Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2001-07-23
2002-07-02
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06412926
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an ink-jet recording head adapted to discharge minute ink droplets from a nozzle to record characters or images, and an ink-jet recording apparatus in which the ink-jet recording head is installed.
BACKGROUND ART
Hitherto, as one of this type of recording heads, an “on-demand type ink-jet recording head” that discharges ink droplets from a nozzle according to printing information has been extensively known, An on-demand type ink-jet recording head has been disclosed in, for example, Japanese Examined Patent Publication (JP-B) No. 53-12138.
FIG. 11
is a sectional view that conceptually shows a basic construction of an ink-jet recording head known as a Caesar type among the on-demand type ink-jet recording heads.
As shown in
FIG. 11
, in the Caesar type recording head, a pressure generating chamber
91
and a common ink chamber
92
are coupled via an ink supply aperture (ink supply passage)
93
at an ink upstream side. At an ink downstream side, the pressure generating chamber
91
and a nozzle
94
are coupled. A bottom plate of the pressure generating chamber
91
shown in the drawing is composed of a diaphragm
95
, and a piezoelectric actuator
96
is provided on the rear surface of the diaphragm
95
.
In such a construction, to perform a printing operation, the piezoelectric actuator
96
is driven to displace the diaphragm
95
on the basis of printing information, thereby suddenly changing the volume of the pressure generating chamber
91
to produce a pressure wave in the pressure generating chamber
91
. The pressure wave causes a part of the ink charged in the pressure generating chamber
91
to be injected outside through the nozzle
94
in the form of an ink droplet
97
. The discharged ink droplet
98
impacts onto a recording medium, such as recording paper, and forms a recording dot. Such a recording dot is repeatedly formed on the basis of the printing information thereby to record a character or an image on the recording medium.
Referring now to FIGS.
12
(
a
) through (
d
) and
FIG. 13
, the relativity between the behaviors of a meniscus and printing performance will be discussed.
FIGS.
12
(
a
) through (
d
) are sectional views illustrating a changing process of a meniscus M of the nozzle
94
in the aforesaid ink droplet discharging process, and
FIG. 13
is a graph showing time-dependent changes of the position of the meniscus M after the ink droplet is discharged. Before the ink droplet
97
is discharged, the meniscus M is set so that it is positioned substantially flush with an aperture surface of the nozzle
94
, as shown in FIG.
12
(
a
). When the piezoelectric actuator
96
is driven and the ink droplet
97
is discharged, the meniscus M moves back into the nozzle
94
according to the amount of the discharged ink, as shown in FIG.
12
(
b
). At this time, if the next discharge is implemented while the meniscus M is still back in the nozzle
94
, as shown in FIG.
12
(
c
), then a discharging condition (a droplet diameter, droplet speed, etc.) changes, or a discharge failure results. Hence, in order to achieve stable continuous discharge, it is important to wait until the meniscus M that has retreated is moved back to the vicinity of its initial position by the action of surface tension, as illustrated in FIG.
12
(
d
), before the next discharge cycle is implemented. More specifically, it is crucial to start the next discharge cycle after a time required for refilling after the ink is discharge has elapsed (refilling time t
r
), as shown in FIG.
13
.
From the descriptions above, it can be understood that a maximum discharging frequency fe of the ink-jet recording head depends on the refilling time t
r
of the head. More specifically, to attain high-speed recording by operating at the maximum discharging frequency fe, it is necessary to shorten the refilling time t
r
so as to satisfy a condition indicated by t
r
<1/fe. To be more specific, the refilling time t
r
can be reduced by increasing a cross-sectional area of the passage system formed of the nozzle
94
, the pressure generating chamber
93
, and the ink supply aperture (ink supply passage)
91
, or by decreasing the viscosity of the ink thereby to decrease a passage resistance.
However, reducing the passage resistance results in a side effect of an increase in an overshoot X
max
of the meniscus M, as shown in
FIG. 13
, although the refilling time t
r
is shortened. More specifically, if the overshoot X
max
is large, then the condition (position or speed) of the meniscus M immediately before the discharge of the ink droplet
97
does not remain constant, leading to an inconvenient problem in that the droplet diameter or the droplet speed (discharging speed) of the droplet
97
varies. Therefore, to secure the accuracy in the droplet diameter or the droplet speed, it is required to control the overshoot X
max
of the meniscus M to a predetermined value or less. Especially to accomplish recording with high image quality by droplet diameter modulation, high accuracy is required of the droplet diameter and the droplet speed. For this reason, the overshoot amount X
max
must be approximately 10 &mgr;m at maximum. A specific measure for suppressing the overshoot X
max
, the cross-sectional area of the passage system may be reduced or the ink viscosity may be increased so as to increase the passage resistance. As mentioned above, however, increasing the passage resistance causes the refilling time t
r
to be prolonged, so that high-speed recording is inconveniently sacrificed.
Thus, in the ink-jet recording head, it is extremely difficult to realize the recording with high image quality performed by droplet diameter modulation, and also high-speed recording at the same time, because the conflicting conditions, namely, the shortened refilling time t
r
and the restrained overshoot X
max
must be satisfied. In the past, however, attempts have been made to realize both the recording with high image quality and high-speed recording by maximizing the reduction in the refilling time and the restraint of the overshoot by devising the shapes of the nozzle or ink supply aperture (the ink supply passage) or the like, and by adjusting the viscosity of the ink.
According to the conventional attempts mentioned above, however, it has been extremely difficult to always achieve the shortened refilling time and the restrained overshoot over a wide operating temperature range of the apparatus. This is because the physical properties of the ink change due to environmental temperatures, and as a result, refilling characteristics markedly change.
As it will be discussed hereinafter, the refilling characteristics of the ink-jet recording head are governed by the inertance (acoustic mass) and the acoustic resistance of the passage system formed of a nozzle, an ink supply aperture (an ink supply passage), a pressure generating chamber, etc., and the acoustic capacitance of a meniscus. Among these factors, the inertance depends on the density of ink, the acoustic resistance depends on the viscosity of ink, and the acoustic capacitance depends on the surface tension of ink. Therefore, if the ink properties (density, viscosity, and surface tension) change according to environmental temperatures, then the characteristic parameters (inertance, acoustic resistance, and acoustic capacitance) of a passage system change accordingly, resulting in a significant change in the refilling characteristics. Actually, when the operating temperature range of the apparatus is 10 to 35° C. (in the vicinity of room temperature), the dependence-on-temperature of the density and the surface tension can be almost ignored, but the temperature-dependent change of the ink viscosity cannot be ignored.
For instance, if the operating temperature of the apparatus is set to 10 to 35° C., then the ink viscosity of a typical water-based ink develops an approximately 2.0-fold to 2.5-fold change. If the environmental temperature is low, then the ink viscosity increases with a resultant increase in
Barlow John
Brooke Michael S
Fuji 'Xerox Co., Ltd.
McGnn & Gibb, PLLC
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