GB1564572A - Jet printing - Google Patents

Description

(54) JET PRINTING (71) We, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION (CSIRO) a body corporate established under the Australian Science and Industry Research Act 1949 - 1968 of Limestone Avenue, Canberra, Commonwealth of Australia, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an apparatus for printing textile and other surfaces using electrostatically controlled streams of droplets.

The decoration of textile and other surfaces by the printing of designs is commonly carried out in a number of different ways. One such printing method hitherto used for wool fabrics is screen printing. This woollen printing process uses expensive equipment, requires the preparation of a plurality of screens for even simple designs, and has associated dye wastage and disposal problems. And because the printed woollen fabric is thus expensive to produce, it has become a high fashion item, which limits the demand to relatively short lengths of printed material, thus denying high volume production economies.

This invention seeks to provide a printing apparatus which is relatively economical and is capable of printing both long and short runs of material with rapid changes of pattern or design being possible.

According to the present invention there is provided jet printing apparatus for printing on a travelling surface, said apparatus comprising at least one linear array of droplet-producing heads, each head being adapted to project droplets on to the surface and having associated charging and deflection means, characterised in that said deflection means is adapted to deflect droplets perpendicular to the line of its respective array, and that the or each array of droplet-producing heads is angled relative to the direction of travel of the surface so that droplets impinging on the surface as a result of the maximum droplet deflection from any one droplet-producing head impinge upon a region of the surface which overlaps with or lies immediately adjacent to the region of the surface on which droplets from the next adjacent droplet-producing head may impinge.

The invention is a development of a printing technique, using electrostatically deflected drops of ink, to the printing of designs on wool, and, where appropriate, other fabrics and surfaces. The basic technique has been proposed in connection with the recording of high frequency signals.

The basis of the apparatus of the invention is as follows: a jet of liquid ink is forced through an orifice under pressure. There is a tendency for such a jet to divide into a succession of discrete droplets. The production of droplets can be ensured and controlled by imposing a vibration upon the liquid ink as it approaches the orifice. Such vibration can conveniently be applied by piezo-electric or magnetostrictive means. By suitable choice of orifice size, jet pressure, and vibration frequency, a jet may be caused to break into a succession of liquid droplets which are very uniform in size, velocity, and inter-droplet spacing.

These droplets are charged by electrostatic induction, conveniently by passing the jet through an annular or U-shaped charging electrode at a suitable electric potential. The electrode is positioned adjacent to the area where the droplets form. The path of these charged droplets can then be controlled by applying an electrostatic field transverse to their path, conveniently applied by means of a pair of parallel plates at a suitable electrical potential difference. If the parallel plates are maintained at constant potentials, the paths of the droplets can be altered by varying the potential on the droplet charging electrode. By this means each droplet can be given an independent trajectory.

If the jet of droplets is directed at a surface to be printed, and the surface is moved relative to the jet and the potential of the charging electrode varied, a continuous line of chosen deflection can be printed.

If a collecting device is so arranged that, at a certain deflection of the jet, the droplets enter the collecting device (typically a gutter) and are directed to waste or to be recirculated, appropriate charging of the droplets will enable an interrupted line of chosen deflection to be printed. The preferred apparatus of the invention may have one or more of the following features: (i) using banks of jets to increase the area covered in a given time.

(ii) supplying the jets with fluids of various colours to obtain multi-coloured effects.

iii) combining interrupted and continuous lines to form a complete pattern on the surface.

iv) using liquid dyestuffs and thus printing on a textile surface.

v) programming the deflecting potential in relation to the movement of the printed surface, for example by using a computer, in order to print a repetitive or non-repetitive pattern.

In the apparatus of the invention the jets, and hence their associated droplet-producing heads, are located in linear arrays at an acute angle to the direction of travel of the surface being printed. We have found that such an arrangement improves the effectiveness and reduces the complexity (and hence cost) of the system.

Additional features, preferably included in practical embodiments of the present invention are (a) using a single deflecting electrode for each jet of drops, rather than a pair of deflecting electrodes, and using a single electrode structure for a plurality of jets; (b) the charging of a drop in the jet periodically, rather than allowing each drop to be charged; (c) arranging pattern program data in parallel banks and starting the processing of corresponding data banks simultaneously; and (d) using a ramp charging technique to electrostatically charge the drops in accordance with the design programme.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1(a) and (b) are, respectively, schematic side and plan views of the jet location, drop charging and deflection arrangements of a jet printing apparatus according to the present invention; Figure 2 illustrates a drop-charging technique for reducing the effects of inter-droplet reaction; Figure 3 shows a circuit arrangement which may be used to effect the drop-charging technique illustrated in Figure 3.

Figure 4 is a schematic diagram used to explain how synchronised data trains may be used to ensure in-phase printing, and Figure 5 depicts the signal voltage waveform gated to the charging electrode.

Referring to Figures la and lb, droplets 13 are formed conventionally by nozzle 12 containing dye (or other suitable liquid) 11 under pressure. The droplets are given an electrostatic charge by a conventional charging electrode 14. After leaving charging electrode 14, the droplets impinge upon fabric 16, which is located in the path of the jet (and not necessarily underneath the jet nozzle, as shown in the accompanying drawings, for the droplet velocity is established by applying pressure to liquid 11 and not by gravity). Deflecting electrodes or plates 15 are located adjacent the path of the droplets, so that charged droplets are deflected by electrical field forces created by plates 15, which are held at different fixed potentials.Plates 15 associated with a number of jets may be electrically interconnected or a pair of plates 15 may be extended to form a common deflecting electrode for a plurality objets arranged in line, and thus mechanical complexity and the effects of stray or fringing fields will be avoided.

It is clearly not possible to print a complete pattern on a surface traversed zerpendicular to a line of such jets, as the deflection of each jet, when the deflection plate is common, is necessarily perpendicular to the line of jets. To avoid the need for numerous arrays of jets extending perpendicular to the direction of travel of the surface being printed, each line or array of jets. in accordance with the present invention, is set at an angle to the direction of travel of the surface so that the transverse component of the maximum droplet deflection at least equals the transverse component of the interjet spacing. In the interests of compactness, this setting at an angle to the traverse of the surface is preferably effected with a number of smaller arrays of jets, arranged in echelon across the width of the surface, as indicated in Figure l(b). Jets of different colours may be included in a single array.

The common deflector plate structure 15 of Figure 1 (b) may comprise a single conducting plate, or it may consist of a plurality of conducting plate regions (for example, metallic foil surfaces supported on a firm substrate) electrically interconnected.

One of the limitations upon the performance of jet printing systems, is due to the effect of the influence on each drop of neighbouring drops. These effects are due partly to aerodynamic interaction and partly to electrostatic interaction. These effects may be alleviated by modifying the charging system and by lowering the drop repetition frequency. The method of overcoming drop-to-drop interaction used in the present jet printing embodiment is illustrated in Figures 2 and 3. Essentially, the technique is to allow a drop to be charged periodically, and to allow all uncharged drops to be collected in the waste gutter 20.To do this, the vibration frequency controlling the drop repetition rate is used as the input frequency to a counting system (suitable designs are well known) arranged to emit a gating pulse after a predeterrnined number "n" of cycles (see Fig. 3). Using conventional circuitry, the charging potential is admitted to the charging electrode only when the gating pulse is emitted. Thus only every nth drop may be given a charge in accordance with the desired pattern. The waste or recirculation conduit - gutter 20 - is arranged so that drops of zero charge enter it. Thus only every nth drop is used in the pattern formation on fabric 16. If n is sufficiently large, the aerodynamic and electrostatic interaction between those drops selected for charging is negligible. In a presently preferred form of this embodiment, n is eight.

Any conventional means of using data stored in a computer to control the charging of drops according to some pattern may be used, but the possibility exists that, due to random perturbations or some incident, the data to a jet or jets will be delayed or advanced by some finite time and then will proceed. If this occurs the pattern from that jet or jets will be out of phase with, i.e., behind or in front of, the remainder of the pattern on the surface, and will remain so indefinitely (for example, until a complete length of cloth is finished).

This undesirable situation can be avoided if the arrangement illustrated in Figure 4 is used.

The data about to be transmitted to each jet or group of jets is held (by known means) in serial groups such that the first group is forwarded serially to the jet, then the second group, and so on. The rate at which data is forwarded is made to synchronize with the rate at which it is required, by controlling it with a clock arrangement controlled, in turn, by the source of vibration frequency associated with droplet production. Thus the utilisation of the second groups of data is made to depend not upon the completion of the processing of the associated first groups of data, as would be the case by conventional means, but upon the elapsing of a predetermined number of cycles of the vibration frequency. By this means the processing of each group of data begins at a determinate point in the overall pattern.Any data in the previous group which has not been processed when the synchronisation pulse is received are dumped. If there is an instance of all data being used before that determinate point, there is no processing activity until the next group is allowed to be processed. Hence any instance of pattern becoming out-of-phase will not last indefinitely, but at the most only for the length of time occupied by one data group. Groups may readily be made sufficiently small for such an effect to be negligible.

A technique for effecting a substantial simplification in the charging arrangements for the various jets and their control by means of a computer is featured in Figure 5. The voltage generated, and thus available for application to all charging electrodes when the gate arrangement described above with reference to Figures 2 and 3 is used, has the shape of a ramp function against time. That is, as shown in Figure 5, it increases uniformly to reach a given value in a given time, then falls to zero and begins to increase uniformly again, and so on. To obtain a desired drop deflection, the data fed from the computer need consist only of on-off information, opening a second gate, to permit charging of a droplet to occur when the ramp voltage attains the necessary value for that deflection.The recurrence frequency of the ramp function (generally a sub-harmonic of the charge permitting gate signal frequency), the droplet recurrence frequency, and the speed of the surface to be printed, may readily be chosen so that there is no visible diminution of pattern continuity incurred by this simplification, even though, normally, only a proportion of the "nth" drops available to be charged will actually acquire a charge and be used in the pattern formation on the surface. A further simplification can be effected if the length of the data groups described in the last preceding paragraph is made to be a small integral submultiple of the periodic time of the ramp function.

In this case, the same frequency-counting means may be used to control the ramp function and the data-group synchronization.

As an example, in one embodiment of the present invention, the following parameters have been adopted: Jet size 80 micron Jet pressure 10 - 12 psi Vibration frequency 30 KlIz Interjet distance 0.5 cm Single jet scan 1.0 cm Cloth speed 1 metre per minute Number of colours 4 Number of levels of droplet charge (i.e. Periodic time of ramp function divided by recurrence time of drops selected for charging) 64 Number of data synchronizations per ramp period 4 this apparatus was capable of printing textile fabrics, in particular woven cloth, with great accuracy and reasonable cost. Change of design could be effected very rapidly by altering the data programme.

WHAT WE CLAIM IS: 1. Jet printing apparatus for printing on a travelling surface, said apparatus comprising at least one linear array of droplet-producing heads, each head being adapted to project droplets on to the surface and having associated charging and deflection means, characterised in that said deflection means is adapted to deflect droplets perpendicular to the line of its respective array, and that the or each array of droplet-producing heads is angled relative to the direction of travel of the surface so that droplets impinging on the surface as a result of the maximum droplet deflection from any one droplet-producing head impinge upon a region of the surface which overlaps with or lies immediately adjacent to the region of the surface on which droplets from the next adjacent droplet-producing head may impinge.

2. Jet printing apparatus as defined in claim 1, in which there is a plurality of arrays of droplet-producing heads in the form of a series of arrays, arranged en echelon across the region over which the surface is adapted travel.

3. Apparatus as defined in either claim 1 or claim 2, in which the deflection means associated with a plurality of droplet-producing heads are electrically interconnected or are formed as a single conducting plate.

4. Apparatus as defined in any preceding claim, in which the printing is controlled by data supplied to each charging means, said data being arranged in a series of data banks, each data bank containing data for a predetermined printing period, the processing of corresponding data banks being started simultaneously.

5. Jet printing apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. Jet size 80 micron Jet pressure 10 - 12 psi Vibration frequency 30 KlIz Interjet distance 0.5 cm Single jet scan 1.0 cm Cloth speed 1 metre per minute Number of colours 4 Number of levels of droplet charge (i.e. Periodic time of ramp function divided by recurrence time of drops selected for charging) 64 Number of data synchronizations per ramp period 4 this apparatus was capable of printing textile fabrics, in particular woven cloth, with great accuracy and reasonable cost. Change of design could be effected very rapidly by altering the data programme. WHAT WE CLAIM IS:

1. Jet printing apparatus for printing on a travelling surface, said apparatus comprising at least one linear array of droplet-producing heads, each head being adapted to project droplets on to the surface and having associated charging and deflection means, characterised in that said deflection means is adapted to deflect droplets perpendicular to the line of its respective array, and that the or each array of droplet-producing heads is angled relative to the direction of travel of the surface so that droplets impinging on the surface as a result of the maximum droplet deflection from any one droplet-producing head impinge upon a region of the surface which overlaps with or lies immediately adjacent to the region of the surface on which droplets from the next adjacent droplet-producing head may impinge.

2. Jet printing apparatus as defined in claim 1, in which there is a plurality of arrays of droplet-producing heads in the form of a series of arrays, arranged en echelon across the region over which the surface is adapted travel.

3. Apparatus as defined in either claim 1 or claim 2, in which the deflection means associated with a plurality of droplet-producing heads are electrically interconnected or are formed as a single conducting plate.

4. Apparatus as defined in any preceding claim, in which the printing is controlled by data supplied to each charging means, said data being arranged in a series of data banks, each data bank containing data for a predetermined printing period, the processing of corresponding data banks being started simultaneously.

5. Jet printing apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.