Image forming apparatus

Details Image forming apparatus Image forming apparatus

1998-11-02

2002-12-10

Pham, Hai (Department: 2861)

Recorders

Markers and/or driving means therefor

With ink supply to marker

C347S103000, C347S158000

Reexamination Certificate

active

06493009

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus applicable to copying machines, facsimile apparatuses, printers and the like.
2. Description of the Related Art
In the conventional electrophotographic image formation process, pixel position shift occurs due to some factors. In a microscopic view, the pixel position shift is positional shift of one toner particle, i.e., scatter of toner, movement faithfully corresponding to electric field of a latent image, or transfer failure; in a macroscopic view, the pixel position shift is a systematic failure as color shift in a copying machine and a printing apparatus.
First, a description will be made on a problem in transfer of toner onto a printing medium by an electric charge q held by the toner and an electrostatic force F=q·E applied among a photoreceptor body, developing electrodes, a sheet, and an intermediate transfer body.
FIG. 34A
shows a spatial electric field caused between a well-regulated rectangular latent image on the photoreceptor body and developing electrodes (The 104th issue of the society of Electrophotography of Japan, Vol. 32 No. 3., 1993). An equipotential line is not completely parallel to the surfaces of the photoreceptor, the developing electrodes and the like, since the photoreceptor body and the developing electrodes are not complete parallel electrodes and an electric force line deflects due to edge effect of edge portions of the rectangular latent image.
FIG. 34
shows a profile of contrast potential in case of shift from the center of the latent image in a process direction (photoreceptor-body rotation direction) X and in an object electrode direction (gap) Y. Note that latent images having a uniform charge density of 100 &mgr;m in length in the process direction are provided at 100 &mgr;m intervals on the photoreceptor body.
FIG. 35
shows the distribution of contrast potential upon scanning in the direction Y. In
FIG. 35
, a solid line a denotes a position at the center of the central latent image; a dotted line b, an intermediate position between the central latent image and its adjacent latent image; and an alternate long and short dashed line c, the result of scanning at a point 70 &mgr;m away from the outer end of the outermost latent image. These calculation results show that the maximum point of the contrast potential lowers as it becomes away from the center of the latent image in the process direction, and the point is becoming shifted between the photoreceptor body and the object electrode. In an actual developing process, development is not started at the latent image center (X=0), but at a point where the potential reaches an effective electric-field threshold value (an effective electric-field threshold value determined by the electric field of the photoreceptor body, the electric charge of toner, and the bias of developing electrode) within a developing nip. Accordingly, the result of development is similar to development shifted in the direction X as shown in FIG.
35
.
The toner receives an electrostatic force for transfer and movement, however, as the contrast potential is low and the development is made at the shifted position, the toner cannot be brought to the original pixel position. This is an inherent problem of electrophotographic technique based on the electrostatic dynamics.
On the other hand, in the macroscopic view, the above technique has problems of color shift and color registration. In a low frequency band, color shift occurs, while in a high frequency band, regular stripes (hereinafter referred to as “banding”) occurs in a process direction, greatly depending on the rotation or moving precision (hereinafter referred to as “motion quality”) of a photoreceptor body, a transfer drum and a sheet conveyance belt generally used in copying machines and printers. The color shift and banding due to bad motion quality appear in a highly precise halftone representation or color characteristic representation as a defect of the image, which greatly degrades the image quality. For this reason, in the conventional copying machines and printers, feedback or feedforward control is performed, or the mechanical precision of constituent parts and assembling is improved so to attain high motion quality in the photoreceptor body and the conveyance belt. Further, various problems of time variation, environmental change, especially change in light quantity of recording-information writing unit due to temperature change, and shift of synchronism among the respective mechanical structures (e.g., in a tandem type copying machine or printer, the ratio between the diameter of a roller for conveying a conveyance belt and the distance between the roller and a photoreceptor body is set to be an integer, however, it changes due to the apparatus frame and temperature expansion of the roller), must be solved. The above problems cause enlargement in apparatus size, complication and a high cost, and poses a limitation on the conventional electrophotographic technique as a printing technique so as not to lower reliability.
However, in the field of printing technique, by solving the problem of pixel position shift, high precision is attained at a cost of high apparatus price. Basically, as a plate is used, pixel position shift in an image writing process as in the electrophotographic printing does not occur. Further, color printing of yellow, magenta, cyan and black is performed in a stable manner from a plate, then a blanket, to a print sheet, by using a physical phenomenon “thixotropy” characteristic of printing ink. Further, the plate, the blanket, a sheet support and conveyance member, having high reliability and durability, are driven by high-precision helical gears and the like. Further, the pixel position shift in high quality printing is 15 to 30 &mgr;m on an average by performing pixel-position shift regulation by an expert operator. However, as the enlargement of apparatus size and high price cannot be avoided, this printing technique is not sufficiently applicable to on-demand printing as a printing technique for business use or office use.
A printing technique Oce3125C, reported in CeBIT '96, is known. This technique visualizes toner by bias development without an electrostatic latent image on a photoreceptor body.
FIG. 36
shows a cross section of an imaging drum. Ring electrodes
70
are provided in 400 dpi in an insulating layer
71
in its axial direction. The surface of the imaging drum is covered with a dielectric layer
69
.
FIG. 37
shows the construction of the entire apparatus. Alphabets “K”, “B”, “R”, “G”, “M”, “C” and “Y” added to respective reference numerals denote respective colors. When a part is representatively used in the description, such alphabet will be omitted. Each imaging drum
72
has a matrix-arranged fixed electrodes for transmitting printing information from a slip ring (not shown). Toner moves from a developing roller
74
, to which an electric field is applied in accordance with the printing information, to the imaging drum
72
and forms an image. The imaging drums
72
, i.e., black imaging drum
72
B to yellow imaging drum
72
Y, are tandem-arranged around a transfer drum
73
, and developing rollers
74
, i.e., black developing roller
74
B to yellow developing roller
74
Y, are arranged at the respective imaging drums. A printing process starts with development. The respective seven color toner images are sequentially formed, each in single layer, on the imaging drums
72
. Next, the respective color toner is transferred onto the intermediate transfer drum
73
without overlapping the respective color toner, and a print sheet is conveyed from a paper cassette
58
, timed with the front end of the image on the intermediate transfer drum
73
. The image on the intermediate transfer drum
73
is transferred by a secondary transfer roller
75
onto the print sheet, and the print sheet is discharged onto a discharge tray
62
.
However, in this technique, pixel position is not contr

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