Image forming apparatus

Abstract

To achieve both prevention of color shift and prevention of delay in an image formation starting time.SOLUTION: An image forming apparatus has an image forming unit that executes image formation on the basis of an image formation execution instruction input thereto; and a correction processing unit 250 that executes color shift correction processing of forming a test pattern image at a predetermined image forming speed and correcting an image forming condition by using a color shift correction value obtained from a result of detection of the test pattern image. The image forming apparatus has a storage unit 253 that stores the image forming speed and temperature specific information during the color shift correction processing. In a case where an image forming speed during the image formation is different from the image forming speed during the color shift correction processing stored in the storage unit, when a result of comparison between the temperature specific information during the image formation and the temperature specific information stored in the storage unit satisfies a condition exceeding a predetermined temperature difference, the correction processing unit executes the color shift correction processing at the image forming speed during the image formation before the execution of the image formation, and when the result of comparison does not satisfy the condition, does not execute the color shift correction processing before the execution of the image formation.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] Conventionally, there has been known an image forming apparatus having an image forming unit that executes image formation in accordance with predetermined image formation conditions based on an input image formation execution instruction, and a correction processing unit that creates a test pattern image for color shift detection at a predetermined image formation speed and executes color shift correction processing to correct the image formation conditions using a color shift correction value obtained from the detection result of the test pattern image.

[0003] For example, Patent Document 1 discloses an image forming apparatus that transfers color toner images on multiple photoconductors developed by multiple developing devices onto an intermediate transfer belt in a superimposed state to form an image. In this image forming apparatus, a color shift correction process is executed at a predetermined execution timing in order to suppress relative positional shift (color shift) of the transferred color toner images in the sub-scanning direction. In the color shift correction process, a test pattern image for detecting color shift is formed on the intermediate transfer belt, the amount of color shift (color shift correction value) is calculated from the result of detecting the test pattern image, and image forming conditions such as the image writing position of each color are corrected based on the calculated amount of color shift. Summary of the Invention [Problem to be solved by the invention]

[0004] However, in conventional color shift correction processes, in an image forming device capable of performing image formation in multiple image formation modes with different image formation speeds, there was a problem in that it was difficult to simultaneously suppress color shift and suppress delays in the start of image formation. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention provides an image forming apparatus having an image forming unit that performs image formation by writing a latent image on each of a plurality of latent image carriers by a latent image writing unit according to predetermined image forming conditions based on an input image formation execution instruction, and developing the latent images on the plurality of latent image carriers by a plurality of developing units to form visible images that are superimposed and transferred onto an intermediate transfer body or a recording material, and a correction processing unit that creates a test pattern image for detecting color shift at a predetermined image forming speed, and performs color shift correction processing to correct the image forming conditions using a color shift correction value obtained from a detection result of the test pattern image, and further includes information specifying the image forming speed and temperature during the color shift correction processing. the correction processing unit has a memory unit that stores therein information about temperature specification information at the time of image formation and temperature specification information at the time of color shift correction processing stored in the memory unit when an image formation speed during image formation executed by an input image formation execution instruction differs from an image formation speed during the color shift correction processing stored in the memory unit, and when the comparison result satisfies a condition that a predetermined temperature difference is exceeded, the color shift correction processing is performed at the image formation speed during image formation before image formation is executed by the image formation execution instruction, and when the comparison result does not satisfy the condition, the color shift correction processing is not performed before image formation is executed by the image formation execution instruction. Effect of the Invention

[0006] According to the present invention, in an image forming apparatus capable of performing image formation in a plurality of image forming modes having mutually different image formation speeds, it is possible to simultaneously suppress color registration and suppress delays in the start of image formation. [Brief description of the drawings]

[0007] [Figure 1] 1 is a schematic configuration diagram showing an image forming apparatus according to an embodiment. [Diagram 2] FIG. 2 is an enlarged configuration diagram showing an imaging unit for Y in the image forming apparatus. [Diagram 3] 4 is a partial configuration diagram for explaining an operation of opening and closing a door of the image forming apparatus; FIG. [Figure 4]FIG. 2 is a control block diagram for image adjustment using a test pattern image in the image forming apparatus. [Diagram 5] FIG. [Figure 6] 5A and 5B are diagrams showing the positions at which pattern images for detecting misregistration are formed in the color misregistration correction process performed by the image forming apparatus while the apparatus is idle; [Figure 7] 5A and 5B are diagrams showing the formation positions of pattern images for detecting misregistration in the color misregistration correction process during a print job of the image forming apparatus; [Figure 8] FIG. 2 is an enlarged configuration diagram showing a first detection sensor of the image forming apparatus; [Figure 9] 10 is a flowchart showing an example of a processing procedure for color misregistration correction processing during and after a print job. [Figure 10] 6 is a flowchart showing an example of a procedure for color misregistration correction processing before a print job in the embodiment. [Figure 11] 10 is a flowchart showing an example of a procedure for color misregistration correction processing before a print job in a modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, an embodiment of an electrophotographic image forming apparatus will be described as an image forming apparatus to which the present invention is applied. First, the basic configuration of the image forming apparatus of this embodiment will be described. FIG. 1 is a schematic diagram showing the configuration of an image forming apparatus according to the present embodiment. FIG. 2 is an enlarged configuration diagram showing the Y imaging unit in the image forming apparatus of this embodiment.

[0009] The image forming apparatus of this embodiment includes four imaging units 6Y, 6M, 6C, and 6K for forming toner images of yellow, magenta, cyan, and black (hereinafter referred to as Y, M, C, and K). These units use toners of different colors as color materials, but otherwise have the same configuration and are replaced when their lifespan is reached.

[0010] Taking the imaging unit 6Y for forming a Y toner image as an example, the imaging unit 6Y has a drum-shaped photoconductor 1Y as a latent image carrier, a drum cleaning device 2Y, a static eliminator, a charging device 4Y, a developing device 5Y, etc. The imaging unit 6Y as an image forming means is detachably attached to the image forming apparatus main body as a unit.

[0011] The charging device 4Y uniformly charges the surface of the photoconductor 1Y, which is rotated by a driving means in the clockwise direction in the figure. The uniformly charged surface of the photoconductor 1Y is exposed and scanned by a laser beam L to bear a Y electrostatic latent image. This Y electrostatic latent image is developed into a Y toner image (visible image) by a developing device 5Y that uses a Y developer containing Y toner and a magnetic carrier. The Y toner image is then primarily transferred onto an intermediate transfer belt 8, which will be described later.

[0012] The drum cleaning device 2Y removes the residual toner adhering to the surface of the photoconductor 1Y after the primary transfer process. The charge removing device removes the residual charge on the photoconductor 1Y after cleaning. This charge removal initializes the surface of the photoconductor 1Y to prepare for the next image formation.

[0013] Similarly, in the imaging units 6M, 6C, and 6K for the other colors, M, C, and K toner images are formed on the photoconductors 1M, 1C, and 1K, respectively, and are primarily transferred onto the intermediate transfer belt 8 in a superimposed manner.

[0014] The developing unit 5Y as a developing means has a developing roll 51Y as a developer carrier arranged so as to be partially exposed from an opening of the casing, and also has two transport screws 55Y arranged in parallel with each other, a doctor blade 52Y, a toner concentration sensor 56Y, and the like.

[0015] The casing of the developing unit 5Y contains a Y developer containing a magnetic carrier and a Y toner. The Y developer is stirred and transported by two transport screws 55Y, and is supported on the surface of the developing roll 51Y while being triboelectrically charged. The layer thickness is then regulated by a doctor blade 52Y and the developer is transported to a development area facing the Y photoconductor 1Y, where the Y toner is attached to the electrostatic latent image on the photoconductor 1Y. This attachment forms a Y toner image on the photoconductor 1Y. In the developing unit 5Y, the Y developer, which has consumed the Y toner during development, is returned to the casing as the developing roll 51Y rotates.

[0016] A partition wall is provided between the two transport screws 55Y. This partition wall separates the inside of the casing into a first supply section 53Y that houses the developing roll 51Y and the transport screw 55Y on the right side in the figure, and a second supply section 54Y that houses the transport screw 55Y on the left side in the figure.

[0017] The transport screw 55Y on the right side in the figure is rotationally driven by a drive means, and supplies the Y developer in the first supply section 53Y to the developing roll 51Y while transporting the Y developer from the front side to the back side in the figure. The Y developer transported to the vicinity of the end of the first supply section 53Y by the transport screw 55Y on the right side in the figure enters the second supply section 54Y through an opening provided in the partition wall.

[0018] In the second supply section 54Y, the transport screw 55Y on the left side in the figure is rotationally driven by a drive means and transports the Y developer sent from the first supply section 53Y in the opposite direction to the transport screw 55Y on the right side in the figure. The Y developer transported to the vicinity of the end of the second supply section 54Y by the transport screw 55Y on the left side in the figure returns to the inside of the first supply section 53Y through the other opening provided in the partition wall.

[0019] The toner concentration sensor 56Y, which is a magnetic permeability sensor, is provided on the bottom wall of the second supply unit 54Y and outputs a voltage value corresponding to the magnetic permeability of the Y developer passing above it. Since the magnetic permeability of a two-component developer containing toner and a magnetic carrier shows a good correlation with the toner concentration, the toner concentration sensor 56Y outputs a voltage value corresponding to the Y toner concentration. The value of this output voltage is sent to the overall control unit 252, which will be described later.

[0020] The overall control unit 252 includes a RAM that stores the Y Vtref, which is a target value of the output voltage from the toner concentration sensor 56Y. In this RAM, data of the M Vtref, the C Vtref, and the K Vtref, which are target values ​​of the output voltage from the toner concentration sensors mounted on the other developing units, are also stored. The Y Vtref is used to drive and control the Y toner conveying device, which will be described later. Specifically, the overall control unit 252 controls the drive of the Y toner conveying device to supply the Y toner to the second supply unit 54Y so that the value of the output voltage from the toner concentration sensor 56Y approaches the Y Vtref. This supply maintains the Y toner concentration in the Y developer in the developing unit 5Y within a predetermined range. For the developing units of the other process units, similar toner supply control is performed using the M, C, and K toner conveying devices.

[0021] In FIG. 1 shown above, an optical writing device 7 is disposed below the imaging units 6Y, 6M, 6C, and 6K as a latent image writing section. The optical writing device 7 optically scans the photoconductors in the imaging units 6Y, 6M, 6C, and 6K with laser light L emitted based on image information. Electrostatic latent images for Y, M, C, and K are formed on the photoconductors 1Y, 1M, 1C, and 1K by this optical scanning. The optical writing device 7 irradiates the photoconductors 1Y, 1M, 1C, and 1K with laser light (L) emitted from a light source via a polygon mirror rotated and driven by a motor, and irradiates the photoconductors 1Y, 1M, 1C, and 1K via multiple optical lenses and mirrors.

[0022] Below the optical writing device 7 in the figure, there is disposed sheet storage means having a sheet storage cassette 26 and a feed roller 27 incorporated therein. The sheet storage cassette 26 stores a plurality of recording sheets P as sheet-like recording materials in a stack, and the feed roller 27 is brought into contact with the topmost recording sheet P of each stack. When the feed roller 27 is rotated counterclockwise in the figure by a drive means, the topmost recording sheet P is sent out toward the sheet supply path 70.

[0023] A pair of registration rollers 28 is disposed near the end of the sheet supply path 70. The pair of registration rollers 28 rotate both rollers to sandwich the recording sheet P, but as soon as the recording sheet P is sandwiched, the rotation of both rollers is temporarily stopped. Then, at an appropriate timing, the rotation of both rollers is resumed to send the recording sheet P toward the secondary transfer nip, which will be described later.

[0024] A transfer unit 15 is disposed above the image forming units 6Y, 6M, 6C, and 6K in the figure, which moves the intermediate transfer belt 8, which serves as an intermediate transfer body, endlessly while stretching it. In addition to the intermediate transfer belt 8, the transfer unit 15 is equipped with a secondary transfer bias roller 19, a cleaning device 10, and the like. It also includes four primary transfer bias rollers 9Y, 9M, 9C, and 9K, a drive roller 12, a cleaning backup roller 13, and a secondary transfer nip entrance roller 14. The intermediate transfer belt 8 is endlessly moved counterclockwise in the figure by the rotational drive of the drive roller 12 while being wound around each of these seven rollers.

[0025] The primary transfer bias rollers 9Y, 9M, 9C, and 9K sandwich the intermediate transfer belt 8, which is moved endlessly in this manner, between the photoconductors 1Y, 1M, 1C, and 1K to form primary transfer nips. A primary transfer bias of the opposite polarity (e.g., positive) to that of the toner is applied to these primary transfer bias rollers. All rollers except for the primary transfer bias rollers 9Y, 9M, 9C, and 9K are electrically grounded.

[0026] As the intermediate transfer belt 8 moves endlessly, it passes through the primary transfer nips for Y, M, C, and K in sequence, and the Y, M, C, and K toner images on the photoconductors 1Y, 1M, 1C, and 1K are primarily transferred onto the intermediate transfer belt 8 in a superimposed manner. As a result, a four-color superimposed toner image (hereinafter referred to as a four-color toner image) is formed on the intermediate transfer belt 8.

[0027] The intermediate transfer belt 8 is sandwiched between the drive roller 12 and a secondary transfer bias roller 19 serving as a contacting / separating member to form a secondary transfer nip. The four-color toner image formed on the intermediate transfer belt 8 is transferred to the recording sheet P at this secondary transfer nip. The four-color toner image is then combined with the white color of the recording sheet P to form a full-color toner image. The secondary transfer bias roller 19 and the drive roller 12 of the intermediate transfer body are generally made of rubber in consideration of the transferability to the recording sheet P.

[0028] Further, the secondary transfer bias roller 19 is provided with a contact / separation mechanism using a spring or the like, so that the secondary transfer bias roller 19 can be brought into contact with and separated from the drive roller 12 of the intermediate transfer belt 8. In order to ensure transferability when the toner image transferred onto the intermediate transfer belt 8 is secondarily transferred to the recording sheet P, the secondary transfer bias roller 19 is brought into contact with the drive roller 12 during transfer onto the recording sheet P.

[0029] On the other hand, when the color misregistration correction process is being performed while the device is idle, the secondary transfer bias roller 19 is separated from the drive roller 12 to prevent contamination of the secondary transfer bias roller 19 with toner and blurring of the test pattern image 42 for detecting color misregistration. The device is idle during a period when no print job is being performed (a period during which no image formation is being performed), such as a period before image formation is started in response to an input print instruction (image formation execution instruction). Note that, when the color misregistration correction process is being performed during a print job, transfer to the recording sheet P is also being performed, so the secondary transfer bias roller 19 is brought into contact with the drive roller 12.

[0030] After passing through the secondary transfer nip, residual toner that has not been transferred to the recording sheet P adheres to the intermediate transfer belt 8. This is cleaned by a cleaning device 10. In addition, the recording sheet P onto which the four-color toner image has been secondarily transferred at the secondary transfer nip is sent to a fixing device 20 via a post-transfer conveying path 71.

[0031] The fixing device 20 forms a fixing nip with a fixing roller 20a having an internal heat source such as a halogen lamp, and a pressure roller 20b which rotates while in contact with the fixing roller 20a at a predetermined pressure. The recording sheet P fed into the fixing device 20 is sandwiched in the fixing nip with its unfixed toner image bearing surface in close contact with the fixing roller 20a. The toner in the toner image is softened by the effects of heat and pressure, and the full-color image is fixed.

[0032] After the recording sheet P on which the full-color image has been fixed in the fixing device 20 leaves the fixing device 20, it approaches a branching point between the discharge path 72 and the pre-reversal transport path 73. A first switching claw 75 is arranged to be swingable at this branching point, and the path of the recording sheet P is switched by the swinging of the first switching claw. Specifically, by moving the tip of the claw in a direction approaching the pre-reversal transport path 73, the path of the recording sheet P is changed to the direction toward the discharge path 72. Also, by moving the tip of the claw in a direction away from the pre-reversal transport path 73, the path of the recording sheet P is changed to the direction toward the pre-reversal transport path 73.

[0033] When the path toward the discharge path 72 is selected by the first switching claw 75, the recording sheet P passes from the discharge path 72 through the pair of discharge rollers 100, is disposed outside the machine, and is stacked on the stack unit 50a provided on the upper surface of the image forming apparatus housing. On the other hand, when the path toward the pre-reversal transport path 73 is selected by the first switching claw 75, the recording sheet P passes through the pre-reversal transport path 73 and enters the nip of the reversal roller pair 21. The reversal roller pair 21 transports the recording sheet P sandwiched between the rollers toward the stack unit 50a, but reverses the rollers just before the trailing end of the recording sheet P enters the nip. Due to this reversal, the recording sheet P is transported in the opposite direction to before, and the trailing end side of the recording sheet P enters the reversal transport path 74.

[0034] The reverse conveying path 74 is shaped to extend while curving from the upper side to the lower side in the vertical direction, and has a first reverse conveying roller pair 22, a second reverse conveying roller pair 23, and a third reverse conveying roller pair 24 in the path. The recording sheet P is reversed upside down by being conveyed while passing through the nips of these roller pairs in sequence. After being reversed upside down, the recording sheet P is returned to the above-mentioned sheet supply path 70 and then reaches the secondary transfer nip again. Then, this time, the recording sheet P enters the secondary transfer nip while its non-image bearing surface is in close contact with the intermediate transfer belt 8, and the second four-color toner image of the intermediate transfer belt 8 is transferred collectively to the non-image bearing surface. After this, the recording sheet P passes through the post-transfer conveying path 71, the fixing device 20, the paper discharge path 72, and the paper discharge roller pair 100, and is stacked on the stack unit 50a outside the machine. By such reverse conveying, a full-color image is formed on both sides of the recording sheet P.

[0035] Between the transfer unit 15 and the stack section 50a above it, a bottle support section 31 is disposed. The bottle support section 31 is equipped with toner bottles 32Y, 32M, 32C, and 32K as toner storage sections that store Y, M, C, and K toner. The Y, M, C, and K toners in the toner bottles 32Y, 32M, 32C, and 32K are appropriately replenished to the developing devices of the image forming units 6Y, 6M, 6C, and 6K by toner conveying devices, respectively. These toner bottles 32Y, 32M, 32C, and 32K are detachable from the image forming apparatus main body, independently of the image forming units 6Y, 6M, 6C, and 6K.

[0036] The reverse conveying path 74 is formed in the opening and closing door, and the opening and closing door has an external cover 61 and a swinging support 62. Specifically, the external cover 61 of the opening and closing door is supported so as to rotate around a first rotation shaft 59 provided in the housing 50 of the image forming apparatus main body. By this rotation, the external cover 61 opens and closes the opening of the housing 50. Also, as shown in FIG. 3, the swinging support 62 of the opening and closing door is exposed to the outside when the external cover 61 is opened, and is supported by the external cover so as to rotate around a second rotation shaft 63 provided in the external cover 61. By this rotation, the swinging support 62 swings with respect to the external cover 61 in a state opened from the housing 50, and the external cover 61 and the swinging support 62 are separated, and the reverse conveying path 74 is exposed. By exposing the reverse conveying path 74 in this way, a jammed sheet in the reverse conveying path 74 can be easily removed.

[0037] FIG. 4 is a functional block diagram for performing image adjustment using a test pattern image. The correction processing section 250 executes color shift correction processing, and is made up of a test pattern forming section 250a, a calculation section 250b, an adjustment section 250c, and an adjustment execution timing control section 250e.

[0038] The test pattern forming unit 250a sends an instruction to form a test pattern image for detecting color misregistration to the overall control unit 252. As a result, the overall control unit 252 controls the optical writing device 7 to form a predetermined test pattern image at a detection position within the area on the intermediate transfer belt where an image corresponding to the recording sheet P is formed (inside the image forming area) and outside the area on the intermediate transfer belt where an image corresponding to the recording sheet is formed (outside the image forming area).

[0039] The calculation unit 250b calculates various amounts of misalignment (color shift correction values) of the test pattern image based on the detection results of the test pattern image transmitted from the test pattern detection unit 251. The amounts of misalignment (color shift correction values) calculated by the calculation unit 250b are stored in the storage unit 253 together with information such as the temperature, time, and image formation speed within the apparatus at the time of misalignment calculation (at the time of color shift correction processing), and the success or failure of the color shift correction processing.

[0040] The adjustment unit 250c performs image adjustment (adjustment of image formation conditions) based on the amount of deviation calculated by the calculation unit 250b. Specifically, image adjustment includes correction of the optical path in the optical system of each color, and correction of the image writing position of each color by changing the pixel clock frequency. Correction of the image writing position of each color can be completed in a relatively short time because these setting values ​​are digitally corrected based on the calculated amount of deviation. On the other hand, correction of the optical path in the optical system of each color takes a long time because the optical system including the light source and the f-θ lens, and the mirror in the optical path are mechanically moved based on the calculated amount of deviation to correct the positions of the optical paths of the optical systems of each color to match each other.

[0041] The adjustment execution timing control unit 250e judges various factors related to the execution timing of the color misregistration correction process, such as the number of sheets in a paper passing job, the amount of skew deviation stored in the memory unit 253, the temperature inside the image forming apparatus, and the elapsed time, and manages the execution timing of the color misregistration correction process. Information necessary for judging various factors related to the execution timing of the color misregistration correction process, such as the number of sheets in a paper passing job, the amount of skew deviation, the temperature inside the image forming apparatus, and the elapsed time, is stored in the memory unit 253.

[0042] The overall control unit 252 outputs a print job start instruction signal to the correction processing unit 250 to instruct the start of image formation for each page and the start of formation of a test pattern image. The overall control unit 252 also transmits information on the number of remaining sheets in a print job and the number of remaining print jobs to the correction processing unit 250. The overall control unit 252 also controls the drive sources that drive the optical writing device 7 and the photoconductors 1Y, 1M, 1C, and 1K based on the image formation conditions adjusted by the correction processing unit 250 to form images.

[0043] A test pattern image 42 for detecting color misregistration shown in FIG. 5 is formed at each detection position in the width direction of the intermediate transfer belt 8 (see FIG. 1). The test pattern image 42 for detecting color misregistration includes first detection images I1C, I1K, I1Y, and I1M arranged at a predetermined interval in the sub-scanning direction, and second detection images I2C, I2K, I2Y, and I2M arranged at a predetermined interval in the sub-scanning direction at the subsequent positions. The direction of the arrow x in the figure is the main scanning direction (photoconductor axial direction). The direction of the arrow y in the figure is the sub-scanning direction (photoconductor surface movement direction). The first detection images I1C, I1K, I1Y, and I1M are formed extending in the main scanning direction x, while the second detection images I2C, I2K, I2Y, and I2M are formed inclined at 45° from the main scanning direction x.

[0044] Fig. 6 shows the positions at which test pattern images for detecting color shift are formed in the color shift correction process while the device is idle. Test pattern images 42 (42a, 42b, 42c) for detecting color shift shown in Fig. 5 are formed at one end, the center, and the other end of the intermediate transfer belt 8 in the width direction (main scanning direction).

[0045] 7 shows the positions at which the test pattern image 42 for detecting color misregistration is formed in the color misregistration correction process during a print job. The test pattern images 42 (42a, 42c) for detecting color misregistration shown in FIG. 5 are formed at one end and the other end in the width direction of the intermediate transfer belt 8. In the color misregistration correction process during a print job, unlike the color misregistration correction process during the device idle, the test pattern image 42b for detecting color misregistration in the center is not formed. This is because, in the color misregistration correction process during a print job, the test pattern image 42 for detecting color misregistration is formed in parallel with the image formation process, and therefore the test pattern image 42 for detecting color misregistration can only be formed at both ends outside the image formation area on the intermediate transfer belt.

[0046] Of the entire area in the circumferential direction of the intermediate transfer belt 8, a detection sensor 150, which is an optical sensor, faces the outer circumferential surface of a region after passing the wrapping position around the drive roller 12 and before entering the pressing position by the pressing roller 11, with a predetermined gap therebetween. This detection sensor 150 has a first detection sensor 150a facing one end of the intermediate transfer belt 8 in the width direction, a second detection sensor 150b facing the center, and a third detection sensor 150c facing the other end (see FIGS. 6 and 7).

[0047] FIG. 8 is an enlarged configuration diagram showing the first detection sensor 150a. The first detection sensor 150a has a light-emitting section 151a that emits light toward the outer peripheral surface of the intermediate transfer belt 8, and a light-receiving section 152a that receives the light reflected by the outer peripheral surface of the belt and outputs a signal according to the amount of received light. A relatively large amount of reflected light is obtained from the portions of the outer peripheral surface of the intermediate transfer belt 8 where the toner of the test pattern image 42 is not attached. In contrast, the amount of reflected light obtained is reduced from the portions where the toner of the test pattern image 42 is attached. Based on this reduction in the amount of reflected light, the test pattern image 42 is detected.

[0048] Although the first detection sensor 150a has been described above, the second detection sensor 150b and the third detection sensor 150c are also configured in the same manner as the first detection sensor 150a. A signal from the light receiving unit of each detection sensor is sent to a test pattern detection unit 251. The test pattern detection unit 251 includes an A / D conversion circuit that converts an analog signal sent from the light receiving unit into a digital signal, and detects the test pattern image 42 based on the fact that the value after digital conversion falls below a predetermined threshold value. Then, the test pattern detection unit 251 immediately outputs a detection signal to the calculation unit 250b of the correction processing unit 250.

[0049] Image position deviations (color deviations) of each color include skew deviation caused by the orientation of the Y, M, and C toner images being tilted relative to the K toner image which is the reference color, sub-scanning direction registration deviation caused by the overall deviation of the formation positions of the Y, M, and C toner images in the sub-scanning direction relative to the formation position of the K toner image, deviation caused by overall magnification error in the main scanning direction, registration deviation in the main scanning direction, etc. Sub-scanning direction registration deviation is a phenomenon in which the formation position of the entire toner image is shifted in the sub-scanning direction from the correct position.

[0050] A specific method for calculating various amounts of deviation when a test pattern image is detected will be described with reference to FIG. The detection sensors 150a, 150b, and 150c arranged at the sensor positions in the figure detect the mark rows of the test pattern image 42 at a predetermined constant sampling time interval. Based on the detection results, the calculation unit 250b (see FIG. 4) calculates the length of the interval between each color of the horizontal line pattern and the interval between the horizontal line pattern and the diagonal line pattern of the corresponding color. Various amounts of deviation can be calculated using the calculated length of each interval.

[0051] In calculating the amount of sub-scanning registration misalignment (amount of color misalignment in the sub-scanning direction), the detection data of the horizontal line pattern is used to calculate the interval values ​​Lck, Lky, Lkm between the pattern of the reference color (K) and each pattern of the target colors Y, M, C, and compared with the pre-stored initial setting interval values ​​Lck0, Lky0, Lkm0. The differences between the detected interval values ​​and the initial setting interval values ​​(Lck-Lck0, Lky-Lky0, Lkm-Lkm0) are then regarded as the amount of sub-scanning registration misalignment of each color Y, M, C relative to the reference color (K).

[0052] In addition, in calculating the amount of main scanning registration deviation (amount of color deviation in the main scanning direction), first, the interval values ​​Lcc, Lkk, Lyy, and Lmm of the horizontal line patterns and diagonal line patterns of each color K to C are calculated. Using these calculated interval values, the difference between the interval value of the reference color (K) and the interval values ​​of the other colors is calculated. That is, the difference in the interval value between K and Y (Lkk-Lyy), the difference in the interval value between K and M (Lkk-Lmm), and the difference in the interval value between K and C (Lkk-Lcc) are calculated. Since the diagonal line pattern is inclined at a predetermined angle with respect to the main scanning direction, when deviation occurs in the main scanning direction, the interval between the horizontal line pattern becomes wider or narrower than the interval of the reference color, and these differences can be regarded as the amount of main scanning registration deviation.

[0053] The skew deviation amount and main scanning magnification error are obtained by combining the detection results of the detection sensors 150a to 150c. The skew deviation amount can be obtained by calculating the difference between the sub-scanning registration deviation amounts calculated from the detection results by the detection sensors 150a and 150c. The main scanning magnification error can be obtained by calculating the difference between the main scanning registration deviation amounts calculated from the detection results by the detection sensors 150a and 150b, and calculating the difference between the main scanning registration deviation amounts calculated from the detection results by the detection sensors 150b and 150c.

[0054] The calculated amounts of misalignment are stored in the storage unit 253 and are used in the color misalignment adjustment process (color misalignment correction process). The color misalignment correction process can be performed, for example, by correcting the emission timing of the light beams corresponding to the respective colors for the photoconductors 1Y, 1M, 1C, and 1K in accordance with the amount of misalignment. It can also be performed by adjusting the inclination of a reflecting mirror that reflects the light beam. The inclination of the reflecting mirror is adjusted by driving a stepping motor attached to the reflecting mirror. It is also possible to perform correction according to the adjustment of the amount of misalignment by changing the image data itself.

[0055] Among the color misregistration amounts, the main scanning registration misregistration and the sub-scanning registration misregistration can be digitally corrected by adjusting the timing of laser light writing on the photoconductor, and the main scanning magnification can be digitally corrected by adjusting the pixel clock. Therefore, adjustment of these color misregistration amounts can be performed during the period when the paper being printed passes the transfer area after the calculation of the misregistration amount is completed in the color misregistration correction process during the print job.

[0056] On the other hand, since skew adjustment requires mechanically operating mirrors in the optical path using a motor or the like to align the positions of the photoconductors for each color, it cannot be performed in a short time such as the period during which the paper passes through the transfer area during printing. Therefore, after the amount of skew is calculated in the color shift correction process during a print job, it is not possible to perform skew adjustment processing subsequently. The skew adjustment processing is performed when the color shift correction process is being executed while the device is idle.

[0057] An example of the procedure for color misregistration correction processing during and after a print job in this embodiment will be described with reference to FIG. FIG. 9 is a flowchart showing an example of a procedure for color misregistration correction processing during and after a print job in this embodiment. First, in the image forming apparatus, when a print instruction (image formation execution instruction) is input to the overall control unit 252 and a print job is started, the overall control unit 252 starts an image forming operation (S1). Then, the overall control unit 252 judges whether the print job based on the input print instruction has ended (S2). If the print job has ended, the process proceeds to step S3, and if not, the process proceeds to step S8.

[0058] If it is determined that the print job has ended (Yes in S2), the adjustment execution timing control unit 250e determines (S3) whether the amount of skew deviation is equal to or greater than a preset threshold value A. If it is equal to or greater than threshold value A, the process proceeds to step S4, and if it is less than threshold value A, the process ends.

[0059] If the skew deviation is equal to or greater than the threshold A (Yes in S3), the adjustment unit 250c sets the color deviation correction processing mode to "color deviation correction processing mode during device idle", and the test pattern forming unit 250a controls the drive sources of the optical writing device 7 and the photoconductors 1Y, 1M, 1C, and 1K via the overall control unit 252 to form the test pattern images 42a, 42b, and 42c. After that, the formed test pattern images 42a, 42b, and 42c are read by the detection sensor 150, and the test pattern detection unit 251 determines whether the test pattern images 42a, 42b, and 42c have been properly read (S5).

[0060] If the test pattern image cannot be read normally (No in S5), the correction failure count in the storage unit 253 is incremented by one (S7), and the process ends. On the other hand, if the test pattern image can be read normally (Yes in S5), the calculation unit 250b calculates the registration misalignment amount, magnification misalignment amount, and skew misalignment amount, and based on the calculation results, the adjustment unit 250c performs image adjustment for the registration misalignment, magnification misalignment, and skew misalignment, and stores the calculation results in the storage unit 253 as color misalignment correction values ​​(S6), and the process ends.

[0061] On the other hand, if it is determined that the print job has not ended (No in S2), the adjustment execution timing control unit 250e determines whether it is time to execute color shift correction processing during the print job (S8). If it is determined that it is time to execute color shift correction processing during the print job (Yes in S8), the process proceeds to step S9, and if it is determined that it is not time to execute the processing (No in S8), the process returns to step S1.

[0062] If it is determined that it is time to perform color misregistration correction processing during the print job (Yes in S8), the adjustment unit 250c sets the color misregistration correction processing mode to "color misregistration correction processing mode during the print job", and the test pattern forming unit 250a controls the drive sources of the optical writing device 7 and the photoconductors 1Y, 1M, 1C, and 1K via the overall control unit 252 to form the test pattern images 42a and 42c (S9). Then, the formed test pattern images 42a and 42c are read by the detection sensor 150, and the test pattern detection unit 251 determines whether the test pattern images 42a, 42b, and 42c have been properly read (S10).

[0063] If the test pattern image cannot be read normally (No in S10), the correction failure count in the storage unit 253 is incremented by one (S11), and the process ends. On the other hand, if the test pattern image can be read normally (Yes in S10), the calculation unit 250b calculates the registration misalignment amount, magnification misalignment amount, and skew misalignment amount, and the adjustment unit 250c performs image adjustment for the registration misalignment and magnification misalignment based on the calculation results (S12), and stores the calculated registration misalignment amount, magnification misalignment amount, and skew misalignment amount in the storage unit 253 as color shift correction values ​​(S13), and the process returns to step 1. As a result, in subsequent image formation during this print job, the overall control unit 252 forms images in a state in which the image adjustment (color shift correction) for the registration misalignment and magnification misalignment has been performed by the adjustment unit 250c (S1).

[0064] The image forming apparatus of this embodiment forms an image at an image forming speed selected from among different image forming speeds (linear speeds of the photoconductor and intermediate transfer belt, also called operating linear speeds) according to a predetermined speed determination condition. Specifically, for example, when the type of recording sheet P used for image formation is thick paper, it is necessary to take a long fixing time for fixing the image by the fixing device 20, so image formation is performed at a slow image forming speed (operating linear speed). Also, for example, if the image forming apparatus is continuously operated at a high image forming speed (operating linear speed), the temperature inside the apparatus will reach the usable temperature threshold due to heat generated by the rotational movement of various moving parts inside the apparatus, so image formation is performed by slowing down the image forming speed in order to extend the continuous operation time.

[0065] In order to reliably suppress color shift at any image formation speed (operating linear speed), when performing image formation at an image formation speed different from the image formation speed during the color shift correction process (when creating the test pattern image 42) at which the shift amount (color shift correction value) stored in the memory unit 253 was calculated, it is desirable to create a test pattern image at the different image formation speed each time and execute a new color shift correction process to calculate the shift amount.

[0066] However, if a new color shift correction process is executed each time an image is formed at an image formation speed different from the speed at which the shift amount (color shift correction value) stored in the memory unit 253 was calculated, a problem occurs in that the start time of image formation (first print time) becomes later each time the image is formed at the different image formation speed.

[0067] It has been found that the main cause of color shift is temperature change in the optical writing device 7. That is, the change in temperature causes deformation of the mirrors and members supporting the lenses in the optical writing device 7, which changes the optical path of the light beam that forms the electrostatic latent image on the photoconductors 1Y, 1M, 1C, and 1K, and this appears on the image as color shift. Therefore, if the environmental conditions (particularly the temperature inside the device) when a print job is executed (when an image is formed) have not changed much since the calculation of the shift amount (color shift correction value) stored in the storage unit 253 (the time when the previous color shift correction process was executed), color shift can be sufficiently suppressed by using the color shift correction value (shift amount stored in the storage unit 253) from the color shift correction process that has already been executed, even if a new color shift correction process is not executed.

[0068] This is also true when the print job is executed at an image formation speed different from the image formation speed when the misalignment amount was calculated (when the color misalignment correction process was performed) stored in the storage unit 253. That is, even in this case, the misalignment amount stored in the storage unit 253 is corrected by a predetermined correction value according to the difference between the image formation speed when the misalignment amount was calculated (when the color misalignment correction process was performed) and the image formation speed of the print job, and the corrected misalignment amount is used as a color misalignment correction value to correct the image formation conditions, thereby making it possible to sufficiently suppress color misalignment.

[0069] On the other hand, if the temperature inside the device when the print job is executed (when the image is formed) has changed to a certain degree (for example, by 5°C or more) from the time when the misalignment amount stored in the memory unit 253 was calculated (when the color misalignment correction process was performed), when the print job is executed at an image formation speed different from the image formation speed when the misalignment amount stored in the memory unit 253 was calculated, the above-mentioned modified correction value cannot sufficiently suppress color misalignment.

[0070] In detail, even when a print job is executed at the same image formation speed as the image formation speed when the misalignment amount stored in memory unit 253 was calculated, if the temperature inside the device when the print job is executed has changed significantly (for example, 10° C. or more) since the calculation of the misalignment amount in memory unit 253, the misalignment amount stored in memory unit 253 may not be able to sufficiently suppress color misalignment, and a new color misalignment correction process must be executed. Conversely, if the change in the temperature inside the device is small (for example, less than 10° C.), color misalignment can be sufficiently suppressed using the misalignment amount stored in memory unit 253 without executing a new color misalignment correction process.

[0071] In contrast, when a print job is executed at an image formation speed different from the image formation speed when the misalignment amount stored in the memory unit 253 was calculated, even if the change in the temperature inside the device is relatively small (for example, even if it is less than 10° C., which does not require execution of a new color misalignment correction process when the print job is executed at the same image formation speed), if the change is to a certain extent (for example, 5° C. or more), it is necessary to execute a new color misalignment correction process. On the other hand, even when a print job is executed at an image formation speed different from the image formation speed when the misalignment amount stored in the memory unit 253 was calculated, if the change in the temperature inside the device is sufficiently small (for example, less than 5° C.), it is possible to sufficiently suppress color misalignment by using a modified correction value using the misalignment amount stored in the memory unit 253, without executing a new color misalignment correction process.

[0072] Based on the above findings, in this embodiment, when a print instruction (instruction to execute image formation) is input, before the print job is started, if the image formation speed of the print job (during image formation) differs from the image formation speed at the time of calculation of the misalignment amount stored in memory unit 253 (during color misalignment correction processing), whether or not to execute color misalignment correction processing is determined based on whether or not the temperature inside the device has changed by a certain amount or more from the time of calculation of the misalignment amount (during color misalignment correction processing).

[0073] Specifically, the temperature specifying information for the print job is compared with the temperature specifying information at the time of calculating the amount of deviation, and if the comparison result satisfies the condition of exceeding a predetermined temperature difference, a new color shift correction process is performed at the image formation speed of the print job before the start of the print job (before image formation is performed). In this case, the color shift is corrected in the print job using the amount of deviation calculated by the new color shift correction process and stored in storage unit 253. On the other hand, if the comparison result does not satisfy the condition, the color shift correction process is not performed before the start of the print job (before image formation is performed). In this case, the color shift is corrected using the amount of deviation stored in storage unit 253.

[0074] FIG. 10 is a flowchart showing an example of a procedure for color misregistration correction processing before a print job in this embodiment. First, in the image forming apparatus, when a print instruction (image formation execution instruction) is input to the overall control unit 252 (S21), before the start of a print job, the overall control unit 252 first determines the image formation speed of the print job in accordance with a predetermined speed determination condition (S22). Specific speed determination conditions include, for example, when the type of recording sheet P specified in the print job is a type corresponding to a low speed (such as cardboard) and when the temperature inside the apparatus detected by the temperature sensor 254 as a temperature detection unit is equal to or higher than the usable temperature threshold, a low image formation speed is determined, and otherwise a high image formation speed is determined. The determined image formation speed is stored in the storage unit 253.

[0075] After that, the overall control unit 252 sends an instruction to the correction processing unit 250 to perform color misregistration correction processing before the print job. As a result, the adjustment execution timing control unit 250e of the correction processing unit 250 first reads out the image formation speed of the current print job stored in the memory unit 253 and the image formation speed at the time of calculation of the misregistration amount (at the time of color misregistration correction processing) stored in association with the misregistration amount stored in the memory unit 253. Then, the adjustment execution timing control unit 250e determines whether or not the image formation speed of the current print job is the same as the image formation speed at the time of calculation of the misregistration amount (at the time of color misregistration correction processing) stored in the memory unit 253 (S23).

[0076] If it is determined in step S23 that the image formation speeds are the same (Yes in S23), the amount of misalignment stored in memory unit 253 is calculated for the image formation speed of the current print job, so no new color misalignment correction processing is performed. In this case, information indicating that the color misalignment correction processing has been completed is sent from correction processing unit 250 to overall control unit 252, and overall control unit 252 starts the print job (S24). As a result, overall control unit 252 starts the image formation operation (S1) in accordance with the procedure shown in FIG. 9.

[0077] On the other hand, if it is determined in step S23 that the image formation speeds are different (No in S23), the temperature identification information of the current print job is compared with the temperature identification information stored in memory unit 253 at the time of calculating the amount of deviation, and it is determined whether the comparison result satisfies the condition of exceeding a predetermined temperature difference (S25).

[0078] The judgment in step S25 can be made based on, for example, the temperature inside the device (particularly the temperature of the optical writing device 7), which is a direct cause of the color shift, that is, the detection result of the temperature sensor 254. In this case, the adjustment execution timing control unit 250e of the correction processing unit 250 reads out, as temperature specifying information, information on the temperature inside the device at the time of calculating the amount of shift (at the time of the color shift correction process) that is stored in association with the amount of shift stored in the memory unit 253. Then, the adjustment execution timing control unit 250e obtains the current temperature inside the device from the temperature sensor 254, and calculates the temperature difference between the temperature inside the device at the time of calculating the amount of shift that was read out and the current temperature inside the device. If this temperature difference exceeds a predetermined temperature difference (for example, 5° C.), it is judged that the condition of step S25 is met (Yes in S25), and if this temperature difference is equal to or less than the predetermined temperature difference, it is judged that the condition of step S25 is not met (No in S25).

[0079] The determination in step S25 is not limited to this. For example, it may be determined that the condition in step S25 is satisfied when the time from the calculation of the amount of misalignment stored in the storage unit 253 (the time of the color misalignment correction process) to the time of the current print job (the time of image formation) exceeds a predetermined time. This is for the following reason.

[0080] That is, since the current time is before the print job, the calculation time (color shift correction processing time) of the misalignment amount stored in the storage unit 253 is usually the time of the color shift correction processing during the previous print job or the time of the color shift correction processing at the end of the previous print job. Therefore, the misalignment amount stored in the storage unit 253 is calculated by the color shift correction processing performed in a state in which the temperature inside the device is increased by the image formation operation of the previous print job. Therefore, the time from the calculation time (color shift correction processing time) of the misalignment amount stored in the storage unit 253 to the time of the current print job (image formation) is a waiting time (non-operating time of the image forming device) during which the image formation operation is not performed, and the longer this time is, the lower the temperature inside the device becomes. For this reason, there is a correlation in which the longer the time from the calculation time (color shift correction processing time) of the misalignment amount stored in the storage unit 253 to the time of the current print job (image formation), the larger the temperature difference between the temperature inside the device when the misalignment amount stored in the storage unit 253 was calculated and the current temperature inside the device.

[0081] Specifically, when the color shift correction process is performed to calculate the amount of shift and store it in the storage unit 253, the time of the color shift correction process is stored as temperature specifying information. Then, the adjustment execution timing control unit 250e of the correction processing unit 250 reads out the time of the calculation of the amount of shift (the color shift correction process) stored in association with the amount of shift stored in the storage unit 253 as temperature specifying information. After that, the adjustment execution timing control unit 250e obtains the current time and calculates the time from the time of the calculation of the read amount of shift to the current time. If this time exceeds a predetermined time, it is determined that the condition of step S25 is met (Yes in S25), and if this time is equal to or less than the predetermined time, it is determined that the condition of step S25 is not met (No in S25).

[0082] If it is determined that the condition of step S25 is satisfied (Yes in S25), a new color shift correction process is executed at the image formation speed of the current print job (S26), and a new shift amount to be used for the current print job is calculated. Specifically, the test pattern forming unit 250a of the correction processing unit 250 controls the driving sources of the optical writing device 7 and the photoconductors 1Y, 1M, 1C, and 1K at the image formation speed of the current print job determined in step S22 by the overall control unit 252, so that the test pattern images 42a, 42b, and 42c are formed. Then, the test pattern detection unit 251 detects the formed test pattern images 42a, 42b, and 42c via the detection sensor 150, and the calculation unit 250b calculates the registration shift amount, magnification shift amount, and skew shift amount, and stores the calculation results in the storage unit 253 as color shift correction values.

[0083] As described above, according to this embodiment, if the image formation speed of the current print job (at the time of image formation) is different from the image formation speed at the time of calculation of the amount of misalignment stored in the storage unit 253 (at the time of color misalignment correction processing), when the condition of step S25 is not satisfied, a new color misalignment correction processing is not performed before the start of the print job. Therefore, it is possible to avoid a delay in the image formation start time (first print time) of the print job, aiming to prioritize shortening the waiting time of the user. Moreover, when the condition of step S25 is not satisfied, the temperature inside the device (particularly the temperature of the optical writing device 7), which is a cause of color misalignment, has not changed much since the time of calculation of the amount of misalignment stored in the storage unit 253 (at the time of color misalignment correction processing), so that the color misalignment can be sufficiently corrected by using the amount of misalignment stored in the storage unit 253.

[0084] On the other hand, when the condition of step S25 is met, the temperature inside the device (particularly the temperature of the optical writing device 7), which is a cause of color misregistration, has changed to a certain degree since the calculation of the misregistration amount stored in the memory unit 253 (the color misregistration correction process), and even if the misregistration amount stored in the memory unit 253 is used, there is a risk that the color misregistration cannot be sufficiently corrected. Therefore, in this embodiment, when the condition of step S25 is met, before the start of a print job, a new color misregistration correction process is performed at the image formation speed for that print job, and the misregistration amount (color misregistration correction value) to be used for that print job is calculated. This makes it possible to sufficiently correct the color misregistration.

[0085] As a result, according to this embodiment, when the image formation speed of the current print job (during image formation) differs from the image formation speed at the time of calculating the amount of misalignment stored in the memory unit 253 (during color misalignment correction processing), it becomes possible to simultaneously suppress color misalignment and suppress delays in the start time of image formation.

[0086] [Modifications] Next, a modified example of the procedure for color misregistration correction processing before a print job in this embodiment will be described. In the procedure of the color misregistration correction process before a print job in this modified example, an arbitrary criterion for determining whether or not to perform a new color misregistration correction process before a print job is added to the procedure of the above-mentioned embodiment. Note that the explanation of the same steps as the procedure of the above-mentioned embodiment will be omitted as appropriate.

[0087] FIG. 11 is a flowchart showing an example of a procedure for color misregistration correction processing before a print job is performed in this modified example. In this modified example, when a print instruction (image formation execution instruction) is input to the overall control unit 252 (S21), before determining the image formation speed of the print job related to the print instruction (S22), first, it is determined whether the image formation of the print job can be executed at a single image formation speed (S31). For example, when only one type of recording sheet P is used in the print job, it is determined that the image formation can be executed at a single image formation speed.

[0088] If it is determined in step S31 that the printing job cannot be performed at a single image formation speed (No in S31), in this modified example, a new color shift correction process is not performed regardless of whether the image formation speed of the current print job (at the time of image formation) is different from the image formation speed at the time of calculation of the amount of misalignment stored in the memory unit 253 (at the time of color shift correction process). In this case, information indicating that the color shift correction process has been completed is sent from the correction processing unit 250 to the overall control unit 252, and the overall control unit 252 starts the print job (S24). As a result, the overall control unit 252 starts the image formation operation (S1) in accordance with the procedure shown in FIG. 9.

[0089] If it is determined that the printing is not possible at a single image formation speed, even if the color shift correction process is performed before the start of the print job, the amount of shift (color shift correction value) obtained by the process corresponds only to some of the image formation speeds in the print job, and does not correspond to other image formation speeds. In this way, allowing a delay in the image formation start time (first print time) of the print job in order to calculate the amount of shift that corresponds only to some of the image formation speeds may be inconvenient for the user.

[0090] According to this modified example, when a single image formation speed is not possible, the shortening of the user's waiting time is prioritized over correcting color misregistration at a portion of the image formation speed in the print job, and a delay in the start of image formation (first print time) for the print job is avoided, thereby improving user convenience.

[0091] In this modification, before determining the image formation speed of the print job related to the print instruction (S22), it is determined whether the image formation of the print job is color printing (S32). If it is determined in step S32 that the image formation is not color printing (No in S32), in this modification, regardless of whether the image formation speed of the current print job (at the time of image formation) is different from the image formation speed at the time of calculation of the amount of deviation stored in the storage unit 253 (at the time of color deviation correction processing), new color deviation correction processing is not executed. In this case, too, information indicating that the color deviation correction processing has been completed is sent from the correction processing unit 250 to the overall control unit 252, and the overall control unit 252 starts the print job (S24). As a result, the overall control unit 252 starts the image formation operation (S1) according to the procedure shown in FIG. 9.

[0092] In the case of non-color printing (for example, in the case of monochrome printing), since a single-color image is formed using a single photoconductor, color shift does not occur in the first place. Therefore, in the case of non-color printing, it is decided early on not to execute a new color shift correction process, and a delay in the start time of image formation for the print job (first print time) is avoided, thereby improving user convenience.

[0093] In this modified example, after the image formation speed of the print job is determined (S22), and before judging whether the image formation speed of the current print job is the same as the image formation speed at the time of calculating the amount of misalignment stored in the storage unit 253 (at the time of color misalignment correction processing) (S23), it is judged whether the image formation speed of the print job has a usage frequency equal to or higher than a predetermined value (S33). This usage frequency is, for example, the usage frequency (%) of the image formation speed in the current print instruction calculated from the image formation speeds in the most recent 10 print instructions (this number is arbitrary). Such information on the usage frequency is stored in the storage unit 253, and the judgment in step S33 is made by referring to this information.

[0094] In the case where it is determined in step S33 that the frequency of use of the image formation speed of the current print job is less than the predetermined value (No in S33), in this modified example, regardless of whether the image formation speed of the current print job (at the time of image formation) is different from the image formation speed at the time of calculation of the amount of misalignment stored in the memory unit 253 (at the time of color misalignment correction processing), a new color misalignment correction process is not executed. In this case, information that the color misalignment correction process has been completed is sent from the correction processing unit 250 to the overall control unit 252, and the overall control unit 252 starts the print job (S24). As a result, the overall control unit 252 starts the image formation operation (S1) in accordance with the procedure shown in FIG. 9.

[0095] If the image formation speed of the current print job is used infrequently, the current print job is an unusual or temporary print job that is not a normal print job that the user frequently performs. In such a print job, performing color shift correction processing before the start of the print job may cause a delay in the image formation start time (first print time) of the subsequent normal print job, which may impair user convenience. In particular, if color shift correction processing is performed before the start of an unusual or temporary print job, it may be necessary to perform color shift correction processing again before the start of the subsequent normal print job, which may significantly impair user convenience.

[0096] According to this modified example, when the image formation speed of the current print job is used infrequently, the shortening of the user's waiting time is prioritized over correcting color misregistration, and a delay in the start of image formation (first print time) for the print job is avoided, thereby improving user convenience.

[0097] In this embodiment, an intermediate transfer type image forming apparatus using an intermediate transfer belt as an intermediate transfer body has been described as an example, but instead of using an intermediate transfer belt, a direct transfer type image forming apparatus using a paper transport belt that holds and transports a recording sheet on its surface that moves endlessly may be used. In this image forming apparatus, Y, M, C, and K toner images formed on the surfaces of photoconductors for Y, M, C, and K are directly superimposed and transferred onto a recording sheet on the paper transport belt to obtain a full-color image. In addition, in this image forming apparatus, a test pattern image is formed on the paper transport belt, and the test pattern image on the paper transport belt is detected by a detection sensor.

[0098] The above description is merely an example, and each of the following aspects provides unique effects. [First aspect] The first aspect is an image forming apparatus having an image forming unit that performs image formation by writing a latent image on each of a plurality of latent image carriers (e.g., photoconductors 1Y, 1M, 1C, and 1K) by a latent image writing unit (e.g., optical writing device 7) according to predetermined image forming conditions based on an input image formation execution instruction (e.g., a print instruction), and developing the latent images on the plurality of latent image carriers by a plurality of developing units (e.g., developers 5Y, 5M, 5C, and 5K) to form visible images that are superimposed and transferred onto an intermediate transfer body or a recording material, and a correction processing unit 250 that creates a test pattern image 42 for detecting color shift at a predetermined image formation speed and performs color shift correction processing to correct the image forming conditions using color shift correction values ​​(e.g., various amounts of shift) obtained from the detection results of the test pattern image, and the color shift correction The correction processing unit has a memory unit 253 that stores the image formation speed and temperature specification information during processing (e.g., temperature inside the device, time, etc.), and when the image formation speed during image formation executed by an input image formation execution instruction is different from the image formation speed during the color shift correction processing stored in the memory unit, the correction processing unit compares the temperature specification information during the image formation with the temperature specification information during the color shift correction processing stored in the memory unit, and when the comparison result satisfies the condition of exceeding a predetermined temperature difference, performs the color shift correction processing at the image formation speed during the image formation before execution of image formation by the image formation execution instruction, and when the comparison result does not satisfy the condition, does not perform the color shift correction processing before execution of image formation by the image formation execution instruction. Conventionally, there is an image forming apparatus capable of performing image formation in a plurality of image forming modes with different image forming speeds. In such an image forming apparatus, in order to reliably suppress color misregistration in any image forming mode (image forming speed), it is desirable to perform a new color misregistration correction process by forming a test pattern image at the different image forming speed each time before performing image formation at an image forming speed different from the image forming speed during the color misregistration correction process (when forming a test pattern image). However, if a new color misregistration correction process is performed each time before performing image formation at a different image forming speed, a problem occurs in that the image formation start time (first print time) when performing image formation at the different image forming speed is delayed each time. Here, if the environmental conditions (particularly the temperature conditions) during image formation have not changed much since the color shift correction process, it is possible to sufficiently suppress color shift by using the color shift correction value from the color shift correction process that has already been performed, without having to perform a new color shift correction process. This is also true when image formation is performed at an image formation speed different from the image formation speed during the color shift correction process. That is, even in this case, it is possible to sufficiently suppress color shift by correcting the color shift correction value from the already performed color shift correction process with a correction value corresponding to the difference in image formation speed from the color shift correction process, and using the corrected correction value. On the other hand, if the environmental conditions (especially the temperature conditions) during image formation have changed to a certain extent since the color shift correction processing, and image formation is performed at an image formation speed different from the image formation speed during the color shift correction processing, the modified correction value described above cannot sufficiently suppress color shift. In this embodiment, based on the above findings, when the image formation speed during image formation is different from the image formation speed during color shift correction processing, whether or not to execute color shift correction processing is determined depending on whether or not the temperature condition during image formation has changed by a certain amount or more since the color shift correction processing. Specifically, the temperature specification information during image formation is compared with the temperature specification information during color shift correction processing, and when the comparison result satisfies the condition of exceeding a predetermined temperature difference, there is a risk that color shift cannot be sufficiently suppressed even if the color shift correction value by the color shift correction processing already executed is used, so a new color shift correction processing is performed at the image formation speed during the image formation before the execution of image formation. This makes it possible to sufficiently suppress color shift. On the other hand, when the comparison result does not satisfy the above condition, color shift can be sufficiently suppressed by using the color shift correction value by the color shift correction processing already executed, so the color shift correction processing is not performed before the execution of image formation. This makes it possible to avoid a delay in the start time of image formation (first print time) caused by performing color shift correction processing before the execution of image formation. As a result, according to this aspect, in an image forming apparatus capable of performing image formation in multiple image formation modes having mutually different image formation speeds, it becomes possible to simultaneously suppress color misregistration and suppress delays in the start time of image formation.

[0099] [Second aspect] The second aspect is characterized in that, in the first aspect, the memory unit stores the time of the color shift correction processing as the temperature identification information, and the correction processing unit uses the time of the image formation as the temperature identification information at the time of the image formation, and when the time from the time of the color shift correction processing to the time of the image formation exceeds a predetermined time, the color shift correction processing is performed as the condition is satisfied, and when the time is equal to or less than the predetermined time, the color shift correction processing is not performed as the condition is not satisfied. According to this, even if the temperature inside the device, which is the main cause of color shift, is not directly detected and the temperature change is understood, the temperature change can be understood from the passage of time, and simple processing can be realized.

[0100] [Third aspect] A third aspect is characterized in that, in the first or second aspect, when there are multiple image formation speeds during image formation performed in response to an input image formation execution instruction, the correction processing unit does not perform the color shift correction processing before image formation is performed in response to the image formation execution instruction, regardless of the comparison result. According to this embodiment, when there are multiple image formation speeds during image formation, it is possible to improve user convenience by prioritizing shortening the user's waiting time over correcting color misregistration at some of the image formation speeds, thereby avoiding delays in the start of image formation (first print time).

[0101] [Fourth aspect] A fourth aspect is characterized in that, in any of the first to third aspects, when the image formation executed in response to an input image formation execution instruction is a monochrome image formation using one latent image carrier among the multiple latent image carriers, the correction processing unit does not perform the color shift correction processing before the image formation is executed in response to the image formation execution instruction, regardless of the comparison result. According to this embodiment, when forming a monochrome image, color shift does not occur in the first place, so it is possible to decide early on not to execute a new color shift correction process, thereby avoiding delays in the start of image formation (first print time) and improving user convenience.

[0102] [Fifth aspect] A fifth aspect is characterized in that, in any of the first to fourth aspects, when the image formation speed of the image formation performed in accordance with the input image formation execution instruction has a frequency of use equal to or lower than a predetermined value, the correction processing unit does not perform the color shift correction processing before the image formation is performed in accordance with the image formation execution instruction, regardless of the comparison result. According to this embodiment, when the image formation speed of the image formation performed in response to the input image formation execution instruction is used infrequently, it is possible to improve user convenience by prioritizing shortening the user's waiting time over correcting color misregistration and avoiding delays in the start time of image formation (first print time).

[0103] [Sixth aspect] A sixth aspect is characterized in that, in any of the first to fifth aspects, the image forming unit has a temperature detection unit (e.g., temperature sensor 254) that detects the temperature inside the device, and when the detected temperature of the temperature detection unit exceeds a specified temperature (e.g., an usable temperature threshold), the image forming unit performs the image formation at an image formation speed that is slower than when the detected temperature of the temperature detection unit does not exceed the specified temperature. According to this, it is possible to suppress a rise in temperature of the latent image writing unit, which causes the color shift to worsen, and to reduce the amount of color shift. [Explanation of symbols]

[0104] 1: Photoreceptor 4: Charging device 5: Developer 6: Imaging unit 7: Optical writing device 8: Intermediate transfer belt 9: Primary transfer bias roller 10: Cleaning device 15: Transcription unit 20: Fixing device 42, 42a, 42b, 42c: Test pattern images 150, 150a, 150b, 150c: detection sensor 151a: Light emitting part 152a: Light receiving part 250: Correction processing section 250a: Test pattern forming unit 250b: Calculation section 250c: Adjustment section 250e: Adjustment execution timing control unit 251: Test pattern detection unit 252: Overall control unit 253: Storage section 254: Temperature sensor P: Recording sheet [Prior art documents] [Patent documents]

[0105] [Patent Document 1] Patent No. 6376445

Claims

1. an image forming section which executes image formation by writing a latent image on each of a plurality of latent image carriers by a latent image writing section in accordance with a predetermined image forming condition based on an input image formation execution instruction, and developing the latent images on the plurality of latent image carriers by a plurality of developing sections to form visible images which are superimposed and transferred onto an intermediate transfer body or a recording material; a correction processing unit that performs a color shift correction process to correct an image forming condition by forming a test pattern image for detecting color shift at a predetermined image forming speed and using a color shift correction value obtained from a detection result of the test pattern image, a storage unit for storing image forming speed and temperature specifying information at the time of the color shift correction processing; The image forming apparatus is characterized in that, when the image formation speed during image formation executed by an input image formation execution instruction is different from the image formation speed during the color shift correction processing stored in the memory unit, the correction processing unit compares the temperature specific information during the image formation with the temperature specific information during the color shift correction processing stored in the memory unit, and when the comparison result satisfies the condition of exceeding a predetermined temperature difference, the color shift correction processing is performed at the image formation speed during the image formation before the image formation is executed by the image formation execution instruction, and when the comparison result does not satisfy the condition, the color shift correction processing is not performed before the image formation is executed by the image formation execution instruction.

2. 2. The image forming apparatus according to claim 1, the storage unit stores a time when the color shift correction process is performed as the temperature specifying information; The image forming apparatus is characterized in that the correction processing unit uses the time of image formation as temperature-specific information at the time of image formation, and when the time from the time of the color shift correction processing to the time of image formation exceeds a predetermined time, it performs the color shift correction processing as it considers the condition to be satisfied, and when the time is equal to or less than the predetermined time, it does not perform the color shift correction processing as it considers the condition to be satisfied.

3. 3. The image forming apparatus according to claim 1, The image forming apparatus is characterized in that, when there are multiple image formation speeds during image formation performed in response to an input image formation execution instruction, the correction processing unit does not perform the color shift correction processing before image formation is performed in response to the image formation execution instruction, regardless of the comparison result.

4. 3. The image forming apparatus according to claim 1, an image forming device characterized in that, when the image formation executed in response to an input image formation execution instruction is a monochrome image formation using one latent image carrier among the plurality of latent image carriers, the correction processing unit does not perform the color shift correction processing before the image formation is executed in response to the image formation execution instruction, regardless of the comparison result.

5. 3. The image forming apparatus according to claim 1, The image forming apparatus is characterized in that, when the image forming speed of the image formation performed in accordance with the input image formation execution instruction is a frequency of use equal to or lower than a predetermined value, the correction processing unit does not perform the color shift correction processing before the image formation is performed in accordance with the image formation execution instruction, regardless of the comparison result.

6. 3. The image forming apparatus according to claim 1, A temperature detection unit detects the temperature inside the device, An image forming apparatus characterized in that, when the detected temperature of the temperature detection unit exceeds a specified temperature, the image forming unit performs the image formation at an image formation speed slower than when the detected temperature of the temperature detection unit does not exceed the specified temperature.

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