JPH0423787B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image forming apparatus, and more specifically, an image forming apparatus that transfers an image on an image carrier carrying a developed image to an intermediate transfer member, and further transfers the image on the intermediate transfer member to a transfer material. The present invention relates to a forming device.

Conventionally, image forming apparatuses, such as transfer-type color electrophotographic apparatuses, have a structure in which images of different colors are sequentially formed on an image bearing member such as a photoreceptor drum, and each color image is finally multiple-transferred onto a transfer material. Various methods have been proposed.

As one of such transfer methods, individual yellow, cyan, and magenta toner images formed on an image bearing member such as a photoreceptor drum or an insulated drum are transferred to an intermediate bearing member such as a drum (hereinafter referred to as a transfer drum). ), and finally on the transfer material,
A method can be considered in which three color toner images are simultaneously transferred onto a transfer drum, which is an intermediate carrier.

However, according to the above method, as a member of the transfer drum that actually carries the toner image, a film-like flexible or relatively hard sheet member (hereinafter referred to as an image-bearing sheet) is made of a synthetic material such as polyester polypropylene or triacetate. When resin films are used, the surface resistance of these films is 1
It has a very high resistance of ×10 ¹⁴ Ω or 1 × 10 ¹⁷ Ω.

When a toner image is repeatedly transferred to such a high-resistance image-bearing sheet, a charge-up phenomenon occurs on the image-bearing surface due to the transfer charge charged on the back side of the support sheet (charges of opposite polarity to the transfer charge are used to carry the image). (accumulation on the sheet surface), the transfer rate decreases as the number of multiple transfers increases. As a result, the toner amount of the second color toner image is lower than that of the first color, and the toner amount of the third color is lower than that of the second color.

In addition, since the charge decay on the surface of the image-bearing sheet is extremely small, electric discharges or air discharges are likely to occur between adjacent conductive members immediately after transfer and during rotation of the transfer drum, causing unevenness in the transferred image. This will cause disturbance.

As a countermeasure to the above-mentioned problem, a method of removing static electricity from the front and back surfaces of the image-bearing sheet using a static eliminating means such as an AC corona discharger is considered, but this method not only complicates the device configuration but also is not sufficiently effective.

In addition, in order to lower the resistance of the image-bearing sheet, a surfactant (ammonium salt aliphatic sulfonate, higher alcohol sulfate salt, alcohol ethylene oxide, betaine, etc.) is mixed into the binder resin that makes up the sheet. By coating the image-bearing sheet, the resistance can be lowered and the charge-up phenomenon and discharge phenomenon of the sheet can be reduced, but even with this treatment, the surface resistance of the image-bearing sheet is still as high as 1×10 ⁹ Ω to 1×10 ¹¹ Ω. , does not solve the problem. Furthermore, this resistance lowering treatment has a significant humidity dependence and is not effective in a low humidity environment of 30% R.H. or less, and there remains the problem that the surface resistance changes significantly over time.

SUMMARY OF THE INVENTION An object of the present invention is to provide a new image forming apparatus that has been improved in view of the above-mentioned problems, and is capable of obtaining high-quality transferred images with a high transfer rate and no transfer unevenness, and is particularly suitable for multiple transfer of color images. It is an object of the present invention to provide an image forming apparatus that is optimal for the image forming apparatus and that can always obtain stable transferred images even when there are environmental changes.

The details of the present invention will be explained below using specific examples with reference to the drawings. FIG. 1 is an explanatory diagram of an example in which the present invention is applied to a color copying machine, which is an image forming apparatus.

In FIG. 1, reference numeral 1 denotes an electrophotographic photosensitive drum having a surface insulating layer, a photoconductive layer, and a conductive substrate, which is rotatably supported on a shaft 2 and rotates in the direction of an arrow 3. When the drum 1 rotates to the normal position, the original placed on the original platen glass 4 is illuminated by an illumination lamp 6 integrated with the first scanning mirror 5.
The reflected light is scanned by a second scanning mirror 7. The reflected light image of the original passes through a lens 8 and a third mirror 9, is separated into colors by a color separation filter 10, and is imaged onto the drum 1 by an exposure section 11.

The drum 1 is equipped with a lamp 13 and an AC charger 14 in advance.
After being charged to a specific polarity by a primary charger 15, the original image is subjected to slit exposure in the exposure section 11. At the same time, the secondary charger 16 performs static charge removal using AC or polarity opposite to the primary charger, and then further exposes the entire surface using the full-surface exposure lamp 17. A latent image is formed.

The electrostatic latent image on the photoreceptor drum 1 is then transferred to a developing device 18.
The image is visualized as a toner image. This developing device 18 has yellow 18 ₁ , magenta 18 ₂ , cyan 1
It consists of three developing units ( ₈₃ ), and a toner image of the required color is obtained by operating the designated developing unit according to the color separation filter used for exposure.

The developed image formed on the photosensitive drum 1 is charged by a pre-transfer charger 19 at the same time as the lamp 20 uniformly exposes the developed image to alternating current or a polarity opposite to the primary charging.

Next, the developed image on the photosensitive drum 1 is transferred to the image-bearing sheet at the transfer position by corona discharge of the first transfer charger 221 in the transfer drum 22, which is rotatably supported on the shaft ₂₁ , which will be described in detail later. A transfer charge having a polarity opposite to that of the toner of the toner image is applied to the back side of the image-bearing sheet 22 ₂ , and the toner on the photosensitive drum ₁ is sequentially transferred in alignment with the surface of the image-bearing sheet 22 2, and finally a multicolor image is formed. It is formed.

After the toner of each color has been transferred, the surface of the photosensitive drum 1 is cleaned by a cleaning device 12 composed of an elastic blade, and the drum 1 proceeds to the next development cycle.

On the other hand, the transfer paper 25 in the cassette 24 is fed into the machine by the paper feed roller 26, and then transferred to the register roller 2.
7, the second transfer charger 28 applies a transfer charge of the same polarity as that of the first transfer charger 22 1 to the back side of the transfer paper 25 at the transfer position, and the transfer charge of the same polarity as that of the first transfer charger 22 ₁ is applied to the back side of the transfer paper ₂₅ at the transfer position. The color images are transferred all at once onto the transfer paper 25. At this time, if corona discharge is performed from inside the transfer drum 22 during transfer using a charger 22 5 of the same polarity as the toner image, for example, a charger 22 ₅ of opposite polarity to the polarity of the second transfer charger 28 or an alternating current.
Even thick toner layers in areas where two or three colors overlap can be transferred to transfer paper extremely efficiently. Therefore, the toner in the lowest layer on the surface of the transfer drum 22 is also sufficiently transferred, resulting in an image with good color balance.

This can be done by making the back side of the image bearing sheet ₂₂ polarity opposite to the toner polarity to repel the toner on the front side, or by eliminating the static charge on the back side of the image bearing sheet 22 to remove the binding force of the toner on the front side, or by removing the binding force of the toner on the front side of the image bearing sheet ₂₂ . It is thought that the corona discharge of the charger ₂₂₅ serves as an opposite pole to the corona discharge of the second transfer charger 28, and this contributes to increasing the discharge efficiency of both chargers.

Furthermore, the improved transfer rate facilitates cleaning of the surface of the image bearing sheet 22 ₂ as a subsequent process. Note that by making the distance on the image bearing sheet 222 from the first transfer charger _{221 to the charger 225 larger} than the maximum length of each development, transfer omission may occur when the charger ₂₂₅ is activated. can be prevented. Immediately after the transfer paper 25 is transferred and charged by the second transfer charger 28, the reverse side of the transfer paper 25 is separated and neutralized by the AC charger 29, and the transfer paper is separated from the transfer drum 22 and delivered to the conveyor belt 30. Furthermore, it is led to a heat roller fixing device 31 where it is pressurized and heated, and then discharged onto a tray 32.

On the other hand, a cleaning device 23 using a fur brush that can come into and out of contact with the surface of the image bearing sheet 22 ₂ of the transfer drum cleans the image bearing surface of the sheet. A cleaning brush 22 ₆ is provided on the back surface of the image bearing sheet 22 ₂ facing the cleaning device 23, and when the cleaning device 23 operates, it comes into contact with the back surface of the image bearing sheet 22 _{2 and} the In addition to supporting this sheet, the back side of the image-bearing sheet 22 ₂ is also cleaned. Note that it is more convenient to use conductive brushes for the cleaning device 23 and the brush 22 ₆ because unnecessary charging is not generated on the front and back sides of the image bearing sheet 22 ₂ .

The color electrophotographic apparatus illustrated in FIG. 1 described above temporarily transfers the developed image carried on the surface of the photoreceptor drum 1 onto the transfer drum 22, and then transfers the obtained multicolor images all at once. A transfer device that transfers the image onto the transfer paper 25 is provided.

As shown in FIG. 2 as a perspective view and as shown in FIG. 3 as a cross-sectional view, the transfer drum 22 of the transfer device of the illustrated embodiment has a cylinder support member 22 with a large cutout leaving a part of the aluminum cylinder. An elastic member 22 ₈ (in FIGS. ₂ and ₃ ) is provided between the supporting member 22 ₇ and the image bearing sheet 22 ₂ . The part with multiple points) is interposed. In the area corresponding to the cylinder notch of this image bearing sheet ₂₂₂ ,
The toner image on the photosensitive drum 1 is transferred. The image-bearing sheet 22 ₂ is an insulating film, for example, a synthetic resin film such as polyester, polypropylene, polyethylene, polystyrene, triacetate, etc., provided with a conductive layer having a surface resistance of 1×10 ⁸ Ω or less.

This conductive layer has a surface resistance with no charge retention when grounded, but since it is provided on an insulating film and is electrically floating, it comes into direct contact with other conductive members. Therefore, the transfer electric field applied to the back surface of the image-bearing sheet during transfer is not blocked. This enables good transfer.

In addition, in order to prevent the transfer electric field from being blocked, the above-mentioned conductive layer is placed at least 1 inch from the end of the insulating film.
It is best to provide it on the inner surface of mm or more. However, even if there is a conductive layer on the entire surface of the insulating film on the image carrier side, the transfer electric field will not be blocked by covering the ends with the insulating film or insulating paint.

When transfer is performed using the transfer drum 22 having the image-bearing sheet 22 ₂ provided with a conductive layer having a surface resistance of 1×10 ⁸ Ω or less, the surface potential of the back surface of the image-bearing sheet 22 ₂ increases due to transfer charging. Number 1,
Even if the potential reaches a high potential of 000 V or more, the electrical charges caused by the discharge that occurs when the photoreceptor drum and transfer drum move apart after transfer are leaked, and the electrical charges are made uniform by surface conduction of the conductive layer. This effect effectively prevents a decrease in transfer efficiency due to the charge-up development and uneven transfer due to peeling discharge.

When the above conductive layer is provided on an insulating film, In ₂ O ₃ , SnO ₂ , TiO ₂ , ZrO ₂ , Cd ₂ SnO ₄ CuI
Semiconductor thin films such as aluminum, palladium, etc.
Metal thin films such as gold and multimetallic thin films such as TiO ₂ /Ag/TiO ₂ are used. In order to laminate such a conductive thin film on the surface of an insulating film, a physical film forming method such as vapor deposition or sputtering, or a chemical film forming method such as a spray method or CVD method can be used. Note that a protective layer that does not impair conductivity can be provided on the surface of the conductive thin film to improve friction resistance. Further, the thickness of the image bearing sheet ₂₂₂ is 25μ to 300μ, preferably
Around 100μ produces a good effect.

By the way, the elastic member 22 ₈ that adheres to the cylinder support member 22 ₇ described in FIGS. 2 and 3 above is made of, for example, a low hardness rubber with a JIS hardness of 40 or less (urethane rubber, ethylene/propylene rubber, chloroprene rubber, etc.). Low hardness materials such as rubber, NBR, natural rubber, silicone rubber, etc.) or rubber and plastic foams (foamed butyl rubber, foamed polyurethane, foamed polyethylene, foamed polystyrene, foamed vinyl chloride, etc.) are used. The thickness of the elastic member ₂₂₈ is preferably in the range of about 0.5 mm to 5 mm.

For example, the transfer drum of the above embodiment has a drum diameter of
200 mm, and the image-bearing sheet is a film in which a tin-doped indium oxide thin film is laminated on a 100-μ thick polyester film substrate.
2 ₈ is 5mm thick foamed plastic rubber (width on one side 12mm)
In this case, when the back surface of the image-bearing sheet 22 ₂ of the transfer drum is charged by the first transfer charger 22 ₁ so that the surface potential becomes 2,000 V, the transfer drum is charged to the photosensitive drum. The pressure is approx.
The transfer weight was 10 g/cm ² and the transfer width was approximately 8 mm. At this time, all of the first, second, and third color toner images on the photosensitive drum could be transferred onto the transfer drum at a transfer rate of 90% or more.

Furthermore, the three-color toner images formed in layers on the transfer drum thus formed are transferred onto the transfer paper at a transfer rate of about 90% or more by the second transfer charger 28 and the charger ₂₂₅ inside the drum 22. I was able to transcribe it.
The multicolor toner image transferred in this manner was a high-quality image without uneven transfer or hollow spots, and free from positional deviation.

For comparison with a case where the present invention is not applied, a transfer drum made of a high-resistance image-bearing sheet without a conductive _layer was used _. Using an AC charger, charge was removed from both sides of the image-bearing sheet each time each color toner was transferred. As a result, the transfer rate of the toner image on the photosensitive drum to the transfer drum was approximately 90% for the first color, approximately 85% for the second color, and approximately 85% for the third color.
It has dropped to 80%. Further, some unevenness in transfer was found due to peeling discharge when the photoreceptor drum and transfer drum were separated.

Furthermore, if the above-mentioned AC chargers 22 ₃ and 22 ₄ were not used to remove static electricity after each transfer, the transfer rate was extremely low, being about 90% for the first color, about 70% for the second color, and less than 40% for the third color. . Further, in this case, the transfer unevenness caused by peeling discharge was so severe that it could not be put to practical use. At this time, the corona discharge voltage of the first transfer charger ₂₂₁ is increased every time each color is transferred, and the surface potential of the back surface of the image bearing sheet is set to 2,000 V for the first color, 2,000 V for the second color, and 2,000 V for the second color.
Even when the voltage was changed to 500V and the voltage for the third color was changed to 3,000V, the above-mentioned decrease in transfer rate was hardly improved.

According to the present invention, by setting the surface resistance of the intermediate carrier onto which the toner image is transferred to 10 ⁸ Ω or less, toner can be transferred multiple times onto the intermediate carrier. Further, stable high transfer efficiency can be maintained even against environmental changes such as humidity. Furthermore, the charge-up phenomenon of the intermediate carrier to which the toner image is transferred a plurality of times is prevented, and at the same time, the adverse effects caused by the discharge phenomenon that occurs when the image carrier and the intermediate carrier are separated after the transfer are prevented.

Note that the scope of application of the present invention is not only the apparatus using the electrophotographic method described above, but also the apparatus using an insulator for the image carrier,
This invention is also effective for recording devices that form latent images on this insulator using signal electrodes.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an embodiment of a color electrophotographic apparatus to which a transfer device according to the present invention is applied, FIG. 2 is a perspective view of a transfer drum used in the transfer device shown in FIG. 1, and FIG. is a sectional view of the transfer drum. In the figure, 1...photosensitive drum, 4...document glass, 5...first scanning mirror, 6...illumination lamp, 7...second scanning mirror, 8...lens, 9
...Third mirror, 10...Color separation filter, 2
2... Transfer drum, 22 ₁ ... First transfer charger,
22 ₂ ... image carrier sheet, 22 ₇ ... support member, 22 ₈ ... elastic member, 28 ... second transfer charger.

Claims (1)

[Scope of Claims] 1. An image forming apparatus that multiple-transfers a developed image formed on an image carrier onto an intermediate image carrier sheet and then transfers the image on the intermediate image carrier sheet to a transfer material, comprising: a transfer charging device that applies a transfer charge to the back side of the intermediate image carrier sheet each time the image is transferred to the intermediate image carrier sheet; and a transfer device that transfers the image on the intermediate image carrier sheet to a transfer material. . An image forming apparatus, wherein the intermediate image carrier has a surface resistance of 10 ₈ Ω or less.