Resistively coated deflection ribbon for an ink jet printing device

A conductive deflection ribbon (52) is provided in an ink jet printing device (10) in which the ribbon (100) is coated with a highly resistive material (101). The deflection ribbon (52) is positioned adjacent a row of orifices (26) in an orifice plate (18) from which are expelled series of droplets (84). Selected ones of the droplets (84) are charged in such a manner they are deflected towards a catching device (54) by an electrostatic field established between the deflection ribbon (52) and the catching device (54). The resistive coating (101) inhibits the production of shorts between the charge plate (50), orifice plate (18) catcher (54) and the ribbon (52).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to ink jet printing devices, and more 
particularly, to ink jet printing devices which utilize a deflection 
ribbon to cause selectively charged drops to be deflected from their 
nominal trajectory. 
2. Prior Art 
The present invention relates more particularly to ink jet recording 
devices of the type which use a deflection ribbon for deflecting charged 
drops into a catching device, such as are disclosed, for example, by 
Mathis U.S. Pat. No. 3,701,998 and Cassill U.S. Pat. No. 3,787,883. These 
devices generally incorporate a fluid supply reservoir which supplies a 
printing liquid to a plurality of orifices disposed in two parallel rows 
and from which are ejected uniform streams of droplets directed towards a 
printing medium. A charge plate is utilized to place a desired level of 
charge on selected ones of the droplets ejected from the orifices. All of 
the droplets then pass immediately adjacent a deflection ribbon which is 
oppositely charged from the droplets so that the selectively charged 
droplets are repelled from the ribbon into an adjacent catcher. The 
remaining uncharged droplets which are unaffected by the electrostatic 
field established by the deflection ribbon then impinge on the recording 
medium in the desired pattern. 
One difficulty which has been discovered in the utilization of such 
printing devices is that due to the development of a fine mist which is 
generated by the printing liquid, the deflection ribbon and other parts of 
the ink jet printing device become sufficiently wet that shorts can occur 
between the charge plate and ribbon or between the catcher and ribbon. 
This situation is aggravated when the printing medium is a fiberous 
material such as paper, since fibers sometimes leave the paper and adhere 
to the ribbon, charge plate, or catchers and thus reduce the clearance 
therebetween and as a result increase the likelihood of shorts across 
these various elements. 
Such shorts result in a loss of deflection voltage which in turn causes 
insufficient deflection of the selectively charged droplets in the 
electrostatic field and will thus cause such droplets to impinge upon the 
recording medium rather than be caught by the catcher, resulting in 
distortion in the intended sequence of printing. 
Because of the possibility of electrical arcing between the deflection 
ribbon and adjacent parts of the printing device, it is necessary to keep 
the voltage on the deflection ribbon relatively low in order to inhibit 
such arcing. This in turn necessitates the use of a higher charge voltage 
on the charge ring in order to induce a higher charge on the selected 
droplets so that the low voltage on the deflection ribbon has sufficient 
effect on the charged droplets to deflect them into the catcher. It is in 
turn much more difficult to switch on and off a higher voltage than it is 
a relatively lower voltage because of the inherent time delay in the 
voltage dropping from its maximum value to substantially zero. This 
produces problems in the charge plate control circuitry and ultimately 
limits the speed at which the printing device can function. 
SUMMARY OF THE INVENTION 
The present invention overcomes the above described difficulties and 
disadvantages associated with such prior art devices by providing a 
deflection ribbon having a resistive coating thereon which substantially 
reduces the incidence of shorts between the deflection ribbon and adjacent 
printing head parts, such as the charging plate and the catcher. The 
degree of resistance of the coating material can be varied substantially, 
and therefore many materials having relatively high resistance would be 
suitable as a coating for the deflection ribbon. In fact, even insulative 
materials have proven to be acceptable for this application. 
In this environment the lower end of the range of resistivity of acceptable 
coating materials is approximately 10.sup.4 ohms-cm resistivity, although 
this is not an absolute lower end of the range since the spacing of the 
various parts of the ink jet printing device from the deflection ribbon 
has an effect on the acceptable threshold of resistivity of materials 
being useful as a coating on the deflection ribbon. 
A further advantage of the use of a resistively coated deflection ribbon is 
that the voltage on the deflection ribbon can be substantially increased 
since arcing between the deflection ribbon and adjacent parts of the ink 
jet printing device is substantially reduced. Since the charge needed on 
the selectively charged droplets is decreased because the electrostatic 
field produced by the deflection ribbon has increased, the charge plate 
voltage can be substantially reduced and still effect the desired 
displacement of the charge droplets into the catcher. This then permits a 
lower charging voltage to be used which has the inherent advantage of 
reducing the decay time on the charged signal, thus permitting an increase 
in the print quality of the device. 
Attempts have been made to reduce or eliminate the possibility of shorting 
between charging electrodes which are subject to a similar environment of 
wetting due to the mist created by the streams of droplets. For example, 
the patents to Van Breemen et al U.S. Pat. No. 4,035,812 and Robertson 
U.S. Pat. No. 3,604,980, both disclose charge plate constructions which 
are provided with resistive material associated with the charging 
electrodes to reduce the possibility of inter-electrode shorting. However, 
this does not eliminate the possibility of shorting between the deflection 
ribbon and the catcher which are downstream from the charging electrodes 
and thus does not directly face the problem with which the present 
invention is concerned.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 1 of the drawings it will be seen that the various 
elements of an ink jet printing head assembly 10 are assembled for support 
by a support bar 12. Assembly thereto is accomplished by attaching the 
elements by means of machine screws (not shown) to a clamp bar 14 which is 
in turn connected to the support bar 12 by means of clamp rods 16. 
The recording head comprises an orifice plate 18 soldered, welded or 
otherwise bonded to fluid supply manifold 20 with a pair of wedge-shaped 
acoustical dampers 22 therebetween. Orifice plate 18 is preferably formed 
of a relatively stiff material such as stainless steel or nickel coated 
beryllium-copper, but is relatively thin to provide the required 
flexibility for direct contact stimulation. Preferably, dampers 22 are 
cast in place and are formed of polyurethane rubber or other suitable 
damping material. 
Orifice plate 18 contains two rows of orifices 26 and is preferably 
stimulated by a stimulator 28 which is threaded into clamp bar 14 to carry 
a stimulation probe 30 through the manifold 20 and into direct contact 
with plate 18. A filter plate 32 is provided to prevent clogging of 
orifices 26. O-rings 34, 36 and 38 are used to prevent leakage from the 
head assembly. Service connections for the recording head include a 
printing fluid supply tube 42, air exhaust and inlet tubes 44 and 46, and 
a tube 48 for connection to a pressure transducer (not shown). 
Other major elements comprising the recording head assembly 10 are a charge 
ring plate 50, an electrically conductive deflection ribbon 52 and a pair 
of catchers 54. Catchers 54 are supported by holders 56 which are fastened 
directly to fluid supply manifold 20. Deflection ribbon 52 is also 
supported by holders 56 and is stretched tightly therebetween by means 
described in detail below. Ribbon 52 extends longitudinally between 
catchers 54 as best shown in FIG. 2. 
Catchers 54 are laterally adjustable relative to ribbon 52. This 
adjustability is accomplished by assembling the head with catchers 54 
resting in slots 68 of holders 56, and urging them mutually inward with a 
pair of elastic bands 70. Adjusting blocks 72 are inserted upwardly 
through recesses 74 and 76 to bear against faces 78 of catchers 54, and 
adjusting screws 80 are provided to drive adjusting blocks 72 and catchers 
54 outwardly against elastic bands 70. 
The fully assembled recording head is shown in cross section in FIG. 2. As 
therein illustrated printing fluid 82 flows downwardly through orifices 26 
forming two rows of streams which break up into two curtains of drops 84. 
Drops 84 then pass through two rows of charge rings 86 in charge ring 
plate 50 and thence into one of the catchers 54 or onto the moving web of 
paper 88. Switching of drops between "catch" and "deposit" trajectories is 
accomplished by electrostatic charging and deflection as hereinafter 
described. Coordinated printing capability is achieved by staggering the 
two rows of streams in accordance with the teachings of Taylor et al U.S. 
Pat. No. 3,560,641. 
Formation of drops 84 is closely controlled by application of a constant 
frequency, controlled amplitude, stimulating disturbance to each of the 
fluid streams emanating from orifice plate 18. Disturbances for this 
purpose may be set up by operating transducer 28 (in FIG. 1) to vibrate 
probe 30 at constant amplitude and frequency against plate 18. This causes 
a continuing series of bending waves to travel the length of plate 18; 
each wave producing a drop stimulating disturbance each time it passes one 
of the orifices 26. Dampers 22 prevent reflection and repropagation of 
these waves. Accordingly, each stream comprises an unbroken fluid filament 
and a series of uniformly sized and regularly spaced drops all in 
accordance with well known techniques. 
As each drop 84 is formed it is exposed to the charging influence of one of 
the charge rings 86. If the drop is to be deflected and caught, an 
electrical charge is applied to the associated charge ring 86 during the 
instant of drop formation. This causes an electrical charge to be induced 
in the tip of the fluid filament and carried away by the drop. As the drop 
traverses the deflecting field set up between ribbon 52 and the face of 
the adjacent catcher, it is deflected to strike and run down the face of 
the catcher, where it is ingested, and carried off. Drop ingestion may be 
promoted by application of a suitable vacuum to the ends 90 of catchers 
54. When drops are to be deposited on the web 88, no electrical charge is 
applied to the associated charge rings. 
Appropriate charges for accomplishment of the above mentioned drop charging 
are induced by setting up an electrical potential difference between 
orifice plate 18 (or any other conductive structure in electrical contact 
with the printing fluid supply) and each appropriate charge ring 86. These 
potential differences are created by grounding plate 18 and applying 
appropriately timed voltage pulses to wires 92 in connectors 94 (only one 
connector illustrated). Connectors 94 are plugged into receptacles 96 at 
the edge of charge plate 50 and deliver the mentioned voltage pulses over 
printed circuit lines 98 to charge rings 86. 
Charge plate 50 is fabricated from insulative material and charge ring 86 
are merely a coating of conductive material lining the surfaces of 
orifices in the charge ring plate. Voltage pulses for the above purpose 
may be generated by circuits of the type disclosed in Taylor et al, and 
wires 92 receiving these pulses may be matched with charge rings 86 on a 
one-to-one basis. 
Deflection of these drops 84 which are to be caught is accomplished by 
setting up appropriate electrical fields between deflection ribbon 52 and 
each of the catchers catchers 54. The preferred arrangement for this 
function is to have catchers 54 and one side of an electrical potential 
source all connected to a common ground. The other side of the potential 
source is then connected to deflection ribbon 52 thereby setting up a pair 
of equal strength, oppositely directed electrical deflection fields. With 
reference to FIG. 2, with the ground at the positive side of the potential 
source, it is necessary that drops 84 be charged negatively in order to be 
caught. However, it is also possible to obtain mutual outward deflection 
of the two curtains of drops 84 by charging the drops positively and 
reversing the terminal on the potential source. 
The deflection ribbon 52 must be relatively thin and straight for proper 
operation in the compactly arranged printing head in which the two 
adjacent rows of orifices are quite close together. Thus, in the preferred 
embodiment the deflection ribbon has a stainless steel core 100 
approximately 0.010 inch thick and is held under tension between and 
parallel to the rows of orifices 26, as shown in FIG. 2. A coating 101 of 
resistive material approximately 0.001-0.003 inch thick on each side of 
the core is provided. 
In the preferred embodiment, the resistivity of this material should be 
approximately at least 10.sup.4 ohms-cm, although this is not an absolute 
lower end of the range since spacing of the various parts of the ink jet 
printing head from the deflection ribbon and thickness of the coating have 
an effect on the acceptable threshold of resistivity of materials being 
useful as a coating on the core material of the deflection ribbon. 
Likewise, the material from which the core is made and its conductivity 
values will also affect the materials which can be used as resistive 
coatings therein. It has further been discovered that the resistive 
material can be insulative in nature and thus insulative materials from 
the upper bound of the range of resistivity of the coating. 
Any resistive or insulative coating having the specified range of 
resistivity indicated above is believed to be satisfactory. However, 
examples of materials which would provide resistivity within the range at 
acceptable thickness, i.e. approximately 0.001 inch, include inorganic 
transition metal oxides of chromium, titanium, molybdenum, etc. or metal 
oxides of aluminium, silicon and iron, and organic materials such as 
polyvinylcarbazole and acrylics. 
It is noteworthy that initially it was believed that the insulative coating 
might not perform satisfactorally due to a build up of an opposite charge 
on the surface of the insulating coating which would not dissipate 
sufficiently fast and thus result in a counteraction to the deflection 
field which could in turn result in insufficient deflection of the charge 
droplets to be caught by the catcher. It was discovered, however, in 
actual operation that this does not occur. It is theorized that one 
possible reason for the insulative coating being acceptable is that the 
surface charge is dissipated due to the "wet" condition existing on the 
ribbon during actual operation. It is believed that this wet condition may 
permit the bleeding off of the opposite charge from the surface of the 
ribbon sufficiently rapidly that an opposite surface charge does not 
accumulate sufficiently to interfere with operation of the deflection 
ribbon. This is only a theory, however, and it is not certain that this is 
actually what occurs. 
It is also noteworthy that although the preferred embodiment of the 
deflection ribbon involves the use of a single stainless steel band, other 
conductive materials could be used for the core section, such as copper. 
In addition, it may be possible to use multiple cores disposed in parallel 
alignment each insulated and independently connected to the potential 
source. 
Because of the relative flexibility of the deflection ribbon due to its 
thinness, means must be provided for holding the ribbon at its ends under 
tension so that the ribbon will be held taught in a vertical plane between 
the rows of droplets. There are obviously many ways in which this can be 
accomplished and FIGS. 3 and 4 schematically set out two examples. FIG. 3 
illustrates an embodiment in which the entire deflection ribbon has been 
coated and then a portion of the resistive coating stripped away along 
each end of the deflection ribbon on the side which will contact the 
electrode 102. The opening in the holder 56 which is secured to the 
printing head in any desired manner is of a general dovetail configuration 
corresponding to the triangular shape of electrode 102 and will thus hold 
the ribbon in place by frictional engagement between the ribbon, the 
electrode and the holder 56. 
A similar arrangement is illustrated in FIG. 4, but in this case the 
resistive coating 101 is stripped from both sides of each end portion of 
the core 100 of deflection ribbon 52 leaving the steel core exposed on 
both sides. An insulative coating 104 is then placed around the inside of 
the opening in holder 56 such as by coating or by use of an insert, and 
then the electrode 102 and the bare portion of the steel core which 
engages the electrode are inserted in a similar dove-tail opening 
configuration to prevent the deflection ribbon from being withdrawn and to 
hold it in tension between the rows of orifices. 
Thus it can be seen that by using a resistively coated deflection ribbon 52 
constructed in accordance with the present invention the possibility of 
shorts from the deflection ribbon to the catcher 54 or to the charge plate 
50 can be substantially reduced due to the resistive coating. This in turn 
permits an increase in the voltage applied to the deflection ribbon and 
thus in turn allows the voltage applied to the charge rings to be reduced. 
Since the charging voltage is constantly switched on and off, the ability 
to reduce this voltage level reduces the switching time and thus permits 
an increase in operating speed of the switching mechanism. Furthermore, 
failure of printing due to shorts is significantly reduced. 
While the form of apparatus herein described constitutes a preferred 
embodiment of this invention, it is to be understood that the invention is 
not limited to this precise form of apparatus, and that changes may be 
made therein without departing from the scope of the invention.