JPH0627837A - Multicolor image forming device - Google Patents

Abstract

PURPOSE:To keep image transfer performance for the high insulation recording sheet of an OHP sheet, a carton, etc., excellent, without setting a voltage applied to the transfer means of each monochromatic image forming unit, extremely high. CONSTITUTION:In a type for successively transferring each toner image on the recording sheet 5 electrostatically attracted to a carrying belt 3, by each monochromatic image forming unit 1, a sheet attracting means 4 is composed of an electrostatic charging means 6 electrostatically charging the carrying belt 3, and a mode selecting means 7 selecting a standard recording mode for setting the output characteristic of the electrostatic charging means 6 in standard specification, with respect to the recording sheet 5 having normal insulation, and a special recording mode for inverting the output polarity of the electrostatic charging means, or lowering an output level, with respect to the high insulation recording sheet 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multicolor image forming apparatus such as a copying machine or a printer for forming a full color image,
In particular, the present invention relates to an improvement of a multicolor image forming apparatus that has an individual monochromatic image forming unit for each color component of a full-color image, and superimposes the color component images formed by the monochromatic image forming units.

[0002]

2. Description of the Related Art Conventionally, this type of multicolor image forming apparatus is of a so-called tandem type. For example, a toner image of each color system of yellow, magenta, cyan, and black is formed by an electrophotographic method to form an image. Four sets of monochromatic image forming units that electrostatically transfer the toner image to the recording sheet side by a transfer device arranged at the transfer site, and sheets that sequentially convey the recording sheet toward the image transfer site of each monochromatic image forming unit. The transport belt and a fixing device that fixes each toner image to the recording sheet when the toner images of the respective color systems are transferred to the recording sheet transported by the sheet transport belt are multiply formed.

In this type of multicolor image forming apparatus, a dielectric film is usually used as the sheet conveying belt, and the sheet conveying belt is charged by a predetermined charger,
Although the recording sheet is electrostatically attracted to the sheet conveying belt, in such a type, since the charge amount of the sheet conveying belt is sequentially increased when the transfer operation of each toner image is repeated, In order to maintain a good image transfer operation, it is necessary to increase the voltage applied to the transfer device of each single-color image forming unit in sequence every time the transfer of each toner image is repeated, so that the transfer electric field for each toner image is surely applied. It becomes necessary (see JP-A-62-191863).

[0004]

In a multicolor image forming apparatus of this type, due to the variety of uses, a transparent sheet for overhead projectors (hereinafter Called OHP sheet)
As shown in FIG. 5, the transfer of each single color image forming unit is higher than that of a normal recording sheet for a recording sheet having a high insulation property such as an OHP sheet or a cardboard. The voltage applied to the container becomes acceleratingly increased each time the transfer of each toner image is repeated. In particular, in the case of an OHP sheet, a voltage exceeding 10 KV is required as a voltage applied to the transfer device of the final-stage monochromatic image forming unit, which inevitably increases the size and cost of the high-voltage power supply. In addition, in order to prevent abnormal discharge, there is a technical problem that the transfer device itself becomes large and expensive.

The present invention has been made in order to solve the above-mentioned technical problems, and can be applied to an OHP sheet, a cardboard, etc. without setting an extremely high voltage applied to the transfer means of each monochromatic image forming unit. The present invention provides a multicolor image forming apparatus capable of maintaining good image transfer performance for a recording sheet having high insulation properties.

[0006]

That is, the present invention is
As shown in FIG. 1, a single-color image forming unit 1 for forming a single-color image corresponding to each color component of a full-color image by electrophotography and transferring it to the recording sheet 5 side at an image transfer site.
(For example, 1a to 1d) and each monochromatic image forming unit 1
(1a to 1d) has a conveyor belt 3 made of a dielectric material that sequentially conveys the recording sheet 5 along a path passing through the image transfer portion, and the sheet suction means 4 keeps the recording sheet 5 on the conveyor belt 3. Assuming a multicolor image forming apparatus provided with a sheet conveying means 2 for electro-adsorption, a charging means 6 for charging the conveying belt 3 and an output characteristic of the charging means 6 for a recording sheet 5 having a normal insulating property. Is set to a standard specification (the output polarity of the charging means 6 is the same as the charge polarity of the conveyor belt 3 by transfer, and the output level is such that the recording sheet 5 can be adsorbed). For the insulating recording sheet 5, the sheet suction means 4 is constituted by the mode selection means 7 in which the special recording mode for reversing the output polarity of the charging means or decreasing the output level is selected. It is characterized in.

[0007] In such technical means, as the charging means 6, if the output characteristics can be changed at least in accordance with the selected mode of the mode selecting means 7, a charging device such as a corotron may be appropriately selected. You can In this case, in order to keep the chargeability of the recording sheet 5 good, the recording sheet 5 may be provided with a grounded conductive roll at a portion facing the charger disposed on one side of the recording sheet 5. It is preferable to ensure that the charged electric charges of 1 spread over the entire area of the sheet.

Further, the mode selecting method of the mode selecting means 7 may be an artificial selection by an operator or the like, or whether the recording sheet 5 is an OHP sheet or a thick paper or other than that. The sheet type detecting means may be used for detection, and the mode may be automatically selected in accordance with the detection signal. Regarding the timing of mode selection by the mode selection means 7, the mode may be switched and selected immediately according to the recording sheet 5 to be used. For example, the charging means may be applied to a portion of the OHP sheet or the like excluding the leading end. It is preferable to switch the output polarity and the output level according to No. 6 because the situation in which the OHP sheet or the like is wrapped around the photosensitive drum or the like can be effectively avoided.

[0009]

According to the above-mentioned technical means, if the recording sheet 5 has a normal insulating property, the mode selecting means 7 selects the standard recording mode, and the output characteristic of the charging means 6 becomes the standard. Set to specifications. Then, the recording sheet 5 is electrostatically adsorbed to the conveyor belt 3 under a normal charging condition, and each monochromatic image forming unit 1 sequentially transfers the toner image of each color to the recording sheet 5 at the image transfer portion. If the recording sheet 5 has a high insulation property, the mode selecting means 7 selects the special recording mode and inverts the output polarity of the charging means 6 or lowers the output level. Then, although the highly insulating recording sheet 5 is different from the normal charging condition, it is surely electrostatically adsorbed on the conveyor belt 3, so that each single-color image forming unit 1 has a toner image of each color at the image transfer portion. Are sequentially transferred to the recording sheet 5. However, in this special recording mode, the initial charging polarity of the recording sheet 5 when the carrier belt 3 is adsorbed is opposite to the transfer charge, or the initial charging amount is smaller than that in the standard recording mode. Even if the voltage is not set so high, the transfer operation of each toner image can be maintained well.
When the special recording mode is applied to the recording sheet 5 having a normal insulating property, the first monochromatic image forming unit 1a is affected by the weakness of the recording sheet 5 and the charging state of the recording sheet 5. When the recording sheet 5 passes through the image transfer portion, the recording sheet 5 becomes the monochromatic image forming unit 1a.
The phenomenon of being wrapped around the photosensitive drum etc. occurred frequently.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the accompanying drawings. Example 1 FIG. 2 shows a so-called tandem type color printer to which the present invention is applied. In the figure, reference numerals 20a to 20d (the whole is indicated by 20) are yellow (Y), magenta (M), cyan (C), and black (K) toner images formed by an electrophotographic process. Each of the monochrome image forming units 20 is a photosensitive drum 21.
A charging corotron 22 for pre-charging the surface of the photosensitive drum 21, a semiconductor laser 23 for writing an electrostatic latent image on the charged photosensitive drum 21, and toners of each color system of yellow, magenta, cyan and black are stored. Developing device 24
And a cleaner 25 for removing the residual toner on the photosensitive drum 21.

Further, reference numeral 30 is a sheet conveying system for conveying the recording sheet 39, which is arranged in parallel with a driving roll 31 which is rotationally driven by a driving motor (not shown) and which is not shown in the drawing. Steering roll 32 whose alignment is adjusted by an adjusting mechanism
And an endless conveyor belt 37 which is stretched between the rolls 31 and 32 and electrostatically holds and conveys the recording sheet 39. In this embodiment, an appropriate number of driven rolls (not shown) are arranged between the drive roll 31 and the steering roll 32 to stably convey the conveyor belt 37.

Further, a transfer corotron 26 is provided on the back side of the conveyor belt 37 corresponding to each monochromatic image forming unit 20.
(Specifically 26a to 26d) are arranged, and high voltage power supply 27 (specifically 27a to 27d) is provided to each corotron wire.
A voltage having a predetermined positive polarity is applied from c), and the transfer corotron 26 electrostatically transfers the toner image formed on the photosensitive drum 21 to the recording sheet 39 on the conveyor belt 37. Furthermore, the recording sheet 39 on the conveyor belt 37 is separated from the conveyor belt 37 at a corner portion corresponding to the steering roll 32, and the separated recording sheet 39 is discharged to the discharge tray 40 via the fixing device 28. Has become.

The portion of the conveyor belt 37 where the recording sheet 39 from the sheet feeding cassette 38 is tacked is the sheet suction device 5 to which the present invention is applied.
0 is provided. In this embodiment, the sheet suction device 50 includes, as shown in FIG. 2, a suction corotron 51 disposed near the back side of the transport belt 37 and a transport belt 37 facing the suction corotron 51. A mode in which the charging condition of the adsorbing corotron 51 is switched and selected according to a mode instruction according to the type of the recording sheet 39 and a conductive tacking roll 52 which is arranged in pressure contact with the front side and moves following the conveyor belt 37 and is grounded. And a selection circuit 53.

FIG. 3 shows a specific example of the mode selection circuit 53. In the figure, a mode selection circuit 53 includes, for example, a mode selection switch 531 that is turned on by an operator when a highly insulating recording sheet 39 such as an OHP sheet or a thick paper is used (hereinafter referred to as OHP mode), and a suction selection switch 531, respectively. A relay 532 having a normally-closed contact 533 and a normally-open contact 534 connected to the corotron wire 511 of the corotron 51, and an excitation coil 535 for opening and closing the contacts 533 and 534, and the mode selection switch 531 ON An excitation power supply 536 for energizing the excitation coil 535 during operation, a positive adsorption power supply 537 connected in series to the normally closed contact 533, and a negative adsorption power supply 538 connected in series to the normally open contact 534. It consists of and.
In this embodiment, the operator manually switches the mode selection switch 531 to select the OHP mode and the normal mode. However, the present invention is not limited to this, and a predetermined sheet type detection is performed. A device (not shown) may detect the thickness and transparency of the recording sheet 39, and the mode selection switch 531 may be automatically switched and selected based on the detection signal.

Next, the operation of the color printer according to this embodiment will be described. Now, when the recording sheet 39 having a normal insulating property is used, the operator may leave the mode selection switch 531 off, and the normal mode is executed in this state. In this normal mode, since the normally closed contact 533 of the sheet suction device 50 is closed, the voltage applied from the positive power source 537 for adsorption of the positive polarity is the corotron wire 511 of the adsorption corotron 51.
Applied to. In this state, the recording sheet 39 from the sheet feeding cassette 38 is electrostatically adsorbed on the conveyor belt 37 by the adsorption corotron 51 that performs positive discharge and the conductive tacking roll 52, and yellow (Y) to black. (K) single-color image forming units 20a-2
The toner images (negatively charged in this embodiment) of the single color image forming units 20a to 20d are transferred by the positive discharge of the transfer corotrons 26a to 26d of the single color image forming units 20a to 20d. Receive.
Then, the recording sheet 39 carrying the toner image passes through the fixing device 28, and the toner images of respective colors are fixed on the recording sheet 39. The voltage applied to the transfer corotron in the transfer process of each color toner image in such an image forming process changes as indicated by the mark ● in FIG. 5, but is set to about 8 KV at the maximum.

On the other hand, when a highly insulating recording sheet 39 such as an OHP sheet is used, the operator may turn on the mode selection switch 531.
In this state, the OHP mode is executed. In the OHP mode, when the mode selection switch 531 is turned on, the excitation coil 535 of the relay 532 is energized to open the normally closed contact 533 and close the normally open contact 534. The voltage applied from the adsorption power source 538 is the corotron wire 51 of the adsorption corotron 51.
1 is applied. In this state, the recording sheet 39 from the sheet feeding cassette 38 is electrostatically adsorbed on the conveyor belt 37 by the adsorption corotron 51 that performs negative discharge and the conductive tacking roll 52, and yellow (Y) to black. (K) single color image forming units 20a to
The single color image forming units 20a to 20d are conveyed to the image transfer unit 20d and positively discharged from the transfer corotrons 26a to 26d of the single color image forming units 20a to 20d.
Of each color toner image (negatively charged in this embodiment). Then, the recording sheet 39 carrying the toner image passes through the fixing device 28, and the toner images of respective colors are fixed on the recording sheet 39. The voltage applied to the transfer corotron in the transfer process of each color toner image in such an image forming process changes as shown by a triangle mark in FIG. 5, but is within about 9 KV at maximum. In order to confirm the effectiveness of this OHP mode, when the normal mode was applied to the OHP sheet, the voltage applied to the transfer corotron in the transfer process of each color toner image changed as shown by a circle in FIG. It was confirmed that up to 12KV would be required.

When the above-mentioned OHP mode is applied to a normal recording sheet 39, the recording sheet 39 often wraps around the photosensitive drum 21 in the single-color image forming unit 20a immediately after the sheet suction device 50. However, it was confirmed that it is difficult to apply the OHP mode to the normal recording sheet 39.

In the OHP mode described above, a so-called transfer drum system (a recording sheet is electrostatically adsorbed on a transfer drum around which a dielectric film is wound, and the transfer drum is rotated a plurality of times, is formed on a photosensitive drum. A method of sequentially transferring the toner images of respective colors to the recording sheet), for example, in order to securely electrostatically attract the recording sheet to a transfer drum having a diameter of 200 mm, 50 gf / cm
An adsorption force of ² or more is required, which corresponds to an effective current of the adsorption corotron 51 of 10 μA or more. However, in the OHP mode, the recording sheet 39 is charged in the opposite polarity in advance, but since substantially the same charges cancel each other out during the transfer process, the attracting force on the recording sheet 39 after transfer becomes extremely small. There is a phenomenon that the recording sheet is peeled off from the transfer drum, leading to a sheet jam accident.
It was confirmed that it is difficult to apply the OHP mode to the transfer drum system in terms of sheet holding property.

Embodiment 2 FIG. 4 shows another embodiment of the color printer to which the present invention is applied. In the figure, in the sheet suction device 50, unlike the first embodiment, the suction power supply 54 for the suction corotron 51 is variably set according to the use mode (normal mode, OHP mode). That is, OH
In the P mode, the adsorption corotron 51 uses the adsorption power supply 5 so as to reduce the discharge current with the same polarity as in the normal mode.
4 is to reduce the applied voltage. According to this embodiment, the charge amount of the recording sheet 39 by the adsorption corotron 51 is reduced in the OHP mode. The toner is securely electrostatically adsorbed on the belt 37, and thereafter, the toner images of the respective colors are transferred at the image transfer portions of the single color image forming units 20, and are discharged through the fixing device 28. The voltage applied to the transfer corotron in the transfer process of each color toner image in such an image forming process changes as shown by the mark □ in FIG. 5, but is within about 10 KV at maximum. Further, when this OHP mode is applied to a normal recording sheet 39, an unfavorable phenomenon is observed as in the case of the first embodiment.
It has been found difficult to use P-mode. In the normal mode, the operation is similar to that of the first embodiment.

[Embodiment 3] FIG. 6 shows the details of a sheet suction device in another embodiment of the color printer to which the present invention is applied. In the figure, the basic structure of the sheet suction device 50 is substantially the same as that of the first embodiment, but unlike the first embodiment, the conveyance belt 37 advances only a few m (about several mm) from the nip area of the tacking roll 52. Sensor 5 for detecting the leading edge of recording sheet 39 on the side
21 is provided, and the sensor-on switch 522 is closed when the leading edge of the recording sheet 39 is detected by the sensor 521.
Is connected in series with the mode selection switch 531.

Therefore, according to this embodiment, for example, OH
Even if the P mode is selected, the charging condition of the suction corotron 51 becomes the normal mode until the leading edge of the recording sheet 39 slightly passes the nip area of the tacking roll 52, and therefore, as shown in FIG. The leading end portion (for the size of m) of the sheet 39 is charged under the charging condition of the normal mode, and thereafter, is charged with the opposite polarity in the OHP mode. At this time,
The recording sheet 3 is positively charged by the transfer corotron 26 at the image transfer portion of each single-color image forming unit 20.
Since the electrostatic attraction force of the leading end portion of the sheet 9 to the conveyor belt 37 is inevitably stronger than the other portions, the leading end portion of the recording sheet 39 is reliably positioned and held on the conveyor belt 37, and the recording sheet 39. Is effectively prevented from being unnecessarily wound around the photosensitive drum 21 of each single-color image forming unit 20.

Further, in this embodiment, as shown in FIG. 8, the tacking roll 52 has a drive motor 523.
The driving speed v _ROLL of the tacking roll 52 is set to be slightly higher than the moving speed v _BELT of the conveyor belt 37. Therefore, in this embodiment, even if the recording sheet 39 contains water and has undulations, the tacking roll 52 forcibly spreads the recording sheet 39 and securely sucks and holds it on the conveyor belt 37. Therefore, even for a recording sheet 39 such as non-uniform water-containing paper, the recording sheet 39 and the conveyor belt 3
Adhesion with No. 7 is maintained good, and the phenomenon of partial toner image transfer omission is eliminated.

[0023]

As described above, according to the present invention, in the type in which the toner images of the respective colors formed by the respective monochromatic image forming units are sequentially superposed on the recording sheets electrostatically attracted to the conveyor belt, the sheet attracting means. By changing the adsorbing conditions of the sheet adsorbing means for a highly insulating recording sheet such as an OHP sheet or a cardboard, an accelerated increase in the voltage applied to each image transferring means can be suppressed. It is possible to maintain good image transfer performance for a highly insulating recording sheet such as an OHP sheet while effectively avoiding an increase in the size of the high-voltage power supply and its high-voltage power supply and an increase in cost.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of a multicolor image forming apparatus according to the present invention.

FIG. 2 is an explanatory diagram showing a first embodiment of a color printer to which the invention is applied.

FIG. 3 is an explanatory diagram showing details of the sheet suction device according to the first embodiment.

FIG. 4 is an explanatory diagram showing details of a sheet suction device according to a second embodiment.

FIG. 5 is a graph showing an image transfer operation in Examples 1 and 2.

FIG. 6 is an explanatory diagram showing details of a sheet suction device according to a third embodiment.

FIG. 7 is an explanatory diagram illustrating a charged state of a recording sheet in a sheet suction device according to a third exemplary embodiment.

FIG. 8 is an explanatory diagram showing the operation of the tacking roll according to the third embodiment.

[Explanation of symbols]

1 (1a to 1d) ... Monochromatic image forming unit, 2 ... Sheet conveying means, 3 ... Conveying belt, 4 ... Sheet adsorbing means, 5 ...
Recording sheet, 6 ... Charging means, 7 ... Mode selecting means

Claims (1)

[Claims] 1. A single-color image forming unit (1: 1a to 1d) for forming a single-color image corresponding to each color component of a full-color image by electrophotography and transferring it to the recording sheet (5) side at an image transfer site. , Each monochrome image forming unit (1:
1a to 1d) has a conveyor belt (3) made of a dielectric material to which a recording sheet (5) is sequentially conveyed along a path passing through the image transfer portion, and the sheet suction means (4) conveys the conveyor belt (3). ) Is provided with a sheet conveying means (2) for electrostatically adsorbing the recording sheet (5), the sheet adsorbing means (4) is a charging means (6) for charging the conveying belt (3). ) And a standard recording mode for setting the output characteristics of the charging means (6) to standard specifications for the recording sheet (5) having a normal insulating property, while the high insulating recording sheet (5) is selected. On the other hand, the multicolor image forming apparatus is characterized by comprising a mode selection means (7) for selecting a special recording mode for reversing the output polarity of the charging means or decreasing the output level.