JP2006153933A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which a switching timing of a transfer bias is accurately controlled to timings when a leading end and a trailing end of a recording medium pass a transfer part. <P>SOLUTION: The image forming apparatus includes: a means for forming a toner image on an image carrier; a transfer means; and a transfer bias control means, and has a rimless image formation mode of causing a front end of the toner image formed on the image carrier to reach the transfer part earlier than that of a recording medium or causing a rear end of the toner image formed on the image carrier to reach the transfer part later than that of a recording medium. A bias condition of the bias applied to the transfer means is switched during a period from the arrival of the leading end of the toner image formed on the image carrier at the transfer part to the arrival of the leading end of the recording medium at the transfer part and during a period from the arrival of the trailing end of the recording medium at the transfer part to the arrival of the trailing end of the toner image formed on the image carrier at the transfer part. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as an electrophotographic laser beam printer (LBP).

  In recent years, with the widespread use of electrophotographic color image forming apparatuses, there has been an increasing demand for speeding up color output in addition to the demand for recording quality of color images. In order to meet this demand, several proposals have been made for image forming methods. Among them, Y (yellow), M (magenta), C (cyan), and Bk (black) toners called tandem types are proposed. There is a color image forming system in which an image is formed on a drum-shaped image carrier, the transfer material conveyed by a transfer conveyance belt as a transfer material carrier is sequentially transferred by each transfer means, and finally fixed. .

  Also, in an ink jet type or bubble jet type printer, so-called “marginless printing” is widely used in which there is no margin at the periphery of the recording medium and an image is formed on the front surface of the recording medium. Also in the electrophotographic laser beam printer (LBP), for such borderless printing, for example, Patent Document 1 proposes a borderless image forming apparatus that forms a toner image with a size protruding from the edge of the recording medium. ing.

  A conventional tandem type color image forming apparatus is described in Patent Document 2, for example.

  In an in-line image forming apparatus, if the machine dimensions deviate from the design value due to assembly errors, part tolerances, thermal expansion of parts, etc. during device production, the main scanning position deviation, sub-scanning position deviation, etc. The resist misalignment will occur.

  In a scanning optical system using a polygon scanner, a main scanning magnification shift is likely to occur due to the positional relationship between the OPC drum and the scanner. In a fixed optical element such as an LED, the exposure beam emitted from the exposure element forms an image on the OPC while having a certain extent from each light emitting point, but the main scanning overall magnification rarely varies greatly. On the other hand, in the polygon scanner which is a scanning optical system, since the exposure beam is scanned radially from the scanner, if the distance relationship between the scanner and the OPC drum has changed, the image magnification in the main scanning direction is the color station. It will be significantly different for each.

  Even if the laser writing position from the BD is made constant for each station, the writing position is likely to change for each color for the same reason, and a positional deviation in the main scanning direction occurs.

  Regarding the main items of registration error, sub-scanning position error, main-scanning position error, and main-scanning magnification, a resist patch is formed on the belt, and two optical sensors are arranged in the main-scanning direction so as to be distributed left and right. , And fine adjustment of the main scanning and sub-scanning writing positions and the image clock for each station makes it possible to perform registration alignment with excellent accuracy and reproducibility.

  By the way, when a toner image is formed with a size that protrudes from the edge of the recording medium and a borderless image is printed, the toner image that protrudes from the edge of the recording medium remains on the transfer conveyance belt 54, and the next image Therefore, it is necessary to remove the toner remaining on the transfer conveyance belt.

  Therefore, in the image forming apparatus capable of printing a borderless image, the toner remaining on the transfer / conveying belt is increased as compared with a normal image forming apparatus. Therefore, a large-capacity container is required to store the toner, and the apparatus Has led to an increase in size.

  Therefore, it is necessary to reduce the amount of toner remaining on the transfer conveyance belt as much as possible.

  Patent Document 3 discloses a method of removing toner protruding from a recording medium by applying a transfer bias having a polarity opposite to normal between paper sheets when the margin is set small.

  An image forming apparatus capable of printing a borderless image also employs such a configuration so that the toner image protruding from the recording medium is not transferred to the transfer conveyance belt 54.

JP 2004-045457 A JP 09-288396 A JP 2002-207373 A

  In the case of forming a borderless image, it is necessary to perform good image transfer at the leading end and the trailing end of the recording medium. For this purpose, a regular transfer bias may be applied at a timing sufficiently earlier than the leading end of the recording medium reaches the transfer portion. Further, after a sufficient time after the rear end portion of the recording medium passes through the transfer portion, it may be switched to a transfer bias of reverse polarity.

  However, when such a configuration is adopted, the toner image of the portion protruding from the recording medium is transferred to the transfer / conveyance belt, and the container for storing the toner becomes large as described above.

  Therefore, in order to achieve both good image transfer at the front end and rear end of the recording medium and prevention of enlargement of the container for containing the removed toner, the transfer bias switching timing and the front end of the recording medium It is desirable that the timing at which the portion and the rear end portion pass through the transfer portion coincide with each other with high accuracy.

  However, since the position of the transfer portion has a deviation due to various tolerances, when the transfer bias switching timing is controlled based on a predetermined timing, the transfer bias switching timing, the leading end portion and the trailing end portion of the recording medium are There is a problem in that the timing of passing through the transfer section is shifted, and unnecessary toner is transferred to the transfer / conveying belt, or a transfer failure occurs at the leading edge and the trailing edge of the recording medium. It was.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of accurately controlling the transfer bias switching timing to the timing at which the leading end portion and the trailing end portion of the recording medium pass through the transfer portion. It is in.

In order to achieve the above object, the present invention comprises means for forming a toner image on an image carrier,
Transfer means for transferring the toner image formed on the image carrier to the recording medium at a transfer portion where the recording medium and the image carrier are in contact;
Transfer bias control means for controlling a bias condition applied to the transfer means at a predetermined timing,
The leading edge of the toner image formed on the image carrier reaches the transfer portion earlier than the leading edge of the recording medium, or the trailing edge of the toner image formed on the image carrier is later than the trailing edge of the recording medium. In an image forming apparatus having a borderless image forming mode that reaches the transfer portion,
The bias condition applied to the transfer means is a period from when the leading end of the toner image formed on the image carrier reaches the transfer unit until the leading end of the recording medium reaches the transfer unit, and Switching is performed between the time when the rear end portion of the recording medium reaches the transfer portion and the time when the rear end portion of the toner image formed on the image carrier reaches the transfer portion.

  According to the present invention, the transfer bias switching timing is controlled based on the result of color registration detection, so that the transfer bias switching timing and the leading edge and the trailing edge of the recording medium due to misalignment of the transfer portion due to various tolerances are controlled. It is possible to correct the deviation of the timing when the end passes through the transfer section, which causes problems such as unnecessary toner being transferred to the transfer conveyance belt and transfer defects at the leading and trailing ends of the recording medium. It is possible to perform good borderless printing without occurrence.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

<Embodiment 1>
FIG. 1 is a longitudinal sectional explanatory view showing the overall configuration of a four-color full-color laser beam printer which is an aspect of a color image forming apparatus according to Embodiment 1 of the present invention.
{Overall configuration of image forming apparatus}
First, the overall configuration of the color image forming apparatus will be described with reference to FIG.

  A color image forming apparatus A shown in FIG. 1 includes photosensitive drums 1 (1a, 1b, 1c, 1d) as four image carriers arranged side by side in the horizontal direction. The photosensitive drum 1 is rotationally driven in a clockwise direction in FIG. 1 by a driving unit (not shown). Around the photosensitive drum 1, the charging device 2 (2a, 2b, 2c, 2d) for uniformly charging the surface of the photosensitive drum 1 in order according to the rotation direction, and a laser beam is irradiated on the photosensitive drum 1 based on image information. A scanner unit 3 (3a, 3b, 3c, 3d) for forming an electrostatic latent image, a developing device 4 (4a, 4b, 4c, 4d) for developing the toner image by attaching toner to the electrostatic latent image, and photosensitive A transfer member 5 (5a, 5b, 5c, 5d) composed of a transfer blade for transferring a toner image on the drum 1 onto a transfer material, and a cleaning device 6 (removing post-transfer toner remaining on the surface of the photosensitive drum 1 after transfer). 6a, 6b, 6c, 6d) and the like are arranged.

  Here, the photosensitive drum 1, the charging device 2, the developing device 4, and the cleaning device 6 for removing the toner are integrally formed into a cartridge to form a process cartridge 7 (7 a, 7 b, 7 c, 7 d), and a color image forming apparatus A is removable.

  Next, the configuration of each unit will be described sequentially.

  The photosensitive drum 1 is configured by applying an organic optical conductor layer (OPC photosensitive member) to an outer peripheral surface of an aluminum cylinder having a diameter of 30 mm, for example. Both ends of the photosensitive drum 1 are rotatably supported by support members, and a driving force from a driving motor (not shown) is transmitted to one end of the photosensitive drum 1 so that the photosensitive drum 1 is rotated clockwise in the figure. The

  Each charging device 2 is a conductive roller formed in the shape of a roller. The roller is brought into contact with the surface of the photosensitive drum 1, and a charging bias voltage is applied to the roller by a power source (not shown). One surface is charged uniformly.

  The scanner unit 3 is composed of an LED array with a short focus imaging lens (not shown) attached to the tip thereof, and the lighting of the LED is controlled according to an image signal by a drive circuit (not shown), which selectively selects the surface of the charged photosensitive drum 1. To form an electrostatic latent image.

  Each of the developing devices 4 (4a, 4b, 4c, 4d) is composed of a developer that stores toners of cyan, magenta, yellow, and black in order from the upstream side in the transfer material conveyance direction (right side in FIG. 1). The When developing the electrostatic latent image on the photosensitive drum 1, the toner in the toner container 4e of the corresponding developing device is fed to the coating roller 4f by the feeding mechanism, and a thin layer of toner is applied to the outer periphery of the rotating developing roller 4g. The toner is charged (frictionally charged). By applying a developing bias between the developing roller 4g and the photosensitive drum 1 on which the electrostatic latent image is formed, toner is attached to the electrostatic latent image and developed as a toner image.

  The cleaning device 6 removes so-called transfer residual toner remaining on the surface of the photosensitive drum 1 without being transferred after the toner developed on the photosensitive drum 1 by the developing device 4 is transferred to the transfer material S.

  A transfer conveyance belt 54, which is an endless belt member that circulates and moves in contact with the four photosensitive drums 1a, 1b, 1c, and 1d, is disposed. The upstream side of the transfer conveyance belt 54 is supported by a driven roller 9 as a first support member, and the downstream side is supported by a drive roller 10 as a second support member, and further moderate tension (usually 10 gf / unit width per unit width). mm). The suction roller 12 is in contact with the transfer conveyance belt 54 on the upstream side in the transfer conveyance direction (the right side in FIG. 1). The transfer conveyance belt 54 serves as a transfer material carrier. When the transfer material is conveyed, a bias voltage is applied to the suction roller 12 to form an electric field with the grounded driven roller 9. In addition, dielectric polarization can be caused between the transfer conveyance belt 54 and the transfer material to generate an electrostatic attracting force therebetween. The driven roller is composed of a metal roller or a low resistance rubber roller.

  As another form of the present embodiment, the transfer / conveying belt 54 may be stretched by only two of the driving roller 10 and the driven roller 9, or by adding one and three.

The transfer conveyance belt 54 is composed of an endless film-like member having a thickness of about 100 to 150 μm and having a volume resistivity of 10 ¹⁰ to 10 ¹⁴ Ωcm. The volume resistivity is a value obtained by applying 100 V at a temperature of 23.5 ° C. and a relative humidity of 60% with a high resistance meter R8340 manufactured by ADVANTEST, using a measurement probe in accordance with JIS-K6911. It is.

  In addition, the resistance values of the transfer members 5 (5a, 5b, 5c, 5d) constituted by the transfer rollers were substantially the same.

  In order to form an image by the image forming apparatus, the transfer material S stored in the cassette 13 mounted at the lower part of the apparatus main body is separated and fed one by one by the feeding roller 14 and the transfer and conveying belt 54 by the conveying roller pair 15. 1 and transported from the upstream side (the right side in FIG. 1) to the downstream side (the left side in FIG. 1), and sequentially transfers the toner images of yellow, magenta, cyan, and black from the respective photosensitive drums 1 in the meantime. Thus, a color image is formed on the transfer material. The transfer material is heat-fixed by passing through a fixing device 16 composed of a pair of heating rollers 16a and a pressure roller 16b, and discharged to a discharge portion 18 at the top of the device by a discharge roller pair 17.

  Next, “marginless” printing and “marginal” printing will be described.

  FIG. 2A illustrates “margined” printing, and FIG. 2B illustrates “marginless” printing. When printing with “border”, the toner image is all contained in the recording medium, and there are peripheral margins of the upper margin (mh), lower margin (mb), left margin (ml), and right margin (mr) around the recording medium. To do. During “edgeless” printing, the toner image reaches the edge of the recording medium, and there is no peripheral margin. FIG. 2B shows a state in which there are no upper margin, lower margin, left margin, and right margin. If one or more margins are zero, “no border” printing is performed.

  In FIG. 3, the toner image formation at the time of “marginless” printing will be described.

  When the user selects borderless printing for the image file on the host computer, the controller resizes the image so that borderless printing is performed for the selected recording medium size.

  FIG. 3A shows the resized toner image size that is actually formed. The vertical axis is Iv and the horizontal axis is Ih. FIG. 3B shows the size of the recording paper, where Pv is vertical and Ph is horizontal. The size relationship between the toner image and the recording medium is set so that Pv <Iv and Ph <Ih. Accordingly, a frame-like toner image as shown in FIG. 3D is left on the transfer conveyance belt as a transfer residue.

  This transfer residual toner is scraped off by a cleaning member and stored in a waste toner container.

  Next, transfer bias control during borderless printing will be described.

  FIG. 4A shows the relationship among the printing range, paper size, and transfer bias application range during borderless printing in the conventional embodiment.

  In the case of forming a borderless image, it is necessary to perform good image transfer at the leading end and the trailing end of the recording medium. For this purpose, a regular transfer bias may be applied at a timing sufficiently earlier than the leading end of the recording medium reaches the transfer portion. Further, after a sufficient time after the rear end portion of the recording medium passes through the transfer portion, it may be switched to a transfer bias of reverse polarity.

  However, when such a configuration is adopted, the toner image of the portion that protrudes from the recording medium is transferred to the transfer conveyance belt 54, and the container for storing the toner has become large as described above. .

  Therefore, during borderless printing, by switching the transfer bias from the reverse polarity to the normal transfer bias before the leading edge of the formed toner image and the leading edge of the paper reach the transfer portion, It is desirable to reduce the range in which the image printed on the portion protruding from the size is transferred to the transfer / conveying belt and reduce the contamination of the transfer / conveying belt (FIG. 4B).

  More preferably, the timing for switching from the reverse polarity transfer bias to the regular transfer bias is made coincident with the timing when the leading edge of the paper reaches the transfer portion so that the image printed on the portion protruding from the paper size is not transferred to the transfer conveyance belt. It is good to make (FIG.4 (c)).

  However, since the position of the transfer portion actually shifts due to various tolerances, it is difficult to match the timing of switching from the reverse polarity transfer bias to the normal transfer bias with the timing at which the leading edge of the paper reaches the transfer portion. The portion of the image that protruded from the paper size was transferred to the transfer / conveying belt, and the transfer / conveying belt was soiled. Further, the waste toner container for collecting the dirt has been enlarged.

  Therefore, in the present embodiment, the position of the transfer portion due to various tolerances is detected, and the transfer bias switching timing is corrected based on the difference, so that the image printed on the portion protruding from the paper size is transferred. The transfer conveyance belt is prevented from being soiled by being transferred to the conveyance belt.

  A method for detecting a transfer position deviation in the present embodiment will be described.

  In this embodiment, the transfer position deviation is detected based on the registration detection result. A resist detection method in this embodiment will be described.

  Problems with a color image forming apparatus having a plurality of image forming units include uneven movement of a plurality of photosensitive drums and conveyor belts due to mechanical accuracy, the photosensitive drum outer peripheral surface and the conveyor belt at the transfer position of each image forming unit For example, the relationship of the amount of movement is different for each color and does not match when the images are overlaid, resulting in color misregistration.

  In particular, in an apparatus having a photosensitive drum and a plurality of image forming units each having a laser scanner as an exposure unit for forming an electrostatic latent image on the photosensitive drum, there is an error in the distance between the laser scanner and the photosensitive drum in each image forming unit. If this error is different between the image forming units, a difference occurs in the scanning width of the laser on the photosensitive drum, resulting in color misregistration.

  An example of color misregistration is shown in FIG. In the figure, an original image position 21 and image positions 22 (22a, 22b, 22c, 22d) when color misregistration occurs are shown. FIGS. 5A, 5B, and 5C show a case where there is a color shift in the main scanning direction (direction perpendicular to the recording material conveyance direction). For the sake of explanation, two lines are separated in the conveyance direction. It is drawn.

  FIG. 5A shows an inclination shift of the main scanning line, which occurs when there is an inclination between the optical unit and the photosensitive drum. For example, the position is corrected in the direction of the arrow by adjusting the position of the optical unit or the photosensitive drum or the position of the lens.

  FIG. 5B shows color misregistration due to variations in the main scanning line width, which is caused by a difference in distance between the optical unit and the photosensitive drum. It tends to occur when the optical unit is a laser scanner. For example, when the image frequency is finely adjusted, that is, when the scanning width is too long, the scanning frequency is corrected by changing the length of the scanning line by increasing the frequency.

  FIG. 5C shows the writing position error in the main scanning direction. For example, if the optical unit is a laser scanner, it is corrected in the direction of the arrow by adjusting the writing start timing from the beam detection position.

  FIG. 5D shows the writing position error in the recording material conveyance direction. For example, the correction is made in the direction of the arrow by adjusting the writing start timing of each color from the detection of the leading edge of the paper.

  In order to correct the color misregistration, the image forming apparatus forms a color misregistration detection pattern for each color as an image for color misregistration detection by the toner image formed in each image forming unit on the transfer conveyance belt 54. Detected by a pair of optical sensors 23a and 23b provided on both sides of the transfer conveyance belt 54 at the downstream side in the recording material conveyance direction (near both ends in the direction orthogonal to the recording material conveyance direction) (FIG. 6). Color misregistration correction means for performing various adjustments as described above according to the misregistration amount is provided. Thereby, the image forming position is corrected.

  FIG. 7 shows an example of the color misregistration detection pattern.

  As shown in FIG. 7, the recording material conveyance direction patterns 30, 31, 32, 33 for detecting the color misregistration amount in the recording material conveyance direction (sub-scanning direction) and the color misregistration amount in the main scanning direction are detected. The main scanning direction patterns 34, 35, 36, and 37 are formed. In this example, the main scanning direction patterns 34, 35, 36, and 37 are formed with an inclination of 45 degrees with respect to the transfer conveyance belt moving direction. Patterns 30 and 34 are black (Bk), 31 and 35 are yellow (Y), 32 and 36 are magenta (M), and 33 and 37 are cyan (C) patterns. Each of the eight toner line images (a, b, c, d, e, f, g, and h) is formed as an average position. In the figure, tsf1 to 4, tmf1 to 4, tsr1 to 4 and tmr1 to 4 indicate the average detection timing of each pattern, and the arrows indicate the moving direction of the conveyor belt 3.

Here, the moving speed of the transfer conveyance belt 54 is vmm / s, black is the reference color, and the black recording material conveyance direction patterns 30, 34 and the respective color patterns 31, 32, 33 and 35, 36, 37 are used. Are assumed to be dsYmm, dsMmm, and dsCmm, respectively. Further, the measured distances between the recording material conveyance direction patterns 30f to 33f on the left and the main scanning direction patterns 34f to 37f when viewed from the upstream side in the movement direction of the conveyance belt are dmfBkmm, dmfYmm, and dmfMmm, respectively. , DmfCmm, and the measured distances between the recording material conveyance direction patterns 30r to 33r on the right side and the main scanning direction patterns 34r to 37r are dmrBkmm, dmrYmm, dmrMmm, and dmrCmm, respectively. When black is a reference color, the positional deviation amount δes of each color in the recording material conveyance direction is
δesY = v × {(tsf2−tsf1) + (tsr2−tsr1)} / 2−dsY
(1)
δesM = v × {(tsf3−tsf1) + (tsr3−tsr1)} / 2−dsM
(2)
δesC = v × {(tsf4-tsf1) + (tsr4-tsr1)} / 2-dsC
(3)
It becomes.

Further, the positional deviation amounts δemf and δemr for the respective colors on the left and right sides in the main scanning direction are:
dmfBk = v × (tmf1-tsf1) (4)
dmfY = v × (tmf2−tsf2) (5)
dmfM = v × (tmf3-tsf3) (6)
dmfC = v × (tmf4-tsf4) (7)
When,
dmrBk = v × (tmr1−tsr1) (8)
dmrY = v × (tmr2−tsr2) (9)
dmrM = v × (tmr3-tsr3) (10)
dmrC = v × (tmr4-tsr4) (11)
From
δemfY = dmfY−dmfBk (12)
δemfM = dmfM−dmfBk (13)
δemfC = dmfC−dmfBk (14)
When,
δemrY = dmrY−dmrBk (15)
δemrM = dmrM−dmrBk (16)
δemrC = dmrC−dmrBk (17)
Thus, the direction of deviation can be determined from the sign of the calculation result.

  Then, the writing position is corrected from δemf, and the main scanning width is corrected from δemr−δemf. When there is an error in the main scanning width, the writing position is calculated not only by δemf but also by taking into account the amount of change in the image frequency that has changed with the main scanning width correction.

  FIG. 8 shows a pattern detection unit (light sensor) 23 of the color misregistration detection means.

  The pattern detection unit 23 includes a light emitting element 41 that is, for example, an LED as the light emitting means, and a light receiving element 42 that is, for example, a photosensor as the light receiving means. The pattern detection unit 23 detects the color misregistration detection patterns 30 to 37 formed on the transfer conveyance belt 54. That is, the light emitting element 41 emits the light emitting light 43, and the light receiving element 42 receives the light receiving light 44 out of the reflected light from the transfer / conveying belt 54 or the color misregistration detection patterns 30 to 37. The light emitting element 41 and the light receiving element 42 are configured by a regular reflection optical system with the transfer conveyance belt 54 as a reflection surface, and a difference in specular reflection light reflectance between the transfer conveyance belt 54 and the color misregistration detection pattern, that is, reflection. The position of the color misregistration detection pattern is detected based on the difference in rate.

  FIG. 9 shows a pattern reading processing unit for processing a pattern detection signal by the color misregistration detection means.

  The pattern reading processing unit obtains the time interval of the position of each color from the detection data, the pattern detection unit 23 configured to include the light emitting element 41, the light receiving element 41, and the like, and the obtained time interval and a preset time interval A calculation unit 231 that calculates a color misregistration amount and a correction value based on the difference between the values, an image output unit 62 that performs image formation according to the calculation result, a timer 233 that performs timing adjustment and various settings of each unit, and a CPU 234. . The arithmetic unit 231, the timer 233, the CPU 234, and the like constitute means for controlling the image forming position of each color image based on the color misregistration detection pattern detection signal.

  Through the above operation, the shift amount δemf from the design value of the transfer position is obtained. From this δemf and the conveyance speed v of the transfer conveyance belt, Δt = δemf ÷ v [sec] is obtained. That is, this value is a deviation from the design value of the timing at which the front end of the paper passes the transfer portion. By correcting the transfer bias switching timing using this Δt, the transfer bias can be switched accurately at the timing when the leading edge of the paper reaches the transfer portion as shown in FIG. It was possible to prevent the protruding toner image from being transferred to the transfer / conveying belt.

  The above description has been given for the leading edge of the paper, but it goes without saying that the same applies to the trailing edge.

<Embodiment 2>
In the present embodiment, a method for accurately detecting the timing at which the paper reaches the first transfer portion will be described.

  In the present embodiment, the leading edge of the paper is detected by detecting the current flowing through the suction roller. That is, when the leading end of the paper enters the suction portion, the electrical load at the suction portion varies, and the value of the current flowing through the suction portion changes. The time when Δs = L ÷ v [sec] obtained from the distance L from the suction portion stored in advance to the first transfer portion and the conveyance speed v of the transfer conveyance belt has elapsed from the time when the current fluctuation occurred. By setting the timing for switching the transfer bias at the transfer portion, the transfer bias can be accurately switched at the leading edge of the paper even at the first transfer portion.

  It also measures and stores the interval from the time when the current value flowing through the suction section changes to the time when the current value flowing through the first transfer section changes during normal bordered image formation, and the first transfer section during borderless printing The timing at which the transfer bias is switched may be corrected.

  In the subsequent transfer section, as in the first embodiment, the transfer bias is applied at the timing corrected by the positional deviation amount of the transfer section measured by the resist detection with reference to the timing of switching the transfer bias in the first transfer section. Switch.

  By adopting the configuration as described above, the transfer bias switching timing can be accurately adjusted to the front end of the paper in all the transfer sections, and both the prevention of contamination of the transfer conveyance belt and the transfer failure at the front end of the paper are achieved. And a good borderless image could be obtained.

1 is a cross-sectional view of a color image forming apparatus according to Embodiment 1 of the present invention. It is a figure explaining borderless printing and bordered printing. It is a figure explaining the image formation at the time of borderless printing. FIG. 6 is a diagram illustrating a correlation between a print range, a paper size, and transfer bias switching. It is explanatory drawing for demonstrating various color shift. It is a schematic diagram which shows embodiment of the arrangement configuration of a pattern detection part. FIG. 5 is a diagram illustrating an example of a color misregistration detection pattern and a minimum value reading pattern. It is a schematic diagram for demonstrating the pattern detection part of a color shift detection means. It is a schematic diagram for demonstrating the circuit structure of the color shift detection pattern reading process part of a color shift detection means.

Explanation of symbols

S transfer material 1 photosensitive drum 2 charging device 3 scanner unit 4 developing device 5 transfer member 6 cleaning device 7 process cartridge 8 transfer conveyance belt 9 driven roller 10 drive roller 11 tension roller 12 suction roller 13 cassette 14 feeding roller 15 conveyance roller pair DESCRIPTION OF SYMBOLS 16 Fixing device 16a Heating roller 16b Pressure roller 17 Discharge roller pair 18 Discharge part 19 Separation roller 20 Density detection sensor 21 Light emitting element 22 Light receiving element 23 Detection position

Claims (4)

Means for forming a toner image on the image carrier;
Transfer means for transferring the toner image formed on the image carrier to the recording medium at a transfer portion where the recording medium and the image carrier are in contact;
Transfer bias control means for controlling a bias condition applied to the transfer means at a predetermined timing,
The leading edge of the toner image formed on the image carrier reaches the transfer portion earlier than the leading edge of the recording medium, or the trailing edge of the toner image formed on the image carrier is later than the trailing edge of the recording medium. In an image forming apparatus having a borderless image forming mode that reaches the transfer portion,
The bias condition applied to the transfer means is a period from when the leading end of the toner image formed on the image carrier reaches the transfer unit until the leading end of the recording medium reaches the transfer unit, and An image forming apparatus that switches between a time when a rear end portion of a recording medium reaches a transfer portion and a time that a rear end portion of a toner image formed on the image carrier reaches the transfer portion.   The image forming apparatus according to claim 1, wherein the bias condition applied to the transfer unit has at least two modes.   The image forming apparatus according to claim 2, wherein at least one of the bias conditions applied to the transfer unit in the two modes has the same polarity as the toner. At least an image carrier, charging means for charging the image carrier to a predetermined polarity, exposure means for forming an electrostatic latent image on the image carrier, and an electrostatic latent image formed on the image carrier A plurality of process devices including developing means for visualizing;
A recording medium conveying means for conveying the recording medium;
The image forming apparatus according to claim 1, further comprising: a transfer unit that transfers an image formed on the image carrier to the recording medium conveyed by the recording medium conveyance unit.
An image forming apparatus characterized in that switching of a bias condition applied to the transfer means is performed based on a result of resist detection.

Images