Oh Say Can UV?

UV technology gives in-plant print shops an opportunity to offer something different in the form of high-quality coatings, special effects, processing of unusual printing stocks and rapid delivery of express orders. UV coatings, for example, have a number of specific advantages, among them: high gloss levels, excellent protective qualities and a pleasant tactile experience. Sheets are dry and ready for finishing as soon as they arrive in the delivery, with no powdering required.

While conventional offset inks dry or cure through their binding agents oxidizing and being absorbed into the printing stock, in UV printing, specially formulated inks are exposed to ultra-violet (UV) radiation, which causes them to harden instantly on top of the substrate. UV coating produces high levels of gloss or dull coating, vivid color and vibrant detail with superior rub resistance and no post-cure dryback—even on soft, uncoated sheets.

On UV-equipped Heidelberg presses, the delivery typically accommodates a three-lamp UV end-of-press dryer, a cold-air cassette, one DryStar combination IR/hot air dryer and one hot air dryer for optimum flow of the coating and maximum gloss. UV dryer cassettes are placed between the printing units to cure the UV ink before the UV coating is applied.

Even without coating, UV inks cure instantly, greatly simplifying printing on nonabsorbent materials such as plastic films or metalized papers. Products can be finished more rapidly, enabling higher throughput and turnaround, even on two-sided jobs. In-line UV printing is notable for the superior results it achieves on difficult substrates, from uncoated paper and board to foil and especially plastic, including synthetic papers, static cling vinyl and lenticular. UV inks also perform exceptionally well on recycled substrates.

Finally, unlike conventional offset inks, energy-curable UV inks are solvent-free and dry via exposure to high-energy electrical power, a process that generates no byproducts and is free of emissions.

That said, UV printing is not for the faint of heart. An emphasis on cost-containment in many in-plant settings typically requires an ROI analysis to determine the practical wisdom of moving to UV. For starters, a correctly equipped UV press usually costs more than a conventional press. Inks, coatings and chemistry can be more expensive. UV blankets cost about the same as conventional, although they tend to be replaced more frequently. Plastic substrates often cost more than $2 per sheet.

Factors to Consider

There are a number of special considerations involved in the decision to go UV. Good communication between the prepress department and the pressroom is essential. Screening curves must be adjusted to account for the additional dot gain that occurs with UV inks, and jobs also must be proofed on the actual substrates on which they will be printed. In addition, UV presses typically operate within much tighter tolerances with respect to ink-water balance and exposure to UV radiation. Press blankets can become embossed. Roller settings must be precise.

Heat—a byproduct of exposing ink, coating and substrates to ultraviolet energy—needs to be removed efficiently from the press. Too-high temperatures will produce distortion in paper and plastic that can cause a job to go off-register. Under cure a job and it won't dry properly. Over cure it and the substrate becomes brittle. Some plastic substrates can discolor when exposed to excessive heat. If the surface tension of a plastic substrate is lower than the surface tension of the ink, the ink won't stick. Printers should work with their ink and coating suppliers to ensure their post press equipment can handle these substrates.

Because UV lamps require additional electricity, it is critical for printers to make sure they can supply enough power to run them. Also, ensure that existing floor pads can bear the weight of these long, heavy presses. Correct planning of the installation is vital to ensure optimum functioning of the complete process. IPG

Errol Moebius is a sheetfed product manager at 
Heidelberg USA.