LASER PRINTING PROCESS

Abstract

Printing process in which a substrate to be printed is disposed opposite an ink carrier having an ink layer, the ink layer being irradiated regionally by a laser beam, said layer accelerating by absorption of the laser beam in the substrate direction, wherein for laser absorption the ink layer is admixed with reflective particles and as soluble polymer having a weight average (Mw) molecular weight of greater than 250 000 g/mol.

Claims

1. A printing process comprising: disposing a substrate to be printed opposite an ink carrier including an ink layer, regionally irradiating the ink layer with a laser beam, the ink layer being accelerated in a direction of the substrate by absorption of the laser beam wherein the ink layer comprises an ink, the ink comprises admixed reflective particles for laser absorption, a solvent, and an additive including a soluble polymer having a weight average (Mw) molecular weight of greater than 250 000 g/mol, the weight average molecular weight being determined according to DIN 55672-2: 2016-3.

2. The printing process according to claim 1, wherein the weight average molecular weight of the soluble polymer ranges from 250 000 g/mol to 2 500 000 g/mol.

3. The printing process according to claim 1, wherein the weight average molecular weight of the soluble polymer ranges from 250 000 g/mol to 1 500 000 g/mol.

4. The printing process according to claim 1, that wherein the ink includes the soluble polymer in an amount between 0.05 to 2 weight %, of the total weight of the ink.

5. The printing process according to claim 1, wherein the ink includes the soluble polymer in an amount between 0.05 to 1 weight %, of the total weight of the ink.

6. The printing process according to claim 1, wherein the ink includes proportion of the soluble polymer in an amount between 0.1 and 0.8 weight %, of the total weight of the ink.

7. The printing process according to claim 1, wherein the soluble polymer comprises a cellulose ester.

8. The printing process according to claim 1, wherein the reflective particles include one or more of metal and a metal-coated carrier material.

9. The printing process according to claim 1, wherein reflective particles have a mean particle size of 0.1 to 10 m.

10. The printing process according to claim 1, wherein the reflective particles: have an L* value in the Lia *b* colour space of more than 50, have an a* and/or b* value in the L*a*b* colour space of less than +/5.

11. (canceled)

12. The printing process according to claim 1, wherein the soluble polymer comprises a cellulose nitrate.

13. The printing process according to claim 1, wherein the soluble polymer comprises a cellulose ether.

14. The printing process according to claim 1, wherein the soluble polymer comprises a hydroxypropylcellulose.

15. The printing process according to claim 1, wherein the soluble polymer comprises a polyurethane.

16. The printing process according to claim 1, wherein the soluble polymer comprises a vinyl polymer.

17. The printing process according to claim 10, wherein the L* value is more than 70.

18. The printing process according to claim 10, herein the L* value is more than 80.

19. The printing process according to claim 10, wherein the a* and/or b* value is less than +/3.

20. The printing process according to claim 19, wherein the L* value is more than 70.

21. The printing process according to claim 19, wherein the L* value is more than 80.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The subject matter of the invention is to be elucidated in more detail below, referring to the drawing of FIG. 1.

[0037] FIG. 1 is a schematic view of a printing machine used for the process of the invention.

DETAILED DESCRIPTION OF THE DRAWING

[0038] FIG. 1 is a schematic view of one exemplary embodiment of a printing machine (1) of the invention.

[0039] The printing machine (1) comprises as ink carrier (4) a circulating ink ribbon.

[0040] The ink ribbon is coated homogeneously and over its full area with ink (2) by the inking unit (8). The ink ribbon subsequently moves in the arrow direction to the printing nip (10). The ink carrier (4) is distanced by a gap from the substrate (6) to be printed. Preferably the width of the gap is adjustable and/or is regulated continuously. This can be done by means, for example, of adaptable distancing rolls (5).

[0041] In the printing nip (10), using a laser scanner (11), a laser beam (3) is focused through the ink carrier (4), which is permeable to the laser light, into the ink (2). The local and targeted heating of parts of the ink (2) by means of the laser beam (3) causes explosive vaporization of a small region of the ink (2), and so a part of the printing ink (2) is transferred from the ink ribbon onto the opposite substrate (6).

[0042] The ink ribbon, controlled by the distancing rolls (5) and the deflection rollers (7), subsequently moves back in the direction of the inking unit (8). On contact between inking unit (8) and the ink ribbon, the ink (2) consumed is replenished.

[0043] The excess ink (2) in the inking unit (8) is collected in the ink trough (9) at the bottom and is added continuously in repetition to the printing operation.

[0044] It is possible to use reflective materials composed of metal and metal-coated polymeric particles.

[0045] A further aspect for improving print quality involves modifying the rheology of the ink to be printed in such a way that disruptive splashes do not form at all or only to a greatly reduced extent. It has been found that the low-level admixing of soluble polymers in the average molecular weight range from about Mw: 250 000 g/mol to about 1 500 000 g/mol has a positive influence on the print behaviour of the ink.

[0046] These admixtures modify what is called the elasticity of the ink. Admixtures of soluble polymers around the lower Mw range (Mw: 10 000 g/mol to approximately 100 000 g/mol) have only a thickening effect and only slight anti-splash properties. Polymers with higher Mw values (>1 500 000 g/mol) lead in contrast to no further improvement in the anti-splash properties, but merely further hinder the solubility.

[0047] Preference is therefore given to using a polymer having a molecular weight (Mw) below 2 500 000 g/mol, more preferably below 1 500 000 g/mol.

[0048] The following listing shows examples of suitable soluble polymer admixtures with various solvents customary within the printing industry, and the amounts typically used in the total ink mixture (in weight %):

TABLE-US-00001 Cellulose Cellulose Cellulose Vinyl Solvent ester nitrate ether Polyurethane polymer Glycol ether Ethoxypropanol 0.1-0.2% 0.1-0.2% 0.1-0.2% 0.05-0.15% 0.2-0.3% Methoxypropanol 0.1-0.2% 0.1-0.2% 0.1-0.2% 0.05-0.15% 0.2-0.3% Alcohol Ethanol 0.2-0.5% 0.2-0.5% 0.2-0.5% 0.1-0.3% 0.15-0.35% Propanol 0.2-0.5% 0.2-0.5% 0.2-0.5% 0.1-0.3% 0.15-0.35% Ester Ethyl acetate 0.2-0.8% 0.2-0.8% 0.2-0.8% 0.2-0.8% 0.2-0.8% Butyl acetate 0.2-0.8% 0.2-0.8% 0.2-0.8% 0.2-0.8% 0.2-0.8% arom. hydrocarbon Toluene 0.1-0.4% 0.1-0.4% 0.1-0.4% Xylene 0.1-0.4% 0.1-0.4% 0.1-0.4% Dialkyl ether Di-n-butyl 0.25-0.5% 0.25-0.5% 0.25-0.5% 0.25-0.5% 0.25-0.5% ether Glycol ester 2-Methoxyethyl 0.1-0.4% 0.1-0.4% 0.1-0.4% 0.1-0.4% 0.1-0.4% acetate Glycol ether Butyl glycol 0.2-0.6% 0.2-0.6% 0.2-0.6% 0.2-0.6% 0.2-0.6%

[0049] By means of the invention it has been possible to reduce the risk of formation of satellites which improves the quality of the printing result. The influence of the molecular weight of the used soluble polymer is additionally shown below by the anti-spray effect of polymer solutions in the above-mentioned printing process:

[0050] Various ink mixtures were printed according to the above-mentioned process. Here it could be observed that in pure printing without additives the printed dot shows a high number of scattered splashes.

[0051] This number of splashes could be significantly reduced by adding a small amount of high molecular soluble additives to the ink.

[0052] In this connection the additives from the chemical substance groups cellulose ester, cellulose nitrate, cellulose ether, polyurethane and also vinyl polymers with the suitable molecular weights of >250,000 Mw with a total proportion between 0.05 and 2% have proved to be excellent anti-spray additives.

[0053] The following tabular evaluation (total proportion in weight %; 3-Ethoxy-1-propanol is used as solvent, use of reflective aluminum particles, the scattered splashes were counted under microscope) provides corresponding experimental examples:

TABLE-US-00002 Cellulose total scattered ether Mw proportion splashes % No polymer 100% (comparative example) 100.000 1% ca. 90% (comparative example) 300.000 1% ca. 70% 800.000 1% ca. 10% 1.200.000 1% ca. 10%

LIST OF REFERENCE NUMERALS RELATING TO FIG. 1

[0054] 1. Printing machine [0055] 2. Ink [0056] 3. Laser beam [0057] 4. Ink carrier [0058] 5. Distancing roll [0059] 6. Substrate [0060] 7. Deflection roller [0061] 8. Inking unit [0062] 9. Ink trough [0063] 10. Printing nip [0064] 11. Laser scanner