Like forming device

Abstract

Even when the ratio of the surface movement speed of the transfer material to the surface movement speed of multiple image carriers is changed, the image positional shift (color shift) on the transfer material between each image of the multiple image carriers is suppressed. [Solution] An image forming apparatus for transferring images formed on a plurality of surface-moving image carriers 5, 6, 7, 8 onto a transfer material (e.g., an intermediate transfer belt 21) so that they overlap each other, comprising: a speed ratio changing means for changing the speed ratio of the surface movement speed of the transfer material to the surface movement speed of the plurality of image carriers from a reference speed ratio; and a transfer timing changing means (e.g., a control unit 100) for changing the start timing of the transfer of an image from at least one of the plurality of image carriers onto the transfer material from the reference speed ratio, so as to reduce the image positional shift on the transfer material between each image of the plurality of image carriers that occurs when the speed ratio is changed from the reference speed ratio.

Description

Technical Field

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

Background Art

[0002] Conventionally, there has been known an image forming apparatus that transfers each image formed on a plurality of image carriers moving on the surface so as to overlap each other on a transfer material.

[0003] For example, Patent Document 1 discloses an image forming apparatus that performs primary transfer of each image formed on four photoreceptors (image carriers) so as to overlap each other on an intermediate transfer member (transfer material), and then performs secondary transfer onto a sheet (recording material) to form an image. This image forming apparatus can form images at a plurality of image forming speeds by changing the rotation speeds of the four photoreceptors and the intermediate transfer member. When this image forming apparatus operates at the first image forming speed, image formation on each photoreceptor is performed based on a predetermined writing timing corresponding to this operation. On the other hand, when this image forming apparatus operates at the second image forming speed, the writing timing is corrected using the speed ratio of the first image forming speed to the second image forming speed. By performing image formation at this corrected writing timing, even when operating at the second image forming speed, image misregistration (color misregistration) on the intermediate transfer member or the paper between each image of the four photoreceptors is prevented.

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, conventionally, there has been a case where the speed ratio of the surface moving speed of the transfer material to the surface moving speed of a plurality of image carriers is changed, and in this case, there has been a problem that color misregistration (image misregistration on the transfer material between each image of a plurality of image carriers) that occurs cannot be suppressed.

Means for Solving the Problems

[0005] To solve this problem, the present invention provides an image forming apparatus for transferring images formed on a plurality of surface-moving image carriers onto a transfer material so that they overlap each other, comprising: a speed ratio changing means for changing the speed ratio of the surface movement speed of the transfer material to the surface movement speed of the plurality of image carriers from a reference speed ratio; and a transfer timing changing means for changing the start timing of the transfer of an image from at least one of the plurality of image carriers onto the transfer material from that at the reference speed ratio, so as to reduce the image positional shift on the transfer material between the images of the plurality of image carriers that occurs as a result of changing the speed ratio from the reference speed ratio. [Effects of the Invention]

[0006] According to the present invention, even if the speed ratio of the surface movement speed of the transfer material to the surface movement speed of multiple image carriers is changed, it is possible to suppress image positional shifts (color shifts) on the transfer material between each image of the multiple image carriers. [Brief explanation of the drawing]

[0007] [Figure 1] A schematic diagram showing the main components of the printer according to this embodiment. [Figure 2] This diagram illustrates how a change in the speed ratio between the surface movement speed of the photoreceptor and the surface movement speed of the intermediate transfer belt results in image positional shifts (color shifts) on the intermediate transfer belt between each image on the photoreceptor. [Figure 3] This diagram illustrates the image position of each main scanning line of the photoreceptor on the intermediate transfer belt before and after changing the speed ratio. [Figure 4] (a) is a timing chart showing the writing start timing of each photoreceptor before correction after changing the surface moving speed of the intermediate transfer belt (after changing the speed ratio). (b) is a timing chart showing the writing start timing of each photoreceptor after correction after changing the surface moving speed of the intermediate transfer belt (after changing the speed ratio). [Figure 5] A flowchart showing the process for correcting the timing of writing to the photoreceptor in the embodiment. [Figure 6] This diagram illustrates an example of a color matching correction pattern for each color formed on the intermediate transfer belt while the intermediate transfer belt is moved across the surface at a reference speed. [Modes for carrying out the invention]

[0008] The following describes one embodiment in which the present invention is applied to a printer as an image forming apparatus. The printer 50 according to this embodiment is a so-called tandem-type intermediate transfer image forming apparatus in which four photoreceptors, which are image carriers, are arranged along the surface movement direction of an intermediate transfer belt, which is an intermediate transfer body, as the transfer material. However, the present invention is not limited to this. For example, it may be a direct transfer image forming apparatus that forms an image by directly transferring an image formed on a plurality of photoreceptors (image carriers) onto a recording material (transfer material) such as recording paper so that the images overlap each other.

[0009] Figure 1 is a schematic diagram showing the main components of the printer 50 according to this embodiment. This printer 50 forms a page image by superimposing four monochrome images (monochrome toner images) of four colors (Y, M, C, K) formed on the surfaces of four photoreceptors 5, 6, 7, and 8 on the surface of an intermediate transfer belt 21. Electrostatic latent images are formed on the surfaces of the four photoreceptors 5, 6, 7, and 8 by optical writing units 1, 2, 3, and 4, which are latent image formation means. The formed electrostatic latent images are transported to a developing area opposite to developing devices 9, 10, 11, and 12, which are developing means, as the photoreceptors rotate in the direction of arrow B in the figure. Each electrostatic latent image on each photoreceptor 5, 6, 7, and 8 is supplied with toner of the respective color by the developing devices 9, 10, 11, and 12, and each electrostatic latent image is converted into a toner image.

[0010] The four photoreceptors 5, 6, 7, and 8 are arranged in a line along their surface movement direction (direction of arrow A in the figure) while in contact with the flat portion of the intermediate transfer belt 21, which is stretched across multiple support rollers. Primary transfer rollers 13, 14, 15, and 16 are located opposite each other on the back side of the intermediate transfer belt where each photoreceptor is in contact. Primary transfer high-voltage power supplies 17, 18, 19, and 20 are connected to each primary transfer roller 13, 14, 15, and 16 for primary transfer of the toner images on each photoreceptor 5, 6, 7, and 8 onto the surface of the intermediate transfer belt 21. The toner images of each color formed on the surface of each photoreceptor 5, 6, 7, and 8 are primary transferred onto the surface of the intermediate transfer belt 21 by each primary transfer roller 13, 14, 15, and 16 so that they overlap each other on the surface of the intermediate transfer belt 21.

[0011] The toner image, which has been primary transferred onto the surface of the intermediate transfer belt 21, is transported to the secondary transfer region as the intermediate transfer belt 21 moves along its surface. In the secondary transfer region, a secondary transfer opposing roller 22, which is a support roller, is provided on the back side of the intermediate transfer belt 21, and a secondary transfer roller 23, which is a transfer member, is provided on the front side of the intermediate transfer belt 21. The secondary transfer roller 23 is rotationally driven by a drive motor 23a.

[0012] The secondary transfer roller 23 is configured to be able to move toward and away from the surface of the intermediate transfer belt 21. During the image forming process, as shown in Figure 1, the intermediate transfer belt 21 and the secondary transfer roller 23 are in contact, and the paper 25 being transported to the secondary transfer area, as indicated by arrow C in the figure, passes through the secondary transfer area while being sandwiched between the intermediate transfer belt 21 and the secondary transfer roller 23. At this time, the toner image formed on the surface of the intermediate transfer belt 21 is secondary transferred onto the paper 25 being transported by the movement of the secondary transfer roller 23, to which a secondary transfer bias has been applied by the secondary transfer high-voltage power supply 24. The toner image transferred onto the paper 25 is fixed onto the paper in a fixing device, and then the paper 25 as recording material is discharged from the machine.

[0013] In this embodiment, for example, the drive roller 27 that drives the intermediate transfer belt 21 may change in diameter due to wear over time or environmental changes such as temperature. In such cases, the surface movement speed of the intermediate transfer belt 21 changes from its original surface movement speed, and the speed ratio of the surface movement speed of the intermediate transfer belt 21 to the surface movement speeds of the four photoreceptors 5, 6, 7, and 8 changes. As a result, in each primary transfer region between each photoreceptor 5, 6, 7, and 8 and the intermediate transfer belt 21, the speed difference between the surface movement speed of the photoreceptor and the surface movement speed of the intermediate transfer belt changes, causing stretching or shrinking (sub-scanning magnification error) in each image on the intermediate transfer belt 21.

[0014] Specifically, for example, as the roller diameter of the drive roller 27 increases with rising temperature, the surface movement speed of the intermediate transfer belt 21 increases. As a result, the surface movement speed of the intermediate transfer belt 21 becomes relatively faster than the movement speed of each image on the photoreceptors 5, 6, 7, and 8, and each image is formed on the intermediate transfer belt 21 in a compressed state in the sub-scanning direction.

[0015] Since the stretching and shrinking (sub-scanning magnification error) of each image on the intermediate transfer belt 21 is consistent across all images, this sub-scanning magnification error can be suppressed by changing the speed ratio of the surface movement speed of the intermediate transfer belt 21 to the surface movement speed of each photoreceptor 5, 6, 7, 8. This speed ratio can be changed, for example, by changing the rotation speed of the drive motor 27a that drives the drive roller 27 of the intermediate transfer belt 21, thereby changing the surface movement speed of the intermediate transfer belt 21.

[0016] Furthermore, for example, the diameter of the secondary transfer roller 23 may change due to wear over time or environmental changes such as temperature. In such cases, the transport speed of the paper 25 transported by the secondary transfer roller 23 (the surface movement speed of the paper 25) changes from the original transport speed, and the speed ratio of the surface movement speed of the paper 25 to the surface movement speed of the intermediate transfer belt 21 changes. As a result, in the secondary transfer region between the intermediate transfer belt 21 and the secondary transfer roller 23, the speed difference between the surface movement speed of the intermediate transfer belt and the surface movement speed of the paper changes, causing stretching or shrinking (sub-scanning magnification error) in the image on the paper 25.

[0017] Such stretching and shrinking of the image on the sheet 25 (sub-scanning magnification error) can be suppressed, for example, by changing the rotational speed of the drive motor 27a that drives the drive roller 27 of the intermediate transfer belt 21 to change the surface movement speed of the intermediate transfer belt 21. That is, by changing the surface movement speed of the intermediate transfer belt 21, the speed ratio of the surface movement speed of the intermediate transfer belt 21 to the surface movement speed of the sheet 25 changes, and the stretching and shrinking of the image on the sheet 25 (sub-scanning magnification error) is suppressed. However, in this case, the speed ratio of the surface movement speed of the intermediate transfer belt 21 to the surface movement speeds of the photoreceptors 5, 6, 7, 8 will be changed.

[0018] Here, as described above, when the speed ratio of the surface movement speed of the intermediate transfer belt 21 to the surface movement speeds of the photoreceptors 5, 6, 7, 8 is changed, image misalignment (color misalignment) on the intermediate transfer belt 21 between each image of the photoreceptors 5, 6, 7, 8 will occur. In the following description, it is assumed that the change in the speed ratio is performed by changing the surface movement speed of the intermediate transfer belt 21 while not changing the surface movement speeds of the photoreceptors 5, 6, 7, 8.

[0019] FIG. 2 is an explanatory diagram showing a state where the speed ratio of the surface movement speed of the intermediate transfer belt 21 to the surface movement speeds of the photoreceptors 5, 6, 7, 8 is changed, and image misalignment (color misalignment) on the intermediate transfer belt 21 between each image of the photoreceptors 5, 6, 7, 8 has occurred. FIG. 3 is an explanatory diagram showing the image positions of each main scanning 1 line of the photoreceptors 5, 6, 7, 8 on the intermediate transfer belt 21 before and after changing the speed ratio (the positions of each image formed so as to overlap each other. For example, the leading edge positions of each image).

[0020] Before the change in the surface movement speed of the intermediate transfer belt 21 (before the change in the speed ratio), that is, when the surface movement speed of the intermediate transfer belt 21 is the reference speed, no color shift occurs as shown in the intermediate transfer belt 21 illustrated on the lower side of FIG. 3. However, for example, when the surface movement speed of the intermediate transfer belt 21 is changed to be increased (when the speed ratio is changed to be increased), as shown in the intermediate transfer belt 21 illustrated on the upper side of FIG. 3, a color shift occurs. This is because the image leading edges Ty, Tm, Tc on the intermediate transfer belt 21 transferred from the upstream photoreceptors 5, 6, 7 reach the transfer positions Pm, Pc, Pk of the downstream photoreceptors 6, 7, 8 earlier than the image leading edges Tm, Tc, Tk on the photoreceptors 6, 7, 8.

[0021] Therefore, in the present embodiment, the transfer start timing of the image of at least one photoreceptor to the intermediate transfer belt 21 is changed (corrected) so that the amount of color shift caused by the change in the surface movement speed of the intermediate transfer belt 21 (the change in the speed ratio) is reduced. Specifically, for example, the writing start timing (image formation start timing) of the electrostatic latent image by the light writing units 1, 2, 3, 4 to each of the photoreceptors 5, 6, 7, 8 with respect to the printing start timing is (corrected).

[0022] FIGS. 4(a) and (b) are timing charts before and after correcting the writing start timing of each of the photoreceptors 5, 6, 7, 8 after the change in the surface movement speed of the intermediate transfer belt 21 (after the change in the speed ratio). Note that FIG. 4(a) is a timing chart before correcting the writing start timing, and FIG. 4(b) is a timing chart after correcting the writing start timing.

[0023] The timing chart shown in Figure 4(a) is for the case before the surface movement speed of the intermediate transfer belt 21 is changed (before the change in the speed ratio), that is, when the surface movement speed of the intermediate transfer belt 21 is the standard speed (when the speed ratio is the standard speed ratio). In other words, in this timing chart, the start timing of writing to each photoreceptor 5, 6, 7, and 8 is determined in relation to the start timing of printing so that color misalignment occurs when the surface movement speed of the intermediate transfer belt 21 is the standard speed.

[0024] In this case, if the surface movement speed of the intermediate transfer belt 21 is changed from the reference speed, and latent image writing (image formation) of each photoreceptor 5, 6, 7, 8 is started at the writing start timing according to the timing chart in Figure 4(a), then, as described above, color shift will occur. Therefore, in this embodiment, the writing start timing to the photoreceptors 5, 6, 7, 8 is changed (corrected) so as to reduce the image position shift (color shift) on the intermediate transfer belt 21 between each image of the photoreceptors 5, 6, 7, 8 caused by this change.

[0025] For example, if the surface movement speed of the intermediate transfer belt 21 is increased (the speed ratio is increased), the writing start timing to each photoreceptor 5, 6, 7, and 8 relative to the printing start timing is changed (corrected) to that of the timing chart in Figure 4(b). As a result, the transfer start timing of each image on the photoreceptors 5, 6, 7, and 8 to the intermediate transfer belt 21 is advanced, and the image positional shift (color shift) on the intermediate transfer belt 21 between each image on the photoreceptors 5, 6, 7, and 8 is reduced or eliminated.

[0026] In this embodiment, the start timing of writing to each photoreceptor 5, 6, 7, and 8 is determined based on the print start timing signal. Specifically, the start timing of writing to the K-colored photoreceptor 8 is determined based on the print start timing signal, and then the start timing of writing to the other photoreceptors 5, 6, and 7 is determined based on the determined start timing of writing to the K-colored photoreceptor 8. The start timing of writing to the other photoreceptors 5, 6, and 7 is determined from the inter-photoreceptor distance (distance between the primary transfer positions of both photoreceptors) Lky, Lkm, Lkc between the K-colored photoreceptor 8 and the other photoreceptors 5, 6, and 7, and the process speed (surface movement speed of the photoreceptors).

[0027] Furthermore, when the surface movement speed of the intermediate transfer belt 21 is changed from the reference speed, the writing start timing of each photoreceptor 5, 6, 7, and 8 (the writing start timing before correction) is multiplied by a correction coefficient corresponding to the changed speed to correct the writing start timing of each photoreceptor. That is, the writing start timing of each photoreceptor 5, 6, 7, and 8 is corrected according to the following equation (1). Corrected write start timing = Uncorrected write start timing × Correction coefficient ... (1)

[0028] The correction coefficient (adjustment parameter) can be determined, for example, from the ratio of the surface movement speed of the intermediate transfer belt 21 before the change (reference speed) to the surface movement speed after the change (changed speed), as shown in equation (2) below. Correction factor = 1 ÷ (Changed speed ÷ Base speed) ... (2)

[0029] Figure 5 is a flowchart showing the flow of the correction process for the writing start timing of each photoreceptor 5, 6, 7, and 8 in this embodiment. When an image forming instruction (print job) is input, the control unit 100 drives the photoreceptors 5, 6, 7, and 8 at a predetermined surface movement speed and controls the drive motor 27a so that the intermediate transfer belt 21 moves across the surface at a reference speed, thereby starting the image forming operation (S1). At this time, the start timing of writing for each photoreceptor 5, 6, 7, and 8 is determined using a reference write start timing value (timing determination parameter) stored in the memory unit of the control unit 100.

[0030] Initially, this reference write start timing value is calculated from an ideal design value to prevent color misalignment. However, over time, it becomes the value of the write start timing corrected by the most recent reference write start timing correction process described later.

[0031] When the conditions for executing the reference write start timing correction process are met, the control unit 100 executes the reference write start timing correction process while the intermediate transfer belt 21 is moved across the surface at a reference speed. In the reference write start timing correction process, first, a pattern image for correcting the reference write start timing (hereinafter referred to as the "color matching correction pattern") is formed on the intermediate transfer belt 21 (S2). Then, the color matching correction pattern formed on the intermediate transfer belt 21 is detected by the pattern detection sensor 26, which is a detection means (S3).

[0032] Figure 6 is an explanatory diagram showing an example of the color matching correction patterns TPy, TPm, TPc, and TPk for each color formed on the intermediate transfer belt 21 while the intermediate transfer belt 21 is moved across the surface at a reference speed. The reference K-color color matching correction pattern TPk is formed by writing at a predetermined reference writing start timing (fixed timing). On the other hand, the other color matching correction patterns TPy, TPm, and TPc are formed on the intermediate transfer belt 21 at predetermined intervals in the sub-scanning direction, according to the reference writing start timing value stored in the memory unit of the control unit 100.

[0033] The start timing of the transfer of the color matching correction patterns TPy, TPm, and TPc of the other colored photoreceptors 5, 6, and 7 to the intermediate transfer belt 21 (the sub-scanning direction position on the intermediate transfer belt 21) relative to the reference color matching correction pattern TPk of the K-colored photoreceptor 8 can change over time. For example, mounting position errors of each photoreceptor 5, 6, 7, and 8 (errors in the distance between photoreceptors), and errors due to changes in component dimensions caused by temperature changes or wear, can occur, which can change the transfer start timing. The reference writing start timing correction process described above is performed to correct such changes over time.

[0034] The control unit 100 corrects the reference writing start timing values ​​of the non-K color photoreceptors 5, 6, and 7 based on the detection results of the color matching correction patterns TPy, TPm, TPc, and TPk formed on the intermediate transfer belt 21 by the pattern detection sensor 26 (S4). Specifically, the control unit 100 calculates pattern correction values ​​(reference alignment parameters) that allow the interval between the color matching correction pattern TPk of the K color photoreceptor 8 and the color matching correction patterns TPy, TPm, and TPc of the photoreceptors 5, 6, and 7 to be set to a predetermined interval. Then, it adds the respective pattern correction values ​​to the reference writing start timing values ​​of the non-K color photoreceptors 5, 6, and 7 to correct the reference writing start timing values ​​of the non-K color photoreceptors 5, 6, and 7. As a result, even if color misalignment occurs due to time-dependent changes such as errors in the distance between photoreceptors or errors due to changes in component dimensions caused by temperature changes or wear, the execution of the reference writing start timing correction process suppresses the occurrence of color misalignment.

[0035] Here, when a predetermined intermediate transfer belt speed change condition is met, the control unit 100 changes the surface movement speed of the intermediate transfer belt 21 without changing the surface movement speed of the photoreceptors 5, 6, 7, 8 (without changing the process speed) (Yes in S5). This predetermined intermediate transfer belt speed change condition includes a condition for performing a process to correct a sub-scanning magnification error, in which the sub-scanning length of the formed image becomes longer or shorter than the original image. In this case, by changing the surface movement speed of the intermediate transfer belt 21 to a speed corresponding to the magnitude of the sub-scanning magnification error, the speed ratio of the surface movement speed of the intermediate transfer belt 21 to the surface movement speed of the photoreceptors 5, 6, 7, 8 or the paper 25 changes, and the stretching or shrinking of the image is corrected.

[0036] However, at this time, the speed ratio of the surface movement speed of the intermediate transfer belt 21 to the surface movement speed of the photoreceptors 5, 6, 7, and 8 is changed, so as described above, image position shift (color shift) occurs on the intermediate transfer belt 21 between each image of the photoreceptors 5, 6, 7, and 8. In order to suppress this color shift, in this embodiment, a correction coefficient is calculated to correct the writing start timing of each photoreceptor 5, 6, 7, and 8 according to the changed surface movement speed of the intermediate transfer belt 21, as described above (S6).

[0037] Subsequently, the control unit 100 corrects the reference writing start timing using the calculated correction coefficient (S7). Then, the control unit 100 drives the photoreceptors 5, 6, 7, and 8 at a predetermined surface movement speed and controls the drive motor 27a so that the intermediate transfer belt 21 moves across the surface at the changed speed, and starts the image forming operation (S8). At this time, the corrected writing start timing corrected in processing step S7 is used for the writing start timing of each photoreceptor 5, 6, 7, and 8.

[0038] In this embodiment, the corrected write start timing for the reference K color is as follows: (3-1) Corrected write start timing (K color) = K-color reference writing start timing × correction coefficient ... (3-1)

[0039] Furthermore, the timing for starting corrected writing in other colors is as follows: (3-2) to (3-4). Corrected write start timing (C color) = (Start timing of writing reference for color C + pattern correction value) × correction coefficient ... (3-2) Corrected write start timing (M color) = (M color reference writing start timing + pattern correction value) × correction coefficient ... (3-3) Corrected write start timing (Y color) = (Y-color reference writing start timing + pattern correction value) × correction coefficient ... (3-4)

[0040] In this embodiment, the corrected write start timing using the correction coefficient is calculated for all four photoreceptors 5, 6, 7, and 8. However, the corrected write start timing may be calculated only for the photoreceptor used for image formation. In this case, the processing load on the control unit 100 can be reduced.

[0041] The above is just one example; each of the following embodiments produces its own unique effects. [First aspect] The first embodiment is an image forming apparatus (e.g., printer 50) that transfers images formed on a plurality of surface-moving image carriers (e.g., photoreceptors 5, 6, 7, 8) onto a transfer material (e.g., intermediate transfer belt 21) so that they overlap each other, and is characterized by comprising: a speed ratio changing means (e.g., control unit 100, drive motor 27a) that changes the speed ratio of the surface movement speed of the transfer material to the surface movement speed of the plurality of image carriers from a reference speed ratio; and a transfer timing changing means (e.g., control unit 100, optical writing units 1, 2, 3, 4) that changes the start timing of the transfer of an image of at least one of the plurality of image carriers (e.g., photoreceptors 5, 6, 7, 8) onto the transfer material from that of the reference speed ratio, so as to reduce the image position shift (color shift) on the transfer material between the images of the plurality of image carriers that occurs when the speed ratio is changed from the reference speed ratio. Conventionally, the speed ratio of the surface movement speed of the material to be transferred to the surface movement speed of multiple image carriers may be changed from the reference speed ratio (speed ratio before change). For example, in an image forming apparatus that transfers images formed on multiple moving image carriers onto a material to overlap each other, various factors may cause images to be formed that are stretched or compressed in the direction of surface movement of the material (sub-scanning direction) compared to the original image. Such stretching or compression of images is called sub-scanning magnification error. When such sub-scanning magnification error occurs, it is possible to cancel out the sub-scanning magnification error by changing the speed ratio of the surface movement speed of the material to be transferred to the material from the reference speed ratio, thereby stretching or compressing the image during image transfer from each image carrier to the material. However, if the speed ratio of the surface movement speed of the material to be transferred to the surface movement speed of multiple image carriers is changed from the reference speed ratio, image positional shifts (color shifts) occur on the material to be transferred between the images of the multiple image carriers. For example, if the speed ratio is changed to be greater than the reference speed ratio (speed ratio before change), the leading edge of the image transferred from the image carrier upstream in the surface movement direction of the material to be transferred will reach the transfer position of the image carrier downstream in the surface movement direction of the material to be transferred before the leading edge of the image on the image carrier downstream in the surface movement direction of the material to be transferred. As a result, image positional shifts (color shifts) occur on the material to be transferred between the images of the multiple image carriers. Therefore, in this embodiment, the timing of the start of image transfer of at least one of the multiple image carriers to the transfer material is changed from that of the reference speed ratio to reduce the image positional shift on the transfer material between each image of the multiple image carriers caused by such a change in the speed ratio. As a result, even if the speed ratio of the surface movement speed of the transfer material to the surface movement speed of the multiple image carriers is changed from the reference speed ratio, the image positional shift (color shift) on the transfer material between each image of the multiple image carriers can be suppressed.

[0042] [Second aspect] The second aspect is characterized in that, in the first aspect, the speed ratio changing means changes the speed ratio so as to reduce the amount of stretching or shrinking (sub-scanning magnification error) in the surface movement direction of the image transferred onto the transfer material. According to this embodiment, even if the speed ratio is changed in order to reduce the amount of stretching or shrinking (sub-scanning magnification error) in the surface movement direction of the image transferred onto the transfer material, the transfer start timing is changed by the transfer timing changing means, and color shift is suppressed.

[0043] [Third aspect] A third embodiment is characterized in that, in the first or second embodiment, the transfer timing changing means changes the timing of the start of image transfer to the transfer material by changing the timing of the start of image formation on the at least one image carrier (e.g., the start of writing timing). According to this, it is possible to change the transcription start timing with a simple configuration.

[0044] [Fourth aspect] The fourth embodiment is characterized in that, in any of the first to third embodiments, the transfer timing changing means determines the start timing of the transfer of the image of at least one image carrier to the transfer material by using a reference alignment parameter (e.g., a reference writing start timing) for aligning the image positions on the transfer material between each image of the plurality of image carriers when the speed ratio is a reference speed ratio, and when the speed ratio changing means changes the speed ratio from the reference speed ratio, it determines the start timing of the transfer of the image of at least one image carrier to the transfer material by using an adjustment parameter (e.g., a correction coefficient) corresponding to the surface movement speed of the transfer material after the change. According to this, the timing of transcription initiation can be changed through simple control.

[0045] [Fifth aspect] The fifth aspect is characterized in that, in the fourth aspect, the reference alignment parameter is an added value (e.g., a pattern correction value) that is added to a timing determination parameter (a reference writing start timing before correction) that determines the timing of the start of the transfer of the image of the at least one image carrier to the transfer material, and the adjustment parameter is a correction coefficient that is multiplied by the reference alignment parameter. According to this, the timing of transcription initiation can be changed with simpler control.

[0046] [Sixth aspect] The sixth aspect is characterized in that, in the fourth or fifth aspect, when the speed ratio changing means changes the speed ratio from the reference speed ratio, the transfer timing changing means determines the transfer start timing for the image carrier used for image formation among the at least one image carrier, but does not determine the transfer start timing for the image carrier not used for image formation. According to this, by not determining the transfer start timing for unnecessary image carriers, the processing load on the transfer timing changing means can be reduced.

[0047] [Seventh aspect] The seventh embodiment is characterized in that, in any of the first to sixth embodiments, the at least one image carrier is all of the plurality of image carriers except for one reference image carrier (e.g., a K-colored photoreceptor 8) (e.g., photoreceptors other than K-colored 5, 6, 7). According to this, the timing of transcription initiation can be changed with simpler control. [Explanation of symbols]

[0048] 1-4: Optical writing unit 5~8: Photoreceptor 9-12: Developing equipment 13-16: Primary transfer roller 17-20: Primary transfer high-voltage power supply 21: Intermediate transfer belt 22: Secondary transfer opposing roller 23: Secondary transfer roller 23a: Drive motor 24: Secondary transfer high-voltage power supply 25: Paper 26: Pattern detection sensor 27: Drive roller 27a: Drive motor 50: Printer 100: Control Unit TPy, TPm, TPc, TPk: Color matching correction patterns [Prior art documents] [Patent Documents]

[0049] [Patent Document 1] Japanese Patent Publication No. 2017-203938

Claims

1. An image forming apparatus that transfers images formed on multiple surface-moving image carriers onto a transfer material so that they overlap each other, A speed ratio changing means for changing the speed ratio of the surface movement speed of the material to be transferred to the surface movement speed of the plurality of image carriers from a reference speed ratio, An image forming apparatus comprising a transfer timing changing means for changing the transfer start timing of an image from at least one of the plurality of image carriers to the transfer material, from that of the reference speed ratio, so as to reduce the image positional shift on the transfer material between each image of the plurality of image carriers that occurs when the speed ratio is changed from the reference speed ratio.

2. In the image forming apparatus according to claim 1, The image forming apparatus is characterized in that the speed ratio changing means changes the speed ratio so that the amount of expansion and contraction in the surface movement direction of the image transferred onto the transfer material is reduced.

3. In the image forming apparatus according to claim 1 or 2, The image forming apparatus is characterized in that the transfer timing changing means changes the timing of the start of image formation on the at least one image carrier, thereby changing the timing of the start of the transfer of the image to the transfer material.

4. In the image forming apparatus according to any one of claims 1 to 3, The transfer timing changing means determines the start timing of the transfer of the image of at least one image carrier to the transfer material by using a reference alignment parameter to match the image positions on the transfer material between each image of the plurality of image carriers when the speed ratio is the reference speed ratio, and when the speed ratio changing means changes the speed ratio from the reference speed ratio, it determines the start timing of the transfer of the image of at least one image carrier to the transfer material by using an adjustment parameter corresponding to the surface moving speed of the transfer material after the change.

5. In the image forming apparatus according to claim 4, The aforementioned reference alignment parameter is an added value that is added to a timing determination parameter that determines the timing of the start of transferring the image of the at least one image carrier to the transfer material. The image forming apparatus is characterized in that the adjustment parameter is a correction coefficient multiplied by the reference alignment parameter.

6. In the image forming apparatus according to claim 4 or 5, The image forming apparatus is characterized in that, when the speed ratio changing means changes the speed ratio from the reference speed ratio, the transfer timing changing means determines the transfer start timing for the image carrier used for image formation among the at least one image carrier, but does not determine the transfer start timing for the image carrier not used for image formation.

7. In the image forming apparatus according to any one of claims 1 to 6, The image forming apparatus is characterized in that the at least one image carrier is all of the plurality of image carriers except for one reference image carrier.

Images