WO2007099274A1

WO2007099274A1 - Deposition apparatus and method of printing

Info

Publication number: WO2007099274A1
Application number: PCT/GB2006/001786
Authority: WO
Inventors: Daniel Arthur Hall
Original assignee: Inkski Ltd
Priority date: 2006-02-09
Filing date: 2006-05-15
Publication date: 2007-09-07
Also published as: GB0602609D0

Abstract

In a deposition apparatus for depositing drops of liquid (for example, ink) on a substrate and in a printing method, a retaining member such as a cylinder (10) retains at least one pair of adjacent droplets (312, 314) of the liquid against the action of an expulsion force (for example, centripetal acceleration). Release of a selected droplet is achieved by actuation of coalescing means (302, 304) which, in response to an input signal, causes the droplets to coalesce into a larger drop which is released from the retaining member by the expulsion force. The release of the drop is achieved using a relatively small amount of induced movement and/distortion of either droplet, so that a drop can be released in a reliable, energy-efficient manner at a precisely determined time.

Description

TITLE: DEPOSITION APPARATUS AND METHOD OF PRINTING

Field of the Invention

This invention relates to Deposition Apparatus, a printer having such apparatus and to a method of printing.

Background to the Invention

The invention is particularly, but not exclusively, applicable to deposition or printing apparatus in which droplets of liquid (for example ink) are projected from a rotating member onto a substrate.

In such an arrangement, the rotating member is typically a cylinder which is rotated about its axis, and which retains the liquid to be deposited on the substrate on its circumferential surface. A selected portion of the retained liquid can be energised or agitated so as to release a droplet which is then projected along an initially tangential trajectory from the cylinder onto the substrate.

In order for the droplet to hit the desired spot on the substrate it is crucial that the droplet is released from the cylinder at the correct time.

Summary of the Invention

According to a first aspect of the invention, there is provided deposition apparatus for depositing drops of liquid on a substrate, the apparatus comprising a retaining member for retaining a pair of adjacent droplets of the liquid against the action of an expulsion force, coalescing means operative, in response to an input signal, to cause the droplets to coalesce so as to forai a larger drop which is released from the retaining member by the expulsion force.

Since the act of coalescing the droplets leads to the release of the drop, the cohesive forces between the droplets which assist the coalescence provide part of the energy needed to release the drops. Accordingly, a relatively small amount induced movement and/or distortion of either droplet can achieve the release. The Applicants thus believe that the coalescing means release the drops in a reliable, energy efficient manner at a precisely predetermined time.

Preferably, the retaining member is operative to retain each droplet at a respective predetermined site on the retaining member.

Preferably, the retaining member is operative to retain a plurality of pairs of said droplets, each being retained at a respective predetermined site on the retaining member, the coalescing means being operative to cause the droplets of a selected pair, or each of a selected group of pairs to coalesce in response to an input signal identifying said pair or group, so as to cause a selected one or more drops to be released.

Thus, in one example, an input signal could identify an individual pair of droplets to be coalesced to cause a release of a corresponding drop. Alternatively, the input signal may identify a group of pairs so that the droplets of each pair in that group are coalesced to form a respective drop. The group could, for example, correspond to a predetermined pattern, for example, a line, to be printed onto the substrate. In such a case, the droplets of the pairs in the selected group are preferably coalesced simultaneously.

Preferably, the coalescing means comprises a plurality of coalescing devices, each associated with a respective pair of sites, so as to be operable to cause the droplets at that pair of sites to coalesce.

In this case, the coalescing devices, or groups thereof, are preferably individually addressable. Thus, in one example, each device may be operated individually so that the droplets to be coalesced can be selected individually.

Alternatively, a group of devices, for example, associated with a row of sites, may be operated together, in response to an input signal selecting that row.

The retaining member may be so arranged that said expulsion force is gravity.

Preferably, however, the apparatus includes means for applying an expulsion force to said droplets and drops.

Such means may, for example, comprise electrostatic and/or magnetic field application means for subjecting the droplets and drops to electrostatic and/or magnetic expulsion forces.

Preferably, however, the means for applying expulsion force comprises a drive for rotating the retaining member, the expulsion force being derived from the centripetal acceleration of said sites. Thus, in this case, the expulsion force can be considered to be the fictitious, centrifugal force experienced by the droplets as a result of their reaction to said acceleration.

Preferably, the retaining member comprises a cylinder rotatable, by the drive, about its axis.

The coalescing means may to advantage comprise means for generating an electrostatic field for urging droplets of a selected pair together.

In this case, the coalescing means may comprise a pair of electrodes.

Where the retaining member has sites for plural pairs of droplets, each coalescing device preferably comprises a respective pair of electrodes between which the associated sites are situated. Thus coalescence of the droplets is achieved by the application of a voltage to the coalescing devices. This results in a lower power requirement than, for example, an arrangement which relies on electrical current to cause either droplet to be heated or otherwise energised in order to coalesce.

Preferably, each site comprises a first zone surrounded by a second zone, the surface characteristics of which are such as to urge liquid towards the first zone. For example, the second zone may be hydrophobic, and the first zone either less hydrophobic than the first zone or hydrophillic.

In this case, the hydrophobic zones may conveniently comprise a common coating on the retaining member.

According to a second aspect of the invention, there is provided printing apparatus comprising deposition apparatus in accordance with the first aspect of the invention, and liquid supply means for supplying the liquid to form the droplets on the retaining member of the deposition apparatus.

The invention also lies in method of printing, the method comprising the steps of feeding liquid to the surface of a retaining member to form a plurality of pairs of adjacent droplets thereon and causing droplets in selected pairs to coalesce so as to be released from the member and projected towards a substrate spaced from the member to form a predetermined pattern on the substrate.

Preferably, the member is rotated, so that coalesce droplets are released and projected towards a substrate as a result of centripetal acceleration.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings wherein:

Figure 1 shows a printing apparatus according to a first embodiment of the invention; Figure 2 shows a perspective view of liquid feeding means for use in the apparatus of Figure 1;

Figures 3(b) and 3(c) show further embodiments of liquid feeding means for use in the apparatus of Figure 1;

Figure 3(d) shows an embodiment of liquid feeding means in combination with droplet absorbing means for use in the apparatus of Figure 1;

Figure 4 shows certain components of the printing in greater detail, and in particular a coalescing device to an enlarged scale;

Figure 5 is a more detailed view, a modified version, of the apparatus shown in Figure 4, showing the coalescing devices for two pairs of droplet sites, again to an enlarged scale;

Figures 6 is a sectional view of part of the cylinder, showing a pair of droplets.

Figure 7 is a view corresponding to Figure 6, and showing the two droplets coalesced;

Figure 8 corresponds to Figures 6 and 7, and shows a drop being returned as a result of said coalescence; and

Figure 9 shows the modified version of printing apparatus, of which Figure 4 provides the more detailed view.

Detailed Description

It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar features in modified and different embodiments.

The printing apparatus of Figure 1 has a cylinder 10 rotatable about its longitudinal axis 12 which as shown is perpendicular to the plane of the figure. In use of the apparatus as illustrated, the cylinder rotates around axis 12 in the direction indicated by arrow 14. The cylinder is rotated by drive means 16 in the form of a motor. Droplets 18 of substantially equal volume are formed on the circumferential surface 20 of the cylinder 10 by liquid feeding means 22. The liquid feeding means comprises a reservoir 24 containing liquid 26 (such as an ink, for example) connected to a tube 28 having an open distal end 30, from which droplets are emitted onto the cylinder.

Typically, the droplets may have a diameter of less than 300 microns - 40 microns for example - and a volume less than 500 pL - 20 pL for example.

Droplet coalescing means (not shown) cause selected droplets to be released from the cylinder, as will be described in more detail below. Control means 34 monitor rotation of the cylinder via an electrical connection 36 to an appropriately located sensor, in the drive means 16, for example, and control operation of the droplet coalescing means.

A coalesced released drop 18' is shown in Figure 1 which has broken contact with the cylinder 10 and is travelling in a direction indicated by arrow 42, approximately tangentially with respect to the circumferential cylinder surface 20. The drop velocity may be approximately equal to the surface velocity of the cylinder and is typically 2m/s or more. Released drops land on a substrate 44 moving relative to the circumferential surface of the cylinder in a direction indicated by arrow 46, to form a desired, predetermined pattern thereon. In a high throughout printing application, the substrate may be moving typically at a speed of around lm/s or greater relative to the cylinder. Droplets 18'" which are not selected for release from the cylinder remain in place for possible selection during the next revolution of the cylinder. Means may be provided to refresh these drops, which are described below. Figure 2 shows the array of droplet forming site pairs mutually spaced apart over the circumferential surface of the cylinder. Each pair of sites is diagranimatically represented as a respective single disc, such as disc 202, but is in fact constituted by a pair of discs spaced apart in the direction of the axis 12. Each disc of the pair has a diameter of around 15 microns and the pairs are spaced around 10 microns apart in the axial direction and around 200 to 300 microns apart in the circumferential direction on the cylinder surface. The cylinder may have a radius less than 4cm, typically around 5 to 6mm and an axial length of 300mm, for example.

In use, the cylinder may rotate with a surface angular acceleration of over 10m/s/s, typically of the order of 68,000m/s/s. This corresponds to a cylinder with a radius of 6mm rotating at approximately 32,000 revolutions per minute. Under these conditions, a released drop may travel with an initial speed of around 20m/s.

The liquid feeding means of Figure 2 comprises a reservoir 24 containing liquid 26 connected by a tube 28 to an ink chamber 204 which defines a narrow open channel 205 positioned so that an ink meniscus 206 forms in contact with the surface of a cylinder 10 rotating around a central axis 12.

Channel 205 may be 100 mm long and 300 microns wide, for example, and positioned 200 microns from the surface of cylinder 10. The channel may typically be defined by edges of two opposing plates.

The liquid feeding means of Figures 3(b) and 3(c) also provide droplet absorbing and recirculation means. The ink chamber 204 contains an ink outlet 207 connected by a tube 208 to a pump 209 which pumps ink around the system, from the ink reservoir to the ink chamber and then back to the ink chamber, thereby recirculating and mixing the ink. The pump 209 is connected by a tube 210 to an inlet 211 to the ink reservoir 26. In this arrangement ink droplets not ejected from the cylinder are absorbed at the surface of meniscus 206 and then recreated as the respective droplet sites move past and away from the meniscus. Ink that has travelled around the cylinder as a surface droplet is mixed with ink from the chamber 204 and recirculated to the ink reservoir 24, maintaining a substantially consistent formulation of liquid droplets on the surface of the rotating cylinder.

In Figure 3(c), a liquid ink reservoir and pump (not shown) supply liquid to an inlet tube 220, supplying a liquid 221 to a chamber 222 containing liquid in contact with absorbent material 223. The absorbent material 223 may be cotton cloth for example. The liquid material is pumped through the absorbent material 223 to another part of the chamber which is connected by a tube 224 back to the liquid ink reservoir such that the liquid ink returns to the reservoir. Liquid 225 from the wetted absorbent material 223 is in contact with the surface 20 of a rotating cylinder 10, such that as the cylinder rotates droplets 18 are formed on the surface and travel around the axis 12 of the cylinder, until they are reabsorbed and mixed with the liquid in the absorbent material. In this way liquid that has travelled around the cylinder as a surface drop is mixed with liquid from the liquid reservoir, maintaining a consistent formulation of liquid drops on the surface of the rotating cylinder.

Figure 3(d) shows an arrangement similar to those of Figures 3 (a) and 3(b) which is modified to include liquid absorbing means separately from the liquid feeding means. A liquid reservoir and pump (not shown) supply liquid via inlets tube 231, 235 and 237 to both the ink feeding chamber 204 and an ink absorbing chamber 233. Like chamber 204, ink absorbing chamber may define an open channel 239 which is arranged to form an ink meniscus 241 in contact with the circumferential surface of cylinder 10.

A flexible member 243 is provided on the ink absorbing chamber 233 adjacent to the channel 239 which has a distal edge close to or in contact with the surface of cylinder 10. Member 243 serves to prevent formation of droplets by the meniscus 241, ensuring that absorbed ink is recirculated instead. The absorbed ink is fed from ink absorbing chamber back to the reservoir via an outlet tube 245.

Figure 4 shows the cylinder 10 in more detail. Figures 6-8 also show the construction of part of the cylinder 10 in more detail. With reference to Figure 6, the cylinder comprises a core 300 of a moderately hydrophillic electrical insulating material, for example, fused silica. Attached to this surface is an array of pairs of electrodes, two of which are shown at 302 and 304. The cylinder and electrodes are, in turn, coated with a hydrophobic electrical insulator 306. Selected areas of the coating 306 are edged away to expose circular patches of the core 300, for example patches 308 and 310, to define an array of hydrophillic sites arranged in pairs of closely adjacent sites. As can be seen from Figure 6, the sites in each pair, in use, retain a corresponding pair of droplets such as the droplets 312 and 314. The material for the outer coating 306 may be PTFE.

Thus, the cylinder 10 has an array of predetermined sites, constituted by the exposed hydrophillic areas of the cylinder, surrounded by a hyrdrophobic zone constituted by the coating 306.

Referring to Figure 4, the motor 16 includes a position detector circuit to determine the rotational position of the cylinder 10. This circuit is operatively connected by line 316 to a programmeable high voltage delayed pulse generator 318. The generator produces a pulse voltage output which is fed to a cylinder coupling circuit 320 by electrical connections 322.

The cylinder coupling circuit 320 may, for example, comprise a mercury slip-ring arrangement which enables the transmission of the voltage pulse from the generator 318 to a conductive copper track 324 on the surface of the core 308. Similarly, the slip-ring arrangement provides an earth connection at 326 to a further track 328 which is also on, and hence rotates with, the cylinder 10. Each of the tracks 328 and 324 is connected to a respective one of the two electrodes 302 and 304 so that the electrode 302 is connected to earth, whilst the electrode 304 is connected to the pulse generator 318. The cylinder coupling circuit 320 includes further similar slip-ring type arrangements to enable the output of the pulse generator on further multiple lines (not shown) to be connected to the corresponding electrodes of the other pairs of electrodes in the array. Each electrode in the array not connected to the pulse generator 318 is connected to earth, also through the coupling circuit 320.

During operation of the device, liquid is deposited onto the cylinder 10 and droplets are formed at the hydrophillic sites such as 308 and 310 (for example, droplets 312 and 314). If, for example, a drop is to be released from the sites 308 and 310, an input signal is fed into the circuitry 318. The timing for the release of the drop is important, and to that end the detector in the motor 16 sends a detection signal to the circuitry 318 when a given angular position of the cylinder 10 is detected. The circuitry 318 is programmed to send a voltage pulse down the line of 322 a predetermined delay after said detection. That voltage pulse is fed to the electrode 304 through the coupling circuit 320. Since the electrode 302 is grounded, the voltage creates an electric field extending from one to the other of the electrodes 302 and 304. This field urges the two droplets 310 and 312 together to form a coalesced drop 316 (Figure 7). The size of this coalesced drop is such that the adhesive forces between the drop and the hydrophillic sites 308 and 310 are not sufficient to resist the centripetal forces exerted on the drop, so that part of the drop detaches from the cylinder to form a free drop 18' (Figure 8) which travels along a tangential trajectory from the cylinder 10 to the substrate 44 (as indicated by arrow 42 in Figure 1).

Figure 5 shows part of a modified version of the device, and more particularly shows a second pair of hydrophillic sites 330 and 332 and associated electrodes 334 and 336. Multiple control signals are required for separate selection of the drop hydrophilic sites. These can be provided by a slip ring or by other means. In addition, the ink supply to the circumferential surface of the cylinder 10 is achieved by means of a spray head 338 which emits an aerosol spray of the ink 340. Any of the spray impacting the hydrophobic coating 306 will rapidly move across the surface of the cylinder 10 until it reaches a hydrophillic site, at which a droplet is formed. The spray head 338 is swept back and forth in the direction of the axis 12. Any excess liquid is quickly expelled from the surface due to the surface's angular acceleration, and this liquid may be captured and recirculated in the ink system.

Figure 9 shows the apparatus of Figure 5 as part of a printing system wherein free liquid drops, for example, drop 340, are produced from the surface of the cylinder 10, each said drop corresponding to a pixel of an image in a data store 342 which provides an input signal to the circuitry 318. The free drops move onto a substrate 344 for the printed image as the substrate is moved past a printing position 346 (i.e. the position at which the drops become detached from the cylinder 10) on the circumference of the cylinder. The selective emission of drops from the cylinder forms an image 348 on the substrate 344.

Claims

1. Deposition apparatus for depositing drops of liquid on a substrate, the apparatus comprising a retaining member for retaining a plurality of adjacent droplets of the liquid against the action of an expulsion force, coalescing means operative, in response to an input signal, to cause the droplets to coalesce so as to form a larger drop which is released from the retaining member by the expulsion force.

2. Apparatus according to claim 1, in which the retaining member is operative to retain each droplet at a respective predetermined site on the retaining member,

3. Apparatus according to claim 1 or claim 2, in which the retaining member is operative to retain a plurality of pairs of said droplets, each being retained at a respective predetermined site on the retaining member, the coalescing means being operative to cause the droplets of a selected pair, or each of a selected group of pairs to coalesce in response to an input signal identifying said pair or group, so as to cause a selected one or more drops to be released.

4. Apparatus according to claim 3, in which the input signal identifies a group of pairs so that the droplets of each pair in that group are coalesced to form a respective drop, the droplets of the pairs in the selected group being coalesced simultaneously.

5. Apparatus according to any of the preceding claims, in which the coalescing means comprises a plurality of coalescing devices, each associated with a respective pair of sites, so as to be operable to cause the droplets at that pair of sites to coalesce.

6. Apparatus according to claim 5, in which the coalescing devices, or groups thereof, are individually addressable.

7. Apparatus according to any of the preceding claims, in which the apparatus includes means for applying an expulsion force to said droplets and drops.

8. Apparatus according to claim 7, in which the means for applying the expulsion force comprises a drive for rotating the retaining member, the expulsion force being derived from the centripetal acceleration of said sites.

9. Apparatus according to claim 8, in which the retaining member comprises a cylinder rotatable, by the drive, about its axis.

10. Apparatus according to any of the preceding claims, in which the coalescing means comprises means for generating an electrostatic field for urging droplets of a selected pair together.

11. Apparatus according to claim 10, in which the coalescing means comprises a pair of electrodes.

12. Apparatus according to any of the preceding claims, in which the retaining member has sites for plural pairs of droplets, each coalescing device comprising a respective pair of electrodes between which the associated sites are situated.

13. Apparatus according to claim 2 or any preceding claim dependent therefrom, in which each site comprises a first zone surrounded by a second zone, the surface characteristics of which are such as to urge liquid towards the first zone.

14. Apparatus according to claim 13, in which the second zone is hydrophobic, and the first zone either less hydrophobic than the first zone or hydrophillic.

15. Apparatus according to claim 14, in which the hydrophobic zones comprise a common coating on the retaining member.

16. Printing apparatus comprising deposition apparatus in accordance with any of the preceding claims, and liquid supply means for supplying the liquid to form the droplets on the retaining member of the deposition apparatus.

17. A method of printing, the method comprising the steps of feeding liquid to the surface of a retaining member to form a plurality of pairs to adjacent droplets thereon and causing droplets in selected pairs to coalesce so as to be released from the member and projected towards a substrate spaced from the member to form a predetermined pattern on the substrate.

18. A method according to claim 17, in which the member is rotated, so that coalesced droplets are released and projected towards a substrate as a result of centripetal acceleration.