Drop marking devices

Abstract: A drop deflection device suitable for use in a drop marking system employs an electret (9). Electrically conductive marking drops (1) such as inks are given an electrical charge. The drops (1) pass between a pair of deflection plates (8, 9) at least one of which is formed by an electret (a dielectric material which has been subjected to corona discharge to create a relatively permanent electrically charged material body). Depending upon the presence or absence of a charge on a particular drop (1), it will be deflected by the field (6) between the plates (8, 9) on to a substrate (11) for marking. If uncharged, the drops (1) continue to a collection device (10) which returns them to the ink reservoir.

Description: This invention relates to drop deflection devices, to drop marking systems such as ink jet printers and the like provided with drop deflection devices, to a method of deflecting electrically charged drops, and to a drop marking system employing such method. Drop deflection devices come in various forms including single drop stream devices and multiple drop stream devices such as the binary array type. Drop marking systems generally employ electrically conductive inks which are supplied to one or more nozzles which have associated therewith a piezoelectric crystal. The ink is forced through the nozzle and, by virtue of the piezoelectric device, discrete drops are formed as the ink leaves the nozzle. The drops next pass through a charging device, such as a charge ring, which selectively imparts electrical charges to them. That is, some drops are charged while others are not. The drops which are charged may have charges of different magnitudes placed thereon. As the drops continue along their initial path, they enter an electric field formed by a drop deflection device which is typically a pair of deflection plates or electrodes that are charged to a high voltage by a power supply. As the drops enter the deflection field formed between the deflection plates, the charged drops are deflected from their original flight path by an amount proportional to the charge which they carry. Thus, uncharged drops are not deflected while highly charged drops are deflected a significant amount. In the usual case, charged drops will be deflected on to a substrate to be marked while uncharged drops are collected for return to an ink reservoir. Additional detail concerning typical ink drop marking systems can be found in U.S. Patent Nos. 4,121,222, 4,319,251 and 4,555,712, hereby incorporated by reference. A problem with ink jet printers of the type described occurs due to the necessity for producing the electric field for deflecting the drops. Typically, this electric field is produced by connecting one deflection plate or electrode to a high voltage power supply while connecting the second deflection plate or electrode to ground. Alternatively, a high voltage power supply of one polarity is connected to one deflection plate or electrode, and a power supply of the opposite polarity is connected to the other deflection plate or electrode. The high voltage power supply required for this purpose is responsible for several problems in the safety and performance of the drop deflection device and the associated drop marking system. Firstly, the high voltage power supply produces electrical noise which disturbs the proper functioning of the other electrical circuits associated with the drop marking system, including the computer systems which determine the drop marking patterns to be placed on the substrate and associated functions. Secondly, arcing between the deflection plates or electrodes, or elsewhere in the high voltage circuit, causes poor print quality and other operational problems. Thirdly, respect to safety, the power supply energy released during arcing between the deflection plates or electrodes can create a fire hazard particularly where the inks used contain flammable solvents as is often the case. Finally the high voltage power supply presents a possible safety hazard due to the potential for electrical shock. It has been proposed by Japanese Patent Application No. 53-105322 to provide a facsimile machine in which a document bearing data for transmission is wound on a drum at a data-transmitting station to be scanned transversely by an electro-optical device, and the transmitted data is printed at a data-receiving station by an ink jet printing head onto paper would on another drum. This proposal relies on the drum at the data-receiving station being driven at the same speed as the drum at the data-transmitting station, and essentially requires the ink jet printing head to be traversed across the paper in synchronism with the electro-optical device at the data-transmitting station. The ink jet printing head is provided with a pair of electrostatic deflection plates and avoids the need for any high voltage power supply by arranging for a grounded electret to be fixed to one of the deflection plates and for the deflection plates to be mounted such that the electret can periodically be contacted by the other deflection plate thereby inducing a high voltage deflection field between the deflection plates. The strength of this high voltage deflection field attenuates rapidly with the passage of time and it is necessary to re-establish the high voltage deflection field at the end of each printed line by moving one of the deflection plates to contact the electret whilst simultaneously closing a switch to earth the moving deflection plate, and then by re-opening the switch and returning the movable deflection plate to its operative position. Whilst this arrangement may be acceptable for a facsimile machine in which the printing head is returned at the end of every line, it is not suitable for use in continuous ink jet printing as the attenuation of the high voltage deflection field would progressively reduce the deflection of the ink drops, adversely affecting print quality. The regularly repeated contacting of the movable deflection plate with the electret would also necessitate shutting down the ink jet which essentially passes between the deflection plates. Japanese Patent Application No. 53-105322 therefore teaches a drop deflection device, for a drop marking system, including a deflection electrode structure incorporating an electret for creating an electric field to deflect selectively charged ink drops from their initial flight path; it also teaches a method of deflecting electrically charged drops including creating an electric field using an electret, and passing the drops through the electric field; it also teaches a drop marking system having means for producing a stream of ink drops, a charging device for imparting an electrical charge to selected ink drops, and a deflection electrode structure incorporating an electret for creating an electric field to deflect charged ink drops from their initial flight path. However, with such drop deflection device, such method of deflecting electrically charged drops, and such drop marking system, it is an essential feature that the electret is grounded and is repeatedly contacted with a movable deflection plate to induce a charge thereon because of the attenuation of the deflection field. According to one aspect of the present invention a drop deflection device has the electret ungrounded and the electric field directly caused by the potential difference between the electret and a deflection electrode. In this manner attenuation of the deflection field is avoided, there is no need to contact the electret with the deflection electrode to induce an electric charge thereon because the electret directly establishes the deflection field, and there is no need to interrupt the ink drops whilst the electret contacts the deflection electrode. The electret and the deflection electrode may be parallely spaced plates, or diverging spaced plates, between which the ink drops are to pass. The deflection plate may be grounded or may be a second ungrounded electret. In the latter case the first ungrounded electret is of opposite polarity to the second ungrounded electret. According to another aspect of the present invention a method of deflecting electrically charged drops includes creating the electric field by using an ungrounded electret. Such method may include creating the electric field between the ungrounded electret and a grounded deflection electrode, or between two ungrounded electrets of different polarity. An electret is a substantially permanent electrically charged material body. Electrets are an electrical analog of a permanent magnet. An electret produces its own electric field without connection to a source of electrical energy. Electrets are well known in the electrical art as, for example, in microphone and speaker technology. The key element of the present invention is the use of an ungrounded electret to produce the electric deflection field in a drop marking system. By employing the invention it is believed to be possible to obtain a number of benefits because of the elimination of the high voltage power supply. These benefits include a reduction of noise, which makes drop placement more accurate, and of the danger of explosion due to arcing. Further the elimination of the high voltage power supply makes the servicing of the device simpler and of course the hazard of electrical shock for service personnel is eliminated. The cost of the device is also reduced. Specific embodiments of the invention will now be described, by way of example only, with reference to the drawings in which:- Figure 1 illustrates a first embodiment of the invention utilizing a single ungrounded electret; Figure 2 illustrates a second embodiment similar to the first embodiment wherein the drops are provided with a charge of opposite polarity; Figure 3 is a third embodiment in which a pair of ungrounded electrets are used to form the deflection field; Figure 4 illustrates a fourth embodiment similar to the Figure 3 embodiment utilizing, however, positively charged droplets; Figure 5 illustrates a fifth embodiment employing a single negatively charged ungrounded electret wherein negatively charged droplets are deflected away from the substrate while uncharged droplets strike the substrate; and Figure 6 illustrates the structure of one form of an electret. Figure 1 illustrates the basic elements of a first embodiment of the invention used in conjunction with a drop marking system. Fluid marking drops 1 emanate from an ink jet nozzle orifice 2 which is located at one end of a typical nozzle housing 3. The drops are acted upon by a piezoelectric device 4 which, in the usual embodiment, surrounds the housing 3 and provides energy to the housing to cause the formation of drops as the ink stream leaves the nozzle 2. The drops 1 are electrically charged by a charging electrode 5 as they leave the nozzle, and are deflected as they pass through an electric deflection field 6 which exists between an upper deflection electrode 8 and a lower deflection electrode 9. In a conventional system, the deflection electrodes 8 and 9 would be formed by a pair of conductors at least one of which would be connected to a power supply while the other conductor might be grounded or connected to a power supply of opposite polarity, thereby creating an electric deflection field between the deflection electrodes as is well known in the art. Drops which are not charged are unaffected by the electric field between the deflection electrodes 8 and 9, and they are caught by an ink catcher 10 which returns the drops to the ink system for reuse. Drops which carry a charge are deflected by the field and caused to be deposited on a substrate 11 to be marked. In this manner, the drops can be placed such that information is recorded on the substrate 11. The deflection electrodes 8 and 9 are usually made in the shape of flat plates, but other shapes and orientations other than parallel are also possible and within the scope of the present invention. According to the present invention, as illustrated in Figure 1, the deflection electrode 9 is made of an ungrounded electret (to be described hereafter) with a negative surface potential in the range of minus 3,000 to minus 6,000 volts. Of course other surface potentials (positive or negative) can be employed depending upon the amount of deflection desired and the characteristics of the ink to be deflected. The electric field 6 which is present in the space between the electret 9 and the deflection electrode 8 acts upon the charged drops and causes them to change the direction of their path through this field, i.e. they are deflected on to the substrate as illustrated. Specifically, the ink drops, which are negatively charged, in this embodiment, are attracted to the grounded deflection electrode 8 which is placed in opposition to the negatively charged electret 9. There is no electrical connection to the electret defining the deflection electrode 9. The electric field 6 is the result of the intrinsic electrical charge distribution of the electret. No additional energy source, such as a power supply, is needed to establish or maintain the electric field 6. Thus, this embodiment is inherently free of the electrical noise, arcing and safety problems, such as shock hazard, which are found in conventional drop marking systems. The drop marking system illustrated in Figure 2, is identical to the Figure 1 system except that the ink drops are positively charged, and the positions of the ungrounded electret 12 and the grounded deflection electrode 8 are reversed. Accordingly, positively charged drops are deflected from their path by the negatively charged electret 12 to cause them to strike the substrate 11. Uncharged droplets still reach the catcher 10. Figure 3 illustrates a third embodiment in which negatively charged drops 1 are deflected into an electric field between a negative surface potential electret 9 and a positive surface potential electret 15. In this embodiment, greater field strength can be produced due to the use of a pair of oppositely charged ungrounded electrets. In such an embodiment, greater deflection of the ink drops can be produced, or greater printing accuracy can be obtained, with the same deflection by moving the substrate closer to the deflection field. In Figure 4 positively charged drops 14 are deflected in the electric field between a positive surface potential electret 16 and a negative surface potential electret 12. This embodiment is identical to the embodiment of Figure 3, with the exception of reversing the polarity of the ink charge and the positions of the positively charged and negatively charged electrets. In Figure 5 negatively charged drops are deflected in to the catcher 10 while uncharged drops 17 are deposited on the substrate. The charged drops are deflected away from a negatively charged electret 8 and toward the grounded electrode 19. The combinations of drop charge polarity and electret surface charge polarity, illustrated in Figures 2-4, also apply to this fifth embodiment in which the charged drops are deflected into a catcher, while uncharged drops are directed towards the substrate. The techniques for manufacturing commercially acceptable electrets are known to those skilled in the art. However, to ensure completeness of this disclosure, the manner in which electrets were prepared for use with the present invention is now described. For additional information concerning electrets, reference is made to the following publication (hereby incorporated by reference): "ELECTROSTATICS AND ITS APPLICATIONS"; A.D. Moore, Editor; copyright 1973, John Wiley & Sons, Inc.; pp. 122-129, and the reference cited therein. Briefly, an electret is a dielectric material which has been processed so that it possesses a permanent electric surface potential, i.e. it will produce its own electric field analogous to the magnetic field carried by a permanent magnet. Figure 6 illustrates the details of an electret which was made and successfully utilised in conjunction with the invention disclosed herein. The electret electrode is formed on a metal plate 30 which is provide as a backing only. Secured to one side of the plate 30 is a length of tetrafluoroethylene tape 32 of approximately four mills thickness. The tape is preferably adhesively secured to the backing plate 30. Alternatively, thick plastic, wax or ceramic could be used to make a self-supporting electret which would not need a supporting structure such as the metal plate of the illustrated embodiment. The assembly thus prepared is then provided with a relatively permanent electrostatic charge in the following manner. A high voltage power supply such as a Spellman Model RHR10PN30 is connected to a sharp edged blade such as a craft knife or the like. The electret is charged by passing the blade near the surface of the tetrofluoroethylene tape 32. A corona discharge is induced with the associated electric field, creating a relatively permanent charge distribution on the tape. At the time that the corona is produced, the tetrafluoroethylene tape 32 is heated to approximately 250°F and then rapidly cooled to approximately minus 40°F, by means of a freezing mist as, for example, from a fluorocarbon spray can. Of course, such method is not critical to practice of the invention, but merely indicates one method in which an electret material can be prepared. Other methods are known in the electret art, and can be utilized as desired. All that is necessary is that the electret or electrets be prepared with an electric charge sufficient to produce the necessary deflection field for the charged ink drops. In producing and using electrets as one or both of the deflection electrodes, it is important to keep the surface of the tetrafluoroethylene tape 32 clean and dry to prevent loss of surface potential.

Claims: A drop deflection device, for a drop marking system, including a deflection electrode structure incorporating an electret for creating an electric field to deflect selectively charged ink drops from their initial flight path, characterised in that the electret (9, 12 or 18) is ungrounded and the electric field is directly caused by the potential difference between the electret (9, 12 or 18) and a deflection electrode (8, 15, 16 or 19). A drop deflection device, as in Claim 1, characterised in that the electret (9, 12 or 18) and the deflection electrode (8, 15, 16 or 19) are parallely spaced plates between which the ink drops are to pass. A drop deflection device, as in Claim 1, characterised in that the electret (9, 12 or 18) and the deflection electrode (8, 15, 16 or 19) are diverging spaced plates between which the ink drops are to pass. A drop deflection device, as in any of Claims 1 to 3, characterised in that the deflection electrode (8 or 19) is grounded. A drop deflection device, as in any of Claims 1 to 3, characterised in that the deflection electrode (15 or 16) is a second ungrounded electret. A drop deflection device, as in Claim 5, characterised in that the first ungrounded electret (9 or 12) is of opposite polarity to the second ungrounded electret (15 or 16). A drop deflection device, as in any preceding claim, characterised in that the, or at least one of the, electrets is formed from a material selected from the group comprising acrylic resins, polyester, terrafluoroethylene, wax and ceramic materials. A drop marking system including a drop deflection device in accordance with any preceding claim. A method of deflecting electrically charged drops, including creating an electric field using an electret, and passing the drops through the electric field, characterised by creating the electric field by using an ungrounded electret. A method, as in Claim 9, characterised by creating the electric field between the ungrounded electret and a grounded deflection electrode. A method, as in Claim 9, characterised by creating the electric field between two ungrounded electrets of different polarity. A drop marking system having means for producing a stream of ink drops, a charging device for imparting an electrical charge to selected ink drops, and a deflection electrode structure incorporating an electret for creating an electric field to deflect charged ink drops from their initial flight path, characterised in that the electret (9, 12 or 18) is ungrounded, and the electric field is directly caused by the potential difference between the electret (9, 12 or 18) and a fixed deflection electrode (8, 15, 16 or 19). A drop marking system, as in Claim 12, characterised in that the fixed deflection electrode (8, 15, 16, or 19) is grounded. A drop marking system, as in Claims 12 or 13, characterised in that the electret (9, 12 or 18) is in the form of a fixed deflection electrode. A drop marking system having means for producing a stream of ink drops, a charging device for imparting an electrical charge to selected ink drops, and a deflection structure incorporating an electret for creating an electric field to deflect charged ink drops from their initial flight path, characterised in that the deflection structure includes two ungrounded electrets (9 and 15, or 12 and 16), and the electric field is directly caused by the potential difference between the two electrets. A drop marking system, as in Claim 15, characterised in that the two electrets (9 and 15, or 12 and 16) are of opposite polarity. A drop marking system, as in Claims 15 or 16, characterised in that the two electrets (9 and 15, or 12 and 16) define a fixed gap for the ink drops.

Inventors: ARWAY GEORGE, ARWAY, GEORGE
Applicants: VIDEOJET SYSTEMS INT, VIDEOJET SYSTEMS INTERNATIONAL, INC.
Application year: 1989
Main classification: B41J2
IPC codes: B41J2