Method and apparatus for extending the environmental operating range of an ink jet print cartridge

An ink jet print cartridge includes an ink reservoir, a print head for ejecting ink from the reservoir and first and second pressure control mechanisms for limiting the reservoir underpressure. The first pressure control mechanism limits reservoir underpressure by controllably introducing replacement fluid (i.e. air or ink) thereto. The second pressure control mechanism limits reservoir underpressure by changing the volume thereof. The two pressure control mechanisms cooperate to regulate the underpressure in the reservoir at a desired value over a broad range of environmental excursions and permit use of a volumetrically efficient package.

FIELD OF THE INVENTION 
The present invention relates to ink jet printing systems, and more 
particularly to a method and apparatus for extending the environmental 
operating ranges of such systems. 
BACKGROUND AND SUMMARY OF THE INVENTION 
Ink jet printers have become very popular due to their quiet and fast 
operation and their high print quality on plain paper. A variety of ink 
jet printing methods have been developed. 
In one ink jet printing method, termed continuous jet printing, ink is 
delivered under pressure to nozzles in a print head to produce continuous 
jets of ink. Each jet is separated by vibration into a stream of droplets 
which are charged and electrostatically deflected, either to a printing 
medium or to a collection gutter for subsequent recirculation. U.S. Pat. 
No. 3,596,275 is illustrative of this method. 
In another ink jet printing method, termed electrostatic pull printing, the 
ink in the printing nozzles is under zero pressure or low positive 
pressure and is electrostatically pulled into a stream of droplets. The 
droplets fly between two pairs of deflecting electrodes that are arranged 
to control the droplets' direction of flight and their deposition in 
desired positions on the printing medium. U.S. Pat. No. 3,060,429 is 
illustrative of this method. 
A third class of methods, more popular than the foregoing, is known as 
drop-on-demand printing. In this technique, ink is held in the pen at 
below atmospheric pressure and is ejected by a drop generator, one drop at 
a time, on demand. Two principal ejection mechanisms are used: thermal 
bubble and piezoelectric pressure wave. In the thermal bubble systems, a 
thin film resistor in the drop generator is heated and causes sudden 
vaporization of a small portion of the ink. The rapidly expanding ink 
vapor displaces ink from the nozzle causing drop ejection. U.S. Pat. No. 
4,490,728 is exemplary of such thermal bubble drop-on-demand systems. 
In the piezoelectric pressure wave systems, a piezoelectric element is used 
to abruptly compress a volume of ink in the drop generator, thereby 
producing a pressure wave which causes ejection of a drop at the nozzle. 
U.S. Pat. No. 3,832,579 is exemplary of such piezoelectric pressure wave 
drop-on-demand systems. 
The drop-on-demand techniques require that under quiescent conditions the 
pressure in the ink reservoir be below ambient so that ink is retained in 
the pen until it is to be ejected. The amount of this "underpressure" (or 
"partial vacuum") is critical. If the underpressure is too small, or if 
the reservoir pressure is positive, ink tends to escape through the drop 
generators. If the underpressure is too large, air may be sucked in 
through the drop generators under quiescent conditions. (Air is not 
normally sucked in through the drop generators because the drop generators 
comprise capillary tubes which are able to draw ink against the partial 
vacuum of the reservoir.) 
The underpressure required in drop-on-demand systems can be obtained in a 
variety of ways. In one system, the underpressure is obtained 
gravitationally by lowering the ink reservoir so that the surface of the 
ink is slightly below the level of the nozzles. However, such positioning 
of the ink reservoir is not always easily achieved and places severe 
constraints on print head design. Exemplary of this gravitational 
underpressure technique is U.S. Pat. No. 3,452,361. 
Alternative techniques for achieving the required underpressure are shown 
in U.S. Pat. No. 4,509,062 and in application Serial No. 07/115,0l3 filed 
Oct. 28, 1987, now 4,791,438, both assigned to the present assignee. In 
the former patent, the underpressure is achieved by using a bladder type 
ink reservoir which progressively collapses as ink is drawn therefrom. The 
restorative force of the flexible bladder keeps the pressure of the ink in 
the reservoir slightly below ambient. In the system disclosed in the 
latter patent application, the underpressure is achieved by using a 
capillary reservoir vent tube that is immersed in ink in the ink reservoir 
at one end and coupled to an overflow catchbasin open to atmospheric 
pressure at the other. The capillary attraction of ink away from the 
reservoir induces a slightly negative pressure in the reservoir. This 
underpressure increases as ink is ejected from the reservoir. When the 
underpressure reaches a threshold value, it draws a small volume of air in 
through the capillary tube and into the reservoir, thereby preventing the 
underpressure from exceeding the threshold value. 
While the foregoing two techniques for maintaining the ink pressure below 
ambient have proven highly satisfactory and unique in many respects, they 
nevertheless have certain drawbacks. The bladder system, for example, is 
not as volumetrically efficient as might be desired. To minimize the 
variability of underpressure as a function of reservoir volume, the 
bladder is desirably of rounded shape. Best volumetric efficiency is 
obtained, however, if the bladder has a rectangular shape. (Even with a 
rounded shape, the underpressure is still a function of the bladder's 
state of collapse and eventually increases to the point that no more ink 
can be drawn therefrom, even though ink in the reservoir is not 
exhausted.) 
The capillary system suffers with environmental excursions. If the ambient 
temperature increases, or if the ambient pressure decreases, the air 
trapped inside the ink reservoir expands. This expansion drives ink from 
the reservoir and out the printhead nozzles where it may contact the user. 
Consequently, it is an object of the present invention to provide an ink 
jet ink reservoir that overcomes these drawbacks of the prior art. 
It is a more particular object of the present invention to extend the 
pressure and temperature range over which a volumetrically efficient ink 
jet ink reservoir can operate without leaking. 
According to one embodiment of the present invention, an ink jet print head 
is provided with an ink reservoir having two portions: a fixed volume 
portion and a variable volume portion. The fixed volume portion can be a 
rigid chamber. The variable volume portion can be a flexible bladder in a 
wall of the rigid chamber. Due to volumetric efficiency considerations, 
the fixed volume portion is desirably larger than the variable volume 
portion. 
Beneath the reservoir is a catchbasin operated at ambient pressure into 
which ink can be pressure driven from the reservoir through a small 
coupling orifice. The coupling orifice serves both to convey ink from the 
reservoir into the catchbasin and to convey fluid (ink or air) from the 
catchbasin back into the reservoir, depending on the pressure 
differential. (Due to its occasional role of introducing air into the 
reservoir, the orifice is sometimes termed a "bubble generator.") 
In normal operation, the partial vacuum left in the reservoir when ink is 
ejected out the print nozzles first causes the flexible bladder portion of 
the reservoir to collapse. After a certain amount of ink is ejected from 
the reservoir, the partial vacuum reaches a point at which it draws air 
into the reservoir from the catchbasin through the small bubble generator 
orifice. The orifice is sized to begin this bubbling action at a desired 
underpressure--five inches of water in the illustrated embodiment. 
Thereafter, as printing continues, the additional underpressure caused by 
the continued ejection of ink is regulated by the introduction of a 
corresponding volume of air through the bubble generator orifice. 
If the ambient temperature rises, causing the air in the reservoir to 
expand (or if the ambient pressure diminishes, with similar effect), the 
bladder starts to restore and expand towards its uncollapsed state so as 
to contain the additional reservoir volume. In so doing, the bladder 
continues to exert the bladder restorative force on the ink, maintaining 
the pressure in the reservoir below ambient to keep the ink in the pen. 
In the foregoing case of rising temperature (or decreasing ambient 
pressure), the bladder restorative force continues to keep the reservoir 
at a pressure slightly below ambient until the reservoir volume has 
increased to fully inflate the bladder. At this point, the bladder can no 
longer serve as a volumetric accumulator and ink is forced to flow through 
the bubble generator orifice into the catchbasin. (Ink is not driven out 
through the print nozzle orifii because these orifii are substantially 
smaller than the bubble generator orifice. Consequently, they require a 
higher reservoir pressure to drive ink therethrough. This higher pressure 
is generally never reached because the bubble generator orifice acts to 
relieve the reservoir pressure before the higher pressure can be 
attained.) 
When the ambient temperature thereafter falls, causing the air pressure in 
the reservoir to diminish (or when the ambient pressure rises, or when ink 
is ejected from the reservoir by printing, all with similar effect), ink 
is drawn from the catchbasin by the pressure differential until it is 
exhausted. Thereafter, the bladder collapses until the partial vacuum in 
the reservoir is sufficient to draw air through the orifice from the 
catchbasin, as described above. 
While the foregoing description has focused on a very particular embodiment 
of an ink jet pen according to the present invention, the invention can 
more generally be described as including: 
a) an ink reservoir; 
b) a print head for ejecting ink from the reservoir and thereby leaving a 
negative pressure therein; 
c) a first pressure control mechanism for limiting the negative pressure in 
the ink reservoir by controllably introducing replacement fluid (i.e. air 
or ink) thereto; and 
d) a second pressure control mechanism for limiting the negative pressure 
in the ink reservoir by changing the volume thereof. 
The foregoing and additional objects, features and advantages of the 
present invention will be more readily apparent from the following 
detailed description, which proceeds with reference to the accompanying 
drawings.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2, an ink jet print head 10 according to one 
embodiment of the present invention includes an ink reservoir 12 having 
two portions. The first portion 14 is of fixed volume and is formed by 
rigid walls 16, 18, 20, 22, 24, etc. The second portion 26 is of variable 
volume and comprises a flexible bladder 27 mounted behind an opening in 
one of the rigid walls. 
Extending downwardly from the fixed volume portion 14 is a well 28 with a 
print head 30 at the bottom thereof. Ink from the reservoir 12 is drawn 
through a filter 32 and into the print head 30 from which it is ejected 
towards the printing medium by thermal or piezoelectric action, as is well 
known in the art. 
Also in the bottom portion of well 28 is a small orifice 36 (FIG. 2) that 
couples the ink reservoir 12 to a catchbasin 38 positioned at the bottom 
of the assembly. Orifice 36 serves both to permit ink to pass from the 
reservoir 12 into the catchbasin 38 and to permit fluid (air or ink) to 
pass from the catchbasin into the reservoir, depending on the pressure 
difference between the two regions. (As noted earlier, this orifice 36 is 
sometimes termed a bubble generator orifice due to its occasional role in 
introducing air bubbles into the reservoir.) The size of the bubble 
generator orifice 36 is selected to be larger than the size of the print 
nozzle orifii so that, in over pressure conditions, ink will 
preferentially flow out the bubble generator orifice 36 instead of out the 
print nozzles. However, the bubble generator orifice 36 is small enough 
that the ink's surface tension prevents it from being gravitationally 
driven therethrough--there must be a driving pressure differential. In the 
illustrated embodiment, the bubble generator orifice diameter is 0.0078 
inches and the print nozzle diameter is 0.0020 inches. Catchbasin 38, to 
which the bubble generator orifice 36 leads, is vented to atmospheric 
pressure by a vent 40 located in the upper sidewall of the catchbasin, 
beneath the platform 24 in which the bladder 26 is mounted. 
In operation, the reservoir 12 is initially filled with ink through an 
opening 42 which is thereafter sealed with a plug 44. When the pen is 
first printed, ink ejected from the print head leaves a corresponding 
partial vacuum or underpressure in the reservoir 12 which causes the 
flexible bladder 27 to begin collapsing. The collapsing of the bladder 
reduces the reservoir volume and thus slows the rate at which the partial 
vacuum builds with continued ejection of ink. 
Despite the bladder's moderating action on reservoir pressure, the 
underpressure nonetheless continues to increase with continued ejection of 
ink. This increase continues until the pressure differential between the 
ink reservoir 12 and the vented catchbasin 38 is sufficient to pull a 
bubble of air through the bubble generator orifice 36 and into the 
reservoir. This bubble of air replaces a volume of ink that has been 
ejected from the reservoir and thereby relieves part of the partial vacuum 
in the reservoir. Thereafter, continued ejection of ink will not further 
collapse the bladder 27 but will instead draw in additional bubbles of air 
through the bubble generator 36. The bubble generator thus acts as a 
pressure regulator that controllably introduces air into the reservoir so 
as to prevent the reservoir pressure from fully attaining ambient. 
FIG. 3 is a chart illustrating the relationship between the reservoir 
underpressure and the ejected ink volume. Before any ink is ejected from 
the reservoir, the reservoir may be at a slight underpressure by reason of 
the restorative force of the flexible bladder pulling on the ink in the 
reservoir. As printing begins, the underpressure builds slowly as the 
bladder collapses, as shown by the solid curve. (If there was no flexible 
bladder present to moderate the underpressure, it would increase much more 
rapidly, as shown by the dashed curve labelled "A".) 
As the ejected ink volume increases, the curve may become somewhat 
irregular, due to the non-linear behavior of the bladder as it folds onto 
itself while collapsing. At the point labelled "B", the underpressure is 
sufficient to start pulling bubbles through the bubble generator orifice 
36 and the underpressure thereafter stabilizes around this "bubble 
pressure" (five inches of water in the illustrative embodiment). The 
underpressure drops suddenly each time a bubble is introduced and then 
increases back up towards the bubble pressure with continued ejection of 
ink. When the bubble pressure is again reached, another bubble is 
introduced and the underpressure falls again. The process continues until 
the reservoir is exhausted of ink. (Line "C" in FIG. 3 represents the 
underpressure that would occur if the bubble generator was omitted. As can 
be seen, the underpressure would rise rapidly and would soon prevent the 
ejection of ink from the pen.) 
While ejection of ink is the principle mechanism causing reservoir 
underpressure to vary, it is not the only one. Environmental factors, such 
as ambient pressure and temperature, also play a role. For example, if the 
ambient pressure outside the reservoir increases, the reservoir 
underpressure (i.e. its partial vacuum relative to ambient) increases as 
well. Similarly, if the ambient temperature decreases, the air inside the 
reservoir contracts according to the ideal gas laws, causing a 
corresponding reduction in net reservoir volume and with it a 
corresponding increase in the reservoir underpressure. In both cases, the 
bladder and bubble generator orifice act as described earlier to 
counteract these changes in reservoir underpressure and regulate the 
underpressure near the desired value. 
Environmental factors can also tend to decrease the reservoir underpressure 
(i.e bring the ink pressure up towards, or even above ambient pressure). 
This can occur, for example, if the ambient pressure falls or if the 
ambient temperature rises. In such cases, the bladder restores and expands 
towards its non-collapsed state to relieve the increased pressure and 
counteract this effect. In so doing, it continues to exert the bladder 
restoring force on the ink to hold it in the reservoir. 
If the ambient pressure continues to fall, or if the ambient temperature 
continues to rise, the bladder will continue to exert its restorative 
force on the ink and maintain it below atmospheric pressure until the 
bladder becomes fully inflated. Thereafter, further increases in ink 
pressure will drive ink through the bubble generator 36 and into the 
catchbasin 38. 
At this point the bladder 27 is fully expanded and the catchbasin 38 
contains ink. When conditions thereafter change and the reservoir 
underpressure increases (i.e. by ejection of ink from the reservoir, by an 
increase ambient pressure, or by a decrease in ambient temperature), the 
pen 10 draws ink through the bubble generator 36 into the reservoir 12 
from the catchbasin 38. Note that the pen in this circumstance operates 
differently than when the catchbasin contains only air. When the 
catchbasin contains only air and the underpressure increases, the 
underpressure is moderated by a collapse of the bladder. If the catchbasin 
contains ink, however, the underpressure is moderated by drawing ink into 
the reservoir from the catchbasin. The difference is attributed to the 
higher pressure differential required to pull a bubble of air into the 
ink-filled reservoir than to pull more ink. The air bubble has surface 
tension that must be overcome before it can bubble into the reservoir. The 
ink from the catchbasin does not. 
Continued ejection of ink from the reservoir (or environmental change that 
tends to increase underpressure) continues to draw ink from the catchbasin 
into the reservoir until the ink in the catchbasin is exhausted. 
Thereafter, the situation is similar to that before the pen has been 
used--the catchbasin is dry and the bladder is fully expanded. Further 
ejection of ink from the pen (or corresponding environmental change) 
causes the bladder to collapse. In its collapsed (or partially collapsed) 
state, the bladder exerts a restorative force on the ink which maintains 
the pressure in the reservoir below ambient. The bladder continues to 
collapse with further ejection of ink until the bladder restorative force 
(i.e. the reservoir underpressure) reaches the point at which air bubbles 
are drawn through bubble generator 36. The process thereafter continues 
substantially as described earlier, with a bubble introduced through the 
bubble generator orifice 36 each time the reservoir underpressure exceeds 
the bubble pressure. 
From FIG. 2 it can be seen that the bubble generator orifice 36 leading to 
the catchbasin is not at the lowest point of the catchbasin. However, the 
catchbasin is desirably formed of plastic that causes the ink thereon to 
bead in an upright geometry under the force of its own surface tension. 
This permits the orifice 36 to drain the catchbasin substantially 
completely despite its elevation above the catchbasin floor. The location 
of the orifice near the corner 46 of the catchbasin also aids in complete 
ink withdrawal since the ink tends to collect in this corner into which it 
was introduced. 
From the foregoing discussion, it will be recognized that one important 
requirement is to design the bladder 27 (i.e. its material and geometry) 
so that its restorative pressure is between the bubble pressure and the 
ambient pressure. That is, the bladder should be designed to collapse over 
a range that includes partial vacuums of between zero and five inches of 
water. If the bladder does not operate in this range, it will be 
ineffective in regulating reservoir pressure since the bubble generator 
would always act to relieve any excessive reservoir underpressure before 
the bladder was prompted to collapse. In the illustrated embodiment, the 
bladder 27 is formed of ethylene propylene diene monomer having a 
thickness of 0.024 inches and a radius of curvature of 0.451 inches. 
In the preferred embodiment, the bladder is not permitted to assume its 
fully hemispherical shape. Such a geometry resists collapsing. Instead, 
the bladder is dimpled, either during fabrication or by a dimpling finger 
48 (FIG. 1). By this arrangement, the bladder can begin collapsing 
immediately as the underpressure increases, and does not require a high 
initial underpressure as would a hemispherical bladder before it begins 
its collapse. 
FIGS. 4 through 5 illustrate alternative embodiments of the present 
invention. In the FIG. 4 embodiment, the variable volume portion of the 
reservoir is formed by a bag 50. Bag 50 has an end piece 52 positioned 
therein and is urged towards a fully open position by a spring 54. The 
spring 54 is biased between the bag end piece 52 and a spring boss 56 in 
the top of the reservoir. Operation of the FIG. 4 embodiment is 
substantially identical to that of the FIGS. 1-2 embodiment except that 
the reservoir underpressure is a more linear function of ejected ink 
volume since the irregular collapsing of a hemispherical bladder is 
avoided. 
FIG. 5 shows another embodiment similar to FIGS. 1,2 and 4 but employing a 
rolling diaphragm 58 as the variable volume portion of the reservoir. The 
rolling diaphragm again behaves substantially linearly in response to 
increases in reservoir underpressure. 
FIG. 6 shows yet another embodiment of the present invention. In this 
embodiment the variable volume portion of the reservoir is positioned 
above, rather than below, the fixed volume portion. The variable volume 
portion here includes a rolling diaphragm 60 in combination with a piston 
62, a fitment 64 and a spring 66. 
In operation, the reservoir 12 is initially filled with ink and the piston 
62 is forced to a fully upward position by spring 66, thereby fully 
stretching diaphragm 60. As ink is ejected from the pen, the reservoir 
underpressure increases. As the underpressure increases, the piston 62 
travels downwardly, with very little friction, until it finally stops in 
contact with a bottom platform 68. Further ejection of ink from the 
reservoir causes air to enter the reservoir through the bubble generator 
36 to regulate the reservoir underpressure. This air accumulates. 
Again, temperature and altitude changes (exogeneous effects) may act on the 
pen, causing the reservoir underpressure to diminish. When this occurs, 
the piston 62 moves vertically upward, acted on by the now unbalanced air 
pressure over piston force and the spring force. This movement causes the 
pen to reestablish a new underpressure equilibrium, just slightly less 
than the prior condition. This process can continue until the 
piston/diaphragm/spring components reach their original uppermost vertical 
position. 
If desired, the pen of FIG. 6 can be equipped with a ball check valve 70 to 
prevent the inadvertent introduction of air into the reservoir. It will be 
recognized that if the pen (or the printer in which it is mounted) is 
inverted, ink will flow away from the bubble generator orifice 36 and may 
permit air to freely enter the reservoir, reducing underpressure to zero. 
This, in turn, may cause a small amount of ink to flow out the pen's 
printing orifii. The unrestricted introduction of air to the reservoir 
also defeats the pen's temperature and elevation compensation capabilities 
by permitting the piston/diaphragm assembly to return to the original, 
extended position, with an air volume in the reservoir. 
To prevent these undesirable conditions, a ball check 72 falls to a seat 74 
provided near the location of the bubble generator whenever the pen is 
inverted, thereby effectively sealing the bubble generator and preserving 
the reservoir underpressure. When the pen is returned to the normal 
position, the ball falls from the seat and permits normal underpressure 
regulation to resume. Although shown in just this FIG. 6 embodiment, the 
ball check valve 70 can be used in any form of the invention. 
Finally, the pen of FIG. 6 is shown as including absorbent foam 76 in the 
catchbasin. This foam captures and retains any ink driven to the 
catchbasin by exogenous effects and prevents any ink from flowing out the 
air vent. At the same time, and at all times, the absorbent foam allows 
air to pass freely between the vent and the bubble generator, thereby 
ensuring normal underpressure regulation. This foam can be used in any 
embodiment and is a last resort to keep ink off of the user. 
The above-described arrangements provide a variety of advantages over the 
prior art. Principal among these is the extended pressure and temperature 
range over which the ink reservoirs can hold ink in the pen. As an added 
benefit, these arrangements permit the catchbasins to be used to store 
part of the initial load of ink, thereby increasing volumetric efficiency. 
Finally, these designs permit essentially all of the ink to be used for 
printing, since none is caught in a tightly collapsed bladder. (Any ink 
that remains in the bladder 27 of FIG. 1 can be dislodged by tilting the 
pen so the ink can flow into the well 28 from which it can be printed.) 
Having described and illustrated the principles of our invention with 
reference to a preferred embodiment and several variations thereof, it 
should be apparent that the invention can be modified in arrangement and 
detail without departing from such principles. For example, while the 
invention has been illustrated with reference to a vent in the upper side 
of the catchbasin, other vent geometries, such as a chimney extending 
upwardly from the floor of the catchbasin as shown in FIG. 6, could 
alternatively be used. Similarly, while the invention has been illustrated 
with reference to a bubble generator orifice coupling the reservoir to the 
catchbasin, a variety of other valve mechanisms, such as the check valve 
disclosed in U.S. Pat. No. 4,677,447, could be substituted therefor. 
In view of the wide range of embodiments and uses to which the principles 
of the present invention can be applied, it should be understood that the 
apparatuses and methods described and illustrated are to be considered 
illustrative only and not as limiting the scope of the invention. Instead, 
our invention is to include all such embodiments as may come within the 
scope and spirit of the following claims and equivalents thereof.