Ink jet print head

Details Ink jet print head Ink jet print head

2000-04-13

2001-11-27

Wong, Peter S. (Department: 2838)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reissue Patent

active

RE037453

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an ink jet print head for discharging ink drops from ink outlets by use of thermal energy.
Description of the Related Art
Recently, in contrast with the wire dot printing methods, non-impact recording method is attracting interest became the recording noise level is negligible. In particular, an ink et recording method is attractive as it permits high-speed recording on ordinary paper without the need of a deposition treatment on the paper side. In the field, therefore, aiming at an optimal ink discharge performance, various approaches have been made, with associated implementations.
In the ink jet recording method, a recording is effected with discharged droplets of recording liquid, called “ink” deposited on a recordable material. This method is categorized into several systems according to the manner in which the drops of recording liquid are formed.
FIG. 1
illustrates a bubble jet recording system as a conventional example. The conventional system includes a substrate
32
provided with a heating resistor
30
, a channel plate member
36
for defining an ink supply path
34
, and an orifice plate
40
formed with an orifice as an ink outlet
38
communicating with the ink supply path
34
. The heating resistor
30
rapidly heats to vaporize a volume of ink supplied on a heating zone surrounding the resistor
30
, causing ink bubbles
42
to grow, exerting pressures therearound so that an ink drop is discharged from the ink outlet
38
, with trailing droplets
50
,
52
as shown in FIG.
2
.
Grown bubbles
42
become deflated as they are cooled by surrounding ink, and fade out with ink vapour therein condensed to be liquidated.
A consumed volume of ink by the discharge is supplemented from an ink pool through the ink supply path
34
, due to capillary forces acting on an ink meniscus
44
retreating inside the ink outlet
38
.
To permit a high-speed recording, it is desirable to repeat a discharge of an ink drop in a short period, supplementing at a high speed a volume of ink consumed during every discharge through the ink outlet
38
.
In a conventional implementation, the diameter of the ink outlet
38
is reduced to have an increased capillary force, and the channel resistance of the ink supply path
34
is reduced.
Thus, ink is supplemented at an increased speed, and with an increased momentum, which causes, as shown in
FIG. 1
, an elongated ink pillar
46
to project from the ink outlet
38
, before it deforms into an ink drop. In the deformation, the elongated ink pillar
46
is broken so that a leading upper portion is changed into a main drop
48
and a trailing lower portion is separated into a number of relatively large low-speed satellites
50
,
52
such as in FIG.
2
. Such satellites adversely affect the printing.
Moreover, as a volume of ink is supplemented with an increased momentum, as shown in
FIG. 3
, an ink meniscus
44
at a top end of the ink outlet
38
has an increased tendency to convex outside and concave inside of the outlet
38
. The meniscus
44
thus vibrates with a reduced damping ratio. That is, the vibration of the meniscus
44
is not readily stopped.
As the ink discharge is repeated in a short period, a subsequent discharge occurs immediately after the supplement of ink, so that it may occur when the ink meniscus
44
starts convexing above the ink outlet. This causes an undesirable deformation of an ink drop and an undesirable development of low-speed satellites, resulting in a reduced quality of recording.
Further, some volume of ink may flood over a surface area around the ink outlet
38
, causing an ink drop to be discharged in an oblique direction, or bubbles to be involved, stopping the discharge, with a reduced reliability of recording.
A probable solution to such problems may include entering subsequent discharge after a sufficient damping of vibration, which however is inconsistent with an intended high-speed recording.
The present invention has been achieved with such points in mind.
SUMMARY OF THE INVENTION
It therefore is an object of the present invention to provide an ink jet print head with criteria such as on an sectional area of an ink outlet and a fluid resistance of an ink supply path to achieve an optimal high-speed ink discharge with an increased reliability and an improved cost effect, without additional elements.
In the present invention, an ink outlet is tapered, with a gradually reduced diameter, toward an orifice plate surface. Supposing a straight aperture of a diameter, it is typical that the quantity Q of an ink drop discharged from a print head of an identical resolution is substantially identical, as well as the volume of a void defined by an ink outlet and an ink meniscus drawn back therein just after a discharge of an ink drop, i.e., the quantity Q
r
of ink to be supplemented.
Letting t
r
be a time for the drawn back ink meniscus to restore to an exit level of the ink outlet, and v be a mean flow velocity in the ink outlet,
v=Q
r
/(A·t
r
),
Letting &rgr; be an ink density, and M be a mean momentum per unit volume,
M=&rgr;·Q
r
/(A·t
r
),
Thus, the larger the diameter of the ink outlet is, the smaller the mean momentum becomes, with a reduced frequency of occurrence of an overshooting ink meniscus.
As the overshooting meniscus convexes like a paraboloid of revolution, letting Q
o
be an overshooting volume of ink and h be an overshooting height or projection of ink,
h=2·Q
o
/A.
Thus, the larger the diameter of the ink outlet is, the smaller the overshooting volume of ink becomes.
For a quantity of ink supplemented in a time the larger the diameter of the ink outlet is, the overshooting ink might have the smaller projection h. However, experiments showed that the projection h of an ink overshoot depends on a sectional configuration of the ink supply path, i.e., a channel resistance or flow resistance thereof.
This fact means that an optimized relationship between an ink outlet sectional area and a channel resistance permits a high-speed recording without low-speed satellites.
The inventors found that a subsequent discharge of ink immediately after a concaved meniscus of the ink has restored to an exit level of an ink outlet can be free from an undesirable deformation of a drop of the ink, when an overshooting height or projection h of the ink falls within a range such that:
 
0<h<0.3(q/A),

0
<h<
0
.
25
(
q/A
),
where q is a quantity in volume of the ink drop, and A is a sectional area at the exit level of the ink outlet.
The present invention is based on this fact.
Thus, to achieve the object, a genus of the present invention provides an ink jet print head comprising a substrate member formed with a heating resistor, an ink path defining member provided on the substrate member, for defining an ink supply path including a heating zone in a vicinity of the heating resistor, and an orifice plate member formed with an ink outlet communicating with the ink supply path and laminated on the substrate member, with the ink path defining member interposed therebetween, the ink jet print head generating heat from the heating resistor to discharge a drop of ink from the ink outlet, the ink supply path having a fluid resistance so that a relationship is established such that
0<h<0.3(q/A)

0
<h<
0
.
25
(
q/A
)
, where q is a quantity o the drop of the ink, A is a sectional area at an exit level of the ink outlet, and h is a maximumal projection that a meniscus of the ink has when it projects from the ink outlet after it has restored the exit level from a retreat position it had after the drop of the ink had been discharged.
According to a species of the genus of the invention, the relationship is established such that:
&pgr;{(3q)/(4&pgr;))
⅔
≦A≦&pgr;(3q)/(2&pgr;)}
⅔
.
This is because of the following reason.
An undesirable overshooting height becomes smaller as the ink outlet has an increased diameter. The ink outlet diameter may preferably be increased.
If the ink outlet has

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