Trying to adjust to the realities of working with UV screen-printing inks? Use this Q&A discussion to clear up any misunderstanding about the inks, the curing process, and other aspects of UV technology.
By Bea Purcell
Most inks are thixotropic. This means that the viscosity changes with applied shear (stirring, floodbar and squeegee action), time, and temperature. Viscosity also decreases with increase in the rate of shear (e.g., speed of floodbar or squeegee). When the temperature gets warmer, the viscosity becomes lower as well. Screen printing inks are formulated to print well on press. However, depending on the press set up and prepress modifications to the ink, printability issues may occur. The viscosity of the ink on press will not be the same as it was in the container.
Ink manufacturers determine a specific range of acceptable viscosities for a particular product. Viscosity measurement is controlled within a specific range of shear force, time, and temperature. With thinner or low-viscosity inks, thickening agents may be added. For thicker or high-viscosity inks, thinners may be added. Contact your ink supplier for specific products and processing information.
What affects the stability or shelf life of UV inks?
One factor that influences stability is ink storage. UV inks normally are packaged in plastic containers instead of metal because plastic containers are permeable to oxygen. Also, an air gap is desirable between the surface of the ink and the container lid. This air gap—specifically, the oxygen in the air—helps in minimizing any premature cross linking within the ink. In addition to the packaging, the temperature at which ink containers are stored plays a big part in maintaining their stability. High temperatures will cause the ink to react and cross link prematurely.
Modifications made to the original ink formula may also affect shelf stability if the remaining ink is stored. Additives, especially catalysts and photoinitiators, may lead to more limited shelf life. Finally, exposure to stray UV light from windows and lighting when a container is left open can affect stability.
What is the difference between in-mold labeling (IML) and in-mold decorating (IMD)?
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