AU602760B2 - Stream deflection jet body for liquid jet printers - Google Patents

Description

PCT

AU-AI-13984/88 Intern, _i a i' a BU 0I INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification International Publication Number: WO 88/ 06525 B41J 3/04 A (43) International Publication Date: 7 September 1988 (07.09.88) (43 (21) International Application Number: PCT/AUS8/00056 (22) International Filing Date: (31) Priority Application Number: (32) Priority Date: 2 March 1958 (02.03.88) Pf0603 2 March 1987 (02.03.87) (33) Priority Country: (74) Agents: DUNCAN, Alan, David et al.; Davies Collison, 1 Little Collins Street, Melbourne, VIC 3000 (A (81) Designated States: AT (European patent), AU, BE (European patent), CH (European patent), DE (Euro-i pean patent), FR (European patent), GB (European patent), IT (European patent), JP, LU (European patent), NL (European patent), SE (European patent),

US.

Published ifflt international search report.

(71) Applicant (for all dcsignated States except US): COM- MONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION [AU/AU]; Lime.ston Avenue, Campbell, ACT 2601 (AU).

(72) Inventors; and Inventors/Applicants (fbr US only) WILLS, Leslie, James [AU/AU]; 62 Dudley Street, Lidcombe, NSW 2141 TURVEY, David, Edward [AU/AU]; 6 Naloura Close, Beecroft, NSW 2119 (AU).

AUSTRALIAN

2 6 SEP F8 I8 PATENT OFFICE

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I-ELI ~tls (54) Title: STREAM DEFLECTION JIT BODY FOR LIQUID JET PRINTERS J~ L flL-fl' 2-.

wCH (57) Abstract A printing head for generating slugs of liquid comprising: a) a liquid stream generating section adapted to receive liquid under pressure and having an orifice for producing a coherent, continuous stream of the liquid; b) an electrode supporting section (17) on which is mounted an electrode the electrode being positioned adjacent to the trajectory of the liquid stream and extending in the direction of flow of the liquid stream and c) a collector section comprising an impingement region which is inclined towards the axis of the liquid stream when the liquid stream impinges thereon and a run-off region which is inclined away from the axis of the liquid stream at the point where the liquid stream impinges upon the impingement region i. ~r 4 'Y i I i 2 'vO 88/06525 PCT/A88100056 1 1 TITLE: "STREAM DEFLECTION JET BODY FOR LIQUID JET PRINTERS" TECHNICAL FIELD OF THE INVENTION This invention concerns apparatus for the generation of slugs of liquid of precise length, such as liquid jet printing apparatus. More particularly it concerns apparatus for producing slugs of liquid from an unbroken, coherent stream of liquid emerging from an orifice.

BACKGROUND TO THE INVENTION In the liquid jet printing field (often termed the ink jet printing field in view of the common use of ink in the jet printers), a wide variety of apparatus is available for controlling the trajectory of liquid jets and the selection of liquid for printing.

Until now it has been generally accepted that accurate and reliable high resolution printing can only be obtained by printing with droplets of liquid and that the deflection and selection of these droplets should be effected after the droplet formation has taken place. However, there have been several previous attempts to control the trajectory, and hence the placement, of liquid streams by causing forces to act on a coherent, unbroken section of the stream issuing from an orifice. These prior art attempts have met with only limited success and most have suffered from one or more major disadvantages.

I.:i Qi *rrr I I C: WO 88/06525 PCT/AU88/00056.

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2 Xn most of the previous examples of stream deflection apparatus, the liquid stream is deflected by an electrostatic field and either the deflected or the undeflected part of the stream is interrupted before it impinges upon the printing substrate. The interruption has been effected either by a baffle or collector arrangement, or by the part of the stream to be collected being brought into contact with a convex deflector surface.

In the specification of U.S. patent No 1,941,001 granted to Clarence W Hansell and assigned to Radio Corporation of. America, an apparatus is described whereby an unbroken liquid stream is attracted by an electrode to which a high voltage has been applied, so that its deflected trajectory is interrupted by a baffle placed between the stream and the printing surface. In this case the stream is permitted to i~pie\ upon the printing surface when in the undeflected trajectory and is interrupted (and thus is prevented from reaching the printing surface) when it is deflected. Since the transition of the stream from one trajectory to the other takes a finite time, the leading edge of the collector intercepts liquid in the transition region between the deflected and the undeflected region. This leads to a build up of liquid on the collector edge. This build-up reduces the selectivity of the collector, and leads to poor resolution of the printing and to fouling on the printed surface.

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c _1 "nPQ- 8 6 PCT/AU88/00056 WOo 88/06525 PCTAU88/00056 3 In another form of apparatus, described by N E Klein and W H Stewart in the specification of U.K. patent No 1,456,458, an air jet from a hollow tube is directed at a liquid stream to deflect an unbroken portion of the stream away from a direct trajectory to the substrate or surface being printed, into a trough or collector which intercept the stream and prevents it reaching the printing surface. In this system the frequency response of the ON-OFF transitions of the stream are restricted by the switching speed of an electro-pneumatic valve which is used to direct the air current against the stream.

This low speed of ,response directly translates to a lower resolution and quality of print than is possible with higher speed systems.

In yet another type of apparatus, which is described in the specification of U.S. Patent No 3,893,623 to Richard A Toupin (assigned to International Business Machines Corporation), a liquid stream is amplitude modulated to produce discrete droplets. A weir, placed at a critical location downstream and adjacent to the trajectory of the stream and droplets, intercepts selected droplets if the diameter of the periodic disturbance on the liquid stream is greater than the necessary value to clear the weir. Toupin's specification discloses the use of a curved surface to capture droplets at the droplet formation point in the liquid stream. However, the sloped collector surface (see Figure 3A of that specification) is not r -,sj WO88/06525 PCTA---U88/00056 I wo 88/06525 :PC7{ 00 6- 4

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i, designed for the high collection efficiency which may be obtained when using the coanda effect to capture liquid.

In an alternative method of jet printing, which uses electrodes placed in close proximity to a coherent unbroken stream and which is described in the specification of US patent No 4,384,296 to Peter A Torpey, the liquid stream is modulated to form discrete droplets and requires further deflection apparatus to properly define exact print positions for the droplets.

In a further disclosure relating to apparatus for steering fluid jets, namely the specification of British patent No 2,041,831 to Graham Francis Stacy, a liquid jet is steered by causing it to come into contact with a convex curved surface. Contact between the liquid stream and the curved surface is effected by mechanical movement of either the convex surface or the jet body, or alternatively by frequency modulation of the jet. This technique has a number of disadvantages. It is very difficult to achieve the required close spacing of the deflecting curved surface relative to the liquid stream, and to simultaneously achieve the required relative 25 displacement of the jet body and the convex surface.

Also, no mechanism is described whereby the undeflected stream can be prevented from reaching the printing surface.

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i; i P ''W.i WO 88/06525 PCT/AU88/00056 DISCLOSURE OF THE PRESEN'T INVENTION It is an object of the present invention to provide apparatus for selecting slugs of liquid from a continuous liquid stream, for use in liquid jet printers, which substantially avoids the disadvantages of the prior art jet -printing systems.

This objective is achieved by constructing a jet body for a jet printer in such a manner that it is a compact structure, having a single electrode and an efficient coanda effect collector, that can be used to establish- slugs of liquid for accurate, controlled printing.

The jet body has a liquid stream generating section which receives liquid under pressure and which has an orifice that enables a coherent continuous stream of the liquid to be established. The stream of liquid passes over the remainder of the jet body, which can be regarded as an elongate structure, first over an electrode whicn has a surface that extends in the direction of flow of the continuous liquid stream, then over a collector section of the jet body. The collector section comprises a coanda effect collector which consists of a surface that includes a small acute angle with the axis of the liquid stream when the liquid stream is directed on to the collector, then slopes away from the direction of movement of the stream further from the liquid stream generating portion,.

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j The electrode is used to deflect a portion of the unbroken liquid stream from its normal trajectory so that either the deflected or the undeflected portions of the stream contact the collector surface and, by virtue of the coanda effect, adhere to it. The collector surface shape ensures that the contacted oortion of the stream is separated from the remainder of the stream. Thus the liquid stream is reduced to a series of liquid slugs of varying length, which can be used for printing purposes. It will be appreciated that slugs having a short length become droplets of liquid.

To effect the deflection of a portion of the unbroken stream of liquid, the electrode is mounted close to the stream of liquid and voltage signal is applied to the electrode as the portion of the liquid stream which is to be deflected flows past the electrode.

The voltage signal applied to the electrode induces a charge of the opposite sign in the region of the fluid stream that is adjacent to the electrode and the resultant attraction causes the portion of the liquid stream to be deflected towards the charged electrode it is passing. The collector surfaceis placed so that it intercepts either the deflected or 25 the undeflected liquid, to generate a required slug of liquid.

Thus, according to the present invention, there is provided a jet body for a liquid jet printer comprising -e 1 i- 1.' a. -7 liquid stream generating means adapted to> receive liquid under pressure and having an orifice therein for producing a coherent, continuous stream of the liquid; an electrode supporting section on which is mounted an electrode, the electrode being positioned adjacent to the trajectory of the liquid stream and extending in the direction of flow of the liquid stream and adapted to receive an electrical signal; and a collector section adjacent to the electrode supporting section, the collector section being adapted to collect at least portions of the liquid stream after passage of the stream over said electrode, the collector section comprising an impingement surface which is inclined towards the axis of the liquid stream when the liquid stream impinges thereon, and a run-off surface which is contiguous with the impingement surface and is inclined away from the axis of the quid stream at the point where the liquid stream impinges upon the impingement surface.

As noted above, liquid from the stream may impinge upon the impingement surface when the stream has been 25 deflected from its normal trajectory, under the influence of a voltage signal applied to the electrode. However, the jet body may be designed so that the liquid stream normally impinges upon the impingement surface of the collector and application of a voltage signal to the electrode is required to deflect the liquid stream to a trajectory which clears the impingement surface of the collector surface.

A scoop collector or wall may be included in the jet body, downstream of the collector section. A vent is preferably included between the liquid stream a

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WO 88/06525 I PCTIAU88/00056 genraing~ section and the electrode supporting Section. The jetk: body may be fabricated from a single block of an electr-icall1y insulating material or it rna' be constructed by assemblinganmbro separately fabricated components.

The electrode is5 preferably curved away from the axis of t"he undefilected liquid stream, and may be arcuate in thne direction verse to direct1ion of f"low of ta G.2Ua 1QS Z 'ea X- A pluralitv of such let bodies may be fabricated from *a si.ngle block, ar a nuimbIer of individual jet bodies may be connected together, to form an array of jet bodies as a pri.nting head for a liquid jet printer.

The present invention also encompasses a jet pri;,ner which includes a pr-inting head that comprises at least one jet body off the present invention.

Embodiments of the invention w~ill now be described, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram (partly schematic) ofE a liquid jet printer having a jet body constructed ini accordance wihthis invention.

Figure 2 is a sectional view (also partly schematic) of the jet body used in the printer of Figure 1.

WO0,88/06525 PCT/AC88/00056 Figure 3 -4s a sctinal.~a of a modified form of -the jet, body Figure 21 Ficue 4is a sectional diagram off another form of jet body ccnstructed in accordance with the present 54 inve nti4o n, in wnich the deflectioQn electrode and the collectcr sur'face are on opposite sides of the liquid Figure 5 is a pers-ppotive set-ch of a preferred shape o-f the electrodeof c:-the jet bodies of: Figures 2 and Figure 6 is a sectional at VI-VI of -the electrode ofFiourc F ig r e 7 1 s a perspective sketch of a printing head for a jet orinter having a poluralityv of jet bodies constructed in accordance with the present invention.

DETATLED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS In the specification Of International patent application No PCT/AU87/00294, apparatus is described whereby accurate printing may be performed by using a travelling wave electrode arrangement and a coanda effect collector surface to select slugs of liquid -for printing. The present invention is an improved firm of that apparatus, with which even higher hvebeen achieved with experimental apparati s manufactured by the present inventors).

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WO 88/06525 PGT I kt18 R-7OW In 1Che embodiments illustrated in F'igures 1, 2, 3 and 7 of the accompanying drawings, the aplication of an asvmmetrical electrostatic force on a coherent, unbroken 1licuild stream causes that stream to def lect a nd contact a collector surface arranged ubstantially pa~rallel 1t-o the undeflected stream or sloping away from the trajectory of the urndefflec-t ed Stream. This is achieved by placing an electrode in close proximity to the stream and applying a voltage signal to the electrode, thereby inducing surface charge on the strea.ma of opposite sign to that on the electrode and causing a deflection of the stream by eTectrostatic attraction. The embodiment illustrated in Figure 4 requires the application of a voltage signal to the electrode to defliect the liquid stream away frogi its normal trajectorym in which it impinges tupon a coll-ector surface.

in kt'h apparatus shvow n in Figure 1, liquid under pressure (created by conventional means) is supplied to the stream generating section 7 of a jet body t N from a liquid reservoir 1 via conduits 2. The liquid 'passes through a filter 3 before entering one side off a cavity in the stream generating section 7. The cavity has a narrow exit orific 4, 4rom whc h liquid leave~s the cavity as a high velocity, continuous and coherent liquid stream 5 of small cross-section. The liquid stream if unaffected b y any applied force, would normally strik,1e a prknting surface or substrate 16 at 'the Point 1S.

1 11 I Afgter traversing a vent 8, the liquid stream 5 from the orifice 4 passes closely above the electrode 6 which is mounted on the electrode supporting section 17 of the jet body. A high voltage signal (typically in the range of from 300 volts to 400 volts, but optionally higher) is applied to the electrode 6, usually as a voltage pulse, by the operation of an electrical signal switching means 18, which is Controlled by a digital data source (not shown).

Whenever a high voltage signal is applied to electrode 6, the stream 5 is attracted to the electrode due to redistribution of oppositely induced charge at the stream surface.

o e Xn the absence of a voltage signal on the electrode 6, the liquid stream 5 clears both the electrode 6 and the impingement surface 9 of a coanda collector of the jet body by the minimum practical spacing, 'which is determined by the precision engineering -tolerances which can be achieved. The tolerance for the electrode to stream spacings should preferably be less than 10 per cent of the dimensions. This tolerance can readily be achieved using current and numerically controlled machines for setting the electrode to stream clearance and forming the collector crest profile and setting its spacing from 'the stream.

When a voltage signal is applied to the electrode 6, the deflected portion of the stream strikes the surface of the collector 10 at its impingement surface 9. Upon contact with the impingement surface 9, the liquid adheres to it by virtue of the coanda effect. The adhering liquid continues to flow in the same general direction, down the slope of the run-off surface of collector 10 to be directed as it leaves the run-off surface, by a scoop collector 13, into a gutter 25. Liquid collected in the gutter 25 is T j WO 8/06525 PCT/AU88/00056 12 returned, via a conduit and under the action of a pump (not shown in the drawings) to the liquid reservoir i.

A liquid slug 12 of the stream which has not been deflected by the electrode 6 escapes collection by the collector 10 and, therefore, is not intercepted by the scoop collector 13 but continues its projection to the printing substrate 16, where it produces marks (indicia) 15. The liquid slugs 14 shown in Figure 1 are additional undeflected portions of the liquid stream which have escaped collection by the collector A more complete understanding of the operation of the jet body of Figure 1 will be acquired with reference to Figure 2.

In the jet body illustrated in Figure 2 (which is that used in the printer of Figure liquid under pressure is supplied to the cavity 19 of the stream generating section 7 by means of an inlet pipe 2.

The liquid stream 5 issues at high velocity from the orifice 4 and passes over the vent 8 and top surface I of the electrode 6. The spacing between the electrode surface and the stream is maintained at the minimum practical value determined by the limitations of precision engineering. As shown in Figure 2, the electrode is curved away from the direction of flow of the liquid stream 5, so that as the liquid stream

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VO 88/06525 PCT/AU88/00056 I I 13 is deflected when a voltage signal is applied to the electrode 6, the spacing between the liquid stream and the electrode 6 remains substantially constant.

The vent 8, which in most cases is open to the atmosphere, between the orifice 4 and the edge of the electrode 6 is necessary in most practical embodiments to prevent the possibility of wall attachment of the liquid of the stream 5 to the adjacent electrode surface.

A high voltage signal, NB applied to the electrode 6 by the signal switching means 18 by a conducting connection, attracts the liquid stream towards the electrode 6 by electrostatic force. When in the undeflected projection, the stream clears the SSUr oce impingement 4 rgq 9 of the collector 10 by the minimum practical value determined by precision engineering limitations and the stream aim stability.

The surface of collector 10 has an initial impact or impingement r-F4s ea 9 beginning towards the top of a convex surface 9A. The impingement region itself makes a small acute angle with the deflected liquid stream, and merges into a generally flat sloping section 10A down which the adhered liquid 11 flows.

S-tra ce- 1Z j The convex shape preceding the impingement/ gie 9 promotes streamlined flow on to the collector surface. Streamlined flow over the collector surface ensures that there is a clean detachment of the produced liquid slug 12 from the stream i i i 3 i 1;; WO 88/06525 PCT/A88/000 14 Where the ink or other liquid of the stream 5 is water-based, the surface of the collector 10 is preferably hydrophylic, although a surface which is simply able to be wetted by the liquid is sufficient in most cases. It has been found that the leading edge of the stream captured on the surface is less turbulent when the surface is either hydrophylic or prewet. One method of ensuring that the surface remains wet is by scouring with a fine abrasive paper. In a prototype collector, made from the commercial plastic DELRIN (trade mark), the scouring was performed with 400 grit abrasive paper.

For optimum performance of the present invention, there .should be a clean collection of the liquid on the4 surface of the collector 10, leaving projected liquid slugs 12, 14 with no residual liquid between the slug and the surface. This performance requirement is similar to the requirement in synchronous droplet printers for the production of droplets which are free of satellite droplets. It can be met by having a wettable collector surface and a gradient appropriate to the stream velocity. The process of attaining residue free operation has been found to be essentially time dependent in the sense that the spatial separation between the stream and the surface occurs in no less than some minimum time determined principally by the diameter of the stream and the physical properties of the liquid.

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The impingement surface of the collector may have either a flat slope or a curved slope, but in both cases the limiting gradient is set by the above-noted requirement that no residue droplets, or tails, be formed between the projected stream and the surface.

The scoop collector 13 serves primarily to arrest collected liquid passing off the run-off surface of the collector and to direct that liquid into the return circulation system. The level of the top of 10 the scoop collector 13 must be such that it does not intercept liquid in the droop 20 at each end of a liquid slug, which is caused by energy imparted to the liquid stream during the collection process.

Typically, this requirement means that there must be a clearance of two stream diameters between the undeflected liquid stream and the top of the scoop collector 13.

In the jet body illustrated in Figure 3, a planar impingement surface 9 for the deflected stream is 20 provided adjacent to, and contiguous with, the flat deflection electrode 6. The orifice 4 produces a liquid stream 5 which is deflected in response to a high voltage signal applied to the electrode 6. The signal applied to the electrode is of strength such that impingement of the stream on the collector surface occurs substantially in the centre of the impingement surface 9. The liquid flattens on contact with the surface 9, and liquid slug separation is residue free as previously described.

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OS S S~ O S 55 r p i i i ri;i WO 88/06525 PCT/AU88/00056 16 The embodiment shown in Figure 3 has manufacturing advantages in that the collector surface comprises two intersecting planes radiused at the intersection.

The inclination of the sloping section is determined empirically as before and has the same surface texture. A small deflection of the stream will cause it to contact the impingement target area 19, which is simply a planar extension from the electrode surface. The small deflections of the stream ensure a smooth non-turbulent attachment of the deflected liauid to the collector surface.

Figure 4 shows a different arrangement of the components which constitute the jet body of the present invention. In this arrangement, the electrode 6 is placed on the opposite side of the liquid stream 5 to the collector 10. In this case, the clearance between the stream 5 and the electrode 6 is maintained as small as possible (as for the embodiments of Figures 1, 2 and 3) but the undeflected stream impinges upon the surface of the collector 10 just before the crest of the convex surface 9A. The undeflected projection of the stream is such that the impacted or intersected area is a minimum for reliable collection. In practice, this is determined mainly by the engineering tolerances on the collector placement, by the stream misalignment and, to a lesser extent, the surface characteristics of the coanda collector. In practice, it has been found that stream intersections of only two micrometres are necessary to collect a i i; NVO 88/06525 PCT/AU88/00056 cI 17 stream having a cross-sectional diameter of 250 microns. This close location of the collector surface relative to the axis of the undeflected liquid stream ensures a high spatial resolution of the system, because the stream moves only a very small distance to clear the collector. The higher the spatial resolution, the more accurately is the slug length determined and the higher the quality of the printing.

A significant advantage of the deflect-to-print arrangement shown in Figure .4 is that the liquid stream is collected without any electrical signal being present on the electrode 6. This feature facilitates start-up procedures for the printer and allows the fluid system to operate in an "idle" condition with the electronic power off.

Figure 7 illustrates a printing head for a liquid jet printer in which three parallel liquid streams 51, 52 and 53 issue from respective orifices in the combined stream generation section 57 of three jet bodies.

Liquid is supplied under pressure to the stream generation sections via a conduit 58. In practice, there will usually be more than three orifices in a linear array contained within a plane substantially parallel to the impingement region 59 of the combined collector sections of the jet bodies. Such printheads can be made in extended widths without interference between adjacent jets, which occurs with modulated droplet printers built in array form.

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Printheads of the type illustrated in Figure 7 require an independent electrical connection to each electrode 56, which is powered by a high voltage switch controlled from a digital data source (not shown in Figure 7).

The present inventors have also developed the theoretical consideration of the ap'plication of the present invention. In general, if the jet body of the present invention (as illustrated in Figures 1, 2 and 3) is used to create a liquid stream of radius r located a distance s from a flat, planar electrode surface, the stream will experience an acceleration a towards the electrode when a potential difference V is applied between the electrode and the stream, and a will be given by the relationship: a 3 2 2 u (cosh u) where and s +r u r 0 is- the dielectric constant of free space, o /D is the density of the liquid.

Using this relationship, it has been found that when a voltage signal of 350 volts is applied to the electrode, for a liquid of density 1.0, the Wo n 88/06 25 P CT/ USS/00056------ W'o .88/06525 ,PCT/AU88/00056 19 accelerations possible for liquid streams which have cross-sectional diameters of 10, 50 and 250 microns, and s values of 10, 20, 50 and 100 microns, are shown in the following table:- Stream Diameters 10 microns 50 microns 250 microns Spacing s Accelerations microns microns 1.01 x 10 g 3.44 x 104g 8.47 x 10 g 9.61 x 10 g 3.33 x 10 g 28.06 x 8.06 x 10 g 2 8.8 x 10 g 3.09 x 10 g 50 microns 100 microns 76.9g 26.6g ii These accelerations are those that are readily achieved without optimising the parameters used (such as increasing the voltage signal until limiting values of field strength are achieved, or using arcuate electrodes).

A constant acceleration of the stream towards the electrode during the application of the voltage signal indicates that if the gap or spacing between the electrode and the stream is to remain constant, then (as indicated above and as shown ii. Figures 2 and 4) the electrode 6 should be curved away from the stream to an extent determined by the stream velocity and the acceleration that is experienced by the stream.

WO 88/06525 PCT/AU88/00056 20 Xt has also been found to be advantageous to give the electrode a concave shape in the direction transverse to the flow direction of the liquid stream, since this shape is more effective in imparting transverse acceleration to the stream. A theoretical analysis of the performance of electrodes of differing transverse shape has indicated that for a constant stream to electrode spacing, a transversely concave electrode will impart roughly double the acceleration to the stream, compared to the acceleration achievable with a flat electrode, when the spacing is microns, and it will impart about 3.5 times the flat electrode acceleration when the stream to electrode spacing is 10 micrometres.

The length of the electrode will be selected to achieve a desired stream deflection, taking into account such factors as the required precision in the length of liquid slugs, the distance downstream of the impingement region of the collector surface, and the precision with which the electrode can be spaced relative to the liquid stream.

Thus the electrodes used in the present invention preferably have the shape illustrated in Figures and 6, which show an electrode which is curved in the direction. of flow of the stream while having an arcuate transve'-se shape.

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In prototype printing heads produced by the present inventors, the electrode length was in the range of from 0.5 to 3.5 nun, with the leading edge of the electrode positioned about 5 mm from the orifice of the stream generating section and the downstream edge of the electrode located about 10 mm from the impingement region of the collector.

Previous designers of liquid jet printers which include collectors working on the coanda effect principle have always asserted that deflection of the liquid stream into two separate paths has been adequate. However, the present inventors have found that with all such collectors, as described in the literature, in the region between the liquid adhered to the surface and the ends of the undeflected droplet, small residue droplets form which neither clear the collector scoop nor enter the scoop chamber, but rather impinge on and build up on the leading edge and uppermost surface of the scoop collector. In the present invention, an empirically determined topology of the collector is preferably adopted to prevent the formation of these residue or satellite droplets.

It is well known in the liquid jet printing field that particular droplet formation conditions prevent the formation of satellite or residue droplets. The analogy in the present invention is that by controlling the rate at which the slug ends separate from the stream attached to the coanda surface, the WO 88/06525 22 formaion of residue droplets can be avoided. The present inventors have ascertained that for a water based ink stream of diameter 250 microns and velocity of 12 metres per second, this is achieved by limiting the inclination of the sloping surface of the collector 10 to a value of 1 in The minimum length of the collector surface can be calculated by determining the minimum separation of the stream from its undeflected trajectory to ensure that it is arrested by the scoop collector 13, while the ends of the droops 20, on the leading and trailing edges of a liquid slug, clear the scoop collector 13. The typical droop extends about one stream diameter below the main region of the liquid slug. Thus, in a typical realisation of the present invention, the minimum slope lehgth is 5 millimetres.

However a further 'imitation is the dynamic retraction of the residue liquid between the stream and the liquid adhered to the collector surface by the coanda effect. This dynamic separation behaviour means that further time is required for full residue free separation to occur, which can readily be provided for by lengthening the sloping surface.

Provision of some tolerance on the length of this surface for changes in liquid properties, in addition to the previous requirements, results in a typical safe final length for the collector surface of 15 mm and a clearance between the main portions of the liquid slugs and the top edge of the scoop collector of 0.5 mm.

i WO 88/06525 PCT/AU88/00056 23 Both single jet and multi-jet printing heads, incorporating the present invention, have been built and operated successfully by the present inventors.

In the single jet prototypes, the fine adjustment of the positions of the electrode and of the collector surface was achieved using micro position translators (available from most optical equipment suppliers).

For the multi-jet prototypes, the printing heads were made using normal manufacturing tolerances, then trimming of the electrode and collector surfaces was carried out by manually scraping these surfaces or by using a purpose-designed trimming tool. Precise control of the slug length was achieved using a feedback system which measured the response of printing head to a predetermined set of input parameters after each pass of the trimming operation.

One problem that did occur in the use of the first prototypes of the present invention when the gap between the electrode and the liquid stream became small (that is, less than about 100 microns when the stream diameter was 250 microns), was found to be due to the condensation of evaporated solvent from the stream on to the electrode. This condensation occurred whenever the electrode was colder than room temperature. Once initiated, the condensation built up because the evaporation of the condensate from the electrode surface cooled the electrode further, thus enhancing the condensation on the electrode.

24 This problem was overcome by the adoption of one of two techniques. In one technique, the electrode was heated, using conduction of heat generated in a resistor mounted alongside (and in contact with) the electrode, or radiation of heat generated in a miniature (300 milliwatts) incandescent lamp. The second technique comprised cooling the liquid before supplying it to the stream generation section of the jet body.

Those skilled in the jet printing art will appreciate that although specific exemplary embodiments of the *present invention have been described above, i modifications to such embodiments can be made without departing from the present inventive concept. They 1 15 will also appreciate that although this invention has it been described with particular reference to its jet i o printing application, the invention is not limited to this application. Other uses of the present invention include the rapid filling of pharmaceutical 20 capsules and the control of the fuel feed in fuel/air S mixing processes (such as those used in firing boilers and certain types of internal combustion engines).

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Claims (12)

1. 2. A printing head for a liquid jet printer, characterised in that it comprises: liquid stream generating means adapted to receive liquid under pressure and having a linear array of a plurality of orifices therein, each said orifice being adapted to produce a coherent stream of the liquid; an electrode supporting section on which is mounted a plurality of electrodes, each said electrode being positioned adjacent to the trajectory of a respective liquid stream from one of said orifices, and each electrode extending in the direction of flow of its associated liquid stream and adapted to receive an electrical signal; and a collector section adjacent to the electrode supporting section, the collection section being adapted to collect at least portions of the liquid stream after passage of the stream over said electrode, the collector section comprising an impingement surface which is inclined towards the axis of the liquid streams when the liquid streams impinge thereon, and a run-off surface which is contiguous with the impingement surface and is inclined away from the axis of the liquid streams at the points where the liquid streams impinge upon the impingement surface. 1 I II 27
2. 3. A jet body as defined in claim 1 or a printing head as defined in claim 2, further characterised in that a scoop collector is positioned downstream of the run-off surface, the scoop collector being positioned to deflect liquid from the liquid stream which has impinged upon the impingement surface and traversed the run-off surface into a collection gutter. o. 4. A jet body or a printing head as defined in g claim 3, including a pump for pumping liquid ~from said gutter to a reservoir from which Sliquid is supplied under pressure to said liquid stream generating means. A jet body or a printing head as defined in any preceding claim, including a vent between said liquid stream generating means and said Selectrode supporting section. i 6. A jet body or a printing head as defined in any preceding claim, in which said or each electrode i o is curved away from the trajectory of the or its associated liquid stream in the direction of 0i flow of the or its associated liquid stream.
3. 7. A jet body or a printing head as defined in any preceding claim, in which said or each electrode is arcuately shaped in the direction which is transverse of the or its associated liquid stream. 28 e g. 0 C. S 0e 0 C
4. 8. A jet body or a printing head as defined in any preceding claim, in which the or each electrode is mounted on the opposite side of the stream or streams to the collector section, and in which, in the absence of a voltage signal applied to the or an electrode, the liquid stream or streams is or are adapted to impinge upon the impingement surface.
5. 9. A jet body or a printing head as defined in any preceding claim, including means to apply voltage signals to said electrode or electrodes. A jet body or a printing head as defined in claim 9, in which said means to apply voltage signals includes switch means adapted to be activated in accordance with predetermined data.
6. 11. A jet body or a printing head as defined in any preceding claim, in which the impingement surface is convex in the region thereof which is adjacent to the electrode supporting section.
7. 12. A jet body or a printing head as defined in any one of claims 1 to 10, in which the impingement surface of the collector section is a planar continuation of said electrode supporting section.
8. 13. A jet body or a printing head as defined in any preceding claim, including means to heat said electrode or electrodes. it1 Ce. so S S C 55 I. i ft 1 S *5 0*e S. S *9 5 S S 29
9. 14. A jet body or a printing head as defined in any one of claims 1 to 12, including means to cool the liquid before it is supplied to said stream generating means. A system for producing slugs of liquid of precise predetermined length or lengths which includes at least one jet body or printing head as defined in any preceding claim.
10. 16. Apparatus as defined in claim 15, in which said system is a liquid jet printing system.
11. 17. A jet body for a liquid jet printer, substantially as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings.
12. 18. A printing head for a liquid jet printer, substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings. DATED this twenty-fourth day of July 1990 COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION by its Patent Attorneys DAVIES COLLISON S SOS *SS.