Consider one other scenario. A job has been color matched and brought to full speed. It’s expected to take 12 hours to produce 650,000-ft. of shippable product, allowing ±10 percent for total waste. Several hours into the run, a color’s density is dropping. This is a common problem resulting from an inability to replace all of the air trapped in a cell with fresh ink. The ink’s density is further diminished by the amount of air that does make it out of the cell aerating the ink. Over time, these micro bubbles add up.
While other options are available, a quick operator response to dropping color density is to increase ink delivery to the chamber to try to force an exchange of air-for-ink in the cell. The seals on the now pressurized chamber begin to leak, prompting a tightening of the chamber to the anilox. Guess what? It seems to have worked. Density has returned. But not for the intended reason. Instead of forcing more ink into an anilox cell:
- The doctor blade has buckled so its side, not its metering edge, is against the anilox
- The blade is no longer sheering ink
- A far wider blade-to-anilox contact area is allowing ink to build up beneath the blade’s side
- Hydraulic blade lift opens the flood gate
- The anilox surface ink film increases to the point that dirty print and dot gain appear
- At the same time, the thicker ink film overwhelms the containment blade. Instead of re-entering the chamber, it falls from the chamber filling the “drip” pan
Wide web pressrooms have reported wasting $300,000+ per press annually to back doctoring. In addition, data collected documents hundreds of hours of lost time a year.
Chamber Designs
Increasing chamber pressure is not the answer to starvation. Instead, the solution has been found by modifying the dimension and shape of anilox cells, ink chemistry and chamber design.
Ink flow is now regulated by pumping ink in both directions. Pumping into and out from the chamber prevents pressure build up. Most newer OEM designs also incorporate at least one air vent to provide air released from returning anilox cells a place to go. Chambers have been fortified to maintain maximum rigidity across the entire anilox, regardless of length. Blades can be set and maintained at optimum contact angles while at low pressure without concern for mid-chamber bowing.
Many pressrooms using low pressure, enclosed inking technology report minimum-to-no starvation concerns, while blade and seal life has increased. They can hold a clean dot at high speeds for multiple shifts without stopping. Ink drip “pans” are there to catch an occasional “drip.”
While there are still many conventional chambered inking systems in use, retrofit low-pressure inking systems are available to replace them. One Sweden-based engineering group, FLEXOART AB, has taken the low-pressure design to the next level. Specifically:
- It increased the chamber’s capacity and widened the gap between containment and doctor blade to improve air-for-ink exchange and to isolate aerated ink away from the anilox
- Custom designs combine a lightweight inner cavity with an outer chamber. The small amount of ink that may make it past the inner seals is captured in the outer chamber and channeled back into circulation, without exposure to the atmosphere or contamination. This design eliminates both the physical and perceived need to increase chamber to anilox pressure
FLEXOART is one of several companies providing flexographic pressrooms with retrofit replacement chambers. It reports having replaced more than 4,000 inking systems on familiar wide and narrow web flexographic presses around the world.
There are still thousands of conventional chambered inking systems in use throughout the industry. Retrofitting them with new low-pressure, no-leak inking systems could be a low-cost, profit-boosting alternative to a new press.
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