JP2007070064A - Image forming device and recording sheet carrying control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress shift in color caused by difference in speed between a sheet feeding device and a carrying belt 9a. <P>SOLUTION: When a leading edge of a sheet reaches a media sensor 21, the thickness of the sheet is detected. When it is the paper with high stiffness, a motor 33 of the sheet feeding device is driven at a rotary drive speed that makes a sheet carrying speed V1 get closer to a carrying speed V3 of the carrying belt 9a as far as the V1 does not exceed the V3. On the other hand, when it is the paper with low stiffness, the motor 33 of the sheet feeding device is driven at a rotary drive speed that makes a sheet carrying speed V2 get closer to the carrying speed V3 of the carrying belt 9a as far as the V2 does not exceed the V3. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus having a function of conveying a recording material by a conveyance belt, and a recording material conveyance control method for controlling conveyance of the recording material.

  2. Description of the Related Art Conventionally, an image forming apparatus, particularly an image forming apparatus that is an electrophotographic system and includes a plurality of image carriers, has been provided with a sheet conveying apparatus having a conveying belt at a transfer position. In such an apparatus, an image forming unit is arranged for each color (magenta, cyan, yellow, black in the case of normal full color) along the conveyance belt.

  The image forming unit includes an image carrier (photosensitive drum and the like) and a developing device that performs development with each color toner.

  With the configuration as described above, the toner images of the respective colors formed on the image carrier are sequentially transferred onto the sheet conveyed while being electrostatically attracted by the conveying belt, and then the image is not fixed on the sheet by the fixing device. The image is fixed.

  Further, a registration roller is provided upstream of the conveying belt in the sheet conveying direction to convey the sheet onto the conveying belt with timing.

  As described above, in an image forming apparatus in which a sheet is adsorbed on a conveyance belt and an image is formed using a plurality of image carriers, a speed difference with a registration roller or a conveyance force when adsorbing the sheet on the conveyance belt. Color shift occurs due to the difference.

  If the speed difference between the registration roller and the conveying belt can be made completely zero, the color shift can be avoided. However, in reality, there are manufacturing tolerances of the diameter of the registration roller, the diameter of the roller that drives the conveyor belt, a difference in wear due to durability, a difference in thermal expansion, and the like, and it is impossible to make the speed difference completely zero.

  Therefore, even if the speed difference between the registration roller and the conveyor belt cannot be reduced to zero at all, in order to avoid the color misregistration, some countermeasures that have been employed in the past are listed first. There is a sheet conveying apparatus having means for releasing the nipping of the registration roller when the conveyed sheet passes the registration roller and reaches the conveyance belt to avoid a speed difference between the registration roller and the conveyance belt.

  In addition, there is a sheet conveying apparatus configured to form a conveying path that can form a loop between the registration roller and the conveying belt.

  Further, as shown in Patent Document 2, an intermediate conveyance roller is installed between the paper feed roller and the registration roller, and a conveyance path is formed so that a loop can be formed between the intermediate conveyance roller and the registration roller. There is a sheet feeding device.

Japanese Patent Laid-Open No. 7-261572 JP 2002-249258 A JP 2002-182518 A

  However, when the conveyed sheet passes the registration roller and reaches the conveyance belt, the nipping of the registration roller is released, and the sheet conveyance apparatus having a means for avoiding the speed difference between the registration roller and the conveyance belt, The following problems occur.

  A mechanism for releasing the nipping of the registration roller is required, which increases the cost, and an impact at the time of nipping and releasing the registration roller may occur on the image as banding. Further, depending on the combination of the sheet conveyance path in the vicinity thereof and the conveyed sheet, the nip of the conveyance roller located further upstream of the registration roller must be released.

  A problem that occurs in the sheet conveying apparatus that forms a conveying path capable of forming a loop between the registration roller and the conveying belt will be described below with reference to FIG.

  When a conveyance path between the registration roller 8d and the conveyance belt 9a is formed so as to secure a loop amount necessary and sufficient to prevent color misregistration, the conveyance is performed between the registration roller 8d and the conveyance depending on the characteristics of the sheet (stretch strength). The conveyance path to and from the belt 9a, that is, the time from when the leading edge of the sheet passes the registration roller 8d to the conveyance belt 9a is different. Since an electrostatic image is drawn on the image carrier in accordance with the timing at which the leading edge of the sheet passes through the registration roller 8d, there is a problem in that the leading edge margin of the image transferred to the sheet varies depending on the characteristics of the sheet (strength of the sheet). Arise.

  This occurs in the sheet feeding apparatus in which an intermediate conveyance roller 8a2 is installed between the sheet feeding roller and the registration roller 8d, and a conveyance path is formed so that a loop can be formed between the intermediate conveyance roller 8a2 and the registration roller 8d. The problem is also shown below using FIG.

  The relationship between the sheet conveyance speed V1 by the intermediate conveyance roller 8a2 and the sheet conveyance speed V2 by the registration roller 8d is V1> V2. The intermediate conveyance roller 8a2 and the registration roller 8d are driven by a normally identical driving source (not shown), for example, a stepping motor, and the difference in conveyance speed is set depending on the size of the diameter.

  The relationship between the sheet conveyance speeds V1 and V2 is V1> V2. The sheet conveyance speed is prevented between the intermediate conveyance roller 8a2 and the registration roller 8d, and the intermediate conveyance roller 8a2 and the registration roller. This is to hold the loop amount once formed with 8d while the sheet is being conveyed.

  In order to reduce the load when the sheet is conveyed by the conveyance belt 9a as much as possible, a one-way clutch is installed on the intermediate conveyance roller 8a2 and the registration roller 8d. When the driving force for rotating the roller in the direction in which the sheet is conveyed is transmitted and the conveying roller is quickly rotated by the sheet, it is possible to follow the rapid rotation with a low load.

  Until the sheet is held between the conveyance belts 9a, the thick thick paper cannot be looped between the intermediate conveyance roller 8a2 and the registration roller 8d, and is therefore conveyed at the conveyance speed V1 of the intermediate conveyance roller 8a2. Since the thin paper with low stiffness can form a loop between the intermediate conveyance roller 8a2 and the registration roller 8d, the thin paper is conveyed at the conveyance speed V2 of the registration roller 8d.

  In order to prevent color misregistration, it is necessary to make the difference between the sheet conveyance speed by the sheet feeding device and the sheet conveyance speed by the conveyance belt 9a as small as possible. However, the sheet conveyance speed by the sheet feeding device varies depending on the thickness of the sheet. And this becomes difficult.

  Further, an intermediate conveyance roller 8a2 is installed between the paper feed roller and the registration roller 8d, and a conveyance path is formed so that a loop can be formed on the sheet between the intermediate conveyance roller 8a2 and the registration roller 8d. In an image forming apparatus provided with a simple sheet feeding device and allowing the user to arbitrarily input the sheet thickness using an operation panel or the like, it is necessary to input each time the user changes the type of the sheet. In addition, if the input is wrong, there arises a problem that an output result having a low image quality with a large color shift is obtained.

  By the way, a sheet feeding device similar to the above that forms a conveyance path that can form a loop between the registration roller 8d and the conveyance belt 9a is provided, and the user can arbitrarily set the sheet thickness on the operation panel or the like. 2. Description of the Related Art An image forming apparatus that can input data is known. In this apparatus, it is necessary to input every time the user changes the sheet type, which is troublesome, and if the input is wrong, there is a problem in that an output result with a low image quality in which the leading margin is different from a desired value is obtained. Arise.

  Accordingly, the present invention provides an image forming apparatus that solves the above-described problems and can suppress color misregistration caused by the speed difference between the sheet feeding apparatus and the conveyance belt regardless of the sheet characteristics. This is the first purpose.

  Another object of the present invention is to provide an image forming apparatus that solves the above-described problems and can suppress variations in the leading edge margin of the image regardless of the sheet characteristics.

  Furthermore, the present invention provides a recording material conveyance control method capable of solving the above-described problems and changing the speeds of the registration roller and the intermediate conveyance roller in accordance with information relating to sheet characteristics. Third purpose.

  The invention according to claim 1 is an image forming apparatus for transferring and fixing a visible image obtained by developing latent images on a plurality of image carriers onto a sheet conveyed by a conveying belt, and storing the sheet Storage means, a registration roller that conveys the sheet to the conveyance belt at a predetermined timing, an intermediate conveyance roller that conveys the sheet from the storage means to the registration roller, and a sheet that is abutted against the registration roller And a loosening means for loosening the sheet between the intermediate conveying roller and a sheet thickness detecting means for detecting the thickness of the sheet; When the sheet conveying speed by the roller is V2, and the sheet conveying speed by the conveying belt is V3, the relationship between the V1, V2, and V3 is The sheet conveyance speed by the intermediate conveyance roller and the registration roller is changed to a predetermined sheet conveyance speed in accordance with information on the thickness of the sheet that satisfies 3 ≧ V1> V2 and is detected by the sheet detection unit. And a sheet conveyance speed control means for controlling.

  According to the first aspect of the present invention, the time until the latent image formation start positions on the plurality of image carriers reach the transfer position is determined by detecting the thickness of the sheet by the sheet detection means and moving the sheet to the transfer position. It can be shorter than the time to reach.

  According to the first aspect of the present invention, there can be provided storage means for storing the thickness of the sheet detected by the sheet detection means.

  According to the first aspect of the present invention, there can be provided means for detecting the presence or absence of a sheet, and the type of sheet can be detected from information obtained from both the sheet detection means and the sheet presence / absence detection means.

  According to a fifth aspect of the present invention, there is provided an image forming apparatus for transferring and fixing a visible image obtained by developing latent images on a plurality of image carriers onto a sheet conveyed by a conveying belt, and storing the sheet Storage means, a registration roller that conveys the sheet to the conveyance belt at a predetermined timing, an intermediate conveyance roller that conveys the sheet from the storage means to the registration roller, and a sheet that is abutted against the registration roller A sheet thickness detecting means for detecting the thickness of the sheet, and information on the sheet thickness detected by the sheet thickness detecting means. And a timing changing means for changing the timing of starting the latent image formation on the image carrier.

  According to a fifth aspect of the present invention, the time until the latent image formation start position on the image carrier reaches the transfer position is that the sheet reaches the transfer position after the thickness of the sheet is detected by the sheet detecting means. It can be shorter than the time until.

  The invention according to claim 5 can further comprise a storage means for storing the thickness of the sheet detected by the sheet detection means.

  In the invention of claim 5, a sheet presence / absence detection unit for detecting the presence / absence of a sheet can be provided, and the type of the sheet can be detected from information obtained from both the sheet detection unit and the sheet presence / absence detection unit.

  According to a ninth aspect of the present invention, there is provided an image forming apparatus for transferring and fixing a visible image obtained by developing a latent image on an image carrier onto a sheet conveyed by a conveying belt, for storing the sheet. Storage means, a registration roller that conveys the sheet to the conveyance belt at a predetermined timing, and the registration roller and the intermediate conveyance so as to form a loop on the sheet between the intermediate conveyance roller and the registration roller. Control means for controlling the speed of the rollers, and detection means for detecting the characteristics of the sheet, wherein the control means speeds of the registration rollers and the intermediate conveyance rollers in accordance with information detected by the detection means It is characterized by changing.

  According to a tenth aspect of the present invention, there is provided a recording material conveyance control method for an image forming apparatus for transferring and fixing a visible image obtained by developing a latent image on an image carrier onto a sheet conveyed by a conveyance belt. The registration roller that conveys the sheet to the conveyance belt at a predetermined timing and the speed of the intermediate conveyance roller that conveys the sheet from the sheet storage means to the registration roller to control the registration roller and the intermediate conveyance roller A loop forming step for forming a loop on the sheet between, and a speed changing step for changing the speeds of the registration roller and the intermediate conveying roller in accordance with information on the characteristics of the sheet in the loop forming step. It is characterized by that.

  According to the present invention, since it is configured as described above, it is possible to suppress color misregistration caused by the speed difference between the sheet feeding device and the conveyance belt regardless of the sheet characteristics.

  Further, according to the present invention, since it is configured as described above, it is possible to suppress variations in the leading edge margin of the image regardless of the sheet characteristics.

  According to the present invention, since it is configured as described above, it is possible to change the speeds of the registration roller and the intermediate conveyance roller in accordance with information relating to the characteristics of the sheet.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
<First Embodiment>
FIG. 3 shows a first embodiment of the present invention. This is an example of a full-color image forming apparatus provided with four image carriers, and this structure is shown in FIG.

  The full-color image forming apparatus shown in FIG. 2 includes four photosensitive drums 1 (1a, 1b, 1c, 1d), and the surface of the photosensitive drum 1 is sequentially arranged around each photosensitive drum 1 in accordance with the rotation direction. Charging means 2 (2a, 2b, 2c, 2d) for uniformly charging, and exposure means 3 (3a, 3b, 3c, 3d) for forming an electrostatic latent image on the photosensitive drum 1 by irradiating a laser beam based on image information ), Developing means 4 (4a, 4b, 4c, 4d) for attaching toner to the electrostatic latent image to visualize the toner image, and transferring means (transfer roller) for transferring the toner image on the photosensitive drum 1 to a transfer material. ) 5 (5a, 5b, 5c, 5d), a cleaning means 6 (6a, 6b, 6c, 6d) for removing the post-transfer toner remaining on the surface of the photosensitive drum 1 after the transfer, etc. It is configured.

  Here, the photosensitive drum 1, the charging unit 2, the developing unit 4, and the cleaning unit 6 for removing the toner are integrally formed as a cartridge to form a process cartridge 7 (7 a, 7 b, 7 c, 7 d).

  Further, the sheet S fed from the feeding unit 8 is conveyed to the image forming unit by a conveying unit 9 constituted by a conveying belt 9a, and each color toner image is sequentially transferred to record a color image, and then a fixing unit. The image is fixed at 10 and discharged to the discharge unit 13 by the discharge roller pairs 11 and 12. When forming a double-sided image, the sheet S is fixed by the fixing unit 10 and discharged by the paper discharge rollers 11 and 12 before being conveyed to the double-sided conveyance path 15 by reversing the paper discharge rollers 11 and 12. (In the direction of arrow A). The sheet S conveyed to the double-sided conveyance path 15 passes through the oblique feeding roller 16 at the front of the apparatus main body, is conveyed vertically downward to the U-turn roller 17, and is conveyed to the image forming unit by the U-turn roller and the registration roller 8d. Is done.

  FIG. 1 shows a paper feeding unit of the full-color image forming apparatus shown in FIG. The resist shutter 14 rotatably attached to the axis of the resist roller 8d is counterclockwise, that is, a force that gives a resistance force when the front end of the sheet comes into contact by a torsion spring (not shown). It depends. When the leading edge of the sheet conveyed by the intermediate conveying roller 8c2 is abutted against the registration shutter 14, the registration shutter 101 is rotated clockwise to apply a resistance force to the leading edge of the sheet, and the skew is corrected. A flag 101a provided on the shutter 101 detects the timing at which the sheet passes by shielding the photo sensor 102 from light.

  In accordance with the detection timing, scanners 3a, 3b, 3c, and 3d (not shown) irradiate the photosensitive drums 1a, 1b, 1c, and 1d, and an electrostatic latent image is formed. This electrostatic latent image is developed by the developing means 4a, 4b, 4c, and 4d, and toner images are formed on the photosensitive drums 1a, 1b, 1c, and 1d.

  The sheet is conveyed by the registration roller 8d at a predetermined timing. Unlike the conventional example shown in FIG. 14, the conveyance direction of the sheet conveyed by the registration roller 8d is equal to a straight line connecting the nip of the registration roller 8d and the nip of the suction roller 9f. Therefore, the conveyance path does not change due to the strength of the paper, and the leading edge margin of the image is also constant. The sheet enters the nip of the suction roller 9f, is electrostatically attracted to the transport belt 9a, and is transported. When the leading edge of the sheet reaches the nip between the photosensitive drum 1a and the transfer roller Tp1, the toner image on the photosensitive drum is transferred to the sheet.

  In the present embodiment, the intermediate conveying roller 8a2 and the registration rollers 8a2 and the registration belt 8a and the registration belt 9a are prevented from causing color misregistration due to the speed difference between the intermediate conveyance roller 8a2 and the registration belt 8a. It is formed between the rollers 8d. That is, the difference between the shortest transport path S1 and the longest transport path S2 is configured to be a necessary and sufficient amount in the sheet passing from the cassette, depending on the nip direction of the intermediate transport roller 8a2 and the shape of the guide 24.

  The conveyance speed V1 of the intermediate conveyance roller 8a2 and the conveyance speed V2 of the registration roller 8d satisfy V1> V2, and the sheet conveyance between the intermediate conveyance roller 8a2 and the registration roller 8d is prevented and the intermediate conveyance is performed. The loop amount once formed between the roller 8a2 and the registration roller 8d is held while the sheet is conveyed.

  The intermediate conveyance roller 8a2 and the registration roller 8d are driven by the same drive source (not shown), for example, a stepping motor, and the difference between the conveyance speeds V1 and V2 is set by the difference between the respective roller diameters.

  The sheet conveyance speed V0 by the sheet feeding device has two values depending on the strength of the sheet.

  In a thick cardboard, the strength of the sheet exceeds the load of a one-way clutch (not shown) for transmitting the driving force only in the sheet conveyance direction installed on the registration roller 8d. No loop is formed with the roller 8d. Accordingly, the sheet conveying speed V0 by the sheet conveying apparatus is equal to the sheet conveying speed V1 by the intermediate conveying roller 8a2.

  In thin paper with low stiffness, the strength of the sheet is less than the load of the one-way clutch for transmitting the driving force only in the sheet conveyance direction installed in the registration roller 8d, so the intermediate conveyance roller 8a2 and the registration roller 8d Does not form a loop between them. Accordingly, the sheet conveying speed V0 by the sheet conveying apparatus is equal to the sheet conveying speed V2 by the registration roller 8d.

  In the present embodiment, a media sensor 21 as a detection unit is disposed in the sheet conveyance path. Below the media sensor 21, upper and lower guides 22 and 23 are arranged so as to sandwich the sheet in order to suppress flapping of the conveyed sheet.

  FIG. 3 shows a schematic diagram of the configuration of the media sensor 21 and the upper and lower guides 22 and 23 and the means for changing the sheet conveying speed according to the thickness of the sheet.

  The media sensor 21 is a sensor that detects the characteristic (thickness in the present embodiment) of the sheet before the sheet is conveyed to the transfer region, and detects the thickness of the sheet during an image forming operation. The result is transmitted as a control signal to the motor controller 32 as a control means via the CPU 31. The motor controller 32 is driven to rotate by sending a drive signal to a motor 33 which is a drive source of the sheet feeding device at a rotational drive speed corresponding to the thickness of the sheet.

  FIG. 4 is a flowchart showing a procedure for changing the sheet conveying speed according to the thickness of the sheet. When the leading edge of the sheet reaches the media sensor 21, the paper thickness is detected (S401). When the sheet is thick paper (S402), the motor 33 of the sheet feeding device is driven at a rotational driving speed that brings the sheet conveying speed V1 as close as possible within a range not exceeding the conveying speed V3 of the conveying belt 9a (S402). S403). On the other hand, when the sheet is thin paper (S402), the motor 33 of the sheet feeding device is driven at a rotational driving speed that brings the sheet conveying speed V2 as close as possible within the range not exceeding the conveying speed V3 of the conveying belt 9a. (S404).

  Until the leading edge of the sheet electrostatically attracted by the conveyance belt 9a reaches the nip between the photosensitive drum 1a and the transfer roller Tp1, the conveyance belt 9a is formed by a loop formed between the intermediate conveyance roller 8a2 and the registration roller 8d. Color misregistration caused by the sheet conveying speed V3 and the sheet feeding apparatus conveying speed V0 can be prevented.

  After the leading edge of the sheet electrostatically attracted by the conveyance belt 9a reaches the nip between the photosensitive drum 1a and the transfer roller Tp1, the holding force acting between the conveyance belt 9a and the sheet is greatly increased. On the other hand, the intermediate conveyance roller 8a2 and the registration roller 8d are provided with a one-way clutch (not shown) for transmitting a driving force only in the sheet conveyance direction, so that the sheet is applied in the direction opposite to the conveyance direction. The load is sufficiently small with respect to the holding force acting between the conveyance belt 9a and the sheet. Therefore, color misregistration caused by the conveyance speed V3 of the conveyance belt 9a and the conveyance speed V0 of the sheet feeding apparatus can be prevented.

In the present embodiment, the driving speed of the motor is controlled with the thickness of the sheet having a basis weight of 105 g / m ² being 0.13 mm as a threshold value. The conveyance speed V1 by the intermediate conveyance roller 8a2 is set to be 1% slower and the conveyance speed V2 by the registration roller 8d is set to be 2% slower than the speed V3 of the conveyance belt 9a.

  By detecting the thickness of the sheet, the difference between the conveyance speed V1 by the intermediate conveyance roller 8a2 and the conveyance speed V2 by the registration roller 8d is 1%, and the conveyance speed V0 of the sheet feeding device is brought close to the speed V3 of the conveyance belt 9a. As a result, the color shift was improved by 25 μm.

  An example of a method for detecting the thickness of the sheet will be described with reference to FIG.

  The media sensor 21 measures the distance to the lower guide 23 with no sheet between the upper and lower guides 22 and 23, and measures the distance to the sheet when there is a sheet between the upper and lower guides.

  The media sensor 21 is an optical reflection type displacement sensor having both a light emitting part and a light receiving part. As shown in FIG. 5, the media sensor 21 includes an LED 301 as a light emitting element and a position detection element 302 (hereinafter referred to as “PSD”) as a light receiving element. The light emitting element 302 irradiates the detection areas 303 and 304 with light, and the reflected light from the detection areas 303 and 304 is detected by the PSD 302 via the lens 305. Reflected light from the measurement object is collected on the PSD 302 by the lens 305, and the focal position of the collected reflected light changes corresponding to the distance to the measurement object.

  More specifically, when the light emitted from the light emitting element 301 is reflected by the detection region 303, it reaches the lens 305 as indicated by a dotted line, and further passes through this lens 305 to the left focal point 306 in the figure of the PSD 302. Condensate. On the other hand, when the light emitted from the light emitting element 301 is reflected by the detection region 304, it reaches the lens 305 as shown by a solid line, and further collects at the right focal position 307 in the figure of the PSD 302 via this lens 305. Shine.

  Therefore, it is possible to detect the distance to the measurement object by processing the output signal from the PSD 302.

  Next, an example in which the means described in Patent Document 3 is used as means for detecting the thickness of the sheet will be described.

  As shown in FIG. 6A, the media sensor 21 includes an LED 402 as a light irradiation unit, a CMOS sensor 34 as a video reading unit, and lenses 405 and 406 as imaging lenses.

  In FIG. 6B, reference numerals 451 to 454 are obtained by arranging the sampling images read into the CMOS sensor 34 as the video reading means and digitally processed in time series.

  For example, when the leading edge of the recording paper passes through the CMOS sensor 34, a sampled video 451 is sampled without the leading edge of the recording paper passing through the CMOS sensor 34, and the next sampled video is Sampled video 412. Yes, the Sample video 452 shows a state in which the leading edge of the recording paper is applied to the CMOS sensor 34.

  In this case, the shadow area of the recording paper varies depending on the thickness of the recording paper.

  In the example of FIG. 6, the area of the shadow is proportional to the thickness of the recording paper for 4 pixels.

  Since the recording paper is being conveyed, the sample image 453 is sequentially sampled every time the image is sampled, and the sample image 454 and the image move.

  In order to further stabilize the accuracy of recording sheet thickness detection with the means described in Patent Document 3, it is preferable to stabilize the position of the leading end of the recording sheet.

  In the example of FIG. 6, two pairs of transport rollers 411 and 421 are arranged in the vicinity of the media sensor 21. The conveyance roller pairs 411 and 421 have the same configuration. The transport roller pair 411 is provided with transport rollers 412 and 413 that rotate as the recording paper advances, and a spring 414 that pressurizes the transport roller 412 from above.

<Second Embodiment>
In the present embodiment, the sheet characteristic (thickness) is detected, and the sheet conveying speed V0 by the sheet feeding apparatus can be optimally changed with respect to the sheet in which the thickness is detected.

  The distance from the end of the sheet passing through the media sensor 21 for detecting the thickness of the sheet until the end of the sheet reaches the transfer position Tp1 is that the laser irradiation position Lp1 to the photosensitive drum 1a reaches the transfer position Tp1. Longer than the distance to do. On the other hand, the sheet conveyance speed and the rotational speed of the photosensitive drum 1a are substantially equal.

  Therefore, the time until the laser irradiation position Lp1 to the photosensitive drum 1a reaches the transfer position Tp1 is longer than the time from when the thickness of the sheet is detected by the media sensor 21 until the sheet reaches the transfer position Tp1. Therefore, even if the sheet thickness is detected for all the conveyed sheets, the throughput is not reduced.

<Third Embodiment>
In this embodiment, as compared with the second embodiment, the time from when the characteristic (thickness) of the sheet is detected by the media sensor 21 until the sheet reaches the transfer position Tp1 is the image carrier. The difference is that the laser irradiation position 1b to the photosensitive drum 1a is shorter than the time required to reach the transfer position Tp1.

  In this case, since the throughput decreases when the sheet thickness is detected for all the conveyed sheets, the decrease in throughput is prevented from the second printing operation using the following means. I made it.

  As shown in FIG. 7, the thickness information of the first sheet of the image forming operation is stored in the ROM 34 as a storage unit via the CPU 31. After the thickness information is obtained once, the thickness of the sheet may be changed. Only in that case, sheet thickness information is detected again, and the motor 33 of the sheet feeding apparatus is driven at a rotational drive speed based on the thickness information stored in the ROM 34 for the second and subsequent sheets thereafter.

  FIG. 8 is a flowchart illustrating a procedure for changing the sheet conveyance speed according to the thickness of the sheet without reducing the throughput. If the leading edge of the sheet reaches the media sensor 21 and the medium is the first sheet (S801), the paper thickness is detected (S802). If the sheet is thick paper (S803), the motor 33 of the sheet feeding device is driven at a rotational driving speed that brings the sheet conveying speed V1 as close as possible without exceeding the conveying speed V3 of the conveying belt 9a (S803). S804). On the other hand, when the sheet is thin paper, the sheet feeding device motor 33 is driven at a rotational driving speed that brings the sheet conveying speed V2 as close as possible within the range not exceeding the conveying speed V3 of the conveying belt 9a (S805). ).

  If the leading edge of the sheet reaches the media sensor 21 and the media is the second or later, the paper thickness is not detected, and the driving speed of the motor 33 remains unchanged.

<Fourth embodiment>
In the present embodiment, a light transmissive sheet (OHT) is detected, and the sheet conveying speed V0 by the sheet feeding device can be optimally changed with respect to OHT.

  OHT can be detected by detecting the sheet thickness by the optical reflection type sensor at the timing when the resist shutter 101 detects the sheet.

  The flag 101a provided on the registration shutter 101 shields the photosensor 102, thereby detecting the physical presence of the sheet. In the light receiving part of the optical reflection type sensor 21, since the sheet is OHT, reflected light is not detected.

  From the information obtained from the above two sensors, it can be detected that the sheet being conveyed is OHT.

  OHT can be detected in the same manner as plain paper. Since OHT is stiff, it is only necessary to control the motor drive speed in the same way as for cardboard.

<Fifth embodiment>
The sheet passing state from the cassette cassette feeding is as shown in FIG. The leading edge of the sheet passes between the two paths S1 and S2 depending on the sheet characteristics (waist strength). Depending on the path that has passed, the time required to reach the nip between the photosensitive drum 1a and the transfer roller Tp1 after the leading edge of the sheet is detected differs, and variations in the leading edge margin occur.

  The strength of the waist of the sheet varies depending on the thickness of the sheet. The greater the thickness, the stronger the sheet, and the closer the conveyance path is to the path S1. On the other hand, the smaller the thickness, the weaker the sheet, and the conveyance path becomes closer to the path S2.

  A media sensor 21 is arranged in the sheet conveyance path. As shown in detail in FIG. 10, the upper and lower guides 22 and 23 are sandwiched between the media sensors 21 so as to sandwich the sheets to be conveyed, as shown in detail in FIG. Has been placed.

  FIG. 10 shows a schematic diagram of means for changing the configuration of the media sensor 21 and the upper and lower guides 22 and 23 and the laser irradiation timing according to the thickness of the sheet.

  The media sensor 21 is a sensor that detects the thickness of the sheet before the sheet is conveyed to the transfer region, and detects the thickness of the sheet during an image forming operation. The result is transmitted as a signal to the CPU 31 which is a control means. The CPU 131 irradiates the laser 133 by sending a control signal to the laser driver 132 that is an oscillation source of the laser 133 at a timing according to the thickness of the sheet.

  FIG. 11 is a flowchart showing a procedure for changing the laser irradiation timing according to the thickness of the sheet. When the leading edge of the sheet reaches the media sensor 21, the paper thickness is detected (S141). When the sheet is thick paper (S142), the laser is replaced at a timing such that the leading edge margin becomes a desired value (S143). On the other hand, when the sheet is thin paper (S142), the laser beam is emitted at an earlier timing than when the sheet is thick (S143).

In the present embodiment, the laser irradiation timing of a sheet having a thickness of 0.13 mm or more in which the thickness of the sheet having a basis weight of 105 g / m ² is 0.13 mm and the laser replacement timing is controlled is the thickness. The leading edge margin is delayed by 1 mm from the laser irradiation timing on a sheet of 0.13 mm or less.

  By detecting the thickness of the sheet, it is possible to suppress variations in the leading edge margin due to the time required to reach the nip between the photosensitive drum 1a and the transfer roller Tp1 after the leading edge of the sheet is detected being different depending on the thickness of the sheet. A marginal variation of 1 mm was obtained.

<Sixth Embodiment>
In the present embodiment, the electrostatic latent image writing timing can be optimally changed with respect to the sheet in which the characteristic (thickness) of the sheet is detected without reducing the throughput.

  The thickness of the sheet is detected by the media sensor 21, that is, the distance from when the leading edge of the sheet passes the media sensor 21 until the leading edge of the sheet reaches the transfer position Tp1 is a laser to the photosensitive drum 1a that is an image carrier. The irradiation position Lp1 is longer than the distance until it reaches the transfer position Tp1. On the other hand, the sheet conveyance speed is substantially equal to the rotation speed of the photosensitive drum 1a, which is an image carrier.

  Therefore, the time until the laser irradiation position Lp1 to the photosensitive drum 1a reaches the transfer position Tp1 is longer than the time from when the thickness of the sheet is detected by the media sensor 21 until the sheet reaches the transfer position Tp1. In this case, even if the sheet thickness is detected for all the conveyed sheets, the throughput is not lowered.

<Seventh embodiment>
The time from when the sheet characteristic (thickness) is detected by the media sensor 21 until the sheet reaches the transfer position Tp1 is until the laser irradiation position 1b to the photosensitive drum 1a as the image carrier reaches the transfer position Tp1. If the time is short, if the sheet thickness is detected for all the conveyed sheets, the throughput decreases.

  Therefore, in the present embodiment, it is possible to prevent a decrease in throughput from the second printing operation as follows.

  The thickness information of the first sheet of the image forming operation is stored in the ROM 134 as a storage unit via the CPU 131 as shown in FIG. 12, and the thickness of the sheet may be changed once the thickness information is acquired. Only in that case, sheet thickness information is detected again, and laser irradiation is performed at a timing based on the thickness information stored in the ROM 134 for the second and subsequent sheets thereafter, and electrostatic latent image formation is started.

  FIG. 13 is a flowchart showing a procedure for changing the laser irradiation timing according to the thickness of the sheet without reducing the throughput. If the leading edge of the sheet reaches the media sensor 21 and the media is the first sheet, the paper thickness is detected (S181). When the sheet is a thick cardboard sheet (S182), when the sheet is a thick cardboard sheet, laser irradiation is performed at a timing such that the leading edge margin becomes a desired value (S184). On the other hand, when the sheet is thin paper (S183), the laser beam is emitted at an earlier timing than when the sheet is thick (S185).

  If the leading edge of the sheet reaches the media sensor 21 and the media is the second or later, the paper thickness is not detected, and the laser irradiation timing remains unchanged.

<Eighth Embodiment>
In the present embodiment, a light transmissive sheet (OHT) is detected, and the electrostatic latent image writing timing can be optimally changed with respect to OHT.

  The OHT can be detected in the same manner as in the fourth embodiment, and the OHT can also be detected in the same way as with plain paper. Since OHT is strong, laser irradiation timing may be controlled in the same manner as for cardboard.

2 is a cross-sectional view illustrating a structure of a paper feed unit of the image forming apparatus according to the first exemplary embodiment of the present invention. FIG. 1 is a cross-sectional view illustrating a structure of an image forming apparatus according to a first embodiment. It is a block diagram which shows the 1st Embodiment of this invention. It is a flowchart which shows the procedure which changes a sheet conveyance speed with the thickness of a sheet | seat. It is explanatory drawing for demonstrating the method to detect the thickness of a sheet | seat. It is a figure which shows the example according to a media sensor. It is a block diagram which shows the 3rd Embodiment of this invention. It is a flowchart which shows the procedure which changes a sheet conveyance speed with the thickness of a sheet | seat. FIG. 10 is a cross-sectional view illustrating a structure of a paper feed unit of an image forming apparatus according to a fifth embodiment of the present invention. It is a block diagram which shows the 5th Embodiment of this invention. It is a flowchart which shows the procedure which changes a laser irradiation timing with the thickness of a sheet | seat. It is a block diagram which shows the 7th Embodiment of this invention. It is a flowchart which shows the procedure which changes a laser irradiation timing with the thickness of a sheet | seat. FIG. 6 is a cross-sectional view illustrating a conventional example of a paper feeding unit of an image forming apparatus.

Explanation of symbols

1a, 1b, 1c, 1d Photosensitive drums 2a, 2b, 2c, 2d Charging means 3a, 3b, 3c, 3d Exposure means 4a, 4b, 4c, 4d Developing means 5a, 5b, 5c, 5d Transfer means 6a, 6b, 6c , 6d Cleaning means 7a, 7b, 7c, 7d Process cartridge 8 Feeding part 8d Registration roller 9 Conveying means 9a Conveying belt 10 Fixing part 11, 12 Paper discharging roller 13 Paper discharging part 14 Paper guide 15 Carrying guide (for cassette paper feeding)
16 Transport guide (for multi-feed)
17 Transport guide (for double-sided transport)
21 Media sensor 8a1 Paper feed roller (for cassette paper feed)
8c1 paper feed roller (for multi paper feed)
8c3 separation pad (for multi-feed)
31, 131 CPU
32 Motor controller 33 Motor 34, 134 ROM
101 Registration shutter 132 Laser driver 133 Laser

Claims (10)

An image forming apparatus that transfers and fixes visible images obtained by developing latent images on a plurality of image carriers onto a sheet conveyed by a conveying belt, and a storage unit for storing the sheet; A registration roller that conveys a sheet to the conveyance belt at a predetermined timing, an intermediate conveyance roller that conveys a sheet from the storage unit to the registration roller, a sheet that is abutted against the registration roller, and the intermediate conveyance roller An image forming apparatus having a relaxing means for relaxing the sheet between
Sheet thickness detecting means for detecting the thickness of the sheet;
When the sheet conveying speed by the intermediate conveying roller is V1, the sheet conveying speed by the registration roller is V2, and the sheet conveying speed by the conveying belt is V3, the relationship between V1, V2, and V3 is V3 ≧ V1. > V2 and a sheet that is controlled by changing the sheet conveyance speed by the intermediate conveyance roller and the registration roller to a predetermined sheet conveyance speed according to information on the thickness of the sheet detected by the sheet detection unit Conveying speed control means;
An image forming apparatus comprising:   2. The time until the latent image formation start positions on the plurality of image carriers reach the transfer position according to claim 1, wherein the sheet is moved to the transfer position after the thickness of the sheet is detected by the sheet detecting means. An image forming apparatus characterized by being shorter than a time required to reach the image forming apparatus.   The image forming apparatus according to claim 1, further comprising a storage unit that stores a thickness of the sheet detected by the sheet detection unit.   2. The image forming apparatus according to claim 1, further comprising means for detecting the presence / absence of a sheet, wherein the type of the sheet is detected based on information obtained from both the sheet detection means and the sheet presence / absence detection means. An image forming apparatus that transfers and fixes visible images obtained by developing latent images on a plurality of image carriers onto a sheet conveyed by a conveying belt, and a storage unit for storing the sheet; A registration roller that conveys a sheet to the conveyance belt at a predetermined timing, an intermediate conveyance roller that conveys a sheet from the storage unit to the registration roller, a sheet that is abutted against the registration roller, and the intermediate conveyance roller An image forming apparatus having a relaxing means for relaxing the sheet between
Sheet thickness detecting means for detecting the thickness of the sheet;
An image forming apparatus, comprising: a timing changing unit that changes a latent image formation start timing on the image carrier in accordance with sheet thickness information detected by the sheet thickness detecting unit.   6. The time until the latent image formation start position on the image carrier reaches the transfer position according to claim 5, the time from when the thickness of the sheet is detected by the sheet detection means until the sheet reaches the transfer position. An image forming apparatus characterized by being shorter than time.   6. The image forming apparatus according to claim 5, further comprising a storage unit that stores the thickness of the sheet detected by the sheet detection unit.   6. The image forming apparatus according to claim 5, further comprising: a sheet presence / absence detection unit configured to detect the presence / absence of a sheet, wherein the type of the sheet is detected based on information obtained from both the sheet detection unit and the sheet presence / absence detection unit. . An image forming apparatus for transferring and fixing a visible image obtained by developing a latent image on an image carrier onto a sheet conveyed by a conveyance belt,
Storage means for storing the sheet;
A registration roller that conveys the sheet to the conveyance belt at a predetermined timing;
An intermediate conveying roller for conveying the sheet from the storage means to the registration roller;
Control means for controlling the speed of the registration roller and the intermediate conveyance roller so as to form a loop on the sheet between the intermediate conveyance roller and the registration roller;
Detecting means for detecting the characteristics of the sheet,
The image forming apparatus, wherein the control unit changes the speeds of the registration roller and the intermediate conveyance roller in accordance with information detected by the detection unit. A recording material conveyance control method of an image forming apparatus for transferring and fixing a visible image obtained by developing a latent image on an image carrier onto a sheet conveyed by a conveyance belt,
By controlling the speed of the registration roller that conveys the sheet to the conveyance belt at a predetermined timing and the intermediate conveyance roller that conveys the sheet from the sheet storage means to the registration roller, the gap between the registration roller and the intermediate conveyance roller is controlled. A loop forming step of forming a loop on the sheet,
And a speed changing step of changing speeds of the registration rollers and the intermediate transport rollers in accordance with information relating to the characteristics of the sheet in the loop forming step.

Images