JPH0980937A - Transfer device for image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent defective transfer from occurring because a transfer roller is not sufficiently charged with transfer charge by controlling the output timing of transfer voltage before a sheet reaches a transfer area based on the detection signal of a sheet sensor. SOLUTION: When the sheet sensor 31 provided on the registration roller 17 of a paper feeding device detects that the leading edge of the sheet S passes the roller 17, a controller 32 controls the output timing of a feed means 20 charging the surface of the transfer roller 19 with the transfer charge and controls the voltage impressed on a feed member 51 in contact with the surface of the roller 19 to be a specified value. Then, the controller 32 controls the output timing of the feed means 20 so that a power source 52 starts output before the sheet S reaches a nip (transfer area) 18, desirably, at timing where the roller 19 can attain at least one revolution before the sheet S reaches the nip 18.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus utilizing an electrophotographic process such as a copying machine, a facsimile and a printer, which transfers a toner image formed on the surface of an image carrier to a sheet which is a recording medium. The present invention relates to a transfer device.

[0002]

2. Description of the Related Art A roller transfer system is widely used in the transfer device of the image forming apparatus. In this, a conductive transfer roller made of an elastic material such as rubber or sponge is provided so as to face the image carrier, and the sheet is conveyed at a nip which is a transfer area formed between the two, and in between,
This is a method in which a transfer voltage is applied to the transfer roller to charge transfer charge on the surface of the transfer roller, and the toner image on the image carrier is transferred onto the sheet. Such a roller transfer type transfer device is excellent in that the transfer efficiency is good even with a low current, the generation of ozone is minute, and the image is faithfully transferred.

However, when the transfer roller is charged with the transfer charge when there is no sheet in the nip due to the forced stop due to the sheet jam or the like, the toner image adheres to or permeates the transfer roller and returns to the normal operation. When this happens, a problem occurs in which the toner image is transferred to the back surface of the sheet and becomes dirty. Further, the electric resistance of the transfer roller is changed due to the surrounding environment such as temperature and humidity, continuous energization, etc., and it is difficult to obtain stable transfer performance. Further, in order to cope with the fluctuation of the electric resistance of the transfer roller, there arises a disadvantage that a high voltage power source is required.

Therefore, as a means for improving the problems of the above-mentioned transfer apparatus of the roller transfer type, for example, as in the transfer apparatus described in Japanese Patent Laid-Open No. 4-27988, a dielectric is formed on the surface of the transfer roller. It has been proposed to cover a dielectric skin part made of a material and to have a means for applying a transfer voltage from the surface side to the skin part. Further, Japanese Patent Application Laid-Open No. 6-138784 discloses a technique similar to the above-mentioned publication in which a dielectric layer is provided on the surface layer of a transfer roller and a transfer charge is charged on the surface of the dielectric layer.

[0005]

However, according to the transfer device described in each of the above publications, the transfer voltage is applied because the transfer charge is charged to the dielectric outer skin portion or the dielectric layer on the surface of the transfer roller. In the initial stage, it is assumed that transfer is performed without sufficient transfer charge. As described above, if the transfer charge is delayed, transfer defects of the sheet may occur remarkably at the leading end of the sheet in the conveying direction.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a transfer device of an image forming apparatus capable of preventing a transfer failure due to insufficient transfer charge of the transfer roller.

[0007]

The present invention has been made to achieve the above object, and transfers a toner image formed on the surface of an image carrier to a sheet fed from a sheet feeding device. A transfer device of an image forming apparatus, comprising: a transfer roller that is disposed so as to face the image carrier and forms a transfer area between the image carrier and the transfer roller; A power supply unit that applies a transfer voltage, a sheet sensor that detects a sheet sent from the sheet feeding device to the transfer area, and a control unit that controls the output timing of the transfer voltage based on a detection signal of the sheet sensor. It is characterized in that the output timing is before the sheet reaches the transfer area.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. A. Configuration of One Embodiment FIG. 1 shows the whole of an electrophotographic printer which is an image forming apparatus in which a transfer device of one embodiment of the present invention is incorporated. The printer 1 has a photosensitive drum 2 which is an image carrier as a center, and a charging device 3, a latent image forming device 4, a developing device 5, a paper feeding device 6, a transfer device 7, a fixing device 8 and a cleaning device around the photosensitive drum 2. The device 9 is arranged in a clockwise arrangement in FIG. Of these devices, the charging device 3, the developing device 5, and the cleaning device 9 are housed in the casing 10 together with the photosensitive drum 2.

The photosensitive drum 2 is adapted to be rotated in the direction of arrow A by an electric motor (not shown). The charging device 3 is composed of a corotron, etc.
The surface of the latent image forming device 4 is normally charged to a negative polarity by the charging device 3, and then the latent image forming device 4 is charged.
Form an electrostatic latent image. The latent image forming device 4 includes a polygon mirror 11, a lens 12, and a reflection mirror 13, converts an input image signal into laser light, passes the laser light through the polygon mirror 11 and lens 12, and then reflects the laser light. The surface of the photosensitive drum 2 is exposed by being reflected by the mirror 13. As a result, an electrostatic latent image corresponding to the image signal is formed on the surface of the photosensitive drum 2.

The developing device 5 includes a developing roller 14 having a sleeve on the outer surface of a magnet roller, and a toner T.
And a toner tank 15 for storing the toner. The toner tank 15 constitutes a part of the casing 10. By the developing device 5, the toner T attached to the peripheral surface of the developing roller 14 is developed on the surface of the photosensitive drum 2 to form a toner image. This toner image is transferred by the transfer device 7 onto the surface of the sheet S fed from the paper feeding device 6.

The sheet feeding device 6 has a cassette (not shown) for stacking and accommodating a large number of sheets S, a sheet feeding roller 16, and a registration roller 17. The sheet S is transferred from the cassette to the sheet feeding roller 16 by the sheet feeding device 6.
After being pulled out one by one, the leading ends are aligned by the registration roller 17 and then sent to the transfer device 7.

The transfer device 7 is a conductive transfer roller 19 which is disposed so as to face the photoconductor drum 2 and forms a nip (transfer region) 18 which is a transfer region between the photoconductor drum 2 and the transfer device 7.
And a power supply unit 20 for charging the transfer roller 19 with a transfer charge. The transfer device 7 transfers the toner image formed on the surface of the photosensitive drum 2 to the surface of the sheet S sent from the paper feeding device 6 to the nip 18.

The sheet S on which the toner image is transferred by the transfer device 7 reaches the fixing device 8 via the conveyance guide 21. The fixing device 8 is composed of a combination of a heating roller 22 and a pressure roller 23.
While passing through the space 3, pressure is applied while being heated, whereby the toner image is fixed on the surface of the sheet S. With this, the printing on the sheet S is completed, and the sheet S is ejected.

The cleaning device 9 is composed of a cleaning blade or the like, and scrapes off the toner remaining on the surface of the photosensitive drum 2 and drops it into the toner recovery tank 24. The tank 24 constitutes a part of the casing 10.

The above is the schematic configuration and operation of the printer 1. Next, the transfer device 7 according to the present invention will be described in detail. FIG. 2 shows the configuration of the transfer device 7. As described above, the transfer device 7 has the conductive transfer roller 19 arranged so as to face the photoconductor drum 2 and the power feeding means 20 for charging the transfer roller 19 with the transfer charge, and further the sheet sensor. 31 and a controller (control means) 32 that receives a signal from the sheet sensor 31 and controls the transfer voltage output timing and the transfer charge.

The transfer roller 19 is driven by the photosensitive drum 2 or independently rotated by an electric motor (not shown). The transfer roller 19 is shown in FIG.
As shown in (b), a conductive shaft 42 serving as a rotation shaft is penetrated through an elastic roller 41 having elasticity such as conductive foam urethane, conductive foam silicon, etc., so that the outer peripheral surface of the elastic roller 41 has conductivity. Dielectric layer 4 with adhesive layer 43 having
4 is a covered structure. The hardness of the elastic roller 41 is
It is preferably about 50 ° (asuka) or less.

The dielectric layer 44 is made of a material such as PVdF having a high relative permittivity (at least 3) and a relatively small time constant of 4.5 sec or less for determining the transfer characteristic of the surface potential. A suitable thickness is about 20 to 100 μm. The dielectric layer 44 needs to be surely adhered to the surface of the elastic roller 41 by the adhesive layer 43. This is because the chargeability of the dielectric layer 44 is kept good, and wrinkles do not occur even if the transfer roller 19 rotates at different peripheral speeds with respect to the photoconductor drum 2, so that it is not damaged. . The adhesive layer 43 is made of, for example, silicon as a base material and is mixed with a conductive material.
It is suitable to have the property of not losing the elasticity required to convey the sheet S even after curing. The electric resistance of the adhesive layer 43 is approximately 10 ⁵ Ω · cm or less, and the thickness is 10 to 100.
μm is appropriate.

The power feeding means 20 has a polarity opposite to that of the toner image on the photosensitive drum 2, that is, + on the surface of the transfer roller 19.
It is used to charge transfer charges of (plus) polarity. While the sheet S is nipped and conveyed between the transfer roller 19 charged with the transfer charge from the power supply unit 20 and the photosensitive drum 2, that is, the toner image on the photosensitive drum 2 is transferred to the sheet. It is designed to be transferred to S.

The power feeding means 20 is composed of a power source 51 and a power feeding member 52 which contacts the surface of the transfer roller 19. For the power feeding member 52, for example, a conductive brush in which a fiber such as rayon is used as a base material and a conductive material is mixed in the base material is suitable for always maintaining a good contact state with the surface of the transfer roller 19. Other than such a conductive brush, a conductive non-woven fabric, a conductive film, a conductive roller or the like can be used.

The minus terminal of the power source 51 is pulled down and connected to the shaft 42 of the transfer roller 19, so that a predetermined voltage is applied to the feeding member 52 from the plus terminal of the power source 51. This allows
The dielectric layer 44 of the transfer roller 19 is charged with a transfer charge determined by the electrostatic capacity of the dielectric layer 44 and the surface potential at that time.

In FIG. 4, the transfer roller 1 is fed by the power feeding means 20.
A transfer voltage is applied to the shaft 42 of the transfer roller 19
A charging circuit 61 for charging transfer charges on the surface of the sheet and a transfer circuit 62 for transferring the toner image on the photosensitive drum 2 to the sheet S by the charging circuit 61 are shown. Nip 18
If the sheet S is not present in the charging circuit 61,
Circulate. When the sheet S is interposed in the nip 18, the charging circuit 61 constitutes a transfer circuit 62 so that the toner image on the photosensitive drum 2 is transferred to the sheet S.

The sheet sensor 31 is provided on the registration roller 17 of the sheet feeding device 6 and detects that the leading edge of the sheet S has passed the registration roller 17. The detection signal of the sheet sensor 31 is input to the controller 32. The controller 32 controls the output of the power feeding unit 20, that is, the output timing of the power source 51, based on the detection signal of the sheet sensor 31, and also controls the voltage applied to the power feeding member 52 to have a predetermined value. Instead of such constant voltage control, the power supply member 5
It is also possible to carry out constant current control for supplying a constant current to 2.

The control mode of the output timing of the power feeding means 20 by the controller 32 is based on starting the output of the power source 51 before the sheet S reaches the nip 18. Preferably, the output is started at the stage where the transfer roller 19 can make at least one rotation before the sheet S reaches the nip 18. In other words, from when the sheet S reaches the nip 18, the transfer roller 19
The output is started before the time corresponding to at least one round of. To do this, from the sheet sensor 31 to the nip 18
Arrival time t ₁ (seconds) and time t ₂ (seconds) required for one rotation of the transfer roller 19 are detected in advance, and at least t ₁ -t ₂ after receiving the detection signal from the sheet sensor 31.
After (seconds), the output of the power supply 51 is started.

The controller 32 also controls the applied voltage so that the transfer charge before the sheet S reaches the nip 18 becomes smaller than that after the transfer. That is, the value of the transfer charge to the transfer roller 19 before the sheet S reaches the nip 18 is made smaller than the value of the required transfer charge when the transfer is actually performed to the sheet S.

B. Operation and Effect of One Embodiment According to the transfer device 7 having the above-described configuration, first, when the sheet S passes the registration roller 17 of the paper feeding device 6,
The sheet sensor 31 detects it and outputs a detection signal to the controller 32. The controller 32 receives the detection signal and starts the output of the power supply 51 after t ₁ -t ₂ (seconds) as described above. By applying the transfer voltage while the transfer roller 19 makes one rotation, the transfer roller 19 is transferred in advance.
The transfer charge is charged all around. That is, the transfer charge is precharged. The transfer charge at the time of this preliminary charging is smaller than the transfer charge at the time of actual transfer. Subsequently, the sheet S reaches the nip 18 and passes through the nip 18, and the toner image on the photosensitive drum 2 is transferred to the sheet S during the passing. The applied voltage is increased when the sheet S reaches the nip 18, and the transfer roller 19 is charged with a required transfer charge larger than the transfer charge up to that point.

According to the transfer device 7 of the above-described embodiment, the transfer charge is precharged on the entire circumference of the transfer roller 19 before the sheet S reaches the nip 18, so that the sheet S reaches the nip 18. Then, from the initial stage of actual transfer, the transfer roller 19 is sufficiently charged with transfer charges. The electric charge charged in the transfer roller 19 is supplied to the power supply member 52.
From the ground, the transfer roller 19 is charged with electric charges for sufficiently supplementing the lost amount.

By the way, when the transfer charges are precharged, the transfer charges are applied to the photosensitive drum 2 without the sheet S intervening in the nip 18. At this time, the electric potential of the photoconductor drum 2 may drop and a so-called charging failure called a transfer memory may occur. It has been experimentally proved that this transfer memory is caused by the transfer charge per unit time, that is, the transfer current rather than the charging time of the transfer charge. However, in the above-described embodiment, since the transfer charge at the time of preliminary charging is set to be smaller than the transfer charge at the time of actual transfer, generation of the transfer memory can be suppressed.

As described above, by precharging the transfer charge to the entire circumference of the transfer roller 19 and preventing the occurrence of the transfer memory, it is possible to prevent the occurrence of a transfer failure at the leading end portion of the sheet S in the conveying direction, and to greatly improve the transferability. Can be improved.

The dielectric layer 44 has a time constant of 4.5 se.
Since it is made of a material that is relatively small, such as c or less,
The transfer charge charged on the transfer roller 19 easily flows out to the ground via the power supply member 52. Therefore, for example, when a forced stop occurs due to a sheet jam or the like, the transfer charge quickly flows from the transfer roller 19 to the ground, so that there is no residual charge, that is, the device is highly safe.

C. Experimental Example of One Embodiment Next, an experimental example based on the above-described one embodiment will be described. (1) Output Timing Start of application of transfer voltage to the transfer roller 19, that is, power supply 5
Transfer was performed by dividing the output start time of 1 under the following conditions. The output of the power supply 51 was started immediately before the sheet S reached the nip 18. The output of the power supply 51 was started when the sheet S came to the position before the nip 18 by a quarter turn of the transfer roller 19. The output of the power supply 51 was started when the sheet S came to the position before the nip 18 by one round of the transfer roller 19. The output of the power supply 51 was started when the sheet S came to the position one round and one quarter front of the transfer roller 19 with respect to the nip 18.

5 to 8 illustrate the output timings of to respectively. And, the degree of the transfer failure occurrence area in (1) to (3) is shown in FIG. In the case of FIG. 5, when the sheet S reaches the nip 18, the pre-charging is not performed at all, so that the transfer defect is likely to occur especially at the leading end of the sheet S at the time of transfer. In the case of FIG. 6, when the sheet S reaches the nip 18, a quarter of the transfer roller 19 with which the leading end of the sheet S contacts
Preliminary charging is applied to the circumference. However, this degree of preliminary charging does not contribute to actual transfer, and is the same as in the case of. In the case of FIG. 7, when the sheet S reaches the nip 18, the entire circumference of the transfer roller 18 is precharged for one round. The transfer failure area was greatly reduced. In the case of (FIG. 8), when the sheet S reaches the nip 18, the transfer roller 18 is preliminarily charged for one round and one quarter. Under these conditions, it was possible to completely suppress the occurrence of transfer defects.

(2) Transfer Current and Transfer Efficiency FIG. 10 shows the relationship between the transfer current and the transfer efficiency in one experimental device of the transfer device 7. Here, the target value of the transfer efficiency is 90%, and in order to reliably achieve this target value,
A transfer current of 3 μA is required. This current value is a value that includes the outflow to the ground. On the other hand, the occurrence of the transfer memory at the time of precharging the transfer charges becomes remarkable when the transfer current is 3 μA, as shown in FIG. That is, a transfer current of 3 μA is required to achieve good transfer efficiency, but a transfer memory is generated when the current value during pre-charging is 3 μA. Therefore, the transfer charge at the precharge may be set to a current value smaller than 3 μA.

At the initial stage of transfer when the sheet S reaches the nip 18 and is transferred to the leading edge of the sheet S, the current value does not reach 3 μA and the transfer efficiency may be insufficient. To be done. However, when the sheet S is not present in the nip 18, no current is consumed other than flowing out to the ground, so that a current of about 1.5 to 2 μA, which is less than 3 μA, is sufficient at the initial transfer stage, as shown in FIG. A high transfer efficiency can be obtained.

As described above, the application of the transfer voltage is started at a position before the transfer roller 19 which corresponds to at least one revolution, and the transfer charge caused by the transfer voltage is initially made smaller than the transfer charge required for the transfer. It has been clarified that the generation of the transfer memory and good transferability can be realized by increasing the required value from the time of actual transfer.

D. Modification Example of the Present Invention The present invention is not limited to the above-described embodiment, and various modifications as described below are possible. (1) The output timing of the power feeding means 20 by the controller 32 is set to be more than one round of the transfer roller 19, for example, two to three rounds before. (2) The sheet sensor 31 is provided not at the registration roller 17 but at another position, for example, the paper feed roller 16 or in the middle of the conveyance path. (3) The present invention is not limited to printers, and can be applied to other image forming apparatuses using electrophotographic processes such as copying machines and facsimiles.

[0036]

As described above, according to the first aspect of the present invention.
According to the invention described in (1), since the transfer voltage output timing by the power feeding unit is set before the sheet reaches the transfer area, the transfer roller is precharged with the transfer charge, and at the time of actual transfer. Since the transfer roller is sufficiently charged with the transfer charge, the occurrence of the transfer failure can be prevented. According to the invention of claim 2, the output timing is from when the sheet reaches the transfer area,
Since it is before the time corresponding to at least one rotation of the transfer roller, the transfer charge is precharged to the entire circumference of the transfer roller, and the transferability is further improved. Claim 3
According to the invention described in (1), the transfer charge before the sheet reaches the transfer area is made smaller than that after the transfer reaches the transfer area, so that the charging failure generated on the image carrier is suppressed.

[Brief description of drawings]

FIG. 1 is an overall side view of a printer incorporating a transfer device according to an embodiment of the present invention.

FIG. 2 is a side view of the transfer device according to the embodiment.

3A and 3B are a front sectional view and a side sectional view, respectively, of a transfer roller.

FIG. 4 is a diagram showing a charging circuit and a transfer circuit of a power feeding unit.

FIG. 5 is a schematic diagram showing a charging state of transfer charges to a transfer roller when the output of the power supply unit is started immediately before the sheet reaches the nip.

FIG. 6 is a schematic diagram showing a charging state of transfer charges to the transfer roller when the output of the power feeding means is started at a position where the sheet is one quarter turn before the transfer roller with respect to the nip.

FIG. 7 is a schematic diagram showing a charging state of transfer charges to the transfer roller when the sheet starts output from the power feeding unit at a position before the transfer roller for one round with respect to the nip.

FIG. 8 is a schematic diagram showing a charging state of transfer charges to the transfer roller when the sheet starts to output from the power feeding unit at a position one round and one quarter front of the transfer roller with respect to the nip.

FIG. 9 is a graph showing a relationship between an output start condition and a transfer failure occurrence area.

FIG. 10 is a graph showing the relationship between transfer current value and transfer efficiency.

FIG. 11 is a graph showing a relationship between a transfer current value and a transfer memory generation level.

FIG. 12 is a graph showing the relationship between the transfer current value and the transfer efficiency of the leading edge of the sheet.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Printer (image forming apparatus), 2 ... Photosensitive drum (image carrier), 6 ... Paper feeding device, 7 ... Transfer device, 18 ... Nip (transfer area), 19 ... Transfer roller, 20 ... Power feeding means, 3
1 ... Sheet sensor, 32 ... Controller (control means).

Claims (3)

[Claims] 1. A transfer device of an image forming apparatus for transferring a toner image formed on a surface of an image carrier to a sheet fed from a paper feeding device, the transfer device being arranged so as to face the image carrier. And a transfer roller that forms a transfer area between the image carrier and a power supply unit that applies a transfer voltage to the transfer roller so as to charge transfer charge, and a sheet sent from the paper feeding device to the transfer area. And a control means for controlling the output timing of the power feeding means based on the detection signal of the sheet sensor, wherein the output timing is before the sheet reaches the transfer area. A transfer device of a characteristic image forming apparatus. 2. The image forming according to claim 1, wherein the output timing is from a time when the sheet reaches the transfer area to a time corresponding to at least one rotation of the transfer roller. Equipment transfer device. 3. The control unit according to claim 1, wherein the transfer charge before the sheet reaches the transfer area is smaller than that after the transfer. A transfer device of the image forming apparatus described.