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
One theory regarding over curing is that the ink’s surface becomes harder and harder (more cross-linked) with subsequent exposure to UV energy. The surface becomes so hard that when another layer of ink is printed and cured over the previous layer, the top layer would experience poor adhesion to the bottom layer. Another theory is the possibility of photo-oxidation on the ink’s surface. Photo-oxidation occurs when UV radiation, in the presence of oxygen, breaks down the chemical bonds on the surface of the ink film. If the molecular bonds on the surface are degraded or broken, adhesion of another layer of ink would be compromised. Over cured ink film may become less flexible and substrate embrittlement also may occur.
Why do some UV inks cure faster than others?
UV inks are formulated to adhere to specific substrates and to meet other specifications required by the application. In UV chemistry, the faster curing an ink is, the less flexible it is once cured. Imagine the ink molecules connecting and forming chains with each other (cross linking) during the curing process until all the molecules are used up. Now imagine these chains of ink molecules with many, many branches, but these branch-es are fairly short. This type of ink would be faster curing but not very flexible. Now imagine a very, very long chain of molecules with very few or no branches. This type of ink would be slower curing but flexible.
Most UV chemistry is optimized for certain applications. For example, an ink that is formulated for membrane-switch overlays would need to adhere to the various substrates used for membrane overlays, including polycarbonate, primed polyester, and sometimes vinyl. In addition, the cured ink film must be compatible with laminating adhesives and be flexible enough to die cut and emboss. Also, the chemistry must such that the ink will not react with the substrate’s surface, which could lead to cracking, shattering, or delamination. Such inks are typically slower curing. In contrast, an ink that is formulated for cardstock or rigid plastics for point-of-purchase displays must adhere to these substrates, but may not need to be very flexible. Inks for these applications are generally faster curing.
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