JP2004012583A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tandem type color image forming apparatus where a toner image is transferred to a recording sheet through an intermediate transfer body, and which realizes the high-efficiency production of a recorded image by shortening the time required for ejecting the first recording sheet when forming a monochrome image and also forms a high-quality recorded image by preventing a ghost image which does not related to image information from being formed on the first recording sheet. <P>SOLUTION: The image forming apparatus is constituted to control applied voltage to an electrifier and developing bias so that the surface potential Vh of a photoreceptor at a developing position may always exceed the developing bias Vdeve in controlling the rise of the surface potential of the photoreceptor and the rise of the developing bias in a developing device when starting image forming operation in the photoreceptor for a color in which at least a path where the toner image reaches the recording sheet is the longest. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus that forms a recorded image using an electrophotographic method, for example, a copying machine or a printer, and in particular, instead of directly transferring a toner image of each color formed on a photoreceptor to a recording sheet, an intermediate image is formed. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus for performing final transfer onto a recording sheet after superimposing on a transfer body, and more particularly, to a so-called tandem type color image forming apparatus provided with a photoreceptor for each color.
[0002]
[Prior art]
Japanese Patent Application Laid-Open No. 2001-75448 discloses a full-color laser beam printer configured to transfer four color toner images onto a recording sheet via an intermediate transfer member. This printer is a so-called tandem-type full-color printer provided with an image forming engine corresponding to each color of yellow Y, magenta M, cyan C and black K. Each image forming engine forms a toner image by an electrophotographic method. Drum, a charger, an exposure unit, and a developing unit for forming a toner image corresponding to image information of each color on the photosensitive drum. The transfer of the toner image from the photosensitive drum of each color to the recording sheet is performed via the first intermediate transfer drum and the second intermediate transfer drum. Further, a pair of first intermediate transfer drums is provided, and yellow and magenta toner images are primarily transferred and superimposed on one of the first intermediate transfer drums, and cyan and black toner images are on the other first intermediate transfer drum. Are transferred and superimposed. The respective color toner images primarily transferred onto the pair of first intermediate transfer drums are secondarily transferred onto the second intermediate transfer drum, and the four color toner images are superimposed on the second intermediate transfer drum. Has become. The four-color multiplex toner images formed on the second intermediate transfer drum in this manner are collectively transferred to a recording sheet by a final transfer roll in contact with the second intermediate transfer drum.
[0003]
Further, as such a tandem type color printer, an image forming engine of each color is arranged along an endless intermediate transfer belt, and superimposes toner images of each color on the intermediate transfer belt, and thereafter, There is also known a printer which is configured so that these toner images are finally transferred to a recording sheet in a lump.
[0004]
In such a tandem-type color printer, when forming a monochrome image instead of a color image, shorten the time until the first recording sheet is discharged, that is, the so-called first print out time (FPOT). Therefore, it can be said that it is preferable to determine the arrangement of the image forming engines for each color so that the path from the black toner image to the final transfer position is the shortest.
[0005]
On the other hand, in such a tandem-type printer, when the timing of applying the voltage of the charger and the timing of applying the developing bias in the developing device are determined for each image forming engine, the processing load related to the control of each device is quadrupled. Therefore, from the viewpoint of suppressing the production cost, it is preferable that the timing of applying these voltages is common to all the image forming engines, and the toner images of the respective colors are accurately overlapped on the intermediate transfer drum or the intermediate transfer belt. Thus, it can be said that it is preferable to change only the exposure start timing in each image forming engine.
[0006]
In this type of printer, when starting the image forming operation, a voltage is applied to the charger at the same time as or slightly after the rotation of the photosensitive drum, and a developing bias voltage is applied to the developing device. Then, it waits for the exposure unit to write the electrostatic latent image on the photosensitive drum at a predetermined writing timing. At this time, the surface potential of the photosensitive drum does not rise immediately, but gradually rises, just after the voltage application to the charger starts. In particular, when a voltage of only the DC component is applied to the charger, it takes a longer time to increase the surface potential of the photosensitive drum than in the case where the DC component is superimposed on the AC component. On the other hand, the developing bias applied between the developing device and the photosensitive drum tends to rise to a predetermined potential sooner than the rise in the surface potential of the photosensitive drum. For this reason, as shown in FIG. 4, at the beginning of the image forming operation, that is, in the preparation stage before actually starting to write the electrostatic latent image, the surface potential of the photosensitive drum reaches a predetermined potential. In some cases, the developing bias may exceed the surface potential of the photosensitive drum.
[0007]
In the so-called reversal development type image formation that is frequently used in laser beam printers and the like, toner adheres to an area where the surface potential of the photosensitive drum is lower than the development bias voltage. When the above-described potential relationship occurs, such an area (shaded area in FIG. 4) is developed by toner, and a belt-like toner image along the rotation axis direction of the photosensitive drum is formed. It is formed on the photosensitive drum. For this reason, when the application of the voltage to the charger of each color image forming engine is started at the same time and the application of the developing bias voltage to the developing unit is also started at the same time, the same image is formed on the photosensitive drum of each image forming engine. At this timing, a belt-shaped toner image is formed, and these toner images are simultaneously primary-transferred to the intermediate transfer drum or the intermediate transfer belt. Therefore, at the beginning of the image forming operation, as shown in FIG. 5, unintentionally, the toner images of the four colors are not overlapped on the intermediate transfer drum or the intermediate transfer belt and are sequentially arranged at predetermined intervals. Will be transcribed.
[0008]
Conventionally, among these belt-shaped toner images, the toner image corresponding to the first sheet of the recording sheet is drawn after the final color of the belt-shaped toner image has passed. Therefore, these belt-shaped toner images have not been a problem. Thus, there is a high demand for high-efficiency production of prints, and shortening of FPOT is a major issue. In particular, there is a great need for shortening FPOT for monochrome images. For this reason, when the four color image forming engines are arranged so that the transfer path of the black toner image is the shortest as described above, among the band-like toner images formed at the start of the image forming operation, as shown in FIG. As described above, the belt-shaped toner image of the color whose transfer path is the longest overlaps the leading edge of the first monochrome recording sheet, and an unintended ghost image of another color is transferred together on the recording sheet. There has occurred.
[0009]
SUMMARY OF THE INVENTION The present invention has been made in view of such a problem, and an object of the present invention is to provide a tandem type color image forming apparatus that transfers a toner image to a recording sheet via an intermediate transfer member when forming a monochrome image. Along with shortening the time required for discharging the first recording sheet to achieve high-efficiency production of a recorded image, preventing the occurrence of a ghost image unrelated to image information on the first recording sheet, Accordingly, it is an object of the present invention to provide a color image forming apparatus capable of forming a high-quality recorded image.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, a color image forming apparatus according to the present invention includes a plurality of image forming engines that form toner images corresponding to image information, and an intermediate unit in which toner images formed by the respective image forming engines are multiplexed and transferred. A transfer body, and a final transfer unit for finally transferring the multiple toner image on the intermediate transfer body to a recording sheet,
Each imaging engine is configured to rotate a photoreceptor that rotates at a predetermined process speed, a contact-type charger that uniformly charges the surface of the photoreceptor, and a uniformly charged surface of the photoreceptor based on image information. In a color image forming apparatus including an exposure unit that forms an electrostatic latent image by exposing, and a developing unit that develops the electrostatic latent image with toner,
At least in the image forming engine in which the path until the toner image reaches the recording sheet is the longest, the control of the rise of the photosensitive member surface potential at the start of the image forming operation and the rise of the developing bias in the developing device is performed at the developing position. The applied voltage and the developing bias of the contact-type charger are controlled so that the surface potential Vh of the photoconductor always exceeds the developing bias Vdev.
[0011]
According to such a technical means, the actual surface potential of the photoconductor and the developing bias do not reverse at the start of the image forming operation. The toner image is not formed on the upper surface, and the writing timing of the toner image to be transferred to the first recording sheet can be advanced without worrying about the existence of the belt-shaped toner image.
[0012]
As described above, in order to specifically realize control such that the surface potential Vh of the photoconductor at the development position always exceeds the development bias Vdev at the start of the image forming operation, the surface potential of the photoconductor is quickly raised. It is necessary to delay the rise of the developing bias.As an example of the former, the voltage applied to the contact-type charger, which is responsible for charging the photoconductor, is temporarily reduced only at the beginning of the image forming operation. It is conceivable to make the voltage higher than the steady state. Examples of the latter method include a method in which the application timing of the developing bias is delayed by a predetermined time from the application timing of the voltage to the charger, or a method in which the developing bias itself is gradually increased to a steady state voltage.
[0013]
As a specific means for quickly raising the surface potential of the photoconductor, an auxiliary charger is provided upstream of the charger in the rotation direction of the photoconductor, and this auxiliary charger is provided only at the beginning of the image forming operation. Alternatively, a voltage may be applied.
[0014]
On the other hand, the image forming engine of this type of image forming apparatus is provided with a cleaning unit for cleaning the photoreceptor upstream of the charger, and the cleaning unit includes the cleaning unit for efficiently collecting toner. A cleaning bias is applied. Therefore, the cleaning unit may be configured to be used as an auxiliary charger. Specifically, only at the beginning of the image forming operation, a bias voltage having the same polarity as the surface potential of the photoconductor is applied to the cleaning means, and the bias voltage is set to be equal to or higher than the discharge start voltage for the photoconductor. In this case, the cleaning means charges the photosensitive drum, and the charging by the charger overlaps with the photosensitive drum, so that the surface potential of the photosensitive member can quickly rise.
[0015]
When the surface potential of the photoreceptor is Vh and the developing bias is Vdev, the difference between the two is preferably controlled within the range of 50V ≦ | Vh−Vdev | ≦ 150V. If the difference is too small, fogging occurs in the developed toner image and the image quality is degraded. On the other hand, if the difference is too large, the carrier in the developer adheres to the photoreceptor, so-called trouble of carrier development occurs. It is.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a color image forming apparatus of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 shows a schematic configuration of a full-color laser beam printer to which the present invention is applied. In addition, the arrow in FIG. 1 has shown the rotation direction of each rotating member.
[0017]
This full-color printer is a so-called tandem type printer having an image forming engine 1 for each color of yellow (Y), magenta (M), black (K), and cyan (C). A toner image corresponding to the image information is formed on the photosensitive drum 10 at a predetermined timing. Further, the toner image formed on each photosensitive drum 10 is primarily transferred to the first intermediate transfer drum 2 and then secondarily transferred from the first intermediate transfer drum 2 to the second intermediate transfer drum 3, and finally The final transfer from the second intermediate transfer drum 3 to the recording sheet P is performed by a final transfer roll 4 in contact with the second intermediate transfer drum 3.
[0018]
The first intermediate transfer drum 2 is provided with a total of two drums, one for each two-color image forming engine, and one yellow and magenta image forming engine 1Y for one first intermediate transfer drum 2a. , 1M, the toner image is transferred to the other first intermediate transfer drum 2a from the black and cyan image forming engines 1K, 1C, and is transferred to the other first intermediate transfer drum 2a, 2b. The color toner images are superimposed. The second intermediate transfer drum 3 is in contact with both the pair of first intermediate transfer drums 2a and 2b, and after the yellow and magenta toner images are transferred from one of the first intermediate transfer drums 2a, The black and cyan toner images are transferred from the other first intermediate transfer drum 2 b, and the four color toner images overlap on the second intermediate transfer drum 3. Further, the recording sheet P to which the toner image has been transferred is discharged to a discharge tray (not shown) via the fixing device 5.
[0019]
Therefore, in the printer of this embodiment, the toner image of each color reaches the final transfer position from the image forming engine to the recording sheet P in the rotation direction of the first intermediate transfer drums 2a and 2b and the second intermediate transfer drum 3. The process distance until the process is performed is longer in the order of black, cyan, yellow, and magenta. To superpose these four color toner images on the second intermediate transfer drum 3, the toner in the magenta image forming engine 1M The image writing timing is the earliest.
[0020]
Each imaging engine 1 includes a photosensitive drum 10, a charging roll 11 for charging the photosensitive drum 10 to a predetermined background potential, and a laser for exposing the photosensitive drum 10 with a light beam Bm modulated with image information. An optical unit (not shown), a developing unit 12 for developing the electrostatic latent image formed on the photosensitive drum 10 by exposure with toner, and unnecessary toner from the surface of the photosensitive drum 10 after the primary transfer of the toner image The first intermediate transfer drums 2 a and 2 b are in contact with the photosensitive drum 10 on the downstream side of the developing device 12. Accordingly, with the rotation of the photosensitive drum 10, the respective steps of charging, exposure, development, and transfer are continuously performed on the surface of the photosensitive drum 10, and a toner image corresponding to image information of each color is formed. The primary transfer is performed on one intermediate transfer drum 2a, 2b.
[0021]
Of the constituent members described above, the photosensitive drum 10, the charging roll 11, and the temporary toner holding member 13 for each color are integrated as a single drum unit 6, and a pair of first intermediate transfer drums 2a, 2b and the second intermediate transfer drum 3 are integrated as a single image transfer unit 7. For example, when the image quality is deteriorated due to deterioration of the photosensitive drum 10 or the like, the drum unit 6 and the charging roll 11 are combined. It is designed to be replaced as it is.
[0022]
The charging roll 11 is formed by covering the periphery of a shaft core member made of a conductive metal with a conductive foamed elastic body. In order to prevent the toner from being clogged in the voids of the foamed elastic body, the charging roll 11 has such elasticity. The circumference of the body is covered with a cylindrical rubber tube having conductivity. Then, a DC voltage of about -900 V is applied to the shaft core member, whereby a discharge is generated in a wedge-shaped minute gap formed between the peripheral surface of the rubber tube and the peripheral surface of the photosensitive drum 10, and the photosensitive drum is discharged. The surface of No. 10 is uniformly charged to, for example, about -370V.
[0023]
The bias voltage applied to the charging roll 11 is preferably a DC component only, but may be a DC component superimposed on an AC component. However, the AC component has a stronger aggressiveness on the surface of the photoconductor drum 10 than the DC component, and the smoothness of the photoconductor drum surface is likely to be lost with use over time. For this reason, in order to maintain the transfer efficiency of the toner image at a high level for a long period of time, a bias voltage consisting only of a DC component is applied to the charging roll 11 to charge the photosensitive drum 10 to a predetermined background portion potential. Is preferred.
[0024]
The developing device 12 employs a so-called magnetic brush developing system. The developing devices of the respective color image forming engines 1Y, 1M, 1K, and 1C are filled with a developer composed of a toner and a carrier of a corresponding color. I have. Each developing device 12 is provided with a developing roller 14 facing the photosensitive drum 10 with a predetermined gap. The developer forms a magnetic brush on the developing roller 14 and rotates with the rotation of the developing roller 14. The photosensitive drum 10 is rubbed. Further, a developing bias voltage in which a DC component is superposed on an AC component is applied to the developing roll 14, and the photosensitive drum 10 is rubbed with a magnetic brush of a developer while applying the developing bias voltage, so The toner adheres only to the image area on the photosensitive drum 10 exposed by the beam Bm, and a toner image is formed. In this embodiment, for example, the image portion potential after exposure of the light beam Bm is set to about -75 V or less, and the developing bias voltage is set to 4 kHz for the AC component, 1.5 kVpp, and -300 V for the DC component.
[0025]
As the toner, it is preferable to use a spherical toner having a shape coefficient MLS2 of 130 or less. Since such a spherical toner has good releasability from the photosensitive drum 10, the transfer efficiency of the primary transfer of the toner image from the photosensitive drum 10 to the first intermediate transfer drums 2a and 2b can be improved. This is optimal for realizing a so-called cleanerless structure in which a cleaner is not provided for the photosensitive drum 10.
[0026]
Further, the toner temporary holding member 13 is a brush roll in which conductive sliding brushes are erected around a metal rotating shaft. In order to prevent toner from adhering to the charging roll 11, the toner temporary holding member 13 is rotated in the rotation direction of the photosensitive drum 10. It is arranged on the upstream side of the charging roll 11. The temporary toner holding member 13 rubs the surface of the photoconductor drum 10 to thereby transfer the toner image to the surface of the photoconductor drum 10 after the primary transfer of the toner image. The discharge products adhered to the photosensitive drum 10 at the time of the electrification are removed, and these substances are prevented from adhering to the surface of the charging roll 11 on the downstream side. As a result of increasing the transfer efficiency of the toner image from the photoconductor drum 10 to the first intermediate transfer drums 2a and 2b, almost no positive toner remaining on the photoconductor drum 10 is generated. Is applied with a bias voltage for positively collecting toner of the opposite polarity. In this embodiment, a DC voltage of -600 V is applied. However, even when such a bias voltage is applied, since the temporary toner holding member 13 mechanically rubs the surface of the photoconductor drum 10, the positive toner slightly remaining on the surface of the photoconductor drum 10 is also removed. The toner adheres to the temporary toner holding member 13.
[0027]
Further, since the transfer efficiency of the toner image is increased, the amount of the positive polarity toner and the reverse polarity toner adhered to the surface of the photosensitive drum 10 after the transfer of the toner image is extremely small. It does not have a mechanism for discharging these unnecessary toners and discharge products removed from the drum 10 out of the process, that is, a detoning mechanism. That is, the temporary toner holding member 13 is used only for temporarily storing unnecessary toner and the like during execution of the print job. Then, unnecessary toner and the like temporarily held are discharged onto the photosensitive drum 10 in a cleaning mode executed between print jobs, and the toner removing members 20 provided on the first intermediate transfer members 2a and 2b are used. By means of 30, it is discharged outside the process. As described above, since it is not necessary to provide each image forming engine 1 with a detoning mechanism for unnecessary toner, each image forming engine 1 is extremely compact, which contributes to downsizing of the printer as a whole.
[0028]
In FIG. 1, only the yellow imaging engine 1Y is described with reference numerals, but the imaging engines 1M, 1K, and 1C for the other colors have exactly the same configuration.
[0029]
On the other hand, the first and second intermediate transfer drums 2 and 3 are made of a metal pipe made of Fe, Al, or the like by using a low-resistance elastic layer (R = 10 ² -10 ³ Ω) and a fluororubber layer having a thickness of 3 to 100 μm as a high release layer on the surface of the low resistance elastic layer (resistance value R = 10 ⁵ -10 ⁹ Ω) and are bonded with a silane coupling agent-based adhesive (primer). The resistance value of the second intermediate transfer drum 3 needs to be set higher than that of the first intermediate transfer drum 2. Otherwise, the second intermediate transfer drum 3 charges the first intermediate transfer drum 2, making it difficult to control the surface potential of the first intermediate transfer drum 2. In this embodiment, the resistance value of the first intermediate transfer drum 2 is 10 ⁸ Ω, and the resistance value of the second intermediate transfer drum 3 is 10 ¹¹ It is set to about Ω.
[0030]
The surface potential required for electrostatically transferring the toner image from the photosensitive drum 10 onto the first intermediate transfer drum 2 is approximately +250 to 500V. The optimum surface potential varies depending on the charge state of the toner, the ambient temperature, and the humidity. However, when the charge amount of the toner is in the range of −20 to −35 μC / g and the environment is normal temperature and normal humidity, the first surface potential is the first. The surface potential of the intermediate transfer drum 2 is preferably about + 400V.
[0031]
The surface potential required for electrostatically transferring the toner image from the first intermediate transfer drum 2 onto the second intermediate transfer drum 3 is approximately +600 to 1200V. The optimum surface potential varies depending on the charged state of the toner, the ambient temperature, and the humidity as in the case of the primary transfer. Further, since what is necessary for the transfer is the potential difference between the first intermediate transfer drum 2 and the second intermediate transfer drum 3, it is necessary to set the value to a value corresponding to the surface potential of the first intermediate transfer drum 2. . As described above, when the charge amount of the toner is in the range of −20 to −35 μC / g, in a normal temperature and normal humidity environment, and the surface potential of the first intermediate transfer drum 2 is about +400 V, 2 The surface potential of the intermediate transfer drum 3 is preferably set to about +810 V, that is, the potential difference between the first intermediate transfer drum 2 and the second intermediate transfer drum 3 is set to about +410 V.
[0032]
Further, toner removing members 20 and 30 are provided for the first intermediate transfer drum 2 and the second intermediate transfer drum 3 to discharge unnecessary toner attached to these intermediate transfer drums out of the process. Have been. Each of the toner removing members 20 and 30 is rotated while being in contact with the intermediate transfer drums 2 and 3, and is formed of a metal recovery roll 21 or 31 to which a bias voltage is applied. The cleaning blades 22 and 32 are provided with metal cleaning blades 22 and 32 in contact with each other. The unnecessary toner on the intermediate transfer drums 2 and 3 is attached to the collection rolls 21 and 31 by electrostatic attraction and then collected by the cleaning blades 22 and 32. It is configured to scrape off the surfaces of the rolls 21 and 31. The scraped-off unnecessary toner is sent to a waste box by an auger (not shown). The collecting rolls 21 and 31 rotate with a peripheral speed difference of about 2% with respect to the peripheral speed of the intermediate transfer drum so that the toner is efficiently transferred from the surfaces of the intermediate transfer drums 2 and 3. ing.
[0033]
During the execution of the print job, the first toner removing member 20 provided for the first intermediate transfer drum 2 removes the opposite polarity toner from the surface of the first intermediate transfer drum 2. This is because the reverse polarity toner generated when the toner image is finally transferred to the recording sheet P and the reverse polarity toner generated when the toner image is secondarily transferred to the second intermediate transfer drum 3 reversely rise in the potential gradient. This is to prevent the toner from adhering to the photosensitive drum 10. In particular, it is important for preventing the magenta and cyan developing units 12 from mixing other color toners.
[0034]
On the other hand, the second toner removing member 30 provided for the second intermediate transfer drum 3 removes the positive toner from the surface of the second intermediate transfer drum 3 during the execution of the print job. When a full-color image is formed, toner images of four colors are formed on the second intermediate transfer drum 3 so as to be superimposed on each other, so that the surface of the second intermediate transfer drum 3 after the final transfer is compared with the first intermediate transfer drum 2 As a result, a large amount of transfer residual toner is likely to be generated. Therefore, unless the positive toner that has not been transferred to the recording sheet P is reliably removed from the second intermediate transfer drum 3, the positive toner travels around the circumference of the second intermediate drum 3, and It is transferred as a ghost image.
[0035]
On the other hand, for the first intermediate transfer drums 2a and 2b and the second intermediate transfer drum 3, the same temporary toner holding members 23 and 33 as those provided for the photosensitive drum 10 are provided, respectively. A temporary toner holding member (hereinafter, referred to as a “first cleaner”) 23 provided for the first intermediate transfer drum 2 is provided between the second transfer portion for the toner image and the first toner removing member 20. 1 The surface of the intermediate transfer drum 2 is rubbed, and during the execution of a print job, a positive voltage toner is captured by applying a bias voltage. Since the positive polarity toner is originally transferred from the first intermediate transfer drum 2 to the second intermediate transfer drum 3, the positive polarity toner captured by the first cleaner 23 is transfer residual toner in the secondary transfer. However, in the secondary transfer, only the two color toner images are transferred to the second intermediate transfer drum 3, so that the transfer residual toner is smaller than in the final transfer. Therefore, the first cleaner 23 having no detoning mechanism is configured to remove the positive polarity toner on the first intermediate transfer drum 2.
[0036]
Further, a temporary toner holding member (hereinafter, referred to as “second cleaner”) 33 provided for the second intermediate transfer drum 3 is provided between the second toner removing member 30 and the first intermediate transfer drum 2a. 2 The surface of the intermediate transfer drum 3 is rubbed, and during the execution of the print job, the positive voltage toner is caught by the application of the bias voltage, similarly to the second toner removing member 30. That is, the positive polarity toner remaining on the second intermediate transfer drum 3 after the final transfer is first removed by the second toner removing member 30 and then further removed by the second cleaner 33, and the generation of a ghost image is carefully performed. Has been prevented.
[0037]
In the thus configured full-color laser beam printer of the present embodiment, when an image processing unit (not shown) receives a print job sent from a computer or the like, the image forming engines 1 of respective colors are simultaneously activated at a predetermined timing. Then, the image forming operation (print job) starts. At this time, in each of the image forming engines 1, a predetermined voltage is applied to the charging roll 11 and the toner temporary holding member 13 at the same time when the photosensitive drum 10 starts rotating, and a developing bias is applied with a delay of 500 msec from this timing. You. However, if the image forming operation is started by applying the same voltage as during the execution of the print job, as shown in FIG. 4, when the surface potential Vh of the photosensitive drum 10 rises, the surface potential Vh A time zone occurs in which the magnitude relationship of the developing bias Vdev is reversed, an unintended belt-shaped toner image is formed on the photoconductor drum 10, and this is transferred to the first intermediate transfer drum 2 and further to the second The image is transferred to the intermediate transfer drum 3. In particular, in the printer of this embodiment, since only the DC component voltage is applied to the charging roll 11, the surface potential of the photosensitive drum 10 rises more slowly than when the AC component is superimposed. Such a belt-like toner image is easily generated in each image forming engine 1.
[0038]
The belt-shaped toner images are simultaneously formed by the image forming engines 1 of the respective colors, and simultaneously primary-transferred to the first intermediate transfer drum 2. The timing at which these toner images are secondary-transferred to the second intermediate transfer drum 3 depends on the timing. The toner image differs depending on the path followed by the toner image, and the belt-shaped toner images of each color are arranged at intervals on the second intermediate transfer drum 3 without overlapping. At this time, the band-like toner image formed by the black image forming engine 1K is located at the forefront on the second intermediate transfer drum 3, while the belt-shaped toner image is located at the rear end. This is a belt-shaped toner image formed by the magenta imaging engine 1M.
[0039]
Therefore, when forming a monochrome monochromatic image, if the toner image to be transferred to the first recording sheet P is formed too early, the corresponding area of the first recording sheet P is changed as shown in FIG. The band-shaped toner image overlaps with the magenta band-shaped toner image, and is transferred to the first recording sheet together with the monochrome image. Accordingly, as shown in FIG. 4, as long as the surface potential Vh of the photosensitive drum 10 and the developing bias Vdev are reversed at the start of the print job, and a band-shaped toner image is generated in each image forming engine 1, As shown in FIG. 3, it is necessary to delay the formation timing of the toner image to be transferred to the recording sheet P so that the magenta band-shaped toner image does not overlap the corresponding area of the first recording sheet P. However, this makes it impossible to increase the FPOT to increase the print production efficiency.
[0040]
Therefore, in the printer of this embodiment, the voltage applied to the charging roll 11 at the beginning of the print job is increased to shorten the rise time of the surface potential of the photosensitive drum 10, and the surface potential always exceeds the developing bias. Like that. More specifically, at the same time as the rotation of the photosensitive drum, a voltage of −940 V larger than that during the print job is applied to the charging roll 11, a developing bias is applied with a delay of 500 msec, and then a delay of 1200 msec from the start of rotation of the photosensitive drum 10. The voltage applied to the charging roll 11 was switched to -900V. Thus, after 1800 msec from the start of rotation of the photosensitive drum 10, the surface potential of the photosensitive drum 10 reaches -370V as the background portion potential, and at the same time, the developing bias also rises to the target -300V. As shown, the developing bias Vdev did not exceed the surface potential Vh of the photosensitive drum 10. As a result, in each image forming engine 1, there is no trouble that a band-shaped toner image having no relation to the image information is formed at the start of the print job. There was no transfer of the magenta belt-like toner image to the second recording sheet P.
[0041]
In order to prevent a belt-shaped toner image from being formed in each image forming engine 1 at the start of a print job, as shown in FIG. Since the diagram showing the rise of the developing bias Vdevet should be always positioned on the lower side, in addition to temporarily increasing the voltage applied to the charging roll 11, for example, while gradually increasing the developing bias itself, stepwise. It is also conceivable to delay the rise time until the target voltage is reached by applying the voltage. Further, it is conceivable to further delay the application start timing of the developing bias from the current 500 msec with respect to the application start timing of the voltage to the charging roll 11.
[0042]
On the other hand, instead of temporarily increasing the voltage applied to the charging roll 11, the bias voltage applied to the temporary toner holding member 13 is temporarily increased, thereby shortening the rise time of the surface potential of the photosensitive drum 10. It is also possible to convert. Specifically, at the start of the print job, the potential difference between the temporary toner holding member 13 and the photosensitive drum 10 is set to exceed the discharge start voltage, and the temporary toner holding member 13 charges the photosensitive drum 10. Set. Usually, although there are uncertain factors such as the thickness of the photosensitive layer applied to the photosensitive drum 10, when the potential difference between both members exceeds about 600 V, discharge starts. What is necessary is just to set the bias voltage of the holding member 13 to about -800 V, and to switch it to -600 V at the time of executing the print job after 1200 mmsec. This can also prevent the belt-shaped toner image from being generated in each image forming engine 1 at the start of a print job.
[0043]
【The invention's effect】
As described above, according to the color image forming apparatus of the present invention, the actual surface potential of the photoconductor and the developing bias do not reverse at the start of the image forming operation, so A belt-shaped toner image is not formed on the photosensitive drum, and the time required for discharging the first recording sheet during monochrome image formation can be shortened, and highly efficient production of a recorded image can be achieved. A ghost image unrelated to image information is prevented from being generated on the first recording sheet, and a high-quality recorded image can be formed.
[Brief description of the drawings]
FIG. 1 is a front view showing a schematic configuration of a color laser beam printer to which the present invention is applied.
FIG. 2 is a timing chart showing a relationship between a surface potential of a photoconductor and a developing bias in the present invention.
FIG. 3 is an explanatory diagram illustrating a relationship between a band-shaped toner image formed on a second intermediate transfer drum and a recording sheet at the start of a print job.
FIG. 4 is a timing chart showing a phenomenon in which the surface potential of a photoconductor and a developing bias are reversed at the start of a print job.
FIG. 5 is an explanatory diagram that points out a problem of the related art.
[Explanation of symbols]
Reference numeral 2 denotes a first intermediate transfer drum, 3 denotes a second intermediate transfer drum, 4 denotes a final transfer roll, 10 denotes a photosensitive drum, 11 denotes a charging roll, 12 denotes a developing device, 13 denotes a temporary toner holding member, and P denotes a recording sheet.

Claims (7)

A plurality of image forming engines for forming toner images corresponding to the image information, an intermediate transfer member on which the toner images formed by the respective image forming engines are multiplex-transferred, and a multiplex toner image on the intermediate transfer member on a recording sheet; And final transfer means for final transfer,
Each imaging engine is configured to rotate a photoreceptor that rotates at a predetermined process speed, a contact-type charger that uniformly charges the surface of the photoreceptor, and a uniformly charged surface of the photoreceptor based on image information. In a color image forming apparatus including an exposure unit that forms an electrostatic latent image by exposing, and a developing unit that develops the electrostatic latent image with toner,
At least in the image forming engine in which the path until the toner image reaches the recording sheet is the longest, the control of the rise of the photosensitive member surface potential at the start of the image forming operation and the rise of the developing bias in the developing device is performed at the developing position Wherein the applied voltage of the contact type charger and the developing bias are controlled so that the surface potential Vh of the photosensitive member always exceeds the developing bias Vdev. 2. The rising control according to claim 1, wherein the surface potential Vh of the photosensitive member and the developing bias Vdev at the developing position are controlled so as to always satisfy 50V ≦ | Vh−Vdev | ≦ 150V. Color image forming apparatus. 2. A color system according to claim 1, wherein the voltage applied to the contact-type charger is temporarily set to a high voltage only at the beginning of the image forming operation in performing the rise control of the photosensitive member surface potential and the developing bias. Image forming device. On the upstream side of the contact-type charger with respect to the rotation direction of the photoconductor, a cleaning unit that rubs the surface of the photoconductor and applies a bias voltage having the same polarity as the surface potential of the photoconductor is provided.
2. A color image forming apparatus according to claim 1, wherein the voltage applied to said cleaning means is temporarily set to a high voltage only at the beginning of the image forming operation in performing the rise control of the photosensitive member surface potential and the developing bias. apparatus. 5. The image forming apparatus according to claim 4, wherein the high voltage temporarily applied to the cleaning unit at the beginning of the image forming operation is equal to or higher than a discharge starting voltage for the photoconductor. 2. A color image forming apparatus according to claim 1, wherein said developing bias is set to rise stepwise to a target voltage when controlling the rise of the photosensitive member surface potential and the developing bias. 2. The color image forming apparatus according to claim 1, wherein a voltage consisting of only a DC component is applied to said contact charger.

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