Ink reservoir with essentially constant negative back pressure

An ink reservoir which incorporates a negative back pressure source coupled to a membrane wall of the reservoir to prevent ink leakage from a reservoir orifice is disclosed. The back pressure is created by either a linear or nonlinear spring which can be either independent of, or integral with, the membrane. The result is freedom from ink leakage and improved quality printing when the reservoir is used in conjunction with an ink pen such as used in ink jet printing.

BACKGROUND OF THE INVENTION 
It has been shown previously that it is important to supply a static 
negative pressure (or head) at the orifices of an ink jet to enhance print 
quality. By doing so, a negative meniscus draws any ink at the orifices 
back into the pen, and provides a cleaner, more uniform ejection surface. 
In a portable or disposable pen, the importance of a negative head is even 
more important, because the ink must be contained even in transit, at any 
altitude, and unde shock and vibration. In the case of a portable 
disposable pen, the only mechanisms holding the ink into the pen when the 
orifices are face down in the vertical direction are surface energy 
related. 
As shown in FIG. 1A, the pressure P1 exerted on the liquid 10 in the 
reservoir 20 by the orifice 30 is related to the radius of curvature, r1, 
and the surface energy of the fluid .gamma.. Thus P1=2.gamma./rl. The 
pressure Pa exerted by the fluid due to an external acceleration such as 
gravity or external shock is related to the fluid density .pi., head 
height h, and acceleration a. Thus Pa=.pi.ah. If the orifice diameter D is 
small enough, an equilibrium condition will be achieved such that ink will 
not flow from the orifices. If the orifice plate wets well in this 
attitude, the contact angle .phi.1 of the fluid, on the orifice surface 
will be insufficient to exert sufficient pressure P2 to sustain Pa as 
shown in FIG. 1B. Thus P2=2.gamma./r2&lt;P1. 
The prior art suggests that an antiwett coating should be applied around 
the orifice, to increase the contact angle .phi.2, as shown in FIG. 1C, 
thus increasing the capillary pressure. In practice this approach has two 
major drawbacks. First, due to a sudden shock (increased a), a blob of ink 
will emerge which may have sufficient radius r to overcome the equilibrium 
condition. Second, and more importantly, most antiwet compounds are 
attacked by the dye in the ink since an important quality of a dye is that 
it chemically bonds itself to a surface. This poisons the antiwet coating 
and drops the contact angle back to a low value. 
Another way to contain the ink in the reservoir includes valves, which 
however are large, clumsy and expensive. 
SUMMARY OF THE INVENTION 
The solution according to the present invention for a portable disposable 
inkjet pen is to mechanically cause a constant negative pressure slightly 
greater than the maximum hydrostatic head. One solution according to the 
present invention is to use a spring to exert a force against a bladder 
membrane which draws back on the ink. The back pressure or suction must 
however remain relatively constant, because below a back pressure equal to 
the pressure exerted by external accelerations (.pi.ah) under some 
conditions the pen will lose ink, and yet above some critical value, the 
print quality deteriorates. Therefore, standard linear springs are only 
suitable for use over a reasonable change of ink volume if a thin 
reservoir (i.e., small h) is used. 
In order to permit the use of more generalized reservoir shapes, the 
present invention also discloses the use of a nonlinear spring exerting a 
force on a bladder mechanism which draws back on the ink with a constant 
pressure across a wide range of deflections. 
Whether a linear or nonlinear spring is used, the spring may be 
incorporated as part of the bladder membrane itself to further minimize 
cost and size. Thus, the bladder membrane can be made of a spring material 
such as silicone rubber, removing the need for connectors and supports 
required to construct a system in which a separate spring is coupled to a 
separate membrane.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 2 shows a block diagram of an embodiment of the present invention. A 
spring 40 coupled to a foundation 15 is used to pull back on a membrane 
bladder 35 by means of linkage 25. The bladder 35 serves to cap pen 50 and 
reservoir 20 containing ink 10 filled to a height h. The reservoir 20 is 
also held motionless relative to the foundation 15. The pen 50 has an 
orifice 30 pointing in the direction of an external acceleration a. 
Adjacent to the orifice 30 is a firing means 60, such as a thermal ink jet 
resistor, which is used to expel droplets 70 of ink 10 through the orifice 
30. 
With such a configuration, the bladder 35 should be a flexible nonporous 
material such as polyethylene, cellophane, or vinyl so that the force Fs 
of the spring 40 can be transferred directly to the ink 10. The spring can 
be conventional coiled spring with Fs=4 grams for a reservoir 20 with ink 
10 having a surface energy .gamma.=40 ergs/sq. cm, and density .pi.=1.18 
gm/cubic centimeters, and an orifice of radius r=40-80 microns. Because of 
the spring force Fs acting against the acceleration pressure Pa no 
substantial quantities of ink 10 will be expelled from orifice 30 except 
under the influence of the firing means 60. 
The spring 40 and bladder 35 can be combined into a single unit by using an 
elastomeric bladder, for example made of silicone rubber or other natural 
or synthetic rubbers with sufficient chemical resistance to the ink 10, 
which can create the spring force Fs directly. Such an integrated bladder 
35 and spring 40 simplifies the construction of the reservoir system by 
eliminating the need for the separate linkage 25 and the separate spring 
40 which must be made of very fine gauge wire so that Fs=4 grams. 
The major disadvantage of such a configuration is that the spring force Fs 
of a conventional spring is proportional to its extension x. Thus 
dFs=K*dx. Hence as the ink 10 is expelled from the reservoir 20, the 
height h of the ink decreases and the spring length x increases and Fs 
increases, thus changing the shape and size of the ink droplets 70 and the 
print quality. This effect can be minimized if the change in height h is 
made small by using a reservoir 20 that is very thin (i.e., h is small) 
while still having a desired volume V. 
A more useful approach is to use a non-linear spring 40 so that the spring 
force Fs is relatively constant over the maximum change of height h. Such 
non-linear springs, as for example a Belleville spring, have a 
force-deflection curve as shown in FIG. 3. As long as the change in force 
dFn across the usable deflection range dx of the spring 40 is greater that 
or equal to the maximum change in ink height h an approximately constant 
back pressure force Fs will be produced which prevents leakage out of 
orifice 30 due to external accelerations a and enhances print quality. 
The non-linear Belleville-like spring approach can also be used as shown in 
FIGS. 4, 5A and 5B to create an integrated bladder and spring to provide 
the desired constant back pressure force Fs. In FIG. 4, a silicone rubber 
dome 200, and a solid ink reservoir 210 are coupled to a housing 220 with 
an orifice 230 which leads to a convention jet printing head (not shown). 
Many shapes may be employed to create the dome 200 to achieve a constant 
back pressure force Fs as long as there are several spring bending moments 
which cancel each other across the desirable range of deflection dx. FIGS. 
5A and 5B show a cross sectional and pictorial view respectively of one 
such Belleville-like bladder dome 200.