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magazine article – Where the rubber meets the chemicals

Thursday, 08 February 2007
By Print 21 Online Article
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The old saying that for the want of a nail the kingdom was lost is a salutary reminder about the importance of paying attention to the smallest detail in order to avoid a more serious outcome. The same lesson applies in printing where quality results depend on the predictable interaction of different elements over time.

It’s an approach that Mitch Mulligan at Böttcher Australia has been arguing from the perspective of consumables usage in the industry and, in particular, the need for all users to ensure compatibility between the various chemistries and their components.
“Typically not enough thought is given to the nature and quality of the chemicals used on the press, their compatibility and, in particular, how they will interact with the other components of the press, especially blankets and rollers,” he says.

“It’s remarkable, given the multi-million dollar investments in press hardware, how some operators will still seek to save a few dollars here and there on consumables without knowing what effect this will have on the press.”

Shrink or swell
In simple terms, whenever a rubber compound comes into contact with chemicals, such as inks and washes, two processes take place: substances leach into the rubber matrix from the chemical, causing the rubber to swell, and plasticisers are extracted from the rubber causing it to shrink.

Both these process take place at the same time and at differing rates depending on various factors such as the operating temperature, the length of exposure and the individual characteristics of the rubber compound. This makes calculating the overall effect on rollers and blankets difficult to determine, but there’s no doubt that the impact on print quality can be profound.

For instance, rubber swelling on a roller increases the dynamic load on that roller causing heat build-up. This in turn leads to a reduction in ink viscosity and can adversely affect the ink-water balance.

“It also means the press requires more energy to drive the rollers than should be the case,” says Mulligan. “It’s like driving with one foot on the brake so you can imagine how much damage it can cause. The end result is increased plate wear and possibly the eventual destruction of the rollers.”

Roller shrinkage causes its own set of problems, not the least being uneven and/or reduced ink transfer. The only way to compensate for this is to reset the rollers, a time-consuming and costly process. It can also cause the rubber compound to become increasingly hard until eventually the rollers have to be replaced.

Life isn’t easy
Life would be straightforward if roller swelling and shrinkage simply cancelled each other out but, of course, nothing is that easy. Both processes may be taking place at the same time on a roller but the net effect is often in one direction, usually shrinkage. That’s because the plasticisers that are extracted out of the rubber cannot be replaced whereas some of the chemicals that leach into the rubber will dissolve out again. Then again, depending on the circumstance, swelling might predominate over shrinkage.

Determining the rate and extent of these changes is the hard part. It’s not sufficient to simply assess each individual chemical and say they will cause such-and-such amount of shrinkage or swelling on a particular rubber compound. What counts is how the chemicals interact on the press.

“Printer’s shouldn’t be using their presses as test equipment. It’s too costly and time-consuming in today’s market,” says Mulligan. “Equally, the worst time to find out there’s something wrong is when it’s too late to do anything about it.”

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