JP2006195082A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing color shift and the lowering of a color tone or the like from occurring by restraining the lowering of the position detecting accuracy of a register mark caused by the change of environment such as temperature or the like. <P>SOLUTION: By constituting the register mark RM of a halftone toner image, the temperature variation of output from a test pattern sensor (optical sensor) 18 for detecting the register mark RM is restrained. As a result, the position detecting accuracy of the register mark RM is improved, so that the color shift and the lowering of the color tone or the like are prevented from occurring. <P>COPYRIGHT: (C)2006,JPO&NCIPI

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 a toner image of a different color, are arranged along the moving direction of a transfer medium, and corrects color misregistration. It is related to the technology.

  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.

  Incidentally, color misregistration is one of the serious problems in an image forming apparatus having a plurality of image forming stations. 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. The position information of the mark is obtained, and the alignment (color misregistration correction) of each toner image is performed based on the position information. Specifically, the registration mark position information is obtained by detecting the edge of the registration mark. That is, the transfer medium on which the registration mark is formed moves, and (1) when the upstream edge portion of the registration registration mark passes through the optical sensor, the output level of the optical sensor crosses a preset threshold level. (2) Conversely, when the downstream edge portion of the registration mark passes through the optical sensor, the position information is detected by the output level of the optical sensor crossing a preset threshold level. Yes.

JP 2004-109617 A (page 4)

  In order to perform color misregistration correction based on registration mark position information with high accuracy in a tandem image forming apparatus, registration mark detection accuracy is important. However, in the prior art, sufficient examination has not been made in this regard, and a strip-shaped solid image is simply adopted as a registration mark. For this reason, when the environment such as temperature changes, the toner density of the registration mark fluctuates, and the change amount of the optical sensor output level accompanying the detection of the edge portion of the registration mark sometimes fluctuates. Such variation in the amount of change greatly affects the sensor output change mode when the upstream edge portion and the downstream edge portion of the registration mark pass through the optical sensor. That is, the output level of the optical sensor at the time of toner detection varies as shown by, for example, the broken line, the solid line, and the alternate long and short dash line in FIG. 6A according to the temperature variation, and as a result, the optical sensor output level crosses the threshold level. Fluctuates as indicated by reference numerals T1 to T3 in FIG. 5A, and there is a problem that the position detection accuracy is lowered. Further, the toner density at the edge portion of the registration mark locally increases due to the so-called edge effect, and therefore, the fluctuation of the toner density becomes large particularly at the edge portion. When the position is detected by detecting the edge portion of the registration mark as described above, the toner density fluctuation at the edge portion is particularly problematic. As described above, fluctuations in the toner density due to changes in the environment such as temperature cause a decrease in registration mark position detection accuracy based on the output signal of the optical sensor, and color misregistration correction cannot be performed satisfactorily. There may be a problem that printing is performed in a state, a desired color tone cannot be obtained, and the like.

  The present invention has been made in view of the above problems, and provides an image forming apparatus capable of suppressing a decrease in registration mark position detection accuracy due to a change in environment such as temperature and preventing occurrence of color misregistration or color tone deterioration. The purpose is to provide.

  In the image forming apparatus according to the present invention, a plurality of image forming stations, each forming a toner image of a different color, are arranged along the moving direction of the transfer medium, and the toner images formed by the plurality of image forming stations are arranged. Image formation that forms resist marks on the transfer medium surface apart in the moving direction, detects each resist mark with an optical sensor, and corrects color misregistration between multiple colors based on an output signal from the optical sensor In the apparatus, in order to achieve the above object, each of the plurality of image forming stations is characterized in that the registration mark is configured by a halftone toner image.

  When the registration mark is composed of only a solid image as in the prior art, the toner density fluctuates due to changes in the environment such as temperature, which is a major obstacle in accurately detecting the registration mark position information. Therefore, in the present invention, the registration mark is composed of a halftone toner image, thereby suppressing fluctuations in toner density and improving the registration mark position detection accuracy. As a result, color misregistration correction can be performed appropriately.

Here, the halftone toner image may be constituted by a two-dimensional halftone pattern. In particular, it is preferable to use the following pattern as the two-dimensional halftone pattern:
(a) a pattern including a plurality of horizontal lines extending substantially parallel to the moving direction of the transfer medium, and the plurality of horizontal lines arranged from the upstream side to the downstream side of the two-dimensional halftone pattern in the moving direction;
(b) a pattern including a plurality of oblique lines extending obliquely with respect to the moving direction of the transfer medium, wherein the plurality of oblique lines are arranged from the upstream side to the downstream side of the two-dimensional halftone pattern in the moving direction;
(c) Lattice pattern.

  When configured in this manner, toner density unevenness (banding) in the moving direction of the transfer medium can be suppressed. The reason is as follows. When the two-dimensional halftone pattern is composed of, for example, only vertical lines extending perpendicularly to the moving direction of the transfer medium, no toner is continuously present on the image carrier in the moving direction of the transfer medium. There are a period in which the image carrier to be carried and the transfer medium are in contact with each other via toner, and a period in which the image carrier is in direct contact without any toner. The friction resistance between the image carrier and the transfer medium is large during the period of contact through the toner, and the friction resistance between the image carrier and the transfer medium is small during the period of contact without the toner. Such a change in frictional resistance becomes a load change for the drive system that drives the image carrier. As a result, the peripheral speed of the image carrier varies and banding occurs. On the other hand, when the registration mark is formed by the two-dimensional halftone pattern of the above (a) to (c), the toner is continuously present in the moving direction of the transfer medium. Banding can be suppressed.

  The halftone toner image is composed of a linear pattern extending in the main scanning direction substantially orthogonal to the moving direction of the transfer medium and a two-dimensional halftone pattern, and the linear pattern is moved in the moving direction with respect to the two-dimensional halftone pattern. In this case, the following effects can be obtained. Conventionally, a so-called edge effect is known in which the toner density at the edge of a two-dimensional image, particularly a solid image, is locally increased. Due to the edge effect, the toner density varies greatly at the edge portion. Therefore, when position detection is performed by detecting the edge portion of the registration mark, the toner density variation at the edge portion due to the edge effect becomes a serious problem. Therefore, in the present invention, by configuring the halftone toner image as described above, the toner density fluctuation in the edge portion of the registration mark is suppressed, and the registration mark position detection accuracy is improved. As a result, color misregistration correction can be performed appropriately.

  Here, as a specific method for obtaining the registration mark position information by the optical sensor, for example, (1) the output signal obtained when the upstream portion of the registration mark in the moving direction of the transfer medium passes through the optical sensor. The position information can be detected based on the change, or (2) the position information can be detected based on the change in the output signal obtained when the downstream portion of the registration mark in the moving direction of the transfer medium passes through the optical sensor. In particular, when the registration mark position information is acquired by the former (1), the halftone toner image is formed by arranging a linear pattern so as to be spaced upstream of the two-dimensional halftone pattern in the moving direction of the transfer medium. Is desirable. As a result, fluctuations in toner density at the registration mark edge portion on the upstream side in the moving direction are suppressed, and the position detection accuracy can be improved. On the other hand, when the position information of the registration mark is acquired by the latter (2), the halftone toner image is formed by arranging the linear pattern separately from the two-dimensional halftone pattern on the downstream side in the transfer medium moving direction. It is desirable to do. As a result, the edge effect on the downstream side in the moving direction is suppressed, and the position detection accuracy can be increased.

  Also, registration marks in which linear patterns are spaced apart from the two-dimensional halftone pattern on the upstream side and downstream side in the movement direction may be used. That is, the halftone toner image is composed of first and second linear patterns and halftone patterns extending in the main scanning direction substantially orthogonal to the moving direction of the transfer medium. The linear patterns may be configured to be spaced apart from each other in this order in the movement direction.

  Note that the two-dimensional halftone pattern is (1) a pattern including a plurality of horizontal lines extending substantially parallel to the moving direction of the transfer medium, and the plurality of horizontal lines in the moving direction from the upstream side to the downstream side of the two-dimensional halftone pattern. (2) The pattern includes a plurality of oblique lines extending obliquely with respect to the moving direction of the transfer medium, and the plurality of oblique lines are downstream from the upstream side of the two-dimensional halftone pattern in the moving direction. (3) It may be configured with a lattice pattern. When configured in this manner, as will be described below, it is preferable because uneven toner density (banding) in the moving direction of the transfer medium can be suppressed.

  Consider a process in which a toner image formed on the surface of an image carrier is transferred to a transfer medium when the two-dimensional halftone pattern is composed of only vertical lines extending perpendicularly to the moving direction of the transfer medium. In this case, since no toner is continuously present on the image carrier in the moving direction of the transfer medium, the period during which the image carrier that carries the toner image and the transfer medium are in contact with each other via the toner and the toner is completely absent. There is a period of direct contact. The friction resistance between the image carrier and the transfer medium is large during the period of contact through the toner, and the friction resistance between the image carrier and the transfer medium is small during the period of contact without the toner. Such a change in frictional resistance becomes a load change for the drive system that drives the image carrier. As a result, the peripheral speed of the image carrier varies and banding occurs. On the other hand, when the two-dimensional halftone pattern is configured as described in paragraphs 0013 (1) to (3), since there is toner continuously in the moving direction of the transfer medium, there is no change in frictional resistance, Banding can be suppressed.

A. First Embodiment FIG. 1 is a diagram showing an embodiment 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 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 monochromatic printing processing 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 copy paper, transfer paper, paper, and an OHP transparent sheet.

  In FIG. 1, an image forming apparatus 1 according to the present embodiment includes a housing body 2, a first opening / closing member 3 that is detachably mounted on the front surface of the housing body 2 (right-hand side surface in FIG. 1), 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 is disposed below the housing body 2 and is driven to rotate by a black drive motor described later, and a driven roller 15 disposed obliquely above the drive roller 14. An intermediate transfer belt 16 that is stretched between the rollers 14 and 15 and driven to circulate in the direction of the arrow D16 in the figure, and a belt cleaner 17 that abuts against the surface of the intermediate transfer belt 16. The driven roller 15 is disposed obliquely above the drive roller 14 (upward on the left hand in FIG. 1). 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 (right downward in FIG. 1) 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 a primary transfer bias generator (not shown), and generate the primary transfer bias at an appropriate timing. A primary transfer bias is applied from the portion.

  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 are moved closer to the photosensitive drum 20, they are brought into contact with the photosensitive drum 20 with the intermediate transfer belt 16 interposed therebetween (solid line in FIG. 1). 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 1). 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. 1, 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 a broken line in FIG. 5, the intermediate transfer belt is executed while only the monochrome printing process is performed 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 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.

  Further, 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 as an “optical sensor” of the present invention. The test pattern sensor 18 is a so-called reflection type optical sensor, and includes a light projecting portion (not shown) that irradiates light toward the surface of the intermediate transfer belt 16, a surface of the intermediate transfer belt 16, and a registration mark described later. A light receiving unit (not shown) for receiving the reflected light is provided. 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. Then, each color toner image on the intermediate transfer belt 16 is positioned based on the output signal from the test pattern sensor 18, and the density of each color toner image is detected to correct the color shift and image density of each color image. The configuration of the registration mark used in this embodiment will be described in detail later.

  In this embodiment, in addition to the sensor 18, a vertical synchronization sensor 60 (FIG. 2) that detects a characteristic part of the intermediate transfer belt 16 (for example, a protrusion protruding in the width direction) is attached to the support frame. ing. For this reason, a vertical synchronization signal (reference signal) is output every time the characteristic portion of the intermediate transfer belt 16 passes the sensor 60.

  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. 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. 1, 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.

  As shown in FIG. 3, the image writing unit 23 uses an array-like writing head 232 in which light emitting diodes 231 are arranged in a line in the axial direction of the photosensitive drum 20, and is spaced apart from the photosensitive drum 20. ing. That is, the plurality of light emitting diodes 231 are arranged in a line in the main scanning direction X substantially orthogonal to the conveyance direction (corresponding to the “movement direction” of the present invention) D16 of the intermediate transfer belt 16, and the drive driver of FIG. When all the light emitting diodes 231 are turned on simultaneously by 526, a one-dot line-like latent image pattern is formed on the photosensitive drum 20 as shown in FIG. Further, when the intermediate transfer belt 16 is moved by n (n = 2, 3,...) Dots while all the light emitting diodes 231 are turned on, a latent image pattern of n dot lines is formed as shown in FIG. It is formed. Then, it is possible to form the “linear pattern” of the present invention by developing the line latent image with toner. Of course, various two-dimensional halftone patterns can be formed by controlling lighting / extinction of each light emitting diode 231 in accordance with a drive command from the drive driver 526.

  The array-like writing head 232 configured as described above has a shorter optical path length than the laser scanning optical system, is compact, can be disposed close to the photosensitive drum 20, and can reduce the size of the entire apparatus. Have advantages. 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. 2). ) 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 transferred to the developing roller at the developing position where the developing roller 33 and the photosensitive drum 20 are in contact with each other by the developing bias applied to the developing roller 33 from the developing bias generator 525 electrically connected to the developing roller 33. The electrostatic latent image formed by the image writing unit 23 is visualized by moving from 33 to the photosensitive drum 20.

  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 to the sheet S is fixed to the sheet S at a predetermined temperature 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 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. 2, 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. 2, 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.

  FIG. 5 is a diagram showing the operation of the image forming apparatus of FIG. In this apparatus, color misregistration correction processing using registration marks is executed at an appropriate timing such as when the power is turned on or cartridge replacement. More specifically, the CPU 521 of the engine controller 52 controls each part of the apparatus as follows in accordance with a program relating to color misregistration correction stored in the ROM 523 to execute registration mark formation / detection and color misregistration correction processing.

  First, in accordance with a control command from the CPU 521 of the engine controller 52, the image forming stations Y, M, C, and K are composed of a plurality of straight lines 2 extending in the main scanning direction orthogonal to the transport direction D16 as shown in FIG. A halftone toner image composed of a three-dimensional halftone pattern is formed as a registration mark RM. The overall size of each registration mark RM, the mutual interval, arrangement and number of registration marks RM formed on the intermediate transfer belt 16 These are arbitrary, and many various aspects have been proposed heretofore. In this embodiment, for example, as shown in FIG. 5, the registration marks RM are spaced apart by a predetermined interval (for example, 0.5 mm) in the transport direction (sub-scanning direction) D16 in the order of K, C, M, and Y. And formed. FIG. 4A shows a yellow registration mark RM (Y), a black registration mark RM (K), a cyan registration mark RM (C), a magenta registration mark RM (M), and a yellow registration mark. Although only RM (Y) is shown, a plurality of RM (Y) are formed for each color.

  Here, the yellow registration mark RM (Y) is enlarged and the configuration thereof will be described with reference to FIG. In this embodiment, the registration mark RM (Y) is constituted by a halftone toner image, and a two-dimensional halftone pattern comprising a plurality of straight lines extending in the main scanning direction perpendicular to the transport direction D16 as the halftone toner image as described above. Is adopted. Although the yellow registration mark RM (Y) has been described, the registration marks RM (K), RM (C), and RM (M) for other toner colors are similarly configured.

  When all or part of the registration mark RM having such a configuration is formed, the test pattern sensor 18 can detect the registration mark RM by turning on the light projecting portion (not shown) of the test pattern sensor (optical sensor) 18. And That is, the registration marks RM (K), RM (C), RM (M), and RM (Y) formed on the intermediate transfer belt 16 as described above move in the transport direction D16 as the intermediate transfer belt 16 moves. Then, it passes through the detection area 181 of the test pattern sensor 18. At this time, the reflected light is received by the light receiving unit (not shown) of the test pattern sensor 18, and the voltage level of the signal output from the test pattern sensor 18 changes according to the amount of received light. The output voltage of the test pattern sensor 18 crosses a preset threshold voltage at a certain timing. Therefore, by detecting the timing, the timing at which each registration mark RM passes can be measured, and the position information of the registration mark RM can be acquired. Specifically, an optical sensor output is compared with a threshold voltage by a comparator (not shown), and switching of the comparator output is detected. Thereby, the position information of the registration mark RM can be acquired. And it becomes possible to obtain | require the mutual space | interval of the registration mark RM based on the positional information. When position information is obtained for all registration marks RM, color misregistration correction is performed based on the position information.

  As described above, according to this embodiment, since each registration mark RM is configured as described above, position information of the registration mark RM can be detected with high accuracy. As a result, color misregistration correction can be performed appropriately. The reason is as follows. That is, when the registration mark is composed of a solid image as in the prior art, the change in the toner concentration due to the change in the environment such as the temperature becomes the change in the optical sensor output level as described above. ), The timing at which the optical sensor output level crosses the threshold level varies as indicated by reference numerals T1 to T3. As a result, the position detection accuracy is lowered, and it is difficult to perform color misregistration correction appropriately. On the other hand, in this embodiment, each registration mark RM is a halftone toner image formed by the two-dimensional halftone pattern shown in FIG. 5A, which is composed of a plurality of straight lines extending in the main scanning direction orthogonal to the transport direction D16. By adopting, the fluctuation of the toner density due to changes in the environment such as temperature can be suppressed, so that the output level of the test pattern sensor (optical sensor) 18 is stabilized. Accordingly, the timing at which the optical sensor output crosses the threshold level is also stabilized as indicated by reference signs T4 to T6 in FIG. As a result, the position detection accuracy of the registration mark RM is improved, and appropriate color misregistration correction is possible.

  In the above embodiment, the two-dimensional halftone pattern shown in FIG. 7A is adopted as the two-dimensional halftone pattern. However, the two-dimensional halftone pattern is not limited to this, and is arbitrary. Can be adopted. For example, the two-dimensional halftone pattern shown in FIGS. 7B to 7H may be used. In particular, when the two-dimensional halftone pattern shown in FIGS. 7D to 7H is used, toner density unevenness (banding) in the transport direction of the transfer medium such as the intermediate transfer belt 16 can be suppressed, which is preferable. The reason is as follows. That is, the two-dimensional halftone patterns in FIGS. 7A to 7C are composed of only straight lines or points extending in the main scanning direction orthogonal to the transport direction D16. The toner exists only discontinuously. Therefore, there are a period in which the photosensitive drum 20 carrying the toner image and the intermediate transfer belt 16 are in contact with each other through the toner, and a period in which the photosensitive drum 20 is in direct contact without any toner. The friction resistance between the two is large during the period of contact through the toner, and the friction resistance between the two is small during the period of contact without the toner. Such a change in frictional resistance is a load change for the drive system that drives the photosensitive drum 20. As a result, the peripheral speed of the photosensitive drum 20 varies and banding occurs. On the other hand, in the two-dimensional halftone patterns in FIGS. 7D to 7H, toner is continuously arranged from the upstream side (right hand side in the figure) to the downstream side (left hand side in the figure). Therefore, the photosensitive drum 20 carrying the toner image and the intermediate transfer belt 16 are always in contact via the toner. Therefore, the frictional resistance does not change as described above, and a constant load is always applied to the drive system that drives the photosensitive drum 20. As a result, the peripheral speed of the photosensitive drum 20 is stabilized and banding is suppressed.

B. Second Embodiment As described in the first embodiment, the registration mark RM is detected by detecting the edge portion of the registration mark RM. In the second embodiment, the registration mark RM is detected particularly in the transport direction D16. An upstream edge portion of the RM is detected. That is, the position information of each of the plurality of registration marks RM is detected based on the change in the output signal obtained when the upstream edge portion of the registration mark RM in the transport direction D16 passes through the test pattern sensor (optical sensor) 18. At the same time, color misregistration is corrected based on these pieces of position information. As shown in FIG. 8A, the halftone toner image constituting the registration mark RM includes a linear pattern LP1 extending in the main scanning direction X substantially orthogonal to the transport direction D16 and an arbitrary two-dimensional halftone pattern. The linear pattern LP1 is made of TP and is arranged so as to be separated from the two-dimensional halftone pattern TP on the upstream side (right hand side in the figure) in the transport direction D16. In the second embodiment, the electrical and optical configurations of the image forming apparatus are exactly the same as those in the first embodiment, and a description thereof will be omitted.

  In the second embodiment, the registration mark RM can be detected with high accuracy by configuring the registration mark RM as described above. As a result, color misregistration correction can be performed appropriately. The reason is as follows. That is, when the halftone toner image constituting the registration mark RM is composed of only the two-dimensional halftone pattern TP and the linear pattern LP1 is not provided, the toner density at the edge of the registration mark RM is locally increased due to the so-called edge effect. Become expensive. Therefore, when the environment such as temperature fluctuates, the toner density fluctuates particularly at the edge portion. The toner density fluctuation at the edge portion becomes a serious problem when position detection is performed by detecting the edge portion of the registration mark RM. On the other hand, in the present embodiment, the linear pattern LP1 extending in the main scanning direction X is arranged on the upstream side in the transport direction D16 with respect to the two-dimensional halftone pattern TP, spaced apart from the two-dimensional halftone pattern TP. is doing. As a result, the edge effect that the edge portion of the registration mark RM has a locally high concentration can be suppressed, and the concentration fluctuation of the edge portion due to a change in the environment such as temperature is suppressed. Thereby, the edge part of the registration mark RM can be detected with higher accuracy.

  In the second embodiment, the linear pattern LP1 is provided on the upstream side with respect to the two-dimensional halftone pattern TP. However, the downstream edge portion of the registration mark is detected in the transport direction D16, and FIG. As shown in (b), the halftone toner image is composed of a linear pattern LP2 extending in the main scanning direction X substantially perpendicular to the transport direction D16 and a two-dimensional halftone pattern TP. The pattern TP may be spaced from the downstream side in the transport direction D16. Further, as shown in FIG. 8C, the halftone toner image is intermediate between the first and second linear patterns LP1 and LP2 extending in the main scanning direction X substantially orthogonal to the conveyance direction D16 of the intermediate transfer belt 18. The first linear pattern LP1, the halftone pattern TP, and the second linear pattern LP2 may be configured to be spaced apart from each other in this order in the transport direction D16. Needless to say, for example, the halftone pattern shown in FIG. 7 can be arbitrarily employed as the two-dimensional halftone pattern TP.

  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, the image writing unit is configured by the array-shaped writing head 232 in which the light emitting diodes 231 are arranged in a row as the light emitting elements. For example, an organic EL element is used as the light emitting element. May be. FIG. 9 is a schematic perspective view of an image writing unit using an organic EL element. In the figure, details of the line head provided in the image writing unit 23 are shown. In the writing head 232 of each image writing unit 23, the light emitting element array composed of the organic EL elements 233 is held in a long housing. The image writing section 23 is driven by a TFT 235 formed on the same glass substrate 234, on which a light emitting section of a light emitting element array composed of organic EL elements 233 is placed on a glass substrate 234. The gradient index rod lens array 236 constitutes an imaging optical system, and a gradient index rod lens 237 arranged in front of the light emitting unit is stacked. The housing covers the periphery of the glass substrate 234 and the side facing the photosensitive drum 20 is open. In this way, light is emitted from the gradient index rod lens 237 to the photosensitive drum 20. A light-absorbing member (paint) is provided on the surface of the housing that faces the end surface of the glass substrate 234.

  FIG. 10 is a cross-sectional view of the image writing unit 23 in the sub-scanning direction. The image writing unit 23 includes a light emitting element array composed of organic EL elements 233 attached facing the rear surface of the gradient index rod lens array 236 in the housing, and an organic EL element 233 in the light emitting element array 233 from the rear surface of the housing. And an opaque cover 238 for shielding the light emitting element array. Further, the cover 238 is pressed against the rear surface of the housing by the fixed plate spring 239, so that the inside of the housing is sealed light-tight. That is, the glass substrate 234 is optically sealed with the housing by the fixed plate spring 239. For this reason, total reflection at the end face of the glass substrate 234 can be prevented and light can be efficiently absorbed. A plurality of fixed leaf springs 239 are provided in the longitudinal direction of the housing.

  A writing head in which elements such as a liquid crystal shutter provided with a backlight are arranged in a line in the axial direction of the photosensitive drum 20 may be used. Of course, you may employ | adopt the image writing part comprised by a laser scanning optical system.

  Furthermore, in the above embodiment, the present invention is applied to the image forming apparatus that forms the registration mark RM on the intermediate transfer belt 16, but the scope of the present invention is not limited to this, and the intermediate transfer drum is not limited thereto. The present invention can be applied to all apparatuses that perform color misregistration processing by forming a resist mark on a transfer medium such as a transfer sheet.

1 is a diagram showing an embodiment 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 schematic diagram showing a schematic configuration of a write head. FIG. 4 is a diagram showing a line latent image formed by the writing head of FIG. 3. The figure which shows the relationship between a registration mark and a test pattern sensor. The figure which shows the relationship between the temperature fluctuation | variation of the optical sensor output at the time of toner detection, and a position detection fluctuation | variation. The figure which illustrated the two-dimensional halftone pattern. FIG. 6 is a diagram illustrating a halftone toner image that suppresses an edge effect. FIG. 5 is a diagram showing another embodiment of the image forming apparatus according to the present invention. FIG. 6 is a view showing still another embodiment of the image forming apparatus according to the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 16 ... Intermediate transfer belt (transfer medium), 18 ... Test pattern sensor (optical sensor), D16 ... Transport direction (moving direction), LP1, LP2 ... Linear pattern, RM, RM (K), RM (Y), RM (M), RM (C): Registration mark, TP: Two-dimensional halftone pattern, X: Main scanning direction, Y, M, C, K: Image forming station

Claims (8)

A plurality of image forming stations, each forming a different color toner image, are arranged along the transfer medium moving direction, and the toner images formed by the plurality of image forming stations are used as registration marks on the surface of the transfer medium. In the image forming apparatus, the registration marks are formed apart from each other in the moving direction, and each registration mark is detected by an optical sensor, and color misregistration between the plurality of colors is corrected based on an output signal from the optical sensor.
Each of the plurality of image forming stations is configured to form the registration mark with a halftone toner image.   The image forming apparatus according to claim 1, wherein the halftone toner image is configured by a two-dimensional halftone pattern.   The two-dimensional halftone pattern is a pattern including a plurality of horizontal lines extending substantially parallel to the moving direction, and the plurality of horizontal lines are arranged from the upstream side to the downstream side of the two-dimensional halftone pattern in the moving direction. The image forming apparatus according to claim 2.   The two-dimensional halftone pattern is a pattern including a plurality of oblique lines extending obliquely with respect to the movement direction, and the plurality of oblique lines are arranged from the upstream side to the downstream side of the two-dimensional halftone pattern in the movement direction. The image forming apparatus according to claim 2.   The image forming apparatus according to claim 2, wherein the two-dimensional halftone pattern is a lattice pattern. Position information of each of the plurality of registration marks is detected based on a change in an output signal obtained when an upstream portion of the registration mark in the moving direction passes through the optical sensor, and based on the plurality of position information Correct color misregistration
The halftone toner image includes a linear pattern extending in a main scanning direction substantially orthogonal to the moving direction and a two-dimensional halftone pattern, and the linear pattern is upstream of the moving direction with respect to the two-dimensional halftone pattern. The image forming apparatus according to claim 1, wherein the image forming apparatus is arranged so as to be spaced apart from each other. Position information of each of the plurality of registration marks is detected based on a change in an output signal obtained when a downstream portion of the registration mark in the moving direction passes through the optical sensor, and based on the plurality of position information Correct color misregistration
The halftone toner image includes a linear pattern extending in a main scanning direction substantially orthogonal to the moving direction and a two-dimensional halftone pattern, and the linear pattern is downstream of the moving direction with respect to the two-dimensional halftone pattern. The image forming apparatus according to claim 1, wherein the image forming apparatus is arranged so as to be spaced apart from each other. The halftone toner image is composed of first and second linear patterns and halftone patterns extending in a main scanning direction substantially orthogonal to the moving direction,
2. The image forming apparatus according to claim 1, wherein the first linear pattern, the first halftone pattern, and the second linear pattern are spaced apart from each other in the order in the moving direction.

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