LASER PRINTING ON CURVED SURFACES

Abstract

The invention relates to a process for printing a substrate (7) containing curved surface sections by using an ink printing assembly with a movable print head (8) comprising an ink carrier (1) having an ink layer, the ink layer being irradiated regionally in such a way that heat bulges are formed in the ink layer which cause the splitting of ink droplets so that the ink printing assembly is working as nozzle-less droplet ejector for ejecting droplets of ink from the ink layer, where the distance between the print head (8) and the curved sections of the substrate (7) is adjusted by moving the print head relative to the substrate by providing the print head with three degrees of freedom in translation, allowing horizontal (Tx), vertical (Tg) and in depth (Tz) translations.

Claims

1. A process for printing a substrate containing curved surface sections by using an ink printing assembly comprising a movable print head comprising an ink carrier having an ink layer, the process comprising: regionally irradiating the ink layer in such a way that heat bulges are formed in the ink layer which cause the splitting of ink droplets so that the ink printing assembly is working as a nozzleless droplet ejector for ejecting droplets of ink from the ink layer, wherein the distance between the movable print head and the curved surface sections of the substrate is adjusted by moving the print head relative to the substrate by providing the print head with three degrees of freedom in translation, allowing horizontal (Tx), vertical (Tg) and in depth (Tz) translations.

2. The process according to claim 1, wherein the ink layer is irradiated by means of a laser.

3. The process according to claim 1, wherein the ink carrier and the ink layer are moved parallel to one another.

4. The process according to claim 1, wherein the movable print head is additionally provided with two degrees of freedom in rotation, which supports and ensures the orientation of the print head by allowing rotations (Rx, Ry) thereof along two perpendicular axes.

5. The process according to claim 2, wherein the laser comprises a switched laser designed as a laser working with a single light wavelength.

6. The process according to claim 2, wherein the ink layer being in contact with the ink carrier is generated with a variable thickness so that the current amount of the ejected ink is adjustable.

7. The process according to claim 2, wherein the current amount of the ejected ink is adjustable by variation of the intensity of the irradiation.

8. The process according to claim 1, wherein the ink layer comprises one or more of absorbing particles and reflective particles, and a soluble polymer having a weight average (Mw) molecular weight of greater than 250 000 g/mol, where the weight average (Mw) of the molecular weight of the soluble polymer is determined according to DIN 55672-2: 2016-3.

9. The process according to claim 8, wherein the soluble polymer has a weight average (Mw) molecular weight of 250 000 g/mol to 2 500 000 g/mol.

10. The process according to claim 8, wherein the proportion of the soluble polymer accounts for between 0.05 to 2 weight %, of the total ink mixture.

11. The process according to claim 8, wherein the absorbing particles comprise carbon black.

12. The process according to claim 1, further comprising drying or thermally curing the ink after printing and/or applying two or more ink layers one above another.

13. (canceled)

14. (canceled)

15. A printing apparatus containing a nozzleless droplet ejector, a movable print head comprising an ink carrier, and an apparatus for moving the movable print head to provide three degrees of freedom in translation, allowing horizontal (Tx), vertical (Tg) and in depth (Tz) translations, the printing apparatus being configured for executing a process for printing a substrate containing curved surface sections, the process comprising: regionally irradiating an ink layer on the ink carrier in such a way that heat bulges are formed in the ink layer which cause the splitting of ink droplets so that the nozzleless droplet ejector ejects droplets of ink from the ink layer, wherein the distance between the movable print head and the curved surface sections of the substrate is adjusted by moving the print head relative to the substrate by providing the print head with the three degrees of freedom in translation.

16. The printing apparatus according to claim 15, wherein the apparatus for moving the print head is robot having an arm connected with the movable print head.

17. The printing apparatus according to claim 15, wherein the apparatus for moving the print head additionally provides two degrees of freedom in rotation, which supports and ensures the orientation of the movable print head by allowing rotations (Rx, Ry) thereof along two perpendicular axes.

18. The process according to claim 1, wherein the ink layer is irradiated by means of a switched laser.

19. The process according to claim 2, wherein the current amount of the ejected ink is adjustable by variation of the laser power.

20. The process according to claim 8, wherein the absorbing particles consist of carbon black.

Description

[0074] The drawing shows in

[0075] FIG. 1 a schematic cross section through the printing head,

[0076] in FIG. 2 a schematic illustration of the spatial controlled printing head and

[0077] in FIG. 3 a schematic illustration of printing on a curved substrate.

[0078] In the print system according to the drawing typically the following components are relevant:

[0079] Ink ribbon, ink, energy beam projector, energy beam, inking unit, writing line, 3D surface, print head and printed ink.

[0080] An ink carrier in cylindrical form (1) is completely and seamlessly coated by a specially designed inking unit (5) with the ink (2) to be printed. An energy beam system (3) located in the ink carrier (1) addresses an energy beam (4) in such a way that the energy beam (4) is able to address a closed line (6). Information is printed in such a way that the energy beam (4) is switched on or off simultaneously with the information to be printed, while the energy beam is addressed on the writing line. One or more energy beams (4) can be used for this purpose. The energy beam (4) can be continuously moved (scanned) across the writing line (6) or by using an array the writing line (6) can also be completely addressed in one step and written by the energy beam (4).

[0081] The inking unit (5) is thereby able to replace the used ink (2) on the ink carrier (1).

[0082] Printing process over a three-dimensional surface:

[0083] The print head (8) is moved over a three-dimensional surface (7) in such a way that the total distance between the writing line (6) and the 3d surface (7) is as small as possible but there is no contact between print head (8) and 3d surface (7). The print head (8) is then moved along an axis over the 3d surface (7). Since the total condition can always change during the movement in one axis, the print head (8) must always be tracked by possible movements in all three spatial axes X,Y,Z and possibly by rotation on the spatial axes X,Y,Z.

[0084] Nevertheless, the print head (8) can only be optimally adjusted approximately to a deformed 3d surface (7) in this way. Thus, depending on the radius of curvature of the 3d surface (7), the conditions for transferring a homogeneous colour film will always change. To compensate for this, the print head (8) is additionally able to transfer different quantities of ink by changing the intensity of the energy beam (4) along the writing line (6). The transfer of different amounts of ink can also be achieved by directly inking the ink carrier (1) to varying degrees, so that an ink film gradient is created on the surface of the ink carrier (1).

[0085] The present invention is additionally illustrated by the following printing example:

[0086] A typical formulation for printing according to the present invention is as follows:

about 0.25% high molecular Ethylcellulose
about 3% Polyvinylbutyral (PVB)
about 6% Carbon Black
about 4% Dispersing Additive (e.g. DisperBYK 102)
about 87% Solvent (e.g. Methoxypropanol)

[0087] This mixture is then used to coat the print head with a 30-40 μm thick film. The print head is then moved to the substrate at different distances and the laser prints the ink. It is important here to reduce the number of splashes by adding, for example.

[0088] Result concerning satellite generating (wet coating thickness is about 30 μm):

TABLE-US-00001 Ethyl- cellulose content Distance 1 mm 2 mm 3 mm 4 mm 5 mm 0.10% 123 145 163 178 201 Number of 0.25% 34 39 42 53 63 spatters; 0.50% 14 21 27 32 41 regarded 0.75% 9 14 19 24 29 reference   1% 4 7 — — — area 0.1 cm.sup.2