What it is
In conventional offset the ink dries in two ways at once: part of it penetrates into the paper, the rest oxidises in air over a matter of hours. That is why freshly printed sheets are dusted with anti-set-off powder, stacked and left to rest before they can be die-cut, laminated or finished. In UV offset the ink does not dry: it cures. It contains photoinitiators which, struck by ultraviolet light, trigger a reaction that binds the molecules together. The change from liquid to solid happens in fractions of a second, under the lamp, at the end of the printing unit.
The letters LED describe how that light is produced. Classic UV uses mercury vapour lamps: they run hot, they have to be switched on in advance, they draw power even when the press is idle and they generate ozone. LEDs emit over a narrow band, switch on and off instantly, run far cooler and produce no ozone. There are three practical consequences. You can print on heat-sensitive substrates, plastic films included. Electricity consumption falls. And the sheet comes off dry and clean, with no powder, so the next operation can start immediately. Then there is a fourth effect, the one you can see with the naked eye: the colour stays on the surface instead of being absorbed, and on uncoated and through-dyed papers the solids hold up, where conventional ink fades.
How to prepare the file
- The colour profile is chosen on the actual printing condition and on the substrate: coated and uncoated have different gamuts, and the customer's expectation is aligned at the proofing stage, not on press.
- Ask prepress for a proof that simulates the chosen substrate, not a standard proof on glossy paper. This is the step that avoids arguments once the print run is finished.
- On synthetic substrates and films, adhesion has to be checked: it depends on the surface treatment of the material, not on the ink. On a new substrate a trial is run before the print run.
- If the job is destined for food contact, the low-migration version of the ink series is required: LED-UV in itself is not automatically low-migration.
- Varnishes, foils and knock-outs in register are declared in prepress and prepared in the file. They cannot be recovered on press.
What can go wrong
- Curing has to be calibrated. Under-curing leaves reactive residues and odour; over-curing stiffens the ink film and can create adhesion problems with varnishes and foils applied afterwards.
- UV ink costs more than conventional and the consumables are dedicated. On simple double-sided jobs on coated paper the advantage may not repay the cost: it is an assessment made job by job, not a rule.
- A fully cross-linked UV ink film behaves differently from a conventional ink in de-inking processes. If a project makes a strong claim about the recyclability of the paper, the question has to be put to the ink supplier and documented.
Frequently asked questions
- Is LED-UV offset greener?
- On some fronts yes: it uses less energy than mercury lamps, it uses no mercury, it produces no ozone and it does away with anti-set-off powder. On others, such as de-inking, it has to be assessed case by case. It is not a green label to be stuck on the job.
- Can LED-UV print on any substrate?
- Almost. The limit is not heat, it is adhesion: on synthetic materials it depends on the surface treatment. On a new substrate you always trial it first.
- Is the colour the same as conventional offset?
- Not identical. The ink stays on the surface, so on absorbent papers the solids are richer and the gamut is wider. That is why the proof has to be made on the actual substrate.