Image forming apparatus

Abstract

To suppress expansion and contraction of an image formed on a sheet in a conveying direction of the sheet, and to suppress damage to the sheet by a pair of registration rollers.SOLUTION: In a case where the image forming apparatus 1 conveys the sheet P whose basis weight is a first basis weight to the secondary transfer nip portion 37, the image forming apparatus 1 controls driving of the separation mechanism 112 to bring the registration roller pair 110 into the separated state, and in a case where the image forming apparatus 1 conveys the sheet P whose basis weight is a second basis weight larger than the first basis weight to the secondary transfer nip portion 37, the image forming apparatus 1 controls driving of the separation mechanism 112 to bring the registration roller pair 110 into the contact state.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus that forms an image on a sheet. [Background technology]

[0002] An electrophotographic image forming apparatus generally performs an image formation process in which a toner image is fixed onto a sheet of paper or other medium through the processes of charging, exposing, developing, transferring, and fixing. Meanwhile, an image forming apparatus using an intermediate transfer method performs an image formation process in which a toner image formed in an image forming unit is primarily transferred onto an intermediate transfer belt, and then the toner image that has been primarily transferred onto the intermediate transfer belt is secondarily transferred onto a sheet that has been fed and transported.

[0003] In such an image forming process, secondary transfer is performed in a secondary transfer nip formed by pressing an inner secondary transfer roller, which stretches the back surface of the intermediate transfer belt, from the front side of the intermediate transfer belt with an outer secondary transfer roller.

[0004] Patent Document 1 discloses a device that applies drive to an outer secondary transfer roller using a drive source such as a motor, thereby reducing transfer misalignment when a toner image is transferred onto a sheet.

[0005] In addition, for sheets transported to the secondary transfer nip portion, it is common to correct the skew of the sheet so that it is parallel to the secondary transfer nip portion using a pair of registration rollers (hereinafter referred to as the "registration roller pair") located immediately before the secondary transfer nip portion.

[0006] In the secondary transfer nip, a conveying force is applied to the sheet by the intermediate transfer belt onto which the toner image has been primarily transferred and the outer secondary transfer roller. During this process, the sheet conveyance speed may fluctuate due to changes in the conveyance force applied to the sheet from the intermediate transfer belt depending on the amount of toner in the toner image. Such fluctuations in the sheet conveyance speed manifest as image expansion and contraction (hereinafter referred to as "magnification fluctuation") in the sheet conveyance direction (the sub-scanning direction of the image). This phenomenon is more pronounced with thicker sheets that have a higher conveyance resistance, but it also occurs with thin sheets of 64 gsm or less. In response to this, the sheet conveyance speed can be stabilized by applying a conveyance force to the sheet from the pair of registration rollers in the secondary transfer nip.

[0007] Furthermore, as shown in Patent Document 2, if the sheet is significantly skewed due to variations in part precision or changes over time, the twist of the sheet that occurs when the leading edge of the sheet is abutted against the nip portion of the pair of registration rollers and the sheet is bent may become large. Conventionally, such a twisted portion of the sheet is stepped on or pressed against the pair of registration rollers, which causes a problem of damage to the sheet.

[0008] This problem is particularly likely to occur with very thin sheets of approximately 64 gsm or less. On the other hand, with relatively thick sheets of approximately 128 gsm or more, the twist described above is less likely to develop and damage to the sheet is less likely to occur. However, because the cost per sheet of thin sheets, which are more likely to be damaged, is low, it is expected that their use will increase in the future.

[0009] In response to this, a conventional method is known in which damage to the sheet is reduced by releasing the nip between the registration roller pair and the sheet immediately after the sheet is transferred from the registration roller pair to the secondary transfer nip portion, thereby releasing the twist of the sheet. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2022-71703 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-87089 Summary of the Invention [Problem to be solved by the invention]

[0011] However, in the conventional method of releasing the nip at the nip portion of the above-mentioned registration roller pair, the sheet cannot receive the conveying force from the registration roller pair, which causes the sheet conveying speed to become unstable and fluctuations in the image magnification occur.

[0012] An object of the present invention is to provide an image forming apparatus that can suppress expansion and contraction of an image formed on a sheet in the sheet transport direction and can suppress damage to the sheet caused by a pair of registration rollers. [Means for solving the problem]

[0013] The image forming apparatus according to the present invention includes an image forming unit that forms a toner image, a first roller member, and a second roller member that contacts the first roller member to form a first nip portion, and a pair of registration rollers that convey a sheet by nipping it in the first nip portion and correct skew of the sheet by causing a leading edge of the sheet in the conveying direction to contact the first nip portion, a separation mechanism that switches the pair of registration rollers between a contact state in which the first roller member and the second roller member contact each other to form the first nip portion and a separation state in which the first nip portion is separated, a belt that carries the toner image formed by the image forming unit, and a second roller member that contacts the belt to contact the belt. a transfer roller that forms a nip portion, and a transfer roller drive unit that drives the transfer roller to rotate, wherein the sheet that is transported after having its skew corrected by the registration roller pair passes through the second nip portion, thereby transferring a toner image to the sheet; and a control unit that controls the drive of the separation mechanism to bring the registration roller pair into the separated state when a sheet with a basis weight of a first basis weight is transported to the second nip portion, and controls the drive of the separation mechanism to bring the registration roller pair into the abutting state when a sheet with a basis weight of a second basis weight that is larger than the first basis weight is transported to the second nip portion. [Effects of the Invention]

[0014] According to the present invention, it is possible to suppress expansion and contraction of an image formed on a sheet in the sheet conveyance direction, and also to suppress damage to the sheet caused by the registration roller pair. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to an embodiment of the present invention. [Figure 2] 1 is a block diagram showing a configuration of an image forming apparatus according to an embodiment of the present invention; [Figure 3]1 is a plan view showing a state before skew of a sheet is corrected by a pair of registration rollers of an image forming apparatus according to an embodiment of the present invention; [Figure 4] 10 is a plan view showing a state after skew of a sheet is corrected by a pair of registration rollers of the image forming apparatus according to the embodiment of the present invention. FIG. [Figure 5] 10 is a plan view showing a state in which conveyance of the sheet is resumed by the pair of registration rollers of the image forming apparatus according to the embodiment of the present invention; FIG. [Figure 6] 1 is a schematic diagram of a portion of an image forming apparatus according to an embodiment of the present invention. [Figure 7] FIG. 4 is a flowchart showing the operation of the image forming apparatus according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments will be described in detail with reference to the drawings.

[0017] <Configuration of image forming device> The configuration of an image forming apparatus 1 according to an embodiment of the present invention will be described in detail with reference to FIGS.

[0018] The image forming apparatus 1 forms an image on a sheet P. The image forming apparatus 1 is exemplified here as a color image forming apparatus using an electrophotographic system.

[0019] Specifically, image forming apparatus 1 includes photoreceptors 11Y, 11M, 11C, and 11K, charging units 12Y, 12M, 12C, and 12K, exposure units 13Y, 13M, 13C, and 13K, and developing units 14Y, 14M, 14C, and 14K. Image forming apparatus 1 also includes an intermediate transfer belt 31, a drive roller 33, a tension roller 34, primary transfer units 35Y, 35M, 35C, and 35K, a secondary transfer unit 36, a secondary transfer external drive motor 39, and an air suction conveyance unit 42. Image forming apparatus 1 also includes a fixing roller pair 50, a post-fixing roller pair 53, a sheet storage 61, a sheet storage 62, a sheet storage 63, a manual feed unit 64, and a paper feed motor 65.

[0020] The image forming apparatus 1 also has a first reversing roller pair 90, a reversing roller pair 91, a double-sided conveying section 92, a discharge roller pair 93, a registration roller pair 110, a registration drive motor (hereinafter referred to as the "registration drive motor") 111, a separation mechanism 112, and a pre-registration roller pair (hereinafter referred to as the "pre-registration roller pair") 120. The image forming apparatus 1 also has a pre-registration drive motor (hereinafter referred to as the "pre-registration drive motor") 121, a conveying roller pair 130, a registration sensor (hereinafter referred to as the "registration sensor") 140, a control section 200, an operation section 203, and a reversing retraction section 300.

[0021] Here, the image forming apparatus 1 has an image forming unit that forms toner images. The image forming unit includes photoreceptors 11Y, 11M, 11C, and 11K, charging units 12Y, 12M, 12C, and 12K, exposure units 13Y, 13M, 13C, and 13K, and developing units 14Y, 14M, 14C, and 14K. The intermediate transfer belt 31, secondary transfer unit 36, and secondary transfer external drive motor 39 constitute a transfer unit. The fixing roller pair 50 and post-fixing roller pair 53 constitute an image fixing unit.

[0022] Photoconductors 11Y, 11M, 11C, and 11K are provided corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (Bk). Photoconductors 11Y, 11M, 11C, and 11K are rotated by drive motors (not shown) that are driven under the control of an image formation control unit 205 (described later) of the control unit 200.

[0023] Charging units 12Y, 12M, 12C, and 12K are provided corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (Bk). Under the control of image formation control unit 205 of control unit 200, charging units 12Y, 12M, 12C, and 12K uniformly charge the surfaces of photoconductors 11Y, 11M, 11C, and 11K.

[0024] Exposure units 13Y, 13M, 13C, and 13K are provided corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (Bk). Exposure units 13Y, 13M, 13C, and 13K are driven under the control of image formation control unit 205 of control unit 200 to form electrostatic latent images on the surfaces of photoconductors 11Y, 11M, 11C, and 11K charged by charging units 12Y, 12M, 12C, and 12K.

[0025] Developing units 14Y, 14M, 14C, and 14K are provided corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (Bk). Under the control of image formation control unit 205 of control unit 200, developing units 14Y, 14M, 14C, and 14K use toner to develop the electrostatic latent images formed on the surfaces of photoconductors 11Y, 11M, 11C, and 11K by exposure units 13Y, 13M, 13C, and 13K. Developing units 14Y, 14M, 14C, and 14K use toner to develop the electrostatic latent images into visible toner images, thereby forming the toner images on the surfaces of photoconductors 11Y, 11M, 11C, and 11K.

[0026] Intermediate transfer belt 31, which serves as a belt, is stretched by rollers such as a drive roller 33, a tension roller 34, and a secondary transfer inner roller 32 (described later) of secondary transfer unit 36, and rotates in the direction of arrow B in FIG. 1. A full-color toner image is formed on intermediate transfer belt 31 by primary transfer units 35Y, 35M, 35C, and 35K, in which the upstream color toner images formed on the surfaces of photoreceptors 11Y, 11M, 11C, and 11K are sequentially superimposed and primarily transferred onto the intermediate transfer belt 31. Intermediate transfer belt 31 carries the formed full-color toner image and transports it to secondary transfer unit 36.

[0027] The drive roller 33 stretches the intermediate transfer belt 31 together with a tension roller 34 and rollers such as the inner secondary transfer roller 32 of the secondary transfer section 36 .

[0028] The tension roller 34 stretches the intermediate transfer belt 31 together with the drive roller 33 and rollers such as the inner secondary transfer roller 32 of the secondary transfer section 36 .

[0029] Primary transfer units 35Y, 35M, 35C, and 35K are provided corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (Bk). Primary transfer units 35Y, 35M, 35C, and 35K apply a predetermined pressure and electrostatic bias to primarily transfer the toner images formed on the surfaces of photoreceptors 11Y, 11M, 11C, and 11K onto intermediate transfer belt 31.

[0030] The secondary transfer unit 36 ​​includes an inner secondary transfer roller 32 and an outer secondary transfer roller 38, which face each other, and serves as a transfer roller. The outer secondary transfer roller 38 comes into contact with the intermediate transfer belt 31, thereby forming a second nip portion, a secondary transfer nip portion 37. The secondary transfer unit 36 ​​applies a predetermined pressure and an electrostatic bias to the sheet P, which is conveyed by the registration roller pair 110 and passes through the secondary transfer nip portion 37, at the secondary transfer nip portion 37, thereby secondarily transferring the toner image formed on the intermediate transfer belt 31. The outer secondary transfer roller 38, which is driven to rotate by a driving force transmitted from an outer secondary transfer drive motor 39, sandwiches the sheet P, onto which the toner image has been secondarily transferred, together with the intermediate transfer belt 31 and conveys the sheet P to the air suction conveyance unit 42.

[0031] The secondary transfer outer drive motor 39, which serves as a transfer roller drive unit, is driven in accordance with an instruction signal input from the control unit 200 by a sheet conveyance control unit 206 (described later), thereby transmitting a driving force to the secondary transfer outer roller 38 and rotating the secondary transfer outer roller 38.

[0032] The air suction conveying section 42 conveys the sheet P conveyed by the secondary transfer section 36 to the fixing roller pair 50.

[0033] The fixing roller pair 50 includes a fixing roller 51 and a pressure roller 52. The fixing roller pair 50 applies pressure and heat to the sheet P conveyed by the air suction conveyance unit 42 using the fixing roller 51 and the pressure roller 52, thereby melting and fixing the toner image formed on the sheet P conveyed by the air suction conveyance unit 42 to the sheet P. The fixing roller pair 50 conveys the sheet P with the melted and fixed toner image to a post-fixing roller pair 53.

[0034] The post-fixing roller pair 53 conveys the sheet P conveyed by the fixing roller pair 50 to the first reversing roller pair 90 or the discharge roller pair 93 .

[0035] Each of the sheet storage cabinets 61, 62, and 63 stores sheets P according to size.

[0036] The sheet storage 61 includes a paper feed section 61a and a size detection mechanism 61d.

[0037] The paper feed unit 61a feeds the sheet P stored in the sheet storage 61 to the pair of conveying rollers 130.

[0038] The size detection mechanism 61d detects the size of the sheet P stored in the sheet storage case 61, and outputs an electric signal corresponding to the detected size of the sheet P to a sensor control unit 207 of the control unit 200, which will be described later.

[0039] The sheet storage 62 includes a paper feed section 62a and a size detection mechanism 62d.

[0040] The paper feed unit 62a feeds the sheet P stored in the sheet storage 62 to the pair of conveying rollers 130.

[0041] The size detection mechanism 62d detects the size of the sheet P stored in the sheet storage case 62, and outputs an electrical signal corresponding to the detected size of the sheet P to the sensor control unit 207 of the control unit 200.

[0042] The sheet storage 63 includes a paper feed section 63a and a size detection mechanism 63d.

[0043] The paper feed unit 63a feeds the sheet P stored in the sheet storage 63 to the pair of conveying rollers 130.

[0044] The size detection mechanism 63d detects the size of the sheet P stored in the sheet storage case 63, and outputs an electrical signal corresponding to the detected size of the sheet P to the sensor control unit 207 of the control unit 200.

[0045] Sheets P are stacked on the manual feed section 64. The manual feed section 64 includes a paper feed section 64a and a size detection mechanism 64d.

[0046] The sheet feeding section 64a feeds the sheets P stacked in the manual feeding section 64 to the pair of registration rollers 110.

[0047] The size detection mechanism 64d detects the size of the sheets P stacked on the manual feed unit 64, and outputs an electrical signal corresponding to the detected size of the sheets P to the sensor control unit 207 of the control unit 200.

[0048] The sheet feed motor 65 is driven in accordance with an instruction signal input from a sheet conveyance control unit 206 (described later) of the control unit 200. When driven, the sheet feed motor 65 transmits a driving force to the roller pair of the sheet feed unit 61 a, the sheet feed unit 62 a, the sheet feed unit 63 a, or the sheet feed unit 64 a, thereby rotating the roller pair of the sheet feed unit 61 a, the sheet feed unit 62 a, the sheet feed unit 63 a, or the sheet feed unit 64 a.

[0049] In the case of double-sided printing in which images are formed on both sides of the sheet P, the first reversing roller pair 90 conveys the sheet P, which has an image formed on its first side and is conveyed by the post-fixing roller pair 53, to the reversing roller pair 91.

[0050] The reversing roller pair 91 temporarily retracts the sheet P being conveyed by the first reversing roller pair 90 using the reversing retraction section 300, and then switches back and conveys the temporarily retracted sheet P to the double-sided conveying section 92. The reversing roller pair 91 switches the first and second sides of the sheet P being conveyed by the first reversing roller pair 90 by switching back and conveying the sheet P.

[0051] The double-sided conveying section 92 is connected to a pair of conveying rollers 130 located upstream of the secondary transfer section 36, and conveys the sheet P, which is switchback-conveyed by the pair of reversing rollers 91, to the pair of conveying rollers 130 again.

[0052] The discharge roller pair 93 discharges the sheet P conveyed by the post-fixing roller pair 53 to the outside of the image forming apparatus 1.

[0053] The registration roller pair 110 includes a registration drive roller (hereinafter referred to as the "registration drive roller") 113 as a first roller member, and a registration driven roller (hereinafter referred to as the "registration driven roller") 114 as a second roller member.

[0054] The registration drive roller 113 rotates when the registration drive motor 111 is driven, and stops rotating when the registration drive motor 111 stops driving.

[0055] The registration driven roller 114 can be moved toward or away from the registration drive roller 113 by being driven by the separating mechanism 112. The registration driven roller 114 comes into contact with the registration drive roller 113 to form a nip portion 115 (see FIG. 6) as a first nip portion between the registration driven roller 114 and the registration drive roller 113, and then moves away from the registration drive roller 113 to release the nip of the nip portion 115. The registration driven roller 114 is rotated by the registration drive roller 113.

[0056] The pair of registration rollers 110 is in a contact state when the registration driven roller 114 comes into contact with the registration drive roller 113, and in a separated state when the registration driven roller 114 separates from the registration drive roller 113.

[0057] In a drive-stop state in which the registration drive roller 113 has stopped rotating, the registration roller pair 110 brings the sheet P conveyed by the pre-registration roller pair 120 into contact with a nip portion 115 to correct the inclination (skew) of the sheet P with respect to the conveying direction A. After correcting the inclination (skew) of the sheet P with respect to the conveying direction A, the registration roller pair 110 makes the sheet P wait.

[0058] In the pair of registration rollers 110, the registration drive roller 113 starts rotating so that the timing at which the toner image formed on the intermediate transfer belt 31 is conveyed to the secondary transfer unit 36 ​​coincides with the timing at which the sheet P is conveyed to the secondary transfer unit 36. When the registration drive roller 113 starts rotating and enters a driving state, the pair of registration rollers 110 sandwiches the waiting sheet P in a nip portion 115 and conveys it to the secondary transfer unit 36.

[0059] The distance between the nip portion 115 of the pair of registration rollers 110 and the secondary transfer nip portion 37 of the secondary transfer portion 36 is shorter than the length of the sheet S of the minimum size in the conveying direction A.

[0060] The registration drive motor 111 is driven in accordance with an instruction signal input from the sheet conveyance control unit 206 of the control unit 200, thereby transmitting a driving force to the registration drive roller 113 of the registration roller pair 110 and causing the registration drive roller 113 to rotate.

[0061] The separation mechanism 112 is driven under the control of the separation control unit 208 of the control unit 200 to separate or contact the registration driven roller 114 with respect to the registration drive roller 113 of the registration roller pair 110. As a result, the separation mechanism 112 switches the registration roller pair 110 between a contact state in which a nip portion 115 is formed and a separation state in which the nip portion 115 is released and the registration rollers are separated. At this time, the separation mechanism 112 separates the registration driven roller 114 from the registration drive roller 113 at a timing according to the thickness of the sheet P.

[0062] The pre-registration roller pair 120 as upstream conveying rollers is provided facing the registration roller pair 110 on the upstream side of the registration roller pair 110 in the conveying direction A of the sheet P. The pre-registration roller pair 120 includes a pre-registration drive roller (hereinafter referred to as "pre-registration drive roller") 122 as a third roller member, and a pre-registration driven roller (hereinafter referred to as "pre-registration driven roller") 123 as a fourth roller member.

[0063] The pre-registration drive roller 122 rotates when the pre-registration drive motor 121 is driven, and stops rotating when the pre-registration drive motor 121 stops driving.

[0064] The pre-registration driven roller 123 contacts the pre-registration drive roller 122 to form a nip portion 124 (see FIG. 1 ) as a third nip portion between the pre-registration driven roller 123 and the pre-registration drive roller 122. The pre-registration driven roller 123 is rotated by the pre-registration drive roller 122. The pre-registration roller pair 120 rotates when the pre-registration drive roller 122 receives a driving force from the pre-registration drive motor 121, and conveys the sheet P conveyed by the conveying roller pair 130 to the registration roller pair 110.

[0065] The pre-registration drive motor 121 is driven in accordance with an instruction signal input from the sheet conveyance control unit 206 of the control unit 200, thereby transmitting a driving force to the pre-registration drive roller 122 and causing the pre-registration drive roller 122 to rotate.

[0066] The conveying roller pair 130 conveys the sheet P fed from the sheet storage case 61 , the sheet storage case 62 , or the sheet storage case 63 to the pre-registration roller pair 120 .

[0067] The registration sensor 140 is provided near the registration roller pair 110 between the registration roller pair 110 and the pre-registration roller pair 120 (see FIGS. 3 to 5). The registration sensor 140 detects the sheet P conveyed by the pre-registration roller pair 120, or the pre-registration roller pair 120 and the registration roller pair 110, under the control of a sensor control unit 207 (described later) of the control unit 200. The registration sensor 140 outputs an electric signal according to the detection result of the sheet P to the sensor control unit 207 of the control unit 200.

[0068] The control unit 200 controls the overall operation of the image forming apparatus 1. The control unit 200 includes a CPU (Central Processing Unit) 201, a memory 202, an image formation control unit 205, a sheet conveyance control unit 206, a sensor control unit 207, and a separation control unit 208.

[0069] The CPU 201 executes predetermined control programs stored in the memory 202 based on electrical signals input from the operation unit 203, thereby controlling the operation of the image formation control unit 205, the sheet conveyance control unit 206, or the sensor control unit 207 and performing various processes. The CPU 201 controls the operation of the separation control unit 208 based on sheet information indicated by the electrical signals input from the operation unit 203 and the detection result of the registration sensor 140 acquired by the sensor control unit 207. The CPU 201 may receive sheet information from a computer 204 connected via a network, and control the operation of the separation control unit 208 based on the received sheet information.

[0070] The CPU 201 controls the operation of the separation control unit 208 to place the registration roller pair 110 in a separated state or a contact state depending on the basis weight of the sheet P conveyed to the secondary transfer nip portion 37. Specifically, when a thin sheet P having a basis weight less than a predetermined value is conveyed to the secondary transfer nip portion 37, the CPU 201 controls the operation of the separation control unit 208 to switch the registration roller pair 110 from a contact state to the separated state when the leading edge of the sheet P in the conveyance direction passes through the registration roller pair 110. Furthermore, when a thick sheet P having a basis weight equal to or greater than a predetermined value is conveyed to the secondary transfer nip portion 37, the CPU 201 controls the operation of the separation control unit 208 to switch the registration roller pair 110 from a contact state to the separated state when the trailing edge of the sheet P in the conveyance direction passes through the registration roller pair 110.

[0071] The memory 202 is a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores various programs and various data in a predetermined storage area.

[0072] The image forming control unit 205 outputs an instruction signal to the image forming unit under the control of the CPU 201, and causes the image forming unit to perform an operation according to the instruction of the instruction signal.

[0073] The sheet transport control unit 206 outputs instruction signals to the paper feed motor 65, the pre-registration drive motor 121, the registration drive motor 111, or the secondary transfer outer drive motor 39 under the control of the CPU 201 to control the transport of the sheet P.

[0074] The sensor control unit 207 controls the operation of the size detection mechanism 61d, the size detection mechanism 62d, the size detection mechanism 63d, the size detection mechanism 64d, or the registration sensor 140. The sensor control unit 207 controls the operation of each detection mechanism or sensor, thereby causing the size detection mechanism 61d, the size detection mechanism 62d, the size detection mechanism 63d, the size detection mechanism 64d, or the registration sensor 140 to start or stop detection. The sensor control unit 207 outputs the detection result indicated by the electrical signal input from the size detection mechanism 61d, the size detection mechanism 62d, the size detection mechanism 63d, the size detection mechanism 64d, or the registration sensor 140 to the CPU 201.

[0075] The separation control unit 208 controls the driving of the separation mechanism 112 under the control of the CPU 201 .

[0076] The operation unit 203 receives input operations by the user, such as sheet information of a sheet on which an image is to be formed or an instruction to execute printing or interrupt printing, and outputs an electrical signal to the CPU 201 according to the received input operations.

[0077] The reversing and retracting section 300 is used when the sheet P being conveyed by the first reversing roller pair 90 is temporarily retracted.

[0078] In the image forming apparatus 1 having the above configuration, the image forming units execute image forming processes for the respective colors Y, M, C, and Bk in parallel. The image forming units also execute the image forming processes at the timing when the toner images are primarily transferred and superimposed on the same position on the intermediate transfer belt 31 in order from the upstream color.

[0079] Furthermore, the sheet P on which an image is formed only on its first side is fed from the paper feed unit 61a, the paper feed unit 62a, or the paper feed unit 63a, passes through the pre-registration roller pair 120, is conveyed to the registration roller pair 110, and is then discharged to the outside by the discharge roller pair 93. Alternatively, the sheet P is fed from the paper feed unit 64a, is conveyed to the registration roller pair 110, and is then discharged to the outside by the discharge roller pair 93.

[0080] Furthermore, after the image is formed on the first side of the sheet P, which has an image formed on both its first and second sides, the sheet P is switchback-conveyed by the reversing roller pair 91, and is again conveyed by the reversing roller pair 91 and the double-sided conveying section 92 toward the conveying roller pair 130. The sheet P conveyed toward the conveying roller pair 130 then passes through the secondary transfer section 36 and the fixing roller pair 50, whereupon an image is formed on the second side, and is then discharged to the outside by the discharge roller pair 93.

[0081] <Skew Correction Operation of Registration Roller Pair> The skew correction operation of the pair of registration rollers 110 of the image forming apparatus 1 according to the embodiment of the present invention will be described in detail with reference to FIGS.

[0082] As shown in FIG. 3, the leading edge of the sheet P, which has passed through the pre-registration roller pair 120 and been conveyed to the registration roller pair 110, in the conveying direction A abuts against the registration roller pair 110 which has stopped rotating.

[0083] Next, the pre-registration roller pair 120 conveys the sheet P, slightly pushing it toward the registration roller pair 110. As a result, a flexure (loop) occurs in the sheet P between the pre-registration roller pair 120 and the registration roller pair 110. The sheet P, which is inclined (skewed) with respect to the conveying direction A when conveyed to the registration roller pair 110, absorbs the skew by generating a difference in the size of the loop in the direction perpendicular to the conveying direction A. As a result, the skew generated in the sheet P is corrected by the leading edge of the sheet P in the conveying direction A moving along the nip portion 115 of the registration roller pair 110, as shown in FIG.

[0084] Next, as shown in FIG. 5, the sheet P whose skew has been corrected is conveyed to the secondary transfer section 36 when the registration drive roller 113 of the registration roller pair 110 starts to rotate.

[0085] Here, a twist occurs in the sheet P due to the difference in the size of the loop in the direction perpendicular to the conveying direction A. If this twist becomes too large compared to the stiffness (rigidity) of the sheet P, when the sheet P is conveyed by the pair of registration rollers 110, the twisted portion of the sheet P is stepped on by the pair of registration rollers 110, causing damage such as wrinkles to the sheet P. In particular, with ultra-thin paper, the stiffness of the sheet P is weak, so even a slight twist can cause damage.

[0086] When a torsion of a certain thickness or more occurs in a sheet, the reaction force exerted by the sheet P on the pair of pre-registration rollers 120 increases. As a result, the pair of pre-registration rollers 120 slides relative to the sheet P, so that the torsion of the sheet P is unlikely to increase and damage to the sheet P is unlikely to occur.

[0087] <Separating Operation of the Registration Roller Pair and Effects of Performing the Separating Operation> The separation operation of the pair of registration rollers 110 of the image forming apparatus 1 according to the embodiment of the present invention and the effect of performing the separation operation will be described in detail with reference to FIG.

[0088] The pair of registration rollers 110 releases the nip at the nip portion 115 by causing the registration driven roller 114 to move away from the registration drive roller 113 by a distance equal to or greater than the thickness of the sheet P, using a separation mechanism 112 driven under the control of a separation control unit 208. Here, the thickness of the sheet P can be expressed by the basis weight of the sheet P.

[0089] Specifically, immediately after the leading edge of thin sheet P, which is less than a predetermined thickness and is susceptible to damage, in the conveying direction A enters secondary transfer nip portion 37, registration roller pair 110 releases the nip of nip portion 115 and releases pressure applied to sheet P by nip portion 115. Here, a thin sheet P having a thickness less than a predetermined thickness is exemplified as a sheet P having a basis weight of less than 64 gsm (grams per square meter). This allows the twist of sheet P to be released before the twisted portion of sheet P is stepped on by registration roller pair 110, thereby suppressing damage to sheet P.

[0090] At this time, the thin sheet P does not receive a conveying force from the registration roller pair 110, but receives a conveying force from the outer secondary transfer roller 38, so the conveying speed can be stabilized. This makes it possible to suppress fluctuations in the magnification of the image formed on the thin sheet P.

[0091] On the other hand, for a sheet P that is thicker than a thin sheet P whose thickness is less than a predetermined value, the contact force with the guide that supports the sheet P is large, and the conveying resistance is large. Therefore, for a thick sheet P, if the conveying force is received only by the outer secondary transfer roller 38, the conveying force may be insufficient and the conveying speed may become unstable. Here, the thick sheet P is exemplified as a sheet P with a basis weight of 128 gsm or more.

[0092] On the other hand, for thick sheets P, twisting due to skew correction is unlikely to occur, so the timing for releasing the nip portion 115 of the registration roller pair 110 is delayed compared to that for thin sheets P. Therefore, for thick sheets P, the nip portion 115 remains substantially sandwiched, and the conveying force is received from both the outer secondary transfer roller 38 and the registration roller pair 110, so the conveying speed can be stabilized. This makes it possible to suppress fluctuations in the magnification of images formed on thick sheets P.

[0093] Here, the variation in image magnification caused by fluctuations in the conveying speed of the sheet P in the secondary transfer nip portion 37 occurs due to differences in the thickness of the sheet P, the surface properties of the sheet P, and the toner density on the intermediate transfer belt 31. The variation in image magnification also occurs when the conveying speed of the sheet P changes due to changes in the conveying force that the sheet P receives from the intermediate transfer belt 31, causing a change in the difference between the speed of the intermediate transfer belt 31 (toner image) and the conveying speed of the sheet P.

[0094] In order to suppress such fluctuations in the conveying speed of the sheet P relative to the intermediate transfer belt 31, the image forming apparatus 1 rotates the outer secondary transfer roller 38 using the outer secondary transfer drive motor 39. As a result, the conveying force that the sheet P receives at the secondary transfer nip portion 37 is increased because the sheet P receives the force not only from the intermediate transfer belt 31 but also from the outer secondary transfer roller 38, and the conveying speed of the sheet P is stabilized. In particular, a thin sheet P can receive a sufficient conveying force because the conveying resistance is small.

[0095] <Operation of image forming device> The operation of the image forming apparatus 1 according to the embodiment of the present invention will be described in detail with reference to FIG.

[0096] 7 when a printing execution command is received in an electrical signal input from the operation unit 203 or a received signal received from the computer 204. At this time, the CPU 201 acquires printing information such as the number of copies to be printed or sheet information in the electrical signal input from the operation unit 203 or the received signal received from the computer 204. The CPU 201 also acquires information on the size of the sheets P stored or stacked in the sheet storage cabinet 61, the sheet storage cabinet 62, the sheet storage cabinet 63, or the manual feed unit 64 from the sensor control unit 207.

[0097] First, the CPU 201 starts a print job (S101).

[0098] Next, the CPU 201 controls the operation of the sheet conveyance control unit 206 to rotate the roller pairs of the sheet feeding unit 61a, 62a, 63a, or 64a that stores or stacks the sheet P of the size of the sheet information, thereby starting to feed the sheet P (S102). As a result, the fed sheet P is conveyed to the pre-registration roller pair 120.

[0099] Next, the CPU 201 acquires the detection result of the registration sensor 140 detecting the leading edge of the sheet P in the conveying direction A from the sensor control unit 207 (S103).

[0100] Next, the CPU 201 controls the operation of the sheet conveyance control unit 206 to stop the driving of the registration drive motor 111 and stop the rotation of the registration drive roller 113 of the registration roller pair 110. By stopping the rotation of the registration drive roller 113, the CPU 201 causes the sheet P to abut against the nip portion 115 of the registration roller pair 110 (S104).

[0101] The sheet P that hits the nip portion 115 is conveyed by a feed amount set by the pre-registration roller pair 120. As a result, the registration roller pair 110 can cause the sheet P, which is in the state shown in FIG. 3 and has rotated, for example, clockwise with respect to the conveying direction A, to bend by a predetermined amount as shown in FIG. 4, thereby correcting the skew of the sheet P.

[0102] Next, the CPU 201 controls the operation of the sheet conveyance control unit 206 to drive the registration drive motor 111. By driving the registration drive motor 111, the CPU 201 transmits the driving force of the registration drive motor 111 to the registration drive roller 113, and the pair of registration rollers 110 starts conveying the sheet P (S105). As a result, the sheet P is conveyed from the pair of registration rollers 110 to the secondary transfer unit 36.

[0103] Next, the CPU 201 controls the operation of the image formation control unit 205 to cause the secondary transfer unit 36 ​​to transfer the toner image formed on the intermediate transfer belt 31 onto the sheet P (S106).

[0104] Next, the CPU 201 determines whether the sheet P is thin or not based on the sheet information of the sheet P acquired from the operation unit 203 or the computer 204 (S107). Specifically, the CPU 201 determines whether the basis weight of the sheet P is less than a predetermined value. For example, the CPU 201 determines whether the basis weight of the sheet P is less than 64 gsm.

[0105] If the sheet P is thin (step S107: Yes), the CPU 201 determines whether the leading edge of the sheet P in the conveying direction A has passed the secondary transfer unit 36 ​​(S108). Specifically, if the basis weight of the sheet P is a first basis weight that is less than a predetermined value, the CPU 201 determines that the sheet P is a thin sheet P, and determines whether the leading edge of the sheet P in the conveying direction A has passed the secondary transfer unit 36. The memory 202 also stores in advance time information related to a predetermined time from when the registration sensor 140 detects the leading edge of the sheet P in the conveying direction A until the leading edge of the sheet P in the conveying direction A passes the secondary transfer unit 36. The CPU 201 determines whether a predetermined time has passed from when the registration sensor 140 detects the leading edge of the sheet P in the conveying direction A by referring to the time information stored in the memory 202.

[0106] The CPU 201 determines whether a predetermined time has elapsed, thereby determining whether the leading edge of the sheet P in the conveying direction A has passed through the secondary transfer unit 36. The predetermined time can be obtained in advance from a preset conveying speed and a distance between the registration sensor 140 and the secondary transfer nip portion 37 of the secondary transfer unit 36.

[0107] If the leading edge of the sheet P in the conveying direction A has not passed through the secondary transfer unit 36 ​​(step S108: No), the CPU 201 repeats the process of step S108.

[0108] On the other hand, when the leading edge of the sheet P in the conveying direction A has passed the secondary transfer unit 36 ​​(step S108: Yes), the CPU 201 controls the operation of the separation control unit 208 to drive the separation mechanism 112. By driving the separation mechanism 112, the CPU 201 separates the registration driven roller 114 from the registration drive roller 113 (S109). This allows the nip of the nip portion 115 of the registration roller pair 110 with respect to the sheet P to be released, and the twist generated in the sheet P between the registration roller pair 110 and the pre-registration roller pair 120 to be released.

[0109] Next, the CPU 201 determines whether the trailing edge of the sheet P in the conveying direction A has passed through the nip portion 115 of the pair of registration rollers 110 based on the detection result of the registration sensor 140 acquired from the sensor control unit 207 (S110). Specifically, the memory 202 stores in advance time information regarding a predetermined time from when the registration sensor 140 detects the trailing edge of the sheet P in the conveying direction A until when the trailing edge of the sheet P in the conveying direction A passes through the pair of registration rollers 110.

[0110] The CPU 201 determines whether a predetermined time has elapsed since the registration sensor 140 detected the trailing edge of the sheet P in the conveying direction A by referring to the time information stored in the memory 202. The CPU 201 determines whether the trailing edge of the sheet P in the conveying direction A has passed the registration roller pair 110 by determining whether a predetermined time has elapsed since the registration sensor 140 detected the trailing edge of the sheet P in the conveying direction A. The predetermined time can be calculated in advance from a preset conveying speed and a distance between the registration sensor 140 and the nip portion 115 of the registration roller pair 110.

[0111] If the trailing edge of the sheet P in the conveying direction A has not passed through the nip portion 115 (step S110: No), the CPU 201 repeats the process of step S110.

[0112] On the other hand, when the trailing edge of the sheet P in the conveying direction A passes through the nip portion 115 (step S110: Yes), the CPU 201 controls the operation of the separation control unit 208 to drive the separation mechanism 112. By driving the separation mechanism 112, the CPU 201 brings the registration driven roller 114 into contact with the registration drive roller 113, thereby releasing the separation between the registration drive roller 113 and the registration driven roller 114 (S111).

[0113] Next, the CPU 201 controls the operation of the image formation control unit 205 to fix the toner image secondarily transferred onto the sheet P in the secondary transfer unit 36 ​​onto the sheet P by the fixing roller pair 50 (S112).

[0114] Next, the CPU 201 determines whether or not it is a double-sided job and the second side is blank (S113).

[0115] If the job is a single-sided job or a double-sided job with printing on both sides (step S113: No), the CPU 201 controls the operation of the sheet conveying control unit 206 to discharge the sheet P with the fixed toner image outside the image forming apparatus 1 using the discharge roller pair 93 (S114).

[0116] Next, the CPU 201 ends the print job (S115) and terminates the operation.

[0117] On the other hand, in the operation of step S113, if it is a double-sided job and the second side is blank (step S113: Yes), the CPU 201 controls the operation of the sheet conveyance control unit 206 to cause the reversing roller pair 91 to switchback convey the sheet P. Furthermore, the CPU 201 causes the double-sided conveyance unit 92 to convey the sheet P to the pre-registration roller pair 120 (S116). Thereafter, the CPU 201 returns to the operation of step S103.

[0118] Furthermore, in the operation of step S107, if the sheet is a thick sheet (step S107: No), the CPU 201 acquires the detection result of the registration sensor 140 from the sensor control unit 207. Specifically, if the basis weight of the sheet P is a second basis weight that is greater than a first basis weight that is equal to or greater than a predetermined value, the CPU 201 determines that the sheet P is a thick sheet P, and acquires the detection result of the registration sensor 140 from the sensor control unit 207. Then, the CPU 201 determines whether the trailing end of the sheet S in the conveying direction A has been detected based on the acquired detection result of the registration sensor 140 (S117).

[0119] When the CPU 201 does not detect the rear end of the sheet S in the conveying direction A (step S117: No), the CPU 201 repeats the operation of step S117.

[0120] On the other hand, when the CPU 201 detects the rear end of the sheet S in the conveying direction A (step S117: Yes), the CPU 201 controls the operation of the separation control unit 208 to drive the separation mechanism 112. By driving the separation mechanism 112, the CPU 201 separates the registration driven roller 114 from the registration drive roller 113 (S118).

[0121] In this way, when a thick sheet P is conveyed, the pair of registration rollers 110 applies a conveying force to the sheet P up to the vicinity of the trailing end of the sheet P in the conveying direction A, thereby stabilizing the conveying speed of the sheet P. Furthermore, when the trailing end of the sheet P in the conveying direction A passes through the nip portion 115 while the sheet P is sandwiched between the pair of registration rollers 110, a sudden speed fluctuation may occur in the sheet P. In response to this, by releasing the nip of the nip portion 115 immediately before the trailing end of the sheet P in the conveying direction A passes through the nip portion 115, it is possible to suppress the sudden speed fluctuation of the sheet P.

[0122] Next, the CPU 201 determines whether the trailing edge of the sheet P in the conveying direction A has passed through the nip portion 115 of the pair of registration rollers 110 based on the detection result of the registration sensor 140 obtained from the sensor control unit 207 (S119).

[0123] If the trailing edge of the sheet P in the conveying direction A has not passed through the nip portion 115 (step S119: No), the CPU 201 repeats the process of step S119.

[0124] On the other hand, when the trailing edge of the sheet P in the conveying direction A has passed through the nip portion 115 (step S119: Yes), the CPU 201 controls the operation of the separation control unit 208 to drive the separation mechanism 112. By driving the separation mechanism 112, the CPU 201 brings the registration driven roller 114 into contact with the registration drive roller 113, thereby releasing the separation between the registration drive roller 113 and the registration driven roller 114 (S120). Thereafter, the CPU 201 performs the operations from step S112 onwards.

[0125] In this way, the timing at which the registration driven roller 114 separates from the registration drive roller 113 is slower for a thick sheet P than for a thin sheet P. This makes it possible to suppress both fluctuations in the conveying speed and damage to the sheet P for both a thin sheet P that has low conveying resistance and is prone to damage, and a thick sheet P that has high conveying resistance and is less likely to be damaged.

[0126] In the above operation, when conveying a thin sheet P, the rotation speed of the pre-registration roller pair 120 arranged upstream of the registration roller pair 110 may be increased in parallel with the operation of releasing the nip at the nip portion 115 of the registration roller pair 110. Specifically, in parallel with the operation of step S109, the CPU 201 controls the operation of the sheet conveyance control unit 206 to increase the rotation speed of the pre-registration roller pair 120 compared to before releasing the nip at the nip portion 115. This makes it possible to compensate for the decrease in conveying force caused by releasing the nip at the nip portion 115.

[0127] Furthermore, when conveyance by the registration roller pair 110 is started, the nip portion 124 of the pre-registration roller pair 120 upstream of the registration roller pair 110 in the conveyance direction A of the sheet P may be released. Specifically, in parallel with the operation of step S109, the CPU 201 releases the nip portion 124 of the pre-registration roller pair 120 by a separation mechanism that moves the pre-registration drive roller 122 and the pre-registration driven roller 123 toward and away from each other. This makes it possible to widen the space in which the twist of the sheet P is released, and to release the twist caused by a larger skew of the sheet P.

[0128] In the above operation, if a sudden speed change does not occur in the sheet P when the trailing end of the sheet P in the conveying direction A passes through the nip portion 115 while the sheet P is being sandwiched by the nip portion 115, the operations from step S117 to step S120 do not have to be performed. In this case, the operations from step S107 to step S112 are skipped.

[0129] Furthermore, in the operation of step S107 described above, all sheets other than thin sheets P are determined to be thick sheets P, but this is not limiting, and a sheet P may be determined to be a thick sheet if its thickness is equal to or greater than a predetermined value. For example, if the CPU 201 determines in the operation of step S107 that the basis weight of the sheet P is equal to or greater than 64 gsm as a first basis weight, the CPU 201 may further determine whether the basis weight of the sheet P is equal to or greater than 128 gsm as a second basis weight that is greater than the first basis weight, and determine that a sheet P with a basis weight of 128 gsm or greater is a thick sheet P. If the CPU 201 determines that the sheet is a thick sheet, the CPU 201 performs the operations of steps S117 to S120.

[0130] In the present embodiment, when a sheet P having a first basis weight is conveyed to the secondary transfer nip portion 37, the separation mechanism 112 is controlled to separate the pair of registration rollers 110. When a sheet P having a second basis weight greater than the first basis weight is conveyed to the secondary transfer nip portion 37, the separation mechanism 112 is controlled to bring the pair of registration rollers 110 into a contact state. This makes it possible to suppress expansion and contraction of the image formed on the sheet P in the conveying direction A of the sheet P, and also to suppress damage to the sheet P caused by the pair of registration rollers 110.

[0131] The present invention is not limited to the above-described embodiment, and it goes without saying that various modifications are possible without departing from the spirit and scope of the present invention.

[0132] Specifically, in the above embodiment, a sheet P having a basis weight of less than 64 gsm is defined as a thin sheet P, but this is not limiting, and a sheet P having a basis weight of less than a value other than 64 gsm may also be defined as a thin sheet P.

[0133] In addition, in the above embodiment, a sheet P having a basis weight of 64 gsm or more or 128 gsm or more is defined as a thick sheet P, but this is not limited to this, and a sheet P having a basis weight of a value other than 64 gsm or 128 gsm or more may also be defined as a thick sheet P. [Explanation of symbols]

[0134] 1. Image forming device 11Y photoreceptor 11M photoreceptor 11C Photoreceptor 11K photoreceptor 31 Intermediate transfer belt 32 Secondary transfer inner roller 36 Secondary transfer unit 37 Secondary transfer nip 38 Secondary transfer outer roller 39 Secondary transfer outer drive motor 110 registration roller pair 111 Resist drive motor 112 Separation mechanism 113 Registration drive roller 114 Register driven roller 115 Nip section 120 Pre-registration roller pair 121 Pre-registered drive motor 140 Resist Sensor 200 control section 203 Operation section

Claims

1. an image forming unit that forms a toner image; a registration roller pair including a first roller member and a second roller member that contacts the first roller member to form a first nip portion, and that conveys a sheet by sandwiching it in the first nip portion, and corrects skew of the sheet by causing a leading edge of the sheet in the conveying direction to abut against the first nip portion; a separation mechanism that switches the pair of registration rollers between a contact state in which the first roller member and the second roller member contact each other to form the first nip portion and a separation state in which the first nip portion is separated; a transfer section including a belt that carries a toner image formed by the image forming section, a transfer roller that contacts the belt to form a second nip section, and a transfer roller drive section that drives to rotate the transfer roller, and transfers the toner image onto the sheet as the sheet that is conveyed after skew correction by the registration roller pair passes through the second nip section; a control unit that controls the driving of the separation mechanism to bring the pair of registration rollers into the separated state when a sheet having a first basis weight is conveyed to the second nip portion, and controls the driving of the separation mechanism to bring the pair of registration rollers into the contact state when a sheet having a second basis weight greater than the first basis weight is conveyed to the second nip portion; An image forming apparatus comprising:

2. The control unit when a sheet having the second basis weight is conveyed to the second nip portion, when a trailing end of the sheet in a conveying direction passes through the pair of registration rollers, driving of the separation mechanism is controlled to switch the pair of registration rollers from the contact state to the separation state.

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

3. an upstream conveying roller that is provided upstream of the pair of registration rollers in the conveying direction and conveys the sheet; The control unit When the first nip portion is separated, the sheet conveying speed by the upstream conveying roller is increased compared to when the first nip portion is not separated.

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

4. a third roller member and a fourth roller member that contacts the third roller member to form a third nip portion, and an upstream conveying roller that is provided opposite the registration roller pair in the conveying direction and that conveys the sheet by nipping it in the third nip portion, and that separates the third nip portion when the first nip portion is separated; 3. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

Images