Print head and ink jet printing apparatus

Details Print head and ink jet printing apparatus Print head and ink jet printing apparatus

2000-08-22

2003-05-06

Nghiem, Michael (Department: 2863)

Incremental printing of symbolic information

Ink jet

Fluid or fluid source handling means

C347S094000

Reexamination Certificate

active

06557989

ABSTRACT:

This application is based on patent application Ser. Nos. 11-236279 filed Aug. 24, 1999 in Japan and 11-236994 filed Aug. 24, 1999 in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a print head and an ink jet printing apparatus for using the print head, and particularly to a configuration for ink refill that is carried out in liquid paths of the print head in associated with ink ejection.
The present invention is applicable to general printing apparatuses, apparatuses such as copy machines, facsimile machines having a communication system, and word processors having a printing section, as well as industrial printing apparatuses combined with various processing apparatus in a compound manner.
2. Description of the Prior Art
Conventional printing apparatuses for printing data on printing medium such as a paper, a cloth, a plastic sheet, an OHP sheet or the like (hereafter simply referred to as “printing paper”) are provided in a form of using a print head of various printing methods, for example, a wire dot method, a thermal-sensitive method, a thermal transfer method and an ink jet method.
The ink jet printing method carries out printing by ejecting an ink from fine openings for ink ejection (hereafter referred to “ejection openings”) of a print head and depositing the ink on printing paper in accordance with printing information. This method has various advantages of enabling printing at a relatively high speed and enabling printing on plain paper easily.
In addition, the ink jet method can be roughly classified depending on an ink droplet forming method and an ejection energy generating method into the continuous method (including a charge grain control method and a spray method) and a on-demand method (including a piezo method, a spark method, and a bubble jet method).
The continuous method is what ejects continuously a charged ink and controls electric fields to deposit only required ink droplets on printing paper. Also, the method collects in an ink receiver part of the ink which is not required for printing. In contrast, the on-demand method is what ejects an ink as required for printing and thus efficiently uses the ink while avoiding ejecting an unnecessary ink to prevent an inside of the apparatus from being stained. On the other hand, the on-demand method employs an ink ejection operation basically including a start and a stop operations of an ink flow, and thus has a lower response frequency for driving of the head than the continuous method. Thus, a number of ejection openings is increased to improve a printing speed as a whole. Based on above points, many of the currently available ink jet printing apparatuses are based on the on-demand method.
A printing apparatus of such an ink jet method has a printing head that comprises ink ejection openings, liquid paths each in communication with a corresponding one of the ink ejection openings and ejection energy generating elements for generating energy in the corresponding liquid path to eject the ink. To carry out printing, the ejection energy-generating element is allowed to generate ejection energy to act on the ink in the corresponding liquid path to generate a pressure therein for ejection, so that the pressure is then used to eject the ink from the ejection opening.
The ink used for the ink jet printing is commonly a printing agent such as a pigment or a dye which is dissolved or dispersed into a solvent such as water, a water-soluble organic solvent, or a non-water-soluble organic solvent.
In an ink ejection operation performed in the print head described above, the pressure generated for ejection is transmitted via the ink in the liquid path both toward the corresponding ejection opening for ejection and toward a liquid chamber that supplies the ink to the liquid path. A part of the pressure which is transmitted toward the ejection opening pushes the ink in the liquid path out from the ink ejection opening to form a flying droplet.
When ejected ink leaves the ink ejection openings in form of droplet, a meniscus which is formed in the liquid path near the ejection opening moves back depending on an amount of the ejected droplet. A tension of the ink (capillary force) which pulls back the meniscus toward the ejection opening causes a filled state of the ink in the liquid path to be returned to that before the ejection after a certain amount of time has passed. This phenomenon is called “refill”, and in actual printing, the above operation is repeated to achieve appropriate refill to enable stable persistent ink ejection.
The refill, however, may fail to be completed before the next ejection due to a cause associated with an ejection frequency or the like, and this incomplete refill may result in inappropriate ejection such as a reduced amount of the ejected ink droplet. As a result, for example, a size of ink dots formed with the ejected ink droplet on a printing medium is reduced to degrade general printing quality and an accuracy with which the ejected ink droplets land on the printing medium, causing blurred, rumpled, striped, or whitened images to be printed.
In printing techniques such as the ink jet printing method which use liquids, the above described problem has been solved by improving structures such as the liquid path or adjusting physical properties of the ink. Mere such improvements or adjustments, however, often fail to sufficiently improve a print head with a large number of ink ejection openings. This problem will be described below with reference to drawings.
FIGS. 20A and 20B
are views showing cross sections of main parts of an ink jet print head as seen from an ink ejection direction.
FIG. 20A
is a view useful in explaining a pressure caused upon ink ejection and acting toward a common liquid chamber, and
FIG. 20B
is a view useful in explaining a pressure required to obtain an appropriate refill state.
A print head
100
comprises a large number of ejection openings (not shown), liquid paths
102
each in communication with a corresponding one of the ejection openings, ejection energy generators
103
each disposed in a corresponding one of the liquid paths
102
, and a common liquid chamber
104
for supplying an ink to each of the liquid paths. The common liquid chamber
104
is in communication with an ink tank (also referred to as an “ink cartridge,” not shown) via an ink supply port
105
and is thus constantly filled with the ink.
As shown in
FIG. 20A
, when the inks are ejected from the large number of ink ejection openings
101
simultaneously or with a delay between ejection timings, a pressure caused by the ejection in each of the liquid paths
102
is transmitted therefrom toward the common liquid chamber
104
. These pressures are integrated together in the common liquid chamber
104
to form a single high pressure. The pressures caused in each liquid path act as forces that push back the ink toward the common liquid chamber
104
as shown by an arrow A, and the sum of these forces is several times as large as that in a print head with a single ejection opening.
In this case, to obtain a proper refill state, a large amount of ink must be rapidly moved toward the ejection openings
101
as shown by an arrow B in
FIG. 20B
, and to change the ink movement direction in this manner, a pressure is required which is sufficient to overcome an initial strong inertia force (total pressure) of the ink such as that described above.
However, a capillary force of the ink which causes the refill in each liquid path
102
is insufficient to instantaneously move a large amount of ink toward the ink ejection openings
101
against the total pressure toward the common liquid chamber
104
. That is, as the above described initial inertia force during the ink movement increases, a larger amount of time is required to allow a meniscus
106
to recover. Then, if the ejection frequency is reduced to allow for the sufficient amount of time for the meniscus recovery, a printing speed will decline.

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