Refill method for ink-jet print cartridge

The ink-jet print cartridge, which has an internal accumulator for maintaining appropriate back pressure within the pen reservoir, and a bubble generator for providing additional regulation, is refilled by a process that provides for the reestablishment of the necessary back pressure upon refilling and that prevents leakage arising as a result of the refilling operation.

TECHNICAL FIELD 
The present invention is directed to a method of refilling the reservoir of 
an ink-jet print cartridge. 
BACKGROUND INFORMATION 
Ink-jet printing generally involves the controlled delivery of ink drops 
from an ink-jet print cartridge reservoir to a printing surface. One type 
of ink-jet printing, known as drop-on-demand printing, employs a print 
cartridge or pen that has a print head that is responsive to control 
signals for ejecting drops of ink from an associated ink reservoir. 
One type of drop-on-demand print head uses a thermal bubble mechanism for 
ejecting drops. A thermal bubble type print head includes a thin-film 
resistor that is heated to cause sudden vaporization of a small portion of 
the ink. The rapid expansion of the ink vapor forces a small amount of ink 
through an associated one of a number of nozzles in the print head. 
Conventional drop-on-demand print heads are effective for ejecting or 
"pumping" ink drops from a pen reservoir, but require mechanisms for 
preventing ink from leaking through the print head nozzles when the print 
head is inactive. Accordingly, drop-on-demand techniques require that the 
fluid in the ink reservoir must be stored in a manner that provides a 
slight back pressure at the print head to prevent ink leakage from the pen 
whenever the print head is inactive. As used herein, the term "back 
pressure" means the partial vacuum within the pen reservoir that resists 
the flow of ink through the print head. Back pressure is considered in the 
positive sense so that an increase in back pressure represents an increase 
in the partial vacuum. Accordingly, back pressure is measured in positive 
terms, such as water column height. 
The back pressure at the print head must be at all times strong enough for 
preventing ink leakage. The back pressure, however, must not be so strong 
that the print head is unable to overcome the back pressure to eject ink 
drops. Moreover, the ink-jet pen must be designed to operate despite 
environmental changes that cause fluctuations in the back pressure. 
A severe environmental change that affects reservoir back pressure occurs 
during air transport of an ink-jet pen. In this instance, ambient 
atmosphere pressure decreases as the aircraft gains altitude and is 
depressurized. As ambient air pressure decreases, a correspondingly 
greater amount of back pressure is needed to keep ink from leaking through 
the print head. Accordingly, the level of back pressure within the pen 
must be regulated during times of ambient pressure drop. 
The back pressure within an ink-jet pen reservoir is also subjected to what 
may be termed "operational effects." One significant operational effect 
occurs as the print head is activated to eject ink drops. The consequent 
depletion of ink from the reservoir increases (makes more negative) the 
reservoir back pressure. Without regulation of this back pressure 
increase, the ink-jet pen will eventually fail because the print head will 
be unable to overcome the increased back pressure to eject ink drops. 
Past efforts to regulate ink-jet reservoir back pressure in response to 
environmental changes and operational effects have included mechanisms 
that may be collectively referred to as accumulators. 
Described in U.S. patent application Ser. No. 07/805,438, U.S. Pat. No. 
5,409,134, which application is owned by the assignee of the present 
application, is a pressure-sensitive accumulator for ink-jet pens. The 
accumulator described in that application provides an accumulator working 
volume that is sufficient for operating the pen notwithstanding extreme 
environmental changes and operational effects on the back pressure within 
the reservoir. The accumulator moves to change the overall volume of the 
reservoir, thereby to regulate back pressure level changes so that the 
back pressure remains within an operating range that is suitable for 
preventing ink leakage while permitting the print head to continue 
ejecting ink drops. 
For example, as the difference between ambient pressure and the back 
pressure within the pen decreases as a result of ambient air pressure 
drop, the accumulator moves to increase the reservoir volume, thereby to 
increase the back pressure to a level, within the range discussed above, 
that prevents ink leakage. Put another way, the increased volume 
attributable to accumulator movement prevents a decrease in the difference 
between ambient air pressure and back pressure that would otherwise occur 
if the reservoir were constrained to a fixed volume as ambient air 
pressure decreased. 
The accumulator also moves to decrease the reservoir volume whenever 
environmental changes or operational effects (for example, ink depletion 
occurring during operation of the pen) cause an increase in the back 
pressure. The decreased reservoir volume attributable to accumulator 
movement reduces the back pressure to a level within the operating range, 
thereby permitting the print head to continue ejecting ink. 
Accumulators are usually equipped with internal or external resilient 
mechanisms that continuously urge the accumulators toward a position for 
increasing the volume of the reservoir. The effect of the resilient 
mechanisms is to retain a sufficient minimum back pressure within the 
reservoir (to prevent ink leakage) even as the accumulator moves to 
increase or decrease the reservoir volume. 
Even with a large-working-volume accumulator as just mentioned, there may 
be instances where the accumulator reaches its maximum working volume (for 
example, to reduce the back pressure within the reservoir as most of the 
ink is depleted during printing) while an appreciable amount of ink 
remains in the reservoir. Continued printing to remove this remaining 
amount of ink could increase the back pressure (which can no longer be 
regulated inasmuch as the accumulator has reached its maximum working 
volume) by a level outside of the operating range, which increase would 
cause the problem of print head failure owing to too high a back pressure 
level. 
To avoid this problem, some ink-jet pens incorporate a "bubble generator." 
A bubble generator is an orifice formed in the ink reservoir to allow 
fluid communication between the interior of the reservoir and the ambient 
atmosphere. The orifice is sized such that the capillarity of the ink 
normally retains a small quantity of ink in the orifice as a liquid seal. 
The geometry of the orifice is such that when the back pressure approaches 
the limit of the operating range of the print head, the back pressure 
overcomes the capillarity of the ink and the liquid seal is broken. 
Ambient air then "bubbles" into the reservoir to reduce the back pressure 
so that the print head can continue to operate. Ideally, when the back 
pressure drops, ink from the reservoir reenters the orifice and reinstates 
the liquid seal. 
In the past, ink-jet pens of the type just described were usually disposed 
of once the reservoir was depleted. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method for refilling the ink 
reservoir of an ink-jet pen that includes an accumulator of the type 
described above. The method is also extendible to pens that include such 
an accumulator in combination with a bubble generator as mentioned above. 
The method may be employed for substantially replacing all the depleted ink 
in the reservoir or for replacing only a portion of the depleted ink. The 
method allows the continued use of the same pen body and print head, so 
that only reservoir ink is replaced, instead of the entire pen. 
The present method permits the refilling of the pen reservoir while 
maintaining or reestablishing a minimum back pressure within the refilled 
reservoir for the proper operation of the pen.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 1-3 show a preferred embodiment of an ink-jet pen 15 to which the 
refill method of the present invention is applicable. It will be 
understood that the term "print cartridge" or "pen" may be interchangeably 
used in this specification. The pen 15 includes an internal accumulator to 
provide compensation for severe environmental changes or operational 
effects on the back pressure within the ink-jet pen reservoir. 
The pen includes a reservoir 24 having rigid side walls that are configured 
to hold a quantity of ink. A well 26 is formed in the bottom of the 
reservoir 24 near one side wall 28 of the pen. A thermal-bubble type print 
head 30 is fit into the bottom wall 32 of the reservoir well 26 for 
ejecting ink drops through nozzles 29 in the print head. The configuration 
of the reservoir walls and print head may be substantially as provided in 
the pen component of an ink-jet printer manufactured by Hewlett-Packard 
Company of Palo Alto, Calif., under the trademark DeskJet. 
An accumulator 20 (FIG. 3) is attached to a cap 40 that is sealed to the 
top of the side walls of the reservoir 24. The accumulator 20 includes an 
expandable bag 42 that is mounted to a spring 44. The bag 42 and spring 44 
are fastened to a fitment 46 that has an upwardly projecting boss 48. The 
boss 48 is sealed to a cylindrically shaped sleeve 47 that is integrally 
formed with the top of the cap 40. 
The bag 42 is fastened to the fitment 46 so that the interior of the bag is 
in fluid communication with the lower end of a central duct 50 that passes 
through the boss 48. The fitment 46 is mounted to the cap 40 of the pen 15 
with the duct 50 arranged so that the upper end 51 of the duct is in fluid 
communication with ambient air. Accordingly, the interior of the bag 42 is 
in fluid communication with ambient air. 
With the accumulator 20 in place, the reservoir 24 is filled with ink 
through a sealable port 41. Preferably, a spherical, resilient 
air-impermeable plug 43 is pressed into the port 41 after ink is added, 
thereby sealing or closing the reservoir to permit establishment of the 
back pressure within the reservoir. A slight back pressure (hereinafter 
referred to as the minimum back pressure) is established within the pen 
reservoir 24. The minimum back pressure is the minimum amount of back 
pressure necessary to keep ink from leaking through the print head 30 when 
the print head is inactive. 
As the pen 15 is used for printing, the air pressure within the reservoir 
24 decreases (hence, the back pressure increases) as ink is depleted. 
During printing, the bag 42 expands (FIG. 6) as a result of the back 
pressure increase. The bag expansion decreases the volume of the reservoir 
24 to maintain the reservoir back pressure within a range such that the 
print head 30 is able to continue ejecting ink from the reservoir 24. If 
the ambient pressure should thereafter decrease (for example, during air 
transport of the pen), the bag 42 will contract to increase the reservoir 
volume so that the back pressure within the reservoir 24, relative to 
ambient, does not drop to a level that permits ink to leak from the print 
head 30. 
Expansion of the bag 42 deflects the spring 44. The elasticity of the 
spring 44 tends to contract the bag 42. The spring 44 and bag 42 are 
configured and arranged to define a back pressure and bag volume 
relationship that maintains the reservoir back pressure within an 
operating range that is suitable for preventing ink leakage, while 
permitting the print head 30 to continue ejecting ink drops. Moreover, the 
accumulator 20 is configured so that the maximum volume of the bag 42, 
that is, the working volume of the accumulator, is large enough to 
maintain the reservoir back pressure within the operating range mentioned 
above, despite severe fluctuations in the pressure of the ambient air. 
Turning now to the particulars of the accumulator 20 formed in accordance 
with the present invention, and with particular reference to FIGS. 3, 5 
and 6, the preferred embodiment of the accumulator spring 44 comprises a 
strip of metal, such as stainless steel, having a thickness of 
approximately 75 microns (.mu.) and a yield strength greater than 5,600 
Kg/cm.sup.2. The spring 44 may be stamped or etched from a flat sheet and 
shaped into the relaxed or undeflected configuration shown in FIG. 3. 
The relaxed configuration of the spring 44 includes a flat base 52 having a 
round main aperture 54 formed therethrough. The spring 44 is bent at each 
edge 56, 58 of the base 52. Elongated slots are formed in the spring 44 at 
each base edge 56, 58 to facilitate bending of the spring 44 at the base 
edges 56, 58. 
The spring 44 is formed to have curved legs 62. One leg 62 extends 
downwardly from each edge 56, 58 of the base 52. Each spring leg 62 is 
formed to have a convex surface 64 facing inwardly toward the convex 
surface 64 of the other leg 62. 
Four access holes 71 are formed in the spring base 52. One hole 71 is 
located near each corner of the base 52. A pair of spaced apart access 
holes 72 are formed through the spring legs 62 beneath and near each base 
edge 56, 58. Four other spaced apart access holes 74 are formed through 
the ends 68 of each spring leg 62. The access holes 71, 72, 74 provide 
means for attaching the bag 42 to the spring 44, as described more fully 
below. 
The bag 42 of the present invention is preferably formed of two thin 
flexible sheets 76, 77 (FIG. 6) that are sealed together at their outer 
edges 78. One sheet, the first sheet 76, has an opening 80 for permitting 
the passage of air into and out of the space between the edge-sealed first 
sheet 76 and second sheet 77. The sheets 76, 77 are shaped slightly larger 
(i.e., in width and length) than the spring 44. Moreover, the portion 79 
of the edge 78 of each sheet that is near the tapered part of the spring 
44 is shaped into a smooth curve. 
Preferably, the first and second sheets 76, 77 are formed of a material 
that can be heat-welded (as at the edges 78) and that is substantially 
impermeable to air. Heat-weldable bag material is preferred because such 
material permits an efficient method for forming the bag 42 and for 
attaching the bag 42 to the spring 44 and fitment 46, as will be described 
more fully below. 
Material that is substantially impermeable to air is preferred as bag 
material so that the back pressure within the pen reservoir 24 is not 
reduced by air that passes into the bag 42 through opening 80 and then 
diffuses through the walls of the bag sheets 76, 77 into the reservoir 24. 
In view of the above, a preferred embodiment of the sheets 76, 77 that make 
up the bag 42 comprises a thin "barrier" film of material such as ethylene 
vinyl alcohol (EVOH) covered with thin outer layers of polyethylene. The 
EVOH film is preferably about 12.mu. thick. The polyethylene layers are 
between 15.mu. and 50.mu. thick. 
The EVOH film provides the desired low-air-permeability property. It is 
contemplated, however, that the barrier film for preventing diffusion of 
air through the bag 42 may be formed of a variety of materials such as 
PVDC (SARAN), nylon, polyester or metal foils, or combinations of such 
materials. 
The polyethylene outer layers of the sheets 76, 77 provide the desired 
heat-weldable property. The use of polyethylene as outer bag layers is 
also advantageous because that material generally includes no cure 
accelerators or plasticizers that might leach into and thereby contaminate 
the ink within the reservoir 24, 
Before the bag 42 is formed by edge-welding the sheets 76, 77, two elements 
are placed between the sheets. One element, hereinafter referred to as a 
"release patch" 82, comprises a thin (approximately 25.mu.) sheet of 
material, such as polyester, having a melting point that is substantially 
higher than the melting point of the polyethylene outer layers of the bag 
sheets 76, 77. The release patch 82 is generally circular shaped and 
positioned beneath the opening 80 in the bag 42. Preferably, the release 
patch 82 includes an adhesive on one side for securing the patch 82 to the 
second sheet 77 of the bag 42. The release patch 82 provides a mechanism 
for facilitating attachment of the bag 42 to the fitment 46, as described 
more fully below. 
The second element that is disposed within the bag 42 is a narrow strip, 
hereinafter referred to as a breather strip 84, of perforated polyethylene 
material having a maximum thickness of approximately 375.mu., such as that 
manufactured by Ethyl VisQueen Film Products under the trademark VISPORE. 
The breather strip 84 provides a mechanism for facilitating movement of 
air into and out of the bag 42, as described more fully below. 
The spring 44 and the bag 42 are attached to the underside of the fitment 
46. More particularly, the fitment 46 is formed of polyethylene having a 
higher melting point than the polyethylene outer layers of the bag sheets 
76, 77 and includes a generally flat base plate 86 having an upwardly 
projecting boss 48. The boss 48 is generally cylindrically shaped and has 
a chamfered upper end 49. The boss 48 includes the internal duct 50 that 
extends completely through the boss. 
The fitment base plate 86 includes two concentric annular mounting rims 88 
that are integrally formed with the base plate 86 to protrude downwardly 
therefrom through the main aperture 54 in the base 52 of the spring 44. 
The mounting rims 88, which surround the lower end 90 of the duct 50 are 
employed for fastening the bag 42 to the fitment 46. To this end, the 
portion of the first bag sheet 76 that surrounds the bag opening 80 is 
pressed through the main aperture 54 in the spring 44 to bear upon the 
mounting rims 88. A heated chuck (not shown) is pressed against the second 
sheet 77 of the bag 42 immediately beneath the mounting rims 88. Heat from 
the chuck is transferred from the second sheet 77 via the release patch 82 
to the interface of the mounting rims 88 and the first sheet 76. The 
mounting rims 88, which, as part of the fitment are formed of polyethylene 
having a higher melting point than the bag, are heated to until the rims 
88 and the first sheet 76 flow together to form a weld. Upon cooling, the 
rims 88 bond with the first layer 76 to form an air-tight seal. 
With the bag 42 sealed to the fitment 46 as just described, the only path 
for air into and out of the bag 42 is through the duct 50 in the fitment 
boss 48. 
It can be appreciated that the release patch 82, in addition to 
transferring heat from the chuck to the interface of the first sheet 76 
and mounting rims 88, separates the first and second sheets 76, 77 in the 
region where the heated chuck is applied. Accordingly, the release patch 
82 prevents the two bag sheets 76, 77 from becoming bonded together at the 
mounting rims 88. 
Preferably, the outermost mounting rim 88 of the fitment 46 is sized to 
have a diameter that is just slightly less than the diameter of the main 
aperture 54 in the spring 44. Accordingly, the spring base 52 fits snugly 
around the outermost rim 88. The effect of this fit is to provide a 
registration mechanism for centering the spring aperture 54 beneath the 
duct 50 in the fitment 46. Moreover, the spring base 52 also includes an 
alignment hole (not shown) formed therethrough that mates with a 
downwardly projecting pin (not shown) in the fitment base plate 86. The 
mating alignment hole and pin provide a supplemental registration 
mechanism to ensure that the spring 44 is properly positioned relative to 
the fitment 46. 
The bag 42 is fastened to the fitment 46 and spring 44 in a manner that 
urges the bag into a contracted or minimum volume state. The preferred 
means for fastening the bag 42 includes heat-welding the bag 42 to the 
fitment through the access holes 71, 72 at the base 52 of the spring 44, 
and securing each end 92 of the bag 42 to a corresponding end 68 of a 
spring leg 62. 
More particularly, the underside of the fitment base plate 86 includes four 
downwardly extending posts, each of which fits through an aligned access 
hole 71 in the corner of the spring base 52. The posts pierce the bag 
sheets 76, 77 as a heated platen (not shown) is pressed against the bag 
sheets 76, 77. The platen then spreads and flattens the ends of the posts 
to effectively form a rivet to attach the bag sheets 76, 77 to the fitment 
base plate 86. This operation is performed while the bag 42 is 
substantially completely contracted. 
Each of two opposing ends of the fitment base plate 86 is formed to have an 
extension 94 that is attached to the base plate 86 by two spaced apart 
hinges 95 (FIG. 6). The hinges 95 are thinner (approximately 250.mu.) than 
the base plate 86 and fold around the associated edges 56, 58 of the 
spring base 52 so that each extension 94 covers a pair of access holes 72 
formed beneath and near each edge 56, 58. Each extension 94 includes on 
its underside an outwardly projecting pair of posts 96. Each of the posts 
96 is sized and arranged to fit through an associated access hole 72. With 
the posts 96 extending through the access holes 72, both sheets 76, 77 of 
the bag 42 are pressed against the pairs of posts 96 at each edge 56, 58. 
The posts 96 are then heat-riveted to the contacting bag sheets 76, 77 in 
a manner as previously described. 
The breather strip 84 within the bag 42 is aligned between adjacent access 
holes 72 in the spring and extends completely around each bent edge 56, 58 
of the spring 44. Accordingly, the breather strip 84 facilitates air 
movement through the bag even though the bag is tightly fastened to the 
edges 56, 58 of the spring base 52 at the access holes 72. Moreover, the 
breather strip 84 ensures that the bag 42 will expand (i.e., the sheets 
76, 77 will move apart) despite condensation within the bag, which 
condensation would tend to stick the sheets 76, 77 together. 
The ends 92 of the bag 42 are wrapped around the ends 68 of the spring legs 
62 so that each portion of the bag that is between the edges 56, 58 and 
the leg ends 68 is pulled firmly against the convex surface 64 of each leg 
62 (FIG. 3). The ends 92 of the bag 42 cover the access holes 74 in the 
leg ends so that when heat is applied to the bag 42 at the access holes 
74, the bag 42 will weld to itself within the holes 74 to secure the bag 
ends 92 to the spring leg ends 68. 
The periphery 55 of the fitment boss 48 is sealed to the sleeve 47 in the 
reservoir cap 40 so that no air can pass between the fitment 46 and the 
cap 40. The cap 40 is then sealed to the reservoir side walls with the 
accumulator 20 suspended inside the reservoir 24. The reservoir 24 is then 
filled with ink, as described earlier. 
As noted earlier, the filled pen 22 is provided with a minimum back 
pressure. Calculated at the print head 30, the minimum back pressure 
should be, for example, 2.5 cm water column. Accordingly, the minimum back 
pressure is established by removing some ink from the filled and sealed 
reservoir. 
The minimum back pressure level establishes the low end of the back 
pressure operating range referred to above. The maximum back pressure or 
upper level of the back pressure operating range is that level (for 
example, 11.5 cm water column) above which the print head 30 would be 
unable to "pump" against for ejecting ink drops. 
As the print head 30 operates to eject ink drops from the reservoir 24, the 
consequent reduction in ink volume in the reservoir increases the back 
pressure. If this increase were not regulated, the back pressure in the 
reservoir 24 would rapidly increase beyond the maximum back pressure, and 
the print head 30 would become inoperative. With the present accumulator 
20, however, the back pressure increase above the minimum level tends to 
expand the bag 42. More particularly, as the back pressure increases, the 
relatively higher-pressure ambient air is drawn through the duct 50 in the 
fitment 46 and into the opening 80 in the bag 42. As the bag 42 expands, 
the first sheet 76 of the bag presses against the spring legs 62 so that 
those legs 62 are deflected out of the relaxed, curved configuration (FIG. 
3) into a reverse bowed configuration (FIG. 6). 
The elasticity of the spring legs 62, which tends to contract the bag 42 
against the convex surfaces 64, is substantially overcome by the expansion 
of the bag 42 that is caused by the increase (over minimum) of the back 
pressure within the reservoir 24. The volume decrease in the reservoir 24 
that is attributable to the expansion of the bag 42 maintains the back 
pressure beneath the maximum back pressure discussed above. 
The bag 42 expands to its maximum volume condition as ink is printed out of 
the pen. During this expansion, the bag 42 maintains the back pressure 
beneath the maximum back pressure level. At the point when the bag 42 of 
the preferred embodiment has expanded to its maximum volume condition, 
about 30% of the pen's ink has been printed out. Any further printing will 
cause a further increase in back pressure, which is relieved by the 
introduction of ambient air into the reservoir 24. To this end, the pen 15 
includes a bubble generator 102 formed in the bottom wall 38 of the 
reservoir 24. 
The bubble generator 102 (FIG. 5) consists of a tubular boss 122 and a 
sphere 124 mounted concentrically within the boss. The outside diameter of 
the sphere 124 is smaller than the inside diameter of the boss 122 to 
define an annular orifice 120 (seen in FIG. 4). In the illustrated 
embodiment, the sphere is maintained within the boss by a number of raised 
ribs 126 formed around the interior of the boss. In this manner the sphere 
124 can be easily press fit into the boss 122 and firmly maintained in 
position by the ribs 126. The raised ribs 126 are sized to provide the 
necessary interference for a press fit to maintain the sphere within the 
boss and provide the necessary clearance from the inside wall of the boss. 
The sphere 124 serves as a capillary member to maintain a quantity of ink 
within the boss 122. As a results even when the pen is oriented such that 
the boss is not submerged in the reservoir ink, a quantity of ink is 
trapped within the boss. Due to the curved surface of the sphere, the gap 
between the exterior surface of the sphere and the inner wall of the boss 
is smallest at the orifice and increases as the distance from the orifice 
increases. This geometry, coupled with the capillarity of the ink, 
constantly urges the trapped quantity of ink toward the orifice--the 
smallest portion of the gap--to provide a robust seal. 
To prevent the trapped quantity of ink from drying or solidifying as a 
result of prolonged exposure to the atmosphere, the bubble generator is 
provided with an inlet labyrinth 130 which serves as a vapor barrier. The 
inlet labyrinth, best seen in FIGS. 4 and 5, is a path through which the 
ambient air must travel before contacting the trapped ink. The proximal 
end 131 of the labyrinth opens to the boss 122 and the distal end 133 
opens to ambient air. The length of the labyrinth is sealed from both the 
ambient and the reservoir. As a result, the humidity within the labyrinth 
varies along its length from approximately 100% at the proximal end 131 to 
approximately ambient at the distal end 133. This humidity gradient serves 
to shield the trapped ink from direct contact with ambient air and prevent 
the trapped ink from drying or solidifying. 
The inlet labyrinth is a path having a semicircular cross section. The 
ratio of the cross sectional area to length of the inlet labyrinth should 
be such that the volume of air in the inlet labyrinth effectively blocks 
convective mass transfer. Diffusive vapor losses are driven by the partial 
pressure gradients through the inlet labyrinth. As indicated by Fick's 
Laws of Diffusion, these losses are proportional to the cross sectional 
area of the inlet labyrinth and inversely proportional to the length of 
the inlet labyrinth. The appropriate dimensions of an inlet labyrinth for 
any particular embodiment can be empirically determined by one skilled in 
the art. 
As best seen in FIGS. 3 and 4, the inlet labyrinth 130 in the illustrated 
embodiment, is a trough 132 molded directly into the external surface of 
the pen wall 38. A cover 134 is attached to the reservoir to seal the 
trough 132 between its ends. A hole 136 through the cover at the distal 
end 133 of the trough 132 provides fluid communication between the trough 
(and the bubble generator) and is an inlet for the ambient atmosphere. The 
circuitous configuration of the trough conserves space and reduces the 
size of the cover. 
The inlet labyrinth 130 also serves as an overflow receptacle. If the pen 
is subject to an environmental change, such as a temperature or altitude 
variation, which causes the fluid volume within the reservoir to expand 
beyond the capacity of the reservoir, the excess ink can exit the 
reservoir via the bubble generator and enter the inlet labyrinth 130. 
Subsequently, when the environmental conditions return to normal, or ink 
is depleted from the reservoir, the excess ink can reenter the reservoir. 
To ensure that excess ink in the labyrinth will completely reenter the 
reservoir, it is preferable that the largest cross-sectional dimension of 
the labyrinth is small enough to allow the ink to form a complete meniscus 
across the cross section at any location along the labyrinth. Otherwise, 
small amounts or beads of ink may become stranded in the labyrinth. In the 
illustrated embodiment, the maximum cross-sectional dimension of the 
labyrinth is approximately 0.89 mm. 
The effectiveness of the illustrated ink-jet pen depends on the appropriate 
sizing of the orifice 120, the boss 122, and the sphere 124 to ensure that 
the liquid seal gives way below the maximum allowable back pressure and is 
reinstated above the minimum allowable back pressure. The exact dimensions 
of the various elements of the ink pen will depend on a number of factors, 
such as the surface energies of the materials, the density and surface 
tension of the ink, the desired range of back pressures, and the shape of 
the orifice. Once these factors are known, the proper dimensions can be 
readily calculated or empirically determined by one skilled in the art. 
If, for example, the desired range of back pressures is from 10 cm to 16 cm 
water column and the ink used has a density of approximately 1 g/cm.sup.3 
and a surface tension of approximately 60.2 dynes/cm stainless steel 
sphere having a diameter of approximately 3.18 mm and a polysulfone boss 
having an inside diameter of between 3.34 mm and 3.39 mm will be 
satisfactory. Of course, each particular embodiment of the invention may 
require different dimensions according to its particular parameters. 
In accordance with the method of the present invention, the pen having the 
accumulator 20 and bubble generator 102 as just described may be refilled 
once depleted. The method is carried out so that a sufficient amount of 
back pressure remains in, or is reestablished in, the reservoir after 
refilling. 
A first preferred approach to refilling or adding ink to a partly depleted 
reservoir 24 includes the step of occluding the fluid communication 
between the interior of the accumulator bag 42 with ambient air by 
blocking the upper end 51 of the duct 50. Any of a number of mechanisms 
can be employed for this occluding step. For instance, a piece of 
vinyl-backed adhesive tape may be pressed over the upper end of the duct 
50. Alternatively, the plugged tip of a syringe may be inserted into the 
duct. Any member sized for providing a substantially air-impermeable 
occlusion of the duct 50 during the refilling operation may be used. 
Once the fluid communication between the accumulator bag interior and 
ambient air is occluded, the spherical plug 43 is removed from its port 41 
to open the reservoir for the purpose of adding ink. The plug 43 may be 
pried out by the use of a stylus or scribe. Alternatively, the plug may be 
forced out of its port 41 and into the reservoir by the use of a punch or 
similar mechanism. Prying out the plug 43 will preserve it for use in 
resealing or closing the reservoir after the filling operation. Punching 
the plug into the reservoir will require replacement with a new plug (or 
other suitable mechanism) for closing the reservoir port 41 once the 
filling operation is completed. 
It will be appreciated by one of ordinary skill that once the reservoir is 
open via the removal of the plug 43, back pressure within the reservoir is 
lost, but restored once the pen is refilled, as described more fully 
below. 
FIG. 7 depicts the filling operation, wherein the plugged tip of a syringe 
148 was first inserted into the duct 50; the spherical plug 43 (not shown) 
was removed as described above; and ink is delivered through the port 41 
to the open reservoir via a suitable conduit 150 from an external supply. 
As a result of the loss of back pressure attendant with opening the 
reservoir for filling, ink will tend to leak from the print head nozzles 
29 while the refilling operation takes place. To prevent this leaking or 
drooling, it is desirable to block the fluid communication between ambient 
air and the interior of the reservoir that is provided by the nozzles. 
This block may be achieved by covering the print head 30 with a 
vinyl-backed tape, or the like, for the purpose of covering the nozzles 29 
of the print head before the reservoir is opened. It is contemplated that 
any one of a number of techniques can be employed for blocking or 
otherwise sealing the print head nozzles during the refill operation. 
Alternatively, such leaking through the nozzles 29 may be permitted, with 
residual ink being wiped from the print head upon completion of the 
refilling operation. 
A loss of back pressure within the reservoir 24 during the refilling 
operation (that is, when the reservoir is open as a result of removal of 
the plug 43) will also permit ink to flow out of the reservoir through the 
bubble generator 102, through the inlet labyrinth 130 to leak from the 
hole 136 at the distal and 133 of the labyrinth. While such leakage may be 
permitted during the refilling operation (and residual ink wiped away upon 
completion of the refilling operation), it is preferred to block the fluid 
communication between ambient air and the interior of the reservoir that 
is provided by the bubble generator orifice 120 and its contiguous inlet 
labyrinth. A preferred method for blocking this fluid communication is to 
apply air-impermeable tape (such as the vinyl-backed tape mentioned above) 
to the cover 134 for the purpose of occluding the hole 136 in the cover. 
It is contemplated that any one of a number of techniques can be employed 
for occluding or otherwise sealing the hole in the cover. 
Once the desired amount of ink is added to the reservoir 24, the reservoir 
is closed, for example, by pressing a spherical plug 43 back into the port 
41. Any of a number of mechanisms may be employed for closing the 
reservoir. For example, a compliant plug made of wax may be used to seal 
the port 41. Such a plug may be easily removed and reused during 
subsequent refill operations. A complaint plug, such as beeswax, conforms 
to the shape of the fill port 41 to provide a robust seal. 
A plug formed of compliant, elastic material is also contemplated for 
sealing the port (hence, closing the reservoir). Moreover, a foam-backed, 
self-adhesive tape may be used, firmly applied to the cap 40 to span and 
seal the port 41. Also, self-tapping set screw may be threaded into the 
port 41 for use as a replacement plug. 
Alternatively, the plug 43 may be replaced (during, for example, the first 
re-fill operation) with a permanently attached conduit, having one end fit 
into the port 41. The other end of the conduit may be detachably connected 
to an ink supply. A small valve or petcock is connected to the conduit and 
is openable for permitting ink to flow from the supply through the 
conduit, and closeable for closing the conduit (hence, closing the 
reservoir). 
Upon closure of the reservoir, fluid communication between the interior of 
the accumulator bag and ambient air is restored as a result of the removal 
of the plugged syringe tip 148. Removal of the syringe tip permits air to 
flow out of the accumulator bag as the bag is contracted by the spring 44. 
In view of the accumulator configuration described above, the contraction 
of the bag provides an increase in reservoir volume by an amount 
sufficient to reestablish a minimum back pressure within the reservoir 24. 
In this regard, is noteworthy that whenever it is necessary to add ink to 
the reservoir, the bag 42 will be in a partly to near-completely expanded 
state (depending upon how much ink has been depleted as explained above), 
and held in that state during the refilling process as a result of 
occluding the duct 50. 
An alternative method to refilling the pen begins with the step of opening 
the pen by removing the plug 43 as mentioned above. This opening step can 
be preceded by the steps of covering the print head nozzles 29 and 
blocking the cover hole 136 to avoid leakage, as described above. With 
this alternative approach, however, the duct 50 leading to the interior of 
the accumulator bag 42 is not occluded before the pen reservoir is opened. 
As a result, the loss of back pressure within the reservoir eliminates 
resistance to bag contraction, thereby permitting the spring 44 to 
completely contract the accumulator bag. 
Next, ink is added to the reservoir, such as through the conduit 150 
mentioned above. Because the accumulator bag 142 is to be inflated after 
ink is added, as described below, it is important that the maximum amount 
of ink that can be stored in the reservoir is not added during the 
refilling process because the subsequent expansion of the bag would have 
the effect of pumping excess ink out of the print head or bubble generator 
orifice. Accordingly, it is preferred that the amount of ink added is no 
greater than the maximum quantity that can be stored in the reservoir, 
minus the maximum expanded volume of the accumulator. 
After the desired amount of ink is added to the pen reservoir, the back 
pressure is reestablished. To this end, the accumulator bag is expanded to 
establish the minimum back pressure. A number of methods may be employed 
for expanding the bag. For example, pressurized air may be directed 
through the duct 50 to the interior of the bag. Air can be directed in 
this manner by a conventional syringe (like syringe 148 but without a 
plugged tip) that has a tip exterior shaped to fit snugly into the duct so 
that the accumulator bags can be forced open via the air pressure 
delivered from the syringe without air leakage between the duct 50 and the 
exterior of the syringe tip. 
Preferably, a controlled amount of air (for example 6.0 cubic centimeters 
at ambient pressure) is delivered to the accumulator bag interior. A 
controlled amount is important for preventing bursting of the bag. 
With the accumulator bag fully expanded or inflated, the reservoir is 
closed by the replacement of the plug 43, or by any other suitable 
mechanism for sealing the port 41. Thereafter, the syringe tip used for 
inflating the accumulator is removed to permit air to flow out of the 
accumulator bag, the resulting contraction of the bag, as before, 
establishing a minimum back pressure in the reservoir. 
As an alternative method for reestablishing back pressure in the reservoir 
after ink has been added, a small volume of ink may be drawn from the 
reservoir through the print head after the reservoir is closed (while the 
accumulator bag remains in fluid communication with ambient air). For 
example, the print head may be temporarily placed in contact with highly 
absorbent paper or cloth having sufficient capillarity for drawing ink 
through the nozzles. A small volume of ink may be otherwise removed, such 
as by printing with the pen for a brief period of time, or by applying a 
sufficient amount of suction to the exterior of the print head. 
Another alternative method for reestablishing back pressure within the 
reservoir after ink has been added is illustrated in FIG. 8. FIG. 8 
depicts the top portion of a print cartridge just after the preferred 
amount of ink has been added to the reservoir. The accumulator duct 50 
remains open, in communication with ambient air. Preferably, hole 136 and 
the print head nozzles have been blocked. 
For the purpose of carrying out the method depicted in FIG. 8, the port 41 
is configured to include spaced apart inwardly projecting ridges 160 on 
top of which the plug 43 may rest as shown in FIG. 8. With the plug in 
this position, a passage for air, shown by arrows 162 extends from the 
interior of the reservoir to outside of the cap 40 of the print cartridge. 
A member serving as an inverted suction cup 164 is then moved downwardly 
against the top of the filled pen to surround the port 41 and define a 
substantially enclosed chamber 166. The chamber 166 is then partially 
evacuated via, for example, a positive displacement pump 168 that removes 
a sufficient volume (for example 6.0 cc) of fluid comprising air, or ink, 
or a combination of air and ink, from the chamber 166, hence from the 
reservoir interior, for establishing the minimum back pressure within the 
print cartridge. With the back pressure so established, a plunger 170 
moves against the plug 43 to force the plug into sealing position within 
the port 41. The vacuum and sealing components 164, 168, 170 are 
thereafter removed. 
The just-described ridges 160 are not necessary and, in the alternative, 
the plug 43 may rest on the top edge of a cylindrically-shaped port and be 
lifted slightly (as the chamber 166 is partially evacuated) from the port 
to provide the fluid passage from the reservoir interior. 
The tape or other mechanism employed for blocking the hole 136 and covering 
the print nozzles 29 is removed prior to operation of the refilled pen. 
As noted above, as nearly all of the ink is depleted from the reservoir the 
back pressure within the reservoir reaches the upper operational limit. In 
such a state, it is possible to employ another alternative method for 
adding ink to the pen reservoir by immersing the nozzles 29 of the print 
head in a supply of ink. The dashed line 152 appearing in FIG. 7 is 
intended to illustrate the upper surface of the ink supply just mentioned. 
With the nozzles so immersed, the reservoir closed, and the accumulator 
bag maintaining fluid communication with ambient air, the high back 
pressure within the reservoir is sufficient for drawing through the 
nozzles and into the reservoir a substantial quantity of ink. 
The back pressure level within the reservoir will gradually decrease until 
reduced back pressure is insufficient for drawing additional ink through 
the nozzles. At that point in time, the immersed print head is removed 
from the external ink supply. The pen should be quickly wiped off and 
replaced in the printer. 
While having described and illustrated the principles of the invention with 
reference to preferred embodiments and alternatives, it should be apparent 
that the invention can be further modified in arrangement and detail 
without departing from such principles. Accordingly, it is understood that 
the present invention includes all such modifications that come within the 
terms of the following claims and equivalents thereof.