Multi-nozzle continuous ink jet printing method

Abstract: A method of printing is described which uses a multi-nozzle continuous ink jet printer 1 which comprises a row of nozzles through which, in use, respective streams of ink 3 are emitted before being broken up into droplets. A charge electrode assembly has a plurality of charge electrodes 4 for charging individual droplets in the streams. Deflection electrodes 5,6 deflect charged droplets according to the charge thereon. An auxiliary charge electrode 8 is used to apply a compensating charge to all droplets in a chosen row causing differential deflection, to allow drops from adjacent rows of droplets to be printed in alignment rather than offset from one another.

Description: The present invention relates to multi-nozzle continuous ink jet printers and, more particularly, to the arrangement and use of charge electrodes for such printers. Multi-nozzle continuous ink jet printers have been developed in order to provide high quality, high speed printing. A row of ink jet nozzles at very close spacings are provided and individual streams of ink issue from each of the nozzles continuously, being broken up into individual droplets automatically, and the individual droplets charged appropriately to cause them to be printed or else deflected into a gutter. Printers of this type are described, for example, in US-A-4613871, US-A-4347519 and US-A-4427986. Such printers are generally known as binary continuous multi-jet ink jet printers. The close spacing between nozzles and thus between individual droplets in the rows of droplets which are formed on break-up of the jets, results in so-called "crosstalk" between individual droplets due to the electrical charge imposed on the droplets by the charge electrodes. Various schemes have been devised in order to compensate for crosstalk, see the above US patent specifications, in order to ensure high quality, ie. high definition, printing. A relatively simple technique for reducing crosstalk problems is to charge alternate droplets in each stream, thus providing a guard droplet between each pair of printable droplets. At the same time, guard droplets are provided between printable droplets in each row, so that every printable droplet has a guard droplet in front of it, behind it and to each side of it. The guard droplets pass into a gutter and are not printed. However, although such a scheme is effective in reducing unwanted crosstalk, the crosstalk compensation results in a staggering or offset between adjacent droplets on the printed substrate, which reduces printing quality. Thus, for example, an ideal straight line of printed drops becomes a sinuous line, thus creating fuzziness and reducing line definition. In US-A-4347519 an operating regime is employed which utilises an auxiliary electrode on each side of the charging electrode plate to compensate for variations in break-up length. In EP-A-0104951 crosstalk compensation is achieved by a DC level shift in the charge voltages applied to printable drops which are arranged to be in groups. The present invention sets out to improve crosstalk compensation. According to the present invention there is provided a method of printing, using a multi-nozzle continuous ink jet printer which comprises a row of nozzles through which, in use, respective streams of ink are emitted before being broken up into droplets, a charge electrode assembly providing a plurality of respective charge electrodes for charging individual droplets in the streams, deflection electrodes for deflecting charged droplets according to the charge thereon, and an auxiliary charge electrode, the method comprising applying a compensating charge to all droplets in a chosen row via said auxiliary charge electrode causing differential deflection of droplets from adjacent rows to allow them to be printed in alignment. By means of the auxiliary electrode, which acts on all droplet streams at once, the auxiliary charge can bring the droplets printed on to the printed substrate from one row, "back into line" with droplets in an immediately preceding or succeeding row. In one specific example, a different compensating charge is applied to 'odd' and 'even' droplets in adjacent rows via the auxiliary electrode, causing differential deflection of droplets from adjacent rows to allow them to be printed in alignment. In another specific example, by means of a 3-phase drive to the auxiliary electrode, differing compensating charges are applied to adjacent rows of droplets causing differential deflection of droplets from adjacent rows, to allow droplets from preceding or succeeding rows of droplets to be printed in alignment. One example of the apparatus and method of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates, diagrammatically, ideal straight line printing of droplets, magnified many times; Figure 2 illustrates how the line becomes staggered due to crosstalk compensation by the insertion of guard drops between printable droplets; Figure 3 provides an illustration of so-called odd and even droplets and their charging regimes; Figure 4 illustrates printing of droplets corrected by auxiliary electrode charging; Figure 5 illustrates a droplet charging regime for printing at lower speeds; Figure 6 illustrates the spacing of corrected droplets; Figure 7 illustrates the application of voltages on the charge electrodes and the auxiliary electrodes in use; Figure 8 illustrates the path of droplets to the substrate; Figure 9 illustrates an alternative charging strategy; Figure 10 illustrates the path of droplets according to the printing strategy of Figure 9; and, Figure 11 illustrates, in side view, the arrangement of the main print head components. It is desirable that drops be capable of being printed from a multi-nozzle print head in a straight line as shown in Figure 1, where the '#' symbols indicate the printed droplets. The line extends across the path of relative movement of the web or substrate and the print head. However, the problems of compensating for crosstalk result, as described above, in connection with US-A-4613871 and US-A-4427986, in a staggering of the printed droplets as shown in Figure 2. The method of the invention can be used to overcome this problem. The method proposed uses an auxiliary electrode to provide different charges on 'odd' and 'even' drops. This allows a different deflection to be given to odd and even drops. The difference in deflection allows drops which are charged at different times (crosstalk correction) to be printed in a straight line on the web as illustrated in Figures 3 & 4. In figures 3 to 6, 'o' indicates a guard drop and '+' or '-' indicates a printed drop. The auxiliary electrode (see Figure 11) stretches across the whole width of the print head, and acts on all jets at once. During the period when odd drops are being printed, the auxiliary electrode is at a higher voltage than when even drops are being printed (see Figure 7 for detail of the auxiliary electrode waveform). This has the effect of putting a different charge on odd and even drops. The charge is very small, as is the difference between odd and even charges. The result of the method is illustrated in Figure 8. If the web is moving at less than the maximum speed, then more drops will be produced than can be used for printing. These drops will be sent to the gutter. Figures 5 & 6 show the effect on printing. Slowing the printing rate places rows of gutter drops between the rows of printed drops, but has no effect on the appearance of the printed result. When the print head is printing "full black" (using all available drops) at full speed, only the guard drops go to the gutter; there are no complete rows of unprinted drops. The preceding discussion refers to the use of one guard drop in between each printed drop. This concept can be extended to the use of two guard drops between each printed drop. Figures 9 & 10 show (similarly to figure 7 & 8) how a 3-phase drive to the auxiliary electrode can be used to correct for the effect of the guard drops. Figure 11 illustrates, in side view, a print head used in the method according to the present invention. The print head 1 has a nozzle plate 2 from which, in use, are emitted streams of droplets in the plane indicated by the line 3. A row of charge electrodes 4 are provided close to the side of the plane of the streams of droplets so that individual drops in the streams can be electrostatically charged (as well known in the art). Guard drops are generated as required and passed in use into a gutter (not shown). A pair of deflector plates (or electrodes) 5, 6 are held, in use, at an appropriate voltage so that charge droplets are deflected in the electric field generated across the electrodes 5, 6 and are deposited onto the substrate 7 moving beneath the print head. In order to carry out the method of the invention the apparatus includes a single auxiliary electrode 8 which extends the entire width of the jets of ink and voltages are applied to the auxiliary electrode 8 as described above.

Claims: A method of printing, using a multi-nozzle continuous ink jet printer (1) which comprises a row of nozzles through which, in use, respective streams of ink are emitted before being broken up into droplets, a charge electrode assembly (4) providing a plurality of respective charge electrodes for charging individual droplets in the streams, deflection electrodes (5,6) for deflecting charged droplets according to the charge thereon, and an auxiliary charge electrode (8), the method comprising applying a compensating charge to all droplets in a chosen row via said auxiliary charge electrode (8) causing differential deflection of droplets from adjacent rows to allow them to be printed in alignment. A method according to claim 1, wherein a different compensating charge is applied to 'odd' and 'even' droplets in adjacent rows via the auxiliary electrode, causing differential deflection of droplets from adjacent rows to allow them to be printed in alignment. A method according to claim 1, wherein, by means of a 3-phase drive to the auxiliary electrode, differing compensating charges are applied to adjacent rows of droplets causing differential deflection of droplets from adjacent rows, to allow droplets from preceding or succeeding rows of droplets to be printed in alignment.

Inventors: WILLIAMS JOHN LLOYD, WILLIAMS, JOHN LLOYD
Applicants: DOMINO PRINTING SCIENCES PLC
Application year: 1996
Main classification: B41J2
IPC codes: B41J2