control method of an image forming apparatus provided with a fixing unit that thermally fixes an image formed by an image forming unit to a sheet while conveying the sheet,
A determination step of determining a density distribution in a direction orthogonal to the sheet conveyance direction of the image to be formed;
A control method for an image forming apparatus, wherein the formed image is rotated 90 degrees and thermally fixed on the fixing unit in accordance with a determination result in the determination step.

Images