WET-TRAPPING OF ENERGY CURABLE FLEXOGRAPHIC INKS AND COATINGS

• US 20120304879A1
• Filed: 01/24/2011
• Published: 12/06/2012
• Est. Priority Date: 01/22/2010
• Status: Active Grant

• Current Assignee: Sun Chemical Corporation
• Original Assignee: Sun Chemical Corporation
• Inventors: Tiessen, John Gerard, Best, Terry, Parris, Juanita, Liu, Xiang Jun
• Granted Patent: US 9,365,064 B2
• Time in Patent Office: Days
• US Class Current: 101/211
• CPC Class Codes: B41M 1/06 Lithographic printing
• CPC Class Codes: B41M 1/18 Printing one ink over another
• CPC Class Codes: B41M 7/0072 using mechanical wave energy, e.g. ultrasonics; using magnetic or electric fields, e.g. electric discharge, plasma
• CPC Class Codes: C09D 11/101 Inks specially adapted for printing processes involving curing by wave energy or particle radiation, e.g. with UV-curing following the printing

Abstract

Methods and apparatus for flexographic color printing, and more particularly for implementing “wet trapping” in flexographic printing using energy curable flexographic liquid inks are presented. Radiation curable inks can be formulated that wet trap over each other without back-trapping onto the plates and anilox units of down-line printing units by controlling the storage modulus of the inklcoating. The storage modulus G′ in the inks can be controlled to ensure that the highest G′ ink is printed ISt down with successive colors being trapped in the order of their (declining) G′ values. An image in wet ink can thus be trapped over a different image in wet ink of a different color without picking the ISt ink back up and redepositing it onto the subsequent plates and rollers in the printing press (back-trapping).

Claims

1. A method for wet trapping energy curable inks on press, comprising:

providing a set of energy curable inks to be successively printed on a printing press;

calculating the storage modulus G′

for each ink in the set as a function of strain over a range of strains; and

ordering the printing of the inks so that the value of G′

for each successive ink laid down is lower than its immediately previously printed ink by at least a defined drop D over a defined range R of strain values.

2. The method of claim 1, wherein D is selected from 1.0 Pascal and 0.9 Pascal.

3. The method of claim 1, wherein R is selected from the following:

from 20% through 50% strain, from 10% through 65% strain, and from 10% through 120% strain.

4. The method of claim 1, wherein the storage modulus of the ink G′

is given by

G′

={σ

_o cos(δ

)}/∈

_o

Where;

σ

_o=amplitude of the stress;

∈

_o=amplitude of the strain; and

δ

=phase angle between them.

5. The method of claim 1, wherein said calculating the storage modulus G′

for each ink in the set includes measuring the rheology of the inks using oscillatory methods.

9. The method of claim 1, further comprising changing the value of G′

for one or more of the inks in said set so as to substantially satisfy the G′

drop criterion over the range of interest R.

10. The method of claim 1, further comprising changing the value of G′

for one or more of the inks in said set so as to change the order of printing of the inks.

11. The method of claim 9, wherein the G′

value of an ink is manipulated to move lower by at least one of the following;

lowering the percentage of pigment in the ink and adding a thickening agent.

12. The method of claim 11, wherein said thickening agent is fumed silica.

13. The method of claim 9, wherein the G′

value of an ink is manipulated to move higher by adding materials that impart pseudo-plastic behavior to the ink.

15. The method of claim 1, wherein D ranges between 1.0 Pascal and 0.0 Pascal at some point(s) across the range of strain from 10% to 65%.

16. The method of claim 1, wherein D is negative at some points across the range of strain from 10% to 65%.

19. The method of claim 9, wherein the G′

value of said one or more the inks is manipulated by using any combination of resins, oligomers, monomers, fillers and additives.

21. The method of claim 19, where the G′

value of the ink is lowered by lowering solids levels in the ink, or using resins that exhibit less cohesive force.

30. The method of claim 1, wherein D varies with each pair of inks in the set.

6. (canceled)

7. (canceled)

8. (canceled)

14. (canceled)

17. (canceled)

18. (canceled)

20. (canceled)

22. (canceled)

23. A method of predicting the ability of two or more inks to successfully wet trap on press, comprising:

printing a first ink with a handproofer over a mask with an aperture having a defined width and a defined length;

leaving the first ink wet on the proofing form;

cleaning the handproofer and applying a second color of ink to it;

printing the second ink directly over the area where the first image had been pulled and extending at least a certain distance D beyond the leading edge of the first image to create a ghost trap region;

passing the handproofer through a UV curing unit; and

evaluating the color density of the first ink in said ghost trap region.

24. The method of claim 23, wherein said aperture is between 8-18 mm in width and between 20-40 mm in length.

25. The method of claim 23, wherein the circumference of the handproofer is equal to the distance D.

26. The method of claim 25, wherein said distance D is 61 mm.

28. The method of claim 23, further comprising:

performing a dry trap control test including each of said wet trapping steps and also including a step of UV curing the first ink after printing with the handproofer before printing the second ink.

29. The method of claim 28, further comprising:

comparing the density of said first down ink in the ghost trap region between the wet trap method and the dry trap control test.

27. (canceled)

31. A method of accurately predicting successful wet trapping behavior of inks, comprising:

calculating the G′

value for each ink; and

ensuring there is a sufficient drop in G′

value between each pair of successive inks to ensure reliably clean trapping.

32. The method of claim 31, wherein a 1.0 Pascal drop at every point along the G′

vs. Strain curve, from a strain value of 10% to a strain value of 65% is a sufficient drop.

33. The method of claim 32, further comprising changing the G′

value of the inks to achieve said sufficient drop in G′

value.

34. The method of claim 33, further comprising at least one of:

adding materials that impart pseudo-plastic behavior to energy curable inks to move G′

values higher,

lowering solids levels in an ink, or using resins that exhibit less cohesive force, to move G′

values of inks lower, and

testing materials or combinations of materials for their effect on G′ and

then adding or removing as needed in order to maintain an optimal spacing of G′

values between successive colors intended to be trapped.