JP7652554B2 - Sheet conveying device and image forming apparatus - Google Patents

Description

The present invention relates to a sheet conveying device that conveys a sheet and an image forming apparatus equipped with the same.

Conventionally, image forming devices that form images on sheets are provided with a skew correction mechanism that corrects the skew of the sheet being conveyed in order to align the sheet with the position of the image formed on the sheet.

In Patent Document 1, a pre-registration roller pair provided upstream in the sheet conveying direction from a registration roller pair that is not driven conveys the sheet a predetermined amount to correct the skew of the sheet. That is, the leading edge of the sheet conveyed by the pre-registration roller pair is abutted against the registration roller pair, and a loop is formed in the sheet between the pre-registration roller pair and the registration roller pair to correct the skew of the sheet.

Japanese Patent Application Publication No. 6-127753

In Patent Document 1, after the skew has been corrected, the registration roller pair and the pre-registration roller pair start conveying the sheet at the same speed by re-driving the registration roller pair in synchronization with the image signal, and the sheet is conveyed to the secondary transfer section. In this case, the sheet conveyed from the registration roller pair to the secondary transfer section is conveyed in a state in which the size of the loop formed between the pre-registration roller pair and the registration roller pair is maintained.

However, the optimal values for the size of the sheet loop for skew correction and the size of the sheet loop to be maintained while being transported from the registration roller pair to the secondary transfer section are different.

In other words, the size of the sheet loop for skew correction is necessary to correct the skew of the sheet before it reaches the pair of registration rollers, and is determined in advance based on the largest amount of skew that should be expected. Furthermore, when loop formation control is performed using a detection signal from a sensor immediately before the pair of registration rollers as a trigger, a skewed sheet will be detected, and the size is determined taking into account this variation.

On the other hand, if the size of the loop of the sheet maintained during transport is too large, the loop will buckle between the pre-registration roller pair and the registration roller pair. Therefore, it is preferable that the size of the loop of the sheet maintained during transport be small to the extent that the sheet is not pulled by the pre-registration roller pair and the registration roller pair.

Therefore, if the sheet is transported from the registration roller pair to the secondary transfer section while maintaining the size of the sheet loop for skew correction, there is a risk that the sheet will buckle due to the large loop between the roller pair.

The object of the present invention is to reduce buckling of the sheet between the pre-registration roller pair and the registration roller pair when the sheet is transported to the secondary transfer section.

A representative configuration of the present invention for achieving the above object is a sheet transporting device that transports a sheet to a transfer transport section that transfers a toner image carried on an image carrier to a sheet, the sheet transporting device having a pair of pre-registration rollers that form a nip portion and transport the sheet by sandwiching it, a pair of registration rollers that are provided downstream in a sheet transport direction from the pair of pre-registration rollers and that form a nip portion and transport the sheet by sandwiching it, and a control section that controls the pair of pre-registration rollers and the pair of registration rollers, the control section transports the sheet by the pair of pre-registration rollers, brings a leading end of the sheet into contact with the nip portion of the pair of registration rollers that are stopped from rotating, and forms a first loop by bending the sheet to one side between the pair of pre-registration rollers and the pair of registration rollers, and controls the pair of pre-registration rollers to control the pair of registration rollers to control ... and, after the sheet has been transported the predetermined distance by the registration roller pair, the sheet transport speed by the pre-registration roller pair is set to a transport speed slower than the sheet transport speed by the registration roller pair until the leading end of the sheet reaches the transfer transport section, forming a second loop on the sheet between the pre-registration roller pair and the registration roller pair that is smaller than the first loop , and, after the sheet has been transported the predetermined distance, the sheet transport speed by the pre-registration roller pair is set to a transport speed slower than the sheet transport speed by the registration roller pair, and the sheet is accelerated to a transport speed faster than the sheet transport speed by the transfer transport section and the same as the sheet transport speed by the registration roller pair at a timing later than the registration roller pair .

According to the present invention, when a sheet is transported to the transfer transport section, the loop of the sheet between the pre-registration roller pair and the registration roller pair is reduced, thereby reducing buckling of the sheet therebetween.

FIG. 1 illustrates a sheet conveying unit in an image forming apparatus. 4A, 4B, and 4C are diagrams showing a sheet skew correction operation; Block diagram of a control unit of an image forming apparatus FIG. 1A is a diagram showing the positions of the leading and trailing ends of a sheet in a sheet conveying section; FIG. 1B is a diagram showing the relationship between the conveying speed of a roller and time; 1A and 1B are diagrams illustrating a loop shape of a sheet caused by a sheet conveying unit; FIG. 13 is a diagram showing a loop shape of a sheet produced by a sheet conveying unit of a comparative example; 1A and 1B are diagrams showing the relationship between the roller conveying speed and time. FIG. 1A is a diagram showing the positions of the leading and trailing ends of a sheet in a sheet conveying section; FIG. 1B is a diagram showing the relationship between the conveying speed of a roller and time; FIG. 1 is a schematic cross-sectional view showing an overall configuration of an image forming apparatus.

The following describes an exemplary embodiment of the present invention with reference to the drawings. However, the components described in the following description are merely examples and do not limit the scope of the present invention.

First, the schematic configuration of an image forming apparatus to which a sheet conveying device is applied will be described with reference to FIG. 9. FIG. 9 is a schematic cross-sectional view showing the schematic configuration of a color digital printer as an example of an image forming apparatus.

The image forming apparatus 1 shown in FIG. 9 is mainly composed of an image forming unit, a secondary transfer unit, and a sheet conveying device (sheet conveying unit).

The image forming section of the image forming device 1 is composed of photoconductors 11 corresponding to the colors yellow (Y), magenta (M), cyan (C), and black (Bk), a charging device 12, an exposure device 13 that functions as an image writing section, and a developing device 14. The image forming section also has an intermediate transfer belt (image carrier) 21, a secondary transfer inner roller 22, and a primary transfer device 25.

In the image forming section, the surface of the photoconductor 11 is uniformly charged in advance by the charging device 12, and the exposure device 13 is driven based on the image information signal to form a latent image on the surface of the rotating photoconductor 11. The electrostatic latent image formed on the surface of the photoconductor 11 is developed by the developing device 14 into a toner image. A predetermined pressure and electrostatic bias are then applied by the primary transfer device 25, and the toner image is transferred onto the intermediate transfer belt 21.

Next, the intermediate transfer belt 21, which is an image carrier, will be described. The intermediate transfer belt 21 is stretched by rollers such as a drive roller 23, a tension roller 24, and a secondary transfer inner roller 22, and is driven to be transported in the direction of arrow B shown in FIG. 9. The image formation processes for each color Y, M, C, and Bk, which are processed in parallel, are performed at a timing that overlaps with the toner image of the upstream color that has been primarily transferred onto the intermediate transfer belt 21. As a result, a full-color toner image is finally formed on the intermediate transfer belt 21, and this toner image on the intermediate transfer belt 21 (image carrier) is transported to the secondary transfer section.

Here, the intermediate transfer belt 21 is exemplified as an image carrier that carries a toner image, but the image carrier is not limited to this. For example, in an image forming device that transfers a toner image formed on a photoreceptor directly to a sheet, the photoreceptor is the image carrier.

The sheets P, which are the transfer material, are stored, for example, by size in paper feed cassettes (sheet storage units) 31, 32 and manual feed section 33, and in order to record an image in the appropriate range, the size of the stored sheets must be recognized by the image forming device body.

The paper feed cassettes 31 and 32 are each equipped with a sheet size detection unit 31d, 32d for detecting the size of the sheets stored therein. The sheet size detection units 31d, 32d are composed of a size detection lever (cam) and multiple sensors or switches. The detection lever (cam) is rotatably provided and slides against a side regulation plate that regulates the position of the sheet in the width direction perpendicular to the conveying direction. The multiple sensors or switches are provided at positions corresponding to the size detection lever in the mounting section where the paper feed cassette is mounted.

Therefore, when the side regulating plate is moved to match the widthwise edge of the sheet, the size detection lever rotates in tandem. When the paper feed cassette is attached to the attachment section in this state, the size detection lever selectively turns ON/OFF the detection element of the sensor or switch located in the attachment section where the paper feed cassette is attached. This causes signals of different patterns to be sent from the sensor or switch to the image forming device main body. The image forming device main body can then recognize the size of the sheets stored in the paper feed cassette based on the signals.

The sheets P are fed one by one from, for example, the paper feed section 31a (or the paper feed section 32a). The sheet feed section (sheet feeder) includes a pair of pre-registration rollers 41 and a pair of registration rollers 42 that correct the skew and send the sheet to the secondary transfer section. Specifically, the sheet P is transported by the pair of pre-registration rollers 41, and its leading edge is abutted against the nip of the pair of registration rollers 42, which is stopped. In this way, the pair of pre-registration rollers 41 loops the sheet P between itself and the pair of stopped registration rollers 42, correcting the skew of the sheet.

The registration roller pair 42 transports the sheet P to the secondary transfer section in accordance with the timing at which the toner image on the intermediate transfer belt 21 is transferred to the sheet P.

The secondary transfer section has a toner image transfer nip formed by the secondary inner transfer roller 22 and the secondary outer transfer roller 44, which face each other via the intermediate transfer belt 21, and transfers the toner image onto the sheet P by applying a predetermined pressure and electrostatic bias. The secondary outer transfer roller 44 is a transfer transport section that transfers the toner image carried on the intermediate transfer belt 21, which is an image carrier, to the sheet P, and is provided downstream in the sheet transport direction from the registration roller pair 42. The secondary outer transfer roller 44 faces the secondary inner transfer roller 22 via the intermediate transfer belt 21 in the secondary transfer section, and transports the sheet by nipping it between the secondary inner transfer roller 22 and the secondary outer transfer roller 44.

After transfer, the sheet P is transported to the fixing device 50, where pressure and heat are applied to melt and fix the toner image to the sheet P. After fixing, the sheet P is transported to the discharge transport section 60, and the transport path is switched by a flapper 64 depending on whether the paper passing mode is single-sided or double-sided. In other words, in the single-sided mode, the flapper 64 switches to the discharge roller 62 side, and the sheet P is discharged by the discharge roller 62 toward the discharge tray 80 and stacked.

On the other hand, in the case of the double-sided mode, the flapper 64 switches to the discharge roller 63 side, and the sheet P is transported by the discharge roller 63 to the reversal roller 71, which then discharges it towards the discharge tray 82. The sheet P is then stopped once with its trailing end remaining a specified distance on the reversal roller 71. The sheet P is then transported again to the registration roller pair 42 via the double-sided conveying section 70 by the reverse rotation of the reversal roller 71, and the image on the second side is formed and fixed in the image forming section.

After fixing, the sheet P passes through the flapper 64 and is discharged by the discharge rollers 62 or the reversing rollers 71 toward the discharge tray 80 or the discharge tray 82, where it is stacked.

Next, a sheet conveying device applied to an image forming apparatus will be described with reference to FIG. 1. FIG. 1 is a cross-sectional view showing a sheet conveying device in an image forming apparatus. The sheet conveying section 40 shown in FIG. 1 is provided in a sheet conveying path that connects paper feed cassettes 31 and 32 and the image forming section in the image forming apparatus 1, and is a sheet conveying device that corrects skewed sheets. FIG. 2 is a schematic diagram for explaining the operation of correcting skewed sheets by the sheet conveying section 40.

The sheet conveying section 40 corrects the skew of the sheet P and conveys the sheet P to the secondary transfer section where the toner image carried on the intermediate transfer belt 21 is transferred. The sheet conveying section 40 has a pair of pre-registration rollers 41 and a pair of registration rollers 42. The sheet conveying section 40 further has a registration sensor 43. The pair of pre-registration rollers 41 and the pair of registration rollers 42 are controlled by the control section 200 (CPU 201) described later.

The pre-registration roller pair 41 is a pair of transport rollers that sandwich and transport the sheet P. The pre-registration roller pair 41 includes a driven roller 41a having a roller made of polyacetal (POM) and a drive roller 41b formed of a rubber roller. The pre-registration roller pair 41 has the driven roller 41a and the drive roller 41b arranged facing each other. The driven roller 41a is pressed against the drive roller 41b by the elastic force of a spring (not shown), and rotates following the drive roller 41b, which is driven to rotate.

The registration roller pair 42 is a pair of conveying rollers that is provided downstream of the pre-registration roller pair 41 in the sheet conveying direction and conveys the sheet P by pinching it between them. The registration roller pair 42 includes a driven roller 42a having a roller made of polyacetal (POM) and a driving roller 42b formed of a rubber roller. The registration roller pair 42 is arranged with the driven roller 42a and the driving roller 42b facing each other. The driven roller 42a is pressed against the driving roller 42b by the elastic force of a spring (not shown), and rotates following the driving roller 42b, which is driven to rotate.

The sheet being conveyed by the pre-registration roller pair 41 is corrected for skew by having the leading edge of the sheet come into contact with the contact area (nip area) where the stationary driven roller 42a and drive roller 42b come into contact.

The registration sensor 43 is a sheet detection unit for detecting the sheet P. The registration sensor 43 is disposed between the pre-registration roller pair 41 and the registration roller pair 42, immediately before the registration roller pair 42. The registration sensor 43 is also disposed in the center (dotted line shown in FIG. 2) in the width direction (thrust direction) perpendicular to the conveying direction of the sheet P (direction of arrow A in FIG. 2).

In FIG. 1, 101 denotes a pre-registration motor, which is a first drive source for driving the drive roller 41b of the pre-registration roller pair 41. 102 denotes a registration motor, which is a second drive source for driving the drive roller 42b of the registration roller pair 42.

Between the pre-registration roller pair 41 and the registration roller pair 42, guides 45 and 46 are arranged to guide the sheet being conveyed. The distance between the guides 45 and 46 is partially widened, and the guides are configured to allow the sheet that hits the nip of the registration roller pair 42 to form a loop.

Specifically, one guide 45 guides one side of the sheet being conveyed. The other guide 46 is provided opposite the one guide 45 and guides the other side of the sheet being conveyed. Here, the one guide 45 is partially spaced apart from the other guide 46. Therefore, the sheet conveyed by the pre-registration roller pair 41 and whose leading edge is brought into contact with the nip portion of the registration roller pair 42 forms a loop between the roller pairs 41 and 42, moving away from the other guide 46 and approaching the one guide 45. In other words, the sheet forms a loop between the roller pairs 41 and 42, bending the sheet toward one side.

Next, the operation of correcting skew of a sheet by the sheet conveying unit 40 will be described with reference to the schematic diagram in FIG. 2.

A skewed sheet is a state in which one side of the leading edge of the sheet in the width direction leads the other side, based on the center of the sheet in the width direction perpendicular to the conveying direction (the dashed line in the figure). Here, as shown in FIG. 2(a), an example is shown in which the left side of the leading edge of the sheet in the width direction leads the right side, in other words, the sheet is skewed clockwise with respect to the conveying direction.

As shown in FIG. 2(a), the skew correction operation will be described when the sheet is skewed clockwise with respect to the conveying direction (the direction of the arrow A). FIG. 2(a) shows the state in which the leading edge of the sheet P has reached the registration sensor 43. When the pre-registration roller pair 41 further rotates and the sheet P is conveyed in the conveying direction, the leading edge on the left side of the sheet P abuts against the nip portion of the registration roller pair 42. At this time, the registration roller pair 42 stops rotating.

Furthermore, when the pre-registration roller pair 41 rotates and the sheet P is transported in the transport direction, as shown in FIG. 2B, the entire leading edge of the sheet P comes into contact with the nip of the registration roller pair 42 across the width. At this time, the leading edge of the sheet P is stopped as the leading corner comes into contact with the nip of the registration roller pair 42, and the sheet gradually forms a loop and rotates so that the corner on the opposite side in the width direction to the corner that comes into contact with the nip approaches the nip. Then, a loop is formed between the registration roller pair 42 and the pre-registration roller pair 41, bending the sheet P to one side. As a result, the entire leading edge of the sheet P in the width direction comes into contact with the nip of the registration roller pair 42, correcting the skew.

Here, when correcting the skew of a sheet, the amount of loop formed in the sheet is appropriately set based on the size, basis weight, etc. of the sheet (hereinafter referred to as sheet information). The control unit 200 shown in FIG. 3 can determine the optimal amount of loop based on at least one of the sheet information specified by the user via the operation unit 203 and the sheet information detected by the sheet size detection units 31d and 32d, or a combination of these.

Then, in the image forming section, a toner image is formed on the intermediate transfer belt 21. Then, in synchronization with this, the registration roller pair 42 and the pre-registration roller pair 41 rotate, and the sheet P is transported to the secondary transfer section with its skew corrected, as shown in FIG. 2(c).

Here, before the leading edge of the sheet reaches the secondary transfer section, the control unit 200 sets the conveying speed of the pre-registration roller pair 41 to a slower speed than that of the registration roller pair 42, thereby reducing the loop of the sheet formed between the roller pairs 41 and 42. This is a characteristic operation of the present invention, and the reason for this will be explained below.

When the registration sensor 43 is disposed at the center in the thrust direction, the loop amount _r1C of the sheet P at the same thrust position as the registration sensor 43 is a predetermined amount as described above. As shown in Fig. 2B, if the loop amount of the leading end of the sheet P is _r1R and the loop amount of the delayed end is _r1F , the relationship of the loop amounts is _r1R > _r1C > _r1F . In other words, the loop amount of the leading side of the sheet P is larger than the preset loop amount _r1C by the amount of the lead.

A predetermined loop space is secured between the guide 45 and the guide 46, assuming the amount of loop of the sheet P formed between the pre-registration roller pair 41 and the registration roller pair 42. However, if the amount of skew of the sheet P before it reaches the registration roller pair 42 is large, the amount of loop may be greater than expected. In this case, if the force received from the sheet P exceeds the force with which the registration roller pair 42 holds the sheet P, the sheet slips at the nip of the registration roller pair 42, causing the sheet to skew or wrinkle. Alternatively, the loop of the sheet may buckle, generating a buckling sound. In addition, the load torque of the pre-registration motor may increase, causing the motor to stop.

In this embodiment, which has been made in consideration of such a situation, after forming a loop amount r _1C required for skew correction on the sheet, the loop amount is reduced to r _2C during the period from when the pair of registration rollers 42 starts conveying to when the sheet is conveyed to the secondary transfer portion. In other words, the control unit 201 conveys the sheet by the pair of pre-registration rollers 41 and makes the leading edge of the sheet abut against the pair of registration rollers 42, which are stopped from rotating. Then, a first loop (loop amount r _1C ) is formed by bending the sheet to one side between the pair of pre-registration rollers and the pair of registration rollers. Furthermore, the control unit 200 sets the conveying speed of the pair of pre-registration rollers 41 to be slower than that of the pair of registration rollers 42 during the period from when the pair of registration rollers 42 starts rotating to when the leading edge of the sheet reaches the secondary transfer portion. Then, a second loop (loop amount r _2C ) smaller than the first loop is formed on the sheet between the pair of pre-registration rollers and the pair of registration rollers. This makes it possible to keep r2R, which is the maximum loop amount on the leading side, within an allowable range, and eliminates the risk of problems such as the aforementioned buckling of the sheet loop.

Next, the control unit of the image forming apparatus 1 equipped with the sheet conveying unit 40 according to this embodiment and the flow of the sheet skew correction operation and loop amount control performed by this control unit will be described with reference to Figures 3 and 4. Figure 3 is a block diagram for explaining the control unit of the image forming apparatus 1. Figure 4(a) is a time series graph of the trailing edge position of the preceding sheet and the leading edge position of the succeeding sheet conveyed by the pre-registration roller pair 41 and the registration roller pair 42. Figure 4(b) shows a drive diagram of the pre-registration motor 101 and the registration motor 102, with the horizontal axis representing time and the vertical axis representing the sheet conveying speed of the pre-registration rollers and the registration rollers driven by the motors.

As shown in FIG. 3, the control unit 200 has various functional units such as a CPU (Central Processing Unit) 201, memory 202, an operation unit 203, an image formation control unit 205, and a sheet transport control unit 206. The CPU 201 executes predetermined control programs and the like to realize various processes performed by the image forming device 1. The memory 202 is, for example, a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores various programs and various data in a predetermined storage area. The operation unit 203 accepts various operations performed by the user, such as various information related to the sheets used by the user for printing (such as size, basis weight, and surface properties), and instructions to start or stop printing.

The image formation control unit 205 issues instructions to the image forming unit including the exposure device 13 and controls image formation. The sheet transport control unit 206 issues instructions to the pre-registration motor 101, registration motor 102, etc. that drive various transport rollers, and controls the transport of the sheet P. The sensor control unit 207 controls the start or stop of detection by the registration sensor 43, etc., and receives the detection results of each of these sensors.

In addition, it may also be configured to receive various information about the sheets to be used for printing via a computer 204 connected via a network, for example.

The CPU 201 of the control unit 200 controls the driving of the motors 101 and 102, and controls the operation of the pre-registration roller and the registration roller, as shown in Figures 4(a) and 4(b). Here, the pre-registration roller refers to the drive roller 41b of the pre-registration roller pair 41, which rotates upon receiving the driving force from the pre-registration motor 101. The registration roller refers to the drive roller 42b of the registration roller pair 42, which rotates upon receiving the driving force from the registration motor 102.

The CPU 201 controls the driving of the pre-registration motor 101 to convey the succeeding sheet by the pre-registration roller pair 41. At this time, the pre-registration roller pair 41 conveys the succeeding sheet at a conveying speed v1, and the registration roller pair 42 stops rotating. The leading edge of the succeeding sheet conveyed by the pre-registration roller pair 41 at the conveying speed v1 is detected by the registration sensor 43 at time t1, and then contacted with the nip portion of the registration roller pair 42, which is stopped at time t2. Then, the CPU 201 stops the driving of the pre-registration motor 101 at time t3 to stop the pre-registration roller pair 41. As a result, a first loop (loop amount Xs shown in FIG. 5A) is formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42, bending the sheet to one side. This first loop is shown as the loop amount Xs in FIG. 4B. The loop amount Xs is the predetermined loop amount of the sheet, and is the area shown by the lattice portion in FIG. 4(b).

After that, the CPU 201 starts driving the pre-registration motor 101 and the registration motor 102 at the same time at time t4 in accordance with the toner image formed on the intermediate transfer belt 21. Then, the pre-registration roller pair 41 and the registration roller pair 42 start conveying the succeeding sheet at the same time.

Then, the CPU 201 sets the pre-registration roller pair 41 to a conveying speed v2 slower than that of the registration roller pair 42 at time t5, between when the registration roller pair 42 starts to rotate and when the leading edge of the succeeding sheet reaches the secondary transfer section. As a result, the loop amount Xs formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42 is reduced by the loop amount Xr. That is, a second loop (loop amount Xs-Xr shown in FIG. 5B) smaller than the first loop (loop amount Xs shown in FIG. 5A) is formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42. Here, the loop amount Xr is the area indicated by the shaded area in FIG. 4B, and indicates the difference in conveying amount between the registration roller pair 42 and the pre-registration roller pair 41.

Regarding the magnitude relationship between this loop amount Xr and the loop amount Xs of the first loop described above, by making Xs>Xr, it is possible to reduce the loop amount Xs-Xr of the second loop while preventing it from becoming zero.

Then, after forming the first loop, when the CPU 201 starts rotating the registration roller pair 42, it accelerates the registration roller pair 42 to a conveying speed v3 that is faster than the conveying speed of the outer secondary transfer roller 44.

On the other hand, the CPU 201 sets the pre-registration roller pair 41 to the same conveying speed as the registration roller pair 42 from the time the registration roller pair 42 starts to rotate until the sheet is conveyed a predetermined distance. In other words, the CPU 201 accelerates the pre-registration roller pair 41 at the same conveying speed as the registration roller pair 42 from the time the registration roller pair 42 starts to rotate until the sheet is conveyed a predetermined distance (from time t4 to time t5).

In the example of operation shown in FIG. 4B, after the CPU 201 has transported the sheet a predetermined distance, as described above, at time t5, the pre-registration roller pair 41 is set to a constant transport speed v2 slower than the registration roller pair 42. Then, at time t6, the pre-registration roller pair 41 is set to a transport speed v3 that is faster than the secondary transfer outer roller 44 and is the same as the registration roller pair. In this way, by setting the pre-registration roller pair 41 to a transport speed v3 faster than the secondary transfer outer roller 44, the productivity of the device can be improved.

The CPU 201 sets the pre-registration roller pair 41 and the registration roller pair 42 to the same conveying speed v4 as the secondary transfer outer roller 44 until the leading edge of the subsequent sheet that has formed the second loop reaches the secondary transfer section (between time t6 and time t7). That is, the pre-registration roller pair 41 and the registration roller pair 42 are set to a conveying speed v3 faster than the secondary transfer outer roller 44 to convey the sheet a certain distance, and then decelerate to reach the same conveying speed v4 as the secondary transfer outer roller 44 by time t7.

As described above, according to this embodiment, the loop amount formed for skew correction is reduced by the time the sheet reaches the secondary transfer section. In other words, the first loop of the sheet formed between the roller pair 41, 42 for skew correction is made into a second loop that is smaller than the first loop by the time the leading edge of the sheet reaches the secondary transfer section. This makes it possible to reduce the buckling noise of the sheet during sheet transport, reduce the torque of the motor that drives the rollers, and improve image printing accuracy with a simple configuration.

The loop shape of the sheet between the roller pair 41, 42 when conveyed by the above-mentioned operation is shown in Figures 5(a) and 5(b). Figure 5(a) is a cross-sectional view showing the loop shape of the sheet at the timing of time t4 in Figure 4. Figure 5(b) is a cross-sectional view showing the loop shape of the sheet at the timing of time t7 in Figure 4. Figure 6 is a cross-sectional view showing the loop shape of the sheet at the timing corresponding to time t7 in a comparative example to which this embodiment is not applied.

The loop amount Xs, which is the size of the loop of the sheet formed between the roller pairs 41 and 42 when the leading edge of the sheet abuts against the registration roller pair 42, is determined from various viewpoints. One viewpoint is the distance for the delayed side of the skewed sheet to reach the registration roller pair 42, that is, the distance based on the amount of skew. Another viewpoint is the variation in the conveyance amount from detection by the registration sensor 43 to reaching the registration roller pair 42. Another viewpoint is the amount by which the leading edge position of the sheet drops due to gravity when the sheet is conveyed vertically upward as in this embodiment. In this way, the loop amount Xs of the sheet between the roller pairs 41 and 42 described above is determined from various viewpoints. Therefore, the required loop amount Xs of the sheet may be excessively large compared to the allowable loop space formed by the guides 45 and 46 between the roller pairs 41 and 42.

As a comparative example, FIG. 6 shows a case where the sheet is transported to the secondary transfer section while maintaining the loop amount Xs formed between the roller pairs 41 and 42. In the comparative example shown in FIG. 6, the sheet between the pre-registration roller pair 41 and the registration roller pair 42 buckles, resulting in a distorted loop shape. In this case, buckling noise may be generated during sheet transport, and buckling marks may appear on the sheet. This is particularly noticeable when the loop increases on the leading side of the skewed sheet, as described above.

On the other hand, according to the operation of this embodiment, as shown in FIG. 5B, after the registration roller pair 42 starts to rotate, the loop amount Xs of the sheet is reduced by the loop amount Xr to make the loop amount Xs-Xr. In other words, the pre-registration roller pair 41 is made to have a slower conveying speed than the registration roller pair 42 during the period from when the registration roller pair 42 starts to rotate until the leading edge of the sheet reaches the secondary transfer section. Then, a second loop (loop amount Xs-Xr) smaller than the first loop (loop amount Xs) is formed in the sheet between the roller pairs 41 and 42. This makes it possible to stably convey the sheet to the secondary transfer section without buckling.

In the comparative example shown in FIG. 6, a case where the basis weight, which is the thickness of the sheet, is relatively small has been described as an example, but the present invention is not limited to this. If the reaction force from the sheet becomes large, the load torque of the conveying motor may become excessive, or the sheet may slip during conveyance by the registration roller pair 42, causing the timing at which the sheet reaches the secondary transfer unit to be shifted. Examples of cases in which the reaction force from the sheet becomes large include when the basis weight of the sheet is relatively large, or when the amount of loop of the sheet is relatively large compared to the space created by the guides 45, 46 between the roller pairs 41, 42.

It is conventional to have a table in which the sheet loop amount Xs is set to a different value depending on the sheet information, i.e., the size, basis weight, shape, and type of the sheet, but the sheet loop amount Xr can also be set to a value according to the sheet information.

Specifically, the types of sheets are compared between plain paper (68 gsm), which is the type of copy paper commonly used in offices, thick paper (300 gsm), and thin paper (52 gsm). Here, thick paper is a sheet that is thicker than plain paper and has higher stiffness than plain paper. Thin paper is a sheet that is thinner than plain paper and has lower stiffness than plain paper.

In this embodiment, the loop amount Xs is the same value (Xsn) for plain paper and thin paper, and is a smaller value (Xsc) for thick paper than for plain paper and thin paper (Xsn>Xsc). This is because, in the case of thick paper, even if a loop is not formed like that of plain paper or thin paper when creating the loop, the leading edge of the sheet receives a reaction force from the loop and obtains a force to abut against the nip portion of the registration roller pair. In other words, because the reaction force of the loop differs greatly depending on the type of sheet, the amount of loop formed Xs also changes accordingly.

The loop amount Xr is set to the same value (Xrn) for plain paper and thick paper, and to a larger value (Xrt) for thin paper than for plain paper and thick paper (Xrn<Xrt). In other words, when a sheet is transported by the registration roller pair, the amount of loop formed between the pre-registration roller pair and the registration roller pair is in the following order: thick paper, plain paper > thin paper > 0. This is due to the stiffness of the sheet. For thin paper, the smaller the loop formed, the less likely the sheet is to buckle.

The above describes sheet information from the perspective of stiffness, but the loop amount may be set in the same manner as above depending on the sheet size, for example, sheet width information in the width direction perpendicular to the sheet transport direction. In other words, if the sheet width is large, the reaction force from the sheet is large and it can be considered to be the same as a sheet with high stiffness, and if the sheet width is small, the reaction force from the sheet is small and it can be considered to be the same as a sheet with low stiffness.

Specifically, for example, when the second loop is formed on the sheet between the pre-registration roller pair 41 and the registration roller pair 42, the size of the second loop may be changed according to the sheet information.

Here, the sheet information is the basis weight of the sheet. In this case, the CPU 201 reduces the size of the second loop formed on the sheet between the pre-registration roller pair and the registration roller pair as the basis weight of the sheet decreases.

Alternatively, the sheet information is the width in a direction perpendicular to the sheet conveying direction. In this case, the CPU 201 reduces the size of the second loop formed on the sheet between the pre-registration roller pair 41 and the registration roller pair 42 as the sheet width becomes smaller.

In this way, when the second loop is formed on the sheet between the pre-registration roller pair 41 and the registration roller pair 42, the size of the second loop is changed according to the sheet information, thereby preventing the sheet from buckling according to the sheet information.

Alternatively, depending on the sheet information, it may be determined whether or not to form the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42.

Here, the sheet information is the basis weight of the sheet. In this case, if the basis weight of the sheet is equal to or less than a threshold value, the CPU 201 forms the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair. On the other hand, if the basis weight of the sheet exceeds the threshold value, the CPU 201 does not form the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42.

Alternatively, the sheet information is the width in a direction perpendicular to the sheet conveying direction. In this case, if the sheet width is equal to or less than a threshold value, the CPU 201 forms the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42. On the other hand, if the sheet width exceeds the threshold value, the CPU 201 does not form the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42.

In this way, depending on the sheet information, it is determined whether or not to form the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42. This not only makes it possible to prevent the sheet from buckling depending on the sheet information, but also improves the productivity of the device.

As a result, by setting the loop amount Xn of the sheet formed between the roller pair 41, 42 to correct skew and the loop amount Xr that is reduced before the leading edge of the sheet reaches the secondary transfer section to values that correspond to the sheet information, it is possible to prevent the sheet from buckling according to the sheet information.

In the above embodiment, the pre-registration roller pair 41 and the registration roller pair 42 start rotating simultaneously at time t4. The reason for this is explained below. The registration roller pair 42 starts rotating at time t4 from a state in which the leading edge of the sheet is abutted against the pair and a loop is formed. At that time, for example, if the pre-registration roller pair 41 rises up slightly ahead of the pair of registration rollers 42, the loop amount increases according to the amount of deviation. This may result in an excessive amount of loop of the sheet. On the other hand, if the pre-registration roller pair 41 rises up slightly behind the pair of registration rollers 42, no particular problem occurs if the amount of loop reduction due to the amount of deviation is sufficiently small compared to the loop amount Xs. However, if this is not the case, if the loop amount is reduced before the sheet sufficiently enters the nip portion of the pair of registration rollers 42, the leading edge position of the sheet whose skew has been corrected may shift again, causing the skew to worsen. For this reason, the pre-registration roller pair 41 and the registration roller pair 42 start rotating simultaneously at time t4.

In the above embodiment, the motor control method for reducing the amount of loop of the sheet is described with reference to FIG. 4, but the present invention is not limited to this. For example, several means are possible, as shown in FIG. 7(a) and FIG. 7(b).

In the control shown in FIG. 7(a), the registration roller and pre-registration roller start up simultaneously at time t4. Then, at time t8, the acceleration of the pre-registration roller is reduced compared to the registration roller. Then, at time t9, the pre-registration roller meets the speed of the registration roller.

That is, in the control shown in FIG. 7A, after the CPU 201 has conveyed the sheet a predetermined distance, at time t8, the CPU 201 sets the conveying speed of the pre-registration roller to a slower speed than that of the registration roller. Then, at a timing (time t9) later than that of the registration roller, the pre-registration roller is accelerated so that it reaches a conveying speed v3 that is faster than that of the outer secondary transfer roller 44 and is the same as that of the registration roller pair.

As a result, the area indicated by the hatched portion in FIG. 7A is the loop reduction amount Xr2, and the loop amount of the sheet at the timing of reaching the secondary transfer unit is the second loop (loop amount Xs-Xr2) which is smaller than the first loop (loop amount Xs).

Furthermore, in the case of the control shown in FIG. 7(b), the pre-registration roller is set to a constant conveying speed v5 at time t10. Then, at time t11, the pre-registration roller merges with the registration roller.

In other words, in the control shown in FIG. 7B, after the CPU 201 has conveyed the sheet a predetermined distance, at time t10 the pre-registration roller is set to a constant conveying speed that is faster than the outer secondary transfer roller 44 and slower than the registration roller. After that, the pre-registration roller is set to the same conveying speed as the registration roller.

As a result, the area indicated by the hatched portion in FIG. 7B is the loop reduction amount Xr3, and the loop amount of the sheet at the timing of reaching the secondary transfer unit is the second loop (loop amount Xs-Xr3) which is smaller than the first loop (loop amount Xs).

The controls shown in FIG. 7(a) and FIG. 7(b) are also examples and are not limited to these. It is sufficient to set the difference in the sheet transport amount from when the registration roller pair and the pre-registration roller pair rise simultaneously until the sheet reaches the secondary transfer section so that the latter is smaller than the former.

Even when controlled as described above, the amount of loop formed for skew correction can be reduced by the time the sheet reaches the secondary transfer section. This makes it possible, with a simple configuration, to reduce the buckling noise of the sheet during sheet transport, reduce the torque of the motor that drives the rollers, and improve image printing accuracy.

In the above embodiment, the sheet is stopped once after it hits the pair of registration rollers 42 to form a loop, and then the conveyance is resumed. However, the present invention is not limited to this. For example, as shown in FIG. 8, the control may be such that the loop is formed immediately before the pair of registration rollers resumes conveyance.

In the control shown in FIG. 8, when the pre-registration roller temporarily stops conveying at time t23, the leading edge of the sheet has not yet reached the registration roller pair 42. The conveying distance from when the pre-registration roller starts conveying at time t24 to time t25 is the first loop (loop amount XS2) of the sheet formed between the roller pair 41, 42. The control after the registration roller and pre-registration roller rise at time t25 is as described above. When the leading edge of the sheet finally reaches the secondary transfer section, the loop amount of the sheet between the roller pair 41, 42 is XS2-Xr.

In other words, in the control shown in FIG. 8, the CPU 201 stops the pre-registration roller at time t23, and then resumes rotation at time t24 before the registration rotation starts at time t23. Then, a first loop (loop amount Xs2) is formed on the sheet between the roller pair 41, 42. The first loop (loop amount Xs2) formed on the sheet between the roller pair 41, 42 may be formed in this manner.

In the above-mentioned embodiment, a printer is exemplified as an image forming apparatus, but the present invention is not limited to this. For example, it may be other image forming apparatuses such as a copier or a facsimile machine, or other image forming apparatuses such as a multifunction machine that combines the functions of these. In addition, an image forming apparatus that uses an intermediate transfer body, transfers toner images of each color to the intermediate transfer body in a sequentially overlapping manner, and transfers the toner images carried on the intermediate transfer body to a transfer material all at once is exemplified, but the present invention is not limited to this. For example, it may be an image forming apparatus that uses a transfer material carrier, and transfers toner images of each color to the transfer material carried on the transfer material carrier in a sequentially overlapping manner. By applying the present invention to a sheet conveying device applied to these image forming apparatuses, it is possible to obtain the same effect.

In the above embodiment, the sheet transport device applied to the image forming apparatus is an integral part of the image forming apparatus, but the present invention is not limited to this. For example, the sheet transport device may be a detachable sheet transport device attached to the image forming apparatus, and the same effect can be obtained by applying the present invention to the sheet transport device.

P...sheet 1...image forming device 11...photoconductor 21...intermediate transfer belt 22...secondary transfer inner roller 23...drive roller 24...tension roller 25...primary transfer device 31, 32...paper feed cassette 31a...paper feed section 31d, 32d...sheet size detection section 33...manual feed section 40...sheet transport section 41...pre-registration roller pair 41a...driven roller 41b...drive roller 42...registration roller pair 42a...driven roller 42b...drive roller 43...registration sensor 44...secondary transfer outer roller 50...fixing device 60...paper discharge transport section 63, 64...transport guide 80...paper discharge tray 200...control section

Claims (11)

A sheet conveying device that conveys a sheet to a transfer conveying section that transfers a toner image carried on an image carrier onto the sheet,
A pair of pre-registration rollers that form a nip portion and sandwich and convey a sheet;
a pair of registration rollers provided downstream of the pair of pre-registration rollers in a sheet conveying direction, the pair of registration rollers forming a nip portion and conveying the sheet by sandwiching the sheet;
a control unit that controls the pre-registration roller pair and the registration roller pair,
the control unit conveys the sheet by the pre-registration roller pair, and causes a leading edge of the sheet to abut against the nip portion of the registration roller pair that is stopped from rotating, thereby forming a first loop by bending the sheet toward one surface side between the pre-registration roller pair and the registration roller pair;
a sheet conveying speed by the pre-registration roller pair is made substantially equal to a sheet conveying speed by the registration roller pair during a period from when the registration roller pair starts conveying the sheet to when the sheet is conveyed a predetermined distance;
and forming a second loop on the sheet between the pre-registration roller pair and the registration roller pair, the second loop being smaller than the first loop, by setting a sheet conveying speed by the pre-registration roller pair to a conveying speed slower than a sheet conveying speed by the registration roller pair during a period from when the registration roller pair has conveyed the sheet by the predetermined distance until the leading edge of the sheet reaches the transfer conveying section;
a sheet conveying device which, after conveying the sheet a predetermined distance, sets the sheet conveying speed by the pre-registration roller pair to a conveying speed slower than the sheet conveying speed by the registration roller pair, and accelerates the sheet to a conveying speed faster than the sheet conveying speed by the transfer conveying section at a timing later than the registration roller pair, so as to reach the same conveying speed as the sheet conveying speed by the registration roller pair. A sheet conveying device that conveys a sheet to a transfer conveying section that transfers a toner image carried on an image carrier onto the sheet,
A pair of pre-registration rollers that form a nip portion and sandwich and convey a sheet;
a pair of registration rollers provided downstream of the pair of pre-registration rollers in a sheet conveying direction, the pair of registration rollers forming a nip portion and conveying the sheet by sandwiching the sheet;
a control unit that controls the pre-registration roller pair and the registration roller pair,
the control unit conveys the sheet by the pre-registration roller pair, and causes a leading edge of the sheet to abut against the nip portion of the registration roller pair that is stopped from rotating, thereby forming a first loop by bending the sheet toward one surface side between the pre-registration roller pair and the registration roller pair;
a sheet conveying speed by the pre-registration roller pair is made substantially equal to a sheet conveying speed by the registration roller pair during a period from when the registration roller pair starts conveying the sheet to when the sheet is conveyed a predetermined distance;
and forming a second loop on the sheet between the pre-registration roller pair and the registration roller pair, the second loop being smaller than the first loop, by setting a sheet conveying speed by the pre-registration roller pair to a conveying speed slower than a sheet conveying speed by the registration roller pair during a period from when the registration roller pair has conveyed the sheet by the predetermined distance until the leading edge of the sheet reaches the transfer conveying section;
a sheet conveying device characterized in that, after conveying the sheet a predetermined distance, the sheet conveying speed by the pre-registration roller pair is set to a conveying speed that is faster than the sheet conveying speed by the transfer conveying section and slower than the sheet conveying speed by the registration roller pair, and then the conveying speed is set to the same as the sheet conveying speed by the registration roller pair. 3. The sheet transport device of claim 1, wherein the control unit starts rotation of the registration roller pair after forming the first loop, sets the sheet transport speed by the registration roller pair to a transport speed faster than the sheet transport speed by the transfer transport unit, and after forming the second loop, sets the sheet transport speed by the pre-registration roller pair to a transport speed that is the same as the sheet transport speed by the registration roller pair and faster than the sheet transport speed by the transfer transport unit, and then sets the sheet transport speed by the pre-registration roller pair and the sheet transport speed by the registration roller pair to the same transport speed as the sheet transport speed by the transfer transport unit until the leading edge of the sheet in the state in which the second loop has been formed reaches the transfer transport unit. 4. The sheet conveying device according to claim 1, wherein the control unit temporarily stops the pre-registration roller pair, and then resumes rotation before the registration roller pair starts to rotate, thereby forming the first loop between the pre-registration roller pair and the registration roller pair. 3. The sheet conveying device according to claim 1, wherein the control unit determines whether or not to form the second loop on the sheet between the pair of pre-registration rollers and the pair of registration rollers in accordance with information about the sheet. 6. The sheet conveying device of claim 5, wherein the sheet information is the basis weight of the sheet, and the control unit forms the second loop on the sheet between the pre-registration roller pair and the registration roller pair when the basis weight of the sheet is equal to or less than a threshold value, and does not form the second loop on the sheet between the pre-registration roller pair and the registration roller pair when the basis weight of the sheet exceeds the threshold value. 6. The sheet conveying device of claim 5, wherein the sheet information is a width in a direction perpendicular to the sheet conveying direction, and the control unit forms the second loop on the sheet between the pre-registration roller pair and the registration roller pair when the sheet width is equal to or less than a threshold value, and does not form the second loop on the sheet between the pre-registration roller pair and the registration roller pair when the sheet width exceeds the threshold value. The sheet conveying device according to any one of claims 1 to 3, characterized in that the control unit changes the size of the second loop in response to sheet information when forming the second loop on the sheet between the pre-registration roller pair and the registration roller pair. The sheet information is the basis weight of the sheet,
9. The sheet conveying device according to claim 8, wherein the control unit reduces a size of the second loop formed in the sheet between the pre-registration roller pair and the registration roller pair in response to a decrease in basis weight of the sheet. The sheet information is a width in a direction perpendicular to the sheet conveying direction,
9. The sheet conveying device according to claim 8, wherein the control unit reduces a size of the second loop formed on the sheet between the pre-registration roller pair and the registration roller pair as the width of the sheet decreases . An image carrier;
a transfer and transport section for transferring the toner image carried on the image carrier onto a sheet;
A sheet conveying device according to any one of claims 1 to 10 ,
An image forming apparatus comprising:

Images