Reversed polarity ink jet imaging

Abstract: A continuous ink jet system includes a linear array of orifices fluidically connected to a fluid supply and pressurization means (18) to produce a linear array of jets, means for stimulating the jets for regular break-up of the jet into a plurality of uniform streams of drops (16), charging means consisting of a linear array of planar conducting elements (12) disposed along the path of motion of the jets, one for each jet, such that placing a preselected voltage on one of the plurality of conducting elements (12) causes an electrostatic charge to be induced on the corresponding jet, the electrostatic charge and the placement of the electrode being such that a charge drop receives a force impulse from the presence of the conducting elements (12), catcher means (22) disposed to catch selected drops with predetermined charge from the charging system, return fluidic means disposed so that drops intercepted by the catcher (22) can be returned through the fluidic system for eventual re-use in forming the jets, means for moving a substrate (30) beneath the catcher (22) means so that drops not intercepted by the catcher means (22) are allowed to impact the substrate (30) to cause printing. The invention provides the improvement of reversing the polarity of the charging means so as to create drops with positive charge.

Description: Technical Field The present invention relates to continuous ink jet imaging and, more particularly, to systems which utilize a linear array of jets at resolutions greater than about 100 jets per inch. Background of the Invention In continuous ink jet printing, ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. The approach for printing with these droplet streams is to selectively charge and deflect certain drops from their normal trajectories. Graphic reproduction is accomplished by selectively charging and deflecting drops from the drop streams and depositing at least some of the drops on a print receiving medium while other of the drops strike a drop catcher device. The continuous stream ink jet printing process is described, for example, in U.S. Pat. Nos. 4,255, 754; 4,698,123 and 4,751,517. With systems which utilize a linear array of jets at resolutions greater than about 100 jets per inch, it is not possible to charge with conventional charge tunnels. Various aspects of flat face charging are treated in the following United States Patent Nos. 4,757,328; 4,636,808; 3,656,980; and 3,604,980. Historically, all ink jet systems have operated with the charge tunnel positive. This is particularly true with ink jet systems which have metal orifices. The basis for this is that when the charge tunnel is positive, the drops are negative. Negative charges induced at the end of the jet result in a dipole layer at the orifice plate with positive charge next to the orifice plate. The goal is to reduce the extraction of (positive) metal ions from the orifice plate. Since like charges repel, it has been generally believed that the positive dipole layer adjacent the orifice plate would inhibit extraction of positive metal ions. Similarly, it has been generally believed that changing the polarity of the drops would have the undesirable result of orifice plate erosion (etching), because the dipole layer at the orifice plate would have negative ions adjacent the orifice plate, enhancing the extraction of the positive metal ions. When it is desired to etch orifice plates, such as to measure the size of the meniscus, the continuous jet is purposely allowed to connect to a negative voltage, causing the extraction of positive metal ions from the orifice plate. In recent ink testing with new ink technology, such as inks containing polymers and pigments, difficulty with build-up of foreign material on the charge lead and on the catcher has been encountered. The build-up usually causes undesirable "spitting drops", that is, a failure to catch drops which should be caught. It is believed that what happens is that an insulating layer of particles is formed on a charge lead, which causes catch drops to be improperly charged, and therefore not caught by the catcher. These drops then either hit the print media, or are deflected into the path of adjoining streams of drops. The net result of this is known as "spitting". Ink drops which should be caught, escape from the printhead. Another typical action is that particles build up on the catcher at the impact line. Since ink is evaporating from the catcher, a line of dried particles is formed. This area of dried particles can also possibly acquire a charge which causes "spitting". It is seen then that there is a need for a continuous ink jet system with an improved charging, particularly systems utilizing high field planar charging. In these systems, the high applied electric field is asymmetric, causing simultaneous charging and deflection. As a consequence, particles which result from the jet disintegration process can be caused to deposit on the charge lead and catcher. Summary of the Invention This need is met by the continuous ink jet system according to present claim 1 wherein the polarity of the charging means is negative so as to create drops with positive charge. In accordance with one aspect of the present invention, a continuous ink jet system includes a linear array of orifices fluidically connected to a fluid supply, pressurization means to produce a linear array of jets, means for stimulating the array of jets for regular break-up of each jet into a plurality of uniform streams of drops, and a linear array of planar conducting elements or strips disposed along the path of motion of the array of jets to provide charging, in use the linear array of planar conducting elements being negatively charged so as to create drops with positive charge. An object of the present invention is to provide a means for charging of systems which utilize a high field planar charging system. It is a further object of the present invention to provide a means for charging of systems which utilize a linear array of jets at resolutions greater than about 100 jets per inch. It is an advantage of the present invention that negatively charged planar conducting elements drastically improve the runnability of a variety of inks. Other objects and advantages of the invention will be apparent from the following description and the appended claims. Brief Description of the Drawings Fig. 1 is a side view of one embodiment of the present invention; and Fig. 2 is an isometric view of the face of the charge plate of Fig. 1 with the ink jets disposed in front of the charge plate. Detailed Description of the Invention It is known in the art that many materials dispersed in aqueous colloids are kept in equilibrium by the fact that they have a negative charge. The negative charge inhibits agglomeration of the dispersed material. In recent ink testing with new ink technology, such as inks containing polymers and pigments, difficulty with build-up of foreign material on the charge lead and on the catcher has been encountered. The build-up usually causes undesirable "spitting drops", that is, drops which fail to return to the catcher device. In an effort to discover whether the charge lead deposit and spitting drops occurred because of the inherent state of charge of ink ingredients, testing was done with "reversed polarity" charging to create drops with positive charge. After extensive testing with a variety of inks, a number of surprising results were discovered. One, after rather extensive testing with a variety of inks, no orifice plate erosion has been noted. Second, inks tested appear to fall into one of two categories. The inks either run better with "reversed polarity" charging, or the inks run equally well with either polarity. Third, a great number of inks tested and rejected with conventional polarity, run very well with reversed polarity. Such unexpected results are so contrary to the teachings of the art, that extensive tests were performed to assure that such results were not merely a fluke. In accordance with the present invention, it has been discovered that there is much greater latitude for ink with reversed polarity. PREPARATION OF INK Example I The following is one embodiment of an ink prepared to test ink runnability using reverse polarity. Component Weight % water 82.50 glycol ether 2.50 polypropylene glycol 0.55 surfactant 0.25 amine 1.80 carbon black pigment 0.83 resin polymer 0.67 dye solution 10.60 corrosion inhibitor 0.10 biocide 0.20 100.00 When the ink composition was formulated as above, the ink was used in a continuous ink jet printer, such as the type manufactured by Scitex Digital Printing, Inc., in Dayton, Ohio. The tests were conducted at room temperature. The ink in the in tank was pumped through a prefilter and a final filter into a printhead. From a linear array of orifices in the printhead, with a linear array of jets at a resolution of 120 jets per inch, an array of ink drop streams were generated. The printhead had an array of charge leads aligned with the drop streams. The charge lead can be either positively charged (conventional) or negatively charged (reverse), to create drops with negative or positive charge, respectively. The ink was recirculated back to the ink tank through a catcher. The ink formulated in accordance with Example I above was tested with both polarities and the charge lead deposit and spitting drops were monitored for each polarity. Approximately 0.8 to 1.0 liters of ink was used in each test. With reverse charge polarity, there was no charge lead deposit for six hours, and no spitting drops for 50 hours. With conventional charge polarity, charge lead deposit was visible in approximately one hour and spitting drops occurred after approximately seven hours. Example II The following is a second embodiment of an ink prepared to test the ink runnability using reverse polarity. Component Weight % water 91.65 surfactant 0.25 amine 1.80 carbon black pigment 3.30 resin polymer 2.70 corrosion inhibitor 0.10 biocide 0.20 100.00 When the ink composition was formulated as above, the ink was used in a continuous ink jet printer, such as the type manufactured by Scitex Digital Printing, Inc., in Dayton, Ohio. The tests were conducted at room temperature. The ink in the ink tank was pumped through a prefilter and a final filter into a printhead. From a linear array of orifices in the printhead, with a linear array of jets at a resolution of 120 jets per inch, an array of ink drop streams were generated. The printhead had an array of charge leads aligned with the drop streams. The charge lead can be either positively charged (conventional) or negatively charged (reverse), to create drops of negative or positive charge, respectively. The ink was recirculated back to the ink tank through a catcher. The ink formulated in accordance with Example II above was tested with both polarities and the charge lead deposit and spitting drops were monitored for each polarity. Approximately 0.8 to 1.0 liters of ink was used in each test. With reverse charge polarity, there was no charge lead deposit for at least eleven hours, and no spitting drops for 32 hours. With conventional charge polarity, charge lead deposit was visible in approximately one hour and spitting drops occurred in approximately two hours. As can be seen by the results of Examples I and II above, the advantage of using reverse charge polarity is that the ink had much better runnability and less charge lead deposit with reverse polarity than with conventional polarity. Referring now to the drawings, the present invention relates to the type of continuous ink jet system illustrated in Fig. 1. A plurality of jets is created at high spatial resolution by a drop generator, which stimulates the natural break-up of jets into uniform streams of droplets. In accordance with the present invention, spatial density of the jets is typically between 150 and 800 jets per inch. Conventional, prior art systems are incapable of charging jets at high spatial resolution. Conventional methods charge the jets by electrostatic induction in a "charge tunnel". The charge tunnel consists of an essentially closed cavity surrounding the jets. The cavity has an internal conducting coating which is electrically connected to an external source of electricity. When a potential is applied to the coating in the charge tunnel, an electrostatic charge is induced on the conducting jets, which are grounded through the system (not shown) which generates the drops. In some alternative embodiments of the older technology, charging is done by means of slots, rather than in tunnels. The slots are generally of the type which are narrow and deep so that the jets are essentially surrounded by the conducting coating in the slot. All of the existing technologies share the same deficiency; a high spatial density of jets cannot be achieved because tunnels, slots and the like take up space. It is known in the art, that tunnels with appropriate mechanical strength cannot be fabricated closer together than about 75 tunnels (or jets) per inch. Slots can be used at up to 100 jets per inch. It is highly desirable to be able to generate jets and independently charge drops at much higher spatial frequencies. For this purpose, the geometry shown in Fig. 1 has been devised. In this case, jets can be placed very close together to achieve high resolution printing. The continuous ink jet system of the present invention allows for segmented phasing across the linear array, wherein all the jets in a small region have a common phase, although the phase is allowed to vary from chip to chip along the array. This technology has been use to make commercial products at 300 jets per inch. The enabling feature is the idea that individual drops can be inductively charged by proximity to the conducting elements. The elements may also be charge electrodes or charge leads. The plurality of charge leads are attached to a planar surface to form a charge plate. The charge plate can be for flat face charging or may be a slotted charge plate. The charge leads can be produced by various photoforming techniques known in the photo-fabrication art. No mechanical features are needed to form the charging surfaces so the spatial frequency at which the conductors can be fabricated is not limited by mechanical considerations. A key difference between the array charging technology shown in Fig. 1, and prior art technology is that the electric field which causes charging in the new technology is asymmetrical. If particulate matter results from the drop formation process, it can be caused to move under the action of the electric field, and will ultimately deposit on the charge lead. This is not the case with tunnel or slot charging, because a particle extracted from a jet during drop formation undergoes a symmetrical electric force. Such particles will therefore either merge with the leading or following drop, or will flow downstream with the drops to be deposited on the substrate, or on the catcher. As ink formulation for ink jet printers has become more sophisticated, various materials are being incorporated into inks which cause the inks to be non-homogeneous. For example, polymer dispersions are incorporated to improve permanence of dye based inks. Also, pigments are used because of their superior image permanence. Particles dispersed in inks can be set free in the process of forming a drop from a jet. While this is a process based on probability, over time such particles can cause a "build-up" of matter on the charge leads, and even on the catcher. In the (likely) event that the buildup is insulating, it can cause the charge lead to lose it's ability to control the charge of the drops. What typically tends to happen is that an insulating layer of particles is formed on a charge lead, which causes improper charging of catch drops. The result is catch drops which are not deflected properly towards the catcher. These drops then either hit the print media, or are deflected into the path of adjoining streams of drops. The net result of this is what is known as "spitting". Ink drops which should have been caught, are allowed to strike the print media. Another typical action is that particles from the ink build up on the catcher at the impact line. Since ink is evaporating from the catcher, a line of dried particles is formed. The area of dried particles can acquire a charge which causes catch drops to be ejected from the printhead in what is termed "spitting". Very often, it has been observed in experiments that changing the polarity of the charging process from use of positive charging leads to the opposite polarity, causes the spitting phenomena to be dramatically and surprisingly reduced. Continuing with Fig. 1 and referring to Fig. 2, a plurality of conducting elements, or charge leads 12, are located on a planar charge plate 14. A plurality of streams of drops 16 are supplied by drop generator 18. A plurality of independently switchable sources 20 of electrostatic potential are supplied to the plurality of charge leads 12. A catcher 22 intercepts the slightly deflected streams of drops. The plurality of streams of drops impacting on the catcher forms a film of ink 26, which in turn forms a flow of ink 24, sucked away from the face of the catcher by a vacuum. Reference number 28 represents the area on the catcher at which the deflected drops impact the catcher and merge together to form a film of ink on the catcher face. The undeflected ink drops then print the image on substrate 30. Continuing with Fig. 2, a plurality of charge leads 12 are situated on the face of the planar charge plate 14. The plurality of drop streams 12 is supplied by drop generator 18 of Fig. 1. Reference number 32 refers to one form of "charge plate build-up", which is a residue of ink deposited on the charge leads 12 below the point at which the jets disintegrate into drops. In operation, Fig. 1 represents the side view of one embodiment of the present invention and Fig. 2 represents an isometric view of the face of the charge plate 14 with the ink jets disposed in front of the charge plate. Drop generator 18 supplies streams of essentially coplanar and colinear drops 16, parallel to the face of the charge plate 14. Each of the drop streams 16 are in linear alignment with a conducting charge lead 12. Prior to disintegration, the streams of ink 16 are electrically conducting. When an electric potential is applied to one of the plurality of charge leads 12 by one of the plurality of voltage sources 20, the last drop which is still connected to the jet in front of that charge lead acquires an electric charge by induction. By applying a predetermined voltage to any of the charge leads in a timed relationship to the jet disintegration process, successive drops can be either charged or uncharged. Typically, at least every other drop is charged so that each charged drop experiences the electrostatic image charge of the plurality of other charged drops. In the ensemble, these electrostatic charges cause an impulsive force of electrostatic attraction to be exerted on the charged drops. In response to the impulsive electrostatic force, the charged drops are attracted towards the face of the catcher 22. Drops which are formed in front of a charge lead which is momentarily at the same potential as the drop stream 16 are not charged. The trajectory of the uncharged drops is not deflected towards the catcher 22, so the uncharged drops move unimpeded towards the print media 30. By appropriate programming of the plurality of charge leads 12 in synchronism with the disintegration of the jet into a stream of drops 16, a desired pattern of drops moving towards the substrate 30 can be produced. When the drop pattern produced is varied in timed relationship with motion of the substrate 30 to the right in Fig. 1, any desired image can be formed on the moving substrate. In conventional systems, a positive voltage is applied to the charge electrodes by the plurality of voltage sources. This results in negatively charged catch drops. When drops are formed from the continuous jet, it can happen that any particles dispersed in the ink may be set free in the jet disintegration process. If it happens that the particle is charged, it can be attracted to the charge electrode by the strong electric field which exists between the jet and the charge electrode. Typically, a particle would not be released every time a drop breaks off from a jet, but on a basis of probabilities, some particles would be released. If the released particles had the appropriate charge, they could then migrate to the charge lead and form a deposit 32. When the dispersed material in the ink is insulating, the deposit 32 on the charge leads 12 effectively insulates the drop charging process from changes in the charge level. The result is that drops which are supposed to be caught, are actually left uncharged or partially charged, so that they escape from the printhead. This results in unwanted drops striking the print substrate 30. When the printhead expels unwanted drops, the result is called "spitting". The improvement described in this invention is the result of careful investigation of the deposit 32 and the spitting defect, as a function of various ink formulations and the polarity of the charge lead during operation. Surprisingly, for a wide variety of ink formulations used in the "flat face" (high jet spatial density) charging embodiment, all inks tested in accordance with the present invention either work better with negative charging voltage, (positively charged drops) or work equally well with either polarity. Industrial Applicability and Advantages The present invention is useful in the field of ink jet printing, and has the advantage of reversing the polarity of the charging means so as to create drops with positive charge. The present invention has the further advantage that reversing the charge polarity results in drastically reduced deposits on the charge lead and catcher. This results in dramatically improved runnability for a variety of ink formulations. While it appears that inks which run with the conventional polarity charging will also run with reversed polarity, many inks which run well with reversed polarity charging will not run with conventional polarity. Further, the present invention has the advantage that reversing the conventional polarity has not had a measurable effect on the life of the orifice plate. The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that modifications and variations can be effected within the scope of the appended claims.

Claims: A continuous ink jet system comprising: a linear array of orifices fluidically connected to a fluid supply; pressurization means to produce a linear array of jets; stimulation means for stimulating jets of the array of jets for regular break-up of each jet into a plurality of uniform streams of drops; and a linear array of planar conducting elements disposed along a path of motion of the array of jets to provide charging, in use the linear array of planar conducting elements being negatively charged so as to create drops with positive charge. A continuous ink jet system as claimed in claim 1, wherein each one of the plurality of the planar conducting elements is associated with a separate corresponding jet. A continuous ink jet system as claimed in claim 2, wherein placing a pre-selected voltage on one of the linear array of conducting elements causes an electrostatic charge to be induced on the corresponding jet. A continuous ink jet system as claimed in claim 3, wherein a charged drop receives a force impulse from the presence of the linear array of charged drops and conducting elements as a result of the electrostatic charge on the drop and placement of the conducting elements. A continuous ink jet system as claimed in any preceding claim and further comprising a catcher disposed to catch selected drops with predetermined charge from the linear array of planar conducting elements. A continuous ink jet system as claimed in claim 5, further comprising a movable substrate located beneath the catcher to receive drops not intercepted by the catcher and cause printing. A continuous ink jet system as claimed in claim 5, further comprising a return fluidic line disposed so that drops intercepted by the catcher means can be returned through the return fluidic line for eventual re-use in forming the array of jets. A continuous ink jet system as claimed in any preceding claim, wherein the spatial density of the jets is between 150 and 800 jets per inch. A continuous ink jet system as claimed in any preceding claim and further comprising segmented phasing across the linear array.

Inventors: BRAUN HILARION, CHEN JING-DEN, BRAUN, HILARION, CHEN, JING-DEN
Applicants: SCITEX DIGITAL PRINTING INC, SCITEX DIGITAL PRINTING, INC.
Application year: 1995
Main classification: B41J2
IPC codes: B41J2