Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1998-12-28
2001-06-05
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S010000, C347S011000, C347S070000
Reexamination Certificate
active
06241345
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an ink jet recording head capable of controlling the diameter of an ink droplet ejected from the ink jet recording head to record a gray scale image. The present invention also relates to a method for controlling the diameter of an ink droplet in an inkjet recording head.
(b) Description of the Related Art
A drop-on-demand ink jet printer ejects ink droplets from ink nozzles of an ink jet recording head only when the ink droplets are requested. Specifically, the ink droplet is ejected from the ink nozzle by impressing a drive voltage to the piezoelectric element to generate a pressure wave in the ink chamber.
On the other hand, a stemmed ink jet recording head, such as proposed in Patent Publication JP-B-49(1974)-9622 for example, ejects ink droplets having variable diameters onto a recording sheet to thereby print a gray scale image such as for photographic data.
FIG. 1
shows a cross section of a conventional ink jet recording head, described in JP-A-51-37541, wherein a combination of a piezoelectric element
185
and a diaphragm
184
generates a pressure wave in a pressure chamber
182
of the ink jet recording head
180
receiving therein liquid ink. The pressure wave is transferred to a first nozzle
181
, where the liquid ink in the ink supply chamber
183
is ejected from a second nozzle
186
due to the pressure wave while forming an ink droplet
188
.
FIGS. 2A and 2B
show examples of dot patterns formed by the conventional ink jet recording head
180
, wherein a single pixel is formed by a matrix of N×N dots
151
. In
FIG. 2A
, the gray scale image is represented by the arrangement of a plurality of dots
151
marked in the matrix, with the diameter of the dots
151
being constant. In this configuration, the number L
1
of gray scale levels are expressed by:
L
1
=N
2
. (1)
A higher resolution and a larger number of gray scale levels, such as for a photographic image, require a larger number (N) of dots
151
for the matrix (or larger matrix size N) in FIG.
2
A. The larger matrix size N also requires a higher resolution for the dot itself due to reduction in the resolution for each pixel.
On the other hand, if the dots have variable dot diameters, such as shown in
FIG. 2B
, the dots by themselves provide gray scale levels. Specifically, assuming that the number of gray scale levels for each dot is n, the number L
2
of gray scale levels in FIG.
2
B can be expressed by:
L
2
=n×N
2
(2)
In the dot pattern of
FIG. 2A
, wherein n=1 in equation (2) due to the constant diameter of the dots
151
and N=3 for the matrix size, the number L
2
of gray scale levels obtained from equation (2)) is L
2
=9. On the other hand, in the dot pattern of
FIG. 2B
wherein n=4 in equation (2)) due to the four levels of the variable dot diameters (
151
a
,
151
b
,
151
c
and
151
d
) and N=3, the number L
2
of gray scale levels obtained from equation (2) is L
2
=36, which is far greater compared to
FIG. 2A
, whereas the resolution for each pixel in
FIG. 2B
is not degraded. In short, the variable dot diameter pattern shown in
FIG. 2B
can increase the number of gray scale levels for the dot pattern without raising the dot resolution.
The control of the dot diameter can be achieved by the amount Q of ink for each ink droplet. The amount Q is expressed by:
Z∝&tgr;×v×A.
(3)
wherein &tgr;, v and A are wave motion period of the pressure wave generated in the pressure chamber
182
, velocity of the ejected ink droplet and the sectional area of the second nozzle
186
, respectively. The velocity (v) of the ink droplet and drive voltage V applied to the piezoelectric element
185
have the following relationship:
v∝V.
(4)
FIG. 3
shows exemplified pressure response characteristics of the ink in the pressure chamber
182
, wherein the peak pressure of the ink in the pressure chamber
182
changes Pa to Pd based on the applied voltages V.
The velocity v of the ejected ink droplet changes based on the pressure, and thus based on the applied voltage, whereas the wave motion period &tgr; does not change. Accordingly, the following relationship:
Q∝V
(5)
can be obtained from relationship (3).
In the ink jet recording head shown in
FIG. 1
, the voltage V applied to the piezoelectric element
185
is changed so as to control the pressure of ink in the pressure chamber
182
, whereby the amount Q of the ink in the ink droplet ejected from the second nozzle
186
is controlled.
It is noted that the change of the velocity v of the ejected ink droplet affects the image quality of the conventional ink jet recording head. This is caused by deviation of the position at which the ink droplet reaches the recording sheet due to the variations of the ratio of the relative velocity between the recording head and the recording sheet to the velocity of the ejected ink droplet.
In addition, when a small ink droplet is ejected, the small ink droplet generally has a lower velocity and tends to stay in the vicinity of the second nozzle, causing stains in the ink jet recording device. This problem may be solved by a recording head proposed in JP-A-51-37541, wherein an air passage
189
is provided outside the pressure chamber
182
and a third nozzle
190
is additionally provided in front of the second nozzle
186
, as shown in FIG.
1
.
In the illustrated example, an airflow
191
flowing out of the third nozzle
190
at a constant velocity is generated by an air pump or an air accumulator installed outside the ink jet recording head
180
. The ink droplets
188
ejected from the second nozzle
186
are lead by the airflow
191
, whereby any ink droplet has a velocity equivalent to the velocity of the air flow
191
. This proposal may solve the problem as described above. However, the proposed ink jet recording head has larger size, complicated structure and larger weight due to provision of the air passage
189
and the air pump or accumulator.
In an alternative of the above proposal, another ink jet recording head is proposed in JP-A-61-100469, wherein it is noted that the wave motion period of the pressure wave is acoustic and inherent to the pressure chamber.
Specifically, it is noted that the amount Q of the ink in the ejected ink droplet can be controlled based on the natural period &tgr; of the ink pressure wave while maintaining the velocity v of the ink droplet at a constant. To obtain different diameters for the ink droplets, a plurality of ink passages having different natural periods are provided in the ink jet recording head, wherein different nozzles eject respective ink droplets having different diameters. The proposed ink jet recording head has, however, drawbacks of increased head size and higher fabrication costs.
Another drop-on-demand ink jet recording head, proposed in JP-A-62-174163, has a configuration wherein one or each of a plurality of piezoelectric elements is attached to the location corresponding to the belly portion between adjacent nodes of one of waves of the natural oscillation modes of the ink in the ink passage. The piezoelectric element thus located is driven to generate a corresponding oscillation mode.
FIG. 4A
shows the configuration proposed in JP-A-62-174163 as mentioned above, wherein the piezoelectric element
172
(shown by a dotted line) is located within an ink passage
171
at the location corresponding to the belly portion sandwiched between adjacent nodes of the wave of the tertiary natural oscillation mode, and
FIG. 4B
shows the wave of the tertiary natural oscillation mode of the ink in the ink passage
171
.
The length of the piezoelectric element
172
is designed equal to the length of the portion of the ink passage
171
corresponding to the belly portion between adjacent nodes of the tertiary natural oscillation mode, and the piezoelectric element
172
is located at the belly portion
175
between these adjacent no
Barlow John
Do An H.
Foley & Lardner
NEC Corporation
LandOfFree
Ink jet recording head controlling diameter of an ink droplet does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Ink jet recording head controlling diameter of an ink droplet, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ink jet recording head controlling diameter of an ink droplet will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2448484