Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2003-04-22
2004-03-16
Nguyen, Thinh (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S015000
Reexamination Certificate
active
06705700
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid discharge head having nozzles for discharging liquid, and a head cartridge and an image forming apparatus that use such a liquid discharge head.
2. Description of the Related Art
Ink discharging methods of the ink jet recording type that have been widely used today may be roughly classified into a method that uses an electro-thermal converter (heater) as a discharge energy generating element that is used for discharging an ink droplet, and a method using a piezoelectric element for the same purpose. In both methods, it is possible to control discharge of an ink droplet using an electrical signal. For example, the principle of the ink droplet discharging method using the electro-thermal converter is that, by giving an electrical signal to an electro-thermal converter, ink near the electro-thermal converter is boiled in an instant and, based on rapid growth of a bubble that is generated due to a phase change of ink thereupon, an ink droplet is discharged at high speed. On the other hand, the principle of the ink droplet discharging method using the piezoelectric element is that, by giving an electrical signal to a piezoelectric element, the piezoelectric element displaces and, based on a pressure generated upon this displacement, an ink droplet is discharged. The former method has merits such that it is not necessary to so much space for the discharge energy generating element, a configuration of an ink jet print head is simple and thus integration of nozzles is easy, and so on.
Recently, following the increasing processing speed of personal computers and the spread of the Internet and so on, the demand for high-speed processing of color images have been increasing more and more, i.e. the demand for quick printing of recorded materials with high fineness and high gradation, that is, so-called extremely high-quality recorded materials, has been increasing, so that printers that can produce high-quality images at high speed have been demanded.
For obtaining a high-quality image with high fineness and high gradation, such a method is suitable that performs recording by discharging a very small ink droplet from each nozzle. On the other hand, for speed-up, it is necessary to discharge ink droplets repeatedly at a short period from nozzles. Further, it is also required that a carriage mounting thereon a print head moves at high speed synchronously with a response frequency of the head. When performing recording by repeatedly discharging a very small ink droplet from each nozzle as described above, a stripe
101
, for example, is generated in an image of a paint-out printing portion, i.e. a solid printing portion
100
, of a bar graph as shown in FIG.
10
. The stripe
101
just corresponds to a transition portion between nth scan and (n+1)th scan.
FIG. 11B
is an enlarged view of this transition portion, and
FIG. 11A
shows the state thereupon wherein ink droplets
102
are discharged from a head
103
. When image data is solid, nozzles SEG
0
to SEG
255
are all driven at a high response frequency. Thus, due to discharge of ink droplets
102
from those nozzles in end regions of the image data, the air with viscosity around the discharged ink droplets moves at a speed substantially equal to that of the ink droplets. Then, the air over the whole discharge port array moves in the same direction as the ink droplets
102
so that a pressure-reduced state is generated in that portion. Accordingly, the air other than the air around the discharged ink droplets moves toward the pressure-reduced portion, so that air flows as shown by arrows in
FIG. 11A
are generated. As a result, a discharge direction of the ink droplets
102
discharged from the nozzles located in end regions of the nozzle array is dethroughted or mis-aligned toward the center of the nozzle array from expected positions due to the air flows, i.e. inward jetting of the ink droplets occurs. Further, due to air flows generated upon movement of a carriage in a main scanning direction upon recording, air flows toward the center of the nozzle array are generated, so that the discharge direction of the ink droplets
102
discharged from those nozzles located in the end regions of the nozzle array are mis-aligned toward the center of the nozzle array due to the air flows. As a result, there has been a problem that hit positions of the ink droplets on a recording medium are mis-aligned to cause the stripe
101
as shown in
FIG. 11B
((dot) mis-alignment). If the discharge amount of ink is increased for preventing the generation of the stripe
101
, waviness is caused on the recording medium due to overflow of ink from the recording medium or absorption of ink into the recording medium, thereby deteriorating a recording image. Particularly, in the image formation with high fineness and high resolution, it is important to reduce graininess and reproduce fine lines, and thus, it is required that a dot size be as small as possible. Accordingly, also from this aspect, it is not desirable. On the other hand, if a period for repetitive discharge of ink droplets is prolonged, the generation of air flows is eased. However, the speed of the printer is lowered, so that it is not possible to satisfy the users' demand for high-speed printing.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a liquid discharge head that is capable of high-speed recording and can reduce generation of a stripe in a recording image, and further provide a head cartridge and an image forming apparatus that use such a liquid discharge head.
As a result of detailed review by the inventors, the following has been made clear.
(a) Not only the ink droplets discharged from those discharge ports located at end portions of the discharge port array are mis-aligned. For example, even in the state where ink droplets
102
are discharged from discharge ports of part of a head
103
as shown in
FIG. 12A
, the ink droplets
102
discharged from the discharge ports located at end portions in the discharge port array direction are mis-aligned toward an intermediate portion of the discharge ports. On the other hand, a stripe
101
between solid portions
100
becomes conspicuous in an image as shown in
FIG. 12B
when it appears at a transition portion between recording scans. Thus, particularly the end portions of the discharge port array causes a problem.
(b) As the number of discharging discharge ports upon recording is reduced, a mis-alignment amount of liquid droplets discharged from discharge ports at end portions of an image is reduced. It is estimated that this is because, due to the reduction in number of the discharging discharge ports, a pressure-reduced state at the intermediate portion of the discharging discharge ports is weakened so that air flows that cause mis-alignment become reluctant to occur. A relationship between the number of discharged liquid droplets and the mis-alignment is shown in
FIG. 6
as a specific example, which will be described later. No mis-alignment in a direction perpendicular to the main scanning direction due to air flows occurs when only one liquid droplet is discharged. As the number of the discharged liquid droplets increases, the mis-alignment amount is increases. It is also estimated that, also in case of reduction in discharged liquid droplet amount, air flows become reluctant to occur inasmuch as a pressure-reduced state-at the intermediate portion of the discharging discharge ports is weakened.
(c) Further, as resolution in a sub-scanning direction for recording is lowered, the mis-alignment amount of liquid droplets discharged from discharge ports at the end portions is reduced.
(d) The present inventors have made detailed review based on the foregoing air flow model, and configured such that a volume of each of discharge ports at end portions is greater than a volume of each of discharge ports at an intermediate portion. As a result, the mis-alignment amount of liquid droplets discharg
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