In the previous article, we talked about 3 reasons why ink drying is abnormal in UV printing. Now, let's move on to the other 3 reasons.

4. Different colors of UV ink have different drying degrees

Experiments show that different colors of UV ink have different absorption of the spectrum. So under the same power of UV light, different colors of UV ink have different drying degrees.

For example, white ink can directly reflect light, black ink will form diffuse reflection of light. Magenta ink has a better absorption effect on light.

For four primary color inks, the difficulty of drying is black, cyan, yellow and magenta from difficult to easy. While gold and silver inks are more difficult to dry than those four inks. What about white ink? It is also difficult to dry due to its reflection effect on light.

Solution

The printing color sequence should be arranged reasonably according to the drying features of different inks. We first print the inks that are more difficult to dry, and then print the inks that are easier to dry. This can solve the problem of poor ink drying to a certain extent.

5. The density of rubber roller is too low

New equipment is more prone to poor ink drying in production. After inspection, it was found that the low-density rubber roller absorbed the photoinitiator in UV ink, resulting in poor ink drying.

The drying of UV ink is mainly achieved by the chemical reaction between photoinitiator and resin. The molecular structure of resin is larger than that of photoinitiator. If the density of rubber roller is too low, many small holes will appear on its surface.

Since the diameter of these small holes is larger than the diameter of the photoinitiator, part of the photoinitiator in UV ink will be absorbed by rubber roller, resulting in insufficient chemical reaction between resin and photoinitiator during the drying process, resulting in poor drying, accompanied by ink drying on the surface of rubber roller.

Solution

The simplest way is to replace the rubber roller. If there is no time to replace the rubber roller, you can remove it, and soak it in UV ink oil for 12 to 24 hours.

The rubber roller will gradually absorb the UV ink oil, and when it reaches saturation, it will be difficult to absorb the photoinitiator in UV ink.

6. Ink dries too fast

When monochrome offset press is used for printing, the imprint of the previous color often dries too fast, forming 'crystallization'. It prevents the next color from being overprinted.

The reason for this is that too much drying oil is added, or the semi-finished products are stacked for too long. This often occurs when printing yellow screen or screens with light background colors.

Solution

If there are not many products that cannot be overprinted, you can use a clean soft cloth dipped in magnesium carbonate powder to gently wipe the surface of each product to roughen surface. Use infrared irradiation to make ink layer expand and become coarser due to heat.

If there are too many products that cannot be overprinted, you can perform humidity adjustment treatment to make ink layer absorb moisture and expand, roughening the surface.

After the surface becomes rough, the ink of next color will be easier to adhere to. To prevent the issue of not being printed, you can add an appropriate amount of non-drying wax auxiliary agent to the yellow ink in advance to prevent the ink layer from crystallizing.

When printing screen with a light background color, you can mix Willy oil and diluent, which can also prevent the next color from not being printed.