JP2004013039A - Tandem color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve such a failure that a toner formed with expensive color toner is wasted when the transfer of a paper is delayed in the midst of continuous-printing in such the constitution that it takes a time and a distance up to transfer to the paper after starting the image formation, as for tandem color copying machine and printer. <P>SOLUTION: A paper detecting means is arranged between a paper feeding position and a registration position, and in the case the paper is not carried in a prescribed time after starting the image formation, it is judged that the transfer of the paper is delayed, and the image forming operation is stopped, and also, the same image is formed again, then, the wasteful toner is reduced. Besides, if the image is formed after judging that the paper is normally transferred, the wasteful consumption of toner caused by the delay of the paper transfer is entirely eliminated. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a tandem color image forming apparatus, and more particularly to image forming timing and sheet conveyance.
[0002]
[Prior art]
In recent years, with the development of color copying machines, tandem color image forming apparatuses have been proposed to increase the copying speed of color copying. This tandem color image forming apparatus has a structure in which a plurality of image forming units are arranged along a sheet conveying direction. Each image forming unit includes a photoconductor drum, a main charging device, an exposing unit, and a developing device. The photoconductor drum is charged by the main charging device, and then exposed on the basis of an image signal to form a photoconductor drum on the photoconductor drum. This is an image forming means using a so-called electrophotography method, which forms an electrostatic latent image on the surface of the image and applies a toner of a developing device to the electrostatic latent image to form a toner image. Further, a cleaning device for the photosensitive drum is provided as needed. The tandem color image forming apparatus is disposed at a position opposite to the belt conveying unit that conveys the sheet by rotating and adsorbing the sheet on the belt surface, and facing the photosensitive drum of each image forming unit via the belt. Transfer means for printing a color image by superimposing toners of each color on paper. There is also a mechanism in which a toner image is primarily transferred onto an intermediate transfer belt instead of paper on which an image is ultimately obtained, and then transferred again to paper at a secondary transfer position.
[0003]
In the tandem color image forming apparatus employing the intermediate transfer belt, the distance from the start of the first image formation to the completion of the secondary transfer is long and takes a long time. For this reason, when performing continuous printing, the distance between sheets cannot be reduced unless image formation is started in advance and a toner image is placed on the intermediate transfer belt irrespective of whether sheet conveyance is normal. For example, as in the case of continuous printing using the tandem full-color image forming apparatus of the related art shown in FIG. 8, the Nth to N + 2nd toner images are sequentially formed at regular intervals, and the paper on which the toner images are transferred is also constant. Paper is sequentially fed at the sheet distance. In this figure, the N-th toner image is being secondary-transferred to the N-th sheet, the (N + 1) -th toner image is almost transferred to the intermediate transfer belt, and the (N + 1) -th sheet is the (N + 1) -th sheet. The toner image is being conveyed on the transfer belt at the timing of secondary transfer. The (N + 2) th toner image is being transferred onto the intermediate transfer belt, and the (N + 2) th sheet is being transported on the vertical transport path.
[0004]
[Problems to be solved by the invention]
However, in this case, the (N + 2) th sheet is larger than a predetermined sheet distance due to slippage in the sheet feeding position or the vertical conveyance path. That is, the paper conveyance is delayed. At this time, although the (N + 2) th toner image is already being formed in the image forming section, the conveyance of the sheet is not in time for the secondary transfer timing of the (N + 2) th toner image. The toner image is wasted. The purpose of the present application is to reduce or eliminate such waste.
[0005]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the first invention of the present application has a plurality of image forming means, and performs primary transfer of a toner image from the plurality of image forming means to an intermediate transfer body, thereby forming a toner image on the intermediate transfer body. In a tandem color image forming apparatus that performs color superposition of an image and secondary-transfers the toner image onto a sheet, a sheet detection sensor is provided on a conveyance path from a sheet feeding position of a sheet cassette on which sheets are stacked to a registration roller position. A tandem color image forming apparatus is provided which includes a sheet conveyance delay determining unit that determines that sheet conveyance is delayed when there is no detection timing of a sheet detection sensor within a predetermined time after the formation start timing.
[0006]
The second invention of the present application is characterized in that when the paper transport delay determining means determines that the paper transport is delayed, the paper transport delay determining means has an image formation stopping means for stopping the image formation of the page without stopping the paper transport. The tandem color image forming apparatus according to the first invention is used.
[0007]
The third invention of the present application has a re-image forming control means for starting the color image formation of the same page as the stopped page again after the timing at which the sheet conveyance is not delayed after the image formation is stopped by the image formation stopping means. A tandem color image forming apparatus according to claim 2 is used.
[0008]
According to a fourth aspect of the present invention, in the tandem color image forming apparatus, the tandem color image forming apparatus includes a normal paper conveyance determining unit for determining whether the paper is normally conveyed. A tandem color image forming apparatus according to any one of claims 1 to 3, further comprising a continuous image forming control means for starting image formation of the n-th page after it is confirmed that the image forming has been performed.
[0009]
[Action]
According to the first aspect of the present invention, in the tandem full-color image forming apparatus employing the intermediate transfer belt, the image formation is started substantially before the paper comes to the registration roller position. By detecting that the conveyance is delayed with respect to the image forming timing, the image forming operation is stopped as in the second invention of the present application, so that wasteful consumption of toner is avoided. Some kind of recovery can be performed, for example, by restarting the image formation at the timing when it is detected that the conveyance is not delayed from the image formation timing as in the invention of the fourth aspect. According to the fourth aspect of the present invention, the waste can be further reduced by starting the image formation at a timing when it is detected that the sheet conveyance is not delayed.
[0010]
【Example】
A tandem color printer employing the present invention will be described with reference to FIG. In FIG. 1, the tandem color printer 1 includes a paper feed unit 2, a vertical transport path 3, a pair of registration rollers 4, a belt transport unit 5, a first image forming unit 6, a second image forming unit 7, and a third image. It comprises a forming unit 8, a fourth image forming unit 9, an intermediate transfer unit 10, a secondary transfer unit 11, a fixing unit 12, a discharge conveyance path 13, a discharge tray 14, and the like.
[0011]
Here, the sheet feeding means 2 employs a multi-stage sheet feeding cassette, and has first to third sheet feeding cassettes (21 to 23). Taking the first paper feed cassette 21 as an example, the first paper feed cassette 21 is composed of a paper stacking means 211, a paper feed roller 212, etc., and the top of a plurality of papers stacked on the paper stacking means 211 according to a paper feed signal. Is fed one sheet at a time.
[0012]
The vertical transport path 3 is a transport path for transporting the paper fed from the paper feed cassettes (21 to 23). A paper detection sensor 31 of the present invention is provided at a predetermined position in the middle of the transport path. The position of the paper detection sensor 31 is controlled depending on the mounting position, and will be described later in detail.
[0013]
The registration roller pair 4 has a role of adjusting the leading edge of the conveyed sheet and a role of starting the sheet with respect to the secondary transfer position in synchronization with the image timing.
[0014]
The belt conveying means 5 is composed of a driving roller 51, a tension roller 52, and a belt member 53 stretched over the two rollers, and is illustrated in order to adsorb a sheet on the surface of the belt member 53. There is no belt charging means. The belt member 53 is wrapped around the drive roller 51 and the tension roller 52, and rotates at substantially the same speed as the paper transport speed while maintaining an appropriate tension. Since the material is close to the fixing means 11, in consideration of heat resistance, elasticity, strength, insulation of the inner surface of the belt, releasability of toner on the outer surface of the belt, etc., the outer peripheral surface of the endless belt made of polyimide is coated with fluorine. Applied is appropriate. Further, since the belt must also serve as the elastic layer, a material having a release layer of a fluorine-based resin such as PTFE on the surface of silicon rubber or EPDM may be used.
[0015]
The first to fourth image forming units (6 to 9) are for yellow, cyan, magenta, and black from the left side in the figure, and all have almost the same configuration. In the black image forming means 9 which consumes a large amount, the supply toner container (not shown) has a larger capacity than those of other colors. Here, the first to fourth image forming means (6 to 9) will be representatively described by the first image forming means 6 with reference to FIG.
[0016]
The first image forming unit 6 includes a first photosensitive drum 61, a first main charging device 62, a first LPH (LED PRINT HEAD) 63, a first developing device 64, a first primary transfer unit. 65, a first cleaning device 66, and a first static elimination lamp 67, which are assembled into a housing made of resin to form one unit, which is attached to the main body. The first photosensitive drum 61 uses an amorphous silicon drum, and is charged by the first main charging device 62 so that the dark potential at the developing position is approximately +450 V. In the first main charging device 62, a thin tungsten wire of 50 to 100 μm is stretched in a longitudinal direction with respect to the drum in a shield case opened toward the first photosensitive drum 61, and a high voltage of about +5 KV is applied thereto. Then, the first photosensitive drum 61 is positively charged. The first LPH 63 irradiates the charged surface of the first photosensitive drum 61 with a light beam corresponding to the image information, whereby an electrostatic latent image is formed on the surface of the first photosensitive drum 61. Although the first LPH 63 is adopted for downsizing the unit, an LSU (Laser Scanner Unit) may be used.
[0017]
Here, the first developing device 64 uses a developer composed of a positively charged toner and a carrier by stirring and mixing so as to have a predetermined concentration, and uses a supply roller 643 having a magnet member inside to supply only the toner of the developer. Is moved to the surface of the developing roller 642, and the toner layer formed on the developing roller 642 is applied to the electrostatic latent image of the portion of the first photosensitive drum 61 irradiated with the light beam, whereby the photosensitive drum 61 is This is a device that forms a toner image (a so-called reversal developing system). The supply roller 643 is composed of a rotating aluminum sleeve and an internal fixed magnet. The developer rubs the developing roller 642 with the rotation of the sleeve, and the bias potential of the supply roller 643 and the bias potential of the developing roller 642 are adjusted. Due to the electric field generated, only the toner is transferred, and a toner layer is formed on the developing roller 642. Since the dark potential (potential when not exposed) of the first photosensitive drum 61 is +450 V and the white plate potential (potential when exposed) is +50 V or less, the developing bias of the developing roller 642 is set to +350 V. I have. In this embodiment, an alternating electric field is applied in addition to the developing bias to increase the developing efficiency. This alternating electric field is preferably about 1 kV of 400 to 2000 Hz. On the other hand, +700 V is applied to the supply roller 642 as a bias potential required to transfer the toner to the developing roller 642.
[0018]
The toner image on the photosensitive drum 61 is transferred to the surface of the intermediate transfer belt 10 at a nip with the first primary transfer means 65. In this embodiment, the first primary transfer means 65 uses a transfer roller, and a high voltage having a polarity opposite to that of the first photosensitive drum 61 is applied. The primary transfer voltage is -1000 V for the first image forming unit, -1100 V for the second image forming unit, and third, as compared with the case where all of the first to fourth image forming units (6 to 9) are set to the same voltage. The setting is preferably such that the absolute value is gradually increased from -1000V to -1200V in the image forming means and -1300V in the fourth image forming means from upstream to downstream. The non-transferred toner on the first photosensitive drum 61 is scraped off by a rubber blade of a first cleaning means 66 in the next process, and a first static elimination lamp is used in order to reduce the residual potential on the surface to make it uniform. After being neutralized by 67, it is ready for the next series of processes.
[0019]
The potential setting for forming an image in this manner is performed by setting the dark potential +450 V, the developing roller bias +350 V, and the primary transfer voltage to be -1000 to -1300 V. These values are the characteristics of the photosensitive drum and the toner. The optimum value varies depending on the performance and environment. In this case, the dark potential, the developing bias potential, and the transfer potential are changed depending on whether or not to form an image. Therefore, the output from the first main charging power supply 621 is turned on / off in the first main charging device 62. And a first developing roller bias switch SW6423 for switching between the eleventh developing roller bias power supply 6421 and the twelfth developing roller bias power supply 6422 in the first developing device 64. And a first supply roller bias switching switch 6433 for switching between an eleventh supply roller bias power supply 6431 and a twelfth supply roller bias power supply 6432. A first primary transfer voltage switching SW 653 for switching between the primary transfer power supply 651 and the twelfth primary transfer power supply 652 is provided. To have. Here, each switch is provided to facilitate understanding of the invention. However, in practice, the voltage of the developing roller bias is controlled so that a plurality of voltages can be output in a stepwise manner. And a primary transfer power source that can variably control the voltage is used.
[0020]
Returning to FIG. The intermediate transfer body unit 10 includes a driving roller 101, a driven roller 102, a counter electrode roller 103 for secondary transfer, and an intermediate transfer belt 101 stretched over these rollers. The toner image primarily transferred to the surface of the intermediate transfer belt 104 is transferred to a sheet at a secondary transfer position. The intermediate transfer belt 104 is made of a polyimide endless belt whose outer peripheral surface is coated with fluorine in consideration of elasticity, strength, insulation properties of the inner surface of the belt, and releasability of toner on the outer peripheral surface of the belt. Appropriate. Further, since the belt must also serve as the elastic layer, a material having a release layer of a fluorine-based resin such as PTFE on the surface of silicon rubber or EPDM may be used. The length of the intermediate transfer belt is generally an integral multiple of the circumference of the photosensitive drums (61 to 91). Further, a belt cleaning unit (not shown) is provided downstream of the secondary transfer position and up to the primary transfer position of the first image forming unit 6.
[0021]
The secondary transfer unit 11 includes a secondary transfer roller 111, a secondary transfer power supply 1111 and a secondary transfer power supply SW1112. The secondary transfer roller 111 has a voltage of 1 to 2 kV during transfer. Is applied. The toner image on the surface of the intermediate transfer belt 104 is secondarily transferred to the sheet surface. The toner image is secondarily transferred to the sheet by an electric field generated by the secondary transfer roller 111 and the counter electrode roller 103.
[0022]
The fixing unit 12 includes a first fixing roller 121, a first fixing heater 1211, a second fixing roller 122, and a second fixing heater 1221. More specifically, the first fixing roller 121 is made of aluminum or the like having good thermal conductivity and coated with fluorine, and has a built-in first fixing heater 1211 to control the temperature to a predetermined temperature required for fixing. . The driving of the first fixing roller 121 may be driven by contact from the second fixing roller 122, but it is preferable that the first fixing roller 121 is driven by the second fixing roller 122 by meshing with a helical gear or the like in order to eliminate the possibility of slip. . The second fixing roller 122 faces and contacts the first fixing roller 121, and is driven by a driving unit (not shown). The fixing roller 122 has a base made of an aluminum roller having good heat conductivity, and its periphery is covered with an elastic layer 1222 in order to secure a fixing nip width. This elastic layer 1222 may be any heat-resistant rubber, but silicon rubber or the like is suitable. In this embodiment, in order to perform color fixing, a second fixing heater 1221 is built in and is controlled to a predetermined temperature required for fixing. Although not shown, each roller is provided with a claw member for peeling off the sheet, and a roller or web for cleaning.
[0023]
Next, the movement of a sheet when printing is performed using all normal colors will be described. The paper fed one by one from the paper feed cassette 2 is adjusted in its leading end by a pair of registration rollers 4 and delivered to the belt transporting means 5 in synchronization with the timing of the image data. While the paper is electrostatically attracted to the belt conveying means 5, the color toner images primarily transferred onto the surface of the intermediate transfer belt 104 from the image forming means (6 to 9) by the rotational movement of the belt are collectively transferred at the secondary transfer position. Then, secondary transfer is performed. The sheet to which the unfixed toner image has been transferred is fixed by being nipped and pressed by a nip formed by the heat roller 121 and the pressure roller 122 of the fixing unit 12. The sheet on which the toner image has been fixed is sequentially discharged to a discharge tray 14 by a discharge conveyance path 13.
[0024]
Normally, a full-color output is rare even for a color copier or a printer. Rather than using all image forming means as described above, an output of only black or a combination of black and red (a combination of magenta and yellow) is provided. There are many outputs by. In the case of only black, the image forming means (6 to 8) is not used, and in the case of outputting only in black and red, the second image forming means 7 (cyan) is not used. The operation of the second image forming means 7, which is not used when outputting only black and red, will be described with reference to FIGS.
[0025]
The first image forming unit 6 and the first primary transfer unit 65 primarily transfer an image of yellow toner onto the surface of the intermediate transfer belt 104, and then the second image forming unit 7 and the second primary transfer unit. No image formation or transfer is performed in the nip with the transfer unit 75, and the red color is formed by superimposing the magenta toner on the portion where the yellow toner has been transferred earlier in the nip between the third image forming unit 8 and the third transfer unit 85. Express. Further, black toner is primarily transferred at the nip between the fourth image forming unit 8 and the fourth transfer unit 85. In this way, only black and red images are formed on the intermediate transfer belt 104.
[0026]
The setting of the potential of the second image forming means 7 containing the cyan toner which does not perform the above-described image formation will be described with reference to FIG. In the second main charging device 72, since the second main charging SW 722 is OFF (at this time, no ozone is generated from the main charging device), the potential is applied to the surface of the second photosensitive drum 71. The surface potential of the second photosensitive drum 71 at the developing position is 0V. Since the second supply roller 743 of the second development device 74 has a voltage of -300 V and the development bias applied to the second development roller is -150 V, the plus toner mixed with the developer is supplied to the second supply roller 743. The toner layer is not formed on the second developing roller 742 because the toner layer is electrically drawn by the second developing roller 743. However, the oppositely charged toner (the negatively charged toner) contained in the second developing device 64 moves to the surface of the second developing roller 742 by the electric repulsion with the second supply roller. Further, the positive toner remaining on the second developing roller 742 is electrically attracted to the developing roller and does not transfer to the surface of the photosensitive drum 71. However, the above-described oppositely charged toner (minusly charged toner) is transferred to the surface of the second photosensitive drum 71 by an electric repulsion with the developing roller. As a result, the oppositely charged toner in the developing device is consumed, resulting in a good result. The bias of each of the second supply roller 743 and the second developing roller 742 is set to a value at which the positive toner does not move to the developing roller when the surface of the photosensitive drum is 0 V, and the carrier of the developer does not fly. The value is set so that the positive toner of the developing roller does not transfer to the surface of the photoconductor. In the nip between the second photosensitive drum 71 and the second primary transfer means 75, no image is placed on the second photosensitive drum 71, so that the toner is not transferred to the sheet. Since the yellow toner transferred to the paper is carried to the second photosensitive drum 71 and is contained therein, the voltage in the second primary transfer is used to attract the toner on the intermediate transfer belt 104 to the belt side. Apply voltage. Since this voltage does not transfer from the photosensitive drum to the sheet, it can play a sufficient role even if it is set lower than the normal transfer voltage, and the generation of ozone can be reduced. In this embodiment, -500 V is applied, but this value may be appropriately set according to conditions and experimental results. Further, the negatively charged toner on the second photosensitive drum 72 is cleaned by the second primary transfer means 75 as it is on the photosensitive drum due to the electric repulsive force. There is no.
[0027]
The transport delay determining means, which is a feature of the present invention, will be described with reference to FIGS. First, in FIG. 5, the Nth to (N + 2) th full-color images are sequentially printed, and the Nth toner image is being secondarily transferred to the Nth sheet without any problem. Are already being conveyed at the timing when the (N + 1) th toner image is transferred. The primary transfer of the (N + 2) th toner image on the intermediate transfer belt 10 is starting, and the (N + 2) th sheet is being transported on the vertical transport path 3. In this embodiment, the paper detecting position of the paper detecting unit 31 is attached to a position where the yellow image forming unit 6 detects the paper on which the image is printed later than the timing when the primary transfer of the image to the intermediate transfer belt is started. That is, it is determined that the conveyance of the (N + 2) th sheet of paper is delayed after a short time has elapsed since the primary transfer of the (N + 2) th toner image onto the intermediate transfer belt 10 is started. Become. Here, it is determined that the (N + 2) th sheet is delayed. In this case, as in the second aspect, when it is determined that the sheet conveyance is delayed, the image forming means 6 to 9 stop image formation and primary transfer of the (N + 2) th sheet as shown in FIG. I do. Subsequently, the printing of the (N) th and (N + 1) th sheets is continued, and the (N + 2) th toner image is collected by cleaning the image forming unit and the intermediate transfer belt. The N + 2nd sheet continues to be transported as it is with a delay and temporarily waits at the position of the registration roller 4. Further, as shown in the third invention, at the timing when the N + 2nd sheet does not have a conveyance delay with respect to the image forming start timing, each of the image forming means 6 to 9 firstly copies the (N + 2) th toner image. And printing on the (N + 2) th sheet. At the position of the sheet detecting means 31, if the image formation is restarted after the sheet conveyance is detected, the timing at which the sheet conveyance is delayed does not occur. In this case, it is detected that the sheet is delayed. However, if even the conveyance is not detected, it is determined that a paper jam has occurred, and the N + 2nd image formation is not restarted. As described above, when it is determined that the paper conveyance is delayed, the image formation is stopped at an early timing after the image formation, so that wasteful consumption of toner is reduced.
[0028]
Another embodiment adopting the fourth invention of the present application will be described with reference to FIG. The black circle 32 in the figure indicates the theoretical position where the leading edge of the paper on which this image is printed passes at the timing when the yellow image forming means 6 starts the primary transfer of the image onto the intermediate transfer belt. Here, the attachment position of the sheet detection unit 31 is set at a position where the yellow image forming unit 6 detects the sheet on which the image is printed earlier than the timing when the primary transfer of the image to the intermediate transfer belt is started. , Can be determined that the conveyance of the (N + 2) th sheet is delayed before the primary transfer of the (N + 2) th toner image onto the intermediate transfer belt 10 is started. The black circle 32 in the figure indicates the theoretical position where the leading edge of the paper on which this image is printed passes at the timing when the yellow image forming means 6 starts the primary transfer of the image onto the intermediate transfer belt. That is, if image formation is started after the sheet detection unit 31 detects a sheet, useless image formation does not have to be performed. However, in such a mounting position, if conveyance is delayed due to slippage in the vertical conveyance path 3 or the like, it is not possible to detect the conveyance. Therefore, it is preferable to provide monitoring by providing a plurality of paper detection means.
[0029]
【The invention's effect】
According to the first aspect of the present invention, in a tandem full-color image forming apparatus employing an intermediate transfer belt, image formation is started substantially before a sheet comes to a registration roller position. By providing a detection sensor and detecting that the conveyance is delayed with respect to the image forming timing, wasteful toner consumption can be reduced by stopping the image forming operation as in the second invention of the present application. Some kind of recovery can be performed, such as restarting image formation at a timing when it is detected that the conveyance is not delayed from the image formation timing as in the third invention. Further, according to the fourth aspect of the present invention, the waste can be further reduced by starting the image formation at a timing when it is detected that the sheet conveyance is not delayed.
[Brief description of the drawings]
FIG. 1 is a schematic view of a tandem color printer employing the present invention.
FIG. 2 is a schematic diagram of a first image forming unit of a tandem color printer employing the present invention.
FIG. 3 is a schematic diagram showing a case where only a black color and a red color are printed by a tandem color printer employing the present invention.
FIG. 4 is a schematic diagram of a second image forming unit which is not used when printing only black and red with a tandem color printer employing the present invention.
FIG. 5 is a schematic diagram of a tandem color printer that employs the present invention and detects a sheet conveyance delay.
FIG. 6 is a schematic diagram of a tandem color printer that employs the present invention, in which a sheet conveyance delay is detected and image formation is stopped.
FIG. 7 is a schematic diagram of a tandem color printer employing the present invention, in which a sheet conveyance delay is detected before image formation starts.
FIG. 8 is a schematic view of a conventional tandem color printer when a sheet conveyance delay occurs.
1: Full color printer
2: Paper feeding means
3: Vertical transport path
31: paper detection means
4: Registration roller pair
5; belt transport means
6; first image forming means (yellow)
7; second image forming means (cyan)
8; Third image forming means (magenta)
9: fourth image forming means (black)
10; Intermediate transfer unit
11; secondary transfer means
12; fixing means
13; discharge conveyance path
14; discharge tray

Claims (4)

A plurality of image forming units are provided, and the toner images are primarily transferred onto the intermediate transfer body from the plurality of image forming units, so that the toner images are overlaid on the intermediate transfer body. In a tandem color image forming apparatus that performs next transfer,
A paper detection sensor is provided in the conveyance path from the paper feeding position of the paper cassette loaded with paper to the registration roller position, and if there is no detection timing of the paper detection sensor within a predetermined time after image formation start timing, paper conveyance will be delayed. A tandem color image forming apparatus, comprising: a sheet conveyance delay determining unit for determining that the sheet is present. 2. The image forming apparatus according to claim 1, further comprising an image forming suspending unit that suspends image forming of the page without stopping the sheet transport when the sheet transport delay determining unit determines that the sheet transport is delayed. Tandem color image forming apparatus. After the image formation is stopped by the image formation stop means, the image forming apparatus further includes a re-image formation control means for starting color image formation of the same page as the stopped page again after a timing at which a sheet conveyance delay does not occur. 3. The tandem color image forming apparatus according to 2. The tandem color image forming apparatus has a normal paper conveyance determining unit for determining whether the paper is normally conveyed, and confirms that the conveyance of the n-th page is normally performed during the continuous printing process. 4. The tandem color image forming apparatus according to claim 1, further comprising: a continuous image forming control unit that starts forming an image of an n-th page after the image forming is performed.

Images