JP2003306250A - Image forming device and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device which can restrict the generation of crinkle of sheets to minimize the generation of missing of images when forming images in both surfaces of the sheet. <P>SOLUTION: In this image forming device, when carrying the sheet S fed from a paper feeding cassette 15 to form an image in one surface thereof, the sheet S is carried at a first speed, and when carrying the sheet fed from a re-carrying route 20 to form an image in a second surface of the sheet S, the sheet S is carried by a second speed lower than the first speed and lower than the carrying speed (process speed) of a sheet carrying belt 7. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a printer,
The present invention relates to an image forming apparatus and an image forming method using an electrophotographic system or an electrostatic recording system such as a copying machine.

[0002]

2. Description of the Related Art In recent years, electrophotographic image forming apparatuses have been
Higher speed, higher functionality, and colorization have been promoted, and various types of printers and copiers that form color images are commercially available. For example, there is known a so-called in-line type image forming apparatus in which image forming means for a plurality of colors are arranged in series and the toner images formed by the respective image forming means are sequentially and multiple-transferred onto a sheet (transfer material). Since this in-line type image forming apparatus is capable of forming a color image at high speed, it is considered to be a main force in future color printers and the like. This in-line type image forming apparatus is configured to convey a sheet while carrying the sheet by a belt-shaped conveying unit (sheet conveying belt), and sequentially transfer multiple toner images of respective colors onto the sheet to obtain a color image. ing.

Further, in the in-line type image forming apparatus, the sheet conveying belt is arranged so as to extend along the arrangement direction of each image forming station and contact each image forming station. The sheet conveyed by the sheet conveying belt is preferably adsorbed stably on the sheet conveying belt in order to enhance transferability of the toner image.
For example, by applying an attraction bias (voltage or current) to the surface of the sheet transport belt, the sheet can be stably attracted on the sheet transport belt.

In the above-described in-line type image forming apparatus, when a toner image is formed on both sides of a sheet, the rear end of the sheet having the image formed on the first side (front side) heat-fixes the toner image on the sheet. After passing through the fixing device for conveying the sheet, the conveying direction of the sheet is reversed so that the second surface (back surface) of the sheet becomes the image forming surface, and the sheet is guided to the re-conveying path and directed to the plurality of image forming units. Will be re-fed.

[0005]

However, when an image is formed on both sides of the sheet when the image is formed on the second side of the sheet, the sheet is once heated and pressed in the fixing device, so that the periphery of the sheet is There is a case where the part becomes wavy, and when the sheet in such a state is conveyed to the sheet conveying belt to which the suction bias is applied, the wavy portion of the sheet is strongly adsorbed to the sheet conveying belt. As a result, as shown in FIG. 5, the corrugated portion Sb of the sheet S is moved toward the rear end of the sheet to induce wrinkles, and the toner is formed in the wrinkle recesses in the sheet (portions where the sheet does not contact the photosensitive drum). The image may not be transferred and the image may be missing. Note that FIG. 5 is for explanation,
This is an exaggerated view of the wrinkles.

Further, since the image formation on the first surface (front surface) is completed and the sheet that has passed through the fixing device once has waviness at both ends in the direction orthogonal to the conveying direction, waviness occurs. Regarding both end portions, the length in the transport direction is longer than the length in the central portion. Therefore, when an image is formed on the second surface (rear surface) of the sheet, the wavy portions generated at both end portions of the sheet are gathered together at the rear end of the sheet and wrinkles occur, and image dropout occurs at the transfer portion. May occur.

An object of the present invention is to provide an image forming apparatus and an image forming method capable of suppressing the occurrence of wrinkles on a sheet and minimizing the occurrence of image loss when forming images on both sides of a sheet. Especially.

[0008]

In order to achieve the above object, the present invention provides a sheet placing means for placing a sheet, a sheet conveying means for conveying a sheet, and a sheet conveyed by the sheet conveying means. Image forming means for forming an image, re-conveying means for conveying the sheet image-formed on the first surface by the image forming means again to the image forming means, the sheet placing means or the re-conveying means Sheet feeding means for feeding the sheet from the sheet to the sheet conveying means, the sheet feeding means for nipping and conveying the sheet together with the sheet conveying means, and the sheet feeding speed by the sheet feeding means. And a control means for controlling the first sheet when feeding the sheet from the sheet placing means to form an image on the first surface of the sheet. When the sheet from the re-conveying unit is fed at a speed so as to form an image on the second surface of the sheet, the sheet is slower than the first speed and the sheet conveying unit conveys the sheet. It is characterized in that the sheet is fed at a second speed which is slower than the predetermined speed.

In order to achieve the above object, the present invention provides a sheet feeding step for feeding a sheet from a sheet placing means for placing a sheet, and a sheet fed in the sheet feeding step as a first step. A first feeding step of feeding at a speed, and a first feeding step of feeding the sheet fed in the first feeding step before the first feeding step ends; A first image forming step of forming an image on the first surface of the sheet being conveyed in the first conveying step; and a first image forming step of forming the image on the first surface of the sheet in the first image forming step. A re-conveying step of re-conveying the sheet to form an image on two sides, and a sheet re-conveyed in the re-conveying step is slower than a first speed and is a sheet in the first conveying step. A second feeding step of feeding at a second speed lower than the conveying speed, and conveying the sheet fed in the second feeding step before the second feeding step is completed. And a second image forming step of forming an image on the second surface of the sheet being conveyed in the second conveying step.

In order to achieve the above object, the present invention includes a sheet placing means for placing a sheet, a sheet conveying means having a belt-like conveying member for conveying the sheet, and the sheet conveying means. Image forming means for forming an image on a conveyed sheet, re-conveying means for conveying the sheet image-formed on the first side by the image forming means to the image forming means again, and the sheet placing means. Alternatively, a sheet feeding unit that feeds the sheet from the re-conveying unit to the sheet conveying unit, and a sheet feeding unit that nips and conveys the sheet together with the sheet conveying unit, and a sheet to the conveying member. The sheet feeding device has a suction means for sucking with a predetermined suction force and a control means for controlling the image forming apparatus, and the control means controls the sheet to form an image on the first surface of the sheet. The predetermined suction force when feeding the sheet from the re-conveying unit is weakened by the predetermined suction force when feeding the sheet from the stacking unit to form an image on the second surface of the sheet. Image formation characterized by the following control.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an overall structure of an image forming apparatus according to an embodiment of the present invention, FIG. 2 shows a control structure of the image forming apparatus, and FIG. 3 shows a first surface of a sheet. 5 illustrates a sheet conveyance state when an image is formed on the sheet.

The image forming apparatus of this embodiment is a full-color image forming apparatus adopting an electrophotographic system, and as shown in FIG. 1, it is arranged in a substantially straight line in the vertical direction to form images of different colors, respectively. Four image forming stations 1a to 1d, and a sheet conveying belt 7 that conveys the sheet S.
With.

Each of the image forming stations 1a-1d has at least a photosensitive drum 2a-2d for carrying a latent image.
Around the photosensitive drums 2a to 2d, charging rollers 3a to 3d for charging the photosensitive drums 2a to 2d to a uniform potential and charging rollers 3a to 3d are uniformly charged. The exposure devices 4a to 4d for irradiating the photosensitive drums 2a to 2d with a laser beam to form an electrostatic latent image, and the latent images formed on the photosensitive drums 2a to 2d correspond to the corresponding colors (magenta). , Developing devices 5a to 5d for developing with cyan, yellow, and black) toners to visualize images, and cleaning devices 6a to 6d for removing residual toner on the photosensitive drums 2a to 2d.

Each of the developing means 5a-5d has a developing sleeve 50a-50d for carrying toner. Each of the developing sleeves 50a to 50d has a corresponding photosensitive drum 2a.
2d to 2d at a predetermined interval or in pressure contact with a predetermined pressure and supported, and at the time of development, the photosensitive drums 2a to 2d.
A developing bias is applied between d and the developing sleeves 50a to 50d.

The sheet conveying belt 7 conveys the sheet S while carrying it along the arrangement direction of the image forming stations 1a to 1d. The sheet conveying belt 7 includes a driving roller 8, a driven roller 9, and belt tension rollers 10, 1.
1 is stretched and is rotatably driven in the direction shown by the arrow in FIG. The sheet conveying belt 7 conveys the sheet S while sequentially contacting the photosensitive drums 2a to 2d of the image forming stations 1a to 1d.

The drive roller 8 is driven by the sheet transfer belt drive motor M1 (see FIG. 2) in the direction indicated by the arrow in FIG. 1 so that the sheet transfer belt 7 rotates, and the driven roller 9 and the belt tension roller 10 are driven. , 11 are rotated along with the rotation of the sheet conveying belt 7. In addition, the photosensitive drum 2
The a to 2d are independently driven by the drum motors M2a to M2d (see FIG. 2).

On the surface of the sheet conveying belt 7 upstream of the image forming station 1a in the transfer material conveying direction,
Adsorption roller 1 arranged so as to face the driven roller 9.
2 is abutted, and the suction roller 12 moves the sheet conveying belt 7
The sheet S passing between and is sandwiched. Suction roller 12
A constant current bias is applied from a constant current power supply 13 (bias means) to the sheet S, which causes the sheet S passing through the suction roller 12 to be charged and the sheet conveying belt 7 to be conveyed.
Is adsorbed on.

Inside the sheet conveying belt 7, transfer rollers 14a to 14d are arranged at positions facing the photosensitive drums 2a to 2d, respectively.
The sheet d is pressed against the corresponding photosensitive drums 2a to 2d via the sheet conveying belt 7. Each transfer roller 14a
To 14d are applied with a transfer bias, whereby the toner images of the respective colors on the photosensitive drums 2a to 2d are transferred onto the sheet S.

The sheets S are placed and stored in a paper feed cassette 15 provided at the lower portion of the apparatus, and are separated and fed one by one from the paper feed cassette 15 by a paper feed roller 16 and fed to a pair of registration rollers 17. To be done. Registration roller pair 17
Feeds the fed sheet S between the sheet conveying belt 7 and the suction roller 12 in synchronism with the image formation on the photosensitive drums 2a to 2d.
The sheet is charged by 2, and is conveyed along each of the image forming stations 1a to 1d while being attracted to the sheet conveying belt 7.

Paper feed roller 16 and registration roller pair 1
The sheet 7 is rotationally driven by the driving force given by the sheet feeding motor M3 (see FIG. 2) to convey the sheet S in the direction of the arrow in FIG. Further, the registration roller pair 17 can convey the sheet S at the same time as the sheet conveying belt 7 at a position facing the suction roller 12. That is, the size of the sheet S conveyed by the image forming apparatus is larger than the distance from the registration roller pair 17 to the suction roller 12,
The sheet is conveyed while being sandwiched by both the sheet conveying belt 7 and the registration roller pair 17 at a position facing the suction roller 12.

Toner images on the photoconductor drums 2a to 2d are sequentially superposed and transferred onto the sheet S conveyed by the sheet conveying belt 7 via the image forming stations 1a to 1d, and the toner images are transferred. The sheet S is sent to the fixing device 18. In the fixing device 18, the sheet S
Is thermally pressed and the toner image is fixed on the sheet S, so that the image is formed on the sheet S. After that, the sheet S is conveyed by the fixing device 18 and the paper discharge roller 22, and is discharged to the discharge tray 19 outside the apparatus.

When images are formed on both sides of the sheet S, the trailing edge of the sheet S passes through the fixing device 18 and the image formation on the first side (front side) of the sheet S is completed, After the trailing edge passes through the fixing device 18, the sheet discharging direction of the sheet S is reversed by the sheet discharging roller 22, and the sheet S is guided to the re-conveying path 20. The sheet S guided to the re-conveyance path 20 is conveyed by a plurality of re-conveyance rollers 21 and re-fed to the registration roller pair 17. Then, an image is formed on the second surface (back surface) of the sheet S in the same procedure as the first surface,
The sheet S is discharged to the discharge tray 19.

A driving force is applied to the fixing device 18 and the discharge roller 22 by the fixing motor M4 (see FIG. 2), and the sheet S is discharged to the discharge tray 19 by rotating the fixing motor M4 in the forward direction. , The sheet S discharged from the fixing device 18 by rotating the fixing motor M4 in the reverse direction.
Can be guided to the re-conveyance path 20 by the paper discharge roller 22. The fixing device 18 has a pair of rollers pressed against each other at a predetermined pressure, one roller is driven by the fixing motor M4, and the other is driven to rotate by the one roller. The sheet S is sandwiched by the pair of rollers, and the sheet discharge roller 22
Is transported to.

Although it is not shown in FIG. 2 which motor is used to drive the re-transport roller 21,
For example, the driving force may be applied by the sheet conveying belt drive motor M1.

Although the overall structure of the image forming apparatus has been described above, the sheet conveying belt driving motor M1, the drum motors M2a to M2d, the sheet feeding motor M3, and the fixing motor M4 used for conveying the sheet S are shown in FIG. It is controlled by the image forming controller 23, and the image forming controller 23 can control each motor independently. In addition, the image forming controller 23
The image processing apparatus is connected to an image processing controller 24 that receives and processes commands and image data from a host device outside the image forming apparatus, and receives information about print modes such as sheet size, type, color, and monochrome received from the host device. It is possible to set the operation mode based on the information received through the image processing controller 24, the received information, and control the sheet conveyance speed based on the set operation mode. For example, if the transport speed is 1 when the type of sheet is plain paper, then OHT (Over Head Tr
(ansparency) can be controlled to 1/2 speed, and glossy glossy paper can be controlled to 1/3 speed.
Further, a sensor for discriminating the type of the sheet may be provided inside the image forming apparatus, and the above-described sheet conveyance speed may be controlled based on the detection result of the sensor.

Further, the image forming controller 23 controls the image forming operation other than the control of the motor. That is, the control of the constant current power source 13 that applies a constant current bias to the attraction roller 12 and the charging rollers 3a to 3d.
The voltage applied to the developing devices 50a to 50d, the irradiation of the laser light emitted from the exposure devices 4a to 4d, and the developing bias applied to the developing sleeves 50a to 50d are controlled.

Problems that may occur when images are formed on both sides of the sheet S in the image forming apparatus having the above-described overall structure and control structure will be described.

When images are formed on both sides of the sheet S, when the images are formed on the first side (front side), the paper feed cassette 15
Since the sheet S on which no image is formed is fed, no deformation such as waviness occurs around the sheet S, and when the suction bias is applied to the sheet S, the entire surface of the sheet S becomes Even when the sheet S is attracted to the sheet transport belt 7 and is transported while being in contact with the photosensitive drums 2a to 2d, the sheet S is hardly wrinkled during the transportation (see FIG. 3).

On the other hand, when an image is formed on the second surface (back surface), the sheet S on which the image is formed on the first surface (front surface) is re-fed, so that the periphery of the sheet S is corrugated or the like. Deformation often occurs (see Figure 4). The deformation such as waviness is caused by the influence of heat and pressure applied when the sheet S passes through the fixing device 18. In such a sheet S, as shown in FIG. 4, undulations are generated at both ends. Therefore, in the length of the sheet S in the transport direction, the length L1 of the sheet at the central portion and the length L2 of the end portion are set. However, L2 is longer than L1. When such a sheet S is conveyed while applying an adsorption bias between the adsorption roller 12 and the driven roller 9, the sheet S is strongly adsorbed to the sheet conveying belt 7 from the leading end thereof, but the length of the central portion is increased. Since there is a difference between the length L1 and the length L2 of the end portion, as the sheet S passes through the portion where the suction roller 12 and the driven roller 9 face each other, L1 and L2 are moved toward the center portion of the rear end of the sheet S. The sheet portions corresponding to the difference may be gathered together and wrinkles may be generated, and there may be a portion at the rear end of the sheet S that cannot be brought into close contact with the sheet conveying belt 7.

The state of the sheet S as described above is shown in FIGS. FIG. 5A and FIG. 5B show the sheet S.
FIG. 6 is a diagram showing a state of the sheet S when the sheet passes through the suction roller 12. As described above with reference to FIG. 5,
This is an exaggerated view of the wrinkles.

Sa in the figure shows a part of the sheet S which has passed through the suction roller 12 and has been sucked by the sheet conveying belt 7, and Sb shows another part of the sheet S before passing through the suction roller 12. It is a thing.

Since a suction bias is applied to the sheet S from the suction roller 12, the corrugated portion of the portion Sa of the sheet S is stretched when the sheet S passes through the suction roller 12 and is brought into close contact with the sheet conveying belt 7. However, in the sheet S having an image formed on the first surface (front surface),
As shown in FIG. 4, since the length L2 of the end portion is longer than the length L1 of the sheet in the central portion, the stretched corrugated portion is collected in the Sb portion that does not pass through the suction roller 12.
Then, as the sheet S passes through the suction roller 12, the Sb portion becomes narrower (see FIG. 5B), but the corrugation, which is the difference between the length L1 of the central portion of the sheet and the length L2 of the edge portion, is generated. Since the portions are collected, as the sheet S passes through the suction roller 12 (as the state of FIG. 5A changes to the state of FIG. 5B), the wavy portion of the sheet S wrinkles in the central portion of the sheet S. Appears. And
When the wrinkle becomes large, the wrinkle passes through the suction roller 12 without being stretched when passing through the suction roller 12. Therefore, the sheet S having wrinkles formed thereon is conveyed on the sheet conveying belt 7. This state is shown in FIG. In addition, in FIG.
A cross section taken along an imaginary plane C is seen from the direction of arrow C shown in FIG.

As shown in FIG. 7, when wrinkles occur at the trailing edge of the sheet S, the wrinkle concave portion A with respect to the photosensitive drum 2a is different from the wrinkle convex portion B in a state in which the wrinkle cannot be adhered to the photosensitive drum 2a. Becomes A suction bias is applied to the sheet S by the suction roller 12, and the concave portion A of the wrinkle is attracted to the sheet conveying belt 7. Therefore, the wrinkle once formed passes through the portion where the suction roller 12 and the driven roller 9 face each other. Even after this, the sheet is conveyed toward the photosensitive drums 2a to 2d while maintaining the unevenness of the wrinkles. Therefore, the photosensitive drum 2
The toner image is less likely to be transferred to the wrinkle concave portion A that cannot be brought into close contact with a to 2d, and so-called image omission occurs in which the image is not normally transferred to the sheet S.

The above problems are caused by the deformation of the sheet S such as waviness when the image is formed on the second surface of the sheet S having the image formed on the first surface (front surface). However, in order to solve this problem, the deformation of the sheet S when forming an image on the second surface of the sheet S is reduced as much as possible.

Therefore, in the present embodiment, the conveying speed (process speed) of the sheet conveying belt 7 during image formation on the first surface and the second surface of the sheet S is kept constant,
When forming an image on the first surface of the sheet S fed from the sheet feeding cassette 15, the feeding speed of the registration roller pair 17 is set to be substantially the same as the feeding speed of the sheet conveying belt 7, and the sheet is fed through the re-conveying path 20. At the time of forming an image on the second surface of the printed sheet S, the feeding speed of the registration roller pair 17 is made slower than the speed of the sheet conveying belt 7. That is, the feeding speed of the registration roller pair 17 at the time of image formation on the second surface of the sheet S is controlled to be slower than the feeding speed at the time of image formation on the first surface of the sheet S.

When the feeding speed of the registration roller pair 17 and the conveyance speed of the sheet conveying belt 7 are substantially the same, the trailing edge of the sheet S is not pulled by the registration roller pair 17 when passing through the suction roller 12. , Suction roller 1
Sa, which is a part of the sheet S that has passed through 2, is in a state of being in close contact with the sheet conveying belt 7, and the corrugated portion of the sheet S is collected in a portion Sb that has not passed through the suction roller 12.

On the other hand, when the feeding speed of the registration roller pair 17 is set lower than the speed of the sheet conveying belt 7 only when forming an image on the second surface of the sheet S, the registration roller pair 17 and the suction roller 12 and The sheet S is pulled between the portion facing the driven roller 9 and the portion facing the driven roller 9. At this time, with respect to the length of the sheet S in the transport direction as described above, the length L1 of the central portion is shorter than the length L2 of both end portions, so that the central portion is mainly pulled.
It becomes difficult for tension to be applied to both ends. Therefore, the sheet S
As for the feed amount of, there will be a difference between the both ends and the central part. That is, the feed amount at both ends of the sheet S is larger than the feed amount at the central portion. Therefore, the corrugated portions of the sheet S are evenly distributed on the sheet conveying belt 7 without being gathered at the trailing edge, so that wrinkles do not occur and it is possible to prevent occurrence of image dropouts. This state is shown in FIG. 8. In the sheet S after passing through the suction roller 12,
The corrugated portions are not collected at the rear end of the sheet S and are evenly distributed, and no wrinkles are formed.

Here, when the speed difference between the feeding speed of the registration roller pair 17 and the conveying speed of the sheet conveying belt 7 is increased, slippage of the sheet S occurs on the sheet conveying belt 7 and this causes color misregistration. Therefore, it is preferable that the feeding speed of the registration roller pair 17 be slowed by, for example, about 0.2% with respect to the carrying speed of the sheet carrying belt 7.

Further, when the image is formed on the first surface of the sheet S, there is no difference in the sheet length between the both end portions and the central portion which would cause wrinkles in the sheet S on the sheet conveying belt 7. Therefore, if a difference is provided between the conveying speed of the sheet conveying belt 7 (the conveying speed of the portion where the suction roller 12 and the driven roller 9 face each other) and the feeding speed of the registration roller pair 17, the sheet S is entirely It will be pulled strongly, and color shift will easily occur. Also, the sheet S
Pulling the whole may cause vertical waviness and induce wrinkles. Therefore, when the image is formed on the first surface of the sheet S, the conveyance speed of the sheet conveyance belt 7 and the feeding speed of the registration roller pair 17 are set to be substantially the same.

As described above, in the present embodiment, the sheet S
Image formation on the first surface of the sheet S fed from the paper feed cassette 15 while keeping the conveyance speed (process speed) of the sheet conveyance belt 7 at the time of image formation on the first surface and the second surface of At times, the feeding speed of the registration roller pair 17 is set to be the same as the feeding speed of the sheet conveying belt 7, and at the time of image formation on the second surface of the sheet S fed through the re-conveyance path 20, the feeding of the registration roller pair 17 is performed. Since the feeding speed is set to be slower than the speed of the sheet conveying belt 7, the corrugated portions of the sheet S are uniformly distributed on the sheet conveying belt 7 without being gathered at the trailing edge when the image is formed on the second surface of the sheet S. Wrinkles do not occur, and it is possible to prevent image dropouts.

In the above description, in the image forming apparatus having the sheet conveying belt 7 for conveying the sheet and the suction roller 12, the conveying speed of the sheet conveying belt 7 (the portion where the suction roller 12 and the driven roller 9 face each other). The wrinkles are prevented by providing a speed difference between the feeding speed of the sheet) and the feeding speed of the registration roller pair 17 that feeds the sheet to the sheet conveying belt 7. However, it is also applicable to other modes. be able to. For example, the photosensitive drum 2a
Since the sheet S may be pulled by the registration roller pair 17 and the portion where the sheet conveying belt 7 and the sheet conveying belt 7 face each other, even when the suction roller 12 is not provided, the conveying speed of the sheet conveying belt 7 and the sheet conveying belt 7 may be increased. By providing a speed difference between the feeding speeds of the registration roller pair 17 that feeds the sheet to the belt 7, wrinkles do not occur on the sheet S, and it is possible to prevent the occurrence of image loss.

Further, in the above description, when the image forming on the second surface of the sheet S is performed, the conveying speed of the registration roller pair 17 is made slower than the conveying speed of the sheet conveying belt 7, so that the wavy portion generated on the sheet S. The sheet conveyor belt 7
Although the sheet is conveyed by weakening the suction force without strongly adhering to the sheet, the wrinkle can be more surely generated by switching the suction bias applied to the sheet S between the first surface and the second surface of the sheet. Can be suppressed. That is, by making the attraction bias applied when the second surface of the sheet S is conveyed by the sheet conveyance belt 7 smaller than the adsorption bias applied when the first side of the sheet S is conveyed by the sheet conveyance belt 7, The suction force to the sheet conveying belt 7 during the conveyance of the second surface of the sheet S can be weakened, and the occurrence of wrinkles can be suppressed more reliably. Note that the suction bias in the transportation of the second surface of the sheet S secures the minimum suction force required to suction the sheet S onto the sheet transport belt 7. Further, although the constant current power supply 13 applies the suction bias (current) to the suction roller 12, the suction voltage bias may be applied to the suction roller 12 from the constant voltage power supply. In this case, the voltage value set on the second surface is made larger than the voltage value set on the first surface.

The operation of forming an image on both sides of the sheet S will be described with reference to the flowchart of FIG. Here, since the operation of forming images on both sides of the sheet S is described, it is assumed that the host device issues an instruction to the image forming apparatus to perform double-sided printing.

In step S901, the image forming controller 23 determines whether or not the print start signal transmitted from the host device such as the host computer via the image processing controller 24 is received, and if the print start signal is received. (YES in step S901), the voltage applied to the charging rollers 3a to 3d, and the exposure units 4a to 4d.
Of laser light emitted by the developing sleeves 50a to 50
After initialization of each part related to image formation such as the developing bias applied to d, the process proceeds to step S902.

In step S902, the image forming controller 23 rotates the paper feed roller 16 to feed the sheet S from the paper feed cassette 15, so that the paper feed motor M
Drive 3

In step S903, the image forming controller 23 sets a predetermined value V1 as the feeding speed of the registration roller pair 17 to the first surface (front surface) of the sheet S.

In step S904, the image forming controller 23 guides the sheet S conveyed by the sheet feeding roller 16 to the image forming stations 1a to 1d.
The sheet feeding motor M3 is driven to start the operation of conveying the sheet S by the registration roller pair 17. The feeding speed of the registration roller pair 17 when the image is formed on the first surface of the sheet S is set to the predetermined value V1 set in step S903.

In step S905, when the sheet S passes through the position where the attraction roller 12 and the driven roller 9 face each other toward the image forming station 1a, the sheet S is fed by the constant current power source 13 via the attraction roller 12. An attraction bias (for example, a constant current bias of 10 μA) is applied.

In step S906, the toner images are sequentially superposed and transferred onto the sheet S attracted to the sheet conveying belt 7. In this embodiment, toner images of four different colors are sequentially transferred to the sheet S. For example, in the image forming station 1a, the cyan toner image is transferred from the photosensitive drum 2a to the sheet S and is conveyed. In the image forming station 1b, the yellow toner image is transferred from the photosensitive drum 2b to the sheet S and is conveyed, and in the image forming station 1c, the magenta toner image is transferred from the photosensitive drum 2c to the sheet S. At the image forming station 1d, a black toner image is transferred from the photoconductor drum 2d to the sheet S and is conveyed, and a color toner image is formed on the sheet S.

The size of the sheet S conveyed by the image forming apparatus is as follows: the pair of registration rollers 17 and the image forming station 1a.
Since the distance between the sheet S and the sheet S is larger than the distance between
Before the trailing edge of the sheet passes the registration roller pair 17, the sheet S is conveyed by the photosensitive drum 2a and the sheet conveying belt 7.

In step S907, the sheet S is conveyed to the fixing device 18 while being nipped by the roller pair. At this time, heat and pressure are applied to the color toner image formed on the sheet, and the color toner image is fixed on the sheet S. Then, the sheet S is the fixing device 18
From the sheet to the sheet discharge roller 22.

In step S908, the image forming controller 23 determines whether or not the image formation on the second surface of the sheet S is completed. If YES, step S911
Then, the sheet discharge roller 22 is driven by the fixing motor M4 to discharge the sheet S to the sheet discharge tray 19. If NO, the process proceeds to step S909 to form an image on the second surface of the sheet S.

In step S909, the image forming controller 23 reverses the sheet S and re-conveys the sheet 20.
After the trailing edge of the sheet S passes through the fixing device 18, the fixing motor M4 is controlled to rotate in the reverse direction so that the sheet S is conveyed to. The sheet S conveyed to the re-conveyance path 20 by the discharge rollers 22 is conveyed again to the registration roller pair 17 by the re-conveyance rollers 21.

In step S910, the image forming controller 23 sets the conveying speed of the registration roller pair 17 when the image is formed on the first surface, before forming the image on the second surface (back surface) of the sheet S. It is set to a predetermined value V2 which is slower (for example, 0.2% slower) than the predetermined value V1. After that, the process returns to step S904, and the same operation as that for the first surface is performed. However, in step S904, since the conveyance speed of the registration roller pair 17 different from that for the first surface is set, the image on the second surface of the sheet S is imaged. At the time of formation, the corrugated portions of the sheet S are not uniformly gathered at the rear end and are uniformly dispersed on the sheet conveying belt 7 so that wrinkles do not occur and it is possible to prevent the occurrence of image dropouts.

In step S905 for applying the suction bias to the second surface of the sheet S, the suction bias (10 μA) applied to the first surface is set to, for example, 5 μA. In order to prevent the occurrence of wrinkles that may occur when an image is formed on the second surface, the sheet S
This is to weaken the suction force of the sheet conveyance belt 7 to the sheet conveyance belt 7.

In the present embodiment, in the case of performing double-sided printing, the feeding speed of the registration roller pair 17 is always controlled as described above, but wrinkles actually occur and image dropout occurs. The case where the phenomenon appears remarkably means that the sheet S is thin paper, for example, 60 to 70 g /
This is the case when using m ² paper. Therefore, a dedicated operation mode using thin paper as the sheet S may be provided, and when the dedicated operation mode is selected, the feeding speed of the registration roller pair 17 described above may be controlled. In addition,
The setting of the dedicated operation mode may be input by the operator from the operation panel 25 provided in the image forming apparatus, or may be set by the operator in the host device and the setting information thereof is set via the image processing controller 24. The image forming controller 23 may be set by receiving it. Specifically, when the dedicated operation mode is not set, the conveyance speed when the sheet S conveyed from the re-conveyance path 20 is conveyed by the registration roller pair 17 is the sheet fed from the sheet feeding cassette 15. The speed at which S is transported is substantially the same. On the other hand, when the dedicated operation mode is set, the conveyance speed when the sheet S conveyed from the re-conveyance path 20 is conveyed by the registration roller pair 17 is the conveyance speed of the sheet S fed from the sheet feeding cassette 15. Make it slower than the speed at which it is used.

As a case where the phenomenon that wrinkles are generated and an image is missing appears remarkably, it may be a humid environment in which the water content of the sheet S is high. The water content of the sheet S is correlated with the resistance value, and when the water content is high, the resistance value of the sheet S becomes low. Therefore, for example, at the time of forming an image on the first surface of the sheet S, the voltage of the constant current (or the current value at the constant voltage) is detected by the suction roller 12 or the like to calculate the resistance value of the sheet S, which is calculated. The above-described control of the feeding speed of the registration roller pair 17 (setting of the operation mode) may be performed according to the resistance value.

The constant current power supply 13 described with reference to FIG. 1 is composed of, for example, a voltage applying section 13a and a current detecting section 13b as shown in FIG.
The voltage value set by No. 3 is applied by the voltage application unit 13a, and the current detection unit 13b detects the current flowing at that time and outputs it to the image forming controller 23. Since it is a constant current power supply, the image forming controller 23 determines the voltage value set in the voltage applying unit 13a so that the current value detected by the current detecting unit 13b becomes a constant value (for example, 18 μA). Since the voltage value set so that the constant current value flows through the sheet S varies depending on the moisture content of the sheet S, etc. as described above, the set voltage value is changed to the sheet S.
The resistance value of can be estimated.

When the resistance value of the sheet S is less than or equal to a predetermined value (for example, 10 ⁷ Ω) (specifically, when the voltage value applied by the voltage applying section 13a indicates the predetermined resistance value of the sheet S). When it is less than the applied voltage value),
By making the feeding speed of the registration roller pair 17 at the time of forming an image on the second surface of the sheet S slower than the feeding speed at the time of forming an image on the first surface of the sheet S, wrinkles occur and an image dropout occurs. It is possible to prevent the phenomenon that

Instead of calculating the resistance value of the sheet S, a humidity sensor (environment sensor) is used to detect the humidity,
The above-mentioned control of the feeding speed of the registration roller pair 17 (setting of the operation mode) may be performed according to the detected humidity. Specifically, when the humidity detected by the humidity sensor is equal to or higher than a predetermined value (for example, 70%), the feeding speed of the registration roller pair 17 when the image is formed on the second surface of the sheet S is set to the sheet S By making it slower than the feeding speed at the time of forming an image on the first side, it is possible to prevent a phenomenon that wrinkles occur and an image is lost.

The control for changing the feeding speed of the registration roller pair 17 depending on whether or not the environment is high humidity is also effective in a low temperature and low humidity environment. That is, the sheet S
In a low-temperature and low-humidity environment where the water content is low, the wrinkles of the sheet S do not occur, and on the contrary, the stiffness of the sheet S becomes relatively strong. Therefore, the conveyance speed of the sheet conveyance belt 7 and the feed of the registration roller pair 17 are increased. The speed difference between the speeds strongly pulls the sheet S, which may deform the toner image on the sheet S and adversely affect color misregistration.

In such a low temperature and low humidity environment, regardless of whether the feeding speed of the registration roller pair 17 is imaged on the first surface or the second surface of the sheet S,
Since the sheet feeding speed by the sheet conveying belt 7 is set to be substantially the same, the occurrence of color misregistration can be minimized.

In the above description, the registration roller pair 17 can convey the sheet S at the same time as the sheet conveying belt 7 at the position facing the suction roller 12. However, the present invention can be applied to an image forming apparatus that does not have the suction roller 12. That is, the pair of registration rollers 17 are used to form the image forming stations 1a to 1a.
The present invention can also be applied to the case where the sheet S can be conveyed at the same time as the sheet conveying belt 7 at a position facing any of the photosensitive drums 2a to 2d of d. The size of the sheet S conveyed by the image forming apparatus may be larger than the distance from the registration roller pair 17 to the image forming station 1a, and the sheet being conveyed by the registration roller pair 17 depending on the size of the sheet S. Although the photosensitive drum that nips and conveys S in cooperation with the sheet conveying belt 7 is different, the photosensitive drum 2a and the sheet conveying belt 7 located at the most upstream side of the sheet S in the conveying direction, and the resist It is conveyed while being sandwiched by the roller pair 17.

[0065]

As described above, according to the present invention,
When the sheet from the re-conveying unit is conveyed to form an image on the second surface (rear surface) of the sheet, the speed is lower than the first speed when the image is formed on the first surface (front surface), and the image forming unit is Since the sheet is conveyed at the second speed lower than the conveying speed, it is possible to suppress the occurrence of wrinkles of the sheet and minimize the occurrence of image omission when forming images on both sides of the sheet. And an image forming method can be provided.

[Brief description of drawings]

FIG. 1 is a sectional view showing an overall configuration of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a control configuration of the image forming apparatus.

FIG. 3 is a diagram illustrating a sheet conveyance state when an image is formed on the first surface of the sheet.

FIG. 4 is a diagram showing a length in a transport direction of a sheet on which an image is formed on a first surface.

FIG. 5 is a diagram showing a state of the sheet S when the sheet S passes through the suction roller 12.

FIG. 6 is a diagram showing a state in which wrinkles are generated at the trailing edge of the sheet during image formation on the second surface of the sheet.

FIG. 7 is a cross-sectional view showing the relationship between the uneven portion of the sheet, the photosensitive drum, and the sheet conveying belt.

FIG. 8 is a diagram illustrating a sheet conveyance state when an image is formed on the second surface of the sheet.

FIG. 9 is a flowchart showing an operation when images are formed on both sides of a sheet.

FIG. 10 is a diagram for explaining a sheet resistance value detection method.

[Explanation of symbols] S sheet M1 sheet conveying belt drive motor M2a, M2b, M2c, M2d Drum motor M3 paper feed motor M4 fixing motor 1 Image forming station 2 photoconductor drum 7 Sheet conveyor belt 8 drive rollers 9 Driven roller 10 Belt tension roller 11 Belt tension roller 12 Adsorption roller 13 constant current power supply 13a Voltage application section 13b Current detector 14 Transfer roller 15 paper cassettes 16 paper feed rollers 17 Pair of registration rollers 18 Fixer 19 Ejection tray 20 Re-transportation route 21 Re-transport roller 22 Paper ejection roller 23 Image forming controller 24 Image processing controller 25 Operation panel

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G03G 15/16 G03G 15/16 3F049 15/20 107 15/20 107 3F100 21/00 384 21/00 384 3F101 (72) Inventor Kazuki Takashima 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Mashiro Saito 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. F-term ( Reference) 2H027 DA14 DA20 DC02 DE07 DE09 EA18 EB06 EC06 ED16 ED24 ED25 EE03 EE07 EF10 FA13 FA35 FB07 FB19 2H028 BA06 BA16 BB02 BB04 2H033 AA46 BA59 BB37 CA04 CA07 CA11 CA16 CA20 CA36 GA19GA23 GA44 FA20 FA01 FA18 FA18 FA18 FA18 FA18 FA18 FA05 GB25 GB30 GB44 HA03 HB12 HB22 JA02 JA16 JA18 JB07 JB16 JB25 JB33 JB39 JB48 JB49 LA19 PA05 PA10 PA23 PA26 PB08 PB14 PB28 2H300 EA08 EA10 EB04 EB07 EB12 ED07 ED09 ED11 ED12 ED13 EF01 EF02 EF06 EF08 EF13 EF14 EF16 EF17 EH16 EJ09 EK01 EK02 EK03 EK06 EK10 FF05 FF16 GG01 GG02 GG08 GG30 GG49 HH19 HH39 PP02 QQ04 QQ10 QQ12 QQ17 QQ29 QQ32 RR08 RR10 RR12 TT03 3F049 AA01 DA12 EA10 LA03 LA07 LB03 3F100 AA02 BA12 BA17 CA15 EA03 EA04 3F101 LA02 LA07 LB03

Claims (17)

[Claims] 1. A sheet placing unit for placing a sheet, a sheet conveying unit for conveying the sheet, an image forming unit for forming an image on the sheet conveyed by the sheet conveying unit, and an image forming unit for forming the image. Re-conveying means for conveying the sheet having the image formed on one surface to the image forming means again, and sheet feeding for feeding the sheet from the sheet placing means or the re-conveying means to the sheet conveying means. A sheet feeding means for nipping and conveying the sheet together with the sheet conveying means, and a control means for controlling a sheet feeding speed by the sheet feeding means, the control means comprising: When feeding the sheet from the sheet placing means to form an image on the first surface of the sheet,
When the sheet is fed at a first speed and the sheet from the re-conveying unit is fed to form an image on the second side of the sheet, the sheet is slower than the first speed,
Further, the image forming apparatus is characterized in that the sheet is fed at a second speed lower than the speed at which the sheet conveying means conveys the sheet. 2. The image forming apparatus according to claim 1, wherein the first speed is substantially the same as a speed at which the sheet conveying unit conveys a sheet. 3. An operation mode setting unit for setting an operation mode of the image forming apparatus, wherein the control unit controls the re-conveying unit when the first operation mode is set by the operation mode setting unit. Sheet is fed at the first speed and the second operation mode is set by the operation mode setting means, the sheet from the re-conveying means is fed at the second speed. The image forming apparatus according to claim 1 or 2, which is characterized in that. 4. The image forming apparatus according to claim 3, wherein the operation mode setting unit sets the operation mode based on information regarding the type of the sheet. 5. A resistance value detecting means for detecting a resistance value of the sheet is provided, and the operation mode setting means sets the first operation mode when the resistance value of the sheet is larger than a predetermined resistance value. The image forming apparatus according to claim 3, wherein the second operation mode is set when the resistance value of the sheet is equal to or less than the predetermined resistance value. 6. A humidity detecting means for detecting humidity is provided, wherein the operation mode setting means sets the first operation mode when the humidity detected by the humidity detecting means is below a predetermined humidity, The image forming apparatus according to claim 3, wherein the second operation mode is set when the humidity detected by the humidity detecting unit is higher than the predetermined humidity. 7. The image forming means includes an image carrier for transferring a toner image onto the sheet, and a fixing means for fixing the toner image onto the sheet. The image forming apparatus according to any one of claims. 8. The fixing device comprises a pair of rollers that rotate while being in contact with each other, and the sheet and the sheet are heated and pressed by the pair of rollers to fix the toner image on the sheet. Image forming device. 9. The sheet carrying means has a belt-shaped carrying member for carrying the sheet, and comprises a bias applying means for applying a bias for attracting the sheet to the carrying member. The image forming apparatus according to any one of claims 1 to 8. 10. The sheet carrying means has a belt-like carrying member for carrying the sheet, and the image forming means is arranged in a carrying direction of the sheet and carried by the carrying member. The image forming apparatus according to claim 1, further comprising a plurality of image carriers for transferring toner images of different colors onto a sheet. 11. A sheet feeding step of feeding a sheet from a sheet placing means for placing a sheet, and a first feeding step of feeding the sheet fed in the sheet feeding step at a first speed. A first transporting step of transporting the sheet fed in the first feeding step before the sheet feeding in the first feeding step is completed, and the first transporting step. A first image forming step of forming an image on the first surface of the sheet being conveyed in step, and an image on the second surface of the sheet imaged on the first surface in the first image forming step. A re-conveying step of re-conveying the sheet to form, a sheet re-conveyed in the re-conveying step is slower than a first speed, and is lower than a speed of conveying the sheet in the first conveying step. The second for feeding in the stomach the second speed
And the second feeding step before the second feeding step is completed.
A second conveying step for conveying the sheet being fed in the feeding step, and a second image forming step for forming an image on the second surface of the sheet being conveyed in the second conveying step. An image forming method comprising: 12. The first speed is substantially the same as the speed at which a sheet is conveyed in the first conveying step and the second conveying step.
1. The image forming method described in 1. 13. The first image forming step and the second image forming step include a transfer step of transferring a toner image onto the sheet, and a fixing step of fixing the toner image onto the sheet. The image forming method according to claim 11 or 12, which is characterized in that. 14. The first image forming step and the second image forming step include a plurality of transfer steps for transferring toner images of different colors to the sheet. 14. The image forming method according to any one of 1 to 13. 15. The first image forming step applies a bias for attracting the sheet to a belt-shaped conveying member for conveying the sheet in the first conveying step. The second image forming step includes a second bias applying step of applying a bias for adsorbing the sheet to the conveying member in the second conveying step. Item 16. The image forming method according to any one of items 11 to 15. 16. A sheet placing means for placing a sheet, a sheet conveying means having a belt-like conveying member for conveying the sheet, and image forming for forming an image on the sheet conveyed by the sheet conveying means. Means, re-conveying means for conveying the sheet imaged on the first side by the image forming means to the image forming means again, the sheet from the sheet placing means or the re-conveying means A sheet feeding unit that feeds to a conveying unit, the sheet feeding unit that nips and conveys the sheet together with the sheet conveying unit, and a suction unit that attracts the sheet to the conveying member with a predetermined suction force, Control means for controlling the image forming apparatus, wherein the control means feeds the sheet from the sheet placing means to form an image on the first surface of the sheet. The image forming method is characterized in that the predetermined suction force when feeding the sheet from the re-conveying device to form an image on the second surface of the sheet is controlled so as to be weaker than the predetermined suction force in combination. apparatus. 17. The attraction means applies a bias for attracting a sheet to the conveying member, and the control means causes the sheet from the sheet placing means to form an image on the first surface of the sheet. 17. The bias applied when feeding the sheet from the re-conveying unit to form an image on the second surface of the sheet is made smaller than the bias applied when feeding the sheet. Image forming device.