JP2007191232A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device for switching back a paper sheet by paper discharge roller pairs capable of preventing jamming caused by the skew of the paper sheet without considerably increasing the number of components and the manufacturing cost. <P>SOLUTION: A plurality of branch claws 61a to 61h which are oscillatingly mounted on a shaft 60 provided vertically with respect to the conveying direction is used for a branching means. The oscillation of the branch claws 61a, 61h provided at least on both sides of the shaft 60 is detected by sensors S<SB>a</SB>, S<SB>h</SB>. The skew amount of a paper sheet P is detected from the detection time difference of the oscillation of the branch claws 61a, 61h provided on both sides of the shaft 60 when the paper sheet P passes through the branch claw 61 is detected. When the detection time difference is not less than the predetermined value, it is determined that the skew amount of the paper sheet is large, and the paper sheet is discharged without any switch-back. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus capable of forming images on both front and back sides of a sheet, and more particularly to an image forming apparatus provided with means for detecting a skew amount of a sheet to be conveyed.

  In recent years, image forming apparatuses capable of forming images on both front and back sides of paper have been widely used from the viewpoint of effective use of resources. In such an image forming apparatus, the paper on which the image is formed on the front surface is reversed and conveyed again to the image forming unit, and an image is formed on the back surface of the paper. As a mechanism for reversing the front and back of a sheet, a mechanism that provides a stack part in the middle of a re-conveying path for re-conveying the sheet to the image forming unit and switches back the sheet here has been widely employed. The stack portion is provided with a pair of regulating members slidable in the paper width direction, and the skew of the paper is corrected by the pair of regulating members.

  However, in response to market demands for smaller and lighter devices, a device that uses a pair of paper discharge rollers for discharging paper to a tray as a switchback mechanism has been put into practical use and is being widely used. According to such an apparatus, it is not necessary to provide a stack part in the re-conveyance path, and the apparatus can be reduced in size and weight. However, since the mechanism for correcting the skew of the paper is eliminated in the re-conveyance path, there is a possibility that problems such as paper jamming in the re-conveyance path and image formation deviation on the back surface of the paper may occur.

Thus, for example, Patent Document 1 proposes that the skew amount of the paper is detected optically, and when the skew amount is equal to or greater than a certain value, it is determined that there is an abnormality, and the image forming conditions thereafter are changed.
JP-A-9-100069

  However, in the proposed technique, it is necessary to provide a plurality of optical sensors according to the paper size in order to reliably detect the skew amount of the paper from the small size to the large size, which increases the number of parts and the manufacturing cost. It also goes against the market demand for smaller size and lighter weight.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus that effectively prevents paper jamming due to paper skew without significantly increasing the number of parts and manufacturing cost. It is what.

  According to the present invention, in an image forming apparatus that can switch back a sheet of an image formed on the front surface by a pair of discharge rollers and transport it to a re-conveying path and form an image on the back surface of the sheet, the sheet is discharged from the fixing device. A branching means that allows the paper to move to the roller pair and prevents the paper switched back by the paper discharge roller pair from returning to the fixing device and leads to the re-conveying path side is provided between the fixing device and the paper discharge roller pair. The branching means has a plurality of branch claws attached to a shaft provided in a direction perpendicular to the transport direction so that the branch claws can swing, and at least the branch claws provided on both sides of the shaft can swing. When the image is formed on the front and back surfaces of the paper detected by the detecting means, the difference in detection time of the swinging of the branching claw provided on both sides of the shaft when the paper on which the front surface image formation has passed passes through the branching means. If is greater than or equal to the specified value Determines that the skew amount is large, the image forming apparatus is provided, which comprises directly discharging the paper without switchback.

  When two or more types of paper with different sizes can be used, branch claws are attached to the positions where both sides of each size of paper contact, and both sides of the conveyed paper come into contact It is desirable to determine the skew amount of the paper by detecting the swing of the paper.

  In the image forming apparatus of the present invention, as a branching unit, a shaft provided in a direction perpendicular to the transport direction and having a plurality of branching claws attached so as to be swingable is provided, and provided at least on both sides of the shaft. Detecting the amount of paper skew from the detection time difference of the swinging of the branching claw provided on both sides of the shaft when the paper passes through the branching means. If the difference in detection time is greater than or equal to a predetermined value, the paper skew amount is judged to be large and the paper is discharged without being switched back. Therefore, the paper skew is not increased without significantly increasing the number of parts and manufacturing costs. It becomes possible to effectively prevent paper jams caused by.

  When two or more types of paper with different sizes can be used, a branching claw is attached to a position where both sides of each size of paper contact, and the branching claw where both sides of the conveyed paper come into contact. If the movement is detected, the skew amount of the paper can be grasped more accurately.

  The image forming apparatus of the present invention will be described below with reference to the drawings. The present invention is not limited to these embodiments.

  FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus of the present invention. In the image forming apparatus of this figure, a paper discharge space 3 having an opening on the front side is formed at a substantially central portion in the vertical direction of the apparatus main body 1. On the bottom surface of the paper discharge space 3, a tray 80 for placing paper to be discharged is formed integrally with the casing. On the other hand, a reading mechanism 91 for reading a document is built in the upper side of the paper discharge space 3, and an image forming mechanism (image forming means) 7 is built in the lower side of the paper discharge space 3.

  The image forming mechanism 7 of the apparatus main body will be described. The surface of the photosensitive drum 70 is uniformly charged positively or negatively by the charging device 71. On the other hand, a document (not shown) is placed on the document placing table 90, and the image data is read by the reading device 91. The read image data is written on the surface of the photosensitive drum 70 by a laser scanner (exposure device) 72, and an electrostatic latent image is formed on the surface of the photosensitive drum 70. Specifically, in the case of the reverse development method, the charge corresponding to the image is removed, and in the case of the regular development method, the charge corresponding to the background is removed to form an electrostatic latent image. Next, the electrostatic latent image on the photosensitive drum 70 is visualized with toner by the developing device 73. At this time, the charging polarity of the toner is the same as the charging polarity of the photosensitive drum 70 in the case of the reversal development method, and is opposite to the charging polarity of the photosensitive drum 70 in the case of regular development.

  On the other hand, the paper P stored in the paper feed cassette 81 is pulled out by the pickup roller 82, sandwiched between the paper feed roller 83 and the separating roller 84, and sent to the conveyance path. Then, the registration roller pair 51 feeds the paper P to the transfer unit at the timing when the toner image on the photosensitive drum 70 reaches the transfer unit. In the transfer portion, the sheet P is sandwiched between the photosensitive drum 70 and the transfer roller 74, and a charge having a polarity opposite to the toner charging polarity is applied to the transfer roller 74. Toner image moves onto the paper P. The toner remaining on the photosensitive drum 70 without moving onto the paper P is removed and collected by the cleaning roller 75. On the other hand, the paper P on which the toner image is placed is conveyed to a pair of fixing rollers (fixing device) 85. Here, the toner image is heated and pressurized by the fixing roller pair 85 and fixed on the paper P. Then, the paper P is sent to the paper discharge roller pair 86 by the branching claw (branching means) 61.

  When image formation is performed only on one side of the sheet, the sheet is discharged to the tray 80 by the discharge roller pair 86 as it is. On the other hand, when forming images on both sides of the paper, the paper discharge roller pair 86 rotates in the reverse direction when the rear end of the paper is held between the paper discharge roller pair 86, and the paper is conveyed by the branching claw 61. It is sent in the direction of the roller 87 (so-called switchback). Then, the sheet is guided to the re-conveying path 50 and sent again to the registration roller pair 51 by the conveying rollers 88 and 89. In the same manner as described above, after an image is formed on the other side surface of the paper, the paper P is sent to the paper discharge roller pair 86 by the branching claw 61 and discharged to the tray 80.

  FIG. 2 is a diagram outlining the movement of the branching claw 61 when the paper P is switched back by the paper discharge roller pair 86. A plurality of branch claws 61a, 61b,..., 61h (which may be referred to as “branch claws 61” as a whole) are swingably attached to the shaft 60. The separation claw 61 is prevented from rotating in a clockwise direction from a normal state (a state where no paper passes) by a stopper (not shown). A sensor S that detects the swinging of the branching claw 61 is disposed so as to face the tip of the branching claw 61 at a distance.

  When the sheet P with the image formed on the front surface reaches the branching claw 61, the branching claw 61 is pushed up in the sheet traveling direction by the sheet P, and the sheet P advances in the direction of the sheet discharge roller pair 86 (FIG. b)). At this time, the sensor S detects that the branch claw 61 has been pushed up by the paper P. When the paper P is sandwiched between the paper discharge roller pair 86, the paper P is sent outward by the paper discharge roller pair 86. When the rear end portion of the paper P is sandwiched between the paper discharge roller pair 86, the rotation direction of the paper discharge roller pair 86 is reversed and the paper P is drawn into the apparatus. At this time, the branch claw 61 is in its original state due to its own weight, that is, the state in which the paper P is prevented from returning in the direction of the fixing roller pair 85 (shown in FIG. 1). Enter the side shown in FIG. (Figure (c))

  FIG. 3 shows a specific layout of the branching claw 61 and the sensor S. In this image forming apparatus, since the sheet is conveyed based on the center, the branching claws 61a, 61b,... 61g, 61h are attached to the shaft 60 so as to be swingable symmetrically in the width direction with the center in the width direction as the center. ing. Sensors are attached to the branching claws where both ends of each sheet come into contact with each other in accordance with the size of the sheet to be conveyed. For example, in the case of “A4” size paper, the branching claws 61a and 61h are in contact with both side edges of the paper P, and in the case of “B5” size paper, the branching claws 61b and 61g are connected to both side edges of the paper P. In the case of “A5” size paper, if the branching claws 61c and 61f come into contact with both side edges of the paper P, sensors are attached to these branching claws, and the other branching claws 61d and 61e are attached to the branching claws 61d and 61e. The sensor is not installed. Note that the number and position of branch claws attached may be determined as appropriate according to the type of paper used in the image forming apparatus.

  The sensor S used in the present invention is not particularly limited as long as it can detect the swing of the branch claw 61, and a conventionally known sensor can be used. For example, by using a PI sensor including a light emitting unit and a light receiving unit, the sensor S is installed so that the branch claw 61 blocks the light passage from the light emitting unit to the light receiving unit. May be received by the light receiving unit to detect the swing of the branching claw 61. Alternatively, the sensor S is installed so that the light from the light emitting part passes through the hole formed in the branching claw 61 and reaches the light receiving part. When the branching claw 61 swings, the light is blocked by the branching claw 61 and the branching claw 61 You may make it detect rocking | fluctuation.

  As a means for detecting the paper size, for example, a pair of cursors (not shown) are used for positioning the paper P in the width direction attached to the paper feed cassette 81 (shown in FIG. 1). When the pair of cursors are moved according to the width of the paper P, the resistance of the resistor attached to the cursor changes. By measuring this change in resistance, the type of paper P (for example, A4, B5, postcard, etc.) is detected. Alternatively, an input unit for inputting the type of paper (for example, thin paper, thick paper, envelope, postcard, recycled paper, etc.) may be provided on the operation panel so that the user can input the type of paper to be used. . Of course, the type detection means and the input unit may be used in combination.

  As described above, when the type of the paper to be used is determined by the detection by the cursor of the paper feed cassette 81 or the input from the operation panel, the branching claws that contact both sides of the paper are specified. The skew amount of the paper is calculated based on the detection signal. The calculation of the skew amount of the paper will be described next.

As shown in FIG. 4A, when the paper is normally transported, the side of the paper on the front side in the transport direction simultaneously contacts the branching claw. On the other hand, as shown in FIG. 6B, when the sheet is conveyed obliquely, one sheet side end at the leading end side in the conveying direction first comes into contact with the branching claw 61a, and the other sheet side end. Finally comes into contact with the branching claw 61h. Sensor S _a of this time, the detection signal S _h shown in FIG. The paper P comes into contact with the branch claw 61a and the detection signal of the sensor Sa is turned on. Then, after t milliseconds, the paper P comes into contact with the branch claw 61h and the detection signal of the sensor Sh is turned on. The greater the inclination of the paper P with respect to the transport direction, the longer the time t from when one paper side edge comes into contact with the branching claw until the other paper side edge comes into contact with the branching claw. Therefore, by measuring the time difference t of the swing of the branching claw 61 detected by the sensor S, the inclination of the paper P can be detected.

  In the present invention, when the time difference t of the swing of the branching claw 61 detected by the sensor S is equal to or larger than a predetermined value, it is determined that the skew amount of the paper is too large, and the paper discharge roller pair 86 (see FIG. 1). The sheet P is discharged as it is to the sheet discharge tray 80 (shown in FIG. 1) without being switched back. Thereby, it is possible to prevent the paper from being jammed in the re-conveying path 50. The time difference t for whether or not the paper P is switched back by the paper discharge roller pair 86 varies depending on the paper size, the paper transport speed, and the like, but the critical value is about 150 to 200 msec for the “A4” size lateral transport.

1 is a schematic diagram illustrating an example of an image forming apparatus according to the present invention. It is a schematic diagram showing the movement of a branch claw and a paper discharge roller pair. It is a perspective view which shows a branch nail | claw and a sensor. FIG. 6 is a schematic diagram illustrating a sheet conveyance state. It is a figure which shows an example of the detection signal of a sensor.

Explanation of symbols

1 Image forming apparatus P Paper S Sensor (detection means)
_{S a, S b, S c} , S f, S g, S h sensor 50 re-feeding path 60 the axis 61 branch claw (branching means)
61a, 61b, 61c, 61d, 61e, 61f, 61g, 61h Branch claw 81 Paper feed cassette 86 Paper discharge roller pair 85 Fixing roller pair (fixing device)

Claims (2)

In the image forming apparatus capable of forming the image on the back side of the paper, the paper image-formed on the front surface is switched back by the pair of discharge rollers and conveyed to the re-conveying path.
The fixing device and the paper discharge roller are provided with a branching unit that allows the paper to be conveyed from the fixing device to the paper discharge roller pair, prevents the paper switched back by the paper discharge roller pair from returning toward the fixing device, and guides the paper to the re-transport path side. Established between the pair,
The branching unit has a plurality of branching claws swingably attached to a shaft provided in a direction perpendicular to the conveying direction.
At least the swinging of the branch claw provided on both sides of the shaft is detected by the detection means,
When image formation is performed on the front and back surfaces of the paper, the difference in detection time of the swinging motion of the branching claws provided on both sides of the shaft when the paper on which front surface image formation has finished passes through the branching unit is a predetermined value or more. And determining that the skew amount of the paper is large and discharging the paper as it is without switching back.   Two or more types of paper with different sizes can be used, and branching claws are attached to the positions where both sides of each size of paper contact, and the branching claw where both sides of the conveyed paper come into contact. The image forming apparatus according to claim 1, wherein swinging is detected to determine a skew amount of the sheet.

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