Defining the optimum ink deposit for UV starts before the ink is applied-Part III

You will find many technical pointers including how to set proper screen tensions, as well as key information regarding curing and adhesion.

Press Set-up
If screens are processed properly for UV printing, many of the potential problems are already under control. We are often asked what are the optimal screen tensions for printing UV ink. As always, consistency is the key. It is preferable to have tensions of at least 20 newtons, however this will depend on frame choice, mesh count, stretching device, etc. Consistency at a level of 20 newtons is much more important than inconsistency in the 30+ range. If consistency at high tensions can be achieved, this will help minimize off-contact and will assist in keeping ink deposits optimal, not heavy. Flood stroke should be consistent and tight to the screen. When the press is not actually printing, keep the screen covered to reduce exposure to light sources, since over the course of a long production run ink can begin to cure in the screen.
Exposing UV inks to light, especially daylight, is a potentially expensive proposition because you could lose the ink in the screen as well as the screen itself. A few minutes of exposure is all it takes for a wet UV ink to start cross linking in the screen, blocking open areas and increasing ink deposit due to build up on the squeegee side of the screen. Ideally, UV filters should be installed above the press to minimize UV exposure to the ink in the screen. Finally, good housekeeping is essential when printing UV ink. Keep your press and the area around it clean and free of foreign particles that can easily clog a fine mesh screen and lead to pinholes in the print itself.

Substrates - Test, test, test
The slightest variations in substrate stock can greatly affect the printability and adhesion of UV inks. It is always recommended to test the substrate prior to full production. Test under the exact conditions in which the full production run will be produced. It is extremely critical when printing with coated plastics to test the number of colors that will actually be printed. Sometimes top coatings will deteriorate under exposure to UV energy. Under certain conditions, this could be the reason that the first color printed will adhere extremely well, while the fourth, fifth or sixth color printed will not. If printability is in question, make the same print on a stock that you are certain is compatible with the ink. If the ink prints fine, it may be that your substrate/ink choice is not compatible. Determining proper cure of a UV ink is critical to adhesion. A properly cured UV ink is completely dry to the touch and has a uniform gloss. To test cure, first try the thumb test. Place your thumb on the ink, apply high downward pressure and twist. If you cannot separate the ink and it feels dry, you have acceptable cure. Next, check adhesion using the cross-hatch test. Cut completely through the printed ink several times at opposite angles using a cross-hatch tool or razor blade. Then, using an aggressive adhesive tape, rub it over the cut lines, apply pressure then lift with a rapid motion. If a significant amount of ink comes off during this test, allow some time to pass and check again later to see if post cure has taken place. If the ink still comes off after another cross-hatch test, it is likely that the ink is not going to adhere to that particular substrate.
The American Society for Testing and Materials (ASTM) has an excellent brochure for details concerning this test. The address is 1916 Race Street, Philadelphia, Pa., 19103 or call 215-299-5400.