UV丝网印刷油墨问题与解答（英文）（二）

清泥

7、Why does polycarbonate film become yellowish when I use UV inks?

Polycarbonate is sensitive to UV wavelengths of less than 320 nanometers. The yellowing on the surface is caused by the breakage of the molecular bonds caused by photo-oxidation. UV energy is absorbed by the molecular bonds of the plastic and generates radicals. The radicals react with oxygen in the atmosphere to generate appearance and physical changes within the plastic.

清泥

8、How can the yellowing on polycarbonate’s surface be avoided or even eliminated?
If UV inks are used to print polycarbonate, the yellowing on the material’s surface may be minimized but may not be completely avoided. The use of additive curing unit bulbs, such as iron and gallium, has been proven to minimize this yellowing. These bulbs emit less UV energy in the shorter wavelengths detrimental to polycarbonate. In addition, curing each ink color properly to minimize exposure of the substrate to UV energy and keeping the curing unit well maintained will go a long way in reducing surface degradation of polycarbonate materials.

清泥

9、Are UV inks considered green, and if so, why?
UV inks are green compared to solvent-based inks. UV-curable inks are consider-ed to be 100% solids, meaning that just about everything in the ink is reacted into the final ink film. Solvent-based inks evaporate solvents into the atmosphere as the ink dries. Solvents are considered volatile organic compounds and not environmentally friendly.

10、What is the unit of measure for the density numbers a densitometer provides?
Optical density numbers are unit-less. Densitometers measure the amount of light reflected or transmitted by a printed substrate (Figure 2). An electronic photocell eye is connected to a meter that mathematically converts percentage of reflected or transmitted light into logarithmic density values. The use of logarithmic scale compresses large measurement numbers into small and manageable values. The basic mathematical formulas are as follows.

清泥

11、What factors affect density?
In screen printing, variables that can greatly affect density values are ink-film thickness (determined by mesh, tension, off-contact, and floodbar and squeegee settings), color, size and number of pigment particles in the ink, and the color of the substrate. Optical density combines the opacity and thickness of the ink film. Opacity is affected by the number and size of the pigment particles and their light-absorbing and light-scattering characteristics.

清泥

12、What is dyne level?
Dyne per centimeter is a unit of mea-surement used to quantify the amount of force created on a surface. The force is caused by the intermolecular attraction of a particular liquid (surface tension) or solid (surface energy). For the sake of practicality, this measurement is referred to as the dyne level.
The dyne level or surface energy of a particular substrate is an indication of the material’s wettability and ink adhesion . Surface energy is a physical property of a substance. Many films and other substrates used in the printing industry have very low dyne levels, such as polyethylene with 31 dynes/cm and polypropylene with 29 dynes/cm, and need to be treated.
Treatment raises the dyne level of a particular film, but only temporarily. So factors such as the amount and time since initial treatment, handling, storage, humidity, dust, and contamination will affect the dyne level when you’re ready to print. Since dyne levels change with time, most printers find it necessary to treat or retreat these films immediately before printing.