JP4710346B2 - Image forming apparatus and color misregistration correction method - Google Patents

Description

  The present invention relates to a so-called tandem color image forming apparatus in which a plurality of image forming stations, each of which forms toner images of different colors, are arranged along the moving direction of a transfer medium. The present invention relates to a technique for correcting color misregistration.

  As this type of image forming apparatus, for example, an apparatus described in Patent Document 1 is known. In this image forming apparatus, an image forming station in which a charging unit, an image writing unit, and a developing unit are arranged around a latent image carrier such as a photosensitive drum is provided for each color along a transfer medium such as a transfer belt. Yes. A toner image formed at each image forming station is superimposed on a transfer medium to form a color image.

  One of the serious problems in an image forming apparatus having a plurality of image forming stations is color misregistration. This occurs when the transfer positions of the toner images formed at different image forming stations onto the transfer medium are shifted from each other, and appear as a change in color tone. Therefore, in order to solve this problem, a reference pattern image (hereinafter referred to as a “registration mark”) for detecting color misregistration is formed on a transfer medium in advance, and each resist mark is detected by an optical sensor. Find the mark position information. Based on the positional information, the relative positional relationship between the toner images formed by the image forming stations can be grasped. Therefore, the registration control amount for correcting the relative color shift of the toner image is conventionally obtained from the position information, and when the color image is actually formed, the formation position of the toner image of each color is based on the registration control amount. Color misregistration is suppressed by adjusting.

JP 2004-109617 A (page 4)

  By the way, when image formation is performed, the operating environment, particularly the internal temperature of the apparatus (so-called internal temperature) may change. As a result, the registration control amount may deviate from the value obtained as described above, and color misregistration may occur. Here, with respect to fluctuations in the resist control amount caused by fluctuations in the in-machine temperature, the in-machine temperature may be measured by a temperature sensor and the resist control quantity may be adjusted based on the detection result as conventionally proposed. However, when double-sided printing is continuously performed, it may not be possible to appropriately cope with the adjustment of the resist control amount according to the in-machine temperature. This is because, for example, in the apparatus described in Patent Document 1, the first image formed on the intermediate transfer belt is transferred to the first surface of the sheet-like recording material and then thermally fixed by the fixing unit. In parallel with the formation of the second image to be printed on the second surface of the recording material on the intermediate transfer belt, the recording material that has undergone thermal fixing is conveyed again to the secondary transfer position. Then, the second image is transferred to the second surface of the recording material at the secondary transfer position to perform double-sided printing. Therefore, at the time of printing an image on the first surface (single-sided printing), a recording material at room temperature is secondarily transferred at the secondary transfer position. However, in order to perform double-sided printing, two recording materials heated by the fixing unit are used. Since the sheet is conveyed to the next transfer position, parts related to transfer such as the intermediate transfer belt, the driving roller, and the secondary transfer roller are locally heated. Among these components, each part of the intermediate transfer belt is locally heated at the secondary transfer position, and then the surface temperature gradually decreases while moving to the first image forming station. Further, the station closest to the secondary transfer position among the image forming stations is affected by heat from the secondary transfer position. For these reasons, the surface temperature of the intermediate transfer belt may be different for each image forming station due to double-sided printing. As a result, the surface temperature of the intermediate transfer belt varies from image forming station to image forming station due to the local thermal effect of the recording material heat-fixed by continuously performing double-sided printing, even though the temperature inside the machine is small. The resist control amount may deviate from the optimum value due to the surface temperature difference.

  The present invention has been made in view of the above problems, and in an image forming apparatus that continuously executes a printing operation including double-sided printing, color misregistration occurs by appropriately adjusting the resist control amount during the continuous printing operation. The purpose is to prevent it effectively.

According to the present invention, a plurality of image forming stations, each forming a different color toner image, are arranged along a moving direction of a transfer medium moving at a predetermined speed, and the toner images formed by the plurality of image forming stations. The plurality of toner images can be superimposed on the transfer medium while controlling the relative positional relationship based on the resist control amount, and a first image formed on the transfer medium can be formed. Is transferred to the first surface of the sheet-like recording material and thermally fixed, and then the second image formed on the transfer medium is transferred to the second surface of the recording material . color shift correction method in an image forming apparatus and the apparatus for performing single-sided printing directly discharging the recording material after the thermal fixing while transferring the first image on the first side of the sheet-shaped recording material It relates.

  In the image forming apparatus configured as described above, when double-sided printing is executed during the continuous printing operation, the second image is transferred to the recording material that has been thermally fixed and heated, and the image is transferred onto the transfer medium. A local temperature rise occurs in the area where the image of the above is transferred to the recording material. As the temperature rises, the surface temperature of the transfer medium differs between the image forming stations, so that the relative positional relationship of the toner images formed by the plurality of image forming stations varies.

Therefore, in the image forming apparatus and the color misregistration correction method according to the present invention, a measurement unit that measures the surface temperature of the transfer medium is provided , and the registration control amount is calculated based on the measurement result of the measurement unit only when duplex printing is performed. It is adjusting. Therefore, the color image formation is performed by the registration control amount which is adjusted to take into account the color shift factor due to the double-sided printing. As a result, even if double-sided printing is included in the continuous printing operation, it is possible to effectively prevent color misregistration from occurring during the continuous printing operation.

Direct impact of the surface temperature of the transfer medium between warmed transfer medium image forming station by both the surface printing are different appears in the relative positional relationship of the toner image between the image forming station. Therefore, the amount of toner image misregistration between image forming stations is obtained based on the surface temperature of the transfer medium, and the resist control amount is adjusted to an appropriate value during continuous printing operation by adjusting the resist control amount according to the amount of misalignment. Thus, color misregistration can be reliably prevented. Here, the most upstream temperature sensor for measuring the surface temperature of the transfer medium at the most upstream station in the moving direction of the transfer medium among the plurality of image forming stations, and the most downstream temperature sensor in the moving direction of the transfer medium. The most downstream temperature sensor that measures the surface temperature of the transfer medium at the most downstream station is provided as a measurement means, and the deviation amount can be obtained from the measurement results of both sensors. By measuring the surface temperature of the transfer medium between the image forming stations farthest in this way, the surface temperature difference of the transfer medium between the image forming stations can be accurately predicted, and the resist control amount can be further increased. The accuracy can be adjusted. Of course, the surface temperature difference of the transfer medium between the image forming stations may be measured by measuring the surface temperature of the transfer medium at each image forming station.

  Also, while the temperature difference between the most upstream surface temperature measured by the most upstream temperature sensor and the most downstream surface temperature measured by the most downstream temperature sensor is zero or relatively small, there is no thermal effect due to duplex printing, or It is thought that there are few. Therefore, the resist control amount may be adjusted only when the temperature difference becomes a predetermined value or more. In this way, useless registration adjustment operations can be omitted. Further, the registration control amount adjustment processing may be performed after each printing operation is performed, or conversely, may be performed before each printing operation.

  Furthermore, as described above, the surface temperature difference of the transfer medium between the image forming stations increases every time double-sided printing is performed during continuous printing operation. Therefore, if double-sided printing is performed in large quantities and continuously, continuous printing is performed. The amount of toner image deviation increases during operation. In such a case, the color image forming operation may be temporarily interrupted during continuous printing while the movement of the transfer medium is continued to newly obtain the registration control amount to update the registration control amount. Of course, instead of interrupting the process for obtaining the registration control amount as described above in the continuous printing operation, the continuous printing operation may be completely stopped and the registration control amount may be newly obtained. In consideration of stability and the like, it is desirable to temporarily interrupt the color image forming operation while moving the transfer medium as described above and execute the registration control amount update process.

A. Regarding the speed fluctuation of the intermediate transfer belt due to double-sided printing In an electrophotographic image forming apparatus, a toner image is formed on a transfer medium such as an intermediate transfer belt, and a recording material stored in a cassette or a manual feed tray is used. The toner image on the transfer medium is transferred to the first surface of the recording material. Further, the toner image is thermally fixed on the recording material. In the case of single-sided printing, the paper is discharged as it is toward the paper discharge tray. On the other hand, in the case of duplex printing, the thermally fixed recording material is conveyed again to the transfer position and the toner image is transferred to the second surface. Therefore, when performing single-sided printing (including printing on the first side of double-sided printing), the toner image is transferred to a recording material close to room temperature, whereas when performing the second side of double-sided printing, heat is applied. There is a possibility that the toner image is transferred to the recording material which is fixed and in a high temperature state, and the temperature rises locally at the transfer position. As described above, since the thermal effect is greatly different between single-sided printing and double-sided printing, the thermal effect in single-sided printing and the thermal effect in double-sided printing were measured and compared.

  FIG. 1 is a graph showing thermal effects in continuous single-sided printing and continuous double-sided printing, and when monochrome printing was continuously performed on A3-sized plain paper using a color laser printer (model number: LP-9000C) manufactured by Seiko Epson. Results are shown. More specifically, the plot in FIG. 5A shows the progress of movement between two points on the transfer medium (intermediate transfer belt) when monochrome double-sided printing is continuously performed on 250 sheets (500 sheets in terms of single-sided printing). The plot in Fig. 8 (b) is a plot of the elapsed time between two points on the intermediate transfer belt (intermediate transfer belt) when 500 sheets of monochrome single-sided printing are continuously performed. It is. In addition, the straight lines in the figure indicate the relationship between the number of printed sheets and the elapsed movement time in double-sided printing and single-sided printing, respectively. As is apparent from the figure, even if single-sided printing is continuously performed, the elapsed time of movement hardly changes, and the speed fluctuation of the intermediate transfer belt is hardly recognized. On the other hand, in double-sided printing, it can be seen that the elapsed time of movement becomes longer in proportion to the number of double-sided printed sheets, and the speed fluctuation of the intermediate transfer belt is reduced. Incidentally, based on the actual measurement result shown in FIG. 1A, the rate of speed fluctuation per double-sided printing was about -0.001 (%).

  As double-sided printing increases in this way, speed fluctuations occur due to the thermal effect caused by double-sided printing, and the relative positional relationship of the toner images formed by each image forming station may be greatly shifted due to the thermal effect. is there. As a result, although a reference pattern image such as a resist mark is formed in advance to obtain a resist control amount and color misregistration correction is performed based on the resist control amount, color misregistration may occur. As a result of examining the main factors of this color misregistration, one of the main factors is that the transfer medium is locally heated by double-sided printing, and the surface temperature of the transfer medium differs depending on the position in the moving direction of the transfer medium. I understood.

  Therefore, it can be understood that the following problem occurs in double-sided printing even in a tandem image forming apparatus that performs double-sided printing. That is, when double-sided printing is performed, the heat-fixed recording material comes into contact with the transfer medium at the secondary transfer position, whereby the transfer medium is locally heated and the surface of the transfer medium between the image forming stations. The temperature may be different. For this reason, the toner image appears in the relative positional relationship between the image forming stations, and as a result, color misregistration due to double-sided printing occurs. The details will be described later.

  Therefore, in the embodiment according to the present invention, the surface temperature of the transfer medium is measured during the continuous printing operation, and the resist control amount is adjusted based on the measurement result. Hereinafter, specific embodiments will be described in detail.

B. Apparatus Configuration and Operation Hereinafter, three embodiments of the image forming apparatus according to the present invention will be described. The apparatus configuration and basic operation of these embodiments are the same, and only the operation is partially different. The configuration and operation of the apparatus common to both will be described, and thereafter, the operation unique to each embodiment will be described.

  FIG. 2 is a diagram showing a basic configuration of the image forming apparatus according to the present invention. FIG. 3 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. The apparatus 1 includes a color printing process for forming a full-color image by superposing four color toners (developers) of black (K), cyan (C), magenta (M), and yellow (Y), and black (K ) To selectively execute a monochrome printing process for forming a monochrome image using only toner. In this image forming apparatus 1, when an image forming command (printing command) is given to the main controller 51 from an external device such as a host computer, the engine controller 52 controls each part of the engine unit EG in accordance with the command from the main controller 51. Then, a predetermined image forming operation is executed, and an image corresponding to the image forming command is formed on a sheet (recording material) S such as plain paper, thick paper, and an OHP transparent sheet.

  2, an image forming apparatus 1 according to the present embodiment includes a housing body 2, a first opening / closing member 3 that is detachably attached to the front surface of the housing body 2 (the right-hand side surface in FIG. 2), and the housing body 2. And a second opening / closing member 4 (also serving as a paper discharge tray) mounted on the upper surface so as to be freely opened and closed. The first opening / closing member 3 is provided with an opening / closing lid 3 a that can be opened and closed on the front surface of the housing body 2. The opening / closing lid 3a can be opened / closed in conjunction with or independently of the first opening / closing member 3.

  In the housing main body 2, an electrical component box 5 containing a power circuit board, a main controller 51, and an engine controller 52 is provided. An image forming unit 6, a blower fan 7, a transfer belt unit 9 and a paper feed unit 10 are also disposed in the housing body 2. On the other hand, on the first opening / closing member 3 side, a secondary transfer unit 11, a fixing unit 12 and a sheet conveying mechanism 13 are disposed. In this embodiment, the consumables in the image forming unit 6 and the paper feeding unit 10 are configured to be detachable from the apparatus main body. The consumables and the transfer belt unit 9 can be removed and repaired or exchanged.

  The transfer belt unit 9 includes a driving roller 14 that is rotationally driven by a driving motor (not shown) disposed below the housing body 2, a driven roller 15 that is disposed obliquely above the driving roller 14, An intermediate transfer belt 16 that is stretched between the rollers 14 and 15 and is circulated and driven in the direction of the arrow D16 in the figure, and a belt cleaner 17 that abuts on the surface of the intermediate transfer belt 16 are provided. The driven roller 15 is disposed obliquely above the drive roller 14 (upward on the left hand in FIG. 2). For this reason, the intermediate transfer belt 16 rotates and moves in the direction D16 while being inclined. Further, the belt surface 16a in which the belt conveyance direction D16 when the intermediate transfer belt 16 is driven is downward (downward right in FIG. 2) is positioned below. In the present embodiment, the belt surface 16a is a belt tension surface (surface pulled by the drive roller 14) when the belt is driven, and the peripheral speed is lower than the peripheral speed of the photosensitive drum (image carrier) 20 of each color. have. In this way, by setting the peripheral speed of the intermediate transfer belt 16 to be slower than the peripheral speed of each photosensitive drum 20, the photosensitive drum 20 is pulled by the intermediate transfer belt 16 in a direction that suppresses rotation. Driving.

The drive roller 14 also serves as a backup roller for the secondary transfer roller 19. A rubber layer having a thickness of about 3 mm and a volume resistivity of 10 ⁵ Ω · cm or less is formed on the peripheral surface of the drive roller 14, and the illustration is omitted by grounding through a metal shaft 2. A conductive path of the secondary transfer bias supplied from the secondary transfer bias generator through the secondary transfer roller 19 is used. Thus, by providing the driving roller 14 with a rubber layer having high friction and shock absorption, it is difficult for the impact when the sheet S enters the secondary transfer portion to be transmitted to the intermediate transfer belt 16, and deterioration of image quality is prevented. can do.

  In the present embodiment, the diameter of the driving roller 14 is smaller than the diameter of the driven roller 15. Thereby, the sheet S after the secondary transfer can be easily peeled by the elastic force of the sheet S itself. The driven roller 15 is also used as a backup roller for the belt cleaner 17. The belt cleaner 17 is provided on the side of the belt surface 16a facing downward in the transport direction, and includes a cleaning blade 17a that removes residual toner and a toner transport member that transports the removed toner, as shown in FIG. Yes. The cleaning blade 17a contacts the intermediate transfer belt 16 at a portion where the intermediate transfer belt 16 is wound around the driven roller 15, and cleans and removes toner remaining on the surface of the intermediate transfer belt 16 after the secondary transfer.

  The driving roller 14 and the driven roller 15 are rotatably supported by a support frame (not shown) of the transfer belt unit 9. Further, a primary transfer roller 21 is provided on the back surface of the belt surface 16a facing downward in the transport direction of the intermediate transfer belt 16 so as to face the photosensitive drum 20 of each image forming station Y, M, C, K described later. . These four primary transfer rollers 21 are rotatably supported with respect to the support frame, and are electrically connected to the primary transfer bias generator 525. From the primary transfer bias generator 525 at an appropriate timing. A primary transfer bias is applied.

  The support frame is rotatable with respect to the housing main body 2 in the arrow direction D21 with the drive roller 14 as a rotation center. Then, by actuating an actuator (not shown), the support frame is rotated to face the photosensitive drums 20 of the yellow (Y), magenta (M), and cyan (C) image forming stations Y, M, and C. The primary transfer roller 21 arranged in this manner approaches the photosensitive drum 20 and moves away from the photosensitive drum 20. For this reason, when the primary transfer rollers 21 for yellow, magenta, and cyan move close to the photosensitive drum 20, they contact the photosensitive drum 20 with the intermediate transfer belt 16 in between (solid line in FIG. 2). This contact position is the primary transfer position, and the toner image is transferred to the intermediate transfer belt 16 at the primary transfer position. Conversely, when the primary transfer rollers 21 for yellow, magenta, and cyan move away from the photosensitive drum 20, the photosensitive drum 20 and the intermediate transfer belt 16 of the image forming stations Y, M, and C are separated from each other (FIG. Dashed line in 2). On the other hand, the primary transfer roller 21 disposed facing the photosensitive drum 20 of the black (K) image forming station K rotates while being in contact with the photosensitive drum 20 with the intermediate transfer belt 16 interposed therebetween. Is configured to do. Therefore, as shown by the solid line in FIG. 2, the color printing process can be executed by positioning all the primary transfer rollers 21 on the photosensitive drum 20 side. On the other hand, as shown by the broken line in FIG. 2, the intermediate transfer belt is only subjected to the monochrome printing process by leaving the primary transfer roller 21 for black and separating the other primary transfer roller 21 from the photosensitive drum 20. 16 is separated from the image forming stations Y, M, and C, and the yellow, magenta, and cyan colors can be set to a non-printing state. Note that the primary transfer roller 21 for black may also be configured to move away from the photosensitive drum 20 as necessary.

  A test pattern sensor 18 is installed on the support frame of the transfer belt unit 9 in the vicinity of the drive roller 14. The test pattern sensor 18 is a so-called reflection type optical sensor, a light projecting unit (not shown) that irradiates light toward the intermediate transfer belt 16, and light reflected by the surface of the intermediate transfer belt 16 and a registration mark. Is provided with a light receiving portion (not shown). Then, light is emitted from the light projecting unit to the registration mark on the intermediate transfer belt 16, while the light from the registration mark is received by the light receiving unit and a signal corresponding to the amount of light received by the light receiving unit is a test pattern sensor 18 is output. The registration control amount can be obtained based on the output signal from the test pattern sensor 18. Since many methods for deriving the resist control amount have been conventionally proposed, the description thereof is omitted here.

  The image forming unit 6 includes image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) that form a plurality (four in this embodiment) of different color images. I have. These image forming stations Y, M, C, and K are arranged in this order along the conveyance direction (movement direction) D16 of the intermediate transfer belt 16, as shown in FIGS. That is, among these image forming stations, the image forming station Y corresponds to the “most upstream station” of the present invention, and the image forming station K corresponds to the “most downstream station” of the present invention. Each image forming station Y, M, C, K is provided with a photosensitive drum 20. A charging unit 22, an image writing unit 23, a developing unit 24, and a photoconductor cleaner 25 are disposed around each photoconductor drum 20. Then, a charging operation, a latent image forming operation, and a toner developing operation are executed by these functional units. In FIG. 2, since the image forming stations of the image forming unit 6 have the same configuration, for convenience of illustration, only some of the image forming stations are denoted by reference numerals, and the other image forming stations are omitted. . Further, the arrangement order of the image forming stations Y, M, C, and K is arbitrary.

  The photosensitive drums 20 of the image forming stations Y, M, C, and K are disposed so as to be in contact with the belt surface 16a facing downward in the transport direction of the intermediate transfer belt 16 at the primary transfer position TR1. Each of these photosensitive drums 20 is connected to a dedicated drive motor and is driven to rotate at a predetermined peripheral speed in the conveyance direction of the intermediate transfer belt 16 as indicated by an arrow D20 in the figure.

  The charging unit 22 includes a charging roller whose surface is made of elastic rubber. The charging roller is configured to be driven to rotate in contact with the surface of the photosensitive drum 20 at a charging position, and at a peripheral speed in the driven direction with respect to the photosensitive drum 20 as the photosensitive drum 20 rotates. Followed rotation. The charging roller is connected to a charging bias generator (not shown), and receives the charging bias from the charging bias generator to charge the surface of the photosensitive drum 20 at the charging position.

  The image writing unit 23 is an array writing in which elements such as a liquid crystal shutter including a light emitting diode and a backlight are arranged in a line in the axial direction of the photosensitive drum 20 (direction perpendicular to the paper surface of FIG. 2). The head is used and is spaced from the photosensitive drum 20. The array-like writing head has a shorter optical path length and is more compact than the laser scanning optical system. Therefore, it can be disposed close to the photosensitive drum 20 and has the advantage that the entire apparatus can be downsized.

  In this embodiment, the photoconductive drum 20, the charging unit 22, the developing unit 24, and the photoconductive cleaner 25 of each of the image forming stations Y, M, C, and K are replaced with replacement cartridges 6Y, 6M, 6C, and 6K (FIG. 3). ) As a unit. Each replacement cartridge 6Y, 6M, 6C, 6K is provided with non-volatile memories 91-94 for storing information relating to the replacement cartridge. The transmission / reception units 53Y, 53M, 53C, and 53K provided in each replacement cartridge and the transmission / reception units 522Y, 522M, 522C, and 522K provided on the main body side are arranged close to each other, and the CPU 521 and the memory of the engine controller 52 are arranged. Wireless communication is performed with 91-94. In this way, information regarding each replacement cartridge is transmitted to the CPU 521, and information in each of the memories 91 to 94 is updated and stored.

  Next, details of the developing unit 24 will be described on behalf of the image forming station K. The developing unit 24 includes a toner storage container 26 for storing toner, two toner agitation supply members 28 and 29 disposed in the toner storage container 26, and a partition member disposed in proximity to the toner agitation supply member 29. 30, a toner supply roller 31 disposed above the partition member 30, a developing roller 33 that contacts the toner supply roller 31 and the photosensitive drum 20 and rotates in a direction indicated by an arrow at a predetermined peripheral speed, and a developing roller And a regulating blade 34 abutting on the bearing 33.

  In each developing unit 24, the toner stirred and carried by the toner stirring supply member 29 is supplied to the toner supply roller 31 along the upper surface of the partition member 30. The toner thus supplied is supplied to the surface of the developing roller 33 via the supply roller 31. The toner supplied to the developing roller 33 is regulated to a predetermined layer thickness by the regulating blade 34 and is conveyed to the photosensitive drum 20. The charged toner is developed at a developing position where the developing roller 33 and the photosensitive drum 20 come into contact with each other by a developing bias applied to the developing roller 33 from a developing bias generator (not shown) electrically connected to the developing roller 33. Moves from the developing roller 33 to the photosensitive drum 20, and the electrostatic latent image formed by the image writing unit 23 is visualized.

  The toner image thus formed is transferred to the surface of the intermediate transfer belt 16 at the primary transfer position TR1. In this embodiment, the temperature sensor 60U (FIG. 2) such as a thermistor is disposed at the position P3 in FIG. 4 at the most upstream station Y among the four image forming stations Y, M, C, K. The upstream temperature sensor 60U can detect the surface temperature (most upstream surface temperature) of the intermediate transfer belt 16 in the vicinity of the primary transfer position TR1. In the most downstream station K, similarly to the most upstream station Y, a temperature sensor 60D (FIG. 2) is arranged at a position P4 in FIG. 4, and the most downstream temperature sensor 60D provides an intermediate position near the primary transfer position TR1. The surface temperature (the most downstream surface temperature) of the transfer belt 16 can be detected. Thus, in this embodiment, the two temperature sensors 60U and 60D function as the “measurement means” of the present invention.

  In this embodiment, a photoreceptor cleaner 25 is provided in contact with the surface of the photoreceptor drum 20 on the downstream side of the primary transfer position TR1 in the rotation direction D20 of the photoreceptor drum 20. The photoconductor cleaner 25 is in contact with the surface of the photoconductor drum 20 to remove the toner remaining on the surface of the photoconductor drum 20 after the primary transfer.

  The sheet feeding unit 10 includes a sheet feeding unit including a sheet feeding cassette 35 in which sheets S are stacked and held, and a pickup roller 36 that feeds the sheets S from the sheet feeding cassette 35 one by one. In the first opening / closing member 3, a registration roller pair 37 that defines the timing of feeding the sheet S to the secondary transfer region TR 2, and a secondary transfer unit that is pressed against the drive roller 14 and the intermediate transfer belt 16. A secondary transfer roller 19, a fixing unit 12, a paper discharge roller pair 39, and a duplex printing conveyance path 40 are provided.

  The secondary transfer roller 19 is provided so as to be able to come into contact with and separate from the intermediate transfer belt 16 and is driven to come into contact with and separate from a secondary transfer roller drive mechanism (not shown). The fixing unit 12 includes a heating roller 45 that includes a heating element such as a halogen heater and is rotatable, and a pressure roller 46 that presses and biases the heating roller 45. The image secondarily transferred onto the sheet S is heat-fixed on the sheet S at a predetermined temperature (for example, 150 ° C. to 180 ° C.) at a nip formed by the heating roller 45 and the pressure roller 46. In the present embodiment, the fixing unit 12 can be disposed in a space formed obliquely above the intermediate transfer belt 16, in other words, in a space opposite to the image forming unit 6 with respect to the intermediate transfer belt 16. Thus, heat transfer to the electrical component box 5, the image forming unit 6, and the intermediate transfer belt 16 can be reduced, and the frequency of performing the color misregistration correction operation for each color can be reduced.

  Further, the sheet S thus subjected to the heat fixing process is conveyed to a second opening / closing member (discharge tray) 4 provided on the upper surface portion of the housing body 2 via the discharge roller pair 39. Further, when images are formed on both sides of the sheet S, when the rear end portion of the sheet S on which the image is formed on one side as described above is conveyed to the reverse position behind the pair of discharge rollers 39. The rotation direction of the discharge roller pair 39 is reversed, whereby the sheet S is conveyed along the duplex printing conveyance path 40. Then, it is put on the conveyance path again before the registration roller pair 37. At this time, the surface of the sheet S to which the image is transferred by contacting the intermediate transfer belt 16 in the secondary transfer region TR2 is transferred first. It is the opposite side of the surface that was made. In this way, images can be formed on both sides of the sheet S.

  In addition, as shown in FIG. 3, the apparatus 1 includes a display unit 54 that is controlled by the CPU 511 of the main controller 51. The display unit 54 is configured by, for example, a liquid crystal display, and in accordance with a control command from the CPU 511, the operation guidance to the user, the progress of the image forming operation, the occurrence of an abnormality in the apparatus, the replacement timing of any unit, and the like. A predetermined message for notification is displayed.

  In FIG. 3, reference numeral 513 denotes an image memory provided in the main controller 51 for storing an image given from an external device such as a host computer via the interface 512. Reference numeral 523 denotes a ROM for storing a calculation program executed by the CPU 521, control data for controlling the engine unit EG, and the like. Reference numeral 524 denotes a RAM for temporarily storing calculation results in the CPU 521 and other data. is there.

  In this apparatus, a registration control amount establishment process (registration mark formation / detection and registration control amount calculation) is performed at an appropriate timing, for example, immediately after the power is turned on, as part of the initial process, and the relative color shift of the toner image is detected. A registration control amount for correction is obtained and stored in the RAM 524. When a color image is actually formed, the color misregistration is prevented by adjusting the formation position of each color toner image from the memory based on the registration control amount.

Here, in the image forming apparatus configured as described above, after continuously performing double-sided printing, the surface temperature of the intermediate transfer belt 16 is changed to four locations P1 to P4 different from each other along the belt conveyance direction D16.
P1: A position near the secondary transfer position TR2.
P2: Position hung around the driven roller 15
P3: a position near the primary transfer position TR1 in the most upstream station Y,
P4: a position near the primary transfer position TR1 in the most downstream station K,
For example, a temperature distribution as shown in FIG. 4B is obtained. That is, at the position P1 in the vicinity of the secondary transfer position TR2, the sheet (recording material) S that has undergone thermal fixing by the fixing unit 12 is conveyed to the secondary transfer position TR2 and secondary transfer is performed. For this reason, heat is applied from the sheet S to the intermediate transfer belt 16 to locally increase the surface temperature of the intermediate transfer belt 16, and the surface temperature T1 at the position P1 becomes relatively high. Then, the intermediate transfer belt 16 is cooled by air while moving toward the driven roller 15, and at the position P2, it is lowered to a temperature T2 slightly lower than the temperature T1. Further, when the heat of the intermediate transfer belt 16 is absorbed by the driven roller 15 and reaches the position P3 of the most upstream image forming station Y, the temperature is lowered to the most upstream surface temperature T3. The surface temperature of the intermediate transfer belt 16 rises while the toner image is primarily transferred at each of the image forming stations Y, M, C, and K, and the most downstream surface temperature at the position P4 in the most downstream image forming station K. Rise to T4.

  As described above, the surface temperature difference of the intermediate transfer belt 16 at the positions Py (= P3), Pm, Pc, and Pk (= P4) in the vicinity of the primary transfer position TR1 in each of the image forming stations Y, M, C, and K is as follows. While it is almost equal to zero during single-sided printing, it gradually differs as double-sided printing increases (FIG. 4C). When double-sided printing is continuously performed, the temperature difference becomes significant, and the toner image may be shifted between the image forming stations.

  Next, the toner image shift due to the difference in the surface temperature of the intermediate transfer belt 16 at the primary transfer position TR1 or in the vicinity thereof will be considered. When there is no temperature difference (= T4−T3), as shown in FIG. 5, the yellow toner image (only the first dot is shown in the figure) formed by the image forming station Y at the timing t1 is the next image. It moves toward the forming station M and reaches the position Pm at the primary transfer timing t2 of the next magenta toner image. For this reason, the yellow toner image and the magenta toner image match. However, when the temperature difference (= T4−T3) occurs as described above, the position of the yellow toner image at the timing t2 is shifted from the position Pm. Therefore, the toner image deviation amount E due to the temperature difference will be examined with reference to FIG.

The moving distance x of the leading dot of the toner image thus moved represents the distance from the position Py (= P3). If the surface temperature of the intermediate transfer belt 16 at the position Px is T, the moving distance x is very small at the position Px. The intermediate transfer belt 16 extends by the thermal expansion amount dE at the length dx. Therefore, if the thermal expansion coefficient of the intermediate transfer belt 16 is μ,
dE = μ (T−T3) dx
It becomes. Further, if the interval between the primary transfer positions TR1 of the image forming stations Y, M, C, and K is L, the temperature T is T = T3 + (T4−T3) × / 3L.
It becomes. Therefore, from these equations, dE = μ (T3 + (T4−T3) x / 3L−T3) dx
= Μ (T4−T3) xdx / 3L
It becomes. Therefore, when this is integrated to obtain the deviation amount E,
E = μ (T4−T3) × ² / 6L
It becomes. Therefore, the yellow toner image formed by the most upstream image forming station Y has a deviation amount Em = μ at the magenta position Pm (x = L), the cyan position Pc (x = 2L), and the black position Pk (x = 3L). (T4−T3) L / 6 (1m),
Deviation Ec = 2μ (T4−T3) L / 3 (1c),
Deviation Ek = 3μ (T4−T3) L / 2 (1k),
It shifts (Fig. 6).

  Accordingly, as shown in FIG. 7, the writing timings of the latent images at the magenta, cyan, and black image forming stations M, C, and K are shifted by an amount corresponding to the shift amount E caused by the temperature difference (this implementation). In the embodiment, the writing timing is advanced), so that the positional deviation of the toner image due to the temperature difference (= T4−T3) can be eliminated (FIG. 8). That is, by adjusting the registration control amount based on the deviation amounts Em, Ec, and Ek, it is possible to eliminate the positional deviation of the toner image due to the temperature difference (= T4−T3).

  Therefore, in the following embodiment, as will be described below, the resist control amount is appropriately set based on the surface temperatures T3 and T4 of the intermediate transfer belt 16 in the uppermost and lowermost image forming units Y and K during the continuous printing operation. The color image is formed with the adjusted resist control amount.

C. First Embodiment FIG. 9 is a flowchart showing a first embodiment of an image forming apparatus according to the present invention. In this apparatus, when an image forming command (printing command) is given to the main controller 51 from an external device such as a host computer, the CPU 511 of the main controller 51 converts the job data into a format suitable for the operation instruction of the engine unit EG, It is sent to the engine controller 52. On the other hand, the engine controller 52 controls each part of the engine unit EG based on job data from the CPU 511 to execute single-sided printing or double-sided printing, and forms an image corresponding to the image signal on the sheet S. Further, not only when a plurality of print commands are included in the image formation command, but also when a plurality of image formation commands are given continuously, the jobs are connected to execute a continuous print operation. The CPU 521 of the engine controller 52 has a function as an “adjustment unit” of the present invention as will be described later, and each unit of the apparatus is configured as follows according to a registration control amount adjustment program stored in the ROM 523 in advance. Color misregistration correction is performed during continuous printing operation.

  In step S1, a printing operation is executed based on the registration control amount stored in the RAM 524. Therefore, the first printing operation is executed based on the registration control amount stored in the RAM 524, and the subsequent printing operation is executed based on the registration control amount appropriately adjusted as described later.

  Next, it is determined whether the printing mode executed in step S1 is double-sided printing or single-sided printing (step S2), and in the case of double-sided printing, steps S3 to S6 are executed to adjust the resist control amount and to perform resist control. The amount is updated to the newly obtained registration control amount. More specifically, a series of processing is executed as follows.

  The surface temperature T3 of the intermediate transfer belt 16 at the most upstream position P3 is measured by the most upstream temperature sensor 60U (step S3), and the surface temperature T4 of the intermediate transfer belt 16 at the most downstream position P4 is measured by the most downstream temperature sensor 60D. Measurement is performed (step S4). Based on the above equations (1m), (1c), and (1k), deviation amounts Em, Ec, Ek accompanying double-sided printing are calculated (step S5). Further, the registration control amount is adjusted based on these deviation amounts Em, Ec, Ek. More specifically, the registration control amount is adjusted so as to advance the writing timing at the image forming stations M, C, and K according to the deviation amounts Em, Ec, and Ek.

  When the registration control amount is adjusted in accordance with double-sided printing in this way, the process proceeds to step S7 to determine whether or not the next printing exists, that is, whether or not it is continuous printing (step S7). In step S1, the printing operation is executed. Therefore, in this printing operation stage, color misregistration correction is performed based on the already adjusted registration control amount, so that a good color image can be formed.

  On the other hand, if it is determined in step S2 that single-sided printing is performed, the process proceeds to step S7 as it is without adjusting the registration control amount as described above to determine whether continuous printing is performed (step S7), and data to be continuously printed. If there is, the process returns to step S1 to execute the printing operation.

  As described above, according to the first embodiment, the most upstream temperature sensor 60U and the most downstream temperature sensor 60D are provided, and the registration control amount is adjusted based on the detection results of the temperature sensors 60U and 60D after performing duplex printing. Therefore, even if a surface temperature difference of the intermediate transfer belt 16 occurs at each image forming station due to double-sided printing, the resist control amount is adjusted according to the temperature difference (= T4−T3). . Therefore, even if double-sided printing is included in the continuous printing operation, it is possible to effectively prevent color misregistration from occurring during the continuous printing operation.

D. Second Embodiment FIG. 10 is a flowchart showing a second embodiment of the image forming apparatus according to the present invention. In the first embodiment, the registration control amount is always adjusted after the double-sided printing is executed. On the other hand, in the second embodiment, the registration control amount is adjusted only when the temperature difference (= T4−T3) is equal to or larger than a predetermined value. In other words, in the second embodiment, following the determination of the temperature difference (= T4−T3) (step S8), each image forming station to the extent that the temperature difference becomes a predetermined value or more and the adjustment of the resist control amount is necessary. It is determined whether or not the surface temperature of the intermediate transfer belt 16 at Y, M, C, and K is different (step S9). When the temperature difference is equal to or greater than the predetermined value in step S9 and it is determined that the registration control amount needs to be adjusted, the registration control amount is adjusted in the same manner as in the first embodiment (steps S5 and S6). On the other hand, when the temperature difference is smaller than the predetermined value, it can be seen that the surface temperature difference of the intermediate transfer belt 16 at each image forming station by double-sided printing is small, and the color misregistration amount due to the temperature difference is small. Therefore, while the color misregistration amount is small, the registration control amount adjustment operation can be omitted and an efficient color misregistration correction operation can be performed.

E. Third Embodiment FIG. 11 is a flowchart showing a third embodiment of the image forming apparatus according to the present invention. In the third embodiment, as shown in the figure, the registration control amount is adjusted before the printing operation is executed. Hereinafter, the third embodiment will be described with reference to FIG.

  In this embodiment, before starting the printing operation, the surface temperature T3 of the intermediate transfer belt 16 at the most upstream position P3 is measured by the most upstream temperature sensor 60U (step S11), and the most downstream temperature sensor 60D is most downstream. The surface temperature T4 of the intermediate transfer belt 16 at the position P4 is measured (step S12). Then, after obtaining the temperature difference (= T4−T3) (step S13), it is determined whether or not the temperature difference is equal to or greater than a predetermined value. If “YES” in step S13, that is, if it is determined that the surface temperatures of the intermediate transfer belts 16 at the image forming stations Y, M, C, and K are different to the extent that adjustment of the resist control amount is necessary. In the same manner as in the first embodiment, the registration control amount is adjusted (steps S15 and S16). On the other hand, when it is determined that the temperature difference is smaller than the predetermined value and the color misregistration amount due to the temperature difference is small, the adjustment of the resist control amount is omitted.

  Thus, after adjusting the registration control amount as necessary, the printing operation is executed (step S17). Further, in the next step S18, it is determined whether or not the next printing exists, that is, whether or not it is continuous printing. When there is data to be continuously printed, the process returns to step S11 and a series of operations (steps S11 to S17). ).

  As described above, according to the third embodiment, the most upstream temperature sensor 60U and the most downstream temperature sensor 60D are provided, and the resist is detected based on the detection results of the temperature sensors 60U and 60D during the continuous printing operation including the duplex printing. Since the printing operation is executed after adjusting the control amount, even if the surface temperature difference of the intermediate transfer belt 16 at each image forming station is caused by the double-sided printing, the temperature difference (= T4−T3). ), The resist control amount is adjusted. Therefore, even if double-sided printing is included in the continuous printing operation, it is possible to effectively prevent color misregistration from occurring during the continuous printing operation.

  In the third embodiment, the adjustment operation of the resist control amount is omitted while the surface temperature difference of the intermediate transfer belt 16 at each image forming station by double-sided printing is small. It can be performed.

<Others>
The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, even if the deviation amount due to double-sided printing increases during the continuous printing operation, the registration control amount is adjusted based on the deviation amount, and the color deviation is corrected based on the adjusted registration control amount. When the thermal effect of double-sided printing becomes large, the continuous printing operation is temporarily interrupted, and the registration control amount establishment process (registration mark formation / detection and registration control amount calculation) is performed. May be updated. Further, this update process (registration control amount establishment process) may be performed after the continuous printing operation is completely stopped, or the color image is printed during continuous printing while the movement of the intermediate transfer belt 16 is continued. The registration operation may be updated by temporarily interrupting the forming operation and obtaining a new registration control amount. Among these, in consideration of operation efficiency, operation stability, and the like, it is desirable to temporarily interrupt the color image forming operation and execute the registration control amount update process as described above.

  In the above embodiment, the surface temperature difference of the intermediate transfer belt 16 between the image forming stations Y, M, C, and K is obtained using the two temperature sensors 60U and 60D. The position and the like are not limited to the above embodiment, and for example, a temperature sensor for measuring the surface temperature of the intermediate transfer belt 16 may be provided in each of the image forming stations Y, M, C, and K. In this case, The surface temperature difference of the intermediate transfer belt 16 between the image forming stations Y, M, C, and K can be obtained with higher accuracy. As a result, the registration control amount can be adjusted more accurately.

The graph which shows the heat influence in continuous single-sided printing and continuous double-sided printing. 1 is a diagram illustrating a basic configuration of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus in FIG. 1. FIG. 2 is a diagram illustrating a temperature distribution of an intermediate transfer belt in the image forming apparatus of FIG. 1. FIG. 4 is a schematic diagram for explaining a toner image shift caused by a surface temperature difference of an intermediate transfer belt. 6 is a graph showing a toner image shift amount caused by a surface temperature difference of an intermediate transfer belt. FIG. 6 is a diagram illustrating a method for correcting a toner image shift caused by a surface temperature difference. FIG. 8 is a graph showing a toner image shift state when corrected by the correction method of FIG. 7. FIG. 1 is a flowchart illustrating a first embodiment of an image forming apparatus according to the present invention. 9 is a flowchart illustrating a second embodiment of the image forming apparatus according to the present invention. 9 is a flowchart illustrating a third embodiment of an image forming apparatus according to the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 9 ... Transfer belt unit, 16 ... Intermediate transfer belt (transfer medium), 52 ... Engine controller (adjustment means), 60D ... Most downstream temperature sensor, 60U ... Most upstream temperature sensor, 521 ... CPU (adjustment) Means), D16 (moving direction of intermediate transfer belt), E, Em, Ec, Ek (shift amount of toner image due to double-sided printing), P3 (most upstream position), P4 (most downstream position), T3 (maximum downstream position) Upstream surface temperature, T4 ... Most downstream surface temperature, S ... Sheet (recording material), Y, M, C, K ... Image forming station

Claims (5)

A plurality of image forming stations, each of which forms a toner image of a different color, are arranged along the moving direction of the transfer medium moving at a predetermined speed, and relative to the toner images formed by the plurality of image forming stations. The color image can be formed by superimposing the plurality of toner images on the transfer medium while controlling the positional relationship based on the resist control amount, and the first image formed on the transfer medium can be formed. Two-sided printing for transferring the second image formed on the transfer medium to the second surface of the recording material after being transferred and thermally fixed to the first surface of the sheet-like recording material, and formed on the transfer medium In an image forming apparatus that performs single-sided printing in which a first image is transferred to a first surface of a sheet-like recording material and thermally fixed, and then the recording material is discharged as it is.
Measuring means for measuring the surface temperature of the transfer medium;
Adjusting means for adjusting the resist control amount based on the measurement result of the measuring means,
The measuring means includes a most upstream temperature sensor for measuring a surface temperature of the transfer medium at the most upstream station located in the most upstream position in the moving direction of the transfer medium among the plurality of image forming stations, and movement of the transfer medium. And a most downstream temperature sensor for measuring the surface temperature of the transfer medium at the most downstream station located in the most downstream in the direction,
The adjusting means is that the surface temperature of the transfer medium is locally increased by the double-sided printing only when double-sided printing is performed, and the surface temperature difference of the transfer medium is generated between the plurality of image forming stations. A deviation amount of the toner image generated between the image forming stations is obtained based on measurement results of the most upstream temperature sensor and the most downstream temperature sensor, and the registration control amount is adjusted according to the deviation amount. Image forming apparatus.   The adjusting means adjusts the registration control amount only when the temperature difference between the most upstream surface temperature measured by the most upstream temperature sensor and the most downstream surface temperature measured by the most downstream temperature sensor becomes a predetermined value or more. The image forming apparatus according to claim 1, wherein adjustment is performed.   The image forming apparatus according to claim 1, wherein the adjustment unit obtains a deviation amount and adjusts a registration control amount according to the deviation amount every time double-sided printing is performed.   The image forming apparatus according to claim 1, wherein the adjustment unit obtains a deviation amount and adjusts a registration control amount according to the deviation amount, and then performs each duplex printing. A plurality of image forming stations, each of which forms a toner image of a different color, are arranged along the moving direction of the transfer medium moving at a predetermined speed, and relative to the toner images formed by the plurality of image forming stations. The color image can be formed by superimposing the plurality of toner images on the transfer medium while controlling the positional relationship based on the resist control amount, and the first image formed on the transfer medium can be formed. Two-sided printing for transferring the second image formed on the transfer medium to the second surface of the recording material after being transferred and thermally fixed to the first surface of the sheet-like recording material, and formed on the transfer medium in the image forming apparatus to perform single-sided printing directly discharging the recording material after the thermal fixing while transferring the first image on the first side of the sheet-shaped recording material, the resist control A color shift correcting method for correcting color misregistration by controlling,
Only when performing duplex printing
The surface temperature of the transfer medium at the most upstream station located in the most upstream in the moving direction of the transfer medium among the plurality of image forming stations is measured by the most upstream temperature sensor, and the most downstream in the moving direction of the transfer medium. A measuring step of measuring the surface temperature of the transfer medium at the most downstream station located at the most downstream temperature sensor;
The amount of deviation of the toner image between the image forming stations caused by the surface temperature of the transfer medium rising locally by the duplex printing and the surface temperature difference of the transfer medium occurring between the plurality of image forming stations. Is determined based on the measurement results of the most upstream temperature sensor and the most downstream temperature sensor, and the adjustment step of adjusting the registration control amount according to the amount of deviation,
A registration process for correcting color misregistration by correcting a relative positional relationship of toner images formed by the plurality of image forming stations based on the adjusted registration control amount;
A method of correcting color misregistration, comprising:

Images