Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2001-07-27
2004-03-16
Pham, Hai (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S010000, C347S068000
Reexamination Certificate
active
06705696
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of driving an inkjet recording head and an inkjet recording apparatus, and specifically, to a driving technique for driving an inkjet recording head for the recording of characters and images by the ejection of minute ink droplets from an ink nozzle in an inkjet recording apparatus.
BACKGROUND ART
As a conventional inkjet recording system, a drop-on-demand type inkjet system is known whereby an electro-mechanical transducer such as a piezoelectric actuator is used to cause a pressure wave (acoustic wave) to be generated in a pressure chamber filled with a liquid ink, so that the pressure wave ejects an ink droplet from a nozzle coupled with the pressure chamber. Such inkjet recording method using the drop-on-demand type inkjet system is disclosed in Japanese Patent Publication No. Sho. 53-12138, for example. An example of the structure of the inkjet recording head of this type is shown in FIG.
22
.
Referring to
FIG. 22
, a pressure chamber
61
is connected with a nozzle
62
for the ejection of ink and an ink supply path
64
for guiding ink from an ink reservoir (not shown) through a common ink chamber
63
. A vibrating plate
65
is mounted on the bottom surface of the pressure chamber.
When an ink droplet is to be ejected, a piezoelectric actuator
66
mounted outside the pressure chamber
61
operates to displace the vibrating plate
65
, whereby the volume within the pressure chamber
61
is changed and thus a pressure wave is generated therein. This pressure wave causes a part of the ink filled in the pressure chamber
61
to be ejected through the nozzle
62
as a flying ink droplet
67
. The flying ink droplet lands on a recording medium such as a recording paper and forms a recorded dot thereon. Such formation of recorded dots are repeated on the basis of image data, thereby recording characters or images on the recording paper.
In order to achieve a high image quality in this type of inkjet recording head, it is necessary to minimize the diameter of the ejected ink droplet (droplet diameter). Specifically, in order to obtain a smooth image with little graininess, the recording dot (pixel) formed on the recording paper must be made as small as possible. For this reason, the diameter of the ink droplet ejected must be minimized in size. Generally, the graininess of the image decreases greatly as the dot diameter becomes 40 &mgr;m or less. As the dot diameter becomes 30 &mgr;m or less, it becomes so difficult to visually recognize the individual dots even in the highlight portion of the image that the image quality improves greatly.
The relationship between the ink droplet diameter and the dot diameter depends on the rate of flight of the ink droplet (droplet velocity), physical properties of the ink (viscosity, surface tension), the type of the recording paper, and so on. Normally, the dot diameter is about twice the size of the ink droplet diameter. Accordingly, in order to obtain a dot diameter of 30 &mgr;m or less, the ink droplet diameter must be set at 15 &mgr;m or less. In the present description, the diameter of the ink droplet (droplet diameter) refers to the diameter of a spherical droplet substituting the total amount of ink (including the satellites) ejected in a single act of ejection.
The most effective way of minimizing the ink droplet diameter is to reduce the nozzle diameter. Practically, however, the nozzle diameter cannot be reduced to less than about 25 &mgr;m, given technical difficulties in the manufacture and the fact that as the nozzle diameter is reduced, the nozzle tends to be clogged. Accordingly, it is impossible to obtain an ink diameter on the order of 15 &mgr;m solely by decreasing the nozzle diameter. To solve this problem, it is known to reduce the droplet diameter of the ejected ink droplet by way of the driving method employed, and some effective methods are proposed.
As one such example, Japanese Patent Laid-open Publication No. Sho. 55-17589 discloses a meniscus control technique whereby the pressure chamber is once expanded immediately before ejection, and then an ink droplet is ejected when the ink meniscus at the nozzle opening is drawn towards the pressure chamber.
FIG. 23
shows an example of the driving waveform for driving the piezoelectric actuator using this technique. In the present description, the relationship between the driving voltage and the piezoelectric actuator operation is such that as the driving voltage increases, the volume of the pressure chamber decreases and, conversely, as the driving voltage decreases, the volume of the pressure chamber increases. Generally, the polarities are often reversed depending on the structure of the piezoelectric actuator and the direction of polarization of the piezoelectric element.
Referring to the driving waveform shown in
FIG. 23
, a voltage fall
71
from V
1
to zero volt expands the volume of the pressure chamber. A subsequent voltage rise
71
from zero volt to V
2
compresses the volume of the pressure chamber to thereby eject an ink droplet. The interval of each of the fall time t
1
and rise time t
2
is generally on the order of 2-10 &mgr;s, which is longer than an inherent period Ta of the conventional piezoelectric actuator.
FIGS.
25
(
a
) to (
d
) illustrate the movement of the ink meniscus at the nozzle opening portion upon application of the driving waveform of FIG.
23
. The ink meniscus has a flat upper portion during the initial state (FIG.
25
(
a
)). As the pressure chamber is expanded immediately before the ejection, the top portion of the ink meniscus assumes a concave shape, as shown in FIG.
25
(
b
). As the pressure chamber is compressed by voltage rise
71
when there is such a concave ink meniscus, a thin liquid column
83
is formed in the center of the ink meniscus as shown in FIG.
25
(
c
). This is followed by the formation of an ink droplet
84
as the tip of the liquid column is separated (FIG.
25
(
d
)). The ink droplet diameter is substantially equal to the thickness of the liquid column thus formed and is smaller than the nozzle diameter. Thus it is possible to eject an ink droplet with a smaller diameter than the nozzle diameter by using such driving method.
As described above, the meniscus control system enables the ejection of an ink droplet with a smaller diameter than the nozzle diameter. However, when such driving waveform as shown in
FIG. 23
is used, the smallest diameter of the droplet that could actually be obtained was about 25 &mgr;m, which is still not good enough to satisfy the need for higher image quality.
FIG. 24
shows another driving waveform as a driving means for enabling the ejection of a smaller droplet. In this waveform shown in
FIG. 24
, a voltage fall
73
draws the ink meniscus immediately prior to the ejection. A subsequent voltage rise
74
compresses the volume of the pressure chamber and thereby causes a liquid column to be formed. A voltage fall
75
separates a droplet from the tip of the liquid column at an early period. A voltage rise
76
suppresses the reverberations of the pressure wave remaining after the ejection of the ink droplet. In other words, the driving waveform of
FIG. 24
is based on the conventional meniscus control system in which a pressure wave control is incorporated for the early separation of the ink droplet and for the suppression of the reverberations. This arrangement allows an ink droplet with a droplet diameter on the order of 20 &mgr;m to be ejected in a stable manner.
However, it was still difficult to eject an ink droplet with an ink diameter of 20 &mgr;m or less easily even by using this improved driving waveform, and particularly an ink diameter of 15 &mgr;m or less was impossible. Thus, there was no driving method that could achieve the ink diameter of 15 &mgr;m or less, which was required for image quality reasons. One of the biggest reasons for this was that in the conventional inkjet recording head, the ink droplet ejection was carried out by the pressure wave that was governed by the acoustic capacity of
Araki Masatoshi
Okuda Masakazu
Dickstein Shapiro Morin & Oshinsky LLP.
Fuji Xerox Co. Ltd
Nguyen Lam
Pham Hai
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