You’ll wonder where the yellow went Rod Urquhart says when printing heavy solid overprints use UCR to avoid setoff problems
By Rod Urquhart
In keeping with my original undertaking to occasionally discuss an industry challenge (read problem) that has passed through the National Printing Laboratory, this month, with acknowledgement to a famous brand of toothpaste, a word about the apparently lost art of UCR.
Recently, the NPL received a series of prints from a local printer with an accompanying complaint about setoff.
As the job was printed in ultra violet cured ink, and initially appeared to be a single colour print, early thoughts turned to lamp failure or old lamp tubes. However, the print seemed to be well and truly dry, and the printer was most adamant that the lamps on the press were in good condition.
[A slight diversion at this point. All UV lamps should be clocked for hours of use, and after 700 hours, the output should be tested with either a calibrated wattmeter, (which is mechanically very difficult due to the safety shielding of most presses), or with a simple visual exposure test such as "Detex" green tape. In the case of the latter, a sample of tape should be stuck onto the sheet or web and exposed at slow, medium, and fast press speeds when the press lamps are new, or when new tubes are installed in the lamp housings. The tape turns from green to burgundy as the amount of UV increases, and an appropriate amount of UV to cure most colours at commercial speeds should turn the tape at least a brown colour. The exposed sample must then be stored in an envelope out of light as a colour standard, and used to compare the UV output as the lamps age. The colour will initially look like the "slow" sample, but progressively the colour will become lighter, more like the "fast’" sample. When green tape stays green, the lamps are overdue to be changed!]
But back to the story.
A closer examination of the problem print showed that the job was not simply a black solid and half tone design, but actually a black composed of four-colour process!
This discovery started a completely different train of thought, as when we checked with the printer, he advised us that the job was in some parts over 320 per cent ink coverage, with black last down.
We asked the obvious one word question, which was of course, "why?" The reply was most enlightening, "because that’s how we received the separations from the agency".
So the real answer to the setoff problem was that the first and probably second down ink layers were not cured because the UV light could not penetrate through the many layers of ink to reach them. This gave rise to a situation where the "foundations" were wet, whilst the top seemed to be dry. When pressure built up as the stack grew, the still wet under layers allowed the ink film to split and give rise to the transfer setoff observed.
[Another digression: ultra violet light has high energy, but very poor penetrating power.
A two or three micron ink film is about the limit that UV can penetrate, and much of the UV energy dissipates very quickly (logarithmically), as the distance into the film increases.
Especially with black, the absorption of UV light is even more extreme. The colour black is by definition one that absorbs all electromagnetic energy, and when printed last wet on wet, black can absorb all the available light and leave any underlying colour(s) without any energy to start them curing.]
I wondered whether in these days of off-the-shelf computer colour separation programs, the ability of these programs to allow under colour removal (UCR) or grey colour replacement (GCR) which is essentially to replace colours with a proportionately greater density of black, had disappeared. So off I went in search of enlightenment.
I met with John Della and his expert staff at PageSet Digital Pre-Press, (a close neighbour to the Monash University campus where the NPL is situated), and asked the question of them.
"UCR is alive and well" they told me, "and because we understand the print process well, we always do it in appropriate circumstances. However, the UCR facility must be selected from the appropriate menu in all current programs before it can be implemented, it is not a default."
From this reply it would seem that the problem stems from so many people preparing DIY colour separations, not all are aware of the practical printing difficulties in such areas as - registration, trapping, conventional ink setting or UV drying, and in general press operation like litho water balance - that can occur when the UCR option is not used and colours are piled on top of one another.
So here’s a big please to all producers of colour separations. When a heavy solid overprint appears, choose the UCR option, know where the yellow, magenta, and cyan went, and in the process get a much better print with more certain setting or drying, so that printers won’t have to bring to the NPL to find out what went wrong!
Rod Urquhart works at Monash University where many aspects of the industry are being studied. He is based in Australia.
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