JP2967357B2 - 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 passing 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 an image carrier to a transfer material by the action of an electric field formed has already been proposed.

Such a transfer means can surely hold the transfer material compared to the case of using a well-known corona discharger, so that a transfer deviation at a transfer portion is less likely to occur, and a transfer bias can be relatively low pressure. It has various advantages such as being compact and compact and not generating ozone.However, on the other hand, when the width of the original is wider than the width of the transfer material, or when printing a thick original such as a book, the width of the transfer material is reduced. The outer part is developed, and the toner in that part adheres to the transfer roller, and this toner scatters and contaminates various parts near the transfer roller,
The back transfer of the subsequent transfer material may occur.

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. 4 shows the relationship between the voltage and current applied to the transfer member and the transfer roller in such a case. This figure shows the range of the voltage and current value 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 a low volume resistance. , And 10 ¹⁰ Ωcm or more requires approximately +1.4 Kv or more.

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

This translates into a charge density of ~ 3 × 10 ^-9 C / cm
² is a value close to the charge density at the time of solid black development of the transfer roller.

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. .

The relationship between the actual current and the cleaning efficiency is as shown in FIG.

FIG. 6 shows the relationship between the voltage applied to the transfer roller and the current using an OPC photosensitive member and a low-resistance transfer roller having a volume resistance of 10 ⁴ Ωcm.

According to this, it can be seen that the current sharply increases from the potential of the transfer roller of about ± 500 V, but this is because discharge does not occur at a low voltage, and the current is only generated by the contact between the photoconductor and the transfer roller. When the current exceeds ± 500 V, discharge starts at a minute gap between the two, and this is considered to be due to the addition of the current due to the contact, and the volume resistance of the transfer roller is 10 ⁷ Ωcm.
In the following cases, when the voltage applied to the roller is about +500 V, the current becomes +0.5 μA, and when the resistance exceeds 10 ⁷ Ωcm,
It is assumed that the voltage for obtaining 0.5 μA gradually increases.

Thus, it can be seen that the optimum cleaning voltage for cleaning the toner on the transfer roller changes depending on the resistance value of the transfer roller.

On the other hand, as a material for the transfer roller, a sponge material such as foamed EPDM has been widely used, and a material having a good release property is used on the surface to increase the cleaning efficiency of the toner on the transfer roller. Rollers having a multi-layer configuration are also used.

However, all such rollers are environmental,
It is known that the resistance value changes by two digits or more at maximum depending on humidity.

For example, in the case of a transfer roller having a surface layer of PVdF conductively formed on the surface of urethane foam rubber, the resistivity in an environment of 23 ° C. and 60% RH is 1 × 10 ⁹ Ωcm, which is 32.5 Ωcm.
° C., to 90% RH at 5 × 10 ⁷ Ωcm, also, 15 ° C., at 10% RH 1 × 10 ⁹ Ωcm, and the in response to changes in such resistivity, optimum cleaning bias at low-humidity + 500V, high It changes remarkably to + 1500V when wet.

However, including the above-mentioned document and Japanese Patent Application Laid-Open No. 51-9840, it has not been considered that the optimum cleaning voltage changes due to the change in the resistivity of the transfer member due to the environment. , As explained above,
The resistance value of the transfer member greatly changes depending on the environment. Even if a cleaning bias is applied, the cleaning effect is reduced, and satisfactory results can be obtained in preventing contamination of the apparatus due to scattering of toner and back contamination of the transfer material. The fact is that it was not obtained.

The present invention has been made to cope with such a situation. In a transfer device of an image forming apparatus using an image carrier and a transfer roller pressed against the image carrier, and applying a cleaning bias when paper is not passed, a cleaning device is provided. It is an object of the present invention to provide a transfer device capable of maintaining a constant current flowing through a transfer roller when a bias is applied and performing good cleaning regardless of environmental changes.

(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 unit for pressing the transfer member, a toner image is formed on the image carrier with the charged toner, and a transfer material is passed through a transfer portion which is a press contact portion between the image carrier and the transfer device; When paper passing through the printer, a transfer bias having a polarity opposite to that of the toner is applied to the transfer unit, and a cleaning bias having the same polarity as the toner is applied to the transfer unit when paper is not passed at the transfer portion when no transfer material is present. In the image forming apparatus, a control unit for controlling the transfer unit at a constant current when a cleaning bias is applied is provided.

With this configuration, it is possible to always maintain a good cleaning function of the transfer roller irrespective of changes in the environment, and this greatly contributes to obtaining a high-quality image.

(Explanation of Embodiment) FIG. 1 is a schematic side view of an image forming apparatus according to an embodiment of the present invention, in which a transfer roller 1 comes into contact with a cylindrical photosensitive member 2 rotating and running in an arrow X direction and a transfer portion. Forming
When the toner image previously formed on the surface of the photoreceptor arrives at the transfer site, a transfer material is supplied to the transfer site at the same time, and a transfer bias is applied to the transfer roller. The toner image on the photoconductor 2 is transferred to the transfer material by the action of the applied electric field.

Specifically, the photoreceptor 2 is a negatively charged OPC photoreceptor having a diameter of 40 mm, and the transfer roller 1 is formed on a core metal having a diameter of 7 mm by foaming EPDM of carbon black dispersion and forming a base layer having a volume resistance of 10 ⁶ Ωcm. A semiconductive treatment is applied to PvdF having a thickness of 100 μm on the surface to form a surface layer having a volume resistance of 10 ⁹ Ωcm.

The process speed was 70 mm / sec, the length of the transfer roller (length in the width direction of the transfer material) was 220 mm, and the toner was a one-component magnetic positive toner.

The circuit shown in the drawing applies a negative polarity transfer bias to the transfer roller at the time of transfer and a positive polarity cleaning bias at the time of cleaning.

The cleaning was performed when the transfer portion had no transfer material and the sheet was not passed. As a result of performing a constant current control of +0.5 μA, it was confirmed that contamination by scattering toner and back contamination of the transfer material could be effectively prevented.

FIG. 2 shows another embodiment, in which constant current control having the same polarity as the transfer bias is performed when paper is not passed, whereby the resistance value of the transfer roller is detected, and based on this value, constant voltage control is performed during paper passing. Is a method for obtaining an optimum cleaning bias voltage in an apparatus that performs the above-described operation, that is, a method for obtaining an optimum cleaning voltage by using constant current control for determining a transfer voltage.

In the figure, the right side shows the relationship between the logarithm of the voltage V _cc when the constant current (-1.5μA), and the logarithm of the resistance R of the transfer roller.

The left side shows the relationship between the logarithm of the transfer roller resistance R and the logarithm of the optimum cleaning voltage _Vc1 .

As can be seen, once _Vcc is determined, _Vc1 can be uniquely determined.

 Next, another embodiment will be described.

In the apparatus as shown in FIG. 1, the current flowing at the time of cleaning varies depending on the process speed, the length of the transfer roller, etc., but the speed is 100 mm / sec or less.
In the case of a small-sized and low-speed apparatus having a roller length of 230 mm or less, the current is less than 1 μA, and the influence of measurement errors and leaks increases, which may make accurate control difficult.

In order to avoid such a situation, it is possible to detect the resistance by increasing the current value once.However, in the case of regular development, this will charge the photoreceptor to the opposite polarity, so it becomes a memory. Is not preferred.

FIG. 3 shows an embodiment capable of coping with such a situation, in which a roller 1a made of a conductive member such as a metal is brought into contact with the transfer roller 1 so that a more accurate resistance detection can be performed from the current flowing through the roller 1a. It was made.

Next, still another embodiment of the present invention will be described.

In the case of an apparatus, such as a color image forming apparatus, which uses a developing device containing two or more kinds of toners of different types and thus different tribos, and prints with a toner of a desired color as needed, It is unavoidable that the optimum cleaning bias voltage is slightly shifted by the toner.

In such a case, a frame is set up for each developing device having a different color, and the sensor of the apparatus main body detects the resistance of the transfer roller and sets an optimum current value, so that an optimum cleaning is performed for each color. It is possible to ignite the bias.

(3) Effects of the Invention As described above, according to the present invention, the image carrier is provided with a transfer unit that is in pressure contact with the image carrier. Means for applying a transfer bias having a polarity opposite to that of the toner when the transfer material is present at the transfer portion, and applying a cleaning bias having the same polarity as the toner when the transfer material is not present. In addition, since a means for controlling the transfer means with a constant current is provided, even if the resistance value of the transfer means fluctuates depending on the environment, a good cleaning effect is always obtained, which greatly contributes to obtaining a high quality image.

[Brief description of the drawings]

FIG. 1 is a schematic side view of a main part of an image forming apparatus showing an embodiment of the present invention, FIG. 2 is an explanatory view showing another embodiment, and FIG. 3 is an image forming apparatus showing still another embodiment. FIG. 4 is a schematic diagram of the configuration near the transfer portion, FIG. 4 is a diagram showing the relationship between the resistance value of the transfer roller, and the range of the current and voltage at which the cleaning effect is maximized. FIG. 5 is the relationship between the current of the transfer roller and the cleaning effect. FIG. 6 is a graph showing the relationship between the voltage applied to the transfer roller and its current. DESCRIPTION OF SYMBOLS 1 ... Transfer roller, 2 ... Photoconductor, 9 ... Error amplification circuit, 10 ... PWM circuit, 11 ... Driver, 12 ... Solenoid.

Claims (1)

(57) [Claims] An image bearing member and a transfer means for pressing the image bearing member are provided. A toner image is formed on the image bearing member by using a charged toner. A transfer material is passed, and a transfer bias having a polarity opposite to that of the toner is applied to the transfer means when the transfer material is present at the transfer portion when the paper is passed. An image forming apparatus for applying a cleaning bias having the same polarity as toner, wherein a control unit for controlling a transfer unit at a constant current when the cleaning bias is applied is provided.