JP6003353B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus.

  In the commercial printing industry, small-lot, multi-variety, variable data printing, etc. is moving from a conventional offset printing machine to POD (Print On Demand) using an image forming apparatus using an electrophotographic method. In order to meet such needs, an electrophotographic image forming apparatus is required to have front and back registration accuracy comparable to an offset printing machine, image uniformity, and the like.

  The causes of misregistration in the image forming apparatus can be broadly classified into vertical and horizontal registration errors, skew errors between the recording medium and the printed image, and image length expansion and contraction during toner image transfer. Further, in an image forming apparatus having a fixing device, a front / back misregistration occurs due to an image magnification error due to expansion / contraction of a recording medium caused by heating by the fixing device.

  In view of this, the image processing apparatus includes means for changing the image magnification of the first and second sides during duplex printing, a means for detecting the size of the recording paper, and a means for calculating an image magnification correction value based on the expansion / contraction ratio of the recording paper. There has been proposed an electrophotographic color printing apparatus that forms an image on the second surface in accordance with the image magnification correction value (see, for example, Patent Document 1).

  However, in the technique of Patent Document 1, the actual image magnification correction value is compared with the expected image magnification correction value, and when the difference is equal to or larger than a predetermined value, the recording paper printed with the expected image magnification correction value is purged. In some cases, printing is interrupted and printing is interrupted.

  The present invention has been made in view of the above, and when the expansion / contraction ratio of the recording medium is out of the allowable range, it is determined whether continuous printing is possible and printing on the recording medium can be continued. An object is to provide a possible image forming apparatus.

  According to the image forming apparatus of one aspect of the present invention, an image forming unit that forms an image on a recording medium, a heating unit that heats the recording medium, and a conveyance distance or a length in the conveyance direction of the recording medium are measured. The expansion / contraction ratio calculation that calculates the expansion / contraction ratio of the recording medium based on the measurement result of the recording medium length measurement means and the recording medium length measurement means before and after the image formation on the one surface of the recording medium by the image forming means And a correction unit that corrects image data formed by the image forming unit based on the expansion / contraction rate, and the heating unit heats the recording medium when the expansion / contraction rate is equal to or greater than an upper limit of an allowable range. And a control unit that causes the recording medium length measuring unit to measure the transport distance or the length in the transport direction of the recording medium and causes the expansion / contraction rate calculation unit to recalculate the expansion / contraction rate of the recording medium.

  According to the embodiment of the present invention, when the expansion / contraction ratio of the recording medium is out of the allowable range, it is determined whether continuous printing is possible and printing on the recording medium can be continued. Can provide.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to a first embodiment. 1 is a schematic cross-sectional view illustrating the configuration of a sheet conveying device of an image forming apparatus according to a first embodiment. 3 is a schematic top view illustrating the configuration of the sheet conveying device of the image forming apparatus according to the first embodiment. FIG. 1 is a block diagram illustrating a functional configuration of an image forming apparatus according to a first embodiment. It is a figure which shows the example of an output of the start trigger sensor in 1st Embodiment, a stop trigger sensor, and a rotary encoder. FIG. 3 is a diagram illustrating a flowchart of a printing process in the image forming apparatus according to the first embodiment. FIG. 10 is a diagram illustrating a flowchart of a printing process in an image forming apparatus according to a second embodiment.

  Hereinafter, embodiments for carrying out the invention will be described with reference to the drawings. In the drawings, the same components are denoted by the same reference numerals, and redundant description may be omitted.

[First embodiment]
<Configuration of image forming apparatus>
FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus 101 according to the first embodiment.

  The image forming apparatus 101 forms an image on a sheet S as a recording medium such as paper or OHP by an image forming unit having a tandem image forming apparatus 54, an intermediate transfer belt 15, and a secondary transfer apparatus 77.

  The intermediate transfer belt 15 is provided in the vicinity of the center of the image forming apparatus 101, and is configured to be wound around a plurality of rollers and rotate clockwise in the drawing. The intermediate transfer belt 15 rotates following the roller 61 that is driven to rotate.

  The tandem image forming apparatus 54 has a plurality of developing devices 53 arranged along the conveyance direction of the intermediate transfer belt 15. An exposure device 55 is provided above the tandem image forming apparatus 54. Each developing device 53 of the tandem image forming apparatus 54 includes a photosensitive drum 71 as an image carrier that carries a toner image of each color.

  A primary transfer roller 81 is provided at a primary transfer position where the toner image is transferred from the photosensitive drum 71 to the intermediate transfer belt 15 so as to face each photosensitive drum 71 with the intermediate transfer belt 15 interposed therebetween.

  The secondary transfer device 77 is provided on the opposite side of the intermediate transfer belt 15 from the tandem image forming device 54 (on the downstream side in the transport direction of the intermediate transfer belt 15). The secondary transfer device 77 transfers the image on the intermediate transfer belt 15 to the sheet S by pressing the secondary transfer roller 14 against a roller 62 as a secondary transfer counter roller and applying a transfer electric field. The secondary transfer device 77 changes the transfer current of the secondary transfer roller 14, which is a transfer condition parameter, according to the type of the sheet S and the like. The secondary transfer roller 14 of the secondary transfer device 77 is provided so as to be able to contact with and separate from the opposing roller 62, and presses the roller 62 via the sheet S only when the toner image is transferred to the sheet S. Is provided.

  Further, the image forming apparatus 101 includes the sheet conveying apparatus 100, and can determine the conveying distance or the length in the conveying direction of the sheet S conveyed by the configuration and method described later.

  The fixing device 50 is an example of a heating unit that heats the sheet S. The fixing device 50 includes a halogen lamp 57 as a heat source, and the pressure roller 52 is pressed against the fixing belt 56 that is an endless belt. The fixing device 50 changes the temperature of the fixing belt 56 and the pressure roller 52, the nip width between the fixing belt 56 and the pressure roller 52, and the speed of the pressure roller 52, which are parameters of the fixing conditions, according to the sheet S. From the secondary transfer device 77 to the fixing device 50, the conveyance belt 41 conveys the sheet S after the image transfer.

  In the image forming apparatus 101, when image data is sent and an image formation start signal is received, a drive motor (not shown) drives the roller 61 to rotate the other plurality of rollers to rotate the intermediate transfer belt 15. Let At the same time, each developing device 53 forms a single color image on each photosensitive drum 71. Then, the single color image formed by the developing device 53 is sequentially superimposed and transferred onto the rotationally driven intermediate transfer belt 15 to form a composite color image.

  Further, the sheet S is selectively rotated by one of the sheet feeding rollers 72 of the sheet feeding table 76, fed out from one of the sheet feeding cassettes 73, conveyed by the conveying roller 74, and abutted against the registration roller 75. Stop. The registration roller 75 corrects the conveying posture of the sheet S and rotates in accordance with the timing at which the composite color image on the intermediate transfer belt 15 reaches the secondary transfer device 77 to convey the sheet S. The composite color image formed on the intermediate transfer belt 15 is transferred onto the surface of the sheet S conveyed to the secondary transfer device 77.

  The sheet S after the image transfer is transported by the transport belt 41 and sent to the fixing device 50, and heat and pressure are applied to melt and fix the transferred image. After the image is fixed on the front side, the sheet S is conveyed to the sheet reversing path 93 and the double-sided conveying path 94 by the branching claw 91 and the flip roller 92 in the case of double-sided printing, and a composite color image is formed on the back side. Is done.

  When the sheet S is reversed, the branching claw 91 guides the sheet S to the sheet reversing path 93 and reverses the sheet S from the front surface to the back surface. In this embodiment, a branch claw and a flip roller are installed as a switchback mechanism below the flip roller 92 and in the path to the purge tray 40. In addition, the sheet S can be conveyed from the reversing path 93 to the duplex conveying path 94 without being reversed. The sheet S that has passed through the fixing device 50 is conveyed to the sheet discharge roller 95 as it is, and the switchback mechanism that reverses the trailing edge of the sheet toward the sheet reversing path 93 and the switchback mechanism are respectively passed. You may convey so that the sheet | seat S may become the same surface.

  In the case of single-sided printing and no sheet reversal, the sheet S is conveyed to the paper discharge roller 95 by the branching claw 91.

  When printing on the sheet S is completed, the sheet S is conveyed to the decurler unit 96 by the paper discharge roller 95, and the decurler unit 96 changes the decurler amount according to the sheet S. The decurler amount is adjusted by changing the pressure of the decurler roller 97, and the sheet S is discharged out of the apparatus by the decurler roller 97. The purge tray 40 is disposed below the reverse paper discharge unit.

  The image forming apparatus 101 according to the present embodiment is configured to transfer the color toner image formed on the intermediate transfer belt 15 to the sheet S. However, the single color toner images formed on the plurality of photosensitive drums 71 are transferred to the sheet S. Alternatively, the image may be transferred directly on the screen. The present invention can also be applied to a monochrome image forming apparatus.

<Configuration of sheet conveying device>
2 and 3 are diagrams illustrating a schematic configuration of the sheet conveying apparatus 100 of the image forming apparatus 101 according to the first embodiment.

  The sheet conveying apparatus 100 conveys the sheet S to the secondary transfer device 77 and measures the conveying distance or the length in the conveying direction of the sheet S. The sheet conveying apparatus 100 is provided immediately upstream of a secondary transfer apparatus 77 as an image forming unit that transfers an image to the sheet S in the conveyance path of the sheet S of the image forming apparatus 101.

  The sheet conveying apparatus 100 includes a driving roller 12 that rotates by receiving a driving force (not shown) such as a driving unit (for example, a motor), and a driven roller 11 that rotates following the sheet S between the driving roller 12.

  As shown in FIG. 3, the length Wr of the driven roller 11 in the width direction orthogonal to the conveyance direction of the sheet S is configured to be smaller than the minimum width Ws of the sheet S to which the sheet conveyance device 100 corresponds. Accordingly, the driven roller 11 does not come into contact with the driving roller 12 when the sheet S is conveyed, and thus the driven roller 11 is driven to rotate only by friction generated between the driven roller 11 and the sheet S. Therefore, when the sheet S is conveyed, the driven roller 11 can measure the conveyance distance of the sheet S more accurately by a method described later without being affected by the driving roller 12.

  As shown in FIGS. 2 and 3, a rotary encoder 18 is provided on the rotating shaft of the driven roller 11 of the sheet conveying apparatus 100. A pulse counting unit (not shown) as a conveyance amount measuring unit detects a slit formed in the rotating encoder disk 18a and counts a pulse signal generated by the encoder sensor 18b, thereby being driven as a conveyance amount of the sheet S. The rotation amount of the roller 11 is measured.

  In this embodiment, the rotary encoder 18 is provided on the rotating shaft of the driven roller 11, but it can also be provided on the rotating shaft of the driving roller 12. Note that the smaller the diameter of the roller to which the rotary encoder 18 is attached, the more the number of pulses to be counted by increasing the number of rotations associated with the sheet conveyance, which is preferable because the conveyance distance of the sheet S can be measured with high accuracy. .

  In addition, the driven roller 11 or the driving roller 12 to which the rotary encoder 18 is attached is preferably composed of a metal roller in order to ensure axial deflection accuracy. By suppressing the rotation of the rotating shaft, it becomes possible to measure the conveyance distance of the sheet S described later with high accuracy.

  Sensors 3 and 4 are provided in the vicinity of the upstream side and the downstream side in the conveyance direction of the sheet S of the driven roller 11 and the driving roller 12. The sensors 3 and 4 detect the passage of the edge of the sheet S being conveyed. For the sensors 3 and 4, for example, a transmissive or reflective optical sensor with high sheet edge detection accuracy can be used. In this embodiment, a reflective optical sensor is used.

  A sensor 3 on the downstream side in the conveyance direction of the sheet S of the driven roller 11 and the driving roller 12 is a start trigger sensor 3 as a downstream detection unit that detects passage of the leading end of the sheet S. Further, the upstream side sensor 4 in the conveyance direction of the sheet S of the driven roller 11 and the driving roller 12 is a stop trigger sensor 4 as upstream side detection means for detecting passage of the rear end portion of the sheet S.

  As shown in FIG. 3, the start trigger sensor 3 and the stop trigger sensor 4 are provided with substantially the same width direction position orthogonal to the sheet S conveyance direction. By providing in this way, it is possible to minimize the influence of the conveying posture of the sheet S (skew with respect to the conveying direction) and more accurately measure the conveying distance of the sheet S.

  In the present embodiment, the two sensors 3 and 4 are arranged at the center position in the width direction orthogonal to the conveyance direction of the sheet S. However, as long as it is within the region through which the sheet S passes, It is also possible to displace them in either direction.

  The distance A shown in FIGS. 2 and 3 is the distance between the start trigger sensor 3 and the driven roller 11 and the drive roller 12 in the conveyance path of the sheet S, and the distance B is the stop trigger sensor 4 and the driven roller 11 and the drive. The distance to the roller 12. The distances A and B are preferably made as small as possible because the pulse count range described later becomes large.

  The driving roller 12 is rotated in the direction of the arrow shown in FIG. 2, and the driven roller 11 is driven and rotated by the driving roller 12 when the sheet S is not conveyed (during idling) and the sheet S is conveyed. Is driven to rotate by the sheet S. When the driven roller 11 rotates, a pulse is generated from the rotary encoder 18 provided on the rotating shaft.

  A pulse counting means (not shown) connected to the rotary encoder 18 starts pulse counting of the rotary encoder 18 when the start trigger sensor 3 detects that the sheet S has been conveyed in the direction of the arrow and the leading end has passed, When the stop trigger sensor 4 detects that the rear end of S has passed, the pulse counting is terminated.

  The sheet transport apparatus 100 has the configuration described above, and can transport the sheet S and obtain the transport distance of the sheet S or the length in the transport direction by a method described later. The image forming apparatus 101 corrects the magnification of the image formed on the back surface of the sheet S based on the expansion / contraction ratio of the sheet S obtained based on the transport distance or the length in the transport direction of the sheet S, so High image can be output.

  The sheet conveying apparatus 100 is preferably provided in the image forming apparatus 101 immediately upstream of the secondary transfer apparatus 77 and as close to the secondary transfer apparatus 77 as possible. The sheet S that has passed through the fixing device 50 at the time of image printing on the first surface contracts by being heated and pressurized, but is deformed in the recovery direction with time. For this reason, the front and back registration accuracy can be further improved by correcting the magnification of the image printed on the sheet S based on the length measurement result of the sheet S immediately before the toner image is transferred.

<Functional configuration of image forming apparatus>
FIG. 4 is a block diagram illustrating a functional configuration of the image forming apparatus 101 according to the first embodiment.

  As shown in FIG. 4, the image forming apparatus 101 includes a start trigger sensor 3, a stop trigger sensor 4, a rotary encoder 18, a control unit 20, a fixing device 50, a tandem image forming apparatus 54, and a secondary transfer device 77.

  The control unit 20 includes a CPU, for example, and includes a pulse counting unit 21, a conveyance distance calculation unit 22, an expansion / contraction rate calculation unit 23, and an image data correction unit 24. The control unit 20 is an arithmetic device that controls operations of the fixing device 50, the tandem image forming device 54, the secondary transfer device 77, and the like by reading a program and data from a storage unit such as a ROM and executing processing. .

  The pulse counting means 21 counts the pulse signal generated from the encoder sensor 18b by the rotation of the encoder disk 18a of the rotary encoder 18 provided on the driven roller 11, and rotates the driven roller 11 as the conveyance amount of the sheet S. Measure the amount.

  The conveyance distance calculation unit 22 determines the conveyance distance or conveyance of the sheet S based on the detection result of the sheet S by the start trigger sensor 3 and the stop trigger sensor 4 and the rotation amount of the driven roller 11 measured by the pulse counting unit 21. Calculate the length of the direction.

  The expansion / contraction rate calculating unit 23 calculates the expansion / contraction rate before and after surface printing of the sheet S based on the calculation result of the conveyance distance or the length in the conveyance direction by the conveyance distance calculation unit 22.

  The image data correction unit 24 corrects the image data to be printed on the sheet S based on the expansion / contraction rate of the sheet S calculated by the expansion / contraction rate calculation unit 23.

<Sheet conveyance distance calculation method>
Next, a method for calculating the transport distance of the sheet S in the image forming apparatus 101 will be described.

  FIG. 5 shows output examples of the start trigger sensor 3, the stop trigger sensor 4, and the rotary encoder 18 in the present embodiment.

  As described above, when the driven roller 11 rotates, the rotary encoder 18 provided on the rotation shaft of the driven roller 11 outputs a pulse signal.

  In the example shown in FIG. 5, after the conveyance of the sheet S is started, the stop trigger sensor 4 detects passage of the leading end of the sheet S at time t1, and the start trigger sensor 3 detects passage of the leading end of the sheet S at time t2. doing.

  Subsequently, the stop trigger sensor 4 detects passage of the rear end portion of the sheet S at time t3, and the start trigger sensor 3 detects passage of the rear end portion of the sheet S at time t4.

  At this time, after the start trigger sensor 3 detects that the leading edge of the sheet S has passed at time t2, until the stop trigger sensor 4 detects that the trailing edge of the sheet S has passed at time t3. The pulse counting means 21 performs the pulse counting of the rotary encoder 18 during the pulse counting time.

  It is assumed that the radius of the driven roller 11 provided with the rotary encoder 18 is r, the number of encoder pulses for one rotation of the driven roller 11 is N, and the number of pulses counted during the pulse count time is n. At this time, the conveyance distance L of the sheet S between the time t2 and the time t3 can be obtained by the following equation (1).

L = (n / N) × 2πr (1)
n: Number of counted pulses N: Number of encoder pulses for one rotation of the driven roller 11 [/ r]
r: radius of the driven roller 11 [mm]
In general, the sheet conveyance speed depends on the mechanical accuracy such as the external accuracy of the roller (particularly the driving roller 12) that conveys the sheet S, the core flutter accuracy, the rotation accuracy of the motor, and the accuracy of the power transmission mechanism such as the gear and belt. fluctuate. Further, since it varies due to a slip phenomenon between the driving roller 12 and the sheet S, a slack phenomenon due to a difference in sheet conveying force or sheet conveying speed between the upstream and downstream conveying means, the pulse cycle of the rotary encoder 18 The pulse width always varies, but the number of pulses does not change.

  Accordingly, the conveyance distance calculation unit 22 provided in the sheet conveyance apparatus 100 can determine the conveyance distance L of the sheet S by the driven roller 11 and the driving roller 12 as the sheet conveyance unit without depending on the sheet conveyance speed according to the equation (1). Can be obtained with high accuracy.

  Here, the expansion / contraction rate calculating unit 23 can obtain the expansion / contraction rate R from the relative ratio of the sheet conveyance distance obtained by the conveyance distance calculation unit 22 by the following equation (2).

R = [(n2 / N) × 2πr] / [(n1 / N) × 2πr] (2)
n1: Number of pulses counted during conveyance of the sheet S before thermal fixing n2: Number of pulses counted during conveyance of the sheet S after thermal fixing Here, an example calculated in the present embodiment will be described below.

In this embodiment, N = 2800 [/ r], r = 9 [mm], and conveyance of the sheet S when the number of pulses counted when the A3 size sheet is conveyed vertically is n1 = 18816. The distance L1 is
L1 = (18816/2800) × 2π × 9 = 380.00 [mm]
It becomes.

Further, when the number of pulses counted again after the thermal fixing of the sheet S is n2 = 18759, the conveyance distance L2 of the sheet S is
L2 = (18759/2800) × 2π × 9 = 378.86 [mm]
The difference between the front and back of the transport distance of the sheet S is
ΔL = 380.00−378.86 = 1.14 [mm]
From the calculation result of the conveyance distance between the front and back of the sheet S, the expansion / contraction rate calculation means 23 calculates the expansion / contraction rate R of the sheet S (relative ratio of the front and back length of the sheet S),
R = 378.86 / 380.00 = 99.70 [%]
Can be obtained as

  Therefore, in this case, since the length of the sheet S in the conveyance direction contracts by about 1 mm due to thermal fixing, if the image lengths of the front and back sides of the sheet S are the same, a front / back misregistration of about 1 mm occurs. Therefore, the image data correction unit 24 corrects the image length to be printed on the back surface of the sheet S based on the expansion / contraction ratio R, thereby improving the front / back registration accuracy.

  In the above-described example, the expansion / contraction rate calculating unit 23 obtains the expansion / contraction rate R based on the transport distances L1 and L2 of the sheet S before and after surface printing of the sheet S calculated by the transport distance calculating unit 22. For example, the ratio of the pulse numbers n1 and n2 counted during conveyance of the sheet S before and after thermal fixing may be obtained as the expansion / contraction rate R.

  For example, in the above-described example, when the number of pulses n1 = 18816 counted when the sheet S before surface printing is conveyed and the number of pulses n2 = 18759 counted when the sheet S after surface printing is conveyed, the expansion / contraction rate calculating means 23 Can obtain the expansion / contraction ratio R as follows.

R = n2 / n1 = 18759/18816 = 99.70 [%]
When the distance a between the start trigger sensor 3 and the stop trigger sensor 4 shown in FIG. 2 is added to the sheet conveyance distance L obtained by Expression (1), the length L in the conveyance direction of the sheet S is obtained.

L = (n / N) × 2πr + a (3)
a: Distance between Start Trigger Sensor 3 and Stop Trigger Sensor 4 In this way, the conveyance distance calculation means 22 of the sheet conveyance apparatus 100 is the conveyance distance L of the sheet S by the sheet conveyance means obtained by the above equation (1). Further, the length of the sheet S in the conveyance direction can be obtained by the equation (3) in which the distance a between the sensors is added.

  Further, the expansion / contraction rate calculating unit 23 can obtain the expansion / contraction rate R from the relative ratio of the length L in the conveyance direction of the sheet S before and after surface printing obtained by the conveyance distance calculation unit 22 by the following equation (4).

R = [(n2 / N) × 2πr + a] / [(n1 / N) × 2πr + a] (4)
As described above, in the sheet conveying apparatus 100, the expansion / contraction rate calculating unit 23 calculates the expansion / contraction rate R based on the conveyance distance or the length in the conveyance direction of the sheet S which is obtained with high accuracy by the conveyance distance calculating unit 22. Is possible.

  Next, an image magnification correction processing procedure based on the expansion / contraction ratio R of the sheet S measured by the expansion / contraction ratio calculation unit 23 will be described.

  The exposure device 55 of the image forming apparatus 101 includes a data buffer unit configured to buffer input image data including a memory, an image data generation unit that generates image data for image formation, and a sheet conveyance direction based on sheet size information. An image magnification correction unit that performs the image magnification correction, a clock generation unit that generates a writing clock, and a light emitting device that irradiates light to the photosensitive drum 71 to form an image.

  The data buffer unit buffers input image data sent from a host device such as a controller, for example, with a transfer clock.

  The image data generation unit generates image data based on the writing clock from the clock generation unit and the pixel insertion / extraction information from the image magnification correction unit. The drive data output from the image data generation unit controls the light emitting device on / off with the length of one cycle of the write clock as one pixel for image formation.

  The image magnification correction unit generates an image magnification switching signal for switching the image magnification based on the expansion / contraction rate calculated by the expansion / contraction rate calculating means 23.

  The clock generation means operates at a high frequency several times the write clock so as to change the clock cycle and to perform image correction such as pulse width modulation, which is a known technique, and basically has a device speed. A write clock is generated at a frequency in accordance with.

  The light emitting device includes any one or more of a semiconductor laser, a semiconductor laser array, a surface emitting laser, and the like, and forms an electrostatic latent image by irradiating light to the photosensitive drum 71 according to drive data.

  In the image forming apparatus 101, as described above, based on the expansion / contraction ratio R of the sheet S before and after the front surface printing obtained by the expansion / contraction ratio calculation unit 23, the data of the sheet S is corrected using the data whose magnification is corrected by the image data correction unit 24. Print an image on the back side. Accordingly, the image forming apparatus 101 can output an image with high accuracy of front and back registration.

<Processing when printing images>
Next, processing at the time of image printing in the image forming apparatus 101 according to the first embodiment will be described based on a flowchart illustrated in FIG.

  When performing double-sided printing on the sheet S in the image forming apparatus 101, first, in step S1, the transport distance or the length in the transport direction of the sheet S before surface printing is measured. The conveyance distance or the length in the conveyance direction is measured by the conveyance distance calculation unit 22 when the sheet S passes through the sheet conveyance apparatus 100 after being fed.

  Next, the surface of the sheet S is printed in step S2. For printing on the sheet S, a toner image is formed by the tandem image forming device 54, the toner image transferred to the intermediate transfer belt 15 is secondarily transferred to the sheet S by the secondary transfer device 77, and then the sheet S is fixed to the fixing device. 50 by heating and pressurizing.

  Subsequently, in step S3, the sheet S is reversely conveyed by the sheet reversing path 93 and the double-sided conveying path 94 after the front surface printing, and when the sheet S passes through the sheet conveying apparatus 100, the conveying distance calculating unit 22 again conveys the sheet S. Alternatively, the length in the transport direction is measured.

  In step S <b> 4, the expansion / contraction rate calculating unit 23 calculates the expansion / contraction rate R <b> 1 before and after surface printing of the sheet S based on the conveyance distance or the length in the conveyance direction of the sheet S calculated by the conveyance distance calculation unit 22.

  Next, in step S5, the expansion / contraction rate calculating means 23 determines whether or not the calculated expansion / contraction rate R1 is within the allowable range AL. If the calculated expansion / contraction rate is within the allowable range AL, the image data correction unit 24 corrects image data to be printed on the back surface of the sheet S based on the calculated expansion / contraction rate R1 in step S6. In step S7, backside printing is executed by the tandem image forming apparatus 54 or the like, and the process ends.

  Here, the allowable range AL of the expansion / contraction rate R1 can be set in advance, for example, 99.5% ≦ AL ≦ 100.5%. Note that an input unit that can set the allowable range AL of the expansion / contraction rate R1 may be provided in the image forming apparatus 101 so that the allowable range AL can be set every time printing is performed.

  If the expansion / contraction rate R1 calculated by the expansion / contraction rate calculating means 23 is outside the allowable range in step S5, whether or not the calculated expansion / contraction rate R1 is greater than or equal to the upper limit value of the allowable range AL in step S8. Determine whether.

  If the calculated expansion / contraction ratio R1 is equal to or greater than the upper limit value of the allowable range AL and the sheet S extends beyond the allowable range AL as compared to before the front surface printing, in step S9, the fixing device. 50, the sheet S is reheated. In this case, the rotation of the intermediate transfer belt 15 on which the toner image to be transferred to the back surface of the sheet S is formed is stopped, and the secondary transfer roller 14 is separated from the opposing roller 62. In this state, the sheet S is conveyed to the fixing device 50 by the sheet conveying device 100 and the conveying belt 41. The fixing device 50 as a heating unit contracts the stretched sheet S by heating and pressurizing the sheet S again. Here, the fixing device 50 may be set so that the pressure applied to the sheet S by the pressure roller 52 is lowered and only heat is applied to the sheet S. Further, in order to heat and shrink the sheet S, a heating unit different from the fixing device 50 may be provided.

  In step S10, the sheet S that has been reheated in step S9 passes through the sheet reversing path 93 and the double-sided conveying path 94 again (however, the front and back of the sheet S are not reversed) to the sheet conveying apparatus 100. Then, the conveyance distance or the length in the conveyance direction is measured.

  In step S8, if the expansion / contraction rate R1 is not equal to or greater than the upper limit value of the allowable range AL but equal to or smaller than the lower limit value of the allowable range AL, the sheet S is discharged to the purge tray 40 in step S15. Then, the next sheet S ′ is fed and conveyed. In step S16, the transport distance before the surface printing or the length in the transport direction of the newly transported sheet S ′ is measured by the sheet transport apparatus 100, and in step S17, the sheet S previously discharged to the purge tray 40 is measured. The front surface of the sheet S ′ is printed with the same data as the front surface printing data printed on the sheet. Next, in step S18, the sheet conveying apparatus 100 measures the conveying distance or the length in the conveying direction of the sheet S after the front surface printing and before the back surface printing in the sheet conveying apparatus 100.

  In step S8, when the expansion / contraction rate R1 calculated in step S4 is equal to or greater than the upper limit value of the allowable range AL, the process proceeds from step S9 and step S10. After performing the process of step S18, the expansion / contraction rate calculating means 23 calculates the expansion / contraction rate R2 in step S11.

  Next, in step S12, it is determined whether the expansion / contraction rate R2 calculated in step S11 is within the allowable range AL. If the expansion / contraction rate R2 is within the allowable range AL, the image data correction means 24 corrects the back side print image data based on the calculated expansion / contraction rate R2 in step S6, and in step S7 the back side Executes printing on and finishes the process.

  If the expansion / contraction rate R2 calculated in step S11 is outside the allowable range AL in step S12, printing is stopped in step S13, and the sheet S or sheet S ′ is placed in the purge tray 40 in step S14. Eject and finish the process.

  As described above, in the image forming apparatus 101 according to the first embodiment, even when the expansion / contraction rate before and after surface printing of the sheet S is equal to or greater than the allowable range, the expansion / contraction rate is within the allowable range. By reheating the sheet S so that the expansion / contraction rate falls within the allowable range again, it is possible to continue printing without wasting the sheet S. In addition, after the reheating, when the expansion / contraction rate is out of the allowable range again or below the lower limit value of the allowable range, the sheet S is discharged, and the sheet S is newly fed and conveyed for printing. By executing, printing can be continued. Further, the image forming apparatus 101 according to the first embodiment can perform printing with high front / back registration accuracy by correcting the image magnification to be printed on the back surface based on the expansion / contraction ratio of the sheet S.

  In addition, when the sheet S in which the expansion / contraction ratio is out of the allowable range is reheated and the expansion / contraction ratio is within the allowable range continues for a specified number of times, the second fixing condition is added to the first normal heat fixing condition. The conditions may be changed in consideration of

[Second Embodiment]
Next, a second embodiment will be described based on the drawings. Note that a description of the same components as those of the above-described embodiment will be omitted.

  FIG. 7 is a diagram illustrating a flowchart of image printing processing in the image forming apparatus 101 according to the second embodiment.

  When performing double-sided printing on the sheet S in the image forming apparatus 101 according to the second embodiment, first, in step S21, the conveyance distance or the length in the conveyance direction of the sheet S before surface printing is measured. Thereafter, in step S22, the surface of the sheet S is printed.

  Subsequently, in step S23, the sheet S is reversely conveyed by the sheet reversing path 93 and the double-sided conveying path 94, and when passing through the sheet conveying apparatus 100, the conveying distance calculating unit 22 again conveys or conveys the sheet S. Measure the length of

  In step S <b> 24, the expansion / contraction rate calculation unit 23 calculates the expansion / contraction rate before and after the front surface printing of the sheet S based on the conveyance distance or the length in the conveyance direction of the sheet S calculated by the conveyance distance calculation unit 22.

  Next, in step S25, the expansion / contraction rate calculating means 23 determines whether or not the calculated expansion / contraction rate is within an allowable range. If the calculated expansion / contraction rate is within the allowable range, in step S26, the image data correcting unit 24 corrects the image data to be printed on the back surface of the sheet S based on the calculated expansion / contraction rate, and step S27. Then, the back side printing is executed by the tandem image forming apparatus 54 and the process is terminated.

  If the expansion / contraction rate calculated by the expansion / contraction rate calculating means 23 is outside the allowable range in step S25, the sheet S is discharged to the purge tray 40 in step S28. Next, in step S29, the conveyance speed is decreased and conveyance of the next sheet S is started. By reducing the conveyance speed, the detection accuracy of the edge part of the sheet S of the start trigger sensor 3 and the stop trigger sensor 4 of the sheet conveyance device 100 is improved, and the conveyance distance calculating unit 22 conveys the sheet S or the length in the conveyance direction. Can be obtained with high accuracy.

  In step S30, the transport distance before the front surface printing or the length in the transport direction of the newly transported sheet S is measured by the sheet transport apparatus 100, and the front surface of the sheet S is printed in step S31. Next, in step S32, the sheet conveying apparatus 100 measures the conveying distance or the length in the conveying direction of the sheet S after the front surface printing and before the back surface printing in the sheet conveying apparatus 100 with the conveying speed lowered. To do. Thereafter, in step S33, the expansion / contraction rate calculating means 23 calculates the expansion / contraction rate of the sheet S that is newly conveyed.

  Next, in step S34, it is determined whether or not the expansion / contraction rate calculated in step S33 is within an allowable range. If the expansion / contraction rate is within the allowable range, the image data correcting unit 24 corrects the back side print image data based on the calculated expansion / contraction rate in step S26, and printing on the back side in step S27. To finish the process.

  If the expansion / contraction rate calculated in step S33 is outside the allowable range in step S34, printing is stopped in step S35, and the sheet S is discharged to the purge tray 40 in step S36 for processing. finish.

  As described above, in the image forming apparatus 101 according to the second embodiment, when the expansion / contraction rate before and after surface printing of the sheet S is out of the allowable range, the conveyance speed is decreased to convey the sheet S. Measure the distance or length in the transport direction. By reducing the conveyance speed of the sheet S, the conveyance distance or the length in the conveyance direction of the sheet S can be measured with high accuracy. Further, by correcting the image magnification to be printed on the back surface based on the expansion / contraction rate of the sheet S, it is possible to continue printing with high front / back registration accuracy.

  It should be noted that the first embodiment and the second embodiment can be implemented in combination. For example, in the first embodiment, when the expansion / contraction rate is equal to or lower than the lower limit value, the processing for reducing the transport speed is set as in the second embodiment. Specifically, for example, in the process shown in FIG. 6, instead of step S15, steps S28 and S29 of the process shown in FIG. 7 may be performed.

  The image forming apparatus according to the embodiment has been described above, but the present invention is not limited to the above-described embodiment, and various modifications and improvements can be made within the scope of the present invention.

3 Start trigger sensor (downstream detection means)
4 Stop trigger sensor (upstream detection means)
11 driven roller (recording medium transport means)
12 Drive roller (recording medium conveying means)
15 Intermediate transfer belt (image forming means)
20 control means 21 pulse counting means (conveyance amount measuring means)
22 Transport distance calculation means 23 Expansion rate calculation means 24 Image data correction means (correction means)
50 Fixing device (heating means)
54 Tandem image forming apparatus (image forming means)
77 Secondary transfer device (image forming means)
100 Sheet conveying device (recording medium length measuring means)
101 Image forming apparatus S sheet (recording medium)

JP 2007-79262 A

Claims (5)

Image forming means for forming an image on a recording medium;
Heating means for heating the recording medium;
A recording medium length measuring means for measuring the conveying distance or the length in the conveying direction of the recording medium;
An expansion / contraction ratio calculating means for calculating an expansion / contraction ratio of the recording medium based on a measurement result of the recording medium length measuring means before and after image formation on one surface of the recording medium by the image forming means;
Correction means for correcting image data formed by the image forming means based on the expansion / contraction rate;
If the expansion / contraction rate is equal to or greater than the upper limit of the allowable range, the recording medium length measuring unit measures the conveyance distance or the length in the conveyance direction after the heating unit heats the recording medium. Control means for causing the expansion / contraction rate calculation means to recalculate the expansion / contraction ratio of the recording medium;
An image forming apparatus comprising: The control unit discharges the recording medium and causes the image forming unit to start image formation on a subsequent recording medium when the expansion / contraction rate is equal to or lower than a lower limit value of the allowable range. The image forming apparatus according to claim 1. The recording medium length measuring means includes
Recording medium conveying means for conveying the recording medium;
A conveyance amount measuring means for measuring a conveyance amount of the recording medium by the recording medium conveyance means;
Downstream detection means for detecting the recording medium on the downstream side of the recording medium conveying means in the recording medium conveyance direction;
Upstream detection means for detecting the recording medium upstream of the recording medium conveying means in the recording medium conveying direction;
2. The conveyance distance calculating unit, and a conveyance distance calculating unit that calculates a conveyance distance of the recording medium based on detection results of the downstream detection unit and the upstream detection unit. The image forming apparatus according to 2. The transport distance calculation means includes
The recording is performed based on the transport amount measured by the transport amount measuring unit between the time when the downstream detection unit detects the recording medium and the time when the upstream detection unit detects the recording medium. The image forming apparatus according to claim 3, wherein a conveyance distance of the medium is calculated. The recording medium conveying means includes
A driving roller for rotational driving;
5. The image forming apparatus according to claim 3, further comprising a driven roller that is driven and rotated by sandwiching and conveying the recording medium with the drive roller . 6.

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