Image forming device

Abstract

To provide an image forming apparatus that can suppress image distortions caused by lateral differences of loop amounts after skew corrections.SOLUTION: An image forming apparatus comprises: an image forming part forming images on a recording material; a registration part having a pair of registration rollers transporting the recording material to the image forming part; a pair of upstream rollers transporting the recording material to the pair of registration rollers; a driving device driving the pair of registration rollers and the pair of upstream rollers; and control means controlling the driving device. The control means is configured to control the driving device to: (i) stop the driving of the pair of upstream rollers after a tip end of the recording material passes the pair of registration rollers and before intrusion into the image forming part; and (ii) restart driving the upstream rollers before the tip end of the recording material intrudes into the image forming part, and is configured so that the pair of registration rollers transports the recording material so as to cause the recording material to slip with respect to the pair of upstream rollers while driving of the pair of upstream rollers is stopped.SELECTED DRAWING: Figure 17

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus for forming an image on a recording material. [Background technology]

[0002] In an image forming apparatus such as a printer, a copier, or a multifunction machine, recording materials (recording media) loaded in a loading section such as a feeding cassette are fed one sheet at a time, conveyed to an image forming section, and discharged outside the apparatus as a finished product after image formation. In the process of conveying the recording material from the loading section to the image forming section, a registration mechanism performs an operation of aligning the leading edge of the recording material with the image formed by the image forming section.

[0003] As a registration mechanism, a so-called leading edge abutting registration mechanism is known, which corrects the skew of the recording material by abutting the leading edge of the recording material against the nip of the registration roller pair in a stopped state or against a shutter member provided immediately before the nip. In the leading edge abutting registration mechanism, after the leading edge of the recording material abuts against the nip of the registration roller pair or the shutter member, the upstream roller pair further conveys the recording material a predetermined amount to bend the recording material (to form a loop). As a result, the leading edge of the recording material is aligned by the nip or the shutter member, so that the skew of the recording material is corrected. Patent Document 1 describes that the loop amount is maintained within a certain range by controlling the conveying speed of each roller pair after conveyance is resumed by a loop roller pair that abuts the leading edge of the paper against the nip of the registration roller pair to form a loop. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-108145 A Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, when a recording material conveyed in a skewed state is corrected by a registration section using a leading edge abutting method, a difference in the amount of loop after the skew correction occurs between one side end and the other side end of the recording material (hereinafter, referred to as the left-right difference in the amount of loop). In addition, not only during skew correction, but also due to misalignment of the conveying rollers constituting the conveying path until the recording material reaches the registration section, or due to non-uniformity in the roller diameter, the left-right difference in the amount of loop of the recording material may occur. When there is a left-right difference in the amount of loop of the recording material between the registration roller pair and the upstream roller pair, the elastic force of the recording material causes a difference in the conveying amount of the recording material between one side and the other side in the width direction of the recording material at the registration roller pair. As a result, there is a possibility that the image formed on the recording material by the image forming section may be distorted.

[0006] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS An object of the present invention is to provide an image forming apparatus capable of suppressing image distortion caused by the difference between the left and right loop amounts after skew correction. [Means for solving the problem]

[0007] One aspect of the present invention includes an image forming unit that forms an image on a recording material, a registration unit located upstream of the image forming unit in a transport direction of the recording material and having a pair of registration rollers that transport the recording material to the image forming unit, an upstream roller pair located upstream of the registration unit in the transport direction and that transports the recording material to the registration roller pair, a drive device that drives the registration roller pair and the upstream roller pair, and a control means that controls the drive device. a reversing means for reversing and conveying the recording material, on whose first surface an image has been transferred in the image forming section, in order to transfer an image to a second surface opposite to the first surface; and a double-sided conveying path along which the recording material reversed by the reversing means is conveyed again toward the registration section; Equipped with the upstream roller pair is disposed on the double-sided conveying path, The control means (i) stops driving the pair of upstream rollers after the leading edge of the recording material has passed the pair of registration rollers and before the leading edge of the recording material enters the image forming unit, and (ii) stops driving the pair of upstream rollers before the leading edge of the recording material enters the image forming unit. versusand a registration roller pair is configured to transport the recording material so that the recording material slips relative to the upstream roller pair during a period in which the driving of the upstream roller pair is stopped. Another aspect of the present invention is a recording medium recording apparatus including an image forming unit that forms an image on a recording material, a registration unit that is located upstream of the image forming unit in a conveying direction of the recording material and has a registration roller pair that conveys the recording material to the image forming unit, an upstream roller pair that is located upstream of the registration unit in the conveying direction and conveys the recording material to the registration roller pair, a drive device that drives the registration roller pair and the upstream roller pair, and a control means that controls the drive device, the drive device including a motor that supplies a drive force for driving the registration roller pair and the upstream roller pair, and a control means that is disposed between the motor and the upstream roller pair and drives the motor and the and a clutch that connects and disconnects the drive transmission to and from the upstream roller pair, wherein the control means is configured to control the drive device so as to (i) stop the drive of the upstream roller pair by changing the clutch from a connected state to a disconnected state after the leading edge of the recording material has passed the registration roller pair and before it enters the image forming section, and (ii) resume the drive of the upstream roller pair before the leading edge of the recording material enters the image forming section, and the registration roller pair is configured to transport the recording material so that the recording material slips relative to the upstream roller pair during the period when the drive of the upstream roller pair is stopped. Another aspect of the present invention is a recording medium recording apparatus including an image forming unit that forms an image on a recording material, a registration unit located upstream of the image forming unit in a conveying direction of the recording material and having a registration roller pair that conveys the recording material to the image forming unit, an upstream roller pair located upstream of the registration unit in the conveying direction and conveying the recording material to the registration roller pair, a drive device that drives the registration roller pair and the upstream roller pair, and a control means for controlling the drive device, wherein the registration unit has an abutment portion against which a leading edge of the recording material abuts, and the control means controls the upstream roller pair to convey the recording material so that the leading edge of the recording material abuts against the abutment portion, and the abutment portion between the upstream roller pair and the upstream roller pair is controlled by the control means. and (ii) controlling the drive device to stop driving the upstream roller pair after the leading edge of the recording material has passed through the registration roller pair and before the leading edge of the recording material enters the image forming unit, and (iii) restarting driving the upstream roller pair before the leading edge of the recording material enters the image forming unit, and the registration roller pair is configured to transport the recording material such that the recording material slips relative to the upstream roller pair during a period in which driving of the upstream roller pair is stopped.

[0008] According to another aspect of the present invention, there is provided an image forming unit for forming an image on a recording material, and a recording medium conveying unit for conveying the recording material, the recording medium conveying unit being disposed upstream of the image forming unit in a conveying direction of the recording material. The above the recording material is conveyed to an image forming unit by a registration unit having a pair of registration rollers which convey the recording material to an image forming unit and an abutment unit against which the leading edge of the recording material abuts; an upstream roller pair which is located upstream of the registration unit in the conveying direction and conveys the recording material to the registration roller pair; a drive device which drives the registration roller pair and the upstream roller pair; and a control means which controls the drive device, wherein the control means causes the upstream roller pair to convey the recording material in a state in which the leading edge of the recording material abuts against the abutment unit, and bends the recording material between the upstream roller pair and the abutment unit to correct skew of the leading edge of the recording material, and then starts conveying the recording material by the registration roller pair, and the control means (i) adjusts the position of the recording material by adjusting ... The above (ii) stopping the driving of the upstream roller pair after the leading edge of the recording material has passed the registration roller pair and before the leading edge of the recording material enters the image forming unit; and versusthe length of the portion of the recording material located between the upstream roller pair and the registration roller pair immediately before the registration roller pair begins to transport the recording material is defined as L0 [mm], the shortest length that the recording material can take on the transport path from the upstream roller pair to the registration roller pair is defined as Lmin [mm], and a target value for the transport speed of the recording material by the registration roller pair is defined as Vreg [mm / sec]; and the length of the period during which the control means stops driving the upstream roller pair is at least longer than (L0-Lmin) / Vreg. Effect of the Invention

[0009] According to the present invention, it is possible to suppress image distortion caused by the difference between the left and right loop amounts after skew correction. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to an embodiment. [Diagram 2] FIG. 2 is a perspective view of an intermediate transfer unit according to the embodiment. [Diagram 3] 3A and 3B are perspective views of a secondary transfer unit according to an embodiment. [Figure 4] 3A to 3C are side views of a secondary transfer unit according to the embodiment. [Diagram 5] 5A and 5B are diagrams for explaining the operation of a registration unit according to the embodiment. [Figure 6] 1A and 1B are diagrams for explaining the transport path of a recording material in the case of single-sided printing and double-sided printing. [Figure 7] FIG. 2 is a block diagram showing a configuration related to transport control of the image forming apparatus according to the embodiment. [Figure 8] FIG. 4 is a perspective view showing an engaged member for positioning the intermediate transfer unit and the secondary transfer unit according to the embodiment. [Figure 9]5A and 5B are diagrams for explaining engagement of an intermediate transfer unit and a secondary transfer unit with an engaged member according to an embodiment. [Figure 10] FIG. 2 is a perspective view of a duplex unit according to the embodiment. [Figure 11] 3A to 3C are side views illustrating the positioning of the duplex unit relative to the apparatus main body according to the embodiment. [Figure 12] FIG. 4 is a cross-sectional view showing a conveying roller and a conveying path provided in the duplex unit according to the embodiment. [Figure 13] FIG. 4 is an enlarged view of the vicinity of a pair of re-feed rollers according to the embodiment. [Figure 14] 5A and 5B are cross-sectional views showing a state after the recording material has been conveyed from the re-feed roller pair to the registration roller pair according to the embodiment. [Figure 15] 13A and 13B are diagrams illustrating a state in which a difference occurs between the left and right loop amounts of the recording material between the pair of registration rollers and the pair of rollers upstream thereof after skew correction. [Figure 16] 6A is a cross-sectional view showing a state in which the driving of the re-feed roller pair according to the embodiment is stopped, and FIG. 6B is a cross-sectional view showing a state after the driving of the re-feed roller pair is resumed. [Figure 17] 5 is a flowchart showing an example of control of the image forming apparatus according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0012] <<General Configuration of Image Forming Apparatus>> 1 is a cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 is a four-color full-color laser printer (electrophotographic image forming apparatus) using an electrophotographic process, which has first to fourth process cartridges PY, PM, PC, and PK as a plurality of cartridges.

[0013] In the embodiment described below, a full-color image forming apparatus in which four process cartridges can be attached and detached is exemplified as an electrophotographic image forming apparatus, but the number of process cartridges is not limited to this and can be appropriately set as necessary. For example, in the case of an image forming apparatus that forms a monochrome image, the number of process cartridges attached to the image forming apparatus is one. In addition, in the embodiment described below, a printer is exemplified as one aspect of the image forming apparatus, but the present technology can also be applied to other image forming apparatuses such as a copier, a facsimile machine, or a multifunction machine that combines the functions of these.

[0014] In the following description and in each drawing, the front side (front face side) of the image forming apparatus 100 is the side where the apparatus opening / closing door (main body door) 31 is disposed. The rear side (rear side) is the opposite side. The front-rear direction is the direction from the rear side of the image forming apparatus 100 toward the front side (front direction) and the opposite direction (rear direction). The left and right are the left and right when the image forming apparatus 100 is viewed from the front side. The left-right direction is the direction from right to left (left direction) and the opposite direction (right direction). The top and bottom are the top and bottom with respect to the vertical direction (direction of gravity) when the image forming apparatus 100 is installed on a horizontal surface. The top direction is the direction from bottom to top, and the bottom direction is the direction from top to bottom.

[0015] Moreover, the "longitudinal direction" of the process cartridges PY to PK or the electrophotographic photosensitive member serving as the image carrier included in each of the process cartridges PY to PK is a direction parallel to the rotation axis direction of the electrophotographic photosensitive member. The "short-side direction" is a direction perpendicular to the longitudinal direction (orthogonal direction). Moreover, one end side in the longitudinal direction is the driving side, and the other end side is the non-driving side. In this embodiment, the longitudinal direction is substantially parallel to the left-right direction, and the right end side in the longitudinal direction is the driving side, and the left end side is the non-driving side.

[0016] <Overall configuration of image forming apparatus> A cartridge storage section 100B is provided inside a main body 100A of the image forming apparatus 100. In this cartridge storage section 100B, four process cartridges PY, PM, PC, and PK are arranged substantially horizontally from the rear side to the front side of the main body 100A and are mounted at predetermined mounting positions. In other words, the image forming apparatus 100 has an in-line configuration and a tandem type configuration. The mounting positions of the process cartridges PY to PK are positions in the cartridge storage section 100B where the process cartridges PY to PK can perform an image forming operation.

[0017] The process cartridges PY to PK contribute to an image forming process for forming an image on a recording material S, and are removably mounted in an apparatus main body 100A of an image forming apparatus 100 for use. Each of the process cartridges PY to PK in this embodiment has a drum-type electrophotographic photosensitive member (hereinafter, referred to as a drum) 1 as an image carrier on which a latent image is formed. And, as electrophotographic image forming process means acting on the drum 1, a charger 2 as a charging means, a developer 3 as a developing means, and a cleaning unit 4 as a cleaning means, and are so-called integrated process cartridges.

[0018] The first process cartridge PY contains yellow toner in the developing device 3 and forms a yellow toner image on the surface of the drum 1. The second process cartridge PM contains magenta toner in the developing device 3 and forms a magenta toner image on the surface of the drum 1. The third process cartridge PC contains cyan toner in the developing device 3 and forms a cyan toner image on the surface of the drum 1. The fourth process cartridge PK contains black toner in the developing device 3 and forms a black toner image on the surface of the drum 1.

[0019] A scanner unit 11, which is an example of an exposure means for exposing the drum 1 of each of the process cartridges PY to PK to form a latent image, is disposed above the process cartridges PY to PK. The scanner unit 11 outputs a laser beam modulated in accordance with image information of each color, and scans and exposes the surface of the drum 1 of each of the process cartridges PY to PK through an exposure window 6 provided on the top surface of the cartridge frame 5.

[0020] A transfer unit (transfer member) is disposed below the process cartridges PY to PK for transferring the toner image formed on the drum 1 in each of the process cartridges PY to PK to the recording material S. The transfer unit (transfer member) in this embodiment is an intermediate transfer unit 12 that performs primary transfer of the toner image formed on the drum 1 to an intermediate transfer body, and then secondarily transfers the toner image from the intermediate transfer body to the recording material S.

[0021] The appearance of the intermediate transfer unit 12 is shown in FIG. 2. The intermediate transfer unit 12 has an endless belt 13 (intermediate transfer belt) made of a dielectric material and having flexibility as an intermediate transfer body. The intermediate transfer unit 12 also has a drive roller 14, a turn roller 15, and a tension roller 16 that wrap and circulate the endless belt 13. The intermediate transfer unit 12 further has a belt cleaning section 12a that slides under the force of the endless belt 13. The drive roller 14 and the turn roller 15 are disposed on the rear side of the device main body 100A. The tension roller 16 is disposed on the front side of the device main body 100A. The part of the endless belt 13 that is stretched between the drive roller 14 and the tension roller 16 is the upper belt part, and the part that is stretched between the tension roller 16 and the turn roller 15 is the lower belt part. The belt cleaning portion 12a is biased against a driving roller 14 and is disposed downstream in the transport direction of the endless belt 13 of a secondary transfer portion Nt described later.

[0022] When each of the process cartridges PY to PK is installed in its installation position, the lower surface of each drum 1 contacts the upper surface of the upper belt portion of the endless belt 13. Four primary transfer rollers 17 are disposed inside the endless belt 13, facing each of the drums 1 with the upper belt portion therebetween. The nip portions between each of the drums 1 and the endless belt 13 are primary transfer portions (primary transfer nip portions) where primary transfer of a toner image is performed.

[0023] A secondary transfer roller 22 is in contact with the drive roller 14 of the intermediate transfer unit 12 with the endless belt 13 sandwiched therebetween. A nip portion between the secondary transfer roller 22 and the endless belt 13 is a secondary transfer portion NtNt (secondary transfer nip portion) where secondary transfer of a toner image is performed. The secondary transfer portion Nt functions as an image forming portion of this embodiment that forms an image on a recording material.

[0024] Fig. 3(a) shows a perspective view of a secondary transfer unit 80 equipped with a secondary transfer roller 22, and Fig. 4(a) to (c) show side views. Fig. 3(a) is a perspective view of the secondary transfer unit 80 as seen from the side of the space (conveyance path) through which the recording material S passes (the front side of the secondary transfer unit 80). Figs. 4(a) to 4(c) are side views of the secondary transfer unit 80 and its periphery as seen in the left-right direction of the image forming apparatus 100 (the longitudinal direction of the drum 1).

[0025] The secondary transfer roller 22 is rotatably held by a conveying guide 80b of the secondary transfer unit 80 via a pair of left and right bearings. The secondary transfer roller 22 is urged by a spring 82 from the conveying guide 80b towards the driving roller 14 via the bearings, and is in pressure contact with the driving roller 14 with the endless belt 13 sandwiched therebetween. As a result, a secondary transfer portion Nt is formed between the secondary transfer roller 22 and the endless belt 13. A voltage (secondary transfer bias) for secondary transfer of the toner image is supplied to the secondary transfer roller 22 from a power supply unit (not shown).

[0026] The image forming mechanism 100C including the above-mentioned process cartridges PY to PK, the scanner unit 11, the intermediate transfer unit 12, and the secondary transfer unit 80 functions as an image forming means (toner image forming means) that forms an image (toner image) on the recording material S.

[0027] As shown in FIG. 1, below the intermediate transfer unit 12, a feeding unit 18 is provided that stocks sheet-shaped recording media (sheet materials: hereinafter, referred to as recording materials) S onto which a toner image is transferred, and feeds (conveys) the recording materials S one by one toward the intermediate transfer unit 12. As the recording materials S, various sheet materials of different sizes and materials can be used, such as paper such as plain paper and thick paper, sheet materials with surface treatments such as plastic films, cloths, and coated paper, and sheet materials of special shapes such as envelopes and index paper. The feeding unit 18 includes a feeding tray 19 that stores the recording materials S, a feeding roller 20, a pair of separation rollers 21, and a registration section 18A including a pair of registration rollers (hereinafter, referred to as a registration roller pair) 20a. The feeding tray 19 is a so-called front loading configuration that can be pulled out from the apparatus main body 100A to the front side and can be inserted from the rear side toward a predetermined mounting position in the apparatus main body 100A.

[0028] The fixing device 23, the discharge roller pair 24, and the reversing roller pair 73 are disposed at the upper rear portion of the apparatus main body 100A. The fixing device 23 is a fixing unit (fixing means, image heating device) that applies heat and pressure to the recording material S to which the toner image has been transferred, thereby fixing the toner image. The fixing device 23 uses a fixing film assembly 23a and a pressure roller 23b, and a fixing nip Nf is formed by both of them. The fixing film assembly 23a has a flexible endless film, a heating element such as a ceramic heater that generates heat when electricity is applied, and a nip forming unit that has a holding member that holds the heating element and is disposed inside the film. The nip forming unit presses the film against the pressure roller 23b, thereby forming a fixing nip Nf between the pressure roller 23b and the film. The discharge roller pair 24 is a discharge means that discharges the recording material S, and the reversing roller pair 73 is a reversing means that reverses and conveys the recording material S to form images on both sides of the recording material S. The upper surface of the apparatus main body 100A is formed as a discharge tray 25 on which the recording material S discharged by the pair of discharge rollers 24 is stacked.

[0029] Downstream in the conveying direction of the fixing nip Nf, a reversing flap 75 is rotatably disposed as a switching member for switching the conveying path of the recording material S between a single-sided conveying path and a double-sided conveying path. The reversing flap 75 can be moved by a drive mechanism (not shown) and can guide the recording material S to either the single-sided conveying path or the double-sided conveying path. The single-sided conveying path is a path that passes through the right side of the reversing flap 75 in FIG. 1 and reaches the discharge roller pair 24 (see also FIG. 6(b)). The double-sided conveying path is a path that passes through the left side of the reversing flap 75 in FIG. 1 and reaches the reversing roller pair 73, and further passes through the rear side of the secondary transfer roller 22 and the fixing device 23 and reaches the registration roller pair 20a (see also FIG. 6(a)).

[0030] In the single-sided transport path, a discharge roller pair 24 is disposed downstream of the inversion flap 75 in the transport direction of the recording material S. The discharge roller pair 24 is composed of a discharge roller 24a and a discharge roller 24b. In the double-sided transport path, a reversing roller pair 73 is disposed downstream of the inversion flap 75 in the transport direction of the recording material S. The reversing roller pair 73 is composed of a reversing roller 73a and a reversing roller 73b. The operations when an image is formed only on the first side of the recording material S (hereinafter referred to as single-sided printing) and when an image is formed on both the first side and the opposite second side of the recording material S (hereinafter referred to as double-sided printing) will be described later.

[0031] In this embodiment, the "roller" constituting the roller pair refers to a drive roller that rotates by receiving a driving force from a drive source via a drive train (transmission mechanism) such as a gear train, and the "roller" refers to a driven roller that rotates (rotates together) with the drive roller. If the drive train can be appropriately arranged, the roller (drive roller) and the roll (driven roller) may be interchanged. Also, both rollers constituting the roller pair may be drive rollers. "The drive source drives the roller pair" means that the drive source drives the drive roller.

[0032] Further rearward of the secondary transfer unit 80, a double-sided unit 70 is disposed. The double-sided unit 70 has a double-sided roller pair 72 and a re-feed roller pair 74, which are arranged in this order from the upstream side to the downstream side in the conveying direction of the recording material S. The double-sided roller pair 72 is made up of a roller (double-sided roller) 72a and a roller (double-sided roller) 72b. The re-feed roller pair 74 is made up of a roller (re-feed roller) 74a and a roller (re-feed roller) 74b. The double-sided roller pair 72 holds the recording material S sent from the reversing roller pair 73 and conveys it to the re-feed roller pair 74. The re-feed roller pair 74 holds the recording material S sent from the double-sided roller pair 72 and conveys it to the registration roller pair 20a.

[0033] In the cartridge storage section 100B, each of the process cartridges PY to PK is located at an installation position where image formation can be performed, and is pressed by a pressing unit 42 to be held in a fixed state at a predetermined positioning portion of the apparatus main body 100A. A drive output portion on the apparatus main body 100A side is connected to a drive input portion of each process cartridge PY to PK held at the installation position (not shown). This allows a predetermined drive force to be transmitted from the drive source of the apparatus main body 100A to the process cartridges PY to PK. A power supply system is conducted from the apparatus main body 100A side to the electrical contacts of the process cartridges PY to PK to appropriately supply a bias required for image formation. The power supply system includes a voltage generation circuit that generates a voltage to be supplied to the charger 2 and the developer 3, and a wiring structure that electrically connects the voltage generation circuit to the electrical contacts of the process cartridges PY to PK positioned at the installation position.

[0034] Image Formation Operation The operation of the image forming apparatus 100 for forming a full-color image on the recording material S is as follows. First, the drums 1 of the first to fourth process cartridges PY, PM, PC, and PK are rotated at a predetermined control speed in the counterclockwise direction indicated by the arrow in FIG. 1. The endless belt 13 is also rotated at a speed corresponding to the speed of the drum 1 in the clockwise direction indicated by the arrow in FIG. 1 (the direction in which the endless belt 1 rotates with the drum 1). The conveying speed of the endless belt 13 defines the transfer speed (the image forming speed in the sub-scanning direction) when the toner image is transferred to the recording material S at the secondary transfer portion Nt, and is also called the process speed. In parallel with the driving of the drum 1 and the endless belt 13, the scanner unit 11 is also separately controlled and driven. The scanner unit 11 is driven by an image signal (also called a video signal) obtained by converting image information inputted from an external computer to the image forming apparatus 100 into component images of each toner color into a time-series signal.

[0035] In synchronization with the driving of the drums 1, etc., a voltage (charging voltage) is supplied to the charger 2 at a predetermined control timing in each of the process cartridges PY to PK, and the charger 2 uniformly charges the surface of the drum 1 to a predetermined polarity and potential. The scanner unit 11 scans and exposes the surface of each drum 1 with laser light modulated according to the image signal of each color. As a result, an electrostatic latent image corresponding to the image signal of the corresponding color is formed on the surface of each drum 1. The formed latent image is developed into a toner image (developer image) by the developer 3.

[0036] By the above-described electrophotographic image forming process operation, a yellow toner image corresponding to the yellow component of the full-color image is formed on the drum 1 of the first process cartridge PY. The toner image is primarily transferred onto the endless belt 13 at the primary transfer section of the process cartridge PY. A magenta toner image corresponding to the magenta component of the full-color image is formed on the drum 1 of the second process cartridge PM. The toner image is primarily transferred onto the endless belt 13 in a superimposed manner on the yellow toner image already transferred onto the endless belt 13 at the primary transfer section of the process cartridge PM. A cyan toner image corresponding to the cyan component of the full-color image is formed on the drum 1 of the third process cartridge PC. The toner image is primarily transferred onto the endless belt 13 in a superimposed manner on the yellow+magenta toner image already transferred onto the endless belt 13 at the primary transfer section of the process cartridge PC. A black toner image corresponding to the black component of the full-color image is formed on the drum 1 of the fourth process cartridge PK. The toner image is primarily transferred onto the endless belt 13 in a superimposed manner on the yellow+magenta+cyan toner image already transferred onto the endless belt 13 at the primary transfer section of the process cartridge PK.

[0037] Thus, an unfixed toner image of four colors, yellow, magenta, cyan, and black, is synthesized (formed) on the endless belt 13. In each of the process cartridges PY to PK, the residual toner remaining on the surface of the drum 1 after the primary transfer of the toner image onto the endless belt 13 is removed by the cleaning unit 4.

[0038] On the other hand, the conveying operation of the recording material S is performed as follows. When an image formation execution instruction (print job) is input to the image forming apparatus 100, the feed motor M1 (FIG. 7) which is a driving source is driven at a predetermined control timing. The driving force causes the feed roller 20 and the separation roller pair 21 to separate and feed the recording material S loaded on the feed tray 19 one by one. The separation roller pair 21 has a conveying roller that conveys the recording material S fed from the feed tray 19 by the feed roller 20, and a separation roller that presses against the conveying roller to form a separation nip. The separation roller is attached to a roller shaft fixed to the apparatus main body 100A, for example, via a torque limiter, and applies a frictional force to the recording material S passing through the separation nip so that only one recording material S in contact with the conveying roller is conveyed. Note that the feed roller 20 and the separation roller pair 21 described above are examples of a feeding means that feeds the recording material S one by one, and other feeding means such as a configuration using a pad-like friction member instead of the separation roller may be used.

[0039] The recording material S sent from the separation roller pair 21 is sent to the registration section 18A. Fig. 5(a) shows the state when the recording material enters the registration section 18A. The registration section 18A includes a registration roller pair 20a and a shutter 20d. The registration roller pair 20a is made up of a registration roller 20b and a registration roller 20c.

[0040] The shutter 20d has an abutting portion 20e against which the leading edge of the recording material S conveyed toward the pair of registration rollers 20a abuts. The shutter 20d opens and closes the conveying path of the recording material S by rotating. The center of rotation of the shutter 20d is coaxial with the registration roller 20c, and when there is no recording material S, the abutting portion 20e blocks the conveying path (see FIG. 5(a)). That is, the closed state of the shutter 20d is a state in which at least one abutting portion 20e is located upstream of the nip portion of the pair of registration rollers 20a in the conveying direction of the recording material S, and restricts the leading edge of the recording material S from entering the nip portion. On the other hand, the open state of the shutter 20d is a state in which the abutting portion 20e retreats from the position that restricts the leading edge of the recording material S from entering the nip portion, and allows the leading edge of the recording material S to enter the nip portion. The shutter 20d of this embodiment has a plurality of abutment portions 20e arranged at predetermined angular intervals in the rotation direction, and in the closed state, one of the abutment portions 20e is located upstream of the nip portion of the pair of registration rollers 20a.

[0041] In this embodiment, the shutters 20d are arranged side by side in the left-right direction (the direction of the rotation axis of the pair of registration rollers 20a, and the direction of the rotation axis of the shutters 20d). Each of the shutters 20d has a plurality of abutment portions 20e, and is arranged so that the positions of the abutment portions 20e are aligned with high precision. The shutters 20d are positioned at a predetermined rotation angle by an elastic body such as a spring so that the shutters 20d maintain a closed state when the recording material S is not being conveyed to the registration section 18A.

[0042] When the recording material S is conveyed to the registration section 18A, the leading edge of the recording material S abuts against the abutment section 20e of the shutter 20d, and the conveyance of the leading edge of the recording material S is regulated. The shutter 20d is set with a biasing force of an elastic body for positioning the shutter 20d so that the shutter 20d does not rotate when the leading edge of the recording material S comes into contact with the shutter 20d. After the leading edge of the recording material S abuts against the abutment section 20e, when the recording material S continues to be conveyed by the upstream roller pair (in this embodiment, the separation roller pair 21 or the re-feed roller pair 74), the recording material S is bent between the upstream roller pair and the registration roller pair 20a. At this time, the leading edge of the recording material S abuts against the multiple abutment sections 20e arranged in the left-right direction, and the skew of the recording material S is corrected. The skew of the recording material S here refers to one end of the leading edge of the recording material S in the left-right direction being ahead of the other end.

[0043] Thereafter, as the bending of the recording material S between the upstream roller pair and the registration roller pair 20a increases, the force acting on the shutter 20d in the conveying direction increases due to the elastic force of the recording material S (the force that stretches to eliminate the bending). When this force exceeds the action of the biasing force of the elastic body for positioning the shutter 20d, the shutters 20d aligned in the left-right direction rotate simultaneously, and the recording material S enters the nip portion of the registration roller pair 20a with the skew of the leading edge corrected (FIG. 5(b)).

[0044] The timing at which the upstream roller pair stops conveying the recording material S and ends the loop formation is the point at which a preset time has elapsed since the pre-registration sensor 52 (FIG. 7) provided upstream of the registration roller pair 20a detects the leading edge of the recording material S. In other words, the upstream roller pair stops after conveying the recording material S an additional distance set in advance after the point at which the leading edge of the recording material S is predicted to hit the shutter 20d based on the detection signal of the pre-registration sensor 52. The amount by which the upstream roller pair conveys the recording material S an additional amount (target loop amount) during skew correction is preset so that the skew of the recording material is sufficiently corrected even when the amount of skew of the recording material is the maximum value expected.

[0045] Thereafter, the pair of registration rollers 20a is driven by a feed motor M1 at a predetermined control timing synchronized with the image forming process in the image forming mechanism 100C, and introduces the recording material S into the secondary transfer portion Nt. In the secondary transfer nip, a secondary transfer bias is supplied to the secondary transfer roller 22, so that the toner image on the endless belt 13 is electrostatically transferred onto the recording material S. In the secondary transfer nip, the recording material S is sandwiched between the endless belt 13 and the secondary transfer roller 22, and is transported to the fixing device 23 according to the transport speed (process speed) of the endless belt 13. As a result, the four-color superimposed toner image on the endless belt 13 is collectively transferred onto the surface of the recording material S in the process of the recording material S being sandwiched and transported through the secondary transfer portion Nt.

[0046] On the other hand, after the toner image is transferred, residual toner that has not been transferred to the recording material S remains on the endless belt 13. The belt cleaning unit 12a slides on the endless belt 13 to remove and collect this residual toner from the belt, thereby preventing the residual toner from being transferred to the next recording material.

[0047] The recording material S is separated from the surface of the endless belt 13 and introduced into the fixing device 23 through a conveying path. Then, while the recording material S is nipped and conveyed between the fixing film assembly 23a and the pressure roller 23b in the fixing nip Nf, the toner image on the recording material S is heated and pressurized. As a result, the toner images of each color are mixed and fixed onto the recording material S.

[0048] The image forming apparatus 100 of this embodiment is capable of double-sided printing. The following describes the operations of single-sided printing and double-sided printing after a toner image is fixed on one side of the recording material S. In the case of single-sided printing, in which an image is formed only on the first side of the recording material S, the recording material S that has had an image transferred to its first side and passed through the fixing device 23 is guided to a single-sided conveyance path by the reversing flap 75 that is positioned at the position (single-sided printing position, discharge position) shown in FIG. 6(b). The recording material S is then discharged from the apparatus main body 100A by the discharge roller pair 24 and stacked on the discharge tray 25.

[0049] On the other hand, in the case of double-sided printing in which an image is formed on both sides of the recording material S, the recording material S, which has an image transferred to a first side and passed through the fixing device 23, is guided to a double-sided conveying path by a reversing flap 75 located at a position (double-sided printing position, reversing position) shown in Fig. 6(a). The reversing roller pair 73 conveys the recording material S a predetermined distance toward the discharge tray 25, and then reverses the rotation direction after the trailing end of the recording material S passes the reversing flap 75 and before it leaves the reversing roller pair 73, and conveys the recording material S to the double-sided unit 70. Thereafter, the recording material S is nipped and conveyed in sequence by the double-sided roller pair 72 and the re-feed roller pair 74 (Fig. 1) of the double-sided unit 70, and is conveyed again to the registration section 18A.

[0050] Then, as in the case of single-sided printing, the leading edge of the recording material S is abutted against the shutter 20d of the registration section 18A by the re-feed roller pair 74 to correct the skew, and then the leading edge of the recording material S enters the nip portion of the registration roller pair 20a and is sandwiched. Thereafter, the registration roller pair 20a is driven by the feed motor M1 at a predetermined control timing synchronized with the image forming process in the image forming mechanism 100C, and introduces the recording material S to the secondary transfer section Nt. At this time, the recording material S is introduced to the secondary transfer section Nt in a state where the first side on which an image has already been formed faces the secondary transfer roller 22, and the second side on which a new image is to be formed faces the endless belt 13. Thereafter, the toner image on the endless belt 13 is transferred to the second side of the recording material S in the same manner as in the case of single-sided printing, and after being subjected to a fixing process in the fixing device 23, the recording material S is discharged to the discharge tray 25 by the discharge roller pair 24 via the single-sided conveyance path as a double-sided printed product.

[0051] In this embodiment, a dedicated motor for rotating the double-sided roller pair 72 and the re-feed roller pair 74 is not provided, and the driving force is obtained from a feed motor M1 (FIG. 7) that generates a driving force for rotating the registration roller pair 20a. In other words, the feed motor M1 is a common driving source for the registration roller pair 20a, the double-sided roller pair 72, and the re-feed roller pair 74. In this way, costs are reduced by driving the registration roller pair 20a and the roller pair upstream of it with one motor (driving source). More specifically, the feed motor M1 drives the registration roller 20b, roller 74a, and roller 72a.

[0052] An electromagnetic clutch 110 is included in a drive train (not shown) that transmits a driving force from the feed motor M1, which is a drive source, to the duplex roller pair 72 and the re-feed roller pair 74. That is, the image forming apparatus 100 of this embodiment is equipped with a drive device that includes the feed motor M1 and the electromagnetic clutch 110. Although it is possible to use a mechanical clutch instead of the electromagnetic clutch 110, the electromagnetic clutch 110 has an advantage that the response delay at the time of connection and disconnection is very short, and the drive state of the re-feed roller pair 74 can be controlled with high time precision.

[0053] Based on a command from the control unit 51 (FIG. 7) of the image forming apparatus 100, the electromagnetic clutch 110 switches between a connected state in which the drive force is transmitted from the feed motor M1 to the duplex roller pair 72 and the re-feed roller pair 74, and a disconnected state in which the drive force is disconnected. During double-sided printing, the electromagnetic clutch 110 is connected, so that the recording material S can be conveyed by the duplex roller pair 72 and the re-feed roller pair 74 using the driving force of the feed motor M1. On the other hand, during single-sided printing, the electromagnetic clutch 110 is disconnected, so that a torque load that does not contribute to the conveyance of the recording material S is not applied to the feed motor M1. This allows a motor with a relatively small output torque to be selected as the feed motor M1, and the amount of power supply during driving (for example, the duty in the case of pulse width modulation) can be kept to a necessary minimum, which allows further cost reduction.

[0054] <Details of the secondary transfer unit> The positioning configuration of the secondary transfer unit 80 in the apparatus main body 100A will be described in detail below. Figures 4(a) to 4(c) are side views of the secondary transfer unit 80 and its related parts as viewed from the left side. Figure 4(a) shows a state in which the secondary transfer unit 80 is positioned in the apparatus main body 100A. Figure 4(b) shows a state in which the positioning of the secondary transfer unit 80 has been released. Figure 4(c) shows a state in which the engaging portion 84a abuts against the main body side engaged portion 100p in the process of changing the secondary transfer unit 80 from the open state to the closed state.

[0055] As shown in FIG. 3(a) and FIG. 4(a), the secondary transfer unit 80 has a frame 80a, a conveying guide 80b, a unit shaft 80c, a secondary transfer roller 22, and a spring 82. The frame 80a is a frame body made of a high-strength member such as sheet metal. The unit shaft 80c is a metal shaft crimped to the frame 80a. The secondary transfer unit 80 is held rotatably about an axis extending in the left-right direction by fitting the unit shaft 80c into a hole (not shown) in the device main body 100A. The front-rear and up-down positioning of the secondary transfer unit 80 is determined by fitting the unit shaft 80c into the hole in the device main body 100A.

[0056] The conveying guide 80b is fixed to the frame 80a. As described above, the secondary transfer roller 22 is rotatably held by the conveying guide 80b via a pair of left and right bearings. The secondary transfer roller 22 is urged by the conveying guide 80b toward the driving roller 14 via the bearings by the spring 82, and is in pressure contact with the driving roller 14 with the endless belt 13 sandwiched therebetween. Since the conveying guide 80b is fixed to the frame 80a, the reaction force from the spring 82 can be received by the rigid frame 80a, and the conveying guide 80b itself is accurately positioned with respect to the apparatus main body 100A.

[0057] A locking mechanism 84 is provided for locking the secondary transfer unit 80 at a predetermined mounting position in the apparatus main body 100A. The locking mechanism 84 includes an engaging portion 84a provided on the secondary transfer unit 80 and a main body side engaged portion 100p provided on the apparatus main body 100A (see FIGS. 4(a) to 4(c)). The engaging portion 84a is rotatably supported by the frame 80a or the conveying guide 80b of the secondary transfer unit 80, and has a groove portion that engages with the main body side engaged portion 100p. When the unit shaft 80c is fitted into a hole in the apparatus main body 100A, the groove portion of the engaging portion 84a engages with the main body side engaged portion 100p, thereby restricting the rotation of the secondary transfer unit 80 about the unit shaft 80c and positioning it in the front-rear direction.

[0058] The engaging portion 84a is provided to rotate integrally with the handle 83, and as shown in Fig. 4(b), the engaging portion 84a is disengaged from the main body side engaged portion 100p by operating the handle 83. When the engaging portion 84a is disengaged from the main body side engaged portion 100p, the secondary transfer unit 80 becomes rotatable about the unit shaft 80c relative to the device main body 100A, as shown in Fig. 4(c).

[0059] As described above, since the secondary transfer roller 22 forms the secondary transfer portion Nt between itself and the drive roller 14, it is desirable to position the secondary transfer unit 80 with high precision relative to the intermediate transfer unit 12. In this configuration, the drive roller 14 of the intermediate transfer unit 12 is positioned with respect to the device main body 100A (not shown), and therefore, by positioning the secondary transfer unit 80 with high precision relative to the device main body 100A, it is possible to prevent misalignment with the intermediate transfer unit 12. The secondary transfer unit 80 is positioned in the left-right direction by an engagement of an engagement protrusion 131 of the transport guide 80b with an engaged member 130 (see FIGS. 8 and 9(a, b)) attached to the frame of the apparatus main body 100A. The engagement protrusion 131 is a rod-shaped protrusion that protrudes forward from the left-right end of the transport guide 80b. In addition to the secondary transfer unit 80, an engagement protrusion 132 (see FIG. 2) of the intermediate transfer unit 12 also engages with the engaged member 130, thereby positioning the intermediate transfer unit 12 in the left-right direction. The engagement protrusion 132 is the lower end of a side plate provided at the left-right end of the intermediate transfer unit 12 and extends in the front-rear direction.

[0060] 8 is a perspective view of the engaged member 130 of the apparatus main body 100A. The engaged member 130 is attached in a state where it is positioned with high precision relative to the frame 101 of the apparatus main body 100A, and has a groove-shaped engaged portion that is a concave shape that opens upward in a U-shape (angular C-shape) and extends in the front-rear direction. In the groove-shaped engaged portion, a first engaged portion 130b with which the intermediate transfer unit 12 engages is provided in the front side portion, and a second engaged portion 130a with which the secondary transfer unit 80 engages is provided in the rear side portion. The height of the engaged member 130 is set low so as to ensure sufficient rigidity.

[0061] 9(a) and 9(b) will be used to explain the positional relationship between the engaged member 130 and the intermediate transfer unit 12 and secondary transfer unit 80 when engaged. Fig. 9(a) shows the engaged member 130 in a state in which the intermediate transfer unit 12 and secondary transfer unit 80 are not engaged. Fig. 9(b) shows the engaged member 130 in a state in which the intermediate transfer unit 12 and secondary transfer unit 80 are engaged, viewed from the same perspective as Fig. 9(a).

[0062] As shown in FIGS. 9(a and 9(b) ), the engaging protrusion 132 of the intermediate transfer unit 12 engages with the first engaged portion 130b, and the engaging protrusion 131 of the secondary transfer unit 80 engages with the second engaged portion 130a. Here, as described above, the engaged member 130 has a groove shape extending in the front-rear direction, and the engaging protrusion 132 of the intermediate transfer unit 12 and the engaging protrusion 131 of the secondary transfer unit 80 also have a shape extending in the front-rear direction. Therefore, when the engaging protrusions 131, 132 of the intermediate transfer unit 12 and the secondary transfer unit 80 are engaged with the engaged member 130, the intermediate transfer unit 12 and the secondary transfer unit 80 are positioned in the left-right direction. At the same time, the clearance (play) in the front-rear direction of the intermediate transfer unit 12 and the secondary transfer unit 80 is absorbed by the shapes of the engaging protrusions 131, 132 and the engaged member 130 extending in the front-rear direction. In this way, the intermediate transfer unit 12 and the secondary transfer unit 80 can be positioned with a minimum protruding shape without the need to extend their shapes from the positions where they are respectively arranged. In addition, the secondary transfer unit 80 and the intermediate transfer unit 12 are positioned relative to the same rigid component (engaged member 130), thereby realizing highly accurate and stable positioning.

[0063] As shown in FIG. 3B, the conveying guide 80b of the secondary transfer unit 80 has a vent hole 133 in a groove that accommodates the secondary transfer roller 22, and hot air between the secondary transfer unit 80 and the fixing device 23 is released. FIG. 3B is a perspective view of the secondary transfer unit 80 shown in FIG. 3A with the secondary transfer roller 22 removed. A plurality of circular vent holes 133 are arranged in the axial direction (left-right direction) of the secondary transfer roller 22 in the groove that can be confirmed when the secondary transfer roller 22 is removed. The air that flows out from the vent hole 133 passes through a vent hole separately opened in the frame 80a as well as through gaps between the components, and escapes to the double-sided unit 70 side. This allows the hot air heated by the fixing device 23 to escape to the double-sided unit 70 side without stagnation between the secondary transfer unit 80 and the fixing device 23. This prevents the temperature in the vicinity of the driving roller 14 in the intermediate transfer unit 12 and the temperature in the vicinity of the belt cleaning section 12a from increasing, thereby reducing the possibility of malfunction due to toner adhesion.

[0064] <Details of the double-sided unit> The following describes the details of the double-sided unit 70. Figure 10 is a perspective view of the double-sided unit 70, and Figures 11(a) to (c) are cross-sectional views of the double-sided unit 70 cut along a plane perpendicular to the left-right direction.

[0065] The duplex unit 70 has a duplex door 71 that can be opened and closed relative to the device main body 100A, and a duplex unit shaft 76 that is fixed to the duplex door 71 and extends in the left-right direction (FIGS. 11(a) to 11(c)). The duplex unit 70 can rotate about the duplex unit shaft 76 relative to the device main body 100A by the duplex unit shaft 76 engaging with a hole 100q provided in the device main body 100A. The hole 100q provided in the device main body 100A is an elongated hole (a hole whose vertical length is longer than its vertical length) that fits snugly with the duplex unit shaft 76 in the front-rear direction and has some play in the up-down direction. By fitting the duplex unit shaft 76 into the hole 100q, the duplex unit 70 is positioned in the front-rear direction and temporarily held in a state with freedom in the up-down direction.

[0066] Further, the double-sided unit 70 has a double-sided engagement shaft 70b (FIGS. 11(a) to 11(c)), which engages with a double-sided engaged portion 23p provided on the fixing device 23. The double-sided engaged portion 23p is shaped like a groove into which the double-sided engagement shaft 70b extending in the left-right direction is fitted so as to hold it from both the top and bottom directions. The double-sided engagement shaft 70b engages with the double-sided engaged portion 23p, so that the double-sided unit 70 is positioned relative to the fixing device 23 in the top and bottom directions. In this way, the upper part of the double-sided unit 70 is positioned relative to the fixing device 23, so that the recording material S sent from the fixing device 23 can be received and transported with high accuracy.

[0067] The duplex unit 70 also has a duplex locking section 70a (FIGS. 11(a) to 11(c)). The duplex locking section 70a is rotatably attached to the duplex door 71, biased in the closing direction by a spring (not shown), and has a hook-shaped tip. The hook-shaped portion engages with an engaged portion provided in the fixing device 23 (the duplex locking section 70a locks), thereby positioning the duplex unit 70. In this embodiment, the duplex locking section 70a is integral with a duplex handle 70c attached to the duplex door 71. When the duplex unit 70 is locked, the user can release the lock by grasping and rotating the duplex handle 70c to open the duplex unit 70 (FIG. 11(b)). When closing the duplex unit 70 from the open state, the duplex door 71 is pushed and rotated (FIG. 11(c)) until it reaches a predetermined position, whereby the duplex locking section 70a is locked and the duplex unit 70 can be closed.

[0068] Furthermore, the duplex unit 70 has a duplex roller pair 72 and a re-feed roller pair 74 as roller pairs that transport the recording material S during duplex printing. The duplex roller pair 72 and the re-feed roller pair 74 are made up of rollers 72a, 74a and rollers 72b, 74b, and the rollers 72b, 74b of each roller pair can be seen in the perspective view of Figure 10. A cross-sectional view showing the positions of each roller is shown in Figure 12.

[0069] 13 shows an enlarged view of the re-feed roller pair 74. As shown in FIGS. 10 and 13, a roller shaft 70d is inserted through the rollers 72b, 74b of each roller pair, and they rotate around the roller shaft 70d. The roller shaft 70d is biased toward the rollers by the biasing member 70e, forming a nip between the rollers 72b, 74b and the rollers 72a, 74a, and obtaining a conveying force for the recording material S. Here, a torsion coil spring is used as the biasing member 70e. A notch 70f is provided in the cylindrical roller shaft 70d, and the biasing member 70e biases the arm of the torsion coil spring against the notch 70f to apply a biasing pressure toward the rollers.

[0070] The position of the roller shaft 70d is determined by the double-sided door 71 in the direction perpendicular to the direction of the force applied to the roller (the conveying direction of the recording material in the nip). The position of the rollers 72b and 74b in the direction of the force applied to the roller is determined by contacting the rollers 74a and 74a. On the other hand, the double-sided door 71 has a sufficient clearance in the left-right direction so as not to impede the force applied to the roller shaft 70d. The roller shaft 70d is positioned in the left-right direction by the force applied member 70e entering the notch 70f of the roller shaft. An enlarged perspective view of the roller shaft 70d and the force applied member 70e is shown in FIG. 13. The arm of the force applied member 70e is positioned in the left-right direction by the double-sided door 71, enters the notch 70f, and forces the roller shaft 70d toward the roller. The width of the notch 70f is approximately equal to the wire diameter of the urging member 70e, thereby regulating the left-right position (axial position) of the roller shaft 70d with some play.

[0071] In this way, when the rollers 72b, 74b and the roller shaft 70d are biased toward the roller side, their axial positioning is performed by the biasing member 70e that operates integrally with the roller shaft 70d, so that the biasing action of the biasing member 70e is not easily hindered. In addition, the biasing member 70e enters the notch 70f of the roller shaft 70d to bias it, thereby restricting the rotation direction of the roller shaft 70d. This makes it possible to prevent wear and abnormal noise caused by the roller shaft 70d rotating integrally with the rollers 72b, 74b.

[0072] <Transport control during double-sided printing> The conveying speed of the recording material S is desirably a constant speed as close as possible to the conveying speed (process speed) of the endless belt 13 so that the toner image transferred to the recording material S at the secondary transfer portion Nt is not disturbed. Meanwhile, the conveying speed of the recording material S by each roller pair varies due to the influence of manufacturing variations in the outer diameter of the roller, the frictional force between the roller and the recording material S, and the biasing force (pressure force) between the roller and the roller facing it. In the following description, the "conveying speed" of the roller pair or the endless belt 13 refers to the peripheral speed of the roller pair or the endless belt 13 in a state in which it is driven at a constant speed during image formation. In contrast, the "moving speed" of the recording material S at the nip portion of the roller pair (including the secondary transfer portion Nt) refers to the speed at which the recording material S actually passes through the nip portion, and is distinguished from the conveying speed of the roller pair or the endless belt 13. When the recording material S is transported without slipping relative to the roller pair or the endless belt 13 , the moving speed of the recording material S in the nip portion coincides with the transport speed of the roller pair or the endless belt 13 .

[0073] When the conveying speed of the recording material S at the registration roller pair 20a is slower than the conveying speed of the endless belt 13, the loop amount of the recording material S between the registration roller pair 20a and the secondary transfer portion Nt decreases as the recording material S is conveyed. Here, the "loop amount" refers to the extra length of the conveying direction length of the portion of the recording material S that is actually located between the nip portions in a state in which the recording material S is sandwiched between the nip portions of the upstream roller pair and the nip portion of the downstream roller pair, with respect to the shortest distance of the conveying path between the nip portions. The loop amount can be measured based on a length measured along one of the side edges of the recording material S, or based on an average value of the lengths measured along both side edges of the recording material S. Hereinafter, the "loop amount" that does not specify one of the side edges refers to the latter average loop amount.

[0074] When the amount of loop between the pair of registration rollers 20a and the secondary transfer portion Nt decreases with the conveyance of the recording material S, and the attitude of the recording material S becomes substantially equal to the shortest path on the conveyance path, tension begins to be generated in the recording material S. In this case, slippage occurs between the recording material S and the endless belt 13 at the secondary transfer portion Nt, and the toner image transferred to the recording material S may be disturbed. In addition, when the trailing end of the recording material S passes through the pair of registration rollers 20a in this state, the force acting on the recording material S in the conveyance direction changes instantaneously, and the moving speed of the recording material S at the secondary transfer portion Nt changes suddenly. As a result, the toner image transferred to the recording material S at the secondary transfer portion Nt may be disturbed. Therefore, it is preferable to adjust the conveyance speed of the pair of registration rollers 20a to be faster than the conveyance speed of the endless belt 13.

[0075] On the other hand, when the conveying speed of the registration roller pair 20a is faster than that of the endless belt 13 and the difference between them is large, the loop amount of the recording material S between the registration roller pair and the secondary transfer portion Nt increases as the recording material S is conveyed. When this loop amount exceeds a certain level, the moving speed of the recording material S at the secondary transfer portion Nt is disturbed by the force received from the rigidity of the recording material S, and the recording material S may slip relative to the endless belt 13. For this reason, it is desirable to set the conveying speed of the registration roller pair 20a to be faster than that of the endless belt 13, but with as small a speed difference as possible.

[0076] In addition, in terms of the relationship between the conveying speeds of the registration roller pair 20a and the re-feed roller pair 74 disposed upstream thereof, if the conveying speed of the re-feed roller pair 74 is slower than the conveying speed of the registration roller pair 20a, the loop amount of the recording material S between the two roller pairs decreases as conveyance proceeds. If the conveying speed of the re-feed roller pair 74 is set to be always slower than the conveying speed of the registration roller pair 20a, tension may be generated in the recording material S between the registration roller pair 20a and the re-feed roller pair 74 during conveyance. If the trailing end of the recording material S passes through the re-feed roller pair 74 while tension is being generated in the recording material S, the moving speed of the recording material S at the secondary transfer portion Nt may fluctuate, and the toner image transferred to the recording material S may become distorted. Furthermore, if the conveying speed of the re-feed roller pair 74 is excessively faster than the conveying speed of the registration roller pair 20a, the moving speed of the recording material S at the secondary transfer portion Nt may fluctuate when the trailing end of the recording material S passes through the re-feed roller pair 74, causing a disturbance in the toner image transferred to the recording material S. For this reason, it is desirable to adjust the conveying speed of the re-feed roller pair 74 to a value equal to or higher than the conveying speed of the registration roller pair 20a, and preferably to a value slightly faster than the conveying speed of the registration roller pair 20a.

[0077] Next, a description will be given of the conveying force acting from the re-feed roller pair 74 and the registration roller pair 20a on the recording material S. The "conveying force" refers to the friction force in the conveying direction applied to the recording material S from the roller pair at the nip portion of the roller pair.

[0078] At the secondary transfer portion Nt, the amount of toner varies depending on the pattern of the image formed on the recording material S, but since the toner acts as a lubricant, the force with which the endless belt 13 and the secondary transfer roller 22 pinch and transport the recording material S at the secondary transfer portion Nt varies. Therefore, in order to maintain the moving speed of the recording material S at the secondary transfer portion Nt substantially the same as the transport speed of the endless belt 13, it is desirable that the fluctuation in the moving speed of the recording material S at the pair of registration rollers 20a be as small as possible. Therefore, it is desirable that the transport force applied to the recording material S by the pair of registration rollers 20a be sufficiently large.

[0079] On the other hand, it is desirable that the re-feed roller pair 74 located upstream of the registration roller pair 20a does not affect the conveying performance of the registration roller pair 20a after sending the recording material S to the registration roller pair 20a. In other words, if the conveying force applied to the recording material S by the re-feed roller pair 74 has a large contribution to the conveying of the recording material S, the conveying speed of the recording material S will suddenly decrease when the trailing end of the recording material S leaves the re-feed roller pair 74, which will lead to a disturbance of the toner image transferred at the secondary transfer portion Nt. Therefore, it is desirable to set the conveying force applied to the recording material S by the re-feed roller pair 74 lower than that of the registration roller pair 20a. Also, from the viewpoint of stably performing the operation of eliminating the difference between the left and right loop amounts of the recording material S described below, it is desirable to set the conveying force applied to the recording material S by the re-feed roller pair 74 lower than that of the registration roller pair 20a.

[0080] In this embodiment, as a method for setting the conveying force applied to the recording material S by the re-feed roller pair 74 to be lower than that of the registration roller pair 20a, a difference is provided in the pressure applied by each roller pair to pinch the recording material S at the nip portion. That is, the spring constant of the above-mentioned urging member 70e is selected so that the pressure (integrated value of nip pressure, total pressure) at the nip portion of the re-feed roller pair 74 is smaller than the pressure at the nip portion of the registration roller pair 20a. The pressure at the nip portion of the re-feed roller pair 74 is preferably 90% or less of the pressure at the nip portion of the registration roller pair 20a. In a configuration example of the image forming apparatus 100 to which this embodiment is applied, the pressure of the registration roller pair 20a is set to 1100±110 [gf], and the pressure of the re-feed roller pair is set to 850±85 [gf].

[0081] (Difference in loop amount between left and right) Next, a description will be given of the difference between the left and right loop amounts of the recording material S that occurs with skew correction in the registration section 18A during double-sided printing, and image distortion caused by the difference between the left and right loop amounts.

[0082] As described above, the recording material S with an image formed on its first side during double-sided printing is guided to the double-sided conveying path, reversed and conveyed by the reversing roller pair 73 (FIG. 1), and then conveyed by the re-feed roller pair 74. Then, the leading edge of the recording material S is abutted against the shutter 20d of the registration section 18A by the re-feed roller pair 74 to correct skew, and then the leading edge of the recording material S enters the nip portion of the registration roller pair 20a and is sandwiched. At this time, since the re-feed roller pair 74 is located upstream of the registration roller pair 20a in the conveying path when an image is formed on the second side during double-sided printing, the re-feed roller pair 74 corresponds to the upstream roller pair in this case.

[0083] Here, let us consider a case where the leading edge of the recording material S is skewed immediately before it is abutted against the shutter 20d by the re-feed roller pair 74. That is, let us consider a case where the leading edge of the recording material S is conveyed in a state where a corner portion Sa1 (FIG. 15) between one side end Sa of the recording material S and the leading edge is ahead of a corner portion Sb1 between the other side end Sb of the recording material S and the leading edge. In this case, immediately after the loop is formed, as shown in FIG. 15, a loop larger than the target value of the loop amount is formed at the side end Sa where the corner portion Sa1 was ahead. On the other hand, a loop smaller than the target value of the loop amount is formed at the side end Sb where the corner portion Sb1 was behind. In this way, a difference occurs between the left and right loop amounts after skew correction between the side end Sa on one side and the side end Sb on the other side of the recording material S depending on the skew amount of the recording material S before skew correction.

[0084] 14A shows a state after the pair of registration rollers 20a starts conveying the recording material S when the leading edge of the recording material S immediately before it hits the shutter 20d is not skewed, as viewed from the left and right (in the direction of the rotation axis of the pair of registration rollers 20a and the pair of re-feed rollers 74). In this case, there is no difference in the amount of loop formed at the side edges Sa, Sb on both sides of the recording material S.

[0085] 14B shows a state after the pair of registration rollers 20a starts conveying the recording material S when the leading edge of the recording material S is skewed just before it hits the shutter 20d, as viewed from the left to right. In this case, the recording material S is conveyed with a large loop formed at the side end Sa on the side where the corner Sa1 hits the shutter 20d first, as described above, and a small loop formed at the side end Sb on the side where the corner Sb1 hits the shutter 20d later.

[0086] In the state of FIG. 14B, the force (elastic force) that the recording material S tries to stretch due to its rigidity (stiffness) differs between the side of one side end Sa of the recording material S and the side of the other side end Sb of the recording material S depending on the amount of loop. The force that the recording material S tries to stretch acts in the direction of conveyance at the nip portion of the pair of registration rollers 20a. That is, the moving speed at which the side end Sa with the relatively large amount of loop passes through the nip portion of the pair of registration rollers 20a may be faster than the moving speed at which the side end Sb with the relatively small amount of loop passes through the nip portion of the pair of registration rollers 20a. In that case, the difference between the left and right in the speed at which the pair of registration rollers 20a feeds the recording material S toward the secondary transfer portion Nt causes a difference between the left and right in the moving speed of the recording material S at the secondary transfer portion Nt. That is, the moving speed at which the side end Sa with the relatively large amount of loop passes through the secondary transfer portion Nt may be faster than the moving speed at which the side end Sb with the relatively small amount of loop passes through the secondary transfer portion Nt.

[0087] As a result, image distortion may occur in which the length of the image transferred to the recording material S at the secondary transfer portion Nt in the conveying direction (sub-scanning direction) of the recording material differs between one side and the other side in the left-right direction (main scanning direction).As a specific example, if a user prints a rectangular image, the recording material S may end up with a trapezoidal image in which the front and rear ends of the recording material S have non-parallel straight lines in the left-right direction.

[0088] In this embodiment, the transport control described below is performed to suppress image distortion caused by the difference in the loop amount between the left and right sides, thereby enabling image formation with higher accuracy.

[0089] That is, in this embodiment, (i) after the leading edge of the recording material S passes through the registration roller pair 20a and before it enters the secondary transfer portion Nt, the electromagnetic clutch 110 is released to stop the driving of the re-feed roller pair 74. Also, (ii) before the leading edge of the recording material S enters the secondary transfer portion Nt, the electromagnetic clutch 110 is connected to resume the driving of the re-feed roller pair 74. Then, during the period when the driving of the re-feed roller pair 74 is stopped, the recording material S is conveyed by the registration roller pair 20a. Then, the recording material S is pulled by the registration roller pair 20a and slips with respect to the re-feed roller pair 74. The above operations (i) and (ii) can be called a temporary stop operation. The control unit 51 controls the driving device including the feeding motor M1 and the electromagnetic clutch 110 so that the temporary stop operation is performed. The state in which the recording material S slips relative to the pair of re-feed rollers 74 refers to a state in which the amount of movement of the recording material S between the roller 74a and the roller 74b exceeds the amount of movement of the surface of the roller 74a.

[0090] In other words, the control means of this embodiment controls the drive device so that (i) the drive of the upstream roller pair is stopped after the leading edge of the recording material has passed the registration roller pair and before it enters the image forming unit, and (ii) the drive of the upstream roller pair is resumed before the leading edge of the recording material enters the image forming unit. As a result, in this embodiment, the registration roller pair is configured to convey the recording material such that a part of the recording material is pulled out from the upstream roller pair while slipping relative to the upstream roller pair during the period when the drive of the upstream roller pair is stopped. Note that the registration roller pair does not need to pull out the trailing edge of the recording material from the upstream roller pair.

[0091] As described in (i) above, when the driving of the re-feed roller pair 74 is temporarily stopped after the recording material S starts to be conveyed by the registration roller pair 20a, the moving speed of the recording material S at the re-feed roller pair 74 decreases or stops. As a result, the loop amount between the re-feed roller pair 74 and the registration roller pair 20a decreases during the period when the driving of the re-feed roller pair 74 is stopped. Then, when the length of the recording material between the re-feed roller pair 74 and the registration roller pair 20a becomes the shortest length that the recording material S can have in the conveying path between the re-feed roller pair 74 and the registration roller pair 20a, tension is generated in the recording material S. Then, the recording material S is pulled by the registration roller pair 20a and slips against the re-feed roller pair 74, and is conveyed as if being pulled out of the re-feed roller pair 74.

[0092] FIG. 16(a) shows a state in which the pair of registration rollers 20a is conveying the recording material S so as to pull it out from the pair of re-feed rollers 74. Even if there is a difference in the amount of loop between the left and right immediately after the loop is formed as shown in FIG. 14(b), the pair of registration rollers 20a conveys the recording material S so as to pull it out from the pair of re-feed rollers 74, and as a result, the difference in the amount of loop between the left and right is eliminated in the state shown in FIG. 16(a). This is because, after the driving of the pair of re-feed rollers 74 is temporarily stopped, the side end Sb with the smaller amount of loop disappears first and starts to be pulled out from the pair of re-feed rollers 74, and then the side end Sa with the larger amount of loop disappears and starts to be pulled out from the pair of re-feed rollers 74. In the state in which the loops of both the side ends Sa and Sb have disappeared as shown in FIG. 16(a), both the side ends Sa and Sb pass through the shortest path between the pair of re-feed rollers 74 and the pair of registration rollers 20a, and the amount of loop is substantially 0 mm.

[0093] Thereafter, before the leading edge of the recording material S enters the secondary transfer portion Nt, the electromagnetic clutch 110 is again brought into a connected state, and the leading edge of the recording material S enters the secondary transfer portion Nt while being conveyed by the pair of registration rollers 20a and the pair of re-feed rollers 74. This state is shown in FIG. 16B. As described above, in this embodiment, the conveying speed of the re-feed roller is slightly faster than the conveying speed of the pair of registration rollers 20a, so the loop amount of the recording material S is slightly increased compared to FIG. 16A. At this time, the difference between the left and right loop amounts is eliminated during the period when the pair of re-feed rollers 74 is stopped driving, so that it is substantially nonexistent or slight. As a result, during the period when the leading edge of the recording material S enters the secondary transfer portion Nt and the toner image is being transferred to the recording material S, the difference between the left and right moving speeds of the recording material S due to the difference between the left and right loop amounts is unlikely to occur. As a result, the distortion of the toner image transferred to the recording material S due to the difference between the left and right moving speeds of the recording material S at the secondary transfer portion Nt is unlikely to occur.

[0094] Now, a case where the timing of disconnecting and reconnecting the electromagnetic clutch 110 is changed will be described. If the electromagnetic clutch 110 is disconnected after the leading edge of the recording material S enters the secondary transfer portion Nt, the loop of the recording material S between the re-feed roller pair 74 and the registration roller pair 20a disappears during the period in which the toner image is transferred to the recording material S. Then, at the point in time when the registration roller pair 20a starts to pull out the recording material S from the re-feed roller pair 74, the moving speed of the recording material S at the registration roller pair 20a and the secondary transfer portion Nt changes suddenly due to the load that the recording material S receives from the re-feed roller pair 74, which is in a stopped state. As a result, there is a possibility that the toner image transferred to the recording material S may be disturbed.

[0095] Next, consider a case where the electromagnetic clutch 110 is released before the leading edge of the recording material S enters the secondary transfer portion Nt, and then re-engaged after the leading edge of the recording material S enters the secondary transfer portion Nt. In this case, the moment the electromagnetic clutch 110 is engaged, the load that the recording material S received from the re-feed roller pair 74, which is in a stopped state, disappears, and the moving speed of the recording material S at the registration roller pair 20a and the secondary transfer portion Nt changes suddenly. As a result, in this case as well, there is a possibility that the toner image transferred to the recording material S may be disturbed.

[0096] In contrast, in this embodiment, the electromagnetic clutch 110 is disconnected and reconnected during the period after the leading edge of the recording material S passes through the pair of registration rollers 20a and before it enters the secondary transfer portion Nt. This makes it possible to eliminate the difference in the loop amount of the recording material S between the left and right sides and suppress image distortion caused by the difference in the loop amount between the left and right sides without causing the above-mentioned inconvenience.

[0097] (Length of the period during which the pair of re-feed rollers is stopped from being driven) Here, an explanation will be given of the length of the period (drive stop time) during which the electromagnetic clutch 110 is released and the drive of the re-feed roller pair 74 is stopped. For simplicity of explanation, the following explanation will be given assuming that the re-feed roller pair 74 stops rotating at the same time that the electromagnetic clutch 110 transitions from the connected state to the released state.

[0098] When the recording material S enters the registration roller pair 20a, the length of the recording material S between the re-feed roller pair 74 and the registration roller pair 20a is L0, and the shortest length that the recording material S can have between the re-feed roller pair 74 and the registration roller pair 20a is Lmin. At this time, since the conveying speed of the registration roller pair 20a is higher than the conveying speed of the re-feed roller pair 74 (0 when the rotation is stopped), the length of the recording material S between the re-feed roller pair 74 and the registration roller pair 20a decreases from L0 during the drive stop period. However, the difference in conveying speed between the registration roller pair 20a and the re-feed roller pair 74 is absorbed by the decrease in the loop amount of the recording material S. Therefore, the force trying to pull the recording material S out of the re-feed roller pair 74 is small, and the slip of the recording material S against the re-feed roller pair 74 is very small.

[0099] When the length of the recording material S between the re-feed roller pair 74 and the registration roller pair 20a reaches Lmin, the difference in conveying speed between the registration roller pair 20a and the re-feed roller pair 74 is no longer absorbed by the reduction in the loop amount of the recording material S. Therefore, the recording material S is pulled by the registration roller pair 20a and the re-feed roller pair 74. As a result, the registration roller pair 20a begins to pull the recording material S out of the re-feed roller pair 74, and the recording material S slips noticeably relative to the re-feed roller pair 74.

[0100] As described above, if there is a difference between the left and right loop amounts of the recording material S immediately after the loop is formed in the registration section 18A (immediately after skew correction), the loop amount at the side end with the smaller loop amount disappears first during the drive stop period of the re-feed roller pair 74. That is, the side end with the smaller loop amount first reaches Lmin in length between the re-feed roller pair 74 and the registration roller pair 20a, and slippage with respect to the re-feed roller pair 74 begins to occur, reducing the difference between the left and right loop amounts. After that, the drive stop period continues, and when the length between the re-feed roller pair 74 and the registration roller pair 20a at the side end with the larger loop amount reaches Lmin, the difference between the left and right loop amounts is substantially eliminated.

[0101] Therefore, in order to reduce the difference between the left and right loop amounts during the drive stop period of the re-feed roller pair 74, a lower limit is set for the length of the drive stop period of the re-feed roller pair 74 so that slippage occurs at least at the side end on the side with the smaller loop amount with respect to the re-feed roller pair 74. Specifically, when the target value (set value) of the conveying speed when the registration roller pair 20a conveys the recording material S toward the secondary transfer portion Nt is Vreg [mm / sec], the length of the drive stop period is set to be at least longer than the following time T0 [sec]. T0=(L0-Lmin) / Vreg

[0102] In the above formula, (L0-Lmin) corresponds to the amount of loop formed in the recording material S for skew correction (the amount of excess recording material S conveyed by the re-feed roller pair 74 after the leading edge of the recording material hits the shutter 20d).

[0103] In order to substantially eliminate the difference between the left and right loop amounts during the drive stop period of the re-feed roller pair 74, the loop amount at the side end with the larger loop amount may be made to disappear during the drive stop period even if the difference between the left and right loop amounts is the expected maximum value. In this case, if the expected maximum value of the difference between the left and right loop amounts is Ldif, the lower limit of the length of the drive stop period of the re-feed roller pair 74 is the next time T1 that is longer than the above-mentioned time T0. Note that Ldif corresponds to the expected maximum value of the skew amount of the recording material S (the distance by which one end of the leading edge of the recording material leads the other end) in the state immediately before it hits the shutter 20d at the registration portion 18A. T1=((L0-Lmin)+Ldif) / Vreg

[0104] In this embodiment, an electromagnetic clutch 110 is used as a means for stopping the rotation of the re-feed roller pair 74. The electromagnetic clutch 110 is brought into a connected state by receiving power from an external source, and is brought into a disconnected state by stopping the power supply. An electromagnetic and mechanical response delay occurs from when the power supply is stopped until the electromagnetic clutch 110 actually transitions from the connected state to the disconnected state. If this is represented as Tc, then the lower limit of the drive stop time T2 of the re-feed roller pair 74 (the control time for the control unit 51 to issue a signal to turn the electromagnetic clutch 110 into the OFF state) taking into account the response delay of the electromagnetic clutch 110 can be expressed as follows: T2 ≥ T1 + Tc

[0105] On the other hand, the upper limit of the drive stop time T2 of the re-feed roller pair 74 will be described. As described above, after the electromagnetic clutch 110 is released to stop the drive of the re-feed roller pair 74, it is preferable to re-connect the electromagnetic clutch 110 and resume the drive of the re-feed roller pair 74 before the leading edge of the recording material S enters the secondary transfer portion Nt. Therefore, the upper limit of the drive stop time T2 of the re-feed roller pair 74 (the drive stop period for the control portion 51 to control the electromagnetic clutch 110) can be expressed as follows. Here, D0 is the distance measured along the conveying path from the registration roller pair 20a to the secondary transfer portion Nt, and Ds is the distance conveyed by the leading edge of the recording material S from the start of the drive of the registration roller pair 20a until the drive of the re-feed roller pair 74 is stopped. T2≦(D0-Ds) / Vreg

[0106] In a small laser beam printer according to a specific configuration example of this embodiment, the following values ​​can be assumed for each parameter, as an example. Vreg 180[mm / sec] L0-Lmin 1.0[mm] Ldif 2.0 [mm] D0 35[mm] Ds 9 [mm] Tc 10 [msec]

[0107] Substituting the above parameters into the above formula gives the following: T1=(1.0+2.0) / 180=0.017[sec]≒17[msec] T2 ≧ T1 + Tc ≒ 27 [msec] T2≦(35-9) / 180≒0.14[sec]=140[msec]

[0108] Therefore, it is desirable to set the drive stop time T2 of the re-feed roller pair 74 in the above configuration example within the following range. 27 [msec] ≤ T2 ≤ 140 [msec]

[0109] From the viewpoint of the load on the recording material S and wear of the re-feed roller pair 74 due to slippage, it is preferable that the period during which the recording material S is pulled out from the re-feed roller pair 74 is as short as possible. Therefore, a more preferable range of the drive stop time T2 of the re-feed roller pair 74 in the above configuration example is 30 msec or more and 50 msec or less.

[0110] As described above, in this embodiment, in order to solve the image distortion caused by the difference between the left and right loop amounts occurring between the re-feed roller pair 74 and the registration roller pair 20a, the difference between the left and right loop amounts is eliminated by temporarily suspending the driving of the re-feed roller pair 74 for a predetermined period of time. This makes it possible to improve image accuracy with a simple configuration and control.

[0111] (Control example) A control method for implementing the above-described operation of image forming apparatus 100 will be described with reference to the flowchart of Fig. 17. This flowchart is implemented by a CPU reading out a control program from a ROM and executing it in control unit 51 as control means of image forming apparatus 100 shown in Fig. 7. The CPU executes the control program using RAM as a work area, and issues a command signal for operating feed motor M1 and electromagnetic clutch 110 based on a detection signal of pre-registration sensor 52, etc.

[0112] When an image forming execution instruction including image information is input from an external computer to the image forming apparatus 100, an image forming job (hereinafter, simply referred to as a job), which is a series of tasks for performing image formation, is started. When the job is started, one sheet of recording material S is fed from a feeding tray S1 (S1), and skew is corrected in a registration section 18A. Next, the pair of registration rollers 20a conveys the recording material S to a secondary transfer section Nt at a timing synchronized with the image forming process by the image forming mechanism 100C, and the recording material S is then conveyed to a fixing device 23, whereby an image is transferred and fixed on the front surface (first surface) of the recording material S (S2).

[0113] The recording material S with an image formed on the first side is guided to a double-sided conveying path, reversed and conveyed by the reversing roller pair 73 (S3), and conveyed toward the registration section 18A by the double-sided roller pair 72 and the re-feed roller pair 74. Then, based on the timing when the pre-registration sensor 52 detects the leading edge of the recording material S (S4: Yes), the feeding motor M1 is once turned off (S6) at the timing when the target loop amount is formed in the recording material S (S5: Yes). As a result, a loop is formed in the recording material S with the leading edge of the recording material S abutting against the shutter 20d, and the skew of the recording material S is corrected. Also, when the loop amount of the recording material S becomes a certain size, the shutter 20d retracts, and the leading edge of the recording material S enters the nip portion of the registration roller pair 20a and is sandwiched by the registration roller pair 20a.

[0114] Thereafter, at a transport timing synchronized with the image forming process by the image forming mechanism 100C for forming an image on the second side of the recording material S (S7: Yes), the feed motor M1 is turned ON (S8), and the pair of registration rollers 20a transports the recording material S toward the secondary transfer portion Nt. At this point, the electromagnetic clutch 110 is ON (connected state), and the pair of re-feed rollers 74 is driven in synchronization with the pair of registration rollers 20a.

[0115] Thereafter, after the start-up of the feeding motor M1 is completed (i.e., after the motor rotation speed reaches the target speed during image transfer), the electromagnetic clutch 110 is turned OFF (disconnected) and the driving of the re-feed roller pair 74 is stopped (S9, S10). After issuing a signal to turn OFF the electromagnetic clutch 110, the control unit waits until the drive stop time T2, which is preset according to the above-mentioned criteria, has elapsed (S11), and turns ON (connected) the electromagnetic clutch 110 again at the point when the drive stop time T2 has elapsed (S12). As described above, during this drive stop period, the loop amount between the re-feed roller pair 74 and the registration roller pair 20a is reduced, and further, the registration roller pair 20a pulls out the recording material S while slipping it relative to the re-feed roller pair 74, thereby eliminating the difference between the left and right loop amounts.

[0116] After the re-feed roller pair 74 resumes driving, the recording material S enters the secondary transfer portion Nt, an image is transferred to the second side of the recording material S, and the image is fixed in the fixing device 23 (S13). The recording material S is then discharged as a product onto the discharge tray 25 by the discharge roller pair 24 (S14). When image formation is instructed on multiple sheets of recording material S, the above series of processes are repeatedly executed to continuously output multiple products.

[0117] (Modification) Incidentally, some image forming apparatuses are configured to temporarily stop conveying the recording material S after the leading edge of the recording material S passes through the pair of registration rollers 20a in order to align the image on the endless belt 13 with the recording material during double-sided printing. In this case, the timing for temporarily releasing the connection of the electromagnetic clutch 110 will be described.

[0118] First, when the conveyance of the recording material is stopped in order to align with the timing of the image as described above, it is preferable not to release the electromagnetic clutch 110. In this case, even if the electromagnetic clutch 110 is released, the registration roller pair 20a is not rotating, so the loop amount of the recording material does not decrease, and the effect of eliminating the difference in the loop amount between the left and right cannot be expected. In addition, because the load of the drive train connected to the re-feed roller pair 74 is eliminated by releasing the electromagnetic clutch 110, there is a possibility that the re-feed roller pair 74 may rotate forward or backward unintentionally. If the re-feed roller pair 74 rotates forward or backward unintentionally, it becomes difficult to control the loop amount of the recording material S, which may result in a result contrary to the elimination of the difference in the loop amount between the left and right. Therefore, it is preferable not to release the electromagnetic clutch 110 while the conveyance of the recording material is stopped.

[0119] In addition, when disconnecting the electromagnetic clutch 110 after restarting the conveyance of the recording material after the timing is adjusted, it is preferable to avoid disconnecting the electromagnetic clutch 110 during acceleration (while the feed motor M1 is starting up) before the conveyance speed of the registration roller pair 20a reaches the preset speed, and to disconnect the electromagnetic clutch 110 after the preset speed is reached. If the feed motor M1 is disconnected during acceleration, the amount of reduction in the loop will vary depending on the way the motor accelerates, which may result in no effect being obtained or excessive pulling.

[0120] (Other embodiments) In this embodiment, a shutter mechanism is provided as a registration mechanism on the upstream side of the pair of registration rollers 20a in the conveying direction, against which the recording material strikes, and the recording material aligned by the shutter mechanism is forced into the pair of registration rollers 20a. This is not limiting, and for example, the pair of registration rollers 20a may be configured to be driven and stopped using a clutch mechanism or the like without providing a shutter mechanism, and the leading edge of the recording material may be struck against the nip portion of the pair of registration rollers 20a in the stopped state to form a loop, thereby performing skew correction. After the skew correction, the pair of registration rollers 20a is rotated again to resume conveyance of the recording material. In this case, the nip portion of the pair of registration rollers 20a corresponds to the "strike portion" against which the leading edge of the recording material strikes.

[0121] In addition, in the present embodiment, a case has been described in which a difference in the amount of loop between the left and right sides occurs due to skew correction of the recording material in the registration unit 18A of the leading edge abutting method, but a difference in the amount of loop between the left and right sides may occur for other reasons. For example, a difference in the amount of loop between the left and right sides of the recording material may occur due to misalignment of the conveying rollers that configure the conveying path until the recording material reaches the registration unit 18A or non-uniformity in the roller diameter. Therefore, the present technology is effective in reducing a decrease in image accuracy caused by a difference in the amount of loop between the left and right sides of the recording material even when a registration unit other than the leading edge abutting method is used.

[0122] In the present embodiment, the electromagnetic clutch 110 is used to connect and disconnect the drive transmission to the re-feed roller pair 74, on the assumption that the registration roller pair 20a and the re-feed roller pair 74, which are the upstream roller pair, are driven by one motor (feed motor M1). This is not limiting, and even if the registration roller pair 20a and the upstream roller pair are driven by separate motors, the drive of the upstream roller pair may be temporarily stopped during the period from when the registration roller pair 20a starts conveying the recording material to when the recording material enters the secondary transfer portion Nt. It is preferable to dispose a ratchet mechanism (one-way clutch) between the upstream roller pair and the motor that drives the upstream roller pair, which allows the upstream roller pair to rotate idly when the recording material moves downstream in the conveying direction while the motor is stopped. This reduces the pull-out load when the registration roller pair 20a pulls out the recording material from the upstream roller pair during the drive stop period of the upstream roller pair to eliminate the difference between the left and right loop amounts.

[0123] In addition, in this embodiment, an electrophotographic image forming unit is exemplified as the image forming mechanism 100C, but the present technology is also applicable to image forming apparatuses equipped with other image forming mechanisms, such as an inkjet type or offset printing type. [Explanation of symbols]

[0124] M1, 110... driving device (feed motor, electromagnetic clutch) / Nt... image forming section (transfer section) / 18A... registration section / 20a... registration roller pair / 20d... shutter / 51... control means (control section) / 74... upstream roller pair (re-feed roller pair) / 100... image forming apparatus

Claims

1. an image forming section for forming an image on a recording material; a registration unit located upstream of the image forming unit in a conveying direction of the recording material, the registration unit having a pair of registration rollers that conveys the recording material to the image forming unit; an upstream roller pair located upstream of the registration portion in the transport direction and configured to transport the recording material to the registration roller pair; a drive device that drives the pair of registration rollers and the pair of upstream rollers; A control means for controlling the driving device; a reversing means for reversing and conveying the recording material having an image transferred onto a first surface thereof in the image forming section in order to transfer an image onto a second surface thereof opposite to the first surface; a double-sided conveying path along which the recording material inverted by the inverting means is conveyed again toward the registration unit; Equipped with the upstream roller pair is disposed on the double-sided conveying path, the control means is configured to control the drive device so as to (i) stop driving the upstream roller pair after the leading edge of the recording material has passed the registration roller pair and before the leading edge of the recording material enters the image forming unit, and (ii) resume driving the upstream roller pair before the leading edge of the recording material enters the image forming unit; an image forming apparatus, characterized in that the registration roller pair is configured to transport the recording material so that the recording material slips relative to the upstream roller pair while the drive of the upstream roller pair is stopped.

2. 2. The image forming apparatus according to claim 1, An image forming apparatus characterized in that, when the drive of the upstream roller pair is resumed, the control means stops the drive of the upstream roller pair so that the length of the portion of the recording material located between the upstream roller pair and the registration roller pair becomes the shortest length that the recording material can have on the transport path from the upstream roller pair to the registration roller pair.

3. 3. The image forming apparatus according to claim 1, the drive device includes a motor that supplies a drive force for driving the registration roller pair and the upstream roller pair, and a clutch that is disposed between the motor and the upstream roller pair and that connects and disconnects drive transmission between the motor and the upstream roller pair, The image forming apparatus according to the present invention, wherein the control means stops driving the pair of upstream rollers by switching the clutch from a connected state to a disconnected state.

4. An image forming section for forming an image on a recording material; a registration unit located upstream of the image forming unit in a conveying direction of the recording material, the registration unit having a pair of registration rollers that conveys the recording material to the image forming unit; an upstream roller pair located upstream of the registration portion in the transport direction and configured to transport the recording material to the registration roller pair; a drive device that drives the pair of registration rollers and the pair of upstream rollers; A control means for controlling the driving device; Equipped with the drive device includes a motor that supplies a drive force for driving the registration roller pair and the upstream roller pair, and a clutch that is disposed between the motor and the upstream roller pair and that connects and disconnects drive transmission between the motor and the upstream roller pair, the control means is configured to control the drive device so as to (i) stop driving the pair of upstream rollers by switching the clutch from a connected state to a disconnected state after the leading edge of the recording material has passed the pair of registration rollers and before the leading edge of the recording material enters the image forming unit, and (ii) resume driving the pair of upstream rollers before the leading edge of the recording material enters the image forming unit; an image forming apparatus, characterized in that the registration roller pair is configured to transport the recording material so that the recording material slips relative to the upstream roller pair while the drive of the upstream roller pair is stopped.

5. 5. The image forming apparatus according to claim 3, The image forming apparatus according to claim 1, wherein the clutch is an electromagnetic clutch.

6. 6. The image forming apparatus according to claim 1, the registration section has an abutment section against which the leading edge of the recording material abuts, the control means causes the upstream roller pair to convey the recording material so that the leading edge of the recording material abuts against the abutting portion, thereby bending the recording material between the upstream roller pair and the abutting portion, thereby correcting skew of the leading edge of the recording material, and then causes the registration roller pair to start conveying the recording material.

1. An image forming apparatus comprising:

7. An image forming section for forming an image on a recording material; a registration unit located upstream of the image forming unit in a conveying direction of the recording material, the registration unit having a pair of registration rollers that conveys the recording material to the image forming unit; an upstream roller pair located upstream of the registration portion in the transport direction and configured to transport the recording material to the registration roller pair; a drive device that drives the pair of registration rollers and the pair of upstream rollers; A control means for controlling the driving device; Equipped with the registration section has an abutment section against which the leading edge of the recording material abuts, the control unit causes the upstream roller pair to convey the recording material so that the leading edge of the recording material abuts against the abutting portion, thereby bending the recording material between the upstream roller pair and the abutting portion, thereby correcting skew of the leading edge of the recording material, and then causes the registration roller pair to start conveying the recording material; the control means is configured to control the drive device so as to (i) stop driving the upstream roller pair after the leading edge of the recording material has passed the registration roller pair and before the leading edge of the recording material enters the image forming unit, and (ii) resume driving the upstream roller pair before the leading edge of the recording material enters the image forming unit; an image forming apparatus, characterized in that the registration roller pair is configured to transport the recording material so that the recording material slips relative to the upstream roller pair while the drive of the upstream roller pair is stopped.

8. 8. The image forming apparatus according to claim 6, An image forming apparatus characterized in that the abutment portion is provided on a shutter that can be in a closed state that prevents the leading edge of the recording material from entering the nip portion of the pair of registration rollers, and an open state that allows the recording material to enter the nip portion.

9. 9. The image forming apparatus according to claim 1, The image forming apparatus is characterized in that, after the control means starts transporting the recording material by the registration roller pair, the transport speed of the registration roller pair is accelerated to the transport speed of the registration roller pair when an image is formed on the recording material in the image forming section, and then the control means stops driving the upstream roller pair.

10. 10. The image forming apparatus according to claim 1, 2. An image forming apparatus according to claim 1, wherein a conveying speed of the recording material by the pair of upstream rollers is equal to or higher than a conveying speed of the recording material by the pair of registration rollers.

11. 11. The image forming apparatus according to claim 1, An image forming apparatus characterized in that a friction force acting on the recording material from the upstream roller pair in a direction along the transport direction of the recording material at the nip portion of the upstream roller pair is smaller than a friction force acting on the recording material from the registration roller pair in a direction along the transport direction of the recording material at the nip portion of the registration roller pair.

12. 12. The image forming apparatus according to claim 1, an image forming apparatus characterized in that the pressure force with which the upstream roller pair pinches the recording material at the nip portion of the upstream roller pair is 90% or less of the pressure force with which the registration roller pair pinches the recording material at the nip portion of the registration roller pair.

13. 13. The image forming apparatus according to claim 1, The rotation of the upstream roller pair is stopped by stopping the driving of the upstream roller pair, an image forming apparatus, characterized in that a portion of the recording material that was in contact with the upstream roller pair when the upstream roller pair stopped is moved downstream of the upstream roller pair during a period in which the drive of the upstream roller pair is stopped.

14. an image forming section for forming an image on a recording material; a registration section that is located upstream of the image forming section in a conveying direction of the recording material and includes a pair of registration rollers that convey the recording material to the image forming section and an abutment section with which a leading edge of the recording material abuts; an upstream roller pair located upstream of the registration portion in the conveying direction and configured to convey the recording material to the registration roller pair; a drive device that drives the pair of registration rollers and the pair of upstream rollers; A control means for controlling the driving device; Equipped with the control means causes the upstream roller pair to convey the recording material in a state in which the leading edge of the recording material abuts against the abutment portion, and corrects skew of the leading edge of the recording material by bending the recording material between the upstream roller pair and the abutment portion, and then causes the registration roller pair to start conveying the recording material; the control means controls the drive device so as to (i) stop driving the pair of upstream rollers after the leading edge of the recording material has passed the pair of registration rollers and before the leading edge of the recording material enters the image forming unit, and (ii) resume driving the pair of upstream rollers before the leading edge of the recording material enters the image forming unit; Immediately before the pair of registration rollers starts to convey the recording material, a length of a portion of the recording material located between the pair of upstream rollers and the pair of registration rollers is defined as L0 [mm]; The shortest length that the recording material can have in the conveying path from the upstream roller pair to the registration roller pair is Lmin [mm], A target value of the conveying speed of the recording material by the pair of registration rollers is Vreg [mm / sec], The image forming apparatus according to claim 1, wherein the length of the period during which the control means stops driving the pair of upstream rollers is at least longer than (L0-Lmin) / Vreg.

15. 15. The image forming apparatus according to claim 1, The image forming apparatus according to claim 1, wherein the length of the period during which the control means stops driving the pair of upstream rollers is 30 msec or more and 50 msec or less.

16. 16. The image forming apparatus according to claim 1, the image forming section includes an image carrier that carries an image, and a transfer section that transfers the image formed on the image carrier to the recording material, The image forming apparatus, wherein the pair of registration rollers transports the recording material toward the transfer portion.

Images