Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2003-04-22
2004-08-10
Nguyen, Thinh (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S043000
Reexamination Certificate
active
06773089
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid discharge head having a nozzle 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
In recent years, owing to the spread of the Internet, digital cameras, and so on, the demand for high-gradation color printing has been increased and, following it, ink jet printers have been improved to have higher performances. As means for obtaining a high-quality print image with high fineness and high gradation, those methods such as (1) and (2) below are considered to be particularly effective.
(1) A volume of ink to be discharged is reduced and a nozzle array pitch is narrowed, thereby to improve the resolution.
(2) With respect to particular color ink, a plurality of nozzle arrays are prepared for discharging, respectively, a plurality of (at least two) color inks that differ in ratio of a contained color material, i.e. in concentration of the color material, and the high-concentration ink and the low-concentration ink are selectively printed in an overlapping manner as required, thereby to improve gradation.
However, for obtaining the high-quality print image with high fineness and high gradation by reducing as much as possible a volume of each ink droplet to be discharged so as to realize the method (1), it is necessary to hit (place) more ink droplets on a recording medium with high accuracy and record them, and thus, nozzles for discharging ink droplets of the stable volume and hitting them on the recording medium with high accuracy as well as a high frequency response of an ink discharge head are required.
Further, for realizing the method (2), nozzle arrays for discharging high-concentration ink and low-concentration ink, respectively, are necessary with respect to particular color ink, so that a configuration of a print head becomes complicated. For discharging such small ink droplets from the nozzles, there has been proposed a print head of the type that causes bubbles growing due to film boiling following heating of ink to communicate with the air via nozzles, as disclosed in, for example, JP-A-04-10940, JP-A-04-10941, and JP-A-04-10742. For distinguishing it from the old bubble jet type that discharges ink droplets without causing bubbles growing due to film boiling to communicate with the air, it is sometimes called the bubble through type.
In the print head of the old bubble jet type that discharges ink droplets without causing bubbles growing due to film boiling to communicate with the air, as the size of an ink droplet discharged from a nozzle is reduced, it is necessary to reduce a sectional area of an ink flow passage communicating with the nozzle, so that there arises a disadvantage that the discharge efficiency is lowered to drop the discharge speed of the ink droplet discharged from the nozzle. If the discharge speed of the ink droplet is lowered, it is possible that a discharge direction thereof becomes unstable, and further, viscosity of the ink is increased following evaporation of moisture while the print head is stopped in operation, and thus the discharge state becomes further unstable to cause initial discharge failure and so on, thus leading to lowering of the reliability.
In contrast, in the print head of the bubble through type in which bubbles communicate with the air, the size of an ink droplet can be determined only from a geometrical shape of a nozzle. Therefore, there are advantages that it is suitable for discharging small ink droplets, it tends to be free of an influence such as temperatures, and the discharge amount of each ink droplet is very stable as compared with the print head of the old bubble jet type. Thus, it is possible to obtain the high-quality print image with high fineness and high gradation relatively easily.
For obtaining the high-quality print image with high fineness and high gradation, a print head configuration in combination of the foregoing methods (1) and (2) is considered to be particularly effective. For obtaining a print image at high speed, it is also effective to shorten a period for discharging ink, or form each recording pixel with ink of a large discharge volume, thereby suppressing the recording density. Therefore, if a discharge volume of low-concentration ink is set to be larger than a discharge volume of high-concentration ink with respect to particular color ink, and an image is recorded on a recording medium by combination of ink droplets of them, the total number of times of discharging ink and a period therefor can be suppressed, so that there can be realized an ink droplet discharge head that is energy saving and highly accurate. Accordingly, the high-quality print image with high fineness and high gradation can be obtained at high speed.
When carrying out printing by discharging an ink droplet of an extremely small amount from one nozzle, the bubble through type is particularly suitable in the ink jet printer as described above.
FIG. 9
shows a discharging state of ink droplets when the ink droplets are continuously discharged from all the nozzles while scan-moving the print head of the ink jet type together with a carriage along a print medium at high speed, thereby performing so-called solid printing relative to the print medium. The scan-moving direction of a print head
101
is perpendicular to the sheet of
FIG. 9
, and nozzles (not shown) are arrayed right and left in the figure. When image data is solid, all the discharge energy generating portions (not shown) corresponding to the respective nozzles are driven at high driving frequencies. Therefore, following motions of ink droplets
103
discharged from the nozzles toward a print medium
102
, the ambient air having viscosity also moves induced by the motions of the ink droplets
103
. As a result, the vicinity of a nozzle surface
104
where the nozzles of the print head
101
open tends to be reduced in pressure as compared with portions around the print head
101
, so that the ambient air flows into the pressure-reduced region as air flows. It has been confirmed that, due to influence of the air flows, the ink droplets
103
discharged from the nozzles, particularly those nozzles located on both end sides in a nozzle array direction, are drawn toward the center in the nozzle array direction, so that the ink droplets
103
are not discharged to expected positions relative to the print medium
102
. Accordingly, a plurality of discharged liquid droplets are drawn toward the center.
FIG. 10
exemplarily shows an image of solid printing that is formed on the print medium when the solid printing is carried out by a plurality of times of scanning movement of a carriage under such a phenomenon. The carriage is scan-moved along with the print head perpendicularly to the sheet of the figure. It is seen that a white stripe
107
is formed between a solid image
105
formed by the previous scanning movement and a solid image
106
formed by the subsequent scanning movement.
It has been found through study of the inventors that such a disadvantage occurs particularly notably in the ink jet printer of the bubble through type wherein the nozzle array pitch is set narrow and ink droplets each of a small amount of 10 pl or less can be discharged at a short period through one driving operation, and the degree of the disadvantage differs depending on a difference in volume of an ink droplet discharged. Table 1 shows data about an end mis-alignment amount (half value of white stripe
107
) when the nozzle array pitch is 1200 dpi (21 &mgr;m).
TABLE 1
Discharge Amount
End Mis-Alignment Amount
(pl)
(&mgr;m)
2
3 or greater
4
20 or greater
The reason therefor is that since the ink droplets are continuously discharged from all the nozzles while scan-moving the print head together with the carriage along the print medium at high speed, various factors such as a shape and a scanning speed of the carriage, a position of the nozzle array, a distance between the nozzle and the print
Inoue Tomoyuki
Mizutani Michinari
Osada Torachika
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Thinh
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