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April 20, 2007
Printing and Diameter Variations
Another recent Q and A:
In running a multi-color rotogravure press, if we have different diameters in our impression roller in different printing units, maybe having variation of maximum up to 3mm in the circumference, because some of the impression roller have been ground once after too much wear.
1) Does it effect the tension in the web?
2) Will it lead to registration problems?
Answer:
My expectation is that the impression roller diameter variations will cause some tension variations, but not necessarily cause registration problems. The answer to why I think this is somewhat complicated, but here's my thought process, in brief.
Multi-color presses typically run in draw control with either a mechanical or electronic drive shaft to synchronize the rotation of the variation printed patterns. In draw control, the tension is determined by the incoming tension (from the unwind) and the speed ratios of the other drive points. Impression roller circumference or diameter variations will directly create speed variations. A 3mm circumference variation on a 10" diameter roller (250mm) would have a speed variation of 0.4 percent, which is a large amount for most papers and films and would create a tension variations.
However, in rotogravure, the inking nip is not necessarily a no slip process. The ink-lubricated gravure cylinder is able to slip (on a microscale) relative to the web, but since it is driven with the lineshaft, all the printing pattern stay in synch without any major effect of small variations in tension goes up or down (talking here about tension offsets present in the steady state, not tension oscillations over time).
-tjw
Posted by Tim Walker at April 20, 2007 09:05 AM
