Image forming device

Abstract

To reduce color shift even when different forces act on a secondary transfer unit through recording media having the same media information.SOLUTION: In a job of continuously feeding recording media from a cassette and forming images, an optical scanner starts writing a second electrostatic latent image after the lapse of a second time obtained by adding a first setting time set according to designation information to a first time from the start of writing a first electrostatic latent image, in order to align the positions in a sub scanning direction of a first toner image and a second toner image that are primarily transferred to an intermediate transfer body to be secondarily transferred to the second and subsequent recording media, and in a job of continuously feeding recording media from a manual tray and forming images, starts writing the second electrostatic latent image after the lapse of a third time obtained by adding a second setting time different from the first setting time set according to the designation information to the first time from the start of writing the first electrostatic latent image, in order to align the positions in the sub scanning direction of the first toner image and the second toner image that are primarily transferred to the intermediate transfer body to be secondarily transferred to the second and subsequent recording media.SELECTED DRAWING: Figure 15

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus having an intermediate transfer member. [Background technology]

[0002] Some color image forming devices that form color images on recording media include multiple image forming units and an intermediate transfer belt. Each of the multiple image forming units includes a photosensitive member and forms a toner image on the photosensitive member. The toner images formed on each photosensitive member by the multiple image forming units are sequentially superimposed and transferred to the intermediate transfer belt at a primary transfer unit formed between each photosensitive member and the intermediate transfer belt. The toner images formed on the intermediate transfer belt are then transferred to the recording media at a nip, which is a secondary transfer unit formed between the intermediate transfer belt and a secondary transfer roller.

[0003] Here, the image forming unit upstream in the direction of movement of the surface of the intermediate transfer belt is referred to as the first image forming unit, and the image forming unit downstream is referred to as the second image forming unit. The toner images formed on the photosensitive elements of each image forming unit are transferred to the intermediate transfer belt at primary transfer units formed between the respective photosensitive elements and the intermediate transfer belt. The timing at which the second image forming unit starts forming an image on the same recording medium relative to the timing at which the first image forming unit forms an image on one recording medium is delayed based on the surface speed of the intermediate transfer belt, the distance between the primary transfer units of the respective photosensitive elements, and the detected amount of color misregistration.

[0004] However, when a recording medium enters the nip portion, which is the secondary transfer portion, the force acting on the secondary transfer portion through the recording medium can cause fluctuations in the surface speed of the intermediate transfer belt. Fluctuations in the surface speed of the intermediate transfer belt can prevent accurate overlay of multiple color toner images on the intermediate transfer belt for images transferred to a subsequent recording medium relative to the recording medium entering the secondary transfer portion, resulting in color misregistration. Color misregistration is particularly severe with recording media with a high basis weight, such as cardboard, so measures to prevent color misregistration when cardboard is passed through are needed.

[0005] In Patent Document 1, measures are taken to avoid the transport influence of thick paper by leaving sufficient paper gaps between successive sheets. Meanwhile, in Patent Document 2, measures are taken to change the timing of exposure writing in the sub-scanning direction, which is the same direction as the movement direction of the surface of the intermediate transfer belt, depending on the detected thickness of the recording medium. Furthermore, in Patent Document 3, measures are taken to change the timing of exposure writing only when a preceding recording medium is detected during successive sheet passing. In other words, in Patent Document 3, the timing of exposure writing is not changed when the paper gap is greater than a certain amount. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-32743 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-42743 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-80399 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when feeding sheets continuously with sufficient spacing, as in Patent Document 1, the number of recording media that can be output within a given time period decreases. Furthermore, the force acting on the secondary transfer unit via the recording medium, which causes color shift when feeding thick paper, differs between recording media fed along a transport path from a cassette to the secondary transfer unit and recording media fed along a transport path from a manual feed tray to the secondary transfer unit. For example, even for recording media of the same thickness, the force acting on the secondary transfer unit via the recording medium may be smaller when fed from a manual feed tray than when fed from a cassette. In this case, the degree of color shift differs even when feeding recording media of the same thickness continuously. Therefore, even with a configuration that changes the timing of the exposure start depending on the thickness of the recording medium, as in Patent Documents 2 and 3, there is a risk of over-correcting color shift when correction of the timing is not necessary. Or, there is a risk of insufficient correction of color shift when correction of the timing is necessary.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to reduce color misregistration when recording media are continuously fed, even when different forces act on a secondary transfer portion via recording media of the same thickness. [Means for solving the problem]

[0009] A representative configuration of the present invention for achieving the above object is an image forming apparatus, comprising: a first photoconductor on which a first toner image is formed; a second photoconductor on which a second toner image of a color different from that of the first toner image formed on the first photoconductor is formed; an optical scanning device that forms a first electrostatic latent image on the first photoconductor and a second electrostatic latent image on the second photoconductor; a developing device that develops the first electrostatic latent image and the second electrostatic latent image with toner of the respective colors to form the first toner image and the second toner image; an intermediate transfer body onto which the first toner image and the second toner image are sequentially transferred so as to be superimposed on each other; a secondary transfer body which forms a transfer nip together with the intermediate transfer body and which secondarily transfers the first toner image and the second toner image superimposed on the intermediate transfer body to a recording medium at the transfer nip; a first feeding unit which is arranged vertically below the optical scanning device and which feeds a recording medium from a cassette capable of storing the recording medium toward the transfer nip; a second feeding unit which is arranged on the side of the image forming apparatus and which feeds a recording medium from a manual feed tray into which the recording medium can be manually inserted toward the transfer nip; Alternatively, the optical scanning device may include a storage unit that stores first designation information that designates the manual feed tray, second designation information that designates media information of the recording medium, and third designation information that designates image formation on one side or both sides of the recording medium, and the optical scanning device may, in the case of a job in which recording media are continuously fed and an image is formed on one side of the recording medium, perform the second designation information after a first time has elapsed since writing of the first electrostatic latent image has started in order to align positions in the sub-scanning direction of the first toner image and the second toner image that are primarily transferred to the intermediate transfer body for secondary transfer onto a first sheet of recording medium. and in the case of a job for forming an image on one side of a recording medium, in order to align the positions in the sub-scanning direction of the first toner image and the second toner image to be primarily transferred to the intermediate transfer body for secondary transfer onto a second or subsequent recording medium, the writing of the second electrostatic latent image is started after a second time has elapsed which is the first time from the start of writing of the first electrostatic latent image plus a first set time set in accordance with the first, second, and third designation information, and the recording medium is continuously fed from the manual feed tray.In the case of a job for forming an image on one side of a recording medium, in order to align the positions in the sub-scanning direction of the first toner image and the second toner image which are primarily transferred to the intermediate transfer body for secondary transfer to a second or subsequent recording medium, writing of the second electrostatic latent image is started after a third time has elapsed which is the first time from the start of writing of the first electrostatic latent image plus a second set time which is different from the first set time set in accordance with the first, second and third designation information. [Effects of the Invention]

[0010] According to the present invention, when recording media are continuously fed, color misregistration can be reduced even when different forces act on the secondary transfer unit via recording media of the same media information. [Brief explanation of the drawings]

[0011] [Figure 1] Schematic cross-sectional view of an image forming apparatus [Figure 2] FIG. 10 is an explanatory diagram illustrating a sheet transport path from a feeding cassette; [Figure 3] FIG. 10 is an explanatory diagram showing a sheet transport path from a feed tray; [Figure 4] FIG. 10 is an explanatory diagram illustrating a sheet transport path that uses a re-transport path; [Figure 5] (a)(b) Schematic diagram of color shift [Figure 6] (a)(b) Graph showing the amount of color misregistration caused by sheets (plain paper, thick paper) [Figure 7] Diagram showing the driving torque fluctuations while sheets (plain paper and thick paper) pass through the secondary transfer unit [Figure 8] Schematic cross-sectional view of a conveying path from a pair of registration rollers to a fixing unit [Figure 9] FIG. 10 is a diagram showing the relationship between the driving torque fluctuation and the pair of conveying rollers sandwiching the sheet. [Figure 10] (a) and (b) are diagrams showing the amount of color misregistration caused by the sheet. [Figure 11] Timing chart for primary and secondary transfer of each color when single-sided continuous paper is fed [Figure 12] (a) and (b) are diagrams showing the amount of color misregistration caused by the sheet. [Figure 13] FIG. 10 is a diagram showing the relationship between media information, feeding units, and correction amounts in single-sided continuous paper feeding. [Figure 14] (a) and (b) are diagrams showing the amount of color misregistration caused by the sheet. [Figure 15] 1 is a flowchart showing a procedure for correcting exposure write timing in the first embodiment. [Figure 16] FIG. 10 is a diagram showing the relationship between media information, transport path, and correction amount in double-sided continuous paper feed. [Figure 17] Timing chart for primary and secondary transfer of each color when double-sided continuous paper is fed [Figure 18] 10 is a flowchart showing a procedure for correcting exposure write timing in the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail by way of example with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the following embodiments may be changed as appropriate depending on the configuration of the device to which the present invention is applied and various conditions, and are not intended to limit the scope of the present invention to those alone.

[0013] Example 1 An image forming apparatus according to this embodiment will be described below with reference to the drawings. As an example of the image forming apparatus, a four-drum full-color electrophotographic image forming apparatus (hereinafter referred to as the image forming apparatus) 201 having an intermediate transfer belt 216 is shown. FIG. 1 is a cross-sectional view of the image forming apparatus 201.

[0014] [Image forming equipment] The image forming apparatus 201 employs a tandem system in which a laser scanner 210, which is an optical scanning device, and multiple photosensitive drums 212a-212d are arranged along an intermediate transfer belt 216, and the image forming processes for each color are performed in parallel. The image forming apparatus 201 employs an intermediate transfer system in which a toner image is transferred onto a recording medium (hereinafter referred to as a sheet) P by passing the sheet P through a secondary transfer unit 218, which is a transfer nip formed between a secondary transfer roller 217 to which a transfer voltage is applied and the intermediate transfer belt 216. The image forming apparatus 201 can continuously transport multiple sheets P in one job and perform continuous image formation (continuous printing) in which images are continuously formed on the multiple sheets P.

[0015] In the following description, one side of the rotation axis direction of the photosensitive drum 212 disposed in the image forming apparatus 201 shown in FIG. 1 is defined as the front side (near side or front side) of the image forming apparatus 201, and the other side is defined as the rear side (rear side or back side) of the image forming apparatus 201. Furthermore, when the photosensitive drum 212d on which the electrostatic latent image for the black toner image is formed is used as a reference, the side on which the photosensitive drum 212a on which the electrostatic latent image for the yellow toner image is formed is defined as the left side. When the photosensitive drum 212a on which the electrostatic latent image for the yellow toner image is formed is used as a reference, the side on which the photosensitive drum 212d on which the electrostatic latent image for the black toner image is formed is defined as the right side. Furthermore, the direction perpendicular to the front-rear and left-right directions defined here and pointing vertically upward is defined as the up direction, and the direction perpendicular to the front-rear and left-right directions defined here and pointing vertically downward is defined as the down direction. The right direction, left direction, up direction, and down direction, excluding the defined front and rear directions, are shown in FIG. 1.

[0016] In the following description, the rotation axis direction of the photosensitive drum 212 is the same as the main scanning direction of the optical scanning device (laser scanner 210), and the rotation direction of the intermediate transfer belt 216 is the same as the sub-scanning direction perpendicular to the main scanning direction of the optical scanning device (laser scanner 210).

[0017] 1, the image forming apparatus 201 has an image forming apparatus main body 201A equipped with an image forming unit 201B that forms an image on a sheet P, and an image reading device 202 that is installed substantially horizontally above the image forming apparatus main body 201A and reads an image of a document. The image forming apparatus 201 has a discharge space D for discharging a recording material formed between the image reading device 202 and the image forming apparatus main body 201A.

[0018] The image forming unit 201B, which is an image forming means, is a four-drum full-color system. The image forming unit 201B includes a laser scanner 210 and four process cartridges 211 that form toner images in four colors: yellow (Y), magenta (M), cyan (C), and black (K). Each process cartridge 211 includes a photosensitive drum 212 that is a photosensitive member, a charger 213 that is a charging means, and a developing unit 214 that is a developing means. Each process cartridge 211 is detachably mounted in the image forming apparatus main body 201A. The image forming unit 201B also includes an intermediate transfer unit 201C arranged above the process cartridges 211 and a fixing unit 220. In FIG. 1, reference numeral 215 denotes a toner cartridge that supplies toner to the developing unit 214 and is replaceably mounted in the image forming apparatus main body 201A.

[0019] In the image forming apparatus 201 shown in FIG. 1, the photosensitive drum 212a is the first photosensitive drum on which a yellow toner image, which is a first toner image, is formed. In this case, the photosensitive drum 212b is the second photosensitive drum on which a magenta toner image, which is a second toner image of a different color from the first toner image formed on the first photosensitive drum, is formed. The photosensitive drum 212b is disposed downstream of the photosensitive drum 212a in the rotation direction of the intermediate transfer belt 216 and is adjacent to the photosensitive drum 212a. In the rotation direction of the intermediate transfer belt 216, the adjacent photosensitive drums on the upstream side and the downstream side are in the relationship of the first photosensitive drum and the second photosensitive drum, respectively, as described above.

[0020] Furthermore, a charger 213 uniformly charges the surface of each photosensitive drum. A laser scanner (optical scanning device) 210 irradiates the uniformly charged surfaces of the photosensitive drums 212a, 212b, 212c, and 212d with light, forming electrostatic latent images corresponding to the respective colors on each photosensitive drum. Here, a single laser scanner that emits multiple light beams to the multiple photosensitive drums is illustrated as an example of the optical scanning device, but a laser scanner may be provided for each photosensitive drum. Furthermore, instead of the laser scanner 210, an LED head may be used as an exposure unit that exposes the photosensitive drum 212. The LED head is an exposure unit that is arranged in a line in the longitudinal direction (direction of the rotation axis) of the photosensitive drum 212 and has multiple light-emitting elements that emit light according to image data to expose the photosensitive drum 212. Furthermore, a developer (developing device) 214 develops the electrostatic line images formed on each photosensitive drum with toner of the respective color to form a toner image.

[0021] The intermediate transfer unit 201C includes an intermediate transfer belt 216 wound around a drive roller 216a and a tension roller 216b. The intermediate transfer belt 216 is rotated in the direction of the arrow by the drive roller 216a, which is driven by a drive unit (not shown). Primary transfer rollers 219 are provided inside the intermediate transfer belt 216 and contact the intermediate transfer belt 216 at positions facing the respective photosensitive drums 212. The primary transfer rollers 219 sequentially transfer negatively charged color toner images on the photosensitive drums 212 onto the intermediate transfer belt 216 in a superimposed manner. Thus, the intermediate transfer belt (intermediate transfer member) 216 rotates in the sub-scanning direction, and the yellow, magenta, cyan, and black toner images are sequentially primary-transferred so as to be superimposed on top of each other.

[0022] A secondary transfer roller 217 that transfers a color image formed on the intermediate transfer belt 216 onto the sheet P is provided at a position facing the drive roller 216a of the intermediate transfer unit 201C. The secondary transfer roller 217 forms a secondary transfer section 218, which is a transfer nip, together with the intermediate transfer belt 216. The secondary transfer roller (secondary transfer body) 217 ​​secondarily transfers the toner image (color image) superimposed on the intermediate transfer belt 216 onto the sheet P at the secondary transfer section 218.

[0023] Furthermore, above the secondary transfer unit 218, there is disposed a fixing unit 220 that fixes the transferred toner image onto the sheet P. Above the fixing unit 220, there are disposed a first pair of discharge rollers 225a, a second pair of discharge rollers 225b, and a duplex reversing unit 201D. The duplex reversing unit 201D is a third feeding unit that reverses the sheet P that has passed through the secondary transfer unit 218 and feeds it again toward the secondary transfer unit 218. The duplex reversing unit 201D is provided with a pair of reversing rollers 222 that can rotate forward and backward, and a re-conveying path R that transports the sheet P, one side of which has an image formed thereon, again to the image forming unit 201B. The re-conveying path R is composed of a plurality of roller pairs that transport the sheet, a guide that guides the sheet, and the like.

[0024] The image forming apparatus main body 201A also includes a first feeding unit 201E that feeds sheets P from a cassette toward the secondary transfer unit 218. The first feeding unit 201E is disposed vertically below the laser scanner 210. In this embodiment, the feeding unit 201E includes first to fourth feeding units 231 to 234, each of which includes feeding cassettes 241 to 244, feeding rollers 251 to 254, conveying roller pairs 261 to 264, and drawing roller pairs 271 to 274. The first feeding unit 231 includes a first feeding cassette 241 that is a recording medium storage unit capable of storing sheets P. The first feeding unit 231 also includes a first feeding roller 251 that feeds sheets P from the first feeding cassette 241, a first conveying roller pair 261 that separates and conveys the fed sheets P one by one, and a first drawing roller pair 271. The sheets P separated and conveyed one by one by the first conveying roller pair 261 are conveyed to the registration roller pair 280 via the first pull-out roller pair 271, and skew is corrected by the registration roller pair 280. Of the multiple feeding units 231 to 234, the feeding unit disposed at the bottom of the main body uses the pull-out roller pair of the feeding unit disposed higher when feeding the sheet. For example, when feeding a sheet P from the third feeding unit 233, the sheet P stored in the third feeding cassette 243 is conveyed to the registration roller pair 280 via the third feeding roller 253, the third conveying roller pair 263, the third pull-out roller pair 273, the second pull-out roller pair 272, and the first pull-out roller pair 271. In FIG. 1, a cassette transport path S is a first transport path of a sheet P from each of the feeding units (feed cassettes) 231 to 234 to the secondary transfer unit 218, and is made up of a plurality of roller pairs, guides, and the like.

[0025] The image forming apparatus main body 201A also has a second feeding unit 201F that feeds sheets P from a feeding tray 245, which is a manual feed tray, toward the secondary transfer unit 218. The second feeding unit 201F is disposed on the side of the image forming apparatus main body 201A and has a feeding tray 245, which is a recording medium mounting unit into which sheets P can be manually fed. The second feeding unit 201F also has a feeding roller 255 that feeds sheets P from the feeding tray 245, a conveying roller pair 265 that separates and conveys the fed sheets P one by one, and a pull-out roller pair 275. The sheets P separated and conveyed one by one by the conveying roller pair 265 are conveyed to a registration roller pair 280 via the pull-out roller pair 275, and skew is corrected by the registration roller pair 280. In FIG. 1, the tray transport path M is a second transport path for the sheet P from the feed tray 245 to the secondary transfer unit 218, and similar to the transport path S, is made up of a plurality of roller pairs, guides, and the like.

[0026] The cassette transport path S, tray transport path M, and re-transport path R all meet in a space upstream of the registration roller pair 280 in the transport direction, and send the transported sheet P to the image forming unit 201B (secondary transfer unit 218). The tray transport path M for the sheet P traveling from the feed tray to the secondary transfer unit 218 meets the cassette transport path S for the sheet P traveling from the cassette to the secondary transfer unit 218, between the cassette and the secondary transfer unit 218. In FIG. 1, the junction J is the junction of the tray transport path M and the cassette transport path S.

[0027] Furthermore, in FIG. 1, a control unit 290 is a control means (controller) that controls the image forming operation, the recording medium feeding operation, and the like.

[0028] Next, the image forming operation of the image forming apparatus 201 will be described. Note that here, an example will be given of a case where a sheet P is fed from the second feeding section 232. First, image information of an original is read by the image reading device 202, and after image processing, this image information is converted into an electrical signal and transmitted to the laser scanner 210 of the image forming section 201B. In the image forming section 201B, the surface of the photosensitive drum 212, the surface of which has been uniformly charged to a predetermined polarity and potential by the charger 213, is sequentially exposed to laser light irradiated from the laser scanner 210. As a result, electrostatic latent images corresponding to images for yellow, magenta, cyan, and black are sequentially formed on the photosensitive drum of each process cartridge 211.

[0029] Thereafter, the electrostatic latent images are developed and visualized with toner of each color, and the toner images of each color on each photosensitive drum are transferred in order onto the intermediate transfer belt 216 in a superimposed manner by a primary transfer bias applied to the primary transfer roller 219. As a result, a toner image is formed on the intermediate transfer belt 216.

[0030] In parallel with this toner image forming operation, the sheet P is sent out from the second conveying roller pair 262 provided in the second feeding section 232. The sent-out sheet P is conveyed to the registration roller pair 280 via the second pull-out roller pair 272 and the first pull-out roller pair 271, and skew is corrected by the registration roller pair 280. After skew correction, the sheet P is conveyed to the secondary transfer section 218 by the registration roller pair 280, and in the secondary transfer section 218, the toner images on the intermediate transfer belt 216 are transferred all at once onto the sheet P by a secondary transfer bias applied to the secondary transfer roller 217. Next, the sheet P onto which the toner images have been transferred is conveyed to the fixing section 220, where the toners of the various colors are melted and mixed by heat and pressure, and the color image is fixed on the sheet P.

[0031] The sheet P on which the image has been fixed is discharged into the discharge space D by the first discharge roller pair 225a or the second discharge roller pair 225b provided above the fixing unit 220, and is stacked on a stacking unit 223 protruding from the bottom surface of the discharge space D. When images are formed on both sides of the sheet P, after the images are fixed, the sheet P is turned over by the reversing roller pair 222 and transported to the re-conveyance path R, and is transported again to the image forming unit 201B (secondary transfer unit 218).

[0032] [Transportation route] Next, the transport path of the sheet P will be described in detail. Figure 2 is a schematic diagram of the second cassette transport path S2 when the sheet P is fed from the second feeding section 232 and discharged by the first discharge roller pair 225a. The second cassette transport path S2 for the sheet P fed from the second feeding section 232 is made up of the second feeding roller 252, the second transport roller pair 262, the second pull-out roller pair 272, the first pull-out roller pair 271, the registration roller pair 280, the secondary transfer section 218, the fixing section 220, and the first discharge roller pair 225a. The sheet P fed from the second feeding cassette 242 is transported from the bottom to the top inside the apparatus by each transport roller pair, as indicated by the arrow in Figure 2.

[0033] The second conveying roller pair 262 is composed of a pair of conveying members, a feed roller and a retard roller, and a nip portion is formed between them. The feed roller and the retard roller are each connected to a feed motor (not shown) and driven to rotate, separating the sheet P sent out by the second feed roller 252 into a single sheet, and feeding the sheet P sandwiched in the nip portion downstream in the conveying direction toward the second pull-out roller pair 272.

[0034] The second pull-out roller pair 272 and the first pull-out roller pair 271 are each composed of a pair of rollers that are a pair of conveying members, and a nip is formed between them. These roller pairs are connected to a conveying motor (not shown) and are driven to rotate, so that the sheet P conveyed from the upstream in the conveying direction is sandwiched between the nip and further conveyed downstream in the conveying direction.

[0035] The pair of registration rollers 280 is composed of a pair of conveying members, a first registration roller and a second registration roller, and a registration nip portion is formed between them. The first registration roller and the second registration roller are each connected to a conveying motor (not shown) and are driven to rotate, and convey the sheet P sandwiched in the registration nip portion toward the secondary transfer unit 218.

[0036] The secondary transfer unit 218 is formed as a nip between the intermediate transfer belt 216, whose inner circumferential surface is supported by the drive roller 216a, and the secondary transfer roller 217. The drive roller 216a and the secondary transfer roller 217 are each connected to an image forming motor (not shown) and driven to rotate, and transfer an image onto the sheet P sandwiched in the secondary transfer unit 218, and transport the sheet P toward the fixing unit 220.

[0037] The fixing unit 220 has a fixing nip formed as a nip between a fixing roller and a pressure roller. The fixing roller and the pressure roller are each connected to a fixing motor (not shown) and are driven to rotate, fixing the toner image on the sheet P held in the fixing nip and transporting the sheet P toward the discharge unit.

[0038] The first discharge roller pair 225a is composed of a pair of rollers, each of which is a pair of conveying members, and a nip is formed between them. These roller pairs are connected to a conveying motor (not shown) and are driven to rotate, and discharge the sheet P conveyed from the upstream side in the conveying direction toward the discharge space D.

[0039] The conveyance path between each nip portion is provided with a guide portion (not shown) that guides the conveyed sheet P, guiding the leading edge of the sheet P conveyed from upstream in the conveyance direction to the nip portion downstream in the conveyance direction. As shown in the figure, the conveyance path is curved, and the sheet P is conveyed in a curved state that follows the space defined by the guide portion. In addition, because there is some space between the guide portions, the sheet P can bend perpendicular to the conveyance direction, and the degree of bending increases or decreases depending on the conveyance speed of the upstream and downstream conveyance roller pairs.

[0040] Fig. 3 is a schematic diagram of tray transport path M when sheet P is fed from feed tray 245, passes through tray transport path M, and is discharged from first discharge roller pair 225a. Fig. 4 is a schematic diagram of the transport path when sheet P, on which an image has been fixed once, is transported to re-transport path R by reversing roller pair 222, has the image fixed again, and is discharged from first discharge roller pair 222a. As shown in Figs. 2, 3, and 4, in the image forming apparatus 201 of this embodiment, sheet P is transported through different transport paths depending on whether the sheet is fed from a cassette or a tray, and whether single-sided printing or double-sided printing is performed. Furthermore, the transport paths are made up of different pairs of transport rollers and guide units, and therefore the shapes of the transport paths also differ.

[0041] [Color misalignment caused by the sheet] 5(a) and 5(b) are schematic diagrams illustrating color shifts that occur on an image in the conveyance direction of sheet P. In the figures, Y1, M1, C1, K1, and Y2, M2, C2, and K2 represent images of each color (yellow, magenta, cyan, and black, respectively) that are arranged at equal positions in the conveyance direction of sheet P in the image information. FIG. 5(a) illustrates a case where there is no color shift, while FIG. 5(b) illustrates a case where there is color shift. Normally, even images of each color that are arranged at equal positions in the conveyance direction as shown in FIG. 5(a) may be transferred with a shift in the conveyance direction due to fluctuations in the conveyance speed of intermediate transfer belt 216, as shown in FIG. 5(b). Here, the shift in the transfer position of each color in the conveyance direction relative to the transfer position of a certain color (e.g., yellow) is referred to as color shift, and the conveyance direction of sheet P is represented as the negative direction of color shift.

[0042] Figures 6(a) and 6(b) show the variation in color shift that occurs in an image when a sheet P of plain paper and thick paper is output. Figure 6(a) shows the case of plain paper, and Figure 6(b) shows the case of thick paper. Both figures show the color shift waveforms when an A3-sized sheet P (420 mm long in the conveying direction) is fed from the second feeding unit (feed cassette) 232 and discharged from the first discharge roller pair 225a. They also show the variation in color shift in the sub-scanning direction (conveying direction) in the images of subsequent sheets (the second sheet (N+1)) and beyond, which are affected by the preceding sheet (the Nth sheet) in a continuous paper feed job. Both figures show the color shift waveforms of magenta (M), cyan (C), and black (K) based on yellow (Y). To highlight only the influence of the sheet P, other influences (such as the rotational period of the photosensitive drum) have been excluded from the calculation. The waveforms in Figures 6(a) and 6(b) show that the amount of misalignment is greater for thick paper than for plain paper. This suggests that the color misalignment is influenced by the sheet P. The continuous paper feed job mentioned above is a job in which sheets P are fed continuously and images are formed on one or both sides of the sheet P.

[0043] Next, the principle of color misregistration caused by the sheet P will be described. First, the primary transfer positions for each color are arranged at regular intervals on the intermediate transfer belt 216. In this configuration, the latent images on the photosensitive drums 212a, 212b, 212c, and 212d for each color are formed at preset time intervals so that they overlap at the same position on the intermediate transfer belt 216. In other words, the latent image on the yellow (Y) photosensitive drum 212a, whose primary transfer position is located upstream in the transport direction of the intermediate transfer belt 216, is formed a certain time earlier than the latent image on the magenta (M) photosensitive drum 212b, which is located downstream thereof.

[0044] When the sheet P is conveyed while being sandwiched in the secondary transfer unit 218, the sheet P and the intermediate transfer belt 216 are in contact with each other, and a force is applied from the sheet P to the intermediate transfer belt 216 in the conveyance direction. As a result, the applied force changes the conveyance speed of the intermediate transfer belt 216, which is constantly driven by the drive roller 216a. As a result, the time from when a toner image of a color located upstream in the conveyance direction of the intermediate transfer belt 216 is primarily transferred to when it reaches the primary transfer position of the color located downstream no longer matches a preset time interval. Therefore, at the downstream primary transfer position, the toner image on the downstream side is transferred out of alignment with the toner image conveyed from the upstream side, resulting in relative misalignment. This is the principle behind the occurrence of color misalignment due to the sheet P. For example, if the conveyance speed of the intermediate transfer belt 216 increases due to the force applied from the sheet P between when a yellow (Y) toner image is primarily transferred to the intermediate transfer belt 216 and when it reaches the primary transfer position of magenta (M), the arrival time becomes shorter than the predetermined interval. As a result, the magenta (M) toner image overlaps the yellow (Y) toner image on the intermediate transfer belt 216, shifting upstream, and the magenta (M) toner image on the sheet P shifts toward the rear end relative to the yellow (Y) toner image, resulting in a color shift in the positive direction.

[0045] The force exerted by sheet P on secondary transfer unit 218, which causes misalignment at the primary transfer unit, can be observed by measuring the fluctuation in drive torque of the drive motor (not shown) that drives drive roller 216a. Figure 7 shows the fluctuation in drive torque when plain paper and thick paper are passed through secondary transfer unit 218 under the conditions shown in Figures 6(a) and 6(b). Figure 7 shows the section of sheet P passing through secondary transfer unit 218, from when its leading edge enters secondary transfer unit 218 until its trailing edge exits, which is thought to affect color misalignment. Comparing the drive torque waveforms for plain paper and thick paper in Figure 7 reveals that the drive torque fluctuation is greater for thick paper. This also suggests that color misalignment is caused by the force exerted by sheet P on secondary transfer unit 218, and that the magnitude of this fluctuation is greater for thick paper.

[0046] Next, the force that the sheet P exerts on the secondary transfer unit 218 will be described using the schematic cross-sectional view of the conveyance path from the registration roller pair 280 to the fixing unit 220 shown in FIG. 8. Examples of external forces and internal stresses acting on the sheet P during conveyance include conveyance forces F1a, F1b, and F1c, reaction forces F2a, F2b, F2c, and F2d, and resultant forces F3a and F3b. Here, the conveyance forces F1a, F1b, and F1c are conveyance forces resulting from the rotational drive of the conveyance roller pairs (here, the registration roller pair 280, the secondary transfer unit 218, and the fixing unit 220) that sandwich the sheet P. The reaction forces F2a, F2b, F2c, and F2d are reaction forces resulting from the stiffness of the sheet P caused by bending of the sheet P in the conveyance path between the nip portions. The resultant forces F3a and F3b are the resultant of normal forces and frictional forces generated when the sheet P comes into contact with and rubs against the guide portions that form the conveyance path. The conveyance force here can be understood as the sum of static and dynamic frictional forces that the sheet P receives from the pair of rotationally driven rollers when the sheet P slides through the nip between the roller pairs. These reaction forces act on the pair of conveyance rollers and guide portions that are in contact with the sheet P, including the secondary transfer unit 218. Therefore, the force that the sheet P exerts on the secondary transfer unit 218 at a given time is affected by the shape of the conveyance path made up of the pair of conveyance rollers and guide portions that nip and convey the sheet P at that time, the posture and stiffness of the sheet P passing through the conveyance path, and the conveyance speed of the pair of conveyance rollers, which contribute to the amount of deflection.

[0047] The figure shows the relationship between the force that sheet P exerts on secondary transfer unit 218 and each pair of conveying rollers that form the conveying path. While being sandwiched between multiple pairs of conveying rollers, sheet P is subjected to conveying forces F1a, F1b, and F1c caused by the rotational drive of the rollers and conveyed downstream in the conveying direction. Therefore, even in the section where sheet P passes through secondary transfer unit 218, sheet P is conveyed in a state where it is sandwiched not only by registration roller pairs 280 and fixing unit 220 disposed before and after secondary transfer unit 218, but also by conveying roller pairs upstream and downstream of those in the conveying direction. Therefore, when considering the forces acting on secondary transfer unit 218, it is preferable to consider the forces acting on sheet P in the conveying path upstream and downstream of secondary transfer unit 218, rather than just the conveying path before and after secondary transfer unit 218.

[0048] Furthermore, FIG. 9 shows the driving torque fluctuations shown in FIG. 7 along with the intervals during which each conveyance roller pair holds the sheet P. Under these conditions, the sheet P is conveyed as follows. After the leading edge of the sheet P sequentially enters the secondary transfer unit 218 and the fixing unit 220, the trailing edge of the sheet P passes through the second conveyance roller pair 262, the second pull-out roller pair 272, the first pull-out roller pair 271, the registration roller pair 280, and the secondary transfer unit 218. Focusing on the relationship between the driving torque waveform and the intervals during which the sheet P is held by each conveyance roller pair in FIG. 9, it can be seen that there are intervals in which the tendency of fluctuations changes before and after the leading edge of the sheet P enters or the trailing edge of the sheet P leaves each conveyance roller pair. Specifically, the driving torque begins to increase gradually when the leading edge of the sheet P enters the secondary transfer unit 218, and the slope of this increase increases as the leading edge enters the fixing unit 220. Furthermore, when the trailing edge of the sheet P passes through the second pull-out roller pair 272, the drive torque increases in a step-like manner. These drive torque fluctuations indicate that the tendency of the force acting on the secondary transfer unit 218 via the sheet P changes depending on the transport roller pair that sandwiches and transports the sheet P. Explaining the phenomenon occurring under the conditions shown in FIG. 9, when the leading edge of the sheet P enters the fixing unit 220, the deflection of the sheet formed in the transport path between the secondary transfer unit 218 and the fixing unit 220 increases according to the difference in transport speed. As a result, the force F2d pushing the sheet P back to the secondary transfer unit 218 from the downstream in the transport direction gradually increases, and the drive torque also gradually increases. Furthermore, when the trailing edge of the sheet P passes through the second pull-out roller pair 272, the transport force (not shown) received from the second pull-out roller pair 272 is lost before and after that. As a result, the force pushing the sheet P from the upstream in the transport direction toward the secondary transfer unit 218 suddenly decreases, and the drive torque also increases in a step-like manner.

[0049] In this way, the force acting on the secondary transfer section 218 via the sheet P is influenced not only by the sections of the conveying path before and after the secondary transfer section 218, but also by sections upstream and downstream in the conveying direction, and is therefore thought to differ for each conveying path.

[0050] Figures 10(a) and 10(b) show color shifts that occur in an image when a cardboard sheet P is output, and the conditions are the same as Figure 6(b) except that the feeding unit is different. Figure 10(a) shows the case where a sheet is fed from the second feeding cassette 242, and Figure 10(b) shows the case where a sheet is fed from the feeding tray 245. Figures 10(a) and 10(b) show that the color shift waveforms caused by the sheet P are different when the feeding unit is different (i.e., when the transport path is different).

[0051] From the above, color misregistration caused by the influence of the sheet P is caused by the force that the sheet P applies to the secondary transfer unit 218, and the degree of this force is greater for thick paper and differs depending on the transport path. Therefore, when taking measures such as shifting the exposure and writing start timing for each color, it is preferable to change the amount of correction for the exposure and writing start timing depending on the basis weight of the sheet P and the transport path.

[0052] Finally, we will explain the range of color shifts caused by sheet P. Color shifts caused by sheet P are caused by a transfer position shift that occurs at the primary transfer section, located upstream of the intermediate transfer belt 216 in the transport direction, while sheet P passes through the secondary transfer section 218. Therefore, the effect of a change in the speed of intermediate transfer belt 216 due to the force exerted by sheet P on the secondary transfer section at a certain time results in color shifts when the toner images of each color are transported from the primary transfer section to the secondary transfer section and then secondary-transferred onto sheet P. Figure 11 is a timing chart of the primary and secondary transfers of each color during continuous single-sided paper feed. The sections of the primary transfer of each color highlighted by dashed lines indicate that secondary transfer is occurring simultaneously during primary transfer. Focusing on the dashed lines in Figure 11 reveals that the patterns differ between the first sheet and the second and subsequent sheets. This indicates that during the primary transfer of the first sheet, only secondary transfer of the sheet P itself is occurring, whereas for the second and subsequent sheets, secondary transfer of the sheet P (hereafter referred to as the preceding sheet) that was fed just before sheet P is occurring. If the distance between successive sheets (the distance between the rear edge of one sheet and the front edge of the next sheet) is the same, the above relationship can be roughly divided into the first sheet with no preceding sheet and the second sheet and subsequent sheets with preceding sheets. Therefore, the effect of color misregistration due to sheet P should also be different between the first sheet and the second sheet and subsequent sheets.

[0053] Figures 12(a) and 12(b) show the color shift caused by each sheet P when cardboard is continuously fed. Figure 12(a) shows the color shift of only the first sheet, while Figure 12(b) shows the color shift of the second to fifth sheets superimposed on each other. Figures 12(a) and 12(b) show a clear difference in the color shift trends between the first sheet and the second to fifth sheets. It can be seen that the color shift of the second to fifth sheets shown in Figure 12(b) fluctuates more than the color shift of the first sheet shown in Figure 12(a), and that the color shift waveforms for each color are similar. This indicates that the presence or absence of a preceding sheet differs between the first sheet and the second and subsequent sheets, as mentioned above, and therefore the color shift caused by sheet P appears differently. Here, measures are taken to reduce color shift from the second and subsequent sheets, which are affected by the preceding sheet, when cardboard is continuously fed.

[0054] Furthermore, even if the media information, such as the basis weight, of the sheets P is the same, if the feeding section for the sheets P is different, the surface speed of the intermediate transfer belt 216 may fluctuate due to the force acting on the secondary transfer section 218 via the sheets P. In this case, too, fluctuations in the surface speed of the intermediate transfer belt make it impossible to accurately overlay multiple color toner images on the intermediate transfer belt for images to be transferred to the sheet (subsequent sheet) following the sheet (preceding sheet) entering the secondary transfer section, resulting in color misalignment. Therefore, even in such cases, measures are needed to address color misalignment of the second and subsequent sheets, which are affected by the preceding sheet, when sheets are continuously fed.

[0055] Here, the feeding unit for the sheet P is a first feeding unit 201E that feeds a sheet from feeding cassettes 241 to 244 toward secondary transfer unit 218, or a second feeding unit 201F that feeds a recording medium from feeding tray 245 toward the transfer nip. Different feeding units means that the first designation information that designates feeding cassettes 241 to 244 or feeding tray 245, which is stored in a memory (not shown) serving as a storage unit of the image forming apparatus, is different.

[0056] The specification information stored in the memory includes, in addition to the first specification information, second specification information that specifies media information of the sheet P, and third specification information that specifies image formation on one side or both sides of the sheet P. The specification information stored in the memory is specified from an operation panel (not shown) of the image forming apparatus or an external device communicably connected to the image forming apparatus. The first specification information may also be information from a detection means such as a sensor that detects the sheet and is provided in the transport path.

[0057] As described with reference to FIG. 1, the feed cassettes 241 to 244 are disposed vertically below the laser scanner 210. The feed tray 245 is disposed on the side of the image forming apparatus main body 201A. Furthermore, in the first feed unit 201E, the feed rollers 251 to 254 are disposed vertically below the secondary transfer unit 218. In the second feed unit 201F, the feed roller 255 is disposed vertically above the junction J and vertically below the secondary transfer unit 218. The junction J is a junction where the tray transport path M and the cassette transport path S merge between the feed cassette 241 and the secondary transfer unit 218.

[0058] Therefore, the cassette transport path S of the sheet P heading from the feeding cassettes 241 to 244 to the secondary transfer unit 218 is different from the tray transport path M of the sheet P heading from the feeding tray 245 to the secondary transfer unit 218. In this case, the posture of the sheet sandwiched in the secondary transfer unit 218 is different between the cassette transport path S and the tray transport path M. In other words, different feeding units also means that the transport paths S and M of the sheet P heading to the secondary transfer unit 218 are different.

[0059] [Laser scanner write timing] In this embodiment, in the case of a job in which sheets P are continuously fed and images are formed on the sheets P, the aforementioned color misregistration is reduced by changing the timing at which the laser scanner 210 starts writing when forming images on the second and subsequent sheets P. This will be explained below. Here, an example of a job in which sheets are continuously fed and an image is formed on one side of the sheet will be explained.

[0060] First, the user specifies the feed unit (feed cassette or feed tray) where the sheet P is set and the media information of the sheet P from the operation panel or an external device. The media information includes at least one of the basis weight, paper thickness, stiffness, and surface condition of the sheet P. This embodiment describes the basis weight as an example of the media information of the sheet, but this is not limited to this, and this embodiment can also be applied to the paper thickness, stiffness, and surface condition. The specification of information also includes the case where the feed unit where the sheet is set is specified by specifying the media information of the sheet P. The user also specifies information for a job to form an image on one or both sides of the sheet.

[0061] The specified information is stored in a memory (not shown) serving as a storage unit of the image forming apparatus 201. The specified information stored in the memory includes first specified information specifying a feeding unit (feed cassette or feed tray), second specified information specifying media information of the sheet P, third specified information specifying image formation on one side or both sides of the sheet P, and the like.

[0062] The control unit 290 sets a predetermined time as a setting time described later in accordance with the specification information stored in the memory, and controls the operation of the laser scanner to change the timing at which the laser scanner starts writing to the timing between drums plus the predetermined time.

[0063] When feeding begins from the feeding unit specified by the user or the feeding unit in which the specified sheet is set, the sheet P is transported along a predetermined transport path. At the same time, in the image forming unit 201B, the laser scanner 210 begins to write electrostatic latent images onto the photosensitive drums 212 of each color in order.

[0064] First, the laser scanner 210 starts writing a first electrostatic latent image of the first sheet P onto the photosensitive drum (first photosensitive member) 212a located most upstream in the rotation direction of the intermediate transfer belt 216.

[0065] Next, the laser scanner 210 starts writing the first second electrostatic latent image onto the photosensitive drum (second photosensitive body) 212b after the drum-to-drum time Ta, which is the first time, has elapsed since starting to write the first electrostatic latent image onto the photosensitive drum (first photosensitive body) 212a. This makes it possible to align the positions in the sub-scanning direction of the toner image (first toner image) on the photosensitive drum 212a, which is primarily transferred onto the intermediate transfer belt 216 for secondary transfer onto the first sheet P, with the toner image (second toner image) on the photosensitive drum 212b.

[0066] Here, the inter-drum time Ta is the time obtained by dividing the horizontal distance L between the photosensitive drums 212a and 212b adjacent to each other in the rotation direction of the intermediate transfer belt 216 (hereinafter referred to as inter-drum distance) by the process speed of the image forming apparatus 201. In this embodiment, the process speed is the set value of the surface speed (first speed) Vb of the intermediate transfer belt 216 before the sheet P enters the secondary transfer unit 218. Note that the process speed may be changed in accordance with media information.

[0067] In this embodiment, the inter-drum distance between the photosensitive drum 212b and the photosensitive drum 212c and the inter-drum distance between the photosensitive drum 212c and the photosensitive drum 212d are the same as the inter-drum distance L between the photosensitive drum 212a and the photosensitive drum 212b.

[0068] Therefore, the laser scanner 210 starts writing the first electrostatic latent image on the photosensitive drum 212c after the elapse of an inter-drum time 2Ta, which is twice the inter-drum time Ta, from the start of writing the first electrostatic latent image on the photosensitive drum 212a.

[0069] In addition, the laser scanner 210 starts writing the first electrostatic latent image on the photosensitive drum 212d after a drum-to-drum time 3Ta, which is three times the drum-to-drum time Ta, has elapsed since starting to write the first electrostatic latent image on the photosensitive drum 212a.

[0070] The laser scanner 210 is also set so that after starting to write the first electrostatic latent image on the photosensitive drum 212a, it starts writing the first electrostatic latent image on the photosensitive drums 212b, 212c, and 212d in sequence after the lapse of inter-drum times Ta, 2Ta, and 3Ta, respectively.

[0071] Then, the laser scanner 210 starts writing the electrostatic latent image for the second sheet onto the photosensitive drum 212a after a time Ts corresponding to the sheet interval has elapsed since starting to write the electrostatic latent image for the first sheet onto the photosensitive drum 212a.

[0072] Here, the start of writing latent images on the second and subsequent sheets by the laser scanner 210 will be described first in the case of a job in which sheets P are continuously fed from a feeding cassette and images are formed on one side of the sheets P. Next, the case of a job in which sheets P are continuously fed from a feeding tray and images are formed on one side of the sheets P will be described.

[0073] First, in the case of a job in which sheets P are continuously fed from a feeding cassette and an image is formed on one side of the sheet P, the laser scanner 210 starts writing electrostatic latent images sequentially onto the photosensitive drums 212 of each color as follows.

[0074] First, the laser scanner 210 starts writing a first electrostatic latent image of the second sheet P onto the photosensitive drum (first photosensitive member) 212a located most upstream in the rotation direction of the intermediate transfer belt 216.

[0075] Next, the laser scanner 210 starts writing the second electrostatic latent image for the second sheet onto the photosensitive drum (first photosensitive body) 212b after a second time, which is the inter-drum time Ta plus a predetermined time (first set time) Tb1, has elapsed from the time when writing of the first electrostatic latent image for the second sheet started onto the photosensitive drum (first photosensitive body) 212a. This makes it possible to align the positions in the sub-scanning direction of the toner image (first toner image) on the photosensitive drum 212a, which is primarily transferred onto the intermediate transfer belt 216 for secondary transfer onto the second sheet P, with the toner image (second toner image) on the photosensitive drum 212b.

[0076] Furthermore, the laser scanner 210 starts writing the second electrostatic latent image on the photosensitive drum 212c after a second time has elapsed, which is the inter-drum time 2Ta plus a predetermined time (first set time) Tb2, from the time when the laser scanner 210 starts writing the second electrostatic latent image on the photosensitive drum 212a.

[0077] In addition, the laser scanner 210 starts writing the second electrostatic latent image on the photosensitive drum 212d after a second time has elapsed, which is the inter-drum time 3Ta plus a predetermined time (first set time) Tb3, from the time when the laser scanner 210 starts writing the second electrostatic latent image on the photosensitive drum 212a.

[0078] The laser scanner 210 is set to start writing the second electrostatic latent image on the photosensitive drum 212a after second times Ta+Tb1, 2Ta+Tb2, and 3Ta+Tb3 have elapsed on the photosensitive drums 212b, 212c, and 212d, respectively.

[0079] Thereafter, until the job is completed, the laser scanner 210 repeats the above operation of starting to write an electrostatic latent image on the photosensitive drum 212a, and then starting to write an electrostatic latent image on the photosensitive drums 212b, 212c, and 212d, respectively, after the second times Ta+Tb1, 2Ta+Tb2, and 3Ta+Tb3 have elapsed.

[0080] On the other hand, in the case of a job in which sheets P are continuously fed from a feed tray and an image is formed on one side of the sheet P, the laser scanner 210 begins to write electrostatic latent images sequentially onto the photosensitive drums 212 of each color as follows.

[0081] First, the laser scanner 210 starts writing a first electrostatic latent image of the second sheet P onto the photosensitive drum (first photosensitive member) 212a located most upstream in the rotation direction of the intermediate transfer belt 216.

[0082] Next, the laser scanner 210 starts writing the second electrostatic latent image for the second sheet onto the photosensitive drum (first photosensitive body) 212a after a third time, which is the inter-drum time Ta plus a predetermined time (second set time) Tc1, has elapsed since starting to write the first electrostatic latent image for the second sheet onto the photosensitive drum (first photosensitive body) 212a. This makes it possible to align the positions in the sub-scanning direction of the toner image (first toner image) on the photosensitive drum 212a, which is primarily transferred onto the intermediate transfer belt 216 for secondary transfer onto the second sheet P, with the toner image (second toner image) on the photosensitive drum 212b.

[0083] Here, the predetermined time Tc1 added to the inter-drum time Ta is a set time for a job in which sheets are continuously fed from a feeding tray, and is a second set time different from the first set time (predetermined time Tb1) for a job in which sheets are continuously fed from a feeding cassette. The predetermined times Tc2 and Tc3 described below are also second set times.

[0084] Furthermore, the laser scanner 210 starts writing the second electrostatic latent image on the photosensitive drum 212c after a third time has elapsed, which is the inter-drum time 2Ta plus a predetermined time (second set time) Tc2, from the time when the laser scanner 210 starts writing the second electrostatic latent image on the photosensitive drum 212a.

[0085] In addition, the laser scanner 210 starts writing the second electrostatic latent image on the photosensitive drum 212d after a third time has elapsed, which is the inter-drum time 3Ta plus a predetermined time (second set time) Tc3, from the time when the laser scanner 210 starts writing the second electrostatic latent image on the photosensitive drum 212a.

[0086] The laser scanner 210 is set to start writing the second electrostatic latent image on the photosensitive drum 212a after a third time Ta+Tc1, 2Ta+Tc2, and 3Ta+Tc3 has elapsed on the photosensitive drums 212b, 212c, and 212d, respectively.

[0087] Thereafter, until the job is completed, the laser scanner 210 repeats the above operation of starting to write an electrostatic latent image on the photosensitive drum 212a, and then starting to write an electrostatic latent image on the photosensitive drums 212b, 212c, and 212d, respectively, after the third times Ta+Tc1, 2Ta+Tc2, and 3Ta+Tc3 have elapsed.

[0088] By doing so, even if the force acting on the secondary transfer unit 218 via the sheet P having the same media information differs depending on the feeding unit, color misregistration can be reduced.

[0089] [Calculation method for the specified time (set time)] Next, a method for calculating the predetermined times Tb1, Tb2, Tb3 and the predetermined times Tc1, Tc2, Tc3 will be described.

[0090] The predetermined times Tb1, Tb2, and Tb3 and the predetermined times Tc1, Tc2, and Tc3 are set according to the process speed of the image forming apparatus, media information of the sheet P, and the transport path (feeding unit) along which the sheet P is transported. The media information of the sheet P includes at least one of the basis weight, paper thickness, rigidity, and surface condition of the sheet P. The transport path along which the sheet P is transported corresponds to the feeding unit. Specifically, the cassette transport path S shown in FIG. 2 is a transport path along which the sheet P is fed from the first feeding unit 201E and discharged by the first discharge roller pair 225a, and corresponds to the first feeding unit. Furthermore, the tray transport path M shown in FIG. 3 is a transport path along which the sheet P is fed from the second feeding unit 201F and discharged by the first discharge roller pair 225a, and corresponds to the second feeding unit. In this embodiment, the first feeding unit 201E has a plurality of feeding units (feed cassettes) 231 to 234, but since all of them follow the same transport path downstream in the transport direction from the first pull-out roller pair 271, the force that sheets P with the same physical properties exert on the intermediate transfer belt is not significantly different, and the amount of color shift is also similar. Therefore, the predetermined time is set to be the same when sheets are fed from the first feeding unit 231 to the fourth feeding unit 234, but this is not limiting, and the predetermined time may be changed even when some of the sheets are transported along the same transport path.

[0091] The predetermined times Tb1, Tb2, and Tb3 are first set times set in accordance with the first, second, and third specified information in order to calculate a second time at which to start writing the second electrostatic latent image on the second sheet in the case of a job in which sheets are continuously fed from a feeding cassette.

[0092] The predetermined times Tc1, Tc2, and Tc3 are second set times different from the first set times set in accordance with the first, second, and third designation information. The predetermined times Tc1, Tc2, and Tc3 are set times for calculating a third time at which to start writing a second electrostatic latent image on a second sheet in the case of a job in which sheets are continuously fed from a feed tray.

[0093] 13 is a table used to determine the predetermined times Tb1, Tb2, Tb3 and the predetermined times Tc1, Tc2, Tc3 from the sheet media information and the feed unit specification information. Here, the predetermined times Tb1, Tb2, Tb3, which are first set times, and the predetermined times Tc1, Tc2, Tc3, which are second set times, are pre-stored in a memory, which is a storage unit. The control unit 290 then determines each predetermined time stored in the memory in accordance with the specification information, and uses each predetermined time to control the timing of exposure and writing by the laser scanner.

[0094] The predetermined times Tb1, Tb2, Tb3 and the predetermined times Tc1, Tc2, and Tc3 are times for correcting the write start timing (image write start time) when the laser scanner 210 starts writing electrostatic latent images onto the photosensitive drums 201b, 201c, and 201d. In the configuration of this embodiment, the predetermined times Tb1, Tb2, and Tb3 are set to equal values, and the predetermined times Tc1, Tc2, and Tc3 are set to equal values, as shown in FIG. 13. However, the values ​​of the predetermined times Tb1, Tb2, and Tb3 and the values ​​of the predetermined times Tc1, Tc2, and Tc3 may be set separately. In this embodiment, the predetermined times Tb1, Tb2, Tb3 and the predetermined times Tc1, Tc2, and Tc3 are set according to the media information and the feeding unit to which the sheet P is fed, as shown in FIG. 13. FIG. 13 illustrates a case in which the media information of the sheet is the basis weight of the sheet.

[0095] For example, when single-sided continuous paper is fed from the first feeding unit 201E (feed cassette) using the cassette transport path S, if the basis weight of the sheet P is equal to or less than a predetermined value (150 g / m² in this case), the predetermined times Tb1, Tb2, and Tb3 are set to 0 (zero) msec. This is because a sheet P with a basis weight of 150 g / m² or less, which is equal to or less than the predetermined value, exerts a small force on the intermediate transfer belt 216 while passing through the secondary transfer unit 218, and therefore no action is required to address this as color misregistration. On the other hand, for a sheet P with a basis weight greater than the predetermined value (150 g / m² in this case), the predetermined times Tb1, Tb2, and Tb3, which are correction times, are set to -0.35 msec. This is because a sheet P with a basis weight greater than the predetermined value exerts a large force on the intermediate transfer belt 216 while passing through the secondary transfer unit 218, and the amount of color misregistration cannot be ignored.

[0096] On the other hand, when single-sided continuous paper is fed from the second feeding unit 201F (feed tray) using the tray transport path M, the predetermined times Tc1, Tc2, and Tc3 are set to 0 (zero) msec if the basis weight of the sheet P is equal to or less than a predetermined value (180 g / m² in this case). For sheets P with a basis weight greater than the predetermined value (180 g / m² in this case), the correction times Tc1, Tc2, and Tc3 are set to -0.35 msec. Here, the time at which the laser scanner 210 starts writing electrostatic latent images to the photosensitive drums 201b, 201c, and 201d is changed only when the basis weight of the recording medium is greater than the predetermined value. However, the predetermined value (150 g / m² in this case) when a feeding cassette is specified differs from the predetermined value (180 g / m² in this case) when a feeding tray is specified. This is because color misregistration for subsequent sheets of the same basis weight varies depending on the transport path.

[0097] In this way, even for recording media of the same basis weight, by changing the correction time depending on the difference in the transport path (feed unit), it is possible to deal with the difference in the degree of color misregistration caused by the sheet P for each transport path and reduce the color misregistration. Note that the relationship between the sheet media information (basis weight) and the specified information of the feed unit (feed cassette or feed tray) and the predetermined times Tb1, Tb2, Tb3 and the predetermined times Tc1, Tc2, Tc3 shown in Fig. 13 is an example and is not limited to this.

[0098] Figures 14(a) and 14(b) show color shifts caused by the second and subsequent sheets of cardboard when the sheets are fed continuously. Figure 14(a) shows a waveform under the same conditions as Figure 6(b) (no correction), while Figure 14(b) shows a waveform under conditions in which the exposure timing for each color was actually corrected based on the media information and the relationship between the feed unit and the specified time shown in Figure 13. The waveforms shown in Figures 14(a) and 14(b) show that the amount of color shift of other colors relative to the reference color is smaller in Figure 14(b), which has been corrected, than in Figure 14(a), which has not been corrected, demonstrating the effectiveness of the correction.

[0099] Here, the positive and negative values ​​of the predetermined time to be corrected will be explained. When the force acting from the sheet P causes the intermediate transfer belt 216 to accelerate, the M, C, and K toner images located downstream of the reference color Y located upstream in the conveyance direction of the intermediate transfer belt 216 are shifted downstream in the conveyance direction. As a result, the other colors are shifted in the positive direction relative to the reference color, resulting in positive color shift, and the predetermined time to be corrected becomes negative (negative correction). On the other hand, when the force acting from the sheet P causes the intermediate transfer belt 216 to decelerate, the M, C, and K toner images located downstream of the reference color Y located upstream in the conveyance direction of the intermediate transfer belt 216 are shifted upstream in the conveyance direction relative to the reference color Y. As a result, the other colors are shifted in the opposite direction to the positive direction relative to the reference color, resulting in negative color shift, and the predetermined time to be corrected becomes positive (positive correction). In this way, the positive and negative values ​​of the predetermined time to be corrected change depending on whether the intermediate transfer belt 216 is accelerated or decelerated by the sheet P. In the configuration of this embodiment, the intermediate transfer belt 216 is accelerated, that is, the predetermined time is negative (negative correction), but this is not the only case, and the positive and negative values ​​may differ for each conveying path depending on the force acting on the secondary transfer section through the sheet.

[0100] Next, the procedure for correcting the exposure timing during image formation in this embodiment will be described using the flowchart shown in FIG. 15. Here, the explanation will be given taking as an example a case where sheets set in the second feeding cassette 242 are continuously fed one-sided. First, when the control unit 290 receives a job for single-sided feeding (S1), the control unit 290 checks the designation information specified in the received job (S2). Here, the designation information includes first designation information that designates a feeding cassette or feeding tray, and second designation information that designates sheet media information. In accordance with the checked designation information, the control unit 290 references the predetermined times Tb1, Tb2, and Tb3 and the predetermined times Tc1, Tc2, and Tc3 stored in memory (S3), and image formation and feeding operations are initiated (S4). The predetermined times Tb1, Tb2, Tb3 and the predetermined times Tc1, Tc2, Tc3 are correction times for correcting the time at which writing of latent images on the photosensitive drums 201b, 201c, and 201d starts, respectively.

[0101] Specifically, if the image to be formed is not the second or subsequent sheet in the continuous paper feed, i.e., if it is the first sheet (S5), writing of the latent image of each color begins at intervals of the inter-drum time Ta (S6), and the image is formed.

[0102] Furthermore, if the image to be formed is the second or subsequent sheet in the continuous sheet feed (S5), the process checks whether the aforementioned designation information indicates sheet feeding from a feed cassette (cassette transport path) (S8). If the sheet is fed from a feed cassette (cassette transport path) (S8), writing of a latent image for each color begins within a second time period obtained by adding one of predetermined times Tb1, Tb2, and Tb3 to the inter-drum time Ta (S9), and an image is formed. On the other hand, if the sheet is not fed from a feed cassette (cassette transport path) (S8), that is, if the sheet is fed from a feed tray (tray transport path), writing of a latent image for each color begins within a third time period obtained by adding one of predetermined times Tc1, Tc2, and Tc3 to the inter-drum time Ta (S10), and an image is formed.

[0103] If there are no sheets P to be output continuously, the job is terminated (S7), and if there are, the process returns to S5, and the job is repeated until there are no sheets P to be output continuously.

[0104] According to this embodiment, when sheets are continuously fed, the timing at which the laser scanner starts writing an electrostatic latent image onto the photosensitive drum is set according to not only the media information of the sheets but also the designated information of the feeding unit, so that color misregistration can be reduced even if the force acting on the secondary transfer unit via sheets of the same thickness differs depending on the feeding unit.

[0105] Example 2 [When transported via a re-transport route] In the first embodiment described above, correction of the write start timing was explained using an example of a job in which sheets are continuously fed and images are formed on one side of the sheets. In this embodiment, correction of the write start timing will be explained using an example of a job in which sheets are continuously fed and images are formed on both sides of the sheets.

[0106] When continuously feeding sheets to form images on both sides thereof, the sheet P fed from the first feeding unit 201E or the second feeding unit 201F has a toner image transferred onto the first side in the secondary transfer unit 218 and then fixed once in the fixing unit 220. The sheet P is then inverted by the double-sided reversing unit 201D and fed again toward the secondary transfer unit 218, where an image is formed on the second side. As described above, the double-sided reversing unit (third feeding unit) 201D is provided with a pair of reversing rollers 222 that can rotate forward and backward, and a re-conveyance path R that transports the sheet P, with an image formed on one side thereof, back to the image forming unit 201B. The path consisting of the roller pair, guides, etc. that constitute the re-conveyance path R is different from the cassette transport path S and the tray transport path M, and therefore the force acting from the sheet P to the secondary transfer unit 218 is also different. Therefore, the effect of the force acting from the preceding sheet on the secondary transfer unit 218 on the color shift of the image formed on the subsequent sheet that is transported to the secondary transfer unit 218 after the preceding sheet also differs. In other words, the amount of correction of the write start timing for the subsequent sheet needs to be changed depending on which transport path, including the re-transport path R, the preceding sheet was transported through.

[0107] FIG. 16 is a table used to determine the predetermined times Tb1, Tb2, and Tb3 based on the media information of the preceding sheet and the transport path through which the sheet has been passed. The predetermined times Td1, Td2, and Td3 are correction times for correcting the time at which writing of latent images begins on the photosensitive drums 201b, 201c, and 201d, respectively. In the configuration of this embodiment, the predetermined times Td1, Td2, and Td3 are set to equal values, the same as the predetermined times Tc1, Tc2, and Tc3 for sheet feeding from the aforementioned feed tray. However, the values ​​of the predetermined times Td1, Td2, and Td3 may be set separately for each transport path of the preceding sheet. Furthermore, the values ​​of the predetermined times Td1, Td2, and Td3 may be set to values ​​different from the values ​​of the predetermined times Tc1, Tc2, and Tc3 for sheet feeding from the aforementioned feed tray. In this embodiment, the predetermined times Td1, Td2, and Td3 are set in accordance with the media information of the preceding sheet and the transport path along which the preceding sheet is transported, as shown in FIG.

[0108] Furthermore, in many configurations in which sheets are continuously fed and images are formed on both sides, image formation is performed with multiple sheets P present simultaneously in the conveyance path in order to increase the number of sheets that can be output per unit time. In this embodiment, we consider a case in which a maximum of three sheets P are present simultaneously in the conveyance path, so-called three-sheet circulation. Note that the number of sheets that can be present simultaneously in the conveyance path varies depending on the main body configuration of the image forming apparatus and the length of the sheets P in the conveyance direction, so the number of circulated sheets is not limited to three and can be set as appropriate.

[0109] FIG. 17 is a timing chart of the primary transfer and secondary transfer of each color during double-sided continuous feeding with a three-sheet circulation. The section of the primary transfer of each color highlighted by a dashed line indicates that the secondary transfer is occurring simultaneously during the primary transfer. In FIG. 17, "front" refers to image formation on the front (one side) of the first sheet, and "back" refers to image formation on the back (other side) of the first sheet. As shown in FIG. 17, the order in which sheets P arrive at the secondary transfer unit 218 during double-sided continuous feeding with a three-sheet circulation is the front side of the first sheet, the front side of the second sheet, the back side of the first sheet, the front side of the third sheet, the back side of the second sheet, etc. It can be seen that this arrival order mixes the front sides of sheets transported from the cassette transport path S or the tray transport path M with the back sides of sheets transported from the re-transport path R.

[0110] 17, there is a sufficient time interval between the front side of the first sheet and the front side of the second sheet, which is the time required to transport the first sheet to the re-transport path R after the image on the front side has been formed.

[0111] As explained in the first embodiment, there is no need to correct the exposure / writing timing of the laser scanner for the first sheet, which is not affected by the preceding sheet, in the case of continuous single-sided paper feeding shown in Fig. 11. Considering this, it can be said that there is also no need to correct the exposure / writing timing of the laser scanner for image formation on the front side of the second sheet in continuous double-sided paper feeding shown in Fig. 17, because there is a sufficient time interval between the front side and the first sheet, which is the preceding sheet. In other words, there is no need to correct the exposure / writing timing for the front side of the second sheet, because secondary transfer of the front side of the first sheet, which is the preceding sheet, is not performed during primary transfer of each color to the front side of the second sheet.

[0112] In this way, if the gap between the preceding and succeeding sheets is greater than a predetermined time, there is no influence from the preceding sheet, and therefore no correction is required for the succeeding sheet following the preceding sheet. Furthermore, if there is a sufficient gap between the preceding and succeeding sheets during continuous paper feed, patch drawing, such as density correction, may be performed by interrupting the continuous paper feed. Even in this case, it is appropriate to determine that the gap between the preceding and succeeding sheets is sufficient, and not perform correction on the succeeding sheet. The predetermined time for determining a sufficient gap between sheets is the time interval in which the secondary transfer of the preceding sheet is completed and the primary transfer of the succeeding sheet begins. The predetermined time for determining a sufficient gap between sheets corresponds to the time obtained by dividing the distance from the primary transfer position, located at the most upstream position in the transport direction on the intermediate transfer belt 216, to the secondary transfer position by the process speed.

[0113] Next, correction of the exposure / writing timing by the laser scanner in this embodiment will be described with reference to Figures 16 to 18. For example, consider a case where sheets in the category of basis weights from 151 g / m to 163 g / m are continuously fed double-sided from the first feeding unit 201E.

[0114] 16 is a table used to determine the predetermined times Tb1, Tb2, Tb3, the predetermined times Tc1, Tc2, Tc3, and the predetermined times Td1, Td2, Td3 from the sheet media information and the feed unit specification information. Here, the predetermined times Tb1, Tb2, Tb3, which are the first set times, the predetermined times Tc1, Tc2, Tc3, which are the second set times, and the predetermined times Td1, Td2, Td3, which are the third set times, are pre-stored in a memory, which is a storage unit. The control unit 290 then determines each of the predetermined times stored in the memory in accordance with the specification information and controls the timing of exposure and writing by the laser scanner using each predetermined time.

[0115] 4, the present embodiment illustrates a configuration in which the re-conveyance path R merges with the tray transport path M upstream of the junction J between the cassette transport path S and the tray transport path M in the transport direction. Therefore, FIG. 16 illustrates a case in which the predetermined times Tc1, Tc2, and Tc3, which are the second set times, and the predetermined times Td1, Td2, and Td3, which are the third set times, are set to the same value. However, this is not limited to this, and should be set appropriately depending on the device configuration, and the third set time may be different from the first and second set times.

[0116] First, for the front of the first sheet and the front of the second sheet, the gap between them and the preceding sheet exceeds the predetermined time, so the timing of exposure and writing by the laser scanner is not corrected. Next, for the back of the first sheet, the gap between it and the front of the second sheet (the preceding sheet) is within the predetermined time, and there is concern about color shift due to the sheet, so the timing of exposure and writing by the laser scanner must be corrected. Since the preceding sheet is the front of the second sheet, which is passed through the cassette transport path S, the predetermined times Tb1, Tb2, and Tb3 are all set to -0.35 msec, referring to Figure 16, and the timing of exposure and writing by the laser scanner is corrected. Next, for the front of the third sheet, the gap between it and the back of the first sheet (the preceding sheet) is also within the predetermined time. Therefore, with regard to the front side of the third sheet, there is concern that the force acting on the secondary transfer unit 218 from the preceding sheet may affect the color misregistration of the image formed on the front side of the third sheet, which is the subsequent sheet transported to the secondary transfer unit 218 after the preceding sheet. Therefore, it is necessary to change the correction amount of the write start timing for the subsequent sheet in response to the preceding sheet being transported via the re-transport path R. Since the preceding sheet is the back side of the first sheet and is transported via the re-transport path R, the predetermined times Td1, Td2, and Td3 are uniformly set to 0 (zero) msec, as shown in FIG. 16, and the exposure write start timing is corrected. In this way, in image formation where the interval between the preceding sheet and the subsequent sheet is within a predetermined time and color misregistration due to the sheet is a concern, the correction time is changed depending on the transport path of the preceding sheet, even if the sheets have the same basis weight. This addresses the difference in the degree of color misregistration caused by the sheet P for each transport path.

[0117] Next, the procedure for correcting the exposure timing during image formation in this embodiment will be described using the flowchart shown in FIG. 18. Here, we will explain an example in which sheets set in the second feeding cassette 242 are continuously fed on both sides. First, when the control unit 290 receives a job for double-sided feeding (S11), the control unit 290 checks the designation information specified in the received job (S12). Here, the designation information includes first designation information specifying the feeding unit, second designation information specifying the sheet media information, and third designation information specifying image formation on one or both sides of the sheet. In accordance with the checked designation information, the control unit 290 references the predetermined times Tb1, Tb2, and Tb3 and the predetermined times Td1, Td2, and Td3 stored in memory (S13), and image formation and feeding operations are initiated (S14). The predetermined times Tb1, Tb2, Tb3 and the predetermined times Td1, Td2, Td3 are correction times for correcting the time at which writing of latent images on the photosensitive drums 201b, 201c, and 201d starts, respectively.

[0118] Specifically, if the paper interval with the preceding paper exceeds a predetermined time (S15), it is confirmed from the aforementioned designation information whether the sheet is being fed from the re-feed path (S18). If the sheet is being fed from the re-feed path (S18), writing of the latent image of each color begins at a fourth time obtained by adding one of predetermined times Td1, Td2, and Td3 to the inter-drum time Ta (S19), and an image is formed. On the other hand, if the sheet is not being fed from the re-feed path (S18), that is, if the sheet is being fed from the first feed unit (cassette transport path), writing of the latent image of each color begins at a second time obtained by adding one of predetermined times Tb1, Tb2, and Tb3 to the inter-drum time Ta (S20), and an image is formed.

[0119] If there are no sheets P to be output continuously, the job is terminated (S17), and if there are, the process returns to S15, and the job is repeated until there are no sheets P to be output continuously.

[0120] According to this embodiment, when sheets are continuously fed, the timing at which the laser scanner starts writing an electrostatic latent image onto the photosensitive drum is set according to not only the media information of the sheets but also the information specifying the feeding unit. Therefore, even if the force acting on the secondary transfer unit 218 via sheets of the same thickness differs depending on the feeding unit, color misregistration can be reduced. [Explanation of symbols]

[0121] J…merging part M, R, S...Transport route P...Seat 201 ...Image forming device 201A...Image forming apparatus main body 201B...Image forming section 201C...Intermediate transfer unit 201D...Double-sided reversal section 201E...First feeding section 201F...Second feeding section 202 ...Image reader 210...Laser scanner 211 ... Process cartridge 212...Photosensitive drum 216...Intermediate transfer belt 216a ... drive roller 216b ... tension roller 217 ... Secondary transfer roller 218 ... Secondary transfer unit 241~244 ... Feeding cassette 245 ... Feeder tray 251~255 ...Feed roller 290...Control unit

Claims

1. An image forming apparatus, comprising: a first photoreceptor on which a first toner image is formed; a second photoconductor on which a second toner image of a different color from the first toner image formed on the first photoconductor is formed; an optical scanning device that forms a first electrostatic latent image on the first photosensitive member and a second electrostatic latent image on the second photosensitive member; a developing device that develops the first electrostatic latent image and the second electrostatic latent image with toner of each color to form the first toner image and the second toner image; an intermediate transfer member that rotates in a sub-scanning direction and onto which the first toner image and the second toner image are sequentially transferred so as to be superimposed on each other; a secondary transfer member that forms a transfer nip together with the intermediate transfer member and secondarily transfers the first toner image and the second toner image superimposed on the intermediate transfer member to a recording medium at the transfer nip; a first feeding unit that is disposed vertically below the optical scanning device and feeds a recording medium from a cassette that can store the recording medium toward the transfer nip; a second feeding unit disposed at a side of the image forming apparatus and configured to feed a recording medium from a manual feed tray into which the recording medium can be manually fed, toward the transfer nip; a storage unit that stores first designation information that designates the cassette or the manual feed tray, second designation information that designates media information of the recording medium, and third designation information that designates image formation on one side or both sides of the recording medium, The optical scanning device In the case of a job in which recording media are continuously fed and an image is formed on one side of the recording media, in order to align the positions in the sub-scanning direction of the first toner image and the second toner image which are primarily transferred to the intermediate transfer body for secondary transfer onto a first sheet of recording media, writing of the second electrostatic latent image is started after a first time has elapsed since writing of the first electrostatic latent image was started; In the case of a job in which recording media are continuously fed from the cassette and an image is formed on one side of the recording media, in order to align the positions in the sub-scanning direction of the first toner image and the second toner image which are primarily transferred to the intermediate transfer body for secondary transfer onto a second or subsequent recording medium, writing of the second electrostatic latent image begins after a second time has elapsed which is the first time from the start of writing of the first electrostatic latent image plus a first set time set in accordance with the first, second, and third designation information; In the case of a job in which recording media are continuously fed from the manual feed tray and an image is formed on one side of the recording media, in order to align the positions in the sub-scanning direction of the first toner image and the second toner image which are primarily transferred to the intermediate transfer body for secondary transfer onto the second and subsequent recording media, writing of the second electrostatic latent image begins after a third time has elapsed which is the first time from the start of writing of the first electrostatic latent image plus a second set time which is different from the first set time set in accordance with the first, second and third designation information. An image forming apparatus characterized by:

2. a third feeding unit that turns over the recording medium that has passed through the transfer nip and feeds it toward the transfer nip again; The optical scanning device In the case of a job in which recording media are continuously fed and images are formed on both sides of the recording media, in order to align the positions in the sub-scanning direction of the first toner image and the second toner image which are primarily transferred to the intermediate transfer body for secondary transfer onto the second or subsequent recording media from the third feed unit, writing of the second electrostatic latent image begins after a fourth time has elapsed, which is the first time from the start of writing of the first electrostatic latent image plus a third set time which is different from the first and second set times set in accordance with the first, second and third designation information.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. the first feeding section is disposed vertically below the transfer nip, the second feeding unit is disposed vertically above a junction where a second transport path for the recording medium from the manual feed tray to the transfer nip joins a first transport path for the recording medium from the cassette to the transfer nip, and vertically below the transfer nip; 3. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. 4. The image forming apparatus according to claim 1, wherein the first set time added to the first time is greater than the second set time.

5. 5. The image forming apparatus according to claim 1, wherein the media information of the recording medium includes at least one of basis weight, paper thickness, rigidity, and surface condition of the recording medium.

6. 5. The image forming apparatus according to claim 1, wherein the time at which the optical scanning device starts writing the second electrostatic latent image is changed when the basis weight of the recording medium is greater than a predetermined value, and the predetermined value when the cassette is specified is different from the predetermined value when the manual feed tray is specified.

Images