Color image forming apparatus

Details Color image forming apparatus Color image forming apparatus

2002-12-05

2004-01-06

Pendegrass, Joan (Department: 2852)

Incremental printing of symbolic information

Electric marking apparatus or processes

Electrostatic

C347S116000, C347S232000, C347S234000

Reexamination Certificate

active

06674454

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a color image forming apparatus using the electrophotography technique. More specifically, the invention relates to a so-called tandem-type color image forming apparatus, wherein images are formed on respective image carriers with light scanned thereon in a primary scanning direction by a rotary polygon mirror while rotating them in a secondary scanning direction, and the formed images are superimposed one after another on an object of transfer to be moved in the secondary scanning direction. In particular, the invention relates to a countermeasure against a misregistration in colors of the images.
As described in, e.g., Japanese Patent Publication Nos. 5-270051A, 6-286226A, and 2000-347116A, a tandem-type color image forming apparatus of single rotary polygon mirror type has been known. A plurality of laser beams are deflected to perform a primary scanning by use of a single rotary polygon mirror, whereby the beams are separated from each other by a beam separation optical system constituted of a reflection mirror or the like. The thus-separated beams are guided to image carriers of corresponding colors, thereby forming images (latent images). As described in, e.g., Japanese Patent Publication Nos. 62-242471A and 7-97244B, there has also been known a tandem-type color image forming apparatus of multiple rotary polygon mirror type, wherein a laser scanning unit, comprising a laser light source, a rotary polygon mirror, and a scanning optical system, is provided for each image carrier. A rotary polygon mirror subjects a laser beam to deflection scanning (primary scanning) on a per-color basis, thereby forming an image on an image carrier of corresponding color.
In a color image forming apparatus of either of the above types, the images (latent images) formed on the respective image carriers are developed by a developer. The images are superimposed one after another on objects of transfer (intermediate transfer members or recording medium, such as paper) to be moved in a direction in which the image carriers are to be rotated (i.e., a secondary scanning direction). If the images formed on the respective image carriers are not correctly registered with each other, a misregistration arises in colors, thus deteriorating picture quality.
To solve the problem, as shown in
FIG. 15
, there has already been proposed a color image forming apparatus (as described in Japanese Patent No. 2608080), wherein a plurality of rotational image carriers
1
(Y, M, C) are subjected to scanning in a primary scanning direction (an axial direction of the image carriers, that is, in the direction of a generating line), thereby forming images on the respective image carriers
1
(Y, M, C). The images are sequentially superimposed on paper S to be moved in a secondary scanning direction in which the image carriers are to rotate, thereby forming a color image.
A scanning period of each scanning surface of a rotary polygon mirror
2
is taken as Ts (see FIG.
16
). A time T
1
is defined as the time lapsing while an image formed on the upstream image carrier
1
(Y) from among the plurality of image carriers, for example,
1
(Y) and
1
(M) is moved from an image formation position
1
a
(Y) to a transfer position
1
b
(M) located between the downstream image carrier
1
(M) and an object of transfer.
A time T
2
is defined as the time lapsing while the image formed on the downstream image carrier
1
(M) from among the plurality of image carriers is moved from an image forming position
1
a
(M) to a transfer position
1
b
(M) at which the image is to be transferred onto an object of transfer.
Under these conditions, the plurality of image carriers
1
(Y, M, C) are arranged such that T
1
−T
2
≅nTs stands. The publication states that the range of “≅” is about ±20% of Ts. Here, “n” represents natural numbers.
In
FIG. 15
, L(Y), L(M), L(C) denote laser beams to be used for subjecting the image carriers
1
(Y),
1
(M),
1
(C) to primary scanning. A belt
4
is an object of transfer to be used for transporting paper S in a secondary scanning direction.
The related-art apparatus yields the following advantage.
Namely, as shown in
FIG. 16A
, each of rotary polygon mirrors
2
(Y),
2
(M),
2
(C) is assumed to be an octagonal prism which is rotated in the direction of the arrow and has scanning surfaces P
1
through P
8
. The image carrier
1
(Y) is sequentially subjected to primary scanning through use of the scanning surfaces P
1
through P
8
of the rotary polygon mirror
2
(Y) (see FIG.
15
), thereby forming images p
1
to p
8
on the image carrier
1
(Y). The images p
1
through p
8
are transferred to the paper S. As shown in
FIG. 16C
, the image carrier
1
(M) is also sequentially subjected to primary scanning through use of the scanning surfaces P
1
through P
8
of the rotary polygon mirror
2
(M) (see FIG.
17
), thereby forming images p
1
through p
8
on the image carrier
1
(M). The images p
1
through p
8
on the paper S and the images p
1
through p
8
formed on the image carrier
1
(M) are substantially in phase with each other at a transfer position
1
b
(M) at which images are to be transferred from the downstream image carrier
1
(M) to an object of transfer. Hence, misregistration of colors is prevented. In this case, there is prevented occurrence of a great misregistration between the images p
1
through p
8
formed on the image carrier
1
(Y) and the images p
1
through p
8
formed on the image carrier
1
(M) with respect to the secondary scanning direction (i.e., the direction in which the object of transfer S is to be moved).
FIGS. 17A and 17B
are views schematically showing writing of an image onto the image carrier
1
performed by the rotary polygon mirror
2
.
As shown in
FIG. 17A
, the rotary polygon mirror
2
, a shaft
2
a
of the mirror
2
, and a support member
2
b
of the mirror
2
involve production errors. A rotation center O
2
of the rotary polygon mirror
2
is inclined with respect to an ideal rotation center O
1
(an angle of inclination is represented by &thgr;).
For this reason, an image to be written (formed) on the image carrier
1
by the laser beam reflected from respective scanning surfaces of the rotary polygon mirror
2
will become deviated from an ideal writing position p
0
(see
FIG. 17B
) with respect to the direction of movement of the image carrier
1
(i.e., the direction designated by an arrow; that is, the secondary scanning direction). Here, reference numeral
3
designates a scanning lens.
A maximum misregistration arises in the secondary scanning direction between images formed by mutually-opposing scanning surfaces (e.g., even-numbered surfaces which are 180° out of phase with each other). For instance, the rotary polygon mirror
2
is assumed to be an octagonal prism (see
FIG. 16A
) having scanning surfaces P
1
through P
8
. As illustrated, maximum misregistration arises between an image p
1
formed by a scanning light L (P
1
) reflected by the scanning surface P
1
and an image p
5
formed by a scanning light L (P
5
) reflected by the scanning surface P
5
. Similarly, maximum misregistration arises between images p
2
and p
6
; between images p
3
and p
7
; and between images p
4
and p
8
.
Such a misregistration in the secondary scanning direction is also caused by inclination of the scanning surfaces with respect to the rotation center O
2
of the rotary polygon mirror
2
.
According to the related-art technique shown in
FIG. 15
, the plurality of image carriers
1
(Y),
1
(M),
1
(C) are arranged such that T
1
−T
2
≅nTs stands. Hence, misregistration between colors becomes unlikely to arise. Specifically, as shown in
FIGS. 16C and 16D
, great misregistration becomes unlikely to arise in the secondary scanning direction between the images p
1
through p
8
formed on the image carrier
1
(Y) and the images p
1
through p
8
formed on the image carrier
1
(M). According to the related-art technique, the related-art apparatus is constructed so as to achieve th

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