JPH0534667B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD The present invention relates to an image forming apparatus that transfers an image formed on an image carrier, such as a copying machine or a printer, using electrophotography.

BACKGROUND TECHNOLOGY In general, in image forming apparatuses that apply electrophotography, an electrostatic latent image formed on an image bearing member such as a photoreceptor drum is converted into a toner image and transferred to a transfer paper guide member such as a timing roller or a guide plate. Therefore, there is a step of transferring this toner image onto the transfer paper guided and conveyed to the transfer position. And for this transcription,
A corona discharge transfer device is widely used, which applies a charge having a polarity opposite to that of the toner to the back surface of a transfer paper by corona discharge.

By the way, at high temperature and high humidity, the resistance of the transfer paper itself decreases, so the charge applied to the transfer paper moves more easily than at room temperature and humidity, and the charge for transfer generated by corona discharge is reduced. The charge flows through the transfer paper to a conductive member such as a guide plate, making it impossible to obtain the amount of charge necessary for transfer. For this reason, the transfer efficiency deteriorates and a transfer defect occurs.

By the way, as shown in Fig. 1, transfer efficiency generally depends on the amount of charge applied to the transfer paper by corona discharge, and the charge that provides the maximum transfer efficiency (indicated by transfer efficiency 1 in Fig. 1) It is understood that there is a quantity.

For this reason, in order to prevent transfer defects due to insufficient charge as described above, conventionally, a charge amount greater than the charge amount that provides the maximum transfer efficiency at room temperature and humidity is applied to the transfer paper. The idea was to compensate for the lack of electrical charge and improve transfer efficiency at high temperatures and high humidity. However, if the amount of charge generated by corona discharge is increased in this way, it is possible to improve transfer efficiency at high temperature and high humidity, but on the other hand, at room temperature and humidity,
Transfer defects started to occur at the trailing edge of the transfer paper.
Specifically, for example, when copying a solid black original, a linear white spot appeared at the trailing edge of the transfer paper.

The inventor of the present invention investigated the cause of this transfer failure and found that it was caused by the following causes.

The cause of this will be explained in detail. First, as shown in FIG. 2a, while the transfer paper P is being held between the timing roller pair 1 and passing through the transfer position T, the distance d between the surface of the transfer paper P and the surface of the photosensitive drum 2 is approximately constant. A substantially constant air layer is formed between the surface of the transfer paper P and the surface of the photoreceptor drum 2. (Needless to say, a toner layer is present in this air layer at the time of transfer.) In other words, the transfer paper P is heated at room temperature by the corona discharge of the transfer charger 5 connected to the transfer power source 4. A larger amount of charge is applied than the amount of charge that provides the maximum transfer efficiency at normal humidity, and a force acts on the transfer paper P to electrostatically attract it to the surface of the photoreceptor drum 2. However, since the transfer paper P is held between the timing roller pair 1 and is also attracted by the suction conveyance section 3, the transfer paper P is conveyed under tension, and the tension force that balances the electrostatic adsorption force is It is applied to the transfer paper P.
As a result, the distance d between the transfer paper P and the surface of the photosensitive drum 2 is kept substantially constant. Therefore, if the output of the transfer power source 4 is constant, the amount of charge applied to the transfer paper P while the transfer paper P is held by the timing roller pair 1 and passes through the transfer position T is: The charge density on the transfer paper P is maintained at a sufficient amount to obtain good transfer efficiency, and the charge density on the transfer paper P becomes almost the same. The amount of charge at this time is assumed to be σ1. (See Figure 1) On the other hand, when the trailing edge of the transfer paper P passes through the timing roller pair 1 and leaves the guide plate 6, the tension on the transfer paper P disappears, and the trailing edge of the transfer paper P moves as shown in Figure 2. As shown in b, it comes to be electrostatically attracted to the surface of the photoreceptor drum 2. FIG. 2b shows a state in which the rear end of the transfer paper P is most strongly attracted to the surface of the photoreceptor drum 2, but as shown in the figure, as the distance d becomes smaller, the transfer paper P and The air layer between the photosensitive drum 2 and the photosensitive drum 2 is reduced. As a result, the amount of charge applied to the transfer paper P increases. The amount of charge at this time is assumed to be σ2. (See Figure 1) From the above, even if the output of the transfer power source 4 is kept constant, the amount of charge σ2 applied to the rear edge of the transfer paper P is the amount of electric charge applied to parts other than the rear edge. σ1
, and the charge density on the transfer paper P becomes non-uniform. Therefore, referring to FIG. 1, the transfer paper P
The transfer efficiency at the rear end is worse than the transfer efficiency at other parts. For this reason, a phenomenon such as the above-mentioned "white spots" occurred.

Purpose of the Invention The purpose of the present invention is to eliminate the above-mentioned inconvenience, that is, the transfer failure that occurs at the trailing edge of the transfer paper. Another object of the present invention is to provide an image forming apparatus having a transfer means capable of applying a corona discharge charge such that stable transfer efficiency can be obtained over the entire surface of a transfer sheet even under high temperature and high humidity.

SUMMARY The present invention achieves the above object in an image forming apparatus according to the present invention, in which the transfer paper is restrained on the upstream and downstream sides of the transfer position to maintain a predetermined distance between the image carrier and the transfer paper. a conveyance means for conveying, a corona discharge means for applying an electric charge to the transfer paper by corona discharge at the transfer position, a power supply means for applying voltage or current for corona discharge to the corona discharge means, The power supply means is configured such that the amount of corona discharge charge to the rear end of the transfer paper after being released from the restraint by the upstream conveyance means is smaller than the amount of corona discharge charge to the portion other than the rear end of the transfer paper. and control means for changing the magnitude of the voltage or current.

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 3 shows, in cross section, an outline of an image forming apparatus to which the present invention is applied, that is, a reader printer that has a printer section consisting of a known powder image transfer type electrophotographic device and is capable of positive-positive and negative-positive copying. It shows.

In the figure, reference numeral 15 denotes a carrier that holds the microfilm flat and is movable on the mount 18.The positive image of the microfilm is illuminated by a lamp 7, and when it is a reader, the first scanning mirror 8,
The image is projected onto a screen 11 via a swinging mirror 9 and a fixed mirror 10. On the other hand, when printing, the swinging mirror 9 is retracted to the position indicated by the broken line 9'.
The first scanning mirror 8 and the second scanning mirror 12 which can be moved together with the first scanning mirror 8 and the second scanning mirror 12, which can be moved together with the first scanning mirror 8, are
The second scanning mirror 12, fixed mirror 13, and slit The film image projected onto the photosensitive drum 2 via the photosensitive drum 14 is scanned in a slit shape.

On the other hand, as mentioned above, the printer section is composed of a powder image transfer type electrophotographic copying device, and around the photosensitive drum 2 is a DC scorotron type charger 16, the aforementioned slit 14, a developing device 17, a transfer charger 5, and a separating device. A charger 19 and a cleaning device 20 are sequentially arranged along the rotation direction of the photosensitive drum 2.

Reference numeral 21 denotes a paper feed cassette for storing copy paper of a predetermined size.The paper feed cassette 21 is attached to the side of the reader printer body so that it can be set in either horizontal or vertical orientation. The transfer paper P is supplied from the paper feed roller 22 to the transfer position T via the roller pair 23 and the timing roller pair 1. As an alternative paper feeding method,
A path from the manual paper feed port 25 to the pair of rollers 23 and the pair of timing rollers 1 via the pair of feed rollers 26 is also prepared.

A fixing device 27 performs a fixing process on the transferred paper and then discharges it out of the machine.

A transfer paper conveyance system including a pair of rollers 23, a pair of timing rollers 1, and the like is directly or indirectly driven together with the photosensitive drum 2 by a main motor (not shown).

The developing device 17 is of a type in which a magnet roller 27' and a developing sleeve 28 are rotated together, and a predetermined developing bias is applied to the developing sleeve 28.

The configuration of the reader printer to which the present invention is applied has been described above. Next, an embodiment of the present invention in this reader printer will be described based on FIG. 4. In the figure, the structure of the transfer charger 5 is exactly the same as that shown in FIGS. 2a and 2b. However, this transfer charger 5 is
It is controlled so that the current applied to the transfer paper P is switched depending on the rear end portion and the portion other than the rear end portion of the transfer paper P, and is connected to the corona wire 30 via a changeover switch SW. Two transfer power supplies 31 and 32 with different output currents and a changeover switch
It has a control circuit 33 that controls switching of the SW. The output current of the transfer power source 31 is the transfer power source 3.
The output current is set smaller than the output current of No. 2. Moreover, the output current of the transfer power source 32 is larger than the amount of corona discharge charge sufficient to obtain good transfer efficiency even under high temperature and high humidity conditions, that is, the amount of charge that provides the maximum transfer efficiency at room temperature and humidity. Transfer the amount of charge to the paper P
is set to a value that can be assigned to Note that the output voltages of the transfer power sources 31 and 32 may be switched and controlled by the changeover switch SW.

Here, at the start of the copying operation, the changeover switch is
SW is connected to terminal A and corona wire 3
0 is applied with a high output current of the transfer power source 32. On the other hand, at the timing when the transfer paper P leaves the guide plates 6, 6, the changeover switch SW is connected to the terminal B by a signal from the control circuit 33, and the low output current of the transfer power source 31 is applied to the corona wire 30. be done. That is, the current applied to the corona wire 30 is controlled corresponding to the rear end of the transfer paper P and the portion other than the rear end, and the amount of corona discharge charge applied to the rear end of the transfer paper P is adjusted to the rear end. The amount of corona discharge charge applied to other parts is smaller than the amount of corona discharge charge applied to other parts. As a result, the amount of charge applied to the entire surface of the transfer paper P is maintained at a sufficient amount of charge to obtain good and stable transfer efficiency, and the charge density on the transfer paper P is maintained over the entire surface of the transfer paper P. evened out.

The inventor has determined that the output current of the transfer power source 31 is
As a result of checking the transferability by setting the output current of the transfer power source 32 to 100 to 200 μA and setting the output current of the transfer power source 32 to 400 μA, the transfer failure of the transfer paper P was resolved, and a good transferred image was obtained over the entire surface of the transfer paper P. was gotten. still,
It goes without saying that the output current value needs to be set appropriately depending on the image forming apparatus such as a reader printer to be used.

In the above embodiment, the amount of corona discharge charge was controlled by switching the two transfer power sources 31 and 32, but it is also possible to control the output voltage or output current with a variable output voltage or output current type transfer power source. The amount of corona discharge charge may be controlled using a control circuit.
Furthermore, it goes without saying that the present invention is widely applicable to image forming apparatuses that apply electrophotography.

Effects As is clear from the above explanation, the voltage applied to the corona discharge means is applied so that the amount of corona discharge charge to the rear end of the transfer paper is smaller than the amount of corona discharge charge to parts other than the rear end of the transfer paper. Or, by changing the magnitude of the current and making the amount of charge on the transfer paper uniform for transfer, it can be applied over the entire surface of the transfer paper, not only at normal environmental temperature and humidity, but also at high temperature and high humidity. Stable transfer efficiency can be obtained,
The conventional problem of poor transfer occurring at the trailing edge of the transfer paper can be completely solved.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between transfer efficiency and the amount of charge on transfer paper due to corona discharge, Figs. 2a and b are sectional views of the conventional transfer device to explain disadvantages in the transfer device, and Fig. 3 4 is a schematic cross-sectional view of a reader printer to which the present invention is applied, and FIG. 4 is a cross-sectional view of a transfer device that is an embodiment of the present invention. 1... Timing roller, 2... Photosensitive drum, 5... Transfer charger, 6... Guide plate, 3
1, 32...Transfer power supply, 33...Control circuit, P
...Transfer paper, SW...changeover switch, T...transfer position.

Claims (1)

[Claims] 1. In an image forming apparatus that transfers an image formed on an image carrier to a transfer paper conveyed to a transfer position, the transfer paper is restrained on the upstream and downstream sides of the transfer position to transfer the image. a conveyance means for conveying the carrier and the transfer paper while maintaining a predetermined distance between them; a corona discharge means for applying an electric charge to the transfer paper by corona discharge at the transfer position; a power supply means for applying a voltage or current of 1, and a corona discharge charge amount to the rear end of the transfer paper after being released from the restraint by the conveyance means on the upstream side of the transfer position to the portion other than the rear end of the transfer paper; and control means for changing the magnitude of the voltage or current of the power supply means so that the amount of corona discharge charge is smaller than the amount of corona discharge charge. 2. The image forming apparatus according to claim 1, wherein the conveyance means applies a tension force to the transfer paper that is balanced with an electrostatic attraction force between the transfer paper and the image carrier. 3. The image forming apparatus according to claim 2, wherein the conveying means provided upstream of the transfer position is a pair of rollers that sandwich the transfer paper. 4. The image forming apparatus according to claim 2, wherein the conveying means provided downstream of the transfer position is a suction conveying section that sucks the transfer paper from the back side.