Electrophotography – Control of electrophotography process – Of plural processes
Reexamination Certificate
2001-12-21
2003-08-05
Chen, Sophia S. (Department: 2852)
Electrophotography
Control of electrophotography process
Of plural processes
C399S046000, C399S057000, C399S058000, C399S072000
Reexamination Certificate
active
06603934
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus that forms a toner image by use of a liquid developer in which toner is dispersed in a solvent.
An image forming apparatus is exemplified by a copying machine which utilizes the electrostatic copying process. This type of copying machine forms an electrostatic latent image on a photosensitive body. The electrostatic latent image corresponds to image information on an original, i.e., an object to be copied, and the image information is sent to the photosensitive body as light-and-shade information. The electrostatic latent image is then visualized by the toner, i.e., a developer. As one method for supplying toner to the electrostatic latent image, the use of the liquid developer in which toner is dispersed in a solvent is known.
In comparison with an image forming apparatus using a dry developer, an image forming apparatus using a liquid developer is advantageous in that the toner it uses is made of small-diameter particles. The use of such toner helps improve the image quality (high image quality), and increases the image forming speed. In addition, the liquid developer is advantageous in that it ensures improved gradation characteristics and enables production of images image that are very high in resolution like printed process. Moreover, the toner has a low melting point, and a toner image can be fixed easily.
However, since the developer is in the liquid state and includes toner particles and a carrier liquid, there is still room for improvement with respect to the method in which the developer is supplied and the method in which a residual developer liquid (a carrier liquid functioning as a solvent) remaining on a photosensitive body is removed. In particular, this holds true for an image formation method in which a color image is obtained by superposing toner images of three or four colors on a photosensitive body and then simultaneously transferring them onto a transfer material. In this image formation method, the developer image of the first color must not mix with the developer images of the next colors after development of the first color. To achieve this, the amount of solvent included in the developer image on the photosensitive body has to be reduced, for example, by non-contact wring, contact wring, drying by air supply, etc.
A wet-type image forming apparatus using a liquid developer has problems in that the image density (image quality) of a toner image formed on the photosensitive body and that of an image (toner) transferred onto a transfer material are not stable.
To solve this problem, a patch of predetermined size (a test image) is formed on a photosensitive body. After the patch is subject to development, the amount of toner attached to the patch is measured, and the measurement is fed back when a toner replenishment operation is performed or when a toner consumption condition (developing condition) is determined. This kind of control is well known in the art.
For example, U.S. Pat. No. 4,082,445 discloses a method in which the amount of toner attached to a photosensitive body is measured by checking the amount of light reflected from a non-image portion of the image bearing member (i.e., the photosensitive body) and the amount of light reflected from a toner layer obtained by developing a latent image and comparing the difference between these amounts with a reference value.
It should be noted, however, that the amount of toner attached to the photosensitive body (i.e., the total amount of toner constituting a toner layer) and the absorption index of light do not vary linearly (non-linear). Let us assume that the surface of the photosensitive body is completely covered with toner particles. In this state of (toner) layer, the reflection factor of light hardly varies without reference to the number of toner layers formed.
In contrast, in the state where the surface of the photosensitive body is partly exposed between toner layer portions, the reflection by the surface of the photosensitive body is inevitably sensed as the reflection factor of the light falling on the toner layer. This being so, the amount of toner attached cannot be accurately measured. It should be also noted that the reflection by the surface of the photosensitive body is dependent on changes in the surface roughness of the photosensitive body, changes in the thickness of the photosensitive layer of the photosensitive body, the occurrence of filming of toner, etc.
With respect to color toner, there may be a case where the thickness of a toner layer cannot be measured, depending upon the combination between the spectral reflection characteristics of the toner and the surface of the photosensitive body. In addition, the wavelength of measurement light must be changed in accordance with the color of toner.
As can be seen from the above, the method shown in U.S. Pat. No. 4,082,445 does not necessarily enable accurate measurement of the amount of toner attached to the photosensitive body.
Jpn. Pat. Appln. KOKAI Publication No. 8-327331 discloses a developer amount measuring method for use in a dry-type image forming apparatus. This method employs a lens that provides different image formation positions in accordance with the thickness of a toner layer. An optical position-detecting element detects the variation (difference) in the image formation positions of the lens.
However, toner particles used in wet-type development are very fine; they are in the range of 0.2 to 1.5 &mgr;m. Even if the amount of toner attaching on a transfer medium changes to such an extent as to change the reflecting density of an image transferred onto that medium, the position where light is focused on the toner layer does not significantly change. Therefore, a change in the amount of toner adhering is hard to detect.
Jpn. Pat. Appln. KOKAI Publication No. 8-87144 shows a method for measuring the thickness of a toner layer. According to the publication, the potential of the toner layer obtained by development is measured to detect the thickness of the toner layer. The publication does not describe anything regarding wet-type development, and in view of the drawings and an embodiment, the publication is considered to relate to dry-type development.
However, the toner used for wet-type development does not contain a high proportion of resin. In comparison with the toner used for dry-type development, the potential of a toner layer formed with the toner for wet-type development is likely to be affected by pigments. The toner used for wet-type development does not produce such a toner layer thickness as shown in Jpn. Pat. Appln. KOKAI Publication No. 8-87144. In particular, when magenta toner is used, there is no substantial potential difference between a toner layer and a latent image. Even if the potential at the toner layer is utilized, the amount of toner attaching cannot be detected with high precision.
As discussed above, none of the techniques shown in the known publications enable accurate detection of the thickness of a toner layer formed on a photosensitive body after development as long as the development is wet-type development using fine toner particles whose average particles diameter is in the range of 0.2 to 1.5 &mgr;m. (In other words, there is no established detection method.) In particular, in the case of wet-type development of color images, wherein toner images of three or four colors are superposed on a photosensitive body and are simultaneously transferred to a transfer medium, the development of each color is followed by the reduction of the amount of solvent contained in the developer image formed on the photosensitive body, so as to prevent color mixing. For example, the non-contact or contact wring of the solvent, the drying of the solvent by air supply, etc. are repeatedly executed, so that the developer image of each color may not mix with the developer images of the subsequent colors after development of each color. For this reason, the sensing of the thickness of a toner layer is much more difficult.
Chen Sophia S.
Foley & Lardner
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