Electrophotography – Image formation – Development
Reexamination Certificate
1999-06-24
2001-10-30
Braun, Fred L. (Department: 2852)
Electrophotography
Image formation
Development
Reexamination Certificate
active
06311034
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a wet type electrophotography apparatus using a non-volatile, high-viscosity liquid toner, and more particularly to a wet type electrophotography apparatus in which blush-free, high-quality images can be developed by feeding and applying a developing solution evenly onto a developing roller and applying a film of prewetting solution evenly onto a photoconductive medium, and a residual toner layer on the developing roller after development can be stably recovered.
2. Description of the Related Art
As for electrophotography apparatus in which an electrostatic latent image is formed on a photoconductive medium (photoconductive drum), a toner is caused to adhere to the charged image, and the powder image is then transferred and thermally fixed onto a printing medium, such as paper, the dry type using a powder toner has been widely employed.
The powder toner, however, tends to be scattered and often involves the problem of poor resolution due to its particles having sizes as large as 7~10 &mgr;m.
In applications requiring high resolution, therefore, a wet type electrophotography apparatus using a liquid toner is usually adopted. The liquid toner is less subject to distortion in toner images and can achieve high resolution because it contains toner particles as small as 1 &mgr;m and has a large charging capacity.
In the conventional wet type electrophotography apparatus, a low-viscosity liquid toner obtained by mixing 1~2% of toner in an organic solvent has been commonly used as the developing solution. This type of developing solution, however, causes environmental concerns because it contains an organic solvent harmful to human bodies and requires a large amount of toner consumption due to low concentration.
It was against this background that a wet type electrophotography apparatus using a high-viscosity, high-concentration developing solution obtained by dispersing a high-concentration toner in silicone oil, etc. was disclosed in International Disclosure Number “WO95/08792.”
The use of this liquid toner eliminates not only the harm to human bodies but also the need for large consumption of developing solution due to high toner concentration.
As disclosed in “International Disclosure Number WO95/08792,” the wet type electrophotography apparatus using high-viscosity, high-concentration liquid toner as the developing solution has employed the same construction as with the dry type apparatus using a powder toner, in which a developing solution is applied to a developing roller or belt, which is brought into contact with a photoconductive medium to cause the toner to deposit on the photoconductive medium on which an electrostatic latent image is formed.
That is, as in the dry type where the powder toner is applied to the developing roller using a purely mechanical contact means, the wet type using a non-volatile, high-viscosity, high-concentration liquid toner also employs a purely mechanical contact means in applying the developing solution to the developing roller. The application of the developing solution is accomplished by finishing the surface which makes mechanical contact with the developing roller to a high-precision finish.
That is, development is carried out by applying a liquid toner onto a developing roller
22
, causing the developing roller
22
to face a photoconductive medium
10
and applying a voltage to the developing roller
22
, as shown in FIG.
25
. At this time, an image area and a non-image area (background area) are formed randomly in accordance with an image pattern on the toner layer on the developing roller
22
after development. Rollers 23~25 in
FIG. 25
are a series of applicator rollers for feeding the developing solution to the developing roller
22
. These rollers are driven by drive motors
31
and
33
, and gears
32
and
34
.
If the image area and the non-image area on the developing roller
22
after development, which have different electrical histories on the toner and different amounts of residual toner, are brought as they are to the contact area at which the developing roller
22
makes contact with the final-stage applicator roller
23
, the evenness of the toner layer formed on the developing roller
22
becomes a problem. Even if the toner layer is formed evenly, the image pattern at the previous rotation of the developing roller
22
may appear on the image in synchronism with the period of the roller due to the electrical history experienced by the toner. Such a phenomenon is a trouble often referred to as the development memory. It is necessary therefore to have a reset mechanism for erasing the history left as the result of a development operation.
A problem characteristic of the liquid developing process is that when a prewetting solution is used as a release agent to prevent blushing, the prewetting solution deposited on the developing roller
22
causes the dilution of the developing solution. To cope with this problem, a rubber blade
37
has heretofore been used to scrape off the residual toner layer on the developing roller
22
.
As a means to solve the trouble called the development memory and the dilution of the developing solution, the use of the rubber blade to scrape off the residual toner layer is effective only when the blade is used to wipe off something on a surface of a relatively hard material, as with the automotive windshield wiper. However, when the rubber blade is used to scrape off the residual toner on a developing roller of a relatively soft developing roller, the contact pressure between both cannot be satisfactorily maintained.
The pressure between the blade and the roller can be considerably increased by forcing the edge of the blade onto the roller in the “doctor” direction (“biting” direction in which the blade faces the roller at an acute angle). However, but when used with a soft roller with a low hardness, the doctor-blade effect is drastically reduced, leading to an increase in the amount of toner left on the roller.
As disclosed in International Disclosure Number “WO95/08792,” when a developing device is constructed using a flexible developing belt, adjustment of the developing belt is extremely difficult, requiring much labor and time. Too tight a developing belt would reduce the flexibility, making the belt too rigid. This makes it impossible to realize the double-layer construction of the liquid toner applied to the developing belt and the prewetting solution layer applied to the photoconductive medium. Too loose a developing belt, on the other hand, would cause a gap between the liquid toner on the developing belt and the prewetting solution layer applied to the photoconductive medium, preventing the movement of the toner. In this way, a developing device using a developing belt would make the adjustment of the developing belt difficult and time-consuming.
When a high-viscosity liquid developing agent is used, the following requirements must be satisfied to form a thin film of the developing agent through the use of a contact roller while maintaining the prewetting solution layer undisturbed. First, the total amount of developing and prewetting agents carried along by the rotation of the photoconductive medium and the developing roller must be passed at a pressure below the contact pressure at the contact area between the developing roller and the photoconductive medium. The lower the hardness of the developing roller the more liquid can be passed. However, the higher the dimensional accuracy of the outside diameter and the wobbling accuracy during rotation of the developing roller, the more stably the pressure on the liquid layer is maintained. In terms of machining, the lower the hardness the more difficult becomes the improvement of machining accuracy.
Second, there must be a prewetting oil layer at all times over the entire region between the developing agent and the surface of the photoconductive medium so as to ensure that the high-viscosity developing agent is kept from directly contacting the sur
Ichida Motoharu
Inamoto Akihiko
Moto Satoru
Nakashima Yutaka
Okano Shigeharu
Braun Fred L.
PFU Limited
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