20 classical problem of UV ink - Part 2

Previous: 20 classical problem of UV ink - Part 1

Let's continue with the previous article and continue to talk about light-curable inks

6. What is the difference between millijoule and milliwatt?

The total energy irradiated to a specific surface in a certain period of time is usually expressed in Joules/cm flat or millijoules/cm2. It is mainly related to the speed of the conveyor belt, the power, quantity, service life, status of the curing lamp, and the shape and condition of the reflector in the curing system.

The power of UV energy or radiant energy irradiated to a specific surface is mainly expressed in watts/square centimeter or milliwatts/square centimeter. The higher the UV energy irradiated to the surface of the substrate, the more energy penetrates into the ink film. Whether it is milliwatts or millijoules, the measurement can only be performed when the wavelength sensitivity of the radiometer meets certain requirements

7. How do we ensure the proper curing of UV inks?

The curing of the ink film when it passes through the curing unit for the first time is very important. Proper curing can minimize the problems of substrate deformation, over-curing, re-wetting and insufficient curing, and can optimize the adhesion between ink and humor or between coating and coating.

The screen printing plant must determine the production parameters before the start of production. In order to test the curing efficiency of UV ink, we can start printing at the lowest speed allowed by the substrate first, and cure the first printed sample. Then, set the power of the curing lamp to the value specified by the ink manufacturer. When dealing with colors that are not easy to cure, such as black and white, we can also increase the parameters of the curing lamp appropriately.

After the printed sheet has cooled, we can use the two-way hatching method to determine the adhesion of the ink film. If the sample can pass the test smoothly, you can increase the conveying speed of the paper by 10 feet per minute, and then print and test until the ink film loses its adhesion to the substrate, and record the conveying belt speed and curing at this time Lamp parameters. Then, according to the characteristics of the ink system or the recommendations of the ink supplier, the speed of the conveyor belt can be reduced by 20-30%.

8. If the colors do not overlap, should I be concerned about over-curing? 

When the surface of an ink film absorbs too much ultraviolet light, the problem of over-curing occurs. If you cannot find a solution to this problem in time, the surface of the ink film will become harder and harder. Of course, as long as we don't perform color overprinting, we don't need to be too concerned about this issue.

However, we need to consider another important factor, which is the printed film or substrate. Ultraviolet light can affect most substrate surfaces and certain plastics that are sensitive to specific wavelengths of ultraviolet light. This sensitivity to specific wavelengths combined with oxygen in the air can cause degradation of the plastic surface. The molecular bonds on the surface of the substrate may be destroyed and cause the adhesion between the UV ink and the substrate to fail. The degradation of the surface function of the substrate is a gradual process, and it is directly related to the ultraviolet light energy it receives.

9. Is UV ink a green ink and why?

Compared with solvent-based inks, UV inks are indeed more environmentally friendly. UV curing ink can become 100% solid, which means that all components in the ink will become the final ink film. The solvent-based ink will discharge the solvent into the atmosphere as the ink film dries. Since solvents are volatile organic compounds, they are harmful to the environment.

10. What is the measurement unit of the density data displayed on the density meter?

The optical density has no unit. A densitometer measures the amount of light reflected or transmitted from a printed surface. The photoelectric eye connected to the densitometer can convert the percentage of reflected or transmitted light into a density value.

11. What factors affect density?

In screen printing, the variables that affect the density value mainly include the thickness of the ink film, the color, the size and number of pigment particles, and the color of the substrate. The optical density is mainly determined by the opacity and thickness of the ink film, and the opacity is affected by the size and number of pigment particles and their light absorption and scattering characteristics.