JP2671044B2 - Image forming device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a latent image formed on an image carrier by an electrophotographic method or an electrostatic recording method, and the latent image is referred to as a visible image by a developing means. The present invention relates to an image forming apparatus that obtains an image by transferring a visible image onto a transfer material by a transfer device, and in particular, has a plurality of developing devices and develops a latent image formed on an image carrier,
The toner image can be preferably embodied in a multicolor image forming apparatus that sequentially transfers the toner images onto a transfer material by a transfer device to obtain an image.

2. Description of the Related Art Conventionally, various multicolor image forming apparatuses have been proposed and are commercially available. An example of a typical color electrophotographic copying machine is shown in FIG.

In FIG. 2, the color electrophotographic copying machine, which will be described in detail later, forms a latent image on the image carrier 3 which is an electrophotographic photosensitive drum in this example, and forms the latent image into a plurality of, for example, magenta and cyan. , Four developing devices 1M containing toner having an average particle diameter of 12 μm, which is made of four types of resin, yellow, and black
A color toner image is formed by the rotary developing device 1 equipped with 1C, 1Y, and 1BK, and these toner images are attached to a drum-shaped transfer device 9 arranged close to the surface of the photosensitive drum 3, and a sheet-like transfer material such as paper is attached. Is superimposed and transferred to obtain a color image.

As a transfer device 9 used in such a color electrophotographic copying machine, a pair of annular members, cylindrical members 9a and 9b in this example, are concentrically arranged as shown in FIG. A transfer drum formed in a cylindrical shape by integrally connecting the two by a connecting member 9c and further covering the entire circumference with a dielectric sheet 93 as a transfer material carrier formed of a dielectric material is used. .

Further, a gripper 91 is arranged on the outer surface of the connecting member 9c of the transfer drum 9, and the transfer material P supplied in time with the toner image on the photosensitive drum 3 is gripped and conveyed.
At each transfer position where the transfer charger 10 is arranged, each time a toner image of each color is transferred onto the transfer material to form a color image, the gripper 91 is released to separate the toner image.
The transfer material is separated from the transfer drum by. The separated transfer material is fed to the fixing unit 17 by the conveying unit 16.

Further, in recent years, with the widespread use of image forming apparatuses such as color electrophotographic copying machines, the applications thereof have expanded to a wide variety, and the demand for image quality thereof has also increased. 2. Description of the Related Art In copying an image such as a general photograph, catalog, or map, it is required to reproduce extremely finely and faithfully without being crushed or interrupted even in a minute portion.

Further, recently, in an image forming apparatus such as a color electrophotographic copying machine using a digital image signal, a latent image is formed by gathering dots having a constant potential, and a solid portion, a halftone portion and a light portion are formed. Is expressed by changing the dot density.

However, in such an image forming apparatus, when the toner particles do not faithfully adhere to the dots and the toner particles protrude from the dots, the toner image corresponding to the dot density ratio of the black portion and the white portion of the digital latent image is formed. There is a problem that gradation cannot be obtained.

Furthermore, in order to improve the image quality, if the dot size is reduced to improve the resolution, the reproducibility of the latent image formed from minute dots becomes more difficult, and the resolution,
In addition, there is a tendency that the image lacks sharpness, particularly the gradation of the highlight portion is poor.

Heretofore, some developers have been provided for the purpose of improving image quality. Toner miniaturization,
As part of this, for example, JP-A-62-157051 proposes to use a color toner having a volume average particle diameter of 1 to 9 μm.

However, when the volume average particle diameter of the toner is reduced, the adhesive force due to the Van der Waals force increases,
Adhesion to the photosensitive drum, which is the image carrier, increases.

This tendency is further increased when an image carrier having a large surface free energy such as containing an organic polymer having a charge transfer layer on the surface is used as the image carrier, so this adhesiveness should be fully considered. Therefore, it has become necessary to perform image formation, especially a transfer process.

SUMMARY OF THE INVENTION In such a color electrophotographic copying machine, a document image is separated into four colors, a magenta image, a cyan image, a yellow image and a black image, and a latent image is formed on the photosensitive drum for each color. The transfer drum must be rotated at least four times because it is formed and transferred to the transfer material through the developing process.

However, depending on the size of the transfer material held on the transfer drum, one transfer of one color is performed by one rotation of the transfer drum, and then, for example, the movement of the developing device cannot be completed at the time of the next rotation, or the original reading optical system cannot be completed. The preparation for the image forming process of the next color is not completed due to the reason that the means cannot return to the start position, and in this case, the transfer drum rotates once more to form the image of the next color. It is necessary to wait for the preparation process for During the rotation of the transfer drum to wait for the completion of the preparatory process for forming the image of the next color, that is, at the time of so-called "idle rotation", the toner image already transferred to the transfer material is transferred to the photosensitive drum again. You will come into contact.

At this time, due to the adhesive force of the toner and the photosensitive drum, the toner may be offset from the toner image on the transfer material to the surface of the photosensitive drum, or the toner may be moved on the transfer material and the image may flow. Then, the problem that the transferred image is disturbed occurred.

Further, as shown in FIG. 4, in the transfer drum 9, the dielectric sheet 93 is pressed in order to evenly disperse the dielectric sheet 93 and to increase the transfer nip to increase the transfer efficiency and obtain a clear transfer image. When the backup member 18 was used in order to do so, the above-mentioned problem remarkably occurred.

Therefore, an object of the present invention is to apply an electric field again to the transfer material during rotation in which transfer is not performed in a state where the transfer material is held and conveyed on the transfer material carrier during a series of transfer steps. The toner on the transfer material is not offset toward the image carrier, the toner on the transfer material is not activated, and the image does not flow.Furthermore, the charge up of the transfer material carrier is minimized. An object of the present invention is to provide an image forming apparatus which can always obtain a high quality image.

Means for Solving the Problems The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image carrier capable of carrying toner images of a plurality of colors, a transfer material carrier carrying a transfer material, and the transfer material carrier carrying the toner images of the plurality of colors on the image carrier. Transfer belt electronic means for sequentially superposing and transferring the transfer material carried on the transfer material at the transfer position, and after transferring the toner image of the first color to the transfer material carried on the transfer material carrier, Before transferring the toner image of the second color, which is the next color of the toner image of the first color, to the transfer material carried by the transfer material carrier.
In the image forming apparatus having a period in which the transfer material having the toner image of the color of 5 passes through the transfer position while being carried by the transfer material carrier, the toner image of the second color is transferred to the transfer material in the period. A second electric current, which is less than a first electric current applied to the transfer charging unit and is of the same polarity as the first electric current for transferring to the transfer material carried on a carrier, is applied to the transfer charging unit. Image forming apparatus.

Preferably, a detection unit for detecting the temperature and humidity inside the device is provided, and the second current is varied according to the detection result of the detection unit.

Embodiment Hereinafter, one embodiment of the present invention will be specifically described with reference to the drawings. In the present embodiment, a full-color electrophotographic copying machine to which the Carlson process is applied will be described as an example, but it goes without saying that the invention can also be applied to an image forming apparatus such as a multi-color recording apparatus or a color recording apparatus of a computer output section. As a process, the electrostatic latent image process,
Of course, for example, the NP process can also be used.

The present invention is preferably embodied in the full-color electrophotographic copying machine described above with reference to FIG. 2, and its configuration and operation will be briefly described.

In this embodiment, the photosensitive drum 3 as an image carrier is rotatably carried in the direction of arrow X (counterclockwise),
The primary charger 4 is located substantially directly above the photosensitive drum 3, the rotary drum type developing device 1 is located on the left side, and the transfer drum 9 as a transfer material carrier, which is a transfer device, is located substantially directly below the transfer drum 9 as indicated by arrow Y.
It is arranged so as to be rotationally driven in the direction (clockwise). Further, the cleaning device 12 is arranged on the right side.

An exposure unit is provided between the primary charger 4 and the developing device 1, and an optical signal is emitted to the photosensitive drum 3 from the polygon mirror unit 8 and the laser reflection mirror 7 arranged above. .

Further, the full-color electrophotographic copying apparatus has a fixing device 17 and a paper feeding device 23 arranged on the right side thereof, and further, a transfer paper conveying system is provided between the transfer drum 9 and the fixing device 17 and the paper feeding device 23. 16 and 20, etc. are arranged.

More specifically, in this embodiment, the photosensitive drum 3 as an image carrier is an OPC photosensitive drum containing an organic polymer having a charge transfer layer on the surface thereof. This photosensitive drum 3 has a diameter of 8
It was set to 0 mm and rotated in the direction of arrow X at a speed of 160 mm / sec.

By the primary charger 4, the surface of the photosensitive drum 3 is -500 to
After being charged to −800V, a laser optical system composed of a polygon mirror unit 8 and a laser reflection mirror 7 arranged above the exposure unit emits image information signals color-separated by an image reading device and an image processing device (not shown). And a latent image is formed.

The developing device is mounted on the rotary support in the direction of its rotation angle.
Four developing devices 1M (magenta development), 1C (cyan development), 1Y (yellow development), and 1BK (black development) are detachably attached at intervals of 90 degrees, and the photosensitive drum 3
The latent image of each color formed on the peripheral surface of is visualized using a corresponding developing device. Therefore, according to the control procedure, the rotary support is rotated by 90 degrees.

The state of FIG. 1 shows a state in which the developing device 1M corresponds to the photosensitive ram 3.

In the present example, the four developing devices each contained a developer containing a mixture of toner and carrier, but the toner was prepared by the following method.

A polyester resin obtained by combining propoxylated bisphenol and fumaric acid, a chromium complex acid of ditertiary butyl salicylic acid, and a suitable amount of dyes and pigments were thoroughly premixed with a Henschel mixer, and then 3 This roll mill was melt-kneaded at least twice, cooled, and then coarsely pulverized to a particle size of about 1 to 2 mm using a hammer mill. Then, it was crushed by an air jet type fine crusher. Further, the obtained finely pulverized product was dispensed by a multi-division dispenser to produce resin particles having a volume average particle diameter of about 8 μm.

In 100 parts of the colorant-containing machine resin particles thus obtained, B
Alumina fine powder with specific surface area of 100 m ² / g by ET method 0.3
And 0.5 part of silica fine powder having a specific surface area of 250 m ² / g by the BET method and having a charge amount of 80 μc / g that was hydrophobized with hexamethyldisilazane were externally added to obtain each color toner.

Further, as the carrier particles, Cu-Zn-Fe ferrite particles surface-coated with a styrene-methyl methacrylate-stilhexyl acrylate copolymer were used.

Therefore, in this embodiment, the latent image on the photosensitive drum 3 is visualized by the developing device 1 having the developer containing the toner and the carrier, using the negatively charged toner by the reversal development method.

The transfer device 9 is a transfer drum having the same structure as described above with reference to FIG. 3, and includes a pair of annular members, in this embodiment, cylindrical members 9a and 9b having a diameter of 160 mm. Concentrically arranged and configured integrally by connecting both by a connecting member 9c, further around the entire circumference, for example, a thickness of 100 ~ 1
A dielectric sheet 93 as a transfer material carrier made of a dielectric material having a volume resistance of about 75 μm and a volume resistance of about 10 ¹³ Ω · cm, such as poly (vinylidene vinylidene) resin (PVdF), has a cylindrical member 9a at its end. The cylindrical transfer drum 9 was formed by sticking on the 9b and the connecting member 9c using an adhesive material or the like.

The transfer drum 9 rotates in synchronization with the photosensitive drum 3, and its rotation speed is 160 mm / sec. Further, a gripper 91 for gripping the transfer material is provided on the outer surface of the connecting member 9c of the transfer drum 9.

Further, inside the transfer drum 9, a transfer charger 10 is provided on the side of the dielectric sheet 93 opposite to the photosensitive drum 3 corresponding to the transfer position.
Was provided. A corona charger is used as the transfer charger 10, for example, a voltage of +3 Kv to +9 Kv is applied, and a transfer current is +25 μA to
It was set to +500 μA. Furthermore, an inner static eliminator 13 and an outer static eliminator 11 for neutralizing the transfer material and the dielectric sheet 93 are provided on the downstream side of the transfer drum 9 in the rotational direction of the transfer position. As these static eliminators, a corona charger having a structure capable of applying a direct current, an alternating current, and a direct current to the alternating current is used.

A separating charging device 14 is provided at a position where the transfer material is separated from the transfer drum 9 at a position downstream of the static eliminator in the rotation direction of the transfer drum.

Further, in the present embodiment, as shown in FIG. 4, in order to evenly disperse the dielectric sheet 93 inside the transfer drum 9, to increase the transfer nip to increase the transfer efficiency and to obtain a clear transfer image. In the vicinity of the transfer charger 10, the dielectric sheet 93 is provided from the inside of the transfer drum 9 toward the photosensitive drum 3.
A backup member 18 is provided to push up. The backup member 18 is made of an elastic material such as polyethylene terephthalate film and applies an appropriate pressure to the dielectric sheet 93.

On the other hand, the transfer material P is fed by the transfer material cassette of the paper feeding device 23 to the transfer device 9 via the transfer material conveying system, and the gripper 91 grips the front end of the transfer material P, and the transfer material carrier 93
The toner images of the respective colors are sequentially transferred at the winding transfer position, and when the transfer of the fourth color is completed, the transfer drum 9 is separated by the separation claw 15 while receiving the charge removal by the discharge chargers 11 and 13 and the separation charger 14. Transfer material transfer system 16
Then, it is sent to the fixing device 17 to fix the image and then discharged to the tray.

The transfer process will be described in more detail using an example of the timing chart shown in FIG.

FIG. 1 shows that when the size of the transfer sheet supported on the transfer drum 9 is long in the rotation direction, for example, when the size of A3 or the like cannot be prepared for the image formation of the next color within the cycle of one rotation of the transfer drum. That is, this is an example of a timing chart in the case where the transfer drum 9 performs "idle rotation", which is rotation in which transfer is not performed.

First, in order to form a magenta (M) image of the first color, corona discharge is performed by the primary charger 4 for a period of time according to the paper size, the surface of the photosensitive drum 3 is uniformly charged, and according to the image information. Laser light is irradiated to form an M image.

On the other hand, at the developing position where the latent image of the M image is formed on the photosensitive drum 3, the M developing device, which has been moved in advance, is on standby, and the developing drive is started to visualize the M image. Development drive
The developer carrying member, which is mounted in the developing device and carries the developer in the developing device and conveys it toward the photosensitive drum, that is, the time t when the developing sleeve rises to a constant rotation speed, from the front end of the image at the developing position in anticipation of time t. Also starts to drive earlier by time t. The transfer drum is in the second rotation at this time.

At the transfer position, the transfer charger 10 is operated in accordance with the transfer timing of the transfer material held on the transfer drum 9 and the transfer member 9c of the transfer drum 9 to perform transfer charging. When the "transfer of the M image" in which the transfer drum makes one rotation and the M image is transferred to the transfer material in this state is completed, the rotary developing device rotates to prepare the C developing device in preparation for the development of the cyan (C) image. To the developing position. At this time, the transfer drum 9 enters the third rotation.

According to the present invention, a toner image is not formed on the photosensitive drum 3 at the time of so-called idle rotation like the third rotation of the transfer drum 9, but the transfer charging is the same as that at the normal transfer. Do on time. The current applied to the transfer charger during idle rotation has the same polarity as during transfer and its magnitude is 25 to 400 μA.
It is preferably 25 to 100 μA. Further, better results were obtained by applying a current equal to or less than the transfer current applied in the next transfer step (in this example, the transfer step for cyan (C) image). These are for the following reasons.

That is, when the transfer drum idles,
By applying an electric field of the same polarity as at the time of transfer from the backside of the dielectric sheet as a transfer material carrier that holds the transfer material that has already been transferred, the toner image transferred on the transfer material is generated by the force of the electric field. It becomes possible to hold it firmly on the transfer material.
This prevents the toner image on the transfer material from being offset from the toner image on the transfer material during idle rotation, or prevents the toner on the transfer material from being moved and causing the image to flow, thus disturbing the transfer image. It is also possible to apply an electric field to such an extent that the transferred toner image on the transfer material can be held during this idle rotation. Therefore, considering the transfer of the next color, the electric field is set so as to minimize the charge-up of the dielectric sheet. Better transfer can be achieved by applying the voltage. Therefore, it is preferable that the transfer current applied during idling is small, that is, about 25 to 100 μA.

At the same time, in the present embodiment, the image exposure is stopped so that the toner image on the transfer material is not electrically offset to the photosensitive drum, but the charging by the primary charger 4 is performed continuously.

When this idle rotation ends and the transfer drum enters the fourth rotation, the same process as above is repeated to transfer the C image to the transfer material.

By carrying out the above steps for the yellow (Y) image and the black (BK) image in the same manner, a transfer image of four colors can be obtained.

The present invention determines the volume average particle diameter of the toner in order to improve the image quality.
When the thickness was 10 μm or less, when the adhesion to the photosensitive drum was increased, a greater effect was exhibited. Further, when a photoreceptor containing an organic polymer having a charge transfer layer on the surface was used as the photoreceptor and the adhesive force was increased, a greater effect than the above was obtained.

Further, in the present embodiment, the transfer charger was used as the charger for applying the electric field at the time of idling, but this is not limited to the transfer charger, and a charger near the transfer area, for example, the present embodiment is used. The inner static eliminator in the example may be used, or the same effect can be obtained by preliminarily applying an electric field during idle rotation by another charger provided upstream of the transfer position in the rotation direction of the transfer drum. .

In this embodiment, all the end portions of the dielectric sheet 93 are fixed to the frame body forming the transfer drum 9 with an adhesive material or the like, but only the front and rear end portions of the dielectric sheet 93 are connected to the connecting member 9c. The same or even better image defect preventing effect can be obtained in the transfer drum which is fixed to the above and has both ends free.

As a result of a further research experiment, the inventors of the present invention provided means for detecting the in-machine environment such as temperature and humidity in the image forming apparatus shown in the above-mentioned embodiment, and idling based on the detected environmental data such as temperature and humidity. By changing the amount of current sometimes applied,
It has been found that even better results are obtained. The detailed configuration will be described below, and the principle thereof will be described.

In this embodiment, the image forming apparatus shown in the above-mentioned embodiment is used, and a member (not shown) for detecting an in-machine environment is provided in the apparatus. After the temperature and humidity data obtained by the member that detects the environment inside the machine is taken into the arithmetic circuit inside the main body and processed by this arithmetic circuit, for example, the temperature is 25 ° C and the relative humidity is 80%. When it was determined that the amount of water was present in the machine, a current of 25 to 100 μA was applied to the transfer material by the transfer charger when the transfer drum was idle, and the toner image on the transfer material was idle in all environments in idle rotation. On the contrary, it was possible to prevent the transfer image from being disturbed because the toner was offset on the surface of the photosensitive drum or the image on which the toner on the transfer material was moved flows.

 It is considered that this is due to the reason described below.

The resistance value of paper normally used as a transfer material changes depending on the atmosphere environment. Generally, the resistance value of paper is high in a low humidity environment, and is low in a high humidity environment. If the resistance value of the paper is large, the charge given by the transfer charger during transfer does not easily decay due to the large resistance value, and the toner can be held on the transfer material for a long time.
On the other hand, when the resistance value of the paper is small, the charge given by the transfer charger at the time of transfer is quickly attenuated, and it has been found that the toner on the transfer material cannot be held for a long time.

On the other hand, in the case of performing multiple transfer, it is preferable to minimize charge-up of the dielectric sheet 93 due to charging such as transfer to the previous color before transferring the second and subsequent colors, which is good transfer for the second and subsequent colors. Is essential to.

Therefore, as described above, in the high humidity environment, the present inventors have almost no influence of charge-up of the dielectric sheet. Therefore, by applying an electric field to the transfer material even in idle rotation, transfer in idle rotation is performed. It was found that offset can be prevented and good image formation can be performed. Also, in low-humidity environment, the effect of charge-up is large, and the image deterioration due to that is large, and as described above, even during idle rotation, the charge due to transfer charging during the previous rotation still remains on the back surface of the transfer material. Therefore, it has been found that, during idle rotation, good image formation without offset can be performed by not supplying the electric charge again, or by making the electric charge smaller than that in a high humidity environment.

Further, in the present embodiment, the environment is divided into two, such as high humidity and low humidity, to switch the charging at the time of idling. However, the environment may be divided into three or more, of course, continuous division. However, it goes without saying that even better results can be obtained by continuously switching the charging during idle rotation.

Further, although the transfer device is described as a transfer drum in the above embodiment, the present invention is not limited to this, and a transfer device using, for example, a belt-shaped transfer material carrier that circulates and moves. Obviously it could be used.

As described above, according to the image forming apparatus of the present invention, after the toner image of the first color is transferred to the transfer material carried by the transfer material carrier, the toner image of the first color is transferred. Second color next to
Before the toner image of the color is transferred to the transfer material carried on the transfer material carrier, the transfer material having the toner image of the first color passes through the transfer position while being carried on the transfer material carrier. During the charging period, the toner image of the second color is less than the first current applied to the transfer charging means for transferring the toner image of the second color to the transfer material carried on the transfer material carrier, and the first current having the same polarity as the first current. Since two currents are applied to the transfer charging means, the toner may be offset from the transfer material toward the image carrier, or the image on the transfer material may be moved by the toner on the transfer material to cause an image defect. While preventing, the charge-up of the transfer material carrier can be minimized, and a high-quality image can always be obtained.

[Brief description of the drawings]

FIG. 1 is a timing chart for explaining the operation of the image forming apparatus according to the present invention. FIG. 2 is a schematic configuration diagram of an embodiment of the image forming apparatus according to the present invention. FIG. 3 is a perspective view of an embodiment of the transfer device. FIG. 4 is an enlarged detailed view of the apparatus at the transfer position. 1: Developing device 3: Image carrier 9: Transfer device 10: Transfer charger

Claims (2)

(57) [Claims] 1. An image carrier capable of carrying toner images of a plurality of colors, a transfer material carrier carrying a transfer material, and the toner images of the plurality of colors on the image carrier being carried on the transfer material carrier. A transfer charging unit for sequentially superposing and transferring the transferred toner onto the transferred transfer material at the transfer position, and after transferring the toner image of the first color to the transfer material carried by the transfer material carrier, Before the transfer of the toner image of the second color, which is the next color of the toner image, to the transfer material carried by the transfer material carrier, the transfer material having the toner image of the first color carries the transfer material. In an image forming apparatus having a period of passing the transfer position while being carried by a body, in the period, the toner image of the second color is transferred to a transfer material carried by the transfer material carrying body. Less than the first current applied to the transfer charging means and having the same polarity as the first current An image forming apparatus characterized by applying a positive second current to the transfer charging means. 2. The image forming apparatus according to claim 1, further comprising a detection unit for detecting temperature and humidity in the apparatus, wherein the second current is varied according to a detection result of the detection unit.