Image forming apparatus and image forming head

Details Image forming apparatus and image forming head Image forming apparatus and image forming head

2001-05-11

2002-12-31

Gordon, Raquel Yvette (Department: 2853)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06499830

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an image forming apparatus and an image forming head for use in, e.g., a printer for computers, facsimiles, copying machines and the like, for driving charged particles from a charged-particle supporting element onto an image receiving medium according to an image signal.
BACKGROUND ART
In this type of image forming apparatuses, an insulating member having a plurality of openings for passing charged particles therethrough is provided between a supporting element for supporting and carrying the charged particles and an opposing electrode, an image receiving medium is provided between the opposing electrode and the insulating member, and a control electrode is provided around each opening. For image formation, a potential difference is provided in advance between the supporting element and the opposing electrode so as to form an electrostatic field for transferring the charged particles from the supporting element toward the opposing electrode. A voltage applied to the control electrodes is controlled to electrostatically open and close the openings, whereby the charged particles are separated from the supporting element onto the image receiving medium through the openings, according to an image signal.
Japanese Laid-Open Publication No. 58-122882 describes that, upon detecting absence of an image receiving medium, spark discharge is generated between control electrodes and an opposing electrode or the like by application of a high voltage to the control electrodes, so as to blow off the toner stuck in openings.
Japanese Laid-Open Publication No. 58-104769 describes that, when image formation is not conducted, an electric field between control electrodes and an image receiving medium is increased so as to remove the toner stagnant in openings toward the image receiving medium.
Japanese Laid-Open Publication No. 58-104771 describes that, during image recording, an electric field between a supporting element and an image receiving medium as well as an electric field in openings are produced in the traveling direction of charged particles toward the image receiving medium, and during non-recording, respective electric fields between the supporting element and a control member and between the control member and the image receiving medium are produced in the opposite direction to that during recording, whereby clogging of the openings with the toner is prevented.
However, in the aforementioned method for removing the charged particles in the openings by spark discharge, an insulating member having the openings may be damaged by the spark discharge if it is formed from a synthetic resin. Moreover, an additional power supply is required to generate the spark discharge, and the charged particles may possibly be fusion-bonded on the insulating member due to heating by the spark discharge.
Moreover, in the method for removing the charged particles in the openings when no image is formed (during non-recording), the charged particles stuck in the openings cannot be removed, if any, during the image formation, i.e., while the charged particles are being sequentially attached to, e.g., a sheet of recording paper according to an image signal. Moreover, the aforementioned methods all require a special voltage-application mode to remove the charged particles in the openings, and also require a special power supply, thereby often increasing the costs.
In other words, it is an object of the present invention to prevent clogging of the openings without voltage control of the control electrodes.
DISCLOSURE OF THE INVENTION
The present invention solves the aforementioned problems by applying ideas to the area of the opening.
More specifically, according to the present invention, an image forming apparatus for forming an image by attaching charged particles to an image receiving medium includes:
a charging means for applying charges to particles for forming an image;
a supporting element for supporting and carrying the charged particles having the charges applied thereto by the charging means;
an opposing electrode provided at a position facing a position on the supporting element to which the charged particles are carried;
an insulating member provided between the supporting element and the opposing electrode and having a plurality of openings for passing the charged particles therethrough;
a control electrode provided around each opening of the insulating member;
a transfer-electrostatic-field forming means for providing a potential difference so as to form between the supporting element and the opposing electrode a transfer electrostatic field for transferring the charged particles on the supporting element toward the opposing electrode; and
a voltage control means for applying a voltage to the control electrodes around the respective openings according to an image signal so as to control passage of the charged particles through the respective openings that is caused by the transfer electrostatic field, wherein
a percentage of an area of the opening to a sum of an area of an extent of the control electrode around the opening and the area of the opening is 8% or more.
The present invention will be specifically described. The inventor arranged ring-shaped control electrodes surrounding respective circular openings, and applied a pulsed driving voltage to the control electrodes so that the control electrodes each have, at a respective opening position, an intermediate potential of the voltage difference between an opposing electrode and a developing sleeve (supporting element), thereby causing charged particles on the developing sleeve to be intermittently driven onto an image receiving medium through the openings. The inventor then observed the resultant driving trace on the developing sleeve (the trace where the charged particles have gone away). This driving trace did not have a circular shape corresponding to the shape of the opening, but a large circular shape corresponding to the outer shape of the control electrode. In other words, dots are not only formed from the charged particles present on a location of the developing sleeve that corresponds to the opening, but the charged particles present on a location of the developing sleeve that corresponds to the control electrode also contributes to the dot formation. This means that not only the charged particles present on the location of the developing sleeve that corresponds to the opening, but also the charged particles present on the location of the developing sleeve that corresponds to the control electrode are separated away from the developing sleeve toward the opening and pass therethrough, in response to application of the driving voltage to the control electrode.
The reason why the charged particles present on the location of the developing sleeve that corresponds to the control electrode move toward the opening is as follows: in response to application of the driving voltage, the charged particles present in the space right above the opening (the space between the location of the developing sleeve corresponding to the opening and the opening location of the insulating member) are discharged through the opening, whereby the charged-particle concentration in the space is reduced. However, as can be seen from the equipotential lines shown in
FIG. 4
, an electrostatic field surrounding the control electrode
19
is produced with its potential progressively increased toward the control electrode
19
. Accordingly, with the reduction in charged-particle concentration right above the opening
16
, the electrostatic field moves the charged particles from a position around the space right above the opening
16
toward the opening
16
.
Thus, not only the charged particles on the location of the developing sleeve that corresponds to the opening
16
, but also the charged particles on the location of the developing sleeve that corresponds to the control electrode
19
are going to pass through the opening
16
. Therefore, when the area of the control electrode
19
is too large as compared to the a

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