Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2002-05-07
2004-02-17
Nguyen, Thinh (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S047000
Reexamination Certificate
active
06692103
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ink jet head for recording text or an image, etc., by jetting ink from a nozzle and an ink jet recording device.
2. Description of Related Art
Among the prior arts, structures disclosed in for example JP S53-12138B or JPH10-193587A are generally well-known. These involve utilizing an electromechanical conversion mechanism such as a piezo-electric actuator for example for a pressure generating mechanism, generating pressure waves in an ink filled pressure generating chamber and using a drop on demand type ink jet recording head for jetting ink droplets from a nozzle communicating with the pressure generating chamber through the pressure waves.
FIG. 11
is a cross-sectional drawing showing in outline the structure of an existing ink jet recording head among the prior arts. Referring to
FIG. 11
, ink pressure generating chamber
111
, nozzle
112
for jetting ink is connected to ink supply passages
114
for channeling ink from an ink tank shown in the lower part of the drawing via common ink passages
113
. Further, vibration plate
115
is provided on the bottom of pressure generating chamber
111
.
When ink droplets are jetted, piezo-electric actuator
116
on the outside of pressure generating chamber
111
causes this vibration plate
115
to be displaced. This in turn creates a change in the volume inside pressure generating chamber
111
thereby generating pressure waves inside that chamber. The effect of these pressure waves causes some of the ink filled in pressure generating chamber
111
to be jet discharged through nozzle
112
, jetting out as ink droplets
117
. These ink droplets
117
expelled in a jet form dots as they contact a recording medium such as paper etc. Repeating the formation of these; recording dots based on image data enables text or an image to be recorded on to the recording medium.
FIGS. 12
a
through
12
f
illustrate how a meniscus of a nozzle portion behaves before and after jetting ink. As shown in
FIG. 12
a
, meniscus
122
that is initially substantially flat, moves towards the outside of nozzle
121
as the pressure generating chamber is compressed causing ink droplets
123
to be jetted (
FIG. 12
b
). As ink droplets are jet expelled the volume of ink inside nozzle
121
decreases forming a concave shape in meniscus
122
. As shown in
FIGS. 12
d
through
12
f
, this concave shaped meniscus
122
gradually regresses towards the opening portion of nozzle
121
due to the action of the elastic surface of the ink, returning back to the original condition before ink was jetted. In this description, this regressing action of a meniscus after ink is jetted is referred to as a “re-fill” and the time taken for a meniscus to first regress back to the opening portion (y=0) after ink is jetted is called “re-fill time (t
r
).”
When ink droplets are continuously jetted, a subsequent ink jetting occurs even before a re-fill is completed leading to inconsistency in diameter and speed of the droplets. If a subsequent jetting of ink is not performed after completion of a re-fill it is not possible to achieve stable and continous ink jetting. In other words, if after ink is jetted more than re-fill time t
r
does not elapse before the next ink jetting that next ink jetting cannot be carried out smoothly. Accordingly, reducing re-fill time t
r
is an important point for increasing maximum jet frequency (i.e. recording speed).
JP S56-93567A, JP S57-107849A and JP S63-13751A for example, disclose ink jet recording heads having a nozzle provided with multiple ink jet ports
131
in nozzle plate
132
as shown in
FIGS. 13
a
and
13
b
as a means for reducing this re-fill time. In other words, this re-fill activity described above is something that operates through pressure arising through the elastic surface of meniscus. As shown in the following formula, the smaller the acoustic capacity c
n
the greater the pressure arising (d
n
is nozzle diameter [m], &sgr; is ink surface elasticity [N/m].
C
n
=
π
d
n
4
64
σ
1
+
16
y
2
d
n
2
(
1
)
As illustrated by that formula, as this meniscus acoustic capacity c
n
is the fourth power of nozzle diameter, nozzle diameter should be made as small as possible in order to reduce acoustic capacity c
n
. However, if the nozzle diameter of a nozzle with a single port is reduced other problems arise such as a decrease in the volume of ink droplets.
Accordingly, as shown in
FIGS. 13
a
and
13
b
, where a nozzle is formed with multiple ink jet ports
131
in which the diameter of the openings is small, extremely minute droplets are jetted from each ink jet port but there is no increase in the total volume of ink droplets expelled, while acoustic capacity c
n
of each ink jet port decreases allowing for an increase in re-fill speed.
Other points of ink jet recording heads having multiple openings to decrease the re-fill time include improved gradient recordability as disclosed in JP H6-286138A, faster printing of a specific pattern such as a bar-code etc. as disclosed in JP S62-196155A for example, or prevention of rear surface recording or improved drying properties through a specified reduction in ink droplet volume as disclosed in JP S56-93567A,
The results of analysis conducted by the inventors of those inventions on experimental ink jet recording heads with multiple ports made it clear however, that in addition to it being difficult to obtain sufficiently reduced re-fill time simply by having a nozzle with a multi-port structure, a multi-port nozzle creates other problems inducing image quality to fall or ink jet properties to deteriorate (deteriorating ink jet efficiency etc.). The results of analysis of multiple assessments conducted on experimental heads and flow analysis to ascertain the causes, clearly showed that problems lay with nozzle structure; length of a nozzle and pitch of ink jet ports etc. and matching with other passage systems. In other words, it became clear that obtaining the optimum nozzle structure and more efficiently matching different passages are indispensable for more effectively bringing out the benefits of a multi-port nozzle. These issues will now be further described with reference to concrete examples.
FIG. 14
shows an example of results obtained investigating the relationship between volume of jetted ink droplets and re-fill time in an existing ink jet recording head having a multi-port nozzle with 5 ink jet ports. Results for an ink jet recording head where a nozzle has multiple ports are plotted with black triangles and these results are compared to those obtained from an ink jet recording head with a mono-port nozzle having just one ink jet port, plotted with circles. For this comparison the total volume of ink droplets used was the same for both types of head.
FIG. 14
clearly shows that when ink droplet volume is 5 pl or less re-fill time of a multi-port nozzle was ½ to ⅔ that of a mono-port nozzle, and the effect of a multi-port nozzle is to decrease re-fill time; increase re-fill speed.
If however ink droplet volume exceeds 5 pl re-fill time is non continuous and increases dramatically, becoming longer than re-fill time for a mono-port nozzle. In other words, it became clear that if ink droplet volume exceeds a specific point, more than just losing the effects of decreased re-fill time, re-fill speed of a multi-port nozzle fell below that of a mono-port nozzle.
Thus, the results of investigating the causes of substantial change in re-fill time in relation to ink droplet volume through conducting experiments and flow analysis etc. clearly show that the condition of the meniscus on a rear surface of a nozzle has a substantial effect. In other words, if the length of an ink jet port is small in relation to the volume of ink droplets discharged, as shown in
FIG. 15
b
, it is clear that a fusion phenomena occurs affecting meniscus
155
at the rear surface of a nozzle. Because this phenomenon causes the
Okuda Masakazu
Otsuka Yasuhiro
Fuji 'Xerox Co., Ltd.
Nguyen Thinh
Scully Scott Murphy & Presser
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