In today's flexographically printed packaging marketplace, there are more and more applications moving toward water-based inks. This is because of the increased technology in water-ink chemistry and also environmentally driven issues. For this reason the possibility of running water and alcohol chemistries on the same machine becomes very real.

If the press has been running alcohol inks and the decision has been made to run water, the press manufacturer should be contacted. In some cases, the print drum or impression roller will need a special coating to prevent rust and pitting. The plate cylinders also may have to undergo some sort of sealing.

Running water-based chemistries requires more CFMs (cubic ft/min) out of the between-color and overhead dryers than alcohol inks require. Water has a vapor pressure, which will quickly saturate the air molecules in the laminar airflow, hindering further evaporation. Higher air velocities will strip away those molecules, allowing for more efficient evaporation.

Because water and alcohol inks have radically different chemistries, great care must be taken to avoid contamination. If even a small amount of water-based ink were to get into alcohol ink, it would turn the alcohol ink into a mixture with the consistency of a not-completely-set Jell-O. If a small amount of alcohol ink contaminates a water-based ink (even though there is some alcohol present in most water ink formulas), a precipitate will be generated. In both these scenarios, clogging of filters and hoses will occur and doctor-blade chambers will become plugged.

It is strongly recommended that at changeover all hoses that come in contact with the ink be replaced—not just flushed. It's also recommended that the press maintain a set of ink pumps dedicated to water inks and another set dedicated to alcohol-based inks. While it represents an added expense, the cost will quickly be justified by the savings in potential downtime in avoiding contamination.

Also, all of the doctor-blade assemblies will need to be completely stripped, cleaned, and new seals and blades installed. Because this can also generate quite a bit of downtime during a changeover, the purchase of multiple spare chambers should be considered.

That takes care of most of the components that will come in contact with the ink, but the anilox roll must not be forgotten. If the same rollers are to be used between water and alcohol, there must be a very good cleaning and verification program in place. Cleaning anilox rolls becomes more of a challenge when they begin to plug with layers of both water and alcohol inks.

Avoiding contamination between the alcohol and water flexo inks is the easy part. The biggest challenge facing the printer who wants to use both ink systems is the re-education of press operators. The ink systems are totally different from each other and so is the way each is handled and controlled at press.

The resolubility reaction of alcohol-based inks is, for the most part, reversible, so it is very forgiving. As long as acetate percentages and viscosities stay within the recommended ranges as supplied by the ink manufacturer, very few plugging or dirt problems occur.

If viscosity needs adjusting, a fair amount of solvent must be added to make the move.

Viscosity shifts are quite fast because of the high evaporation rates of the co-solvents present in the system. In fact, for high-quality, consistent printing, automatic viscosity controls are highly recommended.

What the operator must be keenly aware of, however, are the differing evaporation rates of the various solvents, and the fact that they must be accounted for when adding solvent blends to maintain running viscosity levels.

Water-based inks are not as forgiving. If the resolubility reaction is allowed to progress too far, it is not reversible, and ink will need to be changed, and aniloxes and plates will need to be cleaned. Anilox-roll plugging is much more prevalent in water systems than for alcohol inks. The key to controlling a water-based ink is in the pH measurement; the resolubility reaction depends on it. Viscosity is dependant for the most part on pH.

As was mentioned above, it takes a fair amount of solvent to move the viscosity of an alcohol-based ink. It takes much less solvent to move the viscosity of a water system. For example, to reduce the viscosity of an alcohol ink five seconds it may take up to a quart of solvent. To move the viscosity the same five seconds with a water system, it may take only a couple ounces of a pH-adjusted solution.

Water and alcohol inks can be run on the same machines. To accomplish this, there must be a strong commitment from all involved to make sure the press can handle both systems. Capital will need to be spent in equipment modification and investments in spare chambers and pumps.

To truly be successful, it will require great commitment from management and staff to attain the education necessary to run the different systems and then to efficiently apply what has been learned.