WO1981003149A1

WO1981003149A1 - Control of droplets in jet printing

Info

Publication number: WO1981003149A1
Application number: PCT/AU1981/000049
Authority: WO
Inventors: L Wills; A Stearn
Original assignee: Commw Scient Ind Res Org; L Wills; A Stearn
Priority date: 1980-05-01
Filing date: 1981-05-01
Publication date: 1981-11-12

Abstract

Jet printing apparatus in which a second electrode structure (17, 19; 33, 34) is located between the conventional droplet deflecting electrodes (17, 18; 31, 32) of the apparatus and the surface (15) being printed, to alter the trajectory of scanned droplets (14) so that they strike the surface (15) substantially orthogonally to the plane of the surface. This enables good registration of patterns produced byjet printing on to wide fabrics, carpets and the like, using widely deflected droplets (14), even if the surface (15) being printed is a non-smooth surface or becomes wrinkled as it passes through the printing apparatus.

Description

TITLE: "CONTROL OF DROPLETS IN JET PRINTING"

TECHNICAL FIELD

This invention concerns jet printing. More partic¬ ularly it concerns the jet printing of fabrics, carpets and the like with droplets which are capable of being scanned over a wide angle relative to the undeflected trajectory of drop¬ lets from the generating head of the printer.

BACKGROUND ART

The application of jet, droplet or spray technology to the printing of textile or paper webs, carpets and the like is well known. If the articles to be printed have substantial width, jet printers having banks of droplet generating heads, and associated charging and deflecting electrodes, are required to ensure that proper colour saturation of the articles can be achieved. One form of jet printer that has been constructed for this purpose is described in the specification of Australian patent No. 502,523.

It will be readily apparent that a jet printing equipment which employs a large number of individual droplet generating heads can be constructed (and operated) most econ¬ omically if the droplet deflection that can be achieved is as great as practically convenient. In this way, the number of droplet generating heads is minimised and the computer or microprocessor program used to control the operation of the printer is accordingly simpler to write. However, droplets which are subjected to large deflections will strike the fabric at an inclined angle. When a perfectly flat web is being printed, the printing is effected without problem, but if there should be even a small amount of wrinkling of the fabric being printed, or if the surface being printed is not a perfectly flat surface, then the variation in the position of the point where the droplet which is widely deflected strikes the article can, and does, result in a deteriorated pattern being printed. Since a single flaw in the printing

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of an article can substantially reduce its commercial value, current jet printing equipments use droplets which are scanned through relatively small angles.

DISCLOSURE OF THE INVENTION

It is an objective of the present invention to pr vide an arrangement whereby a jet printer which uses widely scanned droplets can produce quality jet printing with all degrees of droplet deflection.

To achieve this objective, the present invention ensures that the scanned droplets in a jet printing equipmen all strike the article being printed substantially normally. This is done by including, in association with each droplet generating head, a second electrode, located between the deflecting electrode of the jet printer and the surface bein printed, which is charged in such a manner that the accelera tion vector induced on a charged droplet passing the second electrode is in the opposite sense to the acceleration vecto induced by the deflecting electrode. For convenience, the term "collimating electrode" has been adopted for this secon electrode, although, as will be appreciated, the word "collimated" is used in a somewhat special sense that will, nevertheless,be readily understood by persons skilled in this art.

According to the present invention, apparatus for jet printing on to the surface of a fabric or paper web, carp or the like comprises a droplet generating head, droplet char ing electrode means, and droplet deflection means adapted to deflect charged droplets, and is characterised in that second electrode- means are located between the droplet deflecting means and the position of the surface when it is being printed, said second electrode means being adapted to estab- lish an electric field which alters the trajectory of deflect

- XR O droplets so that when they reach said position of the surface, they are moving substantially orthogonal to the surface.

Typically, the means for collimating the deflected droplets is a pair of metal plates, between which a potential difference is established. The plates may be planar or they may be shaped to enable a required electric field structure to be established between them.

In the context of this specification, a single wire conductor is to be regarded as falling within the concept of a plate.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 and 2 illustrate, schematically, and partly in section, two forms of droplet control equipment which may be incorporated into a jet printer.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

In both Figure 1 and Figure 2, a reservoir 10 supplies dye (or ink or the like) via line 11 to a conven¬ tional droplet generating head 12. The droplet generating heads 12 each include a droplet charging electrode 13 which is capable of supplying a predetermined charge to droplets as they leave the head. Electrode 13 is connected to the variable voltage V.. Usually V_ will vary between zero and a positive voltage of up to about 400 volts, so that the stream of- droplets 14 leaving the droplet generating head 12 cc so with a zero or programmed negative charge.

If the droplets 14 carry a charge, they will be

deflected by electrodes, as is well known in this art and as will be described in more detail below, and they will strike a surface 15, which is being printed. If they carry no charge, droplets 14 will follow a straight line trajectory from the droplet generating head 12 to a gutter 16 (which is also termed, in this art, a catcher, a collector or a trap) .

Referring now to Figure 1 alone, when droplets 14 leave the droplet generating head 12, they are projected between a pair of electrodes 17, 18. Electrode 17 is, in the illustrated embodiment, maintained at earth potential. Electrode 18 is connected to a constant voltage VY , which is typically +4kV. An electric field F. is thus established between electrodes 17 and 18 and any charge ^'droplet 14 will be accelerated towards electrode 18 while it is within field F- (that is, while it is the region bounded by dotted lines 22 and 23) .

What has been described so far with reference to Figure 1 is conventional jet printing technology, and it will be appreciated that the.known variations in these features of jet printers may also be included in embodiments of the prese invention.

Ignoring, for the moment, the electrode 19 of

Figure 1, it will be clear that, depending on the charge that is applied to droplets by the charging electrode 13 (which usually surrounds the break-off point of the stream of drople leaving droplet generator 12) , the droplets can be deflected between the trajectories shown in Figure 1 by dashed lines 20 and 21. The droplets following the trajectory of line 21 str the surface 15 at an angle θ.

The embodiment of Figure 1 includes a second pair of electrodes 17, 19. In the illustrated form of the e bodi-

O ment, electrode 17 is an extensive, earthed electrode which forms one electrode of a field-est_.ablishing pair of electrodes in two instances. Electrode 19 is connected to a constant voltage VY which is typically -4kV, so that an electric field F„ is established between electrodes 17 and 19 in the region bounded by dotted lines 24 and 25. While the charged droplets are under the influence of field F_ , they will be accelerated in a sense which is opposite to the acceleration they experi¬ enced while within field F...

The value of voltage V_, the spacing of electrodes 19 and 17, and also the size and shape of electrodes 19 and 17, will normally be selected so that field F effectively cancels the net effect of the deflecting acceleration of field F, . To illustrate such variation, electrode 19 in Figure 1 is shown as a curved electrode, separated from planar electrode 17 by a distance which is greater than the distance between electrodes 17 and 18, but field F„ is an influence on a charged drop for a longer time than field F, ,

Those skilled in this art will recognise that, as shown by trajectory 26, with this arrangement, the voltage V„ applied to electrode 18 may be increased (for example, to +6kV) , thus increasing the maximum deflection of a charged droplet, but a droplet so deflected will always strike surface 15 in a direction which is substantially orthogonal to the surface if voltage V_ is proportionally increased. Alternat¬ ively, if voltages V and V.- are not increased, the maximum value of voltage V, may be increased, with the same overall effect. Of course, the values of voltages V_ and v., and the maximum value of voltage V, may be simultaneously increased, relative- to the maximum values normally used in jet printing.

It will be appreciated that a factor governing the location of electrode 19 is the maximum deflection of a

OMPI droplet by field F, . Electrode 19 must be located so that a droplet experiencing maximum deflection by field F, does not strike electrode 19.

A factor which has to be taken into consideration when programming the voltage V, that is needed to print a - required pattern is that the presence of electrode 19 creates a field F_ between electrodes 18 and 19. This field, F , is a decelerating field (in the direction of field F-.) which wil in practice, have most effect on those droplets 14 which are given the maximum charge, and consequently the maximum defle tion by field F...

It is to minimise the effect of the decelerating field F-. that the alternative electrode arrangement of Figure 2 may be used.

In the embodiment of Figure 2, droplets 14 which are negatively charged are first deflected by field F . estab- lished by electrodes 31 and 32. Electrode 32 is connected to a constant high voltage ₄, of, typically, -7kV, and electrod 31 is earthed. The deflected droplets coast until they reach field F_, established by electrodes 33 and 34, of which elec¬ trode 33 is earthed and electrode 34 is connected to a consta voltage V_g, which (in the case of the illustrated electrode configuration) is approximately equal in magnitude to voltage V., but of the opposite sign.

The droplet generating head 12 is positioned so that the droplet stream is generated as close to electrode 32 as practicable. Care must be taken to avoid the problems which arise when feeding droplets into an electric field clos to a plate connected to a high voltage.

Droplets having maximum charge are deflected to follow the trajectory shown by dashed line 35. Such droplets are affected less than other charged droplets by the field

F, established between electrodes 32 and 34. In the embodiment 6 of Figure 2, field F, will accelerate droplets which come o within its influence. Since droplets having the minimum charge on them will be most likely to pass through the edge of field

F_c , and the lower the charge on a droplet the less the effect b of the field, the electrode arrangement of Figure 2 will result in little, if any, modification of the programming of the voltage V, applied to electrode 13 to compensate for the effect of field F_g.

Electrodes 31 and 33 can,,if required, be a single common electrode, cooperating with electrodes 32 and 34 to establish fields F. and F,..

4 5

The effectivenss of the present invention has been tested by printing straight lines on corrugated cardboard, with the height of the corrugations ranging from 0 to 13mm. The cardboard travelled relative to the printing droplet stream so that the droplets were scanned in a direction parallel to the lines of the corrugations. In each case, in the absence of use of the collimating electrodes, the lines which were printed on the cardboard with droplets which were given little charge were substantially straight, but with only small deflec¬ tions of the droplet stream, .the lines became wavy. With sub¬ stantial deflections, the lines were clearly printed in a zig¬ zag fashion, when viewed from above. When the collimating electrode structure of the present invention was used (in the

^I arrangement illustrated in Figure 1) , all lines were printed as straight lines, even when the droplet deflections were a maximum. ^" "

INDUSTRIAL APPLICABILITY

The present invention is particularly applicable to

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the jet printing of wide fabrics and carpets, where quality printing is required, but where the fabric or other material being printed may become wrinkled while it is being printed. This means that it will be applicable to almost all fabric printing using jet printers, other than printing on thin-widt fabrics. In its applicability to carpets, the present invent ion will be especially useful when printing on carpets having a pile which is not of uniform height.

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Claims

1. Apparatus for jet printing on to the surface (15) of a fabric or paper web, carpet, or the like comprising:

(a) a droplet generating head (12) ;

(b) droplet charging electrode means (13) ; and

(c) droplet deflection means (17,18; 31, 32) adapted to deflect charged droplets; characterised in that: are

(d) second electrode means (17, 19; 33,34) /located between the droplet deflecting means (17, 18; 31, 32) and the position of the surface (15) when it is being printed, said second elec¬ trode means (17, 19; 33,34) being adapted to establish an electric field (F„; F_) which alters the trajectory of deflected droplets so that when they reach said posi¬ tion of the surface (15) , they are moving substantially orthogonal to the surface.

2. Apparatus as defined in claim 1, further character¬ ised in that the second electrode means comprise a pair of metal plates (17, 19; 33,34) .

3. Apparatus as defined in claim 2, in which said plates (17, 19; 33,34) are planar metal plates.

4. Apparatus as defined in claim 2, in which at least one of the pair of metal plates (17, 19; 33,34) is shaped to establish a required electric field (F_; F_g) between the plates

5. . Apparatus as defined in any preceding claim, in which the droplet deflection means and the second electrode means have a common electrode (17) .

6. Apparatus as defined in any one of claims 1 to 4 , in which said second electrode means includes an electrode which is a single wire conductor.

7. Apparatus as defined in any preceding claim, including voltage supply means adapted to provide a D.C. voltage (V_ , V_fi) across electrodes forming said second electrode means.