Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
1998-07-16
2001-03-20
Brase, Sandra (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S261000
Reexamination Certificate
active
06204868
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as digital copiers, laser printers, and the like in which image recording is carried out by scanning the surface of a photoreceptor employing laser beams.
In electrophotographic digital copiers, etc., technology is widely employed in which an electrostatic latent image is written on the surface of a photoreceptor drum, etc., while deflecting laser beams employing a polygonal mirror. The photoreceptor drum of a copier is rotated so that the line speed matches to the conveyance speed of a recording sheet. In digital copiers, the speed of rotation of the polygonal mirror is fixed in accordance with the rotational speed of the photoreceptor drum and pixel-recording density.
Furthermore, when the polygonal mirror is stably rotated at high speed, recording speed and image quality of the digital copier are improved. Accordingly, technology is known in which the rotation shaft is integrated with the polygonal mirror; further, an air bearing, which is one kind of dynamic pressure bearings, is employed on the rotation shaft so as to avoid direct contact of members of the rotation shaft and the bearing each other, allowing the polygonal mirror to be stably rotated at high speed.
Now, because automation and diversification are required to meet requirements for recording sheet output, digital copiers have been increasingly demanded which enable the presentation of a variety of image-processing functions. However, at present, no digital copiers provide the image forming mode in which the speed of rotation of the polygonal mirror is varied.
For example, in analog copiers, when copying is carried out employing thick paper sheets, image forming operation is carried out by decreasing the line speed in order to obtain high quality images. However, in digital copiers, when the line speed is changed, variation in the speed of rotation of the polygonal mirror is required. Accordingly, it has been extremely difficult to provide the image forming mode which outputs good images to thick paper sheets.
The following apparatus has been known in which an electrophotographic image forming apparatus is subjected to mere variation of the speed of rotation of the polygonal mirror. In recent years, an electrophotographic apparatus has been introduced which is employed as a digital copier and a laser printer in one unit. This apparatus is termed a composite apparatus. In such composite apparatuses, the line speed is varied when employed as a digital copier and when employed as a laser printer, and thus the speed of rotation of the polygonal mirror is altered.
Generally, a polygonal mirror is very light and friction caused on the bearings is very small. In an image forming apparatus which is constituted in such a way that the decrease in the rotational speed of the polygonal mirror, which continues to rotate due to inertia is waited, the time until the rotational speed of the polygonal mirror decreases to the predetermined level can be quite long. Particularly, in a high speed apparatus which results in high productivity of image output, a decrease in friction is achieved employing air bearings so as to be in no contact of the polygonal mirror with the portion of the driving source. Accordingly, when the natural decrease in the speed of rotation of the polygonal mirror is waited, the waiting time becomes very long compared with the bearing in which the polygonal mirror is in contact with the portion of the driving source and, for example, several tens of seconds are required for the subsequent image formation. Thick paper sheets may be employed in cases, for example, when a cover is prepared employing a copied sheet of thick paper for copied sheets of normal paper or copied sheets of thick paper are employed as partition sheets and are bound, employing a finisher. In this case, switching from normal paper to thick paper is complicated. If switching from normal paper to thick paper takes several tens of seconds, a long time is required to complete continuous copying operations which decrease the imaging productivity. In addition, when employed upon switching the resolution, during switching, the speed of rotation of the polygonal mirror is occasionally decreased and the waiting time becomes longer.
SUMMARY OF THE INVENTION
An object of the present invention is to decrease a waiting time, in which during the image-writing operation, in the case of varying the speed of rotation of the polygonal mirror in accordance with the variation of the line speed of a photoreceptor, the speed of rotation of the polygonal mirror is reduced in a short period of time.
Furthermore, another object is to improve the image output productivity in the case of performing a series of image forming operations to obtain a plurality of output images onto different kinds of sheets.
Furthermore, the polygonal mirror is light in weight, and rotates at high speed. When a brake shoe, lubricating oil, dust, etc. attach or adhere to it as attached or adhered matter, the rotation balance is not sustained and possibly, normal image formation may not be carried out.
Accordingly, still another object of the present invention is to eliminate the adhesion of foreign matter to the polygonal mirror or rotation shaft, which results in a braking action to decrease the speed of rotation of the polygonal mirror upon increasing the output productivity by shortening the switching time for image formation.
Furthermore, when the speed of rotation of the polygonal mirror is forcibly decreased in a short time instead of decreasing the frequency as the mirror rotates, there may be a period when the rotation of the polygonal mirror is not stabilized. When the image formation is performed during such a period, the image quality may be remarkably deteriorated.
A further object of the present invention is to perform image formation so as to obtain excellent image quality, even though the speed of rotation of a polygonal mirror is forcibly decreased.
The above-mentioned objectives are accomplished by any one of constitutions mentioned below.
(1) An image forming apparatus in which exposure is carried out employing laser beam scanning comprises the following constitutions:
a photoreceptor carrying a latent image;
an exposure means comprising a polygonal mirror by rotating this polygonal mirror, laser beam is deflected so as to scan the above-mentioned photoreceptor;
a means to control the linear speed of the photoreceptor in accordance with the image forming mode;
a braking means to decrease forcibly the speed of rotation of the polygonal mirror, wherein when the line speed of the photoreceptor is herein decreased employing the above-mentioned control means, the above-mentioned braking means decreases the speed of rotation of the polygonal mirror.
(2) laser scanning device in which exposure is carried out employing laser beam scanning comprises the following constitutions:
a laser beam generating means to generate a laser beam;
a polygonal mirror to deflect the laser beam;
a driving means to rotate the polygonal mirror; and
a braking means to decrease the speed of rotation of the polygonal mirror, wherein the braking means decreases forcibly the speed of rotation of the polygonal mirror.
REFERENCES:
patent: 4578689 (1986-03-01), Spencer et al.
patent: 5512927 (1996-04-01), Okamoto
patent: 5589871 (1996-12-01), Aoki et al.
patent: 5900903 (1999-04-01), Fukita
patent: 5946122 (1999-08-01), Itoh et al.
patent: 5995130 (1999-11-01), Akatsu et al.
Kobayashi Hiroshi
Yamauchi Kazumichi
Brase Sandra
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Konica Corporation
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