Electrostatic ink jet system

An improved electrostatic ink jet system is disclosed which provides ink upon demand to print upon plain paper. The demand signal is the application of an increased voltage level over an existing or bias level. The termination of this increased voltage level, of course, stops the ink flow to end the printing operation. The ink jet assembly includes an ink jet of reduced length by the inclusion of a conductive surface at a predetermined point along the ink jet length. The assembly therefore has a unique physical shape which provides improved electrical and mechanical characteristics.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
This invention relates, in general, to electrostatic ink jet printers and, 
in particular, to an ink jet printer cartridge having an ink jet nozzle of 
a predetermined length which cartridge provides a low-head pressure, high 
flow, single filament ink jet that delivers fluid ink to a point in a 
controlled manner. 
B. Prior Art 
In the past, electrostatic ink jets used a long tube as the jet nozzle. 
This long nozzle length restricted fluid flow due to viscous drag. To 
increase this ink flow, the head height of the ink surface could have been 
increased, but this increase not only caused dripping of ink out of the 
jet nozzle tip but caused frequency response problems as well. Likewise, 
the increase in ink flow could have been obtained by using a larger 
diameter ink jet or a lower viscosity ink. In the case of the larger 
diameter jet, control problems were created, whereas low viscosity inks 
were difficult to develop. 
Another problem that existed in the past was the limitation of the writing 
voltage that could be applied. This maximum writing voltage was determined 
by the point where a pair of filaments appeared on the ink meniscus. These 
filaments leave the nozzle at two points where the voltage gradients are 
the same. This limited the voltage that could be applied for control of 
the ink. 
BRIEF DESCRIPTION OF THE INVENTION 
A. Objects 
It is a general object of the present invention to eliminate these and 
other problems of the prior art by providing an on-demand, low-head 
pressure, high-flow, single filament, electrostatic ink jet system which 
does not require the recycling of ink. 
It is a further object of the present invention to provide an ink jet 
nozzle having increased ink flow. 
It is a still further object of the present invention to provide an ink jet 
system having an ink jet assembly with a nozzle having a predetermined 
shortened length. 
It is another object of the present invention to provide an ink jet 
assembly having a metal plate or other field shaping means positioned on 
the nozzle of said assembly close to the end of said assembly. 
It is a still further object of the present invention to provide a low-head 
pressure, high flow, single filament, electrostatic ink jet system having 
a shortened nozzle which significantly increases the ink flow and a metal 
plate positioned on said nozzle close to the jet which moves the voltage 
gradient lines closer together to allow a higher voltage gradient to be 
applied to the ink and still maintain a single filament. 
B. Summary of the Invention 
A low-head pressure, high flow, single filament ink jet assembly is 
disclosed which utilizes a shortened nozzle to provide increased ink flow 
and a metal plate positioned on said nozzle near the jet end thereof to 
allow an increased voltage gradient to be applied to the ink and still 
maintain a single ink filament. 
The ink jet assembly functions in an on-demand mode in response to an 
applied voltage gradient. Since ink does not flow in the absence of the 
voltage gradient, there is no need to recycle ink.

DETAILED DESCRIPTION OF THE INVENTION 
Referring first to FIG. 1, there is shown the prior art configuration of an 
electrostatic ink jet printer. The ink supply 18 is contained in the ink 
reservoir 10. An ink jet 12 is mounted to the reservoir 10. The voltage 
source 14 is connected between the ink jet 12 and a metallic plate 16. 
This voltage source 14 serves as the bias voltage and is approximately 
2000 V in the present preferred embodiment. 
FIG. 2 illustrates the writing signal that is applied between the ink jet 
12 and the metallic plate 16 to cause ink 18 to flow from the ink 
reservoir 10 to the printing paper surface 26. Thus, when a narrow pulse 
writing signal above the threshold voltage level is applied, a short 
duration filament of ink 18 is dispensed from the nozzle 12. If on the 
other hand, the duration of the writing pulse signal is lengthed, a 
filament of longer duration will be dispensed from nozzle 12. 
Returning to FIG. 1, the lines of constant potential (voltage) 20 emanate 
around the ink jet 12 in the configuration shown. The directions of 
voltage gradients are indicated by lines 22 which are locally 
perpendicular. These lines 22 also indicate the axis of a force on a 
charged particle within the field. The paper 26, i.e., the printing 
surface, needs some conductivity and the resistivity suggested in the 
present embodiment is less than 10.sup.14 ohm-centimeters. 
Referring next to FIGS. 3 and 5, there is shown the same ink jet 
configuration as illustrated in FIG. 1 but with the addition of a metal 
ring or washer 24. The mounting of this metal washer near the end of the 
jet gives approximately a 10% increase in the density (darkness) of the 
copy. This improvement is due to the more uniform field resulting from the 
inclusion of the ring 24. Conductive ink filaments tend to diverge in an 
electric field causing a mist of the ink droplets. These droplets are less 
controlled than a single filament and provide a less dense (dark) copy. 
The inclusion of the ring 24 allows a higher electric field 14 to be 
applied to the ink 18, which higher field, in turn, provides a greater 
amount of control over the ink filament. 
In the preferred embodiment of the present invention, the ink reservoir 10 
is formed from a moldable material such as polypropylene which is also 
resistant to chemical reaction with the ink 18. The nozzle 12 is 
fabricated from stainless steel hypodermic-type tubing. 
In FIG. 4, there is shown an alternate embodiment of the present invention. 
In this combination, the washer or ring 24 is now positioned upon the end 
of the reservoir 10, the reservoir 10 containing a much larger ink source 
18 than in the embodiment of FIG. 3. As another alternative to the 
embodiment of FIG. 4, the washer or ring 24 could be eliminated and 
instead the end portion of the ink reservoir 10 connected to the nozzle 12 
could be fabricated from a conductive material. In either case, the length 
of the nozzle 12 has been shortened as much as possible to provide optimum 
ink flow. 
The ink flow is described by the equation: 
##EQU1## 
It has been discovered that there is a typical ink flow for printing based 
upon cosmetics, said flow being at a rate of about 2.7.times.10.sup.-2 
gm/min. 
The frequency response (reaction time) depends upon the mass of the 
meniscus of ink on the end of the jet 12. The less the mass, the faster it 
can respond. Since the density of the fluid is essentially constant, the 
volume of the meniscus must be decreased to increase frequency response. 
This can be done by decreasing the diameter of the jet 12 and decreasing 
the head height. This drastically decreases the mass flow of ink 18 given 
by the above equation. 
To return the mass flow of ink 18 back up to the required value, there are 
two variables still available, the viscosity of the ink 18 and the length 
of the nozzle 12. Since the viscosity of the ink 18 has a practical lower 
limit of 10 centipoises (cps), the length of the jet nozzle 12 must be 
very short. In the past, this length has been approximately 0.400 inches. 
The new improved length in the preferred embodiment is approximately 0.070 
inches. The improvement in the ink 18 flow accomplished by this change is 
0.400/0.070=5.7 times. In other words, the reduction in the length of the 
ink jet nozzle 12 only affects or controls mass flow and has little or 
nothing to do with frequency response. Consequently, it was chosen as the 
element to be improved. 
It will be understood from the foregoing that various changes may be made 
in the preferred embodiment illustrated herein. 
Thus, in place of the metal washer 24, a cylindrical or cone shaped member 
could be mounted axially on the nozzle 12 in order to still further 
improve performance. 
As another variation, a moving drum with paper mounted on it could be 
substituted for the sheet of paper 26. In such a configuration, lines 
could be printed on the drum by extending the duration of the voltage 
pulse applied, thus causing a steady flow of ink 18 to be dispensed from 
the nozzle 12 onto the moving paper 26 surface. 
It is therefore intended that the foregoing material be taken as an 
illustration only and not in a limiting sense, the scope of the invention 
being defined by the following claims.