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
For screen printers, determining production parameters is a must prior to any production run. To test curing effectiveness prior to production, use the production screen, equipment, and ink and start with the lowest possible conveyor speed that the substrate can take, then print and cure one or two good pieces. Set the lamp or lamps at the wattage specified by the ink manufacturer. For slower curing colors, such as black and white, set the lamps on high.
After allowing the print to cool, test for adhesion by using the cross-hatch tape method. If the sample passes, increase the conveyor speed by 10 ft/min, then print, cure and test adhesion again. Keep increasing the conveyor speed at 10 ft/min increments until the adhesion begins to fail. This normally would show as the ink coming off around 5-15% of the cross-hatched area. This is the marginal failure point of the ink film at the particular conveyor speed and lamp setting you used. To set production speed, decrease the conveyor speed 20-30% from the marginal failure point, depending on the ink system, or follow the recommendations of your ink supplier.
Should I be concerned about over curing if the colors do not overlap?
Over curing happens when the surface of an ink film is over exposed to UV energy. The surface becomes harder and harder with continued exposure and, depending on the ink system, this hardening may happen after just one pass or several passes under the curing unit. Over curing is not so much of a concern as long as printed colors do not overlap.
However, a major factor to take in-to consideration is the film or substrate being printed. UV energy affects most surfaces and certain plastics are sensitive to specific wavelengths of UV energy. This sensitivity to certain wavelengths in combination with oxygen in the air can cause degradation on the plastic’s surface. The molecular bonds on the surface may be broken, leading to adhesion failure when UV ink is printed and cured on the material. This degradation of the surface normally happens gradually and is directly related to the amount of UV energy the surface receives. The degradation on the substrate’s surface may reach a point at which ink adhesion is compromised.
What happens when the ink is over cured?
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