Constructions and fabrication methods for drop charge/deflection in continuous ink jet printer

An improved print head construction for continuous ink jet printing of the kind which directs a plurality of ink streams through a drop charge region toward a print zone. The print head includes: (i) charge electrodes located adjacent the drop charge region for selectively applying an information voltage to droplets; (ii) a deflection electrode(s) closely spaced to, and downstream from, the charge electrodes, for applying a deflection field to the droplets; and (iii) a dielectric matrix for integrally embedding the charge and deflection electrodes in closely spaced relation. Methods for fabricating such print heads are also disclosed.

FIELD OF INVENTION 
The present invention relates to print head devices for continuous ink jet 
printers and more particularly to: (i) improved drop charging and drop 
deflecting structures of such devices and (ii) methods of making such 
improved structures. 
BACKGROUND OF INVENTION 
In continuous ink jet printing, print head devices are constructed to: (i) 
form ink streams which break up within a drop charging region into drop 
streams of uniformly sized and spaced ink drops, (ii) selectively impose 
electrical charge on some drops at stream breakup in accord with an 
information signal and (iii) deflect charged drops to a predetermined 
trajectory, which can be either a print trajectory or a "caught" 
trajectory. The "caught", non-printing drops are returned to the ink 
reservoir and recirculated to the print head. 
Most prior art print head assemblies employ drop charge electrodes adjacent 
the drop formation point and drop deflection electrodes that are separate 
of, and spaced downstream from, the charge electrode. U.S. Pat. No. 
3,656,171 discloses a prior art technique wherein a single electrode 
structure extends from the drop breakup region to the drop catch region 
and functions as both the drop charging and drop deflection electrode. 
U.S. Pat. No. 4,636,808 discloses an improvement over the '171 patent 
wherein a "non-extended" charge and deflection electrode that has a path 
length dimension of only about 6 drop spacings or less, provides combined 
drop charging and drop deflecting functions. The '808 patent approach has 
the significant advantage of removing the electrodes from contact with 
charged ink drops while still eliminating the need for a separate 
deflection electrode structure. 
The '808 patent system for combining the functions of drop charging and 
deflection into a single, short electrode works quite well. However, we 
have found that the drop deflections which are produced by that system are 
not uniform and vary based on the charge or non-charge selection as to 
subsequent drops. This can be understood by conceptually separating the 
charging and deflection functions of the combined charge/deflection plate. 
The upper region of that plate, adjacent to the break off point, produces 
the drop charging. The lower region of that plate will have several print 
or catch drops in front of it and serves as a deflection electrode. To 
produce a print drop (e.g. uncharged) the charge voltage on the combined 
charge/deflection plate is switched to 0 volts for 1 stimulation cycle. 
This switching off of the charge voltage also turns off the deflection 
field in front of the lower region of the combined plate and the 
deflections of the drops in front of the lower part of the charge plate 
are thus reduced (in comparison to the deflections imparted when the plate 
is charging a drop). 
The trajectories of charged drops in the '808 system therefore can range 
between a maximum deflection (where the combined plate is charging all 
drops during a drop passage past the plate) and a minimum deflection 
(where the plate is energized only to charge that drop and thereafter 
grounded). While such non-uniformities in deflected drop trajectories of 
the '808 patent system do not prevent good printing operations, they do 
present overall printer design constraints that are undesirable. For 
example, the range of possible trajectories necessitates a relatively high 
charge electrode voltage, i.e. reduces the charging voltage range over 
which proper printing is possible. 
SUMMARY OF THE INVENTION 
Thus one important purpose of the present invention is to provide a print 
head construction that maintains much of the simplicity of the combined 
charge/deflection electrode approach but also eliminates printer design 
problems that evolve from the inconsistent drop deflections of that 
approach. 
In one preferred embodiment, the present invention constitutes in ink jet 
printing apparatus, an improved print head construction comprising: (a) 
means for directing a plurality of ink streams through a drop charge 
region and a deflection region toward a print zone; (b) charging means 
including charge electrodes located adjacent the drop charge region and 
means for selectively applying an information voltage to the charge 
electrodes; (c) deflection electrode means including a deflection 
electrode closely spaced to, and downstream from, the charge electrodes 
and means for applying a deflection voltage to the deflection electrode; 
and (d) dielectric means for integrally embedding the charge and 
deflection electrodes in such closely spaced relation. 
In another aspect the present invention constitutes a method of fabricating 
an improved charge and deflection electrode component for a continuous ink 
jet print head, such method comprising: (a) forming on a substrate a 
photoresist pattern comprising outline boundaries of: (i) a plurality of 
charge electrodes having lead connector portions extending therefrom and 
(ii) a deflection electrode closely spaced to the charge electrodes and 
having a lead connectors portion extending therefrom; (b) electroplating 
within said outline boundaries to form discrete conductive elements within 
the boundaries; (c) bending the substrate to dispose the charge and 
deflection electrode elements at approximately 90% to their lead connector 
portions; (d) removing the photoresist; (e) embedding the exposed portions 
of the electrodes and lead connector portions in a dielectric matrix; and 
(f) removing the substrate to expose face surfaces of the electrodes and 
lead connector portions. 
The advantages provided by constructions in accord with the present 
invention are several. For example, the uniformity of deflection provided 
by such constructions enables a reduction in the requisite charge voltage 
and in some embodiments enables a common voltage source for the charge and 
deflection electrodes. The uniformity of deflection trajectory also causes 
a uniformity of drop impact location on the catcher assembly which 
simplifies catcher constrictions and improves the reliability of ink 
return. Further, the integration of charge and deflection electrode 
structures in accord with the present invention, provides the advantages 
of a separately addressable electrode, while still allowing cleaning of 
such separate electrode by the processes described in U.S. Pat. No. 
4,600,928. In certain embodiments the closely spaced, but electrically 
discrete, electrode structures of the invention can be used to effect 
simplified stimulation adjustment and to provide electrohydrodynamic 
stimulation of the jet stream. In addition, the print head constructions 
in accord with the present invention are simple in fabrication and avoid 
difficult assembly positioning usually necessary to assure accurate 
interspacing between charge and deflection electrode structure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIG. 1, the continuous ink jet printer 10 comprises a 
print head reservoir 12 to which a supply of ink 14 is continuously 
supplied under pressure by a circulation system (not shown). At the bottom 
of reservoir 12 an orifice array 16 is provided so that ink stream 
filaments 18 are directed toward a print zone, e.g., a strip on a sheet 
print media supported on print platen 19. Stimulating vibration is imposed 
on ink egressing orifice 16 (e.g. by a stimulation system such as 
described in U.S. Pat. No. 4,646,104, not shown) to cause the ink 
filaments to break up at a predetermined location into a plurality of 
uniformly sized ink droplets D having a uniform drop spacing "d". The 
location of the drop break off point "p" of the filament 18 is controlled 
by the ink jet printer system (e.g. by regulating the amplitude of the 
vibrations imposed on the ink filament) to be approximately opposite along 
the drop path from the charge electrodes 21. 
In addition to the charge electrodes 21, the lower print head assembly 20 
comprises a drop deflection electrode(s) 22 and a catcher body portion 23 
including a drop impact surface 24 and an ink return channel 25. Referring 
to FIG. 2, as well as FIG. 1, it can be seen that the drop charging and 
drop deflecting assembly 30 comprises an integral construction which can 
be mounted atop the catcher body portion 23. More specifically, the 
plurality of drop charge surfaces 21 each has a respective connector pad 
portion 21b located at a rear region of the assembly and lead portion 21a 
coupling the charge surface portion 21 to the pad portion 21b. Each of 
these portions 21, 21a and 21b preferably are embedded in a matrix M of 
dielectric material, e.g. epoxy resin. Also embedded in matrix M is the 
deflecting field surface 22 of the deflection electrode and its lead and 
connector pad portions 22a and 22b. 
In operation, the pad portions 21b and 22b of assembly 30 are coupled to 
the charge control circuit (denoted generally 40) of the printer 10 and 
improved printing can be effected in accord with the present invention as 
follows. Thus as drops D break off of filaments 18 at points "p" adjacent 
the charge faces 21 of the charge electrodes, circuit 40 selectively 
energizes specific ones of the charge electrodes 21, in accord with a 
print information signal, to selectively charge non-print drops to a 
potential, e.g., V.sub.1. The conductive ink filament is at ground 
potential, and a charge of opposite polarity to voltage V.sub.1 is induced 
on charge drops. 
Control circuit also continuously energizes deflection electrode 22 to a 
predetermined voltage V.sub.2. The continuous deflection field (formed by 
voltage V.sub.2) consistently attracts the oppositely charged droplets to 
a catch trajectory as shown in FIG. 1. Moreover, the proximity of the 
deflection electrode 22 to the grounded jet stream and orifice plate 
enables those ground potential sources to serve as the "ground electrode" 
for a highly effective drop deflecting field, which enables a smaller 
deflection voltage than in prior art systems. Non-charged drops are not 
attracted and pass along the print trajectory toward platen 19, as shown 
and FIG. 1. 
In accord with the present invention, we have found it preferable to have: 
(i) the charge electrode surface to be in the range of about one to three 
drop spacings "d" in length along the drop path. The important limits in 
this regard are that the charge electrode length be adequate to assure 
uniform drop charging across the width of the array but not so long as to 
impair the deflection field (by too distant a spacing of the deflection 
electrode from the grounded orifice plate in ink filament). For the same 
reason it is desirable that the dielectric spacer element have the minimum 
length along the path that will electrically isolate the charge and 
deflection electrodes. One drop spacing or less of length is desirable. 
The deflection electrode is desirably at least about two drop spacings in 
length along the drop path and can be as long as necessary to effect 
proper deflection. Two to three drop spacings in length is usually 
adequate. In one example where the jet stream drop spacing was about 4 
mils, a charge electrode surface of 4 mils and a deflection electrode 
surface of about 10 mils, separated by a dielectric surface of about 3 
mils, was preferred. In one preferred construction the top of charge plate 
21 is spaced about 7 mils from orifice 16 and the faces of charge 
electrode 21, 22 are about 2.5 mils from the center of the ink filament. 
While the charge circuit 40 is shown and described above as having separate 
charge and deflection voltages V.sub.1, V.sub.2, it is preferred in accord 
with the present invention that V.sub.1 and V.sub.2 be approximately equal 
or even more preferably from the same voltage source. D.C. voltages in the 
range of from about 80 to 140 volts are desirable and a voltage of about 
100 volts is preferred with the jet to charge plate spacing of about 2.5 
mils as described above. These voltage parameters will vary depending on 
electrode constructions and jet to charge plate spacing. 
Also, while the deflection electrode means is shown in FIGS. 1 and 2 as 
comprising a single electrode 22, it can also be formed as a plurality of 
parallel electrodes as shown in FIG. 3. The provision of a plurality of 
deflection electrodes is preferred from the fabrication viewpoint of 
enabling more reliable attachment in the dielectric matrix. 
Referring now to FIGS. 3-6, one preferred fabrication method for 
constructing a combined charge/deflection electrode assembly, in accord 
with the present invention, is illustrated. In this method of fabricating 
the assembly, a pattern 104 defining the charge and deflection electrode 
surfaces and their associated connecting structures is formed in a 
covering 102 which is resistant to plating operations and supported on a 
substantially planar foil sheet 100. In FIG. 3, the outlines for the 
charge electrodes and related connection circuits are denoted 105 and the 
outlines for the deflection electrodes and their related connection 
circuits are denoted 103. Preferably, the foil sheet 100 is copper and the 
pattern 104 is formed by initially laminating a photoresistive film, as 
covering 102, to the foil sheet, exposing the photoresistive covering to 
actinic light through a photomask to define the electrode and circuit path 
pattern and removing the portion of the covering corresponding to the 
pattern to expose the surface of the copper foil thereunder. 
The foil sheet is then plated through the exposed pattern 104 with an 
electrically conducting material, preferably nickel, to form charge 
electrodes 21 and the associated connecting circuit structures 21a, 21b 
and deflection electrodes 22 and their associated connecting circuit 
structures 22a, 22b, see FIG. 5. The electrodes and associated circuit 
leads are preferably formed to a thickness which exceeds the thickness of 
the photoresistive covering as shown in FIG. 4. By plating to a thickness 
beyond that of the photoresistive covering, the plated material extends 
above and expands over the upper surface of the covering to form retaining 
flanges 106 on each of the electrodes and circuit leads. The plating 
operation is limited such that no two of the flanges interconnect with one 
another. The remainder of the photoresistive covering is then thoroughly 
removed from the foil sheet such that the electrodes and the connecting 
circuit leads with their retaining flanges extend above the surface of the 
foil sheet as shown in FIGS. 5 and 6. 
The foil sheet is then bent toward the plated material with at least a 
portion of the electrodes being bent and angularly oriented, at 
approximately a 90% angle, relative to the circuit leads as shown in FIG. 
7. The bending step can if desired be performed before removal of 
photoresist layer 102. 
A dielectric matrix is then formed to surround the plated material 
including the retaining flanges as shown in FIG. 8. The matrix is formed 
to have two of its exterior surfaces defined by the interior of the foil 
sheet. The foil sheet is then removed from the matrix and embedded plated 
material, e.g. by etching. This exposes surfaces of the plated material 
electrodes and circuit leads with the remainder embedded within the 
substrate and preferably anchored thereto by means of retaining flanges 
106. The combined assembly with foil sheet 100 removed is as shown in FIG. 
2. 
Preferably, the embedding procedure is effected by clamping the bent foil 
sheet into a mold with the portion of the sheet including the connecting 
circuit leads being clamped to the mold as well as the bent portion of the 
sheet. This ensures flatness of those portions of the substrate after 
formation. The dielectric substrate is then formed by inserting molding 
material, such as an epoxy resin, into the mold and curing the molding 
material to form the dielectric substrate. Charge electrodes formed by 
this technique can have a rounded top edge and in this event the electrode 
length along the path direction can be extended to, e.g., 9 mils so that a 
portion of 4 mils will exist at uniformly close spacing to the ink jet 
filament break off zone. Further details of this preferred procedure for 
forming the integral charge/deflection assembly can be generally as 
described in U.S. Pat. No. 4,560,991. 
FIG. 9 illustrates a charge deflection plate 200 constructed in an 
alternative fabrication process of the present invention. This fabrication 
is effected by plating the charge electrode structure 201 over a 
photoresist pattern as described with respect to FIGS. 3 and 4 and 
removing the photoresist as described with respect to FIGS. 5 and 6. 
Without bending the substrate, the substrate and supported charge 
electrode structures are placed in a molding fixture and a deflection 
electrode 203 held above the charge electrode structure by a spacing that 
defines the insulator gap 204. The elements are then molded in a matrix of 
dielectric material M. After the molding process is completed, support 
substrate is etched off and the face of the charge plate is ground or 
lapped to a flat surface yielding the FIG. 9 structure. The FIG. 9 
fabrication has a very small top radius so that the operative charge 
electrode surface can be only about 3-5 mils. The dielectric spacing 
structure can be about 3 mils and the deflecting electrode surface about 
10-20 mils. In a further alternative embodiment the deflection electrode 
203 can be laminated to the charge electrode surface 201 with a dielectric 
adhesive which defines gap 204. 
Referring now to FIG. 10, the combined charge/deflection plate construction 
of the FIG. 1 embodiment is integrated with an electrohydrodynamic 
stimulator system of the kind described in U.S. Pat. No. 4,220,958. Thus 
power and control 230 is coupled to pump electrodes 231, 232, which are 
separated from ground electrodes 233, 234 by dielectric matrix M. The 
electrodes are separated by one half a drop spacing d/2. The integral 
matrix M also couples charge electrode 21' and deflection electrode 22' 
with intermediate spacer material M so that the drops formed by the 
electrohydrodynamic stimulator system are charged and deflected by the 
integral charge/deflection plate 21', 22'. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variation 
and modifications can be effected within the spirit and scope of the 
invention.