Direct printing apparatus with first and second rollers

Details Direct printing apparatus with first and second rollers Direct printing apparatus with first and second rollers

1999-07-07

2001-09-11

Gordon, Raquel Yvette (Department: 2853)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06286937

ABSTRACT:

This application is based on application No. H10-194160 filed in Japan on Jul. 9, 1998, the content of which is hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to a direct printing apparatus for use in copying machine, printer and so on.
BACKGROUND OF THE INVENTION
Hitherto, as a kind of direct printing apparatus for directly forming an image on a sheet, an apparatus which forms the image by using printing particles such as toner has been known. The direct printing apparatus comprises a developing device for bearing toner to serve development, the developing device having double rollers comprising a first roller and a second roller, a backing electrode opposed to the second roller of the developing device, and a printing head disposed between the backing electrode and the second roller, the printing head having a plurality of apertures and control electrode surrounding the plurality of apertures. To the first roller of the developing device is applied a voltage V
1
, while to the second roller is applied a voltage V
2
. To the backing electrode is applied a voltage V
BE
, thereby an electric field is generated between the second roller and the backing electrode for propelling the toner toward the backing electrode side from the second roller through the apertures. To the control electrode of the printing head is selectively applied either a printing voltage Vb having a reverse polarity to the toner or a non-printing voltage Vw having a same polarity as the toner.
FIG.
4
(A) shows an electric potential distribution between the first roller, the second roller and the control electrode of the conventional direct printing apparatus. When applying the voltage Vb (for example, +300 volts) having a reverse polarity to the toner to the control electrodes, the toner on the second roller to which a voltage of, for example, V
2
=0 is applied (i.e., connected to the earth) is propelled. On the other hand, when applying the voltage Vw (for example, −50 volts) having a same polarity as the toner to the control electrodes, the toner is never propelled. Thus, when the applying voltage Vb, Vw to the control electrodes of the printing head are changed on the basis of a desired image signal, a toner image corresponding to the image signal is formed in the sheet.
If the toner charged quantity is large, the adhesive strength to the second roller becomes large. Therefore, the toner is difficult to be propelled, thereby desired image density is not obtained. So, in the case that the toner charged quantity is large, the following provision has been made. As shown in FIG.
4
(B), the applied voltage V
1
to the first roller is enlarged in comparison to the case that toner charged quantity is small. Thereby, movement of the toner to the second roller from the first roller is accelerated so that adhered toner on the second roller becomes large in quantity, i.e., becomes multilayered. Thus, the adhesive strength of the surface layer toner to the second roller becomes small so that the toner is easy to be propelled, thereby desired image density is obtained.
However, the surface layer of the multilayered toner on the second roller, as shown in FIG.
4
(B), is elevated to the same electric potential (−200 volts) as that of the first roller. Thus, even at the non-printing portion, a few of the toner on the second roller is propelled, causing fog in the background of the image.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been accomplished to solve the aforementioned disadvantages of the prior arts. An object of the present invention is to provide a direct printing apparatus which is possible to form good image without background fog even if toner charged quantity is large.
In order to achieve the aforementioned object, according to the present invention, there is provided a direct printing apparatus for directly depositing printing particles on a print medium to print an image, comprising:
a bearing member for bearing printing particles thereon, the printing particles being charged to a predetermined polarity;
a backing electrode opposed to the bearing member; and
a printing head disposed between the bearing member and the backing electrode, the printing head having a plurality of apertures through which the printing particles can propel and a plurality of control electrodes disposed around the plurality of apertures;
wherein the bearing member comprises a first roller and a second roller which comes into contact with the first roller and is opposed to the backing electrode; and
wherein a voltage having same polarity as the printing particles borne on the bearing means and having electric potential more than that of surface layer of the printing particles is applied to the control electrodes corresponding to non-printing portions.
Preferably, the voltage Vw applied to the control electrodes corresponding to non-printing portions meets the following relationship:
|
Vw−V
2
|≧|
V
1
−
V
2
|
where V
1
is a voltage of the first roller, and V
2
is a voltage of the second roller.
In the direct printing apparatus according to the present invention having above construction, as a voltage having same polarity as the printing particles borne on the bearing means and having electric potential more than that of surface layer of the printing particles is applied to the control electrodes corresponding to non-printing portions, an electric field having a direction to which the printing particles are not propelled to the control electrodes is generated, enabling to form good image without background fog even if toner charged quantity is large.


REFERENCES:
patent: 4814796 (1989-03-01), Schmidlin
patent: 5477250 (1995-12-01), Larson
patent: 5988794 (1999-11-01), Tagaki
patent: 6151475 (2000-11-01), Uno

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