Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1998-04-30
2002-07-02
Fuller, Benjamin R. (Department: 2855)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06412927
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ink ejection device for forming images on a recording medium by electing ink droplets from nozzles according to printing commands.
2. Description of the Related Art
Non-impact type printing devices have recently taken the place of conventional impact type printing devices and are holding an ever-growing share of the market. Of these non-impact type printing devices, ink-ejecting type printing devices have the simplest operation principle, but are still capable of effectively and easily performing multi-gradation and color printing. Of these devices, a drop-on-demand type for ejecting only ink droplets which are used for printing has rapidly gained popularity because of its excellent ejection efficiency and low running cost.
A shear mode type printer using a piezoelectric actuator is one of the drop-on-demand types. Such a printer is disclosed in U.S. Pat. No. 4,879,568. One example of such type of printer is shown in FIGS.
1
(
a
) and
1
(
b
) in which FIG.
1
(
a
) is a cross-sectional view taken along line A—A in FIG.
1
(
b
) an FIG.
1
(
b
) is also a cross-sectional view taken along line B—B in FIG.
1
(
a
).
As shown in FIGS.
1
(
a
) and
1
(
b
), the shear mode type ink ejection device
600
includes a bottom wall
601
, a ceiling wall
602
, and elongated shear mode actuator walls
603
sandwiched therebetween. Each actuator wall
603
includes a lower wall
607
adhesively attached to the bottom wall
601
and an upper wall
605
adhesively attached to the ceiling wall
602
. The upper and lower walls
605
,
607
are polarized in the directions indicated by arrows
609
,
611
, respectively. Alternating pairs of actuator walls
603
form in alternation ink channels
613
therebetween or spaces
615
, which are narrower than the ink channels
613
.
Electrodes
619
and
621
are provided on both side surfaces of each actuator wall
603
. Specifically, the electrode
619
is provided in the ink channel
613
and the electrode
621
is provided in the space
615
. The electrode
621
is also provided on the outer side surface of each of the two outermost actuator walls
603
. The electrode
619
is covered by an insulating layer (not shown) to insulate it from the ink. The electrodes
621
are connected to ground
623
. The electrodes
619
are connected to a control unit
625
in a form of a silicon chip which applies voltages (driving signals) to the electrodes
619
as will be described later.
A nozzle plate
617
is fixedly secured to one end of the actuator walls
603
. The nozzle plate
617
is formed with nozzles
618
at positions corresponding to the ink channels
613
. An ink supplying source (not shown) is connected to the other end of the actuator walls
603
through a manifold
626
. The manifold
626
includes a front wall
627
formed with openings in positions corresponding to the ink channels
613
, and a rear wall
628
for sealing the space between the bottom wall
601
and the ceiling wall
602
. Ink from the ink supplying source is supplied to the manifold
626
or common ink chamber and distributed into the respective ink channels
613
. The front wall
627
prevents ink from the manifold
626
from entering the spaces
615
.
To eject droplets, a voltage from the control unit
625
is applied to the electrode
619
of each ink channel
613
. Pairs of the actuator walls
603
deform outward by the piezoelectric shear effect so that the volume of each ink channel
613
increases. In the example shown in
FIG. 2
, when a voltage E volts is applied to the electrode
619
c
of the ink channel
613
c
, an electric field is developed in the actuator wall
603
e
in the direction indicated by the arrow
631
, and an electric field is developed in the actuator wall
603
f
in the direction indicated by the arrow
632
. Because the electric field directions
631
and
632
are at right angles to the polarization direction
609
,
611
, the actuator walls
603
e
,
603
f
deform outward to increase the volume of the ink channel
613
c
by the piezoelectric shear effect, resulting in a decrease in the pressure in the ink chamber
613
c
, including near the nozzle
618
c.
Application of the voltage E(V) is maintained for a duration of time T, during which time ink is supplied from the ink supplying source. A pressure wave occurring when the ink is supplied from the ink supplying source propagates in the lengthwise direction of the ink channel
613
c
. The duration of time T corresponds to a duration of time required for the pressure wave to propagate once in the lengthwise direction of the ink channel
613
c
. The duration of time T (hereinafter referred to as “pressure wave propagation time”) can be calculated by the following formula:
T=L/a
wherein L is the length of the ink channel
613
; and
a is the speed of sound through the ink filling channel
613
c.
Theories on pressure wave propagation teach that at the moment the duration of time L/a elapses after the application of the voltage, the pressure in the ink channel
613
c
inverts to a positive pressure. The voltage application to the electrode
619
c
of the ink channel
613
c
is stopped in timed relation with this pressure inversion so that the actuator walls
603
e
,
603
f
revert to their initial shape shown in FIG.
1
(
a
).
The pressure generated when the actuator walls
603
e
,
603
f
return to their initial shape is added to the inverted positive pressure so that a relatively high pressure is generated in the ink channel
613
c
. This relatively high pressure ejects an ink droplet
26
from the nozzle
618
c.
In the ink ejection device
600
of the type described above, a dot formed by continuously ejected two or more ink droplets in response to one-dot print command must have an increased density than a dot formed by a single ink droplet. However, when the continuously ejected droplets join together during the flight time toward the recording medium, the density of the dot printed on the recording medium is not as high as it is expected. Because a major part of the joined droplet is absorbed in the recording medium. In this case, the printed dot is almost as large as the dot printed by ejecting a single droplet. However, the print density does not increase despite a large amount of ink consumption.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide an ink ejection device capable of forming a high density dot with two or more ink droplets that are continuously ejected one from another from the same nozzle.
It is another object of the present invention to provide a method for driving such an ink ejection device.
These and other objects of the present invention will be attained by an ink ejection device including a nozzle plate formed with nozzles from which ink is ejected; walls including side walls, a ceiling wall and a bottom wall; and a control unit that drives the actuator in response to a one-dot printing command commanding to print a one dot image on a recording medium. The walls define an ink channel. The ink channel has a volume filled with ink. The nozzle plate is attached to one end of the ink channel. Ink is supplied to the ink channel from another end of the ink channel. The side walls are made from a piezoelectric material and serve as an actuator that applies pressure wave vibrations to the ink in the ink channel. The actuator successively ejects a plurality of ink droplets to print one dot image on the recording medium. The actuator is driven to eject the plurality of ink droplets so that at least two ink droplets ejected from a nozzle do not join together before reaching the recording medium. The plurality of ink droplets form print dot images on the recording medium in a positionally offset relation.
With the ink ejection device thus constructed, the plurality of ink droplets do not join together during their flight times. Therefore, the ink droplets are successively deposited onto the recording medium. Because the ink ejectio
Brother Kogyo Kabushiki Kaisha
Dickens C.
Fuller Benjamin R.
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