DROP DEFLECTOR FOR USE THEREWITH

This invention relates generally to the field of high speed printing of characters

on printing media, such as paper sheets, labels or the like. More specifically, it relates

to non-impact type printing utilizing tiny droplets of electrically conductive ink forced

through a nozzle or orifice under pressure and commonly referred to as drop marking

or ink jet printing. The structure defining the orifice is subjected to ultrasonic vibrations

which are usually produced by a piezoelectric crystal driven by an oscillator circuit and

cause the inkstream to break up at a regular drop rate as it leaves the orifice. The drop rate is proportional to the rate of vibration.

The stream of ink drops is directed past a charge electrode or ring where selected drops are charged. By synchronizing the charge on the ring with the drop formation rate, the charge on each drop is discretely controlled.

The drops are next directed past a drop deflection means comprising a pair of electrodes which have a potential difference of several thousand volts to establish an

electric field between the electrodes. Uncharged ink drops are not deflected by the

electric field and continue along their initial trajectory which is preferably directed

towards a gutter which collects the uncharged drops for recirculation back to the

nozzle. The charged ink drops are deflected from their initial trajectory by the

influence of the electric field and follow trajectories dependent on the level of their respective charge. The result is the ability to direct drops of ink to selected positions

along a line on a printing medium by controlling the electrical charge which a drop receives. This ability is used, in conjunction with movement of the nozzle, to generate predetermined markings, such as alphanumeric characters.

It is well known that, for any given ink drop charge, the angular change in the

trajectory of the ink drop will be a direct function of the electric field intensity between

the deflection electrodes through which the charged ink drop passes, and the time taken for the ink drop to travel therethrough. It is also well known that the size of the printed character is directly related to the maximum obtainable angular change in the

trajectory of the ink drops and the distance of the medium from the drop deflection means. Consequently, to obtain characters or print sizes of a predetermined minimum height, it is necessary to define both the minimum field intensity and the minimum deflection electrode length to achieve the required angular change of trajectory.

For a given character size, any increase of the field intensity will enable a reduction in the length of the deflection electrodes which, in turn, translates into an advantageous decrease in both the overall size of the print head and the spacing

between the ink nozzle and the printing medium. The shortest nozzle to printing

medium spacing generally yields the best quality print. However, the field intensity

may not be increased without limit because inter-electrode electrical arcing occurs at elevated field intensities. One known prior art ink jet printer, for example, utilizes

5,000 volts to energize deflection electrodes spaced in parallel relationship 3/16 inch apart (that is about 0.48 cm), which translates into about 10,500 volts/cm.

Substantially increasing the resulting field intensity can result in arcing.

There is also a problem with ink mist collecting on deflection electrodes. This mist unavoidably forms upon impact of the ink drops with the print medium and/or the

collection gutter. This ink mist has an electrical charge which in a collected form can

influence the deflection field in an uncontrollable manner. It is well-known to utilize

the electrical attraction, between the ink mist having a first charge polarity and the tip

of the high voltage electrode having the opposite polarity, to neutralize the ink mist.

As hereinafter described, it is also known for the drop deflection means to have

its electrodes diverging in the direction of the drop trajectories to provide room for the

diverging drops and to increase the field intensity at the entrance between the electrodes. Due to the closer spacing of the electrodes at the entrance it is also known to insulate the high voltage electrode in the vicinity of the entrance to prevent inter-electrode arcing.

It is accordingly known from the prior art for a drop marking system to include

means for emitting a stream of marking drops, means for charging selected drops, drop deflection means for deflecting the charged drops to follow a trajectory different

from a trajectory followed by the uncharged drops whereby certain drops will mark a

medium whilst other drops will be intercepted by the catcher, the drop deflection

means including a high voltage electrode and a low voltage electrode, means for maintaining the low voltage electrode at ground or nearly at ground, means for

applying a high voltage to the high voltage electrode sufficient to create an electric

field between the electrodes to deflect the charged drops, and insulation material covering part of the high voltage electrode to inhibit arcing between the electrodes.

It is also known from the prior art for a drop deflector, for a drop marking system in which a stream of marking drops are to be emitted and selected drops are to be

charged for deflection along a trajectory different from a trajectory followed by the

uncharged drops, to include a high voltage electrode and a low voltage electrode,

means for maintaining the low voltage electrode at ground or nearly at ground, means for applying a high voltage to the high voltage electrode sufficient to create an electric

field between the electrodes to deflect the charged drops, and insulation material

covering part of the high voltage electrode to inhibit arcing between the electrodes.

According to one aspect of the present invention a drop marking system, or a drop deflector for a drop marking system, has the insulation material covering substantially the entire surface of the high voltage electrode whereby the electric field strength can be increased by operating the high voltage electrode at a voltage which would otherwise cause inter-electrode arcing.

Preferably the electrodes are plates which diverge in the direction of the drop trajectories whereby the electric field will be stronger at the entrance of the drop

deflection means than at its exit.

The means for applying the high voltage to the high voltage electrode may be

arranged to apply the high voltage at the same polarity as the charge to be given to

the selected drops whereby the charged drops will be repelled by the high voltage

electrode towards the low voltage deflection electrode.

Preferably the low voltage electrode has an uninsulated portion positioned to

attract and neutralize the charge on mist produced by charged drops striking the medium.

It is also known from the prior art for a method of deflecting selected charged

drops from a first trajectory to another trajectory to comprise passing a stream of

charged and uncharged drops through an electric field generated between a high

voltage electrode and a low voltage electrode.

According to another aspect of the present invention a method of deflecting selected charged drops from a first trajectory to another trajectory includes insulating the high voltage electrode to enable operation at a voltage which would otherwise cause inter-electrode arcing.

The method may include applying the high voltage to the high voltage electrode with the same polarity as the charge on the charged drops whereby the charged drops will be repelled by the high voltage electrode towards the low voltage electrode.

There is no suggestion from the prior art that the benefits of an increased

intensity electric field and neutralization of ink mist can be obtained by fully insulating

the high voltage electrode to prevent arcing or of using the grounded electrode for ink

mist control. The present invention employs this arrangement by using the high

voltage electrode to repel the drops rather than attract them. The grounded electrode attracts ink mist to neutralize it thereby avoiding any substantial effect on field

strength. This permits the high voltage electrode to be completely insulated to prevent

arcing.

Because of the electrically insulative coating over the high voltage deflection

electrode, the deflection electrodes can be spaced closer than uninsulated or partially insulated electrodes which depend only on avoiding a dielectric breakdown of the air

to maintain a voltage difference that supports an electric field. Higher electrical

potentials can be used resulting in establishment of a greater intensity electric field,

which for the same length of drop travel from the nozzle to the printing medium, increases the deflection amplitude at the printing medium. Conversely a given deflection amplitude at the printing medium can be achieved with a correspondingly shorter length of drop travel.

The present invention therefore provides a print head having deflection electrodes of decreased length.

The invention will now be described, by way of example only, with reference to

the accompanying drawings, in which:-

Figure 1 is a schematic block diagram of a known drop marking system utilized

in an ink jet printer produced by Videojet Systems International, Inc. and sold under

their Trade Mark EXCEL;

Figure 2 is an enlarged diagrammatic view of the ink jet printer head shown in

Figure 1 illustrating details of the drop deflector; and

Figures 3, 4, 5 and 6 are diagrams similar to Figure 2 but illustrating alternative forms of drop deflector, in accordance with the present invention, for use with a drop

marking system such as that shown in Figure 1.

Referring to Figure 1 , a drop marking system has a tank 10 for a supply of

electrically conductive ink which is withdrawn by pump 12 and supplied under pressure

to nozzle 14 via a regulator 16 and a conduit 18 in a conventional manner. The

resistivity of the ink is preferably in the range of 100 ohm-cm to 1500 ohm-cm.

The nozzle 14 has a small orifice 20 through which the ink is emitted. The orifice 20 is preferably defined in a jewel 22 located in the end of the nozzle 14. A piezoelectric transducer 24 is fitted in abutting contact with the nozzle 14 and is driven, generally at an ultrasonic rate, by conventional oscillator/character generator circuitry 26. The ink stream, emitted from the orifice 20 by virtue of the pressure from pump 12, is broken up by the transducer 24 into a series of ink drops 28. The above described operation of the ink nozzle is well known and is disclosed in more detail, for

example, in U.S. Patent No. 3,972,474. The nozzle 14 together with the orifice 20 and

transducer 24 constitute a means for emitting a stream of ink drops along a trajectory

T, aligned with an ink catcher or gutter 36.

A charge electrode or ring 30 is provided along the trajectory T, near the orifice 20 and functions to charge selected individual ink drops 28, as they break off from the stream in the proximity of the ring 30, thus trapping a selected charge on each

selected ink drop. A positive potential is placed on ring 30 to cause a negative charge on the selected ink drops 28 which are identified in Figures 1 and 2 by individual

minus signs. The oscillator/character generator 26 generates a time variant video

signal on line 27, generally in the form of a pulse train of varying amplitude, which is

synchronized with the oscillator drive to the transducer 24 to facilitate the discrete control of the magnitude of the charge acquired by each selected ink drop passing

through the ring 30.

All of the ink drops 28 thereafter pass along the trajectory T ₁ into a spaced deflection system which is indicated generally by arrow 32 and comprises divergent linear deflection electrodes 52 and 54. The electrode 52 is grounded as shown at 53, but the electrode 54 is connected to a means 55 for applying high voltage sufficient to create an electric field between the electrodes 52 and 54 to deflect the selected charged drops 28. All of the uncharged drops 28 are unaffected by this electric field and continue along their initial trajectory T, until intercepted by the gutter 36 and returned through conduit 38 to the ink reservoir. However, the selected negatively charged drops 28 are deflected by the electric field towards the positively charged high

voltage electrode 54 and follow different trajectories, such as T ₂ , which are directed

towards a medium 34, such as paper or other printable material, to be marked by the

selected ink drops.

The deflection or displacement of a given ink drop as it passes through a

uniform electric field is given by the following relationship:-

where:- x is the deflection amplitude at the surface of the print medium; q is the charge on the ink drop;

E _x is the electric field intensity between planar shaped deflection electrodes;

V _j0 is the initial drop velocity; m _d is the mass of the ink drop; and z ₀ is the axial distance from the point of drop formation to the print surface.

Thus the deflection "x" of each drop is in direct proportion to the magnitude of the charge "q" placed thereon. If no charge is placed on the drop, it passes deflection system 32 along a straight, undeflected path into gutter 36 and is not printed on the medium 34.

It is also apparent that the ink drop deflection "x" is directly related to the intensity of the electric field "E _x " which acts upon each charged ink drop as it passes through the deflection system 32 towards the print medium 34. Accordingly any increase in the field intensity "E _x " will result in a linear increase in the ink drop deflection "x". Thus, consistent with maintaining a predetermined character size (i.e. maximum drop deflection), it will be appreciated that the axial distance "z ₀ ^u travelled by the ink drops may be reduced if the electric field intensity "E _x " is correspondingly increased.

As previously indicated, the field intensity "E _x " cannot be increased without limit

due to dielectric breakdown of the air separating the spaced deflection electrodes 52

and 54. Naturally, the presence of moisture, ink droplets and ink mist significantly

reduces the maximum field intensities which can be maintained. Practical maximum

field intensities are approximately 10-15 kv/cm. Such restricted deflection field

intensities limit the maximum drop deflection achievable in a short drop transit space

and, therefore, necessitate longer deflection systems.

Conventional drop deflectors have uninsulated high and low voltage deflection electrodes parallely spaced at a spacing of about 3/16 inch, that is about 0.48 cm, and their performance is limited by the field density that can reliably be created with this

arrangement.

Figure 2 illustrates, in greater detail, the improved deflection system taught by Videojet Systems International, Inc. in which a higher field intensity than conventional is achieved. The linear deflection electrodes 52 and 54 diverge, in the direction of the ink drop'travel, from a drop entrance 58 of relatively close spacing adjacent the ring 30 to a drop exit 60 of maximum spacing adjacent the medium 34 and the gutter 36.

The means 55 places a fixed positive high DC potential, typically about 5000

volts, on the high voltage upper electrode 54 with respect to the grounded voltage

lower electrode 52, in conventional fashion, to generate an electric field 56

therebetween intended to accelerate or attract the selected negatively charged ink

drops 28 upwardly as they pass from the entrance 58 to the exit 60.

The divergence of the deflection electrodes 52 and 54 causes the electric field

56 to be highly non-uniform with a substantially greater field intensity at its entrance

58 than at its exit 60. This non-uniformity defines a region of high field intensity

generally at the entrance 58 which exerts the greatest force for deflecting the charged

ink drops.

The desired high field intensity in the region of the entrance 58 is achieved by

spacing the deflection electrodes 52, 54 substantially closer together in the entrance

region 58 than is conventional. Whilst other known systems generally position the

deflection electrodes at a uniform 3/16 inch (0.48 cm) spacing, the drop entrance 58 of the Figure 2 structure may be reduced to 3/32 inch (0.24 cm) or even 1/16 inch

(0.12 cm), thereby tripling the electric field intensity in this region. The electrodes 52 and 54 diverge to a spacing of between about 3/16 and 5/16 inch (that is 0.48 cm to 0.80 cm) at the drop exit 60.

Elevated electric field intensities of this magnitude would conventionally be impracticable as they would be accompanied by excessive arcing in the narrowed gap

adjacent the entrance 58, particularly in the presence of the conductive ink drops and

mist. The arrangement illustrated in Figures 1 and 2 overcomes this problem by

positioning a dielectric insulation material 62 over the portion of the high voltage

electrode 54 generally in the entrance region of high electric field intensity. The

dielectric insulation material 62 can be Teflon FEP which exhibits a dielectric strength (236 KV/cm) six times that of air, and therefore, a relatively thin 1/10 inch (0.25 cm) layer of this material provides an additional 5,900 volts dielectric strength thereby

doubling the maximum useable electric field intensity.

However, in the prior art device of Figure 2 it is necessary for the high voltage

deflection electrode 54 to have its exit end 66 left uninsulated and exposed to provide

for mist neutralization and collection.

It is well known that ink drops impacting on a print medium create an ink mist

which can accumulate to form an unwanted electric field which, in turn, will deflect charged ink drops from their intended trajectories. In the prior art device of Figure 2, negatively charged ink mist 64 is attracted to the exposed end 66 of the positively charged high voltage electrode 54 (generally 1/8 - 1/4 inch (that is 0.32 - 0.64 cm) of the electrode is left exposed) where, upon contact with that electrode, the mist is electrically neutralized. While the prior art device shown in Figure 2 has been highly satisfactory, there remains the possibility of arcing which, because the ink drops contain volatile solvents, can result in combustion. This possibility is increased by use of the high voltage electrode for controlling mist. As mist builds up, changes in the field occur and evaporation of the solvent decreases the dielectric strength of the air

gap between the plates increasing the likelihood of arcing.

With reference to Figure 3 a stream of ink drops 80 are emitted by a nozzle 84

through a jewelled orifice 86 and selected drops are appropriately charged by charge

electrode 88. The deflection structure consists of a pair of plates defining deflection

electrodes 90 and 92. Uncharged drops pass along trajectory T, to the gutter or

catcher 36 as before while charged drops pass along trajectory T ₂ to mark the

substrate 34. The upper electrode 90 is grounded, or nearly so, to provide the low voltage electrode while the lower electrode 92 is maintained at a negative voltage on the order of 2,000 to 5,000 volts to provide the high voltage electrode. The entire high

voltage electrode 92 is surrounded by an insulating layer 94 which can be of any suitable insulating material such as that previously discussed in connection with the prior art insulating layer 62 in Figure 2. It should particularly be noted that, because the electrode 92 is completely encapsulated, arcing between the electrode cannot

occur under ordinary operating conditions. The ink drops selected for marking the substrate 34 are negatively charged and are therefore repelled by the high voltage electrode 92, as indicated in the drawing, to cause a deflection onto a trajectory, such as T ₂ , to mark the substrate 34.

Because the high voltage electrode is completely encapsulated in insulating material, it cannot be used for mist control. However, this function is provided by the low voltage electrode 90 due to the use of negatively charged drops, that is the same polarity as the high voltage electrode 92. Thus mist, which forms when the drops impact on the substrate 34, is attracted to an end 96 of the grounded electrode 90. Because the high voltage electrode plate 92 is encapsulated in a material of high dielectric strength having very high resistivity (of the order of a thousand volts/mil and

1000 Megohm-cm), shorting the surface of the high voltage electrode 92 to ground, or to the opposite grounded electrode plate 90 will not collapse the electrode field. Furthermore corona currents will not be established because of the high impedance of the encapsulating material 94.

In order to obtain the full benefits of the present invention, the high voltage encapsulated electrode 92 is preferably of the same polarity as the charge on the ink

drops 80. The other electrode 90 is preferably substantially at ground potential. The

charged drops will be repelled from the high voltage electrode 90 rather than, as in the

prior art, attracted thereto. This ensures that the uncontrollable charged satellites 98

constituting the mist will not gather on the high voltage electrode 92 and interfere with

the establishment of the desired field. Instead, the uncontrolled satellites 98 gather

on the end 96 of the opposing grounded electrode 90 and are discharged leaving the field undisturbed. The plates are shown as planar but may have other geometries.

In this manner the present invention permits increased field intensity without adverse inter-electrode arcing, thereby permitting a shorter printhead. This can be accomplished without sacrificing the ink mist neutralizing function. The high voltage electrode is completely encapsulated in an insulating material. The uninsulated electrode is preferably grounded or at a very nominal voltage (say not more than 100 volts) and utilized for the mist neutralization function. Because the ink drops and high voltage electrode are of the same polarity, the high voltage electrode repels the drops to cause deflection, rather than attracting the drops as in the prior art.

Figures 4, 5 and 6 illustrate alternatives to Figure 3 and only the features of difference will be described, the same reference numerals being used to indicate

equivalent parts.

In Figure 4 the polarity of the high voltage supply and the charge on the drops

have been reversed so that they are both positive. The upper low voltage electrode

90 is still grounded, or nearly so.

The arrangement of the electrodes in Figures 3 and 4 can be altered to suit

specific operating requirements. For instance, the gutter 36 could be repositioned to

collect ink drops from one of the deflected trajectories such as T ₂ , and the relative

positions of the electrodes 90, 92 can be reversed.

In Figure 5 the high voltage electrode 92 is of positive polarity whereas the

selected drops are given a negative charge so that they will be attracted from trajectory T, to another trajectory, such as T ₂ or T ₃ , dependent on the level of charge. The end 96 of the low voltage electrode 90 may be kept at a low positive voltage to collect satellites 98 from the vicinity of the catcher 36. Other satellites may

alternatively be removed, if desired, by suction or by using a separate uninsulated electrode of appropriate potential.

In Figure 6 the positions of the low voltage electrode 90 and the high voltage

electrode 92 have been reversed so that the medium 34 will be marked by uncharged drops along trajectory T, and by charged drops along trajectories such as T ₂ . The

drops for recirculation are the most heavily charged drops which pass along trajectory

T ₃ to the catcher 36.

Whilst the general features of the invention are shown in and described with reference to Figure 3, the subsequent Figures 4, 5 and 6 indicate several variations

of the manner in which some features may be re-arranged.

Although the electrodes 90 and 92 illustrated in Figures 3 to 6 are shown as

being the same size as the electrodes 52 and 54 of Figures 1 and 2, it should be

particularly noted the increased field strength provided by the present invention

enables a given drop deflection to be achieved using electrodes which are

correspondingly shorter (in the direction of the trajectories T _v T ₂ , etc) than hitherto

possible. This increases print accuracy and reduces evaporation from the recirculated

drops which have less exposure to the ambient environment.

The relative disposition of the electrodes 90, 92 may be as shown but, if

desired, could be parallely spaced provided the deflected drops can escape through the drop exit 60. However, due to the increased field intensity it is also possible that the angular diverge of the electrodes may also be increased.