JP3008116B2 - Transfer device for image forming device - Google Patents

Description

The present invention relates to an image forming apparatus using an electrostatic transfer process, such as an electrostatic copying machine and a printer, and more particularly to an image forming apparatus using the same. is there.

(Problems to be Solved with the Related Art) A toner image is formed by a charged toner on a running image carrier, and a contact-type transfer member such as a transfer roller that runs synchronously with the image carrier is pressed against the toner image and a pressure nip therebetween. Portion as a transfer portion, while inserting a transfer material such as paper through this, applying a transfer bias of the opposite polarity to the toner to the transfer member,
Therefore, an image forming apparatus configured to transfer a toner image on the image carrier to a transfer material by the action of an electric field formed has already been proposed.

In such an image forming apparatus, the image quality is greatly affected by the charge density (current), and if the charge density is too large, the toner is charged to a polarity opposite to the original charging characteristic, causing “bleeding”, If the particle size is too small, there is a problem in that the toner has a weak adsorbing power and “spattering” of the toner occurs. In order to obtain a good image, it is necessary to give a charge density within a certain range.

By the way, in the paper which is mostly used as a transfer material, the resistance value greatly changes to 4 to 5 digits depending on the environment, particularly humidity, and therefore, in order to suppress the influence of such resistance fluctuation relatively low, It is necessary to increase the resistance of a transfer member such as a transfer roller.

Specifically, when a bias of 1 Kv is applied, 1
By constituting the transfer member with a material having a volume resistance of 0 ¹⁰ Ωcm or more, an image that is practically acceptable can be obtained.

On the other hand, if the resistance of the transfer member is increased, the applied voltage for obtaining the required current increases, and the high-voltage power supply is also increased in size, which is disadvantageous in cost.
When it is suppressed to −6 Kv, the volume resistance of the transfer member such as the transfer roller needs to be suppressed to about 10 ¹¹ Ωcm or less.

From the above results, the volume resistance of the transfer member is 10 ¹⁰ to
This means that it must be limited to an extremely narrow range of 10 ¹¹ Ωcm.

Further, there is a problem of hardness as a characteristic required for the transfer roller.

This is the "missing part" of the image, especially the center of the line drawing
It is necessary as a countermeasure, having the volume resistance as described above, and having little change in resistance due to the environment,
Conventionally, a sponge-shaped roller in which an appropriate filler is dispersed in EPDM and adjusted to a desired volume resistivity has been used.

Such a foamed EPDM has a value of, for example, 6 × 10 ¹¹ Ωcm after being left for 1 week in a low-temperature and low-humidity environment (15 ° C., 10% RH) and 3 × 10 ¹¹ Ωcm after being left for 1 week in a high-temperature and high-humidity environment. It has been measured that they are insensitive to environmental changes.

However, when such a roller is used, for example, when the width of the original is wider than the width of the transfer material, when printing a thick original such as a book, the outer portion of the width of the transfer material is developed, and Toner adheres to the transfer roller,
The toner may be scattered to contaminate various parts in the vicinity of the transfer roller, or to cause back contamination of the subsequent transfer material. To avoid such a situation, as shown in Japanese Patent Application Laid-Open No. 51-9840, a bias for cleaning having the same polarity as that of the toner is applied to the transfer member during non-sheet passing, and the transfer member adheres to the transfer member. One that returns the toner to the image carrier has been proposed.

FIG. 12 shows the relationship between the voltage and current applied to the transfer member and the transfer roller in such a case.

The figure shows the range of the voltage and current values at which the cleaning efficiency is the highest with respect to the change in the volume resistance of the transfer roller.
Regarding the voltage, the maximum effect is obtained at about +500 V in the case of low volume resistance, but almost +1.4 Kv at 10 ¹⁰ Ωcm or more.
I need more.

Regarding the current, in this case, the cleaning efficiency is maximum at about +0.5 μA for all volume resistivity.

This corresponds to a charge density of ~ 3 × 10 ⁹ ΩcmC / cm ²
Is a value close to the charge density due to the current flowing from the transfer roller to the image portion.

When the toner adheres to the transfer roller in a solid black state, it is considered that the cleaning efficiency is increased by applying an electric charge enough to cancel the electric charge, and when the electric charge is excessive, the toner is charged to the opposite polarity and the cleaning efficiency is considered to be reduced. .

FIG. 13 shows the relationship between the voltage applied to the transfer roller and the current when the OPC photosensitive member and a transfer roller having a low resistance of 10 ⁴ Ωcm are used.

According to this, it is understood that when the voltage exceeds about ± 500 V, the current sharply increases.

When the voltage is less than ± 500V, only the current due to the contact between the photoconductor and the transfer roller flows regardless of the discharge.
This is probably due to the addition of discharge current.

From the above results, when the volume resistance is 10 ⁷ Ωcm or less, a current of +0.5 μA is obtained even when the voltage applied to the transfer roller is about 500 V, and when the volume resistance exceeds the above value, +0.5 μA is obtained.
applied voltage for obtaining μA of current gradually increases, 10 ¹⁰
It turns out that +1.4 Kv or more is required for a volume resistance of Ωcm or more.

Therefore, using a transfer roller having a volume resistance of 10 ¹⁰ Ωcm or more and using the above-described proposed method of applying a cleaning bias having the same polarity as that of the toner to the transfer roller when paper is not passed, requires 1.4 V as a high-voltage power supply. A device capable of outputting Kv or more is required, which is disadvantageous in terms of miniaturization and cost.Also, it is conceivable to use the same power supply for transfer and cleaning by dividing it, but this is also 6 Kv or more for transfer and cleaning. Output is required, which is inevitably disadvantageous in terms of downsizing the device.

Of course, lowering the volume resistance of the transfer roller is unsuitable for practical use because it is easily affected by the change in resistance of the transfer material due to environmental fluctuations as described above.

SUMMARY OF THE INVENTION The present invention has been made in order to cope with the above-described situation. In an image forming apparatus using an image carrier and a contact-type transfer transfer member which is in pressure contact with the image carrier, a transfer operation and cleaning of the image carrier are performed. In such a case, by changing the resistance value between the transfer roller surface and the power supply, the inconvenience as described above is eliminated, and the image quality is prevented from deteriorating due to environmental fluctuation.
It is an object of the present invention to provide a transfer device that can always obtain good transferability.

(2) Configuration of the Invention (Technical Means for Solving the Problems, Action of the Invention) In order to achieve the above object, the present invention provides an image carrier,
A transfer member is pressed against the transfer member, and a transfer material is supplied to a transfer portion which is a contact portion between the two, and when the transfer material is present at the transfer portion, the transfer member has a polarity opposite to the charge polarity of the toner. In the image forming apparatus which applies a transfer bias and applies a bias having the same polarity as the charge polarity of the toner when no transfer material is present at the transfer portion, the resistance between the transfer member surface and a bias application power supply is set to A transfer device (1) of an image forming apparatus comprising means for increasing the size of paper when passing paper and reducing the size of paper when paper is not passed, or the transfer device of the image forming apparatus described in (1) above, wherein the transfer member is a transfer roller. (2) or (2), the transfer device (3) of the image forming apparatus, wherein the transfer roller is a roller whose resistance value changes by pressing, or (2). A transfer device of an image forming apparatus transfer roller is a roller that changes the resistance value by heating (4).

With such a configuration, it is possible to always obtain a stable and good transfer property regardless of a change in the characteristics of the transfer material due to environmental fluctuation.

FIG. 1 is a view showing a schematic configuration of a transfer portion of an image forming apparatus according to an embodiment of the present invention, in which a transfer roller 1 is pressed against a cylindrical photoreceptor 2 which rotates in a direction indicated by an arrow. And transcription site N
A transfer material (not shown) is supplied to the transfer portion from the side of the transport path 3 and a transfer bias is applied to the transfer roller at the same time. The toner image transfers to the transfer material.

Thereafter, the transfer material is separated from the photoconductor 2 and transported to a fixing portion (not shown).

The power supply for applying a bias to the transfer roller is a constant voltage of -5.5 Kv for the transfer bias and +500 V for the cleaning. Both switching high voltage relays are used, and reference numeral 9 denotes a high voltage rectifier diode.

Although not shown, a temporary charging device, an image signal applying means, a developing device, and other members necessary for image formation are of course arranged around the photoreceptor 2.

FIG. 2 shows the reverse voltage-current characteristics of the high-voltage rectifier diode 9.

As the transfer roller 1, a foam having a volume resistance adjusted to 10 ⁶ Ωcm by dispersing carbon black in EPDM is used.

The diameter of the photoconductor 2 is 60 mm, the length in the axial direction is 240 mm, the diameter of the transfer roller 1 is 20 mm, and the process speed is 70 mm / sec.

The photoreceptor is an OPC photoreceptor, the primary charging polarity is minus, and the toner charging polarity is plus.

With such a configuration, −0.8 to 2.4 μA during transfer
The transfer was performed with a cleaning bias of about +0.5 μA.

As a result, during the transfer, an appropriate transfer current can be obtained irrespective of the change in the resistance value of the transfer material, and the cleaning bias requires only a low voltage of +500 V.
It was confirmed that it was effective in reducing the size and cost of the device.

FIG. 3 shows another embodiment of the present invention. In this embodiment, the transfer roller itself has a rectifying property.

This figure is a cross-sectional view of the transfer roller. In this case, the photoreceptor employs a reversal developing method using a negatively charged OPC photoreceptor and a negatively charged toner, the transfer bias has a positive polarity, and the cleaning bias has a positive polarity. Has a negative polarity.

The illustrated transfer roller is a conductive base layer 10 made of aluminum having a diameter of 19.9 mm, and reference numeral 11 denotes a carrier transport layer, which has the following structural formula Was coated on the conductive base layer 10 by a dipping method in a solution prepared by dissolving 10 parts of the charge transporting material shown in (1) and 10 parts of a bisphenol Z-type polycarbonate resin in 60 parts of monochlorobenzene to form a 20 mm thick layer.

 Reference numeral 12 denotes a semiconductive layer having the following configuration.

The above solution was applied on the carrier transport layer 11 by a dipping method and dried at 140 ° C. for 30 minutes to form a 20 mm thick.

FIG. 4 shows voltage-current characteristics when the roller transfer roller having the above configuration is used in contact with the photosensitive member.

From this, it was confirmed that a good cleaning effect was obtained at +500 V during cleaning while maintaining high resistance during transfer.

FIG. 5 shows another embodiment of the present invention, and shows a cross section of the transfer roller.

The roller has an elastic conductive layer 14 having a volume resistance of 10 ⁷ Ωcm and a thickness of 5 mm on a core metal 15, and a pressure conductive layer 13 having a thickness of 200 μm provided on the surface thereof. This shows displacement-volume resistance characteristics as shown in FIG.

Using such a transfer roller, the transfer roller is placed in pressure contact with the photoreceptor as exemplified in FIG.

As shown in the figures, a transfer roller 2 is supported by a spring 18 disposed on a bearing base 16 so that the roller 2 is displaced toward the photoreceptor 1 so that the bearing base 16 slides. It is mounted on the moving cam member 17.

With this configuration, the pressing force of the transfer roller 1 against the photoconductor 2 can be changed by displacing the cam member 17 in the direction of the arrow E in the figure.

The degree of the change is adjusted so that the pressing force of the roller 1 when pressed weakly against the photoreceptor 2 is at the position of point A in FIG. 6, and when the roller 1 is strongly pressed, the pressing force is at point B in FIG. .

The transfer roller 1 has a volume resistance of 10 ¹⁰ Ωcm at point A at the time of transfer at this time, and a point B at the time of applying a cleaning bias, and the applied voltage at this time is about +500 V.

FIG. 8 shows another embodiment of the present invention, in which the photoreceptor 2 is an OPC photoreceptor, the transfer roller 1 is a carbon-dispersed EPDM sponge, and the volume resistance is 5 × 10 ¹⁰ Ωcm. It is connected to the transfer power supply 19a by a resistor and shows 10 ¹⁰ Ω when 2 Kv is applied. Reference numeral 19b denotes a cleaning power supply, and switching between the two power supplies uses a high-voltage relay.

FIG. 9 is a sectional view of a transfer roller showing still another embodiment, in which an elastic semiconductive layer 21 is formed on a cored bar 20, and a heating layer 22 and a resistance layer 23 are formed thereon in this order. .

The heat generating layer 22 is formed in a comb shape with a pitch of 500 μm by a PTC heater, and the elastic semiconductive layer 21 and the resistance layer 23 are in contact with each other at the above pitch.

 FIG. 10 is a temperature rise and fall curve of the heating layer 22.

The elastic semiconductive layer 21 is made of carbon-dispersed foamed EPDM and has a thickness of 5 mm and a volume resistance of 10 ⁷ Ωcm, and the resistance layer 23 is a mixture of metal oxides and sintered, and has a thickness of 500 μm.

FIG. 11 is a graph showing the relationship of the ratio R ₂₅ / T between the resistance R _{25 at} 25 ° C. and the resistance value at T ° C. The set value of R ₂₅ is 10 ¹¹
It is about Ωcm, and it can be seen that the resistance value decreases by nearly three digits as the temperature increases.

Using such a roller as a transfer roller,
It is assumed that the resistance of the roller is changed by changing the temperature of the resistance layer 23 by applying a voltage to the roller 22.

(3) Effects of the Invention As described above, according to the present invention, the image carrier is provided with the transfer member pressed against the image carrier, and the transfer material is passed through the transfer portion where both are pressed against each other. In a transfer apparatus in which transfer is performed by applying a transfer bias, the resistance of the circuit section from the transfer member to the power supply is high when the transfer material is passed, and the resistance is high when the transfer material is not present at the transfer portion. , A good cleaning function can be achieved with a low cleaning voltage.
The size and cost of the device can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic side view of an image forming apparatus according to an embodiment of the present invention, particularly showing a transfer site, FIG. 2 is a reverse voltage-current characteristic of a high voltage rectifier diode, and FIG. 3 is an embodiment of the present invention. FIG. 4 is a cross-sectional view of a transfer roller according to the first embodiment, FIG. 5 is a cross-sectional view of a transfer roller according to another embodiment, and FIG. 7 is a side view showing an example of a means for causing the transfer roller to deform, FIG. 8 is a schematic view showing a transfer bias applying means showing still another embodiment, and FIG. 9 is still another example. FIG. 10 is a graph showing the temperature rise and fall of the above, FIG. 11 is a graph showing the relationship between the temperature change and the change of the resistance, and FIG. 12 is the transfer member. Graph showing the relationship between the volume resistance of the sample and the quality of the cleaning effect; FIG. 3 is a graph showing the voltage-current characteristics of the OPC photosensitive member. DESCRIPTION OF SYMBOLS 1 ... Transfer roller, 2 ... Photoconductor, 9 ... High-voltage rectifier diode, 10 ... Conducting base layer, 11 ... Carrier transport layer, 12 ...
... Semiconductive layer, 13 ... Pressurized conductive layer, 14 ... Elastic conductive layer, 16
... bearing base, 17 ... cam member, 18 ... spring, 19 ... resistance, 21 ... elastic semiconductive layer, 22 ... heating layer, 23 ... resistance layer.

Claims (4)

(57) [Claims] An image bearing member and a transfer member pressed against the image carrier are provided, and a transfer material is supplied to a transfer portion which is a contact portion between the image carrier and the transfer member when the transfer material exists at the transfer portion. When a transfer bias having a polarity opposite to the charge polarity of the toner is applied to the member, and the transfer material is not present at the transfer portion, the paper is not fed.
An image forming apparatus for applying a bias having the same polarity as the charging polarity of toner, comprising: means for increasing the resistance between the surface of the transfer member and a bias application power supply when the paper is passed and decreasing the resistance when the paper is not passed. Transfer device. 2. A transfer device for an image forming apparatus according to claim 1, wherein said transfer member is a transfer roller. 3. The transfer device of an image forming apparatus according to claim 2, wherein the transfer roller is a roller whose resistance value changes by pressing. 4. The transfer device of an image forming apparatus according to claim 2, wherein the transfer roller is a roller whose resistance value changes by heating.