|Publication number||CA1070369 A|
|Application number||CA 268535|
|Publication date||22 Jan 1980|
|Filing date||22 Dec 1976|
|Priority date||31 Dec 1975|
|Also published as||CA1070369A1, US3997113|
|Publication number||CA 1070369 A, CA 1070369A, CA 268535, CA-A-1070369, CA1070369 A, CA1070369A|
|Inventors||William B. Pennebaker (Jr.)|
|Applicant||William B. Pennebaker (Jr.), International Business Machines Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Classifications (7), Legal Events (1)|
|External Links: CIPO, Espacenet|
14 BACKGROUND OF THE INVENTION:
1. Field of the Invention:
16 This invention relates generally to apparatus for 17 charging particles and more specifically to apparatus 18 for hiqh frequency A.C. field charging of aerosol 19 particles for employment in an ink mist printing operation.
21 2. Prior Art:
22 The charging of aerosol particles in an ink mist 23 printin~ operation is usually accomplished by a D.C.
24 charging electrode. In such a charging system, genera.lly referred to as electrostatic charging, the 26 aerosol particles are charged by ions which are drawn 27 from a corona source by a D.C. charging electrode. An 28 electric 'ield is produced in the passage between the 29 corona source and the ~.C. charging electrode which effectively draws ions from the corona. The aerosol 31 particles when passing through this passage concentrate ~ . .
~07~369 1 the electric field lines such that ions are drawn
2 to their surfaces.
3 While the electrostatic charging method described
4 above produces a charyed particle suitable for use in ~-a printing operation, the practice of this method 6 has been hampered by precipitation of charged particles 7 to the D.C. charging electrode. As the particles are 8 charged they are also attracted to the D.C. charging 9 electrode. In other words, the same electric field which charges the particles also causes them to 11 precipitate. This precipitation eventually begins to 12 block the passage in which the charging is done, and ~-13 may cause electrical shorts between the electrodes if 14 the particles are electrically conductive. To reduce the amount of charged aerosol particles which precipitate 16 to the D.C. charging electrode, guard flows have been 17 used. Even though guard flows eliminate some 18 precipitation in the D.C. charging system, they have 19 proven to be very ineffective in eliminating a substantial portion of the precipitate to the charging 21 electrode over extended periods of operation or 22 charging.
23 The employment of A.C and transient voltages 24 in electrostatic precipitation operations is known.
Ho~ever, previously known methods and apparatus for 26 electrostatically precipitating particles have 27 utilized A.C. and transient voltages as charging 28 sources for the corona to generate a more intense 29 corona and thus produce pulses of ions to impart an electrical charge to the aerosol particles. It is 31 also known in the electrostatic precipitation art 1C~70369 1 that an R. Fr ripple can be superim'posed on the 2 constant potential supplied to ionizing wi'res in 3 the corona source. This has the effect of increasing 4 corona emission to the charging region. The precipitation of aerosol particles in the examples mentioned above 6 is not retarded, but is increased by superimposing A.C. ;~
7 and transient voltages on potentials applied to the 8 corona source.
9 ~ It is also known to use low frequency A.C.' voltage 10 sources to produce pulses of ions. When using low ~
11 frequency A.C. sources, the aerosol particles are ~-12 charged'directly in the corona. This results in '' 13 pulses of charged particles rather than a continuous 14 stream of uniformly charged particles. ' OBJECT OF THE INVENTIOM~
16 It is therefore an object of this invention to 17 substantially eliminate precipitation of charged 18 particles on charging electrodes in aerosol charging 19 systems.
It is another object o~ this invention to uniformly 21 charge a continuously moving aerosol stream.
22 SUI~*ARY OF THE INVENTION: -.
23 The above objects are accomplished through the 24 use of a high frequency alternating voltage source which is used in place of a standard D.C. voltage to 26 create a charying field. The ions used to charge the 27 aerosol particles are drawn from the corona under 28 the action of a small D.C. electrical field produced 29 by the corona itself. The charging field produced by the high'frequency ~.C. voltage source is maintained 31 at a level sufficient enough to impart a charge to the 1 aerosol particle and at a frequency high enough to 2 avoid repelling or attracting the aerosol particles.
3 An uncharged aerosol, directed through a passage 4 between the corona and the high frequency voltage source, is charged by ions present in the passage.
6 The charged particles are then projected onto a 7 surface to form a discernible representation thereon.
8 The foregoing and other objects, features and g advantages of the invention will be apparent from the following more particular description of the 11 preferred embodiment of the invention, as illustrated 12 in the accompanying drawing. ~-13 BRIEF DESCRIPTION OF THE DRAWING:
14 The FIGURE shows diagrammatically a preferred embodiment of the aerosol charger in accordance with 16 this invention.
17 DESCRIPTION OF THE PREFERRED EMBODIMENT:
18 Referring to the drawing, there is shown a high 19 frequency A.C. field aerosol charger of this invention indicated generally by the numeral 2, including a 21 charging station 17 for charging the uncharged ink 22 aerosol particles 4 as they travel through the 23 charging station. The nebulization process, in 24 which a very fine mist can be generated, is used in this invention to produce aerosol particles 4.
26 The nebulization process is described generally 27 in the following United States patents assigned 28 to the same assignee as the present invention:
29 Patent Number 3,959,798, issued May 25, 1976, and entitled "Selective Wetting Using a Micromist of 31 Particles", by Hochberg, et al.; Patent Number 4,019,188, ` 10~70369 1 issued April 19, 1977, by Hochberg, et al., and entitled 2 "Micromist Jet Printer"; and Patent Number 3,974,769, 3 issued August 17, 1976, by Hochberg, et al., and entitled 4 "Method and Apparatus for Recording Information on a Recording Surface Through the Vse of Nondirected 6 Particles." There are, of course, other ways in which 7 a relatively high density of aerosol particles could 8 be produced. For example, an aerosol could be produced -~
g by spinning liquid sheets and letting the sheets break up into droplets. The commonly used canned dispenser 11 is another possibility for making an aerosol but 12 typically the drops produced in such a dispenser are 13 too large for ink mist printing operations.
14 As shown in the drawing, the ink nebulizer comprises a housing 5 filled with a liquid ink bath 16 with a transducer 3 placed on the lower outer edge 17 thereof. The liquid bath in housing 5 is supplied 18 from ink reservoir 7 through pump 11. The ink reservoir 19 7 is filled from a larger source (not shown) through line 9.
21 The transducer 3, when activated at a frequency 22 on the order of 1 MHz, emits ultrasonic vibrations 23 which act to excite or energize the ink bath in 24 housing 5 producing nebulized ink particles 4 in the upper space of housing 5. The 1 MHz signal produces 26 droplets having a drop size of about three microns 27 in diameter.
28 A carrier stream of air 6 is fed from an air 29 supply 13 t~ugh valve 19 to the upper space of housing 5. The ink aerosol particles 4, when formed, 31 become entrained in the carrier stream of air 6 and .
~070369 1 travel out of housing 5 into aerosol chamber 8. As 2 understood by those skilled in the art, any of a 3 variety of inks may readily be employed for purposes 4 of this invention.
There are three air streams used in this charging 6 system, with air supply 13 serving as a common source 7 for all three streams. As noted, the first air stream 8 6 brings aerosol particles 4 into the charging system 9 from nebulizer 5. Another air stream 36 supplies a small bias flow through corona housing 26. This flow 11 keeps corona housing 26 clean by preventing any aerosol 12 particles 4 from settling therein. A third air stream 13 in the form of a guard flow 10 introduces a layer of 14 clean air into the top and bottom guard flow cavities 12. The guard flow of air 10 encircles carrier stream 16 6 containing aerosol particles 4 and compresses it into 17 a stream or flow 15 of narrow width before it enters 18 orifice 16 leading to charging station 17. This 19 ~ontinues flow of aerosol particles 15 is constrained to the center of passage 18 between the plates 24 and 21 25 in the field electrode assembly. It is understood 22 by those skilled in the art that the air streams as 23 used above could readily be supplied from separate 2~ sources for purposes of this invention.
The guard flow cavities 12 are located within 26 aerosol chamber 8 adjacent charging station 17 and 27 encircle nozzle structure 21. The guard flow of air 28 10 which enters the cavities 12 exits into aerosol 29 chamber 8 through small openings within porous filters 1~. This guard flow of air 10 keeps aerosol particles 31 4 away from the edges of orifice 16 in nozzle 21 by " 1070369 1 shaping aerosol particles 4 into a ribbon or flow 2 15 of narrow width. This continuous flow of aerosol 3 particles 15, still entrained in air stream 6, travels 4 through passage 18 to charging station 17 where the aerosol is charged.
6 The aerosol chamber 8 is also provided with a 7 -lower inclined wall 20 which returns to nebulizer 5, 8 ink 22 formed from aerosol particles which settle to 9 inclined wall 20 under the action o~ gravity.
The charging station 17 is comprised of a high 11 frequency alternating voltage 32, a field electrode 12 assembly 24, an electrically grounded corona housing ;
13 26, a fine wire electrode 28 which forms the corona 14 discharge, and an air flow ca~ity 38 with a porous filter 40 placed therein. A field electrode assembly, 16 comprised of two plates 2a and 25 which are located 17 on the upper and lo~ler inner walls of nozzle 21 and 18 comprised of two plates spaced a small distance 19 apart, provide a passage 18 for aerosol particles 4 to travel through. A high frequency charging 21 field for aerosol particles 4 is formed in this 22 passage 18 between plates 24 and 25 in the field 23 electrode asse~bly. The corona housing 26 and plate 24 25 are located on the lower side of charging station 17. The plate 25 is electrically connected to corona 26 housing 26 and is also grounded. The 1ons produced 27 by corona wire 28 are drawn through opening 27 in 28 corona housing 28 under the action of a s~all D.C.
29 electrical field produced by corona ~ire 28 itself during production of the ions. The high frequency 31 A.C. voltage generator 32 is connected to ?late 24 107~)369 1 which is positioned directly opposite opening 27 2 in corona housing 26. The flow 36 of clean air 3 enters air flow cavity 38 where it then enters 4 corona housing 26 through porous filter 40 which separates corona housing 26 and air flow cavity 38.
6 This small corona flow 36 keeps corona housing 26 -7 clean by preventing any aerosol particle 4 from 8 settling therein. This flow also counteracts the 9 gravitational settling of aerosol particles 4 toward lower plate 25. The effect of corona flow 36 on the 11 deposit flow of aerosol particles 15 can be seen in 12 the area denoted by numeral 44.
13 The potential applied to the corona in this 14 invention is a function of the amount of current to be produced and the type of corona in use. A fine 16 wire electrode 28 with cylindrical symmetry as used 17 in this charging station 17 operates at a potential 18 of about 5-7 kilovolts which is supplied from source 19 30 through resistor 29. For purposes of this invention, the potential of source 30 could be either 21 positive or negative.
22 The frequency used for the high frequency 23 generator 32 should be greater than 1 kilo hertz 24 for air velocities through the charge plates on the order of 300 cm/sec. if precipitation to the charge 26 plates is to be avoided. An optimum charging 27 efficiency is reached at a frequency of about 28 30-100 kilo hertz, and efficiency decreases slowly 29 with increasing frequency above that optimum. The optimum frequency range is reached when ions ~2 31 are no longer drawn to the charge plates, and thus ~07~ 369 1 remain in the charging region for a longer period of 2 time. -3 A small D.C. bias may be added in series with 4 the high frequency A.C. voltage generator 32 to enhance the current drawn from corona wire electrode 6 28. The potential on the D.C. bias would only be a- -7 ~raction of the potential applied to a standard D.C.
8 charging electrode. As this small bias, if added, lS
9 intended only for bias purposes, any charging of aerosol particles 4 is negligible.
11 The charging rate of aerosol particles 4 is a 12 function of time, and the high frequency A.C. fields, 13 associated with the charging, oscillate many times 14 during one particle transit through charging station 17. ~or purposes of clarifying this invention, an 16 high frequency A.C. field will charge aerosol 17 particles to a value that ~70uld be given by a 1~ standard D.C. field. However, the rate at which 19 the high frequency A.C. field will charge is going to be slower than the same D.C. field because the 21 high frequency A.C. field is charging only during 22 the peak portion of the cycle as it approaches its 23 saturation charge. If saturation charge is not 24 reached, less charge will exist on the particle in the hi~h frequency A.C. charging system than would 26 exist in the same D.C. system.
27 A higher charge on an aerosol particle can be 28 obtained in a D.C. field than in an A.C. field up 29 to a point. When higher fields are used, the D.C.
field causes precipitation whereas the high 31 frequency A.C. field does not. (If maintained at _9_ ~0~703~;9 1 a predetermincd frequency). Even though the high 2 frequency A.C. system isn't as effecient as the D.C.
3 charging system, in the sense that a higher field 4 is needed to get the same amount of charge, it does S not cause a problem of precipitation associated with ~`
6 the D.C. system. If less precipitation exists on `-7 ~he charging electrode more charge can be imparted 8 to the particles in transit through the charging "
9 system. ~;
The high frequency A.C. generator 32 produces ``
11 a high field between the electrode assembly 2~ which 12 allows a charging action to take place in the `
13 presence of the high field. This same field if 14 maintained at a sufficiently high frequency does not perturb the trajectory of aerosol particles 16 4 in transit through passage 18. The ions 42 -17 produced by corona wire electrode 28 have a 18 substantially higher mobility than the aerosol 19 particles 4 that are being charged and the aerosol 20 particles 46 that have already been charged. ~his , 21 means that ions 42, drawn from corona wire electrode 22 28, can travel to the aerosol particles during one 23 cycle of the alternating voltage without any 24 perturbation of the path of travel of aerosol particles 4 due to that same A.C. field. In other 26 words, a charging action is taking place in which 27 ions 42 can traverse the distance needed to follow 28 the field lines to aerosol particles 4, without 29 aerosol particles 4 being moved appreciably in that one A.C. cycle.
. ' ' . ~ ~
- 1 0- . .
1 A small portion of aerosol particles 4 precipitate 2 due to space charges that are created during the 3 charging of the uncharged aerosol particles. A space 4 charge is a distribution of point charges in space, produced by a collection of aerosol particles, each 6 with a small charge thereon. If there is a net 7 electric field in the space charge, the individual 8 charged particles encounter a force, and that force 9 always disperses the space charge or dilutes it. The dispersal of the space charge will eventually cause 11 some aerosol particles 4 to precipitate after an 12 extended period of operation. Therefore, the uncharged 13 aerosol particles should stay in charging station 17 14 long enough to acquire an appreciable charge, but the velocity of flows 6, 10, and 36 are sufficient 16 enough to remove the aerosol particles from station 17 17 before they precipitate.
18 The charged aerosol particles 46, after leaving 19 charging station 17, are carried to a surface 48 which has a charged pattern 50 thereon. The 21 surface 48 to which charged aerosol particles 46 22 are projected could be a sheet of dielectric paper, 23 a copier drum, or even a sheet of Mylar*. A charged 24 pattern 50 exists on surface 48 in order for charged aerosol particles 46 to form a discernible 26 representation thereon.
27 STATEMENT OF THE OPERATION:
28 Referring to the drawing, the ink aerosol 29 particles 4 are formed in housing 5 by a nebulization process. The particles 4 are then picked up by an 31 air stream 6 which enters the uppermost part of 32 * Trade Mark A
1 nebulizin~ chamber 5 and are brought to an orifice 2 16 at the inlet to charging station 17. The aerosol 3 particles 4, before entering nozzle 21 are subjected 4 to a guard flow 10 of clean air which forms the aerosol particles into a ribbon or flow 15. This 6 continuous flow of aerosol particles 15, still ~7 entrained in air stream 6, is then carried through 8 passage 18 and charging station 17. An additional 9 flow of clean air 36 is introduced through the corona housing 26 to insure that no particles settle 11 into corona housing 26 and collect on corona wire 12 28 where they can cause problem~ in its operation.
13 A charging field is produced by the high frequency 14 A.C. generator 32 in passage 18 between plates 24 and 25 in the field electrode assembly. A small 16 D.C. field due to the presence of the corona serves 17 to attract ions 42 into the region between plates 18 24 and 25. Once a mixture of ions 42 and aerosol 19 particles 4 is formed in passage 18 the field produced by the high frequency A.C. generator 32 21 will cause ions to be attracted to particles 4.
22 Once particles 4 are charged they are carried 23 out of charging station 17 by moving air strea~
24 6. This air stream 6 is maintained at a velocity which prevents particles 4 from precipitating 26 spontaneously to the charging electrode due to the 27 space ciharge factor. The charged particles 46 28 are then introduced to surface 48 in the form of a 29 low velocity jet. The charged particles 46 impact surface 48 at a very low velocity in order to avoid 1 any wetting action outside of charged pattern SO.
2 The printing process and the development of pattern 3 50 take place by action of the charge on surface 48 4 drawing.in charged particles 46.
While the invention has been shown and described 6 with xeference to a preferred embodiment thereof, it 7 will be ap?reciated by those skilled in the art that 8 variations in form may be made therein without 9 departing from the spirit.and scope of the invention.
|International Classification||G03G15/02, B41J2/215, B41M5/00, G03G15/11, B41J2/015|
|Cooperative Classification||B41J2/215, Y10S101/37|