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Filed under: Coating, Laminating, and Printing --- Tim Walker @ 09:05 AM

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

Filed under: Coating, Laminating, and Printing --- Tim Walker @ 08:56 AM

This question came in to me recently regarding nip uniformity in printing nips. My answer applies to any nipped process.

In our operation for 8 color rotogravure presses, sometimes the pressure in the printing unit nip area varies on both sides of impression roller resulting in the wrinkles/misregistration. To eliminate the problem, we balance the impression roller by different mechanical methods and run the machine. However, we only discover the problem once the machine runs.

Is there anyway of checking the balance of nip pressure on both sides at each printing unit nip, maybe with the help of some mechanical guage putting between the cylinder and impression roller on each side and applying the pneumatic pressure?

My answer:

There are simple and complex ways to measure nipping roller uniformity. Almost all are static measurements, so they don’t indicate variations over time or differences through the roller rotations (unless you take multiple measurements).

The first option many people use is to measure nip footprint length (sometimes called width) in the machine direction. You can measure nip width by inserting Post-It ™ notes from either side of the closed nip, then open the nip to measure the gap between the opposing notes. Many people use knurled aluminum foil (called Sto-Foil) from Stowe-Woodward. There are other nip footprint measuring options, including carbon paper, Fuji Prescale film (sold as Pressurex by Sensor Products), and electronic thin film sensor options from Tekscan and Sensor Products.

Force can be measured indirectly with a frictional device (made of a brass-steel sandwich) and a force gauge or with the Tekscan or Sensor Product devices.

You could also consider installing a load cell in your nipping assembly to measure the force exerted by any external air cylinders (and assume roll weight is constant).

I recommend nip sensing products from both Tekscan (www.tekscan.com), and Sensor Products, Inc. (www.sensorprod.com/nip.php)

I would recommend first trying simple nip width measurement (like the Post-It or carbon paper methods) or the frictional brass-steel system.

-tjw