Pressure control device for an inkjet pen

Details Pressure control device for an inkjet pen Pressure control device for an inkjet pen

2002-05-16

2003-11-11

Vo, Anh T.N. (Department: 2861)

Incremental printing of symbolic information

Ink jet

Fluid or fluid source handling means

C347S086000

Reexamination Certificate

active

06644795

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a pressure control device for an inkjet pen; in particular, a pressure control device that can enhance assembly yield and reduce assembly time.
2. Description of the Related Art
Conventional ink-jet printing generally relies on the controlled delivery of ink droplets from a reservoir of an inkjet pen to a print medium. Among the printing methods for delivering ink drops from the reservoir to the print head, drop-on-demand printing is known as a commonly used method. Drop-on-demand typically uses thermal bubble or piezoelectric pressure wave mechanisms. A thermal bubble type print head includes a thin film resistor that is heated to cause sudden vaporization of a small portion of ink. The vapid expansion of the ink vapor forces a small drop of ink through a print head nozzle. Although drop-on-demand printing is ideal for sending ink drops from a reservoir to the print head, some mechanism must be included to prevent ink leaking out from the print head when the print head is inactive. Such a mechanism usually can build a slight back pressure at the print head to prevent ink leakage from the pen whenever the print head is inactive. Herein, the term “back pressure” represents the partial vacuum within the reservoir. Back pressure is defined in the positive sense so that an increase in back pressure means the degree of partial vacuum has increased.
When the back pressure is established at all times inside the reservoir, ink is prevented from permeating through the print head. However, the back pressure can not be so high that the print head is unable to overcome the back pressure to eject ink drops. Furthermore, as ambient air pressure decreases, a correspondingly greater amount of back pressure is needed to keep ink from leaking. Accordingly, the back pressure within the inkjet pen has to be regulated whenever ambient pressure drops. Also the pressure within the pen is subjected to what may be termed “operational effects”, as the depletion of ink from the reservoir increases the back pressure of the reservoir. Without regulation of this back pressure increase, the inkjet pen will fail soon because the back pressure is too high for the print head to overcome it and eject ink drops.
Conventionally, the back pressure within the reservoir is controlled by mechanism referred to as accumulators. In general, an accumulator includes an elastomeric bag capable of moving between a minimum volume position and a maximum volume position in response to changes in the back pressure within the reservoir. For example, as ambient pressure drops so that back pressure within the reservoir decreases simultaneously, the accumulator will move to increase the volume of the reservoir to thereby increase the back pressure to a level that prevent ink leakage. Another example is depletion occurring during operation of the pen. In such a case, accumulators will move to decrease the volume of the reservoir to reduce the back pressure to a level within operating range, thereby permitting the print head to continue ejecting ink.
However, although accumulators such as elastomeric bags can automatically adjust the volume of the reservoir to keep the back pressure within the operating range, the extent to which elastomeric bags are capable of expanding is quite limited. Consequently, when ink level gradually drops from the print head, the bag may reach its maximum extent and therefore incapable of any further adjustment of the volume of the reservoir. Hence, the back pressure within the reservoir may increase such that ink droplets are prevented from leaving the print head.
To resolve the aforementioned problems, some inkjet pens employ a device called a “bubble generator”. The bubble generator has an orifice through which ambient air can enter the reservoir. The dimension of the orifice is such that ink is trapped within the orifice to seal off the reservoir by capillary effect. When ambient air pressure is high enough to overcome the liquid seal, air can bubble into the reservoir. Therefore, the back pressure within the reservoir can decrease and capillary effect will take over and re-establish the liquid seal again to prevent entrance of more air bubbles.
In general, bubble generators of inkjet pens must satisfy a few conditions. Firstly, the bubble generator must be able to control back pressure precisely. Secondly, the range of fluctuation of the back pressure within the reservoir must be as small as possible. In other words, as air bubbles enter the reservoir leading to a drop in back pressure, the bubble generator must be able to stop the entrance of bubbles soon enough that a suitable back pressure remains inside. Thirdly, the bubble generator must have self-wetting capability. The liquid seal must be able to prevent the entrance of bubbles even when most of the ink within the reservoir is used up, or alternately when the inkjet pen is tilted so much that the bubble generator is no longer immersed below the ink.
Referring to FIG.
1
and
FIG. 2
, a conventional bubble generator
118
according to U.S. Pat. No. 5,526,030 is shown. The bubble generator
118
installed within the reservoir
102
has an orifice
122
and a sphere
124
.
FIG. 2
is a top view showing the surrounding structure of the bubble generator
118
. As shown in
FIG. 2
, the internal side-walls of the orifice
122
contain equidistantly spaced protruding ribs
126
,
128
for centering the sphere
124
. The circular gap
120
between the sphere
124
and the orifice
122
is located where ambient bubbles are produced. Normally, a bubble generator
118
as above is able to meet the demands required for printing with an inkjet pen. In general, the entrance of bubbles into the inkjet pen is determined by surface tension of the ink itself, static pressure of the ink column and the gap
120
between the sphere
124
and the orifice
122
, as shown in FIG.
3
. Usually, the greater the surface tension of the ink or smaller the gap between the sphere
124
and the orifice
122
, the higher will be the back pressure required within the reservoir
102
before air bubbles will start to enter. In addition, the static pressure of the ink column within the reservoir
102
can affect the value of back pressure required before air bubbles begin to enter the reservoir. Therefore, as ink level gradually drops, static pressure of the ink column will decrease leading to the entrance of air bubbles at a smaller back pressure. In summary, major drawbacks of the aforementioned pressure control technique includes:
1. The value of back pressure within the reservoir before the bubble generator starts to function is related to surface tension of the ink used. Since various inks may have different surface tension, the minimum back pressure under which air bubbles can enter the reservoir may be different for each type of ink. Consequently, the gap between the sphere and the orifice must be designed for various inks.
2. The value of back pressure within the reservoir before bubble generator starts to function is also related to the static pressure generated by the column of ink. As ink level within the reservoir drops gradually, static pressure acting on the bubble generator will drop making it easier for air bubbles to enter the reservoir. Often this will lead to a lowering of back pressure within the reservoir, and the adjustable range of the accumulator will be reduced.
3. The gap between the sphere and the orifice has to be precisely engineered to permit the entrance of air bubbles at the correct back pressure within the reservoir. This will increase difficulties in fabricating the reservoir of an ink-jet pen.
FIG. 4
shows another conventional pressure control device
410
for an inkjet pen according to U.S. Pat. No. 6,213,598. During the assembly, a flat spring
330
is welded to the bottom of an inkjet pen
400
so that the flat spring
330
presses a sphere
320
of a bubble generator. Since the flat spring
330
is located at the bottom of the inkjet pen, it is difficult to dis

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