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FLEXO Magazine : June 2011
Technologies & Techniques Spitting Spitting, sometimes referred to as “blow-by,” occurs when small globules of ink pass under the doctor blade, resulting in unwanted spots of ink on the print surface. This is caused by poor rheology or flow and is often associated with the pigment load of the ink, the resultant viscosity or the press speed. Solutions include decreasing press speeds, altering doctor blade composition and thickness and changing to a coarser anilox to allow for lower pigment loads. You can also reduce viscosity, but be sure to consult your ink-maker for recom- mended diluents and dosages. pinholing Pinholing is nearly always the result of poor wetting or ink leveling. It often occurs when trapping one UV ink over another or when printing on a polyolefin film. Common solutions include checking plates, adjusting impression, verifying the dyne level of the substrate (greater than 38 dynes is recommended) or using additives prescribed by the ink vendor. Pinholing of flexo inks over rotary screen white inks is a very common problem. Often this is caused by a mismatch of the rotary screen ink and the flexo inks. If possible, use an additive- free (silicone-free) rotary screen white to improve the wetting of the flexo inks. Adjusting the flexo inks with a recom- mended wetting additive is another option. Finally, increasing the travel distance between the print unit and the UV lamp will provide more time for the inks to flow properly and level over the underlying white. CraCking Cracking usually occurs when the substrate is stressed by temperature changes. If the film is subjected to high tempera- tures, the substrate stretches and then shrinks upon cooling. The solutions are to use more heat-resistant or heat-stable substrates or specially formulated inks with increased flexibility. Inadequate curing contributes to most of the problems discussed here. Curing is critical to the performance of UV flexo inks. Also, remember to consult your ink vendor’s techni- cal experts when faced with especially difficult challenges. Although UV inks are not perfect, they do require less mainte- nance than other kinds of ink, making troubleshooting simpler should issues arise. n About the Author: Dean Radford is the UV research and development manager for Environmental Inks, a member of the Siegwerk Group, in Morganton, NC. He has been with Environmental Inks for 27 years and has served as a research and development manager for water-based and UV-curable technologies. www.flexography.org June 2011 FLeXO 75
Sustainable Spring 2011